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Transcript
Handbook of
Plastic Films
Editor:
Elsayed M. Abdel-Bary
Rapra Technology Limited
Handbook of
Plastic Films
Editor: E.M. Abdel-Bary
rapra
TECHNOLOGY
Rapra Technology Limited
Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom
Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118
http://www.rapra.net
First Published in 2003 by
Rapra Technology Limited
Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
©2003, Rapra Technology Limited
All rights reserved. Except as permitted under current legislation no part
of this publication may be photocopied, reproduced or distributed in any
form or by any means or stored in a database or retrieval system, without
the prior permission from the copyright holder.
A catalogue record for this book is available from the British Library.
Every effort has been made to contact copyright holders of any material
reproduced within the text and the authors and publishers apologise if
any have been overlooked.
ISBN: 1-85957-338-X
Typeset by Rapra Technology Limited
Cover printed by The Printing House, Crewe, UK
Printed and bound by Rapra Technology Limited, Shrewsbury, UK
Contents
1. Technology of Polyolefin Film Production ...................................................... 5
1.1
Introduction ........................................................................................... 5
1.2
Structures of the Polyolefins................................................................... 7
1.2.1
Low-Density Polyethylene (LDPE) ............................................. 7
1.2.2
High-Density Polyethylene (HDPE, MDPE, UHMWPE) ........... 8
1.2.3
Linear Low-Density Polyethylene (LLDPE) ................................ 8
1.2.4
Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE) ... 8
1.2.5
Polypropylene (PP) ..................................................................... 9
1.2.6
Polypropylene Copolymers ........................................................ 9
1.3
Morphology of Polyolefin Films ............................................................ 9
1.4
Rheological Characterisation of the Polyolefins ................................... 10
1.5
1.6
1.4.1
High-Density Polyethylene ....................................................... 10
1.4.2
Linear Low-Density Polyethylene ............................................ 11
1.4.3
Very- and Ultra-Low-Density Polyethylene .............................. 11
1.4.4
Low-Density Polyethylene, Long Branches .............................. 11
1.4.5
Polypropylene .......................................................................... 12
Blown Film Production (Tubular Extrusion) ........................................ 13
1.5.1
Extruder Characteristics .......................................................... 14
1.5.2
Screw Design ........................................................................... 15
1.5.3
Frost-line and Blow Ratio ........................................................ 15
Cast Film Production ........................................................................... 16
1.6.1
Extrusion Conditions ............................................................... 16
1.6.2
Calendering Finishing .............................................................. 17
1.6.3
Extrusion Coating .................................................................... 17
iii
Handbook of Plastic Films
1.7
1.8
1.9
Orientation of the Film ........................................................................ 18
1.7.1
Orientation During Blowing .................................................... 18
1.7.2
Orientation by Drawing........................................................... 18
1.7.3
Biaxial Orientation (Biaxially Oriented PP, BOPP) .................. 18
Surface Properties ................................................................................ 19
1.8.1
Gloss ........................................................................................ 19
1.8.2
Haze ........................................................................................ 20
1.8.3
Surface Energy ......................................................................... 20
1.8.4
Slip........................................................................................... 21
1.8.5
Blocking ................................................................................... 21
Surface Modification ........................................................................... 21
1.9.1
Corona Discharge .................................................................... 21
1.9.2
Antiblocking ............................................................................ 22
1.9.3
Slip Additives ........................................................................... 23
1.9.4
Lubricants ................................................................................ 24
1.9.5
Antistatic Agents ...................................................................... 24
1.10 Internal Additives ................................................................................ 24
1.10.1 Antioxidants ............................................................................ 24
1.10.2 Ultraviolet Absorbers ............................................................... 24
1.11 Mechanical Properties .......................................................................... 25
1.11.1 Tensile Properties ..................................................................... 26
1.11.2 Impact Properties ..................................................................... 28
1.11.3 Dynamic Mechanical Properties .............................................. 29
1.11.4 Dielectric Properties ................................................................. 30
1.12 Microscopic Examination .................................................................... 31
1.12.1 Optical – Polarised Light Effect with Strain ............................. 31
1.12.2 Scanning Electron Microscopy (SEM) – Etching ...................... 31
1.12.3 Atomic Force Microscopy (AFM) ............................................ 31
1.13 Thermal Analysis ................................................................................. 31
1.13.1 Differential Scanning Calorimetry (DSC) ................................. 31
iv
Contents
1.13.2 Temperature-Modulated DSC (TMDSC) ................................. 32
1.14 Infrared Spectroscopy .......................................................................... 32
1.14.1 Characterisation ...................................................................... 32
1.14.2 Composition Analysis of Blends and Laminates....................... 33
1.14.3 Surface Analysis ....................................................................... 33
1.14.4 Other Properties ...................................................................... 34
1.15 Applications ......................................................................................... 35
1.15.1 Packaging ................................................................................ 35
1.15.2 Laminated Films ...................................................................... 36
1.15.3 Coextruded Films .................................................................... 37
1.15.4 Heat Sealing ............................................................................. 38
1.15.5 Agriculture ............................................................................... 38
1.16 Conclusion ........................................................................................... 38
2. Processing of Polyethylene Films ................................................................... 41
2.1
Introduction ......................................................................................... 41
2.2
Parameters Influencing Resin Basic Properties ..................................... 42
2.3
2.2.1
Molecular Weight (Molar Mass) and Dispersity Index ............ 42
2.2.2
Melt Index (Flow Properties) ................................................... 42
2.2.3
Density .................................................................................... 44
2.2.4
Chain Branching ...................................................................... 45
2.2.5
Intrinsic Viscosity .................................................................... 46
2.2.6
Melting Point and Heat of Fusion ............................................ 47
2.2.7
Melt Properties – Rheology ..................................................... 48
2.2.8
Elongational Viscosity ............................................................. 49
2.2.9
Elasticity .................................................................................. 49
Blown Film Extrusion (Tubular Film) .................................................. 50
2.3.1
Introduction ............................................................................. 50
2.3.2
Description of the Blown Film Process ..................................... 50
2.3.3
Various Ways of Cooling the Film ........................................... 51
v
Handbook of Plastic Films
2.4
2.3.4
Extruder Size ........................................................................... 54
2.3.5
Horsepower ............................................................................. 55
2.3.6
Selection of Extrusion Equipment ............................................ 55
Cast Film Extrusion ............................................................................. 57
2.4.1
Description of the Cast Film Process ........................................ 57
2.4.2
Effects of Extrusion Variables on Film Characteristics ............. 58
2.4.3
Effect of Blow-up Ratio on Film Properties ............................. 61
2.5
Processing Troubleshooting Guidelines ................................................ 62
2.6
Shrink Film .......................................................................................... 62
2.6.1
Shrink Film Types .................................................................... 65
2.6.2
Shrink Film Properties ............................................................. 66
2.6.3
The Manufacture of Shrink Film ............................................. 67
2.6.4
Shrink Tunnels and Ovens ....................................................... 70
3. Processing Conditions and Durability of Polypropylene Films ...................... 73
3.1
Introduction ......................................................................................... 73
3.2
Structures and Synthesis ....................................................................... 78
3.3
Film Processing .................................................................................... 85
3.4
Additives .............................................................................................. 85
3.5
Ultraviolet Degradation of Polypropylene ............................................ 86
3.6
3.7
vi
3.5.1
UV Degradation Mechanisms .................................................. 86
3.5.2
Effect of UV Degradation on Molecular Structure
and Properties of PP................................................................. 87
3.5.3
Stabilisation of PP by Additives ............................................... 88
Case Studies ......................................................................................... 90
3.6.1
Materials and Experimental Procedures ................................... 90
3.6.2
Durability-Microstructure Relationship ................................... 91
3.6.3
Durability-Processing Condition Relationship ......................... 94
3.6.4
Durability-Additive Property Relationship ............................... 97
Concluding Remarks ......................................................................... 101
Contents
4. Solubility of Additives in Polymers.............................................................. 109
4.1
Introduction ....................................................................................... 109
4.2
Nonuniform Polymer Structure.......................................................... 109
4.3
Additive Sorption ............................................................................... 110
4.4
Quantitative Data on Additive Solubility in Polymers ....................... 114
4.5
Factors Affecting Additive Solubility ................................................. 118
4.6
4.5.1
Crystallinity and Supermolecular Structure............................ 118
4.5.2
Effect of Polymer Orientation ................................................ 119
4.5.3
Role of Polymer Polar Groups ............................................... 120
4.5.4
Effect of the Second Compound ............................................ 121
4.5.5
Features of Dissolution of High Molecular Weight Additives .. 122
4.5.6
Effect of Polymer Oxidation .................................................. 124
Solubility of Additives and Their Loss ............................................... 125
5. Polyvinyl Chloride: Degradation and Stabilisation ...................................... 131
5.1
Introduction ....................................................................................... 131
5.2
Some Factors Affecting the Low Stability of PVC .............................. 132
5.3
Identification of Carbonylallyl Groups .............................................. 136
5.4
Principal Ways to Stabilise PVC ......................................................... 138
5.5
Light Stabilisation of PVC ................................................................. 144
5.6
Effect of Plasticisers on PVC Degradation in Solution ....................... 145
5.7
‘Echo’ Stabilisation of PVC ................................................................ 151
5.8
Tasks for the Future ........................................................................... 153
6. Ecological Issues of Polymer Flame Retardants ........................................... 159
6.1
Introduction ....................................................................................... 159
6.2
Mechanisms of Action ....................................................................... 160
6.3
Halogenated Diphenyl Ethers – Dioxins ............................................ 162
vii
Handbook of Plastic Films
6.4
Flame Retardant Systems ................................................................... 166
6.5
Intumescent Additives ........................................................................ 168
6.6
Polymer Organic Char-Former ........................................................... 175
6.7
Polymer Nanocomposites .................................................................. 180
7. Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres . 187
7.1
Introduction ....................................................................................... 187
7.2
Interaction of Nitrogen Dioxide with Polymers ................................. 188
7.2.1
Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF ...... 188
7.2.2
Non-Saturated Polymers ........................................................ 191
7.2.3
Polyamides, Polyurethanes, Polyamidoimides ........................ 196
7.3
Reaction of Nitric Oxide with Polymers ............................................ 202
7.4
Conclusion ......................................................................................... 209
8. Modifications of Plastic Films ..................................................................... 213
8.1
Introduction ....................................................................................... 213
8.2
Modification of Mechanical Properties .............................................. 213
8.3
8.4
viii
8.2.1
Orientation ............................................................................ 214
8.2.2
Crystallisation ........................................................................ 214
8.2.3
Crosslinking ........................................................................... 214
Chemical Modifications ..................................................................... 215
8.3.1
Fluorination ........................................................................... 215
8.3.2
Chlorination .......................................................................... 217
8.3.3
Bromination ........................................................................... 217
8.3.4
Sulfonation ............................................................................ 218
8.3.5
Chemical Etching ................................................................... 218
8.3.6
Grafting ................................................................................. 220
Physical Methods Used for Surface Modification............................... 222
8.4.1
Plasma Treatment .................................................................. 222
8.4.2
Corona Treatment ................................................................. 223
Contents
8.5
8.6
Characterisation ................................................................................ 224
8.5.1
Gravimetric Method .............................................................. 224
8.5.2
Thermal Analyses .................................................................. 225
8.5.3
Scanning Electron Microscopy ............................................... 225
8.5.4
Swelling Measurements .......................................................... 226
8.5.5
Molecular Weight and Molecular Weight Distribution .......... 226
8.5.6
Dielectric Relaxation ............................................................. 226
8.5.7
Surface Properties .................................................................. 227
8.5.8
Spectroscopic Analysis ........................................................... 227
8.5.9
Electron Spectroscopy for Chemical Analysis (ESCA)
or X-Ray Photoelectron Spectroscopy (XPS) ......................... 228
Applications ....................................................................................... 228
9. Applications of Plastic Films in Packaging .................................................. 235
9.1
Introduction ....................................................................................... 235
9.2
Packaging Functions .......................................................................... 235
9.3
Flexible Package Forms ...................................................................... 236
9.3.1
Wraps .................................................................................... 237
9.3.2
Bags, Sacks and Pouches ........................................................ 238
9.3.3
Pouch Production .................................................................. 239
9.3.4
Dispensing and Reclosure Features ........................................ 239
9.4
Heat-Sealing ...................................................................................... 240
9.5
Other Uses of Packaging Films........................................................... 241
9.6
Major Packaging Films ...................................................................... 241
9.6.1
Low-Density Polyethylene (LDPE) and Linear
Low-Density Polyethylene (LLDPE) ....................................... 242
9.6.2
High-Density Polyethylene (HDPE) ....................................... 243
9.6.3
Polypropylene (PP) ................................................................. 244
9.6.4
Polyvinyl Chloride (PVC)....................................................... 245
9.6.5
Polyethylene Terephthalate (PET) .......................................... 245
ix
Handbook of Plastic Films
9.6.6
Polyvinylidene Chloride (PVDC) ........................................... 246
9.6.7
Polychlorotrifluoroethylene (PCTFE) ..................................... 247
9.6.8
Polyvinyl Alcohol (PVOH) .................................................... 248
9.6.9
Ethylene-Vinyl Alcohol (EVOH) ............................................ 248
9.6.10 Polyamide (Nylon) ................................................................. 249
9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films ..... 250
9.6.12 Ionomers ................................................................................ 251
9.6.13 Other Plastics ......................................................................... 251
9.7
Multilayer Plastic Films ..................................................................... 252
9.7.1
Coating .................................................................................. 252
9.7.2
Lamination ............................................................................ 253
9.7.3
Coextrusion ........................................................................... 253
9.7.4
Metallisation .......................................................................... 253
9.7.5
Silicon Oxide Coating ............................................................ 254
9.7.6
Other Inorganic Barrier Coatings .......................................... 255
9.8
Surface Treatment .............................................................................. 255
9.9
Static Discharge ................................................................................. 256
9.10 Printing .............................................................................................. 256
9.11 Barriers and Permeation ..................................................................... 257
9.12 Environmental Issues ......................................................................... 261
10. Applications of Plastic Films in Agriculture ................................................ 263
10.1 Introduction ....................................................................................... 263
10.2 Production of Plastic Films ................................................................ 263
10.3 Characteristics of Plastic Films Used in Agriculture ........................... 264
10.4 Stability of Greenhouse Films to Solar Irradiation ............................. 265
10.4.1 Ultraviolet Stabilisers ............................................................. 265
10.4.2 Requirements for Stabiliser Efficiency .................................... 268
10.4.3 Evaluation of Laboratory and Outdoor Photooxidation ........ 271
x
Contents
10.5
10.6
10.7
Other Factors Affecting the Stability of Greenhouse Films .............. 272
10.5.1
Temperature ..................................................................... 272
10.5.2
Humidity .......................................................................... 273
10.5.3
Wind ................................................................................ 273
10.5.4
Fog Formation ................................................................. 273
10.5.5
Environmental Pollution .................................................. 274
10.5.6
Effects of Pesticides .......................................................... 274
Ageing Resistance of Greenhouse Films .......................................... 275
10.6.1
Measurement of Ageing Factors ....................................... 275
10.6.2
Changes in Chemical Structure......................................... 276
Recycling of Plastic Films in Agriculture ......................................... 277
10.7.1
Introduction ..................................................................... 277
10.7.2
Contamination by the Environment ................................. 277
11. Physicochemical Criteria for Estimating the Efficiency of Burn Dressings ... 285
11.1
Introduction .................................................................................... 285
11.2
Modern Surgical Burn Dressings ..................................................... 286
11.3
11.4
11.2.1
Dressings Based on Materials of Animal Origin ............... 286
11.2.2
Dressings Based on Synthetic Materials ............................ 286
11.2.3
Dressings Based on Materials of Vegetable Origin ........... 290
Selection of the Properties of Tested Burn Dressings ....................... 290
11.3.1
Sorption-Diffusion Properties ........................................... 291
11.3.2
Adhesive Properties .......................................................... 292
11.3.3
Mechanical Properties ...................................................... 292
Methods of Investigation of Physicochemical Properties of
Burn Dressings ................................................................................ 292
11.4.1
Determination of Material Porosity ................................. 292
11.4.2
Determination of Size and Number of Pores .................... 293
11.4.3
Estimation of Surface Energy at Material-Medium
Interface ........................................................................... 294
11.4.4
Determination of Sorptional Ability of Materials ............. 294
xi
Handbook of Plastic Films
11.5
11.6
11.7
11.8
11.4.5
Determination of Air Penetrability of Burn Dressings ...... 295
11.4.6
Determination of Adhesion of Burn Dressings ................. 296
11.4.7
Determination of Vapour Penetrability of Burn Dressings .. 296
Results and Discussion .................................................................... 297
11.5.1
Determination of Sorption Ability of Burn Dressings ....... 297
11.5.2
Kinetics of the Sorption of Liquid Media by
Burn Dressings ................................................................. 303
11.5.3
Determination of Vapour Penetrability of Burn Dressings .. 305
11.5.4
Determination of the Air Penetrability of Burn Dressings .. 308
11.5.5
Determination of Adhesion of Burn Dressings ................... 315
The Model of Action of a Burn Dressing ........................................ 318
11.6.1
Evaporation of Water from the Dressing Surface ............. 318
11.6.2
Sorption of Fluid by Burn Dressing from Bulk
Containing a Definite Amount of Fluid ............................ 320
11.6.3
Mass Transfer of Water from Wound to Surroundings ..... 321
Criteria for the Efficiency of First-Aid Burn Dressings .................... 322
11.7.1
Requirements of a First-Aid Burn Dressing ...................... 322
11.7.2
Characteristics of First-Aid Burn Dressings ...................... 322
Conclusion ...................................................................................... 324
12. Testing of Plastic Films ................................................................................ 329
12.1
Introduction .................................................................................... 329
12.2
Requirements for Test Methods ...................................................... 330
12.3
12.4
xii
12.2.1
List of Requirements ........................................................ 330
12.2.2
Interpretation of Test Results ........................................... 330
Some Properties of Plastic Films ...................................................... 332
12.3.1
Dimensions ...................................................................... 332
12.3.2
Conditioning the Samples ................................................. 332
Mechanical Tests ............................................................................. 333
12.4.1
Tensile Testing (Static) ...................................................... 333
12.4.2
Impact Resistance ............................................................. 336
Contents
12.5
12.4.3
Tear Resistance ................................................................. 337
12.4.4
Bending Stiffness (Flexural Modulus) ............................... 339
12.4.5
Dynamic Mechanical Properties ....................................... 339
Some Physical, Chemical and Physicochemical Tests ....................... 340
12.5.1
Density of Plastics ............................................................ 340
12.5.2
Indices of Refraction and Yellowness ............................... 340
12.5.3
Transparency .................................................................... 341
12.5.4
Resistance to Chemicals ................................................... 341
12.5.5
Haze and Luminous Transmittance .................................. 341
12.5.6
Ignition, Rate of Burning Characteristics and
Oxygen Index (OI) ........................................................... 342
12.5.7
Static and Kinetic Coefficients of Friction ........................ 342
12.5.8
Specular Gloss of Plastic Films and Solid Plastics ............. 343
12.5.9
Wetting Tension of PE and PP Films ................................. 344
12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films .. 345
12.5.11 Shrink Tension and Orientation Release Stress ................. 345
12.5.12 Rigidity ............................................................................ 345
12.5.13 Blocking Load by Parallel-Plate Method .......................... 346
12.5.14 Determination of LLDPE Composition by 13C NMR ..... 346
12.5.15 Creep and Creep Rupture ................................................. 346
12.5.16 Outdoor Weathering/Weatherability ................................ 347
12.5.17 Abrasion Resistance ......................................................... 347
12.5.18 Mar Resistance ................................................................. 348
12.5.19 Environmental Stress Cracking......................................... 348
12.5.20 Water Vapour Permeability .............................................. 348
12.5.21 Oxygen Gas Transmission ................................................ 349
12.6
Standard Specifications for Some Plastic Films ................................ 349
12.6.1
Standard Specification for PET Films ............................... 350
12.6.2
Standard Specification for LDPE Films (for General
Use and Packaging Applications) ..................................... 350
12.6.3
Standard Specification for MDPE and General Grade
PE Films (for General Use and Packaging Applications) ... 350
xiii
Handbook of Plastic Films
12.6.4
Standard Specification for OPP Films ............................... 351
12.6.5
Standard Specification for Crosslinkable Ethylene Plastics . 351
13. Recycling of Plastic Waste ........................................................................... 357
13.1
Introduction .................................................................................... 357
13.2
Main Approaches to Plastic Recycling ............................................ 358
13.3
Primary Recycling ............................................................ 358
13.2.2
Secondary Recycling ......................................................... 358
13.2.3
Tertiary Recycling ............................................................ 359
13.2.4
Quaternary Recycling ....................................................... 360
13.2.5
Conclusion ....................................................................... 362
Collection and Sorting .................................................................... 362
13.3.1
Resin Identification .......................................................... 362
13.3.2
General Aspects of Resin Separation ................................ 363
13.3.3
Resin Separation Based on Density .................................. 364
13.3.4
Resin Separation Based on Colour ................................... 365
13.3.5
Resin Separation Based on Physicochemical Properties .... 365
13.4
Recycling of Separated PET Waste .................................................. 367
13.5
Recycling of Separated PVC Waste ................................................. 368
13.6
13.7
xiv
13.2.1
13.5.1
Chemical Recycling of Mixed Plastic Waste ..................... 369
13.5.2
Chemical Recycling of PVC-Rich Waste ........................... 370
Recycling of Separated PE Waste .................................................... 371
13.6.1
Contamination of PE Waste by Additives ......................... 372
13.6.2
Contamination of PE Waste by Reprocessing ................... 372
Recycling of HDPE ......................................................................... 373
13.7.1
Applications for Recycled HDPE ..................................... 373
13.7.2
Rubber-Modified Products ............................................... 373
13.8
Recycling Using Radiation Technology ........................................... 373
13.9
Biodegradable Polymers .................................................................. 374
Preface
The plastic industry continues to grow very rapidly and plays an important role in
many fields such as engineering, medical, agriculture and domestic. It is now very
difficult to find the point at which plastic cannot be considered as an essential
component. The understanding of the nature of plastic films, their production
techniques, applications and their characterisation is essential for producing new
types of plastic films. This handbook has been written to discuss the production and
main uses of plastic films.
Chapter 1 deals with the various types of polyolefins and their suitability for film
manufacture. The rheology, structure and properties of the polymers are discussed in
relation to the type of film manufacturing processes that are most applicable to the
types of polymer. Post-extrusion modifications of the films such as orientation, surface
chemistry and additives are discussed. Characterisation methods used to measure
film mechanical properties; structure and additives are described, as well as other
more specific properties. Finally some particularly important applications that require
special structures or modifications are given.
In Chapter 2, the main parameters influencing resin basic properties are described.
The methods of processing of polyethylene films such as cast film extrusion, blow
extrusion of tubular films are discussed. Effects of extrusion variables on film
characteristics and effect of blow ratio on film properties are considered.
Chapter 3 details the structure, synthesis and film processing of polypropylene. The
effects of some additives and UV stabilisers are discussed.
The solubility of additives plays an important role in determining the efficiency and
the properties of the films as well. For this reason Chapter 4 deals with different
aspects of additives solubility in polymers in relation to the polymer degradation
and stabilisation.
The topic covered in Chapter 5 is the stability of polyvinyl chloride (PVC) films
during procesing and service. The possibility of increasing the intrinsic stability of
PVC during processing with the minimal contents or in total absence of stabilisers
and other additives is discussed.
1
Handbook of Plastic Films
Chapter 6 discusses flame retardants, which as special additives have an important
role in saving lives. These flame retardant system basically inhibit or even suppress
the combustion process by chemical or physical action in the gas or condensed phase.
Conventional flame retardants have a number of negative attributes and the ecological
issues surrounding their applications are driving the search for new polymer flame
retardant systems forward.
Chapter 7 covers thermal and photochemical oxidation of polymers under the
influence of the aggressive, polluting atmospheric gases. Among pollutants, sulfur
dioxide, ozone, nitrogen oxides stand out as the most deleterious impurities of
atmosphere. Thus, this chapter is devoted to consideration of the results obtained in
studies of interactions of nitrogen oxides with polymers.
Chapter 8 discusses the modifications of plastic films to improve their mechanical or
physical properties to meet the requirements of certain applications. This can be
achieved by subjecting the films to mechanical or chemical treatments. A number of
surface modification techniques such as plasma, corona discharge and chemical
treatments have been used.
Chapter 9 deals with applications of plastic films in packaging. A description of the
properties of the most common films used in packaging such as low-density
polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density
polyethylene (HDPE), polypropylene (PP), PVC, polyvinylidene chloride (PVDC),
polyamide (Nylon), and other plastics are given in this chapter.
Chapter 10 deals with the application of plastic films in agriculture. The mechanical
properties suitable that make these films suitable for use in agriculture are discussed.
Stability of these plastic films under the effect of different environmental conditions
is reported. Types of UV stabilisers and their compatibility with plastic are given.
Also, recycling of plastic films used in agriculture is of great importance and finally,
a case study of their reuse as agriculture films is given.
Chapter 11 deals with the principal medical treatment of burns using dressings made
with a polymeric layer or layers. It is difficult to estimate the effectiveness of the new
burn dressings, as their physicochemical properties are not usually presented in
literature. Thus, chapter 11 discusses this subject for the first time. The
physicochemical criteria for estimating the efficiency of burn dressings and the
possibility of using plastic films is given.
Chapter 12 covers the most common test methods generally used for plastic films.
The requirements necessary for the test methods are summarised.
2
Preface
The problem of plastic films recycling is touched on in Chapter 13. The majority of
plastic films are made from polyethylene (LDPE, LLDPE or HDPE) which comprise
approximately 68% of the total film production. Non-polyethylene resins constitute
the remainder of the plastic film. Different types of recycling are given and recycling
of some selected types of films are discussed.
This handbook represents the efforts of many experts in different aspects of plastic
films. Their efforts in preparing contributions to the volume are to be noted and I
take the opportunity to express my heartfelt gratitude for their time and effort. My
gratitude extends also to many colleagues for their kind comments in many aspects.
A special thanks is extended to the staff of Rapra Technology, for the fine production
of this Handbook, particularly Claire Griffiths, Editorial Assistant, Steve Barnfield
who typeset the book and designed the cover and Frances Powers who commissioned
the book and oversaw the whole project.
Elsayed M. Abdel-Bary
January 2003
3
Handbook of Plastic Films
4
1
Technology of Polyolefin Film Production
Robert Shanks
1.1 Introduction
A film is a two-dimensional form of a polymer. A film is typified by a large surface area
to volume ratio. Films are required to exhibit barrier properties to any contaminating
substances that may try to enter, or any desirable substances that may try to leave, across
the film. This property is resistance to diffusion. Since a film is very thin, it must have
high mechanical properties such as tensile strength, impact resistance and tear strength.
The mechanical properties usually depend on molecular structure, molar mass and molar
mass distribution. Visibility through a film is often important, so low haze will be required.
These are the bulk properties of the film [1].
The film will often be required to improve the appearance of an item contained within it,
so surface properties such as gloss and printability are important. The latter property,
printability, is related to a relatively high surface energy to achieve wetting and good
work of adhesion. Suitable surface energy may be achieved through modification.
Protection may also be improved if the friction is low; this property is called slip. When
a film is used to enclose and protect items, it may need to provide adhesion to itself or to
the contents. The immediate form of adhesion is called tack. Subsequently the polymer
must flow to provide complete adhesion.
Manufacture of a film will usually be through an extrusion of the melt, so the melt
rheology must be suited to the manufacturing process. Rheology is controlled by chemical
structure, molar mass and long branches. The way in which the film is extruded, extended
and solidified by cooling will control the microstructure and hence many of the properties.
A summary of the various polyolefins used in film manufacture is provided in Table 1.1.
In this chapter, polyolefin films are reviewed. First, the various types of polyolefins and
their suitability for film manufacture are considered. The rheology, structure and properties
of the polymers are discussed in relation to the type of film manufacturing processes that
are most applicable to the types of polymer. Post-extrusion modifications of the films,
such as orientation, surface chemistry and additives, are discussed. Characterisation
methods used to measure film mechanical properties, structure and additives are described,
as well as other more specific properties. Finally, some important particular applications
that require special structures or modifications are described.
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Handbook of Plastic Films
Table 1.1 Structure, properties and description of the polyolefins
used for film production
Polyolefin
Comonomer
Density
Preparation
method
Mechanical
properties
Comments
High-density or
linear
polyethylene
(HDPE, LPE)
No branches
0.94-0.96
Zeigler-Natta
High tensile
strength, low
impact strength
Brittle film, with
good gas barrier
properties
Low-density
polyethylene
(LDPE)
Random short
and long
branches
0.91
Radical, with
autoclave or
tubular
reactor
Non-Newtonian
melt rheology,
good impact
strength
Good blown
extrusion
characteristics for
flexible films
High-clarity,
glossy film,
difficult to
extrude
Linear lowdensity
polyethylene
(LLDPE)
1-Butene,
1-Hexene,
1-Octene
0.91-0.93
Zeigler-Natta
Intermediate
strength with
elasticity, melt
rheology more
Newtonian than
LDPE
Very-low-density
polyethylene
(VLDPE)
1-Butene,
1-Hexene,
1-Octene
0.89-0.91
Single-site
metallocene
Tough elastic,
moderate strength
High-clarity, very
glossy film, very
thin films possible
VLDPE with
long branches
1-Butene,
1-Hexene,
1-Octene
0.89-0.91
Constrained
geometry
single site
Tough elastic,
moderate
strength, nonNewtonian melt
rheology
Easy to process,
improved melt
strength
Ultra-low-density
polyethylene
(ULDPE),
plastomers
1-Butene,
1-Hexene,
1-Octene
Single-site
metallocene
Elastic, low
tensile strength,
low modulus
Thermoplastic
elastomer, narrow
low temperature
melting, good for
heat seal
Zeigler-Natta
High tensile
strength, brittle,
temperature
resistance
Transparent,
high-strength and
temperature-resistant glossy films
Zeigler-Natta
Tough with high
melting
temperature
(block) or softer
with lower melting
temperature
(random)
Tough films, with
more milky
colour
Single-site
metallocene
Narrow molar
mass distribution,
random
comonomer
distribution and
high isotacticity
Flexible, elastic
transparent films
Polypropylene
Polypropylene
copolymer with
ethylene, block
or random
Polypropylene
and copolymers
with ethylene
6
No branches
Ethylene
Ethylene
<0.89
0.90
0.90
0.90
Technology of Polyolefin Film Production
1.2 Structures of the Polyolefins
1.2.1 Low-Density Polyethylene (LDPE)
Molecular structures of some example polyethylenes are shown in Figure 1.1. LDPE
has rheological properties that are suitable for production of film by the blown film
process [2]. LDPE has some long branches and many short branches. Typically, there
may be three long branches and 30 short branches per molecule. The molar mass is
relatively low, and it has a broad molar mass distribution. The melt strength, or
zero-shear viscosity, and the shear-thinning nature of LDPE enhance processing. The
film has relatively low tensile strength but good impact strength. LDPE films show
good clarity (i.e., low haze) and gloss. The good clarity and gloss result from relatively
low crystallinity. LDPE is polymerised by the high-pressure radical process. There
are two main reactor types, the autoclave and the tubular reactor. The autoclave
tends to provide more branching and broader molar mass distribution. LDPE has a
broad melting range, with a peak melting temperature of 110 °C. The density may
vary from 0.915 to 0.930 g/cm3 for LDPE.
Figure 1.1 Molecular structures for linear and branched polyethylenes (LPE and BPE)
with 100 monomers, four or eight short branches, and one long branch of 40 carbons:
(a) LPE (100); (b) BPE (4, 100); (c) BPE (8, 100); and (d) BPE (8, L1(40), 100)
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Handbook of Plastic Films
1.2.2 High-Density Polyethylene (HDPE, MDPE, UHMWPE)
HDPE has a linear structure, with little or no branching. HDPE is typically formed by
the Ziegler-Natta, Phillips or Unipol processes. Each process involves relatively low
pressure and is catalysed by an organometallic complex with a transition metal.
Polymerisation is usually performed in slurry with a liquid such as heptane, or in the gas
phase with the catalyst in a fluidised bed form. Variations of HDPE are ultra-high molar
mass polyethylene (ultra-high molecular weight polyethylene, UHMWPE), where the
molar mass is of the order of 1,000,000 g/mol, and medium-density polyethylene (MDPE),
where some short branches are introduced by copolymerisation with a 1-alkene, such as
1-butene. HDPE has higher crystallinity and therefore shows higher tensile strength than
LDPE, though its impact strength is deficient for many applications. UHMWPE provides
increased tensile strength due to the longer molecules providing more tie molecules between
crystals. MDPE provides better impact strength because of its reduced crystallinity. HDPE
shows a more Newtonian rheology than LDPE, and so is less suitable for extrusion
processing, by either the blown film or cast film processes [3].
1.2.3 Linear Low-Density Polyethylene (LLDPE)
LLDPE is a copolymer of ethylene and a 1-alkene, typically 1-butene, 1-hexene or 1octene, though branched alkenes such as 4-methyl-1-pentene are also used. These polymers
have densities in the range 0.915-0.930 g/cm3 and they contain 2-7% (w/w) or about 12% mol/mol of the 1-alkene. They are polymerised using multisite catalysts such as
Ziegler-Natta with either a gas-phase or slurry process. Since the boiling temperature of
1-octene is too high for the gas-phase process, the slurry process must be used. The
comonomer composition has a broad distribution, so that some molecules, or segments
of molecules, have few branches while others have many branches. This distribution is
reflected in the broad melting temperature range of the LLDPE. The properties of LLDPE
tend to be in between those of LDPE and HDPE. They have short branches but not long
branches, so that crystallisation-dependent mechanical properties are improved, but
processing rheological properties are inferior to those of LDPE [4].
1.2.4 Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE)
Very- (density 0.89-0.915 g/cm3) (VLDPE) and ultra- (density <0.89 g/cm3) (ULDPE) lowdensity polyethylenes have higher copolymer content. ULDPE are also called polyolefin
elastomers (POE) because of their properties. These polyethylenes have recently been
commercialised as a result of the new metallocene catalyst technology that allows higher
comonomer levels and provides a narrower distribution of comonomer composition as
8
Technology of Polyolefin Film Production
well as of molar mass. These polymers have lower melting temperatures, less crystallinity,
greater toughness and elasticity, but lower tensile strength than other polyethylenes. They
mainly only have short branches, but some varieties also have some long branches [5].
1.2.5 Polypropylene (PP)
Isotactic polypropylene (iPP) is useful for temperature-resistant and glossy film production.
iPP has greater strength and higher melting temperature than any of the other polyolefins.
The crystal size can be made small by rapid cooling and/or nucleating agents, so that
highly transparent, high-gloss films can be produced. The rheological properties are not
ideal for the blown film process, but such processing is used in a two-stage extrusion and
blowing process. Syndiotactic PP (sPP) is now becoming available commercially as a result
of metallocene catalyst polymerisations. sPP provides more elastic films than iPP. iPP has
many advantages over polyethylenes because of its strength, thermal resistance, gloss and
clarity. It is particularly suitable for more durable products [6].
1.2.6 Polypropylene Copolymers
Copolymers of propylene with small amounts of ethylene (0-5% w/w) provide increased
toughness, at the expense of tensile strength. Random copolymers show the greatest property
changes, such as increased elasticity and a decrease in melting temperature. Copolymers with
more block-like structure, where the ethylene is distributed in some of the molecules or
molecular segments, provide a good compromise in properties between toughness and strength.
1.3 Morphology of Polyolefin Films
All the polyolefins are semicrystalline polymers. The crystallinity provides the tensile strength
but reduces the transparency. Larger crystals scatter transmitted light, producing an
opalescent appearance, known as haze. Crystals on the surface reduce the surface smoothness
and cause surface scattering of incident light and reduce the gloss. An example of the
morphology of a polypropylene film is provided in the optical microscope picture in Figure
1.2. Processing conditions can modify the natural tendency of each polyolefin to provide
these crystallisation-dependent properties. Rapid cooling will give smaller crystals. So the
use of cold rollers in the cast film process usually gives smaller crystals and in particular
greater surface smoothness. In the blown film process, the use of a refrigerated air stream
increases the crystallisation rate. Crystallisation is evident as a fogging of the film a short
distance from the extrusion die; this is called the frost-line height. Orientation can be
measured using wide-angle X-ray scattering (WAXS) [7].
9
Handbook of Plastic Films
Figure 1.2 Polarised optical microscope picture of polypropylene blended with 30%
poly(ethylene-co-propylene). The copolymer is a dispersed phase shown by the dark
regions mainly at the edges of the polypropylene spherulites
Orientation of crystals will direct the axes of the crystals, and correspondingly the crystaldependent properties, along the orientation or draw direction. Usually films are oriented,
or drawn, in two orthogonal directions, called biaxial drawing, first parallel to the
extrusion direction, then laterally. Drawing in the extrusion direction involves cooling
the melt until crystallisation takes place, then passing the film between rollers with
increasing differential speed. The lateral drawing depends on the method of manufacture.
When the blown film process is used, orientation is provided during the blowing process.
The cast film process requires a lateral drawing frame called a tenter. The edges of the
film are grasped and the frame moves apart as the film moves forward. Orientation
provides enhanced physical properties in the drawn directions. When the film is biaxially
drawn, the properties are greater in the direction that was drawn last [8].
1.4 Rheological Characterisation of the Polyolefins
1.4.1 High-Density Polyethylene
HDPE consists of linear molecules. The shear stress versus shear rate curve will be
approximately linear except for very high molar mass. The linear relationship is
Newtonian. This means that at high shear rates, as experienced in processing, the viscosity
10
Technology of Polyolefin Film Production
is high and so the force required for extrusion will be high. Another problem is that the
viscosity at low shear rates is not increased. This zero-shear viscosity is related to the
melt strength of the polymer. If the melt strength is low, the molten film may rupture as
it emerges from the extruder as a tube that is then rapidly expanded by a gas pressure.
High melt strength is required to resist rupture and create a dimensionally stable bubble.
The melt strength is less critical in the cast film process, although the film must remain
stable until it reaches the cooling rollers. The force required for extrusion will still be a
problem, since more energy will be needed to extrude a particular mass of polymer, and
this will require more electricity and a more powerful extruder motor. HDPE has high
tensile strength, but low impact and tear strengths, so damage during processing by
tearing is a potential problem. Processing of HDPE can be improved by blending with
other polyethylenes, in particular LDPE.
1.4.2 Linear Low-Density Polyethylene
LLDPE typically have a broad molar mass distribution and a broad distribution of the 1alkene comonomer, or branches. The tensile strength is lower than that of HDPE but
higher than for LDPE. They have improved toughness compared with HDPE. Though
they have short branching comparable with LDPE, they do not have long branches. The
lack of long branches decreases their shear-thinning rheological characteristics compared
with LDPE and so processing is not as efficient. They are often blended with LDPE since
the long branches enhance processing. They have greater tensile strength than LDPE,
but, with their higher crystallinity, they are less transparent.
1.4.3 Very- and Ultra-Low-Density Polyethylene
VLDPE and ULDPE have many short branches distributed along the main chains more
evenly than for LLDPE where single-site catalysts have been used in the polymerisation.
Short branches are not important in the rheology. These polymers will have essentially
Newtonian behaviour. Polymers with very high molar mass have more pronounced shear
thinning, though entanglements between only the main chains are not as effective as
between several long chains such as when long branches are present.
1.4.4 Low-Density Polyethylene, Long Branches
LDPE has long branches that are known to provide non-Newtonian rheological response.
LDPE is shear thinning, so that the power required for extrusion at typical high shear
rates is less than proportional to the shear rate. This makes extrusion of LDPE more
11
Handbook of Plastic Films
Figure 1.3 Parallel plate, continuous shear rheology curve for low-density
polyethylene at 200 °C
economical than for other polyethylenes, and the extruder motor does not need to be as
powerful. At low shear rates, the viscosity rises significantly, so the zero-shear viscosity,
or melt strength, is high. A typical rheology curve for LDPE is shown in Figure 1.3.
LDPE has better bubble strength in the blown film process, so that resistance to bursting
and bubble stability are greater prior to solidification. In the cast film process, the film
will be stable in the molten state between the extrusion die and the cold rollers. The long
branches provide more intermolecular entanglements when the shear rate is low. As the
shear rate increases, the long branches break free of entanglements and the viscosity
decreases markedly. These rheological characteristics are of prime importance during
processing [9].
1.4.5 Polypropylene
Polypropylene has a methyl branch on each monomer unit and so a pseudo-asymmetric
carbon is present. This introduces tacticity, and the isotactic form of polypropylene is
the only one that is suitable for film formation. Polymerisation is performed using ZieglerNatta and several other newer proprietary catalysts. The catalysts have been developed
12
Technology of Polyolefin Film Production
to provide maximum isotactic structure (>99%), so that atactic polypropylene does not
need to be extracted and the mechanical properties are maximised. High catalytic activity
is desired so that residual catalyst does not need to be extracted from the polymer.
Polypropylene has a lower density than most of the polyethylenes (0.905 g/cm3) and a
higher strength. Its melting temperature at 162 °C is significantly higher than that of
HDPE, making it suitable to form retortable and microwave-resistant products. The
glass transition temperature is high, (e.g., 10 °C, but this varies with the crystallinity and
method of measurement) so impact resistance is poor. Impact resistance is improved by
copolymerisation with ethylene. Usually about 5% ethylene is used, and blocky
copolymers are formed, so that the ethylene-containing molecules form an immiscible
dispersed phase in a matrix of homopolymer polypropylene. The toughness is increased
without decreasing the overall melting temperature significantly. Other random
copolymers provide increased toughness and elasticity with decrease in tensile strength
and melting temperature.
Single-site or metallocene-catalysed polypropylenes have narrower molar mass
distribution, though the isotacticity may not be greater. The copolymers with ethylene
have a more even distribution of ethylene, and so a very small proportion of ethylene
will provide a large decrease in melting temperature compared with the traditional
polypropylene copolymers.
1.5 Blown Film Production (Tubular Extrusion)
Formation of film is by extrusion. The extrusion process involves a series of events that
each affect the stability and consistency of the extrudate and hence the film. The processes
in the extruder include feed, melting, mixing, metering and filtration. The die is an annular
shape that produces a tube of polymer. The tube is inflated by air pressure injected inside
at the die. Inflation of the tube makes the film dimensions greater and provides orientation
of the polymer. The tube passes through zones of cooled air, which solidifies the polymer
and controls the crystallisation [10]. A diagram showing the essential features of the
blown film process is shown in Figure 1.4.
In the formation of polypropylene, a two-step tubular orientation process is required.
This is because of the poor melt strength of polypropylene. The film must first be cooled
to enable crystallisation. The film is reheated to be just at the melting temperature and
the tube is blown again before passing through a cooling ring. A comparison of film
orientations in the transverse direction (TD) and machine direction (MD) shows the
properties to be similar if the stretching occurs simultaneously in each direction. In
sequential stretching, the last stretching step predominates, so TD is usually stronger,
13
Handbook of Plastic Films
Figure 1.4 Diagram showing the extrusion blown film process
except for tear strength. Oriented PP (OPP) stretch ratios of 6 x 6 are common. Shrink
films can be prepared from LLDPE and copolymers of ethylene and propylene, but
radiation modification is necessary to partially crosslink the polymer.
1.5.1 Extruder Characteristics
The extruder must be able to process a wide variety of polyethylenes with varying
molar mass, molar mass distribution, comonomer content and comonomer distribution.
Less powerful extruders may only be suitable for LDPE production, since its shearthinning characteristics assist high throughput at lower power. Additives such as
antioxidants, ultraviolet stabilisers, lubricants, slip agents and tackifiers may need to
be included at the extrusion stage, so a facility for separate injection or dry blending of
these additives may be required. The extruder must provide the means to melt and
convey the molten polymer through a die that will produce the film. Typically, a singlescrew extruder will be suitable. There are many types of single-screw extruders, but,
generally, they are best suited to distributive mixing. Distributive mixing is where the
components only need sufficient mixing to provide a uniform melt. Twin-screw extruders
provide more intensive mixing, and so are used when dispersive mixing is required.
Dispersive mixing is where high shear is needed to subdivide a dispersed phase into
smaller particles, where the dispersed phase may be another polymer or a filler with
aggregated particles [11].
14
Technology of Polyolefin Film Production
1.5.2 Screw Design
The extruder screw is usually divided into three zones: feed zone, compression zone and
metering zone. The feed zone conveys the polymer pellets, filler and additives from the
hopper into the main part of the extruder. In the compression zone, the polymer is melted,
mixed with any other components and compressed into a continuous stream of molten
polymer compound. The metering zone provides a uniform flow rate to convey the polymer
to the die. Polyethylenes are semicrystalline polymers with a broad melting range,
particularly if they are copolymers or have random branching, such as LDPE or LLDPE.
The melting or compression zones of the screw must be broad. This is the region where
the depth of flight is decreased to provide the compression. Polyethylenes have a higher
molar mass than other polymers used for extrusion, so the melt viscosity is reasonably
high. Polyolefins have weak intermolecular forces, so the mechanical properties are derived
from a high molar mass and regularity of the chains for close packing. In addition to the
force required for extrusion, the strength of the molten films is important in successful
film formation. Of the polyolefins, polypropylene is the most difficult for film production
because it has relatively low melt strength. Very high molar mass will improve the film
formation, but make the extrusion part of the process more energy-consuming [10].
1.5.3 Frost-line and Blow Ratio
The molten film exits from the extruder through an annular die so that a tube of polymer
is formed. The tube is sealed at the top as it passes between pinch rollers. The tube is
expanded using air pressure. The tube will only expand significantly when the polymer
is molten. The rate at which the polymer exits from the die, the air pressure and the
impingement of external chilled air determine the blow ratio. The blow ratio is the ratio
of the final tube diameter to the diameter of the annulus in the die. This ratio, together
with the width of the slot in the die, determines the film thickness and the transverse
orientation of the film. The film is also oriented in the direction parallel to the die by a
differential between the speed of the polymer exiting the die and the speed of the pinch
rollers pressing the tube flat and feeding it to the auxiliary equipment. The transverse
orientation occurs up until the polymer solidifies and is often the dominant orientation
for properties. The blow ratio able to be used is limited by the melt strength of the
polymer. Linear polymers are more likely to exhibit film rupture in the melted region of
the tube. Polymers with long branches have higher melt strength and so are much better
for production of blown film. The rheology of the polymer is important for other aspects,
since an unstable bubble may be formed [12]. The bubble should be symmetrical about
the centre-line of the die to the pinch rollers. If the film is uneven in thickness or in
solidification, then symmetry will be difficult to control. A thicker portion of the film
will be stronger and will resist blowing and so remain thicker. A thinner portion of the
15
Handbook of Plastic Films
film will expand easier and will become thinner, so this part of the film will bulge outwards
even more. The thinner part of the bubble may even rupture. Differential heater bands
can be placed around the film near the exit from the extruder to provide fine adjustment
of the film temperature as the film is expanded.
The frost-line is the point at which the polymer solidifies by crystallisation. The
transparency of the polymer is decreased on crystallisation, and this is observed as a
sharp transition in the film not very far above the die. The frost-line depends on the
extrusion speed and the temperature, as well as on the cooling air that is directed on to
the polymer tube from the outside. The cooling air is usually refrigerated, and its
temperature, velocity and angle of impingement on to the film may all be varied. Rapid
crystallisation will provide smaller crystals, and so the film will be clearer, apparent in a
low haze, and have a smoother surface, apparent in a high gloss [13].
1.6 Cast Film Production
1.6.1 Extrusion Conditions
Cast film is extruded through a very thin horizontal slit die. The film is drawn from the
extruder by calender rolls. This process does not expand the width nor decrease the
thickness of the film, though the calendering occurs immediately after extrusion. The
extrusion process is the same as for other extrusions. The melted polymer must be
distributed evenly along a slit die, usually using channels in the die. The die is referred
to as a coat hanger or fish tail die. The calender rolls are chilled so that they provide a
melt quenching, giving smaller crystals than the blown film process. The film has a
very smooth surface due to the calendering process [8]. The smooth surface can cause
self-adhesion of the film, called blocking. An antiblocking agent may be added to reduce
the blocking. Cast films will usually have superior gloss and low haze compared with
blown films. Orientation of polypropylene flat film uses a tenter frame (chain) with
clamps in the transverse direction, a quench roll, then reheating rolls followed by tenter
and wind-up roll for the machine direction. The tenter frame is enclosed in an oven
that is used to heat and relax the film [6]. Figure 1.5 provides a schematic illustration
of the cast film process.
Coextrusion is used to make multilayer films by extruding several polymers at the one
time through a single complex die. Each individual polymer will have its own extruder
feeding into a central die. An individual polymer may be included into more than one
layer, yet it only need come from one extruder. Multilayer films are common despite the
complexity of the equipment required for their manufacture. Each layer has a special
16
Technology of Polyolefin Film Production
Figure 1.5 Diagram showing the extrusion cast film process with calendering and
extrusion coating
purpose in the film. The requirements for mechanical protection, diffusion barrier
properties, substrate and interlayer adhesion, and heat shrinkage cannot all be met through
a single polymer. The most suitable polymer for each purpose can be chosen and assembled
into the multilayer structure.
1.6.2 Calendering Finishing
The melted film is usually cooled and pressed to provide a high-quality surface by passing
it through a set of chilled calender rollers immediately after extrusion. The rollers will be
of highly polished steel to provide a smooth glossy surface to the film. Rapid cooling
also assists formation of a glossy surface on the film, since the crystals will be kept small
and crystallisation may be minimised. A series of rollers may be used to provide orientation
by stretching the film in a longitudinal direction.
1.6.3 Extrusion Coating
Extrusion coating is when the melted polymer film is extruded on to an existing film
before passing through the calender rollers. The existing film will be another polymer,
metallic foil or paper. Multiple layers may be formed by extrusion coating both sides of
the primary film or building a multilayer structure by introducing several extrusioncoated layers. Coextrusion can only be used for polymers with similar processing
conditions. Where the processing conditions are different, particularly in the case of
substrates that cannot be melted with the polymer, such as metallic foils and paper, then
extrusion coating is the only choice [6].
17
Handbook of Plastic Films
1.7 Orientation of the Film
1.7.1 Orientation During Blowing
Extrusion blow moulded film usually receives biaxial orientation. During blowing, the diameter
of the extruded tube is increased, and this causes the structure of the film to be oriented
perpendicular to the extrusion direction. Orientation should take place below the melting
temperature when the polymer is crystalline so that the crystals are oriented. The expansion
of the extruded tube will take place while the polymer is entirely melted, so that the effect of
blowing will not provide a level of orientation equivalent to the diameter expansion [14].
At the same time as the film is being expanded by blowing, it is being drawn by pulling along
the axis of extrusion. This provides a parallel orientation. Again, most of the parallel drawing
occurs on the polymer melt between when it leaves the extruder and when it crystallises. The
crystallisation region is called the frost-line. At the frost-line the film will have its maximum
diameter and resist further expansion or drawing compared with the region immediately
before the frost-line. At the frost-line the completely transparent melt becomes foggy due to
crystallisation. The change in opacity depends on the crystallinity of the particular polymer.
Sometimes additional orientation is imparted on the film after the blowing process. This is
the case if the film is to be a shrinkable film. Shrinkable films will contract upon heating. This
is useful for providing tightly fitting wrapping.
1.7.2 Orientation by Drawing
Orientation of a polymer must be performed at a temperature between the glass transition and
melting temperatures. Polyolefins, particularly polypropylene, are normally moderately heated.
The enhancement of tensile properties is directly related to the draw ratio. After drawing, the
film should be further heated to relax or set the structure. This will provide dimensional stability.
If the film is to be heat-shrinkable, then the relaxation is not performed. Some crosslinking
from radiation treatment prior to drawing may be used to increase the plastic memory effect in
the film. Excessive drawing can cause strain hardening because of the introduction of extended
chain crystals at the expense of chain-folded crystals. The strain-hardened film will have a
stiffer or more leathery feel; it will lose elasticity and may have a rougher surface.
1.7.3 Biaxial Orientation (Biaxially Oriented PP, BOPP)
It is desirable to orient films in planar directions, parallel and perpendicular to the flow.
The parallel orientation can be provided by a draw-off faster than the extrusion speed.
18
Technology of Polyolefin Film Production
Figure 1.6 Schematic diagram for biaxial orientation of cast film
The perpendicular direction has been described for blown film production. In extruded
sheet, a frame that attaches to the edges of the film and moves apart must provide the
perpendicular orientation. This is called a tenter frame. The film is often drawn to about
three times its original width. As well as adding strength to the film, a thinner gauge of
film can be produced. The drawing process overcomes the effect of die swell that occurs
as the film leaves the die. Orientation of cast film is illustrated in Figure 1.6.
1.8 Surface Properties
1.8.1 Gloss
Gloss is the reflection of light from a surface. The nature and origin of gloss and haze are
illustrated in Figure 1.7. A high gloss requires a smooth surface. Surface imperfections may
be introduced by the processing. Excessive drawing into the strain-hardening region will
usually reduce the gloss. Blown film usually has a lower gloss, since crystallisation of the
film at the frost-line introduces surface roughness due to the crystals. Rapid crystallisation
of the film by the use of chilled air impinging on the bubble reduces the size of crystals and
improves the gloss. Extrusion cast film passes through chilled rollers after leaving the
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Handbook of Plastic Films
Figure 1.7 Diagram showing the origin of gloss and haze in polymer films
extruder. The rapid cooling and the polished surface of the rollers provide a high-gloss
surface. Extrusion cast films have the higher gloss, but the extrusion blown process produces
film at a lower cost. The rheology of the polymer will contribute to the surface of the film.
Shark skin is the term applied to a rheological problem in the processing [15].
1.8.2 Haze
Haze is an internal bulk property, but, because of the importance of appearance, it has
been considered along with surface properties due to its relation to gloss (see Figure 1.7).
Crystallinity, optical defects, ‘fish eyes’, phase separation of blends, contaminants, gel
particles and dispersion of pigments (carbon black) are structures that increase haze.
Haze is the internal scattering of light. Haze makes it difficult to clearly see an object
through a film as a result of the interference from randomly scattered light reaching the
viewer in addition to light coming straight from the object. Smaller crystals provided by
a nucleating agent will decrease haze. The other phenomena described above can also be
reduced by nucleating agents, better formulation and processing.
1.8.3 Surface Energy
The surface energy of polyolefin films is very low. It is difficult to find other substances
that will adhere to polyolefins. Suitable adhesion can be obtained by melt adhesion of
polyolefins to each other, but only when the polyolefins are very similar. For instance,
polyethylenes have good mutual adhesion. The branched polyethylenes with lower melting
temperature are most used because they can be melted more rapidly and they have suitable
20
Technology of Polyolefin Film Production
rheology to flow on to the adherend. Melt adhesion of films will require higher-melting
layers other than the surface layer, since if more than the surface layer melts then the
structural integrity of the film will be destroyed.
Copolymers of ethylene with vinyl acetate, methyl acrylate, acrylic acid, maleic acid and
many other polar monomers are used to increase the surface energy of polyethylenes to
make them more readily wettable. Similarly, polypropylene can be grafted with maleic
anhydride to increase the adhesion of other substances to it. The surface energy of
polyolefins is also increased through corona discharge treatment.
1.8.4 Slip
Polyolefin films generally have a smooth surface, with the exception of defects and surface
crystal structures. They have a low surface energy and so frictional forces are low. Relative
to their strength, the frictional forces can cause damage to the films. Slip additives can
decrease the frictional forces. The factors that cause poor slip are often desired for other
attributes such as adhesion of printing and adhesion to other surfaces in packaging. Additives
that increase self-adhesion of packaging films will decrease the slip, so that desirable
properties are not universal – they depend on the intended application of the film.
1.8.5 Blocking
The low surface energy and softness of polyethylenes make them self-adhere if pressed
together under a load for a considerable time. This self-adhesion is called blocking. The
polyolefins can flow, or creep, under load, and so mutual adhesion can occur if the
pressure is sufficient or the time of contact is long. This is a significant problem in large
film rolls or when film is stacked in large quantity. Blocking is reduced when the surface
is less smooth, such as when crystallisation or processing conditions cause
microtopographies. A smooth surface with high gloss and clarity is generally preferred,
so that blocking will be a serious problem.
1.9 Surface Modification
1.9.1 Corona Discharge
Processes for modifying the surface properties of plastic films are important. Surface
adhesion can be increased by oxidative treatments such as corona, flame, priming or
subcoating. Corona discharge is most widely used, and surface oxidation of the film
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Handbook of Plastic Films
Figure 1.8 Corona discharge surface treatment system
occurs, giving polar functional groups [16]. A corona discharge treatment facility is
shown in Figure 1.8. Increased polarity will increase the surface energy and enable
wetting by inks or adhesives. Printing is carried out by the flexographic technique,
whereby a rubber roller with raised imprints imparts the ink, or by the rotogravure
process, whereby an engraved steel roller imparts the ink. Film coating, film wetting,
dispersion coating (by an emulsion), solvent coating, barrier and heat seal coatings
are all improved after oxidative surface treatment. Extrusion coating is the process
whereby a film is extruded on to an existing film. Film lamination occurs when existing
films are bonded together with an adhesion layer applied by any of the previously
mentioned methods. Typical are the use of poly(vinylidene dichloride), aluminium
foil and ionomer films, and metallisation by vapour deposition of aluminium.
Coextrusion is the process in which two or more films are extruded and brought
together in the die. Each layer will contribute to specific properties, such as barrier
and adhesive layers.
1.9.2 Antiblocking
Blocking can be reduced by decreasing the surface contact area of the films. Small
particles at the surface can decrease the contact. Particles such as silica, diatomaceous
earth and talc are useful antiblocking agents [17]. To provide the protection efficiently,
they must be included in the polymer during processing. Many of the particles will
be in the bulk of the film, so they will not contribute to antiblocking characteristics.
Those particles that exist at the surface will be active. The particles must be small
enough not to introduce haze when in the interior or decrease gloss when at the
surface. Films that have a rougher surface due to surface crystallites or other
imperfections will have a natural protection against blocking. The mechanism of
antiblock additives is shown in Figure 1.9.
22
Technology of Polyolefin Film Production
Figure 1.9 Mineral particles (silica or talc) prevent blocking, since some particles
become located at the surface and so limit contact of the films
1.9.3 Slip Additives
The coefficient of friction can be reduced by adding slip agents. The requirement for a slip
agent is that it is miscible with the polymer melt, but separates as the polymer is crystallising.
Sometimes the separation will take several hours or days to become complete. The slip agent
should form a very thin layer on the surface. The layer is more effective if it is randomly
structured. The mutual miscibility and separation are important, and long-chain amides
have been found to be best when the chain length is about 22 carbon atoms, whereas 18
carbons is less effective. An amide with a cis double bond is better than the fully saturated
analogue because the cis double bond prevents surface crystallisation of the amide compared
with the saturated amide. Erucamide has thus been found to be the most effective slip agent
for polyolefins [18]. Other fatty amides such as ethylene bis-stearamide, oleamide and
stearamide have been used. The function of a typical slip additive is shown in Figure 1.10.
Figure 1.10 Structure of erucamide (C21H41CONH2, cis-13-docosenamide) slip additive, and
its mechanism for increasing slip. The diagram shows an array of erucamide molecules along
the surface of a film, with the bent cis structure ensuring that they stay irregular
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Handbook of Plastic Films
1.9.4 Lubricants
Lubricants are added to assist with processing. The polymer can be extruded more readily
when there is a layer of lubricant between the polymer and the surfaces within the extruder.
This is particularly important at the die lips, where the lubricant can decrease surface
irregularities such as ‘shark skin’ effects. The melt can maintain laminar flow at higher
extrusion rates with lubricants present. Common lubricants are stearic acid, stearate
salts, paraffin wax and chlorinated paraffins [19]. A fluoropolymer has been reported as
a processing additive for polypropylene [20].
1.9.5 Antistatic Agents
Polyolefins do not absorb water. The dry surface causes a build-up of static electricity.
Antistatic agents are polar substances that are mixed with the polymer and migrate to
the surface after the polymer emerges from the extruder. Polyoxyethylenes are one type
of antistatic agent. When on the surface, they can absorb sufficient water to prevent
static electricity build-up. A problem is that they can attract dust and other materials
that would not normally adhere to the polyolefins. Another problem is that they can be
easily removed by friction or extraction with polar liquids [19].
1.10 Internal Additives
1.10.1 Antioxidants
Immediately the polyolefin is made, it requires protection from oxidation. This is
particularly the case during the processing stages when high temperatures are used.
Hindered phenols and triaryl phosphites are typically used. During the extrusion process,
the polyolefin is then protected from the heat and oxygen. During the lifetime of the
film, other thermal processes may be encountered during printing, packaging of food
and heat sealing. The antioxidants act by removal of radicals from the polymer – by
hydrogen donation in the case of hindered phenols, and by removal of oxygen from
peroxy groups in the case of phosphites [18]. Figure 1.11 shows a typical hindered phenol
antioxidant and a triphenyl phosphite secondary antioxidant.
1.10.2 Ultraviolet Absorbers
Films that are to be used in the sunlight require protection. While the polyolefins do not
absorb ultraviolet light, they do contain many other anomalous functional groups,
additives and impurities that do absorb. Hydroxybenzophenones, hydroxybenzotriazoles
24
Technology of Polyolefin Film Production
Figure 1.11 Structures of common heat and light stabilisers: (a) hindered phenol
antioxidant: (b) tris(2,4-di-tert-butylphenyl) phosphite secondary antioxidant; (c)
hydroxybenzophenone ultraviolet absorber: (d) hindered amine light stabiliser
and tetramethylpiperidines (hindered amine light stabilisers, HALS) are typical compounds
used for ultraviolet stabilisation. Factors such as compatibility with the polyolefin, low
volatility and absence of any colour are just some of the many stringent requirements of
these additives. Hydroxybenzophenones and hydroxybenzotriazoles absorb ultraviolet
light and form an excited electron state that can undergo a radiationless transfer of the
energy to heat, to regenerate the ground state. HALS act by a cyclic mechanism in which
they form a nitroxide that can couple with a radical and then transfer the radical to
terminate another radical and release the nitroxide. HALS are the most effective stabilisers
for polypropylene, but the type of HALS used depends on the application of the
polypropylene and the other components in the composition [17]. Figure 1.11 shows a
typical hindered amine light stabiliser and a hydroxybenzophenone ultraviolet absorber.
1.11 Mechanical Properties
There are many standard test methods to define the performance requirements of polyolefin
films. These are specified in the relevant ASTM, DIN and ISO standards. The material
mechanical properties are used for polymer specification for a particular purpose. There
are many tests available, and each measures a narrowly defined property. Often it is difficult
to predict performance by a material property, and so a product-specific test is designed.
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Handbook of Plastic Films
The mechanical properties can be separated into tensile and impact tests, though there are
many other tests, such as tear testing, abrasion resistance and adhesion tests. The morphology
of the film is of major importance in controlling the mechanical properties [21]. The
morphology of a film is strongly connected with the key processing variables [8].
1.11.1 Tensile Properties
1.11.1.1 Strain Rate and Tensile Properties
The strain rate is important in measuring the tensile properties of films. During packaging
operations, films are often subjected to extremely high strain rates in the machinery.
These processes must be duplicated in the testing procedures where possible. Slow strain
rates may be preferred when material properties are measured to distinguish between
various polyolefin structures. Typical parameters that are obtained from a tensile test are
the modulus, yield stress, break stress and elongation at break. The area under the stressstrain curve is used as a measure of the energy to break.
1.11.1.2 Strain Hardening
When a polyolefin film is considerably extended, the crystal structure orientation will be
significant and the polymer will become harder. Often a strain-hardened film is described
as leathery. The morphology may change from a chain-folded to an extended-chain
configuration. The tie molecules become fully extended so that further strain is limited
before film breakage occurs. Strain hardening is a limit to the useful elongational
performance of a film because the elasticity is lost. A stress-strain curve for linear lowdensity polyethylene showing the yield and strain-hardening regions is shown in Figure 1.12.
1.11.1.3 Stress Relaxation
Stress relaxation is defined as a change in stress when the material is under constant strain.
When a packaging film is stretched around an article, or a large number of articles, the
elasticity of the film will keep the article(s) under constant tension to provide protection and
ease of transport and handling. When stress relaxation occurs, the tension of the packaging
is lost, and the contents are no longer held together. Stress relaxation can be measured with
a standard tensile testing instrument, where the stress is measured over time while the specimen
is held under a constant strain. Polyolefins with high molar mass and high crystallinity will
be the most resistant to stress relaxation. An illustration of the processes involved in stress
relaxation is shown in Figure 1.13; first the amorphous molecules are elongated, then on
relaxation they slide past each other to return to a random-coil conformation.
26
Technology of Polyolefin Film Production
Figure 1.12 Stress-strain curve for linear low-density polyethylene showing the yield stress
after the initial linear elastic region, then the strain-hardening region after 350% strain
Figure 1.13 Schematic for the mechanism of creep and stress relaxation
1.11.1.4 Creep
Creep is the change in strain when the specimen is subjected to a constant stress. Polyolefin
films that are exposed to pressure for extended periods will gradually elongate. Pressures
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Handbook of Plastic Films
are generated inside many packages, and polymer will creep, causing the package to
become larger. Creep is a complementary property to stress relaxation, and so the same
molecular characteristics resist creep. Creep is illustrated in Figure 1.13; under constant
load the molecules gradually slide past each other, resulting in elongation.
1.11.1.5 Burst Strength
When a film experiences a high pressure over a short time, it may burst. This is similar to
elongation at break, except that the film will be under a biaxial tension. A test of burst
strength may be more suitable for a film under pressure than a tensile test. The shortterm nature of the test is in contrast to the long time for creep.
1.11.2 Impact Properties
1.11.2.1 Dart – Puncture Resistance
An impact test is a short-term test; the stress is applied very quickly. The dart test can
involve either a dart falling through a constant distance or a dart propelled by gas pressure.
The shape of the end of the dart is an important factor in the test. A rounded end is
generally used, but the test can be modified to measure the resistance of a film to any
puncture by impact with any particular object. The falling dart test will result in a pass/
fail type result compared with a specification for the load. The instrument can be designed
to measure the deceleration of the dart as it passes through the film. The energy required
to break the film is then calculated from the energy lost by the dart. In this latter situation,
the dart is always required to break the film.
1.11.2.2 Tensile – Tear Strength
Films often break in shear instead of tension. A film can be tested in a tensile instrument
so that the strain is applied under shearing conditions. A cut may be made in the film to
direct the tear. The geometry and conditions of the test are defined so that standard
conditions are used. The Elmendorf tear strength test is used to measure the performance
of films, and it has been related to processing conditions and dart impact strength [22].
1.11.2.3 Tensile Impact
This is a short-term impact test where the specimen is mounted in a pendulum test
instrument so that it receives a rapid tensile force as the pendulum strikes the specimen
28
Technology of Polyolefin Film Production
holder. The tensile impact test is applicable to films, whereas other forms of pendulum
impact tests, such as Izod and Charpy tests, require more rigid specimens. The tensile
impact test can apply a greater strain rate than a typical tensile test instrument.
1.11.3 Dynamic Mechanical Properties
Dynamic mechanical analysis (DMA) measures the properties when an oscillating
stress is applied to the material. The stress and strain are often out of phase, and this
situation can be used to obtain the viscoelastic properties. The viscous or timedependent properties are out of phase with the stress, while the elastic or instantaneous
properties are in phase with the stress. The in-phase property is called the storage
modulus, in that the elastic energy is stored and can be subsequently released when
the stress is removed. The out-of-phase property is called the loss modulus, in that
energy is lost to heat during viscous flow. The properties are usually measured with
temperature and/or frequency. Temperature and frequency can be combined to provide
a time-temperature transposition, so that long-term or very short-term properties
can be measured within real-time limitations [23]. Figure 1.14 shows DMA curves
for polypropylene.
Figure 1.14 DMA curve for polypropylene, showing the storage modulus, E′, loss
modulus, E″, and damping factor, tan δ
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Handbook of Plastic Films
1.11.4 Dielectric Properties
Polar olefin copolymer films, such as poly(ethylene-co-vinyl acetate), poly(ethyleneco-methyl acrylate) and poly(ethylene-co-butyl acrylate), and blends or laminates can
be characterised using dielectric analysis. The dielectric properties of all types of
polyolefins are important because of the electrical applications of these polymers.
However, dielectric analysis can also be performed in an analogous manner to DMA,
in that the mechanical force is replaced by a voltage and the permittivity is measured
while the temperature and/or frequency is varied. The viscoelastic properties of the
polymer can be measured by the dielectric response over a wider range of frequencies
than the mechanical tests. The dielectric test is more sensitive than the mechanical test
when the polymer has more polar groups. Figure 1.15 provides a schematic of the
processes involved in a typical dielectric analysis.
Capacitance (C)
I = current
V = voltage
f = frequency
δ = phase angle
Conductance (R-1)
C=
R−1 =
I sin δ
V 2πf
I cos δ
V
Permittivity (ε′)
ε0 = permittivity of a vacuum,
A = area of electrode
D = distance between electrodes
⎛ C ⎞⎛ 1 ⎞
ε′ = ⎜ ⎟ ⎜
⎟
⎝ ε0 ⎠ ⎝ A / D⎠
Loss factor (ε′′)
ω = angular frequency
σ = conductivity
ε ′′ = ε dipole
+
′′
σ
ωε 0
Figure 1.15 Dielectric analysis of polar ethylene copolymers involves measurement
of the storage (permittivity) and loss (loss factor) components with temperature
30
Technology of Polyolefin Film Production
1.12 Microscopic Examination
1.12.1 Optical – Polarised Light Effect with Strain
The crystallinity of a polyolefin in a film can be viewed with an optical microscope using
polarised light. The film must be very thin, although reflected light can be used for
thicker opaque films. The microscope can be combined with a hot stage to view
crystallisation and melting events.
1.12.2 Scanning Electron Microscopy (SEM) – Etching
Scanning electron microscopy (SEM) can reveal morphology, though usually the sample
must be etched so that amorphous regions are modified more than crystalline, or a
component in a blend is eroded more rapidly. The etching creates a new surface topography
that can be viewed with the SEM. Care must be taken so that the electron beam does not
damage or create artefacts on the surface.
1.12.3 Atomic Force Microscopy (AFM)
The surface of polyolefin films can reveal information about the bulk. Crystal growth at the
surface and other irregularities that may arise from processing or treatments such as corona
discharge can be studied using atomic force microscopy. Variations in the hardness or friction
across the surface as found in blends can be studied to reveal the distribution of components
across the surface. Figure 1.16 shows an atomic force microscope picture of the surface of
linear low-density polyethylene formed by the extrusion blow moulding process.
1.13 Thermal Analysis
1.13.1 Differential Scanning Calorimetry (DSC)
DSC is used to measure the crystallisation and melting temperatures of polyolefins as
well as the enthalpies of crystallisation and melting. The results shown in Figure 1.17 are
used to identify, characterise and measure the crystallinity [24]. The thermal history and
mechanical stresses of a film can be investigated through the melting and crystallisation
response measured by DSC [25]. The crystallisation temperature increases with nucleation,
so that the efficiency of added nucleation agents can be measured. The crystal structure
varies with processing conditions and other treatments such as orientation, and these
can be measured by analysis of the melting of the polyolefin on heating.
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Handbook of Plastic Films
Figure 1.16 Atomic force microscope picture of the surface of linear low-density
polyethylene film prepared by the extrusion blown film process
1.13.2 Temperature-Modulated DSC (TMDSC)
TMDSC is analogous to DMA in that an oscillating force, in this case a temperature
programme, is applied to the sample. The response can be resolved into reversing and nonreversing specific heat capacities. Recrystallisation, rearrangement of the crystals and melting
can be studied simultaneously. This is important for an understanding of the equilibration of
the morphology of the polyolefins after various processing or other thermal treatments [26].
1.14 Infrared Spectroscopy
1.14.1 Characterisation
Infrared spectroscopy is a convenient method to identify polyolefin films. The main classes
of polyolefins can be easily identified. A detailed interpretation of the infrared spectrum
32
Technology of Polyolefin Film Production
Figure 1.17 DSC curves for melting of linear low-density polyethylene and low-density
polyethylene (for clarity, this curve has been shifted upwards by five units)
will enable even very similar structures to be distinguished. The extent of branching can
be measured and crystallinity can be indirectly measured [27].
1.14.2 Composition Analysis of Blends and Laminates
Blends of polyolefins can be identified, and, when the components are known, quantitative
analysis can be performed. The component layers of laminates can be identified after
separation of the layers, or by analysis of the edge of the film using an infrared microscope.
1.14.3 Surface Analysis
Surface additives, such as glyceryl monooleate, polyisobutylene and slip agents, and corona
treatments can be measured using surface infrared spectroscopic analysis. Multiple internal
reflection is the common method, but specular reflectance and grazing angle reflectance
are other useful techniques by which the infrared spectrometer can be used to study
surface chemistry. Figure 1.18 shows a schematic for the surface analysis of a polymer
33
Handbook of Plastic Films
Figure 1.18 Apparatus for multiple internal reflectance (attenuated total reflectance)
infrared spectroscopy for surface analysis of polymer films
film by multiple internal reflection. The angle of the infrared beam to the surface of the
internal reflectance element (typically zinc selenide or germanium) along with the
wavelength of the infrared beam determine the depth of penetration into the surface of
the film. A low depth of penetration will provide spectra more sensitive to the surface
additives or modifications.
1.14.4 Other Properties
1.14.4.1 Thickness
Thickness is known as gauge. Uniformity must be achieved by the manufacturing process,
Generally films are considered to be of ≤250 μm; greater thicknesses are called sheet.
Some films may be 10-20 μm in thickness, and individual layers in multilayer films are
often only 5 μm thick.
1.14.4.2 Moisture Resistance
Polyolefins are nonpolar, so they are particularly efficient at resisting moisture. Their
resistance to liquid water is not necessarily carried on to their resistance to water vapour
or humidity. High-density polyethylene film is the most resistant to water vapour because
gas molecules have difficulty diffusing through the crystalline structure.
34
Technology of Polyolefin Film Production
1.14.4.3 Gas Permeation
Films are used to encase and protect other items. An important property is to resist gas
permeation, in particular, the permeation of oxygen, carbon dioxide and water vapour.
Polyolefins provide very poor barrier layers. Usually a multilayer film that includes another
polymer or other material is required to provide suitable barrier properties. The barrier
properties are increased when higher-crystallinity polymers are used. The density of the
amorphous phase of the polymer has been found to be a guide as to the permeation
resistance [28].
1.14.4.4 Orientation
Orientation of a film may be measured by annealing the film at temperatures below the
melting temperature. Oriented films will shrink more than others. The shrinkage is often
controlled by partial crosslinking of the film. There are many applications in the food
industry for shrink-wrapping of produce. Shrink-wrapping of polyethylene films can be
applied over other packaging materials to group containers into specific quantities.
1.14.4.5 Dimensional Stability
Dimensional stability is usually a consequence of orientation. Under heat treatments
such as may be experienced during packaging of hot foods, sterilisation processes for
foods and heat sealing, the dimensional stability of the film must be maintained. This is
the opposite requirement to shrink-wrap films. Humidity can contribute to changes in
dimension, due to absorption of moisture by other components of a multilayer film or
by the contents of the film package.
1.15 Applications
1.15.1 Packaging
The main application of films is for packaging (Figure 1.19). The functions of a package
can be summarised as: containment, dispensing, preservation, protection, communication
and display. Packaging machinery is diverse, (e.g., vertical fill, shrink-wrap, sleeve-wrap,
stretch-wrap and blister packaging machines), and thus requires many properties of the
film, (e.g., stiffness, stretchability, heat sealability), which in addition must be suitable
for various applications, (e.g., in side weld bags, in bottom seal bags and to hold liquid
35
Handbook of Plastic Films
Figure 1.19 Examples of typical packaging applications for polyolefin films
products). Many of the films are required to be multilayer films to achieve all of the
desired properties [14]. Polyethylenes with new improved properties due to high molar
mass are being used in heavy-duty applications [29].
1.15.2 Laminated Films
The performance requirements for packaging are increasing, with new demands for
protection of the product, speed and ease of packaging and sealing, together with printing,
handling and storage requirements. An individual polymer cannot meet all these
requirements, so multilayer films are necessary. These can be prepared by extrusion of
further layers on to existing films or adhering existing films together. This process is
called lamination. Most of the layers are polymer, but a metal foil (usually aluminium)
may be used. Paper or paperboard is frequently used as substrates for the lamination. A
textile layer may be used, but these are mainly classified separately from laminated films.
36
Technology of Polyolefin Film Production
A basic requirement for a laminated film is good adhesion between the layers. The
materials in the layer will often be chemically different, to provide the diversity of
properties, and so adhesion may not be suitable. In such cases, a separate adhesive layer
must be included between the functional layers. The adhesive layer functions in the same
way as a compatibiliser in polymer blends. Often a copolymer will be used, where each
of the component monomers will contribute to the adhesion with one of the adjacent
layers. Some multilayer films are produced with the adhesion component included as a
blend with a functional layer. In this way the number of individual layers is reduced,
simplifying the lamination process.
The separate layers of the film may consist of the same polymer but in different forms. A
layer could be mineral-filled, pigmented, foamed, oriented, radiation or chemically
crosslinked, include antioxidant or ultraviolet stabilisers, be printed or otherwise modified.
A further protective layer may then be placed over the modified layer, or the modified
layer may be the protective layer.
Lamination of films is often preferred to application of a polymer layer as a lacquer since
the latter includes solvent or an emulsion is used, so that drying and removal of volatiles
is required. The laminated film will often have superior gloss such as when polypropylene
is laminated on to printed paper substrates. Surface spreading by the coating layer and
film thickness will be easier to control by laminating than by solution coating. Ovens
and solvent removal systems can be replaced by calender cooling rollers.
1.15.3 Coextruded Films
Coextruded films contain most of the characteristics of laminated films, except that all
of the layers are formed by extrusion at the same time. This precludes aluminium foil,
paper and textile layers, which must be included by lamination. The coextrusion requires
several extruders with a common die. The die has a complex construction and may be of
either a tubular film type or a cast film linear slit type. The polymer for each layer is fed
into the die by a separate extruder. One extruder per layer may be used or, if more than
one layer contains the same polymer, then its extruder can feed into more than one layer.
The latter connection uses the minimum number of extruders, but is less flexible than
the ‘one extruder per layer’ configuration.
After coextrusion, all of the layers must receive any treatments together. For instance, if
a layer is to be biaxially oriented, then all layers must be oriented. If a layer is to be
radiation-crosslinked, then all of the layers must receive the radiation treatment. Individual
layers cannot be printed, then covered with another protective layer. An advantage is
that the complete multilayer film is produced in one process. The process is not suitable
for polymers with significantly different processing temperatures. Shrinkage stresses may
occur on cooling and so layers may separate.
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Handbook of Plastic Films
1.15.4 Heat Sealing
Polyolefin films are joined by heat sealing, a process whereby the film is partially melted
while pressing against the other component for a well-defined short time. The strength
of adhesion is characterised by the time-temperature-pressure relationship and the
proportion of the polymer melted. Melting will destroy the original crystal structure and
any orientation in the film, so the mechanical properties will be changed. A multilayer
film is preferred so that only the surface adhesive layer is partially melted, while the
structural layer is unaffected. This requires that the surface layer have a lower melting
temperature than the structural layer [30].
Typically, if the structural layer is polypropylene, then the surface layer can be an ethylenepropylene random copolymer, with a small proportion of ethylene. The adhesive layer
may also be grafted with maleic anhydride to provide better adhesion to polar substrates.
If the structural layer is a polyethylene, such as LLDPE, then the adhesive layer can be an
ethylene copolymer, such as poly(ethylene-co-vinyl acetate). Recently VLDPE have been
used as adhesive layers where low surface energy for better wetting is important.
1.15.5 Agriculture
Films for agriculture have become important to protect crops and to protect and bind
products. The very thin films can provide sufficient tensile strength to contain the product,
and with self-adhesion of the film the wrapping process is very efficient. The films may
be pigmented black to protect the contents from sunlight. Pigmented film may be spread
on the ground to control weeds by removal of sunlight and reduce evaporation of water
[31].
1.16 Conclusion
Polyolefin films are complex in their manufacture by the blown film extrusion and
extrusion cast film processes. The films are typically strengthened by biaxial orientation
and can be made very thin. The inert polyolefin surface is often modified by oxidation to
provide polar groups for adhesion, and by the use of additives to provide slip or to
prevent blocking. Specialised properties such as gas barrier, printability, heat sealing and
shrinkability are achieved by coextrusion or lamination of films with different chemical
structures. The diversity of films available may seem simple, but when the details are
considered even a common packaging film involves a complex range of technologies.
Further advances will enable films to possess even more specialised functionality while
being thinner and stronger.
38
Technology of Polyolefin Film Production
References
1.
K.J. McKenzie in Kirk-Othmer, Concise Encyclopedia of Chemical Technology,
Wiley-Interscience, New York, NY, USA, 1999, 845.
2.
J.A. Brydson, Plastics Materials, 4th Edition, Butterworths, London, UK, 1982, 187.
3.
H. Saechtling, International Plastics Handbook, Carl Hanser, Munich, Germany,
1983, 347.
4.
D.Y. Chiu, G.E. Ealer, F.H. Moy and J.O. Buhler-Vidal, Journal of Plastic Film and
Sheeting, 1999, 15, 2, 153.
5.
A.K.C. Mehta, M.C. Chen and C.Y. Lin in Metallocene-Based Polyolefins –
Preparation, Properties and Technology, Eds., J.K. Scheirs and W. Kaminsky, John
Wiley, Chichester, UK, 2000, 463.
6.
E.P. Moore, Polypropylene Handbook, Carl Hanser, Munich, Germany, 1996, 334.
7.
H. Fruitwala, P. Shirodkar, P.J. Nelson and S.D. Schregenberger, Journal of Plastic
Film and Sheeting, 1995, 11, 4, 298.
8.
J.A. Degroot, A.T. Doughty, K.B. Stewart and R.M. Patel, Journal of Applied
Polymer Science, 1994, 52, 3, 365.
9.
F.J. Velisek, Journal of Plastic Film and Sheeting, 1991, 7, 4, 332.
10. F. Rodriguez, Principles of Polymer Systems, 4th Edition, Taylor and Francis,
London, UK, 1996, 451.
11. S.W. Shang and R.D. Kamla, Journal of Plastic Film and Sheeting, 1995, 11, 1, 21.
12. P.J. Carreau, A. Ghaneh-Fard and P.G. Lafleur, Proceedings of ANTECH ‘98,
Atlanta, GA, USA, 1998, Volume 3, 3598.
13. A. Ghaneh-Fard, Journal of Plastic Film and Sheeting, 1999, 15, 3, 194.
14. K.R.J. Osborn and W.A. Jenkins, Plastic Films: Technology and Packaging
Applications, Technomic, Lancaster, PA, USA, 1992, 141.
15. V. Firdaus and P.P. Tong, Journal of Plastic Film and Sheeting, 1992, 8, 4, 333.
16. E. Foldes, A. Toth, E. Kalman, E. Fekete and A. Tomasovszky-Bobak, Journal of
Applied Polymer Science, 2000, 76, 10, 1529.
39
Handbook of Plastic Films
17. R. Gachter and H. Muller, Plastics Additives Handbook, Carl Hanser, Munich,
Germany, 1985, 97.
18. G. Pritchard, Plastics Additives: An A-Z Reference, Chapman and Hall, London,
UK, 1998, 633.
19. J.D. Stepek and H. Daoust, Additives for Plastics, Springer-Verlag, Berlin,
Germany, 1983, 243.
20. S. Amos, Modern Plastics, 2000, 77, 10, 131.
21. D. Ferrer-Balas, M.L. Maspoch, A.B. Martinez and O.O. Santana, Polymer,
2000, 42, 4, 1697.
22. W.D. Harris, C.A.A. Van Kerckhoven and L.K. Cantu, Proceedings of ANTEC
‘91, Montreal, Canada, 1991, 178.
23. K.P. Menard, Dynamic Mechanical Analysis, CRC Press, Boca Raton, FL, USA,
1998, 61.
24. M.M. Jaffe, J.D. Menczel and W.E. Bessey, in Thermal Characterisation of
Polymeric Materials, Ed., E.I. Turi, Academic Press, New York, NY, USA,
1997, 1956.
25. V.B.F. Mathot in Calorimetry and Thermal Analysis of Polymers, Ed., V.B.F.
Mathot, Carl Hanser, Munich, Germany, 1993, 231.
26. J.J. Janimak and G.C. Stevens, Thermochimica Acta, 1999, 332, 2, 125.
27. W. Klopffer, Introduction to Polymer Spectroscopy, Springer-Verlag, Berlin,
Germany, 1984, 53.
28. G. Loeber, Kunststoffe Plaste Europe, 1999, 89, 12, 33.
29. J.J. Wooster and B.A. Cobler, Tappi Journal, 1994, 77, 12, 155.
30. F. Martinez and N. Barrera, Tappi Journal, 1991, 74, 10, 165.
31. O.J. Sweeting, The Science and Technology of Polymer Films, Wiley-Interscience,
New York, NY, USA, 1971.
40
2
Processing of Polyethylene Films
Amin Al-Robaidi
2.1 Introduction
A plastic is solid in its unprocessed and processed states. It is softened enough through
the application of a combination of heat, pressure and mechanical working to be formed
into a variety of shapes such as car bumpers, containers and plastic films. Most
thermoplastic polymers are linear polymers. However, these chains twist and turn around
to form a tangled structure [1].
Commodity polymers are low in cost and high in volume. They are used to a great
extent in our day-to-day lives [2]. Worldwide usage is enormous and is increasing year
by year. Rates of increase up to 5% per year are not unusual. Of these commodity
polymers, polyolefins play the largest role, and therefore their production is of great
importance. Polyolefins represent about 45% of total plastic usage: linear low-density
polyethylene (LLDPE) and low-density polyethylene (LDPE) make up 51% of total
polyolefin use, the rest being 23% polypropylene (PP) and 26% high-density
polyethylene (HDPE). In this chapter, some of the important properties, production
techniques, and chemical and physical phenomena that are of interest for polyolefins
will be explained.
Polyethylenes are made via free-radical and Ziegler-Natta (ZN) processes. The freeradical method, usually at high pressures (up to 3500 atm) and high temperatures (up
to 300 °C), is the older of the two methods. The ZN processes usually have much
milder conditions. Recent developments use metallocene catalysts to produce tailormade polyolefins. The common means of distinction between the various types of
polyethylene is by density. LDPE and LLDPE have a low density below 0.94 g/cm3,
whereas that of HDPE is above 0.94 g/cm3. The density is influenced by polymer
structure. The properties of each type are given in Chapter 1.
LLDPE for film application is characterised by the different processes used to produce
the film and also by end-use application. The processes used to produce the film are
determined by the molecular weight (molar mass) of the resin. For example, typical
melt index ranges of 0.5-2 g/10 min are suitable for blown film, 2-6 g/10 min for slot
cast film, and 5-12 g/10 min for extrusion coating.
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Handbook of Plastic Films
A disadvantage of LLDPE resins is the need to modify existing and available extruders at
the processor site to convert from LDPE to LLDPE products, because of the higher shear
viscosity of LLDPE with narrow molecular weight (molar mass) distribution (MWD).
Owing to its different shear and extensional rheology, LLDPE extrusion and processing
conditions differ from those for LDPE. To optimise its processing, different screw designs
have been developed and air rings have been modified according to the different
extensional rheology.
2.2 Parameters Influencing Resin Basic Properties
Polyethylene resins are generally characterised by three parameters:
(1) Molecular weight distribution (measure of processing ease and product properties),
(2) Melt index (measure of molecular weight) and
(3) Density (measure of branching or rigidity).
These three parameters are considered to be intrinsic properties of polyethylene. The
following section discusses each of the basic parameters influencing resin properties.
2.2.1 Molecular Weight (Molar Mass) and Dispersity Index
In any polymer, there is a distribution of chain lengths or molecular weights. Accordingly,
the molecular weight of polyethylene is not a uniquely defined quantity, but instead
depends on what averaging formula is used. The reader is asked to consult any textbook
on polymer science regarding this item [3, 4].
2.2.2 Melt Index (Flow Properties)
The melt index (MI) is an inverse measure of the length or average size of polyethylene
chains. For a given class of polyethylene, MI can be used to estimate the molecular
weight. By ASTM definition, the melt index is the weight of molten material (in grams)
extruded in 10 min through an orifice at 190 °C under an applied stress of 2.16 kg,
which means a stress of 303 kPa. Only under these conditions is the measurement defined
as the melt index. Measurement under different loads is possible in connection with
international standards, sometimes reported as I2 (flow rate under specific load). Flow
rates under defined loads and other conditions are referred to as flow indices. The common
flow index measured at 190 °C and 690 kPa or I5 (flow rate under a load of 5 kg) is used
42
Processing of Polyethylene Films
for higher molecular weight resin, usually HDPE grades. The common flow index for all
grades is the I21 (HLMI = high-load melt index) measured at 190 °C and 3034 MPa.
In practical terms, the melt index is a guide to the relative level of properties of a material
and the relative ease of flow that can be expected from the resin during fabrication [5,
6]. The melt index is inversely related to the molecular weight. As the molecular weight
increases, the melt index decreases, and vice versa. Since the strength characteristics of
polymers are related to the molecular weight, then melt index can be used as an indicator
of polymer strength. With an increase in melt index, the tensile strength, tear strength,
stress cracking resistance, heat resistance, weatherability, impact strength and shrinkage/
warpage decrease. The modulus of rigidity remains relatively unaffected with melt index
increase For HDPE, the increase in melt index improves the gloss but has relatively little
effect on transparency [4, 7]. With an increase in melt index, the ease of processing also
increases if all other parameters (such as molecular weight distribution) are held constant.
2.2.2.1 Melt Flow Blend Relationship
It is often of interest to calculate the melt flow (MF) of a blend of resins whose individual
MF values are all somewhat different. The literature contains a number of equations for
calculating the viscosity of blends. When blending polyethylene resin, the most frequently
used equation is the arithmetic average of the logarithms (ln or log) of the melt indices,
which is obtained as follows.
Given two polymers, A and B, of the same type and with weight-average molecular
weights Mw(A) and Mw(B), respectively, the weight-average molecular weight of the
mixture is defined as:
Mw(mix) = xAMw(A) + xBMw(B)
(2.1)
where x = weight fraction and so xA + xB = 1. Then the melt index equation for a blend
of two samples A and B is given by:
ln MI = xA ln MIA + xB ln MIB
(2.2)
The melt flow ratio can either be I21/I2 or I21/I5, where the former is typical for injection
moulding and low-density film grades and the latter is typical for higher molecular weight
HDPE film, blow moulding and pipe grades. The melt flow ratio is a rough indicator of
the molecular weight distribution and the shear-thinning characteristic of the polymer.
The higher the melt flow ratio, the broader the expected molecular weight distribution,
with the accompanying increases in shear-thinning behaviour. The melt flow ratio has a
correlation with the molecular weight distribution, but it is not a purely linear relationship
over a broad range [7-9].
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Handbook of Plastic Films
2.2.3 Density
Density can be taken as a measure of the crystallinity of polyethylene. Since branching of
the macromolecular chain affects the solid-state structure or crystallinity of polyethylene,
density is also an indicator of chain branching. In this relation, we have to differentiate
between short-chain branching (SCB) and long-chain branching (LCB). Long-chain
branches are mainly present in LDPE, whereby short-chain branches predominate in
HDPE and LLDPE. Polyethylene (PE) in its solid state is a semicrystalline material. To
first approximation, its structure may be represented as a two-phase composite: a mixture
of a hard crystalline phase (density ~1 g/cm3) and a soft noncrystalline amorphous phase
(density ~0.84 g/cm3). The actual composite density depends on the relative amounts of
crystalline and amorphous phases. Because short-chain branching disturbs the regularity
of the PE chain, the crystallisation process is hindered and, accordingly, the crystallinity
of the solid-state structure is reduced [9]. Thus, with increasing frequency of chain
branching, the crystallinity and density decrease. HDPE, with little or no chain branching,
has a density of approximately 0.94-0.97 g/cm3 and a crystallinity by weight in the range
60-85%. LDPE and LLDPE, with more and longer-chain branches, have a density of
0.9-0.93 g/cm3 and crystallinity by weight of 40-60%. The density range can by classified
as shown below into four categories in accordance with ASTM D-1248 [10]:
(1) Type I (low density)
0.910-0.925 g/cm3
(2) Type II (medium density)
0.926-0.940 g/cm3
(3) Type III (high density)
0.941-0.959 g/cm3
(4) Type IV (very high density)
0.960-0.995 g/cm3
Density is typically measured using a density gradient column. The specimen could be a
pellet or a piece of a plaque produced under controlled cooling conditions. The rate of
cooling affects the crystallinity, and this will affect the density. A plaque quenched in icewater will have a lower density than a slowly cooled plaque. Boiling a slowly cooled
plaque will increase the measured density. As a result of the faster cooling employed, a
fabricated pellet will typically have a density (if unmodified by inorganic additives) lower
than the ASTM density [9, 11].
The density measured is strongly affected by the addition of inorganic additives with
higher density, such as antiblock agents for film-grade resins and other fillers. When
blending materials, the specific volume should be arithmetically averaged:
1/ρ = x1(1/ρ1) + x2 (1/ρ2)
(2.3)
where x is the weight fraction, ρ is density, and 1 and 2 represent samples 1 and 2.
44
Processing of Polyethylene Films
For PE blends, since the densities are to some extent similar, arithmetically averaging the
densities themselves is usually a good approximation. Increasing the density, with all
other parameters held constant, will increase the shrinkage, modulus of elasticity, yield
strength, heat resistance, gloss for HDPE, permeability resistance and hardness. An
increase in density will increase the impact strength, stress cracking resistance, transparency
and tear resistance. For HDPE resin, an increase in density will increase the tensile strength,
and with increase in SCB, the tensile strength will be lower. Weatherability will remain
relatively unaffected.
2.2.4 Chain Branching
Polyethylene produced with the high-pressure technology contains both long-chain
branches (100-200 or more carbon atoms) and short-chain branches. Polyethylene
produced with low-pressure technology has only short-chain branches (1-20 or so carbon
atoms). The two types of branching have different effects. For example, LCB has a
significant effect on melt rheological (flow) properties, whereas short-chain branching
(SCB) has no measurable effects. These differences lead to the definitions of ‘branched’
PE, which contain LCB, and ‘linear’ PE, which essentially do not have any LCB. LCB
affects the following melt flow properties:
(1) Extensional viscosity (‘strain hardening’),
(2) Shear viscosity (‘shear thinning’) and
(3) Elasticity (first normal stress differences).
LCB also affects the solid-state properties due to its influence on the melt flow properties.
Owing to its tendency to strain-harden, the presence of LCB can lead to orientation
effects, and those remain in the resultant solidified PE. This is seen in the blown film
area. SCB, in contrast, has little effect on melt flow properties but plays a major role in
the solid-state properties. SCB will affect the density, modulus of elasticity, heat of fusion,
optical properties, impact resistance, tear resistance and melting point. While branch
frequency (BF) is undoubtedly the most important SCB parameter, branch length and
branch distribution also have important effects. It is well known that, at constant SCB
frequency, the longer the branch, the lower the density. The density difference decreases
as the density of PE approaches the HDPE region.
Homogeneous SCB in the situation in which the branch frequency is the same for all PE
chains and, moreover, the branches are randomly distributed along the main chain.
Heterogeneous branching refers either to branch frequency varying from molecule to
molecule or to a non-random distribution of branches along a given PE chain. At constant
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Handbook of Plastic Films
overall branch frequency, the more heterogeneous the branching, the higher the density.
In high-pressure PE, SCB is homogeneous while LCB increases as the molecular weight
increases [9, 12].
This distribution can be measured by separating the polymer into fractions using
chromatographic techniques. Temperature-rising elution fractionation (TREF) is a method
widely used nowadays to determine the comonomer and branching distribution in
polyolefins. The technique used to measure the distribution is carbon-13 nuclear magnetic
resonance (13C NMR). It gives the sequence distribution along the chain, but averages
the distribution among chains.
SCB can be detected using Fourier transform infrared spectroscopy (FTIR) or 13C NMR.
SCB can be measured using three different units: frequency per 1000 backbone carbon
atoms, weight per cent (wt%) or mole per cent (mol%). If SCB is reported in frequency
per 1000 backbone carbon atoms, it can be converted using the fact that each monomer
or comonomer contributes two carbon atoms to the backbone. Thus, there are 500
monomer/comonomer units making up the 1000 backbone carbon atoms.
2.2.5 Intrinsic Viscosity
The viscosity of a polymer solution or polymer melt, in general, is a measure of the
fluid’s resistance to flow. Intrinsic viscosity is a measure of the hydrodynamic volume of
a polymer molecule in solution. It is sensitive to molecular weight conformation as well
as molecular size [9, 12-16].
For a given set of polyethylene resins produced with the same process, intrinsic viscosity
can be used as an indicator of the average molecular weight of the polymer. Because
intrinsic viscosity is affected by molecular conformation, LDPE (with their long-chain
branching) will assume a hydrodynamic volume in solution smaller than that of a linear
PE of equivalent molecular weight. PE produced by different processes will have different
linear relationships between ln(melt index) and intrinsic viscosity. The molecular weight
distribution may be a factor as well. The dilute solution viscosity test defines what is
known as the viscosity-average molecular weight.
Intrinsic viscosity (dl/g) measurements are normally made by using a three-point zeroconcentration extrapolation. Because it is a zero-concentration measurement, it is
independent of concentration; the intrinsic viscosity is dependent on the solvent used in
the test. Measurements of PE dilute solution viscosity have been made at 140 °C and at
concentrations of 0.1, 0.2 and 0.3 g polymer/100 cm3 decalin.
46
Processing of Polyethylene Films
More recent measurements have been made at temperatures higher than 140 °C with
trichlorobenzene (TCB) at concentrations ranging from 0.025 up to 0.25 g polymer/
100 ml solvent. The latter set of conditions (TCB, 140 °C) corresponds to the more
typical size exclusion chromatography (SEC) conditions used to determine the molecular
weight of PE.
2.2.6 Melting Point and Heat of Fusion
As the temperature rises from ambient temperature, the properties of a PE resin
change from solid to viscous-like material. Some of these changes are due to facilitated
plastic deformation of the crystalline phase. The most part, however, is due to melting
of the crystalline phase.
The melting temperature of PE lamellar crystallites decreases as lamellar thickness
decreases, or as crystallite internal perfection decreases (e.g., due to the partial
incorporation of short branches). Since lamellae vary in thickness and perfection,
the melting of PE occurs over a range of temperatures. The melting point of PE resin
is to some extent a matter of definition. The most popular definition is in terms of a
specified point on a differential scanning calorimeter (DSC) trace. The apparent
melting point will vary with the method of measurement: mechanical response,
dilatometer, X-ray scattering, light scattering or calorimetry. DSC is the most
frequently used method for PE.
The apparent melting point decreases with increasing short-chain branch frequency
(decreasing density), increasing short-chain branch homogeneity, decreasing
crystallisation temperature (faster cooling) or decreasing short-chain branch length.
In the first three cases, the effect is due to decreasing lamellar thickness. In the last
case, the effect is due to decreasing internal perfection of the crystallites. In practical
terms, the melting point can be used as an indicator of the density for a given PE
type. While LDPE may have a melting point of ~120-127 °C, HDPE resins have
melting points in the range of 129-135 °C.
The heat of fusion, ΔHf, is the amount of energy required to accomplish the solid-tomelt phase transition. The heat of fusion is linearly proportional to the density. In
theory, if the heat of fusion of 100% crystalline PE is known, the heat of fusion of
the crystalline fraction of the PE sample can be determined by taking a ratio of the
sample to 100% crystalline. In calculating the per cent crystallinity of a sample, a
value (average of five literature values) of 68.4-69.2 cal/g (286.6-299.0 J/g) is used
as the heat of fusion of 100% crystalline PE.
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Handbook of Plastic Films
2.2.7 Melt Properties – Rheology
For convenience, flow can be classified into shear flow or extensional flow. Shear flows
are those in which the velocity component varies only in a plane normal to its flow
direction. Shear rheology plays an important role in PE extrusion. Extensional flow, in
contrast, is characterised by a velocity component that varies only in its own flow direction.
Elongational viscosity is important in film blowing, for example. It is generally not possible
to predict the extensional rheology of polyethylene from its shear flow results or vice
versa [9, 10]. The viscosity is reported versus either the shear rate or the extension rate.
2.2.7.1 Relation of Viscosity/Shear Rheology
In the melt of a high molecular weight polymer like PE, under equilibrium conditions, the
chain molecules are not stretched out from chain end to chain end, but instead we assume the
so-called ‘random-coil’ configuration, i.e., the contour of a chain resembles a three-dimensional
random walk. The random-coil molecules are mutually interpenetrating, so that the
environment of a given molecule consists mostly of segments of other molecules. These
segments of other molecules collectively form a kind of ‘tube’ in which the molecule of
interest is ensheathed. Flow of the polymer melt involves both the axial motion of molecules
through their respective tubes (‘reptation’) and the lateral translation of the tubes themselves
[7-9]. These processes obviously require the cooperative motion of large numbers of chain
segments, which helps to explain why the viscosity (or resistance to flow) of polymer melts is
orders of magnitude higher than that of small liquid molecules of similar chemistry.
Another difference between a polymer melt and a small liquid molecule is that the viscosity
of PE melts depends on the flow rate [5, 6]. In particular, the shear viscosity of PE melts
decreases with increasing shear rate, a phenomenon known as ‘shear thinning’. Generally
speaking, PE with a broad MWD shear-thins to a greater extent than PE with a narrow
MWD. Shear thinning is of particular importance in the various extrusion processes
(film, sheet, pipe, tubing, profile, blow moulding) as it permits easy flow through the
extruder and die together with shape retention or sag resistance outside the die. Two
polyethylenes that have the same melt index may be processed differently at the higher
shear rates encountered in fabrication steps depending on their level of shear thinning.
PE viscosity decreases with increasing temperature or decreasing molecular weight. For
linear PE over a wide range of MWD, the zero-shear viscosity (i.e., the shear viscosity at
the limit of zero shear rate) is proportional to:
exp(E/RT) Mw3.4
where E is the flow activation energy (5-6 kcal/mol), R is the gas constant, T is absolute
temperature and Mw is the weight-average molecular weight.
48
Processing of Polyethylene Films
At constant MWD, the effect of increasing temperature or decreasing molecular weight
is to shift the logarithmic viscosity versus shear rate vertically towards lower viscosities,
and also horizontally towards higher shear rates [13-15]. The vertical shift is the same as
for the zero-shear viscosities. For a temperature change, the horizontal shift has the same
magnitude as the vertical shift. For a molecular weight change, the horizontal shift is
usually smaller in magnitude than the vertical shift.
The zero-shear viscosity of branched PE is higher than that of linear PE at constant ‘coil
volume’ (i.e., the spherical volume ‘pervaded’ by a random-coil molecule). This is easily
understood in the light of the discussion of melt structure.
In the case of a branched PE molecule, the axial reptation of the main chain through its
tube is necessarily coupled to the lateral translation of the side chains and their tubes
[12]. The effect of this added constraint is to increase the viscosity over that of linear
polymer. The viscosity of branched PE is also more sensitive to temperature than that of
linear PE (apparent flow activation energy 10 kcal/mol versus 6-7 kcal/mol).
2.2.8 Elongational Viscosity
The rheological properties of PE in an elongational type of flow are very different from
those in shear flow. This viscosity is measured under more of a ‘tensile’ mode. At particular
elongation rates, the apparent elongational viscosity does not even reach a steady-state
value within the time required to impose the desire total strain.
Furthermore, while the viscosity of narrow MWD linear PE resin may begin to level off
within this time period, the viscosities of branched or very broad MWD linear PE resins
often take a sharp upturn with increasing strain, similar to a rubber band, which becomes
stiffer the more it is stretched [9, 17]. This phenomenon is known as ‘strain hardening’.
The rubber band analogy is a good one, because strain hardening is more an elastic
phenomenon than a viscous one.
Strain hardening is observed more often for PE resin containing long-chain branching. It
plays an important role in processes that involve highly elongational flows. In film
extrusion, strain hardening limits the draw-down capability of branched or very broad
MWD linear PE; however, it compensates by providing enhanced aerodynamic bubble
stability in blown film. In extrusion coating and slot cast extrusion, strain hardening
provides resistance to neck-in edge waver and draw resonance.
2.2.9 Elasticity
Polymer melts are, in general, viscoelastic. That is, regardless of the mode of deformation
(shear or stretching) imposed on them, they exhibit both viscous and elastic properties.
49
Handbook of Plastic Films
The degree of viscous or elastic behaviour depends on the rate of imposed deformation
and the temperature.
The molecules in a polyethylene melt can be considered as ‘springs’ that prefer a random
conformation, but which, under an applied stress, can become uncoiled and extended.
Thus, in a flowing melt, the molecules are partially uncoiled and extended in the direction
of flow, thereby achieving a balance between the external stress driving the flow on the
one hand and the retractive spring force on the other. Upon removal of the external
stress, the melt will rebound as the molecules relax to their equilibrium random-coil
conformation. The magnitude of this rebound or recoverable strain characterises the
elasticity of the melt.
Melt elasticity increases with increasing flow stress, with increasing elongational
component of the flow, with broadening MWD and with increasing long-chain branch
content. With regard to the last two factors, elasticity is especially sensitive to the presence
of a few large and/or highly branched molecules in the system.
The die swell percentage (ratio of diameter of extrudate/capillary die x 100%) is an
indicator of the elasticity of the polymer melt after having been extruded through the
capillary die. It is not the most accurate measurement, however, since it involves the
physical measurement of the diameter of the extrudate. Elasticity determines the parison
swell characteristics in blow moulding, and the warping characteristics in injection
moulding. In PE film, heat shrinkability, optical clarity and mechanical anisotropy are
directly or indirectly affected by elasticity.
2.3 Blown Film Extrusion (Tubular Film)
2.3.1 Introduction
The most widely used method for film extrusion is the tubular or blown film technique,
which accounts for about 85% of all film production. Cast film extrusion is the other
major process and accounts for about 10-12% of all polyethylene film extrusion [17].
These two methods (blown film and slot cast extrusion) are described in this section
and the next.
2.3.2 Description of the Blown Film Process
To make polyethylene film, solid pellets are first dropped from a feed hopper into the
extruder barrel, melted by subjecting them to heat and pressure, and the melt conveyed by
50
Processing of Polyethylene Films
a rotating screw to the die. After having travelled all along the screw channel, the melt
passes through a screen pack and supporting breaker plate and adapter into the die. The
screen pack filters all contamination and foreign matter and removes them, before finally
the melt is forced through the narrow slit of a die. The screen pack and breaker plate also
help to increase the back-pressure in the barrel to improve mixing of the melt [18, 19]. The
die might be straight or ring-shaped. The resulting thin film has the form of a tube or
‘bubble’. Coming out of the extruder, the film is cooled and is finally rolled up on a core.
As the variety of polymers and blends has steadily increased, and film requirements have
become more demanding, extruder and screw technologies have evolved to the point
where screws are tailor-made for a specific system and application [19-22]. Figure 2.1
depicts a typical decreasing-pitch screw for LLDPE.
Figure 2.1 Typical decreasing-pitch polyethylene screw
In standard tubular blown film extrusion, the hopper feeds the resin to the screw, which
conveys, compresses, shears, melts and pumps the material to the die. In tubular film
extrusion, dies are circular in shape and are bottom or side fed (Figure 2.2).
2.3.3 Various Ways of Cooling the Film
The resin enters the die, flows around the core pin and exits as a thick-walled tube.
While the resin is still in a molten state, air is introduced into the tube through a port in
51
Handbook of Plastic Films
Figure 2.2 Blown film die in different arrangements
the centre of the die to expand the bubble to the desired diameter or lay-flat width. No
additional air is required once the bubble diameter has been reached. By introducing air
through the die-torpedo into the tube of film, the tube can be expanded to two or three
times the diameter of the die. Thus, within limits, many different widths of film can be
obtained from the same die. By varying the speed of the rollers closing the end of the
bubble (the nip rollers), the amount of longitudinal stretching can be varied, and this is
normally used to adjust the thickness of the film to the required value. This process has
several advantages: With only one die, a range of film widths and thicknesses can be
produced as well [23]. A bag can be produced by only one heat seal and two cuts, and
very wide film can be produced (by slitting the tube) with equipment wide enough to
handle only half this width. On the other hand, a drawback of the process is that the rate
of cooling of the film is rather low, particularly if air cooling is used [23, 24]. At high
output rates there are difficulties of controlling bubble movements sufficiently to keep
film thickness between close limits. Further, because the film is nipped between two
rollers at one end of the bubble, the film temperature at this point must be sufficiently
low to prevent ‘blocking’. The use of additives to prevent blocking can allow greater
production speeds to be attained, and improvements in haul-off techniques have given
better control over the swaying and shaking of the bubble.
The cooling rate is becoming the limiting factor on the whole process [25-27]. In this
respect the process compares unfavourably with the slot-die process; also, with air cooling
52
Processing of Polyethylene Films
it is not possible to use shock-cooling and high extrusion temperatures to effect a reduction
in haze, as is possible with other processes.
The bubble might be externally cooled by means of an air ring encircling the base of the
bubble. Cooling air is uniformly distributed and solidifies or quenches the tube. The
collapsing frame serves to collapse the tube into a lay-flat (Figure 2.3), whereupon it
enters the nip rolls for final flattening. The nip rolls seal the air in the bubble and draw
the film upward from the die.
For given extruder output rate and blow-up ratio, the film gauge is controlled by the nip
roll speed. It is desirable to have nip rolls of adjustable height to allow a lower die-to-nip
distance than is normally used for LDPE extrusion [27]. The reduced height provides
greater bubble support and stability, and allows the film to enter the nips when warm,
minimising wrinkles from improper bubble geometry and/or melt temperature and gauge
nonuniformity. Idler rolls guide the lay-flat from that point to the wind-up rolls, which
wind the film on to cores.
Figure 2.3 Air ring cooling the blown film
53
Handbook of Plastic Films
2.3.4 Extruder Size
In size, extruders are available from bench-top models for laboratory work up to 500
mm diameter screws. Screw sections and compression ratio have important functions.
Commonly, the length-to-diameter (L:D) ratios of screws range from 15:1 up to 33:1.
Recent developments consider a mixer at the screw end in the length range of 3D [27,
28]. A number of different mixers are available. The compression ratio is the ratio of the
channel volume of one screw flight in the feed section to that in the metering section. A
compression ratio of about 4:1 is recommended for film extrusion. High compression
forces result in high internal heating, in good mixing of the melt and in pushing back any
traces of air carried forward with the melt. Barrel diameter is determined primarily by
desired output. Knowing the drive motor speed and the reducer ratio of a given extruder,
maximum rpm can be determined. Therefore, the throughput T for any given size extruder
can be calculated, based on the following formulae (where D is the extruder diameter
and h is the depth of the metering section):
(1) Neutral screw
• when D and h are in inches
T = 1.15D2h lb/h rpm
(2.4)
• when D and h are in millimetres
T = 7 × 10–5D2h kg/h rpm
(2.5)
(2) Controlled screw
• when D and h are in inches
T = 1.4D2h lb/h rpm
(2.6)
• when D and h are in millimetres
T = 4.6 × 10–5D2h kg/h rpm
(2.7)
Then it is easy to match dies and extruder capacities [28, 29]. Maximum extruder rpm
can be calculated by dividing gear reduction into rated motor speed.
Some estimates of extruder size and die sizes based on throughput and cooling capacity
are shown in Table 2.1.
54
Processing of Polyethylene Films
Table 2.1 Some estimates of extruder size and die sizes based on throughput
and cooling capacity
Extruder size
Die diameter
inch
cm
inch
cm
1.5
3.81
up to 4
up to 10.16
2.5
6.35
6 to 12
15.2 to 30.5
3.5
8.89
12 to 18
30.5 to 45.7
4.5
11.43
18 to 28
45.7 to 71.1
2.3.5 Horsepower
The horsepower needed to drive the extruder motor is important. Based on a given
pound per hour per horsepower (lb/h hp) relationship, the die, extruder and motor can
be matched to give an optimum equipment arrangement. The examples in Section 2.3.6
give the pound per hour per horsepower values generally expected for 0.2 and 2.0 melt
index resins extruded with neutral and water-cooled screws. Table 2.2 gives the normally
supplied drive motor horsepower ratings for several extruder sizes.
Table 2.2 Extruder size versus horsepower relationships
Extruder size
Drive motor rating
inch
cm
hp
kW
1.5
3.81
5 to 7.5
3.78 to 5.59
2.5
6.35
20 to 40
14.91 to 29.88
3.5
8.89
60 to 100
44.71 to 74.57
4.5
11.43
75 to 150
55.93 to 111.86
2.3.6 Selection of Extrusion Equipment
Selection of blown film equipment should be made in a logical sequence [28]. Before
deciding on specific equipment, the type, size and end-use of the film to be produced
should be determined. Knowing these variables, selection of proper equipment for a
given application is fairly simple. Blow ratio is the tool by which the proper die size
can be selected (Figure 2.4). In general, blow-up ratios of 1.5:1 to 3.5:1 are best suited
for film production.
55
Handbook of Plastic Films
Figure 2.4 Schematic drawing of the blow ratio of film and the blown film width after
the nip rolls have flattened the bubble to a double layer of film
Die size and cooling capacity are the major considerations in the selection of an extruder.
The general ‘rule of thumb’ in the industry is to expect a throughput of 5-7 lb/h inch
(0.9-1.2 kg/h cm) of die circumference. Die capacity and drive motor horsepower, which
depend on several extruder size factors, can then be calculated. Some typical values are
given in Table 2.3 and Table 2.4, respectively.
56
Processing of Polyethylene Films
Table 2.3 Calculated die capacity depending on melt index and type of screw
Die diameter
Output
inch
cm
lb/h
kg/h
4
10.16
75
34.0
6
15.24
110
49.9
8
20.32
150
68.0
10
25.40
190
86.2
12
30.48
220
99.8
16
40.64
300
136.1
Table 2.4 Drive motor horsepower depending on several extruder sizes
Melt index
Screw
2.0
Rate
lb/h hp
kg/h kW
Neutral
8 to 10
13.1 to 16.4
2.0
Cooled
5 to 6
8.2 to 9.8
0.2
Neutral
5 to 6
8.2 to 9.8
0.2
Cooled
3 to 4
9.9 to 6.6
2.4 Cast Film Extrusion
2.4.1 Description of the Cast Film Process
In cast film extrusion, the melt is forced through a flat or slotted die opening, either directly
into a cooling water-bath, or tangentially contacting a highly polished, water-cooled chill
roll (cf. Chapter 1). The flat film is cooled by two or more of these rolls and is carried by
idlers to conventional treating and winding equipment. The film is stretched longitudinally
between the die and the cooling-bath or rollers to the required thickness. Chilled steel
rollers are preferred to a water-bath if the film contains hydrophilic materials such as
antistatic agents, which cause some wetting of the film, with subsequent drying difficulties.
Chilled rolls also allow greater production speeds than a water-bath [22, 25-27].
Flat film dies are often very long and heavy to install or to change [18]. Unfortunately,
there is no close relation between die opening and film thickness. Generally, high gauges
57
Handbook of Plastic Films
require large openings. To produce a film of 25-27 μm thickness, the opening is normally
around 0.5 mm. Usually one of the jaws is adjustable by means of screws so that the die
opening can be reset, using a brass feeler gauge of known thickness and a torque wrench.
Die temperature and resin temperature at the die lands are usually higher than in blown
film dies, ranging up to 300 °C. It is important to keep melt temperature uniform. The
die always has a number of heating zones. To minimise temperature variation and
fluctuations in film quality, the temperature along the die should be kept within very
strict tolerances of 1 °C. To avoid film faults and imperfections, the inside die surface as
well as the polishing rolls must be kept well polished. Slightly surface irregularities will
result in gauge variations and die lines (lengthwise parallel groves).
A regular die cleaning schedule will usually prevent faults in the inner surface. For the
production of very thin films, an air knife is very commonly used. When the hot film is
drawn down on to the first cooled chill roll, it may ‘neck-in’ (shrink) at the edges. Neckin is the difference between the hot melt width at the die lips and the film width on the
chill roll. When film with beading is wound up, the roll will sag in the middle, making it
difficult to use it later for packaging or bag-making. Therefore, such a film must be
trimmed at both edges.
Neck-in will not occur in blown film production, since the bubble has no edges that may
shrink. Another cast film defect, which has something to do with cooling, is ‘puckering’
(a slight bulging across the film recurring at regular intervals). Running the first chill roll
hot may reduce puckering. If the roll runs cold, the film may later warm up in storage
and expand, and the roll may become loose. If the melt flows well, there is little danger
of severe puckering.
The production of packaging bags from flat film requires special machinery to effect
sealing of the sides of the bag as well as the bottom.
A considerable reduction in haze can be obtained by shock-cooling. This is not possible
in the tubular film process except by friction contact with water-cooled metal, with the
consequent marking of the film.
2.4.2 Effects of Extrusion Variables on Film Characteristics
2.4.2.1 Optical
Generally, with increased stock temperatures, higher gloss and lower haze will be obtained
for low-density polyethylene. As the melt becomes hotter or more fluid, molecules will
58
Processing of Polyethylene Films
have more time to align themselves and give a smooth film, which is a prerequisite for
good gloss and low haze. Other problems such as bubble stability may result, however.
In tubular extrusion of high-density polyethylene, the melt temperature increase has a
minimal effect on the optical properties of the inherently hazy film [28-31]. When the
blow-up ratio (BUR) is increased from 1.5:1 to 3:1, the gloss increases and the haze
decreases in low-density polyethylene film, but the effect is negligible in high-density
polyethylene film.
In slot casting and as is observed in tubular film extrusion, increasing the melt temperature
generally improves optical properties, although the degree varies with various high-density
polyethylene resins. Increasing film speed generally results in poorer optical properties –
increased haze and lower gloss.
2.4.2.2 Haze
Haze may be of two kinds: surface roughness caused by melt flow phenomena, and
surface roughness and internal optical irregularities caused by crystallisation. The faster
the film cools between the extrusion die and the freezing point, the less is the haze resulting
from crystallisation, and the greater is the haze resulting from flow. In high-density
polyethylene, in which the haze caused by crystallinity is dominant, shock-cooling can
be used to produce almost haze-free film.
Haze in film was formerly accepted as an unavoidable property of polyethylene, but it
has been shown that commercially available polymers of slightly higher density than the
normal 0.918 g/cm3 give film of lower haze. The higher density of these materials results
in a greater susceptibility to ‘brittle’ tearing, other things being equal, and in greater
gloss, clarity and stiffness.
In soft-goods packaging, haze in the film dulls the colour of the packaged article as seen
through the film, and much of the sales appeal of using a transparent package is lost.
Provided adequate toughness is retained, film of the lowest possible haze is required for
this market.
2.4.2.3 Gloss
Since it has been found that small increases in the density of polymers could result in
better gloss, the packaging market has demanded this property in order to add sparkle to
the package. However, it has been suggested that too much gloss may destroy some of
the appeal of a package by reflecting too much of the shop’s lighting and of the variously
59
Handbook of Plastic Films
coloured goods in the vicinity. A reduction in chill roll temperature generally improves
both haze and gloss in high-density polyethylene films.
2.4.2.4 Impact Strength
To have good impact strength, a balance of orientation between machine direction (MD)
and transverse direction (TD) molecular structure is needed. Therefore, an increase in
blow-up ratio tends to balance this orientation in both directions. As the stock temperature
is increased, impact strength increases and a better balance of orientation (machine
direction versus transverse direction) results. High-density polyethylene has a greater
tendency than low-density polyethylene to orient and show large machine direction versus
transverse direction differences, i.e., splittiness.
The toughness of film is its most important feature in the packaging of agricultural
products (potatoes, carrots, etc.) and building applications. For these uses, the haziness
of the film is not of great importance, and, as toughness and haze are to a certain extent
mutually exclusive, hazy film has been accepted.
The tensile strength of the film is, as expected, mainly dependent on melt flow index, but
the apparent brittleness depends mainly on the density of the polymer: the higher the
density, the more brittle is the film. Brittleness also depends partly on the extrusion
conditions of the film.
Brittleness of film should not be mistaken for the kind of brittleness encountered in
mouldings. Examination of a ‘brittle’ tear does not show a sharp fracture but reveals a
narrow edge of stretched film. A ‘brittle’ film requires much less energy to tear than
does a ‘non-brittle’ one because a smaller area of film is stretched before the tear
occurs. If the stretching is highly localised, a ‘brittle’ tear occurs; if the stretching is
distributed over an area, a ‘non-brittle’ tear results. ‘Brittle’ tears are more likely to
occur if the tearing stress is applied at high speed, such as in an impact, and film is
normally tested under such conditions.
As the density of polyethylene increases, so also does its rigidity. Film made from HDPE
is stiffer in both handling and appearance. In over-wrapping machines designed to handle
stiff ‘Cellophane’ or paper, stiffness exceeding some minimum value may be essential.
Increasing the melt temperature in the slot casting process decreases tensile strength in
the machine direction, but produces film with more balanced orientation and a resulting
increase in impact strength. Impact strength is increased by a reduction in chill roll
temperature in the slot casting process.
60
Processing of Polyethylene Films
2.4.2.5 Blocking
Although blocking is not strictly a property of the film itself, it is one of the most serious
limitations on the high-speed production of film by the blown extrusion process. For higherdensity polymers, blocking is a less serious limitation, because the increased rigidity of the film
prevents the intimate contact obtained between flexible films. In contrast, in low-density
polyethylene extrusion, increased stock temperature can sometimes cause blocking. Excessive
film temperature during the collapsing of the tube will cause the inside surfaces to stick together.
2.4.2.6 Bubble Stability
As the blow-up ratio increases, the blown tube becomes larger and nonuniformities in polymer
flow from the die and cooling air are magnified. With increasing blow-up ratio, the increased
surface area that results is more susceptible to draughts. These external forces tend to make
the bubble waver and cause wrinkling and poor gauge.
2.4.3 Effect of Blow-up Ratio on Film Properties
2.4.3.1 Optical
Up to a certain point as the frost-line is increased, gloss increases and haze decreases. However,
beyond this point, the gloss decreases and the haze increases. The increase in gloss and
decrease in haze from increased frost-line height occur because the molecules tend to become
better oriented and do not allow molecule ‘ends’ to protrude, which would give a ‘rough’
surface. Too long cooling time gives rise to internal haze inherent in polyethylene.
2.4.3.2 Impact Strength
Impact strength is associated with molecular orientation. Thus, on increasing frost-line height
without increasing the blow-up ratio, the orientation in the machine direction increases. This
‘one-direction’ orientation allows the molecules to line up or orient, thereby producing a film
that is ‘splitty’ and poor in impact strength.
2.4.3.3 Bubble Stability
Bubble stability can decrease as stock temperature is increased. Instability results when the
bubble (hot melt) lacks stiffness to resist the forces exerted by the cooling air and draughts.
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Handbook of Plastic Films
Increased frost-line height produces a tube with a longer length of molten or hot material.
This soft material is very susceptible to wavering because of lack of strength, draughts or
variables such as nonuniform cooling. In order to maintain the stable bubble necessary
for good gauge, low frost-lines are desirable.
2.4.3.4 Puckering
Puckering is an expression used in the slot casting process. Puckering is caused by a
nonuniform frost-line on the first chill roll, which produces density variations in the
machine direction. These density differences cause length variations that appear as
small bags, or puckers. Increasing the melt temperature decreases the tendency for
puckering, while reduction in chill roll temperature increases the puckering tendency
[32, 33].
The draw distance (distance between die and chill roll) affects the film properties.
Optimum draw distance for good film production varies with equipment size and
production rates, and must be found experimentally. In general, optimum draw distances
may range from 1-2 inch (2.54-5.08 cm) in laboratory equipment to over 12 inch
(30.48 cm) in commercial equipment.
2.5 Processing Troubleshooting Guidelines
Economical film-making means the production of high-quality film in long trouble-free
runs at the highest possible production rate. Since certain changes in machine conditions
may improve quality while decreasing output, or vice versa, it is frequently necessary to
find some kind of compromise between the two goals – high output and superior quality.
In Table 2.5 some guidelines are given for machine operators to check their machine(s)
and product(s) periodically to prevent unnecessary trouble during film production.
2.6 Shrink Film
Shrink-wrapping use is growing very fast worldwide and especially in Europe, both for
light/small articles and for heavy/huge pallets. Low-cost polyethylene shrink film is
produced on conventional equipment, by the blown extrusion process. No additional
machinery is needed. Only processing conditions and resin characteristics need to be
properly selected, according to the film’s application. The film shrinks because a high
degree of molecular orientation or internal stress was introduced into it during its
manufacture. These stresses are ‘frozen’ in the film by the air cooling. When the film is
62
Processing of Polyethylene Films
Table 2.5 Processing troubleshooting guidelines
Problem
Possible cause and/or solution
Poor gauge
1.
Poor die and/or air ring design
2.
Die and/or air ring need adjustment
Poor optics
Wrinkles
Blocking
3.
Air draughts
4.
Dirt in air ring
5.
Temperature or resin change
1.
Improper resin for property desired
2.
Raise extrusion and die temperatures
3.
Raise frost-line height
4.
Increase blow-up ratio (2, 3, 4 can only be accomplished within
limits of bubble stability and blocking)
1.
Poor gauge control
2.
Misaligned rolls or collapsing tent
3.
Air draughts
4.
Unsupported film length too great
5.
Too much static build-up in film; use static eliminators
6.
Film slip is too low
7.
Film too stiff
8.
Keep a level frost-line
1.
Lower melt temperature
2.
Reduce cooling air temperature
3.
Increase air flow
4.
Increase distance from die to pinch roll if possible
5.
Use additional cooling rings
6.
Reduce primary nip roll pressure
7.
Reduce take-off speed
8.
Treatment too high
9.
Wind-up tension too high
10. Improve air circulation in area of nip rolls
11. Resin may need more antiblock
Low impact
1.
Is the correct resin being used
2.
Increase blow-up ratio
3.
Lower melt temperature
4.
Lower frost-line height
5.
Reduce die opening
6.
Eliminate die and weld lines
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Handbook of Plastic Films
Problem
Table 2.5 Processing troubleshooting guidelines continued
Possible cause and/or solution
Low impact
Film imperfections
(holes, tear-offs,
tube collapse, apple
sauce, fish eyes)
Bouncing
Bubble diameter
control
Poor roll flatness
64
7.
8.
9.
10.
Too high nip roll pressure results in weak crease strength
Check gauges for low caliper spots
Nip rolls may get too hot
Eliminate scratches on film surfaces from collapsing frame,
gussets and rollers
1.
Dirty screens and/or die
2.
Excessive gels in resin
3.
Melt either too hot or too cold
4.
Improper resin for draw-down required
5.
Decrease die opening for improved draw-down
6.
Improve homogenisation by greater back-pressure to screw
7.
Use water cooling of screw for improved resin homogenisation
8.
Improve gauge control
9.
Keep resin and scrap clean
10. Check die land surfaces for imperfections
1.
Frost-line too high or too low
2.
Melt temperature too high or too low
3.
Take-off too slow
4.
Extruder surging
5.
Improper adjustment of air flow from air ring
6.
Guide bars too tight
1.
Nip rolls not completely closed
2.
Nip rolls worn unevenly
3.
Air pressure to nip not holding
4.
Small pinholes in film
5.
Frost-line height change due to melts and/or air temperature change
6.
Frost-line height change due to change in air flow from air ring
7.
Air leakage into bubble because of leaking valve
8.
Air leakage in air lines leading into die for bubble diameter control
9.
Extruder surging
1.
Die and air ring should be rotating or oscillating
2.
Cores should not be out-of-round
3.
Eliminate stray air currents around bubble
4.
Eliminate wrinkles in film at wind-up
5.
Poor gauge control
6.
Film temperature at wind-up too hot, resulting in crushed cores
Processing of Polyethylene Films
Problem
Table 2.5 Processing troubleshooting guidelines continued
Possible cause and/or solution
Drop in roll weight
Insufficient
treatment
Overtreatment
1.
Screens becoming clogged
2.
Screw water temperature getting lower
3.
Flow of water to screw greater
4.
Feed hopper beginning to bridge
5.
Take-off speed has increased
6.
Pre-matted polymer in feed section of screw
1.
Take-off speed too great for treater setting
2.
Treater setting too low
3.
Gap setting between electrode and treater roll too great
4.
Dielectric roll (Mylar, Hypalon, etc.), has pinholes or is too thick
5.
Tune the treating unit
6.
Treater bar has too sharp edge
7.
Check for proper spark gap
8.
Check ink batch and how it is being used
9.
Resin may have too much slip
1.
Reduce treater setting
2.
Film not laying flat on treater roll
3.
Dielectric is loose (Mylar)
4.
Gap setting between electrode and treating rolls too small
reheated to a temperature above its softening point (such as in a shrink-tunnel or
oven), the molecules tend to revert to their entangled unstrained state. Thus, the internal
stresses are released, causing the film to shrink.
2.6.1 Shrink Film Types
Two shrink film types are mostly used today (Table 2.6). At present, bi-oriented shrink
film is more popular than mono-oriented film. In the future, mono-oriented film is expected
to make major gains in the total pallet-wrapping and sleeve-wrapping business. Bi-oriented
film will remain for full overwrap of small and medium-sized packages. The most widely
used film thicknesses are:
•
25-50 μm (thin shrink film) and
•
57-150 μm (heavy-duty shrink film).
65
Handbook of Plastic Films
Table 2.6 Shrinkage (%) of the two major shrink film types
Type
MD
TD
Mono-oriented
60-80
10-20
Bi-oriented
50-60
30-50
2.6.2 Shrink Film Properties
The properties that fully describe a shrink film are the following:
(1) Thickness and thickness uniformity,
(2) Percent shrinkage (MD, TD),
(3) Shrink strength (MD, TD),
(4) Tear resistance (Elmendorf),
(5) Impact strength (dart drop),
(6) Puncture resistance,
(7) Clarity (haze, gloss),
(8) Slip and antiblock, and
(9) UV resistance.
Normally a given application needs only some of these properties to be checked. Two
examples are shown in Table 2.7.
Table 2.7 Shrink film properties to be checked according to application
Display packaging of light articles, with
sharp corners
Wrapping of heavy pallets, to be stored
in the open air
Thickness
Thickness and thickness uniformity
Percent shrinkage
Percent shrinkage
Clarity
Shrink strength
Puncture resistance
Impact strength
Tear resistance
UV resistance
66
Processing of Polyethylene Films
2.6.3 The Manufacture of Shrink Film
To produce film shrinkable in a given direction, molecular chains must be oriented in the
same direction by processing. This orientation is obtained by stretching the film in the
required direction. The greater the film stretching, the higher the molecular orientation
and hence the shrinkage.
Every blown polyethylene film is shrinkable in the machine direction (MD), because it is
normally stretched much more in this direction. In fact, from Figure 2.5 it can be seen
that a film tube having diameter D1, length L1 and thickness T1, in the molten state, is
blown into a film tube having diameter D2, length L2 and thickness T2, in the solid state,
at room temperature.
Figure 2.5 Frost-line and blow-up ratio (BUR) in shrink film
These two film tubes must have the same mass. This condition may be written as
ρ2πD2T2L2 = ρ1πD1T1L1
(2.8)
where ρ1 and ρ2 are the film densities at the relative points. These densities are different
due to the different film temperatures and pressures.
67
Handbook of Plastic Films
A normal thin blown film can be stretched to about 9:1 in the machine direction and to
only 2:1 in the transverse direction. For this reason such a film will shrink very much in
MD and almost nothing in TD. This means that this film is a mono-oriented type shrink
film. Usually a thin shrink film is required to be bi-oriented, i.e., with more balanced
shrinkage. This condition is achieved by properly regulating the processing conditions
shown in Table 2.8.
Table 2.8 Influence of the processing conditions on the shrink
properties of LDPE
Shrinkage
Shrink force
MD
TD
MD
TD
Stronger orientation in the
machine direction
●
❍
●
❍
Larger BUR
❍
●
❍
●
●●●
❍❍❍
●●●
❍❍❍
Higher take-off speed
Neck-type bubble shape
●
●● ●
●●●
Higher frost-line
❍❍❍
●
Production speed
●●●
❍❍❍
Larger die gap speed
●●●
●● ●
Larger length of die gap
❍❍❍
❍❍❍
Thicker film
❍❍❍
❍❍❍
Higher melt temperature
❍❍❍
❍❍❍
❍❍❍
❍❍❍
Higher melt index
❍❍❍
❍❍❍
❍❍❍
❍❍❍
●●●
●
❍
❍❍❍
●●●
❍❍❍
Strong increase
Little increase
Fair decrease
Strong decrease
To obtain a balanced shrink film, the same amount of molecular orientation must be
produced in both MD and TD. First, the stretching ratio in the machine direction (MDSR)
and the blow-up ratio (BUR), in other words, the stretching ratio in the transverse
direction, should be made equal. This corresponds to putting MDSR = BUR.
In practice, the stretching in the machine direction (MD) predominates, due to other
factors (shearing suffered by the melt during its passage through the die, bubble shape,
68
Processing of Polyethylene Films
etc.). Thus, the BUR needed to obtain balanced shrinkage is somewhat higher than the
theoretical value. This corresponds to assuming a slightly higher coefficient (1.1 to 1.2)
in the practical formula for balanced shrinkage.
2.6.3.1 Bubble Shape and Frost-Line
Bubble shape and frost-line are important parameters controlling the shrinkages in TD
and MD. Referring to Figure 2.5, two types of bubble shape can be seen: one with a long
neck (continuous line), corresponding to a higher frost-line; the other with no neck (dotted
line), corresponding to a lower frost-line. The shape with a long neck gives more balanced
shrinkages. This is easy to explain. In fact, for bubbles of this shape, transverse stretching
predominates just below the frost-line, where the film is frozen, and no further relaxation
can take place. Thus, a high level of molecular orientation in TD is retained in the film.
2.6.3.2 Resin Melt Index
Melt index has a remarkable effect on shrink strength and a slight effect on per cent TD
shrinkage and on BUR for balanced shrinkages. In particular, the lower the MI, the
higher the shrink strength. This means that for heavy-duty shrink film a low MI is
preferred. Also, a low MI corresponds to lower BUR for balanced shrinkage and to
higher TD shrinkage.
The MWD has only a slight effect on shrink temperature. In particular, a film obtained
from a resin with narrow MWD (or low swell) shrinks at lower tunnel temperatures.
The presence of slip and antiblock additives has no effect on film shrinkage.
Ethylene-vinyl acetate copolymers (EVA) with low vinyl acetate (VA) content give shrink
films having the following properties:
(1) Faster shrinkage (higher tunnel production),
(2) Lower shrink temperature,
(3) Higher impact resistance (at low temperature),
(4) Higher puncture resistance,
(5) Lower slip and antiblock, and
(6) Higher gas/moisture permeability.
These films find application both for light shrink-wrapping and for heavy pallet wrapping.
69
Handbook of Plastic Films
UV stabilisers are necessary for applications in which the wrapped item (usually a pallet)
is stored in the open air. Stabilisers have no effect on shrinkage. They only affect shrink
film prices.
2.6.4 Shrink Tunnels and Ovens
The shrink-wrapping technique consists of wrapping and heat-sealing the article loosely
in the film. The loose film perimeter should exceed the article’s perimeter by no more
than 7-10%. The package is then conveyed through a shrink tunnel or into an oven.
Many types of heating system are used, the best being that using hot circulating air,
because it gives more uniform heating. The most important properties of a shrink film
from the point of view of an end-user are the following:
(1) Percent shrinkage (MD, TD),
(2) Shrink strength (MD, TD),
(3) Shrink speed and
(4) Shrink temperature.
The last two depend on film thickness and on the nature of item to be packed. In fact,
both these factors affect the amount of heat required to reach the softening point of the
film. Finally, it is worth pointing out that the maximum shrink strength is reached when
the film cools outside the oven and not inside. In fact, inside the oven the film shrinks
with a small shrink force, because it is hot and soft. When it cools rapidly to room
temperature outside the oven, the film shrinks tightly around the article, with a much
higher shrink force.
References
1.
Kunststoff-Handbuch IV: Polyolefine, Carl Hanser, Munich, Germany, 1969.
2.
Kunststoff-Handbuch II: Polyvinylchloride, Carl Hanser, Munich, Germany, 1963.
3.
Plastics Engineering Handbook of the Society of the Plastic Industry, 5th Edition,
Ed., M. L. Berins, Chapman and Hall, London, UK, 1991.
4.
P.J. Lucchesi, S.J. Kurtz and E.H. Roberts, inventors; Union Carbide
Corporation, assignee; US Patent 4,486,377, 1984.
70
Processing of Polyethylene Films
5.
J.C. Miller, R. Wu and G.S. Cielozyk, Tappi Extrusion Conference, Hilton Head
Island, SC, USA, 1985.
6.
J.C. Miller, Tappi Journal, 1984, 67, 6.
7.
A.V. Ramamurthy, Journal of Rheology, 1986, 30, 2, 337.
8.
A.V. Ramamurthy, Proceedings of the 2nd Annual Meeting of the Polymer
Processing Society International, Montreal, Canada, 1986.
9.
J.C. Miller and S.J. Kurtz, Proceedings of the IXth International Congress in
Rheology, 1984.
10. ASTM D1248, Standard Specification for Polyethylene Plastics Extrusion
Materials for Wire and Cable, 2002.
11. W.A. Fraser and G.S. Cieloszyk, inventors; Union Carbide Corporation, assignee;
US Patent 4,243,619, 1981.
12. S.J. Kurtz, T.R. Blakeslee, III and L.S. Scarola, inventors; Union Carbide
Corporation, assignee; US Patent 4,282,177, 1981.
13. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent
4,552,712, 1985.
14. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent
4,554,120, 1985.
15. A.V. Ramamurthy, inventor; Union Carbide Corporation, assignee; US Patent
4,522,776, 1985.
16. D.N. Jones in Proceedings of the 1984 Polymers, Laminations and Coating
Conference, Tappi Press, 1984.
17. W.H. Darnell and E.A.J. Mohl, SPE Journal, 1956, 12, 20.
18. W.D. Mohr, R.L. Saxton and C.H. Jepson, Industrial Engineering and Chemistry,
1957, 49, 1857.
19. S. Eccher and A. Valentinotti, Industrial Engineering and Chemistry, Industrial
Engineering and Chemistry, 1958, 50, 829.
20. B.H. Maddock, SPE Journal, 1959, 15, 383.
71
Handbook of Plastic Films
21. W.D. Mohr, J.P. Clapp and F.C. Starr, SPE Technical Papers, 1961, VII, January.
22. L.F. Street, International Plastics Engineering, 1961, 1, 289.
23. B.H. Maddock, SPE Journal, 1960, 16, 373.
24. B.H. Maddock, SPE Journal, 1961, 17, 369.
25. B.H. Maddock, Proceedings of Pressure Development in Extruder Screws
International Congress, Amsterdam, The Netherlands, 1960, 139.
26. C. Maillefer, Modern Plastics, 1963, 40, 132.
27. B.H. Maddock, SPE Journal, 1964, 20, 1277.
28. W.A. Fraser, L.S. Scarola and M. Concha, Tappi Journal, 1981, 64, 4.
29. J.C. Miller and S.J. Kurtz, Advances in Rheology, 1984, 3, 629.
30. S.J. Kurtz, L.S. Scarola and J.C. Miller, Plastics Engineering, 1982, 38, 6, 45.
31. J.C. Miller, Tappi Journal, 1984, 67, 6.
32. Film Converting Techniques for Linear Low Density Polyethylene, Union
Carbide Corporation, 1985.
33. S.J. Kurtz, Advances in Rheology, 1984, 3, 399.
72
3
Processing Conditions and Durability of
Polypropylene Films
H. Aglan and Y.X. Gan
3.1 Introduction
The durability of polypropylene (PP) films under tensile loadings and ultraviolet (UV)
irradiation is a very important end-use property. In this chapter, an overview of the
structures, synthesis, processing and applications of semicrystalline PP films is
introduced. The UV degradation mechanisms and the effect of UV degradation on the
durability of PP films are then presented. The functions of different additives in PP
films are described. Research findings based on a case study of the durability of several
groups of PP films with additives such as UV stabilisers, antioxidants and colouring
pigments, (e.g. calcium carbonate), are summarised. In the case study, typical PP film
specimens taken from different processing stages are tested to establish the effect of
composition and processing conditions on the durability of PP films. Microstructural
features of the films are identified and correlated with their durability. It has been
found that a lack of proper addition of UV stabiliser and antioxidant agent severely
degrades the durability of PP products. UV-degraded PP woven fabrics made from
stretched PP films totally lost their load-bearing capability and displayed severely
damaged structure with extensive microcracks, voids and dispersed secondary particles.
It has also been found that unstretched PP materials have very good durability under
static tensile loading. The stress-strain behaviour shows several distinct deformation
stages: elastic deformation, yielding and cold flow followed by strain strengthening.
The stress-strain relationship of stretched PP films reveals an elastic deformation stage
followed by limited plastic deformation from which no obvious cold flow was found.
However, stretching results in the drastic decrease in durability of these films. The
durability of the stretched films is less than 70%, while before stretching it was about
600%. Calcium carbonate pigment causes a decrease in tensile strength of stretched PP
tape, while UV stabiliser does not change the strength and durability of PP films
appreciably. A study of the surface morphology of these PP film samples revealed a
similar smooth surface with unidirectional texture. Defects in the form of crevices,
grooves and intruded particles were found on the surface of PP films with calcium
carbonate colouring pigment.
PP is a thermoplastic polyolefin polymer [1]. The structure of PP is stereoregular [2-5].
Crystalline PP was invented in the early 1950s by independent groups in the USA and
73
Handbook of Plastic Films
Europe. It entered the stage of large-scale production in 1957. Prior to 1950,
polypropylene polymer was a branched low molecular weight (molar mass) oil, which
had no significant use. After the discovery of polypropylene, obtained from the TiCl3based first-generation catalyst, at the Polytechnic of Milan in 1954, nothing
revolutionary happened until the discovery of the active MgCl2-supported high-yield
Ziegler-Natta catalysts at the Ferrara Giulio Natta Research Center in 1968. That
event was the beginning of the revolution that brought about the creation of the thirdand fourth-generation catalysts. The Ziegler-Natta achievements made the stereoregular
polymerisation of PP possible. The fourth-generation catalysts are super-active, and
are introducing an innovative and revolutionary new dimension to heterogeneous
catalysis. Because of the specific tailored architecture of the catalyst, it is possible to
give the catalyst the capability of determining the physical shape of the polymer
generated and its external and internal morphology. Thus, the type of specific
distribution within a single PP granule can be precisely controlled. This represented a
real breakthrough for PP synthesis technology. It was possible to design new, versatile,
clean and economical processes to create a new family of materials.
PP is a very useful material for various applications because of its good properties and
processability in large-scale production by extrusion, injection moulding and casting.
Various products can be manufactured from several types of PP, including (1) isotactic,
crystalline PP homopolymers, (2) random copolymers and (3) impact or heterophasic
copolymers. The advantages of PP are that it is lightweight, unaffected by moisture, fireproof, acid-resistant and possesses high stiffness. Structural PP products can maintain
excellent impact capability, high strength, high toughness and good dimensional stability
under service conditions. In addition, PP is cost-effective. The principal forms of PP in
applications include film and sheet, filament and fibres, pipes, profiles and wire coating.
PP has been accepted as a versatile piping material for a considerably long time. The
advantages in this application include resistance to chemicals and corrosive media,
ease and economy of handling and installation, low friction losses, low thermal and
electrical conductivity, high temperature resistance, minimum build-up of soil deposits,
and good outdoor durability in all weather. PP piping is used in industrial drainage
systems, in the chemical processing industry and in the oil industry for handling salt
water and crude oil. PP is also used for internal lining of metal pipes and tanks. PP
pipes can withstand temperatures up to 105 °C. Even under pressure, the service
temperature can be as high as 90 °C.
PP rods are used for the fabrication of prototype and production parts such as gears,
spools, pulleys, casters, etc. Various structural elements possessing high strength,
toughness and surface hardness have been produced from PP for applications requiring
a heat-resistant, noncorrosive material. PP wire and cable coatings possess desirable
74
Processing Conditions and Durability of Polypropylene Films
properties such as surface hardness, crush resistance, high softening temperature, low
dielectric constant and low environmental stress cracking. PP wire coating can be applied
in solid or foamed forms. PP has also found applications in the medical and biochemical
fields. The majority of moulded PP used in medical applications is for implantations
[6, 7], repairs [8-11], membranes [12] and disposable devices such as syringes [13].
Non-woven fabrics are used in items such as surgical masks and gowns. PP can also
serve as a matrix polymer for fabrication of fibre-reinforced composite materials [1420]. In addition, PP can form copolymers and polymer blends [21-25].
PP films are widely used in industry and daily life. Food packaging applications are
very good examples. From the early 1960s to now, PP has been the dominant film
packaging material in the snack food, bakery and candy (sweets) industries. PP films
are heat-sealable and they have the advantages of being grease- and oil-resistant for
the protection of the contents. Snack food packaging is the largest single use of PP
film. It is used because of its excellent moisture barrier properties, stiffness, gloss,
printability and crispness. When needed, special coatings provide excellent oxygen
barrier properties as well. Snack foods that are potato-based are adversely affected by
UV light, a condition that requires an opaque package. Opaque PP films and/or clear
films that are metallised address this need. Snack food packaging consists of more than
one layer. At the very least, a heat-seal layer is required to seal the package. Other
layers include slip films to facilitate processing through the converting line, oxygen
barrier layers for content protection, and adhesive layers to hold it all together. The
introduction of highly flavoured snack foods adds still another requirement to the
package: fragrance retention. The packaging of food products today requires a
sophisticated array of specially engineered films designed specifically for the products
they are chosen to protect.
Opaque PP films are also used to label soft drinks and other beverages. The films
produce an attractive package, do not rip, provide some abrasion resistance, and do
not come off when the bottle is chilled, for example, in a refrigerator or by ice-water.
Cables, wires and capacitors, widely used in the electronics industry, have made use of
special PP films to provide the necessary insulation. Moisture absorption negatively
affects the ability of a plastic to provide insulation. Since PP offers low moisture
absorption and has inherently good insulating properties, it is ideal for this application.
Commercial applications of cast films are growing rapidly. The film is mostly used for
packaging purposes [26], e.g., bread wrap, bakery products, bag liners, grocery bags,
textiles and miscellaneous wrapping and packaging applications. Other applications
include electrical cable wrapping and laminations with substrates such as paper,
cellophane and aluminium foil. Oriented PP film, with high mechanical strength in the
stretching direction, has found some other important applications. The film is slit,
75
Handbook of Plastic Films
stretched in the machine direction and knifed into strands or fibrils. Such types of PP
film have been used for weaving and manufacturing of heavy-duty bags, and many
other industrial and commercial products [27].
Various tapes and pressure-sensitive labels are also made from PP cast films or oriented
films. Cast PP films are also often used as the outer protective layer in diaper (nappy)
construction. The purpose of using PP films is to keep the moisture inside the diaper.
Recently, cast PP films have been used for manufacturing stationary products, including
clear overlays, dividers, photo albums, baseball cards and protector pages. Since PP
films can be easily captured and recycled, the environmental concerns surrounding
polyvinyl chloride (PVC) have accelerated the use of cast PP films in these applications,
many of which were previously served by PVC films.
Because of the outstanding combination of cost, performance, excellent physical
properties, strong and continuous expansion of process versatility, and environmental
friendly processes and materials during manufacturing, use and recycling stages, PP saw
an explosive growth in the amount of production in the worldwide market in its early
stage. The world market for PP has grown from around 1.5 million tons in the 1970s to
about 13 million tons in the early 1990s. The production in 1995 was about 22 million
tons and up to 30 million tons in 2000. Following its explosive early growth, the PP
business has maintained surprising vigour. Its growth rate for US production has remained
above 7% for the past two decades [28]. The recent production of all major plastics and
their growth rates are shown in Tables 3.1 and 3.2, respectively.
Table 3.1 Plastic production in kilograms per capita in 1993 [29]
World region
PP
LDPE
LLDPE
HDPE
PS
PVC
Total
N. America
10.2
9.8
7.5
13.6
7.2
12.6
60.9
W. Europe
9.7
11.2
1.87
7.8
5.3
12.1
47.97
Japan
15.7
7.0
5.03
8.1
10.9
16.7
63.43
Rest of Asia
1.07
0.65
0.62
0.81
0.54
1.7
5.4
Rest of World
0.94
1.95
0.87
1.15
0.92
2.63
8.46
Total
3.25
3.19
1.55
3.08
2.04
4.39
17.5
PP: Polypropylene
LDPE: Low-density polyethylene
LLDPE: Linear low-density polyethylene
HDPE: High-density polypropylene
PS: Polystyrene
76
Processing Conditions and Durability of Polypropylene Films
Table 3.2 Worldwide plastic production growth rate (%) between
1993 and 2000 [29]
World region
PP
LDPE
LLDPE
HDPE
PS
PVC
N. America
3.8
2.2
4.1
3.4
2.9
2.2
W. Europe
5.2
1.5
5.5
4.6
3.3
2.2
Japan
3. 1
2.8
7.8
3.8
3.3
2.6
Rest of Asia
11.2
5.9
11.2
12.2
9.8
8
Rest of World
16.1
7.3
20.4
10.9
11.8
8.3
Total
6.9
3.5
9.7
6.1
5.6
4.6
From the results shown in Table 3.1, it can be seen that the production in weight
per capita for PP is only slightly lower than that for PVC. The three developed
regions, i.e., North America, Western Europe and Japan, consume much more PP
than elsewhere. The other areas of the world consume very little of any plastic.
Table 3.2 shows that linear low-density polyethylene (LLDPE) has the highest growth
rate. PP ranks second and it is one of the fastest-growing plastics in the period
from 1993 to 2000.
The increase in worldwide PP capacity is shown in Table 3.3 by a comparison of
1994 and 1998 figures [28]. It can be seen that the growth in the developing regions
was even more dramatic than that in the three developed regions. Capacity in the
‘Rest of Asia’ region excluding Japan is expected to be the largest producing region
in the world.
Table 3.3 Worldwide polypropylene capacity [28]
1994 capacity
1998 capacity
kilotons
% of total
kilotons
% of total
Growth rate
(% per year)
N. America
5334
26
6313
23
4.3
W. Europe
5568
27
6618
24
4.4
Japan
2649
13
2979
11
3
Rest of Asia
4250
21
7310
27
14.5
Rest of World
2691
13
4321
15
12.6
Total
20492
100
27541
100
7.7
World region
77
Handbook of Plastic Films
3.2 Structures and Synthesis
The propylene molecule is the monomer unit of polypropylene. There are a number of
different ways to link the monomer together, depending on the stereo arrangement. Three
major factors control the stereoregularity of PP [30, 31]:
(1) The first factor is the degree of branching. The molecular chain of PP will be straight
(or linear) if the next monomer unit always attaches to the chain end as shown in
Figure 3.1(a). If the next monomer may add on to the backbone, this results in the
formation of branches, as seen in Figure 3.1(b).
Figure 3.1 Branching
Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.
Copyright 1996, Hanser Publishers.
(2) The pendant methyl sequence can also change the stereoregularity of PP. The addition of
propylene to the growing PP chain can be regiospecific or non-regiospecific as illustrated
in Figure 3.2. We can see that the addition of monomer can be in a head-to-tail manner
(Figure 3.2(a)) or in other ways such as head-to-head or tail-to-tail (Figure 3.2(b)).
Figure 3.2 Regiospecificity
Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.
Copyright 1996, Hanser Publishers.
78
Processing Conditions and Durability of Polypropylene Films
(3) Still another way to control the stereoregularity is the position of the tertiary hydrogen.
As shown in Figure 3.3, there exist two possibilities for the arrangement of the tertiary
hydrogen. If the propylene monomer is always added in the same stereo arrangement,
the alignment of the tertiary hydrogen will be in a same hand way, either right-handed or
left-handed (Figure 3.3(a)). Any change in the stereo arrangement of the adding monomer
can result in an opposite hand distribution of the tertiary hydrogen (Figure 3.3(b)).
Figure 3.3 Chirality
Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.
Copyright 1996, Hanser Publishers.
PP as a commercially used material and in its most widely used form is made with catalysts
that produce crystallisable polymer chains. These give rise to a product that is a
semicrystalline solid with good physical, mechanical and thermal properties. Another form
of PP, produced in much lower volumes as a by-product of semicrystalline PP production
and having very poor mechanical and thermal properties, is a soft, tacky material used in
adhesive, sealants and caulk products. The first product is often referred to as crystallisable
or iPP, while the second type is called noncrystallisable or ‘atactic’ polypropylene (aPP).
In addition to the two commonly defined PPs, isotactic and atactic polypropylene, there
is an intermediate state, which is defined as syndiotactic polypropylene (sPP). The
molecular chain of isotactic PP is linear. The pendant methyl sequence is regiospecific.
That is, the addition of propylene to the growing chain is head-to-tail. In addition, the
same hand arrangement of the tertiary hydrogen can be found in iPP. The regularity of
the isotactic polypropylene allows it to crystallise. The arrangement of carbon atoms in
the main chain of crystallised isotactic PP is in the shape of a helix, when it is viewed
obliquely from one end. Unlike iPP, sPP results from the consistent insertion of the
propylene monomer in the opposite hand from the previous monomer unit. This is a
different type of stereoregularity from that of isotactic PP. Syndiotactic PP is not
commercially significant. Atactic PP can be produced from any one or more of: the
inconsistency in the degree of branching, the change in the pendant sequence, and the
non-stereospecificity. The three types of structure of PP are shown in Figure 3.4.
79
Handbook of Plastic Films
Figure 3.4 Three PP structures
Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p4.
Copyright 1996, Hanser Publishers.
Isotactic PP can crystallise in several forms, which have different density of the structural
(unit) cell. The α form is dominant. Other forms include the β, γ and mesomorphic
(smectic) forms [32]. All of these forms are composed of molecular chains in a helical
conformation with a common repeat distance of 6.5 Å. They differ in unit cell symmetry,
inter-chain packing and structural order [33, 34]. The structure, conditions for
formation, melting behaviour and morphological characterisation of these forms are
discussed by Philips and Wolkowicz [32]. The α-form of isotactic polypropylene
homopolymer is semicrystalline in nature. As with any semicrystalline polymer, the
morphology of α-form iPP exhibits a hierarchy of characteristic scales as shown in
Figure 3.5. The macromorphology of PP can be seen on a visual scale in the range of
millimetres. The morphology of a gross reactor particle in the as-supplied state is shown
in Figure 3.6 at a magnification of x40. The spherical shape of this pure granular PP, as
reported in earlier work by other researchers [35-38], can be seen from the top part of
the micrograph. However, the skin-core structure [39-43] cannot be so easily identified
without using any cross-section slicing. Under carefully controlled optical conditions,
such as small-angle light scattering, a spherulite texture is revealed in a finer scale on
the order of 1-50 μm. The spherulite structure is built up by smaller blocks and lamellae.
At a higher magnification of x500, the lamellar structure of pure PP can be seen, as
illustrated in Figure 3.7. These lamellae are composed of crystallographically ordered
regions. The molecular chains in the crystalline regions are arranged with specific
symmetry, which has been described elsewhere [44-47]. The unit cell of isotactic PP is
monoclinic with a monoclinic angle of about β = 99°. The lattice dimensions of the cell
are a = 6.6 Å and b = 20.8 Å [46,47].
80
Processing Conditions and Durability of Polypropylene Films
Figure 3.5 PP morphology at various scales
Reprinted with permission from Polypropylene Handbook, Ed., E.P. Moore, p114.
Copyright 1996, Hanser Publishers.
Figure 3.6 SEM micrograph of PP granule
81
Handbook of Plastic Films
Figure 3.7 Micrograph of PP showing lamellar structure
The synthesis of isotactic PP has been commercialised through several processes, namely
the Spheripol [48], Exxon (Sumitomo) [49], Mitsui Hypol [50], Unipol [51] and Amoco
[52] processes. Such processes have been summarised by Lieberman and LeNoir [53]. A
flowchart for each process is shown in Figures 3.8-3.12. Because of the application of
fourth-generation catalysts, the removal of catalyst and atactic polymer is not necessary.
The use of hydrocarbon diluent in liquid or gaseous form is prevented. Thus the yield of
the PP products is remarkably increased and the cost of the synthesis of iPP homopolymer
Figure 3.8 Flowchart for the Spheripol process
Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.
Copyright 1995, Gulf Publishing Company.
82
Processing Conditions and Durability of Polypropylene Films
Figure 3.9 Flowchart for the Exxon (Sumitomo) process
Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.
Copyright 1995, Gulf Publishing Company.
Figure 3.10 Flowchart for the Mitsui Hypol process
Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 140.
Copyright 1995, Gulf Publishing Company.
and/or copolymer is reduced. Among the synthesis processes, the Spheripol process for the
production of PP homopolymers or copolymers (as shown in Figure 3.8) has found the
widest application. In this process, catalyst components and monomer are fed to a loop
reactor for homopolymerisation. The high heat removal capability of the loop reactors
83
Handbook of Plastic Films
Figure 3.11 Flowchart for the Unipol process
Reproduced with permission from Hydrocarbon Processing, 1995, 74, 3, 142.
Copyright 1995, Gulf Publishing Company.
Figure 3.12 Flowchart for the Amoco process [52]
allows very large outputs. The operating pressure in the synthesis reactors does not require
excessive wall thickness or special fabrication techniques because of the small diameter of
the loop reactors. Therefore, it is possible to build a large-capacity plant economically.
84
Processing Conditions and Durability of Polypropylene Films
3.3 Film Processing
Polypropylene has been processed into films since the beginning of its production. Development
of new catalysts and innovation in film processing technology have been a tremendous help
in the expansion of this area. Depending on the type of film and the process by which it is
made, the resulting film products can be used for various purposes and applications [54].
The melt stability for processing film products is especially important as compared with
processing moulded bulk products. In film processing, the melt stability is crucial in preventing
rheological changes and maintaining film strength. Additives that control film slippage and
antiblocking properties are also critical in the final stage of film processing.
The most common process used to produce PP films is the chill roll cast method [55].
Another important process is the tenter frame. The chill roll cast method is for nonoriented film production, and the tenter frame method is for production of oriented
films. Blown film processes [56] may also be used for both oriented and non-oriented
film production, but they are not widely used.
3.4 Additives
Stabilisation agents consisting of phenolic and phosphite antioxidants are usually used to
obtain processing stability. The PP polymer for film production also contains other
functionalising additives and groups besides the stabilisers. The two main functionalising
additives are antiblocking and slip agents. These materials are combined to provide the
release of one film from another or from take-off equipment. Since films have large surface
areas and may be wound under tension and at high speeds, there can be substantial static
charges and compressive forces present between film layers. Thus, an antiblock agent,
which is an inert inorganic material, is added to solve these problems. Diatomaceous silica,
calcium carbonate, talc, or glass spheres are commonly used antiblock agents [57-62].
Particle size and dispersability are two important characteristics for antiblocks to provide
the surface separation effect [63, 64]. Depending on the film thickness, antiblock average
particle size can range from less than 1 μm up to 15 μm. Particles or agglomerates larger
than 25 μm can appear as defects. In addition, the presence of agglomerates indicates
that some of the antiblocking agent was not well dispersed, and the normal concentration
may be ineffective. Therefore, the presence of these inorganic antiblock materials at the
film surface can affect the film-handling characteristics.
Slip agents are generally materials that tend to separate from PP and have some inherent
lubricating properties. Since slip agents eventually end up on the film surface, they may
increase the haze of the film. The most widely used slip agents for PP films are fatty
amides, such as erucamide and oleamide.
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Handbook of Plastic Films
3.5 Ultraviolet Degradation of Polypropylene
3.5.1 UV Degradation Mechanisms
Virgin PP obtained directly from a commercial process is very susceptible to UV irradiation
and air oxidation. If stored unstabilised at room temperature, the durability, strength and
physical properties of the PP product deteriorate rapidly over a period of weeks or months
depending on the physical form, temperature, available oxygen, intensity of UV radiation
and other conditions. At elevated temperatures, such as during summer storage, the
degradation process can be accelerated. This uncontrolled degradation is exothermic, and
the released heat and gases can lead to a further increase in the degradation rate [65, 66].
The oxidation processes of PP are considerably complex and depend on a variety of
factors, including oxygen availability, impurities, residual catalyst, crystallinity (or the
content of crystallised portion), storage temperature, air pollutants, radiation exposure
time, chemical exposure, film thickness, loading or stress conditions in the part,
comonomer concentration, and additive type and content. Earlier studies have proven
that the degradation of PP can be divided into three steps: initiation, propagation and
termination [67]. These steps are briefly described in the following three subsections.
3.5.1.1 Initiation of PP degradation
If the PP product is exposed to air, the following reaction proposed by Tudos [68] occurs:
RH + O2 → R• + HOO•
(3.1)
where RH stands for the polypropylene molecules and R• is alkyl radical.
According to Hawkins [69], UV radiation can assist the initiation of the degradation of
PP. In most cases, the products resulting from the oxidation as described in equation
(3.1) can remain separate under steady UV irradiation and exposed to air. However, they
may also recombine to form a hydroperoxide as shown in the following equation:
R• + HOO• → ROOH
(3.2)
3.5.1.2 Propagation of PP degradation
The propagation of the degradation, as indicated by Becker and co-workers [67], occurs
by a series autoxidation scheme as depicted by the following three reactions:
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Processing Conditions and Durability of Polypropylene Films
R• + O2 → ROO•
(3.3)
ROO• + RH → R• + ROOH
(3.4)
R• + R′H → RH + R′•
(3.5)
Since small free-radical fragments are very active, they may contribute to the propagation
of degradation by the following two reactions:
RH + HOO• → R• + H2O2
(3.6)
RH + HO• → R• + H2O
(3.7)
It can be seen from the above reactions that the propagation steps cause the radical sites
to move, but there is no overall increase in the number of radicals.
3.5.1.3 Termination of PP degradation
The last step of the degradation process is termination, where quenching of radicals
occurs. The number of radicals can be reduced by combining two radical sites to form a
non-radical product. Several reactions may lead to the termination, as described in the
following equations [70]:
R• + •R′ → R–R′
(3.8)
RO• + •R → ROR
(3.9)
2ROO• → ROOR + O2
(3.10)
2RO• → ROOR
(3.11)
3.5.2 Effect of UV Degradation on Molecular Structure and Properties of PP
The degradation of PP, initiated either by UV irradiation or through thermal activation,
causes change in crystallisation and melting behaviours of PP [71, 72]. Degradation also
leads to chain scission or cleavage, leading to a decrease in the durability of the films.
Loss in molecular weight (molar mass) also occurs [73]. There are several types of chain
scission found in PP [74]. The most common is a unimolecular scission of carbon- and
oxygen-centred radicals. This cleavage produces several products. The products from
the carbon-centred radicals are an olefin and a new carbon radical. These products can
re-enter the oxidation cycle as PP•, which can be depicted as [67]:
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Handbook of Plastic Films
–CH(CH3)–CH2–CH(CH3)–CH2–CH(CH3)– + R•
→ RH + –CH(CH3)–CH2–C(CH3)–CH2–CH(CH3)–
(3.12)
Accordingly, the chain scission can be expressed as:
–CH(CH3)–CH2–•C(CH3)–CH2–CH(CH3)–
→ –CH(CH3)–CH2–(CH3)C=CH2 + •CH(CH3)–
(3.13)
The resulting olefin is even more susceptible to oxidation than the original PP with a
saturated hydrocarbon structure. If the degradation is initiated by an alkoxy radical, a
carbonyl-containing molecule in the form of –CH2–C(CH3)=O, and another carboncentred radical, can be formed. All of these processes lead to an appreciable loss in
molecular weight of PP.
The reduction in the molecular weight of the PP polymer leads to a change in many of its
corresponding properties. One of the most detrimental is the loss of durability and ductility,
thus a drastic decrease in toughness of the polymer. In addition, the chain scission will
produce products that will tend to cause an increase in the colour of the polymer and the
generation of oxygenated compounds, which will adversely affect the durability, strength
and physical properties of the final PP products.
UV light can accelerate the chain scission processes. In addition, the availability of oxygen
and heat are also key factors in the determination of the degradation kinetics. At PP
processing temperatures, the degradation reaction rate is extremely rapid. The succeeding
extrusion or injection moulding procedure can also result in severe degradation of the PP
polymer. In the solid form, PP is a semicrystalline polymer with a crystalline content that
is normally between 40% and 60%. The crystalline regions are essentially impervious to
oxygen, so the oxidation only occurs in the amorphous region. It has been reported by
Mita [75] that the diffusion rate of oxygen is much slower than the reaction rate, so that
the oxidation process is basically a surface phenomenon [76]. In most cases, the surface
can become dull, crazed, or even powdery. Obviously, unstabilised PP is very prone to
oxidation and degradation in the presence of air. Therefore, adding appropriate stabilisers
is necessary to convert PP into a durable, useful material.
3.5.3 Stabilisation of PP by Additives
Stabilised PP can be obtained by using appropriate additives which can control radical
products or potential radicals. There are many stabilisers and UV antioxidants available,
and they can be classified into two types, i.e., primary and secondary. Some secondary
UV stabilisers in fact also have primary characteristics. The detailed stabilisation
mechanisms are still unknown due to the complicated oxidation intermediates.
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Processing Conditions and Durability of Polypropylene Films
Primary antioxidants are those additives that interfere with the oxidation cycle by reacting
with the formed radicals and interrupting the cycle. Primary antioxidants are also called
radical scavengers. Both hindered phenols (HP) and hindered amines (HA) are effective
primary antioxidants. An HP can react with the radical species generated in the initiation
and propagation stages of degradation. Specifically, the HP is able to transfer its phenolic
hydrogen to the generated radical, causing a non-radical product to be formed. In transferring
the hydrogen, the HP itself becomes a radical known as a hindered phenoxy. This is a
stable radical that will not abstract a hydrogen from the matrix PP polymer. Hindered
phenol enables the radicals to be managed in two different ways. On the one hand, the
initial radical species are effectively removed from participation in the propagation steps;
on the other hand, the abstraction of hydrogen from the HP prevents another initiating
event from occurring on the PP polymer backbone. This step immediately results in at least
one less reactive radical being formed. Regardless of the radical being quenched, the overall
effect of the HP is to delay the oxidation and, eventually, the degradation of the PP. Hindered
phenols are able to terminate more than one radical per phenol moiety. The structure of
HP allows the oxygen-based radical to be delocalised to the carbon atom bearing a
substituent, forming a quinone-like structure [77]. This is why even a very small amount of
HP antioxidant addition can achieve the stabilisation of the degradation for PP.
The shortcoming of the phenolic stabiliser is the development of colour. Some of the
quinone-like structures that are active in the stabilisation process are also intense colour
bodies or colour centres, giving a distinct yellow colour to PP. Even at very low
concentration, the matrix polymer of PP demonstrates an obvious colour change. In
addition, the interaction between HP and catalyst residues can intensify the development
of colour. Further reaction with air pollutants such as nitrogen oxides and sulphur oxides
at room temperature results in increase of colour centres.
Hindered amines have been used as stabilisers against the oxidative degradation initiated by
UV light. More recently, high molecular weight compounds have been shown to be effective
as thermal stabilisers [78]. This class of compounds plays an important role in the commercial
applications and success of PP. Hindered amines act as radical scavengers through the nitroxyl
radical from amines [79, 80]. The stabilisation mechanism of HA can be depicted as:
R′–NH (oxidisation) → R′–NO•
(3.14)
R′–NO• + •R → R′–NO–R
(3.15)
From equations (3.14) and (3.15), it can be seen that the active species is not the amine
functionality. The species that is active is the nitroxyl radical. The oxidation of the amine
leads to the production of nitroxyl groups [80]. The nitroxyl group is regenerative. The
process can be ended by cyclic regeneration. Some of the regenerated products may be
inefficient as radical scavengers; thus, some of the HA stabiliser is lost during exposure [81].
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Handbook of Plastic Films
The secondary stabilisers decompose hydroperoxides and prevent new oxidation cycles
from beginning. This class of compounds is called secondary, because their best
performance occurs in the presence of primary antioxidants. When used in PP by
themselves, secondary antioxidants do not exhibit any appreciable activity. The value of
these compounds comes when they are combined with the correct primary antioxidants.
When the appropriate combination is made, a strong synergistic effect results. The
commonly used secondary antioxidants can be classified into two categories: one is
phosphites, the other is thio compounds. Both the phosphites and thio compounds are
synergistic with the hindered phenols because they attack a source of free radicals, the
hydroperoxides. They can reduce a hydroperoxide to an alcohol. Consequently, the
homolytic cleavage of the ROOH into two radicals can be prevented. Combined with
hindered amines, secondary antioxidants are effective in suppressing UV degradation
[82]. By absorbing the UV radiation before it has a chance to energise a chromophore,
the formation of a radical is prevented. Obviously, it is impossible for the antioxidants to
absorb all the UV light, and thus some radicals are still formed. These radicals are
subsequently neutralised by the primary antioxidant of hindered amine.
3.6 Case Studies
In this work, the UV degradation behaviour of PP films was investigated with emphasis
on their durability, strength and surface morphology. Both unaged and UV-degraded
woven PP fabrics from stretched, knifed and relaxed film tapes were studied to reveal the
degradation effects. Pure PP film and several groups of PP film materials with additives
such as UV stabiliser, antioxidant and colouring pigment (calcium carbonate) were
characterised. Typical PP material specimens taken from different processing stages were
tested to identify the effect of processing conditions on the durability of PP films. Scanning
electron microscopy (SEM) was used to study the microstructural features of the films
and correlate them with the durability.
3.6.1 Materials and Experimental Procedures
3.6.1.1 Materials and Processes
All the PP films used in the current study were first formed using a chill roll film process.
Then, several procedures including water-bathing, air-knifing and stretching were applied
to obtain PP tapes. The fabric was prepared by weaving. Classification of the test samples
taken during processing and for two PP woven fabrics after been exposed to UV
degradation for two weeks are given in Table 3.4.
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Processing Conditions and Durability of Polypropylene Films
Table 3.4 PP film materials taken from different processing conditions and after
two weeks of UV exposure
Number
Name
Processing condition
1
PP1
After extrusion and water-bathing
2
PP3
After air-knifing
3
PP4
Cut from stretching zone
4
PP7
Stretched and relaxed for weaving
5
J
6
A1
Woven fabrics as sack product
UV-degraded PP fabrics
3.6.1.2 Static Tensile Tests
Static tensile tests were performed using a Materials Testing System (MTS 810) equipped
with a 2100 kN load cell. The specimens were gripped between two hydraulic wedge
grips (type 647.10A-01). Static tests were carried out under displacement control
condition at a crosshead speed of 1.5 mm/min. The gauge length was 15 mm. All the
tests were conducted at room temperature of approximately 25 °C. At least three samples
were tested for each material, and the stress-strain behaviour was established based on
the average values. The stress was calculated based on the initial cross-sectional area
before testing.
3.6.1.3 Microscopic Examination
The surface morphology of each film material was examined using a Hitachi S-2150
scanning electron microscope operated at an acceleration voltage of 20 kV. The
micrographs were recorded on Polaroid 55 instant films, and the images were captured
simultaneously with Quartz PCI Version 3.01 image processing software, and stored for
further editing and printing.
3.6.2 Durability-Microstructure Relationship
In order to examine the effect of UV degradation on both the durability and surface
morphology, comparative studies on unaged and UV-degraded woven PP fabrics were
made. The typical stress-strain behaviour of the unaged PP woven fabrics is shown
in Figure 3.13. It can be seen from this curve that the relationship between stress and
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Handbook of Plastic Films
Figure 3.13 Stress-strain behaviour for unaged woven PP fabric material
strain displays almost elastic behaviour in the strain range up to 30%. The calculated
modulus based on this linear portion is about 700 MPa. In the strain range from
30% to 40%, the stress-strain relationship is nonlinear. This indicates that plastic
deformation dominates the behaviour of the PP woven fabric material in this range.
The ultimate tensile strength reached about 200 MPa. After this range, the stress
dropped and the specimen failed. A multi-step fracture behaviour was observed due
to the mechanical interlocking of the woven fabrics. The durability for this woven
fabric is about 63%.
The typical surface morphology of woven PP fabrics shows a striped texture, as
illustrated in Figure 3.14. Parallel lines along the stretching direction can be seen, in
addition to surface defects in the form of longitudinal cracks along the stretching
lines. These cracks were only localised in the area where severely deformed material
exists. The PP materials raised due to the extrusion and stretching processes exhibit
surface crazes.
Unlike the unaged PP woven fabrics, the UV-degraded PP woven fabric sample, A1,
does not possess any durability and load-bearing capability. It was found that, after
aging under intense UV irradiation, the woven fabrics disintegrated. The tensile test
specimens prepared from A1 were so brittle that they could not be tested.
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Processing Conditions and Durability of Polypropylene Films
Figure 3.14 Typical SEM morphology of woven PP fabric material
Figure 3.15 Typical SEM morphology of UV-degraded woven PP fabric material
Microscopic examination of the surface of the UV-degraded PP woven fabric specimen,
A1, shows fraying edge strips, broken pieces and fine particles. Such morphological
features are shown in Figure 3.15. Numerous microcracks can be easily deposited on the
surface. Obviously, severe UV degradation resulted in the formation of these throughthickness cracks. The through-thickness nature of UV-degradation-induced cracks can
explain why the fabric material loses its load-bearing capability totally when such a
condition of degradation is reached.
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Handbook of Plastic Films
3.6.3 Durability-Processing Condition Relationship
The properties of PP films are dependent on the processing conditions. The effect of crystallinity
and orientation of PP crystals on the durability and strength of PP fibres have been studied
[83]. Generally, intensive drawing and stretching result in the orientation of PP crystals and
decrease the crystallinity of PP. Thus the content of the amorphous portion increases. As
indicated by Galanti and Mantell [83], well-oriented PP has a higher durability than that of
poorly oriented PP, and stretched amorphous PP displays a higher strength than that of
regularly crystallised PP. From the view of engineering design and application, comprehensive
studies in this field still need to be carried out. The following section will present the results
of the current investigation on the effect of processing conditions on the durability of PP film.
Two types of PP film, PP1 and PP7, as defined in Table 3.4, were chosen to investigate the
effects of processing conditions. As indicated in Table 3.4, PP1 is the starting film product
and PP7 is the tape ready for weaving. This means that these two films were obtained from
two extreme conditions. Thus, the change in structure and properties from PP1 to PP7 can
reasonably reflect the effect of the entire processing procedure on the durability of PP films.
The stress-strain curve for the PP1 sample, extruded and water-bathed from the pure PP
material, is shown in Figure 3.16(a). The ultimate tensile strength for this sample was about
37 MPa. The durability was about 500%. The stress-strain relationship displayed a high
degree of nonlinear behaviour after an elastic region. The deformation and failure of this
material could be divided into five stages. The first stage is elastic deformation corresponding
to a strain of approximately 10%. The calculated Young’s modulus for the PP1 sample is
about 320 MPa. The second stage is the nonlinear deformation in which both plastic and
elastic deformation can be observed. This region corresponds to a strain range from 10% to
20% approximately. The third stage represents yielding. The maximum yield strength was
about 31 MPa. Considerably large plastic deformation can be observed after the yield point.
The strain at the end of this stage reached 50% with a drop in strength to about 27 MPa. The
fourth stage was the cold flow of the PP, which corresponds to the strain range from 50% to
200%. The fifth stage is the strain hardening of the PP material. With further increase in
strain, the strength of the material increased about 25%. Following the strain-hardening
stage, the material failed catastrophically.
The PP7 tape material underwent a series of processing procedures such as knifing, stretching
and relaxation. From the typical stress-strain behaviour of the PP7, as shown in Figure 3.16(b),
it can be seen that the relationship between stress and strain displays nonlinearity in the
strain range up to 20%. The calculated modulus based on the first linear portion of the curve
is about 900 MPa. In the strain range from 20% to 33%, the stress-strain relationship is
more nonlinear. This indicates that plastic deformation dominates the behaviour of the PP7
tape material in this stage. The ultimate tensile strength reached about 220 MPa. After this
range, the stress dropped and the specimen broke. The durability is about 33%.
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Processing Conditions and Durability of Polypropylene Films
Figure 3.16 Stress-strain behaviour of (a) PP1 and (b) PP7
The difference in the tensile behaviour of these two kinds of PP film materials can be
seen clearly by comparison of Figures 3.16(a) and (b). The unstretched PP tape material
showed much higher durability than the stretched products. However, its strength is
much lower than that of the other. This indicates that stretching followed by knifing can
increase the load capability of the PP material and considerably decrease its durability,
based on the strain to failure on the stress-strain curve.
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Handbook of Plastic Films
A micrograph taken from the PP1 sample (Figure 3.17) shows raised lines along the
extrusion direction. A typical knifed and stretched sample cut from the tape (PP7) was
examined. The microstructure is shown in Figure 3.18. Comparing Figures 3.17 and
3.18, it can be seen that the strip size changed from the original width of about 1 mm to
about 0.3 mm. In some areas in particular, cracking along the strip lines can be seen on
the surface of stretched PP materials.
Figure 3.17 Microstructure of PP1 sample
Figure 3.18 Microstructure of PP7 sample
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Processing Conditions and Durability of Polypropylene Films
3.6.4 Durability-Additive Property Relationship
In order to examine the effect of additive on the durability and load-carrying ability of
PP films, four different materials were manufactured using four different processing
conditions. These PP film materials contain either no additives, a UV stabiliser, a white
colouring pigment (calcium carbonate) or a mixed additive (Amoco 100/03). The four
processing conditions were designated PP1, PP3, PP4 and PP7 are described in Table 3.4.
The stress-strain curves for the four PP1 samples, extruded and water-bathed pure PP
material, are shown in Figure 3.19(a). The PP film with Amoco 100/03 mixed additive
has the highest tensile strength of 60 MPa, while the specimen of PP1 film with UV
stabiliser has the lowest strength of about 35 MPa. The durability for all four materials
exceeds 450%. For the Amoco product, this value reached nearly 700%. The stressstrain relationship displayed a high degree of nonlinear behaviour after an elastic region.
The deformation and failure of these materials could be divided into five stages. The
first stage is elastic deformation corresponding to a strain of approximately 10%. The
second stage is the nonlinear deformation in which both plastic and elastic deformation
can be observed. This region corresponds to a strain range from 10% to 20%
approximately. After this stage, the third stage can be seen. The typical feature in the
third stage is yielding. The yield strength for these materials is in the range from 28 to
35 MPa. Considerably large plastic deformation can be observed after the yield point.
The strain at the end of this stage reached 50%, with a drop in the strength to about
25 MPa. The fourth stage was the cold flow of the PP, which corresponds to the strain
range from 50% to 200%. Again a considerable amount of deformation occurred in
this stage. The total strain due to the plastic deformation in the form of cold flow is
larger than 150%. The fifth stage is the strain strengthening of the PP1 materials. With
further increase in strain, the strength of the materials increased about 25-40%.
Following the strain-hardening stage, the material failed catastrophically, and the stressstrain curves displayed a sharp drop.
The additive-loaded or unloaded PP3 showed the same stress-strain behaviour as found
for the PP1 materials. As shown in Figure 3.19(b), the four PP3 materials also
demonstrated several distinct stages of deformation, including elastic deformation,
yielding, cold flow followed by strain strengthening. However, both the yield strength
and ultimate tensile strength of PP3 materials are a little bit larger than those for the PP1
materials. The typical specimen of PP3 without any additive has a higher yield strength
of about 38 MPa, and its ultimate tensile strength is about 62 MPa. The Young’s modulus
is approximately 350 MPa. Differences in the durability were found from the stressstrain curves for these PP3 materials with different additives. The durability for all these
materials exceeded 400%. The material supplied by Amoco displayed the maximum
durability of 700%. The PP3 with white filler also displayed the same durability as that
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Handbook of Plastic Films
(a)
(b)
Figure 3.19 Stress-strain behaviour of (a) PP1, (b) PP3, (c) PP4 and (d) PP7 film
materials with various additives
for the Amoco product. The specimens with UV stabiliser kept the same durability. It has
the same strain to failure as that for the PP3 material without additive. However, the
ultimate tensile strength of the PP3 with added UV stabiliser showed a considerable
decrease as compared with the samples without any additives.
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Processing Conditions and Durability of Polypropylene Films
(c)
(d)
Figure 3.19 Continued
The PP4 and PP7 materials underwent a series of processing procedures such as knifing,
stretching and relaxation. The four PP4 films were from the stretched zone, while the
PP7 films were in the form of tape, and were ready for weaving. From the typical stressstrain behaviour of the PP4 film materials, as shown in Figure 3.19(c), it can be seen that
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Handbook of Plastic Films
three materials – the PP4 without any additive, with UV stabiliser and with white filler –
have a similar relationship between stress and strain, while the Amoco product displayed
a decrease in both strength and modulus. However, the durability of this film material
reached 68%, which is larger than that of the other three films. All the stress-strain
curves of the four PP4 materials display considerable nonlinearity in the strain range up
to 30%. Beyond this strain range, the stress-strain relationship is more nonlinear. This
indicates that plastic deformation dominates the behaviour of the PP4 films. The ultimate
tensile strength for these PP4 films is in the range from 275 to 375 MPa. The sharp drop
on the stress-strain curves indicated the final rupture of these specimens.
The stretched tape materials, PP7, showed a similar nonlinear deformation behaviour under
static overloading similar to the PP4 film materials. Nevertheless, both the ultimate tensile
strength and the durability of the PP7 materials are smaller than those of the PP4 materials.
The stress-strain curves for the four PP7 materials are shown in Figure 3.19(d). The ultimate
tensile strength is in the range from about 150 to 275 MPa. The durability was in the range
from 20% to about 33%. Moreover, the PP7 samples with calcium carbonate filler show a
tendency in decrease in tensile strength. The ultimate tensile strength for this kind of material
is about 150 MPa, which is only 60% of that of the other three PP7 materials.
The surface morphology of the four groups of PP films with and without additives was
examined. Samples from the PP1 group have a unidirectional texture. Parallel lines along
the extrusion direction can be seen. These lines are raised materials. The surface
morphology for the four PP3 materials was found to be very similar to that of the PP1
materials. Comparing the morphology of PP1 and PP3 with that of PP4 and PP7, it was
Figure 3.20 Micrograph of PP1 containing calcium carbonate pigment
100
Processing Conditions and Durability of Polypropylene Films
found that the strip size changed from the original width of about 1 mm for PP1 and PP3
to less than 0.4 mm for PP4 and PP7. The PP films containing colouring pigment show
crevices and scratches due to the movement of calcium carbonate particles along the
extrusion direction. This is clearly shown in Figure 3.20, a micrograph taken from the
surface of the PP1 film with white colouring pigment, calcium carbonate particles.
3.7 Concluding Remarks
The durability of PP is highly sensitive to ageing under ultraviolet irradiation. Unaged
PP woven fabrics have unidirectional structure and possess good durability and loadbearing capability. The durability is about 60%. The ultimate tensile strength is about
200 MPa. The Young’s modulus reaches 700 MPa. UV degradation causes severe damage
to the structure and drastic decrease in durability and strength of PP film materials.
The load-bearing capability for typical aged PP woven fabrics was totally lost after
two weeks of UV exposure. The surface morphology of the aged PP fabrics was
dominated by numerous microcracks.
The durability of PP film materials is also sensitive to processing conditions. Without
stretching, the films with striped structure demonstrated cold flow followed by strain
strengthening. The durability for this material exceeds 490%. The typical specimen of
PP1 has a yield strength of 31 MPa, and its ultimate tensile strength is about 37 MPa.
The Young’s modulus is approximately 320 MPa. After extensive stretching, knifing and
relaxation, the distance between the parallel raised lines decreased, the durability is reduced
to about 33%, while the load-carrying capability increased remarkably. Limited plastic
deformation without cold flow was found. The sample of the PP7 has a tensile strength
of about 220 MPa, and the Young’s modulus is about 900 MPa.
The durability of the PP films is not sensitive to the addition of additives. UV stabilisers
and antioxidants do not change the durability and morphology of the PP films appreciably.
All the PP film materials have a very smooth surface and possess a similar structure with
unidirectional raised lines. Calcium carbonate can produce crevices during extrusion,
while the durability of the films containing calcium carbonate is almost the same as
those without any whitening additives.
Acknowledgements
This work was supported by the Egyptian Foreign Relations Coordination Unit (FRCU)
of the Supreme Council of Universities/USAID under University Linkage Project II
Grant #93-02-16.
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Jr., Hanser Publishers, Munich, Germany, 1996, 134.
33. A. Turner-Jones, J.M. Aizlewood and D.R. Beckett, Die Makromolekulare
Chemie, 1964, 75, 134.
34. R.L. Miller, Polymer, 1960, 1, 135.
35. R.A. Hutchinson, C.M. Chen and W.H. Ray, Journal of Applied Polymer Science,
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36. P. Galli, J.C. Haylock and T. Simonazzi in Polypropylene: Structure, Blends and
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Netherlands, 1994, 1.
37. R.B. Lieberman and P.C. Barbe, Encyclopedia of Polymer Science and Engineering,
Volume 13, Ed., J.I. Kroschwitz, John Wiley, Chichester, UK, 1988, 464.
38. L. Noristi, E. Marchetti and G. Sgarzi, Journal of Polymer Science, Polymer
Chemistry Edition, 1994, 32, 3047.
39. R. Phillips, G. Herbert, J. News and M. Wolkowicz, Polymer Engineering and
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40. M. Fujiyama, H. Awaya and S. Kimura, Journal of Applied Polymer Science,
1977, 21, 3291.
41. S.S. Katti and J.M. Schultz, Polymer Engineering and Science, 1982, 22, 1001.
42. M. Fujiyama, T. Wakino and Y. Kawasaki, Journal of Applied Polymer Science,
1988, 35, 29.
43. M. Fujiyama and T. Wakino, Journal of Applied Polymer Science, 1991, 43, 97.
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45. Z. Mencik, Journal of Macromolecular Science, 1972, 6, 101.
46. M. Hikosaka and T. Seto, Polymer Journal, 1973, 5, 111.
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Processing Conditions and Durability of Polypropylene Films
47. A. Immirzi, Acta Crystallographica, 1980, 36(B), 2378.
48. Hydrocarbon Processing, 1995, 74, 3, 140.
49. A.M. Jones in Proceedings of Polyolefins V, 5th International SPE RETEC
Conference, Houston, TX, USA, 1987, 33.
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51. Hydrocarbon Processing, 1995, 74, 3, 142.
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Company, assignee; US Patent 3,957,448, 1976.
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Handbook of Plastic Films
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Moore Jr., Hanser Publishers, Munich, Germany, 1996, 178.
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Processing Conditions and Durability of Polypropylene Films
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Polymers, Volume 1, Ed., A. Patsis, Technomic Publishers, Lancaster, PA, USA,
1989, 227.
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107
Handbook of Plastic Films
108
4
Solubility of Additives in Polymers
Alexander Mar’in
4.1 Introduction
Polymer-based materials usually contain, besides the polymer, various low molecular
weight (low molar mass) compounds such as stabilisers, plasticisers, dyes, dissolved
gases, accidental and technological impurities. During exploitation, polymer materials
can come into contact with water, organic liquids, solid substances and foodstuffs,
which could result in the transfer of additives and impurities dissolved in the polymer
to the surroundings, polluting them and decreasing the lifetime of the polymer. On
the other hand, low molecular weight compounds from the surroundings can pass
into the polymer. The distribution of additives between polymers and surroundings
is controlled by processes based on sorption (dissolution) and diffusion. This topic
covers different aspects of additive solubility in polymers in light of polymer
degradation and stabilisation.
4.2 Nonuniform Polymer Structure
A polymeric substance is nonregular. This irregularity may display itself at the molecular,
topological and morphological levels. The molecular irregularity is due to the chain-like
structure of the polymer molecule and the existence of non-equivalency (anisotropy)
along and across the polymer chain. Topological irregularity is due to the existence of
polymer chain ends and various polymer chain entanglements surrounded by relatively
ordered substance in which short-range order is obeyed. Morphological irregularity is
based on the existence of relatively large zones markedly differing in the character of the
arrangement of segments of macromolecules forming these zones and in their physical
properties. In crystalline polymers, this irregularity gives rise to the formation of crystalline
and amorphous regions, fibrils and spherulites.
Gases and additives dissolved in a polymer are mainly present in the amorphous regions
in zones around knots, folds and various chain entanglements where there is a free volume
large enough to hold the molecule [1-8]. The degree of topological irregularity (disorder)
depends on the conditions of polymer synthesis as well as on the conditions of polymer
sample preparation, (i.e., on crystallisation) [9-12].
109
Handbook of Plastic Films
4.3 Additive Sorption
The most important characteristic of the sorption (that is, dissolution) of an additive in a
polymer is its sorption isotherm, i.e., the relationship between the concentration or vapour
pressure of the compound around the polymer and its concentration in the polymer. The
simplest isotherm corresponds to the case of an ideal solution. This isotherm is described
by Henry’s law: the concentration of compound A in the polymer ([A]p) is directly
proportional to its concentration in the surrounding medium ([A]m) or to its pressure Pa:
[A]p = γ*[A]m
or
[A]p = (γ*/RT)Pa = γ*pPa
(4.1)
(4.1a)
where γ* and γ*p are solubility coefficients.
In the case of an ideal solution, d2[A]p/d[A]m2 = 0. That is, dissolution of any compound
A does not change the properties of the polymer medium. In practice, linear isotherms
(4.1) and (4.1a) are observed only at low concentration of a dissolved compound.
Positive and negative deviations from the law (4.1) are possible. In the first case, d2[A]p/
d[A]m2 > 0; and in the second, d2[A]p/d[A]m2 < 0. A positive deviation means that the
sorption of any molecule of A facilitates the sorption of the next one. Two mechanisms
for the positive deviations are possible: (i) an increase in the mobility of the
macromolecules caused by a dissolved compound (for example, plasticising the polymer)
and (ii) the formation of aggregates (clusters) of several A molecules dissolved in the
polymer. Positive deviations may be described by the following equation obtained from
the theory of regular solutions:
μ/RT = ln(P/P0) = ln φ1 + φ2 + χφ22
(4.2)
where P is the vapour pressure of the additive in the system, P0 is the pressure of its saturated
vapour, and χ is the Flory-Huggins parameter (solvent-solute interaction parameter).
Equation (4.2) connects the chemical potential of the solvent (μ) with the volume fractions
of the additive (φ1) and the polymer (φ2). The value of χ can be determined in terms of the
solubility parameters of the additive and of the polymer, δ1 and δ2, respectively:
χ = V1(δ2 – δ2)2/RT
(4.3)
where V1 is the molar volume of sorbate. Equations (4.2) and (4.3) are widely used in
practice and allow one to predict the additive sorption. The values δ1 and δ2 can be
obtained from independent experiments or by simulation. In crystalline polymers, it is
necessary to take into account the volume accessible for the molecules of additive, which
does not always coincidence with the total volume of amorphous phase of the polymer.
110
Solubility of Additives in Polymers
A negative deviation from equation (4.1) corresponds to the case when the polymer possesses
a limited number (concentration) of centres that can sorb one A molecule each. In this
case, with increasing A concentration in the polymer, the number of non-occupied centres
decreases. In many cases the negative deviations can be described by a law analogous to
the Langmuir equation (4.4) or its combination with Henry’s law (4.5) [2-7, 11]:
a[ A]
[A]p = 1 + b[Am]
[A]
p
[ ]
=γ A
m
(4.4)
m
+
[ ]
1+ b[ A ]
aA
m
(4.5)
m
where a and b are constants; the ratio a/b corresponds to limit of the A concentration
in polymer.
The nature of sorption centres may be different. Polymer polar groups interacting with
an additive (for example, due to the formation of hydrogen bonds), as well regions with
a lower density of a polymer substance (the elements of free volume) in the polymer, may
be regarded as such centres. The latter have either a relaxation or a topological nature.
Some authors [3-5] consider sorption centres as microvoids and unrelaxed volume in the
polymer below the glass transition temperature that disappear at high temperature. In
contrast, the centres arising around knots and other chain entanglements are more stable
and can also exist in the polymer melt [7, 8, 12].
Suppose that in a polymer a certain concentration of the same centres Zi is present that
can interact with compound A. Let us also suppose that the sorption of additive proceeds
in two steps. First, the additive forms a true solution, the concentration of A in this
solution being related to its concentration around the polymer by Henry’s law, that is,
[A] = γ[A]m. Then this truly dissolved additive is reversibly sorbed by centres Zi:
K
a
A + Zi ←⎯
⎯
→ AZi
(4.6)
The total concentration of A in the polymer is [A]p = [A] + ∑[AZi] = [A] + [AZa], where
[A] and [AZa] are the concentrations of true dissolved (mobile) and immobile molecules,
respectively. If the additive concentration outside the centres is neglected ([A] << [AZa]),
the sorption isotherm of A will be:
[A]p = [AZa] = γKa[Za][A]m(1 + γKa[A]m)–1
or
1/[A]p = 1/γKa[Za][A]m + 1/[Za]
(4.7)
(4.7a)
111
Handbook of Plastic Films
where γ is the coefficient of true (outside the centres) solubility of A ([A] = γ[A]m).
Formula (4.7) is equivalent to the Langmuir type isotherm (4.4) assuming a = γKa[Za]
and b = γKa.
Formula (4.7) is observed in the case of the sorption of different additives including
antioxidants by polyolefins. The concentration of sorption centres ([Za]) depends on
the type of additive and polymer used, and in most cases remains constant over a wide
temperature range from solid polymer to polymer melt [6, 7, 9, 11-13], indicating the
existence of stable disorder.
The precipitation of polymer from different solvents has been used as a method to
change the concentration of chain entanglements [12]. During precipitation,
macromolecules have to overcome the interaction with molecules of the solvent, which
is more difficult in the case of a ‘good’ solvent, and a polymer sample obtained after
precipitation from a ‘good’ solvent has to possess a less perfect structure. Figure 4.1
shows that the sorption isotherms of 2,6-di-tert-butyl-4-methylphenol (BMP) by
polyethylene (PE) obey equation (4.7a); PE samples precipitated from decane have a
higher concentration of sorption centres ([Za ]) than samples precipitated from
chlorobenzene [12].
In some cases, at high concentration of additive, the sorption isotherms change their
shape and a strong increase in sorption is observed. This dependence can be explained
by means of polymer swelling resulting in changes in the polymer properties and
mechanism of sorption [7-13].
Polymers usually contain different additives present together. The existence in the
polymer of centres capable of sorbing two additives, (i.e. A and B), should result in a
decrease in the concentration of one compound in the presence of another one due to
competition for sorption centres. Formally it may be presented as:
B + AZi ←⎯→ BZi
(4.8)
If ∑[BZi] ≈ [B]p, there should be a linear dependence between the concentrations of the
two compounds: the dissolution of B results in an equivalent decrease of the
concentration of A in the polymer. However, it is not possible to have the complete
replacement of one additive by another one. Substitution is observed only in a limited
range of concentration of both additives, which shows that there are some centres that
do not take part in the substitution process (Figure 4.2) [7, 9, 13].
112
Solubility of Additives in Polymers
Figure 4.1 Sorption isotherms of BMP by PE samples precipitated from chlorobenzene
(1) and decane (2) solutions containing of 1.0% of PE. T = 180 °C
Figure 4.2 Replacement of dibenzoylmethane (DBM) by phenyl benzoate (PB) in
polypropylene from ethanol solution; [DBM]0 = 0.2 mol/l, T = 40 °C
113
Handbook of Plastic Films
4.4 Quantitative Data on Additive Solubility in Polymers
The solubility of an additive corresponds to the concentration at which the additive in
the polymer is present in equilibrium with the additive outside the polymer or with its
saturated vapour. Formally, the solubility of A (SA) corresponds to the point on the sorption
isotherm ([A]p = SA) where the concentration of A in the surroundings is equal to the
concentration of A in its saturated vapour. According to equation (4.7), SA is less than
[Za] and reaches [Za] with increasing temperature [8].
Various methods of measuring additive solubility in polymers have been proposed. The
direct method includes the study of the kinetics of additive dissolution in a polymer
when the additive is present in equilibrium with its saturated vapour or with an additive
introduced on the surface of polymer film [7, 14-17]. For this purpose polymer film with
an additive is kept in a closed vacuum tube or in an inert medium for different periods of
time. Usually, the solubility value corresponds to some plateau on the curve of the
concentration of additive in the polymer versus time. At high temperatures, dissolution
can be accompanied by change in the polymer structure and the solubility will change
with time [8, 16, 17].
To measure additive solubility, the ‘sandwich’ method is also used [18-21]. Polymer film
is placed between films oversaturated with additive. The solubility measured in this way
may be higher than in the case of the free additive method. This is probably due to the
fact that the additive concentration in the oversaturated film does not correspond to the
true equilibrium.
There are some indirect methods of measuring additive solubility. One of them is based on
the measurement of the concentration profile of the additive inside the film [18-23]. This
method makes possible the simultaneous determination of the additive diffusion coefficient.
Another method includes the determination of the temperature dependence of the vapour
pressure of the additive above the pure additive and above the polymer containing a
definite concentration of the additive [24]. The intersection point of the two curves (in
the coordinates lg Pa versus 1/T) corresponds to the temperature at which the additive
concentration in the sample is equal to its solubility. The temperature dependence of the
transparency of polymer films with various additive concentrations [25] allows
measurement of additive solubility. If the additive concentration in the polymer exceeds
its solubility at the given temperature, the excess additive emerges to form crystals or
drops, which sharply decrease the sample transparency. This method is not precise.
Billingham and coworkers [15] considered the solubility of additives in polymers based
on regular solution theory: solubility is defined by the condition that the (negative) free
energy of mixing of the liquid additive with the polymer is equal to the (positive) free
114
Solubility of Additives in Polymers
energy required to convert the crystalline additive into a liquid at the same temperature.
In this case the solubility of the additive in the polymer is represented by:
–ln SA = (ΔHf/RT)(1 – T/Tf) + (1 – V1/V2) + χ
(4.9)
where ΔHf is the heat of fusion of the additive, Tf is its melting temperature, V1 and V2
are the molar volumes of the additive and of the solvent, and χ is the interaction
parameter. According to equation (4.9) a crystal with a higher heat of fusion is expected
to be less soluble in a polymer than one with a lower heat of fusion. The additive
solubility in the polymer can be predicted from data on its solubility in a homologous
set of solvents by extrapolation of the solubility data in the coordinates ln SA versus 1/V2
to the point 1/V2 = 0. This approach does not take into account the features of the
polymer structure.
For the description of the temperature dependence of additive solubility in a polymer,
the van’t Hoff equation:
Ss = Ss0 exp(–ΔH/RT)
(4.10)
is used, where ΔH is the heat of solution. Equation (4.10) is correct in only narrow
temperature ranges. Among the reasons for the violation of this dependence are phase
transitions in the polymer, (i.e., near its melting point), and the dissolved additive. Another
reason is the existence of stable sorption centres whose concentration in the polymer
does not depend on temperature.
The data published on antioxidant solubility in polymers refer mainly to polyolefins,
and markedly differ from one another. These differences are apparently due to differences
in the methods of measuring their solubility and to differences in the structures of the
samples studied. Table 4.1 shows data on the solubility of different stabilisers in polymers.
The solubility of stabilisers decreases with their molecular weight, but there is no simple
dependence between these characteristics. The solubility of phenolic-type stabilisers in
polyolefins and in rubbers is greater than that of aromatic amines with the same molecular
weight. Sulfides are highly soluble in polyolefins, probably due to the presence of aliphatic
groups in their molecules [21]. There is a difference in the solubility in PE of two sterically
hindered amines with close molecular weight (396-423 and 481) [22]; nitroxides are less
soluble than the corresponding amines [23].
As seen from Table 4.1, the solubility of many stabilisers at room temperature is markedly
lower than the concentrations at which the additives are usually added to polymers (0.1–
0.5% by weight). Thus, an excess of a stabiliser added to a polymer often emerges (sweats
out or blooms) from it.
115
Handbook of Plastic Films
Table 4.1 Solubility of stabilisers in polymers
Stabiliser
MW
Polymer*
Temp.
range
(°C)
SA, at 25
°C (%)
lg SA0
ΔH
(kJ/mol)
Ref.
2,6-Di-tert-butyl-4-methylphenol
220
LDPE
iPP
Butadiene
rubber
Chloroprene
rubber
30-72
30-70
1.9
1.75
18†
7.83
6.11
-
43.0
33.5
-
26
26
27
11.3†
-
-
27
2,4,6-Tri-tert-butylphenol
262
LDPE
iPP
30-80
30-50
0.83
0.55
15.78
7.28
90.6
43.2
26
26
2,6-Di-tert-butyl-4-phenylphenol
282
iPP
PMP
40-100
30-50
0.45
0.39
6.11
13.82
36.9
81.3
26
26
3,5-Di-tert-butyl-4-hydroxyphenylpropionic methylate
(Fenosan-1)
292
LDPE
iPP
PMP
PVB (26%)
plasticised
30-90
30-60
30-60
30-60
0.37
0.61
0.05
7.2
8.38
7.34
15.78
10.48
50.3
43.1
97.6
52.6
26
26
26
28
2,2′-Methylenebis(4-methyl6-tert-butyl-phenol)
340
LDPE
LDPE
iPP
iPP
PMP
Butadiene
rubber
30-80
23-90
30-80
50-100
30-90
22-80
0.08
3.5
0.063
1.17
0.012
2.0
3.74
3.34
7.17
4.71
7.75
5.38
27.6
15.9
47.8
26.8
55.3
29
21
21
26
26
26
27
Chloroprene
rubber
-
2.3†
-
-
27
4,4′-Thiobis(6-tert-butylm-cresol) (Santonox)
358
LDPE
23-90
9×10-4
11.11
80.9
21
2,2′-Methylenebis(4-chloro6-tert-butylphenol)
360
iPP
40-100
0.38
4.54
28.5
26
2,2′-Methylenebis(4-ethyl6-tert-butylphenol)
368
LDPE
23-90
0.18
3.95
26.8
21
2,2′-Methylenebis(4-methyl6-α-methylcyclohexyl-phenol)
(Nonox WSP)
420
LDPE
iPP
Butadiene
rubber
Butadieneacrylonitrile
(18%) rubber
23-90
40-90
70-100
0.14
0.17
2.0
4.91
4.06
4.34
32.8
27.6
23.0
21
21
29
70-100
2.2
5.11
27.2
29
LDPE
23-90
0.028
5.20
38.5
21
4,4′-Methylenebis(2,6-di-tertbutylphenol) (Ionox 220)
116
424
Solubility of Additives in Polymers
Table 4.1 Solubility of stabilisers in polymers continued
Stabiliser
MW
Polymer*
Temp.
range
(°C)
SA, at 25
°C (%)
lg SA0
ΔH
(kJ/mol)
Ref.
Octadecyl ester of 3,5-di-tertbutyl-4-hydroxyphenylpropionic acid (Irganox 1076)
530
LDPE
LDPE
52-90
23-52
0.016
7.60
10.97
45.5
72.8
21
21
1,1,3-Tris(5-tert-butyl-4′-hydroxy-2′-methylphenyl)-butane
(Topanol CA)
544
LDPE
LDPE
23-90
50-100
0.105
0.009
6.3
6.66
47.3
49.7
21
21
Bis(3,5-di-tert-butyl-4hydroxy-phenyl) ethoxycarbonyl-ethyl sulphide
(Irganox 1035)
643
LDPE
LDPE
23-74
74-90
0.017
-
9.00
6.78
61.6
46.9
21
21
2,4,6-Tris(3,5-tert-butyl-4hydroxybenzyl)mesitylene
(Ionox 330)
775
LDPE
iPP
23-90
50-100
0.013
0.004
3.04
8.59
28.1
62.9
21
15
Tetramethylene-3-(3′,5′-di-tert-butyl-4′-hydroxy-phenyl)propionate methane
(Irganox 1010)
1178
LDPE
LDPE
iPP
23-90
50-100
50-100
0.005
0.02
0.15
8.60
5.30
4.45
62.4
39.0
31.8
21
21
15
Phenyl-β-naphthylamine
220
LDPE
iPP
PMP
Butadiene
rubber
Butadieneacrylonitrile
(18%) rubber
30-60
60-100
30-60
28-80
0.06
0.048
0.009
1.4
3.62
2.64
4.26
5.91
27.7
22.6
36.1
33.0
26
26
26
24
-
12.1†
-
-
24
Ester of 2,2,6,6-tetramethyl4-piperidinol and stearic acid
(technical grade)
396423
LDPE
-
2-2.2†
-
-
22
Bis(2,2,6,6-tetramethyl-4piperidinyl) sebacate
(Tinuvin 770)
481
LDPE
-
0.1†
-
-
22
Bis(2,2,6,6-tetramethyl-4piperidinyl-1-oxyl) sebacate
511
iPP
iPP
HDPE
LDPE
LLDPE
25-90
100-114
25-90
25-80
25-80
0.008
0.018
0.02
0.041
11.07
2.76
7.88
8.68
7.65
75.0
16.9
55.2
59.5
52.0
23
23
23
23
Dilauryl thiodipropionate
514
LDPE
LDPE
23-40
40-90
0.79
-
9.20
2.15
53.2
10.8
21
21
Distearyl thiodipropionate
682
LDPE
LDPE
23-66
66-90
0.75
-
5.00
0.64
29.3
7.7
21
21
*LDPE: low-density polyethylene; iPP: isotactic polypropylene; PMP: poly(4-methyl-1-pentene); PVB: poly(vinyl
butyral); HDPE: high-density polyethylene; LLDPE: linear low-density polyethylene. † Solubility at 22-23 °C.
117
Handbook of Plastic Films
4.5 Factors Affecting Additive Solubility
4.5.1 Crystallinity and Supermolecular Structure
Additive solubility in nonpolar rubber is greater than that in crystalline polyolefins (see
Table 4.1) because the crystalline regions of polyolefins are not available for additives, and
crystals decrease the plasticising action of dissolved compounds. There is no simple
correlation between polymer crystallinity and additive solubility. The solubility of additives
depends not only on the volume of amorphous fraction but also on its structure. It was
shown [30] that the solubility of diphenylamine and phenyl-β-naphthylamine in solid
polyethylene with different crystallinity is practically constant and only slightly decreases
at high crystallinity of the polymer. The authors attribute this to the irregularity of the
amorphous regions of the polymer, the density of which decreases with increasing polymer
crystallinity. Moisan [31] showed that the solubility of Irganox 1076 in polyethylene at 60
°C only changes weakly with polymer crystallinity in the range from 43 to 57% (density
range 0.92–0.94 g/cm3), but at higher temperatures (70 and 80 °C) the solubility decreases
with polymer crystallinity. It should be noted that crystallinity measured at room temperature
can change considerably with temperature, especially in the polymer premelting region.
The role of the polymer supermolecular structure and polymer prehistory on antioxidant
solubility has been studied [32-37]. It has been shown that the solubilities of
diphenylamine, of the methyl ester of 3,5-di-tert-butyl-β-hydroxypropionic acid and of
2,2′-methylenebis(4-methyl-6-tert-butylphenol) in polyolefins prepared by rapidly cooling
the polymer melt (structure with small spherulites) are higher than in the samples prepared
by slow crystallisation near the polymer melting temperature (the structure with large
spherulites) [32, 33]. The difference in solubility can reach a factor of 2, while the
crystallinity measured by the IR method is practically the same [33].
The precipitation of polymer from different solvents has been used as a method to change
the polymer structure and antioxidant efficiency [34-36]. It was shown that additive
solubility in polypropylene (PP) precipitated from decane (a ‘good’ solvent for PP) was
higher than that in chlorobenzene.
The solubility of phenyl-β-naphthylamine in PP/PE blends and ethylene-propylene
copolymers was studied in solid film and in the melt [37]. It was shown that the solubility
of the antioxidant at 60 °C is practically independent of the composition of the polymer
mixture, whilst the solubility in copolymers has a minimum at a propylene content in the
range near 2% and a wide maximum at 40%. The solubility of different stabilisers in
LDPE, in LDPE/LLDPE blends and in ethylene-vinyl acetate copolymer has been studied
[18]. The additive solubilities in LDPE and in the blend are close, while in ethylene-vinyl
acetate copolymer it is higher, especially in the case of 2,6-di-tert-butyl-4-methylphenol.
118
Solubility of Additives in Polymers
4.5.2 Effect of Polymer Orientation
Orientation drawing of polyolefins results in considerable change in the polymer structure
and additive behaviour: spherulites transform into fibrils; in amorphous zones, the amount
of regular conformers increases and that of irregular conformers decreases [38-42]. The
solubility and diffusion coefficient of additives usually decrease with drawing, but
sometimes these relationships are more complicated [40, 43, 44]. Figure 4.3 shows the
effect of elongation of PE on the solubility of various antioxidants at 60 °C. The
crystallinity of PE determined by differential thermal analysis does not change with
drawing, while the crystallinity determined by IR spectroscopy increases from 36 to
48% for (λ = 0-5.5), showing the change in the conformation set of macromolecules
[44]. Because the orientation drawing can result in deformation and the disappearance
of chain entanglements due to pull-out of macromolecules from knots and other
topological irregularities, one may expect that after further melting the additive solubility
of oriented samples should tend to decrease compared with that of non–oriented ones.
The chain entanglements cannot recover quickly, otherwise a memory of the change in
polymer structure should remain after polymer melting. Figure 4.4 shows the effect of
orientation drawing on the solubility of phenyl benzoate [45]: curve 1 corresponds to
Figure 4.3 Dependence of the solubility of the methyl ether of 3,5-di-tert-butyl4-hydroxyphenylpropionic acid (1), of 2,2′-methylenebis[4-methyl-6(1-methylcyclohexyl)phenol] (2) and the ester of 3,5-di-tert-butyl-4hydroxyphenylpropionic acid and ethylene glycol (3) in polyethylene on the
degree of drawing (l) at 60 °C
119
Handbook of Plastic Films
Figure 4.4 Solubility of phenyl benzoate in polyethylene at 60 °C as a function of degree
of drawing: (1) samples after additional heating, (2) samples without additional heating
samples which after drawing were heated in vacuum to 140 °C followed by fast cooling;
curve 2 corresponds to samples without additional treatment. As can been seen from
Figure 4.4, orientation of the polymer affects the solubility of the additive even if samples
were heated above the polymer melting point.
4.5.3 Role of Polymer Polar Groups
In a polymer containing polar groups, the mechanism of sorption may also include the
interaction of polar groups of the polymer (X) with polar groups of the dissolved additive
(A), as represented [8, 46, 47] by:
A + X ↔ AX
(4.11)
In going from nonpolar to polar polymers, e.g., from polyolefins to aliphatic polyamides
containing –CONH– groups, the polymer density increases as a result of the formation
of hydrogen bonds between these groups. It was shown [8] that, for polymers such as
polyamide-12 (PA-12), polyamide-6,10 and polyamide-6,6 (or polyamide-6; PA-6), the
solubility of phenyl-β-naphthylamine increases in the order polyethylene to polyamide–
12 and decreases with higher concentrations of these groups, whereas the solubilities of
120
Solubility of Additives in Polymers
Figure 4.5 Solubility of phenyl-β-naphthylamine (1) and of phenyl benzoate (2) in
polyethylene and polyamides at 40 °C as a function of the concentration of amide groups
the less polar additives phenyl benzoate and 2,6-di-tert-butyl-4-methylphenol decrease
over the whole range studied (Figure 4.5).
4.5.4 Effect of the Second Compound
The decrease in the solubility of one compound in the presence of another compound
due to competition for sorption centres was mentioned above. In some cases a more
complicated situation is observed. The effect of octamethylcyclotetrasiloxane (OMTS)
introduced into a PE melt on the solubility of phenyl-β-naphthylamine (PNA) in solid PE
is shown in Figure 4.6: the solubility decreases and then passes through a maximum with
OMTS concentration. The content of trans and gauche conformations in the polymer is
also changed, which can be attributed to rearrangement of the sorption centres. OMTS
also affects the polymer melt: the vapour pressure of PNA (the concentration in gas
phase over the polymer) depends on the OMTS concentration in the melt [14].
Plasticisers present in polymers change the polymer structure owing to the increase in
mobility of polymer chains, which affects the solubility and diffusion of additives in the
polymer. The solubility of antioxidants in nonplasticised PVB is low compared with that
in pure plasticiser, i.e., for the ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid
121
Handbook of Plastic Films
Figure 4.6 Solubility of phenyl-β-naphthylamine in polyethylene at 60 °C as a function
of OMTS concentration
and ethyleneglycol: 0.014 and 0.84 mol/kg (0.9% and 53% by weight) respectively [28].
If the solubility in the polymer (SPVB) and in the plasticiser (SPLA), i.e., of antioxidant in
plasticised PVB (Sadd), is considered as a simple sum of the solubilities
Sadd = SPVB + x(SPLA – SPVB)
(4.12)
where x is the percentage of plasticiser in the polymer, one can expect a linear growth of
antioxidant solubility in the polymer with plasticiser concentration (Figure 4.7, dotted
line). Experiment shows that, at low concentrations of the plasticiser (1–5% by weight),
the solubility is higher than what it should be according to equation (4.12); but at higher
concentrations of plasticiser (10–40%), it is less than Sadd (Figure 4.7, solid line) [28].
The effect observed is due to the fact that, at small concentrations, the plasticiser strongly
affects the mobility of macromolecules and, as a result, increases the antioxidant solubility;
at high concentrations, there is solution of the polymer in the plasticiser with strong
polymer–plasticiser interactions, which disturb the antioxidant dissolution.
4.5.5 Features of Dissolution of High Molecular Weight Additives
For additive dissolution, sorption centres should contain an excess volume large enough
to locate the additive molecule. If this volume is less than that necessary for sorption,
122
Solubility of Additives in Polymers
Figure 4.7 Solubility of the methyl ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic
acid as a function of dihexyl adipate content in PVB at 60 °C
dissolution of A can occur only when the rearrangement of this centre results in a change
to the polymer structure. The process of centre rearrangement may be represented by
equation (4.13) followed by polymer swelling:
*
A + Zi → AZi ↔ A + Zi
*
(4.13)
This could be important when large additive molecules are considered. The solubility
of sterically hindered amines with molecular weights from 1364 to 2758 in
polypropylene has been studied [16, 17]. It was shown that the solubility of the stabilisers
in the polymer at 100 °C passes through a maximum with time and depends on the
molecular weight of the stabiliser: the higher the molecular weight of the stabiliser, the
higher its maximum concentration in PP. It was assumed that, at high temperatures,
molecules of larger size are able to change the polymer structure to a greater extent
than those of smaller size, so the apparent solubility may increase with the molecular
weight of the additive as observed experimentally. Thus, the process of dissolution of
high molecular weight additives gives rise to a certain ‘destruction’ of the initial polymer
structure. The decrease in additive solubility with time is probably due to annealing of
the polymer in the presence of additives. Additive dissolution is accompanied by a
change in the polymer crystallinity and in the concentration of irregular conformations
in the amorphous zones of the polymer [16, 17].
123
Handbook of Plastic Films
4.5.6 Effect of Polymer Oxidation
The oxidation reaction first involves the zones with lower polymer packing density
containing polymer chain entanglements and can result in the disappearance of some of
them. This process may be represented as:
Z1 + nO2 → εZ2
(4.14)
where Z1 and Z2 are different types of sorption centres, and ε < 1.
Aliphatic chain scission proceeds in the oxidation reaction according to:
O
O
(4.15)
CH2
CH
CH2
CH2
C
H
+ CH2
As seen from equation (4.15), polar aldehyde groups are formed in this process at the
ends of broken chains. Thus, the newly formed centre Z1 may contain polar groups. In
this case the solubility of polar additives may increase with oxidation. The solubilities of
diphenylamine and phenyl benzoate were studied in polyethylene and in several aliphatic
polyamides after oxidation [47]. Figure 4.8 shows the solubility of phenyl benzoate as a
function of oxidation degree. At low oxidation degrees, up to 0.2–0.3 mol/kg of oxygen,
Figure 4.8 Solubility of phenyl benzoate at 60 °C as a function of amount of oxygen
absorbed during oxidation of polymers: PE, PA-12 and PA-6
124
Solubility of Additives in Polymers
the solubilities of both additives in all polymers studied decrease; at deeper stages, the
solubility in polyamides still decreases, whilst in PE it increases. To explain the
experimental data, we should assume that oxidation results in the decomposition of one
type of sorption centre and the simultaneous formation of other ones. In polyamides the
concentration of polar amide groups is higher compared to those of new ones formed in
oxidation. For this reason we only observe the decrease in additive solubility caused by
polymer oxidation. In nonpolar polyethylene, the effects of both processes are comparable,
and we observe a more pronounced and complicated variation of additive solubilities.
4.6 Solubility of Additives and Their Loss
An additive dissolved in a polymer can transfer from the polymer into the surrounding
medium. This process includes the stages of diffusion of the additive to the surface and
its removal from the surface, (i.e., by sweating out, evaporation or washing out). The
solubility of an additive in the polymer can affect all these stages [48-57].
Any excess of an additive in a polymer (above its solubility) may exude on the polymer
surface, forming either drops or powder; it blooms or sweats out. A quantitative
description of the process of sweating out is difficult, because at high additive
concentrations exceeding its solubility the diffusion coefficient is not constant and the
residual additive concentration after sweating out is greater, the greater its initial
concentration in the polymer.
The evaporation of an additive, A, from a polymer depends on its solubility in the polymer.
In some cases, the rate of additive evaporation, We, is connected with its surface
concentration, [A]sf, and its solubility in the polymer, [A]s, by:
We = Wa[A]sf/[A]s
(4.16)
where Wa is the rate of evaporation of the individual additive. A detailed description of
additive loss due to evaporation may be found elsewhere [51].
High molecular weight additives are not volatile and their diffusion in a polymer at
elevated temperature is very slow, which is why washing out is the principal cause of
undesired loss of stabilisers and other additives from polymeric material used outdoors
or for the flow of liquids in tubes and containers. There are some factors that may
influence the washing out of stabiliser from a polymer, i.e., the additive solubility
and the solvent solubility in the polymer [55]. Owing to low additive solubility, part
of the additive may be present in a polymer in a metastable state or form a separate
phase and it can be lost quickly. On the other hand, the solvent facilitates the migration
of stabilisers passing into the polymer and increasing the segmental mobility of
125
Handbook of Plastic Films
macromolecules. The ability of the solvent to escape the additive is connected with
the solvent solubility in the polymer: the higher the solvent solubility, the higher the
washing-out effect.
The diffusion coefficient of an additive in many cases increases with the additive
concentration in the polymer. The dependence of the diffusion coefficient on diffusant
concentration may be due either to plasticisation of the polymer or to the presence of
sorption centres that bind a part of the diffusing molecules.
Assuming that only mobile molecules of A, outside the sorption centres, participate
in the diffusion, their concentration is represented [9] by:
[ A] =
[ A ]p
Ka ⎛ [ Za ] – [ A]p ⎞⎠
⎝
(4.17)
It is possible to find the theoretical dependence of the diffusion coefficient (D) on the
total concentration of A in the polymer:
Φ = –Dt(d[A]/dx)
D=
[ ]
2
Ka ⎛⎝ [ Za ] – [ A]p ⎞⎠
Dt Z
(4.18)
where Dt is the diffusion coefficient of truly dissolved molecules. Figure 4.9 shows
that D of phenyl-β-naphthylamine in PP depends on the antioxidant concentration
and on the concentration of sorption centres according to equation (4.18).
The results obtained show that features of additive dissolution and diffusion could
be explained by taking into account non-unique additive distributions in the polymer
and the existence of sorption centres around polymer chain entanglements which are
able to hold the additive molecule. The concentration of these entanglements may be
changed either during polymer synthesis or during polymer treatment (orientation,
crystallisation, etc.). The influence of polymer disorder on additive behaviour and
polymer properties should be considered as an important factor in the physical
chemistry and technology of polymeric materials.
126
Solubility of Additives in Polymers
Figure 4.9 Dependence of the diffusion coefficient of phenyl-b-naphthylamine on its
concentration in PP at 60 °C
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Solubility of Additives in Polymers
27. L.S. Feldshtein and A.S. Kuzminsky, Kautchuk i Rezina, 1970, 10, 16.
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Handbook of Plastic Films
43. A. Peterlin, Journal of Macromolecular Science B, 1975, 11, 57.
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Degradation and Stability, 1991, 31, 61.
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Stability, 1993, 40, 365.
48. A.P. Mar’in and Y.A. Shlyapnikov, Polymer Degradation and Stability, 1991,
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130
5
Polyvinyl Chloride: Degradation
and Stabilisation
K.S. Minsker, G.E. Zaikov and V.G. Zaikov
5.1 Introduction
Some aspects of the manufacture of polyvinyl chloride (PVC) that does not contain labile
groups in the backbone are considered. This will provide a drastic increase in the intrinsic
stability of polymeric products, and the possibility of PVC processing with a minimal
content or total absence of stabilisers and other additives. The data presented allow the
creation of rigid, semi-rigid (semi-flexible) and flexible (plasticised) materials and products
with minimal content of chemical additives and increased service lifetime for exploitation
in natural and special conditions.
PVC is one of the most well-known multi-tonnage and practically important polymeric
products. Thousands of rigid, semi-flexible and flexible (plasticised) materials and products
based on PVC are widely used in all spheres of national economies and everyday life.
PVC was first synthesised by E. Baumann in 1872, but its industrial manufacture began
much later – in 1935 in Germany according to the literature data, and in 1930 in the
USA according to the data of the DuPont Company.
Global PVC production is impressive: 220 thousand tons in 1950, about 1.5 million
tons in 1960, more than 3 million tons in 1965, more than 5 million tons in 1970, and its
current production (mid-2002) is estimated to be more than 23 million tons.
A basic problem with PVC is its low stability. Under the action of heat, ultraviolet (UV)
light, oxygen, radiation, etc., it easily disintegrates according to the law of transformation
of adjacent groups with the elimination of hydrogen chloride and the formation of
sequential carbon-carbon double bonds in the macromolecules and the appearance of
undesirable coloration (from yellow to black). Therefore, it is necessary to apply a set of
methods that will lead to the increased stability of PVC itself, and of materials and
products based on it, when exposed to the various factors that occur during synthesis,
storage, processing and use.
It is logical to assume that, among the many aspects causing the low stability of PVC and
the rather short lifetime of materials and products based on it, the most important point
is to understand the reasons for its abnormally high rates of disintegration compared to
low molecular weight models. Researchers in the fields of synthesis and processing of
131
Handbook of Plastic Films
PVC appear to have found this problem rather complex, for, in essence, it is still under
discussion. So far, such workers in industrial research centres in various countries have
not been able to agree upon the identification of the weak site in the structure of the PVC
macromolecule responsible for causing its abnormally low stability.
5.2 Some Factors Affecting the Low Stability of PVC
It used to be thought that the low stability of PVC was connected to the possible presence
of labile groups in the macromolecular structure, which activate polymer disintegration.
These labile groups are distinct from sequences of regular vinyl chloride repeat units:
~CH2CHCl–CH2–CHCl–CH2–CHCl~
The overwhelming majority of researchers believe that such groupings are [1-8]:
(1) Chlorine atoms bonded to tertiary carbon atoms C–Cl (At);
(2) Vicinal chlorine atoms in the macromolecular structure:
~CH2–CHCl–CHCl–CH2~ (Av);
(3) Unsaturated end-groups such as ~CH=CH2 and/or ~CCl=CH2;
(4) β-Chloroallyl groups ~CH2–CH=CH–CHCl~ (Ac);
(5) Oxygen-containing hydroxy and peroxy groups (A0).
However, even after just a brief consideration of the process of PVC disintegration, it is
obvious that there are far fewer labile groups (which can be considered to be the cause of
low PVC stability) in the macromolecules. This is because, on PVC dehydrochlorination,
tertiary chlorine (At) and vicinal (Av) groups turn into β-chloroallyl groups and the
hydroperoxide groups transform into carbonyl groups, as shown in Scheme 5.1.
In addition, PVC research throughout the world has shown that the initial (freshly
synthesised) PVC macromolecules (which are processed in materials and products) do not
contain di- (A2), tri- (A3) and/or polyene (Ap) groups [2, 3, 9-14]. Internal peroxide groups
~CH2–CHCl–O–O–CH2–CHCl~
are not found either, since if they were formed during PVC synthesis they would quickly
disappear as a result of hydrolysis and/or homolytic cleavage of the O–O bond. There
are reliable experimental results, including those obtained during the study of the thermal
132
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
Cl
~H2C
C
At
CH2~
CH3
~H2C
~HC
CH
CH
Cl
Cl
Av
CH2~
O
~H2C
CH~
CH
C
A0
CH2~
– H 2O
OOH
~H2C
Cl
Cl
C
H
C
H
CHCl~
~H2C
– HCl
C
H
C
H
C
H
C
H
CHCl~
Ap
Ax
Scheme 5.1
destruction of fractionated PVC, showing that, although unsaturated end-groups are
present in the structure of polymeric molecules, they do not affect the disintegration rate
of PVC [10, 13-15].
Thus, the process of gross dehydrochlorination of PVC (overall rate constant VHCl) can
be described with sufficient accuracy by Scheme 5.2, where: α0 represents the regular
vinyl chloride ~CH2–CHCl~ groups; KCl, Kt, Kv, Kc and Kp are the rate constants for the
appropriate dehydrochlorination reactions of PVC; and Ktr is the rate constant for the
reaction that terminates polyene growth.
Av
α0
Kv
KCl
At
Kp
Ac
Kc
Ap
A2
Ktr
Kt
A*
Scheme 5.2
133
Handbook of Plastic Films
Following from Scheme 5.2, we have:
VHCl = KCl[a0] + Kc[Ac] + Kp[Ap]
with real values of: KCl = 10–8 to 10–7 s–1 and [α0] = 1 mol/mol PVC; Kt = 10–4 s–1 and
[At] = 10–3 mol/mol PVC; Kc = 10–4 to 10–5 s–1 and [Ac] = 10–4 mol/mol PVC; Kv = 10–3 to
10–4 s–1 and [Av] = 10–5 mol/mol PVC; and Kp = 10–2 s–1 (448 K).
It is obvious that Scheme 5.2 assumes the concept of β-chloroallyl-activated disintegration
of PVC accepted by the majority of researchers, but without real proof [1-5]. However,
this postulate contradicts many experimental facts [16, 17], in particular the following:
(1) Calculated values of VHCl differ greatly from the experimental ones.
(2) The β-chloroallyl activation of PVC disintegration assumes an autoacceleration of
the PVC gross dehydrochlorination process with time [16-18], whereas a linear
dependence is observed experimentally (Figure 5.1). The gross rate constant of PVC
disintegration, according to experimental data and shown in Figure 5.1 at Kc = 10–4
to 10–5 s–1, should contain the term Kp ≅ 10–2 s–1 (at 448 K) from the very beginning
Figure 5.1 Kinetic curves for PVC dehydrochlorination: (1) calculated data for
β-chloroallyl activation; (2) experimental data (448 K, 10–2 Pa)
134
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
Table 5.1 Dehydrochlorination rate constant for thermal destruction of low
molecular weight model compounds
No.
Compound
Temperature range
(K) where compounds
start to degrade at
noticeable rate
Group
index
Decomposition
rate constant
(K s–1)
1
2,4-Dichloropentane
563-593
α0
2.6 × 10–9
2
meso-2,4-Dichloropentane
563-593
α0
1.9 × 10–9
3
3-Ethyl-3-chloropentane
488-553
At
7.9 × 10–6
4
4-Chlorohexene-2
433-463
Ac
5.1 × 10–4
5
4-Chlorodecene-2
438-468
Ac
5.0 × 10–5
6
7-Chlorononadiene-3,5
343-369
Ap
3.4 × 10–2
7
6-Chlorooctadiene-2,4
360-386
Ap
2.6 × 10–2
of PVC thermal destruction. However, according to data obtained on the thermal
disintegration of low molecular weight model compounds [19-21], this is observed
only as a result of the destruction of model compounds containing a chlorine atom in
a β-position to conjugated (C=C)n bonds (at n ≥ 2), i.e., due to the effect of the
adjacent group of the long-range order (Table 5.1). Thus, the concept of β-chloroallyl
activation of PVC dehydrochlorination does not satisfy even a preliminary analysis
of the experimental results, is therefore erroneous and should no longer be considered
a viable theory.
On the basis of theoretical considerations of PVC thermal degradation, and in view of
all the available experimental data, it can be concluded that: even if internal β-chloroallyl
groups (as well as tertiary and vicinal chlorides) are present in the macromolecular
structure, they do not contribute to the process of PVC gross dehydrochlorination as a
result of their sufficient relative stability. It was assumed, and then proved, that the
group that is responsible for the low stability of PVC is an oxovinylene (carbonylallyl)
conjugated dienophile group:
–C(O)–CH=CHCl–CH2–
the double bond of which is activated by the adjacent electrophilic C=O group.
Apparently, this group is present in PVC macromolecules in rather small amounts,
γ ≅ 10–4 mol/mol PVC, but disintegrates at a rather high rate (Kp ≅ 10–2 s–1) with HCl
elimination [14, 17, 22-24].
135
Handbook of Plastic Films
It is extremely important to emphasise that the concept of oxovinylene activation of
PVC disintegration does not contradict any currently known experimental facts. In
addition, new proofs (including original ones) of the existence of basic groups in the
structure of PVC macromolecules have been obtained recently. In particular, oxovinylene
groups in the PVC macromolecule are easily split (under mild conditions) with alkaline
hydrolysis (5% aqueous KOH solution, and 5% solution of PVC in cyclohexanone) [13,
14], which is a characteristic reaction for α,β-unsaturated ketones, as shown in the
following reaction [25]:
O
O
H2O
~CH
CH
C(O)~
+
C
KOH
H3C
C~
(5.1)
H
Using this reaction, it is easy to estimate the content of labile oxovinylene groups (γ0) in
the macromolecular structure by the decrease of viscosity-average molecular weight of
PVC [13-17].
5.3 Identification of Carbonylallyl Groups
It is important to remark that both β-chloroallyl and polyene groups are inert to alkaline
hydrolysis, but easily decomposed on oxidation (in the presence of hydrogen peroxide)
and by ozonolysis [13]. The ozonolysis method allows estimation of the total amount of
internal unsaturated (β-chloroallyl, chloropolyenyl and oxovinylene) groups in the PVC
structure by the decrease of PVC molecular weight. Thus, it is experimentally shown
that practically all the internal unsaturated groups in PVC macromolecules are oxovinylene
ones, and that the PVC dehydrochlorination rate is linearly connected to the content of
internal labile oxovinylene groups in polymeric molecules [14, 26], determined by using
alkaline hydrolysis (Figure 5.2). It is known that PVC synthesised in the absence of
oxygen is always more stable than PVC manufactured industrially. This is due to the
presence of sufficiently stable internal β-chloroallyl (not oxovinylene) groups (oxidative
ozonolysis) in the first type of PVC structure. As a whole, the real process of HCl
elimination during PVC disintegration in the transformation reaction of adjacent groups
is complex, since generally this or that contribution is brought in by all abnormal groups
contained in the PVC structure. However, apparently, the contribution of different
reactions to this process varies and in a number of cases some of them can be neglected.
The kinetic analysis takes into account the real contents of characteristic (including
abnormal) groups in PVC. Also, the rate constants of their disintegration (Table 5.2)
136
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
Figure 5.2 Dependence of PVC dehydrochlorination rate on the content of
carbonylallyl groups in the polymer molecules (448 K, 10–2 Pa)
have precisely shown [14, 17, 24, 27, 28] that the ratio of the appropriate reaction rate
constants is:
KCl : Kc : Kt : Kp ≅ 1 : 100 : 100 : 100,000
and, for this reason, the thermal stability of PVC is determined by the effect of the
adjacent group of the long-range order (conjugation effect). So the total elimination rate
of HCl from PVC is described with sufficient accuracy by the simple equation:
VHCl =
[
d HCl
dt
] = K [α ] + K [ γ ] = V + V
Cl
0
p 0
Cl
p
(5.2)
Even taking into account the participation of tertiary chloride (At) and β-chloroallyl
(Ac) groups in PVC disintegration, the contribution of the expression Vp = Kp[γ0]
comprises about 90% or more of the total gross rate of PVC dehydrochlorination.
This confirms oxovinylene (not β-chloroallyl) activation as the major process in PVC
thermal disintegration.
The development of the concept of oxovinylene activation of PVC thermal destruction
appears to be an important point in the theory and practice of PVC chemistry and
137
Handbook of Plastic Films
Table 5.2 Rate constants of dehydrochlorination of characteristic groupings and
their contents in the initial PVC structure
Contents in PVC
Group
Index
Amount
(mol/mol
PVC)
Rate constant of degradation at 448 K
Index
Value
(s-1)
K. Minsker 1978
E. Sorvik 1984
G. Zimmerman 1984
Kp
10-1-10-2
K. Minsker 1977
W. Starnes 1985
0
K. Minsker 1978
G. Zimmerman 1984
KCl
10-5-10-4
Z. Meyer 1971
B. Troitsky 1973
W. Starnes 1983
~10-3
E. Sorvik 1984
A. Caraculaku 1981
V. Zegelman 1985
Kt
10-4
W. Starnes 1983
Z. Meyer 1971
K. Minsker 1976
Kp
~10-2
Z. Meyer 1971
K. Minsker 1984
-
KCl
10-7-10-8
Z. Meyer 1971
K. Minsker 1972
Authors
Authors
~CO–CH=CH–CHCl~
γ0
~10-4
(~CH2)CCl–CH2–CH2Cl~
ACl0
~CCl–CH2CH2Cl
At0
~CH2–(CH=CH)n>1–CHCl~
Ap
0
~CH2–CHCl–CH2–CHCl~
α0
1
objectively defines the necessity for a new specific approach for studying various aspects
of the destruction and stabilisation of PVC. In particular, studies are needed of the new
characteristic reactions with unsaturated ketones, confirming the presence of oxovinylene
groups in the PVC structure, or the interaction of ~C(O)–CH=CH–CHCl~ groups with
organic phosphites P(OR)3 [29-33] and dienes [34, 35].
5.4 Principal Ways to Stabilise PVC
Organic phosphites react easily in mild conditions (290-330 K) with oxovinylene groups
in the presence of proton donors to yield the stable ketophosphonates:
138
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
~C
CHCl~
O
~C
O
C
H
C
H
CH~
+
P-(OR)3
P(OR)3
O
Cl
C
CH2
O
CH
CHCl~
P(OR)3
(5.3)
The reaction kinetics for the interaction of organic phosphites with oxovinylene groups
are shown in Figure 5.3. The formation of ketophosphonate structures according to reaction
(5.3) results in the disappearance of internal C=C bonds in the PVC structure. As a result,
neither ozonolysis of a polymeric product nor especially alkaline hydrolysis leads to
degradation of macromolecules and, consequently, decrease of PVC molecular weight.
Figure 5.3 The changes in the ~C(O)–CH=CH~ group content in PVC during interaction
with tri(2-ethylhexyl) phosphite (C0 = 10–2 mol/mol PVC): (1) 289 K; (2) 298 K; (3) 448 K
139
Handbook of Plastic Films
It is important to note that organic phosphites do not react with β-chloroallyl groups,
as has been confirmed by the method of competing reactions of organic phosphites
(trialkyl-, arylalkyl- and triarylphosphites) with a mixture (1:1 mol/mol) of methyl
vinyl ketone (model of an oxovinylene group) and 4-chloropentene-2 (model of a βchloroallyl group) at 353 K. Practically, the organic phosphite selectively reacts
quantitatively (with regarding to proton donor) with methyl vinyl ketone, while 4chloropentene-2 is quantitatively allocated after realisation of the reaction, excluding
the small amount (less than 7 wt%) of products of its dehydrochlorination. The main
reaction product (up to 75 wt%) is:
CH3–C(O)–CH2–CH2–P(OR)2
In this reaction, trialkyl- and alkylarylphosphites are more active than triarylphosphites.
Dienophilic oxovinylene groups react with conjugated dienes according to the following
Diels-Alder reaction:
O
O
RHC
~C
C
H
C
H
CHCl~
+
CHR–
C
H
C
H
~C
CH
CH
RHC
CHCl~
CHR–
C
H
C
H
(5.4)
The reactions of PVC with cyclopentadiene, piperylene, isoprene, 5-methylheptatriene1,3,6, etc., proceed in mild conditions (353 K) and result in destruction of internal
unsaturated C=C groups in PVC chains. These reactions are new and have not been
reported before, and are similar to the reaction of PVC with organic phosphites, as
shown in reaction (5.4).
The collection of methods used to increase PVC stability to the action of various factors
(such as heat, light, oxygen, etc.), in terms of storage, processing and use is closely
connected to the level of theoretical development of PVC degradation. Therefore, it is
clear that the significant advances in theoretical developments of the reasons for the
thermal instability of PVC (the presence of oxovinylene groups in the backbone), the
mechanism of the process (the fundamental influence of adjacent groups of the longrange order) and the kinetics of their disintegration were necessary, and have enabled
a new look at the determination of effective methods of PVC stabilisation under thermal
and other influences.
140
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
According to Scheme 5.2 it is impossible (and unnecessary) to increase the stability of
PVC by the reduction of rate VHCl, since this process is rather slow. According to the
experimental data, the rate of PVC statistical dehydrochlorination, VHCl, is constant
(law of randomness) and does not depend on how the polymer was synthesised or its
molecular weight. Hence, it is a fundamental characteristic of PVC, showing that all
parts in clusters ~BXBXBX~ participate similarly in the process of HCl elimination under
the law of randomness. On the other hand, the rate of formation of the conjugated
systems, Vp, differs markedly, since it increases linearly with the content of oxovinylene
groups in the initial PVC macromolecules (γ0) (Figure 5.2).
Thus, the basis of effective PVC stabilisation, which determines processing properties
and the durability of rigid materials and products, is due mainly to the increased selfstability of PVC [17, 36-39]. This can be achieved by chemical stabilisation of the labile
oxovinylene groups present in the initial PVC macromolecules, first of all by studying
specific polymer analogous reactions with either of the reaction centres 1-3:
2
~C
1
C
H
C
H
O
CH~
(5.5)
3
Cl
The conjugation ~C(O)CH=CH~ has to be destroyed and/or the labile chlorine atom has
to be replaced with a more stable adjacent group by interaction with the appropriate
additives (stabilisers). This principle underlies the stabilisation of PVC in real formulations
during manufacture of rigid materials and products, which is called ‘chemical stabilisation’
of PVC [17, 36, 37]. The reactions on centres 1-3 mentioned above are as follows:
C O fragments of oxovinylene chloride groups:
(1) Polymer analogous reactions on
~ CH
R3SiH
R3GeH
O
~C
R3
CH
CH
CHCl~
[5]
Si
O
CH
CH
CHCl~
R'
CH
CH
OH
OH
CH
CH
R"
R'
R'
R"
CH
~C
O
O
CH
CH
C
CHCl~
[40]
(5.6)
R"
141
Handbook of Plastic Films
C C
(2) Polymer analogous reactions on
~C
CH2
fragments of oxovinylene groups:
CH
~C
CHCl~
P(OR)3
O
and/or
P(OR)3
CH~
CH
O
CHCl~
P
O
O
OR
[31-35]
O
R'
CH
CH
CH
CH
~C
R"
CHCl~
CH
CH
CH
R'
[41]
CH
HC
R"
CH
O
C
HC
O
CH
C
CH
C
O
O
[21]
CH
O
C
O
O
CH
~HC
CHCl~
(5.7)
C
H
(3) Polymer analogous reactions on labile
~C(O)
R2Sn(COOR)2
CH
C Cl groups:
CH
CH
CH2–
[16, 17]
Cd(COOR)2
OC(O)R
Zn(COOR)2, etc.
O
R'
CH
CH
(ZnCl2)
142
R"
~C(O)
CH
CH
CH
[43]
O—CHR'—CHClR"
(5.8)
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
The concept of oxovinylene activation of the disintegration of PVC has allowed revealing
new unexpected possibilities for effective stabilisation – not only thermal, but also light
stabilisation – of this polymer. This also allows previously unknown classes of chemical
compounds to be used for its stabilisation, in particular, conjugated diene hydrocarbons,
Diels-Alder reaction adducts, protonic acids, α,β-dicarbonic compounds, etc. [34, 35, 4046]. It has also enabled new real reactions for PVC stabilisation to be revealed, including the
application of known additives that have been used for a long time for PVC stabilisation (for
instance, organic phosphites, epoxy compounds, proton-donating compounds, etc.). So, on
this basis it is possible to manage the PVC ageing process more effectively (Scheme 5.3). The
relation between the chemical structure of additives and their efficiency as stabilisers for
Scheme 5.3
143
Handbook of Plastic Films
PVC gives an opportunity for the scientifically based and economically expedient selection
of the appropriate stabilisers and their synergistic combinations for producing rigid materials
based on PVC.
5.5 Light Stabilisation of PVC
Polymer analogous transformations of the oxovinylene groups in PVC macromolecules
on chemical stabilisation with the appropriate chemical additives lead not only to increased
self-stability of PVC and inhibition of macromolecular crosslinking, but also to a noticeable
increase in the colour stability of PVC.
The transformation of oxovinylene groups as a result of polymer analogous
transformations with chemical additives in the ketophosphonate, cyclohexane, dioxolane,
dihydropyran, etc., structural groups in PVC and the ‘curing’ of labile oxovinylene chloride
groups result in an increase in the optical density of PVC in the UV region of the spectrum.
As a result, these groups act as internal light stabilisers and result in the phenomenon of
self-photostabilisation of PVC [47] (Figure 5.4).
Figure 5.4 Dependence of whiteness retention coefficient Kw in PVC films on exposure
time: (1) unstabilised PVC; and polymer treated with: (2) 2-tris(2-ethylhexyl)
phosphite; (3) 2-ethylhexyl-9,10-epoxy stearate with ZnCl2; (4) piperylene;
(5) cyclopentadiene (295 K, λ = 254 nm, 1 to 1.5 × 1015 photon/s cm2)
144
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
Thus, the determining factor causing the high rate of PVC disintegration and its need for
stabilisation is the presence of abnormal groups, mainly oxovinylene ones, in the structure
of its macromolecules.
5.6 Effect of Plasticisers on PVC Degradation in Solution
In both plasticised (semi-rigid and flexible) PVC materials as well as PVC in solution, the
rates of thermal destruction and effective stabilisation are caused by essentially different
fundamental phenomena in comparison to those involved in the ageing of PVC in the
absence of solvent. The following aspects of both structure and macromolecular dynamics
have a significant influence on the stability of PVC: the chemical nature of the solvent,
its basicity, specific and nonspecific solvation, the concentration of PVC in the solution,
the segmental mobility of macromolecules, the thermodynamic properties of the solvent,
the formation of associates, aggregates, etc. The chemical stabilisation of PVC plays a
less significant role.
As regards PVC destruction in solution, one of the basic reasons for a change in the
kinetic parameters is the nucleophilic activation of the PVC dehydrochlorination
reaction. The process is described by an E2 mechanism. Thus, there is a linear dependence
between PVC thermal dehydrochlorination rate and the relative basicity of the solvent,
B cm–1 (Figure 5.5) [48-50]. The value B cm–1 is evaluated by measuring the shift of a
characteristic band (phenolic OH) at λ = 3600 cm–1 in the IR spectrum due to interaction
with the solvent [51]. It is very important that, in solvents with relative basicity B >
50 cm–1, the rate of PVC dehydrochlorination is always above the rate of PVC
dehydrochlorination without the solvent; while, when B < 50 cm–1, PVC disintegration
rate is always less than that without the solvent. The revealed dependence VHCl = f(B)
is described by the equation:
*
VHCl
= VHCl + k(B − 50)
(5.9)
Inhibition of PVC disintegration in solvents with basicity B < 50 cm–1 is a very interesting
and practically important phenomenon. It has been given the name ‘solvational
stabilisation’ of PVC. However, ignoring the fact that, even at low concentration (2
wt%), PVC solutions should be represented not as solutions with isolated macromolecules,
but rather as structured systems, results in a number of cases of deviation from a linear
dependence of PVC dehydrochlorination rate on the solvent basicity B cm–1. In particular,
an abnormal destruction behaviour of PVC is observed in certain ester-type solvents
(plasticisers) (Figure 5.5, points 25-28), apparently caused by structural changes of the
macromolecules. This has never before been taken into account when working with
PVC solutions.
145
Handbook of Plastic Films
Figure 5.5 Influence of the solvent’s basicity on the rate of thermal dehydrochlorination
in solution: (1) n-dichlorobenzene, (2) o-dichlorobenzene, (3) naphthalene,
(4) nitrobenzene, (5) acetophenone, (6) benzonitrile, (7) di-(n-chlorophenylchloropropyl) phosphate, (8) triphenyl phosphite, (9) phenyl-bis(β-chloroethyl)
phosphate, (10) tri-(n-chlorophenyl) phosphate, (11) 2-ethylhexylphenyl phosphate,
(12) tricresyl phosphate, (13) cyclohexanone, (14) phenyl-bis(β-chloropropyl)
phosphate, (15) tri-β-chloroethyl phosphate, (16) tri-β-chloropropyl phosphate,
(17) di-(2-ethylhexyl) phosphate, (18) 2-ethylhexylnonyl phosphate, (19) tri-(2ethylhexyl) phosphate, (20) tributyl phosphate, (21,25) dibutyl phthalate,
(22,26) di-(2-ethylhexyl) adipate, (23,27) dioctyl phthalate, (24,28) dibutyl sebacate.
Concentration of PVC in solution: (1-24) 0.2 wt%, (25-28) 2 wt%; 423 K, under nitrogen
It was revealed quite unexpectedly that not only ‘polymer-solvent’ interactions, but also
‘polymer-polymer’ interactions, have a significant influence on the rate of PVC
disintegration in solution. It is known that the structure and properties of the appropriate
structural levels depend on the conformational and configurational nature of the
macromolecules, including the supermolecular structure of the polymer, which in turn
determines all the basic (both physical and chemical) characteristics of the polymer.
‘Polymer-polymer’ interaction leads to the formation of structures on the supermolecular
level. In particular, on going to a more concentrated solution, the PVC-solvent system
consistently passes through a number of stages, from isolated PVC macromolecules in
146
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
solution (infinitely dilute solution) to associates and aggregates of macromolecules in
solution. On further increase of PVC concentration, the formation of a spatial fluctuational
net with a structure similar to that of the bulk polymer occurs.
When the polymer concentration in solution increases, the rate of the PVC
dehydrochlorination reaction changes as well, and various types of effect of the solvent
on the PVC disintegration rate in solution are observed depending on the numerical
value of the basicity B cm–1 [52-57]. If the relative basicity of the solvents used is B > 50
cm–1, the polymer degradation rate decreases when its concentration increases. If the
basicity of the employed solvents is B < 50 cm–1, the polymer degradation rate increases
with increasing polymer concentration. In all cases the rate of HCl elimination from the
polymer tends to approach the values of PVC dehydrochlorination rate usually observed
PVC = 5 x 10–8 (mol HCl/mol PVC)/s (Figure 5.6).
in the absence of solvent VHCl
Figure 5.6 The change in PVC dehydrochlorination rate as a function of its
concentration in solution: (1) cyclohexanol, (2) cyclohexanone, (3) benzyl alcohol, (4)
1,2,3-trichloropropane, (5) o-dichlorobenzene, (6) no solvent; 423 K, under nitrogen
147
Handbook of Plastic Films
Equation (5.9) turns into equation (5.10) if one takes into account that the PVC
degradation rate is determined not only by the relative basicity of the solvent, B, but also
by its concentration in solution, C (mol PVC/litre). Also, the degree of ‘polymer-polymer’
interaction (degree of macromolecule structurisation in a solution is given by ΔC = C –
C0, where C0 is the concentration at the beginning of PVC macromolecule association in
solution) is considered:
0
VHCl = VHCl +
A1 (B − 50)
ΔC + d1
(5.10)
where A1 = (0.8 ± 0.2) × 10–9 (mol HCl/mol PVC)/s; and d1 is a dimensionless factor
reflecting the ‘polymer-solvent’ interaction (d1 = 0.5 ± 0.25).
The deviation from the onset of macromolecule association in a solution is taken as an
absolute value, since it can be changed in both directions to more concentrated or more
dilute polymer solution.
Equation (5.10) well describes the change of PVC thermal dehydrochlorination rate as a
function of its concentration in a solution of relative solvent basicity B, irrespective of
the chosen solvent (Figure 5.7).
The observable fundamental effect has significant importance in the production of
plasticised materials and products made from PVC, in particular when esters are used.
Despite the very high basicity of ester-type plasticisers (B = 150 cm–1) in the range of
PVC concentration in solutions above 2%, a noticeable reduction in the degradation
rate of PVC is observed (Figure 5.5, points 25-28), and stabilisation of PVC occurs. This
effect is caused by the formation of dense globules, associates, etc., in the PVC-plasticiser
system. Practically, this allows economic formulations of plasticised materials to be created
from PVC with very low content of metal-containing stabilisers, used as HCl acceptors,
or without their use at all.
Temperature is very important in the formation of heterophase systems. Even at low
concentrations of PVC in ester-type plasticisers (for example, in dioctyl phthalate at C >
0.1 mol/l), true solutions are formed only at temperatures above 400 K. The globular
structure of PVC suspension and the formation of associates are retained at temperatures
up to 430-445 K. In other words, plasticised PVC is able to keep its structural individuality
on a supermolecular level, which is formed during polymer synthesis. Specifically, under
these conditions an ester-type plasticiser behaves not as a highly basic solvent, but as a
stabiliser during PVC thermal degradation due to formation of associates, etc. This leads
to a reduction of the amount of stabiliser, extension of useful lifetime of materials and
products, etc.
148
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
Figure 5.7 The change in PVC dehydrochlorination rate as a function of its
concentration in solution: (1,2) 1,2,3-trichloropropane, (3,4) cyclohexanol; (1,3)
experimental data, (2,4) data calculated using equation (5.10) at A1 = 10–9 and
d1 = 0.8 and 0.7, respectively, at 423 K, under nitrogen
It is necessary to note that the change in the degradation rate of PVC brought about by
association of macromolecules is a general phenomenon and does not depend on how
it was achieved. In particular, a change of character of the dehydrochlorination rate of
PVC in solution is observed, similar to concentrated PVC solutions (Figures 5.6 and
5.7), if a change of PVC structure in solution occurs upon addition of even chemically
inert nonsolvents such as hexane, decane, undecane, polyolefins, polyethylene wax,
etc. [53, 56-59] (Figure 5.8). It is interesting to observe that the degree of relative
change of PVC disintegration rate under the action of a second inert nonsolvent is
much higher than for concentrated PVC solution. This is especially true in the case of
using low-basicity solvents (trichloropropane and dichlorobenzene); it is the result of
more dense formations on the supermolecular level, corresponding associates and
aggregates, and, accordingly, a significant change of PVC destruction rate. The higher
the content of nonsolvent (including inert polymer) in a blend and the lower the
thermodynamic compatibility of the components in a solution, the more structural
149
Handbook of Plastic Films
Figure 5.8 The change in PVC thermodegradation rate on the content of the second
inert polymer in solution of trichloropropane (1,3), dichlorobenzene (2) and
cyclohexanol (4-6) for blends of PVC and polyethylene (1,4), polypropylene (2,5)
and polyisobutylene (3,6); 423 K, under nitrogen
formation takes place in a solution, including that in the presence of polymer blends
(associates, aggregates). Formation of a fluctuational net with participation of
macromolecules is the probable explanation.
The reason for the change in PVC thermal dehydrochlorination rate in the case of its
blends with chemically inert, thermodynamically incompatible polymers is the same. It
is due to the fact that in concentrated PVC solution (structural chemical changes of
polymer in solution), the parameters determining the rate of PVC disintegration will
obviously be similar. Therefore, for PVC thermal destruction, the concentration of the
second polymer blended with PVC and its degree of thermodynamic affinity to PVC, in
addition to the influence of polymer concentration in the solution, the basicity of the
solvent, B cm–1, and ‘polymer-solvent’ interaction forces, have to be taken into account.
In view of these factors equation (5.10) turns into:
0
+
VHCl = VHCl
150
A1 (B − 50)
ΔC + d1 + C + α n
+
A1α 2α n
BC
(5.11)
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
where α is the fraction of the second polymer, varying from 0 to 0.99; n is a
dimensionless parameter describing the degree of thermodynamic affinity of PVC to
the second polymer and varying from 0 (for complete thermodynamic compatibility
of the components) up to a value of ~10 (for complete thermodynamic incompatibility
of the polymers); and d2 is a dimensionless coefficient reflecting the interaction of
the second polymer with the solvent, which equals 2.5 ± 0.1 for the destruction of
PVC blended with polyethylene in dichlorobenzene, trichloropropane and
cyclohexanol.
Observable changes in PVC thermal disintegration rate under the action of a solvent
that is thermodynamically incompatible with PVC or for a concentrated solution of
PVC are caused by the transformation of the solvent from macromolecular globules
of PVC to the structure that existed in the absence of the solvent. This evokes the
unexpected effect of ‘solvent action’, either retardation or acceleration of PVC thermal
disintegration depending on the solvent basicity, B cm–1. A solvent transformation
that accelerates PVC disintegration (B > 50 cm–1) results in a decrease of its interaction
with PVC and leads to a delay in the HCl elimination process, i.e., to stabilisation.
This occurs in the case of both concentrated PVC solutions as well as the addition of
another polymer that is thermodynamically incompatible with PVC. In solvents that
slow down PVC disintegration (B < 50 cm–1) by virtue of low nucleophilicity, the
effect of solvent transformation and the weakening of its interaction with PVC has
the opposite result. In this case an increase of HCl elimination rate from PVC upon
increase of its concentration in solution or by using a chemically inert nonsolvent
occurs. It is obvious that, irrespective of how the changes to the PVC structure in
solution are made, either by increase of its concentration in solution or by addition
of another thermodynamically incompatible inert nonsolvent, the varying structuralphysical condition of the polymer results in a noticeable change of its thermal
dehydrochlorination rate in solution. These effects are caused by structural-physical
changes in the polymer-solvent system, and the previously unknown phenomena can
be classified as ‘structural-physical stabilisation’ (in the case of a reduction in the
gross rate of PVC disintegration in highly basic solvents at B > 50 cm–1) or ‘structuralphysical antistabilisation’ (in the case of an increase in the gross rate of PVC
disintegration in low-basicity solvents with B < 50 cm–1), respectively.
5.7 ‘Echo’ Stabilisation of PVC
Finally, it is necessary to describe one more appreciable achievement in the field of ageing
and stabilisation of PVC in solution. In real conditions the basic reason for the sharp
accelerated ageing of plasticised materials and products is oxidation of the solvent by the
oxygen of the air (Figure 5.9, curve 3).
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Handbook of Plastic Films
Figure 5.9 ‘Echo’ stabilisation of PVC. Elimination of HCl during thermal (argon) (1,2)
and thermo-oxidative (air) (3-5) destruction of PVC in solution of dioctyl sebacate: (1-4)
unstabilised PVC, (5) PVC stabilised with diphenylpropane (0.02 wt%) – ‘echo’
stabilisation; (2,4) PVC with no solvent; 448 K
•
2
RO2 + RH ⎯ ⎯
⎯
→ ROOH + R •
K
3
ROOH ⎯ ⎯
⎯
→ RO• + HO•
K
•
•
(5.12)
K6
RO2 + RO2 ⎯ ⎯
⎯→ inactive products
Peroxides, formed by oxidation of ester-type plasticisers, initiate the disintegration of
macromolecules. In these conditions the rate of PVC destruction increases by two or
more orders of magnitude and is determined by the oxidation stability parameter of the
solvent to oxygen Kef = K2 K03.5 K6−0.5 . Thus, a higher oxidation stability of the solvent (in
particular, an ester-type plasticiser) lowers the degradation rate of semi-rigid and flexible
PVC materials and increases its useful lifetime [60-63]. Inhibition of the oxidation process
of the solvents (including plasticisers) due to the incorporation of stabilisers, antioxidants
or their synergistic compositions slows down the thermo-oxidative disintegration of PVC
in solution (Figure 5.9, curve 5).
Effective inhibition of the oxidation of ester-type plasticisers by oxygen of the air causes
the rate of PVC thermo-oxidative destruction in concentrated solutions to become closer
152
Achievements for Polyvinyl Chloride: Degradation and Stabilisation
to the rate of polymer disintegration. This behaviour is characteristic of the thermal
destruction of PVC in the presence of plasticisers acting as solvent. In other words, it
becomes slower than PVC disintegration in the absence of solvent. This occurs due to a
structural-physical stabilisation. In these cases, inhibition of the solvent oxidation reaction
by using ‘echo’-type antioxidant stabilisers improves PVC stabilisation (Figure 5.9,
curve 5). This fundamental phenomenon of PVC stabilisation in solution and its thermooxidative destruction has been called ‘echo stabilisation’ of PVC [49, 62, 63].
5.8 Tasks for the Future
The creation of high-quality and economic semi-rigid and flexible materials and products
made from PVC, including those where solvents are employed, requires specific approaches
that are essentially different from the principles of manufacture of rigid PVC materials
and products. In particular, consideration and use of the following fundamental
phenomena should be considered: solvational, structural-physical and ‘echo’ stabilisation
of the polymer in solution.
As far as paramount tasks of fundamental and applied research in the field of PVC
manufacture and processing at the beginning of the 21st century are concerned, they are
obviously the following:
(1) The manufacture of industrial PVC that does not contain labile groups in its backbone.
This will provide a drastic increase in the intrinsic stability of polymeric PVC products,
the possibility of processing with the minimal content or total absence of stabilisers
and other chemical additives, and the opportunity to create PVC-based materials
and products with essentially increased useful service lifetime.
(2) Wide use of the latest achievements in the field of destruction and stabilisation of
PVC, in both the presence and the absence of solvents. The phenomena of chemical,
solvational, structural-physical, self- and ‘echo’ stabilisation of PVC will allow the
creation of rigid, semi-rigid and flexible (plasticised) materials and products with
minimal content of chemical additives, and will lead to increased useful service lifetime
under natural and special conditions.
(3) The use of nontoxic and nonflammable products that do not emit toxic and other
poisonous gaseous and liquid products at elevated temperature during the manufacture
and processing of PVC materials and their products.
(4) Complete elimination of all toxic and even low-toxicity (particularly compounds
based on barium, cadmium and lead, etc.), chemical additives from all formulations.
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Handbook of Plastic Films
(5) The search for nontoxic and highly effective inorganic chemical additives, primarily,
stabilisers of zeolite type, modified clays, etc.
At the same time, new ‘surprises’ will undoubtedly be presented to us by this outstanding
polymer puzzle. Certainly, as we look for a plastic for use as a ‘work-horse’ for many
decades, studies on PVC will lead to new stimuli in the development of scientific ideas
and practical development, and the opening-up of new pathways. These will result from
the essential need to delay PVC ageing in natural and special conditions, and to reduce
the amounts of the appropriate chemical additives, down to their complete elimination.
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Chloride, Nauka, Moscow, Russia, 1979, 272.
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Doklady Akademii Nauk SSSR, 1977, 232, 1, 93.
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Series A, 1981, 23, 3, 289.
24. K.S. Minsker, S.V. Kolesov, V.M. Yanborisov, Al.Al. Berlin and G.E. Zaikov,
Vysokomolekulyarnye Soedineniya, Series A, 1984, 26, 5, 883.
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Soedineniya, Series A, 1977, 19, 1, 32.
27. V.M. Yanborisov, S.V. Kolesov, Al.Al. Berlin and K.S. Minsker, Doklady
Akademii Nauk SSSR, 1986, 291, 4, 920.
28. K.S. Minsker, Al.Al. Berlin and V.V. Lisitsky, Vysokomolekulyarnye Soedineniya,
Series B, 1976, 18, 1, 54.
29. K.S. Minsker, N.A. Mukmeneva, Al.Al. Berlin, D.V. Kazachenko, M.Ya.
Yanberdina, S.I. Agadzhanyan and P.A. Kirpichnikov, Doklady Akademii Nauk
SSSR, 1976, 226, 5, 1088.
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Handbook of Plastic Films
30. N.A. Mukmeneva, S.I. Agadzhanyan, P.A. Kirpichnikov and K.S. Minsker,
Doklady Akademii Nauk SSSR, 1977, 233, 3, 375.
31. K.S. Minsker, N.A. Mukmeneva, S.V. Kolesov, S.I. Agadzhanyan, V.V. Petrov and
P.A. Kirpichnikov, Doklady Akademii Nauk SSSR, 1979, 244, 5, 1134.
32. N.A. Mukmeneva, K.S. Minsker, S.V. Kolesov and P.A. Kirpichnikov, Doklady
Akademii Nauk SSSR, 1984, 274, 6, 1393.
33. N.A. Mukmeneva, E.N. Cherezova, L.N. Yamalieva, S.V. Kolesov, K.S. Minsker
and P.A. Kirpichnikov, Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya,
1985, 5, 1106.
34. K.S. Minsker, S.V. Kolesov and V.V. Petrov, Doklady Akademii Nauk SSSR,
1980, 252, 3, 627.
35. K.S. Minsker, S.V. Kolesov, V.V. Petrov and Al.Al. Berlin, Vysokomolekulyarnye
Soedineniya, Series A, 1982, 24, 4, 793.
36. S.V. Kolesov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A,
1983, 25, 8, 1587.
37. K.S. Minsker, Polymer Plastics Technology and Engineering, 1997, 36, 4, 513.
38. K.S. Minsker, S.V. Kolesov and G.E. Zaikov, Journal of Vinyl Technology, 1980,
2, 3, 141.
39. K.S. Minsker, S.V. Kolesov and G.E. Zaikov, Vysokomolekulyarnye Soedineniya,
Series A, 1981, 23, 3, 498.
40. S.R. Ivanova, A.G. Zaripova and K.S. Minsker, Vysokomolekulyarnye
Soedineniya, Series A, 1978, 20, 4, 936.
41. K.S. Minsker, S.V. Kolesov and V.V. Petrov, Doklady Akademii Nauk SSSR,
1982, 268, 3, 632.
42. S. V. Kolesov, V.V. Petrov, V.M. Yanborisov and K.S. Minsker,
Vysokomolekulyarnye Soedineniya, Series A, 1984, 26, 2, 303.
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Series A, 1982, 24, 11, 2329.
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Doklady Akademii Nauk SSSR, 1983, 268, 6, 1415.
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Achievements for Polyvinyl Chloride: Degradation and Stabilisation
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Akhmetkhanov, Plasticheskie Massy, 1983, 12, 39.
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Vysokomolekulyarnye Soedineniya, Series A, 1986, 28, 9, 1885.
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Academic, Chur, Switzerland, 1994, 229.
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Handbook of Plastic Films
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158
6
Ecological Issues of Polymer Flame Retardants
G.E. Zaikov and S.M. Lomakin
6.1 Introduction
The use of polymer flame retardants has an important role in saving lives. The main
flame retardant systems for polymers currently in use are based on halogenated,
phosphorus, nitrogen and inorganic compounds. All of these flame retardant systems
basically inhibit or even suppress the combustion process by chemical or physical action
in the gas or condensed phase. Conventional flame retardants, such as halogenated,
phosphorus or metallic additives, have a number of negative attributes. The ecological
issue of their application demands the search for new polymer flame retardant systems.
Among the new trends in flame retardancy, the following should be pointed out:
intumescent systems, polymer nanocomposites, preceramic additives, low-melting
glasses, different types of char-formers and polymer morphology modification
processing. Brief explanations of the three major types of flame retardant systems
(intumescent systems, polymer nanocomposites and polymer organic char-formers) are
the subject of this overview.
Our environment has a mostly polymeric nature, and all polymers, whether natural or
synthetic, will burn, so the use of polymer flame retardants has an important role in
saving lives. There are four main families of flame retardant chemicals:
(1) Inorganic flame retardants including aluminium trioxide, magnesium hydroxide,
ammonium polyphosphate and red phosphorus. This group represents about 50%
by volume of global flame retardant production [1].
(2) Halogenated flame retardants, primarily based on chlorine and bromine. The
brominated flame retardants (BFR) are included in this group. This group represents
about 25% by volume of global production [1].
(3) Organophosphorus flame retardants are primarily phosphate esters and represent
about 20% by volume of global production [1]. Organophosphorus flame retardants
may contain bromine or chlorine.
(4) Nitrogen-based organic flame retardants are used for a limited number of polymers.
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Handbook of Plastic Films
6.2 Mechanisms of Action
Depending on their nature, flame retardants can act chemically and/or physically in the
solid, liquid or gas phase. They interfere with combustion during a particular stage of
this process, e.g., during heating, decomposition, ignition or flame spread.
Substitution of one type of flame retardant by another consequently means a change
in the mechanism(s) of flame retardancy. Halogen-containing flame retardants act
primarily by a chemical interfering with the radical chain mechanism that takes place
in the gas phase during combustion. High-energy OH and H radicals formed during
combustion are removed by bromine released from the flame retardant. Although
brominated flame retardants are a highly diverse group of compounds, the flame
retardancy mechanism is basically the same for all compounds. However, there are
differences in the flame retardancy performance of brominated compounds, as the
presence of such compounds in the polymer will influence the physical properties of
the polymer. In general, aliphatic bromine compounds are easier to break down and
hence more effective at lower temperatures, but are also less temperature-resistant
than aromatic retardants.
Aluminium hydroxide and other hydroxides act in a combination of various processes.
When heated, the hydroxides release water vapour, which cools the substrate to a
temperature below that required to sustain the combustion processes. The water vapour
liberated also has a diluting effect in the gas phase and forms an oxygen-displacing
protective layer. Additionally, together with the charring products, the oxide forms an
insulating protective layer.
Phosphorus compounds mainly influence the reactions taking place in the solid phase.
By thermal decomposition, flame retardants are converted to phosphorous acid, which
in the condensed phase extracts water from the pyrolysing substrate, causing it to char.
However, some phosphorus compounds may, similarly to halogens, act in the gas phase
as well by a radical trapping mechanism.
Interest in flame retarding polymers goes back to the 19th century with the discovery of
highly flammable cellulose nitrate and celluloid. In more recent times a large volume of
conventional plastics such as phenolics, rigid polyvinyl chloride (PVC) and melamine
resins possess adequate flame retardancy. By the 1970s the major flame retardant polymers
were the thermosets, namely, unsaturated polyesters and epoxy resins that utilised reactive
halogen compounds and alumina hydrate as an additive. There was also a large market
for phosphate esters in plasticised PVC, cellulose acetate film, unsaturated polyesters
and modified polyphenylene oxide. Alumina trihydrate (ATH) was the largest-volume
flame retardant in unsaturated plastics.
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Ecological Issues of Polymer Flame Retardancy
Consumption of halogen-containing flame retardant additives in the 1970s was much
less than that of other additives. The term ‘halogenated flame retardants’ covers a large
number of different organic substances, all with chlorine or bromine in their molecular
structure. Bromine and chlorine have an inhibitory effect on the formation of fire in
organic materials. Flame retardants are added to plastics and textiles in order to comply
with fire safety requirements. The halogenated flame retardant additives include:
(1) Dechlorane Plus,
(2) a chlorinated acyclic (for polyolefins),
(3) tris(dibromopropyl) phosphate,
(4) brominated aromatics,
(5) pentabromochlorocyclohexane and
(6) hexabromocyclododecane (for polystyrene).
A number of chlorinated flame retardant products were produced under the Dechlorane
trade name. The products include:
(1) two moles of hexachlorocyclopentadiene and contained 78% chlorine,
(2) Dechlorane Plus,
(3) a Diels-Alder reaction product of cyclooctadiene and hexachlorocyclopentadiene with
65% chlorine,
(4) a Diels-Alder product with furan and
(5) a product containing both bromine and chlorine with 77% halogen developed for
polystyrene and acrylonitrile-butadiene-styrene (ABS) materials [1].
In 1985-1986 a German study detected brominated dioxins and furans from pyrolysis of
a brominated diphenyl oxide in the laboratory at 510-630 °C [2]. The relevance of these
pyrolysis studies to the real hazard presented by these flame retardants under actual
conditions of use has been questioned. Germany and Holland have considered a ban or
curtailed the use of brominated diphenyl oxide flame retardants because of the potential
formation of highly toxic and potentially carcinogenic brominated furans and dioxins
during combustion [1, 2]. The issue has spread to other parts of Europe, where regulations
have been proposed to restrict their use.
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Handbook of Plastic Films
The chemical stability of the substances – particularly in the cases of polybrominated
biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) – is also the reason why
brominated flame retardants have been the focus of international environmental debate
for many years. PBDE and PBB, which are the most stable of the BFR described, are
widespread in the environment, are bioaccumulated and accumulate in sediments, where
they are degraded only very slowly.
6.3 Halogenated Diphenyl Ethers – Dioxins
Chlorinated dibenzo-p-dioxins and related compounds (commonly known simply as
dioxins) are contaminants present in a variety of environmental media. This class of
compounds has caused great concern to the general public as well as intense interest in
the scientific community. Laboratory studies suggest the probability that exposure to
dioxin-like compounds may be associated with other serious health effects, including
cancer. Conventional laboratory studies have provided new insights into the mechanisms
involved in the impact of dioxins on various cells and tissues and, ultimately, on toxicity
[1]. Dioxins have been demonstrated to be potent modulators of cellular growth and
differentiation, particularly in epithelial tissues. These data, together with the collective
body of information from animal and human studies, when coupled with assumptions
and inferences regarding extrapolation from experimental animals to humans, and from
high doses to low doses, allow a characterisation of dioxin hazards.
Polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF) and
polychlorinated biphenyls (PCB) are chemically classified as halogenated aromatic
hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic
aromatic compounds with similar physical and chemical properties, and the two classes are
structurally similar. Certain of the PCB (the so-called coplanar or mono-ortho coplanar
congeners) are also structurally and conformationally similar. The most widely studied of
these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, often
called simply dioxin, represents the reference compound for this class of compounds. The
structures of TCDD and several related compounds are shown in Figure 6.1 [3].
These compounds are assigned individual toxicity equivalence factor (TEF) values as
defined by the international convention ‘Interim Procedures for Estimating Risks
Associated with Exposures to Mixtures of Chlorinated Dibenzo-p-Dioxins and
Dibenzofurans’ (US Environmental Protection Agency, USEPA, March 1989). Results of
in vitro and in vivo laboratory studies have contributed to the assignment of a relative
toxicity value. TEF are estimates of the toxicity of dioxin-like compounds relative to the
toxicity of TCDD, which is assigned a TEF of 1.0. All chlorinated dibenzodioxins (CDD)
and chlorinated dibenzofurans (CDF) with chlorines substituted in the 2, 3, 7 and 8
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Ecological Issues of Polymer Flame Retardancy
2,3,7,8-Tetrachlorodibenzo-p-dioxin
1,2,3,7,8-Pentachlorodibenzo-p-dioxin
2,3,7,8-Tetrachlorodibenzofuran
2,3,4,7,8-Pentachlorodibenzofuran
3,3′,4,4′,5,5′-Hexachlorobiphenyl
3,3′,4,4′,5′-Pentachlorobiphenyl
Figure 6.1 The structures of dioxin and similar compounds
positions are assigned TEF values [1]. Additionally, the analogous brominated
dibenzodioxins (BDD) and brominated dibenzofurans (BDF) and certain polychlorinated
biphenyls have recently been identified as having dioxin-like toxicity and thus are also
included in the definition of dioxin-like compounds. Generally accepted TEF values for
chlorinated dibenzodioxins and dibenzofurans are shown in Table 6.1 [4].
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Handbook of Plastic Films
Table 6.1 Toxicity equivalence factors (TEF) for CDD and CDF [4]
Compound*
Toxicity equivalence factors, TEF
Mono-, di- and tri-CDD
0
2,3,7,8-TCDD
1
Other TCDD
0
2,3,7,8-PeCDD
0.5
Other PeCDD
0
2,3,7,8-HxCDD
0.1
Other HxCDD
0
2,3,7,8-HpCDD
0.01
Other HPCDD
0
Mono-, di-, and tri-CDF
0
2,3,7,8-TCDF
0.1
Other TCDF
0
1,2,3,7,8-PeCDF
0.05
2,3,4,7,8-PeCDF
0.5
Other PeCDF
0
2,3,7,8-HxCDF
0.1
Other HxCDF
0
2,3,7,8-HpCDF
0.01
Other HPCDF
0
OCDF
0.001
*CDD, chlorinated dibenzodioxin; CDF, chlorinated dibenzofuran.
Prefixes: tetra T, penta Pe, hexa Hx, hepta Hp, octa O.
A World Health Organization/International Program on Chemical Safety meeting held
in the Netherlands in December 1993 considered the need to derive internationally
acceptable interim TEF for the dioxin-like PCB. Recommendations arising from that
meeting of experts suggest that in general only a few of the dioxin-like PCB are likely to
be significant contributors to general population exposures to dioxin-like compounds
[5]. Dioxin-like PCB may be responsible for approximately one-quarter to one-half of
the total toxicity equivalence associated with general population environmental exposures
to this class of related compounds.
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Ecological Issues of Polymer Flame Retardancy
There are 75 individual compounds comprising the CDD, depending on the positioning of
the chlorine(s), and 135 different CDF. These are called individual congeners. Likewise, there
are 75 different positional congeners of the BDD and 135 different congeners of the BDF.
Only seven of the 75 congeners of the CDD or the BDD are thought to have dioxin-like
toxicity; these are ones with chlorine/bromine substitutions in, at least, the 2, 3, 7 and 8
positions. Only 10 of the 135 possible congeners of the CDF or the BDF are thought to have
dioxin-like toxicity; these also are ones with substitutions in the 2, 3, 7 and 8 positions.
While this suggests 34 individual CDD, CDF, BDD or BDF with dioxin-like toxicity, inclusion
of the mixed chloro/bromo congeners substantially increases the number of possible congeners
with dioxin-like activity. There are 209 PCB congeners. Only 13 of these 209 congeners are
thought to have dioxin-like toxicity; these are PCB with four or more chlorines with just one
or no substitution in the ortho position. These compounds are sometimes referred to as
coplanar, meaning that they can assume a flat configuration with rings in the same plane.
Similarly configured polybrominated biphenyls are likely to have similar properties;
however, the database on these compounds with regard to dioxin-like activity has been
less extensively evaluated. Mixed chlorinated and brominated congeners also exist,
increasing the number of compounds considered dioxin-like. The physical/chemical
properties of each congener vary according to the degree and position of chlorine and/or
bromine substitution. Very little is known about the occurrence and toxicity of the mixed
(chlorinated and brominated) dioxin, furan and biphenyl congeners.
In general, these compounds have very low water solubility, high octanol-water partition
coefficients and low vapour pressure, and they tend to bioaccumulate. Although these
compounds are released from a variety of sources, the congener profiles of CDD and
CDF found in sediments have been linked to combustion sources [1].
The Hazards Substance Ordinance in Germany specifies the maximum level of chlorinated
dibenzodioxins and furans that can be present in materials marketed in Germany. This
has been extended to the brominated compounds. The two largest-volume flame
retardants, decabromodiphenyl oxide and tetrabromo-bisphenol A, are said to meet these
requirements [2].
The International Program for Chemical Safety (IPCS) of the World Health Organization
has made several recommendations. Polybrominated diphenyls production (in France)
and use should be limited because of the concern over high persistency, bioaccumulation
and potential adverse effects at low levels. There are limited toxicity data on deca- and
octabromodiphenyls. Commercial use should cease unless safety is demonstrated. For
the polybrominated diphenyl oxides, a Task Group felt that polybrominated
dibenzofurans, and to a lesser extent the dioxins, may be formed. For decabromodiphenyl
oxide, appropriate industrial hygiene measures need to be taken, and environmental
exposure minimised by effluent and emission control. Controlled incineration procedures
should be instituted. For octabromodiphenyl oxide, the hexa- and lower isomers should
165
Handbook of Plastic Films
be minimised. There is considerable concern over persistence in the environment and
accumulation in organisms, especially for pentabromodiphenyl oxide.
There are no regulations proposed or in effect anywhere around the world banning the
use of brominated flame retardants. The proposed EU Directive on the brominated
diphenyl oxides has been withdrawn. Deca- and tetrabromo-bisphenol A as well as other
brominated flame retardants meet the requirements of the German Ordinance regulating
the dioxin and furan content of products sold in Germany [6].
The European search for a replacement for decabromodiphenyl oxide in high-impact
polystyrene (HIPS) has led to consideration of other bromo-aromatics, such as Saytex
8010 from Albemarle, and a heat-stable chlorinated paraffin from Atochem. The former
product is more costly, and the latter, if sufficiently heat-stable, lowers the heat distortion
under load (HDUL) significantly. Neither approach has been fully accepted. In September
1994, the USEPA released a final draft of exposure and risk assessment of dioxins and
dioxin-like compounds [5]. This reassessment finds the risks greater than previously
thought. Based on this reassessment, a picture emerges that tetrachlorodiphenyl dioxins
and related compounds are potent toxicants in animals, with the potential to produce a
spectrum of effects. Some of these effects may occur in humans at very low levels, and
some may result in adverse impacts on human health. The USEPA also concluded that
dioxin should remain classified as a probable human carcinogen [5].
Polymer producers have been seeking non-halogen flame retardants, and the search has been
successful in several polymer systems. Non-halogen flame retardant polycarbonate/ABS blends
are now commercial. They contain triphenyl phosfate or resorcinol diphosfate (RDP) as the
flame retardant. Modified polyphenylene oxide (GE’s Noryl) has used phosfate esters as the
flame retardant for the past 15-20 years, and the industry recently switched from the alkylated
triphenyl phosphate to RDP. Red phosphorus is used with glass-reinforced Polyamide-6,6
(PA-6,6) in Europe, and melamine cyanurate is used in unfilled PA. Magnesium hydroxide is
being used commercially in polyethylene wire and cable. The non-halogen solutions present
other problems, such as poor properties (plasticisers lower the heat distortion temperature),
difficult processing (high loadings of ATH and magnesium hydroxide), corrosion (red
phosphorus) and handling problems (red phosphorus).
In this chapter, we have tried to present the basic trends in the flame retardants hierarchy.
6.4 Flame Retardant Systems
The main flame retardant systems for polymers currently in use are based on halogenated,
phosphorus, nitrogen and inorganic compounds (Figure 6.2). Basically, all these flame
retardant systems inhibit or even suppress the combustion process by chemical or physical
166
Ecological Issues of Polymer Flame Retardancy
action in the gas or condensed phase. To be effective, the flame retardants must be stable
at processing temperatures yet decompose near the decomposition temperature of the
polymer in order for the appropriate chemistry to take place as the polymer decomposes.
Conventional flame retardants, such as halogenated, phosphorus or metallic additives,
have a number of negative attributes. The ecological issue of their application requires
that new polymer flame retardant systems are sought. Among the new trends in flame
retardancy, the use of intumescent systems, polymer nanocomposites, preceramic additives,
low-melting glasses, different types of char-formers and polymer morphology modification
should be noted [1]. However, the close interactions between the different flame retardant
types should be considered in order to achieve synergistic behaviour. A block scheme of
polymer flame retardant systems is given in Figure 6.2.
FLAME RETARDANTS (FR)
HALOGENATED FR
PHOSPHORUS FR
Nitrogen-containing FR
ANTIMONY OXIDE
Mg HYDROXIDE, ALUMINA
TRIHYDRATE, BORON FR
Ecologically friendly flame retardant systems
INTUMESCENT SYSTEMS
POLYMER NANOCOMPOSITES
POLYMER ORGANIC CHARH FORMERS
Preceramics, Low-melting Glass
POLYMER MORPHOLOGY MODIFICATION
Figure 6.2 A block diagram of polymer flame retardant systems
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Handbook of Plastic Films
Brief discussions of the three major types of flame retardant systems (intumescent systems,
polymer nanocomposites and polymer organic char-formers) are presented next.
6.5 Intumescent Additives
Intumescent behaviour, resulting from the combination of charring and foaming of the
surface of burning polymers, is being widely developed for fire retardancy because it is
characterised by a low environmental impact. Among alternative candidates,
considerable attention has been paid to intumescent materials because they provide
fire protection with the minimum of overall fire hazard [7]. Since the first intumescent
coating material was patented in 1938 [8], the mechanism of intumescent flame
retardancy has referred to the formation of a foam that acts as an insulating barrier
between the fire and the substrate. In particular, such intumescence depends significantly
on the ratio of carbon, nitrogen and phosphorus atoms in the compound [7, 9]. Although
intumescent coatings are capable of exhibiting good fire protection for the substrate,
they have several disadvantages, such as water solubility, brushing problems and
relatively high cost [10].
The fire retardation of plastic materials is generally achieved by incorporating fire retardant
additives into the plastic during processing [11, 12]. Since the processing requires that
additives can withstand temperatures up to about 200 °C or more, intumescent systems
with insufficient thermal stability cannot be incorporated into various plastics. The
phosphate-pentaerythritol system has been investigated and developed as an intumescent
material [7]. For example, a systematic study on a mixture of ammonium polyphosphate
and pentaerythritol has shown that intumescence occurs on flaming [13, 14]. Thus, new
intumescent materials with appropriate thermal stability have been synthesised for better
fire retardancy [15].
The most important inorganic nitrogen-phosphorus compound used as an intumescent
flame retardant is ammonium polyphosphate, which is applied in intumescent coatings
and in rigid polyurethane foams. The most important organic nitrogen compounds used
as flame retardants are melamine and its derivatives, which are added to intumescent
varnishes or paints. Melamine is incorporated into flexible polyurethane cellular plastics,
and melamine cyanurate is applied to unreinforced PA. Guanidine sulfamate is used as a
flame retardant for PVC wall coverings in Japan. Guanidine phosphate is added as a
flame retardant to textile fibres, and mixtures based on melamine phosphate are used as
flame retardants for polyolefins or glass-reinforced PA.
All the above-mentioned compounds – ammonium polyphosphate, melamine, guanidine
and their salts – are characterised by an apparently acceptable environmental impact.
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Ecological Issues of Polymer Flame Retardancy
Mechanistic studies in PA-6 with added ammonium polyphosphate (APP), ammonium
pentaborate (NH4B5O8; APB), melamine and its salts have been carried out using combustion
and thermal decomposition approaches [16, 17]. It was shown that APP interacts with PA6 to produce alkylpolyphosphoric ester, which is a precursor of the intumescent char. On
the surface of a burning polymer, APB forms an inorganic glassy layer that protects the
char from oxidation and hinders the diffusion of combustible gases. Melamine and its salts
induce scission of the H–C–C(O) bonds in PA-6, which leads to increased crosslinking and
charring of the polymer [17]. APP added at 10-30 wt% to PA-6 is ineffective in the low
molecular weight (low molar mass) polymer since the limiting oxygen index (LOI) remains
at the level of 23-24 [18] corresponding to non-fire-retarded PA-6. However, APP becomes
very effective at loadings of 40 and 50%, where the LOI increases to 41 and 50, respectively.
A condensed-phase fire retardant mechanism is proposed for APP in PA-6 [18]. In fact,
an intumescent layer is formed on the surface of burning PA-6/APP formulations, which
tends to increase the content of APP.
Thermal analysis has shown that APP destabilises PA-6, since thermal decomposition is
observed at a temperature 70 °C lower than that of pure PA-6 [18]. However, the
intumescent layer effectively protects the underlying polymer from the heat flux. Therefore,
in the conditions of the linear pyrolysis experiments, the formulation PA-6/APP (40%)
decomposes more slowly than pure polymer [18]. These experiments prove the fire
retardant action of the intumescent char. Mechanistic studies of thermal decomposition
in the PA-6/APP system show that APP catalyses the degradation of the polymer and
interacts with it, forming essentially 5-amidopentyl polyphosphate (Scheme 6.1).
On further heating, 5-amidopentyl polyphosphate again liberates polyphosphoric acid
and produces the char. The intumescent shielding layer on the surface of the polymer is
composed of foamed polyphosphoric acid, which is reinforced with the char [18].
Scheme 6.1 Reaction of APP with PA-6
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Handbook of Plastic Films
The effectiveness of APB in high molecular weight PA-6 (Mn = 35,000) is similar to that of
APP as measured by oxygen index [19]. In contrast to APP, APB does not give an intumescent
layer. Instead, a brown-black glassy-like compact layer is formed. As thermal analysis has
shown, APB destabilises PA-6 since the latter decomposes at 50 °C lower. It is likely that
freed boric acid catalyses the thermolysis of the Nylon. In contrast to APP, no other chemical
interaction of PA-6 and APB was found. In fact, the residue obtained in thermogravimetry
in a nitrogen atmosphere for PA-6/APB formulations corresponds to that calculated on the
basis of the individual contributions of PA-6 and APB to the residue [19]. It is likely that a
molten glassy layer of boric acid/boric anhydride accumulates on the surface of burning
polymer, which protects the char from oxidation. This layer reinforced by the char creates
a barrier against diffusion of the volatile fuel from the polymer to the flame, which decreases
the combustibility of PA-6 [19].
A systematic mechanistic study of halogen-free fire retardant PA-6, via the combustion
performance and thermal decomposition behaviour of non-reinforced PA-6 with added
melamine, melamine cyanurate, melamine oxalate, melamine phthalate, melamine
pyrophosphate or dimelamine phosphate, has been reported [20]. Melamine, melamine
cyanurate, melamine oxalate and melamine phthalate promote melt dripping of PA-6,
which increases as the additive concentration increases. These formulations self-extinguish
very quickly in air, and their LOI increase with increasing concentration (Table 6.2) [20].
The melt dripping effect is very strong in the case of melamine phthalate, where a small
amount of the additive (3-10%) leads to large increases in LOI (from 34 to 53).
The combustion behaviour of melamine pyrophosphate and dimelamine phosphate is
different from that of melamine itself and the other melamine salts (Table 6.2). The former
are ineffective at concentrations below 15% and become effective at a loading of 20-30%
because an intumescent char is formed on the surface of burning specimens. The mechanism
Table 6.2 Oxygen indices for high molecular weight PA-6 with added melamine
or its salts (for pure PA-6, LOI = 24) [20]
Additive
Additive concentration (wt%)
3
5
10
15
20
30
Melamine
-
29
31
33
38
39
Dimelamine phosphate
-
23
24
25
26
30
Melamine pyrophosphate
-
24
25
25
30
32
Melamine oxalate
-
28
29
-
33
-
Melamine cyanurate
-
35
37
39
40
40
Melamine phthalate
34
48
53
-
-
-
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Ecological Issues of Polymer Flame Retardancy
of the fire retardant action of both melamine pyrophosphate and dimelamine phosphate is
similar to that of APP, since, analogously with ammonia, melamine volatilises, whereas the
remaining phosphoric acids produce esters with PA-6, which are precursors of the char
[17]. Some part of the freed melamine condenses, probably forming the derivatives melem
and melon [21]. Melamine partially evaporates from the composition PA-6/melamine (30%),
whereas the other part condenses, giving 8% solid residue at 450 °C. However, similar
behaviour with a more thermostable residue is shown by melamine cyanurate. Melamine
pyrophosphate, like dimelamine phosphate [17], gives about 15% of thermostable char.
As mentioned before, it is likely that a glassy layer of molten boric acid and boric anhydride
accumulates on the surface of the burning polymer and protects the char from oxidation.
The glass reinforced by the char creates a barrier against diffusion of the volatile fuel from
the polymer to the flame, which decreases the combustibility of PA-6 [19].
As infrared characterisation of solid residue and high-boiling products has shown [17],
carbodiimide functionalities are formed on thermal decomposition of PA-6 with melamine
and its salts. An unusual mechanism of chain scission of PA-6 through CH2–C(O) bonds
[22] is likely to become operative in the presence of melamines (Scheme 6.2). The resultant
Scheme 6.2 Mechanism of thermal decomposition of PA-6 in the presence of
melamine [22]
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Handbook of Plastic Films
isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to Nalkylisocyanurate. Carbodiimide can also trimerise to N-alkylisotriazine. These
secondary reactions increase the thermal stability of the solid residue and increase the
yield of the char.
In order to understand better the chemical reactions that are responsible for the
intumescent behaviour of APP/pentaerythritol (PER) mixtures, as model examples, a
study of the thermal degradation of pentaerythritol diphosphate (PEDP) was undertaken
[23]. PEDP is a model compound for structures identified in APP/PER mixtures heated
below 250 °C. Five major degradation steps between room temperature and 950 °C
have been identified using thermogravimetric analysis (TGA), and volatile products
are evolved in each step. The formation of the foam reaches a maximum at 325 °C,
corresponding to the second step of degradation; foam formation decreases at higher
temperatures. There are no differences in the TGA or differential scanning calorimetry
(DSC) curves in nitrogen or air up to 500 °C. Above this temperature, thermal oxidation
leads to almost complete volatilisation in a single step, which is essentially completed
at 750 °C. The elucidation of the chemical reactions that occur upon degradation is
easier if each step is studied separately. The separation of the steps is accomplished by
heating to a temperature at which one step goes to completion, and the following
reaction occurs at a negligible rate [23]. The chemical reactions that occur in the first
two steps lead to the initial formation of a char-like structure, which will undergo
subsequent graphitisation.
The first reaction is the elimination of water, with the condensation of OH groups.
This overlaps with the elimination of organics when as little as 28% of the possible
water has been evolved. This involves essentially complete scission of the phosphate
ester bonds and results in a mixture of polyphosphates and a carbonaceous char. Three
mechanisms have been proposed in the literature for this reaction [24, 25]: a freeradical mechanism, a carbonium ion mechanism, and a cyclic cis-elimination mechanism.
The free-radical mechanism has been ruled out because of the lack of effect of freeradical inhibitors on the rate of pyrolysis [25]. The carbonium ion mechanism is
supported by acid catalysis and kinetic behaviour, and may compete with the ciselimination mechanism [24, 25].
The carbonium ion mechanism should occur exclusively if there is no hydrogen atom
on the β-carbon atom, as in PEDP, which is necessary for the cyclic transition state of
the elimination mechanism. The olefin is generated from the thermodynamically most
stable carbonium ion. Hydride migration or skeletal rearrangement may take place to
give a more stable carbonium ion of high reactivity. After ring opening in the ionic
ester pyrolysis mechanism, a second ester pyrolysis reaction occurs, which could also
take place by the cis-elimination mechanism, as shown in Scheme 6.3.
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Ecological Issues of Polymer Flame Retardancy
Scheme 6.3 Ester pyrolysis mechanism [24]
The formation of char can occur by either free-radical or acid-catalysed polymerisation
reactions from the compounds produced in the pyrolysis. For example, the Diels-Alder
reaction followed by ester pyrolysis and sigmatropic (1,5) shifts leads to an aromatised
structure as shown in Scheme 6.4 [24].
Repetition of these steps can eventually build up the carbonaceous char, which is observed.
The reaction pattern shown in Schemes 6.4 and 6.5 should help to provide the structures
observed by spectroscopy in the foamed char [24]. These reactions probably occur in an
irregular sequence and in competition with other processes; the final products are obtained
by some random combination of polymerisation, Diels-Alder condensation, aromatisation,
etc. Ester pyrolysis supplies the chemical structures, which build up the charred material
through relatively simple reactions [24].
In summary, intumescent behaviour resulting from a combination of charring and foaming
of the surface of burning polymers is being widely developed for fire retardancy because
it is characterised by a low environmental impact. However, the fire retardant effectiveness
of intumescent systems is difficult to predict because the relationship between the
occurrence of the intumescence process and the fire protecting properties of the resulting
foamed char is not yet understood.
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Handbook of Plastic Films
Scheme 6.4 Free-radical char formation [24]
174
Ecological Issues of Polymer Flame Retardancy
Scheme 6.5 Acid-catalysed char formation [24]
6.6 Polymer Organic Char-Former
There is a strong correlation between char yield and fire resistance. This follows because
char is formed at the expense of combustible gases and because the presence of a char
inhibits further flame spread by acting as a thermal barrier around the unburned material.
Polymeric additives – poly(vinyl alcohol) (PVOH), systems – that promote the formation
of char in the PVOH/PA-6,6 system have been studied [26]. These polymeric additives
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Handbook of Plastic Films
usually produce a highly conjugated system – aromatic structures that char during thermal
degradation and/or transform into crosslinking agents at high temperatures:
(
CH
CH2)n
OH
CH
CH2
(
CH
CH2)n
CH
CH2
+ H2O
(6.1)
OH
OH
Scission of several carbon-carbon bonds leads to the formation of carbonyl end-groups.
For example, aldehyde end-groups arise from the following reaction:
CH
CH2
OH
CH
CH2
n
CH
CH2
OH
OH
(6.2)
CH
CH2
CH
CH2
n
OH
CH + CH3
CH
O
OH
The identification of a low concentration of benzene among the volatile products of
PVOH has been taken to indicate the onset of a crosslinking reaction proceeding by a
Diels-Alder addition mechanism [27]. Clearly, benzenoid structures are ultimately formed
in the solid residue, and the IR spectrum of the residue also indicated the development of
aromatic structures:
CH2
CH
(a)
CH CH
O
CH
+ CH2
CH
CH
CH
OH
CH2
CH
OH
OH
CH2
CH
CH
CH
CH
CH2
(6.3)
CH
CH
(b)
CH
C
O
176
C
CH2
CH
OH
Ecological Issues of Polymer Flame Retardancy
Acid-catalysed dehydration promotes the formation of conjugated sequences of double
bonds (a), and Diels-Alder addition of conjugated and isolated double bonds in different
chains may result in intermolecular crosslinking, producing structures that form graphite
or carbonisation (b).
In contrast to PVOH, PA-6,6 subjected to temperatures above 300 °C in an inert
atmosphere is completely decomposed. The wide range of degradation products, which
include several simple hydrocarbons, cyclopentanone, water, CO, CO2 and NH3, suggest
that the degradation mechanism is highly complex. Further research has led to the generally
accepted degradation mechanism for aliphatic polyamides [28]:
O
C
O
(CH2)x
C
O
NH
(CH2)y
NH
H2O
C
n
O
(CH2)x
C
OH + NH2
(CH2)y
NH
n
(a)
O
C
O
(6.4)
(CH2)x + C + *NH (*CH2)y + *NH
n
Hydrocarbons,
cyclic ketones, esters,
nitriles, carbon char
O
C
(*CH2)x + CO2 + NH3 + *(CH2)y + *NH
n
The idea of introducing PVOH into PA-6,6 was based on the possibility of hightemperature acid-catalysed dehydration [29]. This reaction can be provided by the acid
products of PA-6,6 degradation hydrolysis, which would promote the formation of
intermolecular crosslinking and char. Such a system has been called ‘synergetic
carbonisation’ because the char yield and flame suppression parameters of the polymer
blend of PVOH and PA-6,6 show significant improvement in comparison with those of
pure PVOH and PA-6,6 separately [30].
An additional improvement to the flame resistance properties of the PVOH/PA-6,6 system
was suggested by means of substitution of pure PVOH by PVOH-ox [poly(vinyl alcohol)
oxidised with potassium permanganate (KMnO4)] [30]. Earlier it was reported that the
oxidation of PVOH in alkaline solutions occurs through the formation of two intermediate
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Handbook of Plastic Films
complexes. The final step of this process was attributed to the formation of polyvinylketone
as a final product of oxidation of the substrate [31]. The fire retardancy approach was
made on the basis of the fire behaviour of PVOH-ox samples. Using cone calorimeter
tests, a dramatic decrease in the rate of heat release and a significant increase in the
ignition time were shown experimentally for the oxidised PVOH in comparison with the
original PVOH (see Table 6.3). One reason for this phenomenon may be the ability of
PVOH oxidised by KMnO4 (polyvinylketone structures) to act as a neutral and/or
monobasic bidentate ligand [32]. Other experimental results (IR and electronic spectra)
provide strong evidence of coordination of the ligand (some metal ions Cd2+, Co2+, Cu2+,
Hg2+, Ni2+) through the monobasic bidentate mode [33]. Based on the above, the following
structure can be proposed for the polymeric complexes (where M = metal):
H
C
C
C
O
O
M
O
O
C
Polymer complex scheme 6A
C
C
H
n
Table 6.3 Cone calorimeter data for PA-6,6/PVOH [30]
Material
PVOH
PVOH-ox*
Heat flux
(kW/m2)
Char yield
(wt%)
Ignition
time (s)
Peak RHR
(kW/m2)
Total heat
release
(MJ/m2)
20
8.8
39
255.5
159.6
35
3. 9
52
540.3
111.3
50
2. 4
41
777.9
115.7
20
30.8
1127
127.6
36.9
35
12.7
774
194.0
103.4
50
9. 1
18
305.3
119.8
*Poly(vinyl alcohol) oxidised with potassium permanganate (KMnO4).
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Ecological Issues of Polymer Flame Retardancy
Cone calorimeter combustion tests for PVOH and PVOH oxidised by KMnO4 (Table 6.3)
clearly indicate the substantial improvement of fire resistance characteristics for PVOH-ox
in comparison with PVOH. PVOH-ox gives about half the peak rate of heat release (peak
RHR, kW/m2), when compared with pure PVOH. Even at 50 kW/m2, the yield of char
residue for PVOH oxidised by KMnO4 was 9.1% [30].
The result of elemental analysis of PVOH-ox indicates the presence of 1.5% of manganese
remaining in this polymeric structure [30]. It has been suggested that the catalytic amount
of chelated manganese structure incorporated in the polymer can provide a rapid hightemperature process of carbonisation followed by formation of char [30].
The sample of PVOH-ox displayed even better flame retardant properties due to the catalytic
effect of the manganese-chelate fragments on the formation of char (Table 6.3). However,
there is a less satisfactory correlation in the determination of total rate of heat release
(Table 6.3) [30]. Although, the cone calorimeter measurements indicated a decrease of
total heat release for PA-6,6/PVOH and PA-6,6/PVOH-ox in comparison with pure PVOH,
the sample of PA-6,6 with PVOH-ox showed a higher value of total heat release than PA6,6 with PVOH (Table 6.3). This fact has been qualitatively explained by the influence of
a catalytic amount of chelated manganese structure incorporated in the polymer on the
smouldering of the polymer samples.
The flame out time for PA-6,6/PVOH-ox is larger than the flame out times of PA-6,6/PVOH
and PA-6,6 alone (Table 6.4). The values of average heat of combustion indicate the exothermal
process of smouldering provided by chelated manganese structures (Table 6.4). Approximately
equal amounts of char yield for PA-6,6/PVOH and PA-6,6/PVOH-ox have been found [30].
Table 6.4 Cone calorimeter data for the heat of combustion and the flame out
time for PA-6,6 compositions at a heat flux of 50 kW/m2
Flame out time (s)
Average heat of
combustion (MJ/kg)
PA-6,6
512
31.5
PA-6,6/PVOH (80/20, wt%)
429
25.1
PA-6,6/PVOH-ox (80/20, wt%)
747
29.5
Composition
The polymer organic char-former (PVOH system) incorporated in PA-6,6 reduced the peak
rate of heat release from 1124.6 kW/m2 (for PA-6,6) and 777.9 kW/m2 (for PVOH) to
476.7 kW/m2 and increased the char yield from 1.4% (for PA-6,6) to 8.7% due to a ‘synergistic’
carbonisation effect. The cone calorimeter was operated at 50 kW/m2 incident flux.
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Handbook of Plastic Films
Cone calorimeter data of PA-6,6 composition with PVOH oxidised by KMnO4 (manganese
chelate complexes) show an improvement in the peak rate of heat release from 476.7
kW/m2 (for PA-6,6/PVOH, 80/20 wt%) to 305.3 kW/m2 (for PA-6,6/PVOH-ox, 80/20
wt%) [30]. On the other hand, the exothermal process of smouldering for PA-6,6/PVOHox compositions has been noted [30]. This reaction is evidently provided by chelated
manganese structures, which increases the total heat release of PA-6,6/PVOH-ox blend
in comparison with PA-6,6/PVOH blend.
6.7 Polymer Nanocomposites
Polymer layered silicate (clay) nanocomposites are materials with unique properties when
compared with conventional filled polymers. Polymer nanocomposites, especially polymerlayered silicates, represent a radical alternative to conventionally filled polymers.
Solventless, melt intercalation of high molecular weight polymers is a new approach to
synthesise polymer-layered silicate nanocomposites. This method is quite general and is
broadly applicable to a range of commodity polymers from nonpolar polystyrene to
strongly polar Nylon. Polymer nanocomposites are thus processable using current
technologies and easily scaled to manufacturing quantities. In general, two types of
structures are possible: (1) intercalated and (2) disordered or delaminated with random
orientation throughout the polymer matrix. Owing to their nanometre size dispersion,
the nanocomposites exhibit improved properties compared to the pure polymers or
conventional composites. The improved properties include increased modulus, decreased
gas permeability, increased solvent resistance and decreased flammability. For example,
the mechanical properties of a PA-6 layered-silicate nanocomposite with a silicate mass
fraction of only 5% show excellent improvement over those for pure PA-6 [34]. The
nanocomposite exhibits 40% higher tensile strength, 68% greater tensile modulus, 60%
higher flexural strength and 126% increased flexural modulus [34].
In the polymer industry there is a need for new, more effective and environmentally
friendly flame resistant polymers. Recent data on the combustion of polymer
nanocomposites indicate that they could be employed for this purpose [35].
There are several proposed mechanisms as to how the layered silicate affects the flame
retardant properties of polymers [35]. The first is increased char layer that forms when
nanocomposites are exposed to flame. This layer is thought to inhibit oxygen transport
to the flame front, as well as gaseous-fuel transport from the polymer, and therefore
reduces the heat release rate of the burning surface. At higher temperatures, the inorganic
additive has the ability to act as a radical scavenger due to adsorption on to Lewis acid
sites. This may interrupt the burning cycle, as radical species are needed to break polymer
chains into fuel fragments. The disordered nanocomposites also inhibit the availability
of oxygen as a combustible ‘fuel’ species by increasing the path length of these species to
180
Ecological Issues of Polymer Flame Retardancy
the flame front. The path length is dramatically increased due to the surface area of the
silicates (approximately 700 m2/g for Na+ montmorillonite). There is also a high possibility
of alumina-silicate solid-phase catalysis of polymer decomposition, which can dramatically
change the overall scheme of the kinetics of the thermal degradation process.
Combustibility of some polymer nanocomposite materials was studied using a cone
calorimeter [36, 37] under irradiation of 35 kW/m2, which is equivalent to that typical of
a small fire [38]. The RHR, which is one of the most important parameters associated with
the flammability and combustion of a material, such as those illustrated in Figure 6.3, can
be evaluated during this test [36, 37].
Figures 6.3-6.5 compare the results obtained for PA-6,6 as such and for intercalated PA6,6 hybrid produced by using a Carver press to mix PA-6,6 with 5 wt% of Cloisite 15A
(montmorillonite modified by ion exchange with dimethyl-ditallow ammonium, a
tetraalkylammonium salt from Southern Clay Products Inc.), in an inert nitrogen
atmosphere at 260 °C for 30 minutes. It can be seen that the RHR displays a lower
maximum peak in the case of the nanocomposite (Figure 6.3), whereas the quantity of
heat released (the area under the RHR curve) is about the same for both products,
suggesting that their thermal degradation mechanisms are the same [37]. The release of
heat by the nanocomposite over a longer period, however, points to its slower degradation.
Figures 6.4 and 6.5 on mass loss and specific extinction area illustrate the advantages of
nanocomposite over initial PA-6,6 fire behaviour.
Figure 6.3 Rate of heat release versus time for PA-6,6 and PA-6,6 nanocomposite at a
heat flux of 35 kW/m2
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Handbook of Plastic Films
Figure 6.4 Mass loss rate versus time for PA-6,6 and PA-6,6 nanocomposite at a heat
flux of 35 kW/m2
Figure 6.5 Specific extinction area (smoke) versus time for PA-6,6 and PA-6,6
nanocomposite at a heat flux of 35 kW/m2
182
Ecological Issues of Polymer Flame Retardancy
During the combustion test of the nanocomposite specimen, the carbon layer that formed
on its surface from the start grew over time and resisted the heat. The formation of a
carbonised layer on the surface of the polymer is a feature of all the nanocomposites
studied so far: the pattern illustrated in Figure 6.6 has been reported for other
nanocomposites based on polystyrene, polyethylene and polypropylene [37]. Examinations
of this residue by X-ray diffraction and transmission electron microscopy (TEM) have
revealed an intercalated nanocomposite structure [37]. The TEM image [37] of the carbon
residue obtained by combustion of a PA-6,6 nanocomposite in Figure 6.6 shows the
intercalation of silicate layers (dark zones) with ‘carbon’ layers (light zones). It should be
emphasised that this intercalated structure was derived from the combustion of a
delaminated hybrid. It is clear that the disordered structure collapsed during the
combustion and was replaced by a self-assembled, ordered structure.
Figure 6.6 TEM image of carbon residue obtained by combustion of PA-6,6
nanocomposite [37]
(Reproduced with permission from J.W. Gilman, T. Kashiwagi, C.L. Jackson,
E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire
Retardancy of Polymers: the Use of Intumescence, Eds., M. Le Bras, G. Camino,
S. Bourbigot and R. Delobel, RSC, Cambridge, UK, 1998. Copyright 1998, RSC.)
References
1.
S.M. Lomakin and G.E. Zaikov, Ecological Aspects of Flame Retardancy, VSP
International Science Publishers, Utrecht, The Netherlands, 1999, 170.
2.
H. Beck, A. Dross, M. Ende, R. Wolf and P. Trubiroha, Bundesgesundheitsblatt,
1991, 34, 564.
183
Handbook of Plastic Films
3.
R.M.C. Theelen in Biological Basis for Risk Assessment of Dioxin and Related
Compounds, Eds., M. Gallo, R. Scheuplein and K. Van der Heijden, Banbury
Report No. 35, Cold Spring Harbor Laboratory Press, Plainview, NY, USA, 1991.
4.
U.G. Ahlborg, G.C. Becking, L.S. Birnbaum, A. Brouwer, H.J.G.M. Derks, M.
Feeley, G. Golor, A. Hanberg, L.C. Larsen, A.K.D. Liam, S.H. Safe, C. Schlatter,
F. Waern, M. Younes and E. Yrjanheikki, Chemosphere, 1994, 28, 6, 1049.
5.
Office of Health and Environmental Assessment Office of Research and
Development, Estimating Exposure to Dioxin-Like Compounds, EPA/600/6-88/
005Ca, Cb, Cc, USEPA, Cinncinnati, OH, USA, 1994.
6.
J. Green, Journal of Fire Sciences, 1996, 14, 426.
7.
C.E. Anderson Jr., J. Dziuk Jr., W.A. Mallow and J. Buckmaster, Journal of Fire
Sciences, 1985, 3, 151.
8.
H. Tramm, C. Clar, P. Kuhnel and W. Schuff, inventors; Ruhrchemie AG,
assignee, US Patent 2,106,938, 1938.
9.
M. Kay, A.F. Price and I. Lavery, Journal of Fire Retardant Chemistry, 1979, 6, 69.
10. D.E. Cagliostro, S.R. Riccitiello, K.J. Clark and A.B. Shimizu, Journal of Fire and
Flammability, 1975, 6, 205.
11. R. Delobel, M. Le Bras, N. Ouassou and F. Alistiqsa, Journal of Fire Sciences,
1990, 8, 85.
12. G. Camino, L. Costa and L. Trossarelli, Polymer Degradation and Stability,
1984, 7, 25.
13. G. Camino, G. Martinasso, L. Costa and R. Gobetto, Polymer Degradation and
Stability, 1990, 28, 17.
14. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability,
1992, 36, 31.
15. H. Heinrich, inventor; Chemie Linz (Deutschland) GmbH, assignee, German
Patent, DE 4,015,490Al, 1991.
16. S.V. Levchik, G. Camino, L. Costa and G.F. Levchik, Fire and Materials, 1995,
19, 1.
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Ecological Issues of Polymer Flame Retardancy
17. S.V. Levchik, G.F. Levchik, A.I. Balabanovich, G. Camino and L. Costa, Polymer
Degradation and Stability, 1996, 54, 217.
18. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability,
1992, 36, 229.
19. S.V. Levchik, G.F. Levchik, A.F. Selevich and A.I. Leshnikovich, Vesti Akademii
Nauk Belarusi, Seryya Khimichnykh, 1995, 3, 34.
20. S.V. Levchik, G.F. Levchik, G. Camino and L. Costa, Journal of Fire Sciences,
1995, 13, 43.
21. L. Costa, G. Camino and M.P. Luda di Cortemiglia in Fire and Polymers:
Hazards Identification and Prevention, ACS Symposium Series No.425, Ed., G.L.
Nelson, American Chemical Society, Washington, DC, USA, 1990, 211.
22. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992,
43, 43.
23. G. Camino, G. Martinasso and L. Costa, Polymer Degradation and Stability,
1990, 27, 285.
24. G. Camino and S. Lomakin in Fire Retardant Materials, Eds., A.R. Horrocks and
D. Price, CRC Press, Boca Raton, FL, 2001, USA.
25. P. Haake and C.E. Diebert, Journal of the American Chemical Society, 1971, 93,
6931.
26. Y. Tsuchiya and K. Sumi, Journal of Polymer Science, 1969, A17, 3151.
27. Polyvinyl Alcohol. Properties and Applications, Ed., C.A. Finch, John Wiley,
London, UK, 1973, 622.
28. B.G. Achhammer, F.W. Reinhard and G.M. Kline, Journal of Applied Chemistry,
1951, 1, 301.
29. S.M. Lomakin and G.E. Zaikov, Khimicheskaia Fizika, 1995, 14, 39.
30. G.E. Zaikov and S.M. Lomakin, Plasticheskie Massy, 1996, 39, 211.
31. R.M. Hassan, Polymer International, 1993, 30, 5.
32. R.M. Hassan, S.A. El-Gaiar and A.M. El-Summan, Polymer International, 1993,
32, 39.
185
Handbook of Plastic Films
33. R.M. Hassan, M.A. El-Gahami and M.A. Abd-Alla, Journal of Materials
Chemistry, 1992, 2, 613.
34. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and
O. Kamigaito, Journal of Materials Research, 1993, 8, 1185.
35. E.P. Giannelis, Advanced Materials, 1996, 8, 1, 29.
36. J.W. Gilman, T. Kashiwagi, M. Nyden, J.E.T. Brown, C.L. Jackson, S. Lomakin,
E.P. Giannelis and E. Manias in Chemistry and Technology of Polymer Additives,
Eds., S. Al-Malaika, A. Golovoy and C.A. Wilkie, Blackwell Science, Oxford,
UK, 1999, 249-265.
37. J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin,
J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of
186
7
Interaction of Polymers with the Nitrogen
Oxides in Polluted Atmospheres
G.B. Pariiskii, I.S. Gaponova and E.Y. Davydov
7.1 Introduction
In this chapter, the mechanisms of the reactions of nitrogen oxides with solid polymers
are considered. Active participants in reactions with nitrogen oxides are double bonds,
the amide groups of macromolecules, alkyl, alkoxy and peroxy radicals, as well as
hydroperoxides. The structure of the reaction front during nitration of rubbers has been
studied using the electron spin resonance (ESR) imaging technique. The reactions with
nitrogen oxides provide a simple way of preparing spin-labelled polymers. The structuralphysical effects on the kinetics and mechanism of reactions of nitrogen dioxide have
been demonstrated by the example of filled polyvinylpyrrolidone (PVP).
Thermal and photochemical oxidation of polymers have been the subject of detailed and
prolonged investigations, because these processes are of major importance for the
stabilisation of polymeric materials. However, since the 1960s, the influence of aggressive
gases in polluted atmospheres on polymer stability has attracted considerable attention
[1]. Among such pollutants in the atmosphere, sulfur dioxide, ozone and the nitrogen
oxides stand out as the most deleterious. However, the pursuance of this research has
run into a number of problems. The interaction of pollutants with polymers involves the
penetration of gases into solids and thus results in a complex kinetic description of the
process. Also, as a rule, these reactions are long term for the concentrations of pollutants
found in the environment. Consequently, other aging processes occur in the actual
conditions of use and storage of polymer materials.
To establish the effect of a given aggressive gas on a particular polymer, the reaction is
generally studied at pollutant concentrations that are much higher than those actually
existing in polluted atmospheres. The results obtained by this means are then linearly
extrapolated to the concentrations of reactants found in the atmosphere. This expedient
is, a priori, ambiguous in view of the fact that the role of the individual stages of a
uniform aging process is changed in conditions of accelerated testing.
The problem of non-equivalent kinetics is inherent to polymer reactions in solids [2]. In
this case particles existing in different surroundings react with different rate constants.
As a result, the most active particles will be removed from the reaction, and the overall
rate constant will decrease with time. On the other hand, relaxation processes in polymers
restore the initial distribution of particles and so their reactivity. Thus the kinetics will
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Handbook of Plastic Films
depend on the relation between the rate of the chemical reaction and the rate of the
relaxation processes [3]. This fact also makes it necessary to reconsider critically the
validity of extending the results of accelerated tests for polymer ageing.
This chapter is devoted to a consideration of the results obtained in studies of the
interactions of nitrogen oxides with polymers. There are eight nitrogen oxides, but only
NO, NO2 and N2O4 are actually important as pollutants. Nitric oxide (NO) exists as a
free radical, but it is reasonably stable in reactions with organic compounds. The
paramagnetic nitrogen dioxide (NO2) is more active compared with NO. This gas is
universally present in equilibrium with its dimer molecule:
2NO2
N2O4
with Kp = 0.141 atm at 298 K [4]. Nitrogen dioxide absorbs light in the near-UV and
visible spectral range. Excited molecules are generated by light with λ > 400 nm. The
dissociation of NO2 into an oxygen atom and NO by light with λ < 365 nm takes place
with a quantum yield near to unity [5].
7.2 Interaction of Nitrogen Dioxide with Polymers
Detailed investigations of the reactions of NO2 with various polymers have been carried
out by Jellinek and co-workers [1, 6]. The degradation of polymer films has been studied
at different pressures of NO2, in mixtures of NO2 with air, under the combined action of
light (λ > 280 nm), O2 and NO2. Based on the data obtained, Jellinek classified all polymers
into three groups:
(1) vinyl polymers – polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethyl
methacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl chloride (PVC) and
polyvinyl fluoride (PVF);
(2) polymers with non-saturation – primarily rubbers;
(3) polyamides, polyurethanes and polyamidoimides.
The presentation of the results in this section will be carried out according to this classification.
7.2.1 Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF
The linear extrapolation of the results of accelerated tests to NO2 concentrations likely
to be found in the atmosphere (1-5 ppm) predicts that polymer properties will be essentially
constant for a long time.
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Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
The first investigations of the interaction of NO2 with PE and PP were performed by
Ogihara and co-workers [7, 8] at 298-383 K and NO2 pressure of 20 kPa. It was
established that NO2 reacts at room temperature with the >C=C< double bonds
originally contained in PE with the formation of dinitro compounds and nitronitrites
by the following reactions:
>C=C< + NO2 → >C•−C(NO2)<
(= R1•)
(7.1)
R1• + NO2 → >C(NO2)−C(NO2)<
(7.2)
R1• + ONO → >C(ONO)−C(NO2)<
(7.3)
Hydrogen atom abstraction does not take place at room temperature. The nitro, nitrite,
nitrate, carbonyl and hydroxyl groups are formed at T > 373 K. The following mechanism
was postulated:
RH + NO2 → R• + HNO2
(7.4)
R• + NO2 → RNO2
(7.5)
R• + ONO → RONO
(7.6)
RONO → RO• + NO
(7.7)
The reactions of RO• radicals lead to the formation of macromolecular nitrates, alcohols
and carbonyl compounds. The activation energy of the NO2 addition to the double bonds
of PE is 8-16 kJ/mol. The activation energies of H atom abstraction are 56-68 kJ/mol in
PE and 60 kJ/mol in PP.
PE, PP, PAN and PMMA change their characteristics slightly at high concentrations
of NO2 (1.3-13 kPa) even under the joint action of pollutant, O2 and UV light [6].
Nitrogen dioxide is capable of abstracting tertiary hydrogen atoms in PS with a low
rate (P = 20-80 kPa), with the formation of nitro and nitrite side groups [reactions
(7.5) and (7.6)]. This process is accompanied by main-chain scission [9, 10]. The
combined action of 0.3 kPa NO2 and light (λ > 280 nm) on PS does not lead to mainchain decomposition in the early stage (10 h), after which the degradation process is
developed with a constant rate. PVC and PVF show a minor loss of chlorine and
fluorine atoms on exposure to NO2 [1, 6].
An attempt to investigate quantitatively the ageing of PS and poly-tert-butyl methacrylate
(P-t-BuMA) has been taken by Huber [11]. The research was performed in a flow system
of air containing 60-900 ppm of NO2 and/or 60-900 ppm SO2 at 300 K under the
simultaneous action of light with λ > 290 nm. The degradation of P-t-BuMA films was
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Handbook of Plastic Films
expressed in terms of the quantity of ruptures per 10,000 monomer units, α. The kinetic
dependence is represented by the equation:
α = (P/Q)[exp(Qt) – 1]
(7.8)
where P and Q are constants. This equation describes an autoaccelerated process.
As Q → 0, so α → Pt, that is, the degradation proceeds with a constant rate. The P and
Q values decrease as the film thickness increases, and yet the P value diminishes more
strongly than Q. Therefore, the accelerated character of the degradation appears more
clearly for thin films. PS degradation in the same conditions proceeds much more slowly
and has a more pronounced autoacceleration (Table 7.1).
Table 7.1 The P and Q values for P-t-BuMA and PS film degradation under the
action of 100 ppm NO2 and light in air
Film thickness (mg/cm2)
P × 104 (h–1)
Q × 104 (h–1)
P-t-BuMA
1.4
0.071
0.026
P-t-BuMA
2.6
0.050
-
P-t-BuMA
2.8
0.041
0.017
PS
1.4
0.034
0.036
Polymer type
The autoaccelerated character of P-t-BuMA degradation was linked to the ester group
decomposition, with isobutylene formation, which gives free radicals in the reaction
with NO2 and thus promotes the degradation process.
The IR spectrum of PS shows peaks corresponding to carbonyl (1686 cm–1) and hydroxyl
(3400 cm–1) groups after exposure to a mixture of NO2 (100 ppm) and air. No bands
connected with the insertion of NO2 into the P-t-BuMA and PS macromolecules were
observed. It is believed that the following sequence of reactions occurs in PS [11]:
RH + NO2 → R• + HNO2
190
(7.9)
R• + O2 → RO2•
(7.10)
RO2• + RH → ROOH + R•
(7.11)
R• + NO2 → RNO2
(7.12)
R• + NO2 → RONO
(7.13)
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
RONO → RO• + NO
(7.14)
ROOH + NO → RO• + •OH + NO
(7.15)
ROOH + hν → RO• + •OH
(7.16)
RO• → degradation + R•
(7.17)
Hydroperoxide decomposition under the action of NO and light gives rise to accelerated
PS degradation.
7.2.2 Non-Saturated Polymers
Among these are primarily rubbers. These polymers are far more sensitive to NO2 action
that the polyolefins. Appreciable degradation of macromolecules as well as moderate
crosslinking were observed for rubbers.
Comprehensive kinetic investigations of butyl rubber (a copolymer of isobutylene with
1.75% isoprene) in an NO2 atmosphere (NO2 pressure 1.33-133 kPa), in a mixture of
NO2 and air, and under the combined action of NO2, O2 and UV light (λ > 280 nm) have
been performed by Jellinek and co-workers [12, 13]. According to the proposed
mechanism, the total number of chain ruptures is made up of three parts: (1) ruptures
that are due to the NO2 interaction only, (2) ruptures that result only from the action of
O2, and (3) ruptures that are caused by the combined action of NO2 and O2.
The kinetic dependence of the degree of degradation, α = (1/DPt − 1/DP0), is described by
the following equation:
α = kef′ t2 + kef″ [NO2][1 − exp(−k3t)]
(7.18)
where DP0 and DPt are the number-average degrees of polymerisation in the original and
degraded macromolecule (at time t). The first term in equation (7.18) is connected with
ruptures of macromolecules due to photolysis of the reaction products (hydroperoxides,
nitro and nitrite groups). The second term describes the degradation for the (NO2 + O2)
system in the absence of light.
It should be noted that the assumed mechanism [12, 13] is very complex, involving a
wealth of elementary reactions, the rate constants of which are unknown in the solid
phase. It is well known that the reaction products can be more active relative to the
nitrogen oxides than the original polymer. In connection with this, the application of
various physical-chemical techniques is extremely important to investigate the
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Handbook of Plastic Films
degradation process. The development of methods to study the movement of the reaction
front across the polymer sample is also required. The use of the ESR technique permits
one to draw additional conclusions on the mechanisms of the interaction of polymers
with nitrogen oxides from the structure of the resulting free radicals and the kinetics of
their formation.
The interaction of polyisoprene (PI) with NO2 gives rise to di-tert-alkylnitroxyl radicals
[14]. The ESR spectra of these radicals show a characteristic anisotropic triplet signal with
a width of 2A||N = 6.2 mT and g|| = 2.0028 ± 0.0005 in the solid polymer, and a triplet with
aN = 1.53 ± 0.03 mT and g = 2.0057 ± 0.0005 in dilute solutions. These macroradicals are
stable in the absence of NO2 during storage for many months in both inert atmosphere and
air. The proposed scheme to explain the formation of these radicals involves three main
stages: (1) generation of N-containing alkyl radicals, (2) synthesis of tertiary macromolecular
nitroso compounds, and (3) spin-trapping of the tertiary alkyl or allyl radicals:
~CH2-C(CH3)=CH-CH2~ + NO2
~C•(CH3)-CH(ONO)-CH2~ + RH
~C•(CH3)-CH(ONO)-CH2~
~C(CH3)(NO)-CH(OH)-CH2~
~C(CH3)(NO)−CH(OH)−CH2∼ + •Rtert → Rtert−N(O•)−Rtert
(7.19)
(7.20)
(7.21)
The reactions of NO2 with double bonds provide a very simple and rapid method for the
synthesis of spin-labelled macromolecules of rubbers. The temperature variation of the
rotational mobility of macromolecules in block PI has been studied using spin-labelled
samples [14]. The temperature dependence of the rotational correlation time τ is described
by τc = τ0 exp(E/RT). The τc values within the fast motion region (τc < 10–9 s) are well
described by the parameters E = 34.7 kJ/mol and log τ0 = −14.2.
The spatial distribution of these macromolecular nitroxyl radicals allows the
estimation of the spatial distribution of the nitration reaction in bulk PI. The
possibilities of the ESR imaging technique to determine the form of the reaction
front of PI nitration has been considered [15]. The ESR imaging spectra were registered
in an inhomogeneous magnetic field on cylindrical samples of 0.4 cm diameter and 1
cm height at NO2 and O2 concentrations of 1 x 10–4 to 2 x10–3 mol/l and 2 x 10–3 to
1.4 x 10–2 mol/l, respectively. The spatial distributions of R2NO• radicals at various
reaction times are shown in Figure 7.1. The width of the distribution varies over 2030% for 740 h. The maximum concentration of nitroxyl radicals is observed in the
superficial layer, and it progressively decreases towards the centre. The width of this
layer is ~1 mm, and radicals are unavailable in the sample centre. The nitroxyl radical
yield with respect to absorbed NO2 molecules is 0.01. The shape and variation of the
distribution in the presence of O2 are the same as in pure NO2, but the reaction front
is narrower. The rate of R2NO• formation in the presence of O2 is much lower than
192
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
Figure 7.1 ESR-imagination of nitroxyl radicals distribution in cylindrical sample
(l = 10 mm, d = 4 mm) of PI in course of interaction with nitrogen dioxide
([NO2]) = 8.8 × 10-4 mole/l; 30 min; 20 °C). The contour lines correspond to vertical
sections with equal [R2NO*]. The concentrations are given in arbitrary units
([R2NO*]max = 0.125 au).
in pure NO2 at the cost of a decay of alkyl radicals in the reactions with O2: W(NO2)/
W(NO2+O2) = 102. The distribution at a fixed distance from the surface is likely
determined by macrodefects in the sample volume, namely, the availability of cracks
and porosity. The front form is determined by the ‘membranous’ regime of the nitration
process rather than by structural changes.
PMMA, which in itself is stable on exposure to NO2, enters into reactions after previous
irradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation of
double bonds as a result of ester group decomposition. The ESR spectrum observed after
exposure of samples to NO2 is shown in Figure 7.2. The spectrum represents the
superposition of the signals of two nitroxyl radicals at low frequencies of rotational
mobility (10–9 s < τc < 10–7 s):
•
Dialkylnitroxyl radicals
~C(CH3)(COOCH3)–N(O•)–C(CH3)(CHO)–CH2~
give an anisotropic triplet signal with hyperfine interaction (HFI) constant A||N = 3.2
± 0.1 mT and g|| = 2.0026 ± 0.0005;
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Handbook of Plastic Films
Figure 7.2 ESR spectrum of nitroxyl radicals generated by NO2 in PMMA
pre-irradiated by UV light at 298 K.
•
Acylalkylnitroxyl radicals
~C(CH3)(COOCH3)−CH(OH)–C(CH3)[N(O•)COOCH3]–CH2~
give a triplet signal that is characterised by A||N = 2.1 ± 0.1 mT and g|| = 2.0027
± 0.0005.
The free-radical process of NO2 interaction with PMMA containing double bonds is
represented by the scheme opposite.
The formation of nitroxyl radicals testifies to the fact that main-chain decomposition by
reaction (7.24) and side-chain ester group cleavage by reaction (7.26) take place in the
polymer. Thus, the interaction of NO2 with double bonds is able to initiate free-radical
reactions of polymer degradation when hydrogen atom abstraction reactions from C–H
bonds are inefficient.
194
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
(7.22)
(7.23)
(7.24)
(7.25)
(7.26)
(7.27)
195
Handbook of Plastic Films
7.2.3 Polyamides, Polyurethanes, Polyamidoimides
Polymers with amide and urethane groups in the macromolecules represent a special
class of materials that are sensitive to NO2. The action of NO2 at pressures of 0.5-2
mm Hg on polyamide-6,6 films with various morphologies has been studied by Jellinek
and co-workers [17, 18]. It was shown that a degradation process takes place. The
degradation of polyamide is a diffusion-controlled reaction and depends on the degree
of crystallinity and the sizes of the crystallites. The process is inhibited by small quantities
of benzaldehyde or benzoic acid. Increase of the degradation rate was observed during
the combined action of NO2, air and UV light. The assumed mechanism of the process
as follows:
~CO–NH~ + NO2 → ~CO–N•~ + HNO2
(7.28)
~CO–N•~ + NO2 → ~CO–N(NO2)~
(7.29)
~CO–N•–CH2~
[~CO–N=CH2 + •CH2~] → chain rupture
(7.30)
There is reason to believe that only a small quantity of amide groups, not linked by the
hydrogen bonds, enter into the reaction. These groups can be interlocked by benzoic
acid with the formation of the following structure:
HO
O
••••••
NH
••••••
CO
C
Ph
Research into the effect of NO2 on polyamide textiles has been described [19]. The exposure
of samples in an NO2 atmosphere of low concentration at room temperature for 100 h
does not lead to a decrease in the whiteness and tensile strength. However, these
characteristics are decreased at higher temperatures. The availability of nitrogen oxides in
the air under the action of UV light results in the additional degradation of textiles.
The conversion of N−H bonds by nitrogen dioxide is also inherent to polycaproamide
(PCA). The UV spectra of PCA films display features of absorption at 390-435 nm during
exposure to NO2 at concentrations of 10–4 to 10–3 mol/l [20]. The absorption bands
were assigned to nitrosamide groups resulting from N−H group conversion. This
196
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
conclusion is confirmed by IR spectroscopy. The intensity of the band with ν = 3293 cm–1,
which is associated with stretching vibrations of the hydrogen-linked N−H groups,
decreases sharply. The intensities of the amide I (ν = 1642 cm–1) and amide II (ν = 1563
cm–1) bands, which are characteristic of PCA, also decrease. Instead of these bands,
absorptions at ν = 1730 cm–1, which corresponds to the absorption of C=O groups, and
at ν = 1504 and 1387 cm–1, which correspond to stretching vibrations of N=O groups of
nitrosamides, appear in PCA. Thus, nitrosation through the amide group is the main
process of PCA transformation in an NO2 atmosphere, which leads to disintegration of
the system of hydrogen bonds. Taking into account the equilibrium:
NO+NO3−
N2 O 4
(7.31)
2NO2
the formation of nitrosamides can be represented as follows:
∼CONHCH2∼ + N2O4 → ∼CON(NO)CH2∼ + HNO3
(7.32)
It was found that the initial rate of nitrosamide group accumulation is proportional to
[NO2]n, where n ≈ 2.
As was shown by ESR, the reaction of NO 2 with N−H bonds also produces
acylalkylnitroxyl macroradicals:
∼CONHCH2∼ + NO2
~CON(ONO)CH2~
ΝΟ2
→ ∼CON•CH2∼ → ~CON(ONO)CH2~
(7.33)
~CON(O•)CH2~ + NO
(7.34)
–HNO2
As well as in PCA, the interaction of NO2 with PVP leads to UV bands characteristic of
the nitrosamide group [20]. The formation of these groups in PVP is associated with
splitting of the side-chain cyclic fragments from the main chain:
PVP + NO2
–HNO2
(R1)
CH2 CHCH2
N
O
(7.35)
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Handbook of Plastic Films
Types of Hot Runner Systems
N
R1
CH2CHCH2
O
+
(7.36)
Thereafter, the reaction of NO 2 with the cyclic double bond gives rise to the
nitrosamide product:
NO2
N
O
O2 N
N
O
R1
+ 2NO2
NO
O NOR
O2N
N
O2 N
O
OR
N
O
(7.37)
+
The ESR spectra observed when NO2 (10–4 to 10–3 mol/l) reacts with PVP represent the
superposition of the signals of acylalkylnitroxyl radicals (A||N = 1.94 mT, g|| = 2.003) and
iminoxyl radicals (A||N = 4.33 mT, A⊥N = 2.44 mT, g|| = 2.0029, g⊥ = 2.0053). The formation
of these iminoxyl radicals is initiated by the hydrogen atom abstraction reaction from
C−H bonds that are in the α-position with respect to the amide group by reaction (7.35)
and the following reaction:
PVP + NO2
–HNO2
(R2)
CH2 CHCH2
N
O
(7.38)
Nitric acid is thought to be the source of nitrogen oxide in the given system:
2HNO2 → H2O + NO2 + NO
198
(7.39)
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
The recombination of NO and R1• initiates the formation of iminoxyl radicals:
R1 + NO
CH2 CHCH2
N
O
–HNO2
CH2 CHCH2
CH2 CHCH2
(7.40)
+NO2
N
N
O
O
ON
The formation of NO explains the production of acylalkylnitroxyl radicals as follows:
R2
CH2 C CH2
N
CH2 C CH2
O
N
NO
O
+ R1 (R2)
CH2 C CH2
(7.41)
R
N
N O
O
An approach based on the analysis of the composition of nitrogen-containing radicals in
PVP depending on the content of filler aerosil has been put forward to elucidate the
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Handbook of Plastic Films
effect of polymer structural-physical organisation [21]. The influence of structural
organisation may be manifest in the rates of iminoxyl and acylalkylnitroxyl radical
formation. Filling gives the possibility of changing the physical structure of the polymer
in interface layers. The decrease in the molecular packing density as a result of filling can
accelerate the rate of reaction (7.41) involving breakage of the pyrrolidone cycle. The
packing density decrease enhances the reaction rate through the promotion of mutual
diffusion of R• macroradicals and nitroso compounds. It is well established that the
quantitative relation between iminoxyl and acylalkylnitroxyl radicals is changed with
the degree of filling.
Formation of a gel fraction has been detected on exposure of polyurethane films to NO2
[21]. Degradation of macromolecules simultaneously takes place in the sol fraction of
the samples. The changes in the destruction degree and the gel-fraction yield with time
are complex to analyse. The gel fraction at 333 K and P(NO2) = 20 mm Hg initially
increases up to 20% and thereafter reduces to nearly zero. The number of scissions in
the sol fraction increases at the beginning, subsequently reduces, and then grows again.
The exposure of films to NO2 is accompanied by the release of CO2 at all temperatures.
The IR spectra in this case show N−H bond (3300 cm–1) consumption. The proposed
mechanism includes the reaction of NO2 with the N−H groups of both the main chain
and the side branches:
~OCO–NH–CH2~ + NO2 → ~OCO–N•–CH2~ + HNO2
(7.42)
~OCO–N(RH)–CH2~ + NO2 → ~OCO–N(R•)–CH2~ + HNO2
(7.43)
The recombination of ~OCO–N•–CH2~ (R1•) and ~OCO–N(R•)–CH2~ (R2•) results in
polymer crosslinking. The conversion of R1• causes macromolecule decomposition and
CO2 release. The exposure of polyurethane films to an NO2 atmosphere or a mixture of
NO2 with air leads to the progressive reduction of the tensile strength limit [22].
The influence of NO2 on the mechanical properties of polyamidoimide films has been
considered at 323 K and P(NO2) = 13 kPa [23]. The temperature dependences of the
storage modulus E′ and loss modulus E″ have been obtained for various times of NO2
exposure. A nonmonotonic decrease of E′ was observed at 473 K, but the maximum of
the E′ temperature dependence appears at approximately the same temperature. Samples
exposed to NO2 for eight days show an increase in E′ at the glass transition temperature
(563 K). The phenomenon is associated with chain breakage and the recombination of
macroradicals giving rise to crosslinking. Chain breakage is supported by results obtained
by the present authors. The ESR spectra of polyamidoimide exposed to an NO2
atmosphere show the formation of iminoxyl radicals with spectral parameters that are
close to those of PVP iminoxyl radicals. The possible mechanism of their formation
includes the main-chain decomposition step as follows:
200
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
NH
O
CO
CO
NO2
N
–HNO2
CO
O
N
CO
CO
N
CO
(7.44)
R + O
N
CO
CO
N
CO
O
N
CO
NO2
O
O
N
CO
N
CO
ON
NO
O
NO
N
CO
RH
O
O
HO
O
N
HO
(7.45)
NO
NOH
CO
NO2
–HNO2
O
N
CO
HO
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Handbook of Plastic Films
7.3 Reaction of Nitric Oxide with Polymers
Nitric oxide is a low-activity free radical and can be used as a ‘counter’ of radicals in gas
and liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitroso
compounds, which are spin traps. Thus, the initiation of free-radical reactions in solid
polymers in the presence of nitric oxide provides further information on their mechanism.
It is well established that at room temperature NO is not able to remove allylic and tertiary
hydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinions
on the capability of NO to react with low molecular weight (low molar mass) dienes and
polyenes. Some authors believe that NO is able to add to dienes and polyenes, for example,
to substituted o-quinonedimethane, phorone and β-carotene, with the formation of free
radicals [27-29]. Another way of looking at these reactions lies in the fact that they can be
initiated by NO2 impurities [25, 26].
This section of the review is concerned with radical reactions in polymers, induced by
photo- and γ-irradiation, in the presence of nitric oxide. Irradiation of powdered PMMA
in an NO atmosphere by the light of a mercury lamp results in the formation of three typesof macromolecular nitroxyl radicals [30]. The radical composition depends on temperature
and the wavelength of the light. If the photolysis of PMMA is performed at room temperature
using unfiltered light from a high-pressure mercury lamp, acylalkylnitroxyl radicals
R1N(O•)C(=O)R2 are formed. The irradiation of samples at 383 K produces, in addition to
acylalkylnitroxyl radicals, dialkylnitroxyl macroradicals R1N(O•)R2. Finally, if PMMA
irradiation is carried out at room temperature using UV light with 260 nm < λ < 400 nm,
the signal of iminoxyl radicals R1C(=NO•)R2 is also observed in the ESR spectrum.
Acetyl cellulose (AC) under action of light at room temperature gives rise to dialkyl- and
acylalkylnitroxyl radicals [30]. The removal of NO from the samples leads to increasing of
components of acylalkylnitroxyl radicals in the ESR spectrum. This phenomenon is probably
connected with the formation of diamagnetic complexes of NO with acylalkylnitroxyl
radicals. Dialkylnitroxyl radicals do not form complexes of this type at 298 K.
The γ-irradiation of PMMA at room temperature, as a photolysis, brings about the formation
of acylalkylnitroxyl radicals [30]. Iminoxyl radicals also arise, but their quantity is essentially
smaller than in AC under γ-irradiation.
The formation of nitroxyl radicals during photolysis as well as in the course of radiolysis
of PMMA and AC in the presence of NO is explained by the following scheme:
polymer
202
γ, hν
→
R1• (R•2)
(7.46)
R1• + NO → R1NO
(7.47)
R2• + R1NO → R1N(•O)R2
(7.48)
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
The structure of nitroxyl radicals is determined by the nature of the free radicals that are
generated by γ- and photo-irradiation of PMMA and AC. Photo-irradiation of PMMA
and AC leads to the formation of •C(O)OCH3 radicals, which give in turn acylalkylnitroxyl
radicals by reactions (7.46)-(7.48). Dialkylnitroxyl radicals arise when two macroradicals
are involved in the reactions with NO.
The free-radical reactions in solid polymers in the presence of NO are of particular
significance for the preparation of spin-labelled polymers. This method has become
particularly important for chemically inert, rigid and insoluble polymers, for instance,
polytetrafluoroethylene (PTFE), because of the difficult problem of introducing spin labels
by chemical reactions of nitroso compounds, nitrons or nitroxyl biradicals [31]. Oriented
PTFE films γ-irradiated at room temperature in air after prolonged NO exposure contain
nitroxyl radicals whose ESR spectra are displayed in Figure 7.3 [32].
Figure 7.3 ESR spectra of perfluoronitroxyl radicals in PTFE films stretched to
fourfold increase in its length at parallel (a) and perpendicular (b) orientation of
magnetic field directions.
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Handbook of Plastic Films
The rotation of the samples leads to changes in angle α between the magnetic field and
stretching directions. At 298 K and α = 0°, the ESR spectrum is a triplet consisting of
quintets with splitting of AN = 0.46 mT and AF = 1.11 mT, and g|| = 2.0060. At α = 90°,
the splittings increase to AN = 1.12 mT and AF = 1.61 mT, and g⊥ = 2.0071. The
radicals observed are nitroxyl radicals with the following structure: ~CF2–N(O•)–CF2~.
A possible mechanism for nitroxyl macroradical synthesis has been suggested [32]. In
an oxygen-containing atmosphere, some of the middle alkyl radicals formed in the
course of γ-irradiation are capable of decomposing with rupture of the main chain as a
result of the high energy transfer to these radicals:
~CF2–CF2–C•F–CF2~ → ~CF2• + CF2=CF–CF2~
(7.49)
In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form
terminal peroxy radicals:
~C•F2 + O2 → ~CF2OO•
(7.50)
Under the action of NO on samples containing neighbouring terminal double bonds and
peroxy radicals, the latter are converted into macromolecular nitrates and nitrites:
~CF2
CF2OO• + NO
~CF2OONO …
(7.51)
~CF2ONO2
~CF2O• + NO2
(7.52)
~CF2O• + NO2
~CF2O• + NO
~CF2ONO
(7.53)
Decomposition of alkoxy radicals in an NO atmosphere causes the synthesis of terminal
nitroso compounds:
NO
~CF2–CF20• → ~C•F2 + CF2O → ~CF2NO
(7.54)
The adjacent terminal double bonds and terminal nitroso compounds formed can enter
into a reaction to synthesise nitroxyl radicals:
~CF2N=O + CF2=CF–CF2~ + NO → ~CF2–N(O•)CF2–CF(NO)–CF2~
204
(7.55)
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
The advantage of the suggested method for the preparation of spin-labelled polymer is
that the nitroxyl free-radical fragment is incorporated in the basic macromolecular chain
without disturbing its orientation.
An analogous investigation of the action of NO on γ-irradiated tetrafluoroethylenehexafluoropropylene copolymer (TFE-HFP) containing 13 mol% of HFP units has been
performed [33]. After exposure of powders and films of TFE-HFP to a dose of 105 Gy in
air, there are three types of stable peroxide macroradicals:
(1) End radicals ~CF2–CF2O2• (denoted ReO2•);
(2) Secondary mid-chain radicals ~CF2–CF(OO•)–CF2~ (denoted RcO2•);
(3) Tertiary mid-chain radicals ~CF2–C(CF3)(OO•)–CF2~ (denoted RtO2•).
Their total concentration is [RO2•] ≈ 3 x 10–3 mol/kg, of which (25 ± 5)% are tertiary
peroxy radicals. Under the action of NO on evacuated samples, the radicals decay to
form peroxy radical conversion products and tertiary nitroso compounds:
~CF2–C(CF3)(NO)–CF2~
Heating these samples in vacuum up to 473 K leads to the formation of nitroxyl radicals
of the type:
~CF2–N(O•)–CF2~
The nitroxyl radicals appear in the temperature range where the tertiary nitroso
compounds decay in vacuum with the generation of tertiary alkyl radicals (Rt•). The first
step of Rt• formation is β-scission by the reaction:
~CF2−C•(CF3)−CF2−CF2~ → ~CF2−C(CF3)=CF2 + •CF2−CF2~
(7.56)
In the presence of NO formed upon decomposition of the tertiary nitroso compounds,
the terminal alkyl radicals can be converted into terminal nitroso compounds, which
react with the adjacent double bonds to form nitroxyl macroradicals:
NO + •CF2−CF2~ → ON−CF2−CF2~
(7.57)
~CF2–C(CF3)=CF2 + ON−CF2−CF2~ → ~CF2–C•(CF3)–CF2–N(O•)–CF2~ →
+X
→ ~CF2–C(CF3)(X)–CF2–N(O•)–CF2~
(7.58)
where X is NO or NO2. Nitrogen dioxide can be formed by the interaction of NO with
RO2• in reaction (7.51).
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Handbook of Plastic Films
One more type of nitroxyl macroradical is observed if a powdered TFE-HFP, γ-irradiated
in air and exposed to NO with subsequent evacuation, is subjected to light irradiation at
λ > 260 nm at 298 K [34]. In this case, a new type of nitroxyl macroradical with the
structure ∼CF2–N(O•)–CF3 was registered. The following scheme provides an explanation
for the radical formation in TFE-HFP under the action of light:
RcO2• + NO → [RcOONO] → RcONO2
(7.59)
ReO2• + NO → [ReOONO] → ReONO2
(7.60)
Rt• + NO → RtNO
(7.61)
RcONO2 + hν → RcO• + NO2
(7.62)
ReONO2 + hν → ReO• + NO2
(7.63)
RtNO + hν → Rt• + NO
(7.64)
RcO• → ~CF2–CFO + •CF2–CF2~
(7.65)
ReO• → ~CF2–CF2• + CF2O
(7.66)
~CF2–CF2• + NO → ~CF2–CF2–NO
(7.67)
~CF2−C•(CF3)−CF2~ + NO2 → ~CF2–C(CF3)(ONO)–CF2~
(7.68)
~CF2–C(CF3)(ONO)–CF2~ + hν → ~CF2–C(CF3)(O•)–CF2~ + NO
(7.69)
~CF2–C(CF3)(O•)–CF2~ → •CF3 + ~CF2–C(=O)–CF2~
(7.70)
•
CF3 + NO → CF3NO
(7.71)
•
CF3 + ON–CF2–CF2~ → CF3–N(O•)–CF2–CF2~
(7.72)
CF3NO + •CF2–CF2~ → CF3–N(O•)–CF2–CF2~
(7.73)
It is obvious that the simultaneous action of light and NO on TFE-HFP results in
macromolecular decomposition.
Polymer hydroperoxides are active participants in degradation processes. The reactions
of nitrogen oxides with these particles are of interest to understand the mechanism of the
influence of pollutants on polymer stability in the course of the oxidation process. The
phenomenon of hydroperoxide decomposition under the action of NO was discussed
206
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
long ago using both macromolecular peroxides and their low molecular weight analogues
[34]. Some authors assumed that the primary stage of peroxide decomposition can be
represented by the reaction [35]:
ROOH + NO → RO• + HONO
(7.74)
Another mechanism [36] suggests that the reaction proceeds with the formation of
peroxide radicals:
ROOH + NO → ROO• + HNO
(7.75)
The kinetics of hydroperoxide decomposition in PP at 298 K and various partial pressures
of NO has been studied in detail [34]. The decomposition kinetics are shown in Figure 7.4.
Figure 7.4 Kinetics of PP hydroperoxide decomposition in NO at various
concentrations (1-3) and NO + NO2 mixture (4): (1) 1.61 × 10-3, (2) 3.22 × 10-3,
(3) 4.13 × 10-3, (4) 3.1 × 10-3 NO and 3.0 × 10-6 NO2 mol/l.
As can be seen, the hydroperoxide consumption rate is initially low and then sharply
increases. The observed character of the kinetic curves cannot be explained by reactions
(7.74) or (7.75). According to the ESR data, the decomposition of PP hydroperoxide in an
207
Handbook of Plastic Films
NO atmosphere gives dialkylnitroxyl radicals. It was shown that the induction periods for
the hydroperoxide decomposition and nitroxyl radical accumulation are very sensitive to
the presence of trace amounts of higher nitrogen oxides. This leads to the conclusion that
the interaction of hydroperoxide with NO is more likely to proceed according to the scheme:
ROOH + N2O3 → [ROONO] + HNO2
ROONO
(7.76)
RONO2
(7.77)
RO• + NO2
(7.78)
RO• + NO2
Alkoxy radicals may decompose or enter into substitution reactions with macromolecules
to form chain Rc• and end Re• alkyl macroradicals, and low molecular weight alkyl
radicals r•, which with NO give nitroso compounds:
RO• → Rc• (Re•, r•)
(7.79)
Rc• (Re•, r•) + NO → RcNO (ReNO, rNO)
(7.80)
The increase in the rate of hydroperoxide decomposition with time can be related to
reactions proceeding with participation of such nitroso compounds:
r′OOH + r″NO → r′O• + r′–N(O•)–OH
(7.81)
r″–N(O•)–OH → r″• + HNO2
(7.82)
The alkyl radicals formed in the system may stimulate hydroperoxide decomposition [37]:
r• (Rc•, Re•) + ROOH → rH (RcH, ReH) + RO2•
RO2• + NO
(7.83)
RO• + NO2
(7.84)
RONO2
(7.85)
ROONO
Another process that can increase the hydroperoxide decomposition rate is the disproportionation
of NO to N2 and NO3• with the participation of nitroso compounds [24]:
208
Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
RNO• + 2NO
R–N=N–ONO2
RONO2 + N2
(7.86)
R• + N2 + NO3
(7.87)
R• + N2 + •ONO2
NO + NO3• → 2NO2
Reactions (7.83)-(7.88) may lead to an increasing NO2 concentration in the system and,
consequently, result in the acceleration of reaction (7.76).
7.4 Conclusion
Nitrogen oxides are capable of influencing the free-radical stages of polymeric material
aging in polluted atmospheres. Nitric oxide is a comparatively low-activity free radical,
and it cannot abstract even labile hydrogen atoms at ordinary temperatures to initiate
the radical degradation process. On the other hand, NO effectively recombines with
free radicals. This reaction is apparently controlled in solid polymers by the gas diffusion
rate, and NO is capable of terminating the oxidation chain by reaction with peroxy
and alkyl macroradicals. The reaction of NO with alkyl radicals gives nitroso
compounds, which are spin traps accepting free radicals. This process can slow down
polymer degradation in the presence of nitrogen oxides in subsequent conversions,
which can break down into alkoxy radicals, effecting the degradation of
macromolecules. In addition, nitric oxide initiates the decomposition of hydroperoxides
resulting from oxidation of polymers.
Nitrogen dioxide is a more active free radical as compared with NO, and is able to
break off the labile hydrogen atoms at room temperature as well as to add to the C=C
bonds of macromolecules, inducing free-radical degradation of polymers. At the same
time, the NO2 radical can inhibit the free-radical reactions giving nitrogen-containing
molecules by the reactions with alkyl, alkoxy and peroxy radicals. The thermal and
photochemical conversions of these products also affect the aging process of polymeric
materials. Nitrogen dioxide is an initiator of the free-radical degradation of polyolefins
at elevated temperatures.
The low stability of polyamides to the action of NO2 is quite surprising, because the
N–H bond of the amide group is rather strong. Therefore, the mechanism of polyamide
degradation connected with hydrogen atom abstraction by NO2 from N–H bonds is
not fully elucidated.
209
Handbook of Plastic Films
References
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N.M. Emanuel and A.L. Buchachenko, Chemical Physics of Polymer
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O.N. Karpukhin, Usppekhi Khimii, 1978, 47, 6, 1119.
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Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres
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Polymer Chemistry, 1972, 10, 6, 1773.
18. H.H.G. Jellinek, R. Yokota and Y. Itoh, Polymer Journal, 1973, 4, 6, 601.
19. H. Herzlinger, B. Kuster and H. Essig, Textile Praxis International, 1989, 44, 6,
574, 655, 661.
20. I.S. Gaponova, E.Y. Davydov, G.G. Makarov, G.B. Pariiskii and V.P. Pustoshnyi,
Polymer Science, Series A, 1998, 40, 4, 309.
21. H.H.G. Jellinek and T.J.Y. Wang, Journal of Polymer Science, Polymer Chemistry
Edition, 1973, 11, 12, 3227.
22. H.H.G. Jellinek, F. Martin and J. Wegener, Journal of Applied Polymer Science,
1974, 18, 6, 1773.
23. H. Kambe and R. Yokota, Proceedings of the 2nd International Symposium
on Degradation and Stabilisation of Polymers, Dubrovnik, Yugoslavia, 1978,
Paper No.39.
24. J.F. Brown, Jr., Journal of the American Chemical Society, 1957, 79, 10, 2480.
25. A. Rockenbauer and L. Korecz, Chemical Communications, 1994, 145.
26. J.S.B. Park and J.C. Walton, Perkin Transactions 2, 1997, 12, 2579.
27. H.-G. Korth, R. Sustmann, P. Lommes, T. Paul, A. Ernst, H. de Groot, L. Hughes
and K.U. Ingold, Journal of the American Chemical Society, 1994, 116, 7, 2767.
28. I. Gabr and M.C.R. Symons, Faraday Transactions, 1996, 92, 10, 1767.
29. I. Gabr, R.P. Patel, M.C.R. Symons and M.T. Wilson, Chemical Communications,
1995, 9, 915.
30. I.S. Gaponova, G.B. Pariiskii and D.Ya. Toptygin, Vysokomolekulyarnye
Soedineniya, Seriya A, 1988, 30, 2, 262.
31. A.M. Wasserman and A.L. Kovarskii, Spinovye Metki i Zondy v Fizikokhimii
Polimerov (Spin Labels and Probes in Physical Chemistry of Polymers), Nauka,
Moscow, Russia, 1986.
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32. I.S. Gaponova and G.B. Pariiskii, Chemical Physics Reports, 1997, 16, 10, 1795
33. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series B, 1998, 40, 11-12, 394.
34. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series A, 1995, 37, 11, 1133.
35. J.R. Shelton and R.F. Kopczewski, Journal of Organic Chemistry, 1967, 32, 9, 2908.
36. D.J. Carlsson, R. Brousseau, C. Zhang and D.M. Wiles, Polymer Degradation
and Stability, 1987, 17, 4, 303.
37. K. Ingold and B. Roberts, Free-Radical Substitution Reactions, John Wiley, New
York, NY, USA, 1972.
212
8
Modifications of Plastic Films
E.M. Abdel-Bary
8.1 Introduction
Modifications of plastic films are generally used to improve mechanical or physical
properties so that the films are suitable for certain applications. This can be achieved by
subjecting the films to mechanical or chemical treatments. Thus, surface treatments modify
the crystalline morphology and surface topography, increase the surface energy and remove
contaminants. Removal of contaminants is necessary for good adhesion of the surface to
other substrates. Other applications, such as printing, decorating, wetting and lamination,
are improved by incorporation of a surfactant to change the surface tension of the
adherents. Also, the presence of polar nitrogen-containing monomers on a polymer film
surface allows one to obtain ionomers for versatile applications. Thus, such films can be
used as anion-exchange membranes in electrodialysis processes, in water desalination
[1], as a carrier for immobilisation of medical products [2], as a separator in alkaline
batteries [3] and in fuel cells, etc.
A number of surface modification techniques, such as plasma, corona discharge and
chemical treatments, have been used to modify polymer surfaces, and the chemical methods
are of particular interest. In this case, adsorption on and adhesion to polymer surfaces
have been modified using many different methods, e.g., oxidation and other chemical
reactions, high-energy irradiation and plasma treatment. In the following sections, we
shall discuss some of the parameters affecting the mechanical and/or physical or
physicochemical characteristics of such films.
8.2 Modification of Mechanical Properties
Improved mechanical properties of plastic films can be realised by changing the following
parameters: orientation, crystallisation and crosslinking. Regarding the orientation
process, the properties of some polymer films can be improved by stretching a film above
its glass transition temperature (Tg). Orientation may be in one direction only (uniaxial
orientation) or in two directions, i.e., in both machine direction and transverse direction
(biaxial orientation). Biaxially oriented film can be further categorised as balanced film,
where orientation is roughly equal in both directions, or as unbalanced. This orientation
of the molecules in thermoplastics is essentially a stretching process, which tends to align
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the molecules in the direction of the stretching force. Once the molecules have been
aligned, the ordered arrangement is frozen in, giving rise to a strained condition.
8.2.1 Orientation
Orientation of plastic films improves some of the physical properties such as tensile
strength, impact strength, clarity and stiffness. In many instances there is also improvement
in gas and water vapour barrier properties. Particularly in the case of polypropylene
(PP), the barrier properties are also improved.
Films that benefit appreciably from orientation include PP, polyethylene terephthalate
(PET) and polyamide (Nylon). Polystyrene (PS) film is brittle material and becomes tough
when biaxially oriented. Another aspect of film orientation is that of shrink-wrapping,
where films such as low-density polyethylene (LDPE) and polyvinyl chloride (PVC) are
stretched at a temperature above their softening points and then cooled to ‘freeze in’ the
consequent orientation of the molecules. When these films are reheated, the molecules
tend to return to their unstretched dimensions. In contrast, heat-setting ‘annealing’ is
used to prevent shrinkage when heating stretched films. If oriented polypropylene, for
example, is heated to about 100 °C immediately after being drawn, it shrinks considerably
unless it is restrained in some way. This can be prevented by heat-setting. The film is
heated, under controlled conditions, and while held under restraint after cooling, the
film will not shrink if heated to below the annealing temperature; the film is said to be
heat-set. The physical and optical properties of the film remain unchanged.
8.2.2 Crystallisation
Crystallisation of polymers occurs as a result of the close approach of molecular chains
in ordered, crystalline areas, which leads to the formation of much stronger intermolecular
forces than in the amorphous areas. The rate of cooling has a significant effect on the
degree of crystallinity and size of crystallites. Thus, rapid quenching of Nylon films
during the casting process produces an amorphous film, whereas slow cooling allows the
formation of crystals. The properties of the final film are highly dependent on the
crystalline state of the polymer. Rapid quenching and consequent inhibition of crystal
growth give a transparent film, which is more easily thermoformed.
8.2.3 Crosslinking
Crosslinking is used to improve the mechanical properties and to obtain an infusible
film. Crosslinked polyethylene film can be achieved by subjecting the film to high-energy
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Modifications of Plastic Films
radiation. When polyethylene film is irradiated, hydrogen and smaller amounts of
methane, ethane and propane gases are liberated, and the polymer becomes increasingly
insoluble as a result of crosslinking of the molecules via C–C bonds. This process slightly
improves the gas and water vapour barrier properties, and the film has good clarity. The
tear strength of the film becomes good, and the resistance to tear initiation and to tear
propagation becomes high.
8.3 Chemical Modifications
Chemical modification of the surface of polymers is an attractive method of improving
the barrier characteristics of polymers that are otherwise considered ideal materials for
packaging. With the exception of low gas barrier properties, polyolefins are extremely
attractive because of their low cost, toughness, processability and excellent water barrier
properties. Surface treatment is ideal for such polymers, because they are easily processed
and made into better barriers by surface modification, either during processing or
afterwards [4, 5].
Chemical reactions of the surface with gases are used to modify the surfaces of existing
polymers without changing bulk properties. This modification can be achieved by reacting
the polymer surface with gases. Modifications of the surface using fluorine, hydrogen
fluoride, sulfur tetrafluoride, chlorine and bromine have been examined.
8.3.1 Fluorination
The manufacture of fluoromonomers and their subsequent polymerisations are hazardous
and difficult. The fabrication of fluoropolymers is also difficult and expensive. For
example, the processing of polytetrafluoroethylene (PTFE) involves costly compaction
and a high-temperature (375 °C) sintering process [6]. Hence, the widespread use of
fluoropolymers is hindered by these considerations. Fluorination of polymers has been
shown to be a successful new route to fluoropolymers. Polymers are fluorinated either
directly or indirectly [7]. In direct fluorination, highly active fluorinating agents such as
fluorine, hydrogen fluoride, or sulfur tetrafluoride convert the polymeric material
completely to a fluorocarbon polymer.
8.3.1.1 Direct Fluorination
Fluorine is a highly active fluorinating agent because of its low dissociation energy. It
forms extremely stable bonds with carbon [8]. Fluorination of polymers by fluorine may
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Handbook of Plastic Films
be divided into two types: bulk fluorination and surface fluorination [9]. The surface
fluorination of polyethylene (PE) film by 10% F2 (diluted with N2) results in a depth of
fluorination ranging between 0 and 50 Å [10]. The extent and depth of fluorination
during surface fluorination of polycarbonate (PC), PS and polymethyl methacrylate
(PMMA) films with F2 diluted with He or N2 increase with reaction time, temperature
and F2 gas pressure [11]. The extents of fluorination for PS, PC and PMMA are as high
as 64.3%, 55.3% and 20% respectively. The relation between depth of fluorination and
reaction time is represented by:
d = Kt1/2
(8.1)
where d is the depth of fluorination and t is the reaction time. The proportionality constant
K depends on the nature of the polymer; for example, the values for K are 13.2 and 5.6
for PS and PC, respectively. Fluorination may be conducted using hydrogen fluoride [7]
and sulfur tetrafluoride [12, 13].
8.3.1.2 Indirect Fluorination
In an effort to overcome the disadvantages of conventional fluorinating agents such as
F2, HF, or SF4, nontoxic fluorocarbons, chlorofluorocarbons and sulfur hexafluoride are
used. These gases cannot be used directly as fluorinating agents. However, when exposed
to high-energy environments such as plasma, glow discharge, or γ-radiation, they generate
active fluorinating agents [14].
Another approach of considerable interest is to modify the surface of existing polymers
without changing the bulk properties. Fluorine attaches to the polymer near the surface
and, because of its bulkiness and polar nature, improves gas and nonpolar liquid barrier
properties [15].
Bulk fluorinated polymers (by F2 under controlled conditions to reduce crosslinking) can
be used for the same purpose in place of fluoropolymers with similar structures prepared
from respective monomers [8]. However, to make it cost-effective, surface fluorination
rather than complete bulk fluorination of fabricated plastic items may be preferred. Such
surface treatments could avoid problems encountered in moulding of fluoropolymers.
Large fabricated plastic items can be given a surface coating of fluorinated polymer (0.1
mm thickness) [16].
Fluorinated plastic surfaces are impervious to most solvents and have good chemical,
solvent and water resistance [16, 17]. Various plastic bottles, containers and tanks are
fluorinated to handle chemicals and solvents safely. Hence, fluorinated plastic containers
are found in use as containers for gasoline (petrol), paint, turpentine, motor oil and
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Modifications of Plastic Films
varnish [18, 19]. However, fluorinated materials may not maintain their barrier properties
after repeated flexing.
Surface fluorination is used to improve the barrier properties of the inner surface of
polyethylene during the blow moulding process for formation of bottles. Polyethylene is
nonpolar and therefore a poor barrier to nonpolar hydrocarbons. Such treatments with
highly polar fluorine significantly improve its barrier properties. Surface-fluorinated
containers are commonly used for gasoline (petrol), herbicides, pesticides and other
products that normally penetrate polyethylene [20].
Mild surface treatment of polyethylene with low concentrations of fluorine can reduce
the permeability of liquid penetrants such as pentane and hexane depending on the
solubility and size of the penetrant [21].
8.3.2 Chlorination
The chlorination reaction is too slow and not practicable, but it results in good barrier
properties with more resistance to flexing. The gas-phase chlorination of the surface of
LDPE has been studied under ambient light [22, 23] as well as in the presence of ultraviolet
(UV) radiation [24]. The resultant surface was reported to consist of C–Cl and C–Cl2
moieties [22, 23]. However, chlorination of the surface of PE leads also to the formation
of allyl chloride and vinyl chloride moieties [24].
8.3.3 Bromination
The introduction of Br moieties on the polyolefin surface opens up a synthetic pathway
to introduce a wide range of specific functional groups on the surface under mild conditions
via nucleophilic substitution of Br moieties by different nucleophiles [25]. The gas-phase
bromination of PE, PP and PS film surfaces by a free-radical photochemical pathway
occurs with high regioselectivity. The surface bromination was accompanied by
simultaneous dehydrobromination resulting in the formation of long sequences of
conjugated double bonds. Thus, the brominated polyolefin surface contains bromide
(Br) moieties in different chemical environments.
As an example, we consider the free-radical mechanism for the bromination of the surface
of PE film. The first step in this reaction is the homolytic bond cleavage of the bromine
molecule into two bromine radicals upon exposure to radiation [26]:
UV
Br2 → 2Br•
(8.2)
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Handbook of Plastic Films
In the second step, the bromine radical abstracts a hydrogen atom from the methylene
unit of LDPE, which results in the formation of a radical centre on the LDPE chain:
–CH2–CH2– + Br• → –•CH–CH2– + HBr
(8.3)
This radical centre further reacts with a bromine molecule to form the C–Br moiety and
a bromine radical:
–•CH–CH2– + Br2 → –CHBr–CH2– + Br•
(8.4)
This bromine radical then reacts with another –CH2– unit [equation (8.3)] and this chain
reaction continues:
–CHBr–CH2– + Br• → –CHBr–•CH– + HBr
(8.5)
The effects of the structure of the polymer on the mechanism of the bromination have been
studied. Since the PS backbone contains 50% benzyl carbon atoms and 50% secondary
carbon atoms, an increased rate of bromination compared to that of PE is expected.
8.3.4 Sulfonation
Sulfonation involves exposure of the polymer surface to SO3/air followed by neutralisation
with NH4OH, NaOH, or LiOH. Chemical reduction of copper, tin, or silver counterions
present from the neutralisation process following sulfonation is called ‘reductive
metallisation’. When combined with a thin protective overcoating of compatible barrier
copolymer, dramatic permeation flux reductions of nearly 200-fold have been reported
[5]. Sulfonation of polystyrene and aromatic polymers can be used to obtain protonconducting polymer electrolytes for use in fuel cells [27]. The aromatic polymers are
easily sulfonated by concentrated sulfuric acid, by chlorosulfonic acid, by pure or
complexed sulfur trioxide, or by acetyl sulfate. Sulfonation with chlorosulfonic acid or
fuming sulfuric acid sometimes causes chemical degradation in these polymers [1].
Surface sulfonation yields excellent gas barrier properties under dry conditions, is relatively
simple and does not affect the mechanical stability of the polymer [5].
8.3.5 Chemical Etching
Chemical treatment is usually used for irregular and, in particular, large articles when
other treatment methods are not applicable. It involves immersion of the article [LDPE
and high-density PE (HDPE)] in an etchant solution such as chromic acid [28],
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Modifications of Plastic Films
permanganate, sulfuric acid [29,30] or chlorosulfonic acid. Reflection infrared (IR)
studies reveal extensive chemical changes on the surface in the case of LDPE but not
HDPE or PP. New bands corresponding to the introduction of –OH, >C=O and SO3H
groups were detected.
Oxidation of PE by sulfuric acid and potassium chlorate [29, 30] has been carried
out. In this case, the free energy of adhesion of the polymer is found to increase
linearly with the surface density of the hydrophilic sites created by oxidation.
The surface tension, polarity, wettability and bondability of fluoropolymer are
improved by sodium etching [31, 32]. The etching solution is the equimolar complex
of sodium and naphthalene dissolved in tetrahydrofuran. X-ray photoelectron
spectroscopy (XPS) shows the complete disappearance of the fluorine peak, the
appearance of an intense oxygen peak, and broadening and shifting of the C 1s peak
to lower binding energy. A significant number of functional groups, such as carbonyl,
carboxyl and C=C unsaturation, are introduced.
The oxidation methods described up to now are heterogeneous in nature, since they
involve chemical reactions between substances located partly in an organic phase
and partly in an aqueous phase. Recently, a technique that is commonly referred to
as phase transfer has come into prominence. This technique involves the use of phasetransferred permanganate (purple hydrocarbon) as an oxidant in a polar medium.
Konar and co-workers [33, 34] have oxidised several polyolefins with the help of
tetrabutylammonium permanganate in a hydrocarbon medium. Characterisation of
the oxidised polyolefins confirmed the introduction of polar functional groups on
the polar surface [35, 36].
Other phase-transfer catalysts, such as tetrabutylammonium bromide,
tetrapentylammonium iodide, dicyclohexyl-18-crown-6 (DC-18-C6) and benzyl
triphenyl phosphonium chloride (BTPC), have been investigated [37]. The results
obtained show that LDPE oxidised using DC-18-C6 and BTPC catalysts has a
relatively greater polar contribution to the total surface free energies than when using
other catalysts. The carboxyl percentage attains 15.0% and 20.0%, respectively [38]
while hydroperoxide attains 22.2% and 15.2%, respectively [36]. When a polymer
is soaked in a heavily oxidative chemical liquid, such as chromic anhydride/
tetrachloroethane, chromic acid/acetic acid or chromic acid/sulfuric acid, and treated
under suitable conditions, polar groups are introduced on the polymer surface [39,
40]. The surface of the polymer is heavily oxidised by nascent oxygen generated
during the reaction as follows:
K2Cr2O7 + 4H2SO4 → Cr2(SO4)3 + K2SO4 + 4H2O + 3[O]
(8.6)
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Handbook of Plastic Films
8.3.6 Grafting
Graft copolymerisation of vinyl monomers on to polymeric materials has been the subject
of extensive studies for about four decades. In spite of the huge number of published
papers and patents, and the interesting results obtained, there has been comparatively
little commercialisation of the grafting process. The reasons for the lack of industrialisation
on a large scale have been partly economic. Among the technical problems, which still
remain to a considerable extent, are the concurrent formation of homopolymer in most
cases and the lack of reproducibility in these largely heterogeneous reactions. In addition,
there is the difficulty of controlling the grafted side chains in the molecular weight (molar
mass) distribution.
There are now a considerable number of methods available for effecting graft
copolymerisation on to preformed polymers, each with its own particular advantages
and disadvantages. Graft copolymerisation is effected, generally, through an initiation
reaction involving attack by a macroradical on the monomer to be grafted. The generation
of the macroradical is accomplished by different means such as:
(1) Decomposition of a weak bond or liberation of an unstable group present in side
groups in the chemical structure of the polymer;
(2) Chain transfer reactions;
(3) Redox reaction;
(4) Photochemical initiation; and
(5) Gamma-radiation-induced copolymerisation.
Grafting using γ-radiation is concentrated on polyolefins and some vinyl polymers and
elastomers, which are usually difficult to graft by chemical means without prior chemical
modification of the substrate.
8.3.6.1 Grafting Using High-Energy Radiation
The surface properties of commercial polymer thin films can be tailored under appropriate
experimental conditions of radiation-induced grafting. The growth in popularity of
radiation as the initiating system for grafting arises from the availability and cost of
ionising radiation. This is due to the introduction of more powerful nuclear reactors.
Apart from its cheapness, radiation is a very convenient method for graft initiation because
it allows a considerable degree of control to be exercised over such structural factors as
220
Modifications of Plastic Films
the number and length of the grafted chains by careful selection of the dose and dose
rate. Thus, the advantages of radiation-chemical methods are:
(1) Ease of preparation as compared to the conventional chemical method;
(2) General applicability to a wide range of polymer combinations (due to the relatively
nonselective absorption of radiation in matter); and
(3) More efficient (and thus more economical) energy transfer provided by radiation
compared to chemical methods requiring heat.
The theory of radiation-induced grafting has received extensive treatment. The direct
effect of ionising radiation in material is to produce active radical sites. The typical steps
involved in free-radical polymerisation are also applicable to graft copolymerisation,
including initiation, propagation and chain transfer. However, the complicating role of
diffusion prevents any simple correlation of individual rate constants to the overall reaction
rate. Among the various methods of radiation grafting, four have received special attention:
(1) Direct radiation grafting of a vinyl monomer on to a polymer;
(2) Grafting on radiation-peroxidised polymer;
(3) Grafting initiated by trapped radicals; and
(4) The intercrosslinking of two different polymers.
Acrylic acid (AA) has been grafted on to PE films using γ-radiation [41, 42]. Gammaradiation grafting of styrene on to PE films has been carried out [43]. The styrene-grafted
films were then sulfonated to form cationic exchange membranes. Rieke and co-workers
described the properties obtained from grafting AA on to HDPE [44]. Their study pursued
the concept of producing thermally sensitive crosslinks that could improve the properties
of PE, (i.e., increase chemical reactivity). In 1977, Toi and co-workers determined the
thermal properties for styrene-grafted HDPE by using γ-radiation [45]. No effect was
observed on the crystallite size and the glass transition temperature after grafting. Ishigaki
and co-workers reported the graft polymerisation of AA on to PE film by the preirradiation method [46, 47]. LDPE and HDPE were irradiated by electron beams of 2-50
Mrad and then immersed in an AA aqueous solution. These products were tested as
semipermeable membranes for water desalination under reverse osmosis conditions [48].
Hydrophilic monomers such as AA or vinylpyridine were grafted on to PE via 60Co γradiation. The hydrophilic monomer-grafted PE could be treated further for
functionalisation, leading to the investigation of a few applications such as separation
membranes, polymeric catalysts and biosensors [49-53].
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Handbook of Plastic Films
8.3.6.2 Photografting
The surface photografting process is based on surface grafting reactions initiated by
ultraviolet radiation. These reactions are efficient and fast. They are limited to the surface
of the polymer without affecting bulk properties, and they give very thin layers (less than
10 nm) of grafted polymer [54]. The grafting of sheets of low-density and high-density
polyethylene with acrylic acid by UV irradiation from a high-pressure mercury lamp
using a batch process has been reported [55]. The surface of HDPE is more difficult to
graft than that of LDPE because of the linear chain structure of HDPE, and consequently
its higher degree of crystallinity, which gives it a rougher surface structure than LDPE.
Surface grafting with acrylic acid, as expected, decreases the contact angle of water,
approaching complete wetting for LDPE.
The molecular mechanism of bulk surface photografting has been given [56]. The primary
grafting given in this mechanism using benzophenone involves initiation and propagation
of short linear chains which is terminated by the addition of ketyl radical. Benzophenone
acts as both initiator and terminator.
The main effects that are important for applications are increased wettability, as mentioned
before, increased adhesion of inks and other substrates, and increased adsorption of
dyes. By grafting of reactive monomers like glycidyl acrylate, the polymer surface is
made reactive to stabilisers, hydrophilic polymers, heparin and other bioactive agents,
which gives functional properties of great interest [57-59]. Biomedical applications are
of particular interest [60]. Examples of other recent publications on surface photografting
include the preparation of polymeric catalyst [61, 62], polyethylene films for studying
electrostatic interactions [63], and films for immobilisation of enzymes [64].
8.4 Physical Methods Used for Surface Modification
Modification techniques using physical methods have been designed to achieve increased
hydrophilicity, chemical modification and attachment of pharmacologically active agents.
These physical methods include plasma treatment, corona treatment, ultraviolet and
gamma radiation.
8.4.1 Plasma Treatment
The implantation process that occurs in plasma treatment is one of the most effective
methods of surface modification of polymeric materials. The plasma activates gas
molecules, such as oxygen and nitrogen. The activated species interact with the polymer’s
surfaces, and then special functional groups, such as hydroxyl, carbonyl, carboxyl, amino
222
Modifications of Plastic Films
and amido groups, are formed at the surface of the polymers. As a result, the implantation
reactions lead to large changes in the surface properties of the polymer; for example, the
polymers change from hydrophobic to hydrophilic. ‘Plasma treatment’ is frequently used
for the improvement of the adhesion and wettability of polymeric materials. A polyethylene
film was treated with a nitrogen plasma, and its surface was inspected by XPS (C 1s and
N 1s core levels) [65]. The original polyethylene film provides a sharp and symmetrical
C 1s core-level spectrum whose peak appears at 285 eV with no N 1s core-level spectrum.
However, the plasma-treated film gives an asymmetrical C 1s spectrum with a tail at
more than 285 eV, and a strong N 1s core-level spectrum. This comparison indicates that
some nitrogen functionalities were generated at the polyethylene film surface through
nitrogen plasma treatment. Similarly, oxygen plasma treatment leads to the formation of
some oxygen functionalities at the surface of polyethylene film [66]. It is clear that plasma
treatment implants atomic residues at the surface of polymeric materials. Carbon
monoxide, carbon dioxide, nitrogen monoxide, nitrogen dioxide and ammonia are used
as plasma gases for hydrophilic surface modification. Polypropylene, polyester,
polystyrene, rubber and polytetrafluoroethylene, among others, but not polyethylene,
have been successfully modified by plasma treatment. The details of the implantation
process are reviewed in the literature [67].
8.4.2 Corona Treatment
In this technique, a sufficiently high-voltage electrical discharge is applied to the surface
of a moving substrate (sheet or film). Pretreatment of films is usually carried out at the
same time as film extrusion, which is an advantage where antistatic and other additives
are present in the film. When film was extruded and stored prior to treatment, it was
found that the additives had bloomed to the surface, and this made it difficult to achieve
an even treatment.
In one method, the film is passed between two electrodes, one of which is a metal blade
connected to a high-voltage, high-frequency generator. The other is an earthed roller,
which is separated from the high-voltage electrode by a narrow gap. The metal electrode
should be slightly narrower in width than the film that is to be treated in order to prevent
direct discharges to the roller. The electrical discharge is accompanied by the formation
of ozone. This oxidises the film surface, rendering it polar. The level of treatment is
governed by the generator output and the speed of throughput.
Both under- and overtreatment should be avoided – the latter causing surface powdering,
brittleness and sealing difficulties. The effect of treatment diminishes with time, and the
treated surface is sensitive to handling and dust pickup. The corona treatment functions
at atmospheric pressure and relatively high temperature. In this case, very significant
surface oxidation occurs [6].
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Handbook of Plastic Films
One simple test for determining whether or not a film has been treated is to run
water over the surface. If the film is untreated, the water will be repelled, whereas a
treated film will retain the liquid for several minutes. Between these two extremes, a
partially treated film will tend to show areas of both good and bad adhesion, so that
the test is only satisfactory for seeing whether or not the surface of the film has been
treated but not for detecting overtreatment.
An improvement on this primitive test method is the peel adhesion test. This is carried
out by applying a specified pressure-sensitive tape to the film surface, using a roller.
The peel strength is then measured with the aid of a tensiometer. The higher the level
of treatment, the higher the peel strength.
The chemical changes occurring on the surface can be detected by using the XPS
technique. This technique enables one to identify the presence of hydroxyl, ether,
ester, hydroperoxide, aldehyde, carbonyl or carboxylic groups in corona dischargetreated polyolefins.
8.5 Characterisation
Characterisation of modified films depends on the method of modification. For instance,
the change in mechanical properties due to stretching can be evaluated by measuring
the changes in mechanical properties using tensile testing machines according to standard
methods. Characterisation of grafted films also differs somewhat from that of physically
treated films. However, the selection of one or other measuring technique depends
generally on the extent of modification.
8.5.1 Gravimetric Method
Graft products are usually characterised by different methods. The first method is the
calculation of graft parameters known as the grafting percentage (GP), grafting efficiency
(GE) and weight conversion percentage (WC). These parameters can be calculated
according to the following equations:
grafting percentage (GP) =
grafting efficiency (GE) =
224
A−B
× 100
B
A−B
× 100
C
(8.7)
(8.8)
Modifications of Plastic Films
weight conversion percentage (WC) =
A
× 100
B
(8.9)
where A, B and C are the weights of the extracted graft product, substrate and monomer,
respectively.
This gravimetric method gives direct and rapid information about the graft reaction.
Other characterisation methods are usually used to detect the changes in physical
properties, which usually result from the changes in the morphology and structures of
the substrates due to grafting.
8.5.2 Thermal Analyses
In polymers having a certain degree of crystallinity, differential scanning calorimetry
(DSC) is used to determine the heat of fusion and, consequently, the changes in the
degree of crystallinity in grafted and ungrafted samples. The changes in the crystallinity
of PE found after grafting include a small 2.5 °C drop in the location of the maximum in
the melting curve and a significant decrease in the area under the melting peak [69].
Similar results were observed in the case of grafting PP and PE/ethylene-vinyl acetate
(EVA) blends [70]. While the decrease in the melting temperature, represented by the
shift in the melting curve, indicates that there is some change in the crystallinity caused
by grafting, comparison of the areas before and after grafting indicates that this may be
a small effect. By assuming that the difference in areas is due only to a difference in the
amount of PE or PP present (in other words, no difference in the degree of crystallinity),
the per cent graft can be calculated from:
%G =
A1 − A2 ρPAN
×
× 100
A2
ρPE
(8.10)
where A1 is the area before grafting, A2 is the area after grafting, and ρ is the density.
8.5.3 Scanning Electron Microscopy
Scanning electron microscopy (SEM) is generally used to detect the topography of a
grafted surface, which usually changes due to monomers grafted on to the surface. In
addition, this method can also be used to detect the depth of grafting into the matrix. If
a binary monomer mixture was used for grafting, scanning electron micrographs help to
detect the grafted monomer distribution by comparison with micrographs of each grafted
monomer separately.
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8.5.4 Swelling Measurements
Equilibrium swelling of grafted samples in a proper solvent helps to detect the presence
of grafted monomer. For instance, polyethylene does not swell to any noticeable extent
in water. However, if polyethylene is grafted with water-soluble polymers such as
polyacrylic acid or polyacrylamide, the equilibrium swelling of the product obtained
increases markedly. Accordingly, the increase in swelling is evidence of grafting. In contrast,
the swellability of natural rubber or styrene-butadiene-rubber vulcanisates in gasoline or
benzene decreases markedly due to grafting with polyacrylonitrile (PAN). This decrease
in swelling, again, is evidence of grafting.
8.5.5 Molecular Weight and Molecular Weight Distribution
It is essential to know the molecular weight (molar mass) distribution of a graft in order
to design functional polymeric membranes precisely by application of radiation-induced
graft polymerisation and to control the grafting process. For example, the length and
density of the polymer chains grafted on to a cellulose triacetate microfiltration membrane
will determine the permeability of liquid through and the adsorptivity of molecules on
the functionalised microfiltration membrane. Thus, the molecular weight distribution of
methyl methacrylate grafted on to cellulose triacetate has been determined by acid
hydrolysis of the substrate. From the gel-permeation chromatogram, the molecular weight
distribution was determined [71]. This method is valid only when it is possible to degrade
the substrate. In the case of grafted natural rubber, for example, ozonolysis is a very
convenient process to use to destroy the natural rubber segments, leaving the plastomer
chains intact [72]. Alternatively, oxidation with perbenzoic acid can be used [73].
Osmometry or solution viscosity may then be used to determine the molecular weight of
the isolated non-rubber fraction.
8.5.6 Dielectric Relaxation
Dielectric relaxation measurements of polyethylene grafted with AA, 2-hydroxyethyl
methacrylate (HEMA) and their binary mixture were carried out in a trial to explore the
molecular dynamics of the grafted samples [74]. Such measurements enable information
to be obtained about their molecular packing and interaction. It was possible to predict
that the binary mixture used yields a random copolymer PE-g-P(AA/HEMA) that is greatly
enriched with HEMA. This method of characterisation is very interesting and is likely to
be developed in different polymer/monomer systems.
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8.5.7 Surface Properties
The surface properties of modified plastic films are very important in industrial
applications. A number of techniques are available for determining the composition of a
solid surface. This is very important in many processes, such as oxidation discoloration,
wear and adhesion. The technique used depends upon such important considerations as
sampling depth, surface information, analysis environment and surface suitability.
The most widely used techniques for surface analysis are Auger electron spectroscopy
(AES), XPS, secondary ion - mass spectroscopy (SIMS), Raman and IR spectroscopy,
and contact angle measurements.
8.5.8 Spectroscopic Analysis
8.5.8.1 Infrared (IR) Spectroscopy
Proof of chemical modification or changes in chemical structure due to physical treatments
such as corona discharge can be followed up by spectroscopic analysis using IR. Thus,
the amount of acrylonitrile grafting on to PE using an electron beam was determined
from the absorbance of the nitrile group at 2240 cm–1 after extraction of homopolymer
[69]. In order to minimise the effects of weighing error, an internal reference method
utilising the methylene absorbance of PE at 730 cm–1 was adopted. Thus, the mass of
PAN in a sample was correlated to the ratio of the absorbance A2240/A730, and the weight
per cent graft defined before was computed from the mass of PAN.
8.5.8.2 X-Ray Fluorescence Spectroscopy (XFS)
This method can be used to detect and characterise the first several hundred nanometres
of depth of a solid. It can be attached to a scanning electron microscope. The main
principle is that energetic electrons bombard the sample, where ionisation takes place.
Ions with an electron vacancy in their atomic core rearrange to a lower energy state,
resulting in the release of electromagnetic energy of a specific wavelength. Analysis of
the wavelengths of the X-radiation emitted identifies the atomic species present.
8.5.8.3 Auger Electron Spectroscopy (AES)
This technique is used to characterise the chemical bonding state of the elements on the
surface. The maximal depth from which Auger electrons can escape is only about 0.30.6 μm. For most materials, AES uses a low-energy, 1-5 keV, electron beam gun for
surface bombardment to minimise surface heating.
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Handbook of Plastic Films
8.5.9 Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photoelectron Spectroscopy (XPS)
In this method the surface is bombarded with low-energy X-rays, which is less disruptive
than an electron beam. The energy is absorbed by ionisation, resulting in the direct
ejection of a core-level electron, i.e., a photoelectron. Hence ESCA is also known as Xray photoelectron spectroscopy. These electrons have an escape depth of less than a
nanometre. Although XPS is less sensitive than AES, it provides a direct measure of the
binding energy of core-level electrons through the relation:
binding energy of ionised core-level electron =
energy of emitted photoelectron – incident X-ray energy
and it gives simpler spectral line shapes than AES.
This technique can be used to distinguish between different elements and different
chemical bonding configurations. It is the most popular surface analytical technique
for providing structural, chemical bonding and composition data for polymeric
systems. All elements, except hydrogen, are readily identified by XPS, since the
different core-level binding energies are highly characteristic. By measuring the relative
peak intensities and dividing them by the appropriate sensitivity factors, one may
find the concentration of different elements on a surface. Moreover, small shifts in
the binding energy of a core level are corroborated by considering the presence of
different functional groups. For example, when a carbon atom is bonded to different
groups of atoms of increasing electronegativity, a systematic shift in the binding energy
of the C 1s peak is observed. The higher the electronegativity of the group, the higher
the binding energy of the C 1s peak.
8.6 Applications
Since bringing about changes in physical properties is often the impetus for grafting, it
is necessary to touch upon this briefly in this section. A number of general reviews on
grafting have also included some discussion on the changes in physical properties that
usually determine the field of applications. Grafting has often been employed to change
the moisture absorption and transport properties of plastic films when hydrophilic
monomers such as acrylamide, acrylic acid and methacrylic acid are grafted. Radiation
grafting of anionic and cationic monomers to impart ion-exchange properties to polymer
films and other structures is rather promising. Thus, grafting of acrylamide and acrylic
acid on to polyethylene and polyethylene/ethylene-vinyl acetate copolymer blend [70]
allows a new product to be obtained with reasonable ion-exchange capacity.
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Modifications of Plastic Films
A number of possible uses of radiation grafting are being explored for microlithography,
diazo printing, and various copying and printing processes.
Radiation grafting for various biomedical applications remains an extremely active field
of development. The grafted side chains can contain functional groups to which bioactive
materials can be attached. These include amine, carboxylic and hydroxyl groups, which
can be considered as centres for further modifications.
Photodegradation of polyethylene waste can be markedly accelerated via its grafting
with acrylamide [70]. In contrast, photostabilisation of polyethylene and polypropylene
can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2hydroxypropoxy)benzophenone using γ-radiation [75]. In this case, the grafted compound,
acting as a UV stabiliser, is chemically bound to the backbone chain of the polymer, and
its evaporation from the surface can be avoided.
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Handbook of Plastic Films
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Modifications of Plastic Films
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27. M. Rikukawa, Progress in Polymer Science, 2000, 25, 10, 1463.
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234
9
Applications of Plastic Films in Packaging
Susan E. Selke
9.1 Introduction
Nearly all products are packaged at some point in their life-cycle. Plastic films are widely
used in packaging, and continue to grow in use as more and more applications switch
from rigid to flexible packages. Flexible packages generally take up much less space than
the rigid structures they replace, especially before they are filled with product. They
commonly require less material, as well. Therefore, switching from rigid to flexible
packaging can provide significant economic savings in warehouse space and
transportation, as well as in package cost. On the other hand, because flexible packaging
does not usually have as much strength as rigid packaging, stronger distribution packaging
may be required. Opening and reclosing of flexible packaging may also be less userfriendly, and consumers may perceive some types of products in flexible packaging as
being lower in quality than equivalent products in rigid or semi-rigid packages.
Common flexible packaging forms include wraps, bags and pouches. In these packages,
plastic films may be used alone or combined with paper and/or metal to serve the basic
packaging functions of containment, protection, communication and utility in the delivery
of quality products to the consumer. While plastic films are most often found in flexible
package structures, they may also be used as a component in rigid or semi-rigid package
structures, for example, as a liner inside a carton, or as lidding on a cup or tray.
The most common film used in packaging is low-density polyethylene (LDPE), defined
broadly to include linear low-density polyethylene (LLDPE). Appreciable amounts of
high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC),
polyvinylidene chloride (PVDC), polyamide (Nylon) and other plastics are also used
9.2 Packaging Functions
Before examining applications of plastic films in packaging, it is useful to take a moment
to consider why we use packages at all, since that will help in evaluation of the advantages
and disadvantages of plastic films as packaging materials. The functions of packaging
can be described in many ways. One simple way of organising them is to consider the
basic packaging functions as containment, protection, communication and utility.
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The containment function of packaging is the most basic. Many types of goods cannot
be readily moved from one place to another unless they are contained in some manner.
This is obvious for liquids and gases, but is also true for many small solid items, for
example, marbles, nails, laundry detergent powder and potato chips (crisps). The package
confines these items in a way that makes it feasible to transport them.
Sometimes the containment function is considered part of the more inclusive protection
function of packaging. An important attribute of many forms of packaging is the ability
of the package to protect the product from some type of damage associated with its
interaction with the environment. In the example above, the marbles and nails must be
protected against exposure to dust and dirt to remain in a condition that will permit
their sale. The laundry detergent needs to be protected from exposure to excess moisture
that could cause caking. The potato chips must be protected against light and oxygen,
which can cause rancidity. In some cases, protection of the environment from the product
is provided by the package. A water-soluble pouch for agricultural chemicals has, as its
prime function, protection of the user from exposure to the hazardous undiluted chemical.
Packages also serve as a vehicle for communication. In most cases, the package must in
some way communicate what it contains. Sometimes this is as simple as being transparent,
so that the user can see what is inside. Since the package is often the primary sales tool,
however, communication needs are usually much more extensive. The package must not
only communicate what is inside, but also act to convince the potential consumer to purchase
the product. Often, there are a number of legally required communications, such as the
amount of product, where and by whom the product is made, required warnings, etc.
Packages also provide utility, either for the end-user or for others who interact with the
package along the supply chain. Utility includes attributes such as a tear strip for opening
and tamper evidence, a zipper closure for resealing, and a hole for use in hanging the
packages on a display.
Individual packages or package elements often provide more than one function,
simultaneously. For example, a stand-up pouch for snack foods provides: containment;
protection of the product against oxygen and moisture; communication of identification,
legally required, and sales messages; opening and reclosure features for consumer utility;
and the ability to stand conveniently on the retailer’s shelf and present a reasonably flat
front panel to catch the consumer’s eye.
9.3 Flexible Package Forms
Flexible packages come in two basic forms: wraps, and bags or pouches. A wrap consists of
plastic film that has not been formed into a package shape. The film is simply wound around
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Applications of Plastic Films in Packaging
the product or products to be contained, and held in place in some fashion. In a bag or
pouch, some shaping of the plastic is done, either before or at the same time as the product is
added. Most often, this shaping is done by heat-sealing the edges of the plastic together.
9.3.1 Wraps
9.3.1.1 Stretch-Wrap
One of the largest uses of plastic films in packaging is in stretch-wrap used for bundling
pallet-loads of products together, in order to unitise them for distribution. The plastic
film, most often linear low-density polyethylene, is stretched as it is wound around the
products and pallet, usually in a spiral fashion. When enough has been applied, the film
is cut, and the tail of the film is adhered to the load, usually by self-cling. When the
stretching force is released, the film’s tendency to return to its unstretched dimensions
causes a restraining force to be exerted on the load, thus unitising it and keeping it from
shifting when the load is moved during distribution.
In addition to its unitising function, stretch-wrap also protects the load against moisture,
dust and abrasion. Stretch-wrap can also be used to provide this protection to single
items, or to unitise smaller than pallet-load quantities of goods.
While stretch-wrap is simple in conception, it may have a fairly complex structure. It is
desirable for each layer of stretch-wrap to stick to the layers below, but it is undesirable
for adjacent shrink-wrapped loads to stick to each other, or to other things with which
they come in contact. Therefore, the stretch-wrap may have a multilayer structure, with
tackifying agents added to the inside layer to enhance cling. Low-density polyethylene,
polyvinyl chloride, ethylene-vinyl acetate and other polymers are used as stretch films, in
addition to LLDPE.
9.3.1.2 Shrink-Wrap
Shrink-wrap is an alternative to stretch-wrap for unitisation. When shrink-wrap is exposed
to a source of heat, the previously aligned (oriented) molecules try to return to the lowerenergy, unoriented, random-coil conformation. The product prevents the film from
returning to its unstretched dimensions, and the force exerted by the material on the
product acts to unitise the load.
For unitising pallet-loads of goods, stretch-wrap is much more common than shrinkwrap, since it generally requires less energy and is more economical. Shrink-wrap is
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more commonly used as a bundling wrap, unitising two or several products (either the
same or different), rather than for full pallet-loads of goods.
Often, shrink-wrap is used for product protection rather than unitisation, in applications
ranging from meat to toys. It can be designed to form a tight enclosure around the entire
product, providing excellent protection against dirt, moisture and abrasion. Usually, the
wrap is formed into a loose pouch before it is shrunk tightly around the product in a
shrink tunnel, where the packaged product is exposed to hot air. LDPE and LLDPE are
common materials for shrink films. PVC and PP are used in lesser quantities, as are some
specialty films.
9.3.2 Bags, Sacks and Pouches
To make a bag, sack or pouch, two or more edges of a plastic film are sealed together,
forming a cavity in which the product can be placed. In most applications, the opening is
then closed so that the product is completely enclosed by the package. In some cases,
such as merchandise sacks, one side remains open.
The terms ‘bag’, ‘sack’ and ‘pouch’ can be confusing. According to some authorities, sacks
are larger than bags, and both refer to packages in which the top is open, while pouches
are smaller, and refer to packages that are totally sealed. However, these definitions do not
conform to common use of these terms, which, in practice, are often used interchangeably.
Common styles of pouches include pillow pouches, three-side-seal pouches and fourside-seal pouches. Pillow pouches are produced by forming the plastic film into a cylinder
and sealing the edges together in what will become the back seam in the finished package.
The bottom of the cylinder is collapsed and sealed, the product introduced, and then the
top seam added. The shape of the filled package resembles a pillow – hence its name.
Three-side-seal pouches are formed, as the name indicates, by folding the film into a
rectangle and sealing the three non-fold sides. In some cases, the fourth side is sealed as
well, for additional strength. Four-side-seal pouches are formed from two pieces of material
that are sealed together on all sides. Therefore, four-side-seal pouches need not be
rectangular in shape. In contrast to pillow and three-side-seal pouches, the front and
back of a four-side-seal pouch may be made from different types of plastic film.
In any of these pouch styles, gussets may be added to expand the capacity of the pouch
without increasing its width or height.
Pouches may be used alone, or may be combined with another package for product
distribution and/or sale. One very common package structure is bag-in-box packages,
which consist of a pouch inside a folding carton or corrugated box.
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Applications of Plastic Films in Packaging
The pouch material may be plastic film alone, or a multilayer material containing paper
and/or aluminium foil. Paper may be used to add strength, rigidity, printability or bulk
to the flexible package. Foil may be incorporated to improve the barrier to permeants
such as oxygen, water vapour, odours or flavours.
In the past several years, stand-up pouches have increasingly been used as substitutes for
cartons or bottles. Stand-up pouches are designed to stand upright on the retail shelf.
Their design involves gussets and special shaping of the bottom panel.
9.3.3 Pouch Production
There are two primary ways of using bags, sacks and pouches for packaging: as preformed
pouches, or in form-fill-seal operations. In a form-fill-seal (FFS) operation, the web stock
(usually preprinted, if applicable) is fed into either a horizontal or vertical FFS machine,
in which it is formed into a pouch, the product added and the final seal formed. If
preformed pouches are used, the packages are formed and an opening left for product
introduction. The product is added to the package in a separate operation, and then the
package is sealed.
Form-fill-seal operations are usually economically advantageous for large-scale
production. Buying of preformed pouches is generally more economical if production
quantities are small, or in cases where the material is difficult to seal and poses quality
control problems.
9.3.4 Dispensing and Reclosure Features
One of the long-standing drawbacks of flexible packaging has been the difficulty of
providing easy-to-use and effective dispensing and reclosure. In the past few years, several
innovations have provided significant improvements in these package attributes.
The most common way to dispense products from flexible packages is to cut or tear the
package open, or to peel open one of the seams. For some products, such as breakfast
cereal in bag-in-box packages, this is a significant source of consumer complaints. The
seals often do not peel easily, and all too often the result is a bag with a split down the
side, spilling cereal into the carton and making it nearly impossible to reclose the pouch
to protect product freshness. Some flexible packages now incorporate zipper closures,
often accompanied by a tear strip for initial opening. Other packages have resealable
flaps, usually located along a seam.
For liquid products, some packages incorporate a threaded spout with a standard threaded
cap. This may be located on the top of the pouch, or on the bottom, depending on the
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product and the package size. In bag-in-box packages, the outer carton may include a
flap through which the spout can be extended for dispensing. For single-serve drinks, it
is common to provide an attached straw (protected from dirt in its own pouch, which is
glued to the side of the drink pouch), along with a designated spot on the package that
has been modified for easy puncturing.
9.4 Heat-Sealing
Heat-sealing is the usual method for producing seals and seams in flexible packaging.
Occasionally, adhesive systems are used. There are a wide variety of types of heat-sealing
systems, but the most common, especially for films, are thermal or bar sealing, and
impulse sealing.
Thermal or bar sealing uses two heated bars that exert pressure on the materials to be
sealed and at the same time conduct heat to the interface, melting the materials. The
pressure ensures good contact between the materials, and assists in interpenetration of
the melted viscous materials at the interface. When sufficient time has elapsed to produce
an initial seal, the materials are released. Therefore, the hot tack of the material is crucial
in forming an adequate seal. The full strength of the seal forms as the material cools, but
the initial strength must be sufficient to maintain the seal integrity while cooling proceeds.
The sealing bars usually have rounded edges to avoid puncturing the material, and often
one bar is fitted with a resilient surface to aid in achieving uniform pressure during
sealing. Usually, the heat-seal jaws are serrated rather than flat, and produce a patterned
seal. In variants of thermal sealing, only one bar is heated and the other is not. Especially
for sealing lidding on containers, the bars may be shaped rather than rectangular,
producing shaped seals. Another variant uses heated rollers rather than bars; the pouch
is sealed as it travels through the rollers.
Impulse sealing also uses two jaws to produce the seal, but heat is generated by flow of
an impulse of electric current through a nichrome wire. The jaws do not remain hot, but
cool down after each electrical impulse. The material being sealed is captured between
the jaws, the current flows to produce heating, and the material remains between the
jaws for a cool-down period before it is released. Cooling may be aided by circulation of
cooling water through the jaws. With impulse sealing, materials do not require as good
hot tack as with thermal sealing. The seal will increase in strength during the cooling
phase, before it is released from the heat-sealer, so it is not as subject to immediate
failure or distortion. On the other hand, the impulse seal is typically much narrower
than the bar seal, and therefore is often not as strong. Impulse sealing is particularly
advantageous for oriented materials, which have a tendency to wrinkle during sealing.
As with bar sealing, the jaws can be shaped to produce shaped seals.
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Applications of Plastic Films in Packaging
Hot wire or knife sealing uses a heated wire or knife to cut and seal films simultaneously.
It is often used to produce thin polyethylene bags for applications such as produce packaging.
The seals are very narrow, often nearly invisible, and relatively weak. In band sealing,
often used for the final seal on filled preformed pouches, the materials to be sealed are
moved through heated bands. Both heating and cooling phases can be provided. Other
types of heat-sealing are used less frequently in production of flexible packaging.
9.5 Other Uses of Packaging Films
Plastics films are sometimes used as components in rigid or semi-rigid packaging structures.
They can serve as liners inside closures for bottles and jars, as lidding on trays or cups, or
can be laminated on paperboard or other materials. While the plastic resins used as
coatings are not produced as stand-alone films, they are deposited on or in packages as
films. Two common packaging applications of plastic films outside the flexible packaging
category are skin packaging and bubble-wrap.
In skin packaging, a product is held tightly to a backing material by a plastic film. Usually,
the backing material consists of a heat-seal coated paperboard. The product is placed on
the board, and then the heated plastic film lowered on to it. A vacuum is drawn through
the backing material, causing the film to form tightly around the product and seal to the
board. Usually, the product is displayed in the retail environment by hanging the backing
from a peg. Obviously, the product must be able to withstand momentary contact with the
hot plastic without damage, and the plastic must not adhere to the product. The coated
backing often requires perforations to permit adequate evacuation of trapped air. In some
cases, films are used that permit sealing to uncoated board. Heavy-duty films sealed to
corrugated board can be used to provide protection to products during distribution, by
physically isolating the skin-packaged product from impacts to the outer container.
Bubble-wrap is a cushioning material produced by forming bubbles of air, of a defined
size, between two plastic films. The bubbles can be various sizes, depending on the enduse of the material. Generally, smaller bubbles are used to protect lighter-weight products,
and larger bubbles are used for heavier products. Bubble-wrap does not provide suitable
protection for products that are very heavy, however.
9.6 Major Packaging Films
A variety of plastic resins are used to make packaging films. Sometimes they are used
alone, and often they are used in combinations that provide the benefits of multiple
materials. The most commonly used packaging resins will be described in this section.
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9.6.1 Low-Density Polyethylene (LDPE) and Linear Low-Density
Polyethylene (LLDPE)
Low-density and linear low-density polyethylene are the most commonly used packaging
films. Low-density polyethylene is produced by a high-temperature, high-pressure process
that results in considerable short- and long-chain branching of the molecules. Linear
low-density polyethylene is produced at temperatures similar to those used for highdensity polyethylene, resulting in linear molecules. The reduction of density comes about
through the use of comonomers that put side groups on the main chain that act like
branching in decreasing crystallinity. In traditional Ziegler-Natta catalyst polymerisations,
these comonomers are butene, hexene or octene. Some of the new family of polyethylenes
using metallocene catalysts incorporate higher alpha-olefins into the polymer structure,
producing longer side groups, which act much like the long-chain branching in highpressure LDPE.
LDPE and LLDPE are soft, flexible materials, with a hazy appearance. At equal density
and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance
and elongation than LDPE. LLDPE based on octene generally has the highest strength,
followed by hexene- and butene-based polymers, in that order. The cost per unit mass of
the materials generally also follows the order octene > hexene > butene. LDPE has better
heat-seal properties than LLDPE. It seals at lower temperatures, seals over a wider
temperature range, and has better hot tack, all of which result, to a great extent, from its
long-chain branching. Metallocene LLDPE containing higher alpha-olefins was designed,
in part, to remedy this disadvantage of LLDPE. Another approach that has commonly
been taken to producing the best mix of properties for a given application is to blend
LLDPE and LDPE.
LDPE and LLDPE are good barriers to water vapour, but are poor barriers to oxygen,
carbon dioxide and many odour and flavour compounds. They have good grease
resistance, and are quite inert. Low-temperature performance is good, as these materials
retain their flexibility at very low temperatures. They soften and melt at moderately
elevated temperatures, so they are not suitable for applications involving significant
exposure to heat.
Some characteristic LDPE and LLDPE properties are presented in Table 9.1. LDPE is
generally the cheapest plastic film, on a per-unit-mass basis. Since LLDPE often permits
considerable down-gauging, it can be the lowest cost alternative on a per-use basis.
Very low-density polyethylene (VLDPE) is LLDPE with a higher concentration of
comonomer, which reduces crystallinity, and consequently density, below the traditional
range for LLDPE, to 0.905-0.915 g/cm3. These materials are very soft films, with excellent
cling but reduced strength.
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Table 9.1 Typical properties of polyethylene films [1, 2]
Property
Polymer
LDPE
LLDPE
HDPE
–120
–120
–120
105-115
122-124
128-138
Glass transition temperature (Tg; °C)
Melting temperature (Tm; °C)
Heat distortion temperature, at
455 kPa (°C)
Density (g/cm3)
40-44
0.915-0.940
62-91
0.915-0.935
0.94-0.97
Tensile modulus (GPa)
0.2-0.5
0.6-1.1
Tensile strength (MPa)
8-31
20-45
17-45
Elongation (%)
100-965
350-850
10-1200
WVTR* at 37.8 °C and 90% RH
(g μm/m2 d)
375-500
125
O2 permeability, at 25 °C
(103 cm3 μm/m2 d atm)
160-210
40-73
*WVTR: Water vapour transmission rate (d = day, 24 h)
RH: relative humidity
9.6.2 High-Density Polyethylene (HDPE)
High-density polyethylene is a linear addition polymer of ethylene, produced at
temperatures and pressures similar to those used for LLDPE, and with only very slight
branching. HDPE films are stiffer than LDPE films, though still flexible, and have poorer
transparency. Their water vapour barrier is better, as is their gas barrier. However,
permeability to oxygen and carbon dioxide is still much too high for HDPE to be suitable
as a barrier for these permeants.
As is the case for LDPE, HDPE is very inert, and has good oil and grease resistance. Using
high molecular weight (high molar mass) resin, HMW-HDPE, which permits considerable
down-gauging, can reduce the cost of HDPE films on a per-use basis. This material is
higher in cost per unit mass, and is also somewhat more difficult to process than lower
molecular weight materials, due to its high viscosity. Another alternative for reducing the
cost of HDPE film is the use of recycled material, often originating in milk cartons.
Because of the distinctly cloudy appearance of HDPE film, a small amount of white
pigment is commonly added to provide an attractive opaque white film. Typical HDPE
properties are shown in Table 9.1.
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9.6.3 Polypropylene (PP)
Polypropylene is a linear addition polymer of propylene; resins used in packaging are
predominantly isotactic. PP has the lowest density of the commodity plastics, 0.890.91 g/cm3. PP films are suitable for high-speed packaging applications that demand a
relatively stiff material, since they are considerably stiffer than HDPE, and also have
much improved clarity. Clarity can be further improved by using copolymer resins
containing some ethylene units, to reduce crystallinity. Another approach to improving
transparency is the use of nucleating agents to reduce average crystallite size. Barrier
properties of PP are comparable to those of HDPE.
Unoriented PP film tends to be somewhat brittle, especially at low temperatures. In
many applications, biaxially oriented film (BOPP) is preferred. Orientation also increases
the stiffness of the film. PP, especially BOPP, does not heat-seal well. Therefore, it is
commonly coated or coextruded with sealants to make heat-sealable films. Typical PP
properties are shown in Table 9.2.
Table 9.2 Typical properties of polypropylene (PP), biaxially oriented
polypropylene (BOPP) and polyvinyl chloride (PVC) films [1-4]
Property
Polymer
PP
BOPP
PVC
Tg (°C)
–10
–10
75-105
Tm (°C)
160-175
160-175
212
Heat distortion temperature, at
455 kPa (°C)
107-121
Density (g/cm3)
0.89-0.91
0.89-0.91
1.35-1.41
Tensile modulus (GPa)
1.1-1.5
1.7-2.4
to 4.1
Tensile strength (MPa)
31-43
120-240
10-55
Elongation (%)
500-650
30-150
14-450
WVTR, at 37.8 °C and 90% RH
(g μm/m2 d)
100-300
100-125
750-15,700
50-94
37-58
3.7-240
O2 permeability, at 25 °C
(103 cm3 μm/m2 d atm)
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Applications of Plastic Films in Packaging
9.6.4 Polyvinyl Chloride (PVC)
Polyvinyl chloride films are formed by combining PVC resin, produced by addition
polymerisation of vinyl chloride, with plasticisers and other additives to produce a flexible
film. Unmodified PVC is quite brittle and difficult to process because of its heat sensitivity.
However, because of its polar nature, PVC has a high affinity for plasticisers, and hence
can be substantially modified. Plasticisers generally consist of high-boiling-point organic
liquids, which serve a lubricating function in the resin. Some soft and flexible PVC films
are approximately 50% plasticiser by weight.
For food packaging uses, plasticisers and other ingredients must be suitable for direct
food contact. The major plasticisers used in such applications are adipates. Often,
epoxidised soybean oil is added as a secondary plasticiser. For non-food use, a wider
range of plasticisers is available. Adipates and phthalates are most common. In addition
to plasticisers, PVC films contain stabilisers, as the resin is heat-sensitive. Oil epoxides
have some stabilising functionality, and in food packaging uses supplement the activity
of calcium, magnesium or zinc stearates. Phosphites may also be used. In non-food
applications, organometallic salts of barium and zinc are commonly used.
The properties of PVC films are strongly influenced by the type and level of modifying
ingredients, especially plasticisers, that have been added. In general, the films are quite
soft and flexible, easy to heat-seal, and have excellent self-cling, toughness, resilience
and clarity. Permeability is relatively high. Both oriented and unoriented films are available.
Properties of PVC film are listed in Table 9.2.
Heavier gauge PVC, sheet rather than film, is often used in thermoformed packaging,
such as in blister packaging.
9.6.5 Polyethylene Terephthalate (PET)
Polyethylene terephthalate is formed by condensation polymerisation of ethylene glycol
and either terephthalic acid or dimethyl terephthalate. It is commonly used in biaxially
oriented form, and has excellent transparency and mechanical properties. Heat-setting
enables the film to be used for extended periods at temperatures ranging from –70 to +150
°C. It can tolerate considerably higher temperatures for short periods, such as in dual
ovenable packaging for frozen foods. PET has good barrier properties, especially for odours
and flavours. The barrier properties can be enhanced by coating with PVDC, or by
metallising, as will be discussed in subsequent sections. Coating or coextrusion is often
used to provide good heat-seal properties. Typical PET properties are listed in Table 9.3.
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Table 9.3 Typical properties of polyethylene terephthalate (PET) films [1, 2, 5]
Property
PET polymer
Unoriented
Oriented
Tg (°C)
73-80
73-80
Tm (°C)
245-265
245-265
Heat distortion temperature, at 455 kPa (°C)
3
Density (g/cm )
38-129
1.29-1.40
1.40
Tensile modulus (GPa)
2.8-4.1
Tensile strength (MPa)
48-72
220-270
Elongation (%)
30-3,000
70-110
WVTR, at 37.8 °C and 90% RH (g μm/m2 d)
390-510
440
O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)
1.2-2.4
1.1
9.6.6 Polyvinylidene Chloride (PVDC)
Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is an excellent
barrier to oxygen, water vapour, odours and flavours. However, its high crystallinity
and sensitivity to heat-induced degradation make it extremely difficult to process.
Therefore, homopolymer PVDC is not used commercially.
Copolymerisation of vinylidene chloride with various amounts and types of comonomers,
usually vinyl chloride, acrylonitrile, methacrylonitrile, methacrylates or alkyl acrylates,
produces a family of PVDC copolymer resins with improved processability, while maintaining
desired barrier properties. Vinylidene chloride content typically ranges from 72 to 94 wt%;
molecular weights range from about 65,000 to 150,000 [6]. In general, the highest barrier
resins are not melt-processable, but instead are applied by solvent or latex coating. Extrudable
resins have undergone more modification, so consequently have somewhat decreased barrier
properties. PVDC films produced for household use are plasticised copolymers, and have
even poorer barrier performance. However, they remain much better barriers than competitive
polyethylene films. Representative properties are shown in Table 9.4.
PVDC copolymer films can be heat-sealed. Therefore, in PVDC copolymer coatings or
coextrusions, the PVDC can serve as a combination barrier and heat-seal layer. However,
the best barrier films generally do not provide the best heat-seal capability, and vice
versa, so when both heat-sealability and barrier are desired, sometimes two differently
formulated PVDC copolymer coatings are applied.
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Applications of Plastic Films in Packaging
Table 9.4 Typical properties of polyvinylidene chloride (PVDC) films [1, 2, 6]
PVDC polymer
Property
Generalpurpose
Highbarrier
Tg (°C)
–15 to +2
–15 to +2
Tm (°C)
160-172
160-172
1.60-1.71
1.73
Tensile modulus (GPa)
0.3-0.7
0.9-1.1
Tensile strength (MPa)
48-100
83-148
Elongation (%)
40-100
50-100
79
20
0.31-0.43
0.031
Heat distortion temperature, at 455 kPa (°C)
Density (g/cm3)
WVTR, at 37.8 °C and 90% RH (g μm/m2 d)
O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)
Nearly all cellophane produced in North America is solvent-coated with PVDC
copolymers. Solvent and latex coatings are also used on plastic sheet for thermoformed
containers, and on blow-moulded plastic bottles. Common substrates include polyolefins,
polyesters, polyamides and styrenics. Coextrusions of PVDC copolymer with polyethylene
or polypropylene are used in shrinkable films for meat, cheese and other moisture- or
oxygen-sensitive foods. Latex coatings of PVDC copolymers are used to provide moisture
resistance, grease resistance and barrier to paper and paperboard packages.
9.6.7 Polychlorotrifluoroethylene (PCTFE)
Polychlorotrifluoroethylene (PCTFE) is another polymer with good barrier
characteristics, especially for water vapour. The homopolymer is very difficult to process
because of its extremely high melt viscosity. A small amount of modification by
copolymerisation yields AlliedSignal Corporation’s trademarked Aclar films, which
contain greater than 95% chlorotrifluoroethylene by weight. These films are considered
the best available transparent moisture barriers for flexible packaging; however, they
are rather expensive.
Aclar films can be used alone, or can be laminated to paper, polyethylene, aluminium
foil or other substrates. The film is heat-sealable, and can be thermoformed. Aclar blister
packages are often used for unit packages for highly moisture-sensitive pharmaceuticals.
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9.6.8 Polyvinyl Alcohol (PVOH)
Polyvinyl alcohol films are unique in several respects. Polyvinyl alcohol polymers are
produced by hydrolysis (more correctly, alcoholysis) of polyvinyl acetate. If fully
hydrolysed, the polymer is readily soluble in water. Controlling the degree of hydrolysis
can produce films that are soluble in hot water but not in cold water. Because PVOH
degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore,
casting from a water solution is used to make film. As produced, the film is amorphous,
but orientation induces some crystallinity.
The water-solubility of PVOH is the major reason for its use in niche markets where this
is a desired attribute. One application is as an inner pouch in packaging of agricultural
or other chemicals, to limit human exposure. The pouch with its contents can be placed
into the dilution and dispensing apparatus, without direct contact between the user and
the chemical. In the water, the pouch dissolves, releasing the chemical. The dissolved
polymer does not clog spray nozzles, and is biodegradable.
Another application is in hospital laundry bags. Here, the hot-water-soluble variety is
used. Soiled laundry is placed in the bags, and then bag and all can be placed into the
washer, so that no contact between the launderer and the potentially infectious linen is
required. Since the polymer does not dissolve in cold water, it will not be affected by
residual liquid in the linens, but will dissolve readily in the hot wash water.
9.6.9 Ethylene-Vinyl Alcohol (EVOH)
Ethylene-vinyl alcohol resins are produced by hydrolysis (alcoholysis) of ethylene-vinyl
acetate random copolymer, analogous to the route for production of polyvinyl alcohol
from polyvinyl acetate. Commercially available materials contain a substantial
percentage of ethylene, typically 27 to 48 mol%. The presence of ethylene renders the
resins melt-processable.
The presence of –OH groups in the structure results in strong intermolecular hydrogen
bonding. While EVOH is a random copolymer, CH2 and CHOH groups are isomorphous;
they fit into the same crystalline structure. Therefore, the polymer crystallises readily.
The combination of strong intermolecular forces and crystallinity makes it an excellent
barrier to gases, odours and flavours. However, the hydrogen bonds also make it a
moisture-sensitive material, and high humidity decreases its barrier capability.
EVOH is most often used as an oxygen barrier. Since, in most applications, it is likely to
be exposed to moisture either from the environment or in the product, it is usually used
as a buried inner layer in a coextruded structure, where a good moisture barrier, often a
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Applications of Plastic Films in Packaging
Table 9.5 Typical properties of ethylene-vinyl alcohol (EVOH) films [1, 2, 7]
EVOH polymer
Property
32 mol%
ethylene
44 mol%
ethylene
Tg (°C)
69
55
Tm (°C)
181
164
Density (g/cm3)
1.19
1.14
Tensile modulus (GPa)
2.6
2.1
Tensile strength (MPa)
77
59
Elongation (%)
230
380
WVTR, at 40 °C and 90% RH (g μm/m2 d)
1535
724
0.0078
0.030
Heat distortion temperature, at 455 kPa (°C)
O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)
polyolefin, protects it. Monolayer EVOH films, oriented or unoriented, are also available,
which can be used alone but are usually combined with other materials by laminating, or
coating. Typical EVOH properties are listed in Table 9.5.
9.6.10 Polyamide (Nylon)
Polyamides, or Nylons, are a family of plastics containing characteristic amide
functionality. They are commonly formed by condensation polymerisation of amino acids,
or of carboxylic acids and amines.
Nylon films are used for specialty applications in packaging, where performance
requirements justify their relatively high cost. Nylons have excellent high-temperature
performance, so can be used, for example, in boil-in-bag packages. Nylons also provide
excellent odour and flavour barrier, and reasonably good oxygen barrier. They are very
poor water vapour barriers, and generally have a tendency to lose some barrier
performance when exposed to large amounts of moisture. However, their performance is
not as water-sensitive as EVOH.
Most Nylons used in packaging have some crystallinity; the amount is heavily dependent
on processing conditions, since Nylons have a narrow window for crystallisation. Films
generally retain good flexibility at low temperatures, and have excellent strength properties.
Owing to their relatively high cost, they are often coextruded with other plastics.
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Table 9.6 Typical properties of polyamide (Nylon) films [1, 2]
Polymer
Property
Nylon-6
Nylon-11
Nylon MXD6
Tg (°C)
60
64
Tm (°C)
210-220
180-190
243
Density (g/cm3)
1.13-1.16
1.03-1.05
1.20-1.25
Tensile modulus (GPa)
0.69-1.7
1.3
3.8-4.1
Tensile strength (MPa)
41-165
55-65
220-230
300
300-400
72-76
3,900-4,300
1,000-2,000
630
0.47-1.02
12.5
0.06-0.26
Heat distortion temperature, at
455 kPa (°C)
Elongation (%)
WVTR, at 40 °C and 90% RH
(g μm/m2 d)
O2 permeability, at 25 °C
(103 cm3 μm/m2 d atm)
Nylon MXD-6: Mitsubishi Gas Chemicals America, Inc., New York, NY, USA
Polyamides manufactured from straight-chain amines and carboxylic acids are typically
named with numbers representing the number of carbons in each of the starting monomers.
For example, Nylon-6,10 is made from a six-carbon amine and a ten-carbon carboxylic
acid. Similarly, polyamides made from amino acids have a number designating the number
of carbons in the acid. When the carbons are not in a straight chain, more complex names
are necessary. Typical properties of some Nylon films are given in Table 9.6. Nylon-6 tends
to be the most-used Nylon packaging film in the USA, and Nylon-11 in Europe.
9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films
Ethylene-vinyl acetate is produced by addition copolymerisation of ethylene and vinyl
acetate. The acetate groups provide polar functionality that increases intermolecular
forces in the film, and, because of the structural irregularity thus introduced, interfere
with crystallisation. These films have excellent transparency, and provide very good heatseal and adhesive properties, with excellent toughness at low temperatures. Typical filmgrade EVA resins contain between 5 and 18% vinyl acetate. Resins designed for use as
an adhesive layer in a multilayer structure are typically at the higher end, and standalone films at the lower end, of this concentration range.
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Applications of Plastic Films in Packaging
Common markets for EVA are poultry and meat wrap, stretch film and ice bags. The
films tend to be sticky, so may require the use of slip and antiblock additives.
Copolymers of ethylene with acrylic acid and with methacrylic acid are also available,
and are commonly called acid copolymer resins. They are characterised by good clarity,
strong adhesion to polar substances such as paper, and also to foil, and low melt and
heat-seal temperatures.
9.6.12 Ionomers
Ionomers are formed by neutralisation of ethylene-acrylic acid or ethylene-methacrylic
acid copolymers containing 7 to 30 wt% acid, to yield sodium or zinc salts. The resulting
ionic bonds function as reversible crosslinks in the polymer, readily disrupted by heat,
but reforming on cooling. Therefore, these materials provide very strong bonding to
numerous substrates. Ionomers can be used for skin packaging to uncoated corrugated
board, for example.
The heat-seal performance of ionomers is outstanding, even permitting sealing through
grease contamination, which makes them ideal for packaging of processed meat. They
have superior hot tack, and excellent melt strength.
Ionomer films have excellent clarity, flexibility, strength and toughness. They can be
used to package sharp objects, which break through many alternative materials when
subject to vibration during distribution. Ionomers have relatively poor gas barrier, and
tend to absorb water readily. They also are relatively high cost compared to films such as
ethylene-vinyl acetate.
9.6.13 Other Plastics
Several other types of plastics are used in packaging films to some extent. Polycarbonate
films have excellent transparency, toughness and heat resistance, but high cost. They
have some use in skin packaging, food packaging where exposure to high temperatures
for in-bag preparation is required, and medical packaging.
Polystyrene is another film with excellent transparency, often used in window envelopes
and window cartons. It has low gas barrier, so can be used for produce where a ‘breathable’
film is required. In heavier gauges, polystyrene is widely used for transparent
thermoformed trays. Expanded polystyrene is used for trays, egg cartons and other
applications where its cushioning properties are desired. In general, these materials are
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Handbook of Plastic Films
classified as sheet, rather than film. Polystyrene film is generally biaxially oriented to
improve its properties, since the unmodified material is too brittle for most applications.
Impact-modified polystyrene sheet that incorporates polybutadiene is often used where
transparency can be sacrificed for impact resistance.
Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionate
and copolymers are also used to a relatively small extent, most often as sheet rather than
film. Their high price and water sensitivity limits their usefulness.
A wide variety of copolymers are available. Some of these have been discussed already. It
is quite common to modify the chemical structure of a polymer to obtain a more desirable
mix of properties. Another way to combine properties is to use blends of polymers.
High-impact polystyrene (HIPS) is actually partially a copolymer and partially a blend
of polybutadiene and polystyrene.
9.7 Multilayer Plastic Films
In many cases, the best combination of packaging attributes at the lowest cost is achieved
by using a combination of materials. Therefore, plastic packaging films are often combined
with one another or with other materials such as paper, aluminium or even glass, through
processes such as coating, lamination, coextrusion and metallisation.
9.7.1 Coating
Coating is commonly used to add a thin layer of a plastic on the surface of another
plastic film or sheet, or, more commonly, on a non-plastic substrate such as paper,
cellophane or foil. The coating may be applied as a solution, a suspension, or a melt.
Common reasons for using coating in flexible packaging are: to impart heat-sealability
for paper, cellophane, foil or plastics that are not themselves easily heat-sealed; to provide
moisture protection for paper or cellophane; to improve barrier properties; and to provide
protection from direct contact of the base material with the product.
Coating with low-density polyethylene is often used on paper to give heat-sealability and
moisture protection, as well as to protect printing from abrasion. It is often used on
aluminium foil to provide heat-sealability and abrasion resistance, and to prevent
interaction between the foil and the product. PVDC copolymer coatings are often used
to improve barrier and heat-sealability.
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Applications of Plastic Films in Packaging
9.7.2 Lamination
Lamination is the process of combining two webs of film together. In flexible packaging
applications, lamination is often used to combine a plastic film with paper or foil, or to
join paper and foil together. A variety of lamination methods are used. When plastic films
are involved, either as a substrate or as an element in the finished structure, the laminating
adhesive is often low-density polyethylene, applied by extrusion, and the process is known
as extrusion laminating. When paper is contained in a flexible package, it is most often
being used for its excellent printability, along with its ability to impart substance and
strength. When aluminium is used, it is most often employed for its excellent barrier to
light and to permeation. Occasionally, it is used primarily for its desirable appearance.
Another significant use of lamination is to produce a web with buried printing. In these
materials, one web is reverse-printed, and is then laminated to a second web, either
made from the same or a different polymer. The printing can be seen through the
transparent plastic, and is protected against abrasion so it maintains a fresh attractive
appearance much better than surface-printed materials.
9.7.3 Coextrusion
Coextrusion results in the production of a multilayer web without requiring initial
production of individual webs and a separate combining step. The melted polymers are
fed together carefully to produce a layered melt, which is then processed in conventional
ways to produce a plastic film or sheet. When only plastics are being used in a flexible
packaging structure, coextrusion is generally preferred to lamination, unless buried
printing is involved. Obviously, coextrusion cannot be used to incorporate nonthermoplastic materials.
A major advantage of coextrusion over lamination is its ability to incorporate very thin
layers of a material, much thinner than those which can be produced as a single web.
This is particularly important for expensive substrates, such as those often used to impart
barrier properties. The amount of the expensive barrier resin used need only be enough
to provide the desired performance. The thinness of the layer is not limited by the need
to produce an unsupported film and handle it in a subsequent lamination step.
9.7.4 Metallisation
Metallisation is a way of applying a thin metal layer on a plastic film (or on paper), as an
alternative to using a lamination with aluminium foil. In commercial packaging practice,
the metal being deposited is almost always aluminium. The process, known as vacuum
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Handbook of Plastic Films
metallising, involves evaporation of aluminium inside a vacuum chamber, and deposition
of the aluminium vapour on a plastic film. The operation is usually done in a batch
mode, with the substrate being metallised and an aluminium wire placed inside the vacuum
chamber. The film is rolled past a chill roll, which removes the heat from the condensing
aluminium, preventing melting of the film. Very high vacuums are needed, which has
retarded the development of continuous metallisation processes, although some are
available. Vaporisation of the aluminium is most often achieved by resistance heating.
Induction and electron-beam heating are used to a lesser extent.
Metallised films have significantly enhanced barrier characteristics, and are usually chosen
for this reason. Cost of metallised film is generally less than that of foil-containing
laminated materials. In snack packaging, for example, metallised film has almost totally
replaced foil laminations.
The barrier performance of metallised film, as initially produced, is somewhat inferior to
foil, and is dependent on the thickness of the deposited metal layer. However, stress
during product distribution can lead to the development of flex cracks in foil, which
then provide a route for gas transfer. Metallised foil, since it retains the flexibility and
other mechanical characteristics of the film substrate, is not usually subject to flex cracking.
Therefore, the barrier characteristics of metallised foil are sometimes superior to those
of foil laminations, at later points in distribution. Also, many oxygen-sensitive products
require better barrier than can be attained with plastic alone, but can be successfully
protected with metallised film. In addition to gas barrier, metallised film provides an
essentially total light barrier.
Occasionally, metallised foil is used for its appearance, rather than for its barrier
characteristics. This is particularly the case when it is used for labels. In many such
applications, however, paper, rather than film, is the metallisation substrate.
9.7.5 Silicon Oxide Coating
One of the disadvantages of metallised film is that the resultant material is opaque, and
is not suitable for use in microwave ovens. The desire for transparent high-barrier coatings
led to the development of glass-like coatings based on silicon oxide, SiO2.
Silicon oxide coatings on film are usually applied in a manner analogous to vacuum
metallising. The silicon oxide is evaporated, using electron-beam heating, and condensed
on the film substrate in a vacuum chamber. The film is very thin, 400 to 1000 Å, and
does not affect the mechanical properties of the material to any significant degree. The
chemical composition of the deposited film depends somewhat on conditions, and is
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Applications of Plastic Films in Packaging
characterised as SiOx, where the value of x is between 1.0 and 2.0. At values close to 1,
the layer imparts a distinct yellowish colour to the film. At values close to 2, it is nearly
colourless. The yellowing is a significant concern in some applications. The SiOx layer
greatly increases barrier of the film, and it is transparent to microwave radiation.
Therefore, it can be used in packages that will be heated in microwave ovens. The most
common substrate is PET, in thickness of 12.5-25 μm, although polypropylene, polystyrene
and polyamides can also be used.
An alternative to evaporative deposition is chemical plasma deposition, in which a siliconcontaining gas such as tetramethyldisiloxane or hexamethyldisiloxane is used as the silica
source. Little heat is required, and the degree of vacuum needed is lower. Therefore,
plasma deposition can be used on heat-sensitive materials such as LDPE and oriented PP.
The coating produced is thinner, and less yellow. Plasma deposition is the method of
choice for SiOx coating of containers, and can also be used for film.
9.7.6 Other Inorganic Barrier Coatings
Processes have also been developed that deposit aluminium oxide coatings on plastic films,
to increase barrier properties. Combinations of SiO and MgO have also been used.
Another type of inorganic barrier coating uses clay nanocomposites, which are deposited on
the film from a solution of PVOH/EVOH copolymer, in a mix of water and isopropyl alcohol,
with nanodispersed 7 nm diameter silica and titanium dioxide particles. Microgravure
equipment is used to coat the solution on to the film substrate. Barrier is reportedly comparable
to that of films metallised with aluminium, but the coatings are transparent.
These materials are all still in relatively early phases of development.
9.8 Surface Treatment
In many packaging applications, it is necessary for something to stick to a plastic film.
This may involve placing a label on a pouch or on a stretch-wrapped pallet, adhering
two films together in a lamination, or, as is often the case, printing the film. Adequate
adhesion requires that secondary bonding forces between the film and the object, such as
the ink, which is to be adhered, be sufficient to retain the material. Historically, this has
been a significant problem for plastic films, since the surface energy of the films is often
low, causing poor adhesion. Several techniques are commonly used to increase the surface
energy of polymers, hence improving adhesion. For films, the most common treatment is
corona discharge.
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Handbook of Plastic Films
In corona discharge treatment, the film surface is exposed to a discharge between grounded
and powered electrodes, at high voltage. The discharge of the electric current ionises the
air in the gap between the electrodes. The ions produced initiate free-radical reactions
with the film surface, causing bond cleavage followed by oxidation. Oxidation of the
surface increases its ability to adhere to substances such as inks and adhesives. The
effectiveness of corona discharge treatment dissipates with time, so ideally it should be
applied within a short time of the subsequent printing. In a roll of corona dischargetreated material that has been stored, the effectiveness of the treatment is likely to be
significantly higher on the inside layers than on the first few outside layers of film.
Other surface treatments that are sometimes applied to film also exist. However, corona
discharge is by far the most frequently encountered.
9.9 Static Discharge
Plastics, because they are nonconductive, are subject to build-up of electrostatic charges.
When such charges build up, the result can range from the attraction of dust and lint to
material handling problems, shocks and sparks. Methods for controlling the build-up of
static charges include charge neutralisation through ionisation of the surrounding air
and incorporation of conductive materials to dissipate the charge.
Antistatic agents can be incorporated into the film as additives, or can be used as a
surface treatment. The agents commonly used include non-ionic ethoxylated alkylamines,
anionic aliphatic sulfonates and phosphates, and cationic quaternary ammonium
compounds. In some cases, humidifying the area can control static, so that a thin layer of
water is absorbed on the film surface, which conducts the charge to ground.
Control of static discharge is especially important for packaging for sensitive electronic
devices. Film designed for such applications, usually polyethylene, is generally pigmented
pink to denote that it contains antistatic agents.
9.10 Printing
In many packaging applications, plastic films are printed to convey information to the user.
When printing is desired, it is usually done on roll stock before packages, such as pouches,
are formed. Printing on formed flexible packages is usually limited to date or lot coding.
Flexography is the printing method used most often for flexible packaging materials. In
this process, a subcategory of relief printing, the printing plates are flexible elastomers,
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Applications of Plastic Films in Packaging
with the images, or printing areas, raised above the nonprinting surrounding areas. Thin,
highly fluid, rapid-drying inks are used. The ink is transferred by a system of rollers to
the top surface of the printing plates, which in turn transfer the ink to the film.
In lithography, the printing image and the background are on the same plane of the thin
metal printing plate. The plate is treated to attract water and repel ink in the non-image
areas, and the reverse in the image areas. A system of rollers is used to transfer both ink
and water to the plate. The image on the plate is then transferred (offset) to a rubberblanket-covered cylinder, and then to the film.
Rotogravure uses copper-plated printing cylinders, which have the image engraved into
the cylinder in the form of tiny cells. The cylinder rotates in an ink bath, filling the cells
with ink. Excess ink is wiped off by a doctor blade, and then the image is transferred to the
film as it is pressed against the printing cylinder by an elastomer-covered impression cylinder.
For printing date and lot codes on formed packages, ink-jet printing is commonly used.
In this process, electrically charged drops of ink are sprayed out of jets, and electrostatically
directed to the desired printing location. This is an impactless form of printing, and is
ideal for printing rapidly changing information such as these codes.
Other types of printing, such as screen printing, as well as variations of the basic processes
described above, are used less frequently for plastic packaging films.
9.11 Barriers and Permeation
As has been discussed, in many packaging applications, protection of the product from
gain or loss of gases or vapours is important. The mechanism by which substances travel
through an intact plastic film is known as permeation. It involves dissolution of the
penetrating substance, the permeant, in the plastic, followed by diffusion of the permeant
through the film, and finally by evaporation of the permeant on the other side of the
film, all driven by a partial pressure differential for the permeant between the two sides
of the film.
The barrier performance of the film is generally expressed in terms of its permeability
coefficient. For one-dimensional steady-state mass transfer, the permeability coefficient
is related to the quantity of permeant, which is transferred through the film as represented
by the equation:
P=
Ql
AtΔp
(9.1)
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Handbook of Plastic Films
where P is the permeability coefficient, Q is the mass of permeant passing through the
material, l is the thickness of the plastic film, A is the surface area available for mass
transfer, t is time, and Δp is the change in permeant partial pressure across the film.
It can be shown that the permeability coefficient, as defined by equation (9.1), is equal to
the product of the Fick’s law diffusion coefficient, D, and the Henry’s law solubility
coefficient, S, in situations where these laws adequately represent mass transfer (ideally
dilute solutions, diffusion independent of concentration):
P = DS
(9.2)
The permeability coefficient, under these circumstances, is a function of temperature,
but is not a function of film thickness or permeant concentration.
While this is a very simplified approach to mass transfer, it is adequate for many packaging
situations. For example, with oxygen-sensitive products, reaction with oxygen is
commonly rapid compared to the rate of transfer, so the oxygen concentration within
the package is relatively constant at nearly zero. Oxygen concentration in the surrounding
air, measured as partial pressure, is constant at approximately 21 kPa. Regardless of the
shape of the flexible package, mass transfer is essentially one-dimensional, through the
thickness of the film. If temperature is constant and P is known, the amount of oxygen
transported through the film in a given period can be easily calculated using equation
(9.1). Conversely, if the sensitivity of the product is known in terms of the maximum
amount of oxygen that can be taken up without resulting in unacceptable product quality,
the time required for that amount of transfer (the product shelf-life) can be calculated.
A similar approach can often be taken for transfer of odour or flavour compounds.
While the diffusivity, and hence the permeability coefficient, of such organic substances
is likely to be concentration-dependent, at the low levels associated with most packaging
situations, the dependence is slight.
Calculating shelf-life when water vapour transmission is involved is more problematic.
In such cases, the partial pressure difference for water vapour between the inside and the
outside of the package is almost never constant. Simplifying assumptions generally used
consider the time for moisture in the product itself and in the product headspace to reach
equilibrium to be small compared to the time required for permeation, and ignore moisture
change in the headspace itself, calculating only moisture gain or loss in the product. The
resulting differential equation is:
dQ 1
= PA( p2 − p1 )
dt
l
258
(9.3)
Applications of Plastic Films in Packaging
where p1 is the partial pressure of water vapour outside the package, p2 is the partial
pressure of water vapour inside the package, and p2 is a function of Q. Solution of this
equation requires knowledge of the moisture sorption isotherm for the product, which
relates the moisture content of the product to the equilibrium relative humidity of the air
in contact with the product, and thus to p2.
In the case where the sorption isotherm at the storage temperature can be approximated
as linear over the range of moisture contents of interest, it can be written as:
W = a + bM
(9.4)
where W is the water activity of the air in equilibrium with the product with moisture
content M (dry weight basis), and a and b are the best-fitting straight-line constants.
Rewriting the basic permeability equation [equation (9.3)] in terms of water activity,
and substituting:
Q = (M – Mi)w
(9.5)
where Mi is the initial moisture content and w is the dry weight of the product gives:
w
dM PAps
=
(W2 − W1 )
dt
l
(9.6)
where W1 and W2 are the water activities at times 1 and 2, and ps is the saturation water
vapour pressure at the storage temperature.
This equation can be integrated, giving the following relationship for moisture gain
or loss:
[
[
]
]
1 ⎛ W2 − W1 t ⎞ PApst
- ln⎜
⎟=
b ⎝ W2 − W1 ⎠
lw
0
(9.7)
Mass transfer characteristics for plastics are often expressed in terms of water vapour
transmission rates (WVTR), rather than permeability coefficients. WVTR reflect the rate
of water vapour transfer under specific conditions, and must be translated to permeability
coefficients for application at different conditions. The relationship between Pwater and
WVTR is the following:
Pwater =
WVTR
Δp
(9.8)
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Handbook of Plastic Films
where Δp is the difference in water vapour partial pressure under the conditions at which
the WVTR was measured. In many cases, this was the ASTM standard condition of 32.2
°C (90 °F) and 90% relative humidity (RH).
When permeability coefficients are not available at the temperature of interest, an
Arrhenius relationship can be used to determine the required value, from the
permeability coefficient at a nearby temperature and the activation energy. The equation
used is the following:
1 ⎤ ⎪⎫
⎪⎧⎛ E ⎞ ⎡ 1
P2 = P1 exp⎨⎜ a ⎟ ⎢ − ⎥ ⎬
⎪⎩⎝ R ⎠ ⎣ T1 T2 ⎦ ⎪⎭
(9.9)
where T1 is the temperature at which P1 is known, T2 is the temperature at which P2 is to
be calculated, Ea is the activation energy, and R is the gas constant.
Care must be taken in applying equation (9.9). The permeability coefficient, as indicated,
is a product of the diffusion coefficient and the Henry’s law solubility constant. Since
these vary in different ways with temperature, equation (9.9) is valid only over reasonably
small temperature ranges. A particular concern is that permeation rates are much higher
above the Tg than below this temperature, and the rate of change with temperature
differs. Therefore, equation (9.9) should never be used to calculate the permeability
coefficient across a temperature range that spans Tg of the plastic.
Permeability coefficients for multilayer plastic film or sheet, either coextrusions or
laminations, can be calculated from the thickness and permeability coefficients of the
individual layers, as follows:
Pt =
lt
∑ (l / P )
i=n
i=1 i
i
(9.10)
where the subscript t indicates the value for the total structure, i indicates the value for
an individual layer, and there are n layers in the structure.
Special care must be taken when the barrier characteristics of a polymer are affected by
the presence of the permeant or of some other substance that may also be permeating.
This situation is most often encountered with water-sensitive plastics, such as ethylenevinyl alcohol, since co-permeation of water vapour and other components of interest,
such as oxygen, may well occur during processing and storage. It may also arise in other
situations, such as co-permeation of organics involved in odour and taste.
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Applications of Plastic Films in Packaging
9.12 Environmental Issues
In recent years, consideration of the environmental effects of packaging decisions has
become more common. While thorough discussion of such issues is beyond the scope of
this chapter, some general observations and conclusions will be made.
It is generally agreed that evaluation of the environmental impacts of a product or package
requires consideration of the total life-cycle of the object. Such ‘cradle to grave’ analysis
is commonly referred to as life-cycle assessment. Usually, when such analyses are carried
out, the most influential life-cycle stage is that of production of the raw materials and
packages, rather than transportation or disposal. Packages that minimise material use
are therefore likely to have reduced environmental impact. Since flexible packaging systems
usually (although not always, since distribution packaging must be included) use less
overall packaging material, they often have reduced environmental impact, compared to
the rigid packaging systems they replace.
In examining the impacts of waste disposal, two general conclusions can be drawn. In
most cases, flexible packaging is less likely to be recovered for recycling than rigid or
semi-rigid packaging. Therefore, a higher proportion of flexible packaging is likely to
require disposal. On the other hand, flexible packaging, as discussed above, usually
means less total material requires handling. Unless recycling rates for the alternatives to
the flexible packages are very high, use of flexible packaging is likely to mean less material
requiring disposal. Also, flexible packages containing plastics are sources of recoverable
energy in appropriate systems.
References
1.
R.J. Hernandez, S.E.M. Selke and J.D. Culter, Plastics Packaging: Properties,
Processing, Applications, and Regulations, Hanser, Munich, Germany, 2000.
2.
S.E.M. Selke, Understanding Plastics Packaging Technology, Hanser, Munich,
Germany, 1997.
3.
D. Kong in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody
and K.S Marsh, Wiley, New York, NY, USA, 1997, 407.
4.
E. Mount and J. Wagner in The Wiley Encyclopedia of Packaging Technology,
Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 415.
5.
J. Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody
and K.S. Marsh, Wiley, New York, NY, USA, 1997, 408.
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Handbook of Plastic Films
6.
P. DeLassus, W. Brown and B. Howell in The Wiley Encyclopedia of Packaging
Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA,
1997, 958.
7.
R. Foster Newton in The Wiley Encyclopedia of Packaging Technology, Eds.,
A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 355.
262
10
Applications of Plastic Films in Agriculture
E.M. Abdel-Bary, A.A. Yehia and A.A. Mansour
10.1 Introduction
The quantity of plastic materials used annually in the world in the agricultural sector
amounts to 2 million tons. About 50% of this is used in protected cultivation greenhouses,
as mulching, for low tunnels, as temporary coverings of structures for fruit trees, etc. [1].
Thin plastic film produced with low investment is economically and technically feasible,
and provides the best cost/benefit ratio for use in greenhouses and low tunnels. The area
covered by both greenhouses and tunnels has been experiencing continual growth. This
growth is expected to appear in many countries where protected cultivation replaces the
traditionally used more expensive glass-clad greenhouses.
Low-density polyethylene (LDPE), ethylene-vinyl acetate (EVA) and linear low-density
polyethylene (LLDPE) films are the most common greenhouse covering films in agriculture.
This chapter looks at the production of polyethylene-based plastic films for protected
cultivation. The mechanical properties that make these films suitable for the use in agriculture
are discussed. The stability of these plastic films under the effects of different environmental
conditions is reported. These include solar irradiation, temperature, humidity, wind, fog
formation and pesticides. Types of ultraviolet (UV) stabilisers and a determination of their
compatibility are given. Also, the recycling of plastic films used in agriculture is of great
importance, and a case study of their recycling as agricultural films is given.
10.2 Production of Plastic Films
LDPE films dominate the market for protected cultivation in the countries of both the
Mediterranean region and worldwide. Most of these contain special additives, which are
used either to enhance the performance of the film in the special conditions met in a
greenhouse, or to prolong its lifetime by minimising the effects of the environment on
the structure of the film.
Advances in the formulation of the LDPE films in use today have led to an expected
lifetime of between one and five cultivating seasons [2]. The expected lifetime is, in fact,
significantly affected by the environmental conditions that the film will face during its
use. The climate of the region, the greenhouse design, the microclimate developing inside
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Handbook of Plastic Films
the greenhouse, the use of agrochemicals and the environmental pollution of the area
can all severely affect the lifetime of the material by inducing ageing of the plastic film to
various degrees. Thus, a film whose lifetime is estimated to be four seasons in NorthCentral Europe will only last two or three seasons in the Mediterranean [2].
The varying requirements for greenhouse systems between different regions because of
different climatic conditions and differences in production methods has led, until very
recently, to a significant variation in approaches, standards and practices adopted or
implemented by the interested National Research Institutions, Commercial Agencies and
relevant industries [3]. Some of the consequences of this differentiation and variability
are reflected in the lack of standardisation concerning the testing methods for covering
materials of greenhouses. Usually, the testing methods used for plastics in general are
also applied to greenhouse covers, despite important functional differences. As a
consequence, quality control data provided by the producers of the covering materials of
greenhouses are usually limited to only a few properties of these materials. In most cases,
it is not possible to reproduce the relevant technical information provided, as this is not
obtained systematically and is available in a somewhat confusing way [3]. The
manufacturing of plastic films used in agriculture is usually carried out by blown film
extrusion (tubular extrusion). Readers are asked to consult Chapters 1 and 2, where the
manufacturing process is given in detail.
10.3 Characteristics of Plastic Films Used in Agriculture
The film products of interest here have been evaluated for their applied efficiency on the
basis of their characteristics and requirements related to mechanical resistance, total
percentage light transmittance (T %) to visible solar and long-wavelength ultraviolet
(UV-A) radiation, useful lifetime and energy-saving potential (greenhouse effect). Visible
solar radiation regulates the nutrition of plants through the ‘chlorophyll function’. Longwavelength ultraviolet radiation favours the formation of pigments and vitamins, which
is advantageous for the quality characteristics of the crop involved, with regard to flavour,
intensity of colour, perfume or smell and good keeping of fruit or vegetables.
With reference to energy saving, values of the total thermal transmittance measured in
W/m2 °C for the different covering materials and their possible combinations have been
estimated. The total thermal transmittance of a covering material indicates the general
heat loss (management, convection, radiation), estimated in watts, through a 1 m2 surface
referred to a difference of 1 °C between the internal and external environmental
temperatures of the prepared covering. These values enable estimation of the theoretical
thermal yield of a manufactured film related to the heating needs for a thermal difference
of 1 °C across the film.
Applications of Plastic Films in Agriculture
The incident heat calculation enables the agricultural operator to choose the proper
covering material with respect to any thermal (°C) and luminous (flux) needs of the
species to be established in the agricultural crop rotation desired. This can be done by
using the results showing the total thermal transmittance (W/m2 °C) and total light
transmittance (T %) of some materials for greenhouse covering [4].
10.4 Stability of Greenhouse Films to Solar Irradiation
The performance and lifetime of the plastic films used as covering materials in protected
cultivation depend strongly on: (a) the original chemical structure of the materials, (b)
the change in the properties of the material brought about by induced ageing, (c) the
type of physical structure used, (d) the climatic conditions of the area where the structure
is installed, and (e) the use of agrochemicals, among other things. A brief description of
the factors affecting the stability of polyethylene (PE) as a greenhouse covering under the
effect of different environmental conditions is given below.
It is well known that photodegradation of many plastic materials occurs on subjecting
these materials to solar radiation with wavelengths of 290-1400 nm [5, 6], the most
energetic part of the solar spectrum. UV radiation in the range 290-400 nm can be
absorbed by the plastic, and this is followed by bond cleavage and depolymerisation,
causing photodegradation.
The photodegradation process of the covering materials of a greenhouse is further
complicated by various interacting factors. The effect of UV radiation combined with
varying temperature, humidity, critical mechanical loads, friction, abrasion, exposure to
agrochemicals, etc., accelerate ageing at various rates. Accordingly, it is difficult to predict
the lifetime of plastic films by laboratory testing of the photostability of films. For instance,
high abrasion of the film by sand or soil particles carried by the wind leads to the formation
of high concentrations of active centres giving rise to an increase in photodegradation.
10.4.1 Ultraviolet Stabilisers
Theoretically, LDPE should be stable under the effect of UV due to its stable structure
and the absence of chromophores. However, during processing, it suffers partial oxidation,
in which carbonyl and hydroxyl groups are formed. Also, it contains some impurities
(photo-absorbing chromophores). Both impart photosensitivity to LDPE films [7, 8].
Special measures are therefore needed in order to protect greenhouse films against solar
radiation and especially its most energetic and therefore harmful portion, namely, UV
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Handbook of Plastic Films
radiation. Inhibition or at least retardation of the reaction responsible for degradation
is, of course, a necessity for successful UV stabilisation. Retardation or protection against
photodegradation can take place by using additives. These additives may retard the
photodegradation of the polymer in three ways, namely, ultraviolet screening, ultraviolet
absorption and excited-state quenching. Thus, stabilisers are often included in the polymer
to provide stability against photooxidation to protect the material from UV light damage.
The effectiveness of a light stabiliser depends on many factors, including its solubility
and concentration in the polymer matrix [9].
10.4.1.1 Ultraviolet Screening
Ultraviolet screening compounds are based on inorganic or organic additives. In this
type of protection, the ultraviolet light is blocked before it can reach the polymer. Screening
is provided by pigments or by reflective coatings. Carbon black is also very effective and
is used to stabilise many outdoor grades of polymers. In this case of UV screeners, any
damage is confined to surface regions because UV penetration is restricted to very short
distances. However, many of the pigments, like chalk, talc, short glass fibres and carbon
black, impart an unattractive appearance, the grey, brown and black colours generally
being unappealing. TiO2 is another common additive, which may act as a screener, but it
may occur in different forms, some of which are chemically active and can promote
photodegradation.
The first class of organic additives for improving the resistance to UV radiation is the UV
absorbers. They act by absorbing the harmful UV radiation above 290 nm, and thus do
not allow it to reach the chromophores present in the chemical structure of LDPE as a
result of processing or as impurities.
Many organic compounds absorb light in the desired region but few act as stabilisers.
Some have little or no effect when added to polymers and may actually increase the rate
of degradation. For a UV absorber to be effective, it must be able to dispose of its excitation
energy without interacting with the polymer in harmful ways and without undergoing
any photochemical reaction that would destroy its effectiveness. Accordingly, a stabiliser
must have a structure that provides a rapid cascade back to the ground state through
thermally excited levels with a quantitative efficiency for return to the ground state not
less than 0.999%, i.e., less than one molecule can be destroyed for every 100,000 molecules
that are excited.
Derivatives of o-hydroxybenzophenone or benzotriazole are examples of UV absorbers.
However, this class of stabilisers seems to perform better in thicker materials and not
well in the thin LDPE greenhouse films [8].
Applications of Plastic Films in Agriculture
10.4.1.2 Excited-State Quenchers
The second class of UV stabilisers is the nickel excited-state quenchers. These quenchers
act by deactivating the excited states of the chromophoric groups responsible for the
photo-initiation by energy transfer, instead of relying on direct absorption of the UV
radiation [8]. With proper selection of the Ni quenchers, the results of the UV stabilisation
are satisfactory. A typical example of a nickel excited-state quencher is nickel
dibutyldithiocarbamate. However, formulations containing such Ni quenchers are
prohibited because of the environmental impact of nickel compounds.
10.4.1.3 Hindered-Amine Light Stabilisers (HALS)
HALS, based on bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, are the most recent and
innovative class of light stabilisers. HALS do not absorb any light above 250 nm and so
cannot be regarded as UV absorbers or as excited-state quenchers. Through oxidation of
the piperidyl group to the nitrosyl group, a radical becomes available that starts a very
efficient cycle of radical scavenging and peroxide decomposition. Thus, HALS are
converted to the corresponding nitroxyl radicals, which are the real species responsible
for polymer stabilisation. Hindered nitroxyl radicals are effective chain breaking
antioxidants that act by trapping alkyl radicals to give hydroxylamines and/or
alkylhydroxylamines – the former regenerates nitroxyl. The overall high efficiency of
HALS as UV stabilisers in polyolefins is attributed to the regeneration of the nitroxyl
radical. The complementary nature of the chain breaking antioxidant mechanisms involved
[10] are shown in Scheme 10.1. From these reactions, nitroxyl and alkoxyl radicals are
formed according to equations (10.1) and (10.3)-(10.5). These radicals act as scavengers
for any radicals formed during UV irradiation [equation (10.2)].
This means that HALS operate as excellent antioxidants. Some of the HALS contain
further antioxidant groups; others are polymeric and less extractable. The main difference
between UV absorbers and HALS is that the former absorb UV radiation and in turn are
destroyed by it, while the latter do not absorb UV radiation and are much more slowly
altered by secondary side reactions.
Thus HALS act as radical traps for radicals produced from photochemical oxidation [8,
9]. They offer an excellent approach to ultraviolet stabilisation and have replaced nickel
quenchers and ultraviolet absorbers in many applications. Highly efficient chemically
resistant light stabiliser systems have been developed. Market demands for extended-life
greenhouses and thinner mulch films require even more powerful stabilisers. New noninteracting chemistries based on alkoxylamine HALS will offer a new generation of
stabilisers for agricultural polyethylene films.
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Handbook of Plastic Films
hν, [O]
N
H
N
O• + P•
N
O
N
ΔH
O•
N
P + POO•
NH + POOH
N• + POO•
(10.1)
O
P
N
(10.2)
O• + POOP
(10.3)
N• + H2O + PO•
(10.4)
NO• + PO•
(10.5)
Scheme 10.1
10.4.2 Requirements for Stabiliser Efficiency
The effectiveness of long-time stabilisation depends not only on the chemical nature but
also on the rate of additive loss, which in turn depends on the compatibility of the
additive with the polymer and is controlled by its volatility, solubility and diffusion
coefficient.
10.4.2.1 Compatibility of the Additive
Compatibility is the main problem for light stabilisers, because light stabilisers are generally
used in concentrations up to 2%. Ideally, stabiliser molecules should be disposed singly
throughout the polymer matrix. This will not generally happen, but compatibility with
the polymer should be sufficient to prevent gross phase separation.
Applications of Plastic Films in Agriculture
Stabilisers are normally dissolved in the polymer melt at the processing temperature.
However, their solubility limit may be exceeded on cooling and this may lead to visually
observable blooming. These processes depend on the nature of the chosen substrate as
well as on its morphology.
Additive diffusion is primarily a consequence of the thermal motion of the polymer
chains above the glass transition temperature and of the related formation and
disappearance of free volumes. If the chains are flexible and move easily, only small
amounts of energy are necessary to move the polymer segments.
With increasing orientation of the polymer chains or on crosslinking and with increasing
crystallinity, the diffusion constant decreases. For this reason, the diffusion of additives
is faster in LDPE than in high-density polyethylene (HDPE). Numerous studies [11-14]
confirm that the diffusion behaviour of UV absorbers depends mainly on polymer structure
and morphology and to a minor extent on additive structure.
The solubility and compatibility of light stabilisers are particularly a problem when highly
polar light stabilisers are used for non-polar plastics such as polyolefin. However, even
in polyurethane, the compatibility of light stabilisers may become a problem.
10.4.2.2 Determination of Compatibility
Stabilisers such as antioxidants, metal deactivators and UV absorbers are added to
polymers to reduce degradation during the manufacturing process and throughout the
lifetime of the polymer products. In order to study the degradation of polymers or the
compatibility between additives and polymers, it is essential to have an analytical method
that can provide both identification and a quantitative measure of additives in the
polymers. Fourier transform infrared (FTIR) spectroscopy [15, 16], UV spectroscopy
[17], near-infrared reflectance gas chromatography, high-performance liquid
chromatography (HPLC) and differential scanning calorimetry (DSC) can all be used as
analytical tools for identification and determination of the concentration of dissolved
stabilisers and their homogeneous distribution. FTIR and UV spectroscopy are the most
important techniques used, as they can be applied directly to the sample without disturbing
the morphology in the solid state. In addition, it is possible to detect any degradation or
changes taking place at earlier stages due to the sensitivity of these tools. Furthermore,
the diffusion coefficient of additives can be estimated by using the disc-stacking technique
[18], where a disc doped with the additives is placed in the middle of a stack of undoped
discs. Diffusion is then allowed to take place at an appropriate temperature for an
appropriate time. Then spectroscopic measurements can be done on different discs to
evaluate the concentration of the additives in each disc. Accordingly, the diffusion
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coefficient can be determined by knowing both the thickness of the discs and the
concentration of the additive by using a characteristic absorption band.
The principle behind the use of near-infrared reflectance analysis is the measurement of
the light reflected by a sample when exposed to light in this region. The logarithm of the
inverse of this reflected light can be related to the concentration of a particular component
found in the sample. The obtained concentration value for different discs can also be
used for determination of either the diffusion coefficient or the solubility of the additive.
The same can be said for gas chromatography or HPLC. On the other hand, differential
scanning calorimetry can be used to determine the exothermic peak of the oxidation of
antioxidants, which allows direct determination of the concentration of the dissolved
antioxidants.
However, the commonly used methods for determination of compatibility do not give
adequate information about the molecular association of the stabilisers, but only
information about the volume concentration of the stabilisers in the polymeric matrix.
This means that it makes no difference whether the stabilisers are present as stacks of
molecules or as single molecules. Such aggregation will lead to lower efficiency of
stabilisers. Of course, FTIR can offer some information from the change in the
perturbation potential that results from polymer-stabiliser and stabiliser-stabiliser
molecular interactions.
However, this needs a very careful study of the sample, and to have references for
molecularly dispersed stabilisers as well as sophisticated calculation for the obtained
spectra. On the other hand, broad-band dielectric spectroscopy [19-22] can be applied
to investigate solubility and compatibility, as it offers an excellent possibility of detecting
the molecular reorientation of stabiliser molecules and segments simultaneously at the
same temperature. Accordingly, the degree of compatibility with most polymeric segments
can be evaluated, where detailed investigation of the molecular dynamics have been
carried out [19-22] for various additives having different shapes, sizes and polarities in
different polymeric matrices.
An empirical relation that determines the dependence of the relaxation frequency
differences between the cooperative process of the additive and the glass process of the
matrix (macro-Brownian cooperative reorientation of the segment associated with the
glass temperature) and the additive length has been given [21]:
Δ log fm = 4 log[(L/d) – 1]
(10.6)
where Δ log fm is the difference between log fm of the cooperative process of the additive
and log fm of the glass process of the matrix; L is the length of the additive; and d is the
Applications of Plastic Films in Agriculture
polymer inter-chain distance. This equation was found to be valid not only for the
relaxation process of an additive in a polymer but also for the δ relaxation process of
side-chain liquid-crystalline polymers and their additives. This equation implies that, if
the molecules are molecularly dispersed, and have a length not longer than 1.8 nm, they
must relax cooperatively with the cooperatively reorienting segments of the glass process
at the same relaxation frequency. However, in the case of Tinuvin P, a commercially
available UV stabiliser (a benzotriazole derivative), the difference in the relaxation
frequency maxima of the stabiliser peak and the glass process of the matrix, Δ log fm, is
greater than three decades of frequency. These results indicate that the reorientations of
the stabiliser are not coupled with the glass process of polystyrene segments, where the
stabiliser molecules can relax locally at higher frequencies, (i.e., faster by a factor of
1000). Furthermore, short additives can relax either cooperatively with the polymeric
segments at the same relaxation frequency as the segments, or locally at higher frequencies.
The ratio of the local contributions to the total relaxation strength (cooperative plus
local) of the additives depends on the size of the stabiliser.
The biodegradation of representative samples of available commercial photo(bio)degradable
polyethylene films was examined with respect to the rate and extent of degradation,
oxidation products and changes in molecular weight both during outdoor exposure and in
laboratory photo-ageing devices with different accelerating factors [23]. Although the rate
of photooxidation was found to depend on the type of degradation system used, all the
samples showed a rapid rate of carbonyl formation, with concomitant reduction in molecular
weight and mechanical properties on exposure to UV light. The photo-fragmented polymers
were shown to be much more hydrophilic in nature compared to the unoxidised analogues,
and photo-fragments of all samples were found to contain high levels of low molecular
weight (low molar mass) bioassimble carboxylic acids and esters.
The recycling behaviour of virgin polyolefins, both as homopolymers and as heterogeneous
polymer blends, which contained 10% of non-oxidised and photooxidised
photo(bio)degradable plastics, has been examined. It was found that the initial mechanical
performance of homogeneous blends was not greatly affected by the presence of nonoxidised degradable materials. However, blends containing degradable films that were
initially partially photooxidised had a much more detrimental effect on the properties of
the recycled blends during processing and weathering; the effect was minimal for
degradable polymers containing the iron-nickel dithiocarbamate system.
10.4.3 Evaluation of Laboratory and Outdoor Photooxidation
Laboratory and outdoor photooxidation of plastic films were evaluated using different
techniques [24]. Thus melt blown, biaxially oriented, unstabilised and stabilised LDPE
films with various thicknesses were exposed in two accelerated artificial weathering devices
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with xenon (Xenotest) and UV-B fluorescent tube (QUV Weatherometer) sources under
controlled temperature and humidity conditions. The structural changes during combined
photo- and thermal degradation have been studied using tensile tensiometric, IR
spectrophotometric and DSC methods. The effects of HALS and film thickness on the
time-dependent changes in elongation, carbonyl group concentration, crystallinity and
the onset temperature (Ton) of the post-fusion DSC oxidation exotherm have been
observed. Photooxidation is accompanied by increased crystallinity, which maximises as
mechanical properties start to deteriorate significantly and the rate of carbonyl group
formation increases. While there is a poor correlation between the reduction in mechanical
properties and increased carbonyl index values, the former correlates well with the DSCderived Ton values for unstabilised and stabilised films. This suggests that thermal analysis
may be used to detect the physicochemical changes occurring in exposed films more
effectively than other techniques such as IR.
However, many problems of premature film failure can occur during their use in
greenhouses, due to their interaction with the agrochemicals used. Both sulfur- and
chlorine-containing agrochemicals inhibit the functioning of HALS, and can have a very
detrimental effect on the life of greenhouse films [25, 26].
The concentration of HALS in LDPE covering films before and after exposure to natural
weathering and accelerated photooxidation conditions has been determined [27]. It has
been found that photostabiliser disappearance above 0.4% up to 600 days is mostly
probably the result of its physical loss during long photooxidation times under both
photooxidative conditions. On the contrary, photostabiliser disappearance in the initial
stage is due to chain scission and the consequent volatilisation and diffusion of these
fragments on the surface.
10.5 Other Factors Affecting the Stability of Greenhouse Films
10.5.1 Temperature
Cyclic temperature changes and the high temperatures developed at the metallic parts of
greenhouse constructions during hot and sunny days lead to increased degradation. One
can observe a lot of damage in the places where the plastic greenhouse films come into
contact with the metallic structural elements, especially when they are not painted. The
temperature at these contact points may reach up to 70 °C and more depending on climatic
conditions. In this case the diffusion of metal ions enhances the degradation process.
Metal particles, especially iron, may catalyse the decomposition of hydroperoxides formed
as a result of oxidation, leading to unnecessarily high rates of degradation. The degradation
Applications of Plastic Films in Agriculture
mechanism of PE films containing additives with metal ions at a simulated composting
temperature has been studied. The hydroperoxide concentration [POOH] in the films
was traced quantitatively by using iodometric potentiometric titration, and compared
with Fourier transform infrared spectrometry (FTIR). The results show that [POOH]
increases during the early stage of degradation, followed by a more or less flat maximum,
before it starts to decrease. At the same time, similar results are obtained by FTIR analysis.
It is also found that the rate laws for the carbonyl index and [POOH] increases seem
more complicated than an exponential-type increase in the early stage of oxidation [28].
Moreover, high temperatures lead to an increase in the rate of reaction for both
photooxidation and chemical oxidation by agrochemicals, and thus to higher degradation
rates. As mentioned before, HALS compounds act as radical traps, and consequently
they also act as heat stabilisers and minimise the effects of high temperature [29].
10.5.2 Humidity
Lower resistance to oxidation and enhancement of degradation occur as a result of
increased humidity as well as rainfall. This is due to the gradual washout of additives
that may bloom on to the surface of plastic films. Besides, the degradation of plastic
films may occur due to hydroxyl radicals or other reactive species generated as a result
of photolysis [30].
10.5.3 Wind
It has been suggested that tearing due to high winds can be a major problem in greenhouses.
Another problem connected with windy areas is the wind load. This load can impose
increased stress on plastic films and lead to premature failure of the film. Abrasion caused
by soil and other particles, which are carried by the wind and impinge on the surface of
the greenhouse film, may also be another problem.
10.5.4 Fog Formation
The term ‘fog’ is used to describe the condensation of water vapour in the form of small
discrete droplets on the surface of transparent plastic films. The physical conditions that
lead to this formation may be summarised as follows [31]:
(1) A fall in temperature of the inside surface of the film to below the dew point of the
enclosed air/water vapour mixture;
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Handbook of Plastic Films
(2) Cooling of the air near the film to a temperature at which it can no longer retain all
the water vapour, so that excess water condenses upon the film;
(3) The difference between the surface tension of condensed water and the critical wetting
tension of the film surface, which causes the water to condense as discrete droplets,
rather than as a continuous film.
A number of undesirable effects may result from fog formation in greenhouse films and
leads to the following:
(1) Light transmission will be reduced where the total internal reflection of incident light
occurs. Consequently, the rate of plant growth will reduce, crop maturity is delayed
and the crop yield decreases;
(2) Light and heat transmission may be focused on delicate plant tissues owing to water
droplets acting as lenses. This causes burning of the plants and crop spoilage.
To prevent fog formation, surface-active agents are usually added to PE during film
production. These compounds are incompatible with the polymer and subsequently
migrate to the film surface where they increase the critical wetting tension. The result is
reduction in contact angle between water and polymer surface, permitting the water to
spread into a more uniform layer [31].
10.5.5 Environmental Pollution
Atmospheric pollution, such as nitrogen oxides, sulfur dioxides, hydrocarbons and
particulates, can enhance the degradation of polymers [32] and must also be taken into
consideration. For instance, infrared studies have revealed that polyethylene reacts with
NO2 at elevated temperature and that chemical attack is observed even at 25 °C. Similarly,
SO2 is rather reactive, especially in the presence of UV radiation, which it readily absorbs
and forms triplet excited sulfur dioxide. This species is capable of abstracting hydrogen
from polymer chains, leading to the formation of macroradicals in the polymer structure,
which in turn can undergo further depolymerisation [33].
10.5.6 Effects of Pesticides
The use of agrochemicals in greenhouses severely affects polymer films [29]. The pesticides
used for the protection of the crop influence the degradation and lifetime of the films.
Usually pesticides have complicated formulations, and contain a number of compounds
besides the active component. They contain sulfur and halogen in their chemical structure.
It is a well-known fact that films are destroyed under the effect of pesticides.
Applications of Plastic Films in Agriculture
Pesticides react with the stabilisers present in the film, decreasing its effect or completely
destroying it. Experimental results clearly show that pesticides with sulfur-containing active
compounds enter into antagonistic interaction with the stabilisers. One simple explanation
[34] is that the interaction of the pesticide and the HALS compound prohibits the latter in
executing its effect. Some sulfur-containing compounds and organic halogenides initiate the
oxidation of PE and bring about rapid deterioration of its mechanical properties. The extent
of this negative effect depends on the molecular weight, dispersion, allotropic modification,
etc., of the elementary sulfur. The introduction of a UV absorber into film considerably
improves the lifetime and light stability of the film. Also, HALS-stabilised greenhouse films
were shown to last 33% longer than Ni-stabilised films in testing under real conditions.
However, the polymer materials used for greenhouse films are changing, and, in particular,
the use of blends is continuously increasing, like the use of additives. These additives are
used in relatively large amounts for different aims, like photooxidation resistance, antifogging, etc. Moreover, the films can absorb fertilisers and pesticides, which can
compromise the use of secondary materials coming from greenhouse covering films in
many applications. UV exposure gives rise to major modifications of the macromolecular
chains, with chain breaking, formation of oxygenated groups, possible formation of
branching and crosslinking, and so on [35-38]. Finally, the reprocessing operations can
induce further degradation due to the thermomechanical treatment in the melt [39-42].
10.6 Ageing Resistance of Greenhouse Films
10.6.1 Measurement of Ageing Factors
Evaluation of the stability and durability of greenhouse films is usually carried out using
laboratory equipment. There is no standard testing scheme for evaluating the degradation
of these properties when the plastic film is used as a greenhouse covering material. This
is due to the fact that there are several interconnected factors that can lead to the
degradation of the mechanical properties. These factors are usually difficult to realise in
the laboratory. Following up the changes that occur in the mechanical properties of
plastic films as a result of ageing is very important in order to throw some light on the
problem and to identify the conditions of the film. However, other properties of the film,
such as physical and chemical properties, are also affected by the degradation, e.g.,
abrasion directly affects light transmittance and also other mechanical properties.
Many research groups have paid attention to this problem and concentrated their efforts
on measuring the effects of various ageing factors on the degradation of plastic materials
[6, 43, 44]. Some of them are concerned with the very specific problem of ageing of
agricultural plastic film [24, 45].
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Handbook of Plastic Films
In principle, all parameters should be covered in order to be able to predict accurately
the performance and lifetime of the material under the particular circumstances that
would occur when the material is used in a greenhouse. The accelerated ageing process
consists of simulating in an intensified manner the most critical parameters that lead to
the degradation of plastics. Several accelerated ageing tests for plastic films have become
commercially available during the past decade. However, their relationship to real outdoor
ageing is extremely questionable. These tests are based on inducing artificial ageing in
the material through an intense UV source coupled with a day-night cycle and a water
spray cycle. Only a few studies have considered the use of pesticides or stress due to
wind loading combined with the effect of UV-induced ageing [25]. An empirical correlation
between the lifetime of films under artificial weathering and in the greenhouse situation
has been given [46]. This standard defines three climatic zones depending on the level of
solar radiation energy that they receive: 70-100, 100-130 or 130-160 kLy/yr, where the
kilolangley is given by 1 kLy = 4.184 kJ/cm2.
However, no special conditions – such as the contact between the film and the metal
parts of the greenhouse structure, and the application of pesticides that can strongly
influence the actual weathering of the plastic films – have been considered. Thus, the
artificial ageing tests can only provide a rough estimate of the actual behaviour of plastic
films when exposed to the real and complicated environmental factors that affect the
plastic during its use [47]. For this reason several researchers have studied the degradation
of plastic films under natural weathering conditions (outdoor tests) [48]. Only a limited
number of tests were performed in greenhouses by this group [48].
10.6.2 Changes in Chemical Structure
Changes in chemical structure resulting from plastic film ageing have been followed using
spectroscopic methods. FTIR is the most frequently used technique [47, 49]. It provides
information about the chemical structure of the macromolecules. For instance, when the
chains are oxidised, carbonyl and OH groups are formed. The additive concentrations and
their changes can also be detected by the same technique. The presence of parts of the
agrochemicals in the film can also be detected spectroscopically. Electron spin resonance
(ESR) is also used to detect the creation of free radicals during degradation [49].
Thermal analysis, such as DSC, is used to study the oxidation process as well as the
changes in the crystallinity of plastic films due to ageing [50].
Gel permeation chromatography (GPC) may be used to evaluate the changes in the
molecular weight and molecular weight distribution of the films [8]. The integrated area
between 1770 and 1690 cm–1 of the absorption band at 1734 cm–1 was used to determine
the concentration of Tinuvin 622 [poly(N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-
Applications of Plastic Films in Agriculture
hydroxypiperidyl succinate)] before and after film exposure. The oxidation degree, i.e.,
carbonyl index (CI), under different oxidation conditions was obtained by calculating
the carbonyl absorption at 1713 cm–1 from the FTIR spectra at various oxidation times
using the spectrum of the unoxidised starting material as a reference. All measured
absorbances should be normalised by the film thickness using the equation:
CI = (A1713/d) x 100
(10.7)
where A1713 is the measured absorbance at 1713 cm–1 at a certain exposure time, and d is
the film thickness in micrometres.
10.7 Recycling of Plastic Films in Agriculture
10.7.1 Introduction
The amount of plastic materials used in agriculture has been continually increasing.
Plastic materials are used for greenhouse covers, mulching, piping, packaging and other
applications. Films used for greenhouses can be considered as an easy source of materials
for recycling. Indeed, large amounts of film can be easily collected and, because of the
homogeneity of the polymers used for this application, the recycling operations can be
relatively easy. However, UV exposure gives rise to major modifications of the
macromolecular chains, with chain breaking, formation of oxygenated groups, possible
formation of branching and crosslinking, and so on [35, 36, 51, 52].
The recycling of post-consumer films for greenhouses is strongly dependent on the initial
structure of the plastic materials and on the processing conditions. Such films contain
small amounts of low molecular weight compounds probably coming from the
photooxidation of the PE molecules and from the absorption of fertiliser and pesticide
residues. The amount of these compounds is small, however, and does not prevent the
use of the recycled materials in many applications. The properties of the secondary
materials deteriorate with the number of extrusion steps, but especially with the increasing
extent of photooxidative degradation. However, the mechanical properties of the recycled
post-consumer film remain relatively good even after many extrusion passes, and such
film is useful for many applications [53].
10.7.2 Contamination by the Environment
Dow Chemicals have actively investigated the recycling of mulch film because the normal
practice of disposal by burning it on the fields is environmentally undesirable. The
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Handbook of Plastic Films
contamination levels in mulch film make its recycling particularly challenging. For
instance, soil contamination can be as high as 30-40%. Furthermore, the soil can contain
up to 3% iron, which is a polyethylene prodegradant [54]. In addition, it was found that
vegetable matter derived from harvested plants could not be removed during the washing
operations [54]. Other contaminants are fumigants, (e.g., methyl bromide), and the
oxidised fractions of LDPE resulting from photodegradation of the mulch film.
Recently effort has been focused on the recycling of LDPE mulch film and greenhouse
film, both of which contain pesticide residues. This area poses special problems that are
difficult to overcome. It has been discovered that organochlorine and organosulfur
pesticide residues can deactivate HALS. This deactivation is believed to occur as a result
of hydrolysis of the pesticides to acidic species that then react with the HALS. This has
implications with respect to the long-term outdoor stability of the recycled product.
During in-service use, PE can become badly degraded and can form low molecular weight
oxygenated products, (e.g., aldehydes, acids, ketones, waxes, etc.). These impurities can
lead to embrittlement of the recycled polymer because low molecular weight oxidised
fractions are segregated from the melt during crystallisation and concentrate at the
spherulite boundaries [55]. The resulting zone, rich in oxidised material, has very low
fracture toughness. Moreover, oxygenated degradation products of PE, such as carbonyl
groups, are active chromophores and can sensitise the reprocessed polymer to
photodegradation.
Plastic waste management, in general, is a global environmental problem. The management
of such waste may be through the famous 4R approach:
•
Reduction (of source material);
•
Reuse;
•
Recycling;
•
Recovery.
The recycling and reuse of plastic waste films generated from greenhouses can share in
solving the problem. The disposal of municipal solid waste has become an environmental
issue of growing concern [56]. It was determined that discarded plastics represent close
to 20% of municipal solid waste on a volume basis [57, 58]. This is due to the high
volume-to-weight ratio of polymeric materials.
The management of plastic waste follows the scheme:
•
Source reduction;
•
Recycling;
Applications of Plastic Films in Agriculture
•
Thermal reduction by incineration;
•
Land-filling.
The most feasible methods for developing countries are source reduction and recycling.
Source reduction is any measure that reduces the volume of plastic waste produced. This
is accomplished through material efficiency, i.e., reducing the quantity of plastic material
used to produce a particular item.
Recycling generally involves the collection of waste plastic materials for reprocessing
[59, 60]. Polyolefin blend technology is of critical importance to various applications,
including greenhouse films. For instance, the LLDPE/LDPE blend is characterised by
reduced haze and better bubble stability. One of the most common blends is LDPE/
ethylene-propylene-diene terpolymer (EPDM) with improved low-temperature flexibility,
rubbery properties, weathering resistance and high-temperature mechanical properties.
The addition of EVA to LDPE has been commercially utilised to improve environmental
stress cracking resistance, toughness, film tearing resistance, flexibility and optical
properties.
Both blending and coextrusion have been employed to deal with the problem of
agricultural plastic film waste. The main goal is to find a solution to the problem of
agricultural plastic waste from greenhouses by recycling and converting the waste into
products usable in the mulch and greenhouse film applications. The proposed solution is
based on the development of multilayer films consisting essentially of a top layer made
from virgin resin and a bottom layer consisting of a blend of recycled PE waste film
material in combination with virgin resin and other ingredients. Evaluation of greenhouse
plastic wastes revealed that it is possible to obtain useful transparent plastic films to be
reused with reduced cost [61].
Multilayer films for greenhouses are a current trend in the industry. LDPE films for the
top layer, stabilised with different concentrations (0.1, 1.0 and 2.5%) of UV quencher,
have been produced in the laboratory by blow extrusion. The effect of natural weathering
on the film properties was investigated over a period of 12 months [62]. Significant
changes in the mechanical properties were observed in the later stages of degradation.
Films stabilised with 0.1% stabiliser crumbled after 12 months of natural weathering,
whereas films with higher concentrations retained their mechanical properties. It is believed
that the inclusion of a UV stabiliser interferes with the crystallisation process and that
the stabiliser particles accumulate in the amorphous matrix. Degradation of the imperfect
crystalline region with its low oxygen permeability proceeds via crosslinking, whereas
chain scission predominates in the amorphous region with excess of oxygen. Films
stabilised with 2.5% UV quencher form a barrier against the transmission of UV radiation
and the bottom layers are less affected by UV radiation. Recycled material can, therefore,
be incorporated at high concentrations into these layers.
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Handbook of Plastic Films
Optimisation of the top layer was based on a fixed concentration of three thermoplastics,
i.e., 80% LDPE, 10% LLDPE and 5% EVA, the remaining 5% being a specially prepared
master batch of LDPE containing 25% UV and heat stabilisers. Different types of UV
stabilisers were taken in different concentrations. These are: Cyasorb 1084 [n-butylaminenickel-2,2′-thio-bis(4-tert-octyl phenolate)] and Chimassorb 81 [2-hydroxy-4-noctoxybenzophenone], acting as UV light absorbers; Chimassorb 944 LD [poly{6-(1,1,3,3tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl amino-hexamethylene-4-(2,2,2,6tetramethylpiperidyl)imine}] and Tinuvin 622 LD, acting as radical scavengers; an energy
transfer agent; and a peroxide decomposer. The data obtained show that the haziness of
all plastics films are within the range required for agricultural films.
It has also been found in the case study [62] that the utilisation of a single UV stabiliser
is less efficient than the utilisation of a two- or three-component UV stabiliser. Thus,
films containing three-component UV stabilisers in addition to a thermal stabiliser (Irganox
1076) can retain at least 94% and 81% of tensile strength and elongation at break,
respectively, after exposure to UV radiation for 600 h. The good resistance of these
plastic films can be attributed to the different mechanisms of action of the utilised
stabilisers. In other words, if the UV absorber Chimassorb 81 is added alone to the
plastic blend, the films retain about 63% of the original elongation; whereas in
combination with Cyasorb 1084, the retained elongation is increased to about 80%.
Consequently, these results indicate the necessity of using a combination of UV absorbers
and radical scavengers [62]. Furthermore, some plastic films of various compositions
were subjected to outdoor weathering tests in two different locations, Cairo and Upper
Egypt. The results obtained indicate that the unprotected films deteriorate completely
within three months, whereas the protected films can withstand almost one year without
a drastic decrease in mechanical properties.
References
1.
J.C. Garnaud in Proceedings of the 13th International Congress of CIPA, Verona,
Italy, 1994.
2.
P.A. Dilara and D. Briassoulis, Journal of Agricultural Engineering Research,
2000, 76, 309.
3.
D. Briassoulis, D. Waaijenberg, J. Gratraud and B. von Elsner, Journal of
Agricultural Engineering Research, 1997, 67, 1.
4.
L. Pacini, Plasticulture, 1999, 117, 25.
5.
M.B. Amin, S.H. Hamid and J.H. Khan, Journal of Polymer Engineering, 1995,
14, 253.
Applications of Plastic Films in Agriculture
6.
S.H. Hamid, A.G. Maadhah and M.B. Amin in Handbook of Polymer
Degradation, Eds., S.H. Hamid, A.G. Maadhah and M.B. Amin, Marcel Dekker,
New York, NY, USA, 1992, 219.
7.
J.F. Rabek, Polymer Photodegradation. Mechanisms and Experimental Methods,
Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994, 73.
8.
F. Gugumus in Developments in Polymer Stabilisation – I, Ed., G. Scott, Applied
Science, London, UK, 1979.
9.
P.P. Klemchuk in Polymer Stabilisation and Degradation, Ed., P.P. Klemchuk,
American Chemical Society, Washington, DC, USA, 1985, 1.
10. S. Al-Malaika, E.O. Omikorede and G. Scott, Journal of Applied Polymer
Science, 1987, 33, 703.
11. R.G. Hauserman and M. Johnson, Journal of Applied Polymer Science, 1976,
20, 2533.
12. M. Johnson and R.G. Hausermann, Journal of Applied Polymer Science, 1977,
21, 3457.
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Fibres and Textiles, York, UK, 1983, Paper No.18.
14. F. Gugumus, Kunststoffe, 1987, 77, 1065.
15. K. Moeller, T.O. Gevert and I. Jakubowicz, Proceedings of the International
Conference on Environmental Science, Mount Prospect, IL, USA, 1990, 635-640.
16. D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C.P. Peck and R.O. Carter, Polymer
Degradation and Stability, 1990, 28, 1, 39.
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Applied Polymer Science, 1994, 54, 1605.
18. J.Y. Moisan, European Polymer Journal, 1980, 16, 979.
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270, 219.
20. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 25.
21. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 17.
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Handbook of Plastic Films
22. A.A. Mansour, Ph.D. Thesis, University of Ulm, Germany, 1992.
23. S. Al-Malaika, S. Chohan, M. Coker, G. Scott, R. Arnaud, P. Dabin, A. Fauve
and J. LeMarie, Journal of Macromolecular Science A, Applied Chemistry, 1995,
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24. M.G. Liu, A.R. Horrocks and M.E. Hall, Polymer Degradation and Stability,
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25. P.C. Powell, Engineering Design Guides, 1979, 19, 1.
26. P. Desriac, Plasticulture, 1991, 89, 1, 9.
27. M. Scoponi, S. Cimmino and M. Kaci, Polymer, 2000, 41, 2, 7969.
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Maadhah and M.B. Amin, Marcel Dekker, New York, NY, USA, 1992, 411.
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1593.
31. ICI Europe, Surfactants, Report, Ciba Speciality, Everberg, Belgium, 1998.
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Washington, DC, USA, 1983, 291-307.
33. W. Schnabel in Polymer Degradation: Principles and Practical Applications,
Hanser International, New York, NY, USA, 1981.
34. E. Epacher and B. Pukanszky, Proceedings of Antec ’99, New York, NY, USA,
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35. F.P. La Mantia, Radiation Physics and Chemistry, 1984, 23, 699.
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47, 211.
37. M. Sebaa, C. Servens and J. Pouyet, Journal of Applied Polymer Science, 1993,
47, 1897.
Applications of Plastic Films in Agriculture
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Polymer Science, 1994, 53, 847.
39. M.K. Loultcheva, M. Proietto, N. Jilov and F.P. La Mantia, Polymer Degradation
and Stability, 1997, 57, 77.
40. A.T.P. Zahavich, B. Latto, E. Takacs and J. Vlachopoulos, Advances in Polymer
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41. M. Marrone and F.P. La Mantia, Polymer Recycling, 1996, 2, 17.
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47. M.R. Kamal and B. Huang in Handbook of Polymer Degradation, Eds., S.H.
Hamid, M.B. Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA,
1992, 127.
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G.E. Zoukov, Journal of Polymer Science, 1983, 21, 1017.
50. L. Peeva and S. Evtimova, European Polymer Journal, 1984, 20, 1049.
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47, 211.
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53, 847.
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M.P. Luda and M. Paci, Polymer Degradation and Stability, 2001, 72, 1, 141.
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Symposia, 1992, 57, 115.
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Sperling, American Chemical Society, Washington, DC, USA, 1986, 2-19.
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Degradation and Stability, 1998, 62, 1, 111.
11
Physicochemical Criteria for Estimating
the Efficiency of Burn Dressings
Klara Z. Gumargalieva and Gennady E. Zaikov
11.1 Introduction
The principal medical treatment of burns is the use of dressings, which often worsen
the effects of the injury. It is difficult to estimate the effectiveness of new burn dressings,
as their physicochemical properties are not usually presented in the literature. This
chapter is devoted to a discussion of this subject for the first time. The authors address
the complexity of physicochemical methods of analysis in order to create criteria for
efficient dressings for a burn wound surface. The characteristics typical of burn wounds
for which dressings are required are shown in Table 11.1.
Table 11.1 Characteristics of burn wounds [3]
Burn degree Image of damage
Physiological process
Burn depth
(mm)
I
Redness and oedema
(medium oedema)
Aseptic inflammatory
process
0
II
Sac formation
Aseptic inflammatory
process
0
III
Damage of skin cover,
exuding wound surface
Skin necrosis, tissue
necrosis
1-2
IV
Exuding wound surface
Full necrosis of tissues,
carbonisation of tissues
2-5
Based on theoretical and experimental data, it was found that the maximal sorptional
ability of a burn dressing is determined by the free volume of the dressing material
calculated from the value of the material density. Kinetic parameters were determined
from the sorption curves. These parameters help in predicting the behaviour of burn
dressings. Criteria for estimating the efficiency of first-aid burn dressings are then
formulated.
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Handbook of Plastic Films
11.2 Modern Surgical Burn Dressings
Dressings for wounds and burns must primarily be protective, sorptional and atraumatic. In
currently used dressings, these properties are provided by a multilayer structure or structural
modifications. Different classifications of dressings can be found in the literature: by material,
by construction or by function [1-3].
The dressings applied in the modern treatment of wounds and burns are subdivided into
three groups according to the material of the layer sorbing the wound exudate. The material
may be of animal origin, synthetic foamed polyurethane or of vegetable origin (Table 11.2).
11.2.1 Dressings Based on Materials of Animal Origin
Typical dressings in this group are collagen sponges. Besides hydrophilic properties,
collagen sponges provide higher sorption of liquid (in the range of 40-90 g/g) [1, 4-9].
The patent literature describes in detail the methods of obtaining collagen dressings for
wounds and burns in the form of sponges and felt [10-13] based on materials of animal
origin. Also, the materials used include that made from biological artificial leathers based
on lyophilised bodies and swine cutis, produced as plates 0.5-0.7 mm thick. However,
these materials possess lower sorptional capacity than collagen dressings.
Dressings called ‘cultivated cutis’ are also obtained from the epithelia of cells of the
patient himself [13]. The shortcoming of biological artificial leathers or bio-dressings is
their expense and, as a rule, their inability to retain their properties on storage.
11.2.2 Dressings Based on Synthetic Materials
The demands for inexpensive raw materials for the production of wound and burn dressings
has led to the production of materials based on synthetic polymers, particularly cellular
polyurethane [10, 14-18]. Cellular polyurethane intended for medical purposes is synthesised
using toluene diisocyanate and polyoxypropyleneglycol [19].
Dressings based on polyurethanes have a pore distribution of about 200-300 pores/cm2,
and allow the regulation of the number and size of pores in layers [20]. Dressings from this
group are prepared as a double layer; the density of the outside layer is high in order to
prevent liquid evaporation and penetration of microorganisms. In rare cases, these dressings
are homogeneous through their thickness.
The influence of the pore size on the sorption properties of polyurethane sponges has
been reported [21], where macroporous sponge with a pore size from 200 up to 2000
μm is completely nourished by exudate under pressure only. In this case, the size of the
pores should be of the order of several micrometres.
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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Table 11.2 Characteristics of dressings in brief
No. Name
Company
Country Structure
Composition
Group 1
1.
Collagen burn
dressing
‘Helitrex’
USA
Dressings uniform by
thickness, dense, pores of
0.01 mm size. It has
gauze cover
Collagen
2.
Collagen
sponge
‘Helitrex’
US A
Similar to No. 1, differs
by big radius of pores,
formed by fibril weaving
of cylindrical form
preferably
Collagen
3.
Collagen
dressing
‘Bayer’
Germany Friable dressing,
possesses rough porous
structure with pore/hole
sizes from 1.5 to 0.1 mm
Collagen
4.
Burn curative
dressing
‘Combutec-2’
USA
Dressing of large-porous
structure with pore size
from 1 to 0.05 mm. Pores
are of cylindrical form
preferably formed by
fibril weaving of collagen
Collagen
5.
Biological
dressing
‘Corretium-2’
US A
Dense, pressed plate.
Fibrillar structure is
observed in dense layers
Collagen
6.
Biological
dressing
‘Corretium-3’
USA
The same as No. 5
Collagen
Group 2
7.
Compositional
burn dressing
‘Biobrant’
USA
Double-layered elastic,
porous dressing, consists
of the upper layer of 0.010.005 mm and flexible
fabric Nylon base. It
represents combination of
hydrophilic components
with elastic silicon films
Silicon, the
main layer,
base made
from
polyamide
8.
Synthetic
dressing
‘Epigard’
USA
Double-layered elastic
porous dressing. Upper
layer is dense, nonporous 0.2 mm thick
The main layer
made from
polyurethane,
the upper one
made from
polypropylene
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Handbook of Plastic Films
Table 11.2 Characteristics of dressings in brief continued
No. Name
Company
Country Structure
Composition
9.
Synthetic burn
dressing
10.
‘Syncrite’
ChSSR
Single-layered dressing on
gauze base with through
large pores, medium
flexible
Polyurethane
Synthetic
‘Syspurderm’
wound dressing
Germany
Dressings of homogeneous Polyurethane
composition, with different
pore distribution: upper
layer is 0.1 mm thick,
possesses small porous
structure with pores of 0.01
mm size; lower layer,
adjoining wound possesses
large pores of 0.05 mm size.
The dressing is ‘elastic’,
accepts a form badly
11.
Synthetic
‘Farmexplant’
wound dressing
PB R
Antiseptic double-layered
dressing. Main
polyurethane layer
possesses pores of 0.1-1.5
mm. Upper layer is 0.1
mm thick, more dense,
non-porous
Polyurethane
12.
Atraumatic
caproic dressing
USSR
Large-cellular dressing on
basis of woven Nylon
Polyamide
USSR
Wound large-cellular
dressing, homogeneous by
its composition
Alginic acid
salts
USSR
Porous cotton balling
dressing with atraumatic
layer
Cellulose
Group 3
13.
Cover for
wound, burns
14.
Needle-pierced
fabric
15.
Wound nonadhering
dressing
288
VNII
medpolymer
‘Algipor’
‘Bayersdorf’
Germany Three-layered dressing of
plaster type with tricot
lower layer. Dressing is of
the sandwich type: upper
layer is crepe paper, main
part is cotton balling, lower
layer is tricot network.
Atraumatic action is
provided by the effect of
dressing ‘bending’
(tunnelling effect)
Main and
upper layers
made from
cellulose,
lower one is
a film of
Dacron or
Nylon
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Table 11.2 Characteristics of dressings in brief continued
No. Name
Company
Country Structure
Composition
16.
Wound
absorbing
dressing
‘Johnson-Joh- USA
nson’
17.
Haemostatic
18.
Wound
dressing
19.
Surgical
dressing
20.
Dressing with
perforated
metallised
layer
Germany First aid dressing with
hydrophobic layer and
lower metallised layer.
Internal layers represent
non-fabric pressed layer
of crepe paper
Main layer
made from
cellulose,
lower layer is
aluminium
spray-coated
21.
Dressing lower
layer is not
metallised
Germany Similar to No. 20
Cellulose with
spray-coated
lower layer
22.
Non-adhesive
dressing
ChSSR
Dense cotton balling
dressing, lower and
upper layers are nonfixed Nylon networks
Cellulose and
polyamide
23.
Series of
experimental
dressings with
various
quantitative
viscose-cotton
composition
USSR
Cellulose or viscose
dressings with
atraumatic layer
Three-layered dressing
with perforated lower
and upper layers 0.01
mm thick, main part is
cotton balling, porous
Main layer
made from
cellulose,
external layers
made from
polypropylene
Sweden
Double-layered dressing
with perforated lower
layer, sewn to the main
layer
Viscose main
layer,
atraumatic
one made
from
polyethylene
‘Mesorb’
France
Cotton balling or viscose
dressing, crepe paper lower and upper layers
Cellulose
‘Kendall’
US A
Similar to No. 16 with
cellulose base and
atraumatic synthetic
lower layer
Cellulose
‘Switin’
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Handbook of Plastic Films
Apart from polyurethanes, other polymeric materials (polyvinyl chloride, Nylon, etc.)
have been used as the sorbing layer [22-25]. This group includes a compositional burn
dressing based on a silicon film, polyamide network and hydrophilic admixture, produced
by Hall Woodroof Co., (USA) [13]. Polyurethane coverings with an atraumatic lower
layer made from polyglycolic acid may be considered as a variety of compositional
dressings [26]. It is characteristic of dressings from this group that they preserve their
high strength properties even after absorption of wound exudate.
A two-component protective dressing ‘Hydron’ was recently applied in the treatment of
burns. It is a film formed on the wound, and consists of a powder of poly(2-hydroxyethyl
methacrylate) dissolved in polyethyleneglycol 400 [2, 27]. Although they possess good
protective properties, ‘Hydron’ dressings have low strength and sorptional capacity.
11.2.3 Dressings Based on Materials of Vegetable Origin
A large number of burn dressings, the so-called ‘cotton balling’, are based on cellulose,
viscose or a combination of the two [28-32]. These dressings differ from each other by
structure and composition of the upper and lower layers. Most often, a sorption layer
based on cellulose is used in complex dressings. Such dressings are usually layered, with
the separate layers being produced from either the same or different materials; the layers
may be fixed mechanically or by using thermoplastic material. To decrease their adhesion
to the wound surface, the lower layer is produced from various fabric and non-fabric
materials (perforated Dacron, polypropylene, pressed paper, metallised fabric material,
etc.). The total sorptional ability of these dressings is defined by the hydrophilicity and
porosity of the basic material and is usually equal to 15-25 g/g.
Data on the action of wound and burn dressings based on another vegetable material –
derivatives of alginic acid – have been reported [1, 33, 34]. Typical ‘Algipor’ specimens
used are based on the mixed sodium-calcium salts of alginic acid as spongy plates of
about 10 mm thickness with high absorption ability.
11.3 Selection of the Properties of Tested Burn Dressings
The data from the literature showed that burn dressings, particularly the first-aid ones,
must perform three main functions [1, 2, 35, 36]:
(1) Absorb the wound exudate, which contains metabolic products and toxins;
(2) Provide optimum water, air and heat exchange between the wound and the atmosphere;
(3) Protect the wound from the penetration of microorganisms from the air.
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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Moreover, the burn dressing must be removable from the wound without further injury
to the patient. Therefore, the following properties of burn dressings have been studied to
determine their efficiency.
11.3.1 Sorption-Diffusion Properties
The sorption-diffusion properties of dressings are extremely important, because they
determine the performance of the three main functions of dressings just mentioned.
11.3.1.1 Water absorption
Water is the main component of the exudate from wounds. At present, there is no opinion
on how fast and to what degree the dressing must absorb the exudate in order to clean
the wound from toxins and metabolic products while at the same time keeping the wound
wet enough to prevent the removal of water from healthy tissue [1, 2, 35, 36].
11.3.1.2 Air penetrability
Sufficient air must be allowed to penetrate the dressing, since an increase of oxygen
concentration helps the healing process.
11.3.1.3 Vapour penetrability
Vapour penetrability of the skin of a healthy man may reach 0.5 mg cm–2 h–1 [37]. Water
loss by evaporation from burns is even higher (Table 11.3). In the absence of technical
data, it may be concluded that high vapour penetrability will lead to ‘drying’ of the
dressing, with a corresponding change in the surface energy of the dressing-wound
Table 11.3 Water losses by evaporation from different types of burns
Surface type
Evaporation (cm3/cm2-h)
Natural skin
1-2
First degree burn
1-2.5
Second degree burn with blisters intact
2.8
Second degree burn with no damage of fermentative layer
37
Third and fourth degree burns
20-31
291
Handbook of Plastic Films
interface. This will promote undesirable removal of water from the tissues, and may
cause the dressing to come off the wound. Low vapour penetrability of the dressing will
lead to the accumulation of liquid under the dressing, which may cause oedema.
11.3.1.4 Microorganism Penetrability
Penetration of microorganisms through the dressing must be blocked to prevent infection.
11.3.2 Adhesive Properties
The adhesive properties of dressings determine their ability to stay attached to the wound.
Thus, the surface energy of the dressing surface facing the wound must always be lower
than that of the wound surface.
11.3.3 Mechanical Properties
Two mechanical properties are important for dressings: (a) flexural rigidity and (b) strength
at break. The former defines the ability of the dressing to mould to the wound profile;
the latter is important since it allows the dressing to be removed from the wound
completely without breaking.
11.4 Methods of Investigation of Physicochemical Properties of
Burn Dressings
11.4.1 Determination of Material Porosity
The porosity of materials (the relation of pore space volume to total volume) is determined
by the following two methods.
(1) By measuring the density, and then using:
l pores
1
l
=
+ mat
P∑ l ∑ Ppores l ∑ Pmat
(11.1)
where Q is the material porosity, ρ is the observable density, and ρ0 is the density of
the material forming the porous medium. The value of ρ is determined by weighing
292
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
a sample of known geometrical size. The value of ρ0 is determined similarly for
samples pressed at 500 GPa.
(2) From photos obtained by a light microscope (MIN-10) we get:
⎡ S pores ⎤
Q=⎢
⎥
⎢⎣ S0 ⎥⎦
3/ 2
(11.2)
where Spores and S0 are total surface area of pores and general surface area of the
material in the field of vision of the microscope, respectively.
11.4.2 Determination of Size and Number of Pores
The number and size of the pores are determined with the help of the MIN-10 microscope
in reflected light. The pore distribution curve (number of pores as a function of radius) is
calculated; typical results are given in Figure 11.1.
Figure 11.1 Typical curves of pore size distribution for various burn dressing materials.
1: Farmexplant; 2: Syncrite; 3: Bayer brown collagen dressing; 4: Syspurderm
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Handbook of Plastic Films
11.4.3 Estimation of Surface Energy at Material-Medium Interface
The surface energy of a material-medium interface is estimated using the wetting angle
of the material surface by the medium. A drop of liquid is applied to the surface of the
material, and the angle is measured between the tangent at the base of the drop and the
material surface. The wetting angle is determined using a horizontal microscope. The
accuracy of angle measurement does not exceed ±1°.
11.4.4 Determination of Sorptional Ability of Materials
The total amount of liquid sorbed by a ‘tiled’ material includes the liquid in macropores
with size over 0.1 μm, that is micropores with size smaller than 0.1 μm, and that in the
material matrix itself (dissolved liquid). The amount of dissolved liquid, and of liquid
filling the micropores, is calculated from the vapour pressure of the sorbed liquid over
the sample (sorption isotherms).
The sorption isotherm for a material with micropores possesses an S-type form (Figure 11.2).
The first part of the curve is connected with the real dissolved liquid, and the second part
with the condensed liquid in micropores.
Figure 11.2 A typical sorption isotherm of a low molecular weight liquid by a
microporous material: part ➀ of the curve represents real dissolving of the liquid by
the material, and ➁ represents condensation of the liquid in the micropores within the
material. Here Δm is the (change in) sorbed liquid mass; and P/P0 is the relative
pressure of liquid in the thermostated vessel (where P0 is the saturated vapour pressure
of the liquid under the conditions used)
294
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Figure 11.3 A schematic diagram of the device for determining the absorbtion
ability of porous materials. 1: sample; 2: perforated plate; 3: thermostated bath;
4: float
The maximal sorption (the amount of liquid, really dissolved and filling micro- and
macropores) is determined using the device shown in Figure 11.3. The device represents
a vessel with liquid medium, in which a float of special perforated square construction is
placed. The float construction is calculated to prevent its sinking. This requires that the
liquid medium does not penetrate through the perforations of the square, but instead
forms a meniscus on the side of the square facing the porous interlayer. The change of
the mass of the porous material is determined from the immersion of the float with the
sample. It is measured using a horizontal microscope.
11.4.5 Determination of Air Penetrability of Burn Dressings
Air penetrability (the volume of air that passes through a specific surface area during a
specific time) was determined using a device specially designed for this purpose. The
device is a cylindrical cell with perforated plate supporting the sample (Figure 11.4). The
air was passed through the cell with the help of an air compressor, equipped with a
manometer and pressure controller. The time required to fill a polyethylene sack (45
litres in volume) with air was measured.
A round form sample was prepared. The sample was then placed on the perforated plate
of the cell. The compressed air passed through the cell pressed on the sample. The time
taken for the polyethylene sack to fill was measured. The method allows determination
of the air penetrability of dry or wet materials.
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Handbook of Plastic Films
Figure 11.4 A schematic diagram of the device for determining the air penetrability of
porous materials. 1: sample; 2: perforated plate; 3: polyethylene sack; 4: manometer;
5: pressure controller
11.4.6 Determination of Adhesion of Burn Dressings
Adhesion of burn dressings was investigated on a modified form of a device previously
described [38]. Thus, a 1 mm thick fibreglass plate, covered with three layers of medical
gauze, was placed into a fibreglass cell having a working surface of 3 × 10 mm2. The cell
was filled with 5 ml of whole blood and 1 ml of 2% thrombin. The dressing to be tested
was then placed on the plate surface for 1 min. The cell containing the sample was
placed into a thermostat at 37 °C for 24 hours. Sample removal was performed at a 90°
angle to the surface of the tested material.
11.4.7 Determination of Vapour Penetrability of Burn Dressings
Vapour penetrability (the mass of water that passes through a specific surface area during
a specific time) was determined using a device described elsewhere [39]. A glass vessel
was filled with a known amount of liquid, e.g., water or aqueous solution of sulfuric
acid. This amount provided a known relative humidity. The investigated sample was
placed on the vessel surface; and a metal ring was set and pressed to the vessel by a
special clamp. The vessel with the contents was weighed and placed into desiccator with
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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
dryer at 37 °C. After measured time periods the vessel was taken out from the desiccator,
weighed and then put back in the desiccator. The amount of water that has passed through
the sample was determined by the mass loss of the vessel contents. The vessel dimensions
used in the experiments were 40 mm diameter and 20 mm height.
11.5 Results and Discussion
11.5.1 Determination of Sorption Ability of Burn Dressings
On applying a dressing to a burn wound, first wetting of the surface layer of the material
occurs, followed by sorption of the wound exudate into the dressing volume. Thus it is
necessary: (1) to know the components of the burn wound exudate, which need to be
sorbed by the material, and the way in which sorption occurs; and (2) to determine the
maximum sorption of the separate components of the exudate by the dressing material.
With respect to the second item, the maximum water sorption of different materials has
been determined previously [40]. For this purpose, the sample was immersed in water,
dried rapidly using filter paper and then weighed. However, this method did not allow
the sorption kinetics to be measured, and the accuracy of the maximum sorption was
low. That is why we have developed the device for continuous measurement of sorption.
The first item mentioned above has not yet been addressed in the published literature.
The exudate from wounds contains water, salts, proteins, damaged cells and various low
and high molecular weight (low and high molar mass) substances in relatively lower
amounts. Table 11.4 shows the approximate composition of oedema liquid in a burn
wound. The composition of oedema liquid changes depending on the burn degree: the
worse the burn, the higher the content of protein and the lower the albumin/globulin
ratio [3]. Similar data for blood plasma are also shown for comparison in Table 11.4.
Table 11.4 Composition of oedema liquid and blood plasma (g/cm3)
Components
Oedema liquid
Blood plasma
Urea
5.1 × 10
5.5 × 10-4
Sugar
5.8 × 10-6
11.0 × 10-6
Protein
3.4 × 10-2
7.2 × 10-2
Salts
1.0 × 10-2
1.0 × 10-2
3. 9
1.5
Albumin/Globulin
-4
297
Handbook of Plastic Films
Sorption of wound exudate may proceed via filling of micro- and macropores, or dissolving
in the material matrix. Let us consider the sorption of the various components of the
wound exudate by the dressing material.
(1) Water fills pores and dissolves in the material matrix. Water solubility is defined by
the material hydrophilicity. The solubility of water, salts and other low molecular
weight substances in polymers is subject to the following rules:
• In hydrophilic polymers, solubility is defined by the size and charge of the low
molecular weight substance;
• In hydrophobic polymers, solubility is defined by vapour pressure (the higher the
vapour pressure, the higher the solubility) [41].
(2) Protein fills pores up to 10–2 m in size and may dissolve only in hydrogels of ‘Hydron’
type with water content over 30% by mass.
(3) Cells fill only open pores over 0.1-0.2 μm in size.
11.5.1.1 Solubility of water in polymers
As mentioned previously, modern burn dressings are heterogeneous materials, usually
consisting of several layers. The upper one exposed to the air is usually more hydrophobic
and less porous than the others. The solubility of water in this layer will define its evaporation
from the dressing surface and the heat exchange between the wound and the surroundings.
Information about solubility of water in various polymers is reported in Table 11.5 [42].
The solubility of water was determined by the sorption method. Extreme values of sorption
at known water vapour pressures were calculated from the sorption curves, and then the
sorption isotherms were constructed using the method described elsewhere [43].
∞
Extreme values of solution φH
at the saturation pressure were determined by extrapolation
2O
∞
of φH 2O to P/Ps = 1. The value φH
of equals the solubility of water in the polymer.
2O
11.5.1.2 Maximum sorption ability of burn dressings
Modern burn dressings are heterogeneous materials that either have large pores or are
fibrillar, and they possess a high free volume. In contact with a wound, the exudate will
fill the free volume of the dressing. The degree of filling is defined by the hydrophilicity
of the material, and the size and geometry of the free volume fraction.
298
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Table 11.5 Solubility of water in various polymers
Solubility (102 g/g)
T (K)
Cellophane
40
303
Viscose fibre
46
303
Cotton
23
303
Cellulose diacetate
18
303
Cellulose triacetate
11.5
303
Polycaproamide
8.5
303
Polyethyleneterephthalate
0.3
303
Polydimethylsiloxane
0.07
308
Poly(2-oxyethylmethacrylate)
40*
310
Polypropylene
0.007
298
Polytetrafluoroethylene
0.01
293
Polyethylene (ρ = 0.923)
0.006
298
Polyurethane
1*
298
Polyvinyl chloride
1.5
307
Polymer
*Measured by the authors.
11.5.1.3 Maximum sorption of water by burn dressings
Sorption of water by burn dressings is measured using a device developed for this purpose
by the present authors. Experiments were performed in the following way. First, different
masses were placed on the perforated plate of the device, and the relative immersion of
the device into the water was measured, in units of the eyepiece graticule of the horizontal
microscope. A calibration curve was then drawn using the coordinates ‘mass’ versus
‘depth of immersion’. The slope coefficient of this calibration curve equals 0.70 ± 0.02 g/
unit.
Then, a sample of a dressing was placed into the device, and the depth of immersion
during time h was measured. The mass of the medium sorbed by the material was
calculated from the correlation:
mc – 0.70h
(11.3)
The extreme value of the mass of sorbed medium was determined at t → ∞.
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Handbook of Plastic Films
Table 11.6 shows the experimental and theoretical [calculated from equation (11.3)]
values of CH∞ 2O , and values of ρ0 determined experimentally and used for the theoretical
calculations. A good correlation was observed between the experimental and theoretical
values of CH∞ 2O for the majority of dressings. This shows that practically the entire free
volume is filled by liquid medium for the contact of dressings with water.
Table 11.6 Experimental and theoretical data of the maximum sorption of
water by burn dressings
∞
CH
2O
Covering name (material)
ρ0 (g/cm3)
Experimental
Theoretical
Helitrex (collagen)
32 ± 2
0.030 ± 0.007
33 ± 2
Helitrex (collagen sponge)
58 ± 3
0.018 ± 0.005
55 ± 3
Collagen dressing
1.8 ± 0.1
0.350 ± 0.07
2.8 ± 0.3
Corretium-2 (collagen)
3.5 ± 0.3
0.300 ± 0.07
3.3 ± 0.3
Corretium-3 (collagen)
2.1 ± 0.2
0.330 ± 0.05
3.0 ± 0.1
Combutec-2 (collagen)
77.0 ± 5.0
0.015 ± 0.005
66.0 ± 3.0
Epigard (foamy polyurethane
10.0 ± 0.3
0.067 ± 0.005
15.0 ± 1.0
Silicon-Nylon composite
7.5 ± 0.2
0.130 ± 0.03
7.7 ± 0.5
Syspurderm (foamy polyurethane)
6.2 ± 0.2
0.140 ± 0.03
7.1 ± 0.5
Syncrite (foamy polyurethane)
20.0 ± 2.0
0.050 ± 0.01
22.0 ± 1.5
Farmexplant (foamy polyurethane)
12.0 ± 0.5
0.064 ± 0.007
15.6 ± 3.0
Johnson & Johnson (cellulose)
11.4 ± 0.5
0.100 ± 0.03
10.0 ± 1.5
Blood-stopping (cellulose)
15.7 ± 0.9
0.080 ± 0.006
12.5 ± 0.7
Tunnelling (cellulose)
4.3 ± 0.2
0.200 ± 0.005
5.0 ± 0.4
Switin (cellulose)
18.0 ± 2.0
0.050 ± 0.005
20.0 ± 1.0
Metallised (cellulose-paper)
12.4 ± 0.7
0.100 ± 0.04
10.0 ± 0.5
Needle-perforated (cellulose-viscose)
28.0 ± 2.5
0.033 ± 0.007
30.0 ± 2.0
100% Viscose
25.0 ± 2.0
0.033 ± 0.007
30.0 ± 2.0
70% cotton + 30% viscose
31.0 ± 3.0
0.030 ± 0.007
33.3 ± 3.0
50% cotton + 50% viscose
25.0 ± 2.0
0.035 ± 0.007
28.5 ± 2.0
30% cotton + 70% viscose
28.0 ± 2.0
0.036 ± 0.007
27.7 ± 2.0
Algipor (vegetable)
30.0 ± 3.0
0.011 ±0.0002
90.0 ± 5.0
300
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
The exception is the ‘Algipor’ dressing, the large pores of which become denser on filling
with water because of the collapse of the pore walls. At the end this leads to the decrease
of the total volume of the dressing. The liquid medium may not fill the whole volume of
the dressing if the material is sufficiently hydrophobic and poorly wetted with water.
To test this assumption, seven collagen materials that differ in production method were
investigated for: density, maximal water sorption, wetting angle and heat of sorption of
water by the material. The latter was determined using a microcalorimeter (LKB 2107) as
follows: A sample of known mass was exposed to vacuum in a thermostated Butch-type cell,
and then an excess amount of water was introduced into the cell, causing the forced filling of
the material volume. The results obtained are presented in Table 11.7 and Figure 11.5.
Table 11.7 Density, maximal water sorption, wetting angle and heat of sorption
of water by different collagens
∞
CH
(g/g)
2O
ρ0
(g/cm3)
Experimental
0.011
Theoretical
ø
(deg)
ΔH
(cal/g)
74
91
170
34.6
0.016
53
62.5
70
25.4
0.013
49
77
90
30.2
0.013
47
77
110
31.9
0.013
8
77
120
31.2
0.014
4
71.4
110
29.8
0.014
30
71.4
50
27.2
Figure 11.5 The dependence of the maximum sorption of water on the heat of
sorption for various collagen materials
301
Handbook of Plastic Films
The following conclusions can be derived from the data presented in Table 11.7:
(1) The experimental value of CH∞ 2O is lower than the ‘theoretical’ one. This may be
explained by two reasons: the decrease of the total volume (as in the case of ‘Algipor’),
and the non-filling of a part of the material free volume by water.
(2) A satisfactory correlation exists between the theoretical values of CH∞ 2O and ΔH. Thus,
the main reason for the difference between the experimental and theoretical values of
∞
CH
is evidently the non-filling of a part of the material free volume by water.
2O
(3) The absence of a correlation between maximal water sorption and wetting angle, defined
on the external surface of the material, shows that the values obtained as mentioned above
do not reflect the real interaction of water with the internal surface of the collagen material.
∞
Figure 11.6 The dependence of C H
on the free volume of various burn dressing
2O
materials: 1, Helitrex collagen dressing; 2, Neutron collagen sponge; 3, Bayer brown
collagen dressing; 4 and 5, Corretium-2 and -3 artificial leathers; 6, Combutec-2; 7,
Epigard synthetic dressing; 8, Syspurderm foamy polyurethane dressing; 9, Syncrite
synthetic dressing; 10, Farmexplant foamy polyurethane dressing; 11, burn face mask;
12, Biobrant compositional dressing; 13, Johnson & Johnson cellulose dressing; 14,
Kendall cellulose dressing; 15, Torcatee non-adhesive cellulose dressing; 16, bloodstopping cellulose dressing; 17, Mesorb cotton balling dressing; 18, dressing with
tunnelling effect; 19, cellulose dressing with non-adhesive synthetic layer; 20, Switin
cotton balling dressing; 21 and 22, metallised dressings; 23 and 24, needle-perforated
fabric with atraumatic layer; 25, 100% viscose; 26 to 29, viscose-cotton balling
302
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Thus, it may be concluded that, for the majority of hydrophilic burn dressings, the
maximum sorption capacity with reference to water may be predicted satisfactorily. For
example, the experimental values of CH∞ 2O correlate well with the free volume part of the
materials (Figure 11.6), the correlation coefficient being 0.96.
11.5.1.4 Maximum sorption of plasma by burn dressings
Sorption of blood plasma by burn dressings was determined by a similar method. Plasma
was obtained by centrifugation of conserved blood. The treatment of the experimental
results was carried out similarly to the case of the investigation of the maximum sorption
∞
of water. The value of C plasma
differs from CH∞ 2O . The difference is not higher than 10%,
∞
are not presented in Table 11.7.
which is why data for C plasma
11.5.2 Kinetics of the Sorption of Liquid Media by Burn Dressings
The study of the kinetics of the sorption of wound exudate by burn dressings is of great
importance for the estimation of their efficiency. There are difficulties in the mathematical
description of the kinetics of the sorption process connected with the absence of a strictly
quantitative description of dressing structure.
11.5.2.1 Structure of burn dressings
Burn dressings are heterogeneous systems, consisting of several component phases. As
general attention in dressings must be paid to the material possessing the maximum
penetrability with reference to the liquid medium, it is necessary to classify the types of
heterogeneous systems. For example, the penetrable parts of the material are placed
under a layer of another weakly penetrable material in such a way that diffusing flow is
perpendicular to the surface layer. This is the case for double-layered dressings with a
dense external layer. The penetrable parts of the material can be dispersed in a continuous
weakly penetrable phase.
Dressings based on collagen and cellulose possess fibrillar structure and the fibres are
randomly placed. In some cases, spatial orientation of fibres is present. The number of
open pores in dressings of this type is large but the open pores possess irregular form and
great tortuosity in the direction of mass transfer. Modern burn dressings are multilayer
with a denser external layer. Table 11.8 shows the mean radius of macropores and their
number per unit area for dressings based on polyurethane.
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Handbook of Plastic Films
Table 11.8 Mean radius R of macropores and their number N per unit area for
dressings based on polyurethane
R (10–2 cm)
N (cm–2)
Epigard
2.2 ± 0.2
370 ± 10
Syspurderm
1.8 ± 0.2
266 ± 5
Syncrite
2.8 ± 0.2
275 ± 5
Farmexplant
2.2 ± 0.2
300 ± 10
Dressing name
Detailed analysis of a number of mathematical models and results of experimental
investigations of heterogeneous systems has been performed by Zaikov [44]. It is
known that the calculation of diffusion coefficients in heterogeneous systems is very
difficult. According to ideas accepted at the present time, the penetration of liquid
into a porous body is ruled by the laws of capillarity. These ideas have been successfully
applied to interpret the penetration of water into paper, leather, fabrics, etc. [45, 46].
An equation that takes into account the real structure of porous bodies was obtained
by Deriagin [47].
11.5.2.2 Kinetics of sorption
The kinetics of sorption of water and blood plasma was investigated using the device
for the maximal sorption of water. Figure 11.7 shows typical kinetic curves of sorption
of water and plasma by various dressings. All curves are satisfactorily described by
the equation:
1/ 2
⎡ Dt ⎤
mt
= 2⎢ 2 ⎥
m∞
⎣ πl ⎦
(11.4)
The following conclusions can be made from the data obtained:
(1) Burn dressings differ significantly in their rates of sorption of liquid media;
(2) The rate of sorption is determined by the pore size and the material hydrophilicity.
304
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Figure 11.7 Curves for the sorption of water and blood plasma by various burn
dressings. 1: water, and 2: plasma, by needle-perforated material; 3: water, and
4: plasma, by Syspurderm polyurethane dressing
11.5.3 Determination of Vapour Penetrability of Burn Dressings
With multilayer dressings, the external layer, which regulates the mass transfer of water
from the wound into the surroundings, is denser than the inner ones. The process of
mass transfer of water through the material layers is often called aqua-, water or vapour
penetrability.
Penetrability and diffusion of water in polymers has been the subject of numerous
investigations. The results given in some reviews and monographs [47, 48] are presented
in Table 11.9. The mass transfer of water molecules in polymers possesses a list of features.
In hydrophobic matrices, the interaction between water molecules and the material matrix
is weak (low solubility). Nevertheless, the interaction of water molecules with each other
stipulates a specific transfer mechanism.
In hydrophilic materials, the interaction between water molecules and the hydrophilic
groups of the material matrix stipulates high solubility of water in the matrix and
increased aqua-penetrability. Consequently, high aqua-penetrability may be a property
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Handbook of Plastic Films
Table 11.9 Penetrability and diffusion of water vapours in polymers [48]
Polymer
T (K)
p
p0
P × 1015
(mol m/m2 s Pa)
D × 1012
(m2/s)
Cellulose
298
1.0
8500
–
Regenerated cellulose
298
0.2
5700
0.1
Cellulose acetate
303
0.5 - 1.0
2000
1.7
Cellulose diacetate
298
1.0
15.7
–
Cellulose triacetate
298
1.0
5.5
–
Ethylcellulose
298
0.84
7950
18
Polydimethylorganosiloxane
308
0.2
14400
7000
Polyethylene (ρ= 0.922)
298
0 - 0.1
30
23
Polyethyleneterephthalate
298
0 - 0.1
58.6
0.39
Polypropylene
298
0 - 0.1
17
24
Polyvinyl chloride
303
–
–
2.3
Polycaproamide
298
0.5
134
0.097
of hydrophobic as well as of hydrophilic materials; however, the causes will be different.
For example, in hydrophilic polydimethylorganosiloxane, the high mobility of water
molecules is stipulated by the high mobility of the chain units in this polymer. That is
why, despite the low solubility of water in polydimethylorganosiloxane, the coefficient
of aqua-penetrability is significant.
In contrast, in regenerated cellulose, the diffusion coefficient is low because only the
dissolved water molecules, which are not connected with the matrix of this polymer,
participate in the mass transfer. In this case the high value of aqua-penetrability is
stipulated by increasing the dissolved water content in the regenerated cellulose, which
increases the fraction of the water molecules participating in mass transfer. This in
turn leads to the increase of both the diffusion coefficient and the penetrability
coefficient.
The mass transfer of water through a porous body is practically equal to that of gases
in a polymer, provided there is no interaction between water molecules and the matrix
of the polymeric material. Since hydrophilic materials, which actively interact with
water molecules, are commonly used for the production of dressings, diffusion should
be considered simultaneously with absorption.
306
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
As a rule, the rate of the absorption process is significantly higher than the diffusion rate.
Therefore, it can be assumed that the absorption equilibrium is immediately reached,
and the concentration of water in the material CH 2O is obtained from the equation:
∂C H 2 O
∂t
= DH 2O
∂2CH 2O
∂x 2
−
a
∂CH
2O
(11.5)
∂t
where DH 2O is the coefficient of water diffusion in the material, x is the diffusion
a
is the concentration of absorbed water.
coordinate, and CH
2O
The concentration of absorbed water can be calculated for particular cases. For example,
if the concentration of functional groups able to link water molecules irreversibly is
limited and equals Cf, we can assume that the bonded water molecules no longer participate
in the diffusion process, but form domains in which fast absorption occurs.
For the case when the concentration of water at one of the surfaces (x = 0) is constant
0
and equals CH
, the reaction zone reaches the second surface of the membrane, which
2O
has thickness l, during the time t [49]. Thus, during time t there will be no water flow
through the surface x = l on the membrane exterior, and then steady-state flow will be set
up immediately. The amount of water passing through the membrane is given by:
mH 2O = DH 2O
ΔCH 2O
l
(11.6)
St
where S is the area of the membrane and
ΔCH 2O
l
is the concentration gradient.
If the solubility of water in the material is ruled by Henry’s law:
CH 2O = σP
(11.7)
where P is the water vapour pressure over the material, then substituting equation (11.7)
into equation (11.6) gives:
mH 2O = DH 2O σ H 2O
ΔP
St
l
(11.8)
Considering the diffusion coefficient DH2O being given by:
PH 2O = DH 2O σ H 2O
(11.9)
307
Handbook of Plastic Films
we obtain:
PH 2O =
mH 2O
(11.10)
ΔP S t
The aqua-penetrability of burn dressings has been determined on the device described in
Section 11.3. The values of the penetrability coefficients were calculated by equation
(11.10). Table 11.10 shows the values of the coefficients of aqua-penetrability PH 2O for
various burn dressings.
Table 11.10 Values of aqua-penetrability coefficients of burn dressings at 37 °C
Dressing name (material)
PH 2 O × 10 9
(mol m/m2 s Pa)
Helitrex (collagen)
1.6 ± 0.1
Helitrex sponge (collagen)
11.0 ± 1.0
Brown dressing (collagen)
6.6 ± 0.6
Syspurderm (foamy polyurethane)
0.8 ± 0.2
Syncrite (foamy polyurethane)
1.2 ± 0.2
Epigard (foamy polyurethane)
4.3 ± 0.4
Farmexplant (foamy polyurethane)
3.3 ± 0.3
Biobrant (silicon-polyamide)
1.6 ± 0.16
Johnson & Johnson (cellulose)
2.0 ± 0.2
Perforated metallised dressing (cellulose)
9.0 ± 0.7
Face mask dressing (cellulose)
2.5 ± 0.2
Burn towel (cellulose)
5.4 ± 0.5
50% cotton + 50% viscose
8.0 ± 0.7
70% cotton + 30% viscose
8.0 ± 0.7
100% viscose
7.0 ± 0.7
11.5.4 Determination of the Air Penetrability of Burn Dressings
As mentioned in Section 11.5.1, active sorption of wound exudate occurs for several minutes
after putting a dressing on a burn wound. Later, the evaporation of water from the external
308
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
side of the dressing proceeds. This leads to a change in the state of the exudate in the
material mass. On the whole, this changes the penetrability of the dressing with respect to
air. In this case, in order for anaerobic conditions not to be created in the wound, it is
necessary to provide optimal air penetrability during the entire period of application.
Data on the penetrability of dressings to dry air are known in the literature. Thus, for
example, it is recommended [50] to determine air penetrability with the help of the
industrially produced VPTM-2 device. This device records automatically the amount of
air passing through a dressing of known area during time t under pressure oscillations of
about 5 mm H2O. However, the application of such a device does not allow investigation
of the air penetrability of dense materials such as foamy polyurethane compositions and,
most importantly, of dressings in the wet state.
The construction and principle of action of a device, developed by the present authors,
that allows thse determination of the air penetrability of any material in any state and
under any conditions were described in Section 11.3.
11.5.4.1 Penetrability of various materials to oxygen and nitrogen
The coefficient of gas penetrability (as well as the coefficient of vapour penetrability) is
calculated according to equation (11.10). Literature data on the penetrability of various
polymers to oxygen and nitrogen are given in Table 11.11. As the data in this table show,
Table 11.11 Penetrability and separation coefficients of gases in polymers.
Polymer
Penetrability coefficient ×1015
(mol m/m2 s Pa)
Separation
coefficient
O2
N2
O2 / N2
Polycaproamide
0.013
0.0033
3.8
Polyvinyl chloride
0.022
0.008
2.8
Polyurethane elastomer
0.032
0.10
3.2
Polyethylene (ρ = 0.922)
0.35
0.13
2.7
Polystyrene
3.13
0.73
2.9
Teflon
2.07
0.67
3.1
Ethylcellulose
3.2
0.93
3.4
Polydimethylsiloxane
168
83.0
2.0
Silicon rubber
200
87.0
2.3
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Handbook of Plastic Films
Table 11.12 Values of penetrability coefficients P (mol m/m2 s Pa), diffusion
D ( m 2/ s ) a n d s o l u b i l i t y σ ( m o l / m 3 P a ) o f g a s e s i n t o p o l y d i m e t h y l s i l o x a n e
at 20 °C [51].
P × 1015
D × 1010
σ × 106
O2
83
23.3
36
N2
164
30
55.6
CO2
720
–
–
Gases
the penetrability of polymers may differ by four orders of magnitude. Special attention
should be paid to the high gas penetrability of polydimethylsiloxane and compositions
based on it, which is the result of the increased solubility of gases in them at high rates of
diffusion (Table 11.12) [51].
11.5.4.2 Penetrability of porous materials filled by a liquid medium
A short list of studies considering the investigation of the gas penetrability of polymeric
membranes in contact with a liquid is given elsewhere [52]. It is observed that the sorption
of liquid by a polymer leads to a decrease in the gas penetrability coefficient in comparison
with that of the liquid-free polymer.
11.5.4.3 Air permeability
Let us consider the mass transfer of air through a porous body in two cases: one in which
the free volume of all the pores is filled by air, and the other with the free volume filled by
a liquid medium. The porous body may be represented as consisting of two phases: the
material forming the body’s matrix, and the free space.
We also assume that pores have cubic form and are disposed within the volume of the
body in such a way that they do not join up with each other. Such a model is sufficient
for porous burn dressings.
Let us determine the total thickness of the body in the direction of mass transfer, the
total thickness of free space occupied by pores, and the total thickness of the layer occupied
by the material.
310
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
•
The total thickness of the body in the direction of mass transfer is given by:
lΣ = V/S
(11.11)
where V and S are the volume and surface area, respectively.
•
The total thickness of the free space occupied by pores is given by:
Qpores = lΣQ1/3 = (V/S)Q1/3
(11.12)
where Qpores = Vpores/V is the porosity.
•
The total thickness of the layer occupied by the material is given by:
lmat = lΣ – lpores = (V/S)(1 – Q1/3)
(11.13)
Thus, air passing through a porous body will overcome the resistance of two layers, each
possessing its own penetrability coefficient with respect to air.
The total penetrability coefficient PΣ of a porous body is thus given by:
l pores
1
l
=
+ mat
P∑ l ∑ Ppores l ∑ Pmat
(11.14)
where Ppores and Pmat are the penetrability coefficients of the porous medium and the
material forming the body’s matrix, respectively.
The following equation can be used to determine the ratio of the penetrability coefficients
of air for the porous body when its pores are filled with liquid and air:
P∑( liq)
P∑(air )
=
( Pmat / Pair ) + 1
ξ( Pmat / Pliq )
(11.15)
where:
ξ = Q1/3/(1 – Q1/3)
(11.16)
Values of Pmat are shown in Table 11.10. Values of Pair and Pliq can be estimated from the
coefficients of diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C
(Table 11.13).
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Handbook of Plastic Films
Table 11.13 Values of the coefficients of penetrability, diffusion and
solubility of oxygen in air, water, plasma and blood at 37 °C
(dimensions as in Table 11.9)
P
D
σ
Air
2.5 × 10-9
2.7 × 10-5
9.4 × 10-5
Water
7.4 × 10-14
3.0 × 10-9*
2.5 × 10-5*
Plasma
–
2.0 × 10-9*
–
Blood
1.4 × 10-14
1.4 × 10-9*
1.0 × 10-5*
Medium
* Values taken from [53]
Values of the penetrability coefficient of oxygen in various media may be calculated
according to the following expression:
P = Dσ
(11.17)
For any material Pair >> Pmat, so we obtain the simpler expression:
P∑( air )
P∑( liq )
≈ξ
Pmat
+1
Pliq
(11.18)
As, for the majority of dressings, ξ >> 1, and Pmat and Pliq are of the same order of
magnitude, the decrease in air penetrability of a dressing when the pores fill with liquid
must be significant.
It has been shown by special experiments that air humidity (from 40 to 100%) does not
practically influence the rate of penetration. The experiments were performed according
to the following scheme. First we determined the time t0 to fill a polyethylene sack, of 45
litre volume, with air in conditions when the sample was not in the cell. This time t0 (a
constant of the device) depended on the pressure in the system (p):
log(1/t0) = –2.00 + 0.44 log p
(11.19)
The time for polyethylene sack filling at p = 100 Pa was selected as the standard. At T =
21 ± 1 °C, it is found that t0 = 16.0 ± 0.1 min.
Subsequently, the time tx to fill the polyethylene sack when the sample was placed into the
cell was similarly determined. It was observed (Figure 11.8) that the dependence of tx on p
312
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Figure 11.8 The dependence of 1/log t on the pressure in the system for dry air.
1: needle-perforated material; 2: collagen sponge; 3: Syspurderm; 4: Syncrite; 5: Switin
cellulose dressing; 6: Farmexplant; 7: Epigard
has the same slope as in equation (11.19) for all investigated dressings in conditions of dry
air penetration:
lg
1
= − Ax + 0.44 lg p
tx
(11.20)
where Ax is a constant depending on the structure and properties of the dressing material.
By bubbling humid air through a dressing saturated by water, the slope increased significantly.
That is why it is necessary to perform several experiments for each dressing at different pressures
in order to extrapolate tx to the pressure of 100 Pa with the required accuracy (Figure 11.9).
The increase in the slope of log(1/tx) versus log p on bubbling air through a dressing saturated
with water was attributed to the change in the material structure of the dressing resulting
from the changes of form and size of the macropores. This is often accompanied by a decrease
of the total volume of the dressing.
The coefficient of air penetrability of the dressing (Px) was calculated according to:
Px =
ml x
S t (t x − t0 )
(11.21)
where m is the polyethylene sack bulk (equal to 2 mol of air at 21 ± 1 °C), and S is the surface
area in contact with the bubbling air (equal to 1.8 × 10–3 m2); and p = 100 Pa. Thus,
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Handbook of Plastic Films
Figure 11.9 The dependence of 1/log t on the pressure in the system for humid air.
1: needle-perforated material; 2: Farmexplant; 3: Combutec-2; 4: Syspurderm;
5: Biobrant (silicon-polyamide) compositional dressing; 6: Epigard
Px = 11
lx
(t x − t0 )
(11.22)
The values of air penetrability coefficients for dry dressings and dressings saturated with
water are shown in Table 11.14. From this table it can be seen that a significant decrease of
air penetrability takes place on saturation with water for all dressings except for ‘Biobrant’.
The penetrability coefficient for dry dressings can be calculated according to the equation:
(
13
Q1 3 1 + Q
=
+
P∑(air )
Pair
Pmat
1
)
(11.23)
Pair is obtained from equation (11.17) using D = 2.7 × 10–5 m2/s and solubility at atmospheric
pressure equal to 45 mol/m3. The value of Pair is 1.2 × 10–3 mol m/m2 s Pa. The values of
PΣ(air) were taken from Table 11.11.Values of Pmat were calculated from equation (11.18).
The values of PΣ(H2O) can be obtained from the equation:
(
1 + Q1 3
Q1 3
=
+
P∑(H 2O) PH 2O
Pmat
1
314
)
(11.24)
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Table 11.14 Coefficients of air penetrability for dry and water-saturated
burn dressings at temperature of 21 ± l °C.
Dressing name (material)
Coefficient of air penetrability
(mol m/m2 s Pa)
Dry
Wet
Helitrex (collagen)
2.7 × 10
Combutec-2 (collagen)
1.1 × 10
0
Epigard (foamy polyurethane)
-4
1.3 × 10
1.3 × 10-5
Syspurderm (foamy polyurethane)
1.3 × 10-4
1.0 × 10-6
Syncrite (foamy polyurethane)
1.1 × 10-3
4.0 × 10-5
Farmexplant (foamy polyurethane)
4.5 × 10-5
0
Biobrant (polyamide + silicon)
-4
1.8 × 10
7.0 × 10-5
Johnson & Johnson (cellulose)
1.6 × 10-4
3.0 × 10-6
Needle-perforated material (cellulose)
1.1 × 10-3
–
-5
0
-3
The calculated values of PΣ(H 2O) fall close to 10–8 mol m/m2 s Pa for the majority of
dressings. This result reveals the extremely low air penetrability for the listed dressings.
For some dressings, the value of PΣ(H 2O) is significantly higher than 10–8 mol m/m2 s Pa.
This can be explained by two effects:
(1) The presence of air flow along the surface of pores (surface flow) [49];
(2) The pressure of channels in the materials that are free of water.
To test these suppositions, additional investigations are required.
11.5.5 Determination of Adhesion of Burn Dressings
Adhesion properties play a key role in dressing performance. The lower layer of a dressing
must be easily wetted, providing good adhesion of the dressing to the wound. Besides,
the surface energy at the dressing-wound interface must be minimal to provide the smallest
trauma on its removal from the wound.
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Handbook of Plastic Films
11.5.5.1 Adhesive strength: theory
Adhesive strength characterises the ability of an adhesive structure to preserve its integrity.
Adhesive strength as well as the strength of homogeneous solids is of kinetic nature.
That is why the rates of surface tension and temperature increase affect the adhesive
strength, and why the scale factors, (i.e., sample dimensions), are also of great importance.
Different theories of adhesion of polymers have previously been suggested [53, 54] as follows:
(1) Mechanical theory (MacBain), according to which the main role is devoted to
mechanical filling of defects and pores of the surface (dressing) by the adhesive (blood);
(2) Adsorption theory (Mac-Loren), considering adhesion as a result of the performance
of molecular interaction forces between contacting phases – according to this theory,
low adhesion, for example, may be reached between a substrate (dressing) with
nonpolar groups and polar adhesive (blood);
(3) Electrical theory (Deriagin), based on the idea that the main factor controlling the
strength of adhesive compounds rests in the double electrical layer that is formed on
the adhesive-substrate interface;
(4) Diffusion theory (Vojytzky), considering the adhesion to be a result of interweaving
of the polymer chains;
(5) Molecular-kinetic theory (Lavrentiev), which assumes that a continuous process of
restoration and breakage of bonds proceeds in the zone of adhesive-substrate contact
– thus, adhesive strength is defined by the difference between the activation energies
for breakage and formation of bonds, and also depends on the correlation between
the amount of segments participating in the formation of bonds and the average
number of molecular bonds per unit contact area.
In recent years, the thermodynamic concept has received the most attention. Thus, the
main role is devoted to the correlation of the surface energies of adhesive and substrate.
The thermodynamic work of adhesion of a liquid to a solid (Wa) is described by the
Dupret-Jung equation:
Wa = γl(1 – cos θ)
(11.25)
where gl is the surface tension of the liquid, and θ is the wetting angle. Substituting
Jung’s equation:
γs-l = γs – γs-l cos θ
316
(11.26)
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
into equation (11.25), we obtain the correlation:
Wa = γs + γl – γs-l
(11.27)
where γs and γs-l are the surface tension of the solid and of the solid-liquid interface,
respectively.
It follows from equation (11.27) that, the higher Wa, the larger are the values of γs and γl
while γs-l are smaller. However, according to equation (11.27), the increase of γs must
lead to the growth of Wa and to an increase of γs-l. That is why the increase of the surface
tension of the substrate is accompanied by the action of two effects. The necessary
condition for adhesive strength is γl >> γs.
Values of γl and Ws −H 2O for different materials are shown in Table 11.15.
Table 11.15 Values of the surface tension and thermodynamic work of
adhesion of various materials [27]
Material
γs (mN/m)
Ws −H 2O
(mN/m)
Polytetrafluoroethylene
18.5
83
Silicon rubber
21.0
78
Polyethylene
31.0
99
Polystyrene
33.0
105
Polymethylmethacrylate
39.0
103
Polyvinyl chloride
39.0
101
Polyethyleneterephthalate
43.0
104
Polycaproamide
46.0
107
Glass
170.0
222
11.5.5.2 Adhesive strength of dressings
The adhesive strength of burn dressings was determined according to the method described
in Section 11.3. Table 11.16 shows the adhesive strength of various burn dressings and
the angle of wetting by water.
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Handbook of Plastic Films
Table 11.16 Adhesive strength (A) and the angle of wetting by water (θ) of
various burn dressings
A (mN/m)
θ (deg)
Corretium (collagen)
220 ± 20
75 ± 2
Syspurderm (foamy polyurethane)
210 ± 20
–
Epigard (foamy polyurethane)
350 ± 50
125 ± 3
Farmexplant (foamy polyurethane)
200 ± 20
130 ± 2
Bern-pack (cellulose)
170 ± 20
–
Biobrant (silicon-polyamide)
70 ± 10
–
Johnson & Johnson (cellulose)
20
–
Blood-stopping non-adhesive dressing (cellulose)
20
–
170 ± 50
–
Dressing name (material)
Dressing with metallised lower layer (cellulose)
11.6 The Model of Action of a Burn Dressing
Three main processes proceed after the application of a dressing to a wound:
(1) Sorption of the wound exudate by the dressing;
(2) Water evaporation from the dressing surface;
(3) Mass transfer of gases through the dressing under conditions of ongoing sorption
and evaporation.
Processes (1) and (3) were analysed in detail in Section 11.4. It was found that sorption
of liquid media (water, plasma) proceeds rapidly and reaches a limiting value (maximal
sorption ability) after several minutes for most dressings, i.e., a time that is significantly
shorter than the time for which the dressing acts (2-3 days).
The mass transfer of gases (oxygen and nitrogen) through the dressing is 2-4 orders of
magnitude slower with wet samples than with the dry ones in similar conditions. Next,
we consider water evaporation from the dressing surface.
11.6.1 Evaporation of Water from the Dressing Surface
Suppose that a dressing is saturated with water in air at 20 °C and 50% humidity. The
temperature of the dressing surface is 32 °C. These conditions are chosen to take into
account the temperature gradient in the matrix of the dressing.
318
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Let us determine the amount of water that evaporates from the surface of the dressing
during a given time period under stationery atmospheric pressure, and when the dressing
surface is completely saturated with water.
The partial pressure of air at 20 °C and 50% relative humidity equals:
PH 2O = 1.26 × 10–3 kg/cm2, Pair = 1.02 kg/cm2
For air at 32 °C in the saturated state:
PH 2O = 4.85 × 10–2 kg/cm2, Pair = 0.98 kg/cm2
The values of density, viscosity, heat conductivity and heat capacity of air at 26 °C equal:
ρ = 1.185 kg/cm3
μ = 1.861 × 10–6 g/m s
λ = 6.1 × 10–6 kcal/m s °C
Cp = 0.24 kcal/°C
After mathematical transformations using the method described elsewhere [55-58], the
following equation for the mass transfer of water in a dressing can be obtained:
W = am
P
( p1 − p2 )
RT ρ av
(11.28)
where am is the coefficient of heat conductivity, ρav is the average value of the mixture
density over and near the surface of the dressing, p1 and p2 are the partial pressure
PH 2O at 37 °C and 20 °C, respectively, R is the universal gas constant, and P is the
normal pressure.
Substituting numerical values for a dressing of 1 m × 1 m size, we obtain:
W = 1.2 × 10–1 g/m2 s
If the dressing surface is not completely occupied by water, we should apply the equation:
W=
Csurf (H 2O)
0
Csurf
(H 2 O )
× 1.2 × 10 −1 g/m 2 s
(11.29)
0
where Csurf (H 2O) and Csurf (H 2O) are the surface concentrations of water on the external
side of the dressing and on the free water surface, respectively.
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Handbook of Plastic Films
11.6.2 Sorption of Fluid by Burn Dressing from Bulk Containing a Definite
Amount of Fluid
Let us consider the case where a burn dressing is applied to a wound containing a definite
amount of liquid. Assume that a dressing membrane of given size (thickness and surface
area S) is in contact with the solution of restricted bulk volume V, which contains a
concentration C0(s-s) of diffusive substance. As the dressing becomes saturated by this
substance, the concentration of the latter in the bulk will decrease.
The solution of the diffusion equation has the following form [55-58]:
m
m∞
= 1−
2a(1 − a)
1 + a + a2 q 2
⎡ 4Dq 2t ⎤
exp⎢ 2 ⎥
⎢⎣ l
⎥⎦
(11.30)
where q is the positive solution of the characteristic equation:
tgq = − aq ; a =
V
(σSl )
where σ is the distribution coefficient of the substance between the membrane and
the solution.
When a sufficient part of the substance in solution is sorbed by the membrane, the value
of a is small and a simpler expression can be used:
⎡
⎤
⎥
mt ≈ m ⎢1 −
12⎥
⎢
⎥
4πDt / l 2
⎣
⎦
∞⎢
a
(
)
(11.31)
From equations (11.30) and (11.31), two important correlations can be obtained. The
sorption ability of the dressing, i.e., the part of the substance sorbed from the solution
under equilibrium conditions, equals:
m∞/m0 = 1/(1 + a)
(11.32)
Thus, for the efficient action of the dressing, it is necessary that the concentration C be
as high as possible in relation to the products of metabolism and toxins. Relating to
water, CH2O ~ 1, it is desirable that the dressing volume (lS) should be close to the volume
of wound exudate (V).
320
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
The time to reach the degree 0.85 of maximum sorption of liquid media by the dressing
equals:
t0.85 = 12
a2l 2
V2
= 12
πD
πDσ 2 S 2
(11.33)
It depends on many parameters, each being able to affect the time of completion of the
sorption process.
11.6.3 Mass Transfer of Water from Wound to Surroundings
Generally, the change in the amount of water under the dressing in the wound ( mH 2O ) is
determined from the correlation derived from equations (11.29) and (11.31):
0
mH 2O = V CH
2O
⎡
⎢
a
− mH O dressing ⎢1 −
)
2 (
⎢
4πDt / l 2
⎣
(
⎤
⎥ Csurf (H 2O)
− 0
× 1.2 × 10 −1 S t
12⎥
C
surf (H 2 O )
⎥
⎦
)
(11.34)
Let us consider the application of the correlation (11.34) for the following case. The
wound characteristics are:
0
0
S = 10–2 m–2, CH
= 106 g/m3, mH
= 50 g
2O
2O
V = 5 × 10–5 m3, l = 10–3 m, σ H 2O = 1
Csurf (H 2O)
0
Csurf
(H 2 O )
= 0.5
Under these conditions:
t0.85 = 12
m∞
m
0
=
25 × 10 −10
π × 10 −9 × 10 −4
(
1 + 5 × 10
= 9.5 × 102 s# (or ~15 min)
1
−5
/ 10 −2 × 10 −3
)
= 0.17#
(or 8.5 g)
321
Handbook of Plastic Films
During the same time the following amount of water will evaporate from the dressing
surface:
mevap ( H2O) < 0.5 × 1.2 × 10 −1 × 950 × 10 −2 = 0.6 g
i.e., the rate of evaporation is significantly (14 times) lower than that of water sorption
by the dressing. All the amount of water from the wound (wound exudate) will evaporate
during the time:
t=
50
0.5 × 1.2 × 10 −1 × 10 −2
= 8.3 × 10 4 s# (or ~23 h)
11.7 Criteria for the Efficiency of First-Aid Burn Dressings
11.7.1 Requirements of a First-Aid Burn Dressing
A first-aid burn dressing must meet the following criteria:
(1) Sorption of the wound exudate, containing products of metabolism and toxic
substances, during the period of dressing action (24-48 h);
(2) Wound isolation from infection of the external medium;
(3) Optimum air and water transfer between wound and surroundings;
(4) Easy removal from the wound, causing no damage to the wound surface.
The characteristics of burn wound dressings, based on the approximate estimations discussed
previously, are listed next. Note that no quantitative data have been reported in the literature.
11.7.2 Characteristics of First-Aid Burn Dressings
11.7.2.1 Sorption ability of dressings
A second- or third-degree burn wound releases on average 5 × 103 g/m2 of exudate. As
may be seen from Table 11.11, the water amount is about 90%. The sorption of different
components of the exudate proceeds at different rates. In this case, the free volume of the
dressing material will be first filled with water. The diffusion of proteins and cells takes
place in space occupied by water.
322
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
Modern burn dressings possess a porosity of 0.9 and almost the entire free volume can be
filled with water (Figure 11.5). The maximum sorption ability for such dressings equals:
CH 2O ≈
ρH 2O
ρ
and the amount of the liquid sorbed per unit area is:
∞
mρ ≈ C H
ρl =
2O
ρH 2O
ρ
ρ l ≈ 106 l g/m 2
because ρH 2O = 106 g/m3.
As a first-aid burn dressing must sorb 5 × 103 g/m3, it follows that:
5 × 103 ≈ 106l
(11.35)
and therefore the thickness of a first-aid burn dressing equals:
l ≈ 5 × 103/106 ≈ 5 × 10–3 m (or 0.5 cm)
Thus, the first criterion for the efficiency of a first-aid burn dressing can be formulated
as follows:
A first-aid burn dressing must use its entire free volume for sorption. This volume
must be 0.9 or more of the total volume of the dressing. Dressing thickness must be
0.5 cm or more. The majority of foreign, (i.e., non-Russian), first-aid dressings fulfil
this criterion.
11.7.2.2 Air penetrability of dressings
The air penetrability of most of the dry dressings ranges between 10–4 and 10–5 mol m/m2 s
Pa (Table 11.14). The air penetrability of the dressings saturated with water is much
lower and decreases to values between 10–6 and 10–5 mol m/m2 s Pa, that is, 0.2-2 dm3/
m2 s. Thus, the second criterion for the efficiency of first-aid burn dressings can be
formulated as follows:
A first-aid burn dressing must possess an air penetrability of 10–5 mol m/m2 s Pa or
higher after the sorption of water. For example, the Biobrant burn dressing fulfils
this criterion.
323
Handbook of Plastic Films
11.7.2.3 Adhesion of dressing to wound
The adhesion strength of dressings with respect to coagulated blood (Table 11.16) varies
in a wide range, but it has the minimum value of ~20 N/m. This value should be accepted
as the optimal one, because it corresponds to the minimal pain and damage on removal
from the surface of natural skin. Thus, the third criterion for the efficiency of first-aid
burn dressing can be formulated as follows:
A first-aid burn dressing must possess an adhesive strength to the wound of 20 N/m
or less after the end of its action. The following burn dressings, for example, fulfil
this criterion: Biobrant, blood-stopping remedy, Johnson & Johnson.
11.7.2.4 Isolation of wound from infection from external medium
It is known that microorganisms causing wound infection do not penetrate through
filters possessing average pores size ~0.5 μm. So the fourth criterion for the efficiency of
first-aid burn dressings is as follows:
A first-aid burn dressing must possess no open pores with average diameter larger
than 5 × 10–7 m (0.5 μm). Moreover, it is implied that first-aid burn dressings possess
sufficient mechanical strength and elasticity in both dry and humid conditions.
11.8 Conclusion
Experimental methods to estimate the main physicochemical properties of burn dressings
were worked out. Based on theoretical and experimental data we found the following:
(1) The maximal sorption ability of a burn dressing equals the free volume of the dressing
material, calculated from the value of the material density.
(2) Water can be used as a model liquid in the study of sorption ability instead of blood
plasma.
(3) Kinetic parameters were determined from the sorption curves. These parameters
showed that first-aid burn dressings markedly differ in the value of the rate of liquid
media sorption at stages close to the sorption limits.
(4) The air penetrability parameter in the wet state decreases abruptly by 2-3 orders of
magnitude for the majority of tested dressings. This is due to the filling of pore space
by the liquid medium.
324
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
(5) Accordingly, it is recommended that the air penetrability parameter should be
determined in the wet state, which represents the common condition of action for
first-aid burn dressings.
(6) The value of the adhesive strength after the end of its action on the wound should
not exceed 20 N/m.
From the data obtained in this study, we formulated the following criteria to estimate the
efficiency of first-aid burn dressings:
•
Maximum sorption ability for water must be at least 10 g/g;
•
Optimal thickness of dressings, fulfilling this value of sorptional capacity, must be
about 5 × 10–3 m (0.5 cm);
•
Adhesive strength must not exceed 20 N/m;
•
Average diameter of open (connected) pores must not exceed 5 × 10–7 m.
References
1.
M.I. Fel’dshtein, V.S. Yakubovich, L.V. Raskina and T.T. Daurova, Polymer
Coatings for Wound and Burn Treatment, Institute of Information, Moscow,
Russia, 1981, 299 (in Russian).
2.
G.B. Park, Biomaterials, Medical Devices and Artificial Organs, 1978, 6, 1.
3.
V. Rudkovsky, V. Nezelovsky, V. Zitkevich and N. Zinkevich, Theory and
Practice of Burn Treatment, Meditsina, Moscow, Russia, 1988, 200 (in Russian).
4.
A. Robin and K.H. Stenzel in Biomaterials, Eds., L. Stark and G. Agarwal,
Plenum Press, New York, NY, USA, 1969, 157.
5.
A. Robin, R.R. Riggio and R.L. Nachman, Transactions of the American Society
of Artificial Internal Organs, 1968, 14, 1669.
6.
H.C. Grillo and I. Gross, Surgical Research, 1962, 2, 69.
7.
J. Oluwasanmi and M. Chapil, Journal of Trauma, 1976, 16, 348.
8.
G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1.
325
Handbook of Plastic Films
9.
J.I. Abbendhaus, R.A. McMahon, J.G. Rosenkranz and I.C. McNeil, Surgical
Forum, 1965, 16, 477.
10. F.J. Richter and C.T. Riall, inventors; American Cyanamid Company, assignee;
US Patent 3,566,871, 1971.
11. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,
NY, USA, 1997.
12. G.E. Zaikov, Degradation and Stabilisation of Polymers, Nova Science
Publishers, New York, NY, USA, 1998.
13. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,
NY, USA, 1995, 286.
14. J.H. Gardner and D.T. Rovee, inventors; Johnson & Johnson, assignee; US Patent
3,521,631, 1970.
15. L.M. Wheeler, inventor; Parke Davis and Company, assignee; US Patent
3,648,692, 1972.
16. No inventors; Johnson & Johnson, assignee; UK Patent 1,309,768, 1973.
17. G.L. Wilks and L.L.J. Samuels, Biomedical Materials Research, 1973, 7, 541.
18. I.A. Agureev, Voenno-Meditsinskii Zhurnal, 1963, 6, 74 (in Russian).
19. P. Lock, inventor; no assignee; French Patent 2,156,068A1, 1973.
20. K. Gorkisch, E. Vaubel and K. Hopf, Proceedings of the 2nd International
Congress on Plastics in Medicine, Amsterdam, The Netherlands, 1973, Paper
No.16.
21. A.L. Iordanskii, G.E. Zaikov and T.E. Rudakova in Transport, Kinetics,
Mechanism, VSP Science Press, Utrecht, The Netherlands, 1993, 288.
22. USSR Certificate No. 245,281, 1969, Bulletin of Certificates, No. 19.
23. W.M. Chardack, M.M. Martin, T.C. Jewett and E.M. Pearce, Plastic and
Reconstructive Surgery, 1962, 30, 554.
24. C.W. Hall, D. Liotta, J.J. Chidoni, V.M.M. Lobo and A. Valente, Journal of
Biomedical Materials Research, 1972, 6, 571.
25. J.J. Guldarian. C. Jelenko, D. Calloway, L. Kalle and M.Lewin, Journal of
Trauma, 1973, 13, 32.
326
Physicochemical Criteria for Estimating the Efficiency of Burn Dressings
26. E.E. Schmitt and R.A. Polistina, inventors; American Cyanamid Company,
assignee; US Patent 3,875,937, 1975.
27. S. Madou, Ed., Polymers for Medicine, Meditsina, Moscow, Russia, 1981, 350
(in Russian).
28. USSR Certificate No. 267,010, Bulletin of Certificates, 1970, No.12.
29. G.E. Zaikov, A.L. Buchachenko and V.B. Ivanov, Ageing of Polymers, Polymer
Composites and Polymer Blends, Nova Science Publishers, New York, NY, USA,
2002.
30. F.C. Moore and L.A. Perkinson, inventors; Moore-Perk Corporation, assignee;
US Patent 3,678,933, 1972.
31. M.G.M. Nilsson, R.G.A.B. Udden, P.E.C. Udden and B.A. Wennerblom,
inventors; Svenska cellulos Aktiebolaget, assignee, US Patent 3,654,929, 1972.
32. H. Kinkel and S. Holzman, Chirurgie, 1965, 36, 535.
33. USSR Certificate No. 6,658,148, Bulletin of Certificates, 1979, No.15.
34. M.I. Kuzin, V.K. Sologub, V.V. Yudenich, Y.B. Monakov, K.S.Minsker and A.A.
Berlin, Khirurgiya, 1979, 8, 86 (in Russian).
35. I.V. Yannas and J.F. Burke, Journal of Biomedical Materials Research, 1980, 14, 65.
36. S. Jacobson and U. Rothenaw, Journal of Plastic and Reconstructive Surgery,
1976, 10, 65.
37. D. Spruit and K.E. Malten, Dermatology, 1966, 132, 115.
38. USSR Certificate No. 685,292, Bulletin of Certificates, 1979, No.34.
39. Textbook on Polymer Materials, Ed., N.A. Plate, Khimiya Publishers, Moscow,
Russia, 1980, 255 (in Russian).
40. G.E. Zaikov, A.L. Iordanskii and V.S. Markin, Diffusion of Electrolytes in
Polymers, VSP Science Press, Utrecht, The Netherlands, 1988, 328.
41. Y.V. Moiseev and G.E. Zaikov, Chemical Resistance of Polymers in Reactive
Media, Plenum Press, New York, NY, USA, 1986, 586.
42. I.A. Barrie in Diffusion in Polymers, Eds., J. Crack and G.S. Park, Academic
Press, London, UK, 1968, 452.
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Handbook of Plastic Films
43. S.I. Papkov and E.Z. Fainberg, Interaction of Cellulose and Cellulose Materials
with Water, Khimiya, Moscow, Russia, 1976, 231 (in Russian).
44. G.E. Zaikov, Chemical and Biochemical Kinetics, Nova Science Publishers, New
York, NY, USA, 2002.
45. S.S. Voyutskii, Physico-Chemical Principles of Fiber Materials – Sorption by
Polymer Dispersions, Khimiya, Leningrad, 1969, 336 (in Russian).
46. D.A. Fridrikhsberg, Course of Colloid Chemistry, Khimiya, Leningrad, 1974, 351
(in Russian).
47. M.I. Al’tshuller and B.V. Deryagin, in Investigations in the Field of Surface Force,
Nauka, Moscow, Russia, 1967, 235 (in Russian).
48. M.M. Mikhailov, Moisture Permeability of Organic Dielectrics, Gosenergoizdat,
Moscow, Russia, 1960, 162 (in Russian).
49. N.I. Nikolaev, Diffusion in Membranes, Khimiya, Moscow, Russia, 1980, 232
(in Russian).
50. Textbook on Textile Materials, Legkaya Industriya, Moscow, Russia, 1974, 342
(in Russian).
51. I.M. Raigorodskii and V.A. Savin, Plasticheskie Massy, 1976, 1, 65 (in Russian).
52. V.N. Manin and A.N. Gromov, Physico-Chemical Resistance of Polymer Materials
During Exploitation, Khimiya, Moscow, Russia, 1980, 247 (in Russian).
53. E. Laifut, Transfer Phenomena in Living Systems, Mir, Moscow, Russia, 1977,
520 (in Russian).
54. V.E. Basin, Adhesion Durability, Khimiya, Moscow, Russia, 1981, 208 (in Russian).
55. L.M. Batuner and M.E. Pozin, Mathematical Methods in Chemical Technology,
Khimiya, Leningrad, 1971, 822 (in Russian).
56. Polymer Analysis and Degradation, Eds., A. Jimenez and G. Zaikov, Nova
Science Publishers, Huntington, NY, USA, 2000, 287.
57. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York,
NY, USA, 1998, 245.
58. A.Y. Polishchuk and G.E. Zaikov, Multicomponent Transport in Polymer Systems
for Controlled Release, Gordon and Breach, New York, NY, USA, 1996, 231.
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12
Testing of Plastic Films
E.M. Abdel-Bary and G. Akovali
12.1 Introduction
Plastics are a very important group of materials. They differ from most of the
‘natural’ materials – such as metals, papers, ceramics, natural fibres – mainly as a
result of their ‘viscoelastic’ behaviour. The word ‘viscoelastic’ is used to describe
behaviour that shows both viscous and elastic characteristics even at ambient
conditions, when stressed. This behaviour is a direct result of the long-chain nature
of the polymeric molecules that constitute the plastic material. Whereas the gross
mechanical behaviour of most ‘natural’ materials under stress could be considered
as elastic or deformation flow, the response of all plastics to stress is a combination
of the two. The ratio of viscous and elastic components, termed ‘damping’, can
vary greatly over quite a narrow temperature range for plastics and it also depends
markedly on the rate of stressing.
One of the most common forms of plastic material is the ‘film’. Test methods for
plastic films have evolved not only from the techniques of the preceding technologies.
The bigger manufacturers and users have also devised their own laboratory
procedures to enable them to control film properties or determine the suitability of
a film for a particular process or application. In addition, research scientists have
published the methods that they have used to study the theoretically interesting
properties of polymers. Standards organisations have attempted to devise standard
test methods acceptable to all branches of the industry. This chapter reports briefly
on the most common test methods generally used for plastic films, according to the
field of applications. Although most countries have their own standards and
standards organisation(s), consideration here will be restricted to tests published
by the American Society for Testing and Materials (ASTM). Anyone interested in
the details of the ASTM tests can find them in ASTM D883 [1], which is one of the
many parts of the ASTM Standards and is available in libraries or directly from the
ASTM or the US Government Printing Office.
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12.2 Requirements for Test Methods
12.2.1 List of Requirements
There are several requirements necessary for a test method to be developed, some of
which are summarised below:
(1) The test method should be rapid, so that results can be used in quality control on
high-output machinery without delaying producing or dispatch.
(2) Results must be reproducible and consistent between different testing stations and
machines. This means that the test should be insensitive to minor variations in specimen
preparation, to wear and to other small differences in test apparatus.
(3) The precision of the results should be no more than is required. The cost of extreme
accuracy is rarely justified in industry, and often a value that is accurate to within a
few per cent will give all the information that is wanted.
(4) It is preferable that the results are scientifically significant. It is imperative that they
are of technological significance and give a meaningful indication of the real-life
performance of the film.
The main advantage of a standard method is that results obtained by its use in different
laboratories can be compared.
12.2.2 Interpretation of Test Results
The main difficulties encountered both in deriving significant tests for polymers and in
interpreting the results are the (relatively rapid) changes in properties with rate of
deformation and, particularly, with temperature.
The mechanical behaviour of conventional materials is fairly insensitive to temperature
in the normal range of ambient and packaging-processing temperature for the films used
in the packaging industry. However, a polymer, being viscoelastic, may change from a
glassy solid through a leathery and then a rubbery stage to a sticky liquid in a temperature
range of less than 100 °C.
This variation can be of practical importance not only for the manufacturer, who is
prevented from using the high temperature sometimes demanded (for example, in print
drying), but also for the designer wishing to provide packages that can be used in
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environments ranging from cold storage at –30 °C to a window display in hot sunshine,
where the temperature can exceed 60 °C.
Viscoelasticity is a complex subject, and all polymers exhibit a similar pattern of
behaviour, the details of which are determined by the chemical nature of the polymer,
its molecular weight (molar mass) and molecular weight distribution, degree of
crystallinity and so on.
Taking polystyrene (PS) as an example of a simple amorphous polymer, one finds
that the elastic modulus is constant in the temperature range up to about 100 °C,
which is the glassy region. Increasing the temperature above 100 °C leads to a drastic
decrease in the elastic modulus, as it exists in the leathery region. Further increase in
temperature has no effect on the elastic modulus as PS falls in the rubbery region. In
all these three regions – glassy, leathery and rubbery – the moduli of commercially
useful polymers are independent of molecular chain length. In the last region, at
temperatures exceeding about 170 °C, the polymer falls in the flow region. The basic
molecular phenomena causing these different types of behaviour are reasonably well
understood. In the glassy region, the long polymer molecule is frozen, with the atoms
vibrating about fixed positions as in any rigid solid. In the leathery (transition) region,
where the modulus changes rapidly with temperature, short-range diffusion of
segments of the polymer chains takes place, but any movement is restricted to
individual atoms of two or three adjacent segments, and the molecule as a whole
does not move. In the rubbery region, the modulus is fairly constant; here the shortrange motions of polymer segments are very fast, and the cooperative movement of
adjacent segments takes place. Entanglements restrict the length of chain that can
move. In the rubbery flow region, the motion of molecules as a whole becomes
important as a result of slippage of the entanglements; while in the region of flow,
changes in the entire molecule take place quicker than the rate of testing, and there is
little elastic recovery at this time-scale. For the last two regions, the modulus depends
on the chain length and its distribution.
The modulus versus temperature curve is also rate- (of testing or stressing) dependent,
since the major changes in the modulus take place when a particular molecular activity
is occurring at large magnitudes at rates faster than the test. This behaviour for the
modulus holds true for any usual mechanical property such as yield strength, breaking
strength, breaking elongation, impact strength (or total breaking energy), etc.
Meanwhile, it is important to consider the parameters in the test, which are important
for the proposed application, e.g., temperature, rate, humidity and geometry, so that
they cover the range met in use. If not, the data of some other standard test should be
able to provide the necessary, and probably the most important, information.
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12.3 Some Properties of Plastic Films
Several standard methods can be used to determine the properties of plastic films.
Properties can be purely physical, physico-chemical, chemical or mechanical. For the
first three, usually the form of the sample does not matter, and the same methods are
used for film samples as well as samples with bar shape. Most of the mechanical tests are
also the same methods that are employed in testing plastics in any form, while some are
specific for the plastic films. Following the procedure described in a relevant standard
for tensile properties, it was shown that it is very important to define appropriately the
whole set of parameters involved in the test. In addition, special adaptation of the
equipment used is required. Harmonisation of the testing methods for impact and initial
tear resistance proved to be more readily obtained. However, some parameters entering
the corresponding measuring procedures had to be adapted. In general, harmonisation
has been achieved regarding the measurement of the specific mechanical properties [2].
Some of the characteristics of these films, usually taken into consideration, are first given
below, followed by the mechanical tests and then other tests.
12.3.1 Dimensions
Measurement of the average thickness of a film is straightforward, and no special problems
should be encountered in their measurement. The accurate measurement of film thickness
is important because the values of some of the other properties – such as tensile strength,
elongation at break, impact resistance, resistance to tear propagation – depend strongly
on the thickness of the material. In general, the thicknesses of plastic films are several
tens of micrometres, (e.g., low-density polyethylene (LDPE) agricultural films usually
range from 50 μm to more than 200 μm, the latter for greenhouse films). The trend is to
reduce thickness to avoid the huge amount of waste at the end of their lifetime.
12.3.2 Conditioning the Samples
In general, the physical and electrical properties of plastics and electrical insulating
materials are strongly influenced by the temperature and stress history of the samples
(used during their preparation) as well as the humidity. In order to make reliable
comparisons, it is necessary to standardise the temperature and humidity conditions
to which plastics are subjected prior to and during testing. Unless otherwise specified
for special polymers, the standard procedure recommended for conditioning samples
prior to testing is described by ASTM D618-61/90 Procedure A [3]. In this method,
for specimens thinner or thicker than 7 mm, condition the specimens for a minimum
40 h immediately prior to testing (or 88 h for the latter, over 7 mm thickness) in the
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standard laboratory atmosphere at 23 °C and 50% relative humidity (RH), whilst
providing adequate air circulation on all sides. This can be achieved by placing the
samples in suitable racks, hanging them from metal clips or laying them on widemesh, wire screen frames with at least 25 mm between the screen and the surface of
the bench.
12.4 Mechanical Tests
12.4.1 Tensile Testing (Static)
Tensile tests are mainly used to determine the tensile strength of a material. Such testing
provides data for research, development and engineering design as well as for quality
control and specification. In tensile testing there are certain difficulties with thin films. It
is essential that the cut edges of the tensile specimen are free from nicks or flaws from
which premature failure could start.
For thinner films, grip surfaces are a problem. Both slippage in the grip and fracture of
the sample at the grips must be avoided. Any technique, such as the use of a thin coating
of rubber on the faces or the use of emery cloth, that prevents slipping in the grips,
prevents grip fractures and does not interfere with the portion of the sample under test,
is acceptable.
From tensile tests, some material characteristics – such as the (tensile) modulus, percent
elongation at break, yield stress and strain, tensile strength and tensile energy to break
values – can be obtained. Tensile properties (static) of plastics are covered in ASTM
D638 (general) [4] and ASTM D882 (films) [5].
12.4.1.1 Tensile Strength
Tensile strength is calculated by dividing the maximum load by the initial crosssectional area of the specimen, and is expressed as force per unit area (usually in
megapascals, MPa).
12.4.1.2 Yield Strength
Yield strength is the load at the yield point divided by the initial cross-sectional area, and
is expressed as force per unit area (MPa), usually to three significant figures.
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12.4.1.3 Tensile Modulus of Elasticity
The tensile modulus of elasticity (or simply the elastic modulus, E) is an index of the
stiffness, while the tensile energy to break (TEB, or toughness) is the total energy absorbed
per unit volume of the specimen up to the point of rupture. The tensile modulus of
elasticity is calculated by drawing a tangent to the initial linear portion of the load versus
extension curve, selecting any point on this tangent, and dividing the tensile force by the
corresponding strain. The results are expressed in MPa, and are usually reported to three
significant figures. Secant modulus (used for cases where no initial linear proportionality
exists between stress and strain) is defined at a designated strain. TEB is calculated by
integrating the energy per unit volume under the stress-strain curve, or by integrating the
total energy absorbed divided by the volume of the original gauge region of the specimen.
TEB is expressed as energy per unit volume (in megajoules per cubic metre, MJ/m3),
usually to two significant figures.
12.4.1.4 Tensile Strength at Break
Tensile strength at break is calculated in the same way as the tensile strength, except that
the load at break is used in place of the maximum load. It should be noted that, in most
cases, tensile strength and tensile strength at break values are identical.
12.4.1.5 Percent Elongation at Break
Percent elongation at break is the extension at the point of rupture divided by the initial
gauge length. It is usually reported to two significant figures.
12.4.1.6 Percent Elongation at Yield
Percent elongation at yield is the extension at the yield point divided by the initial gauge
length of the specimen, usually given to two significant figures.
12.4.1.7 Package Yield of a Plastic Film
A specific ASTM test method (ASTM D4321; [6]) exists for the determination of the
‘package yield’ of plastic films, in terms of area per unit mass of the sample. In this test,
values such as the nominal yield (the target value of the yield as agreed between the user
and supplier), package yield (yield calculated by the standard), nominal thickness (the
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Testing of Plastic Films
target value of the film thickness as agreed between the user and supplier), nominal
density and measured density are defined and obtained. The value of package yield is
important for the manufacturer, because it determines the actual number of units or
packages that can be derived from a given mass of film in a particular application.
12.4.1.8 ASTM D882 Test for Thin Films
In tensile measurements, discrepancies can and usually do occur in the results, either
because of the use of different specimen types with different geometries and/or because
different test speeds are employed in the testing procedure. However, the data from such
tests cannot be considered appropriate for applications whose load time-scales differ
widely from those actually used in the test employed. In fact, the shape of the specimens
suggested may be different depending on the film thickness. They are specified in different
standards (such as ISO 527 for thick films [7-9], and ISO 1184 [9] and ASTM D882 for
films less than 0.25 mm; [5]). A brief description of D882-95a is given next.
A load range is selected such that specimen failure occurs within its upper two-thirds,
for which a few trial runs are recommended. The cross-sectional area, width (to an
accuracy of 0.25 mm) and thickness (to an accuracy of 0.025 mm for thin films with
thicknesses less than 0.25 mm, and for thicker films to an accuracy of 1%) of the
sample are measured at several points. The grip separation rate is set and the test
specimen is placed in the grips and tightened evenly. The machine is started, and load
versus extension values are recorded.
Some characteristic tensile values of different plastic films are presented in the table
given in ASTM D882-95a.
LDPE is one of the weakest films used as the covering of greenhouses, in terms of tensile
strength (11-37.9 MPa) [10]. As the density of polyethylene (PE) increases from LDPE to
high-density polyethylene (HDPE), tensile strength at yield and stiffness values are seen
to increase, while elongation and flexibilities decrease [11]. This is because the crystalline
regions significantly increase the modulus of elasticity and hence the ability of the plastics
to support loads at elevated temperature [12].
Another effect observed from the table in ASTM D882-95a is that of strengthening due to
the molecular orientation imparted during film blowing. This is because, on a molecular
level, tensile properties are higher in the direction of the covalent C–C bond in the chain than
in the transverse direction, which is dominated by the much weaker van der Waals’ bonds.
Since the crystals of LDPE films are preferentially oriented parallel to the machine direction,
load applied in the machine direction may yield higher values of tensile strength than load
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applied perpendicular to that direction. In fact, not only the direction of the film, but also the
process parameters – such as melt temperature, die parameters, blow-up ratio, draw ratio,
frost-line height and cooling conditions – can lead to different mechanical properties between
two films with the same composition [13] (details are given in Chapter 2).
12.4.2 Impact Resistance
Impact values represent the total ability of the material to absorb impact energy, which
is composed of two parts: (a) the energy required to break the bonds, and (b) the work
consumed in deforming a certain volume of the material.
The impact resistance of plastics in general is specified by ASTM D256 [14] as the energy
extracted from standardised pendulum-type hammers with one pendulum swing done either
with milled notched (Izod and Charpy tests) or unnotched samples, for relatively brittle
samples. The results are reported in terms of energy absorbed per unit specimen width.
For tough plastic films, on the other hand, the free-falling dart method is recommended.
There is one specific ASTM standard given for the impact resistance of LDPE measured
by the free-falling dart method (ASTM D1709 [15] or ISO 7765-1 [16] and ISO 7765-2
[17]), which is reported in two different cases, for 260 g and 881 g (for 0.20 mm thick)
film. LDPE has good toughness, which decreases with the density of the material.
ASTM D1790 [17a] and D746 [18] are test methods for the routine determination of the
specific ‘brittleness’ temperature at which plastics exhibit brittle failure under specified
impact conditions. The first method is given for a thin (0.25 mm or less) plastic film, and
the second is for real loading conditions. Thus ways to predict the behaviour of the
material at low temperatures can be made, which is important for plastic films that are
used in variable temperature conditions. The test applies for similar conditions of
deformation, and the brittleness temperature is estimated statistically in the test as that
at which 50% of the specimens would fail.
12.4.2.1 Impact Resistance by Free-Falling Dart Method
The test method ASTM D1709-91 [15] covers the determination of the energy that causes
a plastic film to fail under specified conditions of impact of a free-falling dart. This
energy is expressed in terms of the weight (mass of the missile), falling from a specified
height, that would result in 50% failure of the specimens tested. The impact resistance of
a plastic film, while partly dependent on its thickness, has no simple correlation with
sample thickness.
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Testing of Plastic Films
The specimen for the test should be large enough to extend outside the specimen clamp
gaskets at all points. The specimens will be representative of the film under study, and
should be free from pinholes, wrinkles, folds or other obvious imperfections, unless such
imperfections constitute variables under study.
12.4.2.2 Pendulum Impact Resistance
Like other techniques to measure toughness, this test method (ASTM D256 [14]) provides a
means to determine the parameters of a material at strain rates close to those applicable in
some enduse applications, and the results are more valid than those provided by low-speed
uniaxial tensile tests. The dynamic tensile behaviour of a film is important, particularly when
the film is used as a packaging material. The same uncertainties about correlations with
thickness that apply to other impact tests (such as ASTM D1709 [15]) also apply to this test.
Several impact test methods are used for film samples. It is sometimes desirable to know
the relationships among the test results derived by different methods. A study was conducted
in which films made from two resins [polypropylene (PP) and linear low-density polyethylene
(LLDPE)], with two film thicknesses for each resin, were impacted using ASTM test methods
D1709 [15], D3420 [19] and D4272 [20]. Differences in results between test methods
D1709 and D4272 may be expected, since test method D1709 represents failure-initiated
energy while test method D4272 represents initiation plus completion energy.
12.4.2.3 Hail Resistance
Although impact resistance is a valuable property to measure, the complexity and
multiplicity of events occurring during impact make the value obtained applicable only
under narrow conditions and not suitable for general design purposes. Thus, servicerelated impact tests have been devised for large-volume applications as greenhouse
coverings. According to this method, a complete half of a greenhouse roof is built
horizontally and is randomly shot with Nylon balls. The impact damage is registered
with a camera. Single glass, 4 mm thick, is considered to be the reference material, and
all other materials are compared to that.
12.4.3 Tear Resistance
The tear resistance of a plastic film is a complex function of its ultimate resistance to
rupture. There are different ASTM standards available for the tear resistance of films:
ASTM D1004 [21] is designed to measure the force necessary to initiate tearing at very
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low rates of loading, while ASTM D1938 [22]covers the force necessary to propagate a
tear by a single tear. ASTM D1922 [23] is the determination of the average force to propagate
tearing through a specified length of plastic film by use of an Elmendorf-type tearing tester.
In ASTM D2582 [24], the puncture-propagation tear resistance of films is of interest.
In these tests, two different values are of interest and are measured:
(1) the force required to initiate the tear (ASTM D1004 and ISO 344 [25]);
(2) the force needed to propagate the tear (ASTM D1938, D1922 and ISO 6383-1; [26]).
ISO standards are specific to applications in greenhouses. The second value (the force
needed to propagate the tear) can be considered to be of most interest, because, while it
might occasionally be impossible to prevent a film from tearing in greenhouse applications,
(e.g., when the film is not fastened securely, flaps in the wind, and hits a protruding part
of the structure), it is highly beneficial if the tear propagates with great difficulty. Resistance
to initiation of tear is also important and cannot be neglected in general.
The tear resistance of plastic films is very important with regard to their overall mechanical
behaviour and common failure mechanisms, i.e., for agricultural plastic films. The
resistance to tear propagation for LDPE film is found to vary significantly. The reported
value of resistance to tear propagation is 5-20 N [27]. Possible sources of this variation
are the anisotropy, elongation effects and variable thickness of the tested films, as well as
the use of different speeds during tearing.
12.4.3.1 Propagation Tear Resistance of Plastic Film and Thin Sheeting by
Pendulum Method
This test (ASTM D1922-94a; [23]) covers the determination of the average force to
propagate tearing through a specified length of plastic film. It is widely used in packaging
applications. While it may not always be possible to correlate film tearing data with
other mechanical or roughness properties, the apparatus for this test method provides a
controlled means for tearing specimens at straining rates approximating some of those
found in actual packaging service. Owing to orientation during manufacture, plastic
films and sheeting frequently show marked anisotropy in their resistance to tearing. This
is further complicated by the fact that some films elongate greatly during tearing, even at
the relatively rapid rates of loading encountered in this test method. The degree of this
elongation is dependent in turn on film orientation and the inherent mechanical properties
of the polymer from which it is made. There is no direct relationship between tearing
force and specimen thickness. The tearing force is usually expressed in milli newtons
(mN) or gram-force.
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Testing of Plastic Films
A comparison of propagation tear resistance (Elmendorf tear) in machine direction and
transverse direction of different types of plastic film is given in a table in ASTM D192294a. From the data given in this table,, it can be observed that LLDPE possesses the
highest value of tear resistance in both machine and transverse directions. PP, which
shows a low value of tear resistance in the machine direction, has a higher value in the
transverse direction. The difference between the two directions reflects the degree of
orientation and anisotropy of the material. PS orientation during processing is not
remarkable, and consequently its tear resistance does not differ in the two directions.
12.4.3.2 Puncture-Propagation Tear Resistance
This test method (ASTM D2582-93; [24]) covers the determination of the dynamic tear
resistance of plastic film and film sheeting subjected to enduse snagging-type hazards.
The puncture-propagation tear test measures the resistance of a material to snagging, or,
more precisely, to dynamic puncture and propagation of that puncture resulting in a tear.
Failure due to snagging hazard occurs in a variety of enduses, including industrial bags,
liners and tarpaulins. The tear resistance measured by the instrument in this test is in
newtons (N).
Tear resistance can be measured using a standard drop height of 508 ± 2 mm or a nonstandard drop height (or carriage weight).
12.4.4 Bending Stiffness (Flexural Modulus)
Test methods ASTM D747 [28] and D790 [29] cover the determination of the bending
stiffness of plastic sheets and films. In the test, specimens are subjected to three- or fourpoint bending loads, such as a cantilever beam, and the force and angle of bending are
used to determine the apparent flexural modulus (or bending stiffness) and yield strength.
12.4.5 Dynamic Mechanical Properties
Tests by dynamic mechanical analysis (DMA) provide the elastic and loss moduli as well
as the loss tangent (damping) as functions of temperature, frequency and/or time. These
plots are indicative of the viscoelastic characteristics of the plastic. As the modes of
molecular motion in the specimen change with temperature (or frequency), a
corresponding transition temperature occurs. The most significant transition temperatures
are the glass transition temperature (Tg) and the melting temperature (Tm). In addition,
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there may be a number of sub-glass transition temperatures, which can be very important
in determining the toughness of the material. In the temperature ranges where significant
changes are observed in the modes of molecular motion, a number of mechanical
properties, e.g., elastic modulus, decrease rapidly with increasing temperature (at constant
or near-constant frequency) or increase with increasing frequency (at constant
temperature). Hence DMA tests (provided by ASTM D4065; [30]) provide determination
of transition temperatures, elastic modulus and loss modulus over a range of temperatures
(from –160 °C to degradation), frequencies (0.01 to 1000 Hz) and times, by free vibration
and resonant or nonresonant forced vibration techniques. DMA is usually applied for
materials with elastic modulus from 0.5 MPa to 100 GPa [31].
DMA tests have been shown to be useful to evaluate a number of properties, for example,
(1) degree of phase separation (in multicomponent systems), (2) effects of a certain
processing treatment, and (3) filler type and amount, among others. DMA is very useful
for quality control in general, for specification acceptance and in research, and it can
also be used to determine, e.g., (1) stiffness and its change with temperature, (2) degree
of crystallinity, (3) magnitude of triaxial stress state in rubber phase for rubber-modified
plastics, etc.
DMA tests incorporate laboratory practice for determining the dynamic mechanical
properties of plastic films subjected to various oscillatory deformations on a variety of
instruments (generally called dynamic mechanical analysers, thermomechanical analysers,
mechanical spectrometers or even viscoelastometers).
12.5 Some Physical, Chemical and Physicochemical Tests
12.5.1 Density of Plastics
The density of solid plastics is a conveniently measurable property, which is useful to
follow the occurrence of physical changes, as well as to indicate uniformity among samples.
ASTM D1505 [32] covers the method for density determination through observation of
the level to which a test specimen sinks in a liquid column exhibiting a density gradient,
in comparison with standards of known density.
12.5.2 Indices of Refraction and Yellowness
The refractive index test is useful for controlling the purity and composition of films of
transparent plastics for simple identification purposes, and it is done by use of a
refractometer (ASTM D542; [33]), usually to four significant figures.
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Testing of Plastic Films
For homogeneous, non-fluorescent, nearly colourless transparent and/or nearly white
translucent-opaque plastic films, the yellowness index test is recommended to determine
the degree of yellowness or degree of its change. Yellowness is defined as the deviation in
chroma from whiteness in the dominant wavelength range from 570 to 580 nm relative
to magnesium oxide for CIE Source C. In the test, data are collected using a Hardy GE
type spectrophotometer or an equivalent system. A change in the yellowness index is
taken as a measure of degradation (under exposure to heat, light or other environment)
and has proved to be a very useful parameter for plastic films.
12.5.3 Transparency
The clarity of a film is measured by its ability to transmit light in the visible region.
The regular transmittance of film and sheet materials (defined as the ratio of undiffused
transmitted flux to the incident flux) can be obtained by following ASTM D1746 [34].
12.5.4 Resistance to Chemicals
Plastic films can be subjected to various chemicals and corrosive conditions, and their
resistance to these should be tested. ASTM D543 [35] covers a general test method for
all type of plastic materials. The test follows the changes in weight, dimensions, appearance
and strength properties. As indicated in the test, the choice of type and concentration of
reagent, duration of immersion and temperature are all arbitrary, and this poses the
main limitation of the method.
12.5.5 Haze and Luminous Transmittance
Light scattered from a film can produce a hazy or smoky field when viewed through the
material. Haze is the cloudy or turbid appearance of an otherwise transparent material
as a result of light scattered within or from the surface of the specimen. ASTM D1003
[36] provides a test method for the evaluation of specific light-transmitting and lightscattering properties of transparent plastic films. A hazemeter or a spectrometer [37] is
used, which can provide very useful diagnostic data for the reason for the haze.
In the test, the intensity of the incident light (I1), the total light transmitted by the specimen
(I2), the light scattered by the instrument (I3) and the light scattered by the instrument
and specimen (I4) are all measured. From these, the total transmittance (Tt) is calculated
as Tt = I2/I1; and the diffuse transmittance (Td) is calculated from:
Td =
I 4 − I 3( I2 / I1 )
I1
(12.1)
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From these, the per cent haze is calculated as:
haze =
Td
× 100
Tt
(12.2)
Materials having a haze value greater than 30% are considered diffusing and should
be tested.
12.5.6 Ignition, Rate of Burning Characteristics and Oxygen Index (OI)
Most plastic films are flammable. There are three different ASTM methods available to
test the ignition and rate of burning characteristics and to evaluate the per cent oxygen
necessary to initiate burning, namely the oxygen index. ASTM D635 [38] covers a smallscale laboratory screening procedure to compare the relative rates of burning of selfsupporting plastic films tested in the horizontal position using a burner. ASTM D1929
[39] is for determination of the self-ignition temperature (the lowest initial temperature of
air passing around the specimen at which, in the absence of an ignition source, the selfheating of the specimen leads to ignition) and flash ignition temperature (the lowest initial
temperature of air passing around the specimen at which a sufficient amount of combustible
gas is evolved to be ignited by a small external flame) of plastics by using a hot-air ignition
furnace. The oxygen index test (ASTM D2863; [40]) covers tests that find the minimum
oxygen concentration to support candle-like combustion of plastic film.
12.5.7 Static and Kinetic Coefficients of Friction
The frictional properties of film surfaces may contribute markedly to film behaviour
in packaging machinery and to the stacking properties of sacks. Slip agents are frequently
added to film to improve its frictional behaviour. However, films containing additives
often take considerable time to develop their full properties while the additives diffuse
to the surface, and care must be taken in choosing the time after manufacture to carry
out the test. The ASTM D1894-95 test method [41] covers determination of the
coefficients of starting and sliding friction of plastic film and sheeting, when relative
sliding occurs between the film and other substances under specified test conditions.
The procedure permits the use of a stationary sled with a moving plane film, or the use
of a moving sled with a stationary plane film. The static or starting coefficient of
friction (μs) is related to the force measured to begin movement of the surfaces relative
to each other. The kinetic or sliding coefficient of friction (μk) is related to the force
required to sustain this movement.
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Testing of Plastic Films
Measurements of frictional properties may also be made on a film or sheet specimen
when sliding over itself or over another substance. The coefficient of friction is related to
the slip properties of plastic films, which are of wide interest in packaging applications.
These methods yield empirical data for control purposes in film production. For instance,
slip properties are generated by additives in some plastic films, for example, polyethylene.
These additives have varying degrees of compatibility with the film matrix. Some of
them bloom, or extrude to the surface, lubricating it and making it more slippery. Because
this blooming action may not always be uniform in all areas of the film surface, values
from these tests may be limited in reproducibility. Besides, this blooming action of many
slip additives is time-dependent. For this reason, it is sometimes meaningless to compare
the slip and friction properties of films or sheets produced at different times, unless the
method is designed to study this effect.
Plastic films (not greater than 0.245 mm thick) and sheeting (greater than 0.245 mm
thick) may exhibit different frictional properties in their respective principal directions
due to anisotropy or extrusion effects. Specimens may be tested with their long dimensions
in either the machine direction or transverse direction of the sample, but it is more common
to test the specimen with its long direction parallel to the machine direction. The test
surface must be kept free of dust, lint, fingerprints, or any foreign matter that might
change the surface characteristics of the specimen. The static and kinetic coefficients of
friction (μs and μk, respectively), are calculated from:
μs = As/B
(12.3)
μk = Ak/B
(12.4)
where As is the initial scale reading (g) at which motion just begins, Ak is the average scale
reading (g) obtained during uniform sliding of the film surface and B = sled weight (g).
12.5.8 Specular Gloss of Plastic Films and Solid Plastics
This test (ASTM D2457-90; [42]) covers the measurement of gloss of plastic films, both
opaque and transparent. Specular gloss is defined as the relative luminous reflectance
factor of a specimen in the mirror direction. Specular gloss is used primarily as a measure
of the shiny appearance of film and surfaces. Precise comparisons of gloss values are
meaningful only when they refer to the same measurement procedure and the same general
type of material. In particular, gloss values for transparent films should not be compared
with those of opaque films, and vice versa.
Gloss is a complex attribute of a surface, which cannot be completely measured by any single
number. Specular gloss usually varies with surface smoothness and flatness. The instrument
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Handbook of Plastic Films
used consists of an incandescent light source to produce the incident beam, a means to locate
the surface of the specimen, and a receptor located to receive the required pyramid of rays
reflected by the specimen. The receptor is a photosensitive device that is responsive to visible
radiation. The receptor measurement mechanism should give a numerical indication that is
proportional to the light flux passing the receptor field stops within ±1% of full-scale readings.
Specimen surfaces should have good planarity, since surface warpage, waviness or curvature
may seriously affect test results. The direction of machine marks, or similar texture effects,
should be parallel to the plane of the axes of the two beams. Surface test areas must be kept
free of soiling and abrasion. Gloss is due chiefly to reflection at the surface; therefore,
anything that changes the surface physically or chemically is likely to affect gloss.
12.5.9 Wetting Tension of PE and PP Films
In this test method (ASTM D2578-94 [43]) drops of a series of mixtures of formamide
and cellosolve (ethyleneglycol monoethyl ether) of gradually increasing surface tension
are applied to the surface of the polyethylene or polypropylene film until a mixture is
found that just wets the film surface. The wetting tension of the PE or PP film surface
will be approximated by the surface tension of this particular mixture. The ability of PE
and PP films to retain inks, coating, adhesives, etc., is primarily dependent upon the
character of their surfaces, and can be improved by one of several surface-treatment
techniques mentioned in Chapter 8.
The same treatment techniques have been found to increase the wetting tension of PE or
PP film surfaces in contact with a mixture of formamide and ethyl cellosolve in the presence
of air. It is therefore possible to relate the wetting tension of a PE or PP film surface to its
ability to accept and retain inks, coating, adhesives, etc. The measured wetting tension of
a specific film surface can only be related to acceptable ink, coating, or adhesive retention
through experience. Wetting tension in itself is not a completely reliable measure of ink or
coating retention, or adhesion. A wetting tension of 3.5 × 10-2 N/m or higher has generally
been found to reveal a degree of treatment normally regarded as acceptable for tubular
film made from PE and intended for commercial flexographic printing.
A table showing the measured wetting tension of PE and PP film as a function of the
concentration of a mixture of ethyl cellosolve and formamide is given in ASTM D257894 [43].
Note that a solution is considered to wet a test specimen when it remains intact as a
continuous film of liquid for at least 2 seconds. The reading of the liquid film behaviour
should be made in the centre of the liquid film. Shrinking of the liquid film about its
periphery does not indicate lack of wetting. Breaking of the liquid film into droplets
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Testing of Plastic Films
within 2 seconds does indicate lack of wetting. Too much liquid being placed upon the
film surface may cause severe peripheral shrinkage.
12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films
Unrestrained linear thermal shrinkage, expressed as the percentage of the original dimension,
is defined as ‘the irreversible reduction in linear dimension at elevated temperatures where
no restraint to inhibit shrinkage is present’. During the manufacturing processes, internal
stresses that occur might be locked into the film, which can be released afterwards by
proper heating. The temperature at which shrinkage occurs is mainly related to the processing
techniques employed and may also be related to the phase transition in the base resin. The
magnitude of the shrinkage varies with the temperature. Shrinkage of a particular material
produced by a process may be characterised by the ASTM D2732 test method [44], by
making measurements at several temperatures through the shrinkage range of the material.
The experiment is usually carried out in a constant-temperature liquid bath accurate to
±0.5 °C. It is a prerequisite that the liquid of the bath should not plasticise or react with the
specimen. Polyethyleneglycol, glycerine and water have been found to have wide applicability
for this purpose. Immersion of the sample (100 × 100 mm2) for 10 s has been determined
to be generally adequate for most thermoplastics of up to 50 μm thickness.
Unrestrained linear shrinkage is calculated using:
unrestrained linear shrinkage (%) =
L0 − Lf
× 100
L0
(12.5)
where L0 is the initial length of side (100 mm) and Lf is the length of side after shrinkage.
12.5.11 Shrink Tension and Orientation Release Stress
The ASTM D2838 test [45] measures the maximum force of a totally restrained specimen
and the maximum force of a specimen permitted to shrink a predetermined amount
prior to restraint in a liquid bath at selected temperatures. The results obtained are
especially important and useful for shrink-wrap films and shrink-wrap packaging design.
12.5.12 Rigidity
Rigidity affects the machinability of plastics. It depends mainly on the stiffness of the material,
on its thickness, as well as on a number of other factors such as static electricity, frictional
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Handbook of Plastic Films
properties, etc. The standard test method ASTM D2923 [46] is specific for the rigidity of
polyolefin films and sheeting. In the test, the resistance of the sample to flexure is measured
(by a strain gauge affixed to the end of the sample) and by use of a microammeter connected
to the gauge and calibrated; rigidity is read directly as grams per centimetre of sample width.
12.5.13 Blocking Load by Parallel-Plate Method
Blocking (unwanted adhesion) is a problem with plastic films, which develops during
processing and/or storage, and happens when touching layers of films are in intimate
contact with almost complete exclusion of air between them. Blocking is induced by
increase of temperature and/or pressure. The standard test method provided by ASTM
D3354 [47] simulates the operation of separating blocked films in some enduse
applications. The load (in grams) needed to separate blocked samples [five groups of
specimens each cut to 100 × 180 mm2] is measured by a beam-balance system (similar to
an analytical balance). The test, in summary, is as follows: One sheet of the blocked
specimen is secured to an aluminium block suspended from the end of the balance beam,
while the other end is fixed to another aluminium block fastened to the balance base.
Weight is then added equivalent to 90 ± 10 g/m to the other side of the beam until the
films totally separate (or until they reach 1.905 cm separation). The film-to-film adhesion
is expressed as grams, and the test is limited to maximum 200 g of load.
12.5.14 Determination of LLDPE Composition by 13C NMR
The performance properties of ethylene copolymer plastic films depend on the number
and type of short-chain branches. The ASTM D5017 method [48] allows one to measure
them for ethylene copolymers with propylene, 1-butene, 1-octene and 4-methyl-1-pentene.
For this, the polymer sample (about 1.2 g) is dispersed in a solvent (1.5 ml) and a deuterated
solvent (1.3 ml), put into a 10 mm nuclear magnetic resonance (NMR) tube, and analysed
at high temperatures by using 13C NMR spectroscopy, usually a 13C pulsed Fourier
transform with a field strength of at least 2.35 T. Spectra are recorded under conditions
such that the responses of each chemically different carbon are identical. The integrated
responses for carbons originating from different comonomers are used for calculation of
the copolymer composition. Results are presented as mole per cent alkene and/or branches
per 1000 carbon atoms.
12.5.15 Creep and Creep Rupture
Creep is defined as the increasing strain over time in the presence of a constant stress,
and is expressed as the per cent extension (creep strain per cent). The practical importance
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Testing of Plastic Films
of creep is due to the need (a) to determine the limits of excessive deformation and (b) to
understand creep rupture. The two mechanisms are especially important for LDPE
greenhouse covering materials, because LDPE, with 8.23% creep, has the second highest
creep value of all greenhouse covering films [11]. There is a significant variation in the
creep values of LDPE, which is attributed to the fact that the creep resistance of LDPE
increases with density and with the content of ethylene-vinyl acetate (EVA) in the material’s
composition. Creep is also strongly dependent on the service temperature of the covering
material. ASTM D2990 [49] is a general test method for plastics to characterise creep
and creep rupture. The test is applicable to different loading conditions, (e.g., tensile,
flexural, compressive, etc.), and helps to determine the creep strength and modulus of
standard specimens for use in comparing materials and in design.
12.5.16 Outdoor Weathering/Weatherability
The ASTM D1435 test [50] is used to evaluate the stability of plastic films when exposed
outdoors to the varied influences of the atmosphere and weather. The general climate,
the season, the time of day, the presence or absence of industrial pollutants in the
atmosphere, and annual variations in the weather are the most important factors, and
the results are taken as indicative only. Short-term accelerated exposure tests are also
available by use of a special chamber equipped with a carbon-arc light (ASTM G152
[51] and ASTM G153 [52]), which can indicate the relative outdoor performance, but
cannot be used to predict the absolute long-term performance.
12.5.17 Abrasion Resistance
Abrasion is a surface phenomenon that occurs mechanically, and it is important in the
sense that it can significantly degrade certain physical properties (light transmission,
thermal effect through loss of thickness, etc.), as well as some mechanical properties,
(e.g., impact resistance, tear resistance). As a result it has a direct impact on the functional
characteristics of covering materials. Abrasive damage to transparent plastic films is
judged by following the change of the optical properties (ASTM D1044; [53]) as well as
by volume loss in general (by using abrasion testing machines, ASTM D1242; [54]).
The abrasion resistance of plastic films used in greenhouses is of utmost importance.
Abrasion, in this case, occurs due to the effect of particles carried by the wind, which can
be significant in some areas where greenhouses are built. In this case, abrasion can lead
to the loss of transparency and reduction in mechanical properties much earlier than
expected. Abrasion in general is affected by the exact formulation of the film, and by the
incorporation of (amount and type of) filler, additives and pigments, which can lead to
varying results. Another important factor is that rapid chemical oxidation of the surface
layer may occur due to the buildup of localised high temperatures during abrasion [55].
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Handbook of Plastic Films
It is worth mentioning that abrasion can ultimately lead to increased degradation of the
film, since more active centres for photooxidation are in general created by this procedure.
There is a direct relationship between the density of a PE film and its abrasion resistance:
increase of density increases the abrasion resistance.
12.5.18 Mar Resistance
Test ASTM D673 [56] covers the determination of the extent of resistance of plastic film
surfaces to surface marring, mainly caused by falling abrasive particles. The test simulates
the relatively mild airborne abrasive action that occurs in actual use, and different materials
are ranked according to their relative mar resistances.
12.5.19 Environmental Stress Cracking
The ability of a polymer surface to withstand an aggressive medium under load is
known as environmental stress-cracking resistance (ESCR). Environmental stress
cracking is a characteristic that depends on the nature and level of stresses applied as
well as on the thermal history of the sample and the environment, and is also called
stress corrosion [57]. Under certain conditions of stress and in the presence of certain
environments, environmental stress cracking occurs. For example, in the presence of
soaps, wetting agents and detergents, ethylene plastics may exhibit mechanical failure
by cracking. Typically, increased ESCR is obtained with increased polymer molecular
weight. ASTM D1693 [58] is specific for the environmental stress cracking of ethylene
plastics. A stress crack is an external or internal rupture in the film caused by tensile
stresses less than its short-time mechanical strength. The environment accelerates the
development of stress cracks. The appearance of what seem to be cracks on the surfaces
of transparent polymers develops under tensile stress, with the plane of the craze being
normal to the stress direction. Crazes usually initiate at surfaces but can develop
internally under special circumstances as well. They reflect light in a manner similar to
cracks, and indeed often precede early fracture of the film. In the test, bent specimens,
each having a controlled imperfection on one surface, are exposed to the action of a
surface-active agent, and the proportion of the total number of specimens that crack in
a given time is reported.
12.5.20 Water Vapour Permeability
In the packaging of hygroscopic materials, and particularly in packaging of food, the
permeability properties of the film to water vapour and other gases is very important.
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Testing of Plastic Films
ASTM D3079 [59] and E96 [60]/F372 [61]/F1249 [62] cover standard test methods for
plastic packaging films and plastics for water vapour permeability. These tests are especially
important and are used for packaging plastic films. In the first of these, desiccant- or
product-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constant
temperature, and weighings are repeated to constant rate of moisture gain. Water vapour
permeabilities are reported in grams per 30 days. In the second test method, desiccant- or
product-filled packages are again exposed to a normal atmosphere of 90 ± 2% RH at two
different temperatures for 24 hours and 6 days, respectively. Hence cycling between cold
and hot/moist atmospheres is achieved. In this test, weighings are repeated to constant rate
of moisture gain, and water vapour permeability is reported in grams per cycle. In the third
method mentioned, desiccant- or product-filled packages are exposed to a normal
atmosphere of 90 ± 2% RH at constant temperature for at least 1 month; average rate of
water gain is reported. In the last two methods, infrared (F372) and modulated infrared
(F1249) detection of water vapour transmitted from a moist atmosphere to a dry air stream
is made, which provides a measure of water vapour transmission rates.
12.5.21 Oxygen Gas Transmission
ASTM D1434 [63] and D3985 [64] cover standard test methods for packaging plastic
films and sheeting materials for their oxygen gas transmission. Basically the methods used
can be divided into three types, varying either pressure, volume or concentration. In variablevolume methods, gas is introduced at a high pressure on one side of the film, the chamber
on the other side normally being at atmospheric pressure. The change in volume is followed
as a function of time. A manometer is used to measure the pressure of oxygen transmitted,
from which the rate of transmission at steady state can be calculated. In another method,
a coulometric sensor is used, which measures the rate of oxygen transmitted through a
specimen exposed on one surface to oxygen and on the other to nitrogen.
Considerable experimental difficulties are normally encountered in achieving airtight
seals and in the initial calibration of the instrument to allow for the deadspace in the
filter paper and discs used to support the film
12.6 Standard Specifications for Some Plastic Films
There are several standards available to specify plastic films, such as:
•
ASTM D5047 [65] for polyethylene terephthalate (PET) films;
•
ASTM D4635 [66] for LDPE films;
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Handbook of Plastic Films
•
ASTM D3981 [67] for medium-density polyethylene (MDPE) films;
•
ASTM D2103 [68] for PE films;
•
ASTM D2673 [69] for oriented polypropylene (OPP) films;
•
ASTM D2647 [70] for crosslinkable ethylene plastics.
12.6.1 Standard Specification for PET Films
Specification ASTM D5047 [65] covers biaxially oriented PET films in the range of
1.5-35.5 μm, containing at least 90% PET homopolymer. The thicknesses should be
within ±18% to ±14% of nominal for film tested in accordance with ASTM D374
[71]; while the requirements for the width (within ±1.6 mm and ±3.2 mm of nominal
for rolls up to 1 m or larger, respectively), and weight (within ±10% and ±5% for
orders up to 110 kg or over, respectively), are also given. The film will be tested
appropriately to establish conformance to the critical requirements as agreed by the
purchaser and seller.
12.6.2 Standard Specification for LDPE Films (for General Use and Packaging
Applications)
Specification ASTM D4635 [66] covers unpigmented, unsupported, tubular LDPE films
with densities between 910 and 925 kg/m3 (0.910-0.925 gm/cm3), for general use and
for packaging applications. It is also applicable to polyethylene copolymer (low-pressure
PE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylenevinyl acetate copolymers. The thicknesses are 100 μm or less and the maximum widths
are 3 m. The specification covers dimensional tolerances (including thickness, width,
length and yield), intrinsic quality requirements (density, workmanship, tensile strength,
heat sealability, odour, impact strength, coefficient of friction, optical properties, surface
treatment, etc.), and test methods.
12.6.3 Standard Specification for MDPE and General Grade PE Films (for
General Use and Packaging Applications)
Specification ASTM D3981 [67] is for unpigmented, unsupported, sheet or tubular
MDPE films with densities between 926 and 938 kg/m3 (0.926-0.938 g/cm3), for general
use and for packaging applications. It is also applicable to polyethylene copolymer
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Testing of Plastic Films
(low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers,
including ethylene-vinyl acetate copolymers. The thicknesses are 25-100 μm and the
maximum widths are 3.05 m. The standard excludes heat-shrinkable films. The
specification covers dimensional tolerances (including thickness, width, length and
yield), intrinsic quality requirements (density, workmanship, tensile strength, heat
sealability, odour, impact strength, coefficient of friction, optical properties, surface
treatment, etc.) and test methods.
Specification ASTM D2103 [68] covers general specifications for polyethylene films.
12.6.4 Standard Specification for OPP Films
Specification ASTM D2673 [69] covers OPP films of 10-50 μm thickness with ±10%
of the nominal value, composed of Group 1 or 2 propylene (ASTM D4101 [72]), or a
blend of such Group 1 and/or Group 2 polypropylene with one or more other types of
polymers where the polypropylene fraction is the main component. It must have normal
appearance (be free of gel, streaks, pinholes, particulates, etc., as well as undispersed
raw materials) and it should not block excessively. The average width will be within
–3 to +19 mm of nominal.
If the film yields a minimum tensile strength of 103 MPa in at least one principal
(machine or transverse) direction, it is termed oriented polypropylene (OPP). If the
film is oriented in one (machine or transverse) direction and yields a minimum tensile
strength of 103 MPa in the orientation direction, it is called as uniaxially oriented PP
film. If the film tensile strengths in both the machine and transverse directions exceed
103 MPa, it is biaxially oriented PP. If the film tensile strengths in both the machine
and transverse directions exceed 103 MPa, but do not differ by more than 55 MPa,
and the machine and transverse elongations do not differ by more than 60%, it is
balanced oriented PP.
12.6.5 Standard Specification for Crosslinkable Ethylene Plastics
Specification ASTM D2647 [70] covers crosslinkable ethylene plastics and compounds.
There are mainly two different types: mechanical types (type I) and electrical types (type
II). In the former, mechanical properties (strength, ultimate elongation, elongation
retention after ageing, apparent modulus of rigidity, brittleness temperature) are the
most important in applications.
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Handbook of Plastic Films
References
1.
D833, Terminology Relating to Plastics, 2000.
2.
D. Briassoulis and A. Aristopoulou, Polymer Testing, 2001, 20, 615.
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ASTM D618-00, Standard Practice for Conditioning Plastics for Testing, 2000.
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ASTM D638-02, Standard Test Method for Tensile Properties of Plastics, 2002.
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ASTM D882-02, Standard Test Method for Tensile Properties of Thin Plastic
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6.
ASTM D4321-99, Standard Test Method for Package Yield of Plastic Film, 1999.
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ISO 527-1, Plastics - Determination of Tensile Properties - General Principles,
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8.
ISO 527-2, Plastics - Determination of Tensile Properties - Test Conditions for
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ISO 527-3, Plastics - Determination of Tensile Properties - Part 3: Test
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15. ASTM D1709-01, Standard Test Methods for Impact Resistance of Plastic Film
by the Free-Falling Dart Method, 2001.
16. ISO 7765-1, Plastics Film and Sheeting - Determination of Impact Resistance by
the Free-Falling Dart Method - Part 1: Staircase Methods, 1999.
17. ISO 7765-2, Plastics Film and Sheeting — Determination of Impact Resistance by
the Free-Falling Dart Method — Part 2: Instrumented Puncture Test, 1999.
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Testing of Plastic Films
17a. ASTM D1790-02, Standard Test Method for Brittleness Temperature of Plastic
Sheeting by Impact, 2002.
18. ASTM D746-98e1, Standard Test Method for Brittleness Temperature of Plastics
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19. ASTM D3420-95, Standard Test Method for Pendulum Impact Resistance of
Plastic Film, 1995.
20. ASTM D4272-99, Standard Test Method for Total Energy Impact of Plastic
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21. ASTM D1004-94a, Standard Test Method for Initial Tear Resistance of Plastic
Film and Sheeting, 1994.
22. ASTM D1938-02, Standard Test Method for Tear-Propagation Resistance
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23. ASTM D1922-00a, Standard Test Method for Propagation Tear Resistance of
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24. ASTM D2582-00, Standard Test Method for Puncture-Propagation Tear
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26. ISO 6383-1, Plastics - Film and Sheeting - Determination of Tear Resistance Trouser Tear Method, 1983.
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29. ASTM D790-02, Standard Test Methods for Flexural Properties of Unreinforced
and Reinforced Plastics and Electrical Insulating Materials, 2002.
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31. J.D. Ferry, Viscoelastic Properties of Polymers, 2nd Edition, Wiley, New York,
NY, USA, 1961.
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Handbook of Plastic Films
32. ASTM D1505-98e1, Standard Test Method for Density of Plastics by the
Density-Gradient Technique, 1998
33. ASTM D542-00, Standard Test Method for Index of Refraction of Transparent
Organic Plastics, 2000.
34. ASTM D1746-97, Standard Test Method for Transparency of Plastic Sheeting, 1997.
35. ASTM D543-95 (2001), Standard Practices for Evaluating the Resistance of
Plastics to Chemical Reagents, 2001.
36. ASTM D1003-00, Standard Test Method for Haze and Luminous Transmittance
of Transparent Plastics, 2000.
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38. ASTM D635-98, Standard Test Method for Rate of Burning and/or Extent and
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39. ASTM D1929-96(2001)e1, Standard Test Method for Determining Ignition
Temperature of Plastics, 2001.
40. ASTM D2863-00, Standard Test Method for Measuring the Minimum Oxygen
Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), 2000.
41. ASTM D1894-01, Standard Test Method for Static and Kinetic Coefficients of
Friction of Plastic Film and Sheeting, 2001.
42. ASTM D2457-97, Standard Test Method for Specular Gloss of Plastic Films and
Solid Plastics, 1997.
43. ASTM D2578-99a, Standard Test Method for Wetting Tension of Polyethylene
and Polypropylene Films, 1999.
44. ASTM D2732-01, Standard Specification for Polyethylene (PE) Plastic Tubing, 2001.
45. ASTM D2838-02, Standard Test Method for Shrink Tension and Orientation
Release Stress of Plastic Film and Thin Sheeting, 2002.
46. ASTM D2923-01, Standard Test Method for Rigidity of Polyolefin Film and
Sheeting, 2001.
47. ASTM D3354-96, Standard Test Method for Blocking Load of Plastic Film by
the Parallel Plate Method, 1996.
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Testing of Plastic Films
48. ASTM D5017-96, Standard Test Method for Determination of Linear Low
Density Polyethylene (LLDPE) Composition by Carbon-13 Nuclear Magnetic
Resonance, 1996.
49. ASTM D2990-01, Standard Test Methods for Tensile, Compressive, and Flexural
Creep and Creep-Rupture of Plastics, 2001.
50. ASTM D1435-99, Standard Practice for Outdoor Weathering of Plastics, 1999.
51. ASTM G152-00ae1, Standard Practice for Operating Open Flame Carbon Arc
Light Apparatus for Exposure of Nonmetallic Materials, 2000.
52. ASTM G153-00ae1, Standard Practice for Operating Enclosed Carbon Arc Light
Apparatus for Exposure of Nonmetallic Materials, 2000.
53. ASTM D1044-99, Standard Test Method for Resistance of Transparent Plastics
to Surface Abrasion, 1999.
54. ASTM D1242-95a, Standard Test Methods for Resistance of Plastic Materials to
Abrasion, 1995.
55. V. Shah, Handbook of Plastic Testing Technology, Wiley, New York, NY, USA, 1984.
56. ASTM D673 discontinued not replaced
57. R.P. Kambour and A.S. Holik, Journal of Polymer Science, A-2: Polymer Physics,
1969, 7, 1393.
58. ASTM D1693-01, Standard Test Method for Environmental Stress-Cracking of
Ethylene Plastics, 2001.
59. ASTM D3079-94 (1999), Standard Test Method for Water Vapor Transmission
of Flexible Heat-Sealed Packages for Dry Products, 1999.
60. ASTM E96-00e1, Standard Test Methods for Water Vapor Transmission of
Materials, 2000.
61. ASTM F372-99, Standard Test Method for Water Vapor Transmission Rate of
Flexible Barrier Materials Using an Infrared Detection Technique, 1999.
62. ASTM F-1249-01, Standard Test Method for Water Vapor Transmission Rate
Through Plastic Film and Sheeting Using a Modulated Infrared Sensor, 2001.
63. ASTM D1434-82(1998), Standard Test Method for Determining Gas
Permeability Characteristics of Plastic Film and Sheeting, 1998.
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64. ASTM D3985-02, Standard Test Method for Oxygen Gas Transmission Rate
Through Plastic Film and Sheeting Using a Coulometric Sensor, 2002.
65. ASTM D5047-95, Standard Specification for Polyethylene Terephthalate Film
and Sheeting, 1995.
66. ASTM D4635-01, Standard Specification for Polyethylene Films Made from
Low-Density Polyethylene for General Use and Packaging Applications, 2001.
67. ASTM D3981-95, Standard Specification for Polyethylene Films Made from
Medium-Density Polyethylene for General Use and Packaging Applications,
1995.
68. ASTM D2103-97 Standard Specification for Polyethylene Film and Sheeting, 1997.
69. ASTM D2673-99, Standard Specification for Oriented Polypropylene Film, 1999.
70. ASTM D2647-94 (2000) e1, Standard Specification for Crosslinkable Ethylene
Plastics, 2000.
71. ASTM D374, Standard Test Methods for Thickness of Solid Electrical
Insulation, 1999.
72. ASTM D4101, Standard Specification for Polypropylene Injection and Extrusion
Materials, 2002.
356
13
Recycling of Plastic Waste
E.M. Abdel-Bary
13.1 Introduction
Polymeric materials (plastics and rubbers) comprise a steadily increasing proportion of
the municipal and industrial waste going into landfill. Owing to the huge amount of
plastic wastes and environmental pressures, recycling of plastics has become a predominant
subject in today’s plastics industry. Development of technologies for reducing polymeric
waste, which are acceptable from the environmental standpoint and are cost-effective,
has proven to be a difficult challenge, because of the complexities inherent in the reuse of
polymers. Establishing optimal processes for the reuse/recycling of polymeric materials
thus remains a worldwide challenge in the new century.
Compared with other countries, there is a huge amount of plastic waste in the USA
(taken as a reference), where five main types of polymers dominate in the plastics waste
stream. The highest polymer waste results from low-density polyethylene (LDPE), at 5
million tons per year; high-density polyethylene (HDPE) is second, at 4.1 million tons;
then come polypropylene (PP) at 2.6 million tons, followed by polystyrene (PS) at 2
million tons and polyethylene terephthalate (PET) at 1.7 million tons [1]. These five
polymer types, together with polyvinyl chloride (PVC), also dominate the plastics waste
stream in the European Community [2].
Plastic films find applications in agriculture as well as in plastic packaging, which is a
high-volume market owing to the many advantages of plastics over other traditional
materials. However, such material is also the most visible in the waste stream, and has
received a great deal of public criticism as films have comparatively short life-cycles and
usually are non-degradable [3].
The majority of plastic films are made from LDPE or linear low-density polyethylene
(LLDPE), comprising approximately 68% of the total film production. In addition, HDPE
resins are commonly used in film plastics. Non-polyethylene resins constitute the remainder
of the film plastic types found in the market place. PP, PVC and Nylon resins comprise
the bulk of these other film types. Increasingly, certain multilayer or coextruded films
are used in special applications that seek to combine the performance attributes of two
or more resins for such applications.
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Businesses can save money by reducing their disposal expenses, in the form of both tonnagebased tipping fees and container hauling fees. This is especially evident with plastic films,
where a high volume-to-weight ratio can mean more container pulls per ton hauled.
13.2 Main Approaches to Plastic Recycling
There are four main approaches to recycling plastics (excluding, as not acceptable,
dumping on land or at sea with or without prior treatment) [4]. These are primary,
secondary, tertiary and quaternary recycling.
13.2.1 Primary Recycling
This is the recycling of clean, uncontaminated, single-type waste, and it remains the
most popular as it ensures simplicity and low cost, especially when done ‘in-plant’ and
fed with scrap of controlled history [5]. The recycled scrap or waste is either mixed with
virgin material to assure product quality or used as second-grade material [6]. Primary
recycling is very simple without any precautions except the proper and clean collection
of the waste in the plant.
13.2.2 Secondary Recycling
13.2.2.1 Approaches to Secondary Recycling
There are two main approaches to secondary recycling. One approach is to separate the
plastics from their contaminants and then segregate the plastics into generic types, one or
more of which is then recycled into products produced from virgin or primary recycled
material. The other approach is to separate the plastics from their associated contaminants
and remelt them as a mixture without segregation. The treatment of the plastics-containing
waste streams may include: size reduction by granulators, shredders or crumblers; separation
of plastics from other waste materials and from one another; cleaning; drying; and
compounding [7, 8]. The actual order and number of operations in a particular treatment
system depends on the waste being processed and the desired quality of the final material [9].
13.2.2.2 Mechanical Recycling
Mechanical recycling is mainly related to secondary recycling. The main steps include
separating, sorting and washing to get rid of contamination, especially for plastic films,
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which possess a large surface area and consequently have a large degree of contamination.
In a chemical recycling plant, one should have shredders, metal and mineral separators,
prewashing and granulation, second washing stage, mechanical grinding and dirt removal,
hydrocyclone separation, dewatering and melt processing.
The separation of plastic waste is one of the main factors restricting high performance in
plastic recycling. The separation of plastics into desired categories as well as the elimination
of contaminants is an ongoing technological development process. The aim is to develop
automatic and continuous separation technology to minimise the handling of waste and
to achieve a more efficient recycling process. Probably the best alternative for pure plastic
streams is not to allow them to mix in the first place, neither among themselves nor with
contaminants. If separation starts at the consumer level and at the source point of
collection, there will be fewer difficulties during the recycling.
13.2.3 Tertiary Recycling
Tertiary recycling includes chemical recycling. The terms ‘chemical recycling’ and
‘feedstock recycling’ of plastics are sometimes collectively referred to as ‘advanced recycling
technologies’. In these processes, solid plastic materials are converted into smaller
molecules as chemical intermediates through the use of heat. These chemical intermediates,
usually liquids or gases, but sometimes solids or waxes, are suitable for use as feedstocks
for the production of new petrochemicals and plastics.
The technical and economic feasibility and overall commercial viability of advanced
recycling methods must be considered in each step of the recycling chain, consisting of
collection, processing and marketing. All of them are critical to the success of chemical
and feedstock recycling. Today, most of these technologies remain developmental and
have not yet proven themselves sustainable in a competitive market. Nevertheless, they
remain of considerable interest in their longer-term potential.
The term ‘feedstock recycling’ encompasses chemical recycling but is often applied to the
thermal depolymerisation of polyolefins and substituted polyolefins into a variety of
smaller hydrocarbon intermediates. Fluidised bed pyrolysis investigations of LDPE have
provided data on the suitability of the process and on the influence of the process
conditions on the compatibility of the feedstock produced with the conventional petroleum
feedstock [10]. The gases produced from the pyrolysis of LDPE are mainly hydrogen,
methane, ethane, ethylene, propane, propene, butane and butene. Also, it has been reported
that the thermolysis products of HDPE consist of 80-90 wt% straight-chain alkanes and
1-alkenes. Subsequent hydrogenation of the PE oil resulted in a diesel fuel with high
cetane index and low sulfur and aromatic contents [11].
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In some cases, addition polymers such as polystyrene and polymethyl methacrylate can
be thermally depolymerised back to their corresponding monomers in reasonably high
yield. The term ‘chemical recycling’ is often applied also to the depolymerisation of
certain condensation polymers back to monomers. Examples of these types of plastics
are polyesters, polyamides and polyurethanes. Chemical recycling thus mainly includes
pyrolysis, gasification, hydrogenation, hydrolysis, glycolysis and depolymerisation.
A new reactor system was developed for the recovery of fuels from waste plastic mixtures
in a steam atmosphere. The degradation mechanisms of two polyolefins (PE and PP),
two polyamides (Nylon-6 and Nylon-6,6), polystyrene and three polyesters
(polycarbonate, polybutylene terephthalate and polyethylene terephthalate) in both
nitrogen and steam as the carrier gas have been investigated [12]. The oil produced from
the proposed reactor system was continuously upgraded to produce gasoline and kerosene
over a Raney nickel catalyst in a steam atmosphere.
13.2.4 Quaternary Recycling
Quaternary recycling includes the recovery of the energy content of plastic wastes. Owing
to a lack of other recycling possibilities, incineration (combustion) aimed at the recovery
of energy is currently the most effective way to reduce the volume of organic material.
This may then be disposed of to landfill. Plastics, either thermoplastics or thermosets,
are actually high-yielding energy sources. For example, one litre of heating oil has a net
calorific value of 10,200 kcal, whereas 1 kg of plastics releases 11,000 kcal worth of
energy; for comparison, it should be added that 1 kg of briquettes (blocks of pressed coal
dust) has a net calorific value of 4,800 kcal. It has been estimated that, by burning 1 ton
of organic waste, approximately 250 litres of heating oil could be saved [13]. Clean
incineration of municipal solid waste (MSW) is widely accepted in countries like Sweden
and Germany (50% of total MSW), Denmark (65%), Switzerland (80%) and Japan
(70%) [14]. Although there are very stringent emissions regulations, more than 50 refuse
incineration units are working in Germany.
The energy that can be recovered from the incineration of plastics depends on the type of
plastic. It has been estimated (in kcal/kg) as: 18,720 for PE; 18,343 for PP; 16,082 for
PS; 13,179 for phenol-formaldehyde; 11,362 for foamed polyurethane (PU); 10,138 for
Nylon; 8,565 for polyvinyl acetate (PVAc); 7,516 for PVC; and 7,014 for PU. This energy
is on average 10,000 kcal/kg. Each ton will release about 107 kcal.
However, plastics emit some objectionable gases and form some hazardous compounds.
Thus, recovering energy from plastic waste is not cheap. The main goal must be to avoid
the formation of these hazardous compounds by the correct construction of incinerators
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Recycling of Plastic Waste
and by considering all proper means to avoid pollution. The costs of operating,
maintaining and monitoring an incinerator are quite substantial compared to those for a
conventional power plant. Energy from waste (EFW) is a reliable and renewable source
of energy, especially if the MSW is rich in organic matter. Furthermore, it reduces the
amount of waste to be landfilled at the final stage. Costs involved in developing new
landfills can partly offset the high costs of energy recovery from an EFW facility.
Incinerators with EFW installations are not considered just as power plants, as their
main purpose is to reduce the amount of garbage being landfilled within the purpose of
an integrated waste management system.
Incineration plants should be designed and operated to produce the least amount of
pollution. The use of incineration plants is mandatory for plastic wastes from hospitals
and similar institutions, which is considered as a potential source of disease. Incinerators
do not emit ethane gas, as this gas is completely combusted into CO2 and water, even at
low temperature. However, incinerators have often been associated with dioxin and furan
emissions, which are avoided in modern ones by working at temperatures that are high
enough to decompose such chemicals and prevent them from reaching the ecosystem.
Although dioxin and furan are often perceived as two individual chemical products,
there are in fact 75 congeners of polychlorinated dibenzodioxins (PCDD) and 135
congeners of polychlorinated dibenzofurans (PCDF), each differing in its chemical
configuration and degree of toxicity. The most toxic of the dioxins is 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD). When assessing health risks, the total congeners
are converted to the equivalent TCDD. Dioxins and furans are present in Nature and are
generated by many sources, such as forest fires and as a by-product of a certain chemical
processes and the burning of wood in stoves and fireplaces, barbecues, diesel engines,
power plants, ponds and so on. Scientists can account for about 60% of the dioxins
found in Nature (referred to as ‘background level’), while the source of the remainder is
still unknown. Dioxins enter the human body through the food chain, inhalation and
skin contact. As long as the quantities absorbed are very minute, however, they do not
represent a health hazard.
The complete combustion of organic matter removes all the dioxins present in the garbage
However, during cooling of the flue gases, traces of dioxins are formed. An energy-fromwaste facility acts as a reliable and renewable source of energy. It is a safe method of
reducing the volume of waste dumped in landfills. A considerable reduction in the emission
of greenhouse gases compared to landfills can be achieved. However, further research is
needed to avoid completely this emission.
Recovery of energy from solid waste constitutes the fourth ‘R’ after reducing, reusing
and recycling. Research as the fifth ‘R’ is the key element. Scientists and environmental
scientists have to work together to develop new methods for recycling more products.
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13.2.5 Conclusion
In conclusion, primary recycling is ideal for clean, uncontaminated single-type scrap,
but degradation during service life or reprocessing should be taken under serious
consideration.
Secondary recycling by type can be accomplished by various methods, but the cost
associated with the separation and decontamination of the wastes undoubtedly poses an
inherent obstacle. Dissolution-based techniques seem worth developing, but cannot yet
be considered to be the complete answer. Secondary recycling of plastics mixtures by
remelting is intended to produce downgraded products as a result of incompatibility
problems. Compatibilisation is effective only in specific cases of plastics mixtures.
Tertiary or ‘chemical’ recycling processes involve high levels of investment and succeed
in recovering the chemical products, but negate the value added during the polymerisation.
The latter comment is valid also for the last resort, quaternary recycling (energy recovery
of plastics waste), which can substitute other energy sources and solve disposal problems.
However, it stands strongly accused of undesirable emissions.
The first and most important steps in plastic recycling are collection and sorting, after
which the recycling process depends on the type of plastic and the field of application.
These issues will now be addressed.
13.3 Collection and Sorting
Collection involves gathering lightweight packaging films and other materials. Plastic
packaging from separate collection streams is separated from other lightweight packaging
material and sorted into fractions comprising film containers, mixed plastics and residues.
Identification of the plastic type is one of the most important elements in recycling because
most recycling processes prohibit certain types of plastics. For example, severe problems
appear during processing of recycled resin of unknown origin. Thus, extrusion and injection
moulding require accurate identifications of plastic waste, otherwise a product with bad
appearance and impaired quality, especially poor mechanical properties, is obtained.
13.3.1 Resin Identification
Identification has become more complicated, not only due to the presence of plastic
materials compounded with additives such as plasticisers, stabilisers, flame retardants,
fillers and others, but also due to the presence of polymer blends in the waste. Some time
ago, the Society of the Plastic Industry (SPI) introduced a labelling system for recyclable
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plastic material. It is now common for manufacturers to use this code system printed or
moulded on the product surface for easy plastic identification. The code is a three-sided
triangular arrow with a number in the centre and letters underneath, indicating the type
of resin: 1 for PET; 2 for HDPE; 3 for vinyl polymers, especially PVC; 4 for LDPE; 5 for
PP; 6 for PS; and 7 for others. It is thus easy to develop automated scanning systems that
can read the SPI or other codes. This will help to identify the resin used [15]. Furthermore,
separation of plastic containers has been proposed by printing bar codes on them [16].
Plastic films are more difficult to identify than plastic containers, because most films do
not carry a code, and producers and recyclers need training on how to distinguish between
film types. Sorting generally must occur early in the recovery process, near the initial
point of generation, to be successful.
Optical systems for identification of mixed plastics have been used. A few technologies
originally designed and used in the film and packaging industry were considered earlier.
Electromagnetic scanning equipment was used to recognise chlorine molecules and so to sort
PVC from PET [17]. An X-ray fluorescence (XRF) analyser as a photoelectric sensor was
also used to identify transparent PET, green PET, translucent or natural HDPE, pigmented
HDPE and PVC. The sensor system is connected to an automatic sorting line. Automation of
the process reduces costs and improves the resale value of the separated plastics. Although
dirt does not significantly influence the fluorescence intensity from bottles, paper labels do
reduce the intensity but do not pose an obstacle in detecting vinyl bottles [17]. Paper labels
are virtually blind to X-rays.
Infrared and other spectral separation devices have been reported for the continuous
examination of waste products [18]. However, a satisfactory process for the identification
of plastic products for commercial purposes has yet to be developed.
13.3.2 General Aspects of Resin Separation
Resin separation from contaminants or from undesired materials to obtain the desired
stream can be achieved by a number of processes. These are: magnetic separation for the
removal of ferrous materials; an electrostatic method for nonferrous, mainly aluminium,
separation; air separation via cyclones to separate paper; and flotation tanks or
hydrocyclones used to separate various resins based upon specific gravity. After that the
processed materials are shredded.
Automatic separation of shredded plastic waste is very difficult if the resins have similar
specific gravity. Fortunately, 85% by volume of world plastic consumption is of four
main thermoplastic resins: PE, PP, PVC and PS [19]. In the next sections, some separation
techniques based on different properties will be discussed.
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13.3.3 Resin Separation Based on Density
Air classification can be used to separate plastics on the basis of their bulk densities; thus
film and foamed plastics may be separated from heavier forms of plastic material or
paper [20-22].
The densities of the major thermoplastics give the potential to separate them into types
by a series of float-sink operations [23, 24]. Water may be used to separate PP, LDPE
and HDPE from PS, PVC and PET. A liquid having a density of about 930 kg/m3 may be
used to separate PP and LDPE from HDPE; the PP and LDPE may then be separated
using a liquid having a density of about 910 kg/m3. The PS and PVC may be separated
using a liquid with a density of about 1150 kg/m3 [20]. Blending or filling a plastic may
change its density to the point where it could cause difficulties in the float-sink operation
processes. Labels, residual adhesives, metals and metallic-plastic composites cause similar
difficulties, and therefore processes have been developed to remove these contaminants
before the mixed plastic materials enter a separation system [25]. As an example, a
solvent washing stage, using either tetrachloroethylene- or hexane-related solvents, was
added to the classic water-washing treatment. These solvents were believed to remove
not only the glues but also any toxic organic chemicals that have been stored in beverage
bottles by consumers or are present as additives in plastics and that inadvertently will be
present in the end-products [26].
A different approach for separating mixed plastic wastes by density has been reported
[27, 28]. The process uses the properties of a fluid near its critical point to allow fine
separations at mild temperatures and pressures. The density of the medium can be varied
over a wide range and controlled to a sensitivity of ±0.01 g/cm3. Carbon dioxide is the
most commonly used supercritical fluid and can be compressed to densities in the range
of 1000 kg/m3. Since the separation of non-olefin thermoplastics will require fluid densities
up to approximately 1400 kg/m3, mixtures of carbon dioxide and sulfur hexafluoride, a
very dense supercritical fluid, may be used. By effecting small incremental changes of
pressure, pure CO2 efficiently separated LDPE, HDPE and PP. Separation of green PET,
clear PET and PVC has also been demonstrated, and separation of light- and dark-coloured
HDPE is possible. The different densities exhibited by PET in the neck and the body of
PET bottles can be separated by CO2/SF6. The possibility of separating various components
of wire and cable scrap also exists.
The centrifugal field produced in a hydrocyclone has been extensively used for the
separation of plastics. In a hydrocyclone, the flow rate referred to the separation area
is 100 times higher than in a static float-sink separator. The contamination of plastics
is of only minor relevance in this process compared to flotation. For the separation of
an n-component mixture, n – 1 separation stages (cyclone plants) are necessary.
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Furthermore, continuously operated plants must be equipped with a feeding device
and screens for dehydration purposes [29]. Using hydrocyclones, PS, PET and PVC
can be separated from one another or from polyvinyl alcohol; polyolefins from municipal
solid wastes; PET from PE, PP and paper; and PP car bumpers from metals and other
contaminants [30-32].
13.3.4 Resin Separation Based on Colour
Photoelectric sensors are used for the separation of mixed, whole, or baled plastic
containers. One of the systems uses mechanical means to reduce the baled plastic into
individual bottles and to screen contaminants. After deballing and screening, the containers
are manipulated into a single-line presentation to an optical sensor that performs a threeclass identification: Class 1, dairy HDPE and PP; Class 2, PET and PVC; and Class 3,
mixed colour HDPE containers. Another optical sensor can be used to discriminate green
and amber PET from clear PET containers, PP from dairy HDPE containers, and mixed
colour HDPE according to seven colour classifications. However, reliable identification
of post-consumer containers requires that measurements from much of the container
surface should be ignored. These areas include closures or necks, labels, edges, bottoms
and areas with residue of dirt [33, 34].
13.3.5 Resin Separation Based on Physicochemical Properties
13.3.5.1 Electrification
The separation of mixed plastic wastes can be achieved using high-voltage drums,
taking advantage of their different relative positions in the charging sequence. The
process involves first tribo-electrification of the shredded plastic particles of the mixture
by fluidisation. Subsequently, the electrified mixture is conveyed through an electrostatic
field that separates the individual particles according to the magnitude and the polarity
of the electric charges acquired during the tribo-electrification. When fluidising a mixture
of two shredded plastics, the particles of the plastic with the lower work function
transfer electronic charges to those with the higher work function. For example, the
tribo-electrical contact between PVC and PET results in PVC having a negative charge
and PET a positive charge. In the case of PET/PS mixtures, PET has a negative charge
and PS a positive charge [35]. Mixtures containing more than two plastic species pose
a substantial problem with regard to their charging behaviour. Also, owing to the
various additives contained in different types of resin, the respective positions of the
plastic species are prone to change.
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13.3.5.2 Spectroscopy
Spectroscopy in the near-infrared (NIR) region of the spectrum could be a key to the
rapid identification of various plastics and their subsequent recovery. By illuminating the
sample with light in the near-infrared and measuring the reflected light, a so-called NIR
spectrum of the material is obtained, which contains information about the molecular
vibrations excited by the light energy. For example, the IR vibrational spectra of plastics
show characteristic absorption bands at wavenumbers ν of 1200, 1400, 1700 and 22002500 cm–1 for CH, and at 1300-1500 and 1900-2100 cm–1 for OH. The NH (1500 and
2050 cm–1) and the CO (1730-1740 cm–1) vibrations contain the relevant spectral
information for plastics. Plastic objects, such as beverage bottles, can be dropped through
a vertical tube, and are identified and separated while falling. This also simulates other
transportation possibilities like conveyor belts. From the performance data, it was found
that identification can be achieved within 0.2 s, although several measurements were
needed to avoid mistakes due to dirt or labels. The problem of transparency for satisfactory
measurements could be overcome by reflectance measurements [36].
Other spectroscopic methods are also possible. The identification of several thermoplastics
– such as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PP, PS, high-impact
PS (HIPS) and PVC – can be achieved by Fourier transform IR (FTIR), based on similar
principles [37]. When exposed to γ-radiation, the high molecular weight (high molar
mass) molecules of PVC containing chlorine atoms emit an X-ray return signature easily
visible by an XRF analyser. Polyolefins, which have much lower molecular weights, emit
a lower backscattering signal that barely shows up on the XRF analyser, and so is easily
identified and separated [16, 38]. Bayer have proposed a process for automatic
identification and sorting of post-consumer plastics, in which fluorescent dyes were added
to resins during compounding, a different one for each resin type. These dyes, having
high detection sensitivity, can be added in minute quantities, and so 5 g of dye per ton of
polymer were sufficient for identification by a diode device [39].
13.3.5.3 Selective Dissolution of Polymer Mixtures
Finally, it must be emphasised that solvent recycling of a single-type plastic scrap serves
as a model process providing fundamental knowledge for the development of a selective
dissolution process.
The principle of the selective dissolution of a single polymer in a polymer mixture can be
used to separate the polymers. According to the concept of selective dissolution, one polymer
could be dissolved at a time, and thus dissolution-based processes can deal with mixtures
of polymers. This has an evident impact on the recycling of plastics in municipal solid
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waste. Mixtures of polyolefins, PS, PVC, thermosetting plastics and natural polymers
(rubbers, fibres, paper, etc.) can be separated using certain solvent systems [40]. The mixture
is first treated with xylene at 5-50 °C to dissolve PS, and after separation the mixture is
further heated at 90-150 °C to dissolve PE, leaving PP insoluble. The three main
thermoplastics – polyolefins, PS and PVC – may be separated by dissolving them in a
mixed solvent of xylene (85%) and cyclohexane (15%). From the dissolution, three separate
phases could be obtained containing 99% each of the pure plastics, indicating that excellent
separation can be achieved. [41]. In relevant studies, toluene, xylene and kerosene have
been proposed as suitable solvents for the selective dissolution of LDPE, but the information
given is very limited [42, 43]. Such selective dissolution is accomplished for each of the
polymers of the mixture by heating the waste dispersion at various temperatures.
13.4 Recycling of Separated PET Waste
The worldwide production of PET is above 1 × 106 tonnes per year. With such large
consumption, the effective utilisation of PET waste is of considerable commercial and
technological significance. PET waste may be converted into extruded or moulded articles
after repelletising it. Recently, waste PET films or sheets have been cleaned, crushed,
dried and mixed with LDPE waste [44]. The obtained mix was pelletised and blowextruded into films. The maximum concentration of PET does not exceed 20%. The
films obtained were found to possess very good mechanical properties compared with
LDPE only. Also, the films are expected to possess good printability due to the polar
nature of PET.
PET may be depolymerised to yield raw materials for resin synthesis. Recycling of
segregated waste may be possible by blending in small quantities with virgin monomer,
bis(hydroxyethyl) terephthalate. However, it often lowers the quality of the final product
[45]. It is therefore desirable to break down the polymer into smaller fragments or
oligomers [46].
PET can also be fully depolymerised into dimethyl terephthalate (DMT). However, the
regenerated DMT exhibits a significantly higher carboxyl content, adversely affecting
product quality. It is more economic to convert PET into low molecular weight oligomers
by glycolysis in the presence of a transesterification catalyst [47-49].
When glycolysis is carried out using ethyleneglycol, the oligomers may be directly recycled
into the polycondensation stage in PET manufacture, but this also lowers the product
quality. Glycolysis can be carried out using different glycols, and the oligomers can be
used in the synthesis of unsaturated polyester by reaction with an unsaturated anhydride
[50] or used to synthesise other polymers [51].
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There are two distinct advantages of the process: (1) the PET waste is converted into a
commercial value-added product; and (2) unsaturated polyester resins based on
terephthalic acid (TPA) are obtained without the processing difficulties encountered with
the use of plain TPA.
Yang and Tsai [52] degraded PET fabric waste to glycolysed product by treating the PET
waste with sodium hydroxide using ethyleneglycol or glycerol as the solvent. Compared
with the conventional aqueous alkaline hydrolysis, they found that the degradation rate
in ethyleneglycol increases tenfold. They reported that the kinetics of the alkaline
ethyleneglycol treatment show that the weight loss is linear with respect to time. They
concluded that using ethyleneglycol can greatly shorten the treatment time to achieve
results similar to those with the conventional aqueous system.
A new chemical recycling process for PET using supercritical water has been developed
by Yoshiyuki and co-workers [53]. In this method, the monomers obtained from
supercritical water hydrolysis are the raw materials of each polymer. The purity of the
terephthalic acid obtained from PET is about 99 wt%. It was confirmed that this process
has the advantage of reducing the reaction time and simplicity of the process when
compared with conventional methods such as methanolysis and glycolysis.
13.5 Recycling of Separated PVC Waste
As mentioned before, most of the technologies for the recycling of plastic wastes include
degradative extrusion, pyrolysis, hydrogenation, gasification, glycolysis, hydrolysis,
methanolysis, incineration with HCl recovery, or input as a reducing agent into blast furnaces.
Most of these technologies are still in the research phase, or simply are not suitable for PVCcontaining waste. The latter is particularly true for technologies such as glycolysis and
hydrolysis, which play a role only for well-defined single-waste streams such as PET. Some of
these technologies are currently generally regarded as the most feasible ones for realisation
on a practical scale. However, one group of these technologies is not designed specifically for
PVC waste, but deals with mixed plastic waste (MPW) in general. These technologies mainly
concentrate on recovering the organic part of the MPW. They often have restrictions with
regard to the maximum permissible chlorine (or PVC) input. Other technologies are designed
to deal specifically with PVC waste (chlorine concentrations of well over 10%). They emphasise
recovery of the chlorine fraction in a useful form. Hence, together with the competing
technologies for chemical recycling, three types of technologies have been discussed [54]:
(1) Technologies for chemical recycling of mixed plastic waste;
(2) Technologies for chemical recycling of PVC-rich waste;
(3) Alternatives to chemical recycling (incineration, mechanical recycling).
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Recycling of Plastic Waste
13.5.1 Chemical Recycling of Mixed Plastic Waste
Regarding the chemical recycling of MPW with a PVC content of up to several per
cent, the process consists of two parts, a liquefaction step and an entrained bed
gasifier. In the liquefaction step, the plastic waste is mildly thermally cracked
(depolymerised) into synthetic heavy oil and some condensable and non-condensable
gas fractions. The non-condensable gases are reused in the liquefaction as fuel together
with natural gas.
The heavy oil is filtered to remove large inorganic particles. The oil and condensed gas
are then injected into the entrained gasifier. Also, chlorine-containing gases from the
plastic waste are fed to the gasifier. The gasification is carried out with oxygen and steam
at a temperature of 1200-1500 °C [55]. The products of the process are synthesis gas
(predominantly H2/CO), pure sulfur and NH4Cl.
13.5.1.1 Polymer Cracking Process
In the polymer cracking process, some elementary preparation of the waste plastics feed
is required, including size reduction and removal of most nonplastics. The reactor operates
at approximately 500 °C in the absence of air. The plastics crack thermally under these
conditions to hydrocarbons, which vaporise and leave the bed with the fluidising gas.
The gas has a high content of monomers (ethylene and propylene) and other useful
hydrocarbons, with only some 15% being methane.
13.5.1.2 Conversion Process
The feedstock recycling process was designed to handle the recycling of mixed plastic
waste supplied by the collection system. The conversion of the pretreated mixed plastic
into petrochemical raw materials takes place in a multistage melting and reduction process.
In the first stage the plastic is melted and dehalogenised to preserve the subsequent plant
segments from corrosion. The hydrogen chloride separated out in this process is absorbed
and processed in the hydrochloric acid production plant. Hence, the major part of the
chlorine present in the input, (e.g., from PVC), is converted into saleable HCl. Minor
amounts become available as NaCl or CaCl2 effluent [56]. Gaseous organic products are
compressed and can be used as feedstock in a cracker.
In the subsequent stages the liquefied plastic waste is heated to over 400 °C and cracked
into components of different chain lengths. About 20-30% of gases and 60-70% of oils
are produced and subsequently separated in a distillation column.
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Handbook of Plastic Films
Naphtha produced by the feedstock process is treated in a steam cracker, and the
monomers, (e.g., ethylene, propylene), are recovered. These raw materials are used for
the production of virgin plastic materials. The process is carried out under atmospheric
pressure in a closed system and, therefore, no other residues or emissions are produced.
In sum, the products of the process are:
(1) HCl, which is neutralised or processed in a hydrochloric acid production plant;
(2) Naphtha, to be treated in a steam cracker;
(3) Monomers, (e.g., ethylene, propylene), which can be used for the production of virgin
plastic materials;
(4) High-boiling oils, which can be processed into synthesis gas or conversion coke and
then transferred for further use;
(5) Residues.
13.5.2 Chemical Recycling of PVC-Rich Waste
These processes aim to recover as much as possible of the chlorine present in PVC in a
usable form (HCl or a saleable chloride salt). The two processes in question, which are
discussed below, are:
•
Incineration process;
•
Pyrolysis process.
13.5.2.1 Incineration Process
A plant for the processing of chlorine-containing fluid and solid waste streams is used.
The goal is to process the waste by thermal treatment and to produce HCl using the
energy from the process itself. The plant is based on a rotary kiln and has a capacity of
45 kilotonnes per year, (i.e., not only PVC waste), with a heat production capacity of 25
MW at ca. 7500 production hours per year.
The waste is incinerated in the rotary kiln and a post-combustion chamber, directly after the
rotary kiln, at temperatures of 900-1200 °C. During this treatment HCl is released and
recovered. In this way a continuous production of high-quality HCl can be assured. Also, the
formation of dioxins and furans can be diminished in this way, as the goal of the process is to
oxidise the waste fully, so that no toxic chemicals (dioxins and furans) are formed.
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Recycling of Plastic Waste
13.5.2.2 Pyrolysis Process
In this process the chemical and thermal degradation of the PVC waste takes place in a
reactor at low pressures (200-300 kPa) and moderate temperatures (maximum 375 °C).
Chlorine generated from the PVC reacts with fillers, forming calcium chloride. Simultaneously,
the metal stabilisers that may be present in PVC waste (lead, cadmium, zinc and/or barium)
are converted to metal chlorides. This product consists of over 60% lead and may be purified
and reused. After completion of the reactions, three main intermediate products are formed:
a solid-phase product, a liquid product and a gas-phase product.
In sum the products of the process are:
(1) Calcium chloride (<1 ppm lead), which may be used as thaw salt or for other purposes;
(2) Coke (<0.1 wt% lead and <0.1 wt% chlorine), which may be used as fuel in a
cement kiln;
(3) Metal concentrate (up to 60 wt% lead);
(4) Organic condensate, which may be used as fuel for the process.
To treat the PVC waste, lime and water are needed to run the process. No dioxins, chlorine,
metals or plasticisers are emitted from the process. Also, there are no liquid waste streams
in the process, since all streams are recycled within the system. There is a small volume of
carbon dioxide gas formed by the reaction between lime/limestone and hydrogen chloride.
13.6 Recycling of Separated PE Waste
LDPE recycling is widespread, although not to the same extent as HDPE recycling. The
majority of LDPE that is recycled originates from post-industrial waste such as bundle
shrink-wrap used to stabilise loads on pallets as well as greenhouse films and mulch
films. There is only a limited proportion of recycled LDPE that can be classified as postconsumer recycle (PCR). Contamination in recycled PE can arise from a number of sources:
(1) From multicomponent systems that use dissimilar polymers such as PP closures, from
adhesive-backed paper labels, and even through the incorporation of additives such
as pigments;
(2) During use, (e.g., by the contents of the packaging);
(3) During collection, (e.g., owing to consumers mixing plastic types);
(4) By the environment, (e.g., soil in LDPE mulch film); and
(5) By reprocessing, (e.g., gels and black specks).
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Handbook of Plastic Films
13.6.1 Contamination of PE Waste by Additives
The pigments used in PE mouldings and films are often based on inexpensive metal
oxides. For instance, the common brown, grey and orange pigments are based on
various iron oxides and hydrates that act as prooxidants/prodegradants at the high
temperatures, (i.e., 220 °C), encountered during reprocessing of HDPE. Moreover,
green pigments are usually based on chromium(III) oxide, which can readily catalyse
the thermooxidative degradation of HDPE when present even in trace quantities.
Virgin PE is usually adequately stabilised so that these catalytic compounds do not
cause in-service degradation of the polymer; however, with reprocessing, the
antioxidants are usually consumed and these pigments may then be able to exert
their prodegradant effects [57].
The recycled polymer can also be contaminated by pigmented components in the
feedstock. In the recycling of HDPE bottles by melt processing, a major effort has
been directed toward producing a naturally coloured recycle stream [58]. The major
barrier to overcome to reach this end is the removal of the coloured bottle caps.
LDPE film is extensively used for packaging and for the production of shopping
bags. These films often contain a fatty acid amide lubricant (usually cis-docosenamide)
that can be oxidised during thermal reprocessing of LDPE film. The lubricant readily
undergoes cleavage at the unsaturated site to give a series of aldehydes that have
very low odour thresholds. Such contamination imparts to the recycled material a
rancid odour that may restrict its application potential.
13.6.2 Contamination of PE Waste by Reprocessing
Recycling of HDPE PCR (usually milk bottles) by melt extrusion can lead to
crosslinking during the thermal reprocessing stage since the antioxidant added initially
(during manufacture of the polymer) is consumed [59]. Loosely crosslinked regions
(known as ‘gels’) can act as stress concentrations in film and cause ‘blow-outs’ in
bottles made from recycled HDPE. Another common source of contamination in
recycled HDPE (as well as virgin HDPE) is ‘black specks’. These are small areas of
highly degraded polymer that have been carbonised owing to excessive residence
time in an extruder. These ‘black specks’ typically occur in low-flow regions in the
extruder where ‘hang-ups’ form. Often, such contamination may also appear yellow,
brown, or amber depending on the extent of degradation. Black specks cause a major
problem in the blow-moulding of natural or white bottles because they are aesthetically
undesirable.
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Recycling of Plastic Waste
13.7 Recycling of HDPE
13.7.1 Applications for Recycled HDPE
New markets for recycled HDPE will also emerge as a result of advances in fabrication
technology allowing recycled HDPE to be used in more demanding applications that are
currently the domain of LLDPE and high-modulus HDPE. For instance, there is the
potential use of recycled HDPE in oriented laminates for applications such as pond liners
and moisture barriers. High-performance sheets are obtained by laminating oriented
HDPE webs to equalise cross-web and longitudinal properties [60]. Compared to the
same thickness of monolayer HDPE, the advantages are greatly improved tensile, tear
and puncture strength, and doubling of the moisture barrier properties. Specialised
processing equipment has been developed to handle recycled material [61, 62].
The following are some of the commercial applications of the modified recycled HDPE:
(1) Large mouldings;
(2) Detergent bottles are another major outlet;
(3) Grocery sacks (‘check-out’ bags) are generally produced from high molecular weight
(HMW) HDPE film and have a thickness in the range 15-18 μm, their most critical
performance parameters being tear strength and dart impact strength.
13.7.2 Rubber-Modified Products
Recycled rubber crumb from used car tyres is another class of modifier that is finding
widespread potential for blending with HDPE PCR to yield truly 100% recycled polymeric
materials [44].
Profiles extruded from recycled HDPE are finding increased use in applications such as
decking, fence posts, boundary stakes, road posts, railroad sleepers and similar areas,
replacing wood and concrete [63]. Such applications are generally thick sections and are
ideal outlets for consuming large volumes of HDPE recycle. Furthermore, they are resistant
to rotting, not liable to insect attack, and do not splinter.
13.8 Recycling Using Radiation Technology
Owing to the ability of ionising radiation to alter the structure and properties of bulk
polymeric materials, and the fact that it is applicable to essentially all polymer types,
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Handbook of Plastic Films
irradiation holds promise for impacting the polymer waste problem. The three main
possibilities for use of radiation in this application are:
(1) Enhancing the mechanical properties and performance of recovered materials or
material blends, principally through crosslinking, or through surface modification of
different phases being combined;
(2) Treatment causing or enhancing the decomposition of polymers, particularly through
chain scission, leading to recovery of either low molecular weight mixtures, or
powders, for use as chemical feedstocks or additives;
(3) Production of advanced polymeric materials designed for environmental compatibility.
An overview of the polymer recycling problem describes the major technological obstacles
to the implementation of recycling technologies, and outlines some of the approaches
being taken [64].
13.9 Biodegradable Polymers
The synthetic polymer industry has brought great benefits to modern society. For example,
in the packaging and distribution of foodstuffs and other perishable commodities, the
commercial thermoplastic polymers are hydrophobic and biologically inert, and this has
made them essential to modern retailing [65].
Similarly, in agriculture, plastics have largely replaced glass in greenhouses and cloches,
and they have gained a unique position in the growing of soft fruits and vegetables over
very thin polymer films (mulching films) [66]. The major group of polymers used both in
packaging and in agriculture are the polyolefins, which, due to their resistance to
peroxidation, water and microorganisms, are durable during use.
In the 1970s, it became evident that the very technical advantages which made polymers
so useful were disadvantages when polymer-based products were discarded at the end of
their useful life and in particular when they appeared as litter in the environment. Some
items of plastics packaging waste were found to have very damaging effects on wildlife
[67], and this led to calls from the ‘Green’ movement to return to biologically based
(renewable) polymers.
Materials made from naturally occurring or biologically produced polymers are the only
truly biodegradable ‘plastics’ available. Since living things construct these materials, living
things can metabolise them.
374
Recycling of Plastic Waste
In practice, a relatively small weight proportion of polymeric materials ends up as litter.
In most developed societies, domestic organic waste, including plastics packaging, is
disposed of in sanitary landfill or by incineration. However, burying waste is no longer
an ecologically acceptable way of disposing of consumer wastes [65, 68].
Another approach aimed at the solid waste problem is the development or evaluation of
biodegradable polymers [69]. These are based on a variety of natural products (often
structurally modified to optimise properties), or are laboratory-made polymers with
structures designed to be susceptible to enzymatic attack. A rather large effort has grown
up in this area. Polymer types being studied include cellulose derivatives (such as cellulose
acetate) [70], polysaccharides such as chitin [71], starch and poly(3-hydroxybutrate)
[72]. A related approach to materials that break down under natural environmental
conditions is the development of UV-degradable plastics, designed to decompose in
sunlight should they become ‘litter’ [73]. Examples include a copolymer of ethylene and
carbon monoxide, and modified PET.
Two different applications have emerged over the past two decades for degradable
polymers. The first is where biodegradability is part of the function of the product.
Examples of this are temporary sutures in the body or in controlled release of drugs,
where cost is relatively unimportant. Similarly, in agriculture, very thin films of photobiodegradable polyethylene are used to ensure earlier cropping and to reduce weed
formation [65, 66]. By increasing soil temperature, they also increase crop yields and
ensure earlier harvest. A major ecological benefit of mulching films is the reduction of
irrigation water and fertiliser utilisation [74]. No residues must persist in the soil in
subsequent seasons to make the land less productive by interfering with root growth.
The technology of biodegradable polymers, as a solution to minimising the huge amount
of plastic waste, is developing. There is an ever-widening range of polymers satisfying
the requirements necessary for the numerous applications for which biodegradability of
the materials is essential.
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380
Abbreviations and Acronyms
AA
Acrylic acid
ABS
Acrylonitrile-butadiene-styrene
AC
Acetyl cellulose
AES
Auger electron spectroscopy
AFM
Atomic force microscopy
APB
Ammonium pentaborate
APP
Ammonium polyphosphate
aPP
Atactic polypropylene
ASTM
American Society for Testing and Materials
ATH
Alumina trihydrate
au
Arbitrary units
BDD
Brominated dibenzodioxin(s)
BDF
Brominated dibenzofuran(s)
BF
Branch frequency
BFR
Brominated flame retardant
BMP
2,6-di-tert-butyl-4-methylphenol
BOPP
Biaxially oriented polypropylene
BPE
Branched polyethylene
BTPC
Benzyl triphenyl phosphonium chloride
BUR
Blow-up ratio
CDD
Chlorinated dibenzodioxin(s)
CDF
Chlorinated dibenzofuran(s)
CI
Carbonyl index
DBM
Dibenzoylmethane
DIN
Deutsch Institut für Normung
DMA
Dynamic mechanical analysis
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Handbook of Plastic Films
DMT
Dimethyl terephthalate
DSC
Differential scanning calorimetry
EFW
Energy from waste
EPDM
Ethylene-propylene-diene terpolymer
ESCA
Electron spectroscopy for chemical analysis
ESCR
Environmental stress cracking
ESR
Electron spin resonance
EU
European Union
EVA
Ethylene vinyl acetate
EVOH
Ethylene-vinyl alcohol
FFS
Form-fill-seal
FTIR
Fourier transform infrared spectroscopy
GE
Grafting efficiency
GP
Grafting percentage
GPC
Gel permeation chromatography
HA
Hindered amines
HALS
Hindered amine light stabilisers
HDPE
High-density polyethylene
HDUL
Heat distortion under load
HEMA
2-Hydroxyethyl methacrylate
HFI
Hyperfine interaction
HIPS
High impact polystyrene
HLMI
High-load melt index
HMW
High molecular weight
HP
Hindered phenols
HPLC
High-performance chromatography
IPCS
International Program for Chemical Safety
iPP
Isotactic polypropylene
iPP
Isotactic polypropylene
IR
Infra red
ISO
International Standards Organisation
382
Abbreviations and Acronyms
L:D
Length-to-diameter ratio
LCB
Long chain branching
LDPE
Low-density polyethylene
LLDPE
Linear low-density polyethylene
LOI
Limiting oxygen index
LPE
Linear polyethylene
MD
Machine direction
MDPE
Medium-density polyethylene
MDSR
Machine direction stretching ratio
MF
Melt flow
MI
Melt index
MPW
Mixed plastic waste
MSW
Municipal solid waste
MWD
Molecular weight distribution
NIR
Near infra red spectroscopy
NMR
Nuclear magnetic resonance
NO
Nitric oxide
NO2
Nitrogen dioxide
OMTS
Octamethylcyclotetrasiloxane
OPP
Oriented polypropylene
PA-12
Polyamide-12
PA-6
Polyamide-6
PAN
Polyacrylonitrile
PB
Phenyl benzoate
PBB
Polybrominated biphenyl(s)
PBDE
Polybrominated diphenyl ether(s)
PC
Polycarbonate
PCA
Polycaproamide
PCB
Polychlorinated biphenyl(s)
PCDD
Polychlorinated dibenzodioxin(s)
PCDF
Polychlorinated dibenzofuran(s)
383
Handbook of Plastic Films
PCR
Post-consumer recyclable
PCTFE
Polychlorotrifluoroethylene
PE
Polyethylene
PER
Pentaerythritol
PET
Polyethylene terephthalate
PI
Polyisoprene
PMMA
Polymethyl methacrylate
PMP
Poly(4-methyl-1-pentene)
PNA
Phenyl-β-naphthylamine
PP
Polypropylene
PS
Polystyrene
P-t-BuMA
Poly-tert-butyl methacrylate
PTFE
Polytetrafluoroethylene
PU
Polyurethane
PVAc
Polyvinyl acetate
PVA-ox
Oxidised polyvinyl alcohol
PVB
Poly(vinyl butyral)
PVC
Polyvinyl chloride
PVDC
Polyvinylidene chloride
PVF
Polyvinyl fluoride
PVOH
Polyvinyl alcohol
PVP
Polyvinylpyrrolidone
RDP
Rescorcinol diphosphate
RH
Relative humidity
RHR
Rate of heat release
SCB
Short chain branching
SEC
Size exclusion chromatography
SEM
Scanning electron microscopy
SIMS
Secondary ion-mass spectroscopy
SPI
Society of the Plastics Industry
sPP
Syndiotactic polypropylene
384
Abbreviations and Acronyms
sPP
Syndiotactic polypropylene
TCB
Trichlorobenzene
TCDD
2,3,7,8-Tetrachlorodibenzo-p-dioxin
TD
Transverse direction
TEB
Tensile energy to break
TEF
Toxic equivalence factor
TEM
Transmission electron microscopy
TFE-HFP
Tetrafluoroethylene-hexafluoropropylene copolymer
Tg
Glass transition temperature
TGA
Thermogravimetric analysis
Tm
Melting temperature
TMDSC
Temperature modulated DSC
Ton
Onset temperature
TPA
Terephthalic acid
TREF
Temperature-rising elution fractionation
UHMWPE
Ultra high molecular weight polyethylene
ULDPE
Ultra low-density polyethylene
USEPA
United States Environmental Protection Agency
UV
Ultra violet
VA
Vinyl acetate
VLDPE
Very low-density density polyethylene
WAXS
Wide angle X-ray scattering
WC
Weight conversion percentage
WVTR
Water vapour transmission rate
XFS
X-ray fluorescene spectroscopy
XPS
X-ray photoelectron spectroscopy
XRF
X-ray fluorescence analyser
ZN
Ziegler-Natta
385
Handbook of Plastic Films
386
Contributors
Elsayed M Abdel-Bary
Department of Chemistry
Faculty of Science
Mansoura University
Mansoura
Egypt
Heshmat A Aglan
Department of Mechanical Engineering
Tuskegee University
Tuskegee
AL 36088
USA
Guneri Akovali
Middle-East Technical University
Department of Chemistry
TR-06531 Ankara
Turkey
Amin Al-Robaidi
Jubeiha 11941
PO Box 1628
Amman
Jordan
Evgenii Y Davydov
NM Emanuel Institute of Biochemical Physics
Russian Academy of Sciences
ul. Kosygina 4
Moscow 117977
Russia
Yong X. Gan
Department of Mechanical Engineering
387
Handbook of Plastic Films
Tuskegee University
Tuskegee
AL 36088
USA
Irina S. Gaponova,
Emanuel Institute of Biochemical Physics
Russian Academy of Sciences
ul. Kosygina 4
Moscow 117977
Russia
Klara Z Gumargalieva
N.N. Semenov Institute of Chemical Physics
Russian Academy of Sciences
Moscow
Russia
S.M. Lomakin
Institute of the Biochemical Physics of Russian Academy of Sciences 119991,
Kosygin 4
Russia
Ashraf A Mansour
Department of Chemistry
Faculty of Science
Cairo University
Cairo
Egypt
Alexander Mar´in
Parallel Solutions, Inc.
763 Concord Avenue
Cambridge
MA 02138
USA
Karl S Minsker,
Bashkirian State University
32 Frunze Str.
Ufa
Bashkiriya 450074
Russia
388
Contributors
Georgii B Pariiskii
NM Emanuel Institute of Biochemical Physics
Russian Academy of Sciences
ul. Kosygina 4
Moscow 117977
Russia
Susan E Selke
School of Packaging
Michigan State University
East Lansing
MI 48824-1223
USA
Robert A Shanks
Department of Applied Chemistry
Faculty of Applied Science
RMIT University
GPO Box 2476V
Melbourne
Victoria 3001
Australia
Abbas A Yehia
Department of Polymers and Pigements
National Research Center
Dokki
Cairo
Egypt
Gennady E Zaikov
N.M. Emanuel Institute of Biochemical Physics
Russian Academy of Sciences
4 Kosygin Str.
Moscow 119991
Russia
Vadim G. Zaikov
N. M. Emanuel Institute of Biochemical Physics
4 Kosygin Str.
Moscow 117334
Russia
389
Handbook of Plastic Films
390
Main Index
Index
A
Aclar films 247
Additive dissolution 122, 123, 126
kinetics of 114
Additive solubility 115
additive loss 125
crystallinity 118
factors 118
high molecular weight additives 122
polymer orientation 119
polymer oxidation 124
polymer polar groups 120
quantitative data 114
sorption 110-112
supermolecular structure 118
Additives
antioxidants 14
lubricants 14
slip agents 14
solubility of 109
tackifiers 14
ultraviolet stabilisers 14
Adhesive strength
adsorption theory 316
diffusion theory 316
electrical theory 316
mechanical theory 316
molecular-kinetic theory 316
theory 316
Agricultural films 263
blown film extrusion 264
characteristics of plastic films 264
greenhouses 263
light transmittance 264
polyethylene films
stabilisers 267
Albemarle 166
Algipor 290, 301, 302
Aliphatic chain scission 124
Allied Signal Corporation 247
Amoco 82
Amoco process 84
Antiblocking agents 22, 85
mineral particles 23
Antioxidants
dissolution 122
hindered amines 89
hindered phenols 24, 89
phosphites 90
polyolefins 112
solubility 122
thio compounds 90
triphenyl phosphite 24
Antistatic agents
polyoxyethylenes 24
Ammonium polyphosphate/
pentaerythritol mixtures
intumescent behaviour 172
Applications
agriculture 38, 263
coextruded films 37
heat sealing 38
laminated films 36
packaging 35
Aqua-penetrability 305
Artificial weathering devices 271
Atactic polypropylene 13, 79
Atochem 166
391
Handbook of Plastic Films
B
Biaxial
Biaxially oriented film 18, 213
drawing 10
polypropylene 351
properties of 244
Biobrant 314, 324
burn dressing 323
Biodegradable polymers 374
Blending
agricultural plastic film waste 279
Blow-up ratio 59, 61
Blown film
air ring cooling 53
blow ratio 56
die 52
extrusion process 14
extrusion (tubular film) 50
process 50
cooling the film 51
extruder size 54
extrusion equipment 55
horsepower 55
Tenter frame 19
production 13
Branched polyethylenes 7
molecular structures 7
Burn dressings 285-286
adhesion of 296, 315
adhesive properties 292
adhesive strength 317-318
air penetrability 291, 295, 308, 315,
323
air permeability 310
aqua-penetrability coefficients 308
cellulose 303
characteristics 322
collagen 303
compositional 290
cotton balling 290
degree of filling 298
efficiency of 322
392
evaporation of water from 318
hydrophilic 303
kinetics of sorption 304
materials
free volume of 303
material porosity 292
mechanical properties 292
microorganism penetrability 292
model of action 318
nitrogen penetrability 309
number of pores 293
oxygen penetrability 309
physicochemical properties 292
pore size distribution 293
properties of 290
size of pores 293
solubility of water in polymers 298
sorption ability 297-298, 322
sorption of fluid 320
sorption of liquid media 303
sorption of plasma 303
sorption of water 299-300
sorption-diffusion properties 291
sorptional ability 285
sorptional ability of materials 294
structure of 303
vapour penetrability 291-292, 296,
305
water absorption 291
Butyl rubber
effect of nitrogen oxides on 191
C
Carbonium ion mechanism 172
Carbonylallyl groups
identification of 136
Cast film
biaxial orientation 19
calendering 16
extrusion 57
packaging 75
Index
polypropylene films
applications 76
process
chilled roll 57
production
calendering finishing 17
extrusion coating 17
extrusion conditions 16
tenter frame 16
Char formation
acid-catalysed 175
free-radical 174
Characterisation
dielectric relaxation 226
electron spectroscopy for chemical
analysis 228
gravimetric method 224
molecular weight 226
molecular weight distribution 226
scanning electron microscopy 225
spectroscopic analysis
Auger electron spectroscopy 227
infrared spectroscopy 227
x-ray fluorescence spectroscopy 227
surface properties 227
swelling measurements 226
thermal analyses 225
x-ray photoelectron spectroscopy 228
Charpy tests 29, 336
Chemical modification 215
bromination 217, 218
chemical etching 218
chlorination 217
fluorination 215
bulk 216
direct 215
indirect 216
surface 216
grafting 220
high-energy radiation 220
radiation-induced 221
photografting 222
bulk surface 222
sulfonation 218
Chemical recycling 359
depolymerisation 360
Chimassorb 280
Chromatographic techniques 46
Coextrusion 16, 17, 22
agricultural plastic film waste 279
Collagens 301
maximum sorption of water 301
Corona discharge
surface treatment system 22
Creep
mechanism of 26
Crosslinkable ethylene plastics
standard specifications 351
Cultivated cutis 286
Cyasorb 1084 280
D
Dacron 290
Dehydrochlorination 134
rate constant 135
Density
gradient column 44
Die capacity 57
Die size 55-56
Die swell percentage
polymer melt 50
Dielectric properties 30
Differential thermal analysis 119
Dimensional stability 35
Dioxin
structure of 163
Dow Chemicals 277
Dressings
adhesion of 324
air penetrability 313
animal origin 286
characteristics of 287-289
multilayer 305
penetrability 309
393
Handbook of Plastic Films
polyurethane 304
synthetic materials 286
vegetable origin 290
DuPont Company 131
Dynamic mechanical analysis 339
Dynamic mechanical properties 29, 339
E
Elmendorf tear strength test 28
Energy recovery 361
Erucamide
structure of 23
Ester pyrolysis mechanism 173
Ethylene copolymers
dielctric analysis 30
Ethylene-vinyl alcohol films
oxygen barrier properties 248
typical properties 249
Extruder
blow ratio 15
characteristics 14
compression zone 15
dispersive mixing 14
distributive mixing 14
feed zone 15
frost-line 15, 16
metering zone 15
screw 15
design 15
size 55
twin-screw 14
Extrusion 10
cast film 19
process
calendering 17
extrusion coating 17
tubular extrusion 13
film 51, 59
Exxon 82
process 83
394
F
Feedstock recycling 359
Film 5
brittleness 60
drawing 18-19
extrusion blow moulded 18
extrusion coating 22
extrusion of the melt 5
film lamination 22
gloss 5
lamination 37
manufacture 5
melt adhesion 21
pigmented 38
printing 5, 22
slip agents 23, 342
surface properties 5
tensile strength 60
toughness 60
Film application
linear low-density polyethylene 41
Film blowing 48
Film characteristics
blocking 61
bubble stability 61
extrusion variables 58
gloss 59
haze 59
impact strength 60-61
optical 58
puckering 62
Film extrusion
blown film 50
slot cast extrusion 50
Film orientation
shrink-wrapping 214
Film packaging 75
Film properties
blow ratio 61
gloss 61
haze 61
Index
optical 61
Film samples
impact test methods 337
Film shrinkage 69
Fire retardants
ammonium pentaborate 169
ammonium polyphosphate 168-169
brominated 166
brominated diphenyl oxide 161
char-formers 167
chlorinated 161
chlorinated dibenzo-p-dioxins 162
diagram of 167
dimelamine phosphate 170
environmental impact 168
guanidine sulfamate 168
halogen-containing 159-161
halogen-free 170
halogenated diphenyl ethers - dioxins
162
inorganic 159
intumescent systems 167-168
low-melting glasses 167
mechanism of action 160
condensed-phase 169
melamine pyrophosphate 170
nitrogen-based organic 159
organophosphorus 159
phosphorus 160
polybrominated biphenyls 162, 165
polybrominated diphenyl ethers 162
polymer morphology modification
167
polymer nanocomposites 167
polymer organic char-former 175
preceramic additives 167
Flat film
dies 57
packaging bags 58
Flexible packaging
bags, sacks and pouches 238
dispensing 239
forms 236
heat-sealing 240
pouch production 239
reclosure 239
wraps
shrink-wrap 237
stretch-wrap 237
Fluidised bed pyrolysis 359
Fluoropolymer
sodium etching 219
Free-falling dart method 336
Free-radical processes 41
G
Gas permeation 35
General grade polyethylene films
standard specifications 350
General Electric 166
Greenhouse films
additive 268
ageing factors 275
ageing resistance 275
changes in chemical structure 276
compatibility 269
effects of pesticides 274
environmental pollution 274
excited-state quenchers
nickel dibutyldithiocarbamate 267
fog formation 273
hindered-amine light stabilisers 267,
275
humidity 273
light stabilisers 269
recycling 278
solar irradiation 265
solar radiation 265
stabilisation 265, 268-269, 272
temperature 272
ultraviolet screening
carbon black 266
chalk 266
short glass fibres 266
395
Handbook of Plastic Films
stabilisation 265-266
talc 266
TiO2 266
wind 273
Intrinsic viscosity
polyethylene resin 46
Isotactic polypropylene 9, 12, 79-80
Izod tests 29, 366
H
L
Hall Woodroof Co. 290
Heat stabilisers
structures of 25
High-density polyethylene 10
melt strength 11
recycled 373
applications 373
detergent bottles 373
grocery sacks 373
large mouldings 373
viscosity 11
Hindered amine light stabilisers
hydroxybenzophenones 25
hydroxybenzotriazoles 25
tetramethylpiperidines 25
Hydron 290, 298
Hydroperoxide decomposition 208
Light stabilisers
structures of 25
Linear low-density polyethylene 11
differential scanning calorimetry
curves 33
film
atomic force microscopy of 32
stress-strain curve 27
Linear polyethylene 7
molecular structures 7
Linear low density polyethylene 8
composition
by 13C nuclear magnetic resonance
346
properties 8
shrink films 14
Low-density polyethylene 7, 11
continuous shear rheology curve 12
covering films
hindered-amine ligt stabilising 272
differential scanning calorimetry
curves 33
films
standard specifications 350
high-pressure radical process 7
mulch film
recycling 278
long branches 11
resistance to tear propagation 338
shrink properties 68
Lubricants
chlorinated paraffins 24
paraffin wax 24
stearate salts 24
stearic acid 24
I
Impact properties
dart-puncture resistance 28
tensile impact 28
tensile–tear strength resistance 28
Infrared spectroscopy
characteriastion 32
composition analysis of blends and
laminates 33
surface analysis 33
Inorganic barrier coatings
aluminium oxide 255
clay nanocomposites 255
Internal additives
antioxidants 24
ultraviolet absorbers 24
396
Index
M
Mechanical properties
abrasion resistance 26
adhesion tests 26
impact tests 26
polyolefin films 25
modification of 213
tear testing 26
tensile 26
Mechanical tests
bending stiffness (flexural modulus)
339
free-falling dart method 336
hail resistance 337
impact resistance 336
impact test methods 337
package yield of a plastic film 334
pendulum impact resistance 337
pendulum method 338
percent elongation at break 334
percent elongation at yield 334
propagation tear resistance 338
puncture-propagation tear resistance
339
tear resistance 337
tensile modulus of elasticity 334
tensile strength 333
tensile strength at break 334
tensile testing (static) 333
yield strength 333
Medium-density polyethylene films
standard specifications 350
Melt elasticity 50
Microporous material
typical sorption isotherm 294
Microscopic examination
atomic force microscopy 31
optical–polarised light effect with
strain 31
scanning electron microscopy–etching
31
Mitsui Hypol 82-83
Mixed plastic waste
chemical recycling 369
conversion process 369
feedstock recycling process 369
gasification 369
identification of
electromagnetic scanning 363
optical systems 363
X-ray fluorescence 363
polymer cracking process 369
Modulus 331
Moisture resistance 34
Mulch film 277
recycling 278
Multilayer plastic films 16
coating 252
coextrusion 253
greenhouses 279
lamination 253
metallisation 253
silicon oxide coating 254
N
Nitroxyl radicals 194
Electron spin resonance spectrum 194
structure of 203
Noryl 166
Nylon-6
thermal decomposition 171
O
Orientation 35
biaxial 18
by drawing 18
during blowing 18
machine direction 60
of film 18
transverse direction 60
Oriented polypropylene 351
397
Handbook of Plastic Films
standard specifications 351
Oxygen indices 170
P
Polyamide-6,6
cone calorimeter data 179-180
nanocomposite
carbon residue 183
Polyamide-6,6/Polyvinyl alcohol
cone calorimeter data 178
Packaging 235
Packaging films
acid copolymer films 250
biaxially oriented film 244
cellophane 247
ethylene-vinyl acetate 250
ethylene-vinyl alcohol 248
high-density polyethylene 243
linear low-density polyethylene 242
low-density polyethylene 242
polyamide (Nylon) 249
polychlorotrifluoroethylene 247
polyethylene terephthalate 245
polypropylene 244
polyvinyl alcohol 248
polyvinyl chloride 245
polyvinylidene chloride 246
uses of 241
Packaging materials
barrier 257
cellulose 252
environmental issues 261
high-impact polystyrene 252
ionomers 251
permeation 257
plastics 251
polystyrene 251
printing
flexography 256
ink-jet printing 257
lithography 257
398
rotogravure 257
screen printing 257
static discharge 256
surface treatment
corona discharge 255
Polyethylene
chlorination of 217
films
wetting tension 344
interaction of nitrogen dioxide with
189
long-chain branching 45
melt flow properties 45
melts
shear viscosity 48
oxidation of 219
processing
troubleshooting 63-65
resin
basic properties 42
chain branching 45
density 44, 45
dispersity index 42
elasticity 49
elongational viscosity 49
heat of fusion 47
intrinsic viscosity 46
melt flow blend relationship 43
melt index 42
melt properties 48
melting point 47
molecular weight 42
rheology 48
viscosity/shear rheology 48
short-chain branching 44-45
waste
contamination by additives 372
contamination by reprocessing 372
Perfluoronitroxyl radicals
electron spin resonance spectra of 203
Physical property modification
corona treatment 223
plasma treatment 222
Index
Physicochemical tests
abrasion resistance 347
blocking load
parallel-plate method 346
creep 346
creep rupture 346
density of plastics 340
environmental stress cracking 348
haze transmittance 341
ignition 342
indices of refraction 340
kinetic coefficients of friction 342
luminous transmittance 341
mar resistance 348
orientation release stress 345
outdoor weathering 347
oxygen gas transmission 349
oxygen index 342
rate of burning characteristics 342
resistance to chemicals 341
rigidity 345
shrink tension 345
specular gloss 343
static coefficients of friction 342
transparency 341
water vapour permeability 348
weatherability 347
yellowness 340
Piping material
polypropylene 74
Plastic films
abrasive damage 347
applications 228
artificial ageing tests 276
contamination by the environment
277
crosslinking 213-214
crystallisation 213-214
dimensions 332
gloss 343
grafting 228
greenhouses 263-264
in packaging 235
mechanical properties 213
modification of 213
orientation 213-214
photooxidation 271, 277
physical properties 213
premature failure 272
production of 263
properties of 332
recycling in agriculture 277
removal of contaminants 213
resistance to tearing 338
slip properties 343
specular gloss 343
stability 263, 347
standard specifications 349
tear resistance 338
testing of 329-356
thicknesses 332
unrestrained linear thermal shrinkage
345
Plastic materials
fire retardation 168
photodegradation 265
recycling 279
Plastic production 76
growth rate 77
Plastic recycling
collection 362
mechanical recycling 358
primary recycling 358, 362
quaternary recycling 360
secondary recycling 358, 362
sorting 362
tertiary recycling 359, 362
Plastic surfaces
fluorinated 216
Plastic waste
films
recycling 278
reuse 278
management 278
399
Handbook of Plastic Films
recycling 357
resin identification 362
Plastics
separation of 359
Polymethyl methacrylate
exposure to nitrogen dioxide 193
Polluted atmospheres 187
Polyamide films
typical properties 250
Polyamides
interaction of nitrogen dioxide with
196
Polyamidoimide film
influence of nitrogen dioxide on 200
Polydimethylsiloxane
diffusion of gases 310
penetrability of gases 310
solubility of gases 310
Polyethylene 7, 8
decreasing-pitch screw 51
density 60
high-density 8
high-pressure technology 45
linear low-density 8
low-density 7
low-pressure technology 45
self-adhesion 21
solubility of phenyl benzoate in 120
solubility of phenyl-b-naphthylamine
in 122
surface treatment of 217
ultra-low-density 8
very-low-density 8
Polyethylene films
crosslinked 214
irradiated 215
photo(bio)degradable 271
processing 41
properties of 243
shrink-wrapping 35
Polyethylene resin
intrinsic viscosity 46
400
Polyethylene terephthalate films
properties of 246
standard specifications 350
waste
chemical recycling process 368
depolymerised 367
glycolysis 367-368
recycling of 367
Polyisoprene
interaction with nitrogen dioxide 192
Polymer films
gloss 20
haze 20
surface analysis 34
Polymer mixtures
selective dissolution 366
Polymer nanocomposites
combustibility 181
disordered 180
intercalated 180
Polymer structure
morphological irregularity 109
nonuniform 109
Polymeric materials
graft copolymerisation 220
Polymers
barrier characteristics 215
diffusion of water vapour 306
flame retardancy 159-160
interaction of nitrogen dioxides with
188
interaction of nitrogen oxides with
187-188
nitric oxide 188
nitrogen oxide 188
non-saturated 191
penetrability 306, 310
penetrability of gases 309
photochemical oxidation 187
reaction of nitric oxide with 202
rheology 15
separation coefficients of gases 309
Index
solubility of stabilisers 116-117
solubility of water in 299
thermal oxidation 187
Polyolefin elastomers 8
Polyolefin films 9
crystallisation 9
morphology of 9
packaging applications 36
production 5
Polyolefins 6-7, 10
corona discharge treatment 21
dielectric properties 30
orientation drawing 119
properties 6
rheological characterisation 10
structure of 6-7
virgin
recycling behaviour of 271
Polypropylene 9, 12
additives 88
balanced oriented 351
barrier properties 214
branching 78
calcium carbonate pigment 101
chain scission 87-88
chirality 79
degradation 86
durability-additive property 97
durability-processing condition 94
dynamic mechanical analysis curve 29
films
durability 73
processing conditions 73
hydroperoxide decomposition 207
interaction of nitrogen dioxide with
189
metallocene-catalysed 13
micrograph 82
microstructure 96
morphology 81
regiospecificity 78
rods 74
stress-strain behaviour 94-95, 98-99
stretched tape materials 100
structures 80
properties of 244
two-step tubular orientation 13
ultraviolet degradation 86
uniaxially oriented 351
wire coating 75
worldwide capacity 77
Poly-tert-butyl methacrylate film
degradation 190
Polytetrafluoroethylene
processing of 215
Polyurethane
cellular 286
films
exposure to nitrogen dioxide 200
interaction of nitrogen dioxide with
196
sponges
pore size 286
Polyvinyl chloride
properties of 244
Polyvinylidene chloride films
typical properties 247
Porous materials
determining absorbtion ability 295
determining air penetrability of 296
penetrability 310
Post-consumer films
recycling 277
Polypropylene degradation 87
Polypropylene fabric
scanning electron microscope
morphology 93
of ultraviolet light degraded 93
stress-strain behaviour 92
Polypropylene films 75
additives 85
chill roll cast method 85
durability-microstructure
relationship 91
401
Handbook of Plastic Films
film processing 85
microscopic examination 91
oriented 75
opaque 75
static tensile tests 91
structures 78
surface morphology 100
synthesis 78
ultraviolet degradation behaviour 90
ultraviolet exposure 91
wetting tension 344
Polypropylene granules
Scanninmg electron micrograph 80
Polypropylene woven fabrics
stress-strain behaviour 91
Polypropylene-polyethylene-copolypropylene 10
micrograph 10
Processing
troubleshooting 62
Polystyrene
degradation 191
film
degradation 190
Polyvinyl chloride 134
degradation
effect of plasticisers 145
rate 148
dehydrochlorination 135, 137, 145,
147-148
kinetic curves for 134
rate constants 138
thermal 150
disintegration 132, 134, 136
thermal 151
‘echo’ stabilisation 151-152, 153
global production 131
light stabilisation 144
low stability 132
stabilisation 138, 140-141
thermodegradation rate 150
thermoformed packaging 245
402
Polyvinyl chloride films
plasticisers 245
properties 245
Polyvinyl chloride waste
chemical recycling 368, 370
incineration 368, 370
rotary kiln 370
mechanical recycling 368
pyrolysis process
chemical 371
thermal degradation 371
recycling 368
Polyvinyl alcohol
hospital laundry bags 248
water-solubility 248
Polyvinylpyrrolidone
interaction of nitrogen dioxide with
197
R
Recycling
dioxins 361
energy recovery 360
furans 361
incineration 360
incinerator 361
radiation technology 373
Regenerated cellulose
diffusion coefficient 306
Resin separation
air separation 363
colour 365
density 364
electrification 365
float-sink operations 364
flotation tanks 363
fluidisation 365
Fourier transform IR 366
high-voltage drums 365
hydrocyclone 364
magnetic separation 363
Index
photoelectric sensors 365
physicochemical properties 365
spectroscopy 366
supercritical fluid 364
X-ray fluorescence analyser 366
Rheology 5
Rubber-modified products 373
S
Saytex 8010 166
Separated PE waste
recycling 371
Shock-cooling 58
Short-term tests
dart test 28
impact test 28
Shrink film 18, 62, 65
bi-oriented 65
blow-up ratio 67
bubble shape 69
frost-line 67, 69
manufacture 67
mono-oriented 65
ovens 70
properties 66
resin melt index 69
shrinkage 66
shrink-wrapping 62
tunnels 70
Slip agents
erucamide 23, 85
ethylene bis-stearamide 23
oleamide 23, 85
stearamide 23
Slot casting 59
process
melt temperature 60
Sorption isotherm 110
Spheripol 82
process 82-83
Stabilisation agents
phenolic antioxidants 85
phosphite antioxidants 85
Stress relaxation
mechanism of 26
Surface additives
corona treatments 33
glyceryl monooleate 33
polyisobutylene 33
slip agents 33
Surface modification
antiblocking 22
antistatic agents 24
chemical treatments 213
chlorinated paraffins 24
corona discharge 21, 213
hydrophilic 223
lubricants 24
paraffin wax 24
physical methods 222
plasma 213
slip additives 23
stearate salts 24
stearic acid 24
Surface properties
blocking 21
gloss 19
haze 20
slip 21
surface energy 20
Syndiotactic polypropylene 79
T
Tensile properties
burst strength 28
creep 27, 28
strain hardening 26
strain rate 26
stress relaxation 26
Testing
ASTM D882 335
methods
403
Handbook of Plastic Films
requirements 330
sample conditioning 332
results
interpretation of 330
thin films 335
Tetrafluroroethylene-hexafluoropropylene
gamma-irradiated
action of nitric oxide on 205
Thermal analysis
differential scanning calorimetry 31
temperature-modulated 32
Thermal dehydrochlorination 146
Thickness 34
Thin films
‘neck-in’ 58
Tinuvin 622 LD 280
Toxicity equivalence factors 162-164
CDD 164
CDF 164
U
Ultra-low-density polyethylene 11
Ultraviolet stabilisation 25
Unipol 82
Unipol process 84
Ultraviolet degradation 87
Ultraviolet stabilisers 70
404
V
Very-low-density polyethylene 11
Vinyl polymers
interaction of nitrogen dioxide with
188
Viscoelasticity 331
W
Wound exudate
sorption of by dressings 298
Z
Ziegler-Natta processes 41
ISBN: 1-85957-338-X
Rapra Technology Limited
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TECHNOLOGY
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