Download Chapter 12 Enhancements

Enhancements Chapter 12
Enhancement 12.1.
Polymer Materials
Polymers are used in a wide range of everyday applications, in clothing, housing
materials, appliances, communication devices, and for transportation. Feedstocks for
synthetic polymers are petroleum, coal and natural gas, which are sources of ethylene,
methane, alkenes and aromatics. Polymers have advantages over other types of materials
such as metals and ceramics, because of their low process costs, low weight, high specific
strength and stiffness, and because they are often transparent and tough. The unique
combination of strength, modulus, elongation and impact strength make plastics cost
effective versus metals for many applications.
Polymers are macromolecules formed by joining a large number of small molecules, or
monomers, in a chain. These monomers, small repeat units, react chemically to form long
molecules. The repetition of monomer units can be linear, branched or interconnected to
form three-dimensional networks. Homopolymers are composed of a single repeating
monomer whereas copolymers are often formed from a sequence of two types of
monomers. A wide variety of heteropolymers and copolymers may be formed which have
very different structures and properties. Polymers may be amorphous or crystalline in
nature.
There are three major polymer classes: thermoplastics, thermosets and rubbers or
elastomers. Thermoplastics are among the most common polymers and these materials
are commonly termed ‘plastics’. Linear or branched thermoplastics can be reversibly
melted or can be dissolved in a suitable solvent. Thermoplastics may be crosslinked
during processing so as to provide heat stability and limit flow and melting during use.
Typical crystallizable thermoplastics include polyacetal, polyamide, polycarbonate,
polyethylene and poly(ethylene terephthalate) or polyesters. Glassy thermoplastics
include polystyrene and poly(vinyl chloride).
In thermosets there is a three dimensional network structure, a single highly connected
molecule, which imparts rigidity and intractability. Thermosets are heated to form rigid
structures but once set they do not melt or dissolve. Typical thermosets include epoxies
and phenolics. Polymers with long flexible chains between crosslinks are rubbers and
elastomers which, like the thermosets, cannot be melted. A three-dimensional crosslinked
network that can be stretched and spring back to its original form characterizes
elastomers. Crosslinks are chemical bonds between molecules, such as the vulcanization
of rubber. Examples of elastomers include polybutadiene and styrene-butadiene rubber.
Multiphase polymers, combinations of thermoplastics and elastomers, take advantage of
the ease of fabrication of thermoplastics and the increased toughness of elastomers,
providing toughened engineering thermoplastics.
The background material is organized in two parts. The first part is a discussion of
Polymer Morphology, including especially definition of terms. The second part is an
overview of some Polymer Processes that need to be understood as they play a major role
in formation of polymer structure and morphology.
Enhancement 12.2
Polymer Morphology
In polymer science the term structure refers to the local atomic and molecular details
whereas morphology generally refers to larger forms and organization. Often these terms
are interchanged although the characterization techniques are complementary and differ
for these levels of details. X-ray, electron and optical scattering techniques are commonly
applied to determine the structure of polymers. They permit analysis of the interatomic
ordering and chain packing. The morphology of polymers is determined by a wide range
of optical and electron microscopy techniques, which are the subject of this chapter. The
general morphology of polymers will be summarized below to aid interpretation of
images formed by microscopy techniques.
12.2.1 Amorphous Polymers
Polymers that are glassy or rubbery at room temperature are typically amorphous
polymers – they are noncrystalline. They can form brittle glasses when cooled rapidly
from the melt. The glass transition temperature, Tg or glass-rubber transition is the
temperature above which the polymer is rubbery and can be elongated and below which
the polymer behaves as a glass. Plastic deformation in glassy polymers and in rubber
toughened polymers is due to crazing and shear banding. Crazing is the formation of thin
sheets perpendicular to the tensile stress direction, which contain fibrils and voids. The
fibrils and the molecular chains in them are aligned parallel to the tensile stress direction.
Crazes scatter light and often can be seen by eye as whitened areas. Shear banding is a
local deformation, generally at about 45 to the stress direction, which results in a high
degree of chain orientation. The material in the shear band is more highly oriented than in
the adjacent regions.
12.2.2
Semicrystalline Polymers
Semicrystalline polymers exhibit a melting transition temperature (Tm), a glass transition
temperature (Tg), and crystalline order, as shown by X-ray and electron scattering. The
fraction of the crystalline material is determined by X-ray diffraction, heat of fusion and
density measurements. Major structural units of semicrystalline polymers are the plateletlike crystallites, or lamellae. The dominant feature of melt crystallized specimens is the
spherulite. Keith and Padden (1963), Ward (1975), Bassett (1981, 1984) among many
others have described the formation of polymer crystals and spherulites.
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When a polymer is melted and then cooled it can recrystallize, with process variables
such as temperature, rate of cooling, pressure and additives affecting the nature of the
structures formed. Bulk crystallized material is composed of microscopic units called
spherulites, formed during crystallization under quiescent conditions. The structures
exhibit radially symmetric growth of the lamellae from a central nucleus with the
molecular chain direction perpendicular to the growth direction. The plates branch as
they grow. The molecular chains run perpendicular to the spherulite radius with the
thickness dependent on the molecular weight of the polymer, crystallization conditions
and thermal history. In commercial processes additives are used to control nucleation
density and the resulting properties of the plastic.
A schematic of the spherulite structure is shown in Figure 12.1 (Ward, 1975). The
structure consists of radiating fibrils with amorphous material, additives and impurities
between the fibrils and between individual spherulites. The shape of the growing
spherulite is round; however when spherulites impinge upon one another the resulting
shape tends to be polyhedral. A polarized light micrograph of a thin section of bulk
crystallized nylon, in Figure 12.2, shows the size and shape typical of spherulites.
Spherulites appear bright when viewed in crossed polarizers in a transmitted light
microscope because they are anisotropic and crystalline in nature. Isotropic materials
exhibit the same properties in all directions whereas anisotropic materials exhibit a
variation with direction.
Enhancement 12.3
Polymer Processes
The term plastic is used for commodity polymers whereas polymers with high
performance properties are typically called engineering resins. Commercial resins are
often filled with organic or inorganic fibers, minerals and additives. Plastics and
engineering resins are processed into a wide range of fabricated forms, such as fibers,
films, membranes and filters, moldings and extrudates. New technologies have emerged
resulting in novel polymers with highly oriented structures. High modulus fibers and
films can be produced from extended chain polymers, notably polyethylene, and liquid
crystalline polymers (LCPs). Polymers that exhibit liquid crystallinity in the melt or in
solution often can be processed into materials with ultrahigh performance characteristics.
Commercial processes used to manufacture polymer materials include fiber spinning,
drawing and annealing; film extrusion and stretching; rod extrusion, compounding and
injection and compression molding. Development of relationships between the structure
and properties of the polymers requires an understanding of the effect of these processes
on the morphology and properties. A short summary of two process classes, extrusion of
fibers and films and molding of fabricated plastics follows.
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12.3.1
Extrusion of Fibers and Films
Polymer fibers are found in textiles for clothing and household items such as sheeting and
also for industrial uses, such as cords, rope and belts. A melt or solution spinning process
that results in oriented materials can produce fibers. Spun fibers taken up on a bobbin
may be further oriented by drawing on-line or by a post-treatment process. Polymer films
are used in many applications, including packaging and electronic recording and in
membranes for separation applications. Deformation processes that impart orientation to
polymers can result in anisotropic mechanical properties often with increased stiffness
and strength only in one dimension. Textile fibers formed by melt extrusion or solution
spinning, as shown in the schematic in Figure 12.3 are crystallized and then drawn,
creating a highly oriented structure similar to that described for the drawing of bulk
polymers. Films are also formed by extrusion and drawing processes with uniaxial or
biaxial structures. Increasing the draw ratio is known to increase the stiffness and the
breaking strength by improving the degree of molecular alignment or extension. The
diameter of the microfibrillar texture is also affected by the draw ratio.
Liquid crystalline melts (thermotropic) or solutions (lyotropic) are composed of
sequences of monomers with long rigid molecules. Aromatic polyaramides form liquid
crystal solutions whereas aromatic copolyesters form nematic LCP melts at elevated
temperature. Melt or solution spinning of anisotropic LCPs results in an extended chain
structure in the fiber or film (Figure 12.3). Heat treatment generally improves the
orientation and thus the stiffness and strength properties.
12.3.2
Extrusion and Molding
Polymer morphology in extrudates and moldings is affected by process variables such as
melt and mold temperature, pressure, shear and elongational flow. The resulting
morphology in turn influences the performance and mechanical properties. Pressure
increases, for instance, can increase both the melting temperature and the glass transition
temperature of a polymer, with the result that the polymer solidifies more quickly. In a
crystalline polymer the nucleation density can increase, resulting in a decrease in
spherulite size with increased pressure in injection molding.
Structures that are typically observed in molded parts and extrudates tend to be
anisotropic. Elongation or shear flow in extrusion can result in highly oriented rods at
higher draw ratios and/or smaller diameters. Commonly observed structures with an
oriented skin and less oriented central core – skin-core texture—is often observed due to
temperature variations between the bulk and the core. For instance, extensional flow
along the melt front causes orientation and solidification of the polymer on the colder
mold surface. The skin-core texture results from a rapidly cooled, well-oriented skin and
a slowly cooled randomly oriented core as shown in the schematic in Figure 12.4.
Typically in a semicrystalline polymer there are three zones within the molded part: an
oriented, nonspherulitic skin; a subsurface region with high shear orientation; and a
randomly oriented spherulitic core. Thickness of each zone is a function of factors such
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as melt and mold temperatures. Amorphous polymers show a thin surface oriented skin
on injection molding.
Many major chemical industries are based on toughened plastics, such as ABS
(acrylonitrile-butadiene-styrene) and HIPS (high impact polystyrene). Important issues
are the design of fracture resistant polymers such as compatibility, deformation,
toughening mechanisms and characterization. Particle size distribution and adhesion to
the matrix must be determined by microscopy to develop structure-property relationships.
Rubber toughened polymers are usually either copolymers or polymer blends. Polymer
blends are typically made via compounding, the process used to introduce fillers,
particles or additives to a resin prior to molding. Composites are engineering resins with
particle and/or fibrous fillers, such as mica, talc, glass, carbon or polymer fibers.
Specialty composites, such as those reinforced with carbon fibers, are used in aerospace
applications while short or long glass fiber reinforced resins are used in automobiles,
appliances, housing, etc. Composite properties depend upon the size, shape,
agglomeration and distribution of the filler and its adhesion to the resin matrix. In
addition to extrusion and molding, other processes, such as thermoforming and blow
molding are used to form plastic articles. The process parameters used in these various
manufacturing processes have a major affect on the physical and mechanical properties of
the final engineering resin.
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Table 12.1 Polymer functional groups and stains
______________________________________________________________
Polymers
Stains
______________________________________________________________
Unsaturated hydrocarbons, alcohols,
ethers, amines
Osmium tetroxide
Acids or esters
(a) Hydrazine
(b) Osmium tetroxide
Unsaturated rubber
(resorcinol-formaldehyde-latex)
Ebonite
Saturated hydrocarbons (PE, PP)
Chlorosulfonic acid/
uranyl acetate
Amides, esters and PP
Phosphotungstic acid
Ethers, alcohols, aromatics,
amines, rubber,
bisphenol A and styrene
Ruthenium tetroxide
Esters, aromatic polyamides
Silver sulfide
Acids, esters
Uranyl acetate
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Table 12.2 Specific functional groups, examples and stains
___________________________________________________________________________
Functional group
Examples
Stains
___________________________________________________________________________
--CH—CH--
Saturated hydrocarbons
(PE, PP)
(HDPE)
Chlorosulfonic acid
Phosphotungstic acid
Ruthenium tetroxide
--C=C--
Unsaturated hydrocarbons
(Polybutadiene, rubber)
Osmium tetroxide
Ebonite
Ruthenium tetroxide
--OH, --COH
Alcohols, aldehydes
(Polyvinyl alcohol)
Osmium tetroxide
Ruthenium tetroxide
Silver sulfide
--O--
Ethers
Osmium tetroxide
Ruthenium tetroxide
--NH2
Amines
Osmium tetroxide
Ruthenium tetroxide
--COOH
Acids
Hydrazine, then
Osmium tetroxide
--COOR
Esters
(butyl acrylate)
(polyesters)
(ethylene-vinyl acetate)
Hydrazine, then
Osmium tetroxide
Phosphotungstic acid
Silver sulfide
Methanolic NaOH
--CONH
--CONH--
Amides
(nylon)
Phosphotungstic acid
Tin chloride
Aromatics
Aromatics
Aromatic polyamides
Polyphenylene oxide
Ruthenium tetroxide
Silver sulfide
Mercury trifluoroacetate
Bisphenol A
based epoxies
Epoxy resin
Ruthenium tetroxide
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Figure Titles, Enhancement
Figure 12.1
A schematic of the spherulite structure (Ward, 1975).
Figure 12.2
A thin section of bulk crystallized nylon, in polarized light, reveals
bright, birefringent spherulitic texture (Sawyer and Grubb, 1996).
Figure 12.3
A schematic depicting fiber extrusion of conventional and liquid
crystalline polymers (Sawyer and Grubb, 1996).
Figure 12.4
A schematic diagram of molten resin entering the gate of an injection
molding machine and filling the mold (Sawyer and Grubb, 1996).
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