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EXAMENSARBETE INOM TEKNISK KEMI,
AVANCERAD NIVÅ, 30 HP
STOCKHOLM, SVERIGE 2021
PET recycling - Material
chemistry and performance
aspects
CARL HÖÖG
KTH
SKOLAN FÖR KEMI, BIOTEKNOLOGI OCH HÄLSA
1. Abstract
The recycling and collection of PET bottles has a long tradition in Sweden dating back to 1994 and is
one of the staple recycling industries.
Technology has advanced since then, with new recycling processes to assure food grade certified
recycled PET and manufacturing processes such as Solid-State polymerization to enable the bottle-tobottle mantra. Amidst global warming and climate crisis, the interest in recycling and reducing the use
of fossil fuel to manufacture new bottles is ever-growing. As a result, manufacturers and breweries
want bottles manufactured with higher fractions of recycled PET, and there are already bottles out on
the market made from 100% recycled PET.
In this thesis, the effect that the fraction of recycled PET may have on the mechanical and chemical
properties of the final product was tested. Also, the effect that several recycling cycles may have on
the product was tested.
A lab-scale version of the recycling process used commercially in Sweden by Veolia PET were
carried out. Four cycles of the process were carried out on virgin PET material, resulting in material
batches krPET-1 to krPET-4. Dog bone samples from each recycled batch were manufactured via
injection molding.
All samples were characterized with various instruments and methods such as FT-IR, Tensile testing,
DSC, and intrinsic viscosity testing.
From an environmental standpoint, there are clear advantages to an increase in rPET fraction in PETbottles. Due to issues with manufacturing and the production of samples, only a small sample size was
acquired. All the analyses suffered, as a result, making it hard to draw any definite conclusions
regarding potential disadvantages with a higher rPET fraction.
Table of Contents
1. Abstract .......................................................................................................................................................... 0
2. List of abbreviations ....................................................................................................................................... 2
3. Introduction.................................................................................................................................................... 3
3.1 PET ........................................................................................................................................................... 4
3.2 Degradation ............................................................................................................................................. 6
3.3 Processing ................................................................................................................................................ 8
3.4 Recycled PET ............................................................................................................................................ 8
4. Characterization ........................................................................................................................................... 10
4.1 CIELAB method....................................................................................................................................... 10
4.2 Intrinsic viscosity .................................................................................................................................... 10
4.3 Fourier Transform infrared spectroscopy (FT-IR)................................................................................... 11
4.4 Differential scanning calorimetry ........................................................................................................... 12
4.5 Tensile testing ........................................................................................................................................ 12
5. Materials and methods ................................................................................................................................ 13
5.1 Materials ................................................................................................................................................ 13
5.2 Recycling process ................................................................................................................................... 13
5.3 Production of Samples ........................................................................................................................... 13
5.3 FT-IR ....................................................................................................................................................... 14
5.4 Differential scanning calorimetry ........................................................................................................... 14
5.5 Mechanical Testing ................................................................................................................................ 14
5.6 Intrinsic viscosity .................................................................................................................................... 14
6. Results .......................................................................................................................................................... 15
6.1 FT-IR ....................................................................................................................................................... 15
The results are focused on the areas regarding the C=O carbonyl peak around 1713 ................................ 15
6.2 Instron .................................................................................................................................................... 21
6.3 DSC ......................................................................................................................................................... 22
6.4 Intrinsic viscosity .................................................................................................................................... 24
7. Discussion ..................................................................................................................................................... 24
7.1 Impact of production method ................................................................................................................ 24
7.2 Sample size issues .................................................................................................................................. 25
7.3 Sample Testing ....................................................................................................................................... 25
7.4 Suggestions for further work ................................................................................................................. 25
8. Conclusion .................................................................................................................................................... 26
9. References .................................................................................................................................................... 27
10. Appendix A ................................................................................................................................................. 29
2. List of abbreviations
PET
rPET
SSP
TPA
EG
DMT
BHET
Tm
Tg
Tc
PVC
HCl
HDPE
PP
NaOH
krPET
vePET
FT-IR
DSC
Polyethylene terephthalate
Recycled polyethylene terephthalate
Solid-state processing
Terephthalic acid
Ethylene glycol
Dimethyl terephthalate
Bis(hydroxyethyl) terephthalate
Melting temperature
Glass transition temperature
Crystallization temperature
Polyvinyl chloride
Hydrochloric Acid
High-density polyethylene
Polypropylene
Sodium Hydroxide
KTH Recycled polyethylene terephthalate
Veolia Recycled polyethylene terephthalate
Fourier-Transform Infrared spectroscopy
Differential Scanning Calorimetry
3. Introduction
Polyethylene terephthalate (PET) is a widely employed polymer and one of the most important
engineering polymers. Its application areas span a wide range from fibers to packaging, due to its
many important properties. The thermoplastic polymer is hydrophobic, non-toxic, tough, optically
transparent, and possesses a great barrier capability.
One of the major applications of PET in the packaging industry is PET bottles. Another property
making it excellent for this area is its recyclability. At its early stage recycled PET (rPET) was only
used for other applications than bottles. Due to contaminants, it was hard to meet food-grade
requirements. The processing and recycling of PET also degraded the polymer, making rPET not
suitable for bottle manufacturing. Since then, technology has advanced regarding the cleaning and
sorting processes of PET, but also processing techniques as Solid-State Polymerization (SSP).
Eventually, rPET started being re-incorporated into PET-bottle manufacturing [1].
PET-bottles have a long history with recycling in Sweden, with a recycling rate of 84.1% in 2019.
The collection and recycling of bottles started in 1994, and in 2006 a law was passed which states that
a consumption-ready beverage must be connected to a recycling system.
The fraction of rPET in the production of bottles has steadily increased over the years. There are now
bottles produced of 100% rPET available on the market for consumers.
There are still evident disadvantages with rPET being incorporated into bottles. Some can be noticed
by the naked eye. Lesser optical transparency and a duller color are the most noticeable as a bottle is
emptied of its contents. This may be due to increasing particle contamination according to a recent
study in the Netherlands [2].
As the rPET fraction of bottles is increasing, the question regarding its effect on mechanical and
chemical properties is evolving, due to the high temperatures involved in both the recycling process
and the production of bottles. There is not much research on the area. In one study, they discuss the
effect of incorporating higher rPET fractions on extruded PET sheets. They show some beneficial
effects on mechanical properties such as Young’s modulus and stress at yield at fractions below 40%
of rPET [3].
This study aims to further evaluate the effects of incorporating higher fractions with rPET, and the
effect that several cycles of recycling might have on the mechanical and chemical properties of the
polymer.
3.1 PET
Virgin PET is a versatile thermoplastic polymer being used in a variety of applications from everyday
household items to advanced engineering parts. One of the big application areas for PET is the PETbottles, due to its exceptional barrier properties and high chemical, hydrolytic, and solvent resistance.
In addition to these, it also has a very high melting temperature (260 °C), great mechanical strengths,
and good toughness and abrasion resistance even at temperatures reaching 150-175°C [4].
The intrinsic viscosity [𝜂] of commercial PET, varies from 0.45 to 1.2 𝑑𝑙 𝑔−1 [5].
The repeating unit of PET can be seen in Figure 1.
Figure 1: Poly (ethylene terephthalate) repeating unit structure.
3.1.1 Synthesis
PET may be synthesized via two different starting reactions. The first method is the esterification of
terephthalic acid (TPA) with ethylene glycol (EG) at an elevated temperature of 240-260 C°, at a
pressure between 300-500 kPa with water as a by-product. The other method is the trans-esterification
with dimethyl terephthalate (DMT) and EG with methanol as a by-product. Both methods result in the
in bis(hydroxyethyl) terephthalate (BHET) [5]. Both can be seen in Figure 2.
Figure 2: Esterification and Trans-esterification synthesis of BHET
BHET is then pre-polymerized to a degree of polymerization (DP) of 30, at pressures of 2-3 kPa and
temperature ranges of 250-280 °C. After the pre-polymerization step, the polymer undergoes
polycondensation, see Figure 3, with a catalyst (commonly antimony trioxide) at temperatures
between 270-285 °C. A by-product here is ethylene glycol which is eliminated while forming the PET
chain. After the polycondensation, the DP of PET may be brought up to 100 [5].
Figure 3: Polycondensation of BHET into PET
3.1.2 Solid State Polymerization
At the high temperatures above the melting point of PET, thermal decomposition starts occurring,
which competes with the step-growth reactions of the PET. Due to this, there is an upper limit of the
DP. The molecular weight or intrinsic viscosity at this point is not high enough for PET-bottles. The
intrinsic viscosity requirement for PET assigned to bottle manufacturing is usually in a range of 0.70.81 dl g −1 . To further increase the DP, a process called solid-state polycondensation (SSP) is
performed.
The polymer is kept below its melting point in a solid-state either in vacuum or inert gas, to allow for
the removal of by-products such as water and ethylene glycol. The temperature during the SSP is still
high above the glass transition temperature (𝑇𝑔 ). For bottle-grade PET this process is normally carried
out at a temperature of 210 °C for 15-20 hours.
The removal of these by-products shifts the equilibrium of the reaction towards PET [4]–[8].
3.2 Degradation
The main degradation reactions which affect the PET process are thermal, hydrolytic, and oxidative
degradation. Thermal and hydrolytic degradation can be seen in Figure 4, oxidative in Figure 5.
As degradation reactions occur, the polymer main chain undergoes chain scission reactions. Newly
formed functional groups appear, and a molecular weight loss can be observed due to the loss of
length of the main chain. The material may also go through a color change from clear to yellow to
black depending on the extent of degradation [7].
Thermal degradations occur at high temperatures in the absence of oxygen. Random chain scissions
take place in the ester-linkage of the polymer, resulting in vinyl esters and carboxyl end groups.
The susceptibility for PET to undergo hydrolysis and the rate at which it occurs increases with the
presence of carboxylic groups. Any presence of moisture or water during processing is extremely
disrupting. As PET undergoes hydrolysis the degradation products contain carboxylic and hydroxyl
end groups, further increasing the rate of hydrolysis [7], [9].
Impurities present in the polymer melt may also generate macro-radical sites. If oxygen is present, it
can lead to the production of unstable peroxy-radicals and hydroperoxides which at high temperatures
can further the breakdown of the polymer.
Thermal oxidative degradation may also occur in the presence of oxygen at high temperatures. As the
hydroperoxide is formed at a methylene group in the diester linkage, it may induce further
degradation. The degradation mechanism behind this reaction is not fully understood. Theories
suggest a free radical mechanism that results in radical formation and carboxyl, hydroxyl, and vinyl
ester end groups [7].
The vinyl esters and hydroxyl end groups can undergo recombination, which will maintain the
molecular weight, but a by-product from the recombination is vinyl-alcohol which tautomerizes into
acetaldehyde, Figure 6. Acetaldehyde as a contaminant is very hard to detect, and it may affect food
products negatively.
Acetaldehyde is a very volatile compound and does not pose much of an issue in PET-bottle
production as it can be extracted during the vacuum or drying process of PET. Many other
compounds, carbon monoxide, carbon dioxide, ethylene, benzene, and biphenyl may also be created
due to these degradation mechanisms, most of them from thermal degradation.[4], [9].
Figure 4: Thermal and Hydrolytic degradation of PET
Figure 5: Thermal oxidative degradation of PET
Figure 6: Formation of Acetaldehyde
3.3 Processing
The production of PET-bottles is done via blow molding, either in a single or two-stage injection. The
polymer melt is injection molded at temperatures above 260°C into a mold which is kept at a cold
temperature to produce a PET-preform. Due to the rapid cooling of the polymer, the preform is kept in
an amorphous and transparent state.
In the single-stage method, the preform is immediately transferred into a larger bottle mold and
inflated via an air-blowing unit to the desired size.
More commonly is the two-stage method. The manufactured PET preforms are packed and shipped to
the beverage companies since their lesser volume reduces logistical costs. The beverage companies
may then re-heat the preform above its Tg , and inflate it before filling the bottle with its specific
beverage in a continuous process [5].
3.4 Recycled PET
As PET bottles have been a commonplace item in most households since their inception, it was also
an indicator of the waste problem regarding plastics worldwide. PET is a non-degradable plastic under
normal environmental conditions, having a very slow decomposition rate. It became a large
environmental issue, and still is in some areas of the world. As a result, the recycling industry of PET
was established.
At first, rPET was only used for other applications than bottles, due to the food-grade requirements.
There was no good way to control the contaminants present in the recycled material. However, as the
technology improved, methods to acquire food-grade PET flakes were introduced. This led to the
possibility of a closed bottle-to-bottle recycling system.
In a study from 2012 from Returpack, they concluded that using rPET instead of virgin material can
save as much as 5,26 kWh and 1,3 kg of CO2-emissions per kilogram recycled material [10].
3.4.1 Contamination difficulties
As PET is processed, several contaminants may alter the physical and chemical properties negatively.
As such a rigorous cleaning process must be applied to maintain the food-grade requirements if rPET
is to be reintroduced into the bottle production stream. There is also the issue of additional thermal
degradation, as the recycling process involves high temperatures for prolonged times.
3.4.1.1 Acids
Several contaminants may lead to the production of acids, which in turn may act as catalysts for chain
scission reactions such as hydrolysis [9]. The adhesives used by some bottling companies may
produce rosin, abietic, or acetic acid during the recycling process.
Hydrochloric acid (HCl) may also be produced in cases where labels contain Polyvinyl Chloride
(PVC). These labels are banned in Sweden and are not allowed into the closed bottle-to-bottle feed
loop. However, cases do occur where these bottles may enter the feed loop.
The HCL enables the hydrolytic degradation of PET greatly in addition to chain scission, due to a
catalyzation effect on the reaction between ester bonds and moisture [11].
3.4.1.2 Water
Water, usually in the form of moisture, will induce hydrolysis if it is present during the production of
PET products. To combat hydrolysis, the moisture content when manufacturing PET products must be
kept below 0.002 %. The degradation rate of hydrolysis has been reported to be on an order of at least
a magnitude higher than thermal degradation [9], [12].
When moisture is present in the polymer melt, it will react and reduce viscosity drastically. The
recycling process includes washes with both basic and acidic solutions in addition to washes with
water. Consequently, PET must be always dried before processing. This may lead to issues when
working with rPET depending on the drying temperature due to different contaminants, such as PVC.
At lower temperatures of 120°C, below the degradation temperatures for PVC, the melt viscosity is
affected negatively due to the HCl catalyzed hydrolysis. At higher temperatures, above 230°C, the
melt viscosity is improved in comparison, and the HCl will instead result in black flecks in the
extruded product [9].
3.4.2 Recycling processes
The URRC recycling process used in this study is the same process used commercially in Sweden by
Veolia PET. This process is certified to reach the food-grade requirements needed for rPET.
In Sweden, the logistical task to collect and sort the PET bottles is under Returpack’s jurisdiction.
The recycling process starts with the sorting of PET bottles into clear and colored fractions. Only
clear fractions are suitable for reuse in rPET bottles due to requirements regarding optical
transparency. Bottles are then ground into flakes that are washed in a hot alkaline solution to remove
any remaining labels or contaminants from the PET material.
The caps of PET-bottles are manufactured out of plastic materials such as high-density polyethylene
(HDPE) and polypropylene (PP), these must be sorted out of the feedstock. This is done via a swimsink separation tank. The PET flakes will sink to the bottom due to a higher density, while labels and
cap materials will float to the top.
Following the separation, the flakes are dried and an additional sorting process involving pressurized
air is performed to remove any remaining contaminants like film or label residues.
The flakes are then treated with the patented URRC process, where they are immersed in a sodium
hydroxide (NaOH) solution at high temperatures for a long time to create a chemical etching of the
surface, at a pH around 14, which in turn results in a clean and food-grade PET flake. The flakes are
then dried and further sterilized with vacuum treatment. A rewash in an acidic solution of HCl is done
to neutralize any remaining alkaline substances. The flakes are then put through a final laser sorting
system to remove any foreign materials or unwanted particles [13].
The chemical etching process is essentially a controlled degradation on the surface layer of the
polymer, leaving a new clean layer on top. The chemical reaction, saponification (alkaline hydrolysis)
can be seen in Figure 7 [14].
Figure 7: Saponification of PET
4. Characterization
4.1 CIELAB method
The CIELAB (CIE L*a*b) method is a way to express and measure how the color changes. It is also
one of the current methods of analysis at the recycling facility in Sweden.
It consists of a combination of three values. The L* value defines the lightness of the species, a*
which describes the red/green value, and b* describing the yellow/blue value [15].
These colors are commonly measured by spectrophotometry.
The method can then be used to analyze rPET and determine if the opacity/color of the bottles is
suitable for beverage containers.
4.2 Intrinsic viscosity
The intrinsic viscosity of a polymer is related to the molecular weight of the polymer, and as such is
of great importance to the polymer's mechanical and chemical properties. To measure the intrinsic
viscosity an Ubbelholde viscometer can be used, see Figure 8. A polymer is dissolved in a solvent.
The time it takes for the meniscus of the solution to flow through a capillary, from point A to B, is
measured. The time is then compared to the time it takes for the pure solvent to pass through. The
intrinsic viscosity can be calculated via different equations depending on if the testing was for a single
or several concentrations of a polymer solution.
To evaluate the molecular weight of a polymer from the intrinsic viscosity, the Mark-Houwink
equation is employed.
[η] = K ∗ M a
(Equation 1)
Where K and a are constants that depend on the polymer and solvent and their temperature, and M is
the molecular weight of the polymer [16].
Figure 8: Schematic of an Ubbelholde Viscometer
4.3 Fourier Transform infrared spectroscopy (FT-IR)
By quantifying the absorption of infra-red light by molecules, it is possible with FT-IR to characterize
the chemical structure of the molecules.
FT-IR can be used to detect and analyze the degradation products which occur during the production
and recycling of PET. It is possible to observe as PET degrades into compounds with different
functional groups when comparing the spectra of rPET with virgin PET.
In a study from 2000 [17], two regions of the spectra are observed to evaluate the hydrolytic
degradation of PET. Between 3500-2700 cm−1, there are two peaks of interest around 3445 and
2950 cm−1. These correspond to the -OH and -CH regions, respectively. Sammons et al. note that as
PET degrades there is a large change in the -CH region, as the electronic environment of aliphatic
carbon-hydrogen chains is affected. They also note that an increase of carboxylic and alcoholic end
groups can be observed in the -OH region. This implies that degradation products from thermal
degradation (carboxylic end groups) will be observed in the same region.
In the region around 1700-1750 cm−1 the carbonyl C=O bond can be observed. As degradation
occurs, ester carbonyls will degrade into carboxylic acid carbonyls. An ester carbonyl bond has a
higher absorption peak than carboxylic carbonyls, partly due to a stronger bond. If the ester degrades
into a carboxylic group, the peak in the carbonyl area will broaden or split due to the lower absorption
peak of the carboxyl carbonyl. The broadening of the area will increase as the degradation increases
[18],[19]. Vinyl esters from thermo-oxidative degradation are seen in FT-IR 1780 cm−1 according to
data from Sigma Aldrich [20], however, if these compounds are present there is a high chance the
peaks will overlap with the ester and carboxylic peaks into the larger carbonyl peak.
4.4 Differential scanning calorimetry
Differential scanning calorimetry (DSC) is used in the thermal analysis of compounds. By increasing
the temperature continuously at a steady rate, while measuring the heat flow into a sample and a
reference many important properties can be determined. There are certain factors to consider, as in
most heat flow DSCs the sample may also exchange heat with its surroundings, which is not
accounted for by the sensors. Under the assumption that the sample and the reference exchange the
same losses, and that these losses are constant over calibration runs, the losses can be incorporated in
the calibration equations and handled. It is still an issue though, and the accuracy of the analysis may
suffer as a result [21], [22].
For polymers, properties such as degree of crystallization, the melting (𝑇𝑚 ), glass transition (𝑇𝑔 ) and
crystallization (𝑇𝑐 ) temperatures can be measured and evaluated from the data. Since all of these are
connected to the mechanical and chemical properties, they can be used to evaluate the effect of
incorporating higher fractions of rPET in production.
4.5 Tensile testing
Tensile testing of polymers is one of the most common ways to evaluate certain mechanical
properties. Samples with a standardized size are fastened between two clamps in a universal testing
machine. The clamps are then pulled apart at a constant rate of tension until the sample breaks. The
force which is applied in addition to the elongation of the sample is measured.
As the cross-sectional area of the sample is known, the data can be used to plot a graph of stress
(force/cross-sectional area) versus strain. From this, properties such as Young’s modulus, Tensile
strain, and Stress at break can be evaluated. Most polymers, being viscoelastic, behave elastically
before their yield point (the maximum stress before permanent deformation) and plastically afterward.
5. Materials and methods
5.1 Materials
Virgin PET material was acquired via Petainer, the largest supplier of PET preforms in Sweden. The
material came in two forms, pellets, and granulated preforms. They will be noted as virgin-PET
pellets and virgin-PET preform.
The reference rPET material, vePET, were received from Veolia PET, which is responsible for the
recycling processes of PET in Sweden.
5.2 Recycling process
To acquire KTH recycled PET (krPET) granulated preforms were recycled via the patented URRCprocess from Veolia PET on a lab-scale environment at KTH. The recycling process focused on the
alkaline environments, as no separation of materials (HDPE caps and labels) is required due to the
feedstock used.
The three main areas of the lab-scale recycling process involved:
1. A weak alkaline wash ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss,
followed by drying and subsequently immersed into a stronger alkaline wash
sssssssssssssssssssssssssssssssssssssssssssssss.
2. After the alkaline washes, the material is inserted into an oven
sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
sssssssssssssssssssssssssssssssssss, to achieve the chemical etching process.
3. It is then removed from the oven, neutralized with assssssssssssssss solution, and washed with
more water before drying.
The material was then either used in the production of test samples or recycled up to three times more
with the same procedure.
krPET samples will be noted according to “krPET-number of cycles %fraction of rPET”.
5.3 Production of Samples
Before the production of all samples, the material was dried at 160 °C for 6 hours. The material was
then injection molded to produce dog-bone samples, in a Battenfeld PLUS 250, at an injection
pressure of 120 bar. The dog bone mold had a total length of 150 mm and a gauge length of 60 mm
with a thickness of 3 mm.
The injection molder had four temperature sectors: 40 °C, 240°C, 268°C, 265°C, from hopper to
nozzle. The mold was kept at 40 °C. The average cycle time for the samples was 35 seconds. Due to
complications with samples occasionally getting stuck in the mold and runner, this led to uncertain
residence times of the polymer inside the barrel at high temperatures.
The recycled materials, both krPET and vePET, were mixed with virgin-PET pellets in two weight
fractions, 1:3 and 1:1, in batches of 300 grams. Also, samples with 100% krPET and virgin PET were
made, attempts to produce 100% vePET samples were not successful.
A minimum of 5 samples were made for each batch, except for the 100% rPET batches due to
production issues.
5.3 FT-IR
Samples were taken from each produced batch and tested. FT-IR spectra of PET samples were
procured on a Perkin Elmer Spectrum 100 FT-IR Spectrometer, with a resolution of 4 cm−1 between
the range of 600 to 4000 cm−1 for 16 accumulated scans. All FT-IR spectra were then normalized to
the band at 1410 cm−1 corresponding to the benzene ring, which is a suitable normalization band
[23].
5.4 Differential scanning calorimetry
Shavings were taken from multiple samples of each batch and combined and analyzed in a Mettler
Toledo DSC 3. The shavings were weighed at 15-25 mg and enclosed in 100 µl aluminum cups. An
empty aluminum cup was used as a reference cup.
The cups were then subjected to the following temperature program at a constant rate of 10 K per
minute: 25°C to 260°C, 260°C for ten minutes, 260°C to -20°C, -20°C to 260°C. The temperature
program is designed to remove any thermal history regarding the melting point of the polymer.
From the thermal data the glass transition, crystallization, and melting temperature were acquired. A
degree of crystallization was also calculated with the DSC analysis software. This is done by
comparing the enthalpy of the second melting peak with the theoretical melting enthalpy of 100%
crystalline PET, which is 140,1 J/g.
5.5 Mechanical Testing
All dog-bone samples were tested with an Instron Universal Testing Machine 5944 with a 2 kN load
cell. Due to uneven thickness in samples, the thickness was measured at 5 points along the gauge
length and the average thickness was used for calculation, the average thickness was around 3±0,2
mm. The samples were tested under tensile stress until fracture or a short time after plastic
deformation had occurred at a ramp rate of 10%/min.
5.6 Intrinsic viscosity
Intrinsic viscosity (ƞ) testing was carried in a Schott 501 10-I Ubbelholde viscometer.
500 mg samples from 4 batches of raw material, Virgin PET preforms, krPET-1, krPET-3, and vePET
were dissolved in 15 ml trifluoroacetic acid (TFA).
The solutions and pure solvent were then tested in the viscometer until the flow time of three samples
were within 0.2 seconds from each other. The ƞ-value was then calculated via the Billmeyer equation
[24].
[ƞ] = 0,25 ∗ (ƞ𝑟 − 1 +
3∗ln(ƞ𝑟 )
𝑐
𝑡
, ƞ𝑟 = 𝑡
𝑜
(Equation 2)
Average molecular weight was also calculated from the intrinsic viscosity via the Mark-Houwink
relationship, with the constants K=0,000430 (dl/g), a(TFA)=0,68 [25].
6. Results
6.1 FT-IR
The results are focused on the areas regarding the C=O carbonyl peak around 1713 cm−1 and the -CH
and -OH bonds in the area between 3450cm−1 to 2900cm−1. Due to the small sample size of tested
samples, only observations and possible trends can be discerned, and no distinct conclusions can be
drawn.
For the carbonyl peak, the area under the graph between 1780 to 1630 cm−1 were calculated via the
PerkinElmer Spectrum program and are tabulated in Table 1. The spectra for the 25,50 and 100 %
krPET fractions in this area can be seen in Figure 9-11.
In theory, if the area of a sample has increased or maintained the same value while the height has
decreased compared to a virgin-PET sample, a broadening of the peak downwards will have occurred.
This is due to an increase in carboxylic end-groups because of degradation. We cannot conclude
which type, but the most probable is thermal degradation, due to high temperatures in both the
recycling process and production of samples.
The broadening of the peak may also be visible in the graph.
If the height of a peak has increased, while increasing or maintaining the area, it is harder to conclude
what has occurred. It could be a result of the analysis procedure due to a better contact area with the
FT-IR crystal, or it could mean that it has degraded less than the virgin-PET pellet sample.
The reverse can also be said, if the area decreases with a higher peak it should entail a lesser
degradation. If the area decreases with a lower peak, it is hard to determine the effect.
This applies to the carbonyl area, and not necessarily on the -CH and -OH regions due to
normalization errors.
The area for the Virgin-PET pellet were calculated to 30,17 area units and will be used for
comparison.
The area of samples containing 25% rPET can be seen in Table 1. When comparing these to the area
and graph of Virgin-PET pellet, there is a decrease in the area for all samples except krPET-3 and
vePET. In figure 8 we can also see that only krPET 3 has a higher peak. Also, the peaks for krPET-3
and vePET are broader than virgin-PET pellets. This should entail a larger amount of degradation
products for these two samples. For the other samples, it is harder to conclude any clear results from
the data acquired.
For the samples with 50% rPET, Table 1 shows that the samples with larger areas than virgin-PET
pellets are krPET-3 and krPET-4. In Figure 9, only krPET-3 and krPET-1 show a higher peak, and
compared to the virgin-PET pellets, krPET-3 also has a broader peak which can be seen in the graph.
Following the theory, krPET-4 has an increased area and a lower peak which leads to a broader peak,
which can also be seen in the graph. We can then note that for the 50% rPET fraction, krPET-3 and
krPET-4 have degraded the furthest out of all samples, which is to be expected.
For the samples with 100% rPET, all samples exhibit a larger area compared to virgin-PET albeit
krPET-4 ever so slightly. In Figure 10, we can also note that only krPET-1 has a higher peak. Here we
can note that all krPET samples except krPET-1 have degraded further than virgin-PET pellets.
There is also a trend in this region, which can be seen at very high resolutions. There is a very slight
downward shift of the highest point of the peaks of all samples compared to virgin PET pellets, which
could indicate degradation mechanisms.
The spectra over the -OH and -CH region at 3450cm−1 and 2950 cm−1 respectively, can be seen in
Figure 12-14.
Generally, higher peaks and a larger area in both should imply more degradation products [17].
However, noise and normalization errors might have occurred, which can be seen in Figure 13 for
krPET-2. It is hard to compare the height of peaks when the base absorption differs greatly.
As such, the area is of more use.
The area under the peaks for the regions were calculated and can be seen in Table 2 and Table 3.
The range used for calculations was for -OH: 3500 cm−1 to 3370 cm−1, and for -CH: 3050 cm−1 to
2800 cm−1.
The area of the virgin-PET pellet peak was 0,26 and 2.84 area units for the -OH and -CH regions
respectively.
Starting with samples of 25% rPET in the OH-region. Observing Figure 11, the largest activity
corresponds to krPET-2. This continues in the -CH region, with the addition of krPET-3. In Table 2
and Table 3, all samples have a larger area than virgin-PET pellets in both the -OH and -CH region,
which indicates a larger degradation.
Following with samples of 50% rPET. vePET and krPET-4 account for the largest activity in the OHregion. In this region, all samples except krPET-1 and vePET have a larger area.
For the -CH region all samples except vePET show a smaller area than virgin-PET pellets, indicating
a lower amount of degradation for all krPET samples, contradicting the data from the -OH region.
Finally, for 100% rPET samples which can be seen in Figure 13, there seems to be a kind of noise or
normalization error for the krPET-2 sample as it enters at a high absorption in the -OH region. But by
comparing the area of the samples we can still conclude that only krPET-2 and krPET-3 have a larger
area than the virgin-PET pellet. In the -CH region this trend continues. This could entail a larger
degradation for our krPET-2 and krPET-3 samples, while our krPET-1 and krPET-4 samples seem to
have degraded less.
FT-IR spectra of individual krPET cycles can be found in Appendix A1-A8.
It is hard to note a general trend from these. The regions do not fully correspond to each other. In one
region we might see an increase in degradation, while in another a decrease.
For the -C=O region, there seems to be a trend where an increase in rPET content leads to an increase
in degradation products, across all samples except krPET-3.
For the -CH region, there is a decrease in degradation when increasing the rPET content to 50% from
25%. When further increasing the rPET content to 100%, we see an increase in degradation but still a
lesser one than its 25% counterpart.
In the -OH region it is hard to note any trends, for some samples like krPET-2 an increase in rPET
content leads to an increase in degradation, but for krPET-3 the reverse can be seen.
Table 1: Area of FT-IR peaks between 1780 to 1630 𝑐𝑚 −1 in comparison with Virgin-PET pellets area, 30,17 area units.
Sample
krPET-1
krPET-2
krPET-3
krPET-4
vePET
25% rPET (area unit)
29,49
28,70
33,78
29,55
32,07
50% rPET (area unit)
30,04
29,02
31,37
30,76
28,22
100% rPET (area unit)
30,59
33,94
30,59
30,18
------
Table 2: Area of FT-IR peaks between 3500 to 3370 𝑐𝑚 −1 in comparison with Virgin-PET pellets area, 0,26 area units.
Sample
krPET-1
krPET-2
krPET-3
krPET-4
vePET
25% rPET (area unit)
0,28
0,27
0,32
0,28
0,33
50% rPET (area unit)
0,22
0,28
0,28
0,31
0,26
100% rPET (area unit)
0,25
0,34
0,28
0,26
-----
Table 3: Area of FT-IR peaks between 3050 to 2800 𝑐𝑚 −1 in comparison with Virgin-PET pellets area, 2,84 area units.
Sample
krPET 1
krPET 2
krPET 3
krPET 4
vePET
25% rPET (area unit)
2,97
4,24
3,36
3,04
3,34
50% rPET (area unit)
2,14
2,81
2,58
2,58
3,23
100% rPET (area unit)
2,27
3,85
3,83
2,28
------
Figure 9: Changes in Carbonyl bond for 25 % rPET.
Figure 10: Changes in Carbonyl bond for 50% rPET.
Figure 11: Changes in Carbonyl bond for 100% rPET
Figure 12: Changes in -OH and -CH region for 25% rPET.
Figure 13: Changes in -OH and -CH region for 50% rPET.
Figure 14: Changes in-OH and-CH region for 100% rPET.
6.2 Mechanical testing
Table 4: Mechanical data of all dog-bone samples which underwent plastic deformation.
Sample (n of
replicates)
Average Young’s
Modulus, E (GPa)
Stress at Break
(MPa)
Strain at
Break (%)
krPET-1 25 (2)
krPET-2 25 (4)
krPET-3 25 (4)
krPET-4 25 (5)
vePET 25 (3)
17,40
16,83
16,67
15,62
17,91
534,28
527,83
529,43
510,25
590,93
5,10
5,23
5,11
5,33
5,20
Standard
Deviation
(Young’s
modulus)
0,77
0,25
0,69
1,2
0,4
krPET-1 50 (1)
krPET-2 50 (3)
krPET-3 50 (4)
krPET-4 50 (4)
vePET 50 (5)
15,32
15,91
15,56
15,56
16,79
510,8
497,36
519,26
517,58
548,36
5,3
5,13
5,48
5,22
5,14
----0,72
0,7
0,43
0,40
krPET-1 100 (0)
krPET-2 100 (0)
krPET-3 100 (4)
krPET-4 100 (1)
Virgin Preform (5)
Virgin Pellets (1)
--------17,35
16,2
17,63
17,2
--------555,44
481,6
555,25
500,13
--------5,24
4,68
5,01
4,32
--------0,4
----0,7
-----
All manufactured samples underwent mechanical testing. Many samples fractured in the elastic
region. Since the fracturing may have occurred due to unknown defects or weak points these values
were not included in the graphs or the tabulated data shown.
As a result, several batches have a small sample size, and no real definitive conclusions can be drawn.
As there is only a sample size of 1 for virgin-PET pellets it is difficult to compare data.
A standard deviation for Young’s modulus data was calculated for each sample size. In Table 4 the
mechanical data of all batches can be observed. These data are only taken from samples that
underwent a plastic deformation.
For samples containing 25% rPET, there is a decrease in Young’s modulus as the number of cycles of
recycling increases, in all cases except vePET. This decrease in Young’s modulus stems from a
decrease in stress at break and an increase in strain at break. An implication could be a more ductile
material as rPET material is further recycled and incorporated at higher fractions.
This differs from the study from G. Curtzwiler et al.[3], where an increase in rPET fractions up to
40% resulted in increased Young’s modulus. Their rPET material consisted of washed PET bottle
flakes, however, it is unknown if this is the URRC process. This might entail that the krPET recycling
process is more intensive than its commercial counterpart when it comes to the degrading of the
polymer, however, no direct comparison can be made due to the potential differences in recycling
processes.
An increase of the rPET fraction to 50% results in a decrease of Young’s modulus across all batches,
compared to the 25% fraction.
For krPET-3 100, there is an increase in Young’s modulus, even compared to the 25% fraction. This
can be seen for krPET-4 100 as well, but only with a sample size of 1.
Comparing virgin-PET preform and krPET-3 100, we can see that the change in modulus stems from
an increase in strain at break. This also seems to imply that the recycled material yields a more ductile
product than its virgin PET counterpart.
6.3 DSC
A typical DSC graph is shown in Figure 15 and Figure 16, with Heat flow versus the temperature of
the sample.
The 𝑇𝑔 , 𝑇𝑐 and 𝑇𝑚 are evaluated from the corresponding points in the graph. The area in grey
corresponds to the melting enthalpy and is used to calculate the degree of crystallinity.
Properties calculated from all DSC graphs can be found in Table 5. The thermal graphs of all samples
are similar except krPET-4 50, where a large increase in Tc is seen. This is probably due to a
defective measurement. Across all samples, we can see an increase in Tg as the fraction of rPET
increases, and as the number of cycles increases. As such, there seems to be some effect on Tg by the
recycling process.
We can see that the crystallinity of samples varies between 36,6 % to 26,7% across all samples.
The incorporation of rPET seems to increase the crystallinity compared to its virgin counterparts.
There is a slight trend towards a decrease in crystallinity when increasing the recycled PET fraction
further, something which has been noted in other articles [2]. This can especially be seen when
comparing 25% versus 100% rPET samples. However, further DSC analysis of a larger sample size
would be required to draw such a conclusion.
All DSC thermograms can be found in Appendix A16-A30.
Figure 15: DSC Thermogram of krPET-1 25
Table 5: Thermal Data calculated from DSC analysis.
Sample
Tg (°C)
Tc (°C)
Tm (°C)
krPET-1 25
krPET-2 25
krPET-3 25
krPET-4 25
vePET 25
74.90
74.92
75.26
75.73
75.85
135.17
131.16
134.28
130.57
133.56
247.18
247.64
247.55
246.87
247.48
Crystallinity
(%)
34.34
34.03
36.55
31.75
30.02
krPET-1 50
krPET-2 50
krPET-3 50
krPET-4 50
vePET 50
75.83
76.00
76.11
76.35
73.51
133.84
132.56
131.98
139.07
136.57
246.88
247.54
246.62
245.42
243.10
34.39
33.94
34.56
28.02
29.08
krPET-1 100
krPET-2 100
krPET-3 100
krPET-4 100
Virgin Preform
Virgin Pellets
76.27
76.04
77.79
76.96
76.47
75.97
128.46
125.37
134.22
133.61
138.07
136.57
247.65
243.10
245.6
246.27
248.03
243.10
30.74
29.08
26.71
28.3
31.54
29.08
6.4 Intrinsic viscosity
The intrinsic viscosity of the four batches can be seen in Table 6.
A clear decrease in intrinsic viscosity can be seen for the recycled material. Since krPET compared to
the virgin-PET preform has only been exposed to the actual recycling process and no additional
processing, we can conclude that the large decrease in intrinsic viscosity is solely due to the recycling
process itself. Since there are only two krPET fractions tested, we cannot note with certainty if there is
any specific relationship between the loss of intrinsic value and the number of cycles.
Compared to literature, the requirement for bottle grade PET varies from 0,73-0,8 g/dL [5]. We can
note that all samples except krPET-3 meet these requirements. Comparing krPET-1 and vePET, the
values are somewhat similar, with krPET-1 on the lower end.
Both have gone through commercial injection molding, and thermal degradations which occur at high
temperatures. vePET has also been blow-molded, however since that occurs at low temperatures it
should not induce any substantial degradation. Like the results from the mechanical testing, this could
infer that the krPET recycling process is harder on the material than its commercial counterpart.
The need for processes such as SSP is clear watching the drastic decrease in intrinsic viscosity of
krPET 3. A study from 2006 has shown that the SSP process may increase the intrinsic viscosity of
recycled PET material up to its virgin PET counterpart [26]. As such, if the SSP process is carried out
by the preform manufacturer after each recycling cycle, the increase in cycles should not be a huge
issue in terms of intrinsic viscosity.
The intrinsic viscosity testing could unfortunately not be carried out on all batches due to a shortage
of solvent.
Table 6: Intrinsic viscosity and Molecular weight of samples.
Sample
Virgin Preform
vePET
krPET-1
krPET-3
Ƞ (dL/g)
0,80
0,76
0,74
0,63
Mr (g/mol)
63 728
59 355
57 722
45 512
7. Discussion
7.1 Impact of production method
The production of samples in this study probably had a large impact on the results. As mentioned
above, there were several issues with production, where samples and polymer melt stayed at high
temperatures for long periods. This would result in a much higher degree of thermal degradation
which could be noticed on several samples in terms of color and yellowing of samples. Due to this
issue no CIELAB testing was performed, as there were difficulties picking a relevant sample due to
the large range of colors in a batch.
Effects of the production could also be seen in mechanical testing. Almost 50% of all produced
samples shattered or broke in the elastic region resulting in no valuable data.
There was also a multitude of samples shattering and fracturing when ejected from the mold during
production.
As a result, only a small sample size was tested in mechanical testing and no clear conclusions can be
drawn, and only trends can be indicated.
The large decrease in intrinsic viscosity from the recycling process could be a contributing factor,
however, even when producing samples from virgin-PET pellets the same issues were encountered.
When speaking to bottle manufacturers, they mentioned that different sizes and shapes of the raw
material may produce problems with the mixing of the polymer melt during processing. These
problems have been encountered when incorporating flake material at higher fractions above 50%.
The granulated forms of krPET, all in random shapes, were at least two-three times the size of the
uniform PET pellets. This could be an explanation for the sudden increase in Young’s modulus when
producing 100% krPET samples compared to the 50% krPET ones.
7.2 Sample size issues
The sample size of the FT-IR analysis poses a large problem. From the data regarding the intrinsic
viscosity, a large decrease in molecular weight was acknowledged and in turn, degradation products
should be showing up in FT-IR.
Since there were only FT-IR testing on a single sample from each batch, outliers affect the result
greatly. As the intrinsic viscosity seems to drop drastically for each recycling cycle, we should be
seeing more prominent peaks from krPET-3 and krPET-4. Larger sample sizes and averaging FT-IR
spectra could have possibly counteracted this.
7.3 Sample Testing
To draw any definite conclusions on the effects of the recycling process on degradation, FT-IR and
DSC analysis should have been made on the raw PET material in addition to produced samples from
injection molding. As of now, there is no clear way to deduce the effect the recycling process had on
certain degradations shown in FT-IR and DSC, as all samples have undergone thermal degradation
during processing. If the manufacturing process were foolproof this could have perhaps been
accounted for, but since the residence time of samples is unknown, we cannot determine the effect
that recycling had on final properties.
7.4 Suggestions for further work
When carrying out the recycling process, batches were created individually. It would be better to
make a large batch of krPET-1, and then taking a portion of that to make krPET-2 and so forth. This
would diminish errors or discrepancies that might occur due to handling errors during the recycling
process.
The manufacturing method chosen for the sample production led to many difficulties when analyzing.
Different production methods for samples should perhaps be considered for tests such as FT-IR, DSC,
and CIE-LAB testing, that does not require a specific shape.
FT-IR testing should also be carried out on raw material that has only gone through recycling. It
would make it possible to solely determine the effect of the recycling process. In this thesis, the
thermal degradation which occurs during processing might have overshadowed it.
For both DSC and FT-IR more samples should be tested. Also, a test sample in DSC for each sample
that underwent mechanical testing could give interesting information. This would enable comparing
the mechanical data with parameters like crystallinity and Tg, and their effect.
8. Conclusion
Due to the small sample size, it is not possible to draw any definite conclusions. However, certain
trends have been observed.
The mechanical data from a small sample size seemed to indicate an increase in Young’s modulus as
the rPET material increased to 100%. Whereas a decrease in the same properties was seen when
increasing the content from 25 to 50%.
As the number of recycling cycles increased, a decrease in Young’s modulus could be seen from the
same data, indicating an effect of repeated recycling of PET material. There was also a slight trend
that implied a more ductile material as the rPET content increased.
The same trend could be seen from intrinsic viscosity data, noting a large decrease in intrinsic
viscosity as the material is recycled several times. To what extent and degree were not able to be
measured fully.
Drawing any conclusions from these trends towards the manufacturing of PET-bottles is difficult.
The decrease in intrinsic viscosity should have some effect on the processing, as the raw material
needs to uphold these requirements. SSP can possibly negate these effects.
The decrease in Young’s modulus is at most 10-15% and will most probably not have any noticeable
effect on the mechanical performance of bottles and their capabilities to contain beverages. The trend
towards a more ductile material may result in a less rigid bottle as the rPET fraction and cycles
increase. This might be noticeable when the bottles are empty.
No clear effect of the incorporation that higher fractions of rPET might have on degradation could be
determined from FT-IR. Trends could be noted in the carbonyl region, where an increase in rPET led
to an increase in degradation products. Unfortunately, no distinction can be made from degradation
products that occurred due to the recycling process or the manufacturing of samples. The same trend
could not be seen for the effect of increasing the number of recycling cycles, and more testing would
be needed.
A small decreasing effect on crystallinity and an increase in Tg was seen when increasing the rPET
content, the Tg also seems to increase as the number of recycles increases. A decrease in crystallinity
could be a good thing, making it easier to obtain amorphous bottles. This effect might be negligible
however, due to the rapid cooling of the mold during production.
An increase in Tg could imply in a slightly more resource-intensive processing of the polymer during
production. However, the energy and environmental impact saved by not using virgin PET would
counteract this.
Further testing on larger sample sizes is needed to certify these observations.
Acknowledgments
I would like to extend my sincerest gratitude to my supervisor Ulrica Edlund for all her guidance and
help throughout this project, both in planning and execution.
I would also like to thank Sara Bergendorff and Returpack for the opportunity to work on this project
for them.
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10. Appendix A
Figure A1: in -OH and -CH region for krPET-1 fractions.
Figure A2: Changes in C=O region for krPET-1 fractions.
Figure A3: Changes in -OH and -CH region for krPET-2 fractions.
Figure A4: Changes in C=O region for krPET-2 fractions.
Figure A5: Changes in -OH and -CH region for krPET-3 fractions.
Figure A6:Changes in C=O region for krPET-3 fractions.
Figure A7: Changes in -OH and -CH region for krPET-4 fractions.
Figure A8: Changes in C=O region for krPET-4 fractions.
Figure A9: Tensile Stress versus Strain for krPET-1 fractions
Figure A10: Tensile Stress versus Strain for krPET-2 fractions
Figure A11: Tensile Stress versus Strain for krPET-3 fractions
Figure A12: Tensile Stress versus Strain for krPET-4 fractions
Figure A13: Tensile Stressersus
v
Strain for 25% rPET fractions
Figure A14: Tensile Stress versus Strain for 50% rPET fractions
Figure A15: Tensile Stress versus Strain for 100% rPET fractions.
Figure A16: DSC Thermogram for krPET 1 25
Figure A17: DSC Thermogram for krPET 1 50
Figure A18: DSC Thermogram for krPET 1 100
Figure A19: DSC Thermogram for krPET 2 25
Figure A20: DSC Thermogram for krPET 2 50
Figure A21: DSC Thermogram for krPET 2 100
Figure A22: DSC Thermogram for krPET 3 25
Figure A23: DSC Thermogram for krPET 3 50
Figure A24: DSC Thermogram for krPET 3 100
Figure A25: DSC Thermogram for krPET 4 25
Figure A26: DSC Thermogram for krPET 4 50
Figure A27: DSC Thermogram for krPET 4 100
Figure A28: DSC Thermogram for Virgin-PET preform
Figure A29: DSC Thermogram for vePET 25
Figure A30: DSC Thermogram for vePET 50
Figure A30: DSC Thermogram for virgin-PET pellets
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