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Post-Consumer Poly(ethylene terephthalate) – Properties,
Problems during Reprocessing, and Modification by
Reactive Extrusion
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH
Aachen University zur Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation
vorgelegt von
Dennis Berg, M.Sc.
aus Adenau, Deutschland
Berichter: Universitätsprofessor Dr. rer. nat. Martin Möller
Universitätsprofessor Dr. rer. nat. Andrij Pich
Tag der mündlichen Prüfung: 17.12.2018
Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek verfügbar.
Für Karen…
„Suche nicht andere, sondern dich selbst zu übertreffen.“
Marcus Tullius Cicero (106 - 43 v. Chr.)
II
Eidesstattliche Erklärung
Hiermit versichere ich, Dennis Berg, dass ich die vorliegende Dissertation
selbständig verfasst und keine anderen als die angegebenen Quellen und Hilfsmittel
benutzt habe.
Ferner erkläre ich, dass ich nicht anderweitig mit oder ohne Erfolg versucht habe,
eine Dissertation einzureichen oder mich einer Doktorprüfung zu unterziehen.
Köln, den 26. Dezember 2018
………………………………………….
Dennis Berg
III
Table of Content:
List of Abbreviations: ................................................................................................. VI
Acknowledgment ...................................................................................................... XII
List of Publications................................................................................................... XIV
Summary ................................................................................................................. XVI
Zusammenfassung .................................................................................................. XIX
Chapter 1: Introduction ................................................................................................ 1
1.1 Introduction and Motivation .......................................................................................... 1
1.2 Content of this Thesis .................................................................................................. 2
1.3 Reference .................................................................................................................... 4
Chapter 2: State-of-the-Art .......................................................................................... 5
2.1 PET-Production............................................................................................................ 6
2.1.1 Catalysts ............................................................................................................... 7
2.1.2 Solid State Polycondensation Process .................................................................. 9
2.1.3 Liquid State Polycondensation Process ................................................................13
2.2 Recycling of Poly(ethylene terephthalate) ...................................................................14
2.2.1 Thermal Recycling ................................................................................................14
2.2.2 Chemical Recycling ..............................................................................................15
2.2.3 Mechanical Recycling ...........................................................................................17
2.3 Importance of the Molecular Weight of PET for its Application ....................................19
2.4 Chain Extenders .........................................................................................................20
2.4.1 1,3-Phenylene-bis-oxazoline (1,3-PBO) ................................................................21
2.4.2 N,N’-Carbonylbiscaprolactam (CBC) ....................................................................23
2.5 References .................................................................................................................24
Chapter 3: Reasons for the Discoloration of Post-Consumer Poly(ethylene
terephthalate) during Reprocessing ..................................................... 33
3.1 Introduction .................................................................................................................34
3.2 Experimental Section ..................................................................................................36
3.2.1 Materials ...............................................................................................................36
3.2.2 Heating of Antimony Oxide in Ethylene Glycol ......................................................37
3.2.3 Sample Preparation ..............................................................................................37
3.2.4 Characterization of PET Materials ........................................................................37
3.3. Results and Discussion ..............................................................................................40
IV
3.3.1 On the Graying of Poly(ethylene terephthalate) during Reprocessing ...................40
3.3.2 Yellowing of Poly(ethylene terephthalate) during Reprocessing ............................46
3.4 Conclusions ................................................................................................................59
3.5 References .................................................................................................................60
3.6 Supporting Information ................................................................................................63
3.6.1 On the Graying of Poly(ethylene terephthalate) during Reprocessing ...................63
3.6.2 Detection of Polyamides in Post-Consumer Poly(ethylene terephthalate) .............63
3.6.3 MALDI-ToF-MS Analysis of Poly(ethylene terephthalate) Oligomers ....................68
Chapter 4: Zinc Peroxide Particles as Bleaching Agents to Improve the Color of Post-
Consumer Poly(ethylene terephthalate) .................................................. 69
4.1 Introduction .................................................................................................................70
4.2 Experimental Section ..................................................................................................73
4.2.1 Materials ...............................................................................................................73
4.2.2 Sample Preparation ..............................................................................................73
4.2.3 Grinding of Commercial Zinc Peroxide .................................................................73
4.2.4 Characterization of Zinc Peroxide Particles ..........................................................74
4.2.5 Extrusion ..............................................................................................................74
4.2.6 Characterization of PET Materials ........................................................................75
4.3 Results and Discussion ...............................................................................................78
4.3.1 Characterization of the Zinc Peroxide Particles ....................................................78
4.3.2 Bleaching of Post-Consumer Poly(ethylene terephthalate) with Zinc Peroxide in the
Extrusion Process .........................................................................................................81
4.4 Conclusions ................................................................................................................99
4.5 References ............................................................................................................... 100
Chapter 5: Impact of the Chain Extension of Poly(ethylene terephthalate) with
1,3-Phenylene-bis-oxazoline and N,N’-Carbonylbiscaprolactam by
Reactive Extrusion on its Properties .................................................. 103
5.1 Introduction ............................................................................................................... 104
5.2 Experimental Section ................................................................................................ 107
5.2.1 Materials ............................................................................................................. 107
5.2.2 Extrusion ............................................................................................................ 108
5.2.3 Viscosimetry ....................................................................................................... 108
5.2.4 Size Exclusion Chromatography ......................................................................... 109
5.2.5 Rheology ............................................................................................................ 109
5.2.6 Differential Scanning Calorimetry ....................................................................... 109
5.2.7 Carboxyl End Group Titration ............................................................................. 110
V
5.3 Results and Discussion ............................................................................................. 111
5.3.1 Extrusion Curve .................................................................................................. 111
5.3.2 Inherent Viscosity ............................................................................................... 112
5.3.3 Size Exclusion Chromatography (SEC) .............................................................. 114
5.3.4 Rheology ............................................................................................................ 116
5.3.5 Differential Scanning Calorimetry (DSC) ............................................................. 120
5.3.6 Carboxyl End Group Titration ............................................................................. 126
5.3.7 Combination of 1,3-PBO and CBC ..................................................................... 127
5.4 Conclusions .............................................................................................................. 128
5.5 References ............................................................................................................... 129
Chapter 6: Development of New Masterbatches Containing Chain Extenders for
Poly(ethylene terephthalate) .............................................................. 133
6.1 Introduction ............................................................................................................... 134
6.2 Experimental Section ................................................................................................ 136
6.2.1 Materials ............................................................................................................. 136
6.2.2 Synthesis of the Chain Extender Masterbatches ................................................ 136
6.2.3 Extrusion ............................................................................................................ 136
6.2.4 Analytics ............................................................................................................. 137
6.2.5 Pilot Plant Tests ................................................................................................. 139
6.3 Results and Discussion ............................................................................................. 139
6.3.1 Synthesis and Characterization of Masterbatches Containing Chain Extenders . 139
6.3.2 Compounding of PET with Chain Extender Masterbatches ................................. 152
6.3.3 Pilot Plant Spinning of Post-consumer PET with Added Chain Extender
Masterbatches ............................................................................................................. 163
6.3.4 Pilot Plant Spinning of Post-consumer PET with Added Chain Extender
Masterbatches and Zinc Peroxide ............................................................................... 166
6.4 Conclusions .............................................................................................................. 168
6.5 References ............................................................................................................... 169
6.6 Supporting Information .............................................................................................. 171
6.6.1 NMR Spectra of the 1,3-PBO Masterbatch ......................................................... 171
VI
List of Abbreviations:
1,3-PBO 1,3-Phenylene-bis-oxazoline
3D three-dimensional
ABS Acrylonitrile-butadiene-styrene copolymer
ASTM American Society for Testing and Materials
ATR Attenuated total reflection
BHET Bis(2-hydroxyethyl) terephthalate
BHT 2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene)
c Molar concentration
cat. Catalyst
CBC N,N’-carbonylbiscaprolactam
cf. confer
CIE Commission Internationale de l'Éclairage
cPBT Cyclic poly(butylene terephthalate)
D Germany
Ð Molecular weight distribution
Dist. distilled
DSC Differential scanning calorimetry
DT 1,8,9-Anthracenetriol (Dithranol)
e.g. Exempli gratia
EBA Ethylene butyl acrylate
EDX Energy-dispersive X-ray
EG Ethylene glycol
VII
ELS Evaporative light scattering
endo endothermic
ESCA Electron spectroscopy for chemical analysis
etc. et cetera
EVA Ethylene vinyl acetate
exo exothermic
F Force
FESEM Field-emission scanning electron microscopy
FT-IR Fourier transform infrared spectroscopy
G´ Storage modulus
G´´ Loss modulus
HDPE High-density polyethylene
HFIP 1,1,1,3,3,3-Hexafluoropropane-2-ol
ICP-MS Inductively coupled plasma mass spectrometry
ip in plane
IR Infrared
IV = [η] Intrinsic viscosity
L Lamellar thickness
LDPE Low-density polyethylene
LLDPE Linear low-density polyethylene
LSP Liquid state polycondensation
m Mass
M.Sc. Master of Science
VIII
Mn¯¯ Number average molar mass
Mw¯¯ Weight average molar mass
MALDI-ToF-MS Matrix-assisted laser desorption/ionization coupled with a time of
flight detector and a mass spectrometer
MB Masterbatch
MCT Mercury-Cadmium-Telluride
n.i. not identified
n/a not available
NL Netherlands
NMR Nuclear magnetic resonance
oop out of plane
PA Polyamide
PA 6 Polyamide 6 (Nylon 6)
PA MXD 6 Poly(m-xylene adipamide) (Nylon MXD 6)
PBT Poly(butylene terephthalate)
PC Polycarbonate
PCL Poly-ε-caprolactone
PE Polyethylene
PEF Poly(ethylene furanoate)
PEN Poly(ethylene 2,6-naphthalate)
PET Poly(ethylene terephthalate)
PET-G Poly(ethylene terephthalate) glycol-modified
PIE pulsed ion extraction
IX
PLA Poly(lactic acid)
PMDA Pyromellitic dianhydride
PP Polypropylene
PS Polystyrene
PVC Poly(vinyl chloride)
rer. nat. rerum naturalium
RI refractive index
r-PET reprocessed poly(ethylene terephthalate)
SEC Size exclusion chromatography
SEM Scanning electron microscopy
SSP Solid state polycondensation
SwSP Swollen state polycondensation
t Time
t Titer (volumetric analysis)
Tc Crystallization temperature
TEAC Tetraethylammonium chloride
TEM Transmission electron microscopy
TFA Sodium trifluoroacetate
Tg Glass transition point
TGA Thermogravimetric analysis
Tm Melting temperature
TR Room temperature
UK United Kingdom
X
USA United States of America
V Volume
v-PET Virgin poly(ethylene terephthalate)
w Mass fraction
w-PET Waste poly(ethylene terephthalate)
XPS X-ray photoelectron spectroscopy
β Mass concentration
δ Chemical shift (NMR)
δ Deformation vibration (IR)
ΔHf Melting enthalpy
ε Elongation
η Viscosity
η* Complex viscosity
ηinh. Inherent viscosity
ηred Reduced viscosity
ηrel Relative viscosity
ϑ Temperature
�̅� Wavenumber
ν Stretching vibration
νas antisymmetric stretching vibration
νs symmetric stretching vibration
σ Surface free energy (DSC)
σ Tensile strength (tensile test)
XI
χc Crystallinity
ω Angular frequency
XII
Acknowledgments
This thesis was mainly carried out at DWI – Leibniz Institute for Interactive
Materials e. V. and at the Institute of Technical and Macromolecular Chemistry
(ITMC), RWTH Aachen University. I would like to thank some people who guided me
and contributed to this PhD work. Without their support in any way, this work would
not have been possible.
First of all, I would like to emphasize my gratitude to Prof. Dr. rer. nat. Martin Möller,
my supervisor, for the interesting subject of my PhD Thesis, the opportunity to work
in the institute and for the great scientific supervision.
Furthermore, I thank Dr. rer. nat. Karola Schäfer for the great scientific supervision of
my work. Discussions with her and her comments and suggestions were very helpful
for this work. Thank you very much!
I would like to thank also my second supervisor Prof. Dr. rer. nat. Andrij Pich. Thank
you, for being a part in your working group and the nice discussions in the group
meetings. Furthermore, I am very grateful to Prof. Dr. rer. nat. Markus Albrecht for
being co-reporter of my thesis. Thanks also to Prof. rer. nat. Bernhard Blümich for
being the chairman of my defense.
Moreover, many people in DWI helped me a lot with regard to special analysis.
Therefore, special thanks to Dr. rer. nat. Karola Schäfer for performing the
fluorescence spectroscopy, Dr. rer. nat. Robert Kaufmann for performing the XPS
measurements, Dr. rer. nat. Walter Tillmann for the IR spectroscopy, Dr. rer. nat.
Andrea Körner for the MALDI-ToF analysis, Rainer Haas for plenty GPC
measurements, Birgit Mohr for the ICP/MS analysis, Christian Bergs for performing
the TEM analysis, and Sabrina Mallmann for the FESEM and EDX experiments.
I am also thankful to the motivated project partners of Advansa GmbH (Hamm-
Uentrop/D) (especially Katharina Kowol and Dr. rer. nat. Michael Witschas) and
Umweltdienste Kedenburg GmbH (Beckum/D) within the ResPoSe project for the
efficient discussions.
XIII
Warm thanks are dedicated to the laboratory technicians Marion Arndt-Schaffrath,
Jennifer Hildebrandt, Alexandra Kopp, and Ramona Kloss for their practical support
and the very nice working atmosphere.
I had also a very nice working atmosphere with many colleagues in- and outside of
the institute. I have not found new colleagues there, I found new friends. Thank you
very much to Michael Swaton-Höckels, Alexander Töpel, Sibel Ciftci, Alejandro
Benitez, Alexander Eckert, Annabel Mikosch, Christian Bergs, Daniel Hönders,
Dennis Go, Gent Kapiti, Jason Zografou, Marina Richter, Sjören Schweizerhof,
Stefan Mommer, Tatjana Repenko, Thomas Tigges, Volkan Yavuz, Thorsten Palmer,
Karla Georgi, Dominik Schmitz, Christian Willems, and Thomas Zosel.
To my parents and my brother, I am very grateful for their infinite support starting
from my childhood on. You all gave never up to encourage me to achieve this goal.
Finally, I am the happiest man alive that I found you, Karen. I thank you for
supporting me, lending me always your ears and loving me limitless. I love you!
XIV
List of Publications
Parts of this work are published in:
Articles in Scientific Journals:
Dennis Berg, Karola Schaefer, Andrea Koerner, Walter Tillmann, Martin Moeller,
Michael Witschas; Barriereschichten in Kunststoffverpackungen und
Auswirkungen auf das Recycling; Chemie Ingenieur Technik, 2014, 86, 1613.
Dennis Berg, Karola Schaefer, Andrea Koerner, Robert Kaufmann, Walter Tillmann,
Martin Moeller; Reasons for the Discoloration of Postconsumer Poly(ethylene
terephthalate) during Reprocessing; Macromolecular Materials and Engineering,
2016, 301, 1454.
Dennis Berg, Karola Schaefer, Martin Moeller; Development of New Masterbatches
Containing Chain Extenders for Poly(ethylene terephthalate); Macromolecular
Symposia, 2017, 375, 1600180.
Dennis Berg, Karola Schaefer, Martin Moeller; Impact of the Chain Extension of
Poly(ethylene terephthalate) with 1,3-Phenylene-bis-oxazoline and
N,N’-Carbonylbiscaprolactam by Reactive Extrusion on its Properties; Polymer
Engineering & Science; 2018; Online version.
Articles in Conference Proceedings:
Dennis Berg, Christian T. Bergs, Karola Schaefer, Andrij Pich, Martin Moeller;
Polyester fibres from secondary raw materials – Problems and quality
improvement; Proceedings 8th Aachen-Dresden International Textile Conference,
2014, Dresden/Germany, ed. Doerfel A, ITM/TU Dresden, P62.
Dennis Berg, Karola Schaefer, Martin Moeller; Application of Chain Extenders in
the Production of Polyester Fibres from Post‐Consumer Poly(ethylene
XV
terephthalate); Proceedings 9th Aachen-Dresden International Textile Conference,
2015, Aachen/Germany, ed. Hillmer J, DWI Aachen, P3.
Dennis Berg, Karola Schaefer, Martin Moeller; Development of a new Masterbatch
Containing Chain Extenders for Poly(ethylene terephthalate); Proceedings 10th
Aachen-Dresden International Textile Conference, 2016, Dresden/Germany, ed.
Doerfel A, ITM/TU Dresden, P84.
Poster Presentations:
Dennis Berg, Karola Schaefer, Andrea Koerner, Walter Tillmann, Martin Moeller,
Michael Witschas; Barrier Layers in Plastic Packages and their Impact on the
Recycling; ProcessNet-Jahrestagung und 31. DECHEMA-Jahrestagung der
Biotechnologen, 2014, Aachen/Germany.
Dennis Berg, Christian T. Bergs, Karola Schaefer, Andrij Pich, Martin Moeller;
Polyester Fibres from Secondary Raw Materials – Problems and Quality
Improvement; 8th Aachen-Dresden International Textile Conference, 2014,
Dresden/Germany.
Dennis Berg, Karola Schaefer, Martin Moeller; Application of Chain Extenders in
the Production of Polyester Fibres from Post‐Consumer Poly(ethylene
terephthalate); 9th Aachen-Dresden International Textile Conference, 2015,
Aachen/Germany.
Dennis Berg, Karola Schaefer, Martin Moeller; Development of a new Masterbatch
Containing Chain Extenders for Poly(ethylene terephthalate); 11th International
Symposium Polycondensation, 2016, Moscow and St. Petersburg/Russia.
Dennis Berg, Karola Schaefer, Martin Moeller; Development of a new Masterbatch
Containing Chain Extenders for Poly(ethylene terephthalate); 10th Aachen-
Dresden International Textile Conference, 2016, Dresden/Germany.
XVI
Summary
In this thesis, new methods to improve the properties of discolored post-consumer
poly(ethylene terephthalate) during recycling are presented. Especially for the fiber
production, the use of 100 % post-consumer poly(ethylene terephthalate) is
challenging because of its discoloration and numerous further problems (e.g. low
inherent viscosity). In the fiber industry, white polyesters are indispensable for
various products, which leads to the application of mainly virgin poly(ethylene
terephthalate). Here, new approaches were used, to improve the color of post-
consumer poly(ethylene terephthalate) and to increase its molecular weight,
afterwards. Furthermore, innovative masterbatches containing chain extenders based
on a polyester matrix were synthesized. Finally, application experiments were
performed in reactive extrusion tests in small lab scale extruders and, furthermore, on
a pilot plant spinning device.
Firstly, the cause of the discoloration of post-consumer poly(ethylene terephthalate)
during reprocessing was investigated. The focus was put on the gray and yellow
discoloration of post-consumer poly(ethylene terephthalate) during further
processing. The analysis of post-consumer PET materials with the help of various
analytical methods (e.g. inductively coupled plasma mass spectrometry, X-ray
photoelectron spectroscopy, and MALDI-ToF-MS analysis) revealed that, on the one
hand, the antimony (Sb) content which originates from catalyst residues used in PET
synthesis has high influence on the gray discoloration obtained during reprocessing
of PET due to reduction of Sb3+ to elementary, black-colored Sb0 during heating at
high temperatures, which are required for the melting of PET.
The yellow discoloration of post-consumer PET during repeated heating is partially
due to thermo-oxidative degradation of the polymer in the presence of oxygen due to
the formation of quinones and stilbene quinones. Furthermore, polyamide
contaminants, such as Nylon MXD 6, which is used in barrier layers in PET
packaging materials, were found to cause yellow discoloration of PET. Nylon MXD 6
was found as barrier layer in fruit juice bottles via electron microscopy, MALDI-ToF-
MS, SEC, and IR spectroscopy. After thermal treatment, a yellow discoloration was
observed.
XVII
Thereupon, it was focused on the color improvement of discolored post-consumer
poly(ethylene terephthalate) by bleaching. The bleaching of gray post-consumer
poly(ethylene terephthalate) was performed during reactive extrusion by addition of
microscopic zinc peroxide particles to oxidize the metallic antimony in poly(ethylene
terephthalate). These microscopic zinc peroxide particles were produced by grinding
of commercially available ZnO2 resulting in particles with diameters in the range of
60.7 nm. The bleaching of discolored poly(ethylene terephthalate) materials was
successfully achieved by the addition of small amounts of zinc peroxide (0.1 -
0.2 wt%) to the melt due to its oxygen release.
To compensate an oxidative degradation of poly(ethylene terephthalate) due to the
bleaching with ZnO2, chain extenders were applied during reactive extrusion.
Therefore, the impact of chain extended poly(ethylene terephthalate) with
1,3-phenylene-bis-oxazoline (1,3-PBO) and N,N’-carbonylbiscaprolactam (CBC) on
its properties was studied intensively. An improvement of the properties of
poly(ethylene terephthalate) can be obtained by the addition of small amounts of
these chain extenders. Concentrations up to 0.3 wt % of the chain extenders lead to
quality improvement of poly(ethylene terephthalate). The chain extenders are linked
linearly to the carboxyl and/or hydroxyl terminal groups of poly(ethylene
terephthalate) and its molar mass can be increased without any significant negative
effect on the properties such as crystallinity or rheology of the polymer. At higher
chain extender concentrations, a stronger increase of the storage moduli (G`), loss
moduli (G``), and complex viscosities (η*) was determined, which leads to problems
during fiber manufacturing. Especially, the non-Newtonian behavior increases with an
increasing chain extender dosage. A pronounced shear sensitivity, quantified by the
more pronounced non-Newtonian behavior, is observed at high chain extender
dosages incipient from 0.5 wt% 1,3-PBO and CBC. Also, the crystallinity, especially
the secondary crystallization, is disturbed after addition of higher concentrations of
chain extenders. At higher concentrations of chain extenders (starting from 0.3 wt%
1,3-PBO and 0.5 wt% CBC), the crystallinity and the lamellar thickness distribution of
PET decrease. The secondary crystallization is highly affected after the addition of
chain extenders to the melt. To conclude, concentrations up to 0.3 wt% of the studied
chain extenders proved to be the optimum for spinning of polyester filaments at high
velocities.
XVIII
Additives are usually applied from masterbatches to polymer melts during extrusion
or spinning. Here, novel masterbatches containing 1,3-phenylene-bis-oxazoline or
N,N’-carbonylbiscaprolactam in a non-reactive matrix were synthesized. As non-
reactive matrix a cyclic poly(butylene terephthalate) oligomer which has no reactive
end groups and which can be polymerized in the presence of catalysts such as
butylchlorodihydroxystannane was selected. During reactive extrusion of the chain
extenders with poly(ethylene terephthalate), the matrix cyclic poly(butylene
terephthalate) oligomer polymerizes to high molecular poly(butylene terephthalate)
which is also a polyester with almost the same properties like poly(ethylene
terephthalate). Compared to other non-reactive masterbatch matrices such as
polypropylene, this matrix does not act as a foreign polymer in poly(ethylene
terephthalate). The spinnability of post-consumer poly(ethylene terephthalate) with
the addition of the developed masterbatches was proven by spinning trials on a pilot
plant. Chain extender concentrations up to 0.27 wt% were added and good
spinnability was obtained. The addition of chain extender masterbatches led to an
increase of the mechanical properties of polyester fibers spun from post-consumer
PET. Finally, these spinning tests were also performed with post-consumer
poly(ethylene terephthalate) with addition of a combination of zinc peroxide and the
chain extender masterbatches. It was successfully shown that fibers can be spun and
drawn out from post-consumer poly(ethylene terephthalate) at very high velocities up
to 5000 m ∙ min-1.
In conclusion, discolored post-consumer poly(ethylene terephthalate) can be
bleached and repaired during recycling, which can be an important contribution to
protect the environment, and to save resources and energy.
XIX
Zusammenfassung
Die vorliegende Arbeit befasst sich mit der Entwicklung neuer Methoden, die die
Qualität von verfärbtem Post-Consumer-Polyethylenterephthalat (PET) während des
Recyclingprozesses verbessern. Besonders in der Faserherstellung ist es schwer
möglich, 100% Post-Consumer-PET einzusetzen, da Verfärbungen und viele weitere
Probleme (zum Beispiel zu niedrige inhärente Viskosität) während des Recyclings
auftreten können. Gerade für die Herstellung von Fasern ist es unabdingbar, weiße
bis farblose Polyester zu verwenden, so dass hauptsächlich virgin PET eingesetzt
wird. Hier wurden Versuche zur Aufhellung von verfärbten Post-Consumer PET-
Rezyklaten sowie eine anschließende Molekulargewichtserhöhung durchgeführt.
Darüber hinaus befasst sich die vorliegende Dissertation mit der Herstellung von
neuen innovativen polyesterbasierten Masterbatches, welche aktive
Kettenverlängerer enthalten. Schließlich wurden reaktive Extrusionsversuche sowohl
an kleinen Laborspinnanlagen als auch in größeren Technikumsanlagen
durchgeführt.
Zuerst wurden die Ursachen der Verfärbung von Post-Consumer PET während der
Wiederaufbereitung näher untersucht. Hauptsächlich wurde der Fokus auf eine graue
und gelbe Verfärbung gelegt. Die Analytik des Post-Consumer-PETs (z.B. mittels
Massenspektroskopie mit induktiv gekoppeltem Plasma (ICP-MS),
Röntgenphotoelektronenspektroskopie (XPS) und MALDI-ToF-MS) zeigte auf, dass
metallisches Antimon (Sb), welches aus der Katalyse der PET-Synthese stammt,
einen großen Einfluss auf die Vergrauung von PET hat. Durch das thermische
Wiederaufbereiten von PET (Schmelzen) wird der Katalysator, welches
hauptsächlich dreiwertige Antimonverbindungen (Sb3+) sind, zu elementarem,
schwarzgefärbtem Antimon (Sb0) reduziert.
Zum anderen wurde eine gelbliche Verfärbung bei Post-Consumer-PET
nachgewiesen, die zum Teil auf thermooxidative Spaltprozesse zurückzuführen ist.
Durch wiederholte thermische Behandlung in Anwesenheit von Sauerstoff, entstehen
Chinone und Stilbenchinone, die gelblich gefärbt sind. Weiterhin sind
Polyamidkontaminanten wie Nylon MXD 6, welches als Barriereschicht in PET-
Verpackungsmaterialien eingesetzt wird, als Grund für eine gelbe Verfärbung
aufgezeigt worden. Nylon MXD 6 wurde als Barriereschicht in Fruchtsaftflaschen via
XX
Elektronenmikroskopie, MALDI-ToF-MS, Gelpermeationschromatographie und
Infrarotspektroskopie nachgewiesen. Nach thermischer Behandlung von
Nylon MXD 6 und PET-Verpackungsmaterialien, die Polyamid-basierte
Barriereschichten enthielten, wurde eine Vergilbung beobachtet, die auf das
Polyamid zurückzuführen ist.
Ziel des nächsten Arbeitsschrittes war, eine Farbverbesserung von verfärbten PET
durch Bleiche zu erzielen. Die Bleiche von vergrautem PET wurde in der Schmelze
mittels Zinkperoxidmikropartikeln durchgeführt. Die Zinkperoxidmikropartikel wurden
durch Mahlen aus kommerziell erhältlichem ZnO2 hergestellt. Durch Kryomahlen
konnten Partikel im Durchmesserbereich von 60,7 nm schonend hergestellt werden.
Durch Sauerstofffreisetzung aus den Zinkperoxidpartikeln bei höheren Temperaturen
wurde das metallische Antimon in der PET-Schmelze oxidiert. Die Bleiche der
verfärbten PET-Materialien wurde erfolgreich durch Zuführung von kleinen Mengen
an Zinkperoxid (0,1 - 0,2 Gew.-%) während der Reaktivextrusion durchgeführt.
Zur Kompensation eines oxidativen PET-Abbaus, der durch den Bleichprozess mit
Zinkperoxid entstanden ist, wurden Kettenverlängerer während der reaktiven
Extrusion eingesetzt. Aus diesem Grunde wurde zuerst der Einfluss zweier
Kettenverlängerer auf die Eigenschaften von Polyethylenterephthalat untersucht. Als
Kettenverlängerer wurden 1,3-Phenylenbisoxazolin (1,3-PBO) und
N,N´-Carbonylbiscaprolactam (CBC) ausgewählt. Durch Zugabe von geringen
Mengen der Kettenverlängerer konnte eine Verbesserung der Eigenschaften von
PET erreicht werden. Kettenverlängererkonzentrationen von maximal 0,3 Gew.-%
führen zu einer Qualitätsverbesserung des PETs. Die Kettenverlängerer reagieren
mit den terminalen Carboxyl- und/oder Hydroxylgruppen zu linearen Produkten, ohne
signifikant negative Effekte bei der Kristallinität oder den rheologischen
Eigenschaften hervorzurufen. Erst bei Einsatz höherer
Kettenverlängererkonzentrationen ist ein starker Anstieg der Speicher- und
Verlustmoduli sowie der komplexen Viskosität von PET-Schmelzen festzustellen,
welches zu erhöhten Problemen während der Faserherstellung führen kann. Dies
äußert sich vor allem in einem Anstieg des nichtnewtonschen Verhaltens bei
erhöhten Kettenverlängererdosierungen. Diese ausgeprägte Schersensitivität wurde
bei Zusatz beginnend von 0,5 Gew.-% 1,3-PBO und CBC beobachtet. Zudem wurde
die Kristallinität des PETs, nach Zugabe von hohen Konzentrationen an
XXI
Kettenverlängerer, deutlich gestört. Nach Zusatz von
Kettenverlängererkonzentrationen, beginnend bei 0,3 Gew.-% 1,3-PBO und
0,5 Gew.-% CBC, sinkt der Kristallinitätsanteil und die Lamellendickeverteilung in
PET. Ein großer Effekt war bei der Sekundärkristallisation nach
Kettenverlängererzugabe zu beobachten. Zusammenfassend lässt sich
schlussfolgern, dass geringe Konzentrationen der untersuchten Kettenverlängerer
bis 0,3 Gew.-% geeignet sind, um Polyester bei sehr hohen Geschwindigkeiten zu
verspinnen.
Da Additive in der Technik gewöhnlich in Form von Masterbatches der
Polymerschmelze während des Extrusions- oder Spinnprozesses zugeführt werden,
wurden neuartige Masterbatches in einer nicht-reaktiven Polyestermatrix entwickelt,
die 1,3-Phenylenbisoxazolin (1,3-PBO) und N,N´-Carbonylbiscaprolactam (CBC)
enthalten. Zyklische Polybutylenterephthalat-Oligomere, welche somit keine
reaktiven Endgruppen enthalten, wurden als nicht-reaktive Masterbatchmatrix
verwendet. Diese können wiederum in Anwesenheit von Katalysatoren wie
Butylchlordihydroxystannan zu hochmolekularem Polybutylenterephthalat (PBT)
während der Reaktivextrusion polymerisieren, welches ähnlich gute Eigenschaften
wie PET hat. Im Vergleich zu anderen nicht-reaktiven Matrices, wie zum Beispiel
Polypropylen (PP), wirkt PBT somit nicht als Fremdpolymer in PET. Weiterhin wurde
die Spinnbarkeit von Post-Consumer-PET durch Zugabe der neu entwickelten
Masterbatches in einer Technikumsanlage untersucht. Durch Zusatz von
Kettenverlängererkonzentrationen bis zu 0,27 Gew.-% konnte noch eine gute
Spinnbarkeit von Post-Consumer-PET erzielt werden. Nach Zugabe der
Kettenverlängerer-Masterbatches wurde eine Verbesserung der mechanischen
Eigenschaften des Post-Consumer-PETs festgestellt. Letztlich wurden
Technikumsspinnversuche mit Post-Consumer-PET unter Zugabe von Zinkperoxid in
Kombination mit den Kettenverlängerer-Masterbatches unternommen. Diese
Versuche zeigten erfolgreich, dass Post-Consumer-Polyesterfasern bei sehr hohen
Geschwindigkeiten von 5000 m ∙ min-1 gesponnen und verstreckt werden konnten.
Daraus folgt letztlich, dass verfärbtes Post-Consumer-Polyethylenterephthalat auf
diese Weise aufgehellt und repariert werden kann, welches somit ein wichtiger
Beitrag zur Nachhaltigkeit und zum Umweltschutz ist und sich Ressourcen und
Energie einsparen lassen können.
Chapter 1
1
Introduction
1.1 Introduction and Motivation
In these times, where terms such as sustainability, secondary raw materials, and
recovery are on everybody’s tongue, recycling is more important than ever.
Nowadays, the scarcity of resources and the resources consumption increase more
and more, and the recycling of materials is essential to protect the environment.
Recycling is the recovery of waste materials to the original purpose or other purposes
in any manner. Materials like glass, paperboards, organic materials, metals, or
plastics are commonly subjected to recycling. Especially, the re-usage of plastics
which originate from crude oils is gaining more and more in significance. Many waste
plastic types such as high-density and low-density polyethylene (HDPE and LDPE),
polypropylene (PP), polystyrene (PS), or poly(ethylene terephthalate) (PET) are
recycled today to a large extent, because highly homogeneous separation fractions
can be obtained.
Particularly, poly(ethylene terephthalate) can be won in a very high yield and purity,
because of the deposit system in Germany, since 2003. Thus, the amount of recycled
PET has been rising from year to year as the recollection of post-consumer
poly(ethylene terephthalate) bottles leads to an increased sorting accuracy, too.
Since many years, poly(ethylene terephthalate) (PET) has been one of the most
outstanding polyesters in the world. It is considered as the most important
engineering polymer and it is used in a wide range of products in packaging, fiber,
food, or automotive industry. According to Nicastro and Koehlmann, it is estimated
that the demand for PET is about 22,726 kt worldwide and 3,432 kt in Europe in the
year 2017 [1]. Hence, these high requirement figures lead to a very high interest to
increase the resource efficiency of poly(ethylene terephthalate). Due to its excellent
mechanical, thermal, optical, and chemical properties, the interest in the usage and,
consequently, the re-usage of PET increases more and more. The recycled polymer
should have the same material properties compared to virgin PET, which is often
challenging in the recycling industry. The color of post-consumer PET plays often an
important role after recycling, because a discoloration of recycled PET may occur.
While virgin PET is a white or transparent/colorless material, the recycled material
has often a yellowish or grayish discoloration after reprocessing. This problem leads
Chapter 1
2
to limited application of recycled materials, especially in the fiber industry, where the
color is essential. The clarification of the cause of the discoloration is, therefore,
inevitable. In the next step, this problem has to be solved in this work by increasing
the lightness by preservation of the other excellent properties of PET. This would
lead to resource efficiency during the production of polyester fibers.
In the following chapters, investigations on the elucidation of the reasons for the
discoloration of reprocessed poly(ethylene terephthalate) (PET) are presented.
Furthermore, a novel method to increase the lightness of grayed reprocessed PET
during reactive extrusion has been developed. Moreover, the application of
1,3-phenylene-bis-oxazoline, N,N’-carbonylbiscaprolactam, and combinations thereof
are applied to prevent thermo-oxidative degradation of the polymer chains. The
influence of the chain extension on the molecular structure and the rheological and
thermal properties is analyzed which has not been done so far in this extent. Finally,
innovative masterbatches were synthesized with a non-reactive polyester matrix for
further applications.
1.2 Content of this Thesis
This thesis deals with the improvement of the quality of post-consumer poly(ethylene
terephthalate) (PET). New methods and approaches are applied to improve the
properties of post-consumer poly(ethylene terephthalate). It is focused mainly on the
manufacture of fibers from post-consumer PET and the improvement of their color.
Further studies aim at improving other characteristics of post-consumer PET like its
thermal, rheological, and mechanical properties with additives in novel
masterbatches.
In Chapter 2, the state-of-the-art of poly(ethylene terephthalate) is described. This
chapter deals with the synthesis of PET from the beginnings on up to today. Industrial
production ways are described and a literature review of the PET production, used
catalysts, molecular weight increase, and their influence on the properties of PET is
given. Furthermore, the state-of-the-art of the currently used recycling methods with
their advantages and disadvantages is presented. The focus is put particularly on the
color and other properties of the recycled polymer materials. Applications of PET with
different molecular weights, respectively viscosities are discussed, too. Moreover, the
Chapter 1
3
literature on the application and effectiveness of different chain extenders and their
influence on the polymer are described.
In Chapter 3, the causes of the discoloration of post-consumer poly(ethylene
terephthalate) during reprocessing are analyzed. On the one hand, the gray
discoloration of reprocessed PET and, on the other hand, the yellow discoloration
during repeated thermal treatment is investigated. Foreign contaminants, which may
be responsible for the discoloration of post-consumer PET, are analyzed with various
methods, such as inductively coupled plasma mass spectrometry, X-ray
photoelectron spectroscopy, infrared spectroscopy, size exclusion chromatography,
and matrix-assisted laser desorption/ionization coupled with a time of fly detector and
a mass spectrometer (MALDI-ToF-MS analysis).
In the next chapter (Chapter 4), studies on the bleaching of discolored PET recyclate
are performed. As bleaching agent, inorganic peroxides are chosen because of their
easy and save handling. Microscopic zinc peroxide (ZnO2) particles are produced by
grinding of commercially available ZnO2 (top-down procedure), and added to
discolored PET during reactive extrusion. Zinc peroxide particles in different
concentrations and different particle sizes are used for bleaching. Afterwards, the
bleached polyesters are characterized by thermal and rheological analyses.
Chapter 5 deals with the compensation of the oxidative degradation of poly(ethylene
terephthalate) during the bleaching experiments. For this, chain extenders were
applied in reactive extrusion processes. First of all, the impact of PET which was
chain extended with 1,3-phenylene-bis-oxazoline and N,N’-carbonylbiscaprolactam
on its properties was studied intensively. Characterization methods like plate-plate
rheology, solution viscosimetry, differential scanning calorimetry, size exclusion
chromatography, and carboxyl end group determination were used to receive
information for the best quality improvement of chain extended PET for spinning
experiments at high velocities.
Chapter 6 demonstrates, on the one hand, the synthesis of chain extender containing
masterbatches and, furthermore, spinning experiments on a pilot plant with high
velocities. The transferability onto continuous processes is proven on the pilot plant
scale. The synthesis of masterbatches with incorporated chain extenders in a
Chapter 1
4
non-reactive polyester matrix is described, resulting in still active chain extenders.
These active masterbatches are fully characterized and, afterwards, applied to post-
consumer poly(ethylene terephthalate) in a pilot plant. Finally, spinning tests are also
performed by combination of zinc peroxide and chain extender masterbatches.
1.3 Reference
[1] A. Nicastro, D. Koehlmann, Kunststoffe 2016, 10, 78.
Chapter 2
5
State-of-the-Art
In the early 1930s, first studies on the synthesis of aliphatic polyesters by
polycondensation were published by Wallace Hume Carothers [1-6]. Due to their low
melting points, no usage of these products was found. Later in 1941, the first patent
on the invention of poly(ethylene terephthalate) with a high melting point of 265 °C
and good hydrolytic stability was published by Whinfield and Dickson [7]. After World
War II, Imperial Chemical Industries (ICI) and E. I. du Pont de Nemours and
Company (DuPont) invented polycondensation products with terephthalic acid and
ethylene glycol named Terylen and Dacron, respectively. Afterwards, Trevira
(Hoechst) and Diolen (Vereinigte Glanzwerkstoffe) produced also synthetic fibers
from these raw materials [8]. These products are used as textile fibers until today.
Poly(ethylene terephthalate) (PET) (Scheme 2.1) is one of the most important
technical polymers and the most important polyester. PET is mainly used for fibers
and for food packages such as soft drink packaging or fruit, cheese, and sausage
boxes. Because of its superior properties and its typical characteristics, like high
tensile and impact strength, clarity, good processability, high chemical resistance and
high thermal stability, PET is widely used [9]. In addition, this polyester plays also an
important role in automotive industry for the production of tire cord or seat belts, for
example, due to its excellent mechanical properties [10, 11]. Furthermore, PET is very
interesting for recycling application. The international recycling code of PET is 01
(Scheme 2.1).
Scheme 2.1. Structural formula (left) and international recycling code (right) of
poly(ethylene terephthalate)
O
O
O
O
H
OH
n
Chapter 2
6
2.1 PET-Production
Poly(ethylene terephthalate) is usually synthesized by polycondensation of the raw
materials ethylene glycol and terephthalic acid or dimethyl terephthalate (terephthalic
acid dimethyl ester) by generation of water or methanol as byproducts, respectively
(Scheme 2.2).
Scheme 2.2. Structural formulae of the usual raw materials for the manufacturing of
poly(ethylene terephthalate). a) ethylene glycol, b) terephthalic acid, c) dimethyl
terephthalate.
Terephthalic acid originates from the benzene fraction of the petrochemical industry
where, amongst others, p-xylene is obtained. Terephthalic acid is produced by
oxidizing p-xylene with the help of catalysts. By recrystallization of this product,
purified terephthalic acid is obtained. Similar to terephthalic acid, dimethyl
terephthalate is produced. In contrast to the terephthalic acid production, p-xylene is
oxidized in presence of methanol during dimethyl terephthalate manufacture [12].
Ethylene glycol (ethane-1,2-diol) is the second important raw material for the PET
synthesis. Starting from petroleum gas or petroleum (C2 fraction), ethane is
produced, among other raw materials. Due to oxidization in presence of catalysts,
ethylene oxide is obtained, which can be converted to ethylene glycol by adding
Chapter 2
7
water in an exothermic process. Side products such as diethylene glycol or
triethylene glycol can be evaporated by distillation under vacuum.
Further starting materials can be important in PET manufacturing to achieve other
properties. Co-monomers such as isophthalic acid can be added during PET
synthesis to reduce the crystallinity of PET which is commonly preferred for bottle
manufacturing to obtain transparent products. If crystalline parts are present in PET,
the light rays are refracted so that the product is not transparent anymore. Isophthalic
acid disturbs the crystallinity and transparent PET is achieved. Moreover,
cyclohexane dimethanol or diethylene glycol are co-monomers which increase the
amorphous part of PET.
The synthesis of poly(ethylene terephthalate) proceeds in two general steps. The first
step is the esterification. For the esterification, temperatures of 250 – 285 °C are
needed at a pressure of 1 – 6 bar and a reaction time of 0.5 – 6 h [12]. In that step, the
monomer bis(2-hydroxyethyl) terephthalate (BHET) (Scheme 2.3) is the main product
with a small amount of oligomers. In the second step, PET is produced by
transesterification of BHET and the oligomers with the aid of a catalyst at high
temperatures [13]. The byproducts are evaporated at these high temperatures.
Scheme 2.3. Structural formula of bis(2-hydroxyethyl) terephthalate (BHET).
2.1.1 Catalysts
Antimony trioxide (Sb2O3) is one of the most commonly used catalysts for PET
manufacturing [14-16]. First of all, the Sb3+ compounds react with ethylene glycol (EG)
to antimony-III-glycolate at 150 °C under inert gas atmosphere. In the next step,
antimony-III-glycolate is added to the PET reactor. Sb2O3 is a catalyst which is known
to minimize side reactions [17]. The activation energy of the reaction of
bis(2-hydroxyethyl) terephthalate (BHET) with antimony trioxide as catalyst is lower
than that without addition of any catalyst [18]. Duh proposed a mechanism for the
O
OO
O
OH
HO
Chapter 2
8
effect of the antimony catalyst on the solid state polycondensation of PET
(Scheme 2.4) [19]. He described also that the activation energy decreased from
30.7 kcal ∙ mol-1 to 23.3 kcal ∙ mol-1, when 150 ppm antimony trioxide (Sb2O3) was
used as catalyst. Antimony triacetate and antimony glycoxide are other common
antimony catalysts for PET manufacturing [16, 20].
Scheme 2.4. Proposed mechanism of the action of antimony trioxide (Sb2O3) as
catalyst during synthesis of poly(ethylene terephthalate) from ethylene glycol (EG)
and bis(2-hydroxyethyl) terephthalate (BHET) [19].
More than 90 % of PET is manufactured with the aid of antimony catalysts (like
Sb2O3 or Sb[OOC-CH3]3) [12, 20-23]. Other common catalysts or co-catalysts for the
production of PET are titanium compounds (e.g. titanium tetraisopropoxide) and
germanium compounds (e.g. GeO2) [13, 23-25]. However, the use of titanium alkoxides
as catalysts leads to an undesirable yellowish color of PET [13, 26]. Furthermore, the
presence of titanium catalysts accelerates a yellow discoloration of polyesters during
multiple remolding steps as presented in the case of poly(ethylene furanoate)
O
O
O
O
OH
Sb2O3 + 2 EG
O
O
Sb
O
OH + BHET
O
O
Sb
O
- EG
O
O
OH+
O
O
O
O
O
O
OH
O
OSb
OOH
-
Chapter 2
9
(PEF) [27]. Germanium catalyzed polyesters are produced with application of
germanium dioxide (GeO2) which is mainly used in Japan [26].
The whole synthesis process is presented as an overview in the following scheme
(Scheme 2.5).
Scheme 2.5. Possible synthesis of poly(ethylene terephthalate). Ethylene glycol (EG)
and dimethyl terephthalate react to bis(2-hydroxyethyl) terephthalate (BHET) with
methanol evaporation. In addition, the reaction of antimony trioxide (Sb2O3) with EG
occurs in an extra vessel. The transesterification follows by addition of these two
mixtures. This PET prepolymer is treated, afterwards, to increase the molecular
weight (e.g. in the solid state polycondensation [SSP] process) for further processing
(e.g. to granulate or spin fibers).
2.1.2 Solid State Polycondensation Process
Normal polycondensation techniques stop after reaching low or medium molar
masses. After that, an increase of the molar masses has to be performed by means
Chapter 2
10
of additional processes such as the solid state polycondensation process (SSP
process). The SSP process is a discontinuous process. It is a well-established step to
increase the molecular weight after the synthesis of polyesters and polyamides. In
the case of poly(ethylene terephthalate), it leads to high molar masses by
condensation and evaporation of water, ethylene glycol or low weight oligomers as
byproducts. The increase of the molecular weight follows by reaction of the terminal
groups of the polycondensation products (Scheme 2.6 A + B). Two types of the chain
extension reaction are presented below.
Scheme 2.6 A. Transesterification reaction by release of ethylene glycol or low
molecular weight oligomers in a solid state polycondensation process with
poly(ethylene terephthalate).
Scheme 2.6 B. Esterification reaction by release of water in a solid state
polycondensation process with poly(ethylene terephthalate).
In this process, PET is heated to temperatures between the glass transition
temperature (Tg) and the onset of the melting temperature (Tm) [28, 29]. Established is a
O
O
OHO
O
HO+
O
O
O
O
HO
OH-
O
O
HO+OH
O
H2O-
O
O
O
O
Chapter 2
11
temperature range between 200 °C and 240 °C for this condensation reaction [9, 30].
Antimony trioxide which remains in PET after the synthesis is useful in the SSP
process, too, as Kokkalas presented [31]. Furthermore, vacuum or dry inert gas
stream like nitrogen, helium or carbon dioxide is needed to remove the byproducts [12,
32, 33]. Water vapor in the inert gas has negative impact on the molar mass of PET
during the SSP process [34, 35]. In addition, the atmosphere excludes oxygen and
water in the reactor, which has negative impact on PET due to partial oxidation, chain
cleavage or hydrolysis. The gas atmosphere can be set up in a gas flow (opened
system) or stagnant system (closed system). The advantage of an opened system is
the possibility that byproducts can be easily removed. The flow rate of the inert gas
has also an influence on the molar mass of PET as Zhao et al. and Gao et al.
predicted [32, 36]. A higher flow rate of nitrogen results in a higher molar mass of PET.
Furthermore, a combination of an opened and closed system can be performed [11,
28]. The presence of byproducts may cause degradation reactions which is
counterproductive in this process. Hence, the post polycondensation can also be
performed with the aid of a solvent in the swollen state (swollen state
polycondensation SwSP) as Ma and Agarwal published [37]. The advantage of this
process is the improvement of the diffusion rate of the byproducts such as ethylene
glycol, which may cause cleavage of the product [38, 39].
The reaction time of the solid state polycondensation process is about seven hours or
longer. Generally, the SSP process leads to an increase of the molar mass and a
decrease of volatile compounds resulting in an improvement of the quality of
poly(ethylene terephthalate). The advantage of the SSP process is that the process
temperatures are comparatively low, which impedes side reactions and thermal
cleavage processes. It seems that mainly linear polymers are formed with the help of
the SSP process [28, 40]. Furthermore, the products of the SSP process are more
stable in the heat compared to products originating from the melt. A disadvantage is,
however, the slow reaction rate of this process. This means that long reaction times
are necessary to increase the molar mass of PET because of the reduced chain
mobility during the reaction. Moreover, agglomeration of the SSP products can occur
during this step [41]. Reaction conditions are, for example, 235 °C for 7 h, where the
degree of polymerization of 80 of a PET prepolymer can be increased up to a degree
of polymerization of 145 during the SSP process [28]. The starting materials are PET
Chapter 2
12
flakes with a diameter of 1 mm or powders with a diameter of 100 µm. The particle
size of the starting material has an influence on the molecular weight of the solid
state polycondensed polyester (as demonstrated by intrinsic viscosity
measurements) [11, 42]. The smaller the particles are, the higher is the molar mass of
the PET obtained after the SSP process by using the same parameters (inert gas
flow, reaction temperature, reaction time, and the same starting material) [43]. If a
prepolymer with a higher molar mass is used in the SSP process, the molar mass of
the final product will be much higher. For example, a prepolymer with an intrinsic
viscosity of 0.20 dL ∙ g-1 can reach an intrinsic viscosity of about 0.6 dL ∙ g-1 after the
SSP process with a reaction time of 20 h and a reaction temperature of 210 °C.
Contrarily, a prepolymer of 0.35 dL ∙ g-1 can reach an intrinsic viscosity of approx.
1.1 dL ∙ g-1 by using the same parameters [11]. The chain mobility is limited at higher
COOH concentrations of PET due to the fact that a prepolymer with a higher molar
mass has a lower carboxylic (COOH) end group concentration. The hydrogen bonds
of the COOH end groups interact, the chain mobility decreases, the prepolymer fits
easier into a crystal structure, the COOH end groups are hindered and inactivated for
a chain extension reaction during the SSP process [44-46]. The influence of the
crystallinity of polymers in the SSP process is also described by many authors [46].
The crystallinity of the prepolymer plays an important role during the SSP process
due to the effect of the end groups and diffusion of byproducts during this process [46-
48]. The path through which the byproducts diffuse out of the polymer is longer in a
crystalline structure. Furthermore, the chain mobility is based on the amorphous part
of PET, because the temperature is above the glass transition temperature and
below the melting temperature, where the crystalline part of PET remains as a
crystal. To conclude, a low crystalline part of PET leads to a higher chain mobility in
the SSP process. Thus, the higher the crystallinity of the polymer is, the lower is the
reaction rate of the SSP process.
The SSP process is also performed on post-consumer PET during recycling [9, 49, 50].
The most important advantages of the SSP process are the simplicity of the process
and the requirement of inexpensive equipment by avoiding degradation processes.
On the other hand, the slow reaction rate and the high energy consumption are
disadvantages of this process. This process exhibits high complexity due to the
influence of many parameters (e.g. particle size, molar mass, terminal COOH group
Chapter 2
13
content, and crystallinity of the starting material, inert gas flow, and formation of
byproducts).
2.1.3 Liquid State Polycondensation Process
In contrast to the solid state polycondensation process, the liquid state
polycondensation process (LSP) is a process which is run at higher temperatures. In
this process, the increase of the molecular weight is performed in the melt. One big
advantage of this process is that the spinning process can be carried out directly from
the melt without further cost intensive steps [51, 52]. High performance PET fibers can
be produced in that way, for example for rope manufacturing. The LSP process can
be performed continuously. An increase of the intrinsic viscosity of PET from 0.68 to
1.05 dL ∙ g-1 was obtained after the LSP [53]. High temperatures in the range of 270 °C
to 280 °C are necessary, and this process has to be performed in vacuum to remove
the formed byproducts. An increase of the intrinsic viscosity of PET of 0.01 dL ∙ g-1
per minute can be achieved during LSP, whereas, an increase of 0.01 to 0.02 dL ∙ g-1
per hour is normal for the SSP process [54]. These higher chain extension velocities
lead to a cheaper process. However, due to the requirement of higher temperatures
faster chain cleavage can also occur so that a limitation of the increase of the intrinsic
viscosity is given. Chen et al. investigated the rheological and thermal behavior of
different PET samples treated in the SSP and LSP process [55]. Different intrinsic
viscosities and different carboxylic end groups were determined for PET which was
subjected to these processes. Here, the highest intrinsic viscosity was found for PET
for industrial yarn application which underwent the LSP process. However, the
carboxylic end group concentration of PET is also higher after the LSP process than
in case of PET treated according to the SSP process [55]. Chen et al. found that the
different manufacturing techniques influence the properties of PET such as
crystallinity, rheological behavior or degradation properties in a different way.
Fundamentally, the LSP process is a faster and cheaper process than the SSP
process. However due to the higher temperatures, chain cleavages occur more
quickly and the COOH content increases faster, in spite of, the high intrinsic viscosity.
Chapter 2
14
2.2 Recycling of Poly(ethylene terephthalate)
The PET market is increasing very rapidly during the last decades. In 2015, the
market-share of PET rose by eight percent in Europe. It is estimated that the
worldwide market of PET will amount to 22,726 kt, and to 3,432 kt in Europe in
2017 [56]. The interest in recycling of PET is very great because of its high
consumption and its potential for the fabrication of different products. The collection
rate of post-consumer PET increased in the last twenty years strongly. In Europe, the
growth of the collection rate of post-consumer PET bottles is about 5 to 20 % per
year [25]. In 2011, more than 1.59 million tons of PET were collected for recycling
application. Especially in Germany, the interest in recycling of PET is very great
because of a deposit system for one-way containers like PET bottles and tin cans,
established since 2003. Due to this system, the recollection rate is more than 90% in
Germany. The PET waste can be collected in a high purity without great effort.
Furthermore, for the waste which does not fit into this deposit system another system
exists in Germany. The “Grüner Punkt” is the brand of a company which collects
waste like plastic materials, metals, and paperboards and sorts it into different
categories. The plastic materials are sorted by different types with the aid of infrared
spectroscopy. Here, PET can also be obtained, however, due to the higher efforts
this process is more expensive.
2.2.1 Thermal Recycling
The recycling of polymers like PET can be carried out in various procedures. The
easiest way to recycle PET is the thermal recycling to recover energy [57]. The energy
recovery is often the cheapest recycling method when collection and separation of
PET is too difficult and economically not effective or when hazardous contaminants
are included in the waste polymers. Not only waste PET but other plastics, too, can
be combusted without any elaborate separation process. Burning one ton of plastics
can save 250 liters of heating oil [58]. The incineration converts the plastics into
carbon dioxide and water by emitting thermal energy, which can be transformed into
electrical energy with the aid of turbine generators [59]. However, this recycling
process is ecologically the less beneficial recycling method, because no products can
be used after the incineration for further application.
Chapter 2
15
2.2.2 Chemical Recycling
Thus, there are further ways to recycle PET in a more effective way so that PET can
be used again. The first step of PET recycling is the purification to eliminate
contaminants. Waste PET (e.g. bottles) has to be shredded and passed through an
air classifier to remove light impurities like paper. Next, the PET scraps have to be
washed and separated from polyolefines (e.g. polypropylene stemming from the
bottle caps) by heavy media separation (sink and float separation) due to its density
difference, and finally dried. Post-consumer PET can be reprocessed in different
categories. It can be distinguished between chemical and mechanical recycling. The
chemical recycling methods or solvolysis reactions are degradation processes with
the aid of a variety of chemicals like methanol (methanolysis), ethylene glycol
(glycolysis), water (hydrolysis), ammonia (ammonolysis), amines (aminolysis) etc. [59-
62]. These reactions are based on the cleavage of ester bonds resulting in low
molecular weight products which can be used as educts for further reactions.
For example, the solvent for the hydrolysis to cleave PET is water. This process can
be performed in acidic or in alkaline media. Concentrated sulfuric acid, as an
example, can be used for cleavage to win terephthalic acid back after purification in
high yields and a purity of about 99 % [63]. As alkaline media, sodium carbonate,
sodium hydroxide or potassium hydroxide solution can be used for saponification [60,
64-67]. Catalysts are very helpful for PET cleavage with the aid of alkaline media. As
Abdelaal et al. found, a non-catalyzed alkaline degradation process is less effective
compared to one with usage of zinc sulfate, calcium acetate, or tetraethylammonium
chloride (TEAC) as catalyst in the same alkaline media (here: 5 wt% aqueous NaOH
solution) [67].
Also, neutral hydrolysis can be performed with the help of water steam at a
temperature of 230 °C to 275 °C under high pressure [60, 68, 69]. Supercritical water can
also be used as Goto mentioned [70, 71]. In most of these processes, further steps to
purify the monomers and eliminate contaminants are necessary. For example,
catalyst contaminants or sodium and potassium ions (from the alkaline process) have
to be removed. Moreover, these recovered monomers have to be dried intensively for
further polycondensation steps. Also, mixtures of lower alcohols and water are used
to cleave waste PET to receive terephthalic acid and bis(2-hydroxyethyl)
Chapter 2
16
terephthalate [72]. In conclusion, it has to be considered that these processes are too
expensive for industrial processes.
Furthermore, the methanolysis is a recycling process, where methanol as cleavage
agent is used. Many publications and patents describe the methanolysis by using
sub- and supercritical methanol [60, 73-78]. The rate of the degradation of PET is much
higher in supercritical methanol. Sako, for example, published a process for the
depolymerization of PET into its monomers with the help of supercritical methanol at
a temperature of 300 °C and 2 to 23 MPa pressure where the decomposition rate is
much higher than in subcritical methanol [78]. The final products of the methanolysis
are ethylene glycol and dimethyl terephthalate. High amounts of methanol in relation
to PET (6:1 to 8:1, w/w) are used for the methanolysis [60]. The generated dimethyl
terephthalate can be used as monomer for the PET synthesis or it can be,
furthermore, used to produce terephthalic acid [79].
It is also possible to use other alcohols, for example, ethanol, supercritical ethanol, or
a supercritical ethanol/water mixture, to degrade PET for recycling applications to
diethyl terephthalate with a yield of 98.5 % [67, 70, 80]. Furthermore, this can be used for
the formation of bis(2-hydroxyethyl) terephthalate (BHET) as a new educt for the PET
synthesis.
The glycolysis is also a process to depolymerize poly(ethylene terephthalate) to
reuse the products for further application [60]. In the process, an excess of diols
(usually ethylene glycol) is used. Typical temperatures for the glycolysis are in the
range of 215 °C to 250 °C [81]. Alkylene diols of terephthalic acid such as bis(2-
hydroxyethyl) terephthalate (BHET) are products of the glycolysis or in the presence
of water also terephthalic acid [82]. Other cracking agents such as 1,4-butanediol or
1,3-propanediol can also be used for recycling of polyesters [83]. After the cleavage of
these polyesters, filtration of the residual catalysts, originating from the PET
synthesis, follows. Further catalysts, which are filtered out of the cleaved products,
stem from the cleavage process. For the glycolysis, sodium carbonate, magnesium
salts or zinc acetate are used as catalysts [60, 84, 85]. To decrease the reaction time for
the cleavage, microwave irradiation is also a possibility in this process as described
by Kržan [85, 86].
Chapter 2
17
Further solvolysis processes such as ammonolysis (cleavage by anhydrous ammonia
to terephthalamide) or aminolysis (cleavage reaction with amines or hydrazine to e.g.
terephthalohydrazide) lead to a variety of products [87-90]. Amines such as
methylamine, ethylamine, ethanol amine and butylamine are commonly used;
however, commercial applications of these cleavage products are seldom in chemical
and polymer industry [91-94].
2.2.3 Mechanical Recycling
Mechanical recycling leads to PET which can be used further for different
applications. Separated and washed PET flakes can be molten in an extrusion
process and remold into pellets [59]. During the extrusion process, a melt filtration
eliminates further contaminants such as catalyst residues. However, a complete
removal of these particles is not possible. These mechanically reprocessed pellets
can, for example, be used for fiber production (open-loop). The main problem of this
process is that after each melting and filtration step (due to the higher shear strain)
deterioration of the quality of PET is obtained. Bottle-to-bottle (closed-loop) recycling
is also possible, if the intrinsic viscosity can be increased to an acceptable value for
bottle grade PET ([η] ≈ 0.80 dL ∙ g-1). Bottle-to-bottle recycling gains in importance
over the last years [25]. The increase of the intrinsic viscosity to a value for bottle
grade PET is usually done in the solid state polycondensation process (SSP
process) [49].
However, the solid state polycondensation process is a slow and expensive process
because of its high energy consumption. Besides, further problems arise during
reprocessing of post-consumer PET. Discoloration of recycled PET may cause
problems in the production of white or pastel shade textiles. The origin of this
discoloration may have different sources. Several authors describe the occurrence of
gray discoloration of recycled PET upon repeated heating. Aharoni published some
results on the gray discoloration of PET based on antimony catalysts. In model
experiments, he discovered that free glycols or glycolates which are present in PET
degrade at high temperatures above 200 °C by generation of carbon monoxide and
carbon dioxide [16]. Carbon monoxide is a reducing agent which is oxidized to carbon
dioxide in presence of antimony (III) compounds. These antimony (III) compounds
(e.g. Sb2O3) are reduced to metallic elemental antimony (Sb0). Repeated thermal
Chapter 2
18
treatments like during the SSP process or extrusion processes lead to gray
discoloration of PET. The presence of stabilization agents like phosphite-based light
stabilizers or flame retardants could increase the graying effect because of their
reducing capacity [19]. Further reasons for the graying of recycled PET during
reprocessing could be the presence of carbon particles which are used as reheat-
agents or IR-absorbers or the occurrence of black specs (i.e., degraded polymer
residues attached to the walls of the equipment) [95, 96]. Contamination with dyestuffs,
pigments or other polymers or impurities are further possible reasons for the graying
or discoloration of post-consumer PET during repeated heating [97].
On the other hand, during repeated thermal processing of recycled PET partial
yellowing may occur [98]. This may cause problems in the production of new
materials, e.g. of white or pastel shade textiles.
Studies on the degradation mechanisms of poly(ethylene terephthalate) during
exposure to heat showed that the formation of quinones and stilbene quinones may
contribute to the yellowing of PET [99, 100]. Heat-induced yellowing of post-consumer
PET can be enhanced if co-polymers (e.g. poly(ethylene 2,6-naphthalate), PEN) or
contaminants like foreign polymers (e.g. PVC) are present [97, 101]. The presence of
PVC impurities (PVC amounts of approx. 100 ppm) may cause polymer chain
degradation and discoloration of post-consumer PET [97]. The presence or absence of
additives like process or heat stabilizers (e.g. antioxidants, sterically hindered
amines) in various post-consumer PET materials of different origin may have
influence on the degradation and discoloration of PET during reprocessing [102].
Furthermore, cyclic and linear oligomers possess great practical relevance. They
may diffuse towards the surface of PET films and fibers which affects their surface
properties; furthermore, precipitation of cyclic oligomers causes problems during the
dyeing of polyester fibers [96, 103]. Dulio et al. studied the presence of cyclic and linear
oligomers in recycled PET from post-consumer soft-drink bottles in dependence on
the re-extrusion conditions [103]. Extrusion under vacuum led to decrease of the
overall oligomer content with increasing temperatures. On the other hand, Dulio et al.
found that extrusion at higher temperatures and increased residence time resulted in
an increase of the oligomer concentration and in the formation of larger rings and
longer polymer chains [103].
Chapter 2
19
2.3 Importance of the Molecular Weight of PET for its Application
The molecular weight of poly(ethylene terephthalate) is very important for its
application. The viscosity of polymers depends on their molecular weight. Different
viscosity values such as relative, specific, reduced, inherent, or intrinsic viscosity are
of use in the polymer analysis. These viscosities are determined for diluted polymer
solutions. Melt and complex viscosities, however, are determined in the melt.
Normally, in polyester industries the IV value (intrinsic viscosity) is the process value,
which determines the quality of the polymer. The intrinsic viscosity is determined by
extrapolation of the reduced viscosity (ηred) to a concentration of zero. Often, the
intrinsic viscosity is replaced by the logarithmic inherent viscosity (ηinh.) as it is
obtained by a one plot determination (1).
𝜂𝑖𝑛ℎ. =ln(𝜂𝑟𝑒𝑙)
𝛽=
ln(𝜂
𝜂0)
𝛽=
ln(𝑡
𝑡0)
𝛽 (1)
Here, ηinh. is the inherent viscosity, ηrel the relative viscosity, β the mass
concentration, η the viscosity of the PET solution, η0 the viscosity of the solvent, t the
flow time of the PET solution and t0 the flow time of the solvent.
PETs of different molecular weight are used for different products. For instance, it is
impossible to spin fibers at high velocities from PET of a high molecular weight, due
to its high melt viscosity. On the other hand, high performance materials cannot be
produced with low molecular weight PET because of its worse mechanical properties
compared to high molecular PET. Textile fiber grade PET has low IVs in the range of
0.57 dL ∙ g-1 to 0.65 dL ∙ g-1 to achieve a good and easy spinnability. The good
spinnability results of the low melt viscosity. The melt viscosity of PET with an IV of
0.65 dL ∙ g-1 is 292 Pa ∙ s at a process temperature of 290 °C. In the following table,
some melt viscosities in dependence on the intrinsic viscosity and temperature are
presented by Thiele (Table 2.1) [12].
Chapter 2
20
Table 2.1. Correlation of melt viscosity of poly(ethylene terephthalate) with its intrinsic
viscosity in dependence on the melt temperature [12].
Intrinsic
viscosity 0.65 dL ∙ g-1 0.70 dL ∙ g-1 0.75 dL ∙ g-1 0.85 dL ∙ g-1
Temperature Melt viscosity Melt viscosity Melt viscosity Melt viscosity
280 °C 410 Pa ∙ s 601 Pa ∙ s 859 Pa ∙ s 1640 Pa ∙ s
285 °C 345 Pa ∙ s 506 Pa ∙ s 723 Pa ∙ s 1381 Pa ∙ s
290 °C 292 Pa ∙ s 427 Pa ∙ s 610 Pa ∙ s 1166 Pa ∙ s
Higher melt viscosities are necessary for manufacturing PET bottles. Hence, higher
intrinsic viscosities (in the region of 0.72 dL ∙ g-1 to 0.85 dL ∙ g-1) are needed for bottle
grade PET. The requirements for the mechanical properties are higher to achieve a
good and fast forming during the injection molding process of the bottle preforms and
the blow molding process of the bottles. High performance materials such as tire cord
or safety belts are produced with high molecular PET with intrinsic viscosities of
0.95 dL ∙ g-1 to 1.05 dL ∙ g-1 [11, 12].
2.4 Chain Extenders
To increase to molecular weight of poly(ethylene terephthalate) via reactive
extrusion, chain extenders can be used. Chain extenders are bi- or multifunctional
molecules. The functional groups of the chain extenders are able to link with the
terminal groups of the polymers. In the case of PET, the terminal groups are hydroxyl
(-OH) and carboxyl groups (-COOH). The usage of chain extenders for poly(ethylene
terephthalate) has some advantages and disadvantages. It is easy to apply the chain
extenders during an extrusion process, which has to be performed during remolding.
The application of chain extenders is faster than the SSP or LSP process as it does
not require an additional processing step. Accordingly, the addition of chain
extenders is cheaper compared to the SSP or LSP process in small scales. However,
due to the formation of foreign building blocks during chain extension, an influence
on the properties of the polymer results. Some chain extenders form byproducts or
decompose at high temperatures. These byproduct emissions have to be removed by
vacuum evaporation. Due to reactive functions (such as epoxy groups), some chain
extenders may be hazardous for humans. In the following table, the main advantages
Chapter 2
21
and disadvantages of the usage of chain extenders during extrusion of PET are listed
(Table 2.2).
Table 2.2. Advantages and disadvantages of the usage of chain extenders in a
reactive extrusion process of PET instead of the SSP process.
Advantages Disadvantages
Cheaper compared to the SSP
or LSP process in small scales
Influence on the properties of PET (e.g.
crystallinity, mechanical properties)
Faster than the SSP or LSP
process Byproducts may be generated
Easy application (reactive
extrusion) in small scales Decomposition of chain extenders may occur
Addition of only small amounts
are required
Chain extenders may be hazardous (due to
reactive end groups)
Many publications exist concerning the increase of the molecular weight of polymers
using chain extenders with a variety of functional groups such as bisoxazolines,
bisoxazines, bisepoxides, carboxylic dianhydrides, biscaprolactames, and
diisocyanates. Here, the focus is set on two chain extender types (bisoxazolines (e.g.
1,3-phenylene-bis-oxazoline) and biscaprolactames (e.g.
N,N’-carbonylbiscaprolactam).
2.4.1 1,3-Phenylene-bis-oxazoline (1,3-PBO)
Bisoxazolines such as 1,3-phenylene-bis-oxazoline (1,3-PBO) are addition type chain
extenders. The reaction of 1,3-PBO takes place with the COOH end groups of PET
without evaporating byproducts.
The synthesis of oxazolines has been published by several authors [104-113].
Preferably, oxazolines are synthesized from their corresponding nitriles and 2-
aminoethanol as published for the first time by Witte and Seelinger (Scheme 2.7) [114-
117]. In the case of 1,3-PBO, 1,3-dicyanobenzene (isophthalodinitrile) is the
corresponding starting material. As catalysts, mostly zinc salts such as zinc acetate
are used as weak Lewis acids [107, 111]. The generated byproduct NH3 has to be
removed.
Chapter 2
22
Scheme 2.7. Synthesis of 1,3-phenylene-bis-oxazoline (1,3-PBO) [108, 114, 115].
Oxazolines undergo many ring-opening reactions with a variety of functional groups
like carboxyl groups [118-120]. Furthermore, polymerizations can be performed with
aliphatic dicarboxylic acids (for example, adipic acid or sebacic acid) or aromatic
dicarboxylic acids (for example, terephthalic acid or isophthalic acid) and
bisoxazolines [121-126]. Moreover, reactions with oligomers and polymers can be
performed with the help of bisoxazolines which are chain extension reactions.
Bisoxazolines are commonly used as chain extenders in several polymers with
carboxyl end groups such as polyamides,[127-130] poly(lactic acid) (PLA) [131-133],
poly(butylene terephthalate) (PBT) [134, 135], and poly(ethylene terephthalate)
(PET) [127, 135-142]. In the case of PET, the reaction with the carboxylic acid end groups
is the most important one. The reaction of carboxyl acid groups with oxazolines
results in the formation of ester amide bonds, and many applications are generated
with the aid of this type of reaction [143-146]. In the following scheme, the chain
extension reaction of 1,3-PBO with the carboxyl end groups of PET is presented
(Scheme 2.8). Just small amounts of 1,3-PBO in the range of 0.3 wt% to 0.7 wt% are
needed to achieve good results with regard to high intrinsic viscosities of PET [139].
The chain extension of PET with 1,3-PBO proceeds in a linear way, which is
important in many applications.
Chapter 2
23
Scheme 2.8. Reaction of 1,3-phenylene-bis-oxazoline (1,3-PBO) with the carboxyl
end groups of poly(ethylene terephthalate) (PET) [119, 125, 138].
2.4.2 N,N’-Carbonylbiscaprolactam (CBC)
Beside chain extenders which react with carboxyl end groups, chain extenders which
react with the hydroxyl end groups of PET (e.g., bislactams) have been
investigated [147, 148]. As an example, N,N’-carbonylbiscaprolactam (CBC) is a
commonly known chain extender first published in 1956 by Meyer [149]. Furthermore,
Loontjens et al. published studies using bislactams as chain extenders for a variety of
polymers such as polyurethanes, polyamides, and polyesters [127, 150-156].
N,N’-Carbonylbiscaprolactam is synthesized by the reaction of ε-caprolactam and
phosgene in the presence of a tertiary amine (e.g., triethyl amine) as acid scavenger
(Scheme 2.9).
Scheme 2.9. Synthesis of N,N’-carbonylbiscaprolactam (CBC) by the reaction of
ε-caprolactam and phosgene [154].
The chain extension reaction of CBC occurs at terminal hydroxyl or amine groups. It
is also possible, that CBC reacts with terminal carboxyl groups, but the reaction with
Chapter 2
24
OH or NH2 end groups is faster [154]. In the case of PET, the terminal OH groups react
with CBC via evaporation of ε-caprolactam or ring opening reaction (Scheme 2.10).
Linear chain extension is obtained which is very important, especially for fiber
production to achieve good spinnability. The chain extension takes place within three
minutes and small amounts of 0.1 to 1.0 wt% of chain extender are needed [154].
Scheme 2.10. Potential reactions of N,N’-carbonylbiscaprolactam (CBC) and
hydroxyl end groups of poly(ethylene terephthalate) (PET) [154, 157]. (a [red]) two
substitutions of ε-caprolactam, (b [black]) combination of substitution of
ε-caprolactam and ring opening reaction, (c [blue]) two ring opening reactions.
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N N
O OO
R
PE
T
OH
+
NH
O
RPET
O
O
NH
O
O
PETR
-
N NH
O
O
O
PETR
RPET
O
O
N
O
O
R
PE
T
OH
+
R
PETOH+
NH
O
-
RPET
O
O
RPET
O
RPET
OH+
R
PE
T
OH
R
PE
T
OH
NH
O
-
PET
O
O
HN
O
PETO
OHN
R
R
(a + b)
(b + c)
(a)
(b)
(c)+
(b)
+
Chapter 2
25
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Chapter 3
33
Reasons for the Discoloration of Post-Consumer Poly(ethylene terephthalate) during Reprocessing A
Summary: Gray and/or yellow discoloration may occur during repeated
heating of poly(ethylene terephthalate) (PET). Both phenomena can cause
problems in further application.
In this work, the reasons for the discoloration of PET during reprocessing are
investigated by physical and chemical analysis like colorimetry, size exclusion
chromatography, viscosimetry (ηinh.), inductively coupled plasma mass
spectrometry, X-ray photoelectron spectroscopy, and MALDI-ToF-MS
analysis. It is found that the antimony content which originates from catalyst
residues used in PET synthesis has high influence on the gray discoloration
obtained during reprocessing of PET. Antimony ions are reduced to
elementary antimony during heating to temperatures above 230 °C as proven
by XP spectroscopy.
The yellow discoloration is partially generated by polyamide contaminants
which are used as barrier layers in PET packaging materials like soft drink
bottles. In conclusion, to prevent discoloration of post-consumer PET during
reprocessing different methods like oxidation of gray metallic antimony or
sorting out of polyamide contaminants are needed.
Keywords: Post-consumer poly(ethylene terephthalate), reprocessing,
discoloration, polymer analysis
A. Reproduced with permission from D. Berg, K. Schaefer, A. Koerner, R. Kaufmann,
W. Tillmann, M. Moeller, Macromolecular Materials and Engineering 2016, 301,
1454, DOI: 10.1002/mame.201600313. Copyright WILEY-VCH.
Chapter 3
34
3.1 Introduction
To counteract the discoloration of reprocessed poly(ethylene terephthalate) during
further processing, at first, the reasons for the discolorations have to be evaluated.
As mentioned in Chapter two, poly(ethylene terephthalate) is synthesized by
polycondensation of ethylene glycol with terephthalic acid or terephthalic acid
dimethyl ester by generation of water or methanol as byproducts respectively in two
steps [1]. The first step is the esterification where the monomer bis(2-hydroxyethyl)
terephthalate (BHET) as main product is synthesized which further reacted in the
transesterification step, to PET with the aid of catalysts at high temperatures [2].
Antimony trioxide (Sb2O3) is one of the most frequently used catalysts in this
reaction [3-5]. More than 90 % of the polyesters are produced with the help of
antimony catalysts (like Sb2O3 or Sb[OOC-CH3]3) [1, 6-9]. The catalyst is used for both
steps (esterification and transesterification) in the PET synthesis; it minimizes side
reactions and decreases the activation energy of BHET [10, 11]. The mechanism of the
effect of the antimony catalyst on the solid-state polycondensation process of PET is
presented by Duh [12]. Further, but less frequently used, catalysts for the PET
production are titanium compounds (e.g. titanium tetraisopropoxide) and germanium
compounds (e.g., GeO2) [2, 13]. In general, the used catalysts remain in PET over the
whole life cycle of this polymer.
There is a great interest in the recycling of PET after consumption especially in
Germany, because of its deposit system for one-way-bottles since 2003. The
recycling methods can be distinguished in thermal recycling, chemical recycling, and
mechanical recycling. In this chapter, the mechanical recycling is in the focus.
Separated and washed PET flakes (for example originating from used bottles) can be
molten in an extrusion process and remold into pellets [14]. These mechanically
reprocessed pellets can be used for further products. Often a solid state
polycondensation process (SSP process) has to be performed after the production of
the remold pellets to increase the molecular weight and intrinsic viscosity of PET. In
the mechanical recycling, some problems may arise during and after repeated
reprocessing of PET.
Due to repeated thermal treatments, PET may become more and more gray or
yellow, which could be critical for a variety of applications such as fiber
manufacturing.
Chapter 3
35
During the SSP process, for example, PET becomes often gray. PET is heated to
high temperatures in the range of the glass transition temperature (Tg) and the onset
of the melting temperature (Tm) [15]. Normally, temperatures between 200 °C and
240 °C are used for the condensation reactions over a time of about eight hours or
longer [16]. An oxygen free atmosphere has to be present in this process [1, 17]. In
general, this SSP process leads to an increase of the molar mass and a decrease of
volatile components which results in an improvement of the quality of poly(ethylene
terephthalate). However, due to this thermal treatment discoloration of PET may
occur, which is in addition to the slow and expensive process, because of its high
energy consumption, a disadvantage of this process.
The origin of this discoloration may have different sources. In the literature, some
papers on the identification of the reasons for the discoloration of post-consumer PET
during reprocessing are described.
Aharoni published some results on the gray discoloration of PET induced by
antimony catalysts. He discovered in model experiments that free glycols or
glycolates which are present in PET degrade at high temperatures above 200 °C by
generation of carbon monoxide and carbon dioxide [5]. Carbon monoxide is a
reducing agent which is oxidized to carbon dioxide in the presence of antimony (III)
compounds. These antimony (III) compounds (e.g. Sb2O3) are reduced to metallic
elementary antimony (Sb0). Repeated thermal treatments like the SSP process or
extrusion processes lead to gray discoloration of PET. The presence of stabilization
agents like phosphite-based light stabilizers or flame retardants could increase the
graying effect because of their reducing capacity [12]. Further reasons for the graying
of recycled PET during reprocessing could be the presence of carbon particles which
are used as reheat-agents or IR-absorbers or the occurrence of black specs (i.e.,
degraded polymer residues attached to the walls of the equipment) [18, 19].
Contamination with dyestuffs, pigments or other polymers or impurities are further
possible reasons for the graying or discoloration of post-consumer PET during
repeated heating [20].
On the other hand, during repeated thermal processing of recycled PET partial
yellowing may occur [21]. This may cause problems in the production of new
materials, e.g. of white or pastel shade textiles.
Chapter 3
36
Studies on the degradation mechanisms of poly(ethylene terephthalate) during
exposure to heat showed that the formation of quinones and stilbene quinones may
contribute to the yellowing of PET [22, 23]. Heat-induced yellowing of post-consumer
PET can be enhanced if co-polymers (e.g. poly(ethylene 2,6-naphthalate, PEN) or
contaminants like foreign polymers (e.g. PVC) are present [20, 24]. The presence of
PVC impurities (PVC amounts of approx. 100 ppm) may cause polymer chain
degradation and discoloration of post-consumer PET [20]. The presence or absence of
additives like process or heat stabilizers (e.g. antioxidants, sterically hindered
amines) in various post-consumer PET materials of different origin may have
influence on the degradation and discoloration of PET during reprocessing [25].
In this chapter, the reasons for the gray and yellow discoloration of post-consumer
PET during reprocessing are further clarified with the help of diverse analytical
methods.
3.2 Experimental Section
3.2.1 Materials
Poly(ethylene terephthalate) was provided by different companies. v-PETs are virgin
polyesters for different applications. v-PET 1 is a bottle grade PET having a high
intrinsic viscosity. v-PET 2 and v-PET 3 are fiber grade polyesters manufactured with
different catalysts. v-PET 2 is produced with the aid of titanium catalysts and v-PET 3
with antimony catalysts.
Furthermore, w-PET 1, w-PET 2, and w-PET 3 are materials which have been used
for bottle production and/or food packages, but they were not reprocessed. These
materials were obtained as separated and cleaned waste flakes.
Finally, r-PETs are reprocessed materials, which were treated by solid state
polycondensation to increase their intrinsic viscosity. r-PET 1 is a reprocessed PET
from Japan and r-PET 2 – r-PET 5 are reprocessed materials from Germany.
Furthermore, some PET juice bottles from the market were analyzed.
Concentrated nitric acid, 2,6-di-tert-butyl-4-methylphenol, and methanol were
received from Sigma Aldrich, Taufkirchen, Germany. Antimony (Sb) was obtained
from Alfa Aesar, Karlsruhe, Germany, and antimony oxide (Sb2O3) from abcr,
Karlsruhe, Germany. Chloroform was purchased from J. T. Baker (Deventer, NL) and
1,1,1,3,3,3-hexafluoropropane-2-ol (HFIP) was bought from Fluorochem, Hadfield,
Chapter 3
37
United Kingdom. Sodium trifluoroacetate was obtained from Merck, Darmstadt,
Germany. Dithranol (1,8,9-anthracenetriol; DT) from Sigma-Aldrich was used as
matrix in MALDI-ToF-MS analysis. Nylon 6 (PA 6) was obtained from Sigma Aldrich
and Nylon MXD 6 from Misubishi (Tokyo, Japan).
3.2.2 Heating of Antimony Oxide in Ethylene Glycol
Pure antimony oxide (Sb2O3, 1.5 g) was heated in ethylene glycol (7.5 mL) in a
laboratory autoclave at temperatures > 235 °C for 5 h [5]. After treatment under these
conditions, black residues were obtained which were analyzed by XP spectroscopy.
Heating at lower temperatures (210-220 °C) did not result in the formation of black
precipitates.
3.2.3 Sample Preparation
Before analysis, poly(ethylene terephthalate) bottles or packages were rinsed
repeatedly with warm water; then rinsed with dist. water and finally dried at RT. For
the investigation of the wall composition, pieces were cut out of the PET packages.
The PET packages were shredded with the help of a pair of special scissors, then
minced in a laboratory mixer (Waring Commercial Blendor Mixer with metal beaker,
220 V, Snijders Scientific BV, Tilburg, NL) (several mixing steps with 2 min mixing at
highest frequency and in between cooling for 1 min in an ice bath). In the next step,
the pre-minced PET flakes were ground further in a cryomill to a fine powder.
PET flakes from post-consumer waste were at first manually pre-sorted by removing
colored flakes or contaminants and then ground in a cryomill.
PET flakes were ground with the help of the cryomill Freezer/Mill 6800 (Spex
CertiPrep Model 6800/230, obtained by C3 Analysentechnik GmbH, Haar, Germany).
For size reduction of PET materials, three grinding cycles for 5 or 10 min were
performed in the cryomill with an impact frequency of 10 s-1, in between the samples
were cooled with liquid nitrogen for 5 or 10 min.
3.2.4 Characterization of PET Materials
The color values were measured with the help of the Datacolor Spectraflash SF600
plus CT UV colorimeter (Datacolor, Marl, Germany) using the D65 illuminant and the
10° observer. In each case, fivefold measurements were performed using a special
Chapter 3
38
specimen container (Datacolor) and mean values were calculated. The color values
were calculated using the Datacolor formula based on the CIE-L*a*b*-system. In this
system, L* represents the lightness (L*=0 indicates black, L*=100 white). The
a*-value corresponds to the green-red axis, where negative a*-values document
green and positive a*-values red hues. The b*-value represents the blue-yellow axis,
where negative b*-values document blue and positive b*-values yellow hues.
The antimony concentration of poly(ethylene terephthalate) was determined by
inductively coupled plasma mass spectrometry (ICP-MS). About 0.2 g PET was
digested with concentrated nitric acid (10 mL) in a microwave device (MARS 5, CEM
GmbH, Kamp-Lintfort, Germany). After digestion, the samples were analyzed with an
ICP-MS Plasmaanalyzer 400 (Perkin Elmer GmbH, Rodgau-Juegesheim, Germany)
and the antimony concentration was determined by reference to a calibration curve.
In each case, duplicate determinations were performed.
Size exclusion chromatography (SEC) was applied to determine the molar mass of
the oligomers in PET extracts. PET extracts were dissolved in chloroform. 2,6-Di-tert-
butyl-4-methylphenol (Sigma Aldrich) was used as internal standard and polystyrene
standards (PSS Polymer Standards Service GmbH, Mainz, Germany) were used to
determine the molar mass. The samples were separated on
polystyrene/divinylbenzene columns (PSS Polymer Standards Service GmbH) at
1 mL ∙ min-1 flow rate. A refractive index (RI) detector (RI-2031plus, JASCO Germany
GmbH, Gross-Umstadt, Germany) was used for the detection of the oligomers.
Microscopy measurements of different bottle flakes were performed by field-emission
scanning electron microscopy (FESEM). The bottle flakes were washed with distilled
water, dried and measured with a S-4800 SEM (Hitachi Ltd., Tokyo, Japan) using an
accelerating voltage of 1.0 – 3.0 kV. Cross sections of PET bottles were prepared in
20 µm sections after embedding in an acrylate resin (Technovit® 7100, Heraeus
Kulzer GmbH, Weinheim, Germany) using a Supercut® rotation microtome 2010
(Leica, Nussloch, Germany).
Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray
(EDX) analysis were performed with the help of the Hitachi S-3000 N environmental
scanning electron microscope (ESEM) with energy-dispersive X-ray spectroscopy
(EDAX) detection unit (ESEM/EDAX) using an acceleration voltage of 10-15 kV
(Hitachi High-Technologies Europe GmbH, Tokyo, Japan).
Chapter 3
39
About 5 g poly(ethylene terephthalate) (ground in a cryomill) were extracted with
140 mL solvent at 80 °C for 4 h. As solvent, an azeotropic mixture of chloroform and
methanol (80:20, v:v) was used. After removal of the solvent by evaporation, the
remaining residue was dissolved in chloroform (J. T. Baker) and analyzed by size
exclusion chromatography and infrared spectroscopy.
The IR spectra were measured with a Nicolet 470 FT-IR spectrometer (Thermo
Nicolet, Offenbach, Germany) with a resolution of 4 cm-1. Baseline correction was
performed. Extracts of PET were measured on potassium bromide pellets in
transmission. PET packaging materials were measured in the ATR technique using
germanium or silicon crystals.
X-ray photoelectron spectroscopy (XPS) was performed with the help of an Ultra
AxisTM spectrometer (Kratos Analytical, Manchester, United Kingdom). For recording
the XP spectra, the sample surface was excited with monochromatic Al-Kα1,2
radiation (1486.6 eV) with a total power of 144 W (12 kV x 12 mA). Charge correction
of the spectra was performed via the C 1s photoelectric peak of the aliphatic carbon
(C-C, C-H) which was set to 285.0 eV. The concentration of the elements is given in
atom%. Sample preparation before measurement: PET pellets and flakes were
purified by treatment in ultrapure water for 30 min in an ultrasonic bath, followed by
ultrasonication in propane-2-ol for 30 min for several times. The solvent was removed
after each purification step by decantation, and the ultrasonication was carried on
with fresh propane-2-ol. Finally, the solvent was removed by evaporation.
Mass spectra were measured with a 1 kHz laser UTX MALDI-ToF/ToF mass
spectrometer (matrix assisted laser desorption ionization – time of flight/time of flight)
with pulsed ion extraction (PIE) (Bruker, Bremen, Germany). Both the oligomer
samples and the dithranol (DT) matrix were dissolved in HFIP, with the oligomers
dissolved to a concentration of 5 mg ∙ mL-1 and the DT to a concentration of
20 mg ∙ mL-1. Sample and matrix solution were mixed in a ratio of 1:10. The
cationization solution of 0.1 mol ∙ L-1 sodium trifluoroacetate (TFA) in TA 30
(acetonitrile: TFA 0.1 % in water, 30:70, v/v) was prepared as a thin-layer on the
target plate (ground steel target, Bruker), dried and then 1 µL of the sample/matrix
solution was added on top of it. Mass spectra of positive ions were recorded in the
reflector mode with pulsed ion extraction and a repetition rate of 200 Hz; 3000
spectra were added to a sum spectrum. While recording the sample stage moved in
Chapter 3
40
a random fashion on a sample spot allowing 500 laser shots per position. The
calibration was carried out with an external standard using the Peptide Calibration
Standard 206195 from Bruker Daltonics, Bremen, Germany prepared on a near
neighbor spot. The spectra were baseline subtracted (Top hat algorithm) and mass
peak annotation was obtained with the centroid detection algorithm (width 0.5 at
80 % height). Processing of the MALDI-ToF-MS for identification of oligomer series
was performed with the Polymerix™ Software from Sierra Analytics (Modesto, CA,
USA).
Fluorescence spectroscopy was performed with the help of a Fluoromax-4P
fluorescence spectrometer (Horiba Jobin Yvon GmbH, Unterhaching, Germany).
Excitation and emission spectra of solid poly(ethylene terephthalate) pellets or
containers were recorded after grinding to fine powders by using a special sample
holder for solid materials. Solutions were measured in fluorescence quartz cuvettes.
PET materials and further polymers (PA 6, PA MXD 6) were heated in a muffle
furnace (Nabertherm B180, Lilienthal, Germany) for 40 min at 220 °C either in
presence of air or in nitrogen atmosphere (= roasting test) [26]. After cooling, the
polymers were analyzed with regard to their color, changes in chemical composition
and thermal properties.
3.3. Results and Discussion
3.3.1 On the Graying of Poly(ethylene terephthalate) during Reprocessing
Especially in fiber manufacture, color is a very important topic. Thus, color
measurements are common practice. Here, the color values were calculated using
the CIE-L*a*b*-system. In Figure 3.1, the color values of different PET materials are
depicted.
Chapter 3
41
89.688.5
87.3 87.685.2
89.7
79.1
76.073.3
70.8
-0.2-1.1
-0.5 -1.0 -1.5-0.4
-2.2
-0.8-1.8
-3.1
-0.9
2.4
0.5
2.51.6 1.8
-1.0
2.0
0.6 0.5
v-PET 1
v-PET 2
v-PET 3
w-P
ET 1
w-P
ET 2
r-PET 1
r-PET 2
r-PET 3
r-PET 4
r-PET 5
-4
-2
0
2
4
6
8
80
100
colo
r valu
e
L*-value
a*-value
b*-value
Figure 3.1. Color values (L*, a*, b* according to CIE-L*a*b*) of different virgin
(v-PET), waste (w-PET), and recycled PETs (r-PET).
Especially, the L*-value (lightness value) is important to give information on the
grayness of PET samples. The lower the L*-value, the grayer is the PET sample. All
investigated virgin polyesters have L*-values in the range of 87 – 90, whereas waste
PET samples have L*-values in the range of 85 – 88. Waste PET samples were
sorted with a high amount of white and clear PET flakes. These polyesters were not
reprocessed but used after sorting out of waste plastics. The investigated recycled
poly(ethylene terephthalate)s are samples, which were reprocessed and treated at
higher temperatures. The range of the L*-value of these samples varies from
90 (r-PET 1) which is white to 70 (r-PET 5) which is the grayest sample of the studied
polyesters. Hence, the colorimetric measurements show that the thermal treatment
during reprocessing of PET is one reason for the graying of PET.
The work of Aharoni hinted to a correlation of the graying of poly(ethylene
terephthalate) and the reduction of the antimony catalyst during reprocessing with
formation of elementary black-colored antimony (Equation (1) and (2)) [5].
The experiment of Aharoni was reproduced by heating of antimony oxide (Sb2O3) in
ethylene glycol for 5 h in a laboratory autoclave [5]. Heating at temperatures higher
Chapter 3
42
than 235 °C was required for achieving reduction of Sb2O3 and formation of
elementary antimony (Sb0) which precipitated as black solid.
HO-CH2-CH2-OH + 2 O2 → CO2 + CO + 3 H2O (1)
Sb2O3 + 3 CO → 2 Sb(0) + 3 CO2 (2)
The black residues formed after heating of Sb2O3 for 5 h at 235 °C in ethylene glycol
were analyzed by XP spectroscopy (Table 3.1 and 3.2, Figure 3.2). The XP spectra
of the residues revealed the presence of carbon, oxygen and antimony signals
(Table 3.1). Figure 3.2 shows the high-resolution spectrum of antimony and oxygen.
The Sb 3d level is separated according to the spin orbit splitting in a 3d5/2 and a 3d3/2
component while the O1s peak can be seen on the left-hand side from the Sb 3d5/2
peak. From the shift of the Sb 3d5/2 peaks the presence of pentavalent antimony
cations (Sb5+) and of Sb0 antimony species can be determined. From this fact it can
be concluded that the formed black residues result from Sb0 (Figure 3.2). Table 3.2
shows the binding energies of the different species which are in good agreement with
literature data [27, 28]. This indicates the reduction of Sb2O3 to Sb (and after contact to
air the formation of Sb2O5) or disproportionation of trivalent antimony into pentavalent
and elementary antimony during heating at high temperatures in ethylene glycol.
Table 3.1. XP spectroscopic signals of heated Sb2O3 (heated for 5 h at 235 °C in
ethylene glycol). The binding energies were corrected with regard to the aliphatic
carbon C 1s (C-C, C-H) which was set to 285.0 eV.
Element Binding energy / eV Atomic concentration / atom%
O 1s 532.6 8.3
Sb 3d5/2 531.5 12.0
Sb 3d3/2 540.2
C 1s 285.0 79.7
Chapter 3
43
Figure 3.2. High resolution XP spectrum of antimony and oxygen in heated Sb2O3
(heated at 235°C for 5 h in ethylene glycol).
Table 3.2. Binding energies and speciation of the antimony and oxygen signals of
heated Sb2O3 (heated for 5 h at 235 °C in ethylene glycol) as detected by XP
spectroscopy (cf. Figure 3.2).
Element Binding energy / eV Allocation
Sb 3d5/2
528.3 Sb0
Sb 3d5/2 531.0 Sb2O
5
O 1s 532.6 O
The antimony concentrations of PET were measured by ICP-MS after nitric acid
digestion (Figure 3.3). The antimony concentrations are for each PET sample except
for v-PET 2 higher than 110 ppm which shows, that all of these PET samples were
synthesized with the help of antimony catalysts.
Chapter 3
44
The results show that v-PET 2 is unusual because of its low Sb content. This
polyester is manufactured with the help of titanium catalysts. Also, r-PET 1, which is
a Japanese recyclate, has a low Sb concentration. It is well known that especially in
Japan germanium catalysts are commonly used in PET manufacture in addition to
antimony catalysts, so that the recycled products have lower antimony concentrations
than polyesters from other regions, where only antimony catalysts are used. The
Japanese polyesters are blended with polymers containing other catalysts, so that
lower antimony concentrations are obtained.
123.1
7.0
167.3
204.7
216.7
110.6
133.8
225.9
244.8265.8
v-PET 1
v-PET 2
v-PET 3
w-P
ET 1
w-P
ET 2
r-PET 1
r-PET 2
r-PET 3
r-PET 4
r-PET 5
0
50
100
150
200
250
300
virgin not reprocessed reprocessed
w(S
b)/
ppm
Figure 3.3. Antimony concentration (ppm) in virgin, not reprocessed, and
reprocessed waste poly(ethylene terephthalate) measured by ICP-MS after nitric acid
digestion.
The ICP-MS results reveal a dependence of the gray discoloration of recycled PET
on its antimony content. Antimony (III) oxide (Sb2O3) which is white is commonly
used in PET synthesis as catalyst. An assumption is that the catalyst is reduced to
metallic antimony (Sb0) during the reprocessing of PET. Figure 3.4 shows the
dependence of the antimony content and the L*-value of r-PET (cf. Table S3.1 in the
Supporting Information). The higher the antimony content the grayer is the recycled
PET sample.
Chapter 3
45
70 75 80 85 90
100
120
140
160
180
200
220
240
260
280
300
w(S
b)
/ p
pm
L*-value
Figure 3.4. Antimony content of recycled PET vs. its L*-value (from left to right:
r-PET 5 - r-PET 1). The image shows the corresponding r-PET samples [29].
These results demonstrate a relationship between the antimony concentration and
the color of reprocessed PET (Figure 3.4). However, further contaminants can also
contribute to the gray discoloration of r-PET as evident from the data measured for
r-PET 2 which has a low antimony content and also a low L*-value. A possible reason
for this phenomenon could be that the slightly gray color of r-PET 2 is not only
caused by its Sb content but also by other contaminants (e. g. residual traces of
foreign polymers, former colorants or black specs).
However, no evidence for the presence of carbon particles or black specs in various
post-consumer PET materials was provided in these studies.
It was tried to determine the oxidation of antimony residues in reprocessed
poly(ethylene terephthalate) with the help of XP spectroscopy in the same way as
described for heated antimony oxide, too (cf. Table 3.1 and 3.2, Figure 3.2).
However, the proof that reduction of trivalent antimony in Sb2O3 to elementary
antimony occurs during reprocessing of post-consumer poly(ethylene terephthalate)
in the melt was hampered (a) by the low local concentration of antimony on the PET
Chapter 3
46
surface and (b) by the overlap of oxygen (O 1s) and antimony signals in the XP
spectra. Thus, the proof that elementary antimony is present in PET materials which
were subjected to the SSP process could not be brought forward with the applied
analytical methods.
3.3.2 Yellowing of Poly(ethylene terephthalate) during Reprocessing
Beside gray discoloration, yellowing of recycled poly(ethylene terephthalate) during
reprocessing may also result in reduced quality. This causes in particular problems in
the production of fibers for textiles in white or pastel shades.
The color values in Figure 3.1 show beside the L*-value also the a*- and b*-values.
The b*-value represents the blue or yellow color: -b* is the blue hue and +b* is the
yellow hue. This means the higher the b* value, the higher the yellow color.
All of the studied PET samples have negative a*-values in the range of -0.2 to -3.1
and a slightly green nuance. The PET samples v-PET1 and r-PET 1 have a negative
b*-value (-0.9 and -1.0, respectively) which indicates a slightly blue color. All other
samples have positive b*-values and, thus, yellow nuances. The results show that v-
PET 2 has a highly positive b*-value. This polyester is manufactured with titanium
catalysts and it is well known that titanium catalysts lead to yellowish color of virgin
PET [2, 30]. Furthermore, w-PET 1 has a high b*-value (b* = 2.5). This sample
originates from a waste separation containing some residual yellowish PET
contaminants.
It is known that poly(ethylene terephthalate) is subject to yellowing during heating at
high temperatures (280-300 °C for 4-8 h) in the presence of oxygen due to the
formation of hydroxylated aromatic rings, quinones, and stilbene quinones [23]. The
polyester extrusion process is usually run in vacuo or in nitrogen atmosphere and it
takes only a few minutes. Thus, thermoyellowing should not occur if the extrusion
process of PET is run carefully. This is documented by the color values obtained for
different recycled PET materials as shown in Figure 3.1. The reason why some PET
materials undergo marked yellowing during heating and others not must have
different explanations.
It is supposed that foreign polymers or impurities in PET flakes may contribute to the
yellow discoloration, too. Barrier layers in PET packaging materials can be a further
cause of yellow discoloration in thermally treated PET. Therefore, FESEM images of
Chapter 3
47
cross sections of some fruit juice bottles were taken aiming at the detection of barrier
layers. Figure 3.5 shows an example of a fruit juice bottle including a barrier layer.
Roasting tests (heating at 220 °C for 40 min) show that this bottle yellows during
thermal treatment.
Figure 3.5. FESEM image of a cross section of a fruit juice bottle including a barrier
layer (here: on the left side). (The bar indicates a distance of 20.0 µm).
Furthermore, roasting tests were performed with other PET samples. Roasting
tests are common practice in the plastics recycling industry for the evaluation of
the occurrence of yellowing of polymers during heat exposure [26]. In particular,
the clear fractions of w-PET 1 and w-PET 2 yellowed during exposure to the
roasting test. Figure 3.6 shows bottle flakes of w-PET 1 after roasting. Parts of
the flakes are yellowed, whereas other parts remain clear or white.
Chapter 3
48
Figure 3.6. Partially yellowed PET flakes of w-PET 1 after exposure to roasting test
(40 min at 220°C in the presence of air).
For further characterization of the waste PET materials, the samples w-PET 1 and
w-PET 2 were extracted with an azeotropic mixture of chloroform and methanol
(80:20, v:v) and the extracts were measured by SEC (Figure 3.7) and FT-IR
spectroscopy (Figure 3.8). In both cases oligomers as well as polymers were
detected. The SEC diagrams show next to the low molecular peaks
(< 2,000 g ∙ mol-1) a broad peak in the range of about 40,000 g ∙ mol-1. The high
molecular peak cannot be assigned to PET; as high molecular PET is insoluble in the
used solvent mixture. The SEC peak at high molecular masses should be caused by
the presence of foreign polymers. Further analysis by IR spectrometry is required for
the identification of the polymeric contaminants.
Chapter 3
49
1000 10000 100000
0
1
2
3
4
5
6
7
w(log M
)
M / g mol-1
Figure 3.7. SEC diagram of an extract of w-PET 1 in a mixture of chloroform and
methanol (80:20, v:v). The broad peak at a molar mass of about 3 x 105 g ∙ mol-1
shows that polymers are present.
Various PET bottles and packaging materials were analyzed further by IR and
RAMAN spectroscopy. Beside the PET oligomers also polyamide (PA) and
polycarbonate (PC) were detected by FT-IR spectroscopy (cf. Supporting Information
for further IR data in Figures S3.1-S3.3, Table S3.2).
In the following Table, the characteristic bands of PET, PC, and PA and their
wavenumbers are given (Table 3.3).
Chapter 3
50
Table 3.3. IR signals of poly(ethylene terephthalate) (PET), polycarbonate (PC), and
polyamide (PA) with allocation to vibrations.
Kind of vibration Wavenumber / cm-1 Polymer
(CH2) 2925 PET
(Car(O)C=O) 1774 PC
(C=O) 1727 PET
(CO-NH), Amide I 1641 PA
(N-H), Amide II 1538 PA
(CH2) 1409 PET + PC
(CH2) 1369 PC
(C(O)-O-C) 1266 PET+PC
(C(O)-O-C) 1193 PC
(C(O)-O-C) 1163 PC
(C(O)-O-C) 1099 PET
(Car-H)ip 1016 PET + PC
(Car-H)oop 831 PC
(Car-H)oop 730 PET
4000 3500 3000 2500 2000 1500 1000 500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1410cm-1
(CH2)
1024cm-1
ip(C
ar-H)
730cm-1
oop
(Car-H)
1097cm-1
(C(O)-O-C)
1254cm-1
(C(O)-O-C)1722cm-1
(C=O)
2980-2934cm-1
(CH2)
ab
so
rba
nce
/ cm-1
1638cm-1
(CO-NH)
1520cm-1
(N-H)
Figure 3.8. FT-IR spectrum of an extract of w-PET 1 in a mixture of chloroform and
methanol (80:20, v:v). Gray are the signals of PET oligomers and black are the
Chapter 3
51
polyamide signals. The inset shows a PET bottle with schematic cross section
through the wall of the bottle containing a polyamide barrier layer [29].
In particular, polyamide is known as a polymer with good gas barrier properties.
Poly(m-xylene adipamide) (PA MXD 6) is a widely used additive which reduces the
gas permeability in polymers (Figure 3.9) [31, 32]. The IR spectrum and data of
Nylon MXD 6 are given in the Supporting Information (Figure S3.1, Table S3.2). It is
also well-known that polyamides may yellow during thermal treatment due to
azomethine formation (Scheme 3.1) [38].
Accelerated thermal aging tests (so-called roasting tests) of different polyamides
(PA 6 and PA MXD 6) show also yellow discoloration during thermal treatment in air
and nitrogen atmosphere (cf. Figure 3.10).
(a) PA 6
(b) PA MXD 6
Figure 3.9. Chemical structure of (a) PA 6 and (b) PA MXD 6.
Chapter 3
52
Scheme 3.1. Azomethine formation during heating of polyamides as published by
Karstens and Rossbach [33].
Figure 3.10. Roasting of PA 6 (a) and PA MXD 6 (b) under different conditions
(1: untreated; 2: heated in presence of air (220 °C, 40 min); 3: heated in nitrogen
atmosphere (220 °C, 40 min)).
Chapter 3
53
Their characteristic fluorescence can be used for confirmation of azomethine
structures (excitation at 350 nm, emission at > 405 nm, typically at 405-425 nm)
(Figure 3.11) [33].
Figure 3.11. Emission spectra of untreated and thermally treated PA 6 (220 °C,
40 min in air or in nitrogen atmosphere) (excitation: 350 nm, measured in a solid
sample holder).
Different fractions of recycled PET were subjected to the roasting test and analyzed
by fluorescence spectroscopy. Solutions of the PET fractions in HFIP were prepared
and their fluorescence spectra were measured (Figure 3.12). Poly(ethylene
terephthalate) exhibits intrinsic fluorescence; the fluorescence characteristics depend
on the sample preparation (solid or in solution) and on the concentration [34]. The
solvent HFIP shows no significant fluorescence. The clear parts of the recyclate show
only weak fluorescence in the range of 390-480 nm. After roasting, the fluorescence
of the PET increases, this is most pronounced for the “yellow” parts of the recycled
PET which is markedly yellowed during repeated exposure to heat. The characteristic
emission spectrum with a maximum at 447 nm (after excitation at 350 nm) indicates
Chapter 3
54
that parts of the recycled PET contain PA barrier layers (Figure 3.12). The clear parts
of the recycled PET are not subjected to yellowing during roasting; the corresponding
emission spectrum shows only a broad emission in the range of 370 – 540 nm which
should be due to the intrinsic fluorescence of PET (Figure 3.12). These results show
that the proof of the presence of polyamides in PET recycled materials by
fluorescence spectroscopy after roasting is hampered by the intrinsic fluorescence of
poly(ethylene terephthalate).
Figure 3.12. Emission spectra of HFIP solutions (0.5 or 1 %, m/v) of different
fractions of PET recyclate w-PET 1 before and after roasting (220 °C, 40 min, in air)
in comparison to the solvent HFIP (excitation: 350 nm).
MALDI-ToF-MS analysis was applied to verify which type of polyamide served as
barrier layer in PET containers or wastes. Figure 3.13 shows the characteristic mass
spectrum of the oligomers obtained by HFIP extraction from a PA MXD 6 reference
sample. The binary copolymer PA MXD 6 consists of two alternating comonomers, A
and B with a mass of 134 g ∙ mol-1 for comonomer A and 112 g ∙ mol-1 for
comonomer B, thus resulting in a repeat unit with m/z = 246.7 (cf. Figure 3.13).
Chapter 3
55
Figure 3.13. MALDI-ToF-MS of HFIP-soluble oligomers from a PA MXD 6 reference
sample and general structure of the linear PA MXD 6 molecule.
Two molecular weight distribution series, [mX-A]n and [mX-A]n- OH, each with a
repeating unit of 246.1 Da, were identified in the mass range between 750 and
4000 Da (via Polymerix™ Software from Sierra Analytics, Modesto, CA, USA). About
95 % of the HFIP-extractable PA MXD 6 oligomers were found to be of cyclic nature
(cf. Table 3.4).
Chapter 3
56
Table 3.4. Identified series of PA MXD 6 oligomer species in the HFIP-soluble
fractions of a PA MXD 6 reference sample (MALDI-ToF-MS data processed via
Polymerix™ Software) (cf. Figure 3.13).
Based on this information, the mass spectra of the HFIP soluble oligomers from
various post-consumer PET samples were checked for the presence of PA MXD 6
constituents. The processed mass spectrum of HFIP dissolved oligomers is shown in
Figure 3.14 for the example of a post-consumer PET material (PET juice bottle 2).
Processing of the MALDI-ToF-MS data via Polymerix™ Software revealed a total of
four molecular weight distribution series, each with the ethylene terephthalate
repeating unit of 192.04 Da; traces of PA MXD 6 ions were also detected. For brief
denomination of the ethylene terephthalate oligomers, the nomenclature of
Weidner et al. was adopted with slight modifications [35]. Cyclic and linear PET
oligomer series were identified: S1= [GT] n, S2 = H-[GT] n - OH, S3 = H-[GT] n - GH,
S4 = H-[GGT]1- [GT] n -1- GH. For PA MXD 6 single cyclic oligomers (S6 = [mX-A] n)
were detected (Table 3.5, Figure S3.4, Table S3.3).
These findings are in wide agreement with those of Weidner et al. for technical
poly(ethylene terephthalate) materials [35, 36]. The only exemption here is the lack in
cyclic [GGT]1-[GT]n oligomers, which were detected in small amounts by Weidner
and assigned to stem from diethylene glycol impurities in the production process [35].
In addition to the identified series, there are signal clusters which cannot be assigned
to defined oligomer series. Mass differences between the signals indicate loss of
CO2, CHO, C6H8O6 and water; these signals most likely reflect the loss of PET
oligomers’ molecular integrity as a consequence of strain during recycling.
Series Type Structure
Percent
Series
Percent
Spectrum PD
Mn
[M+Na]+ Label
Total/
100 40.83 1.257 1321.916 Average
S1 linear H-[mX-A]n- OH 4.27 1.74 1.251 1730.326
S2 cyclic [mX-A] n 95.73 39.09 1.276 1303.698
Chapter 3
57
Table 3.5. PET and PA MXD 6 oligomers species in the HFIP-soluble fractions of
post-consumer PET material (PET juice bottle 2) (MALDI-ToF-MS data processed via
Polymerix™ Software).
Series
Label Type Structure
Percent
Series
Percent
Spectrum
PD
Mn
[M+Na]+
Total /
Average
100 17.35 1.151 1166.011
S1 cyclic [GT] n 51.83 8.99 1.233 1064.355
S2 linear H-[GT] n - OH 13.33 2.31 1.204 1306.607
S3 linear H-[GT] n - GH 10.72 1.86 1.336 1462.814
S4 linear H-[GGT]1- [GT] n -1- GH 12.47 2.16 1.254 1146.154
S6 cyclic [mX-A] n 11.65 2.02 1.151 1205.502
Chapter 3
58
Figure 3.14. Series of polyester oligomers and PA MXD 6 traces dissolved from a
post-consumer PET material (PET juice bottle 2) with HFIP; S1: [GT]n,
S2: H-[GT]n - OH, S3: H-[GT]n – GH, S4: H-[GGT]1- [GT]n-1 - GH; S6: [mX-A] n
(MALDI-ToF-MS data processed via Polymerix™ Software).
The overview in Table 3.6 illustrates that cyclic [mX-A]n oligomers with up to n = 4
were unambiguously verifiable in the yellow fraction of the PET recyclate w-PET 1; in
the clear, colorless fraction of the same recyclate, however, no [mX-A]n oligomers
were detected. No PA MXD 6 oligomers were detected in the HFIP/methanol-soluble
fraction of PET juice bottle 1, whereas in the PET juice bottle 2 cyclic [mX-A]n
oligomers with up to n = 5 were clearly proven. This finding corresponds to the
marked yellowing of this recyclate after roasting.
Homopolymer Assignments
S1 R5
983.2S1 R4
791.2
S1 R6
1175.2
S4 R2
673.2
S6 R3
761.4S1 R7
1367.3S2 R4
809.2 S6 R4
1007.5
S4 R4
1057.3S2 R7
1385.3S3 R4
853.2
S4 R7
1633.4
S3 R7
1429.3
S6 R5
1253.7
S3 R5
1045.2
S2 R5
1001.2
S1 R8
1559.3
S3 R6
1237.3
S3 R10
2005.4
S3 R3
661.2
S3 R9
1813.4
S4 R5
1249.3
S4 R3
865.2
S2 R6
1193.3
S2 R8
1577.3
S6 R6
1499.8
S4 R6
1441.3
S1 R10
1943.4
S2 R3
617.1
S3 R8
1621.4
S2 R9
1769.4
S4 R8
1825.4
S6 R7
1745.9
S1 R9
1751.4
S2 R10
1961.4
S6 R8
1992.1
S1 R11
2135.5
S4 R9
2017.5
684.9
1038.8
895.3
1219.9
1898.0
800.0 1000.0 1200.0 1400.0 1600.0 1800.0 2000.0
0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
9000.0
Chapter 3
59
Table 3.6. PA MXD 6 oligomers in the HFIP-soluble fraction of post-consumer PET
materials (identified by MALDI-ToF-MS).
PA MXD 6 Structure w-PET 1
colorless
fraction
w-PET 1
yellow
fraction
PET juice
bottle 1
PET juice
bottle 2
m/z
(Na+ ions)
678.427 n. i. + - + -
686.837 n. i. - + - +
762.971 (mX-A)3 - + - +
1009.657 (mX-A)4 - +/- - +
1256.323 (mX-A)5 - - - +
1502.981 (mX-A)6 - - - -
1749.634 (mX-A)7 - - - -
1996.299 (mX-A)8 - - - -
+ unambiguous proof, - signal is missing, +/ -weak signal, no clear proof, n. i. not
identified.
In summary, aromatic polyamides such as PA MXD 6 are used in PET to improve the
gas barrier properties; they may cause yellow discoloration of PET after repeated
thermal treatment, e.g. during recycling.
3.4 Conclusions
The studies showed that the main influencing factor for the graying of post-consumer
poly(ethylene terephthalate) during reprocessing is the reduction of antimony ions to
elementary antimony as proven by XP spectroscopy. The presence of higher
concentrations of the Sb2O3 catalyst (250 – 300 ppm) in the PET material leads to
increased graying. Increased reduction of antimony oxide is achieved in the solid-
state post-condensation procedure (SSP) during reprocessing of PET. Further
contaminations like foreign polymers or black specs can contribute to the graying of
PET materials, too. In this study, no indication for the presence of black specs in the
investigated post-consumer PET materials was obtained.
Chapter 3
60
During reprocessing of some post-consumer PET materials more or less pronounced
yellowing occurred. Yellowing due to thermo-oxidation of poly(ethylene terephthalate)
should not occur during extrusion in vacuo or in nitrogen atmosphere. As main
reason for marked yellowing, the presence of polyamide-based barrier layers
stemming from PET waste materials, in particular juice bottles has been depicted.
Polyamides are prone to yellowing during exposure to higher temperatures which can
be traced back to the formation of azomethine structures. Furthermore, the presence
of polyamides and PET oligomers in some post-consumer PET flakes was proved by
MALDI-ToF-MS analysis of HFIP/methanol-soluble fractions of PET recyclates.
In conclusion, to prevent discoloration of post-consumer PET during reprocessing
different methods like oxidation of gray metallic antimony or sorting out of polyamide
contaminants are needed.
3.5 References
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[4] R. W. Stevenson, Journal of Polymer Science Part A-1: Polymer Chemistry 1969,
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[5] S. M. Aharoni, Polymer Engineering and Science 1998, 38, 1039.
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Chapter 3
61
[12] B. Duh, Polymer 2002, 43, 3147.
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[20] M. Paci, F. P. La Mantia, Polymer Degradation and Stability 1999, 63, 11.
[21] T. Rieckmann, K. Besse, F. Frei, S. Volker, Macromolecular Symposia 2013,
333, 162.
[22] B. G. Ranby, J. F. Rabek, "Photodegradation, photo-oxidation, and
photostabilization of polymers", New York, Wiley, 1975.
[23] M. Edge, N. Allen, R. Wiles, W. McDonald, S. Mortlock, Polymer 1995, 36, 227.
[24] F. P. La Mantia, L. Botta, M. Morreale, R. Scaffaro, Polymer Degradation and
Stability 2012.
[25] R. Pfaendner, H. Herbst, K. Hoffmann, Macromolecular Symposia 1998, 135, 97.
[26] M. Witschas, Personal communication 2012.
[27] D. Briggs, M. P. Seah, "Practical Surface Analysis, Auger and X-ray
Photoelectron Spectroscopy", John Wiley & Sons, Chichester, Great Britain, 1990.
Chapter 3
62
[28] J. Moulder, W. Stickle, P. Sobol, K. Bomben, "Handbook of X-ray Photoelectron
Spectroscopy ", Perkin Elmer Corporation, Phys. Electr. Division, Eden Prairie, USA,
1992.
[29] D. Berg, "Vergrauung von Polyethylenterephthalat-Rezyklat – Ursachen und
Versuche zur Aufhellung", Masterthesis, DWI an der RWTH Aachen e.V. and
Fachhochschule Aachen, Jülich, 2013.
[30] U. K. Thiele, International Journal of Polymeric Materials 2001, 50, 387.
[31] Y. S. Hu, V. Prattipati, S. Mehta, D. A. Schiraldi, A. Hiltner, E. Baer, Polymer
2005, 46, 2685.
[32] C. Thellen, S. Schirmer, J. A. Ratto, B. Finnigan, D. Schmidt, Journal of
Membrane Science 2009, 340, 45.
[33] T. Karstens, V. Rossbach, Die Makromolekulare Chemie 1990, 191, 757.
[34] N. S. Allen, J. F. Mckellar, Makromolekulare Chemie - Macromolecular
Chemistry and Physics 1978, 179, 523.
[35] S. Weidner, G. Kuhn, U. Just, Rapid communications in mass spectrometry :
RCM 1995, 9, 697.
[36] S. Weidner, G. Kuhn, J. Friedrich, H. Schroder, Rapid Communications in Mass
Spectrometry 1996, 10, 40.
Chapter 3
63
3.6 Supporting Information
3.6.1 On the Graying of Poly(ethylene terephthalate) during Reprocessing
Table S3.1. Antimony content (with standard deviation s obtained for duplicates) and
color values (according to CIE-L*a*b*) of some virgin poly(ethylene terephthalate)
(v-PET) as well as waste (w-PET) and reprocessed PET samples (r-PET).
Sample PET state
Sb-content/
ppm
s (Sb-content)/
ppm L* a* b*
v-PET 1 virgin 123.1 10.0 89.6 -0.2 -0.9
v-PET 2 virgin 7.0 1.1 88.5 -1.1 2.4
v-PET 3 virgin 167.3 11.5 87.3 -0.5 0.5
w-PET 1 waste fraction 204.7 1.3 87.6 -1.0 2.5
w-PET 2 waste fraction 216.7 36.8 85.2 -1.5 1.6
r-PET 1 reprocessed 110.6 4.4 89.7 -0.4 1.8
r-PET 2 reprocessed 133.8 12.3 79.1 -2.2 -1.0
r-PET 3 reprocessed 225.9 4.8 76.0 -0.8 2.0
r-PET 4 reprocessed 244.8 24.0 73.3 -1.8 0.6
r-PET 5 reprocessed 265.8 18.2 70.8 -3.1 0.5
Remark: L* = lightness, L*=0 indicates black, L*=100 white. The a*-value
corresponds to the green-red axis, where negative a*-values document green and
positive a*-values red hues. The b*-value represents the blue-yellow axis, where
negative b*-values document blue and positive b*-values yellow hues.
3.6.2 Detection of Polyamides in Post-Consumer Poly(ethylene
terephthalate)
In the first step, the presence of polyamide-based barrier layers in PET packaging
materials was analyzed by IR spectroscopy. Nylon MXD 6 which is used as additive
for barrier layers in PET packaging materials like juice bottles was analyzed by IR
spectroscopy as reference (Figure S3.1, Table S3.2). IR signals which can be
attributed to polyamides were detected in some juice bottles e.g. in the PET bottle
VP-BLO-1 (Figure S3.2 and S3.3).
Chapter 3
64
4000 3500 3000 2500 2000 1500 1000
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
702
790
912
1031
1092
1145
1255
1351
1422
1492
1537
1638
2867
3071 2930
a
bso
rban
ce
/ cm-1
Nylon MXD 6
3283
Figure S3.1. IR spectrum of Nylon MXD 6 – measured in the ATR technique on a
silicon crystal.
Chapter 3
65
Table S3.2. IR signals of Nylon MXD 6 (cf. Figure S3.1).
IR signal/ cm-1 Allocation to vibrations
3283 (N-H), Amide A
3071 overtone of Amide II
2930 as(CH2)
2867 s(CH2)
1638 (C=O), Amide I
1537 (C-N) and (C-N-H), Amide II
1492 (C=C)ar
1422 (CH2)
1351 (CH2)
1255 (N-H) + (OCN), Amide III
1145 (Car-H)ip
1092 (C-C) or (Car-H)ip
1031 (C-C) or (Car-H)ip
912
790
(Car-H)oop
(Car-H)oop
702 (N-H)
Chapter 3
66
4000 3500 3000 2500 2000 1500 1000
0.0
0.2
0.4
0.6
0.8
727
874
1019
1101
1246
1340
1409
1539
1641
1717
abso
rban
ce
/ cm-1
VS-Blo-2
3294
Figure S3.2. IR spectrum of the PET juice bottle VP-BOL-2 – here: the inner wall of
the bottle was measured in the ATR technique on a germanium crystal.
Chapter 3
67
1660 1640 1620 1600 1580 1560 1540
0.010
0.015
0.020
0.025
0.030
0.035
1539
a
bso
rban
ce
/ cm-1
VS-Blo-2
1641
Figure S3.3. Detailed IR spectrum of the PET juice bottle VP-BOL-2 depicting an
enlarged spectrum with the Amide I band (at 1641 cm-1) and the Amide II band (at
1539 cm-1) (cf. Figure 3.2).
The proof of the presence of polyamide-based barrier layers in PET packaging
materials by IR spectroscopy is hampered by the low layer thickness which results in
weak polyamide signals which overlap with the strong poly(ethylene terephthalate)
signals.
Chapter 3
68
3.6.3 MALDI-ToF-MS Analysis of Poly(ethylene terephthalate) Oligomers
Figure S3.4. PET oligomer species in the HFIP-soluble fractions of v-PET 1 (MALDI-
ToF-MS data processed via Polymerix™ Software).
Table S3.3. PET oligomer species in the HFIP-soluble fractions of v-PET 1 (MALDI-
ToF-MS data processed via Polymerix™ Software).
Series
Label
Type Structure Percent Series Percent Spectrum PD Mn
[M+Na]+
Total/
Average
100 10.8 1.257 1436.358
S1 cyclic [GT]n 30.82 3.33 1.251 1573.524
S2 linear H-[GT]n - OH 22.22 2.4 1.276 1291.227
S3 linear H-[GT]n - GH 21.72 2.34 1.251 1370.261
S4 cyclic [GT]n - G 25.24 2.73 1.252 1453.472
Chapter 4
69
Zinc Peroxide Particles as Bleaching Agents to Improve the Color of Post-Consumer Poly(ethylene terephthalate)*
Summary: Products from reprocessed post-consumer poly(ethylene
terephthalate) (PET) show often gray or yellow discoloration. For some
applications like white or pastel shade textiles or papers, an improvement of
their color is required. In this chapter, small zinc peroxide particles are
developed as bleaching agents for post-consumer PET. Here, the production
of ZnO2 micro- and nano-particles by grinding of commercially available
microscopic ZnO2 is described (top-down procedure). The zinc peroxide
particles are characterized by transmission electron microscopy, and
thermogravimetric analysis (TGA). The oxygen release from the zinc
peroxides is determined by TGA. The ZnO2 particles are applied to the PET
melt during extrusion on the laboratory scale. The produced compounds or
fibers are characterized by microscopy, by the evaluation of their color and
viscosity, by size exclusion chromatography, and by rheology. The results
show that bleaching of discolored PET materials can be achieved, when small
amounts of nano-scaled ZnO2 particles (0.1 - 0.2 wt%) are applied to the melt
during extrusion.
Keywords: Post-consumer poly(ethylene terephthalate), extrusion,
discoloration, bleaching, zinc peroxide particles
Chapter 4
70
4.1 Introduction
In the previous chapters, some reasons for the gray and yellow discoloration of post-
consumer poly(ethylene terephthalate) during reprocessing are described. This
chapter focuses on the improvement of the color of gray reprocessed poly(ethylene
terephthalate) (r-PET) with the help of inorganic bleaching agents.
As forementioned, the gray discoloration of recycled PET may have different sources.
Several authors describe the occurrence of gray discoloration of recycled PET upon
repeated heating [1-5]. Primarily, the gray discoloration of reprocessed PET is caused
by the presence of catalysts used for the polycondensation process during the
synthesis. In the synthesis of poly(ethylene terephthalate) a variety of catalysts are
used, however, more than 90 % of PET is produced with the aid of antimony
compounds [6-8]. Thus, the main problem of gray reprocessed poly(ethylene
terephthalate) (r-PET) is caused by the antimony catalysts. Antimony compounds like
antimony trioxide (Sb2O3), antimony (III) acetate (Sb[OOC-CH3]3), and antimony (III)
glycolate (Sb(C2H4O2)3) are the common catalysts [1, 9-13].
High temperatures and long reaction times which occur during repeated thermal
processes while recycling of PET as e.g. the solid state polycondensation process
(SSP) or in extrusion processes may lead to reduction of antimony (III) compounds
(e.g. Sb2O3) to elementary antimony (Sb0) [1, 4, 14, 15]. The graying effect could be
increased by the presence of additives like phosphite-based light stabilizers or flame
retardants due to their reducing capacity [13]. Furthermore, reasons for the
discoloration of reprocessed PET during thermal treatment could be the presence of
foreign polymers such as poly(vinyl chloride) (PVC) or organic dyestuffs. Moreover,
pigments or carbon particles which are used as IR-absorbers or the occurrence of
black specs (i.e., degraded polymer residues attached to the walls of the equipment)
are further reasons for the gray discoloration of recycled PET [2, 3, 5]. For fiber
production, for example, the problem of gray discoloration of PET has to be solved to
improve its quality.
For fiber applications of post-consumer PET for the fields of textiles or paper white
color is needed. To improve the color of PET recyclates oxidative treatment or
bleaching will be studied in this chapter. For oxidative bleaching of fibers, peroxides
are of use, the most common bleaching agent is hydrogen peroxide. The aim of this
research is to improve the color of fibers from r-PET by the addition of bleaching
Chapter 4
71
agents during melt extrusion. For this process, aqueous solutions of hydrogen
peroxide cannot be used as polyesters are prone to hydrolysis even if trace amounts
of water are present [11]. Organic peroxides like dibenzoyl peroxide are of use in
polymer extrusion processes as polymerization initiators, for molecular weight
adjustment after polymerization (as visbreaking agents), for curing of thermosetting
resins, for crosslinking of elastomers and polyethylene or as reactor additives, e.g. as
antifouling agents [16-18].
Organic peroxides are used as initiators in the polymerization of poly(vinyl chloride),
low density polyethylene, acrylic, styrene, and other thermoplastic polymers. Various
classes of organic peroxides are available, like peroxy(di)carbonates, diacyl
peroxides, peroxy esters, hydroperoxides etc. It is common knowledge that organic
peroxides are thermally unstable compounds; some of them decompose yet at low
temperatures, others decompose with thermal explosion. Thus, great care is required
to work with organic peroxides at higher temperatures. Here, peroxides were to be
applied at higher temperatures (280-300 °C) to the melt of poly(ethylene
terephthalate) during extrusion. No organic peroxides which can be applied at this
temperature to the PET melt without decomposition were available. Thus, inorganic
peroxides were evaluated for their potential as bleaching agents for PET recyclates.
Inorganic peroxides can be applied to the polymer melt as solids or compounded in a
masterbatch. Titanium dioxide is applied during spinning of poly(ethylene
terephthalate) fibers as delustering agent. The application of titanium peroxide to the
polymer melt should lead to the formation of oxygen active agents and titanium
dioxide which can remain in principal in the fiber. However, titanium peroxide is an
unstable compound which has to be synthesized at temperatures of about 0 °C. The
resulting product has to be stored at temperatures below 10 °C over P4O10, and
former studies showed that even at these conditions the titanium peroxide
continuously decomposed with peroxy oxygen release [19]. Zinc peroxide is, in
contrast, very stable at higher temperatures and easy to handle. The addition of zinc
peroxide to polymer melts during extrusion processes is not yet known. In the patent
of Ohno et al., zinc peroxide is used in amounts up to 30 % to expand resins or
thermoplastics to foams for different applications [20, 21]. For these processes,
kneaders, extruders or injection molders are used. Further applications of zinc
peroxide in polymers are the addition of peroxides during vulcanization processes in
Chapter 4
72
rubber mixtures (e.g. carboxylated nitrile rubber) to increase their mechanical
properties [22-24].
A disadvantage of inorganic peroxides like titanium or zinc peroxide is their yellow
color which can lead to yellow discoloration of the polymer. The yellow discoloration
of titanium or zinc peroxide is thought to be due to the presence of active oxygen
species or peroxide complexes on the surface of the peroxides [25-27].
Commercial and macroscopic zinc peroxide is normally used for these applications.
To achieve a good dispersibility and a lower shear force by the same activity of zinc
peroxide the particles were ground in a cryomill. The macroscopic zinc peroxide and
ground zinc peroxide particles were used as bleaching agents for recycled gray PET
materials in the melt. Different publications revealed that zinc peroxide decomposes
at high temperatures (between 190 and 250 °C) into zinc oxide and oxygen [28-30].
The formed oxygen should bleach the black contaminants in r-PET, and mainly,
oxidize the black antimony residues (Sb0) which results in bleaching of grayish PET
recycled materials. In addition, the formed zinc oxide (ZnO) can act as a white
pigment resulting in an additional brightening of PET (Scheme 4.1).
Scheme 4.1. Bleaching of grayish r-PET by oxidation of the antimony catalyst
residues with zinc peroxide at higher temperatures.
Chapter 4
73
4.2 Experimental Section
4.2.1 Materials
Different PET materials were used for bleaching experiments; reprocessed PET
(r-PET) which has been used for bottle production was obtained from the market.
Bottles produced from reprocessed PET were used for caffeine-containing soft drinks
were used for the bleaching experiments.
A blend of zinc peroxide (50-60 wt%) and zinc oxide (40-50 wt%), o-cresol, and
2-bromobenzoic acid were received from Sigma Aldrich, Taufkirchen, Germany.
Chloroform was obtained from VWR, Darmstadt, Germany, and bromophenol blue
from Merck, Darmstadt, Germany. Ethanolic potassium hydroxide solution was
purchased from Fluka, Taufkirchen, Germany, and 1,1,1,3,3,3-hexafluoropropane-2-
ol (HFIP) was bought from Fluorochem, Hadfield, Great Britain.
4.2.2 Sample Preparation
The labels and caps of the PET bottles were removed. The bottles were washed with
warm tap water and, then, with distilled water. The glue of the labels was removed
with acetone. The PET bottles were cut into small pieces with the help of a pair of
scissors, and then minced in a laboratory mixer (Waring Commercial Blendor Mixer
with metal beaker, 220 V, Snijders Scientific BV, Tilburg, The Netherlands). Several
mixing steps with two minutes mixing at highest frequency and in between cooling for
one minute in an ice bath were needed for optimum comminution of the PET
materials.
4.2.3 Grinding of Commercial Zinc Peroxide
Zinc peroxide (Sigma Aldrich) was ground at different periods of time (10, 20, 30, and
40 minutes) to achieve smaller particles. Due to the fact that during the grinding
process heat could decompose the zinc peroxide to zinc oxide and oxygen, a cryomill
(6800 Freezer/Mill, SPEX CertiPrep, Stanmore, Great Britain) was used to cool the
material down with liquid nitrogen so that the oxygen release remained constant. The
grinding was performed at a frequency of 20 Hz, with pre-cooling for 4 min. Two to
eight grinding cycles for 5 min at 20 Hz were performed with in between cooling for
5 min.
Chapter 4
74
4.2.4 Characterization of Zinc Peroxide Particles
The oxygen release amounts were determined via TGA measurements on a Thermo-
Microscale TG 209c from Netzsch-Geraetebau GmbH, Selb, Germany. The samples
were dried for 30 min at 100 °C followed by heating up to 700 °C (heating rate:
8 K ∙ min-1).
The total oxygen content of zinc peroxide was determined by iodometry. For this, a
defined amount of potassium iodide was added to a dispersion of zinc peroxide in
dist. water, the solution was acidified with a 9 molar sulfuric acid solution and the
amount of non-reacted iodide was titrated with a 0.1 molar sodium thiosulfate
solution.
Particle sizes and particle size distributions were determined by TEM imaging on a
Libra 120 from Carl Zeiss AG, Oberkochen, Germany. About 100 separated particles
were measured for each sample followed by standard deviation calculation.
4.2.5 Extrusion
First, PET was dried at 130 °C overnight at least for ten hours. The peroxide
treatment was performed in the PET melt under nitrogen atmosphere. For this, the
Micro 15cc Twin Screw-Extruder (DSM, Geleen, NL) was used (Figure 4.1). About
13 g gray PET were molten in the extruder at 290 °C and mixed with different
amounts of different peroxides for two minutes with 100 rounds per minute (rpm)
screw rotation speed. The DSM Xplore Data Acquisition and Control v1.11 Software
was used to measure the screw force. After the extrusion, the polymers were ground
in a cryomill (6800 Freezer/Mill, SPEX CertiPrep, Stanmore, Great Britain) to achieve
good homogeneity and reproducibility. Three grinding cycles each for 5 min were
performed in the cryomill with an impact frequency of 10 s-1 and the samples were
cooled with liquid nitrogen for 5 min in between the grinding cycles.
Chapter 4
75
Figure 4.1. Micro 15cc Twin Screw-Extruder (DSM, Geleen/NL) (Left: Complete
device, middle: Opened chamber including the screws, right: Scheme of the closed
chamber).
4.2.6 Characterization of PET Materials
The color values were measured fivefold for each sample with the aid of the
Datacolor Spectraflash SF600 plus CT UV colorimeter (Datacolor, Marl, Germany)
using the D65 illuminant and the 10° observer. A special specimen container
(Datacolor) was used. The color values were calculated with the Datacolor formula
based on the CIE-L*a*b* system. In this system, L* represents the lightness (L*=0
indicates black, L*=100 white). The a*-value corresponds to the green-red axis,
where negative a*-values document green and positive a*-values red hues. The
b*-value represents the blue-yellow axis, where negative b*-values document blue
and positive b*-values yellow hues.
The inherent viscosity (ηinh.) of the polymers was measured with the help of an
Ubbelohde viscosimeter (type 0a) (Schott AG, Mainz, Germany). About 0.3300 g
PET was weighed in a 25 mL graduated flask and dissolved in
1,1,1,3,3,3-hexafluoropropane-2-ol (HFIP) (Fluorochem, Hadfield, Great Britain). The
viscosity of this solution was measured at 25 °C. The inherent viscosity was
calculated according to equation (1).
Chapter 4
76
𝜂𝑖𝑛ℎ. =ln(𝜂𝑟𝑒𝑙)
𝛽=
ln(𝜂
𝜂0)
𝛽=
ln(𝑡
𝑡0)
𝛽 (1)
Here, ηinh. is the inherent viscosity, ηrel the relative viscosity, β the mass
concentration, η the viscosity of the PET solution, η0 the viscosity of the solvent, t the
flow time of the PET solution and t0 the flow time of the solvent.
Molecular weights (Mn¯¯ and Mw¯¯) and molecular weight distribution (Ð) of PET were
determined by size exclusion chromatography (SEC). The molecular weight
distribution (Ð) is calculated as follows (2):
Ð =𝑀𝑤̅̅ ̅̅ ̅
𝑀𝑛̅̅ ̅̅̅ (2)
According to Weisskopf, PET was dissolved in HFIP and diluted with chloroform to a
volume concentration of chloroform/HFIP 98:2 vol% [31]. A HPLC pump (PU-
2080plus, Jasco, Groß-Umstadt, Germany) equipped with an evaporative light
scattering detector (PL-ELS-1000, Polymer Laboratories, Amherst, USA) was used.
2,6-Di-tert-butyl-4-methylphenol (c = 250 mg ∙ mL-1) was used as internal standard
and narrow distributed polystyrene standards (PSS Polymer Standards Service
GmbH, Mainz, Germany) were used to achieve calibration. One pre-column (8 mm x
50 mm) and four SDplus gel columns (8 mm x 300 mm, MZ Analysentechnik, Mainz,
Germany) were applied at a flow rate of 1.0 mL ∙ min-1 at 20 °C. The separation
process took place on polystyrene/divinylbenzene columns (50 Å, 100 Å, 1,000 Å,
and 10,000 Å; PSS Polymer Standards Service GmbH, Mainz, Germany). Results
were evaluated using the PSS WinGPC UniChrom software (Version 8.1.1).
The measurements of the rheology were performed with a plate-plate rheometer
(Discovery HR-3 hybrid rheometer, TA Instruments-Waters L.L.C., New Castle,
USA). The polymer was molten at 290 °C and the melt was measured with a gap of
600 μm in a frequency range of 0.1 Hz to 100 Hz with an oscillation of 2 %. The
storage modulus (G´), the loss modulus (G´´), and the complex viscosity (η*) were
calculated.
To determine the carboxyl end groups of PET, titrations with ethanolic potassium
hydroxide (KOH) solution of a concentration of 0.05 mol ∙ L-1 using bromophenol blue
as indicator were performed following ASTM D 7409 - 07ε1 [32]. The titer (t) of the KOH
Chapter 4
77
standard solution was determined with 2-bromobenzoic acid. Approximately 0.8 –
1.5 g PET were dissolved in 20.0 g o-cresol at 80 °C, quenched with chloroform and
titrated against potassium hydroxide standard solution. As blank, 20.0 g o-cresol
mixed with chloroform was also titrated. In each case, triple determinations were
performed. The COOH concentration (in mmol ∙ kg-1) was calculated as follows (3):
𝑐(𝐶𝑂𝑂𝐻) = [𝑉(𝐾𝑂𝐻)−𝑉0(𝐾𝑂𝐻)]∙𝑐(𝐾𝑂𝐻)∙𝑡∙10
3
𝑚(𝑃𝐸𝑇) (3)
The thermal properties of the PET samples were measured by differential scanning
calorimetry using the Netzsch DSC 204 (NETZSCH-Geraetebau GmbH, Selb,
Germany). Between two and ten milligrams of PET were weighed in aluminum pans
before the measurement. The samples were heated to 350 °C with a heating rate of
10 K ∙ min-1 under nitrogen flow. After an isothermal step at 350 °C for two minutes
the sample was cooled down to 20 °C with 10 K ∙ min-1. Then, an isothermal step for
two minutes and heating with 10 K ∙ min-1 to 350 °C was carried on. Subsequently,
the crystallization temperature (Tc) was calculated at the maximum of the exothermic
peak in the cooling curve.
Microscopic measurements of microtome sections of ZnO2 treated poly(ethylene
terephthalate) were performed. First, PET filaments (produced in extrusion
experiments) were embedded in an acrylate resin (Technovit® 7100, based on
HEMA; Heraeus Kulzer GmbH, Weinheim, Germany) for the microtome sectioning.
The microtomy was performed with the help of a Reichert-Jung Supercut 2050
rotational microtome (Leica Microsystems GmbH, Wetzlar, Germany). The obtained
cross sections (thickness = 20 µm) were embedded in an immersion oil on a slide.
The optical microscope Axioplan 2 imaging (Carl Zeiss AG, Oberkochen, Germany)
equipped with a halogen lamp (Hal 100) was used to produce overview images to
evaluate the distribution of the particles in the PET melt.
Electron microscopical analyses (FESEM) were made with a Hitachi SU9000 UHR
FESEM Field-Emission Scanning Electron Microscope (FESEM) (Hitachi, Tokyo,
Japan). The samples were placed on a sample holder with a carbonized adhesive
tape and sputtered with a thin layer of gold to prevent charging effects during
imaging. After sputtering, the samples were put in the microscope. As detector, the
Chapter 4
78
energy-dispersive X-ray spectrometer (EDX) Oxford X-max 80 mm2 detector (Oxford
Instruments, Abingdon, United Kingdom) was used at 10 kV.
4.3 Results and Discussion
4.3.1 Characterization of the Zinc Peroxide Particles
Commercially available zinc peroxide was ground with the help of a cryomill to
produce nano-scaled or micro-scaled particles. The grinding was performed for a
period of time of 10 – 40 min in cycles of 5 min each followed by cooling for 5 min.
The ground ZnO2 particles were characterized by transmission electron microscopy
(TEM), iodometry and thermogravimetric analysis (TGA) with regard to the thermally
induced oxygen release. Transmission electron microscopic measurements were
performed to investigate the sizes of the ground zinc peroxide particles. The images
in Figure 4.2 show a broad distribution of the particle diameters and the presence of
agglomerates. However, it reveals also that the grinding process was successful. As
depicted in Figure 4.2, the ground ZnO2 particles are comminuted in dependence on
the grinding time. While the unground particles are microscopic clusters, where the
particle sizes cannot be measured by evaluation of the TEM images, the ground
particles have sizes lower than 193.5 ±131.8 nm (10 min ground). After 40 min of
grinding, ZnO2 particles with a size of 60.7 ±51.1 nm was obtained. The obtained
particle sizes are presented in Table 4.1.
Chapter 4
79
Figure 4.2. TEM images of the ZnO2 particles obtained by grinding of commercial
ZnO2 for various periods of time in comparison to the unground ZnO2.
The oxygen content of the zinc peroxides was determined by iodometry; the
unground ZnO2 has an average oxygen content of 9.7 %, whereas the ground ZnO2
particles contain 12.0 % O2. The oxygen release from the ground ZnO2 particles was
determined by TGA as described before. As depicted in Figure 4.3, oxygen release
occurs in a temperature range of 195 – 215 °C resulting in the formation of ZnO;
oxygen losses of 3.1 – 3.5 mmol O2 ∙ g-1 sample were measured (Table 4.1). Here,
no significant difference in the oxygen release obtained for the unground and the
smaller ground ZnO2 particles was determined. The main advantage of the
application of micro-scaled ZnO2 particles is their better dispersion in the polymer
melt and smaller particles result in a lower shear strain during the extrusion process.
The lower shear strain, furthermore, results in a less pronounced impairment of the
polymer and a lower yellowing effect.
Chapter 4
80
Figure 4.3. Temperature induced oxygen release from the ZnO2 particles obtained by
grinding of commercial ZnO2 for various periods of time in comparison to the
unground ZnO2 as determined by TGA.
Table 4.1. Temperature induced oxygen release amounts for unground ZnO2, and for
particles obtained by grinding of commercial ZnO2 in dependence on the applied
grinding time as determined by TGA.
Sample Mass loss O2-release/
wt%
O2-release /
mmol ∙ g-1
Particle sizes /
nm
unground 11.2 3.5 n/a
10 min 10.7 3.3 193.5±131.8
20 min 10.9 3.4 120.3±81.7
30 min 10.6 3.3 101.9±85.3
40 min 10.0 3.1 60.7±51.1
n/a = not available
Chapter 4
81
4.3.2 Bleaching of Post-Consumer Poly(ethylene terephthalate) with Zinc
Peroxide in the Extrusion Process
4.3.2.1 Effects of Zinc Peroxides on Poly(ethylene terephthalate)
The bleaching of post-consumer poly(ethylene terephthalate) in the extrusion process
was performed with zinc peroxide. On the Figures 4.4, 4.5, and 4.6, microscopic
images of cross sections of a PET filament are displayed. Figure 4.4 depicts r-PET
extruded without any additives as reference. A few particles are shown on the
images, but, these particles are due to the presence of black specs in the
reprocessed PET. Black specimen (= black specs) in reprocessed PET are mainly
caused by degraded polymers as mentioned in Chapter 3 and by Scheirs [2, 3].
Furthermore, contaminations with foreign polymers, dyestuff, pigments, or other
impurities are further possible reasons black specs [5].
Figure 4.4. Microscopic images of cross sections of reprocessed PET filaments
extruded without any additive as reference.
In Figure 4.5, further microscopic images of cross sections of PET filaments are
presented. Here, a cross section of reprocessed PET filaments extruded with
0.2 wt% zinc peroxide which was 40 minutes ground in a cryomill. In these images,
more particles are visible compared to PET extruded without addition of ZnO2. This
leads to the conclusion that the detected particles are mainly zinc oxide (ZnO)
particles which were formed from ZnO2 during extrusion. It is shown, that the formed
zinc oxide particles are well distributed in the PET melt during the extrusion process.
However, the particle size distribution is broad (cf. Table 4.1) which shows that larger
particles are present in the cross sections, too.
Chapter 4
82
Figure 4.5. Microscopic images of the cross section of reprocessed PET filaments
extruded with 0.2 wt% zinc peroxide (40 min ground) as additive.
Furthermore, in Figure 4.6 microscopic images of the cross section of a reprocessed
PET filament extruded with 1.0 wt% zinc peroxide is shown. Also here, the zinc
peroxide was ground for 40 minutes in the cryomill. In the cross sections of these
PET filaments, some holes are visible which can be attributed to gas bubbles. This
results from the high amount of oxygen which is formed from ZnO2 during reactive
extrusion at high temperatures. Thus, the addition of high amounts of ZnO2 to PET
during extrusion should be avoided as this leads to marked polymer degradation.
However, small amounts of ZnO2 result in a good bleaching effect without significant
polyester degradation.
Figure 4.6. Microscopic images of the cross section of reprocessed PET filaments
extruded with 1.0 wt% zinc peroxide (40 min ground) as additive.
To get evidence that the particles in the microscopic images originate from the added
zinc peroxide, field-emission scanning electron microscopy (FESEM) with an energy-
dispersive X-ray spectrometer (EDX detector) to evaluate the elements was
performed. Figure 4.7 depicts a FESEM image of the cross section of a reprocessed
Chapter 4
83
PET filament extruded without any additive as reference. Here, a few particles are
visible, too, but EDX measurements reveal that these particles are silica particles
which are ubiquitously present in recycled polymers (Figure 4.8).
Figure 4.7. FESEM image of the cross section of reprocessed PET filaments
extruded without any additive as reference.
Figure 4.8. EDX analysis of a particle in a cross section of reprocessed PET
filaments extruded without any additive as reference (top left: FESEM image, top
Chapter 4
84
right: mapping of the found elements (C, O, Si), bottom left: mapping of the Si Kα1
signal, bottom right: EDX spectrum).
Next, a FESEM image of a cross section of reprocessed PET filaments extruded with
0.2 wt% zinc peroxide which was ground for 40 minutes in a cryomill is presented
(Figure 4.9). In this image, ZnO is present as shown in the EDX images and
spectrum (cf. Figure 4.10). The Zn Lα1,2 signals prove that these particles contain
zinc.
Figure 4.9. FESEM image of the cross section of reprocessed PET filaments
extruded with 0.2 wt% zinc peroxide (40 min ground).
Chapter 4
85
Figure 4.10. EDX analysis of a particle in a cross section of reprocessed PET
filaments extruded with 0.2 wt% zinc peroxide (40 min ground) (top left: FESEM
image, top right: mapping of the found elements (C, O, Zn), bottom left: mapping of
the Zn Lα1,2 signal, bottom right: EDX spectrum of the particle).
Further FESEM analyses were performed with PET extruded with 1.0 wt% ZnO2.
Figure 4.11 depicts that more particles are present in the cross section because of
the higher concentration of zinc peroxide applied in the reactive extrusion process.
Further EDX measurements reveal also that these particles contain mainly Zn from
the formed ZnO (Figure 4.12). The Zn Lα1,2 signals of this sample are also presented
in Figure 4.12.
Chapter 4
86
Figure 4.11. FESEM image of the cross section of reprocessed PET filaments
extruded with 1.0 wt% zinc peroxide (40 min ground).
Figure 4.12. EDX analysis of a particle in a cross section of reprocessed PET
filaments extruded with 1.0 wt% zinc peroxide (40 min ground) (top left: FESEM
image, top right: mapping of the found elements (C, O, Zn), bottom left: mapping of
the Zn Lα1,2 signal, bottom right: EDX spectrum of the particle).
Chapter 4
87
Moreover, color measurements of the extruded PET filaments were performed. The
results show that bleaching has been achieved. As depicted in Figure 4.13, the gray
reprocessed PET (r-PET 1) has an L*-value of 79.1; the grayness increases
(L*= 76.2) due to thermal treatment during extrusion under nitrogen atmosphere
without any additive. After addition of ZnO2 during extrusion, the L*-value of the gray
reprocessed PET increases to 81.4. However, the bleaching of PET with zinc
peroxide leads to yellowing of the polymer, too, as documented by its b*-values. The
b*-value represents the blue or yellow color: -b* represents the distance on the blue
axis and +b* the one on the yellow axis. This means higher b*-values correspond
with more intense yellow color. Figure 4.13 shows the results of the color
measurements of recycled gray PET (untreated) and different PET materials which
were extruded with added zinc peroxide which was ground for 40 min. The PET
materials were compounded without peroxides (0 %) as reference or with different
amounts of zinc peroxide (0.1 - 1.0 wt%). The peroxide treated PET samples reveal
an increase of the L*-value in comparison to the reference which indicates a
bleaching effect. However, the increase of the b*-value shows that the polymer
yellows after addition of peroxides. The b*-value, on the contrary, leads to a lower
L*-value. Thus, a good bleaching effect is achievable, when small amounts (0.1 -
0.2 wt%, cf. Figure 4.13) of peroxide are used in the extrusion process. The higher
the b*-value, the worse is the influence on the L*-value. A maximum L*-value is
achieved for reprocessed PET by the addition of 0.2 wt% ZnO2. An addition of
0.3 wt% ZnO2 or more leads to strong increase of the b*-value of PET
(b*= 7.8 - 11.7).
Chapter 4
88
Figure 4.13. Color values (L*-, a*-, and b*-value according to CIE-L*a*b*) of a gray
reprocessed PET (r-PET 1) after treatment with different amounts of zinc peroxide
(0.1 to 1.0 wt%) in the extrusion process. The zinc peroxide was ground for 40 min in
a cryomill before usage and has an average particle diameter of 60.7 ± 51.1 nm. *
___________________________________________________________________
* These measurements were performed in cooperation with my colleague Christian
Bergs. These results are presented in Christian Bergs’ doctoral thesis, too.
Chapter 4
89
The yellow color is an indication for the occurrence of polymer degradation. The first
indication for a thermo-oxidative degradation was observed during the extrusion
experiments. In Figure 4.14, an extrusion curve is given as example. In that case,
PET was compounded with 0.3 wt% ground ZnO2 which has a particle size
distribution of 60.7 ± 51.1 nm. The force (F) of the screws of the extruder was
measured during the compounding process. A direct decrease of the force was
observed after the addition of zinc peroxide, which indicates a decrease of the melt
viscosity and, thus, the occurrence of thermo-oxidative degradation.
Figure 4.14. Extrusion curve of PET compounded with 0.3 wt% ZnO2 (average
particle diameter: 60.7 ± 51.1 nm) measured at 290 °C with 100 rpm screw rotation
speed.
The higher the amount of added peroxides, the shorter are the polymer chains of the
extruded PET. The inherent viscosities reveal the occurrence of polymer degradation
after treatment of PET with ZnO2 (Figure 4.15). While addition of 0.1 wt% ZnO2
causes minor degradation of PET (ηinh.= 0.65 dL ∙ g-1) compared to 0 % ZnO2
(ηinh.= 0.66 dL ∙ g-1), the addition of higher amounts of ZnO2 leads to stronger
degradation (up to 0.50 dL ∙ g-1).
Chapter 4
90
0.75
0.66 0.65 0.64 0.620.58
0.50
untreated 0% 0.1% 0.2% 0.3% 0.5% 1.0%
0.0
0.2
0.4
0.6
0.8
in
h./
dL
g
-1
ZnO2 (60.7 51.1 nm)
Figure 4.15. Inherent viscosities of the peroxide treated (0.1 to 1.0 wt%) polyesters
(r-PET 1, cf. Figure 4.13).*
Size exclusion chromatography (SEC) measurements were also performed and the
results are presented in Figure 4.16. The results reveal that number average molar
mass (Mn¯¯) and weight average molar mass (Mw¯¯) of PET extruded with higher
amounts of zinc peroxide (40 min ground) decreases. The higher the amount of zinc
peroxide is the more oxygen is released. Thus, the thermo-oxidative degradation of
PET increases and its molar mass decreases.
___________________________________________________________________
* These measurements were performed in cooperation with my colleague Christian
Bergs. These results are presented in Christian Bergs’ doctoral thesis, too.
Chapter 4
91
Figure 4.16. Size exclusion chromatography (SEC) results of r-PET treated with zinc
peroxide (40 min ground, having an average particle diameter of 60.7 ± 51.1 nm)
(0.1 wt% – 1.0 wt%) in comparison to untreated PET. The number average molar
mass (Mn¯¯) and weight average molar mass (Mw¯¯) are presented.
Furthermore, the molecular weight distribution (Ð) of PET extruded with higher
amounts of zinc peroxide increased. The untreated PET has a molecular weight
distribution of 1.81 and PET extruded with 1.0 wt% ZnO2 has a broad distribution of
Ð = 3.24. In Table 4.2, the molecular weight distributions and molar masses (Mn¯¯ and
Mw¯¯) are listed. The molecular weight distribution increases except for PET extruded
with 0.2 wt% ZnO2 with increasing addition of zinc peroxide in the extrusion process
and decreasing average molar mass. This happens, because the thermo-oxidative
degradation occurs directly after the addition of zinc peroxide in the extruder at
290 °C and some areas in the melt may not be affected.
Chapter 4
92
Table 4.2. Size exclusion chromatography (SEC) results of r-PET treated with zinc
peroxide (40 min ground; particle size: 60.7 nm ± 51.1 nm) (0.1 wt% – 1.0 wt%) in
comparison to untreated PET. The number average molar mass (Mn¯¯), weight average
molar mass (Mw¯¯), and the molecular weight distribution (Ð) are presented.
PET
sample
Mn¯¯ /
kg ∙ mol-1
Mw¯¯ /
kg ∙ mol-1 Ð
untreated 35.0 63.3 1.81
0.1 wt% 20.0 48.8 2.44
0.2 wt% 17.8 39.9 2.24
0.3 wt% 16.5 41.6 2.52
0.5 wt% 13.2 34.9 2.64
1.0 wt% 7.7 24.8 3.24
Moreover, the rheological behavior of some PET samples was analyzed with the help
of a plate-plate rheometer. In the following Figure 4.17, the storage moduli (G´) of
recycled gray PET (untreated) and different PET materials which were extruded with
zinc peroxide which was ground for 40 min are shown. The extrudates were
compounded with different amounts of zinc peroxide (0.1 - 1.0 wt%). All storage
moduli are higher at higher angular frequencies (ω). In general, when the plates
oscillate faster, more energy is needed and the storage moduli increase. Additionally,
the degree of entanglement of the polymer chains increases at higher angular
frequencies. Moreover, the storage moduli decrease with increase of the zinc
peroxide content in PET. The polymer chains decompose due to the addition of
peroxides in the extrusion process. This results in chain scission; shorter chains lead
to lower storage moduli; as shorter chains are less entangled.
Chapter 4
93
1 10 100 1000
1
10
100
1000
10000
G
´/ P
a
/ rad s-1
untreated
0.1%
0.2%
0.3%
0.5%
1.0%
Figure 4.17. Storage moduli (G´) versus angular frequency (ω) of untreated gray
reprocessed PET (r-PET 1) and r-PET 1 extruded with zinc peroxide (0.1 to 1.0 wt%,
cf. Figure 4.13) (measured at 290 °C).
In Figure 4.18, the loss moduli (G´´) of the extrudates, compounded with different
amounts of zinc peroxide (0.1 - 1.0 wt%), are presented. At higher angular
frequencies, the loss moduli are also higher. The loss modulus is defined as the
energy which is set free due to internal friction. The higher the angular frequency is,
the more energy is generated and the loss modulus increases. It is also recognizable
that PET extruded with higher amounts of zinc peroxide has a lower loss modulus.
The phenomenon is due to the changes of the molar mass of PET. The shorter the
polymer chains are, the lower is the loss of energy due to internal friction, and the
lower is the loss modulus.
Chapter 4
94
1 10 100 1000
10
100
1000
10000
100000
G
´´/ P
a
/ rad s-1
untreated
0.1%
0.2%
0.3%
0.5%
1.0%
Figure 4.18. Loss moduli (G´´) versus angular frequency (ω) of untreated gray
reprocessed PET (r-PET 1) and r-PET 1 extruded with zinc peroxide (0.1 to 1.0 wt%,
cf. Figure 4.13) (measured at 290 °C).
Furthermore, the complex viscosities (η*) were determined by rheological
measurements. In Figure 4.19, the results of the complex viscosities at different
angular frequencies are depicted. These results show that the complex viscosities
are lower at higher angular frequencies due to the higher shear rate. The complex
viscosities are also lower with increase of the amount of zinc peroxide added during
extrusion due to oxidative degradation of PET. In addition, the zero-shear viscosity
decreases from 186 Pa ∙ s (untreated) to 26.8 Pa ∙ s (1.0 wt% zinc peroxide).
Furthermore, the shear thinning behavior of the PET melts changes also. The
measurement of the complex viscosities shows that the viscosity decreases at higher
shear rates at a certain point. This shear thinning point (red arrow) increases at
higher zinc peroxide content from 52 rad ∙ s-1 (untreated) to 508 rad ∙ s-1 (1.0 % zinc
peroxide); this means that PET extruded with higher amounts of zinc peroxide shows
more pronounced Newtonian behavior. The shear thinning point increases with
increasing hydrogen bond content. Addition of higher amounts of zinc peroxide
Chapter 4
95
during extrusion leads to higher amounts of carboxylic end groups in PET. Higher
amounts of carboxylic end groups result in more hydrogen bonds in the PET melt.
Hence, the melt sticks together at higher shear rates until it tears apart.
Figure 4.19. Complex viscosity (η*) versus angular frequency (ω) of untreated gray
reprocessed PET (r-PET 1) and r-PET 1 extruded with zinc peroxide (0.1 to 1.0 wt%,
cf. Figure 4.13) (measured at 290 °C). (The red arrow indicates the shear thinning
point for each curve).
Moreover, the thermal properties of the PET samples were measured by differential
scanning calorimetry (DSC). In Figure 4.20, the results of the crystallization
temperatures (Tc) after cooling from melt are shown.
Chapter 4
96
200
-1
mW
m
g-1
/ °C
untreated Tc= 202°C
0.1% ZnO2 T
c= 201°C
0.2% ZnO2 T
c= 201°C
0.3% ZnO2 T
c= 199°C
0.5% ZnO2 T
c= 198°C
1.0% ZnO2 T
c= 197°C
exo
Figure 4.20. Crystallization behavior (Tc) of a gray reprocessed PET (r-PET 1) after
peroxide treatment in the extrusion process with different amounts of zinc peroxide
(0.1 to 1.0 wt%). The zinc peroxide was ground for 40 min in a cryomill before usage.
These results show that PET extruded with high amounts of ZnO2 crystallizes at
lower temperatures compared with PET which was treated with low amounts of ZnO2.
Normally, polymers with higher molar masses crystallize at colder temperatures
because of the lower chain mobility and higher viscosity [33-35]. Here, the trend is the
other way around. The crystallization of PET depends also on the content of carboxyl
end groups. Two carboxyl end groups can interact via hydrogen bonding. These
interactions of carboxyl terminal groups can indicate an apparently higher molar
mass. In Figure 4.21, the results for the determined carboxyl (COOH) end groups
(in mmol ∙ kg-1) are presented. The concentration of COOH end groups increases
with higher amounts of zinc peroxide added in the extrusion process.
Chapter 4
97
22.6
33.8 34.937.8
49.9
79.2
0% 0.1% 0.2% 0.3% 0.5% 1.0%
0
10
20
30
40
50
60
70
80
c(C
OO
H)/
mm
ol kg
-1
w(ZnO2)/ %
Figure 4.21. Concentration of carboxyl terminal groups of a gray reprocessed PET
(r-PET 1) after peroxide treatment in the extrusion process with different amounts of
zinc peroxide (0.1 to 1.0 wt%). The zinc peroxide was ground for 40 min in a cryomill
before addition to the melt (average particle diameter: about 60.7±51.1 nm). (Here,
triple determinations were performed.)
Furthermore, zinc peroxides with different particle size were applied to the melt of
gray reprocessed PET. Commercial micro-scaled ZnO2 and zinc peroxides ground in
a cryomill at different periods of time were used. With increasing grinding time
smaller zinc peroxide particles were obtained. In Figure 4.22, the results of the color
measurements of PET samples treated with 0.2 wt% zinc peroxide are shown. It is
depicted that smaller particles lead to lower yellowness (cf. b*-value) of the PET
samples extruded with the same concentration of zinc peroxide. As mentioned
before, the b*-value has a negative influence on the L*-value; thus, the samples
which are treated with smaller particles exhibit a brighter color than those treated with
large peroxide particles.
Chapter 4
98
Figure 4.22. Colorimetric results (L*-, a*-, and b*-value according to CIE-L*a*b*) of a
gray reprocessed PET (r-PET 1) after treatment with zinc peroxides with different
particle size in the extrusion process. The same zinc peroxide was ground for 10 to
40 minutes respectively in a cryomill before usage resulting in particles of 193.5 nm
(10 min), 120.3 nm (20 min), 101.9 nm (30 min), and 60.7 nm (40 min ground).
0.2 wt% of zinc peroxide was added to each PET sample during extrusion.
No clear trend was obtained for the inherent viscosities of PET which was extruded
with addition of zinc peroxides with different particle sizes. All measured inherent
viscosities range between 0.59 and 0.64 dL ∙ g-1. The oxygen release of the zinc
peroxides is in the same range as the same zinc peroxide with different particle sizes
was used. Just the inner friction of the particles in the PET melt during extrusion can
influence slightly the reduction of the polymer chains. The larger the particles are, the
higher is the inner friction; hence, the inherent viscosity of PET is lower if unground
zinc peroxide was used during extrusion (Figure 4.23).
Chapter 4
99
0.75
0.66
0.590.62 0.61 0.63 0.64
untreated
extruded
ZnO2 (microscopic)
ZnO2 (193.5 nm)
ZnO2 (120.3 nm)
ZnO2 (101.9 nm)
ZnO2 (60.7 nm)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
in
h./
dL g
-1
Figure 4.23. Inherent viscosities of the polyesters (r-PET 1) treated with zinc peroxide
of different particle size (cf. Figure 4.22). 0.2 wt% of zinc peroxide was added to each
PET sample during the extrusion process.
4.4 Conclusions
Zinc peroxide particles were used as bleaching agents for gray discolored post-
consumer poly(ethylene terephthalate). The micro- and nano-scaled zinc peroxide
particles were produced by grinding of commercially available ZnO2 (top-down
process) with the help of a cryomill. The ZnO2 particles were comminuted in
dependence on the grinding time from 10 minutes to 40 minutes. Particle diameters
of 60.7 ± 51.1 nm were achieved for 40 min ground zinc peroxide, and the oxygen
release was in the same range as in case of unground zinc peroxide. Zinc peroxide
decomposes at temperatures above 190 °C into zinc oxide and oxygen.
Zinc peroxide can be applied as bleaching agent during laboratory-scale extrusion
and spinning of post-consumer PET materials. In dependence on the history of the
PET materials, more or less pronounced improvement of their lightness (L*) was
obtained. Color measurements showed that for gray r-PET L*-values from 79.1 up to
81.4 were achieved. However, the result is hampered by yellowing of the PET
materials which is caused by the yellow color of zinc peroxide originating from
complexes of active oxygen species on the surface of the ZnO2 particles which may
Chapter 4
100
promote the thermal degradation of PET during processing. Addition of small
amounts of zinc peroxides (0.1 - 0.2 wt%) to PET melts in the extrusion process
results in improved lightness and slight yellowness. A maximum L*-value is achieved
for reprocessed PET by the addition of 0.2 wt% ZnO2. The rheological behavior of
PET bleached with ZnO2 reveals that the storage moduli decrease with increase of
the zinc peroxide content in PET. Due to oxidative chain scission resulting in shorter
chains, PET is less entangled. The loss moduli show a decrease with increasing
addition of ZnO2, too, due to the shorter polymer chains. Shorter chains render lower
internal friction during shear strain so that the loss modulus is lower. PET extruded
with higher amounts of zinc peroxide shows more pronounced Newtonian behavior.
Generally, due to the occurrence of cleavage of the PET chains, higher amounts of
carboxylic terminal groups exist and, thus, the hydrogen bonds are more
pronounced, which results in a less pronounced shear sensitivity. Finally, the
crystallization of PET depends also on the content of carboxyl end groups.
Interactions of carboxyl terminal groups indicate an apparently higher molar mass
which results in lower crystallization temperature with addition of increasing ZnO2
amounts to the PET melt.
4.5 References
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[2] J. C. Scheirs, G., "Effect of Contamination on the Recycling of Polymers", in
Recycling of PVC & mixed plastic waste F.P. La Mantia, Ed., University of Palermo,
Palermo, Italy 1996, p. 167
[3] J. Scheirs, T. E. Long, (Eds.), "Modern Polyesters: Chemistry and Technology of
Polyesters and Copolyesters", John Wiley & Sons Ltd, Chichester, Great Britain,
2005.
[4] D. Berg, K. Schaefer, A. Koerner, R. Kaufmann, W. Tillmann, M. Moeller,
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[5] M. Paci, F. P. La Mantia, Polymer Degradation and Stability 1999, 63, 11.
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Chemical Society 2012, 33, 3445.
[7] U. K. Thiele, Chemical Fibers International 2004, 54, 162
Chapter 4
101
[8] F. Ahmadnian, F. Velasquez, K. H. Reichert, Macromolecular Reaction
Engineering 2008, 2, 513.
[9] R. W. Stevenson, H. R. Nettleton, Journal of Polymer Science Part A-1: Polymer
Chemistry 1968, 6, 889.
[10] R. W. Stevenson, Journal of Polymer Science Part A-1: Polymer Chemistry
1969, 7, 395.
[11] U. K. Thiele, "Polyester Bottle Resins Production, Processing, Properties and
Recycling", PETplanet Print Heidelberg/Germany, 2007.
[12] S. B. Maerov, Journal of Polymer Science Part A-Polymer Chemistry 1979, 17,
4033.
[13] B. Duh, Polymer 2002, 43, 3147.
[14] S. N. Vouyiouka, E. K. Karakatsani, C. D. Papaspyrides, Progress in Polymer
Science 2005, 30, 10.
[15] F. Awaja, D. Pavel, European Polymer Journal 2005, 41, 1453.
[16] J. M. Stellman, "Encyclopaedia of occupational health and safety", International
Labour Organization, Geneva, Switzerland, 1998.
[17] S. Al-Malaika, "Reactive Modifiers for Polymers", Chapman & Hall, London,
Great Britain, 1997.
[18] D. Munteanu, in Plastics Additives Handbook, H. Zweifel, Ed., Hanser Fachbuch,
Munich, Germany, 2001, p. 734.
[19] G. V. Jere, C. C. Patel, Zeitschrift fuer Anorganische und Allgemeine Chemie
1962, 319, 175.
[20] DE2914058 C2 (1980), Otsuka Kagaku Yakuhin K.K., invs.: S. Ohno, N.
Aburatani, N. Ueda.
[21] DE2914058 A1 (1980), Otsuka Kagaku Yakuhin K.K., invs.: S. Ohno, N.
Aburatani, N. Ueda.
[22] L. Ibarra, A. Marcos-Fernandez, M. Alzorriz, Polymer 2002, 43, 1649.
[23] US3403136 A (1968), Standard Brands Chem. Ind. Inc., inv. J. J. C. Baker.
[24] DE10056311 A1 (2002), Bayer AG, invs.: W. Obrecht, A. J. M. Sumner.
[25] Y. K. Takahara, Y. Hanada, T. Ohno, S. Ushiroda, S. Ikeda, M. Matsumura,
Journal of Applied Electrochemistry 2005, 35, 793.
[26] A. H. Boonstra, C. A. H. A. Mutsaers, The Journal of Physical Chemistry 1975,
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Chapter 4
102
[27] X. Li, C. Chen, J. Zhao, Langmuir 2001, 17, 4118.
[28] C. Bergs, P. Simon, Y. Prots, A. Pich, RSC Advances 2016, 6, 84777.
[29] W. Chen, Y. H. Lu, M. Wang, L. Kroner, H. Paul, H. J. Fecht, J. Bednarcik, K.
Stahl, Z. L. Zhang, U. Wiedwald, U. Kaiser, P. Ziemann, T. Kikegawa, C. D. Wu, J. Z.
Jiang, The Journal of Physical Chemistry C 2009, 113, 1320.
[30] A. Escobedo-Morales, R. Esparza, A. Garcia-Ruiz, A. Aguilar, E. Rubio-Rosas,
R. Perez, Journal of Crystal Growth 2011, 316, 37.
[31] K. Weisskopf, Journal of Polymer Science Part A-Polymer Chemistry 1988, 26,
1919.
[32] A. S. T. M. International, " D 7409 – 07ε1 Test Method for Carboxyl End Group
Content of Polyethylene Terephthalate (PET) Yarns".
[33] F. Pilati, M. Toselli, M. Messori, C. Manzoni, A. Turturro, E. G. Gattiglia, Polymer
1997, 38, 4469.
[34] B. Gümther, H. G. Zachmann, Polymer 1983, 24, 1008.
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Chapter 5
103
Impact of the Chain Extension of Poly(ethylene terephthalate) with 1,3-Phenylene-bis-oxazoline and N,N’-Carbonylbiscaprolactam by Reactive Extrusion on its Properties A
Summary: In this chapter, the material properties of chain extended
poly(ethylene terephthalate) (PET) by application of two common chain
extenders, 1,3-phenylene-bis-oxazoline (1,3-PBO),
N,N’-carbonylbiscaprolactam (CBC), and combinations thereof are
investigated. The chain extension was performed in one step by a reactive
extrusion process which is important, for example, for fiber production. The
chain extenders are linearly linked to the COOH and/or OH terminal groups of
PET. Furthermore, the influence of the chain extension on the properties of
PET is analyzed by methods such as measurements of the inherent viscosity
and rheology, size exclusion chromatography, differential scanning
calorimetry, and carboxyl end group titration. The results demonstrate that
chain extenders have impact on the properties of PET in dependence on their
chemical composition and concentration. The improvement of the molecular
weight of the obtained compounds was achieved by the addition of small
concentrations of chain extenders (0.2 wt% 1,3-PBO or 0.3 wt% CBC) without
significant negative impact on the properties of PET.
Keywords: Chain extenders, poly(ethylene terephthalate), reactive extrusion,
rheology, thermal properties
A. Reproduced with permission from D. Berg, K. Schaefer, M. Moeller, Polymer
Engineering & Science, 2018 (Online version), doi.org/10.1002/pen.24903. Copyright
John Wiley and Sons.
Chapter 5
104
5.1 Introduction
After the successful bleaching of gray reprocessed poly(ethylene terephthalate) with
the help of inorganic peroxides during reactive extrusion, the molar mass of the
polyester has to be increased again to achieve good processing conditions (e.g. for
fiber manufacturing). To increase the molar mass of PET, usually, the solid state
polycondensation (SSP) process or the liquid state polycondensation (LSP) process
are used. This is also performed during recycling because during repeated recycling
of PET, chain cleavage of the polyester often occurs which results, even for fiber
manufacturing, in too low viscosities [1].
The SSP process is, as mentioned in Chapter 2, a thermal process where the
molecular weight of PET is increased to a value which is needed for a certain
product. At temperatures between the glass transition point (Tg) and the melting
temperature (Tm) (usually at about 200 °C to 240 °C), that means in the solid state,
post polycondensation takes place [1, 2]. During the SSP process, a water and oxygen
free atmosphere (dry inert gas stream or vacuum) has to be ensured. The SSP
process is an additional process which lasts more than seven hours, and high costs
arise because of the high energy consumption [3-5]. In addition, the undesirable gray
color can occur again during the SSP process due to the reduction of the antimony
catalysts (e.g. Sb2O3 or Sb[OOC-CH3]3) to metallic antimony at these conditions [6-8].
An alternative to the SSP process could be the liquid state polycondensation (LSP)
process. The LSP process is a quite new process, where the polycondensation
process is run above the melting temperature. Because of the higher temperature, a
higher rate of condensation reaction occurs. The benefit of this process is to save
energy and time, because of the faster reaction compared to the SSP process [9].
Also, high intrinsic viscosities can be achieved, for example, for high performance
polyester fibers [10-12].
Another approach to prevent polymer degradation by added zinc peroxide is the
application of chain extenders directly in the extrusion process without any additional
process steps. Because of this, chain extenders for PET were studied in this work.
Chain extenders are multifunctional molecules with low molecular weight that connect
the end groups of polymer segments (for example COOH end groups in PET)
together, which results in a higher molecular weight polymer. Bifunctional and tri- or
multifunctional chain extenders are known. Bifunctional molecules extend the
Chapter 5
105
polymer linearly and tri- or multifunctional molecules act as crosslinker (Scheme 5.1
and 5.2).
Scheme 5.1. Principle reaction of bifunctional chain extenders.
Scheme 5.2. Principle reaction of trifunctional chain extenders.
The focus in this chapter is set on bifunctional chain extenders, as, especially for fiber
production, crosslinked polymers are undesirable. Furthermore, highly branched and
crosslinked PET has a negative influence on the rheology [13, 14]. Addition type chain
extenders such as bisepoxy compounds, carboxylic dianhydride, and diisocyanates
are preferred chain extenders, as no byproducts are generated [15, 16]. But, because
of some disadvantages like branching the polyester (in the case of bisepoxy
Chapter 5
106
compounds, carboxylic dianhydride, diisocyanates) or the production of less
thermally stable products (diisocyanates) the chain extenders 1,3-phenylene-bis-
oxazoline (1,3-PBO) and N,N’-carbonylbiscaprolactam (CBC) are used here.
1,3-PBO is also an addition type chain extender, it is a bifunctional heterocyclic
compound from the group of cyclic imino ethers (imidates) which has a general
formula of –N=C–O– (cf. Chapter 2).
1,3-phenylene-bis-oxazoline (1,3-PBO) is commonly used as chain extender for a
variety of polymers with carboxyl end groups such as polyamides [17-20], poly(lactic
acid) (PLA) [21-23], poly(butylene terephthalate) (PBT) [24, 25], and poly(ethylene
terephthalate) (PET) [16, 17, 25-31]. 1,3-Phenylene-bis-oxazoline (1,3-PBO) undergoes a
ring-opening reaction with the terminal carboxyl groups of PET in a linear way [32-34].
The products of the reaction of PET with 1,3-PBO are poly(ester-amides), as
presented in Scheme 5.3. The addition of small amounts of 1,3-PBO are sufficient to
achieve good results with high intrinsic viscosities [28].
Scheme 5.3. Reaction of 1,3-phenylene-bis-oxazoline (1,3-PBO) with the carboxyl
end groups of poly(ethylene terephthalate) (PET) [16, 33, 35].
Beside 1,3-PBO, N,N’-carbonylbiscaprolactam (CBC) was chosen as a further chain
extender to increase the molecular weight of PET. Loontjens et al. published some
papers and patents on CBC as chain extender for different polymers [17, 36-42]. CBC is
a chain extender which has a high affinity to the terminal hydroxyl groups of PET,
because, the reaction rate of CBC with terminal OH groups is faster than with COOH
groups. Also in the case of CBC, the chain extension reaction proceeds via a ring
opening reaction in a linear way (Scheme 5.4). However, evaporation of
ε-caprolactam may occur during the chain extension reaction of CBC with PET
Chapter 5
107
(cf. Scheme 5.4). The chain extension takes place within less than three minutes,
and the addition of small amounts of 0.1 to 1.0 wt% of CBC are effective [40].
Scheme 5.4. Potential reactions of N,N’-carbonylbiscaprolactam (CBC) and hydroxyl
end groups of poly(ethylene terephthalate) (PET) [40]. (a [red]) two substitutions of
ε-caprolactam, (b [black]) combination of substitution of ε-caprolactam and ring
opening reaction, (c [blue]) two ring opening reactions.
Here, the impact of 1,3-phenylene-bis-oxazoline (1,3-PBO),
N,N’-carbonylbiscaprolactam (CBC), and their combinations on the properties of fiber
grade poly(ethylene terephthalate) were investigated. The characteristics of the
obtained chain-extended poly(ethylene terephthalate) were analyzed by viscosity
measurements, size exclusion chromatography, parallel plate rheology, and
differential scanning calorimetry.
5.2 Experimental Section
5.2.1 Materials
PET for fiber production was provided by Maerkische Faser GmbH (Premnitz,
Germany). The fiber grade polyester with an intrinsic viscosity of 0.63 dL ∙ g-1 was
used for the extrusion experiments with the chain extenders.
N N
O OO
R
PE
T
OH
+
NH
O
RPET
O
O
NH
O
O
PETR
-
N NH
O
O
O
PETR
RPET
O
O
N
O
O
R
PE
T
OH
+
R
PETOH+
NH
O
-
RPET
O
O
RPET
O
RPET
OH+
R
PE
T
OH
RP
ET
OH
NH
O
-
PET
O
O
HN
O
PETO
OHN
R
R
(a + b)
(b + c)
(a)
(b)
(c)
+
(b)
+
Chapter 5
108
The solvent 1,1,1,3,3,3-hexafluoropropane-2-ol (HFIP) was purchased from
Fluorochem (Hadfield, United Kingdom) and chloroform was bought from J.T. Baker
(Deventer, Netherlands). 2,6-di-tert-butyl-4-methylphenol, o-cresol, and 2-
bromobenzoic acid were received from Sigma Aldrich, (Taufkirchen, Germany).
Bromophenol blue was received from Merck (Darmstadt, Germany) and ethanolic
potassium hydroxide solution was purchased from Fluka (Taufkirchen, Germany).
Furthermore, N,N’-carbonylbiscaprolactam (CBC) was supplied from DSM (Geleen,
The Netherlands), and 1,3-phenylene-bis-oxazoline (1,3-PBO) was provided from
Adeka Palmarole (Basel, Switzerland).
5.2.2 Extrusion
First of all, PET was dried at 130 °C in an oven overnight at least for ten hours. The
Micro 15cc Twin Screw-Extruder (DSM, Geleen, The Netherlands) was used for the
extrusion experiments to measure the screw force. About 11 g PET were molten at
290 °C with 100 rounds per minute screw rotation speed under nitrogen atmosphere.
Different amounts of the chain extenders were added to PET and mixed for five
minutes in the extruder in a discontinuous process. The applied concentrations of the
chain extenders were 0 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.5 wt%, 1.0 wt%, and 2.0
wt%, respectively. After the compounding process, the samples were ground in a
cryomill (6800 Freezer/Mill, SPEX CertiPrep, Stanmore, United Kingdom) to achieve
good homogeneity and good solubility for further analysis.
5.2.3 Viscosimetry
The inherent viscosity (ηinh.) of the polyester was measured to analyze the influence
of the chain extenders on the polymer molecular weight. About 0.3300 g PET was
weighed in a 25 mL graduated flask and dissolved in HFIP. The viscosity of this
solution was measured in a water bath at 25 °C using an Ubbelohde viscosimeter
(type 0a) (Schott AG, Mainz, Germany). The inherent viscosity was calculated
according to equation (1).
𝜂𝑖𝑛ℎ. =ln(𝜂𝑟𝑒𝑙)
𝛽=
ln(𝜂
𝜂0)
𝛽=
ln(𝑡
𝑡0)
𝛽 (1)
Chapter 5
109
Where, ηinh is the inherent viscosity and ηrel the relative viscosity. β is the mass
concentration, η the viscosity of the PET solution, and η0 the viscosity of the solvent
(here, HFIP). t and t0 are the flow times of the PET solution and the solvent HFIP,
respectively.
5.2.4 Size Exclusion Chromatography
Molecular weights (Mn¯¯ and Mw¯¯) and molecular weight distribution (Ð) of PET samples
were determined by size exclusion chromatography (SEC). PET was dissolved in
HFIP and diluted with chloroform to a volume concentration of chloroform/HFIP
98:2 vol% according to Weisskopf [43]. A HPLC pump (PU-2080plus, Jasco, Tokyo,
Japan) equipped with an evaporative light scattering detector (PL-ELS-1000, Polymer
Laboratories, Amherst, USA) was used.
2,6-Di-tert-butyl-4-methylphenol (c = 250 mg ∙ mL-1) was used as internal standard,
and narrow distributed polystyrene standards (PSS Polymer Standards Service
GmbH, Mainz, Germany) were used to achieve calibration. One pre-column (8 mm x
50 mm) and four SDplus gel columns (8 mm x 300 mm, MZ Analysentechnik, Mainz,
Germany) were applied at a flow rate of 1.0 mL ∙ min-1 at 20 °C. The separation
process was performed on polystyrene/divinylbenzene columns (50 Å, 100 Å,
1,000 Å, and 10,000 Å, PSS Polymer Standards Service GmbH, Mainz, Germany).
Results were evaluated using the PSS WinGPC UniChrom software (Version 8.1.1).
5.2.5 Rheology
The rheology measurements were performed with a parallel plate rheometer
(Discovery HR-3 hybrid rheometer, TA Instruments-Waters L.L.C., New Castle,
USA). PET was molten at 290 °C and measured with a gap of 600 µm in a frequency
range of 0.1 Hz to 100 Hz with 2 % oscillation. The complex viscosities (η*), storage
moduli (G’), and loss moduli (G’’) were calculated.
5.2.6 Differential Scanning Calorimetry
DSC analyses were carried out using the Netzsch DSC 204 (NETZSCH-Geraetebau
GmbH, Selb, Germany). About 10 mg PET were weight into aluminum pans, closed
with a lid and pierced. The samples were heated up to 300 °C with a heating rate of
20 K ∙ min-1 under nitrogen flow and hold there for 10 minutes to delete the thermal
Chapter 5
110
history of the polymer. After that, the pans were cooled down to 200 °C followed by
an isothermal step of 30 min at this temperature. To measure the melting point (Tm)
and melting enthalpy (ΔHf) of PET, the samples were heated up again to 300 °C with
a heating rate of 10 K ∙ min-1, cooled down to 20 °C with a cooling rate of 10 K ∙ min-1
to measure the crystallization by cooling from melt (Tc) and heated up a third time to
110 °C with a heating rate of 20 K ∙ min-1 to determine the glass transition
temperature (Tg).
The crystallinity (χc) of the samples was calculated as follows (2):
𝜒𝐶 = 100% ×𝛥𝐻𝑓
𝛥𝐻𝑓100% (2)
ΔHf100% is the melting enthalpy of a fully crystallized poly(ethylene terephthalate) with
a value of 140 J ∙ g-1 [44-46].
Furthermore, the lamellar thickness distribution of the polymers was calculated with
the aid of an approach of Hoffman, Davis, and Lauritzen using the Gibbs-Thomson
equation (3) [47].
𝐿 = 2𝜎𝑇𝑚
0
∆𝐻𝑓𝑉(𝑇𝑚0−𝑇𝑚)
(3)
Here, L is the lamellar thickness, σ the surface free energy (0.106 J ∙ m-2), Tm0 the
equilibrium melting temperature of an infinite crystal (564 K), Tm the melting
temperature, ΔHfV the melting enthalpy per volume unit of a fully crystallized PET
(2.1 108 J ∙ m-3 at ρ = 1,455 g ∙ cm-3) [45, 46, 48, 49].
5.2.7 Carboxyl End Group Titration
To determine the carboxyl end groups of PET, titrations with ethanolic potassium
hydroxide (KOH) solution with a concentration of 0.05 mol ∙ L-1 using bromophenol
blue as indicator were performed. The titer (t) of the KOH standard solution was
determined with dried 2-bromobenzoic acid. Approximately 0.8 – 1.5 g PET were
dissolved in 20.0 g o-cresol at 80 °C, quenched with chloroform and titrated against
potassium hydroxide standard solution. As blank, 20.0 g o-cresol mixed with
Chapter 5
111
chloroform was also titrated. In each case, triple determinations were performed. The
COOH concentration (in mmol ∙ kg-1) was calculated as follows (4):
𝑐(𝐶𝑂𝑂𝐻) = [𝑉(𝐾𝑂𝐻)−𝑉0(𝐾𝑂𝐻)]∙𝑐(𝐾𝑂𝐻)∙𝑡∙10
3
𝑚(𝑃𝐸𝑇) (4)
5.3 Results and Discussion
5.3.1 Extrusion Curve
The chain extenders were typically pre-mixed with PET (dry blend) and added
afterwards to the extruder. Furthermore, PET was added in a few samples to the
extruder and mixed there with chain extenders to get a first indication of chain
extension reactions. As an example, the extrusion curve is presented in Figure 5.1
where 1,3-PBO was added to the PET melt. The screw force of the extruder reveals
that more force is needed to rotate the screws after the addition of the chain
extender. An increase of the screw force from 1854 N to 2059 N is measured in that
case. This indicates an increase of the melt viscosity and, hence, an increase of the
molecular weight of PET.
Chapter 5
112
Figure 5.1. Extrusion curve of PET with addition of 0.3 % 1,3-phenylene-bis-
oxazoline (1,3-PBO) measured at 290 °C with 100 rpm.
5.3.2 Inherent Viscosity
At first, the inherent viscosities of PET which was extruded with the addition of chain
extenders were determined. In Figure 5.2, the inherent viscosities of virgin PET and
PET extruded with 1,3-phenylene-bis-oxazoline (1,3-PBO) in concentrations of 0, 0.1,
0.2, 0.3, 0.5, 1.0, and 2.0 wt% are presented. Firstly, the inherent viscosity
decreases, when virgin PET is extruded without any chain extender which is due to
the thermal degradation of PET at 290 °C. Then, 1,3-PBO was added to virgin PET in
an extrusion process. Already at small 1,3-PBO concentrations increases of the
inherent viscosity are observed. PET extruded with 0.1 wt% 1,3-PBO has reached
the inherent viscosity of virgin PET. Upon addition of higher amounts of 1,3-PBO, the
inherent viscosity of PET increases also. A maximum inherent viscosity with
0.66 dL ∙ g-1 is reached at 1.0 wt%.
Chapter 5
113
Figure 5.2. Inherent viscosities of virgin PET compounded with 1,3-phenylene-bis-
oxazoline (0.1 wt% – 2.0 wt%) at 290 °C in comparison to virgin and extruded PET
(without chain extender).
Compared to 1,3-PBO N,N’-carbonylbiscaprolactam (CBC) is at small concentrations
less effective; however, small effects are also observable (Figure 5.3). At a
concentration of 0.3 wt% the inherent viscosity of virgin PET is obtained. At high
concentrations the inherent viscosity increases very strongly up to 0.92 dL ∙ g-1 at
2.0 wt% CBC.
Chapter 5
114
Figure 5.3. Inherent viscosities of virgin PET compounded with
N,N’-carbonylbiscaprolactam (0.1 wt% – 2.0 wt%) at 290 °C in comparison to virgin
and extruded PET (without chain extender).
5.3.3 Size Exclusion Chromatography (SEC)
Furthermore, size exclusion chromatography (SEC) measurements of the PET
samples were performed in a chloroform/HFIP (98/2 vol%) solution. In Figure 5.4, the
results of virgin PET and PET extruded with 0.1 wt% - 2.0 wt% 1,3-PBO are given.
While virgin PET has an average molar mass of 14.4 kg ∙ mol-1 (number average
molar mass Mn¯¯) and 40.7 kg ∙ mol-1 (weight average molar mass Mw¯¯) has PET which
was extruded with 1,3-PBO has an average molar mass up to 21.1 kg ∙ mol-1 (Mn¯¯)
and 48.1 kg ∙ mol-1 (Mw¯¯). Furthermore, the molar mass is rather broadly distributed.
The molecular weight distributions are in the range of 2.3 to 2.8. However, this virgin
PET has also a broad molecular weight distribution of 2.8. The increases of the molar
mass and the inherent viscosity of PET indicate that chain extension reactions were
successfully performed with addition of 1,3-PBO in a reactive extrusion process at
290 °C.
Chapter 5
115
Figure 5.4. Size exclusion chromatography (SEC) results of virgin PET compounded
with 1,3-phenylene-bis-oxazoline (0.1 wt% – 2.0 wt%) at 290 °C in comparison to
virgin PET. The number average molar mass (Mn¯¯) and weight average molar mass
(Mw¯¯) are presented.
SEC measurements were also done for PET samples extruded with CBC (0.1 wt% –
2.0 wt%). In Figure 5.5, the SEC results of these samples are presented. These
results reveal strong increases of the average molar masses from 14.4 kg ∙ mol-1 (Mn¯¯)
and 40.7 kg ∙ mol-1 (Mw¯¯) up to 25.4 kg ∙ mol-1 (Mn¯¯) and 83.6 kg ∙ mol-1 (Mw¯¯). Here, the
increases of the molar mass and the inherent viscosity of PET show that a reaction of
the chain extender CBC with PET was successfully realized in a reactive extrusion
process at 290 °C, too.
Chapter 5
116
Figure 5.5. Size exclusion chromatography (SEC) results of virgin PET compounded
with N,N’-carbonylbiscaprolactam (0.1 wt% – 2.0 wt%) at 290 °C in comparison to
virgin PET. The number average molar mass (Mn¯¯) and weight average molar mass
(Mw¯¯ ) are presented.
5.3.4 Rheology
The rheological characteristics of the PET samples extruded with chain extenders are
also determined. With the aid of a parallel plate rheometer the complex viscosity (η*),
storage modulus (G´), and loss modulus (G´´) are measured.
Firstly, the results of the complex viscosity measurements are represented in
Figures 5.6 and 5.7. In Figure 5.6, the results of the η* measurements of PET
extruded with 1,3-PBO are depicted. It reveals that the PET melts have behavior
which is close to a Newtonian-like behavior at an angular frequency ω of 0.5 to
330 rad ∙ s-1. At higher shear rates, shear thickening behavior is obtained. This
behavior is observed for all samples except for PET extruded with 2.0 wt% 1,3-PBO.
In that case, Newtonian-like behavior is observed up to 20 rad ∙ s-1, and it shows
shear thinning behavior at higher angular frequencies. The reason for this effect is
the higher molar mass of the PET chains. Due to the high molar mass, the polymer
chains are highly entangled and disentanglement occurs at high shear rates. In the
other cases, the polymer chains are less entangled and get tangled up at high shear
Chapter 5
117
rates which lead to this shear thickening behavior. The PET samples which are
compounded with 1,3-PBO have larger complex viscosities than the reference
sample extruded without added chain extender. For example, at an angular
frequency of 0.6 rad ∙ s-1, the complex viscosity (η*) increases from 75 Pa ∙ s to
205 Pa ∙ s which indicates chain extension induced by 1,3-PBO.
Figure 5.6. Complex viscosity (η*) of PET extruded with 1,3-phenylene-bis-oxazoline
(1,3-PBO concentration ranging from 0 – 2.0 wt%) measured at 290 °C.
Figure 5.7 presents the results of the complex viscosity (η*) measurements of PET
extruded with CBC. The Newtonian-like behavior is also found for the PET samples
compounded with small amounts of CBC (0 % - 0.5 wt%) up to ω = 330 rad ∙ s-1. The
polymer chains are strongly entangled at high shear rates resulting in increased
complex viscosity. In contrast, PET compounded with 1.0 – 2.0 wt% CBC shows a
direct decrease of the complex viscosity with increase of the angular frequency. Due
to the high molar mass, these polymer chains are highly entangled, and they are
disentangled at high shear rates. The complex viscosities at small angular
frequencies (here: ω = 0.6 rad ∙ s-1) raised from 75 Pa ∙ s (0 % CBC) to 342 Pa ∙ s
(2.0% CBC). This indicates a strong increase of a chain extension by CBC.
Chapter 5
118
Figure 5.7. Complex viscosity (η*) of PET extruded with N,N’-carbonylbiscaprolactam
(CBC concentration ranging from 0 – 2.0 wt%) measured at 290 °C.
Moreover, in the following figures the storage moduli (G’) and the loss moduli (G’’) of
PET treated with the CBC are presented. The storage moduli of PET which was
extruded with CBC are shown in Figure 5.8. At higher angular frequencies (ω), the
storage moduli are also higher, because of the higher shear rates of the melts. The
high shear rate leads to an increased entanglement of the polymer chains and, as a
result, an increased storage modulus. In addition, higher chain extender
concentrations reveal also higher storage moduli. For example, at ω = 0.6 rad ∙ s-1
the storage modulus of PET extruded without chain extender is 2.8 Pa and the
storage modulus of PET extruded with 2.0 wt% CBC is 49 Pa at ω = 0.6 rad ∙ s-1. The
reason for this is that increased CBC concentrations extend the polymer chains and
the entanglement of these chains and, as a result, the storage modulus is also
higher.
Chapter 5
119
Figure 5.8. Storage moduli (G´) measured at 290 °C of PET extruded with
N,N’-carbonylbiscaprolactam (CBC concentration ranging from 0 – 2.0 wt%).
The loss moduli (G´´) are also higher at higher angular frequencies (ω) as depicted in
Figure 5.9. Due to the higher shear rate of the melt, the internal friction of the chains
increases, too. Thus, the loss of energy which is defined as loss modulus (G´´) is also
larger. An increase of the loss modulus due to the addition of the chain extender to
the PET melt is also shown. The loss modulus at ω = 0.6 rad ∙ s-1 rises from 45 Pa
(0 wt% CBC) up to 212 Pa (2.0 wt% CBC). The chain extended polymers have higher
loss moduli because of increased internal friction due to their longer chains after
compounding.
Chapter 5
120
Figure 5.9. Loss moduli (G´´) measured at 290 °C of PET extruded with
N,N’-carbonylbiscaprolactam (CBC concentration ranging from 0 – 2.0 wt%).
5.3.5 Differential Scanning Calorimetry (DSC)
The thermal behavior of the chain extended PET samples was investigated, too. The
results of the differential scanning calorimetry (DSC) measurements of PET samples
are shown in Table 5.1. The glass transition point (Tg), melting points (Tm), melting
enthalpy (ΔHf), crystallinity (χc), and crystallization by cooling from melt (Tc) are listed
in Table 5.1.
Chapter 5
121
Table 5.1. Differential scanning calorimetry (DSC) results of virgin PET and PET
compounds which were extruded without and with different amounts of the chain
extenders CBC or 1,3-PBO.
PET sample Tg/ °C ΔHf/ J ∙ g-1 Tm/Peak/1/ °C Tm/Peak/2/ °C Tc/ °C χc / %
virgin 82 39.0 242 252 187 27.8
0% chain extender 84 36.9 243 252 184 26.4
0.1% 1,3-PBO 83 38.5 243 252 192 27.5
0.2% 1,3-PBO 85 36.7 243 251 185 26.2
0.3% 1,3-PBO 84 37.1 243 251 195 26.5
0.5% 1,3-PBO 84 34.2 243 251 192 24.5
1.0% 1,3-PBO 84 32.9 243 249 192 23.5
2.0% 1,3-PBO 84 31.9 243 249 186 22.8
0.1% CBC 84 37.2 243 252 188 26.6
0.2% CBC 84 37.4 243 252 186 26.7
0.3% CBC 84 38.4 243 252 189 27.4
0.5% CBC 84 36.7 243 252 187 26.2
1.0% CBC 84 34.2 243 250 186 24.4
2.0% CBC 83 30.0 243 - 192 21.4
In the following figures (Figure 5.10 – 5.12), the results of the DSC measurements
are presented. In Figure 5.10, the melting endotherms of virgin PET and PET
extruded with 1,3-PBO with concentrations from 0 wt% to 2.0 wt% are shown. The
melting endotherm represents the transition of the solid semi-crystalline structure to
the molten, amorphous state. The DSC diagram of the investigated virgin PET which
was recorded with a heating rate of 10 K ∙ min-1 up to 300 °C has two maxima and a
broad signal in the range of 235 °C to 260 °C. The two maxima at 242 °C and 252 °C
correspond with two melting endotherms. The multiple melting behavior of PET has
been published by different authors [50-53]. In all cases, annealing at 200 °C for
30 minutes was performed after erasure of the thermal history in the first heating
step. After that, the samples were heated up to 300 °C with a heating rate of
10 K ∙ min-1 to determine the melting characteristics. It is thought that the first melting
endotherm is related to the lamellae formed during crystallization and the second
melting endotherm corresponds to the larger lamellae generated by recrystallization
Chapter 5
122
of the smaller ones which leads to fusion of lamellae [50, 52]. But, it is also possible that
the second melting endotherm is caused by formation of bigger crystalline sequences
from amorphous or partially ordered sequences at the interface of the crystallites.
They may undergo an orientation process due to the annealing step [54].
Here, the lower melting temperature has its maximum at 242 °C, and the higher
melting temperature has a maximum at 252 °C. Figure 5.10 demonstrates that the
second melting area decreases with an increasing 1,3-PBO content of PET together
with an increase of the first melting area. This is most pronounced for PET which was
extruded with 1,3-PBO concentrations of 0.5 wt%. The area under the melting
endotherms is directly related to the crystallinity of PET. Thus, the reaction of PET
with 1,3-PBO disturbs the formation of highly crystalline structures in the polymer
resulting in small crystalline structures.1,3-PBO is a foreign building block which is
inserted into the polyester chains and reduces, as a result, the crystallinity of the
polymer.
220 240 260 280
0.2
0.4
0.6
0.8
1.0
mW
m
g-1
/ °C
virgin
0 %
0.1 %
0.2 %
0.3 %
0.5 %
1.0 %
2.0 %
endo
1,3-PBO
Figure 5.10. Differential scanning calorimetry (DSC) diagrams of virgin PET and PET
extruded with 1,3-phenylene-bis-oxazoline (1,3-PBO) in concentrations of 0 wt% to
2.0 wt%. The melting enthalpy is depicted.
Chapter 5
123
In analogy to PET compounded with 1,3-PBO, the higher melting temperature of PET
extruded with CBC decreases also at higher CBC concentrations (Figure 5.11,
Table 5.1). Here as well, CBC acts as disrupter of the crystalline structure of PET.
The DSC diagram of PET compounded with 2.0 wt% of CBC depicts that the higher
melting area disappears completely (Figure 5.11). This shows that high amounts of
CBC have a stronger influence on the crystallinity of PET than 1,3-PBO.
220 240 260 280
0.2
0.4
0.6
0.8
CBC
mW
m
g-1
/ °C
Virgin
0 %
0.1 %
0.2 %
0.3 %
0.5 %
1.0 %
2.0 %
endo
Figure 5.11. Differential scanning calorimetry (DSC) diagrams of virgin PET and PET
extruded with N,N’ carbonylbiscaprolactam (CBC) in concentrations of 0 wt% to
2.0 wt%. The melting enthalpy is presented.
In general, the crystallinity of the PET samples decreases after extrusion with
1,3-PBO and CBC (Figure 5.12, Table 5.1) due to the disturbance of the crystalline
structure by these chain extenders. Both chain extenders are foreign building blocks
in the polymer chains. The degrees of crystallinity over the whole melt endotherms
are shown in Figure 5.12. It reveals that high chain extender contents decrease the
crystal structure of PET.
Chapter 5
124
Figure 5.12. Degree of crystallinity of virgin PET and PET extruded with chain
extenders (1,3-phenylene-bis-oxazoline or N,N’-carbonylbiscaprolactam) in
concentrations of 0 wt% to 2.0 wt%.
Furthermore, the lamellar thickness distribution of these samples was calculated and
the results are presented in Figure 5.13 (PET compounded with 1,3-PBO) and in
Figure 5.14 (PET compounded with CBC). These figures show that the larger
crystallites decrease after addition of 1,3-PBO and CBC to PET during extrusion.
While virgin PET has a lamellar thickness distribution ranging from 87 Å to 184 Å with
maxima at 117 Å and 145 Å, PET which was extruded with 2.0 wt% 1,3-PBO has a
lamellar thickness distribution of 82 Å to 170 Å and maxima at 118 Å and 136 Å
(Figure 5.13). The lamellae of PET extruded with CBC are even smaller. The
lamellae distribution of PET extruded with 2.0 wt% CBC is in the range of 87 Å to
161 Å and has only one maximum at 118 Å (Figure 5.14). These results demonstrate
that the chain extenders disturb the formation of crystallites. In addition, in the case of
PET compounded with 1,3-PBO the lamellar thickness decreases already at small
amounts of 1,3-PBO (cf. Figure 5.13). The formation of lamellae is promoted by the
hydrogen bonds between the end groups of different PET chains. As 1,3-PBO reacts
Chapter 5
125
with the COOH end groups of PET, less hydrogen bonds and, as a result, less
lamellae are formed. However, the addition of small amounts of CBC during extrusion
of PET has less influence on the lamellar thickness.
60 80 100 120 140 160 180 200 220 60 80 100 120 140 160 180 200 220
145 Å117 Å
en
do
virgin 145 Å
118 Å
0%
144 Å118 Å
en
do
0.1%144 Å118 Å
0.2%
1,3-PBO
142 Å
118 Å
en
do
0.3%
141 Å
118 Å 0.5%
138 Å
118 Å
en
do
L/ Å
1.0%
136 Å
118 Å
L/ Å
2.0%
Figure 5.13. Lamellar thickness distribution (L) of virgin PET and PET extruded with
1,3-phenylene-bis-oxazoline (1,3-PBO concentration from 0 wt% to 2.0 wt%).
Chapter 5
126
60 80 100 120 140 160 180 200 220 60 80 100 120 140 160 180 200 220
145 Å117 Å
en
do
virgin
118 Å
145 Å
118 Å
0%
145 Å145 Å
en
do
0.1%118 Å 0.2%
144 Å118 Å
en
do
0.3%144 Å
118 Å 0.5%
138 Å
118 Å
en
do
L/ Å
1.0%
CBC
118 Å
L/ Å
2.0%
Figure 5.14. Lamellar thickness distribution (L) of virgin PET and PET extruded with
N,N’-carbonylbiscaprolactam (CBC concentration from 0 wt% to 2.0 wt%).
5.3.6 Carboxyl End Group Titration
To validate that 1,3-PBO reacted with the carboxyl end groups, COOH end group
titrations were performed. PET extruded without any chain extender was at first
titrated. The amount of COOH end groups in PET extruded without chain extenders
was about 38.9 ± 0.4 mmol ∙ kg-1. Compared to this, the COOH concentration of PET
extruded with 1.0 wt% CBC decreased slightly to 33.1 ± 0.1 mmol ∙ kg-1. A stronger
decrease is obtained for PET which was extruded with 1.0 wt% of 1,3-PBO for which
a COOH concentration of 22.9 ± 0.3 mmol ∙ kg-1 was determined. These results lead
to the conclusion that the COOH end group concentration decreases after addition of
1,3-PBO. This influences the lamellar thickness distribution as mentioned before.
Chapter 5
127
5.3.7 Combination of 1,3-PBO and CBC
Finally, 1,3-PBO and CBC were combined in the extrusion process with PET. In the
following Figure 5.15, the results of the inherent viscosity measurements are
presented. It is obvious that a combination of these two chain extenders results in an
additional effect. Except in the case of PET extruded with 0.1 wt% 1,3-PBO and
1.0 wt% CBC, higher inherent viscosity results are reached compared to the
application of only one chain extender (cf. Figures 5.2 and 5.3). For example, PET
extruded with 0.1 wt% 1,3-PBO and 0.3 wt% CBC has an inherent viscosity of
0.65 dL ∙ g-1 while for PET extruded with 0.3 wt% CBC an inherent viscosity of
0.63 dL ∙ g-1 was measured (cf. Figure 5.3). A combination of these two chain
extenders is beneficial because of their different reactions with the terminal end
groups of PET. 1,3-PBO reacts with the COOH terminal groups of PET and CBC has
higher reactivity with the OH end groups of PET (cf. Scheme 5.3 + 5.4).
Figure 5.15. Results of inherent viscosity measurements of virgin PET compounded
with 1,3-phenylene-bis-oxazoline (1,3-PBO) and N,N’-carbonylbiscaprolactam (CBC)
in different concentrations in comparison to virgin and extruded PET (without chain
extender) (compare with Figure 5.2 and 5.3).
Chapter 5
128
5.4 Conclusions
High molar mass poly(ethylene terephthalate) (PET) can be obtained by the addition
of chain extenders such as 1,3-phenylene-bis-oxazoline (1,3-PBO) and
N,N’-carbonylbiscaprolactam (CBC), which is very important for example for fiber
production. The chain extenders are linearly linked to the COOH and/or OH terminal
groups of PET. The addition of small amounts (0.1 - 2.0 wt%) of 1,3-PBO and CBC to
the PET melt in the extrusion process results in an increase of the inherent viscosity
which corroborates with increased molar mass which was, furthermore, proven by
size exclusion chromatography. The chain extension reactions occur fast in a
reactive extrusion process at 290 °C which is a one-step process that can be of
interest for recycling applications. Further factors like the reactivity of the chain
extenders with the end groups of PET also play an important role for their
effectiveness.
An increase of the storage moduli (G`), loss moduli (G``), and complex
viscosities (η*) was observed in all cases for PET which was compounded with chain
extender addition. After addition of 2.0 wt% CBC, the complex viscosity of PET
increased up to 267 Pa ∙ s as measured by rheology at 0.6 rad ∙ s-1 and 290 °C. Also,
an increase of 46 Pa for the storage modulus and of 167 Pa for the loss modulus was
measured at 0.6 rad ∙ s-1 and 290 °C. However, for PET which was extruded with
small amounts of the studied chain extenders (0.1 – 0.5 wt%), no marked changes in
its rheological properties were determined. The higher the amount of chain extender
is, the lower are the angular frequencies, where the Newtonian-like behavior ends.
Newtonian-like behavior is observed up to 330 rad ∙ s-1 for PET extruded without
chain extender and up to 20 rad ∙ s-1 for PET extruded with 2.0 wt% 1,3-PBO. In the
case of PET extruded with 2.0 wt% CBC, no Newtonian-like behavior was observed
anymore. Generally, the extended PET chains are higher entangled and, thus, the
disentanglement starts at lower shear rates which results in non-Newtonian behavior.
The thermal properties of PET are not markedly affected by the addition of small
amounts of the investigated chain extenders to the polymer melt in the extrusion
process. However, the crystallinity and the lamellar thickness distribution of PET
decrease after addition of chain extenders to the melt. Especially, the secondary
crystallization is disturbed after addition of chain extenders. Small concentrations of
Chapter 5
129
the chain extenders (0.2 wt% 1,3-PBO or 0.3 wt% CBC) do not have significant
effects on the thermal properties and crystallization of PET.
The COOH terminal group titrations show a slight decrease of the COOH content of
PET compounded with CBC and a strong decrease of the COOH content of PET
compounded with 1,3-PBO. 1,3-PBO has higher reactivity with the COOH terminal
groups of PET than CBC.
Moreover, an additional effect was observed by combining both chain extenders in a
reactive extrusion process. It was shown that 1,3-PBO and CBC are very effective
chain extenders for PET and can be used to replace the SSP process for a variety of
applications.
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Chapter 6
133
Development of New Masterbatches Containing Chain Extenders for Poly(ethylene terephthalate) A
Summary: New masterbatches for poly(ethylene terephthalate) (PET) which
contain chain extenders such as 1,3-phenylene-bis-oxazoline (1,3-PBO)
and/or N,N’-carbonylbiscaprolactam (CBC) were developed. Here, cyclic
poly(butylene terephthalate) (cPBT) which has no reactive end groups was
used as masterbatch matrix. The main advantage of these masterbatches is
their polyester-based matrix, but the incorporated chain extenders are still
active after the synthesis. The masterbatches were analyzed by TGA/FT-IR.
They were applied to poly(ethylene terephthalate) during extrusion and,
afterwards, also during spinning at high velocities on a pilot plant spinning
device. The influence of these masterbatches on the quality of PET was
analyzed by measurement of the screw force during extrusion, by viscosimetry
as a measure of molecular weight, and by rheometry of the extruded
compounds. The fibers spun on the pilot plant were analyzed by viscosimetry
and by tensile strength measurements. The results show that the quality of
polyester fibers spun from post-consumer PET can be improved by applying
the developed masterbatches which contain chain extenders during spinning.
Keywords: Chain extenders, extrusion, masterbatch, polyesters, recycling
A. Reproduced with permission from D. Berg, K. Schaefer, M. Moeller,
Macromolecular Symposia 2017, 375, 1600180, DOI: 10.1002/masy.201600180.
Copyright WILEY-VCH
Chapter 6
134
6.1 Introduction
As mentioned before, the typically used method to increase the molar mass of PET is
the solid state polycondensation (SSP) process [1]. This process is mainly used in
bottle-to-bottle (closed-loop) recycling industries, where high molar masses are
necessary. For the open-loop-recycling (for instance the recycling of bottle to fiber),
lower molar masses are needed. Post-consumer bottle grade PET can be used for
fiber production, where a lower viscosity is needed than for the manufacturing of
bottles. The intrinsic viscosity (IV or [η]) of post-consumer PET as a measure of
molecular weight is often too low, so that a mixture of post-consumer PET and virgin
PET has to be used for fiber production. For the usage of high amounts of post-
consumer poly(ethylene terephthalate) in the fiber production, the molar mass can be
adjusted by the addition of small amounts of chain extenders.
In continuation of the previous studies (cf. Chapter 5), spinning experiments with
addition of the chain extenders 1,3-phenylene-bisoxazoline (1,3-PBO) and/or
N,N’-carbonylbiscaprolactam (CBC) were performed. Both chain extenders extend
poly(ethylene terephthalate) in a linear way, which is necessary for fiber
manufacturing. Branched and cross-linked polyesters lead to blockade of spinning
filters during the spinning process. Furthermore, linear polyesters can be better
drawn. For the usage of additives in pilot plants or in manufacturing facilities, mainly
additives in form of masterbatches are necessary. Masterbatches are highly
concentrated mixtures of polymers (carrier resins) with additives, which can be added
in compounding processes to the raw polymer. The great advantages of using
masterbatches are, firstly, dust free addition of additives due to the compounding with
the carrier resin and, secondly, a better distribution in the raw polymer during
manufacturing. At best, the carrier resin and the polymer are the same polymers to
achieve an optimum distribution without impurities in the final product.
However, the usage of these chain extenders as masterbatch for the compounding of
PET is challenging. It is impossible to compound a PET masterbatch with chain
extenders in a conventional way with common PET. The chain extenders would react
directly with PET during masterbatch manufacturing resulting in their inactivation. A
possible solution for this problem is to compound the chain extenders in other
polymers (e.g., polyolefins) [2, 3]. Polyolefins such as polypropylene (PP) and
polyethylene (HDPE, LDPE and LLDPE) have no reactive end groups and the chain
Chapter 6
135
extenders would be still active. Other non-reactive matrices which are basically used
for chain extender masterbatches are polycarbonate (PC) [4], polystyrene (PS), or
acrylonitrile-butadiene-styrene copolymers (ABS) [5]. Furthermore, thermoplastic
waxes such as ethylene-acrylate copolymers (e.g., ethylene butyl acrylate, EBA),
ethylene vinyl acetate copolymers (e.g., ethylene vinyl acetate, EVA), or polyolefins
(e.g., low density polyethylene wax) are used for high concentrate masterbatches
containing chain extenders [6]. Also, polystyrene-methylmethacrylate copolymers can
be used as non-reactive carrier resins [7]. However, all non-reactive carriers are
foreign polymers which may have negative impact on the thermal and mechanical
properties of PET. Furthermore, reactive resins such as polylactide (PLA) and
poly(ethylene terephthalate) are available [8-10]. Another possibility to produce
masterbatches which contain chain extenders is to use low viscosity PET as matrix,
which has a reduced content of acid end groups [11]. Peeters et al. developed a
masterbatch based on PET which was modified with glycol (PET-G) or poly-ε-
caprolactone (PCL) with pyromellitic dianhydride (PMDA) and pentaerythritol as chain
coupling agent [12]. These reactive carriers (PET-G or PCL), however, can be
processed at temperatures below 250°C and the additives could be mixed without
significant reaction between the additive and the carrier. In patent
US000005536793 A, a further method has been presented to make a masterbatch
with PDMA as matrix. Standard polyester such as PET was used as reactive carrier
resin, too [13]. However, in that case, the chain extender which has been added at
high amounts reacts with the end groups of the carrier. If a higher amount of PDMA
has been applied than the reactive end groups of PET require, then a masterbatch
with still reactive PMDA is available. A disadvantage of this technology is, however,
that the possible compositions of the masterbatches are limited. PMDA is a
branching agent and can crosslink during the masterbatch synthesis resulting in gel
formation. This leads to problems in further processing steps especially during fiber
production. Furthermore, if bifunctional chain extenders are used, very high amounts
of chain extenders are needed to cap all end groups.
In this work, the development of polyester-based masterbatches containing chain
extenders is presented. For this, a cyclic poly(butylene terephthalate) oligomer cPBT
was used. The advantage of this material is that no reactive end groups are present.
Furthermore, the melt viscosity is very low (water-like) which results in a better
Chapter 6
136
distribution in the polymer melt during the spinning process; and finally, cPBT
polymerizes to high molar poly(butylene terephthalate) (PBT) in the presence of
catalysts and does not have negative influences on the good qualities of PET.
Moreover, spinning tests were carried out on a pilot plant at high velocities with the
addition of masterbatches containing 1,3-PBO and/or CBC as chain extenders.
6.2 Experimental Section
6.2.1 Materials
PET for fiber production was provided by Maerkische Faser GmbH (Premnitz,
Germany). The fiber grade polyester has an intrinsic viscosity of 0.63 dL ∙ g-1 and
was used for the extrusion experiments with the masterbatch.
6.2.2 Synthesis of the Chain Extender Masterbatches
120 g of cyclic poly(butylene terephthalate) oligomer (cPBT) (provided by IQ Tec
Germany GmbH, Schwarzheide, Germany) were molten at 190 °C. 30 g of
1,3-phenylene-bis-oxazoline (1,3-PBO) (Adeka Palmarole AG, Basel, Switzerland) or
N,N’-carbonylbiscaprolactam (CBC) (DSM, Geleen, NL) respectively were added to
the melt under continuous stirring. In the last step, butylchlorodihydroxystannane
(Arkema GmbH, Duesseldorf, Germany) was added to the melt as catalyst. In one
case (“MB without cat.”), the last step was not performed. After the addition of the
catalyst, the melt was quenched in an open bowl, cooled with liquid nitrogen and
dried under vacuum.
6.2.3 Extrusion
For the extrusion experiments, the Micro 15cc Twin Screw-Extruder (DSM, Geleen,
NL) was used. The experiments were performed in the melt of PET under nitrogen
atmosphere at 290 °C with 100 rounds per minute screw rotation speed in a
discontinuous process.
For the polymerization of cPBT to PBT, 15 g cPBT was added into the extruder
together with 0.3 wt% of butylchlorodihydroxystannane. Furthermore, chain extension
experiments were performed in the extruder with PET for fiber production. First, the
PET was dried at 130 °C in an oven overnight at least for ten hours. About 11 g PET
were molten in the extruder at 290 °C and mixed with different amounts of the
Chapter 6
137
masterbatches for about five minutes. The concentration of the added masterbatches
was calculated so that the amount of the chain extenders in PET is 0.1 wt%, 0.3 wt%,
0.5 wt%, or 1.0 wt%, respectively. After the extrusion experiments, the samples were
ground in a cryomill (6800 Freezer/Mill, SPEX CertiPrep, Stanmore, UK) to achieve
good homogeneity.
6.2.4 Analytics
6.2.4.1 TGA/FT-IR Analysis
The analysis of the masterbatches was performed with a thermo-gravimetric analyzer
(Perkin Elmer Simultaneous Thermal Analyzer STA 6000, Rodgau-Juegesheim,
Germany) coupled with an infrared spectrometer (Perkin Elmer FT-IR Spectrometer
Frontier, Rodgau-Juegesheim, Germany). About 20 – 30 mg of the sample was
weighed into a ceramic crucible. The samples were heated up to 600 °C under
nitrogen atmosphere with a heating rate of 20 K ∙ min-1. The IR spectra were
measured with a resolution of 4 cm-1 with a Mercury-Cadmium-Telluride detector
(MCT detector).
6.2.4.2 NMR Spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy was performed with a Bruker 400
NMR spectrometer. 1H-NMR spectra were recorded at 400 MHz. The samples were
dissolved in deuterated chloroform (CDCl3) (Deutero GmbH, Kastellaun, Germany).
6.2.4.3 Viscosimetry
The inherent viscosity (ηinh.) of the polymers was measured to analyze the impact of
the chain extenders on the polymer molecular weight. About 0.3300 g PET was
weighed in a 25 mL graduated flask and dissolved in
1,1,1,3,3,3-hexafluoropropane-2-ol (HFIP) (Fluorochem, Hadfield, UK). The viscosity
of this solution was measured at 25 °C using an Ubbelohde viscosimeter (type 0a)
(Schott AG, Mainz, Germany). The inherent viscosity was calculated according to
equation (1).
𝜂𝑖𝑛ℎ. =ln(𝜂𝑟𝑒𝑙)
𝛽=
ln(𝜂
𝜂0)
𝛽=
ln(𝑡
𝑡0)
𝛽 (1)
Chapter 6
138
ηinh. = inherent viscosity
ηrel = relative viscosity
β = mass concentration
η = viscosity of the PET solution
η0 = viscosity of the solvent
t = flow time of the PET solution
t0 = flow time of the solvent
6.2.4.4 Size Exclusion Chromatography
Molecular weights (Mn¯¯ and Mw¯¯) and molecular weight distribution (Ð) of the
masterbatches and PET samples were determined by size exclusion chromatography
(SEC). The masterbatches were dissolved in chloroform and PET was dissolved in
HFIP and diluted with chloroform to a volume concentration of chloroform/HFIP
98:2 vol% according to Weisskopf [14]. A HPLC pump (PU-2080plus, Jasco, Tokyo,
Japan) equipped with an evaporative light scattering detector (PL-ELS-1000, Polymer
Laboratories, Amherst, USA) was used.
2,6-Di-tert-butyl-4-methylphenol (c = 250 mg ∙ mL-1) was used as internal standard,
and narrow distributed polystyrene standards (PSS Polymer Standards Service
GmbH, Mainz, Germany) were used to achieve calibration. One pre-column (8 mm x
50 mm) and four SDplus gel columns (8 mm x 300 mm, MZ Analysentechnik, Mainz,
Germany) were applied at a flow rate of 1.0 mL ∙ min-1 at 20 °C. The separation
process was performed on polystyrene/divinylbenzene columns (50 Å, 100 Å,
1,000 Å, and 10,000 Å PSS Polymer Standards Service GmbH, Mainz, Germany).
Results were evaluated using the PSS WinGPC UniChrom software (Version 8.1.1).
6.2.4.5 Rheology
The rheology measurements were performed with a plate-plate rheometer (Discovery
HR-3 hybrid rheometer, TA Instruments-Waters L.L.C., New Castle, USA). The
polymer was molten at 290 °C and the melt was measured with a gap of 600 µm in a
frequency range of 0.1 Hz to 100 Hz with an oscillation of 2 %. The storage
modulus (G’), the loss modulus (G’’), and the complex viscosity (η*) were calculated.
Chapter 6
139
6.2.5 Pilot Plant Tests
After the application of the masterbatches to virgin PET melts, we tried also to apply
these chain extender masterbatches to post-consumer poly(ethylene terephthalate)
melts (provided by Reiling Unternehmensgruppe, Marienfeld, Germany) in a pilot
plant station to produce fibers at very high velocities. These feasibility tests were
realized at the Thuringian Institute of Textile and Plastics Research (TITK,
Rudolstadt, Germany). Bottle PET flakes which were only sorted, washed, and dried
were molten in the melting spinning device (FET, Leeds, UK) and spun at
5000 m ∙ min-1 under nitrogen flow.
6.2.5.1 Tensile Test
After the spinning tests, the tensile strength of the produced yarns was measured
using the tensile testing device Zwick Z 005 (Zwick/Roell GmbH, Haan, Germany).
The tests were carried out according to DIN EN ISO 2062. The clamping length was
250 mm and the testing speed was 250 mm ∙ min-1 up to fiber breakage.
6.3 Results and Discussion
6.3.1 Synthesis and Characterization of Masterbatches Containing Chain
Extenders
At first, the polymerization of cPBT was studied by melting and mixing with the
catalyst butylchlorodihydroxystannane in the extruder. After two minutes the
extrusion curve showed a strong increase of the screw force (Figure 6.1). This is an
indication that the melt viscosity of cPBT increased and polymerization to
poly(butylene terephthalate) (PBT) was obtained (Scheme 6.1).
Chapter 6
140
Scheme 6.1. Reaction of cPBT with tin organic catalysts (Cat.) such as
butylchlorodihydroxystannane to poly(butylene terephthalate) (PBT).
09:40 09:41 09:43 09:44 09:46 09:47
0
500
1000
1500
2000
2500
F/ N
t/ h:min
cPBT + catalyst at 290°C
Figure 6.1. Extrusion curve of cPBT with addition of 0.3 %
butylchlorodihydroxystannane measured at 290 °C with 100 rpm.
Afterwards, the viscosity of the obtained polymer was determined by measurement of
the inherent viscosity. Figure 6.2 shows the results of the inherent viscosity
measurements. cPBT has a very low inherent viscosity of 0.12 dL ∙ g-1; after thermal
treatment in the extruder without butylchlorodihydroxystannane, the inherent viscosity
decreased to 0.08 dL ∙ g-1. cPBT extruded with 0.3 % butylchlorodihydroxystannane
has an inherent viscosity of 0.84 dL ∙ g-1. This result shows that the cPBT oligomer
polymerized to high molecular PBT. If cPBT is used as carrier in a chain extender
masterbatch, its low viscosity will not disturb in presence of
butylchlorodihydroxystannane since it polymerizes accompanied by an increase of
the viscosity. Furthermore, good distribution of the masterbatch in the PET melt is
Chapter 6
141
achieved, because the masterbatches are polyester-based and have a low viscosity
in the beginning of the compounding process.
0.120.08
0.84
cPBT cPBT without cat. cPBT + 0.3% cat.
0.0
0.2
0.4
0.6
0.8
in
h. /
dL
g
-1
virgin
extruded
Figure 6.2. Inherent viscosities of virgin cPBT, cPBT extruded without and with
addition of 0.3 % butylchlorodihydroxystannane (cat. = catalyst).
The compounded masterbatches were analyzed by TGA/FT-IR to evaluate the
amount of chain extenders which were still active after the synthesis. Figure 6.3 is a
3D overview spectrum of one TGA/FT-IR measurement of the CBC masterbatch. In
the overview, two signals at different temperatures can be recognized (cf. red
arrows). The first weaker signal at about 290 °C is the signal of the chain extender
(here: CBC) and the second more intense signal at approx. 410 °C is the signal of
degradation products of the cPBT matrix.
Chapter 6
142
Figure 6.3. 3D graph of TGA/FT-IR measurement of cPBT masterbatch which
contains N,N’-carbonylbiscaprolactam as chain extender. (The red arrows indicate
the two major FT-IR signals.)
The IR spectra of the masterbatches and chain extenders at different temperatures
are presented in the following figures (Figure 6.4 to Figure 6.6). Figure 6.4 depicts IR
spectra obtained from TGA/FT-IR measurements of 1,3-PBO (black) at 296 °C and of
1,3-PBO masterbatch (red) at 279 °C. Both spectra correspond overall with each
other. For 1,3-PBO, a boiling point of 403.5 °C has been predicted based on
calculations using Advanced Chemistry Development (ACD/Labs) Software V11.02
(© 1994-2017 ACD/Labs). Thus, evaporation of 1,3-PBO at 279 °C can be excluded,
and the IR results of the 1,3-PBO masterbatch at 279 °C can be attributed to the
thermal decomposition of 1,3-PBO.
Chapter 6
143
Figure 6.4. IR spectra obtained by TGA/FT-IR measurements of 1,3-PBO at 296 °C
(black, upper spectrum) and of the 1,3-PBO masterbatch at 279 °C (red, lower
spectrum).
Chapter 6
144
Furthermore, in Figure 6.5 the spectra obtained by TGA/FT-IR measurements of
CBC at 311 °C (black) and of the CBC masterbatch at 293 °C (blue) are given. Also
in these spectra, no significant differences are recognizable. The strong IR-signals at
about 2300 cm-1 can be attributed to carbon dioxide and carbon monoxide formed
during heating (Figure 6.4 and 6.5). The CO2- and CO-Signals are more pronounced
in case of heated cPBT masterbatch compared to the pure chain extenders. This
means that unconverted chain extender is still present after the synthesis of the
masterbatch, and that the chain extenders are still active like before the masterbatch
synthesis. Thus, these masterbatches can be used for further chain extension
experiments.
Chapter 6
145
Figure 6.5. IR spectra obtained by TGA/FT-IR measurements of CBC at 311 °C
(black, upper spectrum, cf. previous page) and of the CBC masterbatch at 293 °C
(blue, lower spectrum).
Also, the IR spectra of cPBT at 415 °C (black) and the spectra of the masterbatches
at temperatures of about 410 °C (1,3-PBO masterbatch at 408 °C [red] and CBC
masterbatch at 412 °C [blue]) in Figure 6.6 reveal that no reaction between cPBT and
the chain extenders are detectable in the IR spectra obtained by TGA/FT-IR
measurements. All spectra are identical, which means that also the carrier cPBT has
still the same chemical structure, and thus, no reaction between cPBT and the chain
extenders is observed by TGA/FT-IR analysis.
Chapter 6
146
0.0
0.2
0.4
0.6
0.0
0.2
0.4
0.6
4000 3500 3000 2500 2000 1500 1000 500
0.0
0.2
0.4
0.6
abso
rba
nce cPBT at 415°C
abso
rba
nce 1,3-PBO MB at 408°C
abso
rba
nce
/ cm-1
CBC MB at 412°C
Figure 6.6. IR spectra obtained by TGA/FT-IR measurements of cPBT at 415 °C, of
the 1,3-PBO masterbatch at 408 °C and of the CBC masterbatch at 412 °C in the gas
phase.
However, the TGA curves in Figure 6.7 show that in the case of the 1,3-PBO
masterbatch small amounts of this chain extender reacted with cPBT during the
synthesis. A possible reason could be, on the one hand, that cPBT which has some
carboxylic end groups reacted with 1,3-PBO or, on the other hand, that some cycles
opened during the melting process during the masterbatch synthesis and reacted
with 1,3-PBO. Due to the induced heat to melt cPBT, some rings can open and few
carboxylic groups are formed. Anyway, the TGA results show, that only 2.3 % of
1,3-PBO reacted with cPBT during the synthesis and 17.7 % of 1,3-PBO are still
active for further chain extension processes. The calculation of the active chain
extender content was done as follows:
cPBT (pure): Mass change = 98.0 %; residual mass = 2.0 %
CBC masterbatch: Mass change of CBC = 18.5 %; residual mass = 3.5 % - 2.0 %
(cPBT) = 1.5 %
Active CBC content in the masterbatch: 18.5 % + 1.5 % = 20 %
Chapter 6
147
1,3-PBO masterbatch: Mass change of 1,3-PBO = 14.5 %; residual mass = 5.2 % -
2.0 % (cPBT) = 3.2 %
Active 1,3-PBO content in the masterbatch: 14.5 % + 3.2 % = 17.7 %
The residual mass obtained after TGA measurement of pure cPBT as char in the
crucible amounts to 2 %. Thus, the higher amounts of the residual masses in the
masterbatches can be attributed to the chain extender.
0 100 200 300 400 500 600
0
20
40
60
80
100
m=2.0%m=3.5%m=5.2%
m1= 18.5% at 290°C
m2= 78.0% at 412°C
m2= 80.3% at 408°C
m1= 14.5% at 276°C
m
/ %
°C
cPBT
1,3-PBO MB
CBC MB
m= 98.0% at 415°C
Figure 6.7. TGA results of the TGA/FT-IR measurements of cPBT, of the 1,3-PBO
masterbatch and of the CBC masterbatch.
Furthermore, size exclusion chromatography (SEC) measurements were performed
using chloroform as eluent to prove that no reaction between cPBT and the chain
extenders occurred. In both cases, no high molar masses are observed. A number
average molar mass (Mn¯¯) of 379 g ∙ mol-1 and a weight average molar mass (Mw¯¯) of
425 g ∙ mol-1 with a molecular weight distribution of 1.1 (Ð) was measured for the
1,3-PBO masterbatch, and Mn¯¯ of 389 g ∙ mol-1 and Mw¯¯ of 435 g ∙ mol-1 with Ð = 1.1
was measured for the CBC masterbatch. Both chromatograms are presented in
Figure 6.8. The highest molar mass, which was detected in these SEC
measurements, is in both cases about 1320 g ∙ mol-1 which is six times the repeating
unit of cPBT.
Chapter 6
148
200 400 600 800 1000 1200 1400
0
1
2
3
4
5
6
7
8
w(log M
)
M / g mol-1
1,3-PBO MB
CBC MB
Figure 6.8. Size exclusion chromatograms (SEC) of 1,3-PBO MB and CBC MB using
chloroform as eluent.
NMR measurements were also performed to evaluate whether a reaction between
the carrier and the chain extenders occurred. In Figure 6.9, the 1H-NMR spectra of
cPBT, CBC, and the CBC MB are presented, exemplarily. The peaks at 1.9 ppm
(-OCH2CH2CH2CH2O- / peak a) and at 4.4 ppm (-OCH2CH2CH2CH2O- / peak b) are
the proton signals of the aliphatic parts of cPBT. The peak at about 8.0 ppm (peak c)
is attributed to the aromatic protons of cPBT. The peaks at 1.7 ppm
(-C(O)CH2CH2CH2CH2CH2N- / peak d), 2.5 ppm (-C(O)CH2CH2CH2CH2CH2N- /
peak e), and 3.7 ppm (-C(O)CH2CH2CH2CH2CH2N- / peak f) can be attributed to the
caprolactam protons of the CBC. The signal at 7.2 ppm corresponds with the
chemical shift of the solvent CDCl3. The 1H-NMR spectra reveal also that no reaction
of the chain extender and the carrier occurred. Also in the case of the 1,3-PBO
masterbatch, no evidence for a reaction of the chain extender with the carrier was
observed by 1H-NMR spectroscopy.
Chapter 6
149
Figure 6.9. Chemical structure of cPBT and CBC, and proton NMR spectra of cPBT
(black), CBC (red) and the CBC masterbatch (blue).
Furthermore, 13C-NMR spectra were recorded. In Figure 6.10, the 13C-NMR spectra
of cPBT (black), CBC (red), and the CBC masterbatch (blue) are presented. The
Chapter 6
150
peaks at 25.9 ppm (-OCH2CH2CH2CH2O- / peak a) and at 64.9 ppm
(-OCH2CH2CH2CH2O- / peak b) are signals of the aliphatic carbons of cPBT. The
aromatic carbon shifts of cPBT are in the range of 129.6 ppm (peak c) and
134.0 ppm (peak d). The shift at 165.7 ppm corresponds with the carbonyl carbon
(C=O / peak e) (cf. Figure 6.10).
Moreover, the peaks at 22.7 ppm (-C(O)CH2CH2CH2CH2CH2N- / peak f), 28.5 ppm
(-C(O)CH2CH2CH2CH2CH2N- / peak g), and 29.6 ppm (-C(O)CH2CH2CH2CH2CH2N- /
peak h), 39.0 ppm (-C(O)CH2CH2CH2CH2CH2N- / peak i) and 47.3 ppm
(-C(O)CH2CH2CH2CH2CH2N- / peak j) can be attributed to the carbons of the
methylene groups of CBC. The shift at 156.8 ppm is allocated to the carbonyl carbon
(N-C(O)-N) in the middle and the shift at 176.5 ppm belongs to the carbonyl carbon of
the caprolactams. The chemical shifts from 76.7 to 77.4 ppm are the peaks of the
solvent CDCl3. The 13C-NMR spectra show no evidence for the occurrence of a
reaction of the chain extender and the carrier. Also in the case of the 1,3-PBO
masterbatch, no evidence for a reaction of the chain extender and the carrier was
observed (cf. Figure S6.1 + S6.2).
Chapter 6
151
Figure 6.10. Chemical structure of cPBT and CBC, and 13C-NMR spectra of cPBT
(black), CBC (red) and the CBC masterbatch (blue).
Chapter 6
152
6.3.2 Compounding of PET with Chain Extender Masterbatches
At first, the masterbatches were applied during compounding using the micro-
extruder. In Table 6.1, an overview of the applied concentrations of the
masterbatches and the concentrations of the incorporated chain extenders is given.
Table 6.1. Overview of the investigated chain extended masterbatch samples and the
concentrations of the incorporated chain extenders.
Sample corresponds
with w(1,3-PBO MB) w(1,3-PBO) w(CBC MB) w(CBC)
MB without cat. Figure 6.9 1.50 wt% 0.27 wt% 5.0 wt% 1.0 wt%
MB + cat. Figure 6.9 1.50 wt% 0.27 wt% 5.0 wt% 1.0 wt%
0.1 % Figure 6.10 0.56 wt% 0.1 wt% - -
0.3 % Figure 6.10 1.69 wt% 0.3 wt% - -
0.5 % Figure 6.10 2.82 wt% 0.5 wt% - -
1.0 % Figure 6.10 5.65 wt% 1.0 wt% - -
0.1 % Figure 6.11 - - 0.5 wt% 0.1 wt%
0.3 % Figure 6.11 - - 1.5 wt% 0.3 wt%
0.5 % Figure 6.11 - - 2.5 wt% 0.5 wt%
1.0 % Figure 6.11 - - 5.0 wt% 1.0 wt%
In Figure 6.9, the results of the inherent viscosities of the compounded PET samples
are presented. Virgin, fiber grade PET with an inherent viscosity of 0.66 dL ∙ g-1 was
used for the chain extension experiments. At first, PET was extruded without any
addition of masterbatches. Its inherent viscosity decreased to 0.61 dL ∙ g-1 due to
thermal degradation. Secondly, virgin PET was mixed during extrusion with a
masterbatch which contained 0.27 % 1,3-PBO (active 1,3-PBO content) and 1.0 %
CBC without or with 0.3 % catalyst (in relation to cPBT). The results show that a
strong increase of the inherent viscosity occurred. The inherent viscosity of PET
which was compounded with the masterbatch mixture, but without catalyst amounts
to 0.79 dL ∙ g-1; and for PET compounded with the masterbatch mixture and with the
catalyst an IV of 0.81 dL ∙ g-1 was measured. The effective functioning of the chain
extenders is detectable in both cases for the same concentrations. The advantage of
Chapter 6
153
the catalyst is that cPBT polymerizes to PBT which does not reduce the viscosity of
PET.
0.660.61
0.79 0.81
PET PET MB without cat. MB + cat.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
in
h./
dL g
-1
virgin
extruded
Figure 6.9. Inherent viscosities of virgin PET and PET extruded without and with
addition of masterbatches. In one masterbatch (MB) 0.3 % (m/m)
butylchlorodihydroxystannane was added as catalyst (cat.).
Furthermore, extrusion experiments were performed with addition of the
masterbatches which contain different concentrations of chain extenders.
Concentrations of 0.1 %, 0.3 %, 0.5 %, and 1.0 % (m/m) of 1,3-PBO or CBC were
applied with PET as masterbatch. In the following figure, extrusion curves of three
selected CBC masterbatch concentrations are presented to show that an increase of
the viscosity was observed (Figure 6.10). The higher the amount of the chain
extender masterbatch is, the higher is the increase of the force. The increase of the
force (F) of the screws indicates an increase of the melt viscosity of the polymer and,
therefore, an enhancement of the molecular weight.
Chapter 6
154
Figure 6.10. Extrusion curves of PET compounded with CBC masterbatches
containing 0.1, 0.5, and 1.0 % (m/m) chain extender. The forces (F/ N) of the screws
are measured at 290 °C with 100 rpm rotation speed.
The subsequent inherent viscosity measurements reveal that the viscosity of PET
increased upon addition of higher amounts of these masterbatches (Figure 6.11 and
Figure 6.12). In the case of the 1,3-PBO masterbatch, 0.565 %, 1.695 %, 2.825 %,
and 5.650 % (m/m) were added during extrusion of PET to achieve 1,3-PBO
concentrations of 0.1 %, 0.3 %, 0.5 %, and 1.0%, respectively. A concentration of
2.825 % of 1,3-PBO MB (containing 0.5 % 1,3-PBO) is required to increase the
inherent viscosity of PET to a value, which is comparable to the inherent viscosity of
virgin PET. At a concentration of 5.650 % 1,3-PBO MB (corresponding to 1.0 %
1,3-PBO), an inherent viscosity of 0.64 dL ∙ g-1 has been achieved, which is a good
value for fiber grade PET (cf. Figure 6.11).
Chapter 6
155
0.6270.606 0.615 0.623 0.627 0.642
virgin 0.0% 0.1% 0.3% 0.5% 1.0%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
in
h./
dL
g-1
1,3-PBO MB
Figure 6.11. Inherent viscosities of virgin PET, PET extruded without and with
addition of 1,3-PBO masterbatches (1,3-PBO concentration ranging from 0 – 1.0 %,
m/m).
In the case of the CBC masterbatch, 0.5 %, 1.5 %, 2.5 %, and 5.0 % (m/m)
masterbatch was added during extrusion of PET to achieve CBC concentrations of
0.1 %, 0.3 %, 0.5 %, and 1.0 %, respectively. Here, smaller amounts of the
masterbatch were needed to increase the inherent viscosity to a level which is
comparable with that of virgin PET. Concentrations between 0.3 % CBC (containing
1.5 % CBC MB) and 0.5 % CBC (with 2.5 % CBC MB) lead to a good value for the
inherent viscosities of the extruded PET. The highest value was achieved after
applying a concentration of 1.0 % CBC (with 5.0 % CBC MB). In this case, an
inherent viscosity of 0.672 dL ∙ g-1 was obtained (cf. Figure 6.12).
Chapter 6
156
0.6270.606 0.615 0.624
0.6450.672
virgin 0.0% 0.1% 0.3% 0.5% 1.0%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
in
h./
dL
g
-1
CBC MB
Figure 6.12. Inherent viscosities of virgin PET, PET extruded without and with
addition of CBC masterbatches (CBC concentration ranging from 0 – 1.0 %, m/m).
Furthermore, size exclusion chromatography (SEC) measurements of the PET
samples were performed in a chloroform/HFIP (98/2 vol%) solution. In Figure 6.13
and Table 6.2, the SEC results of virgin PET and PET extruded with 0.1 % - 1.0 %
1,3-PBO masterbatches are presented. While virgin PET has an average molar mass
of 14.4 kg ∙ mol-1 (number average molar mass Mn¯¯) and 40.7 kg ∙ mol-1 (weight
average molar mass Mw¯¯), PET which was extruded with 1,3-PBO masterbatches has
an average molar mass up to 15.5 kg ∙ mol-1 (Mn¯¯) and 44.8 kg ∙ mol-1 (Mw¯¯). The
molecular weight distribution is in each sample in the same range between 2.6 and
2.9. The increases of the molar mass and the inherent viscosity of PET indicate that
chain extension reactions were successfully performed at 290 °C with addition of
1,3-PBO masterbatches in a reactive extrusion process.
Chapter 6
157
Figure 6.13. Size exclusion chromatography (SEC) results of virgin PET compounded
with 1,3-phenylene-bisoxazoline masterbatches (0.1 % – 1.0 %) in comparison to
virgin PET. The number average molar mass (Mn¯¯) and weight average molar mass
(Mw¯¯) are presented.
Table 6.2. Size exclusion chromatography (SEC) results of virgin PET compounded
with 1,3-phenylene-bisoxazoline masterbatches (0.1 % – 1.0 %) in comparison to
virgin PET. The number average molar mass (Mn¯¯), weight average molar mass (Mw¯¯),
and the molecular weight distribution (Ð) are presented.
PET
sample Mn¯¯ / kg ∙ mol-1 Mw¯¯ / kg ∙ mol-1 Ð
virgin 14.4 40.7 2.8
0 wt% 14.5 41.5 2.9
0.1 wt% 15.6 40.6 2.6
0.3 wt% 16.2 42.7 2.6
0.5 wt% 15.0 43.8 2.9
1.0 wt% 15.5 44.8 2.9
Moreover, SEC measurements were also performed with PET extruded with CBC
masterbatch amounts of 0 % to 1.0 % (m/m) which document that chain extension
reactions were successfully achieved (Figure 6.14, Table 6.3). A molar mass change
Chapter 6
158
from 14.4 kg ∙ mol-1 (number average molar mass Mn¯¯) and 40.7 kg ∙ mol-1 (weight
average molar mass Mw¯¯) to 13.1 kg ∙ mol-1 (number average molar mass Mn¯¯) and
48.4 kg ∙ mol-1 (weight average molar mass Mw¯¯) was measured. The number average
molar mass (Mn¯¯) decreased slightly; however, the molecular weight distribution is
much higher (Table 6.3), due to the carrier. With high amounts of the masterbatch,
also high amounts of the carrier were added. There are a few unpolymerized
percentages of the cyclic poly(butylene terephthalate) oligomer (cPBT) present which
result in a broad molecular weight distribution. Therefore, Mn¯¯ decreases slightly and
Ð increases, but chain extension occurs due to reaction of CBC with PET which
results in an increase of Mw¯¯.
Figure 6.14. Size exclusion chromatography (SEC) results of virgin PET compounded
with N,N’-carbonylbiscaprolactam masterbatches (0.1 % – 1.0 %) in comparison to
virgin PET. The number average molar mass (Mn¯¯) and weight average molar mass
(Mw¯¯) are presented.
Chapter 6
159
Table 6.3. Size exclusion chromatography (SEC) results of virgin PET compounded
with N,N’-carbonylbiscaprolactam masterbatches (0.1 % – 1.0 %) in comparison to
virgin PET. The number average molar mass (Mn¯¯), weight average molar mass (Mw¯¯),
and the molecular weight distribution (Ð) are presented.
PET
sample Mn¯¯ / kg ∙ mol-1 Mw¯¯ / kg ∙ mol-1 Ð
virgin 14.4 40.7 2.8
0 wt% 14.5 41.5 2.9
0.1 wt% 13.6 42.9 3.2
0.3 wt% 14.0 41.7 3.0
0.5 wt% 15.9 46.8 2.9
1.0 wt% 13.1 48.4 3.7
The complex viscosity (η*) was measured with the help of the parallel plate
rheometer (Figure 6.15 and Figure 6.16). Figure 6.15 shows that the PET melts
behave Newtonian-like up to 400 rad ∙ s-1. Above 400 rad ∙ s-1, shear thickening
behavior is observed in four cases. PET with the 1,3-PBO content of 1.0 % shows
shear thinning behavior starting at an angular frequency of about 40 rad ∙ s-1. In that
case, the content of the carrier cPBT in the PET melt is higher and behaves as shear
thinner at higher angular frequencies. The PET samples extruded with the 1,3-PBO
masterbatch have higher complex viscosities than the sample without added chain
extender masterbatch. At an angular frequency of 0.6 rad ∙ s-1, the complex viscosity
increases from 75 Pa ∙ s to 151 Pa ∙ s which indicates the occurrence of chain
extension upon application of this masterbatch (MB containing 1.0 % of 1,3-PBO).
Chapter 6
160
0.1 1 10 100 1000
60
80
100
120
140
160
1.0%
0.5%
0.3%
0%
0.1%
*/
Pa
s
/ rad s-1
0%
0.1%
0.3%
0.5%
1.0%
Figure 6.15. Complex viscosity (η*) of PET extruded with 1,3-PBO masterbatches
(1,3-PBO concentration ranging from 0 – 1.0 %, m/m) (measured at 290 °C).
Figure 6.16 shows that the complex viscosity of PET extruded with the CBC
masterbatch decreases slightly at higher angular frequencies up to 400 rad ∙ s-1. At
higher angular frequencies of more than 400 rad ∙ s-1, strong increase of the complex
viscosity can be observed. At this certain point, the melt has shear-thickening
behavior. The polymer chains are strongly entangled resulting in increased viscosity.
The complex viscosities at small angular frequencies (here: ω = 0.6 rad ∙ s-1) raised
from 75 Pa ∙ s (0 % CBC) to 141 Pa ∙ s (1.0% CBC). This indicates that chain
extension also occurred during compounding of PET with addition of the CBC
masterbatches.
Chapter 6
161
0.1 1 10 100 1000
60
80
100
120
140
160
1.0%
0.5%
0.3%
0.1%
*/
Pa
s
/ rad s-1
0%
0.1%
0.3%
0.5%
1.0%
0%
Figure 6.16. Complex viscosity (η*) of PET extruded with CBC masterbatches (CBC
concentration ranging from 0 – 1.0 %, m/m) (measured at 290 °C).
The rheological behavior of these PET samples was further determined with the help
of the parallel plate rheometer. In the following figures, the storage moduli (G’) and
the loss moduli (G’’) of PET treated with the CBC masterbatch are given. The storage
moduli of PET which was extruded with CBC MB are shown in Figure 6.17. At higher
angular frequencies (ω), the storage moduli are also higher. The higher storage
moduli are a result of the higher shear rates. The higher the angular frequencies are,
the higher are also the shear rates of the samples. This leads to stronger
entanglement of the polymer chains resulting in increase of the storage modulus.
Slight increases of the storage moduli are obtained for the samples with increased
masterbatch concentration. The PET sample extruded without a masterbatch has a
storage modulus in the range of 2.8 Pa at 0.6 rad ∙ s-1 to 30,000 Pa at 630 rad ∙ s-1.
Compared to this, the PET sample which was compounded with 1.0% of CBC
masterbatch has the highest storage modulus ranging from 6.3 Pa at 0.6 rad ∙ s-1 to
40,000 Pa at 630 rad ∙ s-1.
Chapter 6
162
1 10 100 1000
1
10
100
1000
10000
1.0%
0.5%
0.3%0.1%
0%
G´/
Pa
/ rad s-1
0%
0.1%
0.3%
0.5%
1.0%
Figure 6.17. Storage moduli (G’) of PET extruded with CBC masterbatches (CBC
concentration ranging from 0 – 1.0 %, m/m) (measured at 290 °C).
Furthermore, the loss moduli are presented in Figure 6.18. Also, the loss moduli are
higher at higher angular frequencies. Due to the higher shear rate of the melt, the
internal friction of the chains is also higher. Hence, the loss of energy is also higher at
higher shear rates, which is defined as loss modulus. Figure 6.18 shows that an
increase of the loss modulus occurs after addition of higher amounts of the CBC
masterbatch. An increase of the loss modulus from 48 Pa to 88 Pa at ω = 0.6 rad ∙ s-1
and an increase from 57,500 Pa to 65,000 Pa at ω = 630 rad ∙ s-1 is obtained.
Chapter 6
163
0.1 1 10 100 1000
100
1000
10000
1.0%
0.5%
0.3%
0.1%
0%
G´´
/ P
a
/ rad s-1
0%
0.1%
0.3%
0.5%
1.0%
Figure 6.18. Loss moduli (G’’) of PET extruded with CBC masterbatches (CBC
concentration ranging from 0 – 1.0 %, m/m) (measured at 290 °C).
6.3.3 Pilot Plant Spinning of Post-consumer PET with Added Chain Extender
Masterbatches
In the last step, the masterbatches were applied to melts of post-consumer
poly(ethylene terephthalate) in a pilot plant to produce fibers at very high velocities
(5000 m ∙ min-1) (Figure 6.19). During these tests, it was discovered that only small
amounts of chain extenders are needed to produce fibers with good quality at high
velocities. The higher the concentration of the applied chain extenders, the more
viscous is the melt. Melts which are too viscous, cannot be spun and drawn at high
velocities so that the fibers break during drawing. The following scheme shows a
schematic setup of the pilot plant, which was used for these experiments
(Scheme 6.2).
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164
Scheme 6.2. Schematic setup of a pilot plat spinning device, which was used for
spinning experiments at high velocities up to 5000 m ∙ min-1.
Figure 6.19. Pictures of the melting spinning device (FET, Leeds, UK), realized at the
Thuringian Institute of Textile and Plastics Research (TITK, Rudolstadt, Germany)
Chapter 6
165
(Left: Drawing station, middle: Drawing station with fibers, right: Overview of the pilot
plant).
Measurements of the inherent viscosity of PET fibers spun with addition of these
masterbatches show an increase which can be attributed to an increase of the molar
mass as mentioned before. While PET extruded without addition of a chain extender
masterbatch has an inherent viscosity of 0.64 dL ∙ g-1, PET fibers spun with addition
of chain extender masterbatches have higher inherent viscosities (0.67 dL ∙ g-1 –
0.72 dL ∙ g-1, cf. Figure 6.20).
0.640.67 0.67 0.67
0.720.68
extruded
0.09% 1,3-PBO
0.27% 1,3-PBO
0.1% CBC
0.2% CBC
0.09% 1,3-PBO + 0.1% CBC
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
in
h./
dL g
-1
Figure 6.20. Inherent viscosities of the polymer from fibers spun from post-consumer
PET without chain extender (extruded) and with addition of chain extenders (0.09 –
0.27 % 1,3-PBO, 0.1 – 0.2 % CBC, and 0.09 % 1,3-PBO + 0.1 % CBC) on a pilot
plant spinning device with a velocity of 5000 m ∙ min-1.
Afterwards, the produced yarns were measured using the tensile test device Zwick
Z 005 (Zwick/Roell GmbH, Haan, Germany). The tensile strength (σ) and residual
elongation (ε) of the PET fibers were calculated and the results are presented in
Figure 6.21.
The tensile strength of the PET fibers spun with addition of masterbatches is slightly
higher than that of the PET samples spun without addition of additives. The residual
elongation of the produced fibers shows that the PET samples spun with addition of
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166
masterbatches are slightly better drawn than the PET sample spun without additive.
These results reveal that the addition of 1,3-PBO and CBC as masterbatch leads to
fibers with better mechanical properties than PET fibers spun without addition of a
chain extender masterbatch.
39.541.3
39.640.4
42.340.5
22.1 22.221.5 20.9
19.317.9
extruded
0.09% 1,3-PBO
0.27% 1,3-PBO
0.1% CBC
0.2% CBC
0.09% 1,3-PBO + 0.1% CBC
0
10
20
30
40
/ cN
tex
-1
/ %
Figure 6.21. Results of the tensile tests of fibers spun from post-consumer PET on
the pilot plant with a speed of 5000 m ∙ min-1. Blue: The tensile strength of post-
consumer PET fibers spun without (extruded) and with chain extenders (0.09 –
0.27 % 1,3-PBO, 0.1 – 0.2% CBC, and 0.09 % 1,3-PBO + 0.1 % CBC). Orange: The
residual elongation of post-consumer PET fibers spun without and with chain
extenders (0.09 – 0.27 % 1,3-PBO, 0.12 – 0.2 % CBC, and 0.09 % 1,3-PBO + 0.1 %
CBC).
6.3.4 Pilot Plant Spinning of Post-consumer PET with Added Chain Extender
Masterbatches and Zinc Peroxide
Finally, further spinning experiments were performed on the pilot plant. A
combination of zinc peroxide (cf. Chapter 4) and the chain extender masterbatches
were performed to check the spinnability of peroxide treated poly(ethylene
terephthalate). These experiments were successfully performed. While the spinning
tests with the investigated post-consumer PET and zinc peroxide were not feasible,
combinations of post-consumer PET with zinc peroxide and chain extender
masterbatches were successful. Spinning experiments at 4400 m ∙ min-1, as well as,
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167
velocities at 5000 m ∙ min-1 are feasible.
In Figure 6.22, the results of inherent viscosity measurements are presented. The
squared column (post-consumer PET extruded with 0.3 % zinc peroxide) is a
compounded PET which could not be spun and drawn in the pilot plant at high
velocities (4400 m ∙ min 1 and 5000 m ∙ min-1) due to the decrease of the viscosity.
The shaded column (post-consumer PET extruded with 0.27 % 1,3-PBO and 0.1 %
CBC) could not be spun and drawn at 5000 m ∙ min-1 as the melt viscosity was to too
high to achieve a good spinnability. Combinations of zinc peroxide with chain
extender masterbatches (post-consumer PET + 0.3 % zinc peroxide + 0.27 %
1,3-PBO + 0.1 % CBC) could be spun at very high velocities up to 5000 m ∙ min-1.
The results of the inherent viscosities and the spinning tests reveal that neither too
low melt viscosities nor too high melt viscosities are good to spin fibers at high
velocities.
Figure 6.22. Inherent viscosities of the polymer from fibers spun from post-consumer
PET without additive (extruded) and with addition of zinc peroxide (0.3 %), and/or
chain extender masterbatches (0.27 % 1,3-PBO, 0.1 % CBC) on a pilot plant
spinning device with a velocity of 4400 m ∙ min-1 or 5000 m ∙ min-1. The shaded
column is a sample, which cannot be spun at 5000 m ∙ min-1 and the squared column
is a sample, which cannot be spun at both velocities.
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168
The following tensile tests of these spun and drawn fibers are shown in Figure 6.23.
Compared to the fibers spun without addition of additives, the fibers spun with
addition of zinc peroxide and chain extender masterbatches reveal a loss of the
tensile strength (e.g. from 36.9 cN ∙ tex-1 to 29.6 cN ∙ tex-1) and an increase of the
residual elongation (e.g. from 39.8 % to 50.0 %). The tensile strength decreases as a
result of the oxidative degradation of PET due to the action of zinc peroxide. But, the
chain extender masterbatches can repair the degraded polymer chains so that
spinning, even at high velocities, is feasible.
Figure 6.23. Results of the tensile tests of fibers spun from post-consumer PET on
the pilot plant with a speed of 4400 m ∙ min-1 and 5000 m ∙ min-1 respectively. Blue:
The tensile strength of post-consumer PET fibers spun without and with addition of
chain extender masterbatches (0.27 % 1,3-PBO, 0.1 % CBC) and zinc peroxide
(0.3 %). Orange: The residual elongation of post-consumer PET fibers spun without
and with addition of chain extender masterbatches (0.27 % 1,3-PBO, 0.1 % CBC)
and zinc peroxide (0.3 %).
6.4 Conclusions
A simple polyester-based masterbatch which contains chain extenders such as
1,3-phenylene-bis-oxazoline (1,3-PBO) and/or N,N’-carbonylbiscaprolactam (CBC)
was synthesized using cyclic poly(butylene terephthalate) (cPBT) as matrix. The
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169
advantages of cPBT are that no or only small amounts of reactive end groups are
present and that its low viscosity leads to a good distribution in the PET melt during
processing. Afterwards, polymerization of cPBT can be achieved in the compounding
process with the help of catalysts. Contrarily to cPBT, the polymerized PBT is no
viscosity reducer if applied to PET melts. TGA/FT-IR measurements reveal that
2.3 % of the applied 1,3-PBO is bound to cPBT during masterbatch synthesis,
whereas no chemical binding is observed in the case of CBC. Furthermore, no
changes in the chemical structure of the chain extenders during the masterbatch
synthesis were identified. The addition of these masterbatches to melts of virgin PET
(ηinh. = 0.627 dL ∙ g-1) in a laboratory scale extrusion process led to an increase of its
inherent viscosity up to ηinh. = 0.672 dL ∙ g-1. Rheological measurements show also an
increase of the complex viscosity, storage modulus, and loss modulus of these
masterbatch treated polyesters. Furthermore, the developed masterbatches were
applied successfully during extrusion and spinning of post-consumer PET on pilot
plant scale at very high velocities (5000 m ∙ min-1). Under these conditions, polyester
fibers from post-consumer PET flakes can be spun more easily if the developed
masterbatches were added. Chain extension of post-consumer PET was achieved
after extrusion with the developed masterbatches containing 1,3-PBO and/or CBC as
analyzed by viscosimetry. Addition of small amounts of these masterbatches (with a
chain extender concentration of max. 0.27 %) leads to the best spinnability of post-
consumer PET. An increase of the mechanical properties of polyester fibers spun
from post-consumer PET with added chain extenders was obtained, too.
At last, spinning tests were also performed with post-consumer PET with addition of a
combination of zinc peroxide as bleaching agent and chain extender masterbatches.
These feasibility experiments show, that a combination of zinc peroxide and chain
extenders can be used to spin and draw fibers from post-consumer poly(ethylene
terephthalate). Gray post-consumer PET can be bleached and repaired by using this
technology in future.
6.5 References
[1] F. Awaja, D. Pavel, European Polymer Journal 2005, 41, 1453.
[2] WO001995009884 A1 (1995), Eastman Chemical Company, invs.: K. C.
Khemani, J. W. Mercer, R. L. Mcconnell.
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170
[3] US000005801206 A (1998), Eastman Chemical Company, invs.: K. C. Khemani,
J. W. Mercer, R. L. McConnell.
[4] EP000000801108 A2 (1997), Sinco Engineering S.p.A., invs.: H. Al Ghatta, S.
Cobror.
[5] WO002016071126 A1 (2016), Clariant Int. Ltd., invs.: J. Wolf, K. A. Wartig, T. van
den Abbeele, T. Lünstäden.
[6] EP000002343330 A1 (2011), Armacell Enterprise GmbH, inv. J. Li.
[7] US20040147678 A1 (2004), Clariant Int. Ltd., invs.: W. Blasius, V. Karayan, D.
Dodds.
[8] A. Jaszkiewicz, A. K. Bledzki, R. van der Meer, P. Franciszczak, A. Meljon,
Polymer Bulletin 2014, 71, 1675.
[9] A. Jaszkiewicz, A. K. Bledzki, A. Duda, A. Galeski, P. Franciszczak,
Macromolecular Materials and Engineering 2014, 299, 307.
[10] P. Kiliaris, C. D. Papaspyrides, R. Pfaendner, Journal of Applied Polymer
Science 2007, 104, 1671.
[11] US6515044 B1 (2003), Bayer AG, invs.: K. J. Idel, H. J. Dietrich, M. Müller.
[12] US20060293416 A1 (2006), Ciba Specialty Chemicals Corp., invs.: G. Peeters,
M. O'Shea, G. Moad, R. Tozer, D. Simon.
[13] US000005536793 A (1996), Amoco Corp., invs.: G. E. Rotter, W. Chiang, B. C.
Tsai, J. L. Melquist, C. A. Pauer, S. Y. Chen.
[14] K. Weisskopf, Journal of Polymer Science Part A-Polymer Chemistry 1988, 26,
1919.
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171
6.6 Supporting Information
6.6.1 NMR Spectra of the 1,3-PBO Masterbatch
1H-NMR: δ / ppm:
1.9 (-OCH2CH2CH2CH2O- / peak a); 4.4 (-OCH2CH2CH2CH2O- / peak b); 8.0
(aromatic part / peak c) of cPBT
4.0 (=NCH2CH2O- / peak d); 4.4 (=NCH2CH2O- / peak e); 7.4 (aromatic proton in
meta position / peak f); 8.0 (aromatic protons in ortho position / peak g) of 1,3-PBO
(cf. Figure S6.1).
Additional peaks can be attributed to impurities of 1,3-PBO.
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172
Figure S6.1. Chemical structure of cPBT and 1,3-PBO (previous page), and proton
NMR spectra of cPBT (black), 1,3-PBO (red) and the 1,3-PBO masterbatch (blue).
13C-NMR: δ / ppm:
25.9 (-OCH2CH2CH2CH2O- / peak a); 64.9 (-OCH2CH2CH2CH2O- / peak b);
129.6 ppm (non-substituted aromatic carbons / peak c); 134.0 ppm (substituted
aromatic carbons / peak d); 165.7 ppm (carbonyl carbon / peak e) of cPBT
55.0 (=NCH2CH2O- / peak f); 67.7 (=NCH2CH2O- / peak g); 128.0 (aromatic carbon in
ortho position / peak h); 128.1 (aromatic carbon in meta position / peak i); 128.5
(aromatic carbon in ortho position / peak j); 130.8 (substituted aromatic carbon /
peak k); 164.0 (-N=C-O-/ peak l) of 1,3-PBO (cf. Figure S6.2).
The 1,3-PBO peaks in the masterbatch are too weak as the peaks disappear almost
in the background noise.
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173
Figure S6.2. Chemical structure of cPBT and 1,3-PBO, and 13C-NMR spectra of
cPBT (black), 1,3-PBO (red) and the 1,3-PBO masterbatch (blue).
