Degradation and Stabilisation of Aromatic Polyesters

Degradation and Stabilisation of Aromatic Polyesters

Stuart Fairgrieve

Smithers Rapra Update

Degradation and Stabilisation of Aromatic Polyesters Stuart Fairgrieve

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2009 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2009, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.

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ISBN: 978-1-84735-457-0 (Hardback) 978-1-84735-458-7 (ebook)

Typeset by Argil Services Printed and bound by Lightning Source Inc.

C

ontents

1

2

3

Background .......................................................................... 1 1.1

Historical Development .............................................. 1

1.2

Structure and Morphology ......................................... 9

1.2.1

Introduction ................................................9

1.2.2

PET ...........................................................10

1.2.3

PTT ...........................................................11

1.2.4

PBT ...........................................................12

1.2.5

PCT ...........................................................13

1.2.6

PEN...........................................................14

1.2.7

LCP ...........................................................15

Thermal Degradation ......................................................... 21 2.1

Poly(ethylene terephthalate) (PET) ............................ 21

2.2

Poly(butylene terephthalate) (PBT) ........................... 35

2.3

Poly(trimethylene terephthalate) (PTT) ..................... 42

2.4

Other Poly(alkylene terephthalate)s .......................... 44

2.5

Poly(alkylene naphthalate)s (PAN) ........................... 47

2.6

Poly(alkylene phthalate)s and Poly(alkylene isophthalate)s ...................................... 48

2.7

Poly(p-phenylene alkanedioate)s ............................... 48

2.8

Highly Aromatic Polyesters ...................................... 49

Thermo-Oxidative Degradation ......................................... 65 3.1

Poly(ethylene terephthalate) (PET) ............................ 65

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Degradation and Stabilisation of Aromatic Polyesters

4

3.2

Poly(butylene terephthalate) (PBT) ........................... 73

3.3

Poly(trimethylene terephthalate) (PTT) ..................... 77

3.4

Other Aromatic Polyesters ........................................ 80

Photodegradation and Radiation Degradation .................. 85 4.1

Photodegradation and Oxidation of Poly (ethylene terephthalate) (PET)................................... 85

4.2

Photodegradation and Oxidation of Other Poly(alkylene terephthalate)s .................................... 93

4.3

Photodegradation and Oxidation of Poly(alkylene naphthalate)s ...................................... 94

4.4 5

6

7

iv

4.3.1 Formation of a naphthalic acid end group .95 Radiation Degradation ............................................. 96

Chemical Degradation and Recycling ............................... 107 5.1

Hydrolytic Degradation .......................................... 107

5.2

Ester Interchange .................................................... 109

5.3

Aminolysis .............................................................. 110

5.4

Biodegradation ....................................................... 110

5.5

Chemical Recycling ................................................ 112

Thermal and Hydrolytic Stabilisation ............................... 143 6.1

Introduction ........................................................... 143

6.2

Thermal Stabilisation .............................................. 144

6.3

End-capping ........................................................... 153

6.4

Chain Extension ..................................................... 156

6.4.1

Without Additives ...................................156

6.4.2

Chain Extenders ......................................157

Thermo-oxidative Stabilisation......................................... 181 7.1

Introduction ........................................................... 181

7.2

Studies on Antioxidants .......................................... 183

7.3

Potential New Chemistries ...................................... 187

Contents 7.4 8

Patents .................................................................... 188

Stabilisation Against Ultraviolet and Ionising Radiation... 199 8.1

Introduction to Ultraviolet (UV) Stabilisation ......... 199

8.2

UV Screeners and Absorbers ................................... 200

8.3 8.4

8.5

8.2.1

Background .............................................200

8.2.2

Salicyclates ..............................................202

8.2.3

Benzophenones ........................................203

8.2.4

Benzotriazoles. ........................................204

8.2.5

Cinnamates and Related Types ................206

8.2.6

Oxanilides ...............................................209

8.2.7

Cyclic Imino Esters ..................................209

8.2.8

Triazines ..................................................210

8.2.9 Miscellaneous ..........................................211 Excited State Quenching ......................................... 212 Radical Scavengers ................................................. 213 8.4.1

Background .............................................213

8.4.2

HALS ......................................................214

8.4.3 Other Radical Scavengers ........................219 Ionising Radiation Stabilisation .............................. 219

Appendix – Commercial Additive Structures ............................ 241 Abbreviations ........................................................................... 261 Index ........................................................................................ 263

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Degradation and Stabilisation of Aromatic Polyesters

vi

P

reface

While over the years there have been many books and review articles published on the subject of polymer degradation and stabilisation, most have been rather wide-ranging studies, covering either polymers in general, or a range of polymers and/or additives. Even in cases where specific polymers or classes of polymers have been covered, the literature contains few specific reviews of the degradation and stabilisation of condensation polymers such as the aromatic polyesters. There appear to be a number of reasons for this. Firstly, there are the factors of tonnage and range of application. The most used polymers, such as the polyolefins and styrenics, tended to attract the most attention simply as a consequence of their ubiquity. Secondly, there is the factor of rate of deterioration. In general, polyolefins and styrenics degrade noticeably faster than, for example, Nylon 6 or poly(ethylene terephthalate) under equivalent conditions of heat or light exposure, so that the need for effective stablisation against such reactions seemed to be more urgent for the former. A third factor, I would tentatively suggest, was the perception that the degradation and stabilisation of condensation polymers was a study that was somehow more ‘difficult’ due to their very different chemistry compared to that of addition polymers such as the polyolefins. Indeed, in some quarters the view has been expressed that the condensation polymers might not, in fact, require stabilisation, or that the reactions undergone by such polymers during degradation did not lend themselves to being suppressed by known stabilisation pathways.

vii

Degradation and Stabilisation of Aromatic Polyesters As we head towards the second decade of the 21st century, I feel that the points raised in the previous paragraph no longer apply to aromatic polyesters, hence the decision to produce this review of the state of the art of degradation and stabilisation of this class of polymer. In terms of tonnage and use, poly(ethylene terephthalate) is these days virtually a commodity plastic, with widespread, large volume, use in food packaging, beverage bottle and fibres. Other aromatic polyesters have increased in volume production over the last decade or so, such as poly(butylene terephthalate) and poly(ethylene naphthalate). There is also the promise of large quantity use of the most recently commercialised member of the family, poly(trimethylene terephthalate). All such polyesters are also being increasingly pushed into more demanding environments in their applications, including greater heat exposure (for example in hot-fill food packaging applications and ovenable containers) and outdoor applications, where long term stability towards sunlight and oxygen are a prerequisite. The need for a full understanding of the degradation mechanisms, and means of suppressing these, are thus now more required than in the past While some controversies remain, the study of aromatic polyester degradation in the last few years has begun to untangle the mechanisms involved, particularly through the use of analytical techniques previously unavailable. The study of stabilisation has also moved on, with examination of why the established commercial stabilisers (largely developed for use in polyolefins and rubbers) are not as effective in aromatic polyesters, and also attempts to develop new stabilisers specifically for use in such polymers. I have also included in this book information on the chemical recycling of aromatic polyesters, which may be regarded as controlled degradation of these polymers. With the increasing emphasis on conservation of non-renewable resources, the recovery of various useful chemicals from materials which have reached the end of their

viii

Preface useful lives, or are off-specification for their intended use, is a process which needs to be considered. Especially useful is the recovery of starting materials which may then be used to manufacture further polyesters, and the study of the means for carrying out such processes, and their industrial implementation, are important topics in the current ‘green’ climate. Finally, I would like to express my appreciation to the publishing team at iSmithers, Shawbury, for their assistance with the compilation of this book. Especial thanks go to Frances Gardiner for commissioning this work, and to Eleanor Carter for assistance, above and beyond the call of duty, with literature searches and the supply of papers. Dr. Stuart Fairgrieve SPF Polymer Consultants Kidlington July 2009

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x

1

Background

1.1 Historical Development The condensation reaction between carboxylic acid and alcohol to form an ester and water is a fundamental chemical reaction, and has been known at least from when the term ‘organic chemistry’ was coined in the early nineteenth century. From the mid- to latenineteenth century, studies were carried out on the reactions of compounds containing more than one acid group with compounds containing more than one hydroxyl group. These studies produced the first polyester resins, and lead to the development in the early twentieth century of various thermoset polymers such as the alkyd resins. By the late 1920s, researchers had started to try to produce useful thermoplastic materials by the reactions of A-, W-hydroxy acids, or of pairings of diacids with diols. The first systematic study was carried out by Wallace H. Carothers, initially using standard distillation equipment to drive the reaction [1], then using a molecular still to further shift the equilibrium of the reaction to synthesise products of sufficiently high molecular weight to be fabricated into commercial articles such as fibres and films [2]. Other investigators found that the reactions could be enhanced by using catalysts such as salts of alkaline earth or heavy metal ions [3]. These studies appear to have been restricted to reactions between aliphatic diacids and aliphatic diols. The products found were, at this time at least, not commercially viable for several reasons, and Carothers, who was interested in all aspects of condensation polymerisation, quickly discovered more interesting materials: aliphatic polyamides. The Nylons were rapidly

1

Degradation and Stabilisation of Aromatic Polyesters developed based on these studies, and Carothers did not return to polyester study before his death in 1937. Attempts had been made to synthesise polyesters based on phthalic acid as the diacid component, but these products were amorphous, had low softening points, and were rapidly attacked by organic solvents and acids and bases. Research into polyesters made by the reaction of terephthalic acid (or esters thereof) with aliphatic diols, led to the discovery of polyesters of high commercial value: poly(alkylene terephthalate)s [4]. This pairing of diols with terephthalic acid eventually led to the most commercially successful aromatic polyesters, but other synthetic pathways were also investigated towards such products in the early days of polyester development. These included the ‘self-condensation’ of hydroxy acids of the structure -HO-R-Ph-CO2H, where R-OH is para to the acid group and R is -(CH2)- or -(CH2)2- [5], and reactions of aliphatic diacids with 1,4-dihydroxybenzene and similar aromatic diols [6, 7]. Also synthesised about the same time were polyesters based on C2-C6 aliphatic diols and any of the isomeric naphthalene dicarboxylic acids [8]. Research was carried out in commercial laboratories into wholly aromatic polyesters, especially those based on the self-condensation of p-hydroxybenzoic acid, to produce materials of higher potential strength and stability [9–11]. It was found that such homopolymers were extremely refractory materials, and could not be processed by normal techniques used for standard thermoplastics. Further work on aromatic polyesters centred on the condensation of aromatic diacids and dihydric phenols [12], and the condensation of non-fused polynuclear aromatic diacids with non-fused polyaromatic phenols [13, 14] which were expected to be less refractory than the prior art aromatic polyesters. Neither approach provided the desired combination of extreme physical properties with processability. Research centred on copolymers of terephthalic acid, isophthalic acid, bisphenols, and p-hydroxybenzoic acid to provide materials with the

2

Background necessary tractability while retaining improved physical properties and thermal resistance [15–17]. Further variations of this approach elicited several co-polyesters which featured the unusual property of forming anisotropic melts [18, 19], and these led to the development of liquid crystal polyesters (LCP) [20]. Even without considering copolymers, i.e., polymers made by condensing two or more different diacids and/or two or more different diols, it is evident that there are hundreds of possibilities for aromatic polyesters. As is usually the case with potentially large families of polymers, combinations of physical properties, ease of processing, potential applications, and economics limit the number of family members that make it into successful commercial production and use. The first material to be widely commercialised (and still the most successful family member) was poly(ethylene terephthalate) (PET). This was initially utilised in the fibres and textiles industries, where it still remains a major player; later it was used for the manufacture of films, and as an engineering plastic. Its most recent large-scale use is in the production of vast quantities of soft-drinks bottles and packaging applications for other foods and beverages. By the early 1970s, poly(butylene terephthalate) (PBT) gained commercial acceptance as an engineering plastic. While having no really outstanding properties, its balance of properties makes it a useful alternative to more expensive speciality engineering plastics, especially glass-filled grades. While poly(ethylene naphthalate) (PEN) was known from early research, it became a commercially viable product only in the late 1980s when full-scale production of the required precursor dimethyl ester of 2,6-naphthalenedicarboxylic acid began. PEN is used in similar applications to PET (i.e., fibre, film, packaging) where higher temperature resistance and lower gas permeability are required. LCP caused great excitement when they were initially developed, and great things were expected from them. The initial large number of companies producing this class of polyester rapidly dwindled to

3

Degradation and Stabilisation of Aromatic Polyesters a handful, and LCP remain very much high-cost speciality materials. Although some applications have been found in electronics and electrical connectors, they remain to a large extent ‘a solution looking for a problem’. Poly(propylene terephthalate), more often referred to as poly(trimethylene terephthalate) (PTT), was identified from research into aromatic polyesters, but was not commercialised at the time due to difficulty in obtaining pure, low-cost, 1,3-propanediol. Finally introduced into large-scale production in the late 1990s, there are great hopes for commercial application of this polymer, especially as a fibre. In general, it has properties between those of PET and PBT, but has certain unique properties of its own, including superior resilience and wear properties, giving carpets tufted with such fibres physical properties akin to the nylons, and stain resistance similar to PET. Other aromatic polyesters which have been commercialised include poly(1,4-cyclohexylenedimethylene terephthalate) (PCT; used in the production of circuit board components and automotive applications) and the clear, amorphous, polymer poly(1,4-cyclohexylenedimethylene terephthalate-co-isophthalate). The methods and materials used to synthesise most polymers, and especially condensation polymers such as the polyesters, can exert profound effects on the final product, including influencing polymer stability. It is therefore useful to briefly examine the methods used to manufacture aromatic polyesters and to assess the likely effects on the degradation behaviour of the resultant products. Thermoplastic aromatic polyesters need to be produced to a molecular weight of between 12,000 and 60,000 to be useful. The first stage is usually esterification (diacid and diol) or ester-exchange (diester and diol) to provide the first stage product, e.g., a bis(hydroxyalkyl) terephthalate, and potentially some linear oligomeric species. Water or alcohol (usually methanol) is evolved, and removed by fractional distillation. The decision to use the diacid or dimethyl ester in the manufacturing process is usually a matter of balancing questions

4

Background of economics, purity of feedstock, and ease of handling; in fact for poly(alkylene terephthalates), most methods start with the dimethyl ester of terephthalic acid. The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21–23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the esterinterchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. The second (polymerisation) stage is usually achieved in an autoclave, fitted with a stirrer system of suitable power to deal with the viscous melt, under high vacuum and at a temperature above the melting point of the target polymer. During this stage, preventing oxidative degradation of the melt is important, and is achieved by blanketing the vessel with an inert gas. During the polycondenation stage, linear oligomers and the bishydroxyalkyl terephthalate esters undergo a succession of ester-interchange reactions, eliminating the diol which is removed, again under high vacuum, and the molecular weight of the polymer is gradually built up to a suitable level. A catalyst is required for the polymerisation stage of the synthesis: tin and titanium compounds are suitable catalysts for both stages of the reaction but, for the specific case of PET, antimony trioxide (Sb2O3) is a favoured polymerisation catalyst [26]. This catalyst becomes active only at the higher temperature associated with the polymerisation stage of the reaction, and can be added at the beginning of the

5

Degradation and Stabilisation of Aromatic Polyesters ester-interchange reaction. Residual antimony compounds can lead to discoloration of PET due to the presence of Sb3+ ions, and their potential further reduction to Sb0 metal. PET can become more grey or green as a result of this reduction of Sb3+ to Sb0. The finely divided elemental form of antimony can cause Rayleigh scattering of incident light, giving a green tinge to the hot polymer, whereas the grey discoloration is associated with the black coloration of elemental Sb. Paradoxically, this effect may be more pronounced if phosphorus-based stabilisers are added to PET [27]. Titanium catalysts are even more of a problem with PET, leading to a distinct yellow discoloration of the polymer [28], possibly due to reactions with unsaturated chain ends (cf the intense yellow colour of titanium complexes with dienes and aromatic systems). Ge catalysts also suffer from this problem in PET, but to a markedly lesser degree. Such discoloration does not appear to be a problem for PBT and PTT. Discoloration of the final polymer may also result from the manner in which the starting materials have been made. If transition metal catalysts have been used in the manufacture, for example, of terephthalic acid or its ester, residues remaining may subsequently have an effect on the polymer made using such materials. If a polyester is manufactured in the standard melt-condensation manner, a small amount of cyclic oligomer(s) is formed, which is in equilibrium with the polymer [29–32]. This can be extracted, but this is not an economic process, and in any case the equilibrium will be re-established when the polymer is re-melted for fabrication into finished products. The presence of such oligomers, usually at about 1.5 wt%, is not a major problem in most cases, but they can exude to the surface of a polyester product. In the case of fibres, with their very high surface-to-volume ratio, this can interfere with dyeing operations, for example. Even for moulded articles the presence of oligomers at the surface can give trouble in operations which require clean surfaces (e.g., electroless metal plating).

6

Background Because of the reversible nature of the polycondensation reaction, and the high temperature used in the manufacture of polyesters, it is sometimes not possible to generate polymers of sufficiently high molecular weight directly from melt polycondensation. Attempts to do so result in uneconomically long reaction times and/or thermal degradation of the polyester. This results in adverse effects on the usefulness of the product. To prepare polyesters of very high molecular weight, a process known as solid-state post-condensation (SSP) is used [33, 34]. Dried polymer chips of moderate molecular weight are heated to a temperature approximately 20 °C below the polymer melting point, either in a high vacuum or in a stream of hot inert gas, in a device which agitates the solid. Before carrying out the procedure, it is necessary to crystallise the polymer to the highest possible extent to prevent sintering together of the chips. By careful annealing, an optimum melting point of the polymer is achieved, allowing the SSP process to be carried out at the maximum possible temperature and with a reasonably short reaction time. In the SSP process, the volatile by-products of the continuing polycondensation reaction escape by vapour diffusion through the solid chip and are rapidly removed from the chip surface instead of the much slower process of diffusion through a bulk melt. During the polymerisation reaction, various by-products may be formed. In the case of PET there are two main by-products, both of which can result in problems with the use and stability of the polyester. Diethylene glycol (DEG) units can be generated in the polyester chain by dehydration of 2-hydroxyethyl ester chain ends to form an ether link. This process cannot be entirely prevented, but can be minimised only by careful control of the parameters of the polymerisation process. DEG units depress the melting point of PET. They also have an adverse effect on polymer crystallinity, reducing the strength of fibres and oriented films, and increasing the susceptibility of the polyester towards chemical attack and aqueous hydrolysis [35].

7

Degradation and Stabilisation of Aromatic Polyesters The other significant by-product generated during PET manufacture is acetaldehyde, which is produced by thermal degradation processes taking place alongside the polymerisation. Random O-CH2 scission of ester units leaves a vinyl ester end and a carboxyl chain end. The vinyl ester reacts with a polymer end group to form a new polymer link and expels acetaldehyde, the tautomer of the actual leaving group, vinyl alcohol [24]. Although this by-product is highly volatile, its presence even at very low levels (approximately 3 ppm) is sufficient to cause tainting of beverages and foodstuffs packaged in PET. PET bottles are therefore SSP-treated immediately before stretch blow-moulding to keep this contamination to an absolute minimum. Other reactions associated with acetaldehyde formation lead to further problems in the thermal degradation processes of PET (see Section 2.1). PTT acts in a similar manner to PET, forming bis-3-hydroxypropyl ether groups in the polymer chain, and volatile by-products. The volatile product in this case is allyl alcohol, and the more problematical acrolein [36]. In the case of PBT, at least in the first stage of thermal degradation during polymerisation, the principal volatile material is tetrahydrofuran, formed by cyclisation of 1,4-butanediol or by internal cyclisation of C4 ester units [37]. This by-product is largely harmless because it is non-reactive under polymerisation conditions and, being highly volatile, is quickly removed. Polyesters with higher aromatic content such as LCP are made via an alternative route. Because they are phenolic esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavourable reactivities. The usual method is reverse ester exchange or acidolysis reaction [38] where the phenolic hydroxyl groups are acylated with a lower aliphatic acid anhydride, and this ester is heated with an aromatic dicarboxylic acid, with or without catalyst. Many of these polymers are derived from hydroxyacids, and their acetates readily undergo self-condensation in the melt, stoichiometric balance being inherent to the reaction [39, 40].

8

Background In some cases it is possible to directly polycondense acid and phenols evolving water at about 300 °C [41]. This process works well when catalysed with compounds of group IV or V metals; tin salts are preferred, especially dialkyl tin dialkanoates or oxides. The process is not really suitable if p-hydroxybenzoic acid is involved because this can undergo decarboxylation at >200 °C. The polymerisation is not affected because the phenol formed volatilises at the high temperatures involved, but the resultant polyester will contain less than the anticipated mole ratio of hydroxybenzoate-derived units. Hydroxynaphthoic acid does not suffer from this problem, and direct esterification processes may be used if non-benzoate copolymers are required [42]. The use of essentially all-aromatic starting materials, and the volatilisation of potential organic contaminants at the high polymerisation temperatures encountered in such processes, means that few problems are anticipated with instability caused in this manner, with the possible exception of metal catalyst residues. Studies on the effect of polymerisation-induced foreign species on the utility and/or degradation of such high-aromatic (co)polyesters seem to be lacking.

1.2 Structure and Morphology 1.2.1 Introduction Most aromatic polyesters are semi-crystalline materials; they exhibit crystalline and amorphous regions within one article. The processing of the polyester will have a profound effect on the relative ratios of the two morphologies. There is also the possibility, especially in highly oriented specimens such as biaxially oriented films and fibres, that a third morphology may emerge: ‘oriented amorphous’. Polymer morphology will have a direct effect on the degradation and on the potential for stabilisation of the substrate. In most cases,

9

Degradation and Stabilisation of Aromatic Polyesters crystalline regions are denser than the amorphous regions, i.e., the diffusion of impurities, radicals and oxygen will be much faster in amorphous zones than through crystalline regions. This will result in degradation being confined to amorphous regions, at least in the initial stages of degradation or oxidation. It is also known that stabilisers, initially dispersed throughout the polymer melt during processing, will be largely excluded from the growing crystals as the melt cools, and will be further redistributed during later processes such as fibre orientation. This process will, as might be surmised, work in favour of stabilisation of the host polymer because the additives are concentrated in the most vulnerable regions. There is a caveat to this: the concentration of a stabiliser in the amorphous regions of a highly crystallised polyester could theoretically result in saturation and subsequent loss of additive. Though it is outside the scope of this review to fully discuss the morphological complexities of aromatic polyesters, data are provided on the differences in crystallinity and structure of several important aromatic polyesters.

1.2.2 PET The crystal structure of PET is triclinic [43], i.e., none of the cell angles is a right angle, and there is only one repeat unit per unit cell. The cell parameters are: a = 0.456 nm, b = 0.594 nm, c = 1.075 nm, A = 98.5°, B = 118°, and G = 112°. The chain is fully extended (i.e., comparison of c with length calculated from standard bond lengths and angles for the extended trans conformer shows that the unit length is approximately 99% of cell length). The direction of p-disubstitution on the benzene ring makes an angle of approximately 19° with the c-axis, giving a very shallow ‘sawtooth’ appearance. Packing is good, with the benzene rings on adjacent chains virtually eclipsing each other. In general, projections on one molecule fit the hollows on an adjacent one. Early work on morphological changes on tensile testing of PET

10

Background fibres [44] indicated that crystalline strain was small compared with macroscopic overall strain. The conclusion was that crystalline regions possessed a very much higher stiffness than macroscopic stiffness, and that under stress most of the strain could be attributed to distortion of non-crystalline regions.

1.2.3 PTT Two studies [45, 46] on PTT structure were published almost simultaneously. Both determined that the unit cell was triclinic, with the parameters shown in Table 1.1.

Table 1.1 Crystalline structure of poly(trimethylene terephthalate) – Unit cell parameters [45]

[46]

a (Å)

0.464

0.46

b (Å)

0.627

0.62

c (Å)

1.864

1.83

A (°)

98.4

98

B (°)

93.0

90

G (°)

111.1

112

Both sets of authors also concluded that each unit cell contained two repeat units. Comparison of the unit length of the gauche-gauche conformer noted from the diffraction data showed that this is only 76% extended, considerably less than for PET Both sets of authors confirmed that both methylene bonds in the diolderived segment of the chain are in gauche conformation, making the -O-(CH2)3-O- sequence into a helical, shortened configuration. This results in a virtually Z-shaped overall conformation in the a–c plane of the unit cell, making for a considerably deeper sawtooth appearance. In the case of good packing, this provides for the possibility of much

11

Degradation and Stabilisation of Aromatic Polyesters greater steric hindrance to lateral movement of adjacent molecules than would be possible in PET. The crystal lattice of PTT is, as expected from these data, unusually compliant. Recent successful growth of large crystals of PTT [47] has allowed a more precise set of parameters to be obtained. This has confirmed the triclinic nature of the unit cell, which was ambiguous in the light of the value of 90° previously given for B [46]. The new values are: a = 0.453 nm, b = 0.620 nm, c = 1.870 nm, A = 97.6°, B = 93.2°, and G = 110.1°

1.2.4 PBT As with the other polyesters in this series, the unit cell of PBT is triclinic [48], with the parameters: a = 0.48 nm, b = 0.59 nm, c = 1.16 nm, A = 98°, B = 116°, and G = 110°. The unit cell contains one repeat unit. Data show that the methylene section of the chain has the conformation gauche-trans-gauche. Comparison of overall length of this configuration to the unit cell shows that the chain is not fully extended, although not to the same degree as in PTT, being 86% of unit cell length. The chain appears to be folded or ‘crumpled’, rather than the helical configuration noted for PTT. The configuration described above can change, reversibly, to transtrans-trans under low strain. This conformational change also produces, as might be expected, a change in unit cell parameters to: a = 0.47 nm, b = 0.58 nm, c = 1.3 nm, A = 102°, B = 121°, and G = 105°. The angle between the plane of the p-disubstituted phenylene group and the alkylene chain is similar to that of PET, but the sawtooth formed is deeper than PET due to the greater length of the methylene chain. It is nowhere near the deep zigzag of PTT as shown in a–c plane models. Because the conformation angles in the strained form are close to those of PET, it might be expected that this would be the stable form.

12

Background The fact that it is not suggests that each conformation represents an energy minimum, with the relaxed form being the lowest, with the energy barrier between them being sufficiently low for thermal activation to transform most material to the unstrained form at lower temperatures. Tension reverses the relative heights of these energy levels. The volume of the unit cell of the shortened, relaxed, form is smaller than that of the other, indicating a more economically packed, lower energy, form.

1.2.5 PCT First manufactured in 1959 [49], PCT was initially developed as a fibre, particularly for carpets, but has more recently been proposed as a film-forming or moulding plastic. Replacement of a straight chain diol with cyclohexanedimethanol produces a polymer with a regular structure but a stiffer chain than PET. Because cyclohexanedimethanol can exist as all-trans, all-cis, or mixtures of the two isomers, this can affect the structure and stability of the molecule. For example, the all-trans isomer has a melting point of 315–320 °C, whereas all-cis melts at 260–267 °C. Using mixtures of isomers, a range of melting points can be obtained. The polymer is generally noted as having a distinctly improved thermal and UV stability over PET, which means that it is much more suited to outdoor applications. Investigations of the crystal structure of the ‘homopolymers’ [50, 51] showed that both were triclinic, and exhibited the parameters shown in Table 1.2.

Table 1.2 Crystalline structure of of poly(cyclohexanedimethylene terephthalate) - Unit cell parameters a (Å)

b (Å)

c (Å)

A (°)

B (°)

G (°)

Trans

6.37

6.63

14.2

89

47

114

Trans

6.46

6.65

14.2

89

47

115

Cis

6.02

6.01

13.7

89

53

112

13

Degradation and Stabilisation of Aromatic Polyesters Various copolymers based on this chemistry have also been commercialised, including poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalateco-isophthalate). Both of these types are largely amorphous, clear, polymers.

1.2.6 PEN Although known since the 1940s, PEN attained commercial significance only in the late 1980s when large-scale production of the precursor dimethyl-2,6-naphthalene dicarboxylate was achieved. This polyester exhibits higher temperature resistance, UV resistance and tensile strength than PET, and is also a superior barrier to water and oxygen. Structurally, the main difference between PEN and PET is replacement of the phenyl ring with the naphthyl unit. This leads to a stiffer chain, so that the melting point and the glass transition temperature are higher in PEN. Although PEN crystallises at a slower rate than PET, crystallisation is similarly enhanced by orientation, and the barrier properties are much superior to PET. This latter factor has meant that PEN has been proposed as a useful packaging material, especially in hot-fill applications. PEN also finds major applications in fibres [52]. Various copolymers, such as poly(ethylene terephthalate-co-naphthalate) have been produced, as well as other homopolymers in the naphthalate series, especially poly(butylene naphthalate). PEN appears to exist in two crystalline modifications, both triclinic [53, 54], with the parameters shown in Table 1.3.

Table 1.3 Unit Cell Parameters of Poly(ethylene naphthalate) a

b

c

A o

Alpha form

6.51 Å

5.75 Å

13.2 Å

81

Beta form

9.26 Å

15.6 Å

12.7 Å

122o

14

B 144 96o

G o

100o 123o

Background Analysis of the crystal structure of PEN shows that the chains are arranged in such a way that naphthalene rings in one chain are very closely adjacent to -CH2-CH2- units of a neighbouring chain, much closer than in PET. It is therefore conceivable that some additional interaction could take place between these atoms in crystalline regions of this polyester.

1.2.7 LCP Certain aromatic polyesters, containing stiff segments in the chain, exhibit ordered phases which are similar to those seen in smallmolecule liquid crystalline, or mesophasic, substances. In particular, the rod-like molecules are oriented in shear to such an extent that interchain entanglement is small, and the melts consequently exhibit a lower than expected viscosity. On cooling of the melt, the molecules remain oriented, effectively self-reinforcing the polymer in the direction of flow. In general, LCP are synthesised from monomers that are long, flat and rigid along their major axis. Common examples of such mesogens are terephthalic acid, p-hydroxybenzoic acid, hydroquinone, 2-hydroxy-6-napththoic acid, and 4,4´-bisphenols. The ratios of these components may be varied to ‘fine tune’ the properties of the product. With only such aromatic mesogens in place, the polyesters thus formed are highly insoluble, have high melting points, and exhibit a wide temperature range of mesogenic behaviour. The order exhibited by the LCP may vary (in descending degree of ordering) through crystalline, smectic, nematic and isotropic phases. Smectic LCP exhibit what may be referred to as ‘two-dimensional’ ordering, where the polymer chains are largely parallel to each other, creating further order or layering of said mesogens orthogonal to the polymer chain main axis. In nematic ordering, the chains are largely parallel, but the mesogens no longer exhibit the additional ordering associated with the smectic phase. Note that not all LCP exhibit smectic and nematic behaviour.

15

Degradation and Stabilisation of Aromatic Polyesters Wholly aromatic polyesters, while exhibiting some mesophasic behaviour, can be extremely difficult to process by standard methods, and various means for alleviating such concerns have been used: a) Incorporation of flexible spacer units. b) Copolymerisation of several mesogenic monomers of different sizes to give a random or more irregular structure. c) Introduction of units with lateral substituents to disrupt chain symmetry. d) Synthesis of chains with built-in kinks through the use of unsymmetrically linked aromatic units (‘crankshaft’ approach). The homopolymer of p-hydroxybenzoic acid is a largely crystalline polymer, with a very high degree of order. The polymer chains have been shown [61] to form a double helix where the two chains are in a reversed head-to-tail order. The unit cell dimensions are a = 17.8 and c = 18.4, where c is the chain axis, and where the unit cell contains three repeat units.

References 1.

W.H. Carothers, inventor; DuPont de Nemours & Co., assignee; US 2012267, 1935.

2.

W.H. Carothers, inventor; DuPont de Nemours & Co., assignee; US 2071250, 1937.

3.

C.S. Fuller, inventor; Bell Telephone Laboratories Inc., assignee; US 2249950, 1941.

4.

J.R. Whinfield and J.T. Dickson, inventors; DuPont de Nemours & Co., assignee; US 2465319, 1949.

5.

J.G. Cook, J.T. Dickson and A.R. Lowe, inventors; Imperial Chemical Industries Ltd., assignee; US 2471023, 1949.

16

Background 6.

F.C. Wagner, inventor; DuPont de Nemours & Co., assignee; US 2035578, 1936.

7.

J.G.N. Drewitt and J. Lincoln, inventors; Celanese Corporation, assignee; US 2595343, 1952.

8.

J.G. Cook and H.P.W. Huggill, inventors; Imperial Chemical Industries Ltd., assignee; GB 604073, 1948.

9.

J.R. Caldwell, inventor; Eastman Kodak Co., assignee; US 2600376, 1952.

10. D. Aelony and M.M. Renfrew, inventors; General Mills Inc., assignee; US 2728747, 1955. 11. W.K.T. Gleim, inventor; Universal Oil Products Co., assignee; US 3039994, 1962. 12. S.W. Kantor and F.F. Holub, inventors; General Electric, assignee; US 3160602, 1964. 13. A.J. Conix and U.L. Laridon, inventors; Gevaert PhotoProducten NV, asignee; US 3028364, 1962. 14. A.J. Conix, inventor; Gevaert Photo-Producten NV, assignee; US 3317464, 1967. 15. M.H. Keck, inventor; Goodyear Tire & Rubber Co., assignee; US 3133898, 1964. 16. S.G. Cottis and J. Economy, inventors; no assignee; US 3637595, 1972. 17. H. Inata and K. Shoji, inventors; Teijin Ltd., assignee; US 4064108, 1977. 18. T.G. Pletcher, inventor; DuPont de Nemour & Co., assignee; US 3991013, 1976. 19. J.R. Schaefgen, inventor; DuPont de Nemours & Co., assignee; US 4118372, 1978. 17

Degradation and Stabilisation of Aromatic Polyesters 20. W.J. Jackson, G.G. Gebeau and H.F. Kuhfuss, inventors; Eastman Kodak, assignee; US 4153779, 1979. 21. R.E. Wilfong, Journal of Polymer Science, 1961, 54, 385. 22. Inventor unknown; Imperial Chemical Industries Ltd., assignee; FR 1169659, 1959. 23. W.K. Easley, J.K. Lawson and J.B. Ballentine, inventors; Chemstrand Corporation, assignee; CA 573301, 1959. 24. H. Zimmermann and N.T. Kim, Polymer Engineering and Science, 1980, 20, 10, 680. 25. J.L. Adams, inventor; Eastman Kodak, assignee; US 4501878, 1985. 26. C.M. Fontana, Journal of Polymer Science: Polymer Chemistry Edition, 1968, 6, 8, 2343. 27. S.M. Aharoni, Polymer Engineering and Science, 1998, 38, 7, 1039. 28. J.G. Smith, C.J. Kibler and B.J. Sublett, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 4, 7, 1851. 29. L.H. Peebles, M.W. Huffmann and C.T. Ablett, Journal of Polymer Science: Polymer Chemistry Edition, 1966, 7, 2, 479. 30. I. Luderwald, H. Urrutia, H. Herlinger and P. Hirt, Die Angewandte Makromolekulare Chemie, 1976, 50, 163. 31. G.C. East and A.M. Girshab, Polymer, 1982, 23, 3, 323. 32. H. Zeitler, Melliand Textilberichte, 1985, 2, 132. 33. B. Huang and J.J. Walsh, Polymer, 1998, 39, 26, 6991. 34. Y. Ma, U.S. Agarwal, D.J. Sikkema and P.J. Lemstru, Polymer, 2003, 44, 15, 4085. 18

Background 35. W. McMahon, H.A. Birdsall, G.R. Johnson and C.T. Camilli, Journal of Chemical and Engineering Data, 1959, 4, 1, 57. 36. H.L. Traub, P. Hirt, H. Herlinger and W. Oppermann, Die Angewandte Makromolekulare Chemie, 1995, 230, 179. 37. R.M. Lum, Journal of Polymer Science: Polymer Chemistry Edition, 1979, 17, 1, 203. 38. T. Takekoshi, inventor; General Electric, assignee; US 3549593, 1970. 39. G.W. Calundann, inventor; Celanese Corporation, assignee; US 4161470, 1979. 40. A.J. East and G.W. Calundann, inventors; Celanese Corporation, assignee; US 4431770, 1984. 41. A.J. East, inventor; Celanese Corporation, assignee; US 4393191, 1983. 42. A.J. East, inventor; Celanese Corporation, assignee; US 4421908, 1983. 43. D.I. Bower, An Introduction to Polymer Physics, Cambridge University Press, Cambridge, UK, 2002, p.111. 44. W.J. Dumage and L.E. Contois, Journal of Polymer Science, 1958, 28, 275. 45. S. Poulin-Dandurand, S. Perez, J-F. Revol and F. Brisse, Polymer, 20, 4, 419. 46. I.J. Desborough, I.H. Hall and J.Z. Neisser, Polymer, 1979, 20, 5, 545. 47. R.M. Ho, K.Z. Ke and M. Chen, Macromolecules, 2000, 33, 20 7529. 48. I.H. Hall and M.G. Pass, Polymer, 1976, 17, 9, 807.

19

Degradation and Stabilisation of Aromatic Polyesters 49. C.J. Kibler and J.G. Smith, inventors; Eastman Kodak Co., assignee; US 2901466, 1959. 50. C.A. Boye, Journal of Polymer Science, 1961, 55, 1, 275. 51. B. Remillard and F. Brisse, Polymer, 1982, 23, 13, 1960. 52. P. Chen and R. Kotek, Polymer Reviews, 2008, 48, 2, 392. 53. I. Ouchi, M. Hosei and S. Shimotsuma, Journal of Applied Polymer Science, 1977, 21, 3445. 54. G. Wu, Q. Li and J.A. Cuculo, Polymer, 2000, 41, 22, 8139. 55. S.K. Varshney, Journal of Macromolecular Science Macromolecular Reviews, 1986, C26, 4, 551. 56. M. Cox, Liquid Crystal Polymers, Review Report No.4, Rapra Technology, Shrewsbury, UK, 1987. 57. W.J. Jackson, Molecular Crystals and Liquid Crystals, 1989, 169, 1, 23. 58. D. Coates, Liquid Crystal Polymers - Synthesis, Properties and Applications, Review Report No.118, Rapra Technology, Shrewsbury, UK, 2000. 59. A.M. Donald, A.H. Windle and S. Hanna, Liquid Crystalline Polymers, 2nd Edition, Cambridge University Press, Cambridge, UK, 2006. 60. J.M.G. Cowie and V. Arrighi, Polymers: Chemistry and Physics of Modern Material, 3rd Edition, CRC Press, Boca Raton, FL, USA, 2008. 61. J. Economy, R.S. Storm, V.I. Matkovic, S.G. Cottis and B.E. Nowak, Journal of Polymer Science: Polymer Chemistry Edition, 1976, 14, 9, 2207.

20

2

Thermal Degradation

2.1 Poly(ethylene terephthalate) (PET) As the first aromatic polyester of high commercial practicality, PET has been studied extensively. Results of the earliest studies, from 1950 until the late 1960s [1–5], provided (on paper at least) a viable explanation of the features of PET thermal degradation under oxygenfree conditions. In ‘pure’ PET, three general structures are present: A

~Ph-(C=O)-O-CH2-CH2-O-(C=O)-Ph~

‘in chain’

B

~Ph-(C=O)-O-CH2-CH2-OH

‘hydroxyl end’

C

~Ph-(C=O)-OH

‘carboxyl end’

From the high polymeric nature of PET, and the known synthetic route for its manufacture (formation of bis(hydroxyethyl terephthalate) followed by condensation of same), it can be deduced that in the as-produced polymer by far the predominant structure will be A. B will be the next highest, although much less than A; and C will constitute only a very low initial level. PET stability towards all forms of degradation is very much dependent on their being a low content of the carboxyl end-group. From these early studies, it was considered that thermal degradation of PET does not involve a radical (homolytic) pathway, at least at the temperatures generally encountered by this polymer. The initial

21

Degradation and Stabilisation of Aromatic Polyesters reaction undergone by the chain was said to be scission into an acid chain end and an unsaturated chain end, the process taking place via a six-membered ring intermediate in which a hydrogen from a carbon B to ester group is transferred to the ester carbonyl. In the solid state such reactions may or may not proceed but, depending on the conformation of the affected segment of the polyester chain, it is assumed that a PET melt exhibits no order and that the necessary positioning of the atoms in the chain will occur through random motions of a freely rotating polymer chain. Reactions may thus be posited to occur as follows: A

‡

~Ph-(C=O)-OH

+

CH2=CH-O-(C=O)-Ph~

B

‡

~Ph-(C=O)-OH

+

CH2=CH-OH

C

‡

no products

It is immediately obvious from the above speculations that degradation via this route will result in a rapid increase in the number of acid end groups present. For unit B, the vinyl alcohol produced is unstable, and most probably will immediately rearrange to acetaldehyde, i.e., CH2=CH-OH

‡

CH3(C=O)-H

This situation may be exacerbated by a possible reaction of the unsaturated chain end produced from A, via an intermediate, to form yet another acid chain end, and acetylene. While the above reactions are taking place, there will be competition in the melt between chain scission and chain building, and esterexchange reactions will also occur [6]. In the case of the new chain ends created in the above reaction schemes, further chain end reactions could occur, e.g., an unsaturated end group acting as ‘acid’ and a hydroxyl end as ‘alcohol’. This

22

Thermal Degradation would result in formation of a new polymer chain, and expulsion of another molecule of vinyl alcohol, which would again rearrange to form acetaldehyde. Similarly, hydrolysis, by any water present, of the unsaturated end would result in formation of an acid chain end and acetaldehyde. Taking all these reactions into consideration, at least as a first approximation of degradation, i.e., a combination of chain scission, splitting off of small molecules from such chain ends, hydrolysis and (trans)esterification, it may be seen that most reactions taking place result in the creation of acid end groups; also that acetaldehyde is produced as the main small molecule product. The fact that acetaldehyde is so volatile (boiling point (bp) = 21 °C) means that it is easily lost from the bulk of the polymer. If, as is sometimes claimed, acetaldehyde is soluble in the PET melt, an acetalisation reaction may take place between this aldehyde and ethanediol to form 2-methyl1,3-dioxalane and water, or possibly even a chain extending reaction between the aldehyde and two adjacent hydroxyl chain ends. Due to the rapid rearrangement of vinyl alcohol to acetaldehyde, the reactions involving loss of this molecule are essentially irreversible because this rearrangement means, for example, that there is no hydroxyl group available to reform the unsaturated chain end. It has been noted that initial work on the thermal degradation of PET suggested that homolytic (i.e., free radical-based) scission did not have a role, but this was not confirmed and, in any case, certain conditions may exist where such reactions might come into play. At very high temperatures (or under certain conditions in processing machinery where formation of mechanoradicals may occur), homolytic scission of the polyester chain may occur [7–10]. The weakest link in the PET chain would appear to be the sequence carbonyl–oxygen–methylene, which may be expected to homolytically cleave in two possible ways: 1

~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)O. + .CH2CH2O(C=O)Ph~

23

Degradation and Stabilisation of Aromatic Polyesters 2

~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C.=O) + .OCH2CH2O(C=O)Ph~

Even in a polymer melt, the most likely next step to the scissions described above will be recombination of the radicals, in effect an immediate reversal of the reaction. Only in the case where a small, highly mobile, radical species such as oxygen is present is recombination likely to be anything but the predominant reaction. Similar types of scission might also occur in the hydroxyl end groups. Considering the myriad potential reactions which might cascade down from these simple homolytic bond scissions, it can be shown that various small molecule products are possible, as well as grafting, chain extension, and crosslinking reactions. Such small molecule species might include CO, CO2, ethylene, ethanol, water, acetaldehyde and ethylene glycol. So far only ‘pure’ PET has been discussed. Ether linkages also appear in PET, brought about by esterification reactions involving diethylene glycol, which is formed from ethanediol via a dehydrative coupling reaction. Any diethylene glycol present will preferentially react into the polymer because it has the same reactivity as ethanediol but lower volatility. Such moieties cannot be avoided, and appear to be at an equilibrium level of about 1–3% in as-produced PET. While attempts may be made to remove such species, this is a futile exercise because they quickly return to the equilibrium level on further processing of the polymer due to dehydrative coupling between hydroxyl chain ends. Such units may react via the molecular or homolytic pathways noted above. In the former, this will theoretically give various unsaturated ether and etheralcohol species, which may then go on to give polymeric and/or coloured species. In the case of homolytic scission, virtually the same reaction sequences will take place as for normal PET in-chain structures [10].

24

Thermal Degradation As indicated earlier, the other likely contaminants present in assynthesised PET are metallic catalyst residues. In the case of anaerobic thermal degradation, it is posited [11] that the metal will complex with one carbonyl in the in-chain structure, while the other carbonyl is involved in the six-membered intermediate to chain scission. Because there is only one oxygen atom between this complexed carbonyl and the methylene group from which hydrogen is leaving, the resultant shift in electron densities will favour chain scission. If there is another methylene group intruding between the oxygen and the active methylene, such a catalytic effect is much less likely. Two further points may be made: (i) antimony residues did not appear to catalyse chain scission, whereas titanium residues did (as might be expected); and (ii) while confirming the effects of transition metal ions on the thermal degradation of PET, studies of model compounds did not show the same potential effect in poly(butylene terephthalate) (PBT). Buxbaum [5] proposed the following scheme for the main reactions involved in the thermal degradation of PET: ~Ph(C=O)OCH2CH2O(C=O)Ph~ ‡ ~Ph(C=O)OCH=CH2 + HO(C=O)Ph~ ~Ph(C=O)OCH=CH2 ‡ Radical grafting, crosslinking ~Ph(C=O)OCH=CH2 ‡ Vinyl polymer ‡ ~Ph(C=O)OH + Polyene 2 ~Ph(C=O)OH j ~Ph(C=O)O(C=O)Ph~ + CH3(C=O)H CH3(C=O)H ‡ Volatilises or Polyenealdehyde

25

Degradation and Stabilisation of Aromatic Polyesters Gaseous by-products are said to consist of CO, CO2, H2O, CH3(C=O) H, C2H4, 2-methyldioxalane, CH4, and C6H6, with acetaldehyde by far the major product. Cyclic oligomers, especially the trimer, are also formed, although at this stage no concrete evidence of how they were formed was available. Very low concentrations of other non-gaseous products were also detected, including substituted benzoic acids, and biphenyl mono- and di-carboxylic acids. These were present at a total of ̈œ˜Ê ÃÌ>}i]Ê specifically to deactivate catalyst residues [2] UÊ /…ˆœ«…œÃ«…>ÌiÃ]ÊiëiVˆ>ÞÊÌÀˆ«…i˜ÞÌ…ˆœ«…œÃ«…>ÌiÊQÎR UÊ -œ`ˆÕ“Ê«ÞÀœ«…œÃ«…>ÌiÊQ{R UÊ /ÀˆÃ­>ŽÞ>“ˆ˜œ>ŽÞ®«…œÃ«…ˆ˜iÊQxR UÊ
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Degradation and Stabilisation of Aromatic Polyesters UÊ *…œÃ«…>Ìiʈ˜ÊVœ“Lˆ˜>̈œ˜Ê܈̅Ê>Ê«œÞ>“ˆ`iÊQ£ÇR UÊ Ì…Þ‡Ó‡…Þ`œÝÞi̅ޏ‡A‡`ˆi̅ޏ«…œÃ«…œ˜œÃÕVVˆ˜>ÌiÊQ£nR UÊ *…œÃ«…œÀÕÃÊ >Vˆ`Ê iÃÌiÀÃÊ ÜˆÌ…Ê >ÌÊ i>ÃÌÊ œ˜iÊ œÝˆÀ>˜iÊ ÃÕLÃ̈ÌÕi˜ÌÊ Q£™R UÊ iÌ>«…œÃ«…œÀˆVÊ>Vˆ`ÊQÓäR UÊ /Àˆ«…i˜Þ«…œÃ«…ˆÌiÊ>˜`Ê£]Ӈi«œÝއ·«…i˜œÝÞ«Àœ«>˜iÊQÓ£R UÊ /iÌÀ>‡n-alkylphosphonium acetate or halide [22] UÊ ˆ«…i˜Þ«…œÃ«…ˆÌiÃʜÀÊ«…œÃ«…œ˜>ÌiÃÊQÓÎqÓnR UÊ >ÀLœ“i̅œÝÞi̅>˜i«…œÃ«…œ˜ˆVÊ>Vˆ`Ê`ˆ“i̅ޏÊiÃÌiÀÊQәR UÊ *…œÃ«…ˆÌiÃʈ˜Ê}i˜iÀ>]ʈ˜Ê܅œÞÊ>Àœ“>̈VÊ«œÞiÃÌiÀÃÊQÎäqÎÓR UÊ *…œÃ«…ˆÌiÊ œÀÊ «…œÃ«…>ÌiÊ >˜`Ê `ˆi«œÝÞÊ Vœ“«œÕ˜`Ê ˆ˜Ê «œÞ­£]{‡ cyclohexylenedimethylene terephthalate) [33] UÊ
``ˆÌˆœ˜ÊœvÊ«…œÃ«…œÀÕÇL>Ãi`Ê>``ˆÌˆÛiÃÊ̜ʫœÞiÃÌiÀÃÊÃޘ̅iÈÃi`Ê with high-activity catalysts, with addition taking place immediately before melt processing [34] UÊ *…œÃ«…ˆÌiÊ œÀÊ «…œÃ«…>ÌiÊ ÌœÊ ˆ“«ÀœÛiÊ Ì…iÊ Ü…ˆÌi˜iÃÃÊ œvÊ poly(trimethylene terephthalate) (PTT) [35] UÊ /Àˆ>ŽÞ«…œÃ«…œ˜œ>ViÌ>ÌiÊQÎÈR UÊ iÌ>ÊÃ>ÌÃʜvÊ«…œÃ«…œÀˆVÊ>Vˆ`ʈ˜Ê*//ÊQÎÇR UÊ *…œÃ«…œÀÕÃÊVœ“«œÕ˜`Ãʈ˜Ê}i˜iÀ>Ê>ÃÊV>Ì>ÞÃÌÊ`i>V̈Û>̜ÀÃÊ>˜`Ê >ViÌ>`i…Þ`iÊÃÕ««ÀiÃÃiÀÃʈ˜Ê* /Ê«Àœ`ÕV̈œ˜ÊQÎnq{£R UÊ *…œÃ«…œÀÕÃÊVœ“«œÕ˜`Ãʈ˜Ê}i˜iÀ>Ê>ÃÊ>VÀœiˆ˜ÊÃÕ««ÀiÃÃiÀÃʈ˜Ê*//Ê [42, 43]

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Thermal and Hydrolytic Stabilisation The probable key to the continued success of the phosphorus-based additives in aromatic polyesters is their ability to take part in various processes beneficial to the non-oxidative heat stability of their host polymers. They are known hydroperoxide decomposers, and thus could safely destroy such species present in the polyester. They are, for the same reason, excellent secondary antioxidants, especially if used in conjunction with primary antioxidants such as hindered phenols, in a wide variety of polymers. Their ability to react with catalyst residues and prevent these contributing to degradation reactions of the polymer is also important. They would also appear to be capable œvÊÀi>V̈˜}Ê܈̅Ê̅iÊ«œÞiÃÌiÀÊV…>ˆ˜Êi˜`Ã]ʏi>`ˆ˜}Ê̜Êi˜`‡V>««ˆ˜}q>˜`Ê consequent reduction of the amount of volatiles such as acetaldehyde >˜`Ê>VÀœˆi˜qœÀÊiÛi˜Ê«ÀœÛˆ`ˆ˜}ÊV…>ˆ˜ÊiÝÌi˜Ãˆœ˜° Several academic studies have been carried out on the use of phosphorus-based thermal stabilisers with aromatic polyesters Q{{qÈ£R°

…>˜}Ê>˜`ÊVœ‡ÜœÀŽiÀÃÊQ{ÈRÊÃÌÕ`ˆi`Ê̅iÊivviVÌʜvÊ>``ˆ˜}ÊÃÌ>LˆˆÃiÀÃÊ to the polymerisation process of PET synthesised from terephthalic acid using antimony catalysts. Unlike processes starting with dimethyl terephthalate, which use different catalysts, this catalyst system is not inhibited by stabilisers. The authors used triphenyl phosphate, trimethyl phosphate, diethyl-3,5-di-t-butyl-4-hydroxybenzylphosphonate (Irganox‘ 1222; Ciba) and pentaerythritol tetrakis(3,5-di-t-butyl4-hydroxyphenylpropionate) (Irganox‘Ê £ä£äÆÊ ˆL>®°Ê /Àˆ«…i˜ÞÊ phosphate and trimethyl phosphate greatly reduced the carboxyl content of the PET produced, and appeared to also lower diethylene glycol content. The best balance of low carboxyl and high thermal stability was found using triphenyl phosphate at bä°ä{ÊÜ̯° ii˜œÀÊ Q{Ç]Ê xäRÊ ˆ˜ÛiÃ̈}>Ìi`Ê LˆÃ­Ó]{‡`ˆ‡t-butylphenyl) pentaerythritoldiphosphite (Ultranox ‘Ê ÈÓÈÆÊ œÀ}‡7>À˜iÀ®Ê >ÃÊ >Ê process stabiliser in PET. The author stated that the stabilising effect was mainly due to trapping of catalyst residues, and that the additive could also deactivate hydroperoxides. One problem to be overcome with the use of any additive in PET is the high processing temperature,

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Degradation and Stabilisation of Aromatic Polyesters which can lead to chemical and/or physical loss of the additive. The >``ˆÌˆÛiÊÕÃi`ʅiÀi]Ê܅ˆiʓiÌˆ˜}Ê>ÌÊ£ÇäÊc ]ÊÜ>ÃÊÃÌ>LiÊ>˜`Ê«iÀÈÃÌi˜ÌÊ ˆ˜Ê* /Ê>ÌÊ«ÀœViÃȘ}ÊÌi“«iÀ>ÌÕÀiÃÊLiÌÜii˜ÊÓnäÊc Ê>˜`ÊÎÓäÊc °
…>Àœ˜ˆÊ>˜`ÊVœ‡ÜœÀŽiÀÃÊQ{™RʘœÌi`Ê̅>ÌÊ>Àœ“>̈VÊ«…œÃ«…ˆÌiÃÊVœÕ`Ê induce chain extension in PET (for more on this process, see Section 6.4.2). The chain extension in this case was said to take place via a two-stage reaction: a) Rapid reaction of triphenylphosphite with hydroxyl chain ends, with displacement of a phenoxy group. b) Slow reaction of ‘phosphated’ chain ends with acid chain ends to form an ester bond, which extends the polymer and produces diphenylphosphite. ˆiÌâÊQx£]ÊxÓRÊ`i“œ˜ÃÌÀ>Ìi`Ê̅>ÌÊÕÃiʜvÊ«…œÃ«…ˆÌiÃÊ>ÃÊ>``ˆÌˆÛiÃʈ˜Ê the reprocessing of post-consumer scrap PET resulted in reduction of discoloration, lowered development of acid end groups, and reduced acetaldehyde formation. Particular additives investigated were tris(2,4di-t-butylphenyl)phosphite, distearylpentaerythritoldiphosphite and bis(2,4-di-t-butylphenyl)pentaerythritoldiphosphite. The use of aryl phosphites for this purpose was also examined by Solera [53] and Pfaendner and co-workers [54] ˜Ê>˜Ê>À̈Viʈ˜Ê£™™nÊiÝ>“ˆ˜ˆ˜}ʘiÜÊÌÀi˜`Ãʈ˜ÊÃÌ>LˆˆÃˆ˜}Ê>``ˆÌˆÛiÃ]Ê Solera [55] noted that, while phosphites are effective process stabilisers in PET, they can themselves be subject to hydrolytic degradation. A new phosphorus-based stabiliser with increased hydrolytic stability was suggested in the form of 2,2´,2´´-nitrilo[triethyl-tris(3,3´,5,5´tetra-t-butyl-1,1´-biphenyl-2,2´-diyl)]phosphite (Irgafos‘ 12; Ciba).
Å̜˜Ê>˜`ÊVœ‡ÜœÀŽiÀÃÊQxÇRÊ>ÃÃiÃÃi`Ê>Ê܈`iÊÀ>˜}iʜvÊ«…œÃ«…ˆÌiÃÊvœÀÊ their process-stabilising abilities in PET. Additives used were: >®Ê Ó]{]ȇ/Àˆ‡t-butylphenyl-2-butyl-2-ethylpropanediolphosphite L®Ê ˆÃ­Ó]{‡`ˆ‡t-butylphenyl)pentaerythritoldiphosphite

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Thermal and Hydrolytic Stabilisation c) Phenyldidecylphosphite d) Poly(dipropylene glyco)phenylphosphite e) 4,4´-IsopropylidenediphenolC12-15alcoholphosphite f) Tris(dipropylene glycol)phosphite g) Tris(2,4-di-t-butylphenyl)phosphite …®Ê ˆÃ­Ó]{‡`ˆVՓޏ«…i˜Þ®«i˜Ì>iÀÞ̅ÀˆÌœ`ˆ«…œÃ«…ˆÌi 7…ˆiʼL½ÊˆÃÊÜiÊŽ˜œÜ˜Ê>ÃÊ>˜ÊivviV̈ÛiÊ«ÀœViÃÃÊÃÌ>LˆˆÃiÀ]Ê̅iÊ>Õ̅œÀÃÊ found that ‘a’ was best, possibly due to its having similar high activity, but significantly improved hydrolysis resistance. It is also said to be more soluble in the polymer than any of the other additives. Another drawback noted for the use of phosphites in PET is the tendency to form grey discoloration in the presence of antimony catalyst residues by reduction of antimony salts to the metal by the phosphite. Once again, compound ‘a’ exhibited the lowest propensity for this problem of all the additives tested. >ÃVˆ“i˜ÌœÊ>˜`Ê ˆ>ÃÊQxnRÊÃÌÕ`ˆi`Ê̅iʈ˜VÀi>ÃiÃʈ˜Ê“œiVՏ>ÀÊÜiˆ}…ÌÊ vœÕ˜`ʜ˜Ê>``ˆ˜}Ê«…œÃ«…ˆÌiÃÊ̜ÊÀiVÞVi`Ê* /°Ê7…ˆiÊÌÀˆ>Àޏ«…œÃ«…ˆÌiÃÊ were said to be effective in this regard, the authors suggested that alkylphosphites were ineffective, and could even act as prodegredants. -…i}>iÛÊ>˜`ÊVœ‡ÜœÀŽiÀÃÊQÈäRÊvœÕ˜`Ê̅>Ìʈ˜VÀi>Ãi`Ê̅iÀ“>ÊÃÌ>LˆˆÌÞÊ could be achieved by synthesising poly(butylene terephthalate) ­* /®Ê ˆ˜Ê ̅iÊ «ÀiÃi˜ViÊ œvÊ >Ê ÃÌ>LˆˆÃiÀÊ «>VŽ>}iÊ Ü…ˆV…Ê ˆ˜VÕ`i`Ê tris(nonylphenyl)phosphite and sodium hypophosphite.

>Û>V>˜ÌˆÊ>˜`ÊVœ‡ÜœÀŽiÀÃÊQÈ£RÊvœÕ˜`Ê̅>ÌÊ̅iʜ«Ìˆ“>ÊVœ˜`ˆÌˆœ˜ÃÊ vœÀÊV…>ˆ˜ÊiÝÌi˜Ãˆœ˜Ê܈̅ÊÌÀˆ«…i˜Þ«…œÃ«…ˆÌiÊÜiÀiÊ£ÊÜ̯Ê>``ˆÌˆœ˜Ê >˜`Ê>Ê«ÀœViÃȘ}ÊÌi“«iÀ>ÌÕÀiʜvÊÓÈäÊc Êʈ˜Ê* /]Ê܅ˆV…ÊVœ˜`ˆÌˆœ˜ÃÊ

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Degradation and Stabilisation of Aromatic Polyesters resulted in maximum chain extension with minimum subsequent degradation. Two important points were discovered: a) Chemical degradation of the extended PET occurred very easily, even during storage, and was considered to be caused by by-products from the reaction between polyester and additive. Acetone extraction of the offending impurities was effective in improving storage life. b) Recycled PET was shown to be much less reactive to triphenylphosphite than virgin polyester. This was most probably due to the high levels of acid end groups in the recycled polymer. The patent literature contains a number of additives which have been said to be effective thermal stabilisers for aromatic polyesters, and which are not phosphorus-based. Early patents include a series of possibilities researched by two companies in particular: Eastman Kodak and FMC Corporation. Eastman Kodak have claimed the use of hydroquinone derivatives, such as p‡˜ˆÌÀœ«…i˜Þ…Þ`ÀœµÕˆ˜œ˜iʜÀÊx]xA‡LˆÌœÞ…Þ`ÀœµÕˆ˜œ˜iÊQÈÓRÆÊ gentisic acid derivatives such as gentishydrazine or gentisaldehyde QÈÎRÆÊp‡…Þ`ÀœÝÞ>âœLi˜âi˜iÊQÈ{R]ÊÓ]{]x‡ÌÀˆ…Þ`ÀœÝÞ«…i˜œ˜iÃʜvÊv>ÌÌÞÊ >Vˆ`ÃÊQÈxRÆÊ>ÊVœ˜`i˜Ã>̈œ˜Ê«Àœ`ÕVÌʜvʅÞ`ÀœµÕˆ˜œ˜iÊ>˜`Ê>ÞÊ>Vœ…œÊ QÈÈRÆÊ >ŽÞÊ }>>ÌiÃÊ QÈÇRÆÊ }i˜ÌˆÃˆVÊ >Vˆ`Ê `iÀˆÛ>̈ÛiÃÊ Ài>VÌi`Ê ˆ˜ÌœÊ ̅iÊ «œÞiÃÌiÀÊV…>ˆ˜ÊQÈnRÆÊy>Û>˜Ì…Àœ˜iÊQșRÆÊ>˜`ÊvÕÀ>˜œ˜iÊ`iÀˆÛ>̈ÛiÃÊÃÕV…Ê as 5-benzylidene-4-methyl-3-p‡˜ˆÌÀœ«…i˜Þ‡Ó­x®‡vÕÀ>˜œ˜iÊQÇäR° FMC Corporation have claimed an even wider range of potential thermal ÃÌ>LˆˆÃiÀÃ]Ê ˆ˜VÕ`ˆ˜}Ê «…i˜ÞÊ ÃՏvœÝˆ`iÃÊ QÇ£RÆÊ >ˆ«…>̈VÊ œÀ̅œÃˆˆV>ÌiÃÊ QÇÓRÆÊ …Þ`ÀœÝÞV>ÀLޏÌÀˆ>ŽœÝÞȏ>˜iÃÊ QÇÎRÆÊ >“ˆ˜œLi˜âœ«…i˜œ˜iÃÊ QÇ{RÆÊ ÃÌi>À>“ˆ`iÊ QÇxRÆÊ ÃÕLÃ̈ÌÕÌi`Ê >Ž>˜iÊ >˜`Ê >Vœ…œÃÊ ÃÕV…Ê >ÃÊ `ˆ«…i˜Þ“i̅>˜iÊ œÀÊ Li˜âޏ«…i˜ÞV>ÀLˆ˜œÊ QÇÈRÆÊ `ˆÃՏw`iÃÊ ÃÕV…Ê >ÃÊ «…i˜Þ`ˆÃՏw`iʜÀʅiÝޏ`ˆÃՏw`iÊQÇÇRÆÊÃՏvœ˜iÃÊÃÕV…Ê>ÃÊÊ`ˆ…iÝޏÃՏvœ˜iÊ or di-p‡ÌœÞÃՏvœ˜iÊ QÇnRÆÊ ÕÀi>ÃÊ œÀÊ Ì…ˆœÕÀi>ÃÊ QǙRÆÊ >“ˆ˜iÃÊ ÃÕV…Ê >ÃÊ didoceylamine or tri-n‡…iÝޏ>“ˆ˜iÊ QnäRÆÊ >˜`Ê >Àœ“>̈VÊ ˆÃœVÞ>˜>ÌiÃÊ Qn£R

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Thermal and Hydrolytic Stabilisation Many other companies have patented additives which they claim to be useful in preventing thermal degradation of polyesters, and are discussed below. /œÀ>ÞÊ ˜`ÕÃÌÀˆiÃÊ QnÓRÊ ṎˆÃi`Ê >Ê À>˜}iÊ œvÊ ˜ˆÌÀœ}i˜‡Vœ˜Ì>ˆ˜ˆ˜}Ê compounds, especially amides and imides. Preferred additives were benzylphthalimide, terephthaldiamide and phenacetin. They were recommended to be added at the polymerisation stage and incorporated into the polyester chain. ՍˆÊ *…œÌœÊ ˆ“Ê QnÎRÊ `iÃVÀˆLi`Ê Ì…iÊ ÕÃiÊ œvÊ …Þ`ÀœÝއÃÕLÃ̈ÌÕÌi`Ê heterocyclic compounds as heat stabilisers, including 2,2,4-trimethylȇ…Þ`ÀœÝއLJt-butylchroman.
ˆi`Ê …i“ˆV>Ê œÀ«œÀ>̈œ˜Ê Qn{RÊ V>ˆ“Ê ¼Vœ˜ÌÀœ½Ê œÛiÀÊ Ì…iÊ carboxylic acid end content (see Section 6.3) of PET by adding xäqxääÊ ««“Ê `ˆV…œÀœ­`ˆ‡Ó‡«ÞÀˆ`ޏ>“ˆ˜i®Vœ««iÀ­®Ê œÀÊ LˆÃ­`ˆ‡Ó‡ piridylamine)copper(II)chloride to the melt after completion of the polycondensation.

i>˜iÃiÊ œÀ«œÀ>̈œ˜ÊQnxRÊV>ˆ“i`ʈÌÊÜ>ÃÊ«œÃÈLiÊ̜ʅi>ÌÊÃÌ>LˆˆÃiÊ wi`Ê*//ʜÀÊ* /Ê܈̅ʏœÜʏiÛiÃÊ­ä°Óxq£ÊÜ̯®ÊœvÊ>Ê«œÞ>“ˆ`iÊÃÕV…Ê >ÃÊ Þœ˜ÊÈ]Ê Þœ˜ÊÈ]ÈʜÀÊ Þœ˜ÊÈ]£ä° i˜iÀ>Ê iVÌÀˆVÊ œÀ«œÀ>̈œ˜ÊQnÈRÊvœÕ˜`Ê̅>ÌÊ̅iÊ>``ˆÌˆœ˜ÊœvÊÓ>Ê >“œÕ˜ÌÃʜvÊ>Ê«œÞLÕÌ>`ˆi˜i]Ê܈̅Êۈ˜ÞÊVœ˜Ìi˜ÌʜÛiÀÊnä¯ÊÀiÃՏÌi`Ê in substantial melt stabilisation of linear aromatic polyesters such as PET. ˆLiÀʘ`ÕÃÌÀˆiÃʘVœÀ«œÀ>Ìi`ÊQnÇRʘœÌi`Ê>ʓ>ÀŽi`ÊÀi`ÕV̈œ˜Êˆ˜Ê̅iÊ levels of diethylene glycol units in PET when the carboxylic acid salt of a quaternary ammonium base was added to the polymerisation reaction. *œÞ«>Ã̈VÃÊ QnnRÊ >``i`Ê …Þ`>˜Ìœˆ˜Ê ÌœÊ * /]Ê *//Ê œÀÊ * /Ê vœÀÊ Ì…iÊ purpose of reducing evolution of terephthalic acid vapour during processing, thus preventing deposits of this material building up in processing equipment.

151

Degradation and Stabilisation of Aromatic Polyesters

ˆÃ>ˆÊQn™RÊ>˜`Ê ˆL>‡iˆ}ÞÊQ™äRÊV>ˆ“Ê̅iÀ“>ÊÃÌ>LˆˆÃiÀÃÊvœÀÊ>Àœ“>̈VÊ polyesters of the dioxasilepin and dioxasilocin type. These are similar in structure to the Ciba-Geigy biphenyl phosphites, with silicon replacing phosphorus.

i>˜iÃiÊ œÀ«œÀ>̈œ˜ÊQ™£q™ÎRÊÃÕ}}iÃÌi`ÊÃiÛiÀ>Ê>``ˆÌˆÛiÃÊëiVˆwV>ÞÊ targeted at thermal stabilisation of wholly aromatic polyesters, including dimercaptothiadiazoles, 2-mercaptobenzothiazoles, and phenylene oxide oligomers. Õ*œ˜ÌÊ Q™{RÊ œLÃiÀÛi`Ê Ì…>ÌÊ Ì…iÊ >``ˆÌˆœ˜Ê œvÊ £xqÎäääÊ ««“Ê >Ž>ˆÊ metal, alkaline earth, magnesium or calcium salts, preferably at the polymerisation stage, resulted in improved heat resistance in liquid crystalline polyesters. ՘iŽ>Ì>Ê/œ…œŽÕÊÊQ™xq™nRʅi>ÌÊÃÌ>LˆˆÃi`Ê«œÞiÃÌiÀÃÊ܈̅ÊÛ>ÀˆœÕÃÊ formulations composed of tannins, or derivatives thereof. ˆ“Ê ̅>ÌÊ *//Ê V>˜Ê LiÊ …i>ÌÊ ÃÌ>LˆˆÃi`Ê by contacting the polymer with ‘a melt unstable, organic nitrogen-containing stabilising compound’ e.g., 1-aminoethanol, triacetonediamine, 4-aminosalicyclic acid, or ‘a melt stable alcoholate’ such as sorbitol or polyvinyl alcohol.

œV>‡ œ>Ê Q£ä£RÊ `iÃVÀˆLiÊ >Ê «ÀœViÃÃÊ vœÀÊ Ài`ÕVˆ˜}Ê >ViÌ>`i…Þ`iÊ evolution from PET by adding a hydrogen supplier, preferably poly(methylhydro)siloxane and a hydrogenation catalyst such as nickel or palladium. The additives are slurried in oil, coated on PET pellets, and the resultant material extruded.

ˆL>Ê-«iVˆ>ˆÌÞÊ …i“ˆV>ÃÊQ£äÓRÊÃÕ}}iÃÌÊ­Vœ®«œÞ“iÀÃʜvÊӇ«Àœ«i˜œˆVÊ acid esters as aldehyde-reducing additives, especially poly(glycidyl methacrylate). The above survey of intellectual property on thermal stabilisation of aromatic polyesters shows that a great deal of effort has been expended in trying to find suitable additives, but few (if any) of the above non-phosphorus based approaches have been put into commercial practice. 152

Thermal and Hydrolytic Stabilisation As is often the case with patents, little information is provided on any proposed mechanism for the stabilising action of these additives, although it is very likely that many of them will act in some way to control or suppress reactions at chain ends. A more focused discussion of this aspect of polyester stabilisation, along with examples of additives specifically targeted towards this aspect of polyester chemistry, is given in Section 6.3. ,iVi˜ÌÊ ÃÌÕ`ˆiÃÊ …>ÛiÊ ÃÕ}}iÃÌi`Ê Q£äÎ]Ê £ä{RÊ Ì…>˜Ê «œÞ­i̅ޏi˜iÊ naphthalate) (PEN) and liquid crystalline polyesters containing carbon nanotubes surface-modified with carboxylic acid groups not only exhibit increased strength and modulus, but may also exhibit ÃÕLÃÌ>˜Ìˆ>Þʈ“«ÀœÛi`Ê̅iÀ“>ÊÃÌ>LˆˆÌÞ°Ê"̅iÀÊܜÀŽiÀÃÊQ£äxRÊ`i˜ÞÊ this. Further investigations are required to clarify this problem.

6.3 End-capping As discussed in Chapter 2, the chain ends of polyester molecules exert Vœ˜Ãˆ`iÀ>Liʈ˜yÕi˜Viʜ˜Ê̅iÊ̅iÀ“>ÊÃÌ>LˆˆÌÞʜvÊ̅iÃiÊ«œÞ“iÀðÊ˜Ê particular, carboxylic acid chain ends which are present in the assynthesised polymer and which build up with thermal degradation and hydrolytic reactions, can severely restrict the stability of the polymer. It is therefore unsurprising that a great deal of effort has been put into finding additives and processes which can control or eliminate such moieties. Additives of this type are known as ‘end-cappers’, although some are also referred to as ‘hydrolytic stabilisers’. An allegedly very useful type of additive for this purpose is a carbodiimide, i.e., a substance of the general formula R-N=C=N-R Q£äȇ£ÓÓR° Õ“Ê Q£äÈRÊ Ü>ÃÊ œvÊ Ì…iÊ œ«ˆ˜ˆœ˜Ê ̅>ÌÊ ˆ˜ˆÌˆ>̈œ˜Ê œvÊ «ÞÀœÞÈÃÊ œvÊ * /Ê was associated with an ionic process involving an acid-catalysed hydrolysis-type reaction, and that the best approach to stabilising the polymer against damage during processing would be to inhibit this reaction. Using a carbodiimide formulation from Mobay Corporation Q£ä™q£££R]Ê …iÊ vœÕ˜`Ê Ì…>ÌÊ >Ê ä°xÊ ÜÌ¯Ê >``ˆÌˆœ˜Ê Ü>ÃÊ ÃÕvwVˆi˜ÌÊ ÌœÊ 153

Degradation and Stabilisation of Aromatic Polyesters produce a useful effect. It was observed that the temperature at which ̅iÊ`i}À>`>̈œ˜ÊÀ>ÌiÊÀi>V…i`Ê>ʓ>݈“Õ“ÊÜ>Ãʈ˜VÀi>Ãi`ÊLÞÊÓäÊc ]Ê and examination of evolved gases showed that the onset and amount of tetrahydrofuran and carbon dioxide produced were markedly changed. The timing and rate of the later butadiene evolution was not changed significantly. It was concluded that carbodiimide affected the initial reaction, but that further stabilisation would be needed once thermal degradation was well established. /ˆ˜ÌiÊQ£äÇRʘœÌi`Ê̅>ÌÊV>ÀLœ`ˆˆ“ˆ`iÃÊ>Àiʅˆ}…ÞÊÀi>V̈ÛiÊ̜Ü>À`ÃÊ̅iÊ carboxylic acid end groups of PET. The reaction was said to result in the formation of an intermediate adduct which, after rearrangement, leads to amidation of the acid group and the formation of an isocyanate. One caveat was given: liberation of isocyanates from the melt could be a serious drawback to the use of low molecular weight carbodiimides. It was thus suggested that polymeric carbodiimides would be a better option for this reason. For best results, it was suggested that an excess of the additive should be incorporated so that the acid ends formed during degradation of the polymer matrix could be dealt with. Versions of carbodiimides patented up to the present time include «œÞV>ÀLœ`ˆˆ“ˆ`iÃÊQ£ä™]Ê££Ç]Ê££n]Ê£ÓäR]Ê>ÀޏÊV>ÀLœ`ˆˆ“ˆ`iÃÊQ££ä‡ ££xR]Ê >ˆVÞVˆVÊ V>ÀLœ`ˆˆ“ˆ`iÃÊ Q££ä]Ê £££]Ê ££™]Ê £Ó£RÊ >˜`Ê Li˜`ÃÊ œvÊ carbodiimides and polycarbodiimides [122]. Various additives of this class are commercially available. The use of cyclic organic carbonates, especially ethylene carbonate, as end-cappers/hydrolysis stabilisers in aromatic polyesters has been «>Ìi˜Ìi`Ê ˆ˜Ê Û>ÀˆœÕÃÊ vœÀ“ÃÊ LÞÊ >̈œ˜>Ê -Ì>ÀV…Ê Q£ÓÎq£ÓxR]Ê
ˆi`Ê

…i“ˆV>ÊQ£ÓÈ]Ê£ÓÇRÊ>˜`Ê,…œ`ˆ>ÊQ£ÓnR° Formulations by National Starch also include antioxidant species, including phosphorus-based [123], hindered phenols [124] or aromatic amines [125]. Allied Chemical use ethylene carbonate along with an alkali metal iodide, whereas Rhodia enhance the reactivity of the ethylene carbonate by including an alkylphosphonium salt as

154

Thermal and Hydrolytic Stabilisation a catalyst. The reaction of ethylene carbonate with the acid chain end would appear to result in the creation of a hydroxyethyl ester, and the loss of carbon dioxide.

«œÝÞÊ}ÀœÕ«‡Vœ˜Ì>ˆ˜ˆ˜}ÊëiVˆiÃʅ>ÛiÊLii˜Êˆ˜ÛiÃ̈}>Ìi`ÊQ£äÇ]ʣәRÊ >˜`ʅ>ÛiÊLii˜Ê̅iÊÃÕLiVÌʜvÊ«>Ìi˜Ìˆ˜}Ê>V̈ۈÌÞÊQ£Îäq£{£RÊ>ÃÊi˜`‡ cappers in aromatic polyesters. End-capping of target carboxylic acid groups by epoxy moieties results in opening of the oxirane ring and formation of a hydroxyalkyl ester. Epoxides are relatively unreactive towards carboxylic acids, and thus catalysts may be required to drive ̅iÊÀi>V̈œ˜ÊvœÀÜ>À`ÊQ£Îä]Ê£ÎÈR Allied Chemical have patented the use of monoepoxides with various substituent groups such as amides or esters [131, 132], imides [133] and aromatic species [134]. Taiwanese researchers used ethylenestyrene copolymers with epoxy substituents [135]. Most recently, a ˜Õ“LiÀʜvÊVœ“«>˜ˆiÃ]ʈ˜VÕ`ˆ˜}Ê
-ÊQ£ÎÇ]Ê£{£RÊ>˜`ʈÌÃÕLˆÃ…ˆÊ Q£Înq£{äRÊ ÕÃi`Ê i«œÝˆ`ˆÃi`Ê ˜>ÌÕÀ>Ê œˆÃÊ >ÃÊ ÃÌ>LˆˆÃiÀÃÊ ˆ˜Ê >Àœ“>̈VÊ polyesters.
Ê ÃÞÃÌi“>̈VÊ ÃÌÕ`ÞÊ œvÊ V>ÀLœÝޏˆVÊ i˜`Ê }ÀœÕ«Ê V>««ˆ˜}Ê œvÊ * /Ê ˆ˜Ê ÃÕ«iÀVÀˆÌˆV>ÊV>ÀLœ˜Ê`ˆœÝˆ`iÊQ£Ó™Rʅ>ÃÊV>ˆ“i`Ê̜ʫÀœÛˆ`iÊ>Ê«œÞiÃÌiÀÊ with marked improvements in hydrolytic and process stability. Other end-capping approaches have been tried, including 5-hydroxyisophthalic acid to reduce acetaldehyde emissions [142]; benzoyl lactams or benzoylphthalimides as general-purpose endcappers [143]; 2-oxazolines, preferably with fatty acid substituents to control volatility [144]; and end-capping of PTT with a hindered phenolic acid to reduce acrolein emissions during processing. 6ˆ>ˆ˜Ê>˜`ÊVœ‡ÜœÀŽiÀÃÊQ£{ÈRÊiÝ>“ˆ˜i`Ê>Ê܈`iÊÛ>ÀˆiÌÞʜvÊëiVˆiÃÊvœÀÊ potential use as hydroxyl reactive end-cappers suitable for inclusion in PET to minimise acetaldehyde contamination of products. The purpose behind the study was to be able to use cheaper grades of PET for beverage bottles rather than the more expensive solid state post-condensation (SSP)-treated grades normally used (see Section 6.4.1®°Ê iÃÌÊ ÀiÃՏÌÃ]Ê ÜˆÌ…Ê Ì…iÊ >``ˆÌˆÛiÃÊ ÃiiVÌi`]Ê ÜiÀiÊ

155

Degradation and Stabilisation of Aromatic Polyesters obtained with 4-aminobenzoic acid, 3,5-dihydroxybenzoic acid and 4-hydroxybenzoic acid. Good results were obtained under laboratory conditions, but additives were significantly less effective under processing conditions.

6.4 Chain Extension 6.4.1 Without Additives In the polycondensation reaction undergone by polyesters, a point is reached at which the molecular weight achievable in the reaction kettle will reach a plateau value. Any increase in molecular weight over this value will require implementation of a different approach. In a similar way, polyesters which are to be recycled, whether as in-house or as post-consumer scrap, may require other means to convert them into material suitable for further processing into high added-value products. Attempts have been made to increase the molecular weight of post-consumer PET scrap by extrusion with very high degrees of “iÌÊ `i}>ÃȘ}Ê Q£{ÇR]Ê LÕÌÊ ÃÕV…Ê >ÌÌi“«ÌÃÊ >ÀiÊ }i˜iÀ>ÞÊ `œœ“i`Ê ÌœÊ failure by the increased thermal and mechanical stress placed on the polymer by such severe processing, and by the presence of impurities, «>À̈VՏ>ÀÞÊÜ>ÌiÀÊQ£{nRÊ>˜`Ê«œÞۈ˜ÞÊV…œÀˆ`iÊQ£{™R° Known for some time and utilised extensively, especially for the creation of acetaldehyde-free, high molecular weight, polyesters for use in food and drink packaging application, the favoured route towards chain extension without the use of additives is via SSP. The «ÀœViÃÃʅ>ÃÊLii˜ÊÕÃi`ÊvœÀÊۈÀ}ˆ˜Ê* /ÊQ£xäq£xÇR]Ê«œÃ̇Vœ˜ÃՓiÀÊÃVÀ>«Ê * /ÊQ£xn]Ê£x™R]Ê*//ÊQ£ÈäRÊ>˜`Ê* ÊQ£È£R° ˜Ê--*]Ê̅iÊÀiȘʈÃʅi>Ìi`Ê̜Ê>LœÕÌÊÓäÊc ÊLiœÜʈÌÃʓiÌˆ˜}Ê«œˆ˜Ì]Ê during which the polycondensation continues but at a lower rate than in the melt. Side reactions continue as well, but at a lower rate than the polycondensation reactions.

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Thermal and Hydrolytic Stabilisation The process is used extensively for the production of yarns and fibres; monofilaments and strapping; for recycling of scrap polyesters; in production of PET containers and soft-drink bottles; engineering }À>`iÃʜvÊ* /Ê>˜`Ê* /ÊvœÀÊiiVÌÀˆV>]Êi˜}ˆ˜iiÀˆ˜}Ê>˜`Ê>Õ̜“œÌˆÛiÊ applications. The process gives satisfactory results but has several disadvantages: a) High equipment costs. b) High energy consumption. V®Ê /ˆ“iÊVœ˜ÃՓˆ˜}]Ê܈̅Ê̈“i‡ÃV>iÃÊÀ>˜}ˆ˜}ÊLiÌÜii˜Ê£äʅœÕÀÃÊ>˜`Ê >Ãʅˆ}…Ê>ÃÊxäʅœÕÀÃÊ̜Ê>V…ˆiÛiÊ̅iÊÀiµÕˆÀi`ʓœiVՏ>ÀÊÜiˆ}…Ì° d) Many versions of the process, especially batch processing in tumbling reactors, can produce fines which vary in molecular weight from the pellets, and which need to be separated out before melt processing into fabricated articles.

6.4.2 Chain Extenders Following on from the need for increasing the polyester chain length for several important applications, and the discussion undertaken in Section 6.3, there is the possibility of using difunctional species as means of assembling longer chains by reacting each functionality with a different polymer chain. It is also possible to envisage further branched structures through polyfunctional species. Several studies have been carried out on the chain extension of aromatic polyesters through the use of various additive species over ̅iʏ>ÃÌÊÓäqÎäÊÞi>ÀÃÊQ£ÈÓq£™äRÆʓ>˜ÞÊ«>Ìi˜ÌÃʅ>ÛiÊ>ÃœÊLii˜Ê>««ˆi`Ê vœÀʈ˜Ê̅ˆÃÊ>Ài>ÊQ£™£qÓ£äR° The basic concept for a chain extender is a molecule which is polyfunctional (preferably difunctional), and where functionalities

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Degradation and Stabilisation of Aromatic Polyesters can react readily with carboxylic acid and/or hydroxyl chain ends of the host polyester. Selection of a suitable chain extender requires certain properties to be considered: a) The additive should be thermally stable at any required processing temperature. b) The reaction between extender and polymer should take place rapidly but controllably under the conditions of incorporation. c) Reaction should be essentially irreversible under reaction conditions and under likely conditions to which a product made will be subjected. d) Reaction should preferably produce no small-molecule side products. e) If small-molecule side products are produced, these should not induce unwanted side reactions during the processing or in subsequent use, and they should be capable of facile removal from the system by simple processes. f) The new linkage formed should be thermally, photolytically and oxidatively stable; should not interfere with the orientation or crystallisation of the product; and preferably should not contain moieties subject to further unwanted reactions. Early attempts at chain extension took the route of using esters of dicarboxylic acids which had greater reactivity towards the polyester chain ends than simplistic additives such as dimethyl terephthalate Q£™£]Ê£™Î]Ê£™{R]ÊLÕÌʓ>˜ÞʜvÊ̅iʓœÀiÊÀi>V̈ÛiÊëiVˆiÃÊ}>Ûiʘœ˜‡ volatile small-molecule by-products such as phenol, which were `ˆvwVՏÌÊ̜ÊÀi“œÛi°Ê
˜œÌ…iÀÊi>ÀÞÊ>ÌÌi“«ÌÊQ£™ÓRÊÕÃi`Ê`ˆˆÃœVÞ>˜>ÌiÃ]Ê but this approach can give undesirable branching, and the new linkage formed was thermally unstable. Later studies used diisocyanates to V…>ˆ˜‡iÝÌi˜`ÊÀiVÞVi`Ê* /ÊQ£ÇnR°

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Thermal and Hydrolytic Stabilisation -Vˆi˜ÌˆÃÌÃÊ>ÌÊ/iˆˆ˜ÊÌ`°]Ê«>Ìi˜Ìi`ÊQ£™È]Ê£™ÇRÊ>˜`Ê«ÕLˆÃ…i`ÊiÝÌi˜ÃˆÛiÊ ÃÌÕ`ˆiÃʜ˜ÊQ£ÈÓq£ÈÇRÊ̅iÊÕÃiʜvÊLˆÃ­VÞVˆVʈ“ˆ˜œÊi̅iÀîÊ>˜`ÊLˆÃ­VÞVˆVÊ imino esters) as chain extenders for PET.

ޏˆVʈ“ˆ˜œÊi̅iÀÃÊÃÕV…Ê>ÃÊÓ]ÓA‡LˆÃ­Ó‡œÝ>✏ˆ˜i®Ê­ "VÌÊÜˆÌ…Ê carboxylic acid chain ends without production of a leaving group: "Ìi`Ê̅>ÌÊ "VÌi`Ê̜ÊvœÀ“ʏˆ˜i>ÀÊV…>ˆ˜‡iÝÌi˜`i`Ê«œÞ“iÀÃÊ without branching. The authors also proposed that, under thermoœÝˆ`>̈ÛiÊ Vœ˜`ˆÌˆœ˜Ã]Ê Õ˜Ài>VÌi`Ê "ˆ˜ˆ˜}Ê ˆ˜Ê ̅iÊ «œÞ“iÀÊ could act as a stabiliser, reacting with carboxylic acid end groups and preventing decrease in molecular weight, even in the solid state. Conversely, under thermal degradation conditions, i.e., during melt processing, the linkage units formed are less stable than polyester units. *œÞiÃÌiÀÊ ÌÀi>Ìi`Ê ÜˆÌ…Ê "ÃÊ œÜÊ V>ÀLœÝޏˆVÊ >Vˆ`Ê Vœ˜Ìi˜Ì]Ê >ÃÊ might be expected, and considerable improvement in the hydrolytic ÃÌ>LˆˆÌÞÊÜ>ÃʜLÃiÀÛi`°ÊÌÊÜ>ÃʘœÌi`Ê̅>ÌÊ՘Ài>VÌi`Ê "`ÛiÀÃiÞÊ >vviVÌi`ʅÞ`ÀœÞ̈VÊÃÌ>LˆˆÌÞ]Ê>Ì…œÕ}…Ê̅iÊivviVÌÊÜ>Ãʜ˜ÞÊ>LœÕÌÊ£Ó¯Ê as powerful as that of the carboxylic acid groups.

ÞVˆVʈ“ˆ˜œÊiÃÌiÀÃÊÃÕV…Ê>ÃÊÓ]ÓA‡LˆÃ­{‡Î]£‡Li˜âœÝ>∘‡{‡œ˜i®Ê­ ««i>ÀÊ̜ÊiÝiÀÌʏˆÌ̏iÊivviVÌʜ˜Ê̅iÀ“>ÊœÀʅÞ`ÀœÞ̈VÊÃÌ>LˆˆÌÞ°Ê7…ˆiÊ Õ˜Ài>VÌi`Ê ˜Ê >VÌÊ >ÃÊ >Ê Ì…iÀ“>Ê ÃÌ>LˆˆÃiÀ]Ê ˆÌÊ >ÃœÊ >VÌÃÊ >ÃÊ >Ê hydrolysis promoter, with effectiveness similar to that of carboxylic acid end groups.

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Degradation and Stabilisation of Aromatic Polyesters

>À`ˆÊ>˜`ÊVœ‡ÜœÀŽiÀÃÊQ£ÈnRÊṎˆÃi`Ê "ˆ˜‡iÝÌi˜`ÊÀiVÞVi`Ê * /°Ê 7…ˆiÊ ÀiÃՏÌÃÊ Ã…œÜi`Ê Ãœ“iÊ “ˆÌˆ}>̈œ˜Ê œvÊ Ì…iÀ“>Ê >˜`Ê hydrolytic degradation caused by impurities in the scrap feedstock, any meaningful degree of improvement required excessive levels of additive, which resulted in accumulation of unwanted side reactions. Other additives of this type which have been investigated are ‡Ó]ÓA­£]·«…i˜Þi˜i®LˆÃ­Ó‡œÝ>✏ˆ˜i®ÊQÓäxRÊ>˜`ÊÓ]ÓA­£]{‡«…i˜Þi˜i® LˆÃʭӇœÝ>✏ˆ˜i®ÊQ£ÇxR°Ê/…iʏ>ÌÌiÀÊÜ>ÃÊ«>À̈VՏ>ÀÞÊÃÕVViÃÃvՏÊ`ÕiÊ ÌœÊˆÌÃʅˆ}…iÀÊÀi>V̈ۈÌÞÊ̜Ü>À`ÃÊ>Vˆ`ÃÊ̅>˜Ê "ÃiÃÊ in molecular weight appear to have required supplementary chain extenders of different chemistries. Another approach which has been taken by many researchers for chain extension of polyesters is the use of di or poly-epoxy Vœ“«œÕ˜`Ã°Ê ˆi«œÝˆ`iÃ]ÊÕÃÕ>ÞʜvʓœiVՏ>ÀÊÜiˆ}…ÌÊLiÌÜii˜Ê£ääÊ >˜`Ê£Óääʅ>ÛiÊLii˜ÊÕÃi`]ʜvÌi˜Ê܈̅ÊV>Ì>ÞÃÌÃÊÃÕV…Ê>ÃÊÌiÌÀ>Ü`ˆÕ“Ê i̅ޏi˜i`ˆ>“ˆ˜iÌiÌÀ>>ViÌ>ÌiÊQ£™xR]Ê>Ž>ˆÊ“iÌ>Ê…>ˆ`iÃÊQÓääR]ʜÀÊ>Ž>ˆÊ “iÌ>ÊLœÀ>ÌiÃÊQÓäÇR°ÊN,N´-bis(glycidyl ester)pyromellitimides have Lii˜Êˆ˜ÛiÃ̈}>Ìi`ÊQ£Çä]ʣǣR]Ê>Ãʅ>ÛiÊ`ˆ}ÞVˆ`ޏÌiÌÀ>…Þ`Àœ«…Ì…>>ÌiÊ Q£ÇÓR]Ê`ˆ‡ÊœÀÊ«œÞ‡ÃÕLÃ̈ÌÕÌi`Ê`ˆ«…i˜Þ«Àœ«>˜iÊ>˜`ÊÀi>Ìi`ÊÃÕLÃÌ>˜ViÃÊ QÓäxR°Ê ÌÊ …>ÃÊ >ÃœÊ Lii˜Ê ÃÕ}}iÃÌi`Ê Ì…>Ì]Ê vœÀÊ i>ÃiÊ œvÊ …>˜`ˆ˜}Ê >˜`Ê addition, such compounds may be combined with a titanium dioxide V>ÀÀˆiÀÊQÓä™R° ˆ>ViÊ QÓäÈRÊ ÕÃi`Ê >Ê V…>ˆ˜Ê iÝÌi˜`iÀÉLÀ>˜V…iÀÊ L>Ãi`Ê œ˜Ê >Ê >V̜˜iÊ oligomer with cyclohexane rings at each end. Each cyclohexane has two oxirane attached thereto. This is commercially available as Celloxide‘ÊÓän£°
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ÊQÓänRÊ«Àœ«œÃi`Ê}ÞVˆ`ޏʓi̅>VÀޏ>ÌiÊVœ«œÞ“iÀÃÊ>ÃÊ>ÊV…>ˆ˜Ê extending additive. A particular version of this approach is Joncryl‘
,Ê {ÎÈn]Ê >Ê ÃÌÞÀi˜i‡}ÞVˆ`ÞÊ “i̅>VÀޏ>ÌiÊ Vœ«œÞ“iÀ°Ê /…ˆÃÊ >ÃœÊ appears to be the active ingredient in CESA-Extend‘, a masterbatch for chain extension of recycled polyesters jointly developed by

>Àˆ>˜ÌÊ>˜`Êœ…˜Ãœ˜Ê*œÞ“iÀÊQ£n£]Ê£nÈR°

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Thermal and Hydrolytic Stabilisation >À>>L>Žœ«œÕœÃÊ >˜`Ê Vœ‡ÜœÀŽiÀÃÊ Q£ÇÎRÊ V>ÀÀˆi`Ê œÕÌÊ >Ê ÃÌÕ`ÞÊ œvÊ PET chain extension using various bis-epoxides. Additives studied were: UÊ LˆÃ­Î]{‡i«œÝÞVÞVœ…iÝޏ“i̅ޏ®>`ˆ«>Ìi UÊ Î]{‡i«œÝÞVÞVœ…iÝޏ“i̅ޏ‡Î]{‡i«œÝÞVÞVœ…iÝޏV>ÀLœÝޏ>Ìi UÊ £]{‡VÞVœ…iÝ>˜i`ˆ“i̅>˜œÊ`ˆ}ÞVˆ`ޏÊi̅iÀ UÊ £]{‡LÕÌ>˜i`ˆœÊ`ˆ}ÞVˆ`ޏÊi̅iÀ UÊ LˆÃ«…i˜œÊ
Ê`ˆ}ÞVˆ`ޏÊi̅iÀ The main aim of this study was to assess the utility of these relatively cheap, readily available diepoxides. Authors admitted that all were less effective chain extenders than custom-made diepoxides and the bisoxazolines. The cyclic diepoxides appeared more effective than the glycidyl ethers. Low extender concentration and short reaction times favoured the extension by reducing thermal decomposition, and prior drying was necessary to prevent hydrolysis. Resulting polymers showed intrinsic viscosity higher than processed PET, and in some cases equal to that of virgin PET. In addition, carboxylic acid end group levels were reduced to values well below those of virgin PET. ˜Ê >Ê >ÌiÀÊ ÃÌÕ`ÞÊ LÞÊ …>Û>ˆŽ>ÀÊ >˜`Ê 8>˜Ì…œÃÊ Q£näR]Ê Ì…iÊ >Õ̅œÀÃÊ investigated di- and tri-functional additives. The diepoxides used were the commercially oriented additives, diglycidyl ether of bisphenol
Ê ÜˆÌ…Ê “œiVՏ>ÀÊ Üiˆ}…ÌÊ œvÊ >««ÀœÝˆ“>ÌiÞÊ ÎxäÊ ­ «œ˜‘Ê nÓn®Ê >˜`Ê N,N´-bis[3(carbo-2´,3´-epoxypropoxy)phenyl]pyromellitimide. Such additives have long been considered to be suitable chain extenders, with their ability to react with carboxyl and hydroxyl end groups:

>ÀLœÝޏÊʳÊÊi«œÝÞÊÊʇÊÊÊH­ r"®‡"‡ 2‡ ­"®H Þ`ÀœÝޏÊʳÊÊi«œÝÞÊÊʇÊÊÊH 2-CH2-O-CH2‡ ­"®H

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Degradation and Stabilisation of Aromatic Polyesters Note the hydroxyl group on the new linkage; although the authors make no reference to this, it could be the source of unwanted side reactions. The nitrogen-containing additive showed better chain-extending ability. This may be due to its having a much better degradation resistance than the other additive, while the authors also suggest that greater reactivity may be attributed to a tertiary amine in the vicinity of the glycidyl group acting as a built-in catalyst. Tertiary amines are known catalysts for the type of reaction shown above. ˆÃ‡>Vޏ>VÌ>“Ãʅ>ÛiÊ>ÃœÊLii˜Êˆ˜ÛiÃ̈}>Ìi`ÊvœÀÊÕÃiÊ>ÃÊV…>ˆ˜ÊiÝÌi˜`iÀÃ]Ê ˆ˜VÕ`ˆ˜}ÊÌiÀi«…Ì…>œÞ‡LˆÃ‡>ÕÀœ>VÌ>“ÊœÀʇV>«Àœ>VÌ>“ÊQ£™™R]Ê>˜`Ê species synthesised from terephthaloyl chloride, caprolactam and iˆÌ…iÀÊ>ˆ«…>̈VÊ`ˆœÃʜÀÊ`ˆ>“ˆ˜iÃÊQ£nnR°Ê/…iʏ>ÌÌiÀ]Ê`ÕiÊ̜Ê̅iÊ>ŽÞi˜iÊ spacer group contribution of the aliphatic species, would tend to «ÀœÛˆ`iʓœÀiÊyi݈Liʏˆ˜Ž>}iðÊÌÊÜ>ÃÊ>ÃœÊ˜œÌi`Ê̅>ÌÊÃÕV…ÊëiVˆiÃÊ >ÀiÊ̅iÀ“>ÞÊÃÌ>LiÊÕ«Ê̜Ê>Ìʏi>ÃÌÊÓÈäÊc ° Another group of materials which have been extensively investigated and applied as chain extenders are aromatic tetracarboxylic `ˆ>˜…Þ`Àˆ`iÃÊQ£È™]Ê£Ç{]Ê£ÇÈ]Ê£nÓ]Ê£nÇ]ÊÓä£]ÊÓäÓ]ÊÓä{]ÊÓ£äR]ÊÜˆÌ…Ê pyromellitic dianhydride being the most used. This additive is used in a commercial additive Irgamod‘Ê,
ÓäÊ­ ˆL>®]Ê܅ˆV…ʈÃÊ>ʓ>ÃÌiÀL>ÌV…Ê Vœ˜ÃˆÃ̈˜}ʜvÊÇx°x¯Ê«œÞiÃÌiÀ]ÊÓä°x¯Ê«ÞÀœ“iˆÌˆVÊ`ˆ>˜…Þ`Àˆ`i]ÊÓ¯Ê «i˜Ì>iÀÞ̅ÀˆÌœÊ>˜`ÊÓ¯ÊÀ}>˜œÝ‘ 1425. A number of studies have been undertaken to compare the various V…i“ˆÃÌÀˆiÃÊ>Û>ˆ>LiÊvœÀÊ«œÞiÃÌiÀÊV…>ˆ˜ÊiÝÌi˜Ãˆœ˜ÊQ£ÇÇ]ʣǙ]Ê£nÎ]Ê £n{]Ê£n™R°Ê ÝÌi˜`iÀÃÊ܅ˆV…Ê}>ÛiʘœÊLއ«Àœ`ÕVÌÃÊÜiÀiÊÛiÀÞʓÕV…Ê the preferred choice, including diepoxides, bis-2-oxazolines and bisx]ȇ`ˆ…Þ`Àœ‡{‡£]·œÝ>∘iðÊÌÊÜ>ÃʘœÌi`Ê̅>ÌÊÀi>V̈œ˜ÊœvÊi«œÝÞÊ units with acid and hydroxyl chain ends resulted in the creation of secondary hydroxyl groups, which could react further with carboxyl or epoxy groups promoting branching and crosslinking. ˆˆÃœVÞ>˜>ÌiÃÊÜiÀiÊ>ÃœÊVœ˜Ãˆ`iÀi`]Ê>ÃÊÜ>ÃÊ«ÞÀœ“iˆÌˆVÊ`ˆ>˜…Þ`Àˆ`i]Ê although the latter produced severe branching.

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Ê ˜iÜÊ V…>ˆ˜‡iÝÌi˜`iÀÊ >``ˆÌˆÛiÊ Q£nx]Ê £™äRÊ L>Ãi`Ê œ˜Ê ̅iÊ V>}i‡ structured silanol derivative epoxycylohexyl polyhedral oligomeric silsesquioxane has been suggested to be a useful additive.

References 1.

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°Ê,œÌ…ÀœVŽÊ>˜`Ê,°°Ê œ˜}˜i]ʈ˜Ûi˜ÌœÀÃÆÊ/…iÊ,iȘœÕÃÊ *Àœ`ÕVÌÃÊ>˜`Ê …i“ˆV>Ê œ°]Ê>ÃÈ}˜iiÆÊ1-Ó{ÎÇä{È]Ê£™{n°

2.

° °Ê
“LœÀΈÊ>˜`Ê ° °Ê-Àœœ}]ʈ˜Ûi˜ÌœÀÃÆÊ Õ*œ˜ÌÊ`iÊ i“œÕÀÃÊEÊ œ°]Ê>ÃÈ}˜iiÆÊ1-әӣäx£]Ê£™Èä°

3.

° °Ê*ÀiۜÀÃiŽ]ʈ˜Ûi˜ÌœÀÆÊ/…iÊœœ`Þi>ÀÊ/ˆÀiÊEÊ,ÕLLiÀÊ œ°]Ê >ÃÈ}˜iiÆÊ1-ÎÎää{{ä]Ê£™ÈÇ°

4.

7°°ÊiÀ}i˜ÀœÌ…iÀ]ʈ˜Ûi˜ÌœÀÆʈÀiÃ̜˜iÊ/ˆÀiÊEÊ,ÕLLiÀÊ œ°]Ê >ÃÈ}˜iiÆÊ1-Î{ǙΣ™]Ê£™È™°

5.

M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÈ}˜iiÆÊ1-Î{™È£ÎÇ]Ê£™Çä°

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M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÈ}˜iiÆÊ1-ÎxÎΙ™x]Ê£™Çä°

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M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÈ}˜iiÆÊ1-ÎxÎnä{x]Ê£™Çä°

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M.J. Stewart and J.A. Price, inventors; FMC Corporation, >ÃÈ}˜iiÆÊ1-ÎxÇÇÎn£]Ê£™Ç£°

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]Ê >ÃÈ}˜iiÆÊ1-ÎÈ䙣£n]Ê£™Ç£°

£ä°Ê °Ê*ˆâ]Ê°Êœ…iˆÃiÊ>˜`Ê °Ê7iÀ˜iÀ]ʈ˜Ûi˜ÌœÀÃÆÊ>iÊ
]Ê >ÃÈ}˜iiÆÊ1-ÎÈ䙣£™]Ê£™Ç£° 11. M.J. Stewart and O.K. Carlsson, inventors; FMC

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]Ê£™nn]Ê«°ÓÎΰ 51. -° °Ê ˆiÌâʈ˜ÊProceedings of an SPE Conference - New Developments in Plastics Recycling, Charlotte, NC, USA, £™n™]Ê*>«iÀÊ œ°£Î° 52. -° °Ê ˆiÌâʈ˜ÊEmerging Technologies in Plastics Recycling,

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]Ê£™™£]Ê«°£Î{° 53. P. Solera in Proceedings of an SPE RETEC Conference Weathering Well with Colorants and Additives, Orlando, FL, 1-
]Ê£™™Î]Ê«°£È£° 54. R. Pfaendner, H. Herbst, K. Hoffmann and F. Sitek, Die Angewandte Makromolekulare Chemie]Ê£™™x]Ê232]Ê£™Î° 55. P. Solera, Journal of Vinyl and Additive Technology]Ê£™™n]Ê4, Î]Ê£™Ç° xÈ°Ê °Ê*œÃ«ˆÃˆ]Ê7° °Ê>LˆV…iÀÊ>˜`Ê-°Ê iëÕÀiŽÊˆ˜ÊProceedings of a Rapra Technology Conference - Addcon World ‘99,Ê£™™™]Ê Prague, Czechoslovakia, Paper No.22. xÇ°Ê ° °Ê
Å̜˜]Ê7°Ê ˜œÜÊ>˜`Ê/°Ê ii˜Êˆ˜ÊProceedings of an SPE Conference – Antec 2000]Ê"À>˜`œ]Ê]Ê1-
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œÀ«œÀ>̈œ˜]Ê>ÃÈ}˜iiÆÊ1-{xnÓnÇä]Ê£™nÈ° ™£°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{ÈÇnnÓx]Ê£™nÇ° ™Ó°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{näÎÓÎÈ]Ê£™n™° ™Î°Ê J.C. Rosenfeld, inventor; Celanese Corporation, assignee; 1-{nÓ{n™x]Ê£™n™° ™{°Ê ° °Ê7>}}œ˜iÀ]Ê°,°Ê->“ÕiÃÊ>˜`Ê °°Ê
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œ°]Ê>ÃÈ}˜iiÆÊ1-x™£äÎÈÎ]Ê£™™™° 121. 9°Ê“>ňÀœ]Ê°Ê/>Ž>…>ň]Ê °ÊœÀˆiÊ>˜`Ê/°Ê9>“>˜i]Ê ˆ˜Ûi˜ÌœÀÃÆÊ ˆÃň˜LœÊ˜`ÕÃÌÀˆiÃʘV°]Ê>ÃÈ}˜iiÆÊ1-È£ÓÈnÈä]Ê Óäää° 122. -°Ê i˜âˆ˜}iÀ]Ê°Ê>““iÀâ>…]ʰʘ`Ê
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Thermal and Hydrolytic Stabilisation 124. P.C. Georgoudis, inventor; National Starch Chemical

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7

Thermo-oxidative Stabilisation

7.1 Introduction As with all organic polymers, exposure of aromatic polyesters to heat and oxygen can, especially over long time periods, result in degradation of the polymer. This thermo-oxidation manifests as discoloration of materials, loss of physical properties, and complete failure of the substrate. To prevent (or more likely control) such processes it is necessary to incorporate additives which can protect the host polyester against the effects of heat and oxygen: antioxidants. As may be deduced from the discussions undertaken in Chapter 3, the most damaging reactions within the autoxidation cycle posited for polymers are initiation and chain branching, and propagation. These are also the reactions where it may be possible to utilise specific chemical compounds to interfere with the processes involved. Antioxidants are usually roughly divided into two categories: 1. Primary, or chain-breaking antioxidants interfere with the chain propagation step, i.e., the reactions: P. + O2 ‡ POO. POO. + PH ‡ POOH + P. 2. Secondary, or preventative antioxidants react with the hydroperoxides responsible for chain initiation and chain branching.

181

Degradation and Stabilisation of Aromatic Polyesters The primary antioxidants are normally broken down further into the classes of chain-breaking donor (CB-D) and chain-breaking acceptor (CB-A). CB-D additives interact with peroxy radicals, and are by far the commonest class of antioxidant in general use. They are represented by such additives types as hindered phenols and secondary aromatic amines. CB-A additives interact with alkyl radicals but, due to the rapid oxidation of such radicals, these additives are really useful only under low oxygen availability. CB-A types are represented by aromatic nitro and nitroso compounds, and a few speciality ‘stable’ free radicals. Some transformation products of CB-D antioxidants can also act as CB-A species. Secondary antioxidants, often called peroxide decomposers, also may be further categorised as stoichiometric peroxide decomposers and catalytic peroxide decomposers. The former are represented by, for example, phosphites; the latter by organic sulfides and their transformation products. A classic overview of antioxidants and their modes of action is provided by Scott [1]. In considering antioxidants for aromatic polyesters, it should be remembered that most commercial products were originally developed for use in polyolefins and rubbers. Several points must be taken into account when selecting antioxidants for these polyesters: 1. Stability at higher processing temperatures. 2. Unwanted reactivity with host polymer. Conversely, deliberate reactions can be induced to attach antioxidants to the polymer chains, or to build them into chains. 3. Physical loss from substrate during polycondensation or melt processing, depending on at which stage the antioxidant is incorporated into the host. 4. Compatibility with the host, which will depend on chemical and physical factors.

182

Thermo-oxidative Stabilisation Taking poly(ethylene terephthalate) (PET) as an example of an aromatic polyester, this material is less susceptible to oxidation than many other polymers, such as polypropylene (PP), from a chemical reactivity point of view, and from its relative permeability to oxygen (Table 7.1) [2]

Table 7.1 Oxygen Permeability of Selected Polymers Polymer

Permeability (P × 1010 cm3 s–1 mm cm–2 hg–1; 30 °C)

PET

0.22

High-density polyethylene

10.26

PP

23

Low-density polyethylene

15

Polybutadiene

191

Natural Rubber

233

In 1985, Zimmermann [3] was moved to state that ‘Many patents propose the addition of antioxidants to PET, but it is seldom done in technical practice.’ Be that as it may, there will be situations and applications where an antioxidant may be required to boost the innate stability of aromatic polyesters, and an examination of proposed additives will also allow workers in this field to ascertain which adducts have been tried and how successful each category may have been. We will also examine newer additives which, while they may not have been used to date in polyesters, might be candidates for future examination.

7.2 Studies on Antioxidants Only a few articles have been published examining the effects of various antioxidant packages on the thermo-oxidative degradation of aromatic polyesters [3–19]. Angelova and co-workers [4, 5] used differential thermal analysis (DTA) to examine the effects of some antioxidants on PET and model

183

Degradation and Stabilisation of Aromatic Polyesters compounds thereof. They found that a combination of diethyl-3,5di-t-butyl-4-hydroxybenzylphosphonate (Irganox‘ 1222; Ciba) with triphenylphosphine constituted an effective antioxidant package, with a marked synergism between the components. Zimmermann [6] noted that phosphites, while effective thermal stabilisers (see Chapter 6), were also effective antioxidants, especially if used in combination with primary antioxidants such as hindered phenols. Kosinska and co-workers [7] suggested that hindered phenols and secondary aromatic amines could be effective antioxidants in PET. Karayannidis and co-workers investigated the effect of various phosphorus-containing compounds on the thermo-oxidative stability of PET [9]. Additives were incorporated in the reaction kettle after the transesterification stage, but before polycondensation. Additives investigated were: phosphoric acid, tributylphosphate, triphenylphosphate, phenylphosphonic acid, phenylphosphinic acid and sodium phenylphosphinate. The most effective antioxidants were tributylphosphate, phenylphosphonic acid and phenylphosphinic acid. The above authors also investigated the effectiveness of various commercial primary antioxidants in PET [11]. The additives assessed were: UÊ {]{A‡ ˆÃ­A,A-dimethylbenzyl)diphenylamine (Naugard‘ 445; Uniroyal) UÊ £]Î]x‡/Àˆ“i̅ޏ‡Ó]{]ȇÌÀˆÃ­Î]x‡`ˆ‡t-butyl-4-hydroxybenzyl) benzene (Ethanox‘ 330; Ethyl Corporation) U

N,NA‡iÝ>“i̅ޏi˜i‡LˆÃ‡­Î]x‡`ˆ‡t-butyl-4-hydroxycinnamide) (Irganox‘ 1098; Ciba)

U

N,NA‡/Àˆ“i̅ޏi˜i‡LˆÃ‡­Î]x‡`ˆ‡t-butyl-4-hydroxycinnamide) (Irganox‘ 1019; Ciba)

184

Thermo-oxidative Stabilisation Additives were again incorporated in the reaction kettle between the transesterification and polycondensation stages. Due to the high vacuum required during polycondensation, a great deal of the most volatile additive (secondary aromatic amine) was lost. Test results, utilising infra-red spectroscopic assessment and oxidation induction times, demonstrated that 445 and 330 gave the best performance at addition levels between 0.01 and 0.03 wt%, although neither provided full protection to the polyester matrix at such low levels. At loadings between 0.5 and 1.0 wt%, which are more normal levels to attain adequate protection, 1098 and a 1:1 blend of 1098 and 1019 gave superior results. The authors also stated that 1019 exhibited some metal chelating ability, possibly due to the shorter alkylene chain length between the two hindered phenolic ends allowing for better metal ion complexation. Ciba researchers [10, 12, 15] studied the problems inherent in ‘restabilising’ post-consumer recyclates, including PET. The need to at least attempt to take into account the degree of degradation, level of residual stabilisers, and type and quality of contamination that might be present was emphasised. Various combinations of hindered phenolics and phosphorus-based secondary stabilisers were noted as being effective antioxidant packages, including specifically designed commercial combinations such as Recyclostab‘, proprietary mixtures of stabilisers from Ciba. Specific antioxidants recommended (in various combinations) were: UÊ * i ˜ Ì > i À Þ Ì … À ˆ Ì œ  ‡ Ì i Ì À > Ž ˆ à ­ Î ] x ] ` ˆ ‡ t - b u t y l - 4 hydroxyphenylpropionate) (Irganox‘ 1010) UÊÊ Ì…Þi˜i‡LˆÃ­œÝÞi̅ޏi˜i®LˆÃ­Î­x‡t-butyl-4-hydroxyphenyl-mtolyl)propionate) (Iragnox‘ 245) UÊÊ /ÀˆÃ­Ó]{‡`ˆ‡t-butylphenyl)phosphite (Irgafos‘ 168)

185

Degradation and Stabilisation of Aromatic Polyesters Tock [13] suggested that the process stabiliser bis(2,4-di-tbutylphenyl)pentaerythritoldiphosphite (Ultranox‘ 626; GE) was also a moderately effective antioxidant. Feng and Ning [14] recommended the addition of 1,6-hexanediolbis(3(3,5-di-t-butyl-4-hydroxyphenyl)propionate (Irganox‘ 259; Ciba) as an antioxidant for PET. Allen and co-workers [17] investigated commercial antioxidants in poly(ethylene-co-1,4-cyclohexanediemthelyene terephthalate) using the additives: UÊ ˆÃ­Ó]{‡`ˆVՓޏ«…i˜Þ®«i˜Ì>iÀÞ̅ÀˆÌœ`ˆ«…œÃ«…ˆÌiÊ­
Ž>˜œÝ‘ 28; Great Lakes Chemical) UÊ £]Î]x‡/ÀˆÃ­Î]x‡`ˆ‡t-butyl-4-hydroxybenzyl)-1,3,5-triazine2,4,6(1H, 3H, 5H)trione (Irganox‘ 3114; Ciba) UÊ £]Î]x‡/ÀˆÃ­{‡t-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5triazine-2,4,6(1H, 3H, 5H)trione (Irganox‘ 1790; Ciba) Best thermal antioxidant performance was seen with a combination of 0.2 wt% phosphite and 0.2 wt% of either of the hindered phenols. Russian researchers [18, 19] investigated the efficiency of thermal antioxidants in poly(butylene terephthalate) (PBT). Good results were obtained using combinations Irgafos‘ 168 or Ultranox‘ 626 with Irganox‘ 1010 or stearyl-B-(3,5-di-t-butyl-4-hydroxyphenyl) propionate (Irganox‘ 1076; Ciba). From the available studies it would appear that hindered phenols in combination with phosphites, provided both components are sufficiently non-volatile and thermally stable, would be the preferred option for the thermo-oxidative stabilisation of aromatic polyesters. While as yet not fully tested, additives from Polnox Corporation [20– 24] provide some variations on the theme of ‘traditional’ antioxidants which could be useful in aromatic polyesters. Utilising what they say

186

Thermo-oxidative Stabilisation is novel chemistry, this concern has produced molecules which contain either bulky constituents to reduce volatility, or which contain two different stabiliser moieties, e.g., both hindered phenol and aromatic amine. Through the use of novel synthetic methods, this concern has produced two series of additives. One contains an active site and one or more bulky side groups, which renders the additive less susceptible to loss for the polymer matrix through volatilisation. The other type contain two different active stabilising groups, e.g., a hindered phenol and an aromatic amine, holding out the possibility of one dealing with rapid initial oxidation and the other operating over a longer time scale. This could potentially provide a greater overall stabilising effect, or could allow lower levels of additives to be used.

7.3 Potential New Chemistries Although they do not appear to have been investigated in aromatic polyesters, there are a few other antioxidant types which might prove useful in this regard. Benzofuranones [25–27], exemplified by the Ciba product HP136‘ , which is said to be a mixture of 90% 5,7-di-t-butyl-3-(3,4dimethylphenyl)-3H-benzofuran-2-one and 10% 5,7-di-t-butyl-3(2,3-dimethylphenyl)-3H-benzofuran-2-one, are said to be fast-acting radical scavengers, which may effectively trap peroxy and alkyl radicals. Classed as ‘a potent melt processing co-additive’ [25], this is mainly used as a very effective oxidation inhibitor at low levels in combination with hindered phenol and phosphite. The capability of HP-136‘ in reacting with alkyl radicals is said to boost the stabiliser package performance, and there is some suggestion that this additive can also react with phenoxy radicals, thus regenerating the hindered phenolic antioxidant. Commercial packages include: Irganox‘ HP2215 - 15% HP-136, 28% Irganox 1010, 57% Irgafos 168 Irganox‘ HP2225 - 15% HP-136, 42.5% Irganox 1010, 42.5% Irgafos 168 187

Degradation and Stabilisation of Aromatic Polyesters Classed by Marin and co-workers [27] as a moderate CB-D antioxidant, the use of HP-136 ‘ (or the previous packages containing the same) in PET and related polyesters may be worth investigating. Hydroxylamines [25] have the advantage that they are almost completely colourless, unlike the aromatic amines which are coloured, and the hindered phenols which form highly coloured breakdown species. This class of additive is exemplified by N,N-di(hydrogenated tallow)hydroxylamine (Irgastab ‘ FS-042; Ciba). This type of additive appears to operate as a radical scavenger and as a peroxide decomposer. Nitrones formed by oxidation of the hydroxylamine are also said to be stabilisers. Patents claim these additives as useful in reducing aldehyde content in polyester [28, 29], but there does not appear to have been any systematic study of their antioxidant capabilities in aromatic polyesters. Vitamin E (A-tocopherol) [30] is another potential candidate for an antioxidant in polyesters, and is available as a polymer additive from Hoffmann-La Roche as Ronotec‘ 201. It has been shown that this additive can react with peroxy and alkyl radicals, and can trap more than one radical. The potential drawbacks may be the volatility and/ or thermal stability of this additive. As well as being excellent UV stabilisers for several polymer applications, there is ample evidence [31] to suggest that hindred amine light stabilisers can also operate as thermal antioxidants. Discussion of these additives will be undertaken in Chapter 8.

7.4 Patents The patent literature relating to antioxidants specifically for use in aromatic polyesters covers a multitude of additives, but is quite restricted in the class of antioxidant claimed, being largely limited to hindered phenolic derivatives. These include versions with no heteroatoms [32–39], phosphorus-containing [40–47], nitrogen-containing

188

Thermo-oxidative Stabilisation [48, 49], sulfur-containing [50–53] and patents dealing with hindered phenols packaged alongside specific co-additives [54–63]. Aromatic amines are also represented [64–66], whereas single patents have been found claiming the use of phenothiazines [67], sulfides [68] and quinones [69]. Hindered phenolics containing only carbon, hydrogen and oxygen include: a triphenol reaction product of crotonaldehyde and 3-methyl-6-t-butylphenol (C4H7[C6H2(CH3)(C4H9)OH]3) [32]; di- or tri-phenols where the phenolic rings are attached to each other by a -CH2- bridge, cited as useful in PBT or poly(butylene naphthalate) (PBN) [33]; reactable phenols with a -R1COOR2 moiety in the 4-position, again aimed at PBT or PBN [34]; reactive phenols claimed vœÀÊÕÃiʈ˜Ê∘V‡V>Ì>ÞÃi`Ê«œÞ“iÀˆÃ>̈œ˜ÊœvÊ* /]Êi°}°]ʓi̅ޏ­ÎA]xA‡ `ˆˆÃœ«Àœ«Þ‡{A‡…Þ`ÀœÝÞ®Li˜âœ>ÌiÊ QÎxRÆÊ Ài>V̈ÛiÊ «…i˜œÃÊ Ü…iÀiÊ Ì…iÊ stabilising functional group is attached via a -O(CH2)n- bridge to isophthalic acid [36]; hindered phenolic acids or esters specifically aimed at end-capping of poly(trimethylene terephthalate) (PTT) to reduce acrolein formation and improve oxidative stability [37]; Irganox‘ 1010 claimed to be reacted into the polymer backbone in titanium-catalysed polyesters [38]; and antioxidants formed by combining reactive hindered phenols with polyvinyl alcohol or polyacrylic acid [39]. Phosphorus-containing hindered phenols include: UÊ *…œÃ«…œ˜ˆVÊ >Vˆ`Ê iÃÌiÀÃÊ ÃÕV…Ê >ÃÊ {‡…Þ`ÀœÝއÎ]x‡`ˆ‡tbutylbenzylphosphonic acid diethyl ester (Irganox‘ 1222; Ciba) [40]; UÊ ˆ«…i˜œÃÊÃÕV…Ê 9H19PhOP[OPh-t-C3H6Ph(t-Butyl)OH]2 [41]; UÊ O-hydroxycarbyl-substituted hydroxyphenylalkylenephosphonic acid esters of polycarbocyclic polydroxy compounds [42]; UÊ ˆ«…i˜œÃÊÃޘ̅iÈÃi`ÊLÞÊÀi>V̈œ˜ÊœvÊ«…œÃ«…œÀÕÃÊ>Vˆ`Ê>ÃÌiÀÃÊÜˆÌ…Ê pentaerythritol [43];

189

Degradation and Stabilisation of Aromatic Polyesters UÊ -ÌiÀˆV>ÞÊ …ˆ˜`iÀi`Ê ÌÀˆÃQ­…Þ`ÀœÝÞ«…i˜ÞÌ…ˆœ®«…i˜ÞR«…œÃ«…ˆÌiÃ]Ê e.g., P[OPh(t-butyl)(methyl)SPh(t-butyl)(methyl)OH]3 (Hostanox‘ OSP-1; Clariant), said to be especially useful in antimony oxide catalysed polyesters as they prevent formation of antimony metal and its associated grey discoloration [44]; UÊ >VˆÕ“Ê Ã>ÌÊ œvÊ `ˆi̅ޏLˆÃ­­­Î]x‡`ˆ“i̅ޏ‡{‡…Þ`ÀœÝÞ«…i˜Þ® methyl)phosphonate) (Irganox‘ 1425; Ciba) [45]; UÊ
`i…Þ`iÊ ÃÕ««ÀiÃȜ˜Ê ÕȘ}Ê >Ê Vœ“Lˆ˜>̈œ˜Ê œvÊ >Ê «œÞœÊ ÃÕV…Ê as sorbitol or maltose and either Irganox‘ 1222 or 1425 [46]; and UÊ
ÊVœ“Lˆ˜>̈œ˜ÊV…>ˆ˜ÊiÝÌi˜`iÀÊ>˜`Ê>˜ÌˆœÝˆ`>˜ÌÊ«>VŽ>}iʎ˜œÜ˜Ê as Irgamod‘ RA20, which is a masterbatch consisting of 75.5% polyester carried resin, 20.5% pyromellitic dianhydride, 2% pentaerythritol and 2% Irganox‘ 1425 [47]. Nitrogen-containing species include triphenols where the stabilising moieties are attached to an isocyanurate ring, e.g., Irganox‘ 3114 and 1790 [48] and other hindered phenol-substituted N-heterocyales such as N-2-(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxy)ethyl succinimide [49]. Sulfur-containing hindered phenolics include diphenols with alkylthio bridges, e.g., 1,6-bis(3,5-di-t-butyl-4-hydroxyphenylthio) hexane [50]; tri- and tetra-(substituted hydroxyphenylthio) alkanes and cycloalkanes, e.g., 1,1,2,2-tetrakis(3,5-di-t-butyl-4hydroxyphenylthio)ethane and 1,1,3-tris (3-t-butyl-5-t-octyl-4hydroxyphenylthio)-3,5,5-trimethylcyclohexane [51, 53]; and di(substituted hydroxyphenylthio)alkanes and cycloalkanes [52]. Patents claiming the use of hindered phenols in aromatic polyesters along with specific co-additives include: cyclic carbonates and phenols, e.g., ethylene carbonate and 1,3,5-trimethyl-2,4,6tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (Irganox ‘ 1330; Ciba) [54]; combinations with oxetane-substituted phosphites [55]; addition at the polymerisation stage of a synergistic mix of

190

Thermo-oxidative Stabilisation triethyl or tributylphosphite and a hindered phenolic phosphonite [56, 57]; combination of any commercial hindered phenol with Ó]ÓA]ÓAA‡˜ˆÌÀˆœÌÀˆi̅ޏ‡ÌÀˆÃ­Î]ÎA]x]xA‡ÌiÌÀ>‡t‡LÕÌޏ‡£]£A‡Lˆ«…i˜Þ‡Ó]ÓA‡ diylphosphite) (Irgafos‘ 12; Ciba) [58]; addition of primary and secondary antioxidants in reaction kettle before polycondensation, preferably a hindered phenol and a phosphonite, e.g., Irganox‘ 1010 and Sandostab‘ P-EPQ [59, 60]; hindered phenol and phosphite in PTT to reduce acrolein emissions [61]; package consisting of an oxetane-substituted phosphorus-containing hindered phenol and an unsubstituted phosphorus-containing phenol [62]; and poly(cyclohexanedimethylene terephthalate) with an antioxidant package consisting of hindered phenol, phosphite, and a polyamide terpolymer [63]. Patents citing the use of other antioxidants include: combinations of cyclic carbonate and secondary aromatic amine [64]; PBT moulding grades stabilised with diamines such as diphenyl-pphenylenediamine [65]; polymeric diphenylamines made by reacting diphenylamine with dialkylalkenylbenzene or dihydroalkylbenzene [66]; substituted phenothiazines [67]; aliphatic sulfides such as R(OCH2 ,A ,AA-,AAA®n [68]; and combinations of aromatic carbodiimides and quinones [69].

References 1.

Atmospheric Oxidation and Antioxidants, 2nd Edition, Volumes I - III, Ed., G. Scott, Elsevier, Amsterdam, The Netherlands, 1993.

2.

J.A. Brydson, Plastics Materials, 7th Edition, ButterworthHeinemann, Oxford, UK, 1999, p.101.

3.

H. Zimmermann, Developments in Polymer Degradation, 1985, 5, 79.

4.

A. Angelova, S. Woinova and D. Dimitrov, Angewandte Makromolekulare Chemie, 1977, 64, 1, 75.

191

Degradation and Stabilisation of Aromatic Polyesters 5.

S. Woinova, A. Angelova and D. Dimitrov, Angewandte Makromolekulare Chemie, 1977, 64, 1, 81.

6.

H. Zimmermann, Plaste und Kautschuk, 1981, 28, 8, 433.

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W. Kosinska, L. Silin-Boranowska and W. Zielinski, Polimery Tworzywa Wielkoczasteczkowe, 1982, 27, 9, 336.

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G. Capocci and J. Zappia in Proceedings of the SPE Conference - Antec’88, Atlanta, GA, USA, 1988, p.1016.

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G. Karayannidis, I. Sideridou, D. Zamboulis, G. Stalidis, D. Biriakis and A. Wilmers, Angewandte Makromolekulare Chemie, 1993, 208, 117.

10. F.A. Sitek, Modern Plastics International, 1993, 23, 10, 74. 11. G. Karayannidis, I. Sideridou, D. Zamboulis, G. Stalidis, D. Biriakis and A. Wilmers, Polymer Degradation and Stability, 1994, 44, 1, 9. 12. F. Sitek, H. Herbst, K. Hoffmann and R. Pfaendner in Proceedings of a Maack Business Conference - Recycle ’94, Davos, Switzerland, 1994, Paper No.23. 13. P. Tock in Proceedings of an AMI Conference - World Compounding Congress ‘94, Neuss, Germany, 1994, Paper No.6. 14. B. Feng and D. Ning, Huaxue Shijie, 1995, 36, 11, 595. 15. R. Pfaendner, H. Herbst, K. Hoffmann and F. Sitek, Angewandte Makromolekulare Chemie, 1995, 232, 193. 16. H.C. Ashton, W. Enlow and T. Nelen in Proceedings of an SPE Conference - Antec 2000, Orlando, FL, USA, 2000, Paper No.557.

192

Thermo-oxidative Stabilisation 17. N.S. Allen, G. Rivalli, M. Edge, T. Corrales and F. Catalina, Polymer Degradation and Stability, 2002, 75, 2, 237. 18. A.O. Lupezheva, N.I. Mashukov and T.A. Borukaev, International Polymer Science and Technology, 2002, 29, 10, T78. 19. E.V. Kalagina, V.A. Tochin, D.V. Gvozdev, T.N. Vakhtinskaya and T.I. Andreeva, International Polymer Science and Technology, 2004, 31, 8, T50. 20. A.L. Choli, Polymer Preprints, 2006, 47, 2, 292. 21. A.L. Choli, Journal of Macromolecular Science A, 2006, 43, 12, 2001. 22. R. Kumar, S. Yang, V. Kumar and A.L. Choli, inventors; Polnox Corporation, assignee; US2006189824, 2006. 23. A.L. Choli and R. Kumar, inventors; Polnox Corporation, assignee; US2007135539, 2007. 24. A.L. Choli, R. Kumar, T. Canteenwala and V. Kumar, inventors; Polnox Corporation, assignee; US2008249335, 2008. 25. P. Solera, Journal of Vinyl and Additive Technology, 1998, 2, 3, 197. 26. P. Nesvadba, S. Evans, inventors; Ciba Speciality Chemicals Corporation, assignee; US5807505, 1998. 27. A. Marin, L. Greci and P. Dubs, Polymer Degradation and Stability, 2002, 76, 3, 489. 28. P.A. Odorisio, S.M. Andrews, D. Lazzari, D. Simon, R.E. King, M. Stamp, R. Reinicker, M. Tinkl, N. Berthelon, D. Muller and U. Hirt, inventors; Ciba Speciality Chemicals Corporation, assignee; US6908650, 2005.

193

Degradation and Stabilisation of Aromatic Polyesters 29. P.A. Odorisio, S.M. Andrews, D. Lazzari, D. Simon, R.E. King, M. Stamp, R. Reinicker, M. Tinkl, N. Berthelon, D. Muller and U. Hirt, inventors; Ciba Speciality Chemicals Corporation, assignee; US7022390, 2006. 30. Y. Ohkatsu, T. Kajiyama and Y. Arai, Polymer Degradation and Stability, 2001, 72, 2, 303. 31. K. Schwetlick and W.D. Habicher, Polymer Degradation and Stability, 2002, 78, 1, 35. 32. D. Ranson, inventor; ICI, assignee; US3186962, 1965. 33. S. Kawase, H. Inata and T. Shima, inventors; Teijin Ltd., assignee; US3904578, 1975. 34. S. Kawase, H. Inata and T. Shima, inventors; Teijin Ltd., assignee; US3989664, 1976. 35. F.E. Carevic and A. Labriola, inventors; FMC Corporation, assignee; US4011196, 1977. 36. A.W. White and R.S. Beavers, inventors; Eastman Kodak, assignee; US4910286, 1990. 37. D.R. Kelsey, inventor; Shell Oil Co., assignee; US6242558, 2001. 38. K. Tano, U. Marschall and H. Kliesch, inventors; Mitsubishi Polyester Film GmbH, assignee; US6777099, 2004. 39. A.L. Cholli, A. Dhawan and V. Kumar, inventors; University of Massachusetts Lowell, assignee; US7323511, 2008. 40. H. Gysling and H. Peterli, inventors; JR Geigy AG, assignee; US3376258, 1968. 41. C.E. Gleim and R.B. Spacht, inventors; The Goodyear Tire & Rubber Co., assignee; US3386952, 1968.

194

Thermo-oxidative Stabilisation 42. M. Minagawa, Y. Nakahara and M. Takahashi, inventors; Argus Chemical Corporation, assignee; US4145333, 1979. 43. H. Buysh, R. Binsack and D. Rempel, inventors; Bayer AG, assignee; EP48878, 1982. 44. M. Lu and M. Golder, inventors; Ticona LLC, assignee; US6696510, 2004. 45. R. Tsukamoto, N. Hashimoto and T. Hoshi, inventors; Solotex Corporation, assignee; US2006020103, 2006. 46. D. Simon, D. Lazzari, S.M. Andrews and H. Herbst, inventors; Ciba Speciality Chemicals Corporation, assignee; US7205379, 2007. 47. N. Berthelon and D. Muller, inventors; Ciba Speciality Chemicals Holding AG, assignee; EP1882009, 2008. 48. G. Kletacka and P.D. Smith, inventors; BF Goodrich Co., assignee; US3678047, 1972. 49. G. Kletacka and P.D. Smith, inventors; BF Goodrich Co., assignee; US3862130, 1975. 50. M.H. Keck, R.E. Gloth and J.J. Tazuma, inventors; The Goodyear Tire & Rubber Co., assignee; US4330462, 1982. 51. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4560799, 1985. 52. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4611023, 1986. 53. J.D. Spivak and S.D. Pastor, inventors; Ciba-Geigy Corporation, assignee; US4612341, 1986. 54. P.C. Georgoudis, inventor; National Starch, assignee; US3985705, 1976.

195

Degradation and Stabilisation of Aromatic Polyesters 55. D. Freitag, D. Rathmann, U. Hucks, P. Tacke and L. Bottenbruch, inventors; Bayer AG, assignee; EP150497, 1985. 56. J.M. Verheijen and A.M. Marien, inventors; Agfa-Gevaert NV, assignee; EP501545, 1992. 57. J.M. Verheijen and A.M. Marien, inventors; Agfa-Gevaert NV, assignee; US5185426, 1993. 58. A. Schmitter, inventor; Ciba Speciality Chemicals, assignee; US5763512, 1998. 59. X. Huang and L. Dominguez, inventors; Hoechst-Celanese, assignee; US5874515, 1999. 60. X. Huang and L. Dominguez, inventors; Hoechst-Celanese, assignee; US5874517, 1999. 61. D.R. Kelsey, inventor; Shell Oil Co., assignee; US6093786, 2000. 62. M. Bienmuller, K. Idel and P. Friedemann, inventors; Bayer AG, assignee; US2002137823, 2002. 63. C.S. Valentine, H. Zhang and S. Patel, inventors; Voith Paper Patent GmbH, assignee; US7138449, 2006. 64. P.C. Georgoudis, inventor, National Starch, assignee; US3987004, 1976. 65. K. Schlichting, P. Horn and W. Seydl, inventors, BASF AG, assignee; GB1496396, 1977. 66. R.E. Gloth, J.J. Tazuma and M.H. Keck, inventors; The Goodyear Tire & Rubber Co., assignee; US4414348, 1983. 67. C.E. Tholstrup and J.W. Thompson, inventors; Eastman Kodak Co., assignee; US3494886, 1970.

196

Thermo-oxidative Stabilisation 68. J.M. Bohen and J.L. Reilly, inventors; Atochem North America Inc., assignee; US5081169, 1992. 69. V. Ulrich, inventor; Rhein Chemie Rheinau GmbH, assignee; US5130360, 1992.

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Degradation and Stabilisation of Aromatic Polyesters

198

8

Stabilisation Against Ultraviolet and Ionising Radiation

8.1 Introduction to Ultraviolet (UV) Stabilisation Whereas the use of thermo-oxidative stabilisers can be regarded as largely optional for aromatic polyesters unless particularly harsh conditions are likely to be met in service, the light stabilisation of these polymers is more of a requirement in any situation where an article made thereof is going to be exposed to short-wavelength light, or a combination of this with oxygen. Additives formulated with polymers to improve their photostability must carry out one or more of the following functions effectively to protect their host polymer [1, 2]: a) Screening of detrimental wavelengths of light. b) Quenching of electronically excited states. c) Non-radical decomposition of hydroperoxides. d) Complexing of trace metal ions. e) Interception of photo-oxidation products. f) Radical scavenging. g) Transformation into a species capable of fulfilling one or more of the above roles. From what is known of the initial photophysical and photochemical processes involved in polyester photodegradation, it would appear

199

Degradation and Stabilisation of Aromatic Polyesters that option ‘a’ is the most favoured stabilisation route. Option ‘g’, where some form of transformation results in a stabiliser of type ‘a’ is also an option, provided such a transformation is rapid and does not result in the appearance of concomitant pro-degredant species. This is not to say that the other options available for photostabilisation are not worth considering. Most of the other options set out above would be helpful to a greater or lesser extent. Excited state quenching could be useful, especially because it is generally considered that the Norrish Type II chain scission via the excited cyclised transition state occurs via a relatively long-lived triplet state. Peroxide decomposers are helpful in slowing down the autoxidation cycle, whereas catalyst residues or metallic species from other sources (machinery, other additives) are pro-degredant species which it would be useful to suppress. Process stabilisers, such as phosphite (see Chapter 6) can assist with peroxide decomposition and metal ion complexation. Interception of photo-oxidation products and radical scavenging could be achieved by antioxidants (see Chapter 7), although the additives required may not be exactly the same ones used in thermooxidative stabilisation. As was noted in Chapter 4, polyesters such as polyethylene terephthalate (PET) degrade mainly in a thin surface layer under the influence of light and oxygen. This suggests that photostabilisation in the bulk of an article will not be optimally used; at best, portions of the additive distributed in the interior will act as a reservoir to replace stabiliser lost in the outer regions during any of the chemical or physical processes under way.

8.2 UV Screeners and Absorbers 8.2.1 Background A simple approach to light stabilisation of a substrate is to incorporate therein some substance which will reflect or absorb incident light. It would therefore be thought that incorporation of pigments or dyes 200

Stabilisation Against Ultraviolet and Ionising Radiation [3] would be helpful. The prime example of this approach is carbon black, which is well known as an extremely effective UV absorber. Carbon black appears to operate in several ways besides simple screening. Other pigments, ranging from coloured to white, also exhibit some UV screening capability. Specific to aromatic polyesters, it is claimed that titanium dioxide is useable as a light stabiliser [4]. A study by Kashkhozheva and co-workers [5] indicated that Fe2O3, Al2O3, MgO and CaCO3 exhibited some photostabilising effect in poly(butylene terephthalate) (PBT), whereas Guedri and co-workers [6] noted the applicability of ZnO as a screener for poly(ethylene naphthalate) (PEN). Some authors have deduced that dyes, especially aroylbenzimidazoles, provided they are suitably thermally stable [7, 8], and fluorescence brighteners such as Leucopor‘ EGM [9] can photostabilise PET. Naphthol‘ AS dye intermediates have been patented as UV screeners applicable in aromatic polyesters [10]. A number of classes of additives specifically for use as UV absorbers in polymer formulations are available including, among the better known ones, aromatic salicylates [11], o-hydroxybenzophenones [12], 2-hydroxyphenylbenzotriazoles [12, 13], derivatives of cinnamic acid and related materials [14, 15], aromatic oxanilides [16], cyclic imino ester derivatives known variously as benzoxazines or benzoxazones [17, 18] and hydroxyphenyltriazines [19]. Phenyl salicylates are one of the oldest UV stabilisers known. They are not particularly good stabilisers, but they rearrange on exposure to UV radiation to form 2-hydroxybenzophenones, which may be regarded as the active species. The problem with these materials is that the conversion is not rapid or, in some cases, ‘clean’, which can lead to poor performance and some discoloration of the host polymer. These additives have been largely superseded by later developments. o-Hydroxybenzophenones carry out their stabilisation function by absorbing UV energy and dissipating this safely by means of a tautomeric equilibrium involving formation of a six-membered ring

201

Degradation and Stabilisation of Aromatic Polyesters excited state, often referred to as ‘keto-enol tautomerism’, which involves the hydroxyl proton transferring to the carbonyl group, then back again. The result of this mechanism of light absorption and energy dissipation is that it leaves the stabiliser chemically unchanged, and thus capable of undergoing this action many times over. 2-Hydroxyphenylbenzotriazoles and the more recent hydroxyphenyl triazines would appear to operate via largely similar mechanisms. Cinnamic acid derivatives are said to operate via a mechanism in which the excited state dissipates energy by rotating about the double bond, which is weakened towards such a rotation in the excited state. There are also many vibrational energy-dissipation possibilities with cinnamates which can allow safe loss of absorbed energy. Once again, the molecule should be capable of undergoing several of these transformations. Aromatic oxanilides may also dissipate absorbed energy by proton transfer, but this is less clear with these materials. There may also be a contribution from a simple screening effect because these additives have very high extinction coefficients in the region of 280–340 nm Cyclic imino esters were originally developed as PET chain extenders (see Section 6.4.2) but the UV-stabilising ability of some members of the family was noted. The mechanism of energy dissipation is not totally clear, but may involve ring opening and closing cycles in the excited state to achieve a similar effect to the excited state tautomerism of other UV absorbers.

8.2.2 Salicyclates Salicylate-type additives are largely unsuitable for use in aromatic polyesters, but some UV-stabilising ability has been shown by polyesters which contain moieties capable of transformation in the same way as the salicylates: the so-called Photo-Fries rearrangement [20–22]. More recently, re-arrangeable polymers such as poly(phenyl acrylate) and poly(p-methylphenyl acrylate) have been proposed 202

Stabilisation Against Ultraviolet and Ionising Radiation as UV stabilisers in PET [23, 24], but appear to require quite high loadings (5–10 wt%) to be effective. Similar rearrangements have been posited for Ardel‘, a polyarylate made by copolymerising isophthalic acid, terephthalic acid and bisphenol A, which may be used as an additive or coating for other aromatic polyesters [25]

8.2.3 Benzophenones Benzophenone-type UV absorbers have been suggested as viable stabilisers for PET and related polyesters [26–45]. Various additives were claimed for incorporation into a melt processing stage of an aromatic polyester. These include: dihydroxybenzophenones such as 2,2´-dihydroxy-4,4´dimethoxybenzophenone [26]; o-hydroxybibenzoylmethane, especially in poly(cyclohexanedimethylene terephthalate) [27]; bis(2hydroxybenzyl)alkanes, e.g., 1,8-bis(2-hydroxy-5-methylbenzyl) n-octane [28]; a polyester, strictly for use as an additive for other polyesters, consisting of the reaction product of 2,2´,4,4´tetrahydroxybenzophenone and sebacoyl chloride [29]; 2,2´,4,4´tetrahydroxybenzophenone itself [34]; a bis-benzophenone with a bridging group consisting of a -OCH 2-Ph-CH2O- unit [35]; terephthalamide-bis(benzophenone)s [38]; and, most recently, bisbenzophenones with a polyoxyalkylene bridge connecting the two active moieties [45]. It has also been suggested that benzophenones such as 2,2´-dihydroxy4,4´-dimethoxybenzophenone may be incorporated into dyed or non-dyed fibres through the use of a polyhydric alcohol solution of the stabiliser [33]. Later studies [44] noted that this type of UV absorber diffused throughout a fibre cross-section, which was not the case for the other classes investigated. Suggestions have also been made that better overall performance can be achieved by the use of more than one benzophenone [37], or combinations with other classes of stabiliser such as benzotriazoles or cinnamates [36], or in combination with antioxidants [41, 42]. 203

Degradation and Stabilisation of Aromatic Polyesters Due to the reactivity of polyesters, it is claimed that the surface of a fibre or film can be rendered UV-resistant by first treating it with terephthaloyl chloride, then with a benzophenone with available reactive hydroxyl groups [30]. An alternative approach is to incorporate a benzophenone-type stabiliser into the polymer backbone, usually at low levels (0.1–5 wt%). Hoechst AG [32] claimed to achieve this by creating a stabilising moiety in the backbone by reacting ethylene glycol, terephthalic acid and 4-hydroxy-6-t-butylisophthalic acid, but other workers preferred to use a benzophenone stabiliser with a substituent containing two hydroxyl groups, e.g., 2-hydroxy-4(2,3dihydroxypropoxy)benzophenone, to react with other ingredients in the reaction kettle [39, 40, 43], or 2-hydroxy-4(2,3-epoxypropoxy) benzophenone [31].

8.2.4 Benzotriazoles. 2-Hydroxyphenylbenzotriazole-type UV absorbers in aromatic polyesters feature in a number of studies [44, 46–52], and are claimed in many patents [36, 41, 42, 53–65]. General Electric [53] claimed the use of 2(2´-hydroxy-3´,5´-di-tamylphenyl)benzotriazole (Tinuvin‘ 328; Ciba) for UV stabilisation of fire-retarded PET formulations. Adeka Argus [54] patented alkylidene-bis(benzotriazolylphenols) such as 2,2´-methylene-bis(4methyl-6-benzotriazolylphenol) for use in aromatic polyesters, later commercialised as ADK Stab‘ LA31. Heat-shrinkable polyester films [56] have been stabilised with a variety of benzotriazoles, including 2(2´-hydroxy-5´-methylphenyl)benzotriazole (Tinuvin‘ P; Ciba), 2(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol (Tinuvin‘ 234; Ciba) and 2(2´-hydroxy-5´-(1,1,3,3-tetramethylbutyl) phenyl)benzotriazole (Cyasorb‘ UV5411; Cytec). Rhodia Filtec [57] used Tinuvin‘ 234 in pigmented PET formulations, mixing the stabiliser and pigments together before addition to the polymer. Coltro and co-workers [49] studied the leachability of 2-(2´-hydroxy-

204

Stabilisation Against Ultraviolet and Ionising Radiation 3´-t-butyl-5´-methylphenyl)-5-chlorobenzotriazole (Tinuvin ‘ 326; Ciba) from PET bottles, although the additive was mainly present to protect the bottle contents form incident light. Ciba [59] patented benzotriazoles with polyoxyalkylene substituents and bisbenzotriazoles with bridging components of the same type. Begley and co-workers [50] studied the migration of Tinuvin‘ 234 into foodstuffs from PET. Great Lakes [51] produce 2(2-hydroxy-3,5di(1,1-dimethylbenzyl)-2H-benzotriazole (Lowilite‘ 234) and 2,2´methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,3,3-tetramethylbutyl) phenol) (Lowilite‘ 36), both of which are claimed to feature low volatility from the polyester substrate. Andrews [52] studied the use of Tinuvin‘ 234 and octyl-3-(5-chloro2H-benzotriazol-2-yl)-5-(1,1-dimethyl)-4-hydroxybenzene propionate (Tinuvin‘ 109; Ciba) in PET formulations to determine the best point in the process for addition of the stabilisers. It was found that Tinuvin‘ 234 was rapidly lost if added at the polycondensation stage, but that Tinuvin‘ 109, with its potentially reactive substituent, was retained when added at this stage. Both additives gave good stabilising performance if added at the melt processing stage. DuPont [63] claimed a process for stabilisation of dyed poly(trimethylene terephthalate) fibres by adding a speciality benzotriazole in the dye bath (Cibafast‘ USM; Ciba). The structure of this additive was not revealed, but it most probably features extra substituents in the form of sulfonic acid(s) or derivatives thereof, which would allow the additive to be dispersed or dissolved in the aqueous dye bath. Milliken [64] recently patented benzotriazoles with longchain substituents of the type -CH2CH2(C=O)NR-R-O[(C=O) CH2CH2CH2CH2O]nH. These appear to form the basis of the Clearshield‘ product, used as a polyester coating for the protection of a variety of substrates. Benzotriazoles are also available in the form of masterbatches in poly(ethylene terephthalate-co-isophthalate) [48] for extrusion of multilayer films with considerably increased useful outdoor life. 205

Degradation and Stabilisation of Aromatic Polyesters Additive packages containing benzotriazoles along with other co-additives are also known. These include combinations with benzophenones and/or cinnamate-types [36]; with antioxidants [41, 42]; with fatty acid salts of manganese [55]; combinations of polyoxyalkyene-based benzotriazoles and PEN used in PET [61]; and benzotriazole plus poly(isobornyl acrylate) for the protection of polyalkylene naphthalates [62]. Ciba Speciality Chemicals have patented molecules which contain benzotriazole and hindered amine functional groups [60], e.g., 1(2hydroxy-2-methylpropoxy-2,2,6,6-tetramethylpiperidin-4-yl)-3-(5chlorobenzotriazol-2-yl)-5-tbutyl-4-hydroxycinnamate, and have also claimed their efficiency in PET, particularly in the form of fibres. As with benzophenones, attempts have been made to incorporate benzotriazole stabilisers into the polyester chain. Additives investigated include dihydroxy-2-(2-hydroxyphenyl)-2H-benzotriazoles [46], tris(hydroxyphenyl)ethane benzotriazole [47] and diol-functionalised species of the type benzotriazole - CH2N(ROH)2 [58]. A study of the diffusion of benzotriazoles into PET fibre [44] demonstrated that they penetrate only into the outer regions, unlike the benzophenones which diffuse throughout the substrate.

8.2.5 Cinnamates and Related Types Cinnamate-type UV absorbers have been patented by Bayer AG [66, 67] and Eastman Kodak/Eastman Chemical [68–81] for incorporation into polyalkylene terephthalates, mainly as reactive species incorporated into the polymer backbone. More recently low molecular weight additives of this type, which may or may not react into the polymer, have been commercialised by Clariant [82] and BASF AG [83–86]. Intellectual property assigned to Bayer covers benzylidene malonates and benzylidene bis-malonates [66], and includes the use of this type of stabiliser in the manufacture of polyesters [67]. It is likely

206

Stabilisation Against Ultraviolet and Ionising Radiation that the Clariant product, [(4-methoxyphenyl)methylene]dimethyl ester of propendioic acid (Sanduvor‘ PR-25) is based on the same chemistry. The development of this type of chemistry built into polyesters was part of an extensive effort undertaken by Eastman over a number of years to develop various grades of self-coloured, UV-brightened and UV-stabilised polyester fibres. Initial polyesters utilised reactive versions of the simplest cinnamate chemistry, such as: ~OCH2CH2O-PhCH=C(CN)-CO2~

[68]

~O2C-R-C(=C(Ph)(CN))-CO2~

[69]

and

while later versions used a variety of different structures to achieve improved reactivity and UV stabilising function: ~NH-Ph-CH=C(CN)~

[72]

~Heterocycle-CH=C(CN)~

[73]

~PhCH2OPh-CH=C(CN)~

[75]

~S-Ph-CH=C(CN)~

[76]

~C2H4O-Ph-CH=C(CN)~

[77]

~Ph-CH=C(CN)-R-Ph-CH=C(CN)~

[78, 80]

Also noted were PET copolymerised with these structures and with naphthalene dicarboxylic acid [79], and PEN itself with built-in cinnamate-type moieties [81]. Sanduvor‘ PR-25 has been assessed as a UV stabiliser in a variety of polymers, including PET [82], where it is said to be photo-graftable to

207

Degradation and Stabilisation of Aromatic Polyesters the substrate, producing a non-staining, highly absorptive, stabilising system. Comparison with benzophenones and benzotriazoles showed this additive to have much lower colour development and considerably higher UV absorption. It was also shown to outperform the newer phenyltriazines in both respects. BASF products based on cinnamate-type chemistry, classed by the manufacturers as cyanoacrylates, include: Ethyl-2-cyano-3,3-diphenylacrylate

(Uvinul‘ 3035)

2-Ethylhexyl-2-cyano-3,3-diphenylacrylate

(Uvinul‘ 3039)

3-Bis((2-cyano-3,3´-diphenylacryloyl)oxy)-2,2-bis((2-cyano3´,3-diphenylacryloyl)oxyl)methylpropane (Uvinul‘ 3030) Comparison of the absorption spectra of these products with those of benzophenones and benzotriazoles shows a marked difference [85]. The diphenylcyanoacrylates have a maximum absorption at 303 nm, whereas benzophenones have two peaks, at 280 nm and 340 nm, and benzotriazoles at 300 nm and 350 nm. The last two types also exhibit an absorption tail up to 400 nm, giving a yellowish tinge in some substrates. The diphenylcyanoacrylates are also said to be insensitive to metal ions, and to be equally effective under different pH conditions. Uvinul‘ 3030, with its tetrameric functionality and low volatility is likely to be useful in thin polyester structures, such as films and fibres. This additive has also gained FDA approval for food contact use [86]. Allied Signal Incorporated [87] have patented cinnamamides of the general structure Ph[NH-(C=O)-C(CN)=CPh2]2, and have observed that these are especially effective stabilisers for aromatic polyesters. Clariant have noted that a combination of benzylidene bis-malonates with hindered amine light stabilisers showed excellent synergism in UV-stabilising polyalkylene terephthalates [88].

208

Stabilisation Against Ultraviolet and Ionising Radiation

8.2.6 Oxanilides Oxanilide UV stabilisers of the general form ArNH(C=O)(C=O) NHAr have been known for some time in symmetric [89] and asymmetric [90] forms. Both are available commercially from Clariant as Sanduvor‘ EPU and VSU respectively. Also patented were bis-oxalic acid diamides [91] of basic structure ArNH(C=O)(C=O) NH-R-NH(C=O)(C=O)NHAr. While no studies have been found on the efficacy of these additives in aromatic polyesters, their high thermal stability and the absorption peak at 290 nm with a tail only going as far as 350 nm means that they could be useful stabilisers for these polymers. Attempts have also been made to produce dual-functional stabilisers incorporating oxanilide and other functional groups. This was achieved by attaching a phenylbenzotriazole unit to one or both of the phenyl rings of the oxanilide [92], or replacing one of these phenyl rings with a phenylbenzotriazole [93] or a hindered piperidine group [94]. Later patents to General Electric Company [95, 96] claimed oxanilides which could be used to form polymeric stabilisers or could be added to the polymerisation stage of polycarbonates or polyesters to provide built-in stabilisers. An example of this type of structure is HOPh-R-PhNH(C=O)(C=O)NHPh-R-PhOH.

8.2.7 Cyclic Imino Esters Cyclic imino esters were originally developed, at least in terms of plastics additives, as chain extenders for polyesters (see Section 6.4.2). The same company involved in this research, Teijin, discovered that some of these compounds, most notably 2,2´-p-phenylene-bis(3,1benzoxazin-4-one), were effective UV absorbers [97]. It was also noted by the inventors that if the additive reacted with chain ends in a polyester or polyamide, this could result in a diminution of the UV protective ability of the additive. Careful compounding, with as low

209

Degradation and Stabilisation of Aromatic Polyesters a temperature as was practical and with short melt-processing times, was recommended. 2,2´-p-Phenylene-bis(3,1-benzoxazin-4-one) is commercially available from Cytec as Cyasorb‘ 3638. Stabilisers of this type have been claimed as extremely effective UV absorbers for poly(ethylene-co-1,4-cyclohexanedimethylene terephthalate) [98–100], allowing the use of this material in outdoor applications. Developments in novel synthesis methods have improved the inherent stability of this class of additive, paving the way towards their more effective use in aromatic polyesters [101–106] Other additives claimed, besides the aryl-bisbenzoxazinones, include: oligomeric benzoxazin-4-ones end-capped with a phenyl group containing reactive substituents to allow the molecule to be copolymerised into polyesters, especially wholly aromatic polyesters [107]; water-dispersible versions for improvement of light-fastness of textiles, including those based on polyesters [108]; benzoxazinones with bulky side groups [109]; and isobenzoxazinones such as 2,2´-mphenylene-bis(3,1-benzoxazin-4-one) also having an -OR substituent on the 4-position of the bridging phenyl ring, where R is a long-chain alkyl group such as C18H37 [110].

8.2.8 Triazines Triazine-type UV absorbers have been patented in various forms by a number of companies, including Ciba [111–121], Cytec [122–125], Agfa Gevaert [126] and Asahi Denka [127]. Various stabilisers of this type are also commercially available, for example: 2(2-hydroxy-4-hexyloxyphenyl)-4,6-diphenyltriazine (Tinuvin‘ 1577; Ciba) 2(hydroxy-4-octyloxyphenyl)-4,6-bis(2,4-dimethylphenyl) triazine (Cyasorb‘ UV 1164; Cytec)

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Stabilisation Against Ultraviolet and Ionising Radiation Versions with diacid, diol and di-ester substituents have been patented [111] which are suitable for reacting into aromatic polyesters, as have stabilisers with suitable substituents to allow dispersion or dissolution in dye baths for impregnation into fibres or textiles [113, 114, 116]. Other related species include those with biphenylene groups on the 4- and 6-positions on the triazine core [119], naphthalene groups on 4 or on both 4 and 6 [120], 1,2,3,4-tetrahydronaphthalene on 4 and 6 [124], and 2-hydroxynaphthalene on the 2-position [125]. Versions with longer side chains than hexyl or octyl have also been synthesised, and it is claimed that these have increased compatibility with their host polymer [121, 127]. UV1164 has been shown to be particularly effective at protecting PEN [126]. It has also been observed that triazine-type UV absorbers form a particularly good synergistic combination with hindered amine light stabilisers in photostabilisation of a range of polymers [122]. Various studies of the effectiveness of these stabilisers in PET have been carried out [128–131]. Wang and co-workers [128], in an investigation into the kinetics of photo-oxidation of PET, noted that a 2,4,6-triphenyl-1,3,5-triazine derivative improved the lifetime of samples by a factor of 4.5 under the irradiation conditions used. Fechine and co-workers [129–131], in a study on the photo-oxidation of biaxially oriented PET films observed that Tinuvin‘ 1577 was a superior stabiliser to titanium dioxide or carbon black. It was also noted that this stabiliser was equally effective at protecting surface and inner regions of the sample films.

8.2.9 Miscellaneous As research has been carried out into UV absorbers suitable for use in aromatic polyesters, a number of species have been identified which do not easily fit into any of the categories covered above. Amongst such species are: 4-thiazolidone derivatives such as 5-benzal3-(p-B-hydroxyethylephenyl)-2-(p-B-hydroxyethylphenylamino)4-thiazolidone, a reactive UV absorber added to the polyester

211

Degradation and Stabilisation of Aromatic Polyesters reaction kettle [132]; heterocyclic esters such as 4(1,1,3,3tetramethylbutyl)phenyl-4-chloro-5-phenyloxazole-2-carboxylate, which were reasonable stabilisers but themselves highly coloured [133]; 3-benzalphthalides, which may have operated in a similar manner to the cinnamate-type stabilisers due to their PhCH=CR2 structure [134]; various imide types such as phthalimidobenzothiazoles [135] and aromatic diimides [136], although both were coloured species; and benzoxazolyl stilbenes, which again may operate as UV stabilisers via a similar mechanism to the cinnamates [137]. A more promising new additive type is based on 4-hydroxyquinoline3-carboxylic acid derivatives [138], which feature a very high extinction coefficient in the UV, but little or no self-colour. These are claimed to be highly efficient UV absorbers and thus particularly suitable for fibres and films. Similar additives may have been commercialised for use as UV barriers to protect the contents of plastics packaging. Very recently, BASF AG applied for patents on UV absorbers based on 4-cyano-naphthalene-dicarboximide derivatives, such as 4-cyano-N(2,6-diisopropylphenyl)naphthalene-1,8dicarboximide [139], and pyridindione derivatives such as 1,4dimethyl-5-dimethylaminomethylene-2,6-dioxo-3-cyano-1,2,5,6tetrahydropyridine [140]. It remains to be seen how the patenting process will proceed, and whether a commercial product will result.

8.3 Excited State Quenching While in the past the stabilisation of polymers via long-range or short-range energy transfer leading to quenching of excited states created in a polymer by photolysis was considered an important means of photostabilisation, this no longer appears to be the case [2, 141]. Many stabilisers thought to operate via this mechanism have been shown to owe their efficiency to other mechanisms, such as radical scavenging or UV absorption. This is not to say that this mechanism does not feature in the stabilisation of polymers to light. 212

Stabilisation Against Ultraviolet and Ionising Radiation Many stabiliser species operating mainly by other means do show some quenching activity. Researchers at Clemson University [142–145] have shown that inclusion of 0.5–4 mole% of naphthalene dicarboxylate or biphenylene dicarboxylate in a PET synthesis results in polymers with increased UV stability, which they demonstrated was at least partly due to energy transfer processes to the copolymerised units, which could then safely dissipate the energy. Milligan [146] later suggested that such copolymers could be useful in fibres for automotive upholstery applications.

8.4 Radical Scavengers 8.4.1 Background A common feature of degradative processes undergone by polymers exposed to UV radiation is homolytic cleavage of covalent bonds. Thus, regardless of the photoinitiating species or process, or of the precise nature of the subsequent degradation, the polymer thus affected will contain radicals such as alkyl, alkoxy, hydroperoxy and hydroxy, and one or more of these reactive species will be involved in the perpetuation of photodegradation. Photostabilisation of some polymers might therefore be expected to occur via reduction of the numbers or activity of these radical species. Chain breaking – donor antioxidants such as hindered phenols are ineffective under conditions of photo-oxidation due to their rapid consumption in the higher rates of initiation present compared to thermal oxidation, and to photodegredant activity of their derived molecules such as quinones and quinone methides. Although they have limited photo-antioxidant ability when used alone, they can be protected from photolytic destruction by, for example, UV absorbers, and for this reason they can produce good synergistic results with a variety of co-stabilisers.

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Degradation and Stabilisation of Aromatic Polyesters Of available light stabilisers potentially operating via an antioxidant mechanism, the most interesting are the hindered amine light stabilisers (HALS). Initially developed in Japan, these are largely based on active sites comprising 2,2,6,6-tetraalkylpiperidine [147] and 1,2,2,6,6-pentaalkylpiperidine [148]. Many studies have been made to identify the mechanism(s) by which HALS provide such effective UV stabilisation, and these have been reviewed [149–151]. Most authors agree that the >NH moiety is not, in fact, the stabilising species, but that >NO. stable radicals and >NOR hydroxylamine ethers formed during photo-oxidation are the true antioxidant species. These are constantly regenerated; the nitroxyl scavenging alkyl radicals and the hydroxylamine ethers intercepting hydroperoxy radicals and hydroperoxides. Hydroxylamines may also be formed, which are efficient photostabilisers in their own right. Other factors contributing to the stabilising ability of HALS have been postulated. Carbonyl excited state quenching has been put forward as a possible factor [152], although there is also evidence contradicting such a mechanism [153]. It has also been shown that HALS, with their high basicity, are extremely powerful metal ion-chelating agents [153, 154], which could contribute to the stabilisation of a host polymer.

8.4.2 HALS No systematic study of the use of HALS as UV stabilisers in aromatic polyesters has been found in the literature; very few articles even touch on the subject, and these are limited to PBT [157–159]. Without comparisons with other stabiliser types, it is difficult to assess the true capability of the HALS tested by these authors in PBT. It was noted by Borukaev and co-workers [159] that poly[[6[(1,1,3,3tetramethylbutyl)amino]-s-triazine-2,4-diyl]-[(2,2,6,6-tetramethyl4-piperidyl)imino]-1,6-hexamethylene[(2,2,6,6-tetramethyl-4piperidyl)imino]] (Chimmasorb‘ 944; Ciba) was a better UV stabiliser than the dimethylsuccinate polymer with 4-hydroxy-2,2,6,6tetramethyl-1-piperidineethanol (Tinuvin‘ 622; Ciba).

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Stabilisation Against Ultraviolet and Ionising Radiation A potential problem with HALS in aromatic polyesters is the very high basicity of these amines, which may result in reaction with the ester linkages and loss of molecular weight. There are indications that this may be a factor with the use of some HALS in fibre-grade polyesters [160]. Taking the point of view of HALS, there is also the possibility that the additive might react with acid end groups, which may have an adverse effect on additive efficiency. It is clear from a consideration of the antioxidant mechanism of HALS that formation of a full-blown ionic salt of the additive would severely compromise its ability to form the active species involved in stabilising the host polymer. A study of the effects of acid exposure on HALS [161] demonstrated that the stabilising efficiency was destroyed only if additives were exposed to very strong acids such as hydrogen halides or nitric acid. The stabilising function was not significantly lowered in the cases of either CO2/water or formic acid. The patent literature on HALS is voluminous, and some patents include a reference to the potential use of their claimed species in polyesters, although it is clear from reading the embodiment that other polymers are the true targets. Many fewer patents deal with HALS which are specifically claimed as being effective stabilisers for polyesters. A selection of patents in the first category include: piperidine spirooxirane derivatives [162]; low molecular weight polyesters with in-backbone or pendant piperidine substituents [163]; oligomeric esteramides with pendant hindered amine groups [164]; polyoxamate additives prepared by reacting, for example, N,N´-bis(ethoxyoxoacetyl)-N,N´-bis(2,2,6,6-tetramethyl-4piperidyl)-1,6-hexanediamine with 1,4-butanediol [165]; 6(1-hydro2,2,6,6-tetraalkylpiperine-4-oxy)dibenzodioxaphosphepins and dioxaphosphocins [166]; hindered amine-substituted dihydropyridines, such as 2,6-dimethyl-3,5-bis[(1,2,2,6,6-pentamethyl-4-piperidyl) oxycarbonyl)-1,4-dihydropyridine [167]; and polymeric species based on reactions between reactive HALS and diols, diamines or

215

Degradation and Stabilisation of Aromatic Polyesters aminoalcohols [168]. Also noted were additives containing HALS and hydroxylamine functionalities [169], such as N-benzyl-N(2,2,6,6tetramethylpiperidin-4-yl)hydroxylamine. Additives of this dualfunctional type have been claimed to be process and UV stabilisers in a range of polymers. A number of patents have been applied for which utilise existing HALS, specifically in aromatic polyesters, under specified circumstances. Chamassorb‘ 944 may be used in polyester fibres provided the melt temperature is kept as low as possible and residence time is minimised [170]. Polyester yarns have been stabilised through the use of a combination of a benzotriazole and the specific HALS 1-[2[3-(3,5-di-t-butyl-4-hydroxyphenyl)propinyloxy]ethyl]-4-[3-(3,5-dit-butyl-4-hydroxyphenyl)propinyloxy]-2,2,6,6-tetramethylpiperidine [171]. Polyesters of high molecular weight can be produced using a combination of a bisoxazoline chain extender and bis(2,2,6,6tetramethyl-4-piperidinyl)sebacate (Tinuvin‘ 770; Ciba) [172]. It has been claimed that aromatic-aliphatic polyester fibres benefit from the addition of HALS [173]. In antimony-catalysed polycondensation of PET, addition of Tinuvin‘ 622 to the reaction kettle is claimed not only to produce a polymer capable of being spun into UV stable fibres, but also of superior spinnability [174]. PET with improved weather resistance may be achieved by an additive package of 2(5methyl-2-hydroxyphenyl)benzotriazole and a HALS of structure HPCH2CH2(C=O)OCH2CH2PipO(C=O)CH2CH2HP [175], where HP is a hindered phenol and Pip is a tetraalkylpiperidine group. Pigmented polyester fibres may be stabilised with mixtures of HALS and UV absorbers [176], while injection-mouldable compositions benefit from a triple package of UV absorber, antioxidant and HALS [177]. 4-benzoyloxy-2,2,6,6-tetramethylpiperidine has been proposed as a useful UV stabiliser for polyesters [178]. PBT with superior UV stability can be made by reacting appropriate end groups with HALS substituted with -OH, -CO2H or -CO2R groups [179]. A particular stabiliser, bis(2,2,6,6-tetramethylpiperidyl)isophthalamide (Nylostab‘ S-EED; Clariant), while mainly used in polyamides, is also claimed to provide UV stabilisation to polyesters, along with improved processing, and better initial and weathered physical properties

216

Stabilisation Against Ultraviolet and Ionising Radiation [180]. Single or multilayer polyester films are advantageously stabilised with a combination of a HALS, preferably the reaction product of butanedioic acid with 4-hydroxy-2,2,6,6-tetramethyl-1piperidineethanol, and a triazine UV absorber such as Tinuvin‘ 1577 [181]. Cationic dyeable (i.e., sulfonated) PET may be UV stabilised with HALS, particularly tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate (ADKStab‘ LA57; Adeka Argus) [182]. HALS may also be combined with amide-containing aldehyde scavengers such as anthranilamide or 1,6-bis(2-aminobenzamidoyl) hexane in aromatic polyesters [183]. 2,2,6,6-Tetraalkyl- and 1,2,2,6,6-pentaalkyl piperidine-based HALS which have been claimed specifically as stabilisers for aromatic polyesters include: alkylamine derivatives such as pentakis(2,2,6,6tetramethyl-4-piperidinyl)diethylenetriamine-N,N,N´, N´´, N´´pentaacetate [184]; unsymmetrical siloxanes with HALS moieties at one end and a reactive group on the other, e.g., -Si(CH2)3OH group capable of reacting with acid chain ends in the polymer [185]; and HALS diepoxides, which chain-extend and UV-stabilise polyesters and polyamides [186]. Combinations of ‘standard’ HALS types with other stabilisers suitable for use in aromatic polyesters include packages containing hindered phenol, sulfur-based antioxidant and HALS [187]; packages of triazine UV absorbers and tetraalkylpiperidines in engineering polyesters [122]; non-reactive siloxane-based HALS with hindered phenol and phosphite [188]; and bismalonate UV absorbers with HALS, which exhibit very good synergism in stabilising engineering polyesters [88]. There have also been patented stabilisers which contain two functionalities within one molecule, which functionalities are a HALS and a UV absorber. These include oxanilides with one phenyl group replaced with 2,2,6,6-tetramethylpiperidine [94], and additive molecules containing both HALS and a benzotriazole functionality [60, 65].

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Degradation and Stabilisation of Aromatic Polyesters With the potential problem of reaction between hindered amines and polyester, work has been ongoing on identifying means of getting the activity of a HALS into a polyester whilst minimising potential problems. It was found that hydroxylamine ethers could be synthesised which, while being much weaker bases than the parent amines, remain effective UV stabilisers. Ciba-Geigy Corporation [189] developed such materials in the early 1990s and they are now commercially available, e.g., di(1-octyloxy-2,2,6,6-tetramethylpiperidin-4-yl) sebacate (Tinuvin‘ 123; Ciba). Although the original patent does suggest the use of such additives in polyesters, the main target was poly(vinyl chloride), a matrix much more vulnerable to degradation by highly basic additives. Asahi Denka have also applied for patent protection on similar additives [190] based on carbonate structures such as (HE-O(C=O)O)nR, where HE is a hydroxylamine ether group, and n is 1 to 4. Articles describing these ‘NOR’ stabilisers have also been published, describing their use in non-polyester agricultural films [191] and in engineering plastics applications [192], including a commercial product Tinuvin‘ NOR 371 (Ciba). Another newer version of HALS is exemplified by hydroxy-substituted N-alkoxy hindered amines, i.e., molecules with at least one active moiety of the structure >N-OR-(OH)n. A very wide range of such additives has been covered in a family of patents assigned to Ciba Speciality Chemicals [193–198], and claims for applications include aromatic polyesters. These additives are said to have as good as or better UV stabilising ability and antioxidant properties as the earlier HALS. The hydroxyl group(s) is said to impart additional advantages not possible with the NOR types, e.g., antistatic attributes, and better pigment dispersion in polar polymers. A HALS-based additive system has been developed by Clariant based on ‘salt-like’ reaction products of HALS with phosphorus-containing organic acids [199] or carboxylic acids [200]. Despite their salt-like character, these additives are said to stabilise in the normal HALS manner, and additionally provide improved processing, mechanical properties and appearance. They are specifically aimed at polyesters and polyamides. An example of one of these compounds is the

218

Stabilisation Against Ultraviolet and Ionising Radiation reaction product of Nylostab‘ S-EED and diphenylphosphinic acid. The salt-like character also means that these additives may be handled as aqueous dispersions or solutions. Already there have been patents published on the use of such salts in polyesters, in combination with UV absorbers [201].

8.4.3 Other Radical Scavengers Few radical scavengers besides the HALS have been suggested as UV stabilisers for aromatic polyesters. Some time ago, it was noted that certain nitro compounds could be used as UV stabilisers for polyesters, but closer reading of the documents showed this to be limited to certain sulfonated copolyesters [202]. Benzofuran-2-ones have been suggested as UV stabilisers with a good chain breaking – acceptor action, i.e., capable of scavenging alkyl radicals [203, 204, 156], but these are more efficacious as thermal antioxidants and process stabilisers, especially when used in conjunction with hindered phenolic antioxidants.

8.5 Ionising Radiation Stabilisation All hydrocarbon polymers will suffer from severe degradative damage if exposed to high doses and/or long time periods of ionising radiation such as gamma rays. The problem of stabilising such substrates against this powerful radiation is that additives themselves will be liable to damage from radiation. Indeed, an early study into the effect of standard antioxidants on the gamma irradiation of Nylon 6 [205] showed them to have little or no effect on the degradation or oxidation processes. Hindered amines have been noted to have some stabilising effect on the gamma-ray degradation of polymers [206], particularly polypropylene [207], although it was observed that HALS were damaged by the radiation, with the resulting ringopening reactions rendering these additives inactive through time. Certain HALS with active sites based on N-(substituted)-1-(piperazin2-one) structures have been claimed as useful radiation stabilisers in

219

Degradation and Stabilisation of Aromatic Polyesters polypropylene [208]. More recently, stabilisation packages consisting of HALS, phosphite or phosphonate, and either hydroxylamine or nitrone [209] or benzofuranone [210], have been patented for use in polyolefins. These systems are used largely to get away from the use of hindered phenols, whose breakdown products under irradiation would be highly coloured. Jipa and co-workers [211] noted that hindered phenols such as Irganox‘ 1330 (Ciba) could be used in conjunction with pyrene to stabilise LLDPE against gamma-rays. Later studies by Balasa and co-workers [212] showed that other fused ring additives such as perylene and benzathracene could protect polyethylene. A polymer closer in structure to aromatic polyesters which has been the subject of much investigation into potential gamma-ray stabilisers is polycarbonate. Miles Incorporated and Bayer AG have claimed a wide variety of additives to be suitable for this task, including (all optionally alongside polyalkylene ether oligomers): aromatic disulfides [213], aromatic sulfonic acid esters [214], halogenated aromatic acid derivatives [215], disulfide-aliphatic carbonate copolymers [216], dialkyl or dicycloalkyl mono- or poly-sulfides [217], brominated phthalic anhydrides [218], oxirane-substituted phosphides [219], sulfonamides [220], aromatic, fused ring, sulfonamides [221], sulfur and nitrogen-containing heterocycles [222], dicyclohexyl phthalate [223], compounds of structure R-S(=O)2-(CHR)n-S-R [224] and 2-phenyl-1,3-dioxolane derivatives [225]. Exactly how these additives achieve their effect is unclear, and no specific recommendations exist within this intellectual property suggesting their use in aromatic polyesters. More generally applicable gamma-ray stabilisers have been claimed in the form of phosphorus-containing pentaerythritol derivatives, either alone [226] or in conjunction with a benzyl compound of the structure Ar-CHR-O-CHR-Ar [227]. Because the capture of secondary electrons produced by the strike of the gamma-ray photons and their safe deactivation is the most likely means of at least partially ameliorating the potential degradation,

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Stabilisation Against Ultraviolet and Ionising Radiation it is not surprising that highly aromatic species, with their ability to ‘soak-up’ electrons have been investigated. Another additive which might have the potential for capturing secondary electrons is a (semi) conductive polymer, and this approach has been the subject of some studies, including the use of polyaniline in styrene-butadiene rubber [228] and nanofibres of the same in poly(methyl methacrylate) [229], poly(p-sulfanilamide) in poly(methyl methacrylate) [230] and polypyrrole in low-density polyethylene [231]. The polyalkylene terephthalates and polyalkylene naphthalates are, to some extent, self-protecting due to the presence of the aromatic rings, with the latter being noticeably more stable towards gamma irradiation than the former. Recently, Klein and co-workers [232– 234] studied conjugated small molecules as protective species to prevent radiation damage (and the associated increased conductivity) in PET films. Such species, with electron-withdrawing substituents such as nitro or cyano, were reasonably effective in this role, with nitro-substituted fluorenones being the most useful.

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227

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234

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237

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238

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239

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240

A

ppendix – Commercial Additive Structures

The structures provided here are of commercial additives associated in the literature with use in aromatic polyesters, and referred to in the preceding text. However, it should be noted that neither the author or the publisher will accept any responsibility, actual or implied, for any loss, damage, injury or legal action resulting from the use of any of the additives in any formulation or process. Compounders are urged to contact their suppliers and discuss with them the applicability of any additive in a particular formulation or process, and to follow all the health and safety data provided by said suppliers.

ADKStab LA31

241

Degradation and Stabilisation of Aromatic Polyesters

ADKStab LA57

Alkanox 28

242

Appendix – Commercial Additive Structures

Chimmasorb 944

Cyasorb UV1164

243

Degradation and Stabilisation of Aromatic Polyesters

Cyasorb UV5411

Cyasorb 3638

Epon 828

Hostanox OSP-1

244

Appendix – Commercial Additive Structures

HP-136

Irgafos 12

Irgafos 168

245

Degradation and Stabilisation of Aromatic Polyesters

Irganox 245

Irganox 259

Irganox 1010

246

Appendix – Commercial Additive Structures

Irganox 1019

Irganox 1076

Irganox 1098

247

Degradation and Stabilisation of Aromatic Polyesters

Irganox 1222

Irganox 1330

248

Appendix – Commercial Additive Structures

Irganox 1425

Irganox 1790

249

Degradation and Stabilisation of Aromatic Polyesters

Irganox 3114

Irgastab FS-042

Joncryl ADR 4368

250

Appendix – Commercial Additive Structures

Leucopor EGM

Lowilite 36

Naugard 445

251

Degradation and Stabilisation of Aromatic Polyesters

Nylostab S-EED

Ronotec 201

Sandostab P-EPQ

252

Appendix – Commercial Additive Structures

Sanduvor PR-25

Sanduvor EPU

Sanduvor VSU

253

Degradation and Stabilisation of Aromatic Polyesters

Stabilizer 7000

Tinuvin P

Tinuvin 109

254

Appendix – Commercial Additive Structures

Tinuvin 123

Tinuvin 213

Tinuvin 234

255

Degradation and Stabilisation of Aromatic Polyesters

Tinuvin 320

Tinuvin 326

Tinuvin 328

256

Appendix – Commercial Additive Structures

Tinuvin NOR 371

Tinuvin 622

Tinuvin 770

257

Degradation and Stabilisation of Aromatic Polyesters

Tinuvin 1130

Tinuvin 1577

Ultranox 626

258

Appendix – Commercial Additive Structures

Uvinul 3030

Uvinul 3035

Uvinul 3039

259

Degradation and Stabilisation of Aromatic Polyesters

260

A

bbreviations

BHET

Bis(hydroxyethyl)terephthalate

BNZ

2,2´-Bis(4H-3,1-benzoxazin-4-one)

BOZ

2,2´-Bis(2-oxazoline)

bp

Boiling point

CB-A

Chain-breaking acceptor

CB-D

Chain-breaking donor

CEP

Concerted ester pyrolysis

cf

Compare

DEG

Diethylene glycol

DMT

Dimethyl terephthalate

DNaTA

Disodium terephthalate

DTA

Differential thermal analysis

EG

Ethylene glycol

FT-IR

Fourier-transform infrared spectroscopy

GC-MS

Gas chromatography-mass spectrometry

HALS

Hindered amine light stabilisers

HE

Hydroxylamine ether group,

HP

Hindered phenol

IR

Infrared

LCP

Liquid crystal polyesters(s)

MALDI-TOF

Matrix-assisted laser desorption ionisation-time of flight

OX

Oxidation

261

Degradation and Stabilisation of Aromatic Polyesters PAT

Poly(alkylene terephthalate)s

PBN

Poly(butylene naphthalate)

PBT

Poly(butylene terephthalate)

PCT

Poly(1,4-cyclohexylenedimethylene terephthalate)

PEN

Poly(ethylene naphthalate)

PET

Poly(ethylene terephthalate)

PIP

Tetraalkylpiperidine group

PP

Polypropylene

ppm

Parts per million

PTT

Poly(trimethylene terephthalate)

SSP

Solid-state post-condensation

TA

Terephthalic acid

TH

Transesterification/hydrolysis

THF

Tetrahydrofuran

TVA

Thermal volatilisation analysis

UV

Ultraviolet

262

I

ndex

A Alcoholysis 114 Aliphatic polyamides 1 Aminolysis 110 Anaerobic thermal degradation 25 Antioxidants 183 Aromatic oxanilides 202 Aromatic polyesters 2, 4, 9-10, 16, 21, 49, 80, 97, 110, 112, 114115, 118, 143, 144, 147, 150, 152, 154, 181-183, 186, 187, 188, 190, 202-203, 208-211, 214-215, 219-220 depolymerisation 115

B Benzophenone 203-204, 206, 208 Biodegradation 110 Bis(hydroxyethyl)terephthalate 113, 116, 118-119

C Carbon-arc light source 86 Chemical degradation 107 Cinnamates 203 Copolymerisation 16 Cyclic imino esters 202, 209 Cyclic oligomers 26, 33, 39, 40-41, 44, 49, 143

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Degradation and Stabilisation of Aromatic Polyesters

D Depolymerisation 81-82, 112, 120 Diethylene glycol 7 Differential thermal analysis 183 Dimerisation 32 Dimethyl terephthalate 112, 120, 122 Diphenyl cyanoacrylates 208 Disodiumterephthalate 113

E End-capping 153, 155 Ester pyrolysis, concerted 70 Ethanolysis 114 Ethylene glycol 112

F Fourier transform infrared spectroscopy 50, 51 Free-radical breakdown mechanism 40

G Gas chromatography 31 Gas chromatography-mass spectrometry 75, 80-81 Gel formation 73, 95, 98 Glycolysis 113-114, 116, 118, 121

H Hindered amine light stabilisers 214-218, 220 Hydantoin 151 Hydrogen transfer, B C-H 41 Hydrolysis 23, 36, 107, 113-114, 117, 119, 121, 144, 149, 161 depolymerisation 117 Hydrolytic degradation 107, 148 Hydrolytic stabilisation 143 p-Hydroxybenzoic acid 2 Hydroxylamines 188

264

Index

I Infrared spectroscopy 32 Injection moulding 28

M Matrix-assisted laser desorption ionisation-time of flight mass spectrometry 33, 41 Melt processing 143, 146, 157, 182 Methanolysis 116-118

N Norrish I reaction 87 Norrish II reaction 87

O Oxanilide ultraviolet stabilisers 209 Oxidation 70

P Peroxide decomposers 182 Photodegradation 85, 88 Photo-Fries rearrangement 202 Photolysis 89, 94-95, 97-98 Photo-oxidation 87, 89-90, 91, 94-95, 213 Photostabilisation 200, 211-213 Poly(alkylene terephthalate) 44, 47, 48, 93, 94 Polyamides 97 Polyaniline 221 Polyarylates 49, 82, 203 Poly(butylene naphthalate) 80, 95 Poly(butylene terephthalate) 3-4, 6, 8, 12, 35, 36-37, 38-45, 47, 73, 75-77, 78, 93, 95, 98, 108-109, 118, 149, 151, 153, 155, 157, 186, 189, 201 Polycondensation 143, 156, 182, 185 melt 7

265

Degradation and Stabilisation of Aromatic Polyesters Poly(1,4-cyclohexylenedimethylene terephthalate) 4, 13 Polyester, liquid crystal 3-4, 15, 51 Polyester photodegradation 199 Polyester stabilisation 153 Polyester yarns 216 Poly(ethylene naphthalate) 3, 5, 14, 15, 80, 94, 95, 97, 98, 108109, 111, 116, 153, 156, 201, 207 Poly(ethylene terephthalate) 3-8, 10-15, 21-28, 30, 31, 33, 35-38, 40-42, 44-45, 47, 51, 65, 66, 67, 69, 71-78, 85-90, 92, 93, 95, 97, 98, 99, 108, 110-116, 118-122, 143, 146-152, 154, 156-161, 183-184, 185, 189, 200, 202, 204-207, 211, 213, 217 hydrolysis 109 morphology 89 photodegradation 93 photolysis 91-92 photo-oxidation 93 recycling 121 Polymer irradiation 96 Polymerisation 5, 9, 44, 151, 190 Polymer matrix 85, 87, 154, 187 Polymer morphology 9 Polymer recycling 112 Polyolefins 73, 97 Poly(trimethylene terephthalate) 4, 6, 8, 11, 12, 42-44, 77, 79, 108, 109, 146, 151, 155-156, 189, 191 Polyurethanes industry 122 Positron annihilation lifetime spectroscopy 98 Pyrolysis 31, 46, 47, 50, 153 Pyrolysis - mass spectrometry technique 26 direct 31 Pyrolysis-gas chromatography 38, 43

R Radiation degradation 85, 96 Radical scavengers 213, 129 Recycling 107

266

Index

S Salicylates 202 Saponification 113, 115, 119, 122-123 Scission, homolytic 143 Self-condensation 2 Solid-state post-condensation 7

T Tautomerism, keto-enol 202 Tetrahydrofuran 36-37, 40-41, 74, 81 Thermal analysis 98 Thermal degradation 21, 73, 143, 153 Thermal stabilisation 143, 144, 152 Thermal volatilisation analysis 28 Thermogravimetric 51 Thermolysis 97 Thermo-oxidative degradation 65, 80 Thermo-oxidative process 73 Thermo-oxidative stabilisation 181, 199 Thermoset polymers 1 Transesterification 184, 185 Transesterification/hydrolysis 70 Triazines 210

U Ultraviolet absorber 200-203, 206, 210-213, 217 Ultraviolet barriers 212 Ultraviolet degradation 88 Ultraviolet stabilisation 199 Ultraviolet stabiliser 201, 203, 207, 214, 216, 218, 219 Ultraviolet screeners 200-201

V Viscosity, measurements of 98 Volatilisation 187

267

Degradation and Stabilisation of Aromatic Polyesters

X Xenon-arc light source 86

268

Published by iSmithers, 2009

This book provides a comprehensive survey of the degradation and stabilisation processes specific to aromatic polyesters, including thermal, thermo-oxidative, chemical, light and radiation degradation and stabilisation. Current knowledge of all these aspects is discussed and analysed, and some suggestions made as to further studies which might advance the subject. Materials covered include well-known polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate), through the less wellknown poly(alkylene naphthalate)s and liquid crystalline polyesters, to ‘new’ substances such as poly(trimethylene terephthalate). Also covered are the various means of chemically recycling aromatic polyesters into their starting materials and/or other useful chemical feedstock, including current research into improvements in chemistry and economics of such processes, and information on commercial enterprises carrying out such recycling. With over 1000 references to papers and patents, this book provides both a highly detailed source of information on the degradation and stabilisation of aromatic polyesters in itself, and a useful starting point for further study of this topic both by academic and industrial workers in this field. Those researching or manufacturing aromatic polyester formulations for use in fibres, films, packaging, automotive applications and engineering applications will find much to interest them here.

Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 Web: www.rapra.net