Poly(ethylene terephthalate) Film Recycling

W. De Winter

Agfa-Gevaert N.V., Research & Development, Septestraat,B-2640 Mortsel, Belgium

INTRODUCTIONThe impact of man-made polymers on the environment is a problem of high pri-

ority in most industrialised countries. Mainly due to a build-up of disposedwaste in landfills, and due to campaigns in the press about mistakes made in themanagement of waste treatment, public opinion is focusing on this problem. Thefact that the corresponding percentage by volume is higher, due to the low pack-ing density of wastes, makes the problem more visible. Although “plastics” con-stitute not even 10 wt% of the total amount of wastes, both residential andindustrial, found in landfills (see Figure 1), public attention to them is increas-ing. A possible explanation1 of such a reaction suggests that there is a lack ofcompatibility of plastics with the environment, despite the fact that the majorityof products used in present daily life are made of materials which have also beenmanufactured by a chemical process.

The plastic waste in landfills consists of about two-thirds polyolefines, andonly ca. 15 % of styrene polymers, ca.10 % of polyvinyl chloride, and less than10% of all other polymers, including poly(ethylene terephthalate) (PET).

The largest use of PET is in the fiber sector. PET film and PET bottles repre-sents only about 10 % each of the total PET volume produced annually.2 It is alsogenerally known that the total ECO-balance, considering energy consumption,atmospheric and water pollution, as well as solid waste content, is by a factor 2to 5 more favorable for PET film than for its greatest competitors in the packag-ing sector, namely glass and aluminium.3

In addition, PET is one of the largest recycled polymers by volume,4 because itis suitable for practically all recycling methods.1 PET recycling by the followingtechnological processes is discussed below:

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• direct re-use• re-use after modification• monomer recovery• incineration• and re-use in a modified way.

In addition, attention will be given to some other attempts for recycling whichhave not been thoroughly evaluated so far, like biodegradability andphotodegradation.

This paper is limited to the discussion of PET-film recycling. A global review ofPET-recycling in the sectors of fibres, films, and bottles was published earlier.2

2 PET Film Recycling

Figure 1. Composition of landfill-waste (domestic and industrial).

DIRECT RE-USEOver 50 % of the PET film produced in the world is used as a photographic

filmbase. The manufacturers of these materials, mainly Agfa-Gevaert, East-man Kodak, du Pont de Nemours, Fuji, Minnesota Mining & Manufacturing,and Konishiroku have long been interested in PET film recovery. An importantmotivation for the efforts made by these companies is the fact that photographicfilms are usually coated with one or more layers containing some amount ofrather expensive silver derivatives, which have been recovered since the early20th century, when cellulosics were used as a film base. Silver recovery makesPET-base recovery more economical.5,6 In a typical way of operation, PET filmrecycling is coupled with the simultaneous recovery of silver, as represented inFigure 2.

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Figure 2. Combined recovery of silver and PET.

In the first step of the process, photographic emulsion layers containing silverare washed with, for example, NaOH, and after separation, silver is recoveredon one side, and cleaned PET-waste on the other side.2 Important in this processis that the washed PET-film scrap is clean enough to be recovered by directre-extrusion, although careful analysis remains necessary.

Direct recycling of PET-waste in the molten state, before re-extrusion toPET-film, is of course the most economical process thinkable, as recoveredPET-scrap can be substituted for virgin PET-granulate without requiring anyadditional steps. It is well-known that PET in the molten state gives rise simul-taneously to polymer build-up and to polymer degradation, so that reaction con-ditions for this process have to be controlled very carefully in order to obtain anend-product with desired physical, chemical and mechanical properties, likecolor, molecular weight, and molecular weight distribution.

A large number of reaction parameters have to be kept under permanent con-trol (temperature, environmental atmosphere, holding time in a melt state,amount of impurities, type of used catalysts and stabilizers, etc.). The order ofaddition of the PET flakes is very important. A typical flowsheet of abatch-PET-process7 is represented in Figure 3. In such a process, the PET-flakescan be added after polymerization, before the melt enters the film extruderscrew (Figure 3, indication 1). Such a procedure, however, has two main draw-backs:

• a highly viscous melt is difficult to filter (to eliminate possible gels ormicrogels)

• resulting low-boiling or volatile side-products cannot be discarded any-more.

In order to eliminate these disadvantages, several alternative operationmodes have been worked out in the past. A method to add recycled PET during

4 PET Film Recycling

the esterification step (Figure 3, indication 2) has been described by du Pont.8 Insuch a way filtration can take place in the low-viscosity phase, and volatiles canstill be eliminated during the prepolymerisation phase.

Although PET-recycling by direct re-use is by far the most economical process,it is only useful in practice for well characterized PET-wastes, having exactlyknown chemical composition (catalysts, stabilizers, impurities). Therefore, thisprocess is the most suited for the recovery of in-production wastes, but it maynot be ideal for customer-recollected PET-film. An industrial process for X-rayfilm-recycling was worked out by the IPR-company9 and introduced to the mar-ket under the name REPET on the basis of a triple motivation:

• availability of the waste chips on a repetitive basis• suitable purity• very competitive price.

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Figure 3. Batch process flow sheet.

RE-USE AFTER MODIFICATIONSimilar to the method described under direct re-use, in which PET-flakes are

added during the esterification process, PET-polymer is broken down intolow-molecular, low-viscous fractions. Such method could already be viewed as amethod of re-use after modification. Because the intermediate products are notseparated at any moment of the process, the degree of purity of PET-scrap mustbe high.

For PET-wastes having a higher degree of contamination, other technologicalprocesses are applied, including further degradation by either glycolysis,methanolysis, or hydrolysis,10 yielding products which can be isolated. The prin-ciples of chemical processes on which these methods are based are schematicallyrepresented in Figure 4.

6 PET Film Recycling

Figure 4. PET degradation by glycolysis, methanolysis, and hydrolysis.

Glycolysis can be considered as a method for direct re-use, whereasmethanolysis and hydrolysis are mainly taken into consideration for monomerrecovery, as discussed below.

The du Pont Company published11 many details concerning the glycolytic recy-cling of PET. Less costly ingredients than those required for hydrolysis ormethanolysis, and more versatility than direct remelt recycling are quoted asthe reasons for glycolysis choice. Goodyear has also developed the PET recyclingprocess based on glycolysis which is called REPETE.12

Glycolytic recycling of PET, which can be done in a continuous or in a batchprocess, is preferentially performed by addition of a PET waste to a boiling eth-ylene glycol, which leads to the formation of low-molecular weight intermedi-ates and eventually to crystallizable diglycol terephthalate (DGT). The rate ofthe degradation reactions is primarily controlled by varying the holding timeand temperature, which depends on a choice of suitable catalysts (e.g., titaniumderivatives),12,13 and by adjusting the PET/glycol ratio. It is also necessary toavoid side reactions which might occur, e.g., by adding “buffers” or by keepingdown reaction time and temperature.

The low-molecular weight depolymerizates can be introduced directly into apolymerization system,14 preferentially after filtration. In this method, particu-lar care has to be taken in order to avoid glycol ether formation, which may leadto PET of inferior properties. The glycolytic degradation can also be pushed tofurther completion, leading to DGT-recovery, rather than to direct re-use.

In addition to the glycolytic recovery of PET for production of a new PET-film,granulate, or monomer (EG and DGT), alternative methods have been describedfor the preparation of so-called PETGs (i.e., glycol-modified PET), which can beused for different purposes.15,10 Depending on the type of glycol (or polyol) usedfor depolymerization, and on the nature of dicarboxylic acid used for subsequentpolycondensation, the obtained polyester may be used as a saturated polyesterresin (e.g. for films, fibres or engineering plastics), unsaturated polyester resin,mixed with vinyl-type monomers, or alkyd resin, where polycondensation is per-formed in the presence of tri- or poly-functional organic acids.

Although this method for producing unsaturated resin, e.g., for use in regularcastings or in fiber-reinforced laminates, has been thoroughly studied byPET-film manufacturers, it is believed that the method is not currently used inproduction.16

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MONOMER RECOVERYAlthough monomer recovery is the oldest recycling method and can be used to

recover PET-waste having a high degree of impurity, it is regrettable that it isnot the most economical method. The earliest methods of PET synthesizingwere based preferentially on the use of dimethyl terephthalate (DMT), whichcould be better purified than terephthalic acid (TPA), therefore methanolysis isdiscussed before hydrolysis. The chemical principles of both processes are al-ready given in Figure 4.

Methanolysis of PET-wasteThe waste is treated with methanol (in a ratio 1/2 to 1/10), usually under pres-

sure at high temperatures (160-310oC) in the presence of transesterification and(or) depolymerization catalysts.17 Once the reaction is completed, DMT isrecrystallised from the EG-methanol mother liquor, and distilled to obtain poly-merization-grade DMT. Also EG and methanol are purified by distillation. East-man Kodak has been using such a process for recycling of X-ray films for 25years, and it is still improving the process,18 e.g., by using superheated methanolvapor, to allow the use of ever more impure PET-waste. Important factors whichhave to be dealt with in this process are avoiding coloration and keeping downthe formation of ether-glycols.

Hydrolysis of PET-waste19

Although aromatic polyesters are rather resistant to water under atmosphericconditions, compared with other polymers, they can be completely hydrolyzedby water at higher temperatures (and) under pressure. For practical purposes,however, particularly to speed up the process, use has to be made of catalysts.Acidic as well as alkaline catalysts have been studied and worked out in prac-tice.

Figure 5 gives a flow chart of both processes. While both systems are com-pletely realistic, their usefulness under practical production conditions remainscontroversial. As far as acid hydrolysis is concerned, the large acid consumptionand the rigorous requirements of corrosion resistance of the equipment makeprofitability questionable. In addition, the simultaneous (with TPA) regenera-tion of ethylene glycol is difficult, ecologically undesirable (requiring the use oforganic halogenated solvents), and not economical. Concerning alkaline hydro-lysis, the profitability is strongly determined by the necessity of expensive filtra-

8 PET Film Recycling

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Figure 5. Flow chart of acid- and base-catalyzed PET degradation.

tion and precipitation steps. To our knowledge, recycling of PET-waste byhydrolysis is not practiced on a production scale at present. This situation evenpersists in spite of the fact that the majority of newer industrial PET-synthesisplants are based on the TPA-process rather than on the DMT-process.20

INCINERATIONAnother approach which can be used to recycle plastics, particularly when

they contain a large amount of impurities and other combustible solids (if such isa case, it is important to keep them away from landfills), is more recently called“quaternary recycling”, and consists of the energy recovery from the wastes byburning.21 Research along this line has been performed, particularly in Europeand Japan, since the early 1960s. Strong emphasis has been laid on an optimiza-tion of incinerators with regard to higher temperature of their operation and re-duction of the level of air pollution.

PET has a calorific value of ca. 30.2 MJ/kg, which is about equivalent to that ofcoal. It is thus ideally suited for the incineration process. The combustion ofplastics, however, requires 3 to 5 times more oxygen than for conventional incin-eration, produces more soot, develops more excessive heat, and incinerationequipment had to be adapted in order to cope with these problems.

Several processes have been worked out to overcome these technological draw-backs.22-27 Examples include Leidner’s continuous rotary-kiln process, Baliko’sprocess for glass-reinforced PET, Crown Zellerbach Corporation’s combined sys-tem for wood fibre and PET to provide steam to power equipment, and ETH-Zu-rich’s fluidized bed system for pyrolysis, especially of photographic film, i.e., incombination with silver recovery. The latter system raises the additional prob-lem of the formation of toxic halogenated compounds, stemming from the pres-ence of silver halides.

Typical operation conditions take place at temperatures around 700oC. Atlower temperatures, waxy side-products are formed, leading to clogging. Athigher temperatures, in turn, the amount of the desirable fraction ofmononuclear aromatics decreases. A representative sample, pyrolysed underoptimized conditions, yields, in addition to water and carbon, aromatics likebenzene and toluene, and a variety of carbon-hydrogen and carbon-oxygengases. Studies have been performed1 to avoid formation of dioxines and disposalof residual ashes containing heavy metals and other stabilizers.

10 PET Film Recycling

be resolved; however, quite a few residual hurdles will have to be taken25 beforean economically feasible and ecologically accepted industrial technical processwill be available.

BIO- AND PHOTO-DEGRADATIONAlthough there certainly has never been a great incentive for making unstable

polymers, the idea of making photo- or bio-degradable polymers has long ex-isted,28,29 and quite a bit of effort has gone into research along these lines. Forsuch a process, of course, limitations with regard to the percentage of allowableimpurities do not exist.

PhotodegradationSpecial photodegradable polymers30 were synthesized for the purpose of hav-

ing them destroyed after use (e.g., in a landfill). Another approach was the incor-poration of suitable groups (e.g., carbonyl) in the polymer backbone in order tomake polymer photodegradable by sunlight or UV (see Figure 6). A problemarises due to the fact that light exposure conditions on a landfill cannot be regu-lated. The main difficulty, however, seems to be practically insurmountable: it is

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Figure 6. Photodegradable monomers and polymers.31

At present it seems that most problems arising during incineration of PET can

hardly possible to combine rapid degradation upon exposure to light in a landfillafter use with a good light-stability of the film during service. This contradictioin terminis is probably the reason why this method never really caught on.29 An-other problem is a combination of desired properties with favorable economics.

BiodegradationThe main difference between biodegradation and photodegradation lies in the

possibility to create in a landfill an environment completely different from thatencountered under normal storage conditions; e.g., microorganisms which candestroy plastic films may be added to a landfill.

In spite of the fact that substantial research time was spent on studies in thisfield, it is claimed32 that surprisingly little is understood about the molecu-lar-level interaction between polymers and microorganisms. This can be ex-plained by a poorly defined environment (in a landfill), and by a large number ofcomplex parameters involved in the process: methods of evaluation based solelyon changes in physical properties are thus unsuitable for forming conclusions,similar to the evaluations based only on biogas production. Specifically for poly-esters, however, a number of interesting data are available. Esterases (ester-hy-drolyzing enzymes) and also some microorganisms are known to biodegradepolyesters at a reaction rate depending upon the polyester structure.29,33 Whilemany aliphatic polyesters, specifically poly(hydroxy fatty acids) - e.g., theBIOPOL34-36 packaging material commercialized by ICI - are suited forbiodegradation, the aromatic polyesters (e.g., PET) do not possess this prop-erty.32,37-39

Another approach consists of mixing small amounts of biodegradable poly-mers, e.g., polysaccharides, with a regular polymer (e.g., a polyolefin), in order tomake the end-product destroyable as well. Examples of polysaccharides/poly-ethylene have been commercialized.38 Mixtures of starch with other polymers,40

12 PET Film Recycling

including PET, have been studied,34 but no commercialization of the latter mix-ture is known so far. The fact, however, that the starch additive is only needed insmall amounts, which hardly alters the properties of an original polymer, mightshow some promise for future applications. One has to realize, however, that thethermal stability of starch derivatives above 230oC is limited, whereas thePET-film extrusion temperature is in the range of 280oC. There also remainsome controversies about the completeness of the degradation of polymer/starchmixtures.

Although the development of biodegradable plastics is still in progress, it is be-coming evident that the enormous market potential, forecast some years ago, re-quires a real breakthrough in order to be attained.41,42 The main reason for thissetback is probably the fact that organic polymers do not biodegrade fastenough.43,44

CONCLUSIVE REMARKS

• From the data presented in this overview, it seems obvious that there ex-ists a clear hierarchy in PET-film recycling technologies. The most impor-tant criteria of classification are, first of all, the degree of “purity” ofPET-scrap to be handled, and secondly, the economics of the process.

• For the cleanest PET grade, the most economical process, i.e., direct re-usein extrusion, is self-explanatory.

• For less clean PET samples, it is still possible to re-use them after the modi-fication step (partial degradation, e.g., by glycolysis) at a reasonably lowprice.

• More contaminated PET-film waste must be degraded into the startingmonomers, which can be separated and re-polymerized afterwards, ofcourse, at a higher cost. At present, only the methanolysis process is ex-ploited industrially, as opposed to hydrolysis processes, which are kept inreserve.

• Finally, the most heavily contaminated PET-shreds have to be incinerated.Here, however, economics may not be favorable enough for industrial de-velopment. As an alternative, those PET-shreds are brought to a landfill.Perhaps in future more attention will be given to modification of PET-filmsin such a way that they may become biodegradable, if the process can be ac-celerated or if a real breakthrough becomes available.

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36. A. Steinbuchel, Nachr. Chem. Techn. Lab., 39 (10), 1112 (1991).37. P. Klemchuk, Mod. Plastics Int., 82, (Sep-1989).38. J. Evans and S. Sikdar, Chemtech, 38, (Jan-1990).39. K. Joris and E. Vandamme, Technivisie, 179, 5, (1992).40. R. Narayan, Kunststoffe, 79, 1022 (1989).41. N. Holy, Chemtech, 26, (Jan-1991).42. A. Calders, Technivisie, 156, 8 (Nov-1990).43. Anon., Mod. Plast., 20 (1), 72 (1990).44. H. Pearce, Scient. European, 14, (Dec-1990).

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