Consideration of Poly(ethylene terephthalate) - ACS Publications

Chapter 13

Consideration of Poly(ethylene terephthalate) Recycling for Food Use F. L. Bayer, D. V. Myers, and M . J . Gage

Downloaded by UNIV LAVAL on July 9, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch013

Coca-Cola Company, 1 Coca-Cola Plaza NW, Atlanta, GA 30313

The United States used approximately 1.6 billion pounds of PET plastic packaging resins in 1993, with approximately 480 million pounds being recycled. Regulatory and environmental pressures and the need for environmentally sound programs have dictated the need for responsible approaches to recycling. There exist multiple approaches to recycling of PET. This creates complex relationships between establishing suitable recycling technologies and achieving regulatory requirements for food-grade applications. This paper focuses on the interrelationships of the various approaches to generate safe, recycled, food-grade PET and existing or proposed regulatory requirements in the U.S. and Western Europe.

U.S. resin sales in 1993 were 68 billion pounds, with PET accounting for approximately 3 billion pounds (I). Half of that volume - 1.6 billion pounds, went into some form of plastic packaging (2). These numbers dictate a need for responsible waste management through recycling. This need will only increase in the future. Where do we stand today with respect to plastics recycling? Let's focus on plastic packaging, and more specifically, on PET. The 1993 recycling rate of plastic packaging overall was 6.9%, with PET at 28%. If we focus closer on just plastic bottles, then the overall rate is 19.2% with PET at 30%. If we further restrict our view to PET soft-drink bottles, we had a recycling rate of 41% for 1993 (2). Clearly, since the 1960's, public awareness of environmental issues such as solid waste has been increasing. Although plastics constitute a small proportion of the solid waste stream, (about 8 %) (3), plastic food containers, particularly PET soft-drink bottles, have a disproportionately high environmental profile. Consequently, a rash of environmental legislation over the last decade has appeared in the United States,

0097-6156/95/0609-0152$12.00/0 © 1995 American Chemical Society Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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enacted both by individual States and the Federal Government. Notably, this type of legislation is not restricted to the U.S. Internationally, some member countries of the European Union also have introduced mandatory recycling of plastics. Suffice it to say, the concept is catching on worldwide. Any international consumer packaging company is acutely aware of this trend. As a major user of plastic containers for food use, and as a leader in the beverage industry, The Coca-Cola Company has been involved in PET recycling from the start. In fact, we were the first company to introduce food containers using recycled plastic. Based on a cooperative effort with Hoechst Celanese, in January 1991 they received the first "no-objection" letter from the U.S. FDA (4), allowing the use of recycled PET for food-contact applications. Shortly thereafter, in April, The Coca-Cola Company commercially introduced PET soft-drink bottles containing recycled material. Since 1991, the FDA has issued nine " no-objection" letters on the use of recycled plastic for food containers, particularly PET. Since this ACS Symposium is technical, it is most pertinent to focus on the regulatory and technological issues of recycling PET into food containers. From a regulatory standpoint, PET is the first plastics test case that regulators have had to deal with on this issue. It should be mentioned that most regulations worldwide covering the use of plastic packaging in food-contact applications were well in place, and did not anticipate the usage of recycled plastic for food applications. From a technical standpoint, these topics offer a fertile field of discussion because the chemical nature of PET permits a broad array of recycling options (5,6). Therefore, from both a regulatory and technical standpoint, we are in a rapid learning curve on this issue. This paper is structured into three major areas. 1. Review of approaches to PET recycling; 2. Considerations for use of recycled PET ~ technical and regulatory; and 3. An update on the worldwide status of recycled PET being used in the market for food packaging. Approaches to PET Recycling A brief review or reminder of how plastic recycling can be classified is in order. In May 1992 (7), the U.S. FDA prepared a most useful set of guidelines to help those interested in assuring the consumer safety of their recycling processes. In this document, plastic recycling was divided into three classes. Primary recycling is the recycling of plastics which are plant scrap, and have never had consumer exposure. Secondary recycling involves the physical cleaning of post-consumer plastic by physical processes such as washing, vacuum, and heat treatment. Tertiary recycling involves the chemical treatment, usually depolymerization, of the plastic, and later reconstitution to the polymer. Obviously, all plastics can undergo primary and secondary recycling, but only plastics having somewhat labile bonds, such as polyesters, can be recycled through tertiary recycling. Let's review the recycling options for PET resin. As with any plastic,

Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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PLASTICS, RUBBER, AND PAPER RECYCLING

PET can undergo secondary recycling by being physically cleaned. However, being a polyester, PET can also undergo tertiary recycling chemically through depolymerization followed by repolymerization. This attribute of PET enables the monomers and/or the oligomers to be purified and then reassembled, rather than removing any impurities that may be present directly from the polymeric material itself.


PET Manufacturing Before discussing the PET recycling approaches, we should briefly review the manufacturing process for the resin. PET is made by the poly-condensation of ethylene glycol and terephthalic acid or the corresponding methyl ester to result in a linear polymer of alternating ethylene and terephthalate moieties. The free-acid approach involves ester formation between the hydroxyl groups of ethylene glycol and the terephthalate carboxyl groups; whereas in the ester approach, ester interchange occurs between the methanol and the ethylene glycol. In either case, the fundamental monomer of PET, bis-hydroxyethyl terephthalate (BHET), is formed. In a series of condensation steps at high temperatures, polyethylene terephthalate is formed. The molten PET at 280-290° C is extruded, and usually cut into small pellets. The number of repeating units per polymer chain, or degree of polymerization (DP), is critical in terms of the functional use of the resin; and for applications such as soft-drink bottles, where the rate of permeation of CO2 is to be as low as possible, one requires a high DP, or molecular weight, of PET to permit the fabrication of oriented bottles. To increase the DP to as high a value as possible, the process of solid-stating is used. Solid-stating involves the heating under vacuum of the resin, and this process promotes further linear poly-condensation of the polymer to increase the molecular weight, and hence, improve the properties of PET. PET Recycling Options Secondary and tertiary processes of PET recycling are illustrated in Figure 1. Starting with the physical, or secondary, approaches, cleaning with detergents and/or caustic is used routinely by PET and other plastic recyclers, for the cleaning of flake. Generally, this type of cleaning will sanitize the resin, and render it suitable for use in non-food applications. However, the cleaning process might not always be adequate to allow the material to be used directly for some food contact applications ~ a "deeper" cleaning process may be required. Physical cleaning processes having a greater penetration of the resin than that of simple washing offer promise for use in recycling for food applications. There is a patent on the use of super-critical fluid extraction, or SFE, using CO2 (8). Perhaps other suitable non-toxic super-critical extractants could be used to clean PET. Another option involving physical cleaning is the use of solvents having high penetrating power for PET resin. In this connection, a patent also has been issued which uses propylene glycol to clean PET (9.) The last physical approach that could be effective is the use of high


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Figure 1. PET recycling options.


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temperature and vacuum to clean the resin. These conditions of high temperature and high vacuum remove any volatile impurities, and often are used as purification steps in tertiary recycling processes. Secondary Recycling. An ingenious way to use secondary recycled PET is to place the post-consumer recycled resin between two layers of virgin resin, thus making a "post-consumer multilayer PET sandwich," if you will; or by having the recycled layer on the exterior if only two layers are used. Commercially these are termed multilayer approaches (10,11). Migration into the product of any impurities that might be present, even after cleaning of the recycled resin, is considered to be negligible based on data submitted to various regulatory agencies. Not only is the rate and extent of impurity migration very slow, the virgin-resin barrier that exists between the product and the recycled resin assures consumer safety and product quality (12). Extensive testing and the commercial use of the multilayer approach in soft-drink packaging applications have confirmed the feasibility of this approach. Tertiary Recycling. Tertiary recycling, which involves chemical approaches, offers a "very deep" cleaning in the recycling process because the intrinsic structure of the polyethylene terephthalate is destroyed by depolymerization, eliminating all potential of any impurities binding to the polymer chains. Physical methods are then used to separate (purify) the monomers and/or oligomers from any impurities. The chemical approaches can result in total depolymerization to the monomers, or in partial depolymerization to the oligomers. Commercially, PET resin producers conducting recycling receive a ground flake meeting certain specifications for quality (resulting from physically cleaning post-consumer bottles). This material is then depolymerized, purified, and repolymerized into recycled resin/bottles. The agents of depolymerization routinely used to lyse the ester bonds are water (hydrolysis), methanol (methanolysis), and ethylene glycol or diethylene glycol (glycolysis). Basic catalysts are often used to promote hydrolysis or ester interchange. Methanolysis results in the stochiometric formation of dimethyl terephthalate (DMT) and ethylene glycol (EG). The process may be viewed as passing through two steps. Thefirstinvolves dissolution of the PET flake with partial glycolysis followed by methanolysis, yielding DMT and EG. The resulting DMT is purified by crystallization and distillation, to afford a high-purity, recycled monomer. Hydrolysis relies on the use of high pressure and temperature to depolymerize the PET into the monomers of terephthalic acid (TPA) and ethylene glycol. Commercially, hydrolysis is not being used as an approach to producing food-grade recycled PET, predominantly due to costs associated with purification of the recycled TP A. When glycolysis is conducted, the true monomer of the polyester condensation, bis-hydroxy-ethylterephthalate (BHET), along with the oligomers (n=ca. 2 - 10), are formed. After depolymerization, the monomers and/or oligomers are recovered, purified via vacuum distillation, and repolymerized in the presence of ethylene glycol to reform PET.


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The ethylene glycol produced from all three processes (methanolysis, glycolysis and hydrolysis) can also be separated from the reacted mass by distillation, and, if desired, can be rectified to the stage whereby it can be reutilized in the repolymerization step to PET. Because the physical nature of the terephthalate monomers that are formed in the three different chemical depolymerizations differ, the purification steps for the monomers also vary.


Technical and Regulatory Considerations of Recycling One of the most stringent, or " worst case" packaging application is that of a clear (colorless) beverage container. A clear container with recycled PET capable of being used for carbonated soft drinks represents a good example of the most demanding packaging application for three reasons: 1. The food-contact surface is exposed to a liquid food, and if any migration of materialsfromthe resin would occur, it would most likely be under these circumstances. 2. The recycled resin must be of high quality; otherwise it is incapable of being used to form bottles having good CO2 retention, and meeting all of the other important physical characteristics that are necessary for containers of carbonated soft drinks. 3. Package quality is very evident because the bottle is clear (colorless). Any discoloration (yellowing) of the recycled PET resin would be evident at point-of-sale, or certainly after the contents of the bottle have been partially consumed. This may have a negative effect on the consumer's perception of product quality. Concerns about product and package quality are equally as important to the user of recycled plastics/PET in his food packaging as are concerns for consumer safety. U.S. FDA Regulatory Guidelines. The basic guiding principles in regulating any packaging (including recycled plastic) are: (1) the package shall not endanger the consumer through product adulteration by migration of material from the package, and (2) the package will not detract from the organoleptic properties (taste, smell) of the food. These basic tenets are encoded in all worldwide packaging regulations. As stated earlier, most plastic food packaging regulations were in place before the concept of using recycled plastic in food packaging emerged as a result of environmental pressures. The U.S. FDA was first to formally consider this issue and prepared guidelines in 1992 (7) which assist interested parties in testing the safety of their recycling process. Additionally, the recycled plastics would have to comply with all existing regulations promulgated for virgin plastics. The fundamental concept underlying these guidelines is the principle of de minimis, a Latin phrase roughly meaning " below a level of concern," also referred to as a " Threshold of Regulation" (13). How is this concept put into practice? One contaminates 100% of the PET feedstock in the form of either bottles or flake for a period of 2 weeks at 40° C with periodic agitation. Contaminants are then drained from the flake with subsequent processing. This may involve cleaning as in a commercial preprocessor (flaking operation), followed by depolymerization and



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analysis. In the case of tertiary recycling (chemical depolymerization), you can spike the reactor directly with a level of 0.1% of contaminant per weight of flake as an alternative procedure. What type of " contaminants" are used? FDA has established criteria for using contaminants representing the various physical/chemical categories. These would include the categories of polar volatiles, polar non-volatiles, non-polar nonvolatiles, non-polar volatiles and metallics/organometallics. The end results dictate that the material (purified monomer, resin, preforms, or bottles) containing recycled content would meet the de minimis concept. This corresponds to the residual concentration of a contaminant that corresponds to an acceptable upper limit of dietary exposure of 1 ppb. (One-half ppb is the proposed new limit). The maximum residual levels have been established for various plastics based on certain assumptions and calculations involving the density of the material; its thickness; mass-to-surface ratio; the amount of food contact per square inch; the amount of food consumed in this type of container (consumption factor) and the food-type distribution factor. The calculation which is shown in Reference 7 results in a maximum residue level of 430 ppb (215 ppb based on the newly proposed threshold of regulation) for a contaminant in PET corresponding to a total dietary exposure level of 1 ppb. These guidelines are more than adequate to provide for consumer safety. There are four key assumptions that overestimate theriskto the consumer: 1. The assumption that 100% of the material subjected to the regime is contaminated; industry believes that post-consumer PET bottle contamination with toxicants may be as low as 1 in 10,000. 2. The assumption that neat or user-strength solutions are used for contamination; the consumer doesn't always have access to these levels of concentration. 3. The consumption factors used assume that 100% of the food application will use recycled resin. 4. The assumption that the food container will have 100% recycled content. The de minimis concept is a rational approach for establishing risk assessment, and, more importantly, gives industry a tangible basis for evaluating various recycling processes. Unfortunately, de minimis is not universally accepted throughout the world. In Europe, where there is considerable need for recycling due to an abundance of environmental pressure, they take a totally different approach. PET Recycling Status Worldwide European Regulatory Criteria. Most of Europe is governed by the European Union, although national laws exist for all member countries. Historically, about 30 years ago, European regulators began establishing positive lists of substances for manufacturing plastics intended for food packaging, providing specific migration levels for residues of the substance or the maximum amount of the residue in the plastic article. Unfortunately, this led to a broad diversity of regulations covering packaging materials. The European Commission (Union) has worked hard in harmonizing packaging legislation within Europe - or shall we say - bringing "chaos



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into order." The European Union, like the U.S., has no regulation governing the use of recycled PET, and has not established guidelines for recycle food-grade applications. Thus, the current governing regulations have a problem in dealing with certain processes for the recycling of PET for food-contact applications. To understand this dilemma, we must understand the basis of the establishment of guidelines for the manufacture of food-grade PET. The European Union has established a Directive (90/128/EEC) (14) which mandates a "Positive A List" of approved monomers with specific migration limits (SML's) and Global Migration Limits (QML's). The specific migration limits (SML's) establish a residual limit for that monomer on the A list that migrates into the foodstuff or a simulating liquid. The level is generally equal to the Acceptable Daily Intake (ADI) or the Tolerable Daily Intake (TDI) of the substance times 60. The maximum quantity of residual substance (QM) represents the residual level of the substance in the finished product. QM is generally expressed as 10 mg/dm^ of packaging or 60 mg/kg of foodstuff. How does Directive 90/128/EEC affect recycling? First, there are no specific food law regulations covering the use of recycled plastics in the European Union, with the exception of Spain and Italy, which have prohibitive provisions. In principle, tertiary recycling processes resulting in purified monomer complying with the provisions of Directive 90/128/EEC should be authorized. In fact, the Commission and several member states have issued their equivalent of a "noobjection" letter for recycling via methanolysis. Unfortunately, the problem is that the European Union's "Positive A List" approach has difficulty with processes other than tertiary recycling that yield monomer. For example, there is no provision for a barrier layer approach such as multilayer. Presently, the Commission is deliberating over this very issue. Additionally, processes which would have the ability to extract/remove any and all contaminants directly from the flake are excluded. In other words, "Super Cleaned Flake" cannot be regulated because it is not a monomer and, therefore, is obviously excludedfromthe "Positive A List." Conclusion Where do we go from here? Industry and regulatory agencies must continue to work cooperatively to find the appropriate answers to establish workable criteria for regulators, industry, legislators and the consumer. As an example, a recent meeting of U.S. and EU regulators was held in Europe under the auspices of ILSI (the International Life Sciences Institute) to discuss how the "de /w/w/w/s/threshold of regulation" concept can harmonize the approach of risk assessment as it applies to packaging safety. This meeting is an important step toward harmonization of worldwide regulations pertaining to the use of recycled materials in food packaging. The results of this meeting have been reported as positive, and progress is expected to be made in this critical area. The concepts embodied in the 1992 FDA Guidelines have gained some acceptance in assessing the risk of using recycled packaging for food outside the United States. Several processes for recycling PET for food packaging applications


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have been approved by regulatory bodies outside the U.S. In the UK, MAFF (Ministry of Agriculture, Fisheries and Food) approved a methanolysis process in 1992, and shortly thereafter, the same process was approved by the European Union. In 1992, Japan also approved the use of methanolysis for the recycling of PET for soft-drink bottles. In 1993, Australia approved the use of multilayer recycling, followed by New Zealand in 1994. The need and opportunities for recycling of plastics for food packaging have been recognized by industry, and they are working diligently to find meaningful and cost-effective solutions for this issue.


Literature Cited 1. Modern Plastics January 1994, 73-84. 2. Beck, R.W. 1993 National Post-Consumer Plastics Recycling Rate Study. 3. Characterization ofMunicipal Solid Waste in the United States - 1992 Update. Report 530-R-92-019, U.S. Environmental Protection Agency, Washington, DC, July 1992. 4. Rulis, A. Food Additive Master File No. 428, No Objection Letter to R. Simmons, January 9, 1991. 5. Menges, G. International Polymer Sci. & Tech. 1993, 20 (8), T10-T15. 6. Leaversuch, R.D. Modern Plastics 1991, 68 (7), 40-43. 7. Points to Consider for the Use of Recycled Plastics in Food Packaging: Chemistry Considerations. U.S. FDA, Center for Food Safety and Applied Nutrition (HFS-245), Washington, DC, April 1992. 8. U.S. Patent No. 4,764,323, August 16, 1988. Al Ghatta, Cobarr S.p.A., Anagni, Italy. 9. Patent No. 4,680,060, July 14, 1987. Gupta, A.S., Camp, J.T. The Coca-Cola Company, Atlanta, GA. 10. Coleman, E.C. Food Additive Master File No. 518, No Objection Letter to R.A. Simmons, April 14, 1993. 11. Coleman, E.C. Food Additive Master File No. 518, No Objection Letter to R.A. Simmons, May 5, 1994. 12. Begley, T.H., Hollifield, H.C. Food Technology, November 1993, 109-112. 13. Machuga, E.J., Pauli, G.H., Rulis, A.M. Food Control 3 (4), 1992, 180-182. 14. European Community Council Directive No. 90/128/EEC, Council Directive of 23 February 1990 Relating to Plastics Materials and Articles Intended to Come into Use with Foodstuffs. CEC, 1990, Official Journal of the European Communities, L75/19. RECEIVED March 8, 1995


Consideration of Poly(ethylene terephthalate) - ACS Publications

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