Proc. Nati. Acad. Sci. USA Vol. 89, pp. 835-838, February 1992 Colloquium Paper

This paper was presented at a coUoquium entitled "Industrial Ecology," organized by C. Kumar N. Patel, held May 20 and 21, 1991, at the National Academy of Sciences, Washington, DC. Polymer recycling: Opportunities and limitations RICHARD S. STEIN Polymer Research Institute, University of Massachusetts, Amherst, MA 01003

ABSTRACT The disposal of polymer solid waste by means tractors, differences in energy requirements for processing, other than landflhling is necessary. The various approaches- and differences in the care required for maintenance and source reduction, incineration, degradation, composting, and laundering. The choice is indeed not a simple one. recycling-all have their roles and must be employed in an integrated manner. Where appropriate, recycling has ecolog- ical advantages, but its application is dependent upon the INCINERATION feasibility of collection, sorting, and/or compatibilization of Incineration is widely used in Asia, necessitated by the resulting mixtures to produce economically viable products. limited space for landfills. It has not been popular in the The practice should be encouraged by societal or legislative United States, primarily because of concerns about toxic pressure which recognizes that the cost of disposal should be a fumes and ash. These problems could probably be avoided factor in determining the cost of a product. through use of current technology in incinerator design and by employing some degree ofseparation offeedstock so as to The disposal oftrash is a problem confronting our society (1). eliminate "bad actors".* The acceptance of incineration Our available landfills are becoming exhausted. While poly- depends upon the success of these measures. mers compose only about 8% by weight (20% by volume) of A polymer of concern for incineration is poly(vinyl chlo- landfills (2), there is much focus on polymer accumulation ride) (PVC), where hydrochloric acid can be produced with because of their high visibility. This has stimulated the improper incineration. This has prompted possible legislation formulation of considerable restrictive legislation regulating to restrict its use. However, a principal use for PVC is for polymer use. A response has been the consideration of pipe which does not represent a significant waste problem. alternatives for polymer disposal. The principal means are (i) Of course, a principal product of incineration is carbon source reduction, (ii) incineration, (iii) bio- or photodegra- dioxide, which may contribute to the global warming prob- dation, (iv) composting, and (v) recycling. lem. However, since a relatively small fraction of the petro- leum supply is used to produce polymers and only a fraction of that would be incinerated, this source should not be a SOURCE REDUCTION major factor in comparison with carbon dioxide production Means for source reduction are apparent. These involve such arising from the burning of fossile fuels. measures as the elimination of unnecessary packaging and Incineration consumes about 15% of today's solid waste, the packaging of products as concentrates. and a goal of25-30% by the year 2000 has been suggested (2). One approach is the replacement of polymers by alterna- tive materials. This should be done with care, since the BIO- AND PHOTODEGRADATION replacement is not always ecologically desirable and some- There is skepticism as to whether the employment of degrad- times functionally inadequate. One estimate, for example, able polymers will be effective in reducing the buildup of suggests that the abandonment ofplastics in packaging would landfills. Under usual conditions, degradation rates in land- result in a 404% increase in the weight of waste, a 201% fills are too slow (5). Modification of polymers so as to increase in energy consumption in making the alternatives, increase degradation rates often leads to the problem of their and a 212% increase in cost (3). Alternative materials are degrading under normal conditions of use. Furthermore, sometimes heavier, more permeable, more water absorbant, degradation is in opposition to possibilities for future recy- and less strong than their polymer counterparts and thus may cling ofthe polymer. A usual product ofdegradation is carbon not function as well. dioxide, so it parallels incineration in this respect (without the A case in point is the replacement of Styrofoam "ham- advantage of possible energy recovery). The contribution of burger shells" by MacDonald's with a paper-based wrapping. toxic residues to the environment, which is of concern in the We are faced with the decision "paper or plastic" at the consideration of incineration, also must be considered here, supermarket check-out. Several studies have contested the since when a polymer containing such residues degrades, environmental superiority of paper as compared with plastic these are also released. (4) and the choice is not as simple as is commonly portrayed. There is a definite role for degradable polymers. While Another illustration involves the replacement of synthetic ideally, articles such as "six-pack rings" and old fish line and fibers such as nylon and Dacron with "natural degradable" nets should not be carelessly discarded, some such practices fibers such as cotton and wool. A proper analysis of the will always occur, so rendering such articles to be degradable ecological effect would require consideration of the agricul- will reduce the of their harmful to marine tural implications of growing the required amount of cotton possibility being and raising the sheep, the fertilizer needed, the fuel for the Abbreviations: PVC, poly(vinyl chloride); PET, poly(ethylene tere- phthalate); HDPE, high-density polyethylene. The publication costs of this article were defrayed in part by page charge *The Environmental Impact on Municipal Solid Waste Incineration, payment. This article must therefore be hereby marked "advertisement" Findings of the International Symposium on Solid Waste Inciner- in accordance with 18 U.S.C. §1734 solely to indicate this fact. ation, Sept. 26-27, 1989, Washington, DC.

835 Downloaded by guest on September 29, 2021 836 Colloquium Paper: Stein Proc. Natl. Acad. Sci. USA 89 (1992) and wildlife. The state of Maine has recently banned the use Post-Consumer Recycling of Commingled Plastics. Post- of six-pack rings. Furthermore, in addition to the environ- consumer recycling is more difficult. Here, plastics of a mental harm arising from littering, there are cosmetic advan- variety of types are in the hands of consumers after being tages to its reduction through use of degradable polymers. used for a variety of purposes. The consumer may be While the amount of polymer rubbish is not large, it is very motivated to recycle these articles by good citizenship, visible and often leads to unsightly appearance of beaches monetary rewards, or legislation. He/she can usually be able and public areas. Thus, improvement may result from making to separate polymers from nonpolymeric components. If this commonly discarded articles degradable. There is a price to is not done, a certain degree of separation may be accom- pay, however, in that degradation may also occur in normal plished after rubbish collection, making use of the fact that use, so that means for monitoring this are necessary so as to polymers normally have lower densities than other compo- avoid their failure under these conditions. Educational efforts nents and are usually nonmagnetic and nonconducting. Such to reduce littering through instilling good habits are essential separation usually results in obtaining commingledplastic as and may even be more effective than rendering the litter opposed to separated. Such commingled plastics can be degradable. fabricated to make articles such as "plastic lumber," fence One should distinguish between intrinsically degradable posts, and traffic barriers (12, 13). However, they compete polymers and those to which a degradable material is added. with fairly cheap materials, but their cost may be two or three An examp!e of the latter type is polyethylene to which starch times as much. For some applications, such as picnic tables, has been added. The polymers do not disappear on degra- costs may be more competitive (14). Thus, the potential dation of the additive; they just fall apart into small bits. This market is probably limited and may become saturated as may have cosmetic value or may serve to release the contents collection of plastic for recycling increases (information from of a plastic bag to exposure for composting, but it should not Phoenix Industries, Cedar Grove, NJ). A problem with applying commingled plastics to more be considered as a means for disposal of polymers (6). cost be is Intrinsically degradable polymers include poly(lactic acid) demanding applications where their would justified that the interfaces between different polymers are often weak and bacterially synthesized polyalkonates. These convert as a consequence of their usual thermodynamic immiscibility completely to nonpolymeric products on degradation (7-9). (15) leading to little intermolecular penetration. However, So far, their physical properties are not as good as those of there are many less demanding applications where high conventional polymers, but it seems likely that these may be mechanical performance is not required. improved. The principal drawback is cost. Unless this can be The value of commingled plastics may be enhanced significantly lowered, the use of this class of materials will be through strengthening such interfaces by the following. (i) limited to very specialized applications. Copolymerization or grafting, since it is known that copoly- A promising development is the formation of intrinsically mers are more miscible than homopolymers. Grafting is often biodegradable polymers by the thermoforming of starch (10). done with in normal polymer practice, as in the preparation The starting material is sufficiently cheap that there is hope of ABS (acrylonitrile/butadiene/styrene) or HIPS (high- of producing an economically competitive material. impact polystyrene), by grafting a rubbery component to The amount of degradable synthetic polymer in use today polystyrene so as to introduce microphase separation of the is negligible, probably under 1%. This could conceivably be rubbery component which results in energy-dissipating increased to a few percent, which would have a minor effect mechanisms which contribute to their impact strength. One on landfill growth but which could be important if applied to may "impact modify" by this means by carrying out chem- critical situations. ical reactions on the polymer(s) or else by adding another component leading to microphase separation. This benefit can sometimes arise, to some extent, in mixing different COMPOSTING polymers, provided the disadvantage of weak interfaces do While degradation rates are low in landfills, the carrying out not dominate (16). (ii) Functionalization of components of of degradation in compost piles appears to be a reasonable commingled plastics by introducing chemical groups onto prospect (11). Yard and agricultural waste constitutes a much components which attract each other through hydrogen larger part of the solid waste burden than polymers (2) and bonding, donor-acceptor interactions, etc. (iii) The addition composting seems the right approach for these. It would of compatibilizers such as block copolymers which reside at appear feasible to add biodegradable polymers to the com- interfaces and act like emulsifying agents to strengthen the posting mixture. Also cellulosic wastes such as disposable interaction between components. diapers could be accommodated. An infrastructure is needed These approaches are often costly, so it is doubtful to collect wastes of these types in a manner so that they are whether they can provide an economically viable major separated from nondegradable material. impact on the plastic solid waste problem. However, they do To have a major effect such composting would have to be have a limited role, and such efforts should be encouraged. carried out under controlled conditions in centralized facili- Separated Plastics. The market for clean, separated plastics ties. Several studies are in progress concerning how this is much greater. While currently it usually costs more to use might be done. these than virgin polymer, increasing costs for alternative disposal and legislation will undoubtedly render their use more favorable. The problem of separation is resolved for RECYCLING readily identifiable objects such as soda bottles and milkjugs. Recycling is the main theme ofthis article. This implies reuse For soda bottles, the poly(ethylene terephthalate) (PET) of the polymer as polymer. A classification is (i) recycling of polymer is a relatively expensive one, and the return of the industrial scrap (sometimes called "prompt scrap") and (ii) bottles may be encouraged by "bottle bills" requiring de- post-consumer recycling. posits and refunds and by legislation prohibiting their dis- The former primarily involves recovering and reusing posal along with other rubbish. Redemption machines which waste polymer resulting from processing operations. This has read bar codes and make refunds and then grind up the bottle long been practiced by many industrial groups. It is simpler are beginning to appear in supermarkets. than the latter in that polymers are usually of a particular kind There are problems even with PET bottles. They are and they are collected in a central location by experienced sometimes colored, and caps, labels, and bases are often people. The practice is motivated by economics. made of material other than PET. While some degree of Downloaded by guest on September 29, 2021 Colloquium Paper: Stein Proc. Natl. Acad. Sci. USA 89 (1992) 837

separation is possible using techniques such as floatation, the consequence of decisions such as that by MacDonald's to presence of small amounts of contaminants can detract from abandon Styrofoam use. the value of recycle feedstock. It would be desirable, by In addition to the ecological advantages of recycling Sty- either cooperation or legislation, to design such bottles so as rofoam, the educational value of enlisting the efforts of to minimize the number of different components and choose school children in cafeteria recycling procedures cannot be them so as to facilitate separation. FDA restricts the use of ignored. A mission should be to train a generation ofchildren recycled PET to non-food applications. However, there is with positive environmental habits. currently an ample market for applications such as fabrica- It is evident that even if polymers are segregated by tion of carpets and fiberfill (1, 17). chemical species, variations will occur when such polymers PET has been a leader for fraction recycled, with 7% of the from different sources are mixed. For example, they may be 2 billion pounds produced being recycled in 1988 (17), 19o of of differing molecular weight, branching, or tacticity. This soft drink bottles in 1989 and 30% in 1990, with 9% of will affect the rheology of the molten mixture, which is non-packaging PET recycling (18). However, the amount important for its processing. Furthermore, if the polymer used is much less than with polyolefins, PVC, and polysty- crystallizes, the crystallization kinetics and morphology will rene, so its impact on the solid waste stream has not been so be affected (21, 22). Crystallization can be greatly affected by great. However, its use is growing and it is now replacing the presence of nucleating agents which may have been PVC for many container applications. added to some of the sources or may be inadvertently Milk and water jugs have proved a good source of recy- present. clable high-density polyethylene (HDPE). There has been Thus, the processing behavior and product properties of public reluctance toward imposing deposits on these, so their polymers produced from a variable feedstock may be incon- return rate is not as good as for soda bottles. As with PET, sistent and difficult to control. This suggests the need for such recycled HDPE cannot be used for food containers, but analytical methods to monitor properties and to consider they have been extensively used, for example, for containers possibilities for modifying these by adding virgin polymer to for motor oil. [The restriction on food use may be reasonable, the recycle feedstock. since it has been reported that there is enough oil absorption Other than in a relatively few such cases, the separation of from the milk by HDPE to act as a plasticizer and affect future polymers into constituents may prove difficult. The need for recycling (19) and may be a consideration in its further use in sophisticated separation techniques can be minimized if contact with food.] recycling is a consideration in product design. For example, For other polyethylene sources such as shopping bags, the the ability for ready disassembling of a polymer-containing economics of recycling is less favorable, since the amount of product into basic constituents should be a factor in their polymer per article is small, so the cost of collection repre- design. The contribution of polymeric parts ofjunked auto- sents a greater fraction of the value of the recycled polymer. mobiles to the solid waste stream is an increasing burden. Where possible, reuse of such articles should be encouraged. From 1965 to 1995, the amount ofpolymers will increase from Multiple use of a shopping bag, for example, is probably more 2% to 13% of a car's weight (23, 24). Dealing with this effective use of the polymer than would be possible by quantity of material requires means for recovering the plastic collection and recycling. from a junked car in reasonable time. It has been demon- For certain applications, recycling of polyethylene may be strated that a Volkswagen Passet 83 can be disassembled feasible. For example, it is often used as a "mulch" in within 20 min to yield 14.5 kg of polypropylene, 8.7 kg of agriculture or as a temporary covering in building construc- polyethylene, and 5.4 kg of ABS (23, 24). Of course, an tion. Collection and recycling may be effective for such large infrastructure is necessary, probably a computer data base, volume use by knowledgeable consumers. Photodegradation to provide information to the disassembler about procedures is also a viable disposal approach for polyethylene mulch. and constituents for each of the many models of cars and A similar situation may exist for other recycled polymers. instructions for dealing with the recovered polymers. Polypropylene is extensively used industrially and in agri- While efforts are being and should be made to develop culture as strapping or binding. It is also used as outdoor better separation techniques, it seems evident that there will carpet. Nylon is used as indoor carpet. A problem may occur be combinations of polymers which may contain nonpoly- because ofthe employment ofdifferent kinds ofnylon and the meric additives, fillers, and reinforcing fibers such that presence of dyes and other additives. separation is not possible or economically feasible. This In 1988, ofthe 43.3 billion pounds ofpolyolefins (primarily source will grow with the increasing use of polymer-based polyethylene and polypropylene) produced, only 3% was composites in automobiles, housing, etc. Thus alternatives recycled. A goal is to increase this to 9o by 1998 (17). It is with materials. to note that from 1989 to 1990, the recycling of need be considered for dealing such interesting Reduction to Low Molecular may HDPE milk jugs grew from 1.4% to 6.7% (18). Weight Species. Polymers is another polymer for which recycling has met be degraded to low molecular weight species by processes Polystyrene or This with some success. Styrofoam is commonly used for coffee such as pyrolysis, hydrolysis, methanolysis (25, 26). to mixtures of low cups, hamburger shells, insulation, cafeteria trays, etc. Re- could be done for mixed polymers leading cycling ofthis is feasible if sources ofready collection can be molecular weight materials. These may be of sufficiently low identified. School and other cafeterias have often proved viscosity so that they may be separated from insoluble quite cooperative in cleaning and stacking such trays for nonpolymeric additives by means such as filtration. The low centralized collection. The contaminants are usually food molecular weight mixture could be regarded as an organic residues and paper, which can be readily separated by feedstock which could be separated into components by washing and floatation. An example is the collection of used procedures such as fractional distillation. polystyrene foam products from the University of Miami Polymer dissociation is an endothermic process and one Food Service Department and from "fast food" establish- converting a low entropy polymer molecule into higher ments in Ft. Lauderdale, FL, by the Dade Paper Company entropy dissociation products. Since energy conservation and their sale for recycling to the Dart Container Company in should deal with conservation offree energy, which implies Plant City, FL (20). Of the 12 billion pounds of styrenics minimizing entropy production, this aspect is ecologically produced in 1988, again only 3% was recycled, with an 8% undesirable. However, it is a price to pay for permitting goal by 1988 (17). These estimates may be modified as a separation (1). Downloaded by guest on September 29, 2021 838 Colloquium Paper: Stein Proc. Natl. Acad. Sci. USA 89 (1992) The amount ofpolymer solid waste which is recycled today soc. of the Plastic Producing Industry, Frankfurt am Main, is about 16%. A goal would be to at least double that by the F.R.G.), SRPM Study. year 2000 (2). 4. Hocking, M. B. (1991) Science, 504. 5. Rathje, W. J. (1989) The Atlantic Monthly (Dec.), p. 90. 6. Anonymous (1989) N. Y. Times (Oct. 25), p. 1. CONCLUSIONS 7. Brandl, H., Gross, R. A., Lenz, R. W. & Fuller, R. C. (1988) Appl. Environ. Microbiol. 54, 1977. Today, about 69% of the plastic solid waste in the United 8. Smith, C. C. (1988) Trashing Plastic (Univ. of Massachusetts, States ends up in landfills. This cannot continue. There is no Amherst), Vol. 13, p. 30. one alternative which will solve the problem, so multiple 9. Logeton, G. (1989) BioCycle 30, 58. 10. Starch Research Group (1991) The Thermoplastic Processing of approaches must be taken. Source reduction, incineration, Starch (Massachusetts Institute of Technology, Cambridge). employing degradable polymers, and recycling all have their 11. Holusha, J. (1991) N. Y. Times (Feb. 4), Sect. F, p. 4. place, and increases in all of these measures are required 12. Plastics Recycling Foundation (1990) Annual Report, 1990 (27-29, t, t). Source reduction and recycling are preferable in (Plastics Recycling Found., Washington). that they conserve resources and minimize pollution. Rapid 13. Plastics Recycling Foundation, Plastics Recycling: From Vi- is being made sion to Reality (Plastics Recycling Found., Washington). progress in recycling post-consumer plastics 14. Delotta & Touche (1990) Mixed Plastics Product Market Pen- with a growth of45% in 10 plastics markets between 1989 and etration Study. 1990 (18). Composting may be preferred to incineration in 15. Stein, R. S. (1991) in Proceedings of the Symposium on Poly- that it does not lead to air pollution (but could lead to some mer Recycling (Am. Chem. Soc., Philadelphia), in press. sanitation concern). Landfilling is the disposal means of "last 16. Noser, T. J., Morrow, D. W., Renfree, R. W., Van Ness, resort" to be used when none of the other more preferable K. E. & Donaghy, J. J. (1991) Nature (London) 350, 563. 17. Anonymous (1989) Chem. Week (March 29), p. 20. methods are applicable. 18. Anonymous (1991) Modern Plastics 68, 38. Through these means, it appears that the amount of land- 19. Blatz, P. S. (1991) Polymer Preprints 32 (2), 152. filled polymer could be decreased by a factor of 2 by year 20. Anonymous (1990) Sun-Sentinel, Dade County, FL (Dec. 10). 2000. It appears as though meeting these goals will require a 21. Mandelkern, L. (1964) Crystallization of Polymers (McGraw- cooperative effort of the industrial and environmental com- Hill, New York). munities with motivation and regulation provided by govern- 22. Ree, M., Kyu, T. & Stein, R. S. (1987) J. Polym. Sci. Polym. ment. Measures must be consistent with technological and Phys. Ed. 25, 105. economic limitations and a reasoned analysis of the factors 23. Koczek, M. J. (1990) in Fourth European Conference on Com- posite Materials (Elsevier, New York). involved is essential for making the proper choices. 24. Weber, D. A. (1990) in Fourth European Conference on Com- posite Materials (Elsevier, New York). 25. Anonymous (1990) Recycling News 16 (Dec. 4). tWeis, R. S., American Institute of Chemical Engineers Session on 26. Anonymous (1991) Modern Plastics 68 (7), 39-43. Industrial Viewpoints on Processing Recycled Plastics, March 27. Council for Solid Waste Solutions (1991) The Blueprint for 19-22, 1990, Orlando, FL. Plastics Recycling (Council for Solid Waste Solutions, Wash- tFox, E. A., Presentation at an International Conference, April 3, 1991, Davos, Switzerland. ington). 28. Lodge, G. C. & Rayport, J. T. (1991) Recycling Plastics: A Holistic View (Div. of Research, Harvard Graduate School of 1. Anonymous (1990) Modern Plastics (Special Suppl., April). Business Administration, Cambridge, MA), Working Paper. 2. Exxon Chemical (1991) Plastics and the Solid Waste Issue 29. Stone, R. F., Ashford, N. A. & Lomax, G. (1991) The Art ofthe (Exxon Chemical Co.). Possible: The Feasibility ofRecycling Standardsfor Packaging 3. Association of the Plastic Producing Industry (1987) Conse- (Center for Technology, Policy, and Industrial Development, quences ofAbandoning Plastics in the Packaging Sector (As- Massachusetts Institute of Technology, Cambridge). Downloaded by guest on September 29, 2021