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Polymer Recycling: Opportunities and Limitations RICHARD S

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Polymer Recycling: Opportunities and Limitations RICHARD S 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
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