Plastics Recycling: An Overview - ACS Publications - American

Chapter 6

Plastics Recycling:

An Overview

David D. Cornell

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch006

Eastman Chemical Company, P.O. Box 1995, Kingsport, TN 37762

Like other businesses, plastics recycling is subject to similar pressures. Raw materials must be adequate in supply and price. Technology must convert raw materials to useful products. And the worth of products must return value to investors. Because plastics recycling requires capital investment, it must be profitable to be sustained. In the 1990's plastics recycling has thrived as public recycling of post-consumer plastics has created a plentiful, if chaotic, supply of raw material. Technologies have developed to process material as generic resins, mixed resins, regenerated small molecules, and as fuels. The business of plastics recycling, often suffering from capacity imbalances, has found uses for recycled plastics in the same products that use virgin resins. Overview The Business. Plastics recycling is a business. No longer should the activity be seen as a sideline or an avocation. For some it has always been a serious business. As a business, certain characteristics exist and requirements must be met for plastics recycling to be successful. Raw material must be available. Technology and capital investment must be sufficient. Products must bring value-addition to be successfully marketed. And each member of the value chain must create sufficient economic value for the business of plastics recycling to continue. The typical measure of business success, return on invested capital, still applies even for an environmentallyrelevant commercial activity. The chapters of this section discuss business and technical issues of plastics recycling. To its many stakeholders plastics recycling means the recovery of any used plastic items that have been divertedfromdisposal. The diverted material could be pre-consumer or post-consumer in nature. The recycling of post-consumer plastics is a relatively new activity. Unlike the secondary materials market of producer scrap, which has grown for over 50 years, recycling of post-consumer plastics as the public recognizes recycling began in the late 1970's and reached critical mass only about 1990. Uncertainty hampered the growth of post-consumer plastics recycling. Both supply of raw material, collected post-consumer items, and development of 0097-6156/95/0609-0072$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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markets have been chaotic. Even so, the recycle of used plastic botties grew from 363 million pounds in 1990 to 891 million pounds in 1993, an annual growth rate of 35%(1). Government intervention in the United States at the state and local level spurred collection of used packaging by bottle-law deposits and mandated curbside collection of household recyclables. Markets for recycled plastics developed as extensions of traditional uses for wide-specification virgin materials and by government action, such as the option for recycled content in California in 1995 for non-food containers if a recycling rate were not met. The chaotic nature of the business, which was caused, in part, by mixed motivations and uneconomic constraints of the many stakeholders, has resulted in some plastic recyclers prospering and many quitting the business. From 1991 to 1993 113 plastics recycling businesses are estimated to have gone out of business and 248 new players entered the business arena (2). The Technologies. The technology of plastics recycling can be directed at four modes of reuse of resources. Each has its economic role and none is justified as superior to the others except in value creation. Value creation here means the achieved economic and environmental benefits of recycling exceed the economic and environmental costs. As environmental benefits and costs are not commonly denominated into monetary units, value creation frequently reduces to just economic value. Recycling is a means to an end and not properly an end in itself. The purpose of the plastics recycling business is to create sustainable value within the economic context of the society in which it operates. For maximum sustainability that value should be achieved without subsidies or other artificial impositions. At the sametime,recycling is a part of solid waste management which is conducted for its own set of reasons, such as maintenance of public health. As such, some aspects of recycling, such as initial collection, are societal responsibilities and should not be subject to the need for economic value creation. The four modes of plastic recycling are as follows: 1. use as generic plastic, 2. use of mixed plastic, 3. regeneration of raw materials, and 4. use in energy recovery. While a legitimate form of recycling, energy recovery is not here discussed in detail. Energy recovery is often called quaternary recycling. Such recycling recognizes the recoverable energy content of plastics that can be captured during combustion. Polyethylene has been characterized as "natural gas in solid form". This section discusses various aspects of generic plastics recycling, mixed plastic recycling, and regeneration of raw materials by thermal methods. The Technologies - Generic Resin Recycling. Recycling to generic thermoplastic resin typifies the general perception of plastics recycling. Such recycling is also called secondary recycling. The processing begins with receipt of post-consumer items, like used bottles, and continues through cleaning, enhancement, and remelting to saleable pellets or fabrication of saleable items. The necessary steps of collection and sorting of plastic items from solid waste are not detailed in this section. The first post-consumer recycle of generic polymer to gain notice was PET. PET, polyethylene terephthalate, is used to make carbonated beverage bottles.

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


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PLASTICS, RUBBER, AND PAPER RECYCLING

Deposit laws passed in the late 1970's and early 1980's created a critical mass of potentially useful material. Entrepreneurial businesses developed to exploit the opportunities made possible by the supply of material. In 1993, 450 million pounds of post-consumer PET bottles, including 40% of all carbonated soft drink bottles, were recycled in the United States (3). By 1993 the supplyfromnon-deposit systems surpassed the supply from deposit programs. The technology used to process used bottles to saleable generic polymer has been described by the Center for Plastics Recycling Research (4). Commercial processors use that basic scheme of bottle grinding, flake cleaning, and contamination separation or variations that include whole bottle washing. PET bottle recycling has been a successful business, particularly for well capitalized recyclers, such as Wellman Incorporated or Image Industries, who have forward-integrated into value-added uses. Problems have occurred, such as the shortage since 1990 of used bottles and contamination by incompatible materials. PVC and PET are mutually incompatible. By 1995 the quality and consistency of PET recycle permitted uses in higher value applications. Recycled PET is routinely used for carpeting and apparel fiber, thermoformed films for packaging, non-food bottles, and food bottles. PET recycling provides a model for and commentary on other plastics recycling. Initial uses of recycled PET were the lower valued applications for off-specification virgin polymer. With time, investment, and development recycled PET is now used in many higher valued applications once only satisfied by virgin polymer. So long as value creation exceeds cost, the business prospers. Thermoplastics cannot be remelted indefinitely without adverse consequences to the polymer. Both condensation and addition polymers will sustain oxidative and thermal degradation upon repeated processing. Discoloration, changes in molecular weight distribution, and crosslinking may result from repeated remelting. As such, 100% closed loop recycling is not practical. Not only can repeated melting adversely affect polymer, but environmental exposure between melting can magnify the adverse affects (5). Thus, the application's life cycle exposure must be considered in any studies of the consequences of repeated processing on thermoplastics. The Technologies - Mixed Plastics Recycling. Rather than isolate generic resins, some technologists have sought to mix incompatible plastics to form commingled plastic structures. Usually with large cross sections, plastic lumbers utilize a feed mixture that is particularly rich in polyethylenes. The resultant fabrications are typically made by the partial melting of the mixed resin feed during extrusion into a mold. One common process uses a slow, low pressure extrusion to fill light-duty molds. The fabrications often exhibit a mottled surface, which can be an advantage for use in marine decking. The fabrications also can contain voids and cracks and easily warp. The American standards organization, ASTM, has organized producers and users of plastic lumber to allow for consensus definitions, guidelines, and standards to facilitate the successful commercialization of plastic lumber. Since the earliest manufacture of commingled plastic lumber, technologists have recognized that the product lacked the flexural modulus of wood and could fail too easily in tensile, shear, and impact loading. Because of the slow production rate and alternate value of the plastic feedstock, plastic lumber has been priced at a premium to wood. On a lifecycle cost basis, however, for some applications plastic lumber can be economically favored over wood.



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The Technologies - Regeneration of Raw Materials. Regeneration of raw materials is also called tertiary plastics recycling. Condensation polymers, such as PET and nylons, can be depolymerized via the reversible synthesis reactions to initial diacids and diols or diamines. Typical depolymerization reactions are alcoholysis and hydrolysis reactions. Methanolysis of PET recreates dimethyl terephthalate and ethylene glycol. Alcoholysis of PET with ethylene glycol, called glycolysis, results in the recreation of bis-hydroxyethyl terephthalate. Depolymerization of condensation polymers can be nearly stoichiometric in regeneration of initial raw materials. Both PET and nylons raw materials are commercially regenerated from waste or recycled plastic. The recovered raw materials are typically used to create new "virgin" polymer. Because recreation of small molecule raw materials creates species chemically identical to those derived from petrochemical sources, the new "virgin" polymer can be identical to traditional "virgin" polymer. Synthesis reactions for addition polymers, like polyolefins and polystyrene, are not generally reversible. For addition polymers regeneration of raw materials requires thermal depolymerization with recovery of a variable slate of small molecules at variable yields. The thermal processes to regenerate raw materials can also be used to create useful chemicals from condensation polymers, although not usually the original polymer precursors. This section includes chapters on thermal depolymerization of mixed addition and condensation polymers. Organization of the Section The chapters of this section on plastics recycling are organized in three broad categories: 1. Mechanical recycling business for generic commodity resins 2. Mechanical processing technology topics 3. Thermal processing technology for re-creation of raw materials. Mechanical Recycling Business. The first chapters on generic resin recycling address issues of unique and common experiences in recycling post-consumer packaging. Post-consumer plastics are, by experience, inhomogeneous. The recycling processor must accept highly variable feed materials and convert them to uniform generic resins. Prioleau in his chapter points out a common problem to the plastics recycling business, overcapacity. Recyclers who cannot efficiently use all of their production capacity suffer economically. Atkins agrees with the need for more used plastic. In each of their chapters Prioleau for polypropylene, Thompson for polystyrene, and Burnett for PVC all comment on the importance of durable goods as sources of used plastics. Thompson and Burnett also cite the long standing practice of reprocessing pre-consumer scrap. Prioleau also points out two key problems of operating a plastics recycling business: achieving high productivity in bottle grinding and dealing with non-uniform, low bulk density flake. Product drying and pneumatic conveying are made more difficult by the variable nature of the chopped plastic. Atkins and Thompson discuss the use of recycled plastics back into food contact applications. Food contact reuse for used polyethylene food packaging is anticipated; used


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polystyrene can for certain situations be reused in food contact applications. Thompson discusses the unique problems of dealing with low density polystyrene foam. Atkins sounds common themes on the need for responsible packaging design for recyclability and the need for householder cooperation in supplying bottles and avoiding adding inappropriate materials to the recycling stream. Mechanical Processing Technology. The chapters on mechanical processing technology address four questions: 1. What is the nature of commingled, mixed plastic fabrications? 2. What can be done in melt processing to affect the properties of blends of incompatible recycled resins? The melt processing may be with or without compatibilization. 3. How can processing technology deal with networks of crosslinked polymers? Thermoset plastics are rarely recycled as polymers, but frequently as ground filler. 4. How can processing technology allow reuse of food packaging for food packaging again? Section V of this book will more hilly develop this topic. The performance properties of mixed plastic fabrications are highly dependent on the feedstocks used, the preparation of the feedstock, and the processing. The many performance negatives of the product are related to the lack of interfacial transfer of energy between incompatible phases or domains. Because the domains are very large, testing must be conducted on whole items and not typical test coupons. While incompatible resins, like PET and HDPE, are separated efficiently in generic polymer processing, interest is high in being able to combine the two most prevalent bottle resins, PET and HDPE. Simply mixed, PET and HDPE form a weak, low performance mixture. In their chapter Bhakkad and Jabarin explore the consequences of compatibilization and processing variables on PET/HDPE blends. Blends with 80% PET and varying amounts of HDPE and a maleic anhydride-grafted polyolefin show morphology changes due to melt temperature and extruder screw speed. The size of the discontinuous polyolefinic phase affects physical properties and depends on composition of the phase and screw speed. The beneficial effects of the compatibilizer are visually portrayed in adhesion to the PET phase. Improved impact resistance is noted with just the addition of the compatibilizing grafted polyolefin. The work suggests potential for extrusion blowmoldable blends based on the high melt viscosity of the ternary polymer blend. When separation of generic resin items is not practical, the processing needs to convert mixed resins to useful material. Unlike plastic lumber which is usually uncompatibilized, compatibilized mixtures allow for stress transfer across domain boundaries which results in useful physical properties. Even though the most plentiful resin from post-consumer packaging will be a polyethylene, the mixture can contain virtually any thermoplastic plus organic and inorganic non-melting constituents. The business problem is to find compatibilizing systems that result in useful properties at low cost. Because polyethylene dominates the mixture, the compatibilized blend will at best have the relative value of polyethylene. Thus, expensive compatibilization will not be commercialized.



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Thermoset resins do not lend themselves to easy recycle. Thermoset plastics have been ground and used as low-cost, low-performance fillers. Ideally, the polymeric network could be "unzipped" to recover monomelic species. While thermal degradation can create small moleculesfromthe networks, those species are not the original monomers. A value-added process would recover monomers efficiently from crosslinked polymers. In their chapter Shiu et al discuss one route to decompose thermoset polyurethane. In a special case for which thermal or hydrolytic depolymerization is inappropriate, Shiu shows that the application of surfactants, solvent swelling, and ultrasonic irradiation can decompose a polyurethane matrix. Solvent swelling alone is inadequate to break the matrix. The addition of oxidants with the surfactants accelerates the polymer degradation under sonication. The degraded matrix material is then available to recover, purify, and reuse. Reuse of recycled packaging resins back into food packaging represents a major technical and quality accomplishment. The widest use of recycled post-consumer plastic to food contact applications is for PET. In their chapter Bayer et al discuss the growing options to use recycled PET in food contact applications. PET can be cleaned readily for non-food contact applications by standard washing techniques. Additional cleaning techniques for flaked, used bottles can remove much contamination. Use of high temperature and vacuum, as is used to dry PET before melting or to solid-state polymerize PET to higher molecular weight, also removes contamination. Recycled PET can be used in food contact by relying on a barrier layer of virgin PET to prevent contaminants from reaching food at significant amounts. And tertiary recycling of PET through various depolymerization /repolymerization methods is used to allow recycled PET to safely be used for foodcontact applications. Bayer discusses the "threshold of regulation" theory of assuring food safety for indirect food additives, such as packaging resins. Thermal Processing Technology. The chapters on thermal processing to recover raw materials focus on primarily addition polymers in a mix of discarded plastics. The technologies available are discussed. Dealing with highly inhomogeneous materials is a specifically difficult problem. Feeding and slagging reactors are common operational problems. Mackey in his chapter summarizes the forms of chemical and thermal depolymerizations. Thermal depolymerization can be conducted with or without the presence of oxygen or in the presence of hydrogen to produce primarily liquid or gaseous products. Mackey sets the stage for the chapters by Meszaros and Pearson on specific technologies. Mackey points out that a practical limit to mechanical recycling exists and is probably in the 10 to 15% range. Beyond that amount, plastics recycling will require gross treatment, like thermal decomposition. But, as a system to deal with solid waste, thermal depolymerization is less economical than other solid waste management options. Gasification, as discussed by Pearson, does potentially produce a higher valued synthesis gas product, but overall economics are still unattractive. In his chapter Meszaros focuses on a parametric study of plastics thermolysis in an auger kiln to produce liquid naphtha, solid carbonaceous material, and noncondensible gaseous products. The test facility, located at Conrad Industries,



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regularly pyrolizes used auto tires. The study found the product species distribution depends on process temperature, feed rate, reaction residence time and feed type. PET was found to sublime to terephthalic acid which precipitated in the product lines. The study also showed that the process using post-consumer MSW plastics was uneconomical without a substantial subsidy. More work was suggested to better understand the effects of residence times. Meszaros suggests circumstances may exist for which generation of the product slate from used plastics could be economically acceptable. Pearson et al in their chapter studied an approach to deal with highly heterogeneous polymeric feed with a spouted bed reactor. Their approach utilizes an ablative gasification in which high temperature steam is the fluidizing media. In the reducing atmosphere created, synthesis gas (hydrogen and carbon monoxide) are produced. The process was able to handle roughly shredded feed fed uniformly and in slugs. The process is expected to be cheaper than hazardous waste incineration, but still costly vs other solid waste management technologies. Pearson suggests further work is needed to continuously slag the inert solidsfromthe reactor. Final Comment Will plastics recycling ever essentially replace virgin polymer production? While some observers may consider the total replacement of virgin polymer production by recycling to be the desired final state, the thermodynamics dictate otherwise. Recycled polymers will either become exhausted or be expended as yields are less than 100%. Chapters in this section discuss property maintenance for generic plastics recycling and process yields for raw material recovery. This overview suggests technology can recycle post-consumer plastics to valueadded applications. The economics of post-consumer plastics recycling is dependent on the margin spread between the sales price of product and the cost of purchased raw materials. Characteristically, the margin for post-consumer plastics recycling has always been thin. In 1994 and 1995 the observer might think the plastics recycling business were great as selling prices of generic recycled polymer rose to high levels. The reality has been a troubled industry hamstrung by shortages of raw material and high raw material prices that squeeze margins. To address its economic problems the plastics recycling industry has always looked for vertical integration to secure less costly feedstocks and sell higher valued products. Another route to satisfactory economics has been size of capacity. Scale of operation is becoming ever more important as small producers, those selling less than 10,000,000 annual pounds of recycled plastic, will find themselves at distinct disadvantage. As the industry replaces the expensive labor component with capital, more automation and capital investment makes small operations non-competitive. But then the limitations of raw material supply become more critial. Thus, the investor must balance the business among supply of raw material, economic size for the chosen technology, the quality requirements of the sales product, and the economic value of the sales product.


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Literature Cited 1. American Plastics Council, "Leaving a Lighter Footprint, Plastics Makes it Possible", Washington, DC, 1994 2. R.W. Beck and Associates, private communication 3. R.W. Beck and Associates, "1993 National Post-Consumer Plastics Recycling Rate Study", Orlando, Florida; June, 1994 4. Rankin, S., Plastic Beverage Bottle Reclamation Process, Center for Plastics Recycling Research, Rutgers University, New Brunswick, New Jersey 5. Boldizar A., "Simulated Recycling-Repeated Processing and Ageing of LDPE", Swedish National Testing and Research Institute, in R'95 Congress Proceedings, EMPA, Dubendorf, Switzerland, 1995 RECEIVED July 26,

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Plastics Recycling: An Overview - ACS Publications - American

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