Polymer Recycling: An Overview - ACS Symposium Series (ACS

Chapter 1

Polymer Recycling:

An Overview

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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.ch001

Charles P. Rader and Richard F. Stockel

1AdvancedElastomer Systems, L.P., 388 South Main Street, Akron, OH 44311-1058 Tosoh USA, Inc., 1952 Route 22 East, Bound Brook, NJ 08805-1520

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As the twentieth century nears its end, a perceived problem facing industrialized societies is the disposal of our solid wastes (1). Thus, the U.S. generates (2) 207 million tons of municipal solid waste each year. The great majority of this waste is destined for landfills (Figure 1), with significant but much smaller amounts incinerated or recycled.

Figure 1 Disposal of municipal solid waste in USA, 1993.

0097-6156/95/0609-0002$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.

1. RADER AND STOCKEL

Polymer Recycling: An Overview

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In 1993 municipal solid waste had the composition given in Figure 2. An examination of these data finds a major fraction (paper, plastics, rubber/leather) of our solid wastes to be derived from polymeric materials of known composition. Thus, the solution of our solid waste disposal problem is intimately related to the recycle of polymers. All commercial polymers--paper, plastics, rubber -- are subject to recycling.

Figure 2 Composition of municipal solid waste in USA, 1993.

Some cardinal benefits of polymer recycling are a reduction of waste generation, less need for landfills, and a reduction in consumption of resources. Past benefits of polymer recycling have been modest, significant and progressively growing. Almost certainly, these benefits will grow markedly over the next decade. Yet, recycling is an industrial process and, therefore, subject to the constraints of technology and marketplace factors. In 1988, there were about one thousand curbside recycling programs in the U.S. Today, there are more than seven thousand such programs gathering recyclables from over 100 million people. At present, (3) approximately 34.0 percent of paper, 22.0 percent of glass, 26.1 percent of steel, 3.5 percent of plastics and 5.9 percent of rubber and leather are recycled. It is obvious by this low value for plastics recycle that there must be real economic hurdles. The many varieties of polymer fabrications are difficult to sort, clean and reformulate. These are just some of the inherent problems in polymer recycle which need improvement to lower operating costs. Lowering recyclate costs and improving their quality are ongoing problems in polymer recycling.

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


Cellulose, our most abundant polymer, is also the most recycled one, in both the absolute quantity (26.5 million tons, 1993) and percentage (34%) of paper and paper products recovered from municipal solid waste (MSW). A series of papers in this monograph will cover the logistics, technology, engineering and applications of the recycle of paper products. Though a success story in itself, this recycle of paper has generally had a somewhat lower profile than the recycle of plastics, rubber or other industrially significant polymeric materials. The recovery of polymeric materials from containers and packaging constitutes a highly significant recycle effort with polymer systems (2). A series of papers in this monograph is dedicated to the recycle of containers and packaging in contact with foods and beverages. A major polymer application area is in the automotive industry, where the consumption of plastics is increasing due to the governmental push to lower gasoline usage by reducing the overall weight of the vehicle. Compared to the recycle of metals, polymer recycle is still in its infancy. Recently, the major car manufacturers have made (4) a commitment to cooperate with their polymer suppliers to enhance the recyclability of polymers utilized in automobiles. It is estimated that about 1.9 billion pounds of polymer are consumed annually in the manufacture of motor vehicles. If the automobile is to truly become recyclable, this large stream of mixed polymers must be salvaged within reasonable economic bounds. The complexity of this recycle is apparent by virtue of the variety of materials ~ thermoset polyurethanes, filled thermoplastics and a wide variety of rubber articles ~ used in today's new motor vehicle. It is possible that in the future, the marketer of a new motor vehicle (or other assembled device) will be legally obligated (4) to take it back from the consumer when the latter has ceased to use it. This will thus motivate the automotive companies and their suppliers to design for recycle, enabling the rapid dismantling of an exhausted vehicle and the removal of its component plastic, rubber, cellulose and metal parts. It will also provide impetus for the use of fewer different rubber and plastic materials, with a maximum amount of mutual compatibility. A partial solution to the problem of polymer recycle in the automotive and appliance industries will be to increase the use of toughened polyolefins, particularly polypropylene. However, the recyclability of the specialized elastomers used in aggressive environments like under-the-hood applications poses a real technological challenge. Perhaps the standardization of a small number of different rubbers and plastics is the answer. This standardization would offer unique opportunities for the use of thermoplastic elastomers (TPEs) compatible with specific thermoplastics. These particular problems are discussed in this monograph and analyzed by scientists from a major automotive company. The recovery of materials from pneumatic tires is a unique and massive problem now receiving attention on a national scale.




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Also included in this monograph section is a provocative article stating that the first and second laws of thermodynamics dictate that only 25 percent of existing polymer waste can be recycled economically. The article asserts that the remainder of polymeric materials should be incinerated, a conclusion which many at present would consider politically unacceptable. Perhaps it is time for all responsible parties to educate the public concerning this volatile issue. Depolymerization of polymers to their monomers or other petrochemicals appears to offer only niche opportunities, such as the alcoholysis of polyethylene terephthalate (PETE). Yet, there is a significant effort, particularly in Europe, to pyrolyze commingled polymer waste to produce valued petrochemicals. The economic value of depolymerization should be viewed in the light of government requirements and subsidies, such as those in Germany in the mid-1990's. Conditions For Success Of Polymer Recycling While social and political forces can provide the motivation for polymer recycle, they cannot guarantee its ongoing success. This success can only come from satisfying a number of specific conditions, which include: 1.

Recycle must make good business sense. It must obey the laws of economics, just as science and technology must obey the laws of thermodynamics. Those willing to venture their efforts and capital on recycling must ultimately be able to generate a viable business. Quite commonly, the polymer recycler is recurringly confronted with (1) an uncertain raw material supply with gross variation in quality, (2) difficult-topredict governmental policies and regulations and (3) competitive products which are a commodity in nature and subject to broad shifts in the market place.

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Recycle must be based on good science and good technology. If it is not, it will not make good business sense.

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Its practice must be acceptable to a broad majority of consumers and voting citizens. They are the ones who pay its cost and expect to enjoy its benefits.

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It must embrace a suitable compromise of the present with the future. Any reasonable chance of its success requires that at least a part of the present be sacrificed for the benefit of the future. Further, these benefits will likely be reaped, not by those foregoing the present, but by their posterity.

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Recycle practice must be compatible with citizenry life style and population density. Thus, the practice in one community or region could be inappropriate for another. The recycling needs for New Jersey (population density 1042/square mile) will be markedly greater than those of Wyoming (population density 5/square mile). Further, these specific needs will be influenced by climate and physical geography.


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Economics Of Recycle


Key parameters of the feasibility of recycling a polymeric material are the cost and quality of the recyclate relative to that of the corresponding virgin material (VM). If the quality is competitive with that of the VM, the recyclate is then competing head-on with the VM, and price becomes of paramount importance. Polymer recyclates are generally inferior to first-quality virgin resins. They commonly compete with broad-specification virgin resins, which sell at prices 40 to 80 percent of those of the first-quality resins. Table I gives the price (5) of several high-volume polymeric recyclates versus that of the respective VMs. A viable market for recyclate requires a price no higher than that of the V M , and preferably a price significantly lower since recyclate quality can, at best, be equal to or typically somewhat less than that of the VM. TABLE I RECENT YEAR-END U.S. DOLLAR PRICES* PER POUND OF R B T V C I J C n AND VIRGIN POLYMER MATERIALS

YEAR POLYMER MATERIAL

1991

1992

1993

1994

-

-

-

0.07 0.41

Polyethylene terephthalate (PETE) Recycle Virgin

0.48 0.66

0.49 0.66

0.52 0.66

0.52 0.76

High density polyethylene (HDPE) Recycle Virgin

0.38 0.38

0.34 0.36

0.30 0.36

0.42 0.50

Low density polyethylene (LDPE) Recycle Virgin

0.30

0.28 0.39

0.29 0.37

0.34 0.53

Polystyrene (PS) Recycle Virgin

0.40 0.44

0.44 0.47

0.44 0.49

0.44 0.65

Paper pulp Recycle Virgin

"Sources: Reference 5 for PETE, HDPE, LDPE, PS. Dr. Sheryl Baldwin for paper pulp. Prices are those at the end of 1994.



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A major uncertainty for recyclate is the market price of the competitive V M . The immense price difference between recycled and virgin paper pulp indicates clearly the economic incentive for producing recycled paper. This has resulted in paper being the most widely recycled polymeric substance. During 1994, the four plastic resins charted in Table I had a 15 to 43 percent increase in the price of the VMs, giving the recyclate a 16 to 46 percent cost advantage, which could be reversed in subsequent years, based on market conditions. This is an encouraging trend for the recyclates since their direct competition is their respective VMs. It is possible, and has been experienced during business recessions, that the price of recycle resin could exceed that of the corresponding virgin material. The "wild card" in the economics of polymer recycling is the variety of mandates, regulations and economic incentives now emanating from all levels of government - local, state and federal. Though these government pressures reflect the values and desires of our citizenry, it is quite important that they be based on good science and technology, and sound economics. These pressures — aimed to foster the success of recycling - must be derived from good strategic planning and well-thought-out tactics, to produce meaningful motivation and give direction to the recycling effort. Logistics And Technology Much of the focus of the chapters in this monograph is on the technology of polymer recycle. Of comparable importance, however, are the mundane logistics surrounding the use of this technology. Figure 3 depicts the sequential steps in recycling a useful article fabricated from a macromolecular substance (6). A pronounced difficulty in any of these steps can make recycle impractical relative to some other disposal method (incineration, landfill). Virgin polymer resin Fabrlcator Process scrap Regrindlng

Consumer

Discarded article

Landfill

Incineration Collection ~ T ~ Cleaning

Separation

Figure 3 Flow diagram for polymer recycle.



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The first step in the recycling process is collection of the used article ~ be it a newspaper, beverage bottle, automobile tire or milk jug. Concurrently with and subsequent to collection is the separation of the articles by compositional similarity. The Society of the Plastics Industry (SPI) has developed (7) a simple coding system for designating the chemical nature of fabricated plastic bottles. This system is now widely used to mark for recycle a great variety of thermoplastic articles now fabricated in the U.S. A similar system for plastic and TPE automotive parts has been developed (8) by the Society of Automotive Engineers (SAE). Analogous systems for marking rubber articles are currently under development by the SAE Committee on Automotive Rubber Specifications and the International Standards Organization (ISO, Technical Committee 45). All of these codes simply designate the nature of the polymer in the fabricated article, and not its recyclability. These identification codes merely assist the recycling effort. They are not a certification of recyclability. The separation step should result in all of the articles in a given category being compositionally compatible. These parts must now be cleaned to remove impurities, and then ground into pellets for drying (to remove troublesome moisture) and subsequent refabrication. Additives (for specific improvements in performance) may be compounded into the pellets prior to refabrication. It is also not uncommon for pellets of compatible polymer systems to be blended together. The most desirable situation is for the recyclate to have properties and performance essentially those of the virgin material. This has been attained (9) for TPEs but is not common. Some thermoplastics can be recycled to give a resin suitable for direct competition with the virgin material (such as PETE in food contact uses). A more likely prospect is for the recycled materials to have properties and appearance below those of the VMs, thus destining them for lower performance ~ but still practical ~ applications. Such is the case with the different plastic lumbers now in the consumer market as a replacement for wood. Properly employed, these synthetic lumbers can compete with natural wood on a performance/cost basis. Polymer recyclate is often used in less demanding applications for the generic material. Critical to the logistics of polymer recycle are the chemistry and morphology of the discarded article. The recycle of paper is thus the recycle of an aggregate of bonded cellulose fibers, with the reuse involving the removal of ink and other non-cellulose constituents of the paper. Composite materials ~ pneumatic tires, drive belts, reinforced plastics, for example ~ pose major obstacles to their recycle, to such a degree that it is often not practical or economically viable. Thermoset articles ~ conventional vulcanized rubber, melamine plastics, phenolic resins — are far more difficult to recycle than thermoplastic materials (10). Their recycle must involve an irreversible chemical change of the material due to the necessity to cleave the crosslinks between the polymer chains. The recycle of a thermoplastic material, be it a rigid plastic or a rubber, involves a simple change of the physical state of the polymer ~ that of melting, shaping the melt and subsequently cooling it below the melting point (Tm) to enable resolidification. This




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melting/resolidification cycle is reversible and may be carried out several times (up to 5 for PETE or a TPE) with little or no significant loss in properties. Thus, a thermoplastic article is capable of repeated recycling, being limited by thermal and oxidative degradation.


Alternatives To Recycle The recycle of polymeric materials will likely never come close to 100 percent, as aptly pointed out in Stein's chapter of this monograph. Other means of disposal — incineration, landfill ~ will in many cases prove more practical and environmentally friendly than recycle. The best candidate polymers for generic resin recycle are those thermoplastic in nature, such as HDPE, PETE or a TPE. Major success stories in the recycle arena are PETE from soft drink bottles (42% recycle (2)) and high density polyethylene (HDPE) (24% recycle (2)) from milk and water jugs.) The most difficult polymeric materials to recycle are those thermoset in nature ~ especially reinforced composite materials where a rubber or plastic is reinforced with a textile cord or a metal. It is just not practical to separate the polymer matrix from the reinforcement to enable subsequent recycle of the materials. Serumgard and Eastman discuss in their chapter of this monograph the immense logistical and technological problems inherent in the proper disposal of pneumatic tires. A similar argument is valid for rubber drive belts and glass-reinforced plastics. For these difficult-to-recycle polymer systems, incineration is an appealing alternative method of disposal. Approximately 90 percent of the gas and oil consumed in the world (11) is burned to generate energy ~ thermal for warmth and mechanical for transportation. Safe, environmentally acceptable methods of incineration have been developed for most commercial polymeric materials. It offers a ready and reasonable means of disposing of the 3 to 4 billion scrap tires currently stockpiled in the U.S. The least desirable method of discarding a polymeric material is landfilling, which totally fails to harness the utility of the polymer or even its inherent energy content. The undesirability of this method will progressively increase with population density and the cost of fossil fuels. For many communities, especially those in low population areas, landfilling will, by default, remain the most practical means of waste disposal. References 1. 2. 3. 4. 5.

Rubber World, Mar. 1991; Plastics Engineering, Sept. 1990, p. 29. Franklin Associates Limited, Characterization of Municipal Solid Waste in the United States, 1994 Update, Report No. EPA 530-5-94-042, Nov. 1994. National Geographic, July 1994. Schultz, J., Wards Auto World, Dec. 1994, p. 21; Labana, S., Wards Auto World, Feb. 1995, p. 19. Plastics News, Dec. 26, 1994, p. 102.


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Purgly, E.P., Rader, C.P., and Gonzalez, E.A., Rubber and Plastics News, Aug. 31, 1992, p. 15. The SPI Resin Identification Code, Society of the Plastics Industry, Inc., Washington, D.C. SAE J 1344, Marking of Plastic Parts, Society of Automotive Engineers, Warrendale, PA. Alderson, M., and Payne, M.T., Rubber World, May 1993, p. 22. Purgly, E.P., Gonzalez, E.A., and Rader, C.P., Society ofPlastics Engineers, ANTEC '92, Detroit, Michigan, May 1992. Modern Plastics, Sept. 1990, p. 20.

RECEIVED May 2, 1995


Polymer Recycling: An Overview - ACS Symposium Series (ACS

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