Recycling of Rubber - American Chemical Society

Chapter 17

Recycling of Rubber

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch017

Charles P. Rader Advanced Elastomer Systems, L.P., 388 South Main Street, Akron, OH 44311-1058

The recycle of rubber lags far behind that of thermoplastics. This lag results primarily from a great preponderance of rubber articles being fabricated from thermoset materials. As a result, the most facile means of disposing of many used rubber articles, including pneumatic tires, is through incineration to generate energy. This situation will continue until new technology is generated for the fabrication of recyclable rubber articles. Promising technology toward this end is now being offered by thermoplastic elastomers -materials with properties of rubber but which can be processed as a conventional thermoplastic.

The recycle of rubber lags significantly behind that of thermoplastics. Ironically, used rubber articles have been recycled since 1853 (7). The motivation for this recycling, however, was primarily the generation of a useful rubber compounding material — reclaim — with little, if any, interest in reducing the amount of waste discarded to landfills. Since World War II, the trend to higher quality in fabricated rubber articles (especially radial tires) has generated a progressive decrease in the use of rubber reclaim, with the percentage of recycle in 1990 being approximately one-sixth that in 1941 (2). The slow pace of the recycle of rubber arises from the immense dominance of thermoset over thermoplastic rubber. Prior to the 1960's, all commercial rubber was thermoset. Today, in the mid-1990's, all pneumatic tires ~ which consume approximately 51 percent (3) of the rubber used in the world — are fabricated from thermoset rubber. The remaining 49 percent (the non-tire segment of the industry) is presently about 88 percent from thermoset rubber and 12 percent thermoplastic elastomers (TPEs). Thus, approximately 94 percent of the rubber consumed in the world is still thermoset.

0097-6156/95/0609-0196$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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In the non-tire segment, TPEs are making major inroads in replacing thermoset rubber (4). However, their prospects for replacing thermosets in pneumatic tires are quite poor (5). It is highly unlikely that any of the TPEs now commercially available will be able to penetrate significantly the tire segment of the rubber industry. Such a penetration will have to await the development of markedly novel polymer systems for use as TPEs.


Dominance of the Motor Vehicle By far the most important market for rubber articles and their recyclate is provided by the motor vehicle — in the vehicle itself and also in the roadways over which it travels. Pett, Golovoy and Labana, in a previous chapter of this volume, have summarized the impact of the automotive industry on rubber and plastics recycling. These authors provide an inside view of current recycling practice in the U.S. automotive industry and the necessary infrastructure for this recycling in both today's and tomorrow's marketplace. Approximately 75 percent of the materials in today's on-the-road motor vehicle are now recycled, with the polymeric materials — plastics, rubber, textiles, etc. — being greatly eclipsed by the massive quantity of metals recovered. Much stress is placed on the needed logistics and technology for recovering value from the materials in a discarded motor vehicle. Pett, Golovoy and Labana give a most interesting view of the effect of materials recovery and recycling on the design and materials to be used in tomorrow's automobile. An ongoing and perplexing problem in automotive recycle is the recovery of value from the 130-140 pounds of rubber in a modem U.S. car. The proper reuse of this automotive rubber is a major challenge for today's technologists and entrepreneurs. Serumgard and Eastman in their chapter give an update of the massive problem of scrap tire recycle, asserting much reason why today's stockpile of approximately 3 billion scrap tires should largely disappear by the turn of the century. This disappearance will result primarily from incineration to recover the approximately 300,000 BTUs of energy in the average scrap tire, and not from recycle. The recycle of the rubber from a pneumatic tire is just not practical when one considers it as a highly engineered composite of several different compounds reinforced with both textile fiber and steel wire. Just as the motor vehicle provides direction to the rubber recycling effort, so does the road pavement on which the vehicle travels. In his chapter, Smith describes in detail the utilization of the 140 pounds of scrap rubber from a 2400 pound automobile, as a toughening additive in highway asphalt. Fostered by heavy support from the U.S. Department of Transportation, this use of recycled rubber is growing rapidly and will so continue for the rest of the decade. It is safe to conclude that whatever future course the recycle of rubber takes, it will be deeply impacted by the automotive and highway construction industries.

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


Logistics and Technology The required technology for recycling rubber articles (of which the great majority are thermoset) is much more formidable than that for thermoplastics. On the other hand, the logistics are quite comparable. In a motor vehicle, household appliance or other assembled device, disassembly is required to obtain component rubber and plastic parts for recycle. A rapidly growing trend (6) is the design of these devices for quick, economical disassembly. The rubber, plastic and metal parts from this disassembly must then be separated, cleaned and segregated on the basis of their chemical composition and mutual compatibility. The recycle of thermoplastics (and TPEs) simply requires melting and refabrication by molding or extrusion. This recycle involves only reversible physical change (7). The recycle of a thermoset rubber requires an irreversible chemical change, that of devulcanization to remove the chemical crosslinks between the polymer chains. This gives rise to a massive technological barrier to the recycle of the massive amount of fabricated rubber which is thermoset. The most difficult thermoset rubber articles to recycle are those reinforced with a steel wire, textile fiber or both. In this category are pneumatic tires, drive belts, hose and a variety of fabric-reinforced articles. In the past, these articles could be treated chemically to generate reclaim for subsequent use as a compounding agent. The demise of the reclaim industry has rendered impractical all attempts to recycle reinforced rubber articles. Thus, the most practical use of these articles is as a high energy fuel in heat generation, as detailed by Serumgard and Eastman. This end use is still vastly superior to disposal in a landfill. Thermoset rubber articles loaded with carbon black are only slightly easier to recycle than articles reinforced with metal wire or textile fibers. In this case, it is easier to reduce the size of the rubber pieces by cutting, grinding, etc. The chips, pellets or particles thus obtained are still pieces of vulcanized rubber to be used as such (e.g. in asphalt) or treated chemically to cleave the chemical crosslinks. In the case of most vulcanizates, chemical treatment will involve the cleavage of relatively weak C-S and S-S chemical bonds. Warner's chapter in this section provides a complete, up-to-date summary of the chemistry of cleaving these crosslinks to effect devulcanization of thermoset rubber, which can be done by either chemical reagents, electromagnetic radiation or ultrasonics. These methods are primarily of interest in the analysis of rubber vulcanizates, with some ancillary importance in the commercial recycle of thermoset rubber. The most promising technology for recycling rubber parts is through the use of TPEs (8). Payne's chapter in this section points out in detail the recycling advantages of TPEs over conventional thermoset rubbers. Since a TPE is a material with the properties of a rubber and the processability of a thermoplastic, a rubber article fabricated from a TPE can be recycled in essentially the same manner as a conventional thermoplastic. Thus, the recycle of a TPE part involves, 1) collection of parts from the disassembled device, 2) separation of these parts into a mutually compatible group, 3) cleaning of these parts to remove all foreign matter, 4) grinding them into processable pellets, and 5) refabricating the pellets into a useful



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rubber article. This recyclability of TPE parts has recently been demonstrated (9) with rack and pinion steering gear boots from motor vehicles in service for more than five years. Multiple recycle of the same TPE material should be both possible and practical, since the physical properties are essentially unchanged after repeated recycles (10). This multiple recycle of TPEs thus opens up a vista of recycling opportunities. The recycle of automotive rubber parts from TPEs is so similar to that of plastic parts that the Society of Automotive Engineers (SAE) has classified (77) commercial TPE materials generically to enable their segregation into mutually compatible categories for recycle purposes. In this classification scheme, the TPEs are categorized in the same manner as that used for rigid thermoplastics such as polypropylene (PP) and polystyrene. Uses for Rubber Recyclate In today's economy and surroundings, the key reason for recycling rubber articles is to prevent their discard to the environment. For this effort to make good business sense, it is imperative that suitable utility be obtained from the used rubber parts. In many cases ~ pneumatic tires, hose, belting ~ the best use is often incineration, to produce energy which can have a multitude of uses. A quite promising application of used rubber parts is grinding and subsequent reuse of the particulate rubber as a filler in lower-performance thermoset rubber articles or as a toughening agent in asphalt paving. There is still limited use of devulcanized rubber as reclaim, however, the prospects of reclaim as an outlet for recycling are limited, at best. Beck and Klingensmith in their chapter of this volume review the pyrolysis of pneumatic tires to generate a carbon black char for non-demanding uses in rubber compounding. This filler application could replace lower-performance carbon blacks and eliminate the need for discarding to a landfill the rubber articles used as raw material. Perhaps the brightest star on the rubber recycling horizon is TPEs. They are the one hope of recycling rubber articles to a performance level comparable to the virgin rubber part, if proper care is taken during the logistics of recycle. A more practical scenario for TPE recycle is combining different compatible TPEs with each other or with compatible thermoplastics. Thus, a PP/EPDM blend TPE could be combined with a PP/EPDM vulcanizate TPE or with PP thermoplastic to give a useful rubber/PP composition — lower in performance than the original TPE rubber articles, but still useful and reducing the need for landfill disposal. This recycle of TPEs is limited to non-tire articles because these materials are not suitable for pneumatic tire applications. Thus, at least for now, the recycle advantages of TPEs are not available for pneumatic tires.


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The Future for Rubber Recycle As the next 10 to 20 years unfold, a number of positive changes should occur in the recycle of rubber articles. By the turn of the century, the size of the U.S.'s 3 billion plus stockpile of discarded pneumatic tires will be massively reduced and possibly eliminated. The trend toward designing motor vehicles, household appliances and other assembled devices for easy disassembly and subsequent recycle of parts will continue to accelerate and become full blown within the next decade. Marketers of these devices will feel increasing pressure ~ and in some cases outright governmental direction — to take the device back from the consumer at the end of its useful life (6,12). This will generate much incentive for the device manufacturer to design it for easy disassembly. The use of TPEs offers a ready answer for the recycle of many rubber parts. TPEs, now approximately 12 percent of the non-tire rubber market, should grow to about twice this percentage by the early 21 century. Their use in pneumatic tires is highly unlikely during the next decade. Thus, the disposal of scrap tires will continue to be relegated to incineration and certain low performance uses as a rubber compounding material. As the next century dawns, the reutilization of "spent" rubber articles will progressively grow and become an accepted part of our post-industrial-age lifestyle. This reutilization must, 1) be pragmatic, 2) make good business sense, 3) be based on good science and technology, and 4) be socially and politically acceptable. This will enable the future for posterity to be as worthwhile as our past has been. st

References 1.

Goodyear, C., British Patent 2933, December 16, 1853; Miller, G.W., in "Chemistry and Technology of Rubber", C.C. Davis, Ed., Reinhold Publishing Corp., New York, NY, 1937, p. 720.

2.

State of Ohio, Environmental Protection Agency, "Recycling of Used Tires in Ohio", June, 1989.

3.

Rubber Trends. 3rd Quarter 1994, The Economics Intelligence Unit, London.

4.

School, R., in "Elastomer Technology Handbook", N.P. Cheremisinoff, Ed., CRC Press, Inc., Boca Raton, FL, 1993, Chapter 15.

5.

Rader, C.P. and Walker, B.M., in "Handbook of Thermoplastic Elastomers, Second Edition", B.M. Walker and C.P. Rader, Eds., Van Nostrand Reinhold, New York, NY, 1988, p. 10.

6.

Plastics News. March 16, 1992, p. 6.



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7.

Purgly, E.P., Gonzalez, E.A. and Rader, C.P., Society of Plastics Engineers, ANTEC '92, Detroit, MI, May 5, 1992.

8.

Gonzalez, E.A., Purgly, E.P. and Rader, C.P., Presented at the 140 Meeting of the Rubber Division, American Chemical Society, October 8, 1991, Washington, D.C.

9.

Alderson, M. and Payne, M.T., Rubber World. May, 1993, p. 22.

10.

O'Connor, G.E. and Fath, M.A., Rubber World. January, 1982, p. 26.

11.

SAE J 1344, "Marking of Plastic Parts", Society of Automotive Engineers, Warrendale, PA, 1991.

12.

Schultz, J., Wards Auto World. December, 1994, p. 21.

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Recycling of Rubber - American Chemical Society

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