The recycle of plastics and rubber - a contrast.
Recover, recycle, reuse - these are buzz words we have all heard with growing intensity over the past two decades. Virtually everyone is in favor of reusing our plastic and rubber waste; yet only a small minority really has a good perception of the needs and benefits of recycling. Our general direction toward recovering value from used rubber and plastic articles is proper. However, some of our facts and perceptions have been incorrect, resulting in much wasted effort. Yet it is highly apparent that recycling is here to stay and the days of pitch and forget are gone forever.
To avoid confusion, it is appropriate that we first define the terms recovery, recycle (or recycling) and reuse as they are currently used in the rapidly growing field of generating utility from waste plastic and rubber articles:
By recovery we mean the creation of value or usefulness from discarded post-consumer material which otherwise would require disposal as waste (usually to a landfill). This value could accrue from
* primary recycling, the material being in a finer form (ie., chopped or ground);
* secondary recycling, reshaping through molding or extrusion;
* tertiary recycling, chemical decomposition into simpler molecular species (depolymerization of a plastic or rubber); or,
* incineration to yield energy.
By recycle (or recycling) we mean recovery, other than incineration, to give a material more finely divided, reshaped or chemically decomposed into low molecular weight materials.
Finally, reuse is simply a general synonym for recovery.
Process scrap from plastics and rubber fabrication is generally called regrind. It does not qualify as post-consumer waste and is outside the scope of this article.
Over the past two decades, rubber and plastics recycle has made steady progress. Yet, its emphasis has sometimes been on-target, and sometimes off-target (refs. 1 and 2). The accuracy of this emphasis is clearly evidenced by technological and commercial successes in recovering value from spent plastic and rubber articles. Prime examples of these successes are:
* the recycle of PET (polyethylene terephthalate) from soft drink bottles (ref. 3);
* the recycle of HDPE (high density polyethylene) from milk and detergent containers (ref. 4); and
* the incineration of pneumatic tires to provide the needed energy for portland cement manufacture (ref. 5). The lack of accuracy in this emphasis maybe found in the sparse successes to date in recycling copolymers and polystyrene (PS).
The recycling of rubber articles - especially pneumatic tires - has also received emphasis from our hi-tech society; however, the practical response to this emphasis has been markedly different due to the different chemistry, morphology and fabrication of rubber, in contrast to plastics. This article explores the recovery of fabricated rubber articles and contrasts it to that of fabricated plastic articles. This contrast is analyzed in terms of public and political demands, economics, mundane logistical factors and available technology.
The demands on recycling
Immense public and political pressure has been exerted to bring about the recycle of our waste materials. This has resulted in numerous pieces of legislation at the local, state and federal level to promote and, in some cases, force the recycling of polymeric materials. Each of our 50 states has enacted (ref. 6) legislation mandating recycling goals at some future time. In some cases, legislation has generated (ref. 7) major secondary problems such as the great excess of recycled plastic resins in Europe. Further, responsible voices (ref. 8) have begun to speak out against devoting major resources toward plastics recycling.
Regardless of the public pressure exerted or legislation enacted to promote it, there are a number of requirements which rubber and plastics recycling must satisfy if it is to be practiced successfully for an extended period of time. A broad majority of consumers and voting citizens must have the philosophical willingness to sacrifice something today - in the form of personal inconvenience, higher prices, etc. - to obtain certain benefits in the future - a clean, sustainable environment with resources capable of regeneration. We must postpone and reduce consumption today so that those of tomorrow may have something to consume and enjoy. We must also be willing to invest in recycling today to insure our resources of the future will not be depleted.
The success of recycling will also depend on the readiness of our citizens to accept a contraction of their lifestyles. This readiness will vary with population density, physical geography and climate. The recycling needs of Wyoming (five people/ square mile) will differ markedly from those of New Jersey (1,042 people/square mile), and the needs of Bismarck, North Dakota (latitude 47 [degrees] N, near the center of the continent) will differ from those of Miami, Florida (latitude 26 [degrees] N, seashore).
A basic requirement of rubber and plastics recycling is that it be commercially viable and based on sound science and technology. Those willing to venture in this area must be able to ultimately build an on-going business. The recycling entrepreneur must cope with:
* competitive virgin materials which are a commodity in nature and subject to major price variations;
* a raw material supply uncertain in both quantity and quality; and
* governmental policies and regulations most difficult to predict or anticipate.
The laws of economics will be obeyed by those in the recycling business. A recovery product from a rubber or plastic - be it a PET resin, polymer building block, rubber reclaim or energy from scrap tire incineration - must compete directly with that product from virgin resources. Further, the quality of the recyclate will not exceed that of the virgin competitor and likely be inferior to it. Yet, the recyclate must give cost/performance value in the marketplace. Typically, the most direct competition for a recyclate resin is a broad-specification or off-specification resin selling for 40 to 80% of the price of a first quality virgin resin.
Figures 1 and 2 give the price fluctuation (ref. 9) of two thermoplastic recyclates vs. that of virgin resin competitors. The price difference between virgin and recyclate PET makes it apparent why this resin is the shining star of thermoplastic recycling, with the recyclate price being 19 to 29% below that of the virgin material. With PS, on the other hand, the price differential is clearly more uncertain, ranging from 7% above to 44% below the virgin resin. The large differential for the end of 1995 could easily disappear in future years. These economics plus the ready availability of a reliable supply of waste PET articles (soft drink bottles) provide an insight into the commercial success of PET recycle over that of PS.
[Figures 1-2 ILLUSTRATION OMITTED]
Logistics of plastics and rubber recycling
The recycling of rubber and plastics can be divided into logistics and technology. The logistics of recycling are those mundane steps necessary for the process to take place. These steps include collection, separation, segregation, cleaning, grinding and drying. There are essentially no technological barriers to carrying out each of these steps, provided the value added by the recovery operation can justify its cost. It is assumed that the needed technology for each step is readily available and practicable. If it is not, the value added by the recycle/recovery process will decrease.
Figure 3 gives the hierarchy of decreasing desirability of the disposal of a post-consumer rubber or plastic article. The least desirable disposal method is discarding the article (or material). This is a situation where no value is added to the waste material. To be more precise, the value added is negative because of the implicit cost in 1) transporting the article or material to the landfill, and 2) establishing and maintaining the landfill to satisfy environmental requirements. The most desirable recover method is the regeneration of the polymeric material with no chemical change or contamination. This is normally achieved through chopping, grinding or melting with subsequent resolidification. The properties of this recovered material may then be expected to approach those of the article from which it was derived.
[Figure 3 ILLUSTRATION OMITTED]
Collection of post-consumer articles may occur in a variety of ways. Perhaps the most facile method is curbside collection in which the consumers segregate the waste materials by compositions - with plastics kept separate from metals and metals from paper, etc. This is a defacto subsidization of the recycle process by the consumer, who is supplying low-skilled labor. The last decade has initiated (refs. 2 and 10) massive growth in collection programs for rubber and plastic parts, as the data in table 1 clearly show.
Table 1 - growth in polymer recycle in U.S.A. between 1985 and 1992 Years 1985 1992 Collection programs 0 7000 Materials recover facilitation 0 234 Amount PETE recycled, million pounds 100 400 Amount HDPE recycled, million pounds 25 420
Many post-consumer rubber and plastic articles must be removed from an assembled device such as an automobile or household appliance. In many cases, the effort needed for disassembly is simply not justified, with the article being discarded with the assembled device. A relatively recent trend (ref. 11) is "design for recycle," with the assembled device designed for easy, economical disassembly. In Germany (ref. 12), governmental pressure is already being exerted to force design-for-recycle on auto makers.
If the waste articles have not been separated by the consumer, this step must be done in a waste treatment facility. Significant hand labor is required in several of the steps in a typical treatment facility. A modern waste treatment facility will commonly retrieve useful materials - PET, HDPE, paper, ferrous metals, aluminum - from approximately one-third of the garbage from a typical household. The remaining two-thirds - food scraps, composite articles, small assembled devices, etc. - must be disposed of in an approved landfill.
To facilitate separation and subsequent segregation of plastic articles, elaborate marking and identification systems have been developed by SAE (ref. 13) (Society of Automotive Engineers), SPI (ref. 14) (Society of the Plastics Industry) and ISO (ref. 15) (International Standards Organization). For rubber articles, similar systems are currently under development by SAE (Committee on Automotive Rubber Specification) and ISO (Technical Committee 45).
Once the articles are separated by composition, it is important they remain segregated for subsequent processing. Thermoplastic articles are commonly cleaned, ground and then refabricated, by molding or extrusion. The generation of a quality recyclate thermoplastic demands that in its "first life" the resin has not undergone significant degradation (by oxidation, pyrolysis, depolymerization or other chemical attack) or absorbed a significant amount of foreign fluid (solvent, oil, grease, etc.).
The logistics of recycling plastics are quite similar to those for rubber articles. However, a massive difference arises between the technology with which plastic articles and rubber articles will be reprocessed to generate value in the marketplace.
Technology of plastics and rubber processing
The basic underlying cause for plastics being more readily recyclable than rubber lies in the majority of the former being thermoplastic in nature; whereas the great preponderance of the latter is thermoses. In the U.S. (ref. 16), 90% of the plastics produced are thermoplastic - PET, polypropylene (PP), polyethylenes (PE), polyvinyl chloride (PVC), PS, etc. - with the remaining 10% being thermoses - phenol-formaldehyde, urea-formaldehyde, melamine, etc. In marked contrast, 94% of the rubber fabricated in the U.S. (including all pneumatic tires) is thermoses and 6% thermoplastic (ref. 17), though the percentage of the latter is growing rapidly. Further, virtually all of the activity (and success) in plastics recycle is with thermoplastics.
Thermoplastic polymers - be they addition or condensation in nature - are straight chain with distinct melting (or softening) points, and their fabrication is reversible. On the other hand, thermoses polymers have a three-dimensional network and their fabrication is irreversible. Thus the recycle of a thermoplastic or a thermoplastic elastomer (TPE) simply involves a reversible physical change - heating the resin above its melting point ([T.sub.m]), shaping it and then cooling it below [T.sub.m] to obtain the desired recycled article. In contrast, the task of recycling a thermoses rubber (or plastic) is a far more formidable one. The three-dimensional network of the polymer system must be broken down through the cleavage of primary chemical bonds to effect an irreversible change with the resulting material being chemically different from the starting one.
Thus the easy recycle of a thermoplastic material embraces a reversible physical change, that of melting and subsequent resolidification, through which the material can theoretically pass many times. To the contrary, the recycle of a thermoses material embraces an irreversible chemical change which permanently alters the material.
This irreversibility has made recycling of conventional thermoses rubber parts quite difficult. Since 1853, used rubber parts have been recycled (ref. 18) by chemical treatment to give reclaim, a widely used rubber compounding ingredient. Between 1941 and 1985, the usage of reclaim in rubber compounding decreased (ref. 19) from 32 to 5% of new rubber, thus exacerbating the problem of recycling thermoses rubber.
Polymer composites of either rubber or plastics are especially difficult to recycle, due to the complexity of separating the component materials. Though composites of plastics are widely used, those of rubber are much more so. A sizable weight majority of rubber articles now in use - pneumatic tires, belting, hose - consists of one or more rubber compounds reinforced with steel wire and/or textile fiber. The recycle of these articles had proven most difficult. Consequently, a multi-billion stockpile of used tires has accumulated in the U.S., with a crash program now in place (ref. 5) to reduce this stockpile by finding practical uses for these discarded tires. The most promising uses to date (ref. 20) have been;
* incineration (especially in Portland cement plants) to give approximately 300,000 BTUs of heat energy per tire;
* grinding and subsequent addition to asphalt in road paving; and
* shredding and subsequent use in automotive padding and cushions.
The practical recycle of a thermoplastic is thus limited primarily by logistical factors since the technology for its refabrication is both well-established and economical. A thermoplastic recyclate competes directly with the virgin material from which it is derived. Its success in the marketplace will depend directly upon the performance/cost it can deliver relative to that of the virgin material.
In contrast, the recycle of a thermoses rubber is greatly limited by available technology, in addition to logistics. Because this recycle involves irreversible chemical change, the lack of technology will likely continue indefinitely into the future. A prudent focus for this recycling will be to optimize the added value of the thermoses material and/or composite in a rubber article. In the immediate future, this value will come from shredded rubber padding, ground rubber additives, rubber reclaim or incineration for energy generation. All of these uses are more desirable and environmentally compatible than disposal in a landfill.
The promise of TPE recycling
The recycling capability of rubber articles fabricated from TPEs offers a major opportunity for improving the value of these articles after they have given a normal lifetime of service. Unfortunately, TPEs are not suitable for pneumatic tires, which consume approximately one-half of the rubber polymer produced. In the non-tire half of rubber usage, they have broad versatility and have already penetrated virtually all market segments. In 1994, TPEs enjoyed approximately 12% (350,000 metric tons in the U.S.) of the non-tire rubber products market. This share should grow to 20-25% by the end of the next decade (ref. 17).
One of the contributors to this growth of TPEs is their recyclability. Their suitability for refabrication - either as process scrap (regrind) or post-consumer articles - of TPEs is supported by figure 4, which compares the physical properties of a thermoplastic vulcanizate (TPV) TPE after 0, 1, 3 and 5 remoldings (ref. 21).
[Figure 4 ILLUSTRATION OMITTED]
The recycling of a used TPE article is essentially that of a rigid thermoplastic and not that of a thermoses rubber. It is thus subject to the ease, efficiency and economy of thermoplastic processing. This is reflected in many commercial TPEs having recycle classifications in plastics standards documents (ref. 13) and having broad compatibility for recycle. Thus PP-based TPEs - be they a TPV or thermoplastic elastomeric olefin (TEO) (ref. 22) - can be combined with each other and with PP for purposes of recycle.
The practicability of recycling TPEs was shown in a recent study (ref. 23) in which convoluted rack-and-pinion automotive steering gear boots blow molded from a TPV (EPDM/PP, highly crosslinked EPDM phase) were recycled to give properties 80% or more of that of the virgin TPV material. Moreover, dry blending of the recycled TPV with virgin TEO material (EPDM/PP, little or no crosslinking of EPDM phase) was found to enhance the properties of the TEO, especially oil resistance and compression set.
The steering gear boots were selected because:
* They had been in service for more than five years.
* This service was in a demanding environment (automotive, under-the-hood, in direct contact with grease).
* The dismantling and recovery of the parts was logistically feasible, from a plant where the boots were stripped and discarded prior to gear remanufacture.
The logistics of the recycling involve the following steps:
* Disassembly from the gear system.
* Cleaning of the grease laden boots, with high pressure steam and hot water.
* Grinding, 400 rpm, three rotating blades over two fixed ones, to give 1/3 inch particle size.
* Blending with TEO, where applicable.
* Predying, 80 [degree] C (175 [degree] F), three hours, circulating oven.
* Injection molding, thermoplastic equipment and conditions.
From standard injection molded plaques, test specimens were die cut for property testing.
Figures 5 and 6 give the elongation at break and ultimate tensile strength, respectively, of the virgin TPV, recycled TPV and virgin TEO, unaged, air aged and aged in oil. In all cases the tensile strength and ultimate elongation of the recyclate are within 20% of that of the virgin material, after more than five years of useful life of the TPV.
[Figures 5-6 ILLUSTRATION OMITTED]
Two critical performance parameters of a TPV are oil resistance and compression set. The high level of rubber-phase (EPDM) crosslinking renders these properties markedly superior to those of a TEO and competitive with a conventional thermoses rubber. Figure 7 gives a direct comparison of the weight gain resulting from total immersion of the virgin TPV, TPV recyclate and virgin TEO in oil. The recycled TPV is at least equivalent in oil resistance ton's virgin counterpart. Both TPVs are vastly superior to the TEO, as expected. The compression set (figure 8) of the recycled TPV was found equivalent to that of the virgin TPV and superior to that of the TEO.
[Figure 7 ILLUSTRATION OMITTED]
The data in figures 5 through 8 are typical of a database illustrating the recyclability of TPVs to give performance and properties competitive with those of the virgin TPV and with thermoses rubber, which is grossly incapable of such recycle. It is highly probable that other generic classes of TPEs (ref. 23) can be recycled in a manner similar to the TPV in this example. Thus TPEs offer a most fascinating opportunity to further extend practical recycling into the non-tire area of the rubber products market.
The future of plastics and rubber recycling
Beyond question, the recovery and recycling of post-consumer plastics and rubber articles will continue to grow in both size and diversity. This trend is clearly set. Open to much debate, however, is the precise direction this trend will take, since it will be influenced by a number of parameters too difficult to predict from today's vantage point.
One of the parameters is the cost of fossil fuels. As the price of coal and petroleum increase, the cost of the virgin resins competing with recyclate will go up, thus improving the latter's performance/cost competitive position. Come 2005, we do not know whether the price of petroleum will be closer to $20 U.S. per barrel or $40, though the former appears more likely.
A second parameter is the public support for protecting the environment as our population continues to grow. Increasing population density will increase the need to recycle and the public pressure for its actualization. This need and pressure will also increase with the standard of living of numerous nations throughout the world.
The greater ease of recovering value from used thermo-plastic materials will markedly enhance their market position over competitive thermoses materials. Thus the use of thermoplastics will exceed that of thermoses plastics, by a margin greater than the current 90:10 thermoplastic:thermoset ratio. In the non-tire rubber products area, this situation provides TPEs with a most promising growth opportunity, one which will come at the expense of thermoses rubber. A similar situation in the tire area must await the discovery and exploration of novel polymer systems.
Stein (ref. 24) has stressed the existence of a limit to the practical recycle of polymer resins. The rules of the marketplace will ultimately determine where this limit will be, though government legislation and regulation will fine-tune its position.
One clear conclusion will continue to survive, however. Recycling and recovery of post-consumer rubber and plastics are here to stay.
[1.] C.P. Rader, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K. Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D.C., 1995, Chapter 1.
[2.] R.G. Saba and W.E. Pearson, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K. Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D.C., 1995, Chapter 2.
[3.] R.D. Leaversuch, Modern Plastics, July, 1994, p. 48.
[4.] W.K. Atkins, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D.C., 1995, Chapter 10.
[5.] J.R. Serumgard and A.L. Eastman, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K. Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D. C., 1995, Chapter 20.
[6.] Plastics News, January 16, 1995, p. 3.
[7.] Plastics News, March 20, 1995, p. 1.
[8.] R. King, Plastics News, March 13, 1995, p. 12. 9. Plastics News, December 25, 1995, p. 130.
[10.] "Characterization of municipal solid waste in the United States, 1994 update, " Franklin Associates Ltd., Prairie Village, Kansas, 1994
[11.] S. Labana, Wards Auto World, February 1995, p. 19.
[12.] G. Schultz Wards Auto World, December 1994, p. 21; Plastic News, September 7, 1992, p. 10.
[13.] SAE J1344, "Marking of plastic parts," Society of Automotive Engineers, Warrendale, PA.
[14.] "The SPI resin identification code," Society of the Plastics Industry, Inc.
[15.] ISO 11469, "Plastics - generic identification and marking of plastic products," International Standards Organization, Geneva, 1993.
[16.] Plastics World, January 1995, p. 2.
[17.] R. School, in "Elastomer technology handbook" N.P. Cheremisinoff, Ed., CRC Press, Boca Raton, 1993, Chapter 15.
[18.] C. Goodyear, British Patent 2933, December 16, 1853; G.W. Miller in "Chemistry and technology of rubber," C.C. Davis, Ed., Reinhold Publishing Corp., New York, 1937, p. 720.
[19.] Ohio EPA, "Recycling of used tires in Ohio," June, 1989.
[20.] F.G. Smith, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D.C., 1995, Chapter 18.
[21.] G.E. O'Connor and M.A. Fath, Rubber World, December 1981, p. 25.
[22.] M.T. Payne and C.P. Rader, in "Elastomer technology handbook," N.P. Cheremisinoff, Ed., CRC Press, Boca Raton, 1993, Chapter 14.
[23.] M. Alderson and M.T. Payne, Rubber World, May 1993, p.22.
[24.] R.S. Stein, in "Polymer recycle - a pragmatic perspective," C.P. Rader, D.D. Cornell, S.D. Baldwin, G.D. Sadler and R.K Stockel, Eds., ACS Symposium Series, American Chemical Society, Washington, D.C., 1995, Chapter 3.
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|Author:||Lemieux, Marvin A.|
|Date:||May 1, 1997|
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