Many approaches have been tried, but tire-derived fuel remains the largest single market for scrap tires in the U.S.
TDF accounts for more than half of the 299 million scrap tires disposed across the country in 2005, according to the latest Scrap Tire Report from the Rubber Manufacturers Association.
When the RMA releases an updated Scrap Tire Report later this year, TDF is expected to remain the dominant method of reusing scrap tires in the U.S. However, other markets for scrap rubber-some that encompass post-industrial scrap as well as post-consumer rubber-continue, some of them gaining significant market share. These include:
Civil engineering. Since the early 1990s, scrap tire chips of two to 12 inches have found excellent use in such applications as lightweight fill for highway embankments, drainage layers for landfills and aggregate for septic tank leach fields. A couple of early incidents in which tire-chip-filled highway embankments combusted gave the use of scrap rubber in civil engineering adverse early publicity, but further research demonstrated limiting tire chip size and removing as much metal as possible from the chips effectively ended that problem, the RMA said.
Nevertheless, tire-derived aggregate in civil engineering has declined in recent years. In 2005, a total of 49 million scrap tires went to civil engineering projects in 2005, down 6 million from 2003, according to the RMA.
Tire-derived aggregate is often in direct competition with TDF, which is a higher-value-added application for scrap tires than civil engineering, the RMA said. Conflicting state regulations, government policies biased against tire-derived aggregate and some reports of clogging in landfill leachate liners also have contributed to the decline, according to the association.
Civil engineering applications seem to be in permanent decline, according to some rubber recycling experts.
Jerry Swensen, president of Auburndale Recycling Center in Auburndale, Wis., and incoming president of the International Scrap Recycling Institute's Scrap Tire Chapter, believes the recycled rubber market from now on will be a matter of TDF and ground rubber applications. ``Civil engineering and other applications are so minuscule, they don't have a major impact,'' Swensen said.
Ground and crumb rubber. Ground rubber applications were still third behind TDF and civil engineering in 2005, accounting for slightly less than 37.5 million scrap tires, according to the RMA report. But whereas civil engineering applications fell significantly from 2003, ground rubber skyrocketed. In 2003, the RMA reported, ground and crumb rubber accounted for 28.2 million scrap tires, which means the market grew by nearly one-third in two years.
It is a good bet that ground rubber will continue to be a boom market in years to come, industry observers said. It encompasses many different applications, including crumb-rubber modified asphalt, molded goods, athletic turf, rubber mulch, playground fill and fine-mesh engineered powders which can be added to new tires and rubber products.
Despite recent controversy over the putative environmental effects of rubber playground fill and athletic turf, ground rubber in general fulfills the fundamental ideal of recycling: to make a useful new product from a discarded old one.
``With rubber mulch, playground surfacing and asphalt rubber, you're certainly getting the longevity out of rubber,'' said Troy Hess, vice president of Mahantango Enterprises Inc. in Liverpool, Pa., and outgoing president of the ISRI Scrap Tire Chapter.
Retreaded and used tires. ``Retreading is recycling'' has been a battle cry of the Tire Retread Information Bureau from its inception. The RMA does not include retreading in its scrap tire recycling statistics-in the association's words, ``retreading is a viable technology that prolongs tire life and makes a positive contribution toward decreasing scrap tire disposal.''
Nevertheless, there is little question that retreading, in a very real sense, transforms an old tire into a new one. Nearly 16.3 million tires were retreaded in the U.S. in 2005, according to the RMA.
Used tires-those tires that are still usable on vehicles after being removed from initial service-have long been a viable market, particularly among low-income families, though technically they are neither scrap tires nor a new product.
However, used tires have come under increasing fire for alleged safety reasons because of their age and possible internal damage. Groups such as Safety Research & Strategies Inc., the Rehoboth, Mass.-based consumer group with ties to trial lawyers, has petitioned the National Highway Traffic Safety Administration for strict regulation of the used tire market.
Auburndale's Swensen is among those who find this a sad situation.
``Individuals who go out and buy used tires accept a certain liability until someone gets hurt,'' he said. ``Then it's never their fault. So we'll ban used tires, and all the people who are buying the used tires won't be able to afford to pay two, three or four times as much for a new tire.''
Pyrolysis. Pyrolysis-which is also known by other terms such as carbonization, thermal distillation or retort conversion-is not exclusive to tires, but can be performed with any organic material. It is the process under which an organic substance is reduced to its component parts by exposure to heat in a reduced oxygen environment.
Via pyrolysis, scrap tires can be reduced to petroleum, gas, and carbon char. Both the U.S. Environmental Protection Agency and the RMA have fact sheets about pyrolysis on their Web sites.
The EPA said pyrolosis hasn't worked in the U.S. because its products are lower quality than virgin materials.
Since 1985, some 75 pyrolysis projects have been announced, and at least four pyrolysis plants have been built, according to the RMA. However, the group said, ``Pyrolysis appears to be a capital-intensive process for conversion of a solid, nonhazardous material such as scrap tires into a low-grade liquid fuel or solid fuel.''
Other sources, however, cite enormous strides in pyrolysis technology over the past decade. David Forrester, founder and CEO of North Carolina-based TIRES Inc., said at the 2007 Clemson Tire Industry Conference that he had visited at least two pyrolysis facilities he thought had a good chance of becoming commercially viable.
Jesse Klinkhamer, CEO of Vancouver, B.C.-based Klean Industries Inc., believes he holds the key to making a rubber carbonization process commercially viable.
``All of our plants are commercially viable,'' Klinkhamer said. ``We are not a research organization.'' Klean Industries currently has about 500 plants worldwide processing various materials, including tires, through carbonization techniques.
Klean Industries' first North American tire carbonization facility will open in the U.S. before the end of 2008.