Rubber
Rubber was first used to manufacture tires in the mid-nineteenth century. By the turn of the twenty-first century, there were about three billion waste tires stockpiled or clogging leak pollutants intolandfills in the United States alone. Worldwide, about 700 million scrap tires are generated annually. This waste rubber has become a major source of pollution.
Although rubber, made from latex secreted by trees, has been used for thousands of years, it wasn't until 1839, when Charles Goodyear invented vulcanization, that rubber became important for manufacturing. Vulcanization uses sulfur to cross-link the latex fibers into a rubber that is strong, flexible, durable, and resistant to heat and cold. Like natural rubber, modern synthetic rubbers are polymers (long chains of similar molecules) that are cross-linked by vulcanization. Tires consume 60–70% of all rubber produced, twothirds of which is synthetic.
Of the 273 million waste tires generated annually in the United States, about 25% end up in landfills. There they leak pollutants into soil and groundwater and tend to rise to the surface, harming landfill covers. The largest tire dump in the Northeast United States holds an estimated 20–30 million tires. The majority of states now ban tire disposal in landfills and collect disposal fees on tires or require that the tires be chipped or ground before disposal. An additional 800 million tires are stockpiled in the United States, however, and many more are dumped illegally. These mountains of tires can fill up with water and become breeding grounds for rats and mosquitoes. They also can ignite, have a high heat output, and are very difficult to contain. These fires can burn for months or years, producing toxic smoke and oils that pollute the air, water, and soil.
Rubber Recycling
Rubber is difficult to recycle because of its chemical cross-linking. Furthermore most tires contain a mixture of three or four types of synthetic rubbers, as well as natural rubber, other fibers, and steel. Nevertheless in the United States, markets now exist for 76% of newly-scrapped tires, up from 17% in 1990. About 42% of scrapped tires are used for fuel in facilities such as pulp and paper mills and cement kilns. As a fuel, tires are equivalent to oil and produce 25% more energy than coal. Because of new technologies and pollution controls, tire combustion now proceeds at higher temperatures with less air pollution.
About 33 million retread or recapped tires are sold annually in the United States. A retread tire reuses 75% of the old tire and requires 70% less oil than manufacturing a new tire.
In 2001 about 40 million scrap tires were used in the construction of playground equipment, artificial reefs, boat bumpers, crash barriers, stabilizers for slopes, and erosion control for dams. Tire material is used for mulch, mats, septic systems, building products, coatings and sealants, and in hazardous waste containers.
Many new products contain material from recycled tires. In the United States about 24.5 million scrap tires annually are ground up, cut, stamped, or punched for new products. Ground tires are used for running tracks, playgrounds, flooring, and the soles of shoes, or mixed with asphalt for road paving. Rubber crumb is used in products such as athletic turf and auto parts.
In typical rubber recycling, the tires are cut up into small pieces and ground, or frozen in liquid nitrogen and shattered or pulverized. The steel is extracted with magnets and filters separate the rubber from other synthetic fibers. However 15–50% of the original tire remains as a useless rubber-fiber blend that goes to the landfill. The U.S. Department of Agriculture is attempting to develop more efficient methods of separating the rubber from the other fibers.
The development of new tire recycling technologies is an active area of research. Ground-up rubber can be mixed with virgin rubber and vulcanized to form new cross-links, restoring its strength and elasticity. About 5% of scrap tires are recycled in this way. Improved methods for rejuvenating old rubber, without mixing it with newly-manufactured rubber, would enable tire manufacturers to increase the recycled content of new tires.
Another area of research uses heat and pressure to combine the powder from ground-up tires with powder made from asphalt. This asphalt-modified rubber is superior to asphalt for roads, construction materials, and roofing shingles. Likewise, researchers are experimenting with adding rubber crumb to fresh concrete to increase strength and durability. A composite that contains 50% rubber crumb could potentially replace plastics such as polyvinyl chlorides for various applications. Other researchers are studying whether the oils produced by waste tire combustion can be reprocessed into carbon black for use in various products.
Resources
Other
"Arizona State University Research Finds Recycling Cure for Used Tires." ScienceDaily Magazine September 13, 2001 [cited July 7, 2002]. <http://www.sciencedaily.com/rel eases/2001/09/010913074634.htm>.
"Recycling Research Institute." Scrap Tire News Online. 2002 [cited July 7, 2002]. <http://www.scraptirenews.com&#x 003E;.
U.S. Environmental Protection Agency. Jobs Through Recycling. May 31, 2002 [cited July 7, 2002]. <http://www.epa.gov/epaoswer/non -hw/recycle/jtr/comm/rubber.htm> .
U.S. Environmental Protection Agency. Municipal Solid Waste. May 9, 2002 [cited July 7, 2002]. <http://www.epa.gov/msw/tires.ht m>.
"Umass Polymer Scientists Aiming to Turn Scrap Tires into Environmentally Friendly Products." ScienceDaily Magazine March 6, 2002 [cited July 7, 2002]. <http://www.sciencedaily.com/rel eases/2002/03/020306073739.htm>.
Organizations
International Tire & Rubber Association Foundation, Inc., PO Box 37203, Louisville, KY USA 40233-7203 (502) 968-8900, Fax: (502) 964-7859, Toll Free: (800) 426-8835, Email: itra@itra.com, <http://www.itra.com>
Recycled Materials Resource Center, 220 Environmental Technology Building, Durham, NH USA 03824 (603) 862-3957, Email: rmrc@rmrc.unh.edu, <http://www.rmrc.unh.edu>
Rubber Manufacturers Association, 1400 K Street, NW, Washington, DC USA 20005 (202) 682-4800, Email: info@rma.org, <http://www.rma.org>
U.S. Environmental Protection Agency, 1200 Pennsylvania Avenue, NW, Washington, DC USA 20460 (703) 412-9810, Toll Free: (800) 424-9346, Email: public-access@epa.gov, <http://www.epa.gov>
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