Scientists Study Recycling Scrap Tires
AMHERST, Mass. – Researchers at the University of Massachusetts are working on two separate projects aimed at reducing the numbers of used tires clogging the nation's landfills. One team is looking at new methods of recycling old tires into new rubber goods; another is developing a novel substance that is a combination of asphalt and recycled tires, and could be used in products as varied as roadways, construction materials, and roofing shingles. Both research groups are part of the polymer science and engineering department.
"Rubber is one the most useful materials of the modern era, and helped spawn the industrial revolution," said Professor Richard Farris. "It is prized in industry for its strength, elasticity, and wear-resistance. Unfortunately, rubber also represents one of the most difficult recycling problems ever encountered," he added. "One of the biggest pollution problems in this country is scrap tires. Although it's easy to collect, rubber is difficult to recycle. It's chemically cross-linked, and those links will not melt and will not dissolve."
There is an enormous demand for rubber, researchers said. The difficulty in recycling it in an economical way has resulted in a significant environmental problem. Studies estimate that there are approximately 2 billion scrap tires currently piled in U.S. landfills, with more than 273 million additional tires reaching the waste stream each year. "This adds up to approximately 3.6 million tons of waste each year, or 230 pounds of rubber reaching the waste stream per second," noted Drew Williams, a doctoral candidate studying the issue. "Of these 273 million tires, about 170 million are burned for fuel, and 60 million are used in low-tech ways, such as for synthetic turf for athletic fields. The remaining 40 million tires end up in landfills."
When scrap tires pile up in landfills, they can fill with water and provide a breeding ground for mosquitoes and rodents, Williams said. The tires also present a serious fire hazard. Large tire fires in Virginia, Colorado, Washington, Wisconsin, and Minnesota have attracted national attention. The largest tire dump in the Northeast is located in Smithfield, R.I. It contains approximately 20-30 million scrap tires and is an EPA Superfund site.
The team led by Farris is essentially revisiting and improving a process introduced in 1853 by Goodyear. In that process, the reclaimed rubber is ground into a fine powder and mixed with unvulcanized rubber. The mixture is then vulcanized: that is, the material is heated and new cross-links are formed, via the additional sulfur or other reactive materials, in order to restore its strength and elasticity. Five percent of scrap tires are currently used this way, Farris said. But the process has limitations: if the mixture relies on more than 15 percent of the recycled powdered rubber, the resulting product diminishes greatly in quality.
The team has found that subjecting this chemically cross-linked powder, which was derived from scrap tires, to pressures of approximately 1,000 pounds per square inch, and temperatures in the range of 200 degrees Celcius (400 degrees Fahrenheit), the powder sinters together to form a solid rubber object. (Sintering is the process of heating and compacting a powdered material below its melting point).
The process developed at the University is attributed to the thermal energy breaking and re-forming the rubber's chemical bonds, Farris said. This allows scientists to create a rubber material containing 100 percent reclaimed rubber – without greatly compromising the material's quality.
The resulting material typically retains 50-90 percent of the original strength and elasticity, depending upon the chemistry of the starting materials. Farris indicated there are many types of synthetic rubbers, and that a typical tire contains three or four kinds of rubber. The group is also working on chemical additives to enhance this sintering process by suppressing undesirable chemical reactions while promoting others. A patent for the process has been filed.
The team includes Farris, recent Ph.D. graduate Jeremy Morin, Drew Williams, and Amiya Tripathy, a postdoctoral researcher. Among the groups involved in the projects are the University's National Technology Environmental Institute, Chelsea Recycling of Chelsea, Mass., the Center for UMass-Industry Research on Polymers, which is affiliated with the polymer science and engineering department, and the U.S. Environmental Protection Agency.
In a separate but related research project, Williams is developing a material that is a combination of rubber and asphalt. Traditional asphalt is a tar-like substance and is the basis for making roofing shingles and asphalt roads. Two problems with asphalt are that it becomes soft and sticky at high temperatures and very brittle at low temperatures. Williams freezes the asphalt, then easily grinds it into a fine powder before blending it with the rubber powder derived from scrap tires and then subjects the mixture to the sintering process.
The resulting material, which is between 15-40 percent asphalt, is an asphalt alternative that withstands traditional asphalt's tendency to melt or become sticky in hot weather and it remains very flexible even at very low temperatures. This asphalt-modified rubber material is very different than modified asphalts, e.g. asphalts containing small amounts of powdered rubber as an additive, that are used in some parts of the nation as road-paving materials. "These projects really represent 'green' chemistry at its best," said Farris. "We're generating lower amounts of waste, and reclaiming used materials, and all we're adding is heat and pressure."
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