Old tyres feel the heat under pressure
Researchers at the University of Massachusetts are working on two separate projects aimed at reducing the numbers of used tyres clogging the nation's landfills.
One team is looking at new methods of recycling old tyres into new rubber goods; another is developing a substance that is a combination of asphalt and recycled tyres, and could be used in products as varied as roadways, construction materials, and roofing shingles.
'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. It's chemically cross-linked, and those links will not melt and will not dissolve,' he added.
Studies estimate that there are approximately 2 billion scrap tyres currently in US landfills, with more than 273 million additional tyres reaching the waste stream each year. When scrap tyres pile up in landfills, they can fill with water and provide a breeding ground for mosquitoes and rodents, said Drew Williams, a doctoral candidate studying the issue. Tyres also represent a significant fire hazard.
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 unvulcanised rubber.
The mixture is then vulcanised: that is, the material is heated and new cross-links are formed, via the additional sulphur or other reactive materials, in order to restore its strength and elasticity. Five percent of scrap tyres 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 tyres, to pressures of approximately 1,000 pounds per square inch, and temperatures in the range of 200 degrees Celsius (400 degrees Fahrenheit), the powder sinters together to form a solid rubber object.
The process developed at the University is attributed to the thermal energy breaking and re-forming the rubber's chemical bonds, Farris said. This is said to allow 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.
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 grinds it into a fine powder before blending it with the rubber powder derived from scrap tyres 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.
'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.'