Tires die hard -- but scientists are
driving towards solutions
Blame it on Charles Goodyear.
The famed inventor changed the world on that day in 1839 when he accidentally dropped a mixture of India rubber and sulfur onto a hot stove, turning a flimsy gum into the sturdy stuff of modern tires. His mistake essentially fused the rubber molecules' chains together, creating something flexible but nearly indestructible. The process is not unlike knotting together a handful of spaghetti, but Goodyear picked a more eloquent description - vulcanization.
''A tire is basically one big molecule,'' marveled Richard Farris, a polymer scientist at the University of Massachusetts at Amherst. ''Everything is bonded to everything else.''
But the same wonder material that has become a staple of modern society also has created one of the largest solid waste headaches of the 21st century. Unlike paper, which can be turned back into pulp, or plastic, metals and glass, which can be melted down, vulcanized rubber until recently has staunchly resisted recycling. Devulcanizing rubber, the joke goes, is like uncooking a hardboiled egg.
Americans alone scrap more than 280 million tires annually, weighing more than three times the 1.7 million tons of debris removed from the World Trade Center wreckage. And, although experts have found many uses for old tires, according to estimates by the Rubber Manufacturers Association, an industry trade group, an estimated 10 percent to 15 percent still wind up in dumps each year, creating fire hazards and breeding grounds for rats and mosquitoes.
Due to the difficulty of recycling rubber tires, most tire management until now has focused on reusing them - as fodder for road builders, as playground equipment, as a fuel for power stations, and so on. But Farris and other scientists across the country are finally developing techniques to remold old rubber products into entirely new ones.
For instance, Avraam Isayev, a polymer engineering professor at the University of Ohio at Akron, discovered a technique using ultrasound waves to break rubber's molecular bonds. Jeremy Morin, a former graduate student in Farris's lab, found that he could recycle rubber by breaking it into tiny crumbs and subjecting them to extreme heat and pressure.
While the resulting recycled rubber isn't strong enough to make new tires, it's good enough for a variety of uses, such as shoe soles or windshield wipers. ''It's really up to your imagination,'' said Drew Williams, a doctoral candidate at Farris's lab.
It seems that imagination drives the tire disposal industry. Massachusetts banned tires from landfills in 1991, forcing state and local officials to find new ways to get rid of them. Since then, old tires have turned up in an array of new forms. In New England, they have been used mainly in civil engineering and power generation.
As a fuel source, tires are less costly and can generate 25 percent more energy per pound than coal - and they burn fairly cleanly in a controlled incinerator, unlike the uncontrolled fires at tire dumps that can sometimes billow black smoke for days or even longer.
In Sterling, Conn., Exeter Energy's tire-derived fuel facility burns 30,000 tires a day, making it one of the world's biggest tires-to-energy plants. Facility emissions are below state limits. Most tire-derived fuel facilities burn tires in addition to traditional fuels, but the Connecticut plant uses only tires, according to manager Tim Wycherly. Tire-derived fuel facilities are so expensive to operate, however, that they only make economic sense in places such as New England where energy prices are high, said Michael Blumenthal, technical director of the Rubber Manufacturers Association.
Forty-two percent of the nation's tires are burned each year, but many also are ground into crumb rubber and used in civil engineering projects, where they serve as filler in concrete and asphalt. Still more tires are reused as ship bumpers, artificial coral reefs, and paint additives.
But none of these applications address true recycling, or remolding scrap rubber into new products. ''Tires are not made to be recycled,'' Blumenthal said. ''Tires are made for two purposes only - safety and performance. Anything beyond that is a bonus.''
The problem with recycling rubber lies in its chemistry.
Natural rubber is a polymer, or a collection of thousands of repeating molecules bonded together to form long chains. The result - a soft, malleable substance. ''Small molecules are like a bowl of ziti, and polymers [like rubber] are more like a bowl of spaghetti,'' said Williams of Farris's lab. And like pasta, untreated rubber is as flimsy as a cooked noodle.
Goodyear's accident in the 1830s changed all that. Using heat and pressure, vulcanization binds the chains together with a cross-linking agent, which is usually sulfur. And the cross-links are what make rubber so difficult to recycle: The carbon-sulfur bonds prevent the rubber from melting or chemically breaking down.
But some scientists, like Farris's lab at UMass, have found ways to circumvent the problems of vulcanization.
Farris's technique, called high-pressure, high-temperature sintering, uses tens of thousands of pounds per square inch of pressure to squeeze the voids out of crumb rubber and put the particles into intimate contact. Hundreds of degrees of heat then catalyzes a series of reactions that break and reform the rubber bonds, resulting in a new material that can be molded into any shape.
The scientists are able to retain 60 percent of the strength and elasticity of the starting material, and are developing the technology further.
Currently, the lab is investigating various additives and temperature and pressure combinations to help the rubber retain its strength and elasticity, and make the process more commercially viable.
The UMass scientists believe the technology holds promise. While virgin rubber costs more than $1 per pound. crumb rubber costs only 30 cents per pound and is commercially available due to its uses in civil engineering. Additionally, the technique requires no extra chemicals, which keeps costs down, and it can be completed in traditional vulcanization machinery.
''It's a totally green process,'' Farris said.
But Farris's work isn't the only novel approach to rubber recycling. Avraam Isayev of the University of Ohio is perfecting a technique based on ultrasound, or high-energy sound waves above the frequency of human hearing.
In medicine, ultrasound bounces off body parts to create pictures; in the ocean, whales use ultrasonic echo-location as a form of sonar.
But Isayev discovered that the sound waves' energy could also devulcanize rubber by preferentially breaking apart the carbon-sulfur cross-links, which are its weakest bonds. Once devulcanized, the rubber can be treated like virgin rubber and revulcanized into new shapes.
But, like the UMass research, Isayev's is not yet commercially viable. He said that with the most high-powered ultrasound machines, his lab could reach a maximum recycling output of 60 pounds of devulcanized rubber per hour using equipment costing nearly $100,000. He estimated that the technology wouldn't be commercially viable unless it could produce 500 pounds per hour, which requires ultrasound equipment so powerful it doesn't exist.
Isayev said his technology is unique, however, because it is instantaneous. While the work at UMass may take up to an hour to process the rubber crumb, the ultrasound technology is speedy. ''The hour process turns into seconds'' with ultrasound, Isayev said.
Still, one of the largest problems faced by both scientists is that a tire is not just a homogeneous chunk of rubber: Each tire has many different parts made of rubber formulated for different uses. As a result, crumb rubber is a potpourri of compounds, making it hard for Isayev to determine what frequency sound waves to use, and for Farris to set the temperature and pressure for sintering.
''Tires are so complex that someone trying to break them apart faces a tremendous challenge,'' Blumenthal said. ''The simple reality is that it takes all the major markets out there to address and resolve the scrap tire issue. But once the better mousetrap comes along, the world will beat a path to its doorstep. ''