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One Giant Molecule Richard Farris tackles the daunting disposal problem of rubber tires

Chris O'Carroll

DEMONSTRATING THE VERSATILE PROPERTIES of a fascinating material: Richard Farris. (photo by Ben Barnhart) STRETCH IT, BEND IT, SQUEEZE it, twist it, distort it every which way. It takes the punishment and springs back to its original shape. Hurl it at an unyielding surface. It bounces off unharmed. Heat it with 65-mile-an-hour friction against a summertime highway. Instead of puddling in a squishy mass, it remains a tough, resilient solid. Leave it outdoors overnight in the dead of a New England winter. The next morning will find it still steadfastly springy and ready to endure stress without splintering into brittle fragments. More than just the stuff of bathtub duckies and bouncing balls, rubber possesses distinctive physical properties that have made it one of the workhorse materials of the industrialized world for more than a century and a half. For not quite that long, the familiar elastic substance has also been central to the career of Richard Farris. A Distinguished University Professor emeritus, Farris retired last fall from the polymer science and engineering faculty, but remains on-campus with a five-year post-retirement appointment that enables him to continue his rubber research. In his office on the third floor of the Conte Building, and in the adjacent laboratory where he directs the work of his research team, Farris presides over enough rubber to equip a good-sized Greenwich Village fetish store. “It’s a fascinating material,” he says, lifting a shiny black strip of the stuff from his desk and flexing it between his fingers. He adds, however, that the very chemical properties that make rubber so versatile and so scientifically intriguing also render it “a recycling nightmare.” The most visible symbol of this nightmare is the discarded automobile tire. In the United States alone, dumps and landfills are home to an estimated 2 billion tires whose rubber treads are no longer suitable to meet the road. Every year, millions more of the massive rubber doughnuts get tossed on the scrap heap. And unlike glass or metal, this mountain of rubber waste does not cooperate with efforts to melt it down into raw material for a new generation of products. Although rubber will burn – sometimes under controlled conditions in electrical generating plants that use scrap tires in place of coal, sometimes out of control in catastrophic dump fires – it won’t melt without burning. So when a tire or other rubber object reaches the end of its useful life, it cannot be reduced to a liquid state and recast into new manufactured products. It is this issue, rubber’s stubborn resistance to recycling, that has lately been the focus of Farris’ professional efforts. As head of a polymer science team including post-doctoral researcher Amiya Tripathy, recent Ph.D. alumnus Jeremy Morin and graduate student Drew Williams, Farris has been grinding, compressing and chemically tweaking his favorite material to develop a recycling technique that offers a new solution to a problem as old as the modern rubber industry. When European explorers and conquerors ar-rived in South America five centuries ago, they found the natives of the Amazon rain forest using footwear, containers and other artifacts fashioned from a flexible, waterproof material called caoutchouc (pronounced COUCH-hook). This substance, derived from the sap of local trees, soon joined chocolate, cocaine and potatoes on the roster of New World novelties piquing interest across the Atlantic. The chemical knowledge of the 1500’s, Farris says, did not extend to an understanding that this exotic stuff was a polymer, deriving its characteristic texture from the loosely coiled arrangement of its long, chain-like molecules. Nevertheless, as Europe moved toward the Industrial Revolution, scientists and manufacturers did take note of caoutchouc’s special properties, and Brazil developed a modest export trade in the material, supplying a market for such low-tech items as rubber bands and erasers. It was this latter use of the substance that gave rise to the term “rubber.” Around 1770, the English theologian and scientist Joseph Priestly (best known for his contributions to the discovery of oxygen) found that this strangely flexible Brazilian import could be used to rub away pencil markings from a sheet of paper. To this day, the ubiquitous schoolroom artifact that American children call an “eraser” is known to their English counterparts as a “rubber,” a British locution that is second only to the phrase “knock up” as a source of two-people-divided-by-a-common-language giggling. The 18th-century rubber used to make Priestly’s mistake-expunging rubbers was not the vulcanized material of today’s tires, engine belts and hoses, and other everyday implements of an industrialized society. An agricultural product not enhanced by any scientific intervention, the rubber of that day had limited elasticity and was useful across a relatively narrow range of temperatures. “At high temperatures,” Farris explains, “natural rubber becomes soft and sticky, and at low temperatures it becomes stiff and brittle.” Furthermore, since unprocessed rubber straight from the tree contains about 10 percent protein, early rubber goods had a tendency to rot and smell unpleasant if they were not properly stored. These shortcomings, especially the material’s loss of integrity when subjected to intense heat, thwarted the goal of incorporating rubber parts into the machines that were gaining economic importance as the Industrial Revolution picked up momentum. With the invention of the automobile still far in the future, nobody was looking for a promising tire material. But rubber was seen, Farris says, as a likely candidate to replace belts and other leather components in certain engines. Before rubber’s potential for such applications could be realized, however, somebody had to find a way of treating the raw caoutchouc so that it could endure temperature extremes without losing its desirable properties. The solution to this problem fi-nally appeared on the scene in 1839, when Charles Goodyear, a debt-ridden aspiring inventor without a blimp to his name, discovered that by heating a mixture of natural rubber and sulfur he could produce a material capable of withstanding a vastly increased range of temperatures. This heat and sulfur process, dubbed vulcanization after the Roman fire god Vulcan, also rendered rubber impervious to many acids and other solvents, and greatly increased its elasticity, enabling vulcanized rubber objects to retain their shapes, springing back from stresses that would leave natural rubber permanently warped. At the heart of the vulcanization effect are the bonds that sulfur atoms form with the long molecules of raw rubber. These bonds create what polymer scientists call cross-linkages, connecting the rubber molecules to one another in a unified network that is, for all practical purposes, no longer composed of separate molecular units. As Farris puts it, “A tire, or any rubber object, is basically one giant molecule.” Some molecules are harder than others to dismantle, and subsequent generations of scientists have had to grapple with the near-impossibility of putting asunder what vulcanization has joined together. Goodyear was quick to realize that his invention had created a daunting disposal problem. He devised a method of recycling vulcanized rubber by grinding it up, blending the resultant powder (known in the industry as “crumb rubber”) with virgin rubber, then vulcanizing the mixture anew. If the crumb rubber particles used in this process were extremely fine, the second-generation material could be up to 80 percent as strong as all-new rubber. But with coarser particles, the quality dropped to as low as 50 percent. To make matters worse, this method produced satisfactory results only with a lopsided ratio, at least nine to one, of virgin rubber to recycled crumb. Effective though the technique was, it could deal with only a fraction of the rubber industry’s recycling needs. Since Goodyear’s day, the quantity of waste rubber on the planet has grown beyond anything he and his contemporaries could have imagined. Far-flung Asian plantations now cultivate rubber trees once exclusive to the Amazon, and synthetic rubber made from petroleum has greatly increased the world’s supply. Techniques for reusing the material have not come close to keeping pace. “In this country,” Farris estimates, “we throw away about one tire per person every year.” Tens of millions of these discards are burned in generating plants or processed into such low-grade rubber products as artificial turf. But an estimated 40 million tires a year continue to accumulate in dumps and landfills, where they pose a significant fire hazard, and where their concave interiors collect pools of rainwater that make ideal breeding grounds for disease-bearing insects. To address the rubber recycling dilemma, Farris starts, like Goodyear before him, by grinding waste rubber into powder. This in itself is no easy task. “Rubber is a misery to grind,” Farris says. He accomplishes the feat by freezing the material with liquid nitrogen, which pushes its temperature low enough that a degree of brittleness sets in. He then seals chunks of frozen rubber into a small chamber that contains a heavy metal rod, and has a machine agitate the chamber at high speed. The metal rod bashes back and forth like a pestle in a space-age mortar, pulverizing the rubber to dust. The next step is to heat the powdered rubber to about twice the boiling point of water and compress it with a pressure of approximately 1,200 pounds per square inch. The intense pressure brings the surfaces of the minuscule rubber crumbs into intimate contact, squeezing out inter-molecular voids, while the heat causes chemical cross-linkages to break and reform in such a way that the grains of powder actually fuse into a new solid mass. Reconstituted rubber created by this process has about 60 percent of the strength and flexibility of new vulcanized rubber, and Farris has identified chemical additives that he can mix with the powder to increase that figure to 80 percent. In short, with a process that uses entirely recycled rubber, Farris can match the best results that Goodyear achieved with a material composed of nine-tenths new rubber. Surfaces in Farris’ office and lab are littered with sheets of recycled rubber, slabs about the size of a CD case measuring perhaps an eighth of an inch thick. Many of them sport an exaggerated Swiss cheese look, perforated by holes the diameter of a quarter. Farris and his team members punch out samples from the sheets and subject these rubber disks to a variety of stress-strain tests designed to gauge the quality of the material. These tests have shown that rubber recycled by the lab’s grinding, heating and compressing process comes so close to matching the properties of new vulcanized rubber that the recycled material is suitable for all but the most demanding uses. Manufacturers will still choose new rubber to make high-performance products such as tires, but for many everyday rubber items, the recycled material has all the desired attributes. The world manufactures more than 17 million tons of rubber products every year. So the potential market is enormous now that Farris and his team have demonstrated that rubber need no longer be regarded as a nightmare material that refuses to be recycled.

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One Giant Molecule

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