Few consumer products have a poorer environmental image than the car tire. It is hard to escape the bleak sight of coal-colored mountains of used tires that blight the U.S. landscape. But like other parts of the automobile, the tire has been partially reworked during the past decade to make it more ecologically sustainable. As a result of efforts by tire industry chemists and engineers, many of today’s tires roll more easily to save fuel and contain fewer petroleum-based ingredients, both of which shrink this difficult technology's carbon footprint. The work to green the tire further will continue.



When global climate change began to grab headlines some years ago, tire industry researchers analyzed the car tire's total lifetime ("wells-to-wheels") environmental impact to help determine how it might be improved. They found that the aspect of the tire that contributes the most to greenhouse gas emissions—around 86 percent of its effect—is related to the amount of the extra fuel that tires cause engines to burn to overcome the rubber's resistance to rolling, says Forrest Patterson, technical director for passenger cars and light truck tires at Michelin North America in Greenville, S.C. Every 3.8 liters (gallon) of oil that remains unburned keeps 8.2 kilograms (18 pounds) of carbon dioxide out the air.



Tire companies responded initially by developing low rolling resistance tires that generate about 5 percent less friction as they rotate on the pavement, which can boost fuel economy by as much as 4 to 8 percent over regular models. Over the lifetime of a tire, "we could be talking about saving from 10 to 80 gallons [40 to 300 liters] of gasoline" this way, Patterson says. They also set about encouraging motorists to maintain full inflation pressures.



But now tire-makers have turned increasingly to finding renewably sourced raw materials to replace current oil-based ingredients of tires. Depending on the model, anywhere from 15 to 38 liters of petroleum are required to produce a standard tire. Low-oil content tires use various natural, sustainable ingredients as substitutes including chemically toughened natural rubbers, vegetable-based processing oils and fibers made of plant cellulose. They also found nonpetroleum versions of what the tire industry calls fillers—special functional additives that boost, for example, manufacturing processability or durability.



Tire chemistry is more complex than one might expect. "Some 30 or 40 chemicals go into tire rubber compounds, depending on the component—tread, sidewalls, belts, carcass plies and liner," notes James Rancourt, a consulting polymer scientist who heads Polymer Solutions in Blacksburg, Va. By weight, he explains, the tread compounds of a conventional tire contain about 28 percent natural rubber, which comes from latex sap, 28 percent synthetic rubber, which is made from oil, and 28 percent carbon black filler—a sootlike reinforcing agent that is produced by partially burning fossil fuels. The remaining 16 percent comprises different functional agents of various kinds.



The drive to produce low-rolling resistance tires was, in fact, a first step toward making them more ecofriendly because it involved replacing certain petroleum-based substances in tire construction, says Harold Herzlich, president of Las Vegas–based Herzlich Consulting, a former tire industry executive. Manufacturers added modified silica filler—essentially, surface-treated sand microparticles—to replace some of the carbon black reinforcement in standard tires. It lessens the frictional heat that the rubber compound creates as it flexes, stretches and recovers with each turn.



Japan's Sumitomo Rubber Industries introduced several years ago a Dunlop-branded tire line called Enasave 97 that incorporates natural ingredients. Sumitomo engineers not only used silica fillers, they also employed a specially modified natural rubber that grips better than unmodified versions, along with vegetable-derived processing oil, and cellulose-based (rayon) casing fibers, says company spokesperson, Masatoshi Hayashi. In 2008 it unveiled a prototype Enasave tire that contained 97 percent natural compounding ingredients and plans to market a model that will contain no petrochemical materials at all by 2013.



Yokohama Tire has in the meantime developed tires that are 80 percent petroleum-free, says Dan Guiney, director of technical services. The company's dB Super E-spec car tire and its ADVAN ENV-R1 racing tire both use modified natural rubber compounds and processing oil that is derived from orange peels, a waste by-product of fruit juice processing. The company, which introduced the dB Super E-spec orange oil-infused model last year, reportedly charges roughly 30 percent more for the green tire.



Michelin says that it employs sunflower oil in the rubber compound of its premium Primacy MXM4 all-weather tires to improve traction in winter conditions and shorten braking distances in the rain.



Other tire-makers are looking beyond traditional rubber trees to develop new, potentially cheaper and more sustainable crop sources of natural latex such as Russian dandelion, a central Asian weed variety, and guayule, a desert shrub that grows in the U.S. Southwest and Mexico, Herzlich says.



Still other companies are turning to renewable synthetic rubbers made of precursor chemicals that are grown in vats by biotech-modified microorganisms. In 2007 Goodyear entered into a research collaboration with the industrial biochemical firm Genencor of Palo Alto, Calif., (a subsidiary of Danisco, a Denmark-based food ingredient company) to develop microbes that can grow the isoprene monomer, or what the company calls BioIsoprene, says Rich LaDuca, senior director of business development at Genencor. Goodyear will use the BioIsoprene to cook up a synthetic rubber that mimics natural rubber, which is composed of polymerized natural isoprene. The biotech firm will offer to supply the sustainable chemical to other tire manufacturers as well.



A research group at Oregon State University recently reported that microcellulose, or microcrystalline cellulose, which can be made from various plant fibers, provides a promising alternative to the heavy (and costly) silica mineral fillers now used to reduce tires' rolling resistance. The lightweight microcellulose could save even more fuel. More study is needed, however, to confirm the long-term durability of tires that incorporate this natural additive.



In the meantime, the tire industry is trying harder to recycle more of the estimated 300 million used tires that are discarded annually in the U.S. Tire recycling has grown markedly in recent years and small quantities of processed rubber are reused in some tires, but these efforts are limited by the quality of the rubber feedstock, Herzlich says. One company, Georgia-based Lehigh Technologies, has developed a process that freezes the rubber with liquid nitrogen before grinding it into pulverized rubber feedstock that could go into tires.



In time, the ongoing installation of new precision tire-making machinery that can hold much tighter manufacturing tolerances will allow tire-makers to produce tires that have little extra weight beyond the design spec, Herzlich says. "If you take a half ounce out of a tire," he explains, "it saves fuel because it makes it easier to rotate the wheel."



Although the overall environmental legacy of the car tire is still decidedly black, the appearance of fuel-saving designs, biosourced raw materials and more vigorous recycling programs have given its future a distinctly greener hue.