The field of transportation

is ripe for green tech: It accounts for two-thirds of U.S. oil consumption and generates one-third of the nation's greenhouse gas emissions. Most research has focused on alternatives to gasoline-powered internal-combustion engines, but multinational biotechnology company Genencor is working on a more plebian car component: tires.

Making Rubber from Plants

About 250 million tires are sold yearly in the United States. Each one is roughly one-fourth synthetic rubber (the rest consists of natural rubber, steel, nylon, polyester, assorted reinforcing chemicals, waxes, pigments and oils). Synthetic rubber production dates back to the early 1900s and mushroomed into an industry during World War II. Today it takes about seven gallons of oil to make a standard tire—five gallons as feedstock for chemicals that make up synthetic rubber, plus two for the energy required to power the manufacturing process.

Many types of plants, including poplars, oak trees and kudzu, produce isoprene, a volatile hydrocarbon liquid that's a building block of natural rubber. (Isoprene emissions from trees on hot days are a component of smog.) Chemical companies also refine isoprene from petroleum and use it to make synthetic polyisoprene rubber, which has good strength, flexibility and resistance to cold. Synthetic polyisoprene is widely used in tires, as well as other goods like hoses, rubber bands and pipe gaskets.

Working with Goodyear Tire & Rubber, Genencor has developed a way to make isoprene by starting with plant material instead of oil. The company uses E. coli bacteria to break down cellulose-based sugars derived from plant materials like corn, corn cobs or switchgrass, generating natural isoprene as a byproduct.

It's cleaner to make than the oil version. "Some of the microbe cell mass is left over after the microbes catalyze the reaction," says Genencor vice president Carl Sanford, "but that can be recycled, used as fertilizer or burned for energy. There are no highly toxic waste products."

Genencor delivered the first shipment of its BioIsoprene™ to Goodyear early last year, and it was incorporated into two concept tires displayed at the Copenhagen climate change conference in December. In March Genencor announced that it had inserted plant genes for producing isoprene into E. coli bacteria, speeding up the manufacturing process. The company plans to start pilot production of BioIsoprene within two years, and says that bio-based tires could be on the market within three to five years.

The Business Case: High Oil Prices

Genencor's business case relies on making BioIsoprene cost-competitive with isoprene from petroleum, so its prospects hinge partly on world oil prices over the next five years. Global rubber supplies will also be a factor. BioIsoprene could sub in for a portion of the natural rubber already in tires, Sanford says. "And sometimes companies can't get enough petro-derived isoprene, so we may be able to fill the void."

Genencor will no doubt have competition for the plant feedstock: Under the Environmental Protection Agency's Renewable Fuel Standard program, first enacted in 2005 and amended several times since, U.S. oil refiners are required to blend 3 billion gallons of cellulosic biofuels into gasoline annually by 2015, and 16 billion gallons per year by 2022. While federal agencies have estimated that the U.S. could produce as much as a billion tons of biomass crops per year within the next several decades, enough to satisfy the renewable fuel mandate with materials left over, not all experts agree on that number.

It's worth noting, however, that although the global isoprene market isn't trivial—about 1.7 billion pounds per year—replacing that entire amount with plant-based isoprene is a much smaller undertaking than meeting federal biofuel targets. Sanford estimates that every ton of BioIsoprene requires about four tons of feedstock, so it would take only about 3.4 million tons of cellulosic materials to make enough plant-based isoprene for all global applications.

Friction, The Real Oil Guzzler

According to a life-cycle assessment by European tire and rubber manufacturers, the biggest environmental impacts from tires occur not while they're being made, but while they're being used. The main culprit is rolling resistance (friction and other forces that are generated as tires grip the road and dissipate energy as waste heat). Michelin, the world's largest tire manufacturer, estimates that rolling resistance accounts for 9 percent of global oil consumption; in contrast, making tires represents less than 1 percent. Low rolling resistance tires, which are widely available now, can improve drivers' gas mileage by 3 to 4 percent.

"There are three things we must do to move toward a sustainable technology base," says Terry Collins, professor of chemistry and director of the Institute for Green Science at Carnegie Mellon University. "Go solar for energy, move from fossilized to recently dead plant matter for the source materials of our products and reduce and eliminate toxic substances from our technologies." Because oil refining generates high levels of air and water pollution and hazardous wastes, reducing the need for petroleum-based inputs would be reason to call BioIsoprene a green product.

The final verdict, says Collins, depends on what kinds of energy, water and other resources are required to manufacture it commercially. "The devil, and the angels, are in the details," he says.

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