Rubber: there's nothing like the real thing. Manufacturers use synthetic rubber in toys and rubber bands and even passenger car tires, but higher performance products such as truck and aircraft tires demand the natural stuff. The problem is, almost all the natural rubber in the world comes from a single species of tree, Hevea brasiliensis, crops of which cannot be scaled up easily to meet future demand. So the tire industry, seeking a new source of natural rubber, has turned to a species that was last used in World War II: a flowering shrub known as Parthenium argentatum, or guayule.
Threats to the Hevea tree from climate change and disease are driving the push to find new sources of natural rubber. The crop is particularly vulnerable because the plants used commercially are mostly clones of one another. Simply growing more of the trees is not the solution. South American leaf blight, which spreads from tree to tree, makes it impossible to grow Hevea in single-crop fields in Brazil, where the species originated. And Asia, where 93 percent of the world’s natural rubber is produced, is under pressure to preserve rainforest, not clear more of it for crops. "These areas are undergoing changes, which put pressure on the value of land and labor," says Bill Niaura, director of new business development at Bridgestone Americas, a subsidiary of the Japan-based tire giant. "That drives the price up. At a certain threshold it makes sense to look for alternative sources." To that end Bridgestone Americas has opened up an experimental factory in Arizona to extract natural rubber from locally grown guayule.
Guayule has a number of advantages over Hevea. As a desert plant, it requires relatively little water compared with other crops and it can grow in wide swaths of the U.S. In addition, its greater genetic diversity affords better protection against diseases, none of which are transmitted as easily as the leaf blight that attacks Hevea.
Unlike Hevea, guayule can be harvested with machinery. Because Hevea stores rubber just under the bark, growers must extract the tree’s rubber by scoring the tree by hand and allowing the rubber to drip into a bucket. Guayule, in contrast, stores rubber in its cells, which means growers can extract the substance by crushing the whole shrub and separating the particles of rubber from the rest of the biomass with the aid of a solvent—a far more efficient process.
In the 1930s and 1940s the U.S., Germany and the Soviet Union all looked for alternatives to Hevea to use in everything from tires to gas masks. The research became more urgent after Southeast Asia fell to the Japanese, and Brazil’s production was unable to keep up with the demand. The Soviet Union studied a dandelion called Taraxacum kok-saghyz whereas the U.S. focused on guayuleand Nazi scientists researched synthetic compounds.
After the war, Asian rubber came back on the market and the U.S.'s own wartime research hadsucceeded in developing synthetic compounds, so guayule fell by the wayside. Subsequent attempts to revive the use of guayule have focused on small-scale, specialty products; Yulex Corp., for example, partners with manufacturers to make hypoallergenic gloves and a wet suit.
But tire manufacturing consumes some 70 percent of the world's rubber supply, so interest from the industry means the demand will better support guayule development, says Yulex CEO Jeff Martin. "This kind of big capital investment is necessary," he observes. "You need interest from the big companies."
For its part, Bridgestone Americas does not expect to produce enough guayule-based rubber to be commercially viable for several years. Its primary focus for now is to refine the manufacturing techniques.