For the past century, tire manufacturers have sought alternatives to the natural rubber grown on Southeast Asian rubber plantations. Dependence on a single region for the critical raw material has made their supply chains vulnerable to blight, infestation, bad weather, political instability, and, in the 1940s, war.
When Japan cut off rubber supplies in World War II, the U.S. government had to act fast to establish a domestic synthetic rubber industry. The effort not only delivered rubber but also helped establish the modern petrochemical industry.
But there is no true substitute for the real, natural item. To this end, rubber producers have long tried to extract rubber from the guayule—pronounced why-yoo-lee—shrub (Parthenium argentatum), native to the arid southwestern U.S. Efforts to develop guayule, however, have been sporadic, flaring up in times of crisis and then vanishing just as fast.
During the war, the U.S. set up the Emergency Rubber Project to cultivate guayule. The war didn’t last long enough to sustain the program. Only a few hundred tons of rubber were produced from fields in California and wild shrubs in Texas before the project was discontinued (C&EN, March 29, 1948, page 926).
Big tire companies such as Bridgestone and Cooper Tire & Rubber Co. as well as little start-ups such as Yulex are now taking up the cause again. Seeing a payoff from diversifying their sources of natural rubber, they say they are committed to long-term research of ways to economically grow and extract rubber from guayule.
Natural rubber, known chemically as cis-1,4-polyisoprene, is as essential as ever. “A lot of synthetic work was targeted toward making a replacement for the natural product,” says Bill Niaura, director of new business development for Bridgestone Americas. “Those decades of work have shown we’re not that good yet.”
Natural rubber is still a must for truck and airplane tires. Niaura explains that natural rubber strain-crystallizes as a consequence of the stereoregularity of the polymer chain. As those stereoregular chains are pulled—such as when a plane’s tires slam into the runway for landing—they align. Microcrystalline structures form in the rubber, adding reinforcement. “The higher loading that a tire sees, the more natural rubber it is going to have,” Niaura says.
Guayule is 5 to 10% cis-1,4-polyisoprene by weight. But it isn’t as easy to extract rubber from the shrubs as it is from traditional Hevea brasiliensis rubber trees. Latex flows freely within channels in rubber trees. Workers simply tap the plant to let the latex flow into buckets.
In guayule, the rubber is trapped within the plant’s cells and must be extracted after the plant reaches maturity, which takes two or three years. “You cut at the base, as you would with asparagus, and allow the rest of the plant to regrow,” says Colleen M. McMahan, a research chemist at the U.S. Department of Agriculture’s Agricultural Research Service (ARS).
The harvested plant is ground, and the rubber is extracted in one of two ways, according to McMahan. In the solvent-based process, organic solvents dissolve the rubber and are then stripped off, leaving a hard rubber, suitable for tires.
In the aqueous process, which was developed at ARS, the plant matter is run through a centrifuge in an alkali solution. The rubber particles, McMahan says, rise to the top. The product is a latex emulsion, used for “dipped” products such as gloves and balloons.
And it’s such products that take advantage of a key difference between guayule and hevea rubber. Latex from rubber trees is about 2% proteins, some of which cause allergies. Guayule rubber doesn’t contain these proteins.
This quality was the genesis of San Diego-based Yulex, says Chief Executive Officer Jeffrey A. Martin, who founded the company in 2000. Because Yulex was interested in latex products such as gloves and catheters, the company licensed ARS’s process.
Synthetic polymers, such as polyurethane, are currently used to make allergy-free gloves and condoms, but Martin says synthetics lack the “comfort and creature features” of natural-latex-based products. Glove giant Ansell took an equity stake in Yulex as part of a deal to explore this market.
To date, though, Yulex’s commercial hit is the R2 wetsuit, available since 2013 from Patagonia. In this case, guayule rubber is a natural alternative to polychloroprene. Moreover, Martin says, it is lighter and more flexible than the material it is replacing. “That’s huge if you’re a surfer,” he notes.
Yulex makes the latex for the suit, as well as for test-marketing other products, at its pilot plant near Phoenix. Yulex and partners grow guayule in Arizona and Italy.
The company’s future, according to Martin, lies in collaborating with other firms to scale up its technology. One is Versalis, the chemical arm of the Italian oil giant Eni, which plans to build a guayule rubber facility in arid Southern Europe. The guayule crop, grown with Yulex seeds, has already been planted in the region. Versalis hopes to sell the rubber to the Italian tire maker Pirelli.
Indeed, the tire companies involved with guayule have more than niche products such as gloves in mind. They want a large-scale source of natural rubber to compete with hevea rubber, some 11.4 million metric tons of which was consumed globally in 2013, according to the Singapore-based International Rubber Study Group.
Even when tapped by hand from the veins of rubber trees in Thailand and Malaysia, natural rubber can be pretty cheap: $1,710 per metric ton, compared with $2,630 for styrene-butadiene rubber made in the U.S. last year, according to the study group.
Low prices killed earlier guayule programs. “Many of those efforts failed just on the basis that hevea was so inexpensive,” Bridgestone’s Niaura says.
Nevertheless, tire companies have an incentive to press on because they crave supply diversity. “If you look at the future, the beautiful picture would be rubber coming from tropical regions from hevea trees, from arid regions from guayule, and from temperate climates from something like Russian dandelion,” Niaura says.
Guayule must overcome some obstacles to hit the big time. One is finding value for the 95% or so of the guayule plant that isn’t polyisoprene. This is a focus of the work Bridgestone is undertaking at a process research center it opened last September in Mesa, Ariz.
Niaura envisions using the woody material, the bagasse, as a fuel or as a feedstock for cellulosic biofuels. The firm is also exploring markets for the terpene resins that make up roughly 10% of the guayule plant.
The companies trying to commercialize guayule say the most important factor is improving its agronomics—the economics of growing it. Previous efforts to cultivate guayule never fully tamed the wild domestic shrub.
Yulex has tested the germplasm USDA was sending out to research programs around the world. The company found that USDA’s seed could be traced to just five plants that were part of the WWII guayule program. “It is virtually impossible to make any meaningful progress with traditional breeding methods without proper diversity,” Martin says.
The company scoured universities, labs, and private collections to boost the genetic material for its breeding program. It is now testing guayule hybrids in arid regions around the world.
Yulex isn’t the only company that has been wrestling with agronomics. In 2013, Bridgestone planted a 281-acre farm in Eloy, Ariz., where it breeds and tests crops. Niaura says the company hopes to have commercial-scale cultivation and processing under way by the mid-2000s.
Cooper Tire & Rubber Co. is leading a consortium that includes ARS, the guayule research start-up PanAridus, and Arizona State University. The team received a $6.9 million federal grant to define the guayule genome and to plant, irrigate, and harvest the shrub.
“We think this union has the potential to be the difference maker,” says Chuck Yurkovich, Cooper’s head of R&D. The key to a successful effort, he says, is consistent funding to render it invulnerable to the ups and downs of the rubber market. The company, he notes, has already tested guayule-based tires and found performance to be equivalent to their hevea-based counterparts.
Yulex’s Martin argues that the foundational work is nearly complete and that the guayule industry is ready to take the next step. “We have solved all the riddles with respect to processing technology, performance of the material, and finally the question of genetics,” he says. “Now it is just an issue of scale.”