“Guayule can completely replace Hevea and synthetic rubber in most tire areas,” he said, though he added that the impact of strain-induced crystallization in the bead and sidewall areas still is being studied.
Cooper is the leader of the five-year, $6.9 million Biomass Research and Development Initiative (BRDI) funded by the U.S. departments of Agriculture and Energy. The goal of the BRDI is to assess the feasibility of guayule rubber as an ingredient in commercially produced tires.
Cooper Senior Scientist Howard Colvin presented a paper on BRDI's progress at the International Tire Exhibition & Conference in Akron in September, and Mr. Walters' paper was an update on that research as it relates to guayule's performance in various parts of a tire.
“We have been testing guayule in one tire component at a time,” Mr. Walters said. “In the end, we will put everything together to have an all-guayule concept tire by 2017.”
The last all-natural-rubber tire was built in the 1940s, he said, and the obvious differences between tire construction then and now makes it an enormous challenge to make an all-guayule tire that meets modern performance standards.
There are minimal cure differences between guayule and Hevea, though additional specifications are necessary for guayule because of its higher resin content, according to Mr. Walters.
The dynamic and static properties of guayule/Hevea and SR (synthetic rubber)/Hevea compounds are markedly different, he said.
“There's a reason we use synthetic rubber in tires,” he said.
In most areas — including bead and filler assembly, body plies, belt coatings, sidewalls, rim cushions and tread caps/bases — guayule passed all relevant tests, including high speed, endurance and bead unseating, according to Mr. Walters. It also passed Cooper's proprietary endurance and aging/oxidation tests, he added.
The only problem area for guayule was in innerliners, with the strict requirement to maintain inflation pressure, he said.
“It's challenging to make innerliners without halobutyl, especially because of permeability,” Mr. Walters said. “Factory processing is a challenge when you replace synthetic rubber with guayule.”