This Shrub Could Supply Rubber, Insect Repellent and Glue

This Shrub Could Supply Rubber, Insect Repellent and Glue

The sage-green, waist high shrub guayule may look like one of many scraggly bushies that line the hills and valleys in the Chihuahuan Desert of Mexico and the Southwest U.S. Its nondescript stalks are covered in a variety of botanical treasures. These include rubber, which Indigenous people used to make balls for their games hundreds of years ago.

For decades researchers have studied guayule (Partheniumargentatum ) to determine if it could be a commercial source of natural rubber .. The market is dominated by rubber sourced Hevea bristani trees in Southeast Asia. It’s difficult for guayule to compete with these large operations that have lower labor costs. The Southwest is known for its more lucrative crops like alfalfa, cotton and other alfalfa.

But two converging developments could be changing guayule’s prospects. In recent years, global natural rubber supplies have been severely affected by fungal infestations of monocultured Hevea trees. The Southwest’s worst drought in over a millennium has made it difficult for farmers to grow alfalfa, cotton, and other thirsty crops. Guayule is, however, drought-tolerant. It can be grown and harvested for many years without the need to till the soil. This soil is untilled and stores carbon in the soil, which prevents it from eroding.

Researchers in academia and industry are collaborating to increase guayule’s rubber production. They also plan to explore other uses for the plant’s sticky resin and woody material. Catherine Brewer, a New Mexico State University chemical engineer, says that guayule allows a farmer to still grow crops in the desert while still making a living. It also protects the soil and water. “There are very few plants that meet all the criteria .”

.”

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A medical latex glove made of guayule rubber. Credit: Sarah Davis, EnergyEne

Bridgestone Tires has been operating a demonstration-scale processing plant in central Arizona for the past ten years in an effort to demonstrate that guayule can be harvested at a commercial scale for natural rubber. In late August the company announced it would commercialize guayule rubber production by 2030. Although rubber made from petroleum is now the majority of the world’s rubber, it is still essential for certain purposes. For example, plane tires are made from natural rubber. This is better than synthetic rubber for handling impacts like landing on the runway.

Because there is only a small amount guayule currently grown, it would not be able compete with Hevea rubber over a large scale for a while, according to Katrina Cornish who studies alternative rubber sources at Ohio State University. Companies that want to use it will need to start with high-value products, such as the guayulerubber racing tire Bridgestone. Cornish’s own company EnergyEne focuses on premium products made from guayulelatex, which is a soft form of rubber the plant also makes. EnergyEne is developing a radiation-attenuating medical glove, for example. Cornish says that the company has produced “lovely condoms” for the consumer, including those that are Cabernet Sauvignon- or Chardonnay-flavored. Cornish claims Guayule is the best latex, in comparison to other natural and synthetic formulas. It’s strong, stretchy, and soft. It is hypoallergenic unlike latex from Hevea trees. Jason Quinn, a sustainability researcher at Colorado State University, conducted a study in 2020 that found that guayule rubber grown on a typical-size Arizona farm could be cost-competitive with Hevea rubber–though the guayule product’s cost would be toward the higher end of the range of historical Hevea prices.

David Dierig, a Bridgestone plant geneticist, hopes to increase guayule’s profitability by breeding plants with more rubber. His team has mapped the genome of guayule and identified genes associated with higher levels of rubber. Researchers can now select plants that have the potential to produce more rubber and cross them before they reach maturity. This will reduce the time it takes to grow new generations. Dierig states that there is a limit to the amount of rubber a plant can produce. However, it is difficult to determine exactly where that limit is. He says that high-molecular-weight rubber–the kind used for tires–was initially around 2.8 percent of the plant, and he has brought it up to 3.8 percent. “We have to get rubber content to somewhere around 6 percent to be directly competitive [with Hevea],” Dierig says. He adds that selling other products made from the plant’s resin and woody components could help ensure that rubber is competitive before reaching that level.

The woody stems are a more economical way to develop guayule products and to encourage the plant to be grown commercially. Guayule processing involves the grinding of the bushes. The resin and rubber are extracted with a solvent. This leaves behind a dry, sawdust-like material called bagasse. This can be used to make particleboard, such as furniture, or into biofuels for planes and ships. The latter conversion requires pyrolysis (heating the bagasse in oxygen-free conditions). “There is technology to do that. It’s possible, but it takes a lot of energy,” Kim Ogden, who is the leader of a U.S. Department of Agriculture funded research project based at the University of Arizona. She also works with several partner institutions including Bridgestone and Colorado State University. “So I don’t know how great it .”

is economically.

Ogden, along with other researchers, think that guayule is the real moneymaker. The sticky substance is composed of many organic compounds, including essential oil that could be used to make fragrances. It also contains argentins and guayulins that are unique only to guayule. Researchers believe the resin compounds have potential for many lucrative uses. Scientists at the University of Arizona found the resin could be used for plant-based adhesives, potentially replacing some formaldehyde-containing products such as wood glue. A recent study also found that argentatin-derived compounds were toxic toward three types of cancer cells, opening the possibility for their use in drug investigation.

New Mexico State University’s Brewer is leading a team that tests whether the resin compounds can be used as insect repellents. She has conducted experiments to compare cockroaches’ reactions to guayule extracts and their reactions to java oils (which is a well-known repellent for roaches) and acetone (which they don’t mind). She says that some of the resin compounds, including one specific to Guayule, were more effective than the java oils in repelling roaches. “This was quite exciting,” she said. Although the repellent scent seemed to be a deterrent to insects, more research is needed. Brewer is also conducting an experiment with mosquitoes. Brewer believes that a guayule extract could repel them. However, it is possible for these compounds to cause skin irritations or allergies when being launched as a consumer product.

” The resin is a complex mixture of compounds. What will determine how it’s used, according to Brewer, is how much money it costs to separate it. This will affect the value of the products you get. “There’s going to be a sweet spot where there’s just enough processing and purification compared to the uses. We don’t yet know what that sweet spot is .”

Although guayule still has a long way to go before it can be commercialized, the researchers are optimistic. Colorado State University’s Quinn leads economic assessments for numerous emerging products and technologies–and he says only about one in 10 ends up seeming viable. “This,” he says, “is one of those.”

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    Ula Chrobak

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