Nature’s material engineering

The world's main source of rubber is under threat from a fungus, forcing scientists to search for new ways to make the material. Lou Reade reports.

It may be some time since you picked a dandelion and watched the white sap ooze from its broken stem. But this simple action – performed on an industrial scale – might one day come to the car industry's rescue.

The sap found in dandelions is rich in latex – the main ingredient in car tyres, surgical gloves and a host of other products. Latex has always been sourced from a particular tree, the Hevea brasiliensis (otherwise known as the rubber tree), but has come under threat from a number of factors, including a fungus (South American Leaf Blight), insect infestation and climate change. If these continue to adversely affect rubber trees, it could deal a body blow to the global supply of this ubiquitous material.

Rubber was first used on a large scale during the industrial revolution, at a time when another naturally derived product, whale oil, was used as a machine lubricant. Though mineral oils have long since replaced whale oil, rubber is still sourced in the same way.

Imagining a world without rubber is difficult, because in some applications – car tyres, for example – it is irreplaceable. This goes some way to explaining why tyre companies are instrumental in many research projects to find alternative sources of rubber.

EU-Pearls is a pan-European project whose partners are dedicated to finding – and developing – new sources of rubber. It is concentrating efforts on two main sources: the Russian dandelion (Taraxacum kok-saghyz, or TKS) and guayule (Parthenium argentatum), a shrub that thrives in hot, dry conditions. Guayule is a more promising crop for the Mediterranean region, while the Russian dandelion is more suited to northern and eastern Europe.

The project, which began four years ago, recently hit a milestone when one of the industrial partners – Dutch tyre manufacturer Apollo Vredestein – produced the first prototype tyre, using latex produced in Europe from these two plants. The prototypes are now undergoing testing, before the project can progress to full-scale production. The partners hope this will be the first step to setting up European rubber production, in order to reduce reliance on Asia.

One potential route to full industrialisation may be through work led by Dirk Prüfer, of the Institute of Plant Biology and Biotechnology at the University of Münster in Germany. Prüfer estimates that each hectare of land could produce up to 1 tonne of latex per growing season.

His team has identified an enzyme that controls polymerisation of the latex in Russian dandelions. By switching off the enzyme, the latex can flow freely and be collected more easily for industrial use.

"If the plant is cut, the latex flows out instead of being polymerised – and we obtain four to five times the normal amount of material," he says.

Studies have shown that both Russian dandelion and guayale are good alternatives to the rubber tree. One area of interest is to speed up the growth of the Russian dandelion, and increase the content of rubber available for extraction.

EU-Pearls researchers add that latex from these sources wouldn't cause an allergic reaction, which can be a problem with material derived from the traditional source. This would be useful in the medical industry for items such as surgical gloves.

Much of the work on these alternative materials is more akin to agricultural research than materials development because the focus of many scientists is on the traits of each particular strain of plant, and which of them is likely to yield the highest levels of latex.

For example, US researchers at Ohio State University have screened Russian dandelions in an attempt to identify which have the highest latex content. Promising specimens were cross-bred in order to boost latex levels. Of the specimens produced, about 12% had latex levels higher than 10%, which is the minimum needed for commercialisation.

The University's influence on the search for alternative materials goes beyond this single research paper, because one of its departments – the Ohio Agricultural Research and Development Center – coordinates the Program for Excellence in Natural Rubber Alternatives (Penra), a US-wide project similar in scope to EU-Pearls.

One of the partners, the US arm of Japanese tyre giant Bridgestone, recently set up a guayale research farm on a 281-acre site in Arizona – which will supply its new process research centre nearby.

"This is an important step in the realisation of our guayule research activities," says Bill Niaura, director of new business development for Bridgestone Americas. "It brings us closer to our ultimate goal of developing new, domestic and sustainable sources for natural rubber."

The research farm will produce guayule as a commercial crop. Guayale shrubs will be supplied to the nearby process research centre, to be converted into tyre-grade rubber. The research centre is expected to be fully operational in 2014, with trial rubber production starting in 2015.

This move is just a part of the company's strategy to find new sources of rubber – and hang on to the old one. Because, as well as mapping the genome of the Hevea brasiliensis tree (which, it says, may help it to breed new disease-resistant varieties), it has helped to develop tests to diagnose the disease.

It seems unthinkable that these new plant sources could take over, but if the South American Leaf Blight gets the upper hand, the rubber tree could one day go the same way as whale oil.

Source material
The search for new sources of rubber is part of a wider attempt by the chemicals industry to find new ways of making materials using renewable sources, rather than petrochemicals.

Green materials, from bio-derived plastics to speciality chemicals, are on the increase. They are being created through a combination of new and traditional chemistry, with a bit of genetic engineering thrown in.

Brazilian petrochemicals giant Braskem, for example, is making 'green' polyethylene from sugar cane, while many nylon manufacturers are using castor oil as a feedstock.

And you can now make rubber out of starch. DuPont subsidiary Genencor has developed enzymes that convert starch into isoprene gas. This is then supplied to Goodyear, and polymerised into polyisoprene, for use in its 'BioIsoprene' tyre – which it hopes to begin producing commercially next year.

Author
Lou Reade

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