By Jan H. Schut
Over the last 100 years almost all natural rubber has come from tropical rubber trees (Hevea brasiliensis). The global natural rubber supply, which is critical for tire making, is periodically at risk from disease, high prices, and political instability in Africa and Southeast Asia where these trees are grown.
Governments have sponsored research on alternative natural rubber for tires off and on for over 100 years, typically in war time or when rubber prices soar. Now tire makers around the world are driving the development of alternative sources of natural rubber like dandelions and guayule shrubs, working with universities and bio-tech companies. While promising, the alternatives are a lot harder to harvest than latex, which can be tapped and collected right from the rubber tree. The alternative plants also need secondary processing, mechanical-, aqueous-, or solvent-based, to extract rubber-like polymer.
At least one auto maker also wants to modify plastics with alternative rubber, potentially for TPO bumper covers and TPE interior parts. TPOs are mostly copolymers of ethylene octene and ethylene butylene with PP and some copolymers of EPDM/PP, all of which are more stable and compatible with PP than natural rubber, compounders say. Melt blending natural rubber and PP would actually be going back to older technology, but the carbon footprint of natural rubber grown in the U.S. would be attractive.
Alternative rubber plants include Russian or “TKS” dandelions (Taraxacum kok-saghyz), actually from Uzbekistan and Kazakhstan; guayule (Parthenium argentatum) a shrub found in Mexico and the American southwest; sunflowers (Helianthus sp.); prickly lettuce (Letuca serriola); and certain fungi. TKS dandelions would be an annual crop with rubber extracted from the roots. Guayule bushes take two years to mature, but then can be harvested annually, producing rubber in the bark. Both dandelions and guayule produce very high molecular weight polymer like that from rubber trees. Sunflowers are an annual oil seed crop with very low molecular weight latex in the leaves–30,000-80,000 molecular weight vs. 1,300,000-1,500,000 for dandelion, guayule and tree rubber.
“If you were making rubber gloves or condoms, you would want guayule, which is stretchier [than dandelion rubber] and has no allergens. Someone who developed an allergy to Hevea latex would be allergic to dandelion latex, which is almost identical. Sunflower rubber hasn’t been tested yet for allergens,” says Katrina Cornish, research scholar at Ohio State University’s Ohio Agricultural R&D Center in Wooster (oardc.osu.edu). Cornish also heads Program of Excellence for Natural Rubber Alternatives (PENRA), working on both guayule and dandelion rubber. PENRA members include Cooper Tire & Rubber Co., Findlay, OH (www.coopertire.com); Bridgestone Corp., Tokyo, Japan (www.bridgestone.com); Goodyear Tire & Rubber Co., Akron, OH (www.goodyear.com); and Ford Motor Co., Dearborn, MI (corporate.ford.com).
As part of a U.S. Department of Agriculture/Department of Energy grant, Cooper also works with Cornell University, Ithaca, NY (www.cornell.edu); Clemson University, Clemson, SC (www.clemson.edu); and guayule rubber supplier PanAridus LLC, Casa Grande, AZ (www.panaridus.com). Cooper unveiled a tire with multiple guayule rubber components in August 2015 and announced plans to deliver a prototype tire with 100% of natural rubber components made from guayule by 2017.
Bridgestone set up a bio-rubber process research center in Mesa, AZ, and a 281-acre guayule R&D farm in Eloy, AZ, which started up in 2013. The goal of the farm is to produce seed for high-rubber-content guayule and work with independent guayule producers. Bridgestone also has a patented process (U.S. Pat. # 8815965) for “recovering rubber from natural latex” including guayule and TKS. In October this year Bridgestone announced that it had built passenger tires in Japan and in Italy with natural rubber components made 100% from guayule rubber. Bridgestone says the earliest guayule rubber will be in commercial tires is in the 2020s.
Internationally, the tire division of Continental AG in Hanover, Germany (www.continental-corporation.com), an automotive supplier, works with a consortium including the Fraunhofer Institute for Molecular Biology and Applied Ecology in Muenster, Germany (www.ime-fraunhofer.de) and the University of Muenster’s Institute for Plant Biology and Biotechnology (www.taraxagum.com). Apollo Vredestein B.V., Enschede, the Netherlands (www.apollovredestein.com), part of Apollo Tyres Ltd. in Gurgaon, India (www.apollotyres.com), works with the European Union’s EU-PEARLS (www.eu-pearls.eu) alternative rubber project on both guayule and dandelion rubber. Japanese tire maker Sumitomo Rubber Industries Ltd. in Kobe (www.srigroup.com) works on dandelion rubber with Kultevat Inc. in St. Louis, MO (www.kultevat.com), a startup founded in 2010. Kultevat partners with KeyGene N.V., a biotech company in Wageningen, the Netherlands (www.keygene.com), and with the Donald Danforth Plant Center in St. Louis (www.danforthcenter.org).
USING ALTERNATIVE NATURAL RUBBER IN PLASTICS
Both guayule and dandelion rubber could be used to modify PP, says Deborah Mielewski, senior technical leader of sustainable materials and plastics research at Ford, which is working with both alternative rubbers to modify soft TPEs and rigid TPOs. At the Society of Plastic Engineers 2015 TPO Conference last September in Troy, MI, Ford reported testing new generation TPOs that combine recycled PP with guayule and dandelion rubber, reinforced with graphene nanoplatelets to meet specs for things like wheel lips and bumper covers. Guayule is available in larger quantities; dandelion rubber is currently only available in tiny amounts, researchers say.
A patent application (U.S. Pat. Applic. # 20150267015) from Ohio State’s Cornish on “Latex products containing fillers from waste” also reports promising blends of guayule rubber with a cross-linking agent like sulfur and unusual bio fillers like finely ground eggshells and tomato skin waste from catsup making. When the biomass/guayule blend was made into cast film (0.03 to 0.26 mm thick) and tested, it showed tensile strength over 24 MPa; elongation @ break over 750%; flexural modulus of 500%; and elongation of less than 5.5 MPa, the patent application says. Ohio State researchers also blended PLA and PHBV with up to 25% natural rubber, improving the biopolymers’ flexibility.
Guayule rubber also adds insecticidal properties. Fifteen years ago the University of Illinois and U.S. Department of Agriculture applied for a patent (WO # 2001088051) on composites of 70% guayule rubber and 30% HDPE from recycled milk bottles. They tested composites of both ground whole guayule plants and ground guayule bagasse after rubber extraction and found that both showed strong resistance to termites and decay. Only 5% of termites were alive after a week in a jar with a guayule composite sample vs. 100% alive for those in a jar with a piece of pine wood. They also reported that test composites of HDPE with three guayule species and bagasse met or exceeded American Standards for hardboard. The patent concludes that both guayule and guayule bagasse would be useful in wood-filled plastic profiles like lumber, poles, and railroad ties or in plywood and chipboard composites.
Strength of 30/70 HDPE/Guayale vs. HDPE/Wood Flour and Hardboard Standards
|Fiber type||Modulus of rupture
|Modulus of elasticity
|American Standard, 1995
for service hardboard
|American Standard, 1995
for industrial hardboard
*Internal Bond or Tensile Stress Perpendicular to Plane
Source: Univ. of Illinois/USDA Paper for 33rd Annual IRG Meeting, Cardiff, Wales
PanAridus is currently the only commercial producer of guayule rubber, making “several thousand pounds annually” from a plant line that matures in 16-20 months, “and produces more rubber per acre than the tropical rubber tree,” says founder and CEO, Michael Fraley. PanAridus, founded in 2009, uses patent-applied-for technology (U.S. Pat. Applic. # 20150232583) to extract rubber from guayule plants with solvent, reportedly recovering 10% guayule rubber and another 10% lower molecular weight resin for adhesives.
A previous company, Yulex Corp. in Phoenix, AZ (www.yulex.com) founded in 2000, used an aqueous extraction process and sold guayule rubber commercially for medical gloves and wetsuits. Yulex also supplied guayule rubber for Ford’s TPO R&D. Yulex is auctioning its equipment in December and is believed not to be operating.
Kultevat commercially produces dandelion rubber. The company was founded by its president, Daniel Swiger, who was previously a founder of Yulex. In an executive summary in September 2014, Kultevat said it expected to produce 6000 lb of TKS rubber in 2015. Kultevat has patent-applied-for water-based technology (U.S. Pat. Applic. # 2014037059) to extract rubber and sugar for ethanol from shredded dandelion roots.
A Canadian start-up Nova-BioRubber Green Technologies Inc., in Surrey/Vancouver, B.C. (www.novabiorubber.com), plans to produce TKS dandelion rubber using a patented dry mechanical extraction process (U.S. Pat. # 7540438), developed by Kok Technologies Inc., a related company started in 2002. Nova-BioRubber works with National Research Council Canada in Ottawa, Ont. (www.nrc-cnrc.gc.ca), and the Build-in-Canada Innovation Program (buyandsell.gc.ca) to develop its TKS process.
Another start-up biotech company, Edison Agrosciences Inc. in Durham, NC (www.edisonagrosciences.com), plans to produce low molecular weight rubber from sunflower leaves. Edison director of research, Thomas Hohn, says they won’t use leaves from existing oil seed crops, but are developing sunflowers with increased leaf size and rubber concentration that will be grown exclusively for rubber. Ohio State’s Cornish is their chief scientific officer.