Guayule Has Real Rubber in It, and It Grows in the United States

by F.S. Nakayama, Research Chemist U.S. Water Conservation Laboratory, ARS, USDA, Phoenix, AZ; W.W. Schloman, Jr., Instructor, Department of Chemistry, University of Akron, Akron, OH; and S.F. Thames, Distinguished University Research Professor and Professor of Polymer Science, University of Southern Mississippi, Hattiesburg.

North and South America have two plants, hevea and guayule, that provide natural rubber for use in commerce. Many of us are familiar with the hevea tree (Hevea brasiliensis), a native of the Amazon region that is now grown primarily in Southeast Asia, because at present, this plant provides all of the natural rubber used in the world.

History of Guayule

The lesser known guayule shrub (Parthenium argentatum) is a native of north-central Mexico and southwestern Texas. In fact, the Spanish explorers saw Indians in Mexico playing with a bouncing ball made from guayule rubber. Rubber production in those days was a community undertaking; the Indians chewed the bark to separate the rubber from the rest of the plant.

The chemical composition and properties of the rubber from guayule and hevea are similar. Though guayule makes about the same quantity of resin-type compounds that have potentially valuable industrial uses, rubber removal from guayule requires special extraction procedures. The guayule plant, unlike hevea, makes and stores rubber in individual cells of the stem instead of in the stem sap.

From the early 1900's through the 1930's, guayule provided a significant amount of the rubber used to make automobile tires. During World War II, the United States made a major effort to produce guayule rubber to replace the then-unavailable overseas source of hevea. This emergency project produced large amounts of guayule rubber, reaching 10 percent of the Nation's supply and use.

The abrupt closure of the project at the end of the war resulted in the abandonment of the plant nurseries, fields, and processing facilities. Germplasm selections, breeding stock, and supplies of rubber and seeds were also destroyed. The present outlook for the commercialization of guayule is clouded because of its cyclical history of sudden spurts of interest followed by complete neglect.

Why Natural Rubber?

Modern transportation depends heavily on natural rubber, as it is the basic ingredient of tires. Natural rubber is also the ingredient of many elastic products that require high tensile strength, durability, and low heat buildup when flexed. So synthetic rubber from petroleum cannot completely replace natural rubber. In addition, natural rubber is a renewable resource whereas petroleum reserves are not.

Properties of Rubber

Natural rubber is a polymer with a chain of thousands of isoprene molecules (C 5 H 8) linked together. Plants such as goldenrod, dandelion, and milkweed manufacture a short-chain-length polymer, averaging 4,000 or fewer isoprene units. By comparison, the hevea and guayule plants can synthesize long-chain polymers of 25,000 or more units. Only the long-chain polymers have the elastic strength and durability for making tires and rubberbands.

A rubber tire encircling the desert shrub guayule symbolizes the plant's potential as a primary source of natural rubber for the United States. Jack Dykinga /USDA 0584X711-28

Rubber (top), extracted from dried guayule (bottom), at the Northern Regional Research Center, Peoria, IL. Guayule now yields about 5 pounds of rubber per 100 pounds of shrub.

Jack Dykinga/USDA 1284X1802-10

Processing of Rubber

When mass production of automobiles began in the early 1900's, rubber tire manufacturing became a major industry. Guayule rubber was obtained by grinding the shrub in water and skimming off the floating rubber "worms." This flotation procedure was used during the World War II production of guayule rubber. Unfortunately, the rubber from this process contains resinous impurities that make the rubber tacky, soft, and likely to break down with time.

Efforts to improve guayule rubber quality were rekindled in the 1970's, when the petroleum crisis occurred. In the new process, resin in the worms was removed by solvent extraction.

Rubber produced in this way can make high-performance aircraft tires.

Further improvement in rubber quality and efficiency of production came about with the sequential extraction procedure. The sequential process used two different solvents: one to dissolve and remove the resin, and the other to dissolve and separate the rubber. In Salinas, CA, and Peoria, IL, USDA researchers evaluated a variety of such processes.

The latest extraction process uses only one solvent to dissolve both the rubber and the resin. Dissolved rubber in the mixture is then removed by selective coagulation and precipitation. In the late 1980's, Bridgestone/ Firestone, Inc., built a pilot facility in Sacaton, AZ, for extracting rubber with this process. The facility produced 5 metric tons of rubber for tire production and testing before it was closed. Resin and low-molecular-weight rubber fractions from this operation are also undergoing development and testing for possible coproduct applications.

This selective coagulation procedure can separate rubbers of different molecular weights that have different physical properties. Thus, the processor can tailor the end product to the needs of the customer.

Applications of Rubber

The demand for rubber continues to grow. Natural rubber now accounts for 31.5 percent, or 4.7 million tons annually, of the total rubber market worldwide. Petroleum-based synthetic rubber meets the remaining needs. In 1990, the United States imported over 800,000 metric tons of natural rubber, costing nearly $1 billion. About two-thirds of this natural rubber is used for tires and related products. This is not surprising, because a passenger tire is made up of 30 percent natural rubber and an aircraft tire 100 percent. The remaining one-third of natural rubber goes into making other rubber products such as V-belts, conveyor belts, storage tank linings, and hoses. A small amount ends up in footwear and adhesive products.

As expected, performance tests of guayule rubber are primarily directed to transportation. These include light-and heavy-duty truck tires, aircraft tires, military tank track pads, and motor mounts. The tensile-strength, wear, and heat-resistance characteristics of guayule products have met or exceeded those of similar products manufactured from hevea rubber.

Other non-transportation market products are available. Guayule rubber makes a nonstick base for chewing gum. Blending guayule rubber with polyolefins produces a polymer for making extruded or injection-molded products. Finally, chemical modification of guayule rubber has produced new types of polymers. These polymers have properties similar to those of existing thermoplastic elastomers for making various types of molded goods.