Eco Sustainable Village

A native shrub in the sunflower family (Asteraceae) called guayule (Parthenium argentatum) contains a latex sap with polyterpenes similar to those found in Hevea rubber. It is native to Mexico and the southwestern United States. Guayule is a potentially good source of natural rubber, and has been grown on large plantations in arid desert regions.

Rubber and chicle are polyterpenes composed of thousands of C₅H₈ isoprene subunits. They are found in the milky latex sap produced inside special cells called laticifers or laticiferous tubules in the secondary phloem. Laticifers are single cells or strings of cells that form tubes, canals or networks in various plant organs. Plant families which produce copious amounts of milky latex include the euphorbia family (Euphorbiaceae), milkweed family (Asclepiadaceae), mulberry family (Moraceae), and the dogbane family (Apocynaceae). The main source of natural rubber comes from the para rubber tree Hevea brasiliensis (Euphorbiaceae), a rain forest tree native to the Amazon Basin. Ninety percent of all natural rubber comes from the latex sap of this species. Another New World source of natural rubber in the euphorbia family is the ceara rubber tree (Manihot glaziovii). The ceara rubber tree belongs to the same genus as cassava (Manihot esculenta), the source of tapioca and the staff of life for millions of people living in tropical countries where cereals and potatoes will not grow.

In his fascinating article "Are Vascular Plants Inside-Out Lichens" Ecology 69 (1): 17-23, 1988, Peter Atsatt of the University of California, Irvine discusses several types of cells and tissues in vascular plants which resemble fungal hyphae, including pollen tubes, vascular (xylem) tissue, laticifers, and haustoria. Pollen tubes not only resemble the growth of fungal hyphae, but in Pinus, cycads, and Ginkgo they are branched and actually absorb nutrients from the "host's" megasporangium. The latex-producing laticifers found in many members of the Euphorbiaceae, Asclepiadaceae and other dicotyledonous families are very similar to fungal hyphae. Nonarticulated laticifers are elongate, multinucleate cellular tubes that grow throughout the plant body of these families. These laticiferous tubules are the source of the milky latex that is used to make natural chewing gums and rubber.

The trunk and leaf of the ceara rubber tree (Manihot glaziovii), a little-known source of natural rubber latex prior to the 1900s. Like another economically important species in the genus Manihot called cassava (M. esculenta), the leaf is palmately compound.

There are a number of other plant families with rubber-producing species. One of the most famous is the rubber tree (Ficus elastica), commonly grown as an indoor ornamental and outdoors in coastal southern California. The genus Ficus is a member of the mulberry family. It contains about 1,000 species distributed throughout tropical rain forests of the world, all with a milky latex sap. Every species of fig has its own species of pollinator wasp, in one of nature's most complex interrelationships between a plant and an animal.

There are a number of other plant families with rubber-producing species. One of the most interesting latex products comes from the gutta-percha tree (pronounced GUT-a PUR-cha in Webster's Dictionary). Gutta-percha (Palaquium gutta) is tall tree native to rain forests of eastern Asia. It belongs to the sapodilla family (Sapotaceae), along with the chicle or naseberry tree (discussed above). After it is collected from girdled trees, the gutta-percha latex is allowed to coagulate into globs. It is then washed, purified, and molded into bricks for shipping.

Because the latex of gutta-percha contains fewer branched polyterpene chains than Hevea rubber, it is not as elastic. In addition, the polyterpene molecules of gutta-percha have a trans structure, while those of Hevea rubber have a cis structure. This molecular configuration makes gutta-percha more stable, but reduces its elasticity.

Gutta percha rubber is an excellent nonconductor and insulator, and is resistant to seawater. It was used in the past for rubber strands in the core of golf balls and to coat undersea cables. In 1850 the first submarine telegraph cables were laid, and by the end of the nineteenth century over 250, 000 miles of telegraph cable was in use, all insulated with gutta-percha. Today, gutta-percha insulation has largely been replaced by synthetic polymers, such as polyethylene.

Another interesting property of warm gutta-percha is its resistance to shrinkage while cooling. This property was exploited by die sinkers (people who cut and shape dies) to test their dies before making metallic medals. Medals made from gutta-percha are sometimes seen in antique fairs. In the USA and elsewhere, the name gutta-percha is also used to describe a variety dark-colored Victorian molded materials.

One of the most interesting uses for gutta-percha is in dentistry. Slender strands of gutta-percha are commonly used in endodontics as a filling to replace the reamed out nerve core during root canals. The stable, natural properties of gutta-percha make it one of the best materials for this procedure.

In 1839, Charles Goodyear accidentally spilled a mixture of rubber latex and sulfur on a hot stove and discovered that when it cooled, the rubber lost its stickiness and retained its elasticity. The process was named vulcanization after Vulcan, the Roman god of fire. Vulcanization produces cross-linkages (disulfide bonds) between the thousands of isoprene subunits in the rubber latex, improving the elasticity and durability of the rubber. Although synthetic butadiene and styrene polymers are commonly used in the manufacture of tires, natural rubber is still incorporated into the tires to provide more resilience. This is particularly true in radial tires and high impact airplane tires. Because of the great demand for natural Hevea rubber to this day, guayule is considered by some authorities to be a good alternative source of the valuable latex.

Chicle, the milky latex of the sapodilla tree (Manilkara zapota), was once thought to be a good source of natural rubber. In 1866, Antonio Lopez de Santa Anna brought a sample of chicle to New York. There he met Thomas Adams, a man he hoped could produce rubber from chicle. [Santa Anna was a former Mexican president and the famous general who led the Mexican troops in the battle of the Alamo in 1836.] Santa Anna arranged to have two tons of the chicle latex sent to Adams for experimentation. Adams tried unsuccessfully to vulcanize the chicle, but he could not produce a usable elastic product. He had two tons of chicle with no use for it. Then he remembered a comment by Santa Anna that people in Mexico chewed the chicle latex. Adams came up with the idea of sweetening and flavoring the chicle to make chewing gum. At that time, most of the chewing gum was made from sweetened paraffin, so this seemed like a viable alternative to waxy paraffin. Adam's idea marked the beginning of the chewing gum industry and Adams® brand chewing gums which are still sold to this day. One reference referred to chewing gums as a "social nuisance particularly peculiar to the United States," but Adam's discovery produced one of life's simple pleasures. Although natural chicle is still used, most of today's chewing gums are made from a synthetic vinyl gum base.

Although Thomas Adams was certainly one of the early pioneers of chewing gums, it was William Wrigley Jr. who established an international market for the chewing gum industry. In fact, his name became synonymous with chewing gum throughout the twentieth century. He introduced a number of brands and flavors to the industry and became one of the ten wealthiest men in the United States. Some of his acquired assets included the Chicago Cubs baseball team (with a stadium named after him) and Catalina Island off the coast of California. By establishing factories abroad, he stretched his empire to 37 countries. According to Michael Redclift (2004), his factories produced 280 million sticks of gum a week. Chewing gum had truly become part of the American way of life.

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Last modified: 04/08/06