July/August 1997
Just the Facts: A natural milky white or colorless fluid, also called caoutchouc or latex; derived primarily from Hevea Brasiliensis, the rubber tree; elastic, water-repellent, electrically resistant; basic chemical component: isoprene C5H8.
The Indians of tropical Central and South America are credited with discovering the properties and beneficial uses of natural rubber. Today, more than 90 percent of natural rubber comes from Southeast Asia.
Rubber’s name is derived from its ability to rub out pencil marks, first noted in 1770 by English chemist Joseph Priestley. It wasn’t until the 1800s—and particularly the development of vulcanization in 1839—that rubber began to come into its own. As legend has it, vulcanization was discovered when a mixture of rubber, white lead, and sulfur was dropped accidentally on a hot stove. In vulcanization—a reference to Vulcan, the Roman god of fire—crude rubber is combined with sulfur and heated to between 248o and 320oF. This process makes rubber stronger, more elastic, more resistant to temperature changes, and more resistant to abrasion, chemical action, heat, electricity, and more.
In addition to natural rubber, there are synthetic rubbers, which are artificially produced petrochemical elastomers. While there is only one chemical type of natural rubber, there are roughly 20 chemical types of synthetic rubber. In terms of market share, about 70 percent of all rubber used today is synthetic. In 1997, synthetic rubber usage is expected to reach 2.22 million mt, compared with 980,000 mt of natural rubber consumption, according to the Rubber Manufacturers Association (Washington, D.C.).
Applications. For most uses, natural rubber is mixed with a variety of compounding ingredients that modify its characteristics, including carbon black, zinc oxide, benzoic and stearic acid, oil, and sulfur.
Though uncured rubber has relatively few uses compared with vulcanized rubber, it still can be found in rubber-based cements, various types of tapes, and crepe rubber for footwear and insulating blankets. Vulcanized rubber is used in tires—which account for 60 percent of all rubber consumption—conveyor belts, hoses, shock absorbers, balloons, balls, rainwear, diving gear, protective gloves, and many more products.
Recycling. Rubber products, retreading scrap, and particularly scrap tires can be reused and recycled in a variety of ways. The principal use is as a fuel in cement kilns; dedicated scrap tire-to-energy facilities; and pulp and paper mill, utility, and industrial boilers. Scrap rubber and tires are also ground, stamped, punched, and cut for use in products such as rubber-modified asphalt, athletic and recreational surfaces, carpet underlay, railroad ties, dock bumpers, and even new tires. Other end uses center on civil engineering applications such as landfill cover, artificial reefs, clean fill for road embankments, retaining walls, and more. These and other markets helped reuse or recycle about 202 million—or about 75 percent—of the estimated 266 million scrap tires generated in the United States in 1996, according to the Scrap Tire Management Council (Washington, D.C.). While these numbers are encouraging, there are still an estimated 800 million scrap tires in stockpiles around the country, the council notes. •
Just the Facts: A natural milky white or colorless fluid, also called caoutchouc or latex; derived primarily from Hevea Brasiliensis, the rubber tree; elastic, water-repellent, electrically resistant; basic chemical component: isoprene C5H8.