Rubber, whether derived naturally from a tree or synthetically from petroleum products, demonstrates certain properties. These properties led to its use by native American cultures and Western societies since its introduction in the 18th century. Named for its property as an eraser to "rub out" pencil marks, rubber continues today to be widely used.
Make a cut in a rubber tree, or Hevea brasiliensis, and a milky sap oozes out. That sap is latex, produced by laticifers, special cells in the tree. The latex from the rubber tree is elastic. At one time rubber came from wild trees in South America, primarily Brazil. Today almost all rubber is harvested in rubber plantations in southeast Asia. The properties of the rubber latex were discovered by native American cultures, who made rubber balls and used the latex for waterproofing. Today the latex is harvested by cutting the tree each day and collecting the latex in a cup.
Take a rubber band and stretch it. Then release the band. Its ability to extend to long lengths and then return to its original shape demonstrates the elastic property of rubber. According to an answer about rubber bands and elasticity on the U.S. Department of Energy website, the polymer molecules in the rubber band are stacked at rest. When stretched they move out into a line, with the length of the line depending upon the number. Some of the molecules are attached to each other. When you stretch a rubber band too far, you discover this attachment as the band snaps.
Contraction by Heat
Most materials expand when heated. Rubber does just the opposite; it contracts. This occurs because the heat causes the molecules to become tangled with each other. This property is demonstrated by experiments shown by the University of Wisconsin. Rubber bands that have tangled molecules at rest become more so when heated. Remove the heat and the rubber band returns to its original shape, just as it did when the stretching stopped.
Rubber demonstrates resistance to water as well as low temperatures, based upon an article in Info Comm. Rubber is resilient, hard to tear and resists abrasions. It withstands impacts due to its strength and has slow buildup of heat. These properties lead to its use in tires, first for bicycles and then automobiles. When used in latex gloves, especially in the medical profession, it has been shown to create allergies in certain individuals.