Rubber

I

Introduction

Rubber, natural or synthetic substance characterized by elasticity, water repellence, and electrical resistance. Natural rubber is obtained from the milky white fluid called latex, found in many plants; synthetic rubbers are produced from unsaturated hydrocarbons.

II

Natural Rubber

In its natural state, rubber exists as a colloidal suspension in the latex of rubber-producing plants (see Colloid). The most important of these plants are the tree Hevea brasiliensis of the spurge family, and other species in the same genus, which were the sources of the original South American rubber, the commercially important Para rubber. The term Para rubber was then also applied to the product of H. brasiliensis trees cultivated in the rubber plantations of Indonesia, the Malay Peninsula, and Sri Lanka. These trees produce about 90 per cent of all the new natural rubber consumed.

Crude rubber from other plant sources is generally contaminated by an admixture of resins that must be removed before the rubber is suitable for use. Such crude rubbers include gutta-percha and balata, which are products of various tropical trees in the sapodilla family, Sapotaceae. Other, nontropical sources of rubber, which were cultivated for economic reasons during World War II, include two shrublike plants: guayule, Parthenium argentatum, native to Mexico, and the Russian dandelion, Taraxacum kok-saghyz, native to Russian Turkistan.

A

Collection of Latex

To gather the latex from plantation trees, a diagonal cut angled downward is made through the bark; this cut extends one-third to one-half of the circumference of the trunk. The latex exudes from the cut and is collected in a small cup. The amount of latex obtained on each tapping is about 30 ml (about 1 fl oz). Thereafter, a thin strip of bark is shaved from the bottom of the original cut to retap the tree, usually every other day. When the cuttings reach the ground, the bark is permitted to renew itself before a new tapping panel is started. About 250 trees are planted per hectare (100 per acre), and the annual yield for ordinary trees is about 450 kg per hectare (400 lb per acre) of dry crude rubber. In specially selected high-yield trees, the annual yield may range as high as 2225 kg per hectare (2000 lb per acre), and experimental trees that yield 3335 kg per hectare (3000 lb per acre) have been developed. The gathered latex is strained, diluted with water, and treated with acid to cause the suspended rubber particles within the latex to clump together. After being pressed between rollers to consolidate the rubber into 0.6-cm (0.25-in) slabs or thin crepe sheets, the rubber is air- or smoke-dried for shipment.

B

Chemical and Physical Properties

Pure crude rubber is a white or colourless hydrocarbon. The simplest unit of rubber is isoprene, which has the chemical formula C₅H₈. At the temperature of liquid air, which is about -195° C (about -319° F), crude rubber is a hard, transparent solid; from 0° to 10° C (32° to 50° F) it is brittle and opaque, and above 20° C (68° F) it becomes soft, resilient, and translucent. When rubber is mechanically kneaded, or is heated above 50° C (122° F), it becomes plastic and sticky; above 200° C (392° F) it decomposes.

Crude rubber is insoluble in water, alkali, and weak acid; it is soluble in benzene, petroleum, chlorinated hydrocarbons, and carbon disulphide. It is oxidized readily by chemical oxidizing agents, and slowly by atmospheric oxygen.