CAOUTCHOUC, koo'choolc, an elastic, gum-like substance, obtained from the juice of certain tropical trees and shrubs, and commonly known as India-rubber or grubber.' The best caoutchouc comes from the Path region, in Brazil; but supplies are also obtained from Central America and the West Indies, from Africa and from tropical Asia, especially from Ceylon and Malaya. The details of collect ing the juice and preparing it for market vary somewhat according to the locality, and with the nature of the trees or shrubs from which the juice is obtained. In the Amazon region, when the source is a tree, incisions are made in the bark each morning, and the milky sap that exudes is collected in little tin or clay cups that are secured to the tree for the pur pose. At the end of about 10 hours these are emptied into larger collecting vessels, and on the morning of the following day new incisions are made in each tree some eight inches be low the first ones. This process is continued until incisions have been made in the bark from a height of about six feet down to the ground. The poorest quality of sap is obtained from the highest wounds, and the best from the low est ones. To evaporate the juice, a fire is first built of materials that yield dense volumes of smoke. A workman then dips a wooden paddle into the collected sap, after which he holds it in the smoke until the sap solidifies and ac quires a slight tinge of yellow. He then dips the paddle into the sap supply again, repeats the smoldng process, and so proceeds until the paddle is covered with a layer of the dried gum that is perhaps an inch and a half thick. He then slits this layer, removes it from the paddle, hangs it up to dry, and starts a fresh evaporation.
Pure caoutchouc from Path is light-colored below the surface, but superficially it is dark brown from oxidation. It has a specific gravity of about 0.92, and consists chiefly of carbon and hydrogen in the proportion of about 87 per cent of carbon to 13 of hydrogen. Small quantities of oxygen are always present, however, as the best of the Path product contains as much as one-half of 1 per cent of a sort of resin that contains oxygen, and is undoubtedly produced by the oxidation of the gum. In fact, it is known that caoutchouc will oxidize slowly in damp air, even after it is vulcanized, and par ticularly when exposed to the action of light. Caoutchouc consists, apparently, of two differ ent kinds of gum, one of which is fibrous, while the other is viscous, though the two are chemi cally identical. It is slightly soluble in ether, turpentine, chloroform, petroleum, naphtha, benzine and carbon disulphide, the viscous por tion being more soluble than the fibrous part At 250° F. caoutchouc begins to melt, and be comes permanently transformed into a sticky substance which retains its peculiar consistency almost indefinitely. At 400° F. the transforma tion is more complete, and the black, adhesive mass that results makes an excellent lute for sealing glass bottles and jars if it is thoroughly incorporated with 50 per cent of its own weight of dry slaked lime. By careful destructive distillation caoutchouc is resolved into a num ber of hydrocarbon oils that are of interest to the chemist.
As early as 1615 the Spaniards used the crude gum waxing their cloaks, which were made of canvas, so as to make them resist water." But it was not until about 200 years later that caoutchouc began to attract general attention in such ways. At first it was applied to cloth by the aid of heat; but improved meth ods followed the discovery of solvents for the gum, and the invention, by an Englishman named Thomas Hancock (about 1820), of the (masticator,' a machine by which the caout *clone is thoroughly worked over and brought to a uniform consistency. But the greatest step i in the development of the rubber industry was the discovery of the process of vulcanization,— a discovery that appears to have been made in dependently and at about the same time (1843) by Charles Goodyear, of New Haven, Conn., and Thomas Hancock, to whom reference has previously been made. The credit of priority belongs to Goodyear, but Hancock did a great deal to make the discovery a commercial suc cess. Unvulcanized caoutchouc is softened by heat, and is made hard and inelastic by cold; but upon being vulcanized the gum becomes comparatively insensible to ordinary extremes of temperature, and also has its elasticity ma terially increased. The process of vulcanization depends upon the fact that the crude rubber will absorb sulphur, and combine with it at a temperature that is easily attainable without injury to the product. The details of the vul canization differ somewhat, according to the nature of the article that is being manufactured. If sheet rubber is submerged for a few mo ments in melted sulphur at a temperature of 250° F., it absorbs about one-tenth of its weight of that element; but although its color changes somewhat, it is otherwise apparently unaltered. Upon exposure for a somewhat longer time to a temperature of 285° F., however, true com bination of the sulphur and caoutchouc ensues, and the gum is said to become ((vulcanized." It is not necessary that the sulphur should be ac tually melted in order that the sheet rubber may absorb it, for sheets that are laid in powdered sulphur that is heated nearly to its melting point will absorb the proper amount for good vulcanization in the course of a few hours. Vulcanization of rubber sheets can even be brought about without the action of heat, by dipping the sheets for a few seconds in a solu tion of chloride of sulphur in carbon disulphide. It is more common, however, to knead the requisite amount of sulphur directly into the caoutchouc by mechanical means. Tile article to be manufactured is then brought into shape by the action of pressure and moderate heat (or in any other manner), and the final opera tion consists in heating it to the vulcanizing temperature by the aid of a steam bath. Chem ically considered, the process of vulcanization appears to consist in the substitution of one or more sulphur atoms for a portion of the hydro gen of the hydrocarbons of which the caout chouc is composed. For many years chemists have been interested in synthetic rubber to take the place of natural caoutchouc, but the arti ficial product has not yet come into extensive use. See INDIA RUBBER; RUBBER MANUFACTURE.