CAOUT′CHOUC

Syn. India rubber , Elastic gum . India rubber is the concrete juice of the Ficus elastic a , Siphon i a elastic a , the Urceola elastic a , and many other tropical plants. The fresh milky juice is spread over moulds of unbaked clay, and is then exposed to the heat and smoke of a fire, or torches, to dry it, whence it derives its dark colour. Succe ive coats of juice are laid on, and the operation of drying repeated until the bottles acquire sufficient thickne . When it has become thoroughly hard and dry, the clay is beaten out. In this form it is commonly imported. Prop., . The general properties of india rubber, as well as its numerous applications, are well known. The fresh juice has a cream-like appearance and consistence, is coagulated by heat, and is miscible with water, alcohol, and wood naphtha; sp. gr. 1·012 to 1·041; it yields from 18% to 45% of solid caoutchouc, either by heat or evaporation. By excluding it from the air it may be preserved unchanged for a considerable period. Solid caoutchouc has a sp. gr. ranging between ·919 and ·941; it melts at 248° Fahr. into a viscid ma , which does not again harden on cooling; it is unaltered by chlorine, hydrochloric acid, sulphurous acid, fluosilicic acid, ammonia, caustic alkaline lyes (even when boiling), and most similar substances; nitric acid and sulphuric acid act on it only by long contact when concentrated. Some specimens of caoutchouc are harder than gutta perch a itself, and equally inelastic, whilst others never perfectly solidify, but remain in a condition resembling that of birdlime or printers’ varnish. The best solvents of caoutchouc are rectified sulphuric ether (which has been washed with water to remove alcohol and acidity), chloroform, bisulphide of carbon, a mixture of bisulphide of carbon and absolute alcohol (94 of the first to 6 or 7 of the last), and caoutchouc in. All these liquids di olve india rubber rapidly in the cold, and leave it unaltered on evaporation. The first two are, however, too expensive to be generally employed. The others have a disagreeable odour, but are much cheaper than the rest, and po e the advantage of leaving the film of caoutchouc in a firmer and stronger condition than other solvents. Pyrogenous oil of turpentine is another cheap and good solvent. Benzol, rectified mineral or coal-tar naphtha, crude petroleum, and oil of turpentine di olve india rubber by long digestion and trituration (with heat), otherwise they merely form with it a glutinous jelly that dries very slowly and imperfectly, leaving it much reduced in hardne and elasticity. The fats and fixed oils also readily di olve caoutchouc (with heat), forming permanently glutinous solutions or pastes; so also do most of the volatile oils, but the solutions with the majority of them dry with difficulty. One of the most remarkable properties of india rubber is the great amount of heat which is disengaged during its condensation by pre ure or in the exercise of its elasticity. During the proce of kneading the raw caoutchouc in the “masticators,” the cold water thrown in to reduce the temperature soon becomes boiling hot. When no water is added, a temperature so high is often reached as to occasion the melting of the rubber. This is particularly the case during the proce of “dry kneading” with quick-lime. A tube 2 1 ⁄ 4 inches in diameter, impactly secured, was subjected to a force of 200 tons. The result was a compre ion amounting to 1-10th; great heat was evolved, and the exce ive elasticity of the substance caused a fly-wheel weighing five tons to recoil with alarming violence. Mr Brocked on states that he succeeded in raising the temperature of an ounce of water 2° in about fifteen minutes by collecting the heat evolved by the extension of a small thread of caoutchouc. He refers this effect to the change in specific gravity, and contends that the heat thus produced is not due to friction, because the same amount of friction is occasioned in the contraction as in the extension of the substance, and the result of this contraction is to reduce the caoutchouc thus acted upon to its original temperature. The edges and surfaces of india rubber are readily and perfectly joined by mere contact and intense pre ure. On the small scale the edges may be moistened with ether, naphtha, oil of turpentine, or some other solvent, or by long boiling in water, and immediately pre ed tight together and held in contact for some time. Elastic tubes are readily formed of india rubber by cutting it into uniform slips of proper thickne and winding them round rods of polished gla or metal, so that the edges are in close contact or “overlapping.” A piece of tape is then wound round outside it, and the whole boiled in water for 2 or 3 hours, after which time the edges will be found to be sufficiently adherent. A better plan is to immerse the “rubber” in a mixture formed of bisulphide of carbon, 95 parts, and rectified spirit, 5 parts, until it swells into a pasty ma , which may then be moulded into any desired form or pa ed through the die of a tubing machine. For chemical purposes, brewing, ., vulcanised india-rubber tubing has now taken the place formerly occupied by the unprepared material. The once celebrated “Mackintoshes” are made by spreading two or more coats of a paste made of caoutchouc and rectified coal-tar naphtha over the surface of the stuff or cloth, and, when it has become partially dry, pre ing two such surfaces evenly together by pa ing the goods between a pair of cylinders or rollers. The articles are then placed in a stove room for the composition to harden, and to remove the odour of the naphtha. Of late years vulcanised or mineralised rubber (coloured) has been used for this purpose, and being spread on the outside of the stuff instead of the inside forms an ornamental and thoroughly waterproof material. India-rubber thread is prepared by stretching it (previously cut into coarse filaments) to 5 or 6 times its length in boiling water or hot air, in which state it is allowed to cool slowly. This proce is repeated again and again until it reaches 16,000 or 17,000 times its original length, when it is glazed by agitating it with powdered sulphur or French chalk. This thread is readily joined or “pieced,” as it is called, by paring the ends obliquely with a pair of sci ors or a knife, and then pre ing the clean ends strongly together with the fingers. When the coarse filaments from the cutting machine are simply stretched with the moistened thumb and finger in the act of “reeling” to about 8 or 9 times their length, they are said to be “in elasticated,” and are ready to be made into elastic braces, elastic web, and other like elastic ti ues and fabrics in the braiding machine.