RUBBER

is the later from several tropical trees (e.g.) Hevea brazil i ens is, Brazil (Para rubber), Central America, E. and W. Africa, Ceylon, etc. Incisions are made in the bark, beneat. h| which the laticiferous ve els occur, and latex, an emulsion, runs out which on coagulation yields rubber. Production.—In 1921, 295,642 tons were produced. Of this 272,915 tons was plantation rubber and 19,837 tons came from Brazil. American interests have acquired plantations in Sumatra, but the greater part of the rubber producing area is in the hands of British capitalists. Owing to exhaustion of natural resources, Wild rubber is likely to be largely and gradually replaced by plantation rubber. The world’s estimated annual consumption of rubber is (1920) 384,000 tons, whereas in 1875 only 5,000 tons were required. To secure this supply every suitable part of the tropics is being rapidly put under cultivation. his increase is largely due to the demand for motor and cycle righ and the development of electrical lustry. Nature-—The e ential constituent of rubber is caoutchoue (india-rubber), which is present to the extent of 70 to 90 per cent., mixed with resin, proteid matter, ash, and moisture. Caoutchouc is a soft, elastic solid, insoluble in water and alcohol, little acted on by acids and _ alkali but soluble in carbon disulphide, benzene, carbon tetra-chloride, chloroform, etc., forming rubber solutions; it softens above 100° C. and melts at 150 to 200° C.; it consists of unsaturated hydrocarbons of the come position (C;sHs)n, which on destructive distillation yield isoprene. Caoutchouc absorbs oxygen, forming a brittle substance; it also combines with ozone, halogens, and sulphur (vulcanization). Manufacture. — Crude rubber {is washed with hot water, and p between rollers together with sulphur, pigment, and mineral matter, such as whiting or barium sulphate. Vulcanization, which involves combination with the sulphur, is effected by heating the mixture for half an hour to 135-150° C. in closed iron ve els. Chloride of sulphur is sometimes employed for vulcanizing. Vulcanized rubber is harder than caoutchouc, and is unaltered at 160° C. Commercial rubber contains 12 to 60 per cent. caoutchouc, 1 to 2 per cent. combined sulphur, and 25 to 70 per cent. mineral matter. Exce ive vulcanization produces _ vulcanite or ebonite, which can be polished. See Ru er, ARTIFICIAL.