Caoutchouc, B.P. Indiarubber.
Indiarubber (Elastica, U.S.P., Rubber) consists of the prepared latex of Hevea brasiliensis, Muell. Arg. (N.O. Euphorbiaceae), and probably other species indigenous to Brazil; and is known commercially as pure Para rubber. The latex is obtained from the tree by incisions into the bark of the trunk, and coagulated in successive layers on a wooden paddle, by dipping this instrument in the latex and holding it for a few moments in the smoke of burning palm nuts. The coagulated latex is then cut from the paddle, and hung up to dry. Para rubber is imported in flat, elastic masses of varying size, brownish-black externally, paler internally, exhibiting a number of thin layers separated by dark lines. By keeping, it gradually darkens in colour and slowly loses its elasticity. It has a characteristic odour, and is nearly devoid of taste. When heated to 125° it melts, and remains soft and adhesive after cooling. Masticated indiarubber occurs in sheets having a rough surface. It consists of the threads of rubber which have come from the "washing" machine and have been passed through the masticator or kneader, in order that a denser and more uniform product may be obtained. Commercially, indiarubber is obtained from many other plants growing in tropical climates. In Central America, Ecuador, and Peru it is obtained from Castilloa elastica, Cerv. (N.O. Urticaceae), and other species of Castilloa in Madagascar, from species of Landolphia (N.O. Apocynaceae); in Mangabeira, from Hancornia speciosa, Gomez (N.O. Apocynaceae) in Africa, from Funtumia elastica, Staph. (N.O. Apocynaceae), and species of Landolphia. The coagulation is effected by different methods, according to the nature of the latex. In the inferior varieties of rubber the amount of resin present may vary from 10 to 40 per cent.
Insoluble in water, alkalies, or dilute acids. Chloroform, oil of turpentine, carbon bisulphide, mineral naphtha, and benzol, however, have the effect of making it swell and become soft and gelatinous; it is probable that one constituent of the rubber thus passes into solution, while the other remains undissolved, but in a more or less disintegrated condition.
Constituents.—Good Para rubber contains from 40 to 60 per cent. of the hydrocarbon caoutchouc, 30 to 50 per cent. of a gelatinous substance and (1 to 4 per cent. of various impurities, such as resin, fat, colouring matters, and mineral substances. Pure caoutchouc is a white amorphous substance with the empirical formula (C10H16)n, and appears to be an isomer of gutta. When subjected to dry distillation, it yields "oil of caoutchouc"—a mixture of hydrocarbons, including isoprene, caoutchine, and heveene.
Uses.—Indiarubber is a constituent of the basis for self-adhesive plasters, with which medicaments, such as zinc oxide, belladonna, capsicum, etc., are incorporated. Liquor Caoutchouc is a solution of indiarubber in benzol and carbon bisulphide; mixed with powdered mustard seed (freed from fixed oil) it is used in the preparation of Charta Sinapis. Solutions of gutta percha are much to be preferred to those of indiarubber for the application of such medicaments as chrysarobin and resorcin to the skin where close and prolonged contact is required. The indiarubber film is moist and sticky, that of gutta percha dry and hard.
PREPARATIONS.
- Emplastrum Adhaesivum, U.S.P.—ADHESIVE PLASTER, U.S.P.
- Rubber, cut in small pieces, 2; soft paraffin, yellow, 2; lead plaster, 96. Melt the rubber at a temperature not exceeding 150°, add the soft paraffin, and continue the heat until the rubber is dissolved, add the lead plaster, and continue to heat until it becomes liquid. Strain, allow to cool, and stir until it thickens.
- Emplastrum Adhaesivum Elasticum, B.P.C.—RUBBER ADHESIVE PLASTER.
- Indiarubber, with wool fat, balsam of copaiba, orris root, thymol, methyl salicylate, and solvent mineral naphtha.
- Liquor Caoutchouc, B.P.—SOLUTION OF INDIARUBBER.
- Indiarubber, in fine shreds, 5; benzol, 50; carbon bisulphide, 50. Add the indiarubber to the carbon bisulphide, and set aside in a cool place until a jelly is formed; then add the benzol and shake occasionally until a clear solution is obtained. Solution of indiarubber is a reddish-yellow, somewhat viscid liquid. It is used as an adhesive agent in the preparation of mustard leaves, and as a substitute, in certain cases, for collodion.
