This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Caoutchouc is a substance contained in the latex of a large number of plants, especially of those belonging to the natural orders Euphorbiaceoe, Urticaceoe, and Apocynaceoe, but it is by no means confined to these. Indeed, it is so widely distributed that in all cases the latex of plants appears to contain caoutchouc or some similar substance as a constant constituent. Many secrete it in such quantity as to form suitable sources for its commercial extraction. Commercial caoutchouc, however, always consists of caoutchouc associated with fat, resin, etc, and is best termed ' rubber.'
Para rubber, which is regarded as the best variety, is obtained from Hevea brasiliensis, Muller Argoviensis, and probably other species (N.O. Euphorbiaceoe), a large tree widely distributed in the basin of the Amazon and the low ground flanking its tributaries, the Madeira,
Purus, Jurna, etc. The tree is also largely cultivated in Ceylon, the Straits Settlements, etc.
In the collection of the latex, spiral, herring-bone or isolated incisions are made in the bark of the trunk and the latex collected. This latex consists of globules of caoutchouc with small quantities of other bodies held in suspension by proteid substances; it is treated in various ways for the production of rubber.
In Brazil the small tin cups in which the latex has been collected are emptied into calabashes which the collector carries to his hut. Here he has a pole, about the length of a broomstick and thickened for a few decimetres at a little distance from one end, supported horizontally on a cross-bar. Under the thickened portion a clay dome, or sheet iron cone, is placed in which a fire is kindled on to which palm nuts are thrown. The latex is emptied into a large basin and a little is poured by means of a dipper on to the thickened part of the pole. This is now rotated in the dense smoke of the burning nuts. The latex is rapidly coagulated by the acetic acid in the smoke to a layer about 1 mm. thick, the creosote and other tarry substances from the smoke acting as preservatives. The coating and coagulation is repeated again and again until a short, thick, cylindrical mass weighing usually from 3 to 30 kilograms, but sometimes much more, has been obtained. The pole is then removed and the ball hung up to dry. In this way ' ball' Para rubber is obtained. Sometimes a paddle which may be dipped into the latex, is used instead of a pole and the rubber removed by splitting it along one side; this gives ' spade,' ' paddle,' or ' biscuit' Para rubber. Para rubber, therefore, consists of the latex dried after coagulation, and contains in addition to caoutchouc all the other constituents of the latex (except water). The coagulation is probably due to precipitation of part of the albumin by the acetic acid of the smoke. The latex contains about 40 per cent, of caoutchouc, 2.8 per cent, of proteids, and 55 per cent, of water.
Fig. 235. - Sampling Para rubber; in the centre balls of Para rubber cut transversely, showing the central hole.
Para rubber occurs in commerce in elastic, rounded, cylindrical, or flattened masses of varying size and shape, and of characteristic odour. Externally they are of a uniform brownish black colour, but internally they exhibit under a lens a large number of very thin yellowish layers separated by dark lines, each layer representing a coat of coagulated latex. The drug is insoluble in water, alkalies, or dilute acid, but chloroform, oil of turpentine, carbon bisulphide, and benzene make it swell and become soft and gelatinous, the caoutchouc dissolving and leaving a gelatinous substance undissolved. Digestion with alcohol removes from Para rubber about 1.5 per cent, of resin, but inferior rubbers may yield much more.
Para rubber contains from 40 to 60 per cent, of the hydrocarbon caoutchouc (caoutchouc-gutta) which when quite pure is white and corresponds in composition to the empirical formula (C10H16)n; it is a very unstable body and rapidly oxidises. In Para rubber it is associated with 30 to 50 per cent, of the gelatinous substance which probably protects the caoutchouc from change; small quantities of resin, fat, colouring and mineral matter are also present. Good rubber always contains a considerable amount of the gelatinous substance and only small quantities of resin, etc.; in poor rubbers the reverse is the case.
This now forms a very important commercial article, the production of which is rapidly increasing. The latex is coagulated by allowing it to stand for a few hours, with or without the addition of a little acid, and the coagulum is put into a washing-machine consisting of a pair of very heavy corrugated cylinders over which water is poured. The rubber globules are united by this treatment and come from the machine in the form of .thin, crinkled sheets (crepe rubber) which are hung up to dry. Sometimes the coagulum is passed through a machine which presses the rubber into small vermiform pieces; these are afterwards submitted to heavy pressure which converts them into ' block ' rubber. Such rubber is of course free from the proteids and other substances present in the Para rubber. It usually occurs in large thin sheets of a pale yellow or brownish black colour or in translucent dark brown or golden brown blocks about 4 cm. thick.
The keeping properties of rubber are much increased by combining it with sulphur (vulcanisation). This may be effected by heating the rubber with sulphur, or by means of a solution of sulphur chloride in carbon disulphide. Ordinary soft rubber usually contains from 2 to 4 per cent, of sulphur, hard rubber (ebonite) from 20 to 30 per cent.
The description of the numerous other commercial varieties of rubber would be far beyond the scope of this work, but the following may be briefly noticed as illustrating the various methods of preparation and the varying appearance of this important and interesting substance: -
Assam Rubber from Ficus elastica, Roxburgh (N.O. Urticaceoe); the latex is coagulated by boiling and formed into dark red lumps which are compacted into larger masses.
Mangabeira Rubber from Hancornia speciosa, Muller Argoviensis (N.O. Apocynaceoe); collected in Eastern Brazil, the latex being coagulated with alum or salt water. It occurs in large, flat, pinkish white pieces often very wet from presence of salt water.
Central American Rubber from Castilloa elastica, Cervantes (N.O. Urticaceoe); the latex is coagulated with salt or sodium bicarbonate or with an infusion of various plants; the masses of coagulum are pressed together into slabs composed of black, wavy sheets.
The latex of Funtumia (Kickxia) elastica, Stapf (N.O. Apocynaceoe), is coagulated by boiling or by pouring it into cavities in the trunk of the tree. That of Landolphia florida, Bentham (N.O. Apocynaceoe), is smeared by the natives over their bodies, and after drying rolled into balls.
Mozambique Rubber from Landolphia Kirkii, Dyer (N.O. Apocynaceoe), occurs in balls or in sausage-shaped masses consisting of strings of rubber wound round a little wooden spindle.
Ceara Rubber from Manihot Glaziovii, Muller Argoviensis (N.O. Euphorbiaceoe). The bark is shaved off until the latex trickles down the trunk; it is allowed to dry, peeled off in strings and rolled into balls.