Botanical Source and History.—Caoutchouc, as it exudes from the trees, is a milk-like emulsion, wherein globules of caoutchouc proper are suspended in a gummy solution. It is produced by several plants chiefly of the natural orders, Euphorbiaceae, Apocynaceae, Urticaceae, Asclepiadaceae, and Artocarpeae. The finest quality of rubber, that known as Para rubber, comes from the Amazon district, and is produced by the Heveas. The Hevea guyanensis, Aublet (Siphonia elastica, of Persoon; Jatropha elastica, Linné filius) or Syringe-tree, of Brazil, furnishes a portion of the species that are official in the U. S. P. This tree generally averages about 65 feet in height, with a diameter varying from 30 to 40 inches. Frequently the trunk reaches 40 or 50 feet in height before branching. In collecting rubber, the natives fasten a small, swallow-nest-shaped, glazed dish around the base of the tree by means of clay; then, by means of a hatchet, sever the bark immediately above it, and the milky sap immediately exudes, and is collected in the dish below. Twenty trees yield, daily, about 2 pints, continuing this amount for some months. The most favorable period for extracting it is from April to November, during dry weather; the trees having to be wounded afresh every day. Some allow it to coagulate in a small, square box, but this method requires several days with subsequent slicing and pressure, to remove air and water; and some form it into bottles, tubes, etc., by dipping a mold of clay, fastened to the end of a stick, into the fresh juice, and immediately afterward holding it in thick smoke, produced by the combustion of oleaginous seeds. When the first layer has properly solidified, it is dipped again, and so continued until a sufficient thickness has been obtained. The smoke coagulates the milk, and exposure for some time to the sun hardens it. A small quantity of alum, or the addition of acids or salt solutions, accelerates the coagulation of the milk, while ammonia has a contrary effect, and is useful when the milk is required to be kept for some time in a liquid state.
Other South American grades, more or less inferior to Para caoutchouc are known commercially as Pernambuco, Maranham, and Bahia caoutchouc, and are derived mainly from Hancornia speciosa, Gomez, an apocynaceous plant. Very large quantities of rubber are furnished by the Landolphias.L. florida, Bentham; L. gummifera, Lamarck, and L. owariensis, all climbing, apocynaceous plants of West Africa; while in East Africa rubber is yielded by Landolphia Petersiani and L. Kirkii, and various species of Vahea in Madagascar. Other plants of the same order yielding Borneo caoutchouc, are the Urceola elastica, Roxburgh; Urceola esculenta, Bentham (Chavannesia esculenta, DeCandolle), and other species from the Malayan Islands and India. The Willughbeia edulis, Roxburgh, and other apocynaceous plants yield the Chittagong variety. A kind of rubber is derived from Urostigma Vogelii, Miquel (Nat. Ord.—Artocarpeae) of Liberia in West Africa; Ficus indica, Linné, and Ficus religiosa, Linné, yield an inferior caoutchouc.
The Ficus elastica, Roxburgh (Urostigma elastica, Miquel), is the chief source of East India caoutchouc, one variety being called Assam rubber. It has a trunk from 2 to 2 1/2 feet in diameter, and from 40 to 60 feet high. The leaves are alternate, approximated, 3-foliate, articulate at the top of a long slender stalk, convex below, furrowed above, and swelled at its base; the leaflets are smooth, oval, acute, green above and cinereous beneath. The flowers are monoecious. Calyx 5-cleft. The fruit is oblong, greenish, 3-cornered, broadest at base, tricoccus, each coccus opening with 2 valves. The seed is ovate, brownish, variegated with black, with a thin, brittle testa, and a sweet, nut-like, pleasant kernel. It is frequently cultivated as a hot-house plant.
Ceara rubber is the product of Manihot Glaziovii, Mueller, native of the Rio de Janeiro district. This Brazilian species, unlike the others of that country, thrives in dry situations. Good qualities are also obtained from the following South American species: Hevea brasiliensis, Mueller; Hevea discolor, Mueller, both yielding Para rubber; Castilloa Markhamiana, Collins, and Castilloa elastica, Cervantes. The export of rubber from the city of Para (at the mouth of the Amazon River), in the year 1893, exceeded 42,000,000 pounds. An instructive, exhaustive, and admirably illustrated article on rubber and its industry in South America, with a long list of rubber-yielding plants, classified according to their botanical and geographical origin, from the pen of Prof. H. H. Rusby, is to be found in the Druggists' Circular, 1894, p. 173. This valuable paper is too extensive to permit of condensation in our pages, and we commend the original article to persons interested in the subject. Efforts are being made to cultivate caoutchouc-yielding plants in various parts of the world; the chief difficulty with regard to Hevea, according to Prof. Rusby, seems to be in the great length of time required (from 12 to 35 years), to bring the tree to a proper condition of productiveness. In Lagos, on the west coast of Africa, Kickxia Africana, Bentham (an apocynaceous plant, the seeds of which have become known as an adulterant of strophanthus seeds), is being cultivated with success, over 580,000 pounds having been exported during the first half of the year 1895. This plant has advantages over the Landolphias on account of the climbing habits of the latter, which make cultivation difficult (G. M. Beringer, Amer. Jour. Pharm., 1896, p. 212, from Kew Bulletin, 1895).
Description.—Caoutchouc is black when coagulated by smoke, but when pure it is in thin, transparent layers, of a pale-yellow color, destitute both of taste and smell. Upon melting and subsequent cooling it remains in a semi-fluid, adhesive state, undergoing very little change for years, if protected from the action of light, but when exposed to the action of diffused daylight, in the air, it gradually absorbs oxygen, and becomes converted into an inelastic, viscid mass, soluble in alcohol. It is described by the Pharmacopoeia as follows: "In cakes, balls, or hollow, bottle-shaped pieces, externally brown to brownish-black, internally brownish or of lighter tint; very elastic; insoluble in water, diluted acids, or diluted solutions of alkalies; soluble in chloroform, carbon disulphide, oil of turpentine, benzin, and benzol. When heated to about 125° C. (257° F.), it melts, remaining soft and adhesive after cooling. Odor faint, peculiar; nearly tasteless. When pure, or nearly pure, India rubber floats on water"—(U. S. P). It is insoluble in alcohol, softens and swells up by long boiling in water, but resumes its former state on exposure to the air, and is soluble in pure ether, most fixed and volatile oils, and coal-tar naphtha, the latter and oil of turpentine being technically convenient solvents. Hot alcohol and wood alcohol cause it to soften and swell. An excellent solvent for caoutchouc is a mixture composed of 6 parts of alcohol and 94 parts of sulphide of carbon. Its solutions in ether, oil of turpentine, and coal-tar naphtha, by evaporation, leave the gum in an elastic state, and on this principle water-proof cloth is made; the same is said to be the case with its solution in the oils of lavender, sassafras, and cajuput. The fixed oils, in dissolving it destroy its elasticity. Under exposure to heat, caoutchouc first melts and then distills, yielding a mixture of several oily liquids, all of which, as well as pure caoutchouc itself, are hydrocarbons. Atmospheric air, ammonia, hydrochloric and diluted sulphuric acids, exert no influence upon caoutchouc. Rubber tubing, it is said, may be prevented from becoming brittle by keeping it under water, which must be changed from time to time.
Chemical Composition.—India rubber is a hydrocarbon having the general formula (C10H16)x. It is therefore, to be considered a polymer of terpene (C10H16). Upon destructive distillation it yields oily substances, collectively called oil of caoutchouc (caoutchoucin), besides carbonic acid, carbon monoxide, hydrochloric acid, etc. This oil, when subjected to fractional distillation, may be separated into two fractions, one of low and the other of high boiling points. From the former, a hydrocarbon, isopren (C5H8), was isolated by Greville Williams (1860), having a specific, gravity of 0.622, and a boiling point of 37° C. (98.6° F.). Bouchardat (1875) succeeded in obtaining from this oil, by polymerization, oils of higher boiling point, and the composition C10H16; and he even succeeded in obtaining artificial caoutchouc from isopren, which he therefore considers to be its basic principle. From the high boiling fraction Himly (Liebig's Annal., Vol. XXVII, p. 40), isolated the hydrocarbon caoutchin (C10H16), which has a specific gravity of 0.654, and boils at 171° C. (339.8° F.). For a description of substances further evolved from India rubber, see Husemann and Hilger, Pflanzenstoffe, p. 511,
Caoutchoucin, or oil of caoutchouc, is said to be the lightest fluid known, and yet its vapor is denser than the heaviest of the gases. Mixed with alcohol, caoutchoucin dissolves all the resins, especially copal and India rubber, at ordinary temperatures, and it speedily evaporates, leaving them again in the solid state. It mixes with oils in all proportions, and but for its price would be valuable for the solution of resins in the manufacture of varnishes, and to replace turpentines for liquefying oil paints. Being very volatile it must be kept in closed vessels.
VULCANIZED INDIA RUBBER.—When caoutchouc in sheet-form is immersed in a bath of fused sulphur, heated to 121.1° C. (250° F.), it gradually takes up from 12 to 15 per cent of its weight of sulphur, but without undergoing any change in its chemical or physical properties; but if it be then heated for a few minutes to about 150° C. (302° F.), it produces the elastic vulcanized India rubber. The same vulcanized condition can also be produced either by kneading the India rubber with sulphur, and then exposing it to the necessary temperature; or by dissolving the India rubber in any known solvent, as turpentine, previously charged with sulphur. It may also be effected by immersing very thin sheets of caoutchouc in a solution of 1 part of chloride of sulphur in 60 parts of bisulphide of carbon; then simple exposure to the air causes it to take the character of vulcanized caoutchouc without the aid of beat. Thus treated, caoutchouc remains elastic at all moderate temperatures; in its ordinary state it is quite rigid at a temperature of 4.4° C. (40° F.); it is not affected by heat short of the vulcanizing point, and acquires extraordinary powers of resisting compression. It does not readily undergo solution in naphtha or turpentine; the sulphur gradually destroys its elasticity, rendering it brittle and subject to decay.
VULCANITE.—If the vulcanized rubber be exposed to a still higher temperature, 148.8° to 176.6° C. (300° to 350° F.), it assumes a carbonized appearance, becomes black, hard, and like horn, and is termed vulcanite or ebonite, and may be used for most purposes to which horn is adapted. However, it should not come into contact with silver or gold utensils, as the action of the sulphur tarnishes them. Vulcanite becomes negatively electric by friction, is one of the best insulators of electricity known, and appears to resist the action of nearly all solvents. A much higher percentage of sulphur (from 30 to 35 per cent) also enters into the preparation of this hard variety of vulcanized India rubber. For details regarding the manufacture of hard-rubber articles, and the treatment of India rubber in general, we refer the reader to Prof. S. P. Sadtler's most instructive Handbook of Industrial Organic Chemistry, 1895.
Action, and Medical and Surgical Uses.—Caoutchouc is employed for a number of purposes, as rubbing out the writing made by lead-pencils; as a cement or lute by chemists and others, being first fused; for forming tubes of various kinds for surgical and other purposes; and it also enters largely into the preparation of waterproof cloth. Innumerable surgical appliances and instruments are made of soft and hard rubber. It is preferred to metal in many instances where a non-corrosive material is desired, and its durability, lightness, firmness and cheapness are in favor of its selection. Tubes-drainage, stomach, and rectal-syringes, specula, catheters, bougies, pessaries, nipple-shields, dilators, truss-pads, artificial nose, ears, and other parts, orthopaedic and other surgical appliances are constructed from it. Rubber sheets, bed-pans, and when woven into fabrics, rubber cloth, and stockings, bandages, and other appliances for even compression, hot-water bags, etc., are among the many useful articles of which it forms the whole or a part. Indeed, its peculiar character has rendered it useful in various and numerous ways in the arts, sciences, and for domestic purposes. Softened by heat, it has been applied over small bleeding orifices to cheek further hemorrhage; also to arrest toothache, by placing some of it in the abnormal cavity, so as to protect the dental nerve from atmospheric action. Externally, it has been used as an ingredient of adhesive plasters and liniments. Caoutchouc dissolved in oil of origanum or cajuput, and spread upon oil-silk or cloth, and allowed to dry, forms an excellent stimulating plaster for many local difficulties. Skin diseases, as eczema, psoriasis, etc., and burns and ill-conditioned ulcers have been treated by wearing next to the skin thin sheets of rubber or rubber cloth, which prevents the escape of the perspiration, thus rendering the parts moist and soft. Its use in these conditions is not commendable. Swelled testicles have been strapped and compressed by thin bands of caoutchouc, and for even compression and support the rubber stocking and other woven-rubber fabrics are extensively used in varicosities, hydrocele, varicocele, ventral herniae, pendulous abdomen, prolapse of the rectum and other similar conditions. A grain or 2 of caoutchouc has been administered in consumption, repeating it 3 times a day, but its results have not been such as to bring it into general use. It is seldom or never employed internally, and certainly has nothing to recommend it in a medical sense.
Related Preparations.—MARINE GLUE, or CEMENT, is made by digesting from 2 to 4 parts of caoutchouc, cut into small pieces, in 34 parts of coal-tar naphtha, promoting solution by the application of heat, and by agitation. When the solution has the consistence of thick cream, add 62 or 64 parts of powdered shellac, and heat the mixture slowly, constantly stirring it, until complete fusion and combination have been affected. Poor the mixture while still hot on plates of metal, so that it may cool in thin sheets like leather. In using the cement put some of it into an iron vessel, and heat it to about 120° C. (248° F.), and apply it with a brush to the surfaces to be joined.
MINERAL CAOUTCHOUC.—An undetermined substance, differing from petroleum products, and found in deposits a foot or less in depth in Australia, has been named mineral caoutchouc, from its marked resemblance to caoutchouc. Whether it is of animal or mineral origin is at present undetermined.
ARTIFICIAL CAOUTCHOUC is an elastic body produced by acting upon a solution of glue with sodium tungstate and hydrochloric acid. The precipitate formed upon the addition of the latter is highly elastic at 29.5° to 40.5° C. (85° to 105° F.), and becomes solid and brittle upon cooling. The process was discovered, in 1871, by Prof. Sonnenschein (Amer. Jour. Pharm., 1871, p. 471).
King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.