India-rubber

India-rubber, CAOUTCHOU, or GUM ELASTIC, a substance which, on account of its peculiar properties, is extensively used in the arts. It is found in the milky juices of plants, and most abundantly in the natural orders Moraceæ, Artocarpaceæ, Euphorbiaceæ, Apocynaceæ, and Asclepiadaceæ. It exists in the milky juice of plants growing in temperate climates; but it is only in tropical and subtropical countries that it occurs so abundantly as to be of economical importance. The principal South American tree is the Hevea brasiliensis or guianensis, also called Siphonia elastica, or Jatropha elastica, a Euphorbiaceous tree; also the Mexican Castilloa elastica, which is Artocarpaceous. In the East the Ficus elastica (of the order Moraceæ), akin to the Banyan (q.v.), is a tree of noble proportions, the appearance of whose glossy leaves is well known in Europe from small specimens grown in pots as ornamental plants. Various Apocynaceous trees (Willughbeia, Landolphia, Ureola, &c.) yield commercial quantities of rubber in Malaya, Borneo, and Central Africa. The name Caoutchouc is from a Carib or Central American word Cachuchu.

Some of the properties of india-rubber must have been known in America at a very early period, because balls made by the Haytians of the gum of a tree, bouncing better than the wind-balls of Castile, are mentioned by Herrera in his account of Columbus's second voyage. In a book published in 1615 Juan de Torquemada mentions the tree which yields it in Mexico, describes the mode of collecting the gum, and states that it is made into shoes; also that the Spaniards use it for waxing their canvas cloaks to make them resist water. More exact information was furnished by M. de la Condamine in 1735. India-rubber was at first known as Elastic Gum, and received its present name from the discovery (about 1770) of its use for rubbing out black-lead pencil marks, for which purpose it began to be imported into Britain in small quantities about the end of the 18th century, being much valued by artists, and sold at 3s. the cubic half inch. Even before this time its employment for the manufacture of flexible tubes for the use of surgeons and chemists had been successfully attempted; but it was not till 1820 that its employment began to extend beyond the rubbing out of pencil marks. Its application to the manufacture of waterproof cloth first gave it commercial importance. About the same time a method was discovered of fabricating articles of various kinds by casting india-rubber in moulds. Its elasticity and flexibility, its insolubility in water, and its great impenetrability to gases and fluids in general have now been found to adapt it to a great variety of uses; but for by far the greater number of its applications it is now employed in the vulcanised state.

The india-rubber of commerce is obtained most largely from South America, but considerable quantities are also procured from British India, the Indian Archipelago, the west coast of Africa, and the Mauritius. During the year 1888 the imports of this material into Great Britain were as follows:

In 1852 the total imports were only 15,269 cwt.; in 1862, 59,703 cwt.; in 1876, 157,509 cwt.; in 1883, 229,101 cwt.; in 1887, 237,511 cwt.; in 1888, 218,171 cwt.; and in 1889, 236,275 cwt. In 1883 the average price per cwt. was 318s.; in 1885, 220s.; in 1887, 228s.; and in 1894, 216s. The value of imports in 1883 was £3,652,817, and in 1888, £2,555,341. In 1894 the import was 302,451 cwt. (including that from Nyassaland and Congo Free State) valued at £3,272,104. India-rubber goods worth over £1,000,000 are exported from Britain.

India-rubber is sometimes collected by cutting the trees down, which is a very ruinous process, and resorted to mainly that a greater quantity may be obtained. The more usual method, however, is by making simple incisions in the trunks. In a few hours the juice which flows out fills clay basins placed to receive it. It is solidified and dried by various methods—sometimes spread out in thin layers and dried in the sun or the smoke of fires, sometimes (in Central America) coagulated by leaves of a kind of vine. A good tree will yield four ounces of juice daily, and twenty gallons in a season; a gallon producing 2 lb. of good rubber. Adulteration is not uncommon.

Para india-rubber is the best, and commands the highest price in the market (averaging about 3s. per lb.). The other South American kinds are of fair quality. East Indian rubber, though naturally a fine quality, is often injured by adulteration and careless collecting.

Commercial india-rubber is a tough fibrous substance, possessing elastic properties in the highest degree. Reduced to the temperature of freezing water (32° F.) it hardens, and in greater part, if not entirely, loses its elasticity, but does not become brittle. When heated, as by placing in boiling water, it softens and becomes very much more elastic than at ordinary temperatures, though it does not in any degree dissolve in the water. If suddenly stretched to seven or eight times its original length it becomes warm; and if kept in this outstretched form for several weeks it appears to lose in great part its elastic properties, and in this condition is readily cut into those thin threads which are used in the elastic put in bonnets, &c., and the elasticity of which is readily renewed by the application of gentle heat. Of late years, however, elastic thread is usually prepared with vulcanised rubber. Commercial india-rubber is insoluble in water and alcohol, is not acted upon by alkalies or acids, except when the latter are concentrated and heat is applied, but is soluble in ether, chloroform, bisulphide of carbon, naphtha, petroleum, benzol, and the essential oils of turpentine, lavender, and sassafras. Many other essential and fixed oils, when heated with rubber, cause it to soften, and produce thick glutinous compounds, especially linseed-oil, which, in the proportion of $1\backslash frac\{1\}\{2\}$ lb. of the oil to 4 oz. rubber in thin strips of films, yields a solution which, when strained, is of great use in rendering shoes, cloth, &c. waterproof. When heated to 248° F. rubber fuses; and at 600° it is volatilised, at the same time undergoing decomposition, and yields a liquid called Caoutchoucine, possessing great solvent powers over india-rubber and other substances.

To purify the raw material it is boiled for some time in large tanks, which softens it and in some measure releases the solid impurities with which it is often mixed. It is then put through powerful machines which masticate and reduce it to shreds, and while undergoing this operation a stream of water is constantly running over it and thoroughly cleansing it from all impurities. It is then rolled out into thin sheets and hung up to dry in a room heated by artificial means, and thus freed from all moisture. Or, after cleansing, the material undergoes a process of kneading under very heavy rollers, which causes the adhesion of the various pieces of rubber to each other, and ultimately yields a mass or block of rubber in which the condensation is so perfect that all air-holes and other cells and interstices disappear. The block of rubber is then cut under water by powerful knives or shears into sheets, from which bands or thread may be obtained. In the manufacture of square threads mere cutting is had recourse to; and the delicacy of the operation may be understood when it is stated that one pound of rubber will yield 32,000 yards of thread. The round elastic thread is prepared from rubber which has been treated with about double its weight of bisulphide of carbon, containing about 5 per cent. of alcohol, which yields a soft material resembling in consistency bread-dough or putty; and this being squeezed through a series of small holes, produces minute round threads, which are first received on an endless piece of velvet, and ultimately on an endless web of common cloth 500 to 600 yards long, during the transit of the threads across which the solvent or bisulphide of carbon evaporates, and leaves the india-rubber. When it is wished to weave these threads into cloth they are wound upon bobbins, taking care to stretch the rubber as much as possible, so as to deprive it for the time being of its elasticity; and, after it has been woven into the cloth, a hot iron is passed over the fabric, and immediately the rubber resumes its elasticity.

The method for making waterproof clothing or 'Mackintoshes,' the first application of rubber on a large scale, suggested by Professor Syme in 1818, was patented in 1823 by Charles Macintosh of Glasgow. In this manufacture the caoutchouc has to undergo many and varied processes. It is first reduced to a solution with naphtha or other solvent, and it is then amalgamated with other ingredients according to the nature of the material it has to be applied to. It is then spread on the surface of the cloth, a process formerly done by hand, but now by means of spreading machines, which apply it in very thin coats, so thin that with pure para proofing as many as twelve coats are spread to make the cloth air-proof, but so thin is each coat that the twelve only measure one ninety-sixth part of an inch; for ordinary waterproof purposes, however, five or six are generally sufficient. For double textures the cloths are then pressed together between heavy rollers. These cloths are all vulcanised, and this can be performed by a number of processes—by the 'cold' process, by vaporising, by steam, and by dry heat. The garments are then cut out from the cloth, and fastened together by means of pure rubber cement, which makes the edges adhere.

The variety of garments now made up are almost endless, and every year the demand for them in fashionable designs and cloths is increasing.

Vulcanised or Solid India-rubber.—Pure india-rubber is now used only to a limited extent in the arts, but it is applied in the vulcanised state to a very large extent. The remarkable change which caoutchouc undergoes when mixed with sulphur and heated, according to circumstances, from 240° to 310° F., was discovered by Charles Goodyear, in America, in 1834-44, and independently, about the same time, by Mr Thomas Hancock, in England. In the process of vulcanising, the rubber, as a preliminary step, is either torn into shreds or crushed into thin pieces by machinery, and afterwards washed. There are two principal kinds of vulcanised rubber, one hard and horny in its texture, the other soft and elastic. In the case of the former the caoutchouc is mixed with about one-third of its weight of sulphur, and heated for several hours, the temperature finally rising to fully 300° F. For the soft kind of vulcanised rubber, on the other hand, a much smaller proportion of sulphur is required—viz. from 2½ to 10 per cent., and the heat to which it is subjected in the vulcanising chamber is considerably less. Usually, too, with this latter kind, the articles are made before the rubber is heated. The sulphur is commonly added in the ground state, but sometimes the rubber is treated with some solution containing this element, such as the bisulphide of carbon.

Although sulphur is the only essential ingredient required for vulcanising rubber, yet other substances are usually added. Thus, in the case of machinery belting, pipes, and some other articles, the silicate of magnesia (French chalk) is used to prevent adhesiveness. Litharge, or carbonate of lead, again, is frequently mixed with the rubber and sulphur for certain purposes; but there is really a long list of materials more or less used in preparing different qualities of vulcanised caoutchouc, each manufacturer using mixtures the exact nature of which he is careful not to divulge. Asphalt, tar, lime, lampblack, whiting, rosin, sulphide of antimony, and ground cork are some of the ingredients most commonly employed in this way. Belting for machinery and some kinds of tubing are formed of alternate layers of canvas and vulcanised rubber.

Natural caoutchouc, as already stated, is elastic, cohesive, impervious to gases, insoluble in water, and resists many chemical reagents; but it loses its elasticity by cold, softens by heat, and is destroyed by many fixed oils. After being vulcanised caoutchouc has its elasticity greatly increased, is not hardened by cold, and does not soften or become viscid at any temperature short of its absolute decomposition. Besides, it is barely soluble in turpentine, naphtha, and the other solvents of pure caoutchouc; nor does oil readily penetrate or soften it. Very often, however, the natural oil in some cloths, or oils used in manufacture, tend to make the rubber decay, and this has often caused rubber-manufacturers a large amount of trouble.

It would be a hopeless task to attempt to specify the many useful purposes to which vulcanised caoutchouc is applied, even if we had the space to spare. From the year 1843, when it was first made, to the present time the various patented applications of it must be thousands in number. The mere abridgments of the specifications connected with this material, issued by the English Patent Office, form a thick volume. Under the head GOLOSHEs will be found a brief description of the process of making india-rubber shoes. Both coats and shoes of this material have, however, the objectionable property of preventing the escape of moisture from the skin. Belting, buffers, wheel tires, washers, valves, pipes, fire-hose, and other engineering appliances form a large branch of the rubber-trade. For medical and surgical purposes many articles are made of this material. Of such an apparently trivial matter as vulcanised rubber thread one English firm turns out about 3000 lb. per day, and another single small article—viz. tobacco-pouches—is made in another factory at the rate of 3000 per diem.

Hard vulcanised rubber, termed vulcanite, and sometimes ebonite, is made into a great many small articles, such as combs, chains, bracelets, boxes, penholders, paper-knives, knife-handles, buttons, &c., as a substitute for materials like horn, bone, ivory, and jet. Like these substances themselves, it is formed into various objects by moulding, cutting, carving, polishing, and other processes. Vast numbers of these articles are now sold. The black colour of vulcanite ornaments has still a tendency to turn gray, but the brittleness which was a fault of combs made of it a few years ago seems to be overcome. With respect to objects of considerable size, vulcanite has been made into furniture, ornamental tiles, and even rails for railroads and paving for footpaths, for which latter purpose it suits admirably. A kind of vulcanite is now very largely employed as an insulator in electric cables.

India-rubber when melted at 398° F., and mixed with half its weight of slaked lime, forms a useful cement or lute, which can be easily loosened, but it will dry and harden if red lead is added. A very tenacious glue is formed by heating caoutchouc, coal-tar, and shell-lac together. It forms an ingredient in some special kinds of varnishes, and it also improves the lubricating qualities of mineral oils when a small quantity is dissolved in them.

In Great Britain some of the large india-rubber factories employ over 1000 hands, and smaller works are springing up all over the country. The exports are sent all over the world, principally to the Continent, North America, and Australia. The duty levied on this class of goods is prohibitive of the expansion of the trade with the United States. The manufacture of india-rubber is also carried on extensively in the United States and France. In most rubber-factories a large number of the work-people are females; and, as no great skill is required on the part of the operatives engaged in some departments, employment in such works has proved a boon to many persons who have never learned a trade.

The high price of raw india-rubber has led to many attempts to produce a substitute, but none of them equal in durability the pure caoutchouc.