The edges and surfaces of india rubber are readily and perfectly joined by mere contact and intense pressure. On the small scale the edges may be moistened with ether, naphtha, oil of turpentine, or some other solvent, or by long boiling in water, and immediately pressed tight together and held in contact for some time.
Elastic tubes are readily formed of india rubber by cutting it into uniform slips of proper thickness and winding them round rods of polished glass or metal, so that the edges are in close contact or “overlapping.” A piece of tape is then wound round outside it, and the whole boiled in water for 2 or 3 hours, after which time the edges will be found to be sufficiently adherent. A better plan is to immerse the “rubber” in a mixture formed of bisulphide of carbon, 95 parts, and rectified spirit, 5 parts, until it swells into a pasty mass, which may then be moulded into any desired form or passed through the die of a tubing machine. For chemical purposes, brewing, &c., vulcanised india-rubber tubing has now taken the place formerly occupied by the unprepared material.
The once celebrated “Mackintoshes” are made by spreading two or more coats of a paste made of caoutchouc and rectified coal-tar naphtha over the surface of the stuff or cloth, and, when it has become partially dry, pressing two such surfaces evenly together by passing the goods between a pair of cylinders or rollers. The articles are then placed in a stove room for the composition to harden, and to remove the odour of the naphtha. Of late years vulcanised or mineralised rubber (coloured) has been used for this purpose, and being spread on the outside of the stuff instead of the inside forms an ornamental and thoroughly waterproof material.
India-rubber thread is prepared by stretching it (previously cut into coarse filaments) to 5 or 6 times its length in boiling water or hot air, in which state it is allowed to cool slowly. This process is repeated again and again until it reaches 16,000 or 17,000 times its original length, when it is glazed by agitating it with powdered sulphur or French chalk. This thread is readily joined or “pieced,” as it is called, by paring the ends obliquely with a pair of scissors or a knife, and then pressing the clean ends strongly together with the fingers. When the coarse filaments from the cutting machine are simply stretched with the moistened thumb and finger in the act of “reeling” to about 8 or 9 times their length, they are said to be “inelasticated,” and are ready to be made into elastic braces, elastic web, and other like elastic tissues and fabrics in the braiding machine.
Caoutchouc, Vul′canised. Syn. Vulcanised india rubber, Mineralised i. r., Sulphuretted i. r. The discovery of the singular action of sulphur and the mineral sulphides on caoutchouc was made by Mr Charles Goodyear, of New York, in 1842, at which date the manufacture of vulcanised india rubber may be said to have commenced. In 1843 Mr Thomas Hancock patented a process for vulcanised india rubber in these countries, founded on that of Mr Goodyear. A sheet of caoutchouc immersed in melted sulphur absorbs a portion of it, and at the same time undergoes important changes in many of its leading characteristics. So prepared, it is no longer affected by changes of temperature; it is neither hardened by cold nor softened by any heat insufficient to destroy it. It loses its solubility in the solvents of ordinary caoutchouc, whilst its elasticity is greatly augmented, and has become permanent.
The same effect is produced when sulphur is kneaded into caoutchouc in a masticator, or by means of powerful rollers, as well as when common solvents (naphtha, spirit of turpentine, &c.) are charged with a sufficient amount of sulphur in solution to become a compound solvent of the rubber. In these cases articles may be made of any required form before heating them for the change of condition technically termed “vulcanisation.” It is necessary, however, for this purpose that the form should be carefully maintained both before and during the exposure to the heat.
“A vulcanised solid sphere of 21⁄2 inches in diameter, when forced between two rollers 1⁄4 inch apart, was found to maintain its form uninjured. In fact, it is the exclusive property of vulcanised caoutchouc to be able to retain any form impressed upon it, and to return to that form on the removal of any disturbing force which has been brought to act upon it.” (Brockedon.)
Caoutchouc combines with from 12% to 15% of sulphur; the quantity of sulphur added to the naphtha paste should not, therefore, exceed 10% or 12% of its weight.
The temperatures for vulcanisation by the common method range from 320° to 330°; and the period required is one hour or more, according to the temperature. A much lower temperature is, however, sufficient if the duration of the exposure is much extended or the compound mass is softened with any of the common solvents of india rubber.
The process of sulphuring, or mineralisation, is differently conducted in different manufactories. Under Mr Burke’s patent, oxysulphide or amorphous sulphide of antimony (formed by decomposing a solution of crude antimony in a lye of potash or soda with hydrochloric acid) is employed. This powder he combines with either india rubber or gutta percha, or mixtures of them, by kneading in a “masticator”