Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/867

841 INDIA-RUBBER 841 with sulphur and other solid bodies, or for agglomeration into solid masses by means of the masticating machine, an apparatus which consists of a strong cylindrical cast- iron casing, inside which there revolves a metal cylinder with a fluted or corrugated surface. Some of the rub ber having been placed in the annular space between the inner cylinder and the outer casing, the former is made to revolve ; and the continued kneading action to FIG. 7. Roller of Washing Machine. which the rubber is subjected works it into a solid mass, something like a gigantic sausage. Before commencing the mastication it is generally necessary to warm the apparatus by means of steam; but as the operation proceeds the heat produced requires to be moderated by streams of cold water flowing through channels provided for the purpose. The inner cylinder is generally placed somewhat excentric- ally in the outer casing, in order to render the kneading more perfect than would otherwise be the case. To convert the masticated rubber into rectangular blocks, it is first softened by heat, and then forced into iron boxes or moulds. The blocks are cut into thin sheets by means of a sharp knife, which is caused to move to and fro about two thousand times per minute, the knife being kept moistened with water, and the block fed up to it by mechanical means. Cut sheets are largely used for the fabrication of certain classes of rubber goods, these being made by cementing the sheets together with a .solution of rubber in coal-naphtha or benzol. Most articles made of cut sheet rubber would, however, be of very limited utility were they not hardened or vulcanized by the action of sulphur or some compound of that element. After vulcanization, rubber is no longer softened by a moderate heut, a temperature of 1GO C. scarcely affecting it, nor is it rendered rigid by cold, and the ordinary solvents fail to dissolve it. It must, however, be distinctly understood that it is not the mere admixture but the actual combination of sulphur with india-rubber that causes vulcanization. If an article made of cut sheet be immersed for a few minutes in a bath of melted sulphur, maintained at a temperature of 120 C., the rubber absorbs about one- tenth of its weight of that element, and, although somewhat yellowish in colour from the presence of free sulphur, it is still unvulcanized, and unaltered as regards general proper ties. If, however, it be now subjected for an hour or so to a temperature of 140 C., true combination sets in, and vulcanized caoutchouc is the result. When a manufactured article has been saturated with sulphur in the melted- sulphur bath, the heat necessary for vulcanization may be obtained either by high-pressure steam, by heated glycerin, or by immersion in a sulphur-bath heated to about 140 C. In this last case absorption of the sulphur and its intimate combination with the rubber occur simultaneously. Cut sheets, or articles made from them, may be saturated by being laid in powdered sulphur maintained for some hours at about 110 C. Sheets sulphured in this way can be made up into articles and joined together either by warming the parts to be united, or by means of india-rubber solution ; after which the true vulcanization, or &quot;curing&quot; as it is termed, can be brought about in the usual way. Another method of vulcanizing articles made from cut sheet rubber consists in exposing them to the action of chloride of sulphur. Either they are placed in a leaden cupboard into which the vapour is introduced, or they are dipped for a few seconds in a mixture of one part of chloride of sulphur and forty parts of carbon disulphide or purified light petroleum. Vulcanization takes place in this instance without the action of heat ; but it is usual to subject the goods for a short time to a temperature of 40 C. after their removal from the solution, in order to drive off the liquid which has been absorbed, and to ensure a sufficient action of the chloride of sulphur. Treatment with a warm alkaline solution is afterwards advisable, in order to remove traces of hydrochloric acid generated during the process. Another very excellent method of vulcanizing cut sheet goods consists in placing them in a solution of the poly- sulphides of calcium at a temperature of 140 C. Rubber employed for the manufacture of cut sheets is often coloured by such pigments as vermilion, oxide of chromium, FIG. 8. The Mixing Rollers. ultramarine, orpiment, antimony, lamp black, or oxide of zinc, incorporation being effected either by means of the masticator or by a pair of rollers heated internally by steam, and so geared as to move in contrary directions at unequal speed (fig. 8). Most of the rubber now manufactured is not combined with sulphur when in the form of sheets, but is mechanically incorporated with about one tenth of its weight of that substance by means of the mixing rollers, any required pigment or other matter, such as whiting or barium sulphate, being added. The mixed rubber thus obtained is readily softened by heat, and can be very easily worked into any desired form or rolled into sheets by an apparatus known as the calendering machine. Vulcaniza tion is then ensured by exposure for half an hour or more to a temperature of 135-lf&amp;gt;0 3 C., usually in closed iron vessels into which high-pressure steam is admitted (fig. 9). Tubes are gene rally made up around mand rels, and allowed throughout the curing to remain imbedded in pul verized French chalk, which af fords a useful support for many articles that tend to lose their shape during the process. Of late years a consider able amount of tubin - Fin. 9. A Vulranizor. has been made, much in the same way as lead piping, by forcing the mixed rubber through a die, and curing as above&quot; The calendered sheets are generally cured between folds of wet cloth, the markings of which they retain : and hollow articles, such as playing balls or injection bottles, &amp;lt;&amp;gt;re vulcanized in iron or brass moulds, tinned inside and X1T. 106