C. Accelerators of Vulcanization. —Vulcanization of rubber by sulphur alone proceeds at a slow rate and almost invariably suit able materials, called accelerators, are added to the rubber mixes to hasten the process. From the time of Goodyear's experiment, in which he used white lead, until 1906, the only accelerators used were inorganic oxides or hydroxides—litharge, white lead, quick or slaked lime, magnesia. In 1906 George Oenslager, of Akron, 0., discovered that organic amines shortened the time of vulcanization. Aniline was one of the first used and shortly afterward thiocarbanilid and para-amino-dimethyl aniline. In vestigations in this field extended rapidly and more effective or ganic accelerators of vulcanization are now essential ingredients of most rubber mixtures. The activity of many accelerators is improved by the presence in the mix of secondary accelerators, such as zinc oxide, litharge, or magnesia, and some will not func tion in their absence. They are often further assisted by acid materials, such as oleic or stearic acids or pine tar. Pneumatic tires, formerly requiring three hours for vulcanization without accelerators, are now vulcanized by the action of organic accelera tors in less than an hour. Deterioration of rubber may be greatly retarded by the use of certain accelerators.
D. Age-resisters.—Deterioration of vulcanized rubber in stor age or in service may be retarded also by incorporating in the mixture, before vulcanization, 1% to 6% of certain organic chem icals which have practically no effect on the vulcanization rate but greatly retard the rate of oxidation or change in properties of the vulcanized product. Some age-resisters also impart to rubber compounds resistance to deterioration by heat and resistance to cracking under repeated flexure. The age-resisters used in largest volume are secondary amino compounds. These are added as in gredients of the rubber mixtures along with the pigments, acceler ators, and sulphur. Commercial Pale Crepe rubber contains about 4% of natural age-resisters.
E. Other Materials.—A few other materials are used which do not fall under any of the classifications mentioned. Probably the most important of these is "substitute" or "factice." Factice is made either by the action of sulphur chloride on vegetable oils—white substitute—or by the action of sulphur on these oils—brown substitute. White substitute is used almost exclu sively in cold vulcanized goods (see below), brown substitute in hot vulcanized goods. It is estimated that approximately 15o,000 tons of non-rubber materials are used each year in rubber com positions in the United States alone.
the selection of materials as to kinds and relative quantities, can secure vulcanized mixtures of specific properties to resist flexure, abrasion, steam, oil, or acids. Rubber articles have thus been made available for a great variety of industrial uses (see Proper ties of Vulcanized Rubber, below).
Auxiliary Materials in Rubber Manufacture.—Rubber compositions may be fabricated with other materials to secure the peculiar advantages of the rubber stocks, together with the mechanical strength or rigidity of other substances. Cotton fabric, for example, lends strength to conveyor and transmission belting, to hose, and to pneumatic tires. Metal (in the form of wire or tape) or wood is also used to secure improved strength or rigidity. Rubber coated asbestos fibres formed into sheets supply a tough packing for high temperature steam lines. Rubber may be used, moreover, as a coating inside steel tanks, to pro tect them from the action of various acids.
The machinery for processing rubber is of various types. The rubber compositions must be mixed, formed, combined with other materials, fabricated into articles, and vulcanized. By far the greatest quantity of materials is handled in the dry state. Some articles, however, utilize rubber materials in the form of rubber solutions (cements) or of water dispersions (latex mixtures or artificial dispersions). For making cements, metal tanks of 25o to 3,00o gal. capacity fitted with stirrers are used. The rubber or rubber compound is placed in the tank with a small amount of solvent, benzene (benzol) or gasoline, and the contents stirred by power. A heavy dough of the rubber swelled with the solvent first forms. Additional quantities of the solvent are added in small increments, each blending with and thinning the mixture before the addition of the next, until a cement of the desired consistency is obtained. Considerable heat is generated in the early stages of the process and tight lids must be provided to avoid loss of solvent by evaporation. Tightly closed internal mixers which handle the rubber cement in the form of thick doughs are also used for making cements, with great economy in consumption of power and avoidance of solvent losses. The vis cosity of a definite composition of cement of milled rubber de creases with the time of milling, up to a certain point. Conversely, when milled rubber is used more of it must be incorporated into a fixed quantity of solvent to obtain a cement of definite viscosity. Therefore, for spreading purposes, to apply as much rubber as possible with a minimum quantity of solvent, milled rubber mixtures are used. When strong adhesion is required, on the other hand, the rubber is milled as little as possible.