Rubber-Vulcanizing Properties of Colloidal Carbons

Abstract

Abstract 1. Experiments with rubber-sulfur mixings showed that colloidal carbons accelerated vulcanization instead, as generally stated in the literature, of retarding. The fact that this acceleration was found to be independent of the pH value of the carbon suggests that the carbon rubber reinforcement “reaction” and (physical) vulcanization may be related phenomena. 2. When colloidal carbon is used, as in tire treads, in conjunction with other accelerators, disturbances occur due to the selective adsorption properties of the carbon colloid. These disturbances, which are greatest with basic organic accelerators, less with acidic organic accelerators, and least with inorganic accelerators such as litharge, have led to the description of colloidal carbons as quick-or-slow-vulcanizing. 3. The best guide to this “rate of vulcanization” characteristic of rubber carbons is their pH value. 4. Adsorption data are given for cobalt, manganese, lead and zinc soaps, and vary widely for different colloidal carbons. This variation as to the zinc requirements was confirmed by a rubber compounding series. Thus each carbon, depending on its pH and particle size, presents an individual problem in adsorptive requirements, involving fat acid, zinc oxide and accelerator, which should be adjusted in this order. 5. For use with basic accelerators, e.g., D.P.G., colloidal carbons of maximum surface (which involves high fat acid additions with resultant interference with the accelerator) or carbons with low pH (which requires high soluble zinc and therefore high fat acid) are not recommended. Low pH carbons tend to disturb the alkalinity essential to best results, and to adsorb the accelerator. 6. With acid accelerators, e.g., mercaptobenzthiazole, the finer colloidal carbons of moderate to low pH may safely be used. The higher fat acid dosage (soluble zinc) which these carbons require can be freely supplied, since it does not deactivate the accelerator.