Reinforcing Silicas and Silicates

Abstract Since the first intensive literature review on silicas and silicates was published, significant new technology for their use in rubber has developed. Usage in the rubber industry has broadened considerably, and increases in their consumption have occurred during the ensuing fifteen years. From the “soles and heels” fillers of the 1950's, the usage of precipitated silicas and silicates since has gained significantly in quality components of tires and mechanical goods. Potential applications in the immediate future are no longer limited by the “good, but second to carbon black” performance that has prevailed for the past twenty-five years. Besides the traditional unique properties of tear resistance, hardness, stiffness, flex resistance, heat resistance, low modulus, and neutral color imparted to rubber by silicas and silicates, it is now possible to add adhesion, low heat buildup, high resilience, high modulus, and high abrasion resistance. This is part of the new technology which has developed that provides an alternative type of filler to meet the demanding needs of tire components, conveyor belts, automotive mounts, gaskets and seals, and other quality rubber compounds previously inaccessible to silicas. The present review covers the period 1960–1975. Because of the volume of available literature, it became necessary to be quite selective, in a way perhaps that may seem arbitrary. Nevertheless, coverage will be confined largely to that relevant to more general rubber uses. Silicone rubber technology was not covered because of its highly specialized and generally proprietary nature. The definitions used in the previous review were expanded somewhat. Silicas and silicates (henceforth, “silicas”) germane to this review were generally restricted to those which are synthetic and amorphous. However, the size limitations were relaxed somewhat to include those having a specific surface area greater than 25 m 2 /g or an average particle diameter less than 100 nm. Natural products ( e.g. , clays, diatomaceous earth, sand, etc.), as well as their modified forms, were excluded.