ABSTRACT
In classifying the dry fillers used in rubber to increase its usefulness or to make a cost-competitive product, it is convenient to speak of black and nonblack fillers. Nonblack fillers have certain features in common compared to carbon blacks. They have higher specific gravities—from 1.95 for a precipitated hydrated silica to 5.6 for zinc oxide. There are many ways to classify nonblack fillers considering the variety of natural sources and various treatments being used. One of the most widely used nonblack fillers for rubbers is clay. Its use is based on its low comparative cost, versatility, and stiffening properties. Fine particle silica gives the utmost in reinforcement in rubbers of the nonblack fillers; the silicates would probably be in second place. Silica can be made by pyrogenic or precipitation processes. Like clays, calcium carbonates are used at higher loadings than carbon blacks: 50-150 parts is not uncommon.
