Particulate Fillers in Elastomers

Abstract

Fine particulate fillers are widely used in elastomers (indeed are essential for many applications) and are small hard particles usually of carbon or inorganic origin. The main fillers used in general purpose elastomers are carbon black, precipitated silicas, clays, and natural carbonates. There is also significant use of precipitated calcium carbonates and of synthetic flame retardant fillers such as aluminum hydroxide. Fumed silica plays a key role in silicone elastomers while finely ground crystalline silicas are important in specialist high temperature elastomers. Filler reinforcing effects depend very much on the type of elastomer. They are most obvious in noncrystallizing elastomers, such as most synthetic types. Here, the fine filler particles are able to act as tiny crystallites and markedly improve the tensile strength, abrasion resistance, and higher extension modulus. In the crystallizing types, such as natural rubber, the elastomer crystallites which develop on stretching often hide filler effects in laboratory tests, but they are still present and important in many applications. Size, shape, and surface activity are all important factors determining reinforcing ability. Small size, high structure, and high surface activity generally give the best blend of properties. Carbon blacks owe their preeminence to the natural ability of their surface to form strong interaction with hydrocarbon polymers. Other fillers generally require the addition of coupling agents to achieve the same effect.