The Mechanism of Reënforcement of Elastomers by Pigments

Abstract

Abstract Modulus appears to be independent of average particle diameter. With respect to the dependence of modulus on loading, calcium carbonate pigments which are symmetrical follow the equation derived by Guth and Gold for the relation between modulus and volume concentration of symmetrical inelastic particles. Up to about 80 parts by weight loading, HMF black follows a similar equation derived for rod-shaped particles, assuming a value of six for the ratio of the length to the diameter of the particles. The agreement with these theoretical equations is surprisingly good, and holds out the eventual possibility of calculating modulus over a wide loading range from tests at a single loading. Hardness and plasticity increase with loading in qualitative agreement with the equations. The behavior of modulus as a function of loading and average particle diameter of pigments indicates that vulcanized natural or synthetic rubber is not adsorbed in appreciable quantities or with sufficient force to be rendered inelastic at the surface of either calcium carbonate or carbon black pigments. Qualitative predictions based on the hypothesis that tensile strength is a measure of the force required to separate rubber chains from each other rather than to break rubber chains, and that pigment particles act as cross-linking agents holding the chains together, are in agreement with the behavior of calcium carbonate and carbon black pigments of widely different surface area over a considerable loading range.