The Effect of Pigment Particle Size on Some Physical Properties of Rubber Compounds

Abstract

Abstract The importance of flow in rubber on the reinforcing properties of pigmented systems has been emphasized by Park. He suggests that: (1) in the presence of a finely divided pigment, the flow which occurs when a piece of rubber is stretched takes place in the capillary spaces between the pigment particles; (2) some modification of the laws of liquid flow may govern the behavior of rubber with reference to pigments embedded in it, and (3) the forces causing increased stiffness in pigmented rubber compounds are similar to those causing increased resistance to flow of liquids iii tubes of capillary dimensions. Thus increasing fineness of subdivision and the resulting fineness of capillary spaces between the particles should be accompanied by an increase in reinforcing properties. It would be desirable to study the actual stresses around pigment particles in rubber under strain, but so far no suitable microscopic set-up has beem devised. A few years ago the writer, resorting to analogies, measured the strains and stresses around large particles with the assumption that the strains would be relatively the same with small particles. For this study, holes of the desired size and shape were cut in strips of calendered but uncured rubber and fitted with pieces of an uncured semihard rubber compound. After vulcanization squares were marked on the tensile sheets as shown in Fig. 1.