Reinforcement of Elastomers by Silica

Abstract

Abstract The physical characteristics of silica pigments covering a wide range of properties were determined and related by regression analysis to the static and dynamic mechanical properties of SBR vulcanizates reinforced with these silicas. Three morphological parameters were found to determine the vulcanizates' characteristics: the external surface area by CTAB adsorption, the structural index, and the DBP absorption number. The adherence between silica and elastomer was studied by means of swelling in solvents, by determination of the volume increase on deformation, and by scanning and transmission electron micrography. It was found that dewetting, the formation of vacuoles on deformation, is a most important factor, limiting reinforcement by silica at loadings above 40 phr. Equally, at such loadings the pH of the silica has an important influence : the higher the acidity, the less the adherence between pigment particle and elastomer. Silane coupling agents not only increase adherence, but also improve the degree of dispersion of the silica, which is frequently characterized by the presence of large, partly unwetted particle clumps under conventional conditions of incorporation, thus severely limiting the strength properties of the vulcanizates. The curing system employed and the accelerators used are also of paramount significance for adherence.