FILLER ELASTOMER INTERACTIONS .7. STUDY ON BOUND RUBBER

SBR compounds were filled with 17 carbon blacks covering the whole range of rubber grades and tested for bound-rubber content. It was found that the bound-rubber content of a polymer at high loadings is higher for large surface-area carbon blacks. On the other hand, the bound-rubber content per unit of interfacial area in the compound (specific bound-rubber content) decreases with increasing specific surface area and filler loading. This observation was interpreted in terms of interaggregate multiple molecular adsorption, filler agglomeration, and change of molecular weight of rubber during mixing. When the comparison was carried out at critical loading of a coherent mass, the specific bound-rubber content was found to be higher for the high-surface-area products which are characterized by high surface energies. The critical loading of coherent mass of bound rubber also shows a strong surface-area dependence, indicating that large particle carbon blacks give high critical loadings. The measurements of bound rubber at high temperatures for carbon-black-filled compounds and in an ammonia atmosphere for silica-filled compounds suggest that bound rubber is caused essentially by physical adsorption.