Nature of Chemisorptive Mechanisms in Rubber Reinforcement

Abstract

Abstract The present state of research in the field of reinforcing rubber fillers is reviewed and it is pointed out that although a strong school of thought still maintains that the strength of bonding depends on particle size, there are many observations indicating the presence of reactive sites and suggesting the possibility of strong bonds of a chemical nature between filler and rubber matrix. The phenomenon of carbon-gel formation as foremost among these, is discussed as lending fresh support to the idea of chemical bond formation. This effect, known to be most pronounced in the presence of an unsaturated matrix is correlated with the mechanism of vulcanization as proceeding via free radical intermediates. It is suggested that the formation of chemical bonds between pigment and rubber constitutes an integral part of the chemical crosslink formation commenced on the rubber mill and completed by vulcanization. This hypothesis is corroborated by results obtained during studies on the reactivity of pigment surfaces towards simple inorganic and organic free radicals. It is shown that active pigments react with free radicals with the formation of nonionic bonds. Hydrophilic blacks have been prepared by treatment with persulfate. Similarly, electrochemically produced stearate free radicals have been reacted with the surface to form hydrophobic carbons. The “benzidine ”blue free radical, being the monooxidation product of benzidine, reacted easily with less active pigments and thus provided a useful measure of activity. From the experiments with free radicals, the number of active sites for various fillers has been calculated and shown to cover a range of 0.01–3.1 per 100 sq. A˚. Multifunctional polystyrene radicals, obtained in the presence of excess oxygen, yielded crosslinked networks with carbon blacks which resisted solvent extraction. These are considered to be complete analogues to carbon gel. The statistical mechanics of the rubber network has been developed to include crosslink formation as caused by rubber-filler interaction. It is shown that the mechanical properties of a rubber compound that are controlled solely by the network structure are directly proportional to σ/r, the ratio between the number of active sites per unit area and the particle radius. Abrasion does not appear to be only a mechanical phenomenon.