Mechanism of Rubber-To-Brass Adhesion : Effect of Rubber Composition on the Adhesion

Abstract

Abstract A new adhesion model for the bonding of sulfur-vulcanizable rubber compounds to brass has been presented and discussed. The adhesion is explained on the basis of an autocatalytic effect of a thin film of cuprous sulfide formed on the brass surface by the combined action of sulfur and vulcanization accelerator. This film transfers sulfur atoms to rubber molecules, which results in a high rubber surface polarity. This surface bonds to brass by physical interaction. Although minute amounts of cuprous sulfide are a prerequisite, an excess of this compound is detrimental to the adhesion, as thick films are brittle and nonadherent to the brass substrate. Consequently, all parameters which affect the reactivity of a brass surface for cuprous sulfide formation, have an effect on the adhesion. These parameters are brass copper content, brass surface composition, and rubber compound formulation. The action of the various individual rubber ingredients on the reactions at the interface can be simulated by measuring the rate of sulfidation of a standard type of brass in a paraffin solution containing a simple rubber curing system. In spite of the differences between the conditions during actual rubber vulcanization and sulfidation in paraffin, the latter approach yields information which can be used to explain, on a qualitative basis and within the limits of the adhesion model, certain well-known but hitherto unexplained effects, such as the action of cobalt complexes on the adhesion. The method can also be regarded as a simple test for the effect of rubber compounding ingredients on steel cord adhesion.