The rubber-brass adhesion is influenced by the chemical composition and thickness of interfacial layer. The adhesion build-up mechanism involves the chemical reaction of the rubber skim compound and copper of the brass coating, forming CuxS, ZnS and ZnO layers at the rubber-brass interface.
All factors which affect the reactivity of a brass surface for CuxS formation, have an effect on the adhesion but excessive CuxS leads to the loss of adhesion. The adhesion force tends to maximum value with optimum copper content in the plating but better retention of after aging achieves at a lower copper content.
A higher sulfur content is necessary than for ordinary rubber. There is an optimum amount of organic acid cobalt salt, as adhesion diminishes when the rubber is formulated with large amounts.
A high level of unaged adhesion can be obtained if products with a combination of low cobalt and high sulfur content are formed on the brass surface.