Combining Cobalt and Resorcinolic Bonding Agents in Brass-Rubber Adhesion

Abstract

Abstract The bonding of several black-filled natural rubber vulcanizates to brass-plated steel cord was investigated using the TCAT pullout geometry. Compositions using either cobalt naphthenate or an HR bonding system (hexamethoxymethyl melamine plus RFR) had higher pullout forces than a control without these agents. However, the highest pullout forces, both initially and especially after aging, were obtained when the cobalt and HR additives were used together. Results from ATR spectra show that the RFR migrates to the brass surface, and it is proposed that this provides a barrier to attack by moisture of the brass-rubber interfacial region. In support of this idea, moisture uptake is reduced for compositions containing the HR bonding system. Moreover, in the presence of cobalt naphthenate, sulfidation of brass gives a surface topography of substantially higher surface area than seen if this agent is absent. This is expected to promote interlocking between the rubber and the outwardly growing sulfide layer. Finally, the HR resin is able to remove both zinc and cobalt from squalene solution. It is hypothesized that cobalt and the HR resin each have beneficial effects as noted previously, but the subsequent removal of cobalt from the rubber phase by complexing with the HR resin may prevent cobalt, a well-known oxidation catalyst, from promoting degradation of the rubber during aging.