Morphological Study of the Rubber-Brass Adhesion Interphase

Adhesion between rubber and brass-plated steel tire cord is a critical factor which influences the reinforcement effect and the overall performance of tires. The rubber-brass interfacial adhesion is governed by the chemical composition and thickness of the interfacial layer, which is further influenced by the rubber formulation. Recently, one-component resins have been used as adhesion promoters to control degradation of the rubber-brass bonding layer during aging. This paper discusses a comparison of the adhesion improvements due to one-components resins to that of the commonly used two-component resins (HMMM + RF). Grazing incidence X-Ray diffraction (GIXRD) experiments were done on sulfidized polished brass coupons previously bonded to six experimental rubber compounds. New evidence obtained using a synchrotron X-Ray beam showed the presence of previously unseen cobalt-containing phases and non-stoichiometric copper sulfide species at the rubber-brass interface. TOF-SIMS depth profiles indicated the transitions between different interfacial chemical species as a result of aging and rubber formulation. It was confirmed that heat and humidity conditions lead to physical and chemical changes of the rubber-steel tire cord interfacial layer, closely related to the degree of rubber-brass adhesion. The adhesion promoter resins inhibit physical changes in the interfacial layer thus stabilizing it during aging and delaying failure. Tire cord adhesion tests illustrated that the one-component resins improved adhesion after aging using a rubber compound with lower cobalt loading. GIXRD profiles showed that these resins impeded crystallization of the sulfide layer especially after humidity aging. This interfacial study will further enlighten our understanding of the complex nature of the rubber-brass bonding mechanism in steel belts.