Mechanistic Study on the Role of Sulfurized Para-Aramid Short Fibers in Rubber to Brass Adhesion

Abstract

Abstract Adhesion between steel cord and rubber is an important factor determining the durability of tires, conveyer belts, hoses and other rubber based products using steel cord as reinforcement. With the advancement in tire technology, steel reinforced radial tires have captured 90–95% of the tire market in most of the advanced countries and brass coated steel cord has been established as the premium carcass and belt material for radial truck tires. Apart from the use of cobalt salts to improve adhesion, the adhesion characteristics can also be improved through the use of resin systems such as resorcinol/hexamethoxymethyl melamine (R/HMMM), often together with silica as a partial replacement for carbon black. This combination accomplishes several things. It acts as a modulus and hardness enhancer providing a more compatible system with regard to the stiffness of the steel cord compared to that of the cured skim compound. Secondly, the combination can improve resistance to the negative effects of humidity on adhesion. Thirdly, the pro-oxidant effect of cobalt salts is counteracted to some degree. There is, however, a drawback with regard to this resin system; resorcinol generates toxic fumes at high temperatures encountered during mixing and processing. There is demand, therefore, to replace the R/HMMM adhesion system due to health and safety issues. This paper presents the results of studies to achieve adequate steel cord adhesion whilst seeking to improve aged compound characteristics and to reduce compound heat build up through the use of sulfurized para-aramid short fibers. This combination not only improves the properties of steel cord skim compounds, but also can be used effectively to replace the bonding system based on resorcinol and hexamethoxymethylmelamine. Further studies were carried out in squalene model to elucidate the changes in the chemistry of vulcanization both in the presence and absence of sulfurized p-aramid short fibers. Additional studies were carried out on sulfidized brass cords to analyze the adhesion layer's elemental composition using the Electron Dispersive X-ray Spectroscopy (EDX) characterization techniques.