Nonstoichiometric Copper Sulfide Nanostructures at the Brass-Rubber Interface: Implications for Rubber Vulcanization Temperature in the Tire Industry

Brass (which is an alloy of copper and zinc)-coated steel cords (BCSCs) in the form of belts are embedded in a rubber compound in radial tires (beneath the tread) to give stability and strength to the tread region of the tires. The life of the tires also depends on the strength and durability of the bond between the BCSCs and rubber. During the vulcanization process with sulfur, a series of sulfide and oxide nanostructures of copper and zinc are formed at the brass-rubber interface. These nanostructures have a dendritic morphology that can reinforce rubber primarily through mechanical interlocking created through the flow of rubber chains into the dendritic cavities followed by formation of cross-links between rubber chains during vulcanization. The strength and durability of the bonding depend on a number of parameters such as rubber compound formulation, vulcanization temperature (VT) and time, nanostructure thickness (height), and chemical composition of the nanostructures (the so-called adhesion interface). A few methods have been stated in the literature for assessing the chemical composition and thickness of the adhesion interface. However, simple, reliable, and newer methodologies are needed for a better understanding of the same. This paper details a new approach called the "brass mesh experiment"to assess the thickness of the adhesion interface formed under particular vulcanization conditions using microscopy. Raman imaging and spectroscopy were employed to determine the chemical composition of the interface with complementary data from X-ray photoelectron spectroscopy and X-ray diffraction. Using the methodologies, VT optimization was done for a tire compound formulation, and this was verified by the generally accepted pull-out force method. We believe that the methodologies outlined in this paper can trigger further research for a better understanding of the adhesion interface in radial tires.