Influence of the styrene content of thermoplastic styrene–butadiene rubbers in the effectiveness of the treatment with sulfuric acid

Abstract

Three thermoplastic block styrene–butadiene–styrene (TR) rubbers were treated with sulfuric acid to improve their adhesion to polyurethane adhesives. T-peel test, scanning electron microscopy (SEM), contact-angle measurements (water, ethane diol), infra red spectroscopy (ATR-IR) and stress–strain experiments were used to analyze the nature of surface modifications produced in the rubber. The length of the treatment and the neutralization procedure (with and without ammonium hydroxide) were considered in this study. The treatment produced the sulfonation of the butadiene units of the rubber and the creation of highly conjugated CC bonds, which produced a change in the color of the TR rubbers. The treatment also produced a decrease in the tensile strength and the elongation at break of the TR rubbers. This suggests that the treatment with sulfuric acid was not restricted to the rubber surface but also produced a bulk modification. The lower the styrene content in the TR rubber, the more significant modifications were produced on the surface. The styrene content (33–55 wt%) in three thermoplastic styrene–butadiene (TR) rubbers affected the extent, but not the nature of the surface modifications produced by treatment with sulfuric acid. The H 2SO 4 treatment increased the T-peel strength of S1 and S2 rubber/polyurethane adhesive joints and produced a mixed failure mode (adhesion+cohesive failure in the rubber). Sulfonation of the TR rubbers is fast and needs only a 30 s immersion in sulfuric acid to produce high adhesion. Furthermore, the neutralization of the acidic surface with ammonium hydroxide is critical to assure an adequate durability of the adhesive joints.