Surface treatment of styrene–butadiene rubber with carboxylic acids

Solutions of carboxylic acids (fumaric acid, maleic acid, acrylic acid, succinic acid, malonic acid) in ethanol were used for surface treatment of two very different formulations of vulcanized synthetic styrene–butadiene rubbers to improve their adhesion to a polyurethane adhesive. The increase in the T-peel strength of adhesive joints formed between polyurethane and rubber surfaces treated with carboxylic acids depends on the rubber compounding and the characteristics of the carboxylic acid (acidity, solubility in ketones). The chemical composition of the treated surfaces was characterized by means of infra-red spectroscopy, and XPS (ESCA) analysis. The increase in the adhesion properties of styrene–butadiene rubbers treated with carboxylic acids is attributed to the elimination of abhesive substances on the surface of the rubber and to the modification of the failure mode of the joint (from interfacial to interfacial + cohesive failure of rubber). The effectiveness of the surface treatment of rubber with carboxylic acids is ascribed to a migration of the carboxylic acid into the polyurethane adhesive layer once the adhesive joint is formed. It is postulated that the carboxylic acid acts as a compatibilizer between rubber and polyurethane. The difference in effectiveness of the carboxylic acids studied is ascribed to the difference in the extent of rubber–adhesive interfacial interaction which, with increased interaction, allows varying degrees of cohesive failure at greater depths in the rubber.