Bonding of natural rubber to steel: Surface roughness and interlayer structure

This paper is concerned with two aspects of the adhesion produced by the vulcanisation bonding of a simple natural rubber (N.R.) compound to mild steel. Adhesion was measured using a 45 degrees peel test. When the N.R. was bonded, using a proprietary bonding agent (Chemlok 205/220), to "smooth" steel (acid etched) or to "rough" steel (phosphated) high values of peel energy (greater than or equal to 4.5 kJm(-2)), and good environmental resistance to water were obtained, with failure cohesive largely within the rubber. The highest values of peel energy (approximate to 7.5 kJm(-2)) were associated with a phosphated surface which consisted of plate-like crystals which directed the stresses away from the substrate in a way which produced a failure surface within the rubber which showed extensive tearing and cracking. The nature of the layer formed in the interfacial region by interaction between bonding system and rubber was investigated using a chlorinated rubber as a "model compound" representing the adhesive and uncompounded N.R. to represent the rubber. When a blend of the two was heated in air at 150 degrees C, evidence was found of a solid stare chemical reaction in which carbonyl groups were incorporated into the blend which became visually homogeneous. Further evidence points to the relevance of this change to adhesion in rubber-to-metal bonding.