Effect of interfacial bonding on the strength of adhesion of elastomers. II. Dissimilar adherends

Abstract

Abstract Two elastomer layers, differing either in initial degree of crosslinking or in chemical reactivity, were bonded together by a free‐radical crosslinking process. The elastomers employed were polybutadiene (BR) and an ethylene‐propylene copolymer (EPR) differing in the efficiency of crosslinking by dicumyl peroxide by a factor of about 20. When a fully crosslinked sheet of either elastomer was pressed into contact with a partially crosslinked sheet of the same elastomer and the crosslinking then taken to completion, the strength of adhesion under threshold conditions was found to be qualitatively in accord with the predictions of a simple theoretical treatment for the degree of interlinking in terms of the corresponding homogeneous crosslinking reaction. Whereas the theory suggests that the effective degree of interlinking will be one‐half of that generated in a homogeneous system, the experimental results were in accord with a figure of about 70%. When a layer of one elastomer was bonded to a layer of the other in a similar way, the strength of adhesion was found to be relatively high when the initial fully crosslinked layer was BR and relatively low when it was EPR. These results were also in qualitative agreement with theoretical predictions for the degree of chemical interlinking developed between layers differing in chemical reactivity. Thus a general correlation appears to hold between the threshold strength of adhesion and the amount of interlinking.