Effect of interfacial bonding on the strength of adhesion of elastomers. I. Self‐adhesion

Abstract Two partially gelled (crosslinked) layers of elastomer were pressed into intimate contact and the gelation reaction was then taken to completion. By varying the extent of initial gelation, the degree of chemical interlinking was varied from zero, when two fully reacted sheets were pressed together, up to a level characteristic of the final density of molecular linking within each layer, when they were brought together before any reaction had occurred. The strength of adhesion between the layers was measured under threshold conditions, i.e., at low rates of peel, at high temperatures, and, in some instances, with the layers swollen with a compatible liquid. Linear relations were obtained between the threshold work of detachment per unit of interfacial area and the amount of chemical interlinking, deduced from the kinetics of molecular linking within each layer. At any degree of interlinking, ranging from zero to the fully interlinked state, the work of detachment was lower for networks composed of shorter molecular chains, in accordance with the Lake–Thomas theory for the threshold strength of elastomer networks. By extrapolation to the fully interlinked state, the strength of adhesion corresponding to cohesive rupture was inferred. These values agreed with measured tear strength for polybutadiene gelled by a free‐radical process. For a sulfur crosslinking system, and for both free‐radical and sulfur crosslinking of poly(ethylene‐co‐propylene), the threshold tear strength of the elastomer was found to be much higher than the extrapolated value from adhesion measurements. This discrepancy is ascribed to roughness of the tear plane in relatively strong elastomers, in contrast to the smooth separation of flat adhering layers. Adhesion of fully crosslinked sheets was generally low, 1–2 J/m2. Higher values, 5–25 J/m2, were found with sulfur crosslinking systems, especially those yielding a high proportion of polysulfidic crosslinks. Interlinking via polysulfide crosslink interchange reactions is suggested in these cases.