Effect of Interfacial Bonding on the Self-Adhesion of SBR and Neoprene

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Abstract
When two elastomer layers are partially crosslinked in contact with each other, the strength of adhesion increases. This effect has now been studied for simple vulcanizates of SBR and for CR vulcanizates crosslinked with sulfur and/or oxide linkages. Under threshold conditions, e.g., at high temperatures and in the swollen state, when both viscous effects and strain-induced crystallization are virtually eliminated, the work of detachment is small and approximately proportional to the inferred degree of interlinking, rising from a few J/m2 when only Van der Waals' attractions are present, up to 40–70 J/m2 in the fully-interlinked state. No significant differences were observed between SBR and CR vulcanizates, or with different types of crosslinking of CR. The principal structural factors governing the adhesive strength under threshold conditions appear to be the number and length of the molecular strands comprising the network and crossing the interface. The threshold fracture energy is directly proportional to the density of such strands and is smaller for shorter strands, in accord with the theory of Lake and Thomas. Under normal conditions, e.g., at room temperature and in the unswollen state, CR vulcanizates were found to adhere much more strongly for a given degree of interlinking, presumably due to strain-induced crystallization. Indeed, some enhancement of strength persisted even at temperatures of 100–150°C, in the unswollen state.

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