Factors affecting the adhesion between unvulcanised elastomers

The self- and mutual-adhesion of various elastomers were investigated as a function of contact time and temperature by means of peel tests. Peel energies were measured for a peel angle of 90° at ambient temperature (ca. 23°C) at a constant rate. The self-adhesion of unfilled polyisoprene elastomers, both natural and synthetic, with molecular weights less than 5X10s was found to attain its full-strength after a contact time of less than 2 min (the shortest investigated). For synthetic polyisoprenes (IR) of molecular weight 7 X 1O5 and above, the self-adhesion was observed to increase until contact times of 100 h or more were reached. For the IR elastomers studied, the contact time needed for the adhesion to reach the plateau level did not appear to correlate well with predictions based on interdiffusion of the long-chain molecules as the rate-determining process. Compared with the polyisoprenes, the self-adhesion of the unfilled styrene-butadiene polymer (SBR) studied, despite its having a much lower molecular weight, developed more slowly, behaviour consistent with the influence of SBR’s higher glass transition temperature on its flow and diffusion behaviour. The mutual adhesion of natural rubber (NR) with epoxidised natural rubber (ENR) or to SBR, and the mutual adhesion of IR with ENR improved noticeably with time; the locus of failure moved from the interface into the weaker adherend provided sufficient contact time was permitted. As the polymer pairs are all immiscible, having significantly different solubility parameters, and the possibility of significant interactions between polar groups is absent, the development of such strong levels of mutual adhesion is surprising. The present results unexpectedly suggest a substantial difference between the time dependence of the adhesion of IR or NR to ENR, with the synthetic polyisoprene showing a stronger adhesion particularly at shorter contact times; the difference is apparent despite the comparison being made for IR and NR of similar molecular weight. The presence of carbon black filler had little effect on the self-adhesion of SBR. The mutual-adhesion between SBR and NR was increased by the addition of black, though it developed to its plateau level more slowly. The results reported here suggest, somewhat surprisingly, that the influence of carbon black on the self-adhesion of NR, unlike that of SBR, appears to depend upon the type of black.