RELATIONSHIP BETWEEN THE COHESIVE STRENGTH AND TACK OF ELASTOMERS .4. CARBON BLACK FILLED STYRENE-BUTADIENE RUBBER

摘要

The autohesion and cohesion of uncrosslinked SBR (gum and black-filled) have been determined over a broad range of test temperatures and rates using a T-peel geometry. Peeling energies can be time-temperature superposed to form mastercurves using shift factors in accord with the WLF form. Universal constants are appropriate for the gum. While experimental constants were obtained for the black composition. Cohesion for the gum and filled SDR increase continuously with increasing test speed or reduced temperature. On the other hand, autohesion for the gum shows an abrupt transition by decreasing at a critical reduced rate, while autohesion of the filled SBR does nut exhibit the transition. The transition is associated with a viscous-to-elastic response change with Increasing Ra-T; filled SDR has reduced elasticity relative to the gum and hence the transition is not present. By examining relative autohesion, it is seen that the gum undergoes an interfacial-cohesive-interfacial transition response with increasing Ra-T. This is quite different than the behavior found when peeling apart elastomer/hard substrate bonds, such as the SBR/polyester bonds of Gent and Petrich; here, there is sim ply a cohesive-interfacial transition with increasing Ra-T. For elastomer-elastomer junctions there is interpenetration and chain mobility in the interphase formed. At sufficiently low Ra-T, interdiffused chains simply slide by one another giving interfacial failure. With increasing Ra-T entanglement couplings in the interphase become effective in preventing facile flow, and, at this point, failure becomes cohesive; finally, at even higher Ra-T, with a sufficiently elastic response, stresses apparently become concentrated at the interface and failure proceeds there. When an elastomer is bonded to a hard, immobile material, the mechanism of bonding is restricted to surface site adsorption, This reduces elastomeric chain mobility and produces more ''glassy'' dispersive interactions which resist separation relative to tile chains which are held together by ''rubbery'' dispersive forces. Again at sufficiently high Ra-T, with increased elasticity, failure becomes interfacial. [References: 9]