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

The autohesion and cohesion of uncrosslinked SBR (gum and black-filled ) have been determined over a broad range of test temperatures and rat es using a T-peel geometry. Peeling energies can be time-temperature s uperposed to form mastercurves using shift factors in accord with the WLF form. Universal constants are appropriate for the gum. While exper imental constants were obtained for the black composition. Cohesion fo r the gum and filled SDR increase continuously with increasing test sp eed or reduced temperature. On the other hand, autohesion for the gum shows an abrupt transition by decreasing at a critical reduced rate, w hile autohesion of the filled SBR does nut exhibit the transition. The transition is associated with a viscous-to-elastic response change wi th Increasing Ra-T; filled SDR has reduced elasticity relative to the gum and hence the transition is not present. By examining relative aut ohesion, it is seen that the gum undergoes an interfacial-cohesive-int erfacial transition response with increasing Ra-T. This is quite diffe rent than the behavior found when peeling apart elastomer/hard substra te bonds, such as the SBR/polyester bonds of Gent and Petrich; here, t here is sim ply a cohesive-interfacial transition with increasing Ra-T . For elastomer-elastomer junctions there is interpenetration and chai n mobility in the interphase formed. At sufficiently low Ra-T, interdi ffused chains simply slide by one another giving interfacial failure. With increasing Ra-T entanglement couplings in the interphase become e ffective in preventing facile flow, and, at this point, failure become s 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, immobi le material, the mechanism of bonding is restricted to surface site ad sorption, This reduces elastomeric chain mobility and produces more '' glassy'' dispersive interactions which resist separation relative to t ile chains which are held together by ''rubbery'' dispersive forces. A gain at sufficiently high Ra-T, with increased elasticity, failure bec omes interfacial.