Relationship between the Cohesive Strength and Tack of Elastomers: Part IV, 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 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 SBR 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 not exhibit the transition. The transition is associated with a viscous-to-elastic response change with increasing RaT; filled SBR 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 RaT. 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 simply a cohesive-interfacial transition with increasing RaT. For elastomer-elastomer junctions there is interpenetration and chain mobility in the interphase formed. At sufficiently low RaT, interdiffused chains simply slide by one another giving interfacial failure. With increasing RaT entanglement couplings in the interphase become effective in preventing facile flow, and, at this point, failure becomes cohesive; finally, at even higher RaT, 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 the chains which are held together by “rubbery” dispersive forces. Again at sufficiently high RaT, with increased elasticity, failure becomes interfacial.