The Effect of Carbon Black Surface Properties and Structure on Rheometer Cure Behavior

The cure behavior of SBR and NR compounds was examined using the Monsanto Rheometer. The three cure parameters—induction period (td), rate of crosslinking reaction (k2) and maximum increase in torque (ΔL)—were derived from the Rheometer torque curves. The data so obtained were used to calculate Westlinning's αF-values. It was shown that (1) k2 decreases with increasing total acid concentration at the carbon black surface and (2) the value of αF is related to the filler structure and the crosslink density of the rubber matrix. It was also demonstrated that the crosslink density, as measured by ΔL, is influenced by the surface chemistry of carbon black. In SBR stocks the crosslink density increases with increasing total acid concentration, while in NR compounds the reverse is true. These findings were confirmed by examining the tensile properties of regular and oxidized HAF-LS black in SBR and NR compositions. Thus the effect of carbon black surface chemistry on crosslink density can be estimated, for a given recipe, from the value of k2. An empirical relationship between the cure parameters (αF and k2) and filler structure (as measured by extrusion shrinkage) was derived and tested on a wide range of carbon blacks. This relation gives a much better correlation coefficient than the simple relation between extrusion shrinkage and αF alone. Thus by full use of the Rheometer data, the extrusion shrinkage or other structure-related properties of carbon black filled stocks can be predicted.