CARBON BLACK MORPHOLOGY IN RUBBER.

Abstract

The primary properties of carbon black may still be defined in terms of (1) fineness (primary unit size and surface area), (2) structure (primary unit shape), and (3) surface activity. Two simple and effective procedures have been developed for extracting blacks from rubber compounds for EM analysis: (1) thin-layer pyrolysis and (2) dispersed carbon-gel pyrolysis. Both methods are discussed in this paper, with emphasis on the application of the gel procedure to assess rubber performance as a function of carbon black variables. It is demonstrated that the EM image analysis of carbon blacks in specific rubber compounds has greatly expanded the range of useful applications for studies of this type. This dispersed carbon-gel procedure has improved the sampling and test precision at operating speeds that are now reasonably comparable to the simple colloidal procedures for characterizing carbon black. Improved models have been developed for deriving black surface area and intraunit differences that are attributable to carbon black variables. Studies on hysteresis at constant strain (E double prime ) have indicated that the important black variables, in diminishing order of significance, are loading, structure (intraunit occlusion and anisometry), unit size, unit size distribution, and surface activity. For hysteresis at constant energy (resilience), the most important black variables appear to be black loading, unit size, unit size distribution, surface activity, and structure. In terms of tread wear resistance, a somewhat different pattern of carbon black variables is apparent. At constant loading, the most important black properties appear to be specific surface area, surface activity, structure, and unit size distribution.