Vulcanizate Performance as a Function of Carbon Black Morphology

A new nomenclature is proposed for carbon black morphology to be based on actual quantitative electron microscopical measurements. Carbon black is now defined in terms of “paracrystalline graphitic units” which are composed of interlocking “domains of rotational graphitic layer orientation”. The measurement of “unit” or “domain” dimensions replaces the earlier imprecise concepts of structure and particle size. Automated measuring procedures have been developed for determining carbon black unit and domain size functions using a Quantimet Image Analyzing Computer. Unit dimensions are expressed in terms of total projected length (L), average width (W), two-dimensional projected area (A), and form factor (L/W). Domain size (d) measurements have been programmed to give comparable results to earlier carbon black “particle” size determinations using a Zeiss Particle Size Analyzer. The new program enables measurement of similar classes of blacks (e.g., tread grades) under identical instrumental conditions, thus greatly reducing the problem of operator subjectivity. Meaningful correlations have been determined for several vulcanizate properties in terms of the above carbon black parameters. Domain size was found to be the most significant variable in the correlations with resilience, showing a positive relationship. Increasing unit form factor, indicated a depressing effect on resilience and was also the most significant single variable in regard to heat build-up and hardness, showing a positive relationship with both of these properties. A significant inverse correlation with heat build-up and hardness was observed for unit or domain size, when the full range of rubber grade carbon blacks were evaluated. Tensile strength showed an inverse relationship with unit or domain size in the SBR and SBR/BR systems that were studied. In natural rubber, however, there was also a significant depressing effect on tensile strength, attributable to increasing unit form factor. Modulus showed a strong positive correlation with unit length or form factor and an inverse relationship with domain or unit size. High unit form factor was also the most significant variable in reducing extrusion swell. High abrasion resistance (treadwear) was favored by high form factor, low unit size, and low domain size in that order of significance.