Processing in an Internal Mixer as Affected by Carbon Black Properties

This paper has attempted to review current knowledge on mixing fundamentals from a carbon black standpoint. The mixing parameters discussed include volume fill factor, specific energy of mixing, temperature buildup, and flow characteristics. Together, these parameters define the economics of mixing in terms of mixing capacity, energy expenditure, limitations imposed by dump temperature on second-stage operations, and extruder flow requirements. The four parameters in turn were related to basic carbon black properties by way of mixing profiles. Ten currently available tread blacks which exhibit a wide range of tread wear resistance have been characterized in terms of the processability criteria. A processability rating system based on the most simple equipment requirement (a Banbury followed immediately by a second-stage mill or an extruder) was applied to the ten blacks. Comparison of the processability ratings with the tread wear ratings clearly demonstrates that a valid assessment of carbon black utility must include processing cost in addition to performance capability. As in many rubber properties, there is clearly an inverse relationship between performance quality and processability. High-structure, high-surface-area blacks that are desirable from a performance standpoint require the highest processing cost. The question of uniformity was also discussed, and it was concluded that the present specifications controlling carbon black surface area and structure are adequate in minimizing processing variations. The challenge to processing technology from a carbon black standpoint then involves the reduction in the cost of processing high-structure, high-surface-area blacks. At this stage, a quantitative measure of the processing characteristics of the currently available tread blacks at least allows a rational product selection in terms of optimum performance and processing cost. In the meantime, it is evident that significant advances in carbon black technology are still needed if performance optimization is to be reconciled with the economics of processing.