The Influence of Carbon Black, Mixing, and Compounding Variables on Dispersion

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Abstract
A series of response equations relating dispersion and rubber properties to mixing and compounding variables have been developed for formulations based on SBR and EPDM. These provide insight on the relationship of dispersion to rubber performance as carbon black type and loading are widely varied. Optimum conditions have been defined for minimizing Banbury mixing time and power consumption to achieve acceptable dispersion for different grades of carbon black. The different blacks can be classified in terms of their relative tendency to be shear-stress or shear-strain sensitive. The finer, low-DBPA carbon blacks such as N326 are shear-stress sensitive. To achieve high dispersion levels, this type of black requires initially high compound viscosities, i.e., by means of high loadings, withholding all or part of the extender oil until after black incorporation, or by high black/oil ratios. The coarser, high-DBPA carbon blacks such as N650 are more shear-strain dependent and will reach high dispersion levels if enough energy (usually relatively low) is imparted to the batch during Banbury mixing. N650 consistently gave high dispersion levels over a wide range of black/oil levels, regardless of the polymer type or mixing variables. Carbon black dispersibility in SBR-1500 was improved in the direction of increasing DBPA and decreasing surface area. The surface area response was most pronounced when loose oil was added with the black. High oil with the carbon black tends to form harder, more persistent agglomerates which have a greater depressing effect on rubber strength properties such as tensile and fatigue. The effect of carbon black surface area on dispersion was considerably less in SBR-1712 with no loose oil addition. In EPDM, upside-down mixes of different grades of black at varied black/oil loadings produced some notable changes in the dispersion response to carbon black DBPA. At some of the high black/oil ratios, low DBPA was actually beneficial to dispersion. The finer, higher-DBPA blacks such as N351 showed a tendency to form more persistent agglomerates analogous to conventional SBR-1500 mixes in which high amounts of loose oil are added with low-DBPA blacks. These persistent agglomerates are most prone to form in an EPDM upside-down mix when the oil loading is close to the absorptive capacity of the carbon black. The Banbury power profiles of such compounds indicated a high initial peak and a depressed or missing second peak. A normal power profile and a high level of dispersion were achieved with these problem mixes when 25 to 50% of the oil was withheld until after black incorporation. N650 gave excellent dispersion in EPDM at all black/oil loadings and only minimal variations in vulcanizate properties were observed at mixing times ranging from 1 to 5 min.

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