Carbon Black in NR/BR Blends for Truck Tires

Abstract

Abstract A series of ten commercial tread-grade carbon blacks were evaluated in a 60/40 NR/BR truck tire tread formulation. A number of important physical properties and performance criteria were assessed in terms of carbon black surface area and DBPA. Significant response equations were obtained for viscosity, bound rubber, resilience, heat buildup, tear strength, and dynamic properties. Dynamic modulus showed a much greater dependence on DBPA in comparison to previous studies on SBR/BR compounds. In a second designed experiment, a single carbon black (N299) was studied as a function of the NR/BR ratio and the amount of carbon black added to the BR phase. The BR black loading was varied at 30, 60, and 90 phr using separate masterbatches which were blended with NR-black masterbatches to give the same final composition for all of the compounds. Properties such as resilience, heat buildup, fatigue life, and tear strength were all improved in the direction of higher loadings of carbon black in the NR phase. A high loading of black in the BR phase caused low bound-rubber development and poor dispersion. This was found to be related to the viscosity ratio of the separate masterbatches. NR to BR viscosity ratios of about 1 to 3 produced good dispersion and high bound rubber. When the BR masterbatch viscosity was two to three times higher than the NR masterbatch, however, dispersion and bound-rubber development dropped sharply at the same total mixing energy. Low hysteresis properties were found to be most dependent on high bound-rubber development, with polymer phase distribution having a relatively minor influence. In contrast, tear strength and fatigue life reached their maximum levels when the NR was the more continuous polymer phase. High bound rubber also appears to enhance tear strength and fatigue life by improving the microdispersion of the carbon black.