Hydroxyl free radicals were chemisorbed on to the surface of fully reinforcing carbon blacks by reacting them with an aqueous solution of potassium persulfate or of sodium hypochlorite. Both treatments caused a marked decrease in reinforcing ability, from which it was concluded that free radical interaction between these carbon blacks and rubber is a factor in reinforcement. Bound rubber and swelling index, which were determined for commercial carbon blacks covering the full range of particle size used in rubber compounding, were found to be no better related to reinforcing ability than was specific surface. In addition, after hydroxyl free radical treatment, bound rubber and swelling index remained unaltered though the reinforcing ability had been reduced. It was concluded, therefore, that the free radical interaction between carbon black and rubber which contributes to reinforcement occurs mainly during vulcanization, not during milling.
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