Reinforcement and Stearic Acid

Abstract (1) Addition of stearic acid does not increase reinforcement of carbon black mixes to an appreciable extent; however, the work of retraction is considerably augmented, and the hysteresis loss is reduced. (2) The action is two-fold; free stearic acid lubricates, thereby reducing internal friction, while dissolved stearic acid reduces interfacial tension, thereby increasing wetting or true reinforcement. (3) Addition of more than 2 per cent of stearic acid lowers reinforcement, since the lubrication effect decreases that portion of reinforcement due to internal friction. (4) “Blooming” of stearic acid occurs because it decreases surface tension of rubber, and consequently concentrates in the surface. (5) Removal of stearic acid causes complete disappearance of reinforcement, as measured by work of extention to rupture.