Thixotropic Behavior of Carbon Black in Rubber

Abstract The addition of carbon black may cause considerable changes in the physical properties of rubber. Often rubbers compounded with carbon black or other fillers show evidence of thixotropic behavior—their properties depend on the past history of the rubber and may be changed on the one hand by continuous standing and on the other hand by working. This paper describes investigations into the behavior of both unvulcanized and vulcanized rubbers, and discusses the thixotropic build-up and breakdown of colloidal carbon black structures which are shown to exist in rubber. The importance of this build-up and breakdown of structure has not been adequately appreciated in many measurements on compounded rubbers; for example, the terms viscosity, plasticity, and stiffness (modulus) can only be defined when complications due to thixotropy are carefully controlled before and during the actual measurement. A knowledge and understanding of this behavior enables the significance of the results of various tests on these materials to be more correctly assessed and accounts for behavior which has previously appeared to be anomalous. The deficiencies of earlier theories of the reënforcement of rubber produced by carbon black are indicated, and modifications which make allowance for thixotropic filler structure are proposed.