Kinetics and Optimum Phenomenon of Vulcanization

Abstract The fundamental process of vulcanization consists in the combination of rubber with a vulcanizing agent : sulfur, sulfur monochloride, etc. The kinetics of this process may be expressed by monotonous curves. These may be interpreted either as the result of the heterogeneous character of the reaction or as the result of the combination of several homogeneous reactions. At the same time that the rubber combines with the vulcanizing agent, and largely as a consequence of this, a number of its physical-chemical and mechanical properties—solubility, density, tensile strength and other properties—undergo a change. These changes are extremely interesting from the technological point of view. In distinction to the kinetics of the combination of rubber with the vulcanizing agent, the kinetics of these processes can in most cases be represented by curves with a maximum or minimum. Thus, in the vulcanization of crude rubber, the tensile strength and modulus change according to a curve having a maximum; the solubility change follows a curve with a minimum. This character of the change experienced by the principal technical properties of the rubber determines the so-called “vulcanization optimum”. This term refers to that moment in the process of vulcanization when the particular property attains the necessary maximum or minimum, depending on the technical purposes of the vulcanizate.