Dependence of the Fundamental Properties of Unvulcanized and Vulcanized Butadiene-Styrene Rubber Mixtures on the Initial Molecular Weight

Abstract

Abstract 1. The relation of the basic properties of unvulcanized and vulcanized SKS-30A rubber-carbon black mixtures to the initial molecular weight of the rubber was studied. 2. Fractions having molecular weights up to 300,000 possess the best technical properties in their behavior during mill mixing. However, the best dispersion of carbon black is found in the still higher molecular fractions. 3. In the mill mixing of rubber with carbon black, it is principally the highest molecular fractions of the rubber which become bound, with the formation of an insoluble carbon black-rubber gel. It is shown that the thermoplastic properties of rubber-carbon black mixtures depend basically on the molecular weight and, probably, on the structure and quantity of carbon black-rubber gel formed. 4. The basic properties of vulcanizates, viz., tensile strength, relative elongation, heat resistance, and resistance to repeated deformations increase with increase of the molecular weight up to 300,000, but change little with further increase of the molecular weight. 5. The greatest reduction of the properties of vulcanizates is observed at a molecular weight of 80,000 and lower. Addition of a low-molecular fraction to a high-molecular fraction in the proportion of 1:1 greatly decreases the tensile strength and resistance to repeated deformation. 6. The range of molecular weights from 100,000 to 300,000 is, for SKS-30A rubber, the most satisfactory from the viewpoint of obtaining, throughout this range, satisfactory technical properties of mixtures before vulcanization and physical and mechanical properties after vulcanization. 7. The relation which has been established of the basic properties of a vulcanizate to its molecular weight is explained: (1) by the formation of a more regularly constructed network in a vulcanizate of the high-molecular fractions of rubber, which is distinguished from that of the low-molecular fractions by a smaller number of terminal groups not entering into the chain ; and (2) by the different nature of the reaction between carbon black and the high-molecular fractions, resulting in the latter case in a higher value of the energy of inter-molecular reaction.