Bound—Rubber Formation in Diene Polymer Stocks

Abstract

Abstract The phenomenon of insolubilization of rubber by carbon black has been known for at least twenty-seven years. There have been many attempts during this time to establish a relationship between insolubilization, or bound rubber formation, and reinforcement of rubber by carbon black. It was postulated, as far back as 1925, that there was a parallelism between particle size and insolubilization. This in spite of the fact that the methods available for determining particle size of very fine powders at that time were relatively crude. It was postulated at that time that there was a close relationship between the phenomena of vulcanization and reinforcement. This concept has had recurrent periods of popularity since that time. The insolubilization of rubber by carbon black, or other pigments, has been estimated by various techniques by the early investigators in the field. In more recent work, the trend has been toward a very straightforward experimental approach. Bound rubber in an uncured rubber-filler compound is usually determined by static extraction using the same apparatus and techniques used in determining the gel content of unfilled polymers. It follows that, using this method, polymer insoluble because of crosslinking, or gel, cannot be distinguished from polymer insolubilized by incorporation of fine fillers. In investigating the formation of bound rubber, therefore, the presence of gel in the polymer being used, or formation of polymer gel during processing must be taken into account.