The Dissolution of Vulcanized Rubbers and Its Bearing on Problems of Rubber Analysis

Dissolution of a sample of vulcanized rubber is a necessary preliminary to several analytical procedures, e.g., the determination of iodine value or total fillers. If an infallible and convenient agent were available for this process, then doubtless other analytical procedures would be possible of development or improvement and examination of infrared transmission spectra of vulcanized samples would prove a feasible means of identification. Although decalin and nitrobenzene have been widely and satisfactorily used for natural rubber (polyisoprene), there have been several references in the literature to difficulties encountered with the most widely used synthetic rubber, GR-S. Devices to overcome these difficulties include comminution on a roller mill, swelling the sample in strong swelling agents before transferring to the solvent to be used for the dissolution, and grinding on a rubber mill with a plasticizer. All these must be regarded as being of the nature of palliatives, knowledge of the nature of the dissolution process being the real need before the best condition for carrying it out can be decided on. It is shown in this paper that the process is essentially oxidative in nature and, provided that the solvent and rubber are miscible at the temperature of the dissolution, temperature and efficient aeration are the most important rate-determining factors. The nature of the solvent is, of course, of importance with respect to the analytic procedure attempted as all solvents are not suitable for iodine value determination.