Determination of Unsaturation in Butyl Rubber

In a recent publication on unsaturation in Butyl rubber it was pointed out that, of the various methods that had been examined, ozone degradation at the double bonds appears to give the most reliable unsaturation data, the values based on the limiting viscosity of the degraded polymer being in reasonably good agreement with those determined by end-group analyses. This paper describes in some detail the analytical method, together with certain modifications that have been introduced with the aim of facilitating routine work. It was recognized that the earlier procedure had the disadvantage of being too time-consuming, owing mainly to the steps in which the degraded polymer solution is evaporated to dryness, and the residue redissolved in diisobutylene before the viscosity measurement. An obvious possible improvement was to develop a procedure whereby the polymer solution could be ozonized and the viscosity of the degraded solution determined directly, thus eliminating evaporation and transfer to a second solvent. It was first necessary, however, to perform the experiments described below; their significance will be explained in the sequel. A further simplification was introduced through the use of carbon tetrachloride of ordinary reagent grade instead of the highly purified solvent. While this substitution usually leads to viscosity-time curves displaying a small negative slope in the region where the purified solvent gives unchanged viscosity values, a simple extrapolation to zero time gives viscosity data that are satisfactory within the limits inherent in the method. In view of the practical requirements of the method and the magnitude of the experimental error, the extra time and effort involved in further purification of the solvent appear to be unwarranted.