Measurement of the Unsaturation of Butyl Rubbers by the Iodine Index Method

Iodine index is used in the rubber industry to measure the unsaturation of butyl polymers. The procedure involves a complex mixture of iodine, mercuric acetate, and trichloroacetic acid (TCAA), and requires an empirical calibration to convert the iodine index to the degree of unsaturation. In this paper, the chemistry of the iodine index test is discussed and, the factors affecting the measurement, are detailed. Acetyl hypoiodite formed in situ by the reaction of I2 and Hg(OAc)2 is postulated to be the reactive species which iodinates the unsaturation in butyl rubber. Mercuric acetate is not a catalyst, as usually believed, but is consumed in stoichiometric amounts. Hence, care has to be exercised not to use too little of the mercuric acetate when determining the iodine index of polymers with high levels of unsaturation. A large excess of mercuric acetate must also be avoided, as it may form complexes with iodine and, thereby, lower the effective concentrations of both the reactive reagents.