MEASUREMENT OF THE UNSATURATION OF BUTYL RUBBERS BY THE IODINE INDEX METHOD

Iodine index is used in the rubber industry to measure the unsaturatio n of butyl polymers. The procedure involves a complex mixture of iodin e, mercuric acetate, and trichloroacetic acid (TCAA), and requires an empirical calibration to convert the iodine index to the degree of uns aturation. In this paper, the chemistry of the iodine index test is di scussed and, the factors affecting the measurement, are detailed. Acet yl hypoiodite formed in situ by the reaction of I-2 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 belie ved, but is consumed in stoichiometric amounts. Hence, care has to be exercised not to use too little of the mercuric acetate when determini ng the iodine index of polymers with high levels of unsaturation. A la rge excess of mercuric acetate must also be avoided, as it may form co mplexes with iodine and, thereby, lower the effective concentrations o f both the reactive reagents.