EFFECT OF SULFUR DISPERSION ON THE KINETI CS OF VULCANIZATION

Kinetic measurements of vulcanization of natural rubber compounds usin g isothermal DSC with small samples, different temperature and gas rel ease conditions, show that mass transfer of sulphur has an important e ffect on the rate of the cross-linking reaction. In fact, kinetic para meters are different for two samples with the same composition: type 4 4 where all the sulphur is initially finely scattered (1 mum), and 41 in which 75% sulphur is initially in coarse grains (100 mum). At 120-1 50-degrees-C, induction time and degree of cure at the peak of rate of reaction are larger for sample 41. The discrepancies vanish for high temperatures. The rising part of the kinetic curve w(alpha) of the rat e of reaction versus the degree of cure alpha is a single line for sam ple 44 and is made of two lines with different slopes for sample 41. T he study shows that the effect of the size of the sulphur particles, n eglected in recent kinetic models which consider only the chemical asp ect, is of great importance for the cross-linking reaction.