Vulcanization of Elastomers. 28. Vulcanization of Natural and Synthetic Rubbers with Sulfur in the Absence of Accelerators. I

Abstract

Abstract The decrease of sulfur concentration in the reaction of sulfur with natural rubber was studied. The following was found : 1. Sulfur decrease follows the 0.6th order law independent of temperature as long as the sulfur is dissolved in the gum (temperature!). From the rate-temperature relation, an activation energy of 35 kcal/mole is calculated. 2. In an investigation of the kinetics of sulfur concentration at constant temperature but increasing starting concentration, the following two cases can be differentiated : a) As long as the sulfur is soluble in the gum, sulfur decrease still follows the 0.6th order, possibly due to autocatalysis ; the linear relationship between starting rate and starting concentration shows that the process is 1st order with respect to the concentration; this may be the consequence of a thermal, rate determining cleavage of the S8 ring. b) As the sulfur at higher concentration is only incompletely soluble in the gum, conversion curves with points of inflection are found ; this becomes more pronounced at higher concentration. At the start of the reaction one finds an autocatalytic sulfur decrease basically of zero order; in the latter part of the reaction after passing the point of inflection a 0.6th order is observed, as in the range of complete sulfur solubility. 3. A discrepancy between the time law and the concentration-rate relationship is found in pure as well as accelerated sulfur vulcanization ; these conditions are compared and discussed.