The Vulcanization of Highly Elastic Polymers. 81. Sulfur Vulcanization of Ethylene-Propylene Terpolymers Containing Varying Amounts of Double Bonds

The results of unaccelerated and accelerated vulcanizations of ethylene-propylene terpolymers (EPDM) demonstrate that the reaction of sulfur with allyl systems (such as those which the 1,5-polyenes contain at concentrations far exceeding the quantity of sulfur introduced for crosslinking purposes) tends toward an order above unity and eventually towards second order, as the sulfur concentration and the double bond content approach the stoichiometric ratio. This could be shown with unaccelerated vulcanizations of EPDM in the absence of ZnO, as well as those accelerated by zinc mercaptobenzothiazole. This is so, in our opinion, because in the one case autocatalysis by the polysulfidic sulfur is inhibited, and in the other it becomes negligible because of the very rapid reaction rate. According to Scheele and Huischen, the reaction order for sulfur decrease for the unaccelerated vulcanization of nitrile rubber is first order, both with respect to time and (very important!) with respect to concentration (nt=no=1), and the reaction is not subject to autocatalysis because of disulfidic crosslinks. Accordingly, we arrive at the conviction, in view of the relations obtained with the EPDM rubbers, that all the rate laws determined for sulfur decrease in 1,5-polyene vulcanizations with an excess of double bonds have as their basis pseudo-reaction orders or pseudo-molecularities. In spite of these generally revealing differences in the kinetics of sulfur decrease and in its chemistry between the 1,5-polyenes with their excess of double bonds and the EPDM rubbers with double bond content that is commensurate with the sulfur concentration, the fact still remains that the reaction of sulfur with polymers containing allyl groups has all the characteristics of a multistep reaction of of catalysis by an intermediate and that these characteristics are obviously those generally found in reactions between sulfur and any kind of compound that contains allyl groups. Even the great differences observed between the chemical constitution and the macromolecular structure of the various homo- and copolymers of the 1,3-dienes (natural rubber and poly butadiene), of polypentenamer, and of the EPDM rubbers under consideration here, cannot alter that circumstance, since each situation involves an analogous reaction of sulfur with the allyl units which sulfur-crosslinkable rubbers contain. Any quantitative differences noted can be attributed to the concentration ratios of all the participating reactants as well as the chemical constitution and reactivity of the allyl units. Thus one cannot escape the conclusion that the submolecular, molecular, and supermolecular structure of the rubbers, as well as the polymerization degree and the molecular weight distribution, are of greater importance for the physical, mechanical, and technological properties of the vulcanizates than the vulcanization itself, which, incidentally, should always be carried out so as to realize optimum technological properties in order to meet the industrial technological requirements.