Mechanism of Rubber Vulcanization with Sulfur

Abstract

THAT vulcanization of rubber with sulfur always involves a chemical reaction consisting in the addition of sulfur to the double bonds of the rubber molecule has been conclusively established (18, 28). The facts indicate that this addition of sulfur to rubber is an irreversible reaction (31). The temperature coefficient of the reaction is high, increasing about 2.65 fold per 10° C. at ordinary curing temperatures (31). Furthermore, the reaction is apparently exothermic (4, 24). It is noteworthy that catalysts are apparently necessary, since synthetic rubbers prepared from pure materials add sulfur slowly, if at all. The proteins and perhaps the resins in natural rubber undoubtedly serve as accelerators. The curves for combined sulfur vs. time of cure for typical mixes are shown in Figures 1 and 2. Figure 1 is taken from the data of Kratz and Flower (16); the composition and temperature of cure for this mix are shown in Cranor's Table I (9). Figure 2, curve 1, is from Table I of Eaton and Day (10), and curve 2 from data obtained in this laboratory (27, Table I). Superficial inspection of these curves shows extraordinary divergence of type. Figure 1 is a typical fadeaway curve, characteristic of most chemical reactions, where the reaction rate decreases with decreasing concentration of the reacting materials. Curve 1, Figure 2, is an entirely different type, where the rate of sulfur addition is constant until nearly 70 per cent of the initial sulfur has reacted. Curve 2, Figure 2, shows even more complex behavior. Again the rate is constant in the initial portions of the cure. However, following this period, the rate increases markedly but later falls off, approaching zero, to give an S-shaped eurve.

Crystallinity and Size Control of Colloidal Germanium Nanoparticles from Organogermanium Halide Reagents

“…The relevant process is known as vulcanization, which is preceded by the formation of linear diradicals from the homolytic breakup of labile S−S bonds. 57,58 These sulfur radicals are known for having a high affinity toward double bonds, and because oleylamine possesses a long alkyl chain with an unsaturated double bond, the oleylamine and S species can undergo a radical polymerization. This results in the sulfur diradicals establishing bridges between oleylamine molecules and cross-linking them, forming what is called a poly(oleylamine-random-sulfur) which is highlighted in Figure S9 of the Supporting Information.…”

The progress of organic sulfur chemistry

“…In the vulcanization of natural rubber with sulfur it was observed that this process occurred most easily and quickly in the presence of an "accelerator." The first research on accelerators was hit or miss, but now it is known that they function as catalysts in that they unite with free sulfur to form an unstable compound and then pass it on to the rubber molecule in a more reactive form, the accelerator molecule being regenerated and re-used (20). The whole process of vulcanization thus depends upon the inherent unsaturation of low-valent sulfur.…”