Investigations in the Field of Rubber Vulcanization. VII. Influence of Organic Accelerators on the Kinetics of Vulcanization and the Properties of Natural-Rubber Vulcanizates

Abstract

1. Vulcanization accelerators change all parameters of the kinetic strength curve during the vulcanization of natural-rubber mixtures with low sulfur contents. 2. Calculation of the kinetic constants of the fundamental vulcanization equation proposed by Dogadkin, Karmin, and Gol'berg shows that vulcanization accelerators affect both the kinetics of the interaction of rubber with sulfur and the kinetics of the interaction of rubber with oxygen. 3. Direct experiments on the oxidation of rubber have shown that tetramethylthiuram disulfide and diphenylguanidine retard the process of addition of oxygen to rubber, while mercaptobenzothiazole accelerates this process. 4. Data on the rate of plasticization and change in viscosity of rubber solutions during oxidation indicate that tetramethylthiuram disulfide and diphenylguanidine promote the disintegration of molecular chains of rubber during the oxidative destruction of the latter. 5. The activation energy of the process of oxidation of rubber in the presence of mercaptobenzothiazole corresponds to the activation energy calculated from the fundamental vulcanization reaction for the process of oxidative destruction. This provides additional proof of the participation of oxygen in the vulcanization process. 6. It has been established with the aid of the methyl iodide reaction that accelerators increase the bridge-sulfur content of the vulcanizate, which is present in the form of monosulfides, with one sulfur atom connected to an allyl type radical. 7. With increasing temperature, the tensile strength at the vulcanization optimum increases in mixtures containing tetramethylthiuram disulfide, decreases in mixtures containing mercaptobenzothiazole, and remains unchanged in mixtures containing diphenylguanidine. The limiting strength decreases in all cases with increasing temperature. This phenomenon is explained on the basis of the proposed concepts of the character of vulcanization kinetics and of the nature of the vulcanization optimum.

Sulfur Vulcanization of Vinyl-Substituted Polysiloxanes

“…The properties of (MeVi-SiO)4 were published (70) in 1955. The physical properties of (MeViSiO) 4 used in this work are: boiling point, 222°C. at 760 mm. ; nf), 1.4325; d25, 0.9828; viscosity, 3.48 centistokes at 25°C.…”

“…The drop in mechanical strength during the vulcanization process beyond the instant of optimum vulcanization is connected with an increase in cross-link density to a point at which orientation of the molecular chains becomes difficult. Dogadkin (4) and others presented data which gave an excellent comparison of bound sulfur determinations, swelling measurements, and tensile strengths as a function of vulcanization time. Their data substantiate the utility of tensile strength measurements as a means of following vulcanization rates within the previously mentioned limits.…”