The Vulcanization of Butadiene-Styrene Rubber in the Presence of Sulfenamide Accelerators

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Abstract
1. It is established that N,N-diethyl-2-benzothiazolesulfenamide has a structure-forming effect in stocks of butadiene-styrene rubber when these are heated at vulcanization temperatures, even in the absence of the usual vulcanizing agent, sulfur. These stocks, after extraction with acetone, still contain chemically combined sulfur and nitrogen, and the contents of these increase with a longer heating period. 2. The change of the modulus of the above stocks is a linear function of the amount of N,N-diethyl-2-benzothiazolesulfenamide which has reacted. 3. The structure-forming action of N,N-diethyl-2-benzothiazolesulfenamide can be regarded as a polymerization reaction, brought about by the free radicals formed in the thermal decomposition of this compound. 4. It is possible to vulcanize ordinary sulfur-containing butadiene-styrene rubber stocks in the presence of sulfenamide accelerators without the use of the accelerator activator, zinc oxide. 5. The kinetic curves of combination of sulfur and of the change of the equilibrium modulus, for ordinary sulfur stocks containing the sulfenamide accelerators (N,N-diethyl-2-benzothiazolesulfenamide and N-cyclohexyl-2-benzothiazolesulfenamide), is S-shaped, and shows an initial “induction” period in the vulcanization. This latter is especially distinct in stocks containing N,N-diethyl-2-benzothiazolesulfenamide. 6. Multilayer rubber articles made from plied stocks containing sulfenamide accelerators have stronger adhesive bonds. There are two reasons for this increased bond strength : (1) the retardation of the vulcanization kinetics in the initial stage of the vulcanization process, which is characteristic of the accelerators in question, and which has the effect of prolonging the length of time that the plied stocks remain in a semiviscous condition; and (2) the formation of the stronger —C—C— bonds as a result of polymerization reactions, which are stimulated by sulfenamide accelerators. 7. Vulcanizates prepared with sulfenamide accelerators are different from those with a combination of benzothiazolyl disulfide and diphenylguanidine in that the former have a lower heat build-up and a somewhat greater dynamic deformation resistance.

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