The Chemistry of Vulcanization. VII. Role of Zinc Butyrate in the Reaction of Diphenylmethane, Sulfur and 2-Mercaptobenzothiazole

Abstract

Abstract In previous Parts of this series, the accelerating mechanism of thiazole type accelerators, namely, 2-mercaptobenzothiazole (MBT), 2,2′-benzothiazolyl disulfide (MBTS) and zinc salt of 2-mercaptobenzothiazole (ZMBT) in the absence of zinc oxide or zinc soap, was investigated with diphenylmethane (DPM) as a model compound of rubber hydrocarbon. The significance of DPM as a model was discussed in some of the earlier papers. Parts IV, V and VI of this series indicated that 2-mercaptobenzothiazolyl radical generated from accelerators splits the sulfur ring, and that the processes by which accelerators generate the radical differ with each other according to their types. These results were obtained in the absence of zinc oxide or zinc soap. The present study will report the role of zinc butyrate in the reaction involving DPM, sulfur and MBT. Experience in the industry indicates that zinc oxide (or zinc soap) is indispensable to the thiazole type accelerators and that the efficiency of zinc oxide or soap is more prominent in MBT than in MBTS or ZMBT. The results obtained in the previous papers also suggest that zinc oxide or soap may have an influence on the rate at which the accelerator generates 2-mercaptobenzothiazolyl radical, since it is shown in Parts IV, V and VI that the radical has an accelerating effect. Therefore, it may be considered that zinc oxide or zinc soap activates MBT more effectively than does the other thiazole type accelerators in order to produce this radical. As will be seen later in this study, interaction of MBT with zinc butyrate in the absence of sulfur produces ZMBT and butyric acid. The ZMBT will interact with sulfur and generate the 2-mercaptobenzothiazolyl radical as reported in Part VI. The zinc salt thus formed will be dispersed in a state of molecular dispersion in the reaction system, while the same compound prepared in Part VI was not dissolved in DPM even at the reaction temperatures. In this respect the former is considered more effective than the latter. In order to verify the above assumptions the reaction involving DPM, sulfur and MBT in the presence of zinc butyrate were investigated. The reaction products and mechanism were compared with those in the absence of zinc soap. Since zinc butyrate is soluble in the reaction system at the reaction temperatures, a kinetic study also was carried out and compared with that in the absence of zinc soap.