Effect of carboxylic acids on 2-bisbenzothiazole-2,2?-disulfide- and tetramethylthiuram disulfide-accelerated sulfur vulcanization. III. Reaction between accelerators and zinc salts of the acids

Abstract

Stearic and benzoic acids were heated with 2-bisbenzothiazole-2,2'-disul-fide (MBTS)/sulfur/ZnO and tetramethylthiuram disulfide (TMTD)/sulfur/ZnO isothermally at 150°C and in a DSC at 2.5°C/min, in the absence of rubber. The acids readily reacted with ZnO, and at vulcanization temperatures, little or no free acid remains. The zinc salts are less reactive toward the accelerators and accelerator polysulfides than are the acids, although with benzoic acid/ZnO, MBTS formulations produced small amounts of the accelerator-acid complex 2-benzoylthiobenzothiazole and 2-mercaptobenzothiazole, while with TMTD formulations, limited 2-benzoyldimethyldithiocarbamate formation was detected. These small amounts of the reaction can be attributed to the incomplete consumption of benzoic acid, zinc benzoate restricting the access of benzoic acid to ZnO particles. Although conversion of MBTS and TMTD to accelerator-acid complexes is limited, the zinc salts do promote the slow decomposition of MBTS and TMTD above 150°C. It can be concluded that in the presence of ZnO carboxylic acids should not detrimentally effect the vulcanization reaction.