ABSTRACT:The effect of benzoic and stearic acids on 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)-and tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of polyisoprene compounds, containing ZnO, was studied at 150°C. In the presence of ZnO, the acids are rapidly converted to their zinc salts that, unlike the acids, do not attack accelerator polysulfides and, instead, promote vulcanization. The addition of accelerator polysulfides to the chain is not affected by zinc salts, but the rate of crosslinking of pendent groups is facilitated. In MBTS vulcanization, the promotion of crosslinking reactions reduces the number of pendent groups involved in cyclization reactions and higher crosslink densities are attained. In TMTD compounds, crosslink densities are increased on addition of ZnO and of ZnO/carboxylic acids. A mechanism is suggested to account for the promotion of crosslinking by the zinc salts. Bis(2-mercaptobenzothiazolo)zinc(II) and bis(dimethyldithiocarbamato)zinc(II) can act similarly to zinc benzoate or stearate to promote the crosslinking of pendent groups, but are less efficient. With benzoic acid, a limited attack by zinc benzoate on TMTP leads to a slightly lower increase in crosslink density than is achieved with the addition of ZnO alone.

Benzoic and stearic acids are shown to react with 2-bisbenzothiazole-2,2Ј-disulfide (MBTS) and its polysulfides (MBTP) at vulcanization temperatures to form acid-accelerator complexes 2-benzoylthiobenzothiazole (BzM) and stearicthiobenzothiazole (StM), respectively. At higher temperatures MBTS, MBTP, BzM, and StM are decomposed by the acids. Analogous reactions are observed with tetramethylthiuram disulfide (TMTD) and its polysulfides (TMTP). Lesser amounts of acid-accelerator complexes 2-benzoyldimethyldithiocambamate and stearicdimethyldithiocarbamate are formed, but TMTP are more susceptable to decomposition at higher temperatures. The destruction of MBTP and TMTP by carboxylic acids will reduce the concentration of accelerator polysulfides available in compounds, and should impact negatively on vulcanization.

Polyisoprene was vulcanized by 2-bisbenzothiazole-2,2Ј-disulfide (MBTS)/ sulfur and tetramethylthiuram disulfide (TMTD)/sulfur in the absence and presence of benzoic and stearic acids. It was found that the crosslink density of MBTS vulcanizates is halved by the addition of carboxylic acids and this can be explained in terms of the attack of the acids on the accelerator polysulfides. TMTD polysulfides are more reactive toward polyisoprene than are MBTS polysulfides, and their addition to the polymer chain occurs before significant attack by the carboxylic acids can reduce the polysulfide concentration. Consequently, the acids have little effect on the crosslink density of TMTD vulcanizates.

Stearic and benzoic acids were heated with 2-bisbenzothiazole-2,2Ј-disulfide (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 acceleratoracid 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.

Stearic and benzoic acids were heated with 2-bisbenzothiazole-2,2Ј-disulfide (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 acceleratoracid 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.