Tetramethylthiuram disulfide and 2‐mercaptobenzothiazole as binary accelerators in sulfur vulcanization. I. Exchange reactions between the accelerators and sulfur in the absence of rubber

Abstract

SYNOPSISExchange reactions between tetramethylthiuram disulfide, 2-mercaptobenzothiazole, and sulfur were studied by heating powdered mixes to vulcanization temperatures at a programmed rate in a DSC. The reaction was stopped at points along the thermal curve and the mixture was analyzed by HPLC. On dissolution, even unheated samples undergo a sulfide exchange reaction leading to a mixed accelerator, while polysulfides of the thiuram and mixed accelerator form in low concentrations. On heating, higher concentrations of these polysulfides are formed, particularly in the presence of elemental sulfur. Dimethyldithiocarbamic acid, formed in the exchange, influences the product spectrum if it remains trapped in the DSC pan. Tetramethylthiourea is formed only at elevated temperatures when dimethylamine, a degradation product of the acid, is trapped in the DSC pan. A series of reactions is proposed to explain the product spectrum obtained under different conditions. 0

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. II. Vulcanization of rubbers and model compound 2,3‐dimethyl‐2‐butene

“…With TMTD and other accelerators, peaks due to accelerator polysulfides occur at higher retention times than the disulfide accelerator peak. 26,29,30,33 However, these broad peaks were not positively identified. The formation of a range of polysulfidic crosslinked products (TME-S x -TME, x ϭ 1-7, retention times 39 to 100 min) was detected in mixes heated for 2 min and longer (Fig.…”

“…23,28 The contents were frozen and tubes sealed under vacuum. Tubes were fully immersed in an oil bath at 130 or 150°C for various times and residual curatives, reaction intermediates, and products characterized by HPLC 26,28 and NMR. 29 -31 For NMR analysis crosslinked products were separated by TLC and the appropriate band lifted.…”

“…Mixes were compounded on a Brabender Plasticorder and vulcanized in a DSC at a programmed heating rate and isothermally in a press for various times as described earlier. 24,25 The reaction was stopped at various stages, residual curatives were extracted, 24,25 and analyzed by HPLC 25,26 on a C18 Reverse Phase-Bondapak column with 80/20 (v/v) : methanol/water as elution solvent. In gradient elution studies the solvent was changed linearly from 80/20 (v/v) methanol/water to 100% methanol over a period of 120 min.…”

The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

“…Morgan and McGill 25 have shown that, in the absence of sulfur, disulfidic benzothiazole pendent groups are unreactive, as are monothiols. By analogy, it would imply that thiuram and thiol pendent groups involved in reaction 5 must be polysulfidic.…”

“…24 A similar exchange between TMTD and MBT has been demonstrated. 25 In another experiment an equilibrium mixture of sulfur and TMTP was prepared by heating sulfur and TMTD for 5 min at 150°C. The mixture was placed on the lower platen of the setup depicted in Figure 1, with an IR/TMTD/sulfur film adhering to the upper platen.…”