Effect of aerosil on the course of thiuram accelerated sulfur vulcanization

Abstract

AbstractTetramethylthiuram disulfide (TMTD)‐accelerated sulfur vulcanization of natural rubber has been investigated at temperatures from 100°C to 145°C. Continuous measurements in a Vuremo curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. The cure curves as well as their linearized forms (dependences of the logarithm of the extent of vulcanization on the cure time) clearly show that at lower cure temperatures the course of the vulcanization differs significantly from the first‐order rate law. These digressions have been removed by the addition of a highly dispersed silica gel, Aerosil, which simultaneously speeds up the course of the vulcanization up to the value corresponding to the rate of zinc dimethyldithiocarbamate (ZnDMDC)‐accelerated sulfur vulcanization. These results are in accordance with our recent theory supposing that ZnDMDC is the actual accelerator in TMTD‐accelerated sulfur systems. In the presence of Aerosil, the formation of ZnDMDC from TMTD is catalyzed via dispersed silica gel. Support for this view derives from the temperature dependences of vulcanization reactions. The activation energies of TMTD‐accelerated sulfur vulcanizations in the absence (31 kcal/mole) and in the presence of Aerosil (23.5 kcal/mole) correspond exactly to the values calculated from the rate constants of the thiuram decrease in TMTD‐accelerated vulcanization (30 kcal/mole) and from the rate constants of crosslinking in the dithiocarbamate‐accelerated sulfur vulcanization (23 kcal/mole), respectively.