Amidino Thiourea as a Secondary Accelerator in the Sulphur Vulcanization of Natural Rubber Containing Fillers

Abstract

As a part of our study on the mechanism of vulcanization we investigated a number of thiourea derivatives as secondary accelerators in the sulphur vulcanization of natural rubber. In the present study, the accelerator activity of a new binary accelerator system containing unsubstituted amidinothiourea viz. aminoiminomethyl thiourea (ATU) along with different primary accelerators has been examined. Different fillers like carbon black, silica and china clay are incorporated in these mixes under review. Various compositions of ATU are examined for the above vulcanization study. The optimum concentration of this secondary accelerator in different systems has also been derived. The mixes are examined for different cure characteristics and the vulcanizates are evaluated for their physical properties. In order to study the effect of the secondary accelerator on the temperature of vulcanization, these mixes are cured and evaluated at two different temperatures, viz. 120o and at 150oC. The correlation between physical properties and chemical cross links in the different vulcanizates is also examined by determining the cross link density values of the vulcanizates. These binary systems are found to be effective in the filled systems studied. The optimum cure time has been found to be reduced considerably by this secondary accelerator. At the lower temperature i.e., at 120°C, the reduction in cure time is found to be more significant with increase in the concentration of ATU especially when compared with systems containing thiourea. ATU being more nucleophilic than thiourea this points to a nucleophilic reaction mechanism in these vulcanization reactions. The tensile properties of the vulcanizates from these systems are better/comparable to those of the reference mixes. There is favourable increase in many of the physical properties like hardness, compression set, abrasion loss, tear strength etc. The crosslink density values evaluated also support the above conclusions. The non-toxic nature of ATU, which is used in pharmaceutical applications, is an added advantage in this context.