The Synthesis and Mechanism of Action of Organic Accelerators of the Sulphur Vulcanization of Rubber

Abstract

The work was undertaken to lay the foundations for the development of a new class of delayed action accelerators for use in high temperature (above 200°C.) sulphur vulcanization of rubbers. Normal vulcanization temperatures are usually between 130° and 150°C. Initially 2-thiobenzothiazole derivatives were studied as 2-mercaptobenzothiazole is an accelerator. It has been shown that 2-(cyclohex-2'-enyl) thiobenzothiazole isomerised and partially decomposed on heating, to yield 2-mercaptobenzothiazole. It was hoped that the activation energy required to initiate the thermal decomposition at high temperatures would provide greater cure delay than is normally available with conventional accelerators. The synthesis and mechanism of the thermal rearrangement and decomposition of 2-thiobenzothiazoles and benzothiazoline-2-thione derivatives were studied with emphasis being placed on synthesis of the compounds of the general type I and II. The group R was introduced so that it would facilitate thermal decomposition of the type shown on the following page. (chemical formula diagram please see Thesis) The R groups which facilitated complete thermal decomposition were mono-substituted 1,2 diphenylethanes and 1,4 bis substituted tetralins but other alkenyl and aralykl groups were investigated. The work was extended by changing the accelerating species while holding the R group constant. The accelerating species which replaced 2-thiobenzothiazole were dimethyldithiocarbamate, O-methyldithiocarbonate (methylxanthate), and 0,0 dimethl-phosphorodithioate, (see below). (chemical formula diagram please see Thesis) The above compounds were incorporated into a gum vulcanizate and the accelerator activity at 180°, 200°, and 220°C. appraised. The evaluation indicated that the 2-thiobenzothiazole and dimethyldithiocarbamate derivatives possessed accelerator activity at 200°C., while the methyl xanthate and 0,0 dimethylphosphorodithioates derivatives were inactive. The present work has demonstrated that new classes of delayed action accelerators could be developed using facile elimination processes to generate accelerating species.