Studies on sulphur vulcanisation of natural rubber using amidino thiourea

Abstract

Binary accelerator systems are widely used in vulcanisation of rubbers, providing more effective vulcanisation than using single accelerator systems. It has been reported previously that when thiourea is used in binary systems, it favours the cleavage of S–S, S–C, or S–N bonds in primary accelerators such as tetramethylthiuram disulphide (TMTD), mercaptobenzothiazyl disulphide (MBTS), and cyclohexylbenzthiazylsulphenamide (CBS), indicating a nucleophilic reaction mechanism in these vulcanisation reactions. However, there is no conclusive proof for this mechanism. In the present work the results obtained when a derivative of thiourea, namely amidino thiourea (ATU), is used as a secondary accelerator together with TMTD, MBTS, or CBS in the sulphur vulcanisation of natural rubber (NR) are discussed. There is appreciable reduction in the optimum cure time of the different experimental mixes compared with the reference formulations. This study also supports a nucleophilic reaction mechanism in such vulcanisation. Binary systems with varying concentrations of ATU with different primary accelerators have been examined and the optimum quantity of ATU required has been derived. The cure characteristics and tensile and other physical properties of the different mixes are studied in detail. In order to correlate these physical properties with the chemical crosslinks formed, chemical characterisation of the different vulcanisates has also been carried out.