The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Thiourea and its derivatives are important secondary accelerators in this context. It is suggested that thiourea containing binary systems of rubber vulcanization proceed by a nucleophilic reaction mechanism. Amidinothioureas (ATUs), which are derivatives of thiourea, have been investigated extensively as secondary accelerators. One of the aims of this study was to get further proof with regard to the theory of the nucleophilic reaction mechanism in such binary systems. In the present study phenyl substituted ATU was used as a secondary accelerator along with mercaptobenzothizyl disulfide, tetramethyl thiuram disulfide, or cyclohexyl benzthiazyl sulfenamide in the sulfur vulcanization of natural rubber. The results showed an appreciable reduction in the cure time for the mixes containing the ATU compared to the reference mixes. These results are indicative of a nucleophilic reaction mechanism in the vulcanization reaction under review. These vulcanizates also showed comparatively better tensile properties and good retention of these properties after aging. The optimum dosages of the secondary accelerator required for these vulcanization reactions were also derived.

ABSTRACT:In the present study we report the results obtained on the use of rice bran oil (RBO), a naturally occurring nontoxic oil, and its epoxidized variety (epoxidized RBO, or ERBO) in the compounding and vulcanization of different natural rubber-chloroprene rubber (NR-CR) blends. The processability, cure characteristics, and physical properties of the blends prepared with these oils were compared with those of control mixes prepared with aromatic oil. The optimum cure time and scorch time values of the different blends prepared with these oils were found to be lower than those of the respective control blends prepared with aromatic oil. Evaluation of physical properties of the different experimental blends showed that replacement of aromatic oil with these oils did not adversely affect their physical properties. Because RBO contains a good amount of free fatty acids it was tried as a coactivator in addition to its role as a processing aid. The level of these oils required for the blend preparation was optimized in a Brabender plasticorder. Physical properties such as tensile strength, elongation at break, tear strength, swelling index, and abrasion loss, for example, were evaluated for both experimental and control mixes. Comparison of cure characteristics and physical properties of the blends prepared with aromatic oil and with these oils showed that these oils could be used in place of aromatic oil in the above blends. It is also to be noted that aromatic oil is of petroleum origin and is reported to be carcinogenic.

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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.