Accelerators dithiocarbamate-type

The use of accelerators based on secondary amines is being reduced or eliminated. This is because secondary amines can react with nitrogen oxides to form suspected carcinogenic nitrosamines. This is especially a problem with dithiocarbamate-type accelerators. Proposed accelerators that do not give carcinogenic nitrosamine derivatives include... 

It is common practice in the mbber industry for a compounder to use combinations of several accelerators in developing a cure system. Typically these cure systems are comprised of a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary types (kickers) are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (20). 

Rubber. The mbber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural mbber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfudess and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useflil as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

Zinc dithiocarbamates have been used for many years as antioxidants/antiabrasives in motor oils and as vulcanization accelerators in rubber. The crystal structure of bis[A, A-di- -propyldithio-carbamato]zinc shows identical coordination of the two zinc atoms by five sulfur donors in a trigonal-bipyramidal environment with a zinc-zinc distance of 3.786 A.5 5 The electrochemistry of a range of dialkylthiocarbamate zinc complexes was studied at platinum and mercury electrodes. An exchange reaction was observed with mercury of the electrode.556 Different structural types have been identified by variation of the nitrogen donor in the pyridine and N,N,N, N -tetra-methylenediamine adducts of bis[7V,7V-di- .vo-propyldithiocarbamato]zinc. The pyridine shows a 1 1 complex and the TMEDA gives an unusual bridging coordination mode.557 The anionic complexes of zinc tris( V, V-dialkyldithiocarbamates) can be synthesized and have been spectroscopically characterized.558... 

General curing system for EPDM rubbers will be a thiazole (mercaptobenzothiazole or dibenzothiazole disulfide) accelerator with a thiuram and/or a dithiocarbamate. For high heat exposure condition in a process industry, sulfur donor types like tetra methyl thiuram disulfide may replace a larger part or all of the sulfur. 

Frequently, mixtures of accelerators are used. Typically, a benzothiazole type is used with smaller amounts of a dithiocarbamate (thiuram) or an amine type. An effect of using a mixture of two different types of accelerators can be that each activates the other and better-than-expected crosslinking rates can be obtained. Mixing accelerators of the same type gives intermediate or average results. 

Secondary Accelerators. Even bigger changes have come to the secondary curative segment. Three classes of chemicals served historically as secondary accelerators, useful in low levels to speed up sulfenamide cures the thiu-ram (2) and dithiocarbamate (3) types, and the guanidine type (4). Specific examples for these classes are listed in Tables 5 and 6. 

The aliphatic amines are valuable intermediates for chemical synthesis applications, Reaction of ethylamine with cyanuric chloride yields triazine-type herbicides, Diethylamine is used in the preparation of vulcanization accelerators where reaction with carbon disulfide yields a dithiocarbamate. Triethyl-amine is used as an acid acceptor in chemical syntheses and as a salt former in various purification processes. The amine is used as a corrosion inhibitor in aqueous systems, as a catalyst in polyurethane applications, in textile and photographic applications, and in anodic electrocoating. 

The third mode of decomposition is only important in A-type monosulfides (and D-type from the diene model 5) and then only in the presence of catalysts. This is a simple elimination reaction, but its course is often obscured by secondary reactions of the primary products (equation Many accelerators and accelerator transformation products are catalysts for this decomposition. They include zinc benzothiazole-2-thiolate (ZMBT) and zinc dithiocarbamates and their amine complexes, as well as CBS, MBT and MBTS (which would normally not be present in quantity when monosulfide crosslinks are formed during the course of vulcanization). 

This study investigates how to use accelerators zinc mercaptobenzothiazole, diphenylguanidine, and their combination as a substitute for commonly used zinc diethyl dithiocarbamate in the vulcanisation of NR latex, as dithiocarbamates, being secondary amines, produce harmful N-nitrosamines. Network structures of vulcanisates obtained by these accelerators were characterised through total crosslink density and the distribution of crosslink types, and were related with physical and mechanical properties. 19 refs. 

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