Thiuram type accelerator

Cure systems for NBR are somewhat analogous to those of SBR except magnesium carbonate treated sulfur is usually used to aid its dispersion into the polymer [20]. Common accelerator systems include thiazoles, thiouam, thiazole/thiuram, or sulfenamide/thiuram types. Examples of these systems are shown in Table 14.16. 

Compared with common thiuram-type curing accelerators such as TMTD, TETD and TBTD, tetrakis(2-ethylhexyl)thiuramdisulphide (TOTD) shows superior curing characteristics (good scorch safety and faster curing rate) as a secondary curing accelerator for NR sulphur vulcanisation using CBS, and gives the NR vulcanisate with excellent heat resistance because of the increase of thermal stable mono and disulphidic crosslinks. 4 refs. 

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 Chemicals. Sodium nitrite is an important raw material in the manufacture of mbber processing chemicals. Accelerators, retarders, antioxidants (qv), and antiozonants (qv) are the types of compounds made using sodium nitrite. Accelerators, eg, thiuram [137-26-8J, greatly increase the rate of vulcaniza tion and lead to marked improvement in mbber quaUty. Retarders, on the other hand (eg, /V-nitrosodiphenylamine [156-10-5]) delay the onset of vulcanization but do not inhibit the subsequent process rate. Antioxidants and antiozonants, sometimes referred to as antidegradants, serve to slow the rate of oxidation by acting as chain stoppers, transfer agents, and peroxide decomposers. A commonly used antioxidant is A/,AT-disubstituted Nphenylenediamine which can employ sodium nitrite in its manufacture (see Rubber chemicals). 

The W types require additional acceleration and ethylene thiourea (ETU), gives the best balance of all properties. However, the use of this accelerator is increasingly being restricted due to fears of its effects on pregnant women, and more recently men. DETU, thiurams and guanidines can also be used. Sulphur is sometimes used to increase the degree of cure in the W types, but this detracts from the ageing performance of the vulcanisate. 

It is accepted that many widely used latex vulcanisation accelerators - dithiocaibamates, thiurams and thiazoles - are capable of producing Type IV allergic response in certain individuals within the population and may also possess increasingly unacceptable eco-toxic and acute toxicity profiles. Thiurams and dithiocaibamates (derived from secondary amines) can also produce potentially harmful N-nitrosamines. Four safer accelerators developed and commercialised by Robinson Brothers are described. They are designed to reduce or eliminate the impact of the above problems using sustainable technology. At the same time these accelerators produce equivalent technological performances to those conventionally used. 10 refs. 

Type IV reactions are due to chemicals added during manufacture of NRL, which include accelerators, antioxidants, antiozo-nants, emulsifiers, stabilizers, extenders, colorants, retarders, stiffeners, and biocides. Accelerators primarily control the rate, uniformity, and completeness of vulcanization. The most common accelerators include thiurams, carbamates, and mercaptobenzothiazoles. These chemicals are covered in detail in their specific monographs in this volume. 

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. 

NRL is an irritant because of the protein allergens and can cause allergic skin reactions associated with type I (immediate effect) hypersensitivities of hand dermotoses and immunologic contact urticaria . It is also possible to have type IV (delayed effect) hypersensitivities of NRL, due to the chemicals added during its processing (such as accelerators of the thiuram/carbamate/mercaptobenzothiazole types needed for vulcanisation, a number of different antioxidants/antiozonants, emulsifiers, extenders, colorants, retarders, stiffeners, biocides). Either of these types can be serious and even life threatening in some cases [75]. 

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. 

The type of elastomer will influence the rate of cure and the resultant crosslink network. Natural rubber tends to cure faster than SBR. Cure systems containing thiuram accelerators such as tetramethylthiuram disulfide will show short induction times and fast cure rates compared with a system containing diphenylguanidine. 

Thiurams and carbamates have most commonly been responsible for the type-IV reactions due to rubber gloves. Mercaptobenzothiazole was the first benzothiazole accelerator used in gloves, but other derivatives can also be used. The reports on sensitization to thiourea compounds from rubber gloves are few and, for the most part, involve gloves made of neoprene rubber which may also contain diphenyl-guanidine (Estlander et al. 1994a, 1995 Kanerva et al. 

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