Secondary acceleration

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

Fig. 6. Comparison of secondary accelerators where represents Mooney scorch at 121°C and U, optimum (t90) at 153°C A, the primary accelerator is 0.75 phr MBTS and the secondary accelerator is at 0.25 phr B, 0.45 phr TBBS is the primary accelerator and there is 0.2 phr secondary accelerator.

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

Table 14.2 provides comparisons of the different classes of accelerators based on their rates of vulcanization. The secondary accelerators are seldom used alone, but are generally found in combination with primary accelerators to gain faster cures. 

Comparison of Secondary Accelerators in Styrene-Butadiene Rubber (SBR)... 

A fast accelerator for SBR compounds. It is used as either a primary or as a secondary accelerator along with a thiazole type. 

Sulfenamides, made from 170, by reactions involving primary and secondary amines under oxidizing conditions, include A-cyclohexyl-2-benzothiazole sulfenamide (173) and N-oxydiethylene-2-bcnzolhiazolc sulfenamide (2-(morpholinothio)benzothiazole) (174). Sulfenamides release 2-mercapto accelerators and amine during vulcanization. The amine is a secondary accelerator that brings about fast vulcanization after a slow start. Applications include in tyres and conveyor belts. They are not applicable to hot-air vulcanization processes95. 

Most accelerators are organic substances that contain either nitrogen or sulfur, or both. They can be classed as either primary or secondary accelerators and are often used in combinations, depending on the desired final outcome. 

Used as a secondary accelerator with antioxidant, antiozonant, and stabilizing function in synthetic rubber and high polymer materials in the plastics and rubber industries. Mainly used in styrene-butadiene rubber (SBR), chloroprene rubber (CR), epichlorohydrin, and chlorosulfonated polyethylene rubber. Promotes heat-resistance of chlorosulfonated polyethylene rubber, EPDM and CSM and sunshine resistance of CR. 

Used as a secondary accelerator with thiazoles and sulfenamides in NR and SBR compounds. Exhibits better storage stability compared to thiuram and dithiocarbamates but is not as active. Perkacit DPG can be used in latex as a foam stabilizer in the silico-fluoride foam process. 

Use Vulcanization agent without added sulfur, or as a primary or secondary accelerator with sulfur. 

Methasan . [Harwick] Zincdimethyldi-thiocarbamate secondary accelerator for thiazoles arid sulfenamides fast curing accelerator for rubbers and latexes. 

Accelerators are products that increase both the rate of sulfur crosslinking in a rubber compound and crosslink density. Secondary accelerators, when added to primary accelerators, increase the rate of vulcanization and degree of crosslinking, with the terms primary and secondary being essentially arbitrary. A feature of such binary acceleration systems is the phenomenon of synergism. Where a combination of accelerators is synergistic, its effect is always more powerful than the added effects of the individual components. 

Hazardous Decomp. Prods. Heated to decomp., emits very toxic fumes of NOx and SOx Uses Primary vulcanization accelerator, thiazole secondary accelerator for rubber molded and extruded goods accelerator for food-contact rubber articles for repeated use Regulatory FDA 21CFR 177.2600 Manuf./Distrib. Akzo Nobel http //www. akzonobei. com Trade Name Synonyms Akrochem Accelerator Cu.D.D. [Akrochem http //www.akrochem.com], Akrochem Accelerator Cu.D.D.-PM [Akrochem http //www.akrochem.com]-, Cumate t[R-T. Vanderbilt http //www.rtvanderbiit.com]. Methyl Cumate [R.T. Vanderbilt http //www.rtvanderbiitcom], Methyl Cumate Rodform t[R.T. Vanderbilt http //www. rtvanderbiit. com]... 

CH3)2NCS2—]2. A white, crystalline powder used as a fungicide, bacteriostat, and rodent repellent in vinyl compounds, and as a secondary accelerator in rubber curing. 

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. 

Table 7. Comparison of Cure Properties Relative to TMTD of Non-Nitrosamine Secondary Accelerator Alternatives ...

The rubber chemist has reacted vigorously to the challenge of eliminating volatile nitrosamine-generating chemicals in the workplace, even though many had been fixtures of the sulfiir vulcanization process. Non-nitrosamine primary and secondary accelerators and sulfur donors are now available and used extensively. 

It is common practice in the rubber industry for a compoimder to use combinations of several accelerators in developing a cure system. Typically, these cure systems comprise a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary accelerators are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (32). Typically, secondary accelerators are more basic than the primary or major accelerator. 

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