Sulfur vulcanization, accelerators for

The accelerator that has been widely used with metal oxide cures is ethylene thiurea (ETU) or 2-mercaptoimidazoline. Further extensive use of ETU in vulcanization of CR is restricted because of suspected carcinogen. The related compound, thiocarbanahde, used formerly as an accelerator for sulfur vulcanization, has been revived for CR vulcanization other substitute for ETU has been proposed [29,30]. 

TABLE 2.5 Accelerator for Sulfur Vulcanization of Rubbers Accelerator Type and Formula... 

In addition to the use of the above phenolic and amine type antioxidants, peroxide decomposers are used to harmlessly decompose the peroxides, which otherwise could decompose to give free radical propagating species, e.g., R-0 or H-0 . Examples of such peroxide decomposers, which act synergisticaUy with the phenolic or amine antioxidants, are dilauryl- 3, 3-thiodiproprionate and tris(p-nonylphenyl)phosphite. These and others are also listed in Table 4.10, with their chemical structures being listed in Table 4.11. We note that some of the peroxide decomposers are also accelerators for sulfur vulcanization. 

Because of the low level of C=C imsatiu-ation in the polsrmer compared to, eg, natural rubber (NR) and SBR, EPDM needs significant amoimts of highly active accelerators for sulfur vulcanization. Usually, combinations of benzothiazole derivatives or sulfenamides with so-called ultra-accelerators like thiuram compoimds or dithiocarbamates are used. Because of the limited soluhihty of these various accelerators in EPDM versus the large quantities needed to obtain a sufficient level of vulcanization, an EPDM compoimd often contains four or more... 

Other Accelerators. Amine isophthalate and thiazolidine thione, which are used as alternatives to thioureas for cross-linking polychloroprene (Neoprene) and other chlorine-containing polymers, are also used as accelerators. A few free amines are used as accelerators of sulfur vulcanization these have high molecular weight to minimize volatility and workplace exposure. Several amines and amine salts are used to speed up the dimercapto thiadiazole cure of chlorinated polyethylene and polyacrylates. Phosphonium salts are used as accelerators for the bisphenol cure of fluorocarbon mbbers. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Hydrogen peroxide used over a wide pH range has been employed to oxidize sulfur compounds. The scope of the oxidation using hydrogen peroxide includes coupling of thiols to disulfides355 and thiols coupled with amines,356 which are used to produce industrially important compounds. For example, sulfenamides are accelerators for rubber vulcanization (Figure 3.91).357... 

The mixed thiuram disulfides were active as fungicides and accelerators for the vulcanization of rubber with sulfur. 

The conclusions reached in the two previous sections allow the second part of the reaction sequence for sulfur vulcanization outlined in Scheme 1 to be expanded into that shown in Scheme 5 (at least as far as NR and IR are concerned). We now see that the rubber-bound polysulfidic pendent group has not one fate, but three. The two which do not immediately lead to crosslinks both give rise to a loss of crosslinking efficiency (i) the decomposition route by main chain modification and diversion of sulfur, and (ii) the desulfuration route by the removal of accelerator as inactive monosulfidic pendent groups which appear to be reasonably stable except at high temperatures but which, in any case, are not converted into crosslinks. The existence of at least one other route to such pendent groups has been deduced. 

Dithiophosphates. These compounds (13) are made by reaction of an alcohol with phosphoms pentasulfide, then neutralization of the dithiophosphoric acid with a metal oxide. Like xanthates, dithiophosphates contain no nitrogen and do not generate nitrosamines during vulcanization. Dithiophosphates find use as high temperature accelerators for the sulfur vulcanization of ethylene—propylene—diene (EPDM) terpolymers. 

Most accelerators used in the accelerated sulfur vulcanization of other high diene rubbers are not applicable to the metal oxide vulcanization of CR. An exception is the use of so-called mixed-curing system for CR, in which metal oxide and accelerated sulfur vulcanization are combined. Along with the metal oxides, TMTD, DOTG, and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

The tensile strength of NR compounds in the presence of cross-link promoters such as dichlorobenzene is increased as compared to the sulfur-accelerator and peroxide-curing systems. The retention of the maximum tensile strength at elevated temperamres is greater for radiation cured than for chemically vulcanized NR [326,327]. Also reported are a higher abrasion resistance [328] and a lower flex life in the case of radiation-cured system. Effect of phenoxy ethyl acrylate (PEA)... 

Captax (Structure 15.21) is used to the extent of 1% with hevea rubber and accounts for the major part of the over 30,000 t of accelerators used annually in the United States. Other accelerators widely used include 2-mercaptobenzothiazole sulfenamide (Santocure Structure 15.22), used for the vulcanization of SBR dithiocarbamates and thiuram disulfides. Thiuram disulfide (Structure 15.23) is a member of a group called ultra-accelerators, which allow the curing of rubber at moderate temperatures and may be used in the absence of sulfur. 

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene... 

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