Dithiocarbamates cure system

The dithiocarbamates cure system has a long scorch time and cure time, which may not be practical for high-speed production. The heat aging properties obtained are good, but the compression set is poor, making this cure combination a poor candidate for seal applications, but acceptable for extmded, steam-cured parts. 

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). 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

Cure system species, accelerators and their reaction products This class of additive can present problems as they are often thermally labile, reactive and, in some cases, have a degree of ionic character (e.g. zinc dithiocarbamate salts). In these cases LC-MS is a more appropriate technique than GC-MS. It is also easier to use LC-MS with a number of the approved food simulants as they can be injected directly into the instrument, being compatible with the mobile phase. In some cases the reaction products (e.g. aniline from diphenyl guanidine, and benzothiazole from thiazole and sulphonamide accelerators) are stable and so GC and GC-MS can be used. Peroxides are popular curatives for food use rubbers and the stable, breakdown products of these can be easily detected by GC-MS. 

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. 

Methods to prevent this diffusion gradient include preparing a masterbatch of dithiocarbamate accelerators in the polymer (HR, EPDM) and grafting accelerators onto the polymer (EPDM). One advantage of a cure system mismatch is improved ozone crack resistance. In any case, cure system location control is critical. Figure 4.10 shows the diffusion of sulfur from natural rubber to SBR. Two rubber samples are placed in contact with each other and heated. The sulfur concentration is then measured at increasing distances from the interface. In this case, considerable transfer occurs and significant concentrations build up in the SBR phase. 

Sulfur curing systems for butyl include elemental sulfur, thiuram or dithiocarbamate accelerators and thiazole or thiazyldisulfide activators. Zinc oxide or other metallic oxides are necessary to attain satisfactory cure. The relatively minor differences in unsaturation between grades of butyl are significant with sulfur cures, the rate of cure and number of attainable crosslinks increasing with increasing polymer unsaturation, while chemical and ozone resistance decrease. The need for elevated temperature to attain crosslinking limits utility of sulfur cures in adhesives and sealants applications. 

While the sulfenamides are the most common primary accelerators, they are frequently used with secondary accelerators (a.k.a. kickers ) which act as activators for the cure system. The thiurams and dithiocarbamates, which are short on scorch safety when used as primary accelerators, are used at relatively low levels to help activate the cure. 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

As with the peroxide-cured rubbers oxidation is autocatalytic, there is little cross-linking during ageing, little scission occurs in vacuo and the system responds well to conventional antioxidants as well as to dithiocarbamates. 

There is little that needs to be said here about the accelerated sulphur systems except to point out that in order to provide strong fast curing combinations powerful accelerators such as the dithio-carbamates or synergistic dithiocarbamate/benzothiazole derivatives are necessary. 

Combinations of thiuram and thiazole accelerators are commonly used in sulphur cures, especially where cost is a primary consideration and heat resistance is not. Dithiocarbamates give fast cures and good ageing properties, but as they tend to be scorchy a thiazole (MBT or MBTS) is normally used with them to provide processing safety. Sulphur-donor systems based on thiuram/dithiocarbamate combinations and containing very little or no elemental sulphur provide superior ageing properties and low compression set. 

System 2 (Triple 8) S2.0 MBT 1.5 Tellurium diethyl dithiocarbamate (TDEC) 0.8 Dipentamethyl thiuram tetrasulfide (DPTT) 0.8 TMTD 0.8 Excellent physical properties and fast cure Scorchy and expensive... 

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