Sulfenamide accelerator

Sulfenamide accelerators generally requite less fatty acid because they release an amine during the vulcanization process which acts to solubilize the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

Table 4 Delayed Action Benzothiazole-2-sulfenamide Accelerators (37) ...

In the case of sulfenamide accelerators, the inhibitor appears to function by reacting with 2-mercaptobenzothiazole (77MI11506) so suppressing the catalytic pathway (equation 11). This is illustrated by the reaction between 2-mercaptobenzothiazole and IV-cyclo-hexylthiophthalimide (39), one of the most effective PVIs currently in use (equation 12) (73MI11500). It is likely that the reaction between the PVI and amines (equation 13) also makes a contribution to the mechanism of inhibition (77MI11506). 

Besides sulfur, during the vulcanization process other chemical compounds are commonly added. One group of such compounds consists of vulcanization accelerators. Substances such as diphenylguanidine, mercaptobenzothiazole, tetramethythiuram disulfide, N-oxydiethylene-2-benzothiazolylsulfenamide, N-oxydiethylenethiocarbamoyl-N -oxidiethylenesulfenamide, cyclohexylbenzothiazolyl-sulfenamide, etc. are utilized for this purpose. The structures of two sulfenamide accelerators are shown below ... 

Manik, S.P. Banerjee, S. Sulfenamide accelerated sulfur vulcanization of natural rubber in presence and absence of dicumyl peroxide. Rubber Chem. Technol. 1970, 40, 1311. 

UK] Thiocarbamyl sulfenamide accelerator for SBR, EPDM, other rubbers. 

Bhatnagar S.K. and S. Baneijee. 1968. Kinetics of accelerated vulcanization. El. V-cyclohexyl-benzothiazole-2-sulfenamide accelerated sulphur vulcanization of rubbers. J. Inst. Rubber Ind. 2 177-193. 

Thus, one theory for delayed action is the quenching of free radical crosslink precursors by monomeric polysulfides. It has been found that, if bisalkylpolysullides are mixed with uncured rubber stocks, more delay results. It is also been shown that the early reaction products formed by the interaction between accelerator and sulfur (Ac-S -Ac) are inhibitors of crosslink formation. The very substances that give rise to the formation of the crosslink precursor (rubber-Sjc-Ac) inhibit the formation of the crosslinks. We note that other mechanisms for delayed action have been proposed. In the case of acceleration by benzothiazolesulfenamides, the accelerator is depleted in an autocatalytic fashion with the formation of 2-mercaptobenzothiazole (MET). The rate of this depletion is about proportional to the amount of MET present. There is strong evidence, which indicates that the following reactions occur in sulfenamide-accelerated systems ... 

If MET could be taken out of the system as fast as it forms, substantial increases in processing safety would result. Such is the case when the premature vulcanization inhibitor, N-(cyclohexylthio)phthalimide (CTP), is present. This compound (Coran and Kerwood, 1970) and others Uke it react rapidly with MET to form 2-(alkyldithio)benzothiazoles, R-S-S-ET, which are active accelerators but which do not interact rapidly with the sulfenamide accelerator ... 

Sulfenamide accelerators represent the largest class of accelerators consumed on a global basis ... 

N-Oxydiethylene benzothiazole-2-sulfenamide accelerator, dicyandiamide 2-Phenyl imidazole accelerator, dipped goods Dimethyl diphenyl thiuram disulfide accelerator, dyeing... 

Butylene carbonate Ethylene carbonate Propylene carbonate accelerator, elastomers Sodium di-n-butyl dithiocarbamate accelerator, EPDM Thiocarbamyl sulfenamide accelerator, EPDM cures Zinc dibutyl dithiophosphorate accelerator, epoxies... 

For semi-efficient vulcanization systems, intermediate sulfur level of 1-2 phr and 2.5-1 phr of accelerator are often used. The vulcanizates have physical properties intermediate between those of conventional vulcanization and efficient vul-canizationvulcanizates. hi fact, they give some improvements in reversion, ageing resistance and compression set compared with conventional vulcanizationvulca-nizates, but resistance to fatigue and low temperature crystallization is impaired. However, they have higher scorch safely, particularly when sulfenamide accelerators are used in the system. 

The choice of accelerator also has an effect on the quality of adhesion between cord and rubber. The accelerator should not form a stable copper complex which dissolves in the rubber. This would be quite corrosive to the brass plating. In this respect, benzothiazoles and their sulfenamides are much better than dithiocarbamates. DCBS is a particularly good sulfenamide accelerator for rubber-to-brass adhesion. 

Other mechanisms for delayed action have been proposed. In the case of acceleration by benzothiazolesulfenamides, the accelerator is depleted in an autocatal)4ic fashion with the formation of 2-mercaptobenzothiazole (MET). The rate of this depletion is about proportional to the amount of MET present. There is strong evidence that indicates that reactions occur in sulfenamide-accelerated systems as shown in Scheme 5. 

TBBS N-Terf-butyl-2-benzothiazole sulfenamide, accelerator... 

Similarly application of this methodology to a rubber V-shaped belt showed it contained paraffin wax, a light unsaturated oil (wax, antiozonant, disphenylamine/acetone resin antioxidant, fatty acids, rosin acids and N- -butyl 2-benzothiazole (sulfenamide accelerator). 

Rudewicz and Munson [45] used this technique for the direct determination of additives in PP. The technique has also been used to determine oligomers in polyacrylates, PEG, siloxanes and polycarbonates [87], polyglycols [88] and adhesion promoters, primers and additives in the surface of PET film [89], volatile antioxidants in styrene-butadiene rubbers [34, 50], mercaptobenzothiazole sulfenamide accelerator in rubber vulcanisates [90] and divinyl benzene in styrene-divinyl benzene copolymer [91]. 

The nitrosoallq lphenylenediamines, exhibiting a stabilizing action in diene copolymers, simultaneously improve the technology of reprocessing of rubber mixtures in the presence of sulfenamide accelerators. 

Sulfenamide accelerators generally require lower levels of fatty acid because they release an amine during the vulcanization process which acts to solubihze the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. A study of the effect of stearic acid and zinc oxide on a sulfenamide-accelerated, sulfur-cured natural rubber compoimd showed the need for both zinc and fatty acid activators is presented in Figure 11 (52). 

Altenau and co-workers [26,27] used MS to identify quantitatively volatile antioxidants in 0.02-0.03 inch thick samples of synthetic styrene-butadiene rubbers and rubber-type vulcanisates. They extracted the polymer with acetone in a Soxhlet apparatus, removed excess solvent, and dissolved the residue in benzene. Substances identified and determined by this procedure include N-phenyl-P-napthylamine, 6-dodecyl-2,2,4-trimethyl-l,2-dihydroquinolines, trisnonylphenylphosphate, isobutylene-bisphenol, 2-mercaptobenzothiazole sulfenamide (accelerator), N-cyclohexyl-2-benzothiazole... 

Manik, S. R, Baneijee S. (1970). Sulfenamide Accelerated Sulfur Vulcanization of Natural Rubber in Presence and Absence of Dicumyl Peroxide. Rubber Chem. Technol, 43(6), 1311-1326. 

For halobutyl rubbers, zinc oxide is used as the vulcanizing agent with low levels of sulfur stearic acid and sulfenamide accelerators. Bromobutyl rubber shows higher cure reactivity than chlorobutyl rubber. Both CIIR and BUR will cure with zinc oxide, but only BUR will cure with sulfur alone, no zinc oxide or accelerator being necessary. Bromobutyl rubber can be cured with 0.5 phr of sulfur and 1.3 phr of dibenzothiazyl disulfide accelerator, 3 phr of zinc oxide and 1 phr of stearic acid. Levels of sulfur as low as 0.5 phr will give a rapid and reasonable degree of cure. Zinc diethyl dithiocarbomate (ZDC) accelerator can be used in small quantities (0.25-0.75 phr) with zinc oxide for heat resistant products, and to improve compression set. 

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