Sulphenamides oxidation

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. 

A TG-DTA study of the thermochemical processes occurring at vulcanisation temperatures with N-oxydiethylene-2-benzthiazyl sulphenamide and N-cyclohexyl-2-benzthiazyl sulphenamide and their mixtures with sulphur showed the formation of high molecular weight polysulphides [73]. The influence of metallic oxides (Fe203, Sn02) on hot air ageing of one-pack room temperature vulcanised fluorosilicone rubber has been studied by means of TG-DTA [74, 75]. TG-DTA and TG were both applied to study the thermal characteristics of room temperature vulcanised silicone rubber [76]. 

Accelerating agents which are usually metal oxides (MgO, PbO) or organic substances such as thiazoles, mercaptobenzthiazoles (MET) or sulphenamides. Activators are required to make the organic accelerators function effectively. These can typically be zinc oxide and stearic acid. Stearic acid also acts as a lubricant, softener and plasticiser. 

A good example of the competitiveness between chlorine and peroxygen oxidants is the preparation of rubber accelerators such as sulphenamides. Sulphenamides are prepared by the oxidative coupling of a mercaptan with an amine (Fig. 1). 

Sulphenamides can be prepared using a wide variety of oxidising agents. However, commercially, sodium hypochlorite or chlorine are most often used. With the increasing environmental pressure on chlorine oxidants, the industry is looking for alternative clean oxidants. 

H202 circumvents all these problems and in addition gives better quality products (better sulphenamide stability). The main disadvantage at present when compared to chlorine oxidants is a lower yield (typically 77% for NCBS). 

The formation of dialkyl phosphoramidates from trialkyl phosphites and benzothiazol-2 1 sulphenamides has been discussed. (Diaryloxyphosphlnyl)phosphoramidic acids have been prepared by a short conventional sequence.A convenient one-pot synthesis of 1,3-dihydro-l,3,2-diazaphosphole 2-oxides (90)... 

Direct oxidation of sulphenamides or sulphinamides to the sulphonamide have been performed with a variety of oxidants439-443. For example, potassium permanganate oxidation of 2,4-dimethoxy-6-pyrimidine sulphenamide gave the sulphonamide in 64% yield, as shown in equation 108444. In another example, meta-chloroperbenzoic acid was used to oxidize sulfenamides to the sulphonamide442. However, Chiang and collaborators have reported that they were unable to isolate any sulphonamide product after one such attempted oxidation445. 

It may be prepared by the reaetion of 6-ethoxy-benzothiazole with sodium hypochlorite in the presence of sodium hydroxide and ammonia to yield the eorresponding sulphenamide, which upon oxidation with potassium permanganate in acetone forms the official compound. 

Oxidation of diaUcoxy phosphorothioate disulphides with H2O2 produces either the dialkoxy phosphate or the sulphoxide, depending upon conditions employed (9.513). With iodine and caustic potash the products are dialkyl phosphates (RO)2P02 K+, while ammonia gives sulphenamides (9.514). 

Oxidation is carried out with 30% H2O2 at pH 10-10.5, 50°C, in the presence of excess amine to minimise disulphide formation, the sulphena-mide separating as a precipitate or melt [239]. Higher pH tends to overoxidise the thiol to sulphinate etc. Hypochlorite can also be used for this oxidation, the factors affecting oxidant choice being described in section 9.5.1.8.2. Chief commercial products include iV-cyclohexyl-, AT-r-butyl- and A/ -morpholino-2-thiobenzothiazole sulphenamides, and total production is of the same order as the disulphides. 

Accelerators guanidines, thiazoles. sulphenamides, dithiocarbamates, thiuram sulphides, xanthates, aldehydeamines retarders phthalic anhydride, N-nitroso diphenylamine activators zinc oxide/stearic acid... 

Sulphinamides.—A simple synthesis, based on the oxidation of sulphenamides using A-chlorosuccinimide and aqueous KHCO3, involves 5-chlorosulphonium ion intermediates. Oxidation of A-acetyl-sulphenamides with m-chloroperoxybenzoic acid gives the corresponding alkyl- or aryl-sulphinyl-acetamides. ... 

ENR possesses fewer double bonds than NR and therefore would be expected to be more resistant to oxidation. However, the oven air ageing of a conventionally cured ENR vulcanizate (S, 2-5 sul-phenamide accelerator, 0 5 phr) was found to be poor. A rapid hardening occurred on air ageing. This oxidative hardening is not an intrinsic property of ENR since when peroxide and efficient vulcanizing systems (S, 0 3 sulphenamide, 2 4 TMTD, 1 6 phr) were used their ageing characteristics were observed to be similar to those of the corresponding NR vulcanizates (Fig. 16). 

Oxidation of a sulphenamide to a sulphinamide has been described in two recent papers ArSN=CR2 gives ArS(0)N=CR2 with m-chloroperoxy-benzoic acid, and CeF6SNH2 gives CgF6S(0)NH2 with active Mn02 at room temperature. In the latter reaction, oxidation to the sulphonamide takes place at 70... 

A general synthesis of 2-substituted l,2-benzisothiazolin-3-ones from 2-mer-captobenzoic acid involves esterification, halogenation to the sulphenyl halide, conversion into the sulphenamide, and cyclization with a strong base. The synthesis of 3-oxo-3/f-l,2-benzisothiazole 1-oxides by the action of hydrazoic acid on 2-sulphinylbenzoic acids has been extended (see Vol. 2, p. 579). 2-SuI-phinylbenzamides (48) react with hydrazoic acid in polyphosphoric acid to give 3-imino-3//-l,2-benzisothiazole 1-oxides (49) together with the related 5 functional derivatives of (49), and an unusual alkaline ring-scission to (50), were also described. ... 

Sulphur systems. Conventional cure systems for most current, practical, tyre-related and mechanical goods formulations consist of zinc oxide, plus sulphur or sulphur-donors, accelerated with sulphenamides or benzothiazoles. These types of accelerator function initially as retarders but ultimately produce very high states of cure. MBT (mercaptobenzothiazole) is an exception in that it will scorch bromobutyl stocks, but it can be used... 

Journal of Applied Polymer Science 61, No.9, 29th Aug.1996, p.1515-23 SULPHUR VULCANISATION OF POLYISOPRENE ACCELERATED BY BENZOTHIAZOLE DERIVATIVES. IV. REACTION OF POLYISOPRENE WITH N-CYCLOHEXYLBENZOTHIAZOLE SULPHENAMIDE(CBS), SULPHUR AND ZINC OXIDE... 

The disodium salt of hexamethylene bisthiosulphate (HTS) was evaluated as an accelerator in the zinc oxide mediated vulcanisation of tyre sidewall compounds containing a brominated isobutylene-p-methylstyrene copolymer. Rheometer profiles of blends with polybutadiene cured with sulphenamide, sulphur, zinc oxide and HTS showed a characteristic delayed action fast cure kinetic profile, while the same blends cured without HTS exhibited undesirable cure characteristics, with long early cure and fast late cure. Many vulcanisation characteristics and vulcanisate properties were largely dominated by a single cure component. State of cure was dominated by sulphur level, scorch delay and early cure time by HTS loading, and the time to reach t90 by the sulphenamide level. Time of flight secondary ion mass spectroscopy imaging showed that HTS favourably affected the distribution of curatives in blends with SBR. 11 refs. 

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