Dibenzothiazolyl disulfides are known as accelerators in the vulcanization of diene rubbers. Dibenzothiazolyl disulfides or benzothiazole disulfides were originally developed for the safe processing of rubber compounds cured above 142.degree. C. This thiazole accelerator continues to be used widely in compounds of all types for major commercial applications. Its activity and scorch properties can be controlled over a wide range by using various combinations of other accelerators.
Dibenzothiazolyl disulfide is presently prepared from mercaptobenzothiazole (MBT) which has been steam stripped to remove volatiles and then pelletized. This MBT is dissolved in caustic and converted to the sodium salt (II). ##STR1##
The salt solution is then extracted with toluene to remove tars. The salt solution is then combined with sodium nitrite. The NaMBT (II) plus NaNO.sub.3 solution is then combined with a sulfuric acid solution at 60.degree.-65.degree. C. and converted to dibenzothiazolyl disulfide with by-products of Na.sub.2 SO.sub.4 and NO. NO is known to react with secondary amines such as dimethylamine, to form nitrosamines which are carcinogens. Therefore, there is a need for a process to prepare dibenzothiazolyl disulfide which does not pollute the air and water.
It is known to oxidize the sodium salt of mercaptobenzothiazole in a neutral or alkali medium with halogens or ammonium persulfate. In this system the inorganic by-products resulting from the reaction, such as the inorganic salt produced by neutralizing the mineral acid, must be especially treated or discharged as environmental pollutants. It is, therefore, advantageous to discover a relatively pollution free process for the preparation of dibenzothiazolyl disulfide.
U.S. Pat. No. 4,143,045 by J. Tazuma and A. Bergomi discloses a method for preparing dibenzothiazolyl disulfides by oxidizing mercaptobenzothiazole in a substantially neutral saturated aliphatic alcohol solution with hydrogen peroxide.
U.S. Pat. No. 3,904,638 by S. Sagawa et al discloses a process for the purification of crude 2-mercaptobenzothiazole wherein the crude 2-mercaptobenzothiazole is dispersed into an aromatic hydrocarbon, filtering the resultant dispersion, dissolving the collected particles into an aqueous caustic alkali and filtering the resulting alkaline solution to given an aqueous solution of the alkali salt of 2-mercaptobenzothiazole of high quality.
German Offenlegungsschrift No. 2800462 discloses the preparation of 2,2'-benzothiazolyl disulfide by the oxidation of 2-mercaptobenzothiazole sodium salt with chlorine.
German Offenlegungsschrift No. 2355897 discloses the preparation of bis(2)-benzothiazolyl disulfide from 2-mercaptobenzothiazole by oxidation with air in the presence of FeCl.sub.3.
U.S. Pat. No. 2,024,567 disclosed a process of preparing dithiazolyl disulfides which comprises oxidizing a thiazolyl mercaptan with hydrogen peroxide in the presence of an inorganic acid.
U.S. Pat. No. 2,593,761 is concerned with a process for reducing the mercaptan concentration of a sour petroleum distillate which comprises treating said distillate with t-butyl hydroperoxide or cumene hydroperoxide.
The prior art does not suggest or disclose the unique advantages that can be obtained when an amine is used in combination with the peroxide for the oxidation of 2-mercaptobenzothiazole.