Zinc mercaptide

Zinc oxide and certain fatty acids (R—COOH) are also added. Although this mechanism is by no means completely understood, it is proposed that the benzothiazole and zinc oxide give a zinc mercaptide, and this forms a soluble complex with the fatty acid. 

Nucleophilic attack by this species at the a-carbon atom will also be governed by the same steric considerations as in thermal decomposition and hence the inverse relationship of thermal stability and resistance to nucleophilic attack, and anti-wear activity and ease of nucleophilic attack. Further thermal processes involve olefin elimination from alkyl groups and lead to the formation of phosphorus acids. Nucleophilic substitutions of one phosphorus species by another leads to P-O-P structures and zinc mercaptide Zn(SR)2 as a reaction intermediate. Reaction of this mercaptide with dithiophosphate leads to trithiophosphates and eventually tetrathiophosphates. Finally, an oil-insoluble deposit is formed of a mixture of zinc thiophosphate and zinc pyro- and polypyrothiophosphates. 

Aminobenzenethiols and their salts react with anthraquinones yielding nap-hthophenothiazines. A number of substituted e Jo-12,17-o-phenylene-12,17-dihy-dro-naphtho[2,3-u]-l,4-benzothiazino[3,2 -c]phenothiazines and substituted endo-i, 13-o-phenylene-8,13-dihydro-6-chloro-7H-naphtho[2,3-a]phenothiazin-7-ones have been synthesized (83PS(17)85, 82IJC(B)695) by the condensation of endo-9, 0-o-phenylene-2,3-dichloro-9,l0-dihydro-1,4-anthraquinone with substituted 2-amino-thiophenol and their zinc mercaptides, respectively. 

Zinc nitrate (Zn(N03)2, Aldrich) was dissolved in ethanol and an aqueous solution of ammonium hydroxide was added drop by drop until the metal hydroxide was dissolved completely. Then an alcoholic solution of 1-dodecanethiol was added to this system at room temperature, under stirring, and the precipitate was isolated by vacuum filtration. The Zn mercaptide, Zn(SCi2H25)2, obtained was dissolved in chloroform and blended with polystyrene, leading to translucent films after solvent removal. The thermal annealing of zinc mercaptide/polystyrene blends, performed under nitrogen, at 250°C, gave a nanoscopic ZnS phase. 

To improve the efficiency of the vulcanization reaction, various accelerators were developed. Among them are zinc oxide combined with fatty acids, and/or amines. Zinc oxide forms zinc mercaptides like (XS)2ZnL2, where X is an electron-withdrawing substituent and L is a ligand from a carboxyl or an amine group. The function of the ligand is to render the complex soluble. The mercaptide complexes are believed to react with additional sulfur to form zinc perthiomeraptides. 

The initial zinc mercaptide (XI) is sparingly soluble in rubber but is rendered very soluble through co-ordination with nitrogen bases (either present in the raw natural rubber or added as accelerator) or zinc carboxylates examples of complexes involving co-ordination of amine and carboxylate ligands are as follows ... 

In the next stage, the zinc mercaptide complexes (represented below, for simplicity, as XSZnSX) are thought to react with sulphur to form zinc perthiomercaptides (XII) by means of a series of equilibria involving sulphur insertion and interchange ... 

The average values of x and y in the perthiomercaptides will be determined by the relative concentrations of sulphur and zinc mercaptide complex. Zinc perthiomercaptide complexes are believed to be the actual sulphurating agents in the vulcanization process. 

The third step in the vulcanization sequence is considered to be the formation of polysulphide cross-links. The most probable route for the conversion of the rubber-bound intermediate into cross-linked polysulphides is by disproportionation reactions, involving cleavage of S—S bonds these reactions may be initiated by mercaptide ions (XS ) derived from the zinc mercaptide (cf., Section 17.2) ... 

The ratio of BS— to AS— structures is increased substantially by the presence of electron-donating ligands such as carboxylates and amines which can coordinate with the zinc mercaptide and which are either present naturally or deliberately added. 

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