Organotin mercaptides

Additionally, organotin mercaptides can act as antioxidants, as they can sequester free-radical degradation mechanisms (48). The one drawback of mercaptide-based tin stabilizers is the discoloration of the sulfur after exposure to uv-radiation. Special precautions or formulations need to be developed for outdoor apphcations. 

Heat stabiliser Organotin mercaptides/sulfides/carboxylates antimony mercaptides metal carboxylates lead stearate/phosphite/phthalate/sulfate S, Sb, Sn, Ba, Ca, Cd, Mg, Sr, Zn P, Pb, S... 

Organotin mercaptides can also absorb hydrogen chloride. 

The performance of organotin mercaptides is not only based on the amount of tin metal content, but on the organotin species, mercaptide ligand chemistry and organic co-stabiliser (453). The mechanism of organothiotin stabilisation has been studied extensively (221,347, 348, 387, 388). 

Sulfur-containing stabilizers (organotin mercaptides and sulfides). 

Stabilizers containing Sn-S bonds may be of many types, for example organotin mercaptides, organotin mercaptoalcohols, organotin thioglycollates, and organotin polymers containing sulfur. 

Organotin mercaptide stabilizers have an anti-oxidative action. This contributes to the stabilization in as much as the dehydrochlorination is much faster in the presence of air (oxygen) than in the presence of inert gas, and the loss of HCl is noticeably retarded by phenolic antioxidants. 

Organotin mercaptide stabilizers decompose hydrogen peroxides, providing a secondary antioxidant effect (Figure 3.3.4). 

Organotin mercaptide stabilizers also break autoxidation chains, and, compounds of this type are even patented as antioxidants for other plastics. 

Organotin mercaptides are able to bind or to neutralize HCl, which - as already mentioned - has an auto-catalytic effect on dehydrochlorination, especially in presence of oxygen, thus providing a further preventive function (Scheme 3.3.1). 

Scheme 3.3.2 Exchange of labile chlorine atoms in PVC by organotin mercaptide stabilizers ...

The two possible reactions of organotin mercaptide stabilizers, both ending in the formation of dialkyltin dichloride, are shown in Schemes 3.3.1 and 3.3.2. 

Scheme 3.3.3 Proposed mechanism of exchange of chloride atoms in PVC by mercaptide groups of organ-otin stabilizers coordination ofallylic chlorine by organotin mercaptide with subsequent rearrangement to a non-allylic structure...

Treatment with organotin stabilizers improves the color of thermally degraded PVC (100,105). This effect was attributed to the destruction of complexes formed from polyenes and HC1 (100,105) and, in the case of the organotin mercaptides, to the addition of free mer-... 

From these results we conclude that the ligand exchange reaction (I) exists and occurs instantaneously and quantitatively at 25°C in the organotin mercaptide stabilizer systems examined. 

At least one exception in the organotin mercaptides has been noted. In the dimethyltin di( benzyl mercaptide )/dimethyltin dichloride system, regardless of the relative concentrations used, the NMR spectra consisted of broad lines and did not contain any resonances characteristic of the excess component. At 25°C, the reaction appears to be one involving a rapid equilibrium exchange of groups (14). However, at about — 10°C, the resonances of the individual components can be observed owing to a slow rate of exchange (15). 

Organotin compounds that are salts of alkyltin oxides with carboxylic acids (e.g., dioctyltin dilaurate) are usually called organotin carboxylates. Organotin compounds with at least one tin-sulfur bond (e.g., dioctyltin octylthioglycolates) are generally called organotin mercaptides. The latter are considered to be the most efficient and most universal heat stabilizers. The important products which are on the market have the following structures ... 

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