Organotin mercaptide stabilizers

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. 

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. 

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. 

It has been shown in the preceding section that hydroperoxides are involved in the oxidative degradation of poly(vinyl chloride). Many sulphur compounds are known to decompose hydroperoxides into non-radical products and it is likely that organotin mercaptide stabilizers can also react in this way. For example, dimethyltin bis(isooctylthioglycolate) decomposes the model compound, tert-butyl hydroperoxide as follows [14] ... 

Barium/cadmium liquid stabilizers have been standard, but are being replaced with barium/zinc and calcium/zinc liquids. Similar clear coatings have, however, generally used organotin mercaptide stabilizers (except where odor is a consideration). 

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. 

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. 

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-... 

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 ... 

Organotin Stabilizer n Any of an important class of stabilizers for PVC, notable for their high efficiency, compatibility, and imparting of clarity. The family includes sulfides and oxides of tin-alkyls or -aryls, organotins salts of carboxylic acids, organotins mercaptides, and trialkyl or triaryl tin alcoholates. Certain dioctyltin mercaptides and maleate compounds have been approved for food contact use. See also Di-n-Octyltin Maleate Polymer and Di-n-Octyltin-s,s bis(isooctyl mercaptoacetate). 

In addition to the preventive stabilizers, curative effective organotin mercaptides are used to shorten polyene sequences that are able to convert discoloring carbe-nium and onium salts and thus contribute to brightening of PVC color [20]. 

The light stability of a non-specifically UV stabilized PVC formulation is determined by its thermal stabilization. The thermal stabilizers have a sustained influence on UV stability, in particular when other compounds (e.g., phosphites) are added to the mixture. The following list of thermal stabilizers is compiled in the order of their positive influence on UV stability in PVC organotin maleates > Ba/ Cd carboxylates > basic lead phosphites > organotin mercaptides > Ca/Zn carb-oxylates > aminocrotonic acid esters > basic lead sulfates [86]. 

Organotin mercaptides provide only little stabilizing effect. Some inorganic lead compounds can influence weathering resistance positively, especially when added to the mixture with other compounds (e.g., phosphites). Of the known lead salts, basic lead phosphite has the best stabilizing effect. This is attributed to its high UV absorbency without formation of unstable intermediate products [86]. 

Discoloration, however, remains an issue with vinyl wood fiber composites for outdoor application. UV light absorbers for vinyl are discussed in Section 4.5. Hindered amine stabilizers resistant to hydrogen chloride are now available, but require more expensive organotin carboxylate, rather than mercaptide, stabilizers, since they interact with the latter. A useful alternative is to include the hindered amine stabilizer in an acrylic or ASA topcoat over the wood fiber composite. 

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