Organotin compounds for stabilising PVC have been in commercial use forover 40 years and the commercial products currently available are well provenin their respective applications. The tin used in heat stabilisers for PVCaccounts for about 3.5 per cent of the world’s total tin consumption.

Tin stabilisers for PVC compounds are characterised by acentral tin atom, surrounded by alkyl and acidic groups. Mixtures of mono anddialkyl tin salts are used and commercial tin heat stabilisers typically varythe ratio, depending on the performance and property requirements of the finalproduct.

Properties and Applications
There are two main types of tin heat stabilisers for PVC. These are:

1. (a) Thio acid half esters     such as thio-glycollates often known as thiotins or mercaptides.
2. (b) Dicarboxylic half     esters, often referred to as maleates or carboxylates.

PVC tin stabilisersare always based on methyl, butyl or octyl groups. The former are not generallyused in Europe, but are commonly incorporated in a range of compounds,including potable water pipe formulations in theUSA.
Both butyl and octyl tin stabilisers are found in an extensive range ofapplications in Europe. When good outdoorweathering performance or low odour are especially required, it is normal touse maleates or carboxylates, while for all other applications the thiotins arecommonly used. Both butyl and octyl tins provide very good heat stability andcolour control, combined with the option of manufacturing crystal clearproducts. A considerable number of octyl tin stabilisers also have obtainedapproval from official bodies for food contact applications. PVC compounds incorporatingtin heat stabilisers are used in a diverse range of applications includingsheet, bottles, profiles, injection moulded fittings, credit cards, blisterpacks, food containers and display trays. The general range of applications isshown in the table.

Applications for Stabilisers – European Market

++ Major use + Minor use (+) Occasional use (1) Used for potable water pipe (2) Used as astabiliser/’kicker’ for foamed layers in these products

Safety, Health And Environmental Issues
As a result of a four year testing programme and a review of theclassifications according to the EU system, changes were decided in 2006. Alltin stabilisers are classified as toxic and a few are also classified as toxicfor reproduction. Therefore, sufficient precautions have to be taken to avoidingestion, skin absorption and inhalation. The allowable exposure to tinstabiliser vapours in the atmosphere is controlled by national regulations.However, in practice, the typical exposure levels are so low that any risks tohealth are considered to be negligible.

Organotin stabilisers, specifically octyl and methyl tin products, haveextensive national approvals for food contact applications. In the case ofpotable water pipe, tin stabilisers are approved in all European countries andin theUSA.However, in Europe their usage in water pipe has been largely confined toFrance and Belgium, while it is the dominant stabiliser type used for thisapplication in the USA where the regulatory authorities have concluded that theuse of tin stabiliser is safe since there is no leaching of stabiliser to thewater once the initial surface layer of stabiliser has been washed off byflushing. Following a risk assessment, risk reduction measures were implementedin 2010 at EU level. These include a prohibition of butyl tin stabilisers by2012, except for a few applications where use may continue until 2015. Use ofoctyl tin will be limited in a few applications where the products are incontact with the skin. The industry has already phased out tin stabilisers inmost of these applications.

The ‘Forschungszentrum für Umwelt und Gesundheit’ in Germany (GSF) on behalf ofORTEP has published a comprehensive review of the toxic and ecotoxic aspects oftin stabilisers. This concludes that tin stabilisers do not bioaccumulate inthe environment and in the human body.