Organotin Stabilisers

Tin stabilisers for PVC compounds are characterised by a central tin atom, surrounded by alkyl and acidic groups. Mixtures of mono and dialkyl tin salts are used and commercial tin heat stabilisers typically vary the ratio, depending on the performance and property requirements of the final product.

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

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

PVC tin stabilisers are always based on methyl, butyl or octyl groups. The former are not generally used in Europe, but are commonly incorporated in a range of compounds, including potable water pipe formulations in the USA.

Both butyl and octyl tin stabilisers are found in an extensive range of applications in Europe. When good outdoor weathering performance or low odour are especially required, it is normal to use maleates or carboxylates, while for all other applications the thiotins are commonly used.

Both butyl and octyl tins provide very good heat stability and colour control, combined with the option of manufacturing crystal clear products. A considerable number of octyl tin stabilisers also have obtained approval from official bodies for food contact applications.

PVC compounds incorporating tin heat stabilisers are used in a diverse range of applications including sheet, bottles, profiles, injection moulded fittings, credit cards, blister packs, food containers and display trays. The general range of applications is shown in the table.

Applications for Stabilisers - European Market

++ Major use
+ Minor use
(+) Occasional use
(1) Used for potable water pipe
(2) Used as a stabiliser/'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 the classifications according to the EU system, changes were decided in 2006. All tin stabilisers are classified as toxic and a few are also classified as toxic for reproduction.

Therefore, sufficient precautions have to be taken to avoid ingestion, skin absorption and inhalation. The allowable exposure to tin stabiliser vapours in the atmosphere is controlled by national regulations. However, in practice, the typical exposure levels are so low that any risks to health are considered to be negligible.

Organotin stabilisers, specifically octyl and methyl tin products, have extensive national approvals for food contact applications. In the case of potable water pipe, tin stabilisers are approved in all European countries and in the USA.

PVC tin stabilisers are always based on methyl, butyl or octyl groups

However, in Europe their usage in water pipe has been largely confined to France and Belgium, while it is the dominant stabiliser type used for this application in the USA where the regulatory authorities have concluded that the use of tin stabiliser is safe since there is no leaching of stabiliser to the water once the initial surface layer of stabiliser has been washed off by flushing.

Following a risk assessment, risk reduction measures are currently under discussion at EU level. These may include a prohibition of butyl tin stabilisers by 2012, except for a few applications. Use of octyl tin may be limited in a few applications where the products are in contact with the skin.

The industry has already phased out tin stabilisers in most of these applications.

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