Organotin Stabilisers

Organotin stabilisers have been caught up in the elimination of heavy metal discussions. There are also improved analytical techniques now available to detect very low levels of organotin species in different environments (127,183). 

The identification of a perceived hazard, of course, must be related to the chance of that hazard having an effect (the risk) and this is not always appreciated. 

Full information on organotin stabilisers and their safe use over many years is available (138). 

Bisphenol A has been used as an inhibitor at the PVC polymerisation stage and as an antioxidant. There has been some concern regarding possible health hazards and its use has been voluntarily stopped or restricted to levels that do not exceed the predicted no effect 

Nonylphenol can be generated from some liquid heat stabilisers (in addition to many other possible sources) and it has been identified as persistent and oestrogenic (160). 

Organotin stabilisers are very widely used in PVC in North America and in a few European countries, notably France. There are a few associated toxicity issues, because overexposure to methyl tin can affect the nervous system, while butyl and octyl tin could impact on the immune system. The problems associated with organotin antifouling agents in ships are not generally thought to be very relevant to plastics. 

Organotin stabilisers have a strong affinity for PVC, attaching themselves to the chlorine atoms on the outer surfaces of primary polymer particles. They act as primary stabilisers insofar as they are involved with the substitution of the allylic chlorine atoms, and they react with hydrogen chloride to form alkyl thioglycolates that, once produced, act as secondary stabilisers. 

One tin stabihser category consists of thiotin compounds, such as dimethyl tin di(mercaptopropionate) or di(iso-octyl) tin disulfide. One disadvantage is that some of these substances may require extra lubricants and some UV stabilisation. Thio-tin compounds can also suffer from a very strong odom. 

The monoalkyl tin compoimds (R=l) are particularly rapid in their stabilising action, and are favoured in long-life applications such as rigid pipe, as well as certain flexible products. They give optically clear sheeting, including foam sheeting, and are available in both liquid and solid 

Dialkyl tin di(alkyl maleates) constitute a popidar class of tin compound that reacts with the polyene sequences that represent colour development, reducing their length and number by undergoing Diels-Alder reactions, and reducing discoloration. 

If mixed tin stabilisers are believed to be present, for example, a thio-compound and a dialkyltin dialkylcarboxylate, it is advantageons to acidify the acetone filtrate after removal of the dialkyltin oxides with hydrochloric acid, evaporate most of the acetone on a water-bath under an air-jet, cool and extract by shaking with diethyl ether. The ether extract then contains hemiesters, acids, and so on, liberated from the other types of tin stabilisers originally present and can be examined further. Although reaction products produced by 

Simpson and Currell [32] used butanol - glacial acetic acid (97 3) as the mobile phase and sprayed the developed plate with catechol violet solution followed by irradiation of the plate for 10 minutes with UV radiation and a respray with catechol violet solution to detect the separated compounds as blue spots. This procedure can be applied to the detection of organotin compounds in PVC using a preliminary extraction of the additives from 5 g polymer with diethyl ether for eight hours. Ether is removed by evaporation and ethanol added to precipitate polymer which is then filtered off. If more accurate quantitative results are required it is necessary to perform an extraction additional to the diethyl ether extraction by using an azeatrope of carbon tetrachloride - methanol (2 1) for a further four hours. 

Substance Rf value Spot located on spraying Spot located only after UV irradiation and spraying 

Udris [85] discusses the problem of the detection of organotin stabilisers and phthalate ester plasticisers when they are present together in solvent extracts of PVC. He points out that IR spectroscopy whilst it may show the presence of major components of such mixtures may miss the minor ones which may also be present, such as, phenol derivatives and metal carboxylates. These can be separated and identified by TLC followed by IR or UV spectroscopy. 

Metal salts present in tin stabilisers can be identified by diluting the sample with diethyl ether, centrifuging (or filtering) and examining the precipitate by IR spectroscopy and by emission spectroscopy (for metals). 

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KemI (2000) Organotin stabilisers in PVC — Assessment of risks and proposals for risk reduction measures. Stockholm, National Chemicals Inspectorate (Report No. 6/00). 

Progress in the field of polymer/additive analysis in the last three decades can best be illustrated by an old recipe for the direct determination of organotin stabilisers in PVC [142] ... 

Organotin stabilisers in PVC compositions were determined by means of TLC [518]. The determination of inorganic species (down to ppb range) by TLC has been reviewed on several occasions [519,520]. Instrumental methods successful in the quantitative estimation of inorganic TLC zones directly on the plate are densitometry, fluorimetry, radiometry, planimetry and visual methods. In the framework of recycling studies, in particular as to metal stabilisers in PVC, Braun and Richter... 

New York City, 2nd-6th May 1999, p.3569-73. 012 MECHANISM OF ORGANOTIN STABILISATION OF POLYVINYL CHLORIDE. IV. PVC STABILISATION BY ALKYLTIN ALKYL MERCAPTOPROPIONATES Fisch M H Bacaloglu R Dooley T Witco Technical Center (SPE)... 

New York City, 2nd-6th May 1999, p.3564-8. 012 MECHANISM OF ORGANOTIN STABILISATION OF POLYVINYL CHLORIDE. 

Organotin-stabiliser can be extracted from foodstuffs by various chlorinated solvents and hydrocarbons. Interfering substances, especially inorganic tin, were preseparated by paper chromatography After fusion and formation of the tin-catechol complex, chromatography was used for separation of the tin-complex from excess reagent. 

Stabilisers/Antioxidants Phosphite/phosphonites are generally regarded as the most effective stabilisers during processing, protecting both the polymer and the primary antioxidant. The most expensive stabilisers are organotin stabilisers, while lead compounds are the cheapest. 

Organotin stabilisers (containing alkyl groups, notably methyl, butyl or octyl) have been widely used in PVC pipe for many years, and are approved for use in potable water pipe both in the USA and some European countries. This is because they are effective at low dose levels, and the quantities leached out of pipes after the first few flushings are much lower than the recommended maximum human exposure level. 

Crompton has introduced Mark 2000 organotin stabilisers for PVC window frames, to overcome processing problems in high-speed extrusion lines. The company also markets a sulfur-free tin stabiliser called Mark 2289, specifically for PVC window profiles that are subject to intense simshine, as in desert areas of the United States. It is said to protect against heat and UV to some extent as well. 

The main products still include PVC heat stabilisers (including mixed metal and organotin types), epoxy plasticisers, antistatic additives and viscosity modifiers. The organotin stabilisers are made in Greiz, Germany, and the mixed metal stabilisers in Eccles. There is also a site in New Brunswick, USA, and Akcros America has extensive business in North America. Akcros makes polysulfides as well. 

Heavy metals in PVC compoimds, including tin from stabilisers or biocides, have also attracted attention. The implications for heat stabilisers have already been discussed. A decision was made by the Emopean Stabilisers Producers Association to phase out lead stabilisers by 2015. The central issues in the medium-term future are likely to be (a) the rate at which lead is phased out and (b) the future attitude of regulatory authorities to organotin. Organotin stabilisers represented 9% of the Emopean stabiliser market in 2002. (Greenpeace opposes organotin compoimds in PVC products intended for children, as well as lead.)... 

Thus, Frye and co-workers [47] were able to provide evidence by infrared (IR) spectroscopy [46] and radiochemistry [48] of the insertion of ester groupings from barium, cadmium or zinc-carboxylates into PVC after heat treatment. Tin contents and organic residues of organotin stabilisers in heat-treated PVC are indicated by the same authors [47, 49]. Schlimper [50] and Schroder and co-workers [51] proved the direct reaction between PVC and nitrogen-containing organic stabilisers by elemental analytical examinations and infrared and UV spectrometry. Phenolic antioxidants or their decomposition products in part were re-found in PP after oxidative degradation. With rubber vulcanisates hydrochloric acid-resistant amine-rubber compounds have been reported after thermal oxidation in the presence of aromatic amines [52]. 

It is first of all necessary to prepare an extract of the PVC in which the organotin stabiliser... 

Organotin stabilisers Not stated Acetic acid -isopropyl ether (1-5 98-5) 20% Molybdophosphoric acid + ammonia vapour [10]... 

Simpson and Currell [32] use TEC in the determination of additives such as antioxidants (also UV absorbers and organotin stabilisers) in plastic formulations. Comparatively small samples of the plastics materials are required, and, by means of the techniques described, it is... 

Thin-layer liquid chromatography has been employed for the determination of alkylated cresols and amine antioxidants [102, 103] in polybutadiene, phenolic antioxidants in PE [104-106] and PP [106], dilauryl and distearyl thiodipropionate antioxidants in polyolefins, ethylene-vinyl acetate copolymer, acrylonitrile-styrene terpolymer and PS, UV absorbers and organotin stabilisers in polyolefins [102], and accelerators such as guanidines, thiazoles, thiurans, sulfenamides, diethiocarbamides, and morpholine disulfides in unvulcanised rubber compounds. 

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