Flame retardants tin compounds

Tin compounds have been known as flame retardants since the midnineteenth century, when processes based on the in situ precipitation of hydrous tin(iv) oxide were developed to impart flame-resist properties to cotton and other cellulosic materials  

Na2Sn(OH)e (NH4)2S04 Sn02 + Na2S04 + 2NH3 4- 4H2O (2) 

More recently, tin salts have foimd use in flame-retardant treatments for woollen sheepskins and rugs. The active tin species are generally fluoro-stannate-based (e.g. SnF ), and these are electrostatically attracted to the protonated amino groups in the proteinaceous wool structure. 

However, as far as plastics are concerned, commercial interest in the use of tin-based flame retardants has only developed over the past 10 years or so. Although it is estimated that over 600 000 tonnes of chemical additives are used worldwide annually as flame retardants for s)mthetic polymers, recent concerns about the toxic nature of certain additives have led to an intensified search for safer flame retardants. Hence, the generally low toxicity of inorganic tin compounds has been a major factor in their growing acceptance throughout the 1990s as flame retardants and smoke suppressants for plastics, elastomers and other polymeric materials. 

Despite the fact that a wide range of inorganic and organo-tin compounds have been found to exhibit fire-retardant properties (Table 1), only a few 

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Flame letaidancy can be impaited to plastics by incorporating elements such as bromine, chlorine, antimony, tin, molybdenum, phosphoms, aluminum, and magnesium, either duriag the manufacture or when the plastics are compounded iato some useful product. Phosphoms, bromine, and chlorine are usually iacorporated as some organic compound. The other inorganic flame retardants are discussed hereia. 

At the present time, inorganic tin compounds find a relatively small use in natural polymers, particularly as flame-resist treatments for woollen rugs and sheepskins (8,9). Although certain other metal derivatives have received more attention, there has been much interest recently in the potential use of tin chemicals as flame retardants and smoke suppressants for synthetic polymers (10). 

Hence, it is apparent that certain inorganic tin compounds are very effective flame retardants and smoke suppressants for halogenated polymer formulations. Since these additives are generally non-toxic, their potential use as partial or total replacements for existing commercial flame retardants, such as antimony trioxide, is thought to merit serious consideration. 

Tne effectiveness of inorganic tin compounds as flame and smoke retardants, both alone and in combination with inorganic fillers, in several halogen-free thermoplastic, thermosetting and elastomeric substrates, is currently under investigation at the ITRI. 

Certain inorganic tin compounds are effective flame-retardant synergists when incorporated at a 2.5% level into a 50% ATH-filled ethylene-acrylic rubber composition. Tin-containing elastomer formulations retain their flame-retardant superiority at environmental temperatures up to 250°C, and samples containing 2.5% ZnSn(0H)g do not sustain combustion in air at this temperature. 

In general, tin compounds do not exhibit flame-retardant properties in halogen-free polymer systems, unless the composition contains a high inorganic filler loading. However, tin additives often act as smoke suppressants in non-halogenated polymers. 

Apart from ZHS and ZS, little work has generally been undertaken on tin-based Are retardants in nonhalogen polymer systems. However, certain tin(II) compounds have shown excellent flame-retardant and smoke-suppressant properties when incorporated at levels of 20-30% into aromatic polyesters, specifically polybutylene terephthalate (PBT). Hence, tin(ll) oxide, tin(II) oxalate, and tin(II) phosphate have been shown to markedly increase flame retardancy in PBT, whereas, interestingly, tin(IV) oxide is almost totally ineffective in the same polymeric substrate. 

Certain tin compounds, particularly zinc hydroxystannate (ZHS) and zinc stannate (ZS), are effective flame retardants in a wide range of polymeric substrates and offer technical advantages over many competitive additives ... 

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