Tin compounds as flame retardants

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). 

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. 

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 ... 

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. 

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. 

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. 

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. 

There are various additives in PVC plastics, such as antioxidants, light stabilizers, initiators, plasticizers, flame retardants, pigments and others. As initiators, potassium persulfate, benzoyl peroxide, lauryl peroxide, percarbonate and some azo-compounds can be used. The presence of chlorine in the hydrocarbon backbone gives rigidity and toughness to the polymer, but PVC liberates hydrogen chloride when exposed to high temperatures. To prevent this, stabilizers are added to the polymer. There are several kinds of stabilizers on the market. The most important contain lead, tin, calcium and zinc and... 

Many metals, metal salts and metallic compounds are used as additives in plastics. They are used as stabilizers, pigments, fillers, flame retardants and antistatics. The most commonly used metals are aluminum, titanium, lead, zinc, antimony, tin, chromium, and molybdenum. Nickel, copper and zirconium compounds are used to a lesser degree. 

Tin oxide, Sn02, has also been studied as a flame retardant and smoke suppressant in conjunction with chlorine or bromine compounds. Performance was found to be generally less effective than that of the zinc stannates, which also contain less tin, and are hence more cost effective. 

With regard to practical usage, one of the major application areas for ZHS and ZS has been in flexible PVC wire and cable insulation, where the tin additives have been selected because of their combined flame-retardant/smoke-suppressant properties. The grades of ZHS/ZS used must have very low electrolyte levels and must not contain any free zinc oxide, since the presence of the latter compound can result in a phenomenon known as zinc burning, in which the PVC undergoes dehydrochlorination during processing. 

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