Zinc Stannates

By far the most important tin-based fire retardants are the zinc stannates - zinc hydroxystannate (ZHS) and its anhydrous analog, zinc stannate (ZS). Originally developed at ITRI during the mid-1980s, these additives are now being marketed worldwide as non-toxic flame retardants and smoke suppressants for use in a wide range of polymeric materials.  

ZHS is manufactured industrially by the aqueous reaction of sodium (or potassium) hydroxystannate with zinc chloride  

The white precipitate product is washed free of sodium chloride and dried in air at ca. 105 °C. ZS is manufactured by controlled thermal dehydration of ZHS, usually at a temperature in the range 300-400 °C  

Although there is generally little difference in the effectiveness of ZHS and ZS in terms of their fire-retardant properties, ZS is the preferred additive for polymers which are processed at temperatures above ca. 200°C. Some important properties of ZHS and ZS are given in Table 3.5.1. 

The relative tire-retardant efficiency of tin additives, including ZHS and ZS, compared to that of Sb203 

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Antimony trioxide (SbaOj). It is produced from stibnite (antimony sulphide). Some typical properties are density 5.2-5.67 g/cm- pH of water suspension 2-6.5 particle size 0.2-3 p,m specific surface area 2-13 m-/g. Antimony trioxide has been the oxide universally employed as flame retardant, but recently antimony pentoxide (SbaOs) has also been used. Antimony oxides require the presence of a halogen compound to exert their fire-retardant effect. The flame-retarding action is produced in the vapour phase above the burning surface. The halogen and the antimony oxide in a vapour phase (above 315 C) react to form halides and oxyhalides which act as extinguishing moieties. Combination with zinc borate, zinc stannate and ammonium octamolybdate enhances the flame-retarding properties of antimony trioxide. 

Studies at the International Tin Research Institute showed that 2.5% zinc stannate strongly enhanced the flame retardant action of ATH in ethylene-acrylic rubber, and enhanced the char yield (34) (Fig. 5). 

Zinc hydroxystannate and zinc stannate were synthesised at I.T.R.I. according to previously reported procedures (11,12)... 

Antimony trioxide, 8-stannic acid (hydrous tin(IV) oxide), zinc hydroxystannate, and zinc stannate were incorporated into the commercial brominated polyester resin at levels of 1,2,5 and 10% by weight. No processing problems were encountered and the samples cured satisfactorily to give rigid, opaque strips. 

Of the tin additives studied, the anhydrous and hydrated zinc stannates, ZnSnO and ZnSn(OH), respectively, are considerably more effective flame-retardant synergists with the bromine present in the plastic than 8-stannic acid (Figure 1). In line with this observation, oxidic tin-zinc systems have previously been found to exhibit superior flame-retardant properties to tin oxides alone (19-22). In addition, ZnSnO, gives higher values of 01 than Sb20, incorporation levels studied, and, in fact, the 1% ZnSnO - containing plastics outperform samples containing 2% Sb O,. 

Hence, the zinc stannates have been shown to impart"5beneficial properties to this polymer system, in terms of flammability and smoke/carbon monoxide evolution, and the improvements in performance are clearly superior to those exhibited by the commercially available antimony trioxide. 

Incorporation of a 2% level of either ZnSn(OH) or ZnSnO into the resin, leads to marked differences in its thermal degradation profile. The initial decomposition comprises two distinct stages, the major step occurring at a lower temperature than that of the untreated polymer, which may be indicative of promotion of bromine volatilisation by the zinc stannates. Furthermore, the amount of residue burnt off in the char oxidation step is greatly increased in the tin-containing samples, and the temperature at which this process occurs is significantly higher than in the base resin. 

Zinc hydroxystannate and zinc stannate are very effective flame-retardant synergists when incorporated at levels of 1-10% into... 

Inorganic tin compounds, in particular, zinc hydroxystannate and zinc stannate, are effective fire retardants in a number of polymer systems. They appear to have advantages over certain existing commercial additives, namely ... 

Poly (vinyl chloride). PVC is a hard, brittle polymer that is self-extinguishing. In order to make PVC useful and more pliable, plaslict/ers I qv) are added. More often than not the plasticizers are flammable and make Ihe formulation less flame-resistant. The Maine resistance of the polyf vinyl chloride) can he increased by ihe addition of an inorganic llamc-rctardunl synergist, e.g., antimony oxide, mixed metal antimony synergists, zinc borate, molybdenum oxide, zinc stannates. and alumina trihydrale. 

Cusack, P.A. and Hornsby, P.R., Zinc hydroxystannate/zinc stannate-coated fillers Novel flame retardants and smoke suppressants for polymeric materials, J. Vinyl Additive Technol., 5, 21-30, 1999. 

Zinc borate is also an effective SS in combination with phosphate ester plasticiser (130). The combination of ammonium octamolybdate and zinc stannate is also beneficial (435). It has been shown that ATH and MDH functional fillers coated with zinc hydroxystannate give significantly increased combustion resistance and lower levels of smoke evolution (386). Copper (I) complexes have been suggested as smoke suppressants (241). Copper oxides have also been investigated with positive results (60). 

Table 3.5.1 Properties of zinc hydroxystannate and zinc stannate...

Thermal analysis experiments have clearly shown that tin-based fire retardants markedly alter both the initial pyrolysis and the oxidative burn off stages that occur during polymer breakdown These changes have been interpreted as being indicative of an extensive condensed phase action for the tin additive, in which the thermal breakdown of the polymer is altered to give increased formation of a thermally stable carbonaceous char at the expense of volatile, flammable products. The consequent reduction in the amount of fuel supplied to the flame largely accounts for the beneficial smoke-suppressant properties associated with zinc stannates and other tin-based fire retardants. 

Flamtard S (zinc stannate), Flamtard H (zinc hydroxy stannate), Flamtard HB1 (zinc hydroxystannate/zinc borate blend)... 

Zinc stannate is an inorganic flame retardant which can he used hy itself or in combination with aluminum hydroxide or magnesium hydroxide with which it forms synergistic mixtures of high performance flame retardants. It is frequently used as a surface coating on these two fillers. It reduces smoke emission and promotes char formation. 

Figure 13.7 shows the effect of two combinations on smoke release. Addition of zinc stannate or ammonium octamolybdate, AOM, gives better performance than antimony trioxide alone. " Antimony trioxide performance is also improved by brominated flame retardants. Chlorinated hydrocarbons, antimony trioxide, and novel silicone powders form good combinations, reducing the heat release rate and smoke in thermoplastics. ... 

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