In the case of zinc hydroxystannate, the product’s mode of action as a flame retardant is firstly as a result of endothermic dehydration, resulting in the loss of ~19% by weight of water from the ZHS. This water loss cools a potential fire and dilutes the oxygen available to sustain a fire.
In addition to this action, the tin component has been shown to catalyse the formation of a carbonaceous char in which tin-carbon bonds have been observed. This char acts as a barrier between a fire-retarded system containing ZHS and a potential fire, thereby reducing the smoke produced as a result.
In the presence of halogens, there is strong evidence of gas-phase flame retardant behaviour, which further hinders fire propagation.
The mode of action of zinc stannate, which is used for higher temperature applications, is essentially the same as the above, but without the dehydration step.
Zinc hydroxystannate and zinc stannate are effective in a variety of polymer and polymer-based systems and are at least compatible with and their action complimentary to a variety of other flame-retardants.
In many cases, synergies have been reported, i.e. the fire-retardant action of a multicomponent system containing zinc hydroxystannate (ZHS) and zinc stannate (ZS) is greater than the combined effect observed by the addition of each component in isolation.
Zinc hydroxystannate and zinc stannate are non-toxic alternatives, which can be used to completely replace antimony trioxide (Sb2O3) in formulations, whilst maintaining or improving most fire-retardant properties. In addition, the incorporation of zinc hydroxystannate and zinc stannate leads to significant and substantial smoke reduction, whilst ATO often generates additional smoke.
