Fire retardant metal stannate coated inorganic fillers

A process for the preparation of a fire-retardant material which comprises a particulate inorganic filler (other than a tin compound) coated with a layer of a divalent metal hydroxystannate or a divalent metal stannate, which process comprises the steps of:

• (i) forming a slurry of the particulate inorganic filler in an aqueous solution of an alkali hydroxystannate, which also contains a divalent metal oxide dissolved therein and reacting the slurry with a hydrolysing agent;
• or
• (ii) reacting a slurry of the particulate inorganic filler in an aqueous solution of an alkali hydroxystannate with a water-soluble source of a divalent metal, to form a layer of a divalent metal hydroxystannate on the surface of the filler;
• and
• (iii) optionally, heating the filler coated with hydroxystannate in step (i) or step (ii) in order to convert it to the corresponding divalent metal stannate.