Polyurethane foams

Stannous hypophosphite hexahydrate is made by firstly precipitating Sn(OH)2 by the addition of NaOH to a solution of SnSO4 in a mixture of water and H2SO4, filtering off the Sn(OH)2 and dissolving in hot hypophosphorous acid. On cooling stannous hypophosphite crystallizes out.ALSO:Polyurethane foams containing 0.001-5% by weight of a salt of hypophosphorous acid and having improved resistance to discoloration are made by reacting an organic polyisocyanate with a polyhydroxy compound in the presence of a gas generating agent and the salt of hypophosphorous acid. The salt may be a metal salt, e.g. Ca, Zn, Sn or the ammonium salt or a secondary or tertiary amine salt or quaternary ammonium salt. The salt may be incorporated as such in the reaction mixture or may be formed in situ e.g. by adding hypophosphorous acid to a mixture containing a tertiary amine. The polyhydroxy compound may be a polyether, polyester or polyesteramide. The gas generating agent may be water or an inert low-boiling liquid. A phenol may also be incorporated in the foam, e.g. in amount 0.001-5%, as an anti-oxidant. Conventional tertiary amine catalysts and organometallic catalysts may also be used.