Improvements in or relating to polyurethane foams

Foamed polyurethane materials are produced by the interaction of an organic polyisocyanate composition comprising a major proportion of a diarylmethane diisocyanate and at least 5% by weight of a polyisocyanate of functionality greater than two with a hydroxyl group-containing polyether in the presence of a gas-generating agent such as water or a low boiling-point liquid which is inert with respect to the polyurethane foam-forming ingredients, the combined active hydrogen content of the polyether and any water present being such that the weight of polyisocyanate composition required for the interaction is less than the weight of polyether used, the interaction proceeding by the steps of mixing the polyisocyanate composition with an amount of the polyether such that the rate ratio of polyisocyanate composition to polyether is at least equal to one, thus forming a dispersion of polyether in the polyisocyanate composition and thereafter mixing the dispersion with the gas-generating agent and the remaining quantity of polyether and allowing the mixture to react to form a foam. Diphenylmethane diisocyanate and phenyltolyl methane diisocyanate are especially suitable as the diarylmethane diisocyanates. The polyisocyanate of functionality greater than 2 is present as 5% to 50% of the total polyisocyanate; it may be introduced in several ways: either directly to the diaryl methane diisocyanate or by reacting some diaryl methane diisocyanate with a polyhydric polyol or by partially polymerizing diarylmethane diisocyanate to form isocyanurate derivatives or by using polyisocyanates of functionality greater than two which have been formed as biproducts in the formation of diarylmethane diisocyanate by the phosgenation of crude diaminodiarylmethane. Water in quantities of up to 7% by weight of the polyether and low boiling point liquids such as halogenated hydrocarbons can be used as gas generating agents. Surface active agents, catalysts, flame retardants, cross-linking agents, plasticizers, colouring matter and antioxidants can be incorporated into the reaction mixture.