Cured unsaturated polyester-polyurethane hybrid highly filled resin foams

A rigid, lightweight filled open-celled resin foam, comprising:

• (A) a continuous phase having cellular voids therein, wherein said continuous phase comprises an unsaturated polyester-polyurethane polymer resin, wherein said resin forms a matrix comprising an interpenetrating polymer network formed by entanglement of (1) hybrid networks comprising polyester polyurethane chains cross-linked with reactive monomer, (2) modified polyurethane hybrid networks comprising networks obtained by the reaction of a poly- or diisocyanate, an unsaturated polyester polyol, a saturated polyol, and reactive monomer, (3) polymerization products of saturated polyol and poly- or diisocyanate, or (4) mixtures thereof, wherein said resin is formed as the reaction product of an unsaturated polyester polyol, a saturated polyol, a poly- or diisocyanate, and a reactive monomer, wherein said resin forms the walls of said cellular voids;
• and
• (B) fine multisize reinforcing particles selected from the group consisting of fly ash, treated red mud, and mixtures thereof, dispersed in said matrix of said continuous phase and immobilized therein, wherein the diameter of the largest of said multisize particles is no greater than about 33% of the average thickness of said walls between said cellular voids, and wherein the smaller of said multisize particles fill the spaces between the larger particles within said walls.