A phosphorus-containing flame retardant is produced by reacting at a reaction temperature a mixture including a metal or suitable metal compound and a stoichiometric excess relative to the metal or suitable metal compound of an unsubstituted or alkyl or aryl substituted phosphonic or pyrophosphonic acid, wherein the phosphonic or pyrophosphonic acid is in a molten state at the reaction temperature. The chemical composition of the resulting flame retardant product leads to excellent flame retardancy and exhibits high thermal stability. The presently disclosed flame retardants are useful, for example, in polymer compositions, particularly thermoplastics processed at high temperatures, over a wide range of applications.

Polyurethane rigid foams containing at least one phosphorous ester-based flame retardant are returned to the value chain by hydrogenating the polyurethane rigid foam, to obtain a hydrogenation product containing a polyamine and a polyol from the polyurethane rigid foam, and recovering the flame retardant from the hydrogenation product. The hydrogenation is carried out in an organic aprotic solvent, in a hydrogen atmosphere, in the presence of at least one homogeneous transition metal catalyst complex, wherein the transition metal is selected from metals of groups 7, 8, 9 and 10 of the periodic table of elements according to IUPAC, and at a reaction temperature of at least 120 C.

Polyurethane rigid foams containing at least one phosphorous ester-based flame retardant are returned to the value chain by hydrogenating the polyurethane rigid foam, to obtain a hydrogenation product containing a polyamine and a polyol from the polyurethane rigid foam, and recovering the flame retardant from the hydrogenation product. The hydrogenation is carried out in an organic aprotic solvent, in a hydrogen atmosphere, in the presence of at least one homogeneous transition metal catalyst complex, wherein the transition metal is selected from metals of groups 7, 8, 9 and 10 of the periodic table of elements according to IUPAC, and at a reaction temperature of at least 120 C.

A phosphorus-containing flame retardant is produced by reacting at a reaction temperature a mixture including a metal or suitable metal compound and a stoichiometric excess relative to the metal or suitable metal compound of an unsubstituted or alkyl or aryl substituted phosphonic or pyrophosphonic acid, wherein the phosphonic or pyrophosphonic acid is in a molten state at the reaction temperature. The chemical composition of the resulting flame retardant product leads to excellent flame retardancy and exhibits high thermal stability. The presently disclosed flame retardants are useful, for example, in polymer compositions, particularly thermoplastics processed at high temperatures, over a wide range of applications.

A phosphorus-containing flame retardant is produced by reacting at a reaction temperature a mixture including a metal or suitable metal compound and a stoichiometric excess relative to the metal or suitable metal compound of an unsubstituted or alkyl or aryl substituted phosphonic or pyrophosphonic acid, wherein the phosphonic or pyrophosphonic acid is in a molten state at the reaction temperature. The chemical composition of the resulting flame retardant product leads to excellent flame retardancy and exhibits high thermal stability. The presently disclosed flame retardants are useful, for example, in polymer compositions, particularly thermoplastics processed at high temperatures, over a wide range of applications.