Process stabilization of polymer compositions comprising phosphorous containing flame retardants

A method for preparing flame a retardant polymer composition, which method comprises adding to a polymer resin

• from 1% to 50%, by weight based on the total weight of the flame retardant polymer composition, of a flame retardant material comprising one or more compounds of empirical formula (IV)
• wherein
• R is C1-12 alkyl, C6-10 aryl, C7-18 alkylaryl, or C7-18 arylalkyl, wherein said alkyl, aryl, alkylaryl, or arylalkyl are unsubstituted or are substituted by halogen, hydroxyl, amino, C1-4 alkylamino, di-C1-4 alkylamino, C1-4 alkoxy, carboxy or C2-5 alkoxycarbonyl;
• M is a metal,
• y is a number of from 1 to 4 so that M(+)y is a metal cation where (+)y represents the charge formally assigned to the cation, q is 1, 2 or 3, n is 1 or 2, r is 0, 1 or 2 provided that 2(q)+r=n(y);
• wherein the flame retardant material is obtained by thermal conversion of salts of formula (I) before incorporation into the polymer by a process comprising heating at temperatures higher than 200° C. for from about 0.01 hour to about 20 hours one or more than one compound of formula (I)
• wherein R, M and y are as defined above and p is a number of from 1 to 4;
• and one or more compounds selected from the group consisting of hydrotalcite clays, metal borates, metal oxides and metal hydroxides,
• and then melt processing the resulting mixture at elevated temperature.