Imide-containing polyester polyols and intumescent rigid foams

A method comprising formulating a rigid polyurethane or polyisocyanurate foam by reacting:

• (a) a polyisocyanate;
• (b) a polyester polyol comprising recurring units of a combination of a phthalimide-containing polyacid and a phthalimide-containing polyol;
• wherein the phthalimide-containing polyacid is a reaction product of trimellitic anhydride and an amino acid selected from the group consisting of glycine, alanine, phenylalanine, serine, threonine, aspartic acid, and glutamic acid;
• and
• wherein the phthalimide-containing polyol is a reaction product of an aliphatic aminoalcohol, a glycol or polyol, and trimellitic anhydride;
• or a reaction product of a bis(anhydride) and two molar equivalents of an aliphatic aminoalcohol;
• the polyester polyol having a hydroxyl value within the range of 150 to 800 mg KOH/g and an acid value within the range of 0.1 to 5 mg KOH/g;
• (c) water;
• (d) a surfactant;
• (e) a urethane catalyst, an isocyanurate catalyst, or both;
• (f) optionally, a blowing agent;
• and
• (g) optionally, a flame retardant.