Method of forming thermoplastic foams using cell size enlarging agents

A process for making a closed-cell, alkenyl aromatic polymer foam structure having enlarged average cell size, comprising:

• a) heating an alkenyl aromatic polymer material comprising greater than 50 percent by weight alkenyl aromatic monomeric units to form a melt polymer material;
• b) incorporating into the melt polymer material an amount of a substantially non-waxy cell size enlarging agent sufficient to enlarge the average cell size of the foam structure by about 20 percent or more over a corresponding foam structure made without the enlarging agent and wherein the enlarging agent is incorporated in a quantity such that the foam structure has physical properties substantially similar in degree as the corresponding foam structure, the cell size enlarging agent being selected from the group consisting of salts of n-tallow beta amino dipropionate, amine oxides, imidazoline, fatty acid alkanolamides of C.sub.12-18, n-alkyl trimethyl ammonium ohloride, ethoxylated linear alcohols, dioctyl ester sodium sulfosuccinic acid, polyoxy-ethylene sorbitan monopalmitate, diglycol laurate, flourochemical surfactants, coco betaine and silicone compounds;
• c) incorporating a blowing agent into the melt polymer material at an elevated pressure to form a foamable gel;
• d) cooling the foamable gel to an optimum foaming temperature;
• and
• e) extruding the foamable gel through a die into a zone of reduced pressure to form the foam structure.