Ventilated thermoplastic polymer foam filter rods

Claim 1

In a process for the preparation of a plasticized, open cell, foamed cellulose acetate filter rod having a high density substantially water impervious skin structure substantially covering the periphery thereof, said process comprising melt extruding to an eliptical or circular orifice of a screw extruder, a mixture of cellulose acetate, a non toxic taste compatible thermally stable plasticizer having a boiling point of not less than 180°C., a blowing agent which is at least a partial solvent for cellulose acetate, a non toxic, taste compatible, thermally stable plasticizer having a boiling point of not less than 180°C., a flowing agent which is at least a partial solvent for cellulose acetate and a nucleating agent, said melt being substantially free of entrained air, and the extrusion pressure profile being such as to prevent said blowing agent from backing up in said screw extruder, the steps comprising rendering said skin structure air and water pervious by exposure to a solvent selected from the group consisting of acetone, methylene chloride, triacetin, tetrahydrofuran, methyl ethyl ketone, diethylketone, dioxane, alkyl celluloses, low boiling organic acids, low boiling phenols and low boiling aldehydes, the porosity being sufficient to produce a rod ventilation of greater than 50 percent.