Process for producing cigarette filters

In a process wherein a cigarette filter element comprised of a substantially open-cell foamed thermoplastic polypropylene resin is produced by the following steps taken in sequence:

• a. providing a foamable mixture comprised of a thermoplastic polypropylene resin, a volatile blowing agent and a nucleating agent;
• b. extruding said mixture through a die under controlled temperature conditions to produce a strand of foamed polypropylene resin wherein the melt temperature of the resin is maintained in the range of from 160.degree. to 180.degree. C and the temperature within 5 millimeters of the die is maintained in the range of from 155.degree. to 165.degree. C;
• c. passing said strand of foamed resin through sizing means to effect a cross-sectional diameter corresponding to that of a cigarette;
• and
• d. cutting the sized strand into individual filter elements;
• the improvement which comprises:
• i. employing a primary nucleating agent together with a synergist therefor in said foamable mixture, said primary nucleating agent being a particulate compound of a metal selected from Groups IIA, IIIA or IVA of the Mendeleef periodic table of elements and having a particle size in the range of from about 0.5 to 25 microns, and wherein said synergist is an organic compound having a hydrophilic group and a hydrophobic group wherein the hydrophilic group is ionic and the hydrophobic group is a hydrocarbon or a hydrocarbon having one or more hydrogen atoms substituted by a hydroxyl or halogen, said primary nucleating agent being present in said foamable mixture in an amount of from about 0.1 to 40 percent by weight, based on the weight of said polypropylene resin, and said synergist being present in an amount of from about 0.01 to 5.0 percent by weight, based on the weight of said primary nucleating agent;
• and
• ii. controlling the temperature of said extrusion such that the melt temperature of said polypropylene resin is maintained in the range of from about 155.degree. to 180.degree. C and the temperature within 5 millimeters of the die is maintained in the range of from about 150.degree. to 180.degree. C.