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Oriented porous polypropylene films

  • US 4,975,469 A
  • Filed: 03/20/1989
  • Issued: 12/04/1990
  • Est. Priority Date: 03/20/1989
  • Status: Expired due to Term
First Claim
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1. An oriented polymeric porous film with polygonal cells having average diameters of about 5 to about 30 microns and interconnecting pores between the cells having average diameters of about 0.2 to about 20 microns prepared from a polypropylene-based resin having at least 40% crystallinity by a process comprising the steps of:

  • (a) dispersing in said polypropylene-based resin a nucleating agent capable of producing beta-spherulites;

    (b) extruding the nucleating agent-containing polypropylene-based resin into a film;

    (c) cooling said film below the crystallization temperature of said polypropylene-based resin to form at least 20 weight percent of beta-spherulites in the film;

    (d) extracting selectively said beta-spherulites in an amount corresponding to at least 15 weight percent of the polypropylene-based resin from the cooled film with an extraction solvent to form a porous film; and

    (e) orienting the porous film by heating the porous film at a temperature of about 115°

    to about 135°

    C. for a time period of about 2 to about 20 seconds and stretching the heated porous film in at least one direction at a stretch ratio of about 1.5 to about 7.5 to form said oriented polymeric porous film.

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