Patent ID: 7976751
Filing Date: 2011-07-12
Classification: B29C,B29K,C08J,C09K,Y10T

Abstract:
1. A process for producing an expanded porous polytetrafluoroethylene film having a microstructure composed of fine fibrils and nodes connected by the fibrils and having a porosity of 40 to 75%, the process comprising the following steps 1 to 6: (1) an extrusion step 1 of extruding a mixture of unsintered polytetrafluoroethylene powder and a lubricant to prepare an extrudate in the form of a sheet or rod; (2) a rolling step 2 of rolling the extrudate to prepare a rolled sheet; (3) a stretching step 3 of biaxially stretching the rolled sheet in lengthwise and crosswise directions at a total draw ratio being 20 times to 40 times to prepare an expanded porous polytetrafluoroethylene film (A); (4) a sintering step 4 of heating the expanded porous polytetrafluoroethylene film (A) having a porosity of at least 66% to a temperature not lower than the melting point of polytetrafluoroethylene in a state fixed so as not to shrink the film to sinter the film; (5) a cooling step 5 of cooling the sintered expanded porous polytetrafluoroethylene film (A) to room temperature of 10° C. to 30° C.; and (6) a re-rolling step 6 of compressing the cooled expanded porous polytetrafluoroethylene film (A) in a thickness-wise direction of the film at a compression ratio of 1.1 to 4.0 to form an expanded porous polytetrafluoroethylene film (B) having a porosity of 40 to 75% and elastic recovery property in the thickness-wise direction of the film.