Patent ID: 8449660

Claim:
A method for producing a polytetrafluoroethylene porous membrane, comprising the steps of: laminating a first unsintered sheet made of polytetrafluoroethylene having a standard specific gravity of 2.16 or more and a second unsintered sheet made of polytetrafluoroethylene having a standard specific gravity of less than 2.16, and applying a pressure to a resulting laminated body so as to obtain a pressure-bonded article; stretching the pressure-bonded article in a specified direction at a temperature lower than a melting point of polytetrafluoroethylene, and then stretching further the pressure-bonded article in the specified direction at a temperature equal to or higher than the melting point of polytetrafluoroethylene or heating the pressure-bonded article to a temperature equal to or higher than the melting point of polytetrafluoroethylene; and stretching the pressure-bonded article stretched in the specified direction in a width direction perpendicular to the specified direction.