Polytetrafluoroethylene porous membrane, method for producing same, and waterproof air-permeable filter

摘要

A first unsintered sheet made of PTFE having a standard specific gravity of 2.16 or more and a second unsintered sheet made of PTFE having a standard specific gravity of less than 2.16 are laminated, and a pressure is applied to a resulting laminated body so as to obtain a pressure-bonded article. The pressure-bonded article is stretched in a specified direction at a temperature lower than a melting point of PTFE, and then the pressure-bonded article is stretched further in the specified direction at a temperature equal to or higher than the melting point of PTFE or heated to a temperature equal to or higher than the melting point of PTFE. Thereafter, the pressure-bonded article stretched in the specified direction is stretched in a width direction perpendicular to the specified direction at a temperature lower than the melting point of PTFE.