POLYTETRAFLUOROETHYLENE POROUS MEMBRANE, METHOD FOR PRODUCING SAME AND WATERPROOF AIR-PERMEABLE FILTER

摘要

A first unfired sheet composed of a PTFE having a standard specific gravity of not less than 2.16 and a second unfired sheet composed of a PTFE having a standard specific gravity of less than 2.16 are laminated together and a pressure is applied to the thus-obtained laminate, thereby obtaining a pressure-bonded article.  After being stretched in a predetermined direction at a temperature less than the melting point of PTFE, the pressure-bonded article is further stretched in the predetermined direction at a temperature not less than the melting point of PTFE or heated to a temperature not less than the melting point of PTFE.  Then, the pressure-bonded article stretched in the predetermined direction is stretched in the transverse direction perpendicular to the predetermined direction at a temperature less than the melting point of PTFE.