US Patent 7942275 Expanded PFTE membrane and method of making

A method of making a porous membrane includes, in an exemplary embodiment, mixing together a first fine powder PTFE resin and a second fine powder PTFE resin to form a PTFE resin mixture. The first PTFE resin having a characteristic of forming more and longer fibrils than fibrils formed from the second PTFE resin. The second PTFE resin having a characteristic of forming thicker nodes than nodes formed from the first PTFE resin. The method also includes forming a preform from the PTFE resin mixture, extruding the preform into a tape, passing the tape through a plurality of opposing calender rolls to form a calendered tape having a thickness of about 380 μm to about 1200 μm, and stretching the calendered tape in the transverse direction to form a porous membrane having a plurality of nodes and fibrils and having a thickness of at least about 100 μm.