PROCESS FOR PRODUCTION OF POLYTETRAFLUOROETHYLENE POROUS MEMBRANE, FILTER MEDIUM AND FILTER UNIT

摘要

A process for producing a polytetrafluoroethylene (PTFE) porous membrane which exhibits both a high collection efficiency and a low pressure loss though it has a mean pore diameter larger than those of conventional PTFE porous membranes and a membrane thickness greater than those thereof, that is, a process which comprises stretching an unbaked PTFE sheet 5- to 30-fold in a prescribed direction at a temperature of the melting point of PTFE or above, further stretching the resulting sheet 5- to 40-fold in a direction different from the above prescribed direction at a temperature lower than the above melting point, and then heating the sheet at a temperature of the above melting point or above.