| abstract |
A process for producing a multilayer polytetrafluoroethylene porous membrane comprising at least two layers (4,5,6) having different average pore diameters is disclosed, the process comprising the steps of: filling the inside of a cylinder (12) of an extruding mold distinctively with at least two kinds of polytetrafluoroethylene fine powders with each of which a liquid lubricant has been mixed; subsequently paste-extruding the powders to obtain a multilayer extrudate (15), which is then optionally rolled; and then stretching the extrudate or the rolled extrudate at least monoaxially after the liquid lubricant is removed therefrom or without removing the liquid lubricant. |