Method for preparing unsintered polytetrafluoroethylene film and porous film thereof

A method for preparing an unsintered polytetrafluoroethylene film, the method comprising:

• making a mixture by adding a forming aid to polytetrafluoroethylene fine powder obtained by emulsion polymerization of tetrafluorethylene,
• filling the mixture in an extrusion forming die, and extruding the filled mixture from the extrusion forming die to produce an extrusion forming body,
• calendering the extrusion forming body into a film shape with a roll to produce a forming aid-removed film without the forming aid,
• providing two rolls composed of a pair of rollers with one made of metal and the other made of rubber with D type hardness more than 80 as measured by JIS K 6253 coated on a metal shaft core, and
• pinching the forming aid-removed film the rolls at room temperature and compressing the forming aid-removed film with a linear pressure of 50 kg/cm-200 kg/cm so that thickness of the forming aid-removed film is reduced at a reduction rate more than 24.6% and the density is 2.0 g/cm3.