Process for producing microporous PTFE

A process for producing a film from a non-fibrillated, microporous PTFE having a substantially cylindrical outer surface, said process comprising the steps off

• providing thermally and compressively untreated PTFE powder the particles of which are of irregular shape and are of a diameter between 0.20 and 60 microns;
• disagglomerating said powder;
• heating the disagglomerated powder to a temperature having a value close to its microcrystalline melting point to form a thermally treated material;
• cooling the thermally treated material to form a cooled material;
• breaking the cooled material up into granules to form an irregular granulated material;
• forming the irregular granulated material into a bar having a substantially cylindrical outer surface and a central longitudinal axis;
• applying a compressive force to the bar radially inwardly of the bar on the substantially cylindrical outer surface thereof and over the full length thereof;
• relieving the bar of said radial compressive force;
• heating the compressed granulated material to a temperature above its microcrystalline melting point to cause unrestrained expansion of the compressed granulated material in all directions to form a further thermally treated material, the extent of the radial compression being such that, on said subsequent heating of the previously compressed granulated material, the material expands predominantly in a direction opposite to that of the compressive force and the granules fuse together to produce a non-fibrillated material the pores of which, within any cylindrical surface contained within the bar and centered about the central longitudinal axis thereof, are of substantially uniform size and are substantially uniformly distributed within that surface;
• cooling the further thermally treated material;
• and
• skiving the cooled thermally treated material to form a film, the plane of the film being perpendicular to the direction of the radial compressive force whereby the pores within any plane substantially perpendicular to the thickness of the film are of substantially uniform size and are substantially uniformly distributed within that plane and all such parallel planes.