Porous films of polytetrafluoroethylene and process for producing said films

In a process for preparing a porous film of polytetrafluoroethylene having the steps of forming an article from an unsintered polytetrafluoroethylene mixture containing a liquid lubricant;

• stretching the article at least uniaxially while maintaining the temperture of the sheet during stretching at below 327.degree. C, the absolute speed of the sheet being between 1 cm/min and 20 m/min at a stretch ratio of 10 to 100% of the initial length, and then heating the article to a temperature of at least 327.degree. C, the improvement being in said heating step and comprising, providing a moving mirror-surface endless belt positioned over a cooling roll and a heating roll, said rolls rotating whereby each portion of said belt consecutively contacts a portion of the periphery of the heating roll and the cooling, roll, urging the stretched sheet against said endless belt at a first position thereof where said belt is in contact with said cooling roll, peeling off the sheet from said endless belt at a second position thereof where said belt is in contact with said cooling roll, said second position being downstream from said first position in the direction of movement of said belt and being separated therefrom by the path of said belt around a portion of the periphery of said heating roll, whereby said stretched sheet adhered to said belt passes over said heating roll for continuous sintering between adhering to and peeling from said endless belt.