THIN FILM MATERIAL WITH HOLE OF ASYMMETRICAL DIAMETER AND METHOD OF PRODUCING SAME

摘要

Porous film materials composed of polytetrafluoroethylene having an asymmetric structure are produced by stretching the film using a pair of revolving rolls which are heated to a temperature lower than the melting point of the film. Stretching is produced by revolving the two rolls at different speeds which also tends to produce a compressive force in the thickness of the film. The rolls are heated to different temperatures to produce a temperature difference of preferably 50 DEG C. or more. The combination of the compressive force and the temperature gradient in the thickness of the film is believed to produce the asymmetric structure in the resulting porous film materials which are characterized by a fiber structure in the surface which is different from that in the back.