FI771551

A polypropylene film which is useful for electrical devices is produced by biaxially stretching an unstretched polypropylene molded article having, on at least one side, a surface layer having beta -form crystals, of mean diameter over 7 mu m, under such conditions that the stretching temperature at the stretch initiation point is in the range of 145 DEG to 176 DEG C., and the stretch ratio in one direction is less than a factor of 8. The resulting stretched polypropylene film is characterized by excellent mechanical properties, a very dense inner structure, and high air-tightness. The surface of the film has an efficiently roughened, fine, uneven structure containing crater-like patterns or an isotropic network structure and an anisotropic network structure, which is aligned in the extrusion direction. Therefore, the polypropylene film of the present invention can be readily impregnated with insulating oils, and hence can be used in various electrical devices, such as capacitors, electric cables, transformers and the like.