abstract |
The present invention provides a film having high uniformity of thickness, porosity and air permeability. A melt of a polypropylene resin composition containing a β crystal nucleating agent is extruded from a T die and cooled on a chill roll. A sequential biaxial stretching method in which the resulting raw sheet is longitudinally stretched and then laterally stretched, and the degree of orientation of β crystals of the longitudinally stretched sheet is 0.3 by the following method (I) and / or (II). It is a biaxially stretched film obtained by annealing, if necessary, and then transversely stretching: Method (I): Melting a polypropylene resin composition containing needle crystals of a specific β crystal nucleating agent at a temperature not lower than the melting point of the polypropylene resin and lower than the melting temperature of the β crystal nucleating agent in the polypropylene resin melt. And extrude from the T die at the same temperature. (II) Method: The neck-in rate during longitudinal stretching is 25 to 55%. [Selection figure] None |