| abstract |
Disclosed in the present invention is a sequential biaxial stretching method in which a melt of the β-crystal nucleating agent-containing polypropylene resin composition is extruded from a T die, longitudinally stretched a master sheet obtained by cooling on a cooling roll, and then laterally stretched. It is a biaxially stretched film obtained by (I) method and / or (II) method making β-crystal orientation degree of a longitudinal stretch sheet less than 0.3, annealing as needed, and continuing to transversely stretch:n n n (I) method: The polypropylene resin composition containing the needle crystal of the specific (beta) tablet nucleating agent is melt | dissolved at the temperature below the melting temperature with respect to the polypropylene resin melt of (beta) tablet nucleating agent above the melting point of polypropylene resin, Extrude from the T die at the same temperature.n n n (II) Method: The necking ratio at the time of longitudinal stretch is 25 to 55%.n n n The resulting porous polypropylene film has good fracture resistance at the time of manufacture, is excellent in thickness uniformity, high in porosity and air permeability, and is useful for use in battery separators. |