| abstract |
A successive biaxial-orientation process which comprises extruding a molten polypropylene resin composition containing a nucleating agent of β-crystal through a T-die, cooling the extruded melt on a chill roll, stretching the obtained raw sheet in the lengthwise direction, and then stretching the resulting sheet in the widthwise direction to thereby obtain a biaxially oriented film, wherein the degree of orientation of β-crystal of the lengthwise stretched sheet is adjusted to less than 0.3 by the following process (I) and/or the process (II) and the resulting sheet is annealed at need and then stretched in the widthwise direction: Process (I) of melting a polypropylene resin composition containing needle crystals of a specified nucleating agent of β-crystal at such a temperature that is the melting point of the polypropylene resin or above and lower than the lowest temperature at which the nucleating agent can be dissolved in the polypropylene resin and then extruding the resulting resin composition through a T-die at the temperature, and Process (II) of adjusting the neck-in in the lengthwise stretching to 25 to 55 %. The obtained porous polypropylene films are excellent in breakage resistance in manufacturing and thickness uniformity, have high void contents and high air permeability, and are suitable for battery separators. |