(JP7309965-A)      A porous biaxially oriented film comprises a copolymer of ethylene and a 4-8 C -olefin. The copolymer has 1.0-7.5 comonomer residues per 1000 C atoms and has an intrinsic viscosity of 3.5-10.0 dl/g. The film has a porous or microporous structure in which microfibrils of the copolymer are uniformly dispersed. The film pref. loses its air permeability due to loss of porosity when cooled to room temp. after heat treatment at 160 C but still retains the structure based on the microfibrils. The film breaks up at a temp. of not less than 170 C, after having lost its gas permeability.    USE -   The film is used as a battery separator and has increased resistance at high temp., thus inhibiting overheating when an abnormal current flows from the battery due to, e.g. short circuit.    ADVANTAGE -   The film loses its gas permeability at a temp. of 150 C or below, cf. 180 C for known battery separators. The film has a tensile strength of at least 900 kg/cm2 in all directions and a gas permeability of 50-1000 s/100 cm3.    (US5948519-A)      A porous biaxially oriented film comprises a copolymer of ethylene and a 4-8 C -olefin. The copolymer has 1.0-7.5 comonomer residues per 1000 C atoms and has an intrinsic viscosity of 3.5-10.0 dl/g. The film has a porous or microporous structure in which microfibrils of the copolymer are uniformly dispersed. The film pref. loses its air permeability due to loss of porosity when cooled to room temp. after heat treatment at 160 C but still retains the structure based on the microfibrils. The film breaks up at a temp. of not less than 170 C, after having lost its gas permeability.    USE -   The film is used as a battery separator and has increased resistance at high temp., thus inhibiting overheating when an abnormal current flows from the battery due to, e.g. short circuit.    ADVANTAGE -   The film loses its gas permeability at a temp. of 150 C or below, cf. 180 C for known battery separators. The film has a tensile strength of at least 900 kg/cm2 in all directions and a gas permeability of 50-1000 s/100 cm3.