| abstract |
A porous biaxially oriented film comprising a copolymer of an ethylene having an intrinsic viscosity [ eta ] of from 3.5 to 10.0 dl/g and an alpha -olefin having 4 to 8 carbon atoms, the content of the alpha -olefin being in a number of from 1.0 to 7.5 per 1000 carbon atoms and the film retaining the structure which is based upon the microfibrils when it is heat-treated at 160 DEG C. under a constrained state and is then observed at room temperature, and a separator made of this film for batteries. The porous biaxially oriented film exhibits excellent gas permeating property over a practicable temperature range, exhibits excellent mechanical strength and, particularly, tensile strength, and loses gas permeating property when a maximum practicable temperature is exceeded. The film can be favorably used as a separator for batteries. |