| abstract |
[PROBLEMS] To be used suitably for battery exterior applications, particularly for battery exterior applications that can be suitably used for deep drawing shapes of lithium ion secondary batteries having electrolyte (solid, liquid), and for various shapes. A biaxially oriented polyester film is provided. SOLUTION: A polyester film in which 50 layers or more of A layers made of polyester A and B layers made of polyester B are alternately laminated in the thickness direction, and polyester A has a heating rate of 20 ° C. in DSC measurement. The tensile stress σ1 when the amount of heat of crystal fusion at 20 min / min is 20 J / g or more and less than 60 J / g, and the elongation rate is 50% in the stress-elongation curve of polyester B, and the yield point The biaxially oriented polyester is characterized in that the stress ratio A (σ1 / σ2), which is the ratio to the tensile stress σ2 at 1.5, is 1.5 or more, the tensile stress σ2 is less than 100 MPa, and the elastic modulus is less than 4000 MPa. the film. [Selection] Figure 1 |