| 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. A biaxially oriented polyester film characterized in that 50 or more layers of polyester A and polyester B are laminated in the thickness direction and satisfy the following requirements. (1) A total of 55 mol% or more of glycol units in the film is a structural unit derived from 1,4-butanediol, and 55 mol% or more of the total dicarboxylic acid units is a structural unit derived from terephthalic acid. (2) 40 mol% or more of glycol units of polyester A and polyester B are structural units derived from 1,4-butanediol, and 40 mol% or more of dicarboxylic acid units are structural units derived from terephthalic acid. (3) All reflectivities in the wavelength region of 350 nm to 2100 nm are less than 25%. [Selection] Figure 1 |