| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a biaxially-oriented polyester film which can be used suitably as an exterior of a battery, particularly as a deep-drawn exterior of a lithium ion secondary battery having an electrolyte (of a solid or liquid state) and can cope with various shapes and consequently used suitably as a package of a medical product.SOLUTION: The biaxially-oriented polyester film comprises a polyester film obtained by stacking an A layer, which comprises polyester A, and a B layer, which comprises polyester B, alternately in the thickness direction by 50 or more layers. The crystal melting calorie of the polyester A is equal to or higher than 20 J/g or lower than 60 J/g when DSC measurement thereof is performed and the temperature rising rate is 20°C/minute. The polyester B has 1.5 or higher tensile stress ratio A (σ1/σ2), the tensile stressσ2 being lower than 100 MPa and the elastic modulus being lower than 4,000 MPa when a stress-elongation curve is obtained in a tensile test thereof and the tensile stress is defined asσ1 when the elongation rate becomes 50% and that is defined asσ2 at the yield point.</p> |
