Patent ID: 7964265

Claim:
A biaxially oriented polyester film comprising at least three polyester resin layers A1/B/A2, and having a melting point in the range of 246 to 270° C., wherein a major component of the layers A1 and A2 is polyethylene terephthalate and the stress at 100% elongation in the longitudinal direction and width direction of the film at 150° C. and 200° C. meets the following equations 1 and 2: 2≦( F 100 MD +F 100 TD )≦100 (1) 1≦F100 TD ≦60 (2), where F100 MD is the stress at 100% elongation in the longitudinal direction (MPa) of the film, F100 TD is the stress at 100% elongation in the width direction (MPa) of the film and wherein ethylene glycol residue, 1,4-butanediol residue and other glycol residue account for 60 to 90 mol %, 15 to 30 mol % and 1 to 5 mol %, respectively, of the glycol residue of the polyester resin that constitute the layer B, and terephthalic acid residue accounts for 95 mol % or more of the dicarboxylic acid residue of the polyester resin that constitutes the layer B.