BIAXIALLY ORIENTED POLYESTER FILM AND METHOD FOR PREPARING IT

摘要

PROBLEM TO BE SOLVED: To provide a film for flexible circuit base which is excellent in dimensional stability and flatness in use at higher temperatures and is inexpensive. SOLUTION: A method for preparing a biaxially oriented polyester film by performing heat relaxation treatment of at least once of a biaxially oriented film whose heat shrinkage heat-treated at 150 deg.C for 30 min simultaneously satisfies the below described equation (1) and (2) 0.01<=MD<=0.4 (1) -0.1<=TD<=0.1 (2), wherein MD is a heat shrinkage (%) of the film in the machine direction before heat relaxation treatment and TD is a heat shrinkage (%) of the film in the transverse direction before heat relaxation treatment.