Diafoil Company, Ltd.

Masumi Koizumi

Kichinojo Tomitaka

Shigeo Utsumi

Tomoyuki Kotani

A biaxially oriented polyester film simultaneously satisfying the following formulas (I) to (III):

Low-shrinkage polyester film and preparation thereof

The present invention provide a biaxially oriented polyester film for use in a floppy disc, which has a coefficient of thermal expansion (.alpha..sub.T) in film plane of (1.0.+-.0.5).times.10.sup.-5 mm/mm/.degree.C. and a heat shrinkage of not higher than 0.09% after treatment the film for 72 hours under conditions of a temperature of 60.degree. C. and a humidity of 80% RH in any directions in film plane, and simultaneously satisfies the following equations (1) to (5):

Biaxially oriented polyester film for use in floppy disc

A biaxially oriented polyester film for floppy disks which has an in-plane thermal expansion coefficient (.alpha..sub..gamma.) of (1.7.+-.0.8).times.10.sup.-5 /.degree.C. and a shrinkage of not more than 0.03% after 72-hour treatment under the conditions of 60.degree. C. and 80% RH, and which simultaneously satisfies the following relationships (1) to (6):

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Kawasaki, Japan

Masumi Koizumi