| 摘要 |
The present invention provide a biaxially oriented polyester film for use in a floppy disc, which has a coefficient of thermal expansion ( alpha T) in film plane of (1.0+/-0.5)x10-5 mm/mm/ DEG 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 DEG C. and a humidity of 80% RH in any directions in film plane, and simultaneously satisfies the following equations (1) to (5): 1.490</=n alpha </=1.500 (1) 0.155</= DELTA P</=0.168 (2) 1.600</=n</=1.603 (3) 40</=X100</=60 (4) 1.385</= rho </=1.400 (5) wherein n alpha represents a refractive index in film thickness direction, DELTA P represents a degree of planar orientation, &upbar& n represents an average refractive index, X100 represents a crystal size ( ANGSTROM ) of (110) face, and rho represents a density (g/cm3). The biaxially oriented polyester film of the present invention has excellent dimensional stability and flatness and is suitable for use as the base film of floppy disc.
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