BIAXIALLY ORIENTED POLYESTER FILM

摘要

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film suitable for a base film particularly for a heat-sensitive transfer ribbon having excellent productivity while assuring a transfer image having excellent shape reproducibility and gradability. SOLUTION: The biaxially oriented polyester film formed by a simultaneously biaxially stretching method contains a calcium carbonate having 0.1 to 2 wt.% of a mean article size of 0.5 to 4 μm and an aluminum silicate having 0.05 to 1 wt.% of a mean particle size of 0.1 to 2.0 μm. The polyester film further has a refractive index nz of a thickness direction of 1.495 or more and a value obtained by dividing a heat shrinkage factor (HS) (%) of a longitudinal direction at about 200 deg.C by an F-5 value (N/mm<2> ) (F5) of a longitudinal direction measured at the ambient temperature (at about 23 deg.C) of the film of 0.028 or less.