Biaxially oriented polyester film

摘要

A biaxially oriented polyester film which has excellent productivity while ensuring a transferred image having excellent shape reproducibility and gradation and is particularly suitable for use as a base film for a heat-sensitive transfer ribbon. This film is formed by simultaneous biaxial orientation and contains 0.1 to 2 wt % of inert inorganic particles having an average particle diameter of 0.5 to 5 mum. This biaxially oriented polyester film has a value of 0.028 or less obtained by dividing the heat shrinkage factor (HS) (%) in a longitudinal direction at 200° C. by the F-5 value (N/mm<2>) (F5) in the longitudinal direction measured at normal temperature (about 23° C.) of the film.