Biaxially oriented polyester film

Abstract

A biaxially oriented polyester film wherein the microscopic Raman crystallization index Ic measured in the film thickness direction is in a range of 8 cm−1 to 15 cm−1 and the difference between the maximum value and the minimum value of Ic is 1 cm−1 or less. Another biaxially oriented polyester film wherein the relative power (I10TD) of spatial frequency (1/mm) measured along the transverse direction of at least one surface of the polyester film having been heat treated at 100° C. for 24 hours is in a range of −25 to 0 dB. These biaxially oriented polyester films are films good in microplanarity even after high-temperature storage or heat loading during processing. These biaxially oriented polyester films are useful as polyester films for use in a magnetic recording material, an electronic material, a stencil film, a sublimatic ribbon and a package material. In particular, these biaxially oriented polyester films are useful when employed as a base film for a high-density magnetic recording medium, especially, a double layer metal coated digital recording medium according to linear recording system.

Dependencies are inferred from the claim text.