PROCESS FOR THE PREPARATION OF BIAXIALLY ORIENTED POLYESTER FILM

摘要

The present invention relates to an improved process for preparing a biaxially oriented polyester film having excellent surface smoothness and uniform thickness and by drawing an amorphous polyester sheet in the longitudinal direction and then in the transverse direction, wherein the longitudinal drawing process is conducted in three successive steps; (i) a diagonal drawing at a total drawing zone length ranging from 10 to 500 mm, 100 to 140 DEG C and a drawing ratio ranging from 1.1 to 2.0; (ii) a parallel drawing at 100 to 140 DEG C and a drawing ratio ranging from 1.2 to 3.0; and (iii) a diagonal drawing 100 to 140 DEG C and a drawing ratio ranging from 1.5 to 3.0.