PRODUCTION OF BIAXIALLY STRETCHED POLYESTER FILM

摘要

PURPOSE:To produce a biaxially stretched polyester film having high mechanical strength in both of its longitudinal and lateral directions and excellent in thermal dimensional stability in its lateral direction with excellent process stability. CONSTITUTION:A biaxially stretched polyester film stretched by 1.5-2.8 times in its longitudinal direction and by 3.5-5.5 times in its lateral direction and subsequently thermally fixed within a temp. range of (Tg + 20 deg.C))-(Tg + 80 deg.C) is again longitudinally stretched within a temp. range of (Tg + 20 deg.C)-(Tg + 80 deg.C) at first so that the sum stretching magnification becomes 4-7 times. Subsequently, stretching start temp. is set to (T1 + 20 deg.C))-(Tm-80 deg.C) and the temp. at the time of the completion of stretching is set to (Tm-70 deg.C)-(Tm-30 deg.C) while temp. is raised stepwise or continuously and the film is again laterally stretched so that the sum stretching magnification in the lateral direction becomes 5.5 times or more and further subjected to slackening treatment of 2-8% in the lateral direction at 100-160 deg.C).