PREPARATION OF THERMOPLASTIC POLYESTER FILM

摘要

PURPOSE:To obtain a polyester film of which physical properties are uniform in length and width even if at the central part of said film, by successively subjecting the unstretched film to biaxial stretching and multi-stage heat treatment under a specific condition. CONSTITUTION:An amorphous polyester unstretched film is uniaxially stretched 3.0-4.0 times to a longitudinal direction to adjust the crystallization degree thereof to a range of 0.2-0.3 and the resulting uniaxially stretched film is stretched 3.0-4.0 times to a width direction at a glass transition temp. (Tg) or more in a draw ratio 0.83-1.43 times to the stretching draw ratio in the longitudinal direction. After the resulting biaxially stretched film is cooled to Tg or less, it is subjected to first stage heat treatment at a temp. T1 in a range of 200-240 deg.C and succeedingly stretched 1-20% to a width direction at temp. T2 in a second stage treating zone and the treated film is further held at a temp. T3 (T2 and T3 are selected so as to satisfy a condition shown by formula T1>= T2>T3) in a range of 100-200 deg.C by third stage heat treatment.