| 摘要 |
PROBLEM TO BE SOLVED: To obtain a biaxially oriented polyester film which has a high accuracy in laying one on top of another and is excellent in clarity, slipperiness, and windability by specifying the heat shrinkage factor. SOLUTION: When thermally treated at 120 deg.C for 20 sec, this film exhibits heat shrinkage factors simultaneously satisfying the relations represented by formulas: 0.001<=SMD<=0.1, -0.1<=STD<=0.1, -0.05<=S45<=0.05, -0.05<=S135<=0.05, and|S45-S135|<=0.05. In the formulas, SMD is the heat shrinkage factor (%) in the machine direction of the film; and S45 and S135 are heat shrinkage factors (%) at the angles of 45 deg. and 135 deg., respectively, provided the angles of transverse direction and machine direction are 0 deg. and 90 deg., respectively. This polyester film is prepared by drying a polyester containing fine lubricant particles, melt extruding the dried polyester, stretching the resultant unstretched film 2.5-4 times at 70-130 deg.C in the machine direction, stretching 3-6 times at 80-130 deg.C in the transverse direction, thermally setting at 190-240 deg.C, subjecting to a 0.5-5% relaxation treatment at 70-200 deg.C, and releasing from a clamp device when the film temperature reaches 100 deg.C or lower.
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