A biaxially oriented polyester film having excellent impact strength and heat resistance and a method for producing same are provided. A method for producing biaxially stretched polyester film comprising Operation A through Operation D, below. Operation A: Operation in which composition comprising polyester resin for which Tc - (Tg + Tm)/2 is not less than 25° C but not greater than 30° C is melt extruded to obtain unstretched polyester resin composition sheet. Operation B: Operation in which the unstretched polyester resin composition sheet obtained at Operation A is heated so as to be within the range not less than Tg + 5 (° C) of the polyester resin but not greater than Tg + 40 (° C) of the polyester resin, and this is stretched so that the stretching ratio in the machine direction is within the range not less than 2x but not greater than 6x to obtain uniaxially oriented polyester resin composition film. Operation C: Operation in which the uniaxially oriented polyester resin composition film obtained at Operation B is heated so as to be within the range not less than Tc - 46 (° C) of the polyester resin but not greater than Tc + 25 (° C) of the polyester resin, and this is stretched so that the stretching ratio in the transverse direction is within the range not less than 4x but not greater than 8x to obtain biaxially oriented polyester resin composition film. Operation D: Operation in which the biaxially oriented polyester resin composition film obtained at Operation C is made to undergo relaxation within the range not less than 3% but not greater than 20% in the transverse direction of the film as it is heated so as to be within the range not less than Tm - 40 (° C) of the polyester resin but not greater than Tm - 5 (° C) of the polyester resin.
本发明提供了一种具有优异抗冲击强度和耐热性的双向拉伸聚酯薄膜及其生产方法。生产双向拉伸聚酯薄膜的方法包括以下操作 A 至操作 D。操作 A:将 Tc - (Tg + Tm)/2 不小于 25 摄氏度但不大于 30 摄氏度的聚酯
树脂组合物熔融挤出,得到未拉伸聚酯
树脂组合物片材。操作 B:将在操作 A 中获得的未拉伸聚酯
树脂组合片材加热,使其处于不小于聚酯
树脂 Tg + 5(摄氏度)但不大于聚酯
树脂 Tg + 40(摄氏度)的范围内,然后进行拉伸,使机器方向上的拉伸比处于不小于 2 倍但不大于 6 倍的范围内,从而获得单轴拉伸聚酯
树脂组合薄膜。操作 C:将在操作 B 中获得的单轴拉伸聚酯
树脂组合物薄膜加热,使其处于不低于聚酯
树脂 Tc - 46 (°C)但不高于聚酯
树脂 Tc + 25 (°C)的范围内,然后进行拉伸,使横向拉伸比处于不小于 4 倍但不大于 8 倍的范围内,从而获得双轴拉伸聚酯
树脂组合物薄膜。操作 D:在操作 C 中得到的双轴取向聚酯
树脂组合物薄膜在加热过程中,使其在横向方向上发生松弛,松弛率在不小于 3%但不大于 20%的范围内,使其在不小于聚酯
树脂的 Tm - 40 (°C)但不大于聚酯
树脂的 Tm - 5 (°C)的范围内。