| 摘要 |
PROBLEM TO BE SOLVED: To efficiently obtain a biaxially stretched polyester film good in flatness and thermal dimensional stability. SOLUTION: In a method for heat-treating a polyester film in a slackened state, a biaxially stretched polyester film is allowed to reach the maximum temperature Tmax represented by formula (1) Tg+50<=Tmax<=Tg+150 in a state of being suspended within an oven into which no hot air is blown and excessively fed to be slackened and the temperature Tf of the film at the outlet of the oven is set to a range satisfying formula (2) Tg-30<=Tf<=Tg+70. In the formulae (1), (2), Tmax is the maximum temperature ( deg.C) of the film in the oven, Tg is the glass transition temperature ( deg.C) of polyester constituting the biaxially stretched film and Tf is the temperature ( deg.C) of the film at the outlet of the oven. |
