| 摘要 |
PURPOSE:To make the coefficient of linear expansion with humidity of a film smaller and that in all directions on the surface of the film uniform and at the same time improve the widthwise change of a product film by a method wherein a specified simultaneous biaxial stretching is applied to a polyester film which is subjected to sequential biaxially stretching so as to have a specified birefringence. CONSTITUTION:First, polyethylene terephthalate is melt-extruded at 285 deg.C and then quenched so as to make a substantially amorphous sheet. Secondly, the resultant sheet is stretched lengthwise between a pair of nip rolls and cooling rolls which are located apart from the nip rolls by 2mm by one step, with the draw ratio of 3.2-3.3 and then cooled. Thirdly, the resultant sheet is stretched crosswise by the tentering method at 100 deg.C with the draw ratio of 3.1-3.5 and further continuously held at 100 deg.C for 5sec under the state that the width of the sheet is fixed and then cooled down to normal temperatures. Fourthly, the resultant biaxially stretched film is furthermore stretched with a simultaneous biaxially stretching tenter at 150 deg.C with the draw ratio in each direction of 1.5-1.7 and, after that, heat-set under tension at 200 deg.C. As for the polyethylene terephthalate film which is obtained by simultaneously biaxially stretching and then heat-setting a sequentially biaxially stretched film, smaller coefficient of linear expansion with humidity can be realized and the change in all directions on the surface of a film is realized to be made smaller. |
