| 摘要 |
PROBLEM TO BE SOLVED: To increase the strength in the longitudinal direction of a film, and also, increase the rupture ductility in the longitudinal direction by setting the film thickness at a specified value or lower, and setting the elastic modulus in the film longitudinal direction at a specified value or higher, and also, setting the product of the elastic modulus in the film longitudinal direction and the rupture elongation at a specified value or higher. SOLUTION: A completely amorphous undrawn film of which the major component is polyethylene terephthalate, is biaxially drawn, and this biaxial orientation polyester film is produced. In this case, the film thickness is set at 10μm or lower, and the elastic modulus in the film longitudinal direction is set at 6.5 GPa or higher. In addition, the product G×L of the elastic modulus G in the film longitudinal direction and a rupture elongation L% is set to be 500 or higher. In order to produce a film of such a large elastic modulus, the drawing in the longitudinal direction is performed by dividing the drawing into a plurality of times, and the film is drawn to approx. 4.5 times or more in the longitudinal direction as a whole body. By this constitution, when being used by making the film thinner in the longitudinally strengthened film, a stable process can be performed without a breakage or a plastic deformation in the film working process.
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