Process for producing a biaxially oriented polyester film and biaxially oriented polyester film

#The present invention relates to a process for the production of a polyester film and more particularly to such a process that can efficiently form, without involving troublesome film breakage, a polyester film having small thickness and great strength in its lengthwise and widthwise directions.

#Further, this invention relates to a biaxially oriented polyester film that is obtained by the above process.

#Simultaneous biaxial orientation is known in which a thermoplastic film is allowed to simultaneously stretch both lengthwise and widthwise while it is being clamped at its end portions with clips. The technique of orientation stated here is disclosed for instance in Japanese Unexamined Patent Application Publication No. 49-40369 and No. 49-105877.

#The present invention is described hereinbelow with reference to examples.

#Pellets of polyethylene terephthalate (PET), of 78° C. in glass transition temperature Tg, 255° C. in melting point Tm and 0.62 dl/g in intrinsic viscosity, were vacuum-dried at 180° C. for 4 hours, supplied into an extruder and melted therein at 280° C., allowed to pass through a filter and then discharged from a T-die, followed by electrostatic casting onto a cooling drum of 25° C. in surface temperature and by successive solidification thereon. Thus, a 100 μm-thick, 95-mm wide cast film was obtained. This cast film was of a substantially unoriented nature with a birefringence of 0.001.

#The resulting film was cut lengthwise in a length of 95 mm and shaped into a square with all sides of 95 mm. With a biaxial stretching tester (of a standard type) manufactured by Toyo Seiki Ltd., the square film was simultaneously biaxially stretched at 95° C. at a stretch ratio of 3.0 times in each of the lengthwise and widthwise directions, followed by monoaxial widthwise stretching at a stretch ratio of 1.6 times at 150° C. and thereafter by monoaxial lengthwise stretching at a stretch ratio of 15 times at 180° C. Heat treatment was then effected at 210° C. The Young's moduli of the polyester film thus biaxially oriented was 6.5 GPa in the lengthwise direction and 6.5 GPa in the widthwise direction. A biaxially oriented polyester film was obtained in which improved strength was well balanced in the lengthwise and widthwise directions.