Importance

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Abstract

The present invention relates to a process for producing a biaxially oriented polyester film. The process is carried out using the following steps: (a) stretching an amorphous film substantially comprising polyethylene terephthalate 1.2 to 4.0 times in the machine direction by single-stage or multi-stage process to obtain a film having an index of birefringence of 1.times.10.sup.-3 to 2.5.times.10.sup.-2 ; (b) stretching the thus obtained film 1.1 to 3.5 times in the machine direction by single-stage or multi-stage process of 3.0.times.10.sup.-2 to 8.0.times.10.sup.-2 without cooling the film to the glass transition temperature thereof or below; (c) cooling the thus obtained film to the glass transition temperature thereof or below; (d) heating the thus obtained film to the glass transition temperature or higher and stretching the film in the machine direction by a single-stage or multi-stage process at a stretching ratio which brings the overall stretching ratio of 4.0 to 9.0 times; and (e) stretching the thus obtained film at least 3.2 times in the transverse direction. According to the method of the present invention, a biaxially oriented polyester film which is excellent in thickness uniformity and flat and slippery properties can be easily produced and high-speed production thereof is possible, therefore, the present invention has a great industrial value.

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First Claim