Diafoil Company, Ltd.

Daifoil Company, Limited

Kichinojyo Tomitaka

Shigeo Utsumi

Yujiro Fukuda

Tateo Kanesaki

Toshifumi Takisawa

Takatoshi Miki

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.

Process for producing biaxially oriented polyester film

Disclosed herein is a biaxially oriented polyester film for magnetic recording media, containing uniformly carbon black particles and having the following specific properties: ##EQU1## wherein T.sub.900 represents a light transmittance of the film at a wavelength of 900 nm, Ra represents an average surface roughness of the film n.sub.1 represents the number of protuberances of 0.81 to 1.08 .mu.m in height existing on the surface of the film per film area of 25 cm.sup.2, and n.sub.2 represents the number of protuberances of more than 1.08 .mu.m in height existing on the surface of the film per film area of 25 cm.sup.2.

Polyester film for magnetic recording media

A polyester film for magnetic recording media having excellent flatness, slipperiness and adhesiveness of the magnetic layer thereto, said film having minute protrusions on the surface thereof including roughness units each of which consists of a minute protrusion and a recess therearound having a longer diameter of at least 3 .mu.m, wherein the number of the roughness units per square mm, A, (number/mm.sup.2), satisfies the relation (1):

There is disclosed a transparent slippery biaxially stretched polyester film characterized by satisfying the relations represented by the following formulas after being biaxially stretched and heat-set:

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Yokohama, Japan

Kichinojyo Tomitaka