Abstract: A biaxially oriented polyester film simultaneously satisfying the following formulas (I) to (III):.vertline.S.sub.180 -S.sub.120 .vertline..ltoreq.54 (I)T.sub.S.sup.MD .gtoreq.100 (II)Q.sub.120 .ltoreq.0.7 (III)wherein S.sub.180 is shrinking stress (g/mm.sup.2) of the film in the machine direction at 180.degree. C., S.sub.120 is shrinking stress (g/mm.sup.2) of the film in the machine direction at 120.degree. C., T.sub.S.sup.MD is shrinkage initiation temperature (.degree.C.) of the film in the machine direction, and Q.sub.120 is shrinkage (%) of the film in the machine direction after 5 hours' treatment at 120.degree. C., is described. The film has a good dimensional stability due to its low shrinkage, particularly in the machine direction, and substantially free from defect such as wave and wrinkle.It is prepared by biaxially stretching, heat-setting and relaxing an extruded polyester film under particular conditions so as to obtain a biaxially oriented polyester film having these characteristics.

Abstract: The present invention provide a biaxially oriented polyester film for use in a floppy disc, which has a coefficient of thermal expansion (.alpha..sub.T) in film plane of (1.0.+-.0.5).times.10.sup.-5 mm/mm/.degree.C. and a heat shrinkage of not higher than 0.09% after treatment the film for 72 hours under conditions of a temperature of 60.degree. C. and a humidity of 80% RH in any directions in film plane, and simultaneously satisfies the following equations (1) to (5):1.490.ltoreq.n.sub..alpha. .ltoreq.1.500 (1)0.155.ltoreq..DELTA.P.ltoreq.0.168 (2)1.600.ltoreq.n.ltoreq.1.603 (3)40.ltoreq.X.sub.100 .ltoreq.60 (4)1.385.ltoreq..rho..ltoreq.1.400 (5)wherein n.sub..alpha. represents a refractive index in film thickness direction, .DELTA.P represents a degree of planar orientation, n represents an average refractive index, X.sub.100 represents a crystal size (.ANG.) of (110) face, and .rho. represents a density (g/cm.sup.3).

Abstract: A polyester film suitable for a base of high density magnetic recording medium containing barium ferrite as a magnetic material characterized in that the centerline average roughness (Ra) is not more than 0.025 .mu.m and the F.sub.5 value in the longitudinal direction is not less than 14 kg/mm.sup.2 is disclosed wherein the polyester film exhibit excellent better durability when used with barium ferrite as a magnetic material.

Abstract: Disclosed herein is a polyester film containing specific fine spherical silica particles. The film of the present invention is especially suitable as a base film for a magnetic recording medium or dielectric for a capacitor. The films of the present invention have the markedly improved properties such as surface properties, electromagnetic properties, electrical properties, etc. due to the excellent dispersibliity of the particles in the film.

Abstract: A biaxially stretched polyester film which satisfies the following equations (1) to (4):0.155 . .ltoreq..DELTA.P .ltoreq.0.180 (1)1.6050 ..ltoreq.n..ltoreq.1.6085 (2)n .gtoreq.1.622-(0.1.times..DELTA.P) (3).sup..alpha. MD+TD ..ltoreq.3.0 (4)wherein .DELTA.P, n and .sup..alpha. MD+TD respectively respect the degree of surface orientation, the average refractive index and the sum (%) of the shrinkage in the machine and transverse directions after the film is subjected to heat treatment at 105.degree. C. for 30 minutes. The film is not easily curled even if one side thereof is treated with a solvent, and is thus suitable for use involving a step of dipping one surface of a film in a solvent, such as solvent development-type photo resist film and base films for magnetic recording media.

Abstract: A biaxially oriented polyester film for floppy disks which has an in-plane thermal expansion coefficient (.alpha..sub..gamma.) of (1.7.+-.0.8).times.10.sup.-5 /.degree.C. and a shrinkage of not more than 0.03% after 72-hour treatment under the conditions of 60.degree. C. and 80% RH, and which simultaneously satisfies the following relationships (1) to (6):1.492.ltoreq.n.alpha..ltoreq.1.505 (1)0.160.ltoreq..DELTA.P.ltoreq.0.168 (2)1.605.ltoreq.n.ltoreq.1.609 (3)5.ltoreq.X.sub.I .ltoreq.15 (4)60.ltoreq.X.sub.100 .ltoreq.68 (5)1.395.ltoreq..rho..ltoreq.1.405 (6)wherein n.alpha. represents the refractive index of the film in the direction of the thickness, .DELTA.P the degree of planar orientation, n the average refractive index, X.sub.I the X-ray diffraction peak ratio, X.sub.100 the crystal size (A) in the (100) face, and .rho. the density.

Abstract: A biaxially oriented polyester film simultaneously satisfying the following formulas (I) to (III):.vertline.S.sub.180 -S.sub.120 .vertline..ltoreq.54 (I)T.sub.S.sup.MD .gtoreq.100 (II)Q.sub.120 .ltoreq.0.7 (III)wherein S.sub.180 is shrinking stress (g/mm.sup.2) of the film in the machine direction at 180.degree. C., S.sub.120 is shrinking stress (g/mm.sup.2) of the film in the machine direction at 120.degree. C., T.sub.S.sup.MD is shrinkage initiation temperature (.degree.C.) of the film in the machine direction, and Q.sub.120 is shrinkage (%) of the film in the machine direction after 5 hours' treatment at 120.degree. C., is described. The film has a good dimensional stability due to its low shrinkage, particularly in the machine direction, and substantially free from defect such as wave and wrinkle.It is prepared by biaxially stretching, heat-setting and relaxing an extruded polyester film under particular conditions so as to obtain a biaxially oriented polyester film having these characteristics.