Abstract: Disclosed herein is a polyester composition comprising 0.001-5% by weight of particles which have an average particle size of 0.05-5 micrometers, are mainly composed of calcium carbonate and contain 0.001-5% by weight of at least one compound selected from the group consisting of magnesium compounds, strontium compounds and barium compounds.Also disclosed herein is a process for preparing a polyester film by the electrostatic cooling method, which process is characterized by using the polyester composition of the present invention.

Abstract: Disclosed herein is a biaxially oriented polyethylene naphthalate film which shows the haze-increasing rate, represented by the formula:Haze-increasing rate (%)=[(H.sub.2 -H.sub.1)/H.sub.1 .times.100wherein H.sub.1 is the haze before subjecting the film to heat treatment and H.sub.2 is the haze after subjecting the film to heat treatment, of not more than 20% and both the heat shrinkage in the longitudinal direction and the heat shrinkage in the transversal direction of not more than 0.5% when subjecting the film to heat treatment for 2 hours at 150.degree. C.

Abstract: A polyester film suitable for use as a substrate for magnetic recording media, characterized in that said polyester film is a biaxially oriented polyester film having an average refractive index n and a planar orientation .DELTA.P, which respectively satisfy the following equations [I] and [II]:n.gtoreq.1.600 [I].DELTA.P.ltoreq.(1.43.multidot.n)- 2.128 [II]and the surface roughness of the film is 0.015 .mu.m or lower.

Abstract: 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):5<A.ltoreq.2500 (1)and wherein the proportion of the number of the roughness unit to the total number of the protrusions D (%) satisfies the relation (2):0.1.ltoreq.B.ltoreq.50 (2)and wherein the center line average roughness R.sub.2 (.mu.m) and the number of secondary interference fringes measured by the multiple interference method, H.sub.2, (number/mm.sup.2), satisfies the relation (3):R.sub.a .gtoreq.2.times.10.sup.-4.H.sub.2 +0.08 (3)wherein H.sub.

Abstract: 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:0.155.ltoreq..DELTA.P.ltoreq.0.165 (1)1.605.ltoreq.n.ltoreq.1.610 (2).DELTA.n.ltoreq.0.02 (3)7.0.ltoreq.X.sub.I .ltoreq.11.0 (4)0.55.ltoreq.[.eta.].sub.F .ltoreq.0.65 (5) ##EQU1##.DELTA.H.ltoreq.0.5 (7)wherein .DELTA.P, n, .DELTA.n, X.sub.I, [.eta.].sub.F and .DELTA.H ar respectively degree of planar orientation, average refractive index, birefringence, % ratio of the peak value of the (110) face measured by X-ray diffractometry to the peak value of the (100) face, intrinsic viscosity and surface haze of the biaxially stretched film.

Abstract: A process for preparing high tensile strength polyester film especially suitable for manufacturing magnetic recording tapes is disclosed. A raw polyester film is first stretched longitudinally so as to have an index of birefringence of 0.015-0.055, is then further stretched in the same direction at a temperature between 95.degree. C. and 150.degree. C. while the index of birefringence of the film is prevented from exceeding 0.080, is then stretched transversely to 2.5-5 times its width and is further stretched longitudinally and/or transversely to 1.1 times or greater. The product has high tensile strength as well as excellent flatness and slipperiness properties.

Abstract: A process for preparing biaxially stretched polyester films comprising stretching an unstretched polyester film longitudinally so that the .DELTA.n value becomes 0.030-0.055, further stretching the film in the same direction so that the .DELTA.n value does not exceed 0.08 and the n value becomes 1.600 or less, and thereafter stretching the film transversely. The films prepared by the process of this invention are flat and slippery, less breakable and less uneven in thickness, and are especially suitable as the base film for video recording tapes.

Abstract: There is disclosed a polyester film containing from 0.001% to 4% by weight fine particles of a crosslinked polymer having an average diameter of 0.1 to 5.mu., said fine particles being derived by pulverization from a crosslinked polymer having a specific surface area of at least 1 m.sup.2 /g and a pore volume of at least 0.1 ml/g. The films of the present invention are useful as high grade films and afford many advantages in various applications, for example, as magnetic tapes, capacitors, photography, printing plate making, releasing and the like.