Abstract: The present invention relates to a resin composition comprising polyethylene terephthalate and polyethylene naphthalate and a process for preparing the composition. The present invention relates also to a bottle produced from the composition and a process for the preparation of the bottle. The resin composition according to the present invention is prepared by melt-kneading (A) polyethylene terephthalate with (B) polyethylene naphthalate in a proportion of the ethylene naphthalate unit to a total of (A) and (B) of from 5 to 15 mole %, with a kneading extruder in a ratio of an extruder output rate to a screw rotation speed of 0.1 to 1.4 kg/hr·rpm. The present composition gives a bottle superior in transparency and in resistance to heat and pressure despite that it has a transesterification of 20% or lower.

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: 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.