Abstract: The present invention provides a polyester film particularly useful as a base film for a magnetic recording medium such as cassette-type magnetic tape of a digital recording mode and a production process thereof. The polyester film is produced by irradiating UV light onto a surface of a film in a non-stretched stage or a stage prior to the completion of stretching to form fine protrusions on the surface. The fine protrusions on the film surface are specified by the ratio of 10-point average roughness Rz to center line average roughness Ra (Rz/Ra) on the surface (less than 20), the difference in concentration of carboxyl groups between the surface layer part and the inside of the film, and the number of fine protrusions.

Abstract: According to the present invention, a biaxially oriented polyester film is provided which comprises at least one film layer composed predominantly of polypropylene terephthalate and has a heat shrinkage of 0.8% or below after standing at 80° C. for 30 minutes. This film is excellent in wear resistance and hence useful particularly for magnetic recording media.

Abstract: A biaxially-oriented polyester film includes at least two layers and at least one outermost layer is composed of a polyester having a rigid amorphous content of about 10 to 55%. The biaxially-oriented polyester film has a longitudinal Young's modulus of at least about 4.5 GPa. The biaxially-oriented polyester film is suitable for high-density magnetic recording media, such as digital video tapes and high-density data recording tapes.