Abstract: The present invention is directed to a process for producing a biaxially oriented polyester film which comprises: subjecting a polyester film to simultaneous biaxial stretching in a stretching apparatus constructed to simultaneously stretch the film in the lengthwise and widthwise directions thereof; and subsequently subjecting the film to successive biaxial stretching in the lengthwise and widthwise directions thereof in the same apparatus as mentioned above. With this process, a film can be efficiently formed, without film breaking, which film is small in thickness and greatly strong in its lengthwise and widthwise directions.

Abstract: The present invention is directed to a process for producing a biaxially oriented polyester film which comprises: subjecting a polyester film to simultaneous biaxial stretching in a stretching apparatus constructed to simultaneously stretch the film in the lengthwise and widthwise directions thereof: and subsequently subjecting the film to successive biaxial stretching in the lengthwise and widthwise directions thereof in the same apparatus as mentioned above. With this process, a film can be efficiently formed, without film breaking, which film is small in thickness and greatly strong in its lengthwise and widthwise directions.

Abstract: A biaxially oriented polyester film having excellent scratch resistance and chipping resistance by the inclusion of particles (A) having an average primary particle diameter of 5-300 nm, an average degree of aggregation of 5-100 and a Mohs' hardness of 6-10 by the content of 0.01-5 wt. %. The scratch resistance and chipping resistance of the film can be further increased by the inclusion of particles (B) having an average particle diameter of 0.3-3 .mu.m, a Mohs' hardness smaller than that of particles (A) and/or non-incorporated particles.

Abstract: A biaxially oriented polyester film having excellent scratch resistance and chipping resistance by the inclusion of particles (A) having an average primary particle diameter of 5-300 nm, an average degree of aggregation of 5-100 and a Mohs' hardness of 6-10 by the content of 0.01-5 wt. %. The scratch resistance and chipping resistance of the film can be further increased by the inclusion of particles (B) having an average particle diameter of 0.3-3 .mu.m, a Mohs' hardness smaller than that of particles (A) and/or non-incorporated particles.

Abstract: A biaxially oriented polyester film having excellent scratch resistance and chipping resistance by the inclusion of particles (A) having an average primary particle diameter of 5-300 nm, an average degree of aggregation of 5-100 and a Mohs' hardness of 6-10 by the content of 0.01-5 wt. %. The scratch resistance and chipping resistance of the film can be further increased by the inclusion of particles (B) having an average particle diameter of 0.3-3 .mu.m, a Mohs' hardness smaller than that of particles (A) and/or non-incorporated particles.

Abstract: A biaxially oriented polyester film suited as a substrate for magnetic recording media. The film comprises a polyester and inert particles and has at least one surface with protrusions formed by the existence of the inert particles. The surface has a parameter of surface topography .sqroot.(.beta./.sigma.) of 0.1 to 0.7. The average distance between adjacent protrusions is not more than 20 um and the coefficient of friction of the surface and the parameter of surface topography satisfies the following equations (1) and (2): ##EQU1## wherein y is the coefficient of friction, .beta. is mean flatness of protrusions and .sigma. is standard deviation of height distribution of protrusions.