Polyester film and magnetic recording medium using the same

A polyester film having a thickness of 3 to 8 μm and used for a magnetic recording medium, comprising:

• a base layer portion comprising a polyester resin and a polyimide resin;
• a first laminated portion which is provided on one surface of the base layer portion and which contains 0.05 to 1.5 wt % of particles having a weight average particle diameter of 0.05 to 1 μm and a polyester resin or a material composed of a polyester resin and a polyimide resin, the base layer portion and the laminated portion forming a base film;
• and
• a layer which is provided on at least the surface opposite to the first laminated portion of the base film and which contains a water-soluble polymer and inert particles having a weight average particle diameter of 7 to 25 nm at a content of 3,000,000 to 70,000,000/mm2.

A polyester film having a thickness of 3 to 8 μm and used for a magnetic recording medium, comprising:

• a base layer portion comprising a polyester resin and a polyimide resin;
• a first laminated portion which is provided on one surface of the base layer portion and which contains 0.05 to 1.5 wt % of particles having a weight average particle diameter of 0.05 to 1 μm and a polyester resin or a material composed of a polyester resin and a polyimide resin, the base layer portion and the laminated portion forming a base film;
• and
• a second laminated portion which is provided on the base film at the side opposite to the first laminated portion and which contains 0.1 to 3 wt % of particles having a weight average particle diameter of 10 to 50 nm and a polyester resin or a material composed of a polyester resin and a polyimide resin, whereby a structure having at least three layers is formed.

A method for manufacturing a polyester film comprising:

• a step of filtrating a polymer mixture comprising a polyester resin and a polyimide resin through a fiber sintered stainless steel filter having a cut of 1.2 μm or less in an extruder;
• a step of melt-extruding the polymer mixture through an extruding die for forming an unstretched film by using the extruder;
• a step of stretching the unstretched film in the longitudinal direction at a stretching temperature of Tg+10° C. to Tg+30° C., at a stretching ratio of 2.5 to 4.0, and at a stretching rate of 50,000%/minute to 200,000%/minute in at least two stages;
• a step of stretching the stretched film in the width direction at a stretching temperature of Tg+10° C. to Tg+50° C., at a stretching ratio of 3.0 to 4.5, and at a stretching rate of 2,000%/minute to 10,000%/minute;
• a step of further stretching the stretched film in the longitudinal direction at a stretching temperature of Tg+30° C. to Tg+50° C., and at a stretching ratio of 1.2 to 1.8;
• a step of further stretching the stretched film in the width direction at a stretching temperature of Tg+80° C. to Tg+110° C., and at a stretching ratio of 1.2 to 2.0;
• and
• a step of performing heat treatment at a temperature of Tg+100° C. to Tg+125° C. for 0.2 to 10 seconds.