Biaxially oriented polyester film for magnetic recording medium

A biaxially oriented polyester film for a magnetic recording medium, the polyester film comprising:

• (a) synthetic calcium carbonate particles having properties represented by the following formulas (1) to (4) and (13):
• 0.25.ltoreq.d.sub.a .ltoreq.0.75 (1)
• 0.35.ltoreq.W.sub.a.sup.A .ltoreq.1.00 (2)
• 0.5.ltoreq.d.sub.a +0.5W.sub.a.sup.A .ltoreq.0.95 (3)
• 8d.sub.a -W.sub.a.sup.A .gtoreq.1.50 (4)
• d.sub.25 /d.sub.75 .ltoreq.1.8 (13)
• wherein d.sub.a represents an average particle diameter (in .mu.m) of the synthetic calcium carbonate particles in the polyester film, W.sub.a.sup.A represents the content (in wt %) of the synthetic calcium carbonate particles in the polyester film, d.sub.25 represents the particle diameter at the point of 25% of the total accumulated weight and d.sub.75 represents the particle diameter at the point of 75% of the total accumulated weight, when the particles are arranged in order of weight, wherein:
• said synthetic calcium carbonate particles are a mixture of at least two kinds of synthetic calcium carbonate particles each of which have different average particle diameters, wherein the average particle diameter of the kind of synthetic calcium carbonate particles having an average particle diameter larger than d.sub.a is from 1.5.times.d.sub.a to 3.0.times.d.sub.a, and wherein the content of the kind of synthetic calcium carbonate particles having an average particle diameter larger than d.sub.a is from 0.005.times.W.sub.a.sup.A to 0.2.times.W.sub.a.sup.A, and
• said synthetic calcium carbonate particles are produced by surface-treating synthetic calcium carbonate particles with a polycarboxylic acid and treating the surface-treated synthetic calcium carbonate particles with a phosphorus compound represented by the following general formula (12):
• ##STR4## wherein R represents a hydrocarbon group having from 1 to 10 carbon atoms, l is 1 or 2, m is 1 or 2, n is 1 or 2, and the sum of l and m is 3,
• wherein the content of said phosphorus compound is from 0.05 to 10.0 mol %, based on said synthetic calcium carbonate particles;
• and
• (b) aluminum oxide particles having properties represented by the following formulas (5) and (6):
• 0. 005.ltoreq.d.sub.b .ltoreq.0.50 (5)
• 0.15.ltoreq.W.sub.b.sup.A .ltoreq.1.00 (6)
• wherein d.sub.b represents an average particle diameter (in .mu.m) of the aluminum oxide particles in the polyester film, and W.sub.b.sup.A represents the content (in wt %) of the aluminum oxide particles in the polyester film,
• the polyester film having simultaneously properties represented by the following formulas (7) to (9):
• 0.009.ltoreq.Ra.sup.A .ltoreq.0.020 (7)
• 7.0.ltoreq.Rz.sup.A /Ra.sup.A .ltoreq.15 (8)
• 0.80.ltoreq..mu.d.sup.50.spsp.A /.mu.d.sup.1.spsp.A .ltoreq.1.6(9)
• wherein Ra.sup.A and Rz.sup.A represent an average center line roughness (.mu.m) of the exposed surface of the polyester film and a surface roughness (.mu.m) at 10 points on the exposed surface of the polyester film, respectively, and .mu.d.sup.50.spsp.A /.mu.d.sup.1.spsp.A represents the ratio of a dynamic friction coefficient of the exposed surface of the polyester film at the fiftieth running to a dynamic friction coefficient of the exposed surface of the polyester film at the first running, when the polyester film is run repeatedly.