JP2672421B2 - Biaxially oriented polyester film for magnetic recording media - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it is selected from the group consisting of calcium carbonate particles having a specific shape and particle size distribution, aluminum oxide, silicon oxide and titanium oxide. By containing at least one kind of fine particles, and the projections on the film surface formed by these particles are within a specific range, the film has excellent flatness and is easy to handle, coat, and calender. Biaxially oriented polyester film for magnetic recording media with improved processing characteristics such as abrasion resistance, especially useful as a base film for high-grade video tapes with a back coat layer on the side opposite to the magnetic layer Regarding polyester film.

[0002]

2. Description of the Related Art Biaxially oriented polyester film represented by polyethylene terephthalate film is widely used as a base film for magnetic recording media because of its excellent physical and chemical properties.

In a biaxially oriented polyester film for a magnetic recording medium, a protrusion is formed on the surface of the film in order to secure handleability in a series of steps in which the magnetic layer is oriented, dried and the magnetic surface is finished smooth. , And control the size and frequency.

However, when the size and the frequency increase, protrusion of the applied magnetic layer occurs, which causes a problem of deterioration of electromagnetic conversion characteristics. Therefore, the present situation is to provide a magnetic medium compatible with a system having a short recording wavelength such as S-VHS and 8 mm video by using a film having a low production speed and a low slipping property and a winding property. .

Incidentally, a tape for S-VHS or 8 mm video is generally provided with a back coat layer on the side opposite to the magnetic layer. After the tape is formed, the base film itself is slippery and scraped. Is not a problem, and therefore, the slipperiness and scraping property of the base film for use are mainly suitable for the magnetic tape manufacturing process.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic recording medium, especially for magnetic tape applications such as S-VHS and 8 mm video, which can improve productivity in the manufacturing process. It is to provide a biaxially oriented polyester film.

[0007]

The present invention has the following constitution in order to achieve such an object. A biaxially oriented polyester film, wherein the distribution curve of protrusions on the surface of the film (A) has a height X (μm) of 0.02 μm or more and the number of protrusions Y (units / mm ² ) is 50 units / mm. ^In the range of ² or more, the following formula (1) is satisfied, and (B) the number of protrusions of 1 ring or more by the Tl monochromatic light multiple interference reflection microscope is 1/10 cm
Less than ² , and (C) the protrusion has a particle size ratio (major axis / minor axis) of 1.5 to 10 in the film, and an average particle diameter of equivalent area circle of 0.4 to 1.0 μm. Yes and the following formula (2)
0.001 to 0.10% by weight of spindle-shaped calcium carbonate particles having a relative standard deviation of 0.5 or less, a specific surface area of 50 to 120 m ² / g, and a total pore volume of 0.5 to
1.0 ml / g and average particle size 0.02-0.3
0.05 to 1.0 of one or more fine particles selected from the group consisting of 0 μm aluminum oxide, silicon oxide and titanium oxide.
A biaxially oriented polyester film for a magnetic recording medium, wherein the biaxially oriented polyester film is formed by containing the resin in an amount of 10% by weight.

-71.4 X + 5 <log Y <-32.4 X + 5.5 (1) [where X is the distance from the reference level in the height direction (μm)]
And Y is the number of protrusions (pieces / mm ² ) counted when cut at the height X. ]

[0009]

(Equation 2)

The polyester in the present invention is a polyester containing an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. Such polyesters are substantially linear and have film forming properties, especially film forming by melt molding. Examples of the aromatic dicarboxylic acid include terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenoxyethane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid, and anthracene dicarboxylic acid. Can be. As the aliphatic glycol, for example, a polymethylene glycol having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, decamethylene glycol or the like, or an alicyclic group such as cyclohexanedimethanol. Diols and the like can be mentioned.

In the present invention, as the polyester, for example, one having alkylene terephthalate and / or alkylene naphthalate as a main constituent is preferably used.

Among such polyesters, not only polyethylene terephthalate and polyethylene-2,6-naphthalate but, for example, 80 mol% or more of all dicarboxylic acid components are terephthalic acid and / or 2,6.
—A naphthalene dicarboxylic acid, and a copolymer in which 80 mol% or more of all glycol components are ethylene glycol is preferable. At that time, not more than 20 mol% of the total acid component can be the above-mentioned aromatic dicarboxylic acids other than terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and also aliphatic dicarboxylic acids such as adipic acid and sebacic acid. An alicyclic dicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid; Further, 20 mol% or less of the total glycol component may be the above-mentioned glycol other than ethylene glycol, or an aromatic diol such as hydroquinone, resorcin, 2,2-bis (4-hydroxyphenyl) propane, etc .; 1 ，
An aliphatic diol having an aromatic ring such as 4-dihydroxydimethylbenzene; a polyalkylene glycol (polyoxyalkylene glycol) such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like can also be used.

Further, the polyester in the present invention includes:
For example, aromatic oxyacids such as hydroxybenzoic acid, ω-
Also included are those which copolymerize or combine a component derived from an oxycarboxylic acid such as an aliphatic oxyacid such as hydroxycaproic acid at 20 mol% or less based on the total amount of the dicarboxylic acid component and the oxycarboxylic acid component.

Further, in the polyester of the present invention,
An amount in the range that is substantially linear, for example, 2 for the total acid component
Also included are those obtained by copolymerizing a tri- or higher functional polycarboxylic acid or polyhydroxy compound such as trimellitic acid or pentaerythritol in an amount of not more than mol%.

The above polyester is known per se and can be produced by a method known per se.
The above polyester has an intrinsic viscosity of about 0.4 when measured at 35 ° C. as a solution in o-chlorophenol.
Of about 0.9 are preferred.

The biaxially oriented polyester film of the present invention has a large number of fine projections on the film surface, as is clear from the description of the projection distribution which defines the flatness of the film surface.

The large number of fine projections originates from the large number of substantially inert solid microparticles contained according to the invention in dispersed form in the polyester.

The distribution curve of the projections on the surface of the film formed by the particles present in the polyester is the height X
When (μm) is 0.02 μm or more, the number of protrusions Y (pieces / mm ² )
In the range of 50 / mm ² or more, it is important to satisfy the following formula (1).

-71.4 X + 5 <log Y <-32.4 X + 5.5 (1) [where X is the distance in the height direction from the reference level (μm)]
And Y is the number of protrusions (pieces / mm ² ) counted when cut at the height X. ]

If the projection distribution curve deviates from the range of the above formula (1), large projections existing on the film surface for the S-VHS or 8 mm video application will stick out to the magnetic layer, resulting in electromagnetic conversion characteristics. This is not preferable because it causes a decrease, or the magnetic layer becomes too flat and the friction with the head becomes too high, which causes sliding noise to increase.

The number of projections of one ring or more measured by a Tl monochromatic light multiple interference reflection microscope is 1/10 cm.
It is important that it is less than ² . Preferably 0.5 /
It is less than 10 cm ² , more preferably less than 0.2 pieces / 10 cm ² . If the number of protrusions of 1 ring or more becomes 1/10 cm ² or more, the frequency of protrusions forming on the magnetic surface increases when the tape is used for S-VHS or 8 mm video, resulting in a dropout number. Is not preferable because it leads to an increase in

In the present invention, it is important that the projections formed on the surface of the film are composed of specific calcium carbonate particles and one or more fine particles selected from the group consisting of aluminum oxide, silicon oxide and titanium oxide. That is, the particle size ratio (major axis / minor axis) is 1.5 to 10, the average particle diameter of the area circle equivalent diameter is 0.4 to 1.0 μm, and the following formula (2)

[0023]

(Equation 3)

0.001 to 0.10% by weight of spindle-shaped calcium carbonate particles having a relative standard deviation of 0.5 or less, a specific surface area of 50 to 120 m ² / g, and a total pore volume of 0. 5 to 1.0 ml / g and an average particle size of 0.1.
02 to 0.30 μm one or more fine particles selected from the group consisting of aluminum oxide, silicon oxide and titanium oxide.
It is necessary to form projections on the film surface by allowing the presence of 05 to 1.0% by weight in the polyester.

Examples of the method for producing the spindle-shaped calcium carbonate include a carbon dioxide blowing method, a salt metathesis method, and a method for reacting calcium chloride and sodium hydrogen carbonate in the presence of ammonia. For example, a carbon dioxide gas is blown into a calcium hydroxide slurry, a water-soluble polysaccharide or a sulfate is added to the slurry at a carbonation rate of 60 to 70%, and then the carbonation rate is reduced to substantially 100%. However, as long as it is a spindle-shaped calcium carbonate satisfying the above conditions, the production method is not limited. Further, as the crystal form of the spindle-shaped calcium carbonate, calcite,
Examples include aragonite and vaterite. Among these, vaterite calcium carbonate is more preferable.

In the present invention, when the particle diameter ratio of the spindle-shaped calcium carbonate is less than 1.5, the particles do not exhibit stress relaxation in the film stretching direction, the affinity of polyester becomes poor, and the projections formed become sharp. There is a problem that the tape easily comes off when a shocking force such as a die coater is applied during the tape manufacturing process. On the other hand, if the particle size ratio exceeds 10, the slipperiness of the film is deteriorated and shavings are easily generated.

Further, the average particle diameter of the area equivalent circle diameter is 1.
When it exceeds 0 μm, the number of high protrusions of one ring or more increases, resulting in increase of dropout and deterioration of electromagnetic conversion characteristics.
On the other hand, when the average particle diameter of the area equivalent circle diameter is less than 0.4 μm, the magnetic surface becomes too flat when used as a magnetic tape,
Friction with the head becomes too high, which causes sliding noise to increase, which is not preferable.

The relative standard deviation of the particle size is 0.5.
Or less, more preferably 0.4 or less. If the relative standard deviation exceeds 0.5, the heights of the protrusions become uneven, shavings are easily generated, and the electromagnetic conversion characteristics after magnetic tape formation are deteriorated.

Further, if the addition amount is less than 0.001% by weight, the number of projections formed on the film surface becomes insufficient,
The slipperiness is insufficient. On the other hand, if the amount added exceeds 0.10% by weight, the film surface becomes rough and the electromagnetic conversion characteristics after forming a magnetic tape deteriorate. The preferred addition amount is 0.005
It is 0.05% by weight.

The surface of these spindle-shaped calcium carbonates may be modified with various surface treatment agents. The surface treatment agent used for the purpose of improving the affinity with ethylene glycol or polyester is generally applied in an amount of 5% by weight or less based on the particles. Examples of the surface treatment agent include a silane coupling agent and a titanium cup. A ring agent, polyacrylic acid, and the like can be given.

In the present invention, one or more kinds of fine particles selected from the group consisting of aluminum oxide, silicon oxide and titanium oxide used together with the spindle-shaped calcium carbonate are as described above.
It has a specific surface area of 50 to 120 m ² / g and a total pore volume of 0.
5 to 1.0 ml / g and an average particle size of 0.02 to 0.
It is important that the thickness is 30 μm.

If the specific surface area is less than 50 m ² / g, the primary particle diameter becomes large, and the powder tends to come into contact with pass-type rolls or coating rolls in the tape manufacturing process or scraping powder in the calendering process, which is not preferable. . On the other hand, when the specific surface area exceeds 120 m ² / g, the primary particle size becomes too small to form sufficient protrusions, so that the blocking phenomenon easily occurs on the base film roll, and the film unwinding property from the roll is increased. Unfavorably deteriorates. A preferred specific surface area is 60 to 100 m ² / g.

If the total pore volume is less than 0.5 ml / g, the particles are not porous, the affinity with polyester is low, and shavings are likely to occur. On the other hand, if the total pore volume exceeds 1.0 ml / g, the particles become brittle and will collapse during stretching for biaxial orientation, and sufficient protrusions cannot be formed. The preferred total pore volume is 0.6-0.9 ml / g. Also, the average secondary grain
If the diameter is less than 0.02 μm, the particles become fine.
And blocking is likely to occur, which is not preferable. on the other hand
If the average secondary particle size is larger than 0.3 μm, the lubricant aggregates.
Is too large and shavings are easily generated.

The fine particles are known as a method for producing fine particles.
It can be manufactured according to a known method. example
For example, for silicon dioxide, such as sodium silicate
Hydrolyze the silicate to disperse solid primary particles in water.
And then dry the colloidal particles to produce porous particles.
Method (sol-gel method), vaporizing silicon tetrachloride, acid
Manufactured by a method such as combustion decomposition in hydrogen (gas phase method)
Can be. For titanium dioxide,
Sulphates such as benzene are pyrolyzed to separate the acid groups and fired.
Method (liquid phase method), vaporize titanium tetrachloride, and in oxyhydrogen
It can be manufactured by a method such as combustion decomposition (gas phase method).
Wear.

The amount of the fine particles added is 0.05 to 1.0.
%, But if this amount is less than 0.05% by weight, the effect of adding fine particles becomes small, while if it exceeds 1.0% by weight, overlapping of particles is seen and shavings are likely to occur.

The biaxially oriented polyester film of the present invention has, for example, a melting point (Tm: ° C.) to (Tm + 70) ° C.
Melt the polyester at the temperature of
An unstretched film of 0.9 dl / g is obtained, and the unstretched film is uniaxially (longitudinal or lateral) (Tg-10) to
Stretching at a temperature of (Tg + 70) ° C. (however, Tg: glass transition temperature of polyester) at a draw ratio of 2.5 to 5.0,
Next, in the direction perpendicular to the above-mentioned stretching direction (when the first stage stretching is the longitudinal direction, the second stage stretching is the transverse direction), Tg (° C)-
It can be produced by stretching at a temperature of (Tg + 70) ° C. and a draw ratio of 2.5 to 5.0 times. In this case, the area draw ratio is 9
It is preferably ˜22 times, more preferably 12 to 22 times. The stretching means may be either simultaneous biaxial stretching or sequential biaxial stretching.

Further, the biaxially oriented film has (Tg + 7
0) It can be heat-set at a temperature of from 0 ° C. to Tm (° C.).
For example, a polyethylene terephthalate film is preferably heat-set at 190 to 230 ° C. The heat fixing time is, for example, 1 to 60 seconds.

The biaxially oriented polyester film of the present invention has good process suitability even under severe conditions of use in the magnetic tape manufacturing process, and has an extremely excellent surface property, so S-VHS or 8 mm It is extremely useful as a base film for a magnetic recording medium for a system having a short recording wavelength such as video.

Various physical properties and characteristics in the present invention are measured and defined as follows.

(1) Area circle equivalent diameter of spindle-shaped calcium carbonate particles, particle diameter ratio, etc. The following methods are available for measuring the area circle equivalent diameter, particle diameter ratio, etc. of particles. 1) When the average particle diameter and the particle size ratio of the area circle equivalent diameter are determined from the powder. 2) When the average particle diameter and the particle size ratio of the area circle equivalent diameter of the particles in the film are determined.

1) From powders The powders are scattered on an electron microscope sample stand so that the individual particles do not overlap as much as possible, and a gold thin film deposition layer is formed on the surface of the gold sputtering device at a thickness of 200 to 300 angstroms. Formed, scanning electron microscope, for example, 100
Observing at a magnification of 00 to 30,000 and using Luzex 500 manufactured by Nippon Regulator Co., Ltd., the major axis, the minor axis and the area equivalent circle diameter of at least 100 particles are determined. Then, the average particle size and the particle size ratio are calculated from these values.

2) In case of particles in film A small piece of sample film is fixed on a sample stand for a scanning electron microscope, and a sputtering device (JFC-manufactured by JEOL Ltd.) is used.
The film surface is subjected to ion etching under the following conditions using a 1100 type ion etching apparatus). The conditions were as follows: the sample was placed in a bell jar, the degree of vacuum was raised to a vacuum state of about 10 ⁻³ Torr, the voltage was 0.25 kV, and the current was 1
Perform ion etching at 2.5 mA for about 10 minutes. Further, the film surface is subjected to gold sputtering by the same apparatus, observed at a magnification of 10,000 to 30,000 times with a scanning electron microscope, and the average particle diameter and the particle diameter ratio are calculated in the same manner as in 1).

(2) Average particle size of fine particles Shimadzu CP-50 type Centrifugal particle size analyzer (Centrifugal Particle Siz
e Analyser). The particle diameter corresponding to 50 mass% is read from the integrated curve of the particles of each particle diameter calculated based on the obtained centrifugal sedimentation curve and the amount thereof,
This value is used as the average particle size (see Book "Particle Size Measurement Technology", Nikkan Kogyo Shimbun, 1975, pp. 242-247).

(3) Specific surface area of powder, total pore volume BET using Autosoap-1 manufactured by Kantachrome Co., Ltd.
The specific surface area and total pore volume are measured by the method.

(4) Flatness of film surface Ra (center line average roughness) is measured according to JIS B 0601. Using a stylus type surface roughness meter (SURFCOM3B) manufactured by Tokyo Seimitsu Co., Ltd., the radius of the needle is 2 μm and the load is 0.0
Under a condition of 7 g, a chart (film surface roughness curve) is drawn, a portion of the measured length L is extracted from the obtained film surface roughness curve in the direction of the center line, and the center line of the extracted portion is defined as the X axis. When the roughness curve is represented by Y = f (x) with the direction of the vertical magnification as the Y axis, the value (Ra: μm) given by the following equation is defined as the flatness of the film surface.

[0046]

(Equation 4) In the present invention, Ra is expressed as an average value of five pieces measured by removing three pieces from a larger value with the reference length being 0.25 mm.

(5) Protrusion distribution Using a three-dimensional roughness meter (SE-30K) manufactured by Kosaka Laboratory, needle diameter 2 μmR, needle pressure 30 mg, measurement length 1 mm, sampling pitch 2 μm, cutoff 0.25 mm, longitudinal enlargement Magnification 2
The projection profile on the film surface is three-dimensionally (three-dimensionally) imaged under the conditions of 10,000 times magnification, 200 times magnification in the lateral direction, and 150 scanning lines.

When the profile is cut on a plane perpendicular to the thickness direction of the film, the total cross-sectional area of the profiles of the protrusions is 7 of the area of the film measurement region.
A plane with 0% is set as a reference level (0 level), and the distance X (μ is parallel to the plane of the reference level in the height direction of the protrusion.
The number of protrusions counted when cut on a plane separated by m) is Y (pieces / mm ² ). X is sequentially increased or decreased, the number of Y at that time is read, and the projection distribution curve is drawn by plotting it on a graph.

(6) Number of projections of 1 ring or more by a multiple interference reflection microscope Aluminum is uniformly vacuum-deposited on the surface of the film to a thickness of 400 to 500 angstrom or less, and collodion is formed on the opposite non-deposited surface (film surface). Apply, apply and dry. Using a Tl monochromatic light multiple interference reflection microscope (for example, Carl Zeiss JENA), an arbitrary 100 cm ² of the aluminum vapor deposition surface was observed at a magnification of 100 times. The number of protrusions having an interference fringe of 1 ring or more (0.29 μm or more) corresponding to the protrusion height of the protrusions in the microscope field is counted and converted into 10 cm ² .

(7) Blade Abrasion Property It is measured as follows using the apparatus shown in FIG.
In FIG. 1, 1 is an unwind reel, 2 is a tension controller, 3, 5, 6 and 8 are free rollers, 4 is a tension detector, 7 is a blade (blade for industrial razor tester made by US GKI), and 9 is Guide rollers and 10 represent take-up reels, respectively.

The sample slit to ½ inch was applied with a tension of 50 by hitting the blade edge with an angle of 6 degrees.
g, run 100 m at a running speed of 100 m / min, and evaluate the sharpness by the amount of shavings adhering to the blade edge.

This evaluation relates to the impact strength of the protrusions formed on the film surface, and corresponds well to the generation of shavings on a calendar or die coater in the magnetic tape manufacturing process. <Judgment> ◎: Adhesion width of shavings adhering to the blade edge is less than 0.2 mm ◯: Adhesion width of shavings adhering to the blade edge is 0.2 mm or more and less than 0.5 mm △: Adhesion of shavings adhering to the blade edge Width is 0.5 mm or more and less than 1.0 mm x: Shaving powder adhesion width attached to the blade edge is 1.0 mm or more.

[0053]

EXAMPLES The present invention will be further described below with reference to examples.

[0054]

Examples 1 to 3, Comparative Examples 1 to 4 Dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, and antimony trioxide as a polymerization catalyst,
Phosphorous acid was added as a stabilizer, and the additive particles shown in Table 1 were added as a lubricant, followed by polymerization by a conventional method to obtain polyethylene terephthalate having an intrinsic viscosity of 0.62 (orthochlorophenol, 35 ° C.).

The pellets of polyethylene terephthalate were dried at 170 ° C. for 3 hours and then fed to an extruder hopper,
The polymer was melted at a melting temperature of 280 to 300 ° C., and the molten polymer was extruded through a 1 mm slit die onto a rotating cooling drum having a surface finish of about 0.3 s and a surface temperature of 20 ° C.
An unstretched film of 0 μm was obtained.

The unstretched film thus obtained was preheated at 75 ° C., and further 15 mm between low speed and high speed rolls.
It was heated from above by one IR heater having a surface temperature of 900 ° C., stretched 3.6 times, rapidly cooled, and then supplied to a stenter and stretched laterally 3.9 times at 105 ° C. The obtained biaxially oriented film was heat-set at a temperature of 205 ° C. for 5 seconds to obtain a heat-set biaxially oriented film having a thickness of 14 μm.

The characteristics of the biaxially oriented polyester film thus obtained are also shown in Table 1. As a result of examining the distribution of protrusions of the obtained film, Examples 1 to 3 and Comparative Examples 1, 2 and 4 satisfied the formula (1), but Comparative Example 3 had a prescribed range. The number of higher protrusions is larger than that of the other, and the electromagnetic conversion characteristics are at a level of concern as a magnetic recording medium for 8 mm video and S-VHS applications.

In Table 1, there is no practical concern as to the blade sharpness judgment at a level of ◯ or more, but at the level of Δ, shavings are generated in the magnetic tape manufacturing process to cause coating troubles and product defects. Dropouts increase, which is not desirable.

As is clear from Table 1, the magnetic recording medium of the present invention has excellent surface properties for magnetic recording media and extremely excellent abrasion resistance.

[0060]

[Table 1]

[0061]

According to the present invention, it is possible to provide a biaxially oriented polyester film for a magnetic recording medium which has excellent surface flatness and excellent abrasion resistance.

[Brief description of the drawings]

FIG. 1 is a schematic view of an apparatus for measuring blade sharpness of a film.

[Explanation of symbols]

 1 Feeding reel 2 Tension controller 3, 5, 6, 8 Free roller 4 Tension detector (entrance) 7 Blade 9 Guide roller 10 Take-up reel

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location G11B 5/704 G11B 5/704 // B29K 67:00 B29L 7:00 (72) Inventor Manabu Kimura 77, Kitayoshida-cho, Matsuyama City, Ehime Prefecture Matsuyama Office, Teijin Limited

Claims (1)

(57) [Claims] 1. A biaxially oriented polyester film, wherein a distribution curve of projections on the surface of the film (A) has a height X (μm) of 0.02 μm or more and the number of projections Y (pieces / m).
m ² ), in the range of 50 / mm ² or more, the following formula (1) is satisfied, and (B) the number of protrusions of 1 ring or more by the Tl monochromatic light multiple interference reflection microscope is less than 1/10 cm ² . Further, (C) the projection has a particle size ratio (major axis / minor axis) of 1.5 to 10 in the film, an average particle size of the area circle equivalent diameter of 0.4 to 1.0 μm, and the following formula ( 2) 0.001 to 0.10% by weight of spindle-shaped calcium carbonate particles having a relative standard deviation of 0.5 or less and a specific surface area of 50
In ~120m ² / g, the total pore volume is 0.5~1.0ml /
g and an average particle diameter of 0.02 to 0.30 μm, 0.05 to 1.0% by weight of one or more fine particles selected from the group consisting of aluminum oxide, silicon oxide and titanium oxide. A biaxially oriented polyester film for a magnetic recording medium, which is characterized by being formed by: -71.4 X + 5 <log Y <-32.4 X + 5.5 (1) [where X is the distance from the reference level in the height direction (μm)
And Y is the number of protrusions (pieces / mm ² ) counted when cut at the height X. ] [Equation 1]