JPH0959397A - Biaxially oriented polyester film for magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a biaxially oriented polyester film which contains at least inactivated particles and spinel type oxide particles, has excellent slipperiness, shaving resistance, scratching resistance, slitting properties and electromagnetic transducing properties, and is useful as a magnetic tape. SOLUTION: This film contains at least (A) 0.05-0.5wt.% of inactivated particles of 0.3-2.0μm average particle size and (B) 0.005-2.0wt.% of particles of spinel type oxide of 0.002-0.2μm average primary particle size. The component B is MgAl2 O4 and its average aggregation is preferably 1-20 and its specific surface area is preferably 10-150g/m<2> . This polyester film is preferably used in the form of its single layer form or a laminated form of the polyester A layer with the polyester B layer. In a preferred embodiment, the layer A 0.1-3.0μm, preferably 0.5-2.0μm thick is laminated on one surface of the layer B.

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 a magnetic recording medium, and more specifically, it contains at least inert particles and spinel type oxide particles, and has fine projections formed by the particles, so that slipperiness is improved. The present invention relates to a biaxially oriented polyester film for a magnetic recording medium, which is excellent in scratch resistance, slit resistance, and electromagnetic conversion characteristics when formed into a tape while ensuring abrasion resistance.

[0002]

2. Description of the Related Art Biaxially oriented polyester film represented by biaxially oriented polyethylene terephthalate film is
Due to its excellent physical and chemical properties, it is widely used as a base film for magnetic tape.

In this film, scratch resistance,
The slitting property, the sliding property and the abrasion resistance are major factors that affect the workability of the film manufacturing process and the processing process in each application, and further the quality of the product.

Generally, in order to improve the slipperiness of a film, a method of reducing the contact area with a guide roll or the like by providing fine irregularities on the film surface is adopted.

Conventionally, as a method for imparting fine irregularities to the film surface, a method of adding inorganic fine particles such as silicon dioxide, titanium dioxide, calcium carbonate, talc, clay and calcined kaolin to polyester which is a film substrate (for example, Japanese Patent No. 52-32914) or a method for precipitating fine particles containing calcium, lithium or phosphorus in a polymerization system for producing polyester (Japanese Patent Publication No. 52-3).
2914) has been proposed. When these fine particles are formed into a film, fine projections are formed on the surface of the film, and the projections improve the slipperiness of the film.

However, in a polyester film comprising the method of adding the above-mentioned inorganic fine particles, peeling usually occurs at the boundary between the fine particles and the polyester due to biaxial stretching.
Voids are formed around the fine particles. The voids are larger as the fine particles are larger, the stretching area ratio is larger, and the stretching is performed at a lower temperature.

The larger the void, the looser the shape of the protrusion and the higher the coefficient of friction. In addition, small scratches (scratches) on the voids of the biaxially oriented polyester film produced during repeated use cause the particles to fall off, reducing durability and causing shavings. The shavings often cause a loss of a magnetic signal, that is, dropout when used as a magnetic tape, for example. Also, the presence of large voids causes light scattering, which reduces the transparency of the film.

On the other hand, the method of depositing fine particles in the polymerization system of polyester has the advantage that the deposited fine particles and the polyester have a good affinity, but it is difficult to control the size and amount of the particles. However, there is a drawback that the surface irregularities of the obtained film tend to vary.

[0009]

DISCLOSURE OF THE INVENTION The present inventors have solved these inconveniences, and the voids around the inert fine particles are small and the film surface is appropriately roughened, so that the film has improved slipperiness and abrasion resistance. However, by adding spinel type oxide particles at the same time, while securing these characteristics, scratch resistance, slit resistance and electromagnetic conversion characteristics when formed into a tape are also improved, and a biaxial surface that is suitable for applications is used. As a result of earnest research for developing an oriented polyester film, the present invention has been achieved.

Therefore, an object of the present invention is to have small voids, uniform film surface roughness, and uniform surface irregularities, and to provide slipperiness, abrasion resistance, scratch resistance, slit resistance, and electromagnetic properties when used as a tape. It is intended to provide a biaxially oriented polyester film for a magnetic recording medium, which is excellent in conversion characteristics.

[0011]

The present invention has the following constitution in order to achieve the above object.

The film has an average particle size of at least 0.3.
0.05 to 0.5% by weight of inert particles having an average particle size of 0.002 to 0.2 μm.
The biaxially oriented polyester film for magnetic recording media, containing 0.005 to 2.0% by weight of the spinel type oxide particles.

The polyester in the present invention is a polyester having an aromatic dicarboxylic acid as a main acid component and a fatty acid 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, isophthalic acid, 2,6-naphthalenedicarboxylic acid, diphenoxyethanedicarboxylic acid, diphenyldicarboxylic acid,
Examples thereof include diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid and anthracene dicarboxylic acid. Examples of the aliphatic glycol include alkylene glycol having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, decamethylene glycol and the like, and alicyclic diol such as cyclohexanedimethanol. Etc. can be mentioned.

In the present invention, the polyester having alkylene terephthalate and / or alkylene naphthalate as a main constituent is preferable as the polyester.

Among these polyesters, polyethylene terephthalate, polyethylene-2,6-naphthalate, etc., such as total dicarboxylic acid component 8
0 mol% or more is terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and is 80% of the total glycol component.
Copolymers in which more than mol% is ethylene glycol are preferred.

Here, up to 20 mol% of the total acid component can be said aromatic dicarboxylic acids other than terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and also fats such as adipic acid, sebacic acid and the like. Group dicarboxylic acids; alicyclic dicarboxylic acids such as cyclohexane-1,4-dicarboxylic acid and the like. 20 mol% or less of the total glycol component may be the above-mentioned aliphatic glycol other than ethylene glycol, and aromatic diols such as hydroquinone, resorcin, 2,2-bis (4-hydroxyphenyl) propane, and polyethylene; It can also be a polyalkylene glycol (polyoxyalkylene glycol) such as glycol, polypropylene glycol, polytetramethylene glycol or the like.

The polyester of the present invention includes
Aromatic oxyacids such as hydroxybenzoic acid, ω-
Those in which a component derived from an oxycarboxylic acid such as an aliphatic oxyacid such as hydroxycaproic acid is copolymerized or bonded with 20 mol% or less based on the total amount of the dicarboxylic acid component and the oxycarboxylic acid component are also included.

Further, the polyester in the present invention includes:
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 polycarboxylic acid or polyhydroxy compound having a functionality of 3 or more at a mol% or less, such as trimellitic acid or pentaerythritol.

The polyester is known per se and can be produced by a method known per se.

The biaxially oriented polyester film of the present invention has a structure of a single layer film or a laminated film in which a polyester film A layer and a polyester film B layer are laminated. The laminated form is B / A two-layer structure, A /
It is preferable that the B / A has a three-layer structure. Among them, the three-layer structure is more preferable because the waste film generated in the manufacturing process of the polyester film can be reused by mixing it with the B layer of the polyester film. preferable. In the case of the laminated polyester film obtained by the coextrusion method, the polymer of each layer may be the same or different. Among them, the same is preferable.

In the laminated film of the polyester films, the thickness of the polyester film A layer is 0.1 to 3.0 μm, preferably 0.5 to 2.0 μm. If the thickness is more than 3.0 μm, the characteristics are the same as those of the single-layer polyester film. On the other hand, if it is less than 0.1 μm, the particles in the film are likely to fall off and the abrasion resistance is deteriorated, which is not preferable.

The A layer of the single-layer polyester film and the laminated polyester film has an average particle size of 0.3-2.
It contains inert particles of 0 μm, preferably 0.5-1.0 μm. If the inert particles exceed 2.0 μm, the influence of the base film on the surface roughening of the magnetic tape becomes large, and even if the layer A of the laminated film is thinned to reduce the surface roughness of the base film. However, the flatness of the magnetic tape surface cannot be achieved. On the other hand, the number of inert particles is 0.
If the thickness is less than 3 μm, the slipperiness is poor, the tape cannot be wound, and the running of the tape becomes unstable during VTR, and the scraping property of the guide deteriorates.

The A layer of the single-layer polyester film and the laminated polyester film has an average primary particle size of 0.002 to 0.2 μm, preferably 0.01 to 0.
It contains 04 μm spinel type oxide particles. By adding the particles, it is possible to prevent the guide pins from being damaged when the cassette is loaded in the cassette, and the slit property is improved in the film or tape production process. When the above-mentioned particle size of the spinel type oxide particles exceeds 0.2 μm, the surface roughness of the film becomes large and the influence on the surface roughening of the magnetic tape of the base film becomes large, and further, the blade at the time of slitting. Life is reduced and productivity is deteriorated. On the other hand, the average primary particle size of spinel oxide particles is 0.0
If it is less than 02 μm, the damage on the guide pin cannot be controlled and the scratch resistance is reduced.

The spinel type oxide particles are present as particles having a primary particle diameter as they are and / or as secondary particles (aggregated particles) obtained by aggregating these particles, and the agglomeration degree is 1 to 20 in terms of average agglutination degree, and further 1. It is preferably 5 to 20, particularly 1.5 to 10, and especially 2 to 10. Further, the spinel type oxide particles have a specific surface area of 10 to 150 m ² / g, and further 50 to
It is preferably 110 m ² / g, and thereby scratch resistance and slit property can be improved.

The spinel type oxide particles are added in an amount of 0.005 to
The content of 2.0% by weight, preferably 0.1 to 0.5% by weight, and more preferably 0.2 to 0.4% by weight results in good scratch resistance and slit property.
The addition amount of the spinel type oxide particles needs to be within this range.

In the present invention, the spinel type oxide particles are compounds having a regular octahedral structure represented by M ⁿ OM ^m ₂ O ₃ . Here, M ⁿ means Mg ²⁺ , Ca ²⁺ , S
At least one divalent metal element selected from r ²⁺ , Ba ²⁺ , Zn ²⁺ , Fc ^2+, etc., and M ^m is Al ³⁺ , Cr
^It is at least one trivalent metal element selected from ³⁺ , Fe3 ^{+ and the} like. Spinel type oxide particles include M ⁿ and M ^m
May be used alone or in combination of two or more kinds of particles made of a spinel type oxide composed of a combination of the above metal elements, and each of M ⁿ and M ^m , or M ⁿ or M ^m may be two or more kinds of metals. It is also possible to use particles made of a spinel type oxide which is composed of an element and in which each component forms a solid solution. As these medium spinel type oxide particles, MgAl ₂ O ₄ is preferable, and scratch resistance and slit property are more improved.

Examples of the inert particles other than the spinel type oxide particles contained in the layer A of the single-layer film and / or the laminated film in the present invention include (1) silicon dioxide (hydrate, silica sand, quartz, etc.). (2) Alumina of various crystal forms; (3) Silicate containing 30% by weight or more of SiO ₂ content (for example, amorphous or crystalline clay mineral,
Aluminosilicate (including calcined products and hydrates), warm asbestos, zircon, fly ash, etc.) (4) Mg, Z
oxides of n, Zr, and Ti; (5) Ca, and B
(6) Li, Ba, and Ca phosphates (including monohydrogen and dihydrogen salts); (7) Li, Na, and K benzoates; (8) Ca, Ba , Zn, and terephthalates of Mn; (9) Mg, Ca, Ba, Z
n, Cd, Pb, Sr, Mn, Fe, Co, and Ni
Titanates; (10) Ba and Pb chromates; (11) Carbon (eg, carbon black, graphite, etc.); (12) Glass (eg, glass powder, glass beads, etc.); (13) Ca, and Mg carbonate; (1
4) Fluorite; (15) ZnS; (16) Heat-resistant polymer particles (for example, crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked polystyrene particles, crosslinked styrene-acrylic resin particles, crosslinked polyester particles, Teflon particles, polyimide particles) , Polyimide-amide particles, melamine resin particles, etc.) are preferred. More preferably, heat resistant polymer particles, synthetic inorganic particles (eg calcium carbonate,
Silica and the like). These preferably have a narrow particle size distribution, and the relative standard deviation is preferably 0.4 or less, more preferably 0.3 or less, and particularly preferably 0.2 or less.

The total thickness of the polyester film of the present invention is preferably 5 to 25 μm.

The polyester film of the present invention can be obtained by a method which is basically known in the art or accumulated in the art. For example, it can be obtained by first producing a single-layer or laminated unstretched film, and then biaxially orienting the film. Furthermore, the laminated unstretched film can be produced by the conventionally accumulated method for producing a laminated film. For example, a method of laminating a film layer (polyester A layer) forming a surface and a film layer (polyester B layer) forming a core layer in a molten state or a state of being cooled and solidified can be used. More specifically, it can be produced by a method such as co-extrusion or extrusion lamination.

The unstretched film (single-layer unstretched film or laminated unstretched film) laminated by the above method is
Further, the biaxially oriented film can be obtained according to the conventionally accumulated manufacturing method of the biaxially oriented film. For example,
A single-layer unstretched film or a laminated unstretched film is (Tg-10) to (Tg +) in a uniaxial direction (longitudinal direction or transverse direction).
70) ° C. (however, Tg: glass transition temperature of polyester) is stretched at a draw ratio of 2.5 to 7.0, and then in a direction perpendicular to the stretching direction (when the first stage stretching is in the longitudinal direction, The second stage drawing is in the transverse direction) to Tg (° C) to (Tg
It can be produced by stretching at a temperature of +70) ° C at a magnification of 2.5 to 7.0 times. In this case, the area stretching ratio is 9 to 32.
It is preferably doubled, more preferably 12 to 32 times. The stretching means may be a simultaneous biaxial stretching method or a sequential biaxial stretching method. Furthermore, the biaxially oriented film has a (Tg + 70) ° C. to Tm.
It can be heat-set at a temperature of (° C.) (Tm: melting point of polyester). For example, a single layer or laminated polyethylene terephthalate film has a thickness of 190 to 230.
It is preferable to heat-set at ℃. The heat setting time is, for example, 1 to 60 seconds.

The biaxially oriented polyester film of the present invention is excellent in scratch resistance and slit resistance while ensuring slipperiness and abrasion resistance, and is extremely useful as a magnetic recording medium.

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

(1) Average particle size (DP) of inert particles other than spinel oxide CP-50 type Centrifigur particle size analyzer (Centrifugal Paticle Si) manufactured by Shimadzu Corporation
ze Analyzer). 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 remaining amount, and this value is taken as the above average particle diameter. (Book "Particle size measurement technology", published by Nikkan Kogyo Shimbun, 1975, pages 242-24.
7).

(2) Average Primary Particle Size of Spinel Oxide Particles A film containing the particles is cut into a thin section having a thickness of 100 nm in a cross-sectional direction, and a transmission electron microscope (for example, JE manufactured by JEOL Ltd.) is used.
When the particles are observed at a magnification of about 100,000 times using M-1200EX), the smallest particles (primary particles) that cannot be further divided can be observed. Using this photograph, the diameter corresponding to the area circle of each particle was measured for 100 particles using an image analysis device or the like, and the averaged particle diameter was taken as the average particle diameter of the spinel oxide particles. In addition, particle type identification is SE
This can be done using M-XMA or the like.

(3) Average Aggregation Degree of Spinel Oxide Particles About 100 particles observed from the photograph obtained by the measurement of the average primary particle diameter, particularly about agglomerated particles (secondary particles), The average aggregation degree is obtained by counting whether or not the particles are made up of the primary particles and dividing the total sum of the primary particles by the number of the measured particles.

(4) Specific Surface Area of Spinel Oxide Particles BET using Autosorb-1 manufactured by Kantachrome Co., Ltd.
The specific surface area is measured by the method.

(5) Surface roughness (Ra) of the film is a value defined by JIS-B0601 as the center line average roughness (Ra), and in the present invention, the stylus surface roughness of Kosaka Laboratory Ltd. Measure with a meter (SURFCORDER SE-30C). The measurement conditions and the like are as follows. (A) Tip radius of the stylus: 2 μm (b) Measurement pressure: 30 mg (c) Cutoff: 0.25 mm (d) Measurement length: 2.5 mm (e) How to summarize data: Repeated measurement of the same sample six times Excluding one of the largest values, the average value of the remaining five data is displayed.

(6) High-speed pin scraping In an environment of a temperature of 20 ° C. and a humidity of 60%, a film cut into a width of 1/2 inch is bent into a cylindrical shape from a SUS sintered plate, and the surface finish is insufficient (surface roughness). Ra = 0.15 nm) Angle θ = (60/180) π radian (60
°) Contact and run (friction) at a speed of 250 m / min and an entrance tension T ₁ = 50 g. Film is 200
The shavings adhering to the fixed guide pin after traveling m are evaluated. <Judgment of shavings> ◎: No shavings are observed at all ◯: Slightly shavings are seen △: Presence of shavings can be seen at a glance ×: Shavings are heavily attached <Scratch judgment> ◎: No scratches were observed. O: 1 to 5 scratches were observed. Δ: 6 to 15 scratches were observed. X: 16 or more scratches were observed.

(7) Blade shaving In the environment of a temperature of 20 ° C. and a humidity of 60%, a film cut to a width of 1/2 inch is formed at the blade tip of a blade (blade for industrial razor tester made by US GKI) at an angle θ = (6 / 180)
It is brought into contact with π radians (6 °) and run (rubbed) at a speed of 100 m / min and an entrance tension T ₁ = 50 g. The amount of shavings adhering to the blade after the film has run for 100 m is evaluated.

This evaluation relates to the impact strength of the projections 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> ：: The width of the shaving powder adhering to the blade tip is 0.5 mm
Less than ○: The width of the shaving powder adhering to the blade edge is 0.5 mm
Above and less than 1.0 mm △: The width of the shaving powder adhering to the blade tip is 1.0 mm
Above and less than 2.0 mm ×: The width of the shaving powder adhering to the blade tip is 2.0 mm
that's all

(8) Slitting property In an environment of a temperature of 20 ° C. and a humidity of 60%, a film cut into a width of 1/2 inch is formed on the blade edge of a blade (a blade for an industrial razor tester manufactured by US GKI) at an angle θ = (6 / 180)
It is brought into contact with π radians (6 °) and repeatedly run (rubbed) 10 times at a speed of 100 m / min and an entrance tension T ₁ = 50 g. After the film has run, the blade edge of the blade is scanned with a scanning electron microscope (for example, JSM-T30 manufactured by JEOL Ltd.).
The width of wear of the tip of the blade is evaluated by observing at 0).

This evaluation corresponds well to the long and short life of the slitting blade when the film or tape is slit in the polyester film forming step or the tape production step. <Judgment> ◎: The wear width of the blade edge is less than 1.0 μm ◯: 3.0 μ when the wear width of the blade edge is 1.0 μm or more
Less than m △: 5.0 μ when the wear width of the blade edge is 3.0 μm or more
Less than m x: Abrasion width of blade edge is 5.0 μm or more

(9) Electromagnetic conversion characteristics VHS system VTR (BR640 manufactured by Victor Company of Japan, Ltd.)
0) is modified and a 4 MHz sine wave is input to a recording / reproducing head through an amplifier, recorded on a magnetic tape manufactured by the following method and then reproduced, and the reproduced signal is input to a spectrum analyzer. Carrier signal 4 MHz to 0.1
The noise generated at a distance of MHz is measured, and the carrier-to-noise ratio (C / N) is expressed in dB. Using this method, the above-mentioned magnetic tape was measured, and the one obtained in Comparative Example 1 was used as a reference (± 0 dB), and the difference from this magnetic tape was used as the electromagnetic conversion characteristic.

The magnetic tape is manufactured by the following method.
100 parts by weight of γ-Fe ₂ O ₃ (hereinafter, simply referred to as “part”) and the following composition are kneaded and dispersed in a ball mill for 12 hours. Polyester polyurethane 12 parts Vinyl chloride-vinyl acetate-malinic anhydride copolymer 10 parts α-alumina 5 parts Carbon black 1 part Butyl acetate 70 parts Methyl ethyl ketone 35 parts Cyclohexanone 100 parts Further after dispersion fatty acid: oleic acid 1 part Fatty acid: palmitic acid 1 part Fatty acid: ester (amyl stearate) 1 part is added and kneaded for 10 to 30 minutes. Then, 7 parts of a 25% ethyl acetate solution of a triisocyanate compound was added, and 1
The magnetic coating liquid is prepared by dispersing for a long time at high speed.

The obtained coating solution is applied on a polyester film so that the dry film thickness is 3.5 μm (applied to the polyester A ₁ layer side).

Then, after orientation treatment in a DC electromagnetic field, 1
Dry at 00 ° C. After drying, it is subjected to a calendering treatment and slit to a width of 1/2 inch to obtain a magnetic tape.

[0047]

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

Examples 1 to 4 and Comparative Examples 1 to 4 Dimethyl terephthalate and ethylene glycol, manganese acetate as an ester exchange catalyst, antimony trioxide as a polymerization catalyst, phosphorous acid as a stabilizer, and a lubricant as a lubricant. The fine particles shown in the polyester A layer in Tables 1 and 2 were added and polymerized by a conventional method to obtain polyethylene terephthalate used in the polyester film A layer having an intrinsic viscosity of 0.56 (orthochlorophenol, 35 ° C.).

Further, polyethylene terephthalate having an intrinsic viscosity of 0.62 obtained in the same manner as described above except that fine particles were not added as a lubricant and the polyethylene terephthalate for the film A layer were uniformly mixed at a ratio of 1: 1 (weight ratio). To obtain polyethylene terephthalate for use in the polyester film B layer.

Each of these polyethylene terephthalate pellets was dried at 170 ° C. for 3 hours, then fed to two extruder hoppers, melted at a melting temperature of 280 to 300 ° C., and a layer B layer was formed using a multi-manifold type coextrusion die. Layer A is laminated on both sides, surface finish is about 0.3 s, extrusion is performed on a rotating cooling drum with a surface temperature of 20 ° C, and rapid cooling is performed to obtain a thickness of 2
An unstretched laminated film of 00 μm was obtained. Here, the drum surface side was A ₁ layer, and the non-drum surface side was A ₂ layer.

The unstretched laminated film thus obtained is preheated to 75 ° C., and further 15 m between low speed and high speed rolls.
m, and it was stretched 3.2 times by heating with three IR heaters having a surface temperature of 800 ° C. from above, rapidly cooled, then supplied to a stenter, and stretched 4.3 times in the transverse direction at 120 ° 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 polyester film having a thickness of 14 μm.

The thickness of each layer of the coextrusion film is 2
It was adjusted by changing the discharge amount of the extruder of the table, and when the thickness of the polyester A ₁ layer side and the polyester A ₂ layer side were different, the adjustment was performed by narrowing the flow passage on one side. The thickness of each layer was determined by the fluorescent X-ray method and the method of cutting a film into thin pieces and searching for a boundary surface by a transmission electron microscope.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

EFFECTS OF THE INVENTION According to the present invention, the tape has a surface property which is particularly useful for magnetic tape applications, and is sufficiently slippery, scratch resistant, slittable, and tape capable of coping with speeding up of the magnetic tape processing process. It is possible to provide a single-layer and / or laminated polyester film having a good electromagnetic conversion property when the above is performed.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical display location C08K 7/00 C08K 7/00 C08L 67/02 KKF C08L 67/02 KKF G11B 5/704 G11B 5 / 704 // B29K 67:00 105: 16 B29L 7:00 9:00

Claims (6)

[Claims] 1. A film having an average particle size of at least 0.
0.05 to 0.5% by weight of inert particles having a particle size of 3 to 2.0 μm and an average primary particle size of 0.002 to 0.2 μm
A biaxially oriented polyester film for a magnetic recording medium, containing 0.005 to 2.0% by weight of spinel type oxide particles of m. 2. The biaxially oriented polyester film for a magnetic recording medium according to claim 1, wherein the spinel type oxide is MgAl ₂ O ₄ . 3. The average aggregation degree of spinel type oxide particles is 1.
The biaxially oriented polyester film for a magnetic recording medium according to claim 1 or 2, wherein 4. The specific surface area of spinel type oxide particles is 10.
The biaxially oriented polyester film for magnetic recording media according to claim 1, 2 or 3, having a weight ratio of 150 g / m ² . 5. The biaxially oriented polyester film for a magnetic recording medium according to claim 1, wherein the polyester film is a single layer film. 6. The polyester film B layer is laminated with the film according to claim 1 as a polyester film A layer on at least one side thereof, and the polyester film A layer has a thickness of 0.1 to 3.0 μm. A biaxially oriented polyester film for a magnetic recording medium, characterized by being present.