JPH06297661A - Polyester film for magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve travelling and electromagnetic conversion properties as well as abrasion resistance by forming a coated layer, at least, on one surface of a polyester film in a coating step and at the same time, containing specific two different types of particles in the coated layer so that specific three different conditions are satisfied simultaneously. CONSTITUTION:A coated layer is provided, at least, on one surface of polyester film in a polyester film coating step. In addition, aluminum oxide (A) with an average particle diameter of 1mum max. and at least, one type of particle (B) selected from among calcium carbonate, silicon dioxide and a heat-resistant organic particle with a larger average particle diameter than (A), and within a range of 0.05 to 2mum, are contained in the coated layer. Each particle (A) and (B) are contained so that each under-mentioned formula is satisfied simultaneously. That is, 0.1<=Wb<=50. 5<=Wb<=50. 0.1<=Wb/Wa<=20, wherein Wa and Wb are the percentage (weight %) of each particle in the coated layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a magnetic recording medium,
The present invention relates to a polyester film that provides excellent running properties, electromagnetic conversion characteristics, and abrasion resistance. More specifically, the present invention relates to a base film suitable for high-density magnetic recording, which has excellent slipperiness during tape running, has extremely few scratches and abrasion powders, and has suppressed dropouts.

[0002]

2. Description of the Related Art Polyester films are widely used as industrial substrates because of their excellent physical and chemical properties. Among them, the biaxially oriented polyethylene terephthalate film and the polyethylene naphthalate film are now indispensable as base films for magnetic recording media because they are particularly excellent in mechanical strength, dimensional stability and flatness.

By the way, in actual handling of these films, running property and abrasion resistance are required, but this has not been sufficiently achieved in the past. That is, when the film and the base material, for example, a metal pin, come into contact with each other at a high speed, friction and wear between the two become large, so that the film is scratched and abrasion powder is generated.

As a result of intensive studies on the improvement of wear resistance, the present inventors have proposed a method of forming a coating layer containing aluminum oxide particles on the film surface. However, although the present inventors, although such a method is extremely effective for improving the wear characteristics of the film, in terms of film runnability and prevention of tackiness in a heat treatment step such as a calender step during magnetic recording medium production, We found that it was still insufficient.

[0005]

As a result of the inventors of the present invention, as a result of solving these problems, if a coating film in which a specific amount of aluminum oxide particles is added in a specific amount is formed, these problems will be solved. Knowing that the points can be solved all at once,
The present invention has been completed. That is, the gist of the present invention is
At least one surface of the polyester film has a coating layer provided in the film forming process of the polyester film, the coating layer having (A) an aluminum oxide particle having an average particle diameter of 1 μm or less, and (B) an average particle diameter. Is larger than that of (A) and is in the range of 0.05 to 2 μm,
A biaxially stretched polyester film for a magnetic recording medium, characterized in that it contains one or more particles selected from silicon dioxide and heat-resistant organic particles under the condition that the following formulas (1) to (3) are simultaneously satisfied.

0.1 ≦ Wa ≦ 50 (1) 5 ≦ Wb ≦ 50 (2) 0.1 ≦ Wb / Wa ≦ 20 (3) (Wa and Wb are particles A and particles in the coating layer, respectively) The content (% by weight) of B is shown below. The present invention will be described in more detail below.

The polyester referred to in the present invention refers to a polyester obtained from an aromatic dicarboxylic acid such as terephthalic acid or 2,6-naphthalenedicarboxylic acid or an ester thereof and ethylene glycol as a main starting material. It may contain three components. In this case, as the dicarboxylic acid component, for example, isophthalic acid,
Terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid and sebacic acid can be used. As the glycol component, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol or the like can be used. In any case, the polyester referred to in the present invention refers to a polyester in which 80% or more of repeating structural units have ethylene terephthalate units or ethylene-2,6-naphthalate units.

Further, the polyester film of the present invention refers to a polyester film using such polyester as a starting material, and a known method can be adopted as a manufacturing method thereof. For example, after melt-extruding into a sheet form at 270 to 320 ° C., cooling and solidifying at 40 to 80 ° C. to form an amorphous sheet, the area ratio becomes 4 to 20 times in the vertical and horizontal directions at 80 to 150 ° C. As described above, a method of performing sequential biaxial stretching or simultaneous biaxial stretching and heat treatment at 160 to 250 ° C. (for example, the method described in JP-B-30-5639) can be used. Stretching in the machine direction and the transverse direction may be carried out in one step each, or if necessary, in multiple steps,
A heat treatment section for relaxing orientation may be provided during the multi-stage stretching. Further, after biaxial stretching, it may be stretched again before being subjected to the heat treatment step of the next step. This re-stretching
It can be performed either vertically or horizontally.

In the present invention, a coating solution containing at least two kinds of particles (A) and (B) is applied to at least one side of the film in the production process of the polyester film. In this case, the particles (A), That is, the average particle diameter of the aluminum oxide primary particles is 1 μm or less, preferably 0.5 μm or less, and more preferably 0.1 μm or less.

As is well known, aluminum oxide is usually obtained by thermally decomposing alumina hydrates such as dibsite, vialyte, and boehmite. At this time, the type of raw material, hydrolysis conditions, especially temperature. Depending on the condition, α type as well as β, γ, κ, ο, δ, η, χ, ρ
Aluminum oxide having nearly 10 different crystal types, such as the type, is produced. Further, aluminum oxide can also be obtained by a method of burning and hydrolyzing aluminum chloride in an oxyhydrogen flame. In this case, α,
Aluminum oxide of various crystal types such as γ and δ can be obtained.

Each of these aluminum oxides has a unique property. For example, α type has a small specific surface area and shows almost no catalytic activity, whereas γ type and η type usually have
It has a specific surface area of about 50 to 300 m ² / g and has a high catalytic activity. Further, the density can be changed up to about 2.5 to 4 g / cm ² depending on each crystal type, and the hardness is delicately changed.

In the present invention, particularly remarkable effects are exhibited by using aluminum oxide selected from gamma (γ) type, delta (δ) type and theta (θ) type aluminum oxide among the aluminum oxides. The coating layer containing aluminum oxide hardly generates abrasion powder when it comes into contact with either a metal pin or a plastic guide roll or pin, and is extremely excellent in scratch resistance and abrasion resistance. Further, the film having such an aluminum oxide only in the coating layer has a significantly reduced abrasion rate of the slit blade during slitting, as compared with the film compounded in the film. The reason why the wear rate of the slit blade is reduced is that the concentration of such aluminum oxide is probably concentrated only on the surface layer portion of the film so that the amount of particles used in the entire film is less than that of the conventional film. It is considered that this is because it could be significantly reduced to about 05 to 0.2 times.

Although it is not clear why these aluminum oxides exhibit such an effect, particularly excellent wear resistance, it is probably that the particles have a good familiarity with an organic binder and the hardness is appropriate. That seems to be the main reason. Among these aluminum oxides, δ-type crystalline aluminum oxide which is particularly preferably used can be generally obtained by highly flame-hydrolyzing aluminum chloride, but the average particle size thereof is 0.
It is convenient because it is often obtained as 1 μmm or less.

In the present invention, it is preferable to use such aluminum oxide particles dispersed in the primary particles, but the aluminum particles may behave as slightly agglomerated secondary particles as long as they do not adversely affect the surface condition of the film. . However, in this case as well, an apparent average particle size of 0.5 μmm or less is preferably used. In the present invention, if 70% by weight or more, preferably 90% by weight or more of the aluminum oxide contained in the coating layer is γ type, δ type or θ type, the desired effect can be sufficiently exhibited.

By the way, in the present invention, the particles (A) alone are often unsatisfactory as a base film for a magnetic recording medium, and when they are wound in a roll, they are often opposite to the coating film. Since adhesion occurs between the surface and the magnetic layer, the second particles (B) are used together to improve these characteristics. As the particles (B), one or more particles selected from calcium carbonate, silicon dioxide and heat resistant organic particles having an average particle size larger than that of the particles (A) and in the range of 0.05 to 2 μm are used.

These particles may be obtained by any method, and their crystal structure and the like are not particularly limited. That is, as the calcium carbonate particles, natural products may be crushed or synthetic products, and the crystal structure thereof may be any of calcite type, vaterite type and aragonite type. Further, the silicon dioxide particles may be obtained by either a so-called wet method or a dry method, and particularly particles having a uniform particle diameter obtained by a hydrolysis method of alkoxysilane are preferably used.

As the heat resistant organic particles, for example, Japanese Patent Publication No.
Between a monovinyl compound having only one aliphatic unsaturated bond in the molecule and a compound having two or more aliphatic unsaturated bonds in the molecule as a crosslinking agent, as described in JP-A-9-5216. A copolymer can be illustrated. A more specific example of this copolymer is, for example, Japanese Patent Laid-Open No.
Examples thereof include monodisperse particles obtained by an emulsion polymerization method as described in JP-A 9-217755.

Examples of the heat-resistant organic particles include thermosetting phenol resin, thermosetting epoxy resin,
A fine powder of a thermosetting urea resin, a benzoguanamine resin, or a fluorine-based resin such as polytetrafluoroethylene can also be used. In any case, the heat-resistant organic particles as referred to in the present invention refer to particles that can function as so-called blended particles without being substantially dissolved even after heating in the film-forming step after coating.

The average particle of the particles (B) used in the present invention must be larger than that of the particles (A) and not less than 0.05 μm. If this condition is not satisfied, the runnability of the film will not be improved. On the other hand, if the average particle size of the particles (B) exceeds 2 μm, coarse protrusions will frequently occur, and the electromagnetic characteristics will deteriorate when used as a magnetic recording medium. Also,
In the present invention, the content of the particles (A) in the coating layer (W
a) is 0.1 to 50% by weight, preferably 1 to 40% by weight, more preferably 2 to 30% by weight. If this value is less than 0.1% by weight, scratch resistance is not improved,
If it exceeds 5% by weight, the mechanical strength of the coating layer and the adhesion to the base polyester film deteriorate.

The content (Wb) of the particles (B) in the coating layer is 5 to 50% by weight, preferably 7 to 45% by weight, more preferably 10 to 40% by weight. If this value is less than 5%, the running property is not improved, and conversely, if this value exceeds 50% by weight, coarse projections frequently occur on the surface of the coating layer and dropouts increase. Further, in the present invention, the content ratio (Wb / of the particles (B) and the particles (A))
Wa) is 0.1 to 20, preferably 0.3 to 10, and more preferably 2 to 8. When the value of Wb / Wa is less than 0.1, the running property and anti-adhesion are not improved, and when it exceeds 20, the effect of scratch resistance decreases and the dropout increases.

In the present invention, particles (A), (B)
The particle size distribution is not particularly limited, but when it is added in a sharper manner, specifically, from the smaller particle size, the ratio of the particle size of 75% by weight to the particle size of 25% is 2.0 or less, and further 1 Those of 0.5 or less are preferable. Further, the shape of these particles is not particularly limited, but generally, particles having a shape close to a lump or a sphere are preferably used. Specifically, for example, particles having a volume shape of 0.1 to π / 6, further 0.2 to π / 6 as defined in JP-B-53-14583 are preferable.

The specific surface area of the particles used is not particularly limited. For example, average particle size 0.5 μm, density 2 g
A spherical particle having a diameter of / cm ² has a specific surface area of about 6 m ² / g, but a porous particle of about 500 m ² / g can be preferably used. The surface of these particles may be modified with various surface treatment agents for the purpose of improving the affinity with water-based paints.

In the present invention, a coating solution containing these particles and an aqueous resin is applied to at least one surface of the polyester film during the production process (usually after the longitudinal stretching process and before the transverse stretching process is started). It is dried to form a coating film. A water-based resin is used as the binder of the coating layer of the present invention. The water-based resin as used herein means a manufacturing process of a magnetic recording material after being coated on the surface of a polyester film or a magnetic recording material. It has heat resistance and moisture resistance that can withstand the subsequent use environment under high temperature and high humidity, and when this layer is used as an undercoat layer and an overcoat layer such as a magnetic layer or backcoat layer is applied, adhesion with that layer Resins with excellent properties, such as polyester, polyamide, polystyrene, polyacrylate, polycarbonate, polyarylate, polyvinyl chloride, polyvinylidene chloride, polyvinyl butyral, polyvinyl alcohol, polyurethane and other resins, and copolymers and mixtures of these resins. The body can be selected from, but is not limited to. The most preferable resin among these is a polyester resin.

Examples of the components constituting the polyester resin include the following polyvalent carboxylic acids and polyvalent hydroxy compounds. That is, as the polycarboxylic acid, terephthalic acid, isophthalic acid, orthophthalic acid,
Phthalic acid, 4,4'-diphenyldicarboxylic acid, 2,5
-Naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2-potassium sulfoterephthalic acid, 5-sodiumsulfoisophthalic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, glutar Acid, succinic acid, trimellitic acid, trimesic acid, trimellitic anhydride, phthalic anhydride, p-hydroxybenzoic acid, trimellitic acid monopotassium salt and their ester-forming derivatives can be used, and polyhydric hydroxy groups can be used. As the compound, ethylene glycol, 1,2-propylene glycol, 1,3
-Propylene glycol, 1,3-propanediol,
1,4-butanediol, 1,6-hexanediol,
2-methyl-1,5-pentanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, p
-Xylylene glycol, bisphenol A-ethylene glycol adduct, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, dimethylolethyl Sodium sulfonate, potassium dimethylolpropionate and the like can be used.

Usually, one or more of these compounds are appropriately selected and a polyester resin is synthesized by a conventional polycondensation reaction. In addition to the above, JP-A-1
A composite polymer having a polyester component such as a so-called acrylic graft polyester described in JP-A-165633 or a polyester polyurethane in which a polyester polyol is chain-extended with isocyanate can also be preferably used as the polyester resin used in the present invention.

The aqueous resin in the present invention contains 70 wt% of water.
%, A coating agent forcibly dispersed with a surfactant or the like using a liquid as a medium is preferable, but a hydrophilic nonionic component such as polyethers or a cation such as a quaternary ammonium salt is preferable. It is a self-dispersion type coating agent having a functional group, more preferably a water-soluble or water-dispersible resin coating agent having an anionic group.

The resin having an anionic group is a resin having a compound having an anionic group bonded to the resin by copolymerization, grafting, or the like. Sulfonic acid, carboxylic acid, phosphoric acid and their lithium salts and sodium salts. , Potassium salt, ammonium salt and the like. The ratio of the anionic group to the coating solid content is 0.05 to 8% by weight.
Is preferred. If the amount of the anionic group is less than 0.05% by weight, the water solubility or water dispersibility of the resin may be poor, and if the amount of the anionic group exceeds 8% by weight, the coating layer may have water resistance or hygroscopicity. Problems can arise.

The coating solution for obtaining the coating layer in the present invention contains a methylol group or an alkyl group as a cross-linking agent in order to improve the adhesion (blocking property), water resistance, solvent resistance and mechanical strength of the coating layer. Rolled urea compounds, melamine compounds, guanamine compounds, acrylamide compounds, polyamide compounds, epoxy compounds, aziridine compounds, block polyisocyanates, silane coupling agents, titanium coupling agents, zirco-aluminate coupling agents, It may contain a peroxide, a heat- or photoreactive vinyl compound, a photosensitive resin, or the like.

Further, if necessary, an antifoaming agent, a coatability improving agent, a thickener, an antistatic agent, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment and the like may be contained. The effect of the present invention is sufficiently exhibited even in a system containing them. The thickness of the coating layer of the present invention is preferably selected from the range of 0.01 to 1 μm. Coating layer thickness is 0.01μ
If it is less than m, the effect of preventing the falling of large particles in the polyester film may not be exerted, or the strength of the coating film may be lowered, resulting in the falling of the coating film itself. If it exceeds 1 μm, the running property and blocking property may be deteriorated. .

When the coating layer of the coating film of the present invention is coated on one side, a magnetic layer is usually provided on the opposite side of the coating layer. The film of the present invention can be particularly effective when used as a base film for a video tape and also as an audio film. In other words, in such fields, models that have more than double the high-speed dubbing function such as double radio-cassette, compo stereo, etc. have recently become popular, and as a result, tapes and bases have been used during the dubbing process, fast-forwarding and rewinding. The materials come into contact with each other at a higher speed, but in this case, the effect of the present invention is effectively exhibited.

[0031]

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. The measuring methods and definitions of various physical properties and characteristics in the present invention are as follows. In addition, in Examples and Comparative Examples, "part"
"Parts by weight" means "parts by weight". The measuring method used in the present invention is as follows.

(1) Average particle size The particle size of the particles in the coating layer was determined by observing the cross section of the coating layer with a transmission electron microscope. The average particle diameter was defined as the diameter of the equivalent sphere-converted value with a cumulative volume fraction of 50%. The same operation was performed for the dry powder to determine the average particle size.

(2) Centerline average roughness (Ra) The average roughness was measured as follows using a surface roughness measuring device (SE-3F) manufactured by Kosaka Laboratory Ltd. That is, a portion having a reference length L (2.5 mm) is extracted from the film sectional curve in the direction of the center line, and the roughness curve y = f (with the center line of the extracted portion as the x-axis and the longitudinal magnification direction as the y-axis). When represented by x), the value given by the following formula is represented by [μm]. The center line average roughness was represented by the average value of the center line average roughness of the extracted portion obtained from the cross section curves of ten sample film surfaces. The tip radius of the stylus is 2
μm, load 30 mg, cut-off value 0.08 m
m.

[0034]

[Equation 1]

(3) Travelability Two films cut to a width of 15 mm and a length of 150 mm were stacked on a smooth glass plate, a rubber plate was placed on the film, and a load was placed on the rubber plate. The contact pressure of the film was set to 2 g / cm ² , and the films were slid at 20 mm / min to measure the frictional force between the coated surface and the non-coated surface.
The coefficient of friction at the point of 5 mm sliding is the dynamic friction coefficient (F / F
μd) to evaluate the running property.

(4) Abrasion characteristics The hard chrome fixing pin is made to travel for 1000 m while the coated surface of the film is in contact with the chrome fixing pin to visually observe wear white powder adhering to the hard chrome fixing pin. The grade was evaluated. The film speed was 13 m / min, the tension was about 200 g, and the winding angle of the film around the pin was 135 °. A: No white powder generated B: Small white powder generated C: Large white powder generated

(5) Scratch resistance A polyester film was brought into contact with a fixed pin made of hard chrome (diameter 6 mm, surface roughness 3S) with a winding angle of 135 degrees and a tension of 200 g so that the coated surface was in contact, and the speed was 13 m / m.
I ran for 100m in.

Next, aluminum was vapor-deposited on the contact surface of the polyester film with the fixing pin, and the degree of scratches was visually judged and classified into the following ranks. A: Almost no scratches B: Slight scratches are recognized, but satisfactory level C ... Relatively small amount of scratches, few deep scratches D ... Relatively large amount of scratches, deep scratches in some places E ... There are many scratches, often deep scratches

(6) Winding characteristics The appearance of the end surface of the film roll when wound into a roll was judged as follows. A: The end faces are aligned B: The end faces are slightly uneven C: The end faces are significantly uneven

(7) Electromagnetic Properties First, the following magnetic coating material was applied to the uncoated surface of a polyester film to form a magnetic layer so that the film thickness after drying was 5 μm. That is, 200 parts of magnetic fine powder, 30 parts of polyurethane resin, 10 parts of nitrocellulose, 10 parts of vinyl chloride / vinyl acetate copolymer, 5 parts of lecithin, and 100 parts of cyclohexanone.
Parts and 300 parts of methyl ethyl ketone are mixed and dispersed in a ball mill for 48 hours, 5 parts of a polyisocyanate compound is added to form a magnetic paint, which is applied to a polyester film and magnetically oriented before the paint is sufficiently dried and solidified,
It was then dried. Further, this coated film was surface-treated with a super calendar and slit into a 1/2 inch width to obtain a video tape. The following magnetic tape characteristics of this video tape were evaluated at a constant speed using an NV-3700 type video deck manufactured by Matsushita Electric Co., Ltd.

VTR Head Output The VTR head output at a measurement frequency of 4 MHz was measured with a synchroscope, the reference tape was set to 0.0 decibel (dB), and the relative value (dB) was measured. Number of dropouts (DO) A videotape recording a signal of 4.4 MHz was played back, the number of dropouts was measured for about 20 minutes with a dropout counter of Okura Industry Co., Ltd., and converted into the number of dropouts per minute. .

(8) Adhesiveness of Calender Roll A polyester film is passed through a calender roll having a polyester resin roll and a hard chrome plating roll so that the coating layer is in contact with the polyester resin roll, 100 ° C., linear pressure 200 Kgf / cm, speed 20 m. / Se
200 m was passed under the condition of c. At this time, the adhesiveness between the coating layer and the polyester resin roll was observed and evaluated according to the following criteria. A: I didn't stick at all and ran smoothly until the end. B: Weak adhesion is observed between the film and the resin roll. C ... The film and roll are very sticky, and there is a rattling noise or the film meanders.

Example 1 100 parts of dimethyl terephthalate, 60 parts of ethylene glycol and 0.09 part of magnesium acetate tetrahydrate were heated and heated, and methanol was distilled off to carry out a transesterification reaction, which required 4 hours from the start of the reaction. Then, the temperature was raised to 230 ° C. to substantially end the transesterification reaction. Then add 0.04 parts of ethyl acid phosphate,
Furthermore, 0.04 parts of antimony trioxide was added to carry out a polycondensation reaction, and after 4 hours and 30 minutes, polyethylene terephthalate having an intrinsic viscosity of 0.660 was obtained.

After drying the obtained polyester, 290 ° C.
Was melt-extruded to form an amorphous sheet, which was stretched 3.5 times in the machine direction at 90 ° C. Next, a coating solution containing the following particles was applied on one surface of the film by a bar coating method so that the coating thickness after stretching and drying was 0.10 μm.

Table 1 Coating liquid (parts by weight) Aluminum oxide particles (δ type, average particle size 0.02 μm) 0.3 Synthetic calcium carbonate particles (calcite type, average particle size 0.1 μm) 2.4 Polyester resin 7 .3 Water 90.0

Next, the film was stretched in the transverse direction at 110 ° C. by 3.7 times,
Heat treatment was performed at 210 ° C. for 3 seconds to obtain a film having a thickness of 15 μm. The evaluation results of the obtained film are shown in Table 1 below together with those of other examples and comparative examples.

Examples 2 and 3 and Comparative Examples 1 to 3 Films having a coating layer were produced under the same conditions as in Example 1 except that the conditions of the coating agent were changed as shown in Table 1, and the evaluation was performed. .

[0047]

[Table 2]

Any film satisfying the requirements of the present invention has excellent film characteristics and can be suitably used as a base film of a magnetic recording medium. On the other hand, Comparative Example 1 is an example in which the particles (B) are not used in combination, but there are problems in running property, winding property, and calendar adhesion.
The characteristics after magnetic tape formation are also inferior. Comparative Example 2 is an example in which the blending amount of the particles (B) is too large, but it is insufficient in terms of calendar adhesiveness and magnetic tape characteristics. Comparative Example 3 is an example in which the content of the particles (A) in the coating layer is less than the required amount of the present invention, but insufficient results are obtained in wear characteristics, wear resistance and magnetic tape characteristics. It was

[0049]

INDUSTRIAL APPLICABILITY The film of the present invention has excellent surface characteristics and is particularly suitable as a base film for magnetic recording media, and its industrial value is high.

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area B29L 9:00 4F

Claims (1)

[Claims] 1. A polyester film having a coating layer provided on at least one surface of the polyester film in the step of forming the polyester film, the coating layer comprising (A) aluminum oxide particles having an average particle size of 1 μm or less, and ( B) One or more particles selected from calcium carbonate, silicon dioxide and heat resistant organic particles having an average particle size larger than that of (A) and in the range of 0.05 to 2 μm are represented by the following formulas (1) to ( A biaxially stretched polyester film for a magnetic recording medium, which contains 3) at the same time. 0.1 ≦ Wa ≦ 50 (1) 5 ≦ Wb ≦ 50 (2) 0.1 ≦ Wb / Wa ≦ 20 (3) (Wa and Wb are particles A and particles B contained in the coating layer, respectively) Rate (% by weight)