JPH04151231A - Biaxially oriented polyester film for magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve sliding properties, shaving resistance and scratching resistance by containing inactive inorganic grains and heat-resistant high molecular grains having large grain size and alumina grains and colloidal silica grains having small grain size and forming specific projection distribution by said grains. CONSTITUTION:In a biaxially oriented polyester film, the distribution curve of the projections of the surface of the film satisfies formula (A) within a range of height x(mum) of 0.05mum or more and the number (y) (y/mm<2> or more) of projections by grains existing in polyester. A travelling friction coefficient increase is less than 0.15, and the projections are formed in the presence of 0.05-1.0wt.% alumina grains having an independent existence rate of 80% or more in the film and mean grain size of 0.06-0.2mum, 0.05-2.0wt.% colloidal silica grains having mean grain size larger than alumina grains as 0.1-0.3mum, inactive inorganic grains having mean grain size of 0.4-1.5mum and 0.01-1.0wt.% heat- resistant high molecular grains.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it relates to a biaxially oriented polyester film for magnetic recording media, and more particularly, it relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, it relates to a biaxially oriented polyester film for magnetic recording media, and more specifically, The present invention relates to a biaxially oriented polyester film for magnetic recording media, which contains alumina particles and colloidal silica particles of a particle size, and has improved sliding properties, abrasion resistance, and scratch resistance due to a specific protrusion distribution formed by the particles.

[Prior Art] Biaxially oriented polyester films, typified by polyethylene terephthalate films, are widely used as base films for magnetic recording media because of their excellent physical and chemical properties.

In a biaxially oriented polyester film, its slipperiness and abrasion resistance are major factors that affect the workability of the film manufacturing process and processing process, as well as the product quality. If these are insufficient, for example, when a magnetic layer is applied to the surface of a biaxially oriented polyester film and the film is used as a magnetic tape, the friction between the coating roll and the film surface is intense, resulting in the generation of abrasions and scratches on the film surface. Furthermore, when used for VTRs or data cartridges, chips and scratches are generated during the process of winding the tape into a cassette at high speed, which causes LOs.

Particularly in VTR applications, metal guides or plastic guides whose surfaces are not sufficiently finished are sometimes used as guide posts fixed in cassettes in order to reduce costs, but the surfaces of these guide posts are extremely rough and the back For uncoated magnetic tapes, conventional techniques to improve the slipperiness and abrasion resistance of films, such as the addition of inorganic particles such as silicon oxide, titanium dioxide, calcium carbonate, talc, clay, and calcined kaolin (for example, special In the method of precipitating fine particles containing calcium, lithium, or phosphorus in the polymerization system (see, for example, Japanese Patent Publication No. 52-32914), chips and scratches are removed during the process of rolling them into a cassette at high speed. occurs, and the level of Dlo is high, which is a problem. Therefore, there is a strong need for a film that has good abrasion and scratch resistance across all of the conventionally used metal guides with a good surface finish, the newly introduced metal guides with rough surfaces, and plastic guides. is the current situation.

[Objective of the Invention] The object of the present invention is to provide good abrasion resistance and resistance not only to conventionally used metal guides with a good surface finish, but also to newly emerging metal guides and plastic guides with a rough surface finish. The object of the present invention is to provide a biaxially oriented polyester film for magnetic recording media that has scratch resistance and excellent slip properties.

[Structures and Effects of the Invention] According to the present invention, the above object of the present invention is, firstly, to provide a biaxially oriented polyester film, the film being formed by (A) particles present in the polyester; The distribution curve of the protrusions on the surface has a height X (μm) of 0.05
The number of protrusions y (pieces/me2) is 30 pieces/me for μm or more
In the range of 2 or more, the following formula (A) is satisfied, and (B)
The increase in running friction coefficient (Δμk) is less than 0.15, and (C) the independent existence rate of the protrusions in the film is 80%.
Above, 0.05 to 1.0% by weight of alumina particles with an average particle size of 0.06 to 0.2 μm, and 0 colloidal silica particles with an average particle size of 0.1 to 0.3 μm, which is larger than the alumina particles. .05~2.0% by weight and average particle size 0.4~
11. Biaxially oriented polyester film for magnetic recording media, characterized in that it is formed by the presence of 0.01 to 1.0% by weight of inert inorganic particles and/or heat-resistant polymer particles having a diameter of 1.5 μm. 4x+4<log y<-10, Ox+5 -(
A) Here, X is the distance in the height direction from the reference level (μ
m>, and y is the number of protrusions counted when cutting at height X (pieces/m2).

The polyester in the present invention is a polyester containing an aromatic dicarboxylic acid as the main acid component and an aliphatic glycol as the main glycol component. Such polyesters are substantially linear and have film forming properties, particularly by melt molding. Examples of aromatic dicarboxylic acids include terephthalic acid and naphthalene dicarboxylic acid.

Isophthalic acid, diphenoxyethanedicarboxylic acid.

Diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid.

Examples include diphenylketone dicarboxylic acid and anthracene dicarboxylic acid. Examples of aliphatic glycols include polymethylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, and decamethylene glycol, and alicyclic glycols such as cyclohexane dimetatool. Examples include group diols.

In the present invention, polyesters containing, for example, alkylene terephthalate and/or alkylene naphthalate as main constituents are preferably used.

Among such polyesters, for example, polyethylene terephthalate and polyethylene-2,6-naphthalate, for example, 80 mol% of the total dicarboxylic acid component.
A copolymer in which the above is terephthalic acid and/or 2,6-naphthalene dicarboxylic acid and 80 mol% or more of the total glycol component is ethylene glycol is preferred. Up to 20 mol % of the total acid component can then be the above-mentioned aromatic dicarboxylic acids other than terephthalic acid and/or 2,6-naphthalene dicarboxylic acid, and for example adipic acid.

Aliphatic dicarboxylic acids such as sepatic acid; alicyclic dicarboxylic acids such as cyclohexane-1,4-dicarboxylic acid; and the like. In addition, up to 20 mol% of the total glycol component can be the above-mentioned glycols other than ethylene glycol, or aromatic diols such as hydroquinone, resorcinol, 2,2-bis(4-hydroxyphenyl)propane, etc.; , 4-dihydroxydimethylbenzene; and polyalkylene glycols (polyoxyalkylene glycols) such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and the like.

Furthermore, in the polyester of the present invention, a component derived from an oxycarboxylic acid such as an aromatic oxyacid such as hydroxybenzoic acid, an aliphatic oxyacid such as ω-hydroxycaproic acid, and a dicarboxylic acid component and an oxycarboxylic acid component are added. Those copolymerized or combined in an amount of 20 mol% or less based on the total amount are also included.

Furthermore, the polyester in the present invention contains a substantially linear amount of polycarboxylic acid or polyhydroxy compound, such as trimellitic acid, pentamellitic acid, etc., in an amount of 2 mol% or less based on the total acid component. Also included are those copolymerized with erythritol and the like.

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 measured as a solution in 0-chlorophenol at 35°C.
~0.9 is preferred.

The biaxially oriented polyester film of the present invention has a large number of fine protrusions on the film surface, as is clear from the explanation of the protrusion distribution that defines the flatness of the film surface.

According to the invention, these large numbers of fine protrusions originate from a large number of substantially inert solid particles dispersed in the polyester.

The distribution curve of protrusions on the film surface formed by particles present in the polyester has a height x (μm) of 0.05tt m or more and a number of protrusions y (numbers/rNB2) of 30/fIffi12 or more. , the following formula (
It is important to satisfy A).

-11,4x+4<log3'<-10,0x+5
=lA) Here, X is the distance in the height direction from the reference level (μm), and y is the number of protrusions counted when cutting at height X (pieces/mm2).

If the protrusion distribution curve deviates from the range of formula (A), running properties will be insufficient if the film is used as a magnetic tape without a back coat, or the magnetic layer surface will deteriorate due to the presence of large protrusions even if a magnetic layer is provided. It becomes rough and the electromagnetic modification properties become insufficient, making it undesirable for use in magnetic recording media.

In the present invention, the protrusions formed on the film surface are made of particles selected from inert inorganic particles and/or heat-resistant polymer particles, preferably two types or less, and alumina particles and colloidal silica particles. It is essential that at least three types of particles are used. That is, the average particle size is 0.
0.01 to 1.0% by weight of particles selected from inert inorganic particles and/or heat-resistant polymer particles having a size of 4 to 1.5 μm
and 0.05 to 0.05 alumina particles having an independent abundance rate of 80% or more in the film and an average particle size of 0.06 to 0.2 μm.
1.0% by weight and 0.0 colloidal silica particles whose average particle size is larger than alumina particles and 0.1 to 0.3 μm.
5 to 2.0% by weight in the polyester, it is necessary to form protrusions on the film surface.

The particles selected from inert inorganic particles and/or heat-resistant polymer particles in the present invention have an average particle size of 0.4 to 1.5.
μm, preferably 0.5 to 1.0 μm, and the amount added is 0.01 to 1.0% by weight, preferably 0.05 to 0.5
% by weight, the particles can be used in combination, and in this case it is preferable to use two or less types of particles.

In this case, each particle must satisfy the above-mentioned average particle diameter, and the amount added is such that the total amount satisfies the above-mentioned addition amount. If the average particle diameter of these particles is less than 0.4 μm, the height of the protrusions formed will be insufficient, and the resulting film will have insufficient slipperiness. On the other hand, the average particle size is 1.5μ
If it exceeds m, the height of the protrusions formed will become too large and the surface of the film will become too rough for use as a magnetic recording medium, which is not preferable. Also, the amount added is 0.01% by weight
If the amount is less than 1.0% by weight, the film surface will become too rough, making it unsuitable for use in magnetic recording media.

Specific examples of the above-mentioned inert inorganic particles include silicon oxide, calcium carbonate, talc, clay, kaolin, etc. Among them, spherical silica and calcium carbonate are preferred from the viewpoint of slipperiness. Specific examples of the heat-resistant polymer particles include organic polymer particles such as silicone resin particles, crosslinked acrylic particles, crosslinked polystyrene particles, crosslinked polyester particles, Teflon particles, and polyimide particles, among which silicone resin particles is preferred.

In the present invention, in addition to the above-mentioned inert inorganic particles and/or heat-resistant polymer particles, alumina particles having an independent abundance rate of 80% or more in the film and an average particle size of 0.06 to 0.2 μm are used. 0.05 to 1.0% by weight, and 0.05 to 2.0% by weight of colloidal silica particles having an average particle size larger than that of the alumina particles and 0.1 to 0.3 μm, more preferably 0.1 to 1.0% by weight. It is important to add .0% by weight. The alumina particles and colloidal silica particles added form fine protrusions between the high protrusions formed by the inert inorganic particles and/or heat-resistant polymer particles on the film surface.

These two types of minute protrusions are thought to contribute to lowering the coefficient of friction at the bare surface of the film, but according to the inventor's study, their effects are different, and the fine protrusions caused by alumina particles sufficiently It exhibits excellent abrasion and scratch resistance in contact with unfinished metal guides and plastic guides, and the fine protrusions created by colloidal silica particles provide excellent resistance in contact with plastic guides and metal guides with a good surface finish. Excellent abrasion resistance and scratch resistance, and excellent abrasion resistance across all metal guides with a good surface finish, metal guides with poorly finished surfaces, and plastic guides.
It has become clear that in order to obtain scratch resistance, it is necessary for these two types of particles to coexist within a specific range.

The alumina particles used in the present invention must have the above-mentioned properties; if the average particle diameter is less than 0.06 μm, the protrusions formed on the surface of the film will be small and the reduction in the coefficient of friction will be insufficient; on the other hand, if it exceeds 0.2 μm. This is undesirable because the protrusions formed become too large and the abrasiveness during high-speed contact with a friction body, especially a plastic guide, deteriorates. Furthermore, if the amount of alumina particles added is less than 0.05% by weight, the number of protrusions formed on the surface of the film will be insufficient and no improvement in abrasion resistance or scratch resistance can be expected; on the other hand, if the amount exceeds 1.0% by weight, Overlap of alumina particle protrusions occurs in the surface area, deteriorating the abrasion resistance. Further adding 80% of alumina particles
It is also important that more than % is independent in the film,
If this is not satisfied, the abrasion resistance will deteriorate.

The alumina particles used in the present invention are more preferably γ-type crystals having a Mohs hardness of less than 8. When using α-type crystal alumina particles with a Mohs hardness of 9, depending on the additive conditions, the additive particles may be too hard when running for a long time in the manufacturing process of magnetic tapes, etc., and the rolls etc. in the process may be In addition, the metal guide side may be damaged during running of the magnetic tape or the like and winding into the cassette, which may cause abrasions and base clutch failure.

The colloidal silica particles used in the present invention must have an average particle diameter larger than that of the coexisting alumina particles, ranging from 0.1 to 0.0 μm. If this particle size is smaller than the alumina particles,
Poor scratch resistance in contact with metal guides with good surface finish. Furthermore, even if the particle size is less than 0.1 μm, the scratch resistance is poor when in contact with a metal guide with a good surface finish. On the other hand, if this particle size exceeds 0.3 μm,
This is undesirable because the formed protrusions become too large, which worsens the abrasiveness during high-speed contact with a friction body, especially a metal guide whose surface is not sufficiently finished. If the amount added is less than 0.05% by weight, the number of protrusions formed will be insufficient and no improvement in scratch resistance will be observed, while if it exceeds 2.0% by weight, the abrasion resistance will deteriorate. The colloidal silica particles used in the present invention preferably contain Na in a powder weight ratio of 100 to 3000 ppm. When the Na content is outside this range, the scratch resistance becomes poor. The cause of this is not clear, but it is thought to be due to subtle differences in the state of existence of particles in the polyester.

Methods for producing such colloidal silica include dialysis, electrolysis, salting-out ratio method, or ion exchange resin using sodium silicate as a starting material, followed by polymerization. From the viewpoint of uniformity, the ion exchange resin polymerization method is most preferred.

In the present invention, the method of incorporating the above-mentioned inert particles into polyester is not particularly limited, but for example, inert inorganic particles and/or heat-resistant polymer particles, alumina particles, and colloidal silica particles are added to the polyester before polymerization of the polyester. Alternatively, it can be added during polymerization and intimately mixed with the polyester, it can be added when pelletizing the polystill after polymerization and it can be thoroughly mixed in an extruder, or it can be thoroughly mixed with the polyester in an extruder when melt-extruded into a sheet. Examples include a method of kneading, etc. Among these, a method of adding before or during polymerization of polyester is preferred.

The biaxially oriented film of the present invention has an increased running friction coefficient (Δμk
) must be less than 0.15. Δμk is 0.15
If the magnetic tape exceeds this level, the running properties become insufficient, and for example, when a magnetic tape is used repeatedly, the running properties of the tape decrease, and the
If k exceeds 0.3, trouble will occur where the vehicle stops running. Adjustment to Δμ depends on the type of inert particles, the particle size,
This is carried out depending on the amount added, stretching and orientation conditions, etc., but it is particularly preferable to select the characteristics of other inert inorganic particles and/or heat-resistant polymer particles.

The biaxially oriented polyester film of the present invention can be produced by melt-pressing polyester at a temperature of, for example, melting point (Tm:'C) or Tm+70>,'C, and has an intrinsic viscosity of 0.35 to 0.9 dl.
/l of an unstretched film, and the unstretched film was uniaxially (vertical or transverse) (Tg-10) to (Tg+7
0)°C (Tg: glass transition temperature of polyester) at a magnification of 2.5 to 5.0 times, and then in a direction perpendicular to the above stretching direction; The second stage stretching is in the lateral direction) from Tg(”C) to (Tg
It can be produced by stretching at a temperature of 2.5 to 5.0 times at a temperature of +70>"C. In this case, the area stretching ratio is 9 to 2.
It is preferable to increase the amount by 2 times, more preferably from 12 to 22 times. The stretching means may be either simultaneous biaxial stretching or sequential biaxial stretching, but the latter is more preferred.

Furthermore, the biaxially oriented film has (Tg+70>'C~T
It can be heat-set at a temperature of m ('C).

For example, polyethylene terephthalate film is preferably heat-set at 190 to 230°C.

The heat setting time is, for example, 1 to 60 seconds.

The biaxially oriented polyester film of the present invention contains a specific range of inert inorganic particles and/or heat-resistant polymer particles, as well as alumina particles and colloidal silica particles, so it has excellent slipperiness and a good surface finish. It has excellent abrasion resistance and scratch resistance for all metal guides, metal guides with unfinished surfaces, and plastic guides, and is extremely useful for magnetic recording media.

Note that various physical property values and characteristics in the present invention were measured and defined as follows.

(1) Average particle diameter (DP) of the particles Shimadzu CP-50 type Centrifoal Particle Size Analyzer (Centrifoal P
Article 5ize Analyser). From the integrated curve of particles of each particle size and their abundance calculated based on the obtained centrifugal sedimentation curve, the particle size corresponding to 50 mass percent is read, and this value is taken as the above average particle size (Book r particle size measurement (See "Technology" published by Nikkan Kogyo Shimbun, 1975, pp. 1, 242-247).

(2) Film running friction coefficient μk) Measured as follows using the apparatus shown in FIG. In Figure 1, 1
is the unwinding reel, 2 is the tension controller, 3.5
．． 6.8.9 and 11 are free rollers, 4 is tension detector (in Japan), 7 is stainless steel S OS 304
The fixing rod was made of aluminum with an outer diameter of 5 mφ and a surface roughness of Ra = 0. Obzm
), 10 is a tension detector (outlet), 12 is a guide roller, and 13 is a take-up reel.

In an environment with a temperature of 20°C and a humidity of 60%, a film cut to a width of 1/2 inch is attached to a fixed rod 7 at an angle θ = (152/18
1) Contact with π radians (152°) and move (friction) at a speed of 200 pieces per minute. Inlet tension T1 is 3
When the tension controller 2 is adjusted so that the tension controller 2 is adjusted to 5g, the exit tension (T2:g) is 90m.
After running, the outlet tension detector detects the tension, and calculates the running friction coefficient μk using the following formula.

μk = +2.303/θ) log (T2,
/Tl ) = 0.868 log (T2 /35
>(3) Increase in film running friction coefficient (Δμk) A 10 m long film was repeatedly run 50 times at a film moving speed of 2 m/min using the above device for measuring the film running friction coefficient (μk). The friction coefficient of the first time at that time is set to μ and 1. The friction coefficient of the 50th time is set to μ and 5o, and Δμk is calculated by the following formula.

Δ, gk=μ kso −μ kt (4) The flatness Ra (center line average roughness) of the film surface is measured according to JIS B 06 old. Stylus type surface roughness meter (S
Using URFCOM3B), set the needle radius 2. tzm, a chart (film surface roughness curve) is drawn under the conditions of a load of 0.07 g, and from the obtained film surface roughness curve, a part of the measurement length is extracted in the direction of its center line, and the center of this extracted part is When the roughness curve Y = f (X) is expressed with the line as the X axis and the vertical magnification direction as the Y axis, the value given by the following formula (Ra: μm) is defined as the flatness of the film surface. .

In the present invention, Ra
represents.

(5) Protrusion distribution Three-dimensional scale meter manufactured by Koita Research Institute (using + on 5E-30, needle diameter 2 μm R, needle pressure 30 mm, measurement length 1 mm, sampling pitch 2 μm, cutoff 0.25 mm, Wi Large magnification in the force direction 20,000 times, horizontal magnification 200 times.

The profile of protrusions on the film surface is imaged three-dimensionally (stereoscopically) under the condition of 150 scans.

When the profile is cut in a plane perpendicular to the thickness direction of the film, the plane where the total cross-sectional area of the profile of each protrusion is 70% of the area of the measurement area of the film is defined as the reference level (0 level). , the number of protrusions counted when cutting with a plane parallel to the plane of the reference level and separated by a distance of X (μm) in the height direction of the protrusions is expressed as y(
pieces/closed 2). A protrusion distribution curve is drawn by sequentially increasing or decreasing X, reading the number of y at that time, and plotting it on a graph.

(6) Independent abundance ratio of particles The biaxially oriented polyester film obtained by film formation was fixed on a sample stage for a scanning electron microscope, and sputtering equipment manufactured by JEOL ■ was used! Using a JFC-1100 type ion sputtering apparatus, the surface of the film is subjected to ion etching treatment under the following conditions. Install the above sample stage inside the venter! Then, the degree of vacuum is raised to a vacuum state of about 10-3 Torr, and ion etching is performed for about 10 minutes at a voltage of 0.25 kV and a current of 12.5 mA. Furthermore, gold sputtering is applied to the film surface using the same equipment to form a thin gold film layer of about 200A, and measurements are performed at a magnification of 20,000 times using a scanning electron microscope equipped with an XMA.
Obtain the total number of particles a and the number of agglomerated and exfoliated particles b (with voids occurring over the entire width between adjacent particles due to stretching stress during film formation) while confirming that they are alumina particles, and use the following formula. , find the independent abundance ratio of particles.

(7) Na content in colloidal silica powder Colloidal silica powder is completely dissolved using hydrofluoric acid, and Na is quantified by atomic absorption method to determine the Na content in the powder.

(8) Abrasion resistance and scratch resistance In the same device as shown in Figure 1 used to measure the running friction coefficient of the film, the surface was sufficiently finished with a S IJ S 304 fixing rod instead of the stainless steel SU S 304 fixing rod in 7. 6φ tape guide (surface roughness Ra = 0.01
5 μm) is used, method B is the case where a 6φ tape guide (surface roughness Ra-0,151zm) is used, which is made by bending a SUS sintered plate into a cylindrical shape and has an insufficient surface finish. Carbon black-containing polyacetal manufactured by Isseiko Co., Ltd. 6φ
Using method C, a tape guide is used, and the contact angle with the tape guide is 30 degrees, and the machine is run for 200 m at a speed of 300 m/min with an inlet tension of 50 g/1/2'. After running, evaluate the shavings adhering to the guide and the scratches on the tape after running.

Scraping powder judgment> ◎ No scraping powder is seen ○ Scraping powder is seen evenly △ Existence of scraping powder can be seen at a glance × Scraping powder is heavily adhered <Scratch judgment> ◎ No scratches are seen 01 to 5 scratches observed △ 6 to 15 scratches observed x 16 or more scratches observed [Example] The present invention will be described below with reference to Examples.

Examples 1 to 4, Comparative Examples 1 to 6 Dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, antimony trioxide as a polymerization catalyst, phosphorous acid as a stabilizer, and lubricant as shown in Table 1 The additive particles are added and polymerized by a conventional method, and the intrinsic viscosity (orthochlorophenol, 35°C) is 0.
．． 62 polyethylene terephthalate was obtained.

This polyethylene terephthalate pellet was heated to 170°C.
After drying for 3 hours at
The molten polymer is melted at 0 to 300°C and passed through a lsnm slit die to give a surface finish of Oj! This was extruded onto a rotating cooling drum with a surface temperature of 20° C. to obtain an unstretched film of 200 μm.

The unstretched film thus obtained was preheated at 75°C, and further rolled between low speed and high speed rolls from 15 am above.
3. Heating with one IR heater with a surface temperature of 00°C.
Stretched 6 times, rapidly cooled, then fed to a stenter, 1
It was stretched 3.9 times in the transverse direction at 05°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 with a thickness of 14.2 μm.

The properties of the 14.2 μm biaxially oriented polyester film thus obtained are shown in Table 1. In addition, when examining the protrusion distribution of the obtained films, Examples 1 to 4 and Comparative Examples 1 to 5 satisfied the following formula (A) and had excellent slipperiness. 6 is the number of protrusions y (pcs/kaku2
) does not satisfy the relationship of log y < -10, 0x + 5 over the entire range, and the running friction coefficient after repeated running 50 times is as high as 0.31, which is a level of concern for practical use. There is.

-11,4x+4<log y<-10,0x+5
=lAlAs for the abrasion and scratch resistance in Table 1, if it is ○ level or above, there is no practical concern as a magnetic tape, but if it reaches Δ level, it depends on the usage conditions, but scraping powder on the tape guide There is concern that dropouts will increase due to scratches on the tape.

As is clear from Table 1, the metal tape guide according to the present invention has excellent sliding properties for use in magnetic recording media, as well as metal tape guides with a sufficiently finished surface and metal tape guides with an unfinished surface. The film and the plastic tape guide all have excellent abrasion resistance and scratch resistance, and have extremely excellent properties as a biaxially oriented polyester film for magnetic recording media.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus for measuring the running friction coefficient of a film. In the figure, 1 is an unwinding reel 12 which is a tension controller, 4 is a tension detector (inlet), 7 is a fixed rod, 10 is a tension detector (outlet), and 13 is a take-up reel.

Claims (1)

[Claims] 1. A biaxially oriented polyester film, comprising (
A) The distribution curve of protrusions on the film surface formed by the particles present in the polyester has a height x (μm)
is 0.05μm or more and the number of protrusions y (pieces/mm^2) is 3
In the range of 0 protrusions/mm^2 or more, the following formula (A) is satisfied, (B) the increase in running friction coefficient (Δμk) is less than 0.15, and (C) the protrusions are present in the film. 0.05 to 1.0% by weight of alumina particles with an independent abundance rate of 80% or more and an average particle size of 0.06 to 0.2 μm, and an average particle size of 0.1 to 0.3 μm larger than the alumina particles. 0.05 to 2.0% by weight of colloidal silica particles, and 0.01 to 1.0% by weight of inert inorganic particles and/or heat-resistant polymer particles having an average particle size of 0.4 to 1.5 μm. A biaxially oriented polyester film for magnetic recording media, characterized in that it is formed by the presence of. -11.4x+4<logy<-10.0x+5...
(A) Here, 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 cutting at height x. 2. The biaxially oriented polyester film for a magnetic recording medium according to claim 1, wherein the alumina particles are particles of γ type crystal having a Mohs hardness of less than 8. 3. Colloidal silica particles have a powder weight ratio of 100 to 300
The biaxially oriented polyester film for magnetic recording media according to claim 1, containing 0 ppm of Na.