JP2528202B2 - Biaxially oriented polyester film for magnetic recording media - Google Patents

Description

TECHNICAL FIELD The present invention relates to a biaxially oriented polyester film for a magnetic recording medium, more specifically, other inert inorganic particles and / or heat resistant polymer particles, alumina particles and The present invention relates to a magnetic recording medium biaxially oriented polyester film containing titanium oxide particles and having improved slipperiness, abrasion resistance and scratch resistance due to a specific distribution of protrusions formed by the particles.

[Prior Art] Biaxially oriented polyester films represented 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 determine the quality of workability in the film manufacturing and processing steps, and further, 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 used as a magnetic tape, the friction between the coating roll and the film surface is so high that shaving powder is generated or scratches are generated on the film surface. Also, when used for VTRs and data cartridges, shavings and scratches are generated during the high-speed winding process into the cassette, causing D / O.

Particularly in VTR applications, metal guides or plastic guides whose surfaces are not sufficiently finished may be used for guide posts fixed in cassettes for the purpose of cost reduction recently, but the surface of the guide posts is extremely rough, For magnetic tapes without a coating, techniques for improving the slipperiness and abrasion of conventional films, such as silicon oxide, titanium dioxide, calcium carbonate, talc, clay,
A method of adding inorganic particles such as calcined kaolin (see, for example, JP-A-54-57562) or a method of depositing fine particles containing calcium, lithium or phosphorus in a polymerization system (see, for example, JP-B-52-32914). In such a case, shavings and scratches are generated in the process of high-speed winding into the cassette, and the D / O level is high, which is a problem. Therefore, there is a strong demand for a film that has good abrasion resistance and scratch resistance for all metal guides that have been used in the past, such as metal guides with a good surface finish, new metal guides with a rough surface, and plastic guides. Is the current situation.

[Object of the Invention] The object of the present invention is not only to use metal guides having a good surface finish conventionally used, but also to metal guides and plastic guides having a new rough surface finish, which have good abrasion resistance It is intended to provide a biaxially oriented polyester film for a magnetic recording medium which has a scratch property and is excellent in slipperiness.

[Constitution and Effect of the Invention] According to the present invention, the above-mentioned object of the present invention is, first, a biaxially oriented polyester film, wherein (A) a film formed by particles present in the polyester. The distribution curve of the projections on the surface shows that the height x (μm) is 0.05μ
Within the range of m or more and the number of protrusions y (pieces / mm ² ) is 30 pieces / mm ² or more, the following expression (A) is satisfied, and (B) running friction coefficient increase (Δμk) is less than 0.15, and (C) The protrusion is
Independent existence rate in the film is 80% or more, average particle size is 0.06
.About.0.2 .mu.m of alumina particles 0.05 to 1.0% by weight, 0.01 to 1.0% by weight of anatase type titanium oxide particles having an average particle size larger than that of alumina particles of 0.1 to 0.5 .mu.m, and an average particle size of 0.3 to 1.5 .mu.m. Other inert inorganic particles and / or
Biaxially oriented polyester film for magnetic recording medium characterized by being formed by the presence of 0.01 to 1.0% by weight of heat resistant polymer particles -11.4x + 4 <log y <10.0x + 5 (A) where x is a reference Height distance from level (μ
m) and y is the number of protrusions (pieces / mm ² ) counted when cut at height x.

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

In the present invention, as the polyester, for example, those having alkylene terephthalate and / or alkyl naphthalate as a main constituent component are preferably used.

Among such polyesters, for example, polyethylene terephthalate and polyethylene-2,6-naphthalate, of course, 80% or more of the total dicarboxylic acid component is terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and the total glycol component is Is preferably a copolymer in which at least 80 mol% of ethylene glycol is ethylene glycol. At that time, 20 mol% or less of all the acid components is terephthalic acid and / or 2,6-
The above-mentioned aromatic dicarboxylic acids other than naphthalenedicarboxylic acid can be used, and aliphatic dicarboxylic acids such as adipic acid and sebacic acid; cyclohexane-1,4-
It can be an alicyclic dicarboxylic acid such as a dicarboxylic acid. Also, up to 20 mol% of the total glycol component can be the above glycols other than ethylene glycol, or for example hydroquinone, resorcin, 2,2-
Aromatic diols such as bis (4-hydroxyphenyl) propane; Aliphatic diols having an aromatic ring such as 1,4: dihydroxydimethylbenzene; Polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, etc. Oxyalkylene glycol) or the like.

In the polyester of the present invention, a component derived from an oxycarboxylic acid such as an aromatic oxyacid such as hydroxybenzoic acid or an aliphatic oxyacid such as ω-hydroxycaproic acid may be used. Those which are copolymerized or bonded at 20 mol% or less based on the total amount are also included.

Further, in the polyester of the present invention, a polycarboxylic acid or polyhydroxy compound having a functionality of 3 or more, such as trimellitic acid or penta, is used in an amount in a substantially linear range, for example, an amount of 2 mol% or less based on all acid components. A copolymer of erythritol and the like is also included.

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

As the above polyester, the intrinsic viscosity measured at 35 ° C. as a solution in o-chlorophenol is about 0.4 to
A value of about 0.9 is preferred.

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

According to the present invention, these numerous fine projections are derived from a large number of substantially inert solid fine particles dispersedly contained in the polyester.

The distribution curve of the protrusions on the film surface formed by the particles present in the polyester has a height x (μm).
It is important that the following formula (A) is satisfied in the range where the number of protrusions y (number / mm ² ) is 0.05 / μm or more and 30 / mm ² or more.

−11.4x + 4 <log y <10.0x + 5 (A) where x is the distance in the height direction from the reference level (μ
m), and y is the number of protrusions (pieces / mm ² ) counted when cut at the height x.

If the projection distribution curve deviates from the range of the formula (A), the film will not have sufficient running property when used as a magnetic tape without a back coat, and even if a magnetic layer is provided, the presence of large projections causes the magnetic layer surface Is unfavorable for a magnetic recording medium, because the magnetic field becomes rough and the electromagnetic change characteristics become insufficient.

In the present invention, the projections formed on the film surface are:
It is important to use at least three kinds of particles consisting of particles selected from inert inorganic particles and / or heat-resistant polymer particles, preferably two or less, and alumina particles and titanium oxide particles. That is, the average particle size is 0.3 to 1.5
0.01 to 1.0% by weight of particles selected from inert inorganic particles other than alumina particles and titanium oxide particles and / or heat-resistant polymer particles having a particle size of 80 μm, and independent existence rate in the film of 80% or more, average particles 0.05 to 1.0% by weight of alumina particles having a diameter of 0.06 to 0.2 μm and 0.01 to 1.0% by weight of anatase type titanium oxide particles having an average particle size larger than that of alumina particles of 0.1 to 0.5 μm are present in the polyester. Therefore, it is necessary to form protrusions on the film surface.

The particles selected from the inert inorganic particles and / or the heat resistant polymer particles in the present invention have an average particle size of 0.3 to 1.5 μm.
m, preferably 0.5 to 1.0 μm, and the added amount is 0.01 to 1.
It is 0% by weight, preferably 0.05 to 0.5% by weight, but the particles can be used in combination, and in this case, it is preferable to use particles of 2 kinds or less. In this case, it is necessary for each particle to satisfy the above-mentioned average particle diameter, and the total amount of addition should satisfy the above-mentioned addition amount. The average particle size of these particles is 0.3
If it is less than μm, the height of the projections formed is insufficient, and the slipperiness of the obtained film is insufficient. On the other hand, if the average particle size exceeds 1.5 μm, the height of the formed projections becomes too large, and the film surface becomes too rough for use as a magnetic recording medium, which is not preferable. If the addition amount is less than 0.01% by weight, the number of projections formed is insufficient, and the lubricity becomes insufficient. If the addition amount exceeds 1.0% by weight, the film surface becomes too rough, which is suitable for magnetic recording media. Absent.

Specific examples of the inert inorganic particles include silicon oxide,
Examples thereof include calcium carbonate, talc, clay, kaolin, etc. Among them, silicon oxide and calcium carbonate are preferable 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 them, silicone resin particles are preferable. Is preferred.

In the present invention, the inert inorganic particles and / or
In addition to the heat-resistant polymer particles, the independent existence rate in the film is 80% or more, the average particle size is 0.06 to 0.2 μm, and the alumina particles are 0.05 to 1.0% by weight. It is important to add 0.01 to 1.0% by weight, more preferably 0.03 to 0.5% by weight, of anatase type titanium oxide particles having a size of 0.5 μm. The alumina particles and the anatase type titanium oxide particles to be added form fine projections between the high projections formed by the inert inorganic particles and / or the heat resistant polymer particles on the film surface.

It is considered that these two kinds of fine projections contribute to the reduction of the friction coefficient of the background portion of the film, but in the study by the present inventor, the action and effect are different, and the fine projections by the alumina particles are sufficient on the surface. It exhibits excellent abrasion resistance and scratch resistance when contacting unfinished metal guides and plastic guides, and the fine projections due to anatase-type titanium oxide particles allow contact with plastic guides and metal guides with a good surface finish. In order to exert excellent abrasion resistance and scratch resistance, and to obtain excellent abrasion resistance and scratch resistance over all metal guides with a good surface finish, guides where the surface is not sufficiently finished and plastic guides It has become clear that it is necessary for these two types of particles to coexist in a specific range.

The alumina particles used in the present invention need to have the above-mentioned properties.If the average particle size is less than 0.06 μm, the projections formed on the film background are small, and the reduction in the coefficient of friction becomes insufficient. The resulting projections become too large, and the shaving property at the time of high-speed contact with a friction body, particularly a plastic guide, deteriorates, which is not desirable. If the amount of the alumina particles is less than 0.05% by weight, the number of projections formed on the background of the film is insufficient, so that improvement of the abrasion resistance and scratch resistance cannot be expected. The projections of the particles overlap, and the abrasion resistance deteriorates. It is also important that 80% or more of the alumina particles to be added are independent in the film, and if this is not satisfied, the abrasion resistance deteriorates.

The alumina particles used in the present invention are more preferably γ-type crystals having a Mohs hardness of less than 8. In the case of using α-type crystalline alumina particles having a Mohs hardness of 9, depending on the addition conditions, the added particles are too hard when run for a long time in the manufacturing process of a magnetic tape or the like. In some cases, the metal guide side may be damaged when the magnetic tape or the like is run or when the magnetic tape is wound into the cassette, which may cause shavings or a base clutch.

The anatase type titanium oxide particles used in the present invention should have an average particle size larger than that of coexisting alumina particles by 0.1 to 0.5 μm. If the particle size is smaller than the alumina particles, scratch resistance is poor in contact with a metal guide having a good surface finish. Even if the particle size is less than 0.1 μm, the scratch resistance is poor in contact with a metal guide having a good surface finish. On the other hand, if this particle size exceeds 0.5 μm,
The formed protrusions become too large, and the shaving property at the time of high-speed contact with a friction body, particularly a metal guide whose surface is not sufficiently finished, is undesirably deteriorated. If the addition amount is less than 0.01% by weight, the number of projections formed is insufficient and no improvement in scratch resistance is observed, while if it exceeds 1.0% by weight, the abrasion resistance deteriorates. The titanium oxide particles used in the present invention need to be of anatase type. The rutile type has a lower Mohs hardness than the coexisting alumina particles, but is harder than the anatase type, and is undesirable because it may damage a roll or the like in a magnetic tape manufacturing process or damage a metal guide or the like during magnetic tape running. The difference in the damageability of the metal guide due to the hardness of the alumina particles and the titanium oxide particles is unknown, but it is considered to be due to the presence state of the particles in the polyester and the difference in the primary form of the particles.

The anatase type titanium oxide particles used in the present invention are P ₂ O ₅
Contains 0.25% by weight or more of P element in conversion, and in K ₂ O conversion
It is more preferable to contain the element K in an amount of 0.1% by weight or more because coarse particles caused by the titanium oxide particles can be easily removed in the polyester polymerization step.

In the present invention, the method of including the above-mentioned inert particles in the polyester is not particularly limited. For example, an inert inorganic particle and / or a heat-resistant polymer particle, an alumina particle and an anatase type titanium oxide particle are mixed with a polyester. Addition before or during polymerization and intimate mixing with polyester, Addition when pelletizing polyester after polymerization and intimately mixing in extruder, Melt extrusion into sheet form For example, a method of sufficiently kneading may be used. Among these, the method of adding before or during the polymerization of polyester is preferable.

The biaxially oriented film of the present invention has an increased running friction coefficient (Δμ
k) must be less than 0.15. When Δμk is 0.15 or more, the running characteristics become insufficient.For example, when a magnetic tape is used repeatedly, the running property of the tape is reduced, and μk is reduced to 0.
If you exceed 3, you will have trouble running. The adjustment of Δμk is carried out by adjusting the kind, particle size, addition amount and stretching orientation conditions of the inert particles, 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, for example,
Polyester is melt-pressed at a temperature of melting point (Tm: ° C) to (Tm + 70) ° C to obtain an unstretched film having an intrinsic viscosity of 0.35 to 0.9 dl / g, and the unstretched film is uniaxially (longitudinal or transverse) (Tg Temperature of -10) to (Tg + 70) ° C (Tg:
(Glass transition temperature of polyester) at a magnification of 2.5 to 5.0 times, then Tg in the direction perpendicular to the above stretching direction (if the first stage stretching is longitudinal, the second stage stretching is transverse)
It can be manufactured by stretching at a temperature of (° C.) to (Tg + 70) ° C. at a magnification of 2.5 to 5.0 times. In this case, the area stretching ratio is 9 to
It is preferably 22 times, more preferably 12 to 22 times. The stretching means may be either simultaneous biaxial stretching or sequential biaxial stretching, but the latter is more preferred.

Further, the biaxially oriented film is (Tg + 70) ° C. to Tm
It can be heat-set at a temperature of (° C). For example, a polyethylene terephthalate film is preferably heat-set at 190 to 230 ° C. Heat setting time is, for example, 1 to 60
Seconds.

The biaxially oriented polyester film of the present invention contains inert inorganic particles and / or heat-resistant polymer particles in a specific range, alumina particles, and anatase-type titanium oxide particles, and thus has excellent slipperiness and surface finish. Excellent metal guide, excellent metal surface not finished metal guide and plastic guide all excellent abrasion resistance,
It has scratch resistance and is extremely useful for magnetic recording media.

Incidentally, various physical properties and characteristics in the present invention are measured and defined as follows.

(1) Average particle size (DP) Shimadzu CP-50 type Centrifugal particle size analyzer (Centrifugal Particle Siz
e Analyser). The particle size corresponding to 50 mass percent is read from the integrated curve of particles of each particle size and its abundance calculated based on the obtained centrifugal sedimentation curve, and this value is defined as the average particle size described above (Book “Particle Size Measurement Technology, published by Nikkan Kogyo Shimbun, 1975, pp. 242 to 247).

(2) Coefficient of running friction (μk) of the film Measured as follows using the apparatus shown in FIG. In FIG. 1, 1 is an unwinding reel, 2 is a tension controller, 3, 5, 6, 8, 9 and 11 are free rollers, 4 is a tension detector (entrance), and 7 is a stainless steel SUS304 fixing rod ( Outer diameter 5 mmφ, surface roughness Ra = 0.02 μm), 10 is a tension detector (outlet), 12 is a guide roller, and 13 is a take-up reel.

In the environment of temperature of 20 ℃ and humidity of 60%, the film cut into 1/2 inch width is brought into contact with 7 fixed rods at an angle θ = (152/181) π radian (152 °) and the speed is 200 cm / min. Move (rub) with. When the tension controller 2 is adjusted so that the inlet tension T ₁ is 35g, the outlet tension (T ₂ : g) is detected by the outlet tension detector after the film has run 90m, and the running friction coefficient μk is calculated by the following formula. calculate.

.mu.k = a device for measuring (2.303 / θ) log (T 2 / T 1) = 0.868log (T 2/35) (3) running friction coefficient increase of the film (Δμk) running friction coefficient of the film (.mu.k) Using a film moving speed of 2 m / min, a 10 m long film is repeatedly run 50 times. The friction coefficient at the first time is μk ₁ and the friction coefficient at the 50th time is μk ₅₀ , and Δ
Calculate μk.

_{Δμk = μk 50 -μk 1 (4} ) flatness Ra (centerline average roughness) of the film surface is measured according to JIS B 0601. Stylus type surface roughness meter manufactured by Tokyo Seimitsu Co., Ltd. (SURFCOM3
Using B), draw a chart (film surface roughness curve) under the conditions of a needle radius of 2 μm and a load of 0.07 g, and measure the length L from the obtained film surface roughness curve toward the center line. , And the center line of this extracted portion is the X axis, and the direction of the vertical magnification is the Y axis. The roughness curve Y = f (X)
When expressed by, the value (Ra: μm) given by the following equation is defined as the flatness of the film surface.

In the present invention, eight pieces are measured with the reference length set to 0.25 mm, and Ra is expressed as an average value of five pieces obtained by removing three pieces from the larger value.

(5) Protrusion distribution measurement method Using a three-dimensional roughness meter (SE-30K) manufactured by Kosaka Laboratory, needle diameter 2 μmR, needle pressure 30 mg, measurement length 1 mm, sampling pitch 2 μm,
Under the conditions of a cutoff of 0.25 mm, a vertical magnification of 20,000 times, a horizontal magnification of 200 times, and a scanning number of 150, the profile of the projection on the film surface is three-dimensionally imaged.

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

(6) Independent abundance of particles The biaxially oriented polyester film obtained by film formation was fixed to a sample stage for a scanning electron microscope, and a sputtering device manufactured by JEOL Ltd. (JFC-1100 type ion sputtering device) was used. The film surface is subjected to ion etching treatment under the following conditions. Install the sample stage in the benger, raise the vacuum to about 10 ^-3 Torr and raise the voltage to 0.25k.
Perform ion etching at V, current 12.5 mA for about 10 minutes. Furthermore, gold sputtering is performed on the film surface by the same apparatus to form a gold thin film layer of about 200 °, and measurement is performed at a magnification of 20,000 times using a scanning electron microscope with XMA.
The measurement was conducted randomly over 100 fields of view, and the total number of particles a and the number of agglomerated exfoliated particles b (with voids generated over the entire width between adjacent particles due to the stretching stress during film formation) while confirming that the particles were alumina particles. Then, the independent existence ratio of the particles is determined by the following equation.

(7) Scrapability and scratch resistance In the same device as in Fig. 1 used to measure the running friction coefficient of the film, instead of the stainless steel SUS304 fixed rod of 7, the surface of the 6φ tape guide made of SUS304 was sufficiently finished. When using (surface roughness Ra = 0.015 μm) method A, SUS
The case of using a 6φ tape guide (surface roughness Ra = 0.15 μm) in which the surface finish is insufficient by bending the sintered plate into a cylindrical shape is B
Method, using a 6φ tape guide made of Daiichi Seiko's carbon black-containing polyacetal as the C method, and setting the contact angle with the tape guide to 30 degrees, at a speed of 300 m / min and an inlet tension of 50 g / 1/50 Run 200m so as to reach 2 ". Evaluate the shavings adhering to the guide after running and the scratches of the tape after running.

<Judgment of shavings> ◎ No shavings are seen at all ○ Slightly shavings can be seen △ Presence of shavings can be seen at a glance × Shavings are heavily adhered <Scratch judgment> ◎ Scratches are seen at all None ○ 1 to 5 scratches are seen △ 6 to 15 scratches are seen × 16 or more scratches are seen [Examples] 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 was used as a transesterification catalyst, antimony trioxide was used as a polymerization catalyst, phosphorous acid was used as a stabilizer, and the additive particles shown in Table 1 were added as a lubricant. Polymerization was carried out by a conventional method to obtain an intrinsic viscosity (orthochlorophenol , 35 ℃) 0.6
2 polyethylene terephthalate was obtained.

This polyethylene terephthalate pellet is heated to 170 ° C.
After drying for 3 hours in the extruder hopper, melt temperature 280
The melted polymer was extruded through a 1 mm slit die onto a rotating cooling drum having a surface finish of about 0.3 s and a surface temperature of 20 ° C. to obtain a 200 μm unstretched film.

The unstretched film thus obtained is preheated at 75 ° C, and 900 ° C from 15 mm above between low-speed and high-speed rolls.
The sheet was heated by one IR heater having a surface temperature of, stretched 3.6 times, quenched, and then supplied to a stenter, and stretched 3.9 times horizontally at 105 ° C. The obtained biaxially oriented film is
The film was heat-set at a temperature of 05 ° C for 5 seconds to obtain a heat-set biaxially oriented film having a thickness of 14.2 µm.

The properties of the biaxially oriented polyester film of 14.2 μm thus obtained are shown in Table 1. When the distribution of protrusions of the obtained film was examined, Examples 1 to 4 and Comparative Examples 1 to 5 satisfied the following formula (A) and were excellent in slipperiness. 6 is the number of protrusions y (pieces / mm ² )
The relationship of log y <-10.0x + 5 is not satisfied for the entire specified range, and the running friction coefficient after running 50 times is 0.
It is 32, which is a level of practical concern.

-11.4x + 4 <log y <10.0x + 5 (A) As for the judgment of the scraping and scratching properties in Table 1, there is no practical concern as a magnetic tape as long as it is ◯ level or more, but when it becomes Δ level, it is the operating condition. Depending on
An increase in dropout is feared due to shavings on the tape guide and scratches on the tape.

As is clear from Table 1, the metal tape guide according to the present invention has excellent slipperiness for a magnetic recording medium and has a surface which is sufficiently finished, and a metal tape guide whose surface is not sufficiently finished. Also, it has excellent abrasion resistance and scratch resistance over all plastic tape guides, and has extremely excellent characteristics as a biaxially oriented polyester film for magnetic recording media.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus for measuring a running friction coefficient of a film. In the figure, 1 is an unwind reel, 2 is a tension controller, 4 is a tension detector (entrance), 7 is a fixed rod, 10
Is a tension detector (exit), and 13 is a take-up reel.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B29L 7:00 B29L 7:00 (72) Inventor Masami Etsunaka 3-37 Oyama, Sagamihara City, Kanagawa Prefecture No. 19 in Sagamihara Research Center, Teijin Limited (56) Reference JP-A-63-101431 (JP, A) JP-A-2-24823 (JP, A) JP-A-2-214734 (JP, A)

Claims (3)

(57) [Claims] A biaxially oriented polyester film, wherein (A) a distribution curve of projections on the film surface formed by particles present in the polyester has a height x.
(Μm) is 0.05 μm or more and the number of protrusions y (pieces / mm ² ) is 30
In the range of / mm ² or more, the following formula (A) is satisfied,
(B) The running friction coefficient rise (Δμk) is less than 0.15,
And (C) the protrusion has an independent existence rate of 80% in the film.
Above, alumina particles having an average particle size of 0.06 to 0.2 μm 0.0
5 to 1.0% by weight, the average particle size is larger than that of alumina particles.
Anatase type titanium oxide particles having a particle size of 0.1 to 0.5 μm 0.01 to
A magnetic recording medium characterized by being formed by the presence of 1.0% by weight and 0.01 to 1.0% by weight of other inert inorganic particles having an average particle size of 0.3 to 1.5 μm and / or heat resistant polymer particles. Biaxially oriented polyester film. −11.4x + 4 <log y <10.0x + 5 (A) where x is the distance from the reference level in the height direction (μm)
And y is the number of protrusions (pieces / mm ² ) counted when cut at the height x. 2. The biaxially oriented polyester film for a magnetic recording medium according to claim 1, wherein the alumina particles are particles of a γ type crystal having a Mohs hardness of less than 8. 3. The composition according to claim 1, wherein the titanium oxide particles have a composition of P ₂
P element of 0.25% by weight or more in O ₅ conversion and 0.1 in K ₂ O conversion
2. The biaxially oriented polyester film for a magnetic recording medium according to claim 1, wherein the film contains K element by weight or more.