JPH0618071B2 - Biaxially oriented polyester film for magnetic recording media - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a biaxially oriented polyester film for magnetic recording media, and more specifically contains specific spherical silica particles, has excellent abrasion resistance, and has improved slipperiness. And a biaxially oriented polyester film for a magnetic recording medium.

[Prior Art] A polyester film represented by a polyethylene terephthalate film is used as a base film for magnetic recording media such as magnetic tapes and floppy disks because of its excellent physical and chemical properties.

In a polyester film, its slipperiness and abrasion resistance are major factors that affect the workability of the film manufacturing process and the processing process in each application, and further the quality of the product. If these are insufficient, for example, when a magnetic layer is applied to the surface of a polyester film and used as a magnetic tape, friction between the coating roll and the film surface during application of the magnetic layer is severe, and abrasion of the film surface due to this is also severe, In extreme cases, wrinkles, scratches, etc. on the film surface occur. In addition, even after slitting the film after coating the magnetic layer and processing it into audio, video or computer tape, etc., when pulling out from the reel or cassette, winding up and other operations, many guide parts, playback heads etc. Abrasion occurs remarkably between them, and scratches, distortions are generated, and white powdery substances are deposited due to abrasion of the surface of the polyester film and the like, and as a result, loss of magnetic recording signals, that is, a large cause of dropout is often caused.

Generally, in order to improve the slipperiness of the film, a method of reducing the contact area with the guide roll or the like by giving unevenness to the film surface is adopted, and it is roughly classified into (i) contact of the polymer using the film raw material. A method of precipitating inactive fine particles from the residue and a method of (ii) adding inactive inorganic fine particles are used. In general, the larger the size of the fine particles in the raw material polymer, the greater the effect of improving the slipperiness.

On the other hand, in a magnetic recording medium, particularly a high-density magnetic recording tape or a high-density floppy disk, the surface of the base film is required to be as flat as possible from the viewpoint of improving electromagnetic conversion characteristics. However, when the film surface becomes flat, as described above, the slipperiness of the film deteriorates, and various troubles are rejuvenated.

Therefore, a polyester film for a magnetic recording medium is required to satisfy these reciprocal characteristics at the same time.

[Object of the Invention] The present inventors have conducted extensive studies to develop a base film satisfying the above requirements, particularly a base film having a flat film surface and a low friction coefficient and excellent scratch resistance. Reached

Therefore, an object of the present invention is to provide a biaxially oriented polyester film for a magnetic recording medium, in which the film surface has a uniform roughness and the surface has fine irregularities and which is excellent in slipperiness, scratch resistance, and winding property. To provide.

[Structure / Effects of the Invention] According to the present invention, an object of the present invention is to provide a polyester having an average particle size of 0.05 μm or more and less than 0.3 μm and a particle size ratio (major axis / major axis).
This is achieved by a biaxially oriented polyester film for a magnetic recording medium, which comprises 0.01 to 3% by weight of spherical silica particles having a minor axis) of 1.0 to 1.2 and a relative standard deviation of 0.3 or less.

Here, the major axis, the minor axis, and the area equivalent circle diameter of the spherical silica particles are obtained from an image magnified 10,000 times to 30,000 times with a scanning electron microscope after depositing a gold thin film layer on the particle surface, and the average particle size is calculated. Calculate the diameter and particle size ratio by the following formula.

Average particle size = sum of area circle equivalent diameters of measured particles / number of measured particles Particle size ratio = average major axis of silica particles / average minor axis of the particles The relative standard deviation is calculated by the following formula.

Here, Di: area circle equivalent diameter of each particle (μm) D: average value of area circle equivalent diameter n: represents the number of particles.

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, diphenoxyethanedicarboxylic acid, diphenyldicarboxylic acid, diphenyletherdicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenylketonedicarboxylic acid and anthracenedicarboxylic acid. You can Examples of the aliphatic glycol include polymethylene glycol having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and decamethylene glycol, and alicyclic compounds such as cyclohexanedimethanol. Examples thereof include diols.

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

Among such polyesters, for example, polyethylene terephthalate and polyethylene-2,6-naphthalate, as well as, for example, 80 mol% or more of all dicarboxylic acid components are terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and all glycol components are used. A copolymer having 80 mol% or more of ethylene glycol is preferable. At that time, 20% or less of the total acid component is terephthalic acid and / or 2,6-
The above aromatic dicarboxylic acids other than naphthalene dicarboxylic 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. In addition, 20 mol% or less of the total glycol component is
It can be the above glycols other than ethylene glycol. Or for example hydroquinone, resorcin,
Aromatic diols such as 2,2-bis (4-hydroxyphenyl) propane; Aliphatic diols containing an aromatic ring such as 1,4-dihydroxymethylbenzene; Polyethylene glycols such as polyethylene glycol, polypropylene glycol and polytetramethylene glycol It can also be an alkylene glycol (polyoxyalkylene glycol) or the like.

In the polyester used in the present invention, a component derived from an oxycarboxylic acid such as an aromatic oxyacid such as hydroxybenzoic acid; a fatty acid oxyacid such as ω-hydroxycaproic acid, a dicarboxylic acid component and an oxycarboxylic acid component. Those which are copolymerized or bonded at 20 mol% or less based on the total amount of

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, in an amount in a substantially linear range, for example, an amount of 2 mol% or less based on all acid components, It also includes a copolymer of pentaerythritol and the like.

The 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 -chlorophenol at 35 ° C.
0.9 is preferable.

The biaxially oriented polyester film of the present invention has a large number of fine projections on the surface of the film. The large number of fine projections derives from the large number of spherical silica particles contained according to the invention dispersed in the polyester.

Polyesters containing spherical silica particles dispersed therein are usually reacted at a time for forming a polyester, for example, at any time during a transesterification reaction or a polycondensation reaction in the case of a transesterification method or at any time in a direct polymerization method. In addition, spherical silica particles (preferably as a slurry in glycol) can be added to the reaction system. Preferably, the spherical silica particles are added to the reaction system at the initial stage of the polycondensation reaction, for example, until the intrinsic viscosity reaches about 0.3.

In the present invention, the spherical silica particles dispersedly contained in the polyester are spherical silica particles having an average particle diameter of 0.05 μm or more and less than 0.3 μm and a particle diameter ratio (major axis / minor axis) of 1.0 to 1.2. The spherical silica particles have a spherical shape that is very close to a true sphere, and silica particles that have been known as lubricants are ultrafine aggregate particles of about 10 mμ, or aggregates of about 0.2 μm. It is characterized in that it is remarkably different from that of forming a substance (aggregated particles).

The average particle size of the spherical silica particles is preferably 0.10 μm or more
It is less than 0.3 μm, more preferably 0.15 μm or more and less than 0.3 μm, and particularly preferably 0.20 μm or more and 0.28 μm or less.
If the average particle size is less than 0.05 μm, the effect of improving slipperiness and clutch resistance is insufficient, which is not preferable. The particle size ratio of the spherical silica particles is preferably 1.0 to 1.1, more preferably 1.0 to 1.05.

Further, the spherical silica particles preferably have a sharp particle size distribution, and the relative standard deviation showing the steepness of the distribution is 0.3 or less. It is particularly preferably 0.12 or less.

This relative standard deviation is expressed by the following equation.

Here, Di: area circle equivalent diameter (μm) of each particle D : average value of area circle equivalent diameter n: represents the number of particles measured.

Relative standard deviation is 0.3 or less, particularly when using spherical silica particles of 0.12 or less, since the particles are spherical and the particle size distribution is extremely steep, the height of the film surface protrusions becomes extremely uniform, and with the same number of protrusions. Even if there is, slipperiness becomes extremely good as compared with the conventional one.

Spherical silica particles can be produced by
There is no other limitation.

For example, spherical silica particles can be made from ethyl orthosilicate [Si
Hydrolysis of (OC ₂ H ₅ ) ₄ ] results in hydrous silica [Si
(OH) ₄ ] monodisperse spheres are prepared, and the hydrous silica monodisperse spheres are dehydrated to obtain silica-bonded [≡Si-O-Si].
It can be manufactured by three-dimensionally growing [≡]. (Journal of the Chemical Society of Japan '81, NO.9, P.1503).

Si (OC ₂ H ₅ ) ₄ + 4H ₂ O → Si (OH) ₄ + 4C ₂ H ₅ OH ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H ₂ O In the present invention, the addition amount of spherical silica particles is 0.01 to 3.0% by weight based on the polyester, preferably 0.05 to 2.0% by weight, more preferably 0.05 to 1.0% by weight
Is. If the amount added is less than 0.01% by weight, the effect of improving the slipperiness and abrasion resistance will be insufficient, while if it exceeds 3.0% by weight, the surface flatness will decrease, which is not preferable.

The biaxially oriented polyester film of the present invention can be produced according to the conventionally accumulated production method of biaxially oriented film.
For example, by melt-casting a polyester containing spherical silica particles into an amorphous unstretched film, then stretching the unstretched film biaxially, heat-setting, and if necessary subjecting it to relaxation heat treatment. Manufactured. At that time, the film surface characteristics vary depending on the particle size, amount, etc. of the spherical silica particles, and the stretching conditions. The density, heat shrinkage, etc. also change depending on the temperature, the magnification, the speed, etc. during drawing and heat treatment, so the conditions for simultaneously satisfying these characteristics are determined. For example, the stretching temperature is the first stage stretching temperature (for example, the longitudinal stretching temperature: T ₁ ).
Is in the range of (Tg-10) to (Tg + 45) ° C (however, T
g: the glass transition temperature of polyester), the second stage stretching temperature (for example, transverse stretching temperature: T ₂ ) is (T ₁ +5) to
It may be selected from the range of (T ₁ +40) ° C. The stretching ratio is preferably selected from a range in which the uniaxially oriented stretching ratio is 2.5 or more, particularly 3 or more, and the area ratio is 8 or more, particularly 10 or more. Furthermore, the heat setting temperature is 180-250
C., and more preferably from 200 to 230.degree. The thickness of the film is preferably 1-100 μm.

The biaxially oriented polyester film of the present invention preferably has a surface roughness Ra of 0.003 to 0.012 μm. If the surface roughness Ra is too large, it is difficult to maintain the electromagnetic conversion characteristics particularly required for a magnetic recording medium for high image quality, and if it is too small, the slidability is not bad and the film is easily handled and rolled. Removability becomes poor. The surface roughness Ra is preferably 0.003 to 0.009 μm, more preferably 0.004 to 0.007 μm.

Furthermore, in the biaxially oriented polyester film of the present invention, it is preferable that the surface roughness Ra and the film-film static friction coefficient μs have a relationship satisfying the following formula 0.0002 ≦ Ra × μs ² ≦ 0.002. If the value of Ra × μs ² is too small, the escape of air between the film layers becomes poor when the film is wound up by the roll, or the film may meander and the end surface may be easily displaced. On the other hand, if the size is too large, pimples (bulb-shaped projections) occur when wound on a roll, workability and handleability are poor, and wrinkles often occur in the process. From these points,
The value of Ra × μs ² is 0.0002 to 0.0015, and further 0.0003 to 0.
It is preferable to satisfy 0008.

The biaxially oriented polyester film of the present invention is characterized in that it has excellent smoothness and excellent scratch resistance in spite of its flat surface, as compared with the conventional one.

The reason for this is not clear, but the shape of the individual projections on the film surface is sharp and uniform because fine silica particles that are extremely spherical and nearly monodisperse are used.
As a result, the film surface has good flatness, but the coefficient of friction is low, and since it makes uniform contact with other objects and is supported by many projections, it has excellent durability, and as a result, scratches are difficult to enter. It is speculated that

The biaxially oriented polyester film of the present invention is useful as a base film for magnetic recording media, especially high-density magnetic recording media, by taking advantage of such characteristics. Especially when it is used as a base film for high-density video tapes for super high grade and 8mm video, high-density floppy disks, magnetic tapes for high-density computers, etc., it has excellent electromagnetic conversion characteristics, slipperiness, scratch resistance, etc. Is obtained.

[Examples] Hereinafter, the present invention will be further described with reference to Examples.

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

(1) Particle size The particle size can be measured in the following states.

1) When determining the average particle size, particle size ratio, etc. from silica powder 2) When determining the average particle size, particle size ratio, etc. of silica particles in the film.

1) From silica powder: Disperse the silica powder on the electron microscope sample stand so that the individual particles do not overlap as much as possible, and use a gold sputtering device to
A gold thin film vapor-deposited layer is formed on this surface with a thickness of 200Å to 300Å and observed with a scanning electron microscope at a magnification of, for example, 10000 to 30000, and at least one 110 major axis of particles is obtained with Luzex 500 manufactured by Nippon Regulator Co., Ltd. Dli), minor axis (Dsi) and area circle equivalent diameter (Di) are obtained. The major axis (Dl), the minor axis (Ds), and the average particle diameter ( D ) of the silica particles are represented by the number average values represented by the following equations.
Represents

2) In the case of silica particles in the film: A small piece of the sample film is fixed on a sample stand for a scanning electron microscope, and a sputtering device (JFC-1100 manufactured by JEOL Ltd.) is used.
Type ion sputtering apparatus), the film surface was subjected to ion etching treatment under the following conditions. The conditions were such that the sample was placed in a bell jar, the degree of vacuum was raised to a vacuum state of about 10 ⁻³ Torr, and ion etching was carried out for about 10 minutes at a voltage of 0.25 KV and a current of 12.5 mA. Further, the film surface is gold-sputtered by the same apparatus and observed with a scanning electron microscope at a magnification of, for example, 10000 to 30000, and at least 100 particles have a long diameter (Dli) and a short diameter using a Luthesque 500 manufactured by Nippon Regulator Co., Ltd. Diameter (Dsi) and area equivalent circle diameter (D
i) is calculated. Thereafter, the same procedure as 1) above is performed.

(2) Average particle size of particles other than silica particles, particle size ratio, etc. 1) Average particle size Shimadzu CP-50 type Centrifugal Particle Size Analyzer (Centrifugal Particle Size
Analyser). The particle diameter corresponding to 50 mass% is read from the integrated curve of particles of each particle diameter calculated based on the obtained centrifugal sedimentation curve and its abundance, and this value is taken as the above average particle diameter (Book Technology ”, published by Nikkan Kogyo Shimbun, 1975, pp. 242-247).

2) Particle size ratio A small piece of film is fixed and molded with epoxy resin, and a microtome is used to make an ultrathin section with a thickness of about 600Å (cut parallel to the film flow direction). The cross-sectional shape of the lubricant (particles) in the film of this sample was observed with a transmission electron microscope (Hitachi: H-800), and the ratio of the major axis to the minor axis of the lubricant is shown.

3) Relative standard deviation value Calculate the difference particle size distribution from the integrated curve in item 1), and calculate the relative standard deviation based on the following relative standard deviation definition formula.

Here, the respective particle diameters obtained in the Di; 1) term, the average diameter obtained in the D ; 1) term, the number of divisions when obtaining the integrated curve in the n; 1) term φi; the existence of particles of each diameter Represents the probability (mass percentage).

(3) Film surface roughness (Ra) This is a value defined by JIS-B0601 as the center line average roughness (Ra), and in the present invention, a stylus surface roughness meter (SURFCORDER SE of Kosaka Laboratory Ltd.) is used. -30C). The measurement conditions are as follows.

(a) Stylus tip radius: 2 μm (b) Measuring pressure: 30 mg (c) Cut-off: 0.25 mm (d) Measuring length: 0.5 mm (e) Data compilation method The same sample was repeatedly measured 5 times, and the largest Value 1
Exclude one, round off the fourth decimal place of the average value of the remaining four data, and display up to the third decimal place.

(4) Coefficient of friction of film (μk) The angle θ of the film cut into 1/2 inch width to the fixed rod (surface roughness 0.3 μm) in the environment of temperature 20 ° C and humidity 60%
= 152 / 180π radians (152 °), contacting at 200 cm / min
Move (rub) at the speed of. Entrance tension T ₁ is 3
The exit tension (T ₂ : g) when the tension controller is adjusted to be 5 g is detected by the exit tension detector after the film travels 90 m, and the running wear coefficient μk is calculated by the following formula.

Place μk = (2.303 / θ) log (T 2 / T 1) = 0.868 log (T 2/35) (5) static friction coefficient (.mu.s) superposed two glass plates fixed to the underside of the film which combined, Take the film under the stacked films (the film in contact with the glass plate) with a constant speed roll (15
cm / min), fix the detector to one end of the upper film (the end opposite to the take-up direction of the lower film) and detect the film / film pulling force. The thread used at that time has a weight of 1 kg and a lower area of 70 cm.

(6) Scratch judgment A magnetic coating tape (1/2 inch width) was applied using the friction coefficient measuring device of (4) above so that the base film surface of the tape came into contact with the fixed rod at an angle of 152 °, and 5 cm / sec
After traveling 20 m at a speed and repeating this 30 times, the thickness, depth, and number of the scratches that have entered the surface of the 1 / 2-inch width base film were comprehensively judged in the following 5 grades.

<5-level judgment> ◎ No scratches are found on the 1/2 inch width base film. ○ Scratches are not found on the 1/2 inch width base film. △ Scratches are found on the 1/2 inch width base film. X) 1/2 inch width base film has some thick scratches. × x 1/2 inch width base film has many thick and deep scratches. (7) Winding biaxially oriented polyester film In the manufacturing process, the film is rolled up into a roll of 500 mm in width and 4000 m, the appearance of this roll is inspected in detail, and the number of bump-like protrusions with a major axis of 2 mm or more is counted and rated as follows.

0 to 2: ○ 3 to 5: △ 6 or more: × (8) Electromagnetic conversion characteristics of magnetic coating tape A 50% white level signal (100
% White level signal is peak; two; peak voltage is 0.714
The signal obtained by weighting the 100% chroma level signal is recorded in (in volts), and the reproduced signal is measured using a Shiba Soku noise meter: Type 925R. The definition of chroma S / N is as follows according to the definition of Shibasoku.

Here, ES (pp) is the peak-to-peak voltage difference (pp) of the reproduced signal of the white level signal.

ES (pp) = 0.714V (pp) EN (rms) is the square root value of the peak voltage of the reproduction signal of the chroma level signal.

Example 1 Spherical silica having dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, antimony trioxide as a polymerization catalyst, phosphorous acid as a stabilizer, and a lubricant having an average particle diameter of 0.27 μm and a particle diameter ratio of 1.05. The particles were polymerized by a conventional method to obtain polyethylene terephthalate having an intrinsic viscosity of 0.62 (orthochlorophenol, 35 ° C.).

170 ℃ of this polyethylene terephthalate pellets,
After drying for 3 hours, supply to the extruder hopper and melt temperature 280 ~ 3
It was melted at 00 ° C., and the molten polymer was extruded through a 1 mm slit die onto a rotary cooling drum having a surface finish of about 0.3 S and a surface temperature of 20 ° C. to obtain an unstretched film of 200 μm.

The unstretched film thus obtained is preheated at 75 ° C., and further between the low-speed roll and the high-speed roll, 900 mm from above 15 mm.
The film was heated with one IR heater having a surface temperature of ℃ to draw up to 3.6 times, rapidly cooled, and then fed to a stenter and drawn to 3.7 times in the transverse direction at 105 ℃. The obtained biaxially oriented film was heat set at a temperature of 205 ° C. for 5 seconds to obtain a heat set biaxially oriented film having a thickness of 15 μm.

On the other hand, needle-like α-FeOOH containing 5% cobalt
Was heated and reduced with hydrogen to obtain Fe ₂ O ₃ , which was further heated in air to obtain a ferromagnetic iron powder having an average acicular length of 0.2 μm.

100 parts by weight of the above ferromagnetic iron powder (hereinafter simply referred to as "part") and the following composition were kneaded and dispersed in a ball mill for 12 hours.

Polyester polyurethane 12 parts Vinyl chloride-vinyl acetate-maleic anhydride copolymer 10 parts α-alumina 5 parts Carbon black 1 part Butyl acetate 70 parts Methyl ethyl ketone 35 parts Cyclohexanone 100 parts After dispersion, further fatty acid oleic acid 1 part Palmitic acid 1 part 1 part of a fatty acid ester (amyl stearate) was added and kneaded for 15 to 30 minutes, then 7 parts of a 25% ethyl acetate solution of a triisocyanate compound was added, and high-speed shear dispersion was carried out for 1 hour to prepare a magnetic coating solution.

The obtained coating liquid was applied onto the heat-set biaxially oriented film (thickness 15 μm) so that the dry film thickness was 3.5 μm. Then, after orientation treatment in a direct current magnetic field, it was dried at 100 ° C. After drying, it was calendered and slit into a 1/2 inch width to obtain a magnetic tape for video having a thickness of 18.5 μm.

The film thus obtained has a very good winding property and is 5
The coefficient of friction after 0 passes was low, the scratch resistance of the tape was good, and the electromagnetic conversion characteristics were also good.

The characteristics of this film are shown in Table 1.

Example 2 A biaxially oriented film and a magnetic tape were prepared in the same manner as in Example 1 except that the average particle size of the spherical silica particles was changed as shown in Table 1. The quality of this product was extremely good as shown in Table 1.

Comparative Example 1 A biaxially oriented film and a magnetic tape were prepared in the same manner as in Example 1 except that spherical silica particles having different average particle diameters were used. The results are shown in Table 1.

This magnetic tape had poor scratch resistance and windability.

Comparative Examples 2 and 3 Biaxially oriented films and magnetic tapes were prepared in the same manner as in Example 1 except that calcium carbonate (Comparative Example 2) or kaolin (Comparative Example 3) was used as the lubricant. The results are shown in Table 1.

Both had a high coefficient of friction, extremely poor scratch resistance, and were also insufficient in windability and electromagnetic conversion characteristics.

Example 3 A biaxially oriented film and a magnetic tape were produced in the same manner as in Example 1 except that the stretching ratio was changed to 4.5 in the longitudinal direction and 3.6 in the transverse direction.

The characteristics of this film are shown in Table 1.

Example 4 Spherical particles of dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, antimony trioxide as a polymerization catalyst, phosphorous acid as a stabilizer, and a lubricant having an average particle diameter of 0.22 μm and a particle diameter ratio of 1.04. Polymerization was carried out by a conventional method using silica particles to obtain polyethylene terephthalate having an intrinsic viscosity of 0.62 (orthochlorophenol, 35 ° C).

170 ℃ of this polyethylene terephthalate pellets,
After drying for 3 hours, supply to the extruder hopper and melt temperature 280 ~ 3
It was melted at 00 ° C., and this molten polymer was formed and extruded through a 1 mm slit die on a rotary cooling drum having a surface finish of about 0.3 s and a surface temperature of 20 ° C. to obtain an unstretched film.

The unstretched film thus obtained is preheated at 75 ° C. and further heated between one roll at a low speed and a high speed with a 1R heater having a surface temperature of 850 ° C. from 12 mm above, and the film is stretched in the longitudinal direction to 3.6.
Stretched twice, quenched, and then fed to a stenter at 110 ° C
Was stretched 3.7 times in the transverse direction. The obtained biaxially oriented film was heat set at a temperature of 205 ° C. for 5 seconds to obtain a heat set biaxially oriented film having a thickness of 15 μm.

The characteristics of this film are shown in Table 2.

Example 5 A film was prepared in the same manner as in Example 4 except that the average particle size of spherical silica and the addition amount were changed. The characteristics were good as shown in Table 2.

Example 6 A film was prepared in the same manner as in Example 4 except that the centrifugal ratio during film formation was changed to 4.5 times in the longitudinal direction (preheating temperature 70 ° C.) and 3.5 times in the lateral direction (stretching temperature 105 ° C.). Got

The characteristics of this film are shown in Table 2.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihiro Nomi 3-37-19 Oyama, Sagamihara City, Kanagawa Prefecture, Teijin Ltd. Plastics Research Laboratory (56) Reference JP-A-59-171623 (JP, A) JP Sho 63-247913 (JP, A)

Claims (2)

[Claims] 1. A polyester having an average particle size of 0.05 μm or more.
Magnetic recording in which 0.01 to 3.0% by weight of spherical silica particles having a particle size ratio (major axis / minor axis) of 1.0 to 1.2 and a relative standard deviation of 0.3 or less represented by the formula below is less than 0.3 μm. Biaxially oriented polyester film for media. Here, Di: area circle equivalent diameter of each particle (μm) D: average value of area circle equivalent diameter n: represents the number of particles. 2. The surface roughness (Ra) of the film is 0.003 to 0.0.
12 μm, the film-to-film static friction coefficient (μ
The biaxially oriented polyester film according to claim 1, wherein s) and the surface roughness (Ra) satisfy the following formula: 0.0002 ≦ Ra × μs ² ≦ 0.002.