JP2000173045A - Polyester film for magnetic recording medium and magnetic recording tape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a low error rate deposition type magnetic tape for data backup by forming a coating containing a cellulose derivative on one side of a polyester film and setting the number-average molecular weight of the cellulose derivative within a specified range. SOLUTION: One surface of a polyester film is cladded with a coating containing a cellulose derivative having number-average molecular weight of 100,000-500,000. Consequently, rubbing resistance of a ferromagnetic metal thin film vacuum deposited on one surface is enhanced when recording or reproducing operation is performed by means of a magnetic head. Preferably, the cellulose derivative is methyl cellulose or a derivative thereof or ethyl cellulose or a derivative thereof. In order to apply a coating with high adhesion onto the surface of the polyester film, the content of cellulose derivative is set at 10-70 wt.%, preferably at 15-50 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnetic recording medium suitable for producing a high quality ferromagnetic metal thin film type magnetic recording medium for recording digital data, such as a polyester film for a magnetic recording medium, particularly for a data storage tape. The present invention relates to a polyester film for a medium and a magnetic recording tape using the same.

[0002]

2. Description of the Related Art Consumer digital video tapes put into practical use in 1995 are provided by depositing a metal magnetic thin film of Co on a base film having a thickness of 6 to 7 .mu.m by vacuum evaporation and coating the surface with a diamond-like carbon film. Becomes D
In the case of a camera-integrated video using a V mini-cassette, the basic specification (SD specification) has a recording time of one hour.

[0003] This digital video cassette (DVC)
Is the world's first digital video cassette for home use, and a. Despite its small body, it can record a huge amount of information,
b. Since the signal does not deteriorate, the image quality and sound quality do not deteriorate even after many years, c. Enjoys high image quality and high sound quality because it is not disturbed by noise. D. It has such advantages that the image does not deteriorate even after repeated dubbing, and is highly evaluated in the market.

[0004] Also, this deposition type digital video tape,
The use of DVC for computer data backup began in 1996, and tape width was 8m.
m data 8mm (Exabyte Mammoth, Sony AIT
System) Computer data backup devices have been put to practical use, and are popular because of their small size, large capacity and fast data transfer speed.

As the base film, a polyester film, a polymer blend adhered to at least one surface of the film, and a particle size of 50 to 500 are used.
Å consists of a discontinuous coating mainly composed of fine particles, the discontinuous coating contains a water-soluble polyester copolymer,
A polyester film in which fine projections are formed on a discontinuous coating by fine particles (for example, Japanese Patent Publication No. Sho 63-57238), containing fine particles having a particle size of 10 to 300 mμ, and having a height of 50 to 900 mm. A polyester film having surface protrusions formed thereon, and a discontinuous coating mainly composed of a polar polymer having a thickness of 500 ° or less adhered to at least one surface of the film, wherein the height of the fine surface protrusions is A polyester film (for example, Japanese Patent Publication No. 6-51401) which is higher than the height of the coating film is used.

[0006]

However, a magnetic tape made from such a base film has satisfactory image recording and reproduction quality for a video tape, but has an error rate (for a data backup application). (The percentage of data that could not be recorded accurately) tended to be high and was unsatisfactory. In other words, in data backup applications, data is repeatedly recorded and read a large number of times. Therefore, the magnetic metal thin film of Co formed on the base film by the magnetic head tends to be scratched and the error rate tends to be high. there were.

It is an object of the present invention to provide a polyester film for a magnetic recording medium which is suitable for a vapor-deposited magnetic recording tape for data backup with a low error rate.

[0008]

In order to solve the above-mentioned problems, a polyester film for a magnetic recording medium of the present invention has a polyester film coated on one side with a film containing a cellulose derivative. It is characterized by having an average molecular weight of more than 100,000 and not more than 500,000.

Further, the magnetic recording tape according to the present invention is characterized in that a ferromagnetic metal thin film layer is provided on the surface of such a polyester film.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with preferred embodiments. The polyester in the present invention is not particularly limited as long as it is a polyester which becomes a high-strength film by molecular orientation, but is preferably made of polyethylene terephthalate and polyethylene-2,6-naphthalate. 80% or more of the constituent components are ethylene terephthalate and ethylene naphthalate, polyethylene terephthalate, polyethylene-2,
6-naphthalate. Examples of the polyester copolymer component other than ethylene terephthalate and ethylene naphthalate include diol components such as diethylene glycol, propylene glycol, neopentyl glycol, polyethylene glycol, p-xylylene glycol, and 1,4-cyclohexanedimethanol, adipic acid, Sebacic acid, phthalic acid, isophthalic acid, dicarboxylic acid components such as 5-sodium sulfoisophthalic acid, trimellitic acid, polyfunctional dicarboxylic acid components such as pyromellitic acid,
For example, p-oxyethoxybenzoic acid can be used.

The above-mentioned polyester may further comprise at least one of an alkali metal salt derivative of sulfonic acid which is non-reactive with the polyester and a polyalkylene glycol which is substantially insoluble in the polyester, in an amount not exceeding 5% by weight. May be mixed.

One surface A of the polyester film is
It is coated with a coating containing a cellulose derivative having a number average molecular weight of more than 100,000 and not more than 500,000, more preferably 150,000 to 400,000.

Since the ferromagnetic metal thin film formed by vacuum evaporation on the surface A is covered with a coating containing a cellulose derivative having a number average molecular weight of more than 100,000 and 500,000 or less, This is preferable because the scratch resistance due to the magnetic head increases. If the number average molecular weight of the cellulose derivative is 100,000 or less, the coating becomes too soft and easily deformed by a magnetic head, causing scratches. If the number average molecular weight of the cellulose derivative exceeds 500,000, the coating becomes too hard, and the ferromagnetic metal thin film formed by vacuum evaporation on the surface A is easily broken by the magnetic head during recording / reproducing, which is not preferable. . In addition, the oligomer precipitation prevention performance is undesirably reduced.

As the cellulose derivative, methylcellulose or a derivative thereof or ethylcellulose or a derivative thereof is preferable. In particular, methylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, carboxymethylcellulose, hydroxylethylcellulose and the like are preferably used.

In order for the coating to be provided on the surface of the polyester film with good adhesion, it may contain other polymers, but the content of the cellulose derivative is 10 to 70% by weight,
More preferably, the content is 15 to 50% by weight. If the content of the cellulose derivative is not more than 10% by weight, it is not preferable because the polyester film oligomer is deposited on the magnetic layer from inside the polyester film after the magnetic tape is formed. If the content of the cellulose derivative exceeds 70% by weight, the adhesion of the coating film is undesirably reduced. Other polymers include polyvinyl alcohol, tragacanth gum, casein, gelatin, water-soluble polyester,
A polar polymer such as a water-soluble polyester ether copolymer and polyurethane can be used, but not limited thereto.

The surface A of the polyester film and the surface on the opposite side tend to be blocked by the constituents of the film, and a water-repellent substance such as a silicon compound such as silicone and a fluorine compound such as Teflon is contained in the film component in an amount of 0.05 to 2%. 0.4% by weight
It is preferred to add.

Examples of the water-repellent substance include silicon compounds such as polydimethylsiloxane and silicone, and fluorine compounds such as Teflon. The water-repellent substance accounts for 0.05 to 2.4% by weight, more preferably 0.08%, of the coating components.
It is desirable to add about 1.5% by weight. If the content of the water-repellent substance in the coating is less than 0.05% by weight, the anti-blocking effect is not exhibited, which is not preferable. If the content of the water-repellent substance in the coating is higher than 2.4% by weight, the adhesion of the deposited film is undesirably reduced.

The surface A on one side of the polyester film according to the present invention has fine surface projections having a height of 10 to 50 nm of 1 to 100 million / mm ² , more preferably 4 to 8500.
It may be provided at a density of 10,000 / mm ² . Due to the formation of the fine surface projections, the generation of scratches by the magnetic head during recording / reproduction of the ferromagnetic metal thin film formed on the surface A by vacuum deposition is further reduced. If the number of fine surface projections is less than 1,000,000 / mm ² , scratches due to the magnetic head tend to occur on the surface of the ferromagnetic metal thin film during recording and reproduction. If the number of fine surface projections is more than 100 million / mm ² , the magnetic layer of the ferromagnetic metal thin film is too rough, and the error rate of the magnetic tape tends to increase.

The fine surface projections may be formed by adding fine particles to the coating. The fine particles may be spherical, elliptical, rectangular, cubic, or the like. The particle size of the fine particles is preferably from 5 to 50 nm, more preferably from 8 to 30 nm. When the particle size is less than 5 nm, the height of the fine surface projections does not exceed 10 nm, which is not preferable. If the particle size exceeds 50 nm, the height of the fine surface projections may exceed 50 nm, which is not preferable. Here, the particle size refers to the average particle size of the fine particles. Particle types include magnesium oxide, zinc oxide, magnesium carbonate, silicon oxide, silica, calcium carbonate, calcium sulfate, barium sulfate, aluminum oxide, alumina, titanium oxide, calcium, barium, aene, manganese and other acid salts, and polyacrylic acid. Spheres, polystyrene spheres, polysiloxane spheres and the like can be used, but are not limited thereto. A plurality of types of fine particles may be used.

The polyester oligomer in the present invention refers to a lump-shaped substance which precipitates from the inside of the polyester film on the surface as the polyester film is left standing. Refers to polyester-forming monomers, dimers, trimers and the like present in the polyester. When the catalyst residue precipitates, it is also regarded as an oligomer.

The SRa value of one surface A of the polyester film according to the present invention is preferably 5 nm or less. When the SRa value exceeds 5 nm, the roughness of the ferromagnetic metal thin film formed by vacuum evaporation on the surface A becomes large, and the magnetic layer of the ferromagnetic metal thin film becomes too rough, and the output characteristics of the magnetic tape tend to deteriorate. Becomes

The SRa value of one side surface B of the polyester film according to the present invention is such that when a polyester film is formed and then the polyester film is slit to a predetermined width, a product having a good winding form can be easily collected. In order to minimize the transfer of the roughness of the one-side surface B due to the roll-shaped winding after the ferromagnetic thin film is provided on the one-side surface A of the
It is desirable to be in the range of 8 to 35 nm, more preferably 10 to 25 nm. As the SRz value, after forming the polyester film, the roll shape of the roll-shaped product after slitting the polyester film is kept good, and after the ferromagnetic thin film is provided on one surface A of the polyester film, the roll-shaped product is rolled. In order to minimize the transfer of the roughness of the surface B on one side and the occurrence of undulating deformation in the ferromagnetic thin film layer by the removal, 100 to 700 nm, more preferably 140 to 55 nm.
It is desirable to be in the range of 0 nm.

The polyester film of the present invention has a thickness of 7 μm.
m is preferable, and the thickness is more preferably 3 to 5.5.
μm is desirable.

The magnetic recording tape according to the present invention is characterized in that a ferromagnetic metal thin film layer formed by vacuum evaporation is provided on the surface A of the polyester film of the present invention. Known materials can be used and are not particularly limited, but those made of a ferromagnetic material of iron, cobalt, nickel, or an alloy thereof are preferable. The thickness of the metal thin film layer is 100 to 300 nm.

The magnetic recording medium according to the present invention has a back coat layer formed by applying a solution composed of solid fine particles and a binder on the surface B and adding various additives as necessary. Any known solid particulates, binders and additives can be used without any particular limitation. Backcoat layer thickness is 0.3-1.5μ
m.

On the other surface B of the polyester film according to the present invention, that is, on the surface opposite to the above-mentioned surface A, a coarser coating layer containing a lubricant such as silicone is provided or contains finer particles. A layer formed by laminating polyester film layers or a layer further provided with the coating layer thereon is preferably used, but is not particularly limited thereto. The fine particles used here are calcium carbonate, silica,
Alumina, polystyrene and the like are exemplified. The fine particles preferably have an average particle diameter of 100 to 1000 n.
m, more preferably 150 to 900 nm, and the amount of addition is preferably 0.05 to 1% by weight, more preferably 0.08 to 0.8% by weight.

Next, a method for producing the polyester film and the magnetic recording tape according to the present invention will be described. The polyester film of the present invention uses a polyester from which contained particles are removed as much as possible as a surface A side material, and melts, molds,
1. In a normal plastic film manufacturing process consisting of biaxial stretching and heat setting, in the vertical and horizontal directions at 90 to 140 ° C.
Stretched 7 to 5.5 times, 3.5 to 7 times, 190 to 220 ° C
Can be manufactured by performing heat setting at the above temperature and performing the following operation.

On the side A of the smooth polyester film stretched in one direction, 10 to 70% by weight of a cellulose derivative is added.
A coating solution comprising an organic compound is applied to form a coating layer on the surface A side. The coating layer may have either a continuous film shape or a discontinuous film shape. The number average molecular weight of the cellulose derivative is larger than 100,000, is 500,000 or less, preferably 150,000 to 400,000. The number average molecular weight is determined by gel permeation chromatography low angle light scattering photometry.

As the coating components other than the cellulose derivative, polar polymers such as polyvinyl alcohol, tragacanth rubber, casein, gelatin, water-soluble polyester, water-soluble polyester ether copolymer, and polyurethane can be used. It is not limited to.

It is preferable that a water-repellent substance is contained in the coating component in order to prevent blocking. Examples of the water-repellent substance include silicon compounds such as polydimethylsiloxane and silicone, and fluorine compounds such as Teflon. The water-repellent substance is 0.05 to 2.4% by weight, more preferably 0.1 to 2.4% by weight in the coating components. It is desirable to add from 08 to 1.5% by weight.

When the content of the water-repellent substance in the coating component is 0.05
When the amount is less than the weight%, the anti-blocking effect is not exhibited, which is not preferable. If the content of the water-repellent substance in the coating is higher than 2.4% by weight, the adhesion of the deposited film is undesirably reduced.

When it is desired to further increase the durability of the magnetic layer of the magnetic tape, the average particle size in the polyester layer forming the surface A is 30 to 90 nm, preferably 40 to 80 n.
1% by weight or less, preferably 0.8% by weight
In the following, it is preferable to form surface projections on the surface A by containing the compound. Examples of the fine particles include silica, calcium carbonate, alumina, polyacrylic acid spheres, and polystyrene spheres, but are not limited thereto.

Note that the use of the co-extrusion technique allows the layer A
A / B laminated film using a raw material for the layer and a raw material for the layer B positively containing larger fine particles may be melt-extruded to form a film, or the surface A may be formed without using the B layer.
A coating solution containing a lubricant may be applied to the surface B side opposite to the side, and the surface B side may be subjected to easy lubrication treatment. Using the layer B, a coating solution containing a lubricant may be further applied to perform a smooth lubrication treatment on the surface B side.

The biaxial stretching can be performed by, for example, a sequential biaxial stretching method or a simultaneous biaxial stretching method. If desired, the film is stretched again in the vertical or horizontal direction or in the vertical and horizontal directions before heat fixing. It is also possible to use a so-called strengthening type in which the target strength is increased.

The polyester film of the present invention is preferably used as a base film for a magnetic recording medium, particularly when used as a base film for a data storage tape, since excellent results can be obtained. Further, even when used as a base film for digital video tape, extremely excellent results can be obtained, which is preferable.

In the magnetic tape of the present invention, a ferromagnetic metal thin film of Co or the like is formed on one surface A of the polyester base film of the present invention to a film thickness of 100 to 300 nm by vacuum evaporation, and a film of about 10 nm thickness is formed on the metal thin film. The back coat layer is formed by coating a diamond-like carbon film, further applying a lubricant thereon, and applying a solution composed of solid fine particles and a binder on one side surface B and adding various additives as necessary. And create it.

The method of measuring each characteristic value used in the description of the present invention is shown below. (1) SRa value, SRz value Optical stylus type (critical angle focus error detection method) manufactured by Kosaka Laboratory
Was measured using a three-dimensional roughness meter (ET-30HK). SRa value: center line average roughness corresponding to JIS Ra. SRz value: Ten-point average surface roughness corresponding to JIS Rz. Enter the distance between the average of the highest to fifth peak elevations and the average of the deepest to fifth valley floor depths, using the average surface of the portion extracted by the reference area from the roughness surface as the reference surface. What was converted. The test piece was subjected to Al evaporation on the measurement surface. The measurement direction is the width direction, the cutoff value is 0.08 mm, and the measurement length is 0.1
０．２0.25 mm, the feed pitch was 0.2 μm, the measurement speed was 20 μm / s, and the number of measurement lines was 100. The unit was nm.

(2) Tape Characteristics The evaluation of the characteristics of the magnetic tape (data tape) according to the present invention in which a ferromagnetic thin film is provided on the film of the present invention is performed by using a commercially available Ex.
The error rate was determined by using a mammoth with an 8 mm data device manufactured by Abyte. The error rate was determined by measuring the percentage of recording errors when backing up 2 GB data from a hard disk to a tape and the percentage of reading errors when reading. The error rate was obtained at the time of first use after the tape was manufactured, and the value after 100 times of recording and storage and reading.

[0039]

Next, the present invention will be described based on examples. Example 1 A raw material A in which 0.01% by weight of silica having an average particle size of 50 nm was added to polyethylene terephthalate substantially containing no inert particles, and the same polyethylene terephthalate and aluminum silicate having an average particle size of 300 nm were used in 0.1%. The raw material B containing 2% by weight was co-extruded at a thickness ratio of 5: 1, closely adhered to a cooling drum to form a sheet, and longitudinally stretched 3.5 times at 110 ° C. by a roll stretching method.

In the step after the longitudinal stretching, the following aqueous solution was applied to the outside of one surface A. Outer side A: hydroxypropyl methylcellulose: 0.10% by weight (number average molecular weight: 400,000, number average molecular weight was determined by gel permeation chromatography, low angle light scattering photometer. High molecular weight by gel permeation chromatography (GPC)) By separating the chains, incorporating a low-angle light scattering photometer and a differential refractometer, and measuring the light scattering intensity and the refractive index difference of the polymer solution size-sorted by a GPC device, following the elution time. , The molecular weight and content of the solute were sequentially calculated to determine the absolute molecular weight of the polymer.) Water-soluble polyester: 0.30 wt% (70 mol% of terephthalic acid, 30 mol% of 5-sodium sulfoisophthalic acid) 1: 1 copolymer of styrene and ethylene glycol) Aminoethylsilane coupling agent: 0.01% by weight Average particle size 15 nm Finely divided silica: 0.025 wt% polydimethylsiloxane emulsion: 0.002 wt% coating weight (solid concentration): 20 mg / m ²

Thereafter, the film was horizontally moved to 110 ° C. by a stenter.
The film was stretched 5.0 times, heat-set at 210 ° C., wound around an intermediate spool, slit into a small width with a slitter, wound up in a roll around a cylindrical core, and a roll-like polyester film having a thickness of 4.9 μm was obtained.

On the surface A of the polyester film, a cobalt-oxygen thin film having a thickness of 80 nm
Formed in layers. Next, a diamond-like carbon film is formed on the cobalt-oxygen thin film layer by sputtering to a thickness of 10 nm.
And a fluorine-containing fatty acid ester-based lubricant was applied to a thickness of 3 nm. Then carbon black,
5 Backcoat layers made of polyurethane and silicone
It was provided with a thickness of 00 nm, slit to a width of 8 mm with a slitter, wound up on a reel and incorporated into a mammoth cassette to produce a magnetic tape (data tape).

The properties of the obtained polyester film and magnetic tape are shown in Table 1. The SRa value of the A side of the polyester film is 4 nm, and the SRa value of the B side, SR
The z values were 20 nm and 250 nm, respectively.

Example 2 In the production of the polyester film of Example 1,
A magnetic tape having a width of 8 mm was prepared in the same manner as in Example 1 except that silica having an average particle size of 60 nm was removed. Table 1 shows the properties of the obtained polyester film and magnetic tape. In addition, S on the A side of the composite polyester film
The Ra value is 3 nm, and the SRa value and SRz value of the B surface are 2
0 nm and 250 nm.

Example 3 In the production of the base film of Example 2, the polyethylene terephthalate was changed to polyethylene-2,6-naphthalate, the content of aluminum silicate in the raw material B was set to 1.1% by weight, and the longitudinal stretching temperature was changed. 4. Magnification at 135 ° C.
2 times, the transverse stretching temperature and the magnification were 135 ° C and 6.7 times, and the heat treatment was changed at 200 ° C.
A composite polyester film roll having a thickness of 3.8 μm was obtained. Otherwise, a magnetic tape having a width of 8 mm was prepared in the same manner as in Example 1. Table 1 shows the properties of the obtained polyester film and magnetic tape. Incidentally, the SRa value of the A side of the polyester film is 3 nm, the SRa value of the B side,
The SRz values were 20 nm and 250 nm.

Example 4 In the production of the base film of Example 1, the concentration of hydroxypropyl methylcellulose in the aqueous coating solution was adjusted to 0.0
3% by weight. Otherwise, the width is 8 in the same manner as in the first embodiment.
mm magnetic tape was prepared. Table 1 shows the properties of the obtained polyester film and magnetic tape. The SRa value of the A side of the composite polyester film was 2 nm, and the B
The SRa value and SRz value of the surface were 20 nm and 250 nm.

Example 5 In the production of the base film of Example 1, the concentration of hydroxypropylmethylcellulose in the aqueous coating solution was set to 0.3.
8% by weight, and the concentration of the water-soluble polyester was 0.05% by weight. Otherwise, a magnetic tape having a width of 8 mm was prepared in the same manner as in Example 1. Table 1 shows the properties of the obtained polyester film and magnetic tape. The SRa value of the A side of the composite polyester film was 8 nm, and the SRa value of the B side was 8 nm.
Ra value and SRz value were 20 nm and 250 nm.

Example 6 In the production of the base film of Example 1, the concentration of the polydimethylsiloxane emulsion in the aqueous coating solution was adjusted to 0.0
001% by weight. Otherwise in the same manner as in Example 1, a composite polyester film roll having a thickness of 4.9 μm was obtained. Thereafter, a magnetic tape was prepared in the same manner as in Example 1. However, when the film was unwound in the vapor deposition process, the surface A of the film and the opposite surface tended to be blocked, and the processing speed in the vapor deposition process was set to the value obtained in Example 1. 20% lower than 1 to prevent the film from breaking. The SRa value of the A side of the polyester film was 4 nm, and the SRa value and SRz value of the B side were 2 nm.
0 nm and 250 nm.

Example 7 In the production of the base film of Example 1, the concentration of the polydimethylsiloxane emulsion in the aqueous coating solution was adjusted to 0.0
19% by weight. Otherwise in the same manner as in Example 1, a composite polyester film roll having a thickness of 4.9 μm was obtained, and a magnetic tape having a width of 8 mm was prepared. Table 1 shows the properties of the obtained polyester film and magnetic tape. The SRa value of the A side of the polyester film was 4 nm, and the SRa value and SRz value of the B side were 20 nm and 250 nm.

Comparative Example 1 In the production of the base film of Example 1, the number average molecular weight of hydroxypropylmethylcellulose in the aqueous coating solution was 50,000. Other than the above, the thickness is 3.
Obtain a 9 μm composite polyester film roll, width 8 m
m of magnetic tape was prepared. Table 1 shows the properties of the obtained polyester film and magnetic tape. The SRa value of the A side of the polyester film was 4 nm, and the SRa value of the B side was 4 nm.
Ra value and SRz value were 20 nm and 250 nm.

Comparative Example 2 In the production of the base film of Example 1, the number average molecular weight of hydroxypropylmethylcellulose in the coating aqueous solution was set to 600,000. Otherwise in the same manner as in Example 1, a composite polyester film roll having a thickness of 4.9 μm was obtained, and a magnetic tape having a width of 8 mm was prepared. Table 1 shows the properties of the obtained polyester film and magnetic tape. The SRa value of the A side of the polyester film was 4 nm, and the SRa value and SRz value of the B side were 20 nm and 250 nm.

Comparative Example 3 In the production of the base film of Example 1, no aqueous solution was applied. Otherwise in the same manner as in Example 1, a composite polyester film roll having a thickness of 4.9 μm was obtained, and a magnetic tape having a width of 8 mm was prepared. Table 1 shows the properties of the obtained polyester film and magnetic tape. In addition, the SRa value of the A side of the polyester film was 1 nm, and the SRa value and SRz value of the B side were 20 nm and 250 nm.

[0053]

[Table 1]

As is clear from the results shown in Table 1, it is understood that the magnetic tape in which the ferromagnetic metal thin film layer is provided on one surface A of the polyester film according to the present invention can be a data storage tape having a low error rate.

[0055]

As described above, according to the polyester film for a magnetic recording medium of the present invention, the most suitable polyester film to be used for a magnetic recording tape of a ferromagnetic metal thin film deposition type for data backup with a low error rate is provided. Obtainable.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Nao Harada 1-1-1 Sonoyama, Otsu City, Shiga Prefecture Toray Industries, Inc. Shiga Plant F-term (reference) 4F100 AA03 AA20 AB01C AB10 AB15 AJ06B AK41A AK42A BA02 BA03 BA10A BA10C BA26 GB41 JA07B JB06B JG06C JK09 JL00 JM02B JM02C YY00B 5D006 BB01 CB01 CB07 CB08

Claims (8)

[Claims] 1. A polyester for a magnetic recording medium, wherein one side of a polyester film is coated with a film containing a cellulose derivative, and the number average molecular weight of the cellulose derivative is more than 100,000 and 500,000 or less. the film. 2. The polyester film for a magnetic recording medium according to claim 1, wherein the coating contains a water-repellent substance. 3. The method according to claim 1, wherein the content of the cellulose derivative in the coating is 1%.
3. The polyester film for a magnetic recording medium according to claim 1, wherein the amount is in the range of 0 to 70% by weight. 4. The water-repellent substance content in the coating is 0.05
3. The polyester film for a magnetic recording medium according to claim 2, wherein the amount is in the range of about to 2.4% by weight. 4. 5. The polyester film for a magnetic recording medium according to claim 1, wherein the thickness is less than 7 μm. 6. The polyester film for a magnetic recording medium according to claim 1, wherein the polyester is polyethylene terephthalate or polyethylene 2,6-naphthalate. 7. The polyester film for a magnetic recording medium according to claim 1, wherein the polyester film is used for a digital recording type magnetic tape. 8. A magnetic recording tape comprising the polyester film according to claim 1 and a ferromagnetic metal thin film layer provided on one surface of the polyester film.