JP2002370276A - Biaxially oriented polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film useful as a base film of a magnetic recording medium which is not liable to generate an error due to a deviation of a track and which improves output characteristics in a linear track type large capacity digital data storage application having excellent elastic modulus, creep resistant characteristics and delamination resistance. SOLUTION: The biaxially oriented polyester film comprises a copolymer polyethylene-2,6-naphthalenedicarboxylate obtained by copolymerizing a dicarboxylic acid component mainly containing 2,6-naththalenedicarboxylic acid as a main dicarboxylic acid component, ethyleneglycol as a main glycol component and a third component of 1 to 7 mol%. The polyester film satisfies conditions in which (1) a relation between a temperature expansion coefficient αt (×10<-6> / deg.C) of a widthwise direction of the film and a humidity expansion coefficient αh (×10<-6> /%RH) is 20<=(αt+2αh)<=45, and (2) a widthwise dimensional change before and after a process when a load of 32 MPa is applied in a longitudinal direction of the film and the film is treated at 50 deg.C and 90%RH for 72 hours is 0.3% or less, and a film thickness is 3 to 7 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a biaxially oriented polyester film for a magnetic recording medium. More specifically, it is excellent in elastic modulus, dimensional stability and creep resistance, hardly causes blocking between films, and has excellent electromagnetic conversion characteristics and running durability when used as a magnetic recording medium, and is particularly suitable for digital data storage. The present invention relates to a biaxially oriented polyethylene-2,6-naphthalate film.

[0002]

2. Description of the Related Art Polyesters represented by polyethylene terephthalate are widely used today in applications such as fibers, films or resin molded products because of their excellent physical and chemical properties. In particular, biaxially oriented films of polyethylene terephthalate are suitably used as base films in the field of magnetic recording such as audio tapes, video tapes, computer tapes and floppy (registered trademark) disks, which have been remarkably developed in recent years.

[0003] However, with recent advances in miniaturization, weight reduction and high performance of electric and electronic equipment, the characteristics required for base films have become increasingly severe.
For example, in the magnetic recording field, long-time recording, miniaturization,
In order to achieve weight reduction, it is necessary to make the base film thinner, but at the same time, it is important to maintain a stiffness of the film with a higher elastic modulus. For this reason, when the thickness of the base film is extremely thin, the elastic modulus of the conventional base film using polyethylene terephthalate is sometimes insufficient.

[0004] Polyethylene-2,6-naphthalate film is superior to polyethylene terephthalate film in mechanical properties, heat resistance, chemical properties and high glass transition temperature (Tg) and is preferably used in the above applications. . However, a polyethylene-2,6-naphthalate film tends to be inferior in creep resistance as compared with a normal polyethylene terephthalate film (longitudinal stretching under longitudinal tension and contraction in the width direction are large). For this reason, when a biaxially stretched film of, for example, polyethylene-2,6-naphthalate is used for high-density recording, the creep resistance is poor. I will.

[0005] For example, in data storage applications employing a linear track system such as QIC, DLT, and even higher-capacity super DLT and LTO, the track pitch is extremely narrowed in order to realize a higher tape capacity. ,
The dimensional change in the width direction of the tape causes a track shift, which causes an error.
These dimensional changes are due to changes in temperature and humidity, and due to temporal shrinkage in the width direction that occurs when the vehicle is repeatedly run under high tension and high temperature and high humidity. If this dimensional change is large, a track shift is caused, and an error occurs during electromagnetic conversion. In particular, in the latter case, the tape thickness becomes more remarkable as the tape recording capacity is increased, and improvement of the dimensional change is a new problem. This temporal shrinkage in the width direction can be improved by increasing the Young's modulus in the longitudinal direction of the base film. On the other hand, from the viewpoint of polymer properties and film forming properties, the larger the Young's modulus in the longitudinal direction, the more the lateral shrinkage becomes. The upper limit of the Young's modulus in the direction becomes smaller, and as a result, the dimensional change due to the former change in temperature and humidity becomes larger, making it difficult to achieve both.

In particular, in the case of a film having a smooth surface in which high density of magnetic recording is to be achieved, blocking tends to occur when the film is wound into a roll. If the blocking roll-shaped film has poor delamination resistance, it may break at the time of unwinding, and even if it does not break, the material is transferred to the surface where the polyethylene-2,6-naphthalate film contacts by local blocking, There is also the problem of dropout when taped.

[0007]

An object of the present invention is to solve the problems of the conventional polyethylene-2,6-naphthalate film, improve the creep resistance while maintaining a high elastic modulus, and further improve the creep resistance. Improves delamination resistance,
Provides biaxially oriented polyester film that is less likely to cause errors due to track deviation due to dimensional change in tape width, and is useful as a base film for magnetic recording media with improved output characteristics, especially in large-capacity digital data storage applications using linear tracks. Is to do.

[0008]

That is, the present invention relates to the following:
Biaxial orientation comprising copolymerized polyethylene-2,6-naphthalenedicarboxylate obtained by copolymerizing 6-naphthalenedicarboxylic acid as a main dicarboxylic acid component, ethylene glycol as a main glycol component, and a third component at 1 to 7 mol%. It is a polyester film, and the following conditions (1) and (2)

(1) Thermal expansion coefficient α in the width direction of the film
t (× 10 ^-6 / ° C) and humidity expansion coefficient αh (× 10 ^-6 /%
RH) 20 ≦ (αt + 2αh) ≦ 45

(2) A dimensional change in the width direction before and after the treatment when the film is treated at 50 ° C. and 90% RH for 72 hours by applying a load of 32 MPa in the longitudinal direction of the film to satisfy 0.3% or less. Is a biaxially oriented polyester film having a film thickness of 3 to 7 μm.

In a preferred embodiment of the present invention, at least one surface of a film made of polyethylene-2,6-naphthalate has 2,6-naphthalenedicarboxylic acid as a main dicarboxylic acid component, ethylene glycol as a main glycol component, A biaxially oriented polyester film obtained by laminating a film composed of copolymerized polyethylene-2,6-naphthalenedicarboxylate obtained by copolymerizing 1 to 7 mol% of a component, and the following (1) to (2) conditions

(1) Thermal expansion coefficient α in the width direction of the film
t (× 10 ^-6 / ° C) and humidity expansion coefficient αh (× 10 ^-6 /%
RH) 20 ≦ (αt + 2αh) ≦ 45

(2) When a load of 32 MPa is applied in the longitudinal direction of the film and the film is treated at 50 ° C. and 90% RH for 72 hours, the dimensional change in the width direction before and after the treatment satisfies 0.3% or less. Is a biaxially oriented polyester film having a film thickness of 3 to 7 μm. As a further preferred embodiment, the present invention provides copolymerized polyethylene-2,
The third component of 6-naphthalenedicarboxylate is 4,
The above-mentioned biaxially oriented polyester film which is 4′-diphenyldicarboxylic acid, the above biaxially oriented polyester film wherein the third component of the copolymerized polyethylene-2,6-naphthalenedicarboxylate is an ethylene oxide adduct of bisphenol A, The above-mentioned biaxially oriented polyester film having a Young's modulus in the longitudinal direction larger than that in the transverse direction, wherein at least one surface has a surface roughness (WRa) of 0.5 to 1.
The above biaxially oriented polyester film having a thickness of 0 nm and the surface roughness (WRa) on the side on which the magnetic layer is provided are 0.5 to 10 n.
m, the biaxially oriented polyester film having a surface roughness (WRa) of 1 to 20 nm on the nonmagnetic layer side, the biaxially oriented polyester film for a digital recording type magnetic recording medium, and the linear recording type magnetic film. The biaxially oriented polyester film for a recording medium, the biaxially oriented polyester film for a coating type magnetic recording medium, and the biaxially oriented polyester film for a ferromagnetic metal thin film type magnetic recording medium are included.

Hereinafter, the present invention will be described in detail.

[Polyethylene-2,6-naphthalenedicarboxylate copolymer] In the present invention, the polyester constituting the biaxially oriented polyester film is polyethylene-2,6 obtained by copolymerizing 1 to 7 mol% of the third component. -A naphthalenedicarboxylate copolymer. This third component is a component other than 2,6-naphthalenedicarboxylic acid and ethylene glycol, for example, terephthalic acid,
Isophthalic acid, diphenylsulfonedicarboxylic acid, 4,
Aromatic dicarboxylic acids such as 4′-diphenyldicarboxylic acid and benzophenone dicarboxylic acid, aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid and dodecanedicarboxylic acid, hexahydroterephthalic acid, 1,3-adamantanedicarboxylic acid and the like Alicyclic dicarboxylic acid, 1,
3-propanediol, 1,4-butanediol, 1,
6-hexanediol, neopentyl glycol, 1,
Examples thereof include 4-cyclohexanedimethanol, an ethylene oxide adduct of bisphenol A, and p-xylylene glycol. Of these, ethylene oxide adducts of 4,4'-diphenyldicarboxylic acid and bisphenol A are particularly preferred.

Here, the ethylene oxide adduct of bisphenol A is a compound represented by the following formula.

[0017]

Embedded image (Where n + m is a number of 2 to 10, preferably a number of about 4) The copolymerization ratio of the third component is 1 to 7 mol% and 2 to 5 mol% based on all dicarboxylic acid components. Is preferred. If the copolymerization ratio of the third component is less than 1 mol%, the effect of improving the creep resistance and blocking resistance will not be exhibited. The width shrinkage during heat treatment increases,
Both the temperature and humidity expansion coefficients increase, and errors due to track deviation are likely to occur.

[Inert fine particles] The polyethylene-2,6-naphthalenedicarboxylate and the copolymerized polyethylene-2,6-naphthalenedicarboxylate used in the present invention may contain, if necessary, a stabilizer, a coloring agent, and an antistatic agent. Agent,
Additives such as lubricants can be included.

In particular, in order to improve the slip property of the film by roughening the film surface, it is preferable to contain inert solid fine particles as a lubricant. As the inert solid fine particles, preferably, (1) silicon dioxide (including hydrate, silica sand, quartz, etc.); (2) alumina; (3) SiO
Silicates containing 30% by weight or more of ₂ minutes (for example, amorphous or crystalline clay minerals, aluminosilicates (including calcined products and hydrates), hot asbestos, zircon, fly ash, etc.); (4) Oxides of Mg, Zn, Zr, and Ti;
(5) Ca and Ba sulfides; (6) Li, Na and Ca phosphates (including mono- and di-hydrogen salts);
(7) Li, Na and K benzoates; (8) Ca,
Terephthalates of Ba, Zn, and Mn; (9) Mg,
Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe, C
(10) Ba and Pb chromates; (11) carbon (eg, carbon black,
(12) glass (for example, glass powder,
(13) Ca and Mg carbonates;
(14) fluorite and (15) ZnS are exemplified. More preferably, silicon dioxide, silicic anhydride, hydrated silicic acid, aluminum oxide, aluminum silicate (calcined product,
Hydrate, etc.), monolithium phosphate, trilithium phosphate,
Sodium phosphate, calcium phosphate, barium sulfate, titanium oxide, lithium benzoate, double salts of these compounds (including hydrates), glass powder, clay (including kaolin, bentonite, clay, etc.), talc, calcium carbonate, etc. Is exemplified. Particularly preferably, silicon dioxide, titanium oxide,
Calcium carbonate.

Two or more of these may be used, and in this case, those having different average particle sizes may be added in the same amount or different amounts. The average particle size of such fine particles is 0.05 to 0.8 μm
m, and the addition amount is preferably 0.01 to 0.5% by weight based on the total weight of the polyester. However, when used as a laminated film, one layer may not contain inert fine particles.

[Production of Polymer] The polyethylene-2,6-naphthalenedicarboxylate and copolymerized polyethylene-2,6-naphthalenedicarboxylate used in the present invention are usually produced by a known method by a melt polymerization method. Can be.

For example, polyethylene-2,6-naphthalenedicarboxylate is obtained by subjecting 2,6-naphthalenedicarboxylic acid and ethylene glycol to an esterification reaction or dimethyl-2,6-naphthalate and ethylene glycol by an ester exchange reaction. And then subjecting the reaction product to a polycondensation reaction under reduced pressure to produce a polymer having a predetermined degree of polymerization.

The copolymerized polyethylene-2,6-naphthalenedicarboxylate is a polyethylene-polyethylene except that a predetermined ratio of a third component such as 4,4'-diphenyldicarboxylic acid or an ethylene oxide adduct of bisphenol A is used.
It can be produced in the same manner as in the production method of 2,6-naphthalate. At that time, it is preferable to use a catalyst, and other additives can be added.

The intrinsic viscosity of polyethylene-2,6-naphthalenedicarboxylate or copolymerized polyethylene-2,6-naphthalenedicarboxylate is 0.45 to 0.90.
It is preferable that

[Coating Layer] The biaxially oriented polyester film of the present invention may have a coating layer on at least one surface.
The coating layer is preferably composed of a water-soluble resin or a water-dispersible resin. Examples of such a water-soluble resin or a water-dispersible resin include acrylic resin, polyester resin, and acrylic resin.
Examples thereof include ester resins, alkyd resins, phenol resins, epoxy resins, amino resins, polyurethane resins, vinyl acetate resins, and vinyl chloride-vinyl acetate copolymers. Among them, the adhesion of the core layer to the polyester resin, the retention of protrusions, the slipperiness, etc.,
Acrylic resins, polyester resins and acryl-polyester resins are preferred. These may be a homopolymer or a copolymer, and may be a mixture. These water-dispersible resins are preferable.

The water-soluble or water-dispersible acrylic resin includes, for example, acrylic acid esters (alcohol residues such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl). Butyl group, 2
-Ethylhexyl group, cyclohexyl group, phenyl group,
Benzyl group, phenylethyl group, etc.); methacrylic acid ester (the alcohol residue is the same as above);
Hydroxy-containing monomers such as -hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate; acrylamide, methacrylamide, N-methylmethacrylamide, N-methylacrylamide, N-methylolacrylamide, N Amide group-containing monomers such as methylol methacrylamide, N, N-dimethylolacrylamide, N-methoxymethylacrylamide, N-methoxymethylmethacrylamide, N-phenylacrylamide; N, N-diethylaminoethyl acrylate, N, N- Amino group-containing monomers such as diethylaminoethyl methacrylate; glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ester As a main component one or more such epoxy group-containing monomers such as Le, such as styrene sulfonic acid,
Monomers containing a sulfonic acid group or a salt thereof, such as vinyl sulfonic acid and salts thereof (eg, sodium salt, potassium salt, ammonium salt, etc.); crotonic acid, itaconic acid, acrylic acid, maleic acid, fumaric acid and the like A monomer containing a carboxyl group or a salt thereof such as a salt (for example, a sodium salt, a potassium salt, an ammonium salt, etc.); a monomer containing an anhydride such as maleic anhydride or itaconic anhydride; Styrene, vinyl methyl ether, vinyl ethyl ether, vinyl trisalkoxysilane, alkyl maleate monoester, alkyl fumarate monoester, acrylonitrile, methacrylonitrile, alkyl itaconate monoester, vinylidene chloride, vinyl acetate, salt It is preferable to use a combination of one or more monomers such as vinyl as an auxiliary component, and further contain 50 mol% or more of a (meth) acrylic monomer component such as an acrylic acid derivative and a methacrylic acid derivative. And those containing components of methyl methacrylate and ethyl methacrylate are particularly preferable.

The water-soluble or water-dispersible acrylic resin can be self-crosslinked with a functional group in the molecule, or can be crosslinked with a crosslinking agent such as a melamine resin or an epoxy compound.

Examples of the acid component constituting the water-soluble or water-dispersible polyester resin include, for example, terephthalic acid, isophthalic acid, phthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 4,4 ′. -Diphenyldicarboxylic acid, adipic acid, sebacic acid, dodecanedicarboxylic acid, succinic acid, pentasodium sulfoisophthalic acid, 2-potassium sulfoterephthalic acid, trimellitic acid, trimesic acid, trimellitic anhydride, phthalic anhydride, p-hydroxy Examples thereof include polyvalent carboxylic acids such as benzoic acid and monopotassium trimellitate.
Examples of the hydroxy compound component include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, and p- Xylylene glycol, ethylene oxide adduct of bisphenol A, diethylene glycol, triethylene glycol, polyethylene oxide glycol, polytetramethylene oxide glycol, dimethylolpropionic acid, glycerin, trimethylolpropane, sodium dimethylolethylsulfonate, potassium dimethylolpropanoate And the like. A polyester resin can be prepared from these compounds by a conventional method. From the viewpoint of making an aqueous coating composition, it is preferable to use an aqueous polyester resin containing a 5-sodium sulfoisophthalic acid component or a carboxylate group. Such a polyester resin can be a self-crosslinking type having a functional group in the molecule, or can be crosslinked using a curing agent such as a melamine resin or an epoxy resin.

The above-mentioned water-soluble or water-dispersible acrylic-polyester resin is used in the sense that it contains an acrylic modified polyester resin and a polyester-modified acrylic resin, and is obtained by combining an acrylic resin component and a polyester resin component with each other. And includes, for example, a graft type, a block type, and the like. Acrylic polyester resin, for example, adds a radical initiator to both ends of the polyester resin to polymerize the acrylic monomer, or adds a radical initiator to the side chain of the polyester resin to polymerize the acrylic monomer. Alternatively, it can be produced by adding a hydroxyl group to the side chain of the acrylic resin and reacting it with a polyester having an isocyanate group or a carboxyl group at the end to form a comb polymer.

The coating layer may contain inert particles. Examples of the material of the inert particles include polystyrene, polystyrene divinylbenzene, polymethyl methacrylate, methyl methacrylate copolymer, and methyl methacrylate copolymer. Organic materials such as crosslinked products, polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, benzoguanamine resin, silica, alumina, titanium dioxide, kaolin, talc, graphite, calcium carbonate, feldspar, molybdenum disulfide, carbon black, barium sulfate Any of inorganic substances such as the above may be used.

Further, core-shell type particles having a multilayer structure composed of substances having different properties inside and outside may be used.

When inert particles are contained, the average particle size of the inert fine particles is 5 to 100 nm, preferably 10 to 50 nm.
nm. Further, those having a uniform particle size distribution are preferred. If the average particle size is less than 5 nm, the slipperiness deteriorates.
On the other hand, if the average particle size exceeds 100 nm, the particles will fall off and the abrasion resistance will deteriorate. In addition, the spacing with the magnetic head is increased, and it is difficult to provide a magnetic recording medium of a high-density recording type.

The inert particles have a surface projection frequency of 1 in the coating layer.
10⁶~ 110⁹Pieces / mm^TwoTo give the amount
It is contained in the membrane layer. This surface protrusion frequency is 110⁶Pieces/
mm ^TwoIf less, the running durability when used as a magnetic recording medium
Run short. The other 110⁹Pieces / mm^TwoElectromagnetic conversion beyond
Affects properties. Preferred surface projection frequency is 110
⁶~ 110⁸Pieces / mm^Two, More preferably 210⁶~
510⁷Pieces / mm^TwoParticularly preferably 310⁶~ 310
⁷Pieces / mm^TwoIt is.

The coating layer is formed by applying a coating liquid containing inert particles and a water-soluble or water-dispersible resin to at least one surface of the above-mentioned film made of copolymerized polyethylene-2,6-naphthalenedicarboxylate, It can be formed by drying. At this time, the film may be a biaxially oriented film, a uniaxially oriented film or an unstretched film. In the latter case, a stretching treatment for forming a biaxially oriented film is performed after application of the coating liquid.

The coating solution has a solid content of 0.2 to 10% by weight, more preferably 0.5 to 5% by weight, and particularly preferably 0.7 to 3% by weight.
It is preferable that If necessary, other components such as a surfactant, a stabilizer, a dispersant, a UV absorber, a thickener, and the like can be added to the coating liquid.

[Film Thickness] The biaxially oriented polyester film of the present invention has a total thickness of 3 to 7 μm, preferably 3 to 6 μm. If the thickness exceeds 7 μm, the tape thickness becomes too large, for example, the length of the tape to be put in a cassette becomes short, and a sufficient magnetic recording capacity cannot be obtained. On the other hand, if it is less than 3 μm, it cannot withstand the load in the vertical direction, and the elongation becomes large. In the case of a laminated film, the thickness of copolymerized polyethylene-2,6-naphthalate must be 50% or more of the whole.

[Expansion coefficient] The biaxially oriented polyester film of the present invention has a thermal expansion coefficient α in the film width direction.
t (× 10 ^-6 / ° C) and humidity expansion coefficient αh (× 10 ^-6 /%
RH) is 20 ≦ (αt + 2αh) ≦ 45,
Preferably, 25 ≦ (αt + 2αh) ≦ 40, and particularly preferably, 30 ≦ (αt + 2αh) ≦ 40. (Αt + 2
If the value of αh) is greater than 45, the dimensional change due to temperature and humidity changes will increase, causing a track shift, and causing
This will cause playback errors. On the other hand, if it is smaller than 20, the track deviation during repeated running under high temperature and high humidity becomes large.

To obtain such an expansion coefficient, the third
The component to be copolymerized is set to 1 to 7 mol%, sufficient stretching orientation is given in the width direction as well as in the longitudinal direction, and sufficient crystallization treatment is performed.

[Change in Width due to Temperature and Humidity Treatment under Load] The biaxially oriented polyester film according to the present invention has a width of 12.6.
When a load of 32 MPa is applied in the longitudinal direction of a film having a width of 5 mm and the film is treated for 72 hours at a temperature of 50 ° C. and a humidity of 90% RH, the dimensional change in the width direction before and after the treatment is 0.3% or less. Preferably not more than 0.25%, particularly preferably 0.25%
% Or less. If this dimensional change is greater than 0.3%,
When the tape is repeatedly run under high tension and high temperature and high humidity, the dimensional change in the width direction becomes large, causing a track deviation and causing a recording / reproducing error.

In order to keep the width change due to the temperature and humidity treatment under load within the above range, the component to be copolymerized as the third component is set to 1 to 7 mol%, and sufficient stretching orientation is given particularly in the longitudinal direction. Perform sufficient crystallization treatment.

[Young's Modulus] The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the longitudinal direction of 6 GPa or more, more preferably 7 GPa or more, particularly preferably 8 GPa or more. When the Young's modulus in the vertical direction is less than 6 GPa, the strength of the magnetic tape in the vertical direction becomes weak, and if a strong force is applied in the vertical direction during recording / reproducing, the magnetic tape is easily broken.

The Young's modulus in the transverse direction of the film is 4GP.
It is preferably at least a, more preferably at least 5 GPa, particularly preferably at least 6 GPa. When the Young's modulus in the horizontal direction is less than 4 GPa, when a linear track type magnetic tape is used,
A dimensional change in the width direction at the time of temperature and humidity change becomes large, and a recording / reproducing error due to a track shift is likely to occur.

The sum of the longitudinal Young's modulus and the transverse Young's modulus of the biaxially oriented polyester film is 10-2.
It is preferably 0 GPa, more preferably 12 to 16 GPa.

Further, when the magnetic tape is used for a linear track type magnetic tape, the Young's modulus in the vertical direction is preferably larger than the Young's modulus in the horizontal direction from the viewpoint of reducing the elongation in the vertical direction.

If the sum of the Young's modulus in the vertical direction and the Young's modulus in the horizontal direction is less than 10 GPa, the strength of the magnetic tape is weakened, and the tape may be easily broken. As a result, recording / reproducing errors occur due to track deviation, and a satisfactory high-density magnetic medium cannot be obtained. On the other hand, if it exceeds 20 GPa, the draw ratio becomes high during film formation, the film breaks frequently, and the product yield decreases.

[Surface Roughness] The biaxially oriented polyester film in the present invention may be a single layer film or a laminated film. The surface roughness WRa (center plane average roughness) of these films on the side on which the magnetic layer is provided is 0.5 to 10 nm,
Further, it is preferably 1 to 10 nm, particularly preferably 2 to 8 nm. WRz (10-point average roughness) is 30 to
250 nm, even 30-200 nm, especially 30-1
Preferably it is 50 nm. If the surface roughness WRa is larger than 10 nm or WRz is larger than 150 nm, the surface of the magnetic layer becomes rough, and satisfactory electromagnetic conversion characteristics cannot be obtained. On the other hand, if the surface roughness WRa is less than 0.5 nm or WRz is less than 30 nm, the surface becomes too flat, slips on a pass roll or a calender become poor, wrinkles are generated, and the magnetic layer is uniformly formed. It cannot be applied and cannot be calendared properly.

The surface roughness W of the biaxially oriented polyester film on the non-magnetic layer side (for example, on the side on which the back coat layer is coated)
In the case of a single-layer film, Ra (center plane average roughness) and WRz (10 point average roughness) are substantially the same as the surface roughness WRa and WRz on the side on which the magnetic layer is provided. In this case, the values of the surface roughness WRa and WRz are selected from the viewpoint of satisfying the electromagnetic conversion characteristics and the traveling performance.

In the case of a laminated film, the film surface roughness WRa (center plane average roughness) on the nonmagnetic layer side is 1 to 20 n.
m, more preferably 5 to 15 nm, particularly preferably 8 to 12 nm. Further, WRz (10-point average roughness) is preferably 100 to 300 nm, more preferably 100 to 200 nm, particularly preferably 150 to 200 nm. This surface roughness WRa is larger than 20 nm, or WRz is 3
If the thickness is larger than 00 nm, protrusions on the flat layer (magnetic layer) side surface will be pushed up, and transfer of surface irregularities to the magnetic layer surface during winding of the magnetic tape will be large. Characteristics cannot be obtained. On the other hand, when the surface roughness WRa is less than 1 nm or WRz is less than 100 nm, the film slips or the air squeezing property is deteriorated, and when the film is slit, a bump occurs or a vertical wrinkle occurs. , And satisfactory winding properties cannot be obtained.

The surface roughnesses WRa and WRz are determined by adding inert fine particles in the film, for example, IIA and II of the periodic table.
High heat resistance such as inorganic fine particles containing elements B, IVA and IVB (for example, kaolin, alumina, titanium oxide, calcium carbonate, silicon dioxide, etc.), crosslinked silicone resin, crosslinked polystyrene, crosslinked acrylic resin particles, etc. It can be adjusted by incorporating fine particles of a polymer or the like, or by a surface treatment for forming fine irregularities, for example, a coating treatment with a lubricating agent.

When the monolayer film contains inert fine particles, the fine particles preferably have an average particle size of 0.05 to 0.8 μm, more preferably 0.1 to 0.6 μm, and particularly preferably 0.1 to 0.6 μm. It is preferably 0.4 μm. This amount is preferably 0.001 to 1.0% by weight (based on the polymer), more preferably 0.01 to 0.5% by weight (based on the polymer), and particularly preferably 0.02 to 0.3% by weight. (Vs. polymer)
It is preferable that In the case of a laminated film, as long as inert particles are contained on the non-magnetic surface side, inert particles may not be contained on the magnetic surface side.

The inert particles to be contained in the film may be of a single component type or of a multi-component type. In particular, the polymer on the non-magnetic layer side is preferably composed of two components in consideration of compatibility between the electromagnetic conversion characteristics of the tape and the winding property of the film. It is preferable to include inert particles of a multicomponent system or higher. W on film surface
The Ra may be adjusted by appropriately selecting the average particle diameter and the amount of the fine particles from the above ranges.

WRz is preferably adjusted by using means such as sharpening the particle size distribution of fine particles or removing coarse particles.

[Layer Structure] The biaxially oriented polyester film of the present invention may be a single-layer film or a laminated film, but it is easy to achieve both the electromagnetic conversion characteristics of the tape and the winding property of the film. A laminated film is more preferable. As the laminated film, at least one surface of a biaxially oriented polyethylene-2,6-naphthalate film has 2,6-naphthanedicarboxylic acid as a main dicarboxylic acid component, ethylene glycol as a main glycol component, and the aforementioned third component. Of polyesters obtained by copolymerizing 2 to 7 mol% of the above.

[Production of Film] The biaxially oriented polyester film of the present invention can be produced according to a conventionally known method.

For example, when the copolymerized polyethylene-2,6-naphthalate film is a single layer, first, the copolymerized polyethylene-2,6-naphthalate is heated from the die to a temperature of polymer melting point (Tm) + 20 ° C. to (Tm + 70) ° C. After extruding into a film, the mixture is rapidly cooled and solidified at 10 to 90 ° C. to obtain an unstretched film.

Thereafter, the unstretched film is uniaxially (longitudinal or transverse) (Tg-10) to
At a temperature of (Tg + 70) ° C. (where Tg: glass transition temperature of the polymer), the film is stretched at a magnification of 3.0 to 8.0 times, preferably 4.0 to 7.5 times, and then, if necessary, a film is formed. A coating liquid for forming a layer is applied to the film, and thereafter, at a temperature of Tg to (Tg + 70) ° C. in a direction perpendicular to the above direction at a magnification of 2.5 to 7.5 times, preferably 3.0 to 7.5 times.
Stretch at 7.0 times magnification. Further, if necessary, the film may be stretched again in the machine direction and / or the cross direction. That is, two stages, three
Stretching in multiple stages, in four stages, or in multiple stages may be performed. The total stretching ratio is usually 9 times or more, preferably 12 times or more, as the area stretching ratio.
３５35 times, more preferably 15 to 32 times. Subsequently, the biaxially oriented film is formed from (Tg + 70) to (Tm
Heat setting crystallization at a temperature of -10) ° C, for example, 180 to 250 ° C, provides excellent dimensional stability.
The heat setting time is preferably 1 to 60 seconds.

In the case of a laminated film by a coextrusion method, the polyethylene-2,6-naphthalate is laminated in a die such that the copolymerized polyethylene-2,6-naphthalate is laminated on one side or both sides thereof, and then extruded into a film. Preferably, it is extruded at a temperature of melting point (Tm: ° C.) to (Tm + 70) ° C., or is extruded from a die after laminating two or more types of molten polyester, and then quenched and solidified to form a laminated unstretched film. Biaxial stretching and heat treatment are performed under the same method and conditions as in the case to obtain a laminated biaxially oriented film.

[Magnetic Recording Medium] A magnetic recording medium can be manufactured using the biaxially oriented polyester film of the present invention. The embodiment is as follows, for example.

On the surface of the biaxially oriented polyester film or the coating layer of the present invention, iron or needle-like fine magnetic powder (metal powder) containing iron as a main component is added to polyvinyl chloride, vinyl chloride / vinyl acetate copolymer. Or the like, and the magnetic layer thickness is 1 μm or less, preferably 0.1 to 1 μm.
m, and if necessary, by providing a back coat layer on the surface opposite to the magnetic layer surface by a known method, especially in the short wavelength region, electromagnetic waves such as S / N and C / N. A metal-coated magnetic recording medium for high-density recording with excellent conversion characteristics and low dropout and error rate can be obtained.

If necessary, a nonmagnetic layer containing fine titanium oxide particles or the like may be formed on the film layer or the coating layer as an underlayer of the magnetic layer containing the metal powder. It can be dispersed and coated inside.
This metal coated magnetic recording medium is used for recording 8 mm video, Hi8 system, Betacam SP, W-VHS, etc. for recording analog video signals, and DVC Pro, Digital Betacam, DVC for recording digital video signals.
2, D3, D5, Betacam SX, etc., and DLT, LTO, 3570, 359 for digital data recording
It is extremely useful as a magnetic tape medium such as 0, 9840, 9940, data 8 mm, and DDS.

Further, a needle-like fine magnetic powder such as iron oxide or chromium oxide or a plate-like fine magnetic powder such as barium ferrite is coated on the surface of the biaxially oriented polyester film or the coating layer of the present invention with polyvinyl chloride or chloride. It is uniformly dispersed in a binder such as a vinyl / vinyl acetate copolymer, and is applied so that the thickness of the magnetic layer is 1 μm or less, preferably 0.1 to 1 μm. By providing the back coat layer by the method, the oxide-coated magnetic recording for high-density recording is particularly excellent in the output in the short wavelength region, the electromagnetic conversion characteristics such as S / N, C / N, and the dropout and the error rate. It can be a medium.

If necessary, on A or B,
A non-magnetic layer containing fine titanium oxide particles and the like as an underlayer of the magnetic powder-containing magnetic layer may be dispersed in the same organic binder as the magnetic layer and coated. This oxide-coated magnetic recording medium is used for recording digital data.
It is useful as a high-density oxide coated magnetic recording medium such as IC and TRAVAN.

Further, the surface of the biaxially oriented polyester film or the coating film layer of the present invention is coated with iron, iron, or the like by a method such as vacuum evaporation, sputtering, or ion plating.
A ferromagnetic metal thin film layer made of cobalt, chromium, or an alloy or oxide containing these as a main component is formed, and a surface, such as diamond-like carbon (DLC) or the like, if necessary, used, used, or necessary. A protective layer and a fluorinated carboxylic acid-based lubricating layer are sequentially provided, and if necessary, a back coat layer is provided on the surface on the opposite side of the magnetic layer by a known method. /
It is possible to obtain a vapor-deposited magnetic recording medium for high-density recording which has excellent electromagnetic conversion characteristics such as N and has low dropout and error rate. This vapor-deposited magnetic recording medium is a digital video cassette (DV) for recording Hi8 video and digital video signals for recording analog video signals.
C) for recording digital data such as DVcam
It is extremely useful as a magnetic tape medium such as anmoth and AIT.

[0064]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The measuring method used in the present invention is as follows. "Parts" means parts by weight.

(1) Average particle size of inert particles CP-50 type centrifugal particle size analyzer manufactured by Shimadzu Corporation (Centrifugal Particle S)
ize Analyzer). From the integrated curve of particles of each particle size and its abundance calculated based on the obtained centrifugal sedimentation curve, the particle size “equivalent sphere diameter” corresponding to 50 mass percent is read, and this value is defined as the above average particle size. ("Granularity measurement technology", published by Nikkan Kogyo Shimbun, 1975, pages 242 to 242)
247).

(2) Young's Modulus The film was cut into a sample having a width of 10 mm and a length of 15 cm, and the distance between the chucks was set to 100 mm. The film was pulled at a tensile speed of 10 mm / min and a chart speed of 500 mm / min by pulling with an Instron type universal tensile tester. The Young's modulus is calculated from the tangent at the rising edge of the applied load-elongation curve.

(3) Surface roughness (WRa, WRz) Non-contact type three-dimensional roughness meter (NT-2000) manufactured by WYKO
Measurement magnification: 25 ×, measurement area: 246.6 μm × 1
Under the condition of 87.5 μm (0.0462 mm ² ), measurement was performed with the number of measurements (n) of 10 or more, and the center plane average roughness (WR) was measured by the surface analysis software built in the roughness meter.
a) and 10-point average roughness (WRz) are determined.

(A) Center plane average roughness (WRa)

[0069]

(Equation 1)

However, Z _jk is the measurement direction (246.6 μm).
m) and the height on the three-dimensional roughness chart at the j-th and k-th positions in each direction when the direction (187.5 μm) orthogonal thereto is divided into m and n, respectively.

(B) 10-point average roughness (WRz) Five points are taken from the higher peak (HP) and five points from the lower valley (Hv), and the average roughness is defined as WRz.

[0072]

[Number 2] _{WRz = {(H P1 + H} P2 + H P3 + H P4 + H P5)
− (H _V1 + H _V2 + H _V3 + H _V4 + H _V5 )} / 5

(4) Thermal Expansion Coefficient (αt × 10 ⁻⁶ / ° C.) A film sample was cut out to a length of 15 mm and a width of 5 mm in the transverse direction of the film, set on a TMA3000 manufactured by Vacuum Riko, and placed at 30 ° C. in a nitrogen atmosphere at 60 ° C. Minutes, then cool to room temperature. Then 2 ℃ from 25 ℃ to 70 ℃
/ Min, and the sample length at each temperature is measured, and the thermal expansion coefficient (αt × 10 ⁻⁶ ) is calculated from the following equation.

[0074]

Αt = ｛(L2−L1) / (L1 × Δ)
T)｝ × 10 ⁶ +0.5 (Note) Here, L1: Sample length at 40 ° C. (mm) L2: Sample length at 60 ° C. (mm) ΔT: 20 (= 60-40 ° C.) Note: Quartz The coefficient of thermal expansion of glass (× 10 ⁻⁶ ).

(5) Humidity expansion coefficient (αh × 10 ⁻⁶ /% R)
H) A film sample was cut out to a length of 15 mm and a width of 5 mm in the transverse direction of the film, set on a TMA3000 manufactured by Vacuum Riko, and under a nitrogen atmosphere, a humidity of 30% RH and a humidity of 70
While maintaining the% RH constant, the length of the sample at that time is measured, and the humidity expansion coefficient is calculated by the following equation.

[0076]

Αh = ｛(L2−L1) × / (L1 × Δ)
H)｝ × 10 ⁶ where L1: sample length at 30% RH (m)
m) L2: Sample length (mm) at a humidity of 70% RH ΔH: 40 (= 70-30% RH).

(6) Residual shrinkage in the width direction of the film after application of a load in the vertical direction.
The humidity is adjusted in an atmosphere of 50% humidity for 24 hours. In this state, one side of the film (the other is fixed) is weighted so that the load per unit cross-sectional area is 28 MPa, and the width of the film at that time ( L1) is a Keyence laser diameter measuring device (body: 3100 type, sensor: 3060)
Type). Then, under high temperature and high humidity of 50 ° C. × 90% RH, a treatment was carried out for 72 hours (3 days) with one side (the other being fixed) with a weight of 28 MPa, and the weight was removed and the temperature was 25 ° C. After adjusting the humidity in an atmosphere of 50% humidity for 24 hours, a weight of 28 MPa was again attached to one side of the film (the other was fixed), and the width (L2) of the film at that time was measured using a Keyence laser. Diameter (body: 3100 type, sensor: 3060
Type).

From the dimensions before and after the temperature / humidity processing measured above, the width dimension change (αW) before and after the temperature / humidity processing under load is calculated by the following equation.

[0079]

ΑW = {(L2−L1) × / L1} × 100 (%)

(7) Content of diethylene glycol (DEG) The polymer was decomposed using hydrazine hydrate and quantified by gas chromatography. For gas chromatography, Hitachi Model 263-70 was used.

(8) Evaluation of blocking resistance A film was cut into a size of 40 mm in width and 40 mm in width.
Condition humidity for 3 hours in an atmosphere of 0 ° C. and 80%. After 100 sheets of this film are stacked and sandwiched between a pair of metal plates, a predetermined pressure 10 (kgf / cm ² ) and a temperature 40
Press for 1 hour at ° C. In these films, the films sometimes blocked each other, and the blocking property was evaluated on the following scale. A: Each film is not blocked and can be peeled off without delamination. ：: Light delamination has occurred partially (within 10%). Δ: Delamination has occurred as a whole. X: Blocking is severe and peeling cannot be performed.

(9) Track deviation (error rate) The following composition was put into a ball mill and kneaded for 16 hours.
After the dispersion, 5 parts by weight of an isocyanate compound (Desmodur L manufactured by Bayer AG) was added, and the mixture was sheared and dispersed at a high speed for 1 hour to obtain a magnetic paint. Composition of magnetic paint: needle-like Fe particles 100 parts by weight Vinyl chloride-vinyl acetate copolymer 15 parts by weight (SREC 7A manufactured by Sekisui Chemical) 5 parts by weight of thermoplastic polyurethane resin 5 parts by weight of chromium oxide 5 parts by weight of carbon black 2 parts by weight of lecithin Part Fatty acid ester 1 part by weight Toluene 50 parts by weight Methyl ethyl ketone 50 parts by weight Cyclohexanone 50 parts by weight This magnetic paint is applied to the surface of a biaxially oriented polyester film or a coating layer so as to have a coating thickness of 0.5 μm.
Next, orientation treatment was carried out in a DC magnetic field of 2500 gauss, and after heating and drying at 100 ° C., supercalender treatment (linear pressure: 2000 N / cm, temperature: 80 ° C.) was carried out and wound. The wound roll was left in an oven at 55 ° C. for 3 days.

Further, a backcoat layer paint having the following composition was applied to a thickness of 1 μm, dried, and then coated with 6.35 mm (=
1 ′ / 4), a magnetic tape was obtained, and the tape was loaded on a QIC cassette to obtain a cassette tape. Composition of backcoat layer coating composition: carbon black 100 parts by weight thermoplastic polyurethane resin 60 parts by weight isocyanate compound 18 parts by weight (Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.) silicone oil 0.5 parts by weight methyl ethyl ketone 250 parts by weight toluene 50 parts by weight This cassette Using Media Logic M
Using the L4500B, QIC system, the error rate is measured under the following conditions.

Current = 15.42 mA Frequency: 0.25 MHz Location = 0 Threshold: 40.0 Bad / Good / MAX: 1: 1: 1 Tracks: 28 Note that the number of error rates is the number of tracks measured (= 28).
The average value of

The measurement under the conditions 1 and 2 is performed as follows.

Condition 1: After recording at a temperature of 10 ° C. and a humidity of 10% RH, reproducing at a temperature of 45 ° C. and a humidity of 80% RH, a track shift caused by a change in the temperature and humidity is determined by whether an error rate is increased or not. Measure.

Condition 2: After recording at a temperature and humidity of 25 ° C. and 50% RH, the vehicle was repeatedly run at a temperature and humidity of 40 ° C. and 60% RH for 60 hours, and returned to a temperature and humidity of 25 ° C. and 50% RH. After holding for 24 hours, the recording is reproduced, and the rack displacement caused by the width shrinkage due to the traveling at a relatively high temperature and high humidity is measured based on whether or not the error rate has increased.

The evaluation criteria are as follows. ：: No increase in error rate due to track deviation ：: There is an increase in error rate, but there is no practical problem ×: The error rate is remarkably large and there is a practical problem

Example 1 0.02% by weight of calcium carbonate particles having an average particle diameter of 0.4 μm and an average particle diameter of 0.1 μm
The copolymerized polyethylene-2,6-naphthalate shown in Table 1 containing 0.2% by weight of spherical silica particles of
And dried for 5 hours, then fed to an extruder hopper and
C., extruded using a T-shaped extrusion die, and quenched and solidified on a casting drum maintained at a surface finish of 0.3 S and a surface temperature of 60 ° C. to obtain an unstretched film.

The unstretched film is preheated at 125 ° C., and is further placed between low-speed and high-speed rolls by 83 mm from above by 14 mm.
The film was stretched 5.1 times by heating with an infrared heater having a surface temperature of 0 ° C., quenched, and then supplied to a stenter.
The film was stretched 4.8 times in the transverse direction at ℃. Followed by 2
After heat-setting at 20 ° C. for 3 seconds, the film was subjected to a 1.0% relaxation treatment at 190 ° C. in the horizontal direction to obtain a biaxially oriented film having a thickness of 4.5 μm.

Table 1 shows the properties of the obtained film and the magnetic tape prepared by the above method. As is clear from this table, the blocking resistance is good, and the track shift is small.

[Examples 2 and 3] Copolymerized polyethylene-2,6-naphthalate films containing the particles shown in Table 1 were prepared in accordance with Example 1 under the conditions shown in Table 1. Table 1 shows the properties of the obtained film and tape. Good results were obtained as in Example 1.

Example 4 Each polymer was melted with two extruders so that polyethylene-2,6-naphthalate was laminated on both sides of polyethylene-2,6-naphthalate. Copolymer on both surface layers,
The homopolymer was placed in the middle layer and extruded. A biaxially oriented film of 4.5 μm was obtained under the conditions shown in Table 1 below. The layer of copolymerized polyethylene-2,6-naphthalate and the layer of polyethylene-2,6-naphthalate were 1.5 μm each.
/1.5 μm / 1.5 μm.

Example 5 Each of the polymers was melted with two extruders so that polyethylene-2,6-naphthalate was laminated on one side of polyethylene-2,6-naphthalate, and the mixture was melted in a two-layer die. And laminated to obtain an unstretched film. Thereafter, a biaxially oriented film of 4.5 μm was obtained under the conditions shown in Table 1. The copolymer polyethylene-2,6-naphthalate layer and the polyethylene-2,6-naphthalate layer were 3 μm / 1.5 μm, respectively. After the longitudinal stretching in the film forming process, the following coating liquid was applied to form a film layer. Coating layer (magnetic layer forming surface) Resin; Copolymerized polyester (terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid // ethylene glycol / bisphenol A / propion oxide 2 mol adduct = 97/1/2 // 60) / 40) 80 copies
Inert particles; 5 parts of acrylic particles (average particle size: 40 nm)
Surfactant; 15 parts of Inion NS-240 manufactured by NOF Corporation, thickness (after drying); 10 nm Layers (each layer having a thickness of about 0.1 μm) were formed, and a diamond-like carbon (DLC) film and a fluorine-containing carboxylic acid-based lubricating layer were sequentially provided on the surface to form a magnetic layer. The back coat was the same as in Example 1. It was evaluated as a magnetic tape, and the results are shown in Table 1. Good results were obtained as in Example 1.

Example 6 A biaxially oriented film having a thickness of 3.5 μm was obtained by adjusting the discharge amount of an extruder.
It was the same as in Example 1. Table 1 shows the results.

[Comparative Example 1] A polyester was formed as polyethylene-2,6-naphthalate containing no copolymer component under the conditions shown in Table 1. A magnetic tape was prepared and evaluated in the same manner as in Example 1. This film had poor blocking resistance and poor track deviation (condition 2) as a magnetic tape.

[Comparative Example 2] In the production of polyester,
A film was formed under the conditions shown in Table 1 with the amount of 4,4′-diphenyldicarboxylic acid as a copolymer component being 9 mol%. Example
A magnetic tape was prepared and evaluated in the same manner as in 1. This film had good blocking resistance, but had a low Young's modulus, a large width shrinkage after wet heat treatment under a vertical load, and a poor track shift (condition 2).

Comparative Example 3 An unstretched film was obtained in the same manner as in Example 1. This unstretched film is preheated at 125 ° C., and further heated by an infrared heater having a surface temperature of 830 ° C. from 14 mm above between low and high speed rolls.
The film is stretched by one time, quenched, and then supplied to a stenter.
The film was stretched 4.8 times in the transverse direction at 25 ° C. Subsequently, after heat-setting at 190 ° C. for 3 seconds, the film was subjected to 1.0% relaxation treatment at 170 ° C. in the transverse direction to obtain a biaxially oriented film having a thickness of 4.5 μm.

Table 1 shows the properties of the obtained film and the magnetic tape produced by the above method. The width shrinkage after the moisture heat treatment under the vertical load was large, and the track shift (condition 2) was poor.

[Comparative Examples 4 and 5] The same polyester as in Example 2 was used and the particles contained were the same as in Example 1. Films were formed under the conditions shown in Table 1, and magnetic tapes were prepared and evaluated in the same manner as in Example 1. In each case, the track deviation (condition 1 or condition 2) was poor.

Comparative Example 6 A film and a magnetic tape were obtained in the same manner as in Example 1 except that the thickness was changed to 2.5 μm. Table 1 shows the characteristics. Since the film was thin, track deviation occurred under condition 2.

[0102]

[Table 1]

[0103]

According to the present invention, there is provided a biaxially oriented polyester film for a magnetic recording medium which is excellent as a digital data storage tape having excellent delamination resistance, less occurrence of an error rate due to track deviation, and excellent output characteristics. Can be provided.

Continuing from the front page (72) Inventor Ieyasu Kobayashi 3-37-19 Koyama, Sagamihara-shi, Kanagawa Prefecture Teijin DuPont Films Co., Ltd. Sagamihara Research Center F-term (reference) AA26 AH38 QA02 QC06 QG01

Claims (11)

[Claims] 1. A copolymerized polyethylene-2,6-naphthalenediene prepared by copolymerizing 2,6-naphthalenedicarboxylic acid as a main dicarboxylic acid component, ethylene glycol as a main glycol component and a third component in an amount of 1 to 7 mol%. A biaxially oriented polyester film made of a carboxylate and having the following conditions (1) and (2): (1) Thermal expansion coefficient αt (× 10 ⁻⁶ ) in the width direction of the film
/ ° C) and the coefficient of humidity expansion αh (× 10 ⁻⁶ /% RH) are 20 ≦ (αt + 2αh) ≦ 45 (2) A load of 32 MPa is applied in the longitudinal direction of the film,
A biaxially oriented polyester film having a film thickness of 3 to 7 μm, wherein a dimensional change in a width direction before and after treatment at 0 ° C. and 90% RH for 72 hours satisfies 0.3% or less. 2. A film comprising polyethylene-2,6-naphthalate, on at least one side of which a 2,6-naphthalenedicarboxylic acid is a main dicarboxylic acid component, ethylene glycol is a main glycol component, and a third component is 1 to 7 mol. % Copolymerized polyethylene-2,6
A biaxially oriented polyester film obtained by laminating a film made of naphthalene dicarboxylate, and the following conditions (1) and (2): (1) Thermal expansion coefficient αt in the width direction of the film (× 10 ⁻⁶⁾
/ ° C) and the coefficient of humidity expansion αh (× 10 ⁻⁶ /% RH) are 20 ≦ (αt + 2αh) ≦ 45 (2) A load of 32 MPa is applied in the longitudinal direction of the film,
A biaxially oriented polyester film having a film thickness of 3 to 7 μm, wherein a dimensional change in a width direction before and after treatment at 0 ° C. and 90% RH for 72 hours satisfies 0.3% or less. 3. The biaxially oriented polyester film according to claim 1, wherein the third component of the copolymerized polyethylene-2,6-naphthalenedicarboxylate is 4,4′-diphenyldicarboxylic acid. 4. The biaxially oriented polyester film according to claim 1, wherein the third component of the copolymerized polyethylene-2,6-naphthalenedicarboxylate is an ethylene oxide adduct of bisphenol A. 5. The biaxially oriented polyester film according to claim 1, wherein the Young's modulus in the machine direction is larger than the Young's modulus in the transverse direction. 6. The biaxially oriented polyester film according to claim 1, wherein at least one surface has a surface roughness (WRa) of 0.5 to 10 nm. 7. A surface roughness (WRa) on a side on which a magnetic layer is provided.
Is 0.5 to 10 nm, and the surface roughness of the nonmagnetic layer side (WRa)
The biaxially oriented polyester film according to any one of claims 1 to 6, wherein is 1 to 20 nm. 8. The biaxially oriented polyester film according to claim 1, which is for a digital recording type magnetic recording medium. 9. The biaxially oriented polyester film according to claim 1, which is for a magnetic recording medium of a linear recording system. 10. The method according to claim 1, which is for a coating type magnetic recording medium.
10. The biaxially oriented polyester film according to any one of claims 1 to 9. 11. The biaxially oriented polyester film according to claim 1, which is for a ferromagnetic metal thin film type magnetic recording medium.