JP2018150463A - Biaxially oriented polyester film and magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably provide a biaxially oriented polyester film that is smooth and excellent in travelling property and winding property, and gives a high-density magnetic recording medium excellent in electromagnetic conversion characteristics when stored as a magnetic recording medium for a long period of time.SOLUTION: The biaxially oriented polyester film shows, in one of outermost surfaces (A surface) of the biaxially oriented polyester film, a maximum power spectral density (PSD-Xa) of 1,000 to 50,000 nmin a width direction in a region where a wavelength is less than 10 μm, and in the opposite outermost surface (B surface), a maximum power spectral density (PSD-Xb) of 150,000 to 500,000 nmin the width direction in a region where a wavelength is less than 10 μm.SELECTED DRAWING: None

Description

  The present invention relates to a biaxially oriented polyester film that is smooth and excellent in running property and winding property, and further excellent in electromagnetic conversion characteristics when stored as a magnetic recording medium for a long period of time. The present invention relates to a biaxially oriented polyester film that can be suitably used not only for a base film of a recording medium, but also for optics, various release films, and next-generation thermal transfer ribbon films that require high-definition surface properties.

  Biaxially oriented polyester films are used for various applications because of their excellent thermal properties, dimensional stability, mechanical properties, and ease of control of surface morphology, and are particularly useful as supports for magnetic recording media. Are known. Magnetic recording media always require high-density recording. To achieve even higher-density recording, the magnetic layer surface should be made thinner and finer magnetic materials should be used to further improve the smoothness of the magnetic layer surface. Is valid.

  For this reason, in recent magnetic recording medium supports using ferromagnetic hexagonal ferrite powder, which is a fine magnetic powder, the magnetic layer, nonmagnetic layer, backcoat layer, and even the support itself become thinner. The roughening of not only the surface but also the running surface is restricted. When storing in the roll state as a magnetic recording medium in the manufacturing process, the protrusion formed on the running surface is transferred to the magnetic surface, forming a dent on the smooth magnetic layer surface, or the support as the support becomes thinner The large particles contained in the magnetic layer are pushed up to the smooth surface, and a gentle convex undulation is generated on the surface of the magnetic layer, so that the smoothness of the surface of the magnetic layer is deteriorated and the electromagnetic conversion characteristics are deteriorated. To improve the smoothness of the surface of the magnetic layer by reducing the particle size and concentration of the particles contained in the support, and improving the smoothness as an ultra-high-definition surface, the runnability, winding, and durability of the surface are improved. The property becomes insufficient. Also, in order to achieve both surface smoothness and runnability of the support, if an easy-slip layer is provided on the support surface by coating, the durability of the easy-slip layer itself becomes insufficient, and the surface property is lowered, or the in-process roll There is a problem such as deterioration of surface properties due to contamination of the surface and adhesion of scraped material, and the demand for compatibility between running property, winding property, etc. and surface smoothness can be said to be a problem that always arises for high density recording.

  In order to solve the above problems, for example, Patent Document 1 proposes a polyester film having excellent electromagnetic conversion characteristics and error rate characteristics by controlling the background index and the number of protrusions.

  In addition, a polyester film that has fine particles and a coating layer, and controls the spatial frequency density and undele index and surface roughness of the smooth surface of the film, and attempts to achieve both winding properties and electromagnetic conversion characteristics (for example, Patent Document 2, 3) has also been proposed.

  However, as a support for magnetic recording media using ferromagnetic hexagonal ferrite powder, the smoothness of both surfaces of the support is particularly important because the magnetic layer, nonmagnetic layer, and back coat layer are made thinner. Become. Furthermore, since the transfer of the above problem occurs remarkably after long-term storage or storage at high temperature, the back surface is transferred to the magnetic surface after storage as a magnetic recording medium even if the smoothness of the support is controlled in the initial state. There is still a problem that the smoothness is lowered.

JP 2013-2000927 A JP 2001-341265 A JP 2012-153099 A

  As a result of intensive studies in order to solve the above-mentioned object, the present inventors have found that when the surface on the side of the support on which the backcoat layer is provided is rolled up, it is transferred to the surface on which the magnetic layer is formed. The present invention has been found out to be related to the maximum power spectral density in the region of wavelength less than 10 μm.

  An object of the present invention is to solve the above problems. In other words, it is a biaxially oriented polyester film with excellent runnability, slitting properties, and dimensional stability, and has a smooth magnetic layer when used as a magnetic recording medium. Another object of the present invention is to stably provide a biaxially oriented polyester film that becomes a high-density magnetic recording medium excellent in electromagnetic conversion characteristics with a low error rate after long-term storage.

The present invention for solving the above-described problems is characterized by the following configurations.
(1) In the outermost layer surface (A surface) of any one of the biaxially oriented polyester films, the maximum power spectral density (PSD-Xa) in the width direction in the region of a wavelength of less than 10 μm is 1,000 to 50,000 nm ³ A biaxially oriented polyester film having a maximum power spectral density (PSD-Xb) in the width direction of 150,000 to 500,000 nm ³ in a region of a wavelength of less than 10 μm on the outermost surface (B surface) on the opposite surface.
(2) The biaxially oriented polyester film according to (1), wherein the maximum power spectral density in the region of the B-plane with a wavelength of less than 10 μm satisfies the following formula.

0.6 ≦ PSD-Yb / PSD-Xb ≦ 1.2
(However, PSD-Yb (nm ³ ): Maximum power spectral density in the B-plane longitudinal direction.
PSD-Xb (nm ³ ): Maximum power spectral density in the B-plane width direction)
(3) The biaxial as described in (1) or (2), wherein the maximum power spectral density (PSD-Xb1) in the width direction in the region of wavelength B of 1 μm or less on the B plane is in the range of 15,000 to 80,000 nm ³ Oriented polyester film.
(4) The maximum power spectral density in the width direction (PSD-Xb30) (nm ³ ) in the region of the B plane having a wavelength of 10 μm or more and 30 μm or less satisfies the following formula: (1) to (3) Biaxially oriented polyester film.

1.0 ≦ PSD-Xb30 / PSD-Xb ≦ 3.0
(5) The biaxially oriented polyester film according to any one of (1) to (4), which is used for a base film for a magnetic recording medium of a coating type digital recording system.
(6) A magnetic recording medium using the biaxially oriented polyester film according to any one of (1) to (5).

  The biaxially oriented polyester film of the present invention is a biaxially oriented polyester film excellent in running property, slit property, and dimensional stability, and has a smooth magnetic layer when used as a magnetic recording medium and has an environment of temperature and humidity. Biaxially oriented polyester film can be obtained as a high-density magnetic recording medium with small dimensional change due to change and storage, low error rate after long-term storage and excellent electromagnetic conversion characteristics, and optical and various release films Can be suitably used.

  As the polyester used in the present invention, for example, a polyester composed of a polymer having an acid component or a diol component such as an aromatic dicarboxylic acid, an alicyclic dicarboxylic acid or an aliphatic dicarboxylic acid as a structural unit (polymerization unit) is used. Can do.

  Examples of the aromatic dicarboxylic acid component include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 4,4′-diphenyldicarboxylic acid. Acid, 4,4′-diphenyl ether dicarboxylic acid, 4,4′-diphenylsulfone dicarboxylic acid, and the like can be used. Among them, terephthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid can be preferably used. . As the alicyclic dicarboxylic acid component, for example, cyclohexane dicarboxylic acid or the like can be used. As the aliphatic dicarboxylic acid component, for example, adipic acid, suberic acid, sebacic acid, dodecanedioic acid and the like can be used. These acid components may be used alone or in combination of two or more.

  Examples of the diol component include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, , 6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2'-bis (4'- β-hydroxyethoxyphenyl) propane and the like can be used, and among them, ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, diethylene glycol and the like can be preferably used, and ethylene glycol is particularly preferable. etc It can be used. These diol components may be used alone or in combination of two or more.

  The polyester may be copolymerized with a monofunctional compound such as lauryl alcohol or phenyl isocyanate, or a trifunctional compound such as trimellitic acid, pyromellitic acid, glycerol, pentaerythritol, or 2,4-dioxybenzoic acid. A compound or the like may be copolymerized within a range in which the polymer is substantially linear without excessive branching or crosslinking. In addition to the acid component and diol component, the present invention includes aromatic hydroxycarboxylic acids such as p-hydroxybenzoic acid, m-hydroxybenzoic acid, and 2,6-hydroxynaphthoic acid, p-aminophenol, and p-aminobenzoic acid. As long as the effect is not impaired, the copolymerization can be further carried out.

  The copolymerization ratio of the polymer can be examined using NMR method (nuclear magnetic resonance method) or microscopic FT-IR method (Fourier transform microinfrared spectroscopy).

  Polyester having a glass transition temperature of less than 150 ° C. can be suitably used in order to achieve biaxial stretching and to exhibit the effects of the present invention such as dimensional stability. The polyester used in the present invention is preferably polyethylene terephthalate or polyethylene naphthalate (polyethylene-2,6-naphthalate), and may be a copolymer or a modified body thereof or a polymer alloy with another thermoplastic resin. . The polymer alloy here refers to a polymer multi-component system, which may be a block copolymer by copolymerization or a polymer blend by mixing. In particular, the polyester used in the present invention is more preferably polyethylene terephthalate as a main component because a process for increasing the crystallite size and the degree of crystal orientation is easily applied. Here, the main component means 80% by mass or more in the film composition.

  When the polyethylene terephthalate used in the present invention is a polymer alloy, the other thermoplastic resin is preferably a polymer compatible with polyester, more preferably a polyetherimide resin. As the polyetherimide resin, for example, those shown below can be used.

(Wherein R ₁ is a divalent aromatic or aliphatic residue having 6 to 30 carbon atoms, R ₂ is a divalent aromatic residue having 6 to 30 carbon atoms, From the group consisting of alkylene groups having 2 to 20 carbon atoms, cycloalkylene groups having 2 to 20 carbon atoms, and polydiorganosiloxane groups chain-terminated with alkylene groups having 2 to 8 carbon atoms Selected divalent organic group.)
As said R < ₁ >, R < ₂ >, the aromatic residue shown by the following formula group can be mentioned, for example.

  In the present invention, 2,2-bis [4- (2,3-dicarboxyphenoxy) phenyl] propane dianhydride and m-phenylenediamine from the viewpoint of affinity with polyester, cost, melt moldability, and the like, or A polymer having a repeating unit represented by the following formula, which is a condensate with p-phenylenediamine, is preferred.

Or

(N is an integer greater than or equal to 2, Preferably it is an integer of 20-50.)
This polyetherimide is available from SABIC Innovative Plastics under the trade name “Ultem”, “Ultem (registered trademark) 1000”, “Ultem (registered trademark) 1010”, “Ultem (registered trademark) 1040”. , "Ultem (registered trademark) 5000", "Ultem (registered trademark) 6000" and "Ultem (registered trademark) XH6050" series and "Extem (registered trademark) XH" and "Extem (registered trademark) UH" Etc. are known.

  The biaxially oriented polyester film of the present invention preferably has a laminated structure of two or more layers having at least one layer (B layer) containing inert particles having an average particle diameter of 0.050 to 0.30 μm. In this case, the B layer functions as a layer responsible for running properties and is provided as one outermost layer of the film. The other outermost layer has a layered structure of at least two or more layers provided with a layer (A layer) responsible for smoothness containing inert particles having an average particle diameter of 0.01 to 0.10 μm. It is preferable for obtaining the effect. Here, it is preferable to provide a magnetic layer to be described later on the surface on the A layer side (hereinafter also referred to as A surface), in which case the opposite surface (the surface of the B layer, hereinafter also referred to as B surface) is runnability. It will be the side that bears.

The biaxially oriented polyester film of the present invention has a maximum power spectral density (PSD-Xa) in the width direction in a region of a wavelength of less than 10 μm on one of the outermost surface (A surface) of 1,000 to 50,000 nm ^3. It is. Preferably 3,000~40,000nm ^3, still more preferably from 4,000~35,000nm ^3. The power spectral density (Power Spectral Density hereinafter referred to as “PSD”) is a method in which surface roughness profile data is subjected to Fourier transform, frequency analysis is performed, and the intensity at each wavelength is calculated. In the present invention, using an atomic force microscope (AFM), PSD measurement is performed with a measurement visual field of 125 μm × 125 μm, and the maximum PSD is obtained in a region of a wavelength of less than 10 μm. In the present invention, it is preferable to provide a magnetic layer on the outermost layer surface (A surface) side that satisfies PSD-Xa in the above range, so that the smoothness of the magnetic layer surface is increased, and PSD-Xa is in the above range. It is preferable because both running performance and electromagnetic conversion characteristics can be achieved at a high level.

The biaxially oriented polyester film of the present invention has a maximum power spectral density in the width direction (PSD-Xb) of 150,000 to 500,500 in a region of a wavelength of less than 10 μm on the outermost layer surface (B surface) opposite to the A surface. 000nm is ^3. The range is preferably 200,000 to 450,000 nm ³ , more preferably 230,000 to 400,000 nm ³ . The smaller the PSD-Xb, the better the electromagnetic conversion characteristics after long-term storage. However, when the PSD-Xb is too small, the running property and the slit property tend to be lowered. In the present invention, by providing a backcoat layer on the outermost layer surface (B surface) side that satisfies PSD-Xb in the above range, it is possible to achieve both surface smoothness and running property of the backcoat layer, and slit property, The electromagnetic conversion characteristics after long-term storage are improved.

The biaxially oriented polyester film of the present invention has a maximum power spectral density ratio (PSD-Yb / PSD-Xb) of 0.6 ≦ PSD-Yb / PSD-Xb ≦ 1. 2, Preferably it is 0.7-1.0, More preferably, it is 0.75-0.95. This PSD-Xb is the maximum PSD (nm ³ ) in the width direction, and PSD-Yb is the maximum PSD (nm ³ ) in the longitudinal direction. The air layer of the film roll can be controlled by setting the maximum power spectral density ratio (PSD-Yb / PSD-Xb) in the region of a wavelength of less than 10 μm of the present invention within a specific range. In particular, since the maximum power spectral density (PSD-Xb) in the region of the B surface with a wavelength of less than 10 μm defines the value in the width direction of the film (the same value is in the above range), Although the air is entrained with the accompanying airflow, the fluidity of the entrained air in the width direction is increased, and the air can easily escape from the end face of the film. As a result, the grounding (contact) between the films occurs quickly at the end face of the roll, and the remaining air layer plays the role of a cushion layer, and the transfer to the smooth surface (A surface) by the B surface after long-term storage is suppressed and electromagnetic Conversion characteristics are good. Furthermore, when the films are brought into contact with each other at the end face of the roll, the slit property is improved, and the occurrence of winding deviation and wrinkles after long-term storage can be suppressed.

The biaxially oriented polyester film of the present invention preferably has a maximum power spectral density (PSD-Xb1) in the width direction of 15,000 to 80,000 nm ³ in the region having a wavelength of 1 μm or less on the B surface. Is 20,000 to 50,000 nm ³ . If the maximum power spectral density in the width direction of the following areas wavelength 1μm (PSD-Xb1) is running property deteriorates to be less than 15,000 ^3, more than 80,000nm ^3, in the region of less than the wavelength 10μm of the present invention The maximum power spectral density (PSD-Xb) in the width direction may not be controlled within the range of the present invention, and electromagnetic conversion characteristics after long-term storage may be deteriorated.

The biaxially oriented polyester film of the present invention has a maximum power spectral density (PSD-Xb30) (nm ³ ) in the width direction in the region of the above-mentioned B surface at a wavelength of 10 μm or more and a wavelength of 30 μm or less and The power spectral density (PSD-Xb) ratio (PSD-Xb30 / PSD-Xb) is preferably 1.0 to 3.0. The maximum power spectral density (PSD-Xb30) in the width direction in the wavelength region of 10 μm or more and 30 μm or less responds to coarse protrusions due to particle crowding or broad protrusions due to push-up. Electromagnetic conversion characteristics and dropout are likely to deteriorate.

  The PSD-Xa control method described above can be controlled by, for example, the particle diameter and content of particles contained in each of the A and B layers, and the thickness of the A layer. The content of particles having a particle size of 0.01 to 0.1 μm as the particles contained in the A layer is 0.02 to 0.15 mass%, and the thickness of the A layer is 5 times the maximum particle size of the B layer. As mentioned above, Preferably it is 7 to 15 times. When the particle content of the A layer is less than 0.02% by mass, the running property may be deteriorated. Moreover, when it exceeds 0.15 mass%, there exists a tendency for obtaining desired PSD-Xa to become difficult. If the thickness of the A layer is outside the above range, the particles contained in the B layer may form undulations on the surface of the A layer, and the PSD-Xa of the A layer may fall within the scope of the present application. It can be difficult.

  As a method for controlling the above various maximum power spectral densities (PSD-Xb, PSD-Xb30, PSD-Xb1), for example, it is possible to control by the particle diameter, content, thickness of the contained particles in the B layer, and stretching conditions described later. Yes, the greater the particle size, the greater the content, and the greater the thickness, the greater the maximum power spectral density. The particle diameter of the particles to be contained in the B layer is preferably 0.05 to 0.3 μm, and it is possible to use at least two kinds of particles having different average particle diameters in combination with the electromagnetic conversion characteristics and running properties of the present invention. It is preferable for improving the slit property. At this time, it is preferable to use a particle A having an average particle diameter of 0.25 to 0.3 μm for running property and slit property. In this case, the content of the particle A is 0.03 to 0.2. The mass% or less, preferably 0.05 to 0.15 mass% is preferable for controlling the running property, slit property, and maximum power spectral density (PSD-Xb) in the width direction within a desired range. If the average particle diameter of the particles A exceeds 0.3 μm, it may be difficult to obtain a desired maximum power spectral density. When particles B having a particle diameter smaller than particles A are used in combination, particles B having an average particle diameter of less than 0.15 to 0.25 μm are contained in combination with particles A in order to obtain a desired maximum power spectral density. The content of the particles B is preferably less than the content of the particles A, and the blending mass ratio of the particles A and the particles B (particle A / particle B) exceeds 1, It is preferable for controlling the maximum power spectral density (PSD-Xb). If the blending mass ratio of particles A and particles B (particle A / particle B) is less than 1, the maximum power spectral density (PSD-Xb) in the width direction in the region of wavelength less than 10 μm may be too small. In addition, the number of particles B may be too large, and particles A and / or B may be dense. As a result, it becomes easy to form coarse protrusions, and the number of protrusions may be further reduced, so that the electromagnetic conversion characteristics, running properties, and slit properties of the present invention may be deteriorated.

  In addition, when the particles C having an average particle diameter of less than 0.15 μm are further contained, the content is not particularly limited, but is 0.8% by mass or less, preferably 0.1 to 0.5% by mass. A preferable average particle diameter of the particles C is 0.05 to 0.12 μm. It is preferable to add particles C in addition to the particles A and B because the running property and slit property are improved. If the content of the particles C exceeds the upper limit value, coarse protrusions due to aggregation of the particles C may be formed, and thus the maximum power spectral density (PSD-Xb and PSD-Xb1) of the present invention may not be obtained. Caution must be taken.

  The thickness of the B layer is 0.1 to 1 μm, preferably 0.1 to 0.6 μm, more preferably 0.2 to 0.5 μm, and the maximum particle diameter (D) of the particles contained in the B layer is 2 It is preferable to make it not more than double, preferably 1 to 1.7 times or less.

  The above-mentioned maximum power spectral density ratio (PSD-Yb / PSD-Xb) should be such that the stretching conditions and heat treatment temperature of the biaxially oriented polyester film are as described later, in addition to controlling the particles and thickness of the B layer. Is effective.

  Although it does not specifically limit as particle | grains contained preferably in B layer of the biaxially-oriented polyester film of this invention, Both inorganic particles and organic particles can be used. It is preferable to use two or more kinds of particles in combination. Specific types include inorganic particles such as titanium oxide, calcium carbonate, wet silica, dry silica, colloidal silica, alumina silicate, calcium phosphate, barium sulfate, alumina silicate, kaolin, talc, montmorillonite, alumina, zirconia, and the like. Organic particles containing acrylic acid, styrenic resin, silicone, imide, etc., core-shell type organic particles, etc. can be exemplified, but in order to control the maximum power spectral density, the organic particles are monodispersed spherical particles. Particles and colloidal silica are particularly preferred.

  In addition, the particles preferably contained in the A layer of the biaxially oriented polyester film of the present invention are preferably monodispersed spherical particles, but aggregated particles such as alumina and zirconia also have a secondary diameter within 0.15 μm. If it can be controlled, it can be used.

  The center line surface roughness Ra of the B layer side surface containing the above-mentioned particles is preferably 3 to 15 nm, more preferably 3 to 10 nm. Further, the 10-point average roughness Rz on the surface of the B layer side is preferably 60 to 200 nm, more preferably 80 to 180 nm. When the center line surface roughness Ra and the 10-point average roughness Rz are less than the lower limit value, the runnability tends to be poor, and when the center line surface roughness Ra exceeds the upper limit value, dropout tends to deteriorate.

  The biaxially oriented polyester film of the present invention preferably has a humidity expansion coefficient in the width direction of 0 to 8 ppm /% RH. When the humidity expansion coefficient is 8 ppm /% RH or less, when used for a magnetic recording medium, deformation due to humidity change does not increase, and dimensional stability is unlikely to decrease. A more preferred upper limit is 6.5 ppm /% RH, and even more preferred is 6 ppm /% RH. The humidity expansion coefficient is a physical property affected by the degree of tension of the molecular chain, and can be controlled by the ratio of TD stretching 1 and TD stretching 2 as described later. Control is also possible by the ratio. The greater the ratio of TD stretching 1 and TD stretching 2 (TD1 / TD2), the smaller the coefficient of humidity expansion. Further, the higher the TD stretch total magnification, the smaller the humidity expansion coefficient.

  In the present invention, MD refers to the longitudinal direction (longitudinal direction, film forming direction) of the biaxially oriented polyester film, and TD refers to the width direction (lateral direction, film forming direction) of the biaxially oriented polyester film. ).

  The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the width direction of 7 GPa or more, and more preferably 7 to 10 GPa from the viewpoint of controlling the humidity expansion coefficient in the width direction. When the Young's modulus in the width direction is within the above range, when used for a magnetic recording medium, the dimensional stability due to environmental changes at the time of recording / reproducing of the magnetic recording medium tends to be good. The Young's modulus in the width direction can be controlled by the temperature and magnification of TD stretching 1 and 2 described later. In particular, the total TD magnification is affected, and the higher the total TD magnification, the higher the TD Young's modulus.

  The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the longitudinal direction of 3.5 to 8 GPa. When the Young's modulus in the longitudinal direction is within the above range, when used for a magnetic recording medium, the storage stability due to the tension during storage of the magnetic recording medium becomes better. A more preferable range of Young's modulus in the longitudinal direction is 3.8 to 7.5 GPa, and a more preferable range is 4 to 7 GPa. The Young's modulus in the longitudinal direction can be controlled by the MD draw ratio. The MD Young's modulus increases as the MD magnification increases.

  When the biaxially oriented polyester film of the present invention is used as a base film for a magnetic recording medium, it is preferable to provide a back coat layer (hereinafter referred to as BC layer) on the B surface side in order to obtain a high-density magnetic recording medium, In particular, in a magnetic recording medium using a ferromagnetic hexagonal ferrite powder for the magnetic layer, the thickness of the magnetic layer, the nonmagnetic layer, and the BC layer itself is thin, so that the surface of the support (B layer) is placed on the surface of the BC layer. The influence of the protrusion is less likely to occur, and a smooth BC surface is obtained. Therefore, it is possible to obtain an ultra-flat magnetic surface without forming a transfer mark on the magnetic surface, so that excellent electromagnetic conversion characteristics can be exhibited.

  The thickness of the biaxially oriented polyester film of the present invention is preferably in the range of 3.5 to 4.5 μm. When the thickness is smaller than 3.5 μm, the rigidity and dimensional stability are lowered, the tape is not sufficiently stretched, and the electromagnetic conversion characteristics tend to be lowered when the magnetic recording medium is obtained. Moreover, it becomes difficult to suppress the push-up to the smooth surface (A surface) side by B layer surface protrusion. On the other hand, if it is larger than 4.5 μm, the tape length per one tape is shortened, so that it is difficult to reduce the size and capacity of the magnetic tape. As a method for adjusting the thickness, the biaxially oriented film is formed by adjusting the screw discharge amount at the time of melt extrusion of the polymer during the production of the biaxially oriented polyester film and controlling the thickness of the unstretched film from the die. The thickness can be adjusted.

  The biaxially oriented polyester film of the present invention as described above is produced, for example, as follows.

  First, polyester pellets are melted using an extruder, discharged from a die, and then cooled and solidified to form a sheet. At this time, it is preferable to filter the polymer with a fiber-sintered stainless metal filter in order to remove unmelted material in the polymer.

  Various additives, for example, compatibilizers, plasticizers, weathering agents, antioxidants, thermal stabilizers, lubricants, antistatic agents, whitening agents, colorants, as long as they do not impair the characteristics of the present invention. A conductive agent, a crystal nucleating agent, an ultraviolet absorber, a flame retardant, a flame retardant aid, a pigment, a dye, and the like may be added.

  Subsequently, the sheet is stretched biaxially in the longitudinal direction and the width direction. At this time, as a preferable stretching method for improving the maximum power spectral density ratio (PSD-Yb / PSD-Xb) in the region of the wavelength of less than 10 μm and the dimensional stability in the width direction, the film was stretched in the longitudinal direction. Examples include a sequential biaxial stretching method such as stretching in two steps in the width direction later and a stretching method in which stretching is performed in the width direction after simultaneous biaxial stretching, but various maximum power spectral densities of the present invention are set to desired values. In order to control, it is preferable to stretch in the width direction after stretching by the simultaneous biaxial stretching method. For dimensional stability in the width direction, stretching in the width direction is performed in two stages, and the total stretching ratio is set to be larger than the stretching ratio in the longitudinal direction. The maximum power spectral density (PSD-Xb, PSD-Xb30) in the width direction tends to be too large. The biaxially stretched film thus obtained is heat-treated.

  Hereinafter, the production method of the film of the present invention will be described as an example in which polyethylene terephthalate (PET) is used as polyester. In addition, this invention is not limited to a PET film, The thing using another polymer may be used. For example, when a polyester film is formed using polyethylene-2,6-naphthalenedicarboxylate having a high glass transition temperature or a high melting point, it may be extruded or stretched at a temperature higher than the following temperature.

  First, PET pellets are manufactured. PET is manufactured by one of the following processes. That is, (1) A process of obtaining terephthalic acid and ethylene glycol as raw materials, obtaining a low molecular weight PET or oligomer by direct esterification, and then obtaining a polymer by polycondensation reaction using antimony trioxide or a titanium compound as a catalyst. (2) A process in which dimethyl terephthalate and ethylene glycol are used as raw materials to obtain a low molecular weight product by transesterification, and then a polymer is obtained by a polycondensation reaction using antimony trioxide or a titanium compound as a catalyst.

  In order to contain particles in the PET constituting the film, a method is preferred in which particles are dispersed in a predetermined proportion in the form of slurry in ethylene glycol, and this ethylene glycol is added during polymerization. When adding particles, for example, water sol or alcohol sol particles obtained at the time of particle synthesis are added without drying once, so that the dispersibility of the particles is good. It is also effective to mix the aqueous slurry of particles directly with PET pellets and knead them into PET using a vented twin-screw kneading extruder. As a method for adjusting the content of particles, a master pellet of high-concentration particles is prepared by the above method, and this is diluted with PET that does not substantially contain particles at the time of film formation to adjust the content of particles. The method is effective. In this case, adjusting the intrinsic viscosity of the PET containing no particles higher than the intrinsic viscosity of the particle-containing pellets means that the maximum power spectral density ratio (PSD-Yb / PSD-Xb) and surface roughness (Rz) of the present invention. ) Is effective in controlling.

  Next, the obtained PET pellets are dried under reduced pressure at 180 ° C. for 3 hours or more, and then supplied to an extruder heated to 270 to 320 ° C. under a nitrogen stream or under reduced pressure so that the intrinsic viscosity does not decrease. The film is extruded from a slit-shaped die and cooled on a casting roll to obtain an unstretched film. At this time, it is preferable to use various types of filters, for example, filters made of materials such as sintered metal, porous ceramics, sand, and wire mesh, in order to remove foreign substances and altered polymers. In addition, it is extremely preferable to provide a gear pump in order to improve the quantitative supply capability and obtain a desired laminated thickness in order to form the above-described characteristic surface. To laminate the film, a plurality of different polymers may be melt laminated using two or more extruders and manifolds or merge blocks.

  Since simultaneous biaxial stretching is preferable in efficiently controlling the above various maximum power spectral densities to desired values, the simultaneous biaxial stretching method will be described below.

  First, an unstretched film is led to a simultaneous biaxial tenter, and the first stage stretching (MD stretching 1 and TD stretching 1) in the longitudinal direction and the transverse direction at a temperature of 90 to 140 ° C. is 3.0 to Stretch 6.0 times. Subsequently, the second-stage stretching within the temperature range of 170 to 190 ° C. is 1.05 to 1.20 times in the length direction (MD stretching 2), and the width direction (TD stretching 2) is 1.3 to Re-draw at the same time 2.0 times. In order to control the various maximum power spectral densities of the present invention to desired values, the ratio of the first stage MD stretching 1 magnification / TD stretching 1 magnification is set within the range of 0.9 to 1.1, and the second stage MD. It is preferable to stretch at a ratio of 2 stretching ratio / 2 stretching ratio of TD at 0.5 to 0.85.

  Subsequently, the stretched film is heat-set under tension or while relaxing in the width direction. Maximum power spectral density in the longitudinal direction (PSD-Yb) and density ratio (PSD-Yb / PSD-Xb) of the present invention Further, as a heat setting treatment condition for controlling the humidity expansion coefficient in the width direction, the heat setting temperature is 180 to 220 ° C is preferable. When the heat setting temperature exceeds the upper limit, the maximum power spectral density (PSD-Yb), density ratio (PSD-Yb / PSD-Xb) and humidity expansion coefficient in the longitudinal direction of the present invention cannot be controlled within the scope of the present invention. There is. The heat treatment is performed in two or more stages. First, the first stage heat treatment (HS-1) is performed at (second stage stretching temperature + 5) to (second stage stretching temperature + 10) ° C. When the first-stage heat treatment temperature is equal to or higher than the above upper limit, the maximum power spectral density (PSD-Yb) and density ratio (PSD-Yb / PSD-Xb) in the longitudinal direction of the present invention may not be within a specific range. is there. Thereafter, the second stage heat treatment (HS-2) is in the range of ((HS-1) +10) to ((HS-1) +15) ° C., and the third stage heat treatment (HS-3) is ((HS-2) +10. ) To ((HS-2) +20) ° C. The upper limit of the heat setting temperature (HS-3) is more preferably 215 ° C, still more preferably 210 ° C. The lower limit of the heat setting temperature (HS-3) is more preferably 185 ° C, and still more preferably 190 ° C. The heat setting treatment time is preferably in the range of 5 to 30 seconds, and the relaxation rate is preferably 0.3 to 2%. After the heat setting treatment, the gripping clip is released to rapidly cool to room temperature while reducing the tension applied to the film. Thereafter, the film edge is removed and wound on a roll to obtain the biaxially oriented polyester film of the present invention.

  Note that there is a difference between the stretching temperature of the second stage and the heat setting temperature, and if the heat setting temperature is higher than the above range, the surface projection height increases due to shrinkage due to the bowing phenomenon and the winding tension in the MD direction. In particular, the maximum power spectral density (PSD-Yb) in the longitudinal direction may increase. In addition, the film is easy to relax, and it is difficult to obtain the above-described humidity expansion coefficient, which may reduce the dimensional stability. On the other hand, if the heat setting temperature is too low, the crystallinity tends to be low, and the flatness of the base film tends to be lowered and the electromagnetic conversion characteristics tend to be deteriorated in the manufacturing process of the magnetic recording medium.

  Next, the magnetic recording medium is manufactured as follows.

  The magnetic recording medium support (biaxially oriented polyester film) obtained as described above is slit into, for example, a width of 0.1 to 3 m, and conveyed at a speed of 20 to 300 m / min and a tension of 50 to 300 N / m. On the other hand, a non-magnetic paint is applied on one surface by an extrusion coater to a thickness of 0.5 to 1.5 μm, dried, and then a magnetic paint is applied to a thickness of 0.1 to 0.3 μm. Thereafter, the support coated with the magnetic coating material and the nonmagnetic coating material is magnetically oriented and dried at a temperature of 80 to 130 ° C. Next, a back coat is applied to the opposite surface with a thickness of 0.3 to 0.8 μm, calendered, and then wound up. The calendar process is performed using a small test calendar device (metal roll, 7 steps) at a temperature of 70 to 120 ° C. and a linear pressure of 0.5 to 5 kN / cm. Thereafter, aging treatment is performed at 60 to 80 ° C. for 24 to 72 hours, and slitting is performed to a width of 12.65 mm to prepare a pancake. Next, a specific length from this pancake is incorporated into a cassette to obtain a cassette tape type magnetic recording medium.

  Here, examples of the composition of the magnetic paint include the following compositions.

  Hereinafter, when “part” is simply described, it means “part by mass”.

[Magnetic layer forming coating solution]
Barium ferrite magnetic powder 100 parts [plate diameter: 20.5 nm, plate thickness: 7.6 nm, plate ratio: 2.7, Hc: 191 kA / m (≈2400 Oe) saturation magnetization: 44 Am ² / kg, BET specific surface area: 60m ² / g]
12 parts polyurethane resin Mass average molecular weight 10,000
Sulfonic acid functional group 0.5 meq / g
α-Alumina HIT60 (Sumitomo Chemical Co., Ltd.) 8 parts Carbon black # 55 (Asahi Carbon Co., Ltd.) Particle size 0.015 μm 0.5 parts Stearic acid 0.5 parts Butyl stearate 2 parts Methyl ethyl ketone 180 parts Cyclohexanone 100 parts Magnetic layer forming coating solution]
Non-magnetic powder α iron oxide 100 parts Average major axis length 0.09μm, specific surface area by BET method 50m ² / g
pH 7
DBP oil absorption 27-38ml / 100g
Surface treatment layer Al ₂ O ₃ 8% by mass
Carbon black 25 parts CONDUCTEX SC-U (manufactured by Colombian Carbon)
Vinyl chloride copolymer MR104 (manufactured by Nippon Zeon Co., Ltd.) 13 parts Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) 5 parts Phenylphosphonic acid 3.5 parts Butyl stearate 1 part Can be suitably used for computer data backup applications (for example, linear tape recording media (LTO5, LTO6, next-generation LTO tape (LTO7)) and digital image recording applications such as video.

  Examples of the coating type digital recording magnetic recording medium in which the biaxially oriented polyester film of the present invention is suitably used include, for example, a magnetic layer in which a ferromagnetic powder such as barium ferrite is uniformly dispersed in a binder such as a polyurethane resin and magnetic. A coating type magnetic recording medium in which a coating liquid is prepared and a magnetic layer is formed by coating the coating liquid can be exemplified.

  The biaxially oriented polyester film of the present invention can be used as an optical film such as an optical film, a polarizing plate using the same, and an optical compensation film for a liquid crystal display device. With the development of thin and light notebook PCs and thin electronic mobiles in recent years, the demand for thin optical compensation films for liquid crystal display devices has become very strict, especially thin optical films with excellent transparency and runnability. Can be suitably used.

  The biaxially oriented polyester film of the present invention can also be used as a release film. The release film is formed by applying a resin layer having a releasability, such as a silicone resin or an epoxy resin, using a polyester film as a base material. In particular, it is used for various release applications such as green sheet production, liquid crystal polarizing plate release, liquid crystal protective film release, photoresist, and multilayer substrate. In polyester film, it is common to mix fine particles on the film surface to improve processability, such as slipperiness and winding properties, and to form fine protrusions on the film surface. As a result, the release film used is required to have smooth surface properties and running properties without surface defects. Since the biaxially oriented polyester film of the present invention has high-definition surface smoothness and running property, it can be suitably used as a release film for various applications.

(Methods for measuring physical properties and methods for evaluating effects)
The characteristic value measurement method and effect evaluation method in the present invention are as follows.

(1) Maximum power spectral density in the width direction on the A plane (PSD-Xa)
Using an atomic force microscope (AFM), 10 fields of view were measured at different locations. In the sample set, the sample is piezo so that the direction perpendicular to the scanning direction of the cantilever (that is, the Y-axis direction) is the longitudinal direction of the sample film (the longitudinal direction is the direction in which the film travels in the film manufacturing process). Set to Measure. With respect to the obtained image, the maximum 1D-PSD (PSD-X) in the X-axis direction (width direction) at a wavelength of less than 10 μm is determined by the power spectral density of the Off-Line function, and the average value of 10 fields of view is obtained as PSD-Xa. It was. The X-axis is the X-axis direction of the measurement visual field (scanning direction of the cantilever), and the Y-axis indicates the vertical direction of the X-axis, so PSD-X is the sample width direction, and PSD-Y is the sample length. This is the direction value.

Measuring device: NanoScope (R) IIIa version5.31R1
(Manufactured by Digital Instruments)
Cantilever: Silicon single crystal Scanning mode: Tapping mode Scanning range: 125μm
Scanning speed: 0.5Hz
Samples line: 256
Flatten Auto: Order 3
(2) Maximum power spectral density of B-plane (PSD-Xb, PSD-Yb)
Using the apparatus described in (1) above, the opposite surface (B surface) is also measured, and the maximum 1D-PSD (PSD-X) in the X-axis direction and the maximum in the Y-axis direction at a wavelength of less than 10 μm. 1D-PSD (PSD-Y) was calculated | required, and each average value was set to PSD-Xb and PSD-Yb.

  Note that the maximum 1D-PSD (PSD-X) in the X-axis direction (width direction) was determined in the region where the wavelength was 10 μm or more and 30 μm or less, and the average value of 10 fields of view was defined as PSD-Xb30. Similarly, it calculated | required also about the area | region below a wavelength of 1 micrometer, and made average value PSD-Xb1.

(3) Centerline surface roughness Ra, 10-point average roughness Rz
Using the apparatus described in (1) above, the surface roughness was measured for 10 fields of view at different locations. In the sample set, the sample is set in the piezo so that the direction perpendicular to the scanning direction of the cantilever (Y-axis direction) is the longitudinal direction of the sample film (the longitudinal direction is the direction in which the film travels in the film manufacturing process). And measure. About the obtained image, it calculated by Roughness Analysis of Off-Line function, and made the average value Ra and Rz. The conditions were the same as the above (1).

(4) Humidity expansion coefficient in the width direction,
The measurement was performed under the following conditions with respect to the width direction of the film, and an average value of three measurement results was defined as a humidity expansion coefficient in the present invention.

Measuring device: Thermomechanical analyzer TMA-50 manufactured by Shimadzu Corporation (Humidity generator: Humidity atmosphere controller HC-1 manufactured by ULVAC-RIKO)
Sample size: film longitudinal direction 10 mm × film width direction 12.6 mm
Load: 0.5g
Number of measurements: 3 times Measurement temperature: 30 ° C
Measurement humidity: Measured by holding for 6 hours at 40% RH, increasing the humidity to 80% RH in 40 minutes, holding for 6 hours at 80% RH, and measuring dimensional change ΔL (mm) in the width direction of the support To do. The humidity expansion coefficient (ppm /% RH) was calculated from the following equation.

Humidity expansion coefficient (ppm /% RH) = 106 × {(ΔL / 12.6) / (80-40)}
(5) Thickness of the A layer The cross-sectional observation was performed under 10 conditions while changing the location under the following conditions, and the average value of the obtained thickness [nm] was calculated as the thickness of the A layer [nm].

Measuring device: Transmission electron microscope (TEM) Hitachi H-7100FA type Measurement conditions: Acceleration voltage 100 kV
Measurement magnification: 10,000 times Sample preparation: Ultrathin film section method Observation plane: TD-ZD cross section (TD: width direction, ZD: thickness direction)
Number of measurements: 3 points per field of view and 10 fields of view are measured.

(6) Refractive index It measured using the following measuring device according to JIS-K7142 (2008).

Apparatus: Abbe refractometer 4T (manufactured by Atago Co., Ltd.)
Light source: Sodium D line Measurement temperature: 25 ° C
Measurement humidity: 65% RH
Mount solution: methylene iodide (However, methylene iodide was used when the refractive index was 1.74 or more.)
Average refractive index n_bar = ((nMD + nTD + nZD) / 3)
Birefringence Δn = (nMD−nTD)
nMD: Refractive index in the film longitudinal direction nTD: Refractive index in the film width direction nZD: Refractive index in the film thickness direction (7) Young's modulus The Young's modulus of the film was measured according to ASTM-D882 (1997). Instron type tensile tester was used and the conditions were as follows. The average value of five measurement results was defined as the Young's modulus in the present invention.

Measuring instrument: Instron ultra-precision material testing machine MODEL 5848
Sample size:
When measuring Young's modulus in the film width direction Film length direction 2 mm x film width direction 12.6 mm
(The gripping distance is 8mm in the film width direction)
When measuring Young's modulus in the longitudinal direction of the film 2 mm in the film width direction × 12.6 mm in the film longitudinal direction
(Grip interval is 8mm in the longitudinal direction of the film)
Tensile speed: 1 mm / min Measurement environment: temperature 23 ° C., humidity 65% RH
Number of measurements: 5 times.

(8) Total light transmittance, haze, transparency Based on JIS-K 7361-1 (1997) and JIS-K 7136 (2000), it measured using the following measuring device. The five-point transmittance is measured in the longitudinal direction with respect to the center of the support, and the average value of the measurement results is defined as the total light transmittance and haze in the present invention.

Measuring device: Turbidimeter (NDH-5000) manufactured by Nippon Denshoku Industries Co., Ltd. Light source: White LED (5V3W)
Measurement environment: Temperature 23 ° C Humidity 65% RH
Number of measurements: 5 times.

  In addition, about transparency, it judged on the following judgment criteria and set C as the transparency defect.

  A: Haze is 1% or less.

  B: Haze exceeds 1% and is less than 2%.

  C: Haze is 2% or more.

(9) Maximum particle size (D), average particle size (d) of particles contained in the layer, average primary particle size of aggregated particles Observation of film cross section at 10,000 times using a transmission electron microscope (TEM) To do. At this time, when particles of 1 cm or less can be confirmed on the photograph, the TEM observation magnification is changed to 50,000 times for observation. TEM section thickness is about 100 nm, 100 fields of view are measured at different locations, the equivalent circle equivalent diameter is obtained for all dispersed particles photographed in the photograph, the equivalent circle equivalent diameter is plotted on the horizontal axis, and the particle diameter is plotted on the vertical axis. The abundance ratio (volume corresponding to a sphere) of each particle was plotted, and the equivalent circle equivalent diameter of the peak value was defined as the average particle diameter of the particles. Here, when aggregated particles can be confirmed on a photograph observed at 10,000 times, they are not included in the plot. When two or more types of particles having different particle diameters are present in the film, the number distribution of equivalent circle equivalent diameters is a distribution having two or more peaks. In this case, each peak value is defined as the average particle diameter of each particle. The maximum particle size is the particle size of a particle having the maximum particle size in a photograph of 100 fields observed at 10,000 times.

  The average particle diameter of the particles contained in the layer is determined from the equivalent circle diameters obtained above and the volume fraction thereof, the volume average diameter is obtained by the following formula, and this value is defined as the average particle diameter (d). .

d = Σ (di · Nvi) / 100
Here, di is the equivalent circle equivalent diameter, and Nvi is the volume fraction thereof.

  The average primary particle diameter of the aggregated particles is observed at 200,000 times using the above apparatus. For 100 aggregated particles, the equivalent circle equivalent diameter of each primary particle constituting the aggregated particles is obtained and plotted by the same method as described above, and the equivalent circle equivalent diameter of the peak value is determined as the average primary particle diameter of the aggregated particles. To do.

(10) Content of particles (10) -1 Elemental analysis of particles Polyester is removed from the film by a plasma ashing treatment method to expose the particles. The processing conditions are selected such that the polymer is ashed but the particles are not damaged as much as possible. The particles are observed with a scanning electron microscope (SEM), and the particle images are processed with an image analyzer. According to the particle size distribution determined in (9) above, the SEM magnification was set to 30,000, the observation location was changed and 20 fields of view were observed, and all the observed particles were measured using energy dispersive X-ray spectroscopy (EDX). Conduct elemental analysis to clarify the relationship between particles and elements.

(10) -2 Content of Particles The surface of each laminated part is scraped off with a single blade, o-chlorophenol is added to 100 g of scraped powder, and the polymer is dissolved at 100 ° C. for 1 hour with stirring. Next, a rotor RP30 is mounted on a separation ultracentrifuge 40P type manufactured by Hitachi, Ltd., and 30 cc of the solution is injected per cell, and then the speed is gradually increased to 30,000 rpm. The separation of the particles is finished 60 minutes after reaching 30,000 rpm. The supernatant is then removed and the separated particles are collected. After normal temperature o-chlorophenol is added to the collected particles and suspended uniformly, ultracentrifugation is performed. This operation is repeated until the separated particles described later are detected using a differential scanning calorimeter (DSC) until no melting peak corresponding to the polymer is detected. The separated particles thus obtained are vacuum-dried at 120 ° C. for 16 hours, and the value obtained by measuring the mass is taken as the total content of particles, and the ratio (mass%) to this is taken as the content of particles.

(11) Runability Two films with the A side and B side of the film overlapped are placed on a “Teflon” (registered trademark) plate, and a 200 g weight (contact area 40 cm ² ) is placed on the film. Put. One end (moving direction side) of the lower film and the “Teflon” (registered trademark) plate were fixed, and one end (opposite end to the moving direction) of the upper film was fixed to the detector. The static friction coefficient (μs) when the “Teflon” (registered trademark) plate was moved 5 mm at a speed of 2 mm / sec was obtained from the following formula, and the average value of 5 times was defined as the static friction coefficient. Judgment on running performance was as follows.

μs = (Maximum tension at start) / (Load 200g)
A: μs = 0.4 or less B: μs = 0.4 or more, 0.5 or less C: μs = 0.5 or more (12) Slit property When slitting the film to a width of 1 m, the slit speed is changed. The sharpness of the film edge was visually evaluated by the method shown below. In addition, C was judged as a slit property defect.

  AA: Even at a speed of 120 m / min, the end can be slit without distortion.

    A: Distortion occurs at the end only when the speed is 100 m / min or more and less than 120 m / min.

    B: Distortion occurs at the end only when the speed is 80 m / min or more and less than 100 m / min.

    C: Wrinkles are generated on the film surface at a speed of less than 80 m / min, and the edges become distorted.

(13) Electromagnetic conversion characteristics A film slit to a width of 1 m is conveyed with a tension of 200 N, and a magnetic paint and a non-magnetic paint are applied to one surface of a support according to the following and slit to a width of 12.65 mm to create a pancake. To do. Subsequently, a length of 200 m from this pancake was incorporated into a cassette to obtain a magnetic tape.

(Hereinafter, “parts” means “parts by mass.”)
Magnetic layer forming coating solution 100 parts of barium ferrite magnetic powder (plate diameter: 20.5 nm, plate thickness: 7.6 nm,
Plate ratio: 2.7, Hc: 191 kA / m (≈ 2400 Oe)
(Saturation magnetization: 44 Am ² / kg, BET specific surface area: 60 m ² / g)
12 parts polyurethane resin Mass average molecular weight 10,000
Sulfonic acid functional group 0.5 meq / g
α-alumina HIT60 (manufactured by Sumitomo Chemical Co., Ltd.) 8 parts carbon black # 55 (manufactured by Asahi Carbon Co., Ltd.)
Particle size 0.015 μm 0.5 part Stearic acid 0.5 part Butyl stearate 2 parts Methyl ethyl ketone 180 parts Cyclohexanone 100 parts Nonmagnetic powder forming coating solution Nonmagnetic powder α iron oxide 85 parts Average major axis length 0.09 μm, Specific surface area by BET method 50m ² / g
pH 7
DBP oil absorption 27-38ml / 100g
Surface treatment layer Al ₂ O ₃ 8% by mass
15 parts of carbon black “Conductex” (registered trademark) SC-U (manufactured by Colombian Carbon)
Polyurethane resin UR8200 (made by Toyobo Co., Ltd.) 22 parts Phenylphosphonic acid 3 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone 205 parts Cyclohexanone 135 parts Kneaded. The coating solution was pumped through a horizontal sand mill filled with 1.0 mmφ zirconia beads in an amount of 65% with respect to the volume of the dispersed portion, and the liquid was pumped at 2,000 rpm for 120 minutes (substantially residence time in the dispersed portion). ), Dispersed. To the obtained dispersion, polyisocyanate is added to 5.0 parts of the coating for the non-magnetic layer, 2.5 parts to the coating of the magnetic layer, and further 3 parts of methyl ethyl ketone to add a filter having an average pore size of 1 μm. Then, coating solutions for forming the nonmagnetic layer and for forming the magnetic layer were prepared.

  The obtained non-magnetic layer-forming coating solution is coated and dried on a PET film so that the thickness after drying is 0.8 μm, and then the magnetic layer-forming coating solution is dried and the thickness of the magnetic layer is dried. Is applied to a thickness of 0.07 μm, and while the magnetic layer is still wet, it is oriented and dried by a cobalt magnet having a magnetic force of 6,000 G (600 mT) and a solenoid having a magnetic force of 6,000 G (600 mT). I let you. Then, a back coat layer (carbon black average particle size: 17 nm 100 parts, calcium carbonate average particle size: 40 nm 80 parts, α alumina average particle size: 200 nm 5 parts in a polyurethane resin so that the thickness after calendar is 0.5 μm. , Dispersed in polyisocyanate). Next, after calendering with a calendar at a temperature of 90 ° C. and a linear pressure of 300 kg / cm (294 kN / m), it was cured at 65 ° C. for 48 hours and left standing in an environment of 40 ° C. for 7 days. A non-woven fabric and a razor blade were attached to a device having a product delivery and take-up device so that they pressed against the magnetic surface, and the surface of the magnetic layer was cleaned with a tape cleaning device to obtain a magnetic tape.

  A recording head (MIG, gap 0.15 μm, 1.8T) and a reproducing GMR head were attached to a drum tester, and the output of the magnetic tape obtained as described above was measured. The relative speed of the head / tape was set to 5.5 m / sec, a signal having a track density of 16 KTPI and a linear recording density of 400 Kbpi was recorded, and the ratio of output to noise was defined as an electromagnetic conversion characteristic. The result of Example 7 was determined as 0 dB, and 2.0 dB or more was determined as A, less than 2.0 to 0 dB as B, and less than 0 dB as C. A is desirable, but B can also be used practically.

(14) Error rate Using a magnetic tape recorded in the same manner as in (13) above, a commercially available data storage drive with an uncompressed capacity of 6 TB was recorded in an environment of 23 ° C. and 50% RH. Next, the magnetic tape was stored for 7 days in an environment of 50 ° C. and 80% RH. After that, the error rate occurrence state was evaluated by playing back at room temperature for 1 day. The error rate was calculated by the following formula using error information (number of error bits) output from the drive, and evaluated according to the following criteria. Among the following criteria, C is rejected.

Error rate = (number of error bits) / (number of write bits)
AA: Error rate is less than 1.0 × 10 ⁻⁶ A: Error rate is 1.0 × 10 ⁻⁶ or more, less than 1.0 × 10 ⁻⁵ B: Error rate is 1.0 × 10 ⁻⁵ or more, 1 Less than 0 × 10 ⁻⁴ C: Error rate is 1.0 × 10 ⁻⁴ or more

  Based on the following examples, embodiments of the present invention will be described. Here, polyethylene terephthalate is expressed as PET, polyethylene naphthalate is expressed as PEN, and polyetherimide is expressed as PEI.

  (1-a) Preparation of PET pellets: 194 parts by mass of dimethyl terephthalate and 124 parts by mass of ethylene glycol were charged into a transesterification reaction apparatus, and the contents were heated to 140 ° C. and dissolved. Thereafter, 0.3 parts by mass of magnesium acetate tetrahydrate and 0.05 parts by mass of antimony trioxide were added while stirring the contents, and a transesterification reaction was performed while distilling methanol at 140 to 230 ° C. Next, 0.5 parts by mass of a 5% by mass ethylene glycol solution of trimethyl phosphate (0.025 parts by mass as trimethyl phosphate) and 0.3% of a 5% by mass ethylene glycol solution of sodium dihydrogen phosphate dihydrate were obtained. Mass parts (0.015 parts by mass as sodium dihydrogen phosphate dihydrate) were added.

  Addition of trimethyl phosphoric acid in ethylene glycol reduces the temperature of the reaction contents. Therefore, stirring was continued until the temperature of the reaction contents returned to 230 ° C. while distilling excess ethylene glycol. In this way, after the temperature of the reaction contents in the transesterification reactor reached 230 ° C., the reaction contents were transferred to the polymerization apparatus.

  After the transition, the reaction system was gradually heated from 230 ° C. to 275 ° C. and the pressure was reduced to 0.1 kPa. The time to reach the final temperature and final pressure was both 60 minutes. After reaching the final temperature and the final pressure, the reaction was carried out for 2 hours (3 hours after the start of polymerization), and the agitation torque of the polymerization apparatus was a predetermined value (specific values differ depending on the specifications of the polymerization apparatus. The value indicated by polyethylene terephthalate having an intrinsic viscosity of 0.55 was a predetermined value). Then, the reaction system was purged with nitrogen and returned to normal pressure to stop the polycondensation reaction, discharged into cold water in a strand form, and immediately cut to obtain polyethylene terephthalate PET pellets having an intrinsic viscosity of 0.55 (raw material-1).

  Using the rotary vacuum polymerization apparatus, the above PET pellets (raw material-1) were heat-treated at a temperature of 230 ° C. under a reduced pressure of 0.1 kPa for a long time to carry out solid phase polymerization (raw material-1k). The longer the heat treatment time, the higher the intrinsic viscosity. The intrinsic viscosity is 0.60 at a treatment time of 1 hour, and the intrinsic viscosity is 0.70 at 5 hours.

  (1-b) Preparation of PEN pellets: To a mixture of 128 parts by mass of dimethyl 2,6-naphthalenedicarboxylate and 60 parts by mass of ethylene glycol, 0.025 parts by mass of manganese acetate tetrahydrate salt, sodium acetate and 3 water 0.005 part by mass of salt was added, and the ester exchange reaction was carried out while gradually raising the temperature from 150 ° C to 240 ° C. In the middle, when the reaction temperature reached 170 ° C., 0.024 parts by mass of antimony trioxide was added. When the reaction temperature reached 220 ° C., 0.042 parts by mass of 3,5-dicarboxybenzenesulfonic acid tetrabutylphosphonium salt (corresponding to 2 mmol%) was added. Thereafter, a transesterification reaction was carried out, and 0.023 parts by mass of trimethyl phosphoric acid was added. Next, the reaction product is transferred to a polymerization apparatus, heated to a temperature of 290 ° C., subjected to a polycondensation reaction under a high vacuum of 30 Pa, and the stirring torque of the polymerization apparatus is a predetermined value (specifically depending on the specifications of the polymerization apparatus). The value indicated by polyethylene-2,6-naphthalate having an intrinsic viscosity of 0.6 in the present polymerization apparatus was a predetermined value). Therefore, the reaction system was purged with nitrogen and returned to normal pressure to stop the polycondensation reaction, discharged into cold water in the form of strands, and immediately cut to obtain PEN pellets (raw material-1b) having an intrinsic viscosity of 0.6.

  (2-a) Preparation of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass of a 10% by weight water slurry of crosslinked polystyrene particles having an average particle size of 0.30 μm (2 parts by mass as crosslinked polystyrene particles) are supplied, and the vent hole is maintained at a reduced pressure of 1 kPa or less to retain moisture. This was removed to obtain particle-containing pellets (raw material-2a) having an intrinsic viscosity of 0.62 and containing 1% by mass of crosslinked polystyrene particles.

  (2-b) Preparation of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass water slurry of colloidal silica particles having an average particle size of 0.060 μm are supplied by 10 parts by mass (1 part by mass as colloidal silica particles), and the vent hole is maintained at a reduced pressure of 1 kPa or less to keep moisture. This was removed to obtain particle-containing pellets (raw material-2b) having an intrinsic viscosity of 0.62 and containing 1% by mass of colloidal silica particles.

  (2-c) Preparation of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass of a 10% by mass water slurry of colloidal silica particles having an average particle size of 0.10 μm (1 part by mass as colloidal silica particles) are supplied, and the vent hole is maintained at a reduced pressure of 1 kPa or less to keep moisture. Removal was performed to obtain particle-containing pellets (raw material-2c) having an intrinsic viscosity of 0.62 and containing 1% by mass of colloidal silica particles.

  (2-d) Production of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass of water slurry of 10% by mass of colloidal silica particles having an average particle size of 0.12 μm (1 part by mass as colloidal silica particles) are supplied, and the vent hole is maintained at a reduced pressure of 1 kPa or less to keep moisture. This was removed to obtain particle-containing pellets (raw material-2d) having an intrinsic viscosity of 0.62 and containing 1% by mass of colloidal silica particles.

  (2-e) Production of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass water slurry of colloidal silica particles having an average particle size of 0.20 μm are supplied in an amount of 10 parts by mass (1 part by mass as colloidal silica particles), and the vent hole is maintained at a reduced pressure of 1 kPa or less to keep moisture. This was removed to obtain particle-containing pellets (raw material-2e) having an intrinsic viscosity of 0.62 and containing 1% by mass of colloidal silica particles.

  (2-f) Preparation of particle-containing PET pellets: The above-mentioned solid-phase polymerization PET pellets (raw material-1k: treatment time 2 hours) were fed into a bent biaxial kneading extruder of the same direction rotation type heated to 280 ° C. 90 parts by mass and 10 parts by mass of a 10% by mass water slurry of alumina particles having a secondary average particle size of 0.10 μm (1 part by mass as alumina particles) are maintained, and the vent hole is maintained at a reduced pressure of 1 kPa or less to keep moisture. This was removed to obtain particle-containing pellets (raw material-2f) having an intrinsic viscosity of 0.62 and containing 1% by mass of alumina particles.

  (2-g) Production of particle-containing PEN pellets: 90 parts by mass of the PEN pellets (raw material-1b) described above and an average particle size of 0 were added to a co-rotating bent type twin-screw kneading extruder heated to 280 ° C. Supply 10 parts by mass (1 part by mass as crosslinked polystyrene particles) of 10 mass% water slurry of .30 μm crosslinked polystyrene particles, keep the vent hole at a reduced pressure of 1 kPa or less, remove moisture, A particle-containing pellet (raw material -2 g) having an intrinsic viscosity of 0.6% by mass was obtained.

  (2-h) Preparation of particle-containing PEN pellets: 90 parts by mass of the above PEN pellets (raw material-1b) and an average particle size of 0 were placed in a co-rotating vent type biaxial kneading extruder heated to 280 ° C. 10 mass parts (1 part by mass as colloidal silica particles) of 10 mass% water slurry of 0.060 μm colloidal silica particles is supplied, moisture is removed while maintaining the vent hole at a reduced pressure of 1 kPa or less, and colloidal silica particles are 1 A particle-containing pellet (raw material-2h) having an intrinsic viscosity of 0.6% by mass was obtained.

  (2-i) Preparation of particle-containing PEN pellets: 90 parts by mass of the above PEN pellets (raw material-1b) and an average particle size of 0 were placed in a co-rotating bent type twin-screw kneading extruder heated to 280 ° C. 10 parts by weight (1 part by weight as colloidal silica particles) of 10% by weight water slurry of 20 μm colloidal silica particles is supplied, the vent hole is maintained at a reduced pressure of 1 kPa or less, moisture is removed, and the colloidal silica particles 1 Particle-containing pellets (raw material-2i) having an intrinsic viscosity of 0.6% by mass were obtained.

  (2-j) Preparation of particle-containing PEN pellets: 90 parts by mass of PEN pellets (raw material-1b) and secondary average particles were added to a co-rotating bent type twin-screw kneading extruder heated to 280 ° C. Supply 10 parts by mass (1 part by mass as alumina particles) of 10% by mass water slurry of alumina particles having a diameter of 0.10 μm, hold the vent hole at a reduced pressure of 1 kPa or less, remove water, and add 2 parts by mass of alumina particles. % -Containing particle-containing pellets (raw material-2j) having an intrinsic viscosity of 0.6 were obtained.

(3) Preparation of two-component composition (PET / PEI) pellets: Bent twin-screw kneading extruder of the same direction rotation type provided with three kneading paddle kneading sections heated to a temperature of 280 ° C. (manufactured by Nippon Steel Works) , Screw diameter 30 mm, screw length / screw diameter = 45.5), solid phase polymerization PET pellets obtained by the above method (raw material-1k: treatment time 2 hours) and PEI “Ultem” manufactured by SABIC Innovative Plastics, Inc. (Registered trademark) 1010 pellets were supplied, melt extruded at a shear rate of 100 sec ⁻¹ and a residence time of 1 minute, and two-component composition pellets containing 50% by mass of PEI were obtained. In addition, the glass transition temperature of the produced two-component composition pellet was 150 ° C. (raw material-3).

Example 1
In the extruder E1 heated to 280 ° C. using two extruders E1 and E2, 96 parts by mass of PET pellets (raw material-1k) subjected to solid phase polymerization for 4 hours as an A layer raw material, average particle size 4 parts by mass of colloidal silica particle-containing pellets (raw material-2b) having a diameter of 0.06 μm were supplied after drying under reduced pressure at 180 ° C. for 3 hours. Similarly, in the extruder E2 heated to 280 ° C., 61 parts by mass of PET pellets (raw material-1k) used in the A layer as the B layer raw material and colloidal silica particle-containing pellets (raw material— 2b) 20 parts by mass, 8 parts by mass of colloidal silica particle-containing pellets (raw material-2e) having an average particle size of 0.20 μm, and 11 parts by mass of crosslinked polystyrene particle-containing pellets (raw material-2a) having an average particle size of 0.30 μm , And dried under reduced pressure at 180 ° C. for 3 hours. In order to laminate these two layers, the lamination thickness ratio (A layer | B layer) = 8 | 1 was set in the T die and merged so that the B layer side would be the cast drum surface side. While applying an electric charge, the film was closely cooled and solidified to produce a laminated unstretched film.

  The laminated unstretched film was stretched simultaneously at a stretching temperature of 90 ° C. by 3.15 times in the longitudinal direction (MD stretching 1) and 3.1 times in the width direction (TD stretching 1) by a simultaneous biaxial stretching machine. Subsequently, the film was stretched 1.1 times (MD stretching 2) and 1.6 times (TD stretching 2) in the longitudinal direction and the width direction simultaneously at a temperature of 170 ° C. Subsequently, heat treatment was performed at temperatures of 180 ° C., 195 ° C., and 210 ° C. for 5 seconds in the heat treatment zone in the tenter, and further, relaxation treatment was performed in the 0.5% width direction at a temperature of 150 ° C. Subsequently, after uniformly cooling to 25 ° C., the film edge was removed, and the film was wound on a core to obtain a biaxially stretched polyester film having a thickness of 4.5 μm. Film formation stability of the obtained biaxially oriented polyester film was good, and physical properties were evaluated. As shown in the table, the film had excellent characteristics when used as a magnetic tape.

  The table below shows the film configuration of the biaxially oriented polyester film of each example and comparative example, the composition of each layer, the physical properties, the characteristics of the magnetic tape, and the like.

(Example 2)
As shown in the table, a biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 1 except that the laminated thicknesses of the A and B layers were changed.

(Example 3)
Two extruders E1 and E2 were used, and the extruder E1 heated to 280 ° C. had 92 parts by mass of PET pellets (raw material-1k) subjected to solid phase polymerization for 4 hours as the A layer raw material, two components 4 parts by mass of composition pellets (raw material-3) and 4 parts by mass of colloidal silica particle-containing pellets (raw material-2d) having an average particle size of 0.06 μm were dried at 180 ° C. for 3 hours under reduced pressure, and then supplied. Similarly, in the extruder E2 heated to 280 ° C., 52 parts by mass of the PET pellet (raw material-1k) used in the A layer as the B layer raw material, 4 parts by mass of the two-component composition pellet (raw material-3), average 30 parts by mass of alumina particle-containing pellets (raw material-2f) having a particle size of 0.1 μm, 6 parts by mass of colloidal silica particle-containing pellets (raw material-2e) having an average particle size of 0.20 μm, and crosslinked polystyrene particles having an average particle size of 0.30 μm 8 mass parts of containing pellets (raw material-2a) were mix | blended, and it supplied, after drying under reduced pressure at 180 degreeC for 3 hours. In order to laminate these two layers, the lamination thickness ratio (A layer | B layer) = 10 | 1 is combined in the T die, and the B layer side is merged so as to be the cast drum surface side. While applying an electric charge, the film was closely cooled and solidified to produce a laminated unstretched film.

  This laminated unstretched film was stretched at a stretching temperature of 92 ° C. by a simultaneous biaxial stretching machine, and simultaneously stretched 3.15 times in the longitudinal direction (MD stretching 1) and 3.1 times in the width direction (TD stretching 1), Subsequently, the film was simultaneously stretched 1.1 times (MD stretch 2) and 1.6 times (TD stretch 2) in the longitudinal direction and the width direction at a temperature of 180 ° C., respectively. Subsequently, heat treatment was performed at 190 ° C., 200 ° C., and 213 ° C. for 5 seconds in a heat treatment zone in the tenter, and then relaxation treatment was performed in the 0.5% width direction at a temperature of 150 ° C. Subsequently, after uniformly cooling to 25 ° C., the film edge was removed, and the film was wound on a core to obtain a biaxially stretched polyester film having a thickness of 4.4 μm. Film formation stability of the obtained biaxially oriented polyester film was good, and physical properties were evaluated. As shown in the table, the film had excellent characteristics when used as a magnetic tape.

(Example 4)
The stretching ratio in the longitudinal direction of the first stage was changed to 3.3 times (MD stretching 1). Further, as shown in the table, a biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 3 except that the blending amount and the B layer lamination thickness were changed so that the various particle raw materials had a predetermined concentration. It was.

(Example 5)
A biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 4 except that the heat treatment temperature was changed to 195 ° C., 205 ° C., and 218 ° C.

(Example 6)
96 parts by mass of PEN pellets (raw material-1b) and 4 parts by mass of colloidal silica particle-containing pellets (raw material-2h) having an average particle size of 0.06 μm were supplied as the A layer raw material after drying under reduced pressure at 180 ° C. for 3 hours. Similarly, the extruder E2 heated to 280 ° C. contained 50 parts by mass of PEN pellets (raw material-1b) used in the A layer as raw materials for the B layer, and alumina particle-containing pellets (raw material-2j) having an average particle size of 0.1 μm. ) 30 parts by mass, 8 parts by mass of colloidal silica particle-containing pellets (raw material-2i) having an average particle size of 0.2 μm, and 12 parts by mass of crosslinked polystyrene particle-containing pellets (raw material-2 g) having an average particle size of 0.30 μm. , And dried under reduced pressure at 180 ° C. for 3 hours. In order to laminate these two layers, the lamination thickness ratio (A layer | B layer) = 10 | 1 is combined in the T die, and the B layer side is merged so as to be the cast drum surface side. While applying an electric charge, the film was closely cooled and solidified to produce a laminated unstretched film.

  This laminated unstretched film was simultaneously stretched 4.5 times in the longitudinal direction (MD stretching 1) and 4.2 times in the width direction (TD stretching 1) at a temperature of 140 ° C. with a simultaneous biaxial stretching machine. At a temperature of 195 ° C., the film was simultaneously stretched 1.1 times in the longitudinal direction (MD stretching 1) and 1.4 times in the width direction (TD stretching 2). Subsequently, heat treatment was performed at temperatures of 200 ° C., 210 ° C., and 215 ° C. for 5 seconds in the heat treatment zone in the tenter, and then relaxation treatment was performed in the 0.5% width direction at a temperature of 150 ° C. Subsequently, after uniformly cooling to 25 ° C., the film edge was removed, and the film was wound on a core to obtain a biaxially stretched polyester film having a thickness of 4.4 μm.

(Example 7)
The stretching ratio in the width direction is changed to 5.0 times (TD stretching 1), and then the second stage is changed to 1.2 times (TD stretching 2), so that various particle raw materials have predetermined concentrations as shown in the table. A biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 6 except that the blending amount and the lamination thickness ratio were changed.

(Comparative Example 1)
A biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as Example 4 except that the first stage stretching temperature was changed to 80 ° C.

(Comparative Example 2)
Stretching at the same time with the stretching ratio in the longitudinal direction of the first stage (MD stretching 1) being 4 times and the stretching ratio in the width direction (TD stretching 1) being 3.1 times, followed by stretching the second stage at 180 ° C. At the same time, the film was stretched 1.1 times in the longitudinal direction (MD stretching 2) and 1.4 times in the width direction (TD stretching 2) at the same time. Further, as shown in the table, a biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 4 except that the blending amount of various particle raw materials was changed to a predetermined concentration.

(Comparative Example 3)
As shown in the table, a biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 3 except that the blending amount of various particle raw materials was changed to a predetermined concentration.

(Comparative Example 4)
As shown in the table, a biaxially stretched polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 3 except that the blending amount and the lamination thickness ratio were changed so that the various particle raw materials had a predetermined concentration.

(Comparative Example 5)
Biaxially with a thickness of 4.4 μm in the same manner as in Example 3 except that the heat treatment temperature was not divided into multiple stages and was carried out at 230 ° C. for 10 seconds and the amount of each particle raw material was changed to a predetermined concentration. A stretched polyester film was obtained.

(Comparative Example 6)
A biaxially stretched polyester film having a thickness of 4.4 μm was obtained in the same manner as in Example 3 except that the blending amounts of the various particle raw materials of the A layer and the B layer were changed to a predetermined concentration.

(Comparative Example 7)
As shown in the table, a biaxially stretched polyester film having a thickness of 4.3 μm was obtained in the same manner as in Example 1 except that the particle concentration of the A layer and the lamination thickness ratio were changed.

Claims (6)

In the outermost surface (A surface) of any one of the biaxially oriented polyester films, the maximum power spectral density (PSD-Xa) in the width direction in the region of a wavelength of less than 10 μm is 1,000 to 50,000 nm ³ , and the opposite A biaxially oriented polyester film having a maximum power spectral density (PSD-Xb) in the width direction of 150,000 to 500,000 nm ³ in a region of the surface of the outermost layer (B surface) having a wavelength of less than 10 μm. The biaxially oriented polyester film according to claim 1, wherein the maximum power spectral density in a region of the B surface with a wavelength of less than 10 μm satisfies the following formula.
0.6 ≦ PSD-Yb / PSD-Xb ≦ 1.2
(However, PSD-Yb (nm ³ ): Maximum power spectral density in the B-plane longitudinal direction.
PSD-Xb (nm ³ ): Maximum power spectral density in the B-plane width direction) The biaxially oriented polyester film according to claim 1 or 2, wherein the maximum power spectral density (PSD-Xb1) in the width direction in the region of wavelength B of 1 µm or less on the B surface is in the range of 15,000 to 80,000 nm ³ . The biaxially oriented polyester according to any one of claims 1 to 3, wherein a maximum power spectral density (PSD-Xb30) (nm ³ ) in the width direction in the region of the B-plane having a wavelength of 10 µm to 30 µm satisfies the following formula. the film.
1.0 ≦ PSD-Xb30 / PSD-Xb ≦ 3.0 The biaxially oriented polyester film according to any one of claims 1 to 4, which is used for a base film for a magnetic recording medium of a coating type digital recording system. A magnetic recording medium using the biaxially oriented polyester film according to claim 1.