Biaxially oriented polyester film

Abstract

To stably provide a biaxially oriented polyester film that is excellent in runnability and windability and further excellent in dimensional stability and when used as a magnetic recording medium, serves as a high-density magnetic recording medium having a smooth magnetic layer with a small dimensional change due to environmental change of temperature and humidity or due to storage and having a low error rate.SOLUTION: The biaxially oriented polyester film is such that on at least one outermost layer surface of the biaxially oriented polyester film, the number of projections having a height of 60 nm or more is 2,000 to 60,000 / mm2 and the average projection area of cut ends of projections at a height of 60 nm is 0.03 to 0.18 μm2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyester film that has excellent running properties, slitting properties, dimensional stability, and surface properties, and that can be suitably used as a base film for coated magnetic recording media such as data storage. [Background technology]

[0002] Biaxially oriented polyester films are used in a variety of applications due to their excellent thermal properties, dimensional stability, mechanical properties, and ease of control of surface morphology, and are particularly well known for their usefulness as supports for magnetic recording media, etc. Higher recording densities are always required for magnetic recording media, and in order to achieve even higher recording densities, it is effective to further improve the smoothness of the magnetic layer surface by reducing the thickness of the magnetic layer or by using fine particle magnetic materials.

[0003] In recent years, in magnetic recording medium supports using ferromagnetic hexagonal ferrite powder, the thickness reduction of the magnetic layer, non-magnetic layer, backcoat layer, and even the support itself has restricted the roughness of not only the smooth surface but also the running surface. When the magnetic recording medium is stored in a rolled state during the manufacturing process, problems arise such as the protrusions formed on the running surface being transferred to the magnetic surface, forming depressions on the smooth magnetic layer surface, and as the support becomes thinner, large particles contained in the support are pushed up against the smooth surface, causing gentle convex waves on the magnetic layer surface, reducing the smoothness of the magnetic layer surface. If the diameter of the particles contained in the support is reduced to increase the smoothness of the magnetic layer surface and improve the smoothness to create an ultra-high-resolution surface, running ease, winding, and even surface durability become insufficient.

[0004] Therefore, the need for improvement in properties such as compatibility between slitting ability and surface smoothness is a constant challenge in achieving higher density recording.

[0005] To solve the above problems, polyester films containing fine particles have been investigated, controlling the film surface roughness and protrusion height and number to suppress transfer to the magnetic layer surface (e.g., Patent Document 1). However, even if protrusions of a specific height and size are specified on the base film surface on the backcoat layer side, when used in magnetic recording medium supports made of ferromagnetic hexagonal ferrite powder with thin, high-definition magnetic layers and backcoats, the reduction in coarse protrusions remains, and the reduction in smoothness of the magnetic surface due to transfer cannot be resolved. Furthermore, because the laminate thickness on the side where the magnetic layer is not formed is thick, the surface protrusions push up against the opposite side (magnetic layer side), thereby preventing the smoothness of the magnetic layer side from being impaired. Furthermore, as the diameter of the contained particles decreases, the protrusions become lower, reducing the protrusions with a height that contributes to running performance, leaving problems with running performance, winding performance, and surface abrasion resistance. There are also polyester films that combine excellent winding properties and electromagnetic conversion characteristics by controlling the waviness of the polyester film surface within a specific range (e.g., Patent Document 2), and biaxially oriented polyester films that combine running properties and surface properties by specifying the proportion of protrusions with a specific protrusion height (e.g., Patent Document 3). However, when used in magnetic recording medium supports made with ferromagnetic hexagonal ferrite powder, which require a highly precise surface, simply specifying the proportion of protrusions with a specific height reduces the total number of transfer marks on the magnetic layer surface caused by the protrusions on the backcoat layer surface, but it is clear that there are transfer marks that form large depressions, and the smoothness of the magnetic layer surface is still insufficient. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-153100 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-153099 [Patent Document 3] JP 2016-79410 A Summary of the Invention [Problem to be solved by the invention]

[0007] The present inventors have conducted extensive research to achieve the above-mentioned object, and have concluded that simply controlling the roughness of the running surface or the height and number of surface protrusions to suppress the deterioration of the smoothness of the magnetic layer surface due to transfer to the magnetic surface or the protrusions pushing up onto the magnetic surface does not necessarily reduce defects on the magnetic layer surface, and that there is a limit to achieving both high levels of running performance and smoothness.Further research has revealed that there is a distribution in depth and size of transfer marks on the magnetic layer surface, and that in particular, reducing transfer marks that are recessed in large areas regardless of the depth of the transfer marks is important for further smoothness of the magnetic layer surface, and have arrived at this invention.

[0008] The object of the present invention is to provide a biaxially oriented polyester film that solves the above problems, has excellent running properties, slitting properties, and dimensional stability, and when made into a magnetic recording medium, has a smooth magnetic layer, exhibits little dimensional change due to environmental changes such as temperature and humidity, and is subject to storage, and serves as a high-density magnetic recording medium with an excellent frequency of missing pulses. [Means for solving the problem]

[0009] The present invention, which has been made to solve the above problems, is characterized by the following configurations.

[0010] (1) On the surface of at least one outermost layer of a biaxially oriented polyester film, the number of protrusions having a height of 60 nm or more is 25,000 to 60,000 / mm 2 The average area of ​​the cut surface of the protrusions at a height of 60 nm is 0.03 to 0.18 μm 2 and inert particles having an average particle size of 0.01 to 0.3 μm or less. 0.005~0.8% by mass A biaxially oriented polyester film having at least one layer containing the polymer (layer B) and having a laminated structure of two or more layers.

[0011] (2) On the outermost surface (side B) of the film having the characteristics described in (1) above, the area of ​​protrusions at a height of 60 nm is 1 μm 2 More than 100 protrusions / mm2 The biaxially oriented polyester film according to (1) above, which is:

[0012] (3) The biaxially oriented polyester film according to (2) above, wherein the surface roughness (SRa) of side B is 3 to 15 nm.

[0013] (4) The biaxially oriented polyester film according to any one of (1) to (3) above, which has a humidity expansion coefficient in the width direction of 0 to 6.5 ppm / % RH.

[0014] (5) The biaxially oriented polyester film according to any one of (1) to (4) above, wherein the surface roughness (SRa) of the outermost surface (side A) opposite to side B is 0.5 to 5 nm.

[0015] (6) The biaxially oriented polyester film according to any one of (1) to (5) above, which is used as a base film for a coating-type digital recording medium. [Effects of the Invention]

[0016] The biaxially oriented polyester film of the present invention is a biaxially oriented polyester film that has excellent running properties, slitting properties, and dimensional stability, and when made into a magnetic recording medium, it is possible to obtain a biaxially oriented polyester film that has a smooth magnetic layer, exhibits little dimensional change due to environmental changes in temperature and humidity or due to storage, and serves as a high-density magnetic recording medium with an excellent frequency of missing pulses. DETAILED DESCRIPTION OF THE INVENTION

[0017] The polyester used in the present invention may be, for example, a polymer having an acid component such as an aromatic dicarboxylic acid, an alicyclic dicarboxylic acid, or an aliphatic dicarboxylic acid, or a diol component as a constituent unit (polymerization unit).

[0018] Examples of aromatic dicarboxylic acid components that can be used include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, and 4,4'-diphenylsulfonedicarboxylic acid. Of these, terephthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid are preferred. Examples of alicyclic dicarboxylic acid components that can be used include cyclohexanedicarboxylic acid. Examples of aliphatic dicarboxylic acid components that can be used include adipic acid, suberic acid, sebacic acid, and dodecanedioic acid. These acid components may be used alone or in combination.

[0019] Examples of diol components that can be used include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, and 2,2'-bis(4'-β-hydroxyethoxyphenyl)propane. Of these, ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and diethylene glycol are preferred, with ethylene glycol being particularly preferred. These diol components may be used alone or in combination.

[0020] The polyester may be copolymerized with a monofunctional compound such as lauryl alcohol or phenyl isocyanate, or with a trifunctional compound such as trimellitic acid, pyromellitic acid, glycerol, pentaerythritol, or 2,4-dihydroxybenzoic acid, provided that the polymer is substantially linear without excessive branching or crosslinking. In addition to the acid component and diol component, aromatic hydroxycarboxylic acids such as p-hydroxybenzoic acid, m-hydroxybenzoic acid, and 2,6-hydroxynaphthoic acid, as well as p-aminophenol and p-aminobenzoic acid, may also be copolymerized in small amounts that do not impair the effects of the present invention.

[0021] The copolymerization ratio of the polymer can be investigated using NMR (nuclear magnetic resonance) or microscopic FT-IR (Fourier transform microscopic infrared spectroscopy).

[0022] Polyesters with a glass transition temperature of less than 150°C are preferably used because they can be biaxially stretched and can exhibit the effects of the present invention, such as dimensional stability. Polyesters used in the present invention are preferably polyethylene terephthalate or polyethylene naphthalate (polyethylene-2,6-naphthalate), and may also be copolymers or modified products thereof, or polymer alloys with other thermoplastic resins. The term "polymer alloy" as used herein refers to a multi-component polymer system, and may be a block copolymer obtained by copolymerization or a polymer blend obtained by mixing. Polyesters used in the present invention that are primarily composed of polyethylene terephthalate are particularly preferred, as this facilitates the application of processes that increase crystallite size and crystal orientation. Here, "main component" refers to at least 80% by mass of the film composition.

[0023] When the polyethylene terephthalate used in the present invention is used as a polymer alloy, the other thermoplastic resin is preferably a polymer compatible with polyester, more preferably a polyetherimide resin, etc. Examples of the polyetherimide resin that can be used include those shown below.

[0024] [ka]

[0025] (In the above formula, R1 is a divalent aromatic or aliphatic residue having 6 to 30 carbon atoms, and R2 is a divalent organic group selected from the group consisting of a divalent aromatic residue having 6 to 30 carbon atoms, an alkylene group having 2 to 20 carbon atoms, a cycloalkylene group having 2 to 20 carbon atoms, and a polydiorganosiloxane group chain-terminated with an alkylene group having 2 to 8 carbon atoms.) Examples of R1 and R2 include aromatic residues represented by the following formulae:

[0026] [ka]

[0027] In the present invention, from the viewpoints of affinity with polyester, cost, melt moldability, etc., a polymer having a repeating unit represented by the following formula, which is a condensation product of 2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine or p-phenylenediamine, is preferred.

[0028] [ka]

[0029] or

[0030] [ka]

[0031] (n is an integer of 2 or more, preferably an integer of 20 to 50.) This polyetherimide is available from SABIC Innovative Plastics under the "Ultem" trade name and is known by trademarks such as the "Ultem® 1000," "Ultem® 1010," "Ultem® 1040," "Ultem® 5000," "Ultem® 6000," and "Ultem® XH6050" series, as well as "Extem® XH" and "Extem® UH."

[0032] The biaxially oriented polyester film of the present invention preferably has a laminated structure of two or more layers, including at least one layer (Layer B) containing inert particles with an average particle size of 0.01 to 0.3 μm or less. In this case, Layer B functions as a layer responsible for runnability and is provided as one of the outermost layers of the film. A layer (Layer A) responsible for smoothness is provided as the other outermost layer. A laminated structure of at least two layers is preferred to achieve the effects of the present invention.

[0033] 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 backcoat layer (hereinafter referred to as the BC layer) on the surface (side B) of the above-mentioned B layer in order to obtain a high-density magnetic recording medium. Magnetic recording media using ferromagnetic hexagonal ferrite powder in the magnetic layer tend to have thinner base films, magnetic layers, non-magnetic underlayers, and the BC layer itself. In particular, when the BC layer is thinned to 0.4 μm or less, the BC layer surface becomes susceptible to protrusions caused by the support, reducing the smoothness of the BC layer surface. By specifying the number of protrusions of a specific height and the average protrusion area on the B layer surface within the ranges of the present invention, even a thinner BC layer can exhibit excellent electromagnetic conversion characteristics with a low frequency of missing pulses without impairing the surface smoothness.

[0034] The biaxially oriented polyester film of the present invention has a surface of at least one outermost layer having protrusions of 2,000 to 60,000 protrusions per mm with a height of 60 nm or more as determined by atomic force microscope measurement. 2 Preferably, the density is 5,000 to 50,000 particles / mm2 and more preferably 10,000 to 40,000 particles / mm 2 is.

[0035] The number of protrusions with a height of 60 nm or more in the present invention is the number of protrusions at a height of 60 nm from a reference plane determined by atomic force microscope measurement. If the number of protrusions is below the lower limit, running performance deteriorates, resulting in poor slitting performance. If the number of protrusions exceeds the upper limit, for example, when the magnetic recording tape is wound up, the irregularities on the BC layer surface may be transferred to the magnetic layer surface, resulting in an increased number of transfer marks and an increased frequency of missing pulses.

[0036] In addition, the average projection area of ​​the cut surface of the projections at a height of 60 nm on the surface of at least one outermost layer of the biaxially oriented polyester film of the present invention is 0.03 to 0.18 μm 2 Preferably, it is 0.05 to 0.12 μm. 2 is.

[0037] Furthermore, on the surface of at least one outermost layer of the biaxially oriented polyester film of the present invention, the area of ​​a cut surface of a protrusion at a height of 60 nm from a reference plane determined by atomic force microscope measurement is 1 μm 2 The above protrusions are 100 / mm 2 It is preferable that the number of particles is less than 0, and more preferably 0 to 100 particles / mm 2 , and more preferably 30 to 100 pieces / mm 2 More preferably, it is 50 to 80 pieces / mm 2 The protrusion area is 1 μm 2 The number of protrusions is 100 or more per mm 2 If the transfer rate exceeds this value, the number of transfer marks on the surface of the magnetic layer will be too large, the smoothness of the surface of the magnetic layer will be significantly reduced, and missing pulses will tend to occur frequently.

[0038] The protrusion area and average protrusion area of ​​the cut surface of the protrusions at a height of 60 nm are measured using an atomic force microscope and are the area of ​​the cut surface of the protrusions at a height of 60 nm from the reference plane. If the average protrusion area is below the lower limit, the durability of the protrusions may be significantly reduced, and running performance may be impaired. Furthermore, if the average protrusion area exceeds the upper limit, for example, when wound into a magnetic recording tape, the transfer marks on the magnetic layer surface due to the unevenness of the BC layer surface may become large, resulting in large areas of depression, significantly reducing the smoothness of the magnetic layer surface and increasing the frequency of missing pulses. Note that the outermost layer surface having the above characteristics is preferably side B.

[0039] On the outermost surface (side B) of the biaxially oriented polyester film, the number of protrusions (Z) with a height of 10 nm or more is 30,000 / mm 2 ~200,000 pieces / mm 2 More preferably, 40,000 particles / mm 2 ~100,000 pieces / mm 2 The number of protrusions (Z) is 30,000 / mm 2 If the density is less than 200,000 particles / mm, the running property and slitting property tend to be deteriorated. 2 If the thickness exceeds this value, the protrusions will be concentrated in places, causing the surface to become undulated, and when the tape is wound into a magnetic recording tape, the unevenness on the BC layer surface may easily cause transfer of the magnetic layer surface.

[0040] On the outermost surface (side B) of the biaxially oriented polyester film, the protrusion ratio ((X / Z) x 100) is 0.5% or less, preferably 0.3% or less. The number of protrusions (X) is the number of protrusions in the region where the area of ​​the cut surface of a protrusion at a height of 10 nm (hereinafter referred to as the protrusion area) is 20 μm 2 The number of protrusions is equal to or greater than 60 nm in height. The protrusion area and the number of protrusions (X) and (Z) at a height of 10 nm according to the present invention are determined using a scanning white light interference microscope.

[0041] The protrusion area at a height of 10 nm from the reference surface is 20 μm 2Among the above protrusions, the occurrence of missing pulses can be suppressed by controlling the ratio ((X / Z) x 100) of the number of protrusions with a height of 60 nm or more to the number of protrusions with a height of 10 nm or more to a specific value or less. The smaller the protrusion area at 10 nm in height, the better, but as the particle diameter of the particles contained in the film becomes finer, the particles tend to become more densely packed in the film. When the particles become densely packed, the protrusion area at 10 nm in height tends to become larger, and the protrusion height also becomes higher. In particular, when the protrusion area at 10 nm in height is 20 μm 2 These protrusions tend to be large in height, and when the magnetic recording tape is wound up, the depressions that form on the magnetic layer surface tend to become large, which can reduce the smoothness of the magnetic layer surface. Furthermore, if the protrusion ratio ((X / Z) x 100) exceeds 0.5%, the number of protrusions (Z) decreases, which can lead to poor running and slitting properties.

[0042] The surface roughness (SRa) of the outermost surface (side B) of the biaxially oriented polyester film of the present invention is preferably 3 to 15 nm, more preferably 3 to 7 nm. The surface roughness (SRz) is 50 to 160 nm, more preferably 60 to 150 nm. If the surface roughnesses SRa and SRz of side B are below the lower limit, running properties and slitting properties are likely to be poor. If SRa and SRz exceed the upper limit, when a backcoat layer is provided on that surface to form a magnetic recording medium, the frequency of missing pulses due to transfer marks is likely to increase. The surface roughness (SRa) of the outermost surface (side A) opposite side B is preferably 0.5 to 5 nm, more preferably 0.5 to 3 nm. If the surface roughness of side A is outside the range of the present invention, when a magnetic layer is provided on that surface to form a magnetic recording medium, it may be difficult to achieve both running properties, slitting properties, and missing pulses.

[0043] The number and area of ​​the various protrusions measured by the atomic force microscope and scanning white light interference microscope can be controlled by the particle size, amount added, lamination thickness, and stretching conditions of the particles contained in Layer B. It is particularly important to use particles whose average particle size in Layer B is less than 0.4 μm, preferably 0.05 to 0.3 μm.

[0044] The particles preferably contained in Layer B of the biaxially oriented polyester film of the present invention are not particularly limited, but may be either inorganic or organic particles, or two or more types of particles may be used in combination. Specific examples include inorganic particles such as clay, mica, titanium oxide, calcium carbonate, wet silica, dry silica, colloidal silica, calcium phosphate, barium sulfate, alumina silicate, kaolin, talc, montmorillonite, alumina, and zirconia; organic particles containing components such as acrylic acids, styrene resins, silicones, and imides; and core-shell organic particles. To control the protrusion diameter and protrusion density of the present invention, organic particles that are monodispersed spherical particles and colloidal silica are particularly preferred. Furthermore, adding 0.2 to 0.7 mass % of alumina to Layer B is preferred because it efficiently improves runnability and slittability.

[0045] The amount of particles added to Layer B of the biaxially oriented polyester film of the present invention is preferably, for example, 0.005 to 0.8% by mass, and more preferably 0.01 to 0.5% by mass.

[0046] The layer B of the biaxially oriented polyester film of the present invention preferably has a thickness (t) of 1 μm or less, more preferably 0.6 μm or less, and the ratio (t / D) of the thickness (t) to the particle diameter (D) of the largest particle contained in the layer B is preferably set to 1 to 10, more preferably 1.3 to 6.

[0047] The thickness of the biaxially oriented polyester film of the present invention is preferably in the range of 3.5 to 4.6 μm. If the thickness is less than 3.5 μm, the rigidity and dimensional stability will deteriorate, resulting in insufficient tape stiffness and, when used as a magnetic recording medium, tend to deteriorate the electromagnetic conversion characteristics. Furthermore, it will be difficult to suppress protrusions on the surface of Layer B from pushing up toward the smooth surface (Side A). Furthermore, if the thickness is greater than 4.6 μm, the tape length per reel will be short, making it difficult to accommodate the miniaturization and high capacity of magnetic tapes. The thickness can be adjusted by adjusting the screw discharge rate during melt extrusion of the polymer during film formation of the biaxially oriented polyester film, thereby controlling the thickness of the unstretched film from the die, thereby enabling the thickness of the film after biaxial stretching to be adjusted.

[0048] The biaxially oriented polyester film of the present invention preferably has a humidity expansion coefficient in the width direction of 0 to 6.5 ppm / %RH. If the humidity expansion coefficient is 6.5 ppm / %RH or less, when used for magnetic recording media, deformation due to humidity changes will not be significant and deterioration of dimensional stability will be less likely to occur. A more preferred upper limit is 6.0 ppm / %RH, and even more preferably 5.5 ppm / %RH. The humidity expansion coefficient is a physical property affected by the degree of tension in the molecular chain, and as will be described later, can be controlled by the ratio of the TD stretching ratios 1 and 2, and can also be controlled by the ratio between the total TD stretching ratio and the MD stretching ratio.

[0049] In the present invention, MD refers to the longitudinal direction (machine direction) of the biaxially oriented polyester film, and TD refers to the width direction (transverse direction) of the biaxially oriented polyester film.

[0050] The biaxially oriented polyester film of the present invention preferably has a width-direction Young's modulus 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 width-direction Young's modulus is within the above range, when used for a magnetic recording medium, the dimensional stability against environmental changes during recording and reproduction of the magnetic recording medium tends to be good. The width-direction Young's modulus can be controlled by the temperature and ratio of TD stretching 1 and 2 described below. The total TD ratio has a particular effect, and the higher the total TD ratio, the higher the TD Young's modulus.

[0051] 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. If the Young's modulus in the longitudinal direction is within the above range, when used for a magnetic recording medium, the storage stability of the magnetic recording medium due to tension during storage will be improved. A more preferred range for the Young's modulus in the longitudinal direction is 3.8 to 7.5 GPa, and an even more preferred range is 4 to 7 GPa. The Young's modulus in the longitudinal direction can be controlled by the MD stretching ratio. The higher the MD ratio, the higher the MD Young's modulus.

[0052] The biaxially oriented polyester film of the present invention as described above can be produced, for example, as follows.

[0053] First, polyester pellets are melted using an extruder, extruded from a die, cooled, solidified, and formed into a sheet. At this time, it is preferable to filter the polymer through a sintered stainless steel metal filter to remove unmelted material from the polymer.

[0054] Various additives, such as compatibilizers, plasticizers, weathering agents, antioxidants, heat stabilizers, lubricants, antistatic agents, brighteners, colorants, conductive agents, crystal nucleating agents, ultraviolet absorbers, flame retardants, flame retardant assistants, pigments, and dyes, may be added within a range that does not impair the characteristic aspects of the present invention.

[0055] Next, the sheet is simultaneously biaxially stretched in the longitudinal and width directions, and then stretched again in the width direction and heat-treated. In order to improve the number of protrusions, the average protrusion area, and the dimensional stability in the width direction, the stretching process preferably involves multi-stage stretching in the longitudinal and width directions and re-stretching in the width direction. This is preferable because multi-stage stretching in the longitudinal and width directions makes it easier to control the average protrusion area of ​​60 nm in height, and re-stretching in the width direction makes it easier to obtain a high-strength film that is optimal for a magnetic tape with high dimensional stability.

[0056] As a stretching method, a method in which simultaneous biaxial stretching is followed by further stretching in the width direction is preferred. In the sequential biaxial stretching method, stretching stress tends to concentrate on the particle portion when stretching in the width direction after stretching in the longitudinal direction, which tends to result in the formation of voids and may make it impossible to obtain the average protrusion area of ​​the present invention, but it is possible to carry out the method by employing an ingenious method such as multi-stage stretching for stretching in the longitudinal and width directions.

[0057] The film manufacturing method of the present invention will be described below using polyethylene terephthalate (PET) as a representative example of the polyester. Note that the present invention is not limited to PET films, and other polymers may also be used. For example, when a polyester film is made using polyethylene-2,6-naphthalenedicarboxylate, which has a high glass transition temperature or melting point, extrusion and stretching may be performed at temperatures higher than those shown below.

[0058] First, PET pellets are produced. PET is manufactured using one of the following processes: (1) a process in which terephthalic acid and ethylene glycol are used as raw materials and a low-molecular-weight PET or oligomer is obtained by a direct esterification reaction, followed by a polycondensation reaction using antimony trioxide or a titanium compound as a catalyst to obtain a polymer; or (2) a process in which dimethyl terephthalate and ethylene glycol are used as raw materials and a low-molecular-weight substance is obtained by a transesterification reaction, followed by a polycondensation reaction using antimony trioxide or a titanium compound as a catalyst to obtain a polymer.

[0059] To incorporate particles into the PET film, a preferred method is to disperse the particles in ethylene glycol at a predetermined ratio in the form of a slurry and then add this ethylene glycol during polymerization. When adding particles, for example, adding the water or alcohol slurry obtained during particle synthesis without first drying the particles improves particle dispersibility. Here, to control the average protrusion area of ​​the cut surface of protrusions at a height of 60 nm within the range of the present invention, it is important to maintain the particle slurry concentration at 5% or less. When the particles to be added are inorganic particles, it is also highly effective to add glass beads with an average particle size of 0.5 to 0.05 mm to a particle-containing slurry adjusted to a specific concentration and stir at 1,000 to 5,000 rpm for 1 to 5 hours. Next, it is particularly important to directly mix the water slurry with PET pellets and knead them into the PET using a vented twin-screw kneading extruder to obtain a particle content of 0.1 to 0.5% by mass, thereby producing a particle master pellet. A preferred method for adjusting the particle content is to dilute the particle master pellets of a specific concentration prepared by the above method with PET that does not substantially contain particles during film formation to adjust the particle content. In this case, it is particularly important to ensure that the blending ratio of PET pellets that do not substantially contain particles is 50% by mass or less, preferably 35% by mass or less, in order to obtain the average protrusion area of ​​the present application. Furthermore, when using two or more types of particle master pellets, a preferred example for controlling the average protrusion area of ​​the present application is to blend them in advance to a predetermined desired content and then use a two-stage kneaded particle master that is melt-kneaded again.

[0060] The resulting PET pellets are then dried under reduced pressure at 180°C for at least 3 hours. To prevent a decrease in intrinsic viscosity, they are then fed into an extruder heated to 270-320°C under nitrogen or reduced pressure. The pellets are extruded through a slit die and cooled on a casting roll to obtain an unstretched film. To remove foreign matter and altered polymers, various filters, such as high-precision filters made of sintered metal, porous ceramic, sand, or wire mesh, are preferably used. Furthermore, the use of a gear pump to improve quantitative supply and achieve the desired layer thickness is highly desirable for achieving the above-mentioned features. To laminate films, two or more extruders and a manifold or confluence block are used to melt-layer multiple different polymers.

[0061] Next, a method of simultaneously biaxially stretching the unstretched film thus obtained and then stretching it again in the width direction will be described.

[0062] First, an unstretched film is simultaneously biaxially stretched in the MD and TD directions. The stretching temperature varies depending on the type of polymer used, but can be determined based on the glass transition temperature (Tg) of the unstretched film. It is preferably in the range of Tg + 5 to Tg + 20°C, more preferably Tg + 8 to Tg + 15°C. If the stretching temperature is lower than the above range, film breakage occurs frequently, reducing productivity, and it may be difficult to stably stretch the film again in the width direction after simultaneous biaxial stretching. The MD stretch ratio during simultaneous biaxial stretching is 3 to 6 times, preferably 3.1 to 5.0 times. The TD stretch ratio is 3.2 to 6 times, preferably 3.3 to 4.8 times.

[0063] Next, the film is stretched again in the TD direction (TD stretching 2). The stretching temperature for TD stretching 2 is 180 to 205°C, preferably 185 to 200°C. The stretching ratio for TD stretching 2 is preferably 1.4 to 2 times, more preferably 1.5 to 1.8 times. Simultaneous biaxial stretching in the MD direction within a range of 1.05 to 2.0 times, preferably 1.1 to 1.4 times, ensures that the surface protrusions on side B are aligned in the TD direction when the film is stretched again in the TD direction, thereby preventing the average protrusion area of ​​the present invention from becoming too large. This stretched film is heat-set under tension or while relaxed in the width direction. As heat-set conditions, the heat-set temperature is 190 to 230°C, preferably 200 to 230°C, and particularly preferably 215 to 230°C. Increasing the heat treatment temperature to 215°C or higher, preferably 220-230°C, results in the surface protrusions on Side B continuing in the TD direction when re-stretched in the TD, preventing the average protrusion area and protrusion number (X) of the present invention from becoming too large. The heat setting time is preferably in the range of 0.5-10 seconds, with a relaxation rate of 0.3-2%. The film is then introduced into a cooling zone adjusted to 100-150°C and slowly cooled, after which the film edges are cut and wound onto a roll to obtain the biaxially oriented polyester film of the present invention. Furthermore, there is a difference between the stretching temperature in TD stretching 2 and the heat setting temperature. If the heat setting temperature is higher than the above range, the film is likely to relax, making it difficult to achieve the above-mentioned humidity expansion coefficient and reducing dimensional stability. If the heat setting temperature is too low, crystallinity is likely to decrease, which tends to reduce the flatness of the base film during the magnetic recording medium manufacturing process and increase the frequency of missing pulses. Furthermore, if the film is rapidly cooled to room temperature rather than slowly cooled within a specific temperature range after heat setting, it may be difficult to control the average protrusion area of ​​the present invention.

[0064] Next, the magnetic recording medium is manufactured, for example, as follows.

[0065] The magnetic recording medium substrate (biaxially oriented polyester film) obtained as described above is slit into a width of, for example, 0.1 to 3 m. While conveying the substrate at a speed of 20 to 300 m / min and a tension of 50 to 300 N / m, a non-magnetic coating material is applied to one side using an extrusion coater to a thickness of 0.5 to 1.5 μm. After drying, a magnetic coating material is further applied to a thickness of 0.1 to 0.3 μm. The substrate coated with the magnetic and non-magnetic coating materials is then magnetically oriented and dried at a temperature of 80 to 130°C. Next, a backcoat is applied to the opposite side to a thickness of 0.3 to 0.8 μm, calendered, and then wound up. The calendering is performed using a small test calender (metal roll, 7 stages) at a temperature of 70 to 120°C and a linear pressure of 0.5 to 5 kN / cm. The film is then aged at 60 to 80°C for 24 to 72 hours, slit into a width of 12.65 mm, and pancakes are produced. Next, a specific length of the pancake is assembled into a cassette to form a cassette tape type magnetic recording medium.

[0066] Here, the composition of the magnetic paint may be, for example, as follows:

[0067] Hereinafter, when simply "parts" is mentioned, it means "parts by mass."

[0068] [Magnetic layer forming coating liquid] 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 Polyurethane resin 12 parts Mass average molecular weight 10,000 Sulfonic acid functional group 0.5meq / 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 0.5 parts stearic acid Butyl stearate 2 parts 180 parts methyl ethyl ketone 100 parts cyclohexanone [Coating liquid for forming non-magnetic layer] Non-magnetic powder α-iron oxide 100 parts Average major axis length: 0.09 μm, specific surface area by BET method: 50 m 2 / g Average acicular ratio: 7 DBP oil absorption: 27~38ml / 100g Surface treatment layer: Al2O3 8% by mass Carbon black 25 parts Conductex SC-U (manufactured by Colombian Carbon) Vinyl chloride copolymer MR104 (manufactured by Zeon Corporation) 13 parts Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) 5 parts Phenylphosphonic acid 3.5 parts Butyl stearate 1.0 parts Magnetic recording media can be suitably used, for example, for data recording purposes, specifically for backing up computer data (for example, linear tape-type recording media (LTO7, LTO8, next-generation LTO tape (LTO9))) and for recording digital images such as video.

[0069] An example of a coating-type digital recording medium in which the biaxially oriented polyester film of the present invention can be suitably used is a coating-type magnetic recording medium in which a magnetic layer is formed by uniformly dispersing ferromagnetic powder such as barium ferrite in a binder such as polyurethane resin to create a magnetic coating liquid, and then coating the coating liquid.

[0070] (Methods for measuring physical properties and evaluating effects) The methods for measuring the characteristic values ​​and evaluating the effects in the present invention are as follows.

[0071] (1) Number of protrusions with a height of 60 nm or more (pieces / mm 2 ) Measurements were performed in 20 different visual fields using an atomic force microscope (AFM). The sample was set so that the direction perpendicular to the scanning direction of the cantilever (the Y-axis direction) was the longitudinal direction of the sample film (the longitudinal direction is the direction in which the film runs during the film manufacturing process), and measurements were performed under the following AFM measurement conditions. The obtained images were analyzed using the accompanying analysis software (NanoScope Analysis Version 1.40). The obtained height sensor image of the film surface was subjected to only the flattening process described below, and then the Particle Analysis analysis mode was set as follows to automatically determine the reference plane of the film surface. From this reference plane, the threshold height of the protrusions (Threshold Height) was set to 60 nm and the number of protrusions obtained (value in the Mean column - Total Count row) was calculated. The number of protrusions in each of the 20 visual fields was added up to calculate the total number of protrusions in the total area of ​​the 20 visual fields. This total number of protrusions was calculated by dividing the total number of protrusions by 1 mm. 2 The converted value per particle is the number of protrusions with a height of 60 nm or more. [AFM measurement conditions] Equipment: Bruker atomic force microscope (AFM) Dimension Icon with ScanAsyst Cantilever: Silicon nitride probe ScanAsyst Air Scan mode: ScanAsyst Scanning speed: 0.5Hz Scanning direction: Scanning is performed in the width direction of the measurement sample prepared using the method described below. Measurement field of view: 90 μm square Sample lines: 512 LP Deflection BW: 40 kHz Sample preparation: 23°C, 65% RH, left standing for 24 hours AFM measurement environment: 23°C, 65% RH Number of sample measurements: Each sample is measured 20 times, with each sample spaced at least 5 μm apart. [Flatten processing conditions] Flatten Order: 3rd ·Flatten Z Threshholding Direction :No theresholding ·Find Threshold for :the whole image ·Flatten Z Threshold % :0.00 % Mark Excluded Data: Yes [Particle Analysis mode setting conditions] (Detect tab) Threshold Height: 60nm Feature Direction:Above X Axis: Absolute Number Histogram Bins: 512 ·Histogram Filter Cutoff :0.00 nm Min Peak to Peak: 1.00 nm Left Peak Cutoff: 0.00000% Right Peak Cutoff: 0.00000% (Modify tab) ·Beughbirhood Size:3 Number Pixels Off: 1 -Do not perform any dilate / erode operations. (Select tab) ·Image Cursor Mode:Particle Select Bound Particles: Yes ·Non-Representative Particles :Yes ·Height Reference :Relative To Threshold Number Histogram Bins: 50 When calculating the above values, do not select a specific peak or area in the analysis image. Do not select a specific location in any of the histograms: Diameter, Height, and Area.

[0072] (2) Average protrusion area at a height of 60 nm (μm 2 ) In the analysis of (1) above, the average protrusion area (value in the Mean column and the Area row) at a threshold of 60 nm is determined, and the average value of 20 visual fields is taken as the average protrusion area at a height of 60 nm.

[0073] (3) 1 μm at a height of 60 nm 2 Number of protrusions with a protrusion area of ​​more than 2 ) In the analysis of (1) above, select the Area tag of the Select tag and display the Particle Area Histogram. Two guidelines will be displayed. Adjust the left guideline to the X axis of the protrusion area of ​​1 μm. 2 The guideline on the right is set to the maximum value on the X axis at 60 nm height, 1 μm 2 The number of protrusions with an area of ​​1 mm or more is calculated. The number of protrusions in each of the 20 fields of view is added up to calculate the total number of protrusions in the total area of ​​the 20 fields of view. This total number of protrusions is calculated as 1 mm 2 The converted value per 1μm at a height of 60nm is 2 The number of protrusions having a protrusion area of ​​at least this number is specified.

[0074] (4) Surface roughness (SRa) of layer B (side B) In the measurement of (1) above, after only flattening, the roughness analysis mode was selected, Ra was determined under the following setting conditions, and the average value of 20 visual fields was taken as SRa. [Roughness Analysis mode setting conditions] (Stop Band Inputs) Use Threshold: OFF Feature Direction:Above Number Histogram Bins: 512 X Axis: Absolute Boundary Particles: Yes ·Non-Representative Particles :No ·Particles Filter Sigma :1.00 (Peak Inputs) Peak: On ·Perk threshold reference :Zero ·Perk threshold value type :Rms Perk threshold value: 100% Zero Crossing: Off (5) Number of protrusions (X), (Z) Using a scanning white light interference microscope, measurements were taken in 100 different fields of view at different locations. The sample was set so that the direction perpendicular to the X-axis direction of the sample stage (the Y-axis direction) was the longitudinal direction of the sample film (the longitudinal direction is the direction in which the film runs during the film manufacturing process), and measurements were taken under the following measurement conditions. After the following image processing was carried out on all the obtained images using the accompanying analysis software (VS-Viewer), particle analysis was carried out on the entire field of view under the following conditions to determine the number of protrusions with a height of 10 nm or more, and furthermore, the number of protrusions with a cross-sectional area of ​​20 μm 2 The number of protrusions with a height of 60 nm or more from the reference plane was calculated. The total number of protrusions of various heights in each field of view was calculated as the number of protrusions with a height of 10 nm or more (Z), and the cut surface area of ​​the protrusions was calculated as the number of protrusions with a height of 20 μm 2 The number of protrusions (X) having a height of 60 nm or more was defined as the number of protrusions having a height of 60 nm or more.

[0075] <Device>: Hitachi High-Tech Science VS-1540 <Measurement conditions> Objective lens: 50x Wavelength filter: 530white Measuring device: Piezo Measurement mode: Wave Measurement field size: 113μm x 113μm <Image processing conditions> Interpolation: Full Interpolation Filter: Median 3x3 Surface correction: 4th order <Particle analysis> Analysis: sudden analysis Image correction: None Height threshold: 10nm Reference height: Zero plane (6) Humidity expansion coefficient in the width direction (ppm / %RH), dimensional stability Measurement was carried out in the width direction of the film under the conditions below, and the average value of three measurement results was taken as the humidity expansion coefficient in the present invention.

[0076] Measurement equipment: Shimadzu Thermomechanical Analyzer TMA-50 (humidity generator: ULVAC-RIKO Humidity Atmosphere Control Device HC-1) Sample size: 10 mm in the film length direction x 12.6 mm in the film width direction Load: 0.5g Number of measurements: 3 Measurement temperature: 30℃ Measurement humidity: After holding at 40% RH for 6 hours, the dimensions were measured, and the humidity was raised to 80% RH over 40 minutes. After holding at 80% RH for 6 hours, the dimensional change ΔL (mm) in the width direction of the support was measured. The humidity expansion coefficient (ppm / % RH) was calculated using the following formula.

[0077] Humidity expansion coefficient (ppm / %RH) = 10 6 ×{(ΔL / 12.6) / (80-40)} The dimensional stability was evaluated based on the following criteria, with C being judged as poor dimensional stability.

[0078] AA: Humidity expansion coefficient is 5.5 ppm / % RH or less A: Humidity expansion coefficient is over 5.5 ppm / %RH and 6.0 ppm / %RH or less B: Humidity expansion coefficient is greater than 6.0 ppm / %RH and less than 6.5 ppm / %RH C: Humidity expansion coefficient exceeds 6.5 ppm / % RH (7) Lamination thickness (μm) Cross-section observation was performed in 10 different fields under the following conditions, and the average of the thicknesses (μm) was calculated and used as the thickness (μm) of layer A.

[0079] Measurement equipment: Transmission electron microscope (TEM) Hitachi H-7100FA Measurement conditions: Accelerating voltage 100 kV Measurement magnification: 10,000 times Sample preparation: ultrathin sectioning Observation surface: TD-ZD cross section (TD: width direction, ZD: thickness direction) Number of measurements: 3 points per field, 10 fields of view.

[0080] (8) Refractive index Measurements were carried out in accordance with JIS-K7142 (2008) using the following measuring instruments.

[0081] Equipment: Abbe Refractometer 4T (manufactured by Atago Co., Ltd.) Light source: Sodium D-line Measurement temperature: 25℃ Measured humidity: 65%RH Mounting solution: methylene iodide (However, when the refractive index was 1.74 or higher, methylene sulfide iodide was used.) Average refractive index n_bar=((nMD+nTD+nZD) / 3) Birefringence Δn=(nMD-nTD) nMD: Refractive index in the longitudinal direction of the film nTD: Refractive index in the film width direction nZD: Refractive index in the film thickness direction (9) Young's modulus The Young's modulus of the film was measured in accordance with ASTM-D882 (1997). An Instron-type tensile tester was used under the following conditions. The average value of five measurement results was taken as the Young's modulus in the present invention.

[0082] Measurement equipment: Instron ultra-precision material testing machine MODEL5848 Sample size: When measuring Young's modulus in the film width direction Film length: 2mm x film width: 12.6mm (Grip spacing is 8mm across the film) When measuring Young's modulus in the longitudinal direction of a film Film width direction 2mm x film length direction 12.6mm (Grip spacing is 8mm in the longitudinal direction of the film) Pulling speed: 1 mm / min Measurement environment: temperature 23℃, humidity 65%RH Number of measurements: 5.

[0083] (10) Runnability Two films, with the A and B sides of the film overlapping, were placed on the substrate, and a 100 g weight (contact area 40 cm) was placed on the film. 2 ) was placed on the substrate. One end of the lower film (in the direction of movement) was fixed to the substrate, and one end of the upper film (opposite the direction of movement) was fixed to the detector. The static friction coefficient (μs) when the substrate was moved 5 mm at a speed of 2 mm / sec was calculated using the following formula. The measurement was performed five times with different films, and the average value was taken as μs.

[0084] The runnability was judged as follows:

[0085] μs = (starting tension) / (100g load) A:μs=0.5 or less B: μs=0.5 or more, 0.6 or less C: μs=0.6 or more (11) Slitting When the film was slit into a width of 1 m, the slitting speed was changed and the sharpness of the film edge was visually evaluated by the method shown below. Note that C was determined to be poor slitting ability.

[0086] AA: Slitting is possible without distortion of the edges even at a speed of 70 m / min.

[0087] A: Distortion occurs at the edge when the speed is between 60m / min and 70m / min.

[0088] B: Distortion occurs at the edge when the speed is between 50 m / min and 60 m / min.

[0089] C: At a speed of less than 50 m / min, wrinkles appear on the film surface and the edges become distorted.

[0090] (12) Missing pulse occurrence frequency The film slit to a width of 1 m was transported under a tension of 200 N, and one surface of the support was coated with magnetic paint and non-magnetic paint as described below, and then slit to a width of 12.65 mm to create a pancake. A length of 200 m from this pancake was then inserted into a cassette to create a magnetic tape.

[0091] (Hereinafter, "parts" means "parts by mass.") Magnetic layer forming coating liquid Barium ferrite magnetic powder 100 parts (Plate diameter: 20.5nm, plate thickness: 7.6nm, Plate ratio: 2.7, Hc: 191kA / m (≒ 2400Oe) Saturation magnetization: 44Am 2 / kg, BET specific surface area: 60m 2 / g) Polyurethane resin 12 parts Mass average molecular weight 10,000 Sulfonic acid functional group 0.5meq / 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 0.5 parts stearic acid Butyl stearate 2 parts 180 parts methyl ethyl ketone 100 parts cyclohexanone Coating liquid for forming non-magnetic layer Non-magnetic powder α-iron oxide 85 parts Average major axis length: 0.09 μm, specific surface area by BET method: 50 m 2 / g Average acicular ratio: 7 DBP oil absorption: 27~38ml / 100g Surface treatment layer: Al2O3 8% by mass Carbon black 15 parts "Conductex" (registered trademark) SC-U (manufactured by Columbian Carbon Co.) Polyurethane resin UR8200 (manufactured by Toyobo Co., Ltd.) 22 parts Phenylphosphonic acid 3 parts 140 parts cyclohexanone 170 parts methyl ethyl ketone Butyl stearate 1 part 2 parts stearic acid For each of the above coating solutions, the components were kneaded in a kneader. The coating solution was pumped through a horizontal sand mill containing 0.5 mm diameter zirconia beads in an amount that filled 65% of the dispersion section volume, and dispersed at 2,000 rpm for 120 minutes (the time the solution essentially remained in the dispersion section). 5.0 parts of polyisocyanate were added to the resulting dispersion for the non-magnetic layer coating, and 2.5 parts for the magnetic layer coating. 3 parts of methyl ethyl ketone were then added, and the mixture was filtered using a filter with an average pore size of 1 μm to prepare the coating solutions for forming the non-magnetic layer and the magnetic layer, respectively.

[0092] The resulting nonmagnetic layer-forming coating solution was applied to a PET film and dried to a dry thickness of 0.8 μm. The magnetic layer-forming coating solution was then applied to a PET film to a dry thickness of 0.07 μm. While the magnetic layer was still wet, it was aligned and dried using a cobalt magnet with a magnetic force of 6,000 G (600 mT) and a solenoid with a magnetic force of 6,000 G (600 mT). A backcoat layer (100 parts carbon black, average particle size: 17 nm; 80 parts calcium carbonate, average particle size: 40 nm; 5 parts α-alumina, average particle size: 200 nm, dispersed in polyurethane resin and polyisocyanate) was then applied to a dry thickness of 0.35 μm. The film was then calendered at 90°C and a linear pressure of 300 kg / cm (294 kN / m), followed by curing at 65°C for 72 hours. Furthermore, the nonwoven fabric and razor blade were attached to a device having a slit 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.

[0093] The magnetic tape cartridge obtained above (total magnetic tape length 500 m) was set in an IBM LTO-7 drive, and the magnetic tape was run 1,500 times back and forth at a tension of 0.55 N and a running speed of 8 m / sec.

[0094] The magnetic tape cartridge after the above run was set in a reference drive (IBM LTO7 drive), and recording and playback were performed by running the magnetic tape. The playback signal during the run was input into an external AD (Analog / Digital) converter, and a signal where the playback signal amplitude dropped by 80% or more compared to the average (average of measurements across all tracks) was defined as a missing pulse. The frequency of this occurrence was divided by the total length of the magnetic tape to determine the missing pulse occurrence frequency per unit length of magnetic tape (unit: times / m). Tapes with a missing pulse occurrence frequency of 2.5 times / m or less were considered highly reliable, and tapes with a missing pulse occurrence frequency of more than 2.5 times / m were considered defective, and the following criteria were used to determine the quality of the magnetic tape.

[0095] AA: Missing pulse occurrence frequency is 1.5 times / m or less A: Missing pulse occurrence frequency is more than 1.5 times / m to 2.0 times / m or less B: Missing pulse occurrence frequency is over 2.0 times / m to 2.5 times / m or less C: Missing pulse occurrence frequency exceeds 2.5 times / m [Example]

[0096] The following examples will illustrate the embodiments of the present invention, in which polyethylene terephthalate is abbreviated as PET, polyethylene naphthalate as PEN, and polyetherimide as PEI.

[0097] (1) Preparation of PET pellets: 194 parts by mass of dimethyl terephthalate and 124 parts by mass of ethylene glycol were charged into an ester exchange reactor, and the contents were heated to 140°C to dissolve. Then, 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 an ester exchange reaction was carried out while distilling off 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 parts by mass of a 5% by mass ethylene glycol solution of sodium dihydrogen phosphate dihydrate (0.015 parts by mass as sodium dihydrogen phosphate dihydrate) were added.

[0098] When the trimethyl phosphate ethylene glycol solution was added, the temperature of the reaction contents dropped. Therefore, the excess ethylene glycol was distilled off while stirring was continued until the temperature of the reaction contents returned to 230°C. After the temperature of the reaction contents in the transesterification reactor reached 230°C in this way, the reaction contents were transferred to the polymerization reactor.

[0099] 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 final pressure, the reaction was continued for 2 hours (3 hours from the start of polymerization), at which point the stirring torque of the polymerization apparatus reached a predetermined value (the specific value varies depending on the specifications of the polymerization apparatus, but the value indicated by polyethylene terephthalate with an intrinsic viscosity of 0.62 in this polymerization apparatus was used as the predetermined value). The reaction system was then purged with nitrogen and returned to normal pressure to stop the polycondensation reaction, and the mixture was discharged into cold water in the form of strands and immediately cut to obtain PET pellets of polyethylene terephthalate with an intrinsic viscosity of 0.62 (raw material-1).

[0100] (1b) Preparation of PEN pellets: 0.025 parts by mass of manganese acetate tetrahydrate and 0.005 parts by mass of sodium acetate trihydrate were added to a mixture of 128 parts by mass of dimethyl 2,6-naphthalenedicarboxylate and 60 parts by mass of ethylene glycol, and a transesterification reaction was carried out while gradually increasing the temperature from 150°C to 240°C. During the reaction, when the reaction temperature reached 170°C, 0.024 parts by mass of antimony trioxide was added. Furthermore, 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. The transesterification reaction continued, and 0.023 parts by mass of trimethyl phosphate was added. The reaction product was then transferred to a polymerization apparatus, heated to 290°C, and subjected to a polycondensation reaction under a high reduced pressure of 30 Pa until the stirring torque of the polymerization apparatus reached a predetermined value (the specific value differs depending on the specifications of the polymerization apparatus, but the value exhibited by polyethylene-2,6-naphthalate with an intrinsic viscosity of 0.62 in this polymerization apparatus was taken as the predetermined value). At this point, the reaction system was purged with nitrogen and returned to normal pressure to stop the polycondensation reaction, and the product was discharged into cold water in the form of strands and immediately cut to obtain PEN pellets (raw material-1b) with an intrinsic viscosity of 0.62.

[0101] (2-a) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned PET pellets (raw material-1) and 10 parts by mass of a 5% by mass water slurry of cross-linked polystyrene particles with an average particle size of 0.30 μm (0.5 parts by mass as cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove moisture, thereby obtaining particle-containing pellets (raw material-2a) containing 0.5% by mass of cross-linked polystyrene particles and having an intrinsic viscosity of 0.62.

[0102] (2-b) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned solid-state polymerized PET pellets (raw material-1k: processing time 2 hours) and 10 parts by mass of a 20% by mass water slurry of cross-linked polystyrene particles with an average particle size of 0.30 μm (2 parts by mass as cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove moisture, thereby obtaining particle-containing pellets (raw material-2b) containing 2% by mass of cross-linked polystyrene particles and having an intrinsic viscosity of 0.62.

[0103] (2-c) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned solid-state polymerized PET pellets (raw material-1) and 10 parts by mass of a 5% by mass water slurry of cross-linked polystyrene particles with an average particle size of 0.15 μm (0.5 parts by mass of cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw extruder heated to 280 ° C. The vent hole was maintained at a reduced pressure of 1 kPa or less to remove water, and particle-containing pellets (raw material-2c) containing 0.5% by mass of cross-linked polystyrene particles and having an intrinsic viscosity of 0.62 were obtained. Furthermore, particle-containing pellets (raw material-2cb) containing 0.5% by mass of cross-linked polystyrene particles were prepared in the same manner, except that raw material-1 was replaced with PEN pellets (raw material-1b).

[0104] (2-d) Preparation of particle-containing PET pellets: 80 parts by mass of the above-mentioned PET pellets (raw material-1) and 20 parts by mass of a 10% by mass water slurry of cross-linked polystyrene particles with an average particle size of 0.45 μm (2 parts by mass as cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove moisture, thereby obtaining particle-containing pellets (raw material-2d) containing 2% by mass of cross-linked polystyrene particles and having an intrinsic viscosity of 0.62.

[0105] (2-e) Preparation of particle-containing PET pellets: 94 parts by weight of the above-mentioned PET pellets (Feedstock-1) and 6 parts by weight of the water slurry (0.3 parts by weight of colloidal silica particles) containing 5% by weight of colloidal silica particles with an average particle size of 0.060 μm and glass beads with an average particle size of 0.05 mm) were fed into a co-rotating, vented twin-screw extruder heated to 280°C. The slurry was stirred at 1,000 rpm for 2 hours, filtered to remove the glass beads, and then fed into a co-rotating, vented twin-screw extruder. The vent was maintained at a reduced pressure of 1 kPa or less to remove water, yielding particle-containing pellets (Feedstock-2e) containing 0.3% by weight of colloidal silica particles and having an intrinsic viscosity of 0.62. Furthermore, particle-containing pellets (Feedstock-2eb) containing 0.3% by weight of colloidal silica particles and having an intrinsic viscosity of 0.62 were prepared in the same manner, except that Feedstock-1 was replaced with PEN pellets (Feedstock-1b).

[0106] (2-f) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned PET pellets (raw material-1) and a 5% by mass aqueous slurry of colloidal silica particles with an average particle size of 0.10 μm, to which glass beads with an average particle size of 0.05 mm were added, were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C. The mixture was stirred at 1,000 rpm for 2 hours, and then filtered to remove the glass beads. 10 parts by mass of the resulting aqueous slurry (0.5 parts by mass as colloidal silica particles) was then fed. The vent hole was maintained at a reduced pressure of 1 kPa or less to remove the water, yielding particle-containing pellets (raw material-2f) containing 0.5% by mass of colloidal silica particles and having an intrinsic viscosity of 0.62.

[0107] (2-g) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned PET pellets (raw material-1) and a 5% by mass aqueous slurry of colloidal silica particles with an average particle size of 0.20 μm, to which glass beads with an average particle size of 0.05 mm were added, were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C. The slurry was stirred at 3,000 rpm for 2 hours, and then filtered to remove the glass beads. 10 parts by mass of the resulting aqueous slurry (0.5 parts by mass as colloidal silica particles) was then fed. The vent hole was maintained at a reduced pressure of 1 kPa or less to remove the water, and particle-containing pellets (raw material-2g) containing 0.5% by mass of colloidal silica particles and having an intrinsic viscosity of 0.62 were obtained.

[0108] (2-h) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned PET pellets (raw material-1) and 10 parts by mass of a 20% by mass water slurry of colloidal silica particles with an average particle size of 0.20 μm (2 parts by mass as colloidal silica particles) were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove moisture, yielding particle-containing pellets (raw material-2h) containing 2% by mass of colloidal silica particles and having an intrinsic viscosity of 0.62.

[0109] (2-i) Preparation of particle-containing PET pellets: 90 parts by mass of the above-mentioned PET pellets (raw material-1) and a 10% by mass water slurry of alumina particles with an average primary particle size of 0.02 μm, with zirconia beads of an average particle size of 0.5 mm, were added to a co-rotating, vented twin-screw extruder heated to 280 ° C., and the mixture was stirred at 3,000 rpm for 2 hours, after which the zirconia beads were removed. 10 parts by mass (1 part by mass of alumina particles) of the resulting water slurry were added. The vent hole was maintained at a reduced pressure of 1 kPa or less to remove water, and particle-containing pellets (raw material-2i) containing 1% by mass of alumina particles and an intrinsic viscosity of 0.62 were obtained. Furthermore, particle-containing pellets (raw material-2ib) containing 1% by mass of alumina particles and an intrinsic viscosity of 0.62 were obtained by replacing raw material-1 with PEN pellets (raw material-1b).

[0110] (2-j) Preparation of particle-containing PEN pellets: 90 parts by mass of the above-mentioned PEN pellets (raw material-1b) and 10 parts by mass of a 5% by mass water slurry of cross-linked polystyrene particles with an average particle size of 0.30 μm (0.5 parts by mass as cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove moisture, thereby obtaining particle-containing pellets (raw material-2j) containing 0.5% by mass of cross-linked polystyrene particles and having an intrinsic viscosity of 0.62.

[0111] (3) Preparation of two-component composition (PET / PEI) pellets: The PET pellets (raw material-1) obtained by the above method and pellets of PEI “Ultem” (registered trademark) 1010 manufactured by SABIC Innovative Plastics were fed into a co-rotating, vented twin-screw kneading extruder (manufactured by The Japan Steel Works, Ltd., screw diameter 30 mm, screw length / screw diameter = 45.5) equipped with three kneading paddle kneading sections heated to a temperature of 280°C, and the pellets were mixed at a shear rate of 100 sec. -1 The mixture was melt-extruded with a residence time of 1 minute to obtain two-component composition pellets containing 50% by mass of PEI. The glass transition temperature of the two-component composition pellets thus prepared was 150°C (raw material-3).

[0112] Example 1 Two extruders, E1 and E2, were used. Extruder E1 was heated to 280°C and contained 33 parts by mass of PET pellets (raw material-1) and 67 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.06 μm (raw material-2e) as the A layer raw materials, which were dried under reduced pressure at 180°C for 3 hours and then fed. Extruder E2, also heated to 280°C, contained 15 parts by mass of PET pellets (raw material-1), 75 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.20 μm (raw material-2g), and 10 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2a) as the B layer raw materials, which were dried under reduced pressure at 180°C for 3 hours and then fed. To laminate these two layers, the lamination thickness ratio (A layer | B layer) was set to 8 | 1 in a T-die, and they were merged so that the B layer side was facing the cast drum surface.Then, while applying a static charge to the cast drum at a surface temperature of 25°C, the layers were cooled and solidified in close contact, producing a laminated unstretched film.

[0113] This laminated unstretched film was stretched 3.1 times in the machine direction (MD) (MD stretch 1) and 3.5 times in the transverse direction (TD stretch 1) at 88°C in a simultaneous biaxial stretching machine. It was then simultaneously stretched 1.1 times in the machine direction (MD stretch 2) and 1.6 times in the transverse direction (TD stretch 2) in a heating zone at 190°C. It was then heat-treated in the heat treatment zone of the tenter at 210°C for 7 seconds, and further relaxed 0.5% in the transverse direction at 130°C. After uniformly cooling to 25°C, the film edges were removed and wound onto a core to obtain a 4.5 μm-thick biaxially oriented polyester film. The resulting biaxially oriented polyester film exhibited good film formation stability, and its physical properties were evaluated, demonstrating its excellent properties when used as a magnetic tape, as shown in the table.

[0114] The following tables show the raw material compositions, film-forming conditions, physical properties of the biaxially oriented polyester films, characteristics of the magnetic tapes, etc. for each of the Examples and Comparative Examples.

[0115] Example 2 Two extruders, E1 and E2, were used. Extruder E1 was heated to 280°C and contained 29 parts by weight of PET pellets (raw material-1), 4 parts by weight of two-component composition pellets (raw material-3), and 67 parts by weight of pellets containing cross-linked polystyrene particles with an average particle size of 0.06 μm (raw material-2d). These were dried under reduced pressure at 180°C for 3 hours and then fed into extruder E2, also heated to 280°C. Extruder E2 was also heated to 280°C and contained 7 parts by weight of PET pellets (raw material-1), 4 parts by weight of two-component composition pellets (raw material-3), 60 parts by weight of colloidal silica particle-containing pellets with an average particle size of 0.2 μm (raw material-2f), 4 parts by weight of cross-linked polystyrene particle-containing pellets with an average particle size of 0.3 μm (raw material-2a), and 25 parts by weight of alumina particle-containing pellets (raw material-2i). A biaxially oriented polyester film with a thickness of 4.5 μm was obtained in the same manner as in Example 1. The resulting biaxially oriented polyester film had good film formation stability, and when its physical properties were evaluated, it had excellent properties when used as a magnetic tape, as shown in the table.

[0116] Example 3 As shown in the table, the B layer raw materials were changed to 4 parts by mass of two-component composition pellets (raw material-3), 64 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.15 μm (raw material-2c), and 32 parts by mass of pellets containing alumina particles (raw material-2i), and the lamination thickness ratio of A and B layers (A layer | B layer) was set to 12 | 1. Except for this, a biaxially oriented polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 2.

[0117] Example 4 A biaxially oriented polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 3, except that the amount of pellets containing colloidal silica particles (raw material - 2 g) with an average particle size of 0.2 μm was changed to 64 parts by mass and the lamination thickness of Layer B was changed.

[0118] Example 5 The A layer raw materials were 33 parts by weight of PEN pellets (raw material-1b) and 67 parts by weight of pellets containing colloidal silica particles with an average particle size of 0.06 μm (raw material-2eb), which were dried under reduced pressure at 180 °C for 3 hours and then fed. The B layer raw materials were 64 parts by weight of pellets containing cross-linked polystyrene particles with an average particle size of 0.15 μm (raw material-2cb), 6 parts by weight of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2j), and 30 parts by weight of alumina particles (raw material-2ib), which were also heated to 280 °C and then fed to extruder E2, which was also heated to 280 °C, after being dried under reduced pressure at 180 °C for 3 hours. These were laminated in a T-die with a lamination thickness ratio of (A layer | B layer) = 8 | 1, and the layers were joined so that the B layer was facing the casting drum surface. The layers were then cooled and solidified while applying a static charge to the casting drum at a surface temperature of 25 °C, producing a laminated unstretched film.

[0119] This laminated unstretched film was stretched 4 times in the longitudinal direction and 4.5 times in the transverse direction at 125°C in a simultaneous biaxial stretching machine, and then simultaneously stretched 1.2 times in the longitudinal direction and 1.6 times in the transverse direction in a heating zone at 170°C. Subsequently, it was heat-treated at 210°C for 7 seconds in the heat treatment zone of a tenter, and further relaxed 0.5% in the transverse direction at 150°C. After uniformly cooling to 25°C, the film edges were removed and the film was wound onto a core to obtain a 4.5 μm thick biaxially stretched polyester film.

[0120] Example 6 The blending amounts of the raw materials for layer A and the particle types and amounts added for layer B were changed to obtain the results shown in the table. The film was stretched 3.0 times in the machine direction (MD) (MD stretching 1) and 3.5 times in the transverse direction (TD stretching 1) at 88°C in a simultaneous biaxial stretching machine. Subsequently, it was simultaneously stretched 1.05 times in the machine direction (MD stretching 2) and 1.4 times in the transverse direction (TD stretching 2) in a heating zone at 190°C. It was then heat-treated in the heat treatment zone of the tenter at 225°C for 7 seconds, and further relaxed 0.5% in the transverse direction at 150°C. After uniformly cooling to 25°C, the film edges were removed and wound onto a core to obtain a 4.5 μm-thick biaxially oriented polyester film. The resulting biaxially oriented polyester film exhibited good film formation stability, and its physical properties were evaluated, demonstrating its excellent properties when used as a magnetic tape, as shown in the table.

[0121] Example 7 The raw materials for layers A and B were the same as those used in Example 2. This laminated unstretched film was stretched 3.1 times in the machine direction (MD) (MD stretch 1) and 3.5 times in the width direction (TD stretch 1) at 88°C in a simultaneous biaxial stretching machine. It was then simultaneously stretched 1.1 times in the machine direction (MD stretch 2) and 1.6 times in the width direction (TD stretch 2) in a heating zone at 190°C. It was then heat-treated for 7 seconds at 225°C in the heat treatment zone of the tenter, and further relaxed 0.5% in the width direction at 150°C. After uniformly cooling to 25°C, the film edges were removed and the film was wound onto a core to obtain a 4.5 μm-thick biaxially oriented polyester film.

[0122] (Comparative Example 1) The raw materials used for Layer B were a blend of 75 parts by weight of PET pellets (raw material-1), 22.5 parts by weight of pellets containing colloidal silica particles with an average particle size of 0.20 μm (raw material-2h), and 2.5 parts by weight of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2b). These were dried under reduced pressure at 180°C for 3 hours and then fed. To form a two-layer laminate, these materials were placed in a T-die with a lamination thickness ratio (Layer A | Layer B) = 8 | 1. The layers were then joined with Layer B facing the casting drum. The resulting laminate was cooled and solidified while applying a static charge to the casting drum at a surface temperature of 25°C, producing a laminated unstretched film. This laminated unstretched film was stretched 3.5 times in the longitudinal direction at 88°C using a roll-type stretching machine. The resulting uniaxially stretched film was held at both ends with clips and introduced into a tenter preheating zone at 90°C. It was then continuously stretched 3.5 times in the width direction (TD) perpendicular to the machine direction in a heating zone at 90°C (TD stretch 1), and then further stretched 1.6 times in the width direction in a heating zone at 190°C (TD stretch 2). The film was then heat-treated in the tenter heat treatment zone at 190°C for 10 seconds, and then relaxed 0.5% in the width direction at 150°C. After uniformly cooling to 25°C, the film edges were removed and the film was wound onto a core to obtain a 4.5 μm-thick biaxially stretched polyester film.

[0123] (Comparative Example 2) A biaxially oriented polyester film having a thickness of 4.5 μm was obtained in the same manner as in Example 2, except that the B layer raw materials were changed as shown in the table: 55.5 parts by mass of PET pellets (raw material-1), 4 parts by mass of two-component composition pellets (raw material-3), 40 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2a), and 0.5 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.45 μm (raw material-2d).

[0124] (Comparative Example 3) The raw materials used to change the blending amount of the B layer raw materials were 22.5 parts by mass of PET pellets (raw material-1), 4 parts by mass of two-component composition pellets (raw material-3), 66 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.20 μm (raw material-2g), and 7.5 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2b), and a biaxially oriented polyester film with 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 220°C.

[0125] Comparative Example 4 The particle raw materials used for Layer B were 4 parts by mass of two-component composition pellets (Raw Material-3), 66 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.10 μm (Raw Material-2f), and 30 parts by mass of pellets containing alumina particles (Raw Material-2i), and the lamination thickness ratio of Layers A and B (Layer A|Layer B) was set to 3.5|1, and TD stretching 2 was changed to 1.4 times. A biaxially oriented polyester film with a thickness of 4.5 μm was obtained in the same manner as in Example 2, except that slow cooling within a specific temperature range after heat setting was not performed.

[0126] (Comparative Example 5) The B layer raw materials were a mixture of 61.5 parts by mass of PET pellets (raw material-1), 4 parts by mass of two-component composition pellets (raw material-3), 1.5 parts by mass of pellets containing cross-linked polystyrene particles with an average particle size of 0.30 μm (raw material-2b), and 33 parts by mass of alumina particles (raw material-2i), and a biaxially oriented polyester film 4.5 μm thick was obtained in the same manner as in Example 2, except that slow cooling within a specific temperature range was not performed after the heat setting treatment.

[0127] (Comparative Example 6) 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 amount of particles added to the B layer was changed as shown in the table.

[0128] [Table 1]

[0129] [Table 2]

[0130] [Table 3]

Claims

1. The number of protrusions having a height of 60 nm or more on the surface of at least one outermost layer of the biaxially oriented polyester film is 25,000 to 60,000 / mm ２ The average area of ​​the cut surface of the protrusions at a height of 60 nm is 0.03 to 0.18 μm ２ The biaxially oriented polyester film has a laminated structure of two or more layers, and has at least one layer (layer B) containing 0.005 to 0.8 mass % of inert particles having an average particle size of 0.01 to 0.3 μm or less, and layer B is formed by blending particle master pellets having a particle content of 0.1 to 0.5 mass % with 0 to 15 mass % of resin pellets that do not substantially contain particles.

2. On the outermost layer surface (side B) having the characteristics of claim 1, the area of ​​the cut surface of the protrusions at a height of 60 nm is 1 μm ２ More than 100 protrusions / mm ２ 2. The biaxially oriented polyester film of claim 1, wherein:

3. The biaxially oriented polyester film according to claim 2, wherein the surface roughness (SRa) of side B is 3 to 15 nm.

4. The biaxially oriented polyester film according to any one of claims 1 to 3, wherein the humidity expansion coefficient in the width direction is 0 to 6.5 ppm / % RH.

5. 5. The biaxially oriented polyester film according to claim 1, wherein the surface roughness (SRa) of the outermost surface (side A) opposite to side B is 0.5 to 5 nm.

6. The biaxially oriented polyester film according to any one of claims 1 to 5, which is used as a base film for a coating-type digital recording medium.

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