Biaxially oriented polyester film

Abstract

To stably provide a biaxially oriented polyester film having excellent dimensional stability in all temperature-humidity ranges of low temperature-low humidity, low temperature-high humidity, high temperature-low humidity, and high temperature-high humidity and both of excellent surface smoothness and taking-up ability to deal with a thinner support medium.SOLUTION: At least one outermost layer surface (B surface) of a biaxially oriented polyester film has protrusions of a height of 30 nm or more of over 200 thousands / mm2 and 500 thousands / mm2 or less and protrusions of a height of 100 nm or more of 4 thousands / mm2 or more and 10 thousands / mm2 or less. The surface (B surface) has a surface average roughness SaB larger than the surface average roughness SaA of the other outermost layer surface (A surface). The biaxially oriented polyester film has a film thickness of 3.0-4.2 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyester film, and more particularly to a biaxially oriented polyester film 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. Next-generation magnetic recording media will always require higher recording densities.

[0003] Generally, there are three ways to achieve high density recording: increasing the number of tracks, shortening the recording wavelength, and increasing the tape length.

[0004] Increasing the number of tracks narrows the width of each track, so even slight dimensional changes can cause data loss, making it important to control dimensional stability across the tape's width. Such dimensional changes can be divided into irreversible changes, such as thermal shrinkage, and reversible changes, such as expansion and contraction due to temperature and humidity. While it is preferable to eliminate irreversible changes, they can be eliminated through processing such as annealing. In contrast, reversible changes cannot be easily eliminated, and because they cannot be easily eliminated, changes in temperature and humidity during storage can cause the film to expand or contract, displacing the recorded data from its intended location and making it unreadable.

[0005] Furthermore, in order to achieve sufficient electromagnetic conversion characteristics while shortening the recording wavelength, surface smoothness is required, and it is effective to further improve the smoothness of the magnetic layer surface by making the magnetic layer thinner or using fine particle magnetic material.

[0006] Increasing the tape length per reel of magnetic recording media and achieving higher recording densities requires thinner magnetic layers, nonmagnetic layers, backcoat layers, and even the support itself. Thinning the support limits not only the smoothness of the surface but also the roughness of the running surface. When magnetic recording media are stored in roll form during the manufacturing process, protrusions formed on the running surface can transfer to the magnetic surface, forming depressions on the smooth magnetic layer surface. Furthermore, as the support becomes thinner, larger particles contained in the support can be pushed up against the smooth surface, creating a gentle convex undulation on the magnetic layer surface, reducing the smoothness of the magnetic layer surface and generating error signals (missing pulses). Furthermore, when used in next-generation data tapes (e.g., LTO10), heads compatible with high smoothness and high capacity are adopted, and the narrower track density of the head improves sensitivity. It has been discovered that protrusions at levels that were not previously a problem can cause error defects. Accordingly, demands for smoother magnetic layers and backcoat layers are becoming increasingly stringent, and error defects have yet to be reduced. Furthermore, it has been pointed out that as the support becomes thinner, the stiffness of the film decreases, which makes it difficult to wind up, and therefore there is an increasing demand for a support that can achieve both a smooth surface and good winding properties.

[0007] To solve the above problems, polyester films with excellent winding properties, dimensional stability, and error defects have been investigated (for example, Patent Document 1). Also under investigation is a polyester film containing fine particles to control the roughness of the film surface and the height and number of protrusions, thereby suppressing transfer to the magnetic layer surface (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2020-163844 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-200927 Summary of the Invention [Problem to be solved by the invention]

[0009] However, even with the use of thin magnetic layers or fine particle magnetic materials such as ferromagnetic hexagonal ferrite or epsilon iron oxide powder, the support surface has not been able to ensure the smoothness of the magnetic layer surface useful for next-generation data tapes (e.g., LTO10), and the winding properties are insufficient to accommodate thin support thickness. Furthermore, no consideration has been given to thermal expansion, and no film has been developed that exhibits excellent dimensional change across all temperature and humidity ranges, including room temperature and room humidity, low temperature and low humidity, low temperature and high humidity, high temperature and low humidity, and high temperature and high humidity.

[0010] The object of the present invention is to stably provide a biaxially oriented polyester film that has excellent dimensional stability over all temperature and humidity ranges, including low temperature and low humidity, low temperature and high humidity, high temperature and low humidity, and that also has excellent surface smoothness and windability that can accommodate thinner supports. [Means for solving the problem]

[0011] The present invention, which has been made to solve the above problems, is characterized by the following configurations.

[0012] (1) On at least one outermost surface (side B) of the biaxially oriented polyester film, The thickness of the outermost layer (layer B) is less than 0.5 μm, The number of protrusions with a height of 30 nm or more is 200,000 / mm 2 Over 500,000 pieces / mm 2 The number of protrusions with a height of 100 nm or more is 4,000 or more, 10,000 / mm 2 or less, and the average surface roughness SaB of said layer is greater than the average surface roughness SaA of the other outermost layer surface (side A), and the film thickness is 3.0 μm or more and 4.2 μm or less. (2) On the surface of the outermost layer (side B) having the characteristics described in (1), the number of protrusions having a height of 50 nm or more is 30,000 / mm 2 Over 60,000 pieces / mm 2 The biaxially oriented polyester film according to (1), which is: (3) On the surface of the outermost layer (side B) having the characteristics described in (1), the number of protrusions having a height of 60 nm or more is 2,000 / mm 2 Over 30,000 pieces / mm 2 A biaxially oriented polyester film according to (1) and (2), which is: (4) The biaxially oriented polyester film according to any one of (1) to (3), wherein the static friction coefficient μs when the A surface and the B surface are superposed is 0.2 or more and 1.5 or less. (5) A biaxially oriented polyester film according to any one of (1) to (4), wherein the outermost layer (layer B) contains particles S having a number average particle diameter of 0.03 μm or more and less than 0.1 μm and particles L having a number average particle diameter of 0.1 μm or more and less than 0.3 μm in a blending ratio (L / S) of 0.01 to 0.35. ( 6 ) The humidity expansion coefficient in the width direction is 4.5 to 6.5 ppm / % RH, (1) to ( 5 ) The biaxially oriented polyester film according to any one of the preceding claims. ( 7 ) The thermal expansion coefficient in the width direction is -3.0 to 4.0 ppm / ℃, (1) to ( 6 ) The biaxially oriented polyester film according to any one of the preceding claims. ( 8 ) (1) to (( 7 ) The biaxially oriented polyester film according to any one of the preceding claims. [Effects of the Invention]

[0013] The biaxially oriented polyester film of the present invention is a biaxially oriented polyester film that has dimensional stability over all temperature and humidity ranges, including low temperature and low humidity, low temperature and high humidity, high temperature and low humidity, and high temperature and high humidity, as well as excellent surface smoothness and windability that can accommodate thinner film thicknesses.When made into magnetic recording media, particularly next-generation data tapes that are compatible with thinner non-magnetic layers (e.g., tapes from LTO10 onwards), it is possible to obtain a biaxially oriented polyester film that serves as a high-density magnetic recording medium that has a smooth magnetic surface, exhibits little dimensional change due to environmental changes in temperature and humidity or due to storage, and suppresses the occurrence of error defects such as missing pulses. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram of a film dimension measuring device. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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).

[0016] 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.

[0017] 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.

[0018] 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.

[0019] The copolymerization ratio of the polymer can be investigated using NMR (nuclear magnetic resonance) or microscopic FT-IR (Fourier transform microscopic infrared spectroscopy).

[0020] 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.

[0021] 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.

[0022] [ka]

[0023] (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:

[0024] [ka]

[0025] (n is an integer of 2 or more, preferably an integer of 20 to 50.) 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.

[0026] [ka]

[0027] or

[0028] [ka]

[0029] 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."

[0030] On at least one outermost layer surface of the biaxially oriented polyester film of the present invention (hereinafter, such surface may be referred to as "side B" and the layer having such side B may be referred to as "layer B"), the number of protrusions having a height of 30 nm or more is 200,000 / mm 2 Over 500,000 pieces / mm 2 or less, preferably 250,000 to 450,000 particles / mm 2 The number of protrusions with a height of 30 nm or more is 500,000 / mm 2If the number of protrusions with a height of 30 nm or more exceeds the lower limit of 200,000 / mm, they may cause errors or defects when used as a magnetic recording medium. 2 Below this level, it may not be possible to obtain a static friction coefficient μs or winding properties that are suitable for thinner films.

[0031] The number of protrusions having a height of 100 nm or more on at least one outermost layer surface (side B) of the biaxially oriented polyester film of the present invention is 4,000 / mm 2 More than 10,000 pieces / mm 2 Preferably, the number is 4,000 / mm or less. 2 More than 0.8 million pieces / mm 2 The number of protrusions with a height of 100 nm or more is 10,000 / mm 2 If this value is exceeded, when the tape is wound into a next-generation data tape such as LTO10 or later, the irregularities on the surface of the backcoat layer will be transferred to the surface of the magnetic layer, causing transfer marks and reducing the number of missing pulses and error defects.

[0032] The number of protrusions having a height of 30 nm or more or 100 nm or more in the present invention is the number of protrusions at a height threshold of 50 nm or 100 nm from the reference plane determined by atomic force microscope measurement in the measurement method described below.

[0033] The number of protrusions having a height of 50 nm or more on at least one outermost surface (side B) of the biaxially oriented polyester film of the present invention is 30,000 / mm 2 Over 60,000 pieces / mm 2 It is preferable that the number of particles is less than 30,000 / mm 2 Over 55,000 pieces / mm 2 or less, and more preferably 33,000 particles / mm 2 Over 50,000 pieces / mm 2 The number of protrusions with a height of 50 nm or more is 60,000 / mm 2 If the value exceeds the lower limit, errors and defects may increase when used as a next-generation data tape for LTO10 or later. Also, if the value is below the lower limit, the winding properties may be insufficient to accommodate thinner films.

[0034] The number of protrusions having a height of 60 nm or more on at least one outermost layer surface (side B) of the biaxially oriented polyester film of the present invention is 2,000 / mm 2 Over 30,000 pieces / mm 2 It is preferable that the number of particles is 5,000 particles / mm or less, and more preferably 5,000 particles / mm 2 Over 25,000 pieces / mm 2 Less than 8,000 particles / mm 2 Over 20,000 pieces / mm 2 The number of protrusions with a height of 60 nm or more is 30,000 / mm 2 If the value exceeds the lower limit, the error defect may decrease when used as a next-generation data tape for LTO10 or later. Also, if the value is below the lower limit, the winding property may be insufficient to accommodate thinner films.

[0035] The surface roughness of at least one outermost layer surface (side B) of the biaxially oriented polyester film of the present invention is greater than the surface roughness of the other outermost layer surface (hereinafter, this surface may be referred to as side A, and the layer having side A may be referred to as layer A), and the surface roughness (SRaB) of the outermost layer surface (side B) is preferably 2.0 to 8.0 nm, more preferably 2.5 to 7.0 nm. If the surface roughness is below the lower limit, winding properties may deteriorate. If the surface roughness exceeds the upper limit, the number of protrusions having a height of 50 nm or more or 100 nm or more may not be controlled within the range of the present invention.

[0036] Furthermore, the surface roughness (SaRA) of the outermost layer surface (side A) is preferably 0.5 to 3.0 nm, more preferably 0.5 to 2.5 nm. The outermost layer surface (side A) functions as a surface responsible for smoothness, and when used to form a magnetic recording medium, it is preferable to provide a magnetic layer on the side A in order to solve the problems of the present invention. If the surface roughness of the outermost layer surface (side A) is outside the above range, it may be difficult to satisfy missing pulse and error defects when used to form a magnetic recording medium that is compatible with thinner non-magnetic layers.

[0037] The static friction coefficient μs when at least one of the outermost layer surfaces (side B) and the outermost layer surface (side A) of the biaxially oriented polyester film of the present invention is superimposed is preferably 0.2 to 1.5, more preferably 0.2 to 1.2 or less. A static friction coefficient μs within this range is preferred because it stabilizes winding properties in response to thinner films.

[0038] The humidity expansion coefficient in the width direction of the biaxially oriented polyester film of the present invention is preferably 4.5 to 6.5 ppm / %RH, more preferably 5.0 to 6.3 ppm / %RH. If the humidity expansion coefficient is greater than 6.5 ppm / %RH, the film may expand in the width direction in response to humidity changes in the magnetic data recording and playback environment, which may lead to playback defects. If the humidity expansion coefficient is less than 4.5 ppm / %RH, it may be difficult to control the temperature expansion coefficient of the film within the range of the present invention.

[0039] The temperature expansion coefficient in the width direction of the biaxially oriented polyester film of the present invention is preferably -3.0 to 4.0 ppm / °C, more preferably -1.0 to 3.0 ppm / °C. If the temperature expansion coefficient is below -3.0 ppm / °C, the film will shrink significantly in the width direction in response to temperature changes in the environment in which magnetic data is recorded and reproduced. On the other hand, the magnetic head expands in response to temperature changes (typically 7.0 ppm / °C), which can lead to excessive dimensional changes between the two, resulting in playback problems. If the temperature expansion coefficient exceeds 4.0 ppm / °C, the humidity expansion coefficient cannot be controlled within the range of the present invention, and the film will expand in the width direction in response to temperature and humidity changes in the environment in which magnetic data is recorded and reproduced, which can lead to playback problems.

[0040] The heat shrinkage ratio (100°C for 30 minutes) in the width direction of the biaxially oriented polyester film of the present invention is preferably 1.0% or less. If the heat shrinkage ratio in the width direction exceeds 1.0%, the film will shrink in the width direction during the processing step of the magnetic recording medium, which may cause wrinkles to form on the surface of the magnetic recording medium or reduce the smoothness of the magnetic layer surface.

[0041] The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the width direction of 7.0 to 12.0 GPa, and more preferably 7.0 to 11.0 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 against environmental changes during recording and reproduction of the magnetic recording medium tends to be good. The Young's modulus in the width direction 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.

[0042] The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the longitudinal direction of 3.5 to 8.0 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 the tension during storage will be improved. A more preferred range for the Young's modulus in the longitudinal direction is 3.8 to 7.0 GPa, and an even more preferred range is 4.0 to 6.0 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.

[0043] The number of protrusions of various heights, 30 nm or more, on Side B of the present invention can be controlled by adjusting the average particle size and amount of particles contained in Layer B, the widthwise stretching conditions, the heat setting temperature, and the lamination thickness of Layer B. In particular, it is preferable to add 0.01 to 0.15 wt% of particles L, whose number-average particle size is 0.1 μm or more and less than 0.3 μm, preferably 0.1 μm or more and less than 0.2 μm. Furthermore, it is preferable to add 0.4 to 1 wt% of particles S, whose number-average particle size is 0.03 to less than 0.1 μm. It is particularly preferable to use two types of particles L and S with different particle sizes in combination. In this case, it is particularly preferable to combine particles L and S so that the particle size ratio (particles L / particles S) is in the range of 2 to 6, preferably 2.3 to 5, in order to simultaneously suppress errors and ensure winding properties suitable for thin films. Furthermore, adjusting the particle blend ratio (particles L / particles S) to 0.01 to 0.35 is particularly preferred for obtaining a biaxially oriented polyester film of the present invention that combines surface properties and windability compatible with thinner films. If the average particle size or amount of added particles L exceeds the upper limit, the number of protrusions 100 nm or more in height increases, which may worsen error defects when used as a magnetic recording medium. If the amount of added particles B is below the lower limit, the number of protrusions 30 nm or more in height may not be controlled within the range of the present invention, and stable winding compatible with thinner films may not be possible. The lamination thickness of layer B is less than 0.5 μm, preferably 0.3 μm or less, and more preferably 0.20 to 0.28 μm. If the upper limit is exceeded, the number of protrusions 100 nm or more in height may not be controlled within the range of the present invention.

[0044] The thickness of the biaxially oriented polyester film of the present invention is in the range of 3.0 μm or more and 4.2 μm or less. The preferred thickness range is 3.0 to 4.0 μm. If the thickness is less than 3.0 μm, rigidity and dimensional stability deteriorate, resulting in insufficient tape stiffness and poor electromagnetic conversion characteristics when used as a magnetic recording medium. Furthermore, it becomes difficult to suppress the particles contained in Layer B from pushing up toward the smooth surface (Side A). Furthermore, if the thickness is greater than 4.2 μm, the tape length per reel becomes short, making it difficult to respond to the miniaturization and high-capacity trends of magnetic tapes from LTO10 onward. 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 adjustment of the film thickness after biaxial stretching.

[0045] Regarding width direction stretching conditions, high-temperature stretching in two or more stages during width direction re-stretching is effective for controlling the number of protrusions of various heights (30 nm or more) within the range of the present invention. Furthermore, it is also important to perform stepwise high-temperature heat setting in two or more stages within the range of 190 to 235°C. Protrusion height and surface roughness tend to increase with increasing heat setting temperature. However, by employing stepwise high-temperature heat setting, it is possible to suppress increases in protrusion height and surface roughness due to high-temperature heat setting. Furthermore, setting the heat setting temperature too high can easily cause orientation relaxation, which may make it impossible to control the humidity expansion coefficient in the width direction. However, by employing stepwise high-temperature heat setting, orientation relaxation, particularly in the width direction, is gradual, making it possible to control the humidity expansion coefficient in the present invention within a preferred range. Because the film is thin, tearing may occur during high-temperature heat setting. To prevent tearing, it is preferable to perform a width direction relaxation treatment (relaxation) in the range of 0.1 to 0.3% during the first stage of heat setting after re-stretching is completed.

[0046] 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 outermost surface (side B) of the above-mentioned film in order to obtain a high-density magnetic recording medium. Magnetic recording media using ferromagnetic hexagonal ferrite powder or epsilon iron oxide, which are finely divided magnetic particles for high-density recording, in the magnetic layer tend to have thinner base films, as well as thinner magnetic layers, nonmagnetic underlayers, and BC layers themselves. In particular, when the thickness of the BC layer or nonmagnetic underlayer is reduced to 0.5 μm or less, the BC layer surface becomes susceptible to protrusions caused by the support, which reduces the smoothness of the BC layer surface. Furthermore, when the magnetic recording medium is wound up, the function of the nonmagnetic underlayer, which acts as a cushion layer, may not be fully realized. By specifying the number of protrusions of a specific height on the surface of the B layer within the range of the present invention, excellent electromagnetic conversion characteristics with a low frequency of small error defects such as missing pulses can be achieved without compromising the surface smoothness of even thinner BC layers and nonmagnetic underlayers.

[0047] The biaxially oriented polyester film of the present invention as described above can be produced, for example, as follows, but the biaxially oriented polyester film of the present invention is not limited to films obtained by such a method.

[0048] 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.

[0049] 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 features of the present invention.

[0050] The sheet is then biaxially stretched in both the longitudinal and width directions, and then stretched again in the width direction and heat-treated. To control the number of surface protrusions within the preferred range of the present invention and to control the dimensional stability in the width direction within the preferred range of the present invention, the stretching process preferably involves multi-stage stretching in both the longitudinal and transverse directions and re-stretching in the width direction, and further includes multi-stage heat setting. Re-stretching in the width direction is preferred because it facilitates the production of a high-strength film that is optimal for magnetic tape with high dimensional stability. However, re-stretching in the width direction tends to increase the size of voids around the particles and the protrusion height, making it difficult to control the number of protrusions of various heights of 30 nm or more according to the present invention. To obtain the preferred surface characteristics of the present invention, it is preferable to adopt the width direction stretching temperature and stretching ratio described below, as well as multi-stage heat setting temperature, since this not only achieves the characteristic features of the present invention but also improves dimensional stability.

[0051] As the stretching method, a sequential biaxial stretching method in which stretching is performed in the longitudinal direction and then in the width direction in two stages, or a simultaneous biaxial stretching method in which stretching is performed in the width direction afterwards, is preferred.

[0052] The film manufacturing method of the present invention will be described below using polyethylene terephthalate (PET) as a representative example of the polyester. 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 can be carried out at temperatures higher than those shown below.

[0053] 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.

[0054] 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. To control the number of protrusions with a height of 100 nm or more within the range of the present invention, it is important to maintain the particle slurry concentration at 20% by mass or less. When the particles to be added are inorganic, 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.

[0055] Next, it is particularly important to directly mix the aqueous particle slurry with PET pellets and knead them into PET using a vented twin-screw kneading extruder so that the particle content is 0.3 to 1.0 mass % to prepare particle master pellets. 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 contains substantially no particles during film formation to adjust the particle content. This is particularly important because it also leads to reduced film breakage due to high-temperature heat setting. Furthermore, when two or more types of particle master pellets are used, it is preferable to first blend them to a predetermined desired content and then melt-knead them again using a two-stage kneaded particle master pellet to improve particle dispersibility, as this reduces film breakage during stepwise high-temperature stretching.

[0056] 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.

[0057] Next, we will explain a method in which the unstretched film obtained in this manner is stretched in the longitudinal direction (MD stretching) using a longitudinal stretching machine equipped with several rolls by utilizing the difference in peripheral speed of the rolls, and then stretched transversely (TD stretching 1) using a stenter, and then stretched again in the width direction (TD stretching 2).

[0058] 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.

[0059] First, an unstretched film is stretched in the MD direction. 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. A temperature range of Tg + 3 to Tg + 30°C is preferred, and Tg + 5 to Tg + 20°C is more preferred. A stretching temperature lower than this range can result in frequent film tearing, reduced productivity, and difficulty in stably stretching the film again in the width direction after biaxial stretching. The MD stretching ratio is a total stretching ratio of 3.1 to 6.0 times, preferably 3.2 to 5.5 times, and is divided into two or more stages, preferably three stages, for multistage stretching. A preferred example of MD multistage stretching is to perform the first MD stretching 1 at 1.05 to 1.5 times, the second MD stretching 2 at 1.3 to 2.5 times, and the third MD stretching 3 at 1.02 to 1.8 times, which is preferred because it facilitates suppressing an increase in projection height during re-stretching in the width direction.

[0060] Next, TD stretching is carried out using a stenter. The stretching temperature in TD stretching 1 is Tg+10 to Tg+20° C., and the stretching ratio is 3.1 to 6 times, preferably 3.3 to 4.8 times.

[0061] Next, the film is stretched again in the TD direction (TD stretching 2 or TD stretching 3). The stretching temperature for TD stretching 2 is 170 to 185°C, preferably 175 to 180°C. The stretching ratio for TD stretching 2 is preferably 1.05 to 1.2 times. The stretching temperature for TD stretching 3 is 185 to 200°C, preferably 185 to 195°C. The stretching ratio is 1.2 to 1.6 times, preferably 1.3 to 1.5 times. If the stretching ratio or stretching temperature for TD stretching 2 or 3 is outside the range of the present invention, it may not be possible to control the temperature-humidity expansion coefficient within the range of the present invention. The area stretching ratio is 12 to 30 times, preferably 14 to 25 times. If the area stretching ratio is outside the range of the present invention, it may be difficult to achieve both the temperature-humidity expansion coefficient and the area stretching ratio. If the area stretching ratio exceeds 30 times, it may not be possible to control the temperature-humidity expansion coefficient within the range of the present invention. When TD stretching 2 is performed, stepwise change of the stretching temperature to a higher temperature is effective in controlling the number of protrusions of various heights of 30 nm or more within a specific range according to the present invention. In the present application, re-stretching is performed in the TD direction to keep the temperature-humidity expansion coefficient within a specific range, but the height of the surface protrusions tends to further increase by this re-stretching in TD stretching 2 and TD stretching 3, and in particular the height of the protrusions caused by particle L tends to increase. Therefore, when TD stretching 2 and TD stretching 3 are performed, it is important to perform the stretching temperature in a stepwise manner while changing it to a higher temperature in order to control the number of protrusions of various heights of 30 nm or more within the range according to the present invention.

[0062] The stretched film is heat-set under tension or while relaxed in the width direction. Heat-set temperature conditions include a heat-set temperature of 190 to 235°C, preferably 200 to 230°C. Dividing the heat-set temperature into two or more stages for stepwise high-temperature heat-set makes it easier to achieve the desired temperature-humidity expansion coefficient and control the number of protrusions of 30 nm or more within the range of the present invention, and is also preferred from the viewpoints of surface roughness and film breakage. The first-stage heat-set temperature (HS1) is preferably (stretching temperature of TD stretching 2 or 3 + 3°C) to (stretching temperature of TD stretching 2 or 3 + 20°C). The second-stage heat-set temperature (HS2) is preferably in the range of (HS1 + 5°C) to (HS1 + 15°C). The third-stage heat setting temperature (HS3) is preferably in the range of (HS2) to (HS2+10°C), more preferably (HS2+2°C) to (HS2+10°C). The treatment time is preferably in the range of 0.5 to 10 seconds, and the relaxation rate is preferably 0.8 to 2.0%. The film is then introduced into a cooling zone adjusted to 130 to 150°C and slowly cooled. The film edges are then 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 first-stage heat setting temperature. If the heat setting temperature is higher than the above-mentioned 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 tends to decrease, which reduces the flatness of the base film during the magnetic recording medium manufacturing process and tends to deteriorate the electromagnetic conversion characteristics. Furthermore, if the film is rapidly cooled to room temperature without slowly cooling within a specific temperature range after heat setting, the characteristic aspects of the present invention may not be obtained.

[0063] If the heat setting temperature is higher than the above range, the film tends to relax, making it difficult to obtain the humidity expansion coefficient of the present invention, and dimensional stability tends to decrease. Furthermore, the surface smoothness may decrease, making it difficult to obtain the surface roughness of the present invention. On the other hand, if the heat setting temperature is too low, the dimensional stability in the width direction may not be controlled within the range of the present invention, or the crystallinity may decrease, which tends to reduce the flatness of the base film during the manufacturing process of the magnetic recording medium, increasing the frequency of missing pulses and error defects.

[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 of the substrate using an extrusion coater to a thickness of 0.3 to less than 0.5 μm. After drying, a magnetic coating material is 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. Here, the composition of the magnetic paint may be, for example, as follows:

[0066] Hereinafter, when simply referring to "parts", it means "parts by mass".

[0067] [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 42 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 (LTO tape (recording capacity of 20 terabytes or more after LTO10))) and for recording digital images such as video).

[0068] 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.

[0069] (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.

[0070] (1) Number of protrusions with heights of 30 nm or more, 50 nm or more, 60 nm or more, and 100 nm or more (number / mm2 ) 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 images of the film surface were subjected to only the flattening process described below, and then the Particle Analysis analysis mode was set as follows. The reference plane in each measurement visual field is the plane with a height of 0 nm determined under the flattening process conditions described below. The number of protrusions obtained from this reference plane (value in the Mean column - Total Count row) was calculated by setting the threshold protrusion height (Threshold Height) to 30 nm. 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 to be 1 mm2. 2 The number converted per unit is taken as the number of protrusions with a height of 30 nm or more. Next, the threshold value of the protrusion height is set to the desired height (50 nm, 60 nm, 100 nm), and the number of protrusions with various heights or more is determined in the same manner. [AFM measurement conditions] Equipment: Bruker atomic force microscope (AFM) Dimension Icon with ScanAsyst Cantilever: Silicon nitride probe ScanAsyst Air Scan mode: ScanAsyst Scanning speed: 0.8Hz Scanning direction: Scanning is performed in the width direction of the measurement sample prepared using the method described below. Measurement field of view: 50 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 50 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: 30nm 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 -No dilate / erode operations are performed. (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.

[0071] (2) Surface roughness (SRaA, SRaB) After only the flattening process was performed in the measurement of (1) above, the roughness analysis mode was selected and Ra was determined under the following setting conditions: 20 fields of view were measured for each of the A and B surfaces, and the average values were taken as the surface roughness SRaA of the A surface and the surface roughness SRaB of the B surface. [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 (3) Coefficient of static friction (μs) when surfaces A and B are overlapped A film that has been conditioned for 24 hours or more in an environment of 23°C and 65% RH is placed with the A-side facing downwards, and one end of the film (the side opposite the detector) is fixed to the substrate. Another film is placed on top of it with the A-side facing downwards, and one end of the upper film is fixed to the U-gauge of the detector. Next, a 200 g weight (contact area 6.5 x 6.5 cm) is placed on top of the overlapping films. 2 ) was placed on the film, and after 5 seconds the substrate was moved 10 mm at a speed of 2 mm / sec. The static friction coefficient (μs) was calculated using the following formula. The measurement was carried out 10 times with different films in an environment of 23°C and 65% RH, and the average value was used as the static friction coefficient μs when surfaces A and B were overlapped.

[0072] Sample size: MD: 100cm TD: 7.5cm · Movement direction: MD direction μs = (starting tension) / (100g load).

[0073] (4) Humidity expansion coefficient in the width direction (ppm / %RH) 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.

[0074] Measuring device: Okura Industry Co., Ltd. Tape elongation tester 1TTM1 Constant temperature and humidity chamber: Kato Corporation TP Instrumentation Version 1.04 Sample size: Width 10mm x Length 200mm (sample length) Load: 10g Number of measurements: 3 Temperature: 30℃ Humidity conditions: <1> 40%RH for 6 hours <2> Maintain 80%RH for 6 hours (humidity increase rate: 0.7%RH / min) <3> Dehumidify to 40%RH (humidity decrease rate: 1.4%RH / min) and hold for 6 hours <4> Humidification to 80%RH (humidification rate: 0.7%RH / min) Measurement (L 40):Humidity conditions <3> Measure the sample length after keeping it at 40%RH for 6 hours (L 40 ) Measurement (L 80 ):Humidity conditions <4> The sample length was measured when the humidity was increased from 40%RH to 80%RH (humidity increase rate: 0.7%RH / min). 80 ).

[0075] ΔL(mm): Dimensional change amount ΔL(L 80 -L 40 ) The humidity expansion coefficient (ppm / %RH) was calculated using the following formula: Humidity expansion coefficient (ppm / %RH) = 10 6 ×{(ΔL / (sample length before measurement × humidity difference)} Sample length before measurement = 200 (mm) Humidity difference = 40% (40 to 80% RH) Humidity conditions: After holding for 1 hour at 20% RH and then holding for 1 hour at 80% RH, 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.

[0076] Humidity expansion coefficient (ppm / %RH) = 10 6 ×{(ΔL / 200) / (80-40)}.

[0077] (5) Thermal expansion coefficient in the width direction (ppm / ℃) Sample size: 4 mm in the longitudinal direction of the film x 12.6 mm in the transverse direction of the film Measuring device: Micro constant force thermal dilatometer manufactured by Rigaku Co., Ltd. Measurement mode: Constant rate temperature rise measurement Measurement temperature range: 10℃~50℃ Heating rate: 2°C / min Measurement atmosphere: Nitrogen Reference: Quartz glass Load: 0.5g (compression load) Measurement direction: Width direction (TD) Calculation formula: Temperature expansion coefficient = {(ΔL / L0) T1 -(ΔL / L0) T2} / ΔT(T1-T2) *ΔL is the expansion length, L0 is the initial sample length, and ΔT is the temperature difference.

[0078] (6) Lamination thickness (μm) Using a secondary ion mass spectrometer (SIMS), the concentration ratio (M + / C + ) is taken as the particle concentration, and analysis is performed in the thickness direction from the surface to a depth of 5,000 nm. The particle concentration is low in the surface layer due to the surface being an interface, and increases as one moves away from the surface. In the case of the film of the present invention, the particle concentration once reaches a maximum value, but then begins to decrease again. Based on this concentration distribution curve, the depth at which the surface particle concentration is half of its maximum value (this depth is deeper than the depth at which it reaches its maximum value) is determined, and this is taken as the lamination thickness of Layer B. The conditions are as follows:

[0079] (i) Measuring device Secondary ion mass spectrometer (SIMS) PHI Model 6300 (ii) Measurement conditions Primary ion species: Cs + Primary ion energy: 3 keV Static compensation: E-GUN Secondary ion polarity: Positive If there is no maximum particle concentration within a depth of 5,000 nm from the surface, the thickness of Layer A is determined in the same manner from the surface of the opposite side, the thickness of Layer A, and the total thickness minus the thickness of Layer B is taken as the thickness of Layer A. Furthermore, if the particles most abundant within a depth of 5,000 nm from the surface are organic polymer particles, which are difficult to measure using SIMS, the thickness can be determined by measuring the depth profile as described above using XPS (X-ray photoelectron spectroscopy) or IR (infrared spectroscopy) while etching from the surface. Alternatively, the film cross section can be observed using ultrathin sectioning (RuO4 staining) with a transmission electron microscope (Hitachi H-600) at an accelerating voltage of 100 kV to capture the interface and determine its thickness (measurements are performed in the order of priority listed above, and the first measured value is taken as the thickness of the relevant layer). The magnification can be selected depending on the thickness range to be measured and is not particularly limited, but a magnification of 10,000 to 100,000 times is appropriate.

[0080] (7) Film thickness (μm) Measurements were taken at 15 random points using an electronic micrometer, and the average value was taken as the film thickness.

[0081] Measuring device: Electronic micrometer K351C K-402B STAND manufactured by Anritsu Corporation (8) Number-average particle size of particles and average primary particle size of aggregated particles The cross section of the film is observed at 10,000x magnification using a transmission electron microscope (TEM). If particles smaller than 1 cm are observed in the photograph, the TEM magnification is changed to 50,000x. The TEM section thickness is approximately 100 nm, and 100 fields of view are measured at different locations. The equivalent circular diameter is determined for all dispersed particles photographed, and a particle number distribution is plotted with the equivalent circular diameter on the horizontal axis and the number of particles on the vertical axis. The equivalent circular diameter at the peak value is taken as the average particle size. If aggregated particles are observed in the photograph observed at 10,000x magnification, they are not included in the plot. If two or more types of particles with different particle sizes are present in the film, the number distribution of the equivalent circular diameters will have two or more peaks. In this case, each peak value is taken as the average particle size of each particle.

[0082] The average primary particle diameter of agglomerated particles is observed at 200,000 magnification using the above-mentioned device. For 100 agglomerated particles, the equivalent circular diameter of each primary particle constituting the agglomerated particles is determined and plotted in the same manner as above, and the equivalent circular diameter of the peak value is taken as the average primary particle diameter of the agglomerated particles.

[0083] Measurement equipment: Transmission electron microscope (TEM) Hitachi H-7100FA Measurement conditions: Accelerating voltage 100 kV Measurement magnification: 10,000x, 50,000x Sample preparation: ultrathin sectioning Observation surface: TD-ZD cross section (TD: width direction, ZD: thickness direction) (9) Particle content The results were calculated from the amount of particles blended into polyester, the raw material for the film, and are shown in the table.

[0084] It can also be calculated by analyzing the film according to the following method.

[0085] (9)-1 Elemental analysis of particles The polyester is removed from the film using a plasma ashing method to expose the particles. Processing conditions are selected so that the polymer is ashed but the particles are damaged as little as possible. The particles are observed using a scanning electron microscope (SEM), and the particle images are processed using an image analyzer. According to the particle size distribution obtained in (8) above, the SEM magnification is set to 30,000x, and 20 fields of view are observed at different observation points. Elemental analysis is then performed on all observed particles using energy dispersive X-ray spectroscopy (EDX) to clarify the relationship between the particles and elements.

[0086] (9)-2 Particle content The surface of each laminate was scraped off with a single blade, and o-chlorophenol was added to 100 g of scraping powder. The polymer was dissolved at 100°C for 1 hour while stirring. Next, a Hitachi 40P preparative ultracentrifuge equipped with an RP30 rotor was used. 30 cc of the above solution was poured into each cell, and the speed was gradually increased to 30,000 rpm. 60 minutes after reaching 30,000 rpm, particle separation was completed. The supernatant was then removed and the separated particles were collected. Room-temperature o-chlorophenol was added to the collected particles, and the particles were uniformly suspended and then subjected to ultracentrifugation. This process was repeated until no melting peak corresponding to the polymer was detected using a differential scanning calorimeter (DSC). The separated particles thus obtained were vacuum-dried at 120°C for 16 hours, and the mass was measured. This value was used as the total particle content, and the particle content was determined as a percentage (by mass%) of this.

[0087] (10) 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.

[0088] (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.

[0089] The resulting nonmagnetic layer-forming coating solution was applied to a PET film and dried to a dry thickness of 0.4 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 PET film to a dry thickness of 0.3 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.

[0090] The magnetic tape cartridge obtained above (total length of magnetic tape: 900 m) was set in an IBM LTO8 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.

[0091] The magnetic tape cartridge after the above run was set in a reference drive (IBM LTO8 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 60% 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 5 times / m or less were considered highly reliable, and tapes with a missing pulse occurrence frequency of more than 5 times / m were considered defective, and the following criteria were used to determine the quality of the magnetic tape.

[0092] AA: Missing pulse occurrence frequency is 2 times / m or less A: Missing pulse occurrence frequency is more than 2 to 3 times / m or less B: Missing pulse occurrence frequency is more than 3 to 5 times / m or less C: Missing pulse occurrence frequency exceeds 5 times / m.

[0093] (11) Errors and Defects In the evaluation method described in (10) above, a signal in which the playback signal amplitude was reduced by 30% or more compared to the average (average of the measured values for all tracks) was considered an error defect, and the frequency of occurrence was divided by the total length of the magnetic tape to obtain the frequency of error defects per unit length of the magnetic tape (unit: times / m), and judged according to the following criteria.

[0094] AA: Error defect frequency is 4 times / m or less A: Error defect frequency is more than 4 to 6 times / m or less B: Error defect frequency is more than 6 times to 10 times / m or less C: The frequency of error defects exceeds 10 times / m.

[0095] (12) Stability of magnetic tape to environmental changes The magnetic tape prepared in (10) above was set in a film dimension measuring device (Fig. 1) installed in a thermo-hygrostat, and the width of the tape was measured under the following temperature and humidity conditions with a load of 50 g.

[0096] ·Temperature and humidity conditions <1> : 10℃ 10%RH <2> : 10℃ 80%RH <3> : 29℃ 80%RH <4> : 45℃ 24%RH <5> : 45℃ 10%RH Dimensional measurements are <1> , <2> , <3> , <4> , <5> After reaching each temperature and humidity, the tape width was measured after leaving it for 2 hours (each tape width was measured as L <1> , L <2> , L <3> , L <4> , L <5> (Let us assume that.)

[0097] The change in tape width between each condition was calculated, and then a correction was made taking into account the temperature expansion of the magnetic head (7 ppm / °C) (7 ppm / °C minus the temperature difference between each condition), and the change in tape width between conditions was calculated using the following formula.

[0098] Change (1) ΔL <3> -L <1> = |L <3> -L <1> -(7×19)| Change (2) ΔL <5> -L <2> = |L <5> -L <2> -(7×35)| The larger of the above changes (1) and (2) was judged according to the following criteria, and a dimensional change of 450 ppm or more was deemed to have poor environmental stability. A: Less than 350 ppm B: 350 ppm or more and less than 450 ppm C: 450ppm or more.

[0099] (13) Winding property A shuttle test was carried out on a film slit to a width of 1 m, in which the film was wound up 10 times at a speed of 60 m / min, and the number of times that wrinkles or edge distortion occurred was evaluated using the method described below. A grade of C was determined to mean poor winding properties.

[0100] AA: The frequency of wrinkles or distortion of edges is less than 1 / 10.

[0101] A: The frequency of wrinkles or distortion of edges is less than 2 / 10 to 3 / 10 times.

[0102] B: The frequency of wrinkles or distortion of edges is 4 / 10 to 5 / 10 or less.

[0103] C: The frequency of wrinkles or distortion of edges is 6 / 10 or more times. [Example]

[0104] The following examples will illustrate the present invention, in which polyethylene terephthalate is abbreviated as PET and polyetherimide as PEI.

[0105] (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.

[0106] 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.

[0107] 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). (2-a, 2-b) 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 of cross-linked polystyrene particles) were fed into a co-rotating, vented twin-screw extruder heated to 280 ° C., and the vent hole was maintained at a reduced pressure of 1 kPa or less to remove water, resulting in particle-containing pellets (raw material-2a) containing 0.5% by mass of cross-linked polystyrene particles and an intrinsic viscosity of 0.62. Similarly, particle-containing pellets (raw material-2b) containing 0.5% by mass of cross-linked polystyrene particles with an average particle size of 0.13 μm and an intrinsic viscosity of 0.62 were obtained. (2-c) Preparation of particle-containing PET pellets: 94 parts by mass of the above-mentioned PET pellets (raw material-1) and a 20% by mass aqueous slurry of colloidal silica particles with an average particle size of 0.060 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 1,000 rpm for 2 hours, and then filtered to remove the glass beads. 5 parts by mass of the resulting aqueous slurry (1.0 part by mass as colloidal silica particles) were 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-2c) containing 1.0% by mass of colloidal silica particles and having an intrinsic viscosity of 0.62.

[0108] (2-d) 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-2d) containing 0.5% by mass of colloidal silica particles and having an intrinsic viscosity of 0.62 were obtained.

[0109] (2-e) 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 a primary average particle size of 0.02 μm, to which zirconia beads with an average particle size of 0.5 mm were added, were fed into a co-rotating, vented twin-screw kneading extruder heated to 280°C, and the mixture was stirred at 3,000 rpm for 2 hours, and then filtered to remove the zirconia beads. 10 parts by mass of the water slurry (1 part by mass as alumina particles) was then fed, and the vent hole was kept at a reduced pressure of 1 kPa or less to remove the water, yielding particle-containing pellets (raw material-2e) containing 1% by mass of alumina particles and having an intrinsic viscosity of 0.62.

[0110] (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).

[0111] Example 1 Two extruders, E1 and E2, were used. Extruder E1 was heated to 280°C and contained 79 parts by mass of PET pellets (raw material-1), 4 parts by mass of two-component composition pellets (raw material-3), and 17 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.06 μm (raw material-2b) 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 30.3 parts by mass of PET pellets (raw material-1), 4 parts by mass of two-component composition pellets (raw material-3), 41.7 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.06 μm (raw material-2b), and 24 parts by mass of pellets containing colloidal silica particles with an average particle size of 0.20 μm (raw material-2c) 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 12 | 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.

[0112] This laminated unstretched film was stretched in the MD direction at 90°C in three stages under the conditions shown in Table 2: 1.15x in the first stage, 2.4x in the second stage, and 1.16x in the third stage. It was then introduced into a tenter and stretched in the TD direction in three stages: 3.5x in the first stage at 92°C, 1.06x in the second stage at 180°C, and 1.35x in the third stage at 190°C. It was then heat-set (HS1) at 205°C with a relaxation rate of 0.2% for 3 seconds, heat-set (HS2) at 215°C for 3 seconds, and heat-set (HS3) at 220°C with a relaxation rate of 1.2% for 3 seconds, followed by slow cooling to 150°C. The film edges were then removed and the film was wound onto a core to obtain a biaxially oriented polyester film with a thickness of 4.0 μm (A layer thickness / B layer thickness 3.7 / 0.3 μm). 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 Table 4.

[0113] 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.

[0114] (Examples 2 to 5, Comparative Examples 1 to 4) The lamination thickness ratio (A layer|B layer) and the compounding ratio of particle-containing pellets used as the B layer raw material were changed to achieve the B layer lamination thickness and particle addition amount shown in Table 1, and stretching and heat setting were performed under the conditions shown in Table 2.

[0115] [Table 1]

[0116] [Table 2]

[0117] [Table 3]

[0118] [Table 4] [Explanation of symbols]

[0119] 1: Laser transmitter 2: Light receiving part 3: Load detector 4: Load 5-8: Freeroll 9: Magnetic tape 10: Laser light

Claims

1. On at least one outermost layer surface (side B) of the biaxially oriented polyester film, the lamination thickness of the outermost layer (layer B) is less than 0.5 μm, and the number of protrusions with a height of 30 nm or more is 200,000 / mm ２ Over 500,000 pieces / mm ２ The number of protrusions with a height of 100 nm or more is 4,000 or more, and 10,000 or more per mm ２ or less, and the average surface roughness SaB of the layer is greater than the average surface roughness SaA of the other outermost layer surface (side A), and the film thickness is 3.0 μm or more and 4.2 μm or less.

2. The number of protrusions having a height of 50 nm or more on the outermost layer surface (side B) having the characteristics of claim 1 is 30,000 / mm ２ Over 60,000 pieces / mm ２ 2. The biaxially oriented polyester film of claim 1, wherein:

3. On the outermost layer surface (side B) having the characteristics of claim 1, the number of protrusions having a height of 60 nm or more is 2,000 / mm ２ Over 30,000 pieces / mm ２ 3. The biaxially oriented polyester film of claims 1 and 2, wherein:

4. 4. The biaxially oriented polyester film according to claim 1, wherein the static friction coefficient μs when the A surface and the B surface are superposed is 0.2 to 1.

5.

5. 5. The biaxially oriented polyester film according to claim 1, wherein the outermost layer (layer B) contains particles S having a number average particle diameter of 0.03 μm or more and less than 0.1 μm and particles L having a number average particle diameter of 0.1 μm or more and less than 0.3 μm in a blending ratio (L / S) of 0.01 to 0.

35.

6. A biaxially oriented polyester film described in any one of claims 1 to 5, having a humidity expansion coefficient in the width direction of 4.5 to 6.5 ppm / % RH.

7. A biaxially oriented polyester film described in any one of claims 1 to 6, having a temperature expansion coefficient in the width direction of -3.0 to 4.0 ppm / °C.

8. A biaxially oriented polyester film according to any one of claims 1 to 7, used as a base film for a magnetic recording medium of a coating-type digital recording method.

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