Biaxially oriented polyester film

Abstract

To provide a biaxially oriented polyester film suitable for a transfer film, which does not interfere with transferability while having a friction coefficient that provides an excellent film conveying property.SOLUTION: Provided is a biaxially oriented polyester film, at least one surface of which has SRz / SPc of 5.0 or more, where a static coefficient of friction μs between one surface and another surface of the film is 0.10 to 0.50. Here, SRz represents a three-dimensional 10-point average roughness (nm) of the film surface and SPc (number of protrusions per unit area) represents the number of protrusions (number / mm2) per 0.2 mm2 of the film surface, the protrusions having a height exceeding 6.3 nm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a biaxially oriented polyester film that has a friction coefficient excellent for film transport while also exhibiting excellent transferability. [Background technology]

[0002] Biaxially oriented polyester films, such as polyethylene terephthalate and polyethylene-2,6-naphthalate, are used as base films in many applications due to their excellent mechanical properties, heat resistance, dimensional stability, chemical resistance, and cost-effectiveness. One such application is transfer printing in thermal transfer ink ribbons. These thermal transfer ink ribbons are already used in fields such as fax machines and barcode printing due to their excellent cost-effectiveness, maintainability, and ease of operation.

[0003] The demands for clarity and density gradation in these transfer products are increasing year by year, and the required quality and grade levels for the polyester film used as the material are also rising. For example, Patent Document 1 discloses a technique for improving the characteristics of printed materials by controlling the surface roughness of the polyester film. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-144243 [Overview of the project] [Problems that the invention aims to solve]

[0005] While the technology described in the above-mentioned patent document achieves a certain level of improvement in quality and grade, the yield during processing is not sufficient. The processing of film products often involves feeding out the film from a roll, processing it, and winding it up as a processed film. However, the characteristics of the base film and the quality of the film roll can affect each step of the process. Specifically, these include the stability during feeding, the transportability of the film during processing, and the ease of winding the processed film. Common to all of these is the slipperiness between films, and therefore the coefficient of friction of the film. If the coefficient of friction is too low, meandering is likely to occur during film transport, and if the coefficient of friction is too high, air release during film winding deteriorates, and in either case, the quality of the film roll after winding decreases. One factor that affects the coefficient of friction of a film is the surface roughness of the film. Generally recognized trends are that the rougher the film surface, the lower the coefficient of friction, and the smoother the surface, the higher the coefficient of friction. Therefore, in order to maintain the quality of the film roll and to ensure processability in subsequent processes, the surface of the base film needs to be roughened to a certain extent, and optimization of the base film is required.

[0006] On the other hand, in the case of transfer films, the transferability can be hindered by the countless protrusions on the film surface, and a film surface with fewer protrusions and a smoother surface is advantageous for the quality of the transfer product. However, as mentioned above, the base film needs to be roughened to some extent in order to ensure processability in each step of the film processing, and it has been difficult to optimize the film surface roughness and friction coefficient in a way that is advantageous in all aspects for a transfer film.

[0007] In response to the above-mentioned situation, the present invention provides a biaxially oriented polyester film suitable for transfer films that maintains a friction coefficient excellent for film transport without hindering transferability, by suppressing the number of protrusions on the film surface while providing an appropriate protrusion height. [Means for solving the problem]

[0008] To solve the above problems, a preferred embodiment of the present invention has the following configuration. (1) A biaxially oriented polyester film having an SRz / SPc of 5.0 or more on at least one surface, wherein the static friction coefficient μs of one surface and the other surface of the film is 0.10 or more and 0.50 or less. In this case, SRz is the three-dimensional ten-point average roughness (nm) of the film surface, and SPc (number of protrusions per unit area) is the film surface at 0.2 mm. 2 Number of protrusions with a height exceeding 6.3 nm per unit area (number of protrusions / mm 2 ) represents. (2) The biaxially oriented polyester film according to (1), wherein the SRz is 1300 nm or more and 2000 nm or less. (3) The SPc is 100 pieces / mm 2 More than 290 pieces / mm 2 The biaxially oriented polyester film described in (1) or (2) below. (4) A biaxially oriented polyester film according to any one of (1) to (3), wherein the SRz / SPc is 5.0 or higher and the three-dimensional central plane average roughness SRa of the film surface is 40 nm or more and 70 nm or less. (5) A biaxially oriented polyester film according to any one of (1) to (4), wherein the thickness of the film is 2.0 μm or more and 10.0 μm or less. (6) A biaxially oriented polyester film according to any one of (1) to (5), containing three or more particles with different particle compositions. (7) The biaxially oriented polyester film according to (6), which contains one or more organic particles. (8) The biaxially oriented polyester film according to (6) or (7), wherein the sum of the particle content is 0.01% by mass or more and 1.00% by mass or less of the entire film. (9) A biaxially oriented polyester film according to any of (1) to (8) used in a transfer film. [Effects of the Invention]

[0009] According to the present invention, a biaxially oriented polyester film can be obtained that achieves both good film transportability and transferability by suppressing the number of protrusions on the film surface while providing an appropriate protrusion height. [Modes for carrying out the invention]

[0010] The biaxially oriented polyester film used in the present invention is a polyester film that is oriented biaxially due to molecular orientation accompanying stretching. The polyester resin used is preferably polyethylene terephthalate or polyethylene-2,6-naphthalate. These may be polyester copolymers, but it is preferable that 80 mol% or more of the repeating structural units are ethylene terephthalate or ethylene-2,6-naphthalate. Other polyester copolymer components may include diol components such as diethylene glycol, propylene glycol, neopentyl glycol, polyethylene glycol, p-xylene glycol, and 1,4-cyclohexanedimethanol; dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, 5-sodium sulfisoisophthalic acid, and 2,6-naphthalenedicarboxylic acid; or polyfunctional dicarboxylic acid components such as trimellitic acid and pyromellitic acid, or p-hydroxyethoxybenzoic acid. Furthermore, the above-mentioned polyester may be copolymerized or blended with one or more of the following: an alkali metal salt derivative of sulfonic acid that does not react with the polyester, or polyalkylene glycol or aliphatic polyester that is insoluble with the polyester, in an amount not exceeding 5%.

[0011] The polyester film used in the present invention may contain stabilizers, colorants, antioxidants, and other additives as needed.

[0012] The thickness of the polyester film of the present invention is preferably 2.0 μm or more and 10.0 μm or less. More preferably 2.5 μm or more and 5.0 μm or less. By setting the film thickness to be above the above range, for example, transportability can be improved in thermal transfer ribbon applications, and the occurrence of wrinkles during processing can be suppressed. On the other hand, by setting the film thickness to be below the above range, for example, printing sensitivity can be improved in thermal transfer ribbon applications. By setting the film thickness to the above range, for example, the occurrence of print wrinkles in thermal transfer ribbons can be suppressed, and the transferability of the film can be improved.

[0013] In the present invention, the polyester film has an SRz / SPc ratio of 5.0 or higher, calculated from the three-dimensional ten-point average roughness (SRz) of the film surface and the number of protrusions per unit area (SPc). If the SRz / SPc is less than 5.0, either the transferability of the film or the film transportability during processing will be unfavorable, and the effects of the present invention cannot be obtained. Furthermore, if the SRz / SPc is 6.5 or lower, both transferability and transportability can be improved. In this case, the three-dimensional ten-point average roughness (SRz) (unit: nm) is determined by the measurement method described later and is a measurement value from a three-dimensional roughness meter corresponding to the Rz value of JIS-B0601 (1994). The number of protrusions per unit area (SPc) is measured on the film surface at 0.2 mm 2 Number of protrusions with a height exceeding 6.3 nm per unit area (number of protrusions / mm 2 This represents [the value of the value]. Detailed measurement methods will be described later.

[0014] In the polyester film of the present invention, on the film surface where SRz / SPc satisfies 5.0 or more, the three-dimensional ten-point average roughness (SRz) is preferably 1300 nm or more and 2000 nm or less. By setting the three-dimensional ten-point average roughness (SRz) of the film surface where SRz / SPc satisfies 5.0 or more to 2000 nm or less, an effect of suppressing the inhibition of the transferability of the film by large protrusions present on the film surface can be obtained. Further, by setting the three-dimensional ten-point average roughness (SRz) to 1300 nm or more, adhesion between films when the film is wound in a roll state can be suppressed, and an effect of suppressing deterioration of the film conveyance property when the film is unwound from the film roll can be obtained. More preferably, the three-dimensional ten-point average roughness (SRz) is 1300 nm or more and 1800 nm or less. In the polyester film of the present invention, when the ten-point average roughness (SRz) satisfies the above range, good film conveyance property can be maintained while suppressing the inhibition of transferability.

[0015] In addition, in the polyester film of the present invention, on the film surface where SRz / SPc satisfies 5.0 or more, the number of protrusions per unit area (SPc) is 100 / mm 2 or more and 290 / mm 2 or less, preferably 100 / mm 2 or more and 275 / mm 2 or less. By setting the number of protrusions per unit area (SPc) on the surface of the film where SRz / SPc satisfies 5.0 or more to 100 / mm 2 or more, adhesion between films when the film is wound in a roll state can be suppressed, and an effect of suppressing deterioration of the film conveyance property when the film is unwound from the film roll can be obtained. On the other hand, by setting the number of protrusions (SPc) to 290 / mm 2 or less, it is possible to suppress the inhibition of the transferability of the film by the protrusions present on the film surface.

[0016] In addition, for the polyester film of the present invention, the static friction coefficient (μs) measured by overlapping one surface of the film with the opposite surface is 0.10 or more and 0.50 or less. If the static friction coefficient (μs) exceeds 0.50, good film conveyance performance cannot be obtained during film processing. If the static friction coefficient (μs) is less than 0.10, the film is likely to meander during film winding, and the quality of the film roll after winding is likely to be impaired. More preferably, the static friction coefficient (μs) is 0.30 or more and 0.50 or less. When the film satisfies the above range, while the conveyance performance of the film is improved, the quality can be maintained when the film is wound into a roll state, and the conveyance performance when unwound from the film roll can also be improved.

[0017] At this time, it is preferable that the three-dimensional center plane average roughness (SRa) of the film surface with SRz / SPc of 5.0 or more is 40 nm or more and 70 nm or less. In the polyester film of the present invention, when the three-dimensional center plane average roughness (SRa) satisfies the above range, more clearly, good film conveyance performance can be maintained while suppressing the inhibition of transferability. From the same viewpoint, more preferably, the three-dimensional center plane average roughness (SRa) is 40 nm or more and 60 nm or less. The three-dimensional center plane average roughness (SRa) (unit: nm) is obtained by the measurement method described later and is the measurement value by a three-dimensional roughness meter corresponding to the center line average roughness value of JIS-B0601 (1994).

[0018] In the present invention, the method for making the SRz / SPc and static friction coefficient (μs) of the polyester film fall within the above range is not particularly limited, but for example, one method is to include inorganic particles and / or organic particles in the polyester resin constituting the film, and in particular, a method of including three or more types of particles with different particle compositions from among inorganic or organic particles such as silicon dioxide, alumina, calcium carbonate, kaolin, crosslinked polystyrene, crosslinked acrylic resin particles, fluororesin particles, or silicon particles is preferred. The reason for including three or more types of particles with different particle compositions at this time is to simultaneously satisfy the preferred ranges of the SRz, SPc, and static friction coefficient (μs) of the film as specified in the present invention. If the particles are of a single type, particle dispersion within the film is easily homogenized, but when trying to satisfy the preferred range of SRz shown in the present invention, the static friction coefficient (μs) tends to exceed 0.50, making it impossible to simultaneously satisfy the preferred range shown in the present invention. The present invention is highly effective when a state of moderately non-uniform particle dispersion within the film is achieved. However, it is easier to achieve a moderately non-uniform particle dispersion within the film by including three or more types of particles with different particle compositions rather than just two types of particles. In this case, the effect is more easily obtained when the particles used have more different properties from each other, and it is preferable that one or more of the included particles are organic particles.

[0019] The sum of the particle content in the polyester film at this time is preferably 0.01 to 1.00 mass%, and more preferably 0.20 to 0.50 mass%. Setting the particle concentration to 0.01 mass% or higher makes it easy to set the three-dimensional ten-point average roughness (SRz) of the film surface to be above the aforementioned range, and also makes it easy to set the static friction coefficient (μs) to 0.50 or lower. Setting the particle concentration to 1.00 mass% or lower provides the effect of good film formation stability. Furthermore, the average particle diameter of the particles to be included is preferably 0.1 to 3.0 μm for each type of particle. Setting the average particle diameter to 0.1 μm or higher makes it easy to set the three-dimensional ten-point average roughness (SRz) of the film surface to be above the aforementioned range. In addition, since it is not necessary to excessively increase the particle concentration of the film in order to set the three-dimensional ten-point average roughness (SRz) to be above the aforementioned range, the film formation stability of the film can be improved. On the other hand, by keeping the average particle size to 3.0 μm or less, it becomes easy to keep the three-dimensional ten-point average roughness (SRz) of the film surface below the aforementioned range. The average particle size is determined by washing the particles with a solvent that does not melt or dissolve them, adding water to the solution, repeating centrifugation twice, drying the resulting particles, and then measuring the particle size distribution using a laser diffraction particle size distribution analyzer (e.g., Mastersizer 2000 manufactured by Spectris Co., Ltd.), and taking the number average as the average particle size of the particles.

[0020] From the above viewpoint, a particularly preferred embodiment is one in which the product contains 0.25 to 0.35 mass% of silicon dioxide particles with an average particle diameter of 2.0 to 3.5 μm, 0.03 to 0.10 mass% of calcium carbonate particles with an average particle diameter of 0.9 to 1.3 μm, and 0.005 to 0.015 mass% of organic particles with an average particle diameter of 0.2 to 0.4 μm.

[0021] As for the method of adding these particles, when obtaining the biaxially oriented polyester film of the present invention, the polyester resin and particles may be kneaded using an extruder or the like. However, in order to disperse the particles more uniformly, it is preferable to knead the polyester resin with a raw material in which the particles are blended with the polyester resin using an extruder or the like.

[0022] The polyester film of the present invention is not limited to a single-layer film, but may also consist of a composite film with two, three, or more layers.

[0023] The polyester film of the present invention has excellent transportability and transferability, making it suitable for use as a transfer film, particularly for thermal transfer ink ribbons.

[0024] A preferred embodiment of the method for producing the biaxially oriented polyester film of the present invention is described below.

[0025] A polyester raw material containing particles is melt-extruded and formed into a film using a slit-shaped die. This film is then wrapped around a casting drum with a surface temperature of 20-70°C and cooled and solidified to obtain an unstretched film. Subsequently, the unstretched film is stretched 3.0-7.0 times in the longitudinal direction (MD) at 80-130°C to obtain a uniaxially oriented film. By performing multi-stage stretching, a film strongly oriented in the longitudinal direction can be obtained without impairing film-forming properties. In particular, three-stage stretching is preferred. By stretching the first and second stages at 110-130°C for a total of 2-4 times the initial stretch, and the third stage at 110-130°C for 2-3.5 times the initial stretch, and ensuring that the third stage is at a lower temperature than the first and second stages, it becomes easy to produce a polyester film that satisfies the physical properties and effects of the present invention. After that, the uniaxially oriented film is introduced into a tenter and preheated to 100-130°C. Next, the preheated film is stretched 3.0 to 4.5 times in the width direction (TD) to form a biaxially oriented film, and then heat-set (sometimes called HS) at 200 to 220°C. After heat-setting, it is shrunk by 2 to 8% in the width direction at 180 to 220°C, wound into a jumbo roll, then slit to the required film width, and wound onto a paper or resin core to form a film roll. Note that the manufacturing method is not necessarily limited to the method shown here. [Examples]

[0026] The measurement and evaluation methods used in this embodiment are shown below.

[0027] (1) Three-dimensional 10-point mean roughness (SRz) The three-dimensional ten-point average roughness of a polyester film was measured using a SurfCorder ET4000A micro-shape measuring instrument manufactured by Kosaka Laboratory Co., Ltd. The SRz value in this invention is a measurement value obtained with a three-dimensional roughness meter, corresponding to the Rz value in JIS-B0601 (1994). The measurement method was in the width direction, with a cutoff value of 0.25 mm, a measurement length of 0.5 mm, a feed pitch of 5 μm, a stylus load of 10 mg, a measurement speed of 100 μm / s, and 80 measurements. Samples were cut from arbitrary locations, and the measurement was performed on three samples. The ten-point average roughness (SRz) of the film was determined from the average value of the measurements of these three samples.

[0028] (2) Number of protrusions per unit area (SPc) Using the SurfCorder ET4000A micro-shape measuring machine manufactured by Kosaka Laboratory Co., Ltd., the surface of a polyester film was measured to a thickness of 0.2 mm. 2 The number of surface protrusions (SPc) with a protrusion height exceeding 6.3 nm per unit area was counted. The measurement direction was the width direction of the film, with a cutoff value of 0.25 mm, a measurement length of 0.5 mm, a feed pitch of 5 μm, a stylus load of 10 mg, a measurement speed of 100 μm / s, and 80 samples measured. Samples were cut from arbitrary locations, and the measurement was performed on 3 samples. The number of surface protrusions (SPc) of the film was determined from the average value of the measurements of these 3 samples.

[0029] (3) Static friction coefficient (μs) Two films, with one side of the film overlapping the other, are placed on a glass plate, and a 200g weight (contact area 40cm²) is placed on the film. 2 A glass plate was placed in a specific position. One end of the lower film (the side facing the direction of movement) was fixed to the glass, and one end of the upper film (the end opposite to the direction of movement) was fixed to the detector. The coefficient of static friction (μs) when the glass plate was moved 5 mm at a speed of 2 mm / sec was calculated using the following formula. μs = (initial tension) / (load of 200g).

[0030] (4) Average roughness of the three-dimensional central plane (SRa) The average three-dimensional center surface roughness of polyester film was measured using a SurfCorder ET4000A micro-shape measuring instrument manufactured by Kosaka Laboratory Co., Ltd. The measurement method was in the width direction, with a cutoff value of 0.25 mm, a measurement length of 0.5 mm, a feed pitch of 5 μm, a stylus load of 10 mg, a measurement speed of 100 μm / s, and 80 measurements. Samples were cut from arbitrary locations, and the measurement was performed on 3 samples. The average three-dimensional center surface roughness (SRa) of the film was determined from the average value of the measurements of these 3 samples. The average three-dimensional center surface roughness (SRa) (unit: nm) is a measurement value obtained with a three-dimensional roughness meter that corresponds to the centerline average roughness of JIS-B0601 (1994).

[0031] (5) Thickness of polyester film (μm) The thickness (μm) of the polyester film was measured using the micrometer method specified in JIS C2151 (2006) Test Methods for Electrical Plastic Films.

[0032] (6) Film transportability The slitting process of the base film was classified according to the following indicators.

[0033] ◎: Less than 0.3% of slitting failures occurred due to wrinkles in the transported film. ○: Slitting failure due to wrinkles in the transport film occurs in 0.3% or more but less than 1.0% of cases. △: Slitting failure due to wrinkles in the transport film occurs in 1.0% to less than 2.0% of cases. ×: More than 2.0% of slitting failures occurred due to wrinkles in the transported film.

[0034] (7) Transferability As the polyester film of the present invention, a roll with a width of 500 mm and a length of 40,000 m is unwound, and a coating liquid of the following composition (i) is applied to one side at a rate of 0.3 g / m². 2 The material was applied using a direct gravure coater and dried to form a heat-resistant protective layer.

[0035] Composition (i) Silicone resin: 10 parts by mass Toluene: 45 parts by mass Methyl ethyl ketone: 45 parts by mass On the side of the polyester film opposite to the heat-resistant protective layer, the following coating solution (ii) is melted and stirred at 150°C, and the coating amount is 2.0 g / m². 2 To achieve this, a hot melt coater was used to apply the material at approximately 130°C and allowed to dry, forming an easily adhesive layer.

[0036] Composition (ii) Carnauba wax (Carnauba No. 1, manufactured by Toyo Adore Co., Ltd.): 30 parts by mass Paraffin wax (HNP-10, manufactured by Nippon Seiro Co., Ltd.): 35 parts by mass Ethylene vinyl acetate copolymer (MB-11, manufactured by Sumitomo Chemical Co., Ltd.): 10 parts by mass Apply the coating solution with the following composition (iii) to this easy-adhesion layer at a rate of 0.5 g / m². 2 The material was coated using a direct gravure coater and dried to form a heat-meltable ink layer, thereby obtaining a heat transfer ribbon.

[0037] Composition (iii) Carnauba wax: 10 parts by mass Terpene phenol resin: 30 parts by mass Carbon Black: 10 parts by mass Toluene: 100 parts by mass The thermal transfer ribbon obtained above was printed using a thermal transfer printer (Zebra Technologies' "Zebra"® 140 Xi III printer) at energy level 28, and the number of defects [pieces / 100mm] was visually inspected on the printed thermal transfer ribbon. 2 We evaluated the following and classified them according to the following indicators. ◎: Less than 1.2 (Pass, Very Good) ○: 1.2 or higher and less than 2.0 (Pass, Good) △: 2.0 or higher and less than 5.0 (Pass, usable) ×: 5.0 or higher (Failure, practical problems present).

[0038] The present invention will now be described based on the following examples, but the present invention is not limited to these examples.

[0039] (Example 1) Pellet A, which is polyethylene terephthalate containing silicon dioxide particles with an average particle size of 2.6 μm, and Pellet B, which is polyethylene terephthalate containing calcium carbonate particles with an average particle size of 1.1 μm and cross-linked polystyrene particles with an average particle size of 0.3 μm, were melted in an extruder at 285°C. At this time, the content of each particle in the extruder was 0.29 mass% for silicon dioxide particles, 0.07 mass% for calcium carbonate particles, and 0.01 mass% for cross-linked polystyrene particles. The mixture kneaded in the extruder was melt-extruded into a sheet shape from a die, and solidified by adhering it to a rotating cooling drum at 25°C to obtain an unstretched film. Using the difference in peripheral speed of heated rolls, the film was stretched 1.1 times in the longitudinal direction at 123°C (first stretching), then stretched 2.1 times in the longitudinal direction at 123°C (second stretching), and further stretched 2.7 times at 116°C (third stretching), for a total of 6.2 times stretching.

[0040] The film was held at both ends with clips, guided to a tenter, stretched to 3.8 times its original width at 110°C, then heat-treated with hot air at 210°C for 3 seconds, and finally relaxed by 3.0% in the width direction at 150°C to obtain a polyester film with a thickness of 5.0 μm. The evaluation results are shown in Tables 1-3.

[0041] (Examples 2-4, Comparative Examples 1-3) A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the particle types and mixing ratios were changed.

[0042] (Example 5) A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the film thickness was changed.

[0043] [Summary of evaluation results] Examples 1 through 5 all yielded good results in film transportability and transferability evaluation, demonstrating a successful balance between the two. Examples 1 and 2 showed particularly good results.

[0044] Comparative Examples 1-3 did not perform well in either film transportability or transferability, and neither was achieved.

[0045] [Table 1]

[0046] [Table 2]

[0047] [Table 3] [Industrial applicability]

[0048] The biaxially oriented polyester film of the present invention can be suitably used for transfer applications, particularly for thermal transfer ink ribbon applications, but its range of applications is not limited thereto.

Claims

1. A biaxially oriented polyester film having an SRz / SPc of 5.0 or more on at least one surface, wherein the static friction coefficient μs between one surface and the other surface of the film is 0.10 or more and 0.50 or less, and containing 0.25 to 0.35 mass% of silicon dioxide particles with an average particle diameter of 2.0 to 3.5 μm, 0.03 to 0.10 mass% of calcium carbonate particles with an average particle diameter of 0.9 to 1.3 μm, and 0.005 to 0.015 mass% of organic particles with an average particle diameter of 0.2 to 0.4 μm. In this case, SRz is the three-dimensional ten-point average roughness of the film surface (nm), and SPc (number of protrusions per unit area) is the film surface at 0.2 mm. ２ Number of protrusions with a height exceeding 6.3 nm per unit area (number of protrusions / mm ２ ) represents.

2. The biaxially oriented polyester film according to claim 1, wherein the SRz is 1300 nm or more and 2000 nm or less.

3. The aforementioned SPc is 100 pieces / mm ２ 290 pieces / mm or more ２ The biaxially oriented polyester film according to claim 1 or 2, wherein the following applies:

4. A biaxially oriented polyester film according to claim 1 or 2, wherein the average three-dimensional center surface roughness SRa of the film surface, where SRz / SPc is 5.0 or greater, is 40 nm or more and 70 nm or less.

5. The biaxially oriented polyester film according to claim 1 or 2, wherein the thickness of the film is 2.0 μm or more and 10.0 μm or less.

6. A biaxially oriented polyester film according to claim 1 or 2, used in a transfer film.

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