JPH0360301B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a biaxially oriented polyester film, and more particularly to a biaxially oriented polyester film that has both excellent flatness and windability. [Prior Art] Because of its excellent properties, biaxially oriented polyester films are widely used in many applications such as magnetic tapes, electricity, photography, metallization, and packaging. In particular, due to its properties such as high strength and elastic modulus, magnetic recording media, such as video tapes,
Widely used as a base film for audio tapes, computer tapes, floppy disks, etc. In recent years, demands for higher density recording and higher quality have been increasing in these application fields, and with this, there has been an increasing demand for the base polyester film to have a flat surface. However, if the surface becomes flat, the winding of the film in the step of winding the film into a roll deteriorates significantly, and there is a problem that it is difficult to obtain a film roll with a good winding appearance. Defects in the winding appearance of film rolls include bump-like protrusions on the roll, wrinkles in the longitudinal direction of the film, and misalignment of the edges. This is likely to occur when winding up a flat film with high tension for the purpose of increasing the tension. Therefore, the base polyester film is required to have both flatness and winding properties to obtain a good film winding appearance. It has been said that good winding appearance is closely related to the slipperiness of the film. Conventionally, methods for improving the slipperiness of this film include adding inorganic particles such as silicon oxide or calcium carbonate to polyester, or precipitating fine particles containing calcium, lithium, or phosphorus within the polymerization system during polyester synthesis. A method has been proposed. In either method, when polyester is formed into a film, projections are formed on the surface of the film due to fine particles, thereby improving the slipperiness of the film. However, in the method of improving the slipperiness of a film using projections made of fine particles as described above, the slipperiness usually improves as the film surface becomes rougher, but on the other hand, due to the roughening, For example, in magnetic recording media applications, the surface after application of magnetic paint tends to become rough and the electromagnetic conversion characteristics deteriorate. As one of the measures to solve these contradictory problems of flatness and slipperiness, many methods have been proposed that utilize composite inorganic particles in which large-sized particles and small-sized particles coexist. However, these means also have problems, and as they are, it is difficult to meet the demands for higher grades of magnetic recording media, such as higher density and higher quality. The reason for this is that the size of the large particles used in composite inorganic particles is too large for the quality required for high-grade grades, and the larger the particles, the higher the protrusions on the film surface. In applications for magnetic recording media, electromagnetic conversion characteristics deteriorate, and high protrusions are shaved off during the manufacturing process, causing dropouts. In the prior art, as described above, it was difficult to satisfy both flatness and slipperiness at the same time. [Object of the Invention] The present inventor has conducted intensive research to solve the above-mentioned problems and to develop a film that has both flatness and good film winding shape that can be applied to high-quality magnetic recording applications. discovered that if the surface properties of the film had a specific anisotropy, a good winding appearance could be obtained when the film was wound into a roll even if the film had slip properties that were conventionally considered to be insufficient, and the present invention was achieved based on this finding. . An object of the present invention is to provide a base film that can cope with high-density recording and high-quality magnetic recording media. (b) An object of the present invention is to provide a biaxially oriented polyester film that has minute protrusions that do not cause noise such as noises, and (b) has a good film roll appearance. [Configuration/Effects of the Invention] According to the present invention, the main feature of the film surface roughness Ra expressed as center line surface roughness is 0.015 μm or less, and the film surface measured in the lateral direction is The roughness RaTD is in the range of 1.01 to 1.30 times the film surface roughness RaMD measured in the longitudinal direction, and the refractive index in the longitudinal direction nx and the refractive index in the lateral direction ny
The difference (nx−ny) is in the range of 0.030 to 0.070,
Furthermore, the plane orientation index is in the range of 0.15 to 0.25, and the film-to-film friction coefficient μs is 0.4 or more,
This is achieved by a biaxially oriented polyester film characterized in that the coefficient of friction μsTD in the transverse direction is in the range of 0.85 to 0.99 times the coefficient of friction μsMD in the longitudinal direction. The biaxially oriented polyester film of the present invention has a film surface roughness Ra expressed by center line surface roughness of 0.025 μm or less as a film surface property,
and the film surface roughness measured in the lateral direction of the film.
Film surface roughness measured by RaTD in the longitudinal direction
It is in the range of 1.01 to 1.30 times that of RaMD. In other words, the latter characteristic is 1.01≦RaTD/RaMD≦
It can be expressed as 1.30. If the film surface roughness Ra is greater than 0.015 μm, the surface of the base film is too rough and the magnetic surface cannot be made sufficiently flat for use in magnetic recording media, which deteriorates the electromagnetic conversion characteristics, which is not preferable. Preferred surface roughness Ra
is 0.013μm or less, even 0.010μm or less, especially
It is 0.008 or less. Although the lower limit of the surface roughness Ra is not particularly limited, the surface roughness Ra is usually 0.001 μm or more, and preferably 0.002 μm or more from the viewpoint of obtaining a good film winding appearance. Also, if the ratio RaTD/RaMD of the vertical surface roughness RaMD and the horizontal surface roughness RaTD is smaller than 1.01, bump-like protrusions will occur when the base film is rolled up into a roll, resulting in poor rolled appearance. Undesirable. It is particularly preferable that RaTD/RaMD is 1.02 or more, more preferably 1.03 or more. but
If RaTD/RaMD is greater than 1.30, it is not preferable because end face deviation occurs when the base film is wound up into a roll, resulting in poor winding appearance.
RaTD/RaMD is preferably 1.25 or less, more preferably 1.20 or less, and particularly preferably 1.05 or less. Furthermore, the biaxially oriented polyester film has the following properties:
As for the film surface characteristics, the film-to-film friction coefficient μs is 0.4 or more, and the lateral friction coefficient μsTD is greater than the longitudinal friction coefficient μsMD.
It is preferably in the range of 0.85 to 0.99 times. In other words, this latter characteristic means that the ratio of the longitudinal friction coefficient μsMD to the lateral friction coefficient μsTD is 0.85≦
It is expressed as μsTD/μsMD≦0.99. If this ratio is too small, when the base film is rolled up into a roll, it will slip too much in the lateral direction and the end faces will shift, resulting in poor winding appearance, which is not preferable. On the other hand, if this ratio is too large, bump-like protrusions will occur when the base film is rolled up into a roll, resulting in poor winding appearance, which is not preferable.
μsTD/μsMD is further preferably 0.87 or more, particularly preferably 0.89 or more, and further preferably 0.98 or less, particularly 0.95 or less. Also, μsTD,
Both μsMDs are preferably 0.40 or more.
μsTD, if μsMD is less than 0.4, μsTD/
If μsMD is less than 1.2, it is not preferable because end face misalignment occurs. More preferably, μsTD,
Both μsMD are preferably 0.45 or more, particularly 0.55 or more. Although there is no particular upper limit for μsTD and μsMD, they are usually 1.8 or less, and if they are larger than 1.8, bump-like protrusions will occur even if μsTD/μsMD is within the above range, which is not preferable. μsTD,
Both μsMDs are preferably 1.30 or less, more preferably 0.98 or less. The biaxially oriented polyester film of the present invention has the above-mentioned film surface properties, and at the same time, the difference between the refractive index nx in the longitudinal direction and the refractive index ny in the lateral direction (nx -
ny) is in the range of 0.030 to 0.070, and furthermore, the plane orientation index represented by the following formula is in the range of 0.15 to 0.25. Here, the plane orientation index is defined by the following formula. Planar orientation index = nx + ny / 2 - nz [However, nx: refractive index in the longitudinal direction, ny: refractive index in the lateral direction, nz: refractive index in the thickness direction] When (nx - ny) is smaller than 0.030, the surface property is anisotropic. Even if the base film has the above-mentioned surface properties, lump-like protrusions are likely to occur when the base film is rolled up, which is not preferable. (nx−ny) is
It is preferably 0.035 or more, more preferably 0.040 or more, particularly 0.043 or more. On the other hand, when (nx−ny) is larger than 0.070, the stiffness in the horizontal direction becomes too small than the stiffness in the vertical direction, which is undesirable because vertical wrinkles occur when winding up the base film, and the orientation is extremely vertical. This is not preferable because it tends to break frequently during stretching and has poor film-forming properties. (nx-ny) is preferably 0.065 or less, more preferably 0.060 or less, particularly preferably 0.055 or less. Further, if the plane orientation index is smaller than 0.15, it becomes difficult to obtain anisotropy in the surface properties in the longitudinal and lateral directions, which is undesirable. On the other hand, if it is larger than 0.25, breakage tends to occur during film formation, which is not preferable. The plane orientation index is preferably in the range of 0.16 to 0.22, particularly 0.17 to 0.22.
A range of 0.20 is preferred. The polyester referred to in the present invention is a film-forming aromatic polyester containing an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. This aromatic polyester is substantially linear and has film-forming properties by melt molding. Examples of aromatic dicarboxylic acids include terephthalic acid, naphthalene dicarboxylic acid, isophthalic acid, diphenoxyethane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid, and anthracene. Examples include dicarboxylic acids. Examples of aliphatic glycols include polymethylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, and decamethylene glycol, or alicyclic diols such as cyclohexanedimethanol. etc. In the present invention, the polyester preferably contains, for example, alkylene terephthalate and/or alkylene naphthalate as a main component. Among such polyesters, for example, not only polyethylene terephthalate and polyethylene naphthalate, but also terephthalic acid and/or naphthalene dicarboxylic acid account for 80 mol% or more of the total dicarboxylic acid component, and the total glycol component
Particularly preferred are copolymers in which 80 mol% or more is ethylene glycol. In this case, up to 20 mol% of the dicarboxylic acids in the total acid component can be the above-mentioned aromatic dicarboxylic acids, and also include, for example, aliphatic dicarboxylic acids such as adipic acid and sebacic acid;
It can be an alicyclic dicarboxylic acid such as 1,4-dicarboxylic acid. In addition, up to 20 mol% of the total glycol component can be the above-mentioned glycols other than ethylene glycol or polymethylene glycols having side chains, or, for example, hydroquinone, resorcinol, 2,2-bis(4
-Aromatic diols such as (hydroxyphenyl)propane; aliphatic diols containing aromatics such as 1,4-dihydroxymethylbenzene; polyalkylene glycols (polyoxyalkylene glycols) such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. etc. In addition, the polyester referred to in the present invention includes aromatic oxyacids such as hydroxybenzoic acid;
Components derived from oxycarboxylic acids such as aliphatic oxyacids such as hydroxycaproic acid are added to the total amount of dicarboxylic acid components and oxycarboxylic acid components.
It also includes those containing 20 mol% or less. Furthermore, the polyester in the present invention contains a trifunctional or more functional polycarboxylic acid or a polyhydroxy compound, such as trimellitic acid, pentaerythritol, in an amount within a substantially linear range, for example, an amount of 2 mol % or less based on the total acid components. It also includes copolymerized products. The above polyester is known per se, and can be produced by a method known per se. The polyester preferably has an intrinsic viscosity of about 0.4 to about 0.9 as measured as a solution in o-chlorophenol at 35°C. The biaxially oriented polyester film of the present invention has many fine protrusions on its surface. The large number of fine protrusions are derived from the large number of substantially inert solid particles dispersed in the polyester. Polyesters containing a large number of inert solid particles are usually produced at any time during the reaction to form the polyester, for example during the transesterification reaction when using the transesterification method or during the polycondensation reaction or at any time when using the direct polymerization method. It can be prepared by adding inert solid microparticles (preferably as a slurry in glycol) to the reaction system at the time of . Preferably, inert solid particles are added to the reaction system at the beginning of the polycondensation reaction, for example, until the intrinsic viscosity reaches about 0.3. In the present invention, the inert solid fine particles have a short axis diameter of 1 μm or less, a long axis length of 10 μm or less, and a long axis length /
Inert acicular particles having a minor axis diameter in the range of 2 to 10 are preferred. If the minor axis diameter of the particles exceeds 1 μm, the protrusions on the surface will become high, flatness will be lost, and the electromagnetic conversion characteristics will deteriorate for magnetic recording media.
Undesirable. Therefore, the minor axis diameter of the fine particles is 0.7μm
The following is preferable, more preferably 0.4μm or less, especially
It is preferably 0.2 μm or less. The lower limit of the minor axis diameter is not particularly limited, but if it is too small, it will be difficult to obtain the above-mentioned surface properties, so from this point, 0.01 μm or more,
Furthermore, it is preferably 0.03 μm or more, particularly 0.06 μm or more. On the other hand, when the long axis length of the fine particles exceeds 10 μm,
Not only does the flatness and electromagnetic characteristics described above deteriorate, but also frequent breaks occur during stretching for biaxial orientation, which is undesirable. Furthermore, if the ratio of major axis length/minor axis diameter exceeds 10, it is not preferable because if an attempt is made to obtain a biaxially oriented film that satisfies the value of (nx - ny) described above, many breaks occur during stretching. do not have. The ratio of major axis length/minor axis diameter is preferably 8 or less, more preferably 6 or less, particularly preferably
4.5 or less. On the other hand, the ratio of major axis length/minor axis diameter is 2
If it becomes smaller, it becomes difficult to obtain the above-mentioned surface anisotropy, which is not preferable. Major axis length/minor axis diameter is
2.5 or more is preferable, and 3 or more is more preferable. A specific example of inert solid fine particles is iron oxide (γ
−Fe ₂ O ₃ , etc.), cobalt-containing iron oxide (Co−FeOx),
Metal oxides such as chromium dioxide (CrO ₂ ), and
Examples include fine fibrous metal powders such as copper, nickel, and iron, and anisotropic inorganic particles such as calcite and hallosite. These may be further pulverized and used if necessary. In addition, methods include mixing two or more types of inert solid particles, adding silicon oxide, alumina, silicate, and other inorganic particles, and/or adding calcium, lithium, or phosphorus in the polymerization system during polyester synthesis. It may be used in combination with a method of precipitating fine particles containing. In addition, the amount of inert fine particles added is 0.01 to 5.0.
It is preferably 0.1 to 3.0% by weight (based on polyester), particularly 0.2 to 1.0% by weight (based on polyester). If the amount of fine particles increases, the flatness of the film will be lost, and electromagnetic conversion characteristics will deteriorate for use in magnetic recording media, which is not preferable. Furthermore, if the amount of fine particles decreases, although the film becomes flat, it is difficult to obtain a good film winding appearance, which is not preferable. The biaxially oriented polyester film of the present invention can be produced according to conventional methods for producing biaxially oriented films, except for the longitudinal stretching method described below. For example, polyester containing solid fine particles is melt-formed to form an amorphous unstretched film, and then the unstretched film is stretched biaxially, heat-set, and if necessary, subjected to relaxation heat treatment. Manufactured. At this time, the surface characteristics of the film are selected appropriately since they vary depending on the shape, particle size, amount, etc. of the solid fine particles, and also depending on the stretching conditions. For example, the stretching temperature ranges from (Tg-10) to (Tg+45)°C (where Tg is the glass transition temperature of polyester) in the first stage stretching temperature (e.g. longitudinal stretching temperature: T ₁ ) to the second stage stretching temperature. Temperature (e.g. lateral stretching temperature:
T ₂ ) is preferably selected from the range of (T ₁ +10) to (T ₁ +40)°C. In addition, the stretching ratio in the longitudinal direction is 3.5 or more, especially 4 times or more, and the area ratio is 12.
It is preferable to select from a range of 14 times or more, especially 14 times or more. Furthermore, the heat setting temperature is preferably selected from the range of 180 to 250°C, more preferably 200 to 230°C. In producing the biaxially oriented polyester film of the present invention, the longitudinal stretching method is important. That is, in stretching in the longitudinal direction, the stretching speed must be within a specific range, specifically, the stretching speed must be 600%/
It is preferably at least 1 second and at most 10000%/sec.
If the stretching speed is too low, it becomes difficult to obtain a film having anisotropic properties in the longitudinal and transverse directions, which is a feature of the present invention, which is not preferable.
From this point, the stretching speed in the longitudinal direction is 700%/second or more,
More preferably 800%/sec or more, particularly preferably
It is desirable that the rate is 1000%/sec or more. but,
When this stretching speed is greater than 10000%/sec,
Perhaps because the deformation speed was too fast, the stretchability deteriorated,
This is not preferable because it may break during production, making it difficult to obtain a normal film. Longitudinal stretching speed is 10000
%/sec or less, more preferably 5000%/sec or less,
Particularly preferably, it is 1500%/sec or less. Here, the "longitudinal stretching speed" refers to the point at which the film width before longitudinal stretching decreases by 1% to the stretching start point S ₁
The point where the width is the same as the film width after longitudinal stretching is taken as the stretching end point _S2 , and the distance from _S1 to _S2 is ( _S2 - _S1 ) mm, and the speed shown below is . V _MD (%/sec) = X _MD × 100 × V ₁ × 10 ² / (S ₂ − S ₁ ) × 60 [In the formula, V _MD : Longitudinal stretching speed (%/sec) V _MD : Longitudinal stretching Magnification (times) V ₁ : Film speed before longitudinal stretching speed (m/min) S ₂ −S ₁ : Distance from longitudinal stretching start point to end point (mm)] Such longitudinal stretching speed is This can be obtained by adjusting the gap between the pre-stretching heating roll and the post-stretching cooling roll and the film speed. In the longitudinal stretching method that also uses an infrared heater, the length of heating by the infrared heater can also be adjusted. When longitudinal stretching is performed in several stages, the stretching speed of each stage may be within the above range. The biaxially oriented polyester film of the present invention is not particularly limited in its thickness, but it is particularly effective when the film is thin, since it becomes difficult to obtain a good winding shape as the film thickness becomes thinner.
In this sense, the film thickness is preferably 80μ or less. More preferably, it is 25μ or less, particularly preferably 15μ or less. Note that if the film thickness is less than 2μ, the stiffness becomes too small and the winding characteristics change, so it is preferably 2μ or more. Due to its flatness and good winding shape, the biaxially oriented polyester film of the present invention is not only suitable for magnetic recording media, especially for high-density magnetic recording tapes, but also for other general uses such as packaging. Are suitable. [Example] Hereinafter, the present invention will be further explained with reference to Examples. Note that various physical property values and characteristics in the present invention were measured and defined as follows. (1) Particle size The short axis and long axis length of 100 inert particles were measured by electron microscopic observation, and the average value was used. (2) Film surface roughness (Ra) Measured according to JIG B0601. Tokyo Seimitsu Co., Ltd.
A chart (film surface roughness curve) was drawn using a stylus surface roughness meter (SURFCOM 3B) manufactured by Kawasaki Co., Ltd. under the conditions of a needle radius of 2 μ and a load of 0.07 g.
Cut out a portion of measurement length L from the film surface roughness curve in the direction of its center line, and with the center line of this sampled portion as the X axis and the vertical magnification direction as the Y axis, the roughness curve Y = f (x) When expressed as
The value (Ra: μm) given by the following formula is defined as the film surface roughness. Ra=1/L∫ ^L ₀ |f(x)|dx In the present invention, eight measurements were taken using a reference length of 0.25 mm, and Ra was expressed as the average value of the five measurements, excluding three from the largest value. In addition, Ra when the needle is scanned in the vertical direction of the film is RaMD, and RaMD is the value when the needle is scanned in the horizontal direction.
Labeled as RaTD. (3) Friction coefficient (μs) Static friction coefficient μs was measured using a friction coefficient measuring device manufactured by Toyo Tester Co., Ltd. in accordance with ASTM-D1894-63, with the only change being that a load of 1 kg was used. . (4) Young's modulus (Y) Tensilon UTM- manufactured by Toyo Baldwin Co., Ltd.
-500 type was used and measured at 23°C and 50% RH. (5) Winding appearance The film was wound up into a roll with a width of 500 mm and a length of 5000 m, and the appearance of this rolled up roll was inspected in detail and graded from 1st grade to 5th grade. The end face misalignment was graded as follows based on the distance of the end face misalignment in the width direction. Regarding the bump-like protrusions, the number of lump-like protrusions with a major diameter of 2 mm or more as schematically shown in FIG. 1 was counted and graded as follows.

[Table] Furthermore, vertical wrinkles were also graded as follows. Grade 1: You don't want to get wet.Grade 2: You can see faint wavy irregularities with your eyes, but you can't tell when you touch them with your fingers.Grade 3: You can see wavy irregularities with your eyes, and when you touch them with your fingers, you can't tell that there are any irregularities. You can faintly see the unevenness, but if you press it lightly with your finger, the unevenness will collapse (1/8 length
(Less than 1/4 circumference) Grade 4 (Fail) When you touch it with your finger, you can see that it has wavy unevenness, and the unevenness is difficult to crush even when you press with your finger (Length less than 1/4 circumference) Grade 5 (Fail) There is no unevenness even when you press with your finger Items with wavy irregularities that are difficult to crush and whose length is 1/4 or more of the circumference of the roll Example 1 Ethylene glycol (hereinafter abbreviated as EG) 85
In the weight part, the minor diameter is 0.1 μm as inert solid fine particles.
Acicular maghemite (γ-Fe ₂ O ₃ ) with a long axis of 0.4 μm15
After adding parts by weight, mixing and stirring were performed to obtain a slurry. Next, 100 parts by weight of dimethyl terephthalate and
After EG70 weight was subjected to transesterification in a conventional manner using 0.035 parts by weight of manganese acetate tetrahydrate as a catalyst, the above-obtained maghemite (concentration: 0.5% by weight to polymer) was added under stirring. Subsequently, 0.03 parts by weight of trimethyl phosphate and 0.03 parts by weight of antimony trioxide were added, and a polycondensation reaction was carried out in a conventional manner under high temperature vacuum to obtain polyester terephthalate pellets having an intrinsic viscosity of 0.620. Furthermore, the obtained polyester terephthalate (hereinafter abbreviated as PET) pellets were dried at 170°C for 3 hours, then fed to an extruder hopper and melted at a melting temperature of 280 to 300°C, and the molten polymer was passed through a 1 mm slit die to give a surface finish of 0.3S. A 200 μm unstretched film was obtained by extrusion on a rotating cooling drum with a surface temperature of 20°C. The unstretched film thus obtained was heated at 75°C.
by heating at low and high roll surface speeds.
It was stretched 4.3 times and rapidly cooled. The stretching speed at this time is
It was 1400%/second. The film after being stretched in the longitudinal direction was further supplied to a stenter and stretched 3.3 times in the transverse direction at 105°C. The obtained biaxially stretched film was heat-set at a temperature of 205° C. for 5 seconds, and various characteristics of the obtained film were measured. The results were as shown in Table-1. As a result of inspecting the appearance of the roll wound with the film, it was found that there were no end face deviations, nodules, and vertical wrinkles. Example 2 Inert solid fine particles with a short axis of 0.1 μm and a long axis
A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that 1.0% by weight (based on the polymer) of 0.4 μm acicular maghemite was used. The results are shown in Table 1, and the end face deviation, nodules, and vertical wrinkles were all good. Examples 3 to 4 In place of maghemite as inert solid fine particles, hallosite (breadth axis: 0.3 μm, long axis 1.0 μm) or calcite (breadth axis: 0.5 μm, long axis: 2.0 μm)
A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that a predetermined amount of . The results are shown in Table 1, and the end face deviation, nodules, and vertical wrinkles were all good.

【table】

[Table] Comparative Example 1 A biaxially stretched polyester film was obtained in the same manner as in Example 1 except that bulk calcium carbonate (average particle size 0.3 μm) was used instead of maghemite as the inert solid fine particles. The results are shown in Table 1, and although the end face deviation was good, bump-like protrusions were generated and the winding appearance was not good. Comparative Examples 2 to 3 and Example 5 Biaxially stretched polyester films were obtained in the same manner as in Example 1, except that the stretching speed when stretching the unstretched film in the machine direction by 4.3 times was changed. Note that the stretching speed in the longitudinal direction was adjusted by changing the gap between the heating roll and the cooling roll. The results are shown in Table 2. As the stretching speed in the longitudinal direction increases, the anisotropy in the properties in the longitudinal and transverse directions increases, and the overall winding shape becomes better. It became. In the case of Example 5, the properties were good, but since the stretching speed in the longitudinal direction was high, breakage sometimes occurred during stretching, making it difficult to form a stable film.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a diagram schematically showing bump-like protrusions that occur when a film having a small surface anisotropy is wound into a roll.

Claims (1)

[Claims] 1. The surface roughness Ra of the film expressed as center line surface roughness is 0.015 μm or less, and the surface roughness RaMD measured in the longitudinal direction and the surface roughness measured in the lateral direction
The ratio to RaTD is in the range of 1.01≦RaTD/RaMD≦1.30, and the difference between the refractive index nx in the vertical direction and the refractive index ny in the horizontal direction (nx − ny)
is in the range of 0.030 to 0.070, the plane orientation index shown by the following formula is in the range of 0.15 to 0.25, the film-to-film friction coefficient μs is 0.4 or more, and the lateral friction coefficient μsTD is in the longitudinal direction A biaxially oriented polyester film with good windability, characterized by a friction coefficient μsMD in a range of 0.85 to 0.99 times. Planar orientation index = nx + ny / 2 - nz (where, nz: refractive index in the thickness direction nx: refractive index in the longitudinal direction ny: refractive index in the lateral direction) 2 Polyester has a short axis diameter of 1 μm or less and a long axis length of
The biaxially oriented polyester film according to claim 1, which contains at least 0.05% by weight of inert acicular particles having a diameter of 10 μm or less and a ratio of major axis length/minor axis diameter in the range of 2 to 10. 3 The inert acicular particles are γ-iron oxide (γ-Fe ₂ O ₃ ),
The twin shaft according to claim 2, which is at least one member selected from the group consisting of cobalt-containing iron oxide (Co-FeOx), chromium dioxide, fibrous copper, nickel, fine iron powder, calcite, and hallosite. Oriented polyester film.