JPS61261026A - Biaxially oriented polyester film - Google Patents

Abstract

PURPOSE:To obtain a film which is high-quality, applicable to a magnetic recording field and has flatness and a favorable pose of the rolled film, by possessing surface roughness in a specific range and a Young's modulus of more than a specific value. CONSTITUTION:The surface roughness Ra of a film shown by the surface roughness at the center line is less than 0.015mum, which is desirable especially to be less than 0.008mum. The surface roughness RaTD of the film measured in a lateral direction is within a range of 1.05-1.30 times as much as surface roughness RaMD of the film measured in a longitudinal direction, which is desirable especially to be within the range of 1.10-1.25 times as much as that. A Young's modulus YMD in the longitudinal direction is more than 600 kg/cm<2> and a Young's modulus YTD in the lateral direction is within the range of 0.45-0.75 times as much as the Young's modulus YMD in the longitudinal direction. A base film which is capable of corresponding to high density recording of a magnetic recording medium and to increase in its quality by performing in this manner is made obtainable.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a biaxially stretched polyester film, and more particularly to a biaxially stretched polyester film that has both excellent flatness and windability.

[Prior art]

Due to its excellent properties, biaxially stretched polyester films are widely used in many applications such as magnetic tapes, electricity, photography, metallization, and packaging. In particular, because of its properties such as high strength and modulus of elasticity, it is widely used as a base film for magnetic recording media, such as video tapes, audio tapes, computer tapes, floppy disks, etc.

In recent years, these application fields have become highly dense.

The demand for improved product quality is increasing, and along with this, the demand for polyester films to have a flat surface is becoming stronger. however,
When the surface becomes flat, the winding appearance of the film in the process 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 the film roll include (1) Layered protrusions are formed on the roll. ■Wrinkles occur in the longitudinal direction of the film. ■■ is when the edge surface is misaligned and the slipperiness of the film is poor, ■ is when the tension is high <L"C when winding to prevent layered protrusions, and ■ is when winding a flat film. , each of which is likely to occur.

Therefore, the base polyester film is required to have not only flatness but also excellent slipperiness in order to obtain a good film winding appearance.

Conventionally, methods for improving the slipperiness of films include adding inorganic particles such as silicon oxide or calcium carbonate to polyester, or precipitating fine particles containing calcium, lithium, or phosphorus in the polymerization system during polyester synthesis. 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 [7], In magnetic recording media applications, there is a tendency for the surface after application of the magnetic paint to become rough and the electromagnetic conversion characteristics to 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 for composite inorganic particles is too large for the quality required for high-grade grades, and that the protrusions on the surface of the young film become high when the particles are large. Second, the electromagnetic conversion characteristics in magnetic recording media applications are deteriorated, and second, high protrusions are ground down during manufacturing, causing drudgery.

With the prior art, it has been difficult to satisfy the above-mentioned 5K, flatness and slipperiness at the same time.

[Purpose of the invention]

The present inventors have solved the above-mentioned disadvantages and developed a film that has both flatness and good film winding shape that can be applied to high-quality magnetic recording applications. The inventors have discovered that if the properties have a specific anisotropy, a good winding appearance can be obtained when wound up into a roll even if the slip property is conventionally considered to be insufficient, and the present invention has been 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.
Furthermore, the film does not have large protrusions on its surface.
To provide a biaxial polyester film which is flat but has small protrusions that do not cause noise such as droop out, and (b) has a good film roll appearance.

[Structures and Effects of the Invention] According to the present invention, the main feature of the film surface roughness R& expressed by the center line surface roughness is 0.0.
15 μm or less, and the film surface roughness R&TD measured in the horizontal direction is equal to the film surface roughness RaM measured in the vertical direction.
I) is in the range of 1.05 to 1.30 times, and the longitudinal Young's modulus YMD is 600 Kg/mm2 or more,
Furthermore, Young's modulus in the horizontal direction YTD is Young's modulus in the vertical direction YM
The polyester film is made of a biaxially stretched polyester film, which is characterized in that it is 0.45 to 0.75 times as large as D.

The polyester referred to in the present invention is a film-forming aromatic polyester containing aromatic dicarboxylic acid as the main acid component and aliphatic glyfur as the main glyfur component.

The aromatic polyester is substantially linear and has film forming properties, particularly by melt molding. Aromatic dicarboxylic acids include, for example, terephthalic acid,
These include naphthalene dicarboxylic acid, isophthalic acid, diphenoxyethane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid, anthracene dicarboxylic acid, and the like. Aliphatic glycols include, for example, polymethylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, and decamethylene glycol, or fats such as cyclohexane dimetatool. The ring group diono ν etc.

In the present invention, the polyester preferably contains, for example, fullkylene terephthalate and/or alkylene naphthalate as a main component. Among such polyesters, for example, polyethylene tere 7 tally.

Not only polyethylene cutterate, but also copolymers in which 80 moles or more of the total dicarboxylic acid component is terephthalic acid and/or 7 thalene dicarboxylic acid, and 80 mole or more of the total glycol component is ethylene glyfur. preferred. In this case, the dicarboxylic acid of up to 20 mole of the total acid component can be the above-mentioned aromatic dicarboxylic acids, and also, for example, aliphatic dicarboxylic acids such as adipic acid and cehatic acid; alicyclic acids such as cyclohexane-1,4-dicarboxylic acid. dicarboxylic acids and the like.

In addition, 2oMos-Z or less 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(
Aromatic diols such as 4-hydroxyphenyl)propane; aliphatic diols containing aromatics such as 114-dihydroxymethylbenzene; polyalkylene glycols such as polyethylene glycol, polypylene glycol, polytetramethylene glycol, kylene glycol), etc.

In addition, the polyester in the present invention includes a component derived from an oxycarboxylic acid such as an aromatic oxyacid such as hydroxybenzoic acid; an aliphatic oxyacid such as ω-human p-xicaproic acid; a dicarboxylic acid component and an oxycarboxylic acid component; Those containing 20 molti or less based on the total amount of acid components are also included. Furthermore, the polyester in the present invention contains a trifunctional or higher functional polycarboxylic acid or a polyhydroxy compound, such as trimellitic acid, in a substantially linear amount, for example, an amount of 2 mole or less based on the total acid component.

Copolymerized pentaerythritol is also included.

The above polyester is known per se, and can be produced by a method known per se.

The above polyester has an intrinsic viscosity of about 0.4 measured at 35°C as a solution in 0-RF phenol.
~0.9 is preferred.

The biaxially stretched polyester film of the present invention has many fine protrusions on its surface. The large number of fine protrusions are derived from a large number of substantially inert solid particles dispersed and contained 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) into the reaction system at the time of.

Preferably, at the initial stage of the polycondensation reaction, for example, the intrinsic viscosity is about 0.
It is preferable to add inert solid fine particles to the reaction system during the period up to step 3.

In the present invention, the inert solid fine particles are preferably silicon dioxide (hydrate).

(including diatomaceous earth, silica sand, 1 quartz, etc.); ■Alumina; ■S
Silicates containing tow content of 30% by weight or more (for example, amorphous or crystalline clay minerals).

Fluminosilicate (including calcined products and hydrated products), warm asbestos, zircon, fly ash, etc.); 0 Mg r Zn
* Oxides of Zr and Ti: @Ca, and sulfates of B&: @phosphates of Li, Na, and Ca (including monohydrogen salts - dihydrogen salts); ■L ball.

Benzoate of Na and K; ■Ca HBa # Zn
r and Nagi terephthalate: 0 Mg r Ca *
BaIZn, Cd, PdrSr,
Mn*Titanates of Fs, Co and Ni: @Chromates of Ba+ and pb; ■Carbon (e.g. carbon black, graphite, etc.); @Glass (
(e.g. glass powder, glass peas, etc.); ◎Ca and Mg
carbonate; 0 fluorite; and [phase] ZnS. More preferably, anhydrous silicic acid, hydrated silicic acid, aluminum oxide, aluminum silicate (including calcined products, hydrates, etc.), monolithium phosphate, trilithium phosphate, sodium phosphate, and calcium phosphate.

Barium sulfate, titanium oxide, lithium benzoate, double salts of these compounds (including hydrates), glass powder, clay (kaolin, bentonite).

(including white clay, etc.), talc, diatomaceous earth, calcium carbonate, etc. Particularly preferred are silicon dioxide and calcium carbonate.

In any case, the inert solid fine particles preferably have a volume shape factor (f) in the range of 0.08 to π/6. The volume shape factor (f) is determined in °C using the following equation.

■ftm□P (where ■ is the volume of the fine particles, and this volume is the actual pickling value of the sphere calculated from the Stokes average particle diameter determined by the sedimentation method. Also, D is the diameter of the fine particles, and the nucleus The diameter is the arithmetic mean value of the maximum value of each fine particle measured using a microscopic photograph of the fine particles.) In addition, inert solid fine particles have an average particle size of Q, Q2
-0.6 pm, more preferably 0.04-0.5 μm, especially 0.06-0.3 μm, and the amount added is 0.01-2.5% by weight (based on polyester), and 0.05-1.2% by weight (same), especially 0.1-0.6
It is preferable that it is % by weight (same). If the average particle size is larger than the above-mentioned average particle size (and the amount added increases), the flatness of the film will be lost and the electromagnetic conversion characteristics will deteriorate in magnetic recording media, so it is not preferable. If the particle size is smaller or the amount added is less, the film becomes flat, but it is difficult to obtain a good film winding appearance, which is not preferable.Two or more types of the above-mentioned inert solid particles can be used at the same time.

The biaxially stretched polyester film of the present invention has film surface characteristics such as: temperature (°C), film surface roughness Ra expressed by center line surface roughness of 0.015 μm or less, and film surface roughness RITD measured in the transverse direction of the film. Film surface roughness measured in the longitudinal direction: 1.05 to RaMDK
It is in the range of 1.30 times. The latter characteristic is, in other words,
It can be expressed as 1.05≦RaTD/RaMD≦1.30. This film surface roughness Ra is 0.015 μm
If it is thicker, the surface of the base film will be too rough and the magnetic surface cannot be made sufficiently flat for use in magnetic recording media, resulting in poor electromagnetic conversion characteristics, which is not preferable.

The preferred surface roughness (Ra) is 0.010 μm or less, particularly 0.008 μm or less. The lower limit of surface roughness Ra) is not particularly limited, but usually surface roughness Ra) is O
, OO1 nm or more, and preferably 0,002 μm or more from the viewpoint of obtaining a good film winding shape.

Also, the vertical surface roughness IIMD and the horizontal surface roughness Ra
If the ratio RarD/RamD to TD is less than 1.05, layered protrusions will occur when the base film is rolled up into a roll, resulting in an unfavorable winding appearance.
It is particularly preferable that TD/RaMD is 1.07 or more, more preferably 1.10 or more. However, RaTD/RaMD
If it is larger than 1.30, it is undesirable because the end face misalignment occurs when the base film is wound onto a roll, resulting in poor winding appearance.From this point of view, Ra T D / R
It is preferable that a M D is 1.25 or less, and K is 1.20 or less.

Furthermore, 1 the above biaxially stretched polyester film has film surface properties such that the film-to-film friction coefficient μS is 0.5 or more, and the lateral friction coefficient μsTD is O,S with respect to the longitudinal friction coefficient μsMD. O~0.98
Preferably, the range is twice that. In other words, this latter characteristic means that the ratio of the longitudinal friction coefficient μ3MD to the lateral friction coefficient μsTD is 0.80≦μaTD/μsMD≦0.
．． It is represented by 98. If this ratio becomes too small,
When the base film is rolled up into a roll, the base film slips too much in the lateral direction and the end face shifts, resulting in poor winding appearance, which is undesirable. On the other hand, if this ratio is too thick, layered protrusions will occur when the base film is rolled up.
This is not preferable because it makes the roll look bad. From this point, μa
TD/μsMD is further 0°83 or more, especially 0.86
It is preferably greater than or equal to 0.96, and even more preferably less than or equal to 0.96.
It is preferable that the KO is 193 or less. Also, gsT
Both D and psMD are preferably KO, 50 or more. When μ9TD and μsMD are smaller than 0.5, it is not preferable that p s TD / /J m MD becomes 0.98 or less because end face deviation occurs. More preferably, both 4sTD and μsMD are 0.55 or more, particularly 0.60 or more. μ@TD.

Although there is no particular upper limit for μm1MD, it is usually 1.8 or less, and if it is thicker than 1.8, it is not preferable because μ@TD/μsMD is within the above range and layered protrusions occur at °C. In this sense, both pgTD and psMD are preferably 1.30 or less, and K is particularly preferably 0.98 or less.

The biaxially stretched polyester film of the present invention has the above-mentioned film surface properties, and at the same time has Young's modulus YM in the machine direction.
D is 600 Kg/mm2 or more, and the ratio of Young's modulus in the lateral direction to YTD (YTD/YMD) is 0.45≦YT
D/YM is in the range of D≦0.75. This YT
When D/YMD is smaller than 0.45, the stiffness in the horizontal direction becomes too small than the stay 7ness in the longitudinal direction, which causes vertical wrinkles when winding up the base film, which is not preferable. Therefore, YTD/YMD is 0.45
It is desirable that it is more preferably 0.50 or more, particularly preferably 0.55 or more. However, YT
If D/YMD is larger than 0.75, layered protrusions will occur when the base film is rolled up, even if the base film has the above-mentioned surface properties, which is not preferable. Follow 'C, YTD
/YMD is desirably 0.75 or less, more preferably 0.70 or less, particularly preferably 0.65 or less.

On the other hand, even if YTD/YMD is within the above range, YMD
If it is smaller than 5ooKg/mm2, layered protrusions are likely to occur, which is not preferable. From this point,
YMD is 60011/- or more, more preferably [, (is 64
It is desirable that it be 0 to /mj or more, particularly preferably 680 Kg/mm2 or more. The upper limit of Young's modulus YMD in the longitudinal direction is not particularly limited, but is usually 1500 Kg/mm.
2 or less, and it is thicker than tsooK9/- (as described later, the stretchability is poor (as described later), and vertical wrinkles occur when the film is rolled up into a roll shape. (I don't like it.

From this point of view, it is desirable that the YMD is 150011 g/- or less, more preferably t200 Kf/*J or less, particularly preferably 900 Kg/mm2 or less.

The biaxially stretched polyester film of the present invention can be produced according to conventional methods for producing biaxially stretched films, except for one method of stretching in the longitudinal direction, which will be described later. For example, a polyester containing solid fine particles is melt-formed to form an amorphous unstretched film, and then the unstretched film is biaxially stretched and subjected to heat setting (-relaxation heat treatment if necessary). Manufactured by KotoK. At that time, the film surface characteristics are
The shape of the solid fine particles varies depending on the particle size, amount, etc., and also depending on the stretching conditions, so it is selected appropriately. For example, the stretching temperature is such that the first-stage stretching temperature (for example, longitudinal stretching temperature: 'r) is in the range of (Tg-10) to (Tg+45)'C (provided that .

If the second-stage stretching temperature (for example, lateral stretching temperature 2T) is selected from the range of (T, +10) to (T, +, co) °C, good. In addition, the stretching ratio in the longitudinal direction is 3.5
In particular, it is preferable to select from a range in which the area magnification is 4 times or more and the area magnification is 12 times or more, particularly 14 times or more. Furthermore,
Heat setting temperature is 180-250℃, even 200-230℃
It is recommended to select from the range of ℃.

In producing the biaxially stretched polyester film of the present invention, the stretching method in the longitudinal direction 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 1,500 inches/second or higher and 40,000 inches/second.
Preferably, it is less than a second. If this stretching speed is too low, it becomes difficult to obtain a film with 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 1 soo%
It is desirable that the speed is at least 2500%/sec, more preferably at least 2500%/sec, particularly preferably at least 4000%/sec. However, if this stretching speed is higher than 40,000%/sec, it is not preferable because the deformation speed is too fast, and the stretching property deteriorates, and it becomes difficult to obtain a normal film due to trap breakage during production. , machine direction stretching speed is 40000%
It is desirable that the speed is at most 20,000 cm/sec, more preferably at most aoooo%/sec, particularly preferably at most 20,000 chi/sec.

Here, the term ``longitudinal stretching speed'' refers to the point at which the film width decreases by 1% after stretching in the longitudinal direction as the stretching start point S, and the point at which the width becomes the same as the film width after stretching in the longitudinal direction as the stretching end point. s8
Toshi, Sara K.S.

The distance from S to S is (Ss −8+ ) wa, and the speed is shown below.

[Such a longitudinal stretching speed 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 K of heating by the infrared heater can also be adjusted. When longitudinal stretching is performed in several stages, each stage needs to be within the above range.

The biaxially stretched polyester film of the present invention is not particularly limited in its thickness, but it is particularly effective when the film thickness is thin, since it becomes difficult to obtain a better rolled shape as the film thickness becomes thinner. In this sense, the film thickness is 8
It is preferable that it is 0μ or less. More preferable [5 or 2
It is desirable that the thickness be 5μ 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.

Example〕 The present invention will be further explained below with reference to Examples.

Note that various physical property values and characteristics in the present invention were measured and defined as follows.

(1) Average particle size of inert solid fine particles C manufactured by Takashi Seisakusho
P-50111Centrifugal Partl
C. was measured using a cleSize Analyser. The particle size corresponding to 50 mass percent K is read from the cumulative curve of particles of each particle size and their abundance calculated based on the obtained centrifugal sedimentation curve, and this value is used as the above average particle size. (Book Particle Size Measurement Technique), published by Nikkan Kogyo Shimbun, 1975, pp. 1, 242-24
(see 7).

(2) Film surface roughness Ra) JISBO6
Measured according to 01. Using a P-stylus surface roughness meter (SURFCOM3B) manufactured by Tokyo Seimitsu Co., Ltd., the radius of the needle was 2μ.
, a chart (film surface roughness curve) was drawn under the conditions of a load of 0.07 f. A part of measurement length L is extracted from the film surface roughness curve in the direction of its center line, and the center line of this extracted part is set as the X axis, and the direction of vertical magnification is set as the Y axis, and the roughness curve is determined as ), the value (Ra: μm) given by the following formula is defined as the film surface roughness.

In the present invention, Ra is expressed as the average value of 5 measurements after 8 measurements were made with a reference length of 0.25 m, and 3 of the largest values were removed.

Nao, when the needle is scanned in the vertical direction of the film (7) R
A is expressed as RaMn, and the case where the image is scanned in the horizontal direction is expressed as RaTD.

(3) Friction coefficient (μS) Using a friction coefficient measuring device manufactured by Toyo Tester Co., Ltd., ASTM
() The static friction coefficient μs was measured using a method similar to 1894-63, with the only change being that a 1-chest load was used.

(4) Young car (Y) Tenseen UTM-1-500 manufactured by Toyo Baldwin Co., Ltd.
Measurements were made at 23°C and 504RH using a base plate.

(5) Roll up the rolled film A into a roll with a length of 500 m and a length of 5000 m.
Graded from grade to grade 5.

The end face misalignment was graded as follows based on the distance of the end face misalignment in the width direction.

Regarding knob-like protrusions, protrusions with symptoms as schematically shown in FIG. 1 with two or more major axis fins and longitudinal wrinkles were also graded as follows.

1st grade: No wavy surface. 2nd grade: There are faint wavy irregularities, but you can't tell even if you touch them with your finger. 3rd grade: You can see wavy irregularities, and when you touch them with your finger, you can't tell that there are any wavy irregularities. I can faintly see that there is something, but when I press lightly with my finger, the unevenness collapses (length 178 laps or less) Grade 4 (fail) When I press with my finger, I can see that there is a wavy unevenness, and even when I press with my finger, the unevenness collapses. Difficult grade 5 (fail) Items with wavy unevenness that is difficult to crush even when pressed with a finger, and whose length is 1/4 or more of the circumference of the roll Example 1 Ethylene glycol (hereinafter abbreviated as EC) ) to 85 parts by weight, the weight loss rate at 501) °C is i.

After adding 15 parts by weight of calcium carbonate (average particle size: 0.4 m), mixing and stirring were performed to obtain a slurry. The slurry is filtered and the top product is B o o pp.
It was m.

Next, 100 parts by weight of dimethyl terephthalate and EC70
Calcium carbonate (concentration = 0.2% by weight) obtained above after transesterification in a conventional manner using 0.070 parts by weight of manganese tetrahydrate as a catalyst (0.035 parts by weight)
was added under stirring. Subsequently, after adding 0.03 parts by weight of trimethyl phosphate and 0.03 parts by weight of antimony trioxide, a polycondensation reaction was carried out in a conventional manner under high temperature vacuum to obtain an intrinsic viscosity of 0.6.
20 polyethylene terephthalate pellets were obtained.

Furthermore, the obtained polyethylene terephthalate (hereinafter referred to as PET
After drying the pellets at 170℃ for 3 hours, the pellets are fed to an extruder hopper and melted at a melting temperature of 280 to 300℃.The molten polymer is passed through a 1m slit die with a surface finish of about 0.3S and a surface temperature of 20℃. An unstretched film of 200 μm was obtained by extrusion on a rotating cooling drum.

The unstretched film thus obtained was heated at 75°C, stretched 4.8 times at low and high roll surface speeds, and then rapidly cooled. The stretching speed at this time was 8000%/sec. The film after being stretched in the longitudinal direction was further supplied to a stenter and stretched 3.5 times in the transverse direction at 1.05°C. The obtained biaxially stretched film was heat-set at a temperature of 205°C for 5 seconds, and the biaxially stretched heat-set film was then reheated to 120°C to achieve a relaxation rate of 0.5 inches (heating temperature and cooling temperature). The film was relaxed in the longitudinal direction at a speed difference between the two wheels, and the properties of the resulting film were measured. The results were as shown in Table 1 and Example 10.

As a result of inspecting the appearance of the roll wound up with the film, it was found that there were no end face deviations, lamellar protrusions, or vertical wrinkles.

Examples 2 to 3 As inert solid fine particles, silica (average particle size 0.2 μfn) or titanium oxide (
A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that a predetermined amount of polyester (average particle size: 0.3 μm) was used.

The results are shown in Table 1 and Example 2. The results were as in Example 30. The rolls obtained by winding up these films had good end face deviations, layered protrusions, and vertical wrinkles.

Examples 4-6. Comparative Examples 1 and 2 Calcium carbonate (average particle size 0.
A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that 2 ttm ) was used, xlfl (versus the polymer), and the stretching speed in the longitudinal direction was changed. At that time, the stretching speed in the longitudinal direction was adjusted by changing the gap between the heating roller and the cooling roller [7].

The results were as shown in Table-2.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing lamellar protrusions that occur when a film with a low anisotropy of surface properties is rolled up into a roll.

Claims (1)

[Claims] 1. Film surface roughness Ra expressed by centerline surface roughness
is 0.015 μm or less, the film surface roughness RaTD measured in the transverse direction is in the range of 1.05 to 1.30 times the film surface roughness RaMD measured in the longitudinal direction, and the Young's modulus in the longitudinal direction is Y_M_D is 600Kg/mm^2
A biaxially stretched polyester film with good windability, characterized in that the Young's modulus Y_T_D in the transverse direction is in the range of 0.45 to 0.75 times the Young's modulus Y_M_D in the longitudinal direction. 2. The film-to-film friction coefficient μs is 0.5 or more, and the lateral friction coefficient μsTD is the longitudinal friction coefficient μ
The biaxially stretched polyester film according to claim 1, wherein the biaxially stretched polyester film has a sMD of 0.80 to 0.98 times.