JP2000025107A - Biaxially oriented polyester film and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To have high strengths lengthwise and widthwise and realize a favorable frictional resistance and slipperiness by a method wherein the lengthwise Young's modulus of a film and the ratio of the lengthwise static friction coefficient of the film to its widthwise static friction coefficient are set to have specified values. SOLUTION: The lengthwise Young's modulus of a film is set to be 5-10 GPa. The ratio of the lengthwise static friction coefficient of the film to its widthwise static friction coefficient μsMD/μsTD is set to be 1.05-1.25. In order to realize respective desired surface roughnesses on respective surfaces, a lamination structure consisting of two layers or more is employed under the condition that the surface roughnesses of the respective surfaces are respectively set to be 0.008 μm or more and 0.005 μm or less in order to preferably reconcile the outputting characteristics and slipperiness of the film used for a magnetic tape. As the polyester film, a polyethylene terephthalate, a polybutylene naphthalate, a polybutylene terephthalate, a polyethylene naphthalate and their copolymer or the like is preferable. These repeating units of 100 or more and intrinsic viscosities of 0.6 dl/g or more are preferable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially stretched polyester film and a method for producing the same, and more particularly, to a biaxially stretched polyester film suitable for magnetic recording media, for blinter ribbons, for capacitors, for packaging, and for various industrial materials. The present invention relates to a polyester film and a method for producing the same.

[0002]

2. Description of the Related Art Biaxially stretched polyester films are used for various purposes because of their excellent thermal stability, dimensional stability, and mechanical properties, but are particularly useful as base films for magnetic tapes and the like. Sex is well known. In recent years, further reduction in the thickness of the base film has been demanded in order to reduce the weight, size, and long-term recording of equipment. In recent years, there has been a very strong tendency for thin films for thermal transfer ribbons and capacitors. However, when the film is thinned, the mechanical strength becomes insufficient, and the stiffness of the film becomes weak or easily stretched. The conversion characteristics are degraded, and other problems also occur in other applications due to insufficient mechanical strength.

[0003]

SUMMARY OF THE INVENTION The present invention has been achieved as a result of studying to solve the problems in the prior art described above. That is, an object of the present invention is to have high strength in both the longitudinal direction and the width direction, and have excellent wear resistance,
An object of the present invention is to provide a biaxially stretched polyester film having good slipperiness and excellent film formation stability, and a method for producing the same.

[0004]

In order to achieve the above object, the biaxially stretched polyester film of the present invention has a Young's modulus in the longitudinal direction of the film in the range of 5 GPa to 10 GPa, and The ratio μsMD / μsTD of the coefficient of static friction in the width direction is in the range of 1.05 to 1.25.

Further, in the method for producing a biaxially stretched polyester film according to the present invention, the above-mentioned film is converted into a substantially amorphous polyester film, and the birefringence (Δn) of the film is 0.02 or less. It is manufactured by a method characterized by stretching biaxially in the vertical and horizontal directions so that the degree of crystallinity is 10% or less, and then performing biaxial stretching to impart orientation.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail together with preferred embodiments thereof. In the present invention, in order to obtain a film having high strength in both the longitudinal direction and the width direction and having excellent wear resistance, the Young's modulus in the longitudinal direction of the film is in the range of 5 GPa to 10 GPa, And the ratio of the static friction coefficient in the width direction to μsMD /
μsTD needs to be in the range of 1.05 to 1.25, and preferably has a Young's modulus of 6 G in the longitudinal direction of the film.
Pa to 8 GPa, and the ratio μsMD / μsTD of the coefficient of static friction between the film longitudinal direction and the width direction is 1.1 to
1.2.

The Young's modulus in the longitudinal direction of the film is 5 GPa
If it is less than 10, the mechanical strength in the longitudinal direction becomes insufficient in various applications, and it becomes difficult to reduce the film thickness. For example, in magnetic tape applications, the stiffness becomes weak or easily stretched, and the tape is easily damaged. It is preferable that the Young's modulus in the longitudinal direction is high from the viewpoint of the mechanical strength. If the Young's modulus is too high, it is difficult to balance with the strength in the film width direction. For example, in a film in which the Young's modulus in the longitudinal direction of the film is extremely high and the Young's modulus in the film width direction is extremely low, not only the stiffness in the width direction of the film is weakened, but also the unbalance of the orientation on the film surface becomes large. As a result, high wear resistance of the surface cannot be obtained. From such a viewpoint, in the present invention, the upper limit of the Young's modulus in the longitudinal direction is set to 10 GPa.

In addition, while achieving the above Young's modulus, the ratio μsMD / μs of the static friction coefficient between the longitudinal direction and the width direction of the film.
When the TD is in the range of 1.05 to 1.25, a film having a more desirable abrasion resistance in both directions can be obtained. Static friction coefficient ratio μsMD
When / μsTD is smaller than 1.05, the destruction of the projections and the falling off of the particles are likely to occur. Also, the ratio of static friction coefficient μs
When the MD / μsTD is larger than 1.25, slippage occurs too much in the width direction, so that post-processing or the like may cause a problem, and similarly, it is easy to cause protrusion destruction and drop-off of particles. Therefore, in order to obtain a film with balanced wear resistance, the static friction coefficient ratio μsMD / μsTD needs to be in the range of 1.05 to 1.25.

The biaxially stretched polyester film of the present invention preferably has a Young's modulus in the width direction of 7 GPa to 20 GPa in order to obtain a film having high strength in the width direction and excellent wear resistance. More preferably 8GP
a to 15 GPa.

The biaxially stretched polyester film of the present invention preferably has each surface within a desired surface roughness range. For that purpose, the surface roughness of each surface is set to be 0.008 μm or more, and the surface roughness of the opposite surface is set to R.
It is preferable that a is 0.005 μm or less, for example, in order to achieve both output characteristics and slipperiness in a magnetic tape application, more preferably, one-side surface roughness Ra is 0.009.
μm or more, and the surface roughness Ra of the opposite surface is 0.00
4 μm or less. The upper limit of the surface roughness Ra on one side is not particularly limited, but is preferably 0.02 μm or less. The lower limit of the surface roughness Ra on the opposite side is not particularly limited, but is 0.0005 μm. It is preferable that this is the case.

In the present invention, the polyester is a polymer compound having an ester bond in the molecular main chain, and is a polymer obtained by condensation polymerization from a diol and a dicarboxylic acid. Dicarboxylic acids are terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid,
The diol is represented by sebacic acid and the like, and the diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. In the present invention, in particular, polyethylene terephthalate (PET) or a copolymer thereof, polybutylene naphthalate (PBN) or a copolymer thereof, polybutylene terephthalate (PB)
T) or a copolymer thereof, and polyethylene naphthalate (PEN) and a copolymer thereof are preferably used. The repeating unit of these polyesters is 10
It is preferably 0 or more, particularly 150 or more, and the intrinsic viscosity is preferably 0.6 dl / g or more, more preferably 0.7 dl / g or more. In such a case,
Excellent in film-forming stability and preferred. Of course, in these polyesters, known additives, such as lubricants, stabilizers,
Antioxidants, viscosity modifiers, antistatic agents, colorants, pigments, and the like can be arbitrarily compounded.

Although not particularly limited, the polyester film of the present invention is imparted with lubricity, and has good handling properties in the production and processing steps and good running properties when used as a product such as a magnetic tape. Therefore, it is also preferable to include inert particles such as inorganic particles and organic particles. As the inorganic particles, silicon dioxide, calcium carbonate, aluminum oxide, zirconium oxide and the like can be used. As the organic particles, an ethylvinylbenzenedivinylbenzene copolymer, polymethyl methacrylate, and silicone can be used. These inert particles are used alone or in combination of two or more.

As described above, the polyester film of the present invention may be a laminated film having two or more layers. In the case of a laminated film in which two or more layers are laminated, the ratio (d / t) between the average diameter (d) of the particles contained in at least one outer layer and the layer thickness (t) is 0.1 to 10 and Preferably, there is. By doing so, the position at which the particles are contained can be efficiently controlled in the outermost layer,
Desired surface roughness and the like can be easily obtained.

Next, the method for producing the polyester film of the present invention will be specifically described. First, a sufficiently dried polyester raw material pellet is supplied to a known extruder and, after passing through a filter selected as necessary, is extruded into a sheet shape on a rotating metal casting drum by a T-type die, and cooled. It is solidified to obtain a non-oriented film. Alternatively, the undried pellets are supplied to a vented extruder to obtain a non-oriented film in the same manner. The ratio (A / B) between the maximum thickness (A) of the edge portion of the non-oriented film and the thickness (B) of the central portion in the width direction.
Is preferably controlled in the range of 2.0 to 6.0.
In this case, meandering, width fluctuation, stretching unevenness, and breaking frequency in the longitudinal stretching step can be effectively suppressed.

The obtained non-oriented polyester film is stretched biaxially in the vertical and horizontal directions. In the present invention, the vertical means the longitudinal direction of the film, and the horizontal means the width direction. The longitudinal and transverse biaxial stretching is performed by a sequential biaxial system or a simultaneous biaxial system. In the sequential biaxial method, the film is stretched in the longitudinal direction by utilizing the peripheral speed difference between the rolls, and is stretched in the transverse direction using a stenter, although not particularly limited. The order of stretching in the vertical and horizontal directions is not particularly limited. In the simultaneous biaxial method, stretching is performed using a simultaneous biaxial tenter. The stretching temperature in the biaxial stretching in the machine direction is in the range of the glass transition temperature (Tg) to (Tg) + 60 ° C. of the polyester, and the more preferred stretching temperature is (Tg) + 15 ° C. to (Tg) +45.
It is in the range of ° C. The stretching ratio in both the sequential biaxial system and the simultaneous biaxial system is in the range of 1.5 to 8 times, more preferably 2.5 to 6 times, the total stretching ratio in the longitudinal direction and the width direction of the film. Here, the total stretching ratio is a ratio in the longitudinal direction × a ratio in the width direction.

The birefringence (Δn) of the biaxially stretched film obtained in this manner must be in the range of 0 to 0.02, preferably in the range of 0 to 0.01. When the birefringence is within the above range, a film excellent in mechanical strength and abrasion resistance can be obtained not only in the longitudinal direction but also in the lateral direction of the film. When the birefringence exceeds 0.02, the stretchability deteriorates, and a film having high mechanical strength and excellent wear resistance as described above cannot be obtained.

The crystallinity of the film after longitudinal stretching is 1
0% or less, preferably 8% or less, more preferably 6%
Or less, still more preferably 2% or less. If the crystallinity of the film exceeds 10%, the stretchability becomes poor in the subsequent stretching step, and the film is frequently broken during stretching, which is not preferable. By forming a biaxially stretched film that satisfies the above ranges of birefringence and crystallinity at the same time, high mechanical strength and excellent wear resistance as shown in the present invention can be exhibited in the subsequent stretching step.

The biaxially stretched film obtained as described above may be subsequently stretched biaxially again. The longitudinal and transverse biaxial stretching is performed by a sequential biaxial system or a simultaneous biaxial system. The method of sequential biaxial stretching is not particularly limited, but it is usual to stretch in the longitudinal direction by utilizing the peripheral speed difference of the roll, and to stretch in the transverse direction by using a known stenter.
The order of stretching in the vertical and horizontal directions is not particularly limited.

In the simultaneous biaxial system, the film is stretched using a simultaneous biaxial tenter. In this case, the clip may be driven by any of a screw system, a pantograph system, and a linear motor system.

In the present invention, re-stretching can be further performed. This re-stretching can be performed in either the machine direction or the cross direction.

The total stretching ratio in the machine direction and the transverse direction of the biaxially stretched film thus stretched is preferably in the range of 30 to 100 times, more preferably 40 to 8 times.
The range is 0 times.

The biaxially stretched film thus obtained is subjected to a heat treatment in a tensioned state or a relaxed state in order to impart flatness and thermal dimensional stability. The desired Young's modulus in the longitudinal direction of the film is in the range of 5 GPa to 10 GPa, and the ratio of the static friction coefficient between the longitudinal direction and the width direction of the film is μsMD / μs.
A biaxially stretched polyester film having a TD in the range of 1.05 to 1.25 is obtained.

The total thickness of the polyester film in the present invention is not particularly limited, but can be appropriately determined according to the use and purpose as described below as an example. Usually, it is preferably 1 μm or more and 20 μm or less for magnetic materials, 1 μm or more and 6 μm or less for thermal transfer ribbons, and 0.1 μm or less for capacitors.
It is preferably not less than m and not more than 15 μm.

Further, in the present invention, a film whose surface is modified by attaching a resin coat layer represented by urethane, acryl, ester, silicon, wax or the like to the surface of the film may be used. In this case, it is preferable to perform the surface modification in the middle of the film forming line from the viewpoint of reducing the production cost.

[Evaluation of Physical Properties] (1) Birefringence (Δn) The retardation of the film was measured using a Berek compensator under a polarizing microscope, and the birefringence (Δ
n) was determined. Δn = R / d R: retardation d: film thickness

(2) Crystallinity The density of the film was measured by using a sodium bromide aqueous solution according to the density gradient tube method of JIS-K-7112, and the crystallinity and amorphous density of the polyester were measured using the density. The crystallinity (%) was determined from the following equation. Crystallinity = {(density of film−amorphous density) / (crystal density−amorphous density)} × 100 In the case of PET: amorphous density: 1.335 g / cm ³ crystal density: 1.455 g / cm ³

(3) Young's modulus of film A sample film was made 10 mm in width and 100 mm in length using a tensile strength and elongation automatic measuring device “Tensilon Universal Tester UCT-100” manufactured by Orientec Co., Ltd., and the tensile speed was 200 mm. / Min was measured. The Young's modulus was determined from the slope of a straight line connecting the origin of the obtained tension-strain curve and the point at 1% elongation. The measurement was performed in an atmosphere of 25 ° C. and 65% RH.

(4) Intrinsic viscosity Measured at 25 ° C. using o-chlorophenol as a solvent.

(5) Surface roughness Ra (center line average roughness) Surface roughness meter (manufactured by Kosaka Laboratories, high-precision thin-film level measuring device ET)
-10) was measured under the following conditions, and the average of 20 measurements was taken as the Ra value.・ Stylus tip radius: 0.5 μm ・ Stylus load: 5 mg ・ Measurement length: 1 mm ・ Cutoff value: 0.08 mm

(6) Coefficient of static friction μs Using a slip tester (ASTM-D-1894-63), a weight of 200 g and a contact area of 63.5 × 63.5 are used.
mm ² , a moving speed of 150 mm / min, a moving distance of 5 mm, and an initial moving distance of 10 mm.

(7) Abrasion resistance A film slit into a tape having a width of 1/2 inch is repeatedly run on guide pins using a tape running tester (guide material: SUS, surface roughness) :
0.3S). Apply a load of 200 g to one end of the film,
After running 20 passes at a winding angle of 90 ° and a running speed of 3.3 cm / sec, the amount of white powder adhering to the guide pins was evaluated, and the surface of the film was further observed with a differential interference microscope. Was evaluated. Magnification 100
Observation was performed for 10 visual fields at 0 magnification, and the ratio of the destruction of projections and the dropout of particles relative to the total number of projections was evaluated on a three-point scale. -Destruction of protrusions and dropout of grain size are hardly observed. : Excellent ・ Positions where protrusions are broken and particles fall off are less than 2% of the total number of protrusions, and there is almost no white powder adhesion. : Good ・ Positions where protrusions are broken and particles fall off are 2% or more of the total number of protrusions, and there is a white powder adhesion amount. : Bad

(8) Sliding property A film obtained by slitting a film into a tape having a width of 6.35 mm was measured at 20 ° C. and 60% RH using a tape running tester SFT-700 (manufactured by Yokohama System Laboratory Co., Ltd.). After running in an atmosphere, the initial coefficient of friction was determined by the following equation. μk = (2 / π) 1n (T ₂ / T ₁ ) Here, T ₁ is the tension on the input side, T ₂ is the tension on the output side, the guide diameter is 6 mmφ, and the guide material is SUS27 (surface roughness) 0.2S), the winding angle is 90 °, and the running speed is 3.
3 cm / sec. When the dynamic friction coefficient μk obtained by this measurement was 0.3 or less, it was determined that the slip property was good, and when it exceeded 0.3, it was determined that the slip property was poor.

(9) Output Characteristics (C / N) The following composition was applied to the surface of the film of the present invention by a ball mill for 48 hours.
After mixing and dispersing for 6 hours, a kneaded material obtained by adding 6 parts of a curing agent was filtered through a filter, and a magnetic paint was applied by a gravure roll, magnetically oriented, and dried at 110 ° C. Further, a small test calender (styrene / The mixture is cured with a nylon roll (5 steps) at a temperature of 70 ° C. for 48 hours.
Slit the above raw tape to create a pancake,
This pancake was incorporated into a VTR cassette to form a VTR cassette tape. (Magnetic paint composition) Fe: 100 parts Average particle size Length: 0.3 μm Needle ratio: 10/1 Coercive force: 2000 Oe Polyurethane resin: 15 parts Vinyl chloride / vinyl acetate copolymer: 5 parts・ Nitrocellulose resin: 5 parts ・ Aluminum oxide powder (average particle size: 0.3 μm): 3 parts ・ Carbon black: 1 part ・ Lecithin: 2 parts ・ Methyl ethyl ketone: 100 parts ・ Methyl isobutyl ketone: 100 parts ・ Toluene: 100 Part-Stearic acid: 2 parts A commercially available Hi8 VTR (SONY
7MHz ± 1MH using EV-BS3000
C / N measurement of z was performed. This C / N was evaluated as follows by comparing with a commercially available video tape for Hi8. ：: +3 dB or more ：: +2 dB or more and less than +3 dB Δ: +1 dB or more and less than +2 dB ×: less than +1 dB

(10) Break Frequency In the process of producing a biaxially stretched polyester film, the break frequency was determined according to the following criteria. ：: When tearing from the edge does not occur for 48 hours or more 場合: When tearing from the edge does not occur for 24 hours or more ×: When film forming cannot be interrupted for 6 hours or more due to tearing from the edge, film forming stability based on the above criteria For reasons such as properties and yield, a film with a judgment result of ◎ or ○ was judged to be acceptable.

[0035]

EXAMPLES Examples and comparative examples are shown below to make the effect of the present invention clearer. Example 1 A pellet of polyethylene terephthalate (PET) (intrinsic viscosity 0.65) obtained by a known method was heated at 180 ° C.
After drying in vacuum for 3 hours, the mixture was supplied to an extruder heated to 280 ° C., melt-extruded, and discharged from a T-die into a sheet. Further, this sheet was brought into close contact with a cooling drum having a surface temperature of 25 ° C. by electrostatic force and cooled and solidified to obtain a substantially non-oriented film. This film was stretched under the conditions shown in Table 1. First, stretch in the longitudinal direction using the peripheral speed difference of the rolls using a vertical stretching machine in which several rolls are arranged,
Subsequently, transverse stretching was performed with a stenter. After that, stretching for imparting orientation was re-longitudinal-stretched by a roll longitudinal stretching machine, re-laterally stretched by a stenter, subsequently heat-treated and cooled to room temperature. Obtained.

The properties of the obtained film are shown in Table 1.
As shown in Table 1, a high-quality film having excellent wear resistance and high strength in both the longitudinal and width directions was obtained. In addition, a high quality film was stably obtained with little film breakage during film formation.

Examples 2 to 4 and Comparative Examples 1 to 2 Examples 2 to 3 and Comparative Example 1 were produced using the same raw materials as in Example 1 and changing only the stretching conditions. In Example 4 and Comparative Example 2, pellets of polyethylene naphthalate (PEN) (intrinsic viscosity 0.65) obtained by a known method were manufactured using the same drying, extruding and stretching apparatus as in Example 1. It is. The stretching conditions and the properties of the obtained film are as shown in Table 1. When the physical properties of the obtained film are such that the Young's modulus in the film longitudinal direction is in the range of 5 GPa to 10 GPa, and the ratio μsMD / μsTD of the static friction coefficient in the film longitudinal direction and the width direction is in the range of 1.05 to 1.25. Thus, a high-quality film having excellent wear resistance and high strength in both the longitudinal and width directions could be obtained. In addition, a high quality film was stably obtained with little film breakage during film formation. However, when the ratio between the Young's modulus in the longitudinal direction and the coefficient of friction deviated from the range of the present invention, only a film having poor abrasion resistance or extremely poor film-forming stability was obtained.

Examples 5 and 6 First, a raw material (polyester A) to be used on one side was prepared as follows. A water slurry containing silica particles derived from colloidal silica having an average particle size of 0.03 μm was mixed with polyethylene terephthalate pellets and mixed using a vented twin-screw extruder to obtain PET particle pellets. Further, pellets of particle-free polyethylene terephthalate obtained by a known method were obtained. These particle pellets and pellets substantially containing no particles were mixed at a ratio of 1: 3 to obtain a PET polymer pellet having a particle content of 1.0% by weight (polyester A). Furthermore, PET polymer pellets containing divinylbenzene particles (divinylbenzene component 81%) having an average particle diameter of 0.6 μm were prepared as a raw material used for the opposite surface (polyester B). After drying these raw materials (polyesters A and B) in the same manner as in Example 1, they were supplied to extruders 1 and 2, respectively, melt-extruded, merged with a short tube, and laminated into two layers. The thickness of each layer was adjusted by adjusting the rotation speed of a gear pump installed on each line to control the amount of extrusion. This was discharged in the form of a sheet from a T-die, and the sheet was adhered to a cooling drum having a surface temperature of 25 ° C. by electrostatic force to be cooled and solidified to obtain a substantially non-oriented film. This film was stretched under the conditions shown in Table 2. Table 2 shows the physical properties of the obtained film.

The obtained film had a Young's modulus in the longitudinal direction of the film in the range of 5 GPa to 10 GPa as in Example 1, and the ratio μsMD / μsTD of the static friction coefficient between the longitudinal direction and the width direction of the film was 1.05 to 1 .25, it was possible to obtain a high-quality film having excellent abrasion resistance and high strength in both the longitudinal direction and the width direction. In addition, a magnetic layer was formed by the above-described method to examine the output characteristics and the slipperiness. As a result, since the surface roughness Ra of one side and the opposite side of the film was within the range of the present invention, both the output characteristics and the slipperiness were good.

Example 7 First, a raw material (polyester A) used on one side was prepared as follows. A water slurry containing silica particles derived from colloidal silica having an average particle size of 0.03 μm is mixed with polyethylene-2,6-naphthalate pellets, kneaded using a vented twin-screw kneading extruder, and mixed with PEN particles. A pellet was obtained. Further, pellets of particle-free polyethylene-2,6-naphthalate obtained by a known method were obtained. These particle pellets and pellets substantially containing no particles were mixed at a ratio of 1: 3 to obtain a PEN polymer pellet having a particle content of 1.0% by weight (polyester C). Further, PEN polymer pellets containing divinylbenzene particles having an average particle diameter of 0.6 μm (divinylbenzene component 81%) were prepared as a raw material used for the opposite surface (polyester D). After drying these raw materials (polyesters C and D) in the same manner as in Example 1, they were supplied to extruders 1 and 2, respectively, melt-extruded, merged with a short pipe, and laminated into two layers. The thickness of each layer was adjusted by adjusting the rotation speed of a gear pump installed on each line to control the amount of extrusion. This was discharged in a sheet form from a T-die. Further, the sheet is subjected to a surface temperature of 25.
Cooling and solidifying by bringing it into close contact with the cooling drum of
A substantially non-oriented film was obtained. This film was stretched under the conditions shown in Table 2. Table 2 shows the physical properties of the obtained film.

The obtained film had a Young's modulus in the longitudinal direction of the film in the range of 5 GPa to 10 GPa as in Example 1, and the ratio μsMD / μsTD of the static friction coefficient between the longitudinal direction and the width direction of the film was 1.05 to 1 .25, it was possible to obtain a high-quality film having excellent wear resistance and high strength in both the longitudinal direction and the width direction. Further, when the magnetic layer was formed by the above-described method and the output characteristics and the slipperiness were examined, the output characteristics and the slipperiness were good because the surface roughness Ra of one side and the opposite side of the film was within the range of the present invention.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

According to the polyester film of the present invention and the method for producing the same, the Young's modulus in the longitudinal direction of the film is 5%.
GPa to 10 GPa, and the ratio μsMD / μsTD of the static friction coefficient between the film longitudinal direction and the width direction is 1.05 to
Since it is in the range of 1.25, a film having high strength in both the longitudinal direction and the width direction and having excellent abrasion resistance can be obtained. This film, which has high strength in both the longitudinal and width directions and has excellent wear resistance, can be widely used as a biaxially stretched polyester film suitable for use in magnetic recording media, printer ribbons, capacitors, packaging, and the like. is there.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) B29L 7:00 9:00

Claims (7)

[Claims] 1. The film has a Young's modulus in the longitudinal direction of 5 GP.
a in the range of 10 GPa to 10 GPa, and the ratio μsMD / μsTD of the static friction coefficient between the longitudinal direction and the width direction of the film is 1.
A biaxially stretched polyester film, which is in the range of 0.05 to 1.25. 2. The film has a Young's modulus in the width direction of 7 GPa.
The biaxially stretched polyester film according to claim 1, which is in a range of ２０20 GPa. 3. The surface roughness Ra on one side is 0.008 μm or more, and the surface roughness Ra on the opposite side is 0.005 μm.
m or less, and the biaxially stretched polyester film according to claim 1 or 2. 4. A polyester film in a substantially amorphous state is biaxially stretched so that the film has a birefringence (Δn) of 0.02 or less and a crystallinity of 10% or less.
The method for producing a biaxially stretched polyester film according to any one of claims 1 to 3, wherein the stretching for imparting orientation is performed biaxially. 5. When a substantially amorphous polyester film is stretched biaxially in the longitudinal and transverse directions, the stretching temperature is set in the range of glass transition temperature (Tg) to (Tg) + 60 ° C. The method for producing a biaxially stretched polyester film according to claim 4, wherein the stretching is performed in a range of 1.5 times to 8 times. 6. The film is further stretched biaxially in the vertical and horizontal directions after being stretched biaxially in the vertical and horizontal directions, so that the total stretch ratio in the longitudinal direction and the width direction of the film is in the range of 30 to 100 times.
3. The method for producing a biaxially stretched polyester film according to 1. 7. The polyester film according to claim 4, wherein the substantially amorphous polyester film is stretched biaxially in the vertical and horizontal directions so that the crystallinity is 6% or less. A method for producing a biaxially stretched polyester film.