JPH11302408A - Biaxially oriented polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a film having an extremely little separating amt. of low- mol.-wt. matters and a good skew property and curl recoverability, by making it to a biaxially oriented film having the main components of a poly(ethylene terephthalate) and a poly(propylene terephthalate) and also a specific viscosity and a crystallization index. SOLUTION: A biaxially oriented film is made to have an intrinsic viscosity of from more than 0.6 (dl/g) to less than 1.8 (dl/q) and a crystallization index ΔTcg of 70 deg.C or below. A poly (propylene terephthalate) (abbreviated PPT) is of pref. 10 wt.% or more based on the total of a poly(ethylene terephthalate)(PET) and a PPT and the upper limit thereof is 99 wt.% or less. It is desirable that a modulus E (GPa) in the longitudinal direction and a heat- shrinkage degree (S%) in the longitudinal direction in an instance to be kept at 80 deg.C for 30 min should give a relation therebetween: [0.08E-S]>=0.08. A composite film also may be used, in which film, a film having the main component of a polyester is the base layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented polyester film, and more particularly to a biaxially oriented polyester film having an extremely small amount of low molecular weight precipitates and excellent skew characteristics and curl recovery.

[0002]

2. Description of the Related Art Polyester films are used for magnetic recording media by utilizing their excellent mechanical properties, thermal properties, electrical properties, surface properties, optical properties, heat resistance, and chemical resistance. It is widely used for various purposes such as condenser and packaging. During film formation of polyester, or during high-temperature treatment during processing, during transport of the film under high temperature and high humidity, or during storage of the film, low molecular weight substances (hereinafter referred to as oligomers) that precipitate on the surface after the film processing There is a demand for a film quality design that is likely to cause a decrease in performance and that can suppress surface-precipitated oligomers. For example, the deposition of oligomers is suppressed by applying a water-soluble polyester to the surface of a polyester film (for example, Japanese Patent Application Laid-Open No. 62-290535), or the oligomers are suppressed by adding a higher fatty acid ester to the film. Films are known (for example, see JP-A-9-13183).
No. 9).

[0003] On the other hand, in accordance with the needs for high quality and compactness of these application products, required characteristics for polyester films are becoming more and more severe. Among the above-mentioned applications, in particular, for magnetic recording media, the film is made thinner for a longer time or for compactness, the recording density is increased, and the polyester film has a good skew property and curl recovery property. Is desired.

[0004] Conventional polyester films have the property of shrinking by heating. As means for reducing the heat shrinkage of the polyester film, heat treatment at a high temperature, relaxation treatment after stretching, and film-rolling are used. There is known a treatment method (for example, Japanese Patent Application Laid-Open No. 60-103517) in which a wound state is left in an atmosphere at 110 ° C. to 130 ° C. for 6 hours or more.

[0005]

However, the polyester film produced by the above-mentioned conventional technique has, for example, the following problems.

In the case of a film in which a high-performance layer is added to the film by coating for the purpose of suppressing surface-precipitating oligomers, it takes time and effort to prepare a water-soluble polyester coating apparatus and paint preparation, thereby making production more efficient. This is not desirable in that it is performed at low cost. Further, a biaxially oriented polyester film in which a higher fatty acid ester is added to the film has surface defects such as surface dents, which are considered to be due to the fact that the behavior during stretching of the film differs between the polyester and the higher fatty acid ester.

Further, when heat treatment is performed at a high temperature or when relaxation treatment is performed after stretching, mechanical properties such as elastic modulus are reduced accordingly. Further, in the treatment method in which the film is wound into a roll and left in an atmosphere at 110 to 130 ° C. for 6 hours or more, unevenness occurs in the surface layer of the roll and the core of the roll. Deterioration of flatness and the like occur, and as a result, not only the appearance of the roll-shaped film is deteriorated, but also the heat treatment time is long and productivity is not good. Further, when the film is stored in a roll state at high temperature, high humidity, or the like, there is also a problem of curling.

It is an object of the present invention to suppress oligomer precipitation on the film surface and reduce mechanical properties such as elastic modulus without employing a conventional coating method or a method of adding a higher fatty acid ester. Without letting
Another object of the present invention is to provide a biaxially oriented polyester film having excellent skew characteristics and curl recovery without requiring a long processing time or installing special equipment.

[0009]

An object of the present invention is to provide a film comprising polyethylene terephthalate and polypropylene terephthalate as main components and having an intrinsic viscosity of 0.6 (dl / d).
g) or more and less than 1.8 (dl / g), and is achieved by a biaxially oriented polyester film having a crystallization index ΔTcg of 70 ° C. or less.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The biaxially oriented polyester film of the present invention contains polyethylene terephthalate (hereinafter referred to as PET) and polypropylene terephthalate (hereinafter referred to as PPT) as main components. PPT is preferably at least 10% by weight, more preferably at least 15% by weight, even more preferably at least 20% by weight, based on the total amount of PET and PPT (hereinafter a composition comprising both is referred to as polyester A). It is excellent in oligomer deterrence, skew characteristics and curl recovery. Although there is no upper limit to the content of PPT, 99% is required when film strength is required.
% By weight or less, more preferably 90% by weight or less.

The biaxially oriented polyester film of the present invention has an intrinsic viscosity of 0.6 (dl / g) or more and less than 1.8 (dl / g), preferably 0.65 (dl).
/ G) or more and less than 1.6 (dl / g), more preferably 0.65 (dl / g) or more and less than 1.5 (dl / g). With such a range, oligomer deterrence can be obtained, and skew characteristics while maintaining mechanical characteristics such as sufficient elasticity,
Excellent curl recovery. 0.6 intrinsic viscosity
If it is less than (dl / g), the oligomer-suppressing properties will be poor.

The biaxially oriented polyester film of the present invention has a crystallization index ΔTcg represented by a difference between a cold crystallization temperature Tcc and a glass transition temperature Tg of 70 ° C. or less, preferably 6 ° C. or less.
When the temperature is 5 ° C. or lower, more preferably 60 ° C. or lower, the oligomer is excellent in deterrence, skew characteristics, and curl recovery. If the crystallization index ΔTcg exceeds 70 ° C., the skew characteristics and curl recovery properties are particularly deteriorated, which is not good.

The projections on at least one surface of the biaxially oriented polyester film of the present invention may be projections originating from polyester fine crystals. That is, since PPT, which is a highly crystalline polymer, is used, projections can be formed by utilizing the crystallization characteristics of polyester A to provide lubricity and improve workability. Here, as one of the methods for determining whether or not the surface projections are made of fine crystals of polyester, for example, under the projections of interest, etching with a suitable solvent in the film thickness direction, the When the substance that forms the projections remains as an insoluble substance, the particles are externally added particles or internally precipitated particles (I). When there is substantially no insoluble matter remaining, it can be estimated that the cause of forming the projections is fine crystals (II). As the above solvent, for example, a mixed solvent of phenol / carbon tetrachloride (weight ratio: 6/4) is preferably used. In this way the field of view is 1
The frequency of I and the frequency of II when mm ² were determined, and II / (I
+ II) is preferably 50% or more, more preferably 60% or more, when the oligomer-suppressing property, the skew property,
This is desirable because the curl recovery is further improved. The method for determining whether or not the film surface projections are made of crystals is not limited to the above method, and appropriate methods such as SEM-XMA, TEM, plasma etching, measurement of voids in the film, and the like. From the above, an appropriate method can be appropriately selected.

The polyester A may contain particles as long as the effects of the present invention are not impaired. When particles are contained, the content is not particularly limited, but is 1.0% by weight or less, preferably 0.5% by weight or less,
More preferably, 0.3% by weight or less is desirable in view of skew characteristics. The particle size is not particularly limited, but the average particle size is 0.005 to 2 μm, preferably 0.01 to 1.5 μm.
Inert inorganic and organic particles of μm are desirable.

The biaxially oriented polyester film of the present invention has a longitudinal elastic modulus E (GPa) and a longitudinal heat shrinkage S when held at a temperature of 80 ° C. for 30 minutes.
(0.08E-S) is 0.08 or more,
If it is preferably 0.09 or more, more preferably 0.1 or more, the thermal dimensional stability can be improved without significantly lowering the elastic modulus, so that the skew characteristics and curl recovery properties are further improved, which is desirable.

The biaxially oriented polyester film of the present invention preferably has a heat shrinkage in the longitudinal direction of 0.5% or less, more preferably 0.4%, when held at a temperature of 80 ° C. for 30 minutes. % Or less, more preferably 0.3%
It is as follows. If the heat shrinkage is large, for example, when used for a magnetic recording medium, the video signal is recorded on the tape as an oblique track, but the tape expands and contracts (dimension changes) in the running direction for some reason, The length of the track recorded above changes. Therefore, a phenomenon called skew occurs in which the frequency of a signal changes or a tracking error occurs during reproduction, and distortion occurs on a screen. In addition, a processing step of a polyester film, for example, a magnetic layer coating for a magnetic recording medium is performed. If the heat shrinkage is large in the process and the like, wrinkles and poor flatness occur.
Further, the curl amount of the film when stored in a roll state at high temperature and high humidity becomes large. For example, in the case of a magnetic recording medium or the like, the adhesion between the tape and the head becomes poor, and the output decreases.

The biaxially oriented polyester film of the present invention preferably has an elastic modulus in at least one of the longitudinal and width directions of at least 3.5 GPa, more preferably at least 4 GPa, even more preferably at least 5 GPa. For example, when used for a magnetic recording medium, if the elastic modulus of the base film is insufficient, tension is applied to the magnetic head and guide pins during traveling, so that the magnetic tape elongates, which adversely affects the electromagnetic conversion characteristics (output characteristics). give. For magnetic tapes for long-time recording, the base film is made thinner,
It is desirable to improve the elastic modulus in at least one direction.

PE constituting polyester A of the present invention
T can be produced by a known method such as a direct polymerization method using terephthalic acid and ethylene glycol, a transesterification method using dimethyl terephthalate and ethylene glycol, and a polycondensation of diethylene glycol terephthalate. The intrinsic viscosity of PET is 0.45 to 1.6 (d) in view of the mechanical properties of the film and oligomer deterrence.
1 / g).

PPT, which is another polymer constituting the polyester A of the present invention, is obtained by polymerizing, for example, a terephthalic acid component and a 1,3-propylene glycol component, and can be produced by a known method. it can. PPT
Is desirably 0.7 to 2.0 (dl / g) from the viewpoint of the mechanical properties of the film and the property of inhibiting oligomers. Further, additives such as an antioxidant, a heat stabilizer, and an ultraviolet absorber may be added to the extent that the additives are usually added within a range not to impair the object of the present invention. In the present invention, PET
And PPT can be mixed by a known method. For example, a method in which PET pellets and PPT pellets are mixed in a blender and charged into an extruder and melt-mixed in the extruder, or both are melted by separate extruders, and then these molten polymers are mixed and mixed. There is a method, and any method may be used. Furthermore, it is desirable to mix the melt-mixed material with a static mixer because the mixture is more uniformly mixed. If the temperature at the time of mixing both is too low, it is difficult to mix uniformly, and if it is too high, even if both are uniformly mixed, foaming or mechanical strength of the film may be reduced,
The extruder temperature is preferably from 220 to 300 ° C.

The biaxially oriented polyester film of the present invention is a composite film in which a film mainly composed of polyester is used as a base layer (layer B), and the film mainly composed of polyester A (layer A) is arranged on at least one surface thereof. It may be.

The polyester (hereinafter referred to as "polyester B") as a main component of the layer B is not particularly limited, but ethylene terephthalate, ethylene α, β-bis (2-chlorophenoxy) ethane-4,4-dicarboxylate, ethylene naphthalate, etc. It is particularly preferred when at least one structural unit selected from phthalate units is the main constituent. Among them, a polyester containing ethylene terephthalate as a main component is particularly preferable. Two or more polyesters may be mixed or a copolymer may be used as long as the effects of the present invention are not impaired. In addition, PPT
May be included, but usually, the layer B constitutes a base layer and is required to have strength.
It is preferable that the amount is smaller than the T mixing amount. It is desirable that the polyester B does not contain particles, but it may be.

In the case where the biaxially oriented polyester film of the present invention is used as a composite film,
From the viewpoint of curl recovery, it is preferable to set the crystallization parameter ΔTcg of the polyester B larger than that of the polyester A. Since the layer B constitutes a base layer, it is not particularly necessary to suppress oligomers, and any layer can be used as long as it can have a predetermined strength as a base film. By giving the magnitude relationship of the crystallization parameters to the A layer and the B layer, the A layer has a function of suppressing oligomers, and the B layer has a function of clearly exhibiting a desired strength in the entire composite film, which is clearly separated. Can be made.

In the case where the biaxially oriented polyester film of the present invention is used as a composite film having a layer B composed mainly of polyester B and a layer A disposed on at least one surface of the layer B, the thickness of the layer A is based on the total thickness of the film. The ratio is preferably 0.5 or less, more preferably 0.3 or less, and still more preferably 0.2 or less, from the viewpoints of film forming properties, oligomer deterrence, and curl recovery, which are desirable.

The surface roughness of the biaxially oriented polyester film of the present invention is not particularly limited. For example, when the film is used as a base material for a magnetic recording medium, the center line average roughness (Ra) ) Is preferably 50 nm or less, more preferably 3 nm or more and 30 nm or less,
It is more preferable that the thickness be 5 nm or more and 25 nm or less.

Next, the method for producing the biaxially oriented polyester film of the present invention will be described more specifically.

Polyester A is supplied to a known melt extruder at an arbitrary mixing ratio of PPT pellets and PET pellets, extruded into a sheet from a slit die, and cooled and solidified on a casting roll to form an unstretched film. .

When a layer A mainly composed of polyester A is compounded on at least one side of a layer B mainly composed of polyester B, polyester A and polyester B are separately extruded in a known melt extruder. It is extruded into a sheet from a slit-shaped die, and cooled and solidified on a casting roll to produce a composite unstretched film by coextrusion. That is, these polyesters are compounded using two or three extruders, two or three or more multi-layer merging blocks or caps. When the merging block method is used, the composite portion is rectangular, and the difference (absolute value) between the viscosity of the polyesters A and B is 0 to 2000 poise, preferably 0 to 1 poise.
It is effective to keep the film of the present invention stably and industrially in the range of 000 poise.

In the unstretched film, when the draft ratio when the molten polyester is extruded from the die (= flow rate of the die lip polymer / flow rate of the film (polymer) on the casting drum) is 5 to 30, desired film properties are obtained. More desirable to obtain.

In the present invention, the unstretched film obtained by the above method is biaxially stretched and biaxially oriented. As the stretching method, there are known methods such as a sequential biaxial stretching method and a simultaneous biaxial stretching method. Further, the biaxially stretched film may be further stretched in at least one direction.

In the case of sequential biaxial stretching, stretching in the longitudinal direction is performed by:
A method in which the film is divided into three stages, in particular, four or more stages and is stretched 2 to 6 times in the range of 40 to 150 ° C. and at the stretching speed of 1000 to 50,000% / min is effective for obtaining desired film characteristics. . The stretching temperature, speed and stretching ratio in the width direction are 5
0 to 150 ° C, 1000 to 20000% / min, 3 to 10
A double range is preferred. Further, if necessary, the film can be further stretched in at least one of the longitudinal direction and the width direction. Next, the stretched film is heat-set. In this case, the heat treatment conditions are 140 to 240 ° C., preferably 160 to 240 ° C. in any state of relaxation in the width direction, fine stretching, and constant length.
A range of 220 ° C. for 0.5 to 60 seconds is preferable, but it is desirable to use microwave heating in combination with the heat treatment because the film of the present invention can be easily obtained.

The use of the polyester film in the present invention is not particularly limited, but it is particularly useful as a support for magnetic recording, which is particularly important in terms of oligomer suppression and skew characteristics. Also, for capacitors, for photography, for electrical insulation,
Also suitable for packaging.

[0032]

[Measurement method of physical properties and evaluation method of effect] The measurement method of various characteristic values and the evaluation method of effect in the present invention are as follows.

(1) Intrinsic viscosity [η] (unit: dl / g) The value calculated from the following formula from the solution viscosity measured at 25 ° C. in o-chlorophenol is used. That is, ηsp / C = [η] + K [η] ² · C, where ηsp = (solution viscosity / solvent viscosity) −1, C is the weight of dissolved polymer per 100 ml of solvent (g / 100 m
1, usually 1.2), and K is a Haggins constant (assumed to be 0.343). The solution viscosity and the solvent viscosity were measured using an Ostwald viscometer.

(2) Crystallization index ΔTc of polyester A
g Using a film obtained by slitting a film in a tape shape to a width of 1/2 inch, one edge is pressed vertically to the surface of polyester A, and the film is further pushed in by 0.5 mm and run for 20 cm (running tension: 500 g, running speed: 6.7cm /
Seconds). At this time, 10 mg of the shavings on the film surface adhering to the tip of the single blade were collected to obtain a sample. If the amount of shavings was less than 10 mg in one run, the same operation was performed using another film to collect 10 mg of the sample.

The measurement was performed using a DSC (differential scanning calorimeter) type II manufactured by Parking Elma. DSC for 10 mg of sample
After setting in the device and melting at a temperature of 290 ° C for 5 minutes,
Quench in liquid nitrogen. This sample is heated at a rate of 10 ° C./min, and the glass transition point Tg is detected. Continue heating up further,
The crystallization exothermic peak temperature from the glassy state was defined as the cold crystallization temperature Tcc. The difference between Tcc and Tg (Tcc-T
g) is defined as the crystallization index ΔTcg.

(3) Modulus of elasticity According to the method specified in JIS K-7127,
Using a tensile tester manufactured by Toyo Sokki, 25 ° C, 65% RH
Was measured. The sample was cut out into a strip having a width of 10 mm and a length of 200 mm in the measurement direction. The initial distance between the tension chucks was 100 mm, and the tension speed was 300 mm / min.

(4) Heat Shrinkage Measured according to the method specified in JIS C-2318. However, the temperature of the oven was 80 ° C., and the holding time was 30 minutes.

(5) Content of Particles The composition was analyzed by microscopic FT-IR (Fourier transform microscopic infrared spectroscopy), and the ratio of the peak caused by the carbonyl group of the polyester to the peak caused by a substance other than the polyester was determined. The weight percentage of the particles was determined. In order to convert the peak height ratio to a weight ratio, a calibration curve was prepared in advance using a sample whose weight ratio was known, and the polyester ratio to the total amount of the polyester and other substances was determined. An X-ray microanalyzer was used as needed. Also, if you can choose a solvent that dissolves the polyester but not the particles,
The polyester was dissolved and the particles were centrifuged from the polyester and the weight percentage of the particles was determined.

(6) Layer thickness of layer A Using a transmission electron microscope (H-600, manufactured by Hitachi), the cross section of the film was subjected to an ultra-thin section method (R) at an acceleration voltage of 100 kV.
(uO4 staining), the interface is captured, and the layer thickness is determined. The magnification is usually selected according to the thickness of the laminate to be determined, and is not particularly limited, but 10,000 to 100,000 times is appropriate.

(7) Center line average roughness Ra The center line average roughness Ra was measured using a high-precision thin film level difference measuring device ET-10 manufactured by Kosaka Laboratory. The conditions were as follows, and the value was an average value obtained by scanning 20 times in the film width direction and performing measurement.

・ Rib of tip of stylus: 0.5 μm ・ Load of stylus: 5 mg ・ Measuring length: 1 mm ・ Cutoff value: 0.08 mm Ra is defined, for example, by Jiro Nara, “Measurement of surface roughness. Evaluation method ”(General Technology Center, 1983).

(8) Oligomer Deterrence A low molecular weight compound was forcibly deposited on the film surface by leaving it in an oven at 150 ° C. for 30 minutes, aluminum was deposited on the surface, and a 400 × magnification photograph was taken with a differential interference microscope. Then, 25 fields of view are observed. The number of low molecular weight substances in each visual field was counted, and the total number was defined as the number of surface oligomers deposited (pieces / mm ² ). Excellent when the number is less than 80 / mm ² and the size of the low molecular weight substance is smaller than 1 mm on the surface photograph, 8
0 / mm ² and less than 100 / mm ^2, and a good thing size is smaller than 1.5 mm, the number is 100 /
Those that were not less than ² mm2 or larger than 1.5 mm were regarded as defective.

(9) Skew Characteristics A magnetic paint having the following composition is applied to the film with a gravure roll, magnetically oriented, and dried. In addition, a small test calendar device (steel roll / nylon roll,
After performing a calendering process at a temperature of 70 ° C. and a linear pressure of 200 kg / cm, curing is performed at 70 ° C. for 48 hours. The raw tape was slit to 1/2 inch, and a length of 250 m was assembled into a VTR cassette to obtain a VTR cassette tape.

(Composition of magnetic paint) Co-containing iron oxide (BET value 50 m ² / g): 100 parts by weight Eslek A (Sekisui Chemical's vinyl chloride / vinyl acetate copolymer): 10 parts by weight Nipporan 2304 ( Polyurethane elastomer made by Nippon Polyurethane: 10 parts by weight Coronate L (polyisocyanate made by Nippon Polyurethane): 5 parts by weight Lecithin: 1 part by weight Methyl ethyl ketone: 75 parts by weight Methyl isobutyl ketone: 75 parts by weight Toluene: 75 parts by weight -Carbon black: 2 parts by weight-Lauric acid: 1.5 parts by weight The skew (tape distortion) was evaluated by recording a black and white IQ signal and then repeating the reproduction at 45 ° C and 80% RH for 100 hours. The amount of displacement on the monitor after the passage of time was measured, and the deviation of 5 (μsec) or less
c) larger than 15 (μsec) is good, 15
(Μsec) is regarded as defective.

(10) Curl recovery property A film having a width of 50 mm is wound around a plastic cylinder having a diameter of 50 mm and a width of 70 mm for about 10 turns, and is left standing at 50 ° C. and 65% RH for 24 hours. . Then, sample 100mm in the longitudinal direction,
25 ° C, 65% R, with the inside wound up on the glass surface
H for 1 hour, the maximum distance from the glass plate plane to the film was measured, and the curl recovery was determined based on the following criteria.

No curl amount: excellent Curl amount less than 2 mm: good Curl amount 2 mm or more: poor

[0047]

Next, the present invention will be described based on embodiments.

PET pellets 1 polymerized by a conventional method
(Intrinsic viscosity = 0.45, 0.63, 1.55)
Was dried under reduced pressure (3 Torr) at 185 ° C. for 3 hours. Pellets 2 (intrinsic viscosity = 0.7, 0.95, 1.2, 1.9) obtained by subjecting dimethyl terephthalate and 1,3-propanediol to a transesterification reaction and a polycondensation reaction to polymerize PPT.
5) at 140 ° C for 3 hours under reduced pressure (3 Torr)
did. Inert particles were added to PET (average particle size: colloidal silica particles having an average particle size of 0.25 μm, a particle content of 0.4% by weight was added at the time of polymerization, and the intrinsic viscosity was 0.63). Drying under reduced pressure at 185 ° C for 3 hours (3
Torr).

Example 1 After drying the above-mentioned pellets 1 and 2, respectively, 20% by weight of PET pellets 1 containing no particles and 80% by weight of PPT pellets 2 were mixed to obtain polyester A.
After melt-extrusion at 260 ° C. using a known extruder, the film was wound around a casting drum having a surface temperature of 20 ° C. using an electrostatic application casting method, cooled and solidified to form an unstretched film. This unstretched film is treated at a surface temperature of 7
A pre-heat treatment for stretching is performed by passing through four silicone rolls at 0 ° C., and a temperature of 80 ° C. and a stretching speed of 1000 are applied.
2. In the longitudinal direction at 4% or more in the longitudinal direction at 0% / min.
The film is stretched 4 times, and further stretched 4.0 times in the width direction at 85 ° C. at a stretching speed of 2000% / min using a known stenter, and further heat-treated at 200 ° C. for 5 seconds under a constant length.
A biaxially oriented polyester film having a thickness of 12 μm was obtained.

Examples 2 and 3 Comparative Examples 1, 2 and 3 Biaxially oriented polyester films were obtained in the same manner as in Example 1. However, PET pellet 1 and PPT pellet 2,
A biaxially oriented polyester film was obtained by mixing the pellets 3 of the particle-containing PET so as to obtain the polyester A shown in Table 1 and changing the extrusion melting temperature, stretching temperature conditions, and the like. Example 2 was stretched 3.3 times in the longitudinal direction and 3.9 times in the lateral direction, and Comparative Example 2 was stretched 3.3 times in the longitudinal direction.
The film was stretched 3.3 times in the transverse direction. The stretching ratio of Comparative Example 3 was the same as that of Example 1, but the stretching speed was reduced by half.

Example 4 and Comparative Example 4 As polyester A, a mixture of PET pellets 1, PPT pellets 2, and particle-containing PET pellets 3 in the proportions shown in Table 1 was used. The pellets 1 (intrinsic viscosity = 0.63) were supplied to two extruders, respectively, and polyester A was extruded at 265 ° C. in Example 4, and 285 ° C. in Comparative Example 4 by extruder 1.
And the polyester B is extruded at 290
C., melted and laminated by a rectangular merging block (feed block) for three layers, and wound on a casting drum having a surface temperature of 22 ° C. by using an electrostatic application casting method.
After cooling and solidification, a laminated unstretched film having a three-layer structure of A / B / A was produced. Hereinafter, in the same manner as in Example 1, the stretching temperature and the stretching ratio were changed to obtain a biaxially oriented laminated film having a total thickness of 12 μm (one-side thickness of the A layer: 1 μm). Comparative Example 4
The number of protrusions caused by crystals of the film obtained in the above was examined. All protrusions were caused by particles.

Example 5 and Comparative Example 5 As polyester A, PET pellets 1, PPT pellets 2, and particle-containing PET pellets 3 were mixed in the proportions shown in Table 1, and as polyester B, P
Using ET pellet 1 (intrinsic viscosity = 0.63), a laminated unstretched film having a three-layer structure of A / B / A was produced in the same manner as in Example 4. The polyester A of Comparative Example 5 was 270
Melted at ° C. Hereinafter, in the same manner as in Example 1, the stretching temperature and the stretching ratio were changed, and after stretching 1.45 times in the longitudinal direction, a heat treatment was performed at 210 ° C. for 5 seconds under a constant length.
A biaxially oriented film having a thickness of 9 μm (A layer thickness on one side: 0.7 μm) was obtained.

Example 6 As a polyester A, a mixture of a PET pellet 1, a PPT pellet 2, and a particle-containing PET pellet 3 in the proportions shown in Table 1 was used. As the polyester B, a PET pellet 1 ( (Intrinsic viscosity = 0.97) was supplied to each of two extruders. Polyester A was melted at 285 ° C. by extruder 1, and polyester B was melted at 290 ° C. by extruder 2. Similarly,
A laminated unstretched film having a three-layer structure of A / B / A was prepared.
Hereinafter, in the same manner as in Example 1, the stretching temperature was changed,
The film was stretched 3.5 times in the longitudinal direction and 4.5 times in the transverse direction to obtain a biaxially oriented laminated film having a total thickness of 12 μm (one-side thickness of layer A: 1 μm).

Table 2 shows the results of evaluating the films prepared in the above Examples and Comparative Examples. It can be seen that all of the samples belonging to the scope of the present invention are superior to the comparative examples in oligomer inhibition, skew characteristics, and curl recovery.

[0055]

[Table 1]

[Table 2]

[0056]

The biaxially oriented polyester film of the present invention exhibits excellent oligomer deterrence and can reduce the heat shrinkage without lowering the mechanical properties.
Excellent skew characteristics and curl recovery can be obtained. Also, sufficiently good characteristics can be obtained for a magnetic recording medium.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B29K 67:00 B29L 9:00 C08L 67:00

Claims (4)

[Claims] 1. A film mainly composed of polyethylene terephthalate and polypropylene terephthalate, having an intrinsic viscosity of 0.6 (dl / g) or more and less than 1.8 (dl / g), and a crystallization index ΔTcg of 70. A biaxially oriented polyester film having a temperature of not more than ° C. 2. The elastic modulus E (GPa) in the longitudinal direction and the temperature 8
2. The biaxially oriented polyester film according to claim 1, wherein the relationship with the heat shrinkage S (%) in the longitudinal direction when held at 0 ° C. for 30 minutes is 0.08 E−S ≧ 0.08. 3. A biaxially oriented polyester film in which the film according to claim 1 is disposed on at least one surface of a film containing polyester as a main component. 4. A magnetic recording medium using the biaxially oriented polyester film according to claim 1.