JPS6178862A - Biaxially oriented polyester film - Google Patents

Abstract

PURPOSE:To obtain a biaxially oriented polyester film having high modulus and excellent dimensional stability, by blending a specified polyester with polyethylene terephthalate and molding the blend into a film having a specified crystal form. CONSTITUTION:The titled film is obtd. from a blend of a polyester (I) composed of units of formulas I and II and having a flow temp. of 350 deg.C and melt anisotropy forming properties with polyethylene terephthalate. The film is com posed of a blend wherein the molar fraction of component (A) in the polyester (I) is at least 40mol% and blend ratio Xbwt% of the polyester (I) is a value obtd. from formula III wherein Mfmol% is the molar fraction of component A. The film is a biaxially oriented polyester film wherein a coefficient of orienta tion in the crystal face (100) of polyethylene terephthalate is 0.75-0.95 and crystalline size in the direction of the crystal face is 35-65Angstrom .

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a biaxially oriented polyester film, more specifically, a high modulus biaxially oriented polyester film with excellent dimensional stability suitable for magnetic tapes and flexible printed circuit boards. Regarding polyester film.

[Prior art and its drawbacks 1 Biaxially oriented polyethylene terephthalate film is used for the base of magnetic tape and flexible printed circuit boards.

However, as the processing steps for magnetic tapes and flexible printed circuits become more labor-saving and the purposes for which they are used become more sophisticated, film properties such as higher modulus of elasticity and better dimensional stability have come to be required. However, in the case of a biaxially oriented polyethylene terephthalate film, although the elastic modulus increases to some extent when the film is stretched in multiple stages in the film forming process, the dimensional stability is disadvantageously deteriorated.

On the other hand, it is possible to blend a liquid crystal polymer with rigid molecular chains and the ability to form melt anisotropy with a soft and stabilized polymer such as nylon or polyethylene terephthalate to obtain high elasticity or low heat shrinkage. Known about materials and textiles. For example, when wearing nylon, use Pure & App1.
．． CC11e, 55, 5, 819 (1983
): J, Macromol, Sci, -Phy
s.

B17, 4, 591 (1980), or “to fibers”)
I'U 11, 'Mi711) a57-101020
No. 57-101021 discloses the blending effect of liquid crystal polymers. However, biaxial stretching of a film for a polymer system blended with a liquid crystal polymer has not been attempted, and simply blending a liquid crystal polymer with a flexible chain polymer at random may result in poor dispersibility or destruction at the dispersed phase interface. A biaxially stretchable film cannot be obtained.

[Object of the Invention] The present invention aims to eliminate the drawbacks of the above-mentioned biaxially oriented polyethylene terephthalate film and provide a polyester film having a high elastic modulus and excellent dimensional stability (low heat shrinkage rate).

[Structure of the Invention] In order to achieve the above object, the present invention has the following structure, that is, it consists of components (A) and (B) represented by the following general formula, has a flow temperature of 350°C or less, and has melting anisotropy. This film is made by blending polyester (1) having properties-forming ability with polyethylene terephthalate, and the film has a molar fraction of component (A) in polyester (I) of 40 mol% or more, and the value is Mf (mol%),
Blend ratio x of polyester (1), <weight%) is 1≦
It is made of a blend selected from the range of 35～
It is a biaxially oriented polyester film characterized by having a diameter of 65.

0-R-C-(A>(wherein, 〃 is an integer selected from 2, 4.6, R,
R' is 1.3 phenylene, 1,4 phenylene, r, (Polyethylene terephthalate (hereinafter abbreviated as PET), which is one of the components constituting the film of the present invention, is a polymer consisting essentially of O repeating units. Even if components other than ethylene glycol units and terephthalic acid units are co-incorporated in this polymer, up to about 10 mol % is allowed.

The polyester (1), which is another component constituting the film of the present invention, is a polyester having the ability to form melt anisotropy, and is composed of components (A) and (B) represented by the following general formula. ) component has a molar fraction of 40 mol% or more,
It is a polymer with a flow temperature of 350°C or less.

0-R-C-(A) 1○-(CHz)x -OC-R' C(B)II
II O This polyester (1) exhibits optical anisotropy and assumes a liquid crystal structure when the polymer is heated above the flow temperature, if the molar fraction of component (A) is above a certain value. In addition, even if the polymer does not form liquid crystals by heating and melting, if the mole fraction of component (A) is 40 mol% or more, the polymer can be sandwiched between two glass plates and heated at a temperature higher than the flow temperature. When one glass plate is fixed and the other glass plate is slid, and a comparatively small sliding speed of 10 sec-1 or less is applied to the melt, it exhibits flow birefringence and becomes an optically anisotropic liquid. .

Here, the optical anisotropy of the polymer melt can be observed under crossed Nicols using a polarizing microscope equipped with a heating digital camera.

A polymer that exhibits optical anisotropy in a static state or under shear deformation of 10 sec-1 or less when heated above the fluidizing temperature is said to have the ability to form melt anisotropy. Another method for determining the ability to form melt anisotropy is to examine the relationship between shear rate and shear stress using a viscometer.

In other words, when the shear stress of a polymer melt is measured while increasing the shear rate and then while decreasing the shear rate, if the shear stress-shear rate curves do not match in the former and review cases, so-called hysteresis is detected. , the polymer is said to have the ability to form melt anisotropy.

As described above, the polyester blended with PET in the film of the present invention has the ability to form melt anisotropy, and
It is \t'eIi-like that molecules tend to align and orient themselves mutually in a fluid state. In order to maintain this property, the molar fraction of component (A) in the molecule needs to be at least 40 mol%, which allows the biaxially oriented polyester film to exhibit excellent properties. . If the molar fraction of component (A) is less than 40 mol%, the properties such as elastic modulus and heat shrinkage of the biaxially oriented polyester film obtained by blending will be almost the same or inferior to a film made of PET alone. , no improvement in properties due to blending is observed.

As mentioned above, the molar fraction of component (A>) in the polyester to be blended is preferably 40 mol% or more, preferably 50 to 80 mol%. Due to their close flow temperatures and better dispersion in the PET matrix,
The film formability of the film is improved, and the properties of the film are significantly improved.

The copolymerization mode of the components (A) and (B) of the polyester (I) having the ability to form melt anisotropy of the film of the present invention may be either random or block copolymerization, but it is better to blend the random one. Easy to stretch for biaxial orientation
It is preferable for producing melum. In addition, the polyester (1
) flow aS degree is below 350°C, preferably 300'
C or lower. Note that the lower limit of the flow #J hot water temperature is not particularly limited, but is preferably 150°C. Here, the flow temperature refers to the temperature above which the polymer can be fluidized, and in the case of polyesters with little copolymerization, it often coincides with the melting point, but when the copolymerization increases, the melting point is clearly determined by thermal analysis. Although this cannot be confirmed, the existence of a temperature at which the polymer begins to flow under heating is recognized, and in this case, it refers to this temperature.

A flow temperature exceeding 350° C. is not preferable because the melting and mixing temperature and the film forming temperature become high, leading to thermal decomposition and modification of the polymer, and making it uneconomical.

Further, the value of n in the component (B) of the polyester is an integer selected from 2.4.6, but n-2 is particularly preferred from the viewpoint of the elastic modulus of the film. Furthermore, R and R' at c6 in the above formulas (A) and (B) are 1,
3-phenylene, 1.47 blend improves the properties of biaxially oriented films.

As a specific example of the polyester (I) having the ability to form melt anisotropy used in the film of the present invention, (=0.4 to 0.8)
, ('1)-=0.4~0.8), 1=0.4~0.8), (υ=0.4~0.8), (qf=0.6~0.9) Among them, polyesters (a), (b), and (c) are particularly preferable in terms of film properties.

Furthermore, the polyester may be copolymerized with other components as long as it is less than 5 mol%. Other components in this case include terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, α,β-bis(phenoxy)ethane 4.4
'-dicarboxylic acid, 4,4' diphenylcarboxylic acids, or 1,4-cyclohexane dimethacal,
Dioxy compounds such as phenylhydroquinone are used.

When the polyester (1) having melt anisotropy forming the film of the present invention is blended under PE, the preferable range of the blending ratio Xb (wt%) is as follows: It varies depending on IMF (%) and is expressed by the following formula.

1≦Xb　≦-0.8Me+90 Here, the blend ratio Xv (weight %) is defined as:

As the molar fraction of component (A) in the polyester (I) increases, the upper limit of the appropriate blend ratio decreases because the molecular chain becomes rigid and the affinity with PET changes.

When the polyester is blended in an amount exceeding the upper limit of the preferable blending ratio xb specified above, the stretchability of the film decreases, and the resulting biaxially oriented film has poor impact resistance and elastic modulus. Not desirable. Furthermore, when blending is carried out below the lower limit of the above-mentioned Xb range, the degree of improvement in film properties due to blending is not recognized to be of practical significance, which is not preferable.

The biaxially oriented film of the present invention has high elastic modulus, low thermal shrinkage, and impact resistance.
The (100) plane orientation coefficient of T crystal is 0.75 to 0.9
5. The crystal size in the plane direction is 35 to 65 large. If the plane orientation coefficient is less than 0.75, the biaxially oriented film may have poor impact resistance or a low elastic modulus, making it impossible to achieve the object of the present invention. Furthermore, if the plane orientation coefficient exceeds 0.95, the film becomes brittle and cannot have the practically necessary impact resistance. Generally speaking, if the crystal size falls below the lower limit of the range specified above, biaxial orientation will occur. The thermal stability of the carbon lumen becomes poor, and low thermal shrinkage cannot be obtained, and if it exceeds the upper limit, the impact resistance decreases, making it unsuitable for practical use.

In addition, it is preferable to add other types of polymers to each of PET and the polyester (H) having melt anisotropy forming ability that constitute the film of the present invention, or to a blend of both, within a range that does not impede the purpose of the present invention. It may be blended at less than 10% by weight and, if necessary, inorganic and/or organic additives such as antioxidants, heat stabilizers, lubricants, nucleating agents, surface protrusion forming agents, etc., to the extent normally used. May be added.

Next, a method for manufacturing the film of the present invention will be explained.

First, PET used in the present invention is polymerized from ethylene glycol and terephthalic acid or its derivatives by a known method. PET having a reduced viscosity η5p/C of 0.5 or more is used to ensure high strength and elastic modulus of the film.

Polyester (1) having the ability to form melt anisotropy, which is another component used in the present invention, can be synthesized by the following method. That is, alkylene glycol (number of carbons:
2.4 or 6) and terephthalic acid, isophthalic acid, 2
, 6-naphthalene dicarboxylic acid, 2.7-naphthalene dicarboxylic acid, monochloroterephthalic acid, and other dicarboxylic acid esters are transesterified at a temperature of 130 to 260°C in the presence of a catalyst such as a calcium, magnesium, or lithium compound. After cooling, heat at 220 to 300°C under a catalyst such as antimony or germanium compound.
A homopolyester is synthesized by polycondensation under high vacuum.

Next, p-hydroxybenzoic acid, β
- Acetylated didroxycarboxylic acids such as oxy-6-naphthoic acid are blended in a predetermined ratio, and <1
A deacetylation reaction is carried out at 250 to 340°C (0-1 to 10'-2 torr) to obtain a polyester having the ability to form melt anisotropy. In order to keep the mole % of component (A) in the polyester within the range of the present invention, the blending amount of the acetylated product of hydroxycarboxylic acid must be selected appropriately. The thus obtained polyester having the ability to form melt anisotropy may be reheated under high vacuum to a temperature close to the melting point of the polymer to perform in-phase polymerization to increase the degree of polymerization.

Next, polyester (1) with melt anisotropy forming ability and P
Blend ET. Blending is carried out by mixing the powders or pellets of both polymers and then feeding them into a known melt extrusion machine while mixing, but the method is not particularly limited.

However, it is preferable to disperse the polyester in PET as finely as possible, and in order to achieve fine dispersion, it is desirable to use a known static mixer or a screw with a crosstalk zone. However, care must be taken because if the mixture is mixed in a molten state for too long, the polymer composition may change due to the transesterification reaction between PET and the polyester (I), and the expected film properties may not be obtained.

The two polymers thus fed to the melt extruder were 270
The melt is melted at ~320°C, extruded into a sheet through a slit-shaped die, wound around a casting drum whose surface temperature is controlled at 10-80°C, and cooled and solidified to produce an unstretched film. In this case, the t4 voltage application casting method is effective for rapid and uniform cooling.
When polyester (1) that has the ability to form melt anisotropy is melt-blended into 1PET, the blended body often takes a sea-island structure, but the latter that forms the islands have a rod-like or needle-like shape with a large axial ratio. In terms of film properties, it is preferable to For this purpose, the melt discharged from the melt extruder is elongated and the polyester dispersed in islands is drawn out while being cooled and solidified on the casting drum. The degree of elongation of the melt is determined by the cross-sectional area of the slit die of the melt extruder,
When expressed as the value divided by the cross-sectional area perpendicular to the longitudinal direction of the film after casting, that is, the draft ratio, it is 3 to 30.
It is preferable to wind up the melt within a range of twice the draft ratio, so that the biaxially oriented film finally obtained has both excellent elastic modulus and dimensional stability.

Next, this unstretched film is biaxially stretched. As the biaxial stretching method, a known simultaneous biaxial stretching method or a sequential biaxial stretching method can be used. In the case of the sequential biaxial stretching method, it is common to stretch in the longitudinal direction and then in the width direction, but this order may be reversed. The conditions for biaxial stretching are not necessarily constant, depending on the amount of polyester (1) that has the ability to form melt anisotropy in PET, the method of stretching the film, etc.
at a temperature of 70-120°C, preferably 80-100°C,
It is desirable to stretch at a stretching speed of 103 to 105%/min. Further, the preferred stretching ratio for obtaining the planar orientation coefficient and crystal size of the film of the present invention requires that the longitudinal height ratio α and the width direction ratio β satisfy the following formula.

12.5≦α2+β2≦55.0 (however, α>2, β>2) Furthermore, a method of re-stretching a −uniaxially stretched film in at least one direction is effective for increasing the elastic modulus.

Next, this stretched film is heat treated. The heat treatment can be performed by a known method such as in an oven or on a roll. The heat treatment conditions for obtaining the film of the present invention are as follows:
Preferably, the treatment temperature is 180 to 240°C and the treatment time is 0.1 to 120 seconds.

Further, the film of the present invention may be subjected to a known corona discharge treatment (in air, nitrogen, carbon dioxide, etc.) and used.
Further, in order to impart adhesion, moisture resistance, heat sealability, lubricity, surface smoothness, etc., it may be used in the form of a laminate with other polymers or in the form of a coating with an organic and/or inorganic composition.

The characteristic values of the present invention are based on the following measurement method and evaluation criteria.

■ Using the thermomechanical testing apparatus (TMA) of Fluid Temperature Vacuum Riko Co., Ltd.
The temperature at which the needle penetrates 90% or more of the sample thickness using the enetration method was measured, and this was taken as the flow temperature of the polymer. In the penetration test, a cylindrical quartz glass rod with a diameter of 1 mm was stood perpendicularly to the polymer sheet, and the temperature was raised at a rate of 20° C./min while a load of Φ1 g was applied to the glass rod.

■ Elastic modulus, specific elastic modulus J 25℃, 65% using an Instron type tensile tester according to the method specified in 15-7-1702.
The elastic modulus was measured at RH. The elastic modulus of the biaxially oriented film was taken as the arithmetic average value of the elastic modulus in the longitudinal direction and the width direction of the film. If the elastic modulus of the PET film formed under the same conditions and the blended film are Ea and E, respectively, E/
E o was taken as the specific elastic modulus of the blended film.

■ Impact resistance In accordance with the method stipulated in ASTM-D-256, the Charpy impact strength of the film (unit: knee・cIIl/n+
m2) was measured. In addition, the arithmetic mean value was used for the value when the longitudinal direction of the film was set horizontally between two supporting points and when the width direction of the film was set horizontally. When the Charvi impact strength was 20 or more, it was determined that the impact resistance was good, and when it was less than 20, it was determined that the impact resistance was poor.

■ Dimensional stability (thermal shrinkage rate) A sample film was cut out to a width of 10 mm and a length of 250 mm, and two marked lines were placed at an interval of approximately 200 ff 1 m. To accurately measure the interval, this was designated as 8)).

3. At the tip of this sample. 180 with C1 load applied
After leaving the film in a hot air oven at ℃ for 10 minutes, the film was cooled to the lowest temperature and the distance between the marked lines was measured (this was
11). 100X (A-B)/A was determined in the longitudinal direction and width direction of the film, and the arithmetic average thereof was taken as the heat shrinkage rate of the film.

■ Dimensional stability coefficient As the elastic modulus E1 of the film and the heat shrinkage rate δ, δ/(E-4
00) was taken as the dimensional stability coefficient. This value is 25X1
When it was less than 0-3, it was determined that the Young's modulus and thermal shrinkage were well balanced, and when it was 25×10-3 or more, it was determined to be poor.

■ In plane orientation coefficient X-ray diffraction, PET (with diffraction angle 2θ = 25.7°)
100) The intensity of plane diffraction is observed while rotating the sample in azimuth, and if the half width of the peak drawn at that time is Δ, then the plane orientation coefficient is given by 180−Δ)X100/180.

■ Crystal size In X-ray diffraction, PET with diffraction angle 2θ' = 25.7°
The crystal size D was calculated from the diffraction peak of the (100) plane according to the following formula.

λ However, B: Half width of diffraction peak, b: o, 12, λ: C
α ray wavelength for u (1,5418 people) Furthermore, 2 in the chart
The straight line connecting the points showing the scattering intensities at θ=8″ and 2θ=366 was defined as the baseline.

(2) Reduced viscosity η/C PET was dissolved in a mixed solvent of 6 volumes of phenol/4 volumes of tetrachloroethane to a concentration of 0.5 g/dQ, and the reduced viscosity was determined from the viscosity measured at 30°C.

[Effect 1] The present invention is a biaxially oriented film made by blending and dispersing a copolyester having a specific relatively rigid segment in PET, which is a relatively flexible molecular chain, at a specific ratio. The reinforcing effect of dispersion and/or phase dispersion makes it possible to obtain the following excellent effects.

[Effect of the invention 1 ■ A film with high elastic modulus and excellent dimensional stability and impact resistance is obtained.

(■ When made into magnetic tape, it becomes a film with excellent electromagnetic conversion properties.

■ The film has excellent dimensional stability as a flexible printed circuit board.

[Applications] The film of the present invention can be applied to all applications in which biaxially oriented PET films have conventionally been used, but particularly suitable applications include base films for video and audio magnetic tapes, or base films for magnetic disks. , and flexible printed circuit 11? ). Further, although the thickness of the film of the present invention is not specified, a film with a thickness of 1 to 15 μm, preferably 4 to 12 μm, is preferable for use in magnetic tapes, as it is suitable for miniaturization and long-term use, and as a flexible printed circuit board. is 50-150μm
A film having a thickness of .

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

Example 1 PET polymerized according to a conventional method (ηsP/C=O.

The pellets obtained in No. 66) were crushed using a pulverizer to form powder.

On the other hand, 126 parts by weight of p-acetoxybenzoic acid was mixed with 624 parts by weight of PET powder and granules, and the acetic acid depolymerization method (J, Polymer Sci, 14.20
43 (1976)), the ethylene terephthalate-p-oxybenzoate copolymer (p-oxybenzoate copolymer
-containing 70 mol% of oxybenzoate) was polymerized. This polymer was crushed into powder in the same way as PET using a crusher. The polymer was heated to about 310°C using a polarizing microscope equipped with a heating device.
When observed under crossed nicols, it showed optical anisotropy. Further, the flow temperature of the polymer was 350°C or lower.

Next, powders of PET and ethylene terephthalate-p-oxybenzoate copolymer were put into a V-type blender so that the amount of the latter added was 7% by weight, and blended for about 1 hour.

Powder and granules mixed in this way and PET as a comparison
After sufficiently drying the granular material, it was supplied to an extruder with a screw diameter of 35 mm, and melt-extruded into a sheet at 290°C.

At a winding speed with a draft ratio of 6, the melt sheet was wound onto a casting drum with a surface temperature of 20°C using the electrostatic crest method, and cooled and solidified to form a substantially unoriented, unstretched sheet with a thickness of about 1100 u. made a film. This unstretched film was preheated to 80°C and stretched 3.0 times in the longitudinal direction at a stretching temperature of 90°C. Stretching was performed using a difference in peripheral speed between two sets of rolls, and the stretching speed was 50.000%/min.

This -axially stretched film was preheated to 90°C using a stenter and then stretched 3° in the width direction at a stretching temperature of 95°C. The stretching speed in this case was 5,000%/min. Further, this biaxially stretched film was heat-treated at 210° C. for 15 seconds at a constant length to obtain 1 q of film having a thickness of about 11 μm. The specific elastic modulus of this film was good at 1.2, and the dimensional stability coefficient and impact resistance were also good. Also, the plane orientation coefficient is 0.
89, crystal size 52 people.

Examples 2 to 4, Comparative Example 1.2 The ethylene terephthalate-p-oxybenzoate copolymer obtained in Example 1 was blended with PET at the ratio of Example 2゜3.4 in Table 1, and the following were prepared. The film was formed under the following conditions.

First, a substantially unoriented unstretched film with a thickness of about 110 μm produced in the same manner as in Example 1 was stretched at 85°C in the longitudinal and width directions using a film stretcher (manufactured by T, M, Long). Each film was simultaneously biaxially stretched at 3° and 5 times.

The stretching speed in this case was 20.000%/min. Further, this biaxially stretched film was heat-treated at 210° C. for 15 seconds under constant length to obtain a film having a thickness of about 10 μm. The physical properties of these films are shown in Table 1, and it can be seen that all the films have good elastic modulus, and are also excellent in dimensional stability coefficient and impact resistance. However, even if the composition of the polymer having the ability to form melt anisotropy is the same as that of the present invention, if the blend ratio is outside the scope of the present invention as in Comparison 1.2, the elastic modulus, dimensions, etc. At least one of the stability coefficient and impact resistance becomes poor. However, in the case of Comparative Example 2, it was not possible to stretch the film at the ratio A8, and the film was formed by changing the stretching ratios to 1.5 times in the longitudinal direction and 145 times in the width direction.

Fruit Examples 5 and 6, Comparative Examples 3 and 4 Examples 5 and 6 and Comparative Examples 3 and 4 in Table 1 were prepared in the same manner as in Example 1 by changing the proportions of PET and p-acetoxybenzoic acid.
An ethylene terephthalate-p-oxybenzoate copolymer having the polymer composition shown in 4 was synthesized. Here, the polymer of Comparative Example 3 alone did not have the ability to form melt anisotropy.

Furthermore, the flow temperature of all polymers was 350°C or lower.

These copolymerized polyesters were mixed with PET at the blend ratios shown in Table 1, and biaxially stretched films were obtained in the same manner as in Examples 2 and 3. As shown in the table, the physical properties of these films depend on the composition of the copolyester,
If the blend ratio is within the range of the present invention, the properties of the film are good. However, if either the polymer composition or the blend ratio is outside the scope of the present invention, defects in elastic modulus and impact resistance will be observed.

However, in the case of Comparative Example 3.4, it was not possible to stretch the film at the above-mentioned stretching ratio, and the stretching ratio was set to 2.0 times in the longitudinal direction of the film and 2.0 times in the width direction.
The film was formed by changing the magnification to 0 times.

Comparative Example 5 The unstretched film obtained in Example 1 was stretched 4.0 times in the longitudinal direction using a film stretcher at 85° C. with the width fixed. The stretching speed in this case was 5.000%/min. Further, this fixed-width, axially stretched film was heat-treated at 210° C. for 15 seconds under constant length. This film had a planar orientation coefficient of 0.5, a crystal size of 50, and poor impact resistance.

Comparative Example 6 The biaxially stretched film of Example 1 was stretched at a constant length of 11 at 235°C.
0011i heat treatment. Planar orientation coefficient of this film is 0.
97, the crystal size was 70, and the dimensional stability coefficient was good, but the impact resistance was poor.

Example 7, Comparative Example 7.8 624 parts by weight of PET powder and 1 part by weight of β-oxy-6 Naphthotech M1316 were mixed and acetic acid depolymerized under the conditions of Example 1 to obtain the ability to form melt anisotropy. A copolymerized polyester with the following properties was synthesized. The flow temperature of this polymer was below 350°C.

Next, this copolymerized polyester was blended with PET in the proportions shown in Example 7 and Comparative Examples 7 and 8 in Table 1, and unstretched films were prepared in the same manner as in Example 1. Furthermore, simultaneous two-wheel stretching and heat treatment were performed using the methods of Examples 2 to 4 to obtain two-wheel stretched films. However, Comparative Example 8 could not be stretched, and the properties of the unstretched film were evaluated. As shown in Table 1, the physical properties of these films are good if the blend ratio is within the scope of the present invention, but if the blend ratio is outside the scope of the present invention, the dimensional stability coefficient or impact resistance is poor.

Claims (1)

[Claims] (1) Polyester (I) consisting of components (A) and (B) represented by the following general formula, having a flow temperature of 350°C or less and having the ability to form melt anisotropy, is blended with polyethylene terephthalate. It is a film,
The film has a mole fraction of component (A) in polyester (I) of 40 mol% or more, and the value is expressed as Mf (mol%).
Then, the blend ratio of polyester (I) X_b(
% by weight) is a blend selected from the range of 1≦X_b≦-0.8M_f+90, and the plane orientation coefficient of the polyethylene terephthalate crystal (100) plane is 0.75 to 0.97, and the crystal in the plane direction Size 35-6
A biaxially oriented polyester film characterized by having a thickness of 5 Å. ▲There are mathematical formulas, chemical formulas, tables, etc.▼(A) ▲There are mathematical formulas, chemical formulas, tables, etc.▼(B) (In the formula, n is an integer selected from 2, 4, 6, R,
R' is 1,3 phenylene, 1,4 phenylene, 2,6 naphthalene, 2,7 naphthalene, ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼, ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼, ▲ Mathematical formulas, chemical formulas, There are tables, etc. ▼Choose from the list)