JP2022122202A - Biaxially oriented film, and laminate film - Google Patents

Abstract

To provide a biaxially oriented film and a laminate film that are excellent in heat resistance and folding resistance.SOLUTION: A biaxially oriented film contains a polyethylene naphthalate based resin (A) and an amorphous polyester (B) having a glass transition temperature higher than the polyethylene naphthalate based resin (A). The biaxially oriented film has an average hysteresis loss ratio of 46.0% or less in a tensile cycle test to 5% tensile strain in each direction of a longer direction (MD) and a width direction (TD).SELECTED DRAWING: None

Description

The present invention relates to biaxially stretched films and laminated films.

Polyester has excellent properties such as heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, and gas barrier properties.・It is a resin that is widely used for food containers, packaging materials, molded products, films, etc. The main polyester resin is polyethylene terephthalate (hereinafter sometimes referred to as “PET”), which has excellent mechanical properties, electrical properties, chemical resistance, etc., and has a wide range of applications. There are some drawbacks such as foldability.

On the other hand, in recent years, flexible substrates and flexible printed circuits have tended to be used as electronic devices have become smaller and lighter.
In line with this trend, there is a growing demand for flexibility in display applications, and there is a strong demand for films with high heat resistance, excellent restorability, and excellent resistance to repeated bending.

Examples of flexible displays include types such as foldable, bendable, rollable, and stretchable.
A typical example of a flexible display includes a substrate film made of a synthetic resin such as polyimide, and elements such as a thin film transistor (TFT) and an organic light emitting diode (OLED) supported by the substrate film. In addition to films, optical films with excellent flexibility are used as structural members such as front panels of flexible displays, substrate films for touch sensors, and films that protect substrate films (hereinafter referred to as lower protective films).

Thus, in recent years, resin films with excellent flexibility have been studied. For example, Patent Literature 1 discusses a film resistant to repeated bending using a cyclic olefin resin film.

Patent Document 2 proposes a polyimide film as a film having excellent heat resistance and bending resistance.

Further, Patent Document 3 proposes a polyethylene naphthalate resin in which crystallinity is improved without impairing heat resistance by blending an ethylene oxide adduct of a bisphenol compound or a derivative thereof in all alcohol components. .

Furthermore, in Patent Document 4, a modified product improved in heat resistance, impact resistance, etc. is obtained by blending 2,6-naphthalenedicarboxylic acid as an acid component and a bisphenol compound and 1,4-cyclohexanedimethanol as a glycol component. high quality polyester resins have been proposed.

JP 2014-104687 A WO2016/060213 JP-A-8-48759 JP 2004-107559 A

However, the film disclosed in Patent Literature 1 has a low level of resistance to repeated bending and does not meet market requirements.
In addition, since the cyclic olefin-based resin is poor in coatability and adhesiveness, it is difficult to laminate with other members, and it is unsuitable as a member for a flexible display.

In addition, although the polyimide film described in Patent Document 2 has bending resistance, the manufacturing process is a molding method by coating using a solvent, so there are problems such as poor productivity and high cost.

The polyethylene naphthalate resin described in Patent Document 3 improves the crystallization rate by blending an ethylene oxide adduct of a bisphenol compound or a derivative thereof in all the alcohol components, improves troubles during drying of the raw material, and improves molding. It is a product with improved performance.
However, since the crystallization rate is too high, the film formability and stretchability of the film by extrusion molding deteriorate, and there is a problem that it is not suitable for a stretched film.

The modified polyester resin described in Patent Document 4 exhibits high heat resistance, but on the other hand, the temperature at the time of molding must be set high, and concerns about thermal decomposition become apparent, and the moldability and the material after molding properties may be degraded.

The problem to be solved by the present invention is to solve the above problems and to provide a biaxially stretched film excellent in heat resistance and folding endurance.

The inventors of the present invention have completed the present invention as a result of earnest studies to achieve the above-mentioned problems. In one aspect of the present invention, the gist is the following [1] to [12].
[1] A polyethylene naphthalate-based resin (A) and an amorphous polyester (B) having a glass transition temperature higher than that of the polyethylene naphthalate-based resin (A) are included in each of the longitudinal direction (MD) and the width direction (TD) A biaxially stretched film having an average hysteresis loss rate of 46.0% or less when subjected to a tensile cycle test up to 5% tensile strain in the direction of .
[2] The polyethylene naphthalate resin (A) is a copolymer containing a dicarboxylic acid component (a-1) and a diol component (a-2), and 2,6- The biaxially stretched film according to [1] above, which contains naphthalenedicarboxylic acid units, and contains ethylene glycol units and bisphenol A-ethylene oxide adduct units as the component (a-2).
[3] The component (a-2) contains 4 mol% or more and 70 mol% or less of the bisphenol A-ethylene oxide adduct unit and contains 30 mol% or more and 96 mol% or less of the ethylene glycol unit. The biaxially stretched film according to [2].
[4] The biaxially stretched film of any one of [1] to [3] above, wherein the amorphous polyester (B) is a polyarylate.
[5] The above [1], wherein the average value of residual strain is 0.900% or less when a tensile cycle test is performed up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD). The biaxially stretched film according to any one of [4].
[6] The biaxially stretched film of any one of [1] to [5] above, which has a glass transition temperature of 75°C or higher and 150°C or lower.
[7] The biaxially stretched film of any one of [1] to [6] above, which has a crystal melting temperature of 220°C or higher and 300°C or lower.
[8] The biaxially stretched film according to any one of [1] to [7] above, which has a thickness of 1 μm or more and 250 μm or less.
[9] The biaxially stretched film according to any one of [1] to [8] above, which is used for flexible displays.
[10] A laminated film comprising the biaxially stretched film according to any one of [1] to [9] above and an adhesive layer provided on at least one side of the biaxially stretched film.
[11] The laminated film according to [10] above, which has a coating layer between the biaxially stretched film and the adhesive layer.
[12] A flexible display comprising the biaxially stretched film of any one of [1] to [9] above or the laminated film of [10] or [11] above.

The biaxially stretched film and laminated film of the present invention have excellent heat resistance and folding endurance.
Therefore, the biaxially stretched film and laminated film of the present invention can be suitably used for flexible displays and the like.

Profile of a stress-strain curve.

The present invention will be described in detail below. However, the present invention is not limited to the embodiments described below.

[Biaxially stretched film]
A biaxially stretched film (hereinafter sometimes referred to as “this film”) according to an example of an embodiment of the present invention comprises a polyethylene naphthalate (hereinafter sometimes referred to as “PEN”)-based resin (A), An amorphous polyester (B) having a higher glass transition temperature than the polyethylene naphthalate resin (A) is included, and a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD) The average value of the hysteresis loss rate when performing is 46.0% or less.
Since this film is a biaxially stretched film, it can be made into a thin film. Excellent.
In the present invention, the longitudinal direction (MD) of the film refers to the direction in which the film advances in the manufacturing process of the film, that is, the winding direction of the film roll.
Further, the width direction (TD) of the film is a direction parallel to the film surface and perpendicular to the longitudinal direction, ie, a direction parallel to the central axis of the film roll.

The present invention has been completed by finding that the present film having a hysteresis loss rate of a specific value or less has excellent heat resistance and folding resistance as a display film and is suitable for flexible applications, particularly foldable applications. is.
Since the film has a low hysteresis loss rate, it is considered to have excellent restorability and exhibit folding endurance.

1. Physical Properties First, the physical properties of this film will be described.

(1) Hysteresis loss rate This film has an average hysteresis loss rate of 46 when subjected to a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) at 23 ° C. 0% or less, preferably 45.0% or less, more preferably 44.0% or less. Although the lower limit is not particularly limited, it is 0.100% or more. By setting the hysteresis loss ratio to 46.0% or less, the restoring force of the film is increased, and the bending resistance (folding resistance) of the film is maintained within a practical range.

Further, by averaging the hysteresis loss rate in each of the longitudinal direction (MD) and the width direction (TD), it can be used as a characteristic index for the film as a whole.
In addition, the hysteresis loss rate can be adjusted by the type and content of the resin to be used, stretching conditions, and the like.
The hysteresis loss ratio of this film can be measured according to JIS K 7312:1996 by the method described in Examples.
More specifically, when a stress-strain curve graph as shown in FIG. 1 is obtained by a tensile cycle test, the ratio of the area surrounded by abcef to the total area (abcda) is the hysteresis loss rate. is defined as

Furthermore, the difference in hysteresis loss ratio between the longitudinal direction (MD) and the transverse direction (TD) of the film at 23° C. is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably. is 13.0% or less.
When the difference in the hysteresis loss ratio between the longitudinal direction (MD) and the transverse direction (TD) is within the above numerical range, the bending resistance of the film and, in turn, the anisotropy of various properties of the film are reduced. not constrained.
Therefore, there is no need to select a specific direction when using the film as a member or in the manufacturing process such as secondary processing, which has the advantage of avoiding an increase in the workload of workers.
In addition, since problems due to anisotropy only in a specific direction are less likely to occur, the film is excellent in handleability.
As a method of reducing the difference in the hysteresis loss rate between the longitudinal direction (MD) and the transverse direction (TD), for example, a desired value can be obtained by controlling the draw ratio in each direction.

(2) Residual strain The average value of residual strain when performing a tensile cycle test up to 5% tensile strain in each direction (MD) and transverse direction (TD) at 23 ° C. of the film is 0.900% or less. is preferably 0.880% or less, more preferably 0.860% or less, and particularly preferably 0.840% or less. Although the lower limit is not particularly limited, it is 0.100% or more.
By setting the residual strain to 0.900% or less, the restoring force of the film increases and the bending resistance of the film is kept within a practical range. Further, by averaging the residual strain in each of the longitudinal direction (MD) and the transverse direction (TD), it is possible to obtain a characteristic index of the film as a whole. Incidentally, the residual strain can be adjusted by stretching conditions and the like.
The residual strain of this film can be measured according to JIS K 7312:1996 by the method described in Examples.
More specifically, the value of f is defined as the residual strain when the stress-strain curve graph as shown in FIG. 1 is obtained by the tensile cycle test.

Furthermore, the difference in residual strain between the longitudinal direction (MD) and the transverse direction (TD) of the film at 23 ° C. is preferably 0.900% or less, more preferably 0.700% or less, and still more preferably It is 0.500% or less, and particularly preferably 0.350% or less.
When the difference in residual strain in the longitudinal direction (MD) and the transverse direction (TD) is within the above numerical range, the bending resistance of the film and, in turn, the anisotropy of various properties of the film are reduced. do not receive Therefore, there is no need to select a specific direction when using the film as a member or in the manufacturing process such as secondary processing, which has the advantage of avoiding an increase in the workload of workers.
In addition, since problems due to anisotropy only in a specific direction are less likely to occur, the film is excellent in handleability.

(3) Glass transition temperature The glass transition temperature (Tg) of the present film is preferably 75°C or higher and 150°C or lower, more preferably 76°C or higher and 140°C or lower, even more preferably 77°C or higher and 130°C or lower. If the film has a glass transition temperature (Tg) of 75° C. or higher, it can be said that the film is excellent in heat resistance because it is not deformed even when used for displays.
On the other hand, if the glass transition temperature (Tg) is 150° C. or less, it will be suitable for workability.
The glass transition temperature (Tg) of this film is measured using a differential scanning calorimeter (DSC) at a heating rate of 10° C./min according to JIS K7121 (2012).

(4) Crystal melting temperature The crystal melting temperature (Tm) of the present film is preferably 220°C or higher and 300°C or lower, more preferably 221°C or higher and 295°C or lower, and 222°C or higher and 290°C or lower. is more preferable, and 223° C. or more and 285° C. or less is particularly preferable. If the crystalline melting temperature (Tm) of the present film is within the above range, the present film will have an excellent balance between heat resistance and melt extrusion moldability.
Here, the crystalline melting temperature (Tm) of this film is measured using a differential scanning calorimeter (DSC) at a heating rate of 10° C./min according to JIS K7121 (2012).
The crystalline melting temperature (Tm) of the present film can be optimized by adjusting the cooling temperature from the molten state, the draw ratio, the draw temperature, and the heat treatment conditions after drawing in the production of the present film.

(5) Thickness The thickness of the present film is preferably 1 μm or more and 250 μm or less, more preferably 5 μm or more and 200 μm or less. A film strength of 1 μm or more keeps the film strength within a practical range. When the thickness is 250 μm or less, folding endurance is likely to be exhibited. The thickness can be adjusted by film forming and stretching conditions.

2. Ingredients Next, the ingredients constituting the present film will be described.

The present invention generally provides a polyethylene naphthalate resin (A) and a polyethylene naphthalate resin (A) as a polyester resin that exhibits relatively higher yield stress and elastic modulus than those of polyethylene resins and polypropylene resins. The present inventors have found that a film composed mainly of a polyester-based mixed resin containing an amorphous polyester (B) having a high glass transition temperature is excellent in bending resistance.
Even polyester resin, which is a material with high yield stress and elastic modulus, has a problem that deformation occurs when the stress and strain applied by deformation are large, and unresolved strain remains in the material.
However, in the present invention, it has been found that if the hysteresis loss rate of the film is a specific value or less, the restoring force of the film increases and the distortion hardly occurs.
Deformation beyond the elastic deformation region generally results in large strains in the material.
However, even if the deformation is within the elastic deformation region, strain remains in the material, and deformation marks such as creases and wrinkles remain, which may affect the appearance and the material properties themselves.
In other words, the smaller the hysteresis loss rate, the higher the resilience against deformation and the smaller the strain remaining in the material. It is thought that it is difficult to stick and has excellent deformation resistance.
In the present invention, a mixed resin containing a specific polyester-based mixed resin, that is, a polyethylene terephthalate-based resin (A) and an amorphous polyester (B) having a glass transition temperature higher than that of the resin (A) is used. By using this material, the hysteresis loss rate of the film is reduced to a specific value or less, and the restoring force of the film is increased to prevent the material from being distorted, and at the same time, it has heat resistance.

<Polyethylene naphthalate resin (A)>
The polyethylene naphthalate-based resin (A) constituting the present film may be homopolyester, copolyester, or a blend of homopolyester and copolyester.
When it is made of homopolyester, it is obtained by polycondensation of 2,6-naphthalenedicarboxylic acid as the dicarboxylic acid component (a-1) and ethylene glycol as the diol component (a-2).
When homopolyester is used as the PEN resin, the heat resistance of the biaxially stretched polyester film of the present invention can be further improved.

On the other hand, in the case of a copolymerized polyester, 2,6-naphthalene dicarboxylic acid is an essential component as the dicarboxylic acid component (a-1), and other copolymerized components are added as necessary.
Other copolymer components include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3 ,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 3,3′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid and other aromatic dicarboxylic acids; cyclohexanedicarboxylic acid, oxalic acid , malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, aliphatic dicarboxylic acids such as dimer acid; Among these, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferred from the viewpoint of moldability.
These copolymerization components can be used individually by 1 type or in combination of 2 or more types.
In addition, when a copolyester is used as the PEN resin, the folding endurance of the biaxially stretched polyester film of the present invention can be further improved.

The copolymer polyester preferably contains 2,6-naphthalenedicarboxylic acid units in the dicarboxylic acid component (a-1) in an amount of preferably 90 mol% or more, more preferably 92 mol% or more, still more preferably 94 mol% or more, and particularly preferably contains 96 mol % or more, particularly preferably 98 mol % or more, and all (100 mol %) of the dicarboxylic acid component (a-1) may be 2,6-naphthalenedicarboxylic acid.
By setting the content of the 2,6-naphthalenedicarboxylic acid unit in the dicarboxylic acid component (a-1) within the above numerical range, the glass transition temperature and melting point of the copolyester are improved, and thus the heat resistance of the film. improves.

In the copolymer polyester, other copolymer components are preferably added to the dicarboxylic acid component (a-1) at 0 mol% or more and 10 mol% or less, more preferably 0 mol% or more and 8 mol% or less, still more preferably 0 mol%. % or more and 6 mol % or less, particularly preferably 0 mol % or more and 4 mol % or less, particularly preferably 0 mol % or more and 2 mol % or less.
By setting the content of the other copolymerization components in the dicarboxylic acid component (a-1) within the above numerical range, the glass transition temperature and melting point of the copolymerized polyester are improved, and thus the heat resistance of the present film is improved. .

As the diol component (a-2), ethylene glycol is an essential component, and if necessary, other copolymer components such as 1,2-propanediol, 1,3-propanediol and 1,4-butane Diol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, propylene glycol, polyalkylene glycol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, hydroquinone , spiroglycol, 2,2,4,4,-tetramethylcyclobutane-1,3-diol, isosorbide, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol A, bisphenol F or bisphenol compounds such as bisphenol S, derivatives thereof, or ethylene oxide adducts thereof), among which 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferred.
In particular, from the viewpoint of maintaining film strength, it is preferable to use bisphenols.
As bisphenols, it is preferable to use a bisphenol A-ethylene oxide adduct.
These copolymerization components can be used individually by 1 type or in combination of 2 or more types.

The copolymer polyester preferably contains ethylene glycol in the diol component (a-2) at 30 mol% or more and 96 mol% or less, more preferably 40 mol% or more and 95.8 mol% or less, still more preferably 50 mol% or more. 95.6 mol % or less, particularly preferably 60 mol % or more and 95.4 mol % or less, particularly preferably 70 mol % or more and 95.2 mol % or less.
By setting the content of ethylene glycol in the diol component (a-2) within the above numerical range, the crystallinity of the copolyester is maintained and the heat resistance of the present film is improved.

In the copolymer polyester, other copolymer components are preferably added to the diol component (a-2) in an amount of 4 mol% or more and 70 mol% or less, more preferably 4.2 mol% or more and 60 mol% or less, still more preferably 4 4 to 50 mol%, particularly preferably 4.6 to 40 mol%, most preferably 4.8 to 30 mol%.
By setting the content of the other copolymer components in the diol component (a-2) within the above numerical range, the glass transition temperature and melting point of the copolymer polyester are improved, and the heat resistance of the present film is improved. In addition, since the crystallinity can be controlled, the crystallization speed can be slowed down, and the extrusion moldability and stretching processability of the film can be improved. Moreover, when the content is 70 mol % or less, the melting point does not become too high. Therefore, there is no need to set the molding temperature high, and there is no fear of thermal decomposition.

The copolyester constituting the PEN resin (A) contains a dicarboxylic acid component (a-1) and a diol component (a-2), and the component (a-1) is a 2,6-naphthalenedicarboxylic acid unit. and preferably a polyethylene naphthalate copolymer containing ethylene glycol units and bisphenol A-ethylene oxide adduct units as the component (a-2).

The crystal melting temperature (Tm(A)) of the PEN resin (A) is preferably 220°C or higher and 300°C or lower, more preferably 225°C or higher and 290°C or lower, and 230°C or higher and 280°C or higher. ° C. or lower, and particularly preferably 235° C. or higher and 270° C. or lower. If the crystal melting temperature (Tm(A)) of the PEN-based resin (A) is in the range, the PEN-based resin (A) has an excellent balance between heat resistance and melt moldability.
The crystalline melting temperature (Tm(A)) of the PEN resin (A) can be measured at a heating rate of 10°C/min using a differential scanning calorimeter (DSC) according to JIS K7121 (2012). .

The glass transition temperature (Tg(A)) of the PEN resin (A) is preferably 75° C. or higher and 150° C. or lower, more preferably 77° C. or higher and 145° C. or lower, and 80° C. or higher or 140° C. or lower. is more preferable. If the glass transition temperature (Tg(A)) of the PEN-based resin (A) is within this range, the balance between heat resistance and melt moldability is excellent.

<Amorphous polyester (B)>
The amorphous polyester (B) constituting the present film is an amorphous polyester having a glass transition temperature (Tg) higher than that of the PEN resin (A), and is a component that mainly contributes to folding resistance. It is a component that mainly imparts heat resistance to the PEN resin (A).

In general, improvement in heat resistance of a resin composition can be achieved by improving the glass transition temperature (Tg). Here, by mixing an amorphous polyester (B) having a higher glass transition temperature (Tg) than the PEN resin (A), a resin having a higher glass transition temperature (Tg) than the PEN resin (A) alone A composition can be obtained, and a film excellent in heat resistance can be obtained.
In addition, by adding the amorphous polyester (B), which is amorphous, the crystallinity of the PEN resin (A) itself can be relaxed, and breakage during stretching can be suppressed and handleability during processing can be improved. .

The amorphous polyester (B) has, for example, a structure obtained by polycondensation using a combination of at least three dicarboxylic acid components and diol components.
Dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4 - aromatic dicarboxylic acids such as furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid; Aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dimer acid; oxycarboxylic acids such as p-oxybenzoic acid etc.
Diol components include ethylene glycol, diethylene glycol, propylene glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,5-pentadiol, neo Pentyl glycol, polytetramethylene ether glycol, dimer diol, polyalkylene glycol, 2,2-bis(4'-β-hydroxyethoxyphenyl)propane, isosorbite, spiroglycol, bisphenols (bisphenol A, bisphenol F, bisphenol S , a bisphenol compound such as bisphenol TMC, a derivative thereof, or an ethylene oxide adduct thereof).
Among the amorphous polyester (B), from the viewpoint of folding endurance, a non-crystalline polyester containing at least one diol component selected from 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols Crystalline copolyesters are preferred.
Furthermore, from the viewpoint of achieving both heat resistance and folding resistance, a wholly aromatic polyester is more preferable, and among them, a polyarylate (hereinafter sometimes referred to as "PAR") is even more preferable.

The PAR is a polycondensate of a dicarboxylic acid component and a dihydric phenol component. The dicarboxylic acid component constituting PAR is not particularly limited as long as it is a divalent aromatic carboxylic acid, but a mixture of a terephthalic acid component and an isophthalic acid component is particularly preferred.
The mixing ratio (molar ratio) of the terephthalic acid component and the isophthalic acid component is preferably terephthalic acid/isophthalic acid = 99/1 to 1/99, more preferably 90/10 to 10/90, and 80/20 to 20/ 80 is more preferred, 70/30 to 30/70 is particularly preferred, and 60/40 to 40/60 is particularly preferred.
When the mixing ratio of terephthalic acid and isophthalic acid as dicarboxylic acid components is within the above range, PAR is excellent in heat resistance and melt moldability.

The PAR may be obtained by copolymerizing an acid component other than terephthalic acid and isophthalic acid as a dicarboxylic acid component.
Specifically, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, benzophenonedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 3,3′- Aromatic dicarboxylic acids such as diphenyldicarboxylic acid and 4,4'-diphenyletherdicarboxylic acid are preferred.
Moreover, the copolymerization ratio of acid components other than terephthalic acid and isophthalic acid is preferably less than 10 mol % so as not to impair the heat resistance of the PAR resin.

The dihydric phenol component constituting the PAR is not particularly limited as long as it is a dihydric phenol, but bisphenol A (2,2-bis(4-hydroxyphenyl)propane) component, bisphenol TMC (1,1 -bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane) or both bisphenol A and bisphenol TMC.
In general, PAR having excellent melt moldability (fluidity) is obtained by containing a bisphenol A component.
On the other hand, by including the bisphenol TMC component, the glass transition temperature is improved and the PAR is excellent in heat resistance.
Both the bisphenol A component and the bisphenol TMC component are preferably used to balance melt moldability and heat resistance.
In this case, the ratio (mol%) of the bisphenol A component and the bisphenol TMC component is preferably bisphenol A/bisphenol TMC = 99/1 to 1/99, more preferably 90/10 to 10/90, and 80/20 to 20. /80 is more preferred, 70/30 to 30/70 is particularly preferred, and 60/40 to 40/60 is particularly preferred.
By setting the proportions of the bisphenol A component and the bisphenol TMC component within this range, a PAR having an excellent balance between heat resistance and melt moldability can be obtained.

The PAR may be a copolymer of bisphenol A and bisphenols other than bisphenol TMC as a dihydric phenol component.
Specifically, bisphenol AP (1,1-bis(4-hydroxyphenyl)-1-phenylethane), bisphenol AF (2,2-bis(4-hydroxyphenyl)hexafluoropropane), bisphenol B (2, 2-bis(4-hydroxyphenyl)butane), bisphenol BP (bis(4-hydroxyphenyl)diphenylmethane), bisphenol C (2,2-bis(3-methyl-4-hydroxyphenyl)propane), bisphenol E (1 , 1-bis(4-hydroxyphenyl)ethane), bisphenol F (bis(4-hydroxyphenyl)methane), bisphenol G (2,2-bis(4-hydroxy-3-isopropylphenyl)propane), bisphenol M ( 1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene), bisphenol S (bis(4-hydroxyphenyl)sulfone), bisphenol P (1,4-bis(2-(4-hydroxy phenyl)-2-propyl)benzene), bisphenol PH (5,5′-(1-methylethylidene)-bis[1,1′-(bisphenyl)-2-ol]propane), bisphenol Z (1,1 -bis(4-hydroxyphenyl)cyclohexane) and the like.
The copolymerization ratio of the above compounds is preferably less than 10 mol % so as not to impair the heat resistance of PAR.

In order to increase compatibility with the PEN resin (A), PAR contains a mixture of a terephthalic acid component and an isophthalic acid component as a dicarboxylic acid component, and either a bisphenol A component or a bisphenol TMC component as a dihydric phenol component. Alternatively, it is preferred to choose a mixture of bisphenol A and bisphenol TMC.

The glass transition temperature (Tg) of the amorphous polyester (B) is preferably 80° C. or higher and 350° C. or lower, more preferably 100° C. or higher and 340° C. or lower, and 120° C. or higher and 330° C. or lower. is more preferable, 140° C. or higher and 320° C. or lower is particularly preferable, and 160° C. or higher and 300° C. or lower is particularly preferable.
When the glass transition temperature (Tg) of the amorphous polyester (B) satisfies the above range, the glass transition temperature of the biaxially stretched film of the present invention falls within a suitable range, and a film excellent in heat resistance and moldability can be obtained. be done.

The difference in glass transition temperature (Tg) between the PEN resin (A) and the amorphous polyester (B) is preferably 5°C or more, more preferably 20°C or more, and more preferably 35°C or more. is more preferred, 50° C. or higher is particularly preferred, and 65° C. or higher is particularly preferred.
When the difference in glass transition temperature between the PEN resin (A) and the amorphous polyester (B) is within the above range, the glass transition temperature of the biaxially stretched film of the present invention is in a suitable range, and heat resistance and moldability are improved. A film excellent in
Although the upper limit of the difference in glass transition temperature (Tg) between the PEN resin (A) and the amorphous polyester (B) is not particularly limited, it is usually 100°C or less, preferably 90°C or less.

From the viewpoint of achieving both heat resistance and folding resistance, the content of the amorphous polyester (B) is preferably 1 part by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the PEN resin (A). , more preferably 3 to 48 parts by mass, still more preferably 5 to 46 parts by mass, and particularly preferably 10 to 44 parts by mass.
When the content of the amorphous polyester (B) in the present film is 1 part by mass or more, the crystallization rate can be slowed, and thus the stretching processability when stretching the film can be improved.
On the other hand, when the content of the amorphous polyester (B) is 50 parts by mass or less, the crystallinity of the film is maintained and the obtained film has sufficient heat resistance.

In the present invention, the present film may contain resins other than the PEN-based resin (A) and the amorphous polyester (B) as long as the effects of the present invention are not impaired.
Other resins include polystyrene-based resins, polyvinyl chloride-based resins, polyvinylidene chloride-based resins, chlorinated polyethylene-based resins, polyester-based resins other than the above (A) and (B), polycarbonate-based resins, polyamide-based resins, Polyacetal resin, acrylic resin, ethylene vinyl acetate copolymer, polymethylpentene resin, polyvinyl alcohol resin, cyclic olefin resin, polylactic acid resin, polybutylene succinate resin, polyacrylonitrile resin, polyethylene oxide resins, cellulose resins, polyimide resins, polyurethane resins, polyphenylene sulfide resins, polyphenylene ether resins, polyvinyl acetal resins, polybutadiene resins, polybutene resins, polyamideimide resins, polyamide bismaleimide resins, poly Etherimide-based resins, polyetheretherketone-based resins, polyetherketone-based resins, polyethersulfone-based resins, polyketone-based resins, polysulfone-based resins, aramid-based resins, fluorine-based resins, and the like.

Among these, polycarbonate-based resins are preferred. The melt moldability of the PAR is improved by mixing it with a polycarbonate-based resin. That is, since PAR and polycarbonate are compatible, by mixing PAR with polycarbonate, the glass transition temperature of PAR can be lowered while maintaining transparency and mechanical properties, resulting in improved melt moldability. be able to.
When PAR and polycarbonate are mixed, the mixing ratio (mass ratio) is preferably PAR/polycarbonate = 99/1 to 50/50, more preferably 98/2 to 60/40, and further 97/3 to 70/30. Preferably, 96/4 to 80/20 are particularly preferred. If the mixing ratio of PAR and polycarbonate is in such a range, the melt moldability can be improved while maintaining the heat resistance of PAR.

The film may contain particles for the main purpose of imparting lubricity and preventing the occurrence of scratches in each step. The types of particles to be blended are not particularly limited as long as they are particles capable of imparting lubricity. Specific examples include silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, and magnesium phosphate. , inorganic particles such as kaolin, aluminum oxide and titanium oxide, and organic particles such as acrylic resin, styrene resin, urea resin, phenol resin, epoxy resin and benzoguanamine resin. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the production process of a polymer such as polyester can also be used.

In addition, the present film can appropriately contain additives that are generally blended. Examples of the additives include recycled resin generated from trimming loss such as selvages, and pigments such as titanium oxide and carbon black, which are added for the purpose of improving and adjusting molding processability, productivity, and various physical properties of the porous film. , flame retardant, weather stabilizer, heat stabilizer, antistatic agent, melt viscosity improver, cross-linking agent, lubricant, nucleating agent, plasticizer, anti-aging agent, antioxidant, light stabilizer, UV absorber, medium Additives such as blending agents, anti-fogging agents, anti-blocking agents, slip agents, and colorants are included.

3. Manufacturing Method Next, a manufacturing method of the present film will be described.

The method for producing the biaxially stretched film of the present invention will be explained, but the following explanation is an example of the method for producing this film, and this film is not limited to the film produced by such a production method.

A method for producing the present film according to an example of the embodiment of the present invention comprises forming a resin composition containing the PEN resin (A) and the amorphous polyester (B) into a film and biaxially stretching the film. The method.

The method of kneading the PEN-based resin (A), the amorphous polyester (B), other resins, and additives to obtain a resin composition is not particularly limited. , preferably by melt-kneading using an extruder. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to melt-knead using a co-rotating twin-screw extruder.
The kneading temperature must be at least the glass transition temperature (Tg) of all the polymers used, and at least the crystalline melting temperature (Tm) of the crystalline resin for the crystalline resin. Although the higher the kneading temperature is, the easier it is for the transesterification reaction of a part of the polymer to improve the compatibility. However, if the kneading temperature is higher than necessary, the resin will decompose, which is not preferable. For this reason, the kneading temperature is preferably 255° C. or higher and 340° C. or lower, more preferably 260° C. or higher and 330° C. or lower, even more preferably 270° C. or higher and 320° C. or lower, and particularly preferably 280° C. or higher and 310° C. or lower. If the kneading temperature is within such a range, compatibility and melt moldability can be improved without causing decomposition of the polymer.

The obtained resin composition can be molded by a general molding method such as extrusion molding, injection molding, blow molding, vacuum molding, air pressure molding, press molding, etc. to produce a biaxially stretched film. In each molding method, the equipment and processing conditions are not particularly limited.
For example, the present film is preferably produced by the following method.

A substantially amorphous and non-oriented film (hereinafter sometimes referred to as "unstretched film") is produced by an extrusion method from the resin composition obtained by mixing. The unstretched film can be produced by, for example, an extrusion method in which the raw material is melted by an extruder, extruded through a flat die or an annular die, and then quenched to form a flat or annular unstretched film. can. At this time, depending on the case, a laminated structure using a plurality of extruders may be used.

Next, the above unstretched film is stretched in at least one direction in the machine direction (MD) and in the transverse direction (TD) perpendicular thereto, usually 1.1 to 6.0 in terms of stretching effect, film strength, etc. It is stretched 0 times, preferably 1.1 to 6.0 times in each of the vertical and horizontal directions.

As the biaxial stretching method, any of conventionally known stretching methods such as tenter successive biaxial stretching, tenter simultaneous biaxial stretching, and tubular simultaneous biaxial stretching can be employed. For example, in the case of a tenter-type sequential biaxial stretching method, the unstretched film is heated to a temperature range of Tg to Tg + 50 ° C., where Tg is the glass transition temperature of the resin composition, and is stretched in the longitudinal direction by a roll-type longitudinal stretching machine. 1.1 to 6.0 times, and then stretched 1.1 to 6.0 times in the transverse direction within the temperature range of Tg to Tg + 50 ° C. with a tenter type transverse stretching machine. can. In the case of a tenter type simultaneous biaxial stretching method or a tubular type simultaneous biaxial stretching method, for example, in the temperature range of Tg to Tg + 50 ° C., stretching is performed 1.1 to 6.0 times in each axial direction at the same time in the vertical and horizontal directions. can be manufactured by

The biaxially stretched film stretched by the above method is subsequently heat set. Dimensional stability at room temperature can be imparted by heat setting. The treatment temperature in this case is preferably selected within the range of the crystal melting temperature Tm-1 to Tm-120°C of the resin composition. If the heat setting temperature is within the above range, heat setting is sufficiently performed, the stress during stretching is relieved, sufficient heat resistance and mechanical properties are obtained, and troubles such as breakage and whitening of the film surface are prevented. A good film is obtained.

In the present invention, in order to relax the stress of crystallization shrinkage due to heat setting, it is preferable to relax the film in the width direction by 0 to 15%, preferably 1 to 10%, during heat setting. Since the film is sufficiently relaxed and uniformly relaxed in the width direction of the film, the shrinkage rate in the width direction becomes uniform and a film having excellent room-temperature dimensional stability can be obtained. In addition, since the film is relaxed following the contraction of the film, there is no film slack, no fluttering in the tenter, and no breakage of the film.

[Uses of this film]
Since the biaxially stretched film of the present invention is excellent in heat resistance and folding resistance, it can be used as an optical film, a packaging film, various protective films, and the like.
Especially, it can be used suitably for flexible displays from a viewpoint of folding endurance. Specifically, it is preferably used as a structural member for a display such as a front plate, a base film for a touch sensor, or a lower protective film, and particularly preferably as a lower protective film. The lower protective film is a film that protects the back side of the display device.
Also, it is particularly preferred that the display is a foldable display. The display may be used in mobile phones, smart phones, digital cameras, personal computers, and the like. The type of display is not particularly limited, and may be a liquid crystal display, a plasma display, an organic EL display, or a touch panel type display. As the display, an organic EL display is preferable.

[Laminated film]
The laminated film of the present invention has the above biaxially stretched film and an adhesive layer on at least one side of the biaxially stretched film.
The laminate is used as a constituent member of the display, and is preferably laminated with another member via an adhesive layer. Furthermore, it is preferable to provide a flexible display having a structure in which another member is attached via the adhesive layer of the laminate.

<Adhesive layer>
Adhesives for forming the adhesive layer include acrylic adhesives, urethane adhesives, synthetic rubber adhesives, natural rubber adhesives, silicone adhesives, polyester adhesives, and vinyl alkyl ether adhesives. , epoxy adhesives, etc., among which acrylic adhesives, urethane adhesives, and silicone adhesives are preferred. Two or more kinds of materials can be mixed and used, or two or more layers can be used as long as the optical properties are not affected.
The pressure-sensitive adhesive may be of emulsion type, solvent type or non-solvent type, and may be of cross-linking type or non-cross-linking type.
Furthermore, particles such as fillers, additives, and the like can be added.

The (meth)acrylic resin (base polymer) constituting the acrylic pressure-sensitive adhesive preferably includes, for example, butyl (meth)acrylate, ethyl (meth)acrylate, isooctyl (meth)acrylate, (meth)acrylic Polymers or copolymers containing one or more of (meth)acrylic acid esters such as 2-ethylhexyl acid as monomers can be mentioned.
It is preferable to copolymerize a polar monomer with the base polymer. Examples of polar monomers include (meth)acrylic acid, 2-hydroxypropyl (meth)acrylate, hydroxyethyl (meth)acrylate, and (meth)acrylamide. , N,N-dimethylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, monomers having a carboxyl group, a hydroxy group, an amide group, an amino group, an epoxy group, and the like.

The pressure-sensitive adhesive composition may contain only the base polymer, but usually contains a cross-linking agent. Examples of the cross-linking agent include divalent or higher valent metal ions, polyamine compounds, polyepoxy compounds, and polyisocyanate compounds, among which polyisocyanate compounds are preferred.

The urethane-based pressure-sensitive adhesive preferably includes a urethane-based polymer, which is an adhesive polymer obtained by reacting a polyol with a polyisocyanate compound.
Examples of polyols include polyether polyols, polyester polyols, polycarbonate polyols, polycaprolactone polyols, and the like.
Examples of polyisocyanate compounds include diphenylmethane diisocyanate, tolylene diisocyanate, and hexamethylene diisocyanate.

The silicone-based pressure-sensitive adhesive preferably includes a pressure-sensitive adhesive obtained by blending or aggregating a silicone-based polymer that is an adhesive polymer.
Examples of the silicone-based pressure-sensitive adhesive include addition reaction-curable silicone-based pressure-sensitive adhesives and peroxide-curable silicone-based pressure-sensitive adhesives. A type silicone pressure-sensitive adhesive is preferred.
Examples of the curing reaction of the addition reaction-curable silicone-based pressure-sensitive adhesive include a reaction in which a polyalkylhydrogensiloxane composition used for obtaining a polyalkylsilicone-based pressure-sensitive adhesive is cured with a platinum catalyst.

The adhesive layer can be formed by applying the above-described adhesive to a polyester film. Alternatively, the adhesive layer may be formed on a polyester film by applying the above-described pressure-sensitive adhesive to a release sheet and transferring the pressure-sensitive adhesive layer formed on another base material such as a release sheet to the polyester film.
As a method for applying the adhesive, a comma coater, a knife coater, a die coater, a gravure coater, a Meyer bar coater, or the like can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness.

As a curing method for curing the adhesive layer, energy beams such as ultraviolet rays and electron beams, and heat curing methods can be appropriately selected.

<Coating layer>
The laminated film of the present invention can also have a coating layer between the adhesive layer and the biaxially stretched film. Various functions are imparted to the laminated film by having the coating layer.
Functions of the coating layer include hard coat properties, antistatic properties, peelability, easy adhesion properties, printability, UV cut properties, infrared shielding properties, gas barrier properties, and the like. Regarding the formation of the coating layer, it may be provided by in-line coating that treats the film surface during the stretching process, or may be applied outside the system on the film once produced, or offline coating may be employed, or both may be used in combination. good too.

[Flexible display]
The flexible display of the present invention comprises the biaxially stretched film or laminated film described above. The biaxially stretched film and laminated film of the present invention are excellent in heat resistance and folding endurance.
Therefore, the flexible display of the present invention is also excellent in heat resistance and folding resistance, and has high performance as a foldable display device, a bendable display device, a rollable display device and a stretchable display device.

Examples are shown below, but the present invention is not limited by these.

[Evaluation method]
(1) Hysteresis loss rate According to JIS K 7312:1996, the average value of the hysteresis loss rate at 23°C was obtained by the following method.
A tensile tester (manufactured by Shimadzu Corporation, tensile tester AG-1kNXplus) was used as a measuring device. As the test piece, a rectangle having a length of 100 mm in the measurement direction and a width of 10 mm was cut out from this film. Both ends of the test piece in the length direction are chucked at a distance between chucks of 50 mm, and the strain is increased to 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed One cycle of tension A stress-strain curve obtained from the cycle test was obtained. The stress-strain curve takes a profile as shown in FIG. Using the area A2 (abcef), which is the difference between the areas of the obtained curves, it was calculated by Equation 1 below. The test was measured 3 times and the average value was obtained. The tensile cycle test was conducted in the longitudinal direction (MD) and the transverse direction (TD) of the film, respectively, and the average value was obtained.
Hysteresis loss rate = (A2/A1) x 100 (Formula 1)

(2) Residual strain According to JIS K 7312:1996, the average value of residual strain at 23°C was obtained by the following method.
A tensile tester (manufactured by Shimadzu Corporation, tensile tester AG-1kNXplus) was used as a measuring device. As the test piece, a rectangle having a length of 100 mm in the measurement direction and a width of 10 mm was cut out from this film. Both ends of the test piece in the length direction are chucked at a distance between chucks of 50 mm, and the strain is increased to 5% at a crosshead speed of 0.5 mm / min, and then lowered to the initial position at the same speed One cycle of tension From the stress-strain curve obtained from the cycle test, the strain at the point where the stress disappeared was defined as the residual strain. The test was measured 3 times and the average value was obtained. The tensile cycle test was conducted in the longitudinal direction (MD) and the transverse direction (TD) of the film, respectively, and the average value was obtained.

(3) Glass transition temperature The obtained film was heated once to the melting temperature at a heating rate of 10°C/min according to JIS K7121 (2012) using a Diamond DSC (manufactured by PerkinElmer Japan). After that, the temperature was lowered at a heating rate of 10° C./min, and the glass transition temperature was measured during the heating process at a heating rate of 10° C./min.

(4) Crystal melting temperature For the obtained film, using Diamond DSC (manufactured by PerkinElmer Japan), according to JIS K7121 (2012), the crystal melting temperature in the heating process at a heating rate of 10 ° C./min was measured. It was measured.

[Materials used]
<PEN resin (A)>
As PEN resin (A)-1, dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 90 mol%, bisphenol A-ethylene Oxide adduct = 10 mol% was used. The PEN resin (A)-1 had a Tg of 116°C.
As PEN resin (A)-2, dicarboxylic acid component (a-1): 2,6-naphthalene dicarboxylic acid = 100 mol%, diol component (a-2): ethylene glycol = 100 mol%, the PEN resin The Tg of (A)-2 was 119°C.

<Amorphous polyester (B)>
As the amorphous polyester (B), PAR having a carboxylic acid component: terephthalic acid/isophthalic acid (molar ratio) of 50/50 and a bisphenol component: 100 mol % of bisphenol A was used. The amorphous polyester (B) had a Tg of 193°C.

[PET film (C)]
A biaxially stretched PET film with a thickness of 50 μm was used as the PET film (C).

(Example 1)
Add pellet-like (B) at a rate of 10% by mass to pellet-like (A)-1, 90% by mass ((A)-1 is 100 parts by mass, (B) is 11 mass part), after dry blending, melt-kneaded with a Φ40 mm twin screw extruder set at 290 ° C., extruded as a film from a T die with a gap of 1.0 mm, taken up with a cast roll at 120 ° C., cooled and solidified, A film-like material having a thickness of about 460 μm was obtained.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine, heated by an IR heater (the temperature near the IR heater was about 181° C.), and stretched 2.8 times in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.7 times in the width direction (TD) at a preheating temperature of 120°C, a stretching temperature of 127°C, and a heat setting temperature of 190°C. A 3% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

(Example 2)
Add pellet-like (B) at a rate of 20% by mass to pellet-like (A)-1, 80% by mass ((A)-1 is 100 parts by mass, (B) is 25 parts by mass), and after dry blending, a film-like material having a thickness of about 460 μm was obtained in the same manner as in Example 1.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine, heated by an IR heater (the temperature near the IR heater was about 190° C.), and stretched 2.8 times in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.7 times in the width direction (TD) at a preheating temperature of 130°C, a stretching temperature of 135°C, and a heat setting temperature of 190°C. A 3% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

(Example 3)
Add pellet-shaped (B) at a rate of 10% by mass to pellet-shaped (A)-2, 90% by mass ((A)-2 is 100 parts by mass, (B) is 11 parts by mass), and a film-like material having a thickness of about 460 μm was obtained in the same manner as in Example 1, except that the temperature of the cast roll was changed to 121° C. after dry blending.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine, heated by an IR heater (the temperature near the IR heater was about 197° C.), and stretched 2.8 times in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.6 times in the width direction (TD) at a preheating temperature of 130°C, a stretching temperature of 135°C, and a heat setting temperature of 205°C. A 3% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

(Example 4)
Add pellet-like (B) at a rate of 20% by mass to pellet-like (A)-2, 80% by mass ((A)-1 is 100 parts by mass, (B) is 25 parts by mass), and a film-like material having a thickness of about 460 μm was obtained in the same manner as in Example 1, except that the temperature of the cast roll was set to 125° C. after dry blending.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine, heated by an IR heater (the temperature near the IR heater was about 207° C.), and stretched 2.6 times in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.7 times in the width direction (TD) at a preheating temperature of 135°C, a stretching temperature of 140°C, and a heat setting temperature of 205°C. A 3% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

(Comparative example 1)
Table 1 shows the evaluation results of the biaxially stretched PET film (C).

(Comparative example 2)
Using pellet-shaped (A)-2, extruded with a Φ44 mm twin screw extruder (set at 300 ° C.) set at 300 ° C. On a cooling roll whose surface temperature was set to 50 ° C. using an electrostatic contact method. to obtain a cast film.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at a stretching temperature of 133°C. Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 3.7 times in the width direction (TD) at a preheating temperature of 115°C, a stretching temperature of 130°C, and a heat setting temperature of 210°C. A 1.5% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

(Comparative Example 3)
A film-like material having a thickness of about 460 μm was obtained in the same manner as in Example 1 except that pellet-shaped (A)-1 was used, the cast roll was set at 110°C, and the extruder was set at 285°C.
Subsequently, the obtained cast film was passed through a longitudinal stretching machine, heated by an IR heater (the temperature near the IR heater was about 160° C.), and stretched 2.8 times in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film is passed through a transverse stretching machine (tenter) and stretched 4.1 times in the width direction (TD) at a preheating temperature of 125°C, a stretching temperature of 130°C, and a heat setting temperature of 170°C. A 3% relaxation treatment was applied to the film in the tenter. Table 1 shows the results of measurements performed on the obtained film.

As shown in the above Examples, the films of Examples 1 to 4 have high glass transition temperatures (Tg) and crystal melting temperatures (Tm), and are excellent in heat resistance.
In addition, the films of Examples 1 to 4 have clearly lower values of hysteresis loss rate and residual strain than those of Comparative Examples 1 to 3, and have high restorability and excellent folding endurance. It can be said.

Since the biaxially stretched film and laminated film of the present invention are excellent in heat resistance and folding resistance, they can be suitably used for optical films, especially for flexible displays.
Therefore, embodiments of the present disclosure are useful for flexible displays such as foldable display devices, bendable display devices, rollable display devices, and stretchable display devices that take advantage of flexible display panels that can be folded, folded back, and rolled. is.

Claims (12)

Containing a polyethylene naphthalate resin (A) and an amorphous polyester (B) having a higher glass transition temperature than the polyethylene naphthalate resin (A),
A biaxially stretched film having an average hysteresis loss rate of 46.0% or less when subjected to a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD). The polyethylene naphthalate resin (A) is a copolymer containing a dicarboxylic acid component (a-1) and a diol component (a-2),
containing a 2,6-naphthalenedicarboxylic acid unit as the component (a-1),
The biaxially stretched film according to claim 1, comprising ethylene glycol units and bisphenol A-ethylene oxide adduct units as component (a-2). The method according to claim 2, wherein the component (a-2) contains 4 mol% or more and 70 mol% or less of the bisphenol A-ethylene oxide adduct unit and contains 30 mol% or more and 96 mol% or less of the ethylene glycol unit. A biaxially stretched film as described. The biaxially stretched film according to any one of Claims 1 to 3, wherein the amorphous polyester (B) is a polyarylate. Any of claims 1 to 4, wherein the average value of residual strain is 0.900% or less when a tensile cycle test is performed up to 5% tensile strain in each of the longitudinal direction (MD) and the transverse direction (TD). 1. The biaxially stretched film according to item 1. The biaxially stretched film according to any one of claims 1 to 5, which has a glass transition temperature of 75°C or higher and 150°C or lower. The biaxially stretched film according to any one of claims 1 to 6, which has a crystal melting temperature of 220°C or higher and 300°C or lower. The biaxially stretched film according to any one of claims 1 to 7, which has a thickness of 1 µm or more and 250 µm or less. The biaxially stretched film according to any one of claims 1 to 8, which is used for flexible displays. A laminated film comprising the biaxially stretched film according to any one of claims 1 to 9 and an adhesive layer provided on at least one side of the biaxially stretched film. 11. The laminated film according to claim 10, which has a coating layer between said biaxially stretched film and said adhesive layer. A flexible display comprising the biaxially stretched film according to any one of claims 1 to 9 or the laminated film according to claim 10 or 11.

Images