JP7697456B2 - Biaxially oriented film - Google Patents

Description

The present invention relates to a biaxially stretched film with excellent heat resistance and folding resistance.

Polyester has excellent properties such as heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, and gas barrier properties, and is also readily available at a reasonable price, making it a versatile resin that is currently widely used in beverage and food containers and packaging materials, molded products, films, etc. The main polyester resin is polyethylene terephthalate (hereinafter sometimes written as "PET"), which has excellent mechanical properties, electrical properties, chemical resistance, etc. and a wide range of applications, but has drawbacks such as poor heat resistance and folding resistance.

As a material using naphthalenedicarboxylic acid as an acid component, polybutylene naphthalate (PBN) obtained by polymerizing naphthalenedicarboxylic acid and 1,4-butanediol is known. However, since the glass transition temperature of PBN is low at about 75° C. and the melting point is low at about 240° C., there is a problem with heat resistance.
In addition, the crystallization speed is too fast, making it unsuitable for film formation by extrusion molding.

On the other hand, with the increasing need for flexible displays in recent years, there is a strong demand for films that are highly heat resistant, have excellent recovery properties, and are resistant to repeated bending.

For example, Patent Document 1 considers a film made of a cyclic olefin resin that is resistant to repeated bending.

In addition, polyimide film has been proposed as a film with excellent heat resistance and bending resistance (Patent Document 2).

Furthermore, Patent Document 3 proposes a polyethylene naphthalate resin that has improved crystallinity without impairing heat resistance by blending an ethylene oxide adduct of a bisphenol compound or its derivative in the total alcohol component.

Furthermore, Patent Document 4 proposes a modified polyester resin that has improved heat resistance, impact strength, etc., by blending 2,6-naphthalenedicarboxylic acid as the acid component and a bisphenol compound and 1,4-cyclohexanedimethanol as the glycol component.

Furthermore, Patent Document 5 discloses a polyethylene naphthalate resin having a high glass transition temperature by blending N,N-bis-(2-hydroxyethyl)-4-4'-biphthalimide in the total alcohol component.

JP 2014-104687 A International Publication No. 2016/060213 Japanese Patent Application Publication No. 8-48759 JP 2004-107559 A Special table 2015-504105 publication

However, the film disclosed in Patent Document 1 has a low level of resistance to repeated folding, and does not satisfy market demands.
Furthermore, since cyclic olefin resins have poor coating properties and adhesive properties, it is believed that lamination with other materials when used as materials for flexible displays is difficult.

Furthermore, although the polyimide film described in Patent Document 2 is flex-resistant, its manufacturing process involves a molding method using a coating of a solvent, which results in poor productivity and is costly.

The polyethylene naphthalate resin described in Patent Document 3 has an improved crystallization rate, improved problems that occur during drying of the raw material, and improved moldability by blending an ethylene oxide adduct of a bisphenol compound or its derivative into the total alcohol component. However, if the crystallization rate is too fast, the film formability and stretchability of the film produced by extrusion molding deteriorate, making it unsuitable for stretched film.

While the modified polyester resin described in Patent Document 4 exhibits high heat resistance, the temperature during molding must be set high, which may result in concerns about thermal decomposition and a decrease in moldability and the properties of the material after molding.

The polyethylene naphthalate resin described in Patent Document 5 has extremely excellent heat resistance, but because it contains a diol component with a rigid biphthalimide skeleton, there is a possibility that its folding resistance may deteriorate.

The problem to be solved by this invention is to solve the above problems and provide a biaxially stretched film that has excellent folding resistance and heat resistance.

The present inventors have conducted extensive research to achieve the above object, and as a result have completed the present invention. The present invention has the following aspects.
[1] A biaxially oriented film comprising a polyethylene naphthalate copolymer (A), the film having an average hysteresis loss rate of 47.0% or less when subjected to a tensile cycle test up to 5% tensile strain in each of MD and TD.
[2] A biaxially oriented film comprising a polyethylene naphthalate copolymer (A), the film having an average residual strain of 0.900% or less when subjected to a tensile cycle test up to 5% tensile strain in each of the MD and TD directions.
[3] The biaxially stretched film according to the above [1] or [2], wherein the polyethylene naphthalate-based copolymer (A) contains a 2,6-naphthalenedicarboxylic acid unit as the dicarboxylic acid component (a-1), and contains a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit as the diol component (a-2).
[4] The biaxially stretched film according to the above [3], wherein the content of terephthalic acid units as dicarboxylic acid components other than the (a-1) in all dicarboxylic acid components is less than 2 mol%.
[5] The biaxially stretched film according to the above [3] or [4], wherein the content of diol units having a biphthalimide skeleton as diol components other than the (a-2) in all diol components is less than 1 mol%.
[6] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), a dicarboxylic acid component (a-1) and a diol component (a-2), the dicarboxylic acid component (a-1) containing at least a 2,6-naphthalenedicarboxylic acid unit, the diol component (a-2) consisting of a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit, and the content of terephthalic acid units as dicarboxylic acid components other than the (a-1) in the total dicarboxylic acid components is less than 2 mol%.
[7] The biaxially stretched film according to any one of the above [3] to [6], wherein the diol component (a-2) contains 4 mol% or more and 70 mol% or less of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and 30 mol% or more and 96 mol% or less of ethylene glycol units.
[8] The biaxially stretched film according to any one of the above [1] to [7], having a glass transition temperature of 75° C. or higher and 150° C. or lower.
[9] The biaxially stretched film according to any one of the above [1] to [8], having a crystalline melting temperature of 220° C. or higher and 300° C. or lower.
[10] The biaxially stretched film according to any one of [1] to [9] above, having a thickness of 1 μm or more and 250 μm or less.
[11] The biaxially stretched film according to any one of the above [1] to [10], which is for a display.
[12] A foldable display comprising the biaxially stretched film according to any one of [1] to [11] above.

According to the present invention, it is possible to propose a biaxially stretched film having excellent folding resistance and heat resistance.

1 is a 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>
In an embodiment of the present invention, the biaxially stretched film of the first aspect contains a polyethylene naphthalate (hereinafter sometimes referred to as "PEN")-based copolymer (A), and has an average hysteresis loss rate of 47.0% or less when subjected to a tensile cycle test up to 5% tensile strain in each of the MD and TD directions.

The biaxially stretched film of the second embodiment contains a polyethylene naphthalate-based copolymer (A) and has an average residual strain of 0.900% or less when subjected to a tensile cycle test up to 5% tensile strain in each of the MD and TD directions.

The biaxially stretched film of the third embodiment contains a polyethylene naphthalate-based copolymer (A), a dicarboxylic acid component (a-1) and a diol component (a-2), the dicarboxylic acid component (a-1) contains at least a 2,6-naphthalenedicarboxylic acid unit, the diol component (a-2) is composed of two components, a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit, and the content of terephthalic acid units as dicarboxylic acid components other than the (a-1) in all the dicarboxylic acid components is less than 2 mol%.
Since the present film is a biaxially oriented film, it can be made thin and has excellent folding resistance.
In addition, MD means the machine direction of the film, and TD means the direction perpendicular to MD. The biaxially stretched films of the first to third embodiments may be referred to as "the film of the present invention."

The present invention was completed based on the discovery that a biaxially oriented film containing a PEN-based copolymer having a hysteresis loss rate or residual strain of a specific value or less has excellent folding resistance and heat resistance as a display film, and is particularly suitable for foldable applications.
This film has a low hysteresis loss rate or residual strain, which is believed to be why it has excellent recovery properties and folding resistance.
Furthermore, a biaxially stretched film containing a PEN-based copolymer having a specific composition also has excellent folding resistance and heat resistance as a display film, and is particularly suitable for foldable applications.

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

(1) Hysteresis loss rate When the present film is subjected to a tensile cycle test up to 5% tensile strain in each of the MD and TD directions at 23° C., the average hysteresis loss rate is 47.0% or less, more preferably 46.0% or less, even more preferably 45.0% or less, and even more preferably 44.0% or less. The lower limit is not particularly limited, but is 0.100% or more.
When the hysteresis loss rate is 47.0% or less, the restoring force of the film is increased, and the bending resistance (folding resistance) of the film is kept within a practical range.
In addition, the hysteresis loss rate can be used as an index of the overall properties of the film by averaging the hysteresis loss rate in both the MD and TD directions. The hysteresis loss rate can be adjusted by the stretching conditions, etc.
The hysteresis loss rate of the present film can be measured in accordance with JIS K 7312:1996 by the method described in the Examples.
More specifically, when a stress-strain curve such as that shown in FIG. 1 is obtained by a tensile cycle test, the ratio of the area enclosed by abcef to the total area (abcd) is defined as the hysteresis loss rate.

The hysteresis loss rate of the film in the MD and TD directions at 23°C is preferably 47.0% or less, more preferably 46.0% or less, even more preferably 45.0% or less, and even more preferably 44.0% or less. It is preferable that the hysteresis loss rate in one of the MD and TD is within the above numerical range, and it is more preferable that the hysteresis loss rates in both the MD and TD are within the above numerical range.

Furthermore, the difference in hysteresis loss rate between the MD and TD directions of the present film at 23° C. is preferably 20.0% or less, more preferably 15.0% or less, and even more preferably 10.0% or less.
When the difference in hysteresis loss rate between the MD and TD directions is within the above numerical range, the bending resistance of the film and, ultimately, the anisotropy of various film properties are reduced, so that there is no need to select a specific direction when using the film as a component or during secondary processing. In addition, since problems in only a specific direction due to anisotropy are unlikely to occur, the film has excellent handleability.
In addition, the invention related to the first aspect of the present invention requires that the average value of the above-mentioned hysteresis loss rate when a tensile cycle test up to 5% tensile strain in each of the MD and TD directions at 23°C is 47.0% or less, and further preferably satisfies the following residual strain requirement.

(2) Residual Strain The average value of the residual strain when the present film is subjected to a tensile cycle test up to 5% tensile strain in each of the MD and TD directions at 23° C. is 0.900% or less, more preferably 0.890% or less, further preferably 0.880% or less, and further more preferably 0.870% or less. The lower limit is not particularly limited, but is 0.100% or more.
By keeping the residual strain at 0.900% or less, the restoring force of the film is increased, and the bending resistance (folding resistance) of the film is kept within a practical range.
In addition, the residual strain can be used as an index of the overall properties of the film by averaging the residual strain in both the MD and TD directions. The residual strain can be adjusted by adjusting the stretching conditions, etc.
The residual strain of the present film can be measured in accordance with JIS K 7312:1996 by the method described in the Examples.
More specifically, when a stress-strain curve such as that shown in FIG. 1 is obtained by a tensile cycle test, the value of f is defined as the residual strain.

The residual strain in the MD and TD directions of the present film at 23° C. is preferably 0.900% or less, more preferably 0.890% or less, even more preferably 0.880% or less, and still more preferably 0.870% or less.
It is preferable that the residual strain in one of MD and TD is within the above-mentioned numerical range, and it is more preferable that the residual strain in both MD and TD is within the above-mentioned numerical range.

Furthermore, the difference in residual strain between the MD and TD directions of the present film at 23° C. is preferably 0.900% or less, more preferably 0.500% or less, even more preferably 0.200% or less, and still more preferably 0.100% or less.
When the difference in residual strain between the MD and TD directions is within the above numerical range, the bending resistance of the film and, ultimately, the anisotropy of various film properties is reduced, so that there is no need to select a specific direction when using the film as a component or during secondary processing.In addition, since problems in only a specific direction due to anisotropy are unlikely to occur, the film also has excellent handleability.

(3) Glass Transition Temperature The glass transition temperature (Tg) of the present film is preferably from 75° C. to 150° C., more preferably from 76° C. to 140° C., and even more preferably from 77° C. to 130° C. If the Tg is 75° C. or higher, the present film will not deform when used for display purposes, and therefore can be said to have excellent heat resistance.
On the other hand, if the Tg is 150° C. or less, the material is suitable for processability.
The glass transition temperature (Tg) of the present film is measured using a differential scanning calorimeter (DSC) at a heating rate of 10° C./min in accordance with JIS K7121 (2012).

(4) Crystalline melting temperature The crystalline melting temperature (Tm) of the present film is preferably 220°C or higher and 300°C or lower. In particular, it is more preferably 221°C or higher and 295°C or lower, even more preferably 222°C or higher and 290°C or lower, and particularly preferably 223°C or higher and 285°C or lower. If the crystalline melting temperature Tm of the present film is within this range, the present film has an excellent balance between heat resistance and melt extrusion moldability. Here, the crystalline melting temperature Tm is measured for the present film using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min in accordance with JIS K7121 (2012).

(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. By making the thickness 1 μm or more, the film strength is kept within a practical range. By making the thickness 250 μm or less, folding resistance is easily exhibited. The thickness can be adjusted by film formation and stretching conditions.
The thickness of this film was measured at five random locations on the surface with a 1/1000 mm dial gauge, and the average was taken as the thickness.

2. Components Next, the components constituting the present film will be described.

The present invention was made based on the discovery that a biaxially stretched film containing a polyester resin, which generally exhibits a relatively higher yield stress and elastic modulus than polyethylene resins and polypropylene resins, has excellent bending resistance. Even with polyester resin, which is a material with a high yield stress and elastic modulus, if the stress or strain applied by deformation is large, deformation occurs, 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 equal to or less than a specific value, the restoring force of the film becomes large and distortion is less likely to occur.
Generally, deformation that exceeds the elastic deformation region causes large strain in the material, but even deformation within the elastic deformation region may leave residual strain in the material, which can be a cause of deformation marks such as folds and wrinkles, resulting in poor appearance and affecting the material properties themselves.
In other words, the smaller the hysteresis loss rate, the higher the recovery from deformation, and the smaller the strain remaining in the material. Therefore, even when a deformation within the elastic deformation region or a large strain exceeding the elastic deformation region is applied, the material is less likely to leave traces of deformation and is considered to have excellent deformation resistance.

<Polyethylene naphthalate copolymer (A)>
The present film contains a polyethylene naphthalate-based copolymer (A).
Here, in the present invention, "containing" a PEN-based copolymer (A) means that the PEN-based copolymer (A) is contained to the extent that the effects of the present invention are achieved, and the content of the PEN-based copolymer (A) in the present film is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, still more preferably 80% by mass or more, and most preferably 90% by mass or more (including 100% by mass).

Glycol-modified polyethylene terephthalate, which is generally obtained by introducing a copolymerization component into polyethylene terephthalate, is known to have improved impact resistance compared to ordinary polyethylene terephthalate.
On the other hand, there is a concern that the heat resistance may decrease due to the decrease in crystallinity.
Furthermore, since it is necessary to increase the molding temperature by improving the glass transition temperature or melting point, there may be cases where concerns arise about thermal decomposition of the resin.
The present invention relates to the discovery of a biaxially oriented film containing a PEN-based copolymer (A) which can improve impact resistance and hence has excellent bending resistance by introducing a copolymer component into polyethylene naphthalate, and which further has excellent heat resistance and moldability.

The PEN copolymer (A) in the present invention contains a dicarboxylic acid component (a-1) and a diol component (a-2).
As the dicarboxylic acid component (a-1), 2,6-naphthalenedicarboxylic acid is an essential component, and other copolymerization components are added thereto as necessary. Examples of other copolymerization components include aromatic dicarboxylic acids such as 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, benzophenone dicarboxylic acid, 4,4'-diphenyl dicarboxylic acid, 3,3'-diphenyl dicarboxylic acid, and 4,4'-diphenyl ether dicarboxylic acid; aliphatic dicarboxylic acids such as cyclohexane dicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and dimer acid; and oxycarboxylic acids such as p-oxybenzoic 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 may be used alone or in combination of two or more.

On the other hand, as the diol component (a-2), ethylene glycol is an essential component, and other copolymerization components, if necessary, include diethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, or bisphenol S, or derivatives thereof, or ethylene oxide adducts thereof), and among these, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols are preferred. In particular, from the viewpoint of maintaining film strength, it is preferred to use 1,4-cyclohexanedimethanol and bisphenols.
As the bisphenol, it is preferable to use a bisphenol A-ethylene oxide adduct.
These copolymerization components may be used alone or in combination of two or more.

As described above, the PEN-based copolymer of the present invention is a copolymer obtained from PEN obtained from 2,6-naphthalenedicarboxylic acid and ethylene glycol, and contains a copolymer component in at least one of the dicarboxylic acid component (a-1) and the diol component (a-2). By including such a copolymer component, it is possible to further improve the bending resistance, heat resistance, and moldability of PEN.

The PEN copolymer (A) used in the present invention is preferably a polyethylene naphthalate copolymer containing 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1) and a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol units and ethylene glycol units as the diol component (a-2).
The diol component (a-2) is more preferably composed of two components, a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit.

The PEN copolymer (A) 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, even more preferably 94 mol % or more, still more preferably 96 mol % or more, and particularly preferably 98 mol % or more. All of the dicarboxylic acid component (a-1) (100 mol %) may be 2,6-naphthalenedicarboxylic acid.
By adjusting the content of the 2,6-naphthalenedicarboxylic acid unit in the dicarboxylic acid component (a-1) to fall within the above numerical range, the glass transition temperature and melting point of the polyethylene naphthalate polymer are improved, and thus the heat resistance of the present film is improved.
The copolymerization component in the dicarboxylic acid component (a-1) may be copolymerized in an amount of less than 10 mol % for the purpose of improving moldability and heat resistance.
However, from the viewpoint of heat resistance and folding resistance, the content of terephthalic acid, which is a dicarboxylic acid component of polyethylene terephthalate, is preferably less than 2 mol %, more preferably less than 1 mol %, and even more preferably 0 mol %.

The PEN copolymer (A) contains a bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) in an amount of preferably 4 mol % to 70 mol %, more preferably 4.2 mol % to 60 mol %, even more preferably 4.4 mol % to 50 mol %, still more preferably 4.6 mol % to 40 mol %, and particularly preferably 4.8 mol % to 30 mol %.
By adjusting the content of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) to fall within the above numerical range, the glass transition temperature and melting point of the PEN copolymer (A) are improved, and thus the heat resistance of the present film is improved.
Furthermore, since the crystallinity can be controlled, the crystallization rate can be slowed down, and the extrusion moldability and stretch processability of the film can be improved.
In addition, when the content is 70 mol % or less, the melting point does not become too high, so there is no need to set the molding temperature high, and there is no risk of thermal decomposition.

The PEN copolymer (A) contains ethylene glycol in the diol component (a-2) in an amount of preferably 30 mol % or more and 96 mol % or less, more preferably 40 mol % or more and 95.8 mol % or less, even more preferably 50 mol % or more and 95.6 mol % or less, still more preferably 60 mol % or more and 95.4 mol % or less, and particularly preferably 70 mol % or more and 95.2 mol % or less.
By controlling the content of ethylene glycol in the diol component (a-2) within the above numerical range, the crystallinity of the PEN copolymer (A) is maintained, and the heat resistance of the present film is improved.
For the purpose of improving moldability and heat resistance, the PEN copolymer (A) may be copolymerized with less than 10 mol % of a bisphenol A-ethylene oxide adduct or a diol component other than 1,4-cyclohexanedimethanol and ethylene glycol.
However, since a diol component having a rigid biphthalimide skeleton deteriorates folding resistance, its content is preferably less than 1 mol %, more preferably less than 0.5 mol %, and even more preferably 0 mol %.
From the viewpoint of folding endurance, specific examples of diol components other than bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol and ethylene glycol include 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, hydroquinone, bisphenol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol, and isosorbide. Among these, diethylene glycol, 1,3-propanediol, and 1,3-cyclohexanedimethanol are preferred from the viewpoint of moldability.

The crystalline melting heat ΔHm of the PEN copolymer (A) is preferably from 15 to 60 J/g, more preferably from 20 to 50 J/g. If ΔHm(A) is within this range, the PEN copolymer (A) has suitable crystallinity that is excellent in heat resistance, moist heat resistance, melt moldability, and stretch processability.
The crystalline heat of fusion ΔHm(A) of the PEN copolymer (A) can be measured at a heating rate of 10° C./min using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (2012).

The crystalline melting temperature Tm(A) of the PEN copolymer (A) is preferably from 220° C. to 300° C., more preferably from 225° C. to 290° C., further preferably from 230° C. to 280° C., and particularly preferably from 235° C. to 270° C. When the crystalline melting temperature Tm(A) of the PEN copolymer (A) is within this range, the PEN copolymer (A) has an excellent balance between heat resistance and melt moldability.
The crystalline melting temperature Tm(A) of the PEN copolymer (A) can be measured at a heating rate of 10° C./min using a differential scanning calorimeter (DSC) in accordance with JIS K7121 (2012).

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

In the present invention, the film may contain resins other than the PEN copolymer (A) as long as the effects of the present invention are not impaired.
Examples of other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polyester resins, polycarbonate resins, polyamide resins, polyacetal resins, acrylic resins, ethylene vinyl acetate copolymers, polymethylpentene resins, polyvinyl alcohol resins, cyclic olefin resins, polylactic acid resins, polybutylene succinate resins, polyacrylonitrile resins, 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, polyetherimide resins, polyetheretherketone resins, polyetherketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine-based resins.

In addition to the above-mentioned components, the film of the present invention may contain additives that are generally used as appropriate within the range that does not significantly impair the effects of the present invention. Examples of the additives include recycled resins generated from trimming losses such as selvages, inorganic particles such as silica, talc, kaolin, and calcium carbonate, pigments such as titanium oxide and carbon black, flame retardants, weathering stabilizers, heat stabilizers, antistatic agents, melt viscosity improvers, crosslinking agents, lubricants, nucleating agents, plasticizers, antioxidants, antioxidants, light stabilizers, ultraviolet absorbers, neutralizers, antifogging agents, antiblocking agents, slip agents, and colorants, which are added for the purpose of improving and adjusting the moldability, productivity, and various physical properties of the porous film.

In addition, in the present invention, in addition to the additives described above, a coating layer can be provided on the film within a range that does not significantly impair the effects of the present invention. The functions of the coating layer include hard coat properties, antistatic properties, peelability, easy adhesion, printability, UV protection, infrared blocking properties, and gas barrier properties. The coating layer may be formed by in-line coating, which treats the film surface during the stretching process, or by offline coating, which applies a coating outside the system onto a film that has already been produced, or both may be used in combination.

<Manufacturing method of the present film>
A method for producing the biaxially stretched film of the present invention will be described below. However, the following description is one example of the method for producing the present film, and the present film is not limited to films produced by this production method.

The method for producing the film according to one embodiment of the present invention is a method for producing the film by forming a resin composition containing the PEN-based copolymer (A) into a film and biaxially stretching the film.

The method for kneading the PEN copolymer (A), other resins, and additives to obtain a resin composition is not particularly limited, but in order to obtain the resin composition as simply as possible, it is preferable to produce the resin composition 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 equal to or higher than the glass transition temperature of all polymers used, and for crystalline resins, equal to or higher than the crystalline melting temperature of the polymer. The higher the kneading temperature is, the easier it is for a part of the polymer to undergo an ester exchange reaction and the compatibility is improved, but 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 this range, the compatibility and melt moldability can be improved without causing the polymer to decompose.

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

From the resin composition obtained by mixing, a substantially amorphous and unoriented film (hereinafter sometimes referred to as "unstretched film") is produced by an extrusion method. This unstretched film can be produced, for example, by an extrusion method in which the above-mentioned raw materials are melted in an extruder, extruded from a flat die or annular die, and then quenched to form a flat or annular unstretched film. In this case, a laminated structure using multiple extruders may be used in some cases.

Next, the unstretched film is stretched in the film flow direction (machine direction, MD) and in the direction perpendicular to the film flow direction (transverse direction, TD), usually by 1.1 to 5.0 times in at least one direction, and preferably by 1.1 to 5.0 times in each of the machine and transverse directions, in terms of the stretching effect, film strength, etc.

As the method of biaxial stretching, any of the conventionally known stretching methods can be used, such as tenter-type sequential biaxial stretching, tenter-type simultaneous biaxial stretching, tubular-type simultaneous biaxial stretching, etc. For example, in the case of the tenter-type sequential biaxial stretching method, the unstretched film can be produced by heating the unstretched film to a temperature range of Tg to Tg+50° C., where Tg is the glass transition temperature of the resin composition, stretching the film in the longitudinal direction by 1.1 to 5.0 times using a roll-type longitudinal stretching machine, and subsequently stretching the film in the transverse direction by 1.1 to 5.0 times within a temperature range of Tg to Tg+50° C. using a tenter-type transverse stretching machine.
In the case of tenter-type simultaneous biaxial stretching or tubular-type simultaneous biaxial stretching, the film can be produced by stretching the film in the longitudinal and transverse directions simultaneously by 1.1 to 5.0 times in each axial direction, for example, in a temperature range of Tg to Tg+50°C.

The biaxially stretched film stretched by the above method is subsequently heat set.
Heat setting can impart dimensional stability at room temperature. In this case, the treatment temperature is preferably selected in the range of the crystalline melting temperature of the resin composition, Tm-1 to Tm-80°C. If the heat setting temperature is within the above range, heat setting is sufficiently performed, stress during stretching is alleviated, sufficient heat resistance and mechanical properties are obtained, and an excellent film is obtained without problems such as breakage or whitening of the film surface.

In the present invention, in order to alleviate the stress of crystallization shrinkage due to heat setting, it is preferable to perform relaxation in the width direction during heat setting by 0 to 15%, preferably 3 to 10%. Since sufficient relaxation is performed and the film is relaxed uniformly in the width direction, the shrinkage rate in the width direction becomes uniform, and a film with excellent dimensional stability at room temperature can be obtained.
In addition, since the relaxation follows the shrinkage of the film, there is no sagging of the film, no flapping in the tenter, and no breakage of the film.

<Uses of this film>
The biaxially stretched film of the present invention has excellent folding resistance, heat resistance, and transparency, and can be used as a display film, a packaging film, or various protective films. In particular, from the viewpoint of folding resistance, it can be suitably used for a foldable display.

The following examples are given, but the present invention is not limited by them in any way.

(1) Hysteresis Loss Rate According to JIS K 7312:1996, the average value of the hysteresis loss rate at 23° C. was determined by the following method.
The measuring device used was a tensile tester (tensile tester AG-1kNXplus manufactured by Shimadzu Corporation). The test piece was a rectangle cut from the film with a length of 100 mm in the measurement direction and a width of 10 mm. The test piece was chucked at both ends in the length direction with a chuck distance of 50 mm, raised to a strain of 5% at a crosshead speed of 0.5 mm/min, and then lowered to the initial position at the same speed to obtain a stress-strain curve obtained from a tensile cycle test of one cycle. The stress-strain curve had a profile as shown in FIG. 1, and the hysteresis loss rate was calculated from the obtained stress-strain curve using the area A1 (abcda) of the curve obtained by the rising operation and the area A2 (abcef) which is the difference between the area A1 and the area of the curve obtained by the lowering operation, according to the following formula 1. The test was performed three times, and the average value was calculated. The above tensile cycle test was performed in the MD and TD of the film, respectively, and the average value was calculated.
Hysteresis loss rate = (A2 / A1) × 100 (Formula 1)

(2) Residual Strain In accordance with JIS K 7312:1996, the average value of the residual strain at 23° C. was determined by the following method.
The measuring device used was a tensile tester (tensile tester AG-1kNXplus manufactured by Shimadzu Corporation). The test piece was a rectangle cut from the film with a length of 100 mm in the measurement direction and a width of 10 mm. The test piece was chucked at both ends in the length direction with a chuck distance of 50 mm, and the test piece was raised to a strain of 5% at a crosshead speed of 0.5 mm/min, and then lowered to the initial position at the same speed. From the stress-strain curve obtained from one cycle of tensile cycle test, the strain at the point where the stress disappeared was taken as the residual strain. The test was performed three times, and the average value was calculated. The above tensile cycle test was performed in the MD and TD of the film, respectively, and the average value was calculated.

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

(4) Crystal melting temperature (Tm)
The crystalline melting temperature of the obtained film was measured in a heating process at a heating rate of 10° C./min in accordance with JIS K7121 (2012) using a Diamond DSC (manufactured by PerkinElmer Japan).

[PEN copolymer (A)]
The PEN copolymer (A)-1 used was a dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, a diol component (a-2): ethylene glycol = 90 mol %, and a bisphenol A-ethylene oxide adduct = 10 mol %. The Tg of the PEN copolymer (A)-1 was 119°C.
The PEN copolymer (A)-2 used was a dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, a diol component (a-2): ethylene glycol = 95 mol %, and a bisphenol A-ethylene oxide adduct = 5 mol %. The Tg of the PEN copolymer (A)-2 was 120°C.
The PEN copolymer (A)-3 used was a dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, a diol component (a-2): ethylene glycol = 90 mol %, 1,4-cyclohexanedimethanol = 10 mol %. The Tg of the PEN copolymer (A)-3 was 119 ° C.
The PEN copolymer (A)-4 used was a dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol %, a diol component (a-2): ethylene glycol = 80 mol %, 1,4-cyclohexanedimethanol = 20 mol %. The Tg of the PEN copolymer (A)-4 was 119 ° C.

[PET film (B)]
As the PET film (B)-1, a biaxially stretched PET film having a thickness of 50 μm was used.

[PEN film (C)]
As the PEN film (C)-1, a PEN film (Teonex Q51) having a thickness of 50 μm was used.

Example 1
The pellet-like (A)-1 alone was melt-kneaded in a Φ25 mm twin-screw extruder set at 285°C, extruded as a film from a T-die with a gap of 1.0 mm, taken up by a cast roll at 110°C, and cooled and solidified to obtain a film-like material (cast film) with a thickness of about 450 μm.
The obtained cast film was then passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 132° C. The obtained longitudinally stretched film was then passed through a transverse stretching machine (tenter) and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 120 to 125° C., a stretching temperature of 130° C., and a heat setting temperature of 180° C. Thereafter, the film was subjected to a relaxation treatment of 5% in the width direction (TD) while being heat-set in the tenter. The results of measurements performed on the obtained film are shown in Table 1.

Example 2
Using the pelletized (A)-2 alone, a film-like material (cast film) having a thickness of about 450 μm was obtained in the same manner as in Example 1.
The obtained cast film was then passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135° C. The obtained longitudinally stretched film was then passed through a transverse stretching machine (tenter) and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 125 to 130° C., a stretching temperature of 135° C., and a heat setting temperature of 180° C. Thereafter, the film was subjected to a relaxation treatment of 5% in the width direction (TD) while being heat-set in the tenter. The results of measurements performed on the obtained film are shown in Table 1.

Example 3
The pellet-like (A)-3 was melt-kneaded in a Φ25 mm twin-screw extruder set at 285°C, extruded as a film from a T-die with a gap of 1.0 mm, taken up by a cast roll at 113°C, and cooled and solidified to obtain a film-like material (cast film) with a thickness of about 450 μm.
The obtained cast film was then passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135° C. The obtained longitudinally stretched film was then passed through a transverse stretching machine (tenter) and stretched 3.1 times in the transverse direction (TD) at a preheating temperature of 125 to 130° C., a stretching temperature of 135° C., and a heat setting temperature of 180° C. Thereafter, the film was subjected to a relaxation treatment of 5% in the width direction (TD) while being heat-set in the tenter. The results of measurements performed on the obtained film are shown in Table 1.

Example 4
Using the pelletized (A)-4 alone, a film-like material (cast film) having a thickness of about 450 μm was obtained in the same manner as in Example 3.
The obtained cast film was then passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 137° C. The obtained longitudinally stretched film was then passed through a transverse stretching machine (tenter) and stretched 3.3 times in the transverse direction (TD) at a preheating temperature of 120 to 125° C., a stretching temperature of 130° C., and a heat setting temperature of 150° C. Thereafter, the film was subjected to a relaxation treatment of 5% in the width direction (TD) while being heat-set in the tenter. The results of measurements performed on the obtained film are shown in Table 1.

(Comparative Example 1)
The results of evaluation of the biaxially stretched PET film (B)-1 are shown in Table 1.

(Comparative Example 2)
The evaluation results for the PEN film (C)-1 are shown in Table 1.

The films of Examples 1 to 4 have high crystal melting temperatures and glass transition temperatures, and are excellent in heat resistance.
In addition, the hysteresis loss rate and residual strain values are clearly lower than those of the PET film of Comparative Example 1 and the PEN film of Comparative Example 2, and it can be said that the PEN-based copolymer-containing films of Examples 1 to 4 are excellent not only in heat resistance but also in folding resistance.

Claims (11)

A biaxially oriented film for displays , comprising a polyethylene naphthalate copolymer (A), and having an average hysteresis loss rate of 47.0% or less when subjected to a tensile cycle test up to 5% tensile strain in each of MD and TD directions. A biaxially stretched film for displays , comprising a polyethylene naphthalate copolymer (A), and having an average residual strain of 0.900% or less when subjected to a tensile cycle test up to 5% tensile strain in each of MD and TD directions. The polyethylene naphthalate copolymer (A) contains a 2,6-naphthalenedicarboxylic acid unit as the dicarboxylic acid component (a-1), and contains a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit as the diol component (a-2). The biaxially stretched film for displays according to claim 1 or 2. 4. The biaxially stretched film for displays according to claim 3, wherein the content of terephthalic acid units as dicarboxylic acid components other than the (a-1) is less than 2 mol % in all dicarboxylic acid components. 5. The biaxially stretched film for displays according to claim 3 or 4, wherein the amount of diol units having a biphthalimide skeleton as diol components other than the (a-2) is less than 1 mol % in all diol components. Contains a polyethylene naphthalate copolymer (A),
The copolymer (A) contains a dicarboxylic acid component (a-1) and a diol component (a-2),
The dicarboxylic acid component (a-1) contains at least a 2,6-naphthalenedicarboxylic acid unit,
The diol component (a-2) is composed of two components, namely, a bisphenol A-ethylene oxide adduct or a 1,4-cyclohexanedimethanol unit and an ethylene glycol unit.
In the biaxially stretched film for display, the content of terephthalic acid units as dicarboxylic acid components other than the (a-1) is less than 2 mol % in all the dicarboxylic acid components. The diol component (a-2) contains 4 mol% or more and 70 mol% or less of bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit, and 30 mol% or more and 96 mol% or less of ethylene glycol unit. The biaxially stretched film for display according to any one of claims 3 to 6. The biaxially stretched film for displays according to any one of claims 1 to 7, which has a glass transition temperature of 75°C or higher and 150°C or lower. 9. The biaxially stretched film for displays according to any one of claims 1 to 8, which has a crystalline melting temperature of 220°C or higher and 300°C or lower. The biaxially stretched film for displays according to any one of claims 1 to 9, having a thickness of 1 µm or more and 250 µm or less. A foldable display comprising the biaxially stretched film for displays according to any one of claims 1 to 10 .

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