TW202140643A - Biaxial stretch film - Google Patents

Abstract

The biaxially stretched film of the present invention is characterized in that it contains a polyethylene naphthalate copolymer (A), and when the tensile cycle test reaches 5% tensile strain in both MD and TD directions, it has a hysteresis The average loss rate is 47.0% or less. The present invention provides a biaxially stretched film with excellent folding resistance and heat resistance.

Description

Biaxial stretch film

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

Polyester is a resin with excellent heat resistance, weather resistance, mechanical strength, transparency, chemical resistance, gas barrier properties, etc. It also has the advantage of easy availability in terms of price, so it has high versatility and is currently widely used in beverages. , Food containers or packaging materials, molded products, films, etc. The main component of polyester resin is polyethylene terephthalate (hereinafter sometimes referred to as "PET"). It has excellent mechanical properties, electrical properties, chemical resistance, etc., so it has a wide range of uses, but it is resistant to heat and folding. Difficulties exist in aspects such as sex.

In addition, as a resin using naphthalenedicarboxylic acid as an acid component, polybutylene naphthalate (PBN), which is obtained by polymerizing naphthalenedicarboxylic acid and 1,4-butanediol, is known, but its glass transition temperature is 75 It has a low melting point of about 240°C at around ℃, so there is a problem in heat resistance. In addition, since the crystallization rate is too fast, there is a problem that it is not suitable for film formation by extrusion molding.

On the other hand, in recent years, there has been an increasing demand for flexible displays. Among them, there is a strong demand for films with high heat resistance, excellent recovery properties, and excellent resistance to repeated bending.

For example, in Patent Document 1, there is a study of a film produced by using a cyclic olefin resin film that is excellent in resistance to repetitive bending.

Moreover, as a film excellent in heat resistance and bending resistance, a polyimide film is disclosed (Patent Document 2).

In addition, Patent Document 3 proposes a polyethylene naphthalate resin, which is prepared by blending an ethylene oxide adduct of a bisphenol compound or its derivative in all alcohol components without impairing heat resistance. The crystallinity is improved under the following conditions.

In addition, Patent Document 4 proposes a modified polyester resin in which 2,6-naphthalenedicarboxylic acid is blended in an acid component, and a bisphenol compound and 1,4-cyclohexanedimethanol are blended in a diol component. , So that heat resistance, impact resistance, etc. can be improved.

Furthermore, Patent Document 5 discloses a polyethylene naphthalate resin, which is obtained by blending N,N-bis(2-hydroxyethyl)-4-4'-diphthalate in all alcohol components. Imine, and has a high glass transition temperature. Prior art literature Patent literature

Patent Document 1: Japanese Patent Laid-Open No. 2014-104687 Patent Document 2: International Publication No. 2016/060213 Patent Document 3: Japanese Patent Laid-Open No. 8-48759 Patent Document 4: Japanese Patent Laid-Open No. 2004-107559 Patent Document 5: Japanese Patent Special Form 2015-504105

[The problem to be solved by the invention]

However, the film disclosed in Patent Document 1 has a low level of resistance to repeated bending and cannot meet market requirements. In addition, the cyclic olefin-based resin has insufficient coating properties and adhesive properties, and therefore it is considered that it is not easy to be laminated with other members as a member for a flexible display.

In addition, although the polyimide film described in Patent Document 2 has bending resistance, its manufacturing process adopts a forming method using a solvent coating, so the productivity is poor and the cost is also incurred.

Regarding the polyethylene naphthalate resin described in Patent Document 3, the ethylene oxide adduct of a bisphenol compound or its derivative is blended in all alcohol components to increase the crystallization rate and improve the drying time of the raw material. The trouble is to improve the moldability, but if the crystallization speed is too fast, the film forming performance and stretchability of the film during extrusion molding will deteriorate, so it is not suitable for stretched films.

The modified polyester resin described in Patent Document 4 exhibits high heat resistance, but on the other hand, the temperature during molding must be set to a higher temperature, which may cause thermal decomposition and moldability or material properties after molding. reduce.

The polyethylene naphthalate resin described in Patent Document 5 has extremely excellent heat resistance, but because it is blended with a diol component having a rigid two phthalimide skeleton, it has poor folding resistance possibility.

The problem to be solved in the present invention is to solve the above-mentioned problems and provide a biaxially stretched film having excellent folding resistance and heat resistance. [Technical means to solve the problem]

The inventors of the present invention have conducted intensive research in order to achieve the above-mentioned problems, and as a result, they have completed the present invention. The present invention has the following aspects. [1] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and the hysteresis loss rate when subjected to a tensile cycle test of 5% tensile strain in both MD and TD directions The average value is 47.0% or less. [2] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and when a tensile cycle test with a tensile strain of 5% in the MD and TD directions is performed, the residual strain is The average value is 0.900% or less. [3] The biaxially stretched film described in [1] or [2] above, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component ( a-1), containing bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit and ethylene glycol unit as the diol component (a-2). [4] The biaxially stretched film as described in [3] above, wherein the terephthalic acid unit as a dicarboxylic acid component other than the above (a-1) in all the dicarboxylic acid components is less than 2 mol %. [5] The biaxially stretched film as described in the above [3] or [4], wherein among all the diol components, the diol component other than the above (a-2) has diphthalate The diol unit of the amine skeleton is less than 1 mol%. [6] A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and the polyethylene naphthalate copolymer (A) contains a dicarboxylic acid component (a-1) and The diol component (a-2), the dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid units, and the diol component (a-2) contains bisphenol A-ethylene oxide addition Or two components of 1,4-cyclohexanedimethanol unit and ethylene glycol unit, and among all the above-mentioned dicarboxylic acid components, terephthalic acid is the other dicarboxylic acid component other than the above (a-1) The unit is less than 2 mol%. [7] The biaxially stretched film according to any one of [3] to [6] above, wherein the diol component (a-2) contains 4 mol% or more and 70 mol% or less of bisphenol A- Ethylene oxide adducts or 1,4-cyclohexanedimethanol units, and contain 30 mol% to 96 mol% of ethylene glycol units. [8] The biaxially stretched film according to any one of [1] to [7] above, which has a glass transition temperature of 75°C or more and 150°C or less. [9] The biaxially stretched film as described in any one of [1] to [8] above, which has a crystal melting temperature of 220°C or more and 300°C or less. [10] The biaxially stretched film described in any one of [1] to [9] above, which has a thickness of 1 μm or more and 250 μm or less. [11] The biaxially stretched film as described in any one of [1] to [10] above, which is used for displays. [12] A foldable display including the biaxially stretched film described in any one of [1] to [11] above. [Effects of the invention]

According to the present invention, a biaxially stretched film with excellent folding resistance and heat resistance can be disclosed.

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

＜Biaxial stretch film＞ In the embodiment of the present invention, the biaxially stretched film of the first form includes polyethylene naphthalate (hereinafter sometimes referred to as "PEN") copolymer (A), and is performed in both MD and TD The average value of the hysteresis loss rate is 47.0% or less in the tensile cycle test when the tensile strain reaches 5%.

The second form of biaxially stretched film contains polyethylene naphthalate copolymer (A), and the average value of residual strain when the tensile cycle test reaches 5% tensile strain in both MD and TD directions It is 0.900% or less.

The biaxially stretched film of the third form includes a polyethylene naphthalate copolymer (A), and the polyethylene naphthalate copolymer (A) includes a dicarboxylic acid component (a-1) and a diol Component (a-2), and the dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid unit, and the diol component (a-2) contains a bisphenol A-ethylene oxide adduct Or 1,4-cyclohexanedimethanol unit and ethylene glycol unit two components, among all the above dicarboxylic acid components, terephthalic acid unit as other dicarboxylic acid components other than the above (a-1) is not Up to 2 mol%. Since the film is a biaxially stretched film, it can be made into a thin film with excellent folding resistance. Furthermore, MD means the flow direction of the membrane, and TD means the direction perpendicular to the MD. Moreover, the biaxially stretched film of the said 1st-3rd form may be called "this film".

The present invention has discovered that a biaxially stretched film containing a PEN-based copolymer having a hysteresis loss rate or residual strain below a specific value has excellent folding resistance and heat resistance as a film for displays, and is particularly suitable for foldable applications. Since the hysteresis loss rate or residual strain of this film is low, it is considered that it has excellent reversibility and exhibits bending resistance. In addition, a biaxially stretched film including a PEN-based copolymer having a specific composition also has excellent folding resistance and heat resistance as a film for displays, and is particularly suitable for foldable applications.

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

(1) Hysteresis loss rate When the film is subjected to a tensile cycle test to reach 5% tensile strain in both MD and TD directions at 23°C, the average value of the hysteresis loss rate is 47.0% or less, more preferably 46.0% or less, and even more preferably 45.0 % Or less, more preferably 44.0% or less. The lower limit is not particularly limited, but is 0.100% or more. With the hysteresis loss rate being 47.0% or less, and the film's recovery force becomes larger, the film's bending resistance (folding resistance) remains within the practical range. In addition, by setting the hysteresis loss rate as the average value in each direction of MD and TD, it can be used as a characteristic index of the entire film. The hysteresis loss rate can be adjusted by changing the extension conditions. The hysteresis loss rate of the film can be measured according to JIS K 7312: 1996 and by the method described in the examples. More specifically, when the stress-strain curve shown in Fig. 1 is obtained by a tensile cycle test, the ratio of the area surrounded by abcef to the area of the whole (abcda) is defined as hysteresis Loss rate.

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

Furthermore, the difference in the hysteresis loss of the film in the MD and TD directions at 23°C is preferably 20.0% or less, more preferably 15.0% or less, and still more preferably 10.0% or less. By making the difference of the hysteresis loss rate in the MD and TD directions within the above numerical range, the bending resistance of the film and the anisotropy of various film characteristics are reduced, so when the film is used as a member or during secondary processing It is not necessary to select a specific direction, and it is not easy to cause abnormalities in only specific directions caused by anisotropy, so it becomes a film with excellent handling properties. Furthermore, when the first aspect of the invention of the present invention requires a tensile cycle test to reach 5% tensile strain in both MD and TD directions at 23°C, the average value of the hysteresis loss rate is 47.0% or less, which is less than It is better to further satisfy the requirements of residual strain described below.

(2) Residual strain When the film is subjected to a tensile cycle test to reach 5% tensile strain in both MD and TD directions at 23°C, the average residual strain is 0.900% or less, more preferably 0.890% or less, and even more preferably 0.880% Below, it is more preferably 0.870% or less. The lower limit is not particularly limited, but is 0.100% or more. By making the residual strain less than 0.900%, the covering force of the film becomes larger, and the bending resistance (folding resistance) of the film remains within the practical range. In addition, by setting the residual strain as the average value in each direction of MD and TD, it can be used as a characteristic index of the entire film. The residual strain can be adjusted by changing the extension conditions. The residual strain of the film can be measured in accordance with JIS K 7312: 1996 and by the method described in the examples. More specifically, when the stress-strain curve shown in Fig. 1 is obtained by the tensile cycle test, the f-value is defined as the residual strain.

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

Furthermore, the difference between the residual strain in the MD and TD directions of the film at 23°C is preferably 0.900% or less, more preferably 0.500% or less, still more preferably 0.200% or less, and still more preferably 0.100% or less. By making the difference between the residual strain in the MD and TD directions within the above-mentioned numerical range, the bending resistance of the film and the anisotropy of various film properties are reduced, so when the film is used as a member or when performing secondary processing It is not necessary to select a specific direction, and it is not easy to cause abnormalities in only specific directions caused by anisotropy, so it becomes a film with excellent handling properties.

(3) Glass transition temperature The glass transition temperature (Tg) of the film is preferably above 75°C and below 150°C. It is more preferably 76°C or higher and 140°C or lower, and still more preferably 77°C or higher and 130°C or lower. If the Tg is 75°C or higher, the film will not be deformed when it is used for display purposes, so it can be considered that it is excellent in heat resistance. On the other hand, if Tg is 150°C or less, workability is also suitable. The glass transition temperature (Tg) of this film is measured in accordance with JIS K7121 (2012) and using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min.

(4) Crystal melting temperature The crystal melting temperature (Tm) of the film is preferably 220°C or more and 300°C or less. In particular, it is more preferably 221°C or higher and 295°C or lower, still 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 crystal melting temperature Tm of the film is in this range, the heat resistance of the film and the melt-extrusion moldability are excellent in balance. Here, the crystal melting temperature Tm is measured for this film at a heating rate of 10°C/min in accordance with JIS K7121 (2012) and using a differential scanning calorimeter (DSC).

(5) Thickness The thickness of the film is preferably 1 μm or more and 250 μm or less, more preferably 5 μm or more and 200 μm or less. By setting it to 1 μm or more, the film strength can be kept within the practical range. By being 250 μm or less, it is easy to exhibit folding resistance. The thickness can be adjusted by film forming and stretching conditions. Regarding the thickness of the film, use a dial gauge of 1/1000 mm to randomly measure 5 locations in the plane, and set the average value as the thickness.

2. Ingredients Next, the components constituting the film will be described.

The present invention has been completed by discovering the fact that a biaxially stretched film containing a polyester resin, which generally exhibits a relatively higher yield stress or elastic modulus than polyethylene resins and polypropylene resins, is resistant to bending Excellent performance. Even for polyester resins that are materials with high yield stress or high elastic modulus, when the stress or strain applied due to deformation is large, there is a problem that deformation occurs and unremoved strain remains in the material. However, it has been found in the present invention that if the hysteresis loss rate of the film is less than a specific value, the recovery force of the film becomes large and it is difficult to generate strain. It is believed that due to the general deformation that exceeds the elastic deformation area, a large strain is generated in the material. However, even if it is deformation in the elastic deformation area, there may be residual strain in the material, which may cause creases or wrinkles, etc. One of the reasons for the marks of deformation is to cause poor appearance or affect the properties of the material itself. That is, it is believed that the smaller the hysteresis loss rate, the higher the resilience to deformation. Moreover, since the residual strain in the material is smaller, even if the deformation in the elastic deformation region or the greater strain is applied beyond the elastic deformation region In the case, it is not easy to leave deformation marks, and the deformation resistance is excellent.

＜Polyethylene naphthalate copolymer (A)＞ This film contains polyethylene naphthalate copolymer (A). Here, in the present invention, "comprising" the PEN-based copolymer (A) means that the PEN-based copolymer (A) is included in the range where the effect of the present invention is exerted. The PEN-based copolymer (A) in this film is The content is preferably 50% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, still more preferably 80% by mass or more, most preferably 90% by mass or more (including 100% by mass).

It is known that glycol-modified polyethylene terephthalate, which is usually obtained by introducing copolymerization components into polyethylene terephthalate, is more impact resistant than ordinary polyethylene terephthalate. Sexual improvement. On the other hand, there is a concern that heat resistance may be lowered due to the lowering of crystallinity. In addition, because the glass transition temperature or melting point increases, the molding temperature must be increased, so there is a possibility that thermal decomposition of the resin may occur. The present invention has discovered a biaxially stretched film containing a PEN-based copolymer (A), which can be made into a product with improved impact resistance and excellent bending resistance by introducing a copolymer component into polyethylene naphthalate , And also excellent in heat resistance and forming processability.

The PEN-based copolymer (A) in the present invention contains a dicarboxylic acid component (a-1) and a diol component (a-2). As this 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 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'-Diphenyl ether dicarboxylic acid and other aromatic dicarboxylic acids; cyclohexane dicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid Acids, aliphatic dicarboxylic acids such as sebacic acid and dimer acid; hydroxycarboxylic acids such as p-hydroxybenzoic acid, etc. Among them, from the viewpoint of formability, isophthalic acid and 2,5-furandi Carboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid. These copolymerization components can be used individually by 1 type or in combination of 2 or more types.

On the other hand, as the diol component (a-2), ethylene glycol is an essential component, and if necessary, as other copolymer components, diethylene glycol, propylene glycol, 1,4-butanediol, 1,4 -Cyclohexanedimethanol, neopentyl glycol, polytetramethylene ether glycol, dimer alcohol, bisphenols (bisphenol A, bisphenol F or bisphenol S and other bisphenol compounds or their derivatives or the Among them, 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer alcohols, and bisphenols are preferred. In particular, from the viewpoint of maintaining the strength of the film, it is preferable to use 1,4-cyclohexanedimethanol and bisphenols. Furthermore, as the 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.

As described above, the PEN-based copolymer of the present invention is a PEN obtained by 2,6-naphthalenedicarboxylic acid and ethylene glycol containing a dicarboxylic acid component (a-1) and a glycol component (a-2) It is made of at least any copolymer component. By including such a copolymer component, PEN can be further improved in bending resistance, heat resistance, and molding processability.

The PEN-based copolymer (A) used in the present invention preferably contains 2,6-naphthalenedicarboxylic acid unit as the dicarboxylic acid component (a-1), and contains bisphenol A-ethylene oxide adduct or 1 , A polyethylene naphthalate copolymer in which 4-cyclohexanedimethanol units and ethylene glycol units are used as the diol component (a-2). Moreover, it is more preferable that the said diol component (a-2) contains two components of a bisphenol A-ethylene oxide adduct or 1, 4- cyclohexane dimethanol unit and ethylene glycol unit.

The dicarboxylic acid component (a-1) of the PEN-based copolymer (A) preferably contains 90 mol% or more of 2,6-naphthalenedicarboxylic acid units, more preferably 92 mol% or more, and more It is preferably 94 mol% or more, more preferably 96 mol% or more, particularly preferably 98 mol% or more, and all dicarboxylic acid components (a-1) (100 mol%) 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 polyethylene naphthalate polymer are increased , And the heat resistance of the film is improved. In order to improve moldability or heat resistance, the above-mentioned copolymerization component in the dicarboxylic acid component (a-1) may be copolymerized with less than 10 mol%. Among them, from the viewpoint of heat resistance and folding resistance, terephthalic acid, which is the dicarboxylic acid component of polyethylene terephthalate, is preferably less than 2 mol%, more preferably less than 1 mol%. Ear%, more preferably 0 mol%.

Among the diol components (a-2) of the above-mentioned PEN-based copolymer (A), it is preferable to contain 4 mol% or more and 70 mol% or less of bisphenol A-ethylene oxide adduct or 1,4- Cyclohexane dimethanol is more preferably 4.2 mol% or more and 60 mol% or less, still more preferably 4.4 mol% or more and 50 mol% or less, and still more preferably 4.6 mol% or more and 40 mol% or less, More preferably, it is 4.8 mol% or more and 30 mol% or less. By setting the content of the bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol in the diol component (a-2) within the above numerical range, the PEN-based copolymer ( A) The glass transition temperature and melting point are increased, and the heat resistance of the film is improved. In addition, since the crystallinity can be controlled, the crystallization speed can be slowed down, and the extrusion moldability and elongation processability of the film can be improved. In addition, if the content is 70 mol% or less, the melting point will not become too high. Therefore, it is not necessary to set the molding temperature to a higher level, so there is no concern about thermal decomposition.

Among the glycol component (a-2) of the above-mentioned PEN-based copolymer (A), it is preferable to contain 30 mol% or more and 96 mol% or less of ethylene glycol, and more preferably 40 mol% or more and 95.8 mol% Below, it is 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 setting the content of ethylene glycol in the glycol component (a-2) within the above numerical range, the crystallinity of the PEN-based copolymer (A) is maintained, and the heat resistance of the film is improved. Furthermore, in order to improve moldability or heat resistance, the above-mentioned PEN-based copolymer (A) may be reduced to less than 10 mol% to remove bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol Copolymerization with glycol components other than ethylene glycol. Among them, the diol component with a rigid two phthalimide skeleton will deteriorate the folding resistance, so its content is preferably less than 1 mol%, more preferably less than 0.5 mol%, and more Preferably, it is 0 mol%. In addition, from the viewpoint of folding resistance, as diol components other than bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol and ethylene glycol, specific examples 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, spirodiol, 2,2,4,4-tetramethylcyclobutane-1, Among these, 3-diol, isosorbide, etc., are preferably diethylene glycol, 1,3-propanediol, and 1,3-cyclohexanedimethanol from the viewpoint of moldability.

The heat of crystal fusion ΔHm of the PEN-based copolymer (A) is preferably 15 J/g or more and 60 J/g or less, more preferably 20 J/g or more and 50 J/g or less. When ΔHm (A) is in this range, the PEN-based copolymer (A) has moderate crystallinity that is excellent in heat resistance, heat and humidity resistance, melt moldability, and elongation processability. The heat of crystal melting ΔHm(A) of the PEN-based copolymer (A) can be measured in accordance with JIS K7121 (2012) using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min.

The crystal melting temperature Tm(A) of the PEN-based copolymer (A) is preferably 220°C or more and 300°C or less, more preferably 225°C or more and 290°C or less, and still more preferably 230°C or more and 280°C or less, Particularly preferably, it is 235°C or more and 270°C or less. If the crystal melting temperature Tm(A) of the PEN-based copolymer (A) is within this range, the PEN-based copolymer (A) has an excellent balance of heat resistance and melt moldability. The crystal melting temperature Tm(A) of the PEN-based copolymer (A) can be measured in accordance with JIS K7121 (2012) using a differential scanning calorimeter (DSC) at a heating rate of 10°C/min.

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 still 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, within a range that does not impair the effects of the present invention, the film may be allowed to contain other resins other than the PEN-based copolymer (A). Examples of other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polyester resins, polycarbonate resins, and polyamide resins. , Polyacetal resin, acrylic resin, ethylene-vinyl acetate copolymer, polymethylpentene resin, polyvinyl alcohol resin, cyclic olefin resin, polylactic acid resin, polybutylene succinate Ester resin, polyacrylonitrile resin, polyethylene oxide resin, cellulose resin, polyimide resin, polyurethane resin, polyphenylene sulfide resin, polyphenylene ether resin Resin, polyvinyl acetal-based resin, polybutadiene-based resin, polybutene-based resin, polyamide-imide-based resin, polyamide-bismaleimide-based resin, polyetherimide-based resin Resins, polyether ether ketone resins, polyether ketone resins, polyether tungsten resins, polyketone resins, poly tungsten resins, aromatic polyamide resins, fluorine resins, etc.

Furthermore, in the present invention, in addition to the above-mentioned components, the film may appropriately contain additives commonly formulated within a range that does not significantly hinder the effects of the present invention. Examples of the above-mentioned additives include the following substances added to improve and adjust the forming processability, productivity, and physical properties of the porous membrane: recycled resin due to trimming loss of edges, etc., or silica, talc, kaolin, carbonic acid Inorganic particles such as calcium, pigments such as titanium oxide and carbon black, flame retardants, weather resistance stabilizers, heat stabilizers, antistatic agents, melt viscosity modifiers, crosslinking agents, lubricants, nucleating agents, plasticizers, Additives such as anti-aging agents, antioxidants, light stabilizers, ultraviolet absorbers, neutralizers, anti-fogging agents, anti-blocking agents, luster agents and coloring agents.

Furthermore, in the present invention, in addition to the above-mentioned additives, a coating layer may be provided on the film within a range that does not significantly hinder the effects of the present invention. As the function of the above-mentioned coating layer, hard coat properties, antistatic properties, releasability, easy adhesion, printability, UV (Ultraviolet) cut-off properties, infrared ray blocking properties, gas barrier properties, etc. can be cited. Regarding the formation of the coating layer, it can be provided by in-line coating of processing the film surface in the stretching step, so-called off-line coating of out-of-system coating on the temporarily manufactured film can be used, or both can be used in combination.

＜Manufacturing method of this film＞ The manufacturing method of the biaxially stretched film of the present invention will be described, but the following description is an example of the method of manufacturing the film, and the film is not limited to the film manufactured by the manufacturing method.

The manufacturing method of the present film as an example of the embodiment of the present invention is a method in which a resin composition containing the above-mentioned PEN-based copolymer (A) is molded into a film shape and biaxially stretched.

The method of kneading the PEN-based copolymer (A), other resins and additives to obtain a resin composition is not particularly limited, but in order to obtain the resin composition as easily as possible, it is preferable to perform melt kneading by using an extruder And manufacturing. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to perform melt kneading using a co-rotating twin-screw extruder. The mixing temperature is above the glass transition temperature of all polymers used, and for crystalline resins, it must be above the crystal melting temperature of the polymer. When the mixing temperature is higher than the glass transition temperature or crystal melting temperature of the polymer used as much as possible, a part of the polymer is prone to transesterification reaction, and the compatibility is easy to improve. However, if the mixing temperature is higher than necessary, the resin Decomposition occurs, so it is not good. In this case, 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, still 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 in this range, compatibility or melt moldability can be improved without causing polymer decomposition.

The obtained resin composition can be molded by general molding methods such as extrusion molding, injection molding, blow molding, vacuum molding, pressure molding, and pressure molding to produce a biaxially stretched film. In each forming method, the device and processing conditions are not particularly limited. This film is preferably manufactured by the following method, for example.

Using the extrusion method, a substantially amorphous and unaligned film (hereinafter sometimes referred to as "unstretched film") is produced from the resin composition obtained by mixing. Regarding the production of the unstretched film, for example, the following extrusion method can be used: the above-mentioned raw materials are melted by an extruder, extruded from a flat die or a ring die, and then rapidly cooled to form a flat shape. Or ring-shaped unstretched film. At this time, depending on the situation, it can be made into a laminated structure using a plurality of extruders.

Secondly, in terms of the stretching effect and film strength in the direction of flow of the film (longitudinal direction, MD) and the direction at right angles to it (transverse direction, TD), the above-mentioned unstretched film is usually set at 1.1 to in at least one direction. It is extended by 5.0 times, preferably in the range of 1.1 to 5.0 times in the vertical and horizontal biaxial directions.

As the method of biaxial stretching, any of previously known stretching methods such as tentering successive biaxial stretching, tentering simultaneous biaxial stretching, tubular simultaneous biaxial stretching, and the like can be used. For example, in the case of the tentering sequential biaxial stretching method, the glass transition temperature of the above resin composition is set to Tg, and the unstretched film is heated in the temperature range of Tg～Tg+50℃, and the roll is stretched longitudinally. The machine is extended to 1.1-5.0 times in the longitudinal direction, and then stretched to 1.1-5.0 times in the transverse direction in the temperature range of Tg～Tg+50℃ by a tenter-type transverse stretcher, thereby making the film. In addition, in the case of the tentering simultaneous biaxial stretching method or the tubular simultaneous biaxial stretching method, for example, it can be manufactured by simultaneously extending the length and breadth to 1.1 to 5.0 times in each axis direction within the temperature range of Tg～Tg+50°C. .

The biaxially stretched film stretched by the above method can be continuously thermally fixed. By performing heat fixation, dimensional stability at room temperature can be imparted. The treatment temperature at this time is preferably selected from the range of the crystal melting temperature Tm-1 to Tm-80°C of the above-mentioned resin composition. If the heat-fixing temperature is within the above range, the heat-fixing is fully carried out, the stress during stretching is relieved, sufficient heat resistance or mechanical properties can be obtained, and an excellent film without the troubles of breakage or whitening of the film surface can be obtained.

In the present invention, in order to alleviate the stress of crystal shrinkage caused by heat fixation, it is preferable to perform relaxation in the range of 0-15% in the width direction during thermal fixation, and preferably to perform relaxation in the range of 3-10%. . Since the relaxation progresses sufficiently and the relaxation proceeds uniformly in the width direction of the film, a film with uniform shrinkage in the width direction and excellent dimensional stability at room temperature is obtained. In addition, since it relaxes following the shrinkage of the film, there is no sagging of the film, vibration in the tenter, and the film will not break.

＜Use of this film＞ The biaxially stretched film of the present invention is excellent in folding resistance, heat resistance, and transparency, so it can be used as a film for displays, a film for packaging, or various protective films. Among them, from the viewpoint of folding resistance, it can be preferably used for foldable displays. Example

Examples are shown below, but the present invention is not subject to any such restrictions.

(1) Hysteresis loss rate According to JIS K 7312: 1996, the average value of the hysteresis loss rate at 23°C is obtained by the following method. A tensile testing machine (tensile testing machine AG-1kNXplus manufactured by Shimadzu Corporation) was used as the measuring device. The test piece is a rectangle with a length of 100 mm and a width of 10 mm cut from the film in the measuring direction. Clamp the two ends of the test piece in the length direction with a distance between the chucks of 50 mm, raise it at a crosshead speed of 0.5 mm/min until the strain reaches 5%, and then lower it to the initial position at the same speed. One cycle, and the stress-strain curve is obtained based on the tensile cycle test of this one cycle. The stress-strain curve presents the distribution shown in Figure 1. The hysteresis loss rate is based on the obtained stress-strain curve, using the area A1 (abcda) of the curve obtained in the ascending motion, and the area A1 and the curve obtained in the descending motion The difference in area is the area A2 (abcef), and is calculated using the following formula 1. The test was carried out 3 times, and the average value was obtained. The above-mentioned tensile cycle test was carried out on the MD and TD of the film, and the average value was calculated. Hysteresis loss rate = (A2/A1)×100 (Equation 1)

(2) Residual strain According to JIS K 7312:1996, the average value of the residual strain at 23°C is obtained by the following method. A tensile testing machine (tensile testing machine AG-1kNXplus manufactured by Shimadzu Corporation) was used as the measuring device. The test piece is a rectangle with a length of 100 mm and a width of 10 mm cut from the film in the measuring direction. Clamp the two ends of the test piece in the longitudinal direction with a distance of 50 mm between the chucks, raise it at a crosshead speed of 0.5 mm/min until the strain reaches 5%, and then lower it to the initial position at the same speed. This is one cycle, and the stress-strain curve is obtained from the tensile cycle test of this one cycle, and based on the stress-strain curve, the strain at the point where the stress disappears is set as the residual strain. The test was carried out 3 times, and the average value was obtained. The above-mentioned tensile cycle test was carried out on the MD and TD of the film, and the average value was calculated.

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

(4) Crystal melting temperature (Tm) With respect to the obtained film, in accordance with JIS K7121 (2012), a Diamond DSC (manufactured by PerkinElmer Japan) was used to measure the crystal melting temperature during the heating process at a heating rate of 10°C/min.

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

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

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

25 mm雙軸擠出機，對顆粒狀之(A)-1單一成分進行熔融混練，自間隙1.0 mm之T型模頭內以膜之形式擠出，利用110℃之流延輥牽引，進行冷卻固化，從而獲得厚度約450 μm之膜狀物(鑄膜)。 繼而，使獲得之鑄膜通過縱向延伸機，以132℃在縱方向(MD)上延伸3.3倍。繼而，使獲得之縱向延伸膜通過橫向延伸機(拉幅機)，於預熱溫度120～125℃、延伸溫度130℃、熱固定溫度180℃之條件下在橫方向(TD)上延伸3.1倍，其後於拉幅機內一面進行熱固定一面於寬度方向(TD)上進行膜之5%鬆弛處理。對所獲得之膜進行測定，將測定結果示於表1。(Example 1) Using the

25 mm twin-screw extruder, melt and knead the granular (A)-1 single component, extrude it in the form of a film from a T-die with a gap of 1.0 mm, and draw it with a casting roll at 110°C. Cool and solidify to obtain a film (cast film) with a thickness of about 450 μm. Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 132°C. Then, the obtained longitudinally stretched film was passed through a transverse stretcher (tenter), and stretched 3.1 times in the transverse direction (TD) under the conditions of a preheating temperature of 120 to 125°C, a stretching temperature of 130°C, and a heat fixing temperature of 180°C. , And then heat-fixed in the tenter and 5% relaxation of the film in the width direction (TD). The obtained film was measured, and the measurement results are shown in Table 1.

(Example 2) Using granular (A)-2 single component, and using the same method as in Example 1, a film (cast film) with a thickness of about 450 μm was obtained. Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135°C. Then, the obtained longitudinally stretched film was passed through a transverse stretcher (tenter), and stretched 3.1 times in the transverse direction (TD) under the conditions of a preheating temperature of 125-130°C, a stretching temperature of 135°C, and a heat-fixing temperature of 180°C. , And then heat-fixed in the tenter and 5% relaxation of the film in the width direction (TD). The obtained film was measured, and the measurement results are shown in Table 1.

25 mm雙軸擠出機，對顆粒狀之(A)-3單獨成分進行熔融混練，自間隙1.0 mm之T型模頭內以膜之形式擠出，利用113℃之流延輥牽引，並進行冷卻固化，從而獲得厚度約450 μm之膜狀物(鑄膜)。 繼而，使獲得之鑄膜通過縱向延伸機，以135℃在縱方向(MD)上延伸3.0倍。繼而，使獲得之縱向延伸膜通過橫向延伸機(拉幅機)，於預熱溫度125～130℃、延伸溫度135℃、熱固定溫度180℃之條件下在橫方向(TD)上延伸3.1倍，其後於拉幅機內一面進行熱固定一面於寬度方向(TD)上進行膜之5%鬆弛處理。對膜進行測定，將測定結果示於表1。(Example 3) Using the

25 mm twin-screw extruder, melt and knead the granular (A)-3 separate components, extrude it in the form of a film from a T-die with a gap of 1.0 mm, and draw it with a casting roll at 113°C. Cool and solidify to obtain a film (cast film) with a thickness of about 450 μm. Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.0 times in the longitudinal direction (MD) at 135°C. Then, the obtained longitudinally stretched film was passed through a transverse stretcher (tenter), and stretched 3.1 times in the transverse direction (TD) under the conditions of a preheating temperature of 125-130°C, a stretching temperature of 135°C, and a heat-fixing temperature of 180°C. , And then heat-fixed in the tenter and 5% relaxation of the film in the width direction (TD). The film was measured, and the measurement results are shown in Table 1.

(Example 4) Using granular (A)-4 single component, and using the same method as in Example 3, a film (cast film) with a thickness of about 450 μm was obtained. Then, the obtained cast film was passed through a longitudinal stretching machine and stretched 3.3 times in the longitudinal direction (MD) at 137°C. Then, the obtained longitudinally stretched film was passed through a transverse stretcher (tenter), and stretched 3.3 times in the transverse direction (TD) under the conditions of a preheating temperature of 120 to 125°C, a stretching temperature of 130°C, and a heat setting temperature of 150°C. , And then heat-fixed in the tenter and 5% relaxation of the film in the width direction (TD). The obtained film was measured, and the measurement results are shown in Table 1.

(Comparative example 1) The biaxially stretched PET film (B)-1 was evaluated, and the evaluation results are shown in Table 1.

(Comparative example 2) The PEN film (C)-1 was evaluated, and the evaluation results are shown in Table 1.

[Table 1] Table 1 Example 1 Example 2 Example 3 Example 4 Comparative example 1 Comparative example 2 Polyester resin (A)-1 quality% 100 - - - - - (A)-2 quality% - 100 - - - - (A)-3 quality% - - 100 - - - (A)-4 quality% - - - 100 - - PET film (B)-1 quality% - - - - 100 - PEN film (C)-1 quality% - - - - - 100 Extension magnification vertical Times 3.3 3.0 3.0 3.3 - - horizontal Times 3.1 3.1 3.1 3.3 - - Glass transition temperature (Tg) °C 127 128 127 124 78 120 Crystal melting temperature (Tm) °C 239 253 246 228 250 262 thickness μm 50 50 50 50 50 50 Hysteresis loss rate MD % 46.1 42.6 42.6 44.6 49.2 48.0 TD 39.6 44.3 48.0 43.3 69.6 46.8 average value 42.8 43.5 45.3 43.9 59.4 47.4 Residual strain MD % 0.883 0.787 0.795 0.895 0.998 0.947 TD 0.709 0.847 0.970 0.839 1.957 0.862 average value 0.796 0.817 0.883 0.867 1.477 0.905

The films of Examples 1 to 4 have high crystal melting temperature and glass transition temperature, and are excellent in heat resistance. In addition, compared with the PET film of Comparative Example 1 and the PEN film of Comparative Example 2, the value of the hysteresis loss rate or residual strain is also significantly lower. It can be considered that the PEN-based copolymer-containing films of Examples 1 to 4 are not only heat resistant Excellent, and also excellent in folding endurance.

Figure 1 shows the distribution of stress-strain curves.

Claims (12)

A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and the average value of the hysteresis loss rate when the tensile cycle test reaches 5% tensile strain in both MD and TD directions It is 47.0% or less. A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and the average value of the residual strain is Below 0.900%. The biaxially stretched film of claim 1 or 2, wherein the polyethylene naphthalate copolymer (A) contains 2,6-naphthalenedicarboxylic acid unit as the dicarboxylic acid component (a-1), and contains bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit and ethylene glycol unit are used as the diol component (a-2). For the biaxially stretched film of claim 3, in all the dicarboxylic acid components, the terephthalic acid unit as the dicarboxylic acid components other than the above (a-1) is less than 2 mol%. For the biaxially stretched film of claim 3 or 4, among all the diol components, the diol unit having a diphthalimide skeleton as the diol component other than the above (a-2) does not reach 1 mol%. A biaxially stretched film comprising a polyethylene naphthalate copolymer (A), and The above-mentioned copolymer (A) contains a dicarboxylic acid component (a-1) and a diol component (a-2), The above-mentioned dicarboxylic acid component (a-1) contains at least 2,6-naphthalenedicarboxylic acid unit, The above-mentioned glycol component (a-2) contains two components, bisphenol A-ethylene oxide adduct or 1,4-cyclohexanedimethanol unit and ethylene glycol unit, and Among all the above-mentioned dicarboxylic acid components, the terephthalic acid unit as the dicarboxylic acid components other than the above-mentioned (a-1) is less than 2 mol%. The biaxially stretched film according to any one of claims 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 unit, and contains 30 mol% or more and 96 mol% or less of ethylene glycol unit. For the biaxially stretched film of any one of claims 1 to 7, the glass transition temperature is 75°C or more and 150°C or less. For the biaxially stretched film of any one of claims 1 to 8, the crystal melting temperature is 220°C or more and 300°C or less. For example, the biaxially stretched film of any one of claims 1 to 9 has a thickness of 1 μm or more and 250 μm or less. Such as the biaxially stretched film of any one of claims 1 to 10, which is for display purposes. A foldable display is provided with the biaxially stretched film according to any one of claims 1 to 11.

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