Polyethylene terephthalate film, laminated film, and foldable display

Abstract

The purpose of the present invention is to provide a polyethylene terephthalate film for foldable displays, the film having, as a film for foldable displays, a sufficient tensile elastic modulus and being excellent in flexural rigidity. A portable terminal device or an image display device equipped with this foldable display provides a beautiful image, has extensive functionality, and is excellent in convenience such as portability. The present invention relates to a polyethylene terephthalate film for foldable displays, the film being configured such that the sum of the flexural rigidity Fa in the MD direction and the flexural rigidity Fb in the TD direction, as converted for a thickness of 15 μm, is 0.05 gf•cm2 / cm or more. With respect to the film, the transmittance of light having a wavelength of 380 nm is no more than 20%.

Description

Polyethylene terephthalate film, laminated film, and foldable display

[0001] The present invention relates to a polyethylene terephthalate film for foldable displays, which has excellent bending resistance and preferably further has UV absorption properties.

[0002] Polyethylene terephthalate films, which contain polyethylene terephthalate resins as the main resin, have been used in a variety of applications due to their excellent mechanical properties, heat resistance, etc. In recent years, they have increasingly been used as surface protection films for foldable displays that are used by folding. As devices equipped with foldable displays are becoming thinner, thinner polyethylene terephthalate films used in these displays are also being required. However, thinner polyethylene terephthalate films decrease their film strength.

[0003] As a method for increasing the strength of a polyethylene terephthalate film, a method has been reported in which a film is stretched in two directions, i.e., the longitudinal direction and the width direction, and then stretched again in the longitudinal direction and the width direction. For example, Patent Document 1 discloses a method for improving strength without impairing formability by stretching the film in multiple stages in the longitudinal direction and the width direction.

[0004] Japanese Patent Application Laid-Open No. 2002-011786

[0005] The biaxially stretched polyethylene terephthalate film produced by the method described in Patent Document 1 has suppressed elongation deformation due to tension, which is required for its use (magnetic tape), but has the problem of low tensile modulus.

[0006] There has been a demand for a polyethylene terephthalate film for foldable displays that has sufficient tensile modulus in the longitudinal and width directions (directions perpendicular to the longitudinal and thickness directions) and high rigidity even when thinned.

[0007] In addition, as organic EL displays have become more widespread in recent years, it has become necessary to protect the organic EL from ultraviolet rays, and UV absorption performance is required on the front surface of the organic EL display.

[0008] The object of the present invention is to provide a polyethylene terephthalate film for foldable displays that has a sufficient tensile modulus, excellent bending rigidity, and preferably UV absorption performance as a film for foldable displays. As a result of extensive research by the present inventors on polyethylene terephthalate films, the present inventors have found that by stretching a polyester film multiple times at a high temperature below its melting point (Tm), it is possible to elongate the internal molecular chains and increase the crystallinity, thereby obtaining high rigidity.

[0009] The present disclosure typically includes the following aspects: [Item 1] The sum of the longitudinal bending rigidity Fa and the widthwise bending rigidity Fb converted into a thickness of 15 μm is 0.05 gf cm ２ / cm or more. [Item 2] The polyethylene terephthalate film according to Item 1, wherein the size of crystals formed by polymer molecules constituting the film is 3.0 nm or more as determined by small-angle X-ray scattering. [Item 3] The polyethylene terephthalate film according to Item 1 or 2, wherein the bending rigidity in the longitudinal direction is greater than the bending rigidity in the width direction. [Item 4] The polyethylene terephthalate film according to any one of Items 1 to 3, wherein the total light transmittance is 85% or more and the haze is 3% or less. [Item 5] The polyethylene terephthalate film according to any one of Items 1 to 4, wherein the transmittance of light at a wavelength of 380 nm is 20% or less and the b-value is 3.5 or less. [Item 6] A laminate film for a foldable display, comprising a base layer and an easy-adhesion layer, wherein the base layer is the polyethylene terephthalate film according to any one of Items 1 to 5, and the easy-adhesion layer is laminated on at least one surface of the base layer. [Item 7] The laminate film according to Item 6, wherein a hard coat layer is laminated on the easy-adhesion layer. [Item 8] The laminate film according to item 6 or 7, characterized in that it has a total light transmittance of 85% or more and a haze of 3% or less. [Item 9] A foldable display comprising the polyethylene terephthalate film according to any one of items 1 to 5 as a surface protective film. [Item 10] A foldable display comprising the laminate film according to any one of items 6 to 8 as a surface protective film. [Item 11] A mobile terminal device comprising the foldable display according to item 9 or 10.

[0010] The polyethylene terephthalate film and laminate film of the present invention (hereinafter, both films may be collectively referred to as "the film of the present invention") have excellent bending rigidity and therefore good stiffness even when thinned. A film with stiffness can be, for example, a film that does not break when bent and is less likely to maintain deformation when bent. In addition, the film of the present invention has a high elastic modulus, which can suppress deformation and scratches caused by foreign matter. Therefore, it is suitable for use as a surface protection film for foldable displays, which require high quality during processing. The film of the present invention can also protect organic EL modules from ultraviolet rays. Furthermore, since the film of the present invention itself has UV absorption properties, the number of layers does not increase compared to when a UV absorber is coated on the surface of a polyethylene terephthalate film, thereby maintaining visible light transmittance.

[0011] Fig. 1 is a schematic diagram showing the bending radius when the foldable display of the present invention is folded. Fig. 2 is a schematic diagram showing the bending direction of the polyethylene terephthalate film and laminate film for foldable displays of the present invention. Fig. 3 is a schematic diagram for explaining a method for measuring bending rigidity using a pure bending tester. Fig. 4 shows an example of a measurement curve used to explain a method for measuring crystal size obtained by small-angle X-ray scattering.

[0012] As used herein, the phrase "comprising" is intended to encompass the phrases "consisting essentially of" and "consisting of." As used herein, the phrase "PET" is intended to encompass the phrase "polyethylene terephthalate."

[0013] (Display) In the present invention, the term "display" may encompass display devices in general. Examples of display types include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, but bendable LCDs, organic EL displays, or inorganic EL displays are preferred. Bendable organic EL displays and inorganic EL displays are more preferred in that they can reduce the number of layers required for the display, and bendable organic EL displays are even more preferred in that they have a wide color gamut.

[0014] (Foldable Display) A foldable display can include a single continuous display that can be folded in half or otherwise when needed, such as when carrying it around. Folding the display reduces the size of the display by half, improving the portability of a terminal equipped with this display. The bending radius of the foldable display is preferably 5 mm or less, and more preferably 3 mm or less. A bending radius of 5 mm or less allows for a thinner display when folded. The smaller the bending radius, the better, but the more likely it is that fold marks will appear. The bending radius is preferably 0.1 mm or more, but may be 0.5 mm or more, or even 1 mm or more. Even a bending radius of 1 mm can achieve a practically sufficient thinness when folded. The bending radius when folded is obtained by measuring the location indicated by reference numeral 11 in the schematic diagram of FIG. 1 and refers to the inner radius of the folded portion when folded. The surface protective film, described later, may be located on the outer or inner surface of the folded display. Furthermore, foldable displays may be tri-fold or quadruple foldable, or may even be a rollable type. All of these are encompassed by the foldable display of the present invention. The polyethylene terephthalate film for foldable displays and the laminate film for foldable displays of the present invention may be used in any part of a foldable display as long as it is a component of the foldable display. Below, using an organic EL display as an example, a typical configuration of a foldable display and the parts of a foldable display in which the polyethylene terephthalate film and the laminate film of the present invention may be used are described. Note that, throughout this specification, the polyethylene terephthalate film for foldable displays of the present invention may sometimes be simply referred to as a polyester film, and the laminate film for foldable displays of the present invention may sometimes be simply referred to as a laminate film.

[0015] (Foldable Organic EL Display) A foldable organic EL display includes an organic EL module as an essential component, and may further include a circular polarizer, a touch panel module, a surface protective film, a back protective film, and the like, as needed.

[0016] (Organic EL Module) An organic EL module generally comprises an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode. The polyester film or laminate film of the present invention can be used as a substrate on which an electrode is provided and further an electron transport layer, a light-emitting layer, a hole transport layer, and a transparent electrode are provided. The polyester film and laminate film of the present invention are particularly preferably used as a substrate for a transparent electrode. Since the substrate film is required to have high barrier properties against water vapor and oxygen, it is preferable that the polyester film and laminate film of the present invention be provided with a barrier layer such as a metal oxide layer. To improve the barrier properties, the laminate film may have multiple barrier layers. Multiple polyester films each provided with a barrier layer may be used as the substrate film.

[0017] (Protective Film for Organic EL Module) A protective film is preferably provided on the visible side of the organic EL module. It is also preferable to provide a protective film on the non-visible side of the organic EL module. An organic EL module is generally formed on a glass substrate. The glass substrate is a member that supports and protects the organic EL module. However, since glass substrates cannot be bent, they cannot be used in flexible displays such as foldable displays, and instead, foldable films are used as protective members. The polyester film and laminate film of the present invention can be used as a protective film for this organic EL module. A hard coat layer may be provided on the protective film to prevent scratches.

[0018] (Touch Panel Module) The mobile terminal device preferably has a touch panel. In the case of an organic EL display, the touch panel module is preferably disposed above the organic EL display or between the organic EL module and the circular polarizer. The touch panel module may have a transparent substrate such as a film and a transparent electrode disposed thereon. The polyester film and laminate film of the present invention can be used as this transparent substrate. When the polyester film and laminate film of the present invention are used as transparent substrates included in a touch panel module, the polyester film and laminate film are preferably provided with a hard coat layer, a refractive index adjustment layer, or the like.

[0019] (Circular Polarizing Plate) A circular polarizing plate can prevent degradation of image quality due to reflection of external light by components inside a display. A circular polarizing plate can have a linear polarizing plate and a retardation plate. A linear polarizing plate can have a protective film on at least the viewing side of the polarizer. A linear polarizing plate may also have a protective film on the side opposite the viewing side of the polarizer, or a retardation plate may be directly laminated on the polarizer. A retardation plate can be a film in which a retardation layer made of a liquid crystal compound is provided on a resin film having retardation. The polyester film of the present invention can be used as a polarizer protective film, a resin film for a retardation plate, etc. In these cases, the laminate film of the present invention preferably has the slow axis direction of the polyester film at 45 degrees, parallel, or perpendicular to the absorption axis direction of the polarizer. Note that a range of 10 degrees, preferably up to 5 degrees (also referred to as a deviation) from this 45 degrees, parallel, or perpendicular is acceptable.

[0020] (Surface Protective Film) When a display is subjected to an impact from above, the circuits of the organic EL module and touch panel module may be disconnected. For this reason, displays are often provided with a surface protective film. The polyester film and laminate film of the present invention can be used as this surface protective film. Surface protective films include those called cover windows that are incorporated into the outermost surface of the display, and those called after-films that can be attached, peeled off, and replaced by the user. The polyester film and laminate film of the present invention can be used for either. When the polyester film and laminate film of the present invention are used as a surface protective film, the laminate film having a hard coat layer laminated thereon can be provided on the surface of a foldable display so that the side on which the hard coat layer is laminated is the viewing side of the display. The hard coat layer may be provided on both sides of the laminate film. The polyester film and laminate film of the present invention are preferably used as a surface protective film for an organic EL module of a foldable display.

[0021] The polyester film of the present invention contains a polyethylene terephthalate resin as a main resin component, and the content of the polyethylene terephthalate resin in the polyester film of the present invention may be 70 to 100 mass%, or 80 to 100 mass%, or the like, preferably 90 to 100 mass%, and more preferably 95 to 100 mass%.

[0022] The polyethylene terephthalate resin may be a polymer containing ethylene glycol and terephthalic acid as main components. Specifically, the polyethylene terephthalate resin may be a resin in which 80 mol % or more of the repeating units are composed of ethylene terephthalate. The polyethylene terephthalate resin may be copolymerized with other dicarboxylic acid components and glycol components within a range that does not impair the object of the present invention.

[0023] Polyethylene terephthalate resin can be produced by any production method, such as direct polymerization or transesterification. The direct polymerization method is a method in which terephthalic acid and ethylene glycol, and, if necessary, other dicarboxylic acid components and diol components, are directly reacted. The transesterification method is a method in which a dimethyl ester of terephthalic acid (including, if necessary, a methyl ester of another dicarboxylic acid) is transesterified with ethylene glycol (including, if necessary, another diol component).

[0024] The intrinsic viscosity of the resin pellets used as a raw material for the polyethylene terephthalate resin is preferably in the range of 0.57 dl / g to 0.7 dl / g, more preferably 0.58 dl / g to 0.65 dl / g, and the intrinsic viscosity of the polyethylene terephthalate film is preferably in the range of 0.55 dl / g to 0.69 dl / g, more preferably 0.58 dl / g to 0.65 dl / g.

[0025] The polyester film of the present invention may contain various additives as needed in addition to the polyethylene terephthalate resin. Examples of additives include inorganic lubricants such as titanium dioxide, fine silica particles, kaolin, and calcium carbonate, and organic lubricants such as crosslinked polymers of acrylic monomers such as acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters. Other additives include stabilizers, colorants, antioxidants, antifoaming agents, antistatic agents, and ultraviolet absorbers. The additives may be contained alone or in combination of two or more.

[0026] Furthermore, for the purpose of suppressing deterioration of optically functional dyes such as iodine-based dyes, the film of the present invention preferably has a light transmittance of 20% or less at a wavelength of 380 nm. The light transmittance of 380 nm is more preferably 17% or less, even more preferably 16% or less, and particularly preferably 15% or less. A light transmittance of 20% or less can suppress deterioration of the optically functional dye due to ultraviolet light. From the viewpoint of production costs, the light transmittance of 380 nm is preferably 0.1% or more. The light transmittance of the film of the present invention at a wavelength of 380 nm can be 0.1 to 20%, 0.1 to 17%, 0.1 to 16%, or 0.1 to 15%.

[0027] Here, when the film of the present invention is used as a surface protection film for a foldable display, it is preferable that it is colorless and transparent (i.e., has little absorption in the visible light region) from the viewpoint of visibility. However, if an ultraviolet absorber having strong absorption around a wavelength of 380 nm is used in order to reduce the light transmittance at a wavelength of 380 nm, the surface protection film may be colored yellow. Therefore, the light transmittance at a wavelength of 380 nm is more preferably 0.2% or more, even more preferably 0.3% or more, and particularly preferably 0.4% or more. The light transmittance of the film of the present invention for light having a wavelength of 380 nm can be 0.1 to 20%, 0.1 to 17%, 0.1 to 16%, 0.1 to 15%, 0.2 to 20%, 0.2 to 17%, 0.2 to 16%, 0.2 to 15%, 0.3 to 20%, 0.3 to 17%, 0.3 to 16%, 0.3 to 15%, 0.4 to 20%, 0.4 to 17%, 0.4 to 16%, or 0.4 to 15%.

[0028] In the present invention, the transmittance of light having a wavelength of 380 nm is measured in a direction perpendicular to the plane of the film, and can be measured using a spectrophotometer (for example, UV1800 manufactured by Shimadzu Corporation).

[0029] The film of the present invention preferably contains an ultraviolet absorber. Examples of ultraviolet absorbers include triazine-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, benzophenone-based ultraviolet absorbers, benzoxazinone-based ultraviolet absorbers, salicylate-based ultraviolet absorbers, cyanoacrylate-based ultraviolet absorbers, and salicylate-based ultraviolet absorbers. Triazine-based ultraviolet absorbers and benzotriazole-based ultraviolet absorbers are preferably used. Furthermore, the following ultraviolet absorbers can be specifically used as ultraviolet absorbers that are not adversely affected by heat during the production of the film of the present invention.Examples of such ultraviolet absorbers include 2-(2-hydroxy-4-[1-octyloxycarbonylethoxy]phenyl)-4,6-bis(4-phenylphenyl)-1,3,5-triazine, 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-(2H-benzotriazol-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol, octyl-3-[3-tert-butyl]phenol, and octyl-3-[3-tert-butyl]phenol. -4-hydroxy-5-(5-chloro-2H-benzotriazol-2-yl)phenyl]propionate, 2-ethylhexyl-3-[3-tert-butyl-4-hydroxy-5-(5-chloro-2H-benzotriazol-2-yl)phenyl]propionate, 2-(2H-benzotriazol-2-yl)4,6-bis(1-ethyl-1-phenylethyl)phenol, phenol, 2-(5-chloro-2H-benzotriazol-2-yl)-6-(1,1-dimethylethyl)4-methyl, 2,2'-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol), 2-(4,6,-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)roxy]phenol, 2,4-bis(2-hydroxy-4-butyloxyphenyl)-6-(2,4-bis-butyloxyphenicyl)-1,3,5-triazine, benzenepropanoic acid, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxybenzoyl hydroxy-, C7-9 branched and linear alkyl esters 2-(2-hydroxy-5-tert-methylphenyl)-2H-benzotriazole 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol 2,2'-dihydroxy-4-methoxybenzophenone, 2-[4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl]-5-(octyloxy)phenol, 2-(2'-hydroxy-5'-octylphenyl)benzotriazole.

[0030] The concentration of the UV absorber in the polyester film is preferably 0.1% by mass or more, and more preferably 1 to 8% by mass, relative to 100% by mass of the film. At 0.1% by mass or more, a sufficient UV absorption effect can be obtained. The concentration of the UV absorber is preferably 0.1 to 8% by mass, more preferably 0.1 to 6% by mass, more preferably 0.1 to 5% by mass, even more preferably 0.1 to 4% by mass, and particularly preferably 0.1 to 3% by mass, relative to 100% by mass of the film.

[0031] The polyester film may contain particles, and known methods can be used to incorporate particles into the film.

[0032] The film of the present invention may contain an ultraviolet absorber. However, the film of the present invention preferably has a b-value, which indicates a yellowish-blue tint, near zero. The b-value is preferably -1.0 or more and 3.5 or less, and more preferably -0.5 or more and 3.0 or less. In particular, when the film of the present invention is used as a surface protection film for a foldable display, the b-value is preferably -0.5 or more and 3.0 or less. A b-value of 3.5 or less can prevent the film from appearing yellowish, and when applied to a film for a foldable display, problems such as deterioration in image quality and discoloration do not occur, which is preferable. Furthermore, a b-value of -1.0 or more can prevent the film from appearing blue, and is therefore preferable because it prevents the user from feeling a dark impression. The b-value can be determined in accordance with JIS K 7373-2006, and specifically, it is preferably determined by the method described in the Examples.

[0033] (Method for Producing Stretched Film) From the viewpoints of mechanical strength, chemical resistance, heat resistance, etc., the polyester film is preferably an oriented film, more preferably a biaxially oriented film.

[0034] The polyethylene terephthalate resin, optionally blended with a crosslinking agent, can be processed into an unstretched sheet by various methods, and then biaxially stretched to obtain the unstretched sheet. As a method for producing the unstretched sheet, a method using melt extrusion as well as solution casting can be used, and the melt extrusion method is preferred in the present invention.

[0035] The melting temperature of the resin composition in the present invention is preferably 280 to 300° C., more preferably 280° C. When the melting temperature is within the above range, a decrease in the resin molecular weight is suppressed, the resin is sufficiently melted, and an increase in melt viscosity is suppressed, facilitating subsequent extrusion.

[0036] The die temperature for melt extrusion is the same as described above, but is preferably 280 to 300°C, and more preferably 280°C. When the die temperature during melt extrusion is 280°C or higher, the melt viscosity falls within an appropriate range, allowing stable extrusion. By keeping the temperature at 300°C or lower, thermal decomposition of the resin can be suppressed.

[0037] Next, the unstretched film obtained by the above method is biaxially stretched sequentially or simultaneously in the width direction (also referred to as TD) and the longitudinal direction (also referred to as MD). It is important to then stretch the film multiple times (multi-stage stretching) in the width direction and / or the longitudinal direction at a temperature in the range of 100°C to 250°C.

[0038] In the conventional sequential biaxial stretching method, in order to suppress thickness unevenness in the width direction or to obtain dimensional stability, it has been preferable to set the stretching temperature in both the longitudinal and width directions within a range of 10°C higher than the glass transition temperature of the polyethylene terephthalate resin (Tg + 10°C) to 50°C higher than the glass transition temperature (Tg + 50°C), and to set the stretching ratio in both the longitudinal and width directions within a range of 2 to 5. After stretching, in order to reduce the heat shrinkage of the polyester film, it is recommended to carry out a heat setting treatment (heat setting step) within 30 seconds, preferably within 15 seconds, followed by a longitudinal relaxation treatment and a transverse relaxation treatment of 0.5 to 10%.

[0039] However, conventional stretching temperatures induce film breakage due to insufficient heat during high-magnification stretching. Therefore, in the present invention, for example, a uniaxially stretched film stretched in the longitudinal direction at (Tg - 10°C) to (Tg + 50°C) is stretched in the width direction at least once at (Tg + 10°C) to (Tg + 50°C) to orient the molecular chains to a certain extent in the width direction, and then stretched at least once at a higher temperature of (Tg + 60°C) to (Tg + 180°C). In this specification, Tg refers to the glass transition temperature of the polyethylene terephthalate resin. By using such multistage stretching, the present invention can suppress the frequency of breakage during high-magnification stretching and further improve film rigidity. The total stretching ratio in the multistage stretching direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. By setting the total draw ratio in the multi-stage drawing direction within the above range, it is advantageous in that the frequency of film breakage is reduced.

[0040] (1) Sequential Biaxial Stretching In the sequential biaxial stretching method, an unstretched film is stretched in the longitudinal direction by utilizing the speed difference between rolls at a temperature equal to or higher than the glass transition temperature (Tg; 78°C) of the polyethylene terephthalate resin to produce a uniaxially stretched film. For example, the sheet is heated and stretched 1.1 to 6 times between two or more rolls with different peripheral speeds. The heating method used here may be a method using a heated roll or a method using a non-contact heating medium, or both may be used. In this case, the film temperature is preferably set to, for example, 65 to 130°C, preferably (Tg - 10°C) to (Tg + 50°C).

[0041] The uniaxially stretched film is then stretched in multiple stages in the width direction using a tenter. Specifically, for example, when stretching in two stages, the film temperature in the first stage is, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and the film is stretched 1.1 to 4.0 times in the width direction. Then, in the second stage, the film temperature is, for example, 135 to 260°C, preferably (Tg + 60°C) to (Tg + 180°C), and the film is stretched 1.1 to 3.0 times in the width direction. When stretching in two stages, the total stretching ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. Setting the total stretching ratio in the multi-stage stretching direction within the above range is advantageous in that the frequency of film breakage is reduced.

[0042] For example, when stretching in three stages, in the first stage, the film temperature is, for example, 85 to 120°C, preferably (Tg + 10°C) to (Tg + 40°C), and the film is stretched 1.1 to 3.0 times in the width direction. Then, in the second stage, the film temperature is, for example, 125 to 190°C, preferably (Tg + 50°C) to (Tg + 110°C), and the film is stretched 1.1 to 3.0 times in the width direction. Then, in the third stage, the film temperature is, for example, 195 to 260°C, preferably (Tg + 120°C) to (Tg + 180°C), and the film is stretched 1.1 to 3.0 times in the width direction. When stretching in three stages, the total stretch ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. By setting the total stretching ratio in the multi-stage stretching direction within the above range, it is advantageous in that the frequency of film breakage is reduced.

[0043] For example, when stretching is performed in four stages, in the first stage, the film temperature is, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and the film is preferably stretched 1.1 to 3.0 times in the width direction. In the second stage, the film temperature is, for example, 125 to 150°C, preferably (Tg + 50°C) to (Tg + 70°C), and the film is preferably stretched 1.1 to 3.0 times in the width direction. In the third stage, the film temperature is, for example, 155 to 190°C, preferably (Tg + 80°C) to (Tg + 110°C), and the film is preferably stretched 1.1 to 3.0 times in the width direction. In the fourth stretching stage, the film temperature is set to, for example, 195 to 260°C, preferably (Tg + 120°C) to (Tg + 180°C), and more preferably (Tg + 140°C) to (Tg + 180°C), and the film is preferably stretched 1.1 to 3.0 times in the width direction. When stretching in four stages, the total stretching ratio in the width direction can be 9.0 times or less, preferably 1.1 to 9.0 times, more preferably 3.0 to 9.0 times, and even more preferably 6.0 to 9.0 times. Keeping the total stretching ratio in the multi-stage stretching direction within the above range is advantageous in that the frequency of film breakage is reduced.

[0044] After multistage stretching, the film is preferably heat-treated (heat-set) by passing it through a heat-treatment zone (e.g., by passing it in the longitudinal direction) adjusted to a temperature between (Tg + 10°C) and (Tg + 50°C). The heat-set time (the time the film passes through the heat-treatment zone) is, for example, 1 to 30 seconds, preferably 8 to 15 seconds. Rapidly lowering the temperature prevents the molecular chains in the width direction, which have been strained to the maximum by the final stretching, from relaxing due to the temperature drop. The heat-set temperature may be 90°C to 130°C, more preferably 90 to 110°C. Heat-set treatment improves the planarity of the film and prevents a decrease in strength due to temperature drop. Furthermore, during this heat-set treatment, it is also preferable to perform a relaxation treatment in the longitudinal and / or width directions, for example, at a rate of 0.5 to 7.0%, preferably 0.5 to 6.5%. From the viewpoint of improving the planar stability of the film or preventing film shrinkage during storage or processing, the relaxation treatment is advantageous.

[0045] (2) Simultaneous Biaxial Stretching When increasing the rigidity in the longitudinal direction by simultaneous biaxial stretching, the following control is preferred. For example, an unstretched film is held with clips, and the film temperature is adjusted in a heating furnace to, for example, 85 to 130°C, preferably (Tg + 10°C) to (Tg + 50°C), and the film is simultaneously stretched 1.1 to 4.0 times in the longitudinal direction and 1.1 to 2.0 times in the transverse direction. Thereafter, the film is again preferably stretched 1.1 to 1.5 times in the longitudinal direction at, for example, 225 to 260°C, preferably (Tg + 150°C) to (Tg + 180°C). In the case of two-stage stretching, it is preferable to keep the total stretch ratio in the multi-stage stretching direction to 8.0 times or less, as this reduces the frequency of breakage.

[0046] (Film Properties) The thickness of the film of the present invention is preferably 2 to 500 μm, more preferably 10 to 400 μm, even more preferably 10 to 250 μm, and even more preferably 20 to 100 μm. A film thickness of 2 μm or more is advantageous in that the film has sufficient rigidity, is easy to handle, and can protect foldable display components such as organic EL modules and touch panels. Furthermore, a film thickness of 500 μm or less is advantageous in that the film is easy to transport when transported with multiple rolls, the produced film is easy to handle, and the occurrence of cracks due to folding is suppressed.

[0047] The breaking strength of the film of the present invention may be 100 MPa or more, 200 MPa or more, or 350 MPa or more in both the longitudinal and transverse directions. The breaking strength in one direction may be 100 to 1000 MPa, 250 to 1000 MPa, 300 to 1000 MPa, or 350 to 1000 MPa, preferably 250 to 600 MPa, 300 to 600 MPa, or 350 to 600 MPa, and more preferably 250 to 500 MPa, 300 to 500 MPa, or 350 to 500 MPa. The breaking strength in the other direction may be 100 MPa or more, 130 MPa, preferably 100 to 300 MPa, and more preferably 130 to 250 MPa. When the breaking strength is within the above range, the breaking strength is high in both the longitudinal and width directions, so that when processing a foldable display, even if tension is applied in either direction while laminating the polyester film, problems such as stretching and misalignment of the film can be suppressed, which is preferable.

[0048] The sum of the tensile modulus Ea in the longitudinal direction (MD) and the tensile modulus Eb in the transverse direction (TD) of the film of the present invention may be 5.0 GPa or more. The lower limit of the sum of the tensile moduli is preferably 5.0 GPa, more preferably 7.0 GPa, even more preferably 8.0 GPa, and even more preferably 10.0 GPa. The upper limit of the sum of the tensile moduli is preferably 20.0 GPa, more preferably 15.0 GPa, and even more preferably 10.0 GPa. These lower and upper limits can be appropriately combined to form a numerical range. The sum of the tensile moduli may be 5.0 to 10.0 GPa, 5.0 to 15.0 GPa, 5.0 to 20.0 GPa, 7.0 to 10.0 GPa, 7.0 to 15.0 GPa, 7.0 to 20.0 GPa, 8.0 to 10.0 GPa, 8.0 to 15.0 GPa, 8.0 to 20.0 GPa, 10.0 to 15.0 GPa, or 10.0 to 20.0 GPa. When the sum of the tensile moduli is equal to or greater than the lower limit, the film has sufficient rigidity, and wrinkles and warpage of the film can be suppressed. Therefore, even when the film is made thin, sufficient stiffness can be obtained, and scratches and deformation due to foreign matter can be suppressed.

[0049] The film of the present invention is preferably a film having a bending rigidity in the longitudinal direction greater than that in the transverse direction. The sum of the bending rigidity Fa in the longitudinal direction and the bending rigidity Fb in the transverse direction, calculated for a film thickness of 15 μm, is 0.05 gf cm ２ / cm or more, and more preferably 0.06 gf cm ２ / cm or more, and more preferably 0.07 gf cm ２ The preferred lower limit of the sum of the bending rigidities is 0.05 gf cm ２ / cm, and a more preferable lower limit is 0.06 gf cm ２ / cm, and a more preferable lower limit is 0.07 gf cm ２ The preferred upper limit of the sum of the bending rigidities is 0.10 gf cm ２ / cm, and a more preferable upper limit is 0.09 gf cm ２ / cm, and a more preferable upper limit is 0.08 gf cm ２ / cm. If the sum of the bending rigidities is equal to or greater than the lower limit, the film has sufficient stiffness, which is preferable because it can suppress the occurrence of wrinkles and warping when the film is made thinner than the conventional thickness. In addition, since the bending rigidity is excellent, deformation of the film due to pressure from foreign objects or fingers can be suppressed even after it is mounted on a foldable display. From the viewpoint of manufacturing, the upper limit is 0.10 gf cm ２ The sum of the bending rigidities is considered to be 0.05 to 0.10 gf cm. ２ / cm, 0.06 to 0.10 gf cm ２ / cm, 0.07 to 0.10 gf cm ２ / cm, 0.05 to 0.09 gf cm ２ / cm, 0.06 to 0.09 gf cm ２ / cm, 0.07 to 0.09 gf cm ２ / cm, 0.05 to 0.08 gf cm ２ / cm, 0.06 to 0.08 gf cm ２ / cm, 0.07 to 0.08 gf cm ２ / cm.

[0050] The total light transmittance of the polyester film is preferably 85% or more, more preferably 87% or more. Similarly, the total light transmittance of a laminate film having an easy-adhesion layer and, optionally, a hard coat layer provided on the surface of the polyester film is preferably 85% or more, more preferably 87% or more. A total light transmittance of 85% or more for the polyester film and laminate film can ensure sufficient visibility of the foldable display. While a higher total light transmittance for the polyester film is preferable, from the standpoint of manufacturing costs and stable production, a total light transmittance of 99% or less is preferred, and 97% or less may be used. These lower and upper limits can be appropriately combined to form a numerical range. The total light transmittance of the polyester film and laminate film may be 85 to 97%, 85 to 99%, 87 to 97%, or 87 to 99%.

[0051] The surface of the film of the present invention may be smooth or uneven, but since it is used as a surface protection film, deterioration of optical properties due to unevenness is undesirable. The haze of the polyester film is preferably 3.5% or less, more preferably 3% or less, and most preferably 2% or less. Similarly, the haze of a laminate film having an easy-adhesion layer and an optional hard coat layer provided on the surface of the polyester film is preferably 3.5% or less, more preferably 3% or less, and most preferably 2% or less. When the haze of the polyester film and laminate film is 3.5% or less, image visibility can be improved. The lower the haze limit, the better, but from the perspective of stable production, a haze of 0.1% or more is preferable, and 0.3% or more is also acceptable. These lower and upper limits can be appropriately combined to form a numerical range. The haze of the polyester film and laminate film can be 0.1 to 3.5%, 0.1 to 3%, 0.1 to 2%, 0.3 to 3.5%, 0.3 to 3%, or 0.3 to 2%.

[0052] When the film of the present invention is heated at 150°C for 30 minutes, the heat shrinkage is preferably 20% or less in both the longitudinal and transverse directions, more preferably 10% or less in both directions, and even more preferably 7% or less. A heat shrinkage within these ranges is advantageous in that it suppresses poor appearance due to deformation of the film under high heat during post-processing, and suppresses flatness defects such as curling and waviness due to processing involving heating when mounting on a foldable display. While a low heat shrinkage is preferred, from a manufacturing standpoint, a lower limit of 0.01% is preferred. These lower and upper limits can be appropriately combined to form a numerical range. The heat shrinkage may be 0.01 to 20%, 0.01 to 10%, or 0.01 to 7%.

[0053] The longitudinal refractive index Nx and width refractive index Ny of the film of the present invention can be 1.50 or more, preferably 1.60 or more, more preferably 1.61 or more, and even more preferably 1.62 or more. The higher the refractive index, the more molecular chains are aligned in the stretching direction, providing sufficient mechanical strength and preventing problems such as elongation and misalignment during processing. Furthermore, since the mechanical strength is high in both the longitudinal and width directions even after installation in a foldable display, cracks can be prevented from occurring at the bends even when the film is bent in any direction, maintaining good display visibility. From a manufacturing standpoint, the upper limit is preferably 1.75. These lower and upper limits can be combined as appropriate to form a numerical range.

[0054] The refractive index Nx in the longitudinal direction is preferably 1.500 or more, more preferably 1.600 or more, and even more preferably 1.620 or more. The refractive index Ny in the width direction is preferably 1.600 or more, more preferably 1.610 or more. The refractive index Nz in the thickness direction is preferably 1.600 or less, more preferably 1.550 or less. The higher the refractive index in the longitudinal direction and width direction and the lower the refractive index in the thickness direction, the more the molecular chains are aligned in the stretching direction, the sufficient mechanical strength is achieved, and problems such as elongation and displacement during processing can be suppressed, which is preferable. Furthermore, since the mechanical strength is high even after installation in a foldable display, cracks can be suppressed at the bent portion regardless of the direction of folding, and good visibility of the display can be maintained. From a manufacturing perspective, the upper limit of the refractive index in the longitudinal direction and width direction is preferably 1.750. The lower limit of the refractive index Nz in the thickness direction is preferably 1.400. These lower and upper limits can be appropriately combined to form a numerical range. The refractive index Nx in the longitudinal direction may be 1.500 to 1.750, 1.600 to 1.750, or 1.620 to 1.750, the refractive index Ny in the width direction may be 1.600 to 1.750, or 1.610 to 1.750, and the refractive index Nz in the thickness direction may be 1.400 to 1.600, or 1.400 to 1.550.

[0055] The minimum bending radius at which deformation or cracking does not occur when the film of the present invention is continuously bent is preferably 5.0 mm, more preferably 3.0 mm. The smaller the minimum bending radius at which deformation or cracking does not occur, the less likely deformation or cracking will occur even after the film is mounted on a foldable display, and the better the visibility of the display can be maintained.

[0056] The long period of the film analyzed by small-angle X-ray scattering is preferably 10 nm or more, more preferably 12 nm or more, and even more preferably 15 nm or more. The longer the long period, the more the molecular chains connecting the amorphous structure and crystalline structure of the higher-order structure are extended, resulting in sufficient mechanical strength and suppressing problems such as elongation and displacement during processing, which is preferable. From a manufacturing standpoint, the upper limit is preferably 20 nm. These lower and upper limits can be appropriately combined to form a numerical range. The long period can be 10 to 20 nm, 12 to 20 nm, or 15 to 20 nm.

[0057] Regarding the crystals formed by the polymer molecules constituting the polyester film, the crystal size of the film analyzed by small-angle X-ray scattering is preferably 3 nm or more, more preferably 4 nm or more, and even more preferably 5 nm or more. The larger the crystal size, the larger the crystalline structure in the higher-order structure, which suppresses relaxation of the higher-order structure when the film is heated, resulting in sufficient thermal dimensional stability and suppressing deformation of the film under high heat during post-processing, which is preferable. From a manufacturing perspective, the upper limit is preferably 6 nm. These lower and upper limits can be appropriately combined to form a numerical range. The crystal size can be 3 to 6 nm, 4 to 6 nm, or 5 to 6 nm.

[0058] (Laminate film for foldable display) The laminate film of the present invention includes a substrate layer and an easy-adhesion layer. The substrate layer is preferably the polyester film of the present invention.

[0059] (Adhesion-Enhancing Layer) An adhesion-improving layer such as an adhesion-enhancing layer may be formed on at least one surface of the polyester film of the present invention. The presence of this layer improves the adhesion between the polyester film and other layers. The adhesion-enhancing layer can be obtained by applying a coating solution for forming the adhesion-enhancing layer, which will be described later, to one or both sides of an unstretched film or a uniaxially stretched film stretched in the machine direction, drying the film at 75 to 150°C, and further stretching the film in one or two directions. The final coating amount of the adhesion-enhancing layer is 0.05 to 0.20 g / m ２ It is preferable to control the coating amount to 0.05 g / m ２ On the other hand, when the coating amount is 0.20 g / m or more, the adhesiveness is satisfied, which is preferable. ２ If it is less than this, blocking resistance can be obtained, which is preferable.

[0060] Resins that can be contained in the coating liquid used to form the easy-adhesion layer include, without particular limitation, polyester-based resins, polyurethane-based resins, polyether polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. Examples of crosslinking agents that can be contained in the coating liquid for forming the easy-adhesion layer include melamine-based compounds, isocyanate-based compounds, oxazoline-based compounds, epoxy-based compounds, carbodiimide-based compounds, etc. Two or more of these resins or crosslinking agents can also be mixed and used.

[0061] It is preferable to add particles to the adhesion layer to impart lubricity. The average particle size of the particles is preferably 1.1 to 5.0 times the thickness of the adhesion layer. Having an average particle size within this range is advantageous in that the lubricity is improved and the particles are less likely to fall off during handling. The thickness of the adhesion layer is preferably 10 to 1000 nm, more preferably 20 to 500 nm, and even more preferably 50 to 200 nm.

[0062] Examples of particles to be contained in the easy-adhesion layer include inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, and calcium fluoride, and organic polymer particles such as styrene-based polymers, acrylic-based polymers, melamine-based polymers, benzoguanamine-based polymers, and silicone-based polymers. These may be added to the easy-adhesion layer alone, or two or more of them may be added in combination.

[0063] Examples of methods for applying the coating liquid for forming the easy-adhesion layer include reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, and pipe doctor coating, and these methods can be used alone or in combination.

[0064] (Hard Coat Layer) In the laminate film of the present invention, it is preferable that a hard coat layer is laminated on the easy-adhesion layer. In particular, when the laminate film is used as a surface protective film, it is preferable that a hard coat layer is laminated. When an easy-adhesion layer is laminated on one side of the polyester film, it is preferable that the hard coat layer is laminated on the easy-adhesion layer. When easy-adhesion layers are laminated on both sides of the polyester film, the hard coat layer may be laminated on only one of the easy-adhesion layers, or may be laminated on both of the easy-adhesion layers.

[0065] The resin for forming the hard coat layer may be any of (meth)acrylic acid ester, siloxane, inorganic hybrid, acrylate (urethane acrylate, polyester acrylate, etc.), epoxy, etc., and may be a mixture of two or more resins. Among these, (meth)acrylic acid ester and acrylate resins are preferred.

[0066] (Film Thickness) The film thickness of the hard coat layer is preferably 1 to 55 μm. If the film thickness is 1 μm or more, sufficient curing occurs and good pencil hardness is obtained. Furthermore, by making the thickness 55 μm or less, curling of the film due to cure shrinkage of the hard coat is suppressed, and the handling properties of the film can be improved.

[0067] (Coating Method) The coating method for the hard coat layer-forming coating solution can be any method, including a Mayer bar, gravure coater, die coater, knife coater, etc., and can be appropriately selected depending on the viscosity and film thickness. The drying temperature is preferably 60 to 100°C, more preferably 60 to 90°C. If the temperature is 60°C or higher, sufficient drying can be achieved in a short time, and if the temperature is 100°C or lower, deformation of the polyester film can be suppressed, which is preferable.

[0068] (Curing Conditions) The hard coat layer-forming coating liquid applied to the substrate layer can be cured by using energy rays such as ultraviolet rays and electron beams, or by heat.

[0069] (Pencil Hardness) The pencil hardness of the hard coat layer is preferably 2H or more, more preferably 3H or more, and particularly preferably 4H or more. A pencil hardness of 2H or more prevents the layer from easily deforming and does not reduce visibility. Generally, a high pencil hardness of the hard coat layer is preferable, but a hard coat layer having a pencil hardness of 10H or less, or 8H or less, or even 6H or less can be used without any practical problems. The pencil hardness of the hard coat layer may be 2H to 9H, 2H to 8H, 2H to 6H, 3H to 9H, 3H to 8H, or 3H to 6H.

[0070] (Type of hard coat layer) The hard coat layer in the present invention may have other functions added thereto, as long as it can be used for the purpose of increasing the pencil hardness of the surface and protecting the display as described above. For example, hard coat layers having the above-mentioned certain pencil hardness and added functionality such as an antiglare layer, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer are also preferably used in the present invention.

[0071] In another aspect, the present invention is a foldable display comprising the polyester film of the present invention or the laminate film of the present invention. The polyester film of the present invention and the laminate film of the present invention are preferably included in this foldable display as a surface protective film.

[0072] Next, the effects of the present invention will be described using examples and comparative examples. First, the evaluation methods of the characteristic values ​​used in the present invention will be described below.

[0073] (Evaluation Methods) (1) Breaking Strength The breaking strength and breaking elongation of polyester films and laminated films were measured in accordance with JIS-C-2318. A sample was cut into a strip shape with a length of 120 mm and a width of 10 mm in the MD and TD directions of the film using a single-edged razor. The strip sample was then subjected to a test at a speed of 5 mm / min with a chuck distance of 100 mm using an Autograph AG-IS-Loadcell.1KN manufactured by Shimadzu Corporation, and the breaking strength (MPa) in each direction was determined from the load-strain curve.

[0074] (2) Tensile Modulus The tensile modulus of polyester films and laminated films was measured in accordance with JIS-K-7161 and ISO 527-1 and 527-3. A sample was cut into a strip of 200 mm in length and 15 mm in width in the longitudinal and transverse directions of the film using a single-edged razor, and two parallel benchmark lines 50 mm apart were marked in the center of the test piece. Next, the tensile modulus in each direction was determined from the load-strain curve obtained from the strip sample at a test speed of 0.5 mm / min, a chuck distance of 100 mm, and a 0.1-0.3% strain with a 50 mm gap between the benchmark lines using an autograph AGS-IS-Loadcell.1KN manufactured by Shimadzu Corporation.

[0075] (3) Bending rigidity (15 μm thickness equivalent) The bending rigidity of polyester film and laminated film is determined based on the constant curvature (maximum curvature K = ±2.5 cm) in accordance with pure bending theory. －１ ) was applied, and the bending moment (rigidity) was measured automatically. The film was cut into a length of 100 mm in the stretching direction and 150 mm in the perpendicular direction. Next, automatic measurement was performed using a pure bending tester (model number: KES-FB2A) manufactured by Kato Tech Co., Ltd., and the bending rigidity in the MD and TD directions of the film was calculated. An overview of the pure bending tester is shown in Figure 3. The evaluation method in all directions was to determine the bending rigidity at 5° intervals using the measurement method described above, with the longitudinal direction set to 0° and the width direction set to 90°. The minimum value was taken as Fa and the maximum value as Fb. The bending rigidity converted to a thickness of 15 μm was obtained from the following formula. Bending rigidity measurement value (gf cm ２ / cm) × 15 (μm) / thickness of the film to be measured (μm) For example, in the case of Example 4, the film thickness was 25 μm, and the bending rigidity in the MD direction obtained by the above measurement method was 0.080 gf cm ２ / cm, the bending rigidity in the MD direction converted to a thickness of 15 μm is calculated to be 0.048 from the formula 0.080×15 / 25.

[0076] (4) Total Light Transmittance, Haze The haze of the polyester film and the laminated film was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., model NDH5000) in accordance with JIS-7136.

[0077] (5) Bending resistance (bending radius 3.0 mm) A polyester film and a laminate film measuring 20 mm in width and 110 mm in length (the longitudinal direction is the bending direction in the foldable display, and the width direction is the folding direction in the foldable display) and a polyester film and a laminate film measuring 20 mm in width and 110 mm in length (the longitudinal direction is the folding direction in the foldable display, and the width direction is the bending direction in the foldable display) were prepared as samples. Note that the bending direction refers to the direction perpendicular to the folding portion (reference numeral 21) expected in the application of a folding display, as shown by reference numeral 22 on the polyester film (reference numeral 2) in FIG. 2. This film was bent 200,000 times at a bending radius of 3.0 mm using a no-load U-shaped stretch tester (manufactured by Yuasa System Co., Ltd., DLDMLH-FS) at a rate of 1 time per second. After the bending process was completed, the sample was placed on a flat surface with the inner surface of the bend facing down, visually observed, and evaluated according to the following criteria. ◯: No cracks or deformations were observed in the sample. ×: The sample had cracks or creases, or the maximum height of the sample rising from the flat surface was 5 mm or more.

[0078] (6) 150°C Heat Shrinkage The 150°C heat shrinkage was measured in accordance with JIS-2318-1997-5.3.4 (Dimensional Change). When the measurement direction was the transverse direction (TD), polyester film and laminate film samples measuring 190 mm in the transverse direction and 10 mm in the longitudinal direction were prepared. Two marks were then made on the surface of this film, spaced 50 mm apart (100 mm between the marks) from the center to the left and right in the transverse direction, and the distance (A) between the marks was measured. The film was then placed in an oven in a 150°C atmosphere and heat-treated at 150°C ± 3°C for 30 minutes under no load, after which the distance (B) between the marks was measured. The 150°C heat shrinkage was calculated using the following formula: Heat Shrinkage (%) = [(A - B) / A] x 100. On the other hand, when the measurement direction was the longitudinal direction (MD), polyester film and laminate film samples measuring 190 mm in the longitudinal direction and 10 mm in the transverse direction were prepared. Next, two marks were made on the surface of this film at an interval of 50 mm (100 mm between the marks) from the center in the longitudinal direction, and the distance (C) between the marks was measured. The film was then placed in an oven in an atmosphere of 150°C and heat-treated at 150°C ± 3°C for 30 minutes under no load, after which the distance (D) between the marks was measured. The heat shrinkage at 150°C was calculated using the following formula: Heat shrinkage (%) = [(C - D) / C] x 100

[0079] (7) Refractive Index Measurement was performed in accordance with JIS-K-7142. The refractive index was measured using an Abbe refractometer with NaD line light. Methylene iodide was used as the mounting liquid, and the refractive index in the longitudinal direction (Nx), the width direction (Ny), and the thickness direction (Nz) were measured. The measurement was performed at the center of the film width direction.

[0080] (8) Long Period The long period was calculated from the diffraction intensity profile obtained by small-angle X-ray scattering (SAXS) using an X-ray diffractometer. For X-ray evaluation, film samples were cut into 20-30 mm lengths in the multi-stage stretching direction and 8 mm lengths perpendicular to this direction, and several sheets were stacked to a thickness of approximately 100 μm. Silver behenate was used as a standard calibration sample. Using the beamline (BL03XU) at the SPring-8 synchrotron radiation facility, X-rays were incident from the normal direction to the film surface with a wavelength of 1.2 A and a camera length of 1700 mm, and small-angle X-ray scattering (SAXS) measurements were performed. The long period (Lp) was calculated using the analysis software Fit2D and Fityk. The profile I(q) (I: scattering intensity, q: scattering vector) was determined from the diffraction image obtained by X-ray analysis, and the long period (d) was calculated using the following formula:

[0081] (9) Crystal size Using the profile qI(q) obtained by small-angle X-ray diffraction described in (8) above, a Fourier transform is performed using the following formula to obtain the correlation function γ(r) (r = distance), thereby determining the crystal size (r ｃ ) was calculated. Hereinafter, the crystal size (r ｃ As shown in FIG. 4, the smallest r １ The minimum value at γ(r １ ) = γ ｍｉｎ１ ...(A). Next, 0≦r≦r １ In this case, the r at which the second derivative of y = γ(r) becomes 0 is the inflection point r of γ(r). ２ Let r ２ Let the tangent to γ(r) at (A) be y = ar + b...(B). At the intersection of equations (A) and (B), r = r c is the desired crystal size, and γ min１ =ar c +b to r c =(γ min1 -b) / a, r ｃ was calculated.

[0082] (10) Color Tone (b Value) The color tone (b value) was measured using a colorimeter (ZE2000, manufactured by Nippon Denshoku Co., Ltd.) in accordance with JIS K 7373-2006 "Plastics - Determination of yellowness and discoloration index."

[0083] (11) UV Absorption Performance A film sample measuring 8 mm in the transverse direction (TD) × 40 mm in the machine direction (MD) was prepared and placed so that the machine direction (MD) was parallel to the direction of gravity. The light transmittance at a wavelength of 380 nm was measured using a spectrophotometer (Shimadzu Corporation, UV1800).

[0084] (Raw Materials for Coating Solution for Forming Adhesion Layer: Polymerization of Urethane Resin) A four-necked flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer was prepared. 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dimethylolpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as solvents were added to the four-necked flask. These were then stirred in a nitrogen atmosphere at 75°C for 3 hours in the four-necked flask, and it was confirmed that the reaction solution in the four-necked flask had reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisper capable of high-speed stirring, and the temperature was adjusted to 25°C. While stirring and mixing, the isocyanate group-terminated prepolymer was added and dispersed in water. Thereafter, acetonitrile and a portion of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) with a solids content of 35% by mass.

[0085] (Raw Materials for Coating Solution for Forming Adhesion Layer: Polymerization of Water-Soluble Carbodiimide Compound) A flask equipped with a thermometer, nitrogen gas inlet tube, reflux condenser, dropping funnel, and stirrer was prepared. 200 parts by mass of isophorone diisocyanate and 4 parts by mass of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were added to the flask and stirred at 180°C for 10 hours under a nitrogen atmosphere to obtain an isocyanate-terminated isophorone carbodiimide (degree of polymerization = 5). Next, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was gradually added to the mixture at 50°C to obtain a yellow, transparent, water-soluble carbodiimide compound (B) with a solids content of 40% by mass.

[0086] (Preparation of coating liquid for forming easy-adhesion layer) The following components were mixed to prepare a coating liquid for forming an easy-adhesion layer: Water 16.97 parts by mass Isopropanol 21.96 parts by mass Water-soluble polyurethane resin (A) 3.27 parts by mass Water-soluble carbodiimide compound (B) 1.22 parts by mass Particles 0.51 parts by mass (silica sol with an average particle size of 40 nm, solid content concentration 40% by mass) Surfactant 0.05 parts by mass (silicone-based, solid content concentration 100% by mass)

[0087] (Preparation Example of Coating Solution 1 for Forming Hard Coat Layer) 95 parts by weight of pentaerythritol triacrylate (A-TMM-3, manufactured by Shin-Nakamura Chemical Co., Ltd., solid content concentration 100%), 5 parts by weight of a photopolymerization initiator (Irgacure (registered trademark) 907, manufactured by BASF Japan, solid content concentration 100%), and 0.1 parts by weight of a leveling agent (BYK307, manufactured by BYK Japan, solid content concentration 100%) were mixed and diluted with a solvent of toluene / MEK=1 / 1 to prepare coating solution 1 with a concentration of 40%.

[0088] (Preparation Example of Coating Solution 2 for Forming Hard Coat Layer) 50 parts by weight of urethane acrylate (Kyoeisha Chemical Co., Ltd., UA-306H, solids concentration 100%), 45 parts by weight of pentaerythritol triacrylate (Shin-Nakamura Chemical Co., Ltd., A-TMM-3, solids concentration 100%), 5 parts by weight of photopolymerization initiator (BASF Japan Ltd., Irgacure (registered trademark) 907, solids concentration 100%), and 0.1 parts by weight of leveling agent (BYK Japan KK, BYK307, solids concentration 100%) were mixed and diluted with a solvent of toluene / MEK = 1 / 1. This prepared coating solution 2 with a concentration of 40%.

[0089] (Preparation of polyethylene terephthalate pellets) A continuous esterification reactor was used as the esterification reactor, consisting of a three-stage complete mixing vessel equipped with an agitator, a partial condenser, a raw material inlet, and a product outlet. Terephthalic acid (TPA) was supplied at a rate of 2 ton / hr, ethylene glycol (EG) was supplied at 2 moles per mole of TPA, and antimony trioxide was supplied in an amount such that the Sb atom concentration in the resulting PET was 160 ppm. These slurries were continuously supplied to the first esterification reactor of the esterification reactor, and reacted at atmospheric pressure for an average residence time of 4 hours at 255°C. The reaction product in the first esterification reactor was then continuously removed from the system and supplied to the second esterification reactor. Furthermore, EG distilled off from the first esterification reactor was supplied to the second esterification reactor in an amount of 8 mass % relative to the produced polymer (produced PET), and an EG solution containing magnesium acetate in an amount such that the Mg atoms would be 65 ppm relative to the produced PET and an EG solution containing trimethyl phosphate (TMPA) in an amount such that the P atoms would be 20 ppm relative to the produced PET were further added, and the reaction was carried out at atmospheric pressure for an average residence time of 1.5 hours at 260° C. Next, the reaction product in the second esterification reactor was continuously removed from the system and supplied to a third esterification reactor, to which was further added an EG solution containing TMPA in an amount such that the P atoms would be 20 ppm relative to the produced PET, and the reaction was carried out at atmospheric pressure for an average residence time of 0.5 hours at 260° C. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor to carry out polycondensation, and the resulting mixture was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets having an intrinsic viscosity of 0.60 dl / g.

[0090] Example 1 Polyethylene terephthalate pellets (Tg 78°C) were dried under reduced pressure (3 Torr) at 135°C for 8 hours. The polyethylene terephthalate pellets (hereinafter sometimes referred to as PET pellets) were then mixed with ultraviolet-absorbing pellets (UN101 (manufactured by Nippon Pigment Co., Ltd.)), fed to an extruder, and melted at 285°C. The ultraviolet-absorbing pellets were composed primarily of polyethylene terephthalate and contained an organic ultraviolet absorber. The raw material fed to the extruder was melt-extruded into a sheet from the T-die, with the resin temperatures of the extruder's melting section, kneading section, polymer tube, gear pump, polymer tube, and T-die set to 280°C. A sheet was then produced using a cooling roll at 20°C. The sheet was stretched 3.0 times in the longitudinal direction (MD) using a heating roll (105°C). The coating solution for forming an easy-adhesion layer was applied to one side of a uniaxially stretched film by roll coating, and then dried at 80°C for 20 seconds to obtain a uniaxially stretched film having an easy-adhesion layer on one side. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2 The film was then introduced into a tenter, preheated at 90°C, and stretched 3.0 times in the width direction (TD) at 100°C to produce a film. The film was then introduced into the tenter again, preheated at 120°C, and stretched 1.39 times in the width direction at 140°C to produce a film. The film was then introduced into the tenter again, preheated at 140°C, and stretched 1.39 times in the width direction at 170°C to produce a film. The film was then introduced into the tenter again, preheated at 170°C, and stretched 1.39 times in the width direction at 250°C to produce a film. The film was then introduced into a heat treatment zone adjusted to 100°C, and heat-set by flowing the film through this zone for 12 seconds to produce a biaxially stretched polyethylene terephthalate film with a thickness of 21 μm. The amount of the organic ultraviolet absorber blended here was 2% by mass relative to 100% by mass of the biaxially stretched film.

[0091] (Example 2) A sheet was produced in the same manner as in Example 1. The sheet was then stretched 3.4 times in the longitudinal direction using a heated roll (105°C). A uniaxially stretched film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. The uniaxially stretched film was then introduced into a tenter, preheated at 90°C, and stretched 1.68 times in the width direction at 100°C to produce a film. The film was again introduced into the tenter, preheated to 120°C, and stretched 1.68 times in the width direction at 140°C to produce a film. The film was further introduced into the tenter again, preheated to 140°C, and stretched 1.68 times in the width direction at 170°C to produce a film. The film was then introduced into the tenter again, preheated at 170°C, and stretched 1.58 times in the width direction at 250°C, followed by heat setting in the same manner as in Example 1 to obtain a biaxially stretched film with a thickness of 23 μm.

[0092] (Example 3) A uniaxially stretched film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. Next, this uniaxially stretched film was introduced into a tenter, preheated at 90 ° C., and stretched 3.0 times in the width direction at 100 ° C. to produce a film. The film was again introduced into the tenter, preheated to 120 ° C., and stretched 1.39 times in the width direction at 140 ° C. to produce a film. The film was then introduced into the tenter again, preheated to 140 ° C., and stretched 1.39 times in the width direction at 170 ° C. to produce a film. The film was then introduced into the tenter again, preheated at 170 ° C., and stretched 1.47 times in the width direction at 250 ° C., and then subjected to the same heat setting treatment as in Example 1 to obtain a biaxially stretched film having a thickness of 21 μm.

[0093] (Example 4) A sheet was produced in the same manner as in Example 1. Next, a heating furnace capable of simultaneously stretching in the longitudinal direction and the width direction was prepared, and the sheet was stretched in the first zone (90°C) of the heating furnace to 4.0 times in the longitudinal direction and 2.0 times in the width direction. The coating liquid for forming an easy-adhesion layer was applied to one side of a biaxially stretched film by a roll coating method, and then dried at 80°C for 20 seconds to obtain a biaxially stretched film having an easy-adhesion layer on one side. The final coating amount after drying (after biaxial stretching) was 0.06 g / m 2The biaxially stretched film was then stretched 1.5 times in the longitudinal direction in zone 2 of a heating furnace (240°C), thereby obtaining a biaxially stretched film with a thickness of 25 µm.

[0094] (Example 5) Biaxially stretched films were produced in the same manner as in Example 1, except that the mixing ratio of UV-absorbing pellets to PET pellets was changed as shown in Table 1 and the thickness was changed to 50 μm. All of the films in the above examples had a total light transmittance of 85% or more and a haze of 3% or less. Therefore, all of the films in the above examples are suitable for display applications.

[0095] (Comparative Example 1) A uniaxially stretched film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1. Next, this uniaxially stretched film was introduced into a tenter, preheated at 90°C, stretched 4.0 times in the width direction at 100°C, and then heat-set at 230°C to obtain a biaxially stretched film with a thickness of 25 μm.

[0096] (Comparative Example 2) A uniaxially stretched film having an easy-adhesion layer on one side was obtained in the same manner as in Example 1, except that the mixing ratio of the ultraviolet-absorbing pellets to the PET pellets was changed to 0%. Next, this uniaxially stretched film was introduced into a tenter, preheated at 90°C, stretched 4.0 times in the width direction at 100°C, and then heat-set at 230°C to obtain a biaxially stretched polyethylene terephthalate film having a thickness of 25 μm.

[0097]

[0098] The films of Examples 1 to 5 and Comparative Examples 1 and 2 were subjected to various tests, and the results are shown in Table 2.

[0099] The film of the present invention has a high tensile modulus and high bending rigidity, so that even when it is thin, it can maintain a sufficient stiffness and suppress scratches and deformation caused by foreign matter. Furthermore, in one embodiment of the film of the present invention, deterioration of organic EL modules due to ultraviolet rays can be suppressed. The film of the present invention can be used in a wide range of industrial film fields, including surface protection applications for foldable displays, which require high quality, including the above.

Claims

1. The sum of the longitudinal bending rigidity Fa and the widthwise bending rigidity Fb converted to a thickness of 15 μm is 0.05 gf cm ２ / cm or more.

2. The polyethylene terephthalate film according to claim 1, wherein the size of the crystals formed by the polymer molecules constituting the film is 3.0 nm or more as determined by small angle X-ray scattering.

3. The polyethylene terephthalate film according to claim 1, wherein the bending rigidity in the longitudinal direction is greater than the bending rigidity in the transverse direction.

4. The polyethylene terephthalate film according to claim 1, characterized in that the total light transmittance is 85% or more and the haze is 3% or less.

5. The polyethylene terephthalate film according to claim 1, characterized in that the transmittance of light having a wavelength of 380 nm is 20% or less and the b value is 3.5 or less.

6. A laminate film for a foldable display comprising a base layer and an easy-adhesion layer, the base layer being a polyethylene terephthalate film according to any one of claims 1 to 5, and the easy-adhesion layer being laminated on at least one surface of the base layer.

7. The laminate film according to claim 6, wherein a hard coat layer is laminated on the adhesive layer.

8. The laminated film according to claim 6, characterized in that the total light transmittance is 85% or more and the haze is 3% or less.

9. A foldable display comprising the polyethylene terephthalate film according to any one of claims 1 to 5 as a surface protective film.

10. A foldable display comprising the laminate film according to claim 6 as a surface protective film.

11. A mobile terminal device comprising the foldable display according to claim 9.

12. A mobile terminal device comprising the foldable display according to claim 10.

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