Polyester film, polarizer protective film, polarizing plate, and image display device

Abstract

The present invention provides: a polyester film which can cope with an increase in size of an image display device, has excellent visibility at each place in a screen (in particular, can suppress deterioration in visibility due to iridescent speckles at an end part), and suppresses deterioration of a display when used outdoors or the like and exposed to ultraviolet rays; and the like. The polyester of the polyester film is mainly polyethylene terephthalate, the slow axis direction of the polyester film is generally parallel to the MD direction, the in-plane birefringence ΔNxy of the polyester film is 0.06 to 0.2 inclusive, the refractive index of the polyester film in the fast axis direction is 1.58 to 1.63 inclusive, the ultraviolet transmittance of the polyester film at 380 nm is 0% to 30% inclusive, and the ratio of the polarized light transmittance of the polyester film between the MD direction and the TD direction is 0.95 or less.

Description

Polyester film, polarizer protective film, polarizing plate, and image display device

[0001] The present invention relates to a polyester film, a polarizer protective film, a polarizing plate, and an image display device (such as a liquid crystal display device or an organic EL display device).

[0002] Polarizing plates used in liquid crystal display devices (LCDs) typically consist of a polarizer made of polyvinyl alcohol (PVA) or other materials dyed with iodine sandwiched between two polarizer protective films, with triacetyl cellulose (TAC) films typically used as the polarizer protective films. In recent years, thinner polarizing plates have been required as LCDs become thinner and their costs are reduced. However, reducing the thickness of the TAC film used as the protective film for this purpose can result in problems such as insufficient mechanical strength and reduced moisture permeability. Furthermore, TAC films are very expensive, and there is a strong demand for cheaper alternative materials. Furthermore, the use of digital signage, such as LCDs and organic electroluminescence (EL) displays, as outdoor information display devices has been increasing in recent years, requiring UV resistance.

[0003] Although polyester films have superior durability compared to TAC films, unlike TAC films, they have birefringence, which causes a problem of image quality degradation due to optical distortion when used as a polarizer protective film. Specifically, since birefringent polyester films have a predetermined optical anisotropy (retardation), when used as a polarizer protective film, rainbow-like color spots appear when observed from an oblique direction, resulting in degradation of image quality. Therefore, Patent Document 1 addresses this issue by controlling the in-plane retardation of the polyester film within a specific range. Patent Document 1 also discloses a measure to suppress UV degradation of polarizers, such as polyvinyl alcohol (PVA) and iodine, by incorporating an ultraviolet absorber into the polyester film.

[0004] WO2011-162198

[0005] An object of the present invention in one embodiment is to provide a polyester film, a polarizer protective film, a polarizing plate, and an image display device (such as a liquid crystal display device or an organic EL display) that can accommodate larger image display devices such as liquid crystal display devices or organic EL displays (i.e., that have sufficient mechanical strength) and that has excellent visibility at various locations on the screen (especially capable of suppressing deterioration of visibility due to rainbow-like color spots at the edges). Preferably, the object is to provide a polyester film, a polarizer protective film, a polarizing plate, and an image display device (such as a liquid crystal display device or an organic EL display) that further suppresses deterioration of the display when used outdoors or otherwise exposed to ultraviolet rays.

[0006] As a result of extensive research, the present inventors have found that by devising the stretching conditions, etc., it is possible to provide a polyester film, a polarizer protective film, a polarizing plate, and an image display device that can accommodate larger image display devices and have excellent visibility at various locations on the screen (rainbow-like color spots can be suppressed, particularly at the edges), and that it is possible to provide a polyester film, a polarizer protective film, a polarizing plate, and an image display device that further suppresses deterioration of the display when used outdoors or otherwise exposed to ultraviolet rays.Further research led to the completion of the present invention.

[0007] The present invention includes the following embodiments. [Item 1] A polyester film, wherein the polyester of the polyester film is mainly polyethylene terephthalate, the slow axis direction of the polyester film is approximately parallel to the MD direction, the in-plane birefringence ΔNxy of the polyester film is 0.06 to 0.2, the refractive index of the polyester film in the fast axis direction is 1.58 to 1.63, the ultraviolet transmittance of the polyester film at 380 nm is 0% to 30%, and the ratio of the polarized light transmittance in the MD direction to the TD direction of the polyester film is 0.95 or less. [Item 2] The polyester film according to Item 1, wherein the length in the TD direction of the polyester film is 1 m or more, and the difference between the maximum and minimum values ​​of ΔNxy in the TD direction of the polyester film is 0.013 or less. [Item 3] The polyester film according to Item 1 or 2, wherein the NZ coefficient of the polyester film is 1.5 to 2.5. [Item 4] The polyester film according to any one of Items 1 to 3, wherein the thickness of the polyester film is 15 μm or more and 60 μm or less. [Item 5] The polyester film according to any one of Items 1 to 4, wherein the angle between the slow axis direction of the polyester film and the MD direction is 10 degrees or less. [Item 6] The polyester film according to any one of Items 1 to 5, wherein the elastic modulus of the polyester film in the MD direction is 3000 MPa or more. [Item 7] The polyester film according to any one of Items 1 to 6, wherein the retardation of the polyester film is 700 nm or more and 8000 nm or less. [Item 8] The polyester film according to any one of Items 1 to 7, wherein the polyester film is a polarizer protective film. [Item 9] A polarizing plate comprising the polyester film according to any one of Items 1 to 8 laminated on at least one surface of a polarizer, and wherein the angle between the absorption axis direction of the polarizer and the MD direction of the polyester film is 10 degrees or less. [Item 10] A polarizing plate comprising the polyester film according to any one of Items 1 to 8 laminated on one surface of a polarizer, and no film laminated on the other surface of the polarizer.[Item 11] A polarizing plate having the polyester film according to any one of items 1 to 8 laminated on one side of a polarizer and a quarter-wave plate laminated on the other side of the polarizer. [Item 12] An image display device comprising the polarizing plate according to any one of items 9 to 11 and a light source. [Item 13] The image display device according to item 12, which is a liquid crystal display device. [Item 14] The image display device according to item 12, which is an organic EL display. [Item 15] The image display device according to item 12, which is a QLED display.

[0008] The polyester film, polarizer protective film, polarizing plate, and image display device (such as a liquid crystal display device or an organic EL display) of the present invention have excellent uniformity over a wide width, for example, and therefore can provide good visibility at every location on the screen when used in applications involving a large area (especially at the edges, rainbow-like color spots (hereinafter also referred to as "rainbow spots") are suppressed, ensuring good visibility). Furthermore, according to the present invention, deterioration of the display can be suppressed when used outdoors, for example, where the display is exposed to ultraviolet rays. Furthermore, according to the present invention, the film has mechanical strength suitable for thinning, ensuring good processability. Furthermore, according to the present invention, deterioration of visibility due to rainbow spots can be significantly suppressed even when the film is thinned.

[0009] In one embodiment, the polyester film of the present invention preferably satisfies the following (1) to (4): (1) The polyester of the polyester film is mainly polyethylene terephthalate. (2) The slow axis direction of the polyester film is approximately parallel to the MD direction. (3) The in-plane birefringence ΔNxy of the polyester film is 0.06 or more and 0.2 or less. (4) The refractive index of the polyester film in the fast axis direction is 1.58 or more and 1.63 or less.

[0010] In one embodiment, the polyester film of the present invention preferably satisfies the following (5) and (6) in addition to the above (1) to (4): (5) The polyester film has an ultraviolet transmittance at 380 nm of 0% or more and 30% or less, and (6) The ratio of the polarized light transmittance in the MD direction to the TD direction of the polyester film is 0.95 or less.

[0011] In one embodiment, the polyester film of the present invention preferably satisfies the following (7) and (8) in addition to the above (1) to (4): (7) The length of the polyester film in the TD direction is 1 m or more, and (8) The difference between the maximum and minimum values ​​of ΔNxy in the TD direction of the polyester film is 0.013 or less.

[0012] In one embodiment, the polyester film of the present invention preferably satisfies the above (1) to (8).

[0013] The polyester of the polyester film of the present invention is preferably primarily polyethylene terephthalate (PET). Here, "primarily" refers to, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and particularly preferably 99% by mass or more, based on 100% by mass of polyester. The polyester may consist solely of PET (100% by mass of PET), or the PET content may be less than 100% by mass, 99.9% by mass or less, 99.5 parts by mass or less, or 99 parts by mass or less. The PET is not particularly limited as long as it contains terephthalic acid and ethylene glycol as polymerization components, and may contain any copolymerization component. When the total acid components and the total glycol components in the polyester are taken as 100 mol%, the total amount of copolymerization components is preferably 20 mol% or less, more preferably 10 mol% or less, even more preferably 5 mol% or less, and particularly preferably 3 mol% or less. PET has excellent transparency, thermal and mechanical properties, and a large intrinsic birefringence, and its in-plane birefringence can be easily controlled by stretching. Examples of polyesters other than PET include polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polylactic acid (PLA), and copolymer polyesters containing these as main components.

[0014] The slow axis of the polyester film of the present invention is preferably approximately parallel to the MD direction from the viewpoint of suppressing iridescence. The MD direction refers to the running direction (flow direction) during film production and is sometimes referred to as the machine direction. The MD direction is a direction perpendicular to the TD direction. The TD direction refers to the width direction during film production and is sometimes referred to as the transverse direction. "Approximately parallel" means that the angle between the slow axis direction of the polyester film and the MD direction is preferably within 10 degrees, more preferably within 7 degrees, even more preferably within 5 degrees, still more preferably within 3 degrees, and particularly preferably within 2 degrees or 1 degree. The slow axis direction of the polyester film can be determined using a conventional molecular orientation meter (e.g., MOA-6004 Molecular Orientation Meter manufactured by Oji Scientific Instruments Co., Ltd.).

[0015] The in-plane birefringence ΔNxy of the polyester film of the present invention is preferably 0.06 or more, more preferably 0.065 or more, even more preferably 0.07 or more, even more preferably 0.075 or more, and particularly preferably 0.08 or more, from the viewpoint of suppressing iridescence (especially iridescence from an oblique direction). ΔNxy is preferably 0.2 or less, more preferably 0.19 or less, and even more preferably 0.18 or less, from the viewpoint of maintaining strength suitable for protective applications. An even smaller upper limit of ΔNxy is also preferred, and ΔNxy may be, for example, 0.17 or less, 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.1 or less. ΔNxy refers to the absolute value of the difference between the refractive index (nx) in the slow axis direction and the refractive index (ny) in the fast axis direction. In this specification, the wavelength for measuring refractive index is 589 nm.

[0016] The length in the TD direction of the polyester film of the present invention is preferably 1 m or more, more preferably 1.1 m or more, and even more preferably 1.2 m or more. The length in the TD direction is preferably 4 m or less, more preferably 3.5 m or less, even more preferably 3 m or less, and even more preferably 2.7 m or less. In one embodiment, the length in the TD direction is 1 m or more and 4 m or less.

[0017] The difference between the maximum and minimum values ​​of ΔNxy in the TD of the polyester film of the present invention (maximum value - minimum value) is preferably 0.013 or less, more preferably 0.0125 or less, and even more preferably 0.012 or less, from the viewpoint of visibility at various locations on the screen of a large-area display (particularly, suppression of iridescence at the edges). The difference between the maximum and minimum values ​​of ΔNxy in the TD is preferably 0 or more, more preferably 0.001 or more, and even more preferably 0.002 or more, from the viewpoint of stable film formation.

[0018] The refractive index (ny) in the fast axis direction of the polyester film of the present invention is preferably 1.58 or more, more preferably 1.584 or more, even more preferably 1.585 or more, still more preferably 1.588 or more, and particularly preferably 1.59 or more, from the viewpoint of maintaining strength suitable for protective applications, etc. ny is preferably 1.63 or less, more preferably 1.625 or less, even more preferably 1.62 or less, still more preferably 1.615 or less, and particularly preferably 1.61 or less, from the viewpoint of suppressing iridescence (particularly iridescence from an oblique direction). In one embodiment, ny is 1.58 or more and 1.63 or less.

[0019] The refractive index (nx) in the slow axis direction of the polyester film of the present invention is preferably 1.66 or more, more preferably 1.664 or more, even more preferably 1.665 or more, and still more preferably 1.668 or more, from the viewpoint of adjusting ny within a desired range to suppress iridescence (particularly iridescence from an oblique direction). From the viewpoint of adjusting ny within a desired range to maintain strength suitable for protective applications, etc., nx is preferably 1.72 or less, more preferably 1.716 or less, even more preferably 1.715 or less, and still more preferably 1.712 or less. In one embodiment, nx is 1.66 or more and 1.72 or less.

[0020] The refractive index (nz) in the thickness direction of the polyester film of the present invention is preferably 1.5 or more, more preferably 1.505 or more, and even more preferably 1.51 or more. nz is preferably 1.56 or less, more preferably 1.555 or less, and even more preferably 1.55 or less. In one embodiment, nz is 1.5 or more and 1.56 or less.

[0021] The upper limit of the UV transmittance at 380 nm of the polyester film of the present invention is preferably 30% or less in order to suppress deterioration of optically functional dyes such as iodine-based dyes. The lower limit of the UV transmittance is preferably 0% or more, more preferably 0.1% or more. The upper limit of the UV transmittance is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less. These upper and lower limits can be appropriately combined, and the UV transmittance is preferably, for example, 0% or more and 30% or less. If the upper limit of the UV transmittance is 30% or less, deterioration of the optically functional dye due to UV rays can be suppressed. The UV transmittance in the present invention is measured in a direction perpendicular to the plane of the polyester film. The UV transmittance can be measured, for example, using a spectrophotometer (e.g., UV1800 manufactured by Shimadzu Corporation).

[0022] The polyester film of the present invention preferably contains an ultraviolet absorber, and the content of the ultraviolet absorber in the polyester film of the present invention is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.2% by mass or more and 8% by mass or less, even more preferably 0.5% by mass or more and 5% by mass or less, still more preferably 0.8% by mass or more and 4% by mass or less, and particularly preferably 1% by mass or more and 2% by mass or less.

[0023] Examples of the ultraviolet absorber 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, and preferably triazine-based ultraviolet absorbers and benzotriazole-based ultraviolet absorbers. Specific examples of the ultraviolet absorber that is not adversely affected by heat during polyester film production include the following ultraviolet absorbers: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-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-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.

[0024] The lower limit of the ratio R of the polarized light transmittance in the MD direction to the TD direction of the polyester film of the present invention is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, even more preferably 0.75 or more, and particularly preferably 0.8 or more. The upper limit of the ratio R is preferably 0.99 or less, more preferably 0.97 or less, even more preferably 0.96 or less, even more preferably 0.95 or less, and particularly preferably 0.93 or less. These upper and lower limits can be appropriately combined, and the ratio R is, for example, preferably 0.5 or more to 0.99 or less, more preferably 0.6 or more to 0.97 or less, even more preferably 0.7 or more to 0.96 or less, even more preferably 0.75 or more to 0.95 or less, and particularly preferably 0.8 or more to 0.93 or less. The polarized light transmittance is expressed using the polarized light transmittance of two axes perpendicular to each other in the longitudinal direction (MD direction) and the width direction (TD direction) at a wavelength of 390 nm, at which the total light transmittance is 30%. The polarized transmittance can be measured using an ultraviolet-visible spectrophotometer (UV1800 manufactured by Shimadzu Corporation) with a measurement wavelength range of 190 to 1100 nm. The polarized transmittance in each direction can be measured by inserting a polarizing plate (SHC-YL38 manufactured by Nippon Kayaku Co., Ltd.) that transmits ultraviolet wavelengths into the detector side of the spectrophotometer, and measuring the light transmitted through the polarizing plate. In this case, to eliminate the influence of the polarizing plate direction, it is preferable to measure the polarized transmitted light in the longitudinal direction, and then rotate the film 90° to extract the polarized transmitted light in the width direction.

[0025] The UV absorber is oriented in the stretching direction as the polyester film is stretched, and exhibits anisotropy in its transmittance. In the polyester film of the present invention, by orienting it in the MD direction, the molecular chains of the UV absorber are also oriented in the MD direction, and the absorption ability in the MD direction can be increased. When the absorption axis direction of the polarizer is the MD direction, such as in roll-to-roll lamination, the polyester film of the present invention absorbs more UV light in the MD direction, thereby suppressing deterioration of the polarizer.

[0026] From the above viewpoints, it is preferable that the slow axis direction of the polyester film of the present invention is substantially parallel to the MD direction of the polyester film, and that the absorption axis direction of the polarizer is also substantially parallel to the MD direction of the polyester film. The angle between the slow axis direction of the polyester film and the MD direction is preferably 10 degrees or less, more preferably 7 degrees or less, even more preferably 5 degrees or less, still more preferably 3 degrees or less, and particularly preferably 2 degrees or less or 1 degree or less. The angle between the absorption axis direction of the polarizer and the MD direction of the polyester film is preferably 10 degrees or less, more preferably 7 degrees or less, even more preferably 5 degrees or less, still more preferably 3 degrees or less, and particularly preferably 2 degrees or less or 1 degree or less.

[0027] From the viewpoint of exhibiting anisotropy, the angle between the slow axis direction and the MD direction of the polyester film of the present invention is preferably 10 degrees or less, and when it is 5 degrees or less, the anisotropy is exhibited more effectively. Furthermore, from the viewpoint of suppressing deterioration of the polarizer, the film is preferably attached so that the absorption axis direction of the polarizer is 10 degrees or less with respect to the MD direction of the polyester film, and when it is 5 degrees or less, the deterioration of the polarizer can be further suppressed.

[0028] The thickness of the polyester film of the present invention is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, still more preferably 25 μm or more, and particularly preferably 30 μm or more, from the viewpoint of maintaining strength suitable for protective applications, etc. From the viewpoint of producing a polarizing plate of a desired thickness, the thickness is preferably 100 μm or less, more preferably 90 μm or less, even more preferably 80 μm or less, still more preferably 70 μm or less, and particularly preferably 60 μm or less or 55 μm or less. In one embodiment, the thickness is 10 μm or more and 100 μm or less. In a preferred embodiment, the thickness is 15 μm or more and 60 μm or less.

[0029] The Nz coefficient of the polyester film of the present invention is preferably 1.5 or more, more preferably 1.6 or more, and even more preferably 1.7 or more, from the viewpoint of maintaining strength suitable for protective applications, etc. The Nz coefficient is preferably 2.5 or less, more preferably 2.4 or less, and even more preferably 2.3 or less, from the viewpoint of suppressing iridescence (particularly iridescence from an oblique direction). In a preferred embodiment, the Nz coefficient is 1.5 or more and 2.5 or less.

[0030] The Nz coefficient can be determined as follows. The main axis direction (slow axis direction) of the film is determined using a conventional molecular orientation meter (e.g., MOA-6004 molecular orientation meter manufactured by Oji Scientific Instruments Co., Ltd.), and the biaxial refractive indexes of the main axis direction and the direction perpendicular thereto (fast axis direction) (refractive index nx in the slow axis direction, refractive index ny in the fast axis direction, where nx>ny) and the refractive index in the thickness direction (nz) are determined using a conventional Abbe refractometer (e.g., NAR-4T manufactured by Atago Co., Ltd., measurement wavelength 589 nm). The Nz coefficient can be determined by substituting the thus determined nx, ny, and nz into the formula represented by |nx-nz| / |nx-ny|.

[0031] From the viewpoint of suppressing iridescence, the retardation of the polyester film of the present invention is preferably 700 μm or more, more preferably 1000 μm or more, even more preferably 1200 μm or more, still more preferably 1500 μm or more, and particularly preferably 2000 μm or more. The retardation is preferably 8000 nm or less, more preferably 7000 nm or less, even more preferably 6000 nm or less, still more preferably 5500 nm or less, and particularly preferably 5000 nm or less. In a preferred embodiment, the retardation is 700 μm or more and 8000 μm or less.

[0032] The elastic modulus in the MD direction of the polyester film of the present invention is preferably 3000 MPa or more, more preferably 3500 MPa or more, even more preferably 4000 MPa or more, and particularly preferably 4500 MPa or more, from the viewpoint of controlling shrinkage of polarizers such as PVA films (mainly shrinkage in the absorption axis direction) and the resulting warpage of image display devices. The elastic modulus in the MD direction is preferably 10000 MPa or less, more preferably 8000 MPa or less, and even more preferably 7000 MPa or less. In one embodiment, the elastic modulus in the MD direction is 3000 MPa or more and 10000 MPa or less.

[0033] The polyester film of the present invention may be in the form of a roll. The length of the roll is preferably 500 m or more, more preferably 1000 m or more, even more preferably 1500 m or more, and even more preferably 2000 m or more. The length of the roll is preferably 10,000 m or less, more preferably 7,000 m or less, even more preferably 6,000 m or less, and even more preferably 5,000 m or less. In one embodiment, the length of the roll is 500 m or more and 10,000 m or less.

[0034] It is also a preferred embodiment that the polyester film of the present invention contains various additives other than catalysts, provided that the effects of the present invention are not impaired. Examples of additives include inorganic particles, heat-resistant polymer particles, alkali metal compounds, alkaline earth metal compounds, phosphorus compounds, UV absorbers (e.g., benzotriazoles, benzophenones, cyclic iminoesters, etc.), antistatic agents, light stabilizers, flame retardants, heat stabilizers, antioxidants, antigelling agents, surfactants, etc. Furthermore, to achieve high transparency, it is also preferable that the polyester film is substantially free of particles. "Substantially free of particles" means, for example, in the case of inorganic particles, that the content of inorganic elements, when quantified by fluorescent X-ray analysis, is 50 ppm or less, preferably 10 ppm or less, and particularly preferably below the detection limit.

[0035] It is also a preferred embodiment that the surface of the polyester film of the present invention is coated with various hard coats for the purposes of preventing reflection, glare, and scratches.

[0036] Furthermore, in the present invention, the polyester film may be subjected to corona treatment, coating treatment, flame treatment, or the like in order to improve adhesion to a polarizer or various hard coat layers.

[0037] At least one side of the polyester film of the present invention preferably has an easy-adhesion layer composed primarily of at least one selected from polyester resin, polyurethane resin, and polyacrylic resin. Here, "main component" refers to a component that accounts for 50% by mass or more of the solid components constituting the easy-adhesion layer. The coating liquid used to form the easy-adhesion layer is preferably an aqueous coating liquid containing at least one selected from water-soluble or water-dispersible copolymer polyester resin, acrylic resin, and polyurethane resin. Examples of such coating liquids include water-soluble or water-dispersible copolymer polyester resin solutions, acrylic resin solutions, and polyurethane resin solutions disclosed in Japanese Patent Nos. 3,567,927, 3,589,232, 3,589,233, 3,900,191, and 4,150,982.

[0038] The easy-adhesion layer can be obtained, for example, in any step during the polyester film production process, by applying a coating liquid to at least one of the film surfaces and then drying the coating liquid at 100° C. or higher and 150° C. or lower. The final coating amount of the easy-adhesion layer is 0.05 g / m from the viewpoints of adhesion and blocking resistance. ２ 0.2g / m or more ２ When the easy-adhesion layer is provided on both sides of the polyester film, the coating amount of the easy-adhesion layer on each side may be the same or different, and each can be independently set within the above range.

[0039] It is preferable to add particles to the adhesion layer to impart slipperiness. From the viewpoint of preventing falling off, the particles are preferably fine particles having an average particle size of 2 μm or less. Examples of particles to be contained in the 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, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. These may be added to the adhesion layer alone, or two or more types may be added in combination.

[0040] The coating liquid can be applied by any known method, such as reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, wire bar coating, or pipe doctor coating, which can be used alone or in combination.

[0041] The average particle size of the particles is measured by the following method: Particles are photographed with a scanning electron microscope (SEM), and the maximum diameters (the distance between the two most distant points) of 300 to 500 particles are measured at a magnification such that the size of the smallest particle is 2 to 5 mm, and the average value is taken as the average particle size.

[0042] The polyester film of the present invention is preferably a biaxially stretched film. For example, a biaxially stretched film can be produced by melting a polyester resin, extruding the non-oriented polyester into a sheet, stretching the extruded sheet in both MD and TD directions at a temperature equal to or higher than the glass transition temperature, and then heat-treating the extruded sheet.

[0043] When the MD direction is the main stretching direction, it is preferable to stretch the film in the TD direction as well to make it less likely to tear. However, a high stretching ratio in the MD direction is also required to reduce the refractive index in the fast axis direction. Stretching at such a large area ratio can cause the following problems: In sequential biaxial stretching, in which the film is stretched in the MD direction and then in the TD direction, the MD and slow axis directions are likely to be misaligned at the edges of the film due to the bowing phenomenon. In sequential biaxial stretching, in which the film is stretched in the TD direction and then in the MD direction, it is difficult to achieve uniform stretching in the TD direction. In simultaneous biaxial stretching, the film is prone to tearing between the clips, and although the stretching temperature is increased to reduce stress, stretching unevenness (differences in refractive index) is likely to occur.

[0044] The reason why simultaneous biaxial stretching is prone to uneven stretching is thought to be that the film softens rapidly as the temperature rises, and even slight fluctuations in temperature can cause large fluctuations in the film's softness, and sagging can occur, causing the film to flap (especially in wide widths), preventing the stretching force from being applied stably throughout.

[0045] In order to reduce the occurrence of these problems and obtain a polyester film that satisfies the above (1) to (4) and (5) to (6) and / or (7) to (8), it is preferable to carry out the stretching, including stretching in the MD direction, in multiple stages, in which biaxial stretching in the MD direction and the TD direction is carried out in the first half of the stretching, and stretching in the MD direction is carried out mainly in the second half.However, it is also possible to gradually change the stretching direction to be mainly in the MD direction as the stretching progresses without separating the first and second halves of the stretching.

[0046] The biaxial stretching in the MD and TD directions may be sequential biaxial stretching, but in order to further reduce variations in the orientation direction and stretching unevenness and to ensure stable production, simultaneous biaxial stretching is preferred. Note that repeated slight stretching in the MD and TD directions using a simultaneous biaxial stretching machine is also included in the category of simultaneous biaxial stretching.

[0047] The simultaneous biaxial stretching in the MD and TD directions and the stretching mainly in the MD direction may both be performed in the same tenter or in separate tenters. After the simultaneous biaxial stretching in the MD and TD directions is performed in a tenter, the stretching mainly in the MD direction may be performed by roll stretching.

[0048] In the multi-stage stretching, the number of stretching steps (number of stages) is preferably 2 or more. From the viewpoint of facilitating the setting and control of stretching conditions, the number of stretching steps is preferably 10 or less, more preferably 7 or less, even more preferably 5 or less, still more preferably 4 or less, and particularly preferably 3 or less. In one embodiment, the number of stretching steps is 2 or more and 10 or less.

[0049] In the multi-stage stretching, the stretching temperature in the first stretching zone is preferably 90°C or higher, more preferably 95°C or higher, even more preferably 98°C or higher, and even more preferably 100°C or higher, from the viewpoint of film formation stability. Also, the stretching temperature in the first stretching zone is preferably 120°C or lower, more preferably 115°C or lower, even more preferably 112°C or lower, and even more preferably 110°C or lower, from the viewpoint of film formability and suppression of stretching unevenness. In one embodiment, the stretching temperature in the first stretching zone is 90°C or higher and 120°C or lower.

[0050] From the viewpoint of forming a uniform film in the width direction, the areal magnification of the stretching in the first stretching zone is preferably 7 or more, more preferably 7.5 or more, even more preferably 8 or more, still more preferably 8.5 or more, and particularly preferably 9 or more. From the viewpoint of enabling stable film formation without breakage, the areal magnification is preferably 14 or less, more preferably 13.5 or less, even more preferably 13 or less, still more preferably 12.5 or less, and particularly preferably 12 or less. In one embodiment, the areal magnification is 7 or more and 14 or less.

[0051] The stretch ratio in the TD direction in the first stretching zone is preferably 1.4 or more, more preferably 1.5 or more, even more preferably 1.6 or more, still more preferably 1.8 or more, and particularly preferably 2 or more, from the viewpoint of enabling larger sizes to be accommodated by uniform stretching and improving productivity. From the viewpoint of suppressing the refractive index of the fast axis, the stretch ratio in the TD direction is preferably 3.7 or less, more preferably 3.5 or less, even more preferably 3.2 or less, still more preferably 3 or less, and particularly preferably 2.8 or less. In one embodiment, the stretch ratio in the TD direction is 1.4 or more and 3.7 or less.

[0052] From the viewpoint of uniform stretching, the stretch ratio in the MD direction in the first stretching zone is preferably 2 or more, more preferably 2.1 or more, even more preferably 2.2 or more, still more preferably 2.3 or more, and particularly preferably 2.4 or more. From the viewpoint of enabling stable film formation without breakage, the stretch ratio in the MD direction is preferably 5 or less, more preferably 4.9 or less, even more preferably 4.8 or less, still more preferably 4.7 or less, and particularly preferably 4.6 or less. In one embodiment, the stretch ratio in the MD direction is 2 or more and 5 or less.

[0053] In the first stretching zone, the ratio of the stretch ratio in the MD direction to the stretch ratio in the TD direction is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more. The ratio is preferably 3 or less, more preferably 2.5 or less, even more preferably 2.2 or less, and even more preferably 2 or less. In one embodiment, the ratio is 0.5 or more and 3 or less.

[0054] In the multi-stage stretching, the stretching temperature in the second stretching zone is preferably 90°C or higher, more preferably 95°C or higher, even more preferably 98°C or higher, still more preferably 100°C or higher or 105°C or higher, from the viewpoint of film formation stability. Furthermore, the stretching temperature in the second stretching zone is preferably 120°C or lower, more preferably 115°C or lower, even more preferably 112°C or lower, and even more preferably 110°C or lower, from the viewpoint of film formability and suppression of stretching unevenness. In one embodiment, the stretching temperature in the second stretching zone is 90°C or higher and 120°C or lower. Furthermore, in one embodiment, the stretching temperature in the second stretching zone is higher than the stretching temperature in the first stretching zone. The difference (absolute value) between the stretching temperature in the second stretching zone and the stretching temperature in the first stretching zone may be, for example, 1°C or more, 2°C or more, 3°C or more, 4°C or more, or 5°C or more, and may be 20°C or less, 15°C or less, or 10°C or less, or may be 1°C or more and 20°C or less.

[0055] From the viewpoint of appearance, the areal magnification of the stretching in the second stretching zone is preferably 1.1 or more, more preferably 1.2 or more, even more preferably 1.3 or more, still more preferably 1.4 or more, and particularly preferably 1.5 or more. From the viewpoint of film formation stability, the areal magnification is preferably 6 or less, more preferably 5.8 or less, even more preferably 5.6 or less, still more preferably 5.4 or less, and particularly preferably 5 or less. In one embodiment, the areal magnification is 1.1 or more and 6 or less.

[0056] From the viewpoint of appearance, the stretching ratio in the TD direction in the second stretching zone is preferably 0.7 or more, more preferably 0.75 or more, even more preferably 0.8 or more, still more preferably 0.85 or more, and particularly preferably 0.9 or more. From the viewpoint of film formation stability, the stretching ratio in the TD direction is preferably 2 or less, more preferably 1.95 or less, even more preferably 1.9 or less, still more preferably 1.85 or less, and particularly preferably 1.8 or less. In one embodiment, the stretching ratio in the TD direction is 0.7 or more and 2 or less.

[0057] The stretching ratio in the MD direction in the second stretching zone is preferably 1.5 or more, more preferably 1.55 or more, even more preferably 1.6 or more, still more preferably 1.65 or more, and particularly preferably 1.7 or more, from the viewpoint of the balance of the refractive indexes of the slow axis and the fast axis. From the viewpoint of film formation stability, the stretching ratio in the MD direction is preferably 5 or less, more preferably 4.9 or less, even more preferably 4.8 or less, still more preferably 4.7 or less, and particularly preferably 4.6 or less. In one embodiment, the stretching ratio in the MD direction is 1.5 or more and 5 or less.

[0058] In the second stretching zone, the ratio of the stretch ratio in the MD direction to the stretch ratio in the TD direction is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more. The ratio is preferably 3 or less, more preferably 2.5 or less, even more preferably 2.2 or less, and even more preferably 2 or less. In one embodiment, the ratio is 0.5 or more and 3 or less.

[0059] From the viewpoint of uniform stretching, the total TD stretch ratio in the first and second stretching zones is preferably 1.4 or more, more preferably 1.5 or more, even more preferably 1.6 or more, still more preferably 1.8 or more, and particularly preferably 2 or more. From the viewpoint of suppressing the refractive index of the fast axis, the total TD stretch ratio is preferably 3.7 or less, more preferably 3.5 or less, even more preferably 3.2 or less, still more preferably 3 or less, and particularly preferably 2.8 or less. In one embodiment, the total TD stretch ratio is 1.4 or more and 3.7 or less.

[0060] The ratio of the TD stretching ratio in the first stretching zone to the total TD stretching ratio in the first and second stretching zones is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more, from the viewpoint of the balance between the refractive indexes of the slow axis and the fast axis. From the viewpoint of appearance, the ratio is preferably 1.4 or less, more preferably 1.3 or less, even more preferably 1.2 or less, and even more preferably 1.1 or less. In one embodiment, the ratio is 0.5 or more and 1.4 or less.

[0061] From the viewpoint of optical properties, the total MD stretching ratio in the first and second stretching zones is preferably 5 or more, more preferably 5.1 or more, even more preferably 5.3 or more, still more preferably 5.5 or more, and particularly preferably 6 or more. From the viewpoint of film formation stability, the total MD stretching ratio is preferably 7.5 or less, more preferably 7.4 or less, even more preferably 7.3 or less, still more preferably 7.2 or less, and particularly preferably 7.1 or less. In one embodiment, the total MD stretching ratio is 5 or more and 7.5 or less.

[0062] The ratio of the MD stretching ratio in the first stretching zone to the total MD stretching ratio in the first and second stretching zones is preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.3 or more, and even more preferably 0.35 or more, from the viewpoint of the balance of the refractive indexes of the slow axis and the fast axis. From the viewpoint of the balance of the refractive indexes of the slow axis and the fast axis, the ratio is preferably 0.8 or less, more preferably 0.75 or less, even more preferably 0.7 or less, even more preferably 0.65 or less, and particularly preferably 0.63 or less or 0.6 or less. In one embodiment, the ratio is 0.2 or more and 0.8 or less.

[0063] From the viewpoint of the balance between the refractive indices of the slow axis and the fast axis, the ratio of the total MD stretching ratio in the first and second stretching zones to the total TD stretching ratio in the first and second stretching zones is preferably 1.7 or more, more preferably 1.75 or more, even more preferably 1.8 or more, still more preferably 1.9 or more, and particularly preferably 2 or more. From the viewpoint of film formation stability, the ratio is preferably 7.5 or less, more preferably 7.4 or less, even more preferably 7.3 or less, still more preferably 7.2 or less, and particularly preferably 7.1 or less. In one embodiment, the ratio is 1.7 or more and 7.5 or less.

[0064] From the viewpoint of uniform stretching, the total areal stretching ratio of the stretching in the first and second stretching zones is preferably 10 or more, more preferably 11 or more, even more preferably 12 or more, still more preferably 13 or more, and particularly preferably 14 or more. From the viewpoint of film formation stability, the total areal stretching ratio is preferably 30 or less, more preferably 27 or less, even more preferably 25 or less, and still more preferably 23 or less. In one embodiment, the total areal stretching ratio is 10 or more and 30 or less.

[0065] The treatment temperature (heat setting temperature) in the heat treatment after stretching is preferably 150° C. or higher, more preferably 160° C. or higher, and even more preferably 170° C. or higher. The treatment temperature is preferably 250° C. or lower, more preferably 240° C. or lower, and even more preferably 230° C. or lower. In one embodiment, the treatment temperature is 150° C. or higher and 250° C. or lower.

[0066] The treatment time (heat setting time) in the heat treatment after stretching is preferably 1 second or more, more preferably 2 seconds or more. The treatment time is preferably 15 seconds or less, more preferably 14 seconds or less. In one embodiment, the treatment time is 1 second or more and 15 seconds or less.

[0067] It is preferable to carry out a relaxation treatment after heat setting. The relaxation rate is preferably 0.1% or more and 5% or less. The relaxation temperature is preferably 150°C or more and 250°C or less.

[0068] The polyester film of the present invention can be suitably used as a polarizer protective film.

[0069] The present invention encompasses a polarizing plate in which the above-described polyester film (or polarizer protective film) is laminated on at least one surface of a polarizer (for example, a polarizer obtained by dyeing PVA or the like with iodine). In a preferred embodiment, the above-described polarizer protective film is laminated on one surface of the polarizer, and a non-birefringent polarizer protective film or optical compensation film, such as a TAC film, a norbornene film, or an acrylic film, is laminated on the other surface of the polarizer. In another preferred embodiment, the above-described polarizer protective film is laminated on one surface of the polarizer, and no film is laminated on the other surface of the polarizer (no independent film is attached to the polarizer on the other surface of the polarizer). In this embodiment, a coating layer (a hard coat layer, an antiglare layer, an antireflection layer, a low-reflection layer, a moisture-resistant layer (which may be made of either an organic or inorganic material), or a layer having a combination of these functions) may be provided on the surface of the polarizer opposite to the surface on which the above-described polarizer protective film is laminated.

[0070] From the viewpoint of suppressing rainbow spots and warping of the liquid crystal panel, the polarizing plate of the present invention is preferably laminated so that the absorption axis direction of the polarizer and the slow axis direction of the polyester film are substantially parallel to each other. "Substantially parallel" means that the angle formed between the absorption axis direction of the polarizer and the slow axis direction of the polyester film is preferably within 10 degrees, more preferably within 7 degrees, even more preferably within 5 degrees, still more preferably within 3 degrees, and particularly preferably within 2 degrees or 1 degree.

[0071] The present invention encompasses an image display device including the polyester film (or polarizer protective film) described above. The image display device includes a liquid crystal display device, an organic EL display, a QLED display, a micro LED display, and the like, which includes a polarizing plate inside the image display device.

[0072] Generally, a liquid crystal panel is composed of a rear module, a liquid crystal cell, and a front module, in that order from the side facing the backlight source to the side where an image is displayed (the viewing side). The rear module and the front module are generally composed of a transparent substrate, a transparent conductive film formed on the surface of the substrate facing the liquid crystal cell, and a polarizing plate disposed on the opposite side. Here, the polarizing plate is generally disposed on the side facing the backlight source in the rear module, and on the side where an image is displayed (the viewing side) in the front module.

[0073] The liquid crystal display device preferably includes at least a backlight source and a liquid crystal cell disposed between two polarizing plates as its constituent members, and may also include other components, such as a color filter, a lens film, a diffusion sheet, an anti-reflection film, etc.

[0074] The arrangement of the polyester film (or polarizer protective film) of the present invention is not particularly limited, but in the case of a liquid crystal display device having a polarizing plate arranged on the incident light side (light source side), a liquid crystal cell, and a polarizing plate arranged on the exit light side (viewing side), it is preferred that the polarizer protective film on the incident light side of the polarizing plate arranged on the incident light side and / or the polarizer protective film on the exit light side of the polarizing plate arranged on the exit light side be the polyester film of the present invention. In a preferred embodiment, the polarizer protective film on the incident light side of the polarizing plate arranged on the incident light side is the polyester film of the present invention.

[0075] The backlight may be configured as an edge light type having a light guide plate, a reflector, etc. as constituent members, or as a direct type.

[0076] White light-emitting diodes (white LEDs) are preferably used as backlight sources for liquid crystal display devices. White LEDs are phosphor-based devices that emit white light by combining a light-emitting diode that emits blue or ultraviolet light using a compound semiconductor with a phosphor. Phosphors include yttrium-aluminum-garnet-based yellow phosphors and terbium-aluminum-garnet-based yellow phosphors. Among these, white light-emitting diodes that combine a blue light-emitting diode using a compound semiconductor with a yttrium-aluminum-garnet-based yellow phosphor have a continuous and broad emission spectrum and excellent luminous efficiency. Here, a continuous emission spectrum means that there is no wavelength at which the light intensity is zero, at least in the visible light range. Furthermore, the method of the present invention enables the widespread use of low-power white LEDs, thereby achieving energy savings.

[0077] As the backlight light source, a white light source having a peak top of the emission spectrum in each wavelength region of 400 nm or more and less than 495 nm (B region), 495 nm or more and less than 600 nm (G region), and 600 nm or more and less than 780 nm (R region) is also preferred. For example, a white light source using quantum dot technology, a phosphor-type white LED light source using a phosphor having emission peaks in the R (red) and G (green) regions by excitation light and a blue LED, a three-wavelength type white LED light source, a white LED light source combined with a red laser, and others, for example, a white LED light source having a composition formula of K ２ SiF ６ : Mn ４＋ Examples of such white light sources include a white LED light source using a blue LED and a fluoride phosphor (also referred to as "KSF"). These white light sources have attracted attention as backlight sources for wide-color-gamut liquid crystal display devices, and all of them have narrower peak half-widths than conventional light sources using white light-emitting diodes that are light-emitting elements combining a blue light-emitting diode and an yttrium-aluminum-garnet-based yellow phosphor. When using a backlight source using these white light sources, rainbow spots tend to occur more easily when a polyester film having retardation is used as a polarizer protective film, which is a component of a polarizing plate, compared to a backlight source using a white light-emitting diode that is a light-emitting element combining a blue light-emitting diode and an yttrium-aluminum-garnet-based yellow phosphor. However, the polarizer protective film of the present invention can significantly suppress rainbow spots.

[0078] The organic EL element can be appropriately selected from organic EL elements known in the art. The use of an organic EL element is preferred in terms of a wide viewing angle, high contrast, and high-speed response. An organic EL element is typically an emitter (organic electroluminescence emitter) having a structure in which an anode as a transparent electrode, an organic light-emitting layer, and a cathode as a metal electrode are laminated in this order on a transparent substrate. When a voltage is applied between the anode and the cathode, the organic EL cell emits light by recombining holes (positive holes) injected from the anode and electrons injected from the cathode in the organic light-emitting layer.

[0079] Any transparent substrate can be used as the transparent substrate. For example, the transparent substrate can be selected from the group consisting of a glass substrate, a ceramic substrate, a semiconductor substrate, a metal substrate, and a plastic substrate. Specific examples of plastic substrates include conventionally used transparent resin films. The transparent substrate may be provided with a surface treatment layer, if necessary. Examples of the surface treatment layer include a moisture-proof layer, a gas barrier layer, a hard coat layer, and an undercoat layer.

[0080] Examples of materials constituting the anode and cathode include metals, metal oxides, alloys, electrically conductive compounds, and mixtures thereof. Specific examples of materials constituting the anode include conductive transparent materials such as gold, silver, chromium, nickel, copper iodide, indium tin oxide (ITO), tin oxide, and zinc oxide. Specific examples of materials constituting the cathode include magnesium, aluminum, indium, lithium, sodium, cesium, silver, magnesium-silver alloys, magnesium-indium alloys, and lithium-aluminum alloys.

[0081] The thickness of the anode and cathode can be set arbitrarily depending on the materials constituting the anode and cathode. The thickness of the anode can be appropriately set, for example, from 10 nm to 200 nm, preferably from 10 nm to 100 nm. The thickness of the cathode can be appropriately set, for example, from 10 nm to 1000 nm, preferably from 10 nm to 200 nm.

[0082] The organic light-emitting layer is a layer that has the function of providing a site for recombination of holes and electrons when a voltage is applied, thereby emitting light. The organic light-emitting layer contains an organic light-emitting material and may have a single-layer structure or a laminated structure of two or more layers. In the case of a laminated structure, each layer may emit light of a different color. The thickness of the organic light-emitting layer is optional and can be set appropriately within the range of, for example, 3 nm to 3 μm.

[0083] The organic light-emitting material used in the organic light-emitting layer can be appropriately selected from any light-emitting material. Specifically, examples thereof include olefin-based light-emitting materials such as 4,4'-(2,2-diphenylvinyl)biphenyl; anthracene-based light-emitting materials such as 9,10-di(2-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene, 9,10-bis(9,9-dimethylfluorenyl)anthracene, 9,10-(4-(2,2-diphenylvinyl)phenyl)anthracene, 9,10'-bis(2-biphenylyl)-9,9'-bisanthracene, 9,10,9',10'-tetraphenyl-2,2'-bianthryl, and 1,4-bis(9-phenyl-10-anthracene)benzene; and 2,7,2',7'-tetrakis(2,7,2',7'-tetraphenyl-2,2'-bianthryl). The light-emitting material can be appropriately selected from the group consisting of spiro-based light-emitting materials such as (2,2-diphenylvinyl)spirobifluorene; carbazole-based light-emitting materials such as 4,4'-dicarbazolebiphenyl and 1,3-dicarbazolylbenzene; and pyrene-based light-emitting materials such as 1,3,5-tripyreninebenzene.

[0084] The organic EL element may include a sealing member formed to cover the organic EL element, in order to isolate the organic EL element composed of the anode, the organic light-emitting layer, and the cathode on the substrate from the outside air. By providing the sealing member, it is possible to prevent deterioration of the light-emitting characteristics of the organic light-emitting layer due to moisture and oxygen in the outside air.

[0085] The organic EL device may further include any optional components (for example, a hole injection layer, a hole transport layer, an electron injection layer, and / or an electron transport layer) at any appropriate position.

[0086] When an organic EL cell is used as the image display cell, it is preferable to have a polarizing plate on the viewing side. Because the thickness of the organic light-emitting layer is thin, for example, about 10 nm, external light is reflected by the metal electrode and emitted back to the viewing side, which can cause the display surface of the organic EL display device to appear as a mirror when viewed from the outside. To block such specular reflection of external light, it is preferable to provide a polarizing plate on the viewing side of the organic EL cell and further provide a quarter-wave plate between the organic EL cell and the polarizing plate. The polarizing plate described above can be used, and it is preferable that the polyester film of the present invention (or polarizer protective film) is laminated on the viewing side of the polarizer. Another preferable embodiment is to laminate a quarter-wave plate on the polarizer instead of the protective film on the organic EL element side of the polarizer. By combining these viewing-side polarizing plates and a quarter-wave plate to form a circular polarizing plate, the external light specularly reflected by the metal electrode of the organic EL cell is blocked by the circular polarizing plate, thereby suppressing a decrease in visibility of the image display device. A half-wave plate or the like may be further laminated on the organic EL element side or polarizer side of the quarter-wave plate. Preferably, a half-wave plate or the like is laminated on the organic EL element side of the quarter-wave plate with the optical axes of the plates inclined relative to each other, as disclosed in JP-A-10-68816 and JP-A-2017-97379.

[0087] QLED displays are similar to organic EL displays in that they utilize the ability of quantum dots to emit light when electricity is applied, and are attracting attention as next-generation displays.

[0088] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and can be practiced by making appropriate modifications within the scope that is compatible with the spirit of the present invention, and all of these modifications are included in the technical scope of the present invention.

[0089] The physical properties in the following examples were evaluated as follows: (1) Evaluation of the slow axis direction of the film The slow axis direction of the film was measured using a molecular orientation meter (MOA-6004 molecular orientation meter, manufactured by Oji Scientific Instruments Co., Ltd.).

[0090] (2) Δn and Retardation (Re) Retardation is a parameter defined by the product (Δn × d) of the anisotropy of the refractive index of two orthogonal axes on a film (Δn = |nx - ny|) and the film thickness d (nm), and is a measure of optical isotropy and anisotropy. The anisotropy of the biaxial refractive index (Δn) was determined by the following method. Using a molecular orientation meter (MOA-6004 molecular orientation meter manufactured by Oji Measurement Instruments Co., Ltd.), the slow axis direction of the film was determined, and a 4 cm × 2 cm rectangle was cut out so that the slow axis direction was parallel to the long side of the measurement sample, and used as a measurement sample. For this sample, the refractive index of two orthogonal axes (refractive index in the slow axis direction: nx, refractive index in the in-plane direction perpendicular to the slow axis direction (i.e., refractive index in the fast axis direction): ny), and the refractive index in the thickness direction (nz) were measured using an Abbe refractometer (manufactured by Atago Co., Ltd., NAR-4T, measurement wavelength 589 nm), and the absolute value of the difference in refractive index between the two axes (|nx-ny|) was taken as the refractive index anisotropy (Δn). The film thickness d (nm) was measured using an electric micrometer (manufactured by Fine Leaf Co., Ltd., Millitron 1245D) and converted to units in nm. The retardation (Re) was calculated from the product (Δn × d) of the refractive index anisotropy (Δn) and the film thickness d (nm).

[0091] (3) Difference between maximum and minimum values ​​of Δn in the width direction Using a roll 1000 mm long in the TD direction, a total of five samples of 4 cm x 2 cm were cut out from the center position in the TD direction toward each end so that the spacing between the centers of the samples was 200 mm, using the same method as in (2) above, and Δn was measured. The difference between the maximum and minimum values ​​of Δn obtained by the measurements was calculated.

[0092] (4) NZ Coefficient The values ​​of nx, ny, and nz measured by the Abbe refractometer in (2) above were substituted into |nx-nz| / |nx-ny| to determine the NZ coefficient.

[0093] (5) Planar Orientation Degree ΔP The values ​​of nx, ny, and nz measured by the Abbe refractometer in (2) above were substituted into (nx+ny) / 2−nz to determine the planar orientation degree ΔP.

[0094] (6) Ultraviolet transmittance A polyester film sample measuring 8 mm in the transverse direction (TD) × 40 mm in the machine direction (MD) was prepared. The sample was placed so that the machine direction (MD) was parallel to the direction of gravity, and the transmittance at a wavelength of 380 nm was measured using a spectrophotometer (Shimadzu UV1800).

[0095] (7) Ultraviolet transmittance anisotropy (polarized transmittance anisotropy) Ultraviolet transmittance anisotropy was expressed using the ultraviolet transmittance along two perpendicular axes, the longitudinal direction (MD) and the width direction (TD), at a wavelength of 390 nm, at which the sample's total light transmittance was 30%. The wavelength of 390 nm is a wavelength at which anisotropy is easily measured. The ultraviolet transmittance anisotropy was measured using a UV-visible spectrophotometer (Shimadzu Corporation, UV1800) with a measurement wavelength range of 190 to 1100 nm. The transmittance in each stretching direction was measured by inserting a polarizing plate (Nippon Kayaku Corporation, SHC-YL38) that transmits ultraviolet wavelengths into the detector side of the spectrophotometer, and measuring the light transmitted through the polarizing plate. To eliminate the influence of the polarizing plate direction, the polarized transmitted light in the longitudinal direction was measured, and then the film was rotated 90° to extract the polarized transmitted light in the width direction. The transmittance anisotropy (i.e., the ratio of polarized light transmittance in the MD direction to that in the TD direction) can be calculated by the following formula (A): (Transmittance anisotropy) = (longitudinal transmittance (%)) / (widthwise transmittance (%)) (A).

[0096] (8) Optical Evaluation of Large-Area Screens (Liquid Crystal Display Devices) A ​​roll of a polarizer made of iodine and polyvinyl alcohol film produced by uniaxial stretching in the MD direction and a roll of a PET film of each of the Examples described below were laminated together in a roll-to-roll manner so that their MD directions were parallel to each other. A roll of TAC film (manufactured by Fujifilm Corporation, thickness: 40 μm) was also laminated to the other side of the polarizer in a roll-to-roll manner to produce a polarizing plate consisting of a PET film / polarizer / TAC film. The obtained polarizing plates were installed on the incident light side and the exit light side of a liquid crystal display device using a white LED (manufactured by Nichia Chemical, NSPW500CS) as a light source, which was a light-emitting element consisting of a blue light-emitting diode and an yttrium-aluminum-garnet yellow phosphor. The polarizing plate on the incident light side was installed so that the polyester film was on the light source side, and the polarizing plate on the exit light side was installed so that the polyester film was on the viewing side. Five monitors visually observed the polarizing plates of a liquid crystal display device having a large screen (50 inches in this example) from the front and oblique directions, and judged the presence or absence of rainbow spots as follows: (8A) Optical Characteristics (Optical characteristics correspond to Δn (in-plane birefringence)). ∘: Four or more people judged that rainbow spots were not visible when viewing the screen from an oblique direction. Δ: Three or fewer people judged that rainbow spots were not visible when viewing the screen from an oblique direction, and even if rainbow spots were judged to be visible, they were not enough to deteriorate visibility. ×: Three or fewer people judged that rainbow spots were not visible when viewing the screen from an oblique direction, and if rainbow spots were judged to be visible, visibility was deteriorated. (8B) Uniformity (Uniformity corresponds to the difference between the maximum and minimum values ​​of in-plane birefringence in the width direction). ∘: Four or more people judged that rainbow spots were not visible depending on the location on the screen of the large-area display. △: The number of people who judged that rainbows were not visible depending on the location on the screen of the large-area display was 3 or less, and even if it was judged that rainbows were visible, it was not so much that it deteriorated visibility. ×: The number of people who judged that rainbows were not visible depending on the location on the screen of the large-area display was 3 or less, and if it was judged that rainbows were visible, it was seen that visibility was deteriorated. (8C) Overall evaluation (The overall evaluation is a comprehensive evaluation of 8A and 8B above.) ◯: The number of people who judged that no rainbows were visible across the entire screen from any angle when observed is four or more. △: The number of people who judged that no rainbows were visible across the entire screen from any angle when observed is three or less, and even if it was judged that a faint rainbow was visible when observed from an oblique angle across part or the entire screen, it was not so severe that it deteriorated visibility. ×: The number of people who judged that no rainbows were visible across the entire screen from any angle when observed is three or less, and if it was judged that a rainbow was visible across part or the entire screen, it was judged that a deterioration in visibility was observed.

[0097] (9) Durability Test Evaluation of Polarizing Plates A polarizing plate was produced by laminating the PET film and polarizer of each Example described later using a roll-to-roll method. Similarly, a polarizing plate was produced by laminating the PET film and polarizer using a roll-to-roll method in each Comparative Example and Reference Example described later. Here, the angle between the slow axis direction of the PET film and the MD direction was within 10 degrees. The polarizer was laminated so that the absorption axis direction was within 10 degrees with respect to the MD direction of the PET film. PET films absorb more ultraviolet light in the MD direction. Using a UV irradiation tester (Iwasaki Electric Co., Ltd., Eye Super UV Tester, SUV-W151), an irradiation intensity of 100 W / m was applied to each polarizing plate. ２ After 500 hours of irradiation with ultraviolet light under conditions of a black panel temperature of 60°C and a relative humidity of 60% RH or less, the presence or absence of discoloration was visually observed. Those without discoloration were evaluated as having particularly good durability and rated as "Good". Those with slight discoloration were evaluated as having good durability and rated as "Poor". Those with significant discoloration were evaluated as having poor durability and rated as "Poor".

[0098] (10) Film Formability The number of breaks in the first hour after the start of film formation was compared, and the film formability of the film was evaluated as follows: ○: Number of breaks less than 3 times △: Number of breaks 3 or more but less than 6 times ×: Number of breaks 6 or more times

[0099] (Production Example 1 - Polyester A) The temperature of the esterification reactor was increased to 200°C, and 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged. While stirring, 0.017 parts by mass of antimony trioxide, 0.064 parts by mass of magnesium acetate tetrahydrate, and 0.16 parts by mass of triethylamine were charged as catalysts. The temperature was then increased under pressure, and a pressurized esterification reaction was carried out under conditions of a gauge pressure of 0.34 MPa and 240°C. The esterification reactor was then returned to normal pressure, and 0.014 parts by mass of phosphoric acid was added. The temperature was then increased to 260°C over 15 minutes, and 0.012 parts by mass of trimethyl phosphate was added. After 15 minutes, the mixture was dispersed using a high-pressure disperser. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor, and a polycondensation reaction was carried out under reduced pressure at 280°C.

[0100] After the polycondensation reaction was completed, the resin was filtered through a Naslon filter with a 95% cutoff diameter of 5 μm, extruded from a nozzle in the form of a strand, cooled and solidified using cooling water that had been previously filtered (pore diameter: 1 μm or less), and cut into pellets. The resulting polyethylene terephthalate resin (A) had an intrinsic viscosity of 0.62 dl / g, a diethylene glycol (DEG) content of 1.5 mol%, and was substantially free of inert particles and internally precipitated particles. (Hereinafter referred to as PET (A)).

[0101] (Production Example 2 - Polyester B) 10 parts by mass of dried ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazin-4-one) and 90 parts by mass of particle-free PET (A) (intrinsic viscosity of 0.62 dl / g) were mixed and the mixture was kneaded using an extruder to obtain ultraviolet absorber-containing polyethylene terephthalate resin (B) (hereinafter abbreviated as PET (B)).

[0102] (Production Example 3 - Preparation of Adhesion-Modifying Coating Liquid) By carrying out a transesterification reaction and a polycondensation reaction by a conventional method, the dicarboxylic acid component (relative to the total dicarboxylic acid component) was 46 mol% terephthalic acid, 46 mol% isophthalic acid, and 8 mol% sodium 5-sulfonatoisophthalate, and the glycol component (relative to the total glycol component) was 50 mol% ethylene glycol and 50 mol% neopentyl glycol. A water-dispersible sulfonate metal base-containing copolymerized polyester resin was prepared. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, and 0.06 parts by mass of a nonionic surfactant were mixed, and then heated and stirred. When the temperature reached 77 ° C., 5 parts by mass of the water-dispersible sulfonate metal base-containing copolymerized polyester resin was added, and the mixture was stirred until no lumps of resin remained. The resin aqueous dispersion was then cooled to room temperature to obtain a uniform water-dispersible copolymerized polyester resin solution having a solids concentration of 5.0% by mass. Furthermore, 3 parts by mass of aggregated silica particles (Sylysia 310, manufactured by Fuji Silysia Co., Ltd.) were dispersed in 50 parts by mass of water, and then 0.54 parts by mass of an aqueous dispersion of Sylysia 310 was added to 99.46 parts by mass of the water-dispersible copolymer polyester resin liquid, and 20 parts by mass of water was added while stirring to obtain an adhesive property-modifying coating liquid.

[0103] As an example, the case of using a simultaneous biaxial stretching machine to perform two-stage stretching will be described. [Reference Example 1] As the raw material for the intermediate layer of base film, 90 parts by mass of particle-free PET (A) resin pellets and 10 parts by mass of PET (B) resin pellets containing ultraviolet absorber are dried under reduced pressure (1 Torr) for 6 hours at 135 ℃, then fed into extruder 2 (for intermediate layer II layer); PET (A) is dried by conventional method and fed into extruder 1 (for outer layer I layer and outer layer III layer), respectively, and melted at 285 ℃. These two kinds of polymers are respectively filtered through stainless steel sintered filter material (nominal filtration accuracy 10 μm particle 95% cut) and stacked in a two-kind three-layer merging block, extruded into sheet form from a die, and then wound around a casting drum with a surface temperature of 30 ℃ using electrostatic casting method, cooled and solidified, and made an unstretched film. At this time, the discharge rates of the extruders were adjusted so that the ratio of the thicknesses of the I layer, II layer, and III layer was 10:80:10.

[0104] Next, a coating amount of 0.08 g / m2 was applied to both sides of the unstretched PET film by the reverse roll method. ２ After the adhesiveness modifying coating liquid was applied so that the thickness of the coated film became 1 / 2 mm, the coated film was dried at 80° C. for 20 seconds.

[0105] The unstretched film bearing this coating layer was introduced into a simultaneous biaxial stretching machine, and while the edges of the film were held with clips, it was introduced into hot air zone 1 at a first stretching temperature of 100°C, where it was stretched 3.9 times in the machine direction and 2.2 times in the width direction. It was then introduced into hot air zone 2 at a second stretching temperature of 105°C, where it was stretched 1.7 times in the machine direction and 1.0 times in the width direction. While maintaining the stretched width, the film was heat-set at 220°C for 30 seconds to obtain a 40 μm-thick biaxially oriented PET film. This was then wound into a roll (film roll with a length of 1000 mm in the TD direction and 500 m in the MD direction).

[0106] [Examples 1 to 3, 5, and 6, Comparative Examples 1, 5, and 7, and Reference Examples 2 to 4] Biaxially oriented PET films (rolls) were obtained in the same manner as in Reference Example 1, except that the stretching ratios in the machine direction and width direction in hot air zone 1 were changed to the ratios shown in Table 1, and the stretching ratio in the machine direction in hot air zone 2 was changed to the ratio shown in Table 1.

[0107] [Example 4] A biaxially oriented PET film (roll) was obtained in the same manner as in Reference Example 1, except that the stretching ratio in the machine direction of the hot air zone 1 was changed to 4.2 times and the stretching ratio in the width direction was changed to 2.0 times.

[0108] Examples 7 and 8 Biaxially oriented PET films (rolls) were obtained in the same manner as in Example 1, except that the thickness was changed to 60 μm or 30 μm.

[0109] [Example 9] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 1, except that the temperature in hot air zone 1 (first stretching temperature) was changed to 105°C and the temperature in hot air zone 2 (second stretching temperature) was changed to 110°C.

[0110] [Example 10] A biaxially oriented PET film (roll) was obtained in the same manner as in Reference Example 1, except that the stretching ratio in the machine direction in hot air zone 1 was changed to 3.3 times and the stretching ratio in the width direction to 3.0 times, and the stretching ratio in the machine direction in hot air zone 2 was changed to 2.0 times and the stretching ratio in the width direction to 1.1 times.

[0111] [Example 11] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 1, except that 80 parts by mass of particle-free PET (A) resin pellets and 20 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film.

[0112] [Example 12] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 3, except that 80 parts by mass of particle-free PET (A) resin pellets and 20 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film.

[0113] [Example 13] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 6, except that 80 parts by mass of particle-free PET (A) resin pellets and 20 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film.

[0114] [Example 14] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 8, except that 70 parts by mass of particle-free PET (A) resin pellets and 30 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film, and the thickness of the biaxially oriented PET film was set to 30 μm.

[0115] [Example 15] A biaxially oriented PET film (roll) was obtained in the same manner as in Example 1, except that 60 parts by mass of particle-free PET (A) resin pellets and 40 parts by mass of PET (B) resin pellets containing an ultraviolet absorber were used as raw materials for the intermediate layer of the base film, and the thickness of the biaxially oriented PET film was set to 20 μm.

[0116] [Comparative Example 6] A biaxially oriented PET film (roll) was obtained in the same manner as in Reference Example 1, except that the stretching ratio in the machine direction in hot air zone 1 was changed to 3.1 times and the stretching ratio in the machine direction in hot air zone 2 was changed to 1.3 times.

[0117] Reference Example 8 A biaxially oriented PET film (roll) was obtained in the same manner as in Reference Example 1, except that the temperature in hot air zone 1 (first stretching temperature) was changed to 125°C, the stretch ratio in the machine direction was changed to 6.5 times, stretching was not performed in hot air zone 2, and the heat setting temperature was changed to 225°C.

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[0128] Although the above example is an example of two stages, modifications may be made without departing from the concept of the stretching described above, such as adding weak MD stretching or TD stretching before or after the first and second stretching, dividing the first and second stretching into multiple stages, etc. Furthermore, the stretching may not be divided into the first and second halves, and may gradually become dominated by MD stretching as the stretching progresses.

Claims

1. A polyester film, wherein the polyester of the polyester film is mainly polyethylene terephthalate, the slow axis direction of the polyester film is approximately parallel to the MD direction, the in-plane birefringence ΔNxy of the polyester film is 0.06 or more and 0.2 or less, the refractive index of the polyester film in the fast axis direction is 1.58 or more and 1.63 or less, the ultraviolet transmittance of the polyester film at 380 nm is 0% or more and 30% or less, and the ratio of the polarized light transmittance in the MD direction to the TD direction of the polyester film is 0.95 or less.

2. The polyester film according to claim 1, wherein the length of the polyester film in the TD direction is 1 m or more, and the difference between the maximum and minimum values ​​of ΔNxy in the TD direction of the polyester film is 0.013 or less.

3. The polyester film according to claim 1, wherein the NZ coefficient of the polyester film is 1.5 or more and 2.5 or less.

4. The polyester film according to claim 1, wherein the thickness of the polyester film is 15 μm or more and 60 μm or less.

5. The polyester film according to claim 1, wherein the angle between the slow axis direction and the machine direction of the polyester film is within 10 degrees.

6. The polyester film according to claim 1, wherein the modulus of elasticity in the machine direction of the polyester film is 3000 MPa or more.

7. The polyester film according to claim 1, wherein the retardation of the polyester film is 700 nm or more and 8000 nm or less.

8. The polyester film according to claim 7, which is a polarizer protective film.

9. A polarizing plate comprising a polarizer having the polyester film according to claim 8 laminated on at least one surface thereof, wherein the angle between the absorption axis direction of the polarizer and the MD direction of the polyester film is within 10 degrees.

10. A polarizing plate comprising a polarizer having the polyester film according to claim 8 laminated on one side thereof and no film laminated on the other side thereof.

11. A polarizing plate comprising a polarizer having the polyester film according to claim 8 laminated on one side thereof and a quarter-wave plate laminated on the other side thereof.

12. An image display device comprising the polarizing plate according to any one of claims 9 to 11 and a light source.

13. The image display device according to claim 12, which is a liquid crystal display device.

14. The image display device according to claim 12, which is an organic EL display.

15. The image display device according to claim 12, which is a QLED display.

Images