Laminated polyester film, laminated polyester film roll, functional laminated film, laminate, display device, and method for manufacturing polarizing plate

Abstract

The present invention provides a laminated polyester film, a laminated polyester film roll, a functional laminated film, a display device equipped with the laminated polyester film, and a foldable display device, which ensure visibility and reduce color distortion even when the screen is viewed through an optical component with polarizing properties such as polarized sunglasses, when a polarizing plate is manufactured by laminating the film parallel to the longitudinal direction and a polarizer. [Solution] A laminated polyester film comprising two or more layers, including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis direction of the film is 45° or less, and the refractive index difference obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface is 0.010 or more and 0.100 or less.

Description

[Technical Field]

[0001] The present invention relates to a laminated polyester film used in a display device, a laminated polyester film roll, a functional laminated film, a display device equipped with the laminated polyester film, and a foldable display device / image display device. [Background technology]

[0002] Polyester films are widely used as base films in many applications, such as magnetic recording materials and packaging materials, due to their excellent mechanical, electrical, dimensional stability, transparency, and chemical resistance. In recent years, in the fields of flat panel displays and touch panels, the need for thinner polarizer protective films has arisen as display devices become thinner and less expensive. Reducing the thickness of conventional TAC (triacetylcellulose) films does not provide sufficient mechanical strength, and also leads to problems with poor moisture permeability, so replacement with polyester films is being considered.

[0003] Furthermore, the use of large-screen displays outdoors is being considered through the development of display devices for outdoor environments, such as digital signage, and foldable display devices that can be folded and opened. Display devices used in outdoor environments may be viewed by viewers wearing polarized sunglasses to reduce the glare of ambient light. When display devices are viewed while wearing polarized sunglasses, polyester films have faced the challenge of reduced image quality due to optical distortion caused by the orientation of the polymer during stretching.

[0004] To address these challenges, studies have been conducted to suppress interference colors by strongly stretching the polyester film in one direction to create a specific range with a high phase difference (for example, Patent Documents 1-2). Furthermore, studies have been conducted to suppress iridescence when the slow phase axis is parallel to the longitudinal direction of the film and the film is bonded parallel to the polarizer (Patent Document 2). Furthermore, studies have been conducted on using a film structure in which specific resins are laminated to suppress interference colors, thereby setting the in-plane phase difference to a specific range with low values ​​(for example, Patent Documents 3-4). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 6950731 [Patent Document 2] Japanese Patent Publication No. 2023-171426 [Patent Document 3] Japanese Patent Publication No. 2013-200435 [Patent Document 4] Patent No. 6862654 [Overview of the project] [Problems that the invention aims to solve]

[0006] The films described in Patent Documents 1 and 2 suppress color unevenness when used in liquid crystal displays by controlling the in-plane phase difference to a high degree. However, they do not take into account viewing while wearing polarized sunglasses, and visibility through polarized sunglasses is insufficient.

[0007] The film described in Patent Document 3 does not have sufficiently low in-plane phase difference and thickness-direction phase difference, resulting in insufficient elimination of color unevenness. Furthermore, it was not designed to be viewed while wearing polarized sunglasses, resulting in inadequate performance.

[0008] In the film described in Patent Document 4, the in-plane phase difference and thickness-direction phase difference are set to a specific low range, and the variation in the width-direction in-plane phase difference is set to a specific range. As a result, the transverse stretching ratio is higher than the longitudinal stretching ratio, the orientation axis is oriented in the film width direction, and when a polarizing plate is manufactured by laminating the film parallel to the longitudinal direction and the polarizer, the performance is insufficient when viewed while wearing polarizing sunglasses.

[0009] Therefore, the object of the present invention is to provide a laminated polyester film, a laminated polyester film roll, a functional laminated film, a display device equipped with the laminated polyester film, and a foldable display device, which ensure visibility and reduce color distortion even when the screen is viewed through an optical component with polarizing properties such as polarized sunglasses, when a polarizing plate is manufactured by laminating the film parallel to the longitudinal direction and a polarizer. [Means for solving the problem]

[0010] A preferred embodiment of the present invention for solving the above problems is as follows.

[0011] In other words, it is as follows: [1] A laminated polyester film comprising two or more layers, including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis direction of the film is 45° or less, and the refractive index difference (longitudinal direction - width direction), obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface by the prism coupler method, is 0.010 or more and 0.100 or less. [2] A laminated polyester film as described in [1], wherein the width and length is 1600 mm or more, and with the center of the film width direction set as 0 mm, 800 mm wide samples are taken in two directions along the width direction, and these are combined to make a film with a width of 1600 mm, and the angles between the principal orientation axis direction and the longitudinal direction are determined at positions of 50 mm, 150 mm, 250 mm, 350 mm, 450 mm, 550 mm, 650 mm, and 750 mm from both ends in the width direction, and the maximum value among the 16 values ​​obtained is 45° or less. [3] The laminated polyester film according to [1] or [2], wherein the loss tangent tanδ at 10 Hz obtained by dynamic viscoelasticity measurement in the longitudinal direction of the film is 0.2 or more in the range of 60 to 110°C and the temperature at which it is maximum is 90°C or higher, and in the range of 110 to 160°C it is 0.2 or more and the temperature at which it is maximum is 120°C or higher. [4] A laminated polyester film according to any of [1] to [3], wherein the spectral transmittance at 380 nm is 20% or less. [5] A laminated polyester film according to any of [1] to [4], wherein the stress value at 40% elongation in the longitudinal direction is 60 MPa or more and 100 MPa or less. [6] A laminated polyester film according to any of [1] to [5], wherein the phase difference at a wavelength of 590 nm is 400 nm or less. [7] A laminated polyester film according to any of [1] to [6] used for polarizer protection. A laminated polyester film roll as described in any of [8][1] to [6]. A functional laminated film having a functional layer of 2 μm or more laminated directly or via another layer on at least one surface of a laminated polyester film as described in any of [9][1] to [6]. A laminate in which a polarizer is laminated on at least one surface of a laminated polyester film as described in any of

[10] [1] to [6]. A laminate in which the longitudinal direction of the laminated polyester film described in any of

[11] [1] to [6] is parallel to the absorption axis direction of the polarizer. A display device equipped with a laminated polyester film as described in any of

[12] [1] to [6]. A foldable display device equipped with a laminated polyester film as described in any of

[13] [1] to [6].

[14] A method for manufacturing a polarizing plate comprising a laminated polyester film consisting of two or more layers, including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis direction of the film is 45° or less, and the refractive index difference obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface is 0.010 or more and 0.100 or less, wherein the laminated polyester film is laminated in parallel with the absorption axis direction of the polarizer. [Effects of the Invention]

[0012] According to the present invention, by using the laminated polyester film according to the present invention, when a polarizing plate is manufactured by laminating it parallel to the polarizer in the longitudinal direction of the film, it is possible to provide a display device capable of ensuring visibility and reducing coloration even when viewing a screen through an optical member having a polarizing effect such as polarized sunglasses.

Brief Description of Drawings

[0013] [Figure 1-1] View from above during measurement of spectral transmittance [Figure 1-2] View of the measurement sample from the a direction in FIG. 1-1 [Figure 2] Reference diagram for explanation of the measurement test of R40 [Figure 3-1] View before bending in the measurement test of the recovery angle [Figure 3-2] View during bending in the measurement test of the recovery angle [Figure 3-3] View after bending and opening in the measurement test of the recovery angle [Figure 4] Diagram for explaining the sampling position in the film width direction

Embodiments for Carrying Out the Invention

[0014] The laminated polyester film referred to in the present invention refers to a film mainly composed of polyester as the resin constituting the film. The polyester referred to in the present invention is a polymer obtained from a dicarboxylic acid constituent, a diol constituent, or their ester-forming derivatives. The constituent refers to the smallest unit that can be obtained by hydrolyzing the polyester. The main component refers to the resin having the highest content among the resins constituting the film. Preferably, when the total mass of the resins constituting the film is 100% by mass, the resin exceeds 50% by mass, and more preferably 60% by mass or more is contained in the film. Further, as long as the object of the present invention is not inhibited, oxyacids such as hydroxybenzoic acid may be partially used.

[0015] Examples of dicarboxylic acid components that make up the polyester used in the present invention include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 5-sodium sulfisoisophthalic acid, phenylindanedicarboxylic acid, anthracenedicarboxylic acid, and phenantradicarboxylic acid; aliphatic dicarboxylic acids such as malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedionic acid, dimer acid, eicosanedionic acid, pimelic acid, azelaic acid, methylmalonic acid, and ethylmalonic acid; and alicyclic dicarboxylic acids such as adamantanedicarboxylic acid, norbornenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and decalindicarboxylic acid. Among these, aromatic dicarboxylic acids are preferred as the main dicarboxylic acid component. These dicarboxylic acid components are preferably used in combination of two or more types, and in particular, it is preferable to use at least two types, terephthalic acid and isophthalic acid.

[0016] When the sum of all dicarboxylic acid components is taken as 100 mol%, it is preferable that the total amount of terephthalic acid and isophthalic acid exemplified above accounts for 50 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and most preferably 90 mol% or more, with an upper limit of 100 mol%. Using such a polyester improves the heat resistance of the polyester and improves handling during transport when laminating with a polarizer. When the sum of all dicarboxylic acid components is taken as 100 mol%, it is preferable that the total amount of isophthalic acid accounts for 4 mol% or more, more preferably 6 mol% or more, even more preferably 8 mol% or more, with an upper limit of 20 mol% or less, more preferably 16 mol% or less, and even more preferably 14 mol% or less. Using such a polyester is preferable because it makes it easier to control the refractive index and improves visibility when used as a display device.

[0017] Examples of diol components constituting the polyester used in the present invention include ethylene glycol, 1,3-butanediol, 1,4-butanediol, tetramethylene glycol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, paraxylene glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, 1,4:3,6-dianehydroglucitol, 1,2-propanediol, and 1,3-propanediol. Examples include diols, neopentyl glycol, 1,5-pentanediol, 1,7-heptanediol, 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene, 9,9-bis[4-(2-hydroxyethoxy)-3-methylphenyl]fluorene, 9,9-bis[4-(2-hydroxyethoxy)-3,5-dimethylphenyl]fluorene, 2,2-bis(4-hydroxyethoxyphenyl)propane, bisphenol A, spiroglycol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. It is preferable that two or more of these diol components are used in combination, and in particular, it is preferable to use at least one component selected from paraxylene glycol, bisphenol A, 1,4:3,6-dianhydroglucitol, 2,2,4,4-tetramethyl-1,3-cyclobutanediol, and cyclohexanedimethanol as copolymerization components in addition to ethylene glycol.

[0018] When the sum of all diol components is taken as 100 mol%, it is preferable that the total amount of the fluorene derivatives, paraxylene glycol, bisphenol A, 1,4:3,6-dianhydroglucitol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol exemplified above accounts for 5 mol% or more, more preferably 7.5 mol% or more, particularly preferably 15 mol% or more, with an upper limit of preferably 40 mol% or less, more preferably 35 mol% or less, and particularly preferably 30 mol% or less. By using such a copolymerized polyester, the refractive index at 633 nm can be easily controlled, resulting in good visibility when used as a display device, and good handling during transport when bonded with a polarizer. To control R40 to a low level, it is even more preferable to use one or more additional copolymerized components in addition to these.

[0019] Furthermore, polyester may contain resins other than polyester, and polyetherimide is one such resin. However, from the viewpoint of suppressing discoloration caused by polyetherimide, it is preferable that it be contained in an amount of 5% by mass or less relative to the polyester film.

[0020] A preferred embodiment of the laminated polyester film of the present invention is that, in order to achieve good visibility of the mounted image display, it is necessary to have a laminated film with at least two layers. In addition to a two-layer laminated polyester film, a three-layer laminated film or a film with four or more layers may also be used, but from the viewpoint of mass production, it is preferable to have 10 layers or less.

[0021] In the case of a two-layer laminated polyester film, it is important that the polyester composition of each layer, A and B, includes polyester (A) and polyester (B), as described below, with polyester (A) in layer A and polyester (B) in layer B.

[0022] The dicarboxylic acid components for obtaining polyester (A) preferably contain terephthalic acid in an amount of 80 mol% or more, more preferably 90 mol% or more, and particularly preferably 95 mol% to 100 mol%, when the sum of all dicarboxylic acid components contained in polyester (A) is taken as 100 mol%. Furthermore, the diol components for obtaining polyester (A) preferably contain ethylene glycol in an amount of 60 mol% or more, more preferably 70 mol% or more, particularly preferably 80 mol% or more, with an upper limit of preferably 95 mol% or less, more preferably 90 mol% or less, and even more preferably 85 mol% or less. In particular, when the sum of all diol components contained in polyester (A) is taken as 100 mol%, it is preferable that the total amount of the copolymer components exemplified above—fluorene derivatives, paraxylene glycol, bisphenol A, 1,4:3,6-dianhydroglucitol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol—accounts for 5 mol% or more, more preferably 7.5 mol% or more, and especially preferably 15 mol% or more. The upper limit is preferably 20 mol% or less, more preferably 18 mol% or less, and especially preferably 16 mol% or less.

[0023] Preferably, the polyester (A) has a melting peak temperature of 240°C or higher as observed by a differential scanning calorimeter. A melting peak temperature of 240°C or higher tends to improve the mechanical properties of the film. From the viewpoint of processability, a melting peak temperature of 270°C or lower is preferable. The melting peak temperature can be controlled to be within the above range by lowering the total amount of copolymer components. However, as a method of controlling it to be above the preferred lower limit of the total amount of copolymer components, it is preferable to use a mixture of two or more polyester resins, such as a polyester with a melting peak temperature of 240°C or higher and a polyester containing copolymer components. Using a mixture of two or more polyester resins in this way makes it easier to form layer A, which has a high melting peak temperature and a total amount of copolymer components within the preferred range, and is therefore preferable. Adding a catalyst deactivator is also a preferred method for controlling the melting peak temperature to be within the above range. As the catalyst deactivator, a phosphate ester compound containing an alkyl chain with 8 or more carbon atoms is preferred, more preferably a phosphate ester with 14 or more carbon atoms, and among these, phosphate esters with 15 to 20 carbon atoms are particularly preferred from the viewpoint of easily controlling the melting peak temperature to 240°C or higher, minimizing discoloration, and maintaining good transparency. Examples of such compounds include dihexadecyl phosphate, dioctadecyl phosphate, and dinonadecyl phosphate. The catalyst deactivator is preferably present in an amount of 0.01% to 1.00% by mass relative to the mass of polyester (A).

[0024] Polyester (B) preferably contains copolymer components in its dicarboxylic acid components and / or diol components. When the sum of all dicarboxylic acid components contained in polyester (B) is taken as 100 mol%, it is preferable that terephthalic acid is contained in an amount of 80 mol% or more, more preferably 90 mol% or more, particularly preferably 95 mol% or more, with an upper limit of preferably 100 mol% or less, more preferably 98 mol% or less, and even more preferably 96 mol% or less. Furthermore, when the sum of all diol components contained in polyester (B) is taken as 100 mol%, it is preferable that ethylene glycol is contained in an amount of 60 mol% or more, more preferably 70 mol% or more, particularly preferably 80 mol% or more, with an upper limit of preferably 95 mol% or less, more preferably 90 mol% or less, and even more preferably 85 mol% or less. In particular, when the sum of all diol components contained in polyester (A) is taken as 100 mol%, it is preferable that the total amount of the fluorene derivatives, paraxylene glycol, bisphenol A, 1,4:3,6-dianhydroglucitol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol exemplified above accounts for 5 mol% or more, more preferably 7.5 mol% or more, particularly preferably 15 mol% or more, with an upper limit of preferably 20 mol% or less, even more preferably 18 mol% or less, and particularly preferably 16 mol% or less.

[0025] Preferably, polyester (B) has a melting peak temperature of 180°C to 230°C as observed by differential scanning calorimeter. Having a melting peak temperature within this range makes it easier to control the refractive index anisotropy of layer B to a low level. The melting peak temperature can be controlled by the total amount of copolymer components, but as a method to control it within a preferred range of the total amount of copolymer components, it is preferable to use a mixture of two or more polyester resins, such as a polyester with a melting peak temperature of 200°C to 235°C and a polyester containing copolymer components. Using such a mixture of two or more polyester resins makes it easier to form layer B with a melting peak temperature within the preferred range and a preferred total amount of copolymer components, which is preferable.

[0026] By using the above polyester composition, the anisotropy of the refractive index in layer A can be easily controlled by the orientation of the crystals, and the anisotropy of the refractive index in layer B can be controlled to a low level while maintaining mechanical properties. This improves the visibility when used as a display device and the handling during transport when bonded with a polarizer. Furthermore, it also exhibits good resistance to folding.

[0027] In the case of a two-layer laminated polyester film, the thickness of each layer can be any thickness as long as it does not impair the effects of the present invention. The lamination ratio, obtained by dividing the thickness of the laminated polyester film by the thickness of layer A, is preferably 2 to 20, more preferably 3 or more as the lower limit, even more preferably 4 or more, even more preferably 5 or more, and more preferably 18 or less as the upper limit, even more preferably 16 or less, and even more preferably 14 or less.

[0028] In the case of a three-layer laminated polyester film, it is a three-layer laminated polyester film consisting of layer A, layer B, and layer C, with at least one of the surface layers being layer A. When one of the surface layers is layer A, any of the following lamination configurations can be adopted: layer A / layer B / layer C, or layer A / layer C / layer B.

[0029] When adopting a laminated configuration of A / B / C layers, the polyester composition of layer C may be different from or the same as that of layer A, but it is preferable that the polyester compositions of layer C and layer A be the same. It is preferable that layer A contains polyester (A), layer B contains polyester (B), and layer C contains polyester (A) with the same or different composition as layer A. The lamination ratio, obtained by dividing the thickness of the laminated polyester film by the total thickness of layers A and C, is preferably between 2 and 20 in order to obtain good mechanical properties and good visibility when used as a display device. Also, the thickness of layer A T A and C layer T C Regarding T A ≧T CIn this case, the A / C ratio obtained by dividing the thickness of layer A by the thickness of layer C is preferably as close to 1.0 as possible to suppress curling of the laminated polyester film and obtain good flatness, but is preferably 10.0 or less, more preferably 6 or less, even more preferably 4 or less, and even more preferably 2 or less. On the other hand, from the viewpoint of improving folding resistance, the A / C ratio is preferably 1.1 to 10.0, more preferably 1.2 or more, even more preferably 1.3 or more, with an upper limit of preferably 6.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less, in order to distribute the stress concentrated on the folded inner and outer surfaces.

[0030] A preferred embodiment of the laminated polyester film of the present invention requires that the angle between the longitudinal direction of the film and the principal orientation axis of the film be 45° or less in order to achieve good visibility of the mounted image display. The longitudinal direction of the film is the direction of film flow during film formation, for example, the winding direction of the film roll. The principal orientation axis of the film corresponds to the slow axis from the perspective of the vibrational behavior of light, and is sometimes called the slow axis in the film plane. The transmission axes of the polarizer and the analyzer are arranged parallel to each other, the film is placed between the polarizer and the analyzer, and the polarizer and the analyzer are kept in a parallel nicol state and rotated once. The principal orientation axis of the object to be measured can be determined from the change in transmitted light intensity at that time. The angle between the longitudinal direction of the film and the principal orientation axis of the film is a value of 0° or more and 90° or less among the angles formed when a straight line in the longitudinal direction and a straight line in the principal orientation axis direction intersect in the film plane, where 0° indicates that the longitudinal direction and the principal orientation axis direction are parallel, and 90° indicates that the longitudinal direction and the principal orientation axis direction are perpendicular. Having an angle of 45° or less means that the principal orientation axis is in a direction that is 45° or less from the direction parallel to the longitudinal direction. With such a film, when a polarizing plate is manufactured by laminating the film parallel to the longitudinal direction and the polarizer, visibility is good when wearing polarized sunglasses. The method of making the angle between the longitudinal direction of the film and the principal orientation axis of the film 45° or less is adjusted by the stretching ratio or stretching factor in the longitudinal and width directions of the film, and it is important to adjust the stretching ratio in the width direction to be less than or equal to the stretching ratio in the longitudinal direction.

[0031] The laminated polyester film of the present invention has a width and length of 1600 mm or more. With the center of the film width direction set as 0 mm, samples of 800 mm width are taken in two directions along the width direction, and these are combined to make a film with a width of 1600 mm. The angles between the principal orientation axis direction and the longitudinal direction are determined at positions of 50 mm, 150 mm, 250 mm, 350 mm, 450 mm, 550 mm, 650 mm, and 750 mm from both ends in the width direction. Preferably, the maximum value among the 16 values ​​obtained is 45° or less. To explain using Figure 4, the center 401 in the film width direction is set to 0 mm, and 800 mm wide sections 402 are taken in two directions along the width direction to create a 1600 mm wide film 403. The angle between the principal orientation axis and the longitudinal direction is determined at positions 405 (1500 mm width) at 50 mm, 150 mm, 250 mm, 350 mm, 450 mm, 550 mm, 650 mm, and 750 mm from both ends in the width direction. The maximum angle is determined, and this value must be 45° or less. By keeping it within this range, visibility is good at any position on the display surface of the display device, even for larger display devices, which is preferable.

[0032] In one preferred embodiment of the laminated polyester film of the present invention, in order to achieve good visibility of the mounted display device, the refractive index difference (longitudinal direction - width direction), obtained by subtracting the value in the width direction from the value in the longitudinal direction for the refractive index of 633 nm measured from at least one surface using the prism coupler method, must be between 0.010 and 0.100. If the refractive index difference exceeds 0.100, the phase difference tends to increase, which not only worsens visibility but also increases the anisotropy of the film's mechanical properties, making it more prone to tearing and other adverse effects. Furthermore, having a refractive index difference of 0.010 or more makes it easier to control the axis direction of the main orientation, allowing for highly improved visibility. In the prism coupler method, the refractive index of the surface layer on the measurement side of the laminated polyester film can be measured and determined from the lowest angular position where the intensity of reflected light decreases. At this time, the refractive index of extremely thin layers such as easily bonded layers with a thickness of 200 nm or less does not affect the prism coupler method. The method for setting the refractive index difference within the above range is adjusted by the stretching ratio or magnification in the longitudinal and width directions of the film, and it is important to adjust the stretching ratio in the width direction to be less than or equal to the stretching ratio in the longitudinal direction. The refractive index difference has a lower limit of 0.010 or more, preferably 0.020 or more, more preferably 0.030 or more, and an upper limit of 0.100 or less, preferably 0.090 or less, more preferably 0.080 or less, even more preferably 0.070 or less, and even more preferably 0.060 or less.

[0033] The reason why visibility when wearing polarized sunglasses can be improved in polarizing plates manufactured by laminating the film parallel to the longitudinal direction and the polarizer by controlling the principal orientation axis direction of the film and the refractive index difference within the above range is thought to be as follows: The deterioration of visibility when wearing polarized sunglasses is related to the fact that linearly polarized light that has passed through the polarizer of the display device is converted to elliptical polarization by the refractive index difference of the film placed on the viewing side. When the display device is observed from a direction perpendicular to the display surface, the effect on linear polarization is small because the refractive index difference is small, but when the display device is observed from an oblique direction, if the absorption axis in the longitudinal direction of the polarizer is misaligned with the principal orientation axis direction of the film, the effect of the refractive index difference is amplified, resulting in elliptical polarization and a deterioration in visibility.

[0034] In the laminated polyester film of the present invention, it is preferable that the loss tangent (tanδ) at 10 Hz obtained by dynamic viscoelasticity measurement in the longitudinal direction of the film is 0.2 or higher in the range of 60 to 110°C, with the maximum temperature being 90°C or higher, and 0.2 or higher in the range of 110 to 160°C, with the maximum temperature being 120°C or higher. In the present invention, the loss tangent (tanδ) is expressed as the ratio of the storage modulus to the loss modulus, obtained by measuring dynamic viscoelasticity in the measurement temperature range of 20°C to 200°C. The temperature at which the loss tangent is 0.2 or higher and maximum is the temperature involved in the glass transition of the film, and it is preferable that the glass transition temperature originating from the layer with low refractive index anisotropy is observed in the range of 60°C to 110°C, and the glass transition temperature originating from the layer that controls the phase difference by the oriented crystal is observed in the range of 110°C to 160°C. Maintaining excellent heat resistance is possible if the temperature at which the loss tangent is maximum in the 60°C to 110°C range is 90°C or higher, and good mechanical properties are achieved if the temperature at which the loss tangent is maximum in the 110°C to 160°C range is 120°C or higher. Methods for controlling the temperature at which the loss tangent is maximum in the 60°C to 110°C range to be high include raising the glass transition temperature of the polyester resin in the layer with low refractive index anisotropy, or controlling the heat treatment temperature to be low. Methods for controlling the temperature at which the loss tangent is maximum in the 110°C to 160°C range to be high include controlling the stretch ratio to be high and controlling the stretch temperature to be 20°C or lower, which is the highest glass transition temperature of the polyester resin used.

[0035] In the laminated polyester film of the present invention, it is preferable that a glass transition temperature of 60°C to 110°C is observed in one layer, and a glass transition temperature of 110°C to 160°C is observed in at least one other layer. In the case of a three-layer laminated polyester film, it is preferable that a glass transition temperature of 110°C to 160°C is observed in at least one surface layer, and it is more preferable that a glass transition temperature of 110°C to 160°C is observed in both surface layers. Having a laminated structure with the above-described glass transition temperatures makes it easy to control the refractive index of the laminated polyester film and also improves heat resistance, which is preferable. Furthermore, it is preferable that the layer in which a glass transition temperature is observed at 60°C to 110°C is a layer with low refractive index anisotropy, and the layer in which a glass transition temperature is observed at 110°C to 160°C is a layer in which the phase difference is controlled by oriented crystals. The glass transition temperature of each layer can be measured by known methods, for example, by scraping off each layer and determining it at the midpoint temperature of the baseline shift of differential scanning calorimetry.

[0036] The laminated polyester film of the present invention is preferably 20 μm or more and 70 μm or less in thickness. This range is preferable because it improves flexibility and visibility. The preferred lower limit is 20 μm or more, more preferably 25 μm or more, even more preferably 30 μm or more, even more preferably 35 μm or more, and particularly preferably 38 μm or more. The preferred upper limit is 70 μm or less, more preferably 60 μm or less, even more preferably 55 μm or less, even more preferably 50 μm or less, and particularly preferably 48 μm or less.

[0037] In the laminated polyester film of the present invention, the thickness of the layer in which the glass transition temperature is observed between 60°C and 110°C is preferably 20 μm or more and 60 μm or less. Furthermore, the thickness of the layer in which the glass transition temperature is observed between 110°C and 160°C is preferably 0.5 μm or more and 5 μm or less.

[0038] The laminated polyester film of the present invention preferably has a spectral transmittance of 20% or less at a wavelength of 380 nm. More preferably, it is 15% or less, and even more preferably 10% or less. For optical applications, it is required to have low haze and transparency while simultaneously possessing ultraviolet absorption ability. It is particularly preferable to absorb ultraviolet light in the 300 nm to 380 nm range, which is close to visible light, and the indicator for this is a light transmittance of 20% or less at 380 nm. To reduce ultraviolet transmittance, it is effective to add an ultraviolet absorber containing a polymer that has an ultraviolet absorption ability. When adding an ultraviolet absorber, it is preferable to use as little as possible to suppress discoloration and bleed-out, and it is preferable to use an ultraviolet absorber with a high molar extinction coefficient at 380 nm. Benzoxazinon-based ultraviolet absorbers, benzotriazole-based ultraviolet absorbers, and triazine-based ultraviolet absorbers can be used as ultraviolet absorbers. Representative examples of benzoxazinon-based UV absorbers include 2,2'-p-phenylenebis(4H-3,1-benzoxazine-4-one), 2,2'-p-phenylenebis(6-methyl-4H-3,1-benzoxazine-4-one), and 2,2'-p-phenylenebis(6-chloro-4H-3,1-benzoxazine-4-one). Representative examples of benzotriazole-based UV absorbers include 2-(2-hydroxy-5-tert-butylphenyl)-2H-benzotriazole, 2-(2H-benzotriazole-2-yl)-4-methylphenol, 2-(2H-benzotriazole-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol, and 2,2'-methylenebis[6-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol)].Representative examples of triazine-based UV absorbers include 2,4-bis(2-hydroxy-4-butyloxyphenyl)-6-(2,4-bis-butyloxyphenyl)-1,3,5-triazine, 2,4-bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-n-octyloxyphenyl)-1,3,5-triazine, 4,4',4”-(1,3,5-triazine-2,4,6-triyltriimino)tris(2-ethylhexyl)trisbenzoate, and 2,4,6-tris(2-hydroxy-4-hexyloxy-3-methylphenyl)-1,3,5-triazine. UV absorbers can be used alone or in combination of two or more types. However, it is preferable to select from benzoxazinon-based UV absorbers and benzotriazole-based UV absorbers. Furthermore, UV absorbers having a maximum absorption wavelength in the range of 320 nm to 400 nm are preferred. A higher molar extinction coefficient at 380 nm is preferable as it reduces the amount of UV absorber added, but it is preferable to select the UV absorber while also considering coloration due to absorption around 400 nm. It is preferable to include 0.5% to 2% by mass of the UV absorber relative to the laminated polyester film. In the case of a three-layer laminated polyester film, it is preferable to include the UV absorber in the intermediate layer sandwiched between the two layers, as this suppresses the bleeding out of the UV absorber.

[0039] In the present invention, including a phosphorus-based antioxidant along with an ultraviolet absorber is preferable because it reduces coloration due to absorption above 400 nm, does not affect visibility when incorporated into a display device, and allows for low transmittance control at a wavelength of 380 nm. Examples of phosphorus-based antioxidants include triphenyl phosphite, tris(nonylphenyl) phosphite, triisodecyl phosphite, isodecyldiphenyl phosphite, 2-ethylhexyldiphenyl phosphite, tris(2,4-di-tert-butylphenyl) phosphite, 3,9-bis(octadecyloxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, and 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4 Examples include ,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane, tetra-C12-15-alkyl(propane-2,2-diylbis(4,1-phenylene))bis(phosphite), and 2,2'-methylenebis(4,6-di-tert-butylphenyl)-2-ethylhexylphosphite, with 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane being more preferred. The phosphorus-based antioxidant is preferably present in an amount of 0.01% by mass or more and 1.00% by mass or less relative to the laminated polyester film.

[0040] The laminated polyester film of the present invention preferably has a stress (F40) value of 60 MPa or more and 100 MPa or less at a longitudinal elongation of 40%. The F40 value is the stress value in a tensile test, and setting it within the above range is preferable because it provides a film with good toughness, which suppresses the occurrence of defects such as cracking and chipping during the lamination process with a polarizer, thus improving the optical properties as a polarizer.

[0041] The laminated polyester film of the present invention preferably has a phase difference of 400 nm or less at a wavelength of 590 nm. More preferably, it is 300 nm or less, and even more preferably 250 nm or less. The lower limit is preferably 50 nm or more, and more preferably 80 nm or more. By setting it within the above range, good visibility of the image display on which the film is mounted can be obtained. The phase difference can be controlled by adjusting the stretching ratio or stretching factor in the longitudinal and width directions of the film during the film formation process, and can be adjusted by setting the stretching ratio in the width direction to be less than or equal to the stretching ratio in the longitudinal direction.

[0042] The laminated polyester film of the present invention preferably has a phase difference (R40) of 40° oblique at a wavelength of 590 nm, measured at a 40° angle centered perpendicular to the main orientation axis of the film, of 500 nm or less. More preferably, it is 400 nm or less, even more preferably 350 nm or less, with a lower limit of 100 nm or more, and more preferably 150 nm or more. By keeping it within the above range, good visibility of the image display on which this film is mounted can be obtained. The phase difference at a wavelength of 590 nm and the phase difference at 40° oblique at a wavelength can be measured by a phase difference measuring device, such as the KOBRA-WPR manufactured by Oji Instruments Co., Ltd.

[0043] Methods for controlling the 40° phase difference within the above range include controlling the film thickness and the birefringence in the thickness direction of the film. Generally, as the film is oriented in two axes, surface orientation progresses and the 40° phase difference increases. Therefore, it can be controlled by controlling the resin composition as described above and increasing the heat treatment temperature in the film formation process.

[0044] The laminated polyester film of the present invention has a spectral transmittance difference ΔT of 0% or more and 40% or less, as measured by the following evaluation method. (Measurement method) The film was cut to dimensions of 4.0 cm in the longitudinal direction and 5.0 cm in the width direction. The film was then sandwiched between two polarizing plates (Kenis Co., Ltd., thin polarizing film, size S, transmittance 0.43, polarization rate 0.9999, product code 1-115-0820) so that the longitudinal direction of the film and the absorption axis of the polarizing plates were parallel to each other, and used as a measurement sample. The measurement was performed using a Hitachi High-Technologies Corporation (formerly Hitachi High-Technologies Corporation) U-4100 spectrophotometer with the attached angle-adjustable transmission attachment for the U-4100 spectrophotometer, using an 8 mm (H) x 5 mm (W) light source mask, and a direct incidence detector with a mirror surface to measure the light transmittance at wavelengths of 450 to 750 nm. The measurement conditions were set to a scanning speed of 600 nm / min and a sampling pitch of 1 nm, in high-resolution measurement mode. During measurement, the angle of the variable-angle transmission device is set to 30° or 60°, and the sample is positioned so that the angle between the film width direction and the horizontal plane is 30°, with the polarizing plate's transmission axis coinciding with the horizontal plane being defined as 0°. For baseline measurement, only two polarizing plates are used as the sample, stacked with their absorption axes aligned. The angle of the variable-angle transmission device is set to 30° or 60° in the same manner as above, and the sample is positioned so that the angle between the transmission axis and the horizontal plane is 30°, with the polarizing plate's transmission axis coinciding with the horizontal plane being used as the reference point. For angles of 30° and 60° respectively, the average light transmittance at wavelengths of 450-750nm is defined as T30 and T60, and ΔT is calculated by subtracting T60 from T30. In the above evaluation, the longitudinal direction of the film is the winding direction of the film roll.

[0045] From the viewpoint of improving visibility when mounted as a screen protector or polarizer protector, it is more preferable that the ΔT measured by the above measurement method is in the range of 0% to 30%, even more preferable that it is in the range of 0% to 20%, and most preferably that it is in the range of 0% to 15%. In addition, it is most preferable that ΔT be 0% from the viewpoint of improving visibility, but when ΔT approaches 0%, it is necessary to make the film refractive index anisotropy very small, which may result in poor flexibility. For this reason, when considering both visibility and flexibility, it is preferable that ΔT be 2% or more, even more preferable that ΔT be 5% or more, and most preferably that it be 10% or less.

[0046] Next, a preferred embodiment for manufacturing the laminated polyester film of the present invention will be described below, using a three-layer laminated polyester film as an example. However, the present invention is not limited to this example.

[0047] In this invention, the aforementioned polyester (A) and polyester (B) are supplied to separate vented twin-screw extruders and melt-extruded. Preferably, the extruder is kept under a flowing nitrogen atmosphere with an oxygen concentration of 0.7% by volume or less, and the resin temperature is controlled to 240°C to 320°C. Next, foreign matter is removed and the extrusion volume is equalized through filters and gear pumps. Then, in a three-layer confluence block equipped with a rectangular lamination section, polyester (A) is laminated on both surface layers and polyester (B) on the inner layer, and the resulting sheet is discharged from the T-die onto a cooling drum. At this time, the sheet-like polymer is adhered to the casting drum by electrostatic application using electrodes with high voltage to create a water film between the casting drum and the extruded polymer sheet, by setting the casting drum temperature below the glass transition point of the polyester resin to cause adhesion of the extruded polymer, or by a combination of these methods, and then cooled and solidified to obtain an unstretched film. Among these casting methods, when using polyester, the electrostatic application method is preferred from the viewpoint of productivity and flatness.

[0048] Regarding the lamination structure, a laminated polyester film can be produced by co-extruding using two or more extruders as described above. The lamination structure can be set as needed, for example, an A / B lamination structure in which one layer each of layer A and layer B are laminated, a multilayer lamination structure in which multiple layers of layer A and layer B are laminated alternately, or a lamination structure in which three or more types of layers are laminated. When producing a laminated polyester with 101 or more layers, it is preferable to apply a static mixer or feed block to merge the layer A made of the polyester composition of the present invention and the layer B made of the other polyester composition so that they are laminated alternately, and both surface layers become layer B.

[0049] The laminated polyester film of the present invention is preferably biaxially stretched from the viewpoint of heat resistance, dimensional stability, mechanical strength, flatness, and thickness uniformity. It can be obtained by stretching by a sequential biaxial stretching method in which an unstretched film is stretched longitudinally and then stretched in the width direction, or by stretching in the width direction and then stretching in the longitudinal direction, or by a simultaneous biaxial stretching method in which the longitudinal and width directions of the film are stretched almost simultaneously. Alternatively, the film may be further stretched in the longitudinal or width direction after biaxial orientation.

[0050] In the laminated polyester film of the present invention, the following steps (1) to (3) are examples of preferred stretching steps, but are not limited thereto. • Process (1): A process of stretching the material by 3.4 times or more and 4.5 times or less in the longitudinal direction. • Process (2): A process of stretching the material to a width of 3.2 times or more and 4.0 times or less. • Process (3): A process of heat treatment at a temperature between 200°C and 230°C. Regarding step (1): the step of stretching the material by 3.4 times or more and 4.5 times or less in the longitudinal direction, in order to obtain the more remarkable effects of the present invention, a lower limit of 3.4 times or more, preferably 3.5 times or more, more preferably 3.6 times or more, and even more preferably 3.7 times or more is adopted. As an upper limit, a limit of 4.5 times or less, preferably 4.2 times or less, and more preferably 4.0 times or less is adopted. Furthermore, stretching may be performed in a single stretching section to a predetermined magnification, but it is preferable to divide the stretching into two or more stages and stretch to a predetermined magnification. For example, when stretching in three stages, the stretching magnification of the first stage can be 1.02 times or more and 1.2 times or less, the stretching magnification of the second stage can be 1.05 times or more and 1.2 times or less, and the stretching magnification of the third stage can be 3.0 times or more and 4.0 times or less. A stretching method using the difference in rotational speed of a rotating roll is preferred because it makes it easier to control the refractive index anisotropy in the longitudinal direction. The longitudinal stretching temperature is preferably above the glass transition temperature of the layer exhibiting the highest glass transition temperature, and below the glass transition temperature + 20°C. For example, if the glass transition temperature is 90°C, the stretching temperature should be between 90°C and 110°C.

[0051] Regarding step (2): the step of stretching the film to a width of 3.2 times or more and 4.0 times or less, in order to obtain the more remarkable effects of the present invention, the lower limit of the stretching ratio in the width direction is preferably 3.3 times or more, more preferably 3.4 times or more, and the upper limit is preferably 3.8 times or less, more preferably 3.7 times or less. Known methods and apparatus can be used for stretching, but using a tenter apparatus is preferable in that it can improve the uniformity of the film thickness. The stretching temperature in the width direction is preferably above the glass transition temperature of the layer showing the highest glass transition temperature and below the glass transition temperature + 20°C. In addition, it is preferable to set the preheating temperature before stretching to a temperature 5°C to 20°C higher than the stretching temperature, as this can make it easier to align the main orientation axis in the longitudinal direction.

[0052] Regarding step (3): the step of heat treatment at 200°C to 230°C, in order to obtain a more significant effect of the present invention, it is preferable to provide a temperature range lower than that of step (3) before and after the step of heat treatment at 200°C to 230°C. If the temperature of the heat treatment step is too high, the uniformity of the principal orientation axis in the width direction may be poor, and if the heat treatment temperature is too low, it may be difficult to control the refractive index anisotropy, and the effect of improving visibility when used as a display device may not be obtained. By setting the temperature within the above range, good mechanical properties and visibility can be achieved, and it is preferably 210°C to 230°C.

[0053] The temperature in step (3) may include multiple temperature ranges. In addition, stretching and / or relaxation can be performed in the longitudinal and / or widthwise directions in step (3). For example, relaxation of 5% or less can be performed in the longitudinal and / or widthwise directions. The heat treatment time can be arbitrary as long as it does not degrade the properties, preferably 1 second to 60 seconds, more preferably 5 seconds to 40 seconds, and most preferably 10 seconds to 30 seconds. Known methods and apparatus can be used for the heat treatment, but it is preferable to perform the heat treatment in step (3) immediately after the stretching step (2) of the tenter apparatus that stretches in the widthwise direction, as this allows for high control of uniformity in the widthwise direction.

[0054] In this invention, the material may be stretched biaxially and then further stretched in the longitudinal direction. When stretching further after biaxial stretching, the orientation can be deflected in the longitudinal direction, the direction of the principal orientation axis can be made uniform, and unevenness in the plane can be suppressed. When stretching further after biaxial stretching, the stretching ratio is preferably 1.03 times or more and 2 times or less in the longitudinal direction. To obtain a more remarkable effect of this invention, a lower limit of 1.03 times or more is preferred, more preferably 1.04 times or more, and even more preferably 1.05 times or more is adopted. As an upper limit, 2.00 times or less is preferred, more preferably 1.50 times or less, and even more preferably 1.20 times or less is adopted. The stretching method using the difference in rotational speed of rotating rolls is preferred because it is easy to control the refractive index anisotropy in the longitudinal direction. When stretching further after biaxial stretching, the stretching temperature in the longitudinal direction is 100°C or more and 150°C or less.

[0055] In the present invention, the area stretching ratio, which is the sum of the stretching ratios in the longitudinal and width directions, is preferably 10 times or more from the viewpoint of thickness uniformity. Furthermore, in controlling the refractive index difference between the longitudinal and width directions to the above range, it is preferable to set the value obtained by dividing the longitudinal stretching ratio Ls by the width stretching ratio Rs (Ls / Rs) to 1.01 or more, more preferably Ls / Rs to 1.03 or more, even more preferably 1.05 or more, with an upper limit of preferably 1.50 or less, more preferably 1.30 or less, and even more preferably 1.20 or less.

[0056] Furthermore, to improve adhesion to the polarizer, corona treatment can be applied to at least one side, or an easy-adhesion layer can be coated. A preferred method for providing the coating layer in-line within the film manufacturing process is to uniformly apply a coating layer composition dispersed in water onto a film that has been at least uniaxially stretched using a metering wire bar or gravure roll, and then dry the coating while stretching the film. In this case, the thickness of the easy-adhesion layer is preferably 0.05 μm to 0.50 μm. Various additives, such as antioxidants, heat stabilizers, ultraviolet absorbers, infrared absorbers, pigments, dyes, organic or inorganic particles, antistatic agents, and nucleating agents, may also be added to the easy-adhesion layer. The resin preferably used for the easy-adhesion layer is at least one resin selected from acrylic resins, polyester resins, and urethane resins, from the viewpoint of adhesion and handling. Furthermore, off-annealing under conditions of 70°C to 150°C is also preferably used.

[0057] In the polyester film of the present invention, when laminating a surface layer on the outermost surface to impart functions such as hard coating, self-healing, anti-glare, anti-reflective, low-reflectivity, and antistatic properties, it is preferable to use a manufacturing method that forms the surface layer by applying, drying, and curing the aforementioned coating composition. The method of manufacturing the surface layer by application is not particularly limited, but it is preferable to form the surface layer by applying the coating composition to a support substrate or the like using a dip coating method, roller coating method, wire bar coating method, gravure coating method, or die coating method (U.S. Patent No. 2,681,294). Furthermore, among these application methods, the gravure coating method or the die coating method is more preferable as the application method. It is preferable from the viewpoint of economy that the thickness of the surface layer by application be 1 μm or more and 10 μm or less so that the surface properties can be fully expressed.

[0058] Next, to completely remove the solvent by drying the coated liquid film, it is preferable to heat the liquid film during the drying process. Drying methods include heat transfer drying (adhesion to a high-temperature object), convection heat transfer (hot air), radiant heat transfer (infrared rays), and others (microwaves, induction heating). Among these, from the viewpoint of precisely achieving a uniform drying rate in the width direction, methods using convection heat transfer or radiant heat transfer are preferred. Furthermore, a further curing operation (curing process) may be performed by irradiation with heat or energy rays. In the curing process, if curing is performed by heat, the temperature is preferably between room temperature and 200°C, more preferably between 100°C and 200°C, and even more preferably between 130°C and 200°C, from the viewpoint of the activation energy of the curing reaction.

[0059] When cured by active energy rays, it is preferably an electron beam (EB ray) and / or an ultraviolet ray (UV ray) from the viewpoint of versatility. When cured by ultraviolet rays, since oxygen inhibition can be prevented, it is preferable that the oxygen concentration is as low as possible, and it is more preferable to cure in a nitrogen atmosphere (nitrogen purge). When the oxygen concentration is high, the curing of the outermost surface may be inhibited, and the surface curing may be insufficient. Examples of the type of ultraviolet lamp used when irradiating ultraviolet rays include a discharge lamp method, a flash method, a laser method, an electrodeless lamp method, and the like. When ultraviolet curing is performed using a high-pressure mercury lamp which is a discharge lamp method, the ultraviolet illuminance is 100 to 3,000 mW / cm 2 , preferably 200 to 2,000 mW / cm 2 , more preferably 300 to 1,500 mW / cm 2 It is preferable to perform ultraviolet irradiation under the condition that the integrated light amount of ultraviolet rays is 100 to 3,000 mJ / cm 2 , preferably 200 to 2,000 mJ / cm 2 , more preferably 300 to 1,500 mJ / cm 2 It is more preferable to perform ultraviolet irradiation under the condition that the integrated light amount of ultraviolet rays is 100 to 3,000 mJ / cm

[0060] The laminated polyester film of the present invention has controlled refractive index anisotropy and excellent flexibility, thus suppressing interference colors when mounted on display devices that are repeatedly bent, such as display covers or curved surfaces. Taking advantage of these properties, it can be laminated with a PVA sheet (polarizer) created by aligning iodine within PVA to be used as a polarizing plate, preventing scratches on the display device surface without impairing the flexibility of the device. Furthermore, in addition to optical films, it is also preferable to use the present invention as a material film for various cover films and packaging applications, utilizing its properties.

[0061] The present invention relates to a method for manufacturing a polarizer, comprising a laminated polyester film consisting of two or more layers, including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis of the film is 45° or less, and the refractive index difference obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface is 0.010 or more and 0.100 or less, wherein the laminated polyester film is laminated parallel to the longitudinal direction and the absorption axis direction of the polarizer. A polarizer manufactured by the above method can be made visible when used in a display device while maintaining good productivity. The laminated polyester film functions as a polarizer protective film, suppressing scratches on the polarizer, warping due to moisture absorption of the polarizer, and deterioration of polarization characteristics.

[0062] The lamination of the polarizer and the laminated polyester film is carried out by a roll-to-roll process. Preferably, in the polarizer manufacturing method, a polarizer plate is obtained by laminating a long polarizer, which is obtained through a longitudinal stretching process and has an absorption axis in the longitudinal direction, with a long laminated polyester film via an adhesive. The polarizer plate obtained by the manufacturing method of the present invention may or may not include an adhesive layer.

[0063] There are no particular restrictions on the adhesive used to bond the polarizer and the laminated polyester film. For example, acrylic adhesives, urethane adhesives, polyester adhesives, polyvinyl alcohol adhesives, polyolefin adhesives, modified polyolefin adhesives, polyvinyl alkyl ether adhesives, rubber adhesives, vinyl chloride-vinyl acetate adhesives, SEBS (styrene-ethylene-butylene-styrene copolymer) adhesives, ethylene-styrene copolymers and other ethylene-based adhesives, acrylic ester adhesives such as ethylene-(meth)acrylate methyl copolymer and ethylene-(meth)acrylate ethyl copolymer can be used. The polarizer and the laminated polyester film can be bonded together with the adhesive to provide a layer of laminated polyester film on top of the polarizer. In the manufacturing method of this embodiment, an adhesive is used, but in the manufacturing method of the present invention, the adhesive is an optional component. Applying an adhesive between the polarizer and the laminated polyester film is preferable because it can prevent problems such as delamination between the two films, but if sufficient adhesion can be obtained between the polarizer and the laminated polyester film without using an adhesive, the adhesive may not be used.

[0064] The present invention relates to a method for manufacturing a polarizing plate in which a laminated polyester film may be laminated on one side of a polarizer and a laminated polyester film may also be laminated on the other side, or a phase difference film may be laminated. Examples of phase difference films include films made from polyethylene terephthalate, polycarbonate resins, cycloolefin resins, etc. The phase difference film may also be a stretched film. Furthermore, the phase difference film can function as a λ / 4 plate. When laminating on both sides of a polarizer, the films may be laminated simultaneously to form a polarizing plate, or one side may be laminated to form a roll, and then the other side may be laminated in the next step to form a polarizing plate.

[0065] In a display device using the laminated polyester film of the present invention, a polarizing plate and a backlight are mounted in that order from the viewer's side (the side on which the image is displayed; hereinafter sometimes referred to as the "front"). Other components may be mounted in such a display device. For example, an adhesive layer may be provided between the polarizing plate and the display light-emitting element array, and a color filter, lens film, diffusion sheet, anti-reflective film, etc., may be provided as needed. Furthermore, in the following, the side opposite to the viewer's side (the side on which the image is not displayed) may be referred to as the back side.

[0066] The polarizing plate used in the present invention, when applied to an organic electroluminescent display device or the like as a circular polarizing plate used together with an optical film (λ / 4 phase difference film), exhibits the effect of shielding reflected light from metal electrodes of the organic electroluminescent device or the like across all wavelengths of visible light, thereby preventing reflections during observation and improving the representation of black. The circular polarizing plate is composed of a polarizer that converts incident light into linearly polarized light and a phase difference plate that converts linearly polarized light into circularly polarized light.

[0067] The polarizing plate can be any polarizing plate or coated polarizing film used in fields related to display devices, and can be appropriately selected and used. Typical polarizing plates include those made by dyeing a dichroic material such as iodine onto a polyvinyl alcohol film, but are not limited to this, and any known or future developed polarizing plates can be appropriately selected and used.

[0068] The phase difference plate can be appropriately selected and used in fields related to display devices. As the phase difference plate, a plastic film stretched in a specific direction can be used, such as polycarbonate, polyester, polysulfone, polystyrene, and polymethyl methacrylate. While the phase difference plate can be formed from a single birefringent film, it may also be formed by laminating multiple birefringent films to reduce the wavelength dependence of the phase difference value and to function across the entire visible light wavelength range.

[0069] Furthermore, the polarizer and phase difference plate can be bonded together using an acrylic-based transparent adhesive or bonding agent that does not exhibit optical anisotropy.

[0070] Furthermore, to prevent a decrease in visibility due to the reflection of ambient light off its surface, an anti-reflective film can be provided on the surface of the polarizing plate. For example, in addition to directly forming a multilayer film on the surface of the polarizing plate, it is also possible to attach an anti-reflective film. Alternatively, a microstructure such as a moth-eye structure may be provided, or an appropriate anti-glare treatment may be applied.

[0071] The backlight, which is one of the components of a display device using the laminated polyester film of the present invention, can be any backlight used in fields related to display devices, as appropriate. Typical backlights include those using organic electroluminescent elements and those using inorganic electroluminescent elements, but are not limited to these, and any known or future-developed backlights can be appropriately selected and used. In particular, organic electroluminescent elements including a wavelength conversion layer such as quantum dots can be suitably used as backlights because the high visibility-improving effect of the laminated polyester film enables high contrast and the formation of a clear image.

[0072] The foldable display device on which the laminated polyester film of the present invention is mounted is preferably a foldable display device having a bendable portion with a bending diameter of 1 mm or more and 10 mm or less. A foldable display device is one in which a continuous single display device can be folded in half or the like when carried. The more preferable range for the bending diameter is an upper limit of 8 mm or less, more preferably 6 mm or less, and even more preferably 5 mm or less. If the bending diameter is 10 mm or less, it is possible to make it thin when folded. It can be said that the smaller the bending diameter, the better, but the smaller the bending diameter, the more likely it is to leave creases. The bending diameter is preferably 0.1 mm or more, but it may be 1 mm or more. Even with a bending diameter of 1 mm, it is possible to achieve a practically sufficient thinness when carried. [Examples]

[0073] (Methods for measuring characteristics and evaluating effects) The method for measuring the characteristics and evaluating the effects in this invention is as follows.

[0074] (1) Composition of polyester The film was hydrolyzed with alkali, and each component was analyzed by gas chromatography or high-performance liquid chromatography. The composition ratio was determined from the peak area of ​​each component. Dicarboxylic acid components and other components were measured by high-performance liquid chromatography. The measurement conditions could be analyzed using known methods, and an example of the measurement conditions is shown below. Note that the measurements were performed after filtering and separating the inorganic particles. Equipment: Shimadzu LC-10A Column: YMC-Pack ODS-A 150×4.6mm S-5μm 120A Column temperature: 40℃ Flow rate: 1.2ml / min Detector: UV 240nm The diol components and other constituent components can be analyzed using known methods with gas chromatography. An example of measurement conditions is shown below. Equipment: Shimadzu 9A (manufactured by Shimadzu Corporation) Column: SUPELCOWAX-10 Capillary Column 30m Column temperature: 140°C to 250°C (heating rate 5°C / min) Flow rate: Nitrogen 25 ml / min Detector: FID.

[0075] (2) Intrinsic viscosity (IV) The sample to be measured (polyester resin (raw material) or polyester film) is dissolved in 100 ml of orthochlorophenol (solution concentration C (mass of sample / volume of solution) = 1.2 g / 100 ml), and the viscosity of the solution at 25°C is measured using an Ostwald viscometer. The viscosity of the solvent is also measured in the same manner. Using the obtained solution viscosity and solvent viscosity, [η] is calculated using the following formula (3), and the obtained value is taken as the intrinsic viscosity (IV) (unit: dl / g). ηsp / C = [η] + K[η] 2 ·C formula (3) (Here, ηsp = (solution viscosity / solvent viscosity) - 1, and K is the Huggins constant (assumed to be 0.343).)

[0076] (3) Film thickness The film thickness was measured at five arbitrary points on the film using a dial gauge (Mitutoyo Corporation standard dial gauge 2109S-10) in accordance with JIS K7130 (1992) A-2 method. The average value of these measurements was used as the film thickness.

[0077] (4) Front phase difference Re, R40, main orientation axis direction Measurements were performed using a phase difference measuring device ("KOBRA" (registered trademark)-WPR) manufactured by Oji Instruments Co., Ltd. A 3.5 cm x 3.5 cm section of the sample was cut from the center of the film width direction, and the front phase difference Re [nm] and film orientation angle at a wavelength of 590 nm were measured under the following conditions. The obtained orientation angle direction was defined as the principal orientation axis direction.

[0078] Measurement method: High phase difference mode Variance curve parameters: a = 474.44777, b = 2.04281 × 10 7 c=250 Order range at λ=750nm: 1~5 Furthermore, the procedure for measuring R40 will be explained using Figure 2. R40 was defined as the phase difference value obtained by irradiating film sample 201 with a light ray 205 of wavelength 590 nm from an angle of incidence 204 of 40°, with the vertical axis being set to 0° as the reference. This measurement was performed using the same measurement method, dispersion curve parameters, and order range as described above.

[0079] Measurements were taken by cutting five samples from the center of the film width (800 mm position) and the edge of the film width (50 mm position), measuring each sample, and using the average value.

[0080] (5) Stress F40 with elongation of 40% Strip-shaped samples, 150 mm long and 10 mm wide, were cut from the center of the film in the width direction, along the longitudinal and width directions. Measurements were taken using a tensile testing machine according to the method specified in JIS K7127 (1999). The measurements were performed under the following conditions, and measurements were taken for each of the 10 samples, with the average value calculated. Measuring device: Orientec Co., Ltd. automatic film strength and elongation measuring device "Tensilon" (registered trademark) AMF / RTA-100 Sample size: 10mm wide x 50mm long Pulling speed: 300 mm / min Measurement environment: temperature 23℃, humidity 65%RH.

[0081] (6) Refractive index Using a prism coupler (Sairon SPA4000), a single sample film was clamped with a pressing pressure of 0.2 MPa, and measurements were taken in TE (Transverse Electric) mode using a laser beam with a wavelength of 633 nm to obtain a spectrum chart. On the obtained spectrum chart, the point where the detector output dropped sharply was read, and this value was defined as the refractive index. • Prism: GGG (Gadolinium Gallium Garnet. n=1.965).

[0082] (7) Wavelength 380nm transmittance, T30, T60 The film was cut to dimensions of 4.0 cm in the longitudinal direction and 5.0 cm in the width direction. The film was then sandwiched between two polarizing plates (Kenis Co., Ltd., thin polarizing film, size S, transmittance 0.43, polarization rate 0.9999, product code 1-115-0820) so that the longitudinal direction of the film and the absorption axis of the polarizing plates were parallel to each other, and used as a measurement sample. The measurement was performed using a Hitachi High-Technologies Corporation (formerly Hitachi High-Technologies Corporation) U-4100 spectrophotometer with the attached angle-adjustable transmission attachment for the U-4100 spectrophotometer, using an 8 mm (H) x 5 mm (W) light source mask, and a direct incidence detector with a mirror surface to measure the light transmittance at wavelengths of 450 to 750 nm. The measurement conditions were set to a scanning speed of 600 nm / min and a sampling pitch of 1 nm, in high-resolution measurement mode.

[0083] Referring to Figure 1, the light source 101, the measurement sample 102, and the direct incidence detector 103 are set to an angle 104 of the measurement sample at 30° or 60°. The sample is positioned so that the angle 105 between the film width direction and the horizontal plane is 30°, with the transmission axis 106 of the polarizer plate aligned with the horizontal plane 107 being 0°. For baseline measurement, only two polarizer plates are used as the sample, stacked with their absorption axes aligned. The angle of the variable-angle transmission attachment is set to 30° or 60° in the same manner as above. The measurement is performed by positioning the polarizer plate so that the angle between the transmission axis and the horizontal plane is 30°, with the transmission axis aligned with the horizontal plane being the reference point. For angles 30° and 60° of the variable-angle transmission attachment, the average values ​​of light transmittance at wavelengths of 450-750nm are taken as T30 and T60, respectively, and ΔT is calculated by subtracting T60 from T30. In the above evaluation, the longitudinal direction of the film is the winding direction of the film roll.

[0084] (8) Dynamic viscoelasticity measurement (tanδ peak temperature) A 7cm x 1cm section of the sample was cut from the center of the film width direction and placed in a sample holder to create a sample with a measurement length of 2cm and a film width of 1cm. Using a DMS7100 manufactured by Seiko Instruments Inc., the storage modulus E and loss tangent tanδ were measured in tensile mode under the following conditions: room temperature range of 20°C to 240°C, displacement of 10μm, vibration frequency of 10Hz, and heating rate of 2°C / min. Tanδ is calculated as the ratio of the loss modulus E'' to the storage modulus E'. The measurement length was defined as the longitudinal direction of the film. Among the obtained temperature dependence spectra of tanδ, the temperature at which tanδ takes a maximum value was defined as the tanδ peak temperature.

[0085] (9) Visibility from the front and at an angle The polarizer was removed from the display surface of a 27-inch quantum dot organic EL panel, and a circular polarizer sample (functional laminated film / polarizer film / adhesive layer / phase difference film / adhesive layer) was bonded to it via the adhesive layer to obtain an evaluation display device.

[0086] With the panel displaying white, the visibility was observed from a position 1 meter away from the center of the panel surface in the direction normal to the panel surface, with the line of sight aligned, while wearing polarized sunglasses, and the visibility when viewed from the front was judged as follows. A: Interference colors are hardly visible overall. B: Interference colors are visible, making it unsuitable for practical use. Furthermore, with white displayed, the panel was observed from above, below, and from the left and right while wearing polarized sunglasses, and the visibility when viewed from an oblique angle was judged as follows. A: The displayed image was visible from any angle. B: Visibility is slightly reduced at the edges of the frame, depending on the angle. C: Overall, visibility is slightly reduced depending on the viewing angle. D: The displayed image was not visible depending on the viewing angle, making it unsuitable for practical use.

[0087] (10) Overall evaluation of visibility in the width direction If the sample has a width of 1600 mm or more, the center of the film is set to 0 mm, and the sample consists of film with widths extending to 800 mm from each end. Based on the evaluation results from (9) above at 800 mm (center position) and 150 mm (end position) from the end positions of the sample, the overall visibility evaluation was performed as follows. Note that an overall evaluation of ◎, ○, or △ indicates that the film was at an acceptable level and fully usable in display devices. ◎: Both frontal and oblique visibility are rated A. ○: Visibility from the front is A, and visibility from an angle is B. △: Visibility from the front is A, and visibility from an angle is C. ×: Visibility from the front is B, or visibility from an angle is D.

[0088] (11) Flexibility Measurements were performed using a U-shaped stretch tester (DLDMLH-FS, manufactured by Yuasa System Equipment). Referring to Figure 2, a film sample 301 cut to a length of 60 mm and a width of 25 mm was fixed to a tilt clamp with the clamp surface horizontal so that the direction parallel to the long side was the bending direction 302. The hard coat layer was on the inside of the bend, and the central part 303 of the film was bent at a distance of 3 mm between the faces, and the sample was left to stand for 24 hours. After 24 hours, the bend was released and the sample was removed from the device, and left to stand with the bent outside facing downwards, and the angle 304 formed by the film sample was measured. This measurement was repeated 5 times for both a film sample cut with the length of the film as the long side and a film sample cut with the width as the long side, and the arithmetic mean was calculated as the recovery angle. The angle 304 was read with the fully folded state as 0° and the state in which the film recovers to its original unfolded state as 180°. The bending resistance was evaluated according to the following criteria using the measurement result with the smaller recovery angle from the measurement results in the longitudinal and width directions. Bending resistance levels A and B are suitable for use as a foldable display device. A: The recovery angle was 160° or more, and when implemented in a foldable display device, no wrinkles or lifting due to bending occurred. B: The recovery angle is between 140° and 160°, which is sufficient for practical use as a foldable display device. C: The recovery angle is less than 140°, making it usable as an image display device, but unsuitable for use as a foldable display device.

[0089] (Polyester manufacturing) The polyester resin used for film formation was prepared as follows. Note that the mol% notation represents the total amount of each component (dicarboxylic acid and diol) as 100 mol%. Furthermore, the total amounts of the dicarboxylic acid and diol components are equal in molar ratio.

[0090] (Polyester A) A copolymer polyester resin (glass transition temperature: 125°C, intrinsic viscosity: 0.70) in which terephthalic acid is present as the dicarboxylic acid component at 100 mol%, ethylene glycol is present at 29 mol%, diethylene glycol is present at 1 mol%, 1,4-cyclohexanedimethanol is present at 40 mol%, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol is present at 30 mol%.

[0091] (Polyester B) A chip was prepared by mixing 99.5% by mass of a polyester resin (glass transition temperature: 79°C, intrinsic viscosity: 0.70) containing 100 mol% terephthalic acid as the dicarboxylic acid component, 99 mol% ethylene glycol as the diol component, and 1 mol% diethylene glycol as the diol component, with 0.5% by mass of dioctadecyl phosphate using a twin-screw extruder.

[0092] (Polyester C) A copolymer polyester resin (glass transition temperature: 78°C, intrinsic viscosity: 0.85) containing 88 mol% terephthalic acid and 12 mol% isophthalic acid as dicarboxylic acid components, and 99 mol% ethylene glycol and 1 mol% diethylene glycol as diol components.

[0093] (Polyester D) A copolymer polyester resin (glass transition temperature: 118°C, intrinsic viscosity: 0.70) containing 100 mol% terephthalic acid as the dicarboxylic acid component, 29 mol% ethylene glycol as the diol component, 1 mol% diethylene glycol as the diol component, 40 mol% 1,4-cyclohexanedimethanol as the diol component, and 30 mol% 1,4:3,6-dianhydroglucitol as the diol component.

[0094] (Polyester E) A master tip for an ultraviolet absorber was prepared by mixing 89.5% by mass of polyester C with 10.0% by mass of 2,2'-p-phenylenebis(4H-3,1-benzoxazine-4-one) and 0.5% by mass of 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane using a twin-screw extruder.

[0095] (Polyester F) A master tip for an ultraviolet absorber was prepared by mixing 9.5% by mass of polyester C with 10.0% by mass of 2-(2-hydroxy-5-tert-butylphenyl)-2H-benzotriazole and 0.5% by mass of 3,9-bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane using a twin-screw extruder.

[0096] (Easy-to-adhere coating layer) The easy-adhesion coating layer to be laminated onto the surface of the film was prepared as follows.

[0097] Resin solution (a): A solution obtained by mixing 70 parts by mass of a water-soluble coating solution of polyester resin consisting of acid components (terephthalic acid (88 mol%), 5-sodium sulfisoisophthalic acid (12 mol%)) and diol components (ethylene glycol (100 mol%)) with 30 parts by mass of an aqueous dispersion of polyester resin consisting of acid components (terephthalic acid (50 mol%), isophthalic acid (49 mol%), 5-sodium sulfisoisophthalic acid (1 mol%)) and diol components (ethylene glycol (55 mol%), neopentyl glycol (44 mol%), polyethylene glycol (molecular weight: 4000) (1 mol%)). Resin solution (b): Acrylic resin solution copolymerized with 64% by mass of methyl methacrylate, 30% by mass of ethyl acrylate, 5% by mass of acrylic acid, and 1% by mass of acrylonitrile. Crosslinking agent (c): Methylol-based melamine crosslinking agent Crosslinking agent (d): Oxazoline group-containing crosslinking agent Particle (e): Aqueous dispersion of corodile silica particles with a particle size of approximately 150 nm. Particle (f): Aqueous dispersion of corodile silica particles with a particle size of approximately 300 nm. Fluorine-based surfactant (g): DIC Corporation's "Megafac" (registered trademark) F-444.

[0098] These were mixed in a solid content mass ratio of (a) / (b) / (c) / (d) / (e) / (f) / (g) = 42 parts by mass / 5 parts by mass / 19 parts by mass / 20 parts by mass / 4.9 parts by mass / 0.7 parts by mass / 0.1 parts by mass to prepare the easy-adhesion coating layer solution H.

[0099] (Hard coat layer forming coating composition N) The hard coat layer to be laminated on the surface of the film was prepared as follows. Solvent (i): Toluene Polyfunctional urethane acrylate (j): Manufactured by Daicel Ornex Co., Ltd. KRM8655 Pentaerythritol triacrylate mixture (k): Nippon Kayaku Co., Ltd. PET30 Polyfunctional silicone acrylate (l): Manufactured by Daicel Ornex Co., Ltd. EBECRYL1360 Photopolymerization initiator (m): Irgacure 184, manufactured by Ciba Specialty Chemicals.

[0100] These were mixed in a mass ratio of (i) / (j) / (k) / (l) / (m) = 30 parts by mass / 25 parts by mass / 25 parts by mass / 1 part by mass, and diluted with methyl ethyl ketone to prepare a hard coat layer forming coating composition N with a solid content of 40% by mass.

[0101] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0102] (Examples 1-2, Comparative Examples 1-2) The resin components of layers A, B, and C were the polyesters listed in the table. Layers A and C were extruded in a 280°C single-screw extruder, and layer B was extruded in a 300°C single-screw extruder, where they were melted. Then, after passing through an FSS-type leaf disc filter, the materials were metered using a gear pump to achieve a discharge ratio of polyester A / polyester B = 1 / 6. The layers were then laminated in a three-layer confluence block equipped with a rectangular lamination section, with layer B forming the inner layer and layers A and C forming the outer layers on both sides. After extruding the resulting sheet through a T-die, an electrostatic casting method was prepared, and the film was wound onto a 25°C casting drum for cooling and solidification to produce an unstretched film.

[0103] The unstretched film was stretched longitudinally at the temperature and magnification ratio indicated in the table, and then cooled. Next, corona discharge treatment was applied to both sides of the uniaxially oriented film to achieve a wetting tension of 55 mN / m. The easy-adhesion coating solution H was then applied, and the film edges were clipped into a tenter stretching machine for transport. In the tenter stretching machine, the film was stretched at the temperature and magnification ratio indicated in the table, then heat-set at the temperature indicated in the table, and then relaxed by 2% in the longitudinal and width directions at 150°C to obtain a laminated polyester film with a film width of 4000 mm. The thickness of the easy-adhesion coating layer was 100 nm. The evaluation results are shown in the table.

[0104] Next, the aforementioned hard coat layer forming coating composition N was applied to the surface of layer A using a slot die coater, with the flow rate controlled so that the thickness after drying would be 2 μm. The mixture was then dried at 100°C for 1 minute to remove the solvent. Subsequently, the film coated with the hard coat layer was subjected to a high-pressure mercury lamp at a pressure of 300 mJ / cm². 2 A functional laminated film with a hard coat layer was obtained by irradiating it with ultraviolet light. The bending resistance of the obtained functional laminated film was evaluated. The results are shown in the table.

[0105] Next, while applying a PVA-based adhesive to the polarizer film, the polarizer film was laminated to the functional laminated film's hard coat layer and the opposite side of the functional laminated film using a roll-to-roll method, so that the absorption axis of the polarizer film and the longitudinal direction of the functional laminated film were parallel. On the other side of the polarizer film, a silicone release polyethylene terephthalate film was laminated as a release film via an acrylic adhesive layer, thereby obtaining a polarizing plate with a protective film (functional laminated film / polarizer film / adhesive layer / release film).

[0106] A sample of a polarizing plate with a protective film, measuring 1600 mm in total width (800 mm in each of the two width directions, with the center of the width direction set as 0 mm), was sampled and laminated at 550 mm in the longitudinal direction and 300 mm in the width direction, centered at 800 mm (center position) and 150 mm from the edge position. The protective film-covered polarizing plate, with the release film removed, was laminated to one surface of a norbornene-based phase difference film with a front phase difference Re of 140 nm, in an orientation such that the angle between the absorption axis of the polarizer and the slow phase axis of the phase difference film is 45°. An acrylic adhesive layer was applied to the side of the phase difference film opposite to the polarizing plate, creating a circular polarizing plate (functional laminated film / polarizer film / adhesive layer / phase difference film / adhesive layer). The visibility was evaluated using the obtained circular polarizing plate. The results are shown in the table.

[0107] Examples 1 and 2 had an angle of 45° or less between the principal orientation axis and the longitudinal direction, a refractive index difference of 0.010 to 0.100, and exhibited excellent frontal visibility, oblique visibility, and bending resistance.

[0108] Comparative Example 1 had an angle of 45° or more between the principal orientation axis and the longitudinal direction, and the refractive index difference was less than 0.010, resulting in insufficient oblique visibility.

[0109] In Comparative Example 2, the angle between the principal orientation axis and the longitudinal direction was 45° or more at the film width edge, resulting in insufficient oblique visibility and poor bending resistance.

[0110] (Examples 3-20, Comparative Examples 3-4) The resin components of layers A, B, and C were the polyesters listed in the table. Layers A and C were extruded in a 280°C single-screw extruder, and layer B was extruded in a 300°C single-screw extruder, where they were melted. Then, after passing through an FSS-type leaf disc filter, the materials were metered using a gear pump to achieve a discharge ratio of polyester A / polyester B = 1 / 6. The layers were then laminated in a three-layer confluence block equipped with a rectangular lamination section, with layer B forming the inner layer and layers A and C forming the outer layers on both sides. After extruding the resulting sheet through a T-die, an electrostatic casting method was prepared, and the film was wound onto a 25°C casting drum for cooling and solidification to produce an unstretched film.

[0111] The unstretched film was stretched longitudinally at the temperature and magnification ratio indicated in the table, and then cooled. Next, corona discharge treatment was applied to both sides of the uniaxially oriented film to set the film's wetting tension to 55 mN / m. The easy-adhesion coating layer solution H was then applied, and the film edges were clipped into a tenter stretching machine for transport. In the tenter stretching machine, the film was stretched at the temperature and magnification ratio indicated in the table, then heat-set at the temperature indicated in the table, and then relaxed by 2% in the longitudinal and width directions at 150°C. After cooling, a biaxially oriented film was obtained. This biaxially oriented film was stretched longitudinally at the temperature and magnification ratio indicated in the table to obtain a laminated polyester film with a film width of 4000 mm. The thickness of the easy-adhesion coating layer was 100 nm. The evaluation results are shown in the table.

[0112] Next, the aforementioned hard coat layer forming coating composition N was applied to the surface of layer A using a slot die coater, with the flow rate controlled so that the thickness after drying would be 2 μm. The mixture was then dried at 100°C for 1 minute to remove the solvent. Subsequently, the film coated with the hard coat layer was subjected to a high-pressure mercury lamp at a pressure of 300 mJ / cm². 2 A functional laminated film with a hard coat layer was obtained by irradiating it with ultraviolet light. The bending resistance of the obtained functional laminated film was evaluated. The results are shown in the table.

[0113] Next, while applying a PVA-based adhesive to the polarizer film, the polarizer film was laminated to the functional laminated film's hard coat layer and the opposite side of the functional laminated film using a roll-to-roll method, so that the absorption axis of the polarizer film and the longitudinal direction of the functional laminated film were parallel. On the other side of the polarizer film, a silicone release polyethylene terephthalate film was laminated as a release film via an acrylic adhesive layer, thereby obtaining a polarizing plate with a protective film (functional laminated film / polarizer film / adhesive layer / release film).

[0114] A sample of a polarizing plate with a protective film, measuring 1600 mm in total width (800 mm in each of the two width directions, with the center of the width direction set as 0 mm), was sampled and laminated at 550 mm in the longitudinal direction and 300 mm in the width direction, centered at 800 mm (center position) and 150 mm from the edge position. The protective film-covered polarizing plate, with the release film removed, was laminated to one surface of a norbornene-based phase difference film with a front phase difference Re of 140 nm, in an orientation such that the angle between the absorption axis of the polarizer and the slow phase axis of the phase difference film is 45°. An acrylic adhesive layer was applied to the side of the phase difference film opposite to the polarizing plate, creating a circular polarizing plate (functional laminated film / polarizer film / adhesive layer / phase difference film / adhesive layer). The visibility was evaluated using the obtained circular polarizing plate. The results are shown in the table.

[0115] Examples 3 to 20 had an angle of 45° or less between the principal orientation axis and the longitudinal direction, a refractive index difference of 0.010 to 0.100, and exhibited excellent frontal visibility, oblique visibility, and bending resistance.

[0116] Comparative Example 3 showed poor visibility at an angle due to a refractive index difference exceeding 0.100, and also exhibited inferior bending resistance.

[0117] Comparative Example 4 showed that the angle between the principal orientation axis and the longitudinal direction was 45° or more at the film width edge, and the refractive index difference was less than 0.010, resulting in insufficient oblique visibility and poor bending resistance.

[0118] [Table 1]

[0119] [Table 2]

[0120] [Table 3]

[0121] [Table 4]

[0122] [Table 5]

[0123] [Table 6]

[0124] [Table 7]

[0125] [Table 8]

[0126] [Table 9]

[0127] [Table 10]

[0128] [Table 11]

[0129] [Table 12] [Industrial applicability]

[0130] According to the present invention, by using the laminated polyester film according to the present invention, when a polarizing plate is manufactured by laminating the film parallel to the longitudinal direction of the film and a polarizer, it is possible to provide a display device that ensures visibility and reduces color distortion even when the screen is viewed through an optical element with a polarizing effect, such as polarizing sunglasses. [Explanation of symbols]

[0131] 101: Light source 102: Measurement sample 103: Direct Incidence Detector 104: Angle of the angle-adjustable transmissive attachment 105: Angle between the film width direction and the horizontal plane 106: Transmission axis of polarizing plate 107: Horizontal plane 201: Measurement sample 202: Fast axis 203: Vertical axis 204: Angle of incidence 205: Ray of light 301: Measurement sample 302: Bending direction 303: Center of the film 304: Angle of film sample 401: Center in the film width direction 402: Film width 800mm 403: Film width 1600mm 404: Longitudinal direction of the film 405: Positions 50mm, 150mm, 250mm, 350mm, 450mm, 550mm, 650mm, and 750mm from both ends in the width direction.

Claims

1. A laminated polyester film comprising two or more layers, including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis of the film is 45° or less, and the refractive index difference obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface is 0.010 or more and 0.100 or less.

2. The laminated polyester film according to claim 1, wherein the width and length are 1600 mm or more, and with the center of the film width direction set to 0 mm, samples of 800 mm width are taken in two directions along the width direction, and these are combined to make a film of 1600 mm width, and the angles between the principal orientation axis direction and the longitudinal direction are determined at positions of 50 mm, 150 mm, 250 mm, 350 mm, 450 mm, 550 mm, 650 mm, and 750 mm from both ends in the width direction, and the maximum value among the 16 values ​​obtained is 45° or less.

3. The laminated polyester film according to claim 1, wherein the loss tangent tanδ at 10 Hz obtained by dynamic viscoelasticity measurement in the longitudinal direction of the film is 0.2 or more in the range of 60 to 110°C and the temperature at which it is maximum is 90°C or higher, and in the range of 110 to 160°C it is 0.2 or more and the temperature at which it is maximum is 120°C or higher.

4. The laminated polyester film according to claim 1, wherein the spectral transmittance at 380 nm is 20% or less.

5. The laminated polyester film according to claim 1, wherein the stress value at 40% elongation in the longitudinal direction is 60 MPa or more and 100 MPa or less.

6. The laminated polyester film according to claim 1, wherein the phase difference at a wavelength of 590 nm is 400 nm or less.

7. A laminated polyester film according to any one of claims 1 to 6, used for polarizer protection.

8. A laminated polyester film roll according to any one of claims 1 to 6.

9. A functional laminated film having a functional layer of 2 μm or more laminated directly or via another layer on at least one surface of a laminated polyester film according to any one of claims 1 to 6.

10. A laminate in which a polarizer is laminated on at least one surface of a laminated polyester film according to any one of claims 1 to 6.

11. A laminate in which the longitudinal direction of the laminated polyester films according to claims 1 to 6 is parallel to the absorption axis direction of the polarizer.

12. A display device equipped with a laminated polyester film according to any one of claims 1 to 6.

13. A foldable display device equipped with a laminated polyester film according to any one of claims 1 to 6.

14. A method for manufacturing a polarizing plate, comprising a laminated polyester film comprising two or more layers including at least layer A and layer B, wherein the angle between the longitudinal direction of the film and the principal orientation axis of the film is 45° or less, and the refractive index difference obtained by subtracting the value in the width direction from the value in the longitudinal direction for a refractive index of 633 nm measured from at least one surface is 0.010 or more and 0.100 or less, wherein the laminated polyester film is laminated in parallel with the absorption axis direction of the polarizer.

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