Polyester film, and polarizing plate including the same

Abstract

To provide a polyester film that reduces the occurrence of rainbow unevenness when applied to an image display device, and can contribute to an improvement in the durability of a polarizing plate.SOLUTION: In a polyester film of the present invention, a coefficient of linear expansion in a first direction and a coefficient of linear expansion in a second direction orthogonal to the first direction are different from each other, the coefficient of linear expansion in the first direction is lower than the coefficient of linear expansion in the second direction by 1.0×10-5 / °C or more, and the coefficient of linear expansion in the second direction is 7.5×10-5 / °C or less. The polyester film has a slow axis in a direction of -5° to 5° with respect to the first direction.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyester film and a polarizing plate containing the polyester film. [Background technology]

[0002] Image display devices (e.g., liquid crystal displays, organic light-emitting diodes) often have a polarizing plate on at least one side of the display cell, due to their image formation method. In recent years, image display devices have tended to have increasingly diverse functions and applications, and are required to withstand use in harsher environments. Polarizing plates generally have a structure in which a polarizer is sandwiched between two protective films, and triacetylcellulose, acrylic resins, cycloolefin resins, etc., are widely used as protective films. On the other hand, from the viewpoint of durability as described above, it has been proposed to use polyester films with excellent mechanical properties, chemical resistance, and moisture barrier properties, such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), as polarizer protective films (for example, Patent Document 1). However, while polyester films have excellent mechanical properties, they have birefringence, which can cause deterioration of visibility, such as the occurrence of rainbow-like unevenness. In particular, with the recent increase in brightness and color purity of image display devices, this problem of rainbow-like unevenness has become more pronounced.

[0003] On the other hand, polarizing plates made using protective films formed from triacetylcellulose, acrylic resins, or cycloolefin resins, which have been widely used in the past, may develop cracks in the polarizer due to temperature changes. In recent years, with the trend towards thinner image display devices, there has been a demand for thinner polarizers. At the same time, with the increasing number of image display devices that are expected to be used at high temperatures, there is a strong need for polarizing plates that do not develop cracks in the polarizer and have excellent durability. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-271733 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The present invention was made to solve the above-mentioned conventional problems, and its main objective is to provide a polyester film that reduces the occurrence of rainbow unevenness when applied to an image display device and contributes to improving the durability of polarizing plates. [Means for solving the problem]

[0006] The polyester film of the present invention has a different coefficient of linear expansion in a first direction and a second direction perpendicular to the first direction, wherein the coefficient of linear expansion in the first direction is 1.0 × 10⁻⁶ higher than the coefficient of linear expansion in the second direction. -5 The temperature is lower than / ℃, and the coefficient of linear expansion in the second direction is 7.5 × 10 -5 The temperature is below / ℃ and has a slow phase axis in the direction of -5° to 5° with respect to the first direction. In one embodiment, the coefficient of linear expansion in the first direction is 3.0 × 10 -5 It is below / ℃. In one embodiment, the polyester film has a crystallinity of 30% or more as determined by DSC measurement. In one embodiment, the polyester film is formed from polyethylene terephthalate and / or modified polyethylene terephthalate. In one embodiment, the modified polyethylene terephthalate comprises constituent units derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid. According to another aspect of the present invention, a polarizing plate is provided. This polarizing plate comprises a polarizer and the polyester film disposed on at least one side of the polarizer. In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction orthogonal to the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the second direction are both 2.0×10 -5 / °C or less. In one embodiment, the thickness of the polarizer is 20 μm or less. In one embodiment, the polarizing plate further includes an easy-adhesion layer disposed on the polarizer side of the polyester film. In one embodiment, the easy-adhesion layer contains fine particles. In one embodiment, the thickness of the easy-adhesion layer is 0.35 μm or less. In one embodiment, the refractive index of the easy-adhesion layer is 1.55 or less.

Advantages of the Invention

[0007] According to the present invention, by selectively reducing the linear expansion coefficient in a predetermined direction, it is possible to provide a polyester film that generates less rainbow unevenness when combined with a polarizer and can contribute to improving the durability of the polarizing plate.

Brief Description of the Drawings

[0008] [Figure 1] It is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. [Figure 2] It is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention.

Modes for Carrying Out the Invention

[0009] Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0010] A. Polyester Film The polyester film of the present invention is formed such that the coefficient of linear expansion in a first direction is different from the coefficient of linear expansion in a second direction perpendicular to the first direction. Specifically, the coefficient of linear expansion in the first direction is 1.0 × 10⁻⁶ higher than the coefficient of linear expansion in the second direction. -5 / ℃ or lower. Thus, by using polyester having anisotropy in dimensional change, it is possible to laminate it onto a polarizer to effectively protect the polarizer while preventing crack formation in the polarizer. More specifically, polarizers are usually manufactured to have an absorption axis through a stretching process and have anisotropy in dimensional change (for example, dimensional change due to temperature change). If the polarizer and the polyester film are laminated so that the absorption axis of the polarizer and the first direction of the polyester film are substantially parallel, the polyester film and the polarizer can change shape in sync and favorably. As a result, by using the polyester film of the present invention, it is possible to prevent crack formation in the polarizer even in harsh environments such as high temperatures and large temperature changes, and to obtain a polarizer with excellent durability. In one embodiment, the first direction corresponds to the transport direction (MD) when manufacturing the polyester film. The second direction may correspond to TD which is perpendicular to MD. The coefficient of linear expansion can be determined by TMA measurement in accordance with JIS K 7197. The expression "approximately parallel" includes the case where the angle between the two directions is 0°±10°, preferably 0°±7°, and more preferably 0°±5°.

[0011] The coefficient of linear expansion in the first direction is 2.0 × 10⁻⁶ higher than the coefficient of linear expansion in the second direction. -5 It is preferable that the temperature be at least one degree Celsius lower. Within this range, the above effect becomes more pronounced.

[0012] The polyester film of the present invention has a slow axis in the direction of -5° to 5° with respect to the above-mentioned first direction. Within such a range, a polyester film with less occurrence of rainbow unevenness when combined with a polarizer can be obtained. More specifically, as described above, when a polarizer and a polyester film are laminated to form a polarizing plate such that the absorption axis of the polarizer and the first direction are substantially parallel, rainbow unevenness can be effectively prevented.

[0013] The angle formed by the above-mentioned first direction and the slow axis is preferably -3° to 3°, more preferably -1° to 1°, particularly preferably -0.5° to 0.5°, and most preferably 0°. Within such a range, the above-mentioned effect becomes more remarkable.

[0014] The linear expansion coefficient of the above-mentioned polyester film in the first direction is preferably 3.0×10 -5 / °C or less, preferably 0.0×10 -5 / °C to 2.5×10 -5 / °C, and more preferably greater than 0.0×10 -5 / °C and 1.8×10 -5 / °C or less. Within such a range, a polyester film that can effectively protect the polarizer when laminated on the polarizer and prevent crack generation in the polarizer can be obtained.

[0015] The linear expansion coefficient of the above-mentioned polyester film in the second direction is 7.5×10 -5 / °C or less, preferably greater than 2.0×10 -5 / °C and 7.5×10 -5 / °C or less, more preferably 3.5×10 -5 / °C to 5.5×10 -5 / °C, and even more preferably 3.0×10 -5 / °C to 5.0×10 -5 / °C. The linear expansion coefficient in the first direction is 1.0×10 -5By lowering the temperature by more than / ℃ and setting the coefficient of linear expansion in the second direction within the above range, a polyester film can be obtained that can be laminated onto a polarizer to effectively protect the polarizer while preventing crack formation in the polarizer.

[0016] Typically, the polyester film described above may be a stretched film obtained through a stretching process. By appropriately adjusting the manufacturing conditions in the stretching process, the coefficient of linear expansion in the first and second directions (as well as the in-plane phase difference Re(590) described later) can be well controlled, and as a result, a polyester film with excellent properties as a polarizer protective film in terms of rainbow unevenness and durability can be obtained, as described above. Examples of the manufacturing conditions include stretching conditions (stretching temperature, stretching ratio, stretching speed, MD / TD stretching sequence), preheating temperature before stretching, heat treatment temperature after stretching, heat treatment time after stretching, and relaxation rate in the MD / TD direction after stretching. The stretching temperature, stretching ratio, and stretching speed can be appropriately adjusted for each MD / TD direction.

[0017] The in-plane phase difference Re(590) of a polyester film is, for example, greater than 0 nm and less than or equal to 10000 nm. The in-plane phase difference Re(λ) is the in-plane phase difference of the film measured with light of wavelength λ nm at 23°C. Therefore, Re(590) is the in-plane phase difference of the film measured with light of wavelength 590 nm. Re(λ) can be calculated by the formula: Re(λ) = (nx - ny) × d, where d (nm) is the thickness of the film. Here, nx is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., in the direction of the slow axis), and ny is the refractive index in the direction perpendicular to the slow axis in the plane.

[0018] The above-mentioned polyester film preferably has a crystallinity of 30% or more, more preferably 40% or more, and even more preferably 50% or more, as measured by differential scanning calorimetry (DSC). The upper limit of the crystallinity is, for example, 70%. Within this range, a polyester film with excellent heat resistance and mechanical properties, suitable as a polarizer protective film, can be obtained.

[0019] The thickness of the polyester film described above is typically 10 μm to 100 μm, preferably 20 μm to 80 μm, and more preferably 20 μm to 50 μm.

[0020] The total light transmittance of the polyester film described above is preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and particularly preferably 95% or more. The haze of the polyester film described above is preferably 1.0% or less, more preferably 0.7% or less, even more preferably 0.5% or less, and particularly preferably 0.3% or less.

[0021] The moisture permeability of the polyester film is preferably 100 g / m². 2 • Less than 24 hours, more preferably 50 g / m² 2 • Less than 24 hours, and more preferably 15 g / m² 2 • Less than 24 hours. Within this range, polarizing plates with excellent durability and moisture resistance can be obtained.

[0022] The polyester film of the present invention is formed from a polyester resin. The polyester resin can be obtained by condensation polymerization of a carboxylic acid component and a polyol component.

[0023] Examples of carboxylic acid components include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, and alicyclic dicarboxylic acids. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, benzylmalonic acid, 1,4-naphthalic acid, diphenic acid, 4,4′-oxybenzoic acid, and 2,5-naphthalenedicarboxylic acid. Examples of aliphatic dicarboxylic acids include malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyladipic acid, pimelic acid, 2,2-dimethylglutaric acid, azelaic acid, zebacic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, and diglycolic acid. Examples of alicyclic dicarboxylic acids include 1,3-cyclopentanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2,5-norbornanedicarboxylic acid, and adamantanedicarboxylic acid. The carboxylic acid component may be a derivative such as an ester, chloride, or acid anhydride, and includes, for example, dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl terephthalate, and diphenyl terephthalate. The carboxylic acid component may be used alone or in combination of two or more.

[0024] Typical polyol components include dihydric alcohols. Dihydric alcohols include aliphatic diols, alicyclic diols, and aromatic diols. Examples of aliphatic diols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butadiol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,6-hexanediol. Examples of alicyclic diols include 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, spiroglycol, tricyclodecanedimethanol, adamantanediol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Examples of aromatic diols include 4,4′-thiodiphenol, 4,4′-methylenediphenol, 4,4′-(2-norbornylidene)diphenol, 4,4′-dihydroxybiphenol, o-,m- and p-dihydroxybenzene, 4,4′-isopropylidenephenol, 4,4′-isopropylidenebis(2,6-cyclolophenol)2,5-naphthalenediol, and p-xylenediol. The polyol components may be used alone or in combination of two or more.

[0025] Preferably, polyethylene terephthalate and / or modified polyethylene terephthalate are used as the polyester resin, and more preferably, polyethylene terephthalate is used. Using these resins, a polyester film with excellent mechanical properties and minimal iridescence can be obtained. Polyethylene terephthalate and modified polyethylene terephthalate may be used in a blend.

[0026] Examples of modified polyethylene terephthalate include modified polyethylene terephthalate containing constituent units derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid. The proportion of diethylene glycol in the polyol component is preferably greater than 0 mol% and 10 mol% or less, and more preferably greater than 0 mol% and 3 mol% or less. The proportion of 1,4-butanediol in the polyol component is preferably greater than 0 mol% and 10 mol% or less, and more preferably greater than 0 mol% and 3 mol% or less. The proportion of 1,3-propanediol in the polyol component is preferably greater than 0 mol% and 10 mol% or less, and more preferably greater than 0 mol% and 3 mol% or less. The proportion of isophthalic acid in the carboxylic acid component is preferably greater than 0 mol% and 10 mol% or less, and more preferably greater than 0 mol% and 8 mol% or less. Within these ranges, a polyester film with good crystallinity can be obtained. Note that the mol% mentioned above is the mol% relative to the total number of repeating units of the polymer.

[0027] The weight-average molecular weight of the polyester resin is preferably 10,000 to 100,000, and more preferably 20,000 to 75,000. Such a weight-average molecular weight allows for easy handling during molding and enables the production of films with excellent mechanical strength. The weight-average molecular weight can be measured by GPC (solvent: THF).

[0028] In one embodiment, a polyester film with an easy-adhesion layer is provided. The easy-adhesion layer comprises, for example, a water-based polyurethane and an oxazoline-based crosslinking agent. Details of the easy-adhesion layer are described, for example, in Japanese Patent Application Publication No. 2010-55062, the entire description of which is incorporated herein by reference.

[0029] In one embodiment, the easy-adhesion layer includes any suitable fine particles. By forming an easy-adhesion layer containing fine particles, blocking that occurs during winding can be effectively suppressed. The fine particles may be inorganic or organic. Examples of inorganic fine particles include inorganic oxides such as silica, titania, alumina, and zirconia, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate. Examples of organic fine particles include silicone resins, fluororesins, and (meth)acrylic resins. Among these, silica is preferred.

[0030] The particle size (number-mean primary particle size) of the above fine particles is preferably 10 nm to 200 nm, and more preferably 20 nm to 60 nm.

[0031] The thickness of the above-mentioned easy-adhesion layer is preferably 2 μm or less, more preferably 1 μm or less, and even more preferably 0.35 μm or less. Within this range, it is possible to obtain a polyester film with an easy-adhesion layer that does not easily interfere with the optical properties of other components when applied to an image display device.

[0032] In one embodiment, the refractive index of the easy-adhesion layer is preferably 1.45 to 1.60. Within this range, a polyester film with an easy-adhesion layer that does not easily interfere with the optical properties of other components when applied to an image display device can be obtained. In one embodiment, the refractive index of the easy-adhesion layer is 1.54 or higher.

[0033] In one embodiment, the polyester film may have an antiblocking layer on at least one side thereof. The configuration of the antiblocking layer may be the same as that of the easy-adhesion layer described above. Preferably, the antiblocking layer contains the fine particles.

[0034] (Method of manufacturing polyester film) The polyester film described above can be obtained through a molding step of forming a film-forming material (resin composition) containing the polyester resin into a film, and a stretching step of stretching the molded film. Preferably, the stretching step includes a preheating treatment of the film performed before film stretching and a heat treatment performed after film stretching. In one embodiment, the polyester film is provided in a long form (or in a shape cut from a long body).

[0035] The film-forming material may contain additives and solvents in addition to the polyester resin described above. Any suitable additive can be used depending on the purpose. Specific examples of additives include reactive diluents, plasticizers, surfactants, fillers, antioxidants, anti-aging agents, UV absorbers, leveling agents, thixotropic agents, antistatic agents, conductive materials, and flame retardants. The number, types, combinations, and amounts of additives can be appropriately determined depending on the purpose.

[0036] Any suitable molding process can be used to form a film from a film-forming material. Specific examples include compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, cast coating (e.g., casting), calendering, and hot pressing. Extrusion molding or cast coating is preferred because it enhances the smoothness of the resulting film and provides good optical uniformity.

[0037] The film may be stretched using either uniaxial stretching or biaxial stretching.

[0038] In one embodiment, uniaxial stretching is employed as the method for stretching the film, and the film is stretched in the longitudinal direction (MD).

[0039] Biaxial stretching may be sequential or simultaneous. Sequential or simultaneous biaxial stretching is typically performed using a tenter stretcher. Therefore, the stretching direction of the film is typically the length direction (MD) and the width direction (TD).

[0040] In one embodiment, sequential biaxial stretching is employed as the method for stretching the film. It is preferable to perform TD stretching followed by MD stretching to obtain the polyester film. This method mitigates the effects of boeing that occur during TD stretching, making it possible to set an appropriate angle between the first direction (MD) and the slow axis in the polyester film.

[0041] The stretching temperature is preferably between Tg+5°C and Tg+50°C, more preferably between Tg+5°C and Tg+30°C, and even more preferably between Tg+6°C and Tg+10°C, relative to the glass transition temperature (Tg) of the film. By stretching at such temperatures, a polyester film can be obtained in which the direction of the slow axis and the coefficient of linear expansion are well-balanced. Furthermore, a polyester film with excellent transparency can be obtained.

[0042] The stretching ratio in MD is preferably 2 to 7 times, more preferably 2.5 to 6.5 times, and even more preferably 3 to 6 times. Within this range, a polyester film with good crystallinity and excellent durability can be obtained while keeping the coefficient of linear expansion within the desired range.

[0043] The stretching ratio in TD is preferably 1 to 4.5 times, more preferably 1.2 to 4 times, and even more preferably 1.5 to 3.5 times. Within this range, a polyester film with good crystallinity and excellent durability can be obtained while keeping the coefficient of thermal expansion within the desired range.

[0044] The ratio of the stretching ratio in TD to the stretching ratio in MD (MD stretching ratio / TD stretching ratio) is preferably greater than 1 and 7 or less, more preferably 1 to 6, and even more preferably 1 to 3. Within this range, a polyester film with particularly little rainbow unevenness can be obtained. Furthermore, by using the obtained polyester film, crack formation in polarizers can be prevented, and a polarizing plate with excellent durability can be obtained.

[0045] The stretching speed in MD is preferably 5% / sec to 100% / sec, more preferably 8% / sec to 80% / sec, and even more preferably 8% / sec to 60% / sec. Within this range, a polyester film with excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0046] The stretching speed in TD is preferably 5% / sec to 100% / sec, more preferably 8% / sec to 80% / sec, and even more preferably 8% / sec to 60% / sec. Within this range, a polyester film with excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0047] The preheating temperature is preferably 80°C to 150°C, more preferably 90°C to 130°C. The preheating time is preferably 10 seconds to 100 seconds, more preferably 15 seconds to 80 seconds. Within this range, a polyester film with excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0048] The heat treatment temperature is preferably 100°C to 250°C, more preferably 120°C to 200°C, and even more preferably 130°C to 180°C. Within this range, a polyester film with excellent transparency, good crystallinity, and excellent durability can be obtained. The heat treatment time is preferably 2 seconds to 50 seconds, more preferably 5 seconds to 40 seconds, and even more preferably 8 seconds to 30 seconds. Within this range, a polyester film with excellent transparency, good crystallinity, and excellent durability can be obtained.

[0049] B. Polarizing plate Figure 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 comprises a polarizer 10 and a polyester film 20 disposed on one side of the polarizer 10. The polyester film 20 is the polyester film of the present invention as described in Section A above. Any suitable other polarizer protective film may be disposed on the other side of the polarizer, or no polarizer protective film may be disposed. In one embodiment, the polarizer 10 and the polyester film 20 (or another polarizer protective film) are laminated via an adhesive layer 30.

[0050] In one embodiment, the polarizing plate may be applied to an image display device such that the side on which the polyester film is arranged is the viewing side. Furthermore, when the polarizing plate is applied to a liquid crystal display device, the polarizing plate with the polyester film may be arranged on the viewing side of the liquid crystal cell or on the back side.

[0051] Any suitable polarizer can be used as the polarizer. For example, the resin film forming the polarizer may be a single layer resin film or a laminate of two or more layers.

[0052] Specific examples of polarizers composed of a single layer of resin film include hydrophilic polymer films such as polyvinyl alcohol (PVA) films, partially formalized PVA films, and partially saponified ethylene-vinyl acetate copolymer films, which have been subjected to dyeing and stretching treatments with dichroic substances such as iodine or dichroic dyes, as well as polyene-based oriented films such as dehydrated PVA or dehydrochlorinated polyvinyl chloride. Preferably, polarizers obtained by dyeing a PVA film with iodine and uniaxially stretching are used because they have excellent optical properties.

[0053] The above-mentioned iodine dyeing is carried out, for example, by immersing the PVA film in an iodine aqueous solution. The stretching ratio for the above-mentioned uniaxial stretching is preferably 3 to 7 times. Stretching may be performed after the dyeing treatment, or during the dyeing process. Alternatively, dyeing may be performed after stretching. If necessary, the PVA film may be subjected to swelling, crosslinking, washing, drying, etc. For example, immersing the PVA film in water and washing it before dyeing can not only clean dirt and anti-blocking agents from the surface of the PVA film, but also swell the PVA film to prevent uneven dyeing.

[0054] Specific examples of polarizers obtained using a laminate include a laminate of a resin substrate and a PVA-based resin layer (PVA-based resin film) laminated on the resin substrate, or a polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by applying a PVA-based resin solution to a resin substrate, drying it to form a PVA-based resin layer on the resin substrate, and obtaining a laminate of the resin substrate and the PVA-based resin layer; or by stretching and dyeing the laminate to make the PVA-based resin layer a polarizer. In this embodiment, stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, stretching may, if necessary, further include air-stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the aqueous boric acid solution. The resulting resin substrate / polarizer laminate may be used as is (i.e., the resin substrate may be used as a protective layer for the polarizer), or the resin substrate may be peeled off from the resin substrate / polarizer laminate, and any appropriate protective layer may be laminated onto the peeled surface according to the purpose. Details of such a polarizer manufacturing method are described, for example, in Japanese Patent Application Publication No. 2012-73580. The entire description of that publication is incorporated herein by reference.

[0055] The thickness of the polarizer is, for example, 1 μm to 80 μm. In one embodiment, the thickness of the polarizer is preferably 20 μm or less, and more preferably 3 μm to 15 μm. By using the polyester film of the present invention, cracks in the polarizer can be effectively prevented, making it possible to use a thin polarizer even in harsh environments such as high temperatures and large temperature fluctuations.

[0056] The polarizer and the polarizer protective film (polyester film) can be laminated via any suitable adhesive layer. Preferably, the adhesive layer is formed from an adhesive composition containing a polyvinyl alcohol-based resin.

[0057] It is preferable that the absorption axis direction of the polarizer and the first direction (typically the MD) of the polyester film are approximately parallel. By constructing the polarizer plate such that the absorption axis of the polarizer and the first direction of the polyester film are approximately parallel, the polyester film and the polarizer can change shape in a synchronous manner, which is desirable. As a result, cracking of the polarizer is prevented.

[0058] The slow axis angle of the polyester film is preferably such that it matches the angle it makes with the absorption axis of the polarizer, and the angle between the two axes is preferably 0°±10°, more preferably 0°±7°, and even more preferably 0°±5°. Within this range, a polyester film can be obtained that produces less rainbow unevenness when applied to an image display device. Note that the slow axis angle is the angle when the roll flow direction is set to 0°.

[0059] In the above polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in a first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction is preferably 2.0 × 10⁻⁶. -5 It is below / ℃, more preferably 1.5 × 10 -5 The temperature is below / ℃, and more preferably 1.0 × 10 -5 It is below / ℃. Within this range, cracks in the polarizer can be prevented even in harsh environments such as high temperatures and large temperature fluctuations. The lower limit of the absolute value of the difference between the linear expansion coefficient of the polyester film in a first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction is preferably as small as possible, for example, 0.1 × 10 -5 It could be / ℃

[0060] In the above polarizing plate, the absolute value of the difference between the coefficient of linear expansion of the polyester film in a second direction (a direction perpendicular to the first direction) and the coefficient of linear expansion of the polarizer in a direction parallel to the second direction is preferably 2.0 × 10⁻⁶. -5 It is below / ℃, more preferably 1.5 × 10 -5 The temperature is below / ℃, and more preferably 1.0 × 10 -5It is below / ℃. Within this range, cracking of the polarizer can be prevented even in harsh environments such as high temperatures and large temperature fluctuations. The lower limit of the absolute value of the difference between the coefficient of linear expansion of the polyester film in a second direction and the coefficient of linear expansion of the polarizer in a direction parallel to the second direction is preferably as small as possible, for example, 0.1 × 10 -5 It could be / ℃

[0061] In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in a first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in a second direction (perpendicular to the first direction) and the linear expansion coefficient of the polarizer in a direction parallel to the second direction, are both 2.0 × 10⁻⁶. -5 / ℃ or lower (preferably 1.0 × 10) -5 The temperature range is below / °C. Within this range, cracks in the polarizer can be prevented even in harsh environments such as high temperatures and large temperature fluctuations.

[0062] Figure 2 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention. The polarizing plate 200 further comprises an easy-adhesion layer 40 disposed on the polarizer 10 side of the polyester film 20. In one embodiment, the polyester film A with the easy-adhesion layer is placed on the polarizer 10 such that the easy-adhesion layer 40 is on the polarizer 10 side. The easy-adhesion layer described in section A above may be used as the easy-adhesion layer.

[0063] C. Image display device The polarizing plates described above can be applied to image display devices. Typical examples of image display devices include liquid crystal displays and organic electroluminescent (EL) displays. Since image display devices employ configurations well known in the industry, a detailed explanation will be omitted. [Examples]

[0064] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods for each characteristic in the examples are as follows. Unless otherwise specified, "parts" and "%" in the examples are based on weight.

[0065] (1) Orientation angle (direction of the slow phase axis) Samples were prepared by cutting a square 50 mm wide and 50 mm long from the central portion of the polyester film obtained in the examples and comparative examples, with one side parallel to the width direction of the film. The orientation angle θ of this sample was measured using a Müller matrix polarimeter (Axometrics, product name "Axoscan") at a wavelength of 550 nm and 23°C. The orientation angle θ was measured with the sample placed parallel to the measuring stage. (2) Coefficient of linear expansion The linear expansion coefficients of polyester film and polarizer were determined according to JIS K 7197 using a Hitachi High-Tech Science Corporation thermomechanical analyzer "TMA7000". The temperature was increased from 30°C to 150°C at a rate of 10°C / min, and the deformation of the test film at each temperature was measured. The linear expansion coefficient of the film was then determined from the deformation in the temperature range of 30°C to 70°C. A positive value was used when the film dimensions increased (expanded) with increasing temperature, and a negative value was used when the film dimensions decreased (shrinked) with increasing temperature. For polyester films, the coefficients of linear expansion were measured in the MD (first direction) and TD (second direction). For polarizers, the coefficients of linear expansion were measured in the polarizing plate in the direction parallel to the MD and the direction parallel to the TD. (3) Degree of crystallinity The crystallinity of the polyester films used in the examples and comparative examples was measured by differential scanning calorimetry (DSC). The heat of exothermic reaction and heat of fusion observed during heating of the samples to 300°C at a rate of 10°C / min were determined, and the crystallinity was calculated using the following formula. The heat of exothermic reaction and heat of fusion were measured using a TA instruments Q-2000. Crystallinity (%) = (Heat of fusion obtained from measurement - Exothermic amount obtained from measurement) / Heat of fusion of 100% crystallinity polyethylene terephthalate (119 mJ / mg) × 100 (4) Rainbow pattern The liquid crystal cell was removed from an LGD LCD TV "45UH7500," and the polarizing plate on the backlight side was peeled off. The polarizing plates obtained in the examples and comparative examples were attached to the surface of the LCD TV from which the polarizing plate had been removed, using adhesive so that the absorption axis of the polarizer was on the short side of the LCD TV. The liquid crystal cell with the polarizing plates obtained in the examples and comparative examples attached was reinstalled, and the TV was lit up to display white. The illuminated LCD TV was visually inspected in all directions at an extreme angle of 60° to check for the presence or absence of rainbow-like unevenness. The following criteria were used for evaluation. ○: No rainbow pattern was observed. △: Slight rainbow-colored unevenness was observed. ×: Significant rainbow-colored discoloration was observed. (5) Dimensional changes The polyester films used in the examples and comparative examples were cut to 100 mm x 100 mm. Afterward, they were placed in a 100°C oven for 24 hours. The films were then removed, their dimensions were accurately measured again, and the changes in dimensions were confirmed using a metal ruler. The condition of the samples was also visually inspected and evaluated according to the following criteria. ○: No significant contraction of 1 mm or more. ×: Shrinkage of 1 mm or more, or deformation present. (6) Crack test (accelerated heat shock test) The polarizing plates obtained in the examples and comparative examples were evaluated using a thermal shock testing machine (manufactured by ESPEC). The polarizing plates obtained in the examples and comparative examples were cut to 50 mm wide x 150 mm long. At that time, samples were prepared in which the absorption axis direction of the polarizer was parallel to the horizontal direction (short side) of the cut polarizing plate, and samples in which the transmission axis direction of the polarizer was parallel to the horizontal direction (short side) of the cut polarizing plate. The side of the polarizing plate that does not have a protective film (polyester film) laminated on it and 0.5 mm thick alkali-free glass were bonded together using an acrylic adhesive to prepare the samples. The obtained samples were placed in the test area of ​​a thermal shock testing machine, and the temperature inside the test area was lowered from room temperature to -40°C over 30 minutes. Next, the temperature inside the test area was raised to 85°C over 30 minutes, and then lowered again to -40°C over 30 minutes. This process of raising the temperature from -40°C to 85°C and then lowering it back to -40°C constituted one cycle, and after repeating this 100 and 200 cycles, the laminates were removed, visually inspected for the presence or absence of cracks, and evaluated according to the following criteria. ◎: No cracks were observed even after 300 cycles. ○: No cracks were observed after 200 cycles, but cracks appeared after 300 cycles. △: No cracks were observed after 100 cycles, but cracks appeared after 200 cycles. ×: Cracks appeared after 100 cycles.

[0066] [Manufacturing Example 1] Fabrication of a polarizer As the substrate, a long, amorphous isophthalic copolymer polyethylene terephthalate (IPA copolymer PET) film (thickness: 100 μm) with a water absorption rate of 0.75% and a Tg of 75°C was used. One side of the substrate was corona-treated, and an aqueous solution containing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (degree of polymerization 1200, degree of acetoacetyl modification 4.6%, degree of saponification 99.0 mol% or more, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "Gosephymer Z200") in a 9:1 ratio was applied and dried at 25°C to form an 11 μm thick PVA-based resin layer, and a laminate was prepared. The resulting laminate was uniaxially stretched to a 2.0x length in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 120°C (air-assisted stretching). Next, the laminate was immersed for 30 seconds in an insolubilization bath at a liquid temperature of 30°C (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) (insolubilization treatment). Next, the polarizing plates were immersed in a dyeing bath at a liquid temperature of 30°C, adjusting the iodine concentration and immersion time so that the polarizing plates achieved a predetermined transmittance. In this example, the polarizing plates were immersed for 60 seconds in an iodine aqueous solution prepared by mixing 0.2 parts by weight of iodine and 1.5 parts by weight of potassium iodide with 100 parts by weight of water (dyeing treatment). Next, the material was immersed for 30 seconds in a crosslinking bath at a liquid temperature of 30°C (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 3 parts by weight of boric acid with 100 parts by weight of water) (crosslinking treatment). Subsequently, the laminate was immersed in a boric acid aqueous solution at a liquid temperature of 70°C (an aqueous solution obtained by mixing 4 parts by weight of boric acid and 5 parts by weight of potassium iodide with 100 parts by weight of water) and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to achieve a total stretch ratio of 5.5 times (underwater stretching). Subsequently, the laminate was immersed in a washing bath at a liquid temperature of 30°C (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) (washing treatment) to obtain a polarizer with a peelable substrate.

[0067] [Manufacturing Example 2] Manufacturing of Polyester Film A Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, IV value 0.75 dl / g (phenol:1,1,2,2-tetrachloroethane = 6:4 mixed solvent, solution concentration 0.4 g / dl)) was vacuum-dried at 100°C for 10 hours. Then, an amorphous polyester resin film with a thickness of 200 μm was produced using a film-making apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder setting temperature: 280°C), a T-die (width 500 mm, setting temperature: 280°C), a chill roll (setting temperature: 50°C), and a winding machine. The obtained amorphous polyester resin film was simultaneously biaxially stretched using a KAROIV stretcher manufactured by Bruckner to obtain polyester film A (latent axis angle relative to the length direction: -1.3°, in-plane phase Re(590): 142 nm, thickness: 20 μm). The stretching ratio was 5 times in the length direction (MD) and 2 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 30% / sec for both MD and TD. After stretching, the film was heat-treated at 180°C for 10 seconds while maintaining its dimensions.

[0068] [Manufacturing Example 3] Manufacturing of Polyester Film B Polyester film B (latent axis angle relative to the length direction: -0.5°, in-plane phase Re(590): 78 nm, thickness: 17 μm) was obtained in the same manner as in Manufacturing Example 2, except that the stretching ratio was set to 4 times in the length direction (MD) and 3 times in the width direction (TD), and the stretching speed was set to 50% / sec in both the MD and TD directions.

[0069] [Manufacturing Example 4] Manufacturing of Polyester Film C After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification amount: 2.5 mol% (moles relative to the total number of polymer repeating units), diethylene glycol modification amount: 1.0 mol% (moles relative to the total number of polymer repeating units), IV value 0.77 dl / g (phenol:1,1,2,2-tetrachloroethane = 6:4 mixed solvent, solution concentration 0.4 g / dl) at 100°C for 10 hours, an amorphous polyester resin film with a thickness of 200 μm was produced using a film manufacturing apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder setting temperature: 280°C), T-die (width 500 mm, setting temperature: 280°C), chill roll (setting temperature: 50°C), and winding machine. The obtained amorphous polyester resin film was simultaneously biaxially stretched using a KAROIV stretcher manufactured by Bruckner to obtain polyester film C (latent axis angle relative to the length direction: -0.5°, in-plane phase Re(590): 80 nm, thickness: 17 μm). The stretching ratio was 4 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 30% / sec for both MD and TD. After stretching, the film was heat-treated at 180°C for 10 seconds while maintaining its dimensions.

[0070] [Manufacturing Example 5] Manufacturing of Polyester Film D A polyester film D (latent axis angle relative to the length direction: -2.5°, in-plane phase Re(590): 271 nm, thickness: 22 μm) was obtained in the same manner as in Manufacturing Example 2, except that the stretching ratio was set to 3 times in the length direction (MD) and 3 times in the width direction (TD), the stretching speed was set to 2% / sec for both MD and TD, and the stretching treatment was performed at 140°C for 10 seconds after stretching.

[0071] [Manufacturing Example 6] Manufacturing of Polyester Film E A polyester film E (latent axis angle relative to the length direction: -11.9°, in-plane phase Re(590): 54 nm, thickness: 50 μm) was obtained in the same manner as in Manufacturing Example 2, except that the stretching ratio was set to 2 times in the length direction (MD) and 2 times in the width direction (TD), the stretching speed was set to 2% / sec for both MD and TD, and the stretching treatment was performed at 140°C for 10 seconds after stretching.

[0072] [Manufacturing Example 7] Manufacturing of polyester film F A polyester film F (latent axis angle relative to the length direction: -0.6°, in-plane phase Re(590): 2823 nm, thickness: 41 μm) was obtained in the same manner as in Manufacturing Example 2, except that the stretching ratio was set to 6 times in the length direction (MD) and 1 time in the width direction (TD) by fixed-end stretching, the stretching speed was set to 2% / sec for both MD and TD, and the stretching treatment was performed at 140°C for 10 seconds after stretching.

[0073] [Manufacturing Example 8] Manufacturing of Polyester Film G After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification amount: 2.5 mol% (moles relative to the total number of polymer repeating units), diethylene glycol modification amount: 1.0 mol% (moles relative to the total number of polymer repeating units), IV value 0.77 dl / g (phenol:1,1,2,2-tetrachloroethane = 6:4 mixed solvent, solution concentration 0.4 g / dl) at 100°C for 10 hours, an amorphous polyester resin film with a thickness of 100 μm was produced using a film manufacturing apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder setting temperature: 280°C), T-die (width 500 mm, setting temperature: 280°C), chill roll (setting temperature: 50°C), and winding machine. The obtained amorphous polyester resin film was simultaneously biaxially stretched using a KAROIV stretcher manufactured by Bruckner to obtain polyester film G (latent axis angle relative to the length direction: -0.9°, in-plane phase Re(590): 3191 nm, thickness: 38 μm). The stretching ratio was 7 times in the length direction (MD) and 1 time in the width direction (TD) using fixed-end stretching. The stretching temperature was 90°C, and the stretching speed was 10% / sec for both MD and TD. After stretching, the film was heat-treated at 140°C for 10 seconds while maintaining its dimensions.

[0074] [Manufacturing Example 9] Manufacturing of Polyester Film H A polyester film H (late axis angle with respect to the length direction: 3.0°, in-plane phase Re(590): 17 nm, thickness: 50 μm) was obtained in the same manner as in Production Example 8, except that the film thickness was set to 50 μm and no stretching was performed.

[0075] [Manufacturing Example 10] Manufacturing of Polyester Film I After vacuum drying a polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification amount 2.5 mol% (number of moles relative to the total repeating units of the polymer), IV value 0.77 dl / g (phenol:1,1,2,2-tetrachloroethane = 6:4 mixed solvent, solution concentration 0.4 g / dl) at 100°C for 10 hours, an amorphous polyester resin film with a thickness of 170 μm was fabricated using a film manufacturing apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder setting temperature: 280°C), a T-die (width 500 mm, setting temperature: 280°C), a chill roll (setting temperature: 50°C), and a winding machine. This film was uniaxially stretched to a 2.0x length in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 120°C. Next, the material was immersed in water at a liquid temperature of 30°C for 120 seconds, and then uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds while immersed in water at a liquid temperature of 73°C, so that the total stretching ratio was 5.5 times (underwater stretching). The obtained stretched film was heat-treated at 90°C for 10 seconds using a KAROIV stretcher manufactured by Bruckner, to obtain polyester film I (late axis angle with respect to the length direction: -0.2°, in-plane phase Re(590): 3243 nm, thickness: 35 μm).

[0076] [Manufacturing Example 11] Manufacturing of Polyester Film J A stretched film was obtained in the same manner as in manufacturing example 10. The resulting stretched film was heat-treated in a KAROIV stretcher manufactured by Bruckner at 90°C for 10 seconds, and then at 140°C for another 10 seconds, to obtain a polyester film J (late axis angle relative to the length direction: -0.4°, in-plane phase Re(590): 4052 nm, thickness: 35 μm).

[0077] [Example 1] Polyester film A manufactured in manufacturing example 2 was subjected to corona treatment, and an aqueous solution containing 15.2 wt% of "Superflex 210R" (product name from Daiichi Kogyo Seiyaku Co., Ltd.) and 2.7 wt% of "WS-700" (product name from Nippon Shokubai Co., Ltd.) was coated to a thickness of 300 μm after drying, and dried at 80°C for 1 minute to obtain polyester film A with an easy-adhesion layer. A PVA-based resin aqueous solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "Gosephymer® Z-200", resin concentration: 3 wt%) was applied to the polarizer surface of the polarizer with substrate obtained in Manufacturing Example 1, and the polyester film with the easy-adhesion layer was laminated to it. The resulting laminate was heated in an oven maintained at 60°C for 5 minutes. After that, the substrate was peeled off from the PVA-based resin layer to obtain a polarizing plate (polarizer (transmittance 42.3%, thickness 5 μm) / protective film (polyester film)). Note that the polyester film A and the polarizer were laminated so that the MD direction of polyester film A and the absorption axis direction of the polarizer were approximately parallel. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0078] [Example 2] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film B, manufactured in Manufacturing Example 3, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0079] [Example 3] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film C, manufactured in Manufacturing Example 4, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0080] [Comparative Example 1] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film D, manufactured in Manufacturing Example 5, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0081] [Comparative Example 2] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film E, manufactured in Manufacturing Example 6, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0082] [Comparative Example 3] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film F, manufactured in Manufacturing Example 7, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0083] [Comparative Example 4] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film G, manufactured in Manufacturing Example 8, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0084] [Comparative Example 5] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film a (manufactured by Toyobo Co., Ltd., trade name "Cosmoshine A4100", lagging axis angle with respect to the length direction: 90°, in-plane phase Re(590): 7800 nm, thickness: 75 μm) was used instead of polyester film A manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0085] [Comparative Example 6] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film b (manufactured by Mitsubishi Chemical Corporation, product name "T100-J25", lagging axis angle with respect to the length direction: 27°, in-plane phase Re(590): 525 nm, thickness: 25 μm) was used instead of polyester film A manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0086] [Comparative Example 7] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film H, manufactured in Manufacturing Example 9, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0087] [Comparative Example 8] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film I, manufactured in Manufacturing Example 10, was used instead of polyester film A, manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0088] [Comparative Example 9] A polarizing plate was obtained in the same manner as in Example 1, except that polyester film J manufactured in Manufacturing Example 11 was used instead of polyester film A manufactured in Manufacturing Example 2. The obtained polarizing plates were subjected to the evaluations (1) to (6) described above. The results are shown in Table 1.

[0089] [Table 1] [Explanation of symbols]

[0090] 10 Polarizers 20 Polyester film 30 Adhesive layer 40 Easy adhesive layer 100, 200 polarizing plates

Claims

1. A polyester film disposed on one side of a polarizer, the linear expansion coefficient of the polyester film in a first direction is different from the linear expansion coefficient in a second direction perpendicular to the first direction; The linear expansion coefficient of the polyester film in the first direction is 2.0 × 10 smaller than the linear expansion coefficient of the polyester film in the second direction. －５ / ℃ or more lower, the linear expansion coefficient of the polyester film in the first direction is 1.1×10 −5 / °C to 3.0×10 −5 / °C; The linear expansion coefficient of the polyester film in the second direction is greater than 3.3×10 −5 / ° C. and less than 7.5×10 －５ / °C or less, the polyester film has a slow axis in a direction of −5° to 5° with respect to the first direction, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction is 2.0 × 10 -5 / °C or less; the first direction of the polyester film is approximately parallel to the absorption axis of the polarizer; Polyester film.

2. 2. The polyester film according to claim 1, which has a crystallinity of 30% or more as measured by DSC.

3. The polyester film according to claim 1 or 2, wherein the polyester film is formed from polyethylene terephthalate and / or modified polyethylene terephthalate.

4. The polyester film according to claim 3, wherein the modified polyethylene terephthalate contains structural units derived from diethylene glycol, 1,4-butanediol, 1,3-propanediol, or isophthalic acid.

5. A polyester film described in any one of claims 1 to 4, having a thickness of 50 μm or less.

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