Polyester film and polarizing plate comprising the same

Abstract

To provide a polyester film which hardly causes rainbow unevenness when applied to an image display device and can contribute to the improvement of the durability of a polarizing plate.SOLUTION: The polyester film of the present invention has a linear expansion coefficient of 3.0 * 10-5 / °C or less in a first direction, has a linear expansion coefficient of 7.5 * 10-5 / °C to 10.5 * 10-5 / °C in a second direction orthogonal to the first direction, and has a slow axis in a direction of - 5 ° to 5 ° with respect to the first direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Due to their image formation methods, image display devices (e.g., liquid crystal display devices and organic electroluminescence (EL) display devices) often have a polarizing plate disposed on at least one side of the display cell. In recent years, image display devices have tended to have increasingly diverse functions and applications, and are required to be able to withstand use in increasingly harsh environments. Polarizing plates generally have a structure in which a polarizer is sandwiched between two protective films, and triacetyl cellulose, acrylic resins, cycloolefin resins, etc. are widely used as protective films. From the viewpoint of durability, it has been proposed to use polyester films, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), which have excellent mechanical properties, chemical resistance, and moisture barrier properties, as polarizer protective films (e.g., Patent Document 1). However, while polyester films have excellent mechanical properties, they have birefringence, which can cause poor visibility, such as the occurrence of rainbow unevenness. In particular, the problem of rainbow unevenness has become more pronounced as image display devices have become increasingly brighter and more pure.

[0003] On the other hand, polarizing plates that are constructed using protective films formed from triacetyl cellulose, acrylic resins, or cycloolefin resins, which have been widely used in the past, may experience cracks in the polarizer due to temperature changes. In recent years, thinner polarizers are required as image display devices become thinner, and as the number of image display devices that are expected to be used at high temperatures increases, polarizing plates that are free from cracks in the polarizer and have excellent durability are strongly desired. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-271733 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made to solve the above-mentioned conventional problems, and its main object is to provide a polyester film that causes less rainbow unevenness when applied to an image display device and can contribute to improving the durability of the polarizing plate. [Means for solving the problem]

[0006] The polyester film of the present invention has a linear expansion coefficient in a first direction of 3.0×10 -5 / °C or less, and the linear expansion coefficient in a second direction perpendicular to the first direction is 7.5 × 10 -5 / ℃~10.5×10 -5 / ° C. and has a slow axis in the direction of −5° to 5° with respect to the first direction. In one embodiment, the polyester film has a thickness variation in the second direction of 15% or less. In one embodiment, the polyester film has a crystallinity of 30% or more as measured by DSC. According to another aspect of the present invention, there is provided a polarizing plate comprising a polarizer and the polyester film described above disposed on one side of the polarizer. 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 is 2.0 × 10 -5 / °C or less, 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 is 5 × 10 -5 / ℃ or less. In one embodiment, the polarizer has a thickness of 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 easy-adhesion layer has a thickness of 0.35 μm or less. In one embodiment, the refractive index of the easy-adhesion layer is 1.55 or less. [Effects 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 reduces the occurrence of rainbow unevenness when combined with a polarizer and can contribute to improving the durability of the polarizing plate. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0010] A. Polyester film The polyester film of the present invention has a linear expansion coefficient in a first direction of 3.0×10 -5 / °C or less, and the linear expansion coefficient in a second direction perpendicular to the first direction is 7.5 × 10 -5 / ℃~10.5×10 -5 / °C. Thus, by using a polyester having dimensional change anisotropy, laminating the polyester film on a polarizer can effectively protect the polarizer while preventing cracking of the polarizer. More specifically, polarizers are usually manufactured through a stretching process to have an absorption axis and thus have anisotropy in dimensional change (e.g., dimensional change due to temperature change). However, by laminating a polarizer and a polyester film such 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 unison. As a result, by using the polyester film of the present invention, it is possible to obtain a polarizing plate that is excellent in durability and prevents cracking of the polarizer even under harsh environments such as high temperatures and large temperature changes. In one embodiment, the first direction corresponds to the machine direction (MD) during production of the polyester film. The second direction may correspond to TD, which is perpendicular to MD. The linear expansion coefficient can be determined by TMA measurement in accordance with JIS K 7197. The expression "substantially parallel" includes the case where the angle between the two directions is 0°±10°, preferably 0°±7°, and more preferably 0°±5°.

[0011] The linear expansion coefficient of the polyester film in the first direction is preferably 2.8×10 -5 / °C or less, preferably 0.0 × 10 -5 / ℃~2.5×10 -5 / °C, and more preferably 0.5 × 10 -5 / ℃~1.8×10 -5 / °C. Within this range, the above-mentioned effects become more pronounced.

[0012] The linear expansion coefficient of the polyester film in the second direction is preferably 7.5×10 -5 / ℃ greater than 10.5×10 -5 / °C or less, and more preferably 7.5 × 10 -5 / ℃~10×10 -5 / °C, and more preferably 7.5 × 10 -5 / ℃~9.5×10 -5 / °C. Within this range, the above-mentioned effects become more pronounced.

[0013] In one embodiment, the coefficient of linear expansion in the first direction is 7×10 less than the coefficient of linear expansion in the second direction. -5 / °C or more (preferably 7.5 x 10 -5 Within this range, the above-mentioned effects become more pronounced.

[0014] The polyester film of the present invention has a slow axis in the direction of -5° to 5° with respect to the first direction. If the slow axis is in this range, the polyester film can be one that produces little rainbow unevenness when combined with a polarizer. More specifically, when a polarizer and a polyester film are laminated together so that the absorption axis of the polarizer and the first direction are substantially parallel to each other to form a polarizing plate, rainbow unevenness can be effectively prevented.

[0015] The angle between the 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 effects become more pronounced.

[0016] Typically, the polyester film may be a stretched film obtained through a stretching process. By appropriately adjusting the production conditions in the stretching process, the linear expansion coefficients in the first and second directions (as well as the in-plane retardation Re(590) described below) can be well controlled, resulting in a polyester film having excellent properties as a polarizer protective film in terms of iridescent unevenness and durability, as described above. Examples of the production conditions include the stretching conditions (stretching temperature, stretching ratio, stretching speed, MD / TD stretching order), preheating temperature before stretching, heat treatment temperature after stretching, heat treatment time after stretching, and relaxation rates in the MD and TD directions after stretching. The stretching temperature, stretching ratio, and stretching speed can be appropriately adjusted for each of the MD and TD directions.

[0017] The in-plane retardation Re(590) of the polyester film is, for example, greater than 0 nm and not greater than 10,000 nm. The in-plane retardation Re(λ) is the in-plane retardation of the film measured at 23°C using light with a wavelength of λ nm. Therefore, Re(590) is the in-plane retardation of the film measured using light with a wavelength of 590 nm. Re(λ) is 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 maximized (i.e., the slow axis direction), and ny is the refractive index in the in-plane direction perpendicular to the slow axis.

[0018] The polyester film has a crystallinity of preferably 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 having excellent heat resistance and mechanical properties and suitable as a polarizer protective film can be obtained.

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

[0020] The thickness unevenness of the polyester film in the second direction is preferably 15% or less, more preferably 13% or less, and even more preferably 10% or less. In the present invention, by reducing the thickness unevenness, the effect of laminating the polyester film on a polarizer and effectively protecting the polarizer while preventing the occurrence of cracks in the polarizer becomes more pronounced. The influence of thickness unevenness on preventing polarizer cracks is a unique effect of polyester films having anisotropy in the linear expansion coefficient, and the discovery of the relationship between thickness unevenness and polarizer cracks is one of the achievements of the present invention. Furthermore, by reducing the thickness unevenness, a polyester film can be obtained that causes less iridescent unevenness when combined with a polarizer. The smaller the thickness unevenness of the polyester film, the better, and the lower limit thereof is, for example, 3% (preferably 1%, more preferably 0.5%). In this specification, the "thickness unevenness in the second direction" is calculated by measuring the thickness of the polyester film using a continuous thickness measuring device and using the maximum thickness Tmax, minimum thickness Tmin, and average thickness Tave in the second direction (transverse direction, TD) according to the formula {(Tmax-Tmin) / Tave) × 100.

[0021] The polyester film has a total light transmittance of preferably 80% or more, more preferably 85% or more, even more preferably 90% or more, and particularly preferably 95% or more. The polyester film has a haze of 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.

[0022] The moisture permeability of the polyester film is preferably 100 g / m 2 24 hours or less, preferably 50 g / m 2 24 hours or less, and more preferably 15 g / m 2 Within this range, a polarizing plate having excellent durability and moisture resistance can be obtained.

[0023] The polyester film of the present invention is formed from a polyester resin, which can be obtained by condensation polymerization of a carboxylic acid component and a polyol component.

[0024] 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'-hydroxybenzoic 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, zebaic 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 examples thereof include 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.

[0025] Representative examples of the polyol component include dihydric alcohols. Examples of 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, tricyclodecane dimethanol, 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-cyclophenol)2,5-naphthalenediol, and p-xylenediol. The polyol components may be used alone or in combination of two or more.

[0026] As the polyester resin, polyethylene terephthalate and / or modified polyethylene terephthalate are preferably used, and polyethylene terephthalate is more preferably used. By using these resins, a polyester film having excellent mechanical properties and little iridescent unevenness can be obtained. Polyethylene terephthalate and modified polyethylene terephthalate may be used as a blend.

[0027] Examples of modified polyethylene terephthalates include modified polyethylene terephthalates containing structural 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 less than 10 mol%, more preferably greater than 0 mol% and less than 3 mol%. The proportion of 1,4-butanediol in the polyol component is preferably greater than 0 mol% and less than 10 mol%, more preferably greater than 0 mol% and less than 3 mol%. The proportion of 1,3-propanediol in the polyol component is preferably greater than 0 mol% and less than 10 mol%, more preferably greater than 0 mol% and less than 3 mol%. The proportion of isophthalic acid in the carboxylic acid component is preferably greater than 0 mol% and less than 10 mol%, more preferably greater than 0 mol% and less than 8 mol%. Within these ranges, a polyester film with good crystallinity can be obtained. Note that the mol% mentioned above refers to the mol% relative to the total of all repeating units of the polymer.

[0028] 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 for a film to be obtained that has excellent mechanical strength. The weight-average molecular weight can be measured by GPC (solvent: THF).

[0029] In one embodiment, a polyester film with an adhesion layer is provided. The adhesion layer contains, for example, a water-based polyurethane and an oxazoline-based crosslinking agent. Details of the adhesion layer are described, for example, in JP 2010-55062 A. The entire disclosure of this publication is incorporated herein by reference.

[0030] In one embodiment, the adhesive layer contains any appropriate fine particles. By forming an adhesive layer containing fine particles, blocking that occurs during winding can be effectively suppressed. The fine particles may be inorganic fine particles or organic fine particles. 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, fluorine-based resins, and (meth)acrylic resins. Among these, silica is preferred.

[0031] The particle diameter (number average primary particle diameter) of the fine particles is preferably 10 nm to 200 nm, and more preferably 20 nm to 60 nm.

[0032] The thickness of the 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 such a range, a polyester film with an easy-adhesion layer can be obtained that is unlikely to impair the optical properties of other members when used in an image display device.

[0033] 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 can be obtained that is unlikely to impair the optical properties of other members when used in an image display device. In one embodiment, the refractive index of the easy-adhesion layer is 1.54 or more.

[0034] In one embodiment, the polyester film may have an antiblocking layer on at least one side thereof. The antiblocking layer may have the same structure as the easy-adhesion layer described above. Preferably, the antiblocking layer contains the fine particles.

[0035] (Production method of polyester film) The polyester film can be obtained through a molding step of molding a film-forming material (resin composition) containing the polyester resin into a film, and a stretching step of stretching the formed film. Preferably, the stretching step includes a preheating treatment of the film before stretching, and a heat treatment after stretching. In one embodiment, the polyester film is provided in a continuous shape (or cut from a continuous body).

[0036] In addition to the polyester resin, the film-forming material may contain additives and may also contain a solvent. Any appropriate additive may be used depending on the purpose. Specific examples of additives include reactive diluents, plasticizers, surfactants, fillers, antioxidants, antiaging agents, UV absorbers, leveling agents, thixotropic agents, antistatic agents, conductive materials, and flame retardants. The number, type, combination, and amount of additives may be appropriately determined depending on the purpose.

[0037] Any suitable molding method can be used to form a film from the 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), calendar molding, and heat pressing. Extrusion molding and cast coating are preferred because they can improve the smoothness of the resulting film and provide good optical uniformity.

[0038] The film may be stretched uniaxially or biaxially.

[0039] In one embodiment, the film is stretched by uniaxial stretching in the machine direction (MD).

[0040] The biaxial stretching may be sequential biaxial stretching or simultaneous biaxial stretching. Sequential biaxial stretching or simultaneous biaxial stretching is typically performed using a tenter stretching machine. Therefore, the stretching direction of the film is typically the machine direction (MD) and the transverse direction (TD) of the film.

[0041] In one embodiment, the film is stretched by sequential biaxial stretching. Preferably, the polyester film is obtained by TD stretching followed by MD stretching. This reduces the effect of bowing during TD stretching, allowing the angle between the first direction (MD) and the slow axis of the polyester film to be adjusted to an appropriate value.

[0042] The stretching temperature is preferably Tg+5°C to Tg+50°C, more preferably Tg+5°C to Tg+30°C, and even more preferably Tg+6°C to Tg+10°C, relative to the glass transition temperature (Tg) of the film. Stretching at such a temperature makes it possible to obtain a polyester film in which the slow axis direction and linear expansion coefficient are well-balanced and well-controlled. Furthermore, a polyester film with excellent transparency can be obtained.

[0043] The stretching ratio in MD is preferably 2 to 7, more preferably 2.5 to 6.5, and even more preferably 3 to 6. Within this range, a polyester film having good crystallinity and excellent durability can be obtained while keeping the linear expansion coefficient within a desired range.

[0044] The TD stretching ratio is preferably 1 to 4.5, more preferably 1.2 to 4, and even more preferably 1.5 to 3.5. Within this range, a polyester film having good crystallinity and excellent durability can be obtained while keeping the linear expansion coefficient within a desired range.

[0045] 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 equal to or less than 7, 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, it is possible to prevent cracking of the polarizer and obtain a polarizing plate with excellent durability.

[0046] The MD stretching speed 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 having excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0047] The TD stretching speed 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 having excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0048] 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 having excellent optical properties, good crystallinity, and excellent durability can be obtained.

[0049] 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 having 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 having excellent transparency, good crystallinity, and excellent durability can be obtained.

[0050] B. Polarizing plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. Polarizing plate 100 includes a polarizer 10 and a polyester film 20 arranged on one side of polarizer 10. The polyester film of the present invention described in Section A above is used as polyester film 20. Any appropriate separate polarizer protective film may be arranged on the other side of the polarizer, or no polarizer protective film may be arranged. In one embodiment, polarizer 10 and polyester film 20 (or another polarizer protective film) are laminated together via an adhesive layer 30.

[0051] In one embodiment, the polarizing plate may be applied to an image display device such that the side on which the polyester film is disposed is the viewing side. When the polarizing plate is applied to a liquid crystal display device, the polarizing plate including the polyester film may be disposed on the viewing side or the back side of the liquid crystal cell.

[0052] Any appropriate 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.

[0053] Specific examples of polarizers made of a single-layer resin film include hydrophilic polymer films such as polyvinyl alcohol (PVA) films, partially formalized PVA films, and partially saponified ethylene-vinyl acetate copolymer films that have been dyed with iodine or a dichroic substance such as a dichroic dye and stretched, as well as polyene-based oriented films such as dehydrated PVA films and dehydrochlorinated polyvinyl chloride films. Polarizers obtained by dyeing a PVA film with iodine and uniaxially stretching it are preferred because of their excellent optical properties.

[0054] The dyeing with iodine is carried out, for example, by immersing the PVA film in an aqueous iodine solution. The stretching ratio of the uniaxial stretching is preferably 3 to 7 times. The stretching may be carried out after the dyeing treatment or while dyeing. Alternatively, the PVA film may be stretched and then dyed. If necessary, the PVA film may be subjected to a swelling treatment, a crosslinking treatment, a washing treatment, a drying treatment, or the like. For example, by immersing the PVA film in water and washing it before dyeing, it is possible to wash away dirt and antiblocking agents on the surface of the PVA film, and also to swell the PVA film, thereby preventing uneven dyeing.

[0055] Specific examples of polarizers obtained using laminates 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 the resin substrate and drying the resin substrate to form a PVA-based resin layer on the resin substrate, thereby obtaining a laminate of the resin substrate and the PVA-based resin layer, and then stretching and dyeing the laminate to convert the PVA-based resin layer into a polarizer. In this embodiment, stretching typically involves immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, stretching can optionally further include in-air stretching the laminate at an elevated temperature (e.g., 95°C or higher) before stretching in the aqueous boric acid solution. The obtained 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 depending on the purpose may be laminated on the peeled surface. Details of such a method for producing a polarizer are described, for example, in JP 2012-73580 A. The entire disclosure of this publication is incorporated herein by reference.

[0056] 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, cracking of the polarizer can be effectively prevented, and therefore, a thin polarizer can be used even in harsh environments such as high temperatures and large temperature changes.

[0057] 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.

[0058] The absorption axis direction of the polarizer and the first direction (typically MD) of the polyester film are preferably approximately parallel. If a polarizing plate is constructed so 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 deform in a synchronized manner. As a result, cracks in the polarizer are prevented.

[0059] The slow axis angle of the polyester film is preferably as close as possible to the absorption axis direction 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 exhibits little rainbow unevenness when used in an image display device. The slow axis angle is the angle when the roll flow direction is taken as 0°.

[0060] In the 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 -5 / °C or less, and more preferably 1.5 × 10 -5 / °C or less, and more preferably 1.0 × 10 -5 / °C or less. Within this range, cracking of the polarizer can be prevented even under harsh environments such as high temperatures and large temperature changes. The lower limit of 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 is preferably as small as possible. -5 / °C.

[0061] In the polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (the direction perpendicular to the first direction) and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is preferably 5×10 -5 / °C or less, and more preferably 4.5 × 10 -5 / °C or less. Within this range, cracking of the polarizer can be prevented even under harsh environments such as high temperatures and large temperature changes. The lower limit of the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction and the linear expansion coefficient of the polarizer in a direction parallel to the second direction is preferably as small as possible. -5 / °C.

[0062] 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 (a 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 -5 / ℃ or less (preferably 1.0 × 10 -5 / °C or less). If the temperature is in this range, cracking of the polarizer can be prevented even under harsh environments such as high temperatures and large temperature changes.

[0063] 2 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention. Polarizing plate 200 further includes an easy-adhesion layer 40 disposed on the polarizer 10 side of polyester film 20. In one embodiment, polyester film A with an easy-adhesion layer is disposed on polarizer 10 so that the easy-adhesion layer 40 faces polarizer 10. As the easy-adhesion layer, the easy-adhesion layer described in section A above can be used.

[0064] C. Image display device The polarizing plate can be applied to an image display device. Typical examples of the image display device include a liquid crystal display device and an organic electroluminescence (EL) display device. The image display device employs a configuration well known in the art, and therefore a detailed description thereof will be omitted. [Example]

[0065] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring the various properties in the examples are as follows. Unless otherwise specified, "parts" and "%" in the examples are by weight.

[0066] (1) Orientation angle (direction of slow axis) A square specimen measuring 50 mm in width and 50 mm in length was cut from the center of each polyester film obtained in the Examples and Comparative Examples, with one side parallel to the width direction of the film. The specimen was measured for orientation angle θ at a wavelength of 550 nm and 23°C using a Mueller matrix polarimeter (Axometrics, product name "Axoscan"). The orientation angle θ was measured with the specimen placed parallel to the measuring table. (2) Linear expansion coefficient The linear expansion coefficients of the polyester film and polarizer were measured in accordance with JIS K 7197 using a thermomechanical analyzer "TMA7000" manufactured by Hitachi High-Tech Science Corporation, by raising the temperature from 30°C to 150°C at a rate of 10°C / min and measuring the amount of deformation of the test film at each temperature. The linear expansion coefficient of the film was then calculated from the amount of deformation in the temperature range of 30°C to 70°C. Note that 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. The linear expansion coefficients of the polyester film were measured in MD (first direction) and TD (second direction).The linear expansion coefficients of the polarizer were measured in the direction parallel to the MD and the direction parallel to the TD of the polarizing plate. (3) Crystallinity The crystallinity of the polyester films used in the examples and comparative examples was measured by differential scanning calorimetry (DSC). The sample was heated to 300°C at a rate of 10°C / min. The heat of heat and heat of fusion observed during the temperature increase were measured, and the crystallinity was calculated using the following formula. The heat of heat and heat of fusion were measured using a Q-2000 made by TA Instruments. Crystallinity (%) = (heat of fusion obtained by measurement - heat of heat obtained by measurement) / heat of fusion of 100% crystallinity polyethylene terephthalate (119 mJ / mg) × 100 (4) Rainbow unevenness 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 an adhesive, so that the absorption axis of the polarizer was on the short side of the LCD TV. The liquid crystal cell with the attached polarizing plate obtained in the Examples and Comparative Examples was then reinstalled, and the TV was turned on to display white. The LCD TV was turned on and visually inspected in all directions at a polar angle of 60° to check for rainbow unevenness. Evaluation was based on the following criteria. ○: No rainbow unevenness was observed △: Slight rainbow unevenness was observed ×: Significant rainbow unevenness was observed (5) Dimensional change The polyester films used in the examples and comparative examples were cut into 100 mm x 100 mm pieces. After placing them in a 100°C oven for 24 hours, the films were removed and their dimensions were accurately measured again. The dimensions were confirmed with a metal ruler to determine any dimensional changes. The condition of the samples was also visually inspected and evaluated according to the following criteria. ○: No significant shrinkage of 1 mm or more ×: Shrinkage of 1mm or more or deformation (6) Crack test (accelerated heat shock test) The polarizing plates obtained in the examples and comparative examples were evaluated using a thermal shock tester (manufactured by ESPEC). The polarizing plates obtained in the Examples and Comparative Examples were cut to a size of 50 mm wide x 150 mm long. Two samples were prepared: one in which the absorption axis direction of the polarizer was parallel to the horizontal direction (short side) of the cut polarizing plate, and the other 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 not having the protective film (polyester film) laminated thereon was attached to 0.5 mm thick alkali-free glass via an acrylic adhesive to prepare the samples. The obtained sample was placed in the test area of ​​a thermal shock tester, and the temperature in the test area was lowered from room temperature to -40°C over 30 minutes. Next, the temperature in 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 again to -40°C constitutes one cycle, and after 100 or 200 cycles, the laminate was removed and visually inspected for 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 were observed after 300 cycles. Δ: No cracks were observed after 100 cycles, but cracks were observed after 200 cycles. ×: Cracks were observed after 100 cycles. (7) Uneven thickness The thickness of the polyester film used to laminate the polarizing plate was continuously measured across the entire product width (e.g., 1330 mm) at a speed of 0.5 m / s using an offline sheet continuous thickness measuring device manufactured by Yamabun Denki Co., Ltd. The thickness unevenness was calculated from the maximum thickness Tmax, minimum thickness Tmin, and average thickness Tave using the formula {(Tmax-Tmin) / Tave) × 100.

[0067] [Production Example 1] Preparation of Polarizer A The substrate was a long, amorphous isophthalic acid-co-polyethylene terephthalate (IPA-co-PET) film (thickness: 100 μm) with a water absorption rate of 0.75% and a Tg of 75°C. One side of the substrate was subjected to a corona treatment, 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%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z200") in a 9:1 ratio was applied to the corona-treated surface and dried at 25°C to form an 11 μm-thick PVA-based resin layer, producing a laminate. The resulting laminate was uniaxially stretched at its free end to 2.0 times its original size in the machine direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 120°C (auxiliary in-air stretching). Next, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (insolubilizing treatment). Next, the polarizing plate was immersed in a dye bath at a liquid temperature of 30° C., while adjusting the iodine concentration and immersion time so that the polarizing plate would have a predetermined transmittance. In this example, the polarizing plate was immersed for 60 seconds in an iodine aqueous solution obtained by blending 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 sample was immersed in a crosslinking bath (a boric acid aqueous solution obtained by blending 3 parts by weight of potassium iodide and 3 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (crosslinking treatment). Thereafter, the laminate was immersed in an aqueous boric acid solution (aqueous solution obtained by blending 4 parts by weight of boric acid and 5 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 70°C, and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 4.6 times (underwater stretching). Thereafter, the laminate was immersed in a cleaning bath (aqueous solution obtained by blending 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 30°C (cleaning treatment) to obtain a peelable polarizer A with a substrate.

[0068] [Production Example 2] Production 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, and then a 200 μm thick amorphous polyester resin film was produced using a film-forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 280°C), a T-die (width 500 mm, set temperature: 280°C), a chill roll (set temperature: 50°C), and a winder. The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner KAROIV stretching machine to obtain polyester film A (slow axis angle relative to the length direction: -0.2°, in-plane phase Re (590): 98 nm, thickness: 20 μm). The stretching ratio was 5.5 times in the length direction (MD) and 2.0 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 10% / sec in both MD and TD. After the stretching treatment, the film was heat-treated at 180°C for 30 seconds while maintaining the dimensions.

[0069] [Production Example 3] Production of Polyester Film B Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc.; isophthalic acid modification: 2.5 mol% (moles relative to the total of all repeating units of the polymer); diethylene glycol modification: 1.0 mol% (moles relative to the total of all 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)) was vacuum dried at 100°C for 10 hours and then used to produce a 100 μm thick amorphous polyester resin film using a film-forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder temperature setting: 280°C), a T-die (width 500 mm, temperature setting: 280°C), a chill roll (temperature setting: 50°C), and a winder. The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner KAROIV stretching machine to obtain polyester film B (slow axis angle relative to the length direction: -0.5°, in-plane phase Re(590): 159 nm, thickness: 20 μm). The stretching ratio was 5 times in the length direction (MD) and 2 times in the width direction (TD) with fixed ends stretched. The stretching temperature was 95°C, and the stretching speed was 10% / sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 30 seconds while maintaining the dimensions.

[0070] [Production Example 4] Production of Polyester Film C Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification content 2.5 mol% (mol number relative to the total of all polymer repeat units), IV value 0.77 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, and then a 170 μm thick amorphous polyester resin film was produced using a film-forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder temperature setting: 280 °C), a T-die (width 500 mm, temperature setting: 280 °C), a chill roll (temperature setting: 50 °C), and a winder. This film was uniaxially stretched at its free end to 2.0 times its original length between rolls with different peripheral speeds in an oven at 120°C. Next, the film was immersed in water at a temperature of 30°C for 120 seconds, and then, while immersed in water at a temperature of 73°C, it was uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 (underwater stretching). The obtained stretched film was heat-treated at 90°C for 10 seconds using a Bruckner stretching machine KAROIV to obtain polyester film C (slow axis angle relative to the length direction: -0.2°, in-plane phase Re(590): 3243nm, thickness: 35µm).

[0071] [Production Example 5] Production of Polyester Film I Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc.; isophthalic acid modification: 2.5 mol% (moles relative to the total of all repeating units of the polymer); diethylene glycol modification: 1.0 mol% (moles relative to the total of all 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)) was vacuum dried at 100°C for 10 hours and then used to produce a 180 μm thick amorphous polyester resin film using a film-forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd.; screw diameter: 25 mm; cylinder temperature setting: 280°C), a T-die (width: 500 mm; temperature setting: 280°C), a chill roll (temperature setting: 50°C), and a winder. The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner KAROIV stretching machine to obtain polyester film I (slow axis angle relative to the length direction: -0.6°, in-plane phase Re(590): 1312 nm, thickness: 20 μm). The stretching ratio was 6.0 times in the length direction (MD) and 1.5 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 5% / sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 10 seconds while maintaining the dimensions.

[0072] [Production Example 6] Production of Polyester Film II Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, Inc.; isophthalic acid modification: 2.5 mol% (moles relative to the total of all repeating units of the polymer); diethylene glycol modification: 1.0 mol% (moles relative to the total of all 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)) was vacuum dried at 100°C for 10 hours and then used to produce a 180 μm thick amorphous polyester resin film using a film-forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd.; screw diameter: 25 mm; cylinder temperature setting: 280°C), a T-die (width: 500 mm; temperature setting: 280°C), a chill roll (temperature setting: 50°C), and a winder. The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner KAROIV stretching machine to obtain Polyester Film II (slow axis angle relative to the length direction: -0.8°, in-plane phase Re (590): 771 nm, thickness: 32 μm). The stretching ratio was 4.5 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 2% / sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 10 seconds while maintaining the dimensions.

[0073] [Example 1] Polyester film A produced in Production Example 2 was subjected to a corona treatment, and an aqueous solution containing 15.2 wt% of Daiichi Kogyo Seiyaku Co., Ltd.'s product name "Superflex 210R" and 2.7 wt% of Nippon Shokubai Co., Ltd.'s product name "WS-700" was dissolved therein, so that the film thickness after drying would be 300 μm. Polyester film A with an easy-adhesion layer was obtained by coating the film with an easy-adhesion layer at 80°C for 1 minute. An aqueous PVA resin solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z-200", resin concentration: 3 wt %) was applied to the polarizer surface of the substrate-attached polarizer obtained in Production Example 1, and the polyester film with the easy-adhesion layer was then attached to it. The resulting laminate was heated for 5 minutes in an oven maintained at 60°C. Thereafter, the substrate was peeled from the PVA resin layer to obtain a polarizing plate (polarizer (transmittance 42.3%, thickness 5 μm) / protective film (polyester film)). The polyester film A and the polarizer were laminated such that the MD direction of the polyester film A and the absorption axis direction of the polarizer were approximately parallel. The obtained polarizing plate was subjected to the above evaluations (1) to (7). The results are shown in Table 1.

[0074] [Example 2] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film B produced in Production Example 3 was used instead of the polyester film A produced in Production Example 2. The obtained polarizing plate was subjected to the above evaluations (1) to (7). The results are shown in Table 1.

[0075] [Example 3] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film C produced in Production Example 4 was used instead of the polyester film A produced in Production Example 2. The obtained polarizing plate was subjected to the above evaluations (1) to (7). The results are shown in Table 1.

[0076] [Comparative Example 1] A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film I produced in Production Example 5 was used instead of the polyester film A produced in Production Example 2. The obtained polarizing plate was subjected to the above evaluations (1) to (7). The results are shown in Table 1.

[0077] Comparative Example 2 A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film II produced in Production Example 6 was used instead of the polyester film A produced in Production Example 2. The obtained polarizing plate was subjected to the above evaluations (1) to (7). The results are shown in Table 1.

[0078] [Table 1] [Explanation of symbols]

[0079] 10 Polarizer 20 Polyester film 30 Adhesive layer 40 Easy adhesive layer 100, 200 polarizer

Claims

[Claim 1] The linear expansion coefficient in the first direction is 3.0×10 －５ / °C or less, The linear expansion coefficient in a second direction perpendicular to the first direction is 7.5×10 －５ / ℃~10.5×10 －５ / °C, having a slow axis in a direction of −5° to 5° with respect to the first direction; Polyester film.

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