TWI846761B - Polyester film and polarizing plate containing the same - Google Patents

Abstract

The present invention provides a polyester film that has less rainbow fringes when used in an image display device and can help improve the durability of a polarizing plate. The polyester film of the present invention has a linear expansion coefficient of 3.0×10 ^-5 /°C or less in a first direction, a linear expansion coefficient of 3.5×10 ^{-5 /} °C to 7.5× ^{10 -5} /°C in a second direction perpendicular to the first direction, and has a slow axis in a direction of -5° to 5° relative to the first direction.

Description

Polyester film and polarizing plate containing the same

The present invention relates to a polyester film and a polarizing plate comprising the polyester film.

Invention Background

In image display devices (such as liquid crystal display devices and organic EL display devices), due to their image formation methods, a polarizing plate is usually arranged on at least one side of the display unit. In recent years, image display devices have a tendency to have more diverse functions and uses, and are increasingly required to be able to withstand use in more severe environments. Polarizing plates generally have a structure in which a polarizer is sandwiched between two protective films, and the protective films are widely used to be triacetyl cellulose, acrylic resin, cycloolefin resin, etc. On the other hand, from the perspective of durability as described above, there have been proposals to use polyester films such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) that have excellent mechanical properties, chemical resistance, and moisture barrier properties as polarizer protective films (e.g., Patent Document 1). However, although polyester films have excellent mechanical properties, they are relatively prone to deterioration in visibility such as rainbow patterns due to their birefringence. In particular, with the recent increase in brightness and color purity of image display devices, this rainbow pattern problem has become more prominent.

On the other hand, polarizing plates made of triacetyl cellulose, acrylic resin or cycloolefin resin commonly used in the past may have cracks in the polarizing element due to temperature changes. In recent years, with the thinning of image display devices, the polarizing element is required to be thinner. On the other hand, the image display devices that are intended to be used at high temperatures have increased significantly. Among them, there is an urgent need for a polarizing plate with excellent durability that does not cause cracks in the polarizing element.

Prior art documents Patent documents Patent document 1: Japanese Patent Publication No. 8-271733

Summary of the invention Problems to be solved by the invention The present invention is made to solve the above-mentioned previous problems, and its main purpose is to provide a polyester film that produces less rainbow patterns when used in image display devices and can help improve the durability of polarizing plates.

Means for Solving the Problem The linear expansion coefficient of the polyester film of the present invention in the first direction is less than 3.0×10 ^-5 /°C, and the linear expansion coefficient in the second direction perpendicular to the first direction is 3.5×10 ^{-5 /} °C to 7.5×10 ^-5 /°C, and has a slow axis in the direction of -5° to 5° relative to the first direction. In one embodiment, the polyester film is as in claim 1, and the crystallinity of the polyester film measured by DSC is more than 30%. According to another aspect of the present invention, a polarizing plate can be provided. The polarizing plate has a polarizer and the polyester film arranged 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 the 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 below 2.0×10 ^-5 /°C. In one embodiment, the thickness of the polarizer is below 20µm. In one embodiment, the polarizing plate further includes an easy-bonding layer, which is disposed on the aforementioned polarizer side of the polyester film. In one embodiment, the easy-bonding layer includes microparticles. In one embodiment, the thickness of the easy-bonding layer is below 0.35µm. In one embodiment, the refractive index of the easy-adhesion layer is less than 1.55.

Effect of the invention According to the present invention, a polyester film can be provided, which can reduce the generation of rainbow patterns when combined with a polarizer by selectively reducing the linear expansion coefficient in a predetermined direction, and can help improve the durability of the polarizer.

The following describes the preferred embodiments of the present invention, but the present invention is not limited to these embodiments.

A. Polyester film The linear expansion coefficient of the polyester film of the present invention in the first direction is less than 3.0×10 ^-5 /°C, and the linear expansion coefficient in the second direction perpendicular to the first direction is 3.5×10 ^-5 /°C to 7.5×10 ^-5 /°C. In this way, if a polyester with anisotropy in dimensional change is used, it can be layered on a polarizer to effectively protect the polarizer and prevent cracks in the polarizer. In more detail, the polarizer is usually made by extending the step to give it an absorption axis and anisotropy in dimensional change (for example, dimensional change caused mainly by temperature change). If the polarizer and the polyester film are layered in a manner so that the absorption axis of the polarizer is slightly parallel to the first direction of the polyester film, the polyester film and the polarizer will be synchronized and can undergo an ideal shape change. As a result, by using the polyester film of the present invention, a polarizing plate having excellent durability can be obtained which can prevent cracks in the polarizer even in harsh environments such as high temperature and large temperature fluctuations. In one embodiment, the first direction is equivalent to the conveying direction (MD) when the polyester film is manufactured. Furthermore, the second direction can be equivalent to the TD perpendicular to the MD. The linear expansion coefficient can be determined by TMA measurement in accordance with JIS K 7197. In addition, the expression of "substantially parallel" includes the case where the angle between the two directions is 0°±10°, and preferably 0°±7°, and more preferably 0°±5°.

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 /°C to 2.5× ^{10 -5} /°C, and more preferably greater than 0.5×10 ^{-5 /} °C to 1.8×10 ^-5 /°C. Within the above range, the above effect is more significant.

The linear expansion coefficient of the polyester film in the second direction is preferably 3.3×10 ^{-5 /} °C to 7.3× ^{10 -5} /°C. If it is within the above range, the above effect will be more significant.

In one embodiment, the linear expansion coefficient in the first direction is lower than the linear expansion coefficient in the second direction by 1.0×10 ^-5 /°C or more (preferably 2.0×10 ^-5 /°C or more). Within the above range, the above effect is more significant.

The polyester film of the present invention has a slow axis in the direction of -5° to 5° relative to the first direction. If it is within the above range, a polyester film with less rainbow pattern can be produced when combined with a polarizer. More specifically, as described above, when the polarizer and the polyester film are layered in such a way that the absorption axis of the polarizer is slightly parallel to the first direction to form a polarizing plate, rainbow patterns can be effectively prevented.

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°. If it is within the above range, the above effect will be more significant.

Typically, the polyester film may be a stretched film obtained by a stretching step. By properly adjusting the manufacturing conditions in the stretching step, the linear expansion coefficients 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 having excellent properties from the perspective of rainbow patterns and durability as described above can be obtained as a polarizer protective film. The manufacturing conditions may 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, relaxation rate in the MD/TD direction after stretching, etc. The stretching temperature, stretching ratio and stretching speed may be appropriately adjusted according to MD/TD.

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

The crystallinity of the polyester film measured by differential scanning calorimetry (DSC) is preferably 30% or more, more preferably 40% or more, and more preferably 50% or more. The upper limit of the crystallinity is, for example, 70%. If it is within the above range, a polyester film having excellent heat resistance and mechanical properties and suitable for use as a polarizer protective film can be produced.

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

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

The moisture permeability of the polyester film is preferably 100 g/m ² ·24 hr or less, more preferably 50 g/m ² ·24 hr or less, and even more preferably 15 g/m ² ·24 hr or less. If it is within the above range, a polarizing plate with excellent durability and moisture resistance can be produced.

The polyester film of the present invention is formed of polyester resin, which can be obtained by condensation polymerization of carboxylic acid components and polyol components.

Examples of the carboxylic acid component 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-naphthalene dicarboxylic acid, biphenyl dicarboxylic acid, 4,4'-oxybenzoic acid, and 2,5-naphthalene dicarboxylic acid. Examples of aliphatic dicarboxylic acids include malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyl adipic acid, pimelic acid, 2,2-dimethylglutaric acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, and dihydroxyacetic acid. Examples of the alicyclic dicarboxylic acid 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 also be a derivative such as an ester, a chloride, or an anhydride, including, for example, dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 2,6-naphthalene dicarboxylate, dimethyl isophthalate, dimethyl terephthalate, and diphenyl terephthalate. The carboxylic acid component may be used alone or in combination of two or more.

As for the polyol component, representative examples include diols. Examples of diols 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-butadiene, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanedione, 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, spiroglycerol, 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-norbornyl)diphenol, 4,4'-dihydroxybisphenol, o-/m-/p-dihydroxybenzene, 4,4'-isopropylidenephenol, 4,4'-isopropylidenebis(2,6-cyclophenyl)2,5-naphthalenediol, and p-xylenediol. The polyol component may be used alone or in combination of two or more.

The polyester resin is preferably polyethylene terephthalate and/or modified polyethylene terephthalate, and more preferably polyethylene terephthalate. If such resins are used, a polyester film having good mechanical properties and less rainbow pattern can be produced. Polyethylene terephthalate and modified polyethylene terephthalate can be used in combination.

Modified polyethylene terephthalate may include, for example, modified polyethylene terephthalate containing constituent units derived from diethylene glycol, 1,4-butanediol, 1,3-propylene glycol or isophthalic acid. The ratio 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 ratio 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 ratio of 1,3-propylene 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 ratio of isophthalic acid in the carboxylic acid component is preferably greater than 0 mol% and less than 10 mol%, and more preferably greater than 0 mol% and less than 8 mol%. If it is within the above range, a polyester film with good crystallinity can be produced. In addition, the above-mentioned mole % is the total mole % relative to the total repeating units of the polymer.

The weight average molecular weight of the polyester resin is preferably 10,000 to 100,000, preferably 20,000 to 75,000. Such a weight average molecular weight facilitates handling during molding and can produce a film with excellent mechanical strength. The weight average molecular weight can be measured using GPC (solvent: THF).

In one embodiment, a polyester film with an adhesive layer is provided. The easy-adhesion layer contains, for example, a water-based polyurethane and an oxazoline-based crosslinking agent. The details of the easy-adhesion layer are described in, for example, Japanese Patent Publication No. 2010-55062. The entire description of the publication is cited in this specification as a reference.

In one embodiment, the easy-adhesion layer contains any and appropriate microparticles. By forming an easy-adhesion layer containing microparticles, adhesion generated during winding can be effectively suppressed. The above-mentioned microparticles can be inorganic microparticles or organic microparticles. Inorganic microparticles include inorganic oxides such as silicon dioxide, titanium dioxide, aluminum oxide, and zirconium oxide, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, calcium phosphate, etc. Organic microparticles include polysilicone resins, fluorine resins, (meth) acrylic resins, etc. Among them, silicon dioxide is preferred.

The particle size (number average primary particle size) of the above-mentioned microparticles is preferably 10nm~200nm, more preferably 20nm~60nm.

The thickness of the above-mentioned easy-adhesion layer is preferably less than 2µm, more preferably less than 1µm, and more preferably less than 0.35µm. If it is within the above range, a polyester film with an easy-adhesion layer can be produced which is not likely to hinder the optical properties of other components when used in an image display device.

In one embodiment, the refractive index of the above-mentioned easy-adhesion layer is preferably 1.45-1.60. If it is within the above range, a polyester film with an easy-adhesion layer can be produced that is not likely to hinder the optical properties of other components when used in an image display device. In one embodiment, the refractive index of the above-mentioned easy-adhesion layer is greater than 1.54.

In one embodiment, the polyester film has an anti-blocking layer on at least one side thereof. The anti-blocking layer may be formed by the structure of the easy-adhesion layer described above. Ideally, the anti-blocking layer contains the above-mentioned microparticles.

(Production method of polyester film) The polyester film can be obtained by the following steps: a step of forming a film-forming material (resin composition) containing the polyester resin into a film; and a step of stretching the formed film. The stretching step preferably includes: preheating the film before stretching the film, and heat treatment after stretching the film. In one embodiment, the polyester film is provided in a strip shape (or a shape cut from a strip).

The film forming material may contain additives and solvents in addition to the above-mentioned polyester resin. Any appropriate additive may be used according to the purpose. Specific examples of additives include reactive diluents, plasticizers, surfactants, fillers, antioxidants, anti-aging agents, ultraviolet absorbers, leveling agents, thixotropic agents, antistatic agents, conductive agents, and flame retardants. The amount, type, combination, and addition amount of the additives may be appropriately set according to the purpose.

The method of forming a thin film from a thin film forming material can adopt any and appropriate forming method. Specific examples include compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, casting and coating (such as flow casting), calendering, hot pressing, etc. Extrusion molding or casting and coating is preferred because it can improve the smoothness of the obtained film and obtain good optical uniformity.

The film can be stretched in a uniaxial manner or in a biaxial manner.

In one embodiment, the film may be stretched by uniaxial stretching along the longitudinal direction (MD) of the film.

The biaxial stretching can be sequential biaxial stretching or synchronous biaxial stretching. The sequential biaxial stretching or synchronous biaxial stretching is typically performed using a tenter stretching machine. Therefore, the stretching directions of the film are typically the longitudinal direction (MD) and the width direction (TD) of the film.

In one embodiment, the film may be stretched by sequential biaxial stretching. It is preferred to obtain the polyester film by TD stretching followed by MD stretching. In this way, the bowing effect caused by TD stretching can be mitigated, and the angle between the first direction (MD) of the polyester film and the slow axis can be set to an appropriate value.

The stretching temperature should be Tg+5℃~Tg+50℃ relative to the glass transition temperature (Tg) of the film, preferably Tg+5℃~Tg+30℃, and more preferably Tg+6℃~Tg+10℃. Stretching at such a temperature can obtain a polyester film with balanced control of the slow axis direction and linear expansion coefficient. It can also produce a polyester film with good transparency.

The stretching ratio of MD is preferably 2 to 7 times, more preferably 2.5 to 6.5 times, and more preferably 3 to 6 times. If it is within the above range, a polyester film having a linear expansion coefficient within the desired range and having good crystallinity and excellent durability can be produced.

The stretching ratio of TD is preferably 1 to 4.5 times, preferably 1.2 to 4 times, and more preferably 1.5 to 3.5 times. If it is within the above range, a polyester film having a linear expansion coefficient within the desired range and having good crystallinity and excellent durability can be produced.

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 less than 7, preferably 1 to 6, and more preferably 1 to 3. If it is within the above range, a polyester film with very little rainbow pattern can be produced. In addition, if the obtained polyester film is used, cracks in the polarizer can be prevented and a polarizing plate with excellent durability can be obtained.

The stretching speed of MD is preferably 5%/sec to 100%/sec, more preferably 8%/sec to 80%/sec, and even more preferably 8%/sec to 60%/sec. If it is within the above range, a polyester film having excellent optical properties, good crystallinity and excellent durability can be produced.

The stretching speed of TD is preferably 5%/sec~100%/sec, more preferably 8%/sec~80%/sec, and even more preferably 8%/sec~60%/sec. If it is within the above range, a polyester film with excellent optical properties, good crystallinity and excellent durability can be produced.

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. If it is within the above range, a polyester film with excellent optical properties, good crystallinity and excellent durability can be produced.

The temperature of the heat treatment is preferably 100°C to 250°C, more preferably 120°C to 200°C, and more preferably 130°C to 180°C. If it is within the above range, a polyester film having excellent transparency, good crystallinity, and excellent durability can be obtained. The time of the heat treatment is preferably 2 seconds to 50 seconds, more preferably 5 seconds to 40 seconds, and more preferably 8 seconds to 30 seconds. If it is within the above range, a polyester film having excellent transparency, good crystallinity, and excellent durability can be obtained.

B. Polarizing plate Figure 1 is a schematic cross-sectional view of a polarizing plate of one embodiment of the present invention. The polarizing plate 100 has a polarizing element 10 and a polyester film 20 disposed on one side of the polarizing element 10. As the polyester film 20, the polyester film of the present invention described in the above-mentioned item A can be used. An arbitrary and appropriate other polarizing element protective film can be disposed on the other side of the polarizing element, or a polarizing element protective film may not be disposed. In one embodiment, the polarizing element 10 and the polyester film 20 (or another polarizing element protective film) are laminated through an adhesive layer 30.

In one embodiment, the polarizing plate can be applied to an image display device in such a manner that the side with the polyester film is the viewing side. Also, when the polarizing plate is applied to a liquid crystal display device, the polarizing plate with the polyester film can be arranged on the viewing side of the liquid crystal unit or on the back side.

The polarizer can be any appropriate polarizer. For example, the resin film forming the polarizer can be a single-layer resin film or a laminate of two or more layers.

Specific examples of polarizers made of a single layer of resin film include polyvinyl alcohol (PVA) films, partially formalized PVA films, ethylene-vinyl acetate copolymer partially saponified films, and other hydrophilic polymer films that have been dyed and stretched using dichroic substances such as iodine or dichroic dyes, as well as polyene oriented films such as dehydrated PVA films or dehydrogenated polyvinyl chloride films. From the perspective of excellent optical properties, it is preferable to use a polarizer obtained by dyeing a PVA film with iodine and uniaxially stretching it.

The dyeing using iodine can be performed by, for example, immersing the PVA film in an iodine aqueous solution. The stretching ratio of the uniaxial stretching is preferably 3 to 7 times. The stretching can be performed after the dyeing treatment or at the same time as the dyeing. In addition, the dyeing can be performed after the stretching. The PVA film can be subjected to swelling treatment, crosslinking treatment, cleaning treatment, drying treatment, etc. according to the needs. For example, before dyeing, the PVA film is immersed in water for washing, which can not only clean the dirt or anti-adhesive on the surface of the PVA film, but also swell the PVA film, thereby preventing uneven dyeing.

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 laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained by using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by the following procedures: a PVA-based resin solution is coated on the resin substrate, and the PVA-based resin layer is formed on the resin substrate by drying, thereby obtaining a laminate of the resin substrate and the PVA-based resin layer; the laminate is stretched and dyed to make the PVA-based resin layer into a polarizer. In this embodiment, stretching typically includes immersing the laminate in a boric acid aqueous solution and stretching it. Moreover, if necessary, stretching can further include stretching the laminate in the air at a high temperature (e.g., above 95° C.) before stretching in a boric acid aqueous solution. The obtained resin substrate/polarizer laminate can be used directly (i.e., the resin substrate can be used as a protective layer of the polarizer), or the resin substrate can be peeled off from the resin substrate/polarizer laminate and then used after any and appropriate protective layer is applied to the peeled off area layer according to the purpose. The details of the manufacturing method of the polarizer are described in, for example, Japanese Patent Publication No. 2012-73580. The entire description of the publication is cited in this specification as a reference.

The thickness of the polarizer is, for example, 1 μm to 80 μm. In one embodiment, the thickness of the polarizer is preferably less than 20 μm, more preferably 3 μm to 15 μm. If the polyester film of the present invention is used, cracks in the polarizer can be effectively prevented, so that a thin polarizer can be used even in harsh environments such as high temperature and large temperature changes.

The polarizer and the polarizer protection film (polyester film) can be laminated via an arbitrary and appropriate adhesive layer. Ideally, the adhesive layer is formed of an adhesive composition containing a polyvinyl alcohol-based resin.

It is preferred that the absorption axis of the polarizer is slightly parallel to the first direction (MD) of the polyester film. If the polarizing plate is constructed in such a way that the absorption axis of the polarizer is slightly parallel to the first direction of the polyester film, the polyester film and the polarizer will be synchronized and can undergo an ideal shape change. As a result, cracks in the polarizer are prevented.

The more consistent the angle between the slow axis angle of the polyester film and the absorption axis direction of the polarizer, the better. The angle between the two axes is preferably 0°±10°, preferably 0°±7°, and more preferably 0°±5°. If it is within the above range, a polyester film with less rainbow pattern when used in an image display device can be produced. In addition, the slow axis angle is the angle when the roller flow direction is set to 0°.

In the above polarizing plate, 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 2.0×10 ^-5 /°C or less, more preferably 1.5×10 ^-5 /°C or less, and more preferably 1.0×10 ^-5 /°C or less. If it is within this range, the crack of the polarizer can be prevented even in a harsh environment such as high temperature and large temperature change. 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, for example, it can be 0.1×10 ^-5 /°C.

In the above 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 2.0×10 ^-5 /°C or less, more preferably 1.5×10 ^-5 /°C or less, and more preferably 1.0×10 ^-5 /°C or less. If it is within this range, the crack of the polarizer can be prevented even in harsh environments such as high temperature 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 the direction parallel to the second direction is as small as possible, for example, it can be 0.1×10 ^-5 /°C.

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 (direction perpendicular 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 (preferably 1.0×10 ^-5 /°C or less). If they are within this range, cracks in the polarizer can be prevented even in harsh environments such as high temperature and large temperature changes.

FIG2 is a schematic cross-sectional view of another embodiment of the polarizing plate of the present invention. The polarizing plate 200 further includes an easy-adhesion layer 40, which is disposed on the polarizing element 10 side of the polyester film 20. In one embodiment, the polyester film A with an easy-adhesion layer is disposed on the polarizing element 10 in such a manner that the easy-adhesion layer 40 is on the polarizing element 10 side. The easy-adhesion layer may be the easy-adhesion layer described in the above-mentioned item A.

C. Image display device The above-mentioned polarizing plate can be applied to an image display device. Representative examples of image display devices include liquid crystal display devices and organic electroluminescent (EL) display devices. The image display device can adopt a structure well known in the industry, so a detailed description is omitted.

Examples The present invention is specifically described below with examples, but the present invention is not limited to these examples. The measurement methods of various properties in the examples are as follows. In addition, unless otherwise specified, "parts" and "%" in the examples are based on weight.

(1) Orientation angle (direction of slow axis) The central part of the polyester film obtained in the embodiment and the comparative example was cut into a square with a width of 50 mm and a length of 50 mm in such a way that one side was parallel to the width direction of the film to prepare a sample. The sample was measured using a Mueller matrix polarimeter (manufactured by Axometrics, trade name "Axoscan") to measure the orientation angle θ at a wavelength of 550 nm and 23°C. In addition, the orientation angle θ was measured with the sample placed parallel to the measuring table. (2) Linear expansion coefficient The linear expansion coefficient of the polyester film and the polarizer was measured according to JIS K 7197 using a thermomechanical analyzer "TMA7000" manufactured by Hitachi High-Technologies Corporation, with the temperature 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. Then, the linear expansion coefficient of the film is calculated from the deformation in the temperature range of 30°C to 70°C. In addition, as the temperature rises, the film size increases (expands) and the coefficient is positive (plus sign); as the temperature rises, the film size decreases (contracts) and the coefficient is negative (minus sign). The linear expansion coefficients of the polyester film in the MD (first direction) and TD (second direction) are measured. The linear expansion coefficients of the polarizer in the polarizing plate in the direction parallel to the MD and the direction parallel to the TD are measured. (3) Crystallinity The crystallinity of the polyester film used in the examples and comparative examples was measured by differential scanning calorimetry (DSC). The heat release and fusion heat observed during the temperature increase from 10°C/min to 300°C were calculated, and the crystallinity was calculated according to the following formula. The heat release and fusion heat were measured using Q-2000 manufactured by TA instruments. Crystallinity (%) = (measured fusion heat - measured heat release) / fusion heat of 100% crystallinity polyethylene terephthalate (119mJ/mg) × 100 (4) Rainbow pattern The liquid crystal unit was taken out from the liquid crystal TV "45UH7500" manufactured by LGD, and the polarizing plate on the backlight side was peeled off. The polarizing plate obtained in the embodiment and the comparative example was attached to the surface of the liquid crystal TV from which the polarizing plate had been peeled off through an adhesive in such a way that the absorption axis of the polarizer became the short side of the liquid crystal TV. After the liquid crystal unit with the polarizing plates obtained in the embodiment and the comparative example was set up again, the TV was lit with a white screen. At the polar angle of 60° of the lit liquid crystal TV, a full-range visual check was performed to observe whether there was a rainbow pattern. Evaluation was performed according to the following criteria. ○: No rainbow pattern was observed △: Some rainbow pattern was observed ×: Rainbow pattern was clearly observed (5) Dimension change The polyester film used in the embodiment and the comparative example was cut into 100mm×100mm. Then, after being placed in a heating oven at 100℃ for 24 hours, the film was taken out and the size was accurately measured again. The size was confirmed with a ruler and the size change was calculated. The state of the sample was visually confirmed again and evaluated according to the following criteria. ○: No significant shrinkage of more than 1 mm ×: Shrinkage of more than 1 mm or deformation (6) Crack test (thermal shock acceleration test) The polarizing plates prepared 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 into 50 mm in width and 150 mm in length. At this time, the following samples were prepared: samples in which the absorption axis of the polarizer was parallel to the transverse direction (short side) of the cut polarizing plate; and samples in which the transmission axis of the polarizer was parallel to the transverse direction (short side) of the cut polarizing plate. The surface of the polarizing plate without the protective film (polyester film) laminated was bonded to 0.5 mm thick non-alkali glass through an acrylic adhesive to prepare the sample. The obtained sample was placed in the test area of the hot and cold shock tester, and the test area was cooled from room temperature to -40°C in 30 minutes. Then, the test area was heated to 85°C in 30 minutes, and then cooled to -40°C again in 30 minutes. The step of heating from -40°C to 85°C and cooling to -40°C was set as 1 cycle. After repeating 100 cycles and 200 cycles, the laminate was taken out and visually checked 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. △: No cracks were observed after 100 cycles, but cracks occurred after 200 cycles. ×: Cracks occurred after 100 cycles.

[Manufacturing Example 1] Preparation of polarizer A The substrate is a long strip of amorphous isophthalic acid copolymer polyethylene terephthalate (IPA copolymer PET) film (thickness: 100µm) with a water absorption rate of 0.75% and a Tg of 75℃. A corona treatment is applied to one side of the substrate, and an aqueous solution containing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl modified PVA (polymerization degree 1200, acetoacetyl modification degree 4.6%, saponification degree 99.0 mol% or more, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z200") in a ratio of 9:1 is applied to the corona treated surface at 25℃ and then dried to form a PVA resin layer with a thickness of 11µm to produce a laminate. In an oven at 120°C, the obtained laminate is subjected to free-end uniaxial stretching to 2.0 times in the longitudinal direction (long side direction) between rollers with different circumferential speeds (air-assisted stretching). Then, the laminate is immersed in an insolubilization 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 (insolubilization treatment). Then, while immersing in a dyeing bath at a liquid temperature of 30°C, the iodine concentration and immersion time are adjusted so that the polarizing plate reaches a predetermined transmittance. In this embodiment, it is immersed in an iodine aqueous solution obtained 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 for 60 seconds (dyeing treatment). Next, it was immersed in a crosslinking bath (boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 3 parts by weight of boric acid with respect to 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (crosslinking treatment). Then, while the laminate was immersed in a boric acid aqueous solution (boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid and 5 parts by weight of potassium iodide with respect to 100 parts by weight of water) at a liquid temperature of 70°C, it was uniaxially stretched (underwater stretching) in the longitudinal direction (long side direction) between rollers with different peripheral speeds in a manner such that the total stretching ratio became 5.5 times. Then, the laminate was immersed in a cleaning 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) (cleaning treatment), and a polarizer A with a removable substrate was obtained.

[Manufacturing Example 2] Preparation of polarizer B A polarizer B with a removable substrate was prepared in the same manner as in Manufacturing Example 1, except that the stretching ratio in water was set to 4.6 times.

[Production Example 3] Production of polyester film A Polyester resin (polyethylene terephthalate, manufactured by Daejong Polyester Co., Ltd., 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 film forming device equipped with a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 280°C), T-die (width 500 mm, set temperature: 280°C), cooling roll (set temperature: 50°C) and winder was used to produce an amorphous polyester resin film with a thickness of 200 µm. The obtained amorphous polyester resin film was stretched by a stretching machine K manufactured by Brückner Co., Ltd. AROIV was used for synchronous biaxial stretching to produce polyester film A (slow axis angle relative to the longitudinal direction: -1.3°, in-plane phase difference Re(590): thickness 142nm, thickness: 20µm). The stretching ratio was set to 5 times in the longitudinal direction (MD) and 2 times in the width direction (TD). The stretching temperature was set to 90°C, and the stretching speed was set to 30%/sec in both MD and TD. After the stretching treatment, the film was heat treated at 180°C for 10 seconds while maintaining the dimensions.

[Production Example 4] Production of polyester film B Polyester film B (slow axis angle relative to the longitudinal direction: -0.5°, in-plane phase difference Re(590): 78nm, thickness: 17µm) was produced in the same manner as in Production Example 3 except that the stretching ratio was set to 4 times in the longitudinal direction (MD) and 3 times in the width direction (TD), and the stretching speed was set to 50%/sec in both MD and TD.

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

[Manufacturing Example 6] Manufacture of polyester film II A polyester film II (slow axis angle relative to the longitudinal direction: -11.9°, in-plane phase Re(590): 54nm, thickness: 50µm) was prepared in the same manner as in Manufacturing Example 4 except that the stretching ratio was set to 2 times in the longitudinal direction (MD) and 2 times in the width direction (TD), the stretching speed was set to 2%/sec in both MD and TD, and heat treatment was performed at 140°C for 10 seconds after the stretching treatment.

[Manufacturing Example 7] Manufacture of polyester film III A polyester film III (slow axis angle relative to the longitudinal direction: -0.6°, in-plane phase difference Re(590): 2823nm, thickness: 41µm) was manufactured in the same manner as in Manufacturing Example 4 except that the stretching ratio was set to 6 times in the longitudinal direction (MD) and 1 time in the width direction (TD) by fixed-end stretching, the stretching speed was set to 2%/sec in both MD and TD, and heat treatment was performed at 140°C for 10 seconds after the stretching treatment.

[Production Example 8] Production of polyester film IV Polyester resin (polyethylene terephthalate, manufactured by Daejong Polyester Products Co., Ltd., isophthalic acid modification amount 2.5 mol% (relative to the total molar number of the total repeating units of the polymer), diethylene glycol modification amount: 1.0 mol% (relative to the total molar number of the total repeating units of the polymer), IV value 0.77 dl/g (phenol: 1,1,2,2-tetrachloroethane = 6:4 mixed solvent) was prepared. The solution concentration was 0.4 g/dl) and vacuum dried at 100°C for 10 hours. Then, a film forming device 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 cooling roller (setting temperature: 50°C) and a winder was used to produce an amorphous polyester resin film with a thickness of 100 µm. The obtained amorphous polyester resin film was stretched by a stretching machine KAROI manufactured by Brückner Co., Ltd. V was subjected to synchronous biaxial stretching to obtain polyester film IV (slow axis angle relative to the longitudinal direction: -0.9°, in-plane phase difference Re(590): thickness 3191nm, thickness: 38µm). The stretching ratio was set to 7 times in the longitudinal direction (MD) and 1 times in the width direction (TD) by fixed end stretching. The stretching temperature was set to 90°C, and the stretching speed was set to 10%/sec for both MD and TD. In addition, after the stretching treatment, a heat treatment was performed at 140°C for 10 seconds while maintaining the size.

[Manufacturing Example 9] Manufacture of polyester film V A polyester film V (slow axis angle relative to the longitudinal direction: 3.0°, in-plane phase difference Re(590): 17 nm, thickness: 50 µm) was manufactured in the same manner as in Manufacturing Example 7 except that the film thickness was set to 50 µm and no stretching was performed.

[Production Example 10] Production of polyester film VI Polyester resin (polyethylene terephthalate, manufactured by Daejong Polyester Products Co., Ltd., 2.5 mol% (relative to the total molar number of the total repeating units of the polymer) with an IV value of 0.77 dl/g (phenol: 1,1,2,2-tetrachloroethane = 6:4 mixed solvent) was prepared. After vacuum drying at 100°C for 10 hours, a film forming device equipped with a uniaxial 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 cooling roll (setting temperature: 50°C) and a winder was used to produce an amorphous polyester resin film with a thickness of 170µm. The film was uniaxially stretched to 2.0 times in the longitudinal direction (long side direction) between rolls of different circumferential speeds in an oven at 120°C. Then, after being immersed in water at a temperature of 30°C for 120 seconds, the film was uniaxially stretched (underwater stretching) in the longitudinal direction (long side direction) between rollers of different circumferential speeds at a total stretching ratio of 5.5 times while being immersed in water at a temperature of 73°C. The resulting stretched film was heat treated at 90°C for 10 seconds using a stretching machine KAROIV manufactured by Brückner to obtain a polyester film VI (slow axis angle relative to the longitudinal direction: -0.2°, in-plane phase Re(590): 3243nm, thickness: 35µm).

[Manufacturing Example 11] Manufacture of polyester film VII A stretched film was prepared in the same manner as in Manufacturing Example 10. The obtained stretched film was heat treated at 90°C for 10 seconds and then at 140°C for 10 seconds using a stretching machine KAROIV manufactured by Brückner, thereby obtaining a polyester film VII (slow axis angle relative to the longitudinal direction: -0.4°, in-plane phase Re(590): 4052nm, thickness: 35µm).

[Example 1] The polyester film A obtained in Example 3 was subjected to a corona treatment, and an aqueous solution containing 15.2 wt% of "SUPERFLEX 210R" manufactured by Daiichi Kogyo Seiyaku Co., Ltd. and 2.7 wt% of "WS-700" manufactured by Nippon Catalyst Co., Ltd. was applied in a manner such that the film thickness after drying was 300 µm, and dried at 80°C for 1 minute to form an easy-adhesion layer, thereby obtaining a polyester film A with an easy-adhesion layer. A PVA-based resin aqueous solution (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name "GOHSEFIMER (registered trademark) Z-200", resin concentration: 3 wt%) was applied to the surface of the polarizer of the polarizer with substrate obtained in Example 1, and the polyester film with the easy-adhesion layer was attached. The obtained laminate was heated in an oven maintained at 60°C for 5 minutes. The substrate was then peeled off from the PVA resin layer to obtain a polarizing plate (polarizer (transmittance 42.3%, thickness 5µm)/protective film (polyester film)). In addition, the polyester film A and the polarizer were laminated in such a way that the MD direction of the polyester film A and the absorption axis direction of the polarizer were slightly parallel. The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Example 2] A polarizing plate was prepared in the same manner as Example 1 except that the polyester film B prepared in Example 4 was used instead of the polyester film A obtained in Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Example 3] A polarizing plate was prepared in the same manner as Example 1 except that the polarizing element obtained in Example 2 was used instead of the polarizing element with substrate obtained in Example 1. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Example 4] A polarizing plate was prepared in the same manner as Example 2 except that the polarizing element obtained in Example 2 was used instead of the polarizing element with substrate obtained in Example 1. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 1] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film I prepared in Preparation Example 5 was used instead of the polyester film A obtained in Preparation Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 2] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film II prepared in Example 6 was used instead of the polyester film A prepared in Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 3] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film III prepared in Preparation Example 7 was used instead of the polyester film A prepared in Preparation Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 4] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film IV prepared in Preparation Example 8 was used instead of the polyester film A prepared in Preparation Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 5] A polarizing plate was prepared in the same manner as in Example 1 except that polyester film a (manufactured by Toyobo Co., Ltd., trade name "COSMOSHINE A4100", slow axis angle relative to the longitudinal direction: 90°, in-plane phase difference Re(590): 7800nm, thickness: 75µm) was used instead of polyester film A prepared in Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 6] A polarizing plate was prepared in the same manner as in Example 1 except that polyester film b (manufactured by Mitsubishi Chemical Co., trade name "T100-J25", slow axis angle relative to the longitudinal direction: 27°, in-plane phase difference Re(590): 525nm, thickness: 25µm) was used instead of polyester film A prepared in Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 7] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film V prepared in Preparation Example 9 was used instead of the polyester film A prepared in Preparation Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 8] A polarizing plate was prepared in the same manner as in Example 1 except that the polyester film VI prepared in Preparation Example 10 was used instead of the polyester film A prepared in Preparation Example 3. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 9] A polarizing plate was obtained in the same manner as in Example 1 except that the polyester film VII obtained in Production Example 11 was used instead of the polyester film A obtained in Production Example 3. The obtained polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 10] A polarizing plate was prepared in the same manner as in Comparative Example 1, except that the polarizing element obtained in Production Example 2 was used instead of the polarizing element with a substrate obtained in Production Example 1. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 11] A polarizing plate was prepared in the same manner as in Comparative Example 3, except that the polarizing element obtained in Production Example 2 was used instead of the polarizing element with substrate obtained in Production Example 1. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 12] A polarizing plate was prepared in the same manner as in Comparative Example 8, except that the polarizing element obtained in Production Example 2 was used instead of the polarizing element with substrate obtained in Production Example 1. The prepared polarizing plate was subjected to the above-mentioned evaluations (1) to (6). The results are shown in Table 1.

[Table 1]

10: Polarizer 20: Polyester film 30: Adhesive layer 40: Easy-adhesion layer 100, 200: Polarizer A: Polyester film with easy-adhesion layer

FIG1 is a schematic cross-sectional view of a polarizing plate in one embodiment of the present invention. FIG2 is a schematic cross-sectional view of a polarizing plate in another embodiment of the present invention.

Claims (9)

A polarizing plate comprises a polarizer and a polyester film disposed on one side of the polarizer; the linear expansion coefficient of the polyester film in a first direction is 0.5×10 ^{-5 /} °C to 1.8× ^{10 -5} /°C; the linear expansion coefficient of the polyester film in a second direction perpendicular to the first direction is 3.5×10 ^-5 /°C to 7.5×10 ^-5 /°C; the polyester film has a slow axis in a direction of -5° to 5° relative to the first direction; the absorption axis of the polarizer is slightly parallel to the first direction of the polyester film. As in claim 1, the polarizing plate, wherein the crystallinity of the polyester film measured by DSC is above 30%. As in the polarizing plate of claim 1 or 2, wherein the aforementioned polyester film is a biaxially stretched film. A polarizing plate as claimed in claim 1 or 2, wherein the absolute value of the difference between the linear expansion coefficient of the aforementioned polyester film in a first direction and the linear expansion coefficient of the aforementioned 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 below 2.0×10 ^-5 /°C. As in claim 1 or 2, the polarizing plate, wherein the thickness of the polarizing element is less than 20 μm. The polarizing plate of claim 1 or 2 further comprises an easy-bonding layer, which is disposed on the polarizing element side of the aforementioned polyester film. As in claim 6, the polarizing plate, wherein the aforementioned easy-to-bond layer contains microparticles. As in claim 6, the polarizing plate, wherein the thickness of the aforementioned easy-to-bond layer is less than 0.35 μm. As in claim 6, the polarizing plate, wherein the refractive index of the aforementioned easy-to-bond layer is less than 1.55.

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