JP2023159194A - Polyester film, and polarizing plate including the same - Google Patents

Abstract

To provide a polyester film that reduces the occurrence of rainbow unevenness when applied to an image display device, and can contribute to an improvement in the durability of a polarizing plate.SOLUTION: A polyester film has a coefficient of linear expansion of 3.5×10-5/°C or less in a first direction, and a coefficient of linear expansion of 3.5×10-5/°C or less 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: None

Description

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

Image display devices (eg, liquid crystal display devices, organic EL display devices) often include a polarizing plate on at least one side of a display cell, depending on the image forming method. In recent years, the functions and uses of image display devices have tended to become more diverse, and they are required to withstand use in harsher environments. A polarizing plate generally has a structure in which a polarizer is sandwiched between two protective films, and triacetyl cellulose, acrylic resin, cycloolefin resin, etc. are widely used as the protective films. On the other hand, from the viewpoint of durability as mentioned above, polyester films with excellent mechanical properties, chemical resistance, and moisture barrier properties, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), are used as polarizer protective films. It has been proposed (for example, Patent Document 1). However, although polyester films have excellent mechanical properties, they have birefringence, which may cause deterioration in visibility such as rainbow unevenness. In particular, the problem of such rainbow unevenness has become more prominent as image display devices have become more bright and have higher color purity in recent years.

On the other hand, polarizing plates constructed using protective films made from triacetylcellulose, acrylic resins, or cycloolefin resins, which have been widely used in the past, tend to crack due to temperature changes. There is. In recent years, as image display devices have become thinner, there has been a demand for thinner polarizers, and as the number of image display devices that are expected to be used at high temperatures has increased, we have developed polarized light that does not cause cracks in the polarizer and is highly durable. Boards are highly desired.

Japanese Patent Application Publication No. 8-271733

The present invention has been made to solve the above-mentioned conventional problems, and its main purpose is to reduce the occurrence of rainbow unevenness when applied to an image display device, and to contribute to improving the durability of a polarizing plate. Our purpose is to provide polyester films.

The polyester film of the present invention has a linear expansion coefficient of 3.5×10 −5 /°C or less in a first direction, and a linear expansion coefficient of 3.5×10 ⁻⁵ /°C or less in a second direction perpendicular to the first direction. 5×10 ⁻⁵ /° C. or less, and has a slow axis in a direction of −5° to 5° with respect to the first direction.
In one embodiment, the polyester film has a crystallinity of 30% or more as measured by DSC.
According to another aspect of the invention, a polarizing plate is provided. This polarizing plate includes a polarizer and the polyester film disposed on one side of the polarizer.
In one embodiment, the thickness of the polarizer is 20 μm or less.
In one embodiment, the polarizing plate further includes an easily adhesive layer disposed on the polarizer side of the polyester film.
In one embodiment, the easily adhesive layer includes fine particles.
In one embodiment, the thickness of the easily adhesive layer is 0.35 μm or less.
In one embodiment, the refractive index of the adhesive layer is 1.55 or less.

According to the present invention, by selectively reducing the coefficient of linear expansion in a predetermined direction, a polyester film is produced which reduces the occurrence of rainbow unevenness when combined with a polarizer and can contribute to improving the durability of the polarizing plate. can be provided.

1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention.

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

A. Polyester Film The polyester film of the present invention has a linear expansion coefficient of 3.5×10 −5 /°C or less in a first direction, and a linear expansion coefficient of 3.5×10 ⁻⁵ /°C or less in a second direction perpendicular to the first direction. It is 3.5×10 ⁻⁵ /°C or less. If a polyester film having such a coefficient of linear expansion is used, it is possible to effectively protect the polarizer by laminating it on a polarizer and prevent the polarizer from cracking. More specifically, if the polyester film of the present invention is laminated on a polarizer as a polarizer protective film to form a polarizing plate, dimensional changes in the polarizer (for example, dimensional changes due to heat) can be suppressed by the polyester film. Can be done. As a result, by using the polyester film of the present invention, it is possible to prevent cracks in the polarizer even under harsh environments such as high temperatures and large temperature changes, and to obtain a polarizing plate with excellent durability. In one embodiment, the first direction corresponds to the transport direction (MD) when manufacturing a polyester film. Furthermore, the second direction may correspond to TD orthogonal to MD. The linear expansion coefficient can be determined by TMA measurement according to JIS K 7197. Note that 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°.

The polyester film of the present invention has a slow axis in the -5° to 5° direction with respect to the first direction. Within this range, the polyester film can be made with less occurrence of rainbow unevenness when combined with a polarizer. More specifically, as described above, when a polarizing plate is constructed by laminating a polarizer and a polyester film so that the absorption axis of the polarizer and the first direction are approximately parallel, rainbow unevenness becomes effective. can be 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°, most preferably 0°. Within this range, the above effect becomes more significant.

The linear expansion coefficient of the polyester film in the first direction is preferably 3.0×10 ⁻⁵ /°C or less, more preferably 2.5×10 ⁻⁵ /°C or less, and even more preferably 1. It is 5×10 ⁻⁵ /°C or less, particularly preferably 1.3×10 ⁻⁵ /°C or less. Within this range, the above effect becomes more significant. The linear expansion coefficient of the polyester film in the first direction is preferably as small as possible, but its lower limit is, for example, 0.3×10 ⁻⁵ /°C (preferably 0.1×10 ⁻⁵ /°C, more preferably 0 ×10 ⁻⁵ /°C).

The linear expansion coefficient of the polyester film in the second direction is preferably 3.4×10 ⁻⁵ /°C or less, more preferably 2.3×10 ⁻⁵ /°C or less. Within this range, the above effect becomes more significant. The linear expansion coefficient of the polyester film in the first direction is preferably as small as possible, but its lower limit is, for example, 1×10 ⁻⁵ /°C (preferably 0.5×10 ⁻⁵ /°C, more preferably 0.3 ×10 ⁻⁵ /°C).

Typically, the polyester film may be a stretched film obtained through a stretching process. By appropriately adjusting the manufacturing conditions in the stretching process, the linear expansion coefficient in the first direction and the second direction (as well as the in-plane retardation Re (590) described below) can be well controlled. As a result, a polyester film having excellent properties as a polarizer protective film in terms of rainbow unevenness and durability can be obtained as described above. The above manufacturing conditions include 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, MD/TD direction after stretching. Examples include the relaxation rate of The stretching temperature, stretching ratio, and stretching speed can be adjusted appropriately for each MD/TD.

The in-plane retardation Re (590) of the polyester film is, for example, greater than 0 nm and less than 10000 nm. Note that the in-plane retardation Re (λ) is the in-plane retardation of the film measured with light having a wavelength of λ nm at 23°C. Therefore, Re(590) is the in-plane retardation of the film measured with light at a wavelength of 590 nm. Re(λ) is determined by the formula: Re(λ)=(nx−ny)×d, where d (nm) is the thickness of the film. Here, nx is the refractive index in the direction in which the in-plane refractive index is maximum (that is, the slow axis direction), and ny is the refractive index in the in-plane direction orthogonal to the slow axis.

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

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.

The total light transmittance of the polyester film is preferably 80% or more, more preferably 85% or more, still more preferably 90% or more, particularly preferably 95% or more. The haze of the polyester film is preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less, 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, even more preferably 15 g/m ² ·24 hr or less. Within this range, a polarizing plate with excellent durability and moisture resistance can be obtained.

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

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-naphthalic acid, diphenic acid, 4,4'-oxybenzoic acid, and 2,5-naphthalenedicarboxylic acid. Examples of aliphatic dicarboxylic acids include malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyladipic acid, pimelic acid, 2,2-dimethylglutaric acid, azelaic acid, zebacic acid, fumaric acid, Examples include 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, and 2,5-norbornanedicarboxylic acid. acids, adamantane dicarboxylic acid. The carboxylic acid component may be a derivative such as an ester, chloride, acid anhydride, for example dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl terephthalate. and diphenyl terephthalate. The carboxylic acid components may be used alone or in combination of two or more.

A typical example of the polyol component is a dihydric alcohol. 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- Examples include butadiol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,6-hexanediol. It will be done. Examples of alicyclic diols include 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, spiroglycol, tricyclodecanedimethanol, adamantanediol, 2,2,4 , 4-tetramethyl-1,3-cyclobutanediol. Examples of aromatic diols include 4,4'-thiodiphenol, 4,4'-methylene diphenol, 4,4'-(2-norbornylidene) diphenol, 4,4'-dihydroxybiphenol, o-, Mention may be made of m- and p-dihydroxybenzene, 4,4'-isopropylidenephenol, 4,4'-isopropylidenebis(2,6-cyclophenol)2,5-naphthalene diol and p-xylene diol. The polyol components may be used alone or in combination of two or more.

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, it is possible to obtain a polyester film that has excellent mechanical properties and less occurrence of rainbow unevenness. Polyethylene terephthalate and modified polyethylene terephthalate may be used as a blend.

Examples of the modified polyethylene terephthalate include modified polyethylene terephthalate 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 more than 0 mol% and 10 mol% or less, more preferably more than 0 mol% and 3 mol% or less. The proportion of 1,4-butanediol in the polyol component is preferably more than 0 mol% and 10 mol% or less, more preferably more than 0 mol% and 3 mol% or less. The proportion of 1,3-propanediol in the polyol component is preferably more than 0 mol% and no more than 10 mol%, more preferably more than 0 mol% and no more than 3 mol%. The proportion of isophthalic acid in the carboxylic acid component is preferably more than 0 mol% and no more than 10 mol%, more preferably more than 0 mol% and no more than 8 mol%. Within this range, a polyester film with good crystallinity can be obtained. In addition, the mol% described above is mol% with respect to the total of all repeating units of the polymer.

The weight average molecular weight of the polyester resin is preferably 10,000 to 100,000, more preferably 20,000 to 75,000. With such a weight average molecular weight, a film can be obtained that is easy to handle during molding and has excellent mechanical strength. The weight average molecular weight can be measured by GPC (solvent: THF).

In one embodiment, a polyester film with an easily adhesive layer is provided. The easily adhesive layer contains, for example, water-based polyurethane and an oxazoline-based crosslinking agent. Details of the easily adhesive layer are described in, for example, Japanese Patent Laid-Open No. 2010-55062. The entire description of this publication is incorporated herein by reference.

In one embodiment, the easily adhesive layer contains any appropriate fine particles. By forming an easily 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. Can be mentioned. Examples of the organic fine particles include silicone resins, fluorine resins, (meth)acrylic resins, and the like. Among these, silica is preferred.

The particle size (number average primary particle size) of the fine particles is preferably 10 nm to 200 nm, more preferably 20 nm to 60 nm.

The thickness of the adhesive layer is preferably 2 μm or less, more preferably 1 μm or less, and still more preferably 0.35 μm or less. Within this range, it is possible to obtain a polyester film with an easily adhesive layer that does not easily impede the optical properties of other members when applied to an image display device.

In one embodiment, the refractive index of the adhesive layer is preferably 1.45 to 1.60. Within this range, it is possible to obtain a polyester film with an easily adhesive layer that does not easily impede the optical properties of other members when applied to an image display device. In one embodiment, the easily adhesive layer has a refractive index of 1.54 or more.

In one embodiment, the polyester film may be provided with an anti-block layer on at least one side thereof. As the structure of the anti-block layer, the structure of the easy-adhesion layer explained above may be adopted. Preferably, the anti-block layer contains the above-mentioned fine particles.

(Method for producing polyester film)
The above-mentioned polyester film can be obtained through a forming process of forming a film-forming material (resin composition) containing the above-mentioned polyester-based resin into a film shape, and a stretching process of stretching the formed film. Preferably, the stretching step includes a preheating treatment of the film performed before film stretching, and a heat treatment performed after film stretching. In one embodiment, the polyester film is provided in a long form (or in a shape cut out from a long body).

In addition to the polyester resin, the film-forming material may contain additives or a solvent. As the additive, any suitable additive may be employed depending on 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 materials, and flame retardants. can be mentioned. The number, type, combination, amount, etc. of additives can be appropriately set depending on the purpose.

As a method for forming a film from the film-forming material, any suitable forming method may be employed. Specific examples include compression molding method, transfer molding method, injection molding method, extrusion molding method, blow molding method, powder molding method, FRP molding method, cast coating method (e.g. casting method), calendar molding method, hot press method. Laws etc. Extrusion molding or cast coating methods are preferred. This is because the smoothness of the resulting film can be improved and good optical uniformity can be obtained.

The method of stretching the film may be uniaxial stretching or biaxial stretching.

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

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 directions of the film are typically the length direction (MD) and width direction (TD) of the film.

In one embodiment, sequential biaxial stretching is employed as the method for stretching the film. It is preferable to perform MD stretching after TD stretching to obtain the polyester film. In this way, the effect of bowing that occurs during TD stretching can be alleviated, and the angle between the first direction (MD) and the slow axis in the polyester film can be set to an appropriate value. .

The stretching temperature is relative to the glass transition temperature (Tg) of the film, 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. By stretching at such a temperature, it is possible to obtain a polyester film in which the slow axis direction and linear expansion coefficient are controlled in a well-balanced manner. Moreover, a polyester film with excellent transparency can be obtained.

The stretching ratio in MD is preferably 1 to 7 times, more preferably 2.5 to 6.5 times, and even more preferably 3 to 6 times. Within this range, it is possible to obtain a polyester film that has good crystallinity and excellent durability while keeping the coefficient of linear expansion within a desired range.

The stretching ratio in TD is preferably 1 to 7 times, more preferably 1.2 to 4 times, and even more preferably 1.5 to 3.5 times. Within this range, it is possible to obtain a polyester film that has good crystallinity and excellent durability while keeping the coefficient of linear expansion within a desired range.

The ratio of the draw ratio in TD to the draw ratio in MD (MD draw ratio/TD draw ratio) is preferably 1 to 4, more preferably 1 to 2. Within this range, a polyester film with particularly low occurrence of rainbow unevenness can be obtained. Further, by using the obtained polyester film, it is possible to prevent the generation of cracks in the polarizer and obtain a polarizing plate with excellent durability.

The stretching speed in MD is preferably 5%/sec to 100%/sec, more preferably 8%/sec to 80%/sec, and still more preferably 8%/sec to 60%/sec. Within this range, it is possible to obtain a polyester film that has excellent optical properties, good crystallinity, and excellent durability.

The stretching speed in TD is preferably 5%/sec to 100%/sec, more preferably 8%/sec to 80%/sec, and even more preferably 8%/sec to 60%/sec. Within this range, it is possible to obtain a polyester film that has excellent optical properties, good crystallinity, and excellent durability.

The temperature of the preheating treatment is preferably 80°C to 150°C, more preferably 90°C to 130°C. Further, the preheating treatment time is preferably 10 seconds to 100 seconds, more preferably 15 seconds to 80 seconds. Within this range, it is possible to obtain a polyester film that has excellent optical properties, good crystallinity, and excellent durability.

The temperature of the heat treatment is preferably 100°C to 250°C, more preferably 120°C to 200°C, even more preferably 130°C to 180°C. Within this range, it is possible to obtain a polyester film that has excellent transparency, good crystallinity, and excellent durability. 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, it is possible to obtain a polyester film that has excellent transparency, good crystallinity, and excellent durability.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 includes a polarizer 10 and a polyester film 20 disposed on one side of the polarizer 10. As the polyester film 20, the polyester film of the present invention described in Section A above is used. Any other suitable polarizer protective film may be disposed on the other side of the polarizer, or no polarizer protective film may be disposed. In one embodiment, polarizer 10 and polyester film 20 (or another polarizer protective film) are laminated with adhesive layer 30 interposed therebetween.

In one embodiment, the polarizing plate may be applied to an image display device such that the side on which the polyester film is placed is the viewing side. Moreover, when applying the said polarizing plate to a liquid crystal display device, the polarizing plate provided with a polyester film may be arrange|positioned on the viewing side of a liquid crystal cell, and may be arrange|positioned on the back side.

Any suitable polarizer may be employed 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.

Specific examples of polarizers composed of single-layer resin films include hydrophilic polymer films such as polyvinyl alcohol (PVA) films, partially formalized PVA films, and partially saponified ethylene/vinyl acetate copolymer films. Examples include those that have been dyed with dichroic substances such as iodine and dichroic dyes and stretched, and polyene-based oriented films such as dehydrated PVA and dehydrochloric acid treated polyvinyl chloride. Preferably, a polarizer obtained by dyeing a PVA film with iodine and uniaxially stretching is used because it has excellent optical properties.

The above-mentioned staining with iodine is performed, for example, by immersing the PVA-based film in an iodine aqueous solution. The stretching ratio of the above-mentioned uniaxial stretching is preferably 3 to 7 times. Stretching may be performed after the dyeing process or may be performed while dyeing. Alternatively, it may be dyed after being stretched. If necessary, the PVA film is subjected to swelling treatment, crosslinking treatment, washing treatment, drying treatment, etc. For example, by immersing a PVA film in water and washing it with water before dyeing, you can not only wash dirt and anti-blocking agents from the surface of the PVA film, but also swell the PVA film and prevent uneven dyeing. It can be prevented.

Specific examples of polarizers obtained using a laminate include a laminate of a resin base material and a PVA resin layer (PVA resin film) laminated on the resin base material, or a laminate of a resin base material and the resin Examples include polarizers obtained using a laminate with a PVA-based resin layer coated on a base material. A polarizer obtained using a laminate of a resin base material and a PVA-based resin layer coated on the resin base material can be obtained by, for example, applying a PVA-based resin solution to the resin base material and drying it. Forming a PVA-based resin layer thereon to obtain a laminate of a resin base material and the PVA-based resin layer; stretching and dyeing the laminate to use the PVA-based resin layer as a polarizer; obtain. In this embodiment, stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching. Furthermore, the stretching may further include stretching the laminate in air at a high temperature (for example, 95° C. or higher) before stretching in the boric acid aqueous solution, if necessary. The obtained resin base material/polarizer laminate may be used as is (that is, the resin base material may be used as a protective layer for the polarizer), or the resin base material may be peeled from the resin base material/polarizer laminate. However, any appropriate protective layer may be laminated on the peeled surface depending on the purpose. Details of the method for manufacturing such a polarizer are described in, for example, Japanese Patent Application Publication No. 2012-73580. The entire description of this publication is incorporated herein by reference.

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, more preferably 3 μm to 15 μm. If the polyester film of the present invention is used, it is possible to effectively prevent cracks in the polarizer, so it becomes possible to use a thin polarizer even under harsh environments such as high temperatures and large temperature changes.

The polarizer and the polarizer protective film (polyester film) may be laminated via any appropriate adhesive layer. Preferably, the adhesive layer is formed from an adhesive composition containing a polyvinyl alcohol resin.

It is preferable that the absorption axis direction of the polarizer and the first direction (typically MD) or the second direction (typically TD) of the polyester film are approximately parallel; In particular, it is more preferable to be substantially parallel to MD). With this configuration, the polyester film and the polarizer can preferably change their shape in synchronization. As a result, cracks in the polarizer are prevented.

The slow axis angle of the polyester film is preferably the same as the angle made with the absorption axis direction of the polarizer, and the angle between the two axes is preferably 0° ± 10°, more preferably 0° ± 7°. Yes, more preferably 0°±5°. Within this range, a polyester film with less occurrence of rainbow unevenness when applied to an image display device can be obtained. Note that the slow axis angle is an angle when the roll 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 a direction parallel to the first direction is preferably 2.0×10 ^{− 5} /°C or less, more preferably 1.5 x 10 ^-5 /°C or less, even more preferably 1.0 x 10 ^-5 /°C or less. Within this range, cracks in 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, but for example, 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 (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 even more preferably 1.0×10 ⁻⁵ /°C or less. Within this range, cracks in 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, but for example, 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 second The absolute value of the difference between the linear expansion coefficient in the direction (direction perpendicular to the first direction) and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is both 2.0×10 ⁻⁵ /°C or less (preferably 1.0×10 ⁻⁵ /°C or less). Within this range, cracks in the polarizer can be prevented even under harsh environments such as high temperatures and large temperature changes.

FIG. 2 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the invention. The polarizing plate 200 further includes an easily adhesive layer 40 disposed on the polarizer 10 side of the polyester film 20. In one embodiment, the easily adhesive layer-attached polyester film A is placed on the polarizer 10 such that the easily adhesive layer 40 is on the polarizer 10 side. As the easy-adhesive layer, the easy-adhesive layer described in Section A above may be employed.

C. Image Display Device The polarizing plate described above can be applied to an image display device. Typical examples of image display devices include liquid crystal display devices and organic electroluminescence (EL) display devices. Since the image display device employs a configuration well known in the industry, detailed explanation will be omitted.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples. The methods for measuring each characteristic in the examples are as follows. Note that unless otherwise specified, "parts" and "%" in the examples are based on weight.

(1) Orientation angle (direction of slow axis development)
Samples were prepared by cutting the central part of the polyester films obtained in Examples and Comparative Examples into a square shape with a width of 50 mm and a length of 50 mm, with one side parallel to the width direction of the film. This sample was measured using a Mueller matrix polarimeter (manufactured by Axometrics, product name "Axoscan") to measure the orientation angle θ at a wavelength of 550 nm and 23°C. Note that the orientation angle θ was measured with the sample placed parallel to the measurement table.
(2) Linear expansion coefficient The linear expansion coefficient of the polyester film and polarizer was measured at 10°C/min from 30°C to 150°C using a thermomechanical analyzer “TMA7000” manufactured by Hitachi High-Tech Science Co., Ltd. based on JIS K 7197. The amount of deformation of the test film at each temperature was measured. Then, the linear expansion coefficient of the film was determined from the amount of deformation in the temperature range of 30°C to 70°C. Note that the case where the film size increases (expands) as the temperature rises is defined as positive (plus), and the case where the film size decreases (shrinks) as temperature rises is defined as negative (minus).
For the polyester film, linear expansion coefficients in MD (first direction) and TD (second direction) were measured. The linear expansion coefficient of the polarizer in the direction parallel to the MD and the direction parallel to the TD was measured on the polarizing plate.
(3) Crystallinity The crystallinity of the polyester films used in 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 calorific value and heat of fusion observed during heating were determined, and the degree of crystallinity was determined using the following formula. Note that the calorific value and heat of fusion were measured using Q-2000 manufactured by TA instruments.
Crystallinity (%) = (heat of fusion obtained by measurement - calorific value obtained by measurement) / heat of fusion of polyethylene terephthalate with 100% crystallinity (119 mJ/mg) x 100
(4) Rainbow unevenness The liquid crystal cell was taken out from a liquid crystal TV "45UH7500" manufactured by LGD, and the polarizing plate on the backlight side was peeled off. The polarizing plates obtained in 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. Ta. The liquid crystal cells laminated with the polarizing plates obtained in Examples and Comparative Examples were again installed, and the TV was turned on with a white display.
The liquid crystal TV was turned on and visually checked in all directions at a polar angle of 60°, and the presence or absence of rainbow unevenness was observed. Evaluation was made based on the following criteria.
○: Rainbow unevenness was not observed.
△: Rainbow unevenness was slightly observed.
×: Significant rainbow unevenness was observed (5) Dimensional change The polyester film used in the Examples and Comparative Examples was cut into 100 mm x 100 mm. After that, the film was placed in a heating oven at 100° C. for 24 hours, and then the film was taken out, and the dimensions were accurately measured again, and the dimensions were confirmed with a metal ruler to determine the change in dimensions. In addition, the condition of the sample was visually confirmed and evaluated based on the following criteria.
○: No significant shrinkage of 1 mm or more ×: Shrinkage of 1 mm or more or deformation (6) Crack test (heat shock accelerated test)
The polarizing plates obtained in Examples and Comparative Examples were evaluated using a thermal shock tester (manufactured by ESPEC).
The polarizing plates obtained in Examples and Comparative Examples were cut into 50 mm width x 150 mm length. At that time, the absorption axis direction of the polarizer is parallel to the horizontal direction (short side) of the polarizing plate after cutting, and the transmission axis direction of the polarizer is parallel to the horizontal direction (short side) of the polarizing plate after cutting. A sample was prepared. A sample was prepared by bonding the surface of a polarizing plate on which the protective film (polyester film) was not laminated to a 0.5 mm thick alkali-free glass via an acrylic adhesive.
The obtained sample was placed in the test area of a thermal shock tester, and the temperature inside the test area was lowered from room temperature to -40°C over 30 minutes. Next, the temperature inside the test area was raised to 85°C over 30 minutes, and then lowered again to -40°C over 30 minutes. The process of raising the temperature from -40°C to 85°C and lowering the temperature again to -40°C is one cycle, and after repeating 100 cycles and 200 cycles, the laminate is taken out and visually checked for the presence or absence of cracks. Evaluation was made based on 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.
x: Cracks were generated after 100 cycles.

[Production Example 1] Production of Polarizer A As a base material, a long, amorphous isophthalic acid copolymerized polyethylene terephthalate (IPA copolymerized PET) film (thickness: 100 μm) with a water absorption rate of 0.75% and a Tg of 75°C. ) was used. Corona treatment was applied to one side of the base material, and 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) was applied to the corona-treated surface. %, saponification degree of 99.0 mol% or more, manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name "Gosefimer Z200") in a ratio of 9:1 was coated and dried at 25°C to form a 11 μm thick film. A PVA-based resin layer was formed to produce a laminate.
The obtained laminate was uniaxially stretched free end to 2.0 times in the longitudinal direction (longitudinal direction) between rolls having different circumferential speeds in an oven at 120° C. (in-air assisted stretching).
Next, the laminate was immersed for 30 seconds in an insolubilization bath (an aqueous boric acid solution obtained by blending 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30° C. (insolubilization treatment).
Next, the polarizing plate was immersed in a dyeing bath at a liquid temperature of 30° C. while adjusting the iodine concentration and immersion time so that the polarizing plate had a predetermined transmittance. In this example, 100 parts by weight of water was mixed with 0.2 parts by weight of iodine and 1.5 parts by weight of potassium iodide. .
Next, it was immersed for 30 seconds in a crosslinking bath (an aqueous boric acid solution obtained by blending 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. (Crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (an 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. However, uniaxial stretching was performed between rolls having different circumferential speeds in the longitudinal direction (longitudinal direction) so that the total stretching ratio was 5.5 times (underwater stretching).
Thereafter, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 30°C (cleaning treatment), and the peelable base material was attached. Polarizer A was obtained.

[Production Example 2] Production of Polarizer B Polarizer B with a peelable base material was obtained in the same manner as Production Example 1, except that the stretching ratio in the underwater stretching was 4.6 times.

[Production Example 3] Production of polyester film A Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products Co., Ltd., isophthalic acid modification amount 2.5 mol% (mol number based on the total of all repeating units of the polymer), diethylene glycol modification amount: 1.0 mol% (mol number relative to the total of all polymer repeating units), IV value 0.77 dl/g (phenol: 1,1,2,2,-tetrachloroethane = 6:4 mixed solvent solution concentration 0.4 g/dl) at 100°C After vacuum drying for 10 hours, a single screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder temperature setting: 280°C), T-die (width 500 mm, setting temperature: 280°C), chill roll (setting temperature: 50°C) was used. An amorphous polyester resin film having a thickness of 200 μm was produced using a film forming apparatus equipped with a winder and a winder.
The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner stretching machine KAROIV, and polyester film A (slow axis angle to the longitudinal direction: -0.5°, in-plane phase Re (590): 80 nm, thickness: 17 μm) was obtained. The stretching ratio was 4 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was 90° C., and the stretching speed was 30%/sec in both MD and TD. Further, after the stretching treatment, heat treatment was performed at 180° C. for 10 seconds while maintaining the dimensions.

[Production Example 4] Production of polyester film I 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) After drying in vacuum at 100 °C for 10 hours (concentration 0.4 g/dl), a single-screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder temperature setting: 280 °C), T-die (width 500 mm, temperature setting: An amorphous polyester resin film having a thickness of 200 μm was produced using a film forming apparatus equipped with a chill roll (set temperature: 50° C.), a winder (temperature set at 50° C.), and a winder.
The resulting amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner stretching machine KAROIV, resulting in a polyester film I (slow axis angle to the longitudinal direction: -2.5°, in-plane phase Re( 590): 271 nm, thickness: 22 μm) was obtained. The stretching ratio was 3 times in the length direction (MD) and 3 times in the width direction (TD). The stretching temperature was 90° C., and the stretching speed was 2%/sec in both MD and TD. Further, after the stretching treatment, heat treatment was performed at 140° C. for 10 seconds while maintaining the dimensions.

[Production Example 5] Production of polyester film II The stretching ratio was 2 times in the length direction (MD) and 2 times in the width direction (TD), and the stretching speed was 2%/sec in both MD and TD. Polyester film II (slow axis angle with respect to length direction: -11.9°, in-plane phase Re (590): 54 nm , thickness: 50 μm).

[Production Example 6] Production of polyester film III The stretching ratio was 6 times in the length direction (MD) and 1 time in the width direction (TD) by fixed end stretching, and the stretching speed was 2%/2 in both MD and TD. Polyester film III (slow axis angle to the length direction: -0.6°, in-plane phase Re (590): 2823 nm, thickness: 41 μm) was obtained.

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

[Production Example 8] Production of polyester film V Polyester film V (slow axis angle with respect to the length direction: 3 .0°, in-plane phase Re (590): 17 nm, thickness: 50 μm).

[Production Example 9] Production of polyester film VI Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products Co., Ltd., isophthalic acid modification amount 2.5 mol% (mol number based on 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 a single screw extruder (manufactured by Toyo Seiki Co., Ltd., screw Using a film forming apparatus equipped with a T-die (width 500 mm, set temperature: 280°C), a chill roll (set temperature: 50°C), and a winder, a film with a thickness of 170 μm was produced. An amorphous polyester resin film was produced.
This film was uniaxially stretched at its free end by 2.0 times in the longitudinal direction (longitudinal direction) between rolls having different circumferential speeds in an oven at 120°C.
Next, after being immersed in water with a liquid temperature of 30°C for 120 seconds, it was immersed in water with a liquid temperature of 73°C, with a total stretching ratio of 5.5 times in the vertical direction (longitudinal direction) between rolls with different circumferential speeds. Uniaxial stretching was performed (underwater stretching) so that
The obtained stretched film was heat-treated at 90°C for 10 seconds using a Bruckner stretching machine KAROIV, resulting in a polyester film VI (slow axis angle to the longitudinal direction: -0.2°, in-plane phase Re (590)) : 3243 nm, thickness: 35 μm) was obtained.

[Production Example 10] Production of polyester film VII A stretched film was obtained in the same manner as in Production Example 9.
The obtained stretched film was heat-treated at 90°C for 10 seconds and then at 140°C for 10 seconds using a Bruckner stretching machine KAROIV to obtain a polyester film VII (slow axis angle to the longitudinal direction: -0.4°, In-plane phase Re (590): 4052 nm, thickness: 35 μm) was obtained.

[Example 1]
The polyester film A produced in Production Example 3 was subjected to corona treatment, and 15.2 wt% of the product name "Superflex 210R" manufactured by Daiichi Kogyo Seiyaku Co., Ltd. and 2.7 wt% of the product name "WS-700" manufactured by Nippon Shokubai Co., Ltd. were added. A polyester film A with an easily adhesive layer was obtained by coating an aqueous solution in which % was dissolved so that the film thickness after drying would be 300 μm and drying at 80° C. for 1 minute.
On the polarizer surface of the polarizer with a base material obtained in Production Example 1, a PVA-based resin aqueous solution (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name "Gosefimer (registered trademark) Z-200"), resin concentration: 3% by weight ) was applied, and the above-mentioned polyester film with an easily adhesive layer was bonded together. The obtained laminate was heated in an oven maintained at 60° C. for 5 minutes. Thereafter, the base material was peeled off from the PVA-based resin layer to obtain a polarizing plate (polarizer (transmittance 42.3%, thickness 5 μm)/protective film (polyester film)). Note that the polyester film A and the polarizer were laminated so that the MD direction of the polyester film A and the absorption axis direction of the polarizer were substantially 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 obtained in the same manner as in Example 1, except that the polarizer obtained in Production Example 2 was used instead of the base-attached polarizer obtained in Production Example 1. The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative example 1]
A polarizing plate was obtained in the same manner as in Example 1, except that polyester film I produced in Production Example 4 was used in place of Polyester Film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative example 2]
A polarizing plate was obtained in the same manner as in Example 1, except that polyester film II produced in Production Example 5 was used instead of polyester film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

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

[Comparative example 4]
A polarizing plate was obtained in the same manner as in Example 1, except that polyester film IV produced in Production Example 7 was used instead of polyester film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative example 5]
Instead of polyester film A produced in Production Example 3, polyester film a (manufactured by Toyobo Co., Ltd., trade name "Cosmoshine A4100", slow axis angle with respect to the length direction: 90°, in-plane phase Re (590): 7800 nm) A polarizing plate was obtained in the same manner as in Example 1, except that a polarizing plate (thickness: 75 μm) was used.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative example 6]
In place of polyester film A produced in Production Example 3, polyester film B (manufactured by Mitsubishi Chemical Corporation, trade name "T100-J25", slow axis angle with respect to the length direction: 27°, in-plane phase Re (590): A polarizing plate was obtained in the same manner as in Example 1, except that a polarizing plate (525 nm, thickness: 25 μm) was used.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative Example 7]
A polarizing plate was obtained in the same manner as in Example 1, except that polyester film V produced in Production Example 8 was used instead of polyester film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

[Comparative example 8]
A polarizing plate was obtained in the same manner as in Example 1, except that polyester film VI produced in Production Example 9 was used instead of polyester film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above 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 polyester film VII produced in Production Example 10 was used instead of polyester film A produced in Production Example 3.
The obtained polarizing plate was subjected to the above evaluations (1) to (6). The results are shown in Table 1.

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

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

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

10 Polarizer 20 Polyester film 30 Adhesive layer 40 Easy adhesive layer 100, 200 Polarizing plate

Claims (8)

the coefficient of linear expansion in the first direction is 3.5×10 ⁻⁵ /°C or less,
a linear expansion coefficient in a second direction perpendicular to the first direction is 3.5×10 ⁻⁵ /°C or less;
having a slow axis in a direction of -5° to 5° with respect to the first direction;
Polyester film. The polyester film according to claim 1, having a crystallinity of 30% or more as measured by DSC. A polarizing plate comprising a polarizer and the polyester film according to claim 1 disposed on one side of the polarizer. The polarizing plate according to claim 3, wherein the thickness of the polarizer is 20 μm or less. The polarizing plate according to claim 3, further comprising an easily adhesive layer disposed on the polarizer side of the polyester film. The polarizing plate according to claim 5, wherein the easily adhesive layer contains fine particles. The polarizing plate according to claim 5 or 6, wherein the thickness of the easily adhesive layer is 0.35 μm or less. The polarizing plate according to any one of claims 5 to 7, wherein the easily adhesive layer has a refractive index of 1.55 or less.

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