JP7573979B2 - Polyester film and polarizing plate including said polyester film - Google Patents

Description

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

In image display devices (e.g., liquid crystal display devices, organic EL display devices), a polarizing plate is often arranged on at least one side of the display cell due to the image formation method. In recent years, image display devices have tended to have more diverse functions and uses, and are required to be able 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 resins, cycloolefin resins, etc. are widely used as protective films. On the other hand, from the viewpoint of durability as described above, it has been proposed to use polyester films such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) that have excellent mechanical properties, chemical resistance, and moisture blocking properties as polarizer protective films (e.g., Patent Document 1). However, while polyester films have excellent mechanical properties, they have birefringence, which can cause deterioration of visibility, such as the occurrence of rainbow unevenness. In particular, with the recent increase in brightness and color purity of image display devices, the problem of such rainbow unevenness has become more prominent.

On the other hand, polarizing plates constructed using protective films made from triacetyl cellulose, acrylic resins, or cycloolefin resins, which have been widely used in the past, may experience cracks in the polarizer due to temperature changes. In recent years, as image display devices have become thinner, there has been a demand for thinner polarizers, while an increasing number of image display devices are expected to be used at high temperatures, and so there is a strong demand for polarizing plates that are free from cracks in the polarizer and have excellent durability.

Japanese Patent Application Publication No. 8-271733

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

The polyester film of the present invention has a linear expansion coefficient in a first direction of 3.0×10 ⁻⁵ /°C or less, a linear expansion coefficient in a second direction perpendicular to the first direction of 7.5×10 ⁻⁵ /°C to 10.5×10 ⁻⁵ /°C, and a slow axis in a direction at an angle of -5° to 5° with respect to the first direction.
In one embodiment, the polyester film has a thickness variation in the second direction of 15% or less.
In one embodiment, the polyester film has a crystallinity of 30% or more as measured by DSC.
According to another aspect of the present invention, there is provided a polarizing plate comprising a polarizer and the above-mentioned polyester film disposed on one side of the polarizer.
In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in a first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction is 2.0×10 ^-5 /°C or less, and the absolute value of the difference between the linear expansion coefficient of the polyester film in a second direction perpendicular to the first direction and the linear expansion coefficient of the polarizer in a direction parallel to the second direction is 5×10 ^-5 /°C or less.
In one embodiment, the polarizer has a thickness of 20 μm or less.
In one embodiment, the polarizing plate further includes an easy-adhesion layer disposed on the polarizer side of the polyester film.
In one embodiment, the easy-adhesion layer contains fine particles.
In one embodiment, the easy-adhesion layer has a thickness of 0.35 μm or less.
In one embodiment, the easily adhesive layer has a refractive index of 1.55 or less.

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

1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. FIG. 4 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention.

The following describes preferred embodiments of the present invention, 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 in a first direction of 3.0×10 ⁻⁵ /° C. or less, and a linear expansion coefficient in a second direction perpendicular to the first direction of 7.5×10 ⁻⁵ /° C. to 10.5×10 ⁻⁵ /° C. In this way, by using a polyester having anisotropy in dimensional change, it is possible to prevent the occurrence of cracks in the polarizer while effectively protecting the polarizer by laminating it on the polarizer. More specifically, a polarizer is usually manufactured to have an absorption axis through a stretching process and has anisotropy in dimensional change (for example, dimensional change caused by temperature change). However, by laminating a polarizer and a polyester film so that the absorption axis of the polarizer and the first direction of the polyester film are approximately parallel, the polyester film and the polarizer can change their shapes in unison. As a result, by using the polyester film of the present invention, it is possible to obtain a polarizing plate having excellent durability by preventing the occurrence of cracks in the polarizer even under harsh environments such as high temperatures and large temperature changes. In one embodiment, the first direction corresponds to the machine direction (MD) in producing the polyester film. The second direction may correspond to TD perpendicular to MD. The linear expansion coefficient may be determined by TMA measurement according to JIS K 7197. The expression "substantially parallel" includes the case where the angle between the two directions is 0°±10°, preferably 0°±7°, and more preferably 0°±5°.

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 0.5×10 ⁻⁵ /° C. to 1.8× ^{10 −5} /° C. Within such a range, the above-mentioned effects become more remarkable.

The linear expansion coefficient of the polyester film in the second direction is preferably greater than 7.5×10 ⁻⁵ /° C. and equal to or less than 10.5×10 ⁻⁵ /° C., more preferably 7.5×10 ⁻⁵ /° C. to 10×10 ⁻⁵ /° C., and even more preferably 7.5×10 ⁻⁵ /° C. to 9.5×10 ⁻⁵ /° C. Within such a range, the above-mentioned effects become more pronounced.

In one embodiment, the linear expansion coefficient in the first direction is lower than the linear expansion coefficient in the second direction by 7×10 ⁻⁵ /° C. or more (preferably, 7.5×10 ⁻⁵ /° C. or more). In this range, the above-mentioned effect becomes more remarkable.

The polyester film of the present invention has a slow axis in the direction of -5° to 5° with respect to the first direction. Within this range, a polyester film that generates little rainbow unevenness when combined with a polarizer can be obtained. 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 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°. Within such a range, the above effect becomes more pronounced.

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 later) can be well controlled, and as a result, a polyester film having excellent properties as a polarizer protective film in terms of rainbow unevenness and durability as described above can be obtained. The manufacturing conditions include the stretching conditions (stretching temperature, stretching ratio, stretching speed, MD/TD stretching order), preheating temperature before stretching, heat treatment temperature after stretching, heat treatment time after stretching, relaxation rate in the MD/TD directions after stretching, etc. The stretching temperature, stretching ratio, and stretching speed can be appropriately adjusted for each MD/TD.

The in-plane retardation Re(590) of the polyester film is, for example, greater than 0 nm and equal to or less than 10,000 nm. 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 having a wavelength of 590 nm. Re(λ) is calculated by the formula: Re(λ) = (nx - ny) x d, where d (nm) is the thickness of the film. Here, nx is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), and ny is the refractive index in the in-plane direction perpendicular to the slow axis.

The above polyester film preferably has a crystallinity of 30% or more, more preferably 40% or more, and even more preferably 50% or more, as measured by differential scanning calorimetry (DSC). The upper limit of the crystallinity is, for example, 70%. If the crystallinity is within this range, a polyester film having excellent heat resistance and mechanical properties and 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 thickness unevenness of the polyester film in the second direction is preferably 15% or less, more preferably 13% or less, and even more preferably 10% or less. In the present invention, by reducing the thickness unevenness, the effect of preventing the occurrence of cracks in the polarizer while effectively protecting the polarizer by laminating the polyester film on the polarizer becomes more pronounced. The fact that the thickness unevenness affects the prevention of polarizer cracks is a unique effect of polyester films having anisotropy in the linear expansion coefficient, and the discovery of the relationship between thickness unevenness and polarizer cracks is one of the results of the present invention. In addition, by reducing the thickness unevenness, it is possible to obtain a polyester film that generates less rainbow unevenness when combined with a polarizer. The smaller the thickness unevenness of the polyester film, the more preferable it is, and the lower limit is, for example, 3% (preferably 1%, more preferably 0.5%). In this specification, "thickness unevenness in the second direction" is calculated by measuring the thickness of the polyester film using a continuous thickness measuring device, and using the formula {(Tmax-Tmin)/Tave) x 100 from the maximum thickness Tmax, minimum thickness Tmin, and average thickness Tave in the second direction (width direction, TD).

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

The moisture permeability of the polyester film is preferably 100 g/ ^m2 ·24 hr or less, more preferably 50 g/ ^m2 ·24 hr or less, and further preferably 15 g/ ^m2 ·24 hr or less. If it is within such a range, a polarizing plate excellent in durability and moisture resistance can be obtained.

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

Carboxylic acid components include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, and alicyclic dicarboxylic acids. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, benzylmalonic acid, 1,4-naphthalic acid, diphenyl 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, zebacinic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, and diglycolic acid. Examples of alicyclic dicarboxylic acids include 1,3-cyclopentanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 2,5-norbornanedicarboxylic acid, and adamantanedicarboxylic acid. The carboxylic acid component may be a derivative such as an ester, chloride, or acid anhydride, and includes, for example, dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl terephthalate, and diphenyl terephthalate. The carboxylic acid component may be used alone or in combination of two or more kinds.

Representative examples of the polyol component include dihydric alcohols. Examples of the dihydric alcohols include aliphatic diols, alicyclic diols, and aromatic diols. Examples of the aliphatic diols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butadiol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,6-hexanediol. Examples of alicyclic diols include 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, spiroglycol, tricyclodecane dimethanol, adamantane diol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Examples of aromatic diols include 4,4'-thiodiphenol, 4,4'-methylenediphenol, 4,4'-(2-norbornylidene)diphenol, 4,4'-dihydroxybiphenol, o-, m-, and p-dihydroxybenzene, 4,4'-isopropylidenephenol, 4,4'-isopropylidenebis(2,6-cyclophenol)2,5-naphthalenediol, and p-xylenediol. The polyol components may be used alone or in combination of two or more.

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

Examples of 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 not more than 10 mol%, more preferably more than 0 mol% and not more than 3 mol%. The proportion of 1,4-butanediol in the polyol component is preferably more than 0 mol% and not more than 10 mol%, more preferably more than 0 mol% and not more than 3 mol%. The proportion of 1,3-propanediol in the polyol component is preferably more than 0 mol% and not more than 10 mol%, more preferably more than 0 mol% and not more than 3 mol%. The proportion of isophthalic acid in the carboxylic acid component is preferably more than 0 mol% and not more than 10 mol%, more preferably more than 0 mol% and not more than 8 mol%. Within such a range, a polyester film having good crystallinity can be obtained. The mol% described above is the mol% relative 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, and more preferably 20,000 to 75,000. With such a weight average molecular weight, a film that is easy to handle during molding and has excellent mechanical strength can be obtained. The weight average molecular weight can be measured by GPC (solvent: THF).

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

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

The particle diameter (number average primary particle diameter) of the above microparticles is preferably 10 nm to 200 nm, more preferably 20 nm to 60 nm.

The thickness of the easy-adhesion layer is preferably 2 μm or less, more preferably 1 μm or less, and even more preferably 0.35 μm or less. If it is within this range, it is possible to obtain a polyester film with an easy-adhesion layer that is unlikely to impair the optical properties of other components when applied to an image display device.

In one embodiment, the refractive index of the easy-adhesion layer is preferably 1.45 to 1.60. Within this range, a polyester film with an easy-adhesion layer can be obtained that is unlikely to impair the optical properties of other members when applied to an image display device. In one embodiment, the refractive index of the easy-adhesion layer is 1.54 or more.

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

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

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

Any suitable molding method can be used to form a film from the film-forming material. Specific examples include compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, cast coating (e.g., casting), calendar molding, and heat pressing. Extrusion molding and cast coating are preferred because they can increase the smoothness of the resulting film and provide good optical uniformity.

The film may be stretched uniaxially or biaxially.

In one embodiment, the film is stretched uniaxially in the longitudinal direction (MD).

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. Thus, the stretching direction of the film is typically the length direction (MD) and width direction (TD) of the film.

In one embodiment, sequential biaxial stretching is used as the stretching method for the film. It is preferable to obtain the polyester film by performing MD stretching after TD stretching. In this way, it is possible to mitigate the effect of bowing that occurs during TD stretching and to make the angle between the first direction (MD) and the slow axis of the polyester film an appropriate value.

The stretching temperature is preferably Tg+5°C to Tg+50°C relative to the glass transition temperature (Tg) of the film, 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, a polyester film can be obtained in which the slow axis direction and linear expansion coefficient are well-balanced and well-controlled. In addition, a polyester film with excellent transparency can be obtained.

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

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

The ratio of the stretching ratio in TD to the stretching ratio in MD (MD stretching ratio/TD stretching ratio) is preferably greater than 1 and equal to or less than 7, more preferably 1 to 6, and even more preferably 1 to 3. Within this range, a polyester film with particularly little rainbow unevenness can be obtained. Furthermore, by using the obtained polyester film, it is possible to prevent cracks in the polarizer and obtain a polarizing plate with excellent durability.

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

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

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

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

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 arranged on one side of the polarizer 10. The polyester film of the present invention described in the above section A is used as the polyester film 20. Any suitable separate polarizer protective film may be arranged on the other side of the polarizer, or no polarizer protective film may be arranged. In one embodiment, the polarizer 10 and the polyester film 20 (or another polarizer protective film) are laminated via an adhesive layer 30.

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

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

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

The dyeing with iodine is carried out, for example, by immersing the PVA-based film in an aqueous iodine solution. The stretching ratio of the uniaxial stretching is preferably 3 to 7 times. The stretching may be carried out after the dyeing process or while dyeing. Alternatively, the film may be stretched and then dyed. If necessary, the PVA-based film may be subjected to a swelling process, a crosslinking process, a washing process, a drying process, or the like. For example, by immersing the PVA-based film in water and washing it with water before dyeing, it is possible to wash off dirt and antiblocking agents on the surface of the PVA-based film, and also to swell the PVA-based film and prevent 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 polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by applying a PVA-based resin solution to a resin substrate and drying the substrate to form a PVA-based resin layer on the resin substrate to obtain a laminate of a resin substrate and a PVA-based resin layer; and stretching and dyeing the laminate to make the PVA-based resin layer into a polarizer. In this embodiment, stretching typically includes immersing the laminate in an aqueous solution of boric acid and stretching it. Furthermore, stretching may further include air-stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the aqueous solution of boric acid, as necessary. The obtained resin substrate/polarizer laminate may be used as is (i.e., the resin substrate may be used as a protective layer for the polarizer), or the resin substrate may be peeled off from the resin substrate/polarizer laminate, and any suitable protective layer may be laminated on the peeled surface depending on the purpose. Details of the method for producing such a polarizer are described, for example, in JP 2012-73580 A. The entire disclosure 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, and more preferably 3 μm to 15 μm. By using the polyester film of the present invention, cracks in the polarizer can be effectively prevented, making it possible to use a thin polarizer even in harsh environments such as high temperatures and large temperature changes.

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

It is preferable that the absorption axis direction of the polarizer and the first direction (typically MD) of the polyester film are approximately parallel. If the polarizing plate is constructed so that the absorption axis of the polarizer and the first direction of the polyester film are approximately parallel, the polyester film and the polarizer can change shape in a synchronized manner. As a result, cracks in the polarizer are prevented.

The slow axis angle of the polyester film is preferably as close as possible to the angle it forms with the absorption axis direction of the polarizer, and the angle between the two axes is preferably 0°±10°, more preferably 0°±7°, and even more preferably 0°±5°. Within this range, a polyester film can be obtained that produces little rainbow unevenness when applied to an image display device. The slow axis angle is the angle when the roll flow direction is taken as 0°.

In the 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 even more preferably 1.0×10 ⁻⁵ /° C. or less. Within such a range, cracks in the polarizer can be prevented even in 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, and may be, for example, 0.1×10 ⁻⁵ /° C.

In the polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (the direction perpendicular to the first direction) and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is preferably 5×10 ⁻⁵ /° C. or less, and more preferably 4.5×10 ⁻⁵ /° C. or less. Within this range, cracks in the polarizer can be prevented even in 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 the direction parallel to the second direction is preferably as small as possible, and may be, 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 a first direction and the linear expansion coefficient of the polarizer in a direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in a second direction (direction perpendicular to the first direction) and the linear expansion coefficient of the polarizer in a direction parallel to the second direction are both 2.0×10 ⁻⁵ /° C. or less (preferably 1.0×10 ⁻⁵ /° C. or less) Within such a range, cracking of the polarizer can be prevented even in harsh environments such as high temperatures and large temperature changes.

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

C. Image display device The polarizing plate can be applied to an image display device. Representative examples of the image display device include a liquid crystal display device and an organic electroluminescence (EL) display device. The image display device employs a configuration well known in the industry, so a detailed description thereof will be omitted.

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

(1) Orientation angle (direction in which the slow axis appears)
The central portion of the polyester film obtained in the examples and comparative examples was cut into a square shape of 50 mm in width and 50 mm in length, with one side parallel to the width direction of the film, to prepare a sample. 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. 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 the polarizer was measured in accordance with JIS K 7197 using a thermomechanical analyzer "TMA7000" manufactured by Hitachi High-Tech Science Corporation, by increasing the temperature from 30°C to 150°C at a rate of 10°C/min, and measuring the deformation amount of the test film at each temperature. The linear expansion coefficient of the film was then calculated from the deformation amount in the temperature range of 30°C to 70°C. Note that the case where the film dimension increases (expands) with increasing temperature was considered positive, and the case where the film dimension decreases (shrinks) with increasing temperature was considered negative.
The linear expansion coefficients of the polyester film were measured in the MD (first direction) and TD (second direction). The linear expansion coefficients of the polarizer were measured in the direction parallel to the MD and the direction parallel to the TD in the polarizing plate.
(3) Crystallinity The crystallinity of the polyester films used in the examples and comparative examples was measured by differential scanning calorimetry (DSC). The sample was heated to 300°C at a rate of 10°C/min. The heat generation amount and heat of fusion observed during the temperature rise were obtained, and the crystallinity was calculated according to the following formula. The heat generation amount and heat of fusion were measured using a Q-2000 manufactured by TA Instruments.
Crystallinity (%)=(amount of heat of fusion obtained by measurement−amount of heat generated by measurement)/amount of heat of fusion of polyethylene terephthalate with 100% crystallinity (119 mJ/mg)×100
(4) Rainbow unevenness A liquid crystal cell was removed 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 the Examples and Comparative Examples were attached to the surface of the liquid crystal TV from which the polarizing plate had been peeled off, with an adhesive therebetween, so that the absorption axis of the polarizer was on the short side of the liquid crystal TV. The liquid crystal cell to which the polarizing plate obtained in the Examples and Comparative Examples had been attached was again set, and the TV was turned on to display white.
The liquid crystal TV was turned on and visually checked in all directions at a polar angle of 60° to observe whether rainbow unevenness was present or not. Evaluation was made according to the following criteria.
○: No rainbow unevenness was observed
△: Slight rainbow unevenness was observed
×: Rainbow unevenness was significantly observed. (5) Dimensional change The polyester films used in the examples and comparative examples were cut into 100 mm x 100 mm pieces. After that, they were placed in a heating oven at 100°C for 24 hours, and then the films were taken out and the dimensions were accurately measured again, and the dimensions were confirmed with a metal ruler to determine the change in dimensions. The state of the samples was also visually confirmed and evaluated according to the following criteria.
○: No significant shrinkage of 1 mm or more ×: Shrinkage of 1 mm or more or deformation (6) Crack test (accelerated heat shock test)
The polarizing plates obtained in the examples and comparative examples were evaluated using a thermal shock tester (manufactured by ESPEC).
The polarizing plates obtained in the Examples and Comparative Examples were cut to a size of 50 mm wide x 150 mm long. In this case, a sample was prepared in which the absorption axis direction of the polarizer was parallel to the horizontal direction (short side) of the polarizing plate after cutting, and a sample was prepared in which the transmission axis direction of the polarizer was parallel to the horizontal direction (short side) of the polarizing plate after cutting. The surface of the polarizing plate on which the protective film (polyester film) was not laminated was attached to 0.5 mm thick non-alkali glass via an acrylic adhesive to prepare the samples.
The obtained sample was placed in a test area of a thermal shock tester, and the temperature in the test area was lowered from room temperature to -40°C over 30 minutes. Next, the temperature in the test area was raised to 85°C over 30 minutes, and then lowered again to -40°C over 30 minutes. This process of raising the temperature from -40°C to 85°C and lowering the temperature again to -40°C constitutes one cycle, and after repeating this process for 100 cycles and 200 cycles, the laminate was removed and visually checked for the occurrence of cracks, and evaluated according to the following criteria.
⊚: No cracks were observed even after 300 cycles.
◯: No cracks were observed after 200 cycles, but cracks were observed after 300 cycles.
Δ: After 100 cycles, no cracks were observed, but after 200 cycles, cracks were observed.
×: Cracks were generated after 100 cycles.
(7) Thickness Unevenness The thickness of the polyester film used for laminating the polarizing plate was continuously measured in the width direction at 0.5 m/sec over a measurement range of 40 mm and the entire product width (e.g., 1330 mm) using an offline sheet continuous thickness measuring device manufactured by Yamabun Denki Co., Ltd. The thickness unevenness was calculated from the maximum thickness Tmax, the minimum thickness Tmin, and the average thickness Tave according to the formula {(Tmax-Tmin)/Tave)×100.

[Production Example 1] Preparation of Polarizer A A long amorphous isophthalic acid copolymerized polyethylene terephthalate (IPA copolymerized PET) film (thickness: 100 μm) having a water absorption rate of 0.75% and a Tg of 75° C. was used as a substrate. One side of the substrate was subjected to a corona treatment, 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., product name "GOHSEFYMER Z200") in a ratio of 9:1 was applied to the corona-treated surface and dried at 25° C. to form a PVA-based resin layer having a thickness of 11 μm, and a laminate was produced.
The obtained laminate was uniaxially stretched at its free end by 2.0 times in the machine direction (longitudinal direction) between rolls having different peripheral speeds in an oven at 120° C. (auxiliary air stretching).
Next, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30° C. for 30 seconds (insolubilizing treatment).
Next, the polarizing plate was immersed in a dye bath at a liquid temperature of 30° C. while adjusting the iodine concentration and immersion time so that the polarizing plate had a predetermined transmittance. In this example, the polarizing plate was immersed for 60 seconds 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 respect to 100 parts by weight of water (dyeing treatment).
Next, the plate was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 3 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30° C. for 30 seconds (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 respect to 100 parts by weight of water) at a liquid temperature of 70° C., and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds so that the total stretch ratio was 4.6 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) to obtain a peelable polarizer A with a substrate.

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

[Production Example 3] Production of polyester film B A polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, 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 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 100 μm thick amorphous polyester resin film was produced using a film forming device equipped with a single screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 280 ° C.), a T-die (width 500 mm, set temperature: 280 ° C.), a chill roll (set temperature: 50 ° C.) and a winder.
The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner stretching machine KAROIV to obtain polyester film B (slow axis angle with respect to the length direction: -0.5°, in-plane phase Re (590): 159 nm, thickness: 20 μm). The stretching ratio was 5 times in the length direction (MD) and 2 times in the width direction (TD) with fixed end stretching. The stretching temperature was 95°C, and the stretching speed was 10%/sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 30 seconds while maintaining the dimensions.

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

[Production Example 5] Production of polyester film I A polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, 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 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 an amorphous polyester resin film having a thickness of 180 μm was produced using a film forming apparatus equipped with a single screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 280 ° C.), a T-die (width 500 mm, set temperature: 280 ° C.), a chill roll (set temperature: 50 ° C.) and a winder.
The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner stretching machine KAROIV to obtain polyester film I (slow axis angle with respect to the length direction: -0.6°, in-plane phase Re (590): 1312 nm, thickness: 20 μm). The stretching ratio was 6.0 times in the length direction (MD) and 1.5 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 5%/sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 10 seconds while maintaining the dimensions.

[Production Example 6] Production of Polyester Film II A polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, 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 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 180 μm thick amorphous polyester resin film was produced using a film forming device equipped with a single screw extruder (manufactured by Toyo Seiki Co., Ltd., screw diameter 25 mm, cylinder set temperature: 280 ° C.), a T-die (width 500 mm, set temperature: 280 ° C.), a chill roll (set temperature: 50 ° C.) and a winder.
The obtained amorphous polyester resin film was subjected to simultaneous biaxial stretching using a Bruckner stretching machine KAROIV to obtain polyester film II (slow axis angle relative to the length direction: -0.8°, in-plane phase Re (590): 771 nm, thickness: 32 μm). The stretching ratio was 4.5 times in the length direction (MD) and 2 times in the width direction (TD). The stretching temperature was 90°C, and the stretching speed was 2%/sec in both MD and TD. After the stretching treatment, the film was heat-treated at 140°C for 10 seconds while maintaining the dimensions.

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

[Example 3]
A polarizing plate was obtained in the same manner as in Example 1, except that the polyester film C produced in Production Example 4 was used in place of the polyester film A produced in Production Example 2.
The obtained polarizing plate was subjected to the above-mentioned evaluations (1) to (7), and 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 the polyester film I produced in Production Example 5 was used in place of the polyester film A produced in Production Example 2.
The obtained polarizing plate was subjected to the above-mentioned evaluations (1) to (7), and 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 the polyester film II produced in Production Example 6 was used in place of the polyester film A produced in Production Example 2.
The obtained polarizing plate was subjected to the above-mentioned evaluations (1) to (7), and the results are shown in Table 1.

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

Claims (9)

A linear expansion coefficient in a first direction is 0.0×10 ⁻⁵ /° C. to 3.0 ^{×10 −5} /° C.;
a linear expansion coefficient in a second direction perpendicular to the first direction is 7.5×10 ⁻⁵ /° C. to 10.5×10 ⁻⁵ /° C.;
The linear expansion coefficient in the first direction is lower than the linear expansion coefficient in the second direction by 7×10 ⁻⁵ /° C. or more;
A slow axis is in a direction of −5° to 5° with respect to the first direction ,
The thickness variation of the polyester film in the second direction is 15% or less.
Polyester film. 2. The polyester film according to claim 1 , which has a crystallinity of 30% or more as measured by DSC. A polarizing plate comprising a polarizer and the polyester film according to claim 1 or 2 arranged on one side of the polarizer. 4. The polarizing plate according to claim 3, wherein an absolute value of a difference between a linear expansion coefficient of the polyester film in a first direction and a linear expansion coefficient of the polarizer in a direction parallel to the first direction is 2.0×10 ^-5 /°C or less, and an absolute value of a difference between a linear expansion coefficient of the polyester film in a second direction perpendicular to the first direction and a linear expansion coefficient of the polarizer in a direction parallel to the second direction is 5×10 ^-5 /°C or less. The polarizing plate according to claim 3 , wherein the polarizer has a thickness of 20 μm or less. The polarizing plate according to claim 3 , further comprising an easy-adhesion layer disposed on the polarizer side of the polyester film. The polarizing plate according to claim 6 , wherein the easy-adhesion layer contains fine particles. The polarizing plate according to claim 6 , wherein the easy-adhesion layer has a thickness of 0.35 μm or less. The polarizing plate according to claim 6 , wherein the easily adhesive layer has a refractive index of 1.55 or less.

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