CN113444463A

CN113444463A - Polyester film and polarizing plate comprising same

Info

Publication number: CN113444463A
Application number: CN202110320148.3A
Authority: CN
Inventors: 东慎太郎; 北岸一志; 山内健吾; 池田哲朗
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2020-03-27
Filing date: 2021-03-25
Publication date: 2021-09-28
Also published as: JP2024009095A; JP2021157068A; KR20210120843A; TW202142608A

Abstract

A polyester film and a polarizing plate comprising the same. Provided is a polyester film which has little rainbow unevenness generated when applied to an image display device and can contribute to improvement of the durability of a polarizing plate. The polyester film of the present invention has a linear expansion coefficient of 3.0X 10 in the first direction^‑5A linear expansion coefficient of 7.5 × 10 in a second direction perpendicular to the first direction at a temperature of less than or equal to/° C^‑5/℃～10.5×10^‑5A slow axis at-5 deg. to 5 deg. relative to the first direction.

Description

Polyester film and polarizing plate comprising same

Technical Field

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

Background

In image display devices (for example, liquid crystal display devices and organic EL display devices), depending on the image forming system, a polarizing plate is often disposed on at least one side of a display cell. In recent years, the functions and applications of image display devices tend to be further diversified, and it is required that the image display devices can be durable in more severe environments. A polarizing plate generally has a structure in which a polarizing material is sandwiched between two protective films, and cellulose triacetate, acrylic resin, cycloolefin resin, and the like are widely used as the protective film. On the other hand, from the viewpoint of the durability as described above, it has been proposed to use, for example, a polyester film excellent in mechanical properties, chemical resistance, and moisture barrier properties such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) as a polarizer protective film (for example, patent document 1). However, polyester films have birefringence although they are excellent in mechanical properties, and therefore, they may cause deterioration in visual recognition properties such as rainbow unevenness. In particular, with the recent increase in brightness and color purity of image display devices, the problem of rainbow unevenness has become apparent.

On the other hand, in a polarizing plate using a protective film made of conventionally used cellulose triacetate, acrylic resin, or cycloolefin resin, cracks may occur in the polarizer due to a temperature change. In recent years, a polarizing plate is required to be thin as the thickness of an image display device is reduced, and an increase in the number of image display devices used at high temperatures is expected, and development of a polarizing plate having excellent durability without causing cracks in the polarizing plate is strongly desired.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 8-271733

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above conventional problems, and a main object of the present invention is to provide a polyester film which has less rainbow unevenness generated when applied to an image display device and can contribute to improvement of durability of a polarizing plate.

Means for solving the problems

The polyester film of the present invention has a linear expansion coefficient of 3.0X 10 in the first direction^-5A linear expansion coefficient of 7.5 × 10 in a second direction perpendicular to the first direction at a temperature of less than or equal to/° C^-5/℃～10.5×10^-5A slow axis at-5 deg. to 5 deg. relative to the first direction.

In one embodiment, the thickness of the polyester film in the second direction is not all 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, a polarizing plate is provided. The polarizing plate comprises a polarizer and the 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 the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the first direction is 2.0 × 10^-5The absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction perpendicular to the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is 5 × 10 ° C^-5Below/° c.

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 easy adhesion layer has a refractive index of 1.55 or less.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, by selectively reducing the linear expansion coefficient in a predetermined direction, it is possible to provide a polyester film which has less rainbow unevenness generated when combined with a polarizer and which can contribute to improvement of the durability of a polarizing plate.

Drawings

Fig. 1 is a schematic cross-sectional view of a polarizing plate according to 1 embodiment of the present invention.

Fig. 2 is a schematic cross-sectional view of a polarizing plate according to another embodiment of the present invention.

Description of the reference numerals

10 polarizer

20 polyester film

30 adhesive layer

40 easy adhesive layer

100. 200 polarizing plate

Detailed Description

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 3.0X 10 in the first direction^-5A linear expansion coefficient of 7.5 × 10 in a second direction perpendicular to the first direction at a temperature of less than or equal to/° C^-5/℃～10.5×10^-5V. C. If such a polyester having anisotropy with respect to dimensional change is used, the polarizer can be laminated thereon, the polarizer can be effectively protected, and cracks can be prevented from being generated in the polarizer. More specifically, when the polarizer is usually manufactured to have an absorption axis through a stretching process and has anisotropy in dimensional change (for example, dimensional change due to temperature change), if the polarizer and the polyester film are laminated so that the absorption axis of the polarizer is substantially parallel to the first direction of the polyester film, the polyester film and the polarizer can be simultaneously and preferably changed in shape. As a result, if the polyester film of the present invention is used, cracks can be prevented from occurring in the polarizer even in a severe environment such as a high temperature and a large temperature change, and a polarizing plate having excellent durability can be obtained. In 1 embodiment, the first direction corresponds to a conveyance direction (MD) in the production of a polyester film. In addition, the second directionMay correspond to TD orthogonal to MD. The linear expansion coefficient can be determined by TMA measurement according to JIS K7197. The expression "substantially parallel" includes a case where the angle formed by the two directions is 0 ° ± 10 °, preferably 0 ° ± 7 °, and more preferably 0 ° ± 5 °.

The polyester film preferably has a linear expansion coefficient in the first direction of 2.8X 10^-5Lower than/° C, preferably 0.0X 10^-5/℃～2.5×10^-5/. degree.C., more preferably 0.5X 10^-5/℃～1.8×10^-5V. C. If it is in such a range, the above-mentioned effect becomes more remarkable.

The above polyester film preferably has a linear expansion coefficient in the second direction of more than 7.5X 10^-510.5X 10 ℃ and^-5lower than/° C, more preferably 7.5X 10^-5/℃～10×10^-5Further preferably 7.5X 10/. degree.C^-5/℃～9.5×10^-5V. C. If it is in such a range, the above-mentioned effect becomes more remarkable.

In one embodiment, the linear expansion coefficient in the first direction is 7 × 10 smaller than the linear expansion coefficient in the second direction^-5Over/° C (preferably 7.5 × 10. mu.C. smaller)^-5Above/° c). If it is in such a range, the above-mentioned effect becomes more remarkable.

The polyester film of the present invention has a slow axis in a direction of-5 ° to 5 ° with respect to the first direction. When the amount is in this range, a polyester film having little rainbow unevenness generated when combined with a polarizer can be produced. More specifically, when the polarizing plate is configured by laminating a polarizer and a polyester film such that the absorption axis of the polarizer is substantially parallel to the first direction, rainbow unevenness can be effectively prevented.

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

Typically, the polyester film may be a stretched film obtained through a stretching step. By appropriately adjusting the production conditions in the stretching step, the linear expansion coefficients in the first direction and the second direction (and the in-plane retardation Re (590) described later) can be controlled well, and as a result, a polyester film having excellent properties as a polarizer protective film can be obtained from the viewpoints of rainbow unevenness and durability as described above. The production 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, relaxation rate in MD/TD after stretching, and the like. The stretching temperature, stretching magnification and stretching speed may be appropriately adjusted for each MD/TD.

The in-plane retardation Re (590) of the polyester film is, for example, more than 0nm and not more than 10000 nm. The in-plane retardation Re (λ) is an in-plane retardation of the film measured by light having a wavelength λ nm at 23 ℃. Therefore, Re (590) is the in-plane retardation of the film measured by light having a wavelength of 590 nm. When the thickness of the film is denoted by d (nm), Re (λ) is obtained by the formula Re (λ) ═ nx-ny × d. Here, nx is a refractive index in a direction in which the in-plane refractive index is maximized (i.e., the slow axis direction), and ny is a refractive index in a direction orthogonal to the slow axis in the plane.

The crystallinity of the polyester film as 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%. When the amount is in this range, a polyester film which is excellent in heat resistance and mechanical properties and suitable as a polarizer protective film can be obtained.

Typically, the thickness of the polyester film is 10 to 100. mu.m, preferably 20 to 80 μm, and more preferably 20 to 50 μm.

The thickness variation in the second direction of the polyester film is preferably 15% or less, more preferably 13% or less, and still more preferably 10% or less. In the present invention, the effect of effectively protecting the polarizer and preventing the polarizer from cracking by laminating the polyester film to the polarizer is more remarkable by reducing the thickness unevenness. The effect of the thickness unevenness on the prevention of polarizer cracking is a specific effect of a polyester film having anisotropy in linear expansion coefficient, and it was found that the relationship between the thickness unevenness and polarizer cracking is an effect of the present invention. Further, by reducing the thickness unevenness, a polyester film with less rainbow unevenness occurring when combined with a polarizer can be produced. The thickness unevenness of the polyester film is preferably small, and the lower limit thereof is, for example, 3% (preferably 1%, more preferably 0.5%). In the present specification, the "thickness variation in the second direction" is calculated by the equation { (Tmax-Tmin)/Tave) × 100 from the maximum thickness Tmax, the minimum thickness Tmin and the average thickness Tave in the second direction (width direction, TD) by measuring the thickness of the polyester film by a continuous thickness measuring instrument.

The total light transmittance of the polyester film is preferably 80% or more, more preferably 85% or more, further 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, still more preferably 0.5% or less, and particularly preferably 0.3% or less.

The moisture permeability of the polyester film is preferably 100g/m²24hr or less, more preferably 50g/m²24hr or less, more preferably 15g/m²24hr or less. When the amount is in this range, a polarizing plate having excellent durability and moisture resistance can be obtained.

The polyester film of the present invention is formed of a polyester resin. The polyester-based 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 the aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, benzylmalonic acid, 1, 4-naphthalenedicarboxylic acid, diphenic acid, 4' -hydroxybenzoic acid and 2, 5-naphthalenedicarboxylic acid. Examples of the aliphatic dicarboxylic acid include malonic acid, dimethylmalonic acid, succinic acid, glutaric acid, adipic acid, trimethyladipic acid, pimelic acid, 2-dimethylglutaric acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, itaconic acid, thiodipropionic acid, and diglycolic acid. Examples of the alicyclic dicarboxylic acid include 1, 3-cyclopentanedicarboxylic acid, 1, 2-cyclohexanedicarboxylic acid, 1, 3-cyclopentanedicarboxylic acid, 1, 4-cyclohexanedicarboxylic acid, 2, 5-norbornanedicarboxylic acid, and adamantanedicarboxylic acid. The carboxylic acid component can be derivatives such as esters, acid chlorides, anhydrides, and the like, including, 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 2 or more thereof may be used in combination.

As the polyol component, typically, a diol is mentioned. Examples of the diol include aliphatic diols, alicyclic diols, and aromatic diols. Examples of the aliphatic diol include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1, 3-propanediol, 2, 4-dimethyl-2-ethylhexane-1, 3-diol, 2-dimethyl-1, 3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-1, 3-propanediol, 2-ethyl-2-isobutyl-1, 3-propanediol, 1, 3-butanediol, 1, 4-butanediol, 1, 5-pentanediol, 1, 6-hexanediol, 3-methyl-1, 5-pentanediol, and 2,2, 4-trimethyl-1, 6-hexanediol. Examples of the alicyclic diol include 1, 2-cyclohexanedimethanol, 1, 3-cyclohexanedimethanol, 1, 4-cyclohexanedimethanol, spirodiol, tricyclodecanedimethanol, adamantanediol, and 2,2,4, 4-tetramethyl-1, 3-cyclobutanediol. Examples of the aromatic diol include 4,4 '-thiodiphenol, 4' -methylenediphenol, 4 '- (2-norbornylene) diphenol, 4' -dihydroxybiphenol, o-dihydroxybenzene, m-dihydroxybenzene, p-dihydroxybenzene, 4 '-isopropylidene phenol, 4' -isopropylidene bis (2, 6-dichlorophenol), 2, 5-naphthalenediol, and terephthalyl alcohol. The polyol component may be used alone or in combination of 2 or more.

The polyester resin is preferably polyethylene terephthalate and/or modified polyethylene terephthalate, and more preferably polyethylene terephthalate. When these resins are used, a polyester film having excellent mechanical properties and little occurrence of rainbow unevenness can be obtained. The polyethylene terephthalate and the modified polyethylene terephthalate can be blended for use.

Examples of the modified polyethylene terephthalate include modified polyethylene terephthalates containing a structural unit 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 10 mol% or less, more preferably more than 0 mol% and 3 mol% or less. The proportion of isophthalic acid in the carboxylic acid component is preferably more than 0 mol% and 10 mol% or less, more preferably more than 0 mol% and 8 mol% or less. When the amount is in this range, a polyester film having good crystallinity can be obtained. The above-mentioned mol% is a mol% based on the total of all the repeating units of the polymer.

The weight average molecular weight of the polyester resin is preferably 10000 to 100000, and more preferably 20000 to 75000. When the weight average molecular weight is such as this, a film which is easy to handle at the time of 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-to-bond layer may be provided. The easy-adhesion layer contains, for example, an aqueous polyurethane and an oxazoline-based crosslinking agent. The easy adhesion layer is described in detail in, for example, japanese patent application laid-open No. 2010-55062. The entire disclosure of this publication is incorporated herein by reference.

In one embodiment, the easy adhesion layer contains arbitrary and appropriate fine particles. By forming the easy-adhesion layer containing fine particles, blocking occurring at the time of winding can be effectively suppressed. The fine particles may be inorganic fine particles or organic fine particles. Examples of the 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, calcium phosphate, and the like. Examples of the organic fine particles include silicone resins, fluorine resins, and (meth) acrylic resins. Among these, silica is preferable.

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

The thickness of the easy adhesion layer is preferably 2 μm or less, more preferably 1 μm or less, and still more preferably 0.35 μm or less. If the amount is within this range, a polyester film with an easily adhesive layer can be obtained which hardly hinders the optical characteristics of other members when applied to an image display device.

In one embodiment, the easy adhesion layer preferably has a refractive index of 1.45 to 1.60. If the amount is within this range, a polyester film with an easily adhesive layer can be obtained which hardly hinders the optical characteristics of other members when applied to an image display device. In 1 embodiment, the easy adhesion layer has a refractive index of 1.54 or more.

In one embodiment, the polyester film may be provided with an anti-blocking layer on at least one side thereof. The anti-blocking layer can be formed by the above-described easy-adhesion layer. The anti-blocking layer preferably comprises the above-mentioned microparticles.

(method for producing polyester film)

The polyester film can be obtained by subjecting a film-forming material (resin composition) containing the polyester resin to a molding step of molding the film-forming material into a film form and a stretching step of stretching the film obtained by the molding step. The stretching process preferably includes a preheating treatment of the film performed before the film stretching and a heat treatment performed after the film stretching. In 1 embodiment, the polyester film is provided in a long form (or a form cut out from a long form).

The film-forming material may contain an additive in addition to the polyester resin, or may contain a solvent. As the additive, any and appropriate additive can be used according to the purpose. Specific examples of the additives include reactive diluents, plasticizers, surfactants, fillers, antioxidants, anti-aging agents, ultraviolet absorbers, leveling agents, thixotropic agents, antistatic agents, conductive materials, and flame retardants. The amount, kind, combination, addition amount, and the like of the additives can be appropriately set according to the purpose.

As a method for forming a thin film from a thin film-forming material, any and appropriate forming process can be employed. Specific examples thereof include compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, casting coating (for example, casting), calendering, and hot pressing. Extrusion molding or cast coating is preferred. This is because the smoothness of the obtained film can be improved and good optical uniformity can be obtained.

The film may be stretched uniaxially or biaxially.

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

The biaxial stretching may be sequential biaxial stretching or simultaneous biaxial stretching. Typically, the sequential biaxial stretching or simultaneous biaxial stretching is performed using a tenter stretching machine. Thus, the stretching direction of the film is typically the longitudinal direction (MD) and the width direction (TD) of the film.

In one embodiment, as the stretching method of the film, sequential biaxial stretching is employed. Preferably, the polyester film is obtained by TD stretching and then MD stretching. In this way, the influence of bending (bowing) generated during TD stretching can be reduced, and the angle formed between the first direction (MD) of the polyester film and the slow axis can be set to an appropriate value.

The stretching temperature is preferably from Tg +5 ℃ to Tg +50 ℃, more preferably from Tg +5 ℃ to Tg +30 ℃, and still more preferably from Tg +6 ℃ to Tg +10 ℃ as compared with the glass transition temperature (Tg) of the film. By stretching at such a temperature, a polyester film in which the direction of the slow axis and the linear expansion coefficient are controlled in a good balance can be obtained. Further, a polyester film having excellent transparency can be obtained.

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

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

The ratio of the TD stretch ratio to the MD stretch ratio (MD stretch ratio/TD stretch ratio) is preferably greater than 1 and 7 or less, more preferably 1 to 6, and still more preferably 1 to 3. When the amount is within this range, a polyester film having particularly little occurrence of rainbow unevenness can be obtained. In addition, if the polyester film is used, the polarizer can be prevented from cracking, and a polarizing plate with excellent durability can be obtained.

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

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

The temperature of the preheating treatment is preferably 80 to 150 ℃, more preferably 90 to 130 ℃. The time for the preheating treatment is preferably 10 seconds to 100 seconds, and more preferably 15 seconds to 80 seconds. When the amount is in this range, a polyester film having excellent optical properties, good crystallinity and excellent durability can be obtained.

The temperature of the heat treatment is preferably 100 to 250 ℃, more preferably 120 to 200 ℃, and further preferably 130 to 180 ℃. When the amount is in this range, a polyester film having excellent transparency, good crystallinity and excellent durability can be obtained. The time for the heat treatment is preferably 2 seconds to 50 seconds, more preferably 5 seconds to 40 seconds, and still more preferably 8 seconds to 30 seconds. When the amount is in 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 1 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. The polyester film 20 used was the polyester film of the present invention described in the above item A. Any other polarizer protective film may be disposed on the other side of the polarizer, and the polarizer protective film may not be disposed. In embodiment 1, the polarizer 10 and the polyester film 20 (or another polarizer protective film) are laminated via the adhesive layer 30.

In one embodiment, the polarizing plate is applied to an image display device such that a side on which the polyester film is disposed is a visual recognition side. When the polarizing plate is applied to a liquid crystal display device, the polarizing plate provided with a polyester film may be disposed on the visually recognizable side of the liquid crystal cell or on the back side.

As the polarizer, any and appropriate polarizer may be used. 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 the polarizer made of a single-layer resin film include: a polarizing element obtained by subjecting a hydrophilic polymer film such as a polyvinyl alcohol (PVA) -based film, a partially formalized PVA-based film, or an ethylene-vinyl acetate copolymer-based partially saponified film to a dyeing treatment with a dichroic material such as iodine or a dichroic dye and a stretching treatment; and polyene-based oriented films such as dehydrated products of PVA and desalted products of polyvinyl chloride. From the viewpoint of excellent optical properties, a polarizer obtained by uniaxially stretching a PVA-based film dyed with iodine is preferably used.

The iodine-based dyeing is performed by, for example, immersing a 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 performed after the dyeing treatment, or may be performed while dyeing. In addition, it is also possible to dye the fabric after stretching. The PVA-based film is subjected to swelling treatment, crosslinking treatment, washing treatment, drying treatment, and the like as necessary. For example, by immersing the PVA-based film in water and washing it with water before dyeing, not only stains and antiblocking agents on the surface of the PVA-based film can be washed off, but also the PVA-based film can be swollen to prevent uneven dyeing and the like.

Specific examples of the polarizer obtained by using the laminate include: a polarizing plate obtained by using a laminate of a resin base material and a PVA-based resin layer (PVA-based resin film) laminated on the resin base material or a laminate of a resin base material and a PVA-based resin layer formed by coating the resin base material. A polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer formed by applying the resin substrate can be produced as follows: for example, a laminate of a resin substrate and a PVA-based resin layer is obtained by applying a PVA-based resin solution to a resin substrate and drying the solution to form a PVA-based resin layer on the resin substrate; the laminate was stretched and dyed to prepare a polarizing plate from the PVA-based resin layer. In the present embodiment, typically, the stretching includes immersing the laminate in an aqueous boric acid solution and stretching. Further, the stretching may further include, as necessary: before stretching in the aqueous boric acid solution, the laminate is stretched in air at a high temperature (for example, 95 ℃ or higher). The obtained resin substrate/polarizer laminate may be used as it is (that is, 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 an arbitrary and appropriate protective layer suitable for the purpose may be laminated on the peeled surface. The details of the method for producing such a polarizer are described in, for example, japanese patent laid-open No. 2012-73580. 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 1 embodiment, the thickness of the polarizer is preferably 20 μm or less, and more preferably 3 to 15 μm. Since the polyester film of the present invention can efficiently prevent cracks in the polarizer, a thin polarizer can be used even in a severe environment such as a high temperature and a large temperature change.

The polarizer and the polarizer protective film (polyester film) may be laminated via an arbitrary and appropriate adhesive layer. The adhesive layer is preferably formed of an adhesive composition containing a polyvinyl alcohol resin.

The absorption axis direction of the polarizer is preferably substantially parallel to the first direction (typically, MD) of the polyester film. If the polarizing plate is configured such that the absorption axis of the polarizer is substantially parallel to the first direction of the polyester film, the polyester film and the polarizer can be synchronized and preferably changed in shape. As a result, cracks in the polarizing material can be prevented.

The slow axis angle of the polyester film preferably coincides with the angle formed by the absorption axis direction of the polarizer, and the angle formed by both axes is preferably 0 ° ± 10 °, more preferably 0 ° ± 7 °, and still more preferably 0 ° ± 5 °. When the amount is in this range, a polyester film having little rainbow unevenness generated when applied to an image display device can be obtained. The slow axis angle is an angle when the roller moving direction is set to 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 polarizing element in the direction parallel to the first direction is preferably 2.0 × 10^-5Lower than/° C, more preferably 1.5X 10^-5/° C or less, more preferably 1.0X 10^-5Below/° c. In such a range, cracks in the polarizer can be prevented even in a severe environment such as a high temperature and a large temperature change. The lower limit of the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the first direction is preferably small, and may be, for example, 0.1 × 10^-5/℃。

In the polarizing plate, the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (direction orthogonal to the first direction) and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is preferably 5 × 10^-5Lower than/° C, more preferably 4.5X 10^-5Below/° c. In such a range, cracks in the polarizer can be prevented even in a severe environment such as a high temperature and a large temperature change. 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 directionThe lower limit of the absolute value is preferably small, and may be, for example, 0.1X 10^-5/℃。

In one embodiment, the absolute value of the difference between the linear expansion coefficient of the polyester film in the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the first direction, and the absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction (the direction orthogonal to the first direction) and the linear expansion coefficient of the polarizer in the direction parallel to the second direction are both 2.0 × 10^-5Lower than/° C (preferably 1.0X 10)^-5Below/° c). In such a range, cracks in the polarizer can be prevented even in a severe environment such as a high temperature and a large temperature change.

Fig. 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 disposed on the polarizer 10 side of the polyester film 20. In embodiment 1, the easy-adhesive-layer-attached polyester film a is disposed on the polarizer 10 so that the easy-adhesive layer 40 is on the polarizer 10 side. The easy adhesion layer may be the easy adhesion layer described in the above item a.

C. Image display device

The polarizing plate can be applied to an image display device. As typical examples of the image display device, a liquid crystal display device and an organic Electroluminescence (EL) display device can be given. The image display device may have a structure known in the art, and thus, a detailed description thereof will be omitted.

Examples

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods of the characteristics in the examples are as follows. Unless otherwise specified, "part(s)" and "%" in the examples are based on weight.

(1) Orientation angle (slow axis expression direction)

The center portions of the polyester films obtained in examples and comparative examples were cut into square shapes having a width of 50mm and a length of 50mm so that one side of the center portion was parallel to the width direction of the film, to prepare samples. The sample was measured by using a Mueller matrix polarimeter (product name "Axoscan" manufactured by Axometrics) and the orientation angle θ at a wavelength of 550nm and 23 ℃ was measured. The orientation angle θ was measured in a state where the sample was placed in parallel on the measurement table.

(2) Coefficient of linear expansion

The linear expansion coefficients of the polyester film and the polarizer were raised from 30 ℃ to 150 ℃ at a rate of 10 ℃/min using a thermomechanical analyzer "TMA 7000" manufactured by hitachi high tech., based on JIS K7197, and the deformation amounts of the test films at the respective temperatures were measured. The linear expansion coefficient of the film was determined from the amount of deformation in the temperature range of 30 to 70 ℃. Note that, the case where the film size increases (expands) with an increase in temperature is referred to as positive (plus), and the case where the film size decreases (contracts) with an increase in temperature is referred to as negative (minus).

The linear expansion coefficients in the MD (first direction) and TD (second direction) were measured for the polyester film. The linear expansion coefficients of the polarizer in the direction parallel to the MD and the direction parallel to the TD were measured.

(3) Degree of crystallinity

The crystallinity of the polyester films used in examples and comparative examples was measured by Differential Scanning Calorimetry (DSC). The calorific value and the amount of fusion heat observed in the temperature raising process of raising the temperature of the sample to 300 ℃ at 10 ℃/min were determined, and the crystallinity was determined by the following equation. The calorific value and the heat of fusion were measured using Q-2000 manufactured by TAinstinstruments.

Degree of crystallinity (%) (heat of fusion measured-calorific value measured)/heat of fusion of polyethylene terephthalate having degree of crystallinity of 100% (119mJ/mg) × 100

(4) Uneven rainbow pattern

A liquid crystal cell was taken out from a liquid crystal TV "45 UH 7500" 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 liquid crystal TV from which the polarizing plate was peeled off with an adhesive so that the absorption axis of the polarizer was located on the short side of the liquid crystal TV. The liquid crystal cell to which the polarizing plate obtained in example and comparative example was attached was set again, and the TV was turned on in a white display mode.

All-around visual confirmation was performed at an angle of 60 ° of the polar angle of the turned-on liquid crystal TV, and the presence or absence of rainbow unevenness was observed. Evaluation was performed according to the following criteria.

O: no rainbow unevenness observed

And (delta): slight rainbow unevenness was observed

X: uneven rainbow pattern was observed significantly

(5) Dimensional change

The polyester films used in examples and comparative examples were cut into 100mm × 100 mm. Thereafter, the film was placed in a heating oven at 100 ℃ for 24 hours, and then the film was taken out, and the dimensions were measured again accurately, and the dimensions were checked with a metal ruler to determine the dimensional change. The state of the sample was visually confirmed, and evaluated according to the following criteria.

O: without significant shrinkage above 1mm

X: there is shrinkage or deformation of 1mm or more

(6) Crack test (thermal shock acceleration test)

The polarizing plates obtained in examples and comparative examples were evaluated using a thermal shock tester (manufactured by ESPEC corporation).

The polarizing plates obtained in examples and comparative examples were cut to a width of 50mm × a length of 150 mm. At this time, a sample in which the absorption axis direction of the polarizer was parallel to the lateral direction (short side) of the cut polarizing plate and a sample in which the transmission axis direction of the polarizer was parallel to the lateral direction (short side) of the cut polarizing plate were prepared. A sample was prepared by bonding a surface of a polarizing plate on which a protective film (polyester film) was not laminated to alkali-free glass having a thickness of 0.5mm via an acrylic adhesive.

And putting the obtained sample into a testing area of a cold-hot impact testing machine, and cooling the testing area from room temperature to-40 ℃ within 30 minutes. Then, after taking 30 minutes to heat the inside of the test area to 85 ℃, the temperature was again lowered to-40 ℃ taking 30 minutes. The step of raising the temperature from-40 ℃ to 85 ℃ and lowering the temperature again to-40 ℃ was repeated for 100 cycles and 200 cycles as 1 cycle, and then the laminate was taken out, and whether or not cracks were generated was visually checked, and evaluated according to the following criteria.

Very good: even after 300 cycles were repeated, no cracks were observed.

O: after repeating 200 cycles, no cracks were observed, but after repeating 300 cycles, cracks were generated.

And (delta): after repeating 100 cycles, no cracks were observed, but after repeating 200 cycles, cracks were generated.

X: after repeating 100 cycles, cracks were generated.

(7) Uneven thickness

The polyester film used for laminating the polarizing plates was continuously measured for the thickness in the width direction at 0.5 m/sec using an on-line sheet continuous thickness measuring instrument manufactured by Shanwen electric company for the total width (for example, 1330mm) of the product under the condition that the measurement range was 40 mm. The thickness unevenness is obtained from the maximum thickness Tmax, the minimum thickness Tmin and the average thickness Tave by an equation { (Tmax-Tmin)/Tave) × 100.

Production example 1 production of polarizing plate A

As the substrate, a long-sized amorphous isophthalic acid copolymerized polyethylene terephthalate (IPA copolymerized PET) film (thickness: 100 μm) having a water absorption of 0.75% and a Tg of 75 ℃ was used. One surface of the substrate was subjected to corona treatment, and an aqueous solution containing polyvinyl alcohol (polymerization degree of 4200 and saponification degree of 99.2 mol%) and acetoacetyl-modified PVA (polymerization degree of 1200, acetoacetyl-modified degree of 4.6% and saponification degree of 99.0 mol% or more, manufactured by japan synthetic chemical industries, product name "GOHSEFIMER Z200") at a ratio of 9:1 was applied to the corona-treated surface at 25 ℃.

The resulting laminate was uniaxially stretched to 2.0 times along the longitudinal (lengthwise) free end in an oven at 120 ℃ and between rolls at different peripheral speeds (air-assisted stretching).

Next, the laminate was immersed in an insolubilization bath (an aqueous boric acid solution prepared by adding 4 parts by weight of boric acid to 100 parts by weight of water) having a liquid temperature of 30 ℃ for 30 seconds (insolubilization treatment).

Then, the polarizing plate was immersed in a dyeing bath at a liquid temperature of 30 ℃ while adjusting the iodine concentration and the immersion time so that the polarizing plate could have a predetermined transmittance. In this example, an aqueous iodine solution containing 0.2 parts by weight of iodine and 1.5 parts by weight of potassium iodide was immersed in 100 parts by weight of water for 60 seconds (dyeing treatment).

Next, the substrate was immersed for 30 seconds in a crosslinking bath (aqueous boric acid solution prepared by adding 3 parts by weight of potassium iodide and 3 parts by weight of boric acid to 100 parts by weight of water) having a liquid temperature of 30 ℃ (crosslinking treatment).

Thereafter, the laminate was uniaxially stretched (underwater stretching) so that the total stretching ratio was 4.6 times in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds while being immersed in an aqueous boric acid solution (aqueous solution prepared by adding 4 parts by weight of boric acid and 5 parts by weight of potassium iodide to 100 parts by weight of water) having a liquid temperature of 70 ℃.

Thereafter, the laminate was immersed (cleaned) in a cleaning bath (aqueous solution containing 4 parts by weight of potassium iodide per 100 parts by weight of water) at a liquid temperature of 30 ℃.

Production example 2 production of polyester film A

A Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, IV value of 0.75dl/g (concentration of a mixed solvent solution of phenol: 1,1,2, 2-tetrachloroethane: 6: 4: 0.4g/dl)) was dried under vacuum at 100 ℃ 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 Seiko Seisaku-Sho Co., Ltd., screw diameter: 25mm, cylinder set temperature: 280 ℃), T-die (width: 500mm, set temperature: 280 ℃), chill roll (set temperature: 50 ℃) and winder.

The obtained amorphous polyester resin film was simultaneously biaxially stretched by a stretcher KAROIV of Bruckner to obtain a polyester film A (slow axis angle in the longitudinal direction: -0.2 °, in-plane retardation Re (590): 98nm, thickness: 20 μm). The stretching magnification was 5.5 times in the longitudinal direction (MD) and 2.0 times in the width direction (TD). The stretching temperature was set to 90 ℃ and the stretching speeds in both MD and TD were set to 10%/sec. Further, after the stretching treatment, heat treatment was performed at 180 ℃ for 30 seconds while maintaining the dimensions.

Production example 3 production of polyester film B

After Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification amount: 2.5 mol% (mol number based on the total of all the repeating units of the polymer), diethylene glycol modification amount: 1.0 mol% (mol number based on the total of all the repeating units of the polymer), and IV value of 0.77dl/g (concentration of mixed solvent solution of phenol: 1,1,2, 2-tetrachloroethane: 6: 4: 0.4g/dl)) was dried in vacuum at 100 ℃ for 10 hours, a film forming apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki, screw diameter: 25mm, cylinder setting temperature: 280 ℃) a T-die (width: 500mm, setting temperature: 280 ℃), chill roll (setting temperature: 50 ℃) and a winder was used to produce an amorphous Polyester resin film having a thickness of 100. mu.m.

The obtained amorphous polyester resin film was simultaneously biaxially stretched by a stretcher KAROIV of Bruckner to obtain a polyester film B (slow axis angle in the longitudinal direction: -0.5 °, in-plane retardation Re (590): 159nm, thickness: 20 μm). The stretching magnification was 5 times in the longitudinal direction (MD) and 2 times in the width direction (TD) by fixed-end stretching. The stretching temperature was 95 ℃ and the stretching speeds in both MD and TD were 10%/sec. Further, after the stretching treatment, heat treatment was performed at 140 ℃ 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, having an isophthalic acid modification amount of 2.5 mol% (mol number based on the total of all the repeating units of the polymer) and an IV value of 0.77dl/g (concentration of a mixed solvent solution of phenol: 1,1,2, 2-tetrachloroethane: 6: 4: 0.4g/dl)) was dried under vacuum at 100 ℃ 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 Seiko, Seiko, Seiko, Seiko, Seiko, Seiko, Seiko, Seiko.

The film was uniaxially stretched to 2.0 times along the longitudinal (lengthwise) free end in an oven at 120 ℃ and between rolls at different peripheral speeds.

Next, the sheet was immersed in water having a liquid temperature of 30 ℃ for 120 seconds, and then uniaxially stretched (underwater stretch) at a total stretch ratio of 5.5 times in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds while being immersed in water having a liquid temperature of 73 ℃.

The resulting stretched film was heat-treated at 90 ℃ for 10 seconds by means of a stretcher KAROIV made by Bruckner K.K. to obtain a polyester film C (slow axis angle in the longitudinal direction: -0.2 °, in-plane retardation Re (590): 3243nm, thickness: 35 μm).

Production example 5 production of polyester film I

After Polyester resin (polyethylene terephthalate, manufactured by Bell Polyester Products, isophthalic acid modification amount: 2.5 mol% (mol number based on the total of all the repeating units of the polymer), diethylene glycol modification amount: 1.0 mol% (mol number based on the total of all the repeating units of the polymer), and IV value of 0.77dl/g (concentration of mixed solvent solution of phenol: 1,1,2, 2-tetrachloroethane: 6: 4: 0.4g/dl)) was dried in vacuum at 100 ℃ for 10 hours, a film forming apparatus equipped with a single-screw extruder (manufactured by Toyo Seiki, Ltd., screw diameter: 25mm, cylinder setting temperature: 280 ℃ C.), a T die (width: 500mm, setting temperature: 280 ℃ C.), chill roll (setting temperature: 50 ℃ C.) and winder was used to produce an amorphous Polyester resin film having a thickness of 180 μm.

The obtained amorphous polyester resin film was simultaneously biaxially stretched by a stretcher KAROIV of Bruckner to obtain a polyester film I (slow axis angle in the longitudinal direction: -0.6 °, in-plane retardation Re (590): 1312nm, thickness: 20 μm). The stretching magnification was 6.0 times in the longitudinal direction (MD) and 1.5 times in the width direction (TD). The stretching temperature was set to 90 ℃ and the stretching speeds in both MD and TD were set to 5%/sec. After the stretching treatment, heat treatment was performed at 140 ℃ for 10 seconds while maintaining the dimensions.

Production example 6 production of polyester film II

The obtained amorphous polyester resin film was simultaneously biaxially stretched by a stretcher KAROIV of Bruckner to obtain a polyester film II (slow axis angle in the longitudinal direction: 0.8 degrees, in-plane retardation Re (590): 771nm, thickness: 32 μm). The stretching magnification was 4.5 times in the longitudinal direction (MD) and 2 times in the width direction (TD). The stretching temperature was set to 90 ℃ and the stretching speeds in both MD and TD were set to 2%/sec. After the stretching treatment, heat treatment was performed at 140 ℃ for 10 seconds while maintaining the dimensions.

[ example 1]

The polyester film A produced in production example 2 was subjected to corona treatment, and an aqueous solution having 15.2 wt% of a product name "SUPERFLEX 210R" manufactured by first Industrial pharmaceutical Co., Ltd and 2.7 wt% of a product name "WS-700" manufactured by Nippon catalyst Co., Ltd dissolved therein was applied so that the film thickness after drying became 300. mu.m, and the film was dried at 80 ℃ for 1 minute to obtain a polyester film A with an easy adhesive layer.

An aqueous PVA type resin solution (trade name "GOHSEFIMER Z-200", manufactured by japan synthetic chemical industries, inc., and resin concentration: 3 wt%) was applied to the surface of the polarizer with a base material obtained in production example 1, and a polyester film with an easy-adhesion layer was laminated thereon. The resulting laminate was heated in an oven maintained at 60 ℃ for 5 minutes. 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 are laminated such that the MD direction of the polyester film a is substantially parallel to the absorption axis direction of the polarizer.

The obtained polarizing plate was subjected to the above evaluations (1) to (7). 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 instead of the polyester film a produced in production example 2.

[ example 3]

A polarizing plate was obtained in the same manner as in example 1, except that the polyester film C produced in production example 4 was used instead of the polyester film a produced in production example 2.

Comparative example 1

A polarizing plate was obtained in the same manner as in example 1, except that the polyester film I produced in production example 5 was used instead of the polyester film a produced in production example 2.

Comparative example 2

A polarizing plate was obtained in the same manner as in example 1, except that the polyester film II produced in production example 6 was used instead of the polyester film a produced in production example 2.

[ Table 1]

Claims

1. A polyester film having a coefficient of linear expansion in a first direction of 3.0 x 10^-5Below/° c, the temperature of the composition,

linear expansion coefficient of 7.5 × 10 in a second direction orthogonal to the first direction^-5/℃～10.5×10^-5/℃，

Having a slow axis in a direction of-5 to 5 with respect to the first direction.

2. The polyester film according to claim 1, wherein the thickness of the polyester film in the second direction is not all 15% or less.

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

4. A polarizing plate comprising a polarizer and the polyester film according to claim 1 or 2 disposed on one side of the polarizer.

5. The polarizing plate according to claim 4, 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 x 10^-5The absolute value of the difference between the linear expansion coefficient of the polyester film in the second direction perpendicular to the first direction and the linear expansion coefficient of the polarizer in the direction parallel to the second direction is 5 × 10 ° C^-5Below/° c.

6. The polarizing plate of claim 4, wherein the polarizing element has a thickness of 20 μm or less.

7. The polarizing plate of claim 4, further comprising an easy-adhesion layer disposed on the polarizer side of the polyester film.

8. The polarizing plate of claim 7, wherein the easy adhesion layer comprises particles.

9. The polarizing plate according to claim 7, wherein the easy adhesion layer has a thickness of 0.35 μm or less.

10. The polarizing plate according to claim 7, wherein the easy adhesion layer has a refractive index of 1.55 or less.