TWI875866B - Polarizing plate and polarizing plate roll - Google Patents

Abstract

The present invention provides a polarizing plate that is very thin but has good durability. The polarizing plate of the present invention comprises a polarizing element, a first protective layer disposed on one side of the polarizing element, and a second protective layer disposed on the other side of the polarizing element. The first protective layer is formed of a cured product of a coating film of an organic solvent solution of an epoxy resin, and the second protective layer is formed of a resin film. In one embodiment, the thickness of the first protective layer is less than 10 μm, and the glass transition temperature is greater than 90°C.

Description

Polarizing plate and polarizing plate roll

The present invention relates to a polarizing plate and a polarizing plate roll.

In image display devices (such as liquid crystal display devices and organic EL display devices), due to the way the image is formed, a polarizing plate is usually arranged on at least one side of the display unit. In recent years, with the thinning and flexibility of image display devices, there is also a strong demand for thinning polarizing plates. However, the thinner the polarizing plate is, the more obvious the durability problem of reduced optical properties in a heated and humidified environment becomes. Prior art literature Patent literature

Patent document 1: Japanese Patent Publication No. 2015-210474

Problem to be solved by the invention The present invention is made to solve the above-mentioned previous problems, and its main purpose is to provide a polarizing plate that is very thin but has good durability.

Means for solving the problem The polarizing plate of the present invention has a polarizer, a first protective layer disposed on one side of the polarizer, and a second protective layer disposed on the other side of the polarizer. The first protective layer is composed of a cured product of a coating film of an organic solvent solution of an epoxy resin, and the second protective layer is composed of a resin film. In one embodiment, the thickness of the first protective layer is less than 10µm. In one embodiment, the glass transition temperature of the first protective layer is greater than 90°C. In one embodiment, the iodine adsorption amount of the first protective layer is less than 0.4kcps/µm. In one embodiment, the in-plane phase difference Re(550) of the first protective layer is -50nm~+50nm, and the phase difference Rth(550) in the thickness direction is -50nm~+50nm. In one embodiment, the first protective layer is arranged on the display unit side of the image display device, and the second protective layer is arranged on the opposite side of the display unit. In one embodiment, the polarizing plate is arranged on the visual side of the image display device. According to another aspect of the present invention, a polarizing plate roll is provided. The polarizing plate roll is formed by winding the polarizing plate into a roll.

Effect of the invention According to the present invention, by forming one of the protective layers disposed on both sides of the polarizer with a cured product of a coating film of an organic solvent solution of an epoxy resin, a polarizing plate having good durability even though it is very thin can be obtained.

A. Overview of polarizing plate Figure 1 is a schematic cross-sectional view of a polarizing plate of an embodiment of the present invention. The polarizing plate 100 of the illustrated example has a polarizing element 10, a first protective layer 20 disposed on one side of the polarizing element 10, and a second protective layer 30 disposed on the other side of the polarizing element 10. The thickness of the polarizing element 10 is preferably 8µm or less. When the polarizing plate 100 is applied to a liquid crystal display device, it can be disposed on the visual side of the display unit or on the side opposite to the visual side (back side). In either case, the first protective layer 20 can be disposed on the display unit side or on the side opposite to the display unit (outer side). In one embodiment, the polarizing plate 100 is disposed on the viewing side of the display unit (in other words, the image display device), and the first protective layer 20 is disposed on the display unit side. By disposing the first protective layer 20 on the display unit side, a polarizing plate having both excellent optical properties and excellent durability can be realized.

The polarizing plate can be in the form of a long strip or a thin sheet. When the polarizing plate is in the form of a long strip, it is preferably rolled into a roll to make a polarizing plate roll.

Typically, the polarizing plate has an adhesive layer as the outermost layer on one side (typically, the display unit side) and can be attached to the display unit. A surface protection film and/or a carrier film can be temporarily attached to the polarizing plate in a removable manner as required to reinforce and/or support the polarizing plate. When the polarizing plate includes an adhesive layer, a separation piece is temporarily attached to the surface of the adhesive layer in a removable manner to protect the adhesive layer before actual use, and the polarizing plate can be rolled up.

In the embodiment of the present invention, the first protective layer 20 is composed of a cured product of a coating film of an organic solvent solution of an epoxy resin. As long as it is composed as described above, the protective layer can be made very thin (for example, made to be less than 10μm). Moreover, the protective layer can be formed directly on the polarizer (that is, without passing through a bonding agent layer or an adhesive layer). According to the embodiment of the present invention, as described above, the polarizer and the first protective layer are very thin and the bonding agent layer or the adhesive layer can be omitted, so the total thickness of the polarizer can be made very thin. And the adhesion between the polarizer and the protective layer is also good. The second protective layer is composed of a resin film. By forming the second protective layer with a resin film, the handling properties (such as roller transport properties) of the polarizing plate can be ensured.

The total thickness of the polarizing plate is, for example, 50 μm or less, preferably 45 μm or less, more preferably 40 μm or less, and even more preferably 35 μm or less. The lower limit of the total thickness of the polarizing plate may be, for example, 25 μm.

Furthermore, protective layers are arranged on both sides of the polarizer and the first protective layer is formed by a cured product of a coating film of an organic solvent solution of an epoxy resin, thereby realizing a polarizing plate that is very thin but has good durability. Specifically, a polarizing plate in which the degradation of optical properties is suppressed even in a heated and humidified environment can be realized. After the polarizing plate of the present invention is placed in an environment of 85°C and 85%RH for 48 hours, the change ΔTs of the single body transmittance Ts and the change ΔP of the polarization degree P are both very small. The single body transmittance Ts is measured using, for example, an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, product name "V7100"). Polarization P is calculated from the single transmittance (Ts), parallel transmittance (Tp) and orthogonal transmittance (Tc) measured using an ultraviolet visible light spectrophotometer using the following formula. Polarization (P) (%) = {(Tp-Tc)/(Tp+Tc)} ^{1 /2} × 100 In addition, the above Ts, Tp and Tc are the Y values obtained by measuring the 2-degree field of view (C light source) of JIS Z 8701 and performing visual sensitivity correction. In addition, Ts and P are actually the characteristics of the polarizer. ΔTs and ΔP can be obtained by the following formulas. ΔTs(%)=Ts ₄₈ -Ts ₀ ΔP(%)=P ₄₈ -P ₀ Here, Ts ₀ is the single transmittance before placement (initial), Ts ₄₈ is the single transmittance after placement, P ₀ is the polarization before placement (initial), and P ₄₈ is the polarization after placement. ΔTs is preferably 3.0% or less, more preferably 2.7% or less, and more preferably 2.4% or less. ΔP is preferably -0.05%~0%, more preferably -0.03%~0%, and more preferably -0.01%~0%.

As mentioned above, the polarizing plate of the present invention is very thin, so it can be suitable for use in flexible image display devices. Preferably, the image display device has a curved shape (essentially a curved display screen) and/or is flexible or bendable. Specific examples of image display devices include liquid crystal display devices and electroluminescent (EL) display devices (such as organic EL display devices and inorganic EL display devices). Of course, the above description does not prevent the polarizing plate of the present invention from being used in general image display devices.

The following is a detailed description of the polarizer and protective layer.

B. Polarizer Any suitable polarizer can be used as the polarizer. Polarizers are typically made of a laminate with two or more layers. The manufacturing method of polarizers is described in D after the manufacturing method of polarizing plates.

The thickness of the polarizer should be 1µm~8µm, preferably 1µm~7µm, and even more preferably 2µm~5µm.

The boric acid content of the polarizer is preferably 10% by weight or more, preferably 13% by weight to 25% by weight. As long as the boric acid content of the polarizer is within the above range, the ease of adjusting the curling during lamination can be well maintained by the multiplication effect with the iodine content described later, and the curling during heating can be well suppressed, while improving the appearance durability during heating. The boric acid content can be calculated, for example, by the neutralization method using the following formula in the form of the amount of boric acid contained in the polarizer per unit weight. [Mathematical formula 1]

The iodine content of the polarizer is preferably 2% by weight or more, preferably 2% by weight to 10% by weight. As long as the iodine content of the polarizer is within the above range, the iodine content can be multiplied with the above-mentioned boric acid content to maintain the ease of adjusting the curling during lamination and to suppress the curling during heating, while improving the durability of the appearance during heating. The "iodine content" in this specification refers to the amount of all iodine contained in the polarizer (PVA-based resin film). More specifically, iodine exists in the polarizer in the form of iodine ions (I- ⁾ , iodine molecules ( _I2 ), polyiodine ions ( _I3- ^, _I5- ⁾ , etc., and the iodine content in this specification refers to the amount of iodine including all such forms. The iodine content can be calculated using, for example, the calibration curve method of X-ray fluorescence analysis. In addition, polyiodine ions exist in the polarizer in the state of forming a PVA-iodine complex. By forming the complex, absorption dichroism can be exhibited in the wavelength range of visible light. Specifically, the complex of PVA and triiodide ion (PVA・I ₃ ^- ) has an absorption peak near 470nm, and the complex of PVA and pentaiodide ion (PVA・I ₅ ^- ) has an absorption peak near 600nm. As a result, polyiodine ions can absorb light in a wide range of visible light depending on their morphology. On the other hand, iodine ions (I ^- ) have an absorption peak near 230nm, which is substantially unrelated to the absorption of visible light. Therefore, polyiodine ions that exist in the state of complex with PVA are mainly related to the absorption performance of the polarizer.

The polarizer preferably exhibits absorption dichroism at any wavelength of 380nm to 780nm. The single unit transmittance Ts of the polarizer is preferably 40% to 48%, preferably 41% to 46%. The polarization degree P of the polarizer is preferably above 97.0%, preferably above 99.0%, and even more preferably above 99.9%.

C. Protective layer C-1. First protective layer As mentioned above, the first protective layer is composed of a cured product of a coating film of an organic solvent solution of an epoxy resin. The following is a detailed description of the components of the first protective layer, followed by a description of the characteristics of the first protective layer.

C-1-1. Epoxy resin The glass transition temperature (Tg) of epoxy resin is preferably 90°C or higher. As a result, the Tg of the protective layer will be 90°C or higher. As long as the Tg of epoxy resin is 90°C or higher, the polarizing plate including the protective layer obtained from such resin is likely to have excellent durability. The Tg of epoxy resin is preferably 100°C or higher, more preferably 110°C or higher, more preferably 120°C or higher, and particularly preferably 125°C or higher. On the other hand, the Tg of epoxy resin is preferably 300°C or lower, more preferably 250°C or lower, more preferably 200°C or lower, and particularly preferably 160°C or lower. As long as the Tg of epoxy resin is within the above range, the moldability is good.

As long as the epoxy resin has the Tg as described above, any appropriate epoxy resin may be used. Epoxy resins typically refer to resins having an epoxy group in their molecular structure. As epoxy resins, epoxy resins having an aromatic ring in their molecular structure are preferably used. By using epoxy resins having an aromatic ring, an epoxy resin having a higher Tg can be obtained. Aromatic rings in epoxy resins having an aromatic ring in their molecular structure include, for example, benzene rings, naphthalene rings, fluorene rings, and the like. Epoxy resins may be used alone or in combination of two or more. When two or more epoxy resins are used, an epoxy resin containing an aromatic epoxy and an epoxy resin not containing an aromatic epoxy may be used in combination.

Epoxy resins having aromatic rings in the molecule include: bisphenol A diglycidyl ether type epoxy resins, bisphenol F diglycidyl ether type epoxy resins, bisphenol S diglycidyl ether type epoxy resins, resorcinol diglycidyl ether type epoxy resins, hydroquinone diglycidyl ether type epoxy resins, terephthalate diglycidyl ester type epoxy resins, bisphenoxyethanol fluoride diglycidyl ether type epoxy resins, bisphenol fluoride diglycidyl ether type epoxy resins, biscresol fluoride diglycidyl ether type epoxy resins, etc. Epoxy resins having three epoxy groups, such as phenolic epoxy resins, N,N,O-triglycidyl-p- or -m-aminophenol epoxy resins, N,N,O-triglycidyl-4-amino-m- or -5-amino-o-cresol epoxy resins, and 1,1,1-(triglycidyloxyphenyl)methane epoxy resins; epoxy resins having four epoxy groups, such as glycidylamine epoxy resins (e.g., diaminodiphenylmethane type, diaminodiphenylsulfone type, and intercalated diamine type), etc. Furthermore, hexahydrophthalic anhydride type epoxy resin, tetrahydrophthalic anhydride type epoxy resin, dialcic acid type epoxy resin, p-oxybenzoic acid type or other glycidyl ester type epoxy resin may also be used.

The weight average molecular weight of the epoxy resin is preferably 1,000 to 2,000,000, more preferably 5,000 to 1,000,000, more preferably 10,000 to 500,000, particularly preferably 50,000 to 500,000, and most preferably 60,000 to 150,000. The weight average molecular weight can be obtained, for example, by gel permeation chromatography (GPC system, manufactured by Tosoh Corporation) in terms of polystyrene. In addition, tetrahydrofuran can be used as the solvent.

The epoxy equivalent of the epoxy resin is preferably 1000 g/equivalent or more, more preferably 3000 g/equivalent or more, and more preferably 5000 g/equivalent or more. Furthermore, the epoxy equivalent of the epoxy resin is preferably 30000 g/equivalent or less, more preferably 25000 g/equivalent or less, and more preferably 20000 g/equivalent or less. By having the epoxy equivalent in the above range, a more stable protective layer (a protective layer with few residual monomers and fully cured) can be obtained. In addition, in this specification, "epoxy equivalent" means "the mass of the epoxy resin containing 1 equivalent of epoxy group", which can be measured in accordance with JIS K7236.

In the embodiment of the present invention, epoxy resin and other resins can also be used in combination. That is, a mixture of epoxy resin and other resins can also be used for forming the protective layer. Other resins include thermoplastic resins such as styrene resins, polyethylene, polypropylene, polyamide, polyphenylene sulfide, polyetheretherketone, polyester, polysulfone, polyphenylene ether, polyacetal, polyimide, and polyetherimide. The type and blending amount of the resin used in combination can be appropriately set according to the purpose and the desired properties of the resulting film. For example, styrene resins can be used in combination as a phase difference controller.

When epoxy resin and other resins are used together, the content of epoxy resin in the mixture of epoxy resin and other resins is preferably 50 wt% to 100 wt%, more preferably 60 wt% to 100 wt%, more preferably 70 wt% to 100 wt%, and particularly preferably 80 wt% to 100 wt%. When the content is less than 50 wt%, the heat resistance of the protective layer and sufficient adhesion to the polarizer may not be obtained.

C-1-2. Hardener The organic solvent solution of epoxy resin may further contain a hardener. Any suitable hardener may be used in any suitable amount.

C-1-3. Composition and characteristics of the first protective layer As mentioned above, the first protective layer is composed of a cured product of a coating film of an organic solvent solution of an epoxy resin. As long as it is a cured product of the coating film, its thickness can be much thinner than an extruded film. The thickness of the first protective layer is preferably 10µm or less, preferably 7µm or less, more preferably 5µm or less, and even more preferably 3µm or less. The lower limit of the thickness of the first protective layer can be, for example, 1µm. In addition, although it is not clear in theory, the cured product of this coating film shrinks less when the film is formed than the cured product of thermosetting resin or active energy ray curing resin (such as ultraviolet curing resin), and does not contain residual monomers, etc., so it has the advantages of suppressing the degradation of the film itself and suppressing the adverse effects of residual monomers on the polarizing plate (polarizer). In addition, its moisture absorption and moisture permeability are smaller than the cured product of water-based coating films such as aqueous solutions or water dispersions, so it has the advantage of excellent humidification durability. As a result, a durable polarizing plate that can maintain optical properties even in a heated and humidified environment can be achieved. The protective layer of the cured product of the coating film of the epoxy resin organic solvent solution has excellent adhesion to the polarizer. Therefore, even with the above thickness, the polarizer can be protected to the same extent as the protective layer using the conventional film. In addition, even with the above thickness, the heat resistance is still good.

The Tg of the first protective layer is as described in the above section C-1-1 regarding the epoxy resin.

The iodine adsorption of the protective layer should be above 0.4 kcps/µm. In this manual, the iodine adsorption refers to the iodine adsorption measured by the following method. Place the sample (1 cm square, 40 µm thick, single layer film) in a vial, and place a bottle containing iodine solution (staining solution) in the vial and seal it. Then, heat the vial at 65°C for 6 hours and take out the sample from the vial. Use combustion ion chromatography to measure the iodine content in the sample, and use the value obtained as the iodine adsorption.

The first protective layer is preferably substantially optically isotropic. In this specification, "substantially optically isotropic" means that the phase difference at a wavelength of 550nm is -50nm~+50nm. The in-plane phase difference Re(550) is preferably -30nm~+30nm, more preferably -10nm~+10nm, and particularly preferably 0nm~2nm. The phase difference Rth(550) in the thickness direction is preferably -5nm~5nm, more preferably -3nm~3nm, and particularly preferably -2nm~2nm. As long as the Re(550) and Rth(550) of the first protective layer are within the above range, when the polarizing plate including the first protective layer is used in an image display device, it can prevent adverse effects on the display characteristics. In addition, Re(550) is the in-plane phase difference of the film measured at 23°C with light of a wavelength of 550nm. Re(550) can be calculated by the formula: Re(550)=(nx-ny)×d. Rth(550) is the phase difference in the thickness direction of the film measured at 23°C with light of a wavelength of 550nm. Rth(550) can be calculated by the formula: Rth(550)=(nx-nz)×d. Here, nx is the refractive index in the direction where the in-plane refractive index is the largest (i.e., the slow axis direction), ny is the refractive index in the direction orthogonal to the slow axis in the plane (i.e., the fast axis direction), nz is the refractive index in the thickness direction, and d is the thickness of the film (nm).

The higher the light transmittance of the first protective layer at 380 nm when the thickness is 3 μm, the better. Specifically, the light transmittance is preferably 85% or more, more preferably 88% or more, and more preferably 90% or more. As long as the light transmittance is within the above range, the desired transparency can be ensured. The light transmittance can be measured, for example, according to the method of ASTM-D-1003.

The lower the haze of the first protective layer, the better. Specifically, the haze is preferably below 5%, more preferably below 3%, more preferably below 1.5%, and most preferably below 1%. As long as the haze is below 5%, the film can be given a good sense of transparency. Moreover, even when used as a visual side polarizing plate for an image display device, the display content can still be well visually recognized.

The YI of the first protective layer at a thickness of 3µm should be 1.27 or less, preferably 1.25 or less, more preferably 1.23 or less, and particularly preferably 1.20 or less. When the YI is greater than 1.3, the optical transparency may be insufficient. In addition, the YI can be obtained from the color tristimulus values (X, Y, Z) measured using a high-speed integrating sphere spectroscopic transmittance meter (trade name DOT-3C: manufactured by Murakami Color Technology Laboratory) using the following formula. YI=[(1.28X-1.06Z)/Y]×100

The b value (hue scale based on the Hunter colorimetric system) of the first protective layer at a thickness of 3 μm is preferably less than 1.5, and preferably less than 1.0. When the b value is greater than 1.5, an undesirable color tone may appear. In addition, the b value can be obtained, for example, in the following manner: a sample of the film constituting the first protective layer is cut into 3 cm squares, and the hue is measured using a high-speed integrating sphere spectroscopic transmittance meter (trade name DOT-3C: manufactured by Murakami Color Technology Laboratory), and the hue is evaluated according to the Hunter colorimetric system.

The first protective layer (cured product of the coating film) may contain any appropriate additives according to the purpose. Specific examples of additives include ultraviolet absorbers; leveling agents; antioxidants such as hindered phenol, phosphorus, and sulfur; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fibers and carbon fibers; near-infrared absorbers; tris(dibromopropyl) phosphate, triallyl phosphate, etc. Flame retardants such as esters and antimony oxide; antistatic agents such as anionic, cationic, and non-ionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; organic fillers or inorganic fillers; resin modifiers; organic fillers or inorganic fillers; plasticizers; lubricants; antistatic agents; flame retardants, etc. Additives are usually added to the solution when the film is formed. The type, amount, combination, and addition amount of additives can be appropriately set according to the purpose.

The first protective layer may also be provided with an easy-adhesion layer on the polarizer side. The easy-adhesion layer may include, for example, water-based polyurethane and The oxazoline crosslinking agent can improve the adhesion between the first protective layer and the polarizer by forming the easy-adhesion layer.

C-2. Second protective layer As mentioned above, the second protective layer is composed of a resin film. Specific examples of the material constituting the main component of the film include cellulose resins such as triacetate cellulose (TAC), polyester resins such as polyethylene terephthalate (PET), polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyether sulfone resins, polysulfone resins, polystyrene resins, cycloolefin resins such as polynorbornene, polyolefin resins, (meth)acrylic resins, and transparent resins such as acetate resins. In addition, thermosetting resins or ultraviolet curing resins such as (meth) acrylic acid, urethane, (meth) acrylic urethane, epoxy, and silicone can also be cited. Other examples include glassy polymers such as silicone polymers. In addition, polymer films described in Japanese Patent Publication No. 2001-343529 (WO01/37007) can also be used. As the material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted amide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl and nitrile group in the side chain can be used, and for example, a resin composition having an alternating copolymer composed of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer can be cited. The polymer film may be, for example, an extruded product of the above-mentioned resin composition.

When the polarizing plate of the present invention is disposed on the viewing side of the image display device and the second protective layer 30 is disposed on the side of the image display device opposite to the display unit (viewing side), the second protective layer 30 may also be subjected to surface treatments such as hard coating, anti-reflection, anti-sticking, and anti-glare treatments as needed. And/or, the second protective layer 30 may also be subjected to treatments useful for improving visibility when viewed through polarized sunglasses (typically, imparting (elliptical) circular polarization function and imparting ultra-high phase difference) as needed. By performing the above treatments, excellent visibility can be achieved even when viewing the display screen through polarized lenses such as polarized sunglasses. Therefore, the polarizing plate can also be suitably used in image display devices that can be used outdoors.

The thickness of the second protective layer is preferably 10µm~50µm, more preferably 10µm~30µm. In addition, when surface treatment is applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

D. Manufacturing method of polarizing plate D-1. Manufacturing method of polarizing element The manufacturing method of polarizing element described in item B above includes the following steps: forming a polyvinyl alcohol resin layer (PVA resin layer) containing a halogenated substance and a polyvinyl alcohol resin (PVA resin) on one side of a long strip of thermoplastic resin substrate to form a laminate; and sequentially subjecting the laminate to air-assisted stretching treatment, dyeing treatment, underwater stretching treatment and drying shrinkage treatment, wherein the drying shrinkage treatment is to heat the laminate while conveying it in the longitudinal direction, thereby shrinking it by more than 2% in the width direction. The halogenated substance content in the PVA-based resin layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin. The drying and shrinking treatment is preferably carried out using a heating roller, and the temperature of the heating roller is preferably 60°C to 120°C. According to the manufacturing method, a polarizer as described above can be obtained. In particular, after a laminate comprising a PVA-based resin layer containing a halogenated substance is prepared, the laminate is stretched by a multi-stage stretching process including air-assisted stretching and underwater stretching, and the stretched laminate is then heated by a heating roller, thereby obtaining a polarizer having excellent optical properties (represented by single body transmittance and polarization degree) and suppressed optical property variations. Specifically, by using a heated roller in the drying and shrinking step, the laminate can be uniformly shrunk while being transported. This not only improves the optical properties of the resulting polarizer, but also enables stable production of polarizers with excellent optical properties, while suppressing the variation in the optical properties of the polarizer (especially the single-body transmittance). The following describes halides and drying and shrinking treatment. The details of the manufacturing methods other than these are described, for example, in Japanese Patent Publication No. 2012-73580 and Japanese Patent No. 6470455. The entire contents of the publication are cited in this specification for reference.

D-1-1. Halogenated substances The PVA resin layer containing halogenated substances and PVA resins can be formed by applying a coating liquid containing halogenated substances and PVA resins on a thermoplastic resin substrate and drying the coating film. The coating liquid is typically a solution obtained by dissolving the halogenated substances and the PVA resins in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trihydroxymethylpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Of these, water is preferred. The PVA resin concentration of the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. As long as the resin concentration is the above, a uniform coating film that adheres closely to the thermoplastic resin substrate can be formed.

Any appropriate halides may be used as the halides. Examples thereof include iodides and sodium chloride. Examples of iodides include potassium iodide, sodium iodide and lithium iodide. Among these, potassium iodide is preferred.

The amount of halogenated substance in the coating liquid is preferably 5 to 20 parts by weight, preferably 10 to 15 parts by weight, relative to 100 parts by weight of the PVA resin. If the amount of halogenated substance is too much, the halogenated substance may overflow and make the polarizer obtained finally white and cloudy.

Generally speaking, after the PVA resin layer is stretched, the orientation of the polyvinyl alcohol molecules in the PVA resin layer will become higher. However, if the stretched PVA resin layer is immersed in a water-containing liquid, the orientation of the polyvinyl alcohol molecules will be disordered and the orientation will be reduced. In particular, when the laminate of the thermoplastic resin substrate and the PVA resin layer is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin substrate, the tendency of the above orientation to be reduced is very obvious. For example, the stretching of a PVA film monomer in boric acid water is generally performed at 60°C. In contrast, the stretching of a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a relatively high temperature of around 70°C. At this time, the orientation of the PVA at the beginning of the stretching decreases before it rises due to the stretching in water. In contrast, by preparing a laminate of a PVA-based resin layer containing a halogenated substance and a thermoplastic resin substrate, and stretching the laminate at a high temperature in air (auxiliary stretching) before stretching in boric acid water, the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after the auxiliary stretching can be promoted. As a result, when the PVA resin layer is immersed in a liquid, the orientation disorder and the reduction of orientation of the polyvinyl alcohol molecules can be suppressed more than when the PVA resin layer does not contain halides. As a result, the optical properties of the polarizer obtained by immersing the laminate in a liquid through a dyeing process and an underwater stretching process can be improved.

D-1-2. Drying and shrinking treatment The drying and shrinking treatment can be performed by heating the entire area, or by heating the conveying roller (so-called using a heating roller) (heating roller drying method). It is preferred to use both. By using a heating roller to dry it, the heat curling of the laminate can be effectively suppressed, and a polarizer with excellent appearance can be manufactured. Specifically, by drying the laminate along the state of adding a hot roller, the crystallization of the above-mentioned thermoplastic resin substrate can be effectively promoted to increase the crystallinity. Even at a relatively low drying temperature, the crystallinity of the thermoplastic resin substrate can still be well increased. As a result, the rigidity of the thermoplastic resin substrate increases and becomes a state that can withstand the shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. In addition, by using a heating roller, the laminate can be dried while maintaining a flat state, so that not only the generation of curling but also the generation of wrinkles can be suppressed. At this time, the laminate can be shrunk in the width direction through a drying and shrinking treatment to improve the optical properties. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage rate in the width direction obtained by the drying and shrinking treatment of the laminate is preferably 2%~10%, more preferably 2%~8%, and particularly preferably 4%~6%. By using heated rollers, the laminate can be continuously shrunk in the width direction while being conveyed, achieving high productivity.

FIG2 is a schematic diagram showing an example of a drying and shrinking process. In the drying and shrinking process, the laminate 200 is dried while being transported by conveying rollers R1 to R6 and guide rollers G1 to G4 that have been heated to a predetermined temperature. In the example of the figure, the conveying rollers R1 to R6 are arranged to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but, for example, the conveying rollers R1 to R6 may also be arranged to continuously heat only one surface of the laminate 200 (e.g., the surface of the thermoplastic resin substrate).

The drying conditions can be controlled by adjusting the heating temperature of the conveyor roller (temperature of the heating roller), the number of heating rollers, and the contact time with the heating roller. The temperature of the heating roller is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. While the crystallinity of the thermoplastic resin can be increased and the curling can be suppressed, an optical laminate with excellent durability can be produced. In addition, the temperature of the heating roller can be measured by a contact thermometer. In the example of the figure, 6 conveyor rollers are provided, but there is no particular limitation as long as there are plural conveyor rollers. The number of conveyor rollers is usually 2 to 40, and it is preferably 4 to 30. The contact time (total contact time) between the laminate and the heating roller is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating roller can be installed in a heating furnace (such as an oven) or in a general manufacturing line (at room temperature). It is best to install it in a heating furnace equipped with an air supply mechanism. By using the heating roller and hot air drying together, the rapid temperature change between the heating rollers can be suppressed, and the shrinkage in the width direction can be easily controlled. The temperature of hot air drying should be 30℃~100℃. In addition, the hot air drying time should be 1 second~300 seconds. The wind speed of the hot air should be around 10m/s~30m/s. In addition, the wind speed is the wind speed in the heating furnace, which can be measured by a mini fan-type digital anemometer.

It is preferred to perform a cleaning treatment after the water stretching treatment and before the drying shrinkage treatment. The cleaning treatment can be performed by immersing the PVA-based resin layer in a potassium iodide aqueous solution.

According to the above method, a thermoplastic resin substrate/polarizer laminate can be obtained.

D-2. Method for manufacturing polarizing plate A resin film constituting the second protective layer is bonded to the surface of the polarizing element of the laminate obtained in item D-1 above. Then, the thermoplastic resin substrate is peeled off and an organic solvent solution of epoxy resin is applied to the peeled surface to form a coating film, and the coating film is cured to form the first protective layer. According to the above method, a polarizing plate having the structure of the first protective layer (cured product of the coating film)/polarizing element/second protective layer (resin film) can be obtained.

The epoxy resin is as described in the above item C-1-1.

Any suitable organic solvent that can dissolve or uniformly disperse the epoxy resin can be used as the organic solvent. Specific examples of the organic solvent include ethyl acetate, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, and cyclohexanone.

The epoxy resin concentration of the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. As long as the resin concentration is the above, a coating film that adheres closely to the polarizer and is uniform can be formed.

The solution can be applied to any appropriate substrate or to a polarizer. When the solution is applied to the substrate, the cured product of the coating film formed on the substrate is transferred to the polarizer. When the solution is applied to the polarizer, the first protective layer is directly formed on the polarizer by drying (curing) the coating film. It is preferred that the solution is applied to the polarizer to form the first protective layer directly on the polarizer. As long as the structure is as described above, the adhesive layer or the adhesive layer required for transfer can be omitted, so the polarizer can be made thinner. The solution can be applied by any appropriate method. Specific examples include roller coating, spin coating, wire rod coating, dip coating, die coating, curtain coating, spray coating, scraper coating (corner wheel coating, etc.).

The first protective layer can be formed by drying (curing) the coating film of the solution. The drying temperature is preferably below 100°C, more preferably 50°C to 70°C. As long as the drying temperature is within the above range, adverse effects on the polarizer can be prevented. The drying time can be changed according to the drying temperature. The drying time can be, for example, 1 minute to 10 minutes.

According to the above method, a polarizing plate having a structure of a first protective layer (cured product of a coating film)/polarizer/second protective layer (resin film) can be obtained. Alternatively, a thermoplastic resin substrate can be directly used as the second protective layer. In this case, an organic solvent solution of an epoxy resin is applied to the surface of the polarizer of the laminate of the thermoplastic resin substrate/polarizer to form a coating film, and the coating film is cured to form the first protective layer. As a result, a polarizing plate having a structure of a first protective layer (cured product of a coating film)/polarizer/second protective layer (thermoplastic resin substrate) can be obtained.

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

(1) Glass transition temperature Tg The glass transition temperature of the thin film constituting the first protective layer used in the embodiment and the comparative example was measured using a heated TMA analyzer (manufactured by Hitachi High-Tech Science Corporation, product name "TMA-7100C"). The measurement conditions are as follows: load 2g; nitrogen environment (200ml/min); heating from 25°C to 150°C, maintaining at 150°C for 5 minutes, cooling to 25°C, heating again to 150°C, and maintaining at 150°C for 5 minutes; heating rate 2°C/min. (2) Fading A test piece (50mm×50mm) was cut out from the polarizing plate obtained in the embodiment and the comparative example, and the test piece formed two sides opposite to the direction perpendicular to the absorption axis direction of the polarizer and the absorption axis direction. The test piece was adhered to the glass plate with an adhesive so that the first protective layer was on the inside to prepare a test sample. The test sample was placed in an oven at 85°C and 85% RH for 48 hours for heating and humidification. After being arranged in a state of orthogonal polarization with a standard polarizing plate, the fading state of the humidified polarizing plate was visually inspected and evaluated according to the following criteria. No problem: No fading was observed Partial fading: Fading was observed at the end Overall fading: The polarizing plate as a whole was obviously faded (3) Adhesion Test pieces (50 mm × 50 mm) were cut from the polarizing plates obtained in the examples and comparative examples. The test pieces were formed with two sides opposite to the direction perpendicular to the absorption axis direction of the polarizer and the absorption axis direction. Adhesive was applied to the polarizer side of the test piece and attached to a glass plate. Next, a 10×10 square grid was cut out on the protective layer (cured product of the coating film) side with a cutter and adhesive tape (manufactured by Sekisui Chemical Co., Ltd.) was attached to the surface. The adhesive tape was then peeled off and the number of squares that peeled off among the 100 square grids was evaluated.

<Example 1> 1. Preparation of polarizer/resin substrate laminate The resin substrate is a long strip of amorphous isophthalic acid copolymer polyethylene terephthalate film (thickness: 100µm) with a water absorption rate of 0.75% and a Tg of about 75°C. One side of the resin substrate was subjected to a corona treatment. In 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z410") in a ratio of 9:1, 13 parts by weight of potassium iodide was added to prepare a PVA aqueous solution (coating liquid). The PVA aqueous solution was applied to the corona treated surface of the resin substrate and dried at 60°C to form a PVA resin layer with a thickness of 13µm, thereby producing a laminate. The obtained laminate was subjected to free-end uniaxial stretching to 2.4 times in the longitudinal direction (long side direction) between rollers of different circumferential speeds in an oven at 130°C (air-assisted stretching treatment). Then, 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 40°C for 30 seconds (insolubilization treatment). Next, the dyeing bath (an iodine aqueous solution obtained by mixing iodine and potassium iodide at a weight ratio of 1:7 relative to 100 parts by weight of water) with a liquid temperature of 30°C was adjusted while being immersed in it for 60 seconds so that the monomer transmittance (Ts) of the polarizer finally obtained became 41.5%±0.1% (dyeing treatment). Next, it was immersed in a crosslinking bath (an aqueous boric acid solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid relative to 100 parts by weight of water) with a liquid temperature of 40°C for 30 seconds (crosslinking treatment). Then, the laminate was immersed in a boric acid aqueous solution (boric acid concentration 4.0 wt%, potassium iodide 5 wt%) at a liquid temperature of 70°C, and uniaxially stretched in the longitudinal direction (long side direction) between rollers of different circumferential speeds to a total stretching ratio of 5.5 times (underwater stretching treatment). Thereafter, the laminate was immersed in a cleaning bath (aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20°C (cleaning treatment). Thereafter, it was dried in an oven maintained at 90°C while contacting a SUS heating roller maintained at a surface temperature of 75°C for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction after drying and shrinking was 5.2%. According to the above method, a polarizer with a thickness of 5µm was formed on the resin substrate to produce a polarizer/resin substrate laminate.

2. Preparation of polarizing plate A cycloolefin film (ZT-12, manufactured by Zeon Co., Ltd., Japan, with a thickness of 23µm) is bonded to the surface of the polarizing element obtained above as a film constituting the second protective layer through a UV curing adhesive. Specifically, the curing adhesive is applied to a total thickness of 1.0µm and bonded using a roller. Thereafter, UV rays are irradiated from the film side to cure the adhesive. Next, the resin substrate is peeled off to obtain a polarizing plate having a second protective layer (ZT-12)/polarizing element structure.

20 parts of epoxy resin 1 (Mitsubishi Chemical Co., trade name: jER (registered trademark) 1256B40, weight average molecular weight: 40000, epoxy equivalent: 7350) were dissolved in 80 parts of methyl ethyl ketone to obtain an epoxy resin solution (20%). The epoxy resin solution was applied to the surface of the polarizer of the polarizing plate obtained above using a wire rod, and the applied film was dried at 60°C for 5 minutes to form a protective layer in the form of a cured product of the applied film. The thickness of the protective layer was 3µm and the Tg was 98°C. According to the above method, a polarizing plate having a structure of protective layer (cured product of coating film)/polarizer/another protective layer (ZT-12) was obtained. The obtained polarizing plate was subjected to the evaluations of (2) and (3) above.

<Example 2> Except that epoxy resin 2 (Mitsubishi Chemical Co., trade name: jER (registered trademark) YX6954BH30, weight average molecular weight: 36000, epoxy equivalent: 13000) was used instead of epoxy resin 1, a protective layer was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm, and Tg was 130°C. Except for using the protective layer, a polarizing plate was produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 1) Except that an acrylic resin having methyl methacrylate units (manufactured by Kusumoto Chemicals Co., Ltd., product name "B-728") was used instead of epoxy resin 1, a protective layer was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm, and the Tg was 111°C. A polarizing plate having a structure of a protective layer (cured product of a coating film)/polarizer/another protective layer (ZT-12) was obtained in the above manner. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 2) Except that an acrylic resin having a pentyl imide ring (manufactured by Kaneka Corporation, product name "HTX-ZU") is used instead of epoxy resin 1, a protective layer (cured product) is formed in the same manner as in Example 1. The thickness of the protective layer is 3µm and the Tg is 94°C. A polarizing plate is manufactured in the same manner as in Example 1 except that the protective layer is used. The obtained polarizing plate is subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 3) Except that an acrylic resin having a lactone ring structure (manufactured by Nippon Catalyst Co., Ltd., product name "HZ420") is used instead of epoxy resin 1, a protective layer (cured product) is formed in the same manner as in Example 1. The thickness of the protective layer is 3µm, and the Tg is 106°C. Except for using the protective layer, a polarizing plate is produced in the same manner as in Example 1. The obtained polarizing plate is subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 4) Except that an acrylic resin containing ethyl acrylate/methyl methacrylate (manufactured by Kusumoto Chemicals Co., Ltd., product name "B-722") was used instead of epoxy resin 1, a protective layer (cured product) was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm and the Tg was 45°C. A polarizing plate was prepared in the same manner as in Example 1 except that the protective layer was used. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 5) Except that an acrylic resin containing butyl methacrylate/methyl methacrylate (manufactured by Kusumoto Chemicals Co., Ltd., product name "B-734") was used instead of epoxy resin 1, a protective layer (cured product) was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm and the Tg was 60°C. A polarizing plate was prepared in the same manner as in Example 1 except that the protective layer was used. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 6) Except for using a UV-curable acrylic resin (manufactured by Kyoei Chemicals, product name "HPP-A"), a protective layer (cured product) was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm. Except for using the protective layer, a polarizing plate was produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 7) Except for using a UV-curable acrylic resin (manufactured by Toagosei Co., Ltd., product name "D-PEHA"), a protective layer (cured product) was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm. Except for using the protective layer, a polarizing plate was produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

(Comparative Example 8) Except for using UV-curable epoxy resin (manufactured by DAICEL, product name "CELLOXIDE CP2021"), a protective layer (cured product) was formed in the same manner as in Example 1. The thickness of the protective layer was 3µm. Except for using the protective layer, a polarizing plate was manufactured in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 1. The results are listed in Table 1.

[Table 1]

<Evaluation> It is clear from Table 1 that even though the polarizing plate of the embodiment is very thin, the degradation of the optical properties is still suppressed in a heated and humidified environment, so the durability is excellent, and the adhesion with the polarizer is also excellent.

Industrial applicability The polarizing plate of the present invention can be used in image display devices. Examples of image display devices include: portable devices such as PDAs, smart phones, mobile phones, clocks, digital cameras, and portable game consoles; OA devices such as computer monitors, laptops, and copiers; home electrical devices such as video cameras, televisions, and microwave ovens; vehicle-mounted devices such as rear-view monitors, car navigation system monitors, and car stereos; display devices such as digital signs and information guide screens for commercial stores; alarm devices such as surveillance screens; nursing and medical devices such as nursing monitors and medical monitors.

10: Polarizer 20: 1st protective layer 30: 2nd protective layer 100: Polarizer 200: Laminated body G1~G4: Guide rollers R1~R6: Transport rollers

FIG1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. FIG2 is a schematic view showing an example of a drying and shrinking treatment using a heating roller in a method for manufacturing a polarizing plate according to an embodiment of the present invention.

10: Polarizer

20: 1st protective layer

30: Second protective layer

100: Polarizing plate

Claims (8)

A polarizing plate has a polarizer, a first protective layer disposed on one side of the polarizer, and a second protective layer disposed on the other side of the polarizer; The first protective layer is composed of a cured product of a coating film of an organic solvent solution of an epoxy resin, and the epoxy equivalent of the epoxy resin is greater than 3000 g/equivalent and less than 30000 g/equivalent, The second protective layer is composed of a resin film. As in claim 1, the polarizing plate, wherein the thickness of the first protective layer is less than 10µm. The polarizing plate of claim 1 or 2, wherein the glass transition temperature of the first protective layer is above 90°C. The polarizing plate of claim 1 or 2, wherein the iodine adsorption amount of the first protective layer is less than 0.4 kcps/µm. The polarizing plate of claim 1 or 2, wherein the in-plane phase difference Re(550) of the first protective layer is -50nm~+50nm, and the phase difference Rth(550) in the thickness direction is -50nm~+50nm. In the polarizing plate of claim 1 or 2, the first protective layer is disposed on a display unit side of the image display device, and the second protective layer is disposed on an opposite side of the display unit. The polarizing plate of claim 6 is disposed on the viewing side of the image display device. A polarizing plate roll is obtained by winding the polarizing plate of any one of claims 1 to 7 into a roll.

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