JP7724651B2 - Polarizing plate manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a polarizing plate.

Liquid crystal display devices (LCDs) are widely used not only in LCD televisions, but also in personal computers, mobile devices such as cell phones, and in-vehicle applications such as car navigation systems. Typically, LCDs have a liquid crystal panel with polarizing plates attached to both sides of the liquid crystal cell with an adhesive, and the display is achieved by controlling the light from the backlight using the liquid crystal panel. In recent years, organic electroluminescence (EL) display devices, like LCDs, have also become widely used in televisions, mobile devices such as cell phones, and in-vehicle applications such as car navigation systems. In organic EL display devices, a circular polarizer (a laminate including a polarizing element and a λ/4 plate) may be placed on the viewing-side surface of the image display panel to prevent external light from being reflected by the metal electrode (cathode) and appearing as a mirror.

In polarizing plates, polyvinyl alcohol resin adhesives are used to bond the polarizing element and protective film because they provide good adhesion. Patent Document 1 (JP 2009-42383 A) describes how an adhesive in which the weight ratio of polyvinyl alcohol resin and oxyzirconium salt is adjusted to a specific range can improve adhesion and warm water resistance between the polarizing element and protective film.

JP 2009-42383 A

The present invention aims to provide a manufacturing method for a polarizing plate that uses an adhesive containing a polyvinyl alcohol resin to bond a polarizing element and a protective film, which can improve the water resistance of the adhesive layer between the polarizing element and the protective film.

The present invention provides the following exemplary methods for producing a polarizing plate.
[1] A method for producing a polarizing plate having a polarizing element in which a dichroic dye is adsorbed and oriented in a polyvinyl alcohol-based resin layer, and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer, the method comprising:
an adhesive preparation step of preparing an adhesive that forms the adhesive layer;
a bonding step of applying the adhesive to a surface of at least one of the polarizing element and the transparent protective film to bond the polarizing element and the transparent protective film together,
The adhesive preparation step includes:
a preparation step of obtaining an aqueous solution A containing a polyvinyl alcohol-based resin;
a first addition step of adding a first compound to the aqueous solution A to obtain an aqueous solution B;
a second addition step of adding a second compound to the aqueous solution B to obtain an adhesive,
the first compound is at least one selected from the group consisting of urea, a urea derivative, thiourea, and a thiourea derivative, and the second compound is a dialdehyde;
the laminating step is carried out so that the time elapsed since the second compound was added to the aqueous solution B is 100 hours or less.
[2] A method for producing a polarizing plate having a polarizing element in which a dichroic dye is adsorbed and oriented in a polyvinyl alcohol-based resin layer, and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer, the method comprising:
an adhesive preparation step of preparing an adhesive that forms the adhesive layer;
a bonding step of applying the adhesive to a surface of at least one of the polarizing element and the transparent protective film to bond the polarizing element and the transparent protective film together,
The adhesive preparation step includes:
a preparation step of obtaining an aqueous solution A containing a polyvinyl alcohol-based resin;
a first addition step of adding a first compound to the aqueous solution A to obtain an aqueous solution B;
a second addition step of adding a second compound to the aqueous solution B to obtain an adhesive;
Including,
a third compound is also added in at least one of the first addition step and the second addition step;
the first compound is at least one selected from the group consisting of urea, a urea derivative, thiourea, and a thiourea derivative, the second compound is a dialdehyde, and the third compound is a dicarboxylic acid;
the laminating step is carried out so that the time elapsed since the second compound was added to the aqueous solution B is 200 hours or less.
[3] The method for producing a polarizing plate according to [1] or [2], wherein the content of the first compound in the adhesive is 0.1 parts by mass or more and 400 parts by mass or less with respect to 100 parts by mass of the polyvinyl alcohol-based resin.
[4] The method for producing a polarizing plate according to any one of [1] to [3], wherein the content of the second compound in the adhesive is 0.03 parts by mass or more and 20 parts by mass or less per part by mass of the first compound.
[5] The method for producing a polarizing plate according to any one of [1] to [4], wherein the dialdehyde is glyoxal.
[6] The method for producing a polarizing plate according to [2], wherein the content of the third compound in the adhesive is 0.01 parts by mass or more and 400 parts by mass or less with respect to 100 parts by mass of the polyvinyl alcohol-based resin.
[7] The method for producing a polarizing plate according to any one of [1] to [6], wherein the adhesive layer has a thickness of 0.01 μm or more and 7 μm or less.

The polarizing plate manufacturing method according to the present invention can improve the water resistance of the adhesive layer between the polarizing element and the protective film.

The following describes an embodiment of the present invention. The present invention is not limited to the following embodiment.

[Method of manufacturing polarizing plate]
The present invention provides a method for producing a polarizing plate having a polarizing element in which a dichroic dye is adsorbed and oriented in a layer containing a polyvinyl alcohol-based resin (hereinafter also referred to as a "PVA-based resin"), and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer, the method comprising: an adhesive preparation step of preparing an adhesive that forms the adhesive layer; and a lamination step of applying the adhesive to at least one surface of the polarizing element and the transparent protective film to laminate the polarizing element and the transparent protective film. In the method for producing a polarizing plate in which the transparent protective film is laminated on both surfaces of the polarizing element via an adhesive layer, at least the adhesive layer on one surface of the polarizing element, preferably the adhesive layers on both surfaces of the polarizing element, is formed using the adhesive prepared in the adhesive layer preparation step.

<First Aspect>
In a first aspect of the method for producing a polarizing plate according to the present invention, the adhesive preparation step includes a preparatory step of obtaining an aqueous solution A containing a PVA-based resin, a first addition step of adding a first compound to the aqueous solution A to obtain an aqueous solution B, and a second addition step of adding a second compound to the aqueous solution B to obtain an adhesive, and the laminating step is performed so that the elapsed time (hereinafter also referred to as "elapsed time T1") from the addition of the second compound to the aqueous solution B is 100 hours or less. By setting the elapsed time T1 to 100 hours or less, a polarizing plate having an adhesive layer with high water resistance can be produced.

(First compound)
The first compound is at least one selected from the group consisting of urea, urea derivatives, thiourea, and thiourea derivatives. The first compound can be used singly or in combination of two or more. First compounds include water-soluble and poorly water-soluble first compounds, and either type can be used. When using a poorly water-soluble first compound in a water-soluble adhesive, it is preferable to devise a dispersion method to prevent an increase in haze after forming the adhesive layer. The inclusion of the first compound can suppress a decrease in the transmittance of the polarizing element. It is speculated that one of the causes of the decrease in transmittance of the polarizing element is polyenation due to dehydration of the PVA-based resin that constitutes the polarizing element. It is speculated that the first compound contained in the adhesive layer migrates to the polarizing element, thereby suppressing polyenation.

(urea derivative)
A urea derivative is a compound in which at least one of the four hydrogen atoms of a urea molecule is substituted with a substituent. In this case, the substituent is not particularly limited, but is preferably a substituent consisting of carbon, hydrogen, and oxygen atoms.

Specific examples of urea derivatives include mono-substituted ureas such as methylurea, ethylurea, propylurea, butylurea, isobutylurea, N-octadecylurea, 2-hydroxyethylurea, hydroxyurea, acetylurea, allylurea, 2-propynylurea, cyclohexylurea, phenylurea, 3-hydroxyphenylurea, (4-methoxyphenyl)urea, benzylurea, benzoylurea, o-tolylurea, and p-tolylurea.

Disubstituted ureas include 1,1-dimethylurea, 1,3-dimethylurea, 1,1-diethylurea, 1,3-diethylurea, 1,3-bis(hydroxymethyl)urea, 1,3-tert-butylurea, 1,3-dicyclohexylurea, 1,3-diphenylurea, 1,3-bis(4-methoxyphenyl)urea, 1-acetyl-3-methylurea, 2-imidazolidinone (ethylene urea), and tetrahydro-2-pyrimidinone (propylene urea).

Examples of tetrasubstituted ureas include tetramethylurea, 1,1,3,3-tetraethylurea, 1,1,3,3-tetrabutylurea, 1,3-dimethoxy-1,3-dimethylurea, 1,3-dimethyl-2-imidazolidinone, and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone.

(Thiourea derivatives)
A thiourea derivative is a compound in which at least one of the four hydrogen atoms of a thiourea molecule is substituted with a substituent. In this case, the substituent is not particularly limited, but is preferably a substituent consisting of a carbon atom, a hydrogen atom, and an oxygen atom.

Specific examples of thiourea derivatives include mono-substituted thioureas such as N-methylthiourea, ethylthiourea, propylthiourea, isopropylthiourea, 1-butylthiourea, cyclohexylthiourea, N-acetylthiourea, N-allylthiourea, (2-methoxyethyl)thiourea, N-phenylthiourea, (4-methoxyphenyl)thiourea, N-(2-methoxyphenyl)thiourea, N-(1-naphthyl)thiourea, (2-pyridyl)thiourea, o-tolylthiourea, and p-tolylthiourea.

Disubstituted thioureas include 1,1-dimethylthiourea, 1,3-dimethylthiourea, 1,1-diethylthiourea, 1,3-diethylthiourea, 1,3-dibutylthiourea, 1,3-diisopropylthiourea, 1,3-dicyclohexylthiourea, N,N-diphenylthiourea, N,N'-diphenylthiourea, 1,3-di(o-tolyl)thiourea, 1,3-di(p-tolyl)thiourea, 1-benzyl-3-phenylthiourea, 1-methyl-3-phenylthiourea, N-allyl-N'-(2-hydroxyethyl)thiourea, and ethylenethiourea.

Examples of trisubstituted thioureas include trimethylthiourea, and examples of tetrasubstituted thioureas include tetramethylthiourea and 1,1,3,3-tetraethylthiourea.

Among the first compounds, urea derivatives or thiourea derivatives are preferred, with urea derivatives being more preferred, because when used in an image display device with an interlayer filling configuration, they suppress a decrease in transmittance in high-temperature environments and cause little decrease in polarization degree (suppressing cross-drop loss). Among urea derivatives, mono-substituted or di-substituted ureas are preferred, with mono-substituted ureas being more preferred. Di-substituted ureas include 1,1-substituted ureas and 1,3-substituted ureas, with 1,3-substituted ureas being more preferred.

(Second compound)
The second compound is a dialdehyde. The dialdehyde as the second compound can function as a crosslinking agent. Examples of dialdehydes include glyoxal, propanediol (malondialdehyde), butanediol (succinaldehyde), etc. Glyoxal, which has a simple structure and is highly reactive, is particularly preferred. Although glyoxal will be described below, the dialdehyde is not limited to glyoxal and may be any known dialdehyde as described above.

<Adhesive preparation process>
The adhesive preparation process includes a preparation step of obtaining aqueous solution A containing a PVA-based resin, a first addition step of adding a first compound to aqueous solution A to obtain aqueous solution B, and a second addition step of adding a second compound to aqueous solution B to obtain the adhesive.

<Preparation process>
In the preparation step, a PVA-based resin is added to water and stirred to obtain an aqueous solution A. From the viewpoint of adhesiveness, the average polymerization degree of the PVA-based resin is preferably about 100 to 5500, more preferably 1000 to 4500. From the viewpoint of adhesiveness, the average saponification degree is preferably about 85 mol % to 100 mol %, more preferably 90 mol % to 100 mol %.

PVA-based resins containing acetoacetyl groups are preferred because they provide excellent adhesion between the PVA-based resin layer and the protective film and excellent durability. Acetoacetyl-based PVA-based resins can be obtained, for example, by reacting a PVA-based resin with diketene using any method. The degree of acetoacetyl group modification of the acetoacetyl-based PVA-based resin is typically 0.1 mol% or more, and preferably 0.1 mol% to 20 mol%. The resin concentration of aqueous solution A is preferably 0.1 mass% to 15 mass%, and more preferably 0.5 mass% to 10 mass%.

The water used to prepare aqueous solution A is not particularly limited, and may include pure water, ultrapure water, well water, or tap water. However, from the viewpoint of maintaining the uniformity and transparency of the adhesive layer to be formed, pure water or ultrapure water is preferred. Aqueous solution A is subjected to the first addition step. Aqueous solution A may be subjected to the first addition step immediately after preparation, or may be stored for a period of time before being subjected to the first addition step. When storing aqueous solution A, the storage environment is preferably a temperature of 10°C to 40°C and a relative humidity of 30% to 90% RH. The storage period is not particularly limited, but may be, for example, 10 minutes or more, or 60 days or less, preferably 30 days or less, and more preferably 10 days or less.

<First addition step>
In the first addition step, a first compound is added to aqueous solution A to obtain aqueous solution B. The content of the first compound is preferably 0.1 parts by mass or more and 400 parts by mass or less, more preferably 1 part by mass or more and 200 parts by mass or less, and even more preferably 3 parts by mass or more and 100 parts by mass or less, relative to 100 parts by mass of the PVA-based resin. If the content is less than 0.1 parts by mass, the effect of suppressing polyenization of the polarizing element in a high-temperature environment may be insufficient. On the other hand, if the content exceeds 400 parts by mass, the first compound may precipitate after preparation of the polarizing plate, causing an increase in haze. After adding the first compound to aqueous solution A, it is preferable to thoroughly stir the mixture to obtain aqueous solution B.

<Second addition step>
In the second addition step, the second compound is added to aqueous solution B to obtain the adhesive. The content of the second compound is preferably 1 part by mass or more and 60 parts by mass or less, more preferably 1.5 parts by mass or more and 50 parts by mass or less, and even more preferably 2 parts by mass or more and 45 parts by mass or less, relative to 100 parts by mass of the PVA-based resin. If the content is less than 1 part by mass, the effect of improving water resistance may be insufficient. On the other hand, if the content exceeds 60 parts by mass, the preparation stability of the adhesive may be reduced.

In the adhesive, the content of the dialdehyde, the second compound, is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less per part by mass of the urea compound, the first compound. There is no lower limit, but the content is, for example, 0.03 parts by mass or more. When the content ratio of the urea compound to the dialdehyde in the adhesive is within the above range, it becomes easier to achieve both the water resistance improvement effect of the urea compound and the adhesiveness improvement effect of the dialdehyde. It is understood that the adhesiveness improvement effect of the dialdehyde results in improved water resistance. If the dialdehyde content exceeds 20 parts by mass per part by mass of the urea compound, the high-temperature durability improvement effect of the urea compound may not be fully achieved. The content ratio of the urea compound to the dialdehyde in the adhesive can be considered to be the same as the content ratio of the urea compound to the dialdehyde in the adhesive layer.

The adhesive may contain other additives in addition to the PVA-based resin, the first compound, and the second compound. Other additives may include crosslinkers other than dialdehydes, plasticizers, silane coupling agents, antistatic agents, fine particles, and other conventionally known additives. The timing of adding the other additives is not limited, and they can be added during the preparation process, the first addition process, the second addition process, etc.

The adhesive may contain water. The timing of adding water is not limited, and water may be added during the preparation process, first addition process, second addition process, etc.

The adhesive may also contain an organic solvent. Alcohols are preferred as organic solvents due to their miscibility with water, and among alcohols, methanol or ethanol is even more preferred. The methanol concentration in the adhesive is preferably 10% to 70% by mass, more preferably 15% to 60% by mass, and even more preferably 20% to 60% by mass. A methanol concentration of 10% by mass or more can more effectively suppress polyenation of the PVA-based resin in high-temperature environments. Furthermore, a methanol content of 70% by mass or less can suppress deterioration of color. While some urea derivatives have low solubility in water, some have sufficient solubility in alcohol. In this case, one preferred embodiment is to dissolve a urea-based compound in alcohol to prepare an alcohol solution of the urea-based compound, and then add the alcohol solution of the urea-based compound to the aqueous PVA solution to prepare the adhesive.

<Lamination process>
In the lamination step, the adhesive prepared in the adhesive preparation step is applied to at least one surface of the polarizing element and the transparent protective film to bond them together. The lamination step is performed so that the elapsed time (hereinafter also referred to as "elapsed time T1") from the addition of the second compound to aqueous solution B in the second addition step of the adhesive preparation step is 100 hours or less. More specifically, the lamination step involves applying the adhesive to at least one surface of the polarizing element and the transparent protective film, bonding the polarizing element and the transparent protective film together via an adhesive layer, and then curing or drying the adhesive layer. The elapsed time T1 refers to the time elapsed from the addition of the second compound to aqueous solution B in the second addition step to the bonding of the polarizing element and the transparent protective film together via the adhesive layer in the lamination step, and does not include the time required to dry the adhesive layer. By ensuring that the elapsed time T1 is 100 hours or less, an adhesive layer with excellent water resistance can be obtained.

The elapsed time T1 is preferably 90 hours or less, and more preferably 80 hours or less. Since it is desirable to thoroughly stir the second compound after adding it to aqueous solution B, the elapsed time T1 may be, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more.

The thickness of the adhesive when applied can be set to any value, for example, so that an adhesive layer of the desired thickness is obtained after drying. The thickness of the adhesive layer made of adhesive is preferably 0.01 μm or more and 7 μm or less, more preferably 0.01 μm or more and 5 μm or less, even more preferably 0.01 μm or more and 2 μm or less, and most preferably 0.01 μm or more and 1 μm or less. There are no particular limitations on the application method, and various methods such as the roll method, spray method, and dipping method can be used.

After applying the adhesive, the polarizing element and the transparent protective film are laminated together using a roll laminator or similar. After lamination, the laminate is dried to form a dried adhesive layer. The drying temperature is, for example, 5 to 150°C, preferably 30 to 120°C, and the drying time is, for example, 120 seconds or more, preferably 300 seconds or more.

It is preferable to control the temperature during the adhesive preparation process, adhesive application process, and process of laminating the polarizing element and transparent protective film, for example, within the range of 20 to 50°C. By controlling the temperature, the water resistance of the adhesive layer can be further improved.

In a configuration in which transparent protective films are bonded to both sides of a polarizing element via adhesive layers, the adhesive layer on one side of the polarizing element may be formed using an adhesive other than the adhesive prepared in the adhesive preparation process described above. Examples of other adhesives include active energy ray-curable adhesives.

(Active energy ray curing adhesive)
The active energy ray-curable adhesive is an adhesive that cures upon irradiation with active energy rays such as ultraviolet rays, and examples thereof include adhesives containing a polymerizable compound and a photopolymerization initiator, adhesives containing a photoreactive resin, and adhesives containing a binder resin and a photoreactive crosslinking agent. Examples of the polymerizable compound include photopolymerizable monomers such as photocurable epoxy monomers, photocurable acrylic monomers, and photocurable urethane monomers, as well as oligomers derived from these monomers. Examples of the photopolymerization initiator include compounds containing substances that generate active species such as neutral radicals, anion radicals, and cation radicals upon irradiation with active energy rays such as ultraviolet rays.

<Polarizing plate>
The polarizing plate according to the present invention comprises a polarizing element having a polyvinyl alcohol-based resin layer on which a dichroic dye is adsorbed and aligned, and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer. The polarizing plate may have an adjusted moisture content, and preferably has at least one of the following characteristics (a) and (b):
(a) The moisture content of the polarizing element is equal to or greater than the equilibrium moisture content at a temperature of 20° C. and a relative humidity of 30%, and is equal to or less than the equilibrium moisture content at a temperature of 20° C. and a relative humidity of 70%.
(b) The moisture content of the polarizing plate is equal to or greater than the equilibrium moisture content at a temperature of 20° C. and a relative humidity of 30%, and is equal to or less than the equilibrium moisture content at a temperature of 20° C. and a relative humidity of 70%.

<Polarizing element>
Known polarizing elements can be used as the polarizing element in which a dichroic dye is adsorbed and oriented in a layer containing a PVA-based resin (hereinafter also referred to as a "PVA-based resin layer"). Examples of polarizing elements include a stretched film obtained by dyeing a PVA-based resin film with a dichroic dye and uniaxially stretching it, and a stretched layer obtained by using a laminated film having a coating layer formed by applying a coating liquid containing a PVA-based resin to a substrate film, dyeing the coating layer with a dichroic dye, and uniaxially stretching the laminated film. Stretching may be performed after dyeing with the dichroic dye, or may be performed while dyeing, or may be performed after stretching and then dyeing.

PVA resins are obtained by saponifying polyvinyl acetate resins. Examples of polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as copolymers of vinyl acetate with other copolymerizable monomers. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins such as ethylene, vinyl ethers, and unsaturated sulfonic acids.

The PVA-based resin layer is preferably formed from a PVA-based resin with a boron adsorption rate of 5.70% by mass or more. That is, the boron adsorption rate of the PVA-based resin in the raw material stage before dyeing or stretching is 5.70% by mass or more. Using such a PVA-based resin reduces the decrease in transmittance, even when exposed to high-temperature environments, such as temperatures of 105°C. Furthermore, the boron adsorption rate of the PVA-based resin is preferably 10% by mass or less. By using such a PVA-based resin to produce polarizing elements, the boric acid concentration in the boric acid treatment bath can be reduced and the treatment time for the boric acid treatment can be shortened, making it easier to obtain the desired polarizing elements and improving productivity. A boron adsorption rate of 10% by mass or less of the PVA-based resin allows an appropriate amount of boron to be incorporated into the PVA-based resin layer, thereby reducing the shrinkage force of the polarizing element. As a result, when incorporated into an image display device, problems such as peeling between the polarizing plate and other components, such as the front panel, are reduced. The boron adsorption rate of PVA-based resin can be measured by the method described in the examples below.

The boron adsorption rate of a PVA-based resin is a characteristic that reflects the spacing between molecular chains and the crystalline structure of the PVA-based resin. PVA-based resins with a boron adsorption rate of 5.70% by mass or higher are thought to have wider spacing between molecular chains and fewer PVA-based resin crystals than PVA-based resins with a boron adsorption rate of less than 5.70% by mass. This makes it easier for boron, first metal ions, and second metal ions to penetrate into the PVA-based resin layer, presumably making it easier to prevent polyenization in high-temperature environments.

The boron adsorption rate of PVA-based resins can be adjusted, for example, by subjecting the PVA-based resin to pretreatments such as hot water treatment, acidic solution treatment, ultrasonic irradiation treatment, and radiation irradiation treatment before manufacturing polarizing elements. These treatments can increase the spacing between molecular chains in the PVA-based resin or destroy the crystalline structure. Hot water treatments include, for example, immersing the resin in pure water at 30°C to 100°C for 1 to 90 seconds, followed by drying. Acidic solution treatments include, for example, immersing the resin in a boric acid solution with a concentration of 10% to 20% by weight for 1 to 90 seconds, followed by drying. Ultrasonic treatments include, for example, irradiating the resin with ultrasonic waves at a frequency of 20 to 29 kc for 30 seconds to 10 minutes at an output of 200 W to 500 W. Ultrasonic treatments can be performed in a solvent such as water.

The degree of saponification of the PVA-based resin is preferably about 85 mol% or more, more preferably about 90 mol% or more, and even more preferably about 99 mol% or more and 100 mol% or less. The degree of polymerization of the PVA-based resin is, for example, 1,000 to 10,000, preferably 1,500 to 5,000. The PVA-based resin may be modified, such as polyvinyl formal, polyvinyl acetal, or polyvinyl butyral modified with aldehydes.

The thickness of the polarizing element is preferably 3 μm or more and 35 μm or less, more preferably 4 μm or more and 30 μm or less, and even more preferably 5 μm or more and 25 μm or less. A polarizing element thickness of 35 μm or less can suppress the effect of polyenation of the PVA-based resin on degradation of optical properties in high-temperature environments. A polarizing element thickness of 3 μm or more makes it easier to achieve the desired optical properties.

The polarizing element preferably contains a first compound and a second compound. In this embodiment, the polarizing element and the transparent protective film are bonded together using an adhesive layer formed from an adhesive containing the first compound and the second compound. Therefore, it is possible that a portion of the first compound and a portion of the second compound migrated from the adhesive layer and are contained in the polarizing element. The first compound and the second compound in the polarizing element may include compounds added during the manufacturing process of the polarizing element. By providing an adhesive layer containing the first compound and the second compound, the transmittance of the polarizing plate is less likely to decrease even when exposed to a high-temperature environment. Furthermore, by providing an adhesive layer containing the first compound and the second compound, a decrease in the degree of polarization of the polarizing plate can be suppressed even when exposed to a high-temperature environment. When two polarizing plates are arranged in a crossed Nicol relationship, a decrease in the degree of polarization of the polarizing plate is likely to cause light leakage (hereinafter also referred to as "cross leakage"). However, according to the present invention, the degree of polarization is less likely to decrease even when exposed to a high-temperature environment, and therefore cross leakage is also easily suppressed. This is presumably due to the synergistic effect of the first compound and the second compound contained in the polarizing element, which suppresses polyenization of the PVA-based resin.

Methods for incorporating the first compound and the second compound during the production of a polarizing element include immersing the PVA-based resin layer in a treatment solvent containing the first compound and/or the second compound, or spraying, pouring, or dripping the treatment solvent onto the PVA-based resin layer. Among these, the method of immersing the PVA-based resin layer in a treatment solvent containing both the first compound and the second compound is preferred. Specific examples of the first compound and the second compound include those exemplified above as compounds to be incorporated into the adhesive.

The step of immersing the PVA-based resin layer in a treatment solvent containing the first compound and the second compound may be carried out simultaneously with the swelling, stretching, dyeing, crosslinking, washing, and other steps in the method for producing a polarizing element described below, or may be carried out separately from these steps. The step of incorporating the first compound and the second compound into the PVA-based resin layer is preferably carried out after dyeing the PVA-based resin layer with iodine, and more preferably carried out simultaneously with the crosslinking step after dyeing. This method results in little change in hue and minimizes the impact on the optical properties of the polarizing element.

To incorporate the first compound and the second compound into the polarizing element, they may be added both during the manufacturing process of the polarizing element and to the adhesive. For example, an adhesive containing the first compound and the second compound may be used, and at least one of the first compound and the second compound may be added during the manufacturing process of the polarizing element.

Polarizing elements typically contain potassium ions (also referred to as "first metal ions") and preferably also contain metal ions other than potassium ions (also referred to as "second metal ions"). The content of the second metal ions in the polarizing element is preferably 0.05% by mass to 10.0% by mass, more preferably 0.05% by mass to 8.0% by mass, and even more preferably 0.1% by mass to 6.0% by mass. If the second metal ion content of the polarizing element exceeds 10.0% by mass, the polarization degree may decrease in high-temperature, high-humidity environments. Furthermore, if the second metal ion content is less than 0.05% by mass, the durability improvement effect in high-temperature environments may be insufficient. The content of the second metal ions in the polarizing element can be calculated as the mass fraction (mass%) of the metal elements relative to the mass of the polarizing element, for example, by inductively coupled plasma (ICP) atomic emission spectroscopy. The metal element is thought to exist in the polarizing element either as a metal ion or in a state where it forms a crosslinked structure with a component of the polyvinyl alcohol resin, but the content of the second metal ion referred to here is the value as a metal atom.

The second metal ion is not limited as long as it is a metal ion other than potassium ion, but is preferably an ion of a metal other than an alkali metal. In particular, from the standpoint of color adjustment and durability, it is preferable to include at least one metal ion of a transition metal such as cobalt, nickel, zinc, chromium, aluminum, copper, manganese, or iron. Of these metal ions, zinc ions are preferred from the standpoint of color adjustment and heat resistance.

The boron content of the polarizing element is preferably 2.4% by mass or more. The boron content is preferably 3.9% by mass or more and 8.0% by mass or less, more preferably 4.2% by mass or more and 7.0% by mass or less, and even more preferably 4.4% by mass or more and 6.0% by mass or less. If the boron content of the polarizing element exceeds 8.0% by mass, the contraction force of the polarizing element increases, which may result in problems such as peeling between the polarizing element and other components, such as the front panel, when the polarizing element is incorporated into an image display device. Furthermore, if the boron content is less than 2.4% by mass, the desired optical characteristics may not be achieved. The boron content of the polarizing element can be calculated as the mass fraction (mass%) of boron relative to the mass of the polarizing element, for example, using inductively coupled plasma (ICP) emission spectroscopy. Boron is thought to exist in the polarizing element in the form of boric acid or in the form of a crosslinked structure formed between boric acid and the components of the polyvinyl alcohol resin, but the boron content referred to here is the value in terms of boron atoms (B).

When the boron content of the polarizing element is between 2.4% and 8.0% by mass, the decrease in transmittance is further suppressed even when exposed to a high-temperature environment as a component of an image display device with an interlayer filling structure. This is presumably because when the boron content of the polarizing element is between 2.4% and 8.0% by mass, polyenization is less likely to occur even in a high-temperature environment, suppressing the decrease in transmittance.

From the viewpoint of suppressing deterioration of the optical properties of the polarizing element in high-temperature environments, the potassium ion content in the polarizing element is preferably 0.28% by mass or more, more preferably 0.32% by mass or more, and even more preferably 0.34% by mass or more. From the viewpoint of suppressing hue change in high-temperature environments, the potassium ion content is preferably 0.60% by mass or less, more preferably 0.55% by mass or less, and even more preferably 0.50% by mass or less. The potassium ion content can be measured in the same manner as the second metal ion content, and the potassium ion content referred to here is the value in terms of potassium atoms.

(Method for manufacturing polarizing element)
The method for producing a polarizing element is not particularly limited, but typical examples include a method in which a pre-wound PVA type resin film is fed out and stretched, dyed, crosslinked, etc. (hereinafter referred to as "production method 1"), and a method in which a coating liquid containing a PVA type resin is applied to a substrate film to form a PVA type resin layer as a coating layer, and the resulting laminate is stretched (hereinafter referred to as "production method 2").

Production method 1 can be achieved by uniaxially stretching a PVA-based resin film, dyeing the PVA-based resin film with a dichroic dye such as iodine to allow the dichroic dye to adsorb, treating the PVA-based resin film with the adsorbed dichroic dye in an aqueous boric acid solution, and rinsing the film with water after the treatment with the aqueous boric acid solution.

The swelling process is a treatment process in which the PVA-based resin film is immersed in a swelling bath. This process not only removes dirt and blocking agents from the surface of the PVA-based resin film, but also reduces dyeing unevenness by swelling the PVA-based resin film. The swelling bath typically uses a medium primarily composed of water, distilled water, pure water, or other water. The swelling bath may contain surfactants, alcohol, or other additives, as appropriate, according to conventional methods. Potassium iodide may be used in the swelling bath to control the potassium content of the polarizing element. In this case, the concentration of potassium iodide in the swelling bath is preferably 1.5% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

The temperature of the swelling bath is preferably about 10°C to 60°C, more preferably about 15°C to 45°C, and even more preferably about 18°C to 30°C. The immersion time in the swelling bath cannot be determined in general because the degree of swelling of the PVA-based resin film is affected by the temperature of the swelling bath, but is preferably about 5 seconds to 300 seconds, more preferably about 10 seconds to 200 seconds, and even more preferably about 20 seconds to 100 seconds. The swelling process may be carried out once, or multiple times as necessary.

The dyeing process is a treatment process in which the PVA-based resin film is immersed in a dye bath (iodine solution), allowing dichroic dyes such as iodine to be adsorbed and oriented in the PVA-based resin film. The iodine solution is typically preferably an aqueous iodine solution, containing iodine and an iodide as a solubilizing agent. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Of these, potassium iodide is preferred from the perspective of controlling the potassium content in the polarizing element.

The iodine concentration in the dye bath is preferably approximately 0.01% by mass to 1% by mass, and more preferably approximately 0.02% by mass to 0.5% by mass. The iodide concentration in the dye bath is preferably approximately 0.01% by mass to 10% by mass, more preferably approximately 0.05% by mass to 5% by mass, and even more preferably approximately 0.1% by mass to 3% by mass.

The temperature of the dye bath is preferably about 10°C to 50°C, more preferably about 15°C to 45°C, and even more preferably about 18°C to 30°C. The immersion time in the dye bath cannot be determined in general because the degree of dyeing of the PVA-based resin film is affected by the temperature of the dye bath, but is preferably about 10 seconds to 300 seconds, and more preferably about 20 seconds to 240 seconds. The dyeing process may be carried out once, or multiple times as necessary.

The crosslinking process is a treatment step in which the PVA-based resin film dyed in the dyeing process is immersed in a treatment bath (crosslinking bath) containing a boron compound. The boron compound crosslinks the polyvinyl alcohol-based resin film, allowing iodine molecules or dye molecules to be adsorbed to the crosslinked structure. Examples of boron compounds include boric acid, borate salts, and borax. The crosslinking bath is generally an aqueous solution, but it may also be a mixed solution of water and an organic solvent that is miscible with water. From the perspective of controlling the potassium content in the polarizing element, the crosslinking bath preferably contains potassium iodide.

The concentration of the boron compound in the crosslinking bath is preferably approximately 1% by mass to 15% by mass, more preferably approximately 1.5% by mass to 10% by mass, and even more preferably approximately 2% by mass to 5% by mass. When potassium iodide is used in the crosslinking bath, the concentration of potassium iodide in the crosslinking bath is preferably approximately 1% by mass to 15% by mass, more preferably approximately 1.5% by mass to 10% by mass, and even more preferably approximately 2% by mass to 5% by mass.

The temperature of the crosslinking bath is preferably about 20°C or higher and 70°C or lower, and more preferably about 30°C or higher and 60°C or lower. The immersion time in the crosslinking bath cannot be determined in general because the degree of crosslinking of the PVA-based resin film is affected by the temperature of the crosslinking bath, but is preferably about 5 seconds or higher and 300 seconds or lower, and more preferably about 10 seconds or higher and 200 seconds or lower. The crosslinking process may be carried out only once, or multiple times as necessary.

The stretching process is a processing step in which a PVA-based resin film is stretched to a predetermined magnification in at least one direction. Generally, the PVA-based resin film is stretched uniaxially in the conveying direction (longitudinal direction). There are no particular restrictions on the stretching method, and either wet stretching or dry stretching can be used. The stretching process may be carried out once, or multiple times as necessary. The stretching process may be carried out at any stage in the production of a polarizing element.

The treatment bath (stretching bath) used in the wet stretching method typically uses a solvent such as water or a mixture of water and a water-miscible organic solvent. The stretching bath preferably contains potassium iodide to control the potassium content in the polarizing element. When potassium iodide is used in the stretching bath, the concentration of potassium iodide in the stretching bath is preferably approximately 1% to 15% by mass, more preferably approximately 2% to 10% by mass, and even more preferably approximately 3% to 6% by mass. The treatment bath (stretching bath) may contain a boron compound to prevent film breakage during stretching. When a boron compound is contained, the concentration of the boron compound in the stretching bath is preferably approximately 1% to 15% by mass, more preferably approximately 1.5% to 10% by mass, and even more preferably approximately 2% to 5% by mass.

The temperature of the stretching bath is preferably 25 to 80°C, more preferably 40 to 80°C, even more preferably 50 to 75°C, and particularly preferably 65 to 75°C. The immersion time in the stretching bath cannot be determined in general because the degree of stretching of the PVA-based resin film is affected by the temperature of the stretching bath, but is preferably approximately 10 to 800 seconds, more preferably approximately 30 to 500 seconds. The stretching treatment in the wet stretching method may be carried out together with one or more of the following treatment processes: swelling, dyeing, crosslinking, and washing.

Dry stretching methods include, for example, inter-roll stretching, heated roll stretching, and compression stretching. Dry stretching may also be performed in conjunction with a drying process.

The total stretching ratio (cumulative stretching ratio) applied to the polyvinyl alcohol-based resin film can be set appropriately depending on the purpose, but is preferably approximately 2 to 7 times, more preferably approximately 3 to 6.8 times, and even more preferably approximately 3.5 to 6.5 times.

The cleaning process is a treatment process in which the polyvinyl alcohol-based resin film is immersed in a cleaning bath, which allows the removal of foreign matter remaining on the surface of the polyvinyl alcohol-based resin film. The cleaning bath typically uses a medium whose main component is water, such as distilled water or pure water. Furthermore, from the perspective of controlling the potassium content in the polarizing element, it is preferable to use potassium iodide in the cleaning bath. In this case, the concentration of potassium iodide in the cleaning bath is preferably approximately 1% by mass or more and 10% by mass or less, more preferably approximately 1.5% by mass or more and 4% by mass or less, and even more preferably approximately 1.8% by mass or more and 3.8% by mass or less.

The temperature of the cleaning bath is preferably about 5°C to 50°C, more preferably about 10°C to 40°C, and even more preferably about 15°C to 30°C. The immersion time in the cleaning bath cannot be determined in general because the degree of cleaning of the PVA-based resin film is affected by the temperature of the cleaning bath, but is preferably about 1 second to 100 seconds, more preferably about 2 seconds to 50 seconds, and even more preferably about 3 seconds to 20 seconds. The cleaning process may be carried out once, or multiple times as necessary.

Furthermore, it is preferable to include a metal ion treatment step among the above steps, or as a separate step from the above steps. The metal ion treatment step is carried out by immersing the polyvinyl alcohol-based resin film in an aqueous solution containing a metal salt of the second metal ion. The second metal ion is incorporated into the polyvinyl alcohol-based resin film by the metal ion treatment step.

The second metal ion is not limited as long as it is a metal ion other than potassium ion, but is preferably an ion of a metal other than an alkali metal. In particular, from the standpoint of color adjustment and durability, it is preferable to include at least one metal ion of a transition metal such as cobalt, nickel, zinc, chromium, aluminum, copper, manganese, or iron. Of these metal ions, zinc ion is preferred from the standpoint of color adjustment and heat resistance. Examples of zinc salts include zinc halides such as zinc chloride and zinc iodide, zinc sulfate, and zinc acetate.

A metal salt solution is used in the metal ion treatment process. Below, we will explain immersion in a zinc-containing solution as a representative example of a metal ion treatment process using a zinc salt aqueous solution.

The zinc ion concentration in the zinc salt aqueous solution is approximately 0.1 to 10% by mass, preferably 0.3 to 7% by mass. Furthermore, it is preferable to use an aqueous zinc salt solution containing potassium ions and iodine ions, such as potassium iodide, as this facilitates impregnation with zinc ions. The potassium iodide concentration in the zinc salt solution is preferably approximately 0.1 to 10% by mass, more preferably 0.2 to 5% by mass.

When immersing in a zinc-containing solution, the temperature of the zinc salt solution is typically around 15 to 85°C, preferably 25 to 70°C. The immersion time is typically around 1 to 120 seconds, preferably 3 to 90 seconds. When immersing in a zinc-containing solution, the zinc content of the polyvinyl alcohol-based resin film is adjusted to be within the above range by adjusting conditions such as the concentration of the zinc salt solution, the immersion temperature of the polyvinyl alcohol-based resin film in the zinc salt solution, and the immersion time. There are no particular restrictions on when immersion in a zinc-containing solution is performed. Immersion in a zinc-containing solution may be performed alone, or may be performed simultaneously with at least one of the dyeing, crosslinking, and stretching steps by adding zinc salt to the dye bath, crosslinking bath, or stretching bath.

The drying process is a process in which the PVA-based resin film washed in the washing process is dried to obtain a polarizing element. Drying can be carried out by any appropriate method, such as natural drying, air drying, or heat drying.

Production method 2 involves applying a coating liquid containing a PVA-based resin onto a substrate film, uniaxially stretching the resulting laminated film, dyeing the PVA-based resin layer of the uniaxially stretched laminated film with a dichroic dye to adsorb it and form a polarizing element, treating the film with the adsorbed dichroic dye in an aqueous boric acid solution, and rinsing with water after treatment with the aqueous boric acid solution. The substrate film used to form the polarizing element may also be used as a protective layer for the polarizing element. If necessary, the substrate film may be peeled off and removed from the polarizing element.

<Transparent protective film>
The transparent protective film used in this embodiment (hereinafter also simply referred to as "protective film") is attached to at least one surface of the polarizing element via an adhesive layer. This transparent protective film may be attached to one or both surfaces of the polarizing element, but is preferably attached to both surfaces.

The protective film may also have other optical functions, and may be formed into a laminated structure with multiple layers stacked together. From the perspective of optical properties, a thin protective film is preferable, but if it is too thin, the strength decreases and processability becomes poor. The appropriate film thickness is 5 μm or more and 100 μm or less, preferably 10 μm or more and 80 μm or less, and more preferably 15 μm or more and 70 μm or less.

The protective film can be a cellulose acylate film, a film made of a polycarbonate resin, a film made of a cycloolefin resin such as norbornene, a (meth)acrylic polymer film, or a polyester resin film such as polyethylene terephthalate. When protective films are attached to both sides of a polarizing element using a water-based adhesive such as a PVA adhesive, it is preferable that the protective film on at least one side be either a cellulose acylate film or a (meth)acrylic polymer film from the standpoint of moisture permeability, with a cellulose acylate film being particularly preferred.

At least one of the protective films may have a retardation function for purposes such as viewing angle compensation. In this case, the protective film itself may have a retardation function, or may have a separate retardation layer, or a combination of both. The film with a retardation function may be attached directly to the polarizing element via an adhesive, or it may be attached via a pressure-sensitive adhesive or adhesive via another protective film that is attached to the polarizing element.

<Second Aspect>
In a second aspect of the method for producing a polarizing plate according to the present invention, the adhesive preparation step includes a preparatory step of preparing an aqueous solution A containing a PVA-based resin, a first addition step of adding a first compound to the aqueous solution A to prepare an aqueous solution B, and a second addition step of adding a second compound to the aqueous solution B to prepare an adhesive. The laminating step is performed so that the elapsed time (hereinafter also referred to as "elapsed time T2") from the addition of the second compound to the aqueous solution B is 200 hours or less. The second aspect differs from the first aspect in that a third compound is added in at least one of the first addition step and the second addition step, and in the range of the elapsed time T2. Hereinafter, only the differences between the first aspect and the second aspect will be described.

When the third compound is added in the first addition step, the third compound can be added to aqueous solution A before, after, or simultaneously with the addition of the first compound. When the third compound is added in the second addition step, the third compound can be added to aqueous solution B before, after, or simultaneously with the addition of the second compound.

In the second embodiment, by including a third compound in the adhesive, the elapsed time T2 until the lamination step is performed can be set longer than the elapsed time T1 in the first embodiment. In the second embodiment, if the elapsed time T2 is 200 hours or less, an adhesive layer with excellent water resistance can be obtained. This is presumably because the inclusion of a third compound in the adhesive can suppress decomposition of the second compound in the adhesive, thereby improving the pot life of the adhesive.

The elapsed time T2 refers to the time that elapses from the time the second compound is added to aqueous solution B in the second addition step until the time the polarizing element and the transparent protective film are bonded together via the adhesive layer in the bonding step, and does not include the time required to dry the adhesive layer.

The elapsed time T2 is preferably 190 hours or less, and more preferably 180 hours or less. Since it is desirable to thoroughly stir the second compound after adding it to aqueous solution B, the elapsed time T2 may be, for example, 1 minute or more, 5 minutes or more, or 10 minutes or more.

(Third compound)
The third compound is a dicarboxylic acid. Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, tartaric acid, glutamic acid, malic acid, maleic acid, fumaric acid, itaconic acid, muconic acid, 1,4-cyclohexanedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 2,5-pyridinedicarboxylic acid, 3,5-pyridinedicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenylmethanedicarboxylic acid, oxaloacetic acid, methylfumaric acid, and 2,6-pyridinedicarboxylic acid. Among these, citric acid, malic acid, maleic acid, or tartaric acid is preferably used. These dicarboxylic acids may be used alone or in combination of two or more.

The content of the third compound in the adhesive is, for example, 0.01 to 400 parts by weight, preferably 0.05 to 50 parts by weight, and more preferably 0.1 to 10 parts by weight, per 100 parts by weight of the PVA-based resin. If the content is less than 0.01 parts by weight, the pot life may not be sufficiently improved, or the effect of suppressing polyenization of the polarizing element in a high-temperature environment may not be sufficient. On the other hand, if the content exceeds 400 parts by weight, the third compound may precipitate after the polarizing plate is produced.

In the adhesive, the content of the third compound is preferably 0.001 to 100 parts by mass, and more preferably 0.01 to 80 parts by mass, per part by mass of the second compound, dialdehyde. If the content is less than 0.001 part by mass, the pot life improvement effect may be insufficient. On the other hand, if the content exceeds 100 parts by mass, the third compound may precipitate after the polarizing plate is produced.

The third compound may be contained in the polarizing element, similar to the first compound and second compound described in the first embodiment. The method for incorporating the third compound into the polarizing element can be the same as the method for incorporating the first compound and second compound described in the first embodiment.

[Configuration of image display device]
The polarizing plate manufactured as described above is used in various image display devices, such as liquid crystal display devices and organic EL display devices. When an image display device has an interlayer filling configuration in which both surfaces of the polarizing plate are in contact with a layer other than an air layer, specifically a solid layer such as a pressure-sensitive adhesive layer, the transmittance is likely to decrease in a high-temperature environment. An image display device using the polarizing plate of this embodiment can suppress the decrease in transmittance of the polarizing plate in a high-temperature environment, even when the interlayer filling configuration is used. An example of an image display device includes an image display cell, a first pressure-sensitive adhesive layer laminated on the viewing-side surface of the image display cell, and a polarizing plate laminated on the viewing-side surface of the first pressure-sensitive adhesive layer. Such an image display device may further include a second pressure-sensitive adhesive layer laminated on the viewing-side surface of the polarizing plate and a transparent member laminated on the viewing-side surface of the second pressure-sensitive adhesive layer. In particular, the polarizing plate of this embodiment is suitable for use in an image display device having an interlayer filling configuration in which a transparent member is disposed on the viewing side of the image display device, the polarizing plate and the image display cell are bonded together by the first pressure-sensitive adhesive layer, and the polarizing plate and the transparent member are bonded together by the second pressure-sensitive adhesive layer. In this specification, either or both of the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer may be simply referred to as a “pressure-sensitive adhesive layer.” Note that the member used to bond the polarizing plate to the image display cell and the member used to bond the polarizing plate to the transparent member are not limited to a pressure-sensitive adhesive layer and may be an adhesive layer.

<Image display cell>
Examples of image display cells include liquid crystal cells and organic EL cells. The liquid crystal cell may be a reflective liquid crystal cell that utilizes external light, a transmissive liquid crystal cell that utilizes light from a light source such as a backlight, or a semi-transmissive semi-reflective liquid crystal cell that utilizes both external light and light from the light source. When the liquid crystal cell utilizes light from a light source, the image display device (liquid crystal display device) has a polarizing plate disposed on the opposite side of the viewing side of the image display cell (liquid crystal cell), and further has a light source disposed thereon. The polarizing plate on the light source side and the liquid crystal cell are preferably bonded together via an appropriate adhesive layer. The liquid crystal cell may be driven in any type, such as VA mode, IPS mode, TN mode, STN mode, or bend orientation (π type).

A suitable organic EL cell is one in which a light-emitting body (organic electroluminescence light-emitting body) is formed by sequentially laminating a transparent electrode, an organic light-emitting layer, and a metal electrode on a transparent substrate. The organic light-emitting layer is a laminate of various organic thin films, and various layer configurations can be used, such as a laminate of a hole-injection layer made of a triphenylamine derivative or the like and a light-emitting layer made of a fluorescent organic solid such as anthracene, a laminate of such a light-emitting layer and an electron-injection layer made of a perylene derivative or the like, or a laminate of a hole-injection layer, a light-emitting layer, and an electron-injection layer.

<Laminating the image display cell and polarizing plate>
A pressure-sensitive adhesive layer (pressure-sensitive adhesive sheet) is preferably used to bond the image display cell and the polarizing plate. Among these, a method of bonding a polarizing plate with a pressure-sensitive adhesive layer, in which a pressure-sensitive adhesive layer is attached to one side of the polarizing plate, to the image display cell is preferred from the viewpoint of workability, etc. The pressure-sensitive adhesive layer can be attached to the polarizing plate by an appropriate method. Examples of such methods include a method in which a pressure-sensitive adhesive solution of about 10% by mass to 40% by mass is prepared by dissolving or dispersing a base polymer or a composition thereof in a solvent consisting of a single or mixture of appropriate solvents such as toluene or ethyl acetate, and then the solution is directly applied to the polarizing plate by an appropriate spreading method such as a casting method or a coating method, and a method in which a pressure-sensitive adhesive layer is formed on a separator and then transferred to the polarizing plate.

<Adhesive Layer>
The pressure-sensitive adhesive layer may consist of one layer or two or more layers, but preferably consists of one layer. The pressure-sensitive adhesive layer can be composed of a pressure-sensitive adhesive composition whose main component is a (meth)acrylic resin, a rubber resin, a urethane resin, an ester resin, a silicone resin, or a polyvinyl ether resin. Among these, a pressure-sensitive adhesive composition whose base polymer is a (meth)acrylic resin, which is excellent in transparency, weather resistance, heat resistance, etc., is preferred. The pressure-sensitive adhesive composition may be an active energy ray-curable or thermosetting type.

The (meth)acrylic resin (base polymer) used in the pressure-sensitive adhesive composition is preferably a polymer or copolymer containing one or more (meth)acrylic acid esters as monomers, such as butyl (meth)acrylate, ethyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. It is preferable to copolymerize a polar monomer into the base polymer. Examples of polar monomers include monomers having a carboxyl group, hydroxyl group, amide group, amino group, or epoxy group, such as (meth)acrylic acid compounds, 2-hydroxypropyl (meth)acrylate compounds, hydroxyethyl (meth)acrylate compounds, (meth)acrylamide compounds, N,N-dimethylaminoethyl (meth)acrylate compounds, and glycidyl (meth)acrylate compounds.

The PSA composition may contain only the base polymer, but typically also contains a crosslinking agent. Examples of crosslinking agents include divalent or higher metal ions that form metal carboxylates with carboxyl groups, polyamine compounds that form amide bonds with carboxyl groups, polyepoxy compounds or polyols that form ester bonds with carboxyl groups, and polyisocyanate compounds that form amide bonds with carboxyl groups. Of these, polyisocyanate compounds are preferred.

Active energy ray-curable pressure-sensitive adhesive compositions have the property of curing when irradiated with active energy rays such as ultraviolet rays or electron beams. They retain adhesive properties even before exposure to active energy rays, allowing them to adhere to adherends such as films, and they have the property of curing when irradiated with active energy rays, allowing the adhesive strength to be adjusted. Active energy ray-curable pressure-sensitive adhesive compositions are preferably UV-curable. Active energy ray-curable pressure-sensitive adhesive compositions further contain an active energy ray-polymerizable compound in addition to a base polymer and a crosslinking agent. If necessary, they may also contain a photopolymerization initiator, a photosensitizer, etc.

The pressure-sensitive adhesive composition may contain additives such as fine particles for imparting light scattering properties, beads (resin beads, glass beads, etc.), glass fibers, resins other than the base polymer, tackifiers, fillers (metal powders and other inorganic powders, etc.), antioxidants, UV absorbers, dyes, pigments, colorants, defoamers, corrosion inhibitors, and photopolymerization initiators.

The adhesive layer can be formed by applying a diluted solution of the adhesive composition in an organic solvent onto the surface of a substrate film, image display cell, or polarizing plate, and then drying. The substrate film is generally a thermoplastic resin film, and a typical example is a release-treated separate film. The separate film can be, for example, a film made of a resin such as polyethylene terephthalate, polybutylene terephthalate, polycarbonate, or polyacrylate, with a release treatment such as silicone treatment applied to the surface on which the adhesive layer is to be formed.

The adhesive composition may be applied directly to the release-treated surface of the separate film to form an adhesive layer, and this adhesive layer with separate film may then be laminated on the surface of the polarizer. The adhesive composition may be applied directly to the surface of the polarizing plate to form an adhesive layer, and a separate film may then be laminated on the outer surface of the adhesive layer.

When providing a pressure-sensitive adhesive layer on the surface of a polarizing plate, it is preferable to subject the bonding surface of the polarizing plate and/or the bonding surface of the pressure-sensitive adhesive layer to a surface activation treatment such as plasma treatment or corona treatment, with corona treatment being more preferable.

Alternatively, a pressure-sensitive adhesive composition may be applied to a second separate film to form a pressure-sensitive adhesive layer, and a separate film may be laminated on the formed pressure-sensitive adhesive layer to prepare a pressure-sensitive adhesive sheet. After peeling the second separate film from this pressure-sensitive adhesive sheet, the pressure-sensitive adhesive layer with the separate film may be laminated on a polarizing plate. The second separate film used has weaker adhesion to the pressure-sensitive adhesive layer than the separate film and is therefore easier to peel off.

The thickness of the adhesive layer is not particularly limited, but is preferably 1 μm or more and 100 μm or less, more preferably 3 μm or more and 50 μm or less, and may be 20 μm or more.

<Transparent materials>
Examples of transparent members arranged on the viewing side of an image display device include a transparent plate (window layer) and a touch panel. A transparent plate having appropriate mechanical strength and thickness is used as the transparent plate. Examples of such transparent plates include transparent resin plates such as polyimide resins, acrylic resins, and polycarbonate resins, and glass plates. A functional layer such as an anti-reflection layer may be laminated on the viewing side of the transparent plate. Furthermore, when the transparent plate is a transparent resin plate, a hard coat layer may be laminated to increase physical strength, or a low-moisture permeability layer may be laminated to reduce moisture permeability. Examples of touch panels include various touch panels such as resistive, capacitive, optical, and ultrasonic touch panels, as well as glass or transparent resin plates equipped with touch sensor functions. When a capacitive touch panel is used as the transparent member, it is preferable to provide a transparent plate made of glass or a transparent resin plate on the viewing side of the touch panel.

<Bonding of polarizing plate and transparent member>
A pressure-sensitive adhesive or an active energy ray-curable adhesive is preferably used to bond the polarizing plate and the transparent member. When a pressure-sensitive adhesive is used, the pressure-sensitive adhesive can be applied by an appropriate method. Specific application methods include, for example, the application method of the pressure-sensitive adhesive layer used in bonding the image display cell and the polarizing plate described above.

When using an active energy ray-curable adhesive, a suitable method is to place a dam material around the periphery of the image display panel to prevent the adhesive solution from spreading before curing, place the transparent member on the dam material, and then inject the adhesive solution. After the adhesive solution is injected, it is aligned and degassed as necessary, and then irradiated with active energy rays to cause curing.

The present invention will be specifically explained below based on the following examples. The materials, reagents, amounts of substances and their proportions, procedures, etc. shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the present invention is not limited to or by the following examples.

(1) Measurement of the thickness of the polarizing element:
Measurements were made using a digital micrometer "MH-15M" manufactured by Nikon Corporation.

(2) Measurement of luminosity-corrected polarization degree, luminosity-corrected single transmittance, and hue of polarizing plate:
Measurement was carried out using a spectrophotometer equipped with an integrating sphere ("V7100" manufactured by JASCO Corporation, 2-degree field of view; C light source).

(3) Measurement of boron content:
0.2 g of the polarizing element was dissolved in 200 g of a 1.9% by mass aqueous mannitol solution. The resulting aqueous solution was then titrated with a 1 mol/L aqueous sodium hydroxide solution, and the boron content of the polarizing element was calculated by comparing the amount of the aqueous sodium hydroxide solution required for neutralization with a calibration curve.

(4) Measurement of zinc ion content:
Nitric acid was added to a precisely weighed polarizing element, and the solution obtained by acid decomposition using a Milestone General microwave sample pretreatment device (ETHOS D) was used as the measurement solution. The zinc ion content was calculated by quantifying the zinc concentration in the measurement solution using an Agilent Technologies ICP optical emission spectrometer (5110 ICP-OES) and calculating the zinc mass relative to the polarizing element mass.

(5) Measurement of boron adsorption rate of PVA-based resin film:
A PVA-based resin film cut into a 100 mm square was immersed in pure water at 30°C for 60 seconds and then immersed in an aqueous solution containing 5 parts boric acid at 60°C for 120 seconds. The PVA-based resin film removed from the boric acid solution was dried in an oven at 80°C for 11 minutes. The film was then conditioned at 23°C and 55% RH for 24 hours to obtain a boron-containing PVA film. 0.2 g of the resulting boron-containing PVA-based resin film was dissolved in 200 g of a 1.9% by mass aqueous mannitol solution. The resulting aqueous solution was then titrated with 1 mol/L aqueous sodium hydroxide. The amount of sodium hydroxide required for neutralization was compared with a calibration curve to calculate the boron content of the PVA-based resin film. The boron content of the resulting PVA-based resin film was used as the boron adsorption rate of the PVA-based resin film.

(Fabrication of Polarizing Element 1)
A 30 μm thick polyvinyl alcohol resin film with a boron adsorption rate of 5.71% by mass was immersed in pure water at 21.5 ° C. for 79 seconds (swelling treatment), and then immersed in an aqueous solution containing 1.0 mM iodine at 23 ° C. for 151 seconds (dyeing process). Then, the film was immersed in an aqueous solution containing 2.5/4/100 potassium iodide/boric acid/water at a mass ratio of 68.5 ° C. for 76 seconds (first crosslinking process). Subsequently, the film was immersed in an aqueous solution containing 3/5.5/0.6/100 potassium iodide/boric acid/zinc chloride/water at a mass ratio of 45 ° C. for 11 seconds (second crosslinking process, metal ion treatment process). The film was then immersed in a cleaning bath for cleaning (cleaning process) and dried at 38 ° C. (drying process) to obtain a 12 μm thick polarizing element in which iodine was adsorbed and aligned in the polyvinyl alcohol. The stretching was mainly performed in the dyeing step and the first crosslinking step, and the total stretching ratio was 5.85. The obtained polarizing element had a zinc ion content of 0.17% by mass and a boron content of 4.62% by mass.

(Preparation of PVA solution for adhesive)
50 g of a modified PVA resin containing an acetoacetyl group (Gohsenex Z-410, manufactured by Mitsubishi Chemical Corporation) was dissolved in 950 g of pure water, heated at 90°C for 2 hours, and then cooled to room temperature to obtain a PVA solution for adhesive (preparation step).

(Preparation of Adhesive 1 for Polarizing Plate)
Urea and pure water were added to the PVA solution obtained in the preparation step and stirred (first addition step), and 96 hours after the addition of urea, a commercially available glyoxal solution (glyoxal concentration 40% by mass) and pure water were added and stirred to obtain adhesive 1 (second addition step). The composition of adhesive 1 was as shown in Table 1.

(Preparation of Adhesive 2 for Polarizing Plate)
Urea, maleic acid, and pure water were added to the PVA solution (PVA concentration 3.0% by mass) obtained in the preparation step and stirred (first addition step), and 96 hours after the addition of urea, a commercially available glyoxal solution (glyoxal concentration 40% by mass) and pure water were added and stirred to obtain adhesive 2 (second addition step). The composition of adhesive 2 is shown in Table 1.

(Saponification of Cellulose Acylate Film)
A commercially available cellulose acylate film TJ40UL (manufactured by Fujifilm Corporation; film thickness: 40 μm) was immersed in a 1.5 mol/L aqueous NaOH solution (saponification solution) maintained at 55°C for 2 minutes, and then washed with water. The film was then immersed in a 0.05 mol/L aqueous sulfuric acid solution at 25°C for 30 seconds, and then passed through a water washing bath under running water for 30 seconds to neutralize the film. The film was then drained three times using an air knife. After the water was removed, the film was allowed to remain in a drying zone at 70°C for 15 seconds and dried to produce a saponified film.

(Preparation of Polarizing Plates 1a, 1b, and 1c)
Using a roll laminator, saponified cellulose acylate films were laminated onto both sides of polarizing element 1 via adhesive 1 prepared as above and then stored for a predetermined period of time, followed by drying at 80° C. for 5 minutes to obtain polarizing plates 1a, 1b, and 1c, respectively. For polarizing plates 1a, 1b, and 1c, adhesive 1 was stored so that the elapsed time T1 from the addition of glyoxal during the preparation of adhesive 1 to the removal from the roll laminator was the time shown in Table 2.

(Preparation of Polarizing Plates 2a, 2b, and 2c)
Saponified cellulose acylate films were laminated on both sides of polarizing element 1 using a roll laminator via adhesive 2 prepared as above and then stored for a predetermined time, and then dried at 80° C. for 5 minutes to obtain polarizing plates 2a, 2b, and 2c, respectively. For polarizing plates 2a, 2b, and 2c, adhesive 1 was used and stored so that the elapsed time T2 from the addition of glyoxal during the preparation of adhesive 2 to the removal from the roll laminator was the time shown in Table 3.

(Adjustment of moisture content of polarizing plate (polarizing element))
The polarizing plates 1a, 1b, 1c, 2a, 2b, and 2c obtained above were stored for 72 hours at a temperature of 20°C and relative humidity of 30%, 35%, 40%, 45%, 50%, or 55%. The moisture content was measured using the Karl Fischer method after 66, 69, and 72 hours of storage. The moisture content values remained unchanged under all humidity conditions. Therefore, the moisture content of each polarizing plate can be considered to be the same as the equilibrium moisture content in the 72-hour storage environment used in this experimental example. When the moisture content of the polarizing plate reaches equilibrium in a certain storage environment, the moisture content of the polarizing elements in the polarizing plate can also be considered to have reached equilibrium in that storage environment. Furthermore, when the moisture content of the polarizing elements in the polarizing plate reaches equilibrium in a certain storage environment, the moisture content of the polarizing plate can also be considered to have reached equilibrium in that storage environment.

(Adjustment of Moisture Content of Polarizing Plates 1a, 1b, 1c, 2a, 2b, and 2c)
The moisture contents of polarizing plates 1a, 1b, 1c, 2a, 2b, and 2c were adjusted by storing them for 72 hours under conditions of a temperature of 20°C and a relative humidity of 55% so that the moisture contents of polarizing plates 1a, 1b, 1c, 2a, 2b, and 2c would reach the equilibrium moisture content of 55% at 20°C.

<Water resistance evaluation (hot water immersion test)>
The water resistance test in this example was carried out in accordance with the water resistance test described in [0060] of JP-A-2009-025728.
An acrylic adhesive (product number: #7, manufactured by Lintec Corporation) was applied to one side of the polarizing plate prepared above, and the polarizing plate was cut into strips measuring 50 mm x 20 mm with the absorption axis (stretching direction) of the polarizing plate as the long side, and the dimensions in the long side direction were accurately measured. Here, the evaluation sample exhibited a unique color uniformly over the entire surface due to iodine adsorbed to the polarizing element.
One short side of this sample was held with a gripping tool, and about 80% of the length was immersed in a water bath at 60°C and held there for 4 hours. The sample was then removed from the water bath and wiped dry. Immersion in warm water causes the polarizing element of the polarizing plate to shrink. The degree of shrinkage of this polarizing element was evaluated on a three-point scale based on the following criteria by measuring the distance from the edge of the sample (edge of the protective film) to the edge of the shrunken polarizing element at the center of the short side of the sample. The results are shown in Tables 2 and 3.
The distance from the edge of the sample to the edge of the polarizing element is 1 mm or less: A
The distance from the edge of the sample to the edge of the polarizing element is more than 1 mm and 3 mm or less: B
The distance from the edge of the sample to the edge of the polarizing element is greater than 3 mm: C

Polarizing plates containing the first compound (urea) and the second compound (glyoxal) in the adhesive exhibit excellent water resistance when the elapsed time T1 is 100 hours or less.

It can be seen that polarizing plates containing the first compound (urea), the second compound (glyoxal), and the third compound (maleic acid) in the adhesive have excellent water resistance, as the elapsed time T2 is within 200 hours. Furthermore, comparing the results shown in Tables 1 and 2, it can be seen that by including the third compound (maleic acid) in the adhesive in addition to the first compound (urea) and the second compound (glyoxal), the permissible elapsed time can be extended to provide polarizing plates with excellent water resistance.

Claims (7)

A method for producing a polarizing plate , comprising: a polarizing element having a polyvinyl alcohol-based resin layer in which a dichroic dye is adsorbed and oriented; and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer, wherein the polyvinyl alcohol-based resin layer is formed from a polyvinyl alcohol-based resin having a boron adsorption rate of 5.70 mass % or more ,
an adhesive preparation step of preparing an adhesive that forms the adhesive layer;
a bonding step of applying the adhesive to a surface of at least one of the polarizing element and the transparent protective film to bond the polarizing element and the transparent protective film together,
The adhesive preparation step includes:
a preparation step of obtaining an aqueous solution A containing a polyvinyl alcohol-based resin;
a first addition step of adding a first compound to the aqueous solution A to obtain an aqueous solution B;
a second addition step of adding a second compound to the aqueous solution B to obtain an adhesive,
the first compound is at least one selected from the group consisting of urea, a urea derivative, thiourea, and a thiourea derivative, and the second compound is a dialdehyde;
the laminating step is carried out so that the time elapsed since the second compound was added to the aqueous solution B is 100 hours or less. A method for producing a polarizing plate , comprising: a polarizing element having a polyvinyl alcohol-based resin layer in which a dichroic dye is adsorbed and oriented; and a transparent protective film laminated on at least one surface of the polarizing element via an adhesive layer, wherein the polyvinyl alcohol-based resin layer is formed from a polyvinyl alcohol-based resin having a boron adsorption rate of 5.70 mass % or more ,
an adhesive preparation step of preparing an adhesive that forms the adhesive layer;
a bonding step of applying the adhesive to a surface of at least one of the polarizing element and the transparent protective film to bond the polarizing element and the transparent protective film together,
The adhesive preparation step includes:
a preparation step of obtaining an aqueous solution A containing a polyvinyl alcohol-based resin;
a first addition step of adding a first compound to the aqueous solution A to obtain an aqueous solution B;
a second addition step of adding a second compound to the aqueous solution B to obtain an adhesive;
Including,
a third compound is also added in at least one of the first addition step and the second addition step;
the first compound is at least one selected from the group consisting of urea, a urea derivative, thiourea, and a thiourea derivative, the second compound is a dialdehyde, and the third compound is a dicarboxylic acid;
the laminating step is carried out so that the time elapsed since the second compound was added to the aqueous solution B is 200 hours or less. The method for manufacturing a polarizing plate according to claim 1 or 2, wherein the content of the first compound in the adhesive is 0.1 parts by mass or more and 400 parts by mass or less per 100 parts by mass of the polyvinyl alcohol-based resin. The method for manufacturing a polarizing plate according to any one of claims 1 to 3, wherein the content of the second compound in the adhesive is 0.03 parts by mass or more and 20 parts by mass or less per 1 part by mass of the first compound. The method for producing a polarizing plate according to any one of claims 1 to 4, wherein the dialdehyde is glyoxal. The method for manufacturing a polarizing plate according to claim 2, wherein the content of the third compound in the adhesive is 0.01 parts by mass or more and 400 parts by mass or less per 100 parts by mass of the polyvinyl alcohol-based resin. The method for manufacturing a polarizing plate according to any one of claims 1 to 6, wherein the adhesive layer has a thickness of 0.01 μm or more and 7 μm or less.