TWI902932B - Polarizing plate, and imagedisplay device - Google Patents

Abstract

The present invention provides a polarizing plate that suppresses lowering of the transmission in a high temperature environment. Provided is a polarizing plate which has a polarizing element with a dichroic pigment adsorbed and aligned on a polyvinyl alcohol resin layer, and a transparent protecting film that is laminated on at least one side of the aforementioned polarizing element, the aforementioned polarizing element and the transparent protecting film is adhered by an adhesive layer formed by the quaternary ammonium salt, and the water content ration of the aforementioned polarizing element is above the balance humidity content ration at a temperature of 20℃ and a relative humidity of 30% and below the balance humidity content ration at a temperature of 20℃ and a relative humidity of 50%.

Description

Polarizing plate and image display device

This invention relates to a polarizing plate and an image display device.

Liquid crystal displays (LCDs) are used not only in LCD televisions but also widely in portable devices such as computers and mobile phones, as well as in automotive applications such as car navigation systems. Typically, an LCD device consists of a liquid crystal panel with polarizing plates bonded to both sides of the liquid crystal cells using pressure-sensitive adhesive. The liquid crystal panel controls the light from the backlight to display the image. In recent years, similar to LCDs, organic EL displays have also become widely used in portable devices such as televisions and mobile phones, as well as in automotive applications such as car navigation systems. In organic EL displays, to suppress the reflection of external light at the metal electrodes (cathodes) and prevent the image from appearing as a mirror, a circular polarizing plate (a laminate containing polarizing elements and a λ/4 plate) is sometimes placed on the viewing side surface of the image display panel.

As mentioned above, polarizing plates are increasingly used in automotive applications as components of image display devices such as liquid crystal displays (LCDs) and organic EL displays. Compared to portable devices like televisions or mobile phones, polarizing plates used in automotive image display devices are frequently exposed to high-temperature environments, thus requiring those with minimal changes in high-temperature characteristics (high-temperature durability).

On the other hand, to prevent damage to the image display panel from impacts to its outer surface, the construction of a front panel (also known as a "window layer") such as a transparent resin plate or glass plate on the viewing side of the image display panel is becoming increasingly common. Image display devices equipped with touch panels widely adopt the following configuration: a touch panel is provided on the viewing side of the image display panel, and a front panel is provided on the viewing side of the touch panel.

In this configuration, if an air layer exists between the image display panel and transparent components such as the front panel or touch panel, external light reflection will occur due to the reflection of light at the air layer interface, which tends to reduce the image viewing quality. Therefore, the following configuration is often adopted: the space between the polarizing plate disposed on the viewing side surface of the image display panel and the transparent component is filled with a layer other than the air layer, generally a solid layer (hereinafter referred to as "interlayer filler") (hereinafter referred to as "interlayer filling configuration"). The interlayer filler is preferably a material with a refractive index close to that of the polarizing plate or transparent component. The interlayer filler can suppress the reduction in viewing quality caused by interface reflection, and adhesives or UV-curing adhesives can then be used to fix the components together (for example, see Patent 1).

Interlayer filling structures are widely used in portable devices such as mobile phones, which are typically used outdoors. Furthermore, due to increasing demands for visual appeal in recent years, interlayer filling structures are also being considered for use in vehicle applications such as automotive navigation systems. In this type of structure, a front transparent panel is placed on the surface of the image display panel, and an adhesive layer or similar material is used to fill the space between the panel and the front transparent panel.

However, it has been pointed out that the transmittance of the polarizing plate significantly decreases under high-temperature environments when using this configuration. Patent 2 proposes a solution to this problem by reducing the water content per unit area of the polarizing plate to a specific amount or less, and reducing the saturated water absorption of the transparent protective film adjacent to the polarizing element to a specific amount or less, thereby suppressing the decrease in transmittance.

[Previous Technical Documents]

[Patent Documents]

Patent Document 1: Japanese Patent Application Publication No. Hei 11-174417.

Patent Document 2: Japanese Patent Application Publication No. 2014-102353.

However, even with such a polarizing plate, the effect of suppressing transmittance reduction in high-temperature environments is still insufficient. The purpose of this invention is to provide a polarizing plate that can further suppress transmittance reduction in high-temperature environments, and an image display device using such a polarizing plate.

This invention provides polarizing plates and image display devices as exemplified below.

[1] A polarizing plate comprising a polarizing element having a dichroic pigment adsorbed and aligned on a polyvinyl alcohol-based resin layer, and a transparent protective film deposited on at least one side of the aforementioned polarizing element.

The aforementioned polarizing element and the aforementioned transparent protective film are bonded together using an adhesive layer formed with an adhesive containing quaternary ammonium salts.

The aforementioned polarizing element has a moisture content that is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity.

[2] A polarizing plate comprising a polarizing element having a dichroic pigment adsorbed and aligned on a polyvinyl alcohol-based resin layer, and a transparent protective film deposited on at least one side of the aforementioned polarizing element.

The aforementioned polarizing element and the aforementioned transparent protective film are bonded together using an adhesive layer formed with an adhesive containing quaternary ammonium salts.

The aforementioned polarizing plate has a moisture content that is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity.

[3] A polarizing plate as described in [1] or [2], wherein the aforementioned adhesive contains a polyvinyl alcohol resin.

[4] The polarizing plate as described in [3], wherein the content of the aforementioned grade IV ammonium salt in the aforementioned adhesive is between 1 part by weight and 200 parts by weight relative to 100 parts by weight of the aforementioned polyvinyl alcohol resin.

[5] A polarizing plate as described in any one of [1] to [4], wherein the thickness of the aforementioned adhesive layer is 0.01 μm to 7 μm.

[6] A polarizing plate as described in any of [1] to [5], wherein the aforementioned quaternary ammonium salt contains a compound represented by formula (1) below.

[In the formula, ^R1 to ^R4 represent alkyl groups with 1 to 10 carbon atoms, and X represents chlorine, bromine, or iodine.]

[7] A polarizing plate as described in any of [1] to [6], wherein the aforementioned polarizing plate is used in an image display device.

In the aforementioned image display device, a solid layer is disposed in contact with both sides of the aforementioned polarizing plate.

[8] An image display device comprising an image display unit, a first adhesive layer deposited on a viewing-side surface of the image display unit, and a polarizing plate as described in any one of [1] to [7] deposited on the viewing-side surface of the first adhesive layer.

[9] The image display device as described in [8] further comprises a second adhesive layer deposited on the viewing side surface of the aforementioned polarizing plate, and a transparent component deposited on the viewing side surface of the aforementioned second adhesive layer.

[10] The image display device as described in [9], wherein the aforementioned transparent component is a glass plate or a transparent resin plate.

[11] The image display device as described in [9], wherein the aforementioned transparent component is a touch panel.

According to the present invention, a polarizing plate is provided that improves high-temperature durability and, when used in an image display device constructed with interlayer filling, can suppress the decrease in transmittance caused by high temperatures. Furthermore, an image display device is provided that, when using the polarizing plate of the present invention, can suppress the decrease in transmittance in high-temperature environments.

The following describes the embodiments of the present invention, but the present invention is not limited to these embodiments.

[Polarizing plate]

The polarizing plate of this embodiment comprises a polarizing element having a dichroic pigment adsorbed and aligned on a layer containing polyvinyl alcohol-based resin, and a transparent protective film. The polarizing element and the transparent protective film are bonded together by an adhesive layer formed using an adhesive containing quaternary ammonium salts. The polarizing plate of this embodiment has at least one of the features described in (a) and (b).

(a) The moisture content of the polarizing element is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity.

(b) The moisture content of the polarizing plate is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity.

Previous polarizing plates known for their excellent high-temperature durability include those that can suppress transmittance reduction even after being placed at 95°C for 1000 hours. However, even with such polarizing plates, when used in interlayer filling structures, a significant decrease in transmittance is observed in the central area of the polarizing plate after 200 hours at 95°C. In interlayer filling structures, one side of the polarizing plate is bonded to the image display unit, and the other side is bonded to transparent components such as a touch panel or front panel. When image display devices using this interlayer filling structure are exposed to high temperatures, the problem of significantly reduced transmittance of the polarizing plate under high-temperature conditions is particularly prone to occur.

In a polarizing plate where the transmittance is significantly reduced due to the interlayer filling structure, Raman spectrometry shows peaks near 1100 ^cm⁻¹ (derived from the =C=C bond) and near 1500 ^cm⁻¹ (derived from the -C=C- bond), thus it is believed that a polyene structure (-C=C) _n- is formed. The polyene structure is considered to be produced by the polyeneization of the polyvinyl alcohol constituting the polarizing element due to dehydration (Patent 2, paragraph [0012]).

The polarizing plate of this invention improves high-temperature durability. The polarizing plate of this invention is assembled into an image display device using interlayer filling. For example, even when exposed to a high-temperature environment of 105°C for extended periods, it can suppress transmittance reduction; even after storage at 105°C for 48 hours, the transmittance reduction is less than 5%.

<Polarizing element>

A known polarizing element can be used when a polarizing element containing a dichroic dye is adsorbed and aligned onto a layer containing polyvinyl alcohol (hereinafter referred to as "PVA")-based resin (hereinafter also referred to as "PVA-based resin layer"). Examples of polarizing elements include: an extended film obtained by dyeing a PVA-based resin film with a dichroic dye and then uniaxially stretching it; or an extended layer obtained by coating a substrate film with a coating solution containing a PVA-based resin to form a coating layer, using a laminated film having the coating layer, dyeing the coating layer with a dichroic dye, and then uniaxially stretching the laminated film. Stretching can be performed after dyeing with the dichroic dye, or simultaneously with dyeing, or dyeing can be performed after stretching.

PVA-based resins are obtained by saponifying polyvinyl acetate (PVA) resins. Besides homopolymers of vinyl acetate, PVA resins include copolymers of vinyl acetate and other monomers that can be copolymerized with it. Other copolymerizable monomers include, for example, unsaturated carboxylic acids, olefins such as ethylene, vinyl ethers, and unsaturated sulfonic acids.

The saponification degree of PVA-based resins is preferably 85 mol% or higher, more preferably 90 mol% or higher, and even more preferably 99 mol% or higher but less than 100 mol%. The degree of polymerization of PVA-based resins is, for example, 1000 to 10000, preferably 1500 to 5000. PVA-based resins can be modified, for example, into aldehyde-modified polyethylene formaldehyde, polyethylene acetal, and polyethylene butyral.

The thickness of the polarizing element is preferably 3μm to 35μm, more preferably 4μm to 30μm, and even more preferably 5μm to 25μm. By making the thickness of the polarizing element 35μm or less, the effect of polyolefin formation in PVA-based resins on the reduction of optical properties under high-temperature environments can be suppressed. By making the thickness of the polarizing element 3μm or more, it is easier to form a structure that achieves the desired optical properties.

The polarizing element preferably contains quaternary ammonium salt. In this embodiment, the polarizing element and the transparent protective film are bonded together by an adhesive layer formed by an adhesive containing quaternary ammonium salt. It is presumed that a portion of the quaternary ammonium salt is transferred from the adhesive layer and contained in the polarizing element. The quaternary ammonium salt in the polarizing element may be included in the process of manufacturing the polarizing element. By having a polarizing element containing quaternary ammonium salt, the transmittance is not easily reduced even when the polarizing plate is exposed to a high-temperature environment. It is presumed that the quaternary ammonium ions contained in the polarizing element inhibit the polyolefination of PVA-based resins.

(Grade IV ammonium salt)

Quaternary ammonium salts are salts containing quaternary ammonium cations and other anions.

Preferred grade IV ammonium salts are those containing compounds represented by formula (1).

[In the formula, ^R1 to ^R4 represent alkyl groups with 1 to 10 carbon atoms, and X represents chlorine, bromine, or iodine.]

Grade IV ammonium salts can be used alone or in combination with two or more other types.

Quaternary ammonium salts are preferably in which the number of carbon atoms in ^R1 to ^R4 in formula (1) is 2 or more, more preferably 3 or more, and even more preferably 4 or more. Quaternary ammonium salts with alkyl groups having a large number of carbon atoms can further improve the high-temperature durability of polarizing plates.

Quaternary ammonium salts specifically include bases and their salts with tetramethylammonium ion, tetraethylammonium ion, tetrapropylammonium ion, tetrabutylammonium ion, triethylmethylammonium ion, and tributylmethylammonium ion as cations. Anions constituting quaternary ammonium salts can include chloride ions, bromide ions, and iodide ions, with chloride ions being preferred.

Methods for using polarizing elements containing quaternary ammonium salts include: immersing a PVA-based resin layer in a treatment solvent containing quaternary ammonium salts; or spraying, flowing in, or dripping the treatment solvent onto the PVA-based resin layer. The method of immersing the PVA-based resin layer in a treatment solvent containing quaternary ammonium salts is preferred.

In the step of immersing the PVA-based resin layer in a treatment solvent containing quaternary ammonium salts, the swelling, stretching, staining, crosslinking, and washing steps described later in the polarizing element manufacturing method can be performed simultaneously, or they can be separated from these steps. The step of containing quaternary ammonium salts in the PVA-based resin layer is preferably performed after staining the PVA-based resin layer with iodine, and more preferably simultaneously with the crosslinking step after staining. This method can reduce hue changes and minimize the impact of the polarizing element on optical properties.

To ensure that the polarizing element contains quaternary ammonium salts, they can be added during the manufacturing process of the polarizing element and then added to the adhesive.

(Feature(a))

When characteristic (a) is present, the moisture content of the polarizing element is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity. Preferably, the moisture content of the polarizing element is below the equilibrium moisture content at 20°C and 45% relative humidity, more preferably below the equilibrium moisture content at 20°C and 42% relative humidity, and even more preferably below the equilibrium moisture content at 20°C and 38% relative humidity. If the moisture content of the polarizing element is lower than the equilibrium moisture content at 20°C and 30% relative humidity, the polarizing element's handling properties will decrease and it will be prone to breakage. If the moisture content of the polarizing element exceeds the equilibrium moisture content at 20°C and 50% relative humidity, the transmittance of the polarizing element will easily decrease. It is hypothesized that a higher moisture content in the polarizing element facilitates the polyolefin formation of PVA-based resins. The moisture content of the polarizing element refers to the moisture content of the polarizing element within the polarizing plate.

Methods for confirming that the moisture content of a polarizing element falls within the range of equilibrium moisture content at 20°C and 30% relative humidity but below the equilibrium moisture content at 20°C and 50% relative humidity can be exemplified by: storing the polarizing element in an environment adjusted to the aforementioned temperature and relative humidity range, and considering it to be in equilibrium with the environment if its mass remains unchanged for a fixed period of time; or pre-calculating the equilibrium moisture content of the polarizing element in an environment adjusted to the aforementioned temperature and relative humidity range, and comparing the current moisture content of the polarizing element with the pre-calculated equilibrium moisture content, thereby confirming its equilibrium state.

There are no particular limitations on the manufacturing method of polarizing elements having an equilibrium moisture content of 20°C or higher and a relative humidity of 30% or lower than an equilibrium moisture content of 20°C or lower than a relative humidity of 50%. Examples include methods such as storing the polarizing element in an environment adjusted to the aforementioned temperature and relative humidity range for 10 minutes to 3 hours, or methods involving heat treatment at 30°C to 90°C.

Other preferred methods for manufacturing polarizing elements with the aforementioned moisture content include storing a laminate of at least one side of the polarizing element with a protective film, or a polarizing plate composed of polarizing elements, in an environment adjusted to the aforementioned temperature and relative humidity range for 10 minutes to 120 hours, or heating it at 30°C to 90°C. When manufacturing an image display device using interlayer filling, a polarizing plate can be laminated onto an image display unit to form an image display panel. This image display panel can then be stored in an environment adjusted to the aforementioned temperature and relative humidity range for 10 minutes to 3 hours, or heated at 30°C to 90°C, and then bonded to a front panel.

The moisture content of the polarizing element is preferably adjusted during the material stage of constructing the polarizing plate, either using a single polarizing element or a laminate of a polarizing element and a protective film, to achieve the aforementioned value range. Adjusting the moisture content after the polarizing plate is constructed can lead to excessive curling, which can easily cause defects when bonded to the image display unit. By adjusting the polarizing element to achieve the aforementioned moisture content during the material stage before constructing the polarizing plate, and using this polarizing element to construct the polarizing plate, it is easy to construct a polarizing plate with polarizing elements that meet the aforementioned moisture content range. Alternatively, the moisture content of the polarizing elements in the polarizing plate can be adjusted to achieve the aforementioned value range while the polarizing plate is bonded to the image display unit. At this point, the polarizing plate is attached to the image display unit, thus preventing it from curling.

(Feature (b))

When characteristic (b) is present, the moisture content of the polarizing plate is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity. Preferably, the moisture content of the polarizing plate is below the equilibrium moisture content at 20°C and 45% relative humidity, more preferably below the equilibrium moisture content at 20°C and 42% relative humidity, and even more preferably below the equilibrium moisture content at 20°C and 38% relative humidity. If the moisture content of the polarizing plate is below the equilibrium moisture content at 20°C and 30% relative humidity, the polarizing plate's handling properties will decrease, and it will be prone to cracking. If the moisture content of the polarizing plate exceeds the equilibrium moisture content at 20°C and 50% relative humidity, the transmittance of the polarizing element will easily decrease. It is speculated that if the moisture content of the polarizing plate is higher, the polyolefination of PVA-based resins will be easier to achieve.

The method for confirming that the moisture content of a polarizing plate falls within the range of equilibrium moisture content above the equilibrium moisture content at 20°C and 30% relative humidity but below the equilibrium moisture content at 20°C and 50% relative humidity can be as follows: The polarizing plate is considered to be in equilibrium with the environment if it is stored in an environment adjusted to the aforementioned temperature and relative humidity range and its mass remains unchanged within a fixed period of time; or the equilibrium moisture content of the polarizing plate in an environment adjusted to the aforementioned temperature and relative humidity range is calculated in advance, and the moisture content of the polarizing plate is compared with the calculated equilibrium moisture content to confirm its equilibrium.

There are no particular limitations on the manufacturing method of a polarizing plate having an equilibrium moisture content of ≥20°C and 30% relative humidity and ≤50% relative humidity at 20°C. Examples include methods such as storing the polarizing plate in an environment adjusted to the aforementioned temperature and relative humidity range for 10 minutes to 3 hours, or heating it at 30°C to 90°C.

When manufacturing an image display device using interlayer filling, a polarizing plate can be laminated onto the image display unit to form an image display panel. This image display panel is then stored in an environment adjusted to the aforementioned temperature and relative humidity range for 10 minutes to 3 hours, or heated to 30°C to 90°C, before being bonded to the front panel.

(Urea compounds)

The polarizing element may further contain a urea-based compound. A polarizing element containing a urea-based compound can suppress the decrease in transmittance. The urea-based compound may be the same as that which may be contained in the adhesive described later. The method for containing the urea-based compound in the polarizing element can be the same as the method for containing quaternary ammonium salts in the polarizing element. The urea-based compound may be contained in the manufacturing process of the polarizing element, or it may be contained in the adhesive used in the later-described layering of the polarizing element and the transparent protective film.

(Manufacturing method of polarizing element)

There are no particular limitations on the manufacturing method of polarizing elements, but typical methods include: a method of producing a PVA-based resin film pre-wound into a roller shape and then performing stretching, dyeing, cross-linking, etc. (hereinafter referred to as "Manufacturing Method 1"); or a method of forming a PVA-based resin layer by coating a coating solution containing PVA-based resin onto a substrate film, including the step of stretching the resulting laminate (hereinafter referred to as "Manufacturing Method 2").

Manufacturing method 1 can be performed through the following steps: uniaxial stretching of a PVA-based resin film; staining the PVA-based resin film with dichroic pigments such as iodine and adsorbing the dichroic pigments; treating the PVA-based resin film with adsorbed dichroic pigments using a boric acid aqueous solution; and washing with water after treatment with the boric acid aqueous solution.

The swelling step involves immersing the PVA-based resin film in a swelling bath. This step removes dirt and agglomerates from the surface of the PVA-based resin film and causes it to swell, thereby inhibiting uneven staining. The swelling bath typically uses a water-based medium, such as water, distilled water, or purified water. Surfactants, alcohols, etc., can be added to the swelling bath as needed. From the perspective of controlling the potassium content of the polarizing element, potassium iodide can be used in the swelling bath. In this case, the potassium iodide concentration 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 optimal temperature for the swelling bath is between 10°C and 60°C, more preferably between 15°C and 45°C, and even more preferably between 18°C and 30°C. The degree of swelling of the PVA-based resin film is affected by the swelling bath temperature; therefore, the soaking time cannot be generalized, but it is preferably between 5 and 300 seconds, more preferably between 10 and 200 seconds, and even more preferably between 20 and 100 seconds. The swelling step can be performed only once or multiple times as needed.

The staining step involves immersing the PVA-based resin film in a staining bath (iodine solution). This allows for the adsorption and alignment of dichroic pigments such as iodine onto the PVA-based resin film. The iodine solution is typically, preferably, an aqueous iodine solution containing iodine and iodides as a dissolving 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. From the perspective of controlling the potassium content in the polarizing element, potassium iodide is preferred.

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

The optimal temperature for the staining bath is between 10°C and 50°C, more preferably between 15°C and 45°C, and even more preferably between 18°C and 30°C. The degree of staining of the PVA-based resin film is affected by the staining bath temperature; therefore, the immersion time cannot be generalized, but it is preferably between 10 seconds and 300 seconds, and more preferably between 20 seconds and 240 seconds. The staining process can be performed once or multiple times as needed.

The crosslinking step involves immersing the PVA-based resin film, stained in the dyeing step, in a treatment bath (crosslinking bath) containing boron compounds. The boron compounds cause the polyvinyl alcohol-based resin film to crosslink, allowing iodine or dye molecules to adsorb onto the crosslinked structure. Examples of boron compounds include boric acid, borate, and borax. The crosslinking bath is generally an aqueous solution, but it can also be a mixture of water and an organic solvent that is miscible with water. From the perspective of controlling the potassium content in polarizing elements, the crosslinking bath preferably contains potassium iodide.

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

The optimal temperature for the crosslinking bath is between 20°C and 70°C, and more preferably between 30°C and 60°C. The degree of crosslinking of the PVA-based resin film is affected by the crosslinking bath temperature; therefore, the soaking time cannot be generalized, but is preferably between 5 and 300 seconds, and more preferably between 10 and 200 seconds. The crosslinking step can be performed only once or multiple times as needed.

The stretching step is a processing step that stretches the PVA-based resin film by a specific ratio in at least one direction. Generally, this involves uniaxially stretching the PVA-based resin film in the transport direction (longitudinal direction). There are no particular limitations on the stretching method; wet stretching or dry stretching can be used. The stretching step can be performed only once or multiple times as needed. The stretching step can be performed at any stage of the polarizing element manufacturing process.

In the wet stretching method, the treatment bath (stretching bath) can typically use water, or a mixture of water and an organic solvent that is miscible with water. From the viewpoint of controlling the potassium content in the polarizing element, the stretching bath preferably contains potassium iodide. When potassium iodide is used in the stretching bath, the concentration of potassium iodide in the stretching bath is preferably 1% to 15% by mass, more preferably 2% to 10% by mass, and even more preferably 3% to 6% by mass. From the viewpoint of suppressing film rupture during stretching, the treatment bath (stretching bath) may contain boron compounds. When boron compounds are contained, the concentration of boron compounds in the stretching bath is preferably 1% to 15% by mass, more preferably 1.5% to 10% by mass, and even more preferably 2% to 5% by mass.

The optimal temperature for the stretching bath is 25°C to 80°C, more preferably 40°C to 75°C, and even more preferably 50°C to 70°C. The degree of stretching of the PVA-based resin film is affected by the stretching bath temperature; therefore, the soaking time cannot be generalized, but is preferably 10 seconds to 800 seconds, and more preferably 30 seconds to 500 seconds. The stretching treatment in the wet stretching method can be performed together with any one or more of the following steps: swelling, staining, crosslinking, and washing.

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

The total elongation ratio (cumulative elongation ratio) of the polyvinyl alcohol resin film can be appropriately set according to the purpose, but it is preferably 2 to 7 times, more preferably 3 to 6.8 times, and even more preferably 3.5 to 6.5 times.

The cleaning step involves immersing the polyvinyl alcohol (PVA) resin film in a cleaning bath to remove foreign matter residues from the surface of the PVA resin film. The cleaning bath typically uses a water-based medium such as water, distilled water, or purified water. Furthermore, from the perspective of controlling the potassium content in the polarizing element, potassium iodide is preferably used in the cleaning bath. In this case, the potassium iodide concentration in the cleaning bath is preferably 1% to 10% by mass, more preferably 1.5% to 4% by mass, and even more preferably 1.8% to 3.8% by mass.

The ideal temperature for the cleansing bath is between 5°C and 50°C, more preferably between 10°C and 40°C, and even more preferably between 15°C and 30°C. The effectiveness of the PVA resin film removal is affected by the bath temperature; therefore, the soaking time cannot be generalized, but it is preferably between 1 second and 100 seconds, more preferably between 2 seconds and 50 seconds, and even more preferably between 3 seconds and 20 seconds. The cleansing process can be performed once or multiple times as needed.

The drying step involves drying the PVA-based resin film cleaned in the washing step to obtain the polarizing element. Drying can be performed by any suitable method, such as natural drying, forced-air drying, or heated drying.

Manufacturing method 2 can be performed through the following steps: applying a coating solution containing PVA-based resin onto a substrate film; uniaxially stretching to obtain a laminated film; dyeing the PVA-based resin layer of the uniaxially stretched laminated film with a dichroic dye to adsorb and form a polarizing element; treating the film adsorbed with the dichroic dye with a boric acid aqueous solution; and washing with water after treatment with the boric acid aqueous solution. The substrate film used to form the polarizing element can be used as a protective layer for the polarizing element. The substrate film can be peeled off from the polarizing element as needed.

<Transparent Protective Film>

The transparent protective film used in this embodiment (hereinafter also referred to as the "protective film") is adhered to at least one side of the polarizing element using an adhesive. Although the transparent protective film may be adhered to one or both sides of the polarizing element, it is preferred to adhere to both sides.

Protective films can simultaneously possess other optical functions and can also form multi-layered laminated structures. From an optical perspective, thinner protective films are preferable; however, if the film thickness is too thin, the strength will decrease, leading to poorer processability. A suitable film thickness is 5 μm to 100 μm, preferably 10 μm to 80 μm, and even more preferably 15 μm to 70 μm.

The protective film can be made of cellulose esters, polycarbonate resins, cycloalkenyl resins such as norcamphene, (meth)acrylate polymers, or polyester resins such as polyethylene terephthalate. When using water-based adhesives such as PVA adhesives to bond the protective film to both sides of the polarizing element, from a moisture permeability perspective, at least one side of the protective film is preferably either a cellulose ester film or a (meth)acrylate polymer film, and more preferably a cellulose ester film.

At least one protective film can possess phase retardation functionality for purposes such as view angle compensation. In this case, the protective film itself can have phase retardation functionality, or it can have a separate phase retardation layer, or a combination of both. The phase retardation film can be directly bonded to the polarizing element through an adhesive, or it can be bonded to the polarizing element through another protective layer, and then through an adhesive layer.

<Next layer>

The adhesive used to bond the protective film to the adhesive layer of the polarizing element is an adhesive containing quaternary ammonium salts. Water-based adhesives, solvent-based adhesives, and active energy line curing adhesives can be used, but water-based adhesives are preferred, and PVA-based resins are more preferably used. By using an adhesive containing quaternary ammonium salts, the transmittance reduction of the polarizing plate in high-temperature environments can be suppressed.

The thickness of the adhesive coating can be set to any value. For example, it can be set to achieve an adhesive layer of the desired thickness after curing or heating (drying). The thickness of the adhesive layer is preferably 0.01μm to 7μm, more preferably 0.01μm to 5μm, even more preferably 0.01μm to 2μm, and most preferably 0.01μm to 1μm.

The following adhesive specifications describe preferred ranges for use when manufacturing polarizing elements that do not contain quaternary ammonium salts. When the polarizing element contains quaternary ammonium salts, the values should be adjusted appropriately to the following levels. Specific examples of quaternary ammonium salts can be the same as those used in the polarizing element described above. During the drying process of bonding the polarizing element and protective film to form the adhesive layer, a portion of the quaternary ammonium salts may migrate from the adhesive layer to the polarizing element, etc.

When the adhesive is a water-based adhesive containing PVA-based resin, the content of grade 4 ammonium salt in the adhesive is preferably between 1 and 200 parts by weight of PVA-based resin (more preferably between 30 and 150 parts by weight), and even more preferably between 50 and 130 parts by weight. If the content is less than 1 part by weight, the effect of improving high-temperature durability cannot be fully obtained. On the other hand, if the content of grade 4 ammonium salt exceeds 200 parts by weight, crystals will precipitate after drying, resulting in increased haze and other undesirable conditions.

In a configuration where a transparent protective film is laminated to both sides of a polarizing element via an adhesive layer, the adhesive layer on one side of the polarizing element may contain only grade IV ammonium salts, but preferably both sides contain grade IV ammonium salts.

To meet the demand for thinner polarizing plates, a polarizing plate with a transparent protective film on only one side of the polarizing element has been developed. In this configuration, a transparent protective film is also laminated with an adhesive layer containing grade IV ammonium salts. The manufacturing method for such a polarizing plate with a transparent protective film on only one side of the polarizing element is considered as follows: first, a polarizing plate is manufactured by bonding a transparent protective film to both sides with adhesive layers in between, and then the transparent protective film on one side is peeled off. When using this manufacturing method, it is permissible for only one adhesive layer to contain grade IV ammonium salts, but it is preferable that both adhesive layers contain grade IV ammonium salts. When the adhesive layer containing grade IV ammonium salt is used only on one side of the polarizing element, the adhesive layer on the unpeeled film side preferably contains grade IV ammonium salt.

(Water-based adhesive)

Any suitable aqueous adhesive can be used, but it is preferred to use an aqueous adhesive containing PVA-based resin (PVA-based adhesive). From the viewpoint of adhesion, the average degree of polymerization of the PVA-based resin contained in the aqueous adhesive is preferably 100 to 5500, more preferably 1000 to 4500. From the viewpoint of adhesion, the average degree of saponification is preferably 85 mol% to 100 mol%, more preferably 90 mol% to 100 mol%.

The PVA resin contained in the aqueous adhesive is preferably acetoacetyl, because PVA resin layers exhibit excellent adhesion and durability to the protective film. For example, PVA resin can be reacted with diacetone in any manner to obtain PVA resin containing acetoacetyl. The acetoacetyl modification degree of the PVA resin containing acetoacetyl is typically 0.1 mol% or more, preferably 0.1 mol% or more and 20 mol% or less. The resin concentration of the aqueous adhesive is preferably 0.1% by mass or more and 15% by mass, more preferably 0.5% by mass or more and 10% by mass.

Aqueous adhesives may contain crosslinking agents. Commonly known crosslinking agents may be used. Examples of crosslinking agents include water-soluble epoxides, dialdehydes, and isocyanates.

When the PVA resin is a PVA resin containing acetyl acetyl groups, the crosslinking agent is preferably any one of glyoxal, glyoxylate, and hydroxymethyl melamine, more preferably any one of glyoxal and glyoxylate, and even more preferably glyoxal.

Aqueous adhesives may contain organic solvents. From the perspective of miscibility with water, alcohols are preferred organic solvents, with methanol or ethanol being more preferred. The methanol concentration in the aqueous adhesive is preferably 10% by mass to 70% by mass, more preferably 15% by mass to 60% by mass, and even more preferably 20% by mass to 60% by mass. By maintaining a methanol concentration of 10% by mass or more, it is easier to suppress the polyolefination of PVA-based resins under high-temperature environments. Furthermore, by maintaining a methanol content of 70% by mass or less, color deterioration can be suppressed. Some urea derivatives have low solubility in water but conversely, excellent solubility in alcohols. At this point, as a preferred approach, the urea compound can be dissolved in alcohol to prepare an alcoholic solution of the urea compound. This alcoholic solution of the urea compound is then added to the PVA aqueous solution to prepare the adhesive.

(Active Energy Line Hardening Adhesive)

Examples of active energy line curing adhesives include those that cure upon exposure to active energy lines such as ultraviolet light. Examples include adhesives containing polymerizable compounds and photopolymerizable initiators, adhesives containing photoreactive resins, and adhesive resins containing photoreactive crosslinkers. Examples of polymerizable compounds 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 photopolymerizable initiators include compounds containing substances that generate reactive species such as neutral free radicals, anionic free radicals, and cation free radicals upon exposure to active energy lines such as ultraviolet light.

(Urea compounds)

The adhesive may further contain at least one urea compound selected from urea, urea derivatives, thiourea, and thiourea derivatives. By including a urea compound in the adhesive layer formed by the adhesive, high-temperature durability can be further improved. During the formation of the adhesive layer from the adhesive in the drying step during protective film bonding, a portion of the urea compound can migrate from the adhesive layer to polarizing elements, etc. Urea compounds include water-soluble and water-poorly soluble types, but both can be used. When using a water-poorly soluble urea compound in a water-soluble adhesive, a dispersion method should be considered after the adhesive layer is formed to prevent increased haze.

When the adhesive is a water-based adhesive containing PVA-based resin, the amount of urea-based compound added is preferably 0.1 to 400 parts by weight relative to 100 parts by weight of PVA, more preferably 1 to 200 parts by weight, and even more preferably 3 to 100 parts by weight.

(Urea derivatives)

Urea derivatives are compounds in which at least one of the four hydrogen atoms in a urea molecule is substituted with a substituent. There are no particular restrictions on the substituent, but it is preferred to have a substituent formed by a carbon atom, a hydrogen atom, and an oxygen atom.

Specific examples of urea derivatives include monosubstituted 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, ortho-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-imidazolidineone (ethyleneurea), and tetrahydro-2-pyrimidinone (propylurea). Tetrasubstituted ureas include tetramethylurea, 1,1,3,3-tetraethylurea, 1,1,3,3-tetrabutylurea, 1,3-dimethoxy-1,3-dimethylurea, 1,3-dimethyl-2-imidazolidineone, and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone.

(Thiocarbamide derivatives)

Thiourea derivatives are compounds in which at least one of the four hydrogen atoms in a thiourea molecule is substituted with a substituent. There are no particular restrictions on the substituent, but it is preferred to have a substituent formed by a carbon atom, a hydrogen atom, and an oxygen atom.

Specific examples of thiourea derivatives include monosubstituted thioureas such as N-methylthiourea, ethylthiourea, propylthiourea, isopropylthiourea, 1-butylthiourea, cyclohexylthiourea, N-acetylated thiourea, N-allyl thiourea, (2-methoxyethyl)thiourea, N-phenylthiourea, (4-methoxyphenyl)thiourea, N-(2-methoxyphenyl)thiourea, N-(1-naphthyl)thiourea, (2-pyridyl)thiourea, ortho-tolyl thiourea, and p-tolyl thiourea. Examples of 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(ortho-tolyl)thiourea, 1,3-di(p-tolyl)thiourea, 1-benzyl-3-phenylthiourea, 1-methyl-3-phenylthiourea, N-allyl-N'-(2-hydroxyethyl)thiourea, and ethylthiourea. Examples of trisubstituted thioureas include trimethylthiourea, and examples of tetrasubstituted thioureas include tetramethylthiourea and 1,1,3,3-tetraethylthiourea.

Among urea compounds, when used in image display devices with interlayer filling structures, urea derivatives or thiourea derivatives are preferred from the viewpoint of further suppressing the decrease in transmittance under high-temperature environments, and urea derivatives are even more preferred. Among urea derivatives, monosubstituted urea or disubstituted urea are preferred, and monosubstituted urea is even more preferred. Disubstituted urea includes 1,1-substituted urea and 1,3-substituted urea, with 1,3-substituted urea being more preferred.

<Layer containing grade IV ammonium salts>

Grade IV ammonium salts are not limited to being contained in the aforementioned adhesive layer. From the perspective of improving the high-temperature durability of the polarizing plate, they can also be contained in other layers besides the adhesive layer. In polarizing plates with a transparent protective film on only one side, from the perspective of improving physical strength, a hardening layer can be applied to the area opposite to the transparent protective film on the polarizing element.

In this embodiment, the hardened layer contains quaternary ammonium salts and can thus serve as a layer containing quaternary ammonium salts. Normally, such a hardened layer is formed from a hardening composition containing an organic solvent, but paragraphs [0020] to [0042] of Japanese Patent Application Publication No. 2017-075986 describe a method for forming such a hardened layer from an aqueous solution of an active energy line hardening polymer composition. Quaternary ammonium salts are mostly water-soluble, therefore, water-soluble quaternary ammonium salts can be contained in such a composition.

The layer containing a quaternary ammonium salt preferably comprises at least one quaternary ammonium salt and an adhesive. Examples of adhesives include polymer adhesives, thermosetting resin adhesives, and active energy line curing resin adhesives; any of these adhesives is preferred.

The thickness of the layer containing grade IV ammonium salts is preferably 0.1 μm to 20 μm, more preferably 0.5 μm to 15 μm, and even more preferably 1 μm to 10 μm.

[Manufacturing Method of Polarizing Plate]

The method for manufacturing the polarizing plate of this embodiment includes a moisture content adjustment step and a lamination step. In the moisture content adjustment step, when manufacturing the polarizing plate with characteristic (a), the moisture content of the polarizing element is adjusted to a level above the equilibrium moisture content at 20°C and 30% relative humidity and below the equilibrium moisture content at 20°C and 50% relative humidity. The moisture content of the polarizing element can be adjusted according to the aforementioned record of the moisture content of the polarizing element. In the moisture content adjustment step, when manufacturing the polarizing plate with characteristic (b), the moisture content of the polarizing plate is adjusted to a level above the equilibrium moisture content at 20°C and 30% relative humidity and below the equilibrium moisture content at 20°C and 50% relative humidity. The moisture content of the polarizing plate can be adjusted according to the aforementioned record of the moisture content of the polarizing plate. In the lamination step, the polarizing element and the transparent protective layer are laminated, for example, with the aforementioned adhesive layer in between. In the lamination step, for example, the polarizing element and the transparent protective film that have not undergone treatment with grade IV ammonium salts are bonded together using an adhesive containing grade IV ammonium salts. There is no fixed order for the moisture content adjustment and lamination steps; in fact, the moisture content adjustment and lamination steps can be performed simultaneously.

[Composition of an image display device]

The polarizing plate of this embodiment is used in various image display devices such as liquid crystal displays or organic EL displays. The interlayer filling configuration involves bonding both sides of the polarizing plate to layers other than the air layer; specifically, it involves bonding both sides of the polarizing plate to solid layers such as adhesive layers. When the image display device has this interlayer filling configuration, its transmittance tends to decrease in high-temperature environments. In image display devices using the polarizing plate of this embodiment, even with an interlayer filling configuration, the decrease in the transmittance of the polarizing plate in high-temperature environments can be suppressed. The image display device may, for example, have an image display unit, a first adhesive layer deposited on the viewing side surface of the image display unit, and a polarizing plate deposited on the viewing side surface of the first adhesive layer. The image display device may further have a second adhesive layer deposited on the viewing side surface of the polarizing plate, and a transparent component deposited on the surface of the second adhesive layer. In particular, the polarizing plate of this embodiment is suitable for use in an image display device having an interlayer filling structure, wherein a transparent component is disposed on the viewing side of the image display device, the polarizing plate and the image display unit are bonded together with a first adhesive layer, and the polarizing plate and the transparent component are bonded together with a second adhesive layer. In this specification, either or both of the first and second adhesive layers may sometimes be referred to simply as "adhesive layer". Furthermore, the component used for bonding the polarizing plate to the image display unit, and the component used for bonding the polarizing plate to the transparent component, is not limited to an adhesive layer and may be an bonding agent layer.

<Image Display Unit>

Image display units can include liquid crystal units (LCDs) or organic EL units (OLEDs). LCDs can be any of the following: reflective LCDs utilizing external light, transmissive LCDs utilizing light from sources such as backlights, or semi-transmissive/semi-reflective LCDs utilizing both external light and light from a light source. When the LCD utilizes light from a light source, in the image display device (LCD device), a polarizing plate is also arranged on the opposite side of the viewing side of the LCD, and a light source is further arranged thereon. The polarizing plate and the LCD on the light source side are preferably bonded together with a suitable adhesive layer. The driving method of the LCD can be any type, such as VA mode, IPS mode, TN mode, STN mode, or pi-type bending alignment.

Organic electroluminescent (EL) cells are suitable for use on transparent substrates where transparent electrodes, organic light-emitting layers, and metal electrodes are sequentially deposited to form a light emitter (organic electroluminescent emitter). The organic light-emitting layer is a stack of various organic thin films, such as a hole-injection layer composed of triphenylamine derivatives, a light-emitting layer composed of fluorescent organic solids such as anthracene, an electron-injection layer composed of such light-emitting layers and perylene derivatives, or a stack of a hole-injection layer, a light-emitting layer, and an electron-injection layer, etc.

<Lamination between image display unit and polarizing plate>

An adhesive layer (adhesive sheet) is suitable for bonding the image display unit to the polarizing plate. From an operability perspective, a preferred method is to bond the polarizing plate with an adhesive layer on one side of the polarizing plate to the image display unit. The adhesive layer can be applied to the polarizing plate in various suitable ways. Examples include: dissolving or dispersing the base polymer or its components in a solvent composed of a single or mixture of suitable solvents such as toluene or ethyl acetate; preparing an adhesive solution of 10% to 40% by mass; and directly applying the solution to the polarizing plate using a suitable spreading method such as casting or coating; and forming an adhesive layer on a separator and transferring it to the polarizing plate.

<Adhesive layer>

The adhesive layer may consist of one or two or more layers, preferably one layer. The adhesive layer may be composed of an adhesive composition with (meth)acrylate resins, rubber resins, ethyl carbamate resins, ester resins, polysiloxane resins, or polyvinyl ether resins as the main components. Preferably, the adhesive composition is based on an (meth)acrylate resin with excellent transparency, weather resistance, and heat resistance. The adhesive composition may be of the active energy line curing type or the thermosetting type.

The (meth)acrylate resin (base polymer) used in the adhesive composition is preferably a polymer or copolymer with one or more (meth)acrylate monomers, such as butyl (meth)acrylate, ethyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. The base polymer is preferably a copolymer of polar monomers. Examples of polar monomers include (meth)acrylate compounds, 2-hydroxypropyl (meth)acrylate compounds, hydroxyethyl (meth)acrylate compounds, (meth)acrylamide compounds, N,N-dimethylaminoethyl (meth)acrylate compounds, and glycidyl (meth)acrylate compounds, which possess carboxyl, hydroxyl, amide, amino, or epoxy groups.

The adhesive composition may contain only the aforementioned base polymer, but typically further contains a crosslinking agent. Examples of crosslinking agents include, for instance, metal ions with a divalent or higher valence that form a carboxylic acid metal salt with a carboxyl group, polyamine compounds that form amide bonds with a carboxyl group, polyepoxide compounds or polyols that form ester bonds with a carboxyl group, and polyisocyanate compounds that form amide bonds with a carboxyl group. Polyisocyanate compounds are preferred.

Active energy line curing adhesives possess the property of curing upon irradiation with active energy lines such as ultraviolet light or electron beams. They exhibit adhesiveness before irradiation and can adhere tightly to substrates such as films, curing upon irradiation with active energy lines and allowing for adjustment of adhesion strength. Ultraviolet-curing adhesives are preferred. In addition to the base polymer and crosslinking agent, active energy line curing adhesives may further contain polymerizable compounds with active energy lines. Photopolymerization initiators and photosensitizers may be included as needed.

The adhesive composition may contain light-scattering microparticles, beads (resin beads, glass beads, etc.), glass fibers, resins other than the base polymer, adhesives, fillers (metal powders or other inorganic powders, etc.), antioxidants, ultraviolet absorbers, dyes, pigments, colorants, defoamers, corrosion inhibitors, photopolymerization initiators, and other additives.

The adhesive layer is formed by applying a diluted organic solvent solution of the aforementioned adhesive composition to the surface of a substrate film, image display unit, or polarizing plate and then drying it. The substrate film is generally a thermoplastic resin film, a typical example being a release film that has undergone a release treatment. Release films, for example, may be those for which a release treatment, such as polysiloxane treatment, is applied to the adhesive layer forming surface of a film composed of resins such as polyethylene terephthalate, polybutylene terephthalate, polycarbonate, or polyarylate.

An adhesive composition can be directly coated onto the release surface of the release film to form an adhesive layer, and then this adhesive layer with the release film attached can be deposited onto the surface of the polarizer. Alternatively, an adhesive composition can be directly coated onto the surface of the polarizer to form an adhesive layer, and then a release film can be deposited on the outer surface of the adhesive layer.

When an adhesive layer is applied to the surface of a polarizing plate, it is preferable to perform surface activation treatments such as plasma treatment or corona treatment on the bonding surface of the polarizing plate and/or the bonding surface of the adhesive layer, and more preferably, corona treatment.

Alternatively, an adhesive composition can be coated onto the second release membrane to form an adhesive layer. A release membrane can then be deposited on this adhesive layer to form an adhesive sheet. After peeling the second release membrane from this adhesive sheet, the adhesive layer with the release membrane attached is deposited onto the polarizing plate. The second release membrane is one that has a weaker adhesion to the adhesive than the second release membrane and is easier to peel off.

The thickness of the adhesive layer is not particularly limited; for example, it is preferably 1 μm to 100 μm, more preferably 3 μm to 50 μm, and can also be 20 μm or more.

<Transparent Components>

Transparent components configured on the viewing side of an image display device can include transparent panels (windows) or touch panels. The transparent panel can be made of a material with appropriate mechanical strength and thickness. Examples of such transparent panels include transparent resin sheets made of polyimide, acrylic, or polycarbonate resins, or glass. Functional layers such as anti-reflective layers can also be laminated on the viewing side of the transparent panel. Furthermore, when the transparent panel is a transparent resin sheet, a hard coating layer to increase physical strength or a low-permeability layer to reduce moisture permeability can be laminated. The touch panel can be made of various types, including resistive film, capacitive, optical, and ultrasonic touch panels, or a glass plate or transparent resin plate with touch sensor functionality. When using a capacitive touch panel, it is preferable to place a transparent plate made of glass or transparent resin plate on the viewing side of the touch panel.

<Bonding of polarizing plate and transparent component>

For bonding polarizing plates to transparent components, adhesives or active energy line curing adhesives are suitable. When using adhesives, the adhesive application can be done in an appropriate manner. A specific example is the adhesive layer application method used for bonding the image display unit to the polarizing plate, as described above.

When using an active energy line-cured adhesive layer, the following method is suitable for preventing the diffusion of the adhesive solution before curing: A weir is set up to surround the edge of the image display panel; a transparent component is placed on the weir; and the adhesive solution is injected. After injection, alignment and degassing are performed as needed, followed by irradiation with active energy lines for curing.

(Implementation Example)

The present invention is illustrated below with specific examples. The materials, reagents, quantities and ratios, and procedures shown in the following examples can be appropriately modified without departing from the scope of the present invention. Therefore, the present invention is not limited to the following examples.

(Fabrication of polarizing element 1)

A 40 μm thick PVA film composed of PVA with an average degree of polymerization of approximately 2400 and a saponification degree of ≥99.9 moles was dry-stretched uniaxially approximately 5 times. While maintaining tension, it was further immersed in pure water at 60°C for 1 minute, followed by immersion in an aqueous solution of iodine/potassium iodide/water at a weight ratio of 0.05/5/100 at 28°C for 60 seconds. Subsequently, it was immersed in an aqueous solution of potassium iodide/boric acid/water at a weight ratio of 8.5/8.5/100 at 72°C for 300 seconds. After washing with pure water at 26°C for 20 seconds, it was dried at 65°C to obtain a 15 μm thick polarizing element 1 with iodine adsorbed and aligned on the PVA. The thickness of the polarizing element was measured using a digital micrometer "MH-15M" manufactured by Nikon Corporation.

(Next, the preparation of the PVA solution)

50g of modified PVA resin (GOHSENX Z-410, manufactured by Mitsubishi Chemical Co., Ltd.) containing acetyl acetyl groups was dissolved in 950g of pure water. The solution was heated at 90°C for 2 hours and then cooled to room temperature to obtain a bonding agent PVA solution.

Adhesive 1 was prepared by mixing tetraethylammonium chloride, pure water, and methanol in a PVA solution at a PVA concentration of 3.0% by mass, a methanol concentration of 20% by mass, and a tetraethylammonium chloride concentration relative to 75 parts by mass of 100 parts by mass of PVA.

The adhesives were prepared by mixing PVA solution, grade IV ammonium salt, purified water, and methanol to the concentrations shown in Table 1, using the same method, to obtain adhesives 2 to 5. All adhesives were prepared with a PVA concentration of 3.0% by mass and a methanol concentration of 20% by mass.

[Table 1]

All grade IV ammonium salts used reagents from Tokyo Chemical Industry Co., Ltd. The dosages of the grade IV ammonium salts in Table 1 represent the dosages at which the polyolefin inhibition effect of each grade IV ammonium salt is optimal.

(Preparation of transparent protective film)

A commercially available cellulose acetate membrane TD40 (manufactured by FUJIFILM Co., Ltd., 40 μm thick) was immersed in a 1.5 mol/L NaOH aqueous solution (saponification solution) maintained at 55°C for 2 minutes, followed by washing with water. Then, it was immersed in a 0.05 mol/L sulfuric acid aqueous solution at 25°C for 30 seconds, and further washed under running water for 30 seconds to neutralize the membrane. Next, it was dehydrated three times using an air knife, and then dried in a drying area at 70°C for 15 seconds to produce a saponified membrane, which serves as a transparent protective film 1.

(Making of polarizing plates)

A transparent protective film 1 is bonded to both sides of the polarizing element 1 using a roller laminator, with adhesive 1 in between. After bonding, it is dried at 80°C for 5 minutes to obtain the polarizing plate 1. The adhesive layer is adjusted to a thickness of 50 nm on both sides after drying.

Adhesives 2 to 5 are used instead of adhesive 1, and polarizing plates 2 to 5 are otherwise manufactured using the same method as polarizing plate 1.

(Adjustment of moisture content in polarizing plates (polarizing elements))

Polarizing plates 1 to 5 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 determined using the Karl Fischer method after 66, 69, and 72 hours of storage. The moisture content values remained unchanged after 66, 69, and 72 hours under any humidity conditions. Therefore, the moisture content of polarizing plates 1 to 5 can be considered to be the same as the equilibrium moisture content of the storage environment. When the moisture content of a polarizing plate reaches equilibrium with a storage environment, the moisture content of the polarizing element within the polarizing plate can also be considered to be in equilibrium with that storage environment. Furthermore, when the moisture content of the polarizing elements in a polarizing plate reaches equilibrium with a storage environment, the moisture content of the polarizing plate itself can also be considered to have reached equilibrium with that storage environment.

(Fabrication of Optical Laminations)

Polarizing plates 1 to 5 were stored at a temperature of 20°C and relative humidity of 35%, 45%, 50%, or 55% for 72 hours. This resulted in the formation of optical laminates 1 to 9 with the moisture content adjusted as shown in Table 2.

An acrylic adhesive (manufactured by LINTEC Corporation, product number: #7) was applied to both sides of optical laminates 1 to 9 with adjusted moisture content to obtain an optical laminate with an adhesive layer thickness of 25 μm. The optical laminate was then cut into 50 mm × 100 mm pieces with the absorption axis of the polarizing element parallel to its long side. Alkali-free glass (manufactured by Corning Incorporated "EAGLE XG") was laminated onto each adhesive surface to create evaluation samples.

(Evaluation of monomer transmittance after high-temperature durability test (105℃))

The evaluation samples were subjected to a high-pressure autoclave treatment at 50°C and 5 kgf/ ^cm² (490.3 kPa) for 1 hour, followed by 24 hours of storage at 23°C and 55% relative humidity. The transmittance (initial value) was then measured, and the samples were stored at 105°C for 48 to 192 hours, with transmittance measured every 48 hours. Evaluation was based on the time it took for the transmittance to decrease by more than 5% relative to the initial value. The results are shown in Table 2.

If the transmittance drops below 5% after 192 hours: A

For those whose transmittance decreases by more than 5% after 144 hours: B

For those whose throughput decreases by more than 5% after 96 hours: C

Those whose throughput drops to 5% or more after 48 hours: D

[Table 2]

The polarizing plate (optical laminates 1 to 7) comprises a polarizing element and a transparent protective film. The moisture content of the polarizing element is above the equilibrium moisture content at 20°C and 30% relative humidity, and below the equilibrium moisture content at 20°C and 50% relative humidity. The polarizing element and the transparent protective film are bonded together using an adhesive containing grade IV ammonium salts. Even when exposed to a high-temperature environment of 105°C for extended periods, the transmittance of the polarizing plate (optical laminates 1 to 7) does not easily decrease, demonstrating its excellent high-temperature durability.

Claims (8)

A polarizing plate comprising a polarizing element having a dichroic pigment adsorbed and aligned on a polyvinyl alcohol-based resin layer, and a transparent protective film deposited on at least one side of the polarizing element. The polarizing element and the transparent protective film are bonded together by an adhesive layer formed from an adhesive containing polyvinyl alcohol-based resin and quaternary ammonium salt. The content of the quaternary ammonium salt in the adhesive is 30 to 200 parts by mass relative to 100 parts by mass of the polyvinyl alcohol-based resin. The moisture content of the polarizing element is at least the equilibrium moisture content at 20°C and 30% relative humidity, and less than the equilibrium moisture content at 20°C and 50% relative humidity. A polarizing plate comprises a polarizing element having a dichroic pigment adsorbed and aligned on a polyvinyl alcohol-based resin layer, and a transparent protective film deposited on at least one side of the polarizing element. The polarizing element and the transparent protective film are bonded together by an adhesive layer formed from an adhesive containing polyvinyl alcohol-based resin and quaternary ammonium salt. The content of the quaternary ammonium salt in the adhesive is between 30 and 200 parts by mass relative to 100 parts by mass of the polyvinyl alcohol-based resin. The moisture content of the polarizing plate is at least the equilibrium moisture content at 20°C and 30% relative humidity, and less than the equilibrium moisture content at 20°C and 50% relative humidity. The polarizing plate as described in claim 1, wherein the thickness of the aforementioned adhesive layer is 0.01 μm to 7 μm. The polarizing plate as described in any of claims 1 to 3, wherein the aforementioned quaternary ammonium salt contains a compound represented by formula (1) below. [In the formula, ^R1 to ^R4 represent alkyl groups with 1 to 10 carbon atoms, and X represents chlorine, bromine, or iodine]. An image display device includes an image display unit, a first adhesive layer deposited on the viewing side surface of the image display unit, and a polarizing plate as described in any one of claims 1 to 4 deposited on the viewing side surface of the first adhesive layer. The image display device as described in claim 5 further comprises a second adhesive layer deposited on the viewing-side surface of the aforementioned polarizing plate, and a transparent component deposited on the viewing-side surface of the aforementioned second adhesive layer. The image display device as described in claim 6, wherein the aforementioned transparent component is a glass plate or a transparent resin plate. The image display device as described in claim 6, wherein the aforementioned transparent component is a touch panel.