JP7532766B2 - Polarizing plate - Google Patents

Description

The present invention relates to a polarizing plate in which a thermoplastic resin film is laminated to a polarizing film.

Polarizing plates are useful as one of the optical components that make up liquid crystal display devices. Polarizing plates usually have a structure in which a protective film is laminated on at least one side of a polarizing film, and are incorporated into liquid crystal display devices. A widely used method for manufacturing polarizing films is to treat a uniaxially stretched polyvinyl alcohol resin film dyed with a dichroic dye with boric acid, followed by washing and drying.

Usually, a protective film is laminated to the polarizing film immediately after the above-mentioned washing and drying processes. This is because the physical strength of the polarizing film is weak after drying, and once it is rolled up, there is a problem that it is prone to tearing in the processing direction. Therefore, usually, an adhesive is applied to the polarizing film immediately after drying, and protective films are laminated to both sides of the polarizing film simultaneously via this adhesive.

Generally, adhesives used are water-based adhesives whose main component is a water-soluble resin such as polyvinyl alcohol resin, or active energy ray curing adhesives. As an active energy ray curing adhesive, Patent Document 1 describes a photocurable adhesive that combines an alicyclic epoxy compound with an epoxy compound that does not have an alicyclic epoxy group, and is mixed with a photocationic polymerization initiator. It discloses a technology used to bond a polarizing film and a protective film.

JP 2008-257199 A

In recent years, with the diversification of mobile devices, durability in harsher environments is required. It has been found that the polarizing plate described in Patent Document 1 has a problem in that the polarizing plate discolors in harsh environments (for example, when stored in a high-temperature environment for a long period of time).
An object of the present invention is to provide a polarizing plate in which discoloration of the polarizing plate is suppressed even under harsh environments.

The present invention includes the following inventions.
[1] A polarizing plate in which a thermoplastic resin film is attached to at least one surface of a polarizing film via an adhesive layer,
the adhesive layer is a cured product of a photocurable composition containing a cationic polymerizable compound and a photocationic polymerization initiator,
The photocurable composition contains 0.5 to 1.5 parts by mass of a photocationic polymerization initiator based on 100 parts by mass of a cationic polymerizable compound,
The polarizing plate is characterized in that the cationic polymerizable compound contains a difunctional oxetane compound in an amount of 10 to 70% by mass based on the total mass of the cationic polymerizable compound.
[2] The polarizing plate according to [1], wherein the cationically polymerizable compound further contains an aliphatic epoxy compound in an amount of 10 to 70% by mass based on the total mass of the cationically polymerizable compound.
[3] The polarizing plate according to [1] or [2], wherein the thermoplastic resin film has a moisture permeability of 300 g/( ^m2 ·24 hr) or less at a temperature of 40° C. and a relative humidity of 90%.

The polarizing plate of the present invention prevents discoloration of the polarizing plate even in harsh environments.

The present invention will be described in detail below. The polarizing plate of the present invention is a polarizing film having a thermoplastic resin film attached to at least one surface thereof via an adhesive made of a photocurable composition. Each member and component constituting the polarizing plate will be described in turn.

[Polarizing film]
The polarizing film is composed of a polyvinyl alcohol-based resin film in which a dichroic dye is adsorbed and aligned. The polyvinyl alcohol-based resin constituting the polarizing film is obtained by saponifying a polyvinyl acetate-based resin. Examples of polyvinyl acetate-based resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as copolymers of vinyl acetate and other monomers copolymerizable therewith. Examples of other monomers copolymerized with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The saponification degree of the polyvinyl alcohol-based resin is usually 85 to 100 mol%, preferably 98 mol% or more. The polyvinyl alcohol-based resin may be modified, and for example, polyvinyl formal or polyvinyl acetal modified with aldehydes may also be used. The polymerization degree of the polyvinyl alcohol-based resin is usually in the range of 1,000 to 10,000, preferably 1,500 to 5,000.

The polarizing film is manufactured through the steps of uniaxially stretching a polyvinyl alcohol-based resin film, dyeing the polyvinyl alcohol-based resin film with a dichroic dye and allowing the dichroic dye to adsorb, and treating the polyvinyl alcohol-based resin film with the adsorbed dichroic dye with an aqueous boric acid solution.

The uniaxial stretching step may be performed before dyeing with a dichroic dye, simultaneously with dyeing with a dichroic dye, or after dyeing with a dichroic dye. When uniaxial stretching is performed after dyeing with a dichroic dye, this uniaxial stretching may be performed before or during the boric acid treatment. It is also possible to perform uniaxial stretching in these multiple stages. For uniaxial stretching, the film may be stretched between rolls with different peripheral speeds, or may be stretched by sandwiching the film between heated rolls. Dry stretching, in which stretching is performed in the air, or wet stretching, in which stretching is performed in a state swollen with a solvent, may be used. The stretching ratio is usually about 4 to 8 times.

To dye a polyvinyl alcohol-based resin film with a dichroic dye, for example, the polyvinyl alcohol-based resin film may be immersed in an aqueous solution containing the dichroic dye. Specifically, iodine or a dichroic organic dye is used as the dichroic dye.

When iodine is used as the dichroic dye, a method is usually used in which a polyvinyl alcohol resin film is immersed in an aqueous solution containing iodine and potassium iodide to dye it. The iodine content in this aqueous solution is usually about 0.01 to 0.5 parts by weight per 100 parts by weight of water, and the potassium iodide content is usually about 0.5 to 10 parts by weight per 100 parts by weight of water. The temperature of this aqueous solution is usually about 20 to 40°C, and the immersion time in this aqueous solution (dyeing time) is usually about 30 to 300 seconds.

On the other hand, when a dichroic organic dye is used as the dichroic pigment, a method of dyeing a polyvinyl alcohol-based resin film by immersing it in an aqueous solution containing a water-soluble dichroic organic dye is usually adopted. The content of the dichroic organic dye in this aqueous solution is usually about 1×10 ⁻³ to 1×10 ⁻² parts by weight per 100 parts by weight of water. This aqueous solution may contain an inorganic salt such as sodium sulfate. The temperature of this aqueous solution is usually about 20 to 80° C., and the immersion time in this aqueous solution (dyeing time) is usually about 30 to 300 seconds.

The boric acid treatment after dyeing with a dichroic dye is carried out by immersing the dyed polyvinyl alcohol resin film in an aqueous boric acid solution. The content of boric acid in the aqueous boric acid solution is usually about 2 to 15 parts by weight, preferably about 5 to 12 parts by weight, per 100 parts by weight of water. When iodine is used as the dichroic dye, the aqueous boric acid solution preferably contains potassium iodide. The content of potassium iodide in the aqueous boric acid solution is usually about 2 to 20 parts by weight, preferably about 5 to 15 parts by weight, per 100 parts by weight of water. The immersion time in the aqueous boric acid solution is usually about 100 to 1,200 seconds, preferably about 150 to 600 seconds, and more preferably about 200 to 400 seconds. The temperature of the aqueous boric acid solution is usually 50°C or higher, preferably 50 to 85°C.

After the boric acid treatment, the polyvinyl alcohol resin film is usually washed with water. The washing is performed, for example, by immersing the boric acid treated polyvinyl alcohol resin film in water. After washing, a drying process is performed to produce a polarizing film. The temperature of the water in the washing process is usually about 5 to 40°C, and the immersion time is usually about 2 to 120 seconds. The subsequent drying process is usually performed using a hot air dryer or far-infrared heater. The drying temperature is usually 40 to 100°C. The drying time is usually about 120 to 600 seconds.

The thickness of the polarizing film made of polyvinyl alcohol resin thus obtained can be about 10 to 50 μm.

[Thermoplastic resin film]
The polarizing plate of the present invention can be obtained by laminating a thermoplastic resin film onto the polarizing film made of the polyvinyl alcohol-based resin film described above via a photocurable composition, and then curing the photocurable composition.

The thermoplastic resin film is preferably formed from a transparent resin. The thermoplastic resin film may be either an unstretched film or a uniaxially or biaxially stretched film.

The main component of the thermoplastic resin film is preferably at least one resin selected from the group consisting of cellulose-based resins, acrylic-based resins, amorphous polyolefin-based resins, polyester-based resins, and polycarbonate-based resins.

The polyester resin is not particularly limited, but polyethylene terephthalate is particularly preferred in terms of mechanical properties, solvent resistance, scratch resistance, cost, etc. Polyethylene terephthalate means a resin in which 80 mol % or more of the repeating units are composed of ethylene terephthalate, and may contain constituent units derived from other copolymerization components.

Other copolymerization components include dicarboxylic acid components and diol components. Examples of dicarboxylic acid components include isophthalic acid, p-β-hydroxyethoxybenzoic acid, 4,4'-dicarboxydiphenyl, 4,4'-dicarboxybenzophenone, bis(4-carboxyphenyl)ethane, adipic acid, sebacic acid, 5-sodium sulfoisophthalic acid, and 1,4-dicarboxycyclohexane. Examples of diol components include propylene glycol, butanediol, neopentyl glycol, diethylene glycol, cyclohexanediol, ethylene oxide adduct of bisphenol A, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. These dicarboxylic acid components and diol components can be used in combination of two or more types as necessary. It is also possible to use a hydroxycarboxylic acid such as p-hydroxybenzoic acid together with the above dicarboxylic acid components and diol components. As other copolymerization components, a small amount of a dicarboxylic acid component and/or a diol component having an amide bond, a urethane bond, an ether bond, a carbonate bond, or the like may be used.

The polyethylene terephthalate resin means a resin in which 80 mol% or more of the repeating units are composed of ethylene terephthalate, and may contain constituent units derived from other copolymerization components. Examples of other copolymerization components include dicarboxylic acid components such as isophthalic acid, 4,4'-dicarboxydiphenyl, 4,4'-dicarboxybenzophenone, bis(4-carboxyphenyl)ethane, adipic acid, sebacic acid, 5-sodium sulfoisophthalic acid, and 1,4-dicarboxycyclohexane; and diol components such as propylene glycol, butanediol, neopentyl glycol, diethylene glycol, cyclohexanediol, ethylene oxide adduct of bisphenol A, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. These dicarboxylic acid components and diol components can be used in combination of two or more types as necessary. In addition, it is also possible to use hydroxycarboxylic acids such as p-hydroxybenzoic acid and p-β-hydroxyethoxybenzoic acid in combination with the above dicarboxylic acid components and diol components. As other copolymerization components, dicarboxylic acid components and/or diol components containing small amounts of amide bonds, urethane bonds, ether bonds, carbonate bonds, etc. may be used.

The polyethylene terephthalate resin is made into a film, which is then stretched as described above and used as a protective film, making it possible to obtain a rolled polarizing plate that is excellent in mechanical properties, solvent resistance, scratch resistance, cost, etc., and has a reduced thickness.

Polycarbonate resins are polyesters formed from carbonic acid and glycol or bisphenol. Among them, aromatic polycarbonates having diphenylalkane in the molecular chain are preferably used because they have excellent heat resistance, weather resistance, and acid resistance. Examples of such polycarbonates include polycarbonates derived from bisphenols such as 2,2-bis(4-hydroxyphenyl)propane (also known as bisphenol A), 2,2-bis(4-hydroxyphenyl)butane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 1,1-bis(4-hydroxyphenyl)isobutane, and 1,1-bis(4-hydroxyphenyl)ethane.

The polycarbonate-based resin film may be produced by any method, such as a casting method or a melt extrusion method. A specific example of the production method is to dissolve a polycarbonate-based resin in a suitable organic solvent to form a polycarbonate-based resin solution, which is then cast onto a metal support to form a web, which is then peeled off from the metal support, and the peeled off web is then dried with hot air to obtain a film.

The acrylic resin is not particularly limited, but is generally a polymer in which a methacrylic acid ester is the main monomer, and is preferably a copolymer in which a small amount of other comonomer components are copolymerized with this. This copolymer can usually be obtained by polymerizing monofunctional monomers including methyl methacrylate and methyl acrylate in the presence of a radical polymerization initiator and a chain transfer agent. In addition, the acrylic resin can be copolymerized with a third monofunctional monomer.

Examples of third monofunctional monomers that can be copolymerized with methyl methacrylate and methyl acrylate include methacrylic acid esters other than methyl methacrylate, such as ethyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzyl methacrylate, 2-ethylhexyl methacrylate, and 2-hydroxyethyl methacrylate; Examples of the acrylic acid esters include ethyl acrylate, butyl acrylate, cyclohexyl acrylate, phenyl acrylate, benzyl acrylate, 2-ethylhexyl acrylate, and 2-hydroxyethyl acrylate; hydroxyalkyl acrylic acid esters such as 2-(hydroxymethyl)methyl acrylate, 2-(1-hydroxyethyl)methyl acrylate, 2-(hydroxymethyl)ethyl acrylate, and 2-(hydroxymethyl)butyl acrylate; unsaturated acids such as methacrylic acid and acrylic acid; halogenated styrenes such as chlorostyrene and bromostyrene; substituted styrenes such as vinyltoluene and α-methylstyrene; unsaturated nitriles such as acrylonitrile and methacrylonitrile; unsaturated acid anhydrides such as maleic anhydride and citraconic anhydride; and unsaturated imides such as phenylmaleimide and cyclohexylmaleimide. These may be used alone or in combination of different types.

In the case of copolymerizing a polyfunctional monomer, examples of polyfunctional monomers that can be copolymerized with methyl methacrylate and methyl acrylate include, for example, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, nonaethylene glycol di(meth)acrylate, and tetradecaethylene glycol di(meth)acrylate, which are esterified with acrylic acid or methacrylic acid at both terminal hydroxyl groups of ethylene glycol or its oligomers; propylene glycol or its oligomers, which are esterified with acrylic acid or methacrylic acid at both terminal hydroxyl groups of propylene glycol or its oligomers; dihydric alkoxy groups such as neopentyl glycol di(meth)acrylate, hexanediol di(meth)acrylate, and butanediol di(meth)acrylate. Examples of the hydroxyl groups of bisphenol A, alkylene oxide adducts of bisphenol A, or both terminal hydroxyl groups of these halogen-substituted products are esterified with acrylic acid or methacrylic acid; polyhydric alcohols such as trimethylolpropane and pentaerythritol are esterified with acrylic acid or methacrylic acid, and the epoxy groups of glycidyl acrylate or glycidyl methacrylate are ring-opened and added to the terminal hydroxyl groups of these polyhydric alcohols; succinic acid, adipic acid, terephthalic acid, phthalic acid, halogen-substituted dibasic acids, and alkylene oxide adducts of these acids are ring-opened and added to the epoxy groups of glycidyl acrylate or glycidyl methacrylate; aryl (meth)acrylates; aromatic divinyl compounds such as divinylbenzene. Among these, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, and neopentyl glycol dimethacrylate are preferably used.

An acrylic resin having such a composition may be modified by further reacting the functional groups of the copolymer. Examples of such reactions include an intrapolymer chain de-methanol condensation reaction between the methyl ester group of methyl acrylate and the hydroxyl group of methyl 2-(hydroxymethyl)acrylate, and an intrapolymer chain dehydration condensation reaction between the carboxyl group of acrylic acid and the hydroxyl group of methyl 2-(hydroxymethyl)acrylate.

The glass transition temperature of the acrylic resin is preferably in the range of 80 to 120°C. To adjust the glass transition temperature of the acrylic resin to the above range, a method is usually adopted in which the polymerization ratio of the methacrylic acid ester monomer and the acrylic acid ester monomer, the carbon chain length of each ester group and the type of functional group that each ester group has, and the polymerization ratio of the polyfunctional acrylic monomer to the total monomers are appropriately selected.

The acrylic resin may contain known additives as necessary. Examples of known additives include lubricants, antiblocking agents, heat stabilizers, antioxidants, antistatic agents, light fasteners, impact resistance modifiers, and surfactants. However, because transparency is required as a protective film to be laminated to a polarizing film, it is preferable to keep the amount of these additives to a minimum.

The acrylic resin film may be produced by any method, including melt casting, melt extrusion such as the T-die method or inflation method, or the calendar method. Among these, the method of melt extruding the raw resin from, for example, a T-die and contacting at least one side of the resulting film with a roll or belt to produce a film is preferred because it produces a film with good surface properties.

The acrylic resin may contain acrylic rubber particles, which are impact modifiers, from the viewpoint of film-forming properties and impact resistance of the film. The acrylic rubber particles referred to here are particles whose essential component is an elastic polymer mainly composed of acrylic acid ester, and examples thereof include a single-layer structure consisting essentially of this elastic polymer, and a multi-layer structure consisting of this elastic polymer as one layer. An example of such an elastic polymer is a cross-linked elastic copolymer in which an alkyl acrylate is the main component and other copolymerizable vinyl monomers and cross-linking monomers are copolymerized with this. Examples of alkyl acrylates that are the main component of elastic polymers include those with an alkyl group having 1 to 8 carbon atoms, such as methyl acrylate, ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate, and acrylates having an alkyl group having 4 or more carbon atoms are particularly preferred. Other vinyl monomers copolymerizable with this alkyl acrylate include compounds having one polymerizable carbon-carbon double bond in the molecule, more specifically, methacrylic acid esters such as methyl methacrylate, aromatic vinyl compounds such as styrene, vinyl cyanide compounds such as acrylonitrile, etc. Also, crosslinkable monomers include crosslinkable compounds having at least two polymerizable carbon-carbon double bonds in the molecule, more specifically, (meth)acrylates of polyhydric alcohols such as ethylene glycol di(meth)acrylate and butanediol di(meth)acrylate, alkenyl esters of (meth)acrylic acid such as allyl (meth)acrylate, divinylbenzene, etc.

Furthermore, the protective film can be a laminate of a film made of an acrylic resin that does not contain rubber particles and a film made of an acrylic resin that does contain rubber particles. Acrylic resins are readily available commercially, and examples of such products include Sumipex (manufactured by Sumitomo Chemical Co., Ltd.), Acrypet (manufactured by Mitsubishi Rayon Co., Ltd.), Delpet (manufactured by Asahi Kasei Corporation), Parapet (manufactured by Kuraray Co., Ltd.), and Acryviewer (manufactured by Nippon Shokubai Co., Ltd.).

Examples of amorphous polyolefin resins include resins obtained by ring-opening metathesis polymerization using norbornene or its derivatives, which are obtained by the Diels-Alder reaction of cyclopentadiene and olefins, as a monomer, followed by hydrogenation; resins obtained by ring-opening metathesis polymerization using tetracyclododecene or its derivatives, which are obtained by the Diels-Alder reaction of dicyclopentadiene and olefins or methacrylic acid esters, as a monomer, followed by hydrogenation; resins obtained by similar ring-opening metathesis copolymerization using two or more monomers selected from norbornene, tetracyclododecene, their derivatives, and other cyclic polyolefin monomers, followed by hydrogenation; and resins obtained by addition copolymerization of norbornene, tetracyclododecene, or their derivatives with an aromatic compound having a vinyl group. Examples of commercially available amorphous polyolefin resins include "Arton" from JSR Corporation, "ZEONEX" and "ZEONOR" from Zeon Corporation, and "APO" and "Apel" from Mitsui Chemicals, Inc. When forming a film from an amorphous polyolefin resin, a known method such as a solvent casting method or a melt extrusion method is appropriately used for film formation.

The cellulose-based resin is a resin in which at least a portion of the hydroxyl groups in cellulose are esterified with acetate, and may be a mixed ester in which a portion is esterified with acetate and a portion is esterified with another acid. The cellulose-based resin is preferably a cellulose ester-based resin, and more preferably an acetyl cellulose-based resin. Specific examples of acetyl cellulose-based resins include triacetyl cellulose, diacetyl cellulose, cellulose acetate propionate, and cellulose acetate butyrate. Commercially available films made of such acetyl cellulose-based resins include, for example, "FUJITAC TD80", "FUJITAC TD80UF", and "FUJITAC TD80UZ" manufactured by Fujifilm Corporation, and "KC8UX2M" and "KC8UY" manufactured by Konica Minolta Opto Co., Ltd.

A cellulose resin film with optical compensation function can also be used. Examples of such optical compensation films include a film in which a compound having phase difference adjustment function is contained in a cellulose resin, a film in which a compound having phase difference adjustment function is applied to the surface of a cellulose resin, and a film obtained by uniaxially or biaxially stretching a cellulose resin. Examples of commercially available optical compensation films made of cellulose resin include "Wide View Film WV BZ 438" and "Wide View Film WV EA" manufactured by Fujifilm Corporation, and "KC4FR-1" and "KC4HR-1" manufactured by Konica Minolta Opto, Inc.

The thermoplastic resin film attached to one side of the polarizing film may contain an ultraviolet absorbing agent. By placing a protective film containing an ultraviolet absorbing agent on the viewing side of the liquid crystal cell, the liquid crystal cell can be protected from deterioration due to ultraviolet rays.

In the present invention, the above-mentioned thermoplastic resin film is laminated to at least one surface of the polarizing film using a photocurable composition. When a thermoplastic resin film is laminated to only one surface of the polarizing film, for example, a pressure-sensitive adhesive layer for laminating to another member such as a liquid crystal cell can be directly provided on the other surface of the polarizing film.

On the other hand, when thermoplastic resin films are laminated to both sides of a polarizing film, the respective thermoplastic resin films may be of the same type or different types.

The thermoplastic resin film attached to one side of the polarizing film is adhered using a photocurable composition described below, but the thermoplastic resin film attached to the other side of the polarizing film may be adhered using an adhesive layer formed from a different photocurable composition.

Prior to bonding to the polarizing film, the thermoplastic resin film may be subjected to an easy-adhesion treatment such as a saponification treatment, a corona treatment, a primer treatment, or an anchor coating treatment on the bonding surface. In addition, the surface of the thermoplastic resin film opposite to the bonding surface to the polarizing film may have various treatment layers such as a hard coat layer, an anti-reflection layer, or an anti-glare layer. The thickness of the thermoplastic resin film is usually in the range of about 5 to 200 μm, preferably 10 to 120 μm, and more preferably 10 to 100 μm.

The thermoplastic resin film preferably has a moisture permeability of 300 g/( ^m2 ·24 hr) or less, more preferably 200 g/(m2·24 hr) or less, and even more preferably 100 g/( ^m2 ·24 hr) or less, at a temperature of 40 ^° C. and a relative humidity of 90%. A moisture permeability of 300 g/( ^m2 ·24 hr) or less is preferable because it can suppress, for example, a change in transmittance in a high-temperature and high-humidity environment and a change in hue in a high-temperature environment.
When thermoplastic resin films are laminated to both sides of a polarizing film, it is preferable to laminate a thermoplastic resin film having a moisture permeability of 300 g/( ^m2 ·24 hr) or less at a temperature of 40° C. and a relative humidity of 90% to at least the viewing side (outermost surface), and it is more preferable to laminate a thermoplastic resin film having a moisture permeability of 300 g/( ^m2 ·24 hr) or less at a temperature of 40° C. and a relative humidity of 90% to both sides of the polarizing film.

[Adhesive layer]
The polarizing plate of the present invention is obtained by laminating a thermoplastic resin film to one surface of the above polarizing film via an adhesive layer. The adhesive layer is formed from a photocurable composition in which a cationic polymerizable compound and a photocationic polymerization initiator are mixed in a predetermined ratio.

(Cationically polymerizable compound)
The cationically polymerizable compound is a main component of the photocurable composition, and becomes a component that imparts adhesive strength by polymerization and curing. The cationically polymerizable compound includes a bifunctional oxetane compound.

(Bifunctional oxetane compound)
The bifunctional oxetane compound is a compound having two four-membered ring ethers (oxetanyl groups) in the molecule, and examples thereof include the following compounds.

Bis[(3-ethyloxetan-3-yl)methyl]ether, bis[(3-methyloxetan-3-yl)methyl]ether, bis[(oxetan-3-yl)methyl]ether, 1,4-bis[[(3-ethyloxetan-3-yl)methoxy]methyl]benzene, 1,4-bis[(3-ethyloxetan-3-yl)methoxy]benzene, 1,3-bis[(3-ethyloxetan-3-yl)methoxy]benzene, 1,2-bis[(3-ethyloxetan-3-yl)methoxy]benzene, 4,4'-bis[(3-ethyloxetan-3-yl)methoxy]benzene 1,2-bis[(3-ethyloxetan-3-yl)methoxy]biphenyl, 2,2'-bis[(3-ethyloxetan-3-yl)methoxy]biphenyl, 1,1,1-tris[(3-ethyloxetan-3-yl)methoxymethyl]propane, 1,2-bis[(3-ethyloxetan-3-yl)methoxy]ethane, 1,2-bis[(3-ethyloxetan-3-yl)methoxy]propane, 1,4-bis[(3-ethyloxetan-3-yl)methoxy]butane, and 1,6-bis[(3-ethyloxetan-3-yl)methoxy]hexane, etc.

Among these, bis[(3-ethyloxetan-3-yl)methyl]ether is preferred because it provides a high reactivity to the photocurable composition and increases the elastic modulus of the resulting cured product at high temperatures. This increases the durability of the polarizing plate under high temperature conditions and high temperature and humidity conditions.

The bifunctional oxetane compound preferably has a molecular weight of 550 or less, more preferably a molecular weight of 150 to 400, and even more preferably a molecular weight of 150 to 300, in order to provide a photocurable composition with low viscosity and a cured product of the photocurable composition with excellent adhesive strength.

One or more types of bifunctional oxetane compounds may be used.

It is preferable to mix the bifunctional oxetane compound in a ratio of 10 to 70 mass% based on the total mass of the cationic polymerizable compound. If the bifunctional oxetane compound is 10 mass% or less, the curability of the photocurable composition decreases, and if it is 70 mass% or more, the adhesive strength to the thermoplastic resin film decreases. The preferred content of the bifunctional oxetane compound is 15 to 65 mass%, more preferably 25 to 55 mass%, based on the total amount of the cationic polymerizable compound.

(Aliphatic epoxy compounds)
The photocurable composition of the present invention may further contain an aliphatic epoxy compound as a cationic polymerizable compound. When a bifunctional oxetane compound and an aliphatic epoxy compound are used in combination as a cationic polymerizable compound, the storage modulus of the cured product of the photocurable composition can be kept high, while the adhesion between the polarizing film and the protective film can be further improved. The aliphatic epoxy compound referred to here is a compound in which one of the two carbon atoms contained in the epoxy group is bonded to another aliphatic carbon atom. Examples of such compounds include polyglycidyl ethers of alkane polyols and polyglycidyl ethers of polyalkylene glycols.

Examples of polyglycidyl ethers of polyalkylene glycols include diethylene glycol diglycidyl ether, triethylene glycol diglycidyl ether, dipropylene glycol diglycidyl ether, and tripropylene glycol diglycidyl ether.

A specific example of a polyglycidyl ether of an alkane polyol is the compound represented by the following formula (IV):

In the above formula (IV), Z represents an alkylene group having 2 to 10 carbon atoms, and Z may be linear or branched, or may have a ring structure. Z preferably has 2 to 6 carbon atoms, more preferably 4 to 6 carbon atoms.

Examples of the compound represented by the above formula (IV) include ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, 1,3-propanediol diglycidyl ether, 1,4-butanediol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, 1,9-nonanediol diglycidyl ether, cyclohexanedimethanol diglycidyl ether, and 1,4-cyclohexanedimethanol diglycidyl ether.

Other aliphatic epoxy compounds include glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, pentaerythritol polyglycidyl ether and dipentaerythritol polyglycidyl ether, 2-ethylhexyl glycidyl ether, hydroquinone diglycidyl ether, and resorcinol diglycidyl ether.

The aliphatic epoxy compound is preferably a compound represented by formula (IV) because it is easily available and has a large effect of increasing the adhesion between the polarizing film and the thermoplastic resin film. Specifically, compounds in which Z has 4 to 6 carbon atoms, such as 1,4-butanediol diglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether, are preferably used.

Aliphatic epoxy compounds may be used alone or in combination of two or more.

When a bifunctional oxetane compound and an aliphatic epoxy compound are used in combination, the mixing ratio of the two is preferably 10 to 90 mass% of the aliphatic epoxy compound, more preferably 15 to 80 mass%, and even more preferably 15 to 50 mass% based on the total amount of the cationic polymerizable compound. Of course, the total of these does not exceed 100 mass%. By mixing an aliphatic epoxy compound (for example, a compound represented by the above formula (IV)) at a ratio of 10 mass% or more of the total amount of the cationic polymerizable compound, the adhesive strength with the thermoplastic resin film is excellent. In terms of the low viscosity and adhesive strength of the photocurable composition, it is preferable to have a large amount of aliphatic epoxy compound, but if it exceeds 90 mass%, the storage modulus at 80°C of the cured product of the photocurable composition will be low, and the durability of the polarizing plate in which the polarizing film and the protective film are laminated through it will be poor in a thermal shock test.

(Other Cationic Curable Components)
The photocurable composition may contain other cationic curable components, such as aromatic epoxy compounds, hydrogenated epoxy compounds in which the aromatic ring of an aromatic epoxy compound is hydrogenated, alicyclic epoxy compounds, and monofunctional oxetane compounds.

In the present invention, the aromatic epoxy compound refers to a compound in which a glycidyl ether group or a glycidyl ester group is directly bonded to an aromatic ring. Specific examples include bisphenol A type epoxy, bisphenol F type epoxy resin, bisphenol S type epoxy, epoxy resin obtained by polycondensation of bisphenol A, bisphenol F and epichlorohydrin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, bisphenol A novolac type epoxy resin and bisphenol F novolac type epoxy resin.

From the viewpoint of adhesion to thermoplastic resin films, it is preferable that the number of epoxy groups contained in one molecule of the aromatic epoxy compound is 2 or more. For the same reason, the weight average molecular weight (hereinafter referred to as "Mw") of the aromatic epoxy compound is preferably 400 to 50,000, more preferably 1,000 to 10,000, and even more preferably 2,000 to 5,000. In the present invention, Mw means the polystyrene-equivalent Mw measured by gel permeation chromatography (GPC).

Aromatic epoxy compounds may be used alone or in combination of two or more.

When an aromatic epoxy compound is used in combination, the content is preferably 1 to 25% by mass based on the total mass of the cationic polymerizable compound. A content of 1% by mass or more improves adhesion to the thermoplastic resin film. On the other hand, if the content exceeds 25% by mass, the viscosity of the composition increases and the coatability deteriorates. The preferred content is 3 to 20% by mass, more preferably 5 to 15% by mass based on the total mass of the cationic polymerizable compound.

Hydrogenated epoxy compounds are glycidyl ethers of hydrogenated polyhydroxy compounds obtained by selectively hydrogenating aromatic polyhydroxy compounds having at least two phenolic hydroxyl groups in the molecule, which are the raw material for the aromatic epoxy compounds, in the presence of a catalyst under pressure. Specific examples include diglycidyl ethers of hydrogenated bisphenol A, diglycidyl ethers of hydrogenated bisphenol F, and diglycidyl ethers of hydrogenated bisphenol S.

The alicyclic epoxy compound means a compound containing at least two alicyclic epoxy groups in the molecule. The alicyclic epoxy group is a group represented by the following formula (II) (wherein m is an integer of 2 to 5), and specific examples thereof include an epoxycyclopentyl group and an epoxycyclohexyl group.

（式中、Ｒ^１及びＲ^２は、それぞれ独立して、炭素数１～６のアルキル基を表す。）
式（ＩＩＩ）で表される化合物としては、３，４－エポキシシクロヘキシルメチル ３，４－エポキシシクロヘキサンカルボキシレート、３，４－エポキシ－６－メチルシクロヘキシルメチル ３，４－エポキシ－６－メチルシクロヘキサンカルボキシレート等が挙げられる。 The alicyclic epoxy compound is preferably a compound represented by formula (III).

(In the formula, R ¹ and R ² each independently represent an alkyl group having 1 to 6 carbon atoms.)
Examples of the compound represented by formula (III) include 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, 3,4-epoxy-6-methylcyclohexylmethyl 3,4-epoxy-6-methylcyclohexanecarboxylate, and the like.

When an alicyclic epoxy compound is used in combination, the content of the alicyclic epoxy compound is preferably small from the viewpoint of wet heat resistance. Specifically, the content of the alicyclic epoxy compound is preferably less than 30 mass % of the total mass of the cationic polymerizable compound, more preferably less than 15 mass %, and even more preferably 1 to 10 mass %.

(Photocationic Polymerization Initiator)
The photocurable composition undergoes cationic polymerization by irradiation with active energy rays, and is cured to provide an adhesive layer. The photocationic polymerization initiator may be any that generates cationic species or Lewis acid by irradiation with active energy rays such as visible light, ultraviolet light, X-rays, or electron beams, and initiates a polymerization reaction of the cationic polymerizable compound. The photocationic polymerization initiator acts catalytically with light, so that even if it is mixed with the cationic polymerizable compound, it does not affect the storage stability or workability of the cationic polymerizable compound. Examples of the photocationic polymerization initiator include onium salts such as aromatic diazonium salts, aromatic iodonium salts, and aromatic sulfonium salts, and iron-arene complexes.

Examples of aromatic diazonium salts include the following compounds.
Benzenediazonium hexafluoroantimonate,
benzenediazonium hexafluorophosphate,
Benzenediazonium hexafluoroborate, etc.

Examples of aromatic iodonium salts include the following compounds:
diphenyliodonium tetrakis(pentafluorophenyl)borate,
diphenyliodonium hexafluorophosphate,
diphenyliodonium hexafluoroantimonate,
di(4-nonylphenyl)iodonium hexafluorophosphate di(4-alkylphenyl)iodonium hexafluorophosphate,
Triylcumyl iodonium tetrakis(pentafluorophenyl)borate,
di(4-t-butylphenyl)iodonium hexafluorophosphate,
di(4-t-butylphenyl)iodonium hexafluoroantimonate,
(4-methylphenyl)[4-(2-methylpropyl)phenyl]-hexafluorophosphate, and the like.

Examples of the aromatic sulfonium salt include the following compounds.
triphenylsulfonium hexafluorophosphate,
triphenylsulfonium hexafluoroantimonate,
triphenylsulfonium tetrakis(pentafluorophenyl)borate,
4,4'-bis[diphenylsulfonio]diphenyl sulfide bishexafluorophosphate,
4,4'-bis[di(β-hydroxyethoxy)phenylsulfonio]diphenyl sulfide bishexafluoroantimonate,
4,4'-bis[di(β-hydroxyethoxy)phenylsulfonio]diphenyl sulfide bishexafluorophosphate,
7-[di(p-toluyl)sulfonio]-2-isopropylthioxanthone hexafluoroantimonate,
7-[di(p-toluyl)sulfonio]-2-isopropylthioxanthone tetrakis(pentafluorophenyl)borate,
4-phenylcarbonyl-4'-diphenylsulfonio-diphenylsulfide hexafluorophosphate,
4-(p-tert-butylphenylcarbonyl)-4'-diphenylsulfonio-diphenylsulfide hexafluoroantimonate,
4-(p-tert-butylphenylcarbonyl)-4'-di(p-toluyl)sulfonio-diphenyl sulfide tetrakis(pentafluorophenyl)borate 4-(phenylthio)phenyldiphenylsulfonium tris(pentafluoroethyl)trifluorophosphate 4-(phenylthio)phenyldiphenylsulfonium hexafluorophosphate 4-(phenylthio)phenyldiphenylsulfonium trifluoromethanesulfonate, and the like.

Examples of iron-arene complexes include the following compounds:
xylene-cyclopentadienyliron(II) hexafluoroantimonate,
Cumene-cyclopentadienyliron(II) hexafluorophosphate,
Xylene-cyclopentadienyliron(II) tris(trifluoromethylsulfonyl)methanide, etc.

These photocationic polymerization initiators are commercially available. For example, ADEKA OPTOMER SP-100, SP-150, SP-152, SP-170, SP-172 (manufactured by ADEKA Corporation), Photoinitiator 2074 (manufactured by Rhodia), KAYARADD PCI-220, PCI-620 (manufactured by Nippon Kayaku Co., Ltd.), Irgacure 250 (manufactured by Ciba Japan Co., Ltd.), CPI-100P, Examples include CPI-110P, CPI-101A, CPI-200K, CPI-210S (manufactured by San-Apro Co., Ltd.), WPI-113, WPI-116 (manufactured by Wako Pure Chemical Industries, Ltd.), BBI-102, BBI-103, TPS-102, TPS-103, DTS-102, DTS-103 (manufactured by Midori Chemical Co., Ltd.), etc.

These photocationic polymerization initiators may be used alone or in combination of two or more. Among these, aromatic sulfonium salts are particularly preferred because they have ultraviolet absorption properties even in the wavelength region around 300 nm, and therefore have excellent curing properties and can give cured products with good mechanical strength and adhesive strength.

The amount of the photocationic polymerization initiator is 0.5 to 1.5 parts by mass per 100 parts by mass of the cationic polymerizable compound. By adding 0.5 parts by mass or more (more preferably 0.7 parts by mass or more) of the photocationic polymerization initiator per 100 parts by mass of the cationic polymerizable compound, the cationic polymerizable compound can be sufficiently cured, and the resulting polarizing plate is given high mechanical strength and adhesive strength, and is excellent in durability under high temperature and high humidity environments. On the other hand, if the amount is large, the amount of unreacted initiator, the solvent used as necessary to dissolve the initiator, or the decomposition product of the initiator increases, so that the physical properties of the cured product are impaired and the polarizing plate discolors in a high temperature environment. Therefore, the amount of the photocationic polymerization initiator is preferably 1.5 parts by mass or less per 100 parts by mass of the cationic polymerizable compound.

(Other ingredients)
The photocurable composition may contain other components in addition to the cationic polymerizable compound and photocationic polymerization initiator described above. Examples of other components that can be added include photosensitizers, photosensitization assistants, thermal cationic polymerization initiators, chain transfer agents, thermoplastic resins, flow control agents, defoamers, leveling agents, organic solvents, silane coupling agents, and polymers. Depending on the type of protective film to be attached to the polarizing film, it may be preferable to add a photosensitizer and even a photosensitization assistant.

A photosensitizer is a compound that exhibits a maximum absorption at a wavelength longer than the maximum absorption wavelength exhibited by a photocationic polymerization initiator, and promotes the polymerization initiation reaction by the photocationic polymerization initiator. Anthracene-based compounds are preferably used as such photosensitizers. Examples of anthracene-based compounds that can serve as photosensitizers include 9,10-dimethoxyanthracene, 9,10-diethoxyanthracene, 9,10-dipropoxyanthracene, 9,10-diisopropoxyanthracene, 9,10-dibutoxyanthracene, 9,10-dipentyloxyanthracene, and 9,10-dihexyloxyanthracene.

A photosensitizer assistant is a compound that further enhances the action of a photosensitizer. Naphthalene-based compounds are preferably used as such photosensitizer assistants. Examples of naphthalene-based compounds that can serve as photosensitizer assistants include 1,4-dimethoxynaphthalene, 1-ethoxy-4-methoxynaphthalene, 1,4-diethoxynaphthalene, 1,4-dipropoxynaphthalene, and 1,4-dibutoxynaphthalene.

When these other components are added, the amount of each component may be appropriately selected according to the purpose of addition, for example, from a range of 10 parts by mass or less per 100 parts by mass of the cationic polymerizable compound, which is the main component of the photocurable composition.

The viscosity of the photocurable composition in the present invention is not particularly limited, but is preferably 10 to 1000 mPa·s at 25°C. From the viewpoints of coatability and thinning of the coating layer, it is more preferably 15 to 500 mPa·s, and even more preferably 20 to 100 mPa·s. The viscosity in the present invention is measured using an E-type viscometer.

The moisture content of the photocurable composition is preferably 3% by mass or less, more preferably 2% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less, based on the total amount of the photocurable composition. If the moisture content of the photocurable composition exceeds 3% by mass, the curability of the photocurable composition tends to deteriorate. As a result, the adhesion between the thermoplastic resin film and the polarizing film immediately after the polarizing plate is produced decreases, and lifting and peeling tend to occur.

[Preparation of Polarizing Plate]
The thermoplastic resin film is laminated to the polarizing film via an adhesive layer formed from a photocurable composition to form a polarizing plate. This lamination is performed by forming a coating layer of the above-described photocurable composition on one or both of the laminating surfaces of the polarizing film and the thermoplastic resin film, laminating the polarizing film and these protective films via the coating layer, curing the coating layer of the uncured photocurable composition by irradiating it with active energy rays, and fixing these thermoplastic resin films onto the polarizing film.

Various coating methods can be used to form the coating layer of the photocurable composition, such as a doctor blade, wire bar, die coater, comma coater, and gravure coater. It is also possible to continuously supply the polarizing film and the thermoplastic resin film so that the bonding surfaces of the two are on the inside, and cast the photocurable composition between them. Since each coating method has its own optimal viscosity range, it is also a useful technique to adjust the viscosity using a solvent. There is no particular limit to the type of solvent used for this purpose, so long as it dissolves the photocurable composition well without reducing the optical performance of the polarizing film. For example, organic solvents such as hydrocarbons typified by toluene and esters typified by ethyl acetate can be used.

When adhering a polarizing film and a thermoplastic resin film, one or both of the bonding surfaces of the two films may be subjected to an easy-adhesion treatment such as a corona discharge treatment, a plasma treatment, a flame treatment, a primer treatment, or an anchor coating treatment before forming a coating layer of the photocurable composition.

The light source used to irradiate the coating layer of the photocurable composition with active energy rays may be any light source that generates ultraviolet rays, electron beams, X-rays, etc. In particular, light sources having an emission distribution of wavelengths of 400 nm or less, such as low pressure mercury lamps, medium pressure mercury lamps, high pressure mercury lamps, ultra-high pressure mercury lamps, chemical lamps, black light lamps, microwave excited mercury lamps, and metal halide lamps, are preferably used. The irradiation intensity of active energy rays to the photocurable composition is determined for each target composition and is not particularly limited, but it is preferable that the irradiation intensity of the wavelength region effective for activating the initiator is 0.1 to 100 mW/cm ^2. If the light irradiation intensity to the photocurable composition is less than 0.1 mW/cm ² , the reaction time becomes too long, and if it exceeds 100 mW/cm ² , the heat radiated from the lamp and the heat generated during polymerization of the photocurable composition may cause yellowing of the photocurable composition and deterioration of the polarizing film. The light irradiation time of the photocurable composition is controlled for each composition to be cured, and is not particularly limited, but is preferably set so that the cumulative light amount, expressed as the product of the irradiation intensity and the irradiation time, is 10 to 5,000 mJ/cm ^2. If the cumulative light amount of the photocurable composition is less than 10 mJ/cm ² , the generation of active species derived from the initiator may be insufficient, and the resulting adhesive layer may not be sufficiently cured, while if the cumulative light amount exceeds 5,000 mJ/cm ² , the irradiation time becomes very long, which is disadvantageous in improving productivity.

When thermoplastic resin films are laminated to both sides of a polarizing film, the active energy rays may be irradiated from either thermoplastic resin film side. However, for example, if one thermoplastic resin film contains an ultraviolet absorbent and the other thermoplastic resin film does not contain an ultraviolet absorbent, it is preferable to irradiate the active energy rays from the thermoplastic resin film side that does not contain an ultraviolet absorbent in order to effectively utilize the irradiated active energy rays and increase the curing speed.

The thickness of the adhesive layer formed by curing the photocurable composition is usually 20 μm or less, preferably 10 μm or less, and more preferably 5 μm or less. If the adhesive layer becomes too thick, its durability in a heat-resistant environment and in a high-temperature, high-humidity environment tends to deteriorate.

[Laminated optical member]
The polarizing plate of the present invention can be laminated with an optical layer having an optical function other than that of a polarizing plate to form a laminated optical member. Typically, the optical layer is laminated and attached to the thermoplastic resin film of the polarizing plate via an adhesive layer or a pressure-sensitive adhesive layer to form a laminated optical member. Alternatively, for example, a thermoplastic resin film can be laminated to one side of the polarizing film via a photocurable composition according to the present invention, and an optical layer can be laminated and attached to the other side of the polarizing film via an adhesive or a pressure-sensitive adhesive. In the latter case, the adhesive layer for attaching the polarizing film and the optical layer may be a cured product of the photocurable composition defined in the present invention, or an adhesive layer formed from another known adhesive may be used.

Examples of optical layers laminated to a polarizing plate include a reflective layer, a semi-transmissive reflective layer, a light diffusion layer, a light collecting plate, and a brightness enhancement film laminated on the side of the polarizing plate opposite to the side facing the liquid crystal cell for a polarizing plate placed on the rear side of the liquid crystal cell. In addition, for both a polarizing plate placed on the front side of the liquid crystal cell and a polarizing plate placed on the rear side of the liquid crystal cell, there is a retardation plate laminated on the side of the polarizing plate facing the liquid crystal cell.

The reflective layer, semi-transmissive reflective layer, or light diffusion layer are provided to form a reflective polarizing plate (optical component), a semi-transmissive reflective polarizing plate (optical component), or a diffusion polarizing plate (optical component), respectively. The reflective polarizing plate is used in a type of liquid crystal display device that reflects incident light from the viewing side to display, and since a light source such as a backlight can be omitted, it is easy to make the liquid crystal display device thin. The semi-transmissive polarizing plate is used in a type of liquid crystal display device that displays as a reflective type in bright places and with light from a backlight in dark places. For example, the optical component as a reflective polarizing plate can be formed by attaching a foil or a vapor deposition film made of a metal such as aluminum to a protective film on a polarizing film to form a reflective layer. The optical component as a semi-transmissive polarizing plate can be formed by forming the above-mentioned reflective layer as a half mirror, or by adhering a reflective plate containing a pearl pigment or the like and exhibiting light transparency to the polarizing plate. On the other hand, optical components such as diffusion-type polarizing plates have a fine uneven structure formed on the surface using various methods, such as a method of applying a matte finish to a protective film on the polarizing plate, a method of applying a resin containing fine particles, or a method of adhering a film containing fine particles.

Furthermore, an optical component can be formed as a polarizing plate for both reflection and diffusion. In this case, for example, a method can be adopted in which a reflective layer reflecting the fine uneven structure of a diffusive polarizing plate is provided on the fine uneven structure surface of the diffusive polarizing plate. A reflective layer with a fine uneven structure has the advantages of diffusing incident light by diffuse reflection, preventing directionality and glare, and suppressing unevenness in brightness and darkness. In addition, a resin layer or film containing fine particles has the advantage that incident light and its reflected light are diffused when passing through a fine particle-containing layer, suppressing unevenness in brightness and darkness. A reflective layer reflecting a surface fine uneven structure can be formed by directly attaching a metal to the surface of the fine uneven structure by a method such as deposition such as vacuum deposition, ion plating, and sputtering, or plating. The fine particles blended to form the surface micro-uneven structure can be, for example, inorganic fine particles such as silica, aluminum oxide, titanium oxide, zirconia, tin oxide, indium oxide, cadmium oxide, and antimony oxide, each having an average particle size of 0.1 to 30 μm, or organic fine particles such as crosslinked or non-crosslinked polymers.

Light-collecting plates are used for purposes such as light path control, and can be formed as prism array sheets, lens array sheets, or dot-attached sheets.

Brightness enhancement films are used to improve the brightness of liquid crystal display devices. Examples include reflective polarizing separation sheets designed to create anisotropy in reflectance by laminating multiple thin films with different refractive index anisotropy, and circular polarizing separation sheets in which an oriented film of cholesteric liquid crystal polymer or a layer of the oriented liquid crystal is supported on a film substrate.

On the other hand, the retardation plate as the optical layer described above is used for the purpose of compensating for the retardation caused by the liquid crystal cell. Examples include birefringent films made of various stretched plastic films, films in which discotic liquid crystal or nematic liquid crystal is oriented and fixed, and films in which the above-mentioned liquid crystal layer is formed on a film substrate. When a liquid crystal layer is formed on a film substrate, a cellulose-based resin film such as triacetyl cellulose is preferably used as the film substrate.

Examples of plastics that form birefringent films include amorphous polyolefin resins, polycarbonate resins, acrylic resins, linear polyolefin resins such as polypropylene, polyvinyl alcohol, polystyrene, polyarylate, and polyamide. The stretched film can be processed in an appropriate manner such as uniaxial or biaxial. Two or more retardation plates may be used in combination to control optical properties such as broadening the bandwidth.

In laminated optical components, those that include a retardation plate as an optical layer other than a polarizing plate are preferably used because they can effectively provide optical compensation when applied to a liquid crystal display device. The retardation value (in-plane and thickness direction) of the retardation plate can be selected to be optimal depending on the liquid crystal cell to be used.

The laminated optical member can be a laminate of two or three layers or more by combining a polarizing plate with one or more of the above-mentioned various optical layers selected according to the purpose of use. In that case, the various optical layers forming the laminated optical member are integrated with the polarizing plate using an adhesive layer or a pressure-sensitive adhesive layer, and the adhesive or pressure-sensitive adhesive used for that purpose is not particularly limited as long as the adhesive layer or pressure-sensitive adhesive layer is well formed. From the viewpoint of ease of adhesion work and prevention of optical distortion, it is preferable to use a pressure-sensitive adhesive (also called a pressure-sensitive adhesive). The pressure-sensitive adhesive can be one that uses an acrylic polymer, a silicone polymer, polyester, polyurethane, polyether, or the like as a base polymer. Among them, it is preferable to select and use an acrylic pressure-sensitive adhesive that has excellent optical transparency, moderate wettability and cohesive force, excellent adhesion to the substrate, and further has weather resistance and heat resistance, and does not cause peeling problems such as lifting or peeling under heating or humidifying conditions. In acrylic adhesives, an acrylic copolymer with a weight average molecular weight of 100,000 or more is useful as the base polymer, which is a mixture of an alkyl ester of (meth)acrylic acid having an alkyl group with 20 or less carbon atoms, such as a methyl group, ethyl group, or butyl group, and a functional group-containing acrylic monomer such as (meth)acrylic acid or hydroxyethyl (meth)acrylate, so that the glass transition temperature is preferably 25°C or less, more preferably 0°C or less.

The adhesive layer can be formed on the polarizing plate by, for example, dissolving or dispersing the adhesive composition in an organic solvent such as toluene or ethyl acetate to prepare a 10 to 40% by weight solution, which is then directly coated onto the polarizing plate, or by forming an adhesive layer on a protective film in advance, which is then transferred onto the polarizing plate. The thickness of the adhesive layer is determined according to its adhesive strength, but a range of about 1 to 50 μm is appropriate.

The adhesive layer may also contain fillers such as glass fibers, glass beads, resin beads, metal powders, and other inorganic powders, pigments, colorants, antioxidants, antistatic agents, and ultraviolet absorbers, as required. Examples of ultraviolet absorbers include salicylic acid ester compounds, benzophenone compounds, benzotriazole compounds, cyanoacrylate compounds, and nickel complex compounds.

Examples of antistatic agents include ionic compounds, conductive fine particles, and conductive polymers. Among these, appropriate antistatic agents can be used alone or in combination of two or more. Of course, two or more antistatic agents classified as ionic compounds can be used in combination, two or more antistatic agents classified as conductive fine particles can be used in combination, and two or more antistatic agents classified as conductive polymers can be used in combination.

Among the antistatic agents described above, ionic compounds are preferably used because they have excellent compatibility with solvents.

Ionic compounds can be classified into ionic compounds having organic cations, ionic compounds having inorganic cations, ionic compounds having organic anions, and ionic compounds having inorganic anions. Examples of organic cations include pyridinium cations, imidazolium cations, ammonium cations, sulfonium cations, and phosphonium cations. Examples of inorganic cations include lithium and potassium. The cationic component constituting the ionic compound may be an inorganic anion or an organic anion, but is preferably an organic cation from the viewpoint of compatibility with (meth)acrylic resins. On the other hand, the anionic component constituting the ionic compound may be an inorganic anion or an organic anion, but is preferably an anionic component containing a fluorine atom, since it provides an ionic compound with excellent antistatic performance. Examples of the anion component containing a fluorine atom include hexafluorophosphate anion [(PF ₆ ⁻ )], bis(trifluoromethanesulfonyl)imide anion [(CF ₃ SO ₂ ) ₂ N ⁻ ] anion, and bis(fluorosulfonyl)imide anion [(FSO ₂ ) ₂ N ⁻ ] anion.

The laminated optical component can be placed on one or both sides of the liquid crystal cell. Any liquid crystal cell can be used, and a liquid crystal display device can be formed using various liquid crystal cells, such as active matrix drive types such as thin film transistor types, and passive matrix drive types such as super twisted nematic types. An adhesive is usually used to bond the laminated optical component and the liquid crystal cell.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" representing the content or amount used are based on mass unless otherwise specified. The cationic polymerizable compound, photocationic polymerization initiator, and leveling agent used in the following examples are as follows, and are represented by the respective symbols below.

(A) Cationic polymerizable compound (a1) 3-ethyl-3{[(3-ethyloxetane-3-yl)methoxy]methyl}oxetane: available from Toagosei Co., Ltd., trade name "OXT-221". In Table 1 described later, this is abbreviated as "(a1)".
(a2) 1,4-butanediol diglycidyl ether: available from Nagase ChemteX Corporation under the trade name "EX-214L". In Table 1 below, this is abbreviated as "(a2)".

(B) Photocationic Polymerization Initiator (b1) 4-(phenylthio)phenyldiphenylsulfonium hexafluorophosphate: available from San-Apro Co., Ltd., product name "CPI-100P". In Table 1 described later, this is abbreviated as "(b1)". CPI-100P was used as a 50% propylene carbonate solution, and its solid content is shown in Table 1.

[Examples 1 to 3, Comparative Example 1]
[Preparation Examples 1 to 4]
(1) Preparation of Photocurable Compositions Photocurable Compositions 1 to 3 were prepared by mixing the components in the proportions shown in Table 1 and then degassing.

(2) Preparation of polarizing plate
[Example 1]
A polarizing plate was prepared using the photocurable composition 1. A corona discharge treatment was applied to the surface of a 50 μm thick cycloolefin film (manufactured by Zeon Corporation under the trade name "ZEONOR"), and the photocurable composition 1 was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing would be about 1.5 μm. A polyvinyl alcohol-iodine polarizing film was laminated to the surface on which the photocurable composition was applied. A corona discharge treatment was also applied to the lamination surface of a protective film made of an 80 μm thick (meth)acrylic resin (PMMA) containing an ultraviolet absorber (trade name "Technoloy S001", manufactured by Sumitomo Chemical Co., Ltd.), and the same photocurable composition 1 as that used for the cycloolefin film was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing would be about 1.5 μm. A polarizing film having a cycloolefin film laminated to one side thereof prepared above was laminated to the polarizing film side to prepare a laminate. The laminate was irradiated with ultraviolet light from the cycloolefin film side using a belt conveyor-equipped ultraviolet light irradiation device (using a "D bulb" lamp manufactured by Fusion UV Systems) so that the integrated light amount (UVB) was 200 mJ/ ^cm2 , curing the photocurable composition and forming an adhesive layer. In this way, a polarizing plate was produced in which protective films were bonded to both sides of the polarizing film via an adhesive.

[Example 2]
A polarizing plate was prepared using the photocurable composition 1. A corona discharge treatment was applied to the surface of a 50 μm thick cycloolefin film (trade name "ZEONOR" manufactured by Zeon Corporation), and the above photocurable composition 1 was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing would be about 1.5 μm. A polyvinyl alcohol-iodine polarizing film was laminated to the coated surface of the photocurable composition. A corona discharge treatment was also applied to the laminated surface of a 60 μm thick triacetyl cellulose (TAC) film (trade name "TD60UL" manufactured by Fuji Film Corporation) containing an ultraviolet absorber, and the same photocurable composition used for the cycloolefin film was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing would be about 1.5 μm. A polarizing film having a cycloolefin film laminated to one side thereof prepared above was laminated to the polarizing film side to prepare a laminate. The laminate was irradiated with ultraviolet light from the cycloolefin film side using a belt conveyor-equipped ultraviolet light irradiation device (using a "D bulb" lamp manufactured by Fusion UV Systems) so that the integrated light amount (UVB) was 200 mJ/ ^cm2 , curing the photocurable composition and forming an adhesive layer. In this way, a polarizing plate was produced in which protective films were bonded to both sides of the polarizing film via an adhesive.

[Example 3]
A polarizing plate was produced in the same manner as in Example 1, except that the photocurable composition 1 was replaced with the photocurable composition 2.

[Comparative Example 1]
A polarizing plate was produced in the same manner as in Example 1, except that the photocurable composition 1 was replaced with the photocurable composition 3.

The moisture permeability of the thermoplastic resin film at a temperature of 40° C. and a relative humidity of 90% was measured by the cup method specified in JIS Z 0208.
PMMA: 60g/( ^m2・24hr)
TAC: 520g/( ^m2・24hr)
COP: 5.8g/( ^m2・24hr)

(3) Preparation of a polarizing plate with an adhesive layer: A corona treatment was performed on the cycloolefin-based film surface of the polarizing plate prepared in (2) above, and an acrylic adhesive having a thickness of 25 μm was attached thereto using a laminator. The film was then aged for 7 days under conditions of a temperature of 23° C. and a relative humidity of 65%, to obtain a polarizing film with an adhesive.

(4) Evaluation of optical properties of polarizing plate The adhesive-attached polarizing plate prepared in (3) above was cut into a size of 30 mm x 30 mm, and attached to alkali-free glass (trade name "EAGLE XG" manufactured by Corning Incorporated), and the b value of each transmission hue was measured. The measurement was performed using an ultraviolet-visible spectrophotometer "UV-2450" manufactured by Shimadzu Corporation equipped with an optional accessory "film holder with polarizing film", and the transmission spectrum of the polarizing plate in the wavelength range of 380 nm to 780 nm was obtained, and the b value of the transmission hue was calculated using the software "UV-Probe" attached to the spectrophotometer.

(5) Evaluation of Heat Resistance of Polarizing Plate The polarizing plate prepared in (4) above was subjected to a heating test in which the plate was left in a heating environment at a temperature of 90° C. for 48 hours, and the transmission hue b value of the polarizing plate after the test was measured. The measurement and calculation of the b value were performed in the same manner as in (4) above. The absolute value (|Δb|) of the difference in transmission hue before and after the heat resistance test was calculated according to the following formula.

|Δb| = |b value after heating test - b value before heating test|

Next, the "Δb change rate (improvement rate)" (%) of each example was calculated from the obtained |Δb| value based on the |Δb| of Comparative Example 1 according to the following formula. The calculated values of the Δb change rate are shown in Table 1. The larger the Δb change rate, the more excellent the heat resistance.
Δb change rate (%) for each example
= |{(|Δb| of each example)-(|Δb| of Comparative Example 1)}|/(|Δb| of Comparative Example 1)×100
In all of the Examples and Comparative Examples, Δb exhibited a positive value.
The results are shown in Table 1.

From Table 1, it was confirmed that a polarizing plate in which a thermoplastic resin film is attached to at least one surface of a polarizing film via an adhesive exhibits good heat resistance by using an adhesive containing 10 to 70% by mass of a bifunctional oxetane compound in a cationic polymerizable compound and 0.5 to 1.5 parts by mass of a photocationic polymerization initiator per 100 parts by mass of the cationic polymerizable compound.

[Preparation Example 4-10]
(1) Preparation of Photocurable Compositions Photocurable Compositions 4 to 10 were prepared by mixing the components in the proportions shown in Table 2 and then degassing.
The abbreviations in Table 2 represent the compounds described below.
(A) Cationic polymerizable compound (a1) 3-ethyl-3{[(3-ethyloxetane-3-yl)methoxy]methyl}oxetane: available from Toagosei Co., Ltd., trade name “OXT-221”. In Table 2, abbreviated as “(a1)”.
(a2) 1,4-butanediol diglycidyl ether: available from Nagase ChemteX Corporation under the trade name “EX-214L”. In Table 2, this is abbreviated as “(a2)”.
(a3) Neopentyl glycol diglycidyl ether: available from Nagase ChemteX Corporation under the trade name “EX-211L”. In Table 2, this is abbreviated as “(a3)”.
(a4) Pentaerythritol polyglycidyl ether: available from Nagase ChemteX Corporation under the trade name “EX-411”. In Table 2, this is abbreviated as “(a4)”.
(a5) 3',4'-epoxycyclohexylmethyl 3,4-cyclohexanecarboxylate: available from Daicel Chemical Industries, Ltd. under the trade name "Celloxide 2021P". In Table 2, this is abbreviated as "(a5)".
(B) Photocationic Polymerization Initiator (b1) 4-(phenylthio)phenyldiphenylsulfonium hexafluorophosphate: available from San-Apro Co., Ltd., trade name "CPI-100P". Abbreviated as "(b1)" in Table 2. CPI-100P was used as a 50% propylene carbonate solution, and its solid content is shown in Table 2.

[Example 4]
(2) Preparation of polarizing plate The surface of a 50 μm thick cycloolefin resin film [trade name "ZEONOR" manufactured by Zeon Corporation] was subjected to a corona discharge treatment, and the photocurable composition 4 was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing was about 1.5 μm. A polyvinyl alcohol-iodine polarizing film was laminated to the surface on which the photocurable composition 4 was applied. In addition, the laminated surface of a protective film made of a 80 μm thick (meth)acrylic resin (PMMA) [trade name "Technoloy S001" manufactured by Sumitomo Chemical Co., Ltd.] containing an ultraviolet absorber was subjected to a corona discharge treatment, and the photocurable composition 4 was applied to the corona discharge treated surface using a bar coater so that the film thickness after curing was about 1.5 μm. The polarizing film with the cycloolefin film laminated to one side prepared above was laminated to the polarizing film side to the surface on which the photocurable composition 4 was applied, to prepare a laminate. The laminate was irradiated with ultraviolet light from the cycloolefin film side using an ultraviolet light irradiation device with a belt conveyor (the lamp used was a "D bulb" manufactured by Fusion UV Systems) so that the integrated light amount (UVB) was 200 mJ/ ^cm2 , curing the photocurable composition and forming an adhesive layer, thereby producing a polarizing plate in which protective films were attached to both sides of the polarizing film via an adhesive.

[Example 5]
A polarizing plate was prepared in the same manner as in Example 4, except that Photocurable Composition 4 was replaced with Photocurable Composition 5.

[Example 6]
A polarizing plate was prepared in the same manner as in Example 4, except that Photocurable Composition 4 was replaced with Photocurable Composition 6.

[Example 7]
A polarizing plate was prepared in the same manner as in Example 4, except that Photocurable Composition 4 was replaced with Photocurable Composition 7.

[Example 8]
A polarizing plate was produced in the same manner as in Example 4, except that Photocurable Composition 4 was replaced with Photocurable Composition 8.

[Example 9]
A polarizing plate was prepared in the same manner as in Example 4, except that Photocurable Composition 4 was replaced with Photocurable Composition 9.

[Example 10]
A polarizing plate was prepared in the same manner as in Example 4, except that the photocurable composition 4 was replaced with the photocurable composition 10.

(3) Preparation of a polarizing plate with an adhesive layer: A corona treatment was performed on the cycloolefin-based film surface of the polarizing plate prepared in (2) above, and an acrylic adhesive having a thickness of 25 μm was attached thereto using a laminator. The film was then aged for 7 days under conditions of a temperature of 23° C. and a relative humidity of 65%, to obtain a polarizing film with an adhesive.

(4) Evaluation of optical properties of polarizing plate The adhesive-attached polarizing plate prepared in (3) above was cut into a size of 30 mm x 30 mm, and attached to alkali-free glass (trade name "EAGLE XG" manufactured by Corning Incorporated), and the b value of each transmission hue was measured. The measurement was performed using an ultraviolet-visible spectrophotometer "UV-2450" manufactured by Shimadzu Corporation equipped with an optional accessory "film holder with polarizing film", and the transmission spectrum of the polarizing plate in the wavelength range of 380 nm to 780 nm was obtained, and the b value of the transmission hue was calculated using the software "UV-Probe" attached to the spectrophotometer.

(5) Evaluation of Heat Resistance of Polarizing Plate The polarizing plate prepared in (4) above was subjected to a heating test in which the plate was left in a heating environment at a temperature of 90° C. for 48 hours, and the transmission hue b value of the polarizing plate after the test was measured. The measurement and calculation of the b value were performed in the same manner as in (4) above. The absolute value (|Δb|) of the difference in transmission hue before and after the heat resistance test was calculated according to the following formula.
|Δb|=|b value after heating test−b value before heating test|

Next, the "Δb change rate (improvement rate)" (%) of each example was calculated from the obtained |Δb| value based on the |Δb| of Comparative Example 1 according to the following formula. The calculated values of the Δb change rate are shown in Table 2. The larger the Δb change rate, the more excellent the heat resistance.
Δb change rate (%) for each example
= |{(|Δb| of each example)-(|Δb| of Comparative Example 1)}|/(|Δb| of Comparative Example 1)×100
In all of the Examples and Comparative Examples, Δb exhibited a positive value.
The results are shown in Table 2.

(6) Evaluation of wet heat resistance of polarizing plate The polarizing plate prepared in (4) above was subjected to a wet heat resistance test in which the plate was left for 48 hours in a high-temperature and high-humidity environment at a temperature of 80° C. and a relative humidity of 90%, and the luminosity-corrected single transmittance Ty value of the polarizing plate after the test was measured. The measurement and calculation of the Ty value were performed in the same manner as in (4) above. The absolute value (|ΔTy|) of the difference in luminosity-corrected single transmittance Ty before and after the wet heat resistance test was calculated according to the following formula.
|ΔTy|=|Ty value after heating test−Ty value before heating test|
In all of the Examples and Comparative Examples, ΔTy exhibited a positive value.
The results are shown in Table 3.

Claims (3)

A polarizing plate in which a thermoplastic resin film is attached to at least one surface of a polarizing film via an adhesive layer,
the adhesive layer is a cured product of a photocurable composition containing a cationic polymerizable compound and a photocationic polymerization initiator,
The photocurable composition contains 0.5 to 1.5 parts by mass of a photocationic polymerization initiator based on 100 parts by mass of a cationic polymerizable compound,
the cationic polymerizable compound contains a difunctional oxetane compound in an amount of more than 50% by mass and not more than 70% by mass based on the total mass of the cationic polymerizable compound;
The polarizing plate according to claim 1, wherein the photocationic polymerization initiator is an aromatic sulfonium salt . The polarizing plate according to claim 1 , wherein the cationically polymerizable compound further contains an aliphatic epoxy compound in an amount of 10 % by mass or more and less than 50% by mass based on the total mass of the cationically polymerizable compound. The thermoplastic resin film has a moisture permeability of 300 g at a temperature of 40° C. and a relative humidity of 90%.
3. The polarizing plate according to claim 1, wherein the surface roughness is 1/(m2·24 hr) or less.