Polarizing film, polarizing plate, and method for manufacturing polarizing plate

Abstract

Provided are a polarizing plate, and a method for manufacturing the polarizing plate, and a polarizing film capable of satisfactorily suppressing a decrease in the light transmittance of the polarizing plate during high-temperature durability tests. The polarizing film contains polyvinyl alcohol and an iodine-based dye, wherein the peak top temperature of an endothermic peak appearing in a temperature range of 50°C to 150°C is 102°C or higher in differential scanning calorimetry measurement of the polarizing film.

Description

Polarizing film, polarizing plate, and method for manufacturing polarizing plate

[0001] The present invention relates to a polarizing film, a polarizing plate, and a method for producing a polarizing plate.

[0002] Polarizing plates, which have the function of transmitting and blocking light, are fundamental components of liquid crystal displays (LCDs), along with liquid crystals, which change the polarization state of light. Many polarizing plates have a structure in which a protective film, such as a triacetyl cellulose (TAC) film, is laminated to the surface of a polarizing film. The polarizing film that constitutes a polarizing plate is typically a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA") uniaxially stretched and oriented film, to which a dichroic dye, such as an iodine-based dye or a dichroic organic dye, is adsorbed. Such polarizing films are manufactured by uniaxially stretching a PVA film that already contains the dichroic dye, by adsorbing the dichroic dye simultaneously with uniaxial stretching of the PVA film, or by adsorbing the dichroic dye after uniaxial stretching of the PVA film.

[0003] LCDs are used in a wide range of applications, including small devices such as calculators and wristwatches, laptop computers, LCD TVs, mobile phones, and tablet devices. In recent years, LCDs have also been used in in-vehicle image display devices such as car navigation systems and rearview monitors. As a result, LCDs are required to have higher durability in harsher environments than ever before.

[0004] It is known that when an image display device equipped with a polarizing plate is subjected to a long-term high-temperature durability test required for in-vehicle displays, the light transmittance of the polarizing plate decreases. One of the causes of this is thought to be polyenation of the PVA that constitutes the polarizing film, and the decrease in light transmittance of the polarizing plate becomes more pronounced under high-temperature conditions.

[0005] Regarding the problem of polyenization of polarizing plates, Patent Document 1 describes that a polarizing plate with excellent durability and with little decrease in transmittance in a high-temperature environment can be obtained by using a protective film with a small amount of water absorption. Patent Document 2 describes that polyenization can be suppressed by adjusting the thickness of a pressure-sensitive adhesive layer provided between the polarizing plate and a transparent plate or the like.

[0006] JP 2014-102353 A JP 2018-25764 A

[0007] However, according to the studies of the inventors, although the methods described in Patent Documents 1 and 2 tend to suppress the decrease in light transmittance of the polarizing plate, a sufficient effect of suppressing polyenization could not be confirmed in a durability test under a high-temperature environment of 105°C or higher, which has become necessary in recent years.

[0008] The present invention has been made in light of the above circumstances, and aims to provide a polarizing film, a polarizing plate, and a method for manufacturing a polarizing plate that can sufficiently suppress a decrease in the light transmittance of the polarizing plate in a high-temperature durability test.

[0009] As a result of extensive research conducted by the present inventors to achieve the above-mentioned object, they discovered that the thermal stability of the structure, which appears as an endothermic peak in the temperature range of 50°C to 150°C in differential scanning calorimetry of a polarizing film, affects the suppression of polyenization of a polarizing plate. Based on this knowledge, they conducted further research and completed the present invention.

[0010] That is, the present invention relates to: [1] a polarizing film containing polyvinyl alcohol and an iodine-based dye, wherein the polarizing film has an endothermic peak of 102°C or higher in a temperature range of 50°C to 150°C in differential scanning calorimetry; [2] a polarizing plate comprising the polarizing film of [1] and a protective film laminated on at least one surface of the polarizing film, the protective film containing a cellulose ester-based resin; [3] a method for producing a polarizing plate, the method comprising the steps of: (1) laminating a protective film on at least one surface of a polarizing film containing polyvinyl alcohol and an iodine-based dye to obtain a laminate; (2) adjusting the moisture content of the laminate to 4.0% by mass or less; and (3) annealing the laminate by heating at 60°C or higher for 4 hours or more; and [4] a method for producing a polarizing plate of [3], wherein the protective film contains a cellulose ester-based resin.

[0011] According to the present invention, it is possible to provide a polarizing film, a polarizing plate, and a method for producing a polarizing plate, which can sufficiently suppress a decrease in light transmittance of the polarizing plate in a high-temperature durability test.

[0012] Fig. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. Fig. 2 is a schematic cross-sectional view of a laminate (heat resistance test sample) prepared in a heat resistance test in the examples. Fig. 3 is a schematic cross-sectional view of a laminate subjected to an annealing treatment in the examples. Fig. 4 is a DSC curve obtained by differential scanning calorimetry for the polarizing film of Example 1.

[0013] Hereinafter, embodiments of the present invention will be described in detail. In this specification, a numerical range expressed as "A to B" or "from A to B" means that A and B are included as the lower and upper limits, respectively.

[0014] <Polarizing Film> A polarizing film according to one embodiment of the present invention is a polarizing film containing PVA and an iodine-based dye, and in differential scanning calorimetry of the polarizing film, the peak-top temperature of an endothermic peak appearing in the temperature range of 50°C to 150°C is 102°C or higher.

[0015] This polarizing film can sufficiently suppress a decrease in the light transmittance of the polarizing plate in a high-temperature durability test. While the reason for this is unclear, the following is speculated. Polyenization of the PVA contained in the polarizing film is thought to occur through a dehydration reaction in which an iodine-based dye serves as one of the catalysts. Furthermore, when the protective film of a polarizing plate contains a cellulose ester resin, the cellulose ester resin undergoes hydrolysis to generate acid when the polarizing plate is placed in a high-temperature environment for an extended period of time. This acid also catalyzes the dehydration reaction of the PVA, resulting in particularly significant polyenization of the PVA. Meanwhile, the endothermic peak appearing in the temperature range of 50°C to 150°C in differential scanning calorimetry of the polarizing film is thought to be derived from a structure presumed to result from crosslinking of the PVA contained in the polarizing film with boric acid (hereinafter, the "structure presumed to result from crosslinking of PVA with boric acid" may be referred to as a "BCC (Boric Acid Cross-Linked Chain Crystallite) structure"). The BCC structure present in the polarizing film inhibits the dehydration reaction of PVA, and if the BCC structure has high thermal stability, it is believed that the polyenization of PVA is sufficiently suppressed even in high-temperature durability tests. In the polarizing film of the present invention, the peak-top temperature of the endothermic peak is 102°C or higher, and the BCC structure has high thermal stability. Therefore, it is presumed that this polarizing film can sufficiently suppress the decrease in light transmittance of the polarizing plate in high-temperature heat resistance tests.

[0016] The lower limit of the peak top temperature is 102°C, preferably 103°C, more preferably 104°C, and even more preferably 105°C. When the peak top temperature is equal to or higher than the lower limit, the thermal stability of the BCC structure is enhanced, thereby suppressing a decrease in the light transmittance of the polarizing plate in a high-temperature heat resistance test. The upper limit of the peak top temperature may be, for example, 120°C, 110°C, 107°C, or 105°C. As described below, the peak top temperature can be adjusted by the manufacturing conditions of the polarizing film (stretching temperature, drying temperature, moisture content before annealing, annealing conditions, etc.). The peak top temperature is a value measured by the method described in the Examples section below. When multiple endothermic peaks appear in the temperature range of 50°C to 150°C in differential scanning calorimetry of the polarizing film, the lowest endothermic peak is taken as the peak top temperature. Furthermore, it is preferable that only one endothermic peak appears in the temperature range of 50°C to 150°C in differential scanning calorimetry of the polarizing film.

[0017] A polarizing film according to one embodiment of the present invention is typically a film in which an iodine-based dye is adsorbed onto a uniaxially stretched PVA film. In the following description, an unstretched film containing PVA may be referred to as a "PVA film." A polarizing film can be produced by stretching a PVA film that already contains an iodine-based dye, by adsorbing an iodine-based dye simultaneously with stretching the PVA film, or by stretching a PVA film to form a matrix and then adsorbing an iodine-based dye. The components of the polarizing film are described in detail below.

[0018] (PVA) PVA (polyvinyl alcohol) is a polymer consisting of vinyl alcohol units (-CH ２ PVA is a polymer having —CH(OH)— as the main structural unit. In addition to vinyl alcohol units, PVA may also have vinyl ester units and other units.

[0019] As the PVA, those obtained by saponifying polyvinyl esters obtained by polymerizing one or more vinyl esters can be used. Examples of vinyl esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among vinyl esters, those containing a vinyloxycarbonyl group (H) in the molecule are preferred from the viewpoints of ease of production, availability, cost, etc. ２ Compounds having C═CH—O—CO— are preferred, with vinyl acetate being more preferred.

[0020] The polyvinyl ester is preferably one obtained by using only one or more vinyl esters as a monomer, more preferably one obtained by using only one vinyl ester as a monomer, but may also be a copolymer resin of one or more vinyl esters with other monomers copolymerizable therewith, as long as the effects of the present invention are not significantly impaired.

[0021] The upper limit of the proportion of structural units derived from other copolymerizable monomers is preferably 15 mol %, more preferably 10 mol %, even more preferably 5 mol %, and even more preferably 1 mol %, based on the total structural units constituting the PVA.

[0022] Examples of other monomers copolymerizable with vinyl esters include α-olefins having 2 to 30 carbon atoms, such as ethylene, propylene, 1-butene, and isobutene; (meth)acrylic acid or salts thereof; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid or salts thereof, (meth)acrylamidopropyldimethylamine or salts thereof, N N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as (meth)acrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid, or a salt, ester, or acid anhydride thereof; itaconic acid, or a salt, ester, or acid anhydride thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and unsaturated sulfonic acids or a salt thereof.

[0023] The polyvinyl ester may have structural units derived from one or more of the other monomers mentioned above.

[0024] PVA that has not been graft-copolymerized is preferably used. However, PVA may be modified with one or more graft-copolymerizable monomers. Graft copolymerization can be carried out on at least one of polyvinyl ester and PVA obtained by saponifying it. Examples of graft-copolymerizable monomers include unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acids or derivatives thereof; and α-olefins having 2 to 30 carbon atoms. The proportion of structural units derived from graft-copolymerizable monomers in polyvinyl ester or PVA is preferably 5 mol % or less of the total structural units constituting the polyvinyl ester or PVA.

[0025] The lower limit of the degree of polymerization of PVA is preferably 1,000, more preferably 1,500, even more preferably 1,700, and even more preferably 2,000. Having a degree of polymerization of PVA equal to or greater than the lower limit makes it possible to improve the flexibility of the polarizing film. On the other hand, the upper limit of this degree of polymerization is preferably 10,000, more preferably 8,000, even more preferably 5,000, and even more preferably 3,000. Having a degree of polymerization of PVA equal to or less than the upper limit makes it possible to suppress increases in PVA production costs and the occurrence of defects during film formation. The degree of polymerization of PVA refers to the average degree of polymerization measured in accordance with the description of JIS K6726-1994.

[0026] The lower limit of the saponification degree of the PVA is preferably 90 mol%, more preferably 95 mol%, even more preferably 99 mol%, and even more preferably 99.5 mol%, because this improves the wet heat resistance of the polarizing film. The upper limit of this saponification degree is not particularly limited and may be 100% or 99.99 mol%. The saponification degree of the PVA refers to the ratio (mol%) of vinyl alcohol units to the total number of moles of vinyl ester units and vinyl alcohol units. The saponification degree can be measured in accordance with the description of JIS K6726-1994.

[0027] The lower limit of the PVA content in the polarizing film is preferably 50% by mass, and may be 60% by mass or 70% by mass, and the upper limit of the PVA content may be 95% by mass, 90% by mass, or 80% by mass.

[0028] (Iodine-based dye) The iodine-based dye may be a dichroic dye containing iodine element. ３ － , I ５ － and salts containing these anions. ３ － , I ５ － Examples of the counter cations include alkali metal ions such as potassium ions. Iodine-based dyes include, for example, iodine (I ２ The polarizing film can be obtained by contacting iodine-based dyes (i.e., iodine-based dyes) with potassium iodide. One or more iodine-based dyes can be used. The content of the iodine-based dye in the polarizing film is not particularly limited, and it is sufficient that an appropriate amount is contained so that the desired polarizing performance is exhibited.

[0029] (Boron Compound) The polarizing film usually further contains a boron compound. The boron compound may be a component that functions as a crosslinking agent. Part or all of the boron compound may form a crosslinked structure with the PVA. Examples of the boron compound include boric acid and borate salts (borax, etc.), with boric acid being preferred. One or more boron compounds may be used.

[0030] The lower limit of the content of the boron compound in the polarizing film is preferably 1% by mass, more preferably 5% by mass, even more preferably 10% by mass, even more preferably 15% by mass, and particularly preferably 20% by mass. On the other hand, the upper limit of the content of the boron compound is preferably 45% by mass, more preferably 40% by mass, even more preferably 35% by mass, and even more preferably 30% by mass. By ensuring that the content of the boron compound is within the above range, polarization performance, polyenation suppression performance, and the like can be improved. The content of the boron compound may be the content converted to boric acid.

[0031] (Plasticizer) The polarizing film and the PVA film used to produce the polarizing film may contain a plasticizer. By including a plasticizer in the PVA film, the handleability and stretchability of the PVA film can be improved. However, when producing a polarizing film from a PVA film, the plasticizer may be released from the PVA film during processes such as swelling and dyeing. Therefore, the plasticizer does not need to remain in the polarizing film. Polyhydric alcohols are preferably used as the plasticizer. Examples of polyhydric alcohols include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane, with glycerin being preferred. One or more plasticizers can be used.

[0032] The lower limit of the plasticizer content in the PVA film is preferably 2 parts by mass, more preferably 3 parts by mass, and even more preferably 4 parts by mass, per 100 parts by mass of PVA. Meanwhile, the upper limit of this content is preferably 20 parts by mass, more preferably 17 parts by mass, and even more preferably 14 parts by mass, per 100 parts by mass of PVA. Having the plasticizer content in the PVA film equal to or greater than the lower limit improves the stretchability of the PVA film. Meanwhile, having the plasticizer content in the PVA film equal to or less than the upper limit can prevent the plasticizer from bleeding out onto the surface of the PVA film, thereby preventing the PVA film from becoming difficult to handle.

[0033] (Surfactant) Polarized films and PVA films for producing polarized films may contain a surfactant. When a PVA film is produced using a film-forming solution as described below, adding a surfactant to the film-forming solution improves film-forming properties and suppresses unevenness in film thickness. Furthermore, when a metal roll or belt is used for film production, the PVA film can be easily peeled from the metal roll or belt. When a PVA film is produced from a film-forming solution containing a surfactant, the PVA film and the polarized film produced from the PVA film may contain a surfactant. The type of surfactant added to the film-forming solution for producing a PVA film, and thus the type of surfactant contained in the PVA film and polarized film, is not particularly limited. However, from the viewpoint of peelability from the metal roll or belt, anionic surfactants and nonionic surfactants are preferred, and nonionic surfactants are particularly preferred. One or more surfactants may be used.

[0034] Examples of anionic surfactants include carboxylic acid surfactants such as potassium laurate; sulfate ester surfactants such as polyoxyethylene lauryl ether sodium sulfate and octyl sulfate; and sulfonic acid surfactants such as dodecylbenzenesulfonate.

[0035] Examples of nonionic surfactants include alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether.

[0036] When a surfactant is blended into a film-forming solution for producing a PVA film, the lower limit of the surfactant content in the film-forming solution, and therefore the surfactant content in the PVA film or polarized film, is preferably 0.01 parts by mass, more preferably 0.02 parts by mass, per 100 parts by mass of PVA contained in the film-forming solution, PVA film, or polarized film. Meanwhile, the upper limit of the surfactant content is preferably 0.5 parts by mass, more preferably 0.1 parts by mass, per 100 parts by mass of PVA contained in the film-forming solution, PVA film, or polarized film. Having a surfactant content equal to or greater than the lower limit can improve film-forming properties and peelability. Having a surfactant content equal to or less than the upper limit can prevent the surfactant from bleeding out onto the surface of the PVA film, causing blocking and resulting in reduced handleability.

[0037] (Other Components, etc.) The polarizing film may contain other components, such as antioxidants, antifreeze agents, pH adjusters, opacifying agents, color inhibitors, and oils, as needed, in addition to the PVA, iodine-based pigments, boron compounds, plasticizers, and surfactants. However, the content of other components in the polarizing film other than the PVA, iodine-based pigments, boron compounds, plasticizers, and surfactants is preferably 1% by mass or less, and more preferably 0.1% by mass or less. The content of inorganic compounds other than the iodine-based pigments in the polarizing film is also preferably 1% by mass or less, and more preferably 0.1% by mass or less. The other components and inorganic compounds other than the iodine-based pigments may cause defects such as voids in the polarizing film. Therefore, reducing the content of the other components and inorganic compounds other than the iodine-based pigments tends to improve the polarization performance of the polarizing film.

[0038] The upper limit of the thickness of the polarizing film is, for example, 100 μm, preferably 50 μm, more preferably 30 μm, and even more preferably 20 μm. On the other hand, the lower limit of the thickness may be 1 μm, preferably 5 μm, and more preferably 8 μm. By keeping the thickness of the polarizing film within the above range, polarization performance, handling ease, etc. can be improved. The thickness of the polarizing film is the average value of values ​​measured at five points.

[0039] The polarizing film may be a single-layer film or a multi-layer film, but is preferably a single-layer film from the viewpoint of the complexity of lamination work (coating, etc.), cost, etc.

[0040] The polarizing film is not particularly limited in its applications, and can be used for the same applications as conventional polarizing films. The polarizing film is suitable as a component of a polarizing plate used in image display devices, etc. In particular, the polarizing film is suitable for applications requiring high-temperature durability, particularly as a polarizing plate for in-vehicle image display devices.

[0041] <Polarizing Plate> A polarizing plate according to one embodiment of the present invention comprises a polarizing film according to one embodiment of the present invention and a protective film laminated to at least one surface of the polarizing film directly or via another layer (e.g., an adhesive layer). For example, the polarizing plate 1 shown in Fig. 1 comprises a polarizing film 2 and protective films 4 laminated to both surfaces of the polarizing film 2 via adhesive layers 3. That is, the polarizing plate 1 shown in Fig. 1 has a structure in which the first protective film 4, the first adhesive layer 3, the polarizing film 2, the second adhesive layer 3, and the second protective film 4 are laminated in this order.

[0042] In one embodiment of the present invention, the protective film laminated on at least one side of the polarizing film contains a cellulose ester resin. The protective film may be a cellulose ester film containing a cellulose ester resin as a main component. The content of the cellulose ester resin in the protective film is preferably 70% by mass or more, more preferably 90% by mass or more. Examples of cellulose ester resins include cellulose triacetate (triacetyl cellulose: TAC), cellulose diacetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate benzoate, cellulose acetate propionate benzoate, cellulose propionate, cellulose butyrate, cellulose acetate biphenylate, and cellulose acetate propionate biphenylate, with cellulose triacetate being preferred.

[0043] In a polarizing plate having protective films laminated on both sides of a polarizing film directly or via another layer, both of the protective films may contain a cellulose ester resin. Alternatively, one of the two protective films may be a protective film that does not contain a cellulose ester resin. Examples of such protective films include acrylic films, polyester films, and cycloolefin polymer films.

[0044] The polarizing film and the protective film are usually bonded together using an adhesive. Examples of such adhesives include PVA-based adhesives and ultraviolet-curable adhesives, with PVA-based adhesives being preferred. An aqueous solution of PVA can be used as the PVA-based adhesive. The solid or cured product of such an adhesive forms the adhesive layer. Specific and preferred forms of the PVA used as the adhesive are the same as those described above for the specific and preferred forms of the PVA contained in the polarizing film.

[0045] The polarizing plate may have its surface subjected to a surface treatment such as an anti-reflection treatment, an anti-sticking treatment, an anti-glare treatment, or other coating treatment. The polarizing plate may also be used as an optical film by further laminating other optical layers. Examples of other optical layers include a retardation plate and a viewing angle compensation film.

[0046] The polarizing plate can be used in various image display devices such as liquid crystal display devices and organic EL display devices.

[0047] <Method for manufacturing polarizing plate, etc.> Hereinafter, a method for manufacturing a PVA film to obtain a polarizing film, a method for manufacturing a polarizing film using the PVA film, and a method for manufacturing a polarizing plate using the polarizing film will be described in order. Note that, hereinafter, a polarizing film to be used in the method for manufacturing a polarizing plate (a polarizing film that has not been subjected to an annealing treatment) may be referred to as polarizing film (X), and a polarizing film provided in a polarizing plate obtained through the method for manufacturing a polarizing plate (a polarizing film that has been subjected to an annealing treatment) may be referred to as polarizing film (Y).

[0048] (Method for Producing a PVA Film) The method for producing a PVA film is not particularly limited, and a production method that results in a film with uniform thickness and width after film formation can be preferably used. For example, a PVA film can be produced using a film-forming solution in which PVA, which constitutes the PVA film, and optionally one or more of a plasticizer, a surfactant, and other components are dissolved in a liquid medium, or a film-forming solution containing PVA, and optionally one or more of a plasticizer, a surfactant, other components, and a liquid medium, and in which the PVA is melted. When the film-forming solution contains at least one of a plasticizer, a surfactant, and other components, it is preferable that these components are uniformly mixed.

[0049] Examples of liquid media used for preparing the membrane-forming solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine, with water being preferred.

[0050] The volatile content of the membrane-forming solution (the content in the membrane-forming solution of volatile components such as the liquid medium that are removed by volatilization during membrane formation) varies depending on the membrane-forming method, membrane-forming conditions, etc., but the lower limit is preferably 50% by mass, more preferably 60% by mass, and even more preferably 70% by mass. Meanwhile, the upper limit is preferably 96% by mass, more preferably 92% by mass. When the volatile content of the membrane-forming solution is equal to or greater than the lower limit, the viscosity of the membrane-forming solution does not become too high, filtration and degassing during preparation of the membrane-forming solution are smoothly carried out, and it becomes easy to produce a PVA film with few foreign matter or defects. On the other hand, when the volatile content of the membrane-forming solution is equal to or less than the upper limit, the concentration of the membrane-forming solution does not become too low, and it becomes easy to produce a PVA film industrially.

[0051] Examples of film-forming methods for producing a PVA film using a film-forming solution include a cast film-forming method, an extrusion film-forming method, a wet film-forming method, and a gel film-forming method. Of these, the cast film-forming method and the extrusion film-forming method are preferred. Among these, the extrusion film-forming method is more preferred because it can produce a PVA film with uniform thickness and width and good physical properties. The PVA film can be dried or heat-treated as needed.

[0052] The heat treatment temperature is not particularly limited and may be appropriately adjusted depending on the degree of swelling of the PVA film, etc. The upper limit of the heat treatment temperature is preferably 200°C, more preferably 180°C, and even more preferably 150°C, from the viewpoint of suppressing discoloration and deterioration of the PVA film. The lower limit of the heat treatment temperature can be, for example, 80°C.

[0053] The heat treatment time is not particularly limited and may be adjusted appropriately depending on the swelling degree of the PVA film, etc. From the viewpoint of efficiently producing the PVA film, the heat treatment time is preferably 1 to 60 minutes, more preferably 2 to 40 minutes, and even more preferably 3 to 30 minutes.

[0054] (Method for Producing Polarizing Film (X)) The method for producing the polarizing film (X) is not particularly limited, and any conventionally employed method may be used. For example, the polarizing film (X) can be produced by subjecting a PVA film to a swelling treatment, a dyeing treatment, a uniaxial stretching treatment, and, if necessary, further to a crosslinking treatment, a fixing treatment, a drying treatment, a heat treatment, etc. In this case, the order of each treatment, such as the swelling treatment, dyeing treatment, uniaxial stretching treatment, and fixing treatment, is not particularly limited, and one or two or more treatments can be carried out simultaneously. Furthermore, one or two or more of each treatment can be carried out two or more times.

[0055] The swelling treatment can be carried out by immersing the PVA film in water. The lower limit of the water temperature during immersion is preferably 20°C, more preferably 22°C, and even more preferably 25°C. Meanwhile, the upper limit is preferably 40°C, more preferably 38°C, and even more preferably 35°C. The immersion time in water is preferably, for example, 0.1 to 5 minutes. The water used for immersion is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

[0056] The dyeing treatment is carried out using an iodine-based dye. The dyeing may be carried out at any stage before, during, or after uniaxial stretching. Dyeing is generally carried out by immersing the PVA film in a solution (particularly an aqueous solution) containing iodine and potassium iodide as a dye bath. The iodine concentration in the dye bath is preferably 0.01 to 0.5% by mass, and the potassium iodide concentration is preferably 0.01 to 10% by mass. The temperature of the dye bath is preferably 20 to 60°C.

[0057] The crosslinking treatment can be carried out by immersing the PVA film in an aqueous solution containing a crosslinking agent. Crosslinking is introduced into the PVA film, allowing uniaxial stretching at a relatively high temperature. Examples of crosslinking agents that can be used include the boron compounds described above. The concentration of the crosslinking agent in the aqueous solution is preferably 1 to 15% by mass, more preferably 2 to 7% by mass. The temperature of the aqueous solution containing the crosslinking agent is preferably 20 to 60°C.

[0058] The uniaxial stretching treatment may be carried out by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be carried out in an aqueous solution containing a crosslinking agent, or in the dye bath described above or the fixing treatment bath described below. In the case of the dry stretching method, it can be carried out in air. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. The concentration of boric acid in the aqueous boric acid solution is preferably 0.5 to 6.0% by mass, more preferably 1.0 to 5.0% by mass, and even more preferably 1.5 to 4.0% by mass. In addition, the aqueous boric acid solution may contain potassium iodide, and the concentration thereof is preferably 0.01 to 10% by mass.

[0059] The upper limit of the stretching temperature is preferably 60°C, more preferably 55°C, and may be 50°C. By stretching at a temperature equal to or lower than this upper limit, the peak-top temperature of the endothermic peak appearing in the temperature range of 50°C to 150°C in differential scanning calorimetry of the final polarized film (Y) is likely to be high, and the heat of fusion of the endothermic peak is likely to be large. It should be noted that when the peak-top temperature of the endothermic peak appearing in the temperature range of 50°C to 150°C is high and the heat of fusion of the endothermic peak is large, it can be said that the thermal stability of the BCC structure is higher, and therefore the BCC structure is likely to remain during a high-temperature heat resistance test, and polyenization of the PVA is more likely to be suppressed. The lower limit of the stretching temperature is, for example, preferably 20°C, more preferably 30°C.

[0060] The lower limit of the stretching ratio in the uniaxial stretching treatment is preferably 5 times, more preferably 5.5 times, from the viewpoint of the polarizing performance of the resulting polarizing film. The upper limit of the stretching ratio is not particularly limited, but is preferably 8 times.

[0061] In producing the polarized film (X), a fixation treatment is preferably carried out to strengthen the adsorption of the iodine-based dye onto the PVA film. The fixation bath used for the fixation treatment can be an aqueous solution containing one or more boron compounds, such as boric acid and borax. If necessary, an iodine compound, a metal compound, or the like may be added to the fixation bath. The concentration of the boron compound in the fixation bath is generally preferably 0.5 to 15% by mass, and particularly preferably about 1 to 10% by mass. The temperature of the fixation bath is preferably 15 to 60°C, and particularly preferably 20 to 40°C.

[0062] The drying temperature for the polarizing film (X) is preferably 40°C or higher, and more preferably 80°C or higher. Drying at this temperature tends to increase the peak top temperature of the endothermic peak in the resulting polarizing film (Y) when subjected to the annealing treatment described below. The upper limit of the drying temperature is, for example, 100°C, or may be 90°C. The drying time may be, for example, 1 to 20 minutes, or may be 2 to 10 minutes.

[0063] (Method for manufacturing polarizing plate) A method for manufacturing a polarizing plate according to one embodiment of the present invention includes the steps of (1) laminating a protective film on at least one surface of a polarizing film containing PVA and an iodine-based dye to obtain a laminate, (2) adjusting the moisture content of the laminate to 4.0% by mass or less, and (3) annealing the laminate by heating at 60°C or higher for 4 hours or more, in this order.

[0064] In this manufacturing method, a polarizing plate is obtained by annealing a laminate. In other words, the laminate is the polarizing plate before annealing, and the polarizing plate is the laminate after annealing. According to this manufacturing method, a laminate adjusted to a moisture content of 4.0% by mass or less is annealed by heating at 60°C or higher for 4 hours or more, thereby obtaining a polarizing plate in which a decrease in light transmittance is sufficiently suppressed in a high-temperature durability test. This is presumably because the specified annealing treatment enhances the thermal stability of the BBC structure in the polarizing film. The polarizing film (Y) provided in the polarizing plate obtained by this manufacturing method is a polarizing film containing PVA and an iodine-based dye, and the peak-top temperature of an endothermic peak appearing in a temperature range of 50°C to 150°C in differential scanning calorimetry of the polarizing film may be 102°C or higher. That is, this manufacturing method includes a method for manufacturing a polarizing film according to one embodiment of the present invention.

[0065] The polarizing film used in step (1) may be the polarizing film (X) obtained by the above-described method for producing a polarizing film (X). The laminate can be obtained, for example, by laminating a protective film to at least one surface (preferably both surfaces) of the polarizing film (X) using an adhesive. The protective film used in step (1) may be a protective film containing a cellulose ester resin. When using a protective film containing a cellulose ester resin, it is preferable to use one whose surface (the surface to be laminated to the polarizing film (X)) has been saponified to enhance adhesion. The specific forms of the protective film and adhesive used in this production method are the same as those described in the description of the polarizing plate according to one embodiment of the present invention.

[0066] In step (2), the moisture content of the laminate obtained in step (1) is adjusted to 4.0% by mass or less. This step (2) can be performed, for example, by leaving the laminate in an environment adjusted to a predetermined temperature and humidity. The temperature at this time is preferably 5 to 50°C, and more preferably 10 to 35°C. The humidity at this time may be, for example, 5 to 90% RH or 5 to 50% RH. The leaving time may be, for example, 1 to 30 days, or may be 3 to 14 days. The moisture contents of the polarizing film (X) and the protective film used in the laminate may be similar or different. Regardless of the moisture contents of the polarizing film (X) and the protective film used in the laminate, the moisture content of the laminate as a whole reaches equilibrium through the treatment in step (2), and therefore the moisture content of the laminate is adjusted to 4.0% by mass or less.

[0067] In step (3), the laminate, the moisture content of which has been adjusted to 4.0% by mass or less, is subjected to an annealing treatment. The upper limit of the moisture content of the laminate to be subjected to the annealing treatment is preferably 3.8% by mass, more preferably 3.5% by mass, and even more preferably 3.3% by mass. When the moisture content of the laminate to be subjected to the annealing treatment is 4.0% by mass or less, it is easy to adjust the peak top temperature of the endothermic peak in the resulting polarized film (Y) to 102°C or higher. The reason for this is unclear, but it is presumed that if the moisture content during the annealing treatment is high, part of the BCC structure dissolves and becomes unstable. The lower limit of the moisture content is preferably 2.0% by mass, more preferably 2.5% by mass. By setting the moisture content at or above the lower limit, the peak top temperature of the endothermic peak tends to be higher.

[0068] The lower limit of the heating temperature in the annealing treatment is 60°C, preferably 65°C, and more preferably 70°C. By setting the heating temperature to the lower limit or higher, it becomes easy to adjust the peak top temperature of the endothermic peak in the obtained polarizing film (Y) to 102°C or higher. The upper limit of the heating temperature is preferably 90°C, more preferably 85°C, even more preferably 80°C, and even more preferably 75°C. Setting the heating temperature to the above upper limit or lower can suppress deterioration in the hue of the obtained polarizing plate, for example.

[0069] The lower limit of the heating time in the annealing treatment is 4 hours, preferably 6 hours, more preferably 10 hours, and even more preferably 16 hours. By setting the heating time to the above lower limit or more, it is easy to adjust the peak top temperature of the endothermic peak in the resulting polarized film (Y) to 102° C. or higher. From the viewpoint of production efficiency, etc., the upper limit of the heating time is preferably 48 hours, more preferably 24 hours.

[0070] The method for producing a polarizing plate may further include other steps such as a surface treatment step in addition to the above steps (1) to (3).

[0071] Other Embodiments The polarizing film, polarizing plate, and polarizing plate manufacturing method of the present invention are not limited to the above-described embodiments. The polarizing film of the present invention may be used, for example, in a polarizing plate having a protective film that does not contain a cellulose ester resin. The polarizing film and polarizing plate of the present invention may also be manufactured by a method other than the above-described manufacturing method.

[0072] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples in any way. Evaluations were performed according to the methods described below. In the following, the boric acid content in the polarized film was set to 23 to 25% by mass, taking into consideration the effect of the dehydration reaction of PVA caused by boric acid in the polarized film. In each example and comparative example, multiple samples required for the respective evaluations and measurements were prepared under the same conditions.

[0073] [Thickness of PVA Film and Polarizing Film (X)] The thickness of the PVA film obtained in the following Examples or Comparative Examples and the polarizing film (X) before annealing treatment was measured at the center in the width direction of each film using a film thickness meter (DG-5100 manufactured by Ono Sokki Co., Ltd.) It is considered that the thickness of the polarizing film does not change substantially before and after annealing treatment.

[0074] [PVA Film Swelling Degree] A rectangular strip of 50 mm in width and 100 mm in length was cut from the PVA film obtained in the following Examples or Comparative Examples to obtain a test strip. The test strip was immersed in 1,000 g of pure water at 30°C for 30 minutes, after which the moisture on the surface of the test strip was wiped off with filter paper, and the mass Wa (g) of the sample was measured. Subsequently, the test strip was dried in a dryer at 105°C for 16 hours, after which the mass Wb (g) was measured, and the swelling degree was calculated using the following formula. The same measurement was performed three times, and the average value was used. Swelling degree (mass%) = 100 × Wa / Wb

[0075] [Boric Acid Content in Polarizing Film (X)] The mass a (g) of the polarizing film (X) before annealing obtained in the following Examples or Comparative Examples was measured, and the film was heated and dissolved in distilled water to prepare a measurement solution so that the polarizing film (X) was approximately 0.005% by mass. Next, the mass b (g) of the resulting measurement solution was measured, and the boron concentration c (ppm) of the measurement solution was measured using a multi-type ICP optical emission analyzer (Shimadzu Corporation, "ICPE-9000"). The boric acid content (% by mass) in the polarizing film (X) was then calculated using the following formula. In this formula, "61.8" is the molecular weight of boric acid, and "10.8" is the atomic weight of boron. It is believed that the boric acid content in the polarizing film remains unchanged before and after annealing. Boric acid content (% by mass) in polarizing film (X) = [(b × c / 1,000,000) / a] × 100 × (61.8 / 10.8)

[0076] [Peak Top Temperature of Polarizing Film (Y)] After annealing, the polarizing film (Y) obtained in the following Examples or Comparative Examples was conditioned at 23°C and 50% RH for 3 days, and then approximately 2 mg of the polarizing film (Y) was sampled and placed in an aluminum pan for DSC (differential scanning calorimetry) (TA Instruments' "Tzero Low Mass Aluminum Pan"). Measurement was then carried out under the following conditions using a DSC device (TA Instruments' "DSC250"). The temperature at the endothermic peak appearing in the temperature range from 50°C to 150°C was determined as the peak top temperature (°C). The peak top temperature was determined as the temperature corresponding to the measured peak, without taking the baseline into consideration. Measurement temperature range: 25°C to 150°C; Heating rate: 10°C / min; Measurement atmosphere: N ２ 50 mL / min

[0077] [Moisture content of laminate (polarizing plate before annealing treatment)] The mass d (g) of the laminate (polarizing plate to be subjected to annealing treatment) in the following Examples or Comparative Examples was measured. Thereafter, the laminate was dried for 24 hours in a hot air dryer at 105°C. Next, the mass e (g) of the dried laminate was measured, and the moisture content (mass%) of the laminate was calculated using the following formula: Moisture content (mass%) of laminate = 100 - (e / d) x 100

[0078] [Heat Resistance Test] (i) Sample Preparation The polarizing plates obtained in the following examples or comparative examples were subjected to humidity conditioning for 7 days in an environment of 23 ° C. and 50% RH. One test piece measuring 4 cm in length and 4 cm in width was taken from the polarizing plate after humidity conditioning. Next, two glass plates measuring 1 mm in thickness, 10 cm in length, and 10 cm in width, and two adhesive films ("MCS70" manufactured by Mitate Imaging Co., Ltd.) measuring 0.25 μm in thickness, 11 cm in length, and 11 cm in width were prepared. Here, the two adhesive films were previously subjected to humidity conditioning for 7 days in an environment of 23 ° C. and 50% RH. Next, in an environment of 23 ° C. and 50% RH, the adhesive film, test piece (polarizing plate), and adhesive film were laminated one by one on the glass plate using a laminator ("Lamyman IKO-360EII" manufactured by Yubon Co., Ltd.). Next, another glass plate was placed on the adhesive film to obtain a laminate 5 having a laminate structure of "glass plate 6 / adhesive film 7 / polarizing plate 1 / adhesive film 7 / glass plate 6" as shown in Figure 2. Finally, using a vacuum laminator ("1522N" manufactured by Nisshinbo Mechatronics Inc.), the obtained laminate was evacuated at 50°C for 5 minutes, and then pressurized at 10 kPa for 5 minutes to obtain a sample for a heat resistance test.

[0079] (ii) Absorbance Measurement The absorbance of the heat resistance test sample before and after the heat resistance test was measured as follows. For the heat resistance test sample obtained in "(i) Sample Preparation" above, the absorbance (Abs) at a wavelength of 450 nm before the heat resistance test was measured using an ultraviolet-visible spectrophotometer ("UV-2450" manufactured by Shimadzu Corporation). Next, the heat resistance test sample was placed in a metal basket in an upright position so that the inner side of the metal basket did not come into contact with the heat resistance test sample. Next, this metal basket was placed in the center of a 105°C dryer ("DRS620A" manufactured by ADVANTEC Corporation) and heated for 120 hours. Then, the heat resistance test sample was removed from the metal basket, and the absorbance (Abs) of the heat resistance test sample at a wavelength of 450 nm was measured using an ultraviolet-visible spectrophotometer ("UV-2450" manufactured by Shimadzu Corporation).

[0080] [Example 1] (1) Production of PVA Film 100 parts by mass of PVA (degree of polymerization 2,400, degree of saponification 99.95 mol%), 10 parts by mass of glycerin as a plasticizer, 0.03 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and water as a liquid medium were mixed and then heated at 90°C for 4 hours to obtain a PVA aqueous solution with a volatile content of 90% by mass. The obtained aqueous solution was then kept at 85°C for 16 hours to degas, and used as a film-forming solution for PVA film. This film-forming solution was dried on a metal roll at 80°C and then heat-treated in a dryer at 110°C for 10 minutes to obtain a PVA film (swelling degree 200%, thickness 30 μm).

[0081] (2) Production of Polarizing Film (X) A test piece measuring 9 cm in length and 10 cm in width was taken from the PVA film obtained in "(1) Production of PVA Film" above. Next, both longitudinal ends of the test piece were fixed to a stretching jig so that the stretched portion measured 5 cm in length and 10 cm in width. While immersed in water at 30°C for 38 seconds, the test piece was uniaxially stretched in the longitudinal direction at a stretching rate of 12 cm / min to 2.2 times its original length (swelling treatment). Next, while immersed in an aqueous solution (30°C) containing 0.03% by mass of iodine and 3% by mass of potassium iodide for 60 seconds, the test piece after the swelling treatment was uniaxially stretched in the longitudinal direction at a stretching rate of 12 cm / min to 3.3 times its original length (dyeing treatment). The dyed specimen was then immersed in an aqueous solution (temperature: 30°C) containing 3% by mass of boric acid and 3% by mass of potassium iodide for approximately 20 seconds, while being uniaxially stretched in the longitudinal direction at a stretching rate of 12 cm / min to 3.6 times its original length (crosslinking treatment). The crosslinked specimen was then immersed in an aqueous solution (temperature: 53°C) containing 4% by mass of boric acid and approximately 5% by mass of potassium iodide, while being uniaxially stretched in the longitudinal direction at a stretching rate of 12 cm / min to 5.5 times its original length (uniaxial stretching treatment). The uniaxially stretched specimen was then immersed in an aqueous solution (temperature: 22°C) containing 1.5% by mass of boric acid and 3% by mass of potassium iodide for 10 seconds (fixing treatment). Finally, the specimen was dried in a dryer at 80°C for 4 minutes (drying treatment) to obtain a polarized film (X) (thickness: 13 μm) before annealing.

[0082] (3) Production of Polarizing Plates (i) Production of Laminated Plates One test piece measuring 10 cm in length and 5 cm in width was taken from the polarizing film (X) obtained in "(2) Production of Polarizing Film (X)" above. Next, two TAC films ("TG40UL" manufactured by Fujifilm Corporation) with a 10 cm in length and 5 cm in width and a saponification-treated surface were prepared. Furthermore, a PVA-based adhesive was prepared by dissolving 3.5% by mass of PVA (degree of polymerization 2,400, degree of saponification 99.95 mol%) in pure water to prepare a PVA-based aqueous solution. Next, using a laminator ("Lamyman IKO-360EII" manufactured by Ubon Co., Ltd.), one TAC film was laminated on each side of the test piece (polarizing film (X)) via the PVA-based adhesive, producing a laminated plate with a configuration of "TAC film / PVA-based adhesive / polarizing film / PVA-based adhesive / TAC film." The resulting laminate was then fixed in a metal frame and dried in a hot air dryer at 60° C. for 10 minutes.

[0083] (ii) Moisture content adjustment The laminate obtained in "(i) Production of laminate" above was left standing for 7 days in an environment of 20°C and a humidity of 10% RH to 90% RH, thereby adjusting the moisture content of the laminate (polarizing plate before annealing treatment) to 3.0% by mass. Here, the moisture content of the laminate was determined by the method described above.

[0084] (iii) Annealing Treatment As shown in FIG. 3 , the laminate 9 obtained in the "(ii) moisture content adjustment" step was sandwiched between two glass plates 6 that were significantly larger than the laminate 9. The gap between the two glass plates 6 was filled with a silicone sealant 10 to surround the laminate 9, sealing the laminate 9 to prevent the moisture in the laminate 9 from volatilizing, thereby obtaining a laminate 8 to be subjected to the annealing treatment. This laminate was placed in a 70°C dryer ("DKN812" manufactured by Yamato Scientific Co., Ltd.) and annealed for 16 hours. The polarizing plate obtained through the annealing treatment (annealed laminate) was then removed from the annealed laminate. The TAC film was then dissolved and removed from the resulting polarizing plate using dichloromethane to obtain an annealed polarizing film (Y).

[0085] The boric acid content in the polarizing film (X) before annealing obtained in "(2) Production of polarizing film (X)" was measured by the method described above. The peak top temperature of the polarizing film (Y) after annealing obtained in "(3) Production of polarizing plate" was measured by the method described above. The heat resistance test was performed on the polarizing plate obtained in "(3) Production of polarizing plate" by the method described above. The results are shown in Table 1. The obtained DSC curve is shown in Figure 4.

[0086] Examples 2 and 3, Comparative Example 2 PVA films, polarizing films, and polarizing plates were produced and evaluated in the same manner as in Example 1, except that the temperature of the aqueous solution in the uniaxial stretching treatment and the temperature in the drying treatment in the "(2) Production of polarizing film (X)" above, and the moisture content of the laminate (polarizing plate before annealing treatment) in "(ii) Adjustment of moisture content" and the treatment conditions (temperature and time) in "(iii) Annealing treatment" in "(3) Production of polarizing plate" above were changed as shown in Table 1. The results are shown in Table 1.

[0087] Comparative Example 1 A PVA film, a polarizing film, and a polarizing plate were produced and evaluated in the same manner as in Example 1, except that "(iii) annealing treatment" in "(3) Production of polarizing plate" was not performed. The results are shown in Table 1.

[0088] Comparative Example 3 A PVA film, a polarizing film, and a polarizing plate were produced and evaluated in the same manner as in Example 2, except that "(iii) annealing treatment" in "(3) Production of polarizing plate" was not performed. The results are shown in Table 1.

[0089]

[0090] As shown in Table 1, the polarizing plates including the polarizing films (Y) of Examples 1 to 3 had low absorbance at a wavelength of 450 nm in the heat resistance test. The absorbance of the polarizing plates (heat resistance test samples) of Examples 1 to 3 and Comparative Examples 1 to 3 before the heat resistance test was all 0.3, meaning there was no difference in the absorbance before the heat resistance test. This means that the polarizing films (Y) of Examples 1 to 3 were able to sufficiently suppress the decrease in light transmittance of the polarizing plate.

[0091] The polarizing film and polarizing plate of the present invention can be used in image display devices and the like.

[0092] REFERENCE SIGNS LIST 1 Polarizing plate 2 Polarizing film 3 Adhesive layer 4 Protective film 5 Laminate (heat resistance test sample) 6 Glass plate 7 Adhesive film 8 Laminate to be subjected to annealing treatment 9 Laminate plate 10 Silicone-based sealant

Claims

1. A polarizing film containing polyvinyl alcohol and an iodine-based dye, wherein in differential scanning calorimetry of the polarizing film, the peak top temperature of an endothermic peak appearing in the temperature range of 50°C to 150°C is 102°C or higher.

2. A polarizing plate comprising the polarizing film according to claim 1 and a protective film laminated on at least one surface of the polarizing film, the protective film containing a cellulose ester resin.

3. A method for manufacturing a polarizing plate, comprising the steps of: (1) obtaining a laminate by laminating a protective film on at least one side of a polarizing film containing polyvinyl alcohol and an iodine-based dye; (2) adjusting the moisture content of the laminate to 4.0 mass% or less; and (3) annealing the laminate by heating at 60°C or higher for 4 hours or more.

4. The method for producing a polarizing plate according to claim 3, wherein the protective film contains a cellulose ester resin.