Polarizing film, method for producing same, and polarizing plate

Abstract

The present invention enables improvement of the uniform durability of the surface of a polarizing film, furthermore enables introduction of an acid scavenger by a simple method without improving equipment, and provides a polarizing plate excellent in long-term heat resistant durability.　Provided is a polarizing film containing an acid scavenger, the polarizing film including a polyvinyl alcohol-based film, and the elemental nitrogen content in at least one surface of the polyvinyl alcohol-based film being 0.05 mass% or more in a surface layer portion from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film.

Description

Polarizing film, its manufacturing method, and polarizing plate

[0001] The present invention relates to a polarizing film, a method for producing the same, and a polarizing plate. More specifically, the present invention relates to a polarizing film (polyvinyl alcohol-based polarizing film) that can be used to obtain a polarizing plate having excellent long-term heat resistance and durability.

[0002] Liquid crystal display devices have developed remarkably in recent years and are widely used in smartphones, tablets, personal computers, LCD televisions, projectors, in-vehicle panels, etc. The liquid crystal display devices use polarizing plates, which are typically formed by laminating a protective film on a polarizing film in which iodine or a dichroic dye is adsorbed and aligned on a polyvinyl alcohol film. As the applications of liquid crystal display devices have expanded in recent years, the temperature and humidity ranges in which they are used have become wider than ever before. This has created a need for polarizing plates that are even more heat-resistant and durable than ever before and exhibit a high degree of polarization.

[0003] As a method for improving the heat resistance durability of a polarizing plate, for example, Patent Document 1 describes that by using a polyvinyl alcohol film containing a polyvinyl alcohol resin and an acid scavenger, where the absorbance A1 at a wavelength of 210 nm and the average thickness D1 (mm) satisfy A1 / D1≧9, it is possible to produce a polarizing film that can sufficiently suppress a decrease in the light transmittance of the polarizing plate in a high-temperature durability test. Furthermore, Patent Document 2 describes a polarizing film having a layer containing an acid scavenger on at least one surface of the polarizing film.

[0004] International Publication No. 2020 / 184587 Japanese Patent Application Laid-Open No. 2023-140325

[0005] However, in the technology disclosed in Patent Document 1, an acid scavenger is added to a production stock solution for a polyvinyl alcohol film to obtain a polyvinyl alcohol film, and then a polarized film is produced. Since the acid scavenger is ubiquitous throughout the polarized film, there is a problem in that it is not possible to efficiently suppress acid-derived polyenization generated from a protective film present on the surface of the polarized film.

[0006] In Patent Document 2, a polarizing film is obtained that exhibits sufficient heat resistance and durability when used as a polarizing plate. However, it is necessary to apply an acid scavenger to the surface of the polarizing film, which leaves room for improvement, as there are problems with adhesion between the polarizing film and the coating agent, unevenness in the coating film, etc., which prevents uniform performance, and the need to install new equipment for coating.

[0007] Therefore, in this context, the present invention provides a polarizing film that enables uniform improvement in the durability of the polarizing film surface, and further allows an acid scavenger to be introduced by a simple method without any equipment improvements, thereby making it possible to obtain a polarizing plate that has excellent long-term heat resistance and durability.

[0008] In view of these circumstances, the present inventors have conducted extensive research and have discovered that by concentrating the majority of the acid scavenger in the surface layer portion of the inner surface of the polarizing film, it is possible to obtain a polarizing plate having excellent long-term heat resistance and durability.

[0009] That is, the first gist of the present invention is as follows. [1] A polarizing film containing an acid scavenger, the polarizing film comprising a polyvinyl alcohol-based film, wherein the nitrogen content in a surface layer portion of at least one surface of the polyvinyl alcohol-based film extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film is 0.05 mass % or more. [2] A polarizing film containing an acid scavenger, the polarizing film comprising a polyvinyl alcohol-based film, wherein the nitrogen content in a surface layer portion of at least one surface of the polyvinyl alcohol-based film extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film is 95% or more of the total nitrogen content in the polyvinyl alcohol-based film. [3] The polarizing film according to [1] or [2], wherein the acid scavenger is a carbodiimide-based compound. [4] The polarizing film according to [3], wherein the carbodiimide-based compound has a carbodiimide group equivalent of 100 to 1,000. [5] The method for producing a polarizing film according to any one of [1] to [4], in which the polyvinyl alcohol film is passed through a plurality of treatment tanks, and an acid scavenger is blended in a treatment solution of the treatment tank in which the film is last passed at a concentration of 0.5 g / L or more. [6] A polarizing plate having a protective film on at least one surface of the polarizing film according to any one of [1] to [4].

[0010] A polarizing plate obtained using the polarizing film of the present invention is less likely to turn red even in a long-term heat resistance durability test and has excellent heat resistance durability. Furthermore, by using the polarizing film manufacturing method of the present invention, a polarizing film that can efficiently produce a polarizing plate having excellent long-term heat resistance durability can be manufactured.

[0011] Embodiments of the present invention are described in detail below. However, the present invention is not limited to these embodiments. In this specification, "x and / or y (x and y are optional)" means at least one of x and y, and can mean three possibilities: x only, y only, or x and y. In this specification, when "X to Y" (X and Y are optional numbers) is used, unless otherwise specified, it also means "X or more and Y or less," as well as "preferably more than X" or "preferably less than Y." In this specification, when "X or more" (X is optional number) or "Y or less" (Y is optional number) is used, it also means "preferably more than X" or "preferably less than Y." With regard to numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range can also be replaced with the values ​​shown in the examples.

[0012] The polarizing film of the present invention is a polarizing film containing an acid scavenger, and is characterized in that the polarizing film includes a polyvinyl alcohol-based film, and that on at least one surface of the polyvinyl alcohol-based film, the nitrogen element content in a surface layer portion extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film is 0.05 mass% or more.

[0013] The nitrogen element content in the surface layer portion of the polyvinyl alcohol-based film extending from the surface to a depth of 0.5 nm in the thickness direction must be 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more. The upper limit of this content is usually 20% by mass or less, preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. When the nitrogen element content in the surface layer portion of the polyvinyl alcohol-based film extending from the surface to a depth of 0.5 nm in the thickness direction is within the above range, the acid scavenging function of the acid scavenger can be fully exhibited. Here, the surface layer portion extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film refers to the portion extending from one and / or both surfaces of the polyvinyl alcohol-based film to a depth of 0.5 nm in the film thickness direction, and does not include nitrogen elements present in the direction opposite to the film thickness direction from the film surface, i.e., outside the film surface.

[0014] Furthermore, in the polarizing film of the present invention, at least one surface of the polyvinyl alcohol-based film contains 95% or more of the total nitrogen element contained in the polyvinyl alcohol-based film in a surface layer portion extending to a depth of 0.5 nm from the surface in the thickness direction of the polyvinyl alcohol-based film. By incorporating an acid scavenger in the surface layer portion of the polarizing film of the present invention, the polarizing film can be prevented from undergoing polyenization of the polarizer by the acid generated by hydrolysis of components in the protective film of the polarizing plate. If the nitrogen element content in the surface layer portion is too low, the components in the protective film tend to be unable to capture all the acid generated by hydrolysis, making the polarizing film more susceptible to polyenization. Therefore, it is desirable that the polarizing film of the present invention contains 95% or more, preferably 97% or more, and more preferably 98% or more of the total nitrogen element contained in the polyvinyl alcohol-based film in the surface layer portion of the polyvinyl alcohol-based film.

[0015] The method for measuring the nitrogen element content in the surface layer portion is not particularly limited, and any known method can be used as long as it can measure nitrogen element content. For example, the nitrogen element content can be measured by XPS (X-ray photoelectron spectroscopy), SEM-EDX analysis, or TOF-SIMS analysis. The nitrogen element content can be measured by XPS or TOF-SIMS analysis, and more preferably by using XPS and TOF-SIMS analysis in combination.

[0016] In the present invention, the nitrogen element in the surface layer portion of the polyvinyl alcohol-based film extending from the surface to a depth of 0.5 nm in the thickness direction means the nitrogen element derived from the acid scavenger, when the film does not contain any component containing a nitrogen element other than the acid scavenger.

[0017] [Acid Scavenger] The acid scavenger used in the present invention serves to suppress the polyenization of the polarizing film due to the acid generated by hydrolysis of the protective film of the polarizing plate, and to prevent the polarizing film from turning red due to polyenization.

[0018] Representative examples of the acid scavenger include carbodiimide compounds, compounds having an oxazoline group, and compounds having an epoxy group. Other examples include compounds having an amino group and compounds having an oxazine group. Among these, it is preferable to use acid scavengers containing nitrogen elements, such as carbodiimide compounds, compounds having an oxazoline group, compounds having an amino group, and compounds having oxazine, because they react quickly with acid and are highly effective in inhibiting polyenization at high temperatures. Carbodiimide compounds are particularly preferable because they have good affinity when prepared as an aqueous solution. These carbodiimide compounds may be used alone or in combination with other acid scavengers.

[0019] Specific examples of the carbodiimide compound include compounds having a carbodiimide group, such as diisopropylcarbodiimide, dimethylcarbodiimide, dicyclohexylcarbodiimide, dioctylcarbodiimide, diphenylcarbodiimide, t-butylisopropylcarbodiimide, di-t-butylcarbodiimide, di-β-naphthylcarbodiimide, etc. Both low-molecular-weight and high-molecular-weight carbodiimide compounds can be used, and preferably, the carbodiimide compound is water-soluble.

[0020] Although the molecular weight of the carbodiimide compound is not particularly limited, a weight-average molecular weight of 500 or more is preferred, and a weight-average molecular weight of 1,000 or more is even more preferred. On the other hand, the upper limit of the weight-average molecular weight is preferably 200,000 or less, and more preferably 100,000 or less. If the weight-average molecular weight is too low, the number of carbodiimide groups is low, which reduces the effect of suppressing polyenization at high temperatures and tends to weaken the adhesive strength. If the weight-average molecular weight is too high, the dispersibility in water is poor and uniform application tends to be difficult.

[0021] In the carbodiimide compound, the carbodiimide group equivalent is one index representing the amount of carbodiimide groups that react with acid. The carbodiimide group equivalent is the molecular weight per mol of carbodiimide groups and is preferably 100 to 1,000, with the lower limit being preferably 150 or more, more preferably 200 or more, even more preferably 250 or more, and particularly preferably 300 or more. The upper limit is preferably 900 or less, more preferably 800 or less, even more preferably 700 or less, and particularly preferably 600 or less. If the carbodiimide group equivalent is too small, the contribution of the polyenization inhibitory effect at high temperatures tends to be reduced. On the other hand, if the carbodiimide group equivalent is too large, the compound tends to be highly hydrophobic and less compatible with water, making it difficult to handle.

[0022] The acid scavenger may also be called a hydrolysis stabilizer, acid catcher, acid trapping agent, acid scavenger, acid capture agent, etc., but in the present invention, these may be used regardless of the name.

[0023] The polarizing film of the present invention can be produced, for example, by first obtaining a polyvinyl alcohol film using a polyvinyl alcohol resin as a raw material, and then using the polyvinyl alcohol film as a raw sheet, going through polarizing film production steps such as swelling, dyeing, boric acid crosslinking, stretching, washing, drying, etc. Hereinafter, an example of the polarizing film production method will be described in detail in the order of the steps.

[0024] [Materials for Forming Polyvinyl Alcohol-Based Films] First, the polyvinyl alcohol-based resin, which is the material for forming the polyvinyl alcohol-based film, will be described. The polyvinyl alcohol-based resin typically used is an unmodified polyvinyl alcohol-based resin, i.e., a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (typically 10 mol % or less, preferably 5 mol % or less) of a component copolymerizable with vinyl acetate can also be used. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (e.g., salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, and unsaturated sulfonates. Modified polyvinyl alcohol-based resins obtained by chemically modifying hydroxyl groups after saponification can also be used.

[0025] Furthermore, the polyvinyl alcohol resin may also be a polyvinyl alcohol resin having a 1,2-diol structure in a side chain. This polyvinyl alcohol resin having a 1,2-diol structure in a side chain can be obtained, for example, by (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) a method of saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, (iii) a method of saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, or (iv) a method of saponifying a copolymer of vinyl acetate and glycerin monoallyl ether.

[0026] From the viewpoint of manufacturing the polarizing film, the weight-average molecular weight of the polyvinyl alcohol-based resin is 100,000 or more, preferably 110,000 or more, and more preferably 120,000 or more, with an upper limit of 300,000 or less, preferably 280,000 or less, and more preferably 260,000 or less. The weight-average molecular weight ranges, for example, from 100,000 to 300,000. If the weight-average molecular weight is too small, it tends to be difficult to obtain sufficient optical performance when the polyvinyl alcohol-based resin is used as an optical film. If the weight-average molecular weight is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film when manufacturing the polarizing film. The weight-average molecular weight of the polyvinyl alcohol-based resin is a weight-average molecular weight measured by GPC-MALS.

[0027] From the viewpoint of the optical performance of the polarizing film, the average saponification degree of the polyvinyl alcohol resin is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, and particularly preferably 99.8 mol% or more. In other words, if the average saponification degree is too low, the polarizing film produced from the polyvinyl alcohol film tends to have insufficient optical performance. Here, the average saponification degree is measured in accordance with JIS K 6726.

[0028] As the polyvinyl alcohol-based resin, two or more kinds of resins differing in the modified species, the degree of modification, the weight-average molecular weight, the average degree of saponification, etc. may be used in combination.

[0029] Next, an aqueous solution of polyvinyl alcohol resin is prepared using the polyvinyl alcohol resin, which will now be described.

[0030] The aqueous polyvinyl alcohol resin solution is prepared by dissolving the polyvinyl alcohol resin in a solvent such as water. In addition to water, other solvents may be used, such as dimethyl sulfoxide (DMSO), N-methylpyrrolidone, polyhydric alcohols such as glycerin, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and trimethylolpropane, amines such as ethylenediamine and diethylenetriamine, and mixtures thereof.

[0031] In addition to the polyvinyl alcohol resin, the aqueous polyvinyl alcohol resin solution may contain, as necessary, commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, and at least one nonionic, anionic, and cationic surfactant, and the inclusion of these is preferred from the viewpoint of film-forming properties. One selected from these may be used alone, or two or more may be used in combination.

[0032] When the aqueous polyvinyl alcohol resin solution contains a plasticizer, the content thereof is usually 1 part by mass or more, preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, relative to 100 parts by mass of the polyvinyl alcohol resin. The upper limit of the content is 35 parts by mass or less, preferably 30 parts by mass or less, and more preferably 25 parts by mass or less, and the content ranges, for example, from 1 to 35 parts by mass. If the content of the plasticizer is too low, the stretchability during production of the polarizing film tends to decrease, while if the content is too high, the strength of the resulting polyvinyl alcohol film tends to decrease.

[0033] When the aqueous polyvinyl alcohol resin solution contains a surfactant, the content thereof is preferably 0.15 parts by mass or less, particularly preferably 0.1 parts by mass or less, and even more preferably 0.07 parts by mass or less, per 100 parts by mass of the polyvinyl alcohol resin. The lower limit is usually 0.01 parts by mass, and the content ranges, for example, from 0.01 to 0.15 parts by mass.

[0034] From the viewpoint of productivity, the resin concentration of the polyvinyl alcohol-based resin aqueous solution obtained in this manner is 15% by mass or more, preferably 17% by mass or more, more preferably 20% by mass or more, with an upper limit of 60% by mass or less, preferably 55% by mass or less, more preferably 50% by mass or less. The resin concentration ranges, for example, from 15 to 60% by mass. If the resin concentration of this aqueous solution is too low, the drying load increases, tending to reduce productivity, whereas if it is too high, the viscosity becomes too high, making it difficult to produce a uniform solution.

[0035] The resulting aqueous polyvinyl alcohol resin solution is usually subjected to a degassing treatment. Examples of the degassing method include static degassing and degassing using a multi-screw extruder. The multi-screw extruder may be any multi-screw extruder equipped with a vent, and typically a twin-screw extruder equipped with a vent is used.

[0036] [Method for producing polyvinyl alcohol-based film] After the degassing treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into a T-slit die in fixed amounts, and then discharged and cast onto a rotating casting drum to form a film by a continuous casting method. The formed film is then dried to obtain a polyvinyl alcohol-based film. The method for producing this polyvinyl alcohol-based film will now be described.

[0037] The continuous casting method is a technique in which, for example, an aqueous polyvinyl alcohol resin solution is extruded and cast from a T-shaped slit die onto a casting mold such as a rotating casting drum, an endless belt, or a resin film to form a film. The surface temperature of the casting mold, such as the casting drum, is preferably 40 to 99°C, and particularly preferably 60 to 95°C. The film formed in the casting mold is dried by conveying the film while alternately contacting the front and back surfaces of the film with the outer periphery of multiple heated rolls. After drying with the heated rolls, the film may be heat-treated. The heat treatment is preferably carried out at 60 to 150°C, and particularly preferably 80 to 130°C. After the drying and optional heat treatment, both side edges of the film are cut and removed (slit) to form the polyvinyl alcohol film. The polyvinyl alcohol film is then wound around a core tube to form a film roll.

[0038] The thickness of the polyvinyl alcohol film is preferably 5 to 75 μm, particularly preferably 10 to 60 μm from the viewpoint of thinning the polarizing film, and further preferably 15 to 60 μm from the viewpoint of durability.

[0039] The width of the polyvinyl alcohol film is preferably 1 m or more, more preferably 2 m or more, particularly preferably 3 m or more, and even more preferably 3.5 m or more. From the viewpoint of widening the polarizing film, the width is preferably 4 m or more, particularly preferably 5 m or more, and even more preferably 5 to 6 m from the viewpoint of avoiding breakage during production of the polarizing film.

[0040] The length of the polyvinyl alcohol film is preferably 4 km or more, more preferably 4.5 km or more in terms of increasing the area, and particularly preferably 4.5 to 50 km in terms of transport weight.

[0041] [Method for Producing Polarizing Film] Hereinafter, a method for producing the polarizing film according to the embodiment of the present invention obtained by using the polyvinyl alcohol-based film will be described.

[0042] The polarized film is produced by unwinding the polyvinyl alcohol film from the film roll, transporting it horizontally, and passing it through multiple treatment tanks. Specifically, the polarized film is produced by subjecting the polyvinyl alcohol film to processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying. These processes will be described below.

[0043] The swelling step is carried out before the dyeing step. The swelling step is usually carried out by immersing the polyvinyl alcohol film in a treatment solution in a swelling treatment tank. The swelling step not only cleans the surface of the polyvinyl alcohol film but also has the effect of preventing uneven dyeing by swelling the polyvinyl alcohol film. Water is usually used as the treatment solution. As long as the treatment solution is mainly composed of water, it may contain small amounts of additives such as iodide compounds and surfactants, alcohol, etc. The temperature of the treatment solution is usually about 10 to 45°C, and the immersion time in the treatment solution is usually about 0.1 to 10 minutes.

[0044] The dyeing step is usually carried out by immersing the polyvinyl alcohol film in a liquid containing iodine or a dichroic dye in a dyeing bath. An aqueous solution of iodine and potassium iodide is usually used as the liquid, with an iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L being appropriate. A practical dyeing time is about 30 to 500 seconds. The temperature of the liquid is preferably 5 to 50°C. The aqueous solution may contain a small amount of a water-compatible organic solvent in addition to the water solvent.

[0045] The boric acid crosslinking step is typically carried out by immersing the polyvinyl alcohol film in a liquid containing a boron compound such as boric acid or borax in a boric acid treatment bath. The liquid is an aqueous solution or a water-organic solvent mixture, and the concentration of the boron compound in the liquid is approximately 10 to 100 g / L. It is preferable to add potassium iodide to the liquid in order to stabilize the polarization performance. The temperature of the liquid is preferably approximately 30 to 70°C, and the immersion time in the liquid is preferably approximately 0.1 to 20 minutes.

[0046] The stretching step may be carried out independently or during at least some of the swelling step, dyeing step, and boric acid crosslinking step. The total stretching ratio is preferably 3 to 10 times, and more preferably 3.5 to 6 times, in the uniaxial direction. In this case, slight stretching (stretching to a degree sufficient to prevent shrinkage in the width direction or more) may also be carried out in the direction perpendicular to the stretching direction. The temperature around the polyvinyl alcohol film to be stretched during stretching is preferably 40 to 170°C.

[0047] The washing step is carried out, for example, by immersing the polyvinyl alcohol film in water or an aqueous iodide solution such as potassium iodide in a washing treatment tank. The washing step can remove precipitates that form on the surface of the polyvinyl alcohol film. When an aqueous potassium iodide solution is used, the potassium iodide concentration is typically about 1 to 80 g / L. The temperature of the aqueous iodide solution during washing is typically 5 to 50°C, preferably 10 to 45°C. The immersion time in the water or aqueous iodide solution is typically 1 to 300 seconds, preferably 10 to 240 seconds. Washing with water and washing with an aqueous potassium iodide solution may be performed in combination as appropriate.

[0048] The polarizing film of the present invention contains an acid scavenger, which may be added to the treatment solution in any of the treatment tanks in the polarizing film production process. The timing of adding such an acid scavenger to the treatment solution is preferably to the treatment solution in the latter treatment tank, particularly preferably to the treatment solution in the final treatment tank, and particularly preferably, the final treatment tank is a cleaning treatment tank.

[0049] The concentration of the acid scavenger when blended into the processing solution is preferably 0.5 g / L or more, more preferably 1.5 g / L or more, and even more preferably 3.0 g / L or more. On the other hand, the upper limit of the blending concentration of the acid scavenger is not particularly limited as long as it does not impair the function of the polarizing film, but is preferably 20.0 g / L or less, more preferably 15.0 g / L or less. If the concentration is too low, a sufficient amount of the acid scavenger cannot be present on the film surface, which tends to cause polyenization and cause the polarizing plate to turn red.

[0050] The drying step is usually carried out after passing through the final cleaning treatment tank in the above cleaning step, by drying in the atmosphere at an ambient temperature of 40 to 100°C, preferably 70 to 98°C, for 0.5 to 20 minutes.

[0051] The polarization degree of the polarizing film thus obtained is preferably 99.90% or more, particularly preferably 99.92% or more. If the polarization degree is too low, it tends to be difficult to ensure contrast in liquid crystal display devices such as liquid crystal displays.

[0052] Furthermore, the single-piece transmittance of the polarizing film is preferably 42% or more, more preferably 43% or more, and even more preferably 44% or more. If this single-piece transmittance is too low, it tends to be difficult to achieve high brightness in liquid crystal display devices such as liquid crystal displays. The single-piece transmittance is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0053] [Method for Producing Polarizing Plate] Then, an optically isotropic protective film is attached to one or both sides of the obtained polarizing film via an adhesive, thereby obtaining a polarizing plate having a protective film on at least one side.

[0054] Examples of the protective film include films made of acetyl cellulose-based resins such as triacetyl cellulose and diacetyl cellulose, films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, polycarbonate-based resin films, cycloolefin-based resin films, acrylic-based resin films, and films made of chain olefin-based resins such as polypropylene-based resins.

[0055] The polarizing film and the protective film are bonded together by a known method, for example, by uniformly applying a liquid adhesive composition to the polarizing film, the protective film, or both, and then bonding and pressing the two together, followed by heating or irradiating with active energy rays.

[0056] The degree of polarization of the polarizing plate is preferably 99.9% or more, more preferably 99.92% or more, and even more preferably 99.95% or more. If the degree of polarization is too low, it tends to be difficult to ensure contrast in liquid crystal display devices such as liquid crystal displays. The degree of polarization is generally measured as the light transmittance (H 11 The degree of polarization is calculated from the light transmittance (H1) measured at a wavelength λ in a state where the two polarizing plates are superimposed so that their orientation directions are perpendicular to each other, according to the following formula: 11 −H1) / (H 11 + H1) 1 / 2

[0057] Furthermore, the single transmittance of the polarizing plate is preferably 42% or more, more preferably 43% or more. If this single transmittance is too low, it tends to be difficult to achieve high brightness in liquid crystal display devices such as liquid crystal displays. The single transmittance is a value obtained by measuring the light transmittance of the polarizing plate alone using a spectrophotometer.

[0058] The polarizing film and the polarizing plate using the same are excellent in heat resistance durability and polarization performance, and are preferably used in liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game consoles, videos, cameras, photo albums, thermometers, audio equipment, meters for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, anti-reflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, etc.

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. Each physical property was measured as follows.

[0060] <Measurement Conditions> (1) Heat Resistance Durability Test First, adhesive sheets were attached to both sides of the obtained polarizing plate, which was then cut into 3.8 cm squares. This was then attached to a 4.0 cm x 8.0 cm glass plate (1 mm thick), resulting in a laminated structure in the following order: glass plate / adhesive sheet / polarizing plate / adhesive sheet / glass plate. This laminate was then pressed at 50°C for 20 minutes at a pressure of 0.5 MPa using a cylindrical liquid crystal pressure degassing device to obtain a sample for heat resistance durability testing. The prepared heat resistance durability test sample was placed in a dryer at 105°C, and the appearance of the sample after 500 hours and 1000 hours was evaluated according to the following criteria. (Evaluation Criteria) ◯ (very good): No red discoloration was observed in the polarizing plate. △ (good): Part of the polarizing plate was slightly red discolored. × (poor): The entire polarizing plate was deeply red discolored.

[0061] <XPS Analysis> Using an XPS analyzer KRATOS ULTRA2 manufactured by Shimadzu Corporation, bond energies were measured by XPS (X-ray photoelectron spectroscopy), and the elemental composition (mass %) was calculated by conversion from the areas of peaks corresponding to O1s, I3d, B1s, N1s, etc. The detection limit of XPS analysis is less than 0.01 mass %.

[0062] <TOF-SIMS analysis> The acid scavenger in the film was identified using TOF-SIMS (time-of-flight secondary ion mass spectrometry) under the following conditions. The extent to which the acid scavenger was present in the depth direction of the polarizing film was confirmed by etching the film. (TOF-SIMS measurement conditions) Apparatus: Time-of-flight secondary ion mass spectrometer ("TOF-SIMS IV" manufactured by ION-TOF), Primary ion: Bi3 2+ Acceleration voltage: 25 kV Measurement area: 200 μm square (TOF-SIMS measurement conditions for the depth direction) Apparatus: Time-of-flight secondary ion mass spectrometer equipped with a gas cluster ion beam (GCIB) (TRIFT V, manufactured by ION-TOF) Primary ion: Bi3 2+ Acceleration voltage: 25 kV Measurement area: 200 μm square (Etching conditions) Gas used: Ar2500+ Acceleration voltage: 5 kV Irradiation area: 500 μm square Irradiation time: 40 seconds

[0063] Example 1 (Preparation of Polarizing Film and Polarizing Plate) A 30 μm-thick polyvinyl alcohol film was unwound from a roll and conveyed horizontally. It was then immersed in a water bath at 22°C to swell and stretched 1.2 times in the machine direction (MD). It was then immersed in an aqueous solution containing 1.2 g / L of iodine and 25 g / L of potassium iodide at 28°C to dye and stretch 2.2 times in the machine direction (MD). It was then immersed in an aqueous solution (57°C) containing 30 g / L of boric acid and 30 g / L of potassium iodide to uniaxially stretch 5.7 times in the machine direction (MD) while crosslinking with boric acid. Finally, the film was washed with a washing solution (A) consisting of an aqueous potassium iodide solution containing 5 g / L of carbodiimide (carbodiimide group equivalent: 335) dissolved as an acid scavenger at a concentration of 5 g / L. It was then dried at 80°C for 40 seconds to obtain a polarizing film with a total stretch ratio of 5.9 times. A 40 μm-thick triacetylcellulose film was laminated to both sides of the obtained polarizing film using an aqueous polyvinyl alcohol solution as an adhesive, and dried at 80° C. to obtain a polarizing plate with a single transmittance of 43.6% and a polarization degree of 99.99%.

[0064] Example 2 A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the cleaning solution (A) in Example 1 was changed to a cleaning solution (B) in which an acid scavenger was dissolved at a concentration of 10 g / L.

[0065] Comparative Example 1 A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that no acid scavenger was dissolved in the cleaning solution (A).

[0066] Comparative Example 2 A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the cleaning solution (A) in Example 1 was changed to a cleaning solution (C) in which an acid scavenger was dissolved at a concentration of 0.1 g / L.

[0067]

[0068] The polarizing plates obtained in Examples 1 and 2 were obtained using polarizing films in which the nitrogen element content in the surface layer portion extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film was 0.05 mass% or more, or which contained 95% or more of the total nitrogen element contained in the polyvinyl alcohol-based film. Therefore, it can be seen that these polarizing plates have superior long-term heat resistance and durability compared to the polarizing plates of Comparative Examples 1 and 2 in which the nitrogen element content in the surface layer portion extending from the surface to a depth of 0.5 nm in the thickness direction of the film was less than 0.05 mass%, or which contained less than 95% of the total nitrogen element contained in the polyvinyl alcohol-based film.

[0069] Although the above examples show specific embodiments of the present invention, the examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0070] The polarizing plate obtained using the polarizing film of the present invention has excellent long-term heat resistance and durability, and is preferably used for various liquid crystal display devices, in-vehicle applications that require high durability and polarization performance, and display applications for industrial instruments that are required in various environments.

Claims

1. A polarizing film containing an acid scavenger, the polarizing film comprising a polyvinyl alcohol-based film, wherein the nitrogen element content in a surface layer portion of at least one surface of the polyvinyl alcohol-based film extending to a depth of 0.5 nm in the thickness direction from the surface is 0.05 mass % or more.

2. A polarizing film containing an acid scavenger, the polarizing film comprising a polyvinyl alcohol-based film, wherein at least one surface of the polyvinyl alcohol-based film contains 95% or more of the total nitrogen elements contained in the polyvinyl alcohol-based film in a surface layer portion extending from the surface to a depth of 0.5 nm in the thickness direction of the polyvinyl alcohol-based film.

3. The polarized film according to claim 1 or 2, wherein the acid scavenger is a carbodiimide compound.

4. The polarizing film according to claim 3, wherein the carbodiimide compound has a carbodiimide group equivalent of 100 to 1,000.

5. A method for producing a polarized film according to claim 1 or 2, which comprises a step of passing a polyvinyl alcohol film through multiple treatment tanks, in which an acid scavenger is blended in the treatment solution of the final treatment tank at a concentration of 0.5 g / L or more.

6. A polarizing plate having a protective film on at least one surface of the polarizing film according to claim 1 or 2.

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