TWI757444B - Polarizing film, polarizing plate, and methods for production thereof - Google Patents

Abstract

A polarizing film, which is a polarizing film of a PVA film comprising an iodine-containing dichroic dye and cross-linked with a boron compound, the degree of polarization is 99.5% or more, the monomer b value is 3.0 or less, and satisfies the following Formula (1) and Formula (2). This polarizing film has excellent polarizing properties and hue, small shrinkage and excellent dimensional stability; A≦0.9 (1)

B/A≧55 (2).

However, A is the shrinkage rate (%) of the polarizing film heated at 80° C. for 4 hours, and B is the shrinkage stress (N/mm ² ) of the polarizing film heated at 80° C. for 4 hours.

Description

Polarizing film, polarizing plate, and manufacturing method thereof

The present invention relates to a polarizing film, a polarizing plate, and a method for producing the same with a low shrinkage rate of a polyvinyl alcohol film containing an iodine-based dichroic dye.

The polarizing film used in the polarizing plate with the functions of transmitting and shielding light is the basic component of a liquid crystal display (LCD). Most polarizing plates have a structure in which a protective film such as a triacetate cellulose (TAC) film is laminated on the surface of the polarizing film; as the polarizing film constituting the polarizing plate, the mainstream is a polyvinyl alcohol film (hereinafter referred to as "polyethylene"). "Alcohol" (abbreviated as "PVA") is a stretched film that is uniaxially stretched and oriented and has a dichroic dye such as an iodine-based dye (I ₃ ^- or I ₅ ^- , etc.) adsorbed. Such a polarizing film is produced by a method of subjecting a PVA film to a swelling step, a dyeing step, a crosslinking step, a stretching step, an immobilization step, and a drying step, and the like.

In recent years, LCDs have been widely used in mobile applications such as notebook computers and mobile phones. LCDs for such mobile devices are used in various environments. Therefore, there is a need for a polarizing film with low shrinkage at high temperature and excellent dimensional stability.

Patent Documents 1 to 5 describe polarizing films obtained by stretching a PVA film dyed with an iodine-based dye, followed by drying at 50 to 70° C. for 2 to 4 minutes. However, the polarizing film thus obtained has a relatively high shrinkage rate. When such a polarizing film is bonded to a glass plate and used for an LCD, when the LCD is used or stored at a high temperature, there is a problem that the polarizing film shrinks and the glass plate is warped.

prior art literature Patent Literature

Patent Document 1 Japanese Patent Laid-Open No. 2006-65309

Patent Document 2 Japanese Patent Laid-Open No. 2014-197050

Patent Document 3 Japanese Patent Laid-Open No. 2006-267153

Patent Document 4 Japanese Patent Application Laid-Open No. 2013-140324

Patent Document 5 Japanese Patent Application Laid-Open No. 2012-3173

The present invention has been accomplished in order to solve the above-mentioned problems, and it is an object of the present invention to provide a polarizing film having excellent polarization performance and hue, low shrinkage rate, and excellent dimensional stability, and a method for producing the same. Furthermore, it is an object to provide a polarizing plate using such a polarizing film and a manufacturing method thereof.

The above problem can be solved by providing a polarizing film, the polarizing film is a polarizing film of a PVA film formed by containing an iodine-based dichroic dye and cross-linked with a boron compound, and the degree of polarization is 99.5% or more. Monomer The b value (single-piece b value) is 3.0 or less, and satisfies the following formulas (1) and (2): A≦0.9 (1)

B/A≧55 (2).

However, A is the shrinkage rate (%) of the polarizing film heated at 80° C. for 4 hours, and B is the shrinkage stress (N/mm ² ) of the polarizing film heated at 80° C. for 4 hours.

In this case, the value of the monomer b is preferably 1.0 or more. A is also preferably 0.5 or more. The degree of polymerization of the aforementioned PVA is also preferably 1,500 to 6,000. The thickness of the aforementioned polarizing film is also preferably 1 to 30 μm.

The above problem can also be solved by providing a method for producing the aforementioned polarizing film, which comprises a step of dyeing a PVA film with an iodine-based dichroic dye, a step of cross-linking using a boron compound, and a step of stretching Afterwards, an annealing treatment step of heating the PVA film with a fixed stretching direction at 55-65° C. for more than 72 hours is performed.

The polarizing plate formed by laminating the polarizing film and the protective film is a preferred embodiment of the present invention. After the step of dyeing the PVA film with iodine-based dichroic dye, the step of cross-linking with a boron compound, and the step of stretching, the PVA film and the protective film are laminated to obtain a multilayer film, which is then fixed and stretched. The above-mentioned manufacturing method of the polarizing plate in the annealing treatment step of heating the multilayer film at 55-65° C. for more than 72 hours is also a preferred embodiment of the present invention.

The polarizing film of the present invention has excellent polarizing performance and hue, and has small shrinkage rate and excellent dimensional stability. Therefore, the polarizing plate using the aforementioned polarizing film is suitable for high-performance LCDs, especially LCDs used at high temperatures. Moreover, according to the manufacturing method of this invention, such a polarizing film and a polarizing plate can be manufactured simply.

FIG. 1 is a graph showing the shrinkage ratio and the monomer b value of the polarizing films of Examples 1-2, Comparative Examples 1-3, 5-12 and 14-16, and Reference Example 1. FIG.

2 is a graph showing shrinkage stress and shrinkage ratio of polarizing films of Examples 1-2, Comparative Examples 1-3, 5-12, and 14-16, and Reference Example 1. FIG.

The form in which the invention is carried out

The polarizing film of the present invention is a polarizing film comprising a PVA film containing an iodine-based dichroic dye and cross-linked with a boron compound, the degree of polarization is 99.5% or more, the monomer b value is 3.0 or less, and the following Formulas (1) and (2): A≦0.9 (1)

B/A≧55 (2).

However, A is the shrinkage rate (%) of the polarizing film heated at 80° C. for 4 hours, and B is the shrinkage stress (N/mm ² ) of the polarizing film heated at 80° C. for 4 hours.

Generally, if the polarizing performance of the polarizing film containing the PVA film containing an iodine-based dichroic dye is to be improved, the shrinkage rate and shrinkage stress of the polarizing film at high temperature become high. When such a polarizing film is used for an LCD, the glass plate to which this polarizing film is bonded will warp and cause a problem. In particular, LCDs for mobile use are mostly used or stored at high temperatures, and the glass plates used are also relatively thin, so the shrinkage of the polarizing film has become a serious problem.

In order to improve such a problem, if the polarizing film is heat-treated at a high temperature, the shrinkage rate or shrinkage stress will be reduced, but the polarization performance will also be reduced, and the hue will also be deteriorated. In this way, it is difficult to improve the dimensional stability of the polarizing film by reducing the shrinkage rate or shrinkage stress while maintaining excellent polarization performance and hue. The polarizing film of the present invention is used to solve this problem, and is characterized by having excellent polarizing performance and hue, but low shrinkage.

The manufacturing method of such a polarizing film is not particularly limited, and the inventor of the present application successfully produced a polarizing film with such properties for the first time by adopting a new manufacturing method. Specifically, after the step of dyeing the PVA film with an iodine-based dichroic dye, the step of cross-linking with a boron compound, and the step of stretching, the PVA film with a fixed stretching direction is subjected to 55~ The annealing treatment step of heating at 65°C for more than 72 hours can produce a polarizing film with excellent polarization performance and hue, small shrinkage rate and excellent dimensional stability. This production is applicable to the production of various polarizing films including the polarizing film of the present invention. Hereinafter, this production method will be described in detail.

PVA contained in the raw material PVA film used for the production of the polarizing film of the present invention can be obtained by saponifying a polyvinyl ester obtained by polymerizing one or two or more kinds of vinyl esters. Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl trimethyl acetate, vinyl versatic acid, vinyl laurate, hard vinyl Vinyl fatty acid, vinyl benzoate, propylene acetate, etc., among these, vinyl acetate is preferable from the viewpoints of the ease of manufacture, availability, cost, and the like of PVA.

The polyvinyl ester may be obtained by using only one or two or more types of vinyl esters as a monomer, and may be one type or two or more types of vinyl esters within a range that does not impair the effects of the present invention. and copolymers of other monomers with which it can be copolymerized.

Examples of other monomers that can be copolymerized with vinyl esters include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutylene; (meth)acrylic acid or a salt thereof; (methyl) Methyl acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, (meth)acrylate (meth)acrylates of tertiary butyl acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, octadecyl (meth)acrylate, etc.; (meth)acrylate base) acrylamide; N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, N,N-dimethyl (meth) acrylamide, diacetone (methyl) ) acrylamide, (meth) acrylamide propanesulfonic acid or its salt, (meth) acrylamide propyl dimethylamine or its salt, N-methylol (meth) acrylamide or its derivative (meth)acrylamide derivatives of compounds such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, etc. N-vinylamide; methyl vinyl ether, ethyl vinyl ether , n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tertiary butyl vinyl ether, dodecyl vinyl ether, octadecyl vinyl ether and other vinyl ethers; vinyl cyanide such as (meth)acrylonitrile; vinyl halide such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic Acid or its salt, ester or acid anhydride; Iconic acid or its salt, ester or acid anhydride; vinylsilyl compounds such as ethylenetrimethoxysilane; unsaturated sulfonic acid, etc. The said polyvinyl ester may have 1 type or 2 or more types of structural units derived from the said other monomer.

Based on the molar number of the total structural units constituting the polyvinyl ester, the proportion of the structural units derived from other monomers in the polyvinyl ester is preferably 15 mol % or less, more preferably 10 mol % or less, More preferably, it is 5 mol % or less.

In particular, if the other monomer is a monomer such as (meth)acrylic acid, unsaturated sulfonic acid, etc., which has the possibility of promoting the water solubility of the obtained PVA, in order to prevent the PVA from being dissolved in the manufacturing process of the polarizing film, based on the composition of the polymer The molar number of the total structural units of the vinyl ester and the ratio of the structural units derived from these monomers in the polyvinyl ester are preferably 5 mol % or less, more preferably 3 mol % or less.

The PVA used in the present invention may be modified by one kind or two or more kinds of graft-copolymerizable monomers within the range that does not impair the effects of the present invention. Examples of the graft-copolymerizable monomers include unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acids or derivatives thereof; α-olefins having 2 to 30 carbon atoms. The ratio of the structural unit derived from the graft-copolymerizable monomer (the structural unit of the graft-modified portion) in the PVA is preferably 5 mol % or less based on the molar number of the total structural units constituting the PVA.

A part of the hydroxyl groups of the aforementioned PVA may be cross-linked or not. In addition, a part of the hydroxyl groups of the above-mentioned PVA can react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure.

The degree of polymerization of the aforementioned PVA is preferably in the range of 1500-6000, more preferably in the range of 1800-5000, still more preferably in the range of 2000-4000. When the degree of polymerization is 1500 or more, the durability of the obtained polarizing film can be further improved. On the other hand, when the degree of polymerization is 6,000 or less, it is possible to suppress an increase in manufacturing cost, poor step-through properties at the time of film formation, and the like. The polymerization degree of PVA in this specification means the average polymerization degree measured according to the description of JIS K6726-1994. In addition, although the PVA in the polarizing film contains a cross-linked structure generated by a boron compound such as boric acid, if it is dissociated by hydrolysis of boric acid ester or the like, the average degree of polymerization of PVA itself does not substantially change.

From the viewpoint of polarizing properties of the polarizing film, etc., the saponification degree of PVA is preferably 98 mol % or more, more preferably 98.5 mol % or more, and still more preferably 99 mol % or more. If the degree of saponification is less than 98 mol%, PVA may be easily dissolved during the manufacturing process of the polarizing film, and the dissolved PVA will adhere to the film and reduce the polarizing performance of the polarizing film. In addition, the saponification degree of PVA in this specification refers to the vinyl alcohol relative to the total moles of vinyl alcohol units and structural units that can be converted into vinyl alcohol units by saponification (typically vinyl ester units) and vinyl alcohol units possessed by PVA. The proportion of the number of moles in the unit (mol%). The degree of saponification can be measured according to the description of JIS K6726-1994. In addition, the saponification degree of the PVA in the raw material film and the PVA in the obtained polarizing film is substantially the same.

The PVA film is produced using the film-forming stock solution. As a film formation method, a casting film formation method, an extrusion film formation method, a wet film formation method, a gel film formation method, etc. are mentioned, for example. Only one of these film forming methods may be used, or a combination of two or more may be used. Among these film forming methods, the casting film forming method and the extrusion film forming method are preferable because a PVA film with uniform thickness and width and good physical properties can be obtained. The produced PVA film can be dried or heat treated as required.

The film-forming stock solution, for example, can be obtained by mixing the aforementioned PVA with a liquid medium, and one or more of surfactants, plasticizers, additives, and the like can be further added as needed. In the film-forming stock solution, PVA may be in a state of being dissolved in a liquid medium or in a molten state. The above mixing system is preferably carried out under heating.

Examples of the above-mentioned liquid medium used for the preparation of the membrane-forming stock solution include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, Glycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, diethylenetriamine, etc., can be used alone or in two or more. Among them, water is preferable in terms of burden on the environment and recyclability.

The volatile matter ratio of the film-forming stock solution (the content ratio of volatile components such as liquid media removed by volatilization or evaporation in the film-forming stock solution) varies with the film-forming method, film-forming conditions, etc. Generally speaking , preferably in the range of 50 to 95 mass %, more preferably in the range of 55 to 90 mass %, and still more preferably in the range of 60 to 85 mass %. The volatile content permeating the membrane-forming stock solution is 50% by mass or more, the viscosity of the membrane-forming stock solution does not become too high, the filtration or defoaming at the time of preparing the membrane-forming stock solution can be smoothly performed, and it is easy to produce a membrane with fewer impurities or defects. PVA film. On the other hand, when the volatile content permeating the membrane-forming stock solution is 95% by mass or less, the concentration of the membrane-forming stock solution does not become too low, and the PVA thin film can be easily produced industrially.

The membrane-forming stock solution preferably contains a surfactant. By including a surfactant, the film-forming property can be improved, the occurrence of film thickness unevenness can be suppressed, and the film can be easily peeled off from a metal roll or belt used for film-forming. When a PVA film is produced from a film-forming stock solution containing a surfactant, the PVA film may contain a surfactant. The type of the above-mentioned surfactant is not particularly limited, but an anionic surfactant or a nonionic surfactant is preferable from the viewpoint of the releasability of a metal roll or a belt, and the like.

As anionic surfactants, for example, carboxylic acid types such as potassium laurate; sulfate types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acids such as dodecylbenzenesulfonate are suitable. type, etc.

As the nonionic surfactant, for example, alkyl ether types such as polyoxyethylene oleyl ether, alkyl phenyl ether types such as polyoxyethylene octyl phenyl ether, and alkyl phenyl ether types such as polyoxyethylene laurate are suitable. alkyl amine type such as polyoxyethylene lauryl amino ether; alkyl amide type such as polyoxyethylene lauric acid amide; polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; lauric acid Alkolamide type such as diethanolamide and diethanolamide oleate; allylphenyl ether type such as polyoxyalkylene allylphenyl ether, etc.

These surfactants may be used alone or in combination of two or more.

When the membrane-forming stock solution contains a surfactant, the content is preferably 0.01-0.5 mass parts, more preferably 0.02-0.3 mass parts, and particularly preferably 0.05-0.1 mass parts relative to 100 mass parts of PVA contained in the membrane-forming stock solution parts by mass. When the content is 0.01 part by mass or more, film formability and peelability can be further improved. On the other hand, when the content is 0.5 parts by mass or less, it is possible to prevent the surfactant from oozing out to the surface of the PVA film, and blocking occurs and the handling properties are deteriorated.

The content of PVA in the raw material PVA film used in the present invention is preferably 50 to 99% by mass from the viewpoint of the ease of manufacture of the polarizing film. The content is more preferably 75% by mass or more, still more preferably 80% by mass or more, and particularly preferably 85% by mass or more. On the other hand, the content is more preferably 98% by mass or less, still more preferably 96% by mass or less, and particularly preferably 95% by mass or less.

From the viewpoint of improving stretchability, the aforementioned PVA film preferably contains a plasticizer. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, and trimethylolpropane, and the PVA film may be One or more of these plasticizers are contained. Among these, glycerin is preferable from the viewpoint of the effect of improving stretchability.

The content of the plasticizer in the PVA film is preferably 1-20 parts by mass relative to 100 parts by mass of PVA. By making the said content 1 mass part or more, the stretchability of the PVA film can be further improved. On the other hand, when the said content is 20 mass parts or less, it can prevent that a PVA film becomes too soft and the handling property is inferior. The aforementioned content is more preferably 2 parts by mass or more, still more preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more. Moreover, the said content is more preferably 15 parts by mass or less. In addition, although it varies depending on the production conditions of the polarizing film, etc., the plasticizer contained in the PVA film may be eluted during the production of the polarizing film, so the entire amount of the plasticizer does not necessarily remain in the polarizing film.

The aforementioned PVA film may further include antioxidants, antifreeze agents, pH adjusters, concealing agents, anti-coloring agents, oil agents, surfactants and other ingredients as required.

The thickness of the aforementioned PVA film is preferably 5 to 100 μm. Through the thickness of 100 μm or less, a thin polarizing film can be easily obtained. The aforementioned thickness is more preferably 60 μm or less. On the other hand, when the thickness is less than 5 m, in addition to the possibility of making it difficult to manufacture a polarizing film, there is a possibility that uneven dyeing is likely to occur. The thickness of the PVA film is more preferably 7 μm or more. The thickness referred to here means the thickness of the PVA layer in the case of a multilayer film.

The aforementioned PVA film may be a single-layer film, or a multi-layer film having a PVA layer and a base resin layer may also be used. In the case of a single-layer film, the thickness of the film is preferably 20 μm or more, more preferably 30 μm or more, in order to ensure handling properties. On the other hand, in the case of a multilayer film, the thickness of the PVA layer may be 20 μm or less, or 15 μm or less. The thickness of the base resin layer in the multilayer film is usually 20 to 500 μm.

When a multilayer film having a PVA layer and a base resin layer is used as the PVA film, the base resin must be capable of being stretched together with PVA. Polyester or polyolefin resin or the like can be used. Among them, amorphous polyester resins are preferred, and polyethylene terephthalate, or amorphous polyesters obtained by copolymerizing isophthalic acid or 1,4-cyclohexanedimethanol, etc., are suitably used. resin. Preferably, the multilayer film is produced by applying the PVA solution to the base resin film. At this time, in order to improve the adhesiveness between the PVA layer and the base resin layer, the surface of the base resin film may be modified, or an adhesive layer may be formed between the two layers.

The shape of the aforementioned PVA film is not particularly limited, but a long-sized PVA film is preferable because it can be continuously supplied when producing a polarizing film. The length (length in the longitudinal direction) of the long PVA film is not particularly limited, and can be appropriately set according to the application of the polarizing film to be produced, and for example, it can be in the range of 5 to 20000 m.

The width of the aforementioned PVA film is not particularly limited, and can be appropriately set according to the application of the polarizing film to be produced and the like. In recent years, since the area of liquid crystal televisions and liquid crystal screens has been continuously increased, if the width of the PVA film is 0.5 m or more, more preferably 1.0 m or more, it is suitable for these applications. On the other hand, if the width of the PVA film is too large, it tends to be difficult to stretch uniformly when producing a polarizing film with a practical apparatus, so the width of the PVA film is preferably 7 m or less.

Using the PVA film described above as a raw material, the polarizing film of the present invention is produced. Specifically, it is preferable to perform a step of dyeing the PVA film with an iodine-based dichroic dye (hereinafter referred to as a "dyeing step"), and a step of crosslinking the PVA film with a boron compound (hereinafter referred to as a "staining step"). After the “crosslinking step”) and the step of stretching the aforementioned PVA film (hereinafter referred to as the “stretching step”), annealing is performed to heat the PVA film with a fixed stretching direction at 55-65°C for more than 72 hours method of processing steps to manufacture polarizing films.

In the above-mentioned production method, it is preferable to perform the step of swelling the PVA film of the raw material in advance (hereinafter referred to as a "swelling step"), and then perform each of the above steps. It is also preferable that after the dyeing step, the crosslinking step and the stretching step, the step of drying the PVA film (hereinafter referred to as "drying step") is further performed, and then the annealing treatment step is performed. Before the drying step, a step of appropriately cleaning the PVA film can also be performed. In addition, in the above-mentioned manufacturing method, one kind of step may be performed a plurality of times. In addition, a plurality of steps may be simultaneously performed in one bath.

Regarding the aforementioned manufacturing method, it is more preferable to perform the dyeing step, the cross-linking step, the stretching step and the annealing treatment step in sequence, and even more preferably, the swelling step, the dyeing step, the cross-linking step, the stretching step, and the fixing step are performed in sequence. Methods of treatment step, drying step and annealing treatment step. Hereinafter, each step will be described in detail.

In the aforementioned manufacturing method, it is preferable to first perform the step of swelling the PVA film of the raw material. In the swelling step, the PVA film is swelled by immersing in water at 10 to 50°C. The temperature of the water is preferably 20°C or higher, more preferably 40°C or lower. By immersing in water of such a temperature, the aforementioned PVA film can be swelled efficiently and uniformly. The time for immersing the aforementioned PVA film in water is preferably 0.1 to 5 minutes, more preferably 0.5 to 3 minutes. By setting the immersion time as such, the PVA film can be swelled efficiently and uniformly. In addition, the water for immersing the PVA film is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or may be a mixture of water and a water-soluble organic solvent.

In the above-mentioned production method, the step of dyeing the PVA film with an iodine-based dichroic dye is performed. It is preferable to perform the dyeing step after the swelling step. In addition, the dyeing step may be performed before the stretching step described later, or may be performed after the stretching step, but the former is preferable. The dyeing step is generally carried out by immersing the PVA film in a solution (especially an aqueous solution) containing iodine-potassium iodide as a dyeing bath, and this dyeing method is also suitable for use in the present invention. The concentration of iodine in the dyeing bath is preferably 0.01 to 0.5 mass %, and the concentration of potassium iodide is preferably 0.01 to 10 mass %. In addition, the temperature of the dyeing bath is preferably 10 to 50°C, particularly preferably 20 to 40°C. The time for immersing the PVA film in the dyeing bath is preferably 0.1 to 10 minutes, more preferably 0.2 to 5 minutes. The dyeing bath may contain a boron compound, and the content thereof is less than 5 mass % in terms of general boric acid conversion, preferably 1 mass % or less. As the aforementioned boron compound, those used in the crosslinking step to be described later can be used.

In the aforementioned production method, a step of crosslinking the PVA thin film using a boron compound is performed. After the crosslinking step is performed in this way, by performing the annealing treatment step described later, a polarizing film having excellent polarization performance and hue, but a low shrinkage rate can be obtained. The crosslinking step is performed before the annealing treatment step. Moreover, it is preferable to perform a crosslinking process after a dyeing process. When wet stretching is performed at a high temperature, it is preferable to perform the crosslinking step before the stretching step from the viewpoint of preventing the elution of PVA. By using an aqueous solution containing a boron compound as the dyeing bath or stretching bath, the crosslinking step, the dyeing step or the stretching step can also be performed simultaneously. In addition, as a fixing treatment step to be described later, a step of crosslinking the PVA thin film using a boron compound may be performed.

The crosslinking step can be performed by dipping the PVA film in an aqueous solution containing a boron compound. As this boron compound, one type or two or more types of boric acid salts such as boric acid and borax can be used. The concentration of the boron compound in the aqueous solution is preferably 1 to 15 mass % in terms of boric acid, more preferably 2 to 7 mass %. When the said concentration is 1-15 mass %, the said effect can be exhibited, and sufficient stretchability can also be maintained. The aforementioned aqueous solution may also contain auxiliary agents such as potassium iodide. The temperature of the aqueous solution is preferably 20 to 50°C, particularly preferably 25 to 40°C, from the viewpoint of being able to effectively crosslink.

In addition to the stretching step described later, the PVA film may also be stretched in or between the above-mentioned steps. By performing such stretching (pre-stretching), the PVA film can be prevented from being wrinkled. From the viewpoint of polarizing performance when manufacturing polarizing films, etc., based on the original length of the raw PVA film before stretching, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each step) before stretching is higher than Preferably it is 4 times or less. The stretching ratio in the swelling step is preferably 1.05 to 3 times; the stretching ratio in the dyeing step is preferably 3 times or less; and the stretching ratio in the cross-linking step is preferably 2 times or less.

In the manufacturing method of the aforementioned polarizing film, the step of stretching the aforementioned PVA film is performed. The stretching step can be performed by uniaxially stretching the PVA film using 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 boron compound, or can also be carried out in the above-mentioned dyeing bath or crosslinking bath. As the boron compound, the above-mentioned ones used for the crosslinking treatment can be used. In addition, in the case of a dry stretching method, the stretching may be carried out directly at room temperature, or it may be stretched while heating, or it may be carried out in the air using a PVA film after water absorption. Among these, a wet stretching method is preferred, and uniaxial stretching is more preferred in an aqueous solution containing a boron compound. The concentration of the boron compound in the aqueous solution of the boron compound is preferably 0.5 to 6.0 mass % in terms of boric acid, more preferably 1.0 to 5.0 mass %, and particularly preferably 1.5 to 4.5 mass %. Moreover, the said aqueous solution may contain potassium iodide, and its density|concentration is preferably 0.01-10 mass %.

The temperature at which the PVA film is stretched in the stretching step is preferably 30 to 90°C, more preferably 40 to 80°C, and still more preferably 50 to 70°C.

From the viewpoint of polarizing performance when producing a polarizing film, the stretching ratio in the stretching step is preferably 1.2 times or more, more preferably 1.5 times or more, and still more preferably 2 times or more. In addition, based on the original length of the original PVA film before stretching, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each step) including the stretching ratio before the above-mentioned stretching is preferably 5.2 times or more, more preferably 5.5 times or more, and particularly preferably 5.8 times or more. The upper limit of the total stretch ratio is not particularly limited, but the stretch ratio is preferably 8 times or less in order to prevent stretch breakage.

The direction of uniaxial stretching when uniaxially stretching a long PVA film is not particularly limited, and uniaxial stretching in the longitudinal direction or transverse uniaxial stretching in the width direction can be employed. When producing a polarizing film, uniaxial stretching in the longitudinal direction is preferred because of the fact that an excellent polarizing performance can be obtained. The uniaxial stretching in the longitudinal direction can be performed by using a stretching apparatus including a plurality of rolls parallel to each other, and changing the peripheral speed between the rolls. In addition, transverse uniaxial stretching can be performed using a tenter type stretching machine.

In the manufacturing method of the polarizing film, preferably, after the stretching step, a fixing treatment step is performed on the PVA film. Thereby, the adsorption of the iodine-based dichroic dye to the PVA film can be made stronger. As the fixation treatment bath used for the fixation treatment, an aqueous solution containing one or more boron compounds can be used; as the boron compound, the above-mentioned ones used for the crosslinking treatment can be used. In addition, an iodine compound or a metal compound may be added to the fixed treatment bath as required. The concentration of the boron compound in the fixed treatment bath is preferably 1 to 15% by mass. The concentration of the boron compound is more preferably 10% by mass or less. When potassium iodide is contained in the aqueous solution, the concentration thereof is preferably 0.01 to 10% by mass. The temperature of the fixed treatment bath is preferably 15 to 60°C, more preferably 20 to 40°C.

In the production method of the present invention, it is also preferable to perform the step of drying the PVA film after the dyeing step, the cross-linking step and the stretching step, and then the annealing treatment step. When the fixing treatment step is performed, it is preferable to perform the drying step after this step. From the viewpoint of further improving the dimensional stability of the polarizing film, the drying temperature is preferably 30°C or higher, more preferably 40°C or higher, and even more preferably 50°C or higher. On the other hand, from the viewpoint of better polarization performance and hue of the polarizing film, the drying temperature is preferably 100°C or lower, more preferably 80°C or lower. From the viewpoint of further improving the dimensional stability of the polarizing film, the drying time is preferably 10 seconds or more, more preferably 30 seconds or more, and even more preferably 1 minute or more. On the other hand, from the viewpoint of better polarization performance and hue of the polarizing film, the drying time is preferably 30 minutes or less, more preferably 15 minutes or less, still more preferably 10 minutes or less, and particularly preferably 5 minutes or less. It is preferable to perform the drying step in a gas such as air or an inert gas, and the former is more preferable from the viewpoint of easy handling. The humidity when the drying step is performed in air is not particularly limited, but the relative humidity is preferably 35% or less, more preferably 15% or less.

After fixing the stretching direction of the aforementioned PVA film thus obtained, an annealing treatment step of heating at 55 to 65° C. for 72 hours or more is performed. Here, the degree of polarization of the PVA film used in the annealing treatment step is preferably 99.7% or more. By performing the aforementioned annealing treatment step, the shrinkage rate can be reduced while maintaining the excellent polarizing properties and hue of the aforementioned PVA film. Therefore, by performing the aforementioned annealing treatment step on the PVA film with such a high degree of polarization, a polarizing film with excellent polarization performance and hue and low shrinkage rate can be obtained. From the same viewpoint, the monomer b value of the PVA thin film used in the annealing treatment step is also preferably 2.8 or less. When the drying step is performed at 55 to 65° C., the drying step and the annealing treatment step may be performed for a total of 72 hours or more. However, when the roll of the PVA film is annealed as described later, the roll of the PVA film needs to be annealed for 72 hours or more.

In addition, from the viewpoint of further enhancing the above-mentioned effect of the annealing treatment step, the moisture content of the PVA thin film to be applied to this step is preferably 1 to 25% by mass. The said moisture content is more preferably 20 mass % or less, and still more preferably 15 mass % or less.

In the aforementioned annealing treatment step, the aforementioned PVA film needs to be annealed in a state where the stretching direction is fixed. The method at this time is not particularly limited, and after the PVA film is wound in the stretching direction to obtain a film roll, the film roll is heated, or the end of the PVA film in the stretching direction is fixed with a clamp or the like. The method of performing the annealing treatment and the like are preferably the former from the viewpoint of productivity.

The aforementioned annealing treatment steps are performed at 55-65°C. By treating the PVA film at such a low temperature, the shrinkage rate can be reduced while maintaining the excellent polarization properties and hue of the aforementioned PVA film. When the said temperature is less than 55 degreeC, the effect of reducing the shrinkage rate of the said PVA film cannot be acquired. The aforementioned temperature is preferably 57°C or higher. On the other hand, when the said temperature exceeds 65 degreeC, the polarization performance and hue of the said PVA film will deteriorate. The aforementioned temperature is preferably 63°C or lower.

The aforementioned annealing treatment step is preferably performed in a gas such as air or an inert gas, and the former is more preferred from the viewpoint of easy treatment. The humidity when the annealing treatment step is performed in air is not particularly limited, but the relative humidity is preferably 35% or less, more preferably 15% or less.

In the aforementioned annealing treatment step, the aforementioned PVA film is heated for more than 72 hours. By heating the PVA film at a low temperature for a long time in this way, the shrinkage rate can be reduced while maintaining the excellent polarization performance and hue of the PVA film. If the heating time is less than 72 hours, the effect of reducing the shrinkage rate of the PVA film is insufficient. The aforementioned heating time is preferably 85 hours or more, more preferably 100 hours or more. On the other hand, the aforementioned heating time is preferably 300 hours or less, more preferably 200 hours or less.

In order to solve the problem of the warpage of the glass plate caused by the shrinkage of the polarizing film in the LCD, the inventors of the present application have studied the annealing treatment of the polarizing film. Although the shrinkage rate can be reduced by increasing the treatment temperature, the polarization performance or hue will deteriorate, and it is not easy to balance the two. As a result of further investigation by the present inventors, it was unexpectedly found that the shrinkage rate can be reduced while maintaining excellent polarization performance and hue by performing annealing treatment at low temperature for a long time as described above. This mechanism is still unclear, but it is concluded that by annealing at low temperature for a long time, the cross-linking point of PVA generated by the boron compound will increase and the shrinkage rate will decrease. In addition, it is considered that by performing the annealing treatment at a low temperature, since the change of the amorphous part of PVA and the decomposition of the dye can be suppressed, the polarization performance can be maintained. Furthermore, it is considered that by performing the annealing treatment at a low temperature, the thermal decomposition of the PVA film can be suppressed and the hue can be maintained.

The monomer b value of the polarizing film of the present invention thus obtained needs to be 3.0 or less. The polarizing film of the present invention having such a low monomer b value and excellent hue is suitable for use in LCDs and the like. The aforementioned monomer b value is preferably 2.7 or less, more preferably 2.45 or less.

The aforementioned polarizing film needs to satisfy the following formulas (1) and (2): A≦0.9 (1)

B/A≧55 (2).

However, A is the shrinkage rate (%) of the polarizing film heated at 80° C. for 4 hours, and B is the shrinkage stress (N/mm ² ) of the polarizing film heated at 80° C. for 4 hours.

When the shrinkage ratio A of the polarizing film is 0.9% or less, when the polarizing film is used in an LCD, warpage of the glass plate hardly occurs. The polarizing film of the present invention with such a low shrinkage rate A is suitable for LCDs, especially LCDs for mobile devices that are mostly used and stored at high temperatures and use thin glass plates. The shrinkage ratio A is preferably 0.85% or less, more preferably 0.80 or less. On the other hand, the shrinkage rate A is preferably 0.5% or more from the viewpoint of particularly excellent polarization performance and balance of hue and dimensional stability. As a measurement method of the shrinkage rate A of the said polarizing film, the method described in the Example mentioned later was employ|adopted.

When the ratio (B/A) of the shrinkage stress B (N/mm ² ) to the shrinkage ratio A (%) of the polarizing film is 55 or more, the balance between the dimensional stability and the polarizing performance of the polarizing film is good. Generally, if the degree of polarization of the polarizing film is to be increased, the shrinkage stress and the shrinkage rate will increase, and problems such as warpage of the glass plate will occur when used in an LCD. On the other hand, since the polarizing film of the present invention has high shrinkage stress B and low shrinkage rate A, it can have both dimensional stability and polarizing performance, thereby solving this problem. The aforementioned ratio (B/A) is preferably 60 or more. On the other hand, the aforementioned ratio (B/A) is preferably 100 or less.

As a method for measuring the shrinkage ratio A and shrinkage stress B of the polarizing film, the methods described in the examples described later were adopted. In the present invention, the shrinkage ratio and shrinkage stress are measured in the stretching direction of the polarizing film, and when stretching in a plurality of directions, the measurement is carried out in a direction with a high stretching ratio.

The aforementioned polarizing film preferably satisfies the following formula (3): B≧45 (3).

However, B is synonymous with the above formula (2).

When the shrinkage stress B passing through the polarizing film is 45 N/mm ² or more, the polarizing performance can be further improved. As mentioned above, due to the low shrinkage rate A of the polarizing film, when the polarizing film is used in LCD, even when the film shrinks, the dimensional change of the film can still be absorbed by the adhesive layer between the film and the glass plate . Therefore, even if the shrinkage stress B is high, warpage of the glass is less likely to occur. In particular, LCDs used in mobile devices such as portable terminals are smaller in size, and the dimensional changes caused by the shrinkage of the polarizing film are also smaller, so the glass is less likely to warp. The shrinkage stress B is more preferably 47 N/mm ² or more. On the other hand, the shrinkage stress B is preferably 60 N/mm ² or less.

The thickness of the aforementioned polarizing film is preferably 1 to 30 μm. This thin polarizing film is suitable for LCDs, etc., especially LCDs for mobile devices. When the said thickness is less than 1 micrometer, in addition to a possibility that it may become difficult to manufacture a polarizing film, there exists a possibility that dyeing unevenness may arise easily. The aforementioned thickness is preferably 5 μm or more. On the other hand, the aforementioned thickness is more preferably 20 μm or less.

The content of PVA in the aforementioned polarizing film is preferably 50 to 99% by mass. The content is more preferably 75% by mass or more, still more preferably 80% by mass or more, and particularly preferably 85% by mass or more. Moreover, 98 mass % or less is more preferable, 96 mass % or less is still more preferable, and 95 mass % or less is especially preferable.

The content of the boron compound in the polarizing film is preferably 5 to 50 mass % in terms of boric acid. When the said content is less than 5 mass %, there exists a possibility that the effect of reducing a shrinkage rate may become insufficient. The aforementioned content is more preferably 12% by mass or more. On the other hand, when the said content exceeds 50 mass %, there exists a possibility that the shrinkage force of a polarizing film may become too high. The aforementioned content is preferably 30% by mass or less.

The degree of polarization of the aforementioned polarizing film needs to be 99.5% or more. Polarizing films with such a high degree of polarization are suitable for high-performance LCDs. The aforementioned degree of polarization is preferably 99.7% or more, more preferably 99.8% or more.

The monomer b value of the polarizing film needs to be 3.0 or less. Such a polarizing film system with low monomer b value and excellent hue is suitable for high-performance LCD. The aforementioned monomer b value is preferably 2.8 or less, more preferably 2.5 or less. On the other hand, the aforementioned monomer b value is usually 0 or more. The b value of the monomer is preferably 1.0 or more, from the viewpoint that the polarization performance and the balance between hue and dimensional stability are particularly excellent.

The polarizing plate formed by laminating the polarizing film and the protective film is a preferred embodiment of the present invention. The polarizing plate using the aforementioned polarizing film has excellent polarizing properties and hue, and the polarizing film has a low shrinkage rate and excellent dimensional stability, so it is suitable for LCDs, etc., especially LCDs for mobile devices.

The protective film is not particularly limited as long as it is optically transparent and has mechanical strength. For example, triacetate cellulose (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, and polyester film can be used. , Cycloolefin (COP) film, etc. The polarizing plate may be formed by bonding the protective film to one side of the polarizing film, or may be formed by bonding the protective film to both sides of the polarizing film. Moreover, as an adhesive agent used for bonding, a PVA-type adhesive agent, a urethane-type adhesive agent, or an ultraviolet curable adhesive agent etc. are mentioned.

The manufacturing method of the polarizing plate is not particularly limited, and it is preferable to perform the step of dyeing the PVA film with an iodine-based dichroic dye, the step of cross-linking it with a boron compound, and the step of stretching, and then the PVA film is protected. A method of laminating the films to obtain a multi-layer film, and then performing an annealing treatment step of heating the multi-layer film with the stretching direction fixed at 55 to 65° C. for 72 hours or more. According to such a manufacturing method, since a polarizing plate which is bright, has good polarization properties and is excellent in dimensional stability under high temperature conditions can be obtained, it can be applied to the manufacture of various polarizing plates including the polarizing plate of the present invention. The polarizing plate can also be obtained in the same manner as the above-mentioned manufacturing method of the polarizing film, except that the step of laminating a PVA film and a protective film to obtain a multilayer film (hereinafter referred to as a "lamination step") is performed. In the manufacturing method of the polarizing plate, when the drying step is performed, the lamination step may be performed before the drying step, or the lamination step may be performed after the drying step. When the fixing treatment step is performed, it is preferable to perform the lamination step after the fixing treatment step.

The polarizing plate thus obtained is bright, has good polarizing properties, and has excellent dimensional stability under high temperature conditions, so it can be applied to high-performance LCDs, especially LCDs for mobile devices.

[Example]

Hereinafter, the present invention will be described more specifically with reference to examples.

[Calculation of boron content in polarizing film]

The polarizing film (mass Jg) was added to distilled water and heated to obtain an aqueous solution (mass Kg, solid content 0.005 mass %) in which the polarizing film was dissolved. The boron concentration [L (ppm)] in the aqueous solution was measured using a polymorphic ICP emission analyzer (ICP) manufactured by Shimadzu Corporation, and the content of boron element in the polarizing film was determined according to the following formula.

Total boron content in polarizing film (mass %)=[(L× ^10-6 ×K)/J]×100

[Optical properties of polarizing films]

From the center portion of the obtained polarizing film in the width direction, a rectangular sample was taken at 3 cm in the longitudinal direction and 1.5 cm in the width direction of the polarizing film, and a spectrophotometer with integrating sphere (“V7100” manufactured by JASCO Corporation) was used. JIS Z8722 (measurement method of object color) is used to measure the single-piece transmittance (T), the degree of polarization (V) and the single-piece b value after the visual sensitivity is corrected.

[Shrinkage stress of polarizing film]

The shrinkage stress was measured using Autograph AG-X with thermostatic bath and video extensometer TR ViewX120S manufactured by Shimadzu Corporation. The measurement system used a polarizing film conditioned at 20° C. and 20% RH for 18 hours. After the thermostatic bath of Autograph AG-X was brought to 20°C, a chuck was attached to the polarizing film (15 cm in the longitudinal direction and 1.5 cm in the width direction) (the chuck interval was 5 cm). The stretching (speed 1 mm/min) was started at the same time as the temperature increase (10° C./min) of the thermostatic bath to 80°C. After about 3 seconds, the stretching was stopped when the tension reached 2N, and the state was maintained. When the temperature in the thermostatic bath reached 80° C., the tension was measured after 4 hours. At this time, the distance between the chucks changes due to thermal expansion, so the marking stickers are attached to the chucks, and the image-type extensometer TR ViewX120S is used to measure, so that the clips can be corrected according to the movement amount of the marking stickers attached to the chucks. distance between heads. In addition, the value obtained by subtracting the initial tension 2N from the measured value of the tension (N) after 4 hours was used as the shrinkage force (N) of the polarizing film, and the value (N) was divided by the cross-sectional area of the sample (mm ² ) to obtain The value of is defined as shrinkage stress (N/mm ² ).

[Shrinkage rate of polarizing film]

The shrinkage rate was measured using a thermomechanical measuring apparatus (Q400) manufactured by TA Instruments. The measurement system used a polarizing film conditioned at 20° C. and 20% RH for 18 hours. The polarizing film was cut into a measurement sample of 3 cm in the length direction and 0.3 cm in the width direction, and the sample was mounted on the device with a gap of about 2 cm between the clips. After heating the inside of the apparatus from 20°C at 10°C/min to 80°C, the polarizing film was heated at 80°C for 4 hours, and the shrinkage rate was calculated according to the following formula. In addition, a quantitative load of 0.098 (N) was applied between the installation of the sample and the end of the measurement.

Shrinkage (%)=100×(x-y)/x

x: Distance between chucks before heating (cm)

y: distance between chucks after heating (cm)

[Moisture rate]

The PVA thin film was dried at 105° C. for 16 hours, and the moisture content of the PVA thin film was obtained by the following formula from the mass of the PVA thin film before and after drying.

Moisture rate (%)=100×(α-β)/α

α: Mass of PVA film before drying (g)

β: Mass of PVA film after drying (g)

Example 1 [Production of polarizing film]

Using 100 parts by mass of PVA (degree of saponification 99.9 mol%, degree of polymerization 2500), 10 parts by mass of glycerin as a plasticizer, and 0.1 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, And an aqueous solution with a PVA content of 9% by mass was used as a membrane-forming stock solution. This was dried on a metal roll at 80° C., and the obtained film was heat-treated at a temperature of 112° C. for 10 minutes in a hot air dryer to prepare a PVA film having a thickness of 30 μm.

A sample with a width of 5 cm and a length of 11 cm was cut out from the central part of the obtained PVA film in the width direction, and the sample was fixed on a uniaxial stretching jig so that the width of 5 cm and the length of 5 cm could be directed toward the MD ( uniaxial stretching in the direction of the mechanical axis). As a swelling step, this sample was immersed in pure water at 30°C, and uniaxially stretched 1.1 times in the longitudinal direction during this period. Next, as a dyeing step, iodine was adsorbed by being immersed in an aqueous solution containing iodine and potassium iodide in a mass ratio of 1:20 (dyeing bath, temperature 30° C.) for 60 seconds, and uniaxially stretched in the longitudinal direction during this period. Extend up to 2.2 times (2.4 times overall). At this time, the concentration of iodine in the dyeing bath was adjusted so that the transmittance of the polarizing film after drying was 44%. Next, as a cross-linking step, it was immersed in an aqueous solution containing boric acid at a ratio of 2.6 mass % (cross-linking bath, temperature 32° C.), and uniaxially stretched 1.1 times in the longitudinal direction during this period (2.7 times as a whole). ). Then, as a stretching step, it was immersed in an aqueous solution (stretching bath, temperature 60° C.) containing boric acid and potassium iodide at a ratio of 3 mass % and 5 mass %, and uniaxially stretched in the longitudinal direction during this period. 2.2 times (6.0 times overall). Next, as a fixing treatment step, the stretched PVA film was immersed in a boric acid aqueous solution (boric acid concentration 1.5 mass %, potassium iodide concentration 4 mass %, temperature 22° C.) for 10 seconds. Next, as a washing step, it was immersed in an aqueous solution (washing bath, temperature 20° C.) containing potassium iodide at a ratio of 3.5 mass % for 5 seconds. Next, as a drying step, the obtained PVA film was dried in air at 80° C. for 4 minutes. The drying step was performed in a state open to the atmosphere using a hot air dryer. The optical properties, shrinkage ratio, shrinkage stress, thickness, moisture content and boron element content of the PVA film (Reference Example 1) thus obtained before the annealing treatment step were measured. The moisture content of the PVA thin film before the annealing treatment step was 8.1%, and the boron content was 3.46% by mass [the content of boric acid (B(OH) ₃ ) was 19.8% by mass]. Other results are shown in Table 1. In addition, the shrinkage ratio and the monomer b value of the PVA film are plotted in FIG. 1 , and the shrinkage stress and the shrinkage ratio are plotted in FIG. 2 .

The obtained PVA film was cut into 20 cm in the MD direction and 7 cm in the TD direction. Both ends of the PVA film in the MD direction were sandwiched between two sets of stainless steel outer frames so as not to loosen, and the outer frames were further sandwiched by clamps from both outer sides. In this way, both ends in the MD direction of the PVA film are fixed. The PVA film was annealed in air at 60° C. for 120 hours using a thermostat. The annealing treatment is performed in an open state to the atmosphere. The optical properties, shrinkage ratio and shrinkage stress of the polarizing film thus obtained were measured. The results are shown in Table 1. In addition, the shrinkage ratio and the monomer b value of the polarizing film are plotted in FIG. 1 , and the shrinkage stress and the shrinkage ratio are plotted in FIG. 2 . In addition, the content of boron element in the obtained polarizing film and the PVA film before the annealing treatment step is substantially the same.

Example 2, Comparative Examples 1~3, 5~12, 14~16

Production and evaluation of polarizing films were performed in the same manner as in Example 1, except that the temperature and time of the annealing treatment were changed as shown in Table 1. The results are shown in Table 1, FIG. 1 and FIG. 2 .

Example 3 [Production of multilayer films]

In the same manner as in Example 1, a PVA thin film before the annealing treatment step was obtained. By placing a triacetate cellulose (TAC) film coated with PVA paste (PVA content of 3 mass %) on one side on both sides of the aforementioned PVA film and laminating with a laminator, the process was carried out at 60° C. for 10 minutes. drying to obtain the multilayer film before the annealing treatment step.

After the obtained multilayer film before the annealing treatment step was immersed in dichloromethane which is a good solvent for TAC for 1 week, TAC was removed from the PVA film by drying at room temperature for 24 hours in a ventilation room. The optical properties, shrinkage rate, shrinkage stress and thickness of the TAC-removed PVA film (Reference Example 2) were measured. The results are shown in Table 1.

[Annealing treatment of multilayer films]

The annealing treatment step was carried out in the same manner as in Example 1 except that the obtained multilayer film before the annealing treatment step was used to obtain a polarizing film (polarizing plate). After removing TAC from the obtained polarizing film (polarizing plate) in the same manner as described above, the optical properties, shrinkage rate, shrinkage stress, and thickness of the polarizing film from which TAC was removed were measured. The results are shown in Table 1.

Comparative Examples 4 and 13

The polarizing plate was produced and evaluated in the same manner as in Example 3, except that the temperature and time of the annealing treatment were changed as shown in Table 1. These results are shown in Table 1.

Claims (8)

A polarizing film, which is a polarizing film comprising a polyvinyl alcohol film made of iodine-containing dichroic pigments and cross-linked with a boron compound, the degree of polarization is more than 99.5%, and the single-piece b value (single-piece b value) ) is 3.0 or less, and the following formulas (1) and (2) are satisfied: A≦0.9 (1) B/A≧55 (2) However, A is the shrinkage ratio of the polarizing film heated at 80°C for 4 hours (%), B is the shrinkage stress (N/mm ² ) of the polarizing film heated at 80° C. for 4 hours. The polarizing film according to claim 1, wherein the value of the monomer b is 1.0 or more. The polarizing film of claim 1 or 2, wherein A is 0.5 or more. The polarizing film of claim 1 or 2, wherein the degree of polymerization of the polyvinyl alcohol is 1,500 to 6,000. As claimed in claim 1 or 2, the polarizing film has a thickness of 1 to 30 μm. A polarizing plate, which is formed by laminating the polarizing film according to any one of claims 1 to 5 and a protective film. A method for producing a polarizing film as claimed in any one of claims 1 to 5, comprising the steps of dyeing a polyvinyl alcohol film with an iodine-based dichroic dye, cross-linking it using a boron compound, and stretching After this step, an annealing treatment step of heating the polyvinyl alcohol film with a fixed stretching direction at 55-65° C. for more than 72 hours is performed. A method for producing a polarizing plate as claimed in claim 6, wherein after performing the step of dyeing a polyvinyl alcohol film with an iodine-based dichroic dye, the step of cross-linking with a boron compound, and the step of stretching, the polyethylene The alcohol film and the protective film are laminated to obtain a multi-layer film, and then the annealing treatment step of heating the multi-layer film with a fixed stretching direction at 55-65° C. for more than 72 hours is performed.

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