WO2018164196A1

WO2018164196A1 - Polarizing film, polarizing plate, and method for manufacturing same

Info

Publication number: WO2018164196A1
Application number: PCT/JP2018/008826
Authority: WO
Inventors: 達也大園; 辻　嘉久
Original assignee: 株式会社クラレ
Priority date: 2017-03-08
Filing date: 2018-03-07
Publication date: 2018-09-13
Also published as: TWI757444B; JP7199343B2; JPWO2018164196A1; KR20190119127A; CN110383124A; CN110383124B; KR102582196B1; TW201834839A

Abstract

Provided is a polarizing film that is formed of a PVA film which contains an iodine-based dichroic pigment and which is crosslinked by a boron compound, wherein the polarizing film has a polarization degree of 99.5% or greater, a single b value of 3.0 or less, and satisfies formula (1) and formula (2). Such a polarizing film has excellent polarization performance and hue, and has a small contraction rate and excellent dimensional stability. Formulae: (1): A ≤ 0.9; (2): B/A ≥ 55 A is the contraction rate (%) of the polarizing film which has been heated at 80oC for 4 hours, and B is the contraction stress (N/mm2) of the polarizing film which has been heated at 80oC for 4 hours.

Description

Polarizing film, polarizing plate, and production method thereof

The present invention relates to a polarizing film composed of a polyvinyl alcohol film containing an iodine dichroic dye, a polarizing film having a low shrinkage rate, a polarizing plate, and a method for producing them.

A polarizing film used for a polarizing plate having a light transmission and shielding function is a basic component of a liquid crystal display (LCD). Many polarizing plates have a structure in which a protective film such as a cellulose triacetate (TAC) film is bonded to the surface of a polarizing film. As a polarizing film constituting the polarizing plate, a polyvinyl alcohol film (hereinafter referred to as “polyvinyl”). alcohol "and" PVA "and that there is referred) uniaxially stretched iodine to stretched film was oriented dye (I ₃ ^- and I ₅ ^-, etc.) dichroic dye to what has mainstream adsorption of such It has become. Such a polarizing film is manufactured by a method of subjecting a PVA film to a swelling process, a dyeing process, a crosslinking process, a stretching process, an immobilizing process, and a drying process.

In recent years, LCDs are widely used in mobile applications such as notebook computers and mobile phones. Such LCDs for mobile devices are used in various environments. Therefore, a polarizing film having a low shrinkage at high temperatures and excellent dimensional stability is demanded.

Patent Documents 1 to 5 describe polarizing films obtained by stretching a PVA film dyed with an iodine-based dye and then drying at 50 to 70 ° C. for 2 to 4 minutes. However, the polarizing film thus obtained had a high shrinkage rate. When such a polarizing film is laminated on a glass plate and used for an LCD, when the LCD is used or stored at a high temperature, the polarizing film contracts and the glass plate warps. It was.

JP 2006-65309 A JP 2014-197050 A JP 2006-267153 A JP 2013-140324 A JP 2012-3173 A

The present invention has been made in order to solve the above problems, and provides a polarizing film having excellent polarization performance and hue, low shrinkage ratio and excellent dimensional stability, and a method for producing the same. It is the purpose. Moreover, it aims at providing the polarizing plate using such a polarizing film, and its manufacturing method.

The above problem is a polarizing film comprising a PVA film containing an iodine-based dichroic dye and crosslinked with a boron compound, having a polarization degree of 99.5% or more and a simple substance b value of 3.0 or less. It is solved by providing a polarizing film that satisfies the following formulas (1) and (2).
A ≦ 0.9 (1)
B / A ≧ 55 (2)

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

At this time, it is preferable that the simple substance b value is 1.0 or more. It is also preferable that A is 0.5 or more. It is also preferable that the degree of polymerization of the PVA is 1,500 to 6,000. It is also preferable that the polarizing film has a thickness of 1 to 30 μm.

The above-mentioned problem is that the PVA film with the stretching direction fixed at 55 to 65 ° C. after the step of dyeing the PVA film with an iodine dichroic dye, the step of crosslinking with a boron compound and the step of stretching are performed. The problem can also be solved by providing a method for producing the polarizing film in which an annealing treatment step of heating for 72 hours or more is performed.

A polarizing plate formed by laminating the polarizing film and the protective film is a preferred embodiment of the present invention. After performing the process of dyeing a PVA film with an iodine dichroic dye, the process of crosslinking with a boron compound and the process of stretching, the PVA film and a protective film are laminated to obtain a multilayer film, and the stretching direction A method for producing the polarizing plate, in which an annealing treatment step is performed in which the multilayer film to which is fixed is heated at 55 to 65 ° C. for 72 hours or more is also a preferred embodiment of the present invention.

The polarizing film of the present invention has excellent polarizing performance and hue, and has a small shrinkage ratio and excellent dimensional stability. Therefore, the polarizing plate using the polarizing film is suitably used for high-performance LCDs, particularly 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. 3 is a graph plotting the shrinkage rate and the simple substance b value of polarizing films in Examples 1 and 2, Comparative Examples 1 to 3, 5 to 12, and 14 to 16, and Reference Example 1. FIG. 2 is a graph plotting the shrinkage stress and shrinkage rate of polarizing films in Examples 1 and 2, Comparative Examples 1 to 3, 5 to 12, 14 to 16, and Reference Example 1.

The polarizing film of the present invention is a polarizing film comprising a PVA film containing an iodine-based dichroic dye and crosslinked with a boron compound, having a polarization degree of 99.5% or more and a simple substance b value of 3. It is 0 or less and satisfies the following formulas (1) and (2).
A ≦ 0.9 (1)
B / A ≧ 55 (2)

Usually, when an attempt is made to improve the polarizing performance of a polarizing film made of a PVA film containing an iodine-based dichroic dye, the shrinkage rate and shrinkage stress of the polarizing film at high temperatures increase. When such a polarizing film is used for an LCD, the glass plate bonded to the polarizing film is warped, which is a problem. In particular, LCDs used for mobile applications are often used or stored at high temperatures, and the glass plates used are thin, so the shrinkage of the polarizing film has been a major problem.

In order to improve such a problem, when the polarizing film was heat-treated at a high temperature, the shrinkage rate and the shrinkage stress were lowered, but the polarization performance was lowered and the hue was also deteriorated. Thus, it was difficult to increase the dimensional stability of the polarizing film by reducing the shrinkage rate and the shrinkage stress while maintaining excellent polarization performance and hue. The polarizing film of the present invention solves such a point, and is characterized by having a low shrinkage rate while having excellent polarizing performance and hue.

The manufacturing method of such a polarizing film is not particularly limited, but the present inventor succeeded in manufacturing a polarizing film having such performance for the first time by adopting a new manufacturing method. Specifically, after performing a step of dyeing a PVA film with an iodine dichroic dye, a step of crosslinking with a boron compound, and a step of stretching, the PVA film in which the stretching direction is fixed is 55 to 65 ° C. By performing an annealing treatment step of heating for 72 hours or more, a polarizing film having excellent polarization performance and hue, and having a small shrinkage ratio and excellent dimensional stability can be produced. The said manufacture is used suitably for manufacture of various polarizing films including the polarizing film of this invention. Hereinafter, the manufacturing method will be described in detail.

The PVA contained in the raw PVA film used in the production of the polarizing film of the present invention should be obtained by saponifying a polyvinyl ester obtained by polymerizing one or more vinyl esters. Can do. Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among these, vinyl acetate is preferable from the viewpoint of ease of production, availability, and cost of PVA.

The polyvinyl ester may be obtained using only one or two or more kinds of vinyl esters as a monomer, but one or two as long as the effects of the present invention are not impaired. It may be a copolymer of at least one kind of vinyl ester and another monomer copolymerizable therewith.

Examples of other monomers copolymerizable with the vinyl ester include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene and isobutene; (meth) acrylic acid or a salt thereof; (meth) Methyl acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, (meth) (Meth) acrylic acid esters such as t-butyl acrylate, 2-ethylhexyl (meth) acrylate, dodecyl (meth) acrylate, octadecyl (meth) acrylate; (meth) acrylamide, N-methyl (meth) Acrylamide, N-ethyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, diacetone (meth) acrylamid (Meth) acrylamide propanesulfonic acid or a salt thereof, (meth) acrylamidopropyldimethylamine or a salt thereof, (meth) acrylamide derivatives such as N-methylol (meth) acrylamide or a derivative thereof; N-vinylformamide, N-vinylacetamide N-vinyl amides such as N-vinyl pyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether ; Vinyl cyanide such as (meth) acrylonitrile; vinyl halide such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride; Allyl compounds such as ril and allyl chloride; maleic acid or its salts, esters or acid anhydrides; itaconic acid or its salts, esters or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; unsaturated sulfonic acids Can do. Said polyvinyl ester can have a structural unit derived from 1 type, or 2 or more types of an above described other monomer.

The proportion of structural units derived from other monomers in the polyvinyl ester is preferably 15 mol% or less, preferably 10 mol% or less, based on the number of moles of all structural units constituting the polyvinyl ester. More preferred is 5 mol% or less.

In particular, when the other monomer is a monomer that may promote water solubility of the obtained PVA, such as (meth) acrylic acid or unsaturated sulfonic acid, the polarizing film In order to prevent PVA from being dissolved in the production process, the proportion of structural units derived from these monomers in the polyvinyl ester is 5 mol% or less based on the number of moles of all structural units constituting the polyvinyl ester. It is preferable that it is 3 mol% or less.

The PVA used in the present invention may be modified with one or two or more types of graft copolymerizable monomers as long as the effects of the present invention are not impaired. Examples of the graft copolymerizable monomer include unsaturated carboxylic acids or derivatives thereof; unsaturated sulfonic acids or derivatives thereof; α-olefins having 2 to 30 carbon atoms, and the like. The proportion of structural units derived from the graft copolymerizable monomer in PVA (structural units in the graft modified portion) is preferably 5 mol% or less based on the number of moles of all structural units constituting PVA. .

In the PVA, a part of the hydroxyl group may be cross-linked or may not be cross-linked. Moreover, said PVA may react with aldehyde compounds, such as acetaldehyde and a butyraldehyde, etc., and a part of the hydroxyl group may form the acetal structure.

The polymerization degree of the PVA is preferably in the range of 1500 to 6000, more preferably in the range of 1800 to 5000, and still more preferably in the range of 2000 to 4000. When the polymerization degree 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 6000 or less, it is possible to suppress an increase in manufacturing cost, poor process passability during film formation, and the like. The degree of polymerization of PVA in the present specification means an average degree of polymerization measured according to the description of JIS K6726-1994. In addition, although PVA in a polarizing film contains the crosslinked structure by boron compounds, such as a boric acid, if it dissociates by hydrolyzing boric acid ester etc., it will be a substantial change in the average degree of polymerization of PVA itself. There is no.

The degree of saponification of PVA is preferably 98 mol% or more, more preferably 98.5 mol% or more, and further preferably 99 mol% or more from the viewpoint of the polarizing performance of the polarizing film. When the degree of saponification is less than 98 mol%, PVA tends to be eluted during the production process of the polarizing film, and the eluted PVA may adhere to the film and reduce the polarizing performance of the polarizing film. In this specification, the degree of saponification of PVA refers to the total number of moles of structural units (typically vinyl ester units) that can be converted into vinyl alcohol units by saponification and the vinyl alcohol units of PVA. The proportion (mol%) occupied by the number of moles of vinyl alcohol units. The degree of saponification can be measured according to the description of JIS K6726-1994. In addition, PVA in a raw film and PVA in the obtained polarizing film have substantially the same saponification degree.

A PVA film is formed using the stock solution. Examples of the film forming method include a cast film forming method, an extrusion film forming method, a wet film forming method, and a gel film forming method. These film forming methods may be used alone or in combination of two or more. Among these film forming methods, the cast film forming method and the extrusion film forming method are preferable because a PVA film having uniform thickness and width and good physical properties can be obtained. The formed PVA film can be dried or heat-treated as necessary.

The film-forming stock solution can be obtained, for example, by mixing a liquid medium with one or more of the PVA and, if necessary, a surfactant, a plasticizer and an additive. In the film-forming stock solution, PVA may be in a dissolved state in a liquid medium or in a molten state. The mixing is preferably performed under heating.

Examples of the liquid medium used for the preparation of the membrane forming stock solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. , Trimethylolpropane, ethylenediamine, diethylenetriamine and the like, and one or more of them can be used. Among these, water is preferable from the viewpoint of environmental load and recoverability.

The volatile fraction of the film-forming stock solution (content ratio in the film-forming stock solution of volatile components such as liquid media removed by volatilization or evaporation during film formation) varies depending on the film-forming method, film-forming conditions, etc. Specifically, it is preferably in the range of 50 to 95% by mass, more preferably in the range of 55 to 90% by mass, and still more preferably in the range of 60 to 85% by mass. When the volatile fraction of the film-forming stock solution is 50% by mass or more, the viscosity of the film-forming stock solution does not become too high, and filtration and defoaming are smoothly performed during preparation of the film-forming stock solution, and there are few foreign matters and defects. Film production is facilitated. On the other hand, when the volatile fraction of the film-forming stock solution is 95% by mass or less, the concentration of the film-forming stock solution does not become too low, and the production of an industrial PVA film becomes easy.

The film forming stock solution preferably contains a surfactant. By including the surfactant, the film-forming property is improved and the occurrence of uneven thickness of the film is suppressed, and the film is easily peeled off from the metal roll or belt used for film formation. When a PVA film is produced from a film-forming stock solution containing a surfactant, the PVA film may contain a surfactant. Although the kind of said surfactant is not specifically limited, From a viewpoint of the peelability from a metal roll or a belt, an anionic surfactant or a nonionic surfactant is preferable.

As the anionic surfactant, for example, a carboxylic acid type such as potassium laurate; a sulfuric acid ester type such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and a sulfonic acid type such as dodecylbenzene sulfonate are suitable.

Nonionic surfactants include, for example, alkyl ether types such as polyoxyethylene oleyl ether; alkylphenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; polyoxyethylene laurylamino Alkylamine type such as ether; alkylamide type such as polyoxyethylene lauric acid amide; polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as lauric acid diethanolamide and oleic acid diethanolamide; polyoxy An allyl phenyl ether type such as alkylene allyl phenyl ether is preferred.

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

When the film-forming stock solution contains a surfactant, the content thereof is preferably 0.01 to 0.5 parts by weight, preferably 0.02 to 0.00 parts, per 100 parts by weight of PVA contained in the film-forming stock solution. The amount is more preferably 3 parts by mass, and particularly preferably 0.05 to 0.1 parts by mass. When the content is 0.01 parts by mass or more, the film forming property and the peelability are 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 bleeding out on the surface of the PVA film to cause blocking and deterioration in handleability.

The content of PVA in the raw PVA film used in the present invention is preferably 50 to 99% by mass from the viewpoint of ease of production of the polarizing film. The content is more preferably 75% by mass or more, further 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, further preferably 96% by mass or less, and particularly preferably 95% by mass or less.

From the viewpoint of improving stretchability, the PVA film preferably contains a plasticizer. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylol propane. One or more of the agents can be included. 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 to 20 parts by mass with respect to 100 parts by mass of PVA. When the content is 1 part by mass or more, the stretchability of the PVA film can be further improved. On the other hand, when the content is 20 parts by mass or less, it is possible to prevent the PVA film from becoming too flexible and handling properties from being lowered. The content is more preferably 2 parts by mass or more, further preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more. Further, the content is more preferably 15 parts by mass or less. Although depending on the manufacturing conditions of the polarizing film, the plasticizer contained in the PVA film may be eluted when the polarizing film is manufactured, so that the total amount does not always remain in the polarizing film.

The PVA film may further contain components such as an antioxidant, an antifreezing agent, a pH adjuster, a hiding agent, an anti-coloring agent, an oil agent, and a surfactant as necessary.

The thickness of the PVA film is preferably 5 to 100 μm. When the thickness is 100 μm or less, a thin polarizing film can be easily obtained. The thickness is more preferably 60 μm or less. On the other hand, when the thickness is less than 5 μm, it may be difficult to produce a polarizing film, and uneven staining may easily occur. The thickness of the PVA film is more preferably 7 μm or more. The thickness here refers to the thickness of the PVA layer in the case of a multilayer film.

The PVA film may be a single layer film or a multilayer film having a PVA layer and a base resin layer. In the case of a single layer film, the film thickness is preferably 20 μm or more, and 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 can 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 using a multilayer film having a PVA layer and a base resin layer as the PVA film, the base resin must be capable of being stretched together with PVA. Polyester, polyolefin resin, or the like can be used. Among these, an amorphous polyester resin is preferable, and an amorphous polyester resin obtained by copolymerizing polyethylene terephthalate and a copolymer component such as isophthalic acid or 1,4-cyclohexanedimethanol is preferably used. It is preferable to produce a multilayer film 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 both layers.

The shape of the PVA film is not particularly limited, but is preferably a long PVA film because it can be continuously supplied when a polarizing film is produced. The length of the long PVA film (length in the long direction) is not particularly limited, and can be set as appropriate according to the application of the polarizing film to be produced, for example, within a range of 5 to 20000 m. be able to.

The width of the PVA film is not particularly limited, and can be appropriately set according to the use of the polarizing film to be produced. In recent years, since the enlargement of screens of liquid crystal televisions and liquid crystal monitors has progressed, it is suitable for these applications when the width of the PVA film is 0.5 m or more, more preferably 1.0 m or more. On the other hand, if the width of the PVA film is too wide, it tends to be difficult to uniformly stretch the polarizing film when the polarizing film is produced by a device that has been put to practical use. Therefore, the width of the PVA film is 7 m or less. Is preferred.

The polarizing film of the present invention is produced using the PVA film described above as a raw fabric. Specifically, a step of dyeing the PVA film with an iodine dichroic dye (hereinafter sometimes referred to as “dyeing step”), a step of crosslinking the PVA film with a boron compound (hereinafter referred to as “crosslinking”). And the step of stretching the PVA film (hereinafter also referred to as the “stretching step”), the PVA film with the stretched direction fixed thereto is heated at 55 to 65 ° C. for 72 hours. It is preferable to manufacture a polarizing film by the method of performing the annealing process which heats more than time.

In the above production method, it is preferable to perform a step of swelling the original PVA film (hereinafter, sometimes referred to as “swelling step”) before performing the above steps. After performing the dyeing process, the crosslinking process, and the stretching process, it is also preferable to perform the annealing process after further performing a process of drying the PVA film (hereinafter sometimes referred to as “drying process”). Prior to the drying step, a step of appropriately washing the PVA film may be performed. Moreover, in the said manufacturing method, you may perform one type of process in multiple times. Moreover, you may perform a several process simultaneously in one bath.

As the manufacturing method, a method of performing a dyeing step, a crosslinking step, a stretching step, and an annealing treatment step in this order is more preferable, and a swelling step, a dyeing step, a crosslinking step, a stretching step, a fixing treatment step, a drying step, and an annealing treatment step are performed. The method performed in this order is more preferable. Hereinafter, each step will be described in detail.

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

In the manufacturing method, a step of dyeing the PVA film with an iodine dichroic dye is performed. It is preferable to perform the dyeing process after the swelling process. Moreover, although the dyeing | staining process may be performed before the extending process mentioned later and may be performed after an extending process, the former is preferable. The dyeing process is generally performed by immersing the PVA film in a solution (particularly an aqueous solution) containing iodine-potassium iodide as a dyeing bath. In the present invention, such a dyeing method is suitably employed. The The iodine concentration in the dyeing bath is preferably 0.01 to 0.5% by mass, and the potassium iodide concentration is preferably 0.01 to 10% by mass. The temperature of the dyeing bath is preferably 10 to 50 ° C., more 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, but its content is usually less than 5% by mass, preferably 1% by mass or less, in terms of boric acid. As said boron compound, what is mentioned later as what is used for a bridge | crosslinking process is used.

In the manufacturing method, a step of crosslinking the PVA film using a boron compound is performed. After performing the crosslinking step in this manner, a polarizing film having a low shrinkage rate can be obtained while having an excellent polarization performance and hue by performing an annealing treatment step described later. The crosslinking process is performed before the annealing process. Moreover, it is preferable to perform a bridge | crosslinking process after a dyeing process. From the viewpoint of preventing the elution of PVA when performing wet stretching at a high temperature, it is preferable to perform a crosslinking step before the stretching step. By using an aqueous solution containing a boron compound as a dyeing bath or a stretching bath, the crosslinking step and the dyeing step or the stretching step may be performed simultaneously. Moreover, you may perform the process of bridge | crosslinking a PVA film using a boron compound as a fixing process process mentioned later.

The crosslinking step can be performed by immersing the PVA film in an aqueous solution containing a boron compound. As said boron compound, 1 type (s) or 2 or more types, such as boric acids, such as boric acid and borax, can be used. The concentration of the boron compound in the aqueous solution is preferably 1 to 15% by mass, and more preferably 2 to 7% by mass in terms of boric acid. When the concentration is 1 to 15% by mass, the above-described effects can be achieved and sufficient stretchability can be maintained. The aqueous solution may contain an auxiliary agent such as potassium iodide. From the viewpoint of efficient crosslinking, the temperature of the aqueous solution is preferably 20 to 50 ° C., particularly preferably 25 to 40 ° C.

Separately from the stretching process described later, the PVA film may be stretched during or between the above-described processes. By performing such stretching (pre-stretching), wrinkles can be prevented from entering the PVA film. Based on the original length of the original PVA film before stretching, the total stretching ratio of pre-stretching (the ratio obtained by multiplying the stretching ratios in each step) is 4 based on the polarization performance in the case of producing a polarizing film. It is preferable that it is less than 2 times. The draw ratio in the swelling process is preferably 1.05 to 3 times, the draw ratio in the dyeing process is preferably 3 times or less, and the draw ratio in the crosslinking process is preferably 2 times or less.

In the method for producing a polarizing film, a step of stretching the 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 drawing method, it can be carried out in an aqueous solution containing a boron compound, or can be carried out in the above-described dyeing bath or crosslinking bath. As a boron compound, what was mentioned above as what is used for a bridge | crosslinking process can be used. In the case of the dry stretching method, the stretching may be performed at room temperature, may be performed while heating, or may be performed in the air using a PVA film after water absorption. Among these, the wet stretching method is preferable, and it is more preferable to perform uniaxial stretching 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% by mass in terms of boric acid, more preferably 1.0 to 5.0% by mass, and 1.5 to 4.5% by mass. Particularly preferred. The aqueous solution may contain potassium iodide, and the concentration is preferably 0.01 to 10% by 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 even more preferably 50 to 70 ° C.

From the viewpoint of polarizing performance when a polarizing film is produced, the draw ratio in the drawing step is preferably 1.2 times or more, more preferably 1.5 times or more, and 2 times or more. Is more preferable. Moreover, the total draw ratio including the draw ratio of the above-mentioned pre-stretch (the ratio obtained by multiplying the stretch ratio in each step) is 5.2 times or more based on the original length of the original PVA film before stretching. Preferably, it is 5.5 times or more, more preferably 5.8 times or more. The upper limit of the total stretching ratio is not particularly limited, but the stretching ratio is preferably 8 times or less in order to prevent stretching.

There is no particular limitation on the direction of uniaxial stretching when a long PVA film is uniaxially stretched, and uniaxial stretching in the longitudinal direction or transverse uniaxial stretching in the width direction can be employed. In the case of producing a polarizing film, uniaxial stretching in the longitudinal direction is preferable from the viewpoint of obtaining a film having excellent polarizing performance. Uniaxial stretching in the longitudinal direction can be performed by changing the peripheral speed between the rolls using a stretching apparatus including a plurality of rolls parallel to each other. On the other hand, lateral uniaxial stretching can be performed using a tenter type stretching machine.

In the method for producing a polarizing film, it is preferable to perform a fixing treatment step on the PVA film after the stretching step. Thereby, adsorption | suction of the iodine type dichroic dye with respect to a PVA film becomes firm. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing one or more of boron compounds can be used. As the boron compound, those described above as being used for the crosslinking treatment can be used. it can. Moreover, you may add an iodine compound and a metal compound in a fixed treatment bath as needed. 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 is preferably 0.01 to 10% by mass. The temperature of the fixing treatment bath is preferably 15 to 60 ° C, and more preferably 20 to 40 ° C.

In the production method of the present invention, it is also preferable that after the dyeing step, the crosslinking step and the stretching step, the annealing treatment step is performed after the step of drying the PVA film. When performing the fixing treatment step, it is preferable to perform a drying step after the 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 further preferably 50 ° C or higher. On the other hand, from the viewpoint of further improving the polarization performance and hue of the polarizing film, the drying temperature is preferably 100 ° C. or lower, and 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 further preferably 1 minute or more. On the other hand, from the viewpoint of further improving the polarizing performance and hue of the polarizing film, the drying time is preferably 30 minutes or less, more preferably 15 minutes or less, further preferably 10 minutes or less, and particularly preferably 5 minutes or less. The drying step is preferably performed in a gas such as air or an inert gas, and the former is more preferable from the viewpoint of simple treatment. Although the humidity at the time of performing a drying process in the air is not specifically limited, It is preferable that relative humidity is 35% or less, and it is more preferable that it is 15% or less.

After fixing the stretching direction of the PVA film thus obtained, an annealing treatment step of heating at 55 to 65 ° C. for 72 hours or more is performed. Here, it is preferable that the degree of polarization of the PVA film subjected to the annealing process is 99.7% or more. By performing the annealing treatment step, the shrinkage rate can be lowered while maintaining the excellent polarization performance and hue of the PVA film. Therefore, by performing the annealing process on the PVA film having such a high degree of polarization, it is possible to obtain a polarizing film having a low shrinkage ratio while having excellent polarization performance and hue. From the same viewpoint, it is also preferable that the single b value of the PVA film subjected to the annealing treatment step is 2.8 or less. When the drying process is performed at 55 to 65 ° C., the drying process and the annealing process may be performed for a total of 72 hours or more. However, as will be described later, when the PVA film roll is annealed, the PVA film roll needs to be annealed for 72 hours or more.

In addition, from the viewpoint of further enhancing the above-described effect due to the annealing treatment step, it is preferable that the moisture content of the PVA film used in the step is 1 to 25 mass%. The moisture content is more preferably 20% by mass or less, and further preferably 15% by mass or less.

In the annealing process, it is necessary to anneal the PVA film with the stretching direction fixed. The method at this time is not particularly limited, and after winding the PVA film in the stretching direction to obtain a film roll, the method of heating the film roll or the end of the PVA film in the stretching direction was fixed with a clip or the like. A method of annealing, etc. is adopted later, and the former is preferable from the viewpoint of productivity.

The annealing process is performed at 55 to 65 ° C. By treating the PVA film at such a low temperature, the shrinkage rate can be reduced while maintaining the excellent polarization performance and hue of the PVA film. When the temperature is less than 55 ° C., the effect of reducing the shrinkage rate of the PVA film cannot be obtained. The 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 fall. The temperature is preferably 63 ° C. or lower.

The annealing process is preferably performed in a gas such as air or an inert gas, and the former is more preferable from the viewpoint of simple processing. The humidity when the annealing process is performed in air is not particularly limited, but the relative humidity is preferably 35% or less, and more preferably 15% or less.

In the annealing process, the PVA film is heated for 72 hours or more. By thus heating the PVA film at a low temperature for a long time, the shrinkage rate can be lowered while maintaining the excellent polarization performance and hue of the PVA film. When the heating time is less than 72 hours, the effect of reducing the shrinkage rate of the PVA film is insufficient. The heating time is preferably 85 hours or longer, and more preferably 100 hours or longer. On the other hand, the heating time is preferably 300 hours or less, and more preferably 200 hours or less.

In order to solve the problem of the warpage of the glass plate due to the shrinkage of the polarizing film in the LCD, the inventors examined the annealing treatment of the polarizing film. Although the shrinkage ratio is decreased by increasing the treatment temperature, the polarization is reduced. Performance and hue deteriorated, making it difficult to achieve both. As a result of further investigations, the present inventors have surprisingly been able to reduce the shrinkage rate while maintaining excellent polarization performance and hue by annealing at a low temperature for a long time as described above. I found out that I can do it. Although this mechanism is not clear, it is believed that annealing at a low temperature for a long time increases the crosslinking point of PVA with a boron compound and decreases the shrinkage. In addition, it is considered that the polarization performance is maintained by annealing at a low temperature because the change in the amorphous portion of PVA and the decomposition of the dye are suppressed. Furthermore, since the thermal decomposition of the PVA film is suppressed by annealing at a low temperature, the hue is considered to be maintained.

The simple substance b value of the polarizing film of the present invention thus obtained needs to be 3.0 or less. Thus, the polarizing film of the present invention having a low single b value and excellent hue is suitably used in LCDs and the like. The simple substance b value is preferably 2.7 or less, and more preferably 2.45 or less.

The polarizing film needs to satisfy the following formulas (1) and (2).
A ≦ 0.9 (1)
B / A ≧ 55 (2)

When the shrinkage ratio A of the polarizing film is 0.9% or less, the glass plate is hardly warped when the polarizing film is used for an LCD. Thus, the polarizing film of the present invention having a low shrinkage ratio A is suitable for LCDs, in particular, LCDs for mobile devices that are often used or stored at high temperatures and that use thin glass plates. Used. The shrinkage rate A is preferably 0.85% or less, and 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 balance between polarization performance and hue and dimensional stability. As a method for measuring the shrinkage rate A of the polarizing film, the method described in Examples described later is employed.

When the ratio (B / A) of the shrinkage stress B (N / mm ² ) to the shrinkage rate A (%) of the polarizing film is 55 or more, the balance between the dimensional stability and the polarizing performance of the polarizing film is It becomes good. Usually, when the degree of polarization of the polarizing film is increased, the shrinkage stress and the shrinkage rate increase, and problems such as warpage of the glass plate occur when used in an LCD. On the other hand, since the polarizing film of the present invention has a high shrinkage stress B and a low shrinkage ratio A, it is possible to achieve both dimensional stability and polarizing performance, and such problems are solved. The ratio (B / A) is preferably 60 or more. On the other hand, the ratio (B / A) is preferably 100 or less.

As a method for measuring the shrinkage rate A and the shrinkage stress B of the polarizing film, the methods described in Examples described later are employed. In the present invention, the shrinkage rate and the shrinkage stress are measured in the stretching direction of the polarizing film, and when stretching in a plurality of directions, the stretching ratio is measured in a higher direction.

The 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 of the polarizing film is 45 N / mm ² or more, the polarizing performance is further improved. As described above, since the polarizing film has a low shrinkage ratio A, even when the polarizing film is used in an LCD, even when the film is contracted, the dimensional change of the film is caused by the adhesive layer between the film and the glass plate. Is absorbed. Therefore, even if the shrinkage stress B is high, the glass is hardly warped. In particular, LCDs for mobile devices such as mobile terminals are small in size and less dimensional change due to shrinkage of the polarizing film, so that glass warpage is less likely to occur. 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 polarizing film is preferably 1 to 30 μm. Such a thin polarizing film is suitably used for LCDs such as LCDs, especially for mobile devices. When the thickness is less than 1 μm, it may be difficult to produce a polarizing film, and uneven dyeing may easily occur. The thickness is preferably 5 μm or more. On the other hand, the thickness is more preferably 20 μm or less.

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

The content of the boron compound in the polarizing film is preferably 5 to 50% by mass in terms of boric acid. When the content is less than 5% by mass, the effect of reducing the shrinkage rate may be insufficient. The 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 contractile force of a polarizing film may become high too much. The content is more preferably 30% by mass or less.

The degree of polarization of the polarizing film needs to be 99.5% or more. Such a polarizing film having a high degree of polarization is suitably used for a high-performance LCD. The degree of polarization is preferably 99.7% or more, and more preferably 99.8% or more.

The simple substance b value of the polarizing film needs to be 3.0 or less. Such a polarizing film having a low single b value and an excellent hue is suitably used for a high-performance LCD. The simple substance b value is preferably 2.8 or less, more preferably 2.5 or less. On the other hand, the simple substance b value is usually 0 or more. From the viewpoint of particularly excellent balance between polarization performance and hue and dimensional stability, the simple substance b value is preferably 1.0 or more.

A 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 polarizing film has excellent polarization performance and hue, and has excellent dimensional stability due to the low shrinkage rate of the polarizing film. Therefore, the polarizing plate is suitably used for LCDs such as LCDs, especially for mobile devices. .

The protective film is not particularly limited as long as it is optically transparent and has mechanical strength. For example, cellulose triacetate (TAC) film, cellulose acetate / butyrate (CAB) film, acrylic film, polyester film A film, a cyclic olefin (COP) film, etc. can be used. The polarizing plate may be one in which the protective film is attached to one side of the polarizing film, or may be one in which the protective film is attached to both sides of the polarizing film. In addition, examples of the adhesive for bonding include a PVA adhesive, a urethane adhesive, and an ultraviolet curable adhesive.

Although the manufacturing method of the said polarizing plate is not specifically limited, After performing the process of dyeing | staining a PVA film with an iodine type dichroic dye, the process of bridge | crosslinking using a boron compound, and the process of extending | stretching, this PVA film and a protective film, A multilayer film is obtained by laminating the films, and an annealing treatment step is performed in which the multilayer film in which the stretching direction is fixed is heated at 55 to 65 ° C. for 72 hours or more. According to such a production method, a polarizing plate that is bright, has good polarization characteristics, and has excellent dimensional stability even when used under high-temperature conditions can be obtained. It is used suitably for manufacture of this polarizing plate. The polarizing plate is produced in the same manner as the polarizing film production method described above except that the step of obtaining a multilayer film by laminating the PVA film and the protective film (hereinafter sometimes referred to as “lamination step”) is further performed. Obtainable. In the manufacturing method of the said polarizing plate, when performing a drying process, a lamination process may be performed before a drying process and a lamination process may be performed after a drying process. When performing a fixing process, it is preferable to perform a lamination process after performing a fixing process.

The polarizing plate thus obtained is bright, has good polarization characteristics, and is excellent in dimensional stability even when used under high temperature conditions. Therefore, it is suitably used for high performance LCDs, particularly LCDs for mobile devices.

Hereinafter, the present invention will be described more specifically using examples.

[Calculation of boron element content in polarizing film]
The polarizing film (mass Jg) was added to distilled water and then 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 multi-type ICP emission spectrometer (ICP) manufactured by Shimadzu Corporation, and the content of boron element in the polarizing film was determined by the following formula.
Total boron element content in polarizing film (% by mass)
= [(L × 10 ⁻⁶ × K) / J] × 100

[Optical characteristics of polarizing film]
A rectangular sample of 3 cm in the longitudinal direction of the polarizing film and 1.5 cm in the width direction was collected from the center in the width direction of the obtained polarizing film, and a spectrophotometer with an integrating sphere (“V7100” manufactured by JASCO Corporation). ), The single transmittance (T), the degree of polarization (V), and the single b value were measured after correcting the visibility in accordance with JIS Z8722 (measurement method of object color).

[Shrinkage stress of polarizing film]
The shrinkage stress was measured using an autograph AG-X with a thermostatic bath manufactured by Shimadzu Corporation and a video extensometer TR ViewX120S. For the measurement, a polarizing film conditioned at 20 ° C. and 20% RH for 18 hours was used. After the temperature chamber of Autograph AG-X was set to 20 ° C., a polarizing film (length direction: 15 cm, width direction: 1.5 cm) was attached to a chuck (chuck interval: 5 cm). Tensile (speed 1 mm / min) and temperature increase (10 ° C./min) of the thermostat to 80 ° C. were started simultaneously. When the tension reached 2N after about 3 seconds, the tension was stopped and kept in that state. The tension until 4 hours after the temperature in the thermostatic chamber reached 80 ° C. was measured. At this time, because the distance between chucks changes due to thermal expansion, a gauge sticker is attached to the chuck, and the distance between chucks is corrected by the amount of movement of the gauge seal attached to the chuck using the video extensometer TR ViewX120S. Measurements were taken as possible. The value obtained by subtracting the initial tension 2N from the measured value of the tension (N) after 4 hours is the contraction force (N) of the polarizing film, and the value (N) is divided by the sample cross-sectional area (mm ² ). It was defined as shrinkage stress (N / mm ² ).

[Shrinkage rate of polarizing film]
The shrinkage rate was measured using a thermomechanical measuring device (Q400) manufactured by TA Instruments. For the measurement, a polarizing film conditioned at 20 ° C. and 20% RH for 18 hours was used. A measurement sample obtained by cutting the polarizing film in the length direction of 3 cm and the width direction of 0.3 cm was attached to the apparatus so that the distance between the chucks was about 2 cm. The temperature inside the apparatus was increased from 20 ° C. to 80 ° C. at 10 ° C./min, and then the polarizing film was heated by holding at 80 ° C. for 4 hours, and the shrinkage was calculated by the following formula. Note that a constant load of 0.098 (N) was applied from the time when the sample was attached until the end of measurement.
Shrinkage rate (%) = 100 × (xy) / x
x: Distance between chucks before heating (cm)
y: Distance between chucks after heating (cm)

[Moisture percentage]
The PVA film was dried at 105 ° C. for 16 hours, and the moisture content of the PVA film was determined from the mass of the PVA film before and after drying by the following formula.
Moisture content (%) = 100 × (α−β) / α
α: Mass of PVA film before drying (g)
β: mass of the PVA film after drying (g)

Example 1
[Preparation of polarizing film]
100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 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, the content of PVA A 9% by mass aqueous solution was used as a film-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 produce a PVA film having a thickness of 30 μm.

A sample having a width of 5 cm and a length of 11 cm is cut from the central portion of the obtained PVA film in the width direction, and the range of 5 cm and the length of 5 cm is uniaxially stretched in the MD (machine axis) direction during film formation. The sample was fixed to a drawing jig. As a swelling process, this sample was immersed in pure water at 30 ° C., and uniaxially stretched in the length direction 1.1 times during that time. Subsequently, as a dyeing step, iodine is adsorbed by immersing in an aqueous solution (dye bath, temperature 30 ° C.) containing iodine and potassium iodide at a mass ratio of 1:20 for 60 seconds, and 2.2 times ( A total of 2.4 times) was uniaxially stretched in the length direction. At this time, the iodine concentration in the dyeing bath was adjusted so that the transmittance of the polarizing film after drying was 44%. Subsequently, as a crosslinking step, the substrate is immersed in an aqueous solution (crosslinking bath, temperature 32 ° C.) containing 2.6% by mass of boric acid, and the length is 1.1 times (2.7 times as a whole) during that time. Uniaxially stretched in the direction. Subsequently, as a stretching process, the film was immersed in an aqueous solution (stretching bath, temperature 60 ° C.) containing 3% by mass of boric acid and 5% by mass of potassium iodide, and 2.2 times (totally 6. (0 times) was uniaxially stretched in the length direction. Subsequently, 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. Subsequently, as a cleaning step, the substrate was immersed for 5 seconds in an aqueous solution (cleaning bath, temperature 20 ° C.) containing 3.5% by mass of potassium iodide. Subsequently, as a drying process, the obtained PVA film was dried in air at 80 ° C. for 4 minutes. The drying process was performed in a state of being released to the atmosphere using a hot air dryer. The optical properties, shrinkage rate, shrinkage stress, thickness, moisture content, and boron element content of the PVA film (Reference Example 1) before the annealing treatment thus obtained were measured. The moisture content of the PVA film before the annealing treatment step was 8.1%, and the boron content was 3.46% by mass [boric acid (B (OH) ₃ ) content 19.8% by mass]. Other results are shown in Table 1. Moreover, the shrinkage | contraction rate and simple substance b value of the PVA film were plotted in FIG. 1, and the shrinkage stress and the shrinkage | contraction rate were plotted in FIG.

The obtained PVA film was cut so as to be 20 cm in the MD direction and 7 cm in the TD direction. Two stainless steel frames were sandwiched between the ends of the PVA film in the MD direction so as not to be loosened, and the frames were further clipped from both outsides. In this way, both ends of the PVA film in the MD direction were fixed. The PVA film was annealed in air at 60 ° C. for 120 hours using a thermostatic bath. The annealing treatment was performed in a state released to the atmosphere. The optical properties, shrinkage rate and shrinkage stress of the polarizing film thus obtained were measured. The results are shown in Table 1. Further, the shrinkage rate and the simple substance b value of the polarizing film are plotted in FIG. 1, and the shrinkage stress and the shrinkage rate are plotted in FIG. The obtained polarizing film and the PVA film before the annealing treatment step have substantially the same boron element content.

Example 2, Comparative Examples 1 to 3, 5 to 12, 14 to 16
A polarizing film was prepared and evaluated in the same manner as in Example 1 except that the annealing temperature and time were changed as shown in Table 1. The results are shown in Table 1, FIG. 1 and FIG.

Example 3
[Production of multilayer film]
In the same manner as in Example 1, a PVA film before the annealing process was obtained. Cellulose triacetate (TAC) film coated with PVA glue (PVA content is 3% by mass) on one side is placed on both sides of the PVA film and laminated with a laminator, followed by drying at 60 ° C. for 10 minutes Thus, a multilayer film before the annealing treatment step was obtained.

The obtained multilayer film before the annealing treatment step was immersed in methylene chloride, which is a good solvent for TAC, for 1 week, and then dried in a draft at room temperature for 24 hours to remove TAC from the PVA film. The optical properties, shrinkage rate, shrinkage stress and thickness of the PVA film from which TAC was removed (Reference Example 2) were measured. The results are shown in Table 1.

[Annealing of multilayer film]
A polarizing film (polarizing plate) was obtained by carrying out the annealing process in the same manner as in Example 1 except that the obtained multilayer film before the annealing process was used. 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
A polarizing plate was prepared and evaluated in the same manner as in Example 3 except that the annealing temperature and time were changed as shown in Table 1. These results are shown in Table 1.

Claims

A polarizing film comprising a polyvinyl alcohol film containing an iodine-based dichroic dye and crosslinked with a boron compound,
A polarizing film having a polarization degree of 99.5% or more, a simple substance b value of 3.0 or less, and satisfying the following formulas (1) and (2).
A ≦ 0.9 (1)
B / A ≧ 55 (2)
However, A is a shrinkage rate (%) of the polarizing film heated at 80 ° C. for 4 hours, and B is a shrinking stress (N / mm 2 ) of the polarizing film heated at 80 ° C. for 4 hours. It is.
The polarizing film according to claim 1, wherein the simple substance b value is 1.0 or more.
The polarizing film according to claim 1 or 2, wherein A is 0.5 or more.
The polarizing film according to any one of claims 1 to 3, wherein the degree of polymerization of the polyvinyl alcohol is 1,500 to 6,000.
The polarizing film according to claim 1, wherein the polarizing film has a thickness of 1 to 30 μm.
A polarizing plate in which the polarizing film according to any one of claims 1 to 5 and a protective film are laminated.
After performing a step of dyeing a polyvinyl alcohol film with an iodine-based dichroic dye, a step of crosslinking with a boron compound, and a step of stretching, the polyvinyl alcohol film in which the stretching direction is fixed is maintained at 55 to 65 ° C. for 72 hours. 6. The method for producing a polarizing film according to claim 1, wherein the annealing treatment step for heating is performed.
After performing a step of dyeing a polyvinyl alcohol film with an iodine dichroic dye, a step of crosslinking using a boron compound and a step of stretching, a multilayer film is obtained by laminating the polyvinyl alcohol film and a protective film, The method for producing a polarizing plate according to claim 6, wherein an annealing treatment step of heating the multi-layer film in which the stretching direction is fixed at 55 to 65 ° C for 72 hours or more is performed.