WO2021132435A1

WO2021132435A1 - Polyvinyl alcohol film and polarizing film

Info

Publication number: WO2021132435A1
Application number: PCT/JP2020/048365
Authority: WO
Inventors: 亘大橋; 洋平鷹取; 晋三小林; 正博馬場; 結稀藤井
Original assignee: 株式会社クラレ
Priority date: 2019-12-26
Filing date: 2020-12-24
Publication date: 2021-07-01
Also published as: JPWO2021132435A1

Abstract

This polarizing film contains an acetal-modified polyvinyl alcohol containing an acetal structure represented by formula (1) and having an acetalization degree of 1-6 mol%. Such a polarizing film has high polarization performance. Therefore, an LCD panel having excellent image quality can be obtained by using said polarizing film. [In formula (1), R is a hydrogen atom, or a monovalent aliphatic group having 1-6 carbon atoms.]

Description

Polyvinyl alcohol film and polarizing film

The present invention relates to a polarizing film containing acetal-modified polyvinyl alcohol containing a specific acetal structure, and an acetal-modified polyvinyl alcohol film as a raw film thereof.

A polarizing plate having a light transmitting and shielding function is a basic component of a liquid crystal display (LCD) together with a liquid crystal having a light switching mechanism. The polarizing plate is generally a polyvinyl alcohol (hereinafter, may be abbreviated as "PVA") film, which is swelled, then uniaxially stretched and dyed on both sides of a polarizing film, such as a cellulose triacetate (TAC) film. Manufactured by laminating the protective film of.

In recent years, in order to improve the image quality of LCDs such as LCD TVs, LCD projectors, word processor displays, personal computer displays, OA equipment terminal displays, and instrument panel displays for aircraft and automobiles, improvement of the polarizing performance of polarizing films has been required. There is.

For example, Patent Document 1 describes that a polarizing film using PVA having a degree of polymerization of 2,500 or more, preferably 6,000 to 10,000, has excellent optical characteristics. Although the polarization performance is improved by using PVA having a high degree of polymerization, it has been difficult to carry out industrially.

Japanese Unexamined Patent Publication No. 1-105204

An object of the present invention is to provide a polarizing film having excellent polarizing performance and capable of industrial production. Another object of the present invention is to provide a polyvinyl alcohol film capable of obtaining such a polarizing film.

The above problem is solved by providing a polarizing film containing an acetal structure represented by the following formula (1) and containing an acetal-modified PVA having an acetalization degree of 1 to 6 mol%.

[In the formula (1), R is a hydrogen atom or a monovalent aliphatic group having 1 to 6 carbon atoms. ]

At this time, it is preferable that the single transmittance of the polarizing film is 43.95% or more and the degree of polarization is 99.9% or more.

The above object to provide a PVA film containing an acetal structure represented by the following formula (1), containing an acetal-modified PVA having an acetalization degree of 1 to 6 mol%, and a swelling degree of 160 to 240%. It is also solved by that.

At this time, it is preferable that the acetal-modified PVA has a degree of polymerization of 1,000 to 4,000 and a degree of saponification of 99 to 99.99 mol%. It is also preferable that the thickness of the PVA film is 10 to 60 μm. Further, it is also preferable that the PVA film has a retardation of 10 to 40 nm.

Further, the absorbance at a wavelength of 295 nm, which is measured after the following treatments (A), (B) and (C) are performed on the PVA film in this order, is 0.3 or more per 10 μm thickness, and the wavelength is 330 nm. The absorbance in the above is preferably 0.2 or more per 10 μm thickness.
(A) The film is uniaxially stretched twice in the length direction while being immersed in pure water at 30 ° C. for 60 seconds. (B) The film contains 0.04% by mass of iodine and 0.92% by mass of potassium iodide. (C) The film is air-dried at 25 ° C. by adsorbing iodine on the film by uniaxially stretching it 1.2 times in the length direction while immersing it in an aqueous solution at 32 ° C. for 120 seconds.

Further, it is preferable that the PVA film is for optics.

The polarizing film containing the acetal-modified PVA of the present invention has high polarization performance. Therefore, by using the polarizing film of the present invention, an LCD panel having excellent image quality can be obtained. Further, such a polarizing film can be obtained by using the acetal-modified PVA film of the present invention. Moreover, the PVA film can be manufactured industrially.

The polarizing film of the present invention contains an acetal structure represented by the following formula (1) and contains an acetal-modified PVA having an acetalization degree of 1 to 6 mol%.

The acetal-modified PVA can be produced by a method of acetalizing PVA with an aldehyde or the like. The PVA to be subjected to acetalization can be produced by saponifying the polyvinyl ester obtained by polymerizing the vinyl ester. Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl biparate, vinyl versatic acid, vinyl laurate, vinyl stearate, vinyl benzoate, and the like. Select one or more. Among these, vinyl acetate is preferably used from the viewpoints of availability, ease of PVA production, cost and the like. The polymerization temperature is not particularly limited, but when methanol is used as the polymerization solvent, the polymerization temperature is preferably around 60 ° C., which is close to the boiling point of methanol.

As long as the effect of the present invention is not impaired, the polyvinyl ester may be a copolymer of a vinyl ester and another monomer copolymerizable therewith, but the vinyl ester as a monomer The one obtained by using only the monomer is preferable. The vinyl ester used at this time may be two or more kinds, but one kind is preferable.

Examples of other monomers copolymerizable with the vinyl ester include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butyl, 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) (Meta) 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) acrylamide, (meth) acrylamide propanesulfonic acid or a salt thereof, (meth) acrylamidepropyldimethylamine or a salt thereof, N -(Meta) acrylamide derivatives such as methylol (meth) acrylamide or derivatives thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; methylvinyl ether, ethylvinyl ether, n-propylvinyl ether, i- Vinyl ethers such as 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 chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride Vinyl halides such as allyl halides, allyl compounds such as allyl chloride; maleic acid or salts thereof, esters or acid anhydrides; itaconic acid or salts thereof, esters or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; Saturated sulfonic acid or a salt thereof and the like can be mentioned. The polyvinyl ester can have one or more structural units derived from the other monomer.

The ratio of structural units derived from other monomers to 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 preferably, it is more preferably 5 mol% or less.

As the aldehyde used for acetalization, conventionally known aldehydes having 1 to 7 carbon atoms are used. The carbon number of the aldehyde is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. On the other hand, the number of carbon atoms is preferably 6 or less, and more preferably 5 or less. Among them, as the aldehyde, an aldehyde having 4 carbon atoms is particularly preferable, and n-butyraldehyde is most preferable. In the present invention, acetal-modified PVA obtained by using two or more kinds of aldehydes in combination can also be used.

The method for acetalizing the PVA is not particularly limited, and examples thereof include a precipitation method and a solid-liquid reaction method. In the precipitation method, PVA is dissolved in a solvent such as water or acetone, and then a catalyst such as an acid and an aldehyde are added to the obtained solution to carry out an acetalization reaction to precipitate the produced acetal-modified PVA, which is further used as a catalyst. This is a method of obtaining a solid powder by neutralizing the acid used. The solid-liquid reaction method can be carried out in the same manner as the precipitation method, except that a solvent in which PVA before modification is not dissolved is used. In either method, the obtained polyvinyl acetal powder contains impurities such as unreacted aldehydes and salts generated by neutralization. High-purity acetal-modified PVA can be obtained by extracting or evaporating and removing these impurities using a solvent in which they are soluble. Of these, the solid-liquid reaction method is preferable from the viewpoint of productivity.

As the acid catalyst, a known acid can be used, and examples thereof include inorganic acids such as sulfuric acid, hydrochloric acid and nitric acid, and organic acids such as paratoluenesulfonic acid. The amount of the acid catalyst added is not particularly limited, but is usually adjusted so that the final acid concentration in the reaction solution is 0.5 to 5.0% by mass. A predetermined amount of these acid catalysts may be added at one time, but in the case of the precipitation method, it is preferable to add them in an appropriate number of times in order to precipitate and precipitate relatively fine acetal-modified PVA particles. On the other hand, in the case of the solid-liquid reaction method, it is preferable to add a predetermined amount at the beginning of the reaction from the viewpoint of reaction efficiency.

The acetal-modified PVA of the present invention contains an acetal structure represented by the following formula (1).

In formula (1), R is a hydrogen atom or a monovalent aliphatic group having 1 to 6 carbon atoms. The aliphatic group may be a straight chain or a branched chain, but is preferably a straight chain. The aliphatic group preferably has 2 or more carbon atoms, and more preferably 3 or more carbon atoms. On the other hand, when the number of carbon atoms of the aliphatic group exceeds 6, the solubility of acetal-modified PVA in water becomes poor, and film formation becomes difficult. The aliphatic group preferably has 5 or less carbon atoms, and more preferably 4 or less carbon atoms. The aliphatic group preferably has 3 carbon atoms, and most preferably the aliphatic group is an n-propyl group.

The degree of acetalization of the acetal-modified PVA of the present invention needs to be 1 to 6 mol%. When the degree of acetalization is within the above range, a polarizing film having excellent optical performance can be obtained. From the viewpoint of improving the optical performance and reducing the shrinkage stress, the degree of acetalization is preferably 2 mol% or more, more preferably 3.5 mol% or more. On the other hand, if the degree of acetalization exceeds 6 mol%, the solubility in water becomes poor, and it becomes difficult to form a PVA film. The degree of acetalization is preferably 5.5 mol% or less. The degree of acetalization is the above formula (1) with respect to the total of the monomer units (vinyl alcohol unit, vinyl acetate unit, etc., including the vinyl alcohol unit forming the acetal structure) in the acetal-modified PVA. ) Is the ratio (mol%) of vinyl alcohol units forming the acetal structure, and specifically, it is calculated by the method described in Examples using acetal-modified PVA as a sample. The acetal structure represented by the above formula (1) contains two monomer units (vinyl alcohol units).

The saponification degree of the acetal-modified PVA of the present invention is preferably 99 to 99.99 mol%. If the saponification degree is less than 99 mol%, PVA is eluted in the polarizing film manufacturing process and adheres to the film, which may reduce the performance of the polarizing film and the yield. The saponification degree is more preferably 99.3 mol% or more. The degree of saponification is calculated by the method described in Examples using PVA before acetalization as a sample.

The degree of polymerization of the acetal-modified PVA of the present invention is preferably 1,000 to 4,000. If the degree of polymerization is less than 1,000, acetal-modified PVA may elute in the polarizing film manufacturing process and the film may be cut. The degree of polymerization is more preferably 1,500 or more, and even more preferably 2,000 or more. On the other hand, if the degree of polymerization exceeds 4,000, not only the productivity of PVA is lowered, but also the stretchability of the PVA film is deteriorated in the manufacturing process of the polarizing film, so that the PVA film may be easily broken. The degree of polymerization is more preferably 3,500 or less, and even more preferably 3,000 or less. The degree of polymerization is calculated by the method described in Examples using PVA before acetalization as a sample.

The acetal-modified PVA film of the present invention contains the acetal-modified PVA represented by the above formula (1), contains the acetal-modified PVA having an acetalization degree of 1 to 6 mol%, and has a swelling degree of 160 to 240%. It is a thing. The acetal-modified PVA film is suitably used for optics, and specifically, it is preferably used as a raw film used in the production of an optical film. A polarizing film is preferable as the optical film.

It is preferable that the acetal-modified PVA film contains a plasticizer from the viewpoint of improving handleability and stretchability. As the plasticizer, polyhydric alcohol is preferably used, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. The PVA film can contain one or more of these plasticizers. Of these, glycerin is preferable from the viewpoint of further improving the stretchability of the acetal-modified PVA film.

The content of the plasticizer in the acetal-modified PVA film is preferably 3 to 20 parts by mass, more preferably 5 to 17 parts by mass, and 7 to 14 parts by mass with respect to 100 parts by mass of the acetal-modified PVA. It is more preferably a part. When the content is 3 parts by mass or more, the handleability and stretchability of the acetal-modified PVA film are further improved. On the other hand, when the content is 20 parts by mass or less, it is possible to suppress the deterioration of the handleability of the acetal-modified PVA film due to the bleed-out of the plasticizer on the surface.

The content of the acetal-modified PVA in the acetal-modified PVA film is preferably 50 to 100% by mass. The content is more preferably 80% by mass or more, further preferably 85% by mass or more.

The thickness of the acetal-modified PVA film is preferably 10 to 60 μm. If the acetal-modified PVA film is too thin, stretch breakage is likely to occur during the production of the polarizing film. On the other hand, if the acetal-modified PVA film is too thick, stretch spots are likely to occur during the production of the polarizing film. The acetal-modified PVA film is preferably a single-layer film, but it may be a laminate in which an acetal-modified PVA layer and another layer such as a thermoplastic resin film are laminated. In the case of a laminated body, the thickness of the acetal-modified PVA layer is preferably in the above range.

The degree of swelling of the acetal-modified PVA film needs to be 160 to 240%. If the degree of swelling is less than 160%, the tension at the time of stretching becomes too large in the manufacturing process of the polarizing film, and the film cannot be sufficiently stretched. The degree of swelling is preferably 170% or more, more preferably 185% or more. On the other hand, if the degree of swelling exceeds 240%, the water absorption of the PVA film is too high, so that wrinkles and edge curls are likely to occur in the film in the manufacturing process of the polarizing film. Such wrinkles and edge curls cause the film to break during stretching. The degree of swelling is preferably 235% or less, more preferably 230% or less. In order to control the degree of swelling within a predetermined range, for example, the temperature and time for heat-treating the PVA film after film formation may be adjusted within the following range.

The retardation of the acetal-modified PVA film is preferably 10 to 40 nm. If the retardation is less than 10 nm, the dyeing rate at the time of producing the polarizing film becomes slow, so that dyeing spots are likely to occur. The retardation is more preferably 13 nm or more, further preferably 17 nm or more. On the other hand, when the retardation exceeds 40 nm, the film is likely to be cut even at a low draw ratio. The retardation is more preferably 37 nm or less, further preferably 33 nm or less, and particularly preferably 30 nm or less. The PVA film retardation can be measured by the methods described in Examples below.

The method for controlling the retardation of the acetal-modified PVA film within a predetermined range is not particularly limited, and examples thereof include a method of heat-treating the PVA film after adjusting the humidity, and a method of stretching the PVA film by a known method. Above all, the former is preferable from the viewpoint of fixing the generated retardation.

The absorbance at a wavelength of 295 nm, which is measured after the acetal-modified PVA film is subjected to the following treatments (A), (B) and (C) in this order, is 0.3 or more per 10 μm thickness and has a wavelength of 330 nm. The absorbance in the above is preferably 0.2 or more per 10 μm thickness. When the absorbance at each wavelength is equal to or higher than the above value, the decrease in the degree of polarization of the obtained polarizing film is suppressed. The absorbance at a wavelength of 295 nm is more preferably 0.35 or more per 10 μm thickness. Further, the absorbance at a wavelength of 330 nm is more preferably 0.25 or more per 10 μm thickness.
(A) The film is uniaxially stretched twice in the length direction while being immersed in pure water at 30 ° C. for 60 seconds. (B) The film contains 0.04% by mass of iodine and 0.92% by mass of potassium iodide. (C) The film is air-dried at 25 ° C. by adsorbing iodine on the film by uniaxially stretching it 1.2 times in the length direction while immersing it in an aqueous solution at 32 ° C. for 120 seconds.

Examples of the film-forming stock solution used for producing the acetal-modified PVA film include acetal-modified PVA,, if necessary, a film-forming stock solution in which a plasticizer, a surfactant, and other components are dissolved in a liquid medium, and acetal-modified PVA. It can be produced using a film-forming stock solution containing a plasticizer, a surfactant, other components, and a liquid medium, and in which acetal-modified PVA is melted. It is preferable that each component in the film-forming stock solution is uniformly mixed.

The film-forming stock solution contains a surfactant from the viewpoint of suppressing the occurrence of thickness unevenness of the PVA film obtained by improving the film-forming property and improving the peelability of the PVA film from the metal roll or belt. Is preferable. When a PVA film is produced using a film-forming stock solution containing a surfactant, the obtained PVA film may contain the surfactant. The type of the surfactant is not particularly limited, but an anionic surfactant and a nonionic surfactant are preferable, and an anionic surfactant is more preferable, from the viewpoint of facilitating the peeling of the PVA film from the metal roll or the belt. These surfactants can be used alone or in combination of two or more.

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

Examples of the nonionic surfactant include an alkyl ether type such as polyoxyethylene oleyl ether; an alkylphenyl ether type such as polyoxyethylene octylphenyl ether; an alkyl ester type such as polyoxyethylene laurate; and 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 allylphenyl ether type such as alkyleneallylphenyl ether is preferable.

The method for forming the acetal-modified PVA film is not particularly limited, but a cast film forming method, an extrusion film forming method, a wet film forming method, a gel film forming method, or the like is preferable from the viewpoint that a film having a uniform thickness and width can be obtained. , Cast film forming method and extrusion film forming method are more preferable. Above all, the cast film forming method is particularly preferable from the viewpoint that an acetal-modified PVA film having uniform thickness and width and good physical characteristics can be obtained.

Examples of the liquid medium used for the film-forming stock solution include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and trimethylol. Examples thereof include propane, ethylenediamine, and diethylenetriamine, and one or more of these can be used. Of these, water is preferable because it has a small impact on the environment and is recoverable.

The volatile fraction of the film-forming stock solution may be appropriately adjusted according to the film-forming method and the molecular weight of the acetal-modified PVA, but is preferably 50 to 95% by mass, more preferably 60 to 95% by mass, and 70 to 95% by mass. More preferred. If the volatile fraction is less than 50% by mass, the viscosity of the film-forming stock solution becomes too high, which makes it difficult to filter and defoam during preparation, and the resulting acetal-modified PVA film may have foreign substances or defects. On the other hand, if the volatile fraction exceeds 95% by mass, the viscosity of the film-forming stock solution becomes too low, and it becomes difficult to adjust the thickness of the acetal-modified PVA film to a target value or to adjust it with high accuracy. There is a risk.

It is preferable to heat-treat the acetal-modified PVA film obtained by forming the acetal-modified PVA from the viewpoint that the swelling degree and retardation of the PVA film can be easily adjusted within the above ranges. The heat treatment temperature is preferably 100 to 170 ° C. The heat treatment time is preferably 1 to 30 minutes. After the acetal-modified PVA film dried after film formation is adjusted in humidity to adjust the water content (moisture content) to 5 to 15% by mass, heat treatment may be performed with only one axis of the film fixed. At this time, it is preferable to fix the flow direction of the acetal-modified PVA film. When only one axis of the film is fixed and the heat treatment is performed in this way, the same effect as applying gentle stretching to the film can be obtained due to the stress generated by the decrease in water content in the film. Then, by fixing only one axis and performing the heat treatment, retardation is likely to occur. The moisture content of the film after the heat treatment is preferably 1 to 15% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 6% by mass.

The method for producing the polarizing film of the present invention is not particularly limited, but a method of performing a dyeing step and a uniaxial stretching step on the acetal-modified PVA film obtained by the above method is preferable, and the acetal-modified PVA film is compared with the acetal-modified PVA film. A method of performing a swelling step, a dyeing step and a uniaxial stretching step is more preferable. In addition to these steps, the acetal-modified PVA film may be further subjected to a crosslinking step, a fixing treatment step, a drying step, a heat treatment step, and the like. The order of each step is not particularly limited, and one or more steps may be performed at the same time. In addition, one or two or more of each step can be performed twice or more. Hereinafter, each process will be specifically described.

The swelling step can be performed by immersing the acetal-modified PVA film in water. The temperature of water at this time is preferably 20 to 40 ° C, more preferably 25 to 35 ° C. The time of immersion in water is preferably 0.1 to 5 minutes, more preferably 0.5 to 3 minutes. The water 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. The swelling step is preferably performed before the dyeing step.

The dyeing step can be performed by contacting the acetal-modified PVA film with a dichroic dye. As the dichroic dye, an iodine-based dye is preferably used. The dyeing step may be performed before the uniaxial stretching step described later or after the uniaxial stretching step, but the former is preferable. As a dyeing method, a general method, specifically, a method of immersing an acetal-modified PVA film in a dyeing bath is preferably adopted. As the dyeing bath, a solution containing iodine-potassium iodide (particularly an aqueous solution) is used. The concentration of iodine in the solution is preferably 0.01 to 0.5% by mass, and the concentration of potassium iodide is preferably 0.01 to 10% by mass. The temperature of the dyeing bath is preferably 20 to 50 ° C, more preferably 25 to 40 ° C. The time for immersing the acetal-modified PVA film in the dyeing bath is preferably 0.1 to 10 minutes, more preferably 0.2 to 5 minutes.

The cross-linking step can be performed by immersing the acetal-modified PVA film in an aqueous solution containing a cross-linking agent. This makes it possible to more effectively prevent the acetal-modified PVA from dissolving in water during wet stretching at a high temperature. From this point of view, it is preferable that the crosslinking step is performed before the uniaxial stretching step. It is also preferable that the cross-linking step is performed after the dyeing step. As the cross-linking agent, one or more kinds of boron compounds such as borate such as boric acid and borax can be used. The concentration of the cross-linking agent in the aqueous solution is preferably 1 to 15% by mass, more preferably 2 to 7% by mass. Sufficient stretchability can be maintained when the concentration of the cross-linking agent is in the above range. The aqueous solution containing the cross-linking agent may contain an auxiliary agent such as potassium iodide. The temperature of the aqueous solution containing the cross-linking agent is preferably 20 to 50 ° C, more preferably 25 to 40 ° C. When the temperature is in the above range, the acetal-modified PVA is efficiently crosslinked.

Apart from the uniaxial stretching step described later, the acetal-modified PVA film may be uniaxially stretched during or between the steps described above. By performing such stretching (pre-stretching), it is possible to prevent the acetal-modified PVA film from being wrinkled. From the viewpoint of polarization performance, the total draw ratio (magnification obtained by multiplying the draw ratio in each step) is preferably 4 times or less based on the original length of the original PVA film before stretching. The draw ratio in the swelling step is preferably 1.05 to 3 times, the draw ratio in the dyeing step is preferably 3 times or less, and the draw ratio in the cross-linking step is preferably 2 times or less.

The uniaxial stretching step on the acetal-modified PVA film can be performed by uniaxially stretching the 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 boric acid, in the above-mentioned dyeing bath or in the fixing treatment bath described later. In the case of the dry stretching method, stretching may be carried out at room temperature, stretching may be carried out while heating, or the acetal-modified PVA film after water absorption may be used for stretching in the air. Among these, the wet stretching method is preferable, and uniaxial stretching is more preferable in an aqueous solution containing boric acid. The boric acid concentration in the aqueous solution is preferably 0.5 to 6.0% by mass, more preferably 1.0 to 5.0% by mass, and particularly preferably 1.5 to 4.5% by mass. Further, the boric acid aqueous solution may contain potassium iodide, and the concentration thereof is preferably 0.01 to 10% by mass.

From the viewpoint that the shrinkage force of the obtained polarizing film in the stretching direction is significantly reduced and the dimensional stability is further improved, the stretching temperature in the uniaxial stretching step is preferably 30 to 90 ° C, more preferably 40 to 80 ° C, and 50. ~ 70 ° C. is particularly preferable. From the same viewpoint, it is also preferable to use the wet stretching method.

From the viewpoint of the polarization performance of the polarizing film, the stretching ratio in the stretching step is preferably 1.2 times or more, more preferably 1.5 times or more, and further preferably 2 times or more. Further, the total draw ratio including the draw ratio of the above-mentioned pre-stretch (the ratio obtained by multiplying the draw ratio in each step) is 5.8 times or more based on the original length of the acetal-modified PVA film of the raw fabric before stretching. Is preferable. The upper limit of the total draw ratio is not particularly limited, but is preferably 7 times or less in order to prevent stretch breakage.

When a long acetal-modified PVA film is uniaxially stretched, there is no particular limitation on the stretching direction, and the film can be stretched in the length direction (flow direction when the acetal-modified PVA film is manufactured) or in the width direction (horizontal uniaxial stretching). it can. From the viewpoint of further improving the polarization performance, it is preferable to stretch in the length direction. Uniaxial stretching in the length direction can be performed by using a stretching device including a plurality of rolls parallel to each other and changing the peripheral speed between the rolls. On the other hand, the horizontal uniaxial stretching can be performed using a tenter type stretching machine.

After the stretching step, the acetal-modified PVA film may be fixed. This enhances the adsorption of the dichroic dye on the acetal-modified PVA film. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing one or more kinds of boron compounds such as boric acid and borax can be used. If necessary, an iodine compound or a metal compound may be added to the fixing treatment bath. The concentration of the boron compound in the fixing treatment bath is preferably 0.5 to 15% by mass. With such a range, the adsorption of the dichroic dye can be further strengthened. The temperature of the fixing treatment bath is preferably 15 to 60 ° C.

When the acetal-modified PVA film is subjected to the drying step, the drying temperature is preferably 30 to 150 ° C, more preferably 50 to 130 ° C. The acetal-modified PVA film may be dried and heat-treated at 80 to 140 ° C. for 1 to 15 minutes while applying tension to the film when the water content becomes 10% by mass or less.

The polarizing film thus obtained is used as a polarizing plate by laminating a protective film that is optically transparent and has high mechanical strength on both sides or one side thereof. As the protective film, a cellulose triacetate (TAC) film, a cellulose acetate / cellulose acetate (CAB) film, an acrylic film, a polyester film, or the like is used. Examples of the adhesive used at this time include PVA-based adhesives and urethane-based adhesives, and among them, PVA-based adhesives are preferable.

The single transmittance of the polarizing film of the present invention is preferably 43.95% or more, and the degree of polarization is preferably 99.9% or more. When the single transmittance and the degree of polarization are within the above ranges, an LCD panel having excellent image quality can be obtained. Further, from the viewpoint of reducing the warp of the LCD panel to be obtained, the contraction stress of the polarizing film is preferably 59N / mm ² or less, more preferably 54N / mm ² or less, more preferably 50 N / mm ² or less.

The polarizing plate obtained as described above can be used as an LCD component by being coated with an adhesive such as an acrylic material and then bonded to a glass substrate. At the same time, it may be bonded to a retardation film, a viewing angle improving film, a brightness improving film, or the like.

Hereinafter, the present invention will be specifically described with reference to Examples and the like, but the present invention is not limited to these Examples.

[Acetalization degree]
Acetal-modified PVA is dissolved in a mixed solution of N-methylpyrrolidone (NMP) and dimethyl sulfoxide-d6 (DMSO-d6) (N-methylpyrrolidone: DMSO-d6 = 9: 1, mass ratio) and chromium is added thereto. Acetylacetate was added. As a measuring device, a superconducting nuclear magnetic resonance apparatus (“Lambda500”, manufactured by JEOL Ltd.) was used for measurement under the conditions of resonance frequency 13C, 125MHz and temperature 80 ° C. The peak intensity derived from the methine carbon (95 ppm, 103 ppm) bonded to R in the acetal structure represented by the above formula (1), and each of the vinyl alcohol unit, the vinyl ester unit, and the vinyl acetal unit in the main chain. The degree of acetalization was determined from the peak intensity derived from methylene carbon (62 to 75 ppm).

[Degree of polymerization and saponification]
The degree of polymerization and saponification of polyvinyl alcohol (resin before acetalization of acetal-modified PVA) was measured according to JIS-K6726, and the degree of polymerization and saponification of acetal-modified PVA were determined.

[Film swelling degree]
The acetal-modified PVA film was cut to 1.5 g and immersed in 1000 g of distilled water at 30 ° C. After soaking for 30 minutes, the film was taken out, water on the surface was absorbed with a filter paper, and then the mass (We) was measured. Subsequently, the film was dried at 105 ° C. for 16 hours using a dryer, and then its mass (Wf) was measured. From the obtained masses We and Wf, the degree of swelling of the PVA film was determined by the following formula.
Swelling degree (%) = (We / Wf) x 100

[Acetal-modified PVA film retardation Re]
Using the optical material inspection device RETS-1100 manufactured by Otsuka Electronics Co., Ltd., the retardation Re at a measurement wavelength of 550 nm was measured over the entire width at a pitch of 50 mm in the width direction, and the average value was obtained. At this time, rectangular samples (5 cm in the width direction and 10 cm in the flow direction) were continuously collected in the width direction from the acetal-modified PVA film and used for measurement.

[Iodine adsorption]
A rectangular film having a width of 5 cm and a flow direction of 9 cm was collected from the central portion of the acetal-modified PVA film (original fabric) having a thickness of 30 μm in the width direction so that a portion of 5 cm in the width direction and 5 cm in the flow direction could be uniaxially stretched. (A) This film was immersed in pure water at 30 ° C. for 60 seconds, uniaxially stretched twice in the flow direction, and swelled. (B) Subsequently, the film was immersed in an aqueous solution (dyeing treatment bath) at 32 ° C. containing 0.04% by mass of iodine and 0.92% by mass of potassium iodide for 120 seconds and 1.2 times (totally 2. Iodine was adsorbed by uniaxially stretching in the flow direction (4 times). (C) After that, the film was air-dried at 25 ° C. The absorption spectrum of the film at a wavelength of 200 to 800 nm was measured using an ultraviolet-visible spectrophotometer "U-4100" manufactured by Hitachi, Ltd. I ₃ ^- measurement or I ₂ · I ₃ of absorbance at a wavelength of 295nm derived from the ion ^- by the following equation using the measured values of absorbance at a wavelength of 330nm derived from ion was calculated absorbance per thickness of raw 10μm ..
Absorbance per 10 μm thickness of raw fabric = (a / b) × 10
a: Measured value of absorbance of the film subjected to the treatments (A) to (C) at a wavelength of 295 nm or 330 nm b: Thickness of the original fabric (μm)

[Optical performance of polarizing film]
(1) Measurement of transmittance Ts Two rectangular samples with a transmittance of 2 cm in the stretching direction and 1.5 cm in the width direction were taken from the center of the polarizing film, and a spectrophotometer with an integrating sphere (“V7100” manufactured by Nippon Spectral Co., Ltd.). ”) Is used to correct the visibility of the visible light region of the C light source and 2 ° field in accordance with JIS Z8722 (measurement method of object color), and one sample is tilted by + 45 ° with respect to the stretching direction. The transmittance of the light in the case of the above and the transmittance of the light in the case of being tilted by −45 ° were measured, and the average value Ts1 (%) of them was obtained. In the same manner for the other sample, the light transmittance when tilted by + 45 ° and the light transmittance when tilted by −45 ° were measured, and their average value Ts2 (%) was obtained. Then, Ts1 and Ts2 were averaged by the following formula to obtain the transmittance Ts (%) of the polarizing film.
Ts = (Ts1 + Ts2) / 2

(2) Measurement of polarization degree V Light transmittance T∥ (%) and stretching when two samples collected in the measurement of transmittance Ts in (1) are stacked so that their stretching directions are parallel to each other. The light transmittance T⊥ (%) when the lights are overlapped so as to be orthogonal to each other is measured in the same manner as in the case of “(1) Measurement of transmittance Ts” above, and the degree of polarization V (%) is measured by the following formula. Asked.
V = {(T∥-T⊥) / (T∥ + T⊥)} 1/2 × 100

[Film shrinkage stress]
The shrinkage force of the polarizing film was measured using an autograph "AG-X" with a constant temperature bath manufactured by Shimadzu Corporation and a video-type elongation meter "TR ViewX120S". A polarizing film whose humidity was adjusted at 20 ° C./20% RH for 18 hours was used for the measurement. After the constant temperature bath of the autograph "AG-X" was set to 20 ° C, the polarizing film [stretching direction 15 cm, width direction 1.5 cm] was attached to the chuck (chuck interval 5 cm), and at the same time as the tension was started, the constant temperature bath was set to 80 ° C. Started to raise the temperature. The polarizing film was pulled at a speed of 1 mm / min, the tension was stopped when the tension reached 2N, and the tension was measured up to 4 hours later in that state. At this time, since the distance between the chucks changes due to thermal expansion, a marked line sticker is attached to the chuck, and the distance between the chucks is increased by the amount of movement of the marked line sticker attached to the chuck using the video type extensometer "TR ViewX120S". Was measured so that it could be corrected. Since the minimum value of tension occurs at the initial stage of measurement (within 10 minutes from the start of measurement), the minimum value of tension is subtracted from the measured value of tension after 4 hours, and the difference is defined as the contraction force (N) of the polarizing film. The value obtained by dividing the value (N) by the sample cross-sectional area (mm ² ) was defined as the contraction stress (N / mm ² ).

Example 1
<Manufacturing of PVA film>
PVA modified with n-butylaldehyde (acetalization degree 1.6 mol%, R in the above formula (1) is propyl group (carbon number 3), saponification degree 99.5 mol%, polymerization degree 2400) It contains 100 parts by mass of glycerin as a plasticizer, 0.16 parts by mass of diethanolamide lauric acid as a surfactant, and 0.08 parts by mass of sodium polyoxyethylene lauryl ether sulfate, and has a content of 10 parts of acetal-modified PVA. An aqueous solution of mass% was used as the film-forming stock solution. This was cast on a metal roll at 60 ° C. and then dried, and the obtained film was heat-treated at 157 ° C. for 10 minutes to adjust the degree of swelling and retardation Re, and the thickness was increased. A 30 μm acetal-modified PVA film was produced. The swelling degree, absorbance and retardation Re of the obtained acetal-modified PVA film were measured and evaluated. The results are shown in Table 1.

<Manufacturing of polarizing film>
From the central portion of the acetal-modified PVA film thus obtained in the width direction, a rectangular film having a width direction of 5 cm and a flow direction of 9 cm was collected so that a portion 5 cm in the width direction and 5 cm in the flow direction could be uniaxially stretched. This film was immersed in pure water at 30 ° C. for 60 seconds, uniaxially stretched in the flow direction twice, and swelled. Subsequently, the flow direction was increased 1.2 times (2.4 times in total) while being immersed in an aqueous solution (dyeing treatment bath) at 32 ° C. containing 0.04% by mass of iodine and 0.92% by mass of potassium iodide for 120 seconds. It was uniaxially stretched to adsorb iodine (dyeing treatment). Then, boric acid was uniaxially stretched in the flow direction 1.25 times (3 times in total) while being immersed in an aqueous solution (boric acid cross-linking treatment bath) at 32 ° C. containing 2.6% by mass (boric acid cross-linking treatment). ). Further, while immersing in an aqueous solution (uniaxial stretching treatment bath) at 59 ° C. containing 2.8% by mass of boric acid and 5% by mass of potassium iodide, the whole was uniaxially stretched in the flow direction up to 6.0 times (stretching treatment). ). Finally, it was dried at 80 ° C. for 4 minutes to produce a polarizing film. The optical performance of the obtained polarizing film was evaluated. The results are shown in Table 1.

Example 2
PVA with a degree of acetalization of 5 mol% modified with n-butyraldehyde (R in the above formula (1) is a propyl group (carbon number 3), a degree of saponification of 99.5 mol%, a degree of polymerization of 2400). The acetal-modified PVA film and the polarizing film were prepared in the same manner as in Example 1 except that the above was used, and each measurement and evaluation was performed. The results are shown in Table 1.

Example 3
PVA acetal-modified with n-propyl aldehyde (acetalization degree is 4 mol%, R in the above formula (1) is ethyl group (carbon number 2), saponification degree 99.5 mol%, polymerization degree 2400). An acetal-modified PVA film and a polarizing film were prepared in the same manner as in Example 1 except that they were used, and each measurement and evaluation was performed. The results are shown in Table 1.

Comparative Example 1
Acetal-modified PVA film and polarizing film were prepared in the same manner as in Example 1 except that unmodified PVA (saponification degree 99.5 mol%, polymerization degree 2400) was used, and each measurement and evaluation was performed. It was. The results are shown in Table 1.

Comparative Example 2
PVA acetal-modified with n-octylaldehyde (acetalization degree 3.9 mol%, R in the above formula (1) is heptyl group (carbon number 7), saponification degree 99.5 mol%, polymerization An acetal-modified PVA film and a polarizing film were prepared in the same manner as in Example 1 except that the degree 2400) was used, and each measurement and evaluation was performed. The results are shown in Table 1.

The polarizing films of Examples 1 to 3 obtained by using the acetal-modified PVA containing the acetal structure represented by the above formula (1) have a polarization degree of 99. When the single transmittance is 43.97 to 43.99%. It was 9% or more, and the degree of polarization was higher than that of the polarizing film of Comparative Example 1 obtained by using unmodified PVA. Acetal-modified PVA (Comparative Example 2) in which R in the above formula (1) is a monovalent aliphatic group (heptyl group) having more than 6 carbon atoms is insoluble in water and a film-forming stock solution cannot be produced. Therefore, the acetal-modified PVA film could not be formed.

Claims

A polarizing film containing an acetal-modified polyvinyl alcohol having an acetal structure represented by the following formula (1) and having an acetalization degree of 1 to 6 mol%.

[In the formula (1), R is a hydrogen atom or a monovalent aliphatic group having 1 to 6 carbon atoms. ]
The polarizing film according to claim 1, wherein the simple substance transmittance is 43.95% or more and the degree of polarization is 99.9% or more.
A polyvinyl alcohol film containing an acetal structure represented by the following formula (1), containing an acetal-modified polyvinyl alcohol having an acetalization degree of 1 to 6 mol%, and a swelling degree of 160 to 240%.

[In the formula (1), R is a hydrogen atom or a monovalent aliphatic group having 1 to 6 carbon atoms. ]
The polyvinyl alcohol film according to claim 3, wherein the acetal-modified polyvinyl alcohol has a degree of polymerization of 1,000 to 4,000 and a degree of saponification of 99 to 99.99 mol%.
The polyvinyl alcohol film according to claim 3 or 4, which has a thickness of 10 to 60 μm.
The polyvinyl alcohol film according to any one of claims 3 to 5, which has a retardation of 10 to 40 nm.
The absorbance at a wavelength of 295 nm is 0.3 or more per 10 μm, and the absorbance at a wavelength of 330 nm is per 10 μm, which is measured after the following treatments (A), (B) and (C) are performed in this order. The polyvinyl alcohol film according to any one of claims 3 to 6, which is 0.2 or more.
(A) The film is uniaxially stretched twice in the length direction while being immersed in pure water at 30 ° C. for 60 seconds. (B) The film contains 0.04% by mass of iodine and 0.92% by mass of potassium iodide. (C) The film is air-dried at 25 ° C. by adsorbing iodine on the film by uniaxially stretching it 1.2 times in the length direction while immersing it in an aqueous solution at 32 ° C. for 120 seconds.
The polyvinyl alcohol film according to any one of claims 3 to 7, which is for optics.