JP2023056678A - Polyvinyl alcohol film, method of producing polyvinyl alcohol film, stretched film and polarizing film - Google Patents

Abstract

To provide a polyvinyl alcohol film capable of obtaining a polarizing film superior in polarizing performance, a manufacturing method of such polyvinyl alcohol film, and a stretched film and a polarizing film using such polyvinyl alcohol film.SOLUTION: A disclosed polyvinyl alcohol film contains: polyvinyl alcohol (A); and a polymer (B) having structural units derived from maleic anhydride, in which the content of the polymer (B) relative to 100 pts.mass of polyvinyl alcohol (A) is 0.5 pt.mass or more and 9.5 pts.mass or less, and the salification rate of structural units derived from maleic anhydride in the polymer (B) is 0.3 or more and 0.9 or less.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a polyvinyl alcohol film, a method for producing a polyvinyl alcohol film, a stretched film and a polarizing film.

Polarizers, which have the function of transmitting and shielding light, are basic components of liquid crystal displays (LCDs), along with liquid crystals that change the polarization state of light. Many polarizing plates have a structure in which a protective film such as a cellulose triacetate (TAC) film is attached to the surface of the polarizing film in order to prevent the polarizing film from fading or shrinking. there is As the polarizing film constituting the polarizing plate, a stretched film obtained by uniaxially stretching a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be referred to as "PVA") is added with an iodine dye (I ₃ ^- or I ₅ ^- etc.) is the mainstream.

LCDs are used in a wide range of applications such as compact devices such as calculators and wristwatches, smartphones, laptop computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, car navigation systems, and measuring instruments used indoors and outdoors. For these various uses, development of a polarizing film having high polarizing performance is expected. Patent Document 1 describes a polarizing film using PVA having a degree of polymerization of 2600 or more as a polarizing film having excellent durability and a high degree of polarization.

JP-A-1-84203

As in the polarizing film described in Patent Document 1, the use of PVA with a high degree of polymerization tends to increase the polarizing performance. However, when PVA with a high degree of polymerization is used, the productivity may be affected due to thickening of the membrane-forming stock solution. Therefore, it is expected to improve the polarizing performance of the polarizing film by a method other than increasing the weight of PVA.

The present invention has been made based on the above circumstances, and its object is to provide a polyvinyl alcohol film capable of obtaining a polarizing film having excellent polarizing performance, a method for producing such a polyvinyl alcohol film, and a method for producing such a polyvinyl alcohol film. An object of the present invention is to provide a stretched film and a polarizing film using a polyvinyl alcohol film.

The inventions made to solve the above problems are as follows.
[1] Polyvinyl alcohol (A) and a polymer (B) having a structural unit derived from maleic anhydride are included, and the content of the polymer (B) relative to 100 parts by mass of the polyvinyl alcohol (A) is 0.5 parts by mass. A polyvinyl alcohol film having a content of 5 parts by mass or more and 9.5 parts by mass or less, and a salification rate of structural units derived from maleic anhydride in the polymer (B) of 0.3 or more and 0.9 or less;
[2] The polyvinyl alcohol film of [1], in which the polyvinyl alcohol (A) and the polymer (B) form a crosslinked structure;
[3] The polyvinyl alcohol film of [1] or [2], wherein the polymer (B) further has a structural unit derived from an α-olefin;
[4] The polyvinyl alcohol film of [3], wherein the polymer (B) is an alternating copolymer of structural units derived from maleic anhydride and structural units derived from α-olefin;
[5] The polyvinyl alcohol film of any one of [1] to [4], in which some of the structural units derived from maleic anhydride in the polymer (B) form a salt with ammonia or an organic base;
[6] The polyvinyl alcohol film of any one of [1] to [5], wherein the polymer (B) has a weight average molecular weight of 5,000 or more and 200,000 or less;
[7] The polyvinyl alcohol film of any one of [1] to [6], which has a breaking stress of 8 MPa or more and a breaking strain of 150% or more in a 4% by mass boric acid aqueous solution at 50°C;
[8] The polyvinyl of any one of [1] to [7], which has a stress increase of 0.04 MPa/% or more and 0.3 MPa/% or less when the strain is 300% in a 4 mass% boric acid aqueous solution at 50°C. alcohol film;
[9] A step of forming a coating film containing polyvinyl alcohol (A) and a polymer (B) having a structural unit derived from maleic anhydride, and a step of heat-treating the coating film, The content of the polymer (B) with respect to 100 parts by mass of the polyvinyl alcohol (A) is 0.5 parts by mass or more and 9.5 parts by mass or less, and the structural unit derived from maleic anhydride of the polymer (B) is chlorinated A method for producing a polyvinyl alcohol film having a ratio of 0.3 or more and 0.9 or less;
[10] A stretched film formed from the polyvinyl alcohol film of any one of [1] to [8];
[11] A polarizing film formed from the polyvinyl alcohol film of any one of [1] to [8];

According to the present invention, a polyvinyl alcohol film capable of obtaining a polarizing film having excellent polarizing performance, a method for producing such a polyvinyl alcohol film, and a stretched film and a polarizing film using such a polyvinyl alcohol film are provided. can be done.

<Polyvinyl alcohol film>
A polyvinyl alcohol film (PVA film) according to one embodiment of the present invention contains PVA (A) and a polymer (B) having a structural unit derived from maleic anhydride, and The content of the polymer (B) is 0.5 parts by mass or more and 9.5 parts by mass or less, and the salification ratio of the structural units derived from maleic anhydride in the polymer (B) is 0.3 or more and 0.3 parts by mass or more. 9 or less.

The PVA film can be used to produce a polarizing film with excellent polarizing performance. The reason for this is that the polymer (B), which is derived from maleic anhydride and has a partially chlorinated structural unit, has both compatibility and crosslinkability with PVA (A), so that PVA It is presumed that (A) and polymer (B) can form a good crosslinked structure. Specifically, the salified structural unit derived from maleic anhydride can improve the compatibility and water solubility with PVA (A), and the structural unit derived from maleic anhydride and not salified shows good cross-linking reactivity with PVA (A). That is, in the PVA film, PVA (A) and polymer (B) preferably form a crosslinked structure. In addition, since the polymer (B) is usually substantially neutral, it is difficult to affect the decomposition of dichroic dyes such as iodine-based dyes. This is presumed to be a possible factor. Each component and the like will be described in detail below.

(PVA (A))
PVA (vinyl alcohol polymer) (A) is usually the main component of a PVA film. A main component means a component with the largest content on a mass basis. PVA (A) is a polymer having a vinyl alcohol unit ( _--CH.sub.2 --CH(OH)--) as a structural unit. PVA (A) may have vinyl ester units and other structural units in addition to vinyl alcohol units.

The lower limit of the degree of polymerization of PVA (A) is preferably 1,500, more preferably 1,800, and even more preferably 2,000. When the degree of polymerization is at least the above lower limit, the durability of the PVA film and the stretched film and polarizing film obtained from this PVA film can be improved. In addition, when the degree of polymerization is equal to or higher than the above lower limit, the polarizing performance of the polarizing film tends to be further enhanced. On the other hand, the upper limit of the degree of polymerization is preferably 6,000, more preferably 5,000, and even more preferably 4,000. When the degree of polymerization is equal to or less than the above upper limit, it is possible to suppress an increase in production cost and the occurrence of defects during film formation. The degree of polymerization of PVA means the average degree of polymerization measured according to the description of JIS K6726-1994.

The lower limit of the degree of saponification of PVA (A) is preferably 95 mol%, more preferably 96 mol%, and even more preferably 98 mol% from the viewpoint of the water resistance of the PVA film. On the other hand, the upper limit of this degree of saponification may be 100 mol %. The degree of saponification of PVA refers to the ratio (mol% ). The degree of saponification can be measured according to the description of JIS K6726-1994.

The method for producing PVA (A) is not particularly limited. For example, (1) a step of obtaining a vinyl ester polymer and (2) a step of saponifying the obtained vinyl ester polymer, that is, converting the vinyl ester units of the vinyl ester polymer into vinyl alcohol units, PVA (A) can be produced.

The vinyl ester polymer may be a homopolymer consisting only of a vinyl ester monomer, or a copolymer of a vinyl ester monomer and another monomer copolymerizable therewith. There may be.

Vinyl ester-based monomers are not particularly limited, but examples include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, and vinyl caprate. , vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, and the like. From an economical point of view, among these, vinyl acetate is preferable.

Examples of monomers copolymerizable with vinyl ester monomers include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, α-butylene and isobutylene; (meth)acrylic acid or salts thereof; ) methyl acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, (meth) ) t-butyl acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, octadecyl (meth)acrylate and other (meth)acrylic acid esters; (meth)acrylamide; N-methyl (meth) ) acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamidopropanesulfonic acid or salts thereof, (meth)acrylamidopropyldimethylamine or salts thereof, N - (meth)acrylamide derivatives such as methylol (meth)acrylamide or derivatives thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide and N-vinylpyrrolidone; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i- Vinyl ethers such as propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; vinyl cyanides such as (meth)acrylonitrile; vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride Allyl compounds such as allyl acetate and allyl chloride; Maleic acid or its salts, esters or acid anhydrides; Itaconic acid or its salts, esters or acid anhydrides; Vinyl silyl compounds such as vinyltrimethoxysilane and unsaturated sulfonic acid.

The vinyl ester polymer can have structural units derived from one or more of the other monomers. When the vinyl ester-based monomer is subjected to the polymerization reaction, the above-mentioned other monomers are preliminarily present in the polymerization system, or are added to the system during the progress of the polymerization reaction. can be used.

Among the above other monomers, α-olefins are preferred, and ethylene is more preferred. When PVA (A) contains a structural unit derived from an α-olefin, the stretchability of the PVA film is improved. When PVA (A) contains structural units derived from α-olefins, the lower limit of the content of structural units derived from α-olefins is preferably 1 mol%, more preferably 2 mol%, relative to all structural units. . On the other hand, the upper limit of this content is preferably 4 mol %, more preferably 3 mol %. When the content of the structural unit derived from α-olefin is within the above range, the stretchability and the like described above can be more effectively exhibited. Also, the upper limit of the content of structural units derived from other monomers relative to the total structural units of PVA may be preferably 4 mol %, and more preferably 1 mol % or 0.1 mol %.

The polymerization system for polymerizing the vinyl ester monomer may be any system such as batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization. As the polymerization method, known methods such as bulk polymerization method, solution polymerization method, suspension polymerization method and emulsion polymerization method can be applied. Generally, a bulk polymerization method in which polymerization proceeds without a solvent or a solution polymerization method in which polymerization proceeds in a solvent such as alcohol is employed. Emulsion polymerization is also preferred for obtaining a vinyl ester copolymer with a high degree of polymerization. A solvent for the solution polymerization method is not particularly limited, and is, for example, alcohol. Lower alcohols such as methanol, ethanol and propanol can be used as the solvent for the solution polymerization method. The amount of the solvent used in the polymerization system may be selected in consideration of the chain transfer of the solvent according to the desired degree of polymerization of PVA. is preferably in the range of 0.01 to 10, more preferably in the range of 0.05 to 3.

The polymerization initiator used for the polymerization of the vinyl ester monomer may be selected from known polymerization initiators such as azo initiators, peroxide initiators, redox initiators, etc., depending on the polymerization method. good. Examples of azo initiators include 2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(4-methoxy-2,4 -dimethylvaleronitrile). Examples of peroxide-based initiators include peroxydicarbonate-based compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; t-butyl peroxyneodecanate, α acetylcyclohexylsulfonyl peroxide; 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; acetyl peroxide and the like. Potassium persulfate, ammonium persulfate, hydrogen peroxide, or the like may be used in combination with the above initiator as the polymerization initiator. Examples of redox initiators include polymerization initiators obtained by combining the above peroxide initiators with reducing agents such as sodium hydrogensulfite, sodium hydrogencarbonate, tartaric acid, L-ascorbic acid and Rongalite. The amount of the polymerization initiator to be used varies depending on the type of the polymerization initiator and cannot be generally determined, but may be selected according to the polymerization rate. For example, when 2,2'-azobisisobutyronitrile or acetyl peroxide is used as the polymerization initiator, it is preferably 0.01 to 0.2 mol%, and 0.02 to 0.2 mol% with respect to the vinyl ester monomer. 0.15 mole % is more preferred. The polymerization temperature is not particularly limited, but room temperature to about 150° C. is suitable, preferably 40° C. or more and the boiling point of the solvent used or less.

Polymerization of the vinyl ester monomer may be carried out in the presence of a chain transfer agent. Examples of chain transfer agents include aldehydes such as acetaldehyde and propionaldehyde; ketones such as acetone and methyl ethyl ketone; mercaptans such as 2-hydroxyethanethiol; phosphinates such as sodium phosphinate monohydrate. can be done. Among these, aldehydes and ketones are preferably used. The amount of the chain transfer agent to be used can be determined according to the chain transfer coefficient of the chain transfer agent to be used and the desired degree of polymerization of PVA. 1 to 10 parts by mass is preferred.

Saponification of the vinyl ester polymer can be carried out, for example, in a state in which the vinyl ester polymer is dissolved in alcohol or water-containing alcohol as a solvent. Examples of alcohols used for saponification include lower alcohols such as methanol and ethanol, with methanol being preferred. The solvent used for saponification may contain other solvents such as acetone, methyl acetate, ethyl acetate, benzene, etc., for example in proportions of up to 40% by weight of its weight. Catalysts used for saponification are, for example, alkali metal hydroxides such as potassium hydroxide and sodium hydroxide, alkali catalysts such as sodium methylate, and acid catalysts such as mineral acids. Although the temperature for saponification is not limited, it is preferably within the range of 20 to 60°C. When a gel-like product precipitates as the saponification progresses, the product can be pulverized, washed and dried to obtain PVA. The saponification method is not limited to the methods described above, and known methods can be applied.

The lower limit of the content of PVA (A) in the PVA film is preferably 60% by mass, more preferably 70% by mass, even more preferably 80% by mass, and even more preferably 90% by mass. On the other hand, the upper limit of this content is preferably 99% by mass, more preferably 95% by mass. By setting the content of PVA (A) within the above range, the polarizing performance of the obtained polarizing film can be further enhanced.

(Polymer (B))
Polymer (B) has a structural unit (i) derived from maleic anhydride. Structural unit (i) is partially salified. Salification means that a salt is formed by reaction with a base. Of the structural units (i) derived from maleic anhydride, those that are not salified are usually structural units represented by the following formula (1). Of the structural units (i) derived from maleic anhydride, examples of salified structural units include structural units represented by the following formulas (2) and (3).

The structural unit represented by formula (2) is an example of structural unit (i) that forms a salt by reacting with ammonia, which is a base. As in the structural unit represented by formula (2), one of the two carboxy groups may not form a salt. The structural unit represented by formula (3) is an example of structural unit (i) in which a salt is formed by reaction with a base containing sodium ions such as sodium hydroxide. Other chlorinated structural units (i) include organic bases (aniline, pyridine, triethylamine, diethylamine, cyclohexylamine, etc.), alkali metal hydroxides other than sodium, alkaline earth metal hydroxides, and the like. Examples include those that are chlorinated by reaction. When reacting with a base containing a divalent cation such as Mg ²⁺ , the salified structural unit (i) may form a ring structure with two carboxy groups and the divalent cation. Structural unit (i) may be salified with two or more bases.

The lower limit of the salification rate in structural unit (i) is 0.3, preferably 0.4, more preferably 0.5, and even more preferably 0.6. Compatibility with PVA (A), water solubility and the like can be enhanced by setting the salification rate of the structural unit (i) to the above lower limit or higher. On the other hand, the upper limit of the chloride ratio in structural unit (i) is 0.9, preferably 0.8. When the chloride rate of the structural unit (i) is equal to or less than the above upper limit, crosslinkability with PVA (A) can be ensured, and a polarizing film having excellent polarizing performance can be obtained. That is, the carboxylic acid anhydride structure possessed by the structural unit represented by the above formula (1) effectively forms a crosslinked structure with PVA (A) by heating, for example, and PVA (A) is pseudo-high. It is polymerized. By producing a polarizing film using such a PVA film, a polarizing film having excellent polarizing performance can be obtained. Further, in an aqueous solution in which PVA (A) and polymer (B) are mixed, thickening over time is suppressed as compared with an aqueous solution of high-polymerization-degree PVA in which the same level of polarizing performance is exhibited. Although the cause of this is not clear, when the hydroxyl groups of PVA (A) are hydrated, the nucleophilicity of the hydroxyl groups of PVA (A) decreases, and the structural unit represented by the above formula (1) and This is probably because the reaction of PVA (A) is suppressed.

In addition, the "salification rate" means the ratio (mole ratio).

Part of the structural unit (i) preferably forms a salt with ammonia or an organic base, more preferably forms a salt with ammonia. Specifically, structural unit (i) preferably includes a structural unit represented by formula (2) above. When the polymer (B) has such a structural unit (i), the compatibility with the PVA (A) is enhanced, and a highly transparent PVA film can be obtained.

Structural unit (i) is an unmodified structural unit such as the structural unit represented by the above formula (1), and a salified structural unit such as the structural units represented by the above formulas (2) and (3) It may contain other structural units other than the structural unit in the group. Examples of other structural units in such structural unit (i) include structural units in which the structural unit represented by the above formula (1) is modified with a substance other than a base. However, it is preferable that the structural unit (i) is substantially composed of the structural unit represented by the above formula (1) and a salified structural unit. Among the structural units (i), the total content of the structural unit represented by the above formula (1) and the salified structural unit is preferably 80 mol% or more, more preferably 90 mol% or more, and 99 More preferably mol% or more.

The lower limit of the content of the structural unit (i) with respect to the total structural units of the polymer (B) is preferably 10 mol%, more preferably 30 mol%, still more preferably 40 mol%, and even more preferably 45 mol%. . On the other hand, the upper limit of the content of structural unit (i) is preferably 90 mol%, more preferably 70 mol%, even more preferably 60 mol%, and even more preferably 55 mol%. When the content of the structural unit (i) in the polymer (B) is within the above range, compatibility with PVA (A), crosslinkability, water solubility, productivity and the like are further improved.

The polymer (B) preferably further has a structural unit (ii) derived from an α-olefin. By using such a polymer (B), the polarizing performance and the like of the obtained polarizing film can be further enhanced.

Examples of α-olefins forming the structural unit (ii) include α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, α-butylene, isobutylene, 1-pentene and 2-methyl-1-butene. Among these, α-olefins having 2 to 6 carbon atoms are preferred, α-olefins having 3 to 5 carbon atoms are more preferred, and isobutylene (also referred to as isobutene, 2-methylpropene, etc.) is particularly preferred.

The lower limit of the content of the structural unit (ii) with respect to the total structural units of the polymer (B) is preferably 10 mol%, more preferably 30 mol%, even more preferably 40 mol%, and even more preferably 45 mol%. . On the other hand, the upper limit of the content of structural unit (ii) is preferably 90 mol%, more preferably 70 mol%, even more preferably 60 mol%, and even more preferably 55 mol%. When the content of the structural unit (ii) in the polymer (B) is within the above range, compatibility with PVA (A), crosslinkability, water solubility, productivity and the like are further improved.

Polymer (B) is preferably an alternating copolymer of structural unit (i) and structural unit (ii). In such a case, the dispersibility of the polymer (B) is increased, and a highly uniform crosslinked structure can be formed with the PVA (A), so that the polarizing performance of the resulting polarizing film can be further enhanced.

The polymer (B) may further have structural units other than the structural units (i) and (ii). However, the upper limit of the content of other structural units to the total structural units of the polymer (B) is preferably 20 mol %, and more preferably 10 mol %, 3 mol % or 1 mol % in some cases. The polymer (B) is preferably a copolymer consisting essentially of structural units (i) and structural units (ii).

The lower limit of the weight average molecular weight of the polymer (B) is preferably 5,000, more preferably 10,000, even more preferably 20,000. When the weight average molecular weight of the polymer (B) is at least the above lower limit, the effect of improving the polarizing performance by cross-linking with the PVA (A) is more sufficiently obtained. On the other hand, the upper limit of this weight average molecular weight is preferably 200,000, more preferably 170,000, and even more preferably 150,000. When the weight average molecular weight of the polymer (B) is equal to or less than the above upper limit, compatibility with PVA (A), water solubility, etc. are improved, and the transparency, etc. of the PVA film is improved.

Polymer (B) can be synthesized by a known method. For example, homopolymerization of maleic anhydride by a known method, or copolymerization of maleic anhydride and an α-olefin by a known method, followed by reaction of the resulting polymer with a predetermined amount of base. can be done.

A commercial item can also be used as a polymer (B). Commercially available products of the polymer (B) include Isovan (registered trademark)-104 and -110 manufactured by Kuraray Co., Ltd. The polymer (B) can also be obtained by reacting a commercially available product such as a copolymer of maleic anhydride and α-olefin with a base. Examples of commercially available copolymers of maleic anhydride and α-olefin include Isovan (registered trademark)-04, -06, -10 and -18 manufactured by Kuraray Co., Ltd.

The lower limit of the content of polymer (B) to 100 parts by mass of PVA (A) is 0.5 parts by mass, preferably 1 part by mass, more preferably 2 parts by mass, and even more preferably 3 parts by mass. On the other hand, the upper limit of this content is 9.5 parts by mass, preferably 8 parts by mass, more preferably 6 parts by mass. By setting the content of the polymer (B) within the above range, a polarizing film having excellent polarizing performance can be obtained. Specifically, by making the content of the polymer (B) equal to or higher than the above lower limit, the polarizing performance of the obtained polarizing film is improved. On the other hand, by setting the content of the polymer (B) to the above lower limit or less, stretchability and breakage resistance are enhanced, and a polarizing film having excellent polarizing performance can be obtained by sufficient stretching.

The lower limit of the total content of PVA (A) and polymer (B) in the PVA film may be preferably 90% by mass, more preferably 95% by mass, and even more preferably 98% by mass. be.

(Plasticizer)
The PVA film can further contain a plasticizer. By including a plasticizer in the PVA film, it is possible to improve the handleability and stretchability. Preferred plasticizers include polyhydric alcohols, and specific examples include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, trimethylolpropane and the like. One or more of these plasticizers can be used. Among these, glycerin is preferable from the viewpoint of the effect of improving stretchability.

The lower limit of the content of the plasticizer in the PVA film is preferably 1 part by mass, more preferably 3 parts by mass, and even more preferably 5 parts by mass with respect to 100 parts by mass of PVA. By making the content of the plasticizer equal to or higher than the above lower limit, stretchability is further improved. On the other hand, the upper limit of this content is preferably 20 parts by mass, more preferably 17 parts by mass, and even more preferably 15 parts by mass. By making the content of the plasticizer equal to or less than the above upper limit, it is possible to prevent the PVA film from becoming too flexible or bleeding out of the plasticizer to the surface, thereby reducing handleability.

The lower limit of the total content of PVA (A), polymer (B) and plasticizer in the PVA film is preferably 90% by mass, more preferably 95% by mass, and even more preferably 98% by mass. be.

(other additives, etc.)
In addition to the PVA (A), the polymer (B) and the plasticizer, the PVA film further contains a filler, a processing stabilizer such as a copper compound, a weather stabilizer, a colorant, an ultraviolet absorber, a light stabilizer, Antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, antifoaming agents, deodorants, extenders, release agents, release agents, reinforcing agents, cross-linking agents, antifungal agents, Other additives such as preservatives, crystallization rate retardants and the like can be appropriately blended as needed.

However, the upper limit of the content of additives other than PVA (A), polymer (B) and plasticizer in the PVA film may be preferably 1% by mass, more preferably 0.2% by mass. Sometimes. When the content of other additives exceeds the above upper limit, the strength, optical properties, etc. of the PVA film may be affected.

The breaking stress of the PVA film in a 4% by mass boric acid aqueous solution at 50° C. is preferably 8 MPa or more, more preferably 15 MPa or more, further preferably 20 MPa or more, and 25 MPa or more. is even more preferable. In addition, the breaking strain of the PVA film in a 4 mass% boric acid aqueous solution at 50 ° C. is preferably 150% or more, more preferably 200% or more, and further preferably 300% or more, 400% or more is even more preferable. When the breaking stress and breaking strain of the PVA film are at least the above lower limits, stretchability is improved, and a polarizing film with more excellent polarizing performance can be obtained. The upper limit of the breaking stress may be, for example, 100 MPa or 70 MPa. Moreover, the upper limit of the breaking strain may be 1000% or 700%. The above breaking stress and breaking strain are values measured at a distance between chucks of 30 mm and a tension rate of 240%/min.

The degree of stress increase of the PVA film when the strain is 300% in a 4 mass% boric acid aqueous solution at 50 ° C. is preferably 0.04 MPa / % or more, more preferably 0.05 MPa / % or more, and 0.06 MPa. It is particularly preferable that it is above. When the degree of stress increase is at least the above lower limit, the reaction between the PVA (A) and the polymer (B) is particularly sufficient, so that a polarizing film with higher polarizing performance can be obtained. On the other hand, the degree of stress increase at a strain of 300% is preferably 0.3 MPa/% or less, more preferably 0.2 MPa/% or less, and even more preferably 0.1 MPa/% or less. When the degree of increase in stress at a strain of 300% is equal to or less than the above upper limit, stretchability increases and sufficient stretching can be performed, thereby improving the productivity of the polarizing film. The stress increase at 300% strain is a value calculated from a stress-strain curve measured at a chuck distance of 30 mm and a tension rate of 240%/min.

The breaking stress, breaking strain and degree of increase in stress of the PVA film in a 4% by mass boric acid aqueous solution at 50° C. can be adjusted by, for example, the content of the polymer (B). For example, when the content of the polymer (B) is decreased, the breaking stress and breaking strain tend to increase and the degree of stress increase tends to decrease.

The lower limit of the degree of swelling of the PVA film is preferably 160%, more preferably 170%, and even more preferably 180%. When the degree of swelling is equal to or higher than the above lower limit, it is possible to suppress the extreme progress of crystallization and to stably stretch the film up to a high draw ratio. On the other hand, the upper limit of the degree of swelling is preferably 260%, more preferably 240%, and even more preferably 220%. When the degree of swelling is equal to or less than the above upper limit, dissolution during stretching is suppressed, making it possible to stretch even under high-temperature conditions. The degree of swelling of the PVA film is the weight of the PVA film when it is immersed in distilled water at 30°C for 30 minutes, and the weight of the PVA film after being immersed in distilled water at 30°C for 30 minutes and then dried at 105°C for 16 hours. means the percentage of the value obtained by dividing by

The upper limit of the average thickness of the PVA film is preferably 60 µm, more preferably 45 µm, and still more preferably 30 µm. When the average thickness of the PVA film is equal to or less than the above upper limit, a thin polarizing film can be obtained. On the other hand, the lower limit of the average thickness is preferably 5 μm, more preferably 10 μm, still more preferably 15 μm, and even more preferably 20 μm. When the average thickness of the PVA film is at least the above lower limit, the strength of the obtained polarizing film can be increased.

The width of the PVA film is not particularly limited, and can be determined according to the use of the polarizing film to be manufactured. In recent years, the screen size of liquid crystal televisions and liquid crystal monitors has been increasing. Therefore, it is preferable to use the PVA film with a width of 3 m or more. On the other hand, if the width of the PVA film is too large, it tends to be difficult to uniformly uniaxially stretch the film when producing a polarizing film using an apparatus that is in practical use. Therefore, the width of the PVA film is preferably 7 m or less.

The PVA film is suitable as a raw film (or material film) used for producing stretched films such as polarizing films and retardation films. A stretched film is obtained by stretching the PVA film.

The PVA film may be a single layer film or a multilayer film (laminate). Examples of the form of the multilayer film include a film having a PVA layer formed on a thermoplastic resin film by a coating method or the like. A single-layer film is preferable from the viewpoints of more pronounced effects of the present invention, cumbersome lamination (coating, etc.) work, cost of the thermoplastic resin film, and the like.

<Method for producing polyvinyl alcohol film>
The method for producing the PVA film of the present invention comprises
Step (1) of forming a coating film containing PVA (A) and polymer (B), and step (2) of heat-treating the coating film
Prepare.

The content of the polymer (B) with respect to 100 parts by mass of the PVA (A) in the coating film obtained in the step (1) is 0.5 parts by mass or more and 9.5 parts by mass or less. The salification rate in structural units derived from maleic acid is 0.3 or more and 0.9 or less. Specific aspects and preferred aspects of PVA (A) and polymer (B) contained in the coating film obtained in step (1) are the same as PVA (A) and polymer (B) contained in the PVA film described above. is.

In step (1), for example, PVA (A) and polymer (B), and if necessary, one or more of plasticizers, other additives, surfactants described later, etc. It can be carried out by forming a film using a film-forming undiluted solution dissolved in a liquid medium.

Examples of liquid media include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, diethylenetriamine, and the like. be able to. One or more of these liquid media can be used. Among these, water is preferable from the viewpoint of load on the environment and recoverability.

The volatile content of the film-forming stock solution (the content of volatile components such as liquid media that are removed by volatilization or evaporation during film-forming) in the film-forming stock solution varies depending on the film-forming method, film-forming conditions, etc. In general, the lower limit is preferably 50% by mass, more preferably 60% by mass. When the volatile content of the membrane-forming stock solution is at least the above lower limit, the viscosity of the membrane-forming stock solution does not become too high, filtration and defoaming are performed smoothly during preparation of the membrane-forming stock solution, and the PVA film has few foreign substances and defects. facilitating the manufacture of On the other hand, the upper limit of the volatile content is preferably 95% by mass, more preferably 90% by mass. When the volatile content of the membrane-forming stock solution is equal to or less than the above upper limit, the concentration of the membrane-forming stock solution does not become too low, thereby facilitating the industrial production of the PVA film.

The membrane-forming stock solution preferably contains a surfactant. Inclusion of a surfactant improves the film-forming property, suppresses the occurrence of unevenness in the thickness of the PVA film, and facilitates the peeling of the film from the metal roll or belt used for film-forming. When a PVA film is produced from a film-forming stock solution containing a surfactant, the PVA film may contain the surfactant. The type of the surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the viewpoint of releasability from metal rolls and belts.

Examples of anionic surfactants include carboxylic acid type such as potassium laurate; polyoxyethylene lauryl ether sulfate, sodium alkyl sulfate, potassium alkyl sulfate, ammonium alkyl sulfate, triethanolamine alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate , sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, octyl sulfate, etc.; sodium alkylsulfonate, potassium alkylsulfonate, ammonium alkylsulfonate, triethanolamine alkylsulfonate, alkylbenzene sulfone sodium phosphate, disodium dodecyldiphenyl ether disulfonate, sodium alkylnaphthalenesulfonate, disodium alkylsulfosuccinate, disodium polyoxyethylene alkylsulfosuccinate, dodecylbenzenesulfonate, sulfonic acid type; sodium alkyl phosphate, alkyl phosphate Phosphorus such as potassium ester, ammonium alkyl phosphate, triethanolamine alkyl phosphate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxypropylene alkyl ether phosphate, sodium polyoxyethylene alkylphenyl ether phosphate Acid ester type and the like are preferable.

Examples of nonionic surfactants include alkyl ether types such as polyoxyethylene oleyl ether; alkylphenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; Alkylamine 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; Polyoxyalkylene Allyl phenyl ether types such as allyl phenyl ether are preferred.

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

When the membrane-forming stock solution or the obtained PVA film contains a surfactant, the lower limit of the content is preferably 0.01 part by mass with respect to 100 parts by weight of PVA (A) contained in the membrane-forming stock solution or the PVA film. , more preferably 0.02 parts by mass, and even more preferably 0.05 parts by mass. When the content of the surfactant is at least the above lower limit, film formability and peelability are further improved. On the other hand, the upper limit of this content is preferably 0.5 parts by mass, more preferably 0.3 parts by mass, and even more preferably 0.1 parts by mass. When the content of the surfactant is equal to or less than the above upper limit, it is possible to prevent the surfactant from bleeding out to the surface of the PVA film to cause blocking and reduce handleability.

Examples of film-forming methods for film formation using the film-forming stock solution include cast film-forming methods, extrusion film-forming methods, wet film-forming methods, and gel film-forming methods. These film-forming methods may employ only one type or a combination of two or more types. 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.

In step (2), the coating film formed in step (1) is heat-treated. By this heat treatment, the PVA (A) and the polymer (B) in the coating film undergo a cross-linking reaction. By performing such a heat treatment, a PVA film can be obtained from which a polarizing film having excellent polarizing performance can be produced.

The lower limit of the heat treatment temperature in step (2) is preferably 100°C, more preferably 110°C, and even more preferably 120°C. On the other hand, the upper limit of the heat treatment temperature is preferably 180°C, more preferably 160°C, and even more preferably 140°C. This heat treatment can be performed using a hot air dryer or the like.

<Stretched film>
A stretched film according to one embodiment of the present invention is formed from a PVA film according to one embodiment of the present invention. The stretched film is preferably a stretched optical film such as a polarizing film, a retardation film, or a polarizing film. The stretched film may be uniaxially stretched or biaxially stretched, but is preferably uniaxially stretched.

Since the stretched film is obtained by stretching the PVA film according to one embodiment of the present invention, each component contained in the stretched film is the same as the component contained in the PVA film described above. good. However, the stretched film may further contain other components. For example, when the stretched film is a polarizing film, it usually has a dichroic dye or the like adsorbed on its front and back surfaces. Iodine dyes are generally used as dichroic dyes.

<Polarizing film>
A polarizing film according to one embodiment of the present invention is formed from the PVA film according to one embodiment of the present invention. The polarizing film can be obtained by adsorbing a dichroic dye on the front and back surfaces of the PVA film according to one embodiment of the present invention and uniaxially stretching the film, and is a form of stretched film.

As the polarizing performance of the polarizing film, the lower limit of the degree of polarization when the transmittance is 43.7% or more is preferably 99.90%, more preferably 99.92%, and 99.93%. is more preferable. When the degree of polarization is at least the above lower limit, the contrast of the LCD is improved when used in smartphones, notebook computers, liquid crystal televisions, car navigation systems, and the like.

The polarizing film is usually used as a polarizing plate by laminating an optically transparent protective film having mechanical strength on one or both sides thereof. As the protective film, a cellulose triacetate (TAC) film, a cellulose acetate-butyrate (CAB) film, an acrylic film, a polyester film, or the like is used. Moreover, as an adhesive for bonding, a PVA-based adhesive, an ultraviolet curing adhesive, and the like can be mentioned, and a PVA-based adhesive is preferable.

The polarizing plate obtained as described above may be laminated with a retardation film, a viewing angle improving film, a brightness improving film, or the like. The stretched film of the present invention can also be used as the retardation film. The polarizing plate can be used as an LCD component by being coated with an acrylic adhesive or the like and then pasted onto a glass substrate.

<Method for producing stretched film and polarizing film>
A stretched film according to an embodiment of the present invention can be obtained by a production method including the above-described step of stretching the PVA film (stretching treatment). Further, a polarizing film according to one embodiment of the present invention comprises the above-described step of adsorbing a dichroic dye to the PVA film (dyeing treatment) and the step of uniaxially stretching the PVA film (uniaxial stretching treatment). It can be obtained by a manufacturing method. A specific manufacturing method for manufacturing a polarizing film will be described below. In the case of producing a stretched film other than a polarizing film, it can be carried out in the same manner as in the case of producing a polarizing film, except that the dyeing treatment or the like is not carried out.

As a specific method for producing the polarizing film, the PVA film is subjected to a swelling treatment, a dyeing treatment, a uniaxial stretching treatment, and if necessary, a crosslinking treatment, a fixing treatment, a washing treatment, and a drying treatment. , heat treatment, and the like. In this case, the order of each treatment such as swelling treatment, dyeing treatment, cross-linking treatment, uniaxial stretching, and fixing treatment is not particularly limited, and two or more treatments can be performed simultaneously. Also, one or more of each treatment can be performed two or more times.

The swelling treatment can be performed by immersing the PVA film in water. The lower limit of the temperature of water when immersed in water is preferably 20°C, more preferably 22°C, and even more preferably 25°C. On the other hand, the upper limit of this temperature is preferably 40°C, more preferably 38°C, and even more preferably 35°C. Moreover, as a minimum of the time immersed in water, 0.1 minute is preferable and 0.5 minute is more preferable. On the other hand, the upper limit of this time is preferably 5 minutes, more preferably 3 minutes. The water used for immersion is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

The dyeing treatment can be performed by bringing a dichroic dye into contact with the PVA film. As dichroic dyes, iodine dyes and dyes are generally used. The timing of the dyeing treatment may be any stage before the uniaxial stretching treatment, during the uniaxial stretching treatment, or after the uniaxial stretching treatment. When an iodine dye is used, the dyeing treatment is generally carried out by immersing the PVA film in a solution (especially an aqueous solution) containing iodine-potassium iodide as a dyeing bath. The concentration of iodine in the dyeing bath is preferably 0.01% by mass or more and 0.5% by mass or less, and the concentration of potassium iodide is preferably 0.01% by mass or more and 10% by mass or less. Moreover, the lower limit of the temperature of the dyeing bath is preferably 20°C, more preferably 25°C. On the other hand, the upper limit of this temperature is preferably 50°C, more preferably 40°C.

By subjecting the PVA film to a cross-linking treatment, it is possible to effectively prevent elution of PVA into water during wet stretching at a high temperature. From this point of view, the cross-linking treatment is preferably performed before the uniaxial stretching treatment. The cross-linking treatment can be performed by immersing the PVA film in an aqueous solution containing a cross-linking agent. As the cross-linking agent, one or more of inorganic boron compounds such as borate salts such as boric acid and borax, organic boron compounds such as alkylmonoboronic acid and alkyldiboronic acid, and the like can be used. The lower limit of the concentration of the cross-linking agent in the aqueous solution containing the cross-linking agent is preferably 1% by mass, more preferably 2% by mass, and even more preferably 3% by mass. On the other hand, the upper limit of this concentration is preferably 15% by mass, more preferably 7% by mass, and even more preferably 6% by mass. Sufficient stretchability can be maintained when the concentration of the cross-linking agent is within the above range. The aqueous solution containing the cross-linking agent may contain an auxiliary agent such as potassium iodide. The lower limit of the temperature of the aqueous solution containing the cross-linking agent is preferably 20°C, more preferably 25°C. On the other hand, the upper limit of this temperature is preferably 60°C, more preferably 50°C. By setting this temperature within the above range, efficient cross-linking can be achieved.

The uniaxial stretching treatment may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be carried out in an aqueous solution of boric acid, or in the dyeing bath described above or in the fixing treatment bath described later. In the case of the dry stretching method, the uniaxial stretching treatment may be performed at room temperature, the uniaxial stretching treatment may be performed while heating, or the uniaxial stretching treatment may be performed in the air using the PVA film after absorbing water. you can go Among these, the wet stretching method is preferred, and uniaxial stretching treatment in an aqueous boric acid solution is more preferred. The lower limit of the boric acid concentration of the boric acid aqueous solution is preferably 0.5% by mass, more preferably 1.0% by mass, and even more preferably 1.5% by mass. On the other hand, the upper limit of the boric acid concentration is preferably 6.0% by mass, more preferably 5.0% by mass, and even more preferably 4.0% by mass. Further, the boric acid aqueous solution may contain potassium iodide, and its concentration is preferably 0.01% by mass or more and 10% by mass or less.

The lower limit of the stretching temperature in the uniaxial stretching treatment is preferably 30°C, more preferably 40°C, and even more preferably 50°C. On the other hand, the upper limit of the stretching temperature is preferably 90°C, more preferably 80°C, and even more preferably 70°C.

The lower limit of the draw ratio in the uniaxial stretching treatment is preferably 5 times, more preferably 6 times, from the viewpoint of the polarizing performance of the obtained polarizing film. Although the upper limit of the draw ratio is not particularly limited, it is preferably 10 times, and more preferably 8 times, for example.

There is no particular limitation on the direction of uniaxial stretching when a long PVA film is uniaxially stretched. A uniaxial stretching treatment in the longitudinal direction, a transverse uniaxial stretching treatment, or a so-called oblique stretching treatment can be employed, but the uniaxial stretching treatment in the longitudinal direction is preferable because a polarizing film having excellent polarizing performance can be obtained. The uniaxial stretching treatment in the longitudinal direction can be performed by using a stretching device having a plurality of rolls parallel to each other and changing the peripheral speed between the rolls. On the other hand, transverse uniaxial stretching can be performed using a tenter type stretching machine.

In the production of the polarizing film, it is preferable to carry out a fixing treatment after the uniaxial stretching treatment in order to strengthen the adsorption of the dichroic dye (such as an iodine dye) to the PVA film. As the fixing treatment bath used for the fixing treatment, an aqueous solution containing one or more of inorganic boron compounds such as boric acid and borax, organic boron compounds such as alkylboronic acid and alkyldiboronic acid, and the like can be used. . Also, if necessary, an iodine compound or a metal compound may be added to the fixing treatment bath. The lower limit of the concentration of the inorganic boron compound in the fixing treatment bath is preferably 2% by mass, more preferably 3% by mass. On the other hand, the upper limit of this concentration is preferably 15% by mass, more preferably 10% by mass. Adsorption of the dichroic dye can be made stronger by adjusting the concentration within the above range. The lower limit of the temperature of the fixing treatment bath is preferably 15°C. On the other hand, the upper limit of this temperature is preferably 60°C, more preferably 40°C.

The washing treatment is generally carried out by immersing the PVA film in water or the like. At this time, from the viewpoint of improving the polarizing performance, it is preferable that the water or the like used for the cleaning treatment contains an auxiliary agent such as potassium iodide. At this time, the concentration of iodide such as potassium iodide is preferably 0.5% by mass or more and 10% by mass or less. Also, the lower limit of the temperature of water or the like used in the cleaning treatment is generally 5°C, preferably 10°C, and more preferably 15°C. On the other hand, the upper limit of this temperature is generally 50°C, preferably 45°C, more preferably 40°C.

The conditions for the drying treatment are not particularly limited, but the lower limit of the drying temperature is preferably 30°C, more preferably 50°C. On the other hand, the upper limit of the drying temperature is preferably 150°C, more preferably 130°C. By drying at a temperature within the above range, it is easy to obtain a polarizing film with excellent dimensional stability.

By performing a heat treatment after the drying treatment, it is possible to obtain a polarizing film with even better dimensional stability. Here, the heat treatment means a treatment for further heating a polarizing film having a moisture content of 5% or less after drying to improve the dimensional stability of the polarizing film. The heat treatment conditions are not particularly limited, but the heat treatment is preferably performed within the range of 60° C. or more and 150° C. or less. If the heat treatment is performed at a temperature lower than 60°C, the dimensional stabilization effect due to the heat treatment is insufficient. On the other hand, if the heat treatment is performed at a temperature higher than 150° C., the polarizing film may be severely yellowed.

<Other embodiments>
The PVA film, method for producing a PVA film, stretched film, and polarizing film of the present invention are not limited to the above embodiments. For example, although the stretched film is mainly a polarizing film, the stretched film is not limited to a polarizing film. For example, stretched films other than polarizing films, such as retardation films, are also within the scope of the present invention. The PVA film of the present invention may also be used for applications other than stretched film materials such as polarizing films.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples. In addition, each measurement or evaluation method adopted in the following examples and comparative examples is shown below.

[Appearance of membrane-forming stock solution]
Appearance such as transparency of the prepared membrane-forming stock solution was visually evaluated.

[Appearance of PVA film]
The produced PVA film was visually evaluated for appearance such as transparency.

[Breaking tension, breaking strain and stress increment of PVA film]
The breaking stress and breaking strain in a 4% by mass boric acid aqueous solution at 50° C. were measured for the produced PVA film. The distance between chucks was set to 30 mm, the tensile speed was set to 240%/min, and the tensile test was started after 60 seconds of immersion in the boric acid aqueous solution. In addition, the stress-strain curve obtained from the measurement was subjected to differential analysis using Microsoft Excel spreadsheet software, and a moving average of 25-point intervals was performed to calculate the degree of increase in stress at a strain of 300%.

[Optical properties of polarizing film]
In the following examples and comparative examples, a rectangular sample of 4 cm in the length direction of the polarizing film and 2 cm in the width direction was taken from the central part of the obtained polarizing film in the width direction and the length direction, and the spectrophotometer with an integrating sphere was used. Measure the parallel transmittance and cross Nicol transmittance of the polarizing film using a V-7100 (manufactured by JASCO Corporation) and an automatic polarizing film measurement device VAP-7070S (manufactured by JASCO Corporation) equipped with a Gran-Taylor polarizer. bottom. Here, the measurement wavelength range is set to 380 to 780 nm, and the transmittance when the vibration direction of the polarized light incident on the polarizing film through the Glan-Taylor polarizer is parallel to the transmission axis of the polarizing film is the parallel transmittance of the polarizing film. The crossed Nicols transmittance was defined as the case perpendicular to the transmission axis. After that, using the "polarizing film evaluation program" (manufactured by JASCO Corporation), using the above-mentioned parallel transmittance and cross Nicol transmittance so as to comply with JIS Z 8722 (object color measurement method), C Visibility correction was performed in the visible light region of the light source and 2° field of view, and the single transmittance and degree of polarization of the polarizing film were calculated, and these two values were obtained as the optical properties of the polarizing film.

The polymer (B) and the like used in Examples and Comparative Examples are shown below.
Polymer (B-1): "Isovan-104" manufactured by Kuraray (ammonia-modified alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chloride rate 0.7, weight average molecular weight 55,000-65,000)
Polymer (B-2): "Isovan-110" manufactured by Kuraray (ammonia-modified alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chloride rate 0.7, weight average molecular weight 160,000 to 170,000)
Polymer (b-1): "Isovan-04" manufactured by Kuraray (alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chloride ratio 0, weight average molecular weight 55,000 to 65 ,000)
Polymer (b-2): polyacrylic acid (manufactured by Wako Pure Chemical Industries, Ltd., molecular weight of about 150,0000, aqueous solution with polyacrylic acid concentration of 25%)

[Example 1]
(1) 100 parts by mass of PVA (saponified product of homopolymer of vinyl acetate, degree of saponification 99.9 mol%, degree of polymerization 2400), 5 parts by mass of polymer (B-1), 10 parts by mass of glycerin as a plasticizer , and 0.1 part by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant. This film-forming stock solution was dried on a metal roll at 80° C., and the resulting coating film was heat-treated at 128° C. for 10 minutes in a hot air dryer to obtain a PVA film having an average thickness of 30 μm.
By the methods described above, the appearance of the prepared membrane-forming stock solution, and the appearance, breaking stress, breaking strain, and degree of stress increase of the produced PVA film were evaluated or measured. These results are shown in Table 1.
In addition, the prepared film-forming stock solution did not increase in viscosity over time, and had good film-forming properties.

(2) A sample of 5 cm wide by 9 cm long was cut from the central portion of the PVA film obtained in (1) in the width direction so that a range of 5 cm wide by 5 cm long could be uniaxially stretched. This sample was uniaxially stretched 1.1 times in the length direction while being immersed in distilled water at 30°C. Subsequently, an aqueous solution containing 100 parts by mass of potassium iodide to 1 part by mass of iodine while being uniaxially stretched 2.2 times (2.4 times in total) in the length direction (dyeing treatment bath) (Temperature 30° C.) for 60 seconds to adsorb iodine. At this time, the iodine concentration of the dyeing treatment bath was appropriately adjusted so that the obtained polarizing film had a transmittance Ts (%) of 43.7%. Next, while being immersed in an aqueous solution (crosslinking treatment bath) containing 3% by mass of boric acid and 3% by mass of potassium iodide (at a temperature of 30° C.), the lengthwise length is increased by 1.2 times (2.0% in total). 7 times) was uniaxially stretched. Furthermore, while being immersed in an aqueous solution (uniaxial stretching treatment bath) containing 4% by mass of boric acid and 6% by mass of potassium iodide, the total stretching ratio is 6.0 times in the length direction (total stretching ratio) (temperature 60). ° C.) was uniaxially stretched. After that, it was immersed for 5 seconds in an aqueous solution (cleaning treatment bath) containing 3.5% by mass of potassium iodide (temperature: 30°C). Finally, it was dried at 60°C for 4 minutes to obtain a polarizing film.
The polarizing performance (transmittance and degree of polarization) of the obtained polarizing film was evaluated by the method described above. These results are shown in Table 1.

[Example 2]
In the same manner as in Example 1, except that the content of the polymer (B-1) with respect to 100 parts by mass of PVA was 2 parts by mass and the heat treatment temperature was 127 ° C., the film-forming stock solution was prepared, and the PVA film and A polarizing film was produced. In addition, in each example and comparative example, each heat treatment temperature was adjusted so that the degree of swelling of the PVA film was 200%.
By the above-described methods, the appearance of the film-forming stock solution, the appearance of the PVA film, the breaking stress, the breaking strain, the degree of stress increase, and the polarizing performance (transmittance and degree of polarization) of the polarizing film were evaluated or measured. These results are shown in Table 1.

[Example 3]
In the same manner as in Example 1, except that the polymer (B-2) was used instead of the polymer (B-1) and the heat treatment temperature was set to 129° C., the film-forming stock solution was prepared, and the PVA film and A polarizing film was produced.
By the above-described methods, the appearance of the film-forming stock solution, the appearance of the PVA film, the breaking stress, the breaking strain, the degree of stress increase, and the polarizing performance (transmittance and degree of polarization) of the polarizing film were evaluated or measured. These results are shown in Table 1.

[Comparative Example 1]
In the same manner as in Example 1, except that the content of the polymer (B-1) with respect to 100 parts by mass of PVA was 10 parts by mass and the heat treatment temperature was 135 ° C., the film-forming stock solution was prepared, and the PVA film and A polarizing film was produced. Although a film-forming stock solution and a PVA film were obtained, the film was broken by stretching during preparation of the polarizing film, and no polarizing film was obtained.
By the methods described above, the appearance of the undiluted film-forming solution and the appearance, breaking stress, breaking strain and degree of stress increase of the PVA film were evaluated or measured. These results are shown in Table 1.

[Comparative Example 2]
A film-forming stock solution was prepared and a PVA film and a polarizing film were prepared in the same manner as in Example 1, except that the polymer (B-1) was not contained and the heat treatment temperature was set to 120°C.
By the methods described above, the appearance of the film-forming stock solution, the appearance of the PVA film, the breaking stress, the breaking strain and the degree of stress increase, and the polarizing performance (transmittance and degree of polarization) of the polarizing film were evaluated or measured. These results are shown in Table 1.

[Comparative Example 3]
A membrane-forming stock solution was prepared in the same manner as in Example 1, except that the polymer (b-1) was used instead of the polymer (B-1). However, since it was determined that a white precipitate was formed in the film-forming stock solution and a polarizing film could not be obtained, a PVA film and a polarizing film were not produced.

[Comparative Example 4]
In the same manner as in Example 1, except that the polymer (B-2) was used instead of the polymer (B-1) and the heat treatment temperature was set to 120° C., the film-forming stock solution was prepared, and the PVA film and A polarizing film was produced.
The appearance of the film-forming stock solution, the appearance of the PVA film, and the polarizing performance (transmittance and degree of polarization) of the polarizing film were evaluated or measured by the methods described above. These results are shown in Table 1.

As is clear from the above results, the polarizing films obtained from the PVA films of Examples 1 to 3 have a high degree of polarization and excellent polarizing performance. On the other hand, in Comparative Examples 1 to 4, polarizing films with excellent polarizing performance could not be obtained.

The PVA film of the present invention can be suitably used as a material such as a polarizing film, which is a constituent material of LCD.

Claims (11)

Polyvinyl alcohol (A) and a polymer (B) having structural units derived from maleic anhydride,
The content of the polymer (B) with respect to 100 parts by mass of the polyvinyl alcohol (A) is 0.5 parts by mass or more and 9.5 parts by mass or less,
A polyvinyl alcohol film in which the structural unit derived from maleic anhydride in the polymer (B) has a salification rate of 0.3 or more and 0.9 or less. 2. The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol (A) and the polymer (B) form a crosslinked structure. 3. The polyvinyl alcohol film according to claim 1, wherein the polymer (B) further has a structural unit derived from an α-olefin. 4. The polyvinyl alcohol film according to claim 3, wherein the polymer (B) is an alternating copolymer of structural units derived from maleic anhydride and structural units derived from α-olefin. 5. The polyvinyl alcohol film according to any one of claims 1 to 4, wherein part of the structural units derived from maleic anhydride in the polymer (B) form a salt with ammonia or an organic base. The polyvinyl alcohol film according to any one of claims 1 to 5, wherein the polymer (B) has a weight average molecular weight of 5,000 or more and 200,000 or less. The polyvinyl alcohol film according to any one of claims 1 to 6, which has a breaking stress of 8 MPa or more and a breaking strain of 150% or more in a 4% by mass boric acid aqueous solution at 50°C. The polyvinyl alcohol according to any one of claims 1 to 7, which has a stress increase of 0.04 MPa/% or more and 0.3 MPa/% or less when the strain is 300% in an aqueous 4% by mass boric acid solution at 50°C. the film. A step of forming a coating film containing polyvinyl alcohol (A) and a polymer (B) having a structural unit derived from maleic anhydride, and a step of heat-treating the coating film,
The content of the polymer (B) with respect to 100 parts by mass of the polyvinyl alcohol (A) in the coating film is 0.5 parts by mass or more and 9.5 parts by mass or less,
A method for producing a polyvinyl alcohol film, wherein the structural unit derived from maleic anhydride in the polymer (B) has a salification rate of 0.3 or more and 0.9 or less. A stretched film formed from the polyvinyl alcohol film according to any one of claims 1 to 8. A polarizing film formed from the polyvinyl alcohol film according to any one of claims 1 to 8.