Polyvinyl alcohol film, method for producing polyvinyl alcohol film, stretched film, and polarized film

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]

[0001] The present invention relates to a polyvinyl alcohol film, a method for producing a polyvinyl alcohol film, a stretched film, and a polarizing film. [Background technology]

[0002] Polarizing plates, which have the function of transmitting and blocking light, are the basic components of liquid crystal displays (LCDs), along with liquid crystals, which change the polarization state of light. Many polarizing plates have a structure in which a protective film such as a triacetate cellulose (TAC) film is laminated to the surface of the polarizing film to prevent discoloration and shrinkage of the polarizing film. The polarizing film that makes up a polarizing plate is a stretched film made by uniaxially stretching a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be referred to as "PVA") and adding an iodine-based dye (I3 - or I5 - The mainstream is those that adsorb (e.g.,

[0003] LCDs are used in a wide range of applications, including small devices such as calculators and wristwatches, smartphones, laptops, LCD monitors, LCD color projectors, LCD televisions, in-vehicle navigation systems, and measuring instruments used indoors and outdoors. For these various applications, the development of polarizing films with high polarization performance is anticipated. Patent Document 1 describes a polarizing film that uses PVA with a degree of polymerization of 2600 or more, as a polarizing film with excellent durability and a high polarization degree. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 1-84203 Summary of the Invention [Problem to be solved by the invention]

[0005] As in the polarizing film described in Patent Document 1, the use of PVA with a high degree of polymerization tends to improve the polarization performance. However, when a PVA with a high degree of polymerization is used, productivity may be affected due to factors such as an increase in viscosity of the film-forming solution. Therefore, it is hoped that the polarization performance of polarizing films can be improved by methods other than increasing the polymerization degree of PVA.

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a polyvinyl alcohol film from which a polarizing film with excellent polarization performance can be obtained, a method for producing such a polyvinyl alcohol film, and a stretched film and a polarizing film using such a polyvinyl alcohol film. [Means for solving the problem]

[0007] The inventions made to solve the above problems are as follows. [1] A polyvinyl alcohol film comprising polyvinyl alcohol (A) and a polymer (B) having a structural unit derived from maleic anhydride, wherein the content of the polymer (B) per 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 chlorination rate of the structural unit derived from maleic anhydride in the polymer (B) is 0.3 to 0.9; [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 according to [1] or [2], wherein the polymer (B) further has a structural unit derived from an α-olefin; [4] The polyvinyl alcohol film according to [3], wherein the polymer (B) is an alternating copolymer of structural units derived from maleic anhydride and structural units derived from an α-olefin; [5] A polyvinyl alcohol film according to any one of [1] to [4], wherein a portion of the structural units derived from maleic anhydride in the polymer (B) forms a salt with ammonia or an organic base; [6] The polyvinyl alcohol film according to any one of [1] to [5], wherein the weight-average molecular weight of the polymer (B) is 5,000 or more and 200,000 or less; [7] A polyvinyl alcohol film according to 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 mass% boric acid aqueous solution at 50°C; [8] The polyvinyl alcohol film according to any one of [1] to [7], wherein the stress increase rate at a strain of 300% in a 4 mass% boric acid aqueous solution at 50°C is 0.04 MPa / % or more and 0.3 MPa / % or less; [9] A method for producing a polyvinyl alcohol film, comprising the steps of forming a coating film containing polyvinyl alcohol (A) and a polymer (B) having structural units derived from maleic anhydride, and heat-treating the coating film, wherein the content of polymer (B) in the coating film is 0.5 parts by mass or more and 9.5 parts by mass or less per 100 parts by mass of polyvinyl alcohol (A), and the chlorination rate of the structural units derived from maleic anhydride in the polymer (B) is 0.3 to 0.9;

[10] A stretched film formed from the polyvinyl alcohol film of any one of [1] to [8];

[11] [1] to [8] a polarizing film formed from a polyvinyl alcohol film; [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a polyvinyl alcohol film from which a polarizing film with excellent polarization performance can be obtained, a method for producing such a polyvinyl alcohol film, and a stretched film and a polarizing film using such a polyvinyl alcohol film. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Polyvinyl alcohol film> A polyvinyl alcohol film (PVA film) according to one embodiment of the present invention comprises PVA (A) and a polymer (B) having structural units derived from maleic anhydride, in which the content of the polymer (B) relative to 100 parts by mass of the PVA (A) is 0.5 parts by mass or more and 9.5 parts by mass or less, and the salification rate of the structural units derived from maleic anhydride in the polymer (B) is 0.3 to 0.9.

[0010] This PVA film can be used to produce a polarizing film with excellent polarization performance. The reason for this is believed to be that the polymer (B) having structural units derived from maleic anhydride, some of which are salified, has both compatibility with and crosslinkability with the PVA (A), allowing the PVA (A) and polymer (B) to form a favorable crosslinked structure in the film. Specifically, the salified structural units derived from maleic anhydride can enhance compatibility and water solubility with the PVA (A), while the non-salted structural units derived from maleic anhydride exhibit favorable crosslinking reactivity with the PVA (A). That is, in this PVA film, it is preferable that the PVA (A) and polymer (B) form a crosslinked structure. Furthermore, since the polymer (B) is usually substantially neutral, it is unlikely to affect the decomposition of dichroic dyes such as iodine-based dyes, which is believed to be another factor that allows the production of a polarizing film with excellent polarization performance. Each component is described in detail below.

[0011] (PVA(A)) PVA (vinyl alcohol polymer) (A) is usually the main component of a PVA film. The main component refers to the component with the largest content by mass. PVA (A) is a polymer having vinyl alcohol units (-CH2-CH(OH)-) as structural units. PVA (A) may have vinyl ester units or other structural units in addition to vinyl alcohol units.

[0012] 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. A degree of polymerization equal to or greater than the lower limit can improve the durability of the PVA film and the stretched film and polarized film obtained from this PVA film. Furthermore, a degree of polymerization equal to or greater than the lower limit tends to further enhance the polarization performance of the polarized film. 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. A degree of polymerization equal to or less than the upper limit can prevent increases in production costs and defects during film formation. The degree of polymerization of PVA refers to the average degree of polymerization measured in accordance with JIS K6726-1994.

[0013] The lower limit of the saponification degree of PVA (A) is preferably 95 mol%, more preferably 96 mol%, and even more preferably 98 mol%, in terms of the water resistance of the PVA film. Meanwhile, the upper limit of this saponification degree may be 100 mol%. The saponification degree of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of structural units (typically vinyl ester units) that can be converted to vinyl alcohol units by saponification and vinyl alcohol units. The saponification degree can be measured according to the description of JIS K6726-1994.

[0014] The method for producing PVA (A) is not particularly limited. For example, PVA (A) can be produced by (1) obtaining a vinyl ester polymer and (2) saponifying the obtained vinyl ester polymer, i.e., converting the vinyl ester units of the vinyl ester polymer into vinyl alcohol units.

[0015] The vinyl ester polymer may be a homopolymer consisting of only a vinyl ester monomer, or may be a copolymer of a vinyl ester monomer and another monomer copolymerizable therewith.

[0016] The vinyl ester monomer is not particularly limited, but examples thereof include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, etc. From an economical viewpoint, vinyl acetate is preferred among these.

[0017] 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 a salt thereof; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamidopropanesulfonic acid or a salt thereof, and (meth)acrylamidopropyldimethylamine or a salt thereof. N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as (meth)acrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid, or a salt, ester, or acid anhydride thereof; itaconic acid, or a salt, ester, or acid anhydride thereof; vinylsilyl compounds such as vinyltrimethoxysilane; and unsaturated sulfonic acids.

[0018] The vinyl ester polymer may have structural units derived from one or more of the other monomers. The other monomers may be present in the polymerization system beforehand when the vinyl ester monomer is subjected to the polymerization reaction, or may be added to the system during the polymerization reaction.

[0019] Among the other monomers, α-olefins are preferred, and ethylene is more preferred. When PVA (A) contains structural units derived from α-olefins, the stretchability and other properties of the PVA film are improved. When PVA (A) contains structural units derived from α-olefins, the lower limit of the content of the structural units derived from α-olefins is preferably 1 mol%, more preferably 2 mol%, based on the total 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 units derived from α-olefins is within the above range, the above-mentioned stretchability and other properties can be more effectively exhibited. Furthermore, the upper limit of the content of the structural units derived from other monomers based on the total structural units of the PVA may be preferably 4 mol%, more preferably 1 mol% or 0.1 mol%.

[0020] The polymerization method for polymerizing vinyl ester monomers may be any of batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization. Known polymerization methods, such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, can be used. Typically, bulk polymerization, in which polymerization proceeds without a solvent, or solution polymerization, in which polymerization proceeds in a solvent such as alcohol, is used. Emulsion polymerization is also preferred for obtaining vinyl ester copolymers with a high degree of polymerization. The solvent used in solution polymerization is not particularly limited, but alcohol is an example. Examples of alcohols used as solvents in solution polymerization include lower alcohols such as methanol, ethanol, and propanol. The amount of solvent used in the polymerization system can be selected based on the desired degree of polymerization of PVA, taking into account chain transfer of the solvent. For example, when the solvent is methanol, the mass ratio of the solvent to the total monomers contained in the polymerization system {=(solvent) / (total monomers)} is preferably selected within the range of 0.01 to 10, more preferably 0.05 to 3.

[0021] The polymerization initiator used to polymerize the vinyl ester monomer may be selected from known polymerization initiators, such as azo initiators, peroxide initiators, and redox initiators, depending on the polymerization method. Examples of azo initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Examples of peroxide initiators include percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanate and α-cumyl peroxyneodecanate; acetylcyclohexylsulfonyl peroxide; 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; and acetyl peroxide. Potassium persulfate, ammonium persulfate, hydrogen peroxide, or the like may be combined with the above initiators to form a polymerization initiator. Examples of redox initiators include those obtained by combining the above peroxide initiators with reducing agents such as sodium hydrogen sulfite, sodium hydrogen carbonate, tartaric acid, L-ascorbic acid, and Rongalite. The amount of polymerization initiator used varies depending on the type of polymerization initiator and cannot be determined in general terms, but it can be selected based on the polymerization rate. For example, when 2,2'-azobisisobutyronitrile or acetyl peroxide is used as the polymerization initiator, the amount is preferably 0.01 to 0.2 mol %, more preferably 0.02 to 0.15 mol %, relative to the vinyl ester monomer. The polymerization temperature is not particularly limited, but is suitably from room temperature to approximately 150°C, and preferably 40°C or higher and below the boiling point of the solvent used.

[0022] Polymerization of vinyl ester monomers 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; and phosphinates such as sodium phosphinate monohydrate. Among these, aldehydes and ketones are preferred. The amount of chain transfer agent used can be determined depending on the chain transfer coefficient of the chain transfer agent used and the desired degree of polymerization of PVA, but is generally preferably 0.1 to 10 parts by mass per 100 parts by mass of vinyl ester monomer.

[0023] Saponification of a vinyl ester polymer can be carried out, for example, with the vinyl ester polymer dissolved in an alcohol or aqueous alcohol 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, and benzene, for example, at a ratio of 40% by mass or less of the solvent itself. Catalysts used for saponification include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide, alkali catalysts such as sodium methylate, and acid catalysts such as mineral acids. The temperature at which saponification is carried out is not limited, but is preferably within the range of 20 to 60°C. If a gel-like product precipitates as the saponification proceeds, the product can be crushed, washed, and dried to obtain PVA. The saponification method is not limited to the above-described method; known methods can also be used.

[0024] 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 polarization performance of the resulting polarizing film can be further improved.

[0025] (Polymer (B)) The polymer (B) has structural units (i) derived from maleic anhydride. Some of the structural units (i) are salified. Salification refers to the formation of a salt by reaction with a base. Among the structural units (i) derived from maleic anhydride, those that are not salified are usually structural units represented by the following formula (1). Among the structural units (i) derived from maleic anhydride, those that are salified include structural units represented by the following formulas (2) and (3).

[0026] [ka]

[0027] The structural unit represented by formula (2) is an example of a structural unit (i) that has formed a salt by reaction with ammonia, which is a base. As in the structural unit represented by formula (2), one of the two carboxyl groups may not form a salt. The structural unit represented by formula (3) is an example of a structural unit (i) that has formed a salt by reaction with a base containing sodium ions, such as sodium hydroxide. Other examples of the salted structural unit (i) include those that have formed a salt by reaction with organic bases (aniline, pyridine, triethylamine, diethylamine, cyclohexylamine, etc.), hydroxides of alkali metals other than sodium, hydroxides of alkaline earth metals, etc. Note that Mg 2+ When reacted with a base containing a divalent cation, the salified structural unit (i) may form a ring structure with two carboxy groups and the divalent cation. Furthermore, the structural unit (i) may be salified with two or more bases.

[0028] The lower limit of the chlorination rate in the structural unit (i) is 0.3, preferably 0.4, more preferably 0.5, and even more preferably 0.6. When the chlorination rate of the structural unit (i) is equal to or greater than the lower limit, compatibility with PVA (A), water solubility, and the like can be improved. On the other hand, the upper limit of the chlorination rate in the structural unit (i) is 0.9, preferably 0.8. When the chlorination rate of the structural unit (i) is equal to or less than the upper limit, crosslinking with PVA (A) is ensured, resulting in a polarizing film with excellent polarizing performance. 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, for example, heating, thereby artificially increasing the degree of polymerization of PVA (A). When such a PVA film is used to produce a polarizing film, a polarizing film with excellent polarizing performance can be obtained. Furthermore, in an aqueous solution of a mixture of PVA (A) and polymer (B), thickening over time is suppressed compared to an aqueous solution of a high-polymerization PVA that exhibits similar polarizing performance. The reason for this is unclear, but it is thought that when the hydroxyl groups of PVA(A) are hydrated, the nucleophilicity of the hydroxyl groups of PVA(A) is reduced, suppressing the reaction between the structural unit represented by the above formula (1) and PVA(A).

[0029] The "chlorination rate" means the proportion (molar ratio) of structural units (i) (structural units derived from maleic anhydride) that have formed a salt by reaction with a base, i.e., that have become salts.

[0030] A part of the structural unit (i) preferably forms a salt with ammonia or an organic base, more preferably with ammonia. Specifically, the structural unit (i) preferably contains a structural unit represented by the above formula (2). When the polymer (B) has such a structural unit (i), compatibility with the PVA (A) is improved, and a highly transparent PVA film can be obtained.

[0031] The structural unit (i) may contain structural units other than unmodified structural units such as the structural unit represented by the above formula (1) and salified structural units such as the structural units represented by the above formulas (2) and (3). Examples of such other structural units among the structural units (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 preferred that the structural unit (i) is substantially composed of the structural unit represented by the above formula (1) and salified structural units. The total content of the structural unit represented by the above formula (1) and salified structural units in the structural unit (i) is preferably 80 mol % or more, more preferably 90 mol % or more, and even more preferably 99 mol % or more.

[0032] The lower limit of the content of structural unit (i) relative to all structural units in 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 (i) is preferably 90 mol%, more preferably 70 mol%, even more preferably 60 mol%, and even more preferably 55 mol%. When the content of structural unit (i) in polymer (B) is within the above range, compatibility with PVA (A), crosslinkability, water solubility, productivity, etc. are further improved.

[0033] The polymer (B) preferably further contains a structural unit (ii) derived from an α-olefin. By using such a polymer (B), the polarizing performance of the resulting polarizing film can be further improved.

[0034] Examples of the α-olefin 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 called isobutene, 2-methylpropene, etc.) is particularly preferred.

[0035] The lower limit of the content of the structural unit (ii) relative to all structural units in 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 the 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 the PVA (A), crosslinkability, water solubility, productivity, etc. are further improved.

[0036] The polymer (B) is preferably an alternating copolymer of the structural unit (i) and the structural unit (ii). In this case, the dispersibility of the polymer (B) is improved, and a highly uniform crosslinked structure can be formed with the PVA (A), thereby further improving the polarization performance of the resulting polarizing film.

[0037] The polymer (B) may further have structural units other than the structural units (i) and (ii). However, the upper limit of the content of the other structural units relative to the total structural units of the polymer (B) is preferably 20 mol%, and in some cases, more preferably 10 mol%, 3 mol%, or 1 mol%. The polymer (B) is preferably a copolymer substantially composed only of the structural units (i) and (ii).

[0038] The lower limit of the weight-average molecular weight of polymer (B) is preferably 5,000, more preferably 10,000, and even more preferably 20,000. When the weight-average molecular weight of polymer (B) is equal to or greater than the lower limit, the effect of improving the polarizing performance due to crosslinking with PVA (A) is more sufficient. 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 polymer (B) is equal to or less than the upper limit, compatibility with PVA (A), water solubility, etc. are improved, and the transparency of the PVA film is improved.

[0039] The polymer (B) can be synthesized by a known method, for example, by homopolymerizing maleic anhydride by a known method, or by copolymerizing maleic anhydride and an α-olefin by a known method, and then reacting the resulting polymer with a predetermined amount of base.

[0040] Commercially available products can also be used as the polymer (B). Examples of commercially available products of the polymer (B) include ISOBAM (registered trademark)-104 and ISOBAM-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 an α-olefin with a base. Examples of commercially available copolymers of maleic anhydride and an α-olefin include ISOBAM (registered trademark)-04, ISOBAM-06, ISOBAM-10, and ISOBAM-18 manufactured by Kuraray Co., Ltd.

[0041] The lower limit of the content of polymer (B) relative 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. Meanwhile, the upper limit of this content is 9.5 parts by mass, preferably 8 parts by mass, and more preferably 6 parts by mass. By setting the content of polymer (B) within the above range, a polarizing film with excellent polarization performance can be obtained. Specifically, by setting the content of polymer (B) to the above lower limit or more, the polarization performance of the resulting polarizing film is improved. Meanwhile, by setting the content of polymer (B) to the above lower limit or less, stretchability and break resistance are enhanced, and a polarizing film with excellent polarization performance can be obtained by sufficient stretching.

[0042] The lower limit of the total content of PVA (A) and polymer (B) in the PVA film may preferably be 90% by mass, more preferably 95% by mass, and even more preferably 98% by mass.

[0043] (plasticizer) The PVA film may further contain a plasticizer. By including a plasticizer in the PVA film, it is possible to improve the handling property and stretchability. Preferred plasticizers include polyhydric alcohols, and specific examples include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. One or more of these plasticizers may be used. Among these, glycerin is preferred from the viewpoint of improving stretchability.

[0044] The lower limit of the plasticizer content in the PVA film is preferably 1 part by mass, more preferably 3 parts by mass, and even more preferably 5 parts by mass, per 100 parts by mass of PVA. By setting the plasticizer content at or above the 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. Setting the plasticizer content at or below the upper limit can prevent the PVA film from becoming too flexible or the plasticizer from bleeding out onto the surface, thereby preventing deterioration in handleability.

[0045] The lower limit of the total content of the 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.

[0046] (Other additives, etc.) In addition to the PVA (A), polymer (B), and plasticizer, other additives such as fillers, processing stabilizers such as copper compounds, weathering stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, antifoaming agents, deodorizers, extenders, peeling agents, release agents, reinforcing agents, crosslinking agents, mildew inhibitors, preservatives, and crystallization rate retarders may be appropriately blended into the PVA film as needed.

[0047] However, the upper limit of the content of other additives in the PVA film other than PVA (A), polymer (B), and plasticizer is preferably 1% by mass, more preferably 0.2% by mass. If the content of other additives exceeds the upper limit, the strength, optical properties, etc. of the PVA film may be affected.

[0048] The breaking stress of the PVA film in a 4% by mass aqueous solution of boric acid at 50°C is preferably 8 MPa or more, more preferably 15 MPa or more, even more preferably 20 MPa or more, and even more preferably 25 MPa or more. The breaking strain of the PVA film in a 4% by mass aqueous solution of boric acid at 50°C is preferably 150% or more, more preferably 200% or more, even more preferably 300% or more, and even more preferably 400% or more. When the breaking stress and breaking strain of the PVA film are equal to or greater than the above-mentioned lower limits, the stretchability is improved, and a polarizing film with superior polarization performance can be obtained. The upper limit of the breaking stress may be, for example, 100 MPa or 70 MPa. The upper limit of the breaking strain may be 1000% or 700%. The breaking stress and breaking strain are values measured at a chuck distance of 30 mm and a pulling rate of 240% / min.

[0049] The stress increase rate of the PVA film at 300% strain in a 4% by mass boric acid aqueous solution at 50°C is preferably 0.04 MPa / % or more, more preferably 0.05 MPa / % or more, and particularly preferably 0.06 MPa or more. When the stress increase rate is equal to or greater than the lower limit described above, the reaction between the PVA (A) and the polymer (B) is particularly sufficient, resulting in a polarizing film with higher polarization performance. On the other hand, the stress increase rate at 300% strain 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 stress increase rate at 300% strain is equal to or less than the upper limit described above, stretchability is enhanced and sufficient stretching can be achieved, thereby improving the productivity of the polarizing film. The stress increase rate at 300% strain is a value calculated from a stress-strain curve measured at a chuck distance of 30 mm and a pulling rate of 240% / min.

[0050] The breaking stress, breaking strain, and stress increase of the PVA film in a 4% by mass aqueous solution of boric acid at 50° C. can be adjusted, for example, by the content of polymer (B). For example, when the content of polymer (B) is reduced, the breaking stress and breaking strain tend to increase, and the stress increase tends to decrease.

[0051] The lower limit of the swelling degree of the PVA film is preferably 160%, more preferably 170%, and even more preferably 180%. A swelling degree equal to or greater than the lower limit can prevent excessive crystallization and allow the film to be stably stretched to a high magnification. On the other hand, the upper limit of the swelling degree is preferably 260%, more preferably 240%, and even more preferably 220%. A swelling degree equal to or less than the upper limit can prevent dissolution during stretching, allowing the film to be stretched even under high-temperature conditions. The swelling degree of a PVA film refers to the percentage obtained by dividing the mass of the PVA film immersed in distilled water at 30°C for 30 minutes by the mass of the PVA film immersed in distilled water at 30°C for 30 minutes and then dried at 105°C for 16 hours.

[0052] The upper limit of the average thickness of the PVA film is preferably 60 μm, more preferably 45 μm, and even more preferably 30 μm. When the average thickness of the PVA film is equal to or less than the 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, even more preferably 15 μm, and even more preferably 20 μm. When the average thickness of the PVA film is equal to or greater than the lower limit, the strength of the resulting polarizing film can be increased.

[0053] The width of the PVA film is not particularly limited and can be determined depending on the application of the polarizing film to be produced. Given the recent trend toward larger screens for LCD televisions and LCD monitors, a PVA film width of 3 m or more is suitable for these applications. On the other hand, if the PVA film width is too large, it becomes difficult to uniformly perform uniaxial stretching when producing polarizing films using commercially available equipment. Therefore, the width of the PVA film is preferably 7 m or less.

[0054] The PVA film is suitable as a raw film (or material film) used in the production of stretched films such as polarizing films, retardation films, etc. A stretched film can be obtained by stretching the PVA film.

[0055] The PVA film may be a single-layer film or a multi-layer film (laminate). Examples of the multi-layer 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 preferred from the viewpoints of more pronounced effects of the present invention, the complexity of lamination (coating, etc.), and the cost of the thermoplastic resin film.

[0056] <Method of manufacturing polyvinyl alcohol film> The method for producing the PVA film of the present invention comprises the steps of: Step (1) of forming a coating film containing PVA (A) and a polymer (B); and Step (2) of heat-treating the coating film Equipped with.

[0057] The content of polymer (B) in the coating film obtained in step (1) relative to 100 parts by mass of PVA (A) is 0.5 parts by mass or more and 9.5 parts by mass or less, and the chlorination rate of the structural unit derived from maleic anhydride in polymer (B) is 0.3 parts by mass or more and 0.9 parts by mass or less. Specific and preferred embodiments of the PVA (A) and polymer (B) contained in the coating film obtained in step (1) are the same as those of the PVA (A) and polymer (B) contained in the PVA film described above.

[0058] Step (1) can be carried out by forming a membrane using a membrane-forming solution in which, for example, PVA (A) and polymer (B), and, if necessary, one or more of a plasticizer, other additives, and a surfactant (described later) are dissolved in a liquid medium.

[0059] Examples of liquid media include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine. These liquid media can be used alone or in combination. Among these, water is preferred from the viewpoints of environmental load and recoverability.

[0060] The volatile content of the membrane-forming solution (the content of volatile components, such as the liquid medium, in the membrane-forming solution that are removed by volatilization or evaporation during membrane production) varies depending on the membrane-forming method, membrane-forming conditions, etc., but generally, a lower limit of 50 mass% is preferred, with 60 mass% being more preferred. Having a volatile content of the membrane-forming solution at or above the lower limit prevents the membrane-forming solution from becoming too viscous, facilitating filtration and degassing during preparation of the membrane-forming solution, and facilitating the production of PVA films with fewer foreign matter and defects. On the other hand, an upper limit of this volatile content is preferably 95 mass%, with 90 mass% being more preferred. Having a volatile content of the membrane-forming solution at or below the upper limit prevents the membrane-forming solution from becoming too concentrated, facilitating the industrial production of PVA films.

[0061] The film-forming solution preferably contains a surfactant. The inclusion of a surfactant improves film-forming properties, suppresses the occurrence of thickness variations in the PVA film, and facilitates peeling of the film from the metal roll or belt used in film formation. When a PVA film is produced from a film-forming solution containing a surfactant, the PVA film may contain a surfactant. The type of surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the viewpoint of peelability from the metal roll or belt.

[0062] Preferred anionic surfactants include carboxylic acid surfactants such as potassium laurate; sulfate ester surfactants such as polyoxyethylene lauryl ether sulfate, sodium alkyl sulfate, potassium alkyl sulfate, ammonium alkyl sulfate, triethanolamine alkyl sulfate, polyoxyethylene alkyl ether sodium sulfate, polyoxypropylene alkyl ether sodium sulfate, polyoxyethylene alkyl phenyl ether sodium sulfate, and octyl sulfate; sulfonic acid surfactants such as sodium alkyl sulfonate, potassium alkyl sulfonate, ammonium alkyl sulfonate, triethanolamine alkyl sulfonate, sodium alkyl benzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkyl naphthalene sulfonate, disodium alkyl sulfosuccinate, disodium polyoxyethylene alkyl sulfosuccinate, and dodecyl benzene sulfonate; and phosphate ester surfactants such as sodium alkyl phosphate ester, potassium alkyl phosphate ester, ammonium alkyl phosphate ester, triethanolamine alkyl phosphate ester, sodium polyoxyethylene alkyl ether phosphate ester, sodium polyoxypropylene alkyl ether phosphate ester, and sodium polyoxyethylene alkyl phenyl ether phosphate ester.

[0063] Preferred nonionic surfactants include alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether.

[0064] These surfactants can be used alone or in combination of two or more.

[0065] When the film-forming solution or the resulting PVA film contains a surfactant, the lower limit of its content is preferably 0.01 parts by mass, more preferably 0.02 parts by mass, and even more preferably 0.05 parts by mass, relative to 100 parts by mass of PVA (A) contained in the film-forming solution or PVA film. Having a surfactant content equal to or greater than the above-mentioned lower limit further improves film-forming properties and peelability. 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. Having a surfactant content equal to or less than the above-mentioned upper limit can prevent the surfactant from bleeding out onto the surface of the PVA film, causing blocking and resulting in reduced handleability.

[0066] Examples of film-forming methods for forming a film using a film-forming solution include a cast film-forming method, an extrusion film-forming method, a wet film-forming method, and a gel film-forming method. These film-forming methods may be used alone or in combination of two or more. Among these film-forming methods, the cast film-forming method and the extrusion film-forming method are preferred because they can produce PVA films with uniform thickness and width and good physical properties.

[0067] In step (2), the coating film formed in step (1) is heat-treated. This heat treatment causes a crosslinking reaction between the PVA (A) and the polymer (B) in the coating film. By carrying out such heat treatment, a PVA film is obtained that can be used to produce a polarizing film with excellent polarizing performance.

[0068] 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 carried out using a hot air dryer or the like.

[0069] <Stretched film> A stretched film according to one embodiment of the present invention is formed from the 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.

[0070] Since the stretched film is obtained by stretching the PVA film according to one embodiment of the present invention, the components contained in the stretched film may be the same as those contained in the PVA film described above. 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 adsorbed on the front and back surfaces. Iodine-based dyes are commonly used as dichroic dyes.

[0071] <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 is a form of a stretched film, and can be obtained by adsorbing a dichroic dye onto both surfaces of the PVA film according to one embodiment of the present invention and uniaxially stretching the film.

[0072] The polarization performance of the polarizing film is such that the lower limit of the polarization degree when the transmittance is 43.7% or higher is preferably 99.90%, more preferably 99.92%, and even more preferably 99.93%. When the polarization degree is at or above the lower limit, the contrast of the LCD is improved when the film is used in smartphones, laptops, LCD televisions, car navigation systems, etc.

[0073] The polarizing film is usually used as a polarizing plate by laminating an optically transparent and mechanically strong protective film on one or both sides of the film. Examples of protective films that can be used include triacetate cellulose (TAC) film, cellulose acetate butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA adhesives and ultraviolet-curable adhesives, with PVA adhesives being preferred.

[0074] The polarizing plate obtained as described above may further be laminated with a retardation film, a viewing angle improving film, a brightness improving film, etc. The stretched film of the present invention can also be used as the retardation film. The polarizing plate can be coated with an acrylic or other pressure-sensitive adhesive and then laminated to a glass substrate to be used as a component of an LCD.

[0075] <Methods of manufacturing stretched films and polarizing films> The stretched film according to one embodiment of the present invention can be obtained by a manufacturing method including the step of stretching the PVA film (stretching treatment) described above. Furthermore, the polarized film according to one embodiment of the present invention can be obtained by a manufacturing method including the step of adsorbing a dichroic dye to the PVA film (dyeing treatment) and the step of uniaxially stretching the PVA film (uniaxial stretching treatment) described above. A specific manufacturing method for producing a polarized film will be described below. Note that the manufacturing of stretched films other than polarized films can also be carried out in the same manner as the manufacturing of polarized films, except that dyeing treatments and the like are not performed.

[0076] Specific methods for producing the polarized film include subjecting the PVA film to swelling, dyeing, uniaxial stretching, and, if necessary, crosslinking, fixing, washing, drying, heat treatment, etc. In this case, the order of the treatments, such as swelling, dyeing, crosslinking, uniaxial stretching, and fixing, is not particularly limited, and two or more of the treatments can be performed simultaneously. Furthermore, one or more of the treatments can be performed two or more times.

[0077] The swelling treatment can be carried out by immersing the PVA film in water. The lower limit of the water temperature during immersion 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. Furthermore, the lower limit of the time for immersion in water is preferably 0.1 minutes, more preferably 0.5 minutes. On the other hand, the upper limit of this time is preferably 5 minutes, more preferably 3 minutes. The water used for immersion in 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 an aqueous medium.

[0078] The dyeing treatment can be carried out by bringing a dichroic dye into contact with the PVA film. Iodine-based dyes or dyes are generally used as the dichroic dye. The dyeing treatment may be carried out before, during, or after the uniaxial stretching treatment. When an iodine-based dye is used, the dyeing treatment is generally carried out by immersing the PVA film in a solution (particularly an aqueous solution) containing iodine and potassium iodide as a dye bath. The iodine concentration in the dye bath is preferably 0.01% by mass or more and 0.5% by mass or less, and the potassium iodide concentration is preferably 0.01% by mass or more and 10% by mass or less. The lower limit of the temperature of the dye bath is preferably 20°C, more preferably 25°C. The upper limit of this temperature is preferably 50°C, more preferably 40°C.

[0079] By subjecting a PVA film to a crosslinking treatment, it is possible to effectively prevent the PVA from dissolving into water during wet stretching at high temperatures. From this perspective, it is preferable to perform the crosslinking treatment before the uniaxial stretching treatment. The crosslinking treatment can be performed by immersing the PVA film in an aqueous solution containing a crosslinking agent. The crosslinking agent can be one or more of inorganic boron compounds such as boric acid, borax, and other borates, and organic boron compounds such as alkyl monoboronic acids and alkyl diboronic acids. The lower limit of the concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass, more preferably 2% by mass, and even more preferably 3% by mass. Meanwhile, the upper limit of this concentration is preferably 15% by mass, more preferably 7% by mass, and even more preferably 6% by mass. By maintaining the crosslinking agent concentration within the above range, sufficient stretchability can be maintained. The aqueous solution containing the crosslinking agent may contain an auxiliary agent such as potassium iodide. The lower limit of the temperature of the aqueous solution containing the crosslinking 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, crosslinking can be carried out efficiently.

[0080] 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 performed in a boric acid aqueous solution, or in the dye bath described above or the fixing treatment bath described below. In the case of the dry stretching method, the uniaxial stretching treatment may be performed at room temperature, while heating, or in air using a PVA film after water absorption. Among these, the wet stretching method is preferred, and it is more preferred to perform the uniaxial stretching treatment in a boric acid aqueous solution. 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. Meanwhile, 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. The boric acid aqueous solution may also contain potassium iodide, and its concentration is preferably 0.01% by mass or more and 10% by mass or less.

[0081] 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.

[0082] The lower limit of the stretching ratio in the uniaxial stretching treatment is preferably 5 times, more preferably 6 times, from the viewpoint of the polarizing performance of the resulting polarizing film. The upper limit of the stretching ratio is not particularly limited, but may be, for example, preferably 10 times, more preferably 8 times.

[0083] When a long PVA film is uniaxially stretched, the direction of the uniaxial stretching is not particularly limited. Uniaxial stretching in the longitudinal direction, transverse uniaxial stretching, or so-called oblique stretching can be employed, but uniaxial stretching in the longitudinal direction is preferred because it results in a polarizing film with excellent polarization performance. Uniaxial stretching in the longitudinal direction can be performed using a stretching device equipped with multiple rolls parallel to each other, by varying the peripheral speed between the rolls. On the other hand, transverse uniaxial stretching can be performed using a tenter-type stretching machine.

[0084] In producing a polarizing film, a fixing treatment is preferably performed after uniaxial stretching to strengthen the adsorption of the dichroic dye (e.g., iodine-based dye) to the PVA film. The fixing bath used for the fixing treatment can be an aqueous solution containing one or more boron inorganic compounds such as boric acid and borax, and boron organic compounds such as alkylboronic acid and alkyldiboronic acid. If necessary, an iodine compound or a metal compound may be added to the fixing bath. The lower limit of the concentration of the boron inorganic compound in the fixing bath is preferably 2% by mass, more preferably 3% by mass. Meanwhile, the upper limit of this concentration is preferably 15% by mass, more preferably 10% by mass. By keeping this concentration within the above range, the adsorption of the dichroic dye can be strengthened. The lower limit of the temperature of the fixing bath is preferably 15°C. Meanwhile, the upper limit of this temperature is preferably 60°C, more preferably 40°C.

[0085] The cleaning treatment is generally carried out by immersing the PVA film in water or the like. In this case, the water or the like used in the cleaning treatment preferably contains an auxiliary such as potassium iodide in order to improve polarization performance. In this case, the concentration of the iodide such as potassium iodide is preferably 0.5% by mass or more and 10% by mass or less. The lower limit of the temperature of the water or the like used in the cleaning treatment is generally 5°C, preferably 10°C, and more preferably 15°C. Meanwhile, the upper limit of this temperature is generally 50°C, preferably 45°C, and more preferably 40°C.

[0086] 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. Drying at a temperature within the above range makes it easier to obtain a polarizing film with excellent dimensional stability.

[0087] By performing a heat treatment after the drying treatment, a polarizing film with even better dimensional stability can be obtained. Here, heat treatment refers to a treatment in which a polarizing film with a moisture content of 5% or less after the drying treatment is further heated to improve the dimensional stability of the polarizing film. The conditions for the heat treatment are not particularly limited, but it is preferable to perform the heat treatment within the range of 60°C to 150°C. If the heat treatment is performed at a temperature lower than 60°C, the dimensional stabilization effect of the heat treatment will be insufficient. On the other hand, if the heat treatment is performed at a temperature higher than 150°C, the polarizing film may undergo severe yellowing.

[0088] <Other embodiments> The PVA film, PVA film manufacturing method, stretched film, and polarized film of the present invention are not limited to the above-described embodiments. For example, although the stretched film has been described mainly as a polarized film, the stretched film is not limited to a polarized film. For example, stretched films other than polarized films, such as retardation films, are also within the scope of the present invention. Furthermore, the PVA film of the present invention may be used for applications other than as a material for stretched films such as polarized films. [Example]

[0089] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The measurement and evaluation methods used in the following examples and comparative examples are shown below.

[0090] [Appearance of film-forming solution] The prepared film-forming solution was visually evaluated for appearance such as transparency.

[0091] [PVA film appearance] The produced PVA films were visually evaluated for appearance such as transparency.

[0092] [PVA film breaking tension, breaking strain and stress increase] The breaking stress and breaking strain of the manufactured PVA film were measured in a 4% by mass boric acid aqueous solution at 50°C. The distance between the chucks was set to 30 mm, and the tensile speed was set to 240% / min. The tensile test was started after immersion in the boric acid aqueous solution for 60 seconds. In addition, the stress-strain curve obtained from the measurement was subjected to differential analysis using Microsoft Excel spreadsheet software, and the stress increase at 300% strain was calculated by performing a moving average process over a 25-point interval.

[0093] [Optical properties of polarizing film] In the following examples and comparative examples, rectangular samples measuring 4 cm in the length direction and 2 cm in the width direction were taken from the center of the obtained polarizing film in the width and length directions, and the parallel transmittance and crossed Nicol transmittance of the polarizing film were measured using a spectrophotometer V-7100 (manufactured by JASCO Corporation) with an integrating sphere and an automatic polarizing film measuring device VAP-7070S (manufactured by JASCO Corporation) equipped with a Glan-Taylor polarizer. Here, the measurement wavelength range was set to 380 to 780 nm, and the parallel transmittance was defined as the transmittance when the vibration direction of polarized light incident on the polarizing film through the Glan-Taylor polarizer was parallel to the transmission axis of the polarizing film, and the crossed Nicol transmittance was defined as the transmittance when the vibration direction was perpendicular to the transmission axis of the polarizing film. Then, using the "Polarizing Film Evaluation Program" (manufactured by JASCO Corporation), luminosity correction was performed in the visible light region using the parallel transmittance and crossed Nicol transmittance described above in accordance with JIS Z 8722 (measurement method for object color), with a C light source and a 2° field of view, to calculate the single transmittance and degree of polarization of the polarizing film, and these two values were obtained as the optical properties of the polarizing film.

[0094] The polymers (B) and the like used in the examples and comparative examples are shown below. Polymer (B-1): Kuraray's "Isoban-104" (ammonia-modified alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chlorination ratio 0.7, weight-average molecular weight 55,000 to 65,000) Polymer (B-2): Kuraray's "ISOBAM-110" (ammonia-modified alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chlorination ratio 0.7, weight-average molecular weight 160,000 to 170,000) Polymer (b-1): Kuraray's "Isoban-04" (alternating copolymer of structural units derived from maleic anhydride and structural units derived from isobutylene, chlorination rate 0, weight average molecular weight 55,000 to 65,000) Polymer (b-2): Polyacrylic acid (manufactured by Wako Pure Chemical Industries, Ltd., molecular weight approximately 150,0000, aqueous solution of polyacrylic acid concentration 25%)

[0095] [Example 1] (1) An aqueous solution containing 100 parts by weight of PVA (saponified vinyl acetate homopolymer, saponification degree 99.9 mol%, polymerization degree 2400), 5 parts by weight of polymer (B-1), 10 parts by weight of glycerin as a plasticizer, and 0.1 parts by weight of sodium polyoxyethylene lauryl ether sulfate as a surfactant, with a solids concentration of approximately 10% by weight, was prepared and used as a film-forming solution. This film-forming solution was dried on a metal roll at 80°C, and the resulting coating was heat-treated in a hot air dryer at 128°C for 10 minutes to obtain a PVA film with an average thickness of 30 μm. The appearance of the prepared film-forming solution, and the appearance, breaking stress, breaking strain, and stress increase of the produced PVA film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1. Furthermore, the prepared film-forming solution did not increase in viscosity over time and had good film-forming properties.

[0096] (2) A 5 cm wide x 9 cm long sample was cut from the center of the PVA film obtained in (1) above in the width direction so that a 5 cm wide x 5 cm long range could be uniaxially stretched. This sample was immersed in distilled water at 30°C and uniaxially stretched 1.1 times in the length direction. Subsequently, while uniaxially stretching 2.2 times in the length direction (2.4 times overall), the sample was immersed in an aqueous solution (dyeing bath) (temperature: 30°C) containing 100 parts by weight of potassium iodide per 1 part by weight of iodine for 60 seconds to allow iodine absorption. The iodine concentration of the dyeing bath was appropriately adjusted so that the transmittance Ts (%) of the resulting polarized film was 43.7%. Next, while immersed in an aqueous solution (crosslinking bath) (temperature: 30°C) containing 3% by weight of boric acid and 3% by weight of potassium iodide, the sample was uniaxially stretched 1.2 times in the length direction (2.7 times overall). The film was then immersed in an aqueous solution containing 4% by mass of boric acid and 6% by mass of potassium iodide (a uniaxial stretching bath) and uniaxially stretched to a total stretch ratio of 6.0 times (at a temperature of 60°C) in the machine direction. The film was then immersed for 5 seconds in an aqueous solution containing 3.5% by mass of potassium iodide (a washing bath) (at a temperature of 30°C). Finally, the film was dried at 60°C for 4 minutes to obtain a polarized film. The polarizing performance (transmittance and polarization degree) of the obtained polarizing film was evaluated by the above-mentioned methods. The results are shown in Table 1.

[0097] [Example 2] The film-forming solution was prepared and a PVA film and a polarized film were produced in the same manner as in Example 1, except that the content of polymer (B-1) was 2 parts by mass per 100 parts by mass of PVA and the heat treatment temperature was 127° C. In each of the Examples and Comparative Examples, the heat treatment temperature was adjusted so that the swelling degree of the PVA film would be 200%. The appearance of the film-forming solution, the appearance of the PVA film, the breaking stress, the breaking strain and the stress increase rate, and the polarization performance (transmittance and polarization degree) of the polarizing film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1.

[0098] [Example 3] The film-forming solution was prepared and a PVA film and a polarized film were produced in the same manner as in Example 1, except that polymer (B-2) was used instead of polymer (B-1) and the heat treatment temperature was set to 129°C. The appearance of the film-forming solution, the appearance of the PVA film, the breaking stress, the breaking strain and the stress increase rate, and the polarization performance (transmittance and polarization degree) of the polarizing film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1.

[0099] [Comparative Example 1] A film-forming solution was prepared and a PVA film and a polarized film were produced in the same manner as in Example 1, except that the content of polymer (B-1) was 10 parts by mass relative to 100 parts by mass of PVA and the heat treatment temperature was 135° C. Although a film-forming solution and a PVA film were obtained, the film broke during stretching during production of the polarized film, and no polarized film was obtained. The appearance of the film-forming solution, as well as the appearance, breaking stress, breaking strain, and stress increase of the PVA film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1.

[0100] Comparative Example 2 The film-forming solution was prepared and a PVA film and a polarized film were produced in the same manner as in Example 1, except that the polymer (B-1) was not added and the heat treatment temperature was 120°C. The appearance of the film-forming solution, the appearance of the PVA film, the breaking stress, the breaking strain and the stress increase rate, and the polarization performance (transmittance and polarization degree) of the polarizing film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1.

[0101] Comparative Example 3 A film-forming solution was prepared in the same manner as in Example 1, except that polymer (b-1) was used instead of polymer (B-1). However, a white precipitate was formed in the film-forming solution, and it was determined that a polarized film could not be obtained, so production of a PVA film and a polarized film was not carried out.

[0102] Comparative Example 4 The film-forming solution was prepared and a PVA film and a polarized film were produced in the same manner as in Example 1, except that polymer (b-2) was used instead of polymer (B-1) and the heat treatment temperature was set to 120°C. The appearance of the film-forming solution, the appearance of the PVA film, and the polarizing performance (transmittance and polarization degree) of the polarizing film were evaluated or measured by the above-mentioned methods. The results are shown in Table 1.

[0103] [Table 1]

[0104] As is clear from the above results, the polarizing films obtained from the PVA films of Examples 1 to 3 have a high polarization degree and excellent polarization performance. On the other hand, in Comparative Examples 1 to 4, polarizing films with excellent polarization performance could not be obtained. [Industrial Applicability]

[0105] The PVA film of the present invention can be suitably used as a material for polarizing films, which are constituent materials of LCDs.

Claims

1. The composition contains polyvinyl alcohol (A) and a polymer (B) having a structural unit derived from maleic anhydride, the content of the polymer (B) relative 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, the chlorination rate of the structural unit derived from maleic anhydride in the polymer (B) is 0.3 or more and 0.9 or less, The salification ratio is a ratio (molar ratio) of structural units that have formed a salt by reaction with a base to the structural units derived from maleic anhydride, A polyvinyl alcohol film having a polyvinyl alcohol (A) content of 60 mass% or more.

2. 2. The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol (A) and the polymer (B) form a crosslinked structure.

3. 3. The polyvinyl alcohol film according to claim 1, wherein the polymer (B) further has a structural unit derived from an α-olefin.

4. 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 an α-olefin.

5. 5. The polyvinyl alcohol film according to claim 1, wherein a part of the structural units derived from maleic anhydride in the polymer (B) forms a salt with ammonia or an organic base.

6. 6. The polyvinyl alcohol film according to claim 1, wherein the weight average molecular weight of the polymer (B) is 5,000 or more and 200,000 or less.

7. 7. The polyvinyl alcohol film according to claim 1, which has a breaking stress of 8 MPa or more and a breaking strain of 150% or more in a 4% by mass aqueous solution of boric acid at 50°C.

8. 8. The polyvinyl alcohol film according to claim 1, wherein the stress increase rate at a strain of 300% in a 4% by mass aqueous solution of boric acid at 50°C is 0.04 MPa / % or more and 0.3 MPa / % or less.

9. A method for producing a polyvinyl alcohol film having a polyvinyl alcohol (A) content of 60 mass% or more, comprising: 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 Equipped with the content of the polymer (B) in the coating film relative 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, the chlorination rate of the structural unit derived from maleic anhydride in the polymer (B) is 0.3 or more and 0.9 or less, The salification rate is the proportion (molar ratio) of structural units that form a salt by reaction with a base among the structural units derived from maleic anhydride.

10. A stretched film formed from the polyvinyl alcohol film according to any one of claims 1 to 8.

11. A polarizing film formed from the polyvinyl alcohol film according to claim 1 .

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