Polyvinyl alcohol film and polarizing film using the same

Abstract

Provided are: a poly(vinyl alcohol) (PVA) film from which a polarizing film having satisfactory separability from supports and having little optical unevenness can be produced; and a polarizing film obtained from such PVA film.　This PVA film is a water-insoluble PVA film wherein, when one of the two surfaces orthogonal to the thickness direction of the PVA film is referred to as a first surface, then the first surface has a proportion (Na1S) of sodium element in all the elements, determined by analysis by X-ray photoelectron spectroscopy, of 0.3-1.5 mol% and a plane lying at a depth of 0.01 μm from the first surface has a proportion (Na1B) of sodium element in all the elements, determined by analysis by X-ray photoelectron spectroscopy, of 0.3 mol% or less.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol film capable of producing a polarizing film having good releasability from a support during film formation and little optical unevenness, and a polarizing film using the same.

Background Art

[0002] A polarizing plate having a light transmission and shielding function is a basic component of a liquid crystal display (LCD) together with a liquid crystal having a light switching function. The application fields of this LCD have also expanded from small devices such as calculators and wristwatches in the early stage of development to various fields such as notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal TVs, in-vehicle navigation systems, mobile phones, and measuring instruments used indoors and outdoors in recent years.

[0003] A polarizing plate is manufactured by laminating a protective film such as a cellulose triacetate (TAC) film or a cellulose acetate butyrate (CAB) film on the surface of a polarizing film. The polarizing film is produced by uniaxially stretching a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be referred to as "PVA") after dyeing treatment, or uniaxially stretching while dyeing treatment, or dyeing treatment after uniaxially stretching to produce a dyed uniaxially stretched film, and this uniaxially stretched film is generally produced by immobilizing treatment with a boron compound. In addition, the immobilizing treatment with this boron compound may be performed simultaneously with uniaxial stretching or dyeing treatment.

[0004] For the PVA film used for such applications, various additives such as a plasticizer are blended to improve various physical properties, or a surfactant is added to improve the releasability from a support such as a metal roll or a metal belt used during film formation.

[0005] Furthermore, with the increasing demand for PVA film in recent years, there is a need to improve productivity. While productivity can be improved by increasing the film-forming speed of PVA film, this can result in insufficient release of the PVA film from the support during film formation. As a result, surface roughness and defects are more likely to occur in the PVA film, and optical inconsistencies are more likely to occur in the polarizing film obtained from the PVA film. Therefore, to improve the release properties of the PVA film from the support during film formation, it is conceivable to add more surfactant to the PVA film-forming solution. However, adding a large amount of surfactant leads to the problem of surfactant-induced optical inconsistencies when manufacturing polarizing films using the resulting PVA film.

[0006] To solve the problem of peeling PVA films from the support during film formation, Patent Document 1 describes adding a fluorine-containing surfactant to the PVA film formation solution. Patent Document 2 describes coating a cast substrate with a fluorine-containing resin to form a strong fluorine-based resin film. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2006-307059 [Patent Document 2] Japanese Patent Publication No. 2006-305924 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the method described in Patent Document 1 uses a fluorine-containing surfactant, which is a halogen substance, making it difficult to address environmental pollution. Furthermore, the method described in Patent Document 2 involves forming a fluorine-based resin film on a support such as a roll or belt, which is a cast substrate. This results in high costs for forming or maintaining the fluorine-based resin film, which tends to increase the manufacturing cost of the PVA film.

[0009] Therefore, the present invention aims to provide a PVA film with good peelability from a support and a polarizing film using such a PVA film. Furthermore, the present invention aims to provide a PVA film that can be used to manufacture a polarizing film with minimal optical unevenness and a polarizing film using such a PVA film. [Means for solving the problem]

[0010] As a result of diligent research, the inventors have found that the above problem can be solved by, in a non-water-soluble PVA film, designating one of two surfaces perpendicular to the thickness direction of the PVA film as the first surface, analyzing the first surface by X-ray photoelectron spectroscopy to determine the proportion of sodium elements in the total elements within a specific range, and analyzing a surface at a depth of 0.01 μm from the first surface by X-ray photoelectron spectroscopy to determine the proportion of sodium elements in the total elements within a specific range. Based on this finding, the inventors further researched and completed the present invention.

[0011] Furthermore, after diligent research, the inventors discovered that in a non-water-soluble PVA film, when one of two surfaces perpendicular to the thickness direction of the PVA film is designated as the first surface, the above problem can be solved by analyzing the surface at a depth of 0.01 μm from the first surface using X-ray photoelectron spectroscopy to specify the proportion of sodium elements in the total elements. Based on this finding, the inventors conducted further research and completed the present invention.

[0012] In other words, the present invention relates to the following [1] to [8]. [1] A polyvinyl alcohol film that is not water-soluble, wherein when one of two surfaces perpendicular to the thickness direction of the polyvinyl alcohol film is designated as the first surface, the proportion of sodium elements in the total element (Na1S) determined by analyzing the first surface by X-ray photoelectron spectroscopy is 0.3 to 1.5 mol%, and the proportion of sodium elements in the total element (Na1B) determined by analyzing a surface at a depth of 0.01 μm from the first surface by X-ray photoelectron spectroscopy is 0.3 mol% or less; [2] The polyvinyl alcohol film according to [1], wherein, of the two surfaces perpendicular to the thickness direction of the polyvinyl alcohol film, the surface opposite the first surface is designated as the second surface, and the percentage of sodium elements in the total elements (Na2S) determined by analyzing the second surface by X-ray photoelectric spectroscopy is 0.3 to 1.5 mol%, and the percentage of sodium elements in the total elements (Na2B) determined by analyzing a surface at a depth of 0.01 μm from the second surface by X-ray photoelectric spectroscopy is 0.3 mol% or less; [3] A polyvinyl alcohol film according to [1] or [2], comprising a sodium sulfate type surfactant or a sodium sulfonate type surfactant, wherein the sodium element is derived from the sodium sulfate type surfactant or the sodium sulfonate type surfactant; [4] The polyvinyl alcohol film according to [3], wherein the sodium sulfate type surfactant or the sodium sulfonate type surfactant is at least one selected from the group consisting of sodium alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, sodium alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkylnaphthalene sulfonate, disodium alkyl sulfosuccinate, and disodium polyoxyethylene alkyl sulfosuccinate; [5] The polyvinyl alcohol film according to [3] or [4], wherein the molecular weight of the sodium sulfate type surfactant or the sodium sulfonate type surfactant is 200 to 10,000, and the content of the sodium sulfate type surfactant or the sodium sulfonate type surfactant is 0.02 to 0.4 parts by mass per 100 parts by mass of polyvinyl alcohol contained in the polyvinyl alcohol film; [6] A polyvinyl alcohol film according to any of [1] to [5] above, which is a film for manufacturing optical films; [7] The polyvinyl alcohol film according to [6], wherein the optical film is a polarizing film; [8] A method for producing a polarizing film using a polyvinyl alcohol film as described in any of [1] to [7] above; Regarding. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a PVA film with good peelability from a support and a polarizing film using such a PVA film. Furthermore, according to the present invention, it is possible to provide a PVA film that can be manufactured to produce a polarizing film with less optical uniformity and a polarizing film using such a PVA film. [Brief explanation of the drawing]

[0014] [Figure 1] This is a perspective view of the PVA film of the present invention. [Figure 2] This is a view of the PVA film of the present invention from the thickness direction. [Modes for carrying out the invention]

[0015] The present invention will be described in detail below.

[0016] <PVAフィルム> (Percentage of sodium element in all elements) Fig. 1 is a perspective view of the PVA film. Fig. 2 is a view of the PVA film of the present invention as seen from the thickness direction. In the present invention, as shown in Fig. 1 and Fig. 2, one of the two surfaces orthogonal to the thickness direction 2 of the PVA film is defined as the first surface 3. In the present invention, the ratio (Na1S) of the sodium element to all elements obtained by analyzing the first surface 3 of this PVA film 1 by X-ray photoelectron spectroscopy (hereinafter sometimes referred to as XPS) is preferably 0.3 to 1.5 mol%. When Na1S is less than 0.3 mol%, the peelability from the support during film formation of the PVA film 1 becomes insufficient. Na1S is preferably 0.3 mol% or more, more preferably 0.5 mol% or more, and still more preferably 0.6 mol% or more. On the other hand, when Na1S exceeds 1.5 mol%, surface defects are likely to occur during film formation of the PVA film 1, and as a result, optical unevenness is likely to occur in an optical film such as a polarizing film obtained. Na1S preferably does not exceed 1.5 mol%, and more preferably does not exceed 1.3 mol%. By appropriately adjusting the content of the plasticizer of the PVA film, the content of the surfactant, the volatile fraction of the film-forming stock solution, the thickness of the PVA film, the surface temperature of the support during the production of the PVA film, the drying temperature, and the heat treatment temperature, Na1S can be set to 0.3 to 1.5 mol%.

[0017] In the present invention, it is preferable that the percentage of sodium elements (Na1B) in the total elements, as determined by analyzing the surface at a depth of 0.01 μm from the first surface 3 of the PVA film 1 by XPS, is 0.3 mol% or less. Here, the surface at a depth of 0.01 μm from the first surface is also a surface perpendicular to the thickness direction 2 of the PVA film. If Na1B exceeds 0.3 mol%, defects are likely to occur on the surface of the PVA film 1 when manufacturing optical films such as polarizing films, and as a result, optical unevenness is likely to occur in the resulting optical films such as polarizing films. Na1B may be 0 mol%, that is, below the detection limit of the XPS measuring device. The detection limit of a typical XPS measuring device is usually around 0.1 mol%. It is preferable that Na1B be 0.3 mol% or less, and more preferably 0.25 mol% or less. By appropriately adjusting the plasticizer content of the PVA film, the surfactant content, the volatile fraction of the film-forming solution, the thickness of the PVA film, and the surface temperature of the support during the manufacturing of the PVA film, the drying temperature, and the heat treatment temperature, the Na1B content can be reduced to 0.3 mol% or less.

[0018] In the present invention, as shown in FIGS. 1 and 2, of the two surfaces orthogonal to the thickness direction 2 of the PVA film, the surface facing the first surface 3 is defined as the second surface 4. In the present invention, the ratio (Na2S) of the sodium element determined by analyzing the second surface 4 of the PVA film 1 by XPS to all elements is preferably 0.3 to 1.5 mol%. That is, in either of the two surfaces orthogonal to the thickness direction of the PVA film 1, the ratio (Na1S, Na2S) of the sodium element determined by analyzing by XPS to all elements is preferably 0.3 to 1.5 mol%. When Na2S is less than 0.3 mol%, the peelability from the support during film formation of the PVA film 1 becomes insufficient. Na2S is preferably 0.3 mol% or more, more preferably 0.5 mol% or more, and still more preferably 0.6 mol% or more. On the other hand, when Na2S exceeds 1.5 mol%, surface defects are likely to occur during film formation of the PVA film 1, and as a result, optical unevenness is likely to occur in an optical film such as a obtained polarizing film. Na2S preferably does not exceed 1.5 mol%, and more preferably does not exceed 1.3 mol%. By appropriately adjusting the content of the plasticizer of the PVA film, the content of the surfactant, the volatile fraction of the film-forming stock solution, the thickness of the PVA film, the surface temperature of the support during the production of the PVA film, the drying temperature, and the heat treatment temperature, Na2S can be set to 0.3 to 1.5 mol%.

[0019] In the present invention, it is preferable that the proportion of sodium elements (Na2B) to the total elements, as determined by analyzing the surface at a depth of 0.01 μm from the second surface of the PVA film 1 by XPS, is 0.3 mol% or less. That is, it is preferable that the proportion of sodium elements (Na1B, Na2B) to the total elements, as determined by analyzing the surface at a depth of 0.01 μm from either of the two surfaces perpendicular to the thickness direction 2 of the PVA film by XPS, is 0.3 mol% or less. Here, the surface at a depth of 0.01 μm from the second surface is also a surface perpendicular to the thickness direction 2 of the PVA film. If Na2B exceeds 0.3 mol%, defects are likely to occur on the surface of the PVA film 1 when manufacturing optical films such as polarizing films, and as a result, optical unevenness is likely to occur in the resulting optical films such as polarizing films. Na2B may be 0 mol%, i.e., below the detection limit of the XPS measuring device. Na2B is preferably 0.3 mol% or less, and more preferably 0.2 mol% or less. By appropriately adjusting the plasticizer content of the PVA film, the surfactant content, the volatile fraction of the film-forming solution, the thickness of the PVA film, and the surface temperature of the support during the manufacturing of the PVA film, the drying temperature, and the heat treatment temperature, the Na2B content can be reduced to 0.3 mol% or less.

[0020] (XPS) In this invention, the amounts of sodium and other elements on the first or second surface of the PVA film, or on a surface 0.01 μm deep from the first or second surface, are measured by XPS. XPS is a method of identifying and quantifying elements present on a sample surface and analyzing their chemical bonding state by irradiating the sample surface with X-rays to excite the inner-shell electrons of atoms and detecting the kinetic energy of the photoelectrons emitted. In this invention, the elements measured by XPS are carbon (1s orbital electrons), nitrogen (1s orbital electrons), oxygen (1s orbital electrons), sodium (1s orbital electrons), silicon (2p orbital electrons), phosphorus (2p orbital electrons), and sulfur (2p orbital electrons). These elements are quantified, and the ratio of sodium to their total amount is defined as Na1S, Na1B, Na2S, or Na2B.

[0021] XPS also allows for analysis in the depth direction by etching the film surface with C60 (Buckminsterfullerene) or argon clusters. In this invention, C60 was used to etch the film surface for 30 seconds under conditions of an acceleration voltage of 10kV, a sample current of 20nA, and a scanning range of 0.5 × 2.0 mm, exposing a surface to a depth of approximately 0.01 μm, and Na1B or Na2B was quantified.

[0022] (Physical properties) The PVA film of the present invention is preferably water-insoluble. Because the PVA film is water-insoluble, when uniaxial stretching is performed in an aqueous solution during the manufacture of optical films such as polarizing films, the PVA film can be stretched without breaking during uniaxial stretching, even at high maximum stretching speeds. Here, in the present invention, water-insoluble means the following: <1> ~ <4> This refers to the situation where, when a PVA film is immersed in 30°C water (deionized water) using the procedure described above, the PVA film does not completely dissolve, and some parts remain undissolved.

[0023] <1> The PVA film is placed in a constant temperature and humidity chamber adjusted to 20°C and -65%RH for at least 16 hours to allow it to adjust its humidity. <2> After cutting out a rectangular sample measuring 40mm in length and 35mm in width from a humidity-controlled PVA film, the sample is sandwiched and fixed between two 50mm x 50mm plastic plates, each with a rectangular window (hole) measuring 35mm in length and 23mm in width, so that the length of the sample is parallel to the length of the window and the sample is positioned approximately in the center of the window's width. <3> Add 300 mL of deionized water to a 500 mL beaker and stir with a magnetic stirrer equipped with a 3 cm long bar at a rotation speed of 280 rpm while adjusting the water temperature to 30 °C. <4> the above <2> The sample, fixed to a plastic plate, is immersed in deionized water in a beaker for 1000 seconds, taking care not to allow it to come into contact with the rotating bar of the magnetic stirrer.

[0024] (PVA) In the PVA film of the present invention, the PVA can be a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing vinyl ester monomers. Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versaticate. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0025] Vinyl ester polymers are preferably polymers obtained using only one or more vinyl ester monomers as monomers, and more preferably polymers obtained using only one vinyl ester monomer as monomers. The vinyl ester polymer may also be a copolymer of one or more vinyl ester monomers with other monomers copolymerizable thereto.

[0026] Other monomers include, for example, ethylene; olefins with 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, methac Methacrylic acid esters such as i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidepropanesulfonic acid or its salts, acrylamidopropyldimethylamine or its salts, N-methylolacrylamide or its derivatives. Examples of derivatives include: methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or its salts, methacrylamidepropyldimethylamine or its salts, N-methylolmethacrylamide or its derivatives; 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 acrylonitrile and methacrylonitrile; 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 its salts, esters, or acid anhydrides; itaconic acid or its salts, esters, or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. Furthermore, vinyl ester polymers may have structural units derived from one or more of these other monomers.

[0027] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol% or less, and more preferably 8 mol% or less, based on the total number of moles of structural units constituting the vinyl ester polymer.

[0028] The degree of polymerization of PVA is preferably 200 or higher, more preferably 300 or higher, and even more preferably 500 or higher. By setting the degree of polymerization of PVA to be above the lower limit mentioned above, it is possible to ensure the mechanical strength of the resulting PVA film while preventing excessive crystallization of the PVA. On the other hand, the degree of polymerization of PVA is preferably 8,000 or lower, more preferably 6,000 or lower, and even more preferably 4,000 or lower.

[0029] The degree of polymerization of PVA refers to the average degree of polymerization measured in accordance with the description in JIS K 6726-1994. That is, the degree of polymerization (Po) is calculated by the following formula (1).

[0030] Degree of polymerization Po = ([η]×10 4 (8.29) (1 / 0.62) (1)

[0031] In formula (1) above, η is the intrinsic viscosity (deciliters / g) measured in water at 30°C after resaponification and purification of PVA.

[0032] The degree of saponification of PVA is preferably 90 mol% or higher, more preferably 95 mol% or higher, even more preferably 99 mol% or higher, and particularly preferably 99.8 mol% or higher. A degree of saponification of PVA of 99 mol% or higher makes the resulting PVA film 1 more likely to be water-insoluble. The definition of water-insoluble is as described above.

[0033] The degree of saponification of PVA refers to the ratio (mol%) of moles of vinyl alcohol units to the total number of moles of structural units that can be converted to vinyl alcohol units by saponification (typically vinyl ester monomer units) and vinyl alcohol units. The degree of saponification of PVA can be measured in accordance with the description in JIS K 6726-1994.

[0034] PVA may contain one type of PVA alone, or it may contain two or more types of PVA with different degrees of polymerization, saponification, and modification.

[0035] The PVA content in the PVA film of the present invention is not necessarily limited, but is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0036] (Plasticizer) The PVA film of the present invention preferably contains a plasticizer. By including a plasticizer, the PVA film can be given flexibility equivalent to that of other plastic films, and the breakage of the PVA film during the film-forming and stretching processes can be suppressed.

[0037] Examples of plasticizers include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. These plasticizers may be used individually or in combination of two or more. Among these, ethylene glycol or glycerin are preferred as plasticizers, and glycerin is more preferred, for reasons such as being less likely to bleed out onto the surface of the PVA film.

[0038] The plasticizer content in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of PVA. On the other hand, the plasticizer content is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of PVA. When the plasticizer content is within the above range, it becomes easier to adjust the proportion of sodium elements (Na1S, Na2S, Na1B, Na2B) in the total elements of the PVA film to within the above range. This allows for sufficient improvement of mechanical properties such as impact strength. Furthermore, it prevents the PVA film from becoming too flexible, reducing its handling properties, and prevents the plasticizer from bleeding out onto the surface of the PVA film.

[0039] (Surfactants) In the present invention, it is preferable that the proportion of sodium elements in the total elements on the first or second surface of the PVA film (Na1S, Na2S), or the proportion of sodium elements in the total elements on the surface at a depth of 0.01 μm from the first or second surface (Na1B, Na2B), is within a specific range. That is, the PVA film of the present invention is characterized by the presence of sodium elements on the surface and inside the film. In the present invention, it is preferable that these sodium elements are contained in a sodium sulfate type surfactant or a sodium sulfonate type surfactant. That is, it is preferable that the PVA film of the present invention contains a sodium sulfate type surfactant or a sodium sulfonate type surfactant.

[0040] Examples of the sodium sulfate-type surfactant include alkyl sodium sulfate, polyoxyethylene alkyl ether sodium sulfate, polyoxypropylene alkyl ether sodium sulfate, and polyoxyethylene alkylphenyl ether sodium sulfate. The alkyl group is preferably one having 8 to 20 carbon atoms, with a lauryl group being more preferred.

[0041] Examples of the sodium sulfonate-type surfactant include sodium alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkylnaphthalene sulfonate, disodium alkyl sulfosuccinate, and disodium polyoxyethylene alkyl sulfosuccinate. The alkyl group is preferably an alkyl group having 8 to 20 carbon atoms, with a dodecyl group being more preferred.

[0042] Such surfactants may be used individually or in combination of two or more. That is, it is preferable that the sodium sulfate type surfactant or sodium sulfonate type surfactant is at least one selected from the group consisting of sodium alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, sodium alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkylnaphthalene sulfonate, disodium alkyl sulfosuccinate, and disodium polyoxyethylene alkyl sulfosuccinate. Sodium polyoxyethylene alkyl ether sulfate is preferable because it is easily present on the surface of the PVA film 1, resulting in good peelability of the PVA film 1 from the support during film formation.

[0043] Furthermore, the product may contain surfactants other than the sodium sulfate type surfactant or sodium sulfonate type surfactant mentioned above. Nonionic surfactants are preferred as surfactants other than sodium sulfate type surfactants or sodium sulfonate type surfactants because they are excellent in reducing surface abnormalities during film formation of PVA films, alkanolamide type surfactants are more preferred, and dialkanolamides of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids with 8 to 30 carbon atoms, etc.) (e.g., lauric acid diethanolamide, etc.) are even more preferred.

[0044] The molecular weight of the sodium sulfate-type surfactant or sodium sulfonate-type surfactant is preferably 200 or more, and more preferably 250 or more. The molecular weight of the sodium sulfate-type surfactant or sodium sulfonate-type surfactant is preferably 10,000 or less, and more preferably 5,000 or less. By having a molecular weight within the above range, it is possible to obtain a film with a small number of surfactant aggregates on the surface of the PVA film, and with good peelability and film surface quality.

[0045] In the PVA film of the present invention, the content of the sodium sulfate type surfactant or sodium sulfonate type surfactant is preferably 0.02 parts by mass or more, more preferably 0.03 parts by mass or more, and even more preferably 0.05 parts by mass or more, per 100 parts by mass of PVA. The content of the sodium sulfate type surfactant or sodium sulfonate type surfactant is preferably 0.4 parts by mass or less, more preferably 0.3 parts by mass or less, even more preferably 0.2 parts by mass or less, and particularly preferably 0.1 parts by mass or less, per 100 parts by mass of PVA. When the surfactant content is within the above range, it becomes easier to adjust the proportion of sodium elements in the total elements in the PVA film (Na1S, Na2S, Na1B, Na2B) to be within the above range. Furthermore, when the surfactant content is within the above range, it is possible to prevent adhesion between PVA films (hereinafter sometimes referred to as "blocking"). Furthermore, it is possible to prevent the surfactant from bleeding out to the surface of the PVA film or deterioration of the appearance of the PVA film due to the aggregation of surfactants. Furthermore, the PVA film of the present invention may contain surfactants other than the sodium sulfate type surfactant or sodium sulfonate type surfactant. In that case, the total amount of surfactants in the PVA film is preferably 0.04 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.15 parts by mass or more, per 100 parts by mass of PVA. The total amount of surfactants in the PVA film is preferably 10 parts by mass or less, more preferably 1 part by mass or less, and even more preferably 0.5 parts by mass or less.

[0046] (Other ingredients) In addition to PVA, the PVA film of the present invention may contain components such as water-soluble polymers, moisture, antioxidants, ultraviolet absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, antifungal agents, and other polymer compounds within a range that does not interfere with the effects of the present invention. The proportion of the total mass of PVA, surfactants, plasticizers, and other components other than PVA in the total mass of the PVA film is preferably 60 to 100% by mass, more preferably 80 to 100% by mass, and even more preferably 90 to 100% by mass.

[0047] <Method for manufacturing PVA film> The method for manufacturing the PVA film of the present invention is not particularly limited, and for example, any of the following methods can be adopted. Such methods include adding a solvent, additives, etc. to PVA to homogenize the film-forming stock solution, and then using a casting film-forming method, a wet film-forming method (a method of discharging into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method of once cooling and gelling the film-forming stock solution and then extracting and removing the solvent), or a method of forming a film by a combination of these, or a melt extrusion film-forming method or an inflation molding method in which the film-forming stock solution obtained using an extruder or the like is extruded from a T-die or the like to form a film. Among these, as the method for manufacturing the PVA film, the casting film-forming method and the melt extrusion film-forming method are preferred. By using these methods, a homogeneous PVA film can be obtained with good productivity. Hereinafter, the case of manufacturing the PVA film using the casting film-forming method or the melt extrusion film-forming method will be described.

[0048] When manufacturing the PVA film of the present invention using a casting method or a melt extrusion method, first, a film-forming stock solution containing PVA, a solvent, and, if necessary, additives such as plasticizers is prepared. Next, this film-forming stock solution is flowed (supplied) in a film-like manner onto a rotating support such as a metal roll or a metal belt. This forms a liquid film of the film-forming stock solution on the support. The liquid film solidifies into a film when heated on the support and the solvent is removed. Examples of methods for heating the liquid film include heating the support itself with a heat transfer medium or the like, or blowing hot air onto the opposite side of the liquid film that is in contact with the support. The solidified long film (PVA film) is peeled from the support, dried using a drying roll or drying oven as necessary, and further heat-treated as necessary, before being wound into a roll.

[0049] During the drying process (solvent removal process) of the liquid coating flowed onto the support, and the subsequent drying process of the PVA film, the PVA crystallizes while being heated. The rate of crystallization at this time is influenced not only by the proportion of structural units derived from other monomers in the PVA, the degree of polymerization of the PVA, the degree of saponification of the PVA, and the plasticizer content, but also by the moisture content, temperature, and draw (tensile elongation in the flow direction) of the PVA.

[0050] Normally, the drying of PVA film proceeds by the evaporation of volatile components from the open film surface that is not in contact with the support or drying rolls. Therefore, during the drying process, a concentration distribution of volatile components such as moisture occurs in the thickness direction of the PVA film, and depending on the temperature and drawing conditions at the time, a distribution of sodium elements occurs in the thickness direction of the PVA film. This distribution of sodium elements, that is, the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B), can be adjusted by the surface temperature of the support, the contact time with the support, the temperature and amount of hot air, and the temperature of the drying rolls or drying oven. Therefore, by appropriately adjusting each of the above factors, the distribution of sodium elements can be adjusted.

[0051] The volatile content of the film-forming stock solution (the concentration of volatile components such as solvents that are removed by volatilization or evaporation during film formation) is preferably 50% by mass or more, and more preferably 55% by mass or more. The volatile content of the film-forming stock solution is preferably 90% by mass or less, and more preferably 80% by mass or less. When the volatile content is within the above range, the viscosity of the film-forming stock solution can be adjusted to a suitable range, thereby improving the film-forming properties of the liquid coating flowed onto the support and making it easier to obtain a PVA film with a uniform thickness. Furthermore, when the volatile content is within the above range, it becomes easier to adjust the proportion of sodium elements in the total elements on the first surface of the obtained PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) to be within the above range. The film-forming stock solution may contain a dichroic dye as needed. The volatile content of the film-forming stock solution refers to the value obtained by the following formula (2).

[0052] Volatile fraction (mass%) of the film-forming solution = {(Wa-Wb) / Wa} × 100 (2)

[0053] In formula (2) above, Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of the film-forming stock solution after drying Wa (g) in an electric dryer at 105°C for 16 hours.

[0054] There are no particular limitations on the method for preparing the film-forming stock solution. Examples include dissolving PVA and additives such as plasticizers and surfactants in a solvent in a dissolution tank, or melt-kneading hydrated PVA together with additives such as plasticizers and surfactants using a single-screw or twin-screw extruder.

[0055] The film-forming solution generally passes through the die lip of a die such as a T-die and flows in a film-like manner onto a support such as a metal roll or metal belt. On the support, the solvent evaporates from the side of the flowed film-like solution that is not in contact with the support (hereinafter sometimes referred to as the free side), while virtually no evaporation occurs from the side that is in contact with the support (hereinafter sometimes referred to as the touch side). As a result, a distribution occurs in the thickness direction of the film where the solvent concentration is lower on the free side and higher on the touch side. Therefore, the solidification of PVA also proceeds first from the free side.

[0056] The surface temperature of the support onto which the film-forming solution is applied is preferably 65°C or higher, more preferably 70°C or higher, and even more preferably 75°C or higher. The surface temperature of the support onto which the film-forming solution is applied is preferably 110°C or lower, more preferably 100°C or lower, and even more preferably 95°C or lower. When the surface temperature is within the above range, the drying of the liquid film applied to the support and the segregation of sodium elements near the surface of the film proceed at an appropriate rate, making it easier to adjust the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) to the above range. As a result, the film surface of the PVA film can be formed normally, and the peelability from the support is good.

[0057] The PVA film is dried on a support to a volatile content of preferably 5-50% by mass (solvent removal), then peeled from the support and further dried as necessary. The drying method is not particularly limited and includes passing through a drying oven or contacting with drying rolls. When drying the PVA film using multiple drying rolls, it is preferable to alternately contact one surface of the PVA film with the drying rolls. This allows for adjustment of the proportion of sodium element in the total elements of the PVA film on both sides of the PVA film (two surfaces perpendicular to the thickness direction). In this case, the number of drying rolls is preferably 3 or more, more preferably 4 or more, and even more preferably 5 or more. The number of drying rolls is preferably 30 or less, more preferably 25 or less, and even more preferably 20 or less.

[0058] The temperature of the drying oven or the surface temperature of the drying roll is preferably 40°C or higher, more preferably 45°C or higher, and even more preferably 50°C or higher. The temperature of the drying oven or the surface temperature of the drying roll is preferably 120°C or lower, more preferably 110°C or lower, and even more preferably 100°C or lower. By setting the temperature of the drying oven or the surface temperature of the drying roll within the above range, it becomes easier to adjust the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) to within the above range.

[0059] After peeling the PVA film from the drying roll, it can be heat-treated. The heat treatment is performed by alternately bringing one side of the PVA film into contact with multiple heat treatment rolls (for example, two heat treatment rolls). The surface temperature of the heat treatment rolls is preferably 70°C or higher, and more preferably 80°C or higher. The surface temperature of the heat treatment rolls is preferably 150°C or lower, and more preferably 140°C or lower. By setting the surface temperature of the heat treatment rolls within the above range, it becomes easier to adjust the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) to within the above range.

[0060] The PVA film manufactured in this way is then subjected to further processes such as humidity control treatment and trimming of both ends (edges) of the film, as needed, before being wound into a roll on a cylindrical core and packaged in a moisture-proof container to become the final product.

[0061] The volatile content of the PVA film ultimately obtained through the series of processes is not necessarily limited. The volatile content of the PVA film is preferably 1% by mass or more, and more preferably 2% by mass or more. The volatile content of the PVA film is preferably 5% by mass or less, and more preferably 4% by mass or less.

[0062] The thickness of the PVA film ultimately obtained through the series of processes is not necessarily limited. The thickness of the PVA film is preferably 10 μm or more, and more preferably 20 μm or more. The thickness of the PVA film is preferably 90 μm or less, and more preferably 80 μm or less. As the PVA film becomes thicker, the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) tend to decrease. As the PVA film becomes thinner, the proportion of sodium elements in the total elements on the first surface of the PVA film (Na1S, Na1B) and the proportion of sodium elements in the total elements on the second surface of the PVA film (Na2S, Na2B) tend to increase. The thickness of the PVA film can be adjusted by the concentration of the film-forming solution, the roll temperature, etc.

[0063] <Method for manufacturing optical films> The PVA film of the present invention is preferably a film for manufacturing optical films. That is, the PVA film of the present invention is suitably used as a raw material film when manufacturing optical films. Examples of optical films include polarizing films, viewing angle improving films, phase difference films, and brightness improving films, but polarizing films are preferred. Below, a method for manufacturing a polarizing film will be specifically described as an example of a method for manufacturing an optical film.

[0064] Polarizing films can typically be manufactured using PVA film as the base film, through processing steps such as swelling, dyeing, crosslinking, stretching, and fixing. Specific examples of processing solutions used in each step include swelling solution used in swelling, dyeing solution used in dyeing, crosslinking solution used in crosslinking, stretching solution used in stretching, fixing solution used in fixing, and washing solution used in washing.

[0065] The following describes in detail each processing step that can be used in the manufacturing method for producing polarizing films. In the manufacturing method for polarizing films, one or more of the following processes may be omitted, the same process may be performed multiple times, or different processes may be performed simultaneously.

[0066] (Washing treatment before swelling treatment) It is preferable to perform a cleaning treatment on the PVA film before performing the swelling treatment on the PVA film. Such a cleaning treatment before the swelling treatment can remove anti-blocking agents and other substances adhering to the PVA film, and prevent contamination of each treatment solution in the polarizing film manufacturing process with anti-blocking agents and other substances. The cleaning treatment is preferably performed by immersing the PVA film in the cleaning treatment solution, but it can also be performed by spraying the cleaning treatment solution onto the PVA film. For example, water can be used as the cleaning treatment solution. The temperature of the cleaning treatment solution is preferably in the range of 20 to 40°C. A temperature of 20°C or higher makes it easier to remove anti-blocking agents and other substances adhering to the PVA film. Also, a temperature of 40°C or lower prevents a part of the surface of the PVA film from dissolving and causing the films to stick together, which reduces handling ease. The temperature of the cleaning treatment solution is more preferably 22°C or higher, even more preferably 24°C or higher, and particularly preferably 26°C or higher. Furthermore, the temperature of the cleaning solution is more preferably 38°C or lower, even more preferably 36°C or lower, and particularly preferably 34°C or lower.

[0067] (Swelling treatment) The swelling treatment can be carried out by immersing the PVA film in a swelling treatment solution such as water. The temperature of the swelling treatment solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 24°C or higher. The temperature of the swelling treatment solution is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 36°C or lower. The immersion time in the swelling treatment solution is preferably, for example, 0.1 minutes or more, more preferably 0.5 minutes or more. The immersion time in the swelling treatment solution is preferably, for example, 5 minutes or less, and even more preferably 3 minutes or less. The water used as the swelling treatment solution is not limited to pure water, but may be an aqueous solution in which various components such as boron-containing compounds are dissolved, or a mixture of water and an aqueous medium. The type of boron-containing compound is not particularly limited, but boric acid or borax is preferred from the viewpoint of ease of handling. When the swelling treatment solution contains a boron-containing compound, its concentration is preferably 6% by mass or less from the viewpoint of improving the stretchability of the PVA film.

[0068] (Dialysis treatment) The dyeing treatment is preferably carried out using an iodine-based dye as a dichroic dye, and the dyeing can be performed at any of the following stages: before, during, or after the stretching treatment. The dyeing treatment is preferably carried out by immersing the PVA film in a solution (preferably an aqueous solution) containing iodine-potassium iodide as the dyeing solution. The concentration of iodine in the dyeing solution is preferably in the range of 0.005 to 0.2% by mass, and the mass ratio of potassium iodide / iodine is preferably in the range of 20 to 100. The temperature of the dyeing solution is preferably 20°C or higher, and more preferably 25°C or higher. The temperature of the dyeing solution is preferably 50°C or lower, and more preferably 40°C or lower. The dyeing solution may also contain a boron-containing compound such as boric acid as a crosslinking agent. If the PVA film used as the base film is pre-containing a dichroic dye, the dyeing treatment can be omitted. Furthermore, the PVA film used as the base film can be pre-treated to contain boron-containing compounds such as boric acid and borax.

[0069] (Crosslinking treatment) In the manufacture of polarizing films, it is preferable to perform a crosslinking treatment after the dyeing treatment in order to strengthen the adsorption of dichroic dyes onto the PVA film. The crosslinking treatment can be carried out by using a solution (preferably an aqueous solution) containing a crosslinking agent as the crosslinking treatment solution and immersing the PVA film in the crosslinking treatment solution. As the crosslinking agent, one or more boron-containing compounds such as boric acid and borax can be used. If the concentration of the crosslinking agent in the crosslinking treatment solution is too high, the crosslinking reaction tends to proceed too much, making it difficult to perform sufficient stretching in the subsequent stretching treatment. Also, if the concentration of the crosslinking agent in the crosslinking treatment solution is too low, the effect of the crosslinking treatment tends to be reduced. For this reason, the concentration of the crosslinking agent in the crosslinking treatment solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0070] To suppress the elution of dichroic dyes from the PVA film after dyeing, the crosslinking solution may contain an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking solution is too high, the heat resistance of the resulting polarizing film tends to decrease for reasons unknown. Conversely, if the concentration of the iodine-containing compound in the crosslinking solution is too low, the effect of suppressing the elution of dichroic dyes tends to decrease. The concentration of the iodine-containing compound in the crosslinking solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the iodine-containing compound in the crosslinking solution is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0071] If the temperature of the crosslinking solution is too high, the dichroic dye tends to dissolve, leading to uneven dyeing in the resulting polarized film. Conversely, if the temperature is too low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 25°C or higher. The temperature of the crosslinking solution is preferably 45°C or lower, more preferably 40°C or lower, and even more preferably 35°C or lower.

[0072] In addition to the stretching process described later, the PVA film may be stretched during or between each of the above-mentioned processes. Such stretching (pre-stretching) can prevent wrinkles from forming on the surface of the PVA film. The total stretching ratio of the pre-stretching (the ratio obtained by multiplying the stretching ratios in each process) is preferably 4 times or less, and more preferably 3.5 times or less, based on the original length of the PVA film roll before stretching, from the viewpoint of the polarization performance of the resulting polarizing film. The total stretching ratio of the pre-stretching is preferably 1.5 times or more, and more preferably 2 times or more, based on the original length of the PVA film roll before stretching, from the viewpoint of the polarization performance of the resulting polarizing film.

[0073] The stretching ratio in the swelling treatment is preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.4 times or more. The stretching ratio in the swelling treatment is preferably 3 times or less, more preferably 2.5 times or less, and even more preferably 2.3 times or less.

[0074] The stretching ratio in the dyeing process is preferably 2 times or less, more preferably 1.8 times or less, and even more preferably 1.5 times or less. The stretching ratio in the dyeing process is preferably 1.1 times or more, more preferably 1.15 times or more, and even more preferably 1.2 times or more.

[0075] The stretch ratio in the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and even more preferably 1.3 times or less. The stretch ratio in the crosslinking treatment is preferably 1.05 times or more, more preferably 1.1 times or more, and even more preferably 1.15 times or more.

[0076] (Stretching process) The stretching process may be carried out by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, a solution containing a boron-containing compound such as boric acid (preferably an aqueous solution) is used as the stretching solution, and the stretching can be carried out in the stretching solution, or in a dyeing solution or a fixation solution described later. In the case of the dry stretching method, the stretching can be carried out in air using the PVA film after water absorption. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. When the stretching solution contains a boron-containing compound, the stretchability of the PVA film can be improved, so the concentration of the boron-containing compound in the stretching solution is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and even more preferably 2.5% by mass or more. Furthermore, since the stretchability of the PVA film can be improved, the concentration of the boron-containing compound in the stretching solution is preferably 7% by mass or less, more preferably 6.5% by mass or less, and even more preferably 6% by mass or less.

[0077] It is preferable to include an iodine-containing compound such as potassium iodide in the stretching solution. If the concentration of the iodine-containing compound in the stretching solution is too high, the resulting polarizing film tends to have a strong bluish hue. Also, if the concentration of the iodine-containing compound in the stretching solution is too low, the heat resistance of the resulting polarizing film tends to decrease for reasons unknown. The concentration of the iodine-containing compound in the stretching solution is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. The concentration of the iodine-containing compound in the stretching solution is preferably 8% by mass or less, more preferably 7.5% by mass or less, and even more preferably 7% by mass or less.

[0078] If the temperature of the stretching solution is too high, the PVA film tends to melt and soften, making it prone to breakage. Conversely, if the temperature of the stretching solution is too low, the stretchability of the PVA film tends to decrease. The temperature of the stretching solution is preferably 50°C or higher, more preferably 52.5°C or higher, and even more preferably 55°C or higher. The temperature of the stretching solution is preferably 70°C or lower, more preferably 67.5°C or lower, and even more preferably 65°C or lower. The preferred range of stretching temperature when the stretching process is performed by the dry stretching method is also as described above.

[0079] Since a higher stretching ratio in the stretching process yields a polarizing film with superior polarization performance, the stretching ratio in the stretching process is preferably 1.2 times or more, more preferably 1.5 times or more, and even more preferably 2 times or more. Furthermore, from the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio (the ratio obtained by multiplying the stretching ratios in each process), including the stretching ratio of the pre-stretching process described above, is preferably 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more, based on the original length of the raw material PVA film before stretching. There is no particular upper limit to the stretching ratio, but since stretching fracture is more likely to occur if the stretching ratio is too high, it is preferable to keep it at 8 times or less.

[0080] There are no particular restrictions on the method of uniaxial stretching; uniaxial stretching in the longitudinal direction or transverse uniaxial stretching in the width direction can be employed. When manufacturing polarizing films, uniaxial stretching in the longitudinal direction is preferred because it yields films with superior polarization performance. Uniaxial stretching in the longitudinal direction can be performed by using a stretching device equipped with multiple rolls that are parallel to each other and changing the peripheral speed between each roll.

[0081] In the present invention, there are no particular restrictions on the maximum stretching speed (% / min) when the stretching process is performed by uniaxial stretching, but it is preferably 200% / min or more, more preferably 300% / min or more, and even more preferably 400% / min or more. Here, the maximum stretching speed refers to the fastest stretching speed in a stage when the stretching process of the PVA film is performed in two or more stages using three or more rolls with different peripheral speeds. If the stretching process of the PVA film is performed in one stage without dividing it into two or more stages, the stretching speed in that stage becomes the maximum stretching speed. Furthermore, the stretching speed refers to the increase in the length of the PVA film per unit time compared to the length of the PVA film before stretching. For example, a stretching speed of 100% / min is the speed at which the PVA film is deformed to twice its length in one minute from its length before stretching. A higher maximum stretching speed is preferable because it allows the stretching process (uniaxial stretching) of the PVA film to be performed at a higher speed, resulting in improved productivity of the polarizing film. On the other hand, if the maximum stretching speed becomes too high, excessive tension may be applied locally to the PVA film during the stretching process (uniaxial stretching), making stretching fracture more likely. From this perspective, it is preferable that the maximum stretching speed does not exceed 900% / min.

[0082] (Fixed process) In the manufacture of polarizing films, it is preferable to perform a fixation treatment to strengthen the adsorption of dichroic dyes onto the PVA film. The fixation treatment can be carried out by using a solution (preferably an aqueous solution) containing one or more boron-containing compounds such as boric acid and borax as the fixation treatment solution, and immersing the PVA film (preferably the stretched PVA film) in the fixation treatment solution. If necessary, the fixation treatment solution may also contain iodine-containing compounds or metal compounds. The concentration of the boron-containing compound in the fixation treatment solution is preferably 2% by mass or more, and more preferably 3% by mass or more. The concentration of the boron-containing compound in the fixation treatment solution is preferably 15% by mass or less, and more preferably 10% by mass or less. The temperature of the fixation treatment solution is preferably 15°C or higher, and more preferably 25°C or higher. The temperature of the fixation treatment solution is preferably 60°C or lower, and more preferably 40°C or lower.

[0083] (Washing process after dyeing) It is preferable to perform a washing treatment on the PVA film after the dyeing treatment, preferably after the stretching treatment. The washing treatment is preferably carried out by immersing the PVA film in a washing solution, but it can also be carried out by spraying the washing solution onto the PVA film. For example, water can be used as the washing solution. The water is not limited to pure water and may contain an iodine-containing compound such as potassium iodide. The washing solution may also contain a boron-containing compound, in which case the concentration of the boron-containing compound is preferably 2.0% by mass or less.

[0084] The temperature of the cleaning solution is preferably in the range of 5 to 40°C. A temperature of 5°C or higher can suppress the breakage of the PVA film due to the freezing of water. Furthermore, a temperature of 40°C or lower improves the optical properties of the resulting polarizing film. The temperature of the cleaning solution is more preferably 7°C or higher, and even more preferably 10°C or higher. Furthermore, the temperature of the cleaning solution is more preferably 38°C or lower, and even more preferably 35°C or lower.

[0085] Specific methods for manufacturing polarizing films include dyeing, stretching, and crosslinking and / or fixing treatments applied to a PVA film. A preferred example is a method in which the PVA film is subjected to swelling, dyeing, crosslinking, stretching (especially uniaxial stretching), and washing in this order. The stretching treatment may also be performed in any of the preceding treatment steps, or in two or more stages.

[0086] A polarizing film can be obtained by drying the PVA film after each of the above processes. There are no particular restrictions on the drying method; for example, a contact method in which the film is brought into contact with a heated roll, a method of drying in a hot air dryer, and a floating method in which the film is dried with hot air while suspended in the air can be used.

[0087] <Polarizing plate> The polarizing film obtained as described above is preferably used as a polarizing plate by laminating a protective film that is optically transparent and has mechanical strength to one or both sides of it. Suitable protective films include cellulose triacetate (TAC) film, cycloolefin polymer (COP) film, cellulose acetate / butyrate (CAB) film, acrylic film, and polyester film. Suitable adhesives for lamination include PVA-based adhesives and urethane-based adhesives, but PVA-based adhesives are preferred.

[0088] The polarizing plate obtained as described above can be laminated with an acrylic-based adhesive and then bonded to a glass substrate to be used as an LCD component. It may also be bonded at the same time with a phase difference film, a viewing angle enhancement film, a brightness enhancement film, etc. [Examples]

[0089] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way by the following examples.

[0090] (1) XPS measurement of PVA film (1-1) Measurement of the proportion of sodium element in the total element (Na1S and Na2S) The PVA films obtained in the following examples or comparative examples were cut to a size of 5 mm x 5 mm and set on the measurement base of the XPS measuring device via conductive double-sided tape. Then, the first surface and the second surface of the PVA film (two surfaces perpendicular to the thickness direction of the PVA film) were measured using XPS under the following measurement conditions, and XPS spectra were obtained.

[0091] (XPS measurement conditions) Measuring device: Ohi Quantera SXM (ULVAX―PHI.INC.) Analysis software: Multi Pack ver9.0 (ULVAX-PHI.INC.) X-ray source: Monochromatic Al Kα (1486.6eV) X-ray beam diameter: 100 μmφ (25 W, 15 kV) Measurement range: 100 μm × 300 μm Signal acquisition angle: 45° Charge neutralization conditions: Neutralization electron gun, Ar+ ion gun Vacuum degree: 1×10 -6 Pa Measured elements (peaks of excited inner-shell atoms used for quantification): B(1s), C(1s), N(1s), O(1s), Na(1s), Si(2p), P(2p), S(2p)

[0092] The obtained XPS spectra were analyzed using the above-mentioned analysis software to determine the proportion of sodium elements (Na1S and Na2S) in the total elemental composition on the first and second surfaces of the PVA film. Here, S stands for surface.

[0093] (1-2) Measurement of the proportion of sodium element in the total element (Na1B and Na2B) The PVA films obtained in the following examples or comparative examples were cut to a size of 5 mm x 5 mm, and the first and second surfaces of the PVA film (two surfaces perpendicular to the thickness direction of the PVA film) were etched to a depth of 0.01 μm in the thickness direction of the PVA film in the above-mentioned XPS measuring apparatus under the following conditions. Then, XPS measurements and analyses were performed on the surface at a depth of 0.01 μm from the first or second surface of the PVA film under the same conditions as above, and the proportion of sodium elements (Na1B and Na2B) in the total elements at the surface at a depth of 0.01 μm from the first or second surface was determined.

[0094] (Etching conditions) Processing conditions: Acceleration voltage 10kV Sample current: 20mA Scanning area: 0.5mm x 2.0mm Etching rate: 20 nm / min Etching material: C60 (Buckminsterfullerene)

[0095] (2) Evaluation of the peelability of the PVA film from the support In the following examples or comparative examples, the delamination state of the PVA film from the support was visually observed and evaluated according to the following criteria after the PVA film was fabricated. Peelability evaluation criteria: A: The peeling position is horizontal to the width direction of the PVA film, and there are no wrinkles or stretching on the surface of the PVA film. B: The peeling position is horizontal to the width direction of the PVA film, but wrinkles and stretching have occurred on the surface of the PVA film. C: The peeling area is wavy in the width direction of the PVA film, and wrinkles and stretching have occurred on the surface of the PVA film.

[0096] (3) Evaluation of optical uniformity of polarizing film The optical uniformity of the polarizing films obtained in the following examples or comparative examples was observed visually and evaluated according to the following criteria. A: Optical inconsistencies are hardly noticeable. B: Optical inconsistencies are visible. C: Optical inconsistencies are clearly visible.

[0097] <Example 1> A film-forming stock solution (volatile fraction 66% by mass) was prepared by melt-mixing 100 parts by mass of PVA (saponification degree 99.9 mol%, degree of polymerization 2400), 12 parts by mass of glycerin as a plasticizer, 0.08 parts by mass of sodium polyoxyethylene lauryl ether sulfate (average molecular weight: 430) as a surfactant, 0.16 parts by mass of lauric acid diethanolamide, and 217.6 parts by mass of water in a melt extruder. Next, this film-forming stock solution was extruded in a film-like manner from a T-die onto a support (surface temperature 80°C) to form a liquid coating on the support. After the film-forming stock solution was poured onto the support and the PVA film was peeled off the support, the PVA film was further dried between the first drying roll and the final drying roll (19th drying roll) immediately before the heat treatment roll, so that one side of the PVA film alternately contacts each drying roll, and then peeled off from the final drying roll. At this time, the surface temperature of each drying roll from the first drying roll to the final drying roll was set to 75°C. Furthermore, the PVA film was peeled from the final drying roll and heat-treated so that one side of the PVA film alternately contacted the other side of each heat-treatment roll. At this time, the heat treatment was carried out using two heat-treatment rolls, and the surface temperature of both heat-treatment rolls was set to 90°C to obtain a PVA film (thickness 60 μm, width 1200 mm).

[0098] XPS measurements were performed on the obtained PVA film. The results showed that the first surface of the PVA film contained 0.7 mol% Na1S and 0.2 mol% Na1B. The second surface of the PVA film contained 0.6 mol% Na2S and 0.1 mol% Na2B. The first surface of the PVA film was the side that was in contact with the support material. The second surface of the PVA film was the side that was not in contact with the support material (free surface).

[0099] The obtained PVA film was slit to a width of 650 mm, and polarizing films were continuously manufactured by performing swelling, dyeing, crosslinking, stretching, washing, and drying treatments in this order. The swelling treatment was performed by uniaxial stretching to 2.00 times its length while immersed in pure water (swelling treatment solution) at 25°C. The dyeing treatment was performed by uniaxial stretching to 1.26 times its length while immersed in a potassium iodide / iodine dyeing solution (dyeing treatment solution) at a temperature of 32°C (potassium iodide / iodine (mass ratio) 23, iodine concentration 0.03~0.05 mass%). In this dyeing treatment, the iodine concentration in the dyeing treatment solution was adjusted within the range of 0.03~0.05 mass% so that the transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was in the range of 43.5% ± 0.2%. The crosslinking treatment was performed by uniaxially stretching the material 1.19 times in length while immersing it in a 32°C boric acid aqueous solution (crosslinking treatment solution) (boric acid concentration 2.6 mass%). The stretching treatment was performed by uniaxially stretching the material 2.00 times in length while immersing it in a 55°C boric acid / potassium iodide aqueous solution (stretching treatment solution) (boric acid concentration 2.8 mass%, potassium iodide concentration 5 mass%). The maximum stretching speed in this stretching treatment was 400% / min. The washing treatment was performed by immersing the material in a 22°C potassium iodide / boric acid aqueous solution (washing treatment solution) (potassium iodide concentration 3-6 mass%, boric acid concentration 1.5 mass%) for 12 seconds without stretching. The drying treatment was performed by hot air drying at 80°C for 1.5 minutes without stretching to obtain a polarizing film. At this time, the peelability evaluation of the PVA film from the support was "A", and the optical uniformity evaluation of the polarizing film was "A". The results are shown in Table 1.

[0100] <Example 2> Except for changing the type of PVA to PVA with a saponification degree of 99.0 mol% and a degree of polymerization of 2400, and setting the surface temperature of each drying roll from the first drying roll to the final drying roll to 70°C and the surface temperature of the heat treatment roll to 85°C, the PVA film and polarizing film were manufactured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0101] <Example 3> Except for changing the type of PVA to PVA with a saponification degree of 99.9 mol% and a degree of polymerization of 4100, setting the surface temperature of each drying roll from the first drying roll to the final drying roll to 85°C, the surface temperature of the heat treatment roll to 97°C, and changing the thickness of the PVA film to 30 μm, the PVA film and polarizing film were manufactured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0102] <Comparative Example 1> The PVA film and polarizing film were manufactured and evaluated in the same manner as in Example 1, except that the surfactant sodium polyoxyethylene lauryl ether sulfate was not used. The results are shown in Table 1.

[0103] <Comparative Example 2> The PVA film and polarizing film were manufactured and evaluated in the same manner as in Example 1, except that the amount of sodium polyoxyethylene lauryl ether sulfate, a surfactant, was changed to 0.45 parts by mass. The results are shown in Table 1.

[0104] [Table 1]

[0105] Based on the above results, the PVA film of the present invention can be used to produce a polarizing film with good peelability from the support and minimal optical uniformity, using only a small amount of surfactant without the use of fluorine-containing surfactants. [Explanation of symbols]

[0106] 1 PVA film 2. Thickness direction of PVA film 3. First surface 4. Second surface

Claims

1. A non-water-soluble polyvinyl alcohol film, When one of the two surfaces perpendicular to the thickness direction of the polyvinyl alcohol film is designated as the first surface, The proportion of sodium elements (Na₂S) in the total elements, as determined by analyzing the first surface by X-ray photoelectron spectroscopy, is 0.5 to 1.5 mol%, The proportion of sodium elements (Na₂B) in the total elements, determined by analyzing a surface 0.01 μm deep from the first surface using X-ray photoelectric spectroscopy, is 0.3 mol% or less. Contains sodium sulfate type surfactant or sodium sulfonate type surfactant, A polyvinyl alcohol film in which the sodium element is derived from the sodium sulfate type surfactant or the sodium sulfonate type surfactant.

2. When, of the two surfaces of the polyvinyl alcohol film perpendicular to the thickness direction, the surface facing the first surface is designated as the second surface, The percentage of sodium (Na₂S) in the total elements, determined by analyzing the second surface by X-ray photoelectric spectroscopy, is 0.3 to 1.5 mol%, The polyvinyl alcohol film according to claim 1, wherein the proportion of sodium elements (Na2B) in the total elements, as determined by analyzing a surface at a depth of 0.01 μm from the second surface by X-ray photoelectric spectroscopy, is 0.3 mol% or less.

3. The polyvinyl alcohol film according to claim 1 or 2, wherein the sodium sulfate type surfactant or the sodium sulfonate type surfactant is at least one selected from the group consisting of sodium alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, sodium alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkylnaphthalene sulfonate, disodium alkyl sulfosuccinate, and disodium polyoxyethylene alkyl sulfosuccinate.

4. The polyvinyl alcohol film according to any one of claims 1 to 3, wherein the molecular weight of the sodium sulfate type surfactant or the sodium sulfonate type surfactant is 200 to 10,000, and the content of the sodium sulfate type surfactant or the sodium sulfonate type surfactant is 0.02 to 0.4 parts by mass per 100 parts by mass of polyvinyl alcohol contained in the polyvinyl alcohol film.

5. A polyvinyl alcohol film according to any one of claims 1 to 4, which is a film for manufacturing optical films.

6. The polyvinyl alcohol film according to claim 5, wherein the optical film is a polarizing film.

7. A method for producing a polarizing film, comprising dyeing, stretching, and drying a polyvinyl alcohol film according to any one of claims 1 to 6.

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