JP7840874B2 - Polyvinyl alcohol film and polarizing film using the same - Google Patents

Description

The present invention relates to a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA" and "polyvinyl alcohol film" as "PVA film") which is suitably used in the manufacture of polarizing films, and a polarizing film obtained using said PVA film.

Liquid crystal display (LCD) displays are used in a wide range of applications, including small devices such as calculators and watches, laptop computers, LCD monitors, LCD color projectors, LCD televisions, in-car navigation systems, mobile phones, and measuring instruments used both indoors and outdoors. Polarizing films, which have light transmission and shielding functions, along with liquid crystals that have light switching functions, are fundamental components of LCDs.

The mainstream polarizing films are those in which dichroic dyes such as iodine-based dyes ( _I3- ^, _I5- ^, etc.) are adsorbed onto a matrix (a stretched film that has been uniaxially stretched and oriented) made of uniaxially stretched PVA film. Such polarizing films are manufactured by uniaxially stretching a PVA film that already contains dichroic dyes, by adsorbing dichroic dyes simultaneously with uniaxial stretching of a PVA film, or by adsorbing dichroic dyes after uniaxial stretching of a PVA film.

In recent years, with the expansion of LCD applications, there has been a demand for increased efficiency in polarizing film manufacturing. While increasing the line speed of the manufacturing line, i.e., increasing the stretching speed of the PVA film, is effective for efficiently producing polarizing films, there has been a problem in increasing the stretching speed as desired. If the stretching speed is too fast, the higher-order structural changes of the PVA molecules accompanying stretching cannot keep up, and stretching breakage easily occurs even at low stretching ratios, making it difficult to obtain a polarizing film with high polarization performance.

The present invention was made to solve the above problems, and aims to provide a PVA film that can be produced as a polarizing film even when stretched at a high stretching speed without impairing high polarization performance. Furthermore, it aims to provide a polarizing film made from such a PVA film.

Gel permeation chromatography (GPC) is a commonly used analytical method for determining the molecular weight and degree of polymerization of resins. Generally, it involves measuring the relative molecular weight using a calibration curve derived from standard substances with known molecular weights.

The detectors typically used are differential refractive index detectors (RI detectors), which detect the difference in refractive index between the mobile phase solvent and the sample. However, it is also possible to use other detectors, such as ultraviolet-visible absorption detectors (UV detectors) and light scattering detectors (LS detectors), in combination to evaluate physical properties.

In recent years, a method has been established for calculating the molecular weight and degree of saponification of polyvinyl alcohol-based materials using both an RI detector and a UV detector (Reference Patent Document: Japanese Patent Application Publication No. 9-196905). This method utilizes the phenomenon that the UV detection intensity increases as the amount of hydroxyl groups (degree of saponification) of PVA decreases, because the residual acetate groups of polyvinyl alcohol absorb UV light. On the other hand, since the concentration-dependent RI detection intensity is almost independent of the degree of saponification, it is possible to calculate the degree of saponification at each molecular weight by determining the ratio of the peak heights of the molecular weight distribution curves obtained from the RI detector and the UV detector, respectively.

Based on the knowledge gained from the GPC analysis of various PVA films for each molecular weight, the inventors diligently conducted further studies and found that the above-mentioned problem can be solved by using a PVA film in which the ratio of UV detection intensity to RI detection intensity at a specific molecular weight satisfies a certain value. Based on this finding, the inventors further investigated and completed the present invention.

In other words, the present invention is
[1] A polyvinyl alcohol film containing polyvinyl alcohol having a degree of saponification of 98 mol% or more and a degree of polymerization of 1200 or more and 8000 or less, wherein the polyvinyl alcohol is measured by gel permeation chromatography at a measurement temperature of 40°C using hexafluoroisopropanol with 20 mM sodium trifluoroacetate added as the mobile phase solvent, and the result satisfies the following formula (1).
0.017 ≤ Huv²/Hri² ≤ 0.030 (1)
(However, Huv2 is the detection intensity at an absorption wavelength of 210 nm for a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a UV-Vis absorbance detector (UV detector) of a gel permeation chromatograph, and Hri2 is the detection intensity at a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a refractive index detector (RI detector) of a gel permeation chromatograph.)
[2] The polyvinyl alcohol film according to [1], wherein the polyvinyl alcohol is measured by the gel permeation chromatograph and satisfies the following formula (2).
Huv1/Hri1-Huv2/Hri2≦-0.002 (2)
(However, Huv1 is the maximum detection intensity at an absorption wavelength of 210 nm obtained using a UV detector, and Hri1 is the maximum detection intensity obtained using an RI detector.)
[3] The polyvinyl alcohol film according to [1] or [2], wherein the value obtained by dividing Hri2 by Hri1, Hri2/Hri1, is 0.2 or less;
[4] A polyvinyl alcohol film according to any one of [1] to [3] above, wherein the polyvinyl alcohol comprises polyvinyl alcohol (A) having a degree of polymerization of 1500 or more and 3500 or less and a degree of saponification of 98 mol% or more, and polyvinyl alcohol (B) having a degree of polymerization of 50 or more and 800 or less and a degree of saponification that is 1 mol% or more and 5 mol% or less lower than that of polyvinyl alcohol (A);
[5] The polyvinyl alcohol film according to [4], wherein the blend ratio of the polyvinyl alcohol (A) to the polyvinyl alcohol (B) is in the range of polyvinyl alcohol (A):polyvinyl alcohol (B) = 75:25 to 97:3 by mass;
[6] An optical film, which is a polyvinyl alcohol film according to any of [1] to [5] above;
[7] A method for producing a polyvinyl alcohol film according to any one of [1] to [6] above, characterized by using polyvinyl alcohol that satisfies formula (1);
[8] A polarizing film made from the polyvinyl alcohol film described in [6] above;
This can be achieved.

By using the PVA film of the present invention, a polarizing film with high polarization performance can be obtained even when stretched at a high stretching speed.

The polyvinyl alcohol film of the present invention satisfies the following formula (1) when measured by gel permeation chromatography using hexafluoroisopropanol with 20 mM sodium trifluoroacetate added as the mobile phase solvent at a measurement temperature of 40°C.
0.017 ≤ Huv²/Hri² ≤ 0.030 (1)

However, Huv2 is the detection intensity at an absorption wavelength of 210 nm for a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a UV-Vis absorbance detector (UV detector) of a gel permeation chromatograph. Hri2 is the detection intensity at a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a differential refractive index detector (RI detector) of a gel permeation chromatograph.

Furthermore, it is preferable that the PVA film of the present invention satisfies the following formula (2) when the PVA contained in the film is analyzed by GPC.
Huv1/Hri1-Huv2/Hri2≦-0.002 (2)

However, Huv1 is the maximum detection intensity at an absorption wavelength of 210 nm obtained using a UV detector, and Hri1 is the maximum detection intensity obtained using an RI detector.

As described above, the detection intensity by GPC using an RI detector is almost independent of the degree of saponification of PVA, whereas the detection intensity at the absorption wavelength of 210 nm using a UV detector in GPC becomes stronger as the degree of saponification decreases. Therefore, by calculating the ratio of the detection intensities of the UV detector and the RI detector at each point in the molecular weight distribution curve and plotting it against the molecular weight, a distribution curve of the degree of saponification as a function of molecular weight for PVA can be obtained. It is presumed that the absorption at a wavelength of 210 nm originates from the acetate group.
In equation (1), Huv2/Hri2 is the ratio of detection intensities of the UV detector and the RI detector at a molecular weight equivalent to 10 to the power of 4.3, i.e., approximately 20,000. A molecular weight of 20,000 corresponds to a degree of polymerization of approximately 450, assuming a molecular weight of 44 for vinyl alcohol units.
In equation (2), the first term Huv1/Hri1 on the left side is the ratio of the detection intensities of the UV detector and the RI detector at the highest molecular weight on the molecular weight distribution curve, i.e., the peak top. Therefore, equation (2) means that the degree of saponification of the relatively low molecular weight PVA component with a molecular weight of approximately 20,000 is lower than that of the PVA component at the peak top of the molecular weight distribution curve.

Furthermore, the "polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3" used to define Huv2 and Hri2 above does not necessarily refer to a molecular weight that perfectly matches 10 to the power of 4.3, but rather includes polymethyl methacrylate equivalent molecular weights with a base-10 logarithm of 4.25 to 4.35.

The reason why low saponification of low molecular weight components results in good high-speed stretchability is not entirely clear, but it is presumed to be as follows.
In the production of polarizing films using PVA film as a raw material, the PVA single crystals present in the raw PVA film absorb water, causing the folded and arranged PVA molecular chains to unravel, which then stretch and crystallize into an oriented form. The lower the molecular weight of the PVA incorporated into the single crystal, the more easily it can absorb water, making crystal unraveling more likely and thus a more likely starting point for changes in the crystal structure. We believe that if the degree of saponification of this low molecular weight PVA is low, unraveling can occur even more easily due to the disorder of the crystal structure, and that changes in the crystal structure can occur in a short time even during high-speed stretching.

The Huv2/Hri2 value is 0.017 or higher, preferably 0.019 or higher, and more preferably 0.021 or higher. If the Huv2/Hri2 value is less than 0.017, the film tends to break easily when stretched at high speed, and sufficient high-speed stretchability cannot be obtained. The Huv2/Hri2 value is 0.030 or lower, preferably 0.029 or lower, and more preferably 0.028 or lower. If the Huv2/Hri2 value exceeds 0.030, the polarization performance of the polarizing film tends to decrease.

The value of Huv1/Hri1 - Huv2/Hri2 is preferably -0.002 or less, more preferably -0.0025 or less, even more preferably -0.003 or less, and particularly preferably -0.0035 or less. If the value of Huv1/Hri1 - Huv2/Hri2 exceeds -0.002, it tends to break easily when stretched at high speed, and sufficient high-speed stretchability cannot be obtained. On the other hand, the lower limit of the value of Huv1/Hri1-Huv2/Hri2 is not necessarily limited, but if the value of Huv1/Hri1-Huv2/Hri2 is too small, the polarization performance of the resulting polarizing film tends to decrease. Therefore, the value of Huv1/Hri1-Huv2/Hri2 is preferably -0.03 or higher, more preferably -0.02 or higher, even more preferably -0.015 or higher, and particularly preferably -0.010 or higher.

In the present invention, it is preferable that Hri2/Hri1, which is the value obtained by dividing Hri2 by Hri1, is 0.2 or less. If Hri2/Hri1 exceeds 0.2, there is a risk that foreign matter will adhere to the polarizing film. This foreign matter is presumed to be aggregated low molecular weight PVA that has eluted into the processing solution in the polarizing film manufacturing process. It is more preferable that Hri2/Hri1 be 0.19 or less, and even more preferable that it be 0.18. On the other hand, there is no particular lower limit to Hri2/Hri1, but if it is too low, the manufacturing cost of PVA tends to increase, so it is preferable that Hri2/Hri1 be 0.1 or more, more preferably 0.12 or more, and even more preferable that it be 0.14 or more.

In the present invention, there are no particular limitations on the method for obtaining a PVA film satisfying formula (1) and/or formula (2). For example, one method is to blend PVA(A) with a high degree of polymerization and a high degree of saponification with PVA(B) with a low degree of polymerization and a low degree of saponification. However, the PVA film of the present invention only needs to satisfy formula (1) and/or formula (2), and may contain only one type of PVA.

[GPC measurement]
In the present invention, Huv1, Huv2, Hri1, and Hri2 can be determined by the following method.
Generally, there are two types of molecular weights that can be determined from GPC measurements: relative molecular weight and absolute molecular weight. In this invention, however, the relative molecular weight calculated from GPC measurements using a differential refractive index detector (RI detector) and an ultraviolet-visible absorption detector (UV detector) is used. In GPC measurements, it is necessary to completely dissolve the measurement sample in the solvent. By using a column suitable for each mobile phase solvent, it is possible to calculate the relative molecular weight in each solvent from the analysis software in the measurement device. In this invention, a PVA film containing PVA can be analyzed under the following measurement conditions. Hexafluoroisopropanol (HFIP) is used as the mobile phase solvent because it can completely dissolve the film.
The relative molecular weight distribution curves obtained from the RI detector and UV detector are calculated using polymethyl methacrylate (PMMA) as a calibration curve standard and the analysis software attached to the analyzer. Hri1 and Huv1 are the maximum detection intensities of the RI detector and UV detector before conversion to the molecular weight distribution curve, at the molecular weight (Mp) that shows the peak top of this molecular weight distribution curve. Here, the detection intensities of the RI detector and UV detector before conversion to the molecular weight distribution curve are the measured intensities before correction by the analysis software, and are the measured intensities of the measurement data obtained from the RI detector and UV detector themselves. Furthermore, Hri2 and Huv2 are the detection intensities of the RI detector and UV detector before conversion to the molecular weight distribution curve at the molecular weight (Mp = ^10⁴⁧ ) where the base-10 logarithm of the relative molecular weight is 4.3. The measurement wavelength used for the UV detector is 210 nm.

[Polyvinyl alcohol]
The PVA film in this invention contains PVA. As the PVA, a vinyl ester polymer obtained by polymerizing vinyl ester monomers and then saponifying it can be used. Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versaticate, etc., and among these, vinyl acetate is preferred from the viewpoint of availability, cost, and PVA productivity.

The vinyl ester polymer described above is preferably obtained using only one or more vinyl ester monomers as monomers, and more preferably obtained using only one vinyl ester monomer as monomers. However, it may also be a copolymer of one or more vinyl ester monomers and other monomers copolymerizable thereto.

Other monomers copolymerizable with such vinyl ester monomers include, for example, ethylene; olefins having 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, Methacrylic acid esters such as n-propyl methacrylate, 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 Acrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or its salts, methacrylamidepropyldimethylamine or its salts, N-methylolmethacrylamide or its derivatives, and other methacrylamide derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, and other N-vinylamides; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether Examples include vinyl ethers such as 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. The above vinyl ester polymers may have structural units derived from one or more of these other monomers.

The proportion of structural units derived from the other monomers in the vinyl ester polymer is not particularly limited as long as it does not hinder the objectives of the present invention, but it is preferably 15 mol% or less, and more preferably 5 mol% or less, based on the number of moles of all structural units constituting the vinyl ester polymer.

In the present invention, the degree of polymerization of PVA is preferably 1200 or higher, more preferably 1500 or higher, more preferably 2000 or higher, and particularly preferably 2500 or higher. The degree of polymerization of PVA is preferably 8000 or lower, more preferably 6000 or lower, even more preferably 5000 or lower, and particularly preferably 4000 or lower. If the degree of polymerization of PVA is less than 1200, the polarization performance may be impaired. On the other hand, if the degree of polymerization of PVA exceeds 8000, the viscosity of the aqueous solution or molten PVA may increase, making film formation difficult. Here, the degree of polymerization refers to the average degree of polymerization measured in accordance with the description in JIS K6726-1994, and is determined by the following formula from the intrinsic viscosity [η] (unit: deciliters/g) measured in water at 30°C after resaponification and purification of PVA(A).
Po = ([η]×10 ³ /8.29) ^(1/0.62)

In this invention, the degree of saponification of PVA is 98 mol% or higher. If the degree of saponification of PVA is less than 98 mol%, sufficient polarization performance may not be obtained. The degree of saponification of PVA is preferably 99 mol% or higher, more preferably 99.5 mol% or higher, and particularly preferably 99.9 mol% or higher. Here, 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 (typically vinyl ester monomer units) and vinyl alcohol units that can be converted into vinyl alcohol units by saponification. The degree of saponification of PVA can be measured in accordance with the description in JIS K6726-1994. If multiple PVAs with different degrees of saponification are mixed, the degree of saponification of the PVA mixture can be determined by calculating the weighted average of the saponification degrees of these multiple PVAs.

The PVA used in the production of the PVA film of the present invention is not particularly limited as long as it has a degree of saponification of 98 mol% or more, a degree of polymerization of 1200 or more and 8000 or less, and satisfies formula (1) in GPC analysis. For example, in the saponification of vinyl ester polymers, vinyl esters adjacent to the saponified vinyl ester units are preferentially saponified (the saponification reaction proceeds in a chain reaction), so a method can be used to increase the degree of modification of polymerization ends and side chains that are easily cleaved during saponification by appropriately adjusting the type of monomer other than vinyl ester, polymerization solvent, polymerization temperature, chain transfer agent, etc., which makes it difficult to increase the degree of saponification. Another example is a method of blending PVA (A) with a high degree of polymerization and a high degree of saponification with PVA (B) with a low degree of polymerization and a low degree of saponification. Of these methods, the method of blending PVA (A) with a high degree of polymerization and a high degree of saponification with PVA (B) with a low degree of polymerization and a low degree of saponification is preferred from the viewpoint of cost and the degree of freedom of the obtained PVA.

In the method of blending PVA(A), which has a high degree of polymerization and a high degree of saponification, with PVA(B), which has a low degree of polymerization and a low degree of saponification, the degree of polymerization of PVA(A) is preferably 1500 or higher, more preferably 1800 or higher, even more preferably 2000 or higher, and particularly preferably 2300 or higher. The degree of polymerization of PVA(A) is preferably 3500 or lower, more preferably 3300 or lower, and even more preferably 3000 or lower. If the degree of polymerization of PVA(A) is less than 1500, the polarization performance may be impaired. Also, if the degree of polymerization of PVA(A) exceeds 3500, the viscosity of the aqueous solution or molten PVA may increase, making film formation difficult.

The degree of saponification of PVA(A) is preferably 98 mol% or higher. If the degree of saponification of PVA(A) is less than 98 mol%, it is difficult to achieve a degree of saponification of 98 mol% or higher for the PVA after blending with PVA(B). The degree of saponification of PVA(A) is more preferably 99 mol% or higher, even more preferably 99.5 mol% or higher, and particularly preferably 99.9 mol% or higher.

On the other hand, the degree of polymerization of PVA(B) is preferably 50 or higher, more preferably 100 or higher, even more preferably 200 or higher, and particularly preferably 300 or higher. The degree of polymerization of PVA(B) is preferably 800 or lower, more preferably 700 or lower, even more preferably 600 or lower, and particularly preferably 500 or lower. If the degree of polymerization of PVA(B) is less than 50, the amount of PVA that dissolves into the processing solution during the polarizer manufacturing process increases, and when this aggregates due to boric acid crosslinking, etc., it tends to adhere to the polarizing film, resulting in blue to brown foreign matter defects (hereinafter sometimes abbreviated as "blue spots"). Also, if the degree of polymerization of PVA(B) exceeds 800, there is a risk that the improvement in high-speed stretchability will be insufficient.

The degree of saponification of PVA(B) is preferably 1 mol% or more lower than that of PVA(A), more preferably 1.2 mol% or more lower, even more preferably 1.4 mol% or more lower, and particularly preferably 1.6 mol% or more lower. The degree of saponification of PVA(B) is preferably lower than that of PVA(A) by no more than 5 mol%, more preferably by no more than 4 mol%, and even more preferably by no more than 3 mol% lower. If the degree of saponification of PVA(B) is higher than that of PVA(A), or lower than that of PVA(A) but the difference is less than 1 mol%, it tends to become difficult to obtain PVA that satisfies formula (1). Also, if the degree of saponification of PVA(B) is more than 5 mol% lower than that of PVA(A), there is a tendency for the amount of blue particles adhering to the polarizing film to increase.

PVA(A) and PVA(B) may each be a single type of PVA, as long as formula (1) is satisfied, or they may be a blend of two or more types of PVA with different degrees of polymerization, saponification, or modification.

The blend ratio of PVA(A) and PVA(B) in this invention is not necessarily limited, but it is preferably in the range of PVA(A):PVA(B) = 75:25 to 97:3 by mass. The ratio of PVA(A) is preferably 75% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more. If the ratio of PVA(A) is less than 75% by mass, the polarization performance of the resulting polarizing film tends to decrease. On the other hand, the ratio of PVA(A) is preferably 97% by mass or less, more preferably 95% by mass or less, and even more preferably 93% by mass or less. If the ratio of PVA(A) exceeds 97% by mass, it tends to become difficult to obtain PVA that satisfies formula (1).

The PVA content in the PVA film of the present invention is preferably 75% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the total mass of the PVA film.

[Surfactants]
The PVA film of the present invention may contain a surfactant. By including a surfactant, known effects can be obtained, such as preventing adhesion to metal supports such as drums during the film-forming process, improving the slipperiness of the PVA film, and suppressing the occurrence of wrinkles in film rolls wound into long lengths.

The surfactant content in the PVA film of the present invention is preferably 0.001 parts by mass or more, more preferably 0.005 parts by mass or more, and even more preferably 0.01 parts by mass or more, per 100 parts by mass of PVA. The surfactant content in the PVA film is preferably 1 part by mass or less, more preferably 0.8 parts by mass or less, and even more preferably 0.5 parts by mass or less, per 100 parts by mass of PVA. If the surfactant content is less than 0.001 parts by mass, the above effects may not be sufficiently obtained. Furthermore, if the surfactant content exceeds 1 part by mass, discoloration and a decrease in transparency of the film tend to occur. Also, streaky defects may occur on the film surface.

The types of surfactants are not particularly limited, but examples include anionic surfactants and nonionic surfactants.

Examples of the above anionic surfactants include carboxylic acid-type surfactants such as potassium laurate;
Sulfate ester types such as octyl sulfate;
Examples include sulfonic acid types such as dodecylbenzenesulfonate.

Examples of the nonionic surfactants mentioned above include alkyl ether types such as polyoxyethylene oleyl ether;
Alkylphenyl ether types such as polyoxyethylene octylphenyl ether;
Alkyl ester types such as polyoxyethylene laurate;
Alkylamine types such as polyoxyethylene laurylamino ether;
Alkylamide types such as polyoxyethylene lauric acid amide;
Polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether;
Alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide;
Examples include allylphenyl ether types such as polyoxyalkylene allylphenyl ether.

Of these, nonionic surfactants are preferred from the viewpoint of being excellent in reducing abnormalities on the film surface during film formation, alkanolamide-type surfactants are more preferred, and dialkanolamides such as diethanolamide of aliphatic carboxylic acids such as saturated or unsaturated aliphatic carboxylic acids having 8 to 30 carbon atoms are even more preferred. Note that one surfactant may be used alone, or two or more may be used in combination.

[Plasticizer]
The PVA film of the present invention may contain a plasticizer. Because PVA films are more rigid than other plastic films, they may not have sufficient impact strength, process passability during secondary processing, etc. However, these disadvantages can be improved by containing the above-mentioned plasticizer in the PVA film.

Examples of plasticizers include polyhydric alcohols. Examples of polyhydric alcohols include ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Of these, ethylene glycol and glycerin are preferred from the viewpoint of improving the stretchability of the PVA film. These plasticizers may be used individually or in combination of two or more.

The plasticizer content in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, per 100 parts by mass of PVA. The plasticizer content in the PVA film of the present invention is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of PVA. If the plasticizer content is less than 1 part by mass, the above effect may not be obtained, and if it exceeds 30 parts by mass, the PVA film may become too flexible and its handling properties may decrease.

[Other optional components]
The PVA film of the present invention may further contain other optional components besides PVA, surfactants, and plasticizers, as long as they do not impair the effects of the present invention. Examples of such other optional components include water, antioxidants, ultraviolet absorbers, lubricants, colorants, preservatives, fungicides, and other polymer compounds other than those mentioned above.

The content of the above-mentioned other optional components in the PVA film of the present invention is preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the total mass of the PVA film.

[Method for manufacturing PVA film]
The PVA film of the present invention can be manufactured using PVA that satisfies formula (1) above, by employing conventionally known methods. Conventionally known methods include, for example, casting, wet film formation, dry-wet film formation, gel film formation, melt extrusion film formation, and methods combining these methods. Of these, the melt extrusion film formation method is preferred from the viewpoint of obtaining a PVA film with high transparency and low coloration.

The PVA film of the present invention can be manufactured, for example, as follows by the above melt extrusion method. First, PVA chips are immersed in distilled water at 10°C to 50°C for 10 to 48 hours, then centrifugal dehydration is performed to obtain water-containing PVA chips with a volatile content of 30% to 90% by mass. Appropriate amounts of plasticizers such as glycerin, surfactants, solvents such as water, etc., are added to the above water-containing PVA chips and mixed. This mixture is fed into a twin-screw extruder, and melt extrusion film formation is carried out continuously by conventionally known methods. Specifically, the above mixture is heated and melted in a twin-screw extruder at a maximum temperature of 100°C to 200°C, cooled to 80°C to 120°C in a heat exchanger to obtain a film-forming stock solution. This film-forming stock solution is discharged from a T-die at 80°C to 120°C, cast onto a metal drum at 75°C to 115°C, and dried to obtain a film with a moisture content of 10% to 40% by mass. After peeling this film from the metal drum, it can be passed through a hot air drying oven at 50°C to 100°C to produce the PVA film of the present invention.

There are no particular restrictions on the thickness of the PVA film of the present invention, but when used as a raw material for a polarizing film, it is preferable that the average thickness be in the range of 5 to 150 μm. The average thickness of the PVA film can be determined by measuring the thickness at any 10 locations (for example, any 10 locations on a straight line drawn in the width direction of the PVA film) and taking the average value of these measurements. The film width of the PVA film can be set to a size appropriate for the application.

The prepared PVA film can usually be wound onto a cylindrical core in a roll shape using conventionally known methods to produce a film roll. A specific method for manufacturing the above-mentioned film roll is, for example, to slit both ends of the PVA film in the width direction by 0.5 cm to 20 cm, and then, using a conventionally known winding machine, to wind the film onto a cylindrical core with a film tension of 1.0 kgf/cm to 10 kgf/cm, thereby producing a PVA film roll.

The outer diameter of the core (or the diameter of its circumscribed circle in the case of a rectangular tube) is preferably 10 cm or more. If the outer diameter is less than 10 cm, the film roll may sag under its own weight and wrinkle. The length of the core may be the same as the width of the PVA film, or it may be longer than the film width, but it is preferably 10 cm or more longer than the film width. If the length of the core is shorter than the film width, breakage is likely to occur from the end in the film width direction during stretching, and uniform stretching may be difficult. Furthermore, the core is preferably in a cylindrical shape with an outer surface made of metal or plastic to prevent wrinkles during winding.

The length of the PVA film wound into a roll is preferably 1,300 m or more. A length of less than 1,300 m of PVA film wound into a roll is undesirable because it results in significant losses due to film roll switching during the polarizing film manufacturing process. While there is no particular upper limit on the length of the PVA film wound into a roll, if it is too long, the film roll becomes too heavy and difficult to handle. Alternatively, it may cause problems such as the film roll bending and wrinkles forming in the film. Therefore, the length of the PVA film is preferably 20,000 m or less.

PVA film wound into a roll is preferably stored and transported in a suspended state, supported by moisture-proof packaging and with the weight supported by the entire core of the film roll or both ends of the core. Preferred methods for storing and transporting in the suspended state include placing the cores protruding from both ends of the roll on a support, suspending the cores protruding from both ends of the roll by a support, inserting a part of the support into the core, placing a rod-shaped jig inserted into the core on a support, and suspending a rod-shaped jig inserted into the core by a support. Of these, the method of placing the cores protruding from both ends of the roll on a support is more preferred. Furthermore, PVA has high hygroscopicity, and if stored and transported in environments other than low humidity conditions, it is highly likely to easily absorb moisture and swell, causing wrinkles in the film. Therefore, if storage and transport in such environments are anticipated, sufficient moisture-proof packaging is necessary.

The volatile content of the PVA film ultimately obtained through this series of processes is not necessarily limited, but it is preferable that the volatile content of the PVA film be 1% by mass or more and 5% by mass or less.

[Method for manufacturing optical films]
The applications of the PVA film of the present invention are not particularly limited, but for example, it can be used as a base 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, as an example of a method for manufacturing optical films, a method for manufacturing a polarizing film will be specifically described.

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 cleaning solution used in cleaning.

The following describes in detail each processing step that can be used in the manufacturing method for polarizing films. Note that 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.

(Washing treatment before swelling treatment)
It is preferable to perform a washing treatment on the PVA film before performing the swelling treatment. Such a washing treatment before the swelling treatment can remove anti-blocking agents and other substances adhering to the PVA film, and prevent contamination of each processing solution in the polarizing film manufacturing process with anti-blocking agents and other substances. The washing treatment is preferably performed by immersing the PVA film in the washing treatment solution, but it can also be performed by spraying the washing treatment solution onto the PVA film. For example, water can be used as the washing treatment solution. The temperature of the washing 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. A temperature of 40°C or lower prevents a part of the surface of the PVA film from dissolving and sticking together, which would reduce the handling properties. The temperature of the washing treatment solution is more preferably 22°C or higher, even more preferably 24°C or higher, and particularly preferably 26°C or higher. The temperature of the washing solution is more preferably 38°C or lower, even more preferably 36°C or lower, and particularly preferably 34°C or lower.

(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, and 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 solution contains a boron-containing compound, from the viewpoint of improving the stretchability of the PVA film, it is preferable that the concentration of the boron-containing compound in the swelling solution be 6% by mass or less.

(Dyeing process)
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. The potassium iodide/iodine (mass) ratio 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.

(Crosslinking treatment)
In the manufacture of polarizing films, a crosslinking treatment can be performed after the dyeing treatment for purposes such as strengthening 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 far, making it difficult to perform sufficient stretching in the subsequent stretching treatment. Conversely, 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. Therefore, 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.

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, although the reason is unknown. Also, 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. For the reasons above, 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. Furthermore, 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.

If the temperature of the crosslinking solution is too high, the dichroic dye tends to dissolve, and uneven dyeing is likely to occur in the resulting polarizing film. Conversely, if the temperature of the crosslinking solution is too low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking solution is preferably in the range of 20°C to 45°C. 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.

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, 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 more preferably 3.5 times or less. The total stretching ratio of the pre-stretching is preferably 1.5 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. The stretching ratio in the swelling process is preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.4 times or more, based on the original length of the PVA film. In the swelling treatment, the stretching ratio is preferably 3 times or less, more preferably 2.5 times or less, and even more preferably 2.3 times or less, based on the original length of the PVA film. In the dyeing treatment, the stretching ratio is preferably 2 times or less, more preferably 1.8 times or less, and even more preferably 1.5 times or less, based on the original length of the PVA film. In the dyeing treatment, the stretching ratio is even more preferably 1.1 times or more, based on the original length of the PVA film. In the crosslinking treatment, the stretching ratio is preferably 2 times or less, more preferably 1.5 times or less, and even more preferably 1.3 times or less, based on the original length of the PVA film. In the crosslinking treatment, the stretching ratio is preferably 1.05 times or more, based on the original length of the PVA film.

(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 fixing 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 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, as this can improve the stretchability of the PVA film. When the stretching solution contains a boron-containing compound, 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, as this can improve the stretchability of the PVA film.

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. Conversely, if the concentration of the iodine-containing compound 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.

If the temperature of the stretching solution is too high, the PVA film tends to melt, soften, and become easily broken. Conversely, if the temperature of the stretching solution is too low, the stretchability 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 treatment is performed by the dry stretching method is also as described above.

In the stretching process, a higher stretching ratio is preferable to obtain a polarizing film with superior polarization performance, so a stretching ratio of 1.2 times or more is preferable, a stretching ratio of 1.5 times or more is preferable, and a stretching ratio of 2 times or more is even preferable. Furthermore, 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 preferable to 5.5 times or more, a stretching ratio of 5.7 times or more is preferable, and a stretching ratio of 5.9 times or more is even preferable, based on the original length of the raw material PVA film before stretching, from the viewpoint of the polarization performance of the resulting polarizing film. There is no particular upper limit to the stretching ratio, but since stretching breakage is more likely to occur if the stretching ratio is too high, it is preferable that the stretching ratio is 8 times or less.

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 parallel to each other and changing the peripheral speed between each roll.

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 the two or more stages 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 refers to 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 for faster stretching (uniaxial stretching) of the PVA film, 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.

(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.

(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.

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

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.

A polarizing film can be obtained by drying the PVA film after it has undergone the above-described 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.

(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.

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

The PVA film of the present invention can be suitably used as an optical film. Specifically, it can be suitably used as a raw material for optical films such as polarizing films, phase difference films, and special light-gathering films with few optical defects. However, it can also be used for other purposes, such as packaging materials, water-soluble films for laundry bags, and release films when manufacturing artificial marble.

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

(1) Relative molecular weight analysis by GPC measurement The method for analyzing the relative molecular weight of PVA film by GPC analysis is shown below.
<Sample Preparation>
A film sample of approximately 5 mg of PVA film was taken and accurately weighed. To the sample, hexafluoroisopropanol (HFIP) containing 1 mL of 20 mM sodium trifluoroacetate per 1 mg of film was added and heated at 40°C for 3 hours to dissolve. GPC analysis was performed using this solution under the following conditions. The relative molecular weight distribution curve was calculated by converting the relative molecular weight obtained from the RI detector and UV detector using the analysis software attached to the analyzer. The analysis was performed three times on the same sample, and the average value was used as the analysis result.
The detection intensities of the RI detector and UV detector at the peak-top molecular weight (Mp) of the obtained relative molecular weight distribution curve were defined as Hri1 and Huv1, respectively, before conversion to the molecular weight distribution curve. Furthermore, the detection intensities of the RI detector and UV detector at the molecular weight at the point where the logarithm of the relative molecular weight conversion (with 10 as the lowest value) was 4.3 were defined as Hri2 and Huv2, respectively, before conversion to the molecular weight distribution curve. Using these values, the values of Huv2/Hri2, Huv1/Hri1 - Huv2/Hri2, and Hri2/Hri1 were calculated.

<GPC analysis conditions>
Measurement device: HLC-8320GPC (manufactured by TOSOH Corporation, column length 15 cm, column diameter 4.6 mm)
Analysis software: Empower (manufactured by Waters Inc.)
Sample concentration: 0.1 mg/mL
Mobile phase solvent: Hexafluoroisopropanol with 20 mM sodium trifluoroacetate added. Injection volume: 10 μL
Flow rate: 0.2mL/min
Measurement temperature: 40℃
Sample dissolution conditions: 40°C x 3 hours. Filter filtration: 0.45 μm PTFE filter. Column: GMMHR-H(S) (manufactured by TOSOH Corporation), 2 columns. Detector: RI detector and UV detector attached to the instrument (measurement wavelength 210 nm).
Instrument calibration standard: PMMA (Agilent, 4 ml tri-pack (90 vials), Agilent EasyVial (GPC/SEC Calibration Standards), PMMA with MP values of 2,210,000, 1,020,000, 538,500, 265,300, 146,500, 72,000, 26,550, 13,900, 72,900, 18,40, 885, 550)

(2) High-speed stretchability In the following examples or comparative examples, uniaxial stretching was performed continuously for 20 minutes during the stretching process when manufacturing polarizing films. The number of stretch breaks that occurred during this 20 minutes of continuous stretching was measured and the stretch break frequency (times/20 min) was evaluated.

(3) Blue particles In the following examples or comparative examples, when the polarizing film was manufactured continuously for 20 minutes, the blue particles adhering to the surface of the polarizing film were visually observed and evaluated according to the following criteria.
A: No blue objects were found.
B: A small amount of blue specks were observed, but at a level that does not pose any practical problems.
C: Blue particles were observed at a level that poses a practical problem.

(4) Polarization Degree Two 1.5 cm × 1.5 cm square samples were taken from the center of the width direction of the polarizing film obtained in the following examples or comparative examples, parallel to the orientation direction of the polarizing film. For each sample, a Hitachi V-7100 spectrophotometer (with integrating sphere) was used to perform luminous efficiency correction in the visible light region with a C light source and a 2-degree field of view, in accordance with JIS Z8722 (Method for measuring object color). For one polarizing film sample, the light transmittance when tilted at 45 degrees and when tilted at -45 degrees with respect to the stretch axis direction was measured, and the average value (Y1) was calculated. For the other polarizing film sample, the light transmittance when tilted at 45 degrees and when tilted at -45 degrees was measured in the same manner as above, and the average value (Y2) was calculated. The transmittance (Y) (%) of the polarizing film was calculated by averaging Y1 and Y2 obtained above.
The light transmittance (Y∥) when the two polarizing film samples collected above were stacked with their orientation directions parallel, and the light transmittance (Y⊥) when they were stacked with their orientation directions perpendicular, were measured using the same method as the transmittance measurement method described above, and the degree of polarization (V) (%) was calculated from the following formula.
Polarization degree (V) (%) = {(Y∥ - Y⊥) / (Y∥ + Y⊥)} ^1/2 × 100

[Example 1]
<Manufacturing of PVA film>
A film-forming stock solution (volatile fraction 66% by mass) was prepared by melt-mixing 90 parts by mass of PVA (A) with a degree of saponification of 99.9 mol% and a degree of polymerization of 2500, 10 parts by mass of PVA (B) with a degree of saponification of 98.5 mol% and a degree of polymerization of 500, 12 parts by mass of glycerin as a plasticizer, 0.2 parts by mass of lauric acid diethanolamide as a surfactant, 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 85°C) to form a liquid coating on the support. On the support, hot air at 90°C was blown at a speed of 7.5 m/s over the entire non-contact surface of the liquid coating with the support to dry it and obtain a PVA film (moisture content 25% by mass). Next, the PVA film was peeled from the support and further dried between the first drying roll and the final drying roll (19th drying roll) immediately preceding the heat treatment roll, with one side of the PVA film alternately in contact with each drying roll. After drying, it was peeled 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 70°C. Furthermore, the PVA film was peeled from the final drying roll and heat treated with heat, with one side of the PVA film alternately in contact with each heat treatment roll. At this time, the heat treatment was performed using two heat treatment rolls, and the surface temperature of both heat treatment rolls was set to 100°C. The obtained film was trimmed at both ends to a width of 900 mm and wound into a roll on a cylindrical core.

<Manufacturing and evaluation of polarizing films>
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 potassium iodide/iodine dyeing solution (dyeing treatment solution) at a temperature of 32°C (potassium iodide/iodine (mass ratio) of 23, iodine concentration in the range of 0.03 to 0.05 mass%). In this dyeing treatment, the iodine concentration in the dyeing treatment solution was adjusted within the range of 0.03 to 0.05 mass% so that the single-layer 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% by 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% by mass, potassium iodide concentration 5% by mass). The maximum stretching speed in this stretching treatment was 400%/min. The washing treatment was performed by immersing the material for 12 seconds without stretching in a 22°C potassium iodide/boric acid aqueous solution (washing treatment solution) (potassium iodide concentration 3-6% by mass, boric acid concentration 1.5% by mass). The drying treatment was performed by hot air drying at 80°C for 1.5 minutes without stretching to obtain a polarizing film. When the manufacturing of the polarizing film was continued for 20 minutes, there were zero instances of stretching and breaking during that time, and no blue spots were observed on the surface of the polarizing film.
The obtained PVA film and polarizing film were evaluated using the method described above. The results are shown in Table 1.

[Comparative Example 1]
In Example 1, a PVA film was manufactured in the same manner as in Example 1, except that the amount of PVA(A) was 100 parts by mass and the amount of PVA(B) was 0 parts by mass, i.e., PVA was used only as PVA(A).
When a polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1, eight stretch fractures occurred in 20 minutes, making it impossible to stably manufacture a polarizing film. Therefore, blue spot evaluation and polarization degree measurement of the polarizing film were not performed. The GPC analysis results of the PVA film are shown in Table 1.

[Example 2 and Comparative Example 2]
In Example 1, a PVA film was manufactured in the same manner as in Example 1, except that the amounts of PVA(A) and PVA(B) were 80 parts by mass, 20 parts by mass, and 65 parts by mass, and 35 parts by mass, respectively. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1. The evaluation results are shown in Table 1.

[Example 3 and Comparative Example 3]
In Example 1, a PVA film was manufactured in the same manner as in Example 1, except that the degree of saponification of PVA(B) was changed to 93.1 mol% and 99.8 mol%, respectively. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1. The evaluation results are shown in Table 1.

[Example 4]
A PVA film was manufactured in the same manner as in Example 1, except that the degree of polymerization of PVA(A) was changed to 3400. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1. The evaluation results are shown in Table 1.

[Example 5]
A PVA film was manufactured in the same manner as in Example 1, except that the degree of polymerization of PVA(B) was changed to 1200. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1. The evaluation results are shown in Table 1.

[Example 6]
In Example 4, a PVA film was manufactured in the same manner as in Example 4, except that the degree of saponification of PVA(B) was changed to 97.6 mol%. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 4. The evaluation results are shown in Table 1.

[Comparative Example 4]
A PVA film was manufactured in the same manner as in Example 1, except that the degree of saponification of PVA(A) was changed to 97.0 mol%. A polarizing film was manufactured using the obtained PVA film in the same manner as in Example 1. The evaluation results are shown in Table 1.

Claims (6)

A polyvinyl alcohol film containing polyvinyl alcohol having a degree of saponification of 98 mol% or more and a degree of polymerization of 1200 or more and 8000 or less, wherein the polyvinyl alcohol was measured by gel permeation chromatography at a measurement temperature of 40°C using hexafluoroisopropanol with 20 mM sodium trifluoroacetate added as the mobile phase solvent, and the following formula (1) was satisfied:
The polyvinyl alcohol comprises polyvinyl alcohol (A) having a degree of polymerization of 1500 or more and 3500 or less, and a degree of saponification of 98 mol% or more, and polyvinyl alcohol (B) having a degree of polymerization of 50 or more and 800 or less, and a degree of saponification that is 1 mol% or more and 5 mol% or less lower than that of polyvinyl alcohol (A).
A polyvinyl alcohol film in which the blend ratio of the polyvinyl alcohol (A) and the polyvinyl alcohol (B) is in the range of polyvinyl alcohol (A):polyvinyl alcohol (B) = 75:25 to 97:3 by mass (excluding cases in which polyvinyl alcohol (A) is unmodified polyvinyl alcohol and polyvinyl alcohol (B) is modified polyvinyl alcohol) .
0.017 ≤ Huv²/Hri² ≤ 0.030 (1)
(However, Huv2 is the detection intensity at an absorption wavelength of 210 nm for a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a UV-Vis absorbance detector (UV detector) of a gel permeation chromatograph, and Hri2 is the detection intensity at a polymethyl methacrylate equivalent molecular weight with a base-10 logarithm of 4.3, obtained using a differential refractive index detector (RI detector) of a gel permeation chromatograph.) The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol is measured by the gel permeation chromatograph and satisfies the following formula (2).
Huv1/Hri1-Huv2/Hri2≦-0.002 (2)
(However, Huv1 is the maximum detection intensity at an absorption wavelength of 210 nm obtained using a UV detector, and Hri1 is the maximum detection intensity obtained using an RI detector.) The polyvinyl alcohol film according to claim 1 or 2, wherein the value obtained by dividing Hri2 by Hri1, Hri2/Hri1, is 0.2 or less. A polyvinyl alcohol film according to any one of claims 1 to 3 , which is an optical film. A method for producing a polyvinyl alcohol film according to any one of claims 1 to 4 , characterized in that polyvinyl alcohol that satisfies the above formula (1) is used. A polarizing film made from the polyvinyl alcohol film described in claim 4 .