Polyvinyl alcohol-based film, polarizing membrane, and polarizing plate

Abstract

Provided is a polyvinyl alcohol-based film that exhibits excellent curl resistance and stretchability, that elutes less amounts of low molecular weight components, and that also has excellent optical characteristics.　In the polyvinyl alcohol-based film, the value of a slope SL0-30 obtained by linearly approximating the change between a long period before water immersion and a long period at 30 seconds after being immersed in 35°C water with respect to an immersion time as determined by small-angle X-ray scattering measurement satisfies 0.208<SL0-30<0.270.

Description

Polyvinyl alcohol-based film, polarizing film, and polarizing plate

[0001] The present invention relates to a polyvinyl alcohol-based film. More specifically, it relates to a polyvinyl alcohol-based film, a polarizing film, and a polarizing plate that are excellent in curl resistance, elution property, and stretchability.

[0002] Generally, a polarizing film is produced by unwinding a polyvinyl alcohol-based film as a raw web from a roll, conveying it in the flow direction (MD direction), swelling it with water (including warm water), followed by processes such as dyeing with iodine, stretching to orient the iodine, and boric acid crosslinking to fix the oriented state. Therefore, defects occurring during these processes significantly reduce the productivity of the polarizing film. For example, in the swelling process, if the polyvinyl alcohol-based film absorbs or desorbs water and curls, folds or wrinkles occur at the end portions in the width direction of the film, making it impossible to produce the polarizing film with good productivity. Also, due to folds or wrinkles in the polyvinyl alcohol-based film, the film may break without being uniformly stretched, causing production interruption. Furthermore, in the swelling process, if impurities elute from the polyvinyl alcohol-based film and contaminate the swelling tank, the contamination will spread throughout the subsequent processes. In the dyeing process and the boric acid crosslinking process as well, if impurities elute from the polyvinyl alcohol-based film, not only will the polarization performance of the resulting polarizing film decrease, but it will also require a great deal of labor for filtering and replacing the chemical solutions used in each process. Examples of the impurities include low-molecular-weight components such as low-molecular-weight polyvinyl alcohol-based resins (including oligomers) present in the polyvinyl alcohol-based film. In particular, low-molecular-weight substances with a molecular weight of 50,000 or less are likely to elute in water and tend to easily form low-molecular-weight iodine complexes that reduce the degree of polarization.

[0003] The polyvinyl alcohol-based film, which is the raw sheet of the polarizing film, is generally produced by dissolving a polyvinyl alcohol-based resin as a raw material in water and performing continuous casting from such an aqueous solution (film-forming stock solution). Specifically, an aqueous solution of a polyvinyl alcohol-based resin is discharged and cast onto a casting mold such as a casting drum or an endless belt to form a film. After the obtained film is peeled off from the casting mold, it is dried and heat-treated while being conveyed in the flow direction (MD direction). Among these steps, the drying step is an important step for controlling the curl resistance, elution property, and stretchability of the polyvinyl alcohol-based film. If the drying in such a drying step is insufficient, there is a tendency for breakage and an increase in eluted resin during the production of the polarizing film.

[0004] In addition, the polarizing plate obtained using the polyvinyl alcohol-based film has a light transmission and shielding function, and together with a liquid crystal having a light switching function, it is a basic component of a liquid crystal display (LCD). The application fields of this LCD have also expanded widely from small devices such as calculators and wristwatches in the early stage of development to laptop computers, word processors, liquid crystal projectors, in-vehicle navigation systems, liquid crystal TVs, personal phones, and measuring instruments used indoors and outdoors in recent years. Along with such an expansion of the application fields of LCDs, a polyvinyl alcohol-based film having physical properties superior to those of conventional products in a thin film is required.

[0005] For example, Patent Document 1 discloses a polyvinyl alcohol-based film that can produce a polarizing film having a high absorbance in the long wavelength region (for example, the visible light region of 680 nm or more) and excellent polarization characteristics, and having a crystal long period of 14.5 to 16.0 nm in water.

[0006] Furthermore, Patent Document 2 discloses a polyvinyl alcohol-based film that can suppress the occurrence of swelling wrinkles during the swelling process when manufacturing an optical film, even when it is thin, and wherein the crystal long period Ds, which is determined from small-angle X-ray scattering measurements performed in a water / methanol mixed solvent (volume ratio: 2 / 8), and the crystal long period Da, which is determined from small-angle X-ray scattering measurements performed before immersion in the mixed solvent, satisfy 0.15 ≤ (Ds - Da) / Da < 0.3.

[0007] International Publication No. 2013 / 146147, International Publication No. 2022 / 004535

[0008] Although the film described in Patent Document 1 has high absorbance in the visible light region at 700 nm, it suffers from poor curl resistance during polarizing film manufacturing and excessive elution of low molecular weight components from the film, both of which are unsatisfactory. Furthermore, while the film described in Patent Document 2 has excellent optical performance and swelling properties, it exhibits high tension during film stretching, resulting in insufficient handling during polarizing film manufacturing.

[0009] Therefore, against this background, the present invention provides a polyvinyl alcohol-based film that exhibits excellent curl resistance and stretchability, low elution of low molecular weight components, and excellent optical properties.

[0010] However, in view of these circumstances, the inventors have conducted extensive research and have found that in polyvinyl alcohol-based films, the slope S is the slope of the linear approximation of the change in the long period before water immersion and the long period up to 30 seconds of immersion in 35°C water, which is determined by small-angle X-ray scattering. Ｌ0-30 We found that when the value of is within a specific range, a polyvinyl alcohol-based film can be obtained that exhibits excellent curl resistance, stretchability, low elution of low molecular weight components, and excellent optical properties.

[0011] In other words, the present invention has the following aspects: [1] The slope S when the change in the long period before immersion in water and the long period up to 30 seconds of immersion in 35°C water, obtained by small-angle X-ray scattering, is linearly approximated. Ｌ0-30 The value of is 0.208 < S Ｌ0-30A polyvinyl alcohol-based film satisfying <0.270. [2] The slope S when the change with respect to long-period immersion time from 60 to 300 seconds in 35°C water, determined by small-angle X-ray scattering, is linearly approximated. Ｌ60-300 The above S Ｌ0-30 ΔS subtracted from Ｌ The value of 0.206 < ΔS Ｌ [1] A polyvinyl alcohol-based film satisfying <0.268. [3] A polyvinyl alcohol-based film according to [1] or [2] having a film thickness of 5 to 50 μm. [4] A polyvinyl alcohol-based film according to any one of [1] to [3] for manufacturing polarizing films. [5] A polarizing film made using a polyvinyl alcohol-based film according to any one of [1] to [4]. [6] A polarizing plate comprising the polarizing film according to [5] and a protective film provided on at least one side of the polarizing film.

[0012] The polyvinyl alcohol-based film of the present invention exhibits excellent curl resistance and stretchability, low elution of low molecular weight components, and superior optical properties. Therefore, the polyvinyl alcohol-based film of the present invention is particularly useful as a base film for thin polarizing films.

[0013] Figure 1(a) is a schematic diagram illustrating the measurement conditions for the curl angle, and Figure 1(b) is a diagram illustrating the curl angle.

[0014] The present invention will be described below based on examples of embodiments for carrying out the present invention. However, the present invention is not limited to the embodiments described below.

[0015] In this specification, "x and / or y (where x and y are arbitrary configurations)" means at least one of x and y, and means three cases: only x, only y, and x and y. When expressing as "X to Y" (where X and Y are arbitrary numbers) in this specification, unless otherwise specified, it includes the meaning of "X or more and Y or less" and also the meaning of "preferably exceeding X" or "preferably less than Y". When expressing as "X or more" (where X is an arbitrary number) or "Y or less" (where Y is an arbitrary number) in this specification, it also includes the meaning of "preferably exceeding X" or "preferably less than Y". Regarding the numerical ranges described stepwise in this specification, the upper limit value or lower limit value of a numerical range at a certain step can be arbitrarily combined with the upper limit value or lower limit value of a numerical range at another step. Also, in the numerical ranges described in this specification, the upper limit value or lower limit value of that numerical range can also be replaced with the value shown in the examples. In this specification, "film" means including "tape" and "sheet". In this specification, "main component" means a component that greatly affects the characteristics of the object, and the content of this component is usually 50% by mass or more in the object, preferably 55% by mass or more, more preferably 60% by mass or more, still more preferably 70% by mass or more, and may be 100% by mass.

[0016] The polyvinyl alcohol-based film according to one embodiment of the present invention (hereinafter sometimes referred to as "this polyvinyl alcohol-based film") has a slope S when linearly approximating the change with respect to the immersion time of the long period before water immersion and the long period up to 30 seconds of water immersion, which is obtained by small-angle X-ray scattering. Ｌ0-30 The value of which satisfies 0.208 < S Ｌ0-30 < 0.270.

[0017] This polyvinyl alcohol-based film measures the long period before water immersion and the long period of the film immersed in water at 35°C for a predetermined time according to the method described later by small-angle X-ray scattering, and the slope S of the straight line when representing the change of the long period with respect to the immersion time by an approximate straight line Ｌ0-30 The value of is preferably greater than 0.208, preferably 0.210 or more, more preferably 0.212 or more, and still more preferably 0.214 or more. Also, SＬ0-30 The upper limit of the value is less than 0.270, preferably 0.265 or less, more preferably 0.260 or less, and even more preferably 0.255 or less. Ｌ0-30 As the value of is within the above range, this polyvinyl alcohol-based film, despite being a thin film, exhibits excellent curl resistance and stretchability, low elution of low molecular weight components, and excellent optical properties. Ｌ0-30 This refers to the slope of the straight line obtained by plotting the long period from before water immersion (0 seconds) to 30 seconds after water immersion, obtained by measuring small-angle X-ray scattering, on a graph with the long period on the Y axis and the immersion time on the X axis, and representing it with an approximate straight line. Typically, polyvinyl alcohol-based films have randomly distributed crystalline and amorphous regions. The "long period" refers to the average length of one period in the repeating period of these crystalline and amorphous regions. It is known that this long period changes depending on the film manufacturing conditions, etc. That is, S Ｌ0-30 If the value of is too small, the initial curl will worsen due to insufficient swelling, reducing transportability during polarizing plate manufacturing and making it more prone to breakage. Also, the polyvinyl alcohol-based film will not swell sufficiently, increasing the tension during stretching and reducing its stretchability. Ｌ0-30 If the value is too high, the initial curl may be good, but excessive swelling will cause the curl to persist, leading to reduced transportability during polarizer manufacturing, increased elution of low molecular weight components, and a decrease in optical properties.

[0018] This polyvinyl alcohol-based film was subjected to small-angle X-ray scattering when the film was immersed in water at 35°C for a predetermined time, and the resulting ΔS Ｌ The value of is preferably greater than 0.206, more preferably 0.208 or greater, even more preferably 0.210 or greater, and particularly preferably 0.212 or greater. Ｌ The upper limit is preferably less than 0.268, more preferably 0.265 or less, even more preferably 0.260 or less, and particularly preferably 0.255 or less, and within that range, for example, 0.206 < ΔS Ｌ <0.268, etc. ΔS ＬIf the value of is too small, the difference in swelling rate becomes small, so the film cannot swell sufficiently, the tension during stretching increases, the stretchability tends to decrease, and the optical properties also tend to deteriorate. Ｌ If the value of is too large, the difference in rate between the initial and late stages of swelling becomes large, which tends to reduce the transportability during polarizer manufacturing, make them more prone to breakage, and also tends to reduce optical properties. Here, "ΔS Ｌ " is the slope (S) of the line obtained by plotting the long period from 60 seconds to 300 seconds after immersion in 35°C water, which is obtained by measuring small-angle X-ray scattering, on a graph with the long period on the Y axis and the immersion time on the X axis, and representing it with an approximate straight line. Ｌ60-300 ) to the above S Ｌ0-30 The value subtracted from (S Ｌ0-30 -S Ｌ60-300 )

[0019] This polyvinyl alcohol-based film was subjected to small-angle X-ray scattering when the film was immersed in water at 35°C for a predetermined time. Ｌ：ave The value of is usually greater than 0.1040, preferably 0.1060 or higher, and more preferably 0.1080 or higher. Ｌ：ave The upper limit is usually less than 0.1355, preferably 0.1350 or less, and more preferably 0.1345 or less, and within that range, for example, 0.1040 < S Ｌ：ave <0.1355, etc. Ｌ：ave If the value of is too small, the swelling rate is slow, and the film cannot swell sufficiently, which increases the tension during stretching, leading to a decrease in stretchability and increased susceptibility to breakage, as well as a tendency for the optical properties to deteriorate. Also, S Ｌ：ave If the value of is too large, the swelling rate is fast, which tends to reduce transportability and make breakage more likely during polarizer manufacturing. Also, the amount of low molecular weight components that leach out increases, which tends to reduce optical properties. Here, "S Ｌ：ave " refers to the aforementioned S Ｌ0-30 and S Ｌ60-300 This is the average value.

[0020] The small-angle X-ray scattering described above can be measured by the following method. After storing the polyvinyl alcohol-based film at 23°C and 65% humidity for 24 hours or more, cut it into multiple pieces measuring 3 cm x 3.8 cm. Stack the multiple films on a measurement mount so that the total thickness is approximately 180 μm, and use this as the sample before water immersion (0 seconds). Also, after storing the polyvinyl alcohol-based film at 23°C and 65% humidity for 24 hours or more, cut it to a size of 5 mm wide x 70 mm long, with the longitudinal direction (the direction of the 70 mm length) being the MD direction (flow direction). Immerse this cut film in 35°C warm water for a predetermined time to allow it to swell. Fold the films in the longitudinal direction and stack them on a measurement cell so that the total thickness of the films immersed for the predetermined time is approximately 500 μm, and use these as the samples after water immersion for each immersion time. Note that the total film thickness of approximately 180 μm and approximately 500 μm includes not only 180 μm and 500 μm, but also thicknesses within ±15% of 180 μm and 500 μm, respectively. Small-angle X-ray scattering measurements will be performed using these samples under the following measurement conditions. [Measurement conditions] ・Measurement device Aichi Synchrotron Radiation Center BL8S3 ・Measurement conditions Transmission measurement X-ray: CuKα line Wavelength: 1.5 Å Detector: PILATUS 2M (Measurement area = 253.7 × 288.8 mm) ２ Pixel size: 172 μm x 172 μm Camera length: 4 m X-ray exposure time: 40 seconds (in water), 120 seconds (before immersion in water) Ambient temperature: Room temperature (25°C)

[0021] In the aforementioned small-angle X-ray measurement, scattering from the polyvinyl alcohol-based film is superimposed by scattering from air and water in the X-ray-passing portion, so it is necessary to correct for scattering as background. Therefore, in the aforementioned small-angle X-ray measurement, the scattering intensity due to air and water scattering is calculated separately from the scattering intensity obtained by measuring the sample, and this is subtracted from the scattering intensity obtained by measuring the sample to perform the correction. From the obtained measured values, a correlation function can be derived and the crystal long period can be calculated. However, if the correlation function is derived using the measured values ​​directly, the effects of truncation of the integration range and statistical noise are reflected, resulting in a correlation function with a large shape and small vibrations added. To remove such vibrational structures, least-squares fitting is performed. From the correlation function obtained by Fourier transforming the results of this least-squares fitting, indices for the long period and crystal thickness can be derived. Furthermore, by subtracting the crystal thickness index from the obtained long period, an index for amorphous thickness can be derived. Furthermore, by integrating the scattering intensity with respect to the scattering vector q in the azimuthal direction from the scattering intensity image measured by the two-dimensional detector, the relationship between the scattering vector q and the one-dimensional profile of the scattering intensity I(q) can be derived, thereby obtaining a scattering curve.

[0022] This polyvinyl alcohol-based film includes a crystalline portion and an amorphous portion, and is composed of the crystalline portion and amorphous portion, and the slope S when the change with respect to the immersion time of the long period before immersion in water and the long period up to 30 seconds of immersion in 35°C water, which is determined by small-angle X-ray scattering, is linearly approximated Ｌ0-30 The value of is 0.208 < S Ｌ0-30 This is a polyvinyl alcohol-based film that satisfies the requirement of <0.270.

[0023] This polyvinyl alcohol-based film S Ｌ0-30 Methods for controlling the S of the polyvinyl alcohol film to a predetermined range include, for example, adjusting the amount and type of plasticizer contained in the aqueous solution of polyvinyl alcohol resin, adjusting the modified type, amount of modification, molecular weight and degree of saponification of the raw material polyvinyl alcohol resin, controlling the drying temperature and drying state of the film during film formation, and methods that combine these methods. In particular, in the present invention, the S of the polyvinyl alcohol filmＬ0-30 As a method for controlling these factors within a predetermined range, a method of controlling the drying temperature during film formation and the drying state of the film is preferred.

[0024] The film thickness of this polyvinyl alcohol-based film is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. The upper limit of the film thickness is preferably 50 μm or less, more preferably 45 μm or less, even more preferably 40 μm or less, and particularly preferably 35 μm or less, with a range of, for example, 5 to 50 μm. Even with film thicknesses within the above range, this polyvinyl alcohol-based film exhibits excellent curl resistance and stretchability, as well as excellent optical properties with suppressed elution of low molecular weight components.

[0025] The tension of the polyvinyl alcohol-based film is preferably 150 N or less, more preferably 140 N or less, and even more preferably 135 N or less. The lower limit of the tension of the polyvinyl alcohol-based film is not particularly limited, but is preferably 80 N or more, more preferably 90 N or more, and its range is, for example, 80 to 150 N. If the tension is too high, there is a tendency for a large amount of low molecular weight components to leach out due to film breakage or structural destruction during polarizing film production. The tension can be measured, for example, by the stretchability test method described in the examples below.

[0026] The amount of this polyvinyl alcohol-based film leached into water (ppm / m²) 2 ) is 400 ppm / m 2 Preferably, it is less than 390 ppm / m 2 The following applies. When the amount of elution into water falls within the aforementioned range, the amount of low molecular weight components eluted into water during polarizing film production is reduced, chemical contamination during polarizing film production can be suppressed, and the degradation of polarization performance due to the adhesion of eluted low molecular weight components tends to be prevented. The amount of elution can be measured, for example, by the method described in the examples below.

[0027] The manufacturing method of this polyvinyl alcohol-based film will be described in more detail below, in order of the steps, but this polyvinyl alcohol-based film is not limited to these embodiments.

[0028] This polyvinyl alcohol-based film is preferably manufactured through the following steps (A) to (C), and preferably through step (D) as needed. Step (A): A step of preparing an aqueous solution of polyvinyl alcohol-based resin. Step (B): A step of casting the aqueous solution of polyvinyl alcohol-based resin into a cast mold to form a film. Step (C): A step of drying the formed film. Step (D): A step of heat-treating the obtained film.

[0029] <Step (A)> Step (A) is a step in which an aqueous solution of polyvinyl alcohol-based resin is prepared. First, the polyvinyl alcohol-based resin and the aqueous solution of polyvinyl alcohol-based resin, which are materials for the polyvinyl alcohol-based film, will be described. In this embodiment, the polyvinyl alcohol-based resin that constitutes the polyvinyl alcohol-based film is usually an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of a component copolymerizable with vinyl acetate can also be used. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (e.g., salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. In addition, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl groups after saponification can also be used. These can be used individually or in combination of two or more.

[0030] Furthermore, a polyvinyl alcohol-based resin having a 1,2-diol structure in its side chain can also be used. Such a polyvinyl alcohol-based resin having a 1,2-diol structure in its side chain can be obtained, for example, by (i) saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, (iii) saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, or (iv) saponifying a copolymer of vinyl acetate and glycerol monoallyl ether.

[0031] The weight-average molecular weight of polyvinyl alcohol-based resins is typically 100,000 or more, preferably 110,000 or more, and more preferably 120,000 or more. The upper limit of the weight-average molecular weight is typically 300,000 or less, preferably 280,000 or less, and more preferably 260,000 or less, with a range of, for example, 100,000 to 300,000. If the weight-average molecular weight is too low, it tends to be difficult to obtain sufficient optical performance when using polyvinyl alcohol-based resins as optical films, and if it is too high, it tends to be difficult to stretch when manufacturing polarizing films using polyvinyl alcohol-based films. The weight-average molecular weight of the polyvinyl alcohol-based resins is the weight-average molecular weight measured by the GPC-MALS method.

[0032] The average degree of saponification of the polyvinyl alcohol-based resin used in this embodiment is preferably 98 mol% or higher, particularly preferably 99 mol% or higher, even more preferably 99.5 mol% or higher, and especially preferably 99.8 mol% or higher. If the average degree of saponification is too low, sufficient optical performance tends not to be obtained when the polyvinyl alcohol-based film is used as a polarizing film. Here, the average degree of saponification is measured in accordance with JIS K 6726.

[0033] In this embodiment, two or more polyvinyl alcohol-based resins with different characteristics such as modified species, degree of modification, weight-average molecular weight, and average degree of saponification may be used in combination.

[0034] In addition to the polyvinyl alcohol resin aqueous solution, it is more preferable from the standpoint of film-forming properties to optionally include commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, as well as at least one nonionic, anionic, and cationic surfactant. These can be used individually or in combination of two or more.

[0035] If the aqueous solution of the polyvinyl alcohol resin contains a plasticizer, its content is usually 1 part by mass or more, preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of the polyvinyl alcohol resin. The upper limit of the content is usually 35 parts by mass or less, preferably 30 parts by mass or less, and more preferably 25 parts by mass or less, and the range is, for example, 1 to 35 parts by mass. If the plasticizer content is too low, the stretchability during the production of the polarizing film tends to decrease, and if it is too high, the strength of the resulting polyvinyl alcohol film tends to decrease.

[0036] If the aqueous solution of the polyvinyl alcohol-based resin contains a surfactant, its content is preferably 0.15 parts by mass or less, more preferably 0.1 parts by mass or less, and even more preferably 0.07 parts by mass or less, per 100 parts by mass of the polyvinyl alcohol-based resin. The lower limit of the content is usually 0.01 parts by mass, and its range is, for example, 0.01 to 0.15 parts by mass.

[0037] The resin concentration of the polyvinyl alcohol-based resin aqueous solution obtained in this manner is preferably 15% by mass or more, more preferably 17% by mass or more, and even more preferably 20% by mass or more. The upper limit of the resin concentration is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less, with a range of, for example, 15 to 60% by mass. If the resin concentration of such an aqueous solution is too low, the drying load increases, which tends to reduce production capacity, and if it is too high, the viscosity becomes too high, which tends to make uniform dissolution difficult.

[0038] Next, the obtained polyvinyl alcohol-based resin aqueous solution is subjected to degassing treatment. Degassing methods include static degassing and degassing using a multi-screw extruder. Any multi-screw extruder with a vent will suffice, and a twin-screw extruder with a vent is usually used.

[0039] <Process (B)> Process (B) is a process of forming a film by casting an aqueous solution of polyvinyl alcohol-based resin into a casting mold. After the degassing treatment, a fixed amount of the aqueous solution of polyvinyl alcohol-based resin is introduced into a T-type slit die, discharged and cast onto a rotating casting drum, and a film is formed by a continuous casting method.

[0040] The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-type slit die is usually 70°C or higher, preferably 80°C or higher. The upper limit of the resin temperature is usually 100°C or lower, preferably 98°C or lower, and the range is, for example, 70 to 100°C. If the resin temperature of such a polyvinyl alcohol-based resin aqueous solution is too low, it tends to flow poorly, and if it is too high, it tends to foam.

[0041] The viscosity of the aforementioned polyvinyl alcohol-based resin aqueous solution is preferably 50 Pa·s or more, and more preferably 70 Pa·s or more, at the time of discharge. Furthermore, the upper limit of the viscosity is preferably 200 Pa·s or less, and more preferably 150 Pa·s or less, with a range of, for example, 50 to 200 Pa·s. If the viscosity of such an aqueous solution is too high, it tends to result in poor flow, and if it is too low, it tends to make film formation by casting difficult.

[0042] The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-type slit die to the casting drum is typically 0.2 m / min or more, preferably 0.4 m / min or more, and more preferably 0.6 m / min or more. The upper limit of the discharge rate is typically 5 m / min or less, preferably 4 m / min or less, and more preferably 3 m / min or less, within a range such as 0.2 to 5 m / min. If the discharge rate is too slow, productivity tends to decrease, and if it is too fast, casting tends to become difficult.

[0043] The diameter of the cast drum is preferably 2 m or more, more preferably 2.4 m or more, and even more preferably 2.8 m or more. The upper limit of the diameter is usually 5 m or less, preferably 4.5 m or less, and more preferably 4 m or less, with a range of, for example, 2 to 5 m. If the diameter of the cast drum is too small, the drying section on the cast drum becomes shorter, which tends to make it difficult to increase the speed, and if it is too large, transportability tends to decrease.

[0044] The width of the casting drum is preferably 3 m or more, more preferably 3.5 m or more, even more preferably 4 m or more, particularly preferably 4.5 m or more, and especially preferably 5 to 8 m. If the width of the casting drum is too small, productivity tends to decrease.

[0045] The surface temperature of the cast drum is preferably 93°C or lower in upper limit, as this suppresses foaming during drying and results in a film with excellent appearance. More preferably 91°C or lower, even more preferably 89°C or lower, and particularly preferably 86°C or lower. Furthermore, the lower limit of the surface temperature is preferably 40°C or higher in terms of excellent peelability when peeling the film obtained from the cast drum. More preferably 45°C or higher, even more preferably 50°C or higher, and particularly preferably 60°C or higher. In other words, the surface temperature range of the cast drum is, for example, 40 to 93°C.

[0046] Furthermore, the moisture content of the film when peeled from the cast drum is preferably 30% by mass or less, more preferably 28% by mass or less, even more preferably 26% by mass or less, and particularly preferably 25% by mass or less. If the moisture content is too high, peeling will be poor during film formation, and breakage is more likely to occur. Furthermore, the moisture content of the film when peeled is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 19% by mass or more. If the moisture content is too low, the film tends to curl easily. In other words, the range of moisture content of the film when peeled is, for example, 10 to 30% by mass.

[0047] <Process (C)> Process (C) is a process of heating and drying the film that has been prepared. In process (B), the film peeled off from the cast mold (prepared film) is conveyed in the flow direction (MD direction) using a nip roll or the like, and is dried by alternately bringing the front and back surfaces of the film into contact with multiple heat rolls.

[0048] The aforementioned heat rolls are, for example, rolls with a diameter of 0.2 to 2 m whose surfaces are hard chrome plated or mirror-finished, and it is preferable to use 2 to 30 rolls, preferably 10 to 25 rolls, for drying.

[0049] The surface temperature of the heat roll is not particularly limited, but is usually 30°C or higher, preferably 40°C or higher. The upper limit of the surface temperature is usually 150°C or lower, preferably 140°C or lower, and the range is, for example, 30 to 150°C. If the surface temperature of the heat roll is too low, drying tends to be insufficient, and if it is too high, it tends to dry too much, leading to appearance defects such as waviness.

[0050] In this embodiment, it is preferable that the moisture content of the film at 60% of the total residence time in process (C) is 5 to 10% by mass, and that at least one of the heat rolls in contact with the film is 100°C or higher at the point after 60% of the residence time. This polyvinyl alcohol-based film can be produced by adjusting the moisture content of the film and the drying temperature (heat roll temperature) at 60% of the residence time, as described above. Ｌ0-30 ΔS Ｌ S Ｌ：ave The value of can be made to a desired number. For example, by increasing the moisture content of the film and / or the heat roll temperature, the S Ｌ0-30 ΔS Ｌ S Ｌ：ave The value of can be increased, and by lowering the moisture content of the film and / or the heat roll temperature, the S Ｌ0-30 ΔS Ｌ S Ｌ：ave The value of can be reduced. By using such drying conditions, the aforementioned S Ｌ0-30 ΔS Ｌ S Ｌ：aveThere is a tendency to obtain polyvinyl alcohol-based films that are within a specific range. In this specification, "total residence time" is the time required from the moment the formed film comes into contact with the first heat roll until the moment it leaves the last heat roll.

[0051] The moisture content of the film at the 60% residence time is preferably 5% by mass or more, more preferably 5.2% by mass or more, and even more preferably 5.4% by mass or more. The upper limit of the moisture content is preferably 10% by mass or less, more preferably 9.8% by mass or less, and even more preferably 9.6% by mass or less, with a range of, for example, 5 to 10% by mass. If the moisture content is too high, the running performance during polarizing plate manufacturing tends to deteriorate, and if it is too low, the amount of low molecular weight components that leach out tends to increase.

[0052] The moisture content of the film at the point where the residence time reaches 60% is calculated by taking the point when the film contacts the first heat roll in process (C) as 0 seconds, and the time at which the film leaves the final heat roll in process (C) as t seconds, and determining the moisture content of the film at 0.6 × t seconds. The point at which the residence time reaches 60% can be easily identified by marking the film surface and measuring the time.

[0053] Furthermore, the moisture content can be measured by the following method: Cut the film from the center in the width direction, and measure the initial mass W of the film. ０ (g) is measured. Next, the film is dried in a vacuum dryer at 83°C for 20 minutes, and the mass W after drying is measured. １ Measure (g). Initial mass W ０ (g) and the mass W after drying １ From (g), the moisture content W is obtained by the following formula (1). ２ Calculate the moisture content (by mass). ２ (mass%) = (W ０ -W １ ) / W ０ ×100 ... (1)

[0054] At the point after 60% of the residence time, the temperature of at least one of the heat rolls in contact with the film is preferably 100°C or higher, more preferably 105°C or higher, even more preferably 110°C or higher, particularly preferably 111°C or higher, and especially preferably 112°C or higher. The upper limit of the heat roll temperature is usually 140°C or lower, preferably 130°C or lower, and more preferably 120°C or lower, with a range such as 100 to 140°C. Furthermore, it is preferable that the heat roll temperature is the temperature of the hottest heat roll in contact with the film at the point after 60% of the residence time.

[0055] <Step (D)> Step (D) is a step in which the obtained film is heat-treated using hot air. Step (D) is performed by heat-treating the film that has gone through step (C) in, for example, a hot air drying oven. The upper limit of the heat treatment temperature is preferably 99°C or less, more preferably 98°C or less, even more preferably 97°C or less, and particularly preferably 96°C or less. The lower limit of the heat treatment temperature is preferably 60°C or higher, more preferably 62°C or higher, even more preferably 64°C or higher, and particularly preferably 65°C or higher. That is, the range of the heat treatment temperature is, for example, 60 to 99°C. If the heat treatment temperature is too high, the tension during polarizing film production tends to increase, and if it is too low, the amount of elution of low molecular weight components during polarizing film production tends to increase, and wrinkles and folds tend to occur in the film during the swelling process. The heat treatment time is preferably 20 to 100 seconds, and particularly preferably 40 to 70 seconds.

[0056] Up to this point, we have described a method for producing a polyvinyl alcohol-based film by preparing an aqueous solution of polyvinyl alcohol-based resin, casting this solution onto a rotating casting drum (drum-type roll), and then forming and drying the film by the casting method. However, it is also possible to cast the aqueous solution of polyvinyl alcohol-based resin onto a resin film or a metal belt, and then form and dry the film.

[0057] [Polyvinyl alcohol-based film] Thus, a polyvinyl alcohol-based film is obtained through the above steps (A) to (C), and optionally step (D), and is finally wound onto a roll to become the final product.

[0058] The length of the polyvinyl alcohol-based film is preferably 4 km or more from the viewpoint of increasing the area of ​​the polarizing film, and more preferably 5 to 50 km from the viewpoint of transport mass.

[0059] The width of the polyvinyl alcohol-based film is preferably 1 m or more, more preferably 2 m or more, even more preferably 3 m or more, and particularly preferably 3.5 m or more. The upper limit of the film width is usually 7 m or less, and the range is, for example, 1 to 7 m.

[0060] The polyvinyl alcohol-based film obtained by this manufacturing method is useful for optical applications because it has excellent curl resistance and stretchability, low elution of low molecular weight components, and excellent optical properties. In particular, it is very useful as a base film for polarizing films, and the following describes a method for manufacturing polarizing films and polarizing plates made from this polyvinyl alcohol-based film.

[0061] [Method for Manufacturing Polarizing Film] A polarizing film according to one embodiment of the present invention is manufactured by unwinding the polyvinyl alcohol-based film from a roll and transporting it horizontally, followed by processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

[0062] The swelling process is performed before the dyeing process, for example, by immersing the film in a swelling solution (swelling bath). The swelling process not only cleans dirt from the surface of the polyvinyl alcohol-based film, but also prevents uneven dyeing by swelling the film. In the swelling process, water is usually used as the swelling solution. The swelling solution may contain additives such as iodide compounds, surfactants, and alcohol, as long as the main component is water. The temperature of the swelling bath is usually around 10 to 45°C, and the immersion time in the swelling bath is usually around 0.1 to 10 minutes.

[0063] The aforementioned dyeing process is carried out by contacting the film with a liquid containing iodine or a dichroic dye (dyeing solution), for example, by immersing it in the dyeing solution. Typically, an aqueous solution of iodine-potassium iodide is used as the dyeing solution, with a suitable iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L. In addition to water, a small amount of a water-compatible organic solvent may be included in the dyeing solution. A practical dyeing time is approximately 30 to 500 seconds. A dyeing temperature of 5 to 50°C is preferred.

[0064] The boric acid crosslinking process is carried out using boric acid or a boron compound such as borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 10 to 100 g / L, and it is preferable to include potassium iodide in the solution for stabilizing the polarization performance. The processing temperature is preferably about 30 to 70°C, and the processing time is preferably about 0.1 to 20 minutes, and stretching may be performed during the process as needed.

[0065] The stretching step preferably involves stretching the film 3 to 10 times in a uniaxial direction, and particularly preferably 3.5 to 7 times. At this time, some stretching (to the extent that shrinkage in the width direction is prevented, or more) may also be performed in the direction perpendicular to the stretching direction. The stretching temperature is preferably 40 to 170°C. Furthermore, the stretching ratio only needs to be set within the above range in the end, and the stretching operation may be performed not only in one stage but multiple times in the manufacturing process.

[0066] The aforementioned washing step is performed, for example, by immersing the film in an iodide aqueous solution such as water or potassium iodide, which can remove precipitates that form on the surface of the film. When using a potassium iodide aqueous solution, the potassium iodide concentration is preferably about 10 to 1000 g / L. The temperature during the washing process is usually 5 to 50°C, preferably 10 to 45°C. The processing time is usually 1 to 300 seconds, preferably 10 to 240 seconds. Note that washing with water and washing with a potassium iodide aqueous solution may be performed in combination as appropriate.

[0067] The drying process is carried out, for example, by drying in a dryer at 40 to 100°C for 0.1 to 10 minutes.

[0068] Thus, a polarizing film is obtained, and the degree of polarization of such a polarizing film is preferably 99.90% or higher, more preferably 99.99% or higher. If the degree of polarization is too low, the contrast in the liquid crystal display tends to decrease. The degree of polarization is generally measured by the light transmittance (H) measured at wavelength λ with two polarizing films superimposed so that their orientation directions are in the same direction. 11 The degree of polarization is calculated from the light transmittance (H1) measured at wavelength λ when the two polarizing films are superimposed so that their orientation directions are perpendicular to each other, according to the following formula (2): Polarization degree = [(H 11 -H1) / (H 11 +H1) 1 / 2 ... (2)

[0069] Furthermore, the transmittance of the polarizing film in this embodiment is preferably 43% or higher. If the transmittance is too low, it tends not to be possible to achieve high brightness in the liquid crystal display. The transmittance of the polarizing film is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0070] Next, a method for manufacturing a polarizing plate using the polarizing film of this embodiment will be described. The polarizing film of this embodiment is suitable for manufacturing a polarizing plate with less color unevenness and excellent polarization performance.

[0071] [Method for Manufacturing Polarizing Plates] Polarizing plates are manufactured by laminating an optically isotropic resin film as a protective film to one or both sides of the polarizing film of this embodiment via an adhesive. Examples of protective films include films made of acetylcellulose resins such as triacetylcellulose and diacetylcellulose, films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate, polycarbonate resin films, cycloolefin resin films, acrylic resin films, and films made of chain-like olefin resins such as polypropylene resins.

[0072] The bonding method is carried out by known techniques, for example, by uniformly applying a liquid adhesive composition to the polarizing film, the protective film, or both, then bonding the two together, pressing them together, and irradiating them with heat or active energy rays.

[0073] Furthermore, a polarizing film can also be made by applying a curable resin, such as a urethane resin, acrylic resin, or urea resin, to one or both sides of the polarizing film, curing it to form a cured layer, and thus creating a polarizing plate. In this way, the cured layer replaces the protective film, allowing for a thinner film. These curable resins can be used individually or in combination of two or more types.

[0074] Polarizing films and polarizing plates obtained using this polyvinyl alcohol-based film have excellent polarization performance and are preferably used in portable information terminals, personal computers, televisions, projectors, signage, electronic desktop calculators, electronic clocks, word processors, electronic paper, game consoles, video players, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, foldable displays, anti-reflective layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

[0075] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention. In the examples, "parts" means by mass.

[0076] <Example 1> (Preparation of polyvinyl alcohol-based film) 2,000 kg of polyvinyl alcohol-based resin with a weight-average molecular weight of 142,000 and a degree of saponification of 99.8 mol%, 5,000 kg of water, and 220 kg of glycerin as a plasticizer were added, and the mixture was heated to 140°C while stirring and dissolved under pressure. Then, water was added to adjust the concentration to 25% by mass, and an aqueous solution of polyvinyl alcohol-based resin in which each component was uniformly dissolved was obtained. Next, the aqueous solution of polyvinyl alcohol-based resin was supplied to a twin-screw extruder with a vent to remove bubbles, the aqueous solution temperature was raised to 95°C, and the solution was extruded and cast from the T-type slit die outlet onto a rotating cast drum heated to a surface temperature of 80°C to form a film. The moisture content of the obtained film when peeled off the cast drum was 24% by mass. The obtained film was peeled off the cast drum and heated and dried using multiple hot rolls. During the heating and drying process, the moisture content of the film was set to 6% by mass at 60% of the total residence time, and the temperature of the hottest heat roll in contact with the film after 60% of the residence time was set to 110°C. Subsequently, a floating dryer was used to heat-treat the film by blowing 90°C hot air from both sides. Finally, both ends were slit and the film was wound onto a roll to obtain a polyvinyl alcohol-based film with a thickness of 45 μm, a width of 5 m, and a length of 5 km.

[0077] (Manufacturing of polarizing film) The obtained polyvinyl alcohol-based film was unwound from the roll and transported horizontally while being immersed in a water bath at 30°C to swell, and stretched to 1.7 times its original size in the flow direction (MD direction) relative to the original roll. Next, it was dyed by immersing it in an aqueous solution (at 30°C) with a composition of 0.5 g / L iodine and 30 g / L potassium iodide, and stretched to 2.7 times its original size in the flow direction (MD direction) relative to the original roll. Furthermore, it was immersed in an aqueous solution (at 55°C, the treatment tank is called the stretching tank) with a composition of 40 g / L boric acid and 30 g / L potassium iodide to crosslink with boric acid, and uniaxially stretched to 6.0 times its original size in the flow direction (MD direction) relative to the original roll. Finally, it was washed with an aqueous potassium iodide solution and dried at 70°C for 2 minutes to obtain a polarizing film with a total stretch ratio of 6.0 times.

[0078] <Example 2> The process was carried out in the same manner as in Example 1, except that the moisture content of the film at 60% of the total residence time in the heating and drying process was changed to 7% by mass, and the temperature of the hot roll that the film contacts at the highest temperature after 60% of the residence time was changed to 115°C. A polyvinyl alcohol-based film was obtained using the same procedure as in Example 1. A polarizing film was also obtained using the same procedure as in Example 1.

[0079] <Comparative Example 1> A polyvinyl alcohol-based film was obtained by following the same procedure as in Example 1, except that the moisture content of the film at 60% of the total residence time in the heating and drying process was changed to 5% by mass, and the temperature of the hot roll that the film contacts at the highest temperature after 60% of the residence time was changed to 80°C. A polarizing film was also obtained using the same procedure as in Example 1.

[0080] <Comparative Example 2> A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the moisture content of the film at 60% of the total residence time in the heating and drying process was changed to 3% by mass, and the temperature of the hot roll that the film contacts at the highest temperature after 60% of the residence time was changed to 80°C. A polarizing film was also obtained using the same procedure as in Example 1.

[0081] In Examples 1 and 2 and Comparative Examples 1 and 2, the moisture content of the film at a residence time of 60% was determined by the following method.

[0082] [Moisture content of the film at 60% residence time] At 60% residence time in the heating and drying process, the film was sampled from the center in the width direction, and the initial mass W ０ (g) was measured. Next, the mass W of the film after drying it in a vacuum dryer at 83°C for 20 minutes was measured. １ From (g), the moisture content W is obtained by the following formula (1). ２ The moisture content (by mass) was calculated. ２ (%) = (W ０ -W １ ) / W ０ ×100 ... (1) Furthermore, the moisture content was determined by measuring the moisture content of three films (n=3) and taking the average value as the moisture content of the film at a residence time of 60%.

[0083] The polyvinyl alcohol-based films obtained from Examples 1 and 2, and Comparative Examples 1 and 2, were subjected to small-angle X-ray scattering measurements, curl tests, elution tests, and stretchability tests. The results are shown in Table 1 below.

[0084] [Small-angle X-ray scattering measurement] Polyvinyl alcohol-based film was stored at 23°C and 65% humidity for more than 24 hours, and then cut into multiple pieces measuring 3 cm x 3.8 cm. Next, these cut pieces of film were stacked on a measurement mount so that the total thickness was approximately 180 μm, and this was used as the sample before water immersion (0 seconds). In addition, polyvinyl alcohol-based film was stored at 23°C and 65% humidity for more than 24 hours, and then cut into pieces measuring 5 mm wide x 70 mm long, with the longitudinal direction being the MD direction (flow direction). Next, this cut film was immersed in 35°C warm water for a predetermined time (0 to 300 seconds) to swell. The films that had been immersed for the predetermined time were folded along the longitudinal direction and stacked on a measurement cell so that the total thickness of the films was approximately 500 μm, and these were used as the samples after water immersion for each immersion time. Small-angle X-ray scattering measurements were performed using these samples under the following measurement conditions. [Measurement Conditions] • Measurement device: Aichi Synchrotron Radiation Center BL8S3 • Measurement conditions: Transmission measurement, X-ray: CuKα, Wavelength: 1.5 Å, Detector: PILATUS 2M (Measurement area = 253.7 × 288.8 mm) ２ Pixel size: 172 μm x 172 μm Camera length: 4 m X-ray exposure time: 40 seconds (in water), 120 seconds (before immersion in water) Ambient temperature: Room temperature (25°C)

[0085] In the aforementioned small-angle X-ray measurement, scattering from the polyvinyl alcohol-based film is superimposed by scattering from air and water in the X-ray-passing portion, so it is necessary to correct for scattering as background. Therefore, in the aforementioned small-angle X-ray measurement, the scattering intensity due to air and water scattering, etc., was calculated separately from the scattering intensity obtained from measuring the sample, and this was subtracted to correct the scattering intensity obtained from measuring the sample. The long period was derived from the correlation function obtained by least-squares fitting of the obtained values ​​and Fourier transform of the results. The derived long periods at each immersion time (0, 10, 30, 60, 150, 300 seconds) were plotted on a graph with the long period on the Y axis and the water immersion time on the X axis, and the slope S of the three points from 0 seconds to 30 seconds after immersion was defined. Ｌ0-30 S is the slope of three points from immersion time of 60 seconds to 300 seconds. Ｌ60-300 ΔS Ｌ S Ｌ：ave They sought it.

[0086] [Curl Test] A test piece (test piece 1) measuring 10 cm on the long side and 4 cm on the short side was cut from the center of the width direction of the obtained polyvinyl alcohol-based film. At this time, the long side of the test piece was the length direction (MD direction) of the film, and the short side was the width direction (TD direction) of the film. Next, as shown in Figure 1(a), the center of the lower edge of the long side of the test piece 1 was clamped with a (first) clip (KOKUYO "Eyeball Clip Extra Small"; jaw width 20 mm; mass 2.1 g) 2a, and a wire 2b was wrapped around the clip 2a. A weight 2 with a total mass of 5 g was made using the clip 2a and wire 2b, and a test specimen consisting of the test piece 1 and the weight 2 was prepared. Subsequently, the center of the upper edge of the long side of the test piece 1 was clamped with a (second) clip (same as above) 3a, and a string 3b was tied to the clip 3a (a suspension jig 3 consisting of the clip 3a and the string 3b was attached to the center of the upper edge of the long side of the test piece 1). Next, with the test piece with the clip 3a attached suspended by the string 3b, the entire test piece was immersed in 30°C water 4a in the water tank 4. The maximum angle of curl in the short side direction of the test piece at both ends of the film in the center of the long side direction, observed at the beginning of the immersion, was read as the initial curl [α1(°)]. Also, for convenience, a minus sign was added to the direction in which the initial curl occurred in order to distinguish the direction of the curl. Next, the time [t0 (seconds)] from when the test piece was immersed in water until the curl angle in the short side direction returned to 0° after the initial curl was read. Furthermore, the curl angle in the short side direction at 180 seconds after immersion was read as the final curl [α2(°)]. The curl was observed visually from the top of a water tank 4 (not shown), as shown in Figure 1(b). Note that in Figure 1(b), the weight 2, suspension jig 3, and water tank 4 are not shown for clarity of the configuration. The curl angle [α(°)] is the angle between the flat portion of the test piece 1 and the tangential direction of the edge of the curled portion in the short side direction. The initial curl [α1(°)], final curl [α2(°)], and time to return to 0° [t0(seconds)] were evaluated according to the following evaluation criteria.(Evaluation Criteria) ・Initial curl [α1] ○ (very good): Greater than -90° × (poor): Less than or equal to -90° ・Final curl [α2] ○ (very good): Greater than -90° and less than +90° × (poor): Less than or equal to -90°, or greater than or equal to +90° ・Time to return to 0° [t0] ○ (very good): Less than 25 seconds × (poor): 25 seconds or more ・Overall evaluation ○ (very good): Initial curl [α1], final curl [α2], and time to return to 0° [t0] all receive a ○ rating × (poor): One or more of the following items receive a × rating.

[0087] [Leaching Test] The obtained polyvinyl alcohol-based film was conditioned at 23°C and 50% RH for 24 hours, and then measured at 100 mm x 100 mm (0.01 m). ２ Five test pieces were cut out. Each test piece was immersed in 1 L of ion-exchanged water at 50°C for 1 minute, and this process was repeated five times to obtain an eluate. 10 mL of the eluate was mixed with 10 mL of a color-developing reagent (500 g of ion-exchanged water, 7.4 g of potassium iodide, 0.65 g of iodine, 10.6 g of boric acid) at room temperature. The absorbance at a wavelength of 690 nm was measured using a spectrophotometer (Shimadzu UV-3600Plus), and the concentration (ppm) of the polyvinyl alcohol-based resin was calculated from a pre-prepared calibration curve. The elution amount (ppm / m²) was then calculated by converting it to area. ２ The following evaluation criteria were used to determine the level of quality: (Evaluation Criteria) ○ (very good): 400 ppm / m ２ Below × (poor): 400ppm / m ２ bigger

[0088] [Stretchability Test] A film roll was obtained by slitting the obtained polyvinyl alcohol-based film at a width of 400 mm from the center position in the width direction. The film was unwound from the roll and continuously conveyed horizontally while being immersed in a water bath at 30°C to swell, and stretched in the flow direction (MD) to 1.7 times the original roll. Next, it was immersed in an aqueous solution (at 30°C) containing 0.5 g / L iodine and 30 g / L potassium iodide to stain it, and stretched in the flow direction (MD direction) to 2.7 times the original roll. Furthermore, it was immersed in an aqueous solution (at 55°C) containing 40 g / L boric acid and 30 g / L potassium iodide to crosslink with boric acid, and uniaxially stretched in the flow direction (MD direction) to 6.0 times the original roll. The tension (N) was measured at this point. The measured tension (N) was evaluated according to the following evaluation criteria. (Evaluation Criteria) ○ (very good): 150N or less △ (good): Higher than 150N but 200N or less × (poor): Higher than 200N

[0089]

[0090] From the results in Table 1, the polyvinyl alcohol-based films of Examples 1 and 2 have a slope S when the change in immersion time with respect to the long period before water immersion and the long period up to 30 seconds of immersion in 35°C water is approximated by a straight line. Ｌ0-30 Because the value of was within a specific range, it exhibited excellent curl resistance and stretchability, and the amount of low molecular weight components leached out was also kept low. In contrast, the polyvinyl alcohol-based film of Comparative Example 1 had S Ｌ0-30 Because the value was outside the specified range, both curl resistance and stretchability were poor, and the amount of low molecular weight components leached out was high. In addition, the polyvinyl alcohol-based film of Comparative Example 2 had S Ｌ0-30 Because the value was outside a specific range, the curl resistance was poor and the amount of low molecular weight components leached out was high. However, the polarizing film obtained using the polyvinyl alcohol-based film of the example was free of foreign matter defects and had excellent polarization performance.

[0091] While the above embodiments illustrate specific forms of the present invention, these embodiments are merely illustrative and should not be interpreted restrictively. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0092] This polyvinyl alcohol-based film is preferably used as a raw material for manufacturing polarizing films used in portable information terminals, personal computers, televisions, projectors, signage, electronic calculators, electronic clocks, word processors, electronic paper, game consoles, video players, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices such as instruments in automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, foldable displays, rollable televisions, rollable displays, anti-reflective coatings for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

[0093] 1. Test specimen 2. Weight 2a. Clip 2b. Wire 3. Hanging jig 3a. Clip 3b. String 4. Water tank 4a. Water

Claims

1. The slope S when linearly approximating the change in long-period values ​​with respect to immersion time, both before immersion and up to 30 seconds of immersion in 35°C water, as determined by small-angle X-ray scattering. Ｌ0-30 The value of is 0.208 < S Ｌ0-30 A polyvinyl alcohol-based film that satisfies <0.

270.

2. The slope S when linearly approximating the change in immersion time with respect to long-period immersion time from 60 to 300 seconds in 35°C water, as determined by small-angle X-ray scattering. Ｌ60-300 The above S Ｌ0-30 ΔS subtracted from Ｌ The value of 0.206 < ΔS Ｌ A polyvinyl alcohol-based film according to claim 1, satisfying <0.

268.

3. The polyvinyl alcohol-based film according to claim 1 or 2, wherein the film thickness is 5 to 50 μm.

4. A polyvinyl alcohol-based film according to claim 1 or 2, for use in manufacturing polarizing films.

5. A polarizing film made using the polyvinyl alcohol-based film described in claim 1 or 2.

6. A polarizing plate comprising a polarizing film according to claim 5 and a protective film provided on at least one side of the polarizing film.

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