Polyvinyl alcohol film and method for producing same

Abstract

The polyvinyl alcohol film is a polyvinyl alcohol film containing polyvinyl alcohol, the average value X (nm) of the lamellar thickness of the polyvinyl alcohol, which is obtained by immersing the polyvinyl alcohol film in pure water at 25 DEG C and performing small-angle X-ray measurement, and the average value Y (nm) of the lamellar thickness of the polyvinyl alcohol, which is obtained by performing small-angle X-ray measurement on the polyvinyl alcohol film in air at 25 DEG C, satisfy the following formulae (1) and (2). By using this polyvinyl alcohol film to produce a polarizing film, it is possible to efficiently obtain a polarizing film by reducing the breakage of the end of the film at the first half of the stretching step and the breakage of the film at the second half of the stretching step. (1) (X-Y) / Y * 100 < = 28, and (2) X < = 4.00.

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol film and a method for manufacturing the same. Background Art

[0002] A polarizing plate having light-transmitting and light-shielding functions and liquid crystal that changes the polarization state of light are both basic components of a liquid crystal display (LCD). LCDs are gradually used in a wide range of devices such as small devices like calculators and watches, laptop computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal TVs, in-vehicle navigation systems, mobile phones, and measuring devices used indoors and outdoors.

[0003] A polarizing plate is usually manufactured as follows: A polyvinyl alcohol film (hereinafter sometimes abbreviated as "PVA") is swollen, dyed, crosslinked, uniaxially stretched, and if necessary, further fixed with a boron compound or the like. After manufacturing the polarizing film, a protective film such as a cellulose triacetate (TAC) film is pasted on the surface of the polarizing film. The optical properties such as the contrast coefficient of an LCD significantly depend on the polarization properties such as the degree of polarization of the polarizing film. Therefore, a PVA film having high stretchability has been proposed to improve the polarization properties.

[0004] For example, in Patent Document 1, as a PVA film that can reduce the generation of wrinkles during stretching, has a low stretching stress, reduce breakage during stretching, and can manufacture a thin polarizing film with a good yield, the following PVA film is described: a PVA film having a swelling degree of 190 to 230% and a thickness of 40 μm or less, and having a swelling degree of 260% or more after being stretched to a stretching ratio of 3 times in water at 30°C.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: International Publication No. 2014 / 050697 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In recent years, due to the increasing demand for LCDs, it has been sought to manufacture a polarizing film, which is one of the components, with good efficiency. In order to improve the manufacturing efficiency of the polarizing film, it is necessary to suppress the breakage of the end of the PVA film in the treatment bath at a lower temperature (20 to 30°C) in the first half of the stretching process (swelling, dyeing, crosslinking processes) and the breakage of the PVA film in the treatment bath at a higher temperature (50 to 60°C) in the second half of the stretching process (uniaxial stretching process). As a method for suppressing the breakage of the end of the PVA film in the first half of the stretching process, a method of increasing the crystallinity of the PVA film and reducing the swelling degree by heat treatment can be considered. However, since the stretching stress of the obtained PVA film is high, it is likely to break in the second half of the stretching process, which becomes a problem.

[0010] As a result of research by the present inventors, even in the case of the PVA film described in Patent Document 1, breakage sometimes occurs due to dissolution in the latter half of the stretching process, and it is sometimes difficult to suppress breakage of the end portion of the PVA film in the first half of the stretching process and sufficiently reduce breakage of the PVA film in the latter half of the stretching process.

[0011] The present invention has been made to solve the above problems, and an object thereof is to provide a PVA film and a method for producing the same that can reduce breakage of the end portion of the PVA film in the first half of the stretching process and breakage of the PVA film in the latter half of the stretching process when producing a polarizing film, and can obtain a polarizing film with good efficiency.

[0012] Means for Solving the Problems

[0013] The present inventors repeatedly conducted in-depth research and found that: by performing small-angle X-ray measurement on a PVA film under specific conditions and using a PVA film in which the average value of the lamellar thickness of PVA in the PVA film satisfies a specific range, the above problems can be solved. And based on this insight, further repeated research was conducted, and thus the present invention was completed.

[0014] That is, the present invention relates to the following technical solutions:

[0015] [1] A PVA film, which is a PVA film containing PVA, and the average value X (nm) of the lamellar thickness of the aforementioned PVA obtained by immersing the aforementioned PVA film in pure water at 25°C and performing small-angle X-ray measurement and the average value Y (nm) of the lamellar thickness of the aforementioned PVA obtained by performing small-angle X-ray measurement on the aforementioned PVA film in air at a temperature of 25°C satisfy the following formulas (1) and (2);

[0016] (X - Y) / Y × 100 ≤ 28 (1)

[0017] X ≤ 4.00 (2)

[0018] [2] The polyvinyl alcohol film according to [1], wherein the swelling degree A (mass%) measured by immersing in pure water at 45°C and the swelling degree B (mass%) measured by immersing in pure water at 30°C satisfy the following formulas (3) and (4);

[0019] A ≥ 270 (3)

[0020] 1.30 ≤ A / B ≤ 1.35 (4)

[0021] [3] The PVA film according to claim [1] or [2], having a thickness of 20 to 80 μm;

[0022] [4]The manufacturing method of the PVA film has: a film-forming step of obtaining a film by drying a film-forming stock solution containing PVA using a drying roll; and a heat treatment step of heat-treating the aforementioned film using a heat treatment roll. The surface temperature of the aforementioned drying roll in the aforementioned film-forming step is lower than 96.0 °C, the average drying speed is 1.10 - 1.20 mass% / second, the moisture content of the aforementioned film supplied to the aforementioned heat treatment step is less than 15.0 mass%, the surface temperature of the aforementioned heat treatment roll in the aforementioned heat treatment step is 96.0 °C or higher, and the difference in the moisture content of the aforementioned film before and after the aforementioned heat treatment is 2.00 - 5.00 mass%.

[0023] Advantages of the Invention

[0024] When using the PVA film of the present invention to manufacture a polarizing film, the breakage of the end of the PVA film in the first half of the stretching step and the breakage of the PVA film in the second half of the stretching step are reduced, and a polarizing film can be obtained with good efficiency. The manufacturing method of the present invention is applicable to the manufacture of such a PVA film. Detailed Embodiments

[0025] <PVA Film>

[0026] The PVA film of the present invention is a PVA film containing PVA. The average value X (nm) of the lamellar thickness of the aforementioned PVA obtained by immersing the aforementioned PVA film in pure water at 25 °C and performing small-angle X-ray measurement and the average value Y (nm) of the lamellar thickness of the aforementioned PVA obtained by performing small-angle X-ray measurement on the aforementioned PVA film in air at a temperature of 25 °C satisfy the following formula (1). It should be noted that in the present invention, the lamellar thickness refers to the thickness of the crystal part of the lamellar structure, and the lamellar structure refers to a layered structure formed by crystals and amorphous PVA.

[0027] (X - Y) / Y × 100 ≤ 28 (1)

[0028] X ≤ 4.00 (2)

[0029] In the PVA film of the present invention, as shown in the above formula (1), the ratio ((X - Y) / Y × 100) of the difference between the average value X of the lamellar thickness and the average value Y of the lamellar thickness to the average value Y of the lamellar thickness must be 28% or less. From the viewpoint of suppressing the breakage of the end of the PVA film in the first half of the stretching step when manufacturing a polarizing film, the ratio is more preferably 27.5% or less, and further preferably 27% or less. On the other hand, from the viewpoint of moderately reducing the tensile stress and breakage of the PVA film in the second half of the stretching step when manufacturing a polarizing film, the ratio is preferably 10% or more, more preferably 15% or more, further preferably 20% or more, and particularly preferably 25% or more.

[0030] In the PVA film of the present invention, as shown in the above formula (2), the average value X of the lamellar thickness of PVA obtained by immersing the PVA film in pure water at 25 °C and performing small-angle X-ray measurement must be 4.00 nm or less. From the viewpoint of moderately maintaining the tensile stress of the PVA film in the latter half of the stretching process during the production of the polarizing film and reducing the breakage caused by dissolution, the average value X of the lamellar thickness is preferably 3.60 nm or more, more preferably 3.70 nm or more, further preferably 3.80 nm or more, and particularly preferably 3.85 nm or more. On the other hand, from the viewpoint of moderately reducing the tensile stress of the PVA film in the latter half of the stretching process during the production of the polarizing film and reducing breakage, the average value X of the lamellar thickness must be 4.00 nm or less, preferably 3.98 nm or less, more preferably 3.95 nm or less, and further preferably 3.92 nm or less.

[0031] The reason for the above-mentioned effects of the PVA film of the present invention by satisfying the above formulas (1) and (2) is speculated as follows. First, the average value X of the lamellar thickness of PVA obtained by immersing the PVA film in pure water at 25 °C and performing small-angle X-ray measurement refers to the average value of the lamellar thickness of PVA remaining without complete dissolution in pure water at 25 °C. On the other hand, the average value Y of the lamellar thickness of PVA obtained by performing small-angle X-ray measurement on the PVA film in air at a temperature of 25 °C refers to the average value of the lamellar thickness of PVA contained in the PVA film before immersion in pure water at 25 °C. Therefore, the ratio ((X - Y) / Y×100) of the difference between the average value X of the lamellar thickness and the average value Y of the lamellar thickness to the average value Y of the lamellar thickness refers to the ratio of the change amount (X - Y) of the average value of the lamellar thickness of PVA before and after immersing the PVA film in pure water at 25 °C to the average value Y of the lamellar thickness of PVA before immersing the PVA film in pure water at 25 °C. The larger this ratio, the more it means that the crystalline component of PVA contained in the PVA film dissolves more in pure water at 25 °C. Therefore, it can be considered that if the above ratio is too large, the crystalline component of PVA contained in the PVA film will dissolve excessively in pure water at 25 °C. As a result, in the first half of the stretching process during the production of the polarizing film, the PVA film swells excessively and the end of the PVA film is likely to break. In addition, if the average value X of the lamellar thickness is too large, in the latter half of the stretching process during the production of the polarizing film, the stress of the PVA film in the stretching tank is likely to increase due to an increase in the amount of the crystalline component of PVA that does not dissolve in the stretching tank, and the PVA film is likely to break. Therefore, it can be considered that in the PVA film of the present invention, by satisfying the above formulas (1) and (2), the above-mentioned effects are achieved.

[0032] In the PVA film of the present invention, from the viewpoint of moderately maintaining the tensile stress of the PVA film during stretching in the latter half of the stretching process when manufacturing a polarizing film and reducing breakage due to dissolution, the average value Y of the lamellar thickness of PVA obtained by performing small-angle X-ray measurement on the PVA film in air at a temperature of 25°C is preferably 3.00 nm or more, more preferably 3.02 nm or more, and still more preferably 3.04 nm or more. On the other hand, from the viewpoint of moderately reducing the tensile stress of the PVA film and reducing breakage in the latter half of the stretching process when manufacturing a polarizing film, the average value Y of the aforementioned lamellar thickness is preferably 3.40 nm or less, more preferably 3.30 nm or less, still more preferably 3.20 nm or less, and particularly preferably 3.10 nm or less.

[0033] (Small-angle X-ray scattering measurement)

[0034] The measurement principle of small-angle X-ray scattering measurement (hereinafter sometimes referred to as SAXS measurement) is as follows: When irradiating a sample with X-rays, the X-rays scattered by the electrons around the atoms are measured in the form of a function of the scattering angle and analyzed. SAXS measurement is a method of measuring the scattered X-rays in the small-angle region where 2θ < 10° in particular and evaluating the material structure, and can generally evaluate structures with sizes on the order of several nm to several tens of nm. By using this information, the long period of the PVA lamellae, the lamellar thickness, etc. of PVA can be obtained.

[0035] A specific implementation method of SAXS measurement for obtaining the average values X and Y of the lamellar thickness of PVA contained in the PVA film of the present invention will be described. It should be noted that by substituting the obtained average values X and Y of the aforementioned lamellar thickness, the aforementioned ratio ((X - Y) / Y × 100) in the above formula (1) can be calculated.

[0036] [Average value X of the lamellar thickness of PVA]

[0037] 《1》SAXS measurement

[0038] Cut out multiple film samples with a size of MD (flow direction) × TD = 2 cm × 1 cm from the central part in the width direction (TD) of the PVA film obtained from the following examples or comparative examples. After sufficiently conditioning the film samples in air at a temperature of 23°C and a humidity of 50%, stack them in the same orientation so that no bubbles enter between the film samples until the thickness reaches about 750 μm, and immerse them in pure water (distilled water) with a mass ratio of about 1000 times at 25°C for 24 hours. Then, place the film samples in a cuvette with the MD in the vertical direction and perform transmission measurement using a small-angle X-ray scattering measurement device "NANOPIX" (manufactured by Rigaku Corporation).

[0039] It should be noted that the structure of the cuvette is as follows: A KAPTON film with a thickness of 7.5 μm is used as the window material on the incident light side and the reflected light side, and the interval between the window materials is set to about 1.5 mm so that the measurement sample can be sealed in water. If this cuvette is used, the sample can be placed in water in the normal measurement configuration of the above-mentioned device.

[0040] [Measurement conditions]

[0041] X-ray: CuKα ray

[0042] Wavelength: 0.15418 nm

[0043] Output power: 40 kv - 30 mA

[0044] First slit:

[0045] Guard slit:

[0046] Detector: Semiconductor two-dimensional detector "HyPix-6000"

[0047] Pixel size: 100 μm × 100 μm

[0048] Camera length: 960 mm

[0049] X-ray exposure time: Continuous measurement for 5 minutes each (taking 4 shots)

[0050] Ambient temperature (pure water temperature): 25 °C

[0051] 《2》Analysis method of SAXS

[0052] In the measurement of small-angle X-ray scattering, since the scattering from devices such as slits, the air in the X-ray passing part, and the cuvette will overlap with the scattering of the sample, it is necessary to correct these scatterings as the background. In the present invention, the correction is performed by subtracting the scattering intensity when only pure water at 25 °C is poured into the cuvette and measured from the scattering intensity obtained from the measurement sample (film sample).

[0053] Furthermore, based on the scattering intensity image measured by the two-dimensional detector, the scattering intensity with respect to the scattering vector q is integrated along the azimuthal direction to obtain a one-dimensional curve of the scattering vector q and the scattering intensity I(q). Then, for q being 1.7 nm -1The scattering intensity of the above region is fitted by the least squares method based on the constant c, and the obtained constant c is used as the baseline and subtracted from the scattering intensity. After performing a Lorentz correction on the obtained one-dimensional curve, a least squares fit is performed using a Gaussian function, and the resulting scattering curve is Fourier-transformed to derive a correlation function K(z) defined as in the following formula (3).

[0054] K(z) ∝ ∫[0→∞]{q 2 ·I(q) ·cos(qz)} ·dq (3)

[0055] The z-coordinate value of the maximum point of the correlation function K(z) derived by the above operations is defined as the lamellar long period. In addition, the value calculated from the intersection of the line with a slope of 0 passing through the minimum point of the correlation function K(z) and the line obtained by fitting in the region where z is small is defined as the average value of the lamellar thickness. Specifically, in the present invention, the intersection of the line with a slope of 0 passing through the minimum point of the correlation function K(z) and the line obtained by fitting the correlation function K(z) using the least squares method in the range where z is 1.0 to 2.5 nm is obtained, and the z-coordinate value of this intersection is set as the average value X of the lamellar thickness of PVA.

[0056] [Average value Y of the lamellar thickness of PVA]

[0057] 《1》SAXS measurement

[0058] Multiple film samples with a size of MD (flow direction) × TD = 2 cm × 1 cm are cut out from the central part in the width direction (TD) of the PVA film obtained in the following Examples or Comparative Examples. After fully conditioning the film samples in air at a temperature of 23°C and a humidity of 50%, they are overlapped along the same orientation so that no bubbles enter between the film samples until the thickness reaches about 750 μm. Then, with the MD of the film samples in the vertical direction, they are placed in a cuvette in air at a temperature of 23°C and a humidity of 50%, and immediately thereafter, transmission measurement is performed using a small-angle X-ray scattering measurement device "NANOPIX" (manufactured by Rigaku Corporation) in a measurement chamber at a temperature of 25°C.

[0059] It should be noted that the structure of the cuvette is as follows: A 7.5-μm-thick KAPTON film is used as the window material on the incident light side and the reflected light side, and the interval between the window materials is set to about 1.5 mm.

[0060] [Measurement conditions]

[0061] X-ray: CuKα ray

[0062] Wavelength: 0.15418 nm

[0063] Output power: 40 kv - 30 mA

[0064] First slit:

[0065] Protective slit:

[0066] Detector: Semiconductor two-dimensional detector "HyPix-6000"

[0067] Pixel size: 100μm × 100μm

[0068] Camera length: 960mm

[0069] X-ray exposure time: Continuous measurement for 5 minutes each (taking 4 shots)

[0070] Ambient temperature: 25°C (temperature of the measurement chamber)

[0071] 《2》Analysis method of SAXS

[0072] In the "《2》Analysis method of SAXS" of the above-mentioned "average value X of the lamellar thickness of PVA", correction is performed by subtracting the scattering intensity measured when measuring the empty cuvette (a cuvette filled only with air at 25°C) from the scattering intensity obtained from the measurement sample (film sample). Except for this, the same operation is performed to obtain the average value Y of the lamellar thickness of PVA.

[0073] The swelling degree A (% by mass) measured by immersing the PVA film of the present invention in pure water at 45°C and the swelling degree B (% by mass) measured by immersing it in pure water at 30°C preferably satisfy the following formulas (3) and (4). By using such a PVA film to manufacture a polarizing film, the breakage of the film end in the first half of the stretching process and the breakage of the film in the second half of the stretching process can be further reduced, and a polarizing film can be obtained with better efficiency. Satisfying the above formulas (3) and (4) means that there are PVA crystals of appropriate size in the PVA film and the distribution of crystal sizes is narrow, which is considered to contribute to the above effects.

[0074] A≥270 (3)

[0075] 1.30≤A / B≤1.35 (4)

[0076] The swelling degree A measured by immersing the PVA film of the present invention in pure water at 45°C is preferably 270% by mass or more. The swelling degree is an index indicating the water retention ability when the PVA film is immersed in water, and can be obtained as a percentage by dividing the mass of the PVA film directly (without stretching) after immersing in the specified water for 30 minutes by the mass after drying at 105°C for 16 hours after immersion. Specifically, it can be measured by the method described in the examples below. By making the swelling degree A 270% by mass or more, the tensile stress of the PVA film is moderately reduced in the latter half of the stretching process when manufacturing the polarizing film, and breakage is further reduced. On the other hand, from the viewpoint of further reducing breakage due to dissolution of the PVA film in the latter half of the stretching process, the swelling degree A is preferably 300% by mass or less, more preferably 290% by mass or less, further preferably 285% by mass or less, and particularly preferably 278% by mass or less.

[0077] The swelling degree B measured by immersing the PVA film of the present invention in pure water at 30°C is preferably 190% by mass or more. By making the swelling degree B 190% by mass or more, the dye can be adsorbed onto the PVA film with good efficiency in the dyeing process, which is the first half of the stretching process when manufacturing the polarizing film. The swelling degree B is more preferably 195% by mass or more, further preferably 200% by mass or more, particularly preferably 203% by mass or more, and most preferably 205% by mass or more. On the other hand, from the viewpoint of further reducing breakage at the ends of the film in the first half of the stretching process, the swelling degree B is preferably 250% by mass or less, more preferably 230% by mass or less, further preferably 220% by mass or less, still further preferably 215% by mass or less, and particularly preferably 210% by mass or less.

[0078] The swelling degree A (% by mass) measured by immersing the PVA film of the present invention in pure water at 45°C and the swelling degree B (% by mass) measured by immersing it in pure water at 30°C preferably satisfy the following formula (4). Thereby, breakage at the ends of the film in the first half of the stretching process and breakage of the film in the second half of the stretching process when manufacturing the polarizing film are further reduced.

[0079] 1.30 ≤ A / B ≤ 1.35 (4)

[0080] The strain curing point measured by immersing the PVA film of the present invention in pure water at 50°C is preferably 180 to 230%. The strain curing point is measured as follows. Immerse the aforementioned PVA film in water at 50°C for 1 minute. Thereafter, while uniaxially stretching along the longitudinal direction (mechanical flow direction) at a prescribed speed, continuously measure the tensile stress (MPa) with respect to the strain (tensile ratio) (%). With respect to the strain, plot the value (ΔS) obtained by dividing the change amount of the tensile stress in 0.05 seconds by the change amount of the strain in 0.05 seconds. In the range of strain from 100% to 400%, the strain showing the minimum value of ΔS is taken as the strain curing point. Specifically, use the method for measuring the strain curing point described in the examples below. By making the strain curing point of the aforementioned PVA film within the above range, the breakage of the film in the latter half of the stretching process for manufacturing a polarizing film can be further reduced as follows. When the aforementioned strain curing point is less than 180%, the film is likely to break easily in the first half and the latter half of the stretching process. The aforementioned strain curing point is more preferably 185% or more, further preferably 195% or more, still further preferably 205% or more, and particularly preferably 215% or more.

[0081] The aforementioned strain curing point is the reference strain at which the orientation crystallization of PVA contained in the PVA film and the high orientation of the amorphous part start by the stretching process during the manufacture of a polarizing film. The smaller the value of the aforementioned strain curing point, the easier it is for the tension of the PVA film to increase in the latter half of the stretching process during the manufacture of a polarizing film, and the easier it is for the tensile stress in water at 50°C described below to increase. On the other hand, the larger the value of the aforementioned strain curing point, the easier it is for the tension of the PVA film to decrease in the latter half of the stretching process during the manufacture of a polarizing film, and the easier it is for the tensile stress in water at 50°C described below to decrease. Therefore, by adjusting the aforementioned strain curing point to the above range, during the latter half of the stretching process for manufacturing a polarizing film, an appropriate tension can be applied to the PVA film, and film breakage caused by high tensile stress and film breakage caused by dissolution in the latter half of the stretching process can be suppressed.

[0082] The tensile stress of the aforementioned PVA film in water at 30°C is preferably 10.0 to 14.0 MPa. Thereby, the breakage of the end portion of the film in the first half of the stretching process of the polarizing film is further reduced. The aforementioned tensile stress is more preferably 13.0 MPa or less, further preferably 12.5 MPa or less, and particularly preferably 12.0 MPa or less. The aforementioned tensile stress is more preferably 10.5 MPa or more, further preferably 11.0 MPa or more. The tensile stress of the aforementioned PVA film is measured by the method described in the examples below. The tensile stress of the PVA film in water at 30°C is measured by the method described in the examples below.

[0083] The tensile stress of the aforementioned PVA film in water at 50°C is preferably 1.30 to 1.65 MPa. Thereby, the breakage of the film in the latter half of the stretching process during the production of the polarizing film is further reduced. The aforementioned tensile stress is more preferably 1.35 MPa or more. On the other hand, the aforementioned tensile stress is more preferably 1.60 MPa or less, further preferably 1.55 MPa or less, still further preferably 1.53 MPa or less, and particularly preferably 1.48 MPa or less. The tensile stress of the PVA film in water at 50°C is measured by the method described in the following examples.

[0084] [PVA]

[0085] Examples of the PVA constituting the PVA film of the present invention include PVA obtained by saponifying a polyvinyl ester obtained by polymerizing one or more of vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among these vinyl esters, from the viewpoints of ease of production, availability, and cost of PVA, vinyl acetate is preferred.

[0086] The polyvinyl ester is preferably obtained by using only one or more vinyl esters as monomers, more preferably by using only one vinyl ester as a monomer. As long as the effects of the present invention are not impaired, it may be a copolymer of one or more vinyl esters and other monomers capable of copolymerizing therewith.

[0087] As other monomers capable of copolymerizing with vinyl esters, for example, α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, isobutene, etc.; (meth)acrylic acid or its salts; (meth)acrylic acid methyl ester, (meth)acrylic acid ethyl ester, (meth)acrylic acid n-propyl ester, (meth)acrylic acid isopropyl ester, (meth)acrylic acid n-butyl ester, (meth)acrylic acid isobutyl ester, (meth)acrylic acid tert-butyl ester, (meth)acrylic acid 2-ethylhexyl ester, (meth)acrylic acid dodecyl ester, (meth)acrylic acid octadecyl ester, etc. (meth)acrylic acid esters; (meth)acrylamide, N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, diacetone(meth)acrylamide, (meth)acrylamide propane sulfonic acid or its salts, (meth)acrylamide propyl dimethylamine or its salts, N-hydroxymethyl(meth)acrylamide or its derivatives, etc. (meth)acrylamide derivatives; N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, etc. N-vinylamides; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc. vinyl ethers; vinyl cyanides such as (meth)acrylonitrile; vinylidene dihalides such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride; allyl compounds such as allyl acetate, allyl chloride; maleic acid or its salts, esters or anhydrides; itaconic acid or its salts, esters or anhydrides; vinyl silyl compounds such as vinyltrimethoxysilane; unsaturated sulfonic acids, etc. The above polyvinyl esters may have structural units derived from one or more of these other monomers.

[0088] The proportion of the structural units derived from other monomers in the polyvinyl ester is preferably 15 mol% or less, more preferably 10 mol% or less, and still more preferably 5 mol% or less based on the total number of moles of all the structural units constituting the polyvinyl ester. In particular, when other monomers such as (meth)acrylic acid and unsaturated sulfonic acids are monomers that are likely to promote the water solubility of the resulting PVA, in order to prevent the dissolution of PVA when the resulting PVA film is used as a raw material film for manufacturing a polarizing film, etc., the proportion of the structural units derived from these monomers in the polyvinyl ester is preferably 5 mol% or less, more preferably 3 mol% or less based on the total number of moles of all the structural units constituting the polyvinyl ester.

[0089] The PVA may be PVA modified with one or more graft copolymerizable monomers as long as the effects of the present invention are not impaired. As the graft copolymerizable monomers, for example, unsaturated carboxylic acids or their derivatives; unsaturated sulfonic acids or their derivatives; α-olefins having 2 to 30 carbon atoms, etc. are exemplified. The proportion of the structural units derived from the graft copolymerizable monomers in the PVA is preferably 5 mol% or less based on the total number of moles of all the structural units constituting the PVA.

[0090] Part of the hydroxyl groups in PVA can be crosslinked or uncrosslinked. In addition, part of the hydroxyl groups in PVA can react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure, or can not react with these compounds and do not form an acetal structure.

[0091] The degree of polymerization of PVA is preferably 1,500 to 6,000, more preferably 1,800 to 5,000, and further preferably 2,000 to 4,000. If the degree of polymerization is less than 1,500, there is a tendency for the durability of the polarizing film manufactured using the obtained PVA film to deteriorate. On the other hand, if the degree of polymerization exceeds 6,000, there is a tendency for the manufacturing cost to increase, the process qualification during film formation to be poor, and the shrinkage stress of the obtained polarizing film to increase. It should be noted that the degree of polymerization of PVA in this specification refers to the average degree of polymerization measured according to the description in JIS K6726-1994.

[0092] From the viewpoint of the water resistance of the polarizing film manufactured using the obtained PVA film, the saponification degree of PVA is preferably 98.0 mol% or more, more preferably 98.5 mol% or more, and further preferably 99.0 mol% or more. If the saponification degree is less than 98.0 mol%, there is a tendency for the water resistance of the polarizing film to deteriorate. It should be noted that the saponification degree of PVA in this specification refers to the proportion (mol%) of the number of moles of the vinyl alcohol unit relative to the total number of moles of the structural unit (typically a vinyl ester unit) that can be converted into a vinyl alcohol unit by saponification and the vinyl alcohol unit in PVA. The saponification degree can be measured according to the description in JIS K6726-1994.

[0093] [Surfactant]

[0094] The PVA film of the present invention preferably contains a surfactant. By containing a surfactant, the thickness unevenness of the PVA film is suppressed, and it is easy to peel off from the roll and the belt when manufacturing the PVA film. The type of the surfactant is not particularly limited, and from the viewpoint of the peelability when peeling off from the roll, the belt, etc. during the manufacture of the PVA film, an anionic surfactant or a nonionic surfactant is preferred.

[0095] As the anionic surfactant, for example, carboxylic acid types such as potassium laurate; sulfate ester types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; sulfonic acid types such as dodecylbenzenesulfonate, etc. are preferred.

[0096] Examples of the nonionic surfactant include alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octyl phenyl ether; alkyl ester types such as polyoxyethylene laurate; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide types such as polyoxyethylene lauryl amide; polypropylene glycol ether types such as polyoxyalkylene allyl phenyl ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; allyl phenyl ether types such as polyoxyalkylene allyl phenyl ether, etc.

[0097] These surfactants can be used alone or in combination of two or more.

[0098] The content of the surfactant in the PVA film of the present invention is preferably 0.01 to 0.5 parts by mass, more preferably 0.02 to 0.3 parts by mass, and further preferably 0.05 to 0.1 parts by mass based on 100 parts by mass of PVA. By making the content of the surfactant within the above range, the film-forming property and peelability of the PVA film can be maintained, and adhesion and reduction in processability due to the exudation of the surfactant to the surface of the PVA film can be prevented.

[0099] [Plasticizer]

[0100] The PVA film of the present invention preferably contains a plasticizer. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. The PVA film of the present invention can contain at least one of these plasticizers. Among these, as described in the manufacturing method of the PVA film described later, the plasticizer is preferably at least one selected from the group consisting of glycerin, diglycerin, and propylene glycol, and the plasticizer more preferably contains glycerin at least.

[0101] The content of the plasticizer in the PVA film of the present invention is preferably 1 to 20 parts by mass, more preferably 3 to 17 parts by mass, and further preferably 5 to 15 parts by mass based on 100 parts by mass of PVA. By making the content of the plasticizer within the above range, the stretchability of the PVA film can be further improved, and the PVA film can be prevented from being too soft and the processability from being reduced.

[0102] [Other additives]

[0103] The PVA film of the present invention can further contain components such as an antioxidant, an antifreeze, a pH regulator, a masking agent, an anti-coloring agent, and an oil agent as needed.

[0104] [Shape, etc.]

[0105] The shape of the PVA film of the present invention is not particularly limited. From the aspect of being able to continuously and smoothly manufacture a more uniform PVA film and being able to be continuously used even when manufacturing a polarizing film using the same, etc., a long strip film is preferred. The length (the length in the mechanical flow direction) of the long strip film is not particularly limited and can be appropriately set according to the use, etc. For example, it can be set within the range of 5 to 30,000 m.

[0106] The width of the PVA film of the present invention is not particularly limited and can be appropriately set according to the use of the PVA film, the polarizing film manufactured therefrom, etc. In recent years, from the viewpoint that the large screen of liquid crystal televisions and liquid crystal monitors is being promoted, if the width of the PVA film is set to 3 m or more, more preferably 4 m or more, it is suitable for these uses. On the other hand, if the width of the PVA film is too large, it is likely to be difficult to uniformly uniaxially stretch itself when manufacturing a polarizing film using a device that has been put into practical use. Therefore, the width of the PVA film is preferably 7 m or less.

[0107] In the present invention, the thickness of the PVA film is preferably 20 to 80 μm. By making the thickness of the PVA film 80 μm or less, the drying speed when manufacturing the aforementioned PVA film and polarizing film becomes higher. The aforementioned thickness is more preferably 60 μm or less, further preferably 55 μm or less, still further preferably 50 μm or less, and particularly preferably 48 μm or less. On the other hand, by making the thickness of the PVA film 20 μm or more, the breakage of the PVA film in the first half and the second half of the stretching process can be effectively reduced. The aforementioned thickness is more preferably 30 μm or more.

[0108] The PVA film of the present invention is preferably an optical film. That is, the PVA film of the present invention is preferably used as a raw material film used in the manufacture of optical films such as polarizing films and retardation films. Among these, according to the PVA film of the present invention, the breakage of the film end in the first half of the stretching process and the film breakage in the second half of the stretching process can be reduced, and a polarizing film with excellent optical properties can be obtained. Therefore, it is preferably used as a raw material film used in the manufacture of polarizing films.

[0109] <Manufacturing method of PVA film>

[0110] There is no particular limitation on the manufacturing method of the PVA film of the present invention, and existing well-known methods such as wet film formation method, gel film formation method, casting film formation method, and extrusion film formation method can be adopted. By adopting the manufacturing method of the PVA film described below, the PVA film of the present invention can be efficiently manufactured.

[0111] The manufacturing method of the PVA film of the present invention includes: a film-forming step of obtaining a film by drying a film-forming stock solution containing PVA using a drying roll; and a heat treatment step of heat-treating the aforementioned film using a heat treatment roll. The surface temperature of the aforementioned drying roll in the aforementioned film-forming step is lower than 96.0 °C, and the average drying speed is 1.10 to 1.20 mass% / second. The moisture content of the aforementioned film supplied to the aforementioned heat treatment step is less than 15.0 mass%. The surface temperature of the aforementioned heat treatment roll in the aforementioned heat treatment step is 96.0 °C or higher, and the difference in the moisture content of the aforementioned film before and after the heat treatment is 2.00 to 5.00 mass%.

[0112] In the above manufacturing method of the PVA film, as the film-forming stock solution containing PVA (hereinafter sometimes abbreviated as the film-forming stock solution), a stock solution obtained by dissolving PVA in water or a stock solution obtained by melting PVA containing PVA and water can be used. The specific and suitable forms of PVA contained in the film-forming stock solution are the same as those of PVA contained in the above PVA film.

[0113] As long as it is within the range that does not impair the effects of the present invention, volatile components other than water (liquid media such as those removed by volatilization and evaporation during film formation) can be contained in the film-forming stock solution. As volatile components other than water, water-soluble organic solvents (such as alcohols and ketones) that can be dissolved in water in any proportion can be exemplified. When the film-forming stock solution contains volatile components other than water, its content is preferably 10 mass parts or less relative to 100 mass parts of water. From the viewpoints of environmental burden and recyclability, the film-forming stock solution preferably substantially does not contain volatile components other than water.

[0114] The moisture content of the film-forming stock solution varies depending on the film-forming method, film-forming conditions, etc., and is preferably 50 to 95 mass%, more preferably 55 to 90 mass%, and further preferably 60 to 85 mass%. If the moisture content of the film-forming stock solution is too low, there is a tendency that the viscosity of the film-forming stock solution becomes too high, making it difficult to filter and defoam when preparing the film-forming stock solution, and it is difficult to manufacture a PVA film with few foreign substances and defects. On the other hand, if the moisture content of the film-forming stock solution is too high, there is a tendency that the concentration of the film-forming stock solution becomes too low and it is difficult to industrially manufacture a PVA film.

[0115] Here, the moisture content of the film-forming stock solution in this specification is the value obtained by the following formula.

[0116] Moisture content of the film-forming stock solution (mass%) = {(a - b) / a} × 100

[0117] In the above formula, a represents the mass (mass parts) of the film-forming stock solution, and b represents the mass (mass parts) of the a mass parts of the film-forming stock solution after drying in an electric drying oven at 105 °C for 16 hours.

[0118] The casting dope preferably contains a surfactant. The specific and suitable ways of containing the surfactant in the casting dope are the same as those of the surfactant contained in the above PVA film.

[0119] The casting dope preferably contains a plasticizer. The specific and suitable ways of containing the plasticizer in the casting dope are the same as those of the plasticizer contained in the above PVA film.

[0120] In the method for manufacturing the above PVA film, a film-forming apparatus having drying rollers and heat treatment rollers with parallel rotating shafts can be used. The aforementioned film-forming apparatus has known ejection devices (casting devices) such as a T-die, a hopper plate, an I-die, and a lip coater die. The drying roller can be one or more. When multiple drying rollers are used, all the rollers must satisfy the temperature conditions described below.

[0121] In the aforementioned film-forming process, a film is obtained by casting a casting dope containing PVA on a drying roller having a surface temperature lower than 96.0 °C and drying it. It can be considered that by making the surface temperature lower than 96.0 °C, the generation of bubbles in the film and the excessive formation of crystal nuclei can be suppressed. From the viewpoint of making the distribution of crystal sizes in the PVA film narrower, the surface temperature of the drying roller is preferably 80 °C or higher, more preferably 82 °C or higher, further preferably 88 °C or higher, and particularly preferably 90 °C or higher.

[0122] In the aforementioned film-forming process, the average drying rate of the cast casting dope must be 1.10 to 1.20% by mass / second. By drying the casting dope relatively quickly in this way, the distribution of crystal sizes in the PVA film can be narrowed. The mechanism at this time is not yet clear, but it can be considered that by drying the casting dope relatively quickly, a relatively small number of crystal nuclei are formed, and they grow evenly. The aforementioned average drying rate is preferably 1.13% by mass / second or higher, more preferably 1.15% by mass / second or higher. The aforementioned average drying rate refers to the value obtained by dividing the difference in the moisture content (mass%) between the casting dope and the PVA film after the film-forming process and before the heat treatment process by the contact time (seconds) between the PVA film and the drying roller (when multiple drying rollers are used, it is the sum of the contact times with each drying roller). Specifically, it is obtained by the method described in the examples.

[0123] In the above-mentioned film-forming process, after drying the PVA film until the moisture content is less than 15.0% by mass, the film is subjected to a heat treatment process. It is considered that by making the moisture content of the PVA film subjected to the heat treatment process after the above-mentioned film-forming process less than 15.0% by mass, the rapid growth of crystals and the enlargement of the crystal size distribution can be suppressed. The moisture content is preferably 13.0% by mass or less, more preferably 11.0% by mass or less, and further preferably 10.0% by mass or less. On the other hand, from the viewpoint of growing crystals to a specified size to impart hot water resistance, the moisture content of the PVA film subjected to the above-mentioned heat treatment process is preferably 5.0% by mass or more, more preferably 7.0% by mass or more, and further preferably 8.0% by mass or more. The moisture content (%) of the PVA film is calculated using the following formula based on the mass c (g) of the film and the mass d (g) after drying the film in a dryer at 105 °C for 16 hours. Specifically, it is calculated using the method described in the examples below.

[0124] Moisture content (%) = {(c - d) / c} × 100

[0125] In the above-mentioned heat treatment process, the above-mentioned PVA film with a moisture content less than 15.0% by mass is heat-treated using a heat treatment roller. The heat treatment roller can be one or more. When using multiple heat treatment rollers, all the rollers must satisfy the temperature conditions described below.

[0126] The surface temperature of the above-mentioned heat treatment roller must be 96.0 °C or higher. By setting the surface temperature to 96.0 °C or higher, crystal growth can be sufficiently promoted. The surface temperature is preferably 98.0 °C or higher, more preferably 99.0 °C or higher. On the other hand, if the heat treatment temperature is too high, the crystals may grow excessively. Therefore, the surface temperature of the above-mentioned heat treatment roller is preferably 110.0 °C or lower, more preferably 108.0 °C or lower, and further preferably 103.0 °C or lower.

[0127] In the above-mentioned heat treatment process, the difference in moisture content of the above-mentioned film before and after heat treatment is preferably 2.00 to 5.00% by mass. By making the difference in moisture content 2.00% by mass or more, crystal growth can be moderately promoted. The difference in moisture content is more preferably 2.30% by mass or more, further preferably 2.50% by mass or more, still further preferably 2.60% by mass or more, and particularly preferably 2.70% by mass or more. On the other hand, it is considered that by making the difference in moisture content of the above-mentioned film before and after heat treatment 5.00% by mass or less, excessive crystal growth can be suppressed. The difference in moisture content is more preferably 4.50% by mass or less, further preferably 4.00% by mass or less, still further preferably 3.50% by mass or less, and particularly preferably 3.00% by mass or less.

[0128] <Method for manufacturing a polarizing film>

[0129] The PVA film according to the present invention can reduce the breakage of the film ends in the first half of the stretching process (swelling, dyeing, crosslinking) and the breakage of the film in the second half of the stretching process (uniaxial stretching), and can obtain a polarizing film with good efficiency. Therefore, it is preferably used as the raw material film for manufacturing a polarizing film. The method for manufacturing a polarizing film by using the PVA film of the present invention as the raw material film is not particularly limited, and any method that has been conventionally used can be adopted. As such a method, for example, a method of subjecting the PVA film to swelling, dyeing, crosslinking, uniaxial stretching, and further performing a fixing treatment, drying, heat treatment, etc. as needed can be cited. In this case, the order of the treatments such as swelling, dyeing, uniaxial stretching, and fixing treatment is not particularly limited, and one or more of the treatments can also be performed simultaneously. In addition, one or more of the treatments can also be performed two or more times.

[0130] Swelling can be carried out by immersing the PVA film in water. As the temperature of the water when immersed in water, it is preferably 20 to 40 °C, more preferably 22 to 38 °C, and further preferably 25 to 35 °C. In addition, as the time of immersion in water, for example, it is preferably 0.1 to 5 minutes, more preferably 0.5 to 3 minutes. It should be noted that the water when immersed in water is not limited to pure water, and can be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

[0131] Dyeing can be carried out using iodine. As the dyeing stage, it can be any stage before uniaxial stretching, during uniaxial stretching, or after uniaxial stretching. Dyeing is usually carried out by immersing the PVA film in a solution (especially an aqueous solution) containing iodine-potassium iodide as a dyeing bath. This dyeing method is also suitable in the present invention. The iodine concentration in the dyeing bath is preferably 0.01 to 0.5% by mass, and the potassium iodide concentration is preferably 0.01 to 10% by mass. In addition, the temperature of the dyeing bath is preferably 20 to 50 °C, more preferably 25 to 40 °C.

[0132] By subjecting the PVA film to a crosslinking treatment, when wet stretching is carried out at a high temperature, the dissolution of PVA into water can be effectively prevented. From this viewpoint, the crosslinking treatment is preferably carried out before the uniaxial stretching treatment. The crosslinking treatment can be carried out by immersing the PVA film in an aqueous solution containing a crosslinking agent. As the above-mentioned crosslinking agent, one or more of boron inorganic compounds such as borates such as boric acid and borax can be used. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass or more, more preferably 2% by mass or more, and further preferably 3% by mass or more. On the other hand, the aforementioned concentration is preferably 15% by mass or less, more preferably 7% by mass or less, and further preferably 6% by mass or less. By making the concentration within the above range, the stretchability of the PVA film can be sufficiently maintained. The aqueous solution containing the crosslinking agent may contain an auxiliary agent such as potassium iodide. The temperature of the aqueous solution containing the crosslinking agent is preferably 20 °C or higher, more preferably 25 °C or higher. On the other hand, this temperature is preferably 50 °C or lower, more preferably 40 °C or lower. By setting the temperature within the above range, the PVA film can be crosslinked with good efficiency.

[0133] The uniaxial stretching can be carried out by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it can be carried out in an aqueous solution containing boric acid, or in the above-mentioned dyeing bath or the subsequent fixing treatment bath described below. In addition, in the case of the dry stretching method, the water-absorbed PVA film can be used and carried out in the air. Among these, the wet stretching method is preferred, and it is more preferably carried out in an aqueous solution containing boric acid for uniaxial stretching. The concentration of boric acid in the boric acid aqueous solution is preferably 0.5 to 6.0% by mass, more preferably 1.0 to 5.0% by mass, and particularly preferably 1.5 to 4.0% by mass. In addition, the boric acid aqueous solution may contain potassium iodide, and its concentration is preferably 0.01 to 10% by mass. The stretching temperature in the uniaxial stretching is preferably 30 to 90 °C, more preferably 40 to 80 °C, and particularly preferably 50 to 70 °C. In addition, from the viewpoint of the polarization performance of the obtained polarizing film, the stretching ratio in the uniaxial stretching is preferably 5 times or more, more preferably 5.5 times or more, and particularly preferably 6 times or more. The upper limit of the stretching ratio is not particularly limited, and the stretching ratio is preferably 8 times or less.

[0134] When manufacturing a polarizing film, in order to firmly adsorb a dye (such as iodine) to the PVA film, a fixing treatment is preferably carried out. As the fixing treatment bath used in the fixing treatment, an aqueous solution containing one or more of boron compounds such as boric acid and borax can be used. In addition, an iodine compound or a metal compound can be added to the fixing treatment bath as needed. The concentration of the boron compound in the fixing treatment bath is preferably 1 to 15% by mass, more preferably 1.5 to 10% by mass. The temperature of the fixing treatment bath is preferably 15 to 60 °C, more preferably 25 to 40 °C.

[0135] Drying is preferably carried out at 30 to 150 °C, more preferably at 50 to 130 °C. If a tension is applied to the polarizing film at the time when the moisture content of the polarizing film becomes 10% or less by drying and heat treatment is carried out at about 80 to 120 °C for about 1 to 5 minutes, a polarizing film having more excellent dimensional stability, durability, etc. can be obtained.

[0136] The polarizing film obtained by the above operation is usually used as a polarizing plate by pasting a protective film on both sides or one side thereof. As the protective film, a protective film that is optically transparent and has mechanical strength can be cited. Specifically, for example, a triacetyl cellulose (TAC) film, a cellulose acetate-butyrate (CAB) film, an acrylic film, a polyester film, etc. are used. In addition, as the adhesive for pasting, a PVA adhesive, a urethane adhesive, etc. can be cited, and among them, a PVA adhesive is suitable.

[0137] The polarizing plate obtained by the above operation can be pasted on a glass substrate after coating an adhesive such as an acrylic resin and used as a component of an LCD. At the same time, a retardation film, a viewing angle improvement film, a brightness enhancement film, etc. can also be pasted.

[0138] The thickness of the polarizing film is not particularly limited. For example, it can be set to 30 μm or less, and further can be set to 25 μm or less. From the viewpoints of reducing the shrinkage stress of the polarizing film and the polarizing plate obtained by using it and preventing warping of the laminated thin glass, the thickness of the polarizing film is preferably 20 μm or less, more preferably 15 μm or less. On the other hand, an overly thin polarizing film tends to be difficult to manufacture and process. Therefore, the thickness of the polarizing film is preferably 3 μm or more.

[0139] Examples

[0140] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples at all. Evaluation, etc. are carried out according to the methods shown below.

[0141] [SAXS measurement]

[0142] SAXS measurement is carried out by the above method, and the average values X and Y of the lamellar thickness of PVA obtained by immersing the PVA film in pure water at 25 °C and performing small-angle X-ray measurement are obtained and substituted into the aforementioned X and Y, and thereby the ratio (X - Y) / Y × 100 is calculated.

[0143] [Swelling degree (A) of PVA film]

[0144] Rectangular samples with a width of 50 mm and a length of 100 mm were cut from the PVA films obtained in the following Examples and Comparative Examples in the width direction to obtain short strip-shaped samples. The samples were immersed in 1000 g of pure water at 45 °C. After 30 minutes of immersion, the moisture on the surface of the samples was wiped off with filter paper, and the mass Wa (g) of the samples was measured. Then, the aforementioned samples were dried in a dryer at 105 °C for 16 hours, and the mass Wb (g) was measured. The swelling degree (A) was calculated using the following formula. It should be noted that the same measurement was performed 3 times, and the average value was used.

[0145] Swelling degree (A) (mass %) = 100 × Wa / Wb

[0146] [Swelling degree (B) of PVA film]

[0147] The temperature of pure water was set to 30 °C, and except for this, the same operations as in the above "Swelling degree (A) of PVA film" were performed to obtain the swelling degree (B) of the PVA film.

[0148] [Moisture content Ma of PVA film before heat treatment]

[0149] In the following Examples and Comparative Examples, short strip-shaped samples were obtained by cutting the PVA film after film formation and before heat treatment in the width direction of 50 mm and the length direction of 100 mm. The mass Wc (g) of the sample was measured. After drying the sample in a dryer at 105 °C for 16 hours, its mass Wd (g) was measured. Based on the obtained masses Wc and Wd, the moisture content Ma (%) of the PVA film before heat treatment was calculated using the following formula.

[0150] Ma (%) = {(Wc - Wd) / Wc} × 100

[0151] [Average drying rate of PVA film in film formation process]

[0152] Using the moisture content (mass %) of the film-forming stock solution, the contact time (seconds) with the drying roller (the sum of the contact times with each drying roller), and Ma (%) obtained in the above "Moisture content Ma of PVA film before heat treatment" in the following Examples and Comparative Examples, the average drying rate (mass % / second) of the PVA film in the film formation process was calculated using the following formula.

[0153] Average drying rate (mass % / second) = {(Moisture content of film-forming stock solution - Ma) / (Contact time)}

[0154] [Moisture content Mb of PVA film after heat treatment, Moisture content difference (Ma - Mb)]

[0155] In the following Examples and Comparative Examples, a heat-treated PVA film was used. Except for this, the same operations as those for the "moisture content Ma of the PVA film before heat treatment" were performed to determine the moisture content Mb of the heat-treated PVA film. In addition, the difference (Ma - Mb) (mass %) between the moisture content Ma (mass %) of the PVA film before heat treatment and the aforementioned moisture content Mb (mass %) was determined.

[0156] [Tensile stress of PVA film (30 °C)]

[0157] A short strip sample with a width of 30 mm and a length of 100 mm was cut out from the central part in the width direction of the PVA film obtained in the following Examples and Comparative Examples. In order to perform uniaxial stretching along the length direction, it was clamped in a tensile fixture with a chuck spacing of 30 mm and immersed in pure water at 30 °C for 1 minute. Thereafter, while being immersed in pure water at 30 °C, uniaxial stretching was performed along the length direction at a stretching speed of 72 mm / min (240% / min), and the stress with respect to the stretching ratio was continuously measured. At this time, the stress when the strain (stretching ratio) of the PVA film was 400% was defined as the tensile stress. It should be noted that in the measurement of stress, Autograph ("AG-I" manufactured by Shimadzu Corporation) was used, and in addition, the stress was obtained by dividing the measured tension by the cross-sectional area (thickness × width (30 mm)) of the sample before stretching. The same measurement was performed 3 times, and the average value was defined as the tensile stress of the PVA film (30 °C).

[0158] [Tensile stress of PVA film (50 °C)]

[0159] The temperature of pure water was set to 50 °C. Except for this, the same operations as those for the "tensile stress of PVA film (30 °C)" were performed to determine the tensile stress of the PVA film (50 °C).

[0160] [Strain curing point of PVA film (50 °C)]

[0161] In the above "tensile stress of PVA film (50 °C)", the value (ΔS) obtained by dividing the change amount of the tensile stress in 0.05 seconds by the change amount of the strain (stretching ratio) in 0.05 seconds when plotting the tensile stress against the strain. The strain at which the minimum value of ΔS appears in the range of strain from 100% to 400% was defined as the strain curing point.

[0162] [Evaluation of breakage at the end after water bath immersion treatment]

[0163] A short strip sample with a width of 50 mm and a length of 100 mm was cut from the central part in the width direction of the PVA film obtained from the following Examples and Comparative Examples. Next, in order to perform uniaxial stretching along the length direction, the sample was clamped in a stretching jig so that the chuck pitch was 50 mm and immersed in a water bath at 30 °C containing distilled water. After immersion in the water bath, the sample was immediately uniaxially stretched along the length direction at a stretching speed of 50 mm / min (100% / min) for 1 minute. After such uniaxial stretching, the sample was immediately lifted from the water bath at a speed of 50 mm / s, and it was visually evaluated whether breakage occurred at the ends at this time. The sample with end breakage was evaluated as "B", and the sample without end breakage was evaluated as "A".

[0164] [Example 1]

[0165] A film-forming stock solution was prepared containing 100 parts by mass of PVA (saponified product of a homopolymer of vinyl acetate, degree of polymerization 2,400, saponification degree 99.95 mol%), 10 parts by mass of glycerol as a plasticizer, 0.1 part by mass of sodium lauryl polyoxyethylene ether sulfate as a surfactant, and 223 parts by mass of water with a water content of 67% by mass. The film-forming stock solution was cast on a first drying roll with a surface temperature of 92 °C, and drying and heat treatment were carried out in such a way that the surface of the PVA film in contact with the first drying roll and the surface not in contact with the first drying roll alternately faced each drying roll or heat treatment roll. Specifically, the PVA film peeled from the first drying roll was dried by the second drying roll and the third drying roll until the water content of the PVA film became 9.40% by mass (that is, the water content Ma of the PVA film before heat treatment was 9.40% by mass), and then it was brought into contact with the first heat treatment roll and the second heat treatment roll, thereby performing heat treatment to obtain a PVA film with a thickness of 45 μm. At this time, the surface temperatures of the second drying roll and the third drying roll were 92 °C, the total contact time of the PVA film with the first drying roll to the third drying roll was 49.2 seconds, and the average drying speed of the PVA film until before heat treatment was 1.17% / second. In addition, the surface temperatures of the first heat treatment roll and the second heat treatment roll were 100 °C, and the water content Mb of the PVA film after heat treatment was 6.57% by mass. For the obtained PVA film, the swelling degree (A), swelling degree (B), strain curing point (50 °C) of the PVA film, tensile stress (30 °C), and tensile stress (50 °C) were measured by the above method. The ratio (A / B) of the swelling degree (A) to the swelling degree (B) was calculated. In addition, using this PVA film, the evaluation of end breakage after the water bath immersion treatment was carried out according to the above method. The results are shown in Tables 1 and 2.

[0166] [Examples 2, Comparative Examples 1 - 3]

[0167] As shown in Table 1, change the surface temperatures of the first drying roller to the third drying roller, the average drying rate, and the surface temperature of the heat treatment roller. Except for this, perform the same operations as in Example 1 to obtain a PVA film. For the obtained PVA film, measure the swelling degree (A), swelling degree (B), strain curing point (50 °C) of the PVA film, tensile stress (30 °C), and tensile stress (50 °C) using the above method. Calculate the ratio (A / B) of the swelling degree (A) to the swelling degree (B). In addition, use this PVA film to evaluate the end breakage after the water bath immersion treatment according to the above method. The results are shown in Table 2.

[0168] [Table 1]

[0169]

[0170]

[0171] As shown in Table 2, the tensile stresses of the PVA films of Examples 1 to 2 in pure water at 50 °C are sufficiently small. Therefore, it can be confirmed that the breakage of the PVA film in the latter half of the stretching process when manufacturing the polarizing film is reduced. In addition, the PVA films of Examples 1 to 2 did not experience end breakage after being immersed in a water bath at 30 °C. Therefore, it can be confirmed that the end breakage of the PVA film in the first half of the stretching process when manufacturing the polarizing film is reduced.

[0172] On the other hand, as shown in Table 2, the tensile stresses of the PVA films of Comparative Examples 1 and 3 in pure water at 50 °C are large. Therefore, it can be confirmed that there is a possibility of breakage of the PVA film in the latter half of the stretching process when manufacturing the polarizing film. In addition, the tensile stress of the PVA film of Comparative Example 2 in pure water at 50 °C is sufficiently small. Therefore, it can be confirmed that although the breakage of the PVA film in the latter half of the stretching process when manufacturing the polarizing film is reduced, since end breakage occurred after immersion treatment in a water bath at 30 °C, there is a possibility of end breakage of the PVA film in the first half of the stretching process when manufacturing the polarizing film.

Claims

1. A polyvinyl alcohol film, which is a polyvinyl alcohol film containing polyvinyl alcohol, The average value X (nm) of the lamellar thickness of the polyvinyl alcohol obtained by immersing the polyvinyl alcohol film in pure water at 25 °C and performing small-angle X-ray measurement and the average value Y (nm) of the lamellar thickness of the polyvinyl alcohol obtained by performing small-angle X-ray measurement on the polyvinyl alcohol film in air at a temperature of 25 °C satisfy the following formulas (1) and (2). (X - Y) / Y × 100 ≤ 28 (1) X ≤ 4.00 (2).

2. The polyvinyl alcohol film according to claim 1, wherein, The swelling degree A (% by mass) measured by immersing in pure water at 45°C and the swelling degree B (% by mass) measured by immersing in pure water at 30°C satisfy the following formulas (3) and (4). A≥270 (3) 1.30 ≤ A / B ≤ 1.35 (4).

3. The polyvinyl alcohol film according to claim 1 or 2, having a thickness of 20 to 80 μm.

4. A method for manufacturing a polyvinyl alcohol film, comprising: A film-forming step of obtaining a film by drying a film-forming stock solution containing polyvinyl alcohol using a drying roll; and A heat treatment step of heat-treating the film using a heat treatment roll, The surface temperature of the drying roll in the film-forming step is lower than 96.0 °C, and the average drying speed is 1.10 to 1.20 mass% / second, The moisture content of the film supplied to the heat treatment step is less than 15.0 mass%, The surface temperature of the heat treatment roll in the heat treatment step is 96.0 °C or higher, and the difference in the moisture content of the film before and after the heat treatment is 2.00 to 5.00 mass%.

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