Polyvinyl alcohol film and method for producing optical film using the same

Abstract

The present invention provides a PVA film capable of producing an optical film having excellent optical properties and low shrinkage stress at high temperatures, and a method for producing an optical film using such a PVA film. For the polyvinyl alcohol film of the present invention, the long crystallization period Ds determined by small-angle X-ray scattering in a water / methanol mixed solvent (volume ratio: 2 / 8) and the long crystallization period Da determined by small-angle X-ray scattering before immersion in the mixed solvent satisfy the following formula: 0.3 ≤ (Ds-Da) / Da < 0.5.

Description

Technical Field

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

[0002] Polarizing plates, which transmit and block light, are essential components of liquid crystal displays (LCDs), along with liquid crystals that change the polarization state of light. Polarizing plates are typically manufactured by dyeing and stretching a polyvinyl alcohol film (hereinafter sometimes referred to as "PVA") (uniaxially stretching), and optionally fixing it with a boron compound or other agent to create a polarizing film. A protective film, such as a triacetyl cellulose (TAC) film, is then attached to the surface of the polarizing film.

[0003] LCDs are widely used in small devices such as calculators and watches, laptop computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, car navigation systems, mobile phones, and measuring equipment used indoors and outdoors. In response to the recent trend toward higher performance of LCDs, higher performance is also required for polarizing plates, a component of LCDs. Specifically, there is a demand for polarizing plates with superior optical properties and excellent dimensional stability at high temperatures. Therefore, the polarizing film used in the polarizing plate is also required to have superior optical properties (polarization properties) and low shrinkage stress at high temperatures.

[0004] However, it's not easy to improve the optical performance (polarization performance) of polarizing films while reducing shrinkage stress at high temperatures. This is because, generally speaking, the polarization performance and shrinkage stress of polarizing films exhibit a trade-off relationship. Specifically, improving the polarization performance of a polarizing film increases its shrinkage stress, while reducing its shrinkage stress decreases its polarization performance.

[0005] Patent Document 1 describes a method for producing a polarizing film having excellent polarization performance and low shrinkage stress by using a PVA film containing PVA having an average degree of polymerization of 2500 to 3500 and employing a predetermined cross-linking and stretching process. However, even when producing a polarizing film using the method described in Patent Document 1, it is sometimes difficult to achieve a good balance between polarization performance and shrinkage stress in the resulting polarizing film, depending on the film-forming conditions of the PVA film. Furthermore, there is room for improvement in terms of the crystal structure of the PVA film.

[0006] On the other hand, Patent Document 2 describes that by using a PVA film whose long period and amorphous portion thickness, as determined by small-angle X-ray scattering, are within a specific range, a polarizing film having excellent in-plane uniformity of polarization performance (polarization degree, single transmittance) and capable of handling large-area and high-precision applications can be obtained. Furthermore, Patent Document 3 describes that by using a PVA film whose long period of crystallization in water is within a specific range, a polarizing film having high absorbance and high polarization degree in the long wavelength region can be manufactured. However, Patent Documents 2 and 3 do not take into account the shrinkage stress of the polarizing film, and of course, do not consider balancing the polarization performance and shrinkage stress of the polarizing film.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-142347

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-188655

[0011] Patent Document 3: WO2013 / 146147 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] Therefore, an object of the present invention is to provide a PVA film capable of producing an optical film having excellent optical performance and low shrinkage stress at high temperatures, and a method for producing an optical film using such a PVA film.

[0014] Means used to solve problems

[0015] The present inventors focused on the crystal structure of PVA films before and after swelling, particularly the changes in the long crystal period. As a result of intensive research, they discovered that by setting the ratio of the change in the long crystal period before and after immersion in a water / methanol mixed solvent to the long crystal period before immersion within a specific range, an optical film with excellent optical properties and low shrinkage stress at high temperatures can be obtained. Based on this finding, further research was conducted, leading to the completion of the present invention.

[0016] That is, the present invention relates to the following [1] to [7].

[0017] [1] A PVA film, wherein the long crystal period Ds obtained by small-angle X-ray scattering measurement in a water / methanol mixed solvent (volume ratio: 2 / 8) and the long crystal period Da obtained by small-angle X-ray scattering measurement before immersion in the aforementioned mixed solvent satisfy the following formula.

[0018] 0.3≤(Ds-Da) / Da<0.5

[0019] [2] The PVA film according to [1], wherein the aforementioned crystalline long period Da is 10.0 to 12.5 nm.

[0020] [3] The PVA film according to [1] or [2], wherein the PVA contained in the aforementioned PVA film contains an ethylene unit, and the content thereof is 1 to 8 mol%.

[0021] [4] The PVA film according to any one of [1] to [3], and the average thickness of the film is 15 to 60 μm.

[0022] [5] The PVA film according to any one of [1] to [4], and it is a raw material film for manufacturing an optical film.

[0023] [6] A method for manufacturing an optical film, wherein the PVA film according to any one of [1] to [5] is uniaxially stretched.

[0024] [7] The method for manufacturing an optical film according to [6], which includes a swelling step of swelling the PVA film.

[0025] Advantages of the Invention

[0026] Through the present invention, a PVA film capable of manufacturing an optical film with excellent optical properties and small shrinkage stress at high temperatures, a method for manufacturing an optical film using such a PVA film, and an optical film can be provided. Description of the Drawings

[0027] Figure 1 It is a reference diagram when setting the scattering vector q (nm

[0032] ,

[0031] ,

[0030] ,

[0029] , Figure 2 , ,

[0027] , , , , , -1 , , ,

[0028] , Figure 1 ) in the scattering curve obtained by small-angle X-ray scattering measurement.

[0028] Figure 2 It is a diagram obtained by plotting the degree of polarization at a monomer transmittance of 44.0% against the shrinkage stress for the polarizing films obtained in Examples 1 to 3 and Comparative Examples 1 to 3. Detailed Description of the Invention

[0029] Hereinafter, the present invention will be specifically described.

[0030] <PVA Film>

[0031] (Small-Angle X-Ray Scattering Measurement)

[0032] For the PVA film of the present invention, the crystalline long period Ds obtained by small-angle X-ray scattering measurement in a water / methanol mixed solvent (volume ratio: 2 / 8) and the crystalline long period Da obtained by small-angle X-ray scattering measurement before immersion in the aforementioned mixed solvent satisfy the formula 0.3 ≤ (Ds - Da) / Da < 0.5.

[0033] In small-angle X-ray scattering (SAXS), when a sample is irradiated with X-rays, the diffraction generated by the scattering or interference of the X-rays by electrons surrounding the atoms is analyzed. In particular, by analyzing diffraction occurring at low angles (2θ < 10°), the structure of the sample being measured can be evaluated. Typically, structures ranging from a few nanometers to tens of nanometers can be evaluated. For example, the long crystal period (the average distance between randomly occurring crystals in a polymer) of the sample being measured can be evaluated.

[0034] Generally speaking, when X-rays are incident on a regularly arranged material, they scatter. The scattered X-rays interfere with each other, intensifying in specific directions. According to the Bragg theorem, where d is the lattice spacing, θ is the Bragg angle, and λ is the wavelength of the X-rays, diffracted X-rays resulting from this scattering are observed only in directions that satisfy the Bragg theorem: 2d·sinθ=nλ.

[0035] There are crystalline parts (lamellae) and amorphous parts in the PVA film. The crystalline part is a state in which the molecular chains of PVA are folded, and the amorphous part is a state in which the molecular chains of PVA are loose without being folded. Moreover, in the present invention, the average value of the distance between the crystals of the crystalline parts (lamellae) is set as the long crystal period. The long crystal period can be obtained based on the diffraction peak of the scattering curve obtained by performing small-angle X-ray scattering measurement on the PVA film. And it is generally known that: when the PVA film is a raw material film for optical film manufacturing, the diffraction peak derived from the long crystal period appears at the scattering vector q (nm -1 ) is 0.5nm -1 nearby.

[0036] (Calculation of the long crystallization period)

[0037] In the present invention, a PVA film, which is the object of small-angle X-ray scattering measurement, is manufactured as a measurement sample as follows. First, the PVA film to be measured is cut into multiple pieces of 2 cm × 1 cm in a manner that does not distinguish between the width direction (TD direction) and the mechanical flow direction (MD direction). After the cut PVA film is stored at a temperature of 20°C and a humidity of 65% for 24 hours, 10 pieces are stacked in a measurement dish to serve as a measurement sample. Here, the long crystallization period obtained based on the scattering curve obtained when the measurement sample is subjected to small-angle X-ray scattering measurement in air (temperature of 20°C and humidity of 65%) is the long crystallization period Da described later. In addition, the PVA film obtained by similarly cutting multiple pieces of 2 cm × 1 cm in size is immersed in a water / methanol mixed solvent (volume ratio: 2 / 8) for 24 hours, and then 10 pieces are stacked in a measurement dish filled with the mixed solvent to serve as a measurement sample. Here, the long crystallization period obtained based on the scattering curve obtained when the measurement sample is subjected to small-angle X-ray scattering measurement is the long crystallization period Ds described later.

[0038] In preparing the measurement sample, a 7.5 μm-thick KAPTON film was used as the window material on both the incident and reflected light sides of the measurement dish, with the gap between the windows set to approximately 1.5 mm. This structure allows the measurement sample to be hermetically sealed within the measurement dish. Furthermore, using this measurement dish allows the PVA film to be placed in the mixed solvent using the typical measurement setup in the measurement apparatus described below.

[0039] In the present invention, small-angle X-ray scattering measurement was performed using a nanometer-scale X-ray structure evaluation device "NanoViewer" (manufactured by Rigaku Corporation). The measurement conditions are as follows.

[0040] Transmittance measurement

[0041] X-rays: CuKα rays

[0042] Wavelength: 0.15418nm

[0043] Output power: 40kV-20mA

[0044] First slit: φ0.4mm

[0045] Second slit: φ0.2mm

[0046] The third slit: φ0.45mm

[0047] Detector: Semiconductor two-dimensional detector PILATUS-100K (measurement area = 33.5 × 83.8 mm)

[0048] Pixel size: 0.172mm square

[0049] Camera length: 1004.51mm

[0050] Beam stopper diameter: 4mm

[0051] X-ray exposure time: 1 hour

[0052] Measurement mode: Normal measurement

[0053] Ambient temperature: room temperature (20°C)

[0054] In the small-angle X-ray scattering measurement of PVA film, the scattering from the PVA film overlaps with the scattering from the slit and other devices, the air in the portion where the X-rays pass, and the solvent inside the dish. Therefore, it is necessary to correct these scatterings as background. Therefore, correction is performed by subtracting the scattering intensity of the above background calculated separately from the scattering intensity obtained by measuring the measurement sample. Furthermore, based on the scattering intensity image of the small-angle X-ray scattering measured by the two-dimensional detector, the scattering vector q (nm) is calculated. -1 ) is integrated along the azimuth direction to derive the scattering vector q(nm -1 ) and the one-dimensional curve of the scattering intensity I(q) to obtain the scattering curve.

[0055] As described above, in the small-angle X-ray scattering measurement of the PVA film, the diffraction peak originating from the long-period crystal of the PVA film appears in the scattering curve at the scattering vector q (nm -1 ) is 0.5nm -1 In the present invention, the scattering vector q (nm -1 ) value to calculate the crystal long period Ds and crystal long period Da. Here, the peak is at the scattering vector q (nm -1 ) is within the range of 0.2 or more and 1.0 or less, and the scattering curve becomes an inflection point that is convex upward (refer to Figure 1 ).

[0056] According to the scattering vector q(nm -1 ) values, the formulas for calculating the long crystal period Ds and the long crystal period Da are shown below.

[0057] Crystallization long period (nm) = 2π / q

[0058] In the present invention, it is important that the long crystal period Ds obtained by small-angle X-ray scattering measurement in a water / methanol mixed solvent (volume ratio: 2 / 8) and the long crystal period Da obtained by small-angle X-ray scattering measurement before immersion in the aforementioned mixed solvent satisfy the formula 0.3≤(Ds-Da) / Da<0.5.

[0059] Here, (Ds-Da) / Da refers to the rate of increase of the long crystal period before and after immersion in a water / methanol mixed solvent (volume ratio: 2 / 8). By performing small-angle X-ray scattering measurement in a water / methanol mixed solvent (volume ratio: 2 / 8), it is possible to evaluate the initial state of crystal dissolution when the PVA film is immersed in water. (Ds-Da) / Da (hereinafter sometimes referred to as the "rate of increase of the long crystal period") is preferably less than 0.5, more preferably less than 0.4. (Ds-Da) / Da is preferably greater than 0.3, more preferably greater than 0.32. When the rate of increase of the long crystal period is too large, in the swelling process when manufacturing the optical film, the distance between the lamellae in the PVA film is easily expanded due to the swelling of the PVA film in a solvent such as water. That is, it can be considered that in the amorphous part between the lamellae in the PVA film, the interaction between the molecular chains of PVA is easily reduced. As a result, the tensile stress applied to the PVA film during the stretching process in optical film manufacturing fails to fully contribute to the orientation of the PVA molecular chains, potentially leading to reduced optical performance of the resulting optical film. On the other hand, it is conceivable that if the rate of increase in the long crystallization period is too low, the amorphous portion of the PVA film will have difficulty swelling in solvents such as water during the swelling process in optical film manufacturing. In other words, during the stretching process in optical film manufacturing, the lamellae in the PVA film are difficult to dissolve, and tensile stress tends to concentrate on the crystalline portions. Consequently, the PVA molecular chains in the amorphous portion of the PVA film are not fully oriented, and the resulting optical film may not achieve both optical performance and shrinkage stress.

[0060] In the present invention, the long crystal period Da of the PVA film obtained by small-angle X-ray scattering measurement before immersion in a water / methanol mixed solvent (volume ratio: 2 / 8) is preferably 10.0 nm or more. In addition, the long crystal period Da is preferably 12.5 nm or less. Here, the long crystal period Da is obtained by small-angle X-ray scattering measurement before immersion in a water / methanol mixed solvent (volume ratio: 2 / 8), that is, in air (temperature 20°C, humidity 65%). The lower limit of the long crystal period Da is more preferably 11.0 nm. The upper limit of the long crystal period Da is more preferably 12.3 nm. If the long crystal period Da is less than 10.0 nm, it is presumed that the lamellae in the PVA film have a small thickness. If the thickness of the lamellae is small, the microcrystals of the lamellae in the PVA film are easily dissolved when the PVA film is immersed in a solvent such as water during the swelling process when manufacturing the optical film. As a result, there is a risk that the PVA film becomes soft and wrinkles are easily generated in the PVA film during the swelling process. On the other hand, if the long crystal period Da is greater than 12.5 nm, it is speculated that the PVA film has a crystal structure with a large lamellae thickness or a long distance between lamellae and a large number of amorphous parts. In the former case, the lamellae in the PVA film will not be fully dissolved during the swelling process when manufacturing the optical film, and the tensile tension of the PVA film will easily become high during the stretching process when manufacturing the optical film. As a result, there is a risk that the shrinkage stress of the resulting optical film will become high. In the latter case, during the swelling process when manufacturing the optical film, when the PVA film is immersed in a solvent such as water, the amorphous parts in the PVA film will easily trap water. As a result, there is a risk that the PVA film becomes soft, the tensile tension of the PVA film will become low during the stretching process when manufacturing the optical film, and the optical performance of the resulting optical film will be insufficient.

[0061] In the present invention, the long crystal period Ds determined by small-angle X-ray scattering in a water / methanol mixed solvent (volume ratio: 2 / 8) is preferably 12.0 nm or more. In addition, the long crystal period Ds is preferably 18.0 nm or less. The lower limit of the long crystal period Ds is more preferably 13.0 nm. The upper limit of the long crystal period Ds is more preferably 17.0 nm. If the long crystal period Ds exceeds 18.0 nm, there is a concern that in the swelling process when manufacturing the optical film, when the PVA film is immersed in a solvent such as water, the molecular chain of the PVA is easily extended, and the heat and moisture resistance of the resulting optical film deteriorates. On the other hand, if the long crystal period Ds is less than 12.0 nm, in the swelling process when manufacturing the optical film, when the PVA film is immersed in a solvent such as water, water is not easy to enter the amorphous part in the PVA film. As a result, there is a concern that in the stretching process when manufacturing the optical film, the tensile tension of the PVA film becomes high, and the shrinkage stress of the resulting optical film becomes high.

[0062] In the present invention, the methods for adjusting the long crystal period Ds and the long crystal period Da and controlling the increase rate of the long crystal period to the above-mentioned range include: (1) a method for adjusting the crystallization state of PVA by controlling the spraying, drying and heating conditions when manufacturing the film; (2) a method for adjusting the degree of interaction between the molecular chains of PVA according to the type of PVA (degree of saponification, amount of modification, etc.) and adjusting the expansion of the amorphous part in the PVA film; (3) a method for adjusting the size of the lamellae by adding a plasticizer, etc.; (4) a method for adjusting the cross-linking structure between the molecular chains of PVA by adding a cross-linking agent, etc. and adjusting the crystallization state or the expansion of the amorphous part of PVA; and methods for adjusting by a combination thereof.

[0063] When the increase rate of the long crystallization period is controlled to the above range by the method (1) above, the ejection conditions during film production are such that, for example, the volatility of the film-forming stock solution is preferably set to 10% by mass or more and 40% by mass or less. In addition, the shear rate at the outlet of the film-forming ejection device is preferably 75 s -1 More than 1000s -1 . In addition, regarding the drying conditions when manufacturing the film, for example, the surface temperature of the support on which the film-making stock solution is to be cast is preferably 60°C or higher and preferably 100°C or lower. The temperature of the hot air blown to the non-contact surface side of the PVA film on the support is preferably 50°C or higher and preferably 150°C or lower. The temperature of the drying furnace or the average temperature of the drying roller (the average value of the surface temperature of the drying roller) is preferably 40°C or higher and preferably 110°C or lower. Regarding the heating conditions when manufacturing the film, for example, the surface temperature of the heat treatment roller is preferably 135°C or lower.

[0064] When the increase rate of the long crystal period is controlled to be within the above range by the method (2) above, the saponification degree of PVA is preferably, for example, 85 mol% or more and 95 mol% or less. In addition, the modification amount of PVA (the proportion of modification with monomers other than polyvinyl alcohol units) is preferably, for example, 0.3 mol% or more and 8 mol% or less.

[0065] When the increase rate of the long crystal period is controlled to the above range by the method (3) above, the content of the plasticizer is preferably 2 parts by mass or more and 20 parts by mass or less relative to 100 parts by mass of PVA. In addition, ethylene glycol, glycerol, diethylene glycol, and diglycerol are preferably used as the plasticizer.

[0066] (PVA)

[0067] As the PVA contained in the PVA film of the present invention, a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer can be used. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among the above, vinyl acetate is preferred as the vinyl ester monomer.

[0068] The vinyl ester polymer is not particularly limited, but is preferably a polymer obtained using only one or more vinyl ester monomers as monomers, and more preferably a polymer obtained using only one vinyl ester monomer as a monomer. It should be noted that the vinyl ester polymer may be a copolymer of one or more vinyl ester monomers and one or more other monomers copolymerizable with the vinyl ester monomer.

[0069] As this other monomer, ethylene is preferably used. That is, the PVA contained in the PVA film of the present invention preferably contains ethylene units. In addition, the content of ethylene units is preferably 1 mol% or more based on the molar number of all structural units constituting the vinyl ester polymer. In addition, the content of ethylene units is preferably 8 mol% or less, more preferably 5 mol% or less. By making the content of ethylene units within the above range, it is possible to take into account both the optical properties and shrinkage stress of the resulting optical film. The reason for this is not necessarily clear, but it can be speculated that this is because: in the dyeing process when manufacturing the optical film, the dye in the dyeing process is easily adsorbed to the hydrophobic ethylene units in the PVA film, and the orientation of the dye adsorbed due to the tensile stress applied to the PVA film is improved.

[0070] Examples of the other monomers include, in addition to ethylene, olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutylene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid or its salts, acrylamidopropyldimethylamine or its salts, N-hydroxymethylacrylamide or acrylamide derivatives such as those derived from acrylamide; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid or its salts, methacrylamidopropyldimethylamine or its salts, N-hydroxymethylmethacrylamide or its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or its salts, esters, or anhydrides; itaconic acid or its salts, esters, or anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; isopropenyl acetate, etc. In addition, the vinyl ester polymer may have structural units derived from one or two or more of the above-mentioned other monomers.

[0071] The proportion of the structural units derived from monomers other than ethylene is not necessarily limited as long as it does not hinder the effects of the present invention. It may be preferably 15 mol% or less, more preferably 5 mol% or less, further preferably 1 mol% or less, and even more preferably 0.1 mol% or less, based on the number of moles of all structural units constituting the vinyl ester polymer.

[0072] The degree of polymerization of PVA is not particularly limited, but is preferably 1000 or higher. Furthermore, the degree of polymerization of PVA is preferably 8000 or lower. From the perspective of improving the optical properties and moist heat resistance of the resulting optical film, the degree of polymerization of PVA is more preferably 1500 or higher, and more preferably 2000 or higher. On the other hand, from the perspective of improving the productivity of PVA, the upper limit of the degree of polymerization is more preferably 5000 or lower, and even more preferably 4000 or lower.

[0073] Here, the degree of polymerization refers to the average degree of polymerization measured in accordance with JIS K 6726-1994. Specifically, in the present invention, the degree of polymerization (Po) is determined by re-saponifying and refining the residual acetic acid groups in PVA and then calculating the intrinsic viscosity [η] (dL / g) measured in water at 30°C using the following formula.

[0074] Degree of polymerization Po = ([η] × 10 4 / 8.29) (1 / 0.62)

[0075] In the present invention, the lower limit of the saponification degree of PVA is preferably 98.7 mol%, more preferably 99.0 mol%, even more preferably 99.5 mol%, particularly preferably 99.8 mol%, and preferably 99.9 mol%. A saponification degree exceeding this lower limit provides an optical film having excellent optical properties and resistance to moist heat. While the upper limit of the saponification degree is not particularly limited, it is preferably 99.99 mol% or less from the perspective of PVA productivity.

[0076] Here, the saponification degree of PVA refers to the ratio (mol %) of the molar number of vinyl alcohol units relative to the total molar number of structural units (typically vinyl ester monomer units) and vinyl alcohol units that can be converted into vinyl alcohol units by saponification. The saponification degree of PVA can be measured according to the description of JIS K 6726-1994.

[0077] The PVA film of the present invention may contain one type of PVA alone, or may contain two or more types of PVAs having different polymerization degrees, saponification degrees, modification degrees, and the like.

[0078] The upper limit of the content ratio of PVA in the PVA film is not particularly limited. On the other hand, the lower limit of the content ratio of PVA is preferably 50% by mass, more preferably 80% by mass, and even more preferably 85% by mass.

[0079] (Plasticizer)

[0080] The PVA film of the present invention preferably contains a plasticizer. By including a plasticizer in the PVA film, the stretchability of the PVA film can be improved during the stretching process during the manufacture of the optical film. The plasticizer is preferably a polyol. Examples of the polyol include ethylene glycol, glycerol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Of these, glycerol is preferred for its effect on improving stretchability. One plasticizer may be used alone or in combination of two or more.

[0081] Adjusting the plasticizer content in PVA film can adjust the amount and size of lamellae. While this depends on the primary structure of the PVA molecular chain, PVA films containing a small amount of plasticizer generally facilitate crystal growth upon heat treatment compared to PVA films without plasticizer. This is presumably because a small amount of plasticizer facilitates the mobility of PVA molecules within the film, resulting in an energetically more stable crystal structure. Furthermore, as crystal growth progresses, the lamellae tend to increase in size and the long crystallization period (Da). On the other hand, excessive amounts of plasticizer in the PVA film can hinder crystal growth. This is presumably because the amount of plasticizer that interacts with the hydroxyl groups of the PVA molecules increases, weakening the interactions between PVA molecules. Furthermore, the presence of plasticizer in the PVA film tends to increase water entrapment in the amorphous portion of the film, leading to a higher rate of increase in the long crystallization period (Da).

[0082] From the viewpoint of adjusting the lamella size in the PVA film to an appropriate range, adjusting the long crystal period Da and the rate of increase of the long crystal period, the content of the plasticizer is preferably 2 parts by mass or more relative to 100 parts by mass of PVA. In addition, the content of the plasticizer is preferably 20 parts by mass or less relative to 100 parts by mass of PVA. When the content of the plasticizer is less than 2 parts by mass or exceeds 20 parts by mass relative to 100 parts by mass of PVA, there is a tendency for the lamella size in the PVA film to become too small. As a result, there is a risk that the long crystal period Da becomes smaller and the rate of increase of the long crystal period of the PVA film is outside the specified range. The content of the plasticizer is more preferably 5 parts by mass or more relative to 100 parts by mass of PVA, and further preferably 8 parts by mass or more. In addition, the content of the plasticizer is more preferably 17 parts by mass or less relative to 100 parts by mass of PVA, and further preferably 15 parts by mass or less.

[0083] (Surfactant)

[0084] The PVA film of the present invention preferably contains a surfactant. By using a film-forming stock solution containing a surfactant to produce the PVA film, the film-forming properties of the PVA film are improved. As a result, uneven thickness of the PVA film is suppressed, and the PVA film can be easily peeled from the metal rolls and belts used for film production. When the PVA film is produced from a film-forming stock solution containing a surfactant, the resulting PVA film contains the surfactant.

[0085] The type of surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the viewpoint of peeling properties of the PVA film from a metal roll or a belt.

[0086] Examples of the anionic surfactant include carboxylic acid-type surfactants such as potassium laurate; sulfate-ester-type surfactants such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acid-type surfactants such as dodecylbenzenesulfonate.

[0087] Examples of the nonionic surfactant include alkyl ether types such as polyoxyethylene oleyl ether; alkyl phenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkylamine types such as polyoxyethylene lauryl amino ether; alkylamide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether types such as polyoxyalkylene allylphenyl ether.

[0088] The surfactant may be used alone or in combination of two or more.

[0089] When the PVA film includes a surfactant, the lower limit of the content of the surfactant in the PVA film is preferably 0.01 mass parts, more preferably 0.02 mass parts, and more preferably 0.05 mass parts relative to PVA 100 mass parts. By setting the content of the surfactant to more than the above lower limit, the film forming property and the peeling property of the PVA film are further improved. On the other hand, the upper limit of the content of the surfactant in the PVA film is preferably 0.5 mass parts, more preferably 0.3 mass parts, and more preferably 0.2 mass parts relative to PVA 100 mass parts. By setting the content of the surfactant to below the above upper limit, it is possible to suppress the surfactant from oozing out to the surface of the PVA film and causing adhesion, and it is possible to suppress the reduction in handleability.

[0090] (Other additives, etc.)

[0091] The PVA film of the present invention may further contain fillers, processing stabilizers such as copper compounds, weathering stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, defoamers, deodorants, extenders, peeling agents, release agents, reinforcing agents, cross-linking agents, mildew inhibitors, preservatives, crystallization rate retarders and other additives as needed.

[0092] The total proportion of PVA, plasticizer, and surfactant in the PVA film of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 99% by mass or more. The PVA film of the present invention is substantially composed of PVA, plasticizer, and surfactant, and thus, when a polarizing film is produced using the PVA film of the present invention, a polarizing film having excellent polarization performance can be obtained.

[0093] (Shape, physical properties, etc.)

[0094] The PVA film of the present invention is a so-called raw material film for producing an optical film, which is used as a material for an optical film. However, the PVA film of the present invention is not limited to being in a roll shape.

[0095] The average thickness of the PVA film of the present invention is not particularly limited. The lower limit of the average thickness of the PVA film is preferably 1 μm, more preferably 5 μm, further preferably 10 μm, and particularly preferably 15 μm. By making the average thickness above the above lower limit, the breakage of the PVA film can be suppressed in the stretching process when manufacturing the optical film. On the other hand, the upper limit of the average thickness is preferably 60 μm, more preferably 50 μm, further preferably 45 μm, and further preferably 35 μm. By making the average thickness below the above upper limit, the effect of the present invention can be fully exerted. It should be noted that "average thickness" refers to the average value of the thicknesses measured at any 5 locations (hereinafter, the same applies to the average thickness).

[0096] The PVA film of the present invention may be a single-layer film formed by one PVA layer, or a multilayer film including one PVA layer. Among them, when the PVA film of the present invention is used as a raw material film for the manufacture of a polarizing film, etc., it is preferably a single-layer film. When the PVA film of the present invention is included in the form of a PVA layer of a multilayer film, the lower limit of the average thickness of the PVA layer is preferably 1 μm, more preferably 5 μm, further preferably 10 μm, and particularly preferably 15 μm. By making the average thickness above the above lower limit, the breakage of the PVA film can be suppressed during the stretching process when manufacturing the optical film. In addition, the upper limit of the average thickness of the PVA layer is preferably 60 μm, more preferably 50 μm, further preferably 45 μm, and further preferably 35 μm. By making the average thickness below the above upper limit, the effect of the present invention can be fully exerted.

[0097] A multilayer film refers to a film having more than two layers. The number of layers of the multilayer film may be 5 or less layers, or 3 or less layers. Examples of the multilayer film include laminated films having a laminated structure of a substrate resin layer and a PVA layer. The average thickness of the substrate resin layer is preferably, for example, 20 μm or more. In addition, the average thickness of the substrate resin layer is preferably, for example, 500 μm or less. The substrate resin layer in the multilayer film is preferably capable of uniaxial stretching together with the PVA layer. As the resin constituting the substrate resin layer, polyester, polyolefin, etc. may be used. Among them, amorphous polyester is preferred, and polyethylene terephthalate and amorphous polyesters obtained by copolymerizing polyethylene terephthalate with copolymerization components such as isophthalic acid and 1,4-cyclohexanedimethanol are suitable. An adhesive layer may be provided between the substrate resin layer and the PVA layer.

[0098] The width of the PVA film of the present invention is not particularly limited and can be determined according to its use and the like. For example, the lower limit of the width of the PVA film is preferably 3 m. By setting the lower limit of the width to 3 m, it is suitable for the use of liquid crystal televisions and liquid crystal monitors, which have been increasingly larger in size in recent years. On the other hand, the upper limit of the width of the PVA film is preferably 7 m. By setting the upper limit of the width to 7 m, when manufacturing optical films such as polarizing films using commercially available devices, the stretching process (uniaxial stretching process) can be effectively carried out.

[0099] From the viewpoints of the productivity and optical properties of the obtained optical film, etc., the swelling degree of the PVA film of the present invention is preferably 140% or more. In addition, the swelling degree of the PVA film of the present invention is preferably 400% or less. The lower limit of the swelling degree is more preferably 170%, further preferably 180%, and particularly preferably 190%. In addition, the upper limit of the swelling degree is more preferably 220%, further preferably 210%. The swelling degree of the PVA film can be adjusted to a smaller value by, for example, increasing the heat treatment temperature of the PVA film.

[0100] Here, the "swelling degree of the PVA film" refers to the value obtained by the following formula.

[0101] Swelling degree (%) = 100 × N / M

[0102] In the formula, N represents the mass (g) of the sample taken from the PVA film after being immersed in distilled water at 30°C for 30 minutes and then removing the surface water. M represents the mass (g) of the sample after drying in a dryer at 105°C for 16 hours.

[0103] The PVA film of the present invention is usually a substantially non-stretched film (non-stretched film, un-stretched film). In addition, the in-plane retardation of the PVA film is preferably 100 nm or less, more preferably 50 nm or less. Usually, by performing a stretching process (uniaxial stretching process or biaxial stretching process) on the PVA film of the present invention, etc., an optical film can be obtained.

[0104] Based on the PVA film of the present invention, an optical film with excellent optical properties and low shrinkage stress at high temperatures can be manufactured. Examples of the optical film that can be manufactured using the PVA film of the present invention include polarizing films, retardation films, viewing angle improvement films, brightness enhancement films, etc., and a polarizing film is preferred.

[0105] <Method for manufacturing PVA film>

[0106] In the present invention, the film-making method of the PVA film can be made by the following method: using a film-making stock solution obtained by adding a solvent, an additive, etc. to PVA and homogenizing it, a method of making a film by using a casting film method, a wet film-making method (spraying into a poor solvent), a dry-wet film-making method, a gel film-making method (a method of temporarily cooling and gelling the film-making stock solution, extracting and removing the solvent to obtain a PVA-based polymer film), or a combination thereof; using an extruder, etc. to obtain the above-mentioned film-making stock solution, and extruding it from a T-die, etc. to make a film, an arbitrary method such as a melt extrusion film-making method, an inflation molding method, etc. Among these, the casting film-making method and the melt extrusion film-making method can obtain a homogeneous film with good productivity, so they are preferred. Below, the casting film-making method or the melt extrusion film-making method of the PVA film is described.

[0107] When producing PVA film using cast film or melt extrusion methods, the film-forming stock solution is cast onto a support such as a metal roll or belt into a film-like shape. The film is then heated to remove the solvent, causing it to solidify and form a film. The solidified film is then peeled from the support and dried using a drying roll or oven, if necessary. It is then heat-treated and wound up to produce a long roll of PVA film.

[0108] The film-making stock solution (hereinafter sometimes referred to as "PVA film") cast on the support undergoes crystallization during the period of being heated and dried on the support and in the subsequent drying process. In particular, the film-making stock solution is heated in a state of high volatility (water content), so that the mobility of the molecular chains of PVA in the film-making stock solution (PVA film) becomes higher, and crystallization occurs. As a result, there is a tendency that the amount of lamellae in the PVA film increases and the long crystallization period Da decreases. Here, if the drying speed is too fast, there is a tendency that the crystallization of the PVA film will not be fully carried out, the amount of lamellae will decrease, and the long crystallization period Da will increase. On the other hand, if the drying speed becomes slower, there is a tendency that crystal growth of the PVA film occurs, the size of the lamellae increases, and the long crystallization period Da increases. In addition, if too much heat is applied, there is a tendency that the size of the lamellae in the PVA film increases, the long crystallization period Da becomes too large, or the rate of increase of the long crystallization period becomes too small.

[0109] The volatility of the film-forming stock solution (the proportion of volatile components such as solvent removed by volatilization or evaporation during film formation) is preferably 50% by mass or greater, more preferably 55% by mass or greater. Furthermore, the volatility of the film-forming stock solution is preferably 90% by mass or less, more preferably 80% by mass or less. If the volatility is less than 50% by mass, the viscosity of the film-forming stock solution may increase, making it difficult to form a PVA film. On the other hand, if the volatile component concentration exceeds 90% by mass, the viscosity decreases, and the thickness uniformity of the PVA film may be impaired.

[0110] Here, the "volatile fraction of the membrane-forming stock solution" in the present invention refers to the volatile fraction determined by the following formula.

[0111] Volatile fraction of film-forming stock solution (mass %) = {(Wa-Wb) / Wa} × 100

[0112] (Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of the film-forming stock solution obtained by drying Wa (g) in an electric drying machine at 105°C for 16 hours.)

[0113] The method for adjusting the film-forming stock solution is not particularly limited, and examples thereof include a method of dissolving PVA and additives such as plasticizers and surfactants in a dissolving tank, etc.; a method of melt-kneading the water-containing PVA using a single-screw extruder or a twin-screw extruder, and melt-kneading it together with the plasticizer, surfactant, etc.;

[0114] When a PVA film is produced by a cast film method or a melt extrusion film method, the film-forming stock solution is cast from a film-shaped ejecting device onto a support such as a metal roller or a metal belt into a film-like shape, and the solvent is removed by heating, thereby solidifying and forming a film.

[0115] The rate of increase of the long crystallization period of the PVA film of the present invention before and after being immersed in a water / methanol mixed solvent (volume ratio: 2 / 8) is within a specific range. It can be inferred that the rate of increase is affected by the degree of entanglement of the molecular chains of PVA in the amorphous part between the lamellae in the PVA film. Therefore, by adjusting the shear rate at the outlet of the film-forming liquid ejecting device subjected to a strong shear force, the rate of increase of the long crystallization period of the PVA film can be controlled. From this point of view, the shear rate at the outlet of the film-forming liquid ejecting device is preferably 75s -1 More than 100s, more preferably -1 More than 125s, more preferably -1 In addition, the shear rate at the outlet of the film-shaped ejection device is preferably 1000s -1 Less than, preferably 900s -1 Below, more preferably 800s -1 By making the shear rate below the upper limit, the rate of increase of the long crystal period can be prevented from becoming too small. On the other hand, by making the shear rate above the lower limit, the rate of increase of the long crystal period can be prevented from becoming too large.

[0116] In the present invention, in the case of a general T-die or I-die, the shear velocity at the outlet of the film-forming ejection device refers to the shear velocity at the wall surface of the film-forming liquid flow path at the die lip, and can be calculated using the following formula.

[0117] γ=6Q / Wh 2

[0118] Here, γ is the shear velocity at the wall (s -1 ), W refers to the width of the die lip (cm), h refers to the opening of the die lip (cm), and Q refers to the speed at which the film-forming solution is ejected from the die lip (cm 3 / s).

[0119] The surface temperature of the support for casting the film-making stock solution is preferably 50°C or higher. In addition, the surface temperature of the support for casting the film-making stock solution is preferably 110°C or lower. When the surface temperature is less than 50°C, there is a tendency that the time required for drying the film-making stock solution becomes longer and the productivity decreases. On the other hand, when the surface temperature exceeds 110°C, there is a tendency that abnormalities are easily generated on the film surface of the PVA film due to foaming, etc. In addition, by quickly drying the film-making stock solution, there is a tendency that the crystallization of the PVA film does not proceed sufficiently, and the amount of lamellae and the long crystallization period Da become larger. From the viewpoint of easily adjusting the increase rate of the long crystallization period of the PVA film, the surface temperature of the support is preferably 60°C or higher, more preferably 65°C or higher. In addition, the surface temperature of the support is preferably 100°C or lower, more preferably 95°C or lower.

[0120] While the PVA film is being heated on the support, hot air at a speed of 1 to 10 m / s can be evenly blown across the entire non-contact surface of the PVA film to adjust the drying rate. From the perspectives of drying efficiency and uniformity, the temperature of the hot air blown onto the non-contact surface is preferably 50°C or higher, more preferably 70°C or higher. Furthermore, from the perspectives of drying efficiency and uniformity, the temperature of the hot air blown onto the non-contact surface is preferably 150°C or lower, more preferably 120°C or lower.

[0121] The PVA film is preferably dried on the support to a volatile fraction of 5 to 50% by mass, then peeled off and further dried as needed. The drying method is not particularly limited, and methods of contact with a drying furnace or a drying roller can be mentioned. When drying using multiple drying rollers, it is preferred to make one side and the other side of the film contact the drying rollers alternately, because both sides are uniform. The number of drying rollers is preferably 3 or more, more preferably 4 or more, and even more preferably 5 or more. In addition, the number of drying rollers is even more preferably 30 or less. The upper limit of the temperature of the drying furnace or the average temperature of the drying roller (the average value of the surface temperature of the drying roller) is preferably 110°C, more preferably 100°C, more preferably 90°C, and even more preferably 85°C. If the temperature of the drying furnace or the average temperature of the drying roller is too high, there is a tendency for crystal growth of the PVA film to proceed and the size of the lamellae in the PVA film to increase. As a result, there is a risk that the long crystal period Da becomes larger and the rate of increase of the long crystal period becomes too small. On the other hand, the lower limit of the temperature of the drying furnace or the average temperature of the drying roller is preferably 40°C, more preferably 45°C, and even more preferably 50°C. If the temperature of the drying furnace or the average temperature of the drying rollers is too low, the crystal growth of the PVA film may be insufficient, and the crystallite size in the PVA film may be reduced. As a result, there is a risk that the long crystal period Da may become too small or the rate of increase of the long crystal period may become too large.

[0122] The dried PVA film can be further heat-treated as needed. Heat treatment can be used to adjust the strength, swelling, birefringence, and other properties of the PVA film. The surface temperature of the heat treatment roller used for heat treatment is preferably 60°C or higher. Furthermore, the surface temperature of the heat treatment roller is preferably 135°C or lower, more preferably 130°C. Excessively high surface temperatures of the heat treatment roller can result in excessive heat application, increasing the size of the PVA film's lamellae, increasing the long crystal period Da, or decreasing the rate of increase in the long crystal period.

[0123] The PVA film thus produced can be further subjected to humidity control treatment, cutting of both ends (sides) of the film, etc. as needed, and then wound into a roll on a cylindrical core and moisture-proof packaged to form a product.

[0124] The volatile fraction of the PVA film finally obtained by the above series of treatments is not necessarily limited, and the volatile fraction of the PVA film is preferably 1% by mass or more, more preferably 2% by mass or more. In addition, the volatile fraction of the PVA film is preferably 5% by mass or less, more preferably 4% by mass or less.

[0125] It should be noted that when the PVA film of the present invention is a multilayer film, the multilayer film can be produced by, for example, applying a film-forming stock solution onto a substrate resin film (substrate resin layer). In this case, in order to improve the adhesion between the PVA layer and the substrate resin layer, the surface of the substrate resin film may be modified or an adhesive may be applied to the surface of the substrate resin film.

[0126] <Method for Manufacturing Optical Film>

[0127] The method for producing an optical film of the present invention comprises a step of uniaxially stretching the PVA film. Hereinafter, as an example of the method for producing an optical film, a method for producing a polarizing film will be described in detail.

[0128] As a method for manufacturing a polarizing film, there can be cited a method having the following steps: a dyeing step of dyeing the PVA film separately, a stretching step of uniaxially stretching it, a swelling step of further swelling it as needed, a cross-linking step of cross-linking it, a fixing step of fixing it, a washing step of washing it, a drying step of drying it, a heat treatment step of heat treatment, etc. In this case, there is no particular limitation on the order of the steps, and the steps can be performed in the order of, for example, a swelling step, a dyeing step, a cross-linking step, a stretching step, a fixing step, etc. In addition, one or more steps can be performed simultaneously, and each step can be performed two or more times. In particular, the PVA film of the present invention can suppress the occurrence of swelling wrinkles in the swelling step, and is therefore useful as a film used in a method for manufacturing a polarizing film having a swelling step.

[0129] The swelling process can be carried out by immersing the PVA film in a swelling treatment bath containing water, etc. The temperature of the swelling treatment bath is preferably above 20°C, more preferably above 22°C, and further preferably above 25°C. In addition, the temperature of the swelling treatment bath is preferably below 55°C, more preferably below 50°C, and further preferably below 45°C. In addition, the immersion time in the swelling treatment bath is, for example, preferably above 0.1 minutes, more preferably above 0.5 minutes. The immersion time in the swelling treatment bath is, for example, preferably below 5 minutes, more preferably below 3 minutes. It should be noted that the water used in the swelling treatment bath 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.

[0130] The dyeing process can be carried out by contacting the PVA film with a solution containing a dichroic dye (dyeing treatment bath). As a dichroic dye, an iodine-based dye is generally used. As the timing of dyeing, it can be any stage before uniaxial stretching, during uniaxial stretching, and after uniaxial stretching. As a dyeing treatment bath, a solution containing iodine-potassium iodide is preferably used, and the solution is preferably an aqueous solution. The iodine concentration in the dyeing treatment bath is preferably 0.01% by mass or more. In addition, the iodine concentration is preferably 0.5% by mass or less. The concentration of potassium iodide is preferably 0.01% by mass or more. In addition, the concentration of potassium iodide is preferably 10% by mass or less. In addition, the temperature of the dyeing treatment bath is preferably 20°C or more, more preferably 25°C or more. The temperature of the dyeing treatment bath is preferably 50°C or less, more preferably 40°C or less. The dyeing time is preferably 0.2 minutes or more. In addition, the dyeing time is preferably 5 minutes or less.

[0131] By performing a crosslinking step to crosslink the PVA in the PVA film, dissolution of the PVA into water during wet stretching at high temperatures can be suppressed. From this perspective, the crosslinking step is preferably performed after the dyeing step and before the stretching step. The crosslinking step can be performed by immersing the PVA film in an aqueous solution containing a crosslinking agent (a crosslinking bath). As the crosslinking agent, one or more boron compounds such as boric acid and borate salts such as borax can be used. The concentration of the crosslinking agent in the crosslinking bath is preferably 1% by mass or greater, more preferably 1.5% by mass or greater, and even more preferably 2% by mass or greater. Furthermore, the concentration of the crosslinking agent is preferably 15% by mass or less, more preferably 7% by mass or less, and even more preferably 6% by mass or less. By maintaining the crosslinking agent concentration within the above range, the stretchability of the PVA film can be adequately maintained. The crosslinking bath may contain potassium iodide, etc. The temperature of the crosslinking bath is preferably 20°C or greater, more preferably 25°C or greater. Furthermore, the temperature of the crosslinking bath is preferably 60°C or less, more preferably 55°C or less. By maintaining the temperature within the above range, the PVA film can be crosslinked efficiently.

[0132] The stretching process of uniaxially stretching the PVA film 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 (stretching treatment bath), or in the above-mentioned dyeing treatment bath or the fixing treatment bath described later. In addition, in the case of the dry stretching method, stretching can be performed directly at room temperature (25°C), or it can be stretched while heating, or the PVA film after absorbing water can be used and performed in the air. Among these, from the aspect of being able to stretch with high uniformity along the width direction, the wet stretching method is preferred, and uniaxial stretching in a stretching treatment bath is more preferred. The boric acid concentration in the stretching treatment bath is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and further preferably 1.5% by mass or more. In addition, the boric acid concentration in the stretching treatment bath is preferably 6.0% by mass or less, more preferably 5.0% by mass or less, and further preferably 4.0% by mass or less. In addition, the stretching treatment bath may contain potassium iodide, and the concentration of potassium iodide is preferably 0.01% by mass or more. In addition, the concentration of potassium iodide is preferably 10% by mass or less. The stretching temperature in uniaxial stretching is preferably 30° C. or higher, more preferably 40° C. or higher, and further preferably 50° C. or higher. The stretching temperature in uniaxial stretching is preferably 90° C. or lower, more preferably 80° C. or lower, and further preferably 75° C. or lower.

[0133] From the perspective of polarization performance of the obtained polarizing film, the stretching ratio in uniaxial stretching (the total stretching ratio of stretching from the non-stretched PVA film) is preferably 5 times or more, more preferably 5.5 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] The maximum tensile stress of the PVA film during uniaxial stretching is preferably 50 N / mm 2 Below, more preferably 25N / mm 2 Below, more preferably 15N / mm 2 Below, particularly preferably 10 N / mm 2 Below. Here, the maximum tensile stress refers to the value obtained by dividing the tensile tension of the PVA film carried between the adjacent rollers in the stretching treatment bath by the cross-sectional area of ​​the PVA film. At this time, the tensile tension of the PVA film can be measured using a tension roller set between the adjacent rollers in the stretching treatment bath. When more than three rollers are used in the stretching treatment bath, the maximum value is used. In addition, the cross-sectional area of ​​the PVA film is obtained based on the unstretched PVA film provided before the polarizing film is produced. By reducing the maximum tensile stress, a polarizing film with small shrinkage stress can be obtained. In addition, the maximum tensile stress is usually 1N / mm 2 above.

[0135] The uniaxial stretching direction of a long PVA film is not particularly limited; both uniaxial stretching along the long strip and transverse uniaxial stretching can be employed. From the perspective of obtaining a polarizing film with excellent polarization performance, uniaxial stretching along the long strip is preferred. Uniaxial stretching along the long strip can be performed using a stretching apparatus equipped with multiple parallel rollers and varying the circumferential speeds of the rollers. Transverse uniaxial stretching, on the other hand, can be performed using a tenter-type stretching machine.

[0136] When manufacturing a polarizing film, in order to make the dichroic dye (iodine-based dye, etc.) firmly adsorbed on the PVA film, a fixing treatment process can be performed after the stretching process. As the fixing treatment bath used in the fixing treatment, an aqueous solution containing one or more boron compounds such as boric acid and borax can be used. In addition, iodine compounds and metal compounds can be added to the fixing treatment bath as needed. The concentration of the boron compound in the fixing treatment bath is preferably 2% by mass or more, more preferably 3% by mass or more. In addition, the concentration of the boron compound in the fixing treatment bath is preferably 15% by mass or less, more preferably 10% by mass or less. By setting the concentration of the boron compound to within the above range, the adsorption of the dichroic dye can be made stronger. The temperature of the fixing treatment bath is preferably 15°C or more, more preferably 25°C or more. In addition, the temperature of the fixing treatment bath is preferably 60°C or less, more preferably 40°C or less.

[0137] The cleaning process is generally carried out by immersing the film in distilled water, pure water, an aqueous solution, or the like. At this time, from the viewpoint of improving the polarization performance of the obtained polarizing film, it is preferred to use an aqueous solution (cleaning treatment bath) containing an iodide such as potassium iodide as an auxiliary agent. The concentration of the iodide is preferably 0.5% by mass or more. In addition, the concentration of the iodide is preferably set to 10% by mass or less. In addition, the temperature of the cleaning treatment bath is preferably 5°C or more, more preferably 10°C or more, and further preferably 15°C or more. In addition, the temperature of the cleaning treatment bath is preferably 50°C or less, more preferably 45°C or less, and further preferably 40°C or less. By setting the temperature of the cleaning treatment bath to the above range, the polarization performance of the obtained polarizing film can be further improved.

[0138] The conditions of the drying step are not particularly limited. However, the drying temperature of the PVA film is preferably 30°C or higher, more preferably 50°C or higher. Furthermore, the drying temperature of the PVA film is preferably 150°C or lower, more preferably 130°C or lower. Drying within the above-mentioned temperature range facilitates obtaining a polarizing film having excellent dimensional stability at high temperatures.

[0139] It should be noted that optical films other than phase difference films and polarizing films can also be produced by a method comprising uniaxially stretching the PVA film of the present invention. Specific production methods can employ conventionally known methods in addition to using the PVA film of the present invention.

[0140] When the optical film of the present invention is a polarizing film, the dichroic ratio (R) of the polarizing film is preferably 100 or greater. The dichroic ratio (R) is more preferably 150 or greater, and even more preferably 160 or greater. The dichroic ratio (R) is, for example, preferably 350 or less, and preferably 300 or less.

[0141] The calculation method of the dichroic ratio (R) of the polarizing film is as follows. First, the relationship between the transmittance (T') after excluding surface reflection and the single transmittance (T) is expressed by the following formula (a). At this time, the refractive index of the polarizing film is set to 1.5, and the reflectivity of the surface is set to 4%. On the other hand, the relationship between the transmittance (T'), the polarization degree (V) and the dichroic ratio (R) is expressed by the following formula (b). Therefore, by measuring the single transmittance (T) and the polarization degree (V), using their values ​​to solve the following formulas (a) and (b), the dichroic ratio (R) of the polarizing film can be calculated.

[0142] T'=T / (1-0.04) 2 ···(a)

[0143] R={-ln[T'(1-V)]} / {-ln[T'(1+V)]}···(b)

[0144] Based on the dichroic ratio (R) of the polarizing film, the polarization degree (V) at a predetermined single transmittance (T) can be calculated. First, the transmittance (T') at a predetermined single transmittance (T) is obtained from the above formula (a). And, by solving the formula obtained by substituting the transmittance (T') and the dichroic ratio (R) into the following formula (c) obtained by modifying the above formula (b), the polarization degree (V) at a predetermined single transmittance (T) can be obtained. In the present invention, this method is used to calculate the polarization degree (V) when the single transmittance (T) of the polarizing film is 44.0%.

[0145] T'=[1-V] 1 / (R-1) / [1+V] R / (R-1) ...(c)

[0146] The polarizing film obtained by the above operation is usually made into a polarizing plate for use by pasting an optically transparent and mechanically strong protective film on both sides or one side thereof. As the protective film, triacetate cellulose (TAC) film, cycloolefin polymer (COP) film, cellulose acetate-butyrate (CAB) film, acrylic film, polyester film, etc. can be used. In addition, as the adhesive for pasting, PVA-based adhesives, urethane-based adhesives, acrylate-based ultraviolet curing adhesives, etc. can be listed. That is, the polarizing plate has a polarizing film and a protective film directly laminated on one side or both sides of the polarizing film or laminated with the help of an adhesive layer.

[0147] Polarizing plates can be used as components of LCDs by, for example, applying an acrylic adhesive and then attaching them to a glass substrate. A retardation film, a viewing angle improvement film, a brightness enhancement film, etc. can also be attached to the polarizing plate.

[0148] Example

[0149] The present invention will be described in detail below by way of examples, etc. However, the present invention is not limited in any way by the following examples. It should be noted that the evaluation items and methods used in the following examples and comparative examples are as follows.

[0150] (1) Crystallization long periods Ds and Da of PVA film

[0151] The PVA films obtained in the following examples and comparative examples were subjected to small-angle X-ray measurement before and after being immersed in a water / methanol mixed solvent (volume ratio: 2 / 8). Specifically, the PVA film was first cut into multiple pieces of 2 cm × 1 cm in a manner that did not distinguish between the width direction (TD direction) and the mechanical flow direction (MD direction). After the cut PVA film was stored at a temperature of 20°C and a humidity of 65% for 24 hours, 10 pieces were stacked in a measuring dish to prepare a measurement sample. The measurement sample was subjected to small-angle X-ray scattering measurement in air (temperature 20°C, humidity 65%) to obtain a scattering curve. According to the scattering vector q (nm -1 ) value, the crystal long period Da is calculated as follows.

[0152] Crystallization long period Da (nm) = 2π / q

[0153] In addition, the PVA films obtained in the following examples and comparative examples were cut into multiple pieces of 2 cm × 1 cm in a manner that did not distinguish between the width direction (TD direction) and the machine flow direction (MD direction). After the cut PVA films were immersed in a water / methanol mixed solvent (volume ratio: 2 / 8) for 24 hours, 10 pieces were stacked in a measuring dish filled with the mixed solvent to serve as a measurement sample. The measurement sample was subjected to small-angle X-ray scattering measurement to obtain a scattering curve. According to the scattering vector q (nm) at the peak of the scattering curve, the scattering vector q (nm) was obtained.-1 ) value, the crystal long period Ds is calculated as follows.

[0154] Crystallization long period Ds (nm) = 2π / q

[0155] (2) Swelling degree of PVA film

[0156] The swelling degrees of the PVA films obtained in the following Examples and Comparative Examples were determined by the above-described method.

[0157] (3) Production of polarizing film

[0158] Polarizing films were prepared using the PVA films obtained in the following examples and comparative examples. First, the PVA film was stretched 2.0 times in the MD direction in pure water at 25°C (swelling treatment bath), and then stretched in a 32°C aqueous solution containing 0.03% by mass of iodine and 0.7% by mass of potassium iodide (dyeing treatment bath) so that the total stretching ratio became 2.4 times. Next, it was stretched in a 32°C aqueous solution containing 2.6% by mass of boric acid (cross-linking treatment bath) so that the total stretching ratio became 3.0 times, and then stretched in a 58°C aqueous solution containing 2.8% by mass of boric acid and 5.0% by mass of potassium iodide (stretching treatment bath) until the total stretching ratio reached 6.0 times. Next, it was immersed in a 22°C aqueous solution containing 1.5% by mass of boric acid and 2.5% by mass of potassium iodide (cleaning treatment bath) for 5 seconds, and dried in a drying oven at 80°C for 4 minutes.

[0159] (4) Maximum tensile stress of PVA film during polarizing film production

[0160] In the production of the polarizing film, the stretching tension of the PVA film held between adjacent rollers in a stretching treatment bath is measured using a tension roller placed therebetween, and the value obtained by dividing the measured value by the cross-sectional area of ​​the PVA film is defined as the maximum tensile stress. In this case, the cross-sectional area of ​​the PVA film is determined using an unstretched PVA film prior to polarizing film production.

[0161] (5) Optical properties of polarizing films

[0162] A rectangular measurement sample measuring 4 cm in the machine direction (MD) and 1.5 cm in the width direction (TD) of the resulting polarizing film was collected from the center of the film. This sample was measured using a spectrophotometer with an integrating sphere ("V7100" manufactured by JASCO Corporation) using a C illuminant and a 2° field of view in the visible light region according to JIS Z8722 (Measurement of Object Color). The single-element transmittance and polarization degree were then measured. The polarization degree was calculated using the aforementioned method, with a single-element transmittance of 44.0%.

[0163] (6) Shrinkage stress of polarizing film at high temperature

[0164] The shrinkage stress was measured using an Autograph AG-X with a constant temperature bath manufactured by Shimadzu Corporation. During the measurement, a polarizing film (15 cm in the length direction and 1.5 cm in the width direction) humidified at 20°C / 20% RH for 18 hours was mounted on a chuck (chuck spacing of 5 cm), and the constant temperature bath was heated to 80°C at the same time as the stretching began. The polarizing film was stretched at a speed of 1 mm / min, and the stretching was stopped when the tension reached 2 N. The tension was measured in this state until 4 hours later. At this time, the distance between the chucks changed due to the thermal expansion of the shaft, so the marking labels were attached to the chucks, and a video extensometer TR ViewX120S was used to measure the distance between the chucks in a manner that could correct the movement of the marking labels attached to the chucks. It should be noted that the value obtained by subtracting the initial tension of 2 N from the measured value of the tension after 4 hours was used as the shrinkage force of the changed film, and the value obtained by dividing the value by the cross-sectional area of ​​the sample was defined as the shrinkage stress (N / mm 2 ).

[0165] <Example 1>

[0166] A film-forming stock solution containing 100 parts by mass of PVA (99.9 mol% saponification degree, 2400 degree of polymerization) obtained by saponifying polyvinyl acetate, 14 parts by mass of glycerin as a plasticizer, 0.1 parts by mass of lauric acid diethanolamide as a surfactant, and 73% by mass of water was prepared. The filtrate of the film-forming stock solution was filtered through a T-die at 207 s. -1 The film is sprayed onto a support (surface temperature 90°C) at a shear rate of 100°C and dried by blowing hot air of 85°C at a rate of 5 m / s on the entire surface of the support that is not in contact with the support. Next, it is peeled off from the support, and one side and the other side of the PVA film are alternately contacted with each drying roller. After further drying between the first drying roller and the final drying roller (the 19th drying roller) located just in front of the heat treatment roller, it is finally peeled off from the drying roller. At this time, the average value of the surface temperature of each drying roller from the first drying roller to the final drying roller is made to be 70°C. Finally, after heat treatment using a heat treatment roller with a surface temperature of 104°C, it is wound into a roll to obtain a PVA film (thickness 45μm, width 3.3m).

[0167] Small-angle X-ray scattering (SAXS) analysis of the resulting PVA film revealed a long crystal period Da of 12.6 nm, a long crystal period Ds of 16.8 nm, and a long crystal period increase rate of 0.33. Furthermore, the degree of swelling of the PVA film was measured to be 197%.

[0168] Furthermore, the obtained PVA film was used to produce a polarizing film. At this time, the maximum tensile stress was 9.0 N / mm 2 The optical properties of the polarizing film were measured and the calculated results showed that the transmittance of the single film was 43.8%, the polarization degree was 99.935%, and the polarization degree was 99.873% when the transmittance of the single film was 44.0%. In addition, the shrinkage stress of the polarizing film was measured and the result was 43.7N / mm 2 .

[0169] <Example 2, Example 3 and Comparative Examples 1 to 4>

[0170] A PVA film was obtained in the same manner as in Example 1 except that the type of PVA, amount of plasticizer, and production conditions were changed as shown in Table 1. In Table 1, ethylene modification with an ethylene unit content of 3 mol% is abbreviated as "ΔEt3".

[0171] The evaluation results of the obtained PVA films and polarizing films are shown in Table 1. In addition, for the polarizing films obtained in Examples 1 to 3 and Comparative Examples 1 to 3, the polarization degree when the single transmittance is 44.0% is plotted against the shrinkage stress in a graph shown in Table 1. Figure 2 The PVA film of Comparative Example 4 dissolved in distilled water at 30°C, so the degree of swelling could not be measured. Furthermore, the PVA film of Comparative Example 4 dissolved in pure water at 25°C (swelling treatment bath), so a polarizing film could not be produced.

[0172] [Table 1]

[0173]

[0174] According to Table 1 and Figure 2 It was found that the polarizing film produced from the PVA film of the present invention has excellent optical properties and low shrinkage stress at high temperatures.

[0175] Description of Reference Numerals

[0176] 1 Peak of the scattering curve

[0177] 2 Auxiliary lines when setting the scattering vector q at the peak of the scattering curve

[0178] 3 Scattering vector q at the peak of the scattering curve

Claims

1. A polyvinyl alcohol film, wherein the long crystal period Ds determined by small-angle X-ray scattering measurement in a water / methanol mixed solvent having a volume ratio of 2 / 8 and the long crystal period Da determined by small-angle X-ray scattering measurement before immersion in the mixed solvent satisfy the following formula: 0.3≤(Ds-Da) / Da<0.5, The saponification degree of the polyvinyl alcohol is 98.7 mol% or more and 99.99 mol% or less, The polyvinyl alcohol film contains 2 parts by mass or more and 20 parts by mass or less of a plasticizer relative to 100 parts by mass of polyvinyl alcohol.

2. The polyvinyl alcohol film according to claim 1, wherein The aforementioned long crystal period Da is 10.0 to 12.5 nm.

3. The polyvinyl alcohol film according to claim 1 or 2, wherein The polyvinyl alcohol contained in the polyvinyl alcohol film contains ethylene units, and the content of the ethylene units is 1 to 8 mol%. The polyvinyl alcohol film according to claim 1 or 2, which has an average thickness of 15 to 60 μm. The polyvinyl alcohol film according to claim 1 or 2, which is a raw material film for producing an optical film.

6. A method for producing an optical film, wherein: The polyvinyl alcohol film according to any one of claims 1 to 5 is uniaxially stretched. 7 . The method for producing an optical film according to claim 6 , comprising a swelling step of swelling the polyvinyl alcohol film.

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