JP7601876B2 - Polyvinyl alcohol film and method for producing optical film using same - Google Patents

Description

The present invention relates to a polyvinyl alcohol film and a method for producing an optical film using the same.

Polarizing plates, which have the function of transmitting and blocking light, are a basic component of liquid crystal displays (LCDs) along with liquid crystals, which change the polarization state of light. Polarizing plates are generally manufactured by producing a polarizing film by subjecting a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as "PVA") to a dyeing process, a stretching process (uniaxial stretching process), and, if necessary, a fixing process using a boron compound, etc., and then laminating a protective film such as a triacetate cellulose (TAC) film onto the surface of the polarizing film.

LCDs are now used in a wide range of applications, including small devices such as calculators and watches, laptops, LCD monitors, LCD color projectors, LCD televisions, car navigation systems, mobile phones, and measuring instruments used both indoors and outdoors. In response to the recent increase in performance of LCDs, there is a demand for improved performance in the polarizing plates, a component of LCDs. Specifically, there is a demand for polarizing plates that have better optical performance and excellent dimensional stability even at high temperatures. Therefore, there is a demand for polarizing films used in polarizing plates that have better optical performance (polarizing performance) and low shrinkage stress at high temperatures.

However, it is not easy to reduce the shrinkage stress at high temperatures while improving the optical performance (polarization performance) of polarizing films. This is because there is generally a trade-off between the polarization performance and shrinkage stress of polarizing films. In other words, if you try to increase the polarization performance of a polarizing film, the shrinkage stress increases, and if you try to decrease the shrinkage stress, the polarization performance decreases.

Patent Document 1 describes that a polarizing film with excellent polarization performance and small shrinkage stress can be obtained by using a PVA film containing PVA with an average degree of polymerization of 2500 to 3500 and adopting a specified cross-linking and stretching process. However, even if a polarizing film is produced using the method described in Patent Document 1, it may be difficult to achieve both the polarization performance and shrinkage stress of the resulting polarizing film depending on the film-forming conditions of the PVA film, and there is room for improvement from the perspective of the crystal structure of the PVA film.

On the other hand, Patent Document 2 describes that by using a PVA film with a specific range of long period and amorphous thickness determined by small-angle X-ray scattering, a polarizing film with excellent in-plane uniformity of polarization performance (degree of polarization, single transmittance) and compatible with large area and high definition can be obtained. Patent Document 3 describes that by using a PVA film with a specific range of crystal long period in water, a polarizing film with high absorbance in the long wavelength range and high polarization degree can be produced. However, Patent Documents 2 and 3 do not take into consideration the shrinkage stress of the polarizing film, and naturally do not take into consideration the compatibility of the polarization performance and shrinkage stress of the polarizing film.

JP 2017-142347 A JP 2006-188655 A WO2013/146147 publication

Therefore, the present invention aims to provide a PVA film that can be used to produce 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.

The inventors conducted extensive research focusing on the change in the crystal structure, particularly the long crystal period, before and after swelling of the PVA film, and discovered that by setting the ratio of the amount of 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 performance and small shrinkage stress at high temperatures can be obtained. Based on this knowledge, further research led to the completion of the present invention.

That is, the present invention relates to the following [1] to [7].
[1] A PVA film, in which a crystalline long period Ds determined by small-angle X-ray scattering measurement performed in a water/methanol mixed solvent (volume ratio: 2/8) and a crystalline long period Da determined by small-angle X-ray scattering measurement performed before immersion in the mixed solvent satisfy the following formula:
0.3≦(Ds-Da)/Da<0.5
[2] The PVA film according to [1], wherein the crystal long period Da is 10.0 to 12.5 nm.
[3] The PVA film according to [1] or [2], wherein the PVA contained in the PVA film contains an ethylene unit and the content of the ethylene unit is 1 to 8 mol %.
[4] The PVA film according to any one of [1] to [3], wherein the average thickness of the film is 15 to 60 μm.
[5] The PVA film according to any one of [1] to [4], which is a raw film for producing an optical film.
[6] A method for producing an optical film, comprising uniaxially stretching the PVA film according to any one of [1] to [5].
[7] The method for producing the optical film according to [6], further comprising a swelling step of swelling a PVA film.

The present invention provides a PVA film that can be used to produce an optical film having excellent optical performance and low shrinkage stress at high temperatures, a method for producing an optical film using such a PVA film, and the optical film.

FIG. 2 is a reference diagram for setting the peak-top scattering vector q (nm ⁻¹ ) in a scattering curve obtained by small-angle X-ray scattering measurement. 1 is a graph in which the polarization degree when the single transmittance is 44.0% is plotted against the shrinkage stress for the polarizing films obtained in Examples 1 to 3 and Comparative Examples 1 to 3.

The present invention is described in detail below.

<PVA film>
(Small angle X-ray scattering measurement)
In the PVA film of the present invention, the long crystalline period Ds determined by small-angle X-ray scattering measurement performed in a water/methanol mixed solvent (volume ratio: 2/8) and the long crystalline period Da determined by small-angle X-ray scattering measurement performed before immersion in the mixed solvent satisfy the formula: 0.3≦(Ds−Da)/Da<0.5.

In small-angle X-ray scattering measurements, when a sample is irradiated with X-rays, the diffraction that occurs when the X-rays are scattered or interfered with by electrons around the atoms is analyzed. In particular, by analyzing the diffraction that appears in the low-angle region of 2θ<10° or less, the structure of the sample to be measured can be evaluated, and it is generally believed that structures of a few nm to a few tens of nm in size can be evaluated. For example, the crystal long period of the sample to be measured (the average value of the distance between crystals randomly present in a polymer) can be evaluated.

Generally, when X-rays are incident on a material with a regular arrangement, the X-rays are scattered. The scattered X-rays interfere with each other and reinforce each other in specific directions. According to Bragg's equation, diffracted X-rays due to this scattering can only be observed in directions that satisfy the Bragg equation of 2d sinθ = nλ, where d is the lattice spacing, θ is the Bragg angle, and λ is the wavelength of the X-rays.

In a PVA film, there are crystalline parts (lamellar crystals) in which the molecular chains of PVA are folded, and amorphous parts in which the molecular chains of PVA are unfolded. In the present invention, the average intercrystalline distance between the crystalline parts (lamellar crystals) is taken as the crystal long period. This crystal long period can be determined from the diffraction peak of the scattering curve obtained by subjecting the PVA film to small-angle X-ray scattering measurement. When the PVA film is a raw film for producing an optical film, it is generally known that the diffraction peak resulting from this crystal long period appears when the scattering vector q (nm ^-1 ) is in the vicinity of 0.5 nm ^-1 .

(Calculation of Crystal Long Period)
In the present invention, the PVA film to be measured for small-angle X-ray scattering was prepared as follows to be used as a measurement sample. First, the PVA film to be measured was cut into multiple pieces of 2 cm x 1 cm in size, regardless of the width direction (TD direction) or the machine direction (MD direction). After storing the cut PVA film for 24 hours under conditions of a temperature of 20°C and a humidity of 65%, 10 sheets were stacked in a measurement cell to be used as a measurement sample. Here, the crystal long period Da, which is described later, is determined from the scattering curve obtained when the measurement sample was subjected to small-angle X-ray scattering measurement in air (temperature 20°C, humidity 65%). In addition, the PVA film cut into multiple pieces of 2 cm x 1 cm in size was immersed in a water/methanol mixed solvent (volume ratio: 2/8) for 24 hours, and then 10 sheets were stacked in a measurement cell filled with the mixed solvent to be used as a measurement sample. Here, the crystal long period obtained from a scattering curve obtained by performing small angle X-ray scattering measurement on this measurement sample is the crystal long period Ds described later.

In preparing the measurement sample, the measurement cell used 7.5 μm thick Kapton films as the window materials on the incident light side and the reflected light side, with the distance between the window materials being approximately 1.5 mm. This structure allows the measurement sample to be sealed in the measurement cell. In addition, by using this measurement cell, the PVA film to be measured can be placed in the mixed solvent in the normal measurement arrangement in the measurement device described below.

In the present invention, small-angle X-ray scattering measurements were performed using a nanoscale X-ray structure evaluation device "Nano Viewer" (manufactured by Rigaku Corporation). The measurement conditions are as follows:

Transmission measurement X-ray: CuKα ray Wavelength: 0.15418nm
Output: 40kV-20mA
First slit: φ0.4 mm
Second slit: φ0.2 mm
Third slit: φ0.45 mm
Detector: Semiconductor two-dimensional detector PILATUS-100K (measurement area = 33.5 x 83.8 mm)
Pixel size: 0.172mm square Camera length: 1004.51mm
Beam stopper diameter: 4 mm
X-ray exposure time: 1 hour Measurement mode: normal measurement Environmental temperature: room temperature (20°C)

In the small-angle X-ray scattering measurement of the PVA film, scattering from the PVA film is overlapped with scattering from the slits and other devices, the air through which the X-ray passes, and the solvent inside the cell, and therefore it is necessary to correct these scatterings as background. Therefore, the correction was performed by subtracting the above-mentioned background scattering intensity calculated separately from the scattering intensity obtained by measuring the measurement sample. Furthermore, from the scattering intensity image of the small-angle X-ray scattering measured by the two-dimensional detector, the scattering intensity with respect to the scattering vector q (nm ^-1 ) was integrated in the azimuth direction, and the relationship between the scattering vector q (nm ^-1 ) and the one-dimensional profile of the scattering intensity I (q) was derived to obtain a scattering curve.

As described above, in the small-angle X-ray scattering measurement of a PVA film, a diffraction peak derived from the crystalline long period of the PVA film appears in the scattering curve where the scattering vector q (nm ^-1 ) is about 0.5 nm ^-1 . In the present invention, the crystalline long period Ds and the crystalline long period Da were calculated from the value of the scattering vector q (nm ^-1 ) at the peak top of this diffraction peak. Here, this peak top is an inflection point where the scattering curve becomes upwardly convex when the scattering vector q (nm ^-1 ) is in the range of 0.2 to 1.0 (see FIG. 1).

From the value of the scattering vector q (nm ⁻¹ ) obtained above, the crystal long period Ds and the crystal long period Da are calculated using the following formulas.
Crystal long period (nm) = 2π/q

In the present invention, it is important that the crystalline long period Ds determined from small-angle X-ray scattering measurement performed in a water/methanol mixed solvent (volume ratio: 2/8) and the crystalline long period Da determined from small-angle X-ray scattering measurement performed before immersion in the mixed solvent satisfy the formula: 0.3≦(Ds−Da)/Da<0.5.

Here, (Ds-Da)/Da means the rate of increase in 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), the state of crystal dissolution at the beginning of swelling when the PVA film is immersed in water can be evaluated. (Ds-Da)/Da (hereinafter sometimes referred to as the "rate of increase in the long crystal period") is preferably less than 0.5, more preferably less than 0.4. (Ds-Da)/Da is preferably 0.3 or more, more preferably 0.32 or more. If the rate of increase in the long crystal period is too large, the PVA film will swell with a solvent such as water in the swelling process when manufacturing the optical film, and the distance between the lamellar crystals in the PVA film will tend to increase. In other words, it is considered that the interaction of the molecular chains of PVA is likely to become smaller in the amorphous parts present between the lamellar crystals in the PVA film. As a result, in the stretching process for producing an optical film, the stretching stress applied to the PVA film cannot sufficiently contribute to the orientation of the molecular chains of PVA, and the optical performance of the obtained optical film may be deteriorated. On the other hand, if the increase rate of the crystal long period is too small, it is considered that the amorphous part in the PVA film is difficult to swell with a solvent such as water in the swelling process for producing an optical film. That is, in the stretching process for producing an optical film, the lamellar crystals in the PVA film are difficult to dissolve, and the stretching stress is likely to concentrate on the crystalline part. As a result, the molecular chains of PVA are not sufficiently oriented in the amorphous part of the PVA film, and the optical performance and shrinkage stress of the obtained optical film may not be compatible.

In the present invention, the crystal long period Da of the PVA film, determined by small-angle X-ray scattering measurement performed before immersion in a water/methanol mixed solvent (volume ratio: 2/8), is preferably 10.0 nm or more. The crystal long period Da is preferably 12.5 nm or less. Here, the crystal long period Da is determined by performing 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 crystal long period Da is more preferably 11.0 nm. The upper limit of the crystal long period Da is more preferably 12.3 nm. If the crystal long period Da is less than 10.0 nm, it is estimated that the crystal structure has a small thickness of the lamellar crystals in the PVA film. If the thickness of the lamellar crystals is small, the microcrystals of the lamellar crystals in the PVA film are easily dissolved when the PVA film is immersed in a solvent such as water in the swelling process when manufacturing an optical film. As a result, the PVA film becomes soft, and wrinkles may easily occur in the swelling process. On the other hand, if the crystal long period Da is greater than 12.5 nm, it is estimated that the thickness of the lamellar crystals in the PVA film is large, or the distance between the lamellar crystals is long, resulting in a crystal structure with many amorphous parts. In the former case, the lamellar crystals in the PVA film are not sufficiently dissolved in the swelling process when producing the optical film, and the stretching tension of the PVA film is likely to be high in the stretching process when producing the optical film. As a result, the shrinkage stress of the obtained optical film may be high. In the latter case, when the PVA film is immersed in a solvent such as water in the swelling process when producing the optical film, the amorphous parts in the PVA film are likely to take in water. As a result, the PVA film becomes soft, and the stretching tension of the PVA film is low in the stretching process when producing the optical film, and the optical performance of the obtained optical film may be insufficient.

In the present invention, the crystal long period Ds obtained from the small-angle X-ray scattering measurement performed in a water/methanol mixed solvent (volume ratio: 2/8) is preferably 12.0 nm or more. The crystal long period Ds is preferably 18.0 nm or less. The lower limit of the crystal long period Ds is more preferably 13.0 nm. The upper limit of the crystal long period Ds is more preferably 17.0 nm. If the crystal long period Ds exceeds 18.0 nm, the molecular chain of PVA is likely to spread when the PVA film is immersed in a solvent such as water in the swelling process when producing an optical film, and the moist heat resistance of the obtained optical film may be deteriorated. On the other hand, if the crystal long period Ds is less than 12.0 nm, water is unlikely to enter the amorphous part of the PVA film when the PVA film is immersed in a solvent such as water in the swelling process when producing an optical film. As a result, the stretching tension of the PVA film is high in the stretching process when producing an optical film, and the shrinkage stress of the obtained optical film may be high.

In the present invention, methods for adjusting the crystal long period Ds and the crystal long period Da and controlling the rate of increase of the crystal long period within the above range include (1) a method for adjusting the crystalline state of PVA by controlling the extrusion, drying, and heating conditions when forming a film, (2) a method for adjusting the degree of interaction between PVA molecular chains depending on the type of PVA (degree of saponification, amount of modification, etc.) and adjusting the spread of the amorphous part in the PVA film, (3) a method for adjusting the size of lamellar crystals by adding a plasticizer, etc., (4) a method for adjusting the crosslinked structure between PVA molecular chains by adding a crosslinking agent, etc., and adjusting the crystalline state of PVA or the spread of the amorphous part, and a method of adjustment using a combination of these.

When the increase rate of the crystal long period is controlled to the above range by the above method (1), the discharge conditions when forming the film are, for example, preferably set to 10% by mass or more and 40% by mass or less of the volatile content of the film-forming solution. Also, the shear rate at the outlet of the film-like discharge device is preferably 75 s ^-1 or more and 1000 s ^-1 . Also, the drying conditions when forming the film are, for example, preferably set to 60° C. or more and 100° C. or less of the surface temperature of the support on which the film-forming solution flows. The temperature of the hot air blown on the non-contact side of the PVA film on the support is preferably 50° C. or more and 150° C. or less. The temperature of the drying oven or the average temperature of the drying roll (average value of the surface temperature of the drying roll) is preferably 40° C. or more and 110° C. or less. The heating conditions when forming the film are, for example, preferably set to 135° C. or less of the surface temperature of the heat treatment roll.

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

When the increase rate of the crystal long period is controlled to the above range by the above method (3), the content of the plasticizer is preferably 2 parts by mass or more and 20 parts by mass or less per 100 parts by mass of PVA. In addition, it is preferable to use ethylene glycol, glycerin, diethylene glycol, or diglycerin as the plasticizer.

(PVA)
The PVA contained in the PVA film of the present invention may be a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer. 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.

The vinyl ester polymer is not particularly limited, but is preferably a polymer obtained using only one or more vinyl ester monomers as a monomer, and more preferably a polymer obtained using only one vinyl ester monomer as a monomer. 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.

As the other monomer, ethylene is preferred. That is, the PVA contained in the PVA film of the present invention preferably contains an ethylene unit. The content of the ethylene unit is preferably 1 mol% or more based on the number of moles of all structural units constituting the vinyl ester polymer. The content of the ethylene unit is preferably 8 mol% or less, more preferably 5 mol% or less. By having the content of the ethylene unit in the above range, the optical performance and shrinkage stress of the obtained optical film can be compatible. The reason is not necessarily clear, but it is presumed that in the dyeing process when manufacturing the optical film, the dye during the dyeing process is easily adsorbed to the hydrophobic ethylene unit in the PVA film, and the orientation of the adsorbed dye is enhanced by the stretching stress applied to the PVA film.

Other monomers include, in addition to ethylene, olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, Methacrylic acid esters such as i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylic acids such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, N-methylolmethacrylamide or a derivative thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, 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 a salt, ester, or acid anhydride thereof; itaconic acid or a salt, ester, or acid anhydride thereof; vinyl silyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. The vinyl ester polymer may have structural units derived from one or more of the other monomers mentioned above.

The proportion of structural units derived from monomers other than ethylene is not necessarily limited as long as it does not interfere with the effects of the present invention, but it is preferably 15 mol % or less, more preferably 5 mol % or less, even more preferably 1 mol % or less, and in some cases even more preferably 0.1 mol % or less, based on the number of moles of all structural units constituting the vinyl ester polymer.

The degree of polymerization of PVA is not particularly limited, but is preferably 1,000 or more. In addition, the degree of polymerization of PVA is preferably 8,000 or less. From the viewpoint of improving the optical performance and moist heat resistance of the obtained optical film, the degree of polymerization of PVA is more preferably 1,500 or more, and more preferably 2,000 or more. On the other hand, the upper limit of the degree of polymerization is more preferably 5,000 or less, and even more preferably 4,000 or less, from the viewpoint of improving the productivity of PVA.

Here, the degree of polymerization means the average degree of polymerization measured in accordance with the description of JIS K 6726-1994. That is, in the present invention, the degree of polymerization (Po) is calculated from the intrinsic viscosity [η] (dl/g) measured in water at 30°C after resaponifying the residual acetate groups of PVA and purifying it, according to the following formula:
Degree of polymerization Po = ([η]×10 ⁴ /8.29) ^(1/0.62)

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%. By having a saponification degree equal to or greater than the above lower limit, an optical film having excellent optical performance and moist heat resistance can be obtained. On the other hand, there is no particular limit to the upper limit of the saponification degree, but from the viewpoint of PVA productivity, it is preferably 99.99 mol% or less.

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

The PVA film of the present invention may contain one type of PVA alone, or may contain two or more types of PVA differing from each other in degree of polymerization, degree of saponification, degree of modification, etc.

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

(Plasticizer)
The PVA film of the present invention preferably contains a plasticizer. By containing a plasticizer in the PVA film, the stretchability of the PVA film can be improved in the stretching step when producing an optical film. The plasticizer is preferably a polyhydric alcohol. Examples of the polyhydric alcohol include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Among these, glycerin is preferred from the viewpoint of the effect of improving the stretchability. The plasticizer can be used alone or in combination of two or more kinds.

By adjusting the content of the plasticizer in the PVA film, the amount or size of the lamellar crystals can be adjusted. Although it depends on the primary structure of the molecular chain of the PVA, in general, crystal growth is more likely to proceed by heat treatment in a PVA film containing a small amount of plasticizer than in a PVA film that does not contain a plasticizer. This is presumed to be because a small amount of plasticizer makes the PVA molecules in the PVA film more mobile and makes it easier to form a more energetically stable crystal structure. As the crystal growth of the PVA film progresses, the size of the lamellar crystals in the PVA film increases, and the crystal long period Da tends to increase. On the other hand, if the PVA film contains an excessive amount of plasticizer, crystal growth is more likely to be inhibited. This is presumed to be because the amount of plasticizer that interacts with the hydroxyl groups of the PVA molecules increases, weakening the interaction between the PVA molecules. In addition, if the PVA film contains a plasticizer, the amorphous part in the PV film is more likely to take up water, and the increase rate of the crystal long period tends to increase.

From the viewpoint of adjusting the size of the lamellar crystals in the PVA film to an appropriate range and adjusting the crystal long period Da and the increase rate of the crystal long period, the content of the plasticizer is preferably 2 parts by mass or more per 100 parts by mass of PVA. In addition, the content of the plasticizer is preferably 20 parts by mass or less per 100 parts by mass of PVA. In both cases where the content of the plasticizer is less than 2 parts by mass and more than 20 parts by mass per 100 parts by mass of PVA, the size of the lamellar crystals in the PVA film tends to become too small. As a result, the crystal long period Da becomes small, and there is a risk that the increase rate of the crystal long period of the PVA film will be outside the specified range. The content of the plasticizer is more preferably 5 parts by mass or more per 100 parts by mass of PVA, and even more preferably 8 parts by mass or more. In addition, the content of the plasticizer is more preferably 17 parts by mass or less per 100 parts by mass of PVA, and even more preferably 15 parts by mass or less.

(Surfactant)
The PVA film of the present invention preferably contains a surfactant. By producing a PVA film using a film-forming solution containing a surfactant, the film-forming properties of the PVA film are improved. As a result, the occurrence of thickness unevenness in the PVA film is suppressed, and the PVA film is easily peeled off from a metal roll or belt used in film formation. When a PVA film is produced from a film-forming solution containing a surfactant, the resulting PVA film contains the surfactant.

The type of surfactant is not particularly limited, but anionic surfactants and nonionic surfactants are preferred from the viewpoint of the peelability of the PVA film from the metal roll or belt.

Examples of anionic surfactants include carboxylic acid types such as potassium laurate; sulfate ester types such as polyoxyethylene lauryl ether sulfate and octyl sulfate; and sulfonic acid types such as dodecylbenzenesulfonate.

Examples of nonionic surfactants 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; alkyl amine types such as polyoxyethylene lauryl amino ether; alkyl amide 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 allyl phenyl ether.

Surfactants may be used alone or in combination of two or more.

When the PVA film contains a surfactant, the lower limit of the surfactant content in the PVA film is preferably 0.01 parts by mass, more preferably 0.02 parts by mass, and even more preferably 0.05 parts by mass, relative to 100 parts by mass of PVA. By setting the surfactant content to the above lower limit or more, the film-forming and peeling properties of the PVA film are further improved. On the other hand, the upper limit of the surfactant content in the PVA film is preferably 0.5 parts by mass, more preferably 0.3 parts by mass, and even more preferably 0.2 parts by mass, relative to 100 parts by mass of PVA. By setting the surfactant content to the above upper limit or less, it is possible to suppress the surfactant from bleeding out onto the surface of the PVA film, causing blocking and reducing handleability.

(Other additives, etc.)
The PVA film of the present invention may further contain additives, such as a filler, a processing stabilizer such as a copper compound, a weather resistance stabilizer, a colorant, an ultraviolet absorber, a light stabilizer, an antioxidant, an antistatic agent, a flame retardant, other thermoplastic resins, a lubricant, a fragrance, an antifoaming agent, a deodorant, an extender, a peeling agent, a releasing agent, a reinforcing agent, a crosslinking agent, a mildew inhibitor, a preservative, and a crystallization rate retarder, as necessary.

In the PVA film of the present invention, the total proportion of PVA, plasticizer, and surfactant is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and in some cases even more preferably 99% by mass or more. Since the PVA film of the present invention is substantially composed of PVA, plasticizer, and surfactant, when a polarizing film is produced using the PVA film of the present invention, a polarizing film with excellent polarization performance can be obtained.

(shape, physical properties, etc.)
The PVA film of the present invention is used as a material for optical films, that is, a raw film for producing optical films. However, the PVA film of the present invention is not limited to a rolled film.

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, even more preferably 10 μm, and particularly preferably 15 μm. When the average thickness is equal to or greater than the above lower limit, breakage of the PVA film can be suppressed during the stretching process in manufacturing the optical film. On the other hand, the upper limit of the average thickness is preferably 60 μm, more preferably 50 μm, even more preferably 45 μm, and even more preferably 35 μm. When the average thickness is equal to or less than the above upper limit, the effects of the present invention can be fully exhibited. The term "average thickness" refers to the average value of thicknesses measured at any five points (hereinafter the same applies to the average thickness).

The PVA film of the present invention may be a single layer film consisting of one PVA layer, or may be a multilayer film containing one PVA layer. However, when the PVA film of the present invention is used as a raw film for manufacturing a polarizing film, it is preferable that the film is a single layer film. When the PVA film of the present invention is included as 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, even more preferably 10 μm, and particularly preferably 15 μm. When the average thickness is equal to or greater than the above lower limit, breakage of the PVA film can be suppressed in the stretching process when manufacturing an optical film. In addition, the upper limit of the average thickness of the PVA layer is preferably 60 μm, more preferably 50 μm, even more preferably 45 μm, and even more preferably 35 μm. When the average thickness is equal to or less than the above upper limit, the effect of the present invention can be fully exhibited.

A multilayer film is a film having two or more layers. The number of layers of a multilayer film may be five or less, or three or less. Examples of multilayer films include laminated films having a laminated structure of a base resin layer and a PVA layer. The average thickness of the base resin layer is preferably, for example, 20 μm or more. The average thickness of the base resin layer is preferably, for example, 500 μm or less. The base resin layer in a multilayer film is preferably one that can be uniaxially stretched together with the PVA layer. Resins that constitute the base resin layer can be polyester, polyolefin, etc. Among them, amorphous polyesters are preferred, and amorphous polyesters obtained by copolymerizing polyethylene terephthalate with copolymerization components such as isophthalic acid and 1,4-cyclohexanedimethanol are preferably used. An adhesive layer may be provided between the base resin layer and the PVA layer.

The width of the PVA film of the present invention is not particularly limited and can be determined according to the application. For example, the lower limit of the width of the PVA film is preferably 3 m. A lower limit of 3 m makes it suitable for use in liquid crystal televisions and liquid crystal monitors, which have become 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 optical films such as polarizing films are manufactured using commercially available equipment, stretching (uniaxial stretching) can be performed efficiently.

From the viewpoint of the productivity and optical performance of the optical film obtained, the swelling degree of the PVA film of the present invention is preferably 140% or more. 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%, even more preferably 180%, and particularly preferably 190%. The upper limit of the swelling degree is more preferably 220%, and even more preferably 210%. The swelling degree of the PVA film can be adjusted to a smaller value, for example, by increasing the temperature of the heat treatment of the PVA film.

Here, the "swelling degree of the PVA film" refers to a value calculated by the following formula.
Swelling degree (%) = 100 x N/M
In the formula, N represents the mass (g) of a sample taken from a PVA film after immersing the sample in distilled water at 30° C. for 30 minutes and removing the surface water, and M represents the mass (g) of the sample after drying the sample in a dryer at 105° C. for 16 hours.

The PVA film of the present invention is usually a film that is not substantially stretched (non-stretched film, unstretched film). The in-plane retardation of the PVA film is preferably 100 nm or less, more preferably 50 nm or less. Usually, an optical film can be obtained by subjecting the PVA film of the present invention to a stretching process (uniaxial stretching process or biaxial stretching process).

The PVA film of the present invention can be used to produce an optical film that has excellent optical performance and small shrinkage stress at high temperatures. Examples of optical films that can be produced using the PVA film of the present invention include polarizing films, retardation films, viewing angle improving films, and brightness improving films, and polarizing films are preferred.

<Production method of PVA film>
In the present invention, the PVA film can be produced by any method, such as a casting film production method, a wet film production method (discharge into a poor solvent), a dry-wet film production method, a gel film production method (a method in which the film production stock solution is cooled and gelled, and then the solvent is extracted and removed to obtain a PVA-based polymer film), or a combination of these methods, or a melt extrusion film production method or inflation molding method in which the film production stock solution is obtained using an extruder or the like and extruded from a T-die or the like to produce a film. Among these, the casting film production method and the melt extrusion film production method are preferred because they can produce a homogeneous film with good productivity. Hereinafter, the casting film production method and the melt extrusion film production method of the PVA film will be described.

When a PVA film is produced by the casting method or the melt extrusion method, the film-forming solution is cast in the form of a film onto a support such as a metal roll or a metal belt, and is heated to remove the solvent, thereby solidifying the film. The solidified film is peeled off from the support, dried with a drying roll or a drying oven as necessary, and further heat-treated as necessary, and wound up to obtain a long roll of PVA film.

The film-forming solution (hereinafter sometimes referred to as "PVA film") dripped onto the support undergoes crystallization while being heated and dried on the support and in the subsequent drying process. In particular, by heating the film-forming solution in a state of high volatile content (moisture content), the mobility of the molecular chains of PVA in the film-forming solution (PVA film) increases, and crystallization proceeds. As a result, the amount of lamellar crystals in the PVA film tends to increase, and the crystal long period Da tends to decrease. Here, if the drying speed is too fast, the crystallization of the PVA film does not proceed sufficiently, the amount of lamellar crystals tends to decrease, and the crystal long period Da tends to increase. On the other hand, if the drying speed is too slow, the crystal growth of the PVA film proceeds, the size of the lamellar crystals tends to increase, and the crystal long period Da tends to increase. In addition, if too much heat is applied, the size of the lamellar crystals in the PVA film tends to increase, and the crystal long period Da tends to become too large or the increase rate of the crystal long period tends to become too small.

The volatile content of the film-forming solution (the proportion of volatile components such as solvents that are removed by volatilization or evaporation during film formation) is preferably 50% by mass or more, more preferably 55% by mass or more. The volatile content of the film-forming solution is preferably 90% by mass or less, more preferably 80% by mass or less. If the volatile content is less than 50% by mass, the viscosity of the film-forming solution increases, making it difficult to form a PVA film. On the other hand, if the volatile content concentration exceeds 90% by mass, the viscosity decreases and the uniformity of the thickness of the PVA film is easily impaired.

Here, the "volatile content of the membrane-forming solution" in the present invention refers to the volatile content calculated by the following formula.
Volatile content of film-forming solution (mass%)={(Wa−Wb)/Wa}×100
(In the formula, Wa represents the mass (g) of the film-forming solution, and Wb represents the mass (g) of the film-forming solution Wa (g) after drying it in an electric dryer at 105° C. for 16 hours.)

There are no particular limitations on the method for preparing the film-forming solution, and examples of the method include dissolving PVA and additives such as plasticizers and surfactants in a dissolution tank, or melt-kneading the water-containing PVA together with plasticizers, surfactants, etc. using a single-screw or twin-screw extruder.

When a PVA film is produced by the casting method or the melt extrusion method, the film-forming solution is cast in the form of a film from a film-forming extrusion device onto a support such as a metal roll or metal belt, and is heated to remove the solvent, causing the film to solidify.

The PVA film of the present invention has an increase rate of the crystal long period before and after immersion in a water/methanol mixed solvent (volume ratio: 2/8) within a specific range. It is presumed that this increase rate is affected by the degree of entanglement of the molecular chains of PVA in the amorphous portion present between the lamellar crystals in the PVA film. Therefore, the increase rate of the crystal long period of the PVA film can be controlled by adjusting the shear rate at the outlet of the film-like discharge device where the film-forming solution is subjected to a strong shear force. From this viewpoint, the shear rate at the outlet of the film-like discharge device is preferably 75 s ^-1 or more, more preferably 100 s ^-1 or more, and even more preferably 125 s ^-1 or more. In addition, the shear rate at the outlet of the film-like discharge device is preferably 1000 s ^-1 or less, more preferably 900 s ^-1 or less, and even more preferably 800 s ^-1 or less. By keeping the shear rate below the above upper limit, it is possible to prevent the increase rate of the crystal long period from becoming too small. On the other hand, by ensuring that the shear rate is equal to or higher than the above lower limit, it is possible to prevent the rate of increase in the crystal long period from becoming too large.

In the present invention, the shear rate at the outlet of the film-like extrusion device refers to the shear rate at the wall surface of the film-forming liquid flow path at the die lip in the case of a general T-die or I-die, and can be calculated by the following formula.
γ=6Q/ ^Wh2
Here, γ is the shear rate at the wall (s ⁻¹ ), W is the width of the die lip (cm), h is the opening of the die lip (cm), and Q is the extrusion rate of the film-forming solution from the die lip (cm ³ /s).

The surface temperature of the support on which the film-forming solution flows is preferably 50°C or higher. The surface temperature of the support on which the film-forming solution flows is preferably 110°C or lower. If the surface temperature is less than 50°C, the time required to dry the film-forming solution tends to be long, and productivity tends to decrease. On the other hand, if the surface temperature exceeds 110°C, abnormalities tend to occur on the film surface of the PVA film due to foaming, etc. In addition, the film-forming solution is dried rapidly, so that the crystallization of the PVA film does not proceed sufficiently, the amount of lamellar crystals decreases, and the crystal long period Da tends to increase. From the viewpoint of making it easier to adjust the rate of increase in the crystal long 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.

The drying speed may be adjusted by uniformly blowing hot air at a speed of 1 to 10 m/sec onto the entire area of the non-contact side of the PVA film while heating the PVA film on the support. From the viewpoints of drying efficiency and uniformity of drying, the temperature of the hot air blown onto the non-contact side is preferably 50°C or higher, and more preferably 70°C or higher. From the viewpoints of drying efficiency and uniformity of drying, the temperature of the hot air blown onto the non-contact side is preferably 150°C or lower, and more preferably 120°C or lower.

The PVA film is dried on the support preferably to a volatile content of 5 to 50% by mass, and then peeled off and further dried as necessary. There are no particular limitations on the drying method, and examples include contacting the film with a drying oven or drying roll. When drying with multiple drying rolls, it is preferable to alternately contact one side and the other side of the film with the drying roll to make both sides uniform. The number of drying rolls is preferably 3 or more, more preferably 4 or more, and even more preferably 5 or more. In addition, the number of drying rolls is even more preferably 30 or less. The upper limit of the temperature of the drying oven or the average temperature of the drying roll (average surface temperature of the drying roll) is preferably 110°C, more preferably 100°C, more preferably 90°C, and even more preferably 85°C. If the temperature of the drying oven or the average temperature of the drying roll is too high, the crystal growth of the PVA film proceeds and the size of the lamellar crystals in the PVA film tends to increase. As a result, the crystal long period Da increases, and there is a risk that the increase rate of the crystal long period becomes too small. On the other hand, the lower limit of the temperature of the drying oven or the average temperature of the drying roll is preferably 40° C., more preferably 45° C., and even more preferably 50° C. If the temperature of the drying oven or the average temperature of the drying roll is too low, the crystal growth of the PVA film tends to be insufficient, and the size of the lamellar crystals in the PVA film tends to be small. As a result, there is a risk that the crystal long period Da becomes too small or the increase rate of the crystal long period becomes too large.

The dried PVA film can be further heat-treated as necessary. By performing heat treatment, the strength, swelling degree, birefringence, etc. of the PVA film can be adjusted. The surface temperature of the heat treatment roll for heat treatment is preferably 60°C or higher. In addition, the surface temperature of the heat treatment roll is preferably 135°C or lower, and more preferably 130°C. If the surface temperature of the heat treatment roll is too high, the amount of heat applied is too large, and the size of the lamellar crystals in the PVA film tends to become large, and the crystal long period Da tends to become too large or the increase rate of the crystal long period tends to become too small.

The PVA film produced in this manner may be further subjected to humidity conditioning treatment, both ends (edges) of the film may be cut, etc., if necessary, and then the film may be wound into a roll on a cylindrical core and then moisture-proof packaged to complete the product.

Although the volatile content of the PVA film finally obtained by the above-mentioned series of processes is not necessarily limited, the volatile content of the PVA film is preferably 1% by mass or more, and more preferably 2% by mass or more. The volatile content of the PVA film is preferably 5% by mass or less, and more preferably 4% by mass or less.

When the PVA film of the present invention is a multilayer film, the multilayer film can be produced, for example, by applying a film-forming solution onto a base resin film (base resin layer). At this time, in order to improve the adhesion between the PVA layer and the base resin layer, the surface of the base resin film may be modified or an adhesive may be applied to the surface of the base resin film.

<Method of Manufacturing Optical Film>
The method for producing an optical film of the present invention includes a step of uniaxially stretching the PVA film described above. Hereinafter, a method for producing a polarizing film will be specifically described as an example of the method for producing an optical film.

The method for producing a polarizing film includes a dyeing process for dyeing a PVA film, a stretching process for uniaxial stretching, and, if necessary, a swelling process for swelling, a crosslinking process for crosslinking, a fixing process for fixing, a washing process for washing, a drying process for drying, a heat treatment process for heat treatment, etc. In this case, the order of each process is not particularly limited, but can be, for example, the swelling process, the dyeing process, the crosslinking process, the stretching process, and the fixing process. In addition, one or more processes can be performed simultaneously, and each process can be performed twice or more times. In particular, the PVA film of the present invention can suppress the occurrence of swelling wrinkles in the swelling process, and is therefore useful as a film to be used in a method for producing a polarizing film having a swelling process.

The swelling process can be performed by immersing the PVA film in a swelling treatment bath containing water or the like. The temperature of the swelling treatment bath is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 25°C or higher. The temperature of the swelling treatment bath is preferably 55°C or lower, more preferably 50°C or lower, and even more preferably 45°C or lower. The time of immersion in the swelling treatment bath is preferably, for example, 0.1 minutes or more, and more preferably 0.5 minutes or more. The time of immersion in the swelling treatment bath is preferably, for example, 5 minutes or less, and more preferably 3 minutes or less. The water used in the swelling treatment bath is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and an aqueous medium.

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

By carrying out a crosslinking process to crosslink the PVA in the PVA film, it is possible to effectively prevent the PVA from dissolving into water during wet stretching at high temperatures. From this viewpoint, it is preferable to carry out the crosslinking process after the dyeing process and before the stretching process. The crosslinking process can be carried out by immersing the PVA film in an aqueous solution (crosslinking treatment bath) containing a crosslinking agent. As the crosslinking agent, one or more boron compounds such as boric acid, borax, and other borates can be used. The concentration of the crosslinking agent in the crosslinking treatment bath is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. In addition, 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 having the concentration of the crosslinking agent within the above range, the stretchability of the PV film can be sufficiently maintained. The crosslinking treatment bath may contain potassium iodide, etc. The temperature of the crosslinking treatment bath is preferably 20°C or more, and more preferably 25°C or more. The temperature of the crosslinking treatment bath is preferably not higher than 60° C., and more preferably not higher than 55° C. By setting the temperature within the above range, the PVA film can be efficiently crosslinked.

The stretching process for uniaxially stretching the PVA film may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, it may be performed in an aqueous solution (stretching treatment bath) containing boric acid, or in the dyeing treatment bath described above or the fixing treatment bath described below. In the case of the dry stretching method, the stretching may be performed at room temperature (25°C), or may be performed while heating, or may be performed in air using the PVA film after water absorption. Among these, the wet stretching method is preferred because it can be stretched with high uniformity in the width direction, and uniaxial stretching in a stretching treatment bath is more preferred. The concentration of boric acid in the stretching treatment bath is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more. In addition, the concentration of boric acid in the stretching treatment bath is preferably 6.0% by mass or less, more preferably 5.0% by mass or less, and even more preferably 4.0% by mass or less. The stretching bath may contain potassium iodide, and the concentration of potassium iodide is preferably 0.01% by mass or more. The concentration of potassium iodide is preferably 10% by mass or less. The stretching temperature in the uniaxial stretching is preferably 30° C. or more, more preferably 40° C. or more, and even more preferably 50° C. or more. The stretching temperature in the uniaxial stretching is preferably 90° C. or less, more preferably 80° C. or less, and even more preferably 75° C. or less.

The stretching ratio in uniaxial stretching (total stretching ratio from the unstretched PVA film) is preferably 5 times or more, more preferably 5.5 times or more, from the viewpoint of the polarizing performance of the resulting polarizing film. There is no particular upper limit to the stretching ratio, but the stretching ratio is preferably 8 times or less.

The maximum stretching stress of the PVA film in uniaxial stretching is preferably 50 N/mm2 ^{or less} , more preferably 25 N/ ^mm2 or less, even more preferably 15 N/mm2 or ^less , and particularly preferably 10 N/mm2 or ^less . Here, the maximum stretching stress is a value obtained by dividing the stretching tension of the PVA film applied between adjacent rolls in the stretching bath by the cross-sectional area of the PVA film. At this time, the stretching tension of the PVA film can be measured by a tension roll installed between adjacent rolls in the stretching bath, and when three or more rolls are used in the stretching bath, the maximum value among them is used . In addition, the cross-sectional area of the PVA film is determined from the unstretched PVA film before being used for the production of the polarizing film. By reducing the maximum stretching stress, a polarizing film with a small shrinkage stress can be obtained. In addition, the maximum stretching stress is usually 1 N/mm2 or ^more .

There is no particular restriction on the direction of uniaxial stretching when uniaxially stretching a long PVA film, and uniaxial stretching in the long direction or uniaxial stretching in the transverse direction can be used. Uniaxial stretching in the long direction is preferred because it results in a polarizing film with excellent polarization performance. Uniaxial stretching in the long direction can be performed by using a stretching device equipped with multiple rolls parallel to each other and changing the peripheral speed between each roll. On the other hand, uniaxial stretching in the transverse direction can be performed using a tenter-type stretching machine.

In the manufacture of polarizing films, a fixing process can be performed after the stretching process in order to strengthen the adsorption of the dichroic dye (iodine-based dye, etc.) to the PVA film. The fixing bath used for the fixing process can be an aqueous solution containing one or more boron compounds such as boric acid and borax. If necessary, an iodine compound or a metal compound may be added to the fixing bath. The concentration of the boron compound in the fixing bath is preferably 2% by mass or more, more preferably 3% by mass or more. The concentration of the boron compound in the fixing bath is preferably 15% by mass or less, more preferably 10% by mass or less. By setting the concentration of the boron compound within the above range, the adsorption of the dichroic dye can be strengthened. The temperature of the fixing bath is preferably 15°C or more, more preferably 25°C or more. The temperature of the fixing bath is preferably 60°C or less, more preferably 40°C or less.

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

The conditions for the drying process are not particularly limited, but the drying temperature for the PVA film is preferably 30°C or higher, and more preferably 50°C or higher. The drying temperature for the PVA film is preferably 150°C or lower, and more preferably 130°C or lower. Drying at a temperature within the above range makes it easier to obtain a polarizing film with excellent dimensional stability at high temperatures.

In addition, optical films other than polarizing films, such as retardation films, can also be manufactured by a method including a step of uniaxially stretching the PVA film of the present invention. As for the specific manufacturing method, a conventionally known method can be adopted, except that the PVA film of the present invention is used.

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

The method for calculating the dichroic ratio (R) of a polarizing film is as follows. First, the relationship between the transmittance (T') excluding surface reflection and the single-piece transmittance (T) is shown by the following formula (a). In this case, the refractive index of the polarizing film is assumed to be 1.5, and the reflectance at the surface is assumed to be 4%. Meanwhile, the relationship between the transmittance (T'), the degree of polarization (V), and the dichroic ratio (R) is shown by the following formula (b). Therefore, the single-piece transmittance (T) and the degree of polarization (V) are measured, and then the dichroic ratio (R) of the polarizing film can be calculated by solving the following formulas (a) and (b) using these values.
T'=T/(1-0.04) ² ...(a)
R={-ln[T'(1-V)]}/{-ln[T'(1+V)]} ...(b)

From the dichroic ratio (R) of the polarizing film, the degree of polarization (V) at a given single transmittance (T) can be calculated. First, the transmittance (T') at a given single transmittance (T) is calculated from the above formula (a). Then, the degree of polarization (V) at a given single transmittance (T) can be calculated by solving the following formula (c) obtained by modifying the above formula (b) by substituting the transmittance (T') and the dichroic ratio (R). In the present invention, the degree of polarization (V) at a given single transmittance (T) of the polarizing film was calculated by this method when the single transmittance (T) of the polarizing film was 44.0%.
T'=[1-V] ^1/(R-1) /[1+V] ^R/(R-1) ...(c)

The polarizing film obtained in the above manner is usually used as a polarizing plate by laminating an optically transparent and mechanically strong protective film on both or one side of the polarizing film. Examples of protective films that can be used include cellulose triacetate (TAC) film, cycloolefin polymer (COP) film, cellulose acetate butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA adhesives, urethane adhesives, and acrylate ultraviolet-curing adhesives. In other words, a polarizing plate has a polarizing film and a protective film laminated directly or via an adhesive layer on one or both sides of the polarizing film.

The polarizing plate can be used as a part of an LCD by, for example, coating it with an acrylic or other adhesive and then bonding it to a glass substrate. In addition, the polarizing plate may also be bonded with a retardation film, a viewing angle improving film, a brightness improving film, etc.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples. The evaluation items and methods used in the following examples and comparative examples are as follows.

(1) Long crystal periods Ds and Da of PVA film
Small-angle X-ray measurement was performed on the PVA films obtained in the following Examples and Comparative Examples before and after immersion in a water/methanol mixed solvent (volume ratio: 2/8). Specifically, first, the PVA film was cut into multiple pieces of 2 cm x 1 cm size without distinction between the width direction (TD direction) and the machine direction (MD direction). The cut PVA films were stored for 24 hours under conditions of a temperature of 20°C and a humidity of 65%, and then 10 sheets were stacked in a measurement cell to obtain a measurement sample. Small-angle X-ray scattering measurement was performed on the measurement sample in air (temperature 20°C, humidity 65%) to obtain a scattering curve. The crystal long period Da was calculated from the value of the scattering vector q (nm ^-1 ) at the peak top of this scattering curve according to the following formula.
Crystal long period Da (nm) = 2π/q

In addition, the PVA films obtained in the following Examples and Comparative Examples were cut into multiple pieces with a size of 2 cm x 1 cm, regardless of the width direction (TD) or machine direction (MD). The cut PVA films were immersed in a water/methanol mixed solvent (volume ratio: 2/8) for 24 hours, and then 10 pieces were stacked in a measurement cell filled with the mixed solvent to obtain a measurement sample. Small-angle X-ray scattering measurement was performed on the measurement sample to obtain a scattering curve. The crystal long period Ds was calculated from the value of the scattering vector q (nm ^-1 ) at the peak top of this scattering curve according to the following formula.
Crystal long period Ds (nm) = 2π/q

(2) Swelling Degree of PVA Film The swelling degrees of the PVA films obtained in the following Examples and Comparative Examples were determined by the above-mentioned method.

(3) Preparation of Polarizing Film A polarizing film was prepared using the PVA film obtained in the following Examples and Comparative Examples. First, the PVA film was stretched 2.0 times in the MD direction in pure water (swelling treatment bath) at 25 ° C., and then stretched to a total stretch ratio of 2.4 times in an aqueous solution (dyeing treatment bath) at 32 ° C. containing 0.03 mass% iodine and 0.7 mass% potassium iodide. Next, the film was stretched to a total stretch ratio of 3.0 times in an aqueous solution (crosslinking treatment bath) at 32 ° C. containing 2.6 mass% boric acid, and further stretched to a total stretch ratio of 6.0 times in an aqueous solution (stretching treatment bath) at 58 ° C. containing 2.8 mass% boric acid and 5.0 mass% potassium iodide. Next, the film was immersed in an aqueous solution (cleaning treatment bath) at 22 ° C. containing 1.5 mass% boric acid and 2.5 mass% potassium iodide for 5 seconds, and dried in a drying oven at 80 ° C. for 4 minutes.

(4) Maximum stretching stress of PVA film in the production of polarizing film In the production of the polarizing film, the stretching tension of the PVA film applied between adjacent rolls in the stretching treatment bath was measured by a tension roll placed between them, and the maximum stretching stress was calculated by dividing the measured value by the cross-sectional area of the PVA film. The cross-sectional area of the PVA film was determined from the unstretched PVA film before it was used in the production of the polarizing film.

(5) Optical performance of polarizing film A rectangular measurement sample of 4 cm in the machine direction (MD) and 1.5 cm in the width direction (TD) of the obtained polarizing film was taken from the center of the width direction (TD). The measurement sample was measured using an integrating sphere spectrophotometer ("V7100" manufactured by JASCO Corporation) in accordance with JIS Z8722 (measurement method for object color) after performing luminosity correction in the visible light region with a C light source and a 2° visual field, and the single transmittance and polarization degree were measured. The polarization degree of a single transmittance of 44.0% was calculated by the above-mentioned method.

(6) Shrinkage stress of polarizing film at high temperature The shrinkage stress was measured using an autograph AG-X with a thermostatic chamber manufactured by Shimadzu Corporation. For the measurement, a polarizing film (length 15 cm, width 1.5 cm) that had been conditioned at 20°C/20% RH for 18 hours was attached to a chuck (chuck interval 5 cm), and the temperature of the thermostatic chamber was raised to 80°C at the same time as the start of tension. The polarizing film was pulled at a speed of 1 mm/min, and the tension was stopped when the tension reached 2 N, and the tension was measured in this state for up to 4 hours. At this time, since the distance between the chucks changes due to the thermal expansion of the shaft, a marking seal was attached to the chuck , and the measurement was performed using a video extensometer TR ViewX120S so that the distance between the chucks could be corrected by the amount of movement of the marking seal attached to the chuck. The shrinkage force of the modified film was determined by subtracting the initial tension of 2 N from the measured tension after 4 hours, and the value was divided by the cross-sectional area of the sample to define the shrinkage stress (N/mm ² ).

Example 1
A film-forming stock solution with a volatile content of 73% by mass was prepared, consisting of 100 parts by mass of PVA (saponification degree 99.9 mol%, polymerization degree 2400) 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 water. The film-forming stock solution was filtered and discharged in the form of a film onto a support (surface temperature 90°C) from a T-die at a shear rate of 207 s ^-1 , and dried by blowing hot air at 85°C at a speed of 5 m/sec onto the entire non-contact surface of the support. Next, the film was peeled off from the support, and further dried between the first drying roll and the final drying roll (19th drying roll) immediately before the heat treatment roll so that one side and the other side of the PVA film alternately contact each drying roll, and then peeled off from the final drying roll. At this time, the average value of the surface temperature of each drying roll from the first drying roll to the final drying roll was set to 70°C. Finally, the film was heat-treated with a heat-treatment roll having a surface temperature of 104° C., and then wound into a roll to obtain a PVA film (thickness: 45 μm, width: 3.3 m).

Small angle X-ray scattering measurements were performed on the obtained PVA film, and the crystal long period Da was found to be 12.6 nm, the crystal long period Ds was found to be 16.8 nm, and the increase rate of the crystal long period was 0.33. In addition, the swelling degree of the PVA film was measured and found to be 197%.

Furthermore, a polarizing film was produced using the obtained PVA film. At this time, the maximum stretching stress was 9.0 N/ ^mm2 . The optical performance of the obtained polarizing film was measured, and the single transmittance was 43.8%, the polarization degree was 99.935%, and the polarization degree was calculated to be 99.873% when the single transmittance was 44.0%. In addition, the shrinkage stress of the polarizing film was measured, and it was 43.7 N/ ^mm2 .

<Example 2, Example 3, and Comparative Examples 1 to 4>
A PVA film was obtained in the same manner as in Example 1, except that the type of PVA, the amount of plasticizer, and the production conditions were changed as shown in Table 1. In Table 1, ethylene-modified films with an ethylene unit content of 3 mol% are abbreviated as "ΔEt3".

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

As is clear from Table 1 and Figure 2, the polarized film produced from the PVA film of the present invention has excellent optical performance and low shrinkage stress at high temperatures.

1 Peak top of scattering curve 2 Auxiliary line for setting scattering vector q at the peak top of the scattering curve 3 Scattering vector q at the peak top of the scattering curve

Claims (7)

A polyvinyl alcohol film, in which the crystalline long period Ds determined by small-angle X-ray scattering measurement performed in a water/methanol mixed solvent (volume ratio: 2/8) and the crystalline long period Da determined by small-angle X-ray scattering measurement performed before immersion in the mixed solvent satisfy the following formula:
0.3≦(Ds-Da)/Da<0.5 The polyvinyl alcohol film described in claim 1, wherein the crystal long period Da is 10.0 to 12.5 nm. 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 thereof is 1 to 8 mol %. A polyvinyl alcohol film according to any one of claims 1 to 3, wherein the average thickness of the film is 15 to 60 μm. The polyvinyl alcohol film according to any one of claims 1 to 4, which is a raw film for the production of optical films. A method for producing an optical film, comprising uniaxially stretching a polyvinyl alcohol film according to any one of claims 1 to 5. The method for producing an optical film according to claim 6 , further comprising a swelling step of swelling the polyvinyl alcohol film.