Polyvinyl alcohol-based film, polarization film using same, and polarization plate

Abstract

The present invention provides a polyvinyl alcohol-based film from which it is possible to obtain a polarization film having high polarization performance even when formed into a thin film, particularly a polarization film excelling in a high degree of polarization and in orthogonal transmittance on long-wavelength and short-wavelength sides, in a well-balanced manner. The polyvinyl alcohol-based film has a film thickness of 5-35 μm, a crystal long period shorter than 10 nm as measured with a small-angle X-ray scattering method, and an amorphous thickness of 6.6 nm or smaller as measured with a small-angle X-ray scattering method.

Description

Polyvinyl alcohol film, and polarizing film and polarizing plate using the same

[0001] The present invention relates to a polyvinyl alcohol film, and more particularly to a thin polyvinyl alcohol film from which a polarizing film having excellent polarizing performance can be obtained, and a polarizing film and a polarizing plate made using the same.

[0002] Polyvinyl alcohol films have been used in a variety of applications due to their excellent transparency, including polarizing films, which are used as basic components of liquid crystal displays. In recent years, their use has expanded to include machinery that requires high quality and high reliability.

[0003] Under these circumstances, as screens of liquid crystal televisions, multifunctional mobile terminals and the like are becoming brighter, more precise, larger in area and thinner, there has been a demand for polyvinyl alcohol-based films for producing polarizing films with excellent in-plane uniformity of polarization performance (degree of polarization and single-unit transmittance).

[0004] For example, Patent Document 1 focuses on the crystalline state of the film and proposes a polyvinyl alcohol film having a long period of 20 nm or less as determined by small-angle X-ray scattering, a thickness of an amorphous portion of 15 nm or less, and a film thickness of 30 to 70 μm.

[0005] Japanese Patent Application Laid-Open No. 2006-188655

[0006] However, although the polyvinyl alcohol-based film obtained by the technique disclosed in Patent Document 1 provides a polarizing film having excellent in-plane uniformity of polarization performance (degree of polarization, single transmittance) to accommodate larger areas and higher definitions, when used as a polarizing film having thinner thicknesses in recent years, it does not exhibit sufficient polarization performance or breaks during production.

[0007] In view of this background, the present invention provides a polyvinyl alcohol-based film that can provide a polarizing film that has high polarization performance, particularly a high degree of polarization and crossed transmittance at long and short wavelengths, in a well-balanced manner even when made thin.

[0008] However, in view of the above circumstances, the present inventors have conducted extensive research and have found that by using a polyvinyl alcohol-based film having a crystal long period within a specific range as a thin polyvinyl alcohol-based film, it is possible to obtain a polarizing film that has excellent polarization performance, in particular a high degree of polarization and crossed transmittances at long and short wavelengths in a well-balanced manner.

[0009] That is, the present invention has the following aspects. [1] A polyvinyl alcohol-based film having a thickness of 5 to 35 μm, wherein the polyvinyl alcohol-based film has a crystalline long period of less than 10 nm or more as measured by small-angle X-ray scattering and an amorphous thickness of 6.6 nm or less as measured by small-angle X-ray scattering. [2] The polyvinyl alcohol-based film according to [1], wherein the polyvinyl alcohol-based resin contained in the film has a degree of saponification of 98% or more. [3] The polyvinyl alcohol-based film according to [1] or [2], which is used for producing a polarizing film. [4] A polarizing film comprising the polyvinyl alcohol-based film according to any one of [1] to [3]. [5] A polarizing plate comprising the polarizing film according to [4] and a protective film provided on at least one surface of the polarizing film.

[0010] The polyvinyl alcohol film of the present invention exhibits high polarizing performance even when made thin, and in particular, it is possible to obtain a polarizing film that has a high degree of polarization and a well-balanced cross transmittance on the long wavelength and short wavelength sides.

[0011] The polyvinyl alcohol film of the present invention is a polyvinyl alcohol film having a thickness of 5 to 35 μm, and is most characterized by having a crystalline long period of less than 10 nm as measured by small-angle X-ray scattering and an amorphous thickness of 6.6 nm or less as measured by small-angle X-ray scattering.

[0012] The value of the crystalline long period of such a polyvinyl alcohol-based film, as measured by small-angle X-ray scattering, must be less than 10 nm, preferably 9.9 nm or less, particularly preferably 9.8 nm or less, and even more preferably 9.7 nm or less. The lower limit of the crystalline long period is usually 5 nm or more, preferably 6 nm or more, more preferably 7 nm or more, and even more preferably 8 nm or more. When the value of the crystalline long period, as measured by small-angle X-ray scattering, is within this range, a polarizing film can be obtained that exhibits a high degree of polarization and a well-balanced cross transmittance at long and short wavelengths. When a polyvinyl alcohol-based film is made thinner, such as to have a thickness of 35 μm or less, as in the present invention, the stretching conditions (e.g., stretch ratio, stretching temperature, boric acid concentration, etc.) must be changed to prevent breakage during production of the polarizing film, compared to when the film is made thick. This tends to result in a decrease in polarizing performance. In the present application, even when the film is thinned to a thickness of 35 μm or less, by controlling the crystalline long period and amorphous thickness of the polyvinyl alcohol-based film to specific values, it is possible to obtain a polarizing film that has high polarization performance and a well-balanced crossed transmittance at both the long wavelength and short wavelength sides.

[0013] Small-angle X-ray scattering (SAXS) is a technique that analyzes the diffraction that occurs when X-rays are irradiated onto a sample and scattered and interfered with by electrons surrounding the atoms. This analytical technique evaluates the crystalline structure of a material by measuring X-rays that appear in the low-angle region of 2θ<10° or less, and is generally capable of evaluating structures on the order of a few nanometers to a few tens of nanometers in size. Using this diffraction information, it is possible to determine the long-period structure, lamellar structure, etc. of the crystal.

[0014] X-rays incident on and scattered by a regularly arranged material interfere with each other and are reinforced in specific directions. According to the Bragg equation, when d is the interstitial distance, θ is the Bragg angle, and λ is the wavelength of the X-ray, diffracted X-rays can only be observed in directions that satisfy the equation 2d sin θ = nλ.

[0015] It is known that in polyvinyl alcohol-based films, a diffraction peak resulting from the repeating structure of lamellar crystals appears when the long-period peak q (nm-1) is approximately 0.5. Furthermore, by deriving a correlation function from the scattering curve, the crystal long period, crystalline thickness, and amorphous thickness in the polyvinyl alcohol-based film can be determined.

[0016] Examples of methods for controlling the value of the crystalline long period of the polyvinyl alcohol-based film measured by small-angle X-ray scattering within a predetermined range include adjusting the molecular weight or degree of saponification of the raw material polyvinyl alcohol-based resin, controlling the crystallinity or orientation state of the polymer in the process of forming a film from an aqueous polyvinyl alcohol-based resin solution, and controlling the drying temperature during film formation and the drying state of the film thereafter. In the present invention, preferred methods for controlling the value of the crystalline long period of the polyvinyl alcohol-based film measured by small-angle X-ray scattering within a predetermined range include controlling the temperature of the casting mold in the film-forming process when producing the polyvinyl alcohol-based film, and controlling the moisture content of the film within a specific range when the casting mold is peeled off.

[0017] The amorphous thickness of such a polyvinyl alcohol-based film measured by small-angle X-ray scattering is 6.6 nm or less, preferably 6.55 nm or less, particularly preferably 6.5 nm or less, and even more preferably 6.45 nm or less. The lower limit of the amorphous thickness is usually 3 nm or more, preferably 4 nm or more. When the amorphous thickness measured by small-angle X-ray scattering is within this range, a polarizing film having a high degree of polarization and a well-balanced orthogonal transmittance at long and short wavelengths can be obtained. In particular, when the crystalline long period measured by small-angle X-ray scattering as specified in the present invention is less than 10 nm and the amorphous thickness is within the above range, a polarizing film having a particularly well-balanced orthogonal transmittance at long and short wavelengths can be obtained from a thinner polyvinyl alcohol-based film.

[0018] The method for producing the polyvinyl alcohol film of the present invention will be described in more detail below in the order of steps, but the polyvinyl alcohol film of the present invention is not limited to these embodiments.

[0019] In this specification, when an expression "X to Y" (X and Y are any numbers) is used, it means "X or more and Y or less" unless otherwise specified, and also includes the meaning "preferably larger than X" or "preferably smaller than Y." Furthermore, in this specification, when an expression "X or more" (X is any number) or "Y or less" (Y is any number) is used, it also includes the meaning "preferably larger than X" or "preferably smaller than Y."

[0020] In the present specification, when numerical ranges are described in stages, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described herein, the upper or lower limit of the numerical range can also be replaced with a value shown in the examples. Furthermore, in the present specification, parts and percentages based on mass are synonymous with parts and percentages based on weight.

[0021] The polyvinyl alcohol-based film is preferably produced through the following steps (A) to (C), and preferably also through step (D) as necessary. Step (A): A step of preparing an aqueous polyvinyl alcohol-based resin solution. Step (B): A step of casting the aqueous polyvinyl alcohol-based resin solution into a casting mold to form a film. Step (C): A step of drying the formed film. Step (D): A step of heat-treating the obtained film.

[0022] <Step (A)> Step (A) is a step of preparing an aqueous solution of a polyvinyl alcohol resin. First, the polyvinyl alcohol resin and aqueous solution of the polyvinyl alcohol resin, which are the materials for the polyvinyl alcohol film, will be described. In the present invention, the polyvinyl alcohol resin constituting the polyvinyl alcohol film is typically an unmodified polyvinyl alcohol resin, i.e., a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol % or less, preferably 5 mol % or less) of a component copolymerizable with vinyl acetate can also be used. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (e.g., salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. Modified polyvinyl alcohol resins obtained by chemically modifying hydroxyl groups after saponification can also be used. These can be used alone or in combination of two or more.

[0023] Furthermore, the polyvinyl alcohol resin may also be a polyvinyl alcohol resin having a 1,2-diol structure in a side chain. Such polyvinyl alcohol resins having a 1,2-diol structure in a side chain can be obtained, for example, by (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) a method of saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, (iii) a method of saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, or (iv) a method of saponifying a copolymer of vinyl acetate and glycerin monoallyl ether.

[0024] The weight-average molecular weight of the polyvinyl alcohol-based resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If the weight-average molecular weight is too small, it tends to be difficult to obtain sufficient optical performance when the polyvinyl alcohol-based resin is used to form an optical film, while if the weight-average molecular weight is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film when a polarizing film is produced using the polyvinyl alcohol-based resin. The weight-average molecular weight of the polyvinyl alcohol-based resin is a weight-average molecular weight measured by GPC-MALS.

[0025] The average saponification degree of the polyvinyl alcohol resin used in the present invention is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, and particularly preferably 99.8 mol% or more. If the average saponification degree is too low, sufficient optical performance tends to be difficult to obtain when the polyvinyl alcohol film is used as a polarizing film. Here, the average saponification degree in the present invention is measured in accordance with JIS K 6726.

[0026] As the polyvinyl alcohol resin used in the present invention, two or more kinds of resins differing in the modified species, the degree of modification, the weight average molecular weight, the average degree of saponification, etc. may be used in combination.

[0027] In terms of film-forming properties, it is more preferable that the aqueous polyvinyl alcohol resin solution contains, in addition to the polyvinyl alcohol resin, a commonly used plasticizer such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, or trimethylolpropane, or at least one nonionic, anionic, or cationic surfactant, as needed. These may be used alone or in combination of two or more.

[0028] The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by mass, particularly preferably 17 to 55% by mass, and further preferably 20 to 50% by mass. If the resin concentration of such an aqueous solution is too low, the drying load increases, which tends to reduce production capacity, while if it is too high, the viscosity tends to be too high, making it difficult to achieve uniform dissolution.

[0029] Next, the obtained aqueous polyvinyl alcohol resin solution is subjected to a degassing treatment. Examples of the degassing method include static degassing and degassing using a multi-screw extruder. The multi-screw extruder may be any multi-screw extruder equipped with a vent, and typically a twin-screw extruder equipped with a vent is used.

[0030] <Step (B)> Step (B) is a step of casting an aqueous solution of a polyvinyl alcohol-based resin into a casting mold to form a film. After the degassing treatment, the aqueous solution of a polyvinyl alcohol-based resin is introduced into a T-shaped slit die in fixed amounts, and then extruded and cast onto a rotating casting drum to form a film by a continuous casting method.

[0031] The resin temperature of the aqueous polyvinyl alcohol resin solution at the outlet of the T-shaped slit die is preferably 70 to 100° C., particularly preferably 80 to 98° C. If the resin temperature of the aqueous polyvinyl alcohol resin solution is too low, it tends to have poor flowability, and if it is too high, it tends to foam.

[0032] The viscosity of the aqueous polyvinyl alcohol resin solution during discharge is preferably 50 to 200 Pa s, and particularly preferably 70 to 150 Pa s. If the viscosity of the aqueous solution is too high, the flow tends to be poor, whereas if the viscosity is too low, casting the solution into a film tends to be difficult.

[0033] The discharge speed of the aqueous polyvinyl alcohol resin solution discharged from the T-shaped slit die onto the casting drum is preferably 0.2 to 5 m / min, particularly preferably 0.4 to 4 m / min, and further preferably 0.6 to 3 m / min. If the discharge speed is too slow, productivity tends to decrease, while if it is too fast, casting tends to become difficult.

[0034] The diameter of the casting drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and further preferably 2.8 to 4 m. If the diameter is too small, the drying zone on the casting drum becomes short, making it difficult to increase the speed, whereas if the diameter is too large, transportability tends to decrease.

[0035] The width of the casting drum is preferably 3 m or more, more preferably 3.5 m or more, particularly preferably 4 m or more, further preferably 4.5 m or more, and particularly preferably 5 to 8 m. If the width of the casting drum is too small, productivity tends to decrease.

[0036] The upper limit of the surface temperature of the cast drum is preferably 55°C or less, in order to suppress foaming during drying and obtain a film with excellent appearance, particularly preferably 50°C or less, even more preferably 47°C or less, and particularly preferably 45°C or less. The lower limit is preferably 25°C or more, in order to obtain excellent releasability when the formed film is peeled from the casting mold, more preferably 30°C or more, even more preferably 35°C or more, and particularly preferably 38°C or more. That is, the surface temperature of the cast drum is preferably 25 to 55°C, more preferably 30 to 50°C, even more preferably 35 to 47°C, and even more preferably 38 to 45°C.

[0037] The moisture content of the film upon peeling from the casting mold is preferably 20% by mass or less, particularly preferably 18% by mass or less, even more preferably 17% by mass or less, and especially preferably 16% by mass or less. If the moisture content is too high, peeling may be poor during OPL film formation, and breakage may be more likely to occur. The moisture content immediately after peeling is preferably 5% by mass or more, particularly preferably 7% by mass or more, and even more preferably 9% by mass or more. If the moisture content is too low, the film may be more likely to curl. That is, the moisture content is preferably 5 to 20% by mass, more preferably 7 to 18% by mass, and even more preferably 9 to 17% by mass. In particular, in the present invention, by setting the temperature of the casting drum within the above-mentioned preferred temperature range and setting the moisture content of the film upon peeling from the casting mold within the above-mentioned range, a polarizing film can be obtained that exhibits a high degree of polarization and a well-balanced cross transmittance at both long and short wavelengths, even when the film thickness is 35 μm or less, or even thinner.

[0038] <Step (C)> Step (C) is a step of heating and drying the formed film. The film (the formed film) peeled from the casting drum is transported in the machine direction (MD direction) using nip rolls or the like, and the front and back surfaces of the film are dried by alternately contacting them with multiple heated rolls. The heated rolls are, for example, rolls with a diameter of 0.2 to 2 m and whose surfaces have been hard chrome-plated or mirror-finished, and drying is preferably carried out using typically 2 to 30 rolls, preferably 10 to 25 rolls.

[0039] The surface temperature of the heat roll is not particularly limited, but is usually 30 to 150° C., preferably 40 to 120° C., more preferably 40 to 100° C., and even more preferably 40 to 90° C. If the surface temperature is too low, drying tends to be insufficient, while if the surface temperature is too high, drying tends to be excessive, which tends to result in poor appearance such as waviness.

[0040] <Step (D)> Step (D) is a step of heat-treating the obtained film using hot air. The film that has undergone step (C) may be heat-treated, for example, in a hot air drying oven. The upper limit of the heat treatment temperature is preferably 190°C or lower, particularly preferably 180°C or lower, even more preferably 175°C or lower, and especially preferably 170°C or lower. The lower limit is preferably 100°C or higher, particularly preferably 105°C or higher, even more preferably 110°C or higher, and especially preferably 115°C or higher. If the heat treatment temperature is too high, the dyeability of the polarizing film during production tends to decrease. If the heat treatment temperature is too low, the film is likely to wrinkle or fold during the swelling step during production, which tends to deteriorate the appearance of the polarizing film. The heat treatment time is preferably 20 to 100 seconds, particularly preferably 40 to 70 seconds.

[0041] Up to this point, a method for producing a polyvinyl alcohol-based film has been described in which an aqueous solution of a polyvinyl alcohol-based resin is prepared, the aqueous solution is cast onto a rotating cast drum (drum-shaped roll), a film is formed by a casting method, and the film is dried. However, it is also possible to cast an aqueous solution of a polyvinyl alcohol-based resin onto a resin film or a metal belt, and then form a film and dry the film.

[0042] [Polyvinyl Alcohol Film] Thus, a polyvinyl alcohol film is obtained through the above steps (A) to (C) and, if necessary, step (D), and is finally wound up into a roll to become a finished product.

[0043] The upper limit of the thickness of the polyvinyl alcohol-based film of the present invention must be 35 μm or less, preferably 31 μm or less, more preferably 30 μm or less, and particularly preferably 25 μm or less. From the viewpoint of production stability of the polarizing film, the lower limit is preferably 5 μm or more, particularly preferably 10 μm or more, and even more preferably 15 μm or more. That is, the thickness of the polyvinyl alcohol-based film of the present invention is preferably 5 to 35 μm, more preferably 10 to 31 μm, and even more preferably 15 to 25 μm. When the film is thin, the crystalline long period and amorphous thickness are made smaller than those of conventional techniques, and further, the crystalline long period and amorphous thickness are set within the ranges specified in the present invention, thereby enabling the film to have optical properties that maintain a high degree of polarization while also exhibiting an excellent balance of cross transmittances on the long wavelength side and the short wavelength side.

[0044] The length of the polyvinyl alcohol film is preferably 4 km or more from the viewpoint of increasing the area of ​​the polarizing film, and particularly preferably 5 to 50 km from the viewpoint of transport mass.

[0045] The width of the polyvinyl alcohol film is preferably 1 m or more, more preferably 2 m or more, particularly preferably 3 m or more, and even more preferably 3.5 m or more. The upper limit of the film width is usually 7 m or less.

[0046] The polyvinyl alcohol-based film obtained by the production method of the present invention is useful for optical purposes, and is particularly useful as a raw film for a polarizing film. Hereinafter, a method for producing a polarizing film and a polarizing plate made of the polyvinyl alcohol-based film will be described.

[0047] [Method for Producing Polarizing Film] The polarizing film of the present invention is produced by unwinding the polyvinyl alcohol film obtained by the production method of the present invention from a roll and transporting it horizontally, followed by processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

[0048] The swelling step is carried out before the dyeing step. The swelling step not only cleans the surface of the polyvinyl alcohol-based film from dirt, but also has the effect of preventing uneven dyeing by swelling the polyvinyl alcohol-based film. In the swelling step, water is usually used as the treatment liquid. As long as the treatment liquid is mainly composed of water, it may contain additives such as iodide compounds and surfactants, alcohol, etc. The temperature of the swelling bath is usually about 10 to 45°C, and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

[0049] The dyeing process is carried out by bringing the film into contact with a liquid containing iodine or a dichroic dye. An aqueous solution of iodine and potassium iodide is usually used, with an iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L being appropriate. A practical dyeing time is about 30 to 500 seconds. The temperature of the treatment bath is preferably 5 to 50°C. The aqueous solution may contain a small amount of an organic solvent that is compatible with water in addition to the water solvent.

[0050] The boric acid crosslinking step is carried out using a boron compound such as boric acid or borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 10 to 100 g / L, and it is preferable to have potassium iodide coexist in the solution from the viewpoint of stabilizing the polarization performance. The treatment temperature is preferably about 30 to 70°C, and the treatment time is preferably about 0.1 to 20 minutes. If necessary, a stretching operation may be carried out during the treatment.

[0051] In the stretching step, the film is preferably stretched uniaxially by 3 to 10 times, preferably 3.5 to 7 times. At this time, slight stretching (stretching to a degree sufficient to prevent shrinkage in the width direction, or more) may also be performed in the direction perpendicular to the stretching direction. The temperature during stretching is preferably 40 to 170°C. Furthermore, the final stretching ratio need only be set within the above range, and the stretching operation may be performed not only in one step but also multiple times during the production process.

[0052] The washing step is carried out, for example, by immersing the film in water or an aqueous iodide solution such as potassium iodide, and can remove precipitates that form on the surface of the film. When using an aqueous potassium iodide solution, the potassium iodide concentration may be approximately 10 to 1,000 g / L. The temperature during the washing treatment is usually 5 to 50°C, preferably 10 to 45°C. The treatment time is usually 1 to 300 seconds, preferably 10 to 240 seconds. Note that washing with water and washing with an aqueous potassium iodide solution may be performed in combination as appropriate.

[0053] The drying step is carried out, for example, using a dryer at 40 to 100° C. for 0.1 to 10 minutes.

[0054] A polarizing film is thus obtained, and the polarization degree of the polarizing film is preferably 99.90% or more, more preferably 99.99% or more. If the polarization degree is too low, the contrast of the liquid crystal display tends to decrease. The polarization degree is generally determined by the light transmittance (H １１ ) and the light transmittance (H １ ) is calculated according to the following formula (1): １１ -H １ ) / (H １１ +H １ ) １／２ ...(1)

[0055] Furthermore, the single transmittance of the polarizing film of the present invention is preferably 41% or more, more preferably 43% or more. If the single transmittance is too low, it tends to be difficult to achieve high brightness in liquid crystal displays. The single transmittance is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0056] Next, a method for producing a polarizing plate of the present invention using the polarizing film of the present invention will be described. The polarizing film of the present invention is suitable for producing a polarizing plate with little color unevenness and excellent polarizing performance.

[0057] [Method for Manufacturing Polarizing Plate] The polarizing plate of the present invention includes a protective film provided on at least one side of the polarizing film of the present invention. The polarizing plate is manufactured by laminating an optically isotropic resin film as a protective film to one or both sides of the polarizing film of the present invention via an adhesive. Examples of the protective film include films made of acetyl cellulose-based resins such as triacetyl cellulose and diacetyl cellulose, films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, polycarbonate-based resin films, cycloolefin-based resin films, acrylic-based resin films, and films made of linear olefin-based resins such as polypropylene-based resins.

[0058] The lamination method is carried out by a known method, for example, by uniformly applying a liquid adhesive composition to the polarizing film, the protective film, or both, and then laminating and pressing the two together, followed by heating or irradiating with active energy rays.

[0059] A polarizing plate can also be produced by applying a curable resin such as a urethane resin, an acrylic resin, or a urea resin to one or both sides of the polarizing film of the present invention and curing the resin to form a cured layer. In this way, the cured layer serves as a substitute for the protective film, making it possible to further reduce the thickness of the polarizing plate.

[0060] The polarizing film and polarizing plate obtained using the polyvinyl alcohol-based film of the present invention have excellent polarization performance, and are preferably used in liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game consoles, videos, cameras, photo albums, thermometers, audio equipment, meters for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, foldable displays, anti-reflection films for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, etc.

[0061] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts" and "%" are based on mass.

[0062] <Measurement conditions>

[0063] [Small-angle X-ray scattering measurement] (Measurement of crystal structure) - After storing the sample film at 23°C and 65% humidity for 24 hours or more, it was cut into several pieces of 3 x 3.8 cm. Several pieces were stacked on a measurement mount so that the total thickness was 180 μm and used for measurement. - Measurement equipment: Aichi Synchrotron Light Center BL8S3 - Measurement conditions: Transmission measurement X-ray: CuKα ray wavelength: 1.5 Å Detector: PILATUS 2M (measurement area = 253.7 x 288.8 mm ２ ) Pixel size: 172 μm x 172 μm Camera length: 4 m X-ray exposure time: 120 seconds Environmental temperature: room temperature Data analysis: In small-angle X-ray measurements, scattering from the polyvinyl alcohol film is superimposed on scattering from the air in the X-ray passage, and therefore scattering must be corrected as background. In the measurement, the scattering intensity due to the above was calculated separately from the scattering intensity obtained by measuring the sample and subtracted from the scattering intensity obtained by measuring the sample. Furthermore, from the scattering intensity image measured with the two-dimensional detector, the scattering intensity relative to the scattering vector q was integrated in the azimuthal direction to derive the relationship between the scattering vector q and the one-dimensional profile of the scattering intensity I(q), thereby obtaining a scattering curve. Calculation of long period, crystalline thickness, and amorphous thickness: To calculate the crystalline long period, we decided to derive the correlation function from experimental values. However, if the correlation function is derived directly using experimental values, the effects of truncation of the integration range and statistical noise will be reflected, resulting in a correlation function with a large shape plus small oscillations. To remove such vibrational structures, we performed least-squares fitting and derived the function. The fitting results were Fourier-transformed to obtain correlation functions, from which we derived indices of long-period size and crystalline thickness. Furthermore, we derived an index of amorphous thickness by subtracting the index of crystalline thickness from the long-period size.

[0064] A test piece measuring 3 cm in length and 3 cm in width was cut out from the obtained polarizing film, and the polarization degree (%), single transmittance (%), and crossed transmittance (%) at 430 nm and 700 nm were measured using an automatic polarizing film measuring device (manufactured by JASCO Corporation: VAP-8010).

[0065] Example 1 (Production of Polyvinyl Alcohol-Based Film) 500 kg of polyvinyl alcohol-based resin with a weight-average molecular weight of 156,000 and a degree of saponification of 99.8 mol%, 1,500 kg of water, 56 kg of glycerin as a plasticizer, and 0.7 kg of sodium dodecyl sulfonate as a surfactant were placed in a dissolver, and the mixture was heated to 140°C with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol-based resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol-based resin solution was continuously discharged and cast into a casting mold heated to 44°C using a T-shaped slit die, followed by drying. The moisture content of the resulting film upon removal from the casting mold was 10% by mass. The resulting film was then dried using a heated roll and then heat-treated using a floating dryer. Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol-based film (thickness 20 μm).

[0066] (Production of Polarizing Film) The resulting polyvinyl alcohol film was unwound from the roll and conveyed horizontally. While immersed in a water bath at 25°C to swell, it was stretched in the machine direction (MD) to 1.6 times its original size. The film was then dyed by immersing in an aqueous solution at 28°C containing 30 g / L of potassium iodide, adjusting the amount of iodine so that the final polarizing film would have a single-piece transmittance of 43.6%. The film was then stretched in the machine direction (MD) to 2.2 times its original size. The film was then stretched in the machine direction (MD) to 5.4 times its original size while immersed in an aqueous solution at 50°C containing 40 g / L of boric acid and 24 g / L of potassium iodide. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C for 40 seconds to obtain a polarizing film. The physical properties of the resulting polarizing film are shown in Table 1.

[0067] Example 2 A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the temperature of the casting mold was changed to 40°C and the moisture content of the film when the casting mold was peeled off was changed to 15% by mass, and a polarizing film was also obtained in the same manner as in Example 1. The obtained polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0068] Comparative Example 1 A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the temperature of the casting mold was changed to 66°C, and a polarizing film was also obtained in the same manner as in Example 1. The obtained polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0069] Comparative Example 2 A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the temperature of the casting mold was changed to 57°C. Further, a polarizing film was obtained in the same manner as in Example 1, except that the film was stretched 5.35 times the original roll. The obtained polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0070] Comparative Example 3 A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the temperature of the casting mold was changed to 48°C and the moisture content of the film when the casting mold was removed was changed to 15% by mass. Further, a polarizing film was obtained in the same manner as in Example 1, except that the film was stretched 5.25 times the original roll. The obtained polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0071] Comparative Example 4: A dissolver was charged with 2,250 kg of a polyvinyl alcohol-based resin having a weight-average molecular weight of 156,000 and a saponification degree of 99.8 mol%, 6,300 kg of water, 300 kg of glycerin as a plasticizer, and 3.2 kg of sodium dodecyl sulfonate as a surfactant. The mixture was heated to 140°C with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol-based resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol-based resin solution was continuously discharged and cast into a casting mold heated to 73°C using a T-shaped slit die, followed by drying. The resulting film had a moisture content of 11% by mass upon removal from the casting mold. The resulting film was then dried using a heated roll and heat-treated using a floating dryer. Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol-based film (thickness: 20 μm). A polarizing film was then obtained in the same manner as in Example 1. The resulting polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0072] Comparative Example 5 A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 4, except that the temperature of the casting mold was changed to 69°C, and a polarizing film was further obtained in the same manner as in Example 1. The obtained polyvinyl alcohol-based film and polarizing film were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0073]

[0074] It is clear that the polyvinyl alcohol-based films of Examples 1 and 2, whose crystalline long periods and amorphous thicknesses measured by small-angle X-ray scattering were within the ranges specified in the present invention, were able to provide polarizing films with high polarization performance, particularly a high degree of polarization and well-balanced orthogonal transmittances at both long and short wavelengths, even when thinned. On the other hand, the polyvinyl alcohol-based films of Comparative Examples 1 to 5, whose crystalline long periods measured by small-angle X-ray scattering were outside the ranges specified in the present invention, were inferior in both the degree of polarization and the orthogonal transmittances at both long and short wavelengths.

[0075] Although the above examples show specific embodiments of the present invention, the examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0076] A polarizing film using a polyvinyl alcohol-based film obtained by the production method of the present invention has excellent polarization performance, and is preferably used in liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game consoles, videos, cameras, photo albums, thermometers, audio equipment, meters for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, foldable displays, rollable televisions, rollable displays, anti-reflection films for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, etc.

Claims

1. A polyvinyl alcohol-based film having a thickness of 5 to 35 μm, a crystalline long period of less than 10 nm as measured by small-angle X-ray scattering, and an amorphous thickness of 6.6 nm or less as measured by small-angle X-ray scattering.

2. The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol resin contained in the film has a degree of saponification of 98% or more.

3. The polyvinyl alcohol film according to claim 1 or 2, which is used for producing a polarizing film.

4. A polarizing film comprising the polyvinyl alcohol film according to claim 1 or 2.

5. A polarizing plate comprising the polarizing film according to claim 4 and a protective film provided on at least one surface of the polarizing film.

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