Poly(vinyl alcohol)-based film, and polarizing film and polarizing plate obtained using same

Abstract

In order to provide a poly(vinyl alcohol)-based film which exhibits excellent stretchability and can suppress breaking during production of a polarizing film even if the film is made thinner, the present invention provides a poly(vinyl alcohol) film which has a thickness of 5-50 μm, and in which the ratio (TD / MD) of the elastic modulus in the machine direction (MD) and the elastic modulus in the transverse direction (TD) is 0.95 or more when elastic modulus is measured in water at 30ºC.

Description

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

[0001] The present invention relates to a polyvinyl alcohol-based film, and more particularly to a polyvinyl alcohol-based film that has excellent stretchability and can prevent breakage during production of a polarizing film, improves transportability during production of the polarizing film, and has low shrinkage force, and thus can prevent bending of the polarizing plate and cracking of the polarizing film, particularly under high-temperature conditions, after being processed into a polarizing plate, as well as a polarizing film and a polarizing plate including the polyvinyl alcohol-based film.

[0002] Polyvinyl alcohol films have been used in a variety of applications as films with excellent transparency, including polarizing films, which are used as basic components of liquid crystal displays. In recent years, their use has expanded to devices that require 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 that have excellent stretchability during the production of polarizing films, particularly when made into thin films, and that enable the stable production of polarizing films.

[0004] For example, Patent Document 1 proposes a polyvinyl alcohol-based film having a length of 4 km or more, and a tensile modulus X in the machine direction (MD direction) of 5 to 12 MPa when the film has a moisture content of 9% by mass.

[0005] WO2017 / 195813

[0006] However, although the polyvinyl alcohol-based film obtained by the technique disclosed in Patent Document 1 is said to be able to prevent breakage during the production of a thin polarizing film, there is still room for improvement in the production conditions for thin polarizing films, which have become more advanced in recent years.

[0007] Under these circumstances, the present invention provides a polyvinyl alcohol-based film that exhibits excellent stretchability and is capable of preventing breakage during the production of a polarizing film even when made thin, and further exhibits low shrinkage force and favorable transportability during the production of a polarizing film.

[0008] However, the present inventors have conducted extensive research in light of these circumstances and have found that, when the ratio of the elastic modulus in the machine direction (MD) to the elastic modulus in the width direction (TD) (TD / MD) measured in water, which is closer to the time required for producing a polarizing film than the elastic modulus in gas as has been conventionally focused on, falls within a specific range, it is possible to obtain a polyvinyl alcohol-based film that exhibits excellent stretchability and suppresses breakage during production of a polarizing film even when made thin, exhibits good transportability during production of a polarizing film, and further has low shrinkage force, and is capable of suppressing bending of the polarizing plate and cracking of the polarizing film after processing into a polarizing plate, particularly under high-temperature environments.

[0009] That is, the present invention has the following aspects. [1] A polyvinyl alcohol-based film having a thickness of 5 to 50 μm, wherein the ratio (TD / MD) of the elastic modulus in the machine direction (MD) to the elastic modulus in the cross direction (TD) is 0.95 or more when the elastic modulus in water is measured at 30° C. [2] The polyvinyl alcohol-based film according to [1], having a thickness of 40 μm or less. [3] A polarizing film comprising the polyvinyl alcohol-based film according to [1] or [2]. [4] A polarizing plate comprising the polarizing film according to [3] and a protective film provided on at least one side of the polarizing film.

[0010] The polyvinyl alcohol-based film of the present invention has excellent stretchability and can suppress breakage during production of a polarizing film even when made thin, and has good transportability during production of a polarizing film. Furthermore, it has low shrinkage force and can suppress bending of the polarizing plate and cracking of the polarizing film, particularly under high-temperature conditions, after being processed into a polarizing plate.

[0011] The polyvinyl alcohol film of the present invention is a polyvinyl alcohol film having a thickness of 5 to 50 μm, and is most characterized in that the ratio of the elastic modulus in the machine direction (MD) to the elastic modulus in the cross direction (TD) (TD / MD) is 0.95 or more when the elastic modulus is measured in water at 30° C.

[0012] When the polyvinyl alcohol film is measured for its elastic modulus in water at 30°C, the ratio (TD / MD) of the elastic modulus in the machine direction (MD) to the elastic modulus in the width direction (TD) must be 0.95 or more, preferably 1 or more, particularly preferably 1.05 or more, and even more preferably 1.09 or more. The upper limit of the TD / MD ratio is usually 3 or less, preferably 2.5 or less, and even more preferably 2 or less. When the ratio (TD / MD) of the elastic modulus in the machine direction (MD) to the elastic modulus in the width direction (TD) is measured for its elastic modulus in water at 30°C, the ratio is within this range, and even a thin film can have a good balance between the elastic modulus in the MD and TD directions during underwater stretching, resulting in a film with excellent stretchability. In particular, when the film is thinner, a TD / MD value within this range not only provides a film with excellent stretchability, but also reduces the shrinkage force of the polarizing film after processing into a polarizing plate, thereby preventing bending of the polarizing plate and cracking of the polarizing film.

[0013] Examples of methods for controlling the ratio (TD / MD) of the elastic modulus in the machine direction (MD) to the elastic modulus in the transverse direction (TD) within a predetermined range when the elastic modulus in water is measured at 30°C include a method of adjusting the molecular weight or degree of saponification of the polyvinyl alcohol resin used as a raw material, a method of controlling the crystallinity or orientation state of the polymer in the process of forming a film from an aqueous polyvinyl alcohol resin solution, and a method of controlling the drying temperature during film formation and the drying state of the film thereafter. In the present invention, preferred methods are a method of controlling the moisture content of the film at the time of peeling from the cast mold within a specific range in the film formation process in the production of a polyvinyl alcohol film, which will be described later, a method of controlling the temperature of a heat roll, and a method of controlling the temperature of a floating dryer.

[0014] The value of the elastic modulus in the machine direction (MD) when measuring the elastic modulus in water at 30° C. is preferably 3 to 20 MPa, particularly preferably 4 to 15 MPa, and even more preferably 5 to 12 MPa. If this value is too low, the film tends to lose stiffness and reduce transportability, while if it is too high, the tension becomes high and breakage tends to occur more easily.

[0015] The value of the elastic modulus in the machine direction (MD) when measuring the elastic modulus in water at 30° C. is preferably 3.5 to 25 MPa, particularly preferably 4.5 to 22 MPa, and even more preferably 6 to 20 MPa. If this value is too low, the tension in the MD direction will be high and breakage will tend to occur more easily, while if this value is too high, the degree of swelling in the width direction will tend to decrease and the width yield will tend to decrease.

[0016] The method for producing a 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.

[0017] 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."

[0018] 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.

[0019] 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 using multiple heated rolls. Step (D): A step of heat-treating the obtained film.

[0020] <Step (A)> Step (A) is a step of preparing an aqueous solution of a polyvinyl alcohol resin. The polyvinyl alcohol resin and aqueous solution of the polyvinyl alcohol resin, which are the materials for the polyvinyl alcohol film, are described below. 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, and unsaturated sulfonates. Modified polyvinyl alcohol resins obtained by chemically modifying hydroxyl groups after saponification can also be used. These can be used alone or in combination.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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 mass average molecular weight, the average degree of saponification, etc. may be used in combination.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] <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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] The surface temperature of the casting drum is preferably 50 to 100° C., particularly preferably 55 to 90° C., and further preferably 60 to 80° C. If the temperature is too low, drying tends to be insufficient, and if it is too high, foaming tends to occur.

[0035] The rotation speed of the casting drum is preferably 1 to 50 m / min, particularly preferably 2 to 40 m / min, and further preferably 3 to 35 m / min. If the rotation speed is too slow, productivity tends to decrease, while if it is too fast, drying tends to be insufficient.

[0036] The moisture content of the film when peeled from the casting mold is preferably 5 to 20% by mass, particularly preferably 5 to 17% by mass, further preferably 5 to 13% by mass, and particularly preferably 5 to 9% by mass. If the moisture content is too high, the film tends to be less releasable from the casting mold, whereas if the moisture content is too low, the film tends to curl easily.

[0037] <Step (C)> Step (C) is a step of drying the formed film using multiple heated rolls. 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 the multiple heated rolls. The heated rolls are, for example, rolls with a diameter of 0.2 to 2 m 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.

[0038] The surface temperature of the heat roll is not particularly limited, but is usually preferably 30 to 150° C., more preferably 40 to 140° C. If the surface temperature is too low, drying tends to be insufficient, whereas if the surface temperature is too high, drying tends to be excessive, which tends to result in poor appearance such as undulation.

[0039] In the present invention, in step (C), when the entire drying process is defined as the time from when the formed film contacts the first heated roll to when it leaves the last heated roll, it is preferable that the temperature of at least one of the heated rolls with which the film comes into contact, within the first 30% of the number of heated rolls used in the entire drying process, is 100°C or higher, particularly preferably 103°C or higher, and even more preferably 105°C or higher. That is, when the number of heated rolls used in the entire drying process is, for example, 10, it is preferable that the temperature of at least one of the first to third heated rolls is 100°C or higher, particularly preferably 103°C or higher, and even more preferably 105°C or higher. In the present invention, particularly by combining the preferred range of the moisture content of the film when peeled from the cast mold with the drying conditions of the heated rolls, it becomes easier to set the ratio of the elastic modulus in the machine direction (MD) to the elastic modulus in the cross direction (TD) within a predetermined range when measuring the elastic modulus in water.

[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 (floating dryer). The upper limit of the heat treatment temperature is preferably 120°C or lower, particularly preferably 100°C or lower, even more preferably 80°C or lower, and especially preferably 60°C or lower. The lower limit is preferably 10°C or higher, particularly preferably 15°C or higher, even more preferably 20°C or higher, and especially preferably 25°C or higher. That is, the heat treatment temperature is, for example, preferably 10 to 120°C, more preferably 15 to 100°C, even more preferably 20 to 80°C, and especially preferably 25 to 60°C. If the heat treatment temperature is too high, the dyeability of the polarizing film during production tends to decrease, whereas 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 and, further, tends to increase the shrinkage force of the polarizing film after processing into a polarizing plate. The heat treatment time is preferably 20 to 100 seconds, and 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 film of the present invention thus obtained must be 50 μm or less, preferably 40 μm or less, particularly preferably 35 μm or less, and even more 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.

[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 applications, particularly 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 but also prevents color unevenness due to dyeing by swelling the polyvinyl alcohol-based film. In the swelling step, water is typically used as the treatment liquid. If the treatment liquid is primarily water, it may contain additives such as iodide compounds and surfactants, alcohol, etc. The temperature of the treatment liquid when the film is immersed is typically about 10 to 45°C, and the immersion time in the treatment liquid is typically about 0.1 to 10 minutes. In this specification, the term "major component" refers to a component that significantly affects the properties of the target object. The content of the component is typically 50% by mass or more, preferably 55% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and may even be 100% by mass.

[0049] The dyeing process is carried out by bringing the film into contact with a liquid containing iodine or a dichroic dye. Typically, an aqueous solution of iodine and potassium iodide is 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 liquid with which the film is brought into contact 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] Furthermore, the shrinkage force when the polyvinyl alcohol film of the present invention is processed into a polarizing film is preferably 8 N or less, more preferably 6 N or less, particularly preferably 5 N or less, and even more preferably 4.5 N or less. The lower limit is preferably as low as possible, and the shrinkage force may be 0 N or more. By controlling the shrinkage force within the above range, bending of the polarizing plate and cracking of the polarizing film can be suppressed, especially in a high-temperature environment, after the film is processed into a polarizing plate.

[0057] Next, a method for producing a polarizing plate including the polarizing film of the present invention will be described. The polarizing plate of the present invention comprises the polarizing film and a protective film on at least one surface of the polarizing film. That is, the polarizing film of the present invention is suitable for producing a polarizing plate with little color unevenness and excellent polarization performance.

[0058] [Method for manufacturing polarizing plate] The polarizing plate of the present invention 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 a film made of an acetyl cellulose-based resin such as triacetyl cellulose or diacetyl cellulose, a film made of a polyester-based resin such as polyethylene terephthalate, polyethylene naphthalate, or polybutylene terephthalate, a polycarbonate-based resin film, a cycloolefin-based resin film, an acrylic-based resin film, and a film made of a chain olefin-based resin such as a polypropylene-based resin.

[0059] 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.

[0060] Alternatively, a polarizing plate can be fabricated 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 and curing the resin to form a cured layer. In this way, the cured layer serves as a substitute for the protective film, thereby enabling a thinner polarizing plate to be fabricated.

[0061] The polarizing film and polarizing plate comprising 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.

[0062] 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 by mass unless otherwise specified.

[0063] <Measurement Conditions> [Measurement of Underwater Elastic Modulus] A test piece was cut from the obtained polyvinyl alcohol-based film at the center of the film width direction, measuring TD (5 mm) x MD (25 mm). The measurement width of the test piece was measured using a digital microscope. The thickness of the test piece was measured at three arbitrary points using a contact film thickness meter, and the average value was used as the measured thickness. Next, the test piece was fixed to a viscoelasticity measuring device (manufactured by IT Measurement Control Co., Ltd., "DVA-225") so that the gripping distance at both ends of the short side (TD side) was 15 mm. The measurement conditions were then set to a dynamic strain of 0.3%, a frequency of 10 Hz, and an upper limit elongation of 70%, and measurement of the elastic modulus was initiated. The elastic modulus was first measured in air for 100 seconds, and then measured in water at 30°C for 550 seconds. Regarding the measurement results, the average value of the elastic modulus for 200 to 550 seconds from the start of measurement in water at 30°C was calculated, and this was defined as the MD elastic modulus in water at 30°C (the same measurement was performed three times, and the average value was used). A test piece having a size of MD (5 mm) x TD (25 mm) was cut out from the center of the polyvinyl alcohol film in the film width direction, and the value measured according to the above method was defined as the TD elastic modulus.

[0064] [Measurement of limit stretch ratio] The obtained polyvinyl alcohol film was stretched 1.8 times while immersed in a water bath at a water temperature of 25 ° C. Next, it was stretched 2.3 times in a dyeing bath (28 ° C) containing 0.5 g / L of iodine and 30 g / L of potassium iodide. It was further stretched 3.2 times in a boric acid treatment bath (38 ° C) containing 11 g / L of boric acid and 24 g / L of potassium iodide. It was further immersed in a stretching bath (54 ° C) containing 30 g / L of boric acid and 42 g / L of potassium iodide, and uniaxially stretched at 1.5 mm / sec. The total stretch ratio relative to the raw film at the time of breakage was taken as the limit stretch ratio.

[0065] [Shrinkage Force Measurement] The resulting polyvinyl alcohol film was stretched 1.6 times while immersed in a water bath at 25°C, and then stretched 2.2 times in a dyeing bath (28°C) containing 0.8 g / L of iodine and 25 g / L of potassium iodide. The film was then stretched 2.6 times in a boric acid treatment bath (38°C) containing 10 g / L of boric acid and 30 g / L of potassium iodide. The film was further stretched 5.4 times in a stretching bath (56°C) containing 30 g / L of boric acid and 30 g / L of potassium iodide. The film was then immersed in a water bath at 30°C and a bath containing 80 g / L of potassium iodide (30°C), and then dried at 75°C to obtain a polarizing film. A test piece measuring TD (5 mm) x MD (50 mm) was cut from the resulting polarizing film. Next, the test piece was fixed to a dynamic mechanical analyzer (DMAQ800, manufactured by TA Instruments) so that the gripping distance at both ends of the short side (TD side) was 15 mm. After pulling with 0.001 N to remove slack from the sample, the temperature was raised from room temperature to 80°C at a rate of 20°C / min, and the tension after holding for 120 min was recorded as the contraction force.

[0066] Example 1: 500 kg of polyvinyl alcohol resin with a mass 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 resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously discharged and cast into a casting mold heated to 72°C using a T-shaped slit die, followed by drying. The moisture content of the resulting film immediately after peeling from the casting mold was 7% by mass. The resulting film was then dried using multiple heated rolls under the drying conditions listed in Table 1 (Step (C)). In this case, the entire drying process was defined as the time from when the film contacted the first heated roll to when it left the last heated roll. The process of drying using the heated rolls that comprised the first 30% of the number of heated rolls used in the entire drying process was defined as the "heat roll front stage," the process of drying using the heated rolls that comprised the first 40% of the number of heated rolls from the front stage of the heated rolls was defined as the "heat roll middle stage," and the process of drying using the heated rolls that comprised the last 30% of the number of heated rolls from the middle stage of the heated rolls (until the end) was defined as the "heat roll rear stage" (the same applies to the following examples and comparative examples). The heat roll temperatures in each stage of step (C) listed in Table 1 are the temperatures of the hottest heat rolls among the heated rolls provided in each stage. After step (C), the film was treated at 30°C using a floating dryer (step (D)). Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol film (thickness 20 μm). The resulting polyvinyl alcohol film was used to measure its underwater elastic modulus and critical stretch ratio. The evaluation results are shown in Table 1.

[0067] Example 2 A polyvinyl alcohol film was obtained under the same conditions as in Example 1, except that the temperature of the casting mold was changed to 66° C. The obtained polyvinyl alcohol film was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0068] Comparative Example 1: 500 kg of polyvinyl alcohol resin with a mass average molecular weight of 156,000 and a saponification degree 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 resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously extruded and cast into a casting mold heated to 66°C using a T-shaped slit die, followed by drying. The resulting film had a moisture content of 10% by mass immediately after peeling from the casting mold. The resulting film was then dried using multiple heated rolls under the drying conditions listed in Table 1 (step (C)) and then heat-treated at 130°C using a floating dryer (step (D)). Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol film (thickness 20 μm). The obtained polyvinyl alcohol film was subjected to measurement of the underwater elastic modulus and the limit stretch ratio. The evaluation results are shown in Table 1.

[0069] Comparative Example 2: 500 kg of polyvinyl alcohol resin with a mass average molecular weight of 156,000 and a saponification degree 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 resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously extruded and cast into a casting mold heated to 57°C using a T-shaped slit die, followed by drying. The resulting film had a moisture content of 10% by mass immediately after peeling from the casting mold. The resulting film was then dried using multiple heated rolls under the drying conditions listed in Table 1 (step (C)) and then heat-treated at 130°C using a floating dryer (step (D)). Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol film (thickness 20 μm). The obtained polyvinyl alcohol film was subjected to measurement of the underwater elastic modulus and the limit stretch ratio. The evaluation results are shown in Table 1.

[0070] Comparative Example 3: 500 kg of polyvinyl alcohol resin with a mass average molecular weight of 156,000 and a saponification degree 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 resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously extruded and cast into a casting mold heated to 48°C using a T-shaped slit die, followed by drying. The resulting film had a moisture content of 15% by mass immediately after peeling from the casting mold. The resulting film was then dried using multiple heated rolls under the drying conditions listed in Table 1 (step (C)) and then heat-treated at 130°C using a floating dryer (step (D)). Finally, both ends of the film were slit and wound up to obtain a roll-shaped polyvinyl alcohol film (thickness 20 μm). The obtained polyvinyl alcohol film was subjected to measurement of the underwater elastic modulus and the limit stretch ratio. The evaluation results are shown in Table 1.

[0071] Comparative Example 4: 500 kg of polyvinyl alcohol resin with a mass average molecular weight of 156,000 and a saponification degree 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 resin solution with a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously extruded and cast into a casting mold heated to 40°C using a T-shaped slit die, followed by drying. The resulting film had a moisture content of 15% by mass immediately after peeling from the casting mold. The resulting film was then dried using multiple heated rolls under the drying conditions listed in Table 1 (step (C)) and then heat-treated at 130°C using a floating dryer (step (D)). Finally, both ends of the film were cut off along slits and wound up to obtain a roll-shaped polyvinyl alcohol film (thickness 20 μm). The obtained polyvinyl alcohol film was subjected to measurement of the underwater elastic modulus and the limit stretch ratio. The evaluation results are shown in Table 1.

[0072]

[0073] The polyvinyl alcohol-based films of Examples 1 and 2, in which the ratio of the elastic modulus in the machine direction (MD) to the elastic modulus in the transverse direction (TD) (TD / MD) when measured in water at 30°C was within the range specified in the present invention, exhibited excellent stretch ratios and a good balance of the elastic modulus in water between the MD and TD directions even when thinned, and were thus able to produce polyvinyl alcohol-based films with excellent transportability and reduced shrinkage force after processing into a polarizing film. When this polarizing film was processed into a polarizing plate, the low shrinkage force prevented bending of the polarizing plate and cracking of the polarizing film, particularly in high-temperature environments, and was expected to provide excellent durability. On the other hand, the polyvinyl alcohol-based films of Comparative Examples 1 to 4 were found to be outside the range specified in the present invention and therefore exhibited poor stretchability and transportability.

[0074] 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.

[0075] A polarizing film having 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 50 μm, in which the ratio (TD / MD) of the elastic modulus in the machine direction (MD) to the elastic modulus in the cross direction (TD) when measured in water at 30°C is 0.95 or more.

2. The polyvinyl alcohol film according to claim 1, having a thickness of 40 μm or less.

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

4. A polarizing plate comprising the polarizing film according to claim 3 and a protective film provided on at least one surface of the polarizing film.

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