Polyvinyl alcohol film, polarizing film using same, polarizing plate, and method for producing polyvinyl alcohol film

Abstract

In order to provide: a polyvinyl alcohol film with which it is possible to efficiently produce a polarizing film in which wrinkles are less likely to occur during the production of the polarizing film, and which makes it possible to obtain a polarizing film having little optical unevenness; a polarizing film using the polyvinyl alcohol film; and a polarizing plate, the present invention provides a polyvinyl alcohol film having a film thickness of 45 μm or less, wherein, when the retardation in the film surface is measured at a pitch of 10 mm in the film width direction, the standard deviation (σ) calculated from all values of the measurement points is 4 nm or less.

Description

Polyvinyl alcohol film, polarizing film and polarizing plate using the same, and method for producing polyvinyl alcohol film

[0001] The present invention relates to a polyvinyl alcohol film, and more specifically to a polyvinyl alcohol film that is less likely to wrinkle during production, allows efficient production of a polarizing film with little optical unevenness, and relates to a method for producing the same, and a polarizing film and a polarizing plate using the polyvinyl alcohol film.

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

[0003] Under these circumstances, polarizing films with uniform optical properties in the width direction are required as the screens of liquid crystal televisions, multifunctional mobile terminals, etc. become brighter, more precise, larger in area, and thinner. Furthermore, in the production of polarizing films, the thinner films are likely to make handling more difficult, so there is a demand for polyvinyl alcohol films that can be easily stretched.

[0004] Generally, polyvinyl alcohol films are produced by a continuous casting method from an aqueous solution of polyvinyl alcohol resin. Specifically, the aqueous solution of polyvinyl alcohol resin is cast into a casting mold such as a casting drum or an endless belt, and the resulting film is peeled from the casting mold and then transported in the machine direction (MD) using nip rolls or the like while being dried using a heated roll or a floating dryer. During this transport process, the film is pulled in the machine direction (MD), so the polyvinyl alcohol polymer tends to be oriented in the MD.

[0005] On the other hand, polarizing films are produced by swelling a polyvinyl alcohol film, which serves as the base film, in water (including warm water), dyeing it with a dichroic dye such as iodine, and stretching it. In this stretching step, the dyed film is stretched in the machine direction (MD) to highly orient the dichroic dye in the film. In order to improve the polarization performance of the polarizing film, it is important that the polyvinyl alcohol film, which serves as the base film, exhibits good stretchability in the machine direction (MD) and does not break during the stretching step.

[0006] As an example of a polyvinyl alcohol film with improved stretchability and resistance to breakage, Patent Document 1 proposes "a polyvinyl alcohol film having a width of 3 m or more, a length of 1,000 m or more, and a thickness of 15 to 65 μm, in which the retardation values ​​measured at a longitudinal pitch of 15 m and a transverse pitch of 10 mm satisfy specific formulas (1) to (4)." Patent Document 2 also proposes a polyvinyl alcohol film having a width of 3 m or more produced by a casting method, characterized in that the retardation is 15 nm or less and the difference between the retardation at the end and the retardation at the center in the transverse direction is 5 nm or less.

[0007] Patent Document 1: WO2019 / 054487 Patent Document 2: JP2006-291173A

[0008] However, although Patent Document 1 improves general stretchability (resistance to breakage), it is insufficient because it does not mention the occurrence of wrinkles or uneven appearance in the width direction, which are problems when stretching a thin polyvinyl alcohol film. Furthermore, Patent Document 2 focuses on improving color unevenness of the film, but does not particularly pay attention to the effect on stretchability of the difference in retardation between the end portions and the center portion of the polyvinyl alcohol film in the width direction, and also uses a special manufacturing method in which a heat cycle treatment is performed during the production of the polyvinyl alcohol film.

[0009] Here, it is thought that thinned polyvinyl alcohol films are prone to partial stretching during the conveying process, leading to a state in which nonuniformity in retardation occurs in the direction perpendicular to the width direction. Furthermore, while thinning polyvinyl alcohol films allows for the production of thinner polarizing films, the resulting thin film is known to lead to poor running properties during the stretching process. In other words, polyvinyl alcohol films with uniform retardation in the width direction are uniformly oriented in the width direction, allowing for uniform stretching, which is expected to reduce nonuniformity in the direction perpendicular to the width direction. Furthermore, uniform orientation is thought to reduce variation in the degree of swelling of the polyvinyl alcohol film in the width direction, resulting in a smooth stretching process.

[0010] In view of this background, the present invention provides a polyvinyl alcohol film that is less likely to wrinkle during production, allows efficient production of a polarizing film, and allows for the production of a polarizing film with little optical unevenness, as well as a polarizing film and a polarizing plate using the polyvinyl alcohol film.

[0011] However, the present inventors have conducted extensive research in light of these circumstances and have found that by using a polyvinyl alcohol film with small variations in retardation in the width direction, even a thin, wide polyvinyl alcohol film is less likely to wrinkle during production of a polarizing film, allowing for efficient production of a polarizing film with little optical unevenness.

[0012] That is, the present invention has the following aspects. [1] A polyvinyl alcohol film having a film thickness of 45 μm or less, wherein when in-plane retardation is measured at 10 mm intervals in the film width direction, the standard deviation (σ) calculated from all measured retardation values ​​is 4 nm or less. [2] The polyvinyl alcohol film according to [1], having a film thickness of 35 μm or less. [3] A method for producing a polyvinyl alcohol film according to [1] or [2], comprising a film-forming step of continuously discharging and casting an aqueous solution of a polyvinyl alcohol resin into a casting mold to form a film, a drying step of drying the formed film, and a heat-treating step of heat-treating the dried film, wherein the film is formed in the film-forming step so that the value of parameter H represented by the following formula (1) is 16 or less. [4] A polarizing film obtained using the polyvinyl alcohol film according to [1] or [2]. [5] A polarizing plate comprising the polarizing film according to [4] and a protective film provided on at least one side of the polarizing film.

[0013] The polyvinyl alcohol film of the present invention can provide a polarizing film with little optical unevenness, and also can produce polarizing films with high yields since wrinkles are less likely to occur during production of the polarizing film.

[0014] The present invention will be described in detail below, but the present invention is not limited to the embodiments described below.

[0015] In the present invention, when the expression "x to y" (x and y are arbitrary numbers) is used, unless otherwise specified, it includes not only the meaning of "x or more and y or less," but also the meaning of "preferably greater than x" or "preferably less than y." Furthermore, when the expression "X or more" (X is an arbitrary number) or "Y or less" (Y is an arbitrary number) is used, it also includes the meaning of "preferably greater than X" or "preferably less than Y." Furthermore, in this specification, the term "film" also includes "tape" and "sheet." Furthermore, with regard to the numerical ranges described in stages in this specification, the upper or lower limit of a numerical range in one stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range can also be replaced with the values ​​shown in the examples.

[0016] When the in-plane retardation of the polyvinyl alcohol film of the present invention is measured at 10 mm intervals in the film width direction, the standard deviation (σ) calculated from all the values ​​at the measurement points is 4 nm or less.

[0017] When the in-plane retardation of the polyvinyl alcohol film of the present invention is measured at 10 mm intervals in the film width direction, the standard deviation (σ) calculated from the values ​​at all measurement points must be 4 nm or less, preferably 3 nm or less, particularly preferably 2.8 nm or less, and even more preferably 2.5 nm or less. Furthermore, when the in-plane retardation is measured at 10 mm intervals in the film width direction, the lower limit of the standard deviation (σ) calculated from the values ​​at all measurement points is usually 0.1 nm or more, preferably 0.2 nm or more, particularly preferably 0.3 nm or more, and even more preferably 1 nm or more. When the in-plane retardation is measured at 10 mm intervals in the film width direction, the standard deviation (σ) calculated from the values ​​at all measurement points is 4 nm or less, so that the polarizing film is less likely to wrinkle during production, allowing for efficient production of the polarizing film, and a polarizing film with little optical unevenness can be obtained from the polyvinyl alcohol film. In other words, when the retardation in the film plane is measured at 10 mm intervals in the film width direction, if the standard deviation (σ) calculated from the values ​​of all measurement points exceeds the upper limit, the film formation stability becomes significantly unstable and the effects of the present invention are not achieved.

[0018] The retardation in the present invention is measured by unwinding 50 mm from a polyvinyl alcohol film roll in the film flow direction, cutting out a measurement sample for the entire width in the direction perpendicular to the film flow direction, and measuring the retardation at 590 nm using a retardation measuring device. The measurement is performed sequentially from a position 10 mm from one end of the measurement sample in the width direction to the other end in the width direction at 10 mm intervals, and the standard deviation (σ) is calculated from the measured values.

[0019] When the retardation in the film plane is measured at 10 mm intervals in the film width direction, methods for controlling the standard deviation (σ) calculated from the values ​​of all measurement points within a predetermined range include, for example, a method of adjusting the molecular weight or degree of saponification of the polyvinyl alcohol resin used as a raw material, a method of washing the polyvinyl alcohol resin to improve purity, a method of adjusting the concentration of an aqueous polyvinyl alcohol resin solution, 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 state of the film during and after film formation.In the present invention, as a method for controlling the drying state in the film formation process when producing a polyvinyl alcohol film, a method of forming a film so that the value of parameter H represented by the following formula (1) is 16 or less, or a method of controlling the orientation state of the polymer, is preferred.

[0020] The maximum value of the in-plane retardation of the polyvinyl alcohol film of the present invention, measured at intervals of 10 mm in the width direction of the film, is preferably 80 nm or less, particularly preferably 70 nm or less, and further preferably 65 nm or less. When the maximum value is within the above range, the film has excellent stretchability in the stretching step during production of the polarizing film.

[0021] In the present invention, the minimum retardation value when the retardation in the plane of the polyvinyl alcohol film is measured at intervals of 10 mm in the width direction of the film is preferably 0 or more, particularly preferably 0.1 nm or more, further preferably 1 nm or more, particularly preferably 10 nm or more, and more preferably 20 nm or more. When the minimum value is within the above range, a polarizing film having an excellent, uniform appearance can be easily obtained when made into a polarizing film.

[0022] The average retardation value of the polyvinyl alcohol film of the present invention measured in the film plane at 10 mm intervals in the film width direction is preferably 1 to 60 nm, particularly preferably 2 to 55 nm, further preferably 3 to 50 nm, and particularly preferably 10 to 50 nm. When the average value is within the above range, a film that is uniform in the width direction and has few wrinkles is easily obtained.

[0023] 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.

[0024] The polyvinyl alcohol film of the present invention is preferably produced through the following steps (A) to (C), and preferably also through step (D) as necessary. Step (A) is a step of preparing an aqueous polyvinyl alcohol resin solution. Step (B) is a step of casting the aqueous polyvinyl alcohol resin solution into a cast mold to form a film (film-forming step). Step (C) is a step of heating and drying the formed film by contacting it with multiple heated rolls (drying step). Step (D) is a step of heat-treating the obtained film using hot air (heat-treating step).

[0025] <Step (A)> Step (A) is a step of preparing an aqueous polyvinyl alcohol resin solution. First, the polyvinyl alcohol resin and aqueous polyvinyl alcohol resin solution, 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.

[0026] Furthermore, a polyvinyl alcohol resin having a 1,2-diol structure in a side chain can also be used as the polyvinyl alcohol resin. Such a polyvinyl alcohol resin 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.

[0027] The weight-average molecular weight of the polyvinyl alcohol 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 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 film when a polarizing film is produced using the polyvinyl alcohol resin. The weight-average molecular weight of the polyvinyl alcohol resin is a weight-average molecular weight measured by GPC-MALS.

[0028] 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.

[0029] 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.

[0030] The polyvinyl alcohol resin is used to prepare an aqueous solution of polyvinyl alcohol resin, which will be used as a film-forming solution. The resin concentration of the aqueous solution of polyvinyl alcohol resin is usually 5 to 70% by weight, preferably 10 to 60% by weight.

[0031] 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.

[0032] The resin concentration of the aqueous polyvinyl alcohol resin solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and even more preferably 20 to 50% by weight. If the resin concentration of such an aqueous solution is too low, the drying load increases, which tends to reduce productivity, while if it is too high, the viscosity becomes too high, which tends to make it difficult to achieve uniform dissolution.

[0033] 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.

[0034] <Step (B)> In step (B), the aqueous polyvinyl alcohol resin solution is cast into a casting mold to form a film (film-forming step). After the degassing treatment, the aqueous polyvinyl alcohol resin solution 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.

[0035] 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.

[0036] The viscosity of the aqueous polyvinyl alcohol resin solution during extrusion 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.

[0037] 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.

[0038] In the present invention, it is preferable to carry out film formation in the film formation step so that the value of parameter H represented by the following formula (1) is 16 or less, particularly preferably 14 or less, and even more preferably 12 or less, in order to achieve stable film formation. The lower limit of parameter H is 3 or more, preferably 5 or more, and more preferably 7 or more. That is, the parameter H is preferably 3 to 16, more preferably 5 to 14, and even more preferably 7 to 12.

[0039] 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.

[0040] 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.

[0041] The rotation speed of the casting drum is preferably 3 to 50 m / min, particularly preferably 7 to 40 m / min, and further preferably 10 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.

[0042] The moisture content of the film immediately after peeling from the casting mold is preferably 18% or less, more preferably 17% or less, particularly preferably 16% or less, even more preferably 14% or less, and particularly preferably 12% or less. If the moisture content is too high, the film tends to be subjected to high tension during production of the polarizing film, making it more susceptible to breakage. Furthermore, the moisture content immediately after peeling is preferably 3% or more, particularly preferably 5% or more, and even more preferably 7% or more. If the moisture content is too low, the film tends to curl easily.

[0043] <Step (C)> Step (C) is a step of heating and drying the formed film (drying step). The film (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, 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.

[0044] 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.

[0045] The upper limit of the hot roll drying time is preferably 60 seconds or less, particularly preferably 55 seconds or less, even more preferably 50 seconds or less, and especially preferably 45 seconds or less. The lower limit is preferably 20 seconds or more, particularly preferably 25 seconds or more, even more preferably 27 seconds or more, and especially preferably 30 seconds or more. If the hot roll drying time is too long, the dyeability during production of the polarizing film tends to decrease, while if it is too fast, the film is likely to wrinkle or fold during the swelling step during production of the polarizing film, and the appearance of the polarizing film tends to deteriorate.

[0046] <Step (D)> Step (D) is a step of heat-treating the obtained film using hot air (heat treatment step). The film that has undergone step (C) may be heat-treated, for example, using a floating dryer. The upper limit of the heat treatment temperature is preferably 150°C or lower, particularly preferably 140°C or lower, even more preferably 135°C or lower, and especially preferably 130°C or lower. The lower limit is preferably 100°C or higher, particularly preferably 103°C or higher, even more preferably 105°C or higher, and especially preferably 110°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, resulting in a deterioration in the appearance of the polarizing film. The heat treatment time is preferably 20 to 100 seconds, particularly preferably 40 to 70 seconds.

[0047] [Polyvinyl Alcohol Film] Thus, a polyvinyl alcohol film is obtained through the above steps (A) to (D), and is finally wound up on a roll to become a finished product.

[0048] The upper limit of the thickness of the polyvinyl alcohol film of the present invention thus obtained must be 45 μm or less, preferably 35 μm or less, and more preferably 30 μm or less. From the viewpoint of production stability of the polarizing film, the lower limit is preferably 10 μm or more, particularly preferably 15 μm or more, and more preferably 20 μm or more. That is, the thickness of the polyvinyl alcohol film is preferably 10 to 45 μm, more preferably 15 to 35 μm, and even more preferably 20 to 30 μm.

[0049] The length of the polyvinyl alcohol film is preferably 4 km or more in terms of increasing the area of ​​the polarizing film, and particularly preferably 5 to 50 km in terms of transport weight.

[0050] 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.

[0051] Furthermore, the polyvinyl alcohol film of the present invention preferably has a ratio (E / C) of the weight swelling degree of the film measured at the widthwise ends (E) to the weight swelling degree of the film measured at the center (C) of 0.995 to 1.005, more preferably 0.996 to 1.004, particularly preferably 0.997 to 1.003, and even more preferably 0.998 to 1.002. By setting the ratio of the weight swelling degrees of the film at the widthwise ends to the center within the above range, it is possible to further suppress variation in swelling of the polyvinyl alcohol film in the width direction, achieve more uniform orientation, and further facilitate the stretching process.

[0052] The widthwise ends of the film are positions 400 mm inward (towards the center) from the widthwise ends of the film (two positions at both ends in the widthwise direction). The center of the film is a position on a line connecting both widthwise ends of the film and equidistant from both ends. Therefore, the weight swelling degree (E) of the film measured at the widthwise ends of the film and the weight swelling degree (C) of the film measured at the center of the film are measured by taking samples from each of the above positions. The film weight swelling degree (E) at the widthwise ends of the film is the average of the values ​​at the two widthwise ends.

[0053] The weight swelling degree can be calculated as follows. First, a 10 cm x 10 cm piece of film is cut out from the center of the position where the sample is to be obtained, and the film is immersed in an ion-exchange water tank adjusted to 30°C for 15 minutes. Next, the film is taken out, spread out on a filter paper (5A), and a separately prepared filter paper (5A) is placed on the film. Further, a 15 cm x 15 cm x 0.4 cm piece (density 4.4 g / cm) is placed on the filter paper (5A). ２) is placed on the film for 5 seconds, and the water adhering to the film surface is absorbed and removed by the filter paper (5A). The film from which the adhering water has been removed is quickly placed in a weighing bottle, and the film weight is measured, and this film weight is defined as the film weight A when swollen. This series of operations up to this point is performed in an environment of 23°C and 50% RH. Then, the film from which the film weight A has been measured is placed in a dryer at 105°C and left to stand for 16 hours to remove the water content in the film. The film from which the water content has been removed is quickly placed in a weighing bottle, and the film weight is measured, and this film weight is defined as the film weight B after drying. The weight swelling degree (%) can be calculated from the film weight A when swollen and the film weight B after drying according to the following formula: Weight swelling degree (%) = A / B x 100

[0054] The polyvinyl alcohol 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 film will be described.

[0055] [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.

[0056] The swelling step is carried out before the dyeing step. The swelling step not only cleans the surface of the polyvinyl alcohol film but also prevents uneven dyeing by swelling the polyvinyl alcohol 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 treatment liquid is usually about 10 to 45°C, and the immersion time in the treatment liquid is usually about 0.1 to 10 minutes.

[0057] 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 approximately 30 to 500 seconds. The temperature of the aqueous solution is preferably 5 to 50°C. The aqueous solution may contain a small amount of a water-compatible organic solvent in addition to the aqueous solvent.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] The drying step is carried out, for example, using a dryer at 40 to 100° C. for 0.1 to 10 minutes.

[0062] 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 (2): １１ -H １ ) / (H １１ +H １ ) １／２ ...(2)

[0063] 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.

[0064] 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.

[0065] [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.

[0066] The protective film can be attached to the polarizing film by a known method, for example, by uniformly applying a liquid adhesive composition to the polarizing film, the protective film, or both of them, and then attaching and pressing them together, followed by heating or irradiating them with active energy rays.

[0067] 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 film to be obtained.

[0068] The polarizing film and polarizing plate obtained using the polyvinyl alcohol 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, 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 layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, etc.

[0069] 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 weight.

[0070] Example 1 (Production of Polyvinyl Alcohol Film) 1,000 kg of polyvinyl alcohol resin with a weight average molecular weight of 156,000 and a saponification degree of 99.8 mol%, 3,000 kg of water, 115 kg of glycerin as a plasticizer, and 0.25 kg of sodium dodecyl sulfonate as a surfactant were placed in a 5,000 L dissolver, and the mixture was heated to 140°C with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution adjusted to a resin concentration of 26%. The aqueous polyvinyl alcohol resin solution was continuously extruded and cast into a casting mold through a T-shaped slit die, and dried to a parameter H calculated by the following formula (1) of 10.4. Hot air was then blown against both sides of the drying roll and the film to produce a polyvinyl alcohol film (thickness 30 μm, film width 3.3 m) with a moisture content of 1% or less.

[0071] (Preparation of Polarizing Film) The resulting polyvinyl alcohol film was unwound from the roll and, while conveying horizontally, immersed in a water bath at 25°C to swell and stretched in the machine direction (MD) to 1.6 times its original size. During this swelling process, no folds or wrinkles were observed in the film. The amount of iodine was then adjusted so that the final polarizing film would have a single-unit transmittance of 43.5%. The film was then immersed in an aqueous solution (28°C) containing 30 g / L of potassium iodide, dyed, and stretched in the machine direction (MD) to 2.3 times its original size. The film was then immersed in an aqueous solution (59°C) containing 30 g / L of boric acid and 30 g / L of potassium iodide, and uniaxially stretched in the machine direction (MD) to 5.9 times its original size while crosslinking with boric acid. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C for 40 seconds to obtain a polarizing film.

[0072] Example 2 A polyvinyl alcohol film (thickness: 30 μm, film width: 3.3 m) was obtained in the same manner as in Example 1, except that the parameter H was produced to have the value shown in Table 1. A polarizing film was also obtained in the same manner as in Example 1.

[0073] Example 3 (Production of Polyvinyl Alcohol Film) 1,000 kg of polyvinyl alcohol resin with a weight average molecular weight of 123,000 and a saponification degree of 99.8 mol%, 3,000 kg of water, 115 kg of glycerin as a plasticizer, and 0.25 kg of sodium dodecyl sulfonate as a surfactant were placed in a 5,000 L dissolver, and the mixture was heated to 140 °C with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution adjusted to a resin concentration of 29%. The aqueous polyvinyl alcohol resin solution was continuously discharged and cast into a casting mold through a T-shaped slit die, and dried to a parameter H of 15.1. Hot air was then blown against both sides of the drying roll and the film to produce a polyvinyl alcohol film (thickness 30 μm, film width 3.1 m) with a moisture content of 1% or less.

[0074] (Preparation of Polarizing Film) The resulting polyvinyl alcohol film was unwound from the roll and, while conveying horizontally, immersed in a water bath at 25°C to swell and stretched in the machine direction (MD) to 1.6 times its original size. During this swelling process, no folds or wrinkles were observed in the film. The amount of iodine was then adjusted so that the final polarizing film would have a single-piece transmittance of 43.5%. The film was then immersed in an aqueous solution (28°C) containing 30 g / L of potassium iodide, dyed, and stretched in the machine direction (MD) to 2.3 times its original size. The film was then immersed in an aqueous solution (57°C) containing 30 g / L of boric acid and 30 g / L of potassium iodide, and uniaxially stretched in the machine direction (MD) to 5.9 times its original size while crosslinking with boric acid. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C for 40 seconds to obtain a polarizing film.

[0075] Comparative Example 1 A polyvinyl alcohol film (thickness: 30 μm, film width: 3.3 m) was obtained in the same manner as in Example 1, except that the parameter H was produced to have the value shown in Table 1. A polarizing film was also obtained in the same manner as in Example 1.

[0076] Comparative Example 2 A polyvinyl alcohol film (thickness: 30 μm, film width: 3.1 m) was obtained in the same manner as in Example 3, except that the parameter H was produced to have the value shown in Table 1. A polarizing film was also obtained in the same manner as in Example 1.

[0077] The properties (retardation values, parameter H shown in formula (1)) of the polyvinyl alcohol films obtained in Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 1. The standard deviation σ (nm) of retardation was calculated based on the following measurement conditions.

[0078] <Standard deviation σ (nm) of retardation> The retardation of the obtained polyvinyl alcohol film was measured across the entire width at 10 mm intervals in the film width direction, and the standard deviation (σ) was calculated from the obtained total retardation value. Specifically, after the drying process, the edge portions were cut off, and then the polyvinyl alcohol film roll was unwound by 50 mm in the flow direction of the film and cut across the entire width in the perpendicular direction to prepare a measurement sample (50 mm × total width mm). The measurement sample was subjected to retardation measurement at 590 nm using a retardation measurement device (KOBRA-WFD0 (manufactured by Oji Scientific Instruments)). The retardation measurement was performed sequentially from a position 10 mm inward (toward the center) from one width direction end of the measurement sample toward the other width direction end at 10 mm intervals, and the standard deviation (σ) was calculated from the obtained total retardation value.

[0079] <Weight swelling degree> Samples are taken from the end (400 mm from the end of the film) and the center (center of the film width) on the same line in the width direction of the film, and the weight swelling degree of each is measured. Here, the weight swelling degree of the center of the film is designated as C. The weight swelling degree E of the end of the film is designated as the average value of the weight swelling degrees at both ends in the width direction of the film.

[0080] Specifically, the sample was cut into a size of 10 cm x 10 cm and immersed in an ion-exchange water bath adjusted to 30° C. for 15 minutes. Next, the film was taken out, spread on a filter paper (5A), and the filter paper (5A) was placed on the film. A 15 cm x 15 cm x 0.4 cm (4.4 g / cm) sample was then placed on top of the film. ２ ) is placed on the film for 5 seconds, and the water adhering to the film surface is absorbed and removed by the filter paper (5A). The film is quickly placed in a weighing bottle, and its weight is measured, which is defined as the film weight A when swollen. The above operation is carried out in an environment of 23°C and 50% RH. The film is then left to stand in a dryer at 105°C for 16 hours to remove the water in the film, after which the film is taken out and quickly placed in a weighing bottle, and its weight is measured, which is defined as the film weight B after drying. The weight swelling degree can be calculated from the obtained film weight A when swollen and film weight B after drying using the following formula: Weight swelling degree (%) = A / B x 100

[0081] The weight swelling ratios were determined based on the above-mentioned measurement conditions and calculation method, and the weight swelling ratios (E / C) were calculated. The results are shown in Table 1. The polarizing films obtained in Examples 1 to 3 and Comparative Examples 1 and 2 were also evaluated for optical unevenness and wrinkles during production of the polarizing films. The results are also shown in Table 1.

[0082] <Optical unevenness> Two polarizing plates were placed parallel to each other on a backlight, and the prepared polarizing film was then placed between the two polarizing plates. The appearance of the polarizing film was visually observed in the perpendicular direction and evaluated based on the following criteria: (Evaluation criteria) ○ (very good): There was almost no difference in brightness between light and dark within the polarizing film. △ (good): There was slight difference in brightness between light and dark within the polarizing film. × (poor): There was a large difference in brightness between light and dark within the polarizing film.

[0083] <Wrinkles during polarizing film production> The resulting polyvinyl alcohol film was unwound from a 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 (based on the original web) (swelling step). The amount of iodine was then adjusted so that the final polarizing film would have a single-piece transmittance of 43.5%. The film was then immersed in an aqueous solution (28°C) containing 30 g / L of potassium iodide to dye it and stretched in the machine direction (MD) to 2.3 times its original size (based on the original web). It was then immersed in an aqueous solution (59°C) containing 30 g / L of boric acid and 30 g / L of potassium iodide to crosslink it with boric acid and stretched uniaxially in the machine direction (MD) to 5.9 times its original size (based on the original web). Finally, the film was washed with the potassium iodide aqueous solution and dried at 80°C for 40 seconds to obtain a polarizing film. Among the series of steps in the production of a polarizing film, the appearance of the polyvinyl alcohol film running on a guide roll installed above the outlet of the water tank used in the swelling step was visually observed and evaluated based on the following criteria: (Evaluation criteria) ○ (very good): No wrinkles. △ (good): Several wrinkles were observed. × (poor): Many wrinkles were observed.

[0084]

[0085] The results shown in Table 1 show that when the in-plane retardation of the polyvinyl alcohol film was measured at 10 mm intervals in the film width direction, the polyvinyl alcohol films of Examples 1 to 3 had standard deviations (σ) calculated from all the values ​​at the measurement points that were within the range specified in the present invention, making it possible to efficiently produce polarizing films that are less likely to wrinkle during production and have less optical unevenness.On the other hand, when the in-plane retardation of the polyvinyl alcohol films of Comparative Examples 1 and 2 was measured at 10 mm intervals in the film width direction, the standard deviations (σ) calculated from all the values ​​at the measurement points were outside the range specified in the present invention, confirming the occurrence of optical unevenness and wrinkles in the polarizing film.

[0086] 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.

[0087] A polarizing film using the polyvinyl alcohol film obtained by the production method of the present invention has excellent polarization performance, and is preferably used for liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, 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 film having a film thickness of 45 μm or less, in which, when the retardation in the film plane is measured at 10 mm intervals in the film width direction, the standard deviation (σ) calculated from all measured retardation values ​​is 4 nm or less.

2. The polyvinyl alcohol film according to claim 1, having a thickness of 35 μm or less.

3. A method for producing a polyvinyl alcohol film according to claim 1 or 2, comprising a film-forming step of continuously discharging and casting an aqueous solution of polyvinyl alcohol resin into a cast mold to form a film, a drying step of drying the formed film, and a heat treatment step of heat treating the dried film, wherein the film-forming step is carried out so that the value of parameter H, represented by the following formula (1), is 16 or less.

4. A polarizing film obtained by using 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 side of the polarizing film.

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