Polyvinyl alcohol film and polarizing film

Abstract

The following is provided as a polyvinyl alcohol film in which the elution amount of a polyvinyl alcohol resin into water is small during polarizing film manufacturing, contamination of a chemical tank during polarizing film manufacturing is suppressed, a polarizing film with few foreign matter defects can be obtained with good productivity, and the film is excellent in drawing processability during polarizing film manufacturing.　A polyvinyl alcohol film satisfying formula (1) in a section from a surface layer of each film surface to a depth of 1 μm on both surfaces of the polyvinyl alcohol film, and having a weight swelling rate of 185%-210%.　(1): | K | ≥ 3.5 × 10-5 (where | K | is an absolute value of the ratio (inclination) of the change in the magnitude of birefringence from the surface layer to a depth of 1 μm).

Description

Polyvinyl alcohol film, polarizing film

[0001] The present invention relates to a polyvinyl alcohol-based film, and more specifically to a polyvinyl alcohol-based film that causes little elution of impurities into water during the production of a polarizing film, suppresses contamination of a chemical tank during the production of a polarizing plate, and further has excellent stretchability during the production of a polarizing film, allowing polarizing films to be obtained with high productivity.

[0002] Liquid crystal display devices have developed remarkably in recent years and are now widely used in smartphones, tablets, personal computers, LCD televisions, projectors, in-vehicle panels, etc. Such liquid crystal display devices use polarizing films, which are mainly polyvinyl alcohol-based films with iodine adsorbed and aligned. With the recent trend toward higher resolution, higher brightness, larger size, and thinner screens, there is a demand for polarizing films that are wider and longer than conventional products, have better polarization performance, are free of color unevenness, and are available in a wide and long size.

[0003] Polarizing films are typically produced by unwinding a raw polyvinyl alcohol film from a roll and transporting it in the machine direction (MD). The film is then swollen in water (including warm water), followed by dyeing with iodine, stretching to orient the iodine, and boric acid crosslinking to fix the orientation. Problems that arise during these processes significantly reduce the productivity of polarizing films. For example, if impurities leach out of the polyvinyl alcohol film during the swelling process and contaminate the swelling tank, the contamination spreads throughout subsequent processes. Also, if impurities leach out of the polyvinyl alcohol film during the dyeing and boric acid crosslinking processes, not only will the polarization performance of the resulting polarizing film deteriorate, but the filtration and replacement of chemical solutions used in each process will require significant effort. Examples of such impurities include low-molecular-weight polyvinyl alcohol resins (including oligomers) present in the polyvinyl alcohol film. Low-molecular-weight resins with a molecular weight of 50,000 or less are particularly prone to leach out in water and to form low-molecular-weight iodine complexes that reduce the polarization degree. Furthermore, the eluted low-molecular-weight polyvinyl alcohol resin can form associations with iodine or boric acid during the polarizing film manufacturing process, resulting in foreign matter. These foreign matter adhere to the surface of the polarizing film during processing, causing foreign matter defects in the final polarizing film.

[0004] On the other hand, a polyvinyl alcohol-based film, which is a raw material, is generally produced by dissolving a polyvinyl alcohol-based resin as a raw material in water and continuously casting the resulting aqueous solution (film-forming solution). Specifically, the aqueous solution of the polyvinyl alcohol-based resin is discharged and cast onto a casting mold such as a casting drum or an endless belt to form a film, and the resulting film is peeled from the casting mold and then dried and heat-treated while being transported in the flow direction (MD direction). However, if the type of polyvinyl alcohol-based resin as a raw material is not selected or the production conditions during the production process are not appropriately adjusted, the amount of polyvinyl alcohol-based resin eluted during the production of a polarizing film tends to increase.

[0005] As methods for improving the above-mentioned problems, for example, a method for obtaining a polyvinyl alcohol-based film with a reduced amount of elution by using a vinyl alcohol-based polymer having a viscosity-average degree of polymerization (P) of 4000 to 8000, a degree of saponification of 99.50 to 99.97 mol%, and a content of 1,2-glycol bond units within a specific range, and which shows specific thickening properties and absorbance when dissolved in an aqueous solution (see Patent Document 1), and a method for obtaining a polyvinyl alcohol-based film with a reduced amount of elution by focusing on the temperature conditions of a hot roll during the drying step in the production of a polyvinyl alcohol-based film (see Patent Document 2), etc. have been proposed.

[0006] JP 2009-221462 A

[0007] However, in the technique disclosed in the above Patent Document 1, when a 10 cm square film is immersed in 1 liter of water at 50° C. for 4 hours, the concentration of the polyvinyl alcohol resin eluate is 10 to 50 ppm. 2 This translates to 1000 to 5000 ppm / m 2 Therefore, in order to manufacture polarizing films with high production efficiency in response to the recent demand for polarizing films, it has been necessary to further reduce the amount of resin elution. Furthermore, the technology disclosed in Patent Document 2 mentioned above has a polyvinyl alcohol-based resin elution amount of 900 ppm / m when immersed in water at 50° C. for 1 minute. 2 As will be described below, in response to the recent trend toward higher transmittance and higher polarization degree of polarizing films, it has become necessary to further reduce the amount of resin elution in order to produce polarizing films at a high stretching ratio, which is a condition under which polyvinyl alcohol-based resins are more likely to elute.

[0008] Under these circumstances, the present invention provides a polyvinyl alcohol-based film that reduces the amount of polyvinyl alcohol-based resin eluted into water during production of a polarizing film, suppresses contamination of chemical tanks during production of a polarizing film, and enables production of polarizing films with few foreign matter defects with good productivity, and further has excellent stretchability during production of a polarizing film.

[0009] However, the present inventors have focused on the rate of change in birefringence magnitude in a section 1 μm deep from the surface of a polyvinyl alcohol-based film and the weight swelling degree of the film, and have found that when these values ​​are within a specific range, the above-mentioned problems can be solved and a polarizing film with few foreign matter defects can be produced with high productivity.

[0010] That is, the gist of the present invention is the following [1] to [4]. [1] A polyvinyl alcohol-based film, in which the following formula (1) is satisfied in a section of both surfaces of the polyvinyl alcohol-based film at a depth of 1 μm from the surface layer of each of the film surfaces, and the weight swelling degree of the polyvinyl alcohol-based film is 185% or more and 210% or less. |K| ≧ 3.5 × 10 -5 ... (1) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm.) [2] The polyvinyl alcohol-based film according to [1], wherein at least one film surface of the polyvinyl alcohol-based film satisfies the following formula (2) in a section from the surface of the film surface to a depth of 1 μm: |K|≧5×10 -5 ... (2) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm.) [3] The polyvinyl alcohol-based film according to [1] or [2], having a thickness of 10 to 70 μm. [4] A polarizing film using the polyvinyl alcohol-based film according to any one of [1] to [3].

[0011] The polyvinyl alcohol film of the present invention focuses on the rate of change in the magnitude of birefringence within 1 μm of the surface layer of the polyvinyl alcohol film and the weight swelling degree of the film, and by setting these values ​​within specific ranges, crystals on the surface of the film can be sufficiently grown, thereby reducing the amount of polyvinyl alcohol resin eluted from the film surface into chemical solutions during polarizing film production. This suppresses contamination of chemical solution tanks during polarizing film production, allowing for highly productive production of polarizing films with few foreign matter defects. Furthermore, the film has a good balance between the crystalline content and the amorphous content, and when swollen in water, the film has a hardness suitable for stretching, making it less likely to break. Therefore, the film is useful as a raw film that also has excellent stretchability during polarizing film production.

[0012] The present invention will be described in detail below, but the present invention is not limited to the following embodiments.

[0013] As used herein, "x and / or y (x and y are any configuration)" refers to at least one of x and y, and can mean three things: x only, y only, or x and y. In this specification, when "X to Y" (X and Y are any numbers) is used, unless otherwise specified, it also means "X or more and Y or less," as well as "preferably greater than X" or "preferably less than Y." In this specification, when "X or more" (X is any number) or "Y or less" (Y is any number) is used, it also means "preferably greater than X" or "preferably less than Y." For numerical ranges described in stages in this specification, 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 the values ​​shown in the examples.

[0014] In this specification, the term "film" also includes "tape" and "sheet." In this specification, the term "main component" refers to a component that has a significant effect on the properties of the target object, and the content of the component is usually 50% by mass or more, preferably 55% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, and may be 100% by mass.

[0015] A polyvinyl alcohol film according to one embodiment of the present invention (hereinafter sometimes simply referred to as a "polyvinyl alcohol film") is a polyvinyl alcohol film that satisfies the following formula (1) in sections of 1 μm depth from the surface layer on both surfaces of the polyvinyl alcohol film: |K|≧3.5×10 -5 ... (1) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm.)

[0016] Here, in this embodiment, the "absolute value (|K|) of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm" is a value measured by the following method. (Measurement Method) A strip measuring MD×TD = 5 mm × 10 mm is cut from the center of the polyvinyl alcohol film in the width direction (TD) at an arbitrary position in the machine direction (MD) of the polyvinyl alcohol film. Then, both sides of the strip are sandwiched between 100 μm-thick PET films, which are then sandwiched between a microtome measurement jig and attached to a microtome device. Next, the strip cut out above is sliced ​​at 10 μm intervals parallel to the machine direction (MD) of the strip to prepare slices (MD×TD = 5 mm × 10 μm) for observation. Next, the slice was placed on a glass slide with the slice surface facing up so that the slice surface could be observed, and sealed with a cover glass and tricresyl phosphate (refractive index 1.557). Retardation was measured using a two-dimensional photoelastic evaluation system "PA-micro" (manufactured by Photonic Lattice). With the retardation distribution of the slice displayed on the measurement screen of the "PA-micro," a line segment X perpendicular to the original surface of the polyvinyl alcohol film was drawn across the slice from one surface to the other, and line analysis was performed on this line segment X to obtain retardation distribution data in the thickness direction of the slice. Note that observation was performed using a 40x objective lens, and the average retardation value was adopted with a line width of 3 pixels. The obtained retardation distribution data in the thickness direction of the slice was divided by the slice thickness of 10 μm to obtain the distribution of birefringence Δn in the thickness direction of the slice. Each birefringence distribution was aligned to 1,000 points using cubic spline interpolation. The rate of change (slope) of the magnitude of birefringence from the film surface in contact with the cast surface and the film surface not in contact with the cast mold to a point corresponding to 1 μm from each of the retardation distribution data aligned to 1,000 points was calculated by the least squares method, and the absolute value of each calculated slope was taken.

[0017] For both film surfaces of the polyvinyl alcohol film, the absolute value (|K|) of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm was 3.5×10-5 It is necessary that the density is equal to or greater than 3.7×10 -5 More preferably, 4×10 -5 More preferably, 4.5×10 -5 More preferably, 5×10 -5 When the absolute value of the rate of change (slope) of the magnitude of birefringence from the surface to a depth of 1 μm on both film surfaces of the polyvinyl alcohol film satisfies the above range, a polyvinyl alcohol film with a small amount of elution can be obtained. The upper limit of this value is usually 4×10 -4 Below 3 × 10, preferably -4 Particularly preferably, 2 × 10 -4 The above range is preferable in terms of excellent stretchability and dyeability during the production of the polarizing film.

[0018] The polyvinyl alcohol film must have a weight swelling degree of 185% or more and 210% or less, and the lower limit of the weight swelling degree is preferably 188% or more, particularly preferably 190% or more, even more preferably 193% or more, and especially preferably 195% or more. The upper limit of the weight swelling degree is preferably 208% or less, particularly preferably 206% or less, even more preferably 204% or less, and especially preferably 199% or less. By setting the weight swelling degree of the polyvinyl alcohol film within the above range, the crystalline content and amorphous content are well balanced, and the film has a hardness suitable for stretching when swollen in water, making the film less likely to break, and thus a polyvinyl alcohol film with excellent stretch processability during the production of a polarizing film can be obtained.

[0019] Here, the weight swelling degree in this embodiment is a value measured by the following method. (Measurement Method) A film is cut into a size of 10 cm x 10 cm and immersed in an ion-exchange water tank adjusted to 30°C for 15 minutes. Next, the film is taken out, spread on a filter paper (5A), and the filter paper (5A) is placed on top of the film. A 15 cm x 15 cm x 0.4 cm (4.4 g / cm) piece of the film is placed on top of the filter paper (5A). 2) SUS plate is placed on the film for 5 seconds to remove any adhering water on the film surface. 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. Next, the film is 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 degree of weight swelling is then calculated using the following formula based on the film weight A when swollen and the film weight B after drying: Weight swelling degree (%) = A / B x 100

[0020] For both film surfaces of such a polyvinyl alcohol-based film, examples of methods for controlling the absolute value (|K|) of the rate of change (slope) of the magnitude of birefringence from the surface to a depth of 1 μm and the weight swelling degree within specific ranges include a method for controlling the degree of polymerization of the polyvinyl alcohol-based resin, a method for controlling the concentration of the aqueous solution of the polyvinyl alcohol-based resin in the process of discharging and casting the aqueous solution of the polyvinyl alcohol-based resin into a casting mold during film formation, a method for controlling the temperature and speed of the casting mold, a method for controlling the temperature and air volume of drying with hot air from the film surface side that does not contact the casting mold, a method for controlling the moisture content of the film after hot roll drying during film production, a method for controlling the tension in the MD direction and TD direction during hot roll drying during film production, a method for controlling the heat treatment temperature during film production, and a method for controlling the film conveyance speed.These values ​​can be controlled within specific ranges by using these methods alone or in combination of two or more.

[0021] Among these, in the step of forming a film by discharging and casting an aqueous solution of a polyvinyl alcohol-based resin into a casting mold, a method of controlling the temperature and speed of the casting mold, a method of controlling the temperature and air volume of hot air drying from the side of the film surface that is not in contact with the casting mold, a method of controlling the moisture content of the film after hot roll drying during film production, a method of controlling the tension in the MD and TD directions during hot roll drying during film production, and a method of controlling the heat treatment temperature during film production are preferred, and in particular, a method of controlling the heat treatment temperature during film production and a method of controlling the moisture content of the film after hot roll drying during film production are even more preferred.

[0022] Furthermore, it is preferable that at least one film surface of the polyvinyl alcohol-based film satisfies the following formula (2) in a section of 1 μm depth from the surface layer of the film surface, and more preferably |K|≧5.5×10 -5 , more preferably |K|≧6×10 -5 , particularly preferably |K|≧7×10 -5 When the absolute value of the rate of change (slope) of the magnitude of birefringence from the surface to a depth of 1 μm on at least one film surface of the polyvinyl alcohol-based film satisfies the above range, a polyvinyl alcohol-based film with a smaller amount of elution tends to be obtained. |K|≧5×10 -5 ... (2) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm.)

[0023] 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 solution of a polyvinyl alcohol-based resin. Step (B) A step of casting the aqueous solution of the polyvinyl alcohol-based resin into a casting mold to form a film. Step (C) A step of drying the formed film. Step (D) A step of heat-treating the obtained film using hot air.

[0024] <Step (A)> Step (A) is a step of preparing an aqueous polyvinyl alcohol resin solution (solution preparation step). First, the polyvinyl alcohol resin and aqueous polyvinyl alcohol resin solution, which are materials for the polyvinyl alcohol film, will be described. In this embodiment, 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. In addition, 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.

[0025] 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.

[0026] The weight-average molecular weight of the polyvinyl alcohol-based resin is preferably 80,000 to 300,000, particularly preferably 90,000 to 280,000, and even more preferably 100,000 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.

[0027] The polyvinyl alcohol-based resin preferably has a polydispersity (weight average molecular weight / number average molecular weight) of 1.85 to 2.30, particularly preferably 1.90 to 2.20, and even more preferably 1.95 to 2.10. If the polydispersity is too small, stretching tends to be difficult when a polarizing film is produced using the polyvinyl alcohol-based film, while if the polyvinyl alcohol-based resin is too large, sufficient optical performance tends to be difficult to obtain when the polyvinyl alcohol-based resin is used as an optical film. The weight average molecular weight and number average molecular weight when measuring the polydispersity of the polyvinyl alcohol-based resin are weight average molecular weights and number average molecular weights measured by GPC-MALS.

[0028] The average saponification degree of the polyvinyl alcohol resin is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.3 mol% or more, and particularly preferably 99.6 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 this embodiment is measured in accordance with JIS K 6726.

[0029] As the polyvinyl alcohol-based resin, 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] In order to obtain a desired molecular weight distribution, the polyvinyl alcohol resin is preferably obtained by controlled radical polymerization using a control agent added during the polymerization of vinyl acetate. The control agent added during such polymerization is not particularly limited, but from the viewpoint of controllability, it is preferable to use an organic cobalt complex as the control agent.

[0031] The polyvinyl alcohol-based resin is used to prepare an aqueous polyvinyl alcohol-based resin solution, which serves as a film-forming solution. The resin concentration of the aqueous polyvinyl alcohol-based resin solution is usually 5 to 70% by mass, and preferably 10 to 60% by mass.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] <Step (B)> Step (B) is a step (film-forming 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 discharged and cast onto a rotating casting drum to form a film by a continuous casting method.

[0036] The resin temperature of the aqueous polyvinyl alcohol resin solution at the outlet of the T-shaped slit die is preferably 80 to 100° C., particularly preferably 85 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.

[0037] 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.

[0038] 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.

[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 4 m or more, particularly preferably 4.5 m or more, further preferably 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 surface temperature of the casting drum is preferably 40 to 99° C., and particularly preferably 60 to 95° C. If the surface temperature is too low, drying tends to be insufficient, and if it is too high, foaming tends to occur.

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

[0044] The film (the film formed above) peeled from the casting drum is transported in the machine direction (MD) 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.

[0045] The surface temperature of the heat roll is not particularly limited, but is usually preferably 50 to 150° C., more preferably 60 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.

[0046] The moisture content of the film immediately after the drying process using such a heated roll is preferably 0.5% or more in terms of transportability and storage stability of the polyvinyl alcohol film and processability during production of a polarizing plate, and is particularly preferably 1% or more, further preferably 2% or more, particularly preferably 3% or more, and more preferably 4% or more. The upper limit is preferably 10% or less, particularly preferably 9% or less, further preferably 8% or less, and particularly preferably 7% or less.

[0047] <Step (D)> Step (D) is a step of heat-treating the obtained film using hot air (heat treatment step). The film that has been subjected to step (C) may be heat-treated, for example, using a floating dryer. The upper limit of the heat treatment temperature is preferably 200°C or lower, particularly preferably 180°C or lower, even more preferably 160°C or lower, and especially preferably 140°C or lower. The lower limit is preferably 50°C or higher, particularly preferably 80°C or higher, even more preferably 100°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, which tends to deteriorate the appearance of the polarizing film. The heat treatment time is preferably 20 to 100 seconds, particularly preferably 40 to 70 seconds.

[0048] [Polyvinyl Alcohol Film] Thus, a polyvinyl alcohol film is obtained through the above steps (A) to (C) and, if necessary, the above step (D), and is finally wound up into a roll to become a finished product.

[0049] The thickness (μm) of the polyvinyl alcohol-based film is preferably 10 μm or more in terms of production stability of the polarizing film, particularly preferably 15 μm or more, further preferably 20 μm or more, and particularly preferably 25 μm or more, and the upper limit is preferably 80 μm or less, particularly preferably 70 μm or less in terms of reducing warping of a liquid crystal display using a polarizing film produced from the polyvinyl alcohol-based film, more preferably 50 μm or less, and particularly preferably 40 μm or less.

[0050] 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 weight.

[0051] The width of the polyvinyl alcohol-based film is preferably 3 m or more in terms of widening the polarizing film, more preferably 4 m or more, particularly preferably 5 m or more, and even more preferably 5 to 6 m in terms of avoiding breakage during production of the polarizing film.

[0052] The amount of polyvinyl alcohol resin eluted from the polyvinyl alcohol film was 19 mg / cm 3 It is preferable that the concentration is 15 mg / cm or less, and particularly preferable that the concentration is 15 mg / cm 3 More preferably, 13 mg / cm 3 The lower the amount of elution, the better, and the lower limit is usually 0 mg / cm 3 The amount of polyvinyl alcohol resin eluted can be measured, for example, by the method described in the Examples below.

[0053] The stretching ratio at break of the polyvinyl alcohol film is preferably 7.0 times or more, particularly preferably 7.1 times or more, and even more preferably 7.2 times or more. There is no particular upper limit, but it is usually 15 times or less. The stretching ratio at break can be measured, for example, by the method described in the Examples below.

[0054] The polyvinyl alcohol-based film obtained by the above-mentioned production method is useful for optical purposes, and is particularly useful as a raw film for a polarizing film. Hereinafter, a method for producing a polarizing film and a polarizing plate made of the polyvinyl alcohol-based film will be described.

[0055] Thus, the polyvinyl alcohol film of this embodiment is obtained, and the polyvinyl alcohol film of this embodiment is suitably used as a polyvinyl alcohol film for producing a polarizing film.

[0056] [Method for Producing Polarizing Film] A polarizing film according to one embodiment of the present invention (hereinafter, may be simply referred to as a "polarizing film") is produced by unwinding the polyvinyl alcohol-based film obtained by the above-described production method from a roll, transporting the film in a horizontal direction, and subjecting the film to steps of swelling, dyeing, boric acid crosslinking, stretching, washing, drying, and the like.

[0057] The swelling step is carried out before the dyeing step. The swelling step not only cleans the surface of the polyvinyl alcohol-based film from dirt, but also has the effect of preventing uneven dyeing by swelling the polyvinyl alcohol-based film. In the swelling step, water is usually used as the treatment liquid. As long as the treatment liquid is mainly composed of water, it may contain additives such as iodide compounds and surfactants, alcohol, etc. The temperature of the swelling bath is usually about 10 to 45°C, and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

[0058] The dyeing process is carried out by bringing the film into contact with a liquid containing iodine or a dichroic dye. An aqueous solution of iodine and potassium iodide is usually used, with an iodine concentration of 0.1 to 2 g / L and a potassium iodide concentration of 1 to 100 g / L being appropriate. A practical dyeing time is about 30 to 500 seconds. The temperature of the treatment bath is preferably 5 to 50°C. The aqueous solution may contain a small amount of an organic solvent that is compatible with water in addition to the water solvent.

[0059] 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.

[0060] 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 70°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.

[0061] 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.

[0062] The drying step is carried out, for example, using a dryer at 40 to 100° C. for 0.1 to 10 minutes.

[0063] 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 The degree of polarization is calculated from the light transmittance (H1) measured at a wavelength λ in a state where the two polarizing films are superimposed so that their orientation directions are perpendicular to each other, according to the following formula (3): 11 −H1) / (H 11 + H1) 1 / 2 ...(3)

[0064] Furthermore, the single transmittance of the polarizing film is preferably 40% or more. If the single transmittance is too low, it tends to be difficult to achieve high brightness in the liquid crystal display. The single transmittance is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0065] Next, a method for producing a polarizing plate according to one embodiment of the present invention using a polarizing film will be described. The polarizing film of this embodiment is suitable for producing a polarizing plate with little color unevenness and excellent polarization performance.

[0066] [Method for Manufacturing Polarizing Plate] A polarizing plate according to one embodiment of the present invention is produced by laminating an optically isotropic resin film as a protective film on one or both sides of a polarizing film via an adhesive. Examples of the protective film include films made of acetyl cellulose-based resins such as triacetyl cellulose and diacetyl cellulose, films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, polycarbonate-based resin films, cycloolefin-based resin films, acrylic-based resin films, and films made of linear olefin-based resins such as polypropylene-based resins.

[0067] 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.

[0068] 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.

[0069] The polarizing film and polarizing plate obtained using the polyvinyl alcohol-based film of the present embodiment 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 layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication devices, medical devices, building materials, toys, etc.

[0070] 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" refers to parts by mass.

[0071] <Measurement Conditions> <Absolute Value (|K|) of the Rate of Change (Slope) of Birefringence Magnitude from the Surface to a Depth of 1 μm> A strip measuring MD×TD=5 mm×10 mm was cut from the center of the polyvinyl alcohol film in the width direction (TD) at an arbitrary position in the machine direction (MD) of the polyvinyl alcohol film. The strip was then sandwiched between 100 μm-thick PET films on both sides, which were then further sandwiched between a microtome measurement jig and attached to a microtome device. The strip was then sliced ​​parallel to the machine direction (MD) of the strip at 10 μm intervals to prepare slices (MD×TD=5 mm×10 μm) for observation. Next, to allow observation of the slice surface, the slice was placed on a glass slide with the slice surface facing upward and sealed with a cover glass and tricresyl phosphate (refractive index 1.557). Retardation was measured using a two-dimensional photoelastic evaluation system "PA-micro" (manufactured by Photonic Lattice, Inc.). With the retardation distribution of the slice displayed on the measurement screen of the "PA-micro," a line segment X perpendicular to the original surface of the polyvinyl alcohol film was drawn across the slice from one surface to the other, and line analysis was performed on this line segment X to obtain retardation distribution data in the thickness direction of the slice. Note that observation was performed using a 40x objective lens, and the average retardation value was adopted with a line width of 3 pixels. The obtained retardation distribution data in the thickness direction of the slice was divided by the slice thickness of 10 μm to obtain the distribution of birefringence Δn in the thickness direction of the slice. Each birefringence distribution was aligned to 1,000 points using cubic spline interpolation. The rate of change (slope) of the magnitude of birefringence from the film surface in contact with the cast mold and the film surface not in contact with the cast mold to a point corresponding to 1 μm from each of the film surfaces of the retardation distribution data aligned at 1,000 points was calculated by the least squares method. The absolute value of each calculated slope was taken, and the absolute value |K| of the rate of change (slope) of the magnitude of birefringence from the surface layer to a depth of 1 μm was defined.

[0072] <Weight swelling degree> A polyvinyl alcohol film was cut into a size of 10 cm x 10 cm and immersed for 15 minutes in an ion-exchange water tank adjusted to 30° C. Next, the film was taken out, spread on a filter paper (5A), and the filter paper (5A) was placed on the film. 2 ) SUS plate was placed on the film for 5 seconds to remove water adhering to the film surface. The film was quickly placed in a weighing bottle and its weight was measured, which was defined as the film weight A when swollen. The above operation was carried out in an environment of 23°C and 50% RH. Next, the film was left to stand in a dryer at 105°C for 16 hours to remove the water in the film, after which the film was taken out and quickly placed in a weighing bottle and its weight was measured, which was defined as the film weight B after drying. The film weight A when swollen and the film weight B after drying were used to calculate the degree of weight swelling (%) from the following formula: Weight swelling (%) = A / B x 100

[0073] <Stretching Breaking Ratio> The stretching breaking ratio measurement is a test in which a film is dyed and stretched in the machine direction until it breaks, and the resulting breakage ratio is evaluated. Specifically, a 40 mm (machine direction) x 50 mm (width direction) test piece was cut out from a polyvinyl alcohol-based film and attached to a stretching jig (chuck distance: 20 mm) so that the machine direction was the stretching direction. The sample was immersed in a water bath at 25°C for 100 seconds, and then immersed in an aqueous solution of 0.6 g / L iodine and 25 g / L potassium iodide at 28°C for 36 seconds to perform iodine dyeing. Subsequently, the sample was immersed in an aqueous solution of 35 g / L boric acid and 35 g / L potassium iodide at 57°C, and stretched in the machine direction at a stretching speed of 2.25 cm / min until the film broke. The stretching ratio at break was taken as the stretching breaking ratio.

[0074] <Moisture content> A test piece measuring 100 mm in width and 100 mm in length was cut out from the polyvinyl alcohol film sampled immediately after the drying process using a heated roll, and the moisture content (mass%) was calculated from the mass A (g) before drying and the mass B (g) after drying for 16 hours in a dryer at an atmospheric temperature of 105°C using the following formula: Moisture content (mass%) = (A - B) / A x 100

[0075] <Elution amount> [Test method] After conditioning the polyvinyl alcohol film at 23°C and 50% RH for 24 hours, a 100 mm x 100 mm (0.01 m 2 ) test pieces were cut out, and all of them (total 0.05 m 2 ) was immersed in 1 L of ion-exchanged water at 50°C for 1 minute to obtain an eluate. 10 mL of this eluate was mixed with 10 mL of a color-developing reagent (ion-exchanged water 500 g, potassium iodide 7.4 g, iodine 0.65 g, boric acid 10.6 g) at room temperature (23°C), and the absorbance at a wavelength of 690 nm was measured using a spectrophotometer (Shimadzu Corporation, UV-3100PC). The amount of eluted polyvinyl alcohol resin per volume (mg / cm) was calculated from a calibration curve prepared in advance. 3 ) was calculated. [Evaluation] ○ (Very Good): The amount of elution was 15 mg / cm 3 Below △ (Good): Elution amount is 15mg / cm 3 Larger than 19 mg / cm 3 Less than × (Poor): Elution amount is 19 mg / cm 3 Greater than

[0076] Example 1: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 160,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 26% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 5.2% by mass. The film was then heat-treated at 121°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0077] Example 2: 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 160,000 and a degree of saponification of 99.8 mol% was added with 9 parts by mass of glycerin and water, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 26% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 6.0% by mass. The film was then heat-treated at 124°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0078] Example 3: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 160,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 26% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 5.7% by mass. The film was then heat-treated at 120°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0079] Example 4: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 160,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 26% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 5.2% by mass. The film was then heat-treated at 115°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0080] Example 5: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 145,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 28% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 4.9% by mass. The film was then heat-treated at 115°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0081] Comparative Example 1: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 145,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 28% by mass. The aqueous polyvinyl alcohol resin solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 4.9% by mass. The film was then heat-treated at 90°C using a floating dryer to obtain a polyvinyl alcohol film with a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0082] Comparative Example 2: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 130,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 29% by mass. The aqueous polyvinyl alcohol resin solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 4.8% by mass. The film was then heat-treated at 50°C using a floating dryer to obtain a polyvinyl alcohol film with a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0083] Comparative Example 3: 100 parts by mass of polyvinyl alcohol resin having a weight-average molecular weight of 130,000 and a degree of saponification of 99.8 mol% was mixed with 6 parts by mass of glycerin and water, and the mixture was heated to 140°C and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 28% by mass. The aqueous polyvinyl alcohol resin solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 5.7% by mass. The film was then heat-treated at 50°C using a floating dryer to obtain a polyvinyl alcohol film with a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0084] Comparative Example 4: 12 parts by mass of glycerin and water were added to 100 parts by mass of polyvinyl alcohol resin having a weight average molecular weight of 160,000 and a degree of saponification of 99.8 mol%, and the mixture was heated to 140°C and dissolved under pressure to obtain a polyvinyl alcohol resin aqueous solution having a resin concentration of 26% by mass. The polyvinyl alcohol resin aqueous solution was extruded from a T-shaped slit die onto a casting drum and dried with a heated roll so that the film had a moisture content of 3.2% by mass. The film was then heat-treated at 128°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 45 μm. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0085]

[0086] The polyvinyl alcohol-based films of Examples 1 to 5 had absolute values ​​(|K|) of the rate of change (slope) of birefringence magnitude from the surface layer of each surface to a depth of 1 μm within the range specified by the present invention, and therefore had a small amount of polyvinyl alcohol-based resin eluted during polarizing film production, did not contaminate the treatment bath in the polarizing film production process, and had few foreign matter defects in the resulting polarizing film, demonstrating good productivity.On the other hand, the polyvinyl alcohol-based films of Comparative Examples 1 to 3 had absolute values ​​(|K|) of the rate of change (slope) of birefringence magnitude from the surface layer of each surface to a depth of 1 μm within the range specified by the present invention, and therefore contaminate the treatment bath in the polarizing film production process, caused many foreign matter defects in the resulting polarizing film, and demonstrated poor productivity.

[0087] Furthermore, the polyvinyl alcohol films obtained above in Examples 3 and 4 and Comparative Example 4 were evaluated according to the method for measuring the stretching ratio at break. The results are shown in Table 2.

[0088]

[0089] The polyvinyl alcohol-based films of Examples 3 and 4 have weight swelling degrees within the range specified by the present invention, large stretching break ratios, excellent stretchability during production of polarizing films, and good productivity. On the other hand, the polyvinyl alcohol-based film of Comparative Example 4 has a weight swelling degree outside the range specified by the present invention, and therefore is inferior in stretchability during production of polarizing films.

[0090] 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.

[0091] The polarizing film or polarizing plate made of the polyvinyl alcohol-based film 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 machines, videos, cameras, photo albums, thermometers, audio equipment, meters for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, anti-reflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication devices, medical devices, building materials, toys, etc.

Claims

1. A polyvinyl alcohol-based film, in which the following formula (1) is satisfied in a section of 1 μm deep from the surface layer of each of the film surfaces on both surfaces of the polyvinyl alcohol-based film, and the weight swelling degree of the polyvinyl alcohol-based film is 185% or more and 210% or less. |K| ≧ 3.5 × 10 -5 ... (1) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface to a depth of 1 μm.) 2. The polyvinyl alcohol-based film according to claim 1, wherein at least one of the film surfaces of the polyvinyl alcohol-based film satisfies the following formula (2) in a section of 1 μm deep from the surface layer of the film surface: |K|≧5×10 -5 ... (2) (where |K| is the absolute value of the rate (slope) of change in the magnitude of birefringence from the surface layer to a depth of 1 μm.) 3. The polyvinyl alcohol film according to claim 1 or 2, having a thickness of 10 to 70 μm.

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