Polyvinyl alcohol film and polarizing film

Abstract

The present invention provides a polyvinyl alcohol-based film in which the amount of polyvinyl alcohol-based resin eluted into water is small during the production of a polarizing film, contamination of a chemical solution tank during the production of a polarizing film is suppressed, and a polarizing film having few foreign matter defects can be obtained with good productivity. Provided is a polyvinyl alcohol film in which the interval from the surface layer to the depth of 1 [mu] m of each of both surfaces of the polyvinyl alcohol film satisfies the following formula (1). S / F > = 5 * 10-7 (1) (where S is the absolute value of the ratio (slope) of birefringence changes from the surface layer to a depth of 1 [mu] m, and F is the thin film thickness ([mu] m). ).

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol-based film, and more particularly to a polyvinyl alcohol-based film that has less elution of impurities into water when manufacturing a polarizing film, suppresses contamination of a chemical solution tank when manufacturing a polarizing plate, and can obtain a polarizing film with good productivity. Background Art

[0002] In recent years, the development of liquid crystal display devices has been active, and they are widely used in smartphones, tablet computers, personal computers, liquid crystal televisions, projectors, in-vehicle panels, and the like.

[0003] A polarizing film is used in the above liquid crystal display device, and as the polarizing film, a film obtained by adsorbing and orienting iodine on a polyvinyl alcohol-based film is mainly used. In recent years, with the high definition, high brightness, large size, and thinness of the screen, a polarizing film with more excellent polarization performance, no color unevenness, and wide-width long strip is required.

[0004] Generally, a polarizing film is manufactured as follows: A polyvinyl alcohol-based film as a blank is unrolled from a roll, conveyed along the longitudinal direction (MD direction), swollen in water (including hot water), and then subjected to processes such as iodine-based dyeing, stretching for orienting iodine, and boric acid crosslinking for fixing the orientation state. Defects occurring in these processes can significantly reduce the productivity of the polarizing film. For example, in the swelling process, if impurities elute from the polyvinyl alcohol-based film and contaminate the swelling tank, the contamination will spread to the entire subsequent processes. In the dyeing process and the boric acid crosslinking process, if impurities elute from the polyvinyl alcohol-based film, not only the polarization performance of the obtained polarizing film is reduced, but also a great deal of effort is required for filtering and exchanging the chemical solutions used in each process. As the above impurities, low-molecular-weight polyvinyl alcohol-based resins (including oligomers) present in the polyvinyl alcohol-based film can be cited. In particular, low-molecular-weight substances with a molecular weight of 50,000 or less are easily eluted into water, and tend to form low-molecular-weight iodine complexes that reduce the degree of polarization.

[0005] Furthermore, the eluted low-molecular-weight polyvinyl alcohol-based resin forms an aggregate with iodine and boric acid in the polarizing film manufacturing process and may become a foreign substance. This foreign substance adheres to the surface of the polarizing film being processed, thereby becoming a foreign substance defect in the finally obtained polarizing film.

[0006] On the other hand, a polyvinyl alcohol-based film as a blank is usually manufactured as follows: a polyvinyl alcohol-based resin as a raw material is dissolved in water, and the above aqueous solution (film-forming stock solution) is manufactured by a continuous casting method. Specifically, it is manufactured as follows: the aqueous solution of the polyvinyl alcohol-based resin is discharged and cast onto a casting mold such as a casting drum or a endless belt to form a film, and after the obtained film is peeled off from the casting mold, it is transported in the longitudinal direction (MD direction), and dried and heat-treated to manufacture. However, if the selection of the type of the polyvinyl alcohol-based resin as a raw material and the adjustment of the manufacturing conditions in the manufacturing process are not appropriate, there is a tendency for an increase in the polyvinyl alcohol-based resin eluted when manufacturing a polarizing film.

[0007] As a method for improving the above problems, for example, the following methods have been proposed: a method of obtaining a polyvinyl alcohol-based film with a reduced elution amount by using a vinyl alcohol-based polymer having a viscosity-average degree of polymerization (P) of 4000 to 8000, a saponification degree of 99.50 to 99.97 mol%, and the content of 1,2-diol bond units within a specific range and showing specific thickening characteristics and absorbance when forming an aqueous solution (see Patent Document 1); a method of obtaining a polyvinyl alcohol-based film with a reduced elution amount by focusing on the hot roll temperature conditions in the drying process when manufacturing a polyvinyl alcohol-based film (see Patent Document 2).

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-221462

[0011] Patent Document 2: WO2017 / 204271 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] However, in the disclosed technology of 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 eluate of the polyvinyl alcohol-based resin is 10 to 50 ppm, but when converted to per m 2 film, 1000 to 5000 ppm / m 2 of the polyvinyl alcohol-based resin will elute. In order to manufacture a polarizing film with high production efficiency to meet the demands of polarizing films in recent years, a further reduction in the resin elution amount is required.

[0014] In addition, in the disclosed technology of the above Patent Document 2, the elution amount of the polyvinyl alcohol-based resin when immersed in water at 50 °C for 1 minute is 900 ppm / m 2However, in order to meet the requirements of high transmittance and high polarization degree of polarizing films in recent years, and to manufacture polarizing films at a high draw ratio under conditions where polyvinyl alcohol-based resins are more likely to dissolve, a further reduction in the resin dissolution amount is required.

[0015] Therefore, in the present invention, in such a background, there is provided: a polyvinyl alcohol-based thin film that has a small amount of polyvinyl alcohol-based resin dissolved in water when manufacturing a polarizing film, suppresses the contamination of the chemical solution tank during the manufacture of the polarizing film, and can produce a polarizing film with few foreign matter defects with good productivity.

[0016] Means for Solving the Problems

[0017] Thus, the present inventors focused on the ratio of the change in birefringence magnitude between the surface layers of 1 μm of the polyvinyl alcohol-based thin film, and found that: when the value obtained by dividing its absolute value by the film thickness is within a specific range on both surfaces of the thin film, the above-mentioned problems can be solved, and a polarizing film with few foreign matter defects can be manufactured with high productivity.

[0018] That is, the gist of the present invention lies in the following [1] to [4].

[0019] [1] A polyvinyl alcohol-based thin film, wherein for both surfaces of the polyvinyl alcohol-based thin film, the interval from the surface layer of each of the aforementioned thin film surfaces to a depth of 1 μm satisfies the following formula (1).

[0020] |S| / F ≥ 5×10 -7 …(1)

[0021] (Here, |S| is the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness of the thin film (unit: μm).)

[0022] [2] The polyvinyl alcohol-based thin film according to [1], wherein for at least one thin film surface of the aforementioned polyvinyl alcohol-based thin film, the interval from the surface layer of the aforementioned thin film surface to a depth of 1 μm satisfies the following formula (2).

[0023] |S| / F ≥ 9×10 -7 …(2)

[0024] (Here, |S| is the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness of the thin film (unit: μm).)

[0025] [3] The polyvinyl alcohol-based thin film according to [1] or [2], wherein the film thickness (F) of the thin film is 10 to 60 μm.

[0026] [4] A polarizing film that uses the polyvinyl alcohol-based thin film according to any one of [1] to [3].

[0027] Effects of the Invention

[0028] The polyvinyl alcohol-based film of the present invention focuses on the ratio of the change in the birefringence magnitude within the surface layer of 1 μm from both surfaces of the polyvinyl alcohol-based film, and sets the value obtained by dividing the absolute value thereof by the film thickness to a specific range on both surfaces of the film. As a result, the crystals in the surface portion of the film grow sufficiently, and thus, the amount of the polyvinyl alcohol-based resin dissolved from the film surface into the chemical solution during the production of the polarizing film can be reduced, and the contamination of the chemical solution tank during the production of the polarizing film is suppressed. Therefore, it is useful as a blank film for producing a polarizing film with few foreign matter defects at high productivity. Detailed Description of the Invention

[0029] Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

[0030] In this specification, "x and / or y (where x and y are arbitrary components)" means at least one of x and y, and means three cases: only x, only y, and x and y.

[0031] In this specification, when expressed as "X to Y" (where X and Y are arbitrary numbers), unless otherwise specified, it includes the meaning of "X or more and Y or less" and the meaning of "preferably greater than X" or "preferably less than Y".

[0032] In this specification, when expressed as "X or more" (where X is an arbitrary number) or "Y or less" (where Y is an arbitrary number), it also includes the meaning of "preferably greater than X" or "preferably less than Y".

[0033] Regarding the numerical ranges described stepwise in this specification, the upper limit value or the lower limit value of a certain stepwise numerical range can be arbitrarily combined with the upper limit value or the lower limit value of other stepwise numerical ranges. In addition, within the numerical ranges described in this specification, the upper limit value or the lower limit value of the numerical range can also be replaced with the values shown in the examples.

[0034] In addition, in this specification, "film" includes the meanings of "strip" and "sheet".

[0035] In this specification, "main component" means a component that has a great influence on the characteristics of the object, and the content of this component is usually 50% by mass or more in the object, preferably 55% by mass or more, more preferably 60% by mass or more, further preferably 70% by mass or more, and may be 100% by mass.

[0036] The polyvinyl alcohol-based film of one embodiment of the present invention (hereinafter, sometimes simply referred to as "polyvinyl alcohol-based film") is a polyvinyl alcohol-based film that satisfies the following formula (1) in the range from the surface layer to a depth of 1 μm from both surfaces of the polyvinyl alcohol-based film.

[0037] |S| / F ≥ 5 × 10 -7 …(1)

[0038] (Here, |S| is the absolute value of the ratio (slope) of the change in the birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness (unit: μm).)

[0039] Here, the "absolute value of the ratio (slope) of the change in the birefringence magnitude from the surface layer to a depth of 1 μm (|S|)" in the present embodiment refers to the value measured by the following method.

[0040] (Measurement method)

[0041] At an arbitrary position in the machine direction (MD) of the polyvinyl alcohol-based film, a fragment of size MD × TD = 5 mm × 10 mm is cut out from the central part in the transverse direction (TD) of the polyvinyl alcohol-based film. Then, both sides of this fragment are clamped with a PET film having a thickness of 100 μm, and it is further clamped in a jig for slicer measurement and installed in a slicer device.

[0042] Then, the previously cut fragment is cut at 10-μm intervals parallel to the machine direction (MD) of the fragment to produce a slice for observation (MD × TD = 5 mm × 10 μm).

[0043] Next, the slice is laid down in a manner that allows the cutting surface to be observed and placed on a glass slide with the cutting surface facing up, sealed with a glass cover slip and tricresyl phosphate (refractive index 1.557), and the retardation value is measured using a two-dimensional photoelastic evaluation system "PA-micro" (manufactured by Photonic Lattice Co., Ltd.).

[0044] With the retardation value distribution of the slice displayed on the measurement screen of "PA-micro", a line segment X perpendicular to the surface of the original polyvinyl alcohol-based film is drawn from one surface to the other surface in a manner that crosses the slice, and line analysis is performed on this line segment X to obtain retardation value distribution data in the thickness direction of the slice. It should be noted that observation is performed using a 40-fold objective lens, the line width is set to 3 pixels, and the average value of the retardation values is used.

[0045] The obtained retardation value distribution data in the thickness direction of the slice is divided by the slice thickness of 10 μm to obtain the distribution of the birefringence Δn in the thickness direction of the slice. The distributions of the respective birefringences are arranged into 1000 points using cubic spline interpolation. For the retardation value distribution data arranged into 1000 points, the ratio (slope) of the change in the birefringence magnitude equivalent to 1 μm is calculated from the film surface in contact with the casting surface and the film surface not in contact with the casting mold using the least squares method, and the value obtained by taking the absolute value of each calculated slope is used.

[0046] For the two film surfaces of the above-mentioned polyvinyl alcohol-based film, the value (|S| / F) obtained by dividing the absolute value (|S|) of the ratio (slope) of the change in birefringence from the surface layer to a depth of 1 μm by the film thickness (F) of the film must be 5×10 -7 or more, preferably 5.5×10 -7 or more, particularly preferably 6×10 -7 or more, more preferably 6.5×10 -7 or more, particularly preferably 7×10 -7 or more, particularly further preferably 7.5×10 -7 or more.

[0047] For the two film surfaces of the polyvinyl alcohol-based film, a polyvinyl alcohol-based film with a low elution amount can be obtained by satisfying the above range for the value obtained by dividing the absolute value of the ratio (slope) of the change in birefringence from the surface layer to a depth of 1 μm by the film thickness.

[0048] In addition, as the upper limit value of the above value, it is usually 5×10 -6 or less, preferably 4×10 -6 or less, particularly preferably 3×10 -6 or less.

[0049] Regarding the absolute value (|S|) of the ratio (slope) of the change in birefringence from the surface layer to a depth of 1 μm, in terms of reducing the pollution caused by the elution of the polyvinyl alcohol-based resin in each tank during the production of the polarizing film, the lower limit value is preferably 5×10 -6 or more, particularly preferably 1×10 -5 or more, more preferably 1.5×10 -5 or more, particularly preferably 2×10 -5 or more. In addition, in terms of stretchability and dyeability during the production of the polarizing film, the upper limit value is preferably 4×10 -4 or less, particularly preferably 2.5×10 -4 or less, more preferably 1.5×10 -4 or less.

[0050] In addition, regarding the above-mentioned film thickness F (μm), in terms of the production stability of the polarizing film, the lower limit value is preferably 10 μm or more, particularly preferably 15 μm or more, more preferably 20 μm or more, particularly preferably 25 μm or more. In addition, in terms of reducing the warping of the liquid crystal display using the polarizing film made of the polyvinyl alcohol-based film, ideally, the upper limit value is preferably 80 μm or less, particularly preferably 60 μm or less, more preferably 50 μm or less, particularly preferably 40 μm or less.

[0051] As a method for controlling the value (|S| / F) obtained by dividing the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm on both film surfaces of the above-mentioned polyvinyl alcohol-based film by the film thickness within a specific range, the following methods can be cited: 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 onto a casting mold to form a film; a method for controlling the temperature and speed of the casting mold; a method for controlling the temperature and air volume of hot air-based drying from the film surface side that does not contact the casting mold; a method for controlling the film moisture content after hot roll drying during film production; a method for controlling the tension in the MD direction and TD direction during hot roll drying when manufacturing the film; a method for controlling the heat treatment temperature during film production; a method for controlling the conveying speed of the film; etc. By using these individually or in combination of two or more, the above value can be controlled within a specific range.

[0052] Among these, the following methods are preferred: a method for controlling the temperature and speed of the casting mold in the process of discharging and casting the aqueous solution of the polyvinyl alcohol-based resin onto a casting mold to form a film; a method for controlling the temperature and air volume of hot air-based drying from the film surface side that does not contact the casting mold; a method for controlling the film moisture content after hot roll drying during film production; a method for controlling the tension in the MD direction and TD direction during hot roll drying when manufacturing the film; a method for controlling the heat treatment temperature during film production. Particularly, the following methods are further preferred: a method for controlling the heat treatment temperature during film production; a method for controlling the film moisture content after hot roll drying during film production.

[0053] Furthermore, for at least one film surface of the polyvinyl alcohol-based film, in the range from the surface layer of the film surface to a depth of 1 μm, it is preferably to satisfy the following formula (2), and particularly preferably |S| / F ≥ 9.5×10 -7 , and more preferably |S| / F ≥ 1×10 -6 .

[0054] For at least one film surface of the polyvinyl alcohol-based film, by satisfying the above range for the value obtained by dividing the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm by the film thickness, there is a tendency to obtain a polyvinyl alcohol-based film with less elution amount.

[0055] |S| / F ≥ 9×10 -7 …(2)

[0056] (Here, |S| is the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness (unit: μm).)

[0057] The polyvinyl alcohol-based film is preferably manufactured through the following processes (A) to (C), and preferably through process (D) as needed.

[0058] Process (A) is a process of preparing an aqueous solution of a polyvinyl alcohol-based resin.

[0059] Process (B) is a process of casting an aqueous solution of a polyvinyl alcohol-based resin onto a casting mold and forming a film.

[0060] Process (C) is a process of drying the formed film.

[0061] Process (D) is a process of heat-treating the obtained film with hot air.

[0062] <Process (A)>

[0063] Process (A) is a process of preparing an aqueous solution of a polyvinyl alcohol-based resin (preparation process).

[0064] First, the polyvinyl alcohol-based resin and the aqueous solution of the polyvinyl alcohol-based resin, which are the materials of the above polyvinyl alcohol-based film, will be described.

[0065] In the present embodiment, as the polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based film, an unmodified polyvinyl alcohol-based resin, that is, a resin manufactured by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate, is usually used. As needed, 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 the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (such as ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. In addition, a modified polyvinyl alcohol-based resin obtained by chemically modifying the saponified hydroxyl group can also be used. They can be used alone or in combination of two or more.

[0066] In addition, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain can also be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is obtained, for example, by the following methods: (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 ethylene carbonate; (iii) a method of saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane; (iv) a method of saponifying a copolymer of vinyl acetate and glycerol monoallyl ether.

[0067] The weight-average molecular weight of the polyvinyl alcohol resin is preferably from 80,000 to 300,000, particularly preferably from 90,000 to 280,000, and further preferably from 100,000 to 260,000. If the above weight-average molecular weight is too small, when forming an optical film from the polyvinyl alcohol resin, there is a tendency that sufficient optical properties are not easily obtained. If it is too large, when manufacturing a polarizing film using the polyvinyl alcohol film, there is a tendency that stretching becomes difficult. It should be noted that the weight-average molecular weight of the above polyvinyl alcohol resin is the weight-average molecular weight measured by the GPC-MALS method.

[0068] The dispersity (weight-average molecular weight / number-average molecular weight) of the polyvinyl alcohol resin is preferably from 1.85 to 2.30, particularly preferably from 1.90 to 2.20, and further preferably from 1.95 to 2.10.

[0069] If the above dispersity is too small, when manufacturing a polarizing film using the polyvinyl alcohol film, there is a tendency that stretching becomes difficult. If it is too large, when forming an optical film from the polyvinyl alcohol resin, there is a tendency that sufficient optical properties are not easily obtained.

[0070] It should be noted that the weight-average molecular weight and number-average molecular weight when measuring the dispersity of the above polyvinyl alcohol resin are the weight-average molecular weight and number-average molecular weight measured by the GPC-MALS method.

[0071] 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 small, when forming a polarizing film from the polyvinyl alcohol film, there is a tendency that sufficient optical properties cannot be obtained.

[0072] Here, the average saponification degree in this embodiment is measured according to JIS K 6726.

[0073] As the polyvinyl alcohol resin, two or more kinds with different modification substances, modification amounts, weight-average molecular weights, average saponification degrees, etc. can also be used in combination.

[0074] For the polyvinyl alcohol resin, in order to make the molecular weight distribution a desired value, it is preferably obtained by adding a control agent during the polymerization of vinyl acetate and controlling radical polymerization. The control agent added during the above polymerization is not particularly limited. From the viewpoint of control, a method using an organic cobalt complex as the control agent is preferred.

[0075] Using the above polyvinyl alcohol resin, an aqueous solution of the polyvinyl alcohol resin is prepared into a film-forming stock solution. The resin concentration of the aqueous solution of the polyvinyl alcohol resin is usually 5 to 70% by mass, preferably 10 to 60% by mass.

[0076] In the polyvinyl alcohol-based resin aqueous solution, in terms of film-forming properties, in addition to the polyvinyl alcohol-based resin, if necessary, it is more preferably contained with commonly used plasticizers such as glycerol, diglycerol, triglycerol, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, and at least one surfactant of nonionic, anionic, and cationic types. They can be used alone or in combination of two or more.

[0077] The resin concentration of the thus obtained polyvinyl alcohol-based resin aqueous solution 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 the above aqueous solution is too low, the drying load becomes large, so there is a tendency for the production capacity to decrease. If it is too high, the viscosity becomes excessively high, and there is a tendency for uniform dissolution to become difficult.

[0078] Next, the obtained polyvinyl alcohol-based resin aqueous solution is defoamed. As the defoaming method, methods such as static defoaming and defoaming based on a multi-screw extruder can be cited. As the multi-screw extruder, any multi-screw extruder having an exhaust port can be used, and a twin-screw extruder having an exhaust port is usually used.

[0079] <Process (B)>

[0080] Process (B) is a process (film-forming process) of casting the aqueous solution of the polyvinyl alcohol-based resin on a casting mold and forming a film.

[0081] After the above defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced little by little in a constant amount into a T-slot die head, discharged and cast onto a rotating casting drum, and formed into a film by a continuous casting method.

[0082] The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-slot die head is preferably 80 to 100 °C, particularly preferably 85 to 98 °C.

[0083] If the resin temperature of the above polyvinyl alcohol-based resin aqueous solution is too low, there is a tendency for poor flow, and if it is too high, there is a tendency for foaming.

[0084] The viscosity of the above polyvinyl alcohol-based resin aqueous solution at the time of discharge is preferably 50 to 200 Pa·s, particularly preferably 70 to 150 Pa·s.

[0085] If the viscosity of the above aqueous solution is too high, there is a tendency for poor flow, and if it is too low, there is a tendency for casting film formation to become difficult.

[0086] The discharge speed of the polyvinyl alcohol-based resin aqueous solution discharged from the T-slot die head to the casting drum is preferably 0.2 to 5 m / minute, particularly preferably 0.4 to 4 m / minute, and further preferably 0.6 to 3 m / minute.

[0087] If the above discharge speed is too slow, there is a tendency for productivity to decrease, and if it is too fast, there is a tendency for casting to become difficult.

[0088] The diameter of the above casting drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and further preferably 2.8 to 4 m.

[0089] If the above diameter is too small, the drying section on the casting drum becomes short, and thus there is a tendency for the speed not to increase easily. If it is too large, there is a tendency for the transportability to decrease.

[0090] The width of the above 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 especially preferably 5 to 8 m.

[0091] If the width of the casting drum is too small, there is a tendency for productivity to decrease.

[0092] The rotation speed of the above casting drum is preferably 3 to 50 m / minute, particularly preferably 7 to 40 m / minute, and further preferably 10 to 35 m / minute.

[0093] If the above rotation speed is too slow, there is a tendency for productivity to decrease, and if it is too fast, there is a tendency for drying to become insufficient.

[0094] The surface temperature of the above casting drum is preferably 40 to 99 °C, particularly preferably 60 to 95 °C.

[0095] If the above surface temperature is too low, there is a tendency for poor drying to occur, and if it is too high, there is a tendency for foaming to occur.

[0096] <Process (C)>

[0097] Process (C) is a process (drying process) of heating and drying the film formed above.

[0098] The film peeled off from the casting drum (the film formed above) is dried as follows: It is transported in the flow direction (MD direction) using a roll or the like, and the front and back surfaces of the film are alternately brought into contact with a plurality of hot rolls for drying. The hot roll is, for example, a roll with a diameter of 0.2 to 2 m whose surface has been subjected to hard chromium plating or mirror finishing. Ideally, 2 to 30 rolls are usually used, and preferably 10 to 25 rolls are used for drying.

[0099] The surface temperature of the above hot roll is not particularly limited, and is usually 50 to 150 °C, and further preferably 60 to 140 °C. If the above surface temperature is too low, there is a tendency for poor drying to occur, and if it is too high, it becomes overly dry, and there is a tendency for appearance defects such as bending to occur.

[0100] In terms of the transportability, storage stability, and processability during the production of polarizing plates of the polyvinyl alcohol-based film, the lower limit of the moisture content of the film immediately after the drying process using the above-mentioned hot roller is preferably 0.5% or more, particularly preferably 1% or more, further preferably 2% or more, especially preferably 3% or more, and more preferably 4.5% or more.

[0101] In addition, the upper limit is preferably 10% or less, particularly preferably 9% or less, further preferably 8% or less, and especially preferably 7% or less.

[0102] <Process (D)>

[0103] Process (D) is a process of heat-treating the obtained film with hot air (heat-treatment process), and the film that has undergone the above-mentioned Process (C) can be heat-treated using, for example, a floating dryer or the like.

[0104] The upper limit of the temperature of the above-mentioned heat treatment is preferably 200°C or less, particularly preferably 180°C or less, further preferably 160°C or less, and especially preferably 140°C or less. In addition, the lower limit is preferably 50°C or more, particularly preferably 80°C or more, further preferably 100°C or more, and especially preferably 110°C or more.

[0105] If the heat treatment temperature is too high, the dyeability during the production of the polarizing film tends to decrease. If it is too low, the film is likely to wrinkle or fold during the swelling process in the production of the polarizing film, and the appearance of the polarizing film tends to deteriorate.

[0106] In addition, the heat treatment time is preferably 20 to 100 seconds, particularly preferably 40 to 70 seconds.

[0107] 〔Polyvinyl alcohol-based film〕

[0108] In this way, through the above-mentioned Processes (A) to (C) and, if necessary, the above-mentioned Process (D), a polyvinyl alcohol-based film is obtained and finally wound around a roller to become a product.

[0109] Regarding the length of the polyvinyl alcohol-based film, from the perspective of increasing the area of the polarizing film, it is preferably 4 km or more, and from the perspective of the transport weight, it is particularly preferably 5 to 50 km.

[0110] Regarding the width of the polyvinyl alcohol-based film, in terms of widening the width of the polarizing film, it is preferably 3 m or more, more preferably 4 m or more, particularly preferably 5 m or more, and further preferably 5 to 6 m from the perspective of avoiding breakage during the production of the polarizing film.

[0111] The elution amount of the polyvinyl alcohol-based resin in the polyvinyl alcohol-based film is preferably 19 mg / cm 3 or less, particularly preferably 15 mg / cm 3Hereinafter, it is further preferably 13 mg / cm 3 Hereinafter. In addition, the lower the dissolution amount, the more preferable, and the lower limit value is usually 0 mg / cm 3 .

[0112] The dissolution amount of the above-mentioned polyvinyl alcohol-based resin can be measured, for example, by the method described in the following examples.

[0113] The polyvinyl alcohol-based film obtained by the above manufacturing method is useful for optical applications. In particular, it is very useful as a blank film for a polarizing film. Hereinafter, the manufacturing methods of the polarizing film and the polarizing plate formed from the polyvinyl alcohol-based film will be described.

[0114] [Manufacturing method of polarizing film]

[0115] The polarizing film (hereinafter sometimes simply referred to as "polarizing film") of one embodiment of the present invention is manufactured as follows: The above-mentioned polyvinyl alcohol-based film obtained by the above manufacturing method is unwound from a roll, conveyed in the horizontal direction, and passed through processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying, thereby manufacturing.

[0116] The swelling process is carried out before the dyeing process. Through the swelling process, the dirt on the surface of the polyvinyl alcohol-based film can be washed, and in addition, the polyvinyl alcohol-based film is swollen, thereby having the effect of preventing uneven dyeing, etc. In the swelling process, water is usually used as the treatment liquid. As long as the main component of this treatment liquid is water, additives such as iodine compounds and surfactants, and alcohols can also be added. 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.

[0117] 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-potassium iodide is usually used. Appropriately, the concentration of iodine is 0.1 to 2 g / L, and the concentration of potassium iodide is 1 to 100 g / L. A dyeing time of about 30 to 500 seconds is practical. The temperature of the treatment bath is preferably 5 to 50 °C. In the aqueous solution, in addition to the water solvent, a small amount of an organic solvent compatible with water can also be contained.

[0118] The boric acid crosslinking process is carried out using boron compounds such as boric acid and 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. In terms of stabilizing the polarization performance, it is preferable to make potassium iodide coexist in the liquid. The treatment temperature is preferably about 30 to 70 °C, the treatment time is about 0.1 to 20 minutes, and in addition, a stretching operation can also be carried out during the treatment as needed.

[0119] In the stretching step, it is preferable to stretch the film 3 to 10 times, more preferably 3.5 to 7 times, in a uniaxial direction. At this time, slight stretching may also be performed in the direction perpendicular to the stretching direction (to prevent shrinkage in the width direction or more). The temperature during stretching is preferably 40 to 70°C. Further, as long as the final stretching ratio is set within the aforementioned range, the stretching operation can be carried out not only in one stage but also multiple times in the manufacturing process.

[0120] The cleaning step can be carried out, for example, by immersing the film in an aqueous solution of iodide such as water or potassium iodide to remove the precipitates generated on the surface of the film. The potassium iodide concentration when using an aqueous potassium iodide solution is preferably about 10 to 1000 g / L. The temperature during the cleaning 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. It should be noted that water cleaning and cleaning based on an aqueous potassium iodide solution can be appropriately combined and carried out.

[0121] The drying step can be carried out, for example, by using a dryer and drying at 40 to 100°C for 0.1 to 10 minutes.

[0122] The polarizing film thus obtained preferably has a polarization degree of 99.90% or more, more preferably 99.99% or more. If the polarization degree is too low, there is a tendency for the contrast in the liquid crystal display to decrease.

[0123] It should be noted that the polarization degree is usually calculated as follows: According to the transmittance (H 11 ) measured at wavelength λ in a state where two polarizing films are overlapped with their orientation directions being the same, and the transmittance (H1) measured at wavelength λ in a state where two polarizing films are overlapped with their orientation directions being orthogonal to each other, it is calculated according to the following formula (3).

[0124] Polarization degree = [(H 11 - H1) / (H 11 + H1)] 1 / 2 …(3)

[0125] Furthermore, the single - sheet transmittance of the polarizing film is preferably 40% or more. If the single - sheet transmittance is too low, there is a tendency for the high brightness of the liquid crystal display to become impossible to achieve.

[0126] The single - sheet transmittance is a value obtained by measuring the transmittance of a single polarizing film using a spectrophotometer.

[0127] Next, a method for manufacturing a polarizing plate according to an embodiment of the present invention using the polarizing film will be described.

[0128] The polarizing film of this embodiment is suitable for manufacturing a polarizing plate with less color unevenness and excellent polarization performance.

[0129] [Method for manufacturing a polarizing plate]

[0130] A polarizing plate according to an embodiment of the present invention is manufactured as follows: A resin film that is optically isotropic is adhered to one or both sides of a polarizing film with an adhesive as a protective film. As the protective film, for example, a film formed of an acetyl cellulose-based resin such as triacetyl cellulose or diacetyl cellulose, a film formed of a polyester-based resin such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, a polycarbonate-based resin film, a norbornene-based resin film, an acrylic-based resin film, and a film formed of a chain olefin-based resin of a polypropylene-based resin can be mentioned.

[0131] The bonding method is carried out by a known method. For example, it is carried out as follows: After a liquid adhesive composition is uniformly coated on the polarizing film, the protective film, or both of them, the two are bonded and pressed, and heated and irradiated with active energy rays.

[0132] It should be noted that a curable resin such as a urethane-based resin, an acrylic-based resin, or a urea resin is coated on one or both sides of the polarizing film and cured to form a cured layer, and a polarizing plate can also be formed. In this way, the above-mentioned cured layer can replace the above-mentioned protective film to achieve thinning.

[0133] The polarizing film and the polarizing plate obtained by using the polyvinyl alcohol-based film for manufacturing the polarizing film of this embodiment have excellent polarization performance and are preferably used for liquid crystal display devices such as mobile information terminals, personal computers, televisions, projectors, signs, electronic calculators, electronic clocks, word processors, electronic papers, game machines, video recorders, cameras, photo albums, thermometers, audio equipment, automobiles, and meters of machinery, as well as 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.

[0134] Examples

[0135] Hereinafter, examples are given to explain the present invention more specifically, but the present invention is not limited to the following examples as long as it does not exceed its gist.

[0136] It should be noted that "parts" in the examples refer to mass basis.

[0137] <Measurement conditions>

[0138] <Absolute value (|S|) of the ratio (slope) of the change in the birefringence magnitude from the surface layer to a depth of 1 μm>

[0139] At an arbitrary position in the machine direction (MD) of the polyvinyl alcohol-based film, a fragment with a size of MD×TD = 5 mm×10 mm is cut out from the central part in the transverse direction (TD) of the polyvinyl alcohol-based film. Then, both sides of this fragment are clamped with a PET film having a thickness of 100 μm, and it is further clamped in a jig for slicer measurement and installed in a slicer device.

[0140] Then, the cut-out fragment is cut at intervals of 10 μm parallel to the machine direction (MD) of the fragment to produce a slice for observation (MD×TD = 5 mm×10 μm).

[0141] Next, the slice is laid down in a manner that enables the cutting surface to be observed with the cutting surface facing upward on a glass slide, sealed with a glass cover slip and tricresyl phosphate (refractive index 1.557), and the retardation value is measured using a two-dimensional photoelastic evaluation system "PA-micro" (manufactured by Photonic Lattice).

[0142] With the retardation value distribution of the slice displayed on the measurement screen of "PA-micro", a line segment X perpendicular to the surface of the original polyvinyl alcohol-based film is drawn from one surface to the other surface in a way that cuts across the slice, and line analysis is performed on this line segment X to obtain the retardation value distribution data in the thickness direction of the slice. It should be noted that observation is carried out using a 40-fold objective lens, the line width is set to 3 pixels, and the average value of the retardation values is used.

[0143] The obtained retardation value distribution data in the thickness direction of the slice is divided by the thickness of the slice, 10 μm, to obtain the distribution of birefringence Δn in the thickness direction of the slice. The distributions of each birefringence are arranged to 1000 points using cubic spline interpolation. For the retardation value distribution data arranged to 1000 points, the ratio (slope) of the change in the birefringence magnitude equivalent to 1 μm is calculated from the film surface in contact with the casting surface and the film surface not in contact with the casting mold on the film surface using the least squares method. The absolute value of each calculated slope is taken as the absolute value |S| of the ratio (slope) of the change in the birefringence magnitude from the surface layer to a depth of 1 μm.

[0144] <Moisture content>

[0145] A test piece with a width of 100 mm×a length of 100 mm is cut out from the polyvinyl alcohol-based film sampled just after the drying process using a hot roll. From the mass A (g) before drying and the mass B (g) after drying in a dryer with an atmosphere temperature of 105 °C for 16 hours, the moisture content (mass%) is calculated according to the following formula.

[0146] Moisture content (mass%) = (A - B) / A×100

[0147] <Dissolution amount>

[0148] [Test Method]

[0149] After conditioning the polyvinyl alcohol-based film at 23 °C and 50% RH for 24 hours, cut out 5 test pieces of 100 mm × 100 mm (0.01 m 2 ). Immerse all of them (total 0.05 m 2 ) in 1 L of ion-exchanged water at 50 °C for 1 minute to obtain a leachate. At room temperature (23 °C), mix 10 mL of a color-developing reagent (500 g of ion-exchanged water, 7.4 g of potassium iodide, 0.65 g of iodine, 10.6 g of boric acid) with 10 mL of the above leachate, and then use a spectrophotometer (manufactured by Shimadzu Corporation, UV-3100PC) to measure the absorbance at a wavelength of 690 nm. Calculate the leaching amount per unit volume (mg / cm 3 ) of the polyvinyl alcohol-based resin from a pre-prepared standard curve.

[0150] [Evaluation]

[0151] ○(Very good): The leaching amount is 15 mg / cm 3 or less

[0152] △(Good): The leaching amount is greater than 15 mg / cm 3 and 19 mg / cm 3 or less

[0153] ×(Poor): The leaching amount is greater than 19 mg / cm 3

[0154] <Example 1>

[0155] For 100 parts by mass of a polyvinyl alcohol-based resin with a weight-average molecular weight of 160,000 and a saponification degree of 99.8 mol%, add 12 parts by mass of glycerol and water, heat up to 140 °C and carry out pressure dissolution to obtain a polyvinyl alcohol-based resin aqueous solution with a resin concentration of 26% by mass. Discharge this polyvinyl alcohol-based resin aqueous solution from a T-shaped slit die onto a casting drum, and dry it with a hot roll so that the moisture content of the film becomes 5.5% by mass. Then, perform heat treatment at 127 °C using a floating dryer to obtain a polyvinyl alcohol-based film with a thickness of 30 μm. The characteristics of the obtained polyvinyl alcohol-based film are shown in Table 1.

[0156] <Example 2>

[0157] For 100 parts by mass of a polyvinyl alcohol resin having a weight average molecular weight of 145,000 and a saponification degree of 99.8 mol%, 12 parts by mass of glycerol and water were added, and the temperature was raised to 140 °C for pressure dissolution to obtain an aqueous polyvinyl alcohol resin solution having a resin concentration of 28% by mass. The aqueous polyvinyl alcohol resin solution was discharged from a T-slit die onto a casting drum and dried with a hot roll so that the moisture content of the film became 6.6% by mass. Thereafter, heat treatment was performed 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.

[0158] <Example 3>

[0159] For 100 parts by mass of a polyvinyl alcohol resin having a weight average molecular weight of 130,000 and a saponification degree of 99.8 mol%, 12 parts by mass of glycerol and water were added, and the temperature was raised to 140 °C for pressure dissolution to obtain an aqueous polyvinyl alcohol resin solution having a resin concentration of 29% by mass. The aqueous polyvinyl alcohol resin solution was discharged from a T-slit die onto a casting drum and dried with a hot roll so that the moisture content of the film became 4.6% by mass. Thereafter, heat treatment was performed at 104 °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.

[0160] <Comparative Example 1>

[0161] For 100 parts by mass of a polyvinyl alcohol resin having a weight average molecular weight of 130,000 and a saponification degree of 99.8 mol%, 12 parts by mass of glycerol and water were added, and the temperature was raised to 140 °C for pressure dissolution to obtain an aqueous polyvinyl alcohol resin solution having a resin concentration of 29% by mass. The aqueous polyvinyl alcohol resin solution was discharged from a T-slit die onto a casting drum and dried with a hot roll so that the moisture content of the film became 4.2% by mass. Thereafter, heat treatment was performed at 90 °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.

[0162] <Comparative Example 2>

[0163] For 100 parts by mass of a polyvinyl alcohol resin having a weight average molecular weight of 130,000 and a saponification degree of 99.8 mol%, 12 parts by mass of glycerol and water were added, and the temperature was raised to 140 °C for pressure dissolution to obtain an aqueous polyvinyl alcohol resin solution having a resin concentration of 29% by mass. The aqueous polyvinyl alcohol resin solution was discharged from a T-slit die onto a casting drum and dried with a hot roll so that the moisture content of the film became 2.1% by mass. Thereafter, heat treatment was performed at 90 °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.

[0164] <Comparative Example 3>

[0165] To 100 parts by mass of a polyvinyl alcohol resin having a weight-average molecular weight of 130,000 and a saponification degree of 99.8 mol%, 12 parts by mass of glycerol and water were added, and the temperature was raised to 140°C for pressure dissolution to obtain an aqueous polyvinyl alcohol resin solution having a resin concentration of 29% by mass. The aqueous polyvinyl alcohol resin solution was discharged from a T-shaped slit die onto a casting drum and dried with a hot roll so that the moisture content of the film became 2.0% by mass. Thereafter, heat treatment was performed at 50°C using a floating dryer to obtain a polyvinyl alcohol film having a thickness of 30 μm. The characteristics of the obtained polyvinyl alcohol film are shown in Table 1.

[0166] [Table 1]

[0167]

[0168] It can be seen that for the polyvinyl alcohol films of Examples 1 to 3, the value (|S| / F) obtained by dividing the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm on each surface by the film thickness is within the range defined in the present invention. When manufacturing a polarizing film, the amount of polyvinyl alcohol resin elution is small, the treatment tank is not contaminated during the polarizing film manufacturing process, the obtained polarizing film has few foreign matter defects, and the productivity is good.

[0169] On the other hand, for the polyvinyl alcohol films of Comparative Examples 1 to 3, the value (|S| / F) obtained by dividing the absolute value of the ratio (slope) of the change in birefringence magnitude from the surface layer to a depth of 1 μm on each surface by the film thickness is lower than the lower limit value defined in the present invention. Therefore, the treatment tank is contaminated during the polarizing film manufacturing process, the obtained polarizing film has many foreign matter defects, and the productivity is poor.

[0170] The specific embodiments of the present invention are shown in the above examples, but the above examples are merely simple examples and are not to be construed in a limiting sense. Various modifications that are obvious to those skilled in the art are within the scope of the present invention.

[0171] Industrial Applicability

[0172] The polarizing film and polarizing plate formed of the polyvinyl alcohol film of the present invention have excellent polarization performance and are preferably used for liquid crystal display devices such as mobile information terminals, personal computers, televisions, projectors, signs, electronic desk calculators, electronic clocks, word processors, electronic papers, game machines, video recorders, cameras, photo albums, thermometers, audio equipment, automobiles, and meters of machinery, as well as for anti-reflection layers for sunglasses, anti-glare glasses, 3D glasses, wearable displays, and 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, wherein, For the two surfaces of the polyvinyl alcohol-based film, the region from the surface layer to a depth of 1 μm of each film surface satisfies the following formula (1). |S| / F≥5×10 -7 …(1) Here, |S| is the absolute value of the slope, which is the ratio of the change in the birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness of the film, with the unit of the film thickness being μm.

2. The polyvinyl alcohol-based film according to claim 1, wherein, For at least one film surface of the polyvinyl alcohol-based film, the region from the surface layer to a depth of 1 μm of the film surface satisfies the following formula (2). |S| / F≥9×10 -7 …(2) Here, |S| is the absolute value of the slope, which is the ratio of the change in the birefringence magnitude from the surface layer to a depth of 1 μm, and F is the film thickness of the film, with the unit of the film thickness being μm.

3. The polyvinyl alcohol-based film according to claim 1 or 2, wherein, The film thickness F of the film is 10 to 60 μm.

4. A polarizing film that uses the polyvinyl alcohol-based film described in claim 1 or 2.

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