Polyvinyl alcohol-based film for polarizing film production, method for producing polyvinyl alcohol-based film for polarizing film production, polarizing film

Abstract

Provided is a polyvinyl alcohol-based film for polarizing film production, which has a small amount of elution of polyvinyl alcohol-based resin in water during production of a polarizing film, and which can be used to produce a polarizing film with excellent stretchability and few foreign matter defects at a good production rate. The polyvinyl alcohol-based film for polarizing film production has an elastic modulus in water at 50°C of 0.7 to 1.8 MPa.

Description

Technical Field

[0001] This invention relates to polyvinyl alcohol-based films for manufacturing polarizing films, and more specifically, to polyvinyl alcohol-based films useful as blank films for manufacturing polarizing films, which have low impurity elution in water during the manufacturing of polarizing films and excellent stretchability, thus enabling the production of polarizing films with low foreign matter defects with good productivity. Background Technology

[0002] In recent years, liquid crystal display (LCD) devices have seen significant development and are widely used in smartphones, tablets, personal computers, LCD TVs, projectors, automotive panels, and more. These LCD devices utilize polarizing films, primarily those obtained by oriented iodine adsorption onto polyvinyl alcohol (PVA) films. In recent years, with the increasing demands for higher resolution, higher brightness, larger size, and thinner screens, there is a need for polarizing films with superior polarization performance compared to existing products, free of color spots, and with a wide and long width.

[0003] Typically, polarizing films are manufactured as follows: a polyvinyl alcohol (PVA) film, serving as a preform, is rolled out from a roller, conveyed along its length (MD direction), and swollen in water (including hot water). It then undergoes processes such as iodine dyeing, stretching for iodine orientation, and boric acid crosslinking to fix the orientation. Defects occurring in these processes can significantly reduce the productivity of polarizing films. For example, during the swelling process, if impurities are washed off from the PVA film and contaminate the swelling tank, the contamination can spread to subsequent processes. Similarly, if impurities are washed off from the PVA film during the dyeing and boric acid crosslinking processes, not only will the polarization performance of the resulting polarizing film decrease, but the filtration and exchange of chemical solutions used in each process will require considerable labor. Examples of such impurities include low-molecular-weight PVA resins (including oligomers) present in the PVA film, particularly those with a molecular weight below 50,000, which are easily washed off in water. Furthermore, these impurities tend to form low-molecular-weight iodine complexes that reduce polarization.

[0004] Furthermore, the low molecular weight polyvinyl alcohol resin after elution may form an association with iodine and boric acid in the polarizing film manufacturing process, becoming a foreign substance. This foreign substance adheres to the surface of the polarizing film during processing, thus becoming a foreign defect in the final polarizing film.

[0005] On the other hand, polyvinyl alcohol (PVA) films used as preforms are typically manufactured as follows: PVA resin, which serves as the raw material, is dissolved in water, and the film is produced by continuous casting from the aqueous solution (film-forming solution). Specifically, the manufacturing process involves: discharging the aqueous solution of PVA resin into a casting mold such as a casting drum or ring belt and casting the film by casting; peeling the resulting film from the casting mold; conveying it along its length direction (MD direction); and simultaneously drying and heat-treating. Among these processes, the drying process is crucial for controlling the elution components from the PVA film. If the drying method in the aforementioned drying process is inappropriate, there is a tendency for an increase in the amount of PVA resin eluted during the manufacture of polarizing films.

[0006] As an improvement to the above-mentioned problem, for example, a polyvinyl alcohol film with reduced elution of polyvinyl alcohol resin in water has been proposed (see Patent Document 1); a polyvinyl alcohol film mixed with polyvinyl alcohol resins of different molecular weights has been proposed (see Patent Document 2).

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2009-221462

[0010] Patent Document 2: Japanese Patent Application Publication No. 2018-135426 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] However, in the technology disclosed in Patent Document 1, the eluent concentration of polyvinyl alcohol resin when a 10cm square film is immersed in 1 liter of water at 50°C for 4 hours is 10-50 ppm. But if the film... 2 Conversion would elute 1000–5000 ppm / m 2 Polyvinyl alcohol-based resins, in order to manufacture polarizing films with high production efficiency, require a further reduction in the amount of resin eluted, based on the demand for polarizing films in recent years.

[0013] In addition, although the polyvinyl alcohol-based film with high ultimate tensile ratio and good stretchability is obtained in the technology disclosed in the aforementioned patent document 2, the low molecular weight polyvinyl alcohol-based resin component after mixing is washed off in the polarizing film manufacturing process, causing chemical solution contamination and an increase in foreign matter defects. Therefore, it is difficult to achieve both the improvement of stretchability and the reduction of foreign matter defects.

[0014] Therefore, in this context, the present invention provides a polyvinyl alcohol film for manufacturing polarizing films that can produce polarizing films with good productivity, low elution of polyvinyl alcohol resin in water during manufacturing, excellent tensile properties, and few foreign matter defects.

[0015] Solution for solving the problem

[0016] Therefore, the inventors focused on the relationship between the elastic modulus of polyvinyl alcohol films in water at 50°C and the elution amount and tensile properties of polyvinyl alcohol resin in water, and found that when the elastic modulus of polyvinyl alcohol films in water at 50°C is within a specific range, the above-mentioned problems can be solved, and polarizing films with fewer foreign matter defects can be manufactured with high productivity, thus completing the present invention.

[0017] That is, the main idea of ​​the present invention is in the following [1] to [4].

[0018] [1] A polyvinyl alcohol film for manufacturing polarizing film, wherein the elastic modulus in water at 50°C is 0.7 to 1.8 MPa.

[0019] [2] The polyvinyl alcohol film used for manufacturing polarizing film according to [1] has a thickness of 5 to 50 μm.

[0020] [3] A method for manufacturing a polyvinyl alcohol-based film for polarizing film manufacturing, comprising the following steps: a film forming step, wherein an aqueous solution of polyvinyl alcohol-based resin is continuously discharged into a casting mold and cast to form a film; a drying step, wherein the film is dried; and a heat treatment step, wherein the dried film is heat treated.

[0021] When the moisture content of the film immediately after being peeled from the casting mold in the aforementioned film-making process is set as A [mass%], and the moisture content of the film 35 seconds after being peeled from the casting mold in the aforementioned drying process is set as B [mass%], the drying is carried out in a manner that satisfies the following formula (1).

[0022] 0.23≤B / A≤0.45…(1)

[0023] [4] A polarizing film obtained by manufacturing a polyvinyl alcohol-based film using any one of the polarizing films described in [1] to [3].

[0024] The effects of the invention

[0025] The polyvinyl alcohol-based film used in the manufacturing of polarizing films of the present invention has a low elution amount of polyvinyl alcohol resin and excellent tensile properties. Therefore, it is useful as a preform film for manufacturing polarizing films with few foreign matter defects with high productivity. Detailed Implementation

[0026] The present invention will now be described in detail. However, the present invention is not limited to the embodiments described below.

[0027] It should be noted that, in this invention, "main component" refers to the component that has a significant impact on the properties of the object. The content of this component is typically 40% by mass or more in the object, preferably 45% by mass or more, and more preferably 50% by mass or more. Furthermore, when the component with the highest mass percentage in the object is large, accounting for 50% by mass or more, it is envisioned that it accounts for 55% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more (including 100% by mass).

[0028] In addition, in this invention, X and / or Y (X and Y can be any configuration) means at least one of X and Y, and refers to the three meanings of only X, only Y, and X and Y.

[0029] The most significant feature of the polyvinyl alcohol-based film for manufacturing polarizing film of the present invention (hereinafter, sometimes simply referred to as "polyvinyl alcohol-based film") is that it is a polyvinyl alcohol-based film with an elastic modulus of 0.7 to 1.8 MPa in water at 50°C.

[0030] The elastic modulus of the polyvinyl alcohol film used in the manufacture of the above-mentioned polarizing film in water at 50°C is preferably 0.74 to 1.75 MPa, more preferably 0.78 to 1.70 MPa, even more preferably 0.82 to 1.65 MPa, particularly preferably 0.85 to 1.63 MPa, and especially preferably 0.87 to 1.60 MPa.

[0031] By ensuring that the elastic modulus in water at 50°C meets the above-mentioned range, it is possible to balance the reduction of elution volume with the improvement of stretchability. Therefore, polarizing films with fewer foreign matter defects can be obtained with good productivity.

[0032] It should be noted that the elastic modulus of polyvinyl alcohol films used in polarizing film manufacturing, produced by conventional manufacturing methods, in water at 50°C is not within the above range, and is typically 2.05 to 2.65 MPa.

[0033] If the elastic modulus in water at 50°C is below the lower limit, the tensile properties improve, but the tension during stretching decreases, increasing foreign matter defects in the polarizing film and resulting in reduced productivity. Conversely, if it is above the upper limit, the elution rate decreases, reducing foreign matter defects in the polarizing film, but the tension during stretching becomes excessively high, leading to reduced productivity due to decreased tensile properties, and ultimately, production losses due to breakage.

[0034] In this invention, after determining the elastic modulus in the flow direction (MD) and the elastic modulus in the width direction (TD) of water at 50°C according to the following method, the value obtained by taking their root mean square as the elastic modulus in water at 50°C.

[0035] (Determination of the elastic modulus of MD in water at 50℃)

[0036] A test piece was cut from a polyvinyl alcohol film that had been stored at 23°C and 50% RH for 24 hours, with dimensions of TD (5 mm) × MD (25 mm) at the center of the film width. The measurement width of the test piece was measured using a digital microscope. The thickness of the test piece was measured at any three points using a contact thickness gauge, and the average value was taken as the measured thickness.

[0037] Next, the test piece was fixed in the viscoelasticity measuring apparatus (IT Measurement Control Co., Ltd., "DVA-225") with the clamp spacing at both ends of the short side (TD side) set to 15 mm. The dynamic strain was set to 0.3%, the frequency to 10 Hz, and the upper limit elongation to 80%, and the elastic modulus was measured. The measurement was conducted as follows: first, the elastic modulus in the atmosphere was measured for 100 seconds, followed by the elastic modulus in water at 50°C for 600 seconds.

[0038] For the measurement results, the average value of the elastic modulus in water at 50°C for 200 to 600 seconds after the start of the measurement was obtained. The value obtained after thickness correction after the measurement was taken as the elastic modulus of MD in water at 50°C (the same measurement was performed 3 times and the average value was used).

[0039] It should be noted that the thickness after measurement refers to the average value of the thickness of the test piece after the viscoelasticity test in water was measured using a contact film thickness gauge at three points.

[0040] (Determination of the elastic modulus of TD in water at 50℃)

[0041] Cut out a test piece to the size of TD (25 mm) × MD (5 mm). Otherwise, perform the same measurement as the elastic modulus measurement of MD to determine the elastic modulus of TD in water at 50°C.

[0042] As a method to control the elastic modulus in water at 50°C to a specific range, the following methods can be cited: adjusting the molecular weight and saponification degree of the polyvinyl alcohol resin used as raw material; introducing trace amounts of modifying groups into the polyvinyl alcohol resin; cleaning the polyvinyl alcohol resin to improve its purity; controlling the crystallinity and orientation state of the polymer in the process of forming a film from an aqueous solution of polyvinyl alcohol resin; controlling the drying state of the film after film formation and neutralization; etc., by using two or more of these methods individually or in combination, the elastic modulus can be controlled to a specific range. Among these, the method of controlling the drying state of the film after film formation and neutralization is preferred, and the method of controlling the drying state of the film using the specific drying conditions described later is even more preferred.

[0043] Furthermore, the entanglement density of the polyvinyl alcohol resin in the polyvinyl alcohol-based film of the present invention is preferably greater than 0.8 × 10⁻⁶. - 4 mol / cm 3 .

[0044] The entanglement density of the polyvinyl alcohol resin in the above-mentioned polyvinyl alcohol-based film is more preferably 0.85 × 10⁻⁶. - 4 mol / cm 3 Above, with a preferred value of 0.9×10 -4 mol / cm 3 Above, further optimization of 1.0×10 -4 mol / cm 3 By satisfying the above-mentioned range, there is a tendency to obtain a polarizing film with high polarization performance with good productivity due to the reduction in the amount of polyvinyl alcohol resin eluted and the reduction in foreign matter defects caused by the entanglement effect.

[0045] If the lower limit of the entanglement density of the polyvinyl alcohol-based resin is too low, there is a tendency for the foreign matter defects in the polarizing film to increase due to the increase in the amount of polyvinyl alcohol-based resin eluted, and the polarization performance to become more easily reduced.

[0046] On the other hand, the upper limit of the entanglement density of the polyvinyl alcohol resin in the above-mentioned polyvinyl alcohol-based film is preferably 3.0 × 10⁻⁶. -4 mol / cm 3 The following is a preferred option: 2.8 × 10⁻⁶ -4 mol / cm 3 The following is a further preferred option: 2.6 × 10 -4 mol / cm 3 The following, especially preferred, is 2.4 × 10⁻⁶. -4 mol / cm 3 the following.

[0047] If the upper limit of the entanglement density of the polyvinyl alcohol resin is too high, the entanglement of the polyvinyl alcohol resin will increase excessively, resulting in increased tension applied to the polyvinyl alcohol film during the polarizing film manufacturing process, making it more prone to breakage.

[0048] The entanglement density of the above polyvinyl alcohol resin can be calculated according to the following formula (1).

[0049]

[0050] (E is the elastic modulus in water, k) B (where is the Boltzmann constant and T is the measurement temperature.)

[0051] As a method to control the entanglement density of the aforementioned polyvinyl alcohol resin within a specific range, the following methods can be cited: adjusting the molecular weight and saponification degree of the polyvinyl alcohol resin used as raw material; introducing trace amounts of modifying groups into the polyvinyl alcohol resin; cleaning the polyvinyl alcohol resin to improve its purity; controlling the crystallinity and orientation state of the polymer in the process of forming a film from an aqueous solution of polyvinyl alcohol resin; controlling the drying state of the film after film formation; etc. By using two or more of these methods individually or in combination, the entanglement density of the polyvinyl alcohol resin can be made to be within a specific range.

[0052] Among these, the preferred method is to control the drying state of the film after film formation, and in particular, the preferred method is to use specific drying conditions described later to control the drying state of the film during the drying process.

[0053] The polyvinyl alcohol-based film for manufacturing polarizing films of the present invention uses polyvinyl alcohol resin as the main component. Hereinafter, the manufacturing method of the polyvinyl alcohol-based film for manufacturing polarizing films of the present invention will be described.

[0054] Typically, polyvinyl alcohol (PVA) films are manufactured through the following steps: a film-forming step in which an aqueous solution of PVA resin is continuously discharged into a casting mold and cast to form a film; a drying step in which the film is dried; and a heat treatment step in which the dried film is heat-treated. The manufacturing method of the present invention, as described below, is characterized by the following feature: when the moisture content of the film immediately after being peeled from the casting mold in the aforementioned film-forming step is set as A [mass %], and the moisture content of the film 35 seconds after being peeled from the casting mold in the aforementioned drying step is set as B [mass %], B / A is set to a specific range.

[0055] <Film-making process>

[0056] The polyvinyl alcohol (PVA) resin used in this invention is typically a resin obtained by saponifying unmodified PVA resin, i.e., polyvinyl acetate obtained by polymerizing vinyl acetate. Depending on the requirements, a resin obtained by saponifying a copolymer of vinyl acetate with a small amount (usually 10 mol% or less, preferably 5 mol% or less) of a component capable of copolymerizing with vinyl acetate may also be used. Examples of components capable of copolymerizing with vinyl acetate include unsaturated carboxylic acids (e.g., containing salts, esters, amides, nitriles, etc.), olefins with 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, and unsaturated sulfonates. Alternatively, a modified PVA resin obtained by chemically modifying the saponified hydroxyl groups may also be used.

[0057] In addition, polyvinyl alcohol resins having a 1,2-diol structure in the side chain can also be used as polyvinyl alcohol resins. The aforementioned polyvinyl alcohol resins having a 1,2-diol structure in the side chain can be obtained, for example, by methods such as: saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene; saponifying and decarboxylating a copolymer of vinyl acetate and vinylene carbonate; saponifying and deketating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane; saponifying a copolymer of vinyl acetate and glyceryl monoallyl ether; etc.

[0058] The weight-average molecular weight of polyvinyl alcohol (PVA) resins is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If the weight-average molecular weight is too low, the polarization of the polarizing film tends to decrease; if it is too high, stretching during the manufacture of the polarizing film tends to become more difficult. It should be noted that the weight-average molecular weight of the PVA resins mentioned above is determined using the GPC-MALS method.

[0059] The average saponification degree of the polyvinyl alcohol resin used in this invention is preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, and especially preferably 99.8 mol% or more. If the above-mentioned average saponification degree is too low, there is a tendency for the polarization degree of the polarizing film to decrease.

[0060] Here, the average degree of saponification in this invention is determined according to JIS K 6726.

[0061] As the polyvinyl alcohol resin used in this invention, two or more polyvinyl alcohol resins with different modifiers, modification amounts, weight-average molecular weights, and average saponification degrees can also be used together.

[0062] An aqueous solution of the above-mentioned polyvinyl alcohol resin was prepared as a film-forming stock solution.

[0063] First, the polyvinyl alcohol-based resin is washed with water and then dehydrated using a centrifuge or similar device to preferably produce a polyvinyl alcohol-based resin wet filter cake with a water content of 50% by mass or less. If the water content is too high, it tends to become difficult to form the desired aqueous solution concentration.

[0064] The above-mentioned polyvinyl alcohol-based resin wet filter cake is dissolved in warm water and hot water to prepare a polyvinyl alcohol-based resin aqueous solution.

[0065] There are no particular limitations on the preparation method of polyvinyl alcohol-based resin aqueous solution. For example, it can be prepared using a heated multi-screw extruder. Alternatively, the aforementioned polyvinyl alcohol-based resin wet filter cake can be added to a dissolving vessel equipped with upper and lower circulating flow generating stirring blades, and water vapor can be blown into the vessel to dissolve and prepare an aqueous solution of the desired concentration.

[0066] In addition to the polyvinyl alcohol resin, the aqueous solution of the polyvinyl alcohol resin also contains commonly used plasticizers such as glycerol, diglycerol, triglycerol, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, as well as nonionic, anionic, and / or cationic surfactants, which are preferred for the film-forming properties of polyvinyl alcohol films. These substances can be used alone or in combination of two or more.

[0067] The resin concentration of the polyvinyl alcohol-based resin aqueous solution obtained in this way is preferably 15-60% by mass, particularly preferably 17-55% by mass, and even more preferably 20-50% by mass. If the resin concentration of the above aqueous solution is too low, the drying load will increase, and therefore, there is a tendency to reduce production capacity. If it is too high, the viscosity will become excessively high, making it difficult to form a uniform solution.

[0068] In the aforementioned film-forming process, the above-mentioned polyvinyl alcohol-based resin aqueous solution is discharged into a rotating casting mold and cast to form a film, and then the obtained film is peeled off from the casting mold.

[0069] Here, examples of casting molds include casting drums (drum-shaped rollers), ring belts, and resin films, but from the perspective of wide width, long length, and excellent uniformity of film thickness, casting drums are preferred.

[0070] The following explanation will take the case where the casting mold is the casting drum as an example.

[0071] First, the obtained polyvinyl alcohol-based resin aqueous solution is subjected to degassing treatment. Examples of degassing methods include static degassing and degassing using a multi-screw extruder with a vent. As for the multi-screw extruder with a vent, a twin-screw extruder with a vent is typically used.

[0072] After degassing, a polyvinyl alcohol-based resin aqueous solution is introduced into a T-slit mold in a constant amount each time, discharged onto a rotating casting drum and cast, and film is formed according to the continuous casting method.

[0073] The temperature of the polyvinyl alcohol-based resin aqueous solution at the T-slit die outlet is preferably 80–100°C, and particularly preferably 85–98°C. If the temperature of the above-mentioned polyvinyl alcohol-based resin aqueous solution is too low, it tends to have poor flowability; if it is too high, it tends to foam.

[0074] The viscosity of the above-mentioned polyvinyl alcohol-based resin aqueous solution is preferably 50-200 Pa·s, and particularly preferably 70-150 Pa·s. If the viscosity of the above-mentioned aqueous solution is too low, it tends to have poor flowability; if it is too high, it tends to have difficulty in casting.

[0075] The discharge rate of the polyvinyl alcohol-based resin aqueous solution from the T-slit mold to the casting drum is preferably 0.1–5 m / min, particularly preferably 0.2–4 m / min, and even more preferably 0.3–3 m / min. If the discharge rate is too slow, productivity tends to decrease; if it is too fast, casting becomes difficult.

[0076] As casting drums, they are typically made by applying a metal plating to the surface of stainless steel (SUS), which is mainly composed of iron, to prevent scratches. Examples of metal plating include chromium plating, nickel plating, and zinc plating, which can be used alone or in layers of two or more. Among these, from the perspective of the durability of the drum surface, chromium plating is preferred for the outermost surface.

[0077] The diameter of the casting drum is preferably 2-5m, particularly preferably 2.4-4.5m, and even more preferably 2.8-4m. If the diameter is too small, the drying length will be insufficient, making it difficult to achieve high speed; if the diameter is too large, the transportability will be reduced.

[0078] The width of the casting drum is preferably 4-7m, and particularly preferably 5-6m. If the width of the casting drum is too narrow, there is a tendency to reduce productivity; if the width is too wide, there is a tendency to increase the equipment load.

[0079] The rotational speed of the casting drum is preferably 3 to 50 m / min, particularly preferably 4 to 40 m / min, and even more preferably 5 to 35 m / min. If the rotational speed is too slow, the productivity tends to decrease; if it is too fast, the peelability of the film obtained by peeling it from the casting mold tends to decrease.

[0080] The surface temperature of the casting drum is preferably 40–99°C, more preferably 50–98°C, even more preferably 60–97°C, particularly preferably 70–96°C, and especially preferably 80–95°C. If the surface temperature is too low, the peelability of the film obtained by peeling it from the casting mold for film formation tends to decrease; if the surface temperature is too high, foaming tends to occur.

[0081] The film is thus formed, and the resulting film should be peeled off from the casting drum, but the moisture content of the film at this time is defined as A [mass %].

[0082] Methods for adjusting the moisture content A [mass %] of the film immediately after being peeled from the casting mold are not particularly limited to the following specific examples. Examples include: adding organic solvents, plasticizers, and surfactants to an aqueous solution of polyvinyl alcohol (PVA) resin; adjusting the resin concentration of the PVA resin aqueous solution; adjusting the temperature of the PVA resin aqueous solution; adjusting the discharge rate of the PVA resin aqueous solution; adjusting the rotation speed and surface temperature of the casting drum; irradiating with infrared radiation; processing under a reduced pressure atmosphere; supplying air; blowing hot air; adjusting the hot air temperature and speed; processing under a pressurized atmosphere; processing under a humidified atmosphere; blowing steam; etc.

[0083] It should be noted that the moisture content A [mass %] is determined according to the method described later.

[0084] <Drying Process>

[0085] Next, the drying process for drying the obtained film will be explained.

[0086] In this invention, when the moisture content of the film immediately after being peeled from the casting mold is set as A [mass%] and the moisture content of the film after being peeled from the casting mold for 35 seconds is set as B [mass%], and drying is carried out in a manner that satisfies the following formula (1), it is preferable in that the elastic modulus in water at 50°C can be adjusted to a suitable range. Regarding the value of B / A, it is particularly preferable that it satisfies the following formula (1'), and even more preferably that it satisfies the following formula (1”).

[0087] 0.23≤B / A≤0.45…(1)

[0088] 0.235≤B / A≤0.40…(1')

[0089] 0.24≤B / A≤0.35…(1”)

[0090] If the B / A value is too low, there is a tendency for the elastic modulus in water at 50°C to decrease excessively; if it is too high, there is a tendency for the elastic modulus in water at 50°C to increase excessively.

[0091] It should be noted that, from the point of view of production stability, the manufacturing method of polyvinyl alcohol-based films for general polarizing film manufacturing prefers drying conditions that are milder than the manufacturing method of the present invention, and the B / A ratio is usually around 0.49 to 0.55, which does not satisfy the above formula (1).

[0092] As for methods to control the film moisture content B [mass %] 35 seconds after the film is peeled from the casting mold (casting drum), they are not limited to the following specific examples. For example, the following methods can be cited: adding organic solvents, plasticizers, and surfactants to an aqueous solution of polyvinyl alcohol resin; contacting the film with multiple metal heating rollers; introducing it into a heating oven; irradiating it with infrared light; treating it under a reduced pressure atmosphere; supplying air; blowing hot air; adjusting the hot air temperature and hot air velocity; treating it under a pressurized atmosphere; treating it under a humidified atmosphere; blowing water vapor; etc.

[0093] Furthermore, in this invention, the drying process is preferably carried out in a manner where the time it takes for the film moisture content A [mass %] immediately after being peeled from the casting mold to reach 25% is less than 35 seconds. By ensuring that the time for the film moisture content A [mass %] to reach 25% is less than 35 seconds, the entanglement density of the polyvinyl alcohol resin can be controlled within a suitable range, reducing the leaching of the polyvinyl alcohol resin and decreasing the tendency for foreign matter defects in the polarizing film. For the aforementioned elapsed time, 34 seconds or less is particularly preferred, and more preferably 33 seconds or less.

[0094] If the above-mentioned time is too long, there is a tendency for the entanglement density of polyvinyl alcohol resin to become excessively small.

[0095] Furthermore, from the viewpoint of reducing equipment load, it is preferable to perform drying in a manner where the aforementioned drying time is 5 seconds or more, and particularly preferably 10 seconds or more.

[0096] The drying conditions within the range described above, used to bring the moisture content of the aforementioned film to 25% of the moisture content A [mass %] of the film immediately after being peeled from the casting mold, are not particularly limited to the following specific examples. For example, the following methods can be cited: adding organic solvents, plasticizers, and surfactants to an aqueous solution of polyvinyl alcohol resin; alternating contact of the surface and back of the film with multiple metal heating rollers; introducing the film into a heated oven; irradiating the film with infrared light; treating the film under reduced pressure; blowing air onto the film; blowing hot air onto the film; adjusting the hot air temperature and hot air velocity; treating the film under pressurized atmosphere; treating the film under humidified atmosphere; blowing water vapor onto the film; etc.

[0097] Specifically, for example, in a method where the surface and back of a film are alternately contacted with multiple metal heating rollers (hereinafter sometimes simply referred to as "heat rollers"), the heat rollers are not particularly limited as long as they are made of metal and have a heating function. Preferably, they are rollers with a diameter of 0.2 to 2 μm that have undergone hard chrome plating or mirror finishing. The number of heat rollers is typically 2 to 30, preferably 10 to 25. Preferably, the surface temperature of the heat rollers is heated to 40°C or higher and then adjusted. It should be noted that the upper limit of the surface temperature of the heat rollers is typically 120°C.

[0098] <Heat Treatment Process>

[0099] In this invention, it is preferable to perform heat treatment on the dried film. To ensure uniform drying on both sides of the film, heat treatment from both sides is particularly preferred. Examples of heat treatment methods include: blowing hot air onto both sides of the film using a floating dryer; and irradiating both sides of the film with near-infrared light using infrared lamps.

[0100] The heat treatment temperature is preferably 50–150°C, and particularly preferably 70–120°C. The heat treatment time is not particularly limited; when using a floating dryer, it is preferably 10–100 seconds, and particularly preferably 20–80 seconds. It should be noted that the heat treatment method in this invention is designed to be a method without the use of hot rollers.

[0101] The moisture content of the film after the heat treatment process is preferably less than 5.0% by mass, particularly preferably 0.1 to 4.5% by mass, further preferably 0.2 to 4.0% by mass, and especially preferably 0.3 to 3.5% by mass.

[0102] If the moisture content is too high, the resulting polyvinyl alcohol film tends to be poorly dried.

[0103] For films that have undergone heat treatment as needed after drying, the two ends in the width direction are cut and rolled into a product (polyvinyl alcohol film).

[0104] Regarding the length of the polyvinyl alcohol film thus obtained, from the perspective of maximizing the area of ​​the polarizing film, it is preferable to be 5 km or more, and from the perspective of transport quality, it is particularly preferable to be 5 to 50 km.

[0105] Regarding the width of polyvinyl alcohol-based films, in terms of widening the polarizing film, 4m or more is preferred, and 5m or more is particularly preferred. From the perspective of avoiding breakage during the manufacture of polarizing films, 5 to 6m is even more preferred.

[0106] In addition, the thickness of the polyvinyl alcohol film is preferably 5 to 50 μm, and in terms of thinning, it is particularly preferred to be 10 to 50 μm, and even more preferably 15 to 45 μm.

[0107] It should be noted that the method for manufacturing polyvinyl alcohol films is described using the example of casting drums (drum-shaped rollers) as casting molds, but casting tapes or resin films can also be used as casting molds.

[0108] Thus, a polyvinyl alcohol-based thin film for manufacturing polarizing films according to the present invention is obtained.

[0109] The polyvinyl alcohol-based film for manufacturing polarizing film of the present invention exhibits low polyvinyl alcohol resin leaching during water immersion and excellent tensile properties, therefore, it is particularly preferred as a raw material for polarizing film.

[0110] The following describes a method for manufacturing a polarizing film using a polyvinyl alcohol-based film for polarizing film manufacturing according to the present invention.

[0111] The polarizing film of the present invention is manufactured as follows: the above-mentioned polyvinyl alcohol film for manufacturing polarizing film is transported horizontally from the roll and manufactured through processes such as swelling, dyeing, boric acid crosslinking, stretching, cleaning, and drying.

[0112] The swelling process is performed before the dyeing process. Through the swelling process, contaminants on the surface of the polyvinyl alcohol (PVA) film used in polarizing film manufacturing can be cleaned. Furthermore, the swelling of the PVA film helps prevent uneven dyeing. Water is typically used as the treatment solution in the swelling process. The solution only needs to be primarily water, but small amounts of iodinated compounds, surfactants, alcohols, etc., can also be added. The temperature of the swelling bath is usually around 10–45°C, and the immersion time is usually around 0.1–10 minutes.

[0113] The dyeing process is carried out by contacting the film with a liquid containing iodine or a dichroic dye. An aqueous solution of iodine and potassium iodide is typically used; preferably, the concentration of iodine is 0.1–2 g / L and the concentration of potassium iodide is 1–100 g / L. A dyeing time of approximately 30–500 seconds is practical. The temperature of the treatment bath is preferably 5–50°C. In addition to the aqueous solvent, the aqueous solution may contain a small amount of water-soluble organic solvent.

[0114] The boric acid crosslinking process is carried out using boron compounds such as boric acid and borax. The boron compounds are used in the form of aqueous solutions or water-organic solvent mixtures at a concentration of approximately 10–100 g / L. The presence of potassium iodide in the liquid is preferred for stabilizing polarization properties. The preferred processing temperature is approximately 30–70°C, and the processing time is approximately 0.1–20 minutes. Stretching operations can be performed during the process if necessary.

[0115] The stretching process is preferably performed along a single axis at a stretch ratio of 3 to 10 times, more preferably 3.5 to 6 times. Alternatively, some stretching can be performed along a direction perpendicular to the stretching direction (to prevent shrinkage in the width direction, or even more). The stretching temperature is preferably 30 to 170°C. Furthermore, the stretching ratio can be ultimately set within the aforementioned range, and the stretching operation is not limited to being performed in a single stage; it can be performed at any stage within the manufacturing process.

[0116] The cleaning process, for example, involves immersing a polyvinyl alcohol-based film used in the manufacture of polarizing films in an aqueous solution of iodides such as water or potassium iodide, which can remove precipitates formed on the surface of the film. The concentration of potassium iodide in the aqueous solution can be approximately 1–80 g / L. The cleaning temperature is typically 5–50°C, preferably 10–45°C. The processing time is typically 1–300 seconds, preferably 10–240 seconds. It should be noted that water cleaning and cleaning using an aqueous solution of potassium iodide can be appropriately combined.

[0117] The drying process only needs to be carried out in the atmosphere at 40-80°C for 1-10 minutes.

[0118] Furthermore, the polarization degree of the polarizing film is preferably 99.8% or higher, more preferably 99.9% or higher. If the polarization degree is too low, there is a tendency to make it impossible to ensure the contrast ratio in the liquid crystal display.

[0119] It should be noted that the polarization is calculated as follows: based on the transmittance (H) measured at wavelength λ when two polarizing films are typically overlapped in such a way that their orientations are aligned. 11 The transmittance (H1) measured at wavelength λ, with the two polarizing films overlapping in a manner in which their orientation directions are orthogonal to each other, is calculated according to the following formula.

[0120] [(H] 11 -H1) / (H 11 +H1)〕 1 / 2

[0121] Furthermore, the transmittance of the polarizing film of the present invention is preferably 42% or more, more preferably 43% or more. If the transmittance of the film is too low, it tends to become impossible to achieve high brightness in the liquid crystal display.

[0122] The transmittance of a single film is a value obtained by measuring the transmittance of the polarizing film individually using a spectrophotometer.

[0123] Thus, the polarizing film of the present invention is obtained. However, the polarizing film of the present invention is suitable for manufacturing polarizing plates with low foreign object defects and no color spots with good productivity.

[0124] The manufacturing method of the polarizing plate of the present invention will be described below.

[0125] The polarizing film of the present invention has an optically isotropic resin film bonded to one or both sides of its surface with an adhesive to serve as a protective film, thus forming a polarizing plate. Examples of protective films include films or sheets of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cyclic olefin polymers, cyclic olefin copolymers, polystyrene, polyethersulfone, polyarylate, poly-4-methylpentene, and polyphenylene ether.

[0126] The bonding method can be carried out using known techniques, such as uniformly coating a liquid adhesive composition onto a polarizing film, a protective film, or both, then bonding and pressing the two together, followed by heating or irradiation with active energy rays.

[0127] In addition, for the purpose of thinning, polarizing film can also be formed by coating one or both sides of it with curable resins such as urethane resin, acrylic resin, and urea resin and curing it, instead of the above-mentioned protective film.

[0128] The polarizing film and polarizing plate formed from the polyvinyl alcohol-based thin film for manufacturing polarizing film of the present invention have excellent polarization performance and are preferably used in portable information terminal devices, personal computers, televisions, projectors, information billboards, desktop electronic calculators, electronic clocks, word processors, electronic paper, game consoles, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices for automobiles or machinery, sunglasses, anti-glare glasses, stereo glasses, wearable displays, anti-reflective layers for display components (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical devices, building materials, toys, etc.

[0129] Example

[0130] The following examples illustrate the present invention in more detail, but the present invention is not limited to the following examples as long as it does not deviate from its spirit.

[0131] It should be noted that in the example, "part" refers to the quality standard.

[0132] <Example 1>

[0133] (Preparation of polyvinyl alcohol-based films)

[0134] 2000 kg of polyvinyl alcohol (PVA) resin with a weight-average molecular weight of 132,000 and an average saponification degree of 99.8 mol%, 5000 kg of water, and 220 kg of glycerol as a plasticizer were added. The mixture was stirred and heated to 140°C under pressure to dissolve the resin. The concentration was then adjusted to 25% to obtain a uniformly dissolved PVA resin aqueous solution. Next, this PVA resin aqueous solution was fed to a twin-screw extruder with a vent and degassed. The aqueous solution temperature was set to 95°C, and the film was discharged from a T-slit die outlet into a rotating casting drum (discharge speed 2.5 m / min) for film casting. Then, the film moisture content was adjusted while drying, with the ratio of the moisture content A [mass %] of the film immediately after peeling from the casting die and the moisture content B [mass %] of the film after peeling from the casting drum for 35 seconds set to 0.248. Finally, the film was heat-treated using a floating dryer, with hot air at 90°C blown from both sides. Finally, the two ends were cut and wound into a roll to obtain a polyvinyl alcohol film with a thickness of 45 μm, a width of 5 m, and a length of 5 km. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

[0135] (Manufacturing of polarizing film)

[0136] The obtained polyvinyl alcohol-based film was unwound and conveyed horizontally while being immersed in a water bath at 30°C to swell. Simultaneously, the original roll was stretched to 1.7 times its original length along the flow direction (MD). Next, it was immersed in an aqueous solution (30°C) containing 0.5 g / L iodine and 30 g / L potassium iodide, and while dyeing, the original roll was stretched to 2.7 times its original length along the flow direction (MD). Then, it was immersed in an aqueous solution (55°C, referred to as the stretching bath) containing 40 g / L boric acid and 30 g / L potassium iodide, and while boric acid crosslinking, the original roll was uniaxially stretched to 5.7 times its original length along the flow direction (MD). Finally, it was washed with a potassium iodide aqueous solution and dried at 70°C for 2 minutes to obtain a polarizing film with a longitudinal stretch ratio of 5.7 times. The characteristics of the obtained polarizing film are shown in Table 1.

[0137] <Examples 2-3>

[0138] As shown in Table 1, the B / A value was changed from the moisture content A [mass %] of the film immediately after being peeled from the casting mold and the moisture content B [mass %] of the film after being peeled from the casting mold for 35 seconds. Otherwise, a polyvinyl alcohol-based film was obtained in the same manner as in Example 1. The characteristics of the obtained polyvinyl alcohol-based film and the polarizing film obtained in the same manner as in Example 1 are shown in Table 1.

[0139] <Example 4>

[0140] A polyvinyl alcohol (PVA) resin with a weight-average molecular weight of 156,000 was used, and the B / A value was changed as shown in Table 1, based on the moisture content A [mass %] of the film immediately after peeling from the casting mold and the moisture content B [mass %] of the film after peeling from the casting mold for 35 seconds. Otherwise, a PVA film was obtained in the same manner as in Example 1. The properties of the obtained PVA film and the polarizing film obtained in the same manner as in Example 1 are shown in Table 1.

[0141] <Comparative Examples 1-2>

[0142] As shown in Table 1, the B / A value was changed from the moisture content A [mass %] of the film immediately after being peeled from the casting mold and the moisture content B [mass %] of the film after being peeled from the casting mold for 35 seconds. Otherwise, a polyvinyl alcohol-based film was obtained in the same manner as in Example 1. The characteristics of the obtained polyvinyl alcohol-based film and the polarizing film obtained in the same manner as in Example 1 are shown in Table 1.

[0143] <Elastic modulus in water at 50℃ (MPa)>

[0144] After determining the elastic modulus in the flow direction (MD) and the elastic modulus in the width direction (TD) of water at 50°C using the following method, the value obtained from their second root mean square is taken as the elastic modulus in water at 50°C.

[0145] (Determination of the elastic modulus of MD in water at 50℃)

[0146] Test pieces were cut from a polyvinyl alcohol film that had been stored at 23°C and 50% RH for 24 hours, with dimensions of TD (5 mm) × MD (25 mm) centered on the width of the film. The measurement width of the test pieces was measured using a digital microscope. The thickness of the test pieces was measured at any three points using a contact film thickness gauge, and the average value was taken as the measured thickness.

[0147] Next, the test piece was fixed to the viscoelasticity measuring device (IT Measurement Control Co., Ltd., "DVA-225") with the clamps at both ends of the short side (TD side) spaced 15 mm apart. The measurement conditions were set to dynamic strain 0.3%, frequency 10 Hz, and upper limit elongation 80%, and the elastic modulus measurement was started. The measurements were as follows: first, the elastic modulus in the atmosphere was measured for 100 seconds, and then the elastic modulus in water at 50°C was measured for 600 seconds.

[0148] For the measurement results, calculate the average value of the elastic modulus from 200 to 600 seconds after the start of the measurement in water at 50°C. The value obtained after correcting for the thickness after the measurement is set as the elastic modulus of MD in water at 50°C (the same measurement is performed 3 times and the average value is used).

[0149] It should be noted that the thickness after measurement refers to the average value of the thickness of the test piece at three points after the viscoelasticity of the water was measured using a contact film thickness gauge.

[0150] (Determination of the elastic modulus of TD in water at 50℃)

[0151] The test piece was cut out to a size of TD (25 mm) × MD (5 mm). Otherwise, the elastic modulus of TD in water at 50°C was determined in the same manner as the elastic modulus of MD.

[0152] <Moisture Ratio B / A>

[0153] [Moisture content A [mass %] of the film immediately after being peeled from the casting mold]

[0154] For the film just peeled from the casting mold, three films were sampled at three points along the width (equivalent to 20%, 50% (center), and 80% of the film's full width, with one end set to 0%, the center to 50%, and the other end to 100%), and the initial mass A0 (g) of each film was measured. Then, the moisture content A2 [mass %] was calculated from the mass A1 (g) of these films after drying them in a vacuum dryer at 83°C for 20 minutes, according to the following formula (i).

[0155] Moisture content A2 [mass %] = 100 × (A0 - A1) / A0…(i)

[0156] The average moisture content of the total 9 films (3 locations × 3 films) obtained in the above formula (i) is taken as the moisture content A [mass %] of the film just peeled from the casting mold.

[0157] [Moisture content B [mass %] of the film after 35 seconds of peeling from the casting mold]

[0158] For the film at the location 35 seconds after being peeled from the casting mold, three films were sampled at three points in the width direction (corresponding to 20%, 50% (center), and 80% of the film width when one end is set to 0%, the center to 50%, and the other end to 100%), and the initial mass B0 (g) of each film was measured. Then, the moisture content B2 [mass %] was calculated from the mass B1 (g) of these films after drying them in a vacuum dryer at 83°C for 20 minutes according to the following formula (ii).

[0159] Moisture content B2 [mass %] = 100 × (B0 - B1) / B0…(ii)

[0160] The average moisture content of the total 9 films (3 locations × 3 films) obtained in the above formula (ii) is taken as the moisture content B [mass %] of the film after being peeled from the casting mold for 35 seconds.

[0161] It should be noted that 35 seconds after peeling from the casting mold refers to the time taken from the location where the moisture content A [mass %] of the film was calculated immediately after peeling from the casting mold, starting from 0 seconds. The location 35 seconds after peeling from the casting mold can be easily determined by marking the film surface beforehand.

[0162] < Resin elution rate (ppm / m) 2 >

[0163] The obtained polyvinyl alcohol film was conditioned at 23°C and 50% RH for 24 hours, and then cut into 100mm × 100mm (0.01m) pieces. 2 Five test pieces were prepared, and each of the five test pieces was immersed in 1L of ion-exchange water at 50°C for 1 minute to obtain the eluent. 10mL of a colorimetric reagent (containing 500g of ion-exchange water, 7.4g of potassium iodide, 0.65g of iodine, and 10.6g of boric acid) was added to 10mL of the eluent. After mixing at room temperature (23°C), the absorbance at a wavelength of 690nm was measured using a spectrophotometer (Shimadzu Corporation, "UV-3600Plus"). The concentration (ppm) of the polyvinyl alcohol resin was calculated from a pre-prepared standard curve, and the area was converted to obtain the elution volume (ppm / m²). 2 ).

[0164] (Evaluation Criteria)

[0165] ○ (very good): 900ppm / m 2 the following

[0166] ×(good): Greater than 900ppm / m 2

[0167] <Stretchability>

[0168] The obtained polyvinyl alcohol film was unwound and conveyed horizontally while being immersed in a water bath at 30°C for swelling. Simultaneously, the original roll was stretched to 1.7 times its original length along the flow direction (MD). Next, it was immersed in an aqueous solution (30°C) containing 0.5 g / L iodine and 30 g / L potassium iodide, and while dyeing, the original roll was stretched to 2.7 times its original length along the flow direction (MD). Then, it was immersed in an aqueous solution (55°C, referred to as the stretching tank) containing 40 g / L boric acid and 30 g / L potassium iodide, and while boric acid crosslinking occurred, the original roll was uniaxially stretched to 5.7 times its original length along the flow direction (MD). The tension (N) at this point was measured. This tension was divided by the width (mm) of the polarizing film being processed at the outlet of the stretching tank, and the resulting value was taken as the tension (N / mm) at 5.7 times stretching.

[0169] For the tension (N / mm) at 5.7 times the tensile strength, the tensile properties are evaluated according to the following evaluation criteria.

[0170] (Evaluation Criteria)

[0171] ○ (very good): Below 0.70 N / mm

[0172] ×(good): Greater than 0.70 N / mm

[0173] <Pollution of the treatment tank>

[0174] 10 mL of chemical solution was collected from the stretching tank before polarizing film processing and from the stretching tank after processing the 5000 μm polyvinyl alcohol film. 10 mL of colorimetric reagent (containing 500 g of ion-exchanged water, 7.4 g of potassium iodide, 0.65 g of iodine, and 10.6 g of boric acid) was added to each chemical solution sample. After mixing at room temperature, the absorbance at a wavelength of 690 nm was obtained using a spectrophotometer (Shimadzu Corporation, "UV-3600Plus").

[0175] The difference in absorbance of the chemical solution before and after polarization film processing is defined as ΔI. The larger ΔI is, the more the chemical solution in the stretching tank is contaminated. On the other hand, the smaller ΔI is, the less chemical solution contamination occurs. The evaluation is carried out according to the following evaluation criteria.

[0176] (Evaluation Criteria)

[0177] 〇(very good):0≤ΔI<0.040

[0178] △(good): 0.040≤ΔI<0.060

[0179] ×(poor): 0.060≤ΔI

[0180] <Number of foreign matter defects in polarizing film>

[0181] A test piece measuring 30cm in length and 30cm in width was cut from the obtained polarizing film and fixed 10cm in front of the screen. Light was projected onto the polarizing film in a dark room using a projector, and the number of defects projected onto the screen was counted.

[0182] (Evaluation Criteria)

[0183] 〇 (very good): 0-2

[0184] △(good): 3-4

[0185] ×(poor): 5 or more

[0186] <Pigmentation>

[0187] Cut a test piece with a length of 30cm and a width of 30cm from the obtained polarizing film. Clamp it at a 45° angle between two polarizing plates in a cross-prism configuration (single plate transmittance 43.5%, polarization 99.9%). Observe the optical color spot in transmission mode using a light box with a surface illuminance of 14000lx and evaluate it according to the following criteria.

[0188] (Evaluation Criteria)

[0189] ○ (very good): No blemishes

[0190] ×(poor): Has pigmentation

[0191] [Table 1]

[0192]

[0193] It is known that the elastic modulus of the polyvinyl alcohol films in water at 50°C in Examples 1 to 4 is within the range defined by the present invention. The amount of polyvinyl alcohol resin washed off during the manufacture of polarizing film is small, and the tensile properties are also excellent. Therefore, the polarizing film manufacturing process does not cause pollution of the processing tank, and the obtained polarizing film has few foreign matter defects and no color spots. Therefore, the productivity is good.

[0194] On the other hand, the elastic modulus of the polyvinyl alcohol film of Comparative Example 1 in water at 50°C was lower than the lower limit value defined in this invention. Therefore, contamination of the processing tank occurred during the polarizing film manufacturing process, resulting in numerous foreign matter defects and color spots in the obtained polarizing film, thus leading to poor productivity. Furthermore, the elastic modulus of the polyvinyl alcohol film of Comparative Example 2 in water at 50°C exceeded the upper limit value defined in this invention. Therefore, its tensile properties deteriorated significantly, reducing the yield of the polarizing film, and color spots, attributed to uneven strain during stretching, also occurred, resulting in poor productivity.

[0195] The above embodiments illustrate specific aspects of the present invention, but these embodiments are merely examples and not intended to be limiting. Various modifications that will be apparent to those skilled in the art are within the scope of this invention.

[0196] Industrial availability

[0197] The polarizing film and polarizing plate formed from the polyvinyl alcohol-based thin film for manufacturing polarizing film of the present invention have excellent polarization performance and are preferably used in portable information terminal devices, personal computers, televisions, projectors, information billboards, desktop electronic calculators, electronic clocks, word processors, electronic paper, game consoles, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices for automobiles or machinery, sunglasses, anti-glare glasses, stereo glasses, wearable displays, anti-reflective layers for display components (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical devices, building materials, toys, etc.

Claims

1. A polyvinyl alcohol-based film for manufacturing polarizing films, characterized in that, The elastic modulus in water at 50℃ is 0.7~1.8MPa. The elastic modulus in the flow direction (MD) and the elastic modulus in the width direction (TD) of water at 50℃ are calculated, and the value obtained by their square root is taken as the elastic modulus in water at 50℃.

2. The polyvinyl alcohol-based film for manufacturing polarizing film according to claim 1, characterized in that, The thickness of the polyvinyl alcohol-based film used in the manufacture of the polarizing film is 5~50μm.

3. A method for manufacturing a polyvinyl alcohol-based thin film for polarizing film manufacturing according to claim 1 or 2, characterized in that, The process includes the following steps: a film-forming step, in which an aqueous solution of polyvinyl alcohol resin is continuously discharged into a casting mold and cast to form a film; a drying step, in which the film is dried; and a heat treatment step, in which the dried film is heat-treated. When the moisture content of the film immediately after being peeled from the casting mold in the film-making process is set as A [mass%], and the moisture content of the film 35 seconds after being peeled from the casting mold in the drying process is set as B [mass%], the drying process shall be carried out in a manner that satisfies the following formula (1). 0.23≤B / A≤0.45 …(1).

4. A polarizing film, characterized in that, It is obtained by manufacturing a polyvinyl alcohol-based film using the polarizing film described in claim 1 or 2.

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