JP7447054B2 - Polyvinyl alcohol film, its manufacturing method, and optical film using the same - Google Patents

Description

The present invention relates to a polyvinyl alcohol (PVA) film that can be used as an optical film, particularly as a polarizing film.

Polyvinyl alcohol (PVA) film is a kind of hydrophilic polymer, which has properties such as transparency, mechanical strength, water solubility, and good processability, and is especially used in packaging materials or optical films for electronic products. Widely used in polarizing films.

When manufacturing a polarizing film, a PVA film is placed in a boric acid aqueous solution and stretched and dyed.The dye molecules diffuse into the PVA molecules of the PVA film and are regularly arranged, resulting in a polarizing film. absorbs light components parallel to the arrangement direction, and transmits light components in the perpendicular direction, resulting in polarized light characteristics.

A good polarizing film has characteristics such as uniform color, less color spots, and no wrinkles, and can provide excellent optical properties. In order to improve the optical properties of polarizing films, conventional techniques have improved the optical properties by changing the structure of polyvinyl alcohol or adding functional groups (e.g. cationic groups) to change the viscosity and degree of saponification. ing.

Although the known technology has already provided methods for improving optical properties when manufacturing polarizing films using PVA films, in the prior art, when manufacturing large-sized optical films using PVA films, it is difficult to dye them. Phenomena such as non-uniformity of color, density unevenness, color spots, or color non-uniformity during stretching often occur, increasing the difficulty of optical film production.

In order to solve the above problems, the present invention aims to provide a polyvinyl alcohol (PVA) film, which after being treated with 0.2 wt% boric acid, is subjected to differential scanning calorimetry (DSC). ) is >210°C, and the melting point difference (ΔTm) at any two points of the PVA film is <3.2°C.

In a preferred embodiment, the PVA film after treatment with 0.2 wt% aqueous boric acid has a crystallization temperature (T _c ) of <153.5° C. as measured by DSC.

In a preferred embodiment, the crystallization temperature difference (ΔT _c ) at any two points of the PVA film after treatment with 0.2 wt % boric acid aqueous solution is <3.85°C.

In a preferred embodiment, the standard deviation of the amount of retardation that the PVA film has is <3 nm.

In a preferred embodiment, after treating 0.12 g of PVA film with a 0.2 wt% boric acid aqueous solution, the boron content of the aqueous solution after dissolving in water is 16-19 ppm.

Another object of the present invention is to provide an optical film, which is made with the PVA film of the present invention.

In preferred embodiments, the optical film is a polarizing film.

In a preferred embodiment, the polarizing film has a degree of polarization >99.8.

Another object of the present invention is to provide a method for producing the PVA film of the present invention, which comprises: (a) stirring a polyvinyl alcohol-based resin at a heating rate of 4 to 20°C/hour; (b) casting the casting solution into a casting drum; and (c) forming a PVA film through drying.

In a preferred embodiment, the rotation speed of the casting drum is between 3 and 7 m/min.

In a preferred embodiment, the drying step (c) is performed using heated rollers or a floating dryer.

In a preferred embodiment, the maximum/minimum air flow experienced by the film along its width within the floating dryer is ≦3.0.

In a preferred embodiment, the polyvinyl alcohol casting solution further contains a plasticizer, and the plasticizer is 3 to 30 parts by weight based on 100 parts by weight of the polyvinyl alcohol resin.

In a preferred embodiment, the polyvinyl alcohol resin content in the polyvinyl alcohol casting solution is 10-60 wt%.

In a preferred embodiment, the degree of polymerization of the polyvinyl alcohol resin is between 800 and 10,000.

Compared with the known technology, the PVA film of the present invention has a uniform color after dyeing, does not cause color unevenness problems such as density unevenness and color spots, and effectively overcomes the drawbacks of producing optical films with PVA. This method improves the yield rate in the production of optical films, and can be particularly used to produce large-sized polarizing films.

It is a conceptual diagram in which three test pieces with an area of 10 cm*10 cm were cut out on the left, middle, and right sides of a PVA film.

The following embodiments are not intended to unduly limit the invention. Those skilled in the art to which this invention pertains will be able to make modifications and changes to the embodiments discussed herein without departing from the spirit or scope of the invention; Immediately.

The terms "1" and "kind" as used herein refer to the presence of one or more (ie, at least one) grammatical object.

In the present invention, MD is the machine direction, that is, the longitudinal direction of the PVA film, and TD is the transverse direction, that is, the transverse direction of the PVA film.

The method for producing a polyvinyl alcohol (PVA) film of the present invention includes the steps of preparing a polyvinyl alcohol casting solution, then casting the polyvinyl alcohol casting solution into a casting drum, and forming a polyvinyl alcohol polymer film through drying. including.

Specifically, the method for manufacturing a PVA film includes the following steps. In a dissolution tank, polyvinyl alcohol resin is dissolved in a solution (eg, water) to form a polyvinyl alcohol casting solution. The polyvinyl alcohol casting solution may optionally be filtered with a filter. The polyvinyl alcohol casting solution is then cast into the casting drum using a gear pump and a coating machine (eg, a T-die coater). Finally, after peeling off the PVA film formed on the drum, a PVA film is obtained through a series of heating roller and/or drying heat treatments.

In preparing the polyvinyl alcohol casting solution, the polyvinyl alcohol resin is melted in a melting tank, the melting temperature in the melting tank being >100°C, preferably >110°C, more preferably >120°C; Specifically, the temperature is, for example, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C or 140°C, but the present invention is not limited thereto. The temperature of the dissolution tank is preferably increased at a rate of 4 to 20°C per hour, preferably 5 to 15°C, more preferably 6 to 9°C, and specifically, for example, 4.0°C/hr. 5.0°C/hr, 6.0°C/hr, 7.0°C/hr, 8.0°C/hr, 9.0°C/hr, 10°C/hr, 11°C/hr, 12°C/hr, 13°C/hr, 14°C/hr, 15°C/hr, 16°C/hr, 17°C/hr, 18°C/hr, 19°C/hr or 20°C/hr, etc. If the temperature increase rate is too fast , caking tends to occur in the polyvinyl alcohol resin, resulting in incomplete dissolution. After heating to the desired melting temperature, the polyvinyl alcohol casting solution is continued to be stirred for 2 to 4 hours, preferably 3 hours, and the stirring direction is changed at least twice per hour, but preferably 3 hours. For example, after rotating clockwise for 20 minutes, it is rotated counterclockwise for 20 minutes. The above-mentioned reversal of direction during the stirring process can enhance the dissolution effect and prevent clusters from remaining in the polyvinyl alcohol casting solution.

In preparing the polyvinyl alcohol casting solution, the content of polyvinyl alcohol resin is 10 to 60 wt%, preferably 15 to 40 wt%, more preferably 20 to 30 wt%, specifically, for example 10, 15 wt%. , 20, 25, 30, 35, 40, 45, 50, 55, 60 wt%, etc. If the content of the polyvinyl alcohol resin is insufficient, the viscosity of the polyvinyl alcohol casting solution will become too low, the drying load will become excessively large, and the film forming efficiency in PVA film preparation will deteriorate. On the other hand, if the content of polyvinyl alcohol resin is too high, the polyvinyl alcohol resin will be difficult to dissolve and clusters will remain easily, which will deteriorate the uniformity of the retardation of the PVA film and reduce the uniformity of dyeing in the subsequent manufacturing process. It affects sexuality.

The above-mentioned polyvinyl alcohol resin is obtained by forming a polyvinyl ester resin by polymerizing vinyl ester resin monomers and then performing a saponification reaction. Among them, vinyl ester resin monomers include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate, but the present invention is not limited to these and is suitable. is vinyl acetate. Moreover, a copolymer of an olefin compound or an acrylate derivative and the above-mentioned vinyl ester resin monomer can also be used. The olefin compounds include ethylene, propylene, butylene, etc., but the present invention is not limited thereto. Acrylate derivatives include, but are not limited to, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, or n-butyl acrylate.

The degree of saponification of the polyvinyl alcohol resin is 90% or more, preferably 99% or more, which provides good optical properties. Specifically, for example, 90, 91, 92, 93, 94, 95 , 96, 97, 98, 99, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, 99.9%, etc. The degree of polymerization of polyvinyl alcohol is between 800 and 10,000, preferably between 2,200 and 10,000, specifically, for example, 800, 900, 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000. .

In addition to the polyvinyl alcohol resin, the casting solution can also contain a plasticizer to enhance the processability of film formation. Examples of plasticizers that can be used include ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. Polyhydric alcohols include, but are not limited to, ethylene glycol, propylene glycol or glycerol, with ethylene glycol and glycerol being preferred. The amount of plasticizer added is usually between 3 and 30 parts by weight, preferably between 7 and 20 parts by weight, based on 100 parts by weight of the polyvinyl alcohol resin. 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, For example, 30 parts by weight. If the content of plasticizer is insufficient, crystals are likely to form in the formed PVA film, which will affect the dyeing effect in subsequent processing. On the contrary, if the content of plasticizer is too high, the mechanical properties of the PVA film will be impaired.

The equipment used in the PVA film manufacturing method includes a dissolution tank, a filter, a coating machine, and a transport pipe connected from the dissolution tank to the front of the coating machine, and preferably these facilities are covered with a heat insulating device. The heat insulating device may be a metal heating element (heating wire) or a jacket containing a liquid such as oil or water. By uniformly heating the surface (surface) and maintaining a warm state, it is possible to prevent the surface temperature of the equipment or piping from being lost and forming gels or clusters in the polyvinyl alcohol in the polyvinyl alcohol casting solution. In addition, the insulating temperature should not be too high, otherwise a part of the polyvinyl alcohol casting solution will dehydrate or gel, forming a golden brown or black gel, and the PVA film after coating in the subsequent process. surface quality and uniformity. The heat retention temperature of the polyvinyl alcohol casting solution in coating molding is 80 to 120°C, preferably 90 to 110°C, more preferably 90 to 100°C, and specifically, for example, 80, 85, 90, 95 , 100, 105, 110, 115, 120°C, etc.

When the polyvinyl alcohol casting solution is cast into the casting drum, the rotational speed of the casting drum is about 3-7 m/min, preferably 4-6 m/min. If the drum speed is too slow, the casting solution tends to be excessively dry, resulting in uneven phase difference and melting point distribution. On the other hand, if the speed of the drum is too high, the casting solution will not be sufficiently dried and the releasability will be reduced. In a preferred embodiment, the temperature of the drum is set at 85 to 90°C, specifically, for example, 85, 86, 88, 87, 88, 89, 90°C, etc., and if the drum temperature is too high, , the casting solution on the drum is likely to cause foaming phenomenon.

After the PVA film that is first formed on the casting drum is peeled off from the drum, it is dried to form a PVA film.The drying process can be performed on a heating roller or on a floating dryer. be able to. The number of heating rollers and floating dryers is not particularly limited and can be adjusted as necessary. However, in a preferred embodiment, the maximum/minimum temperature ratio of the drying chamber (ie, dryer) is 2.0 to 2.4. If the temperature ratio of the drying chamber is too large, the degree of crystallinity tends to become non-uniform, and the reaction with boric acid becomes non-uniform when the PVA film is used to produce an optical film. In addition, the temperature difference between adjacent drying chambers is preferably 65°C or less, more preferably 60°C or less, and optimally 50°C. If the temperature difference between adjacent drying chambers is too large, the phase difference distribution tends to become uneven. Furthermore, the air flow rate that the PVA film that is formed along the width direction (i.e., perpendicular to the machine direction) receives in the floating dryer is not particularly limited, but the maximum/minimum air flow ratio must be controlled to 3.0 or less. Specifically, it is, for example, 0, 0.5, 1.0, 1.5, 2.0, 2.5, or 3.0. Areas with a large amount of airflow tend to absorb moisture, resulting in higher dryness, but within this airflow ratio range, it is possible to control the amount of air that touches each position of the PVA film at the same rate and dry it evenly. be. Furthermore, if the air flow rate ratio is too large, problems such as non-uniform reaction with boric acid and non-uniform retardation occur when the PVA film is used for manufacturing optical films.

In the present invention, "treating with a 0.2 wt% boric acid aqueous solution" means (a) Putting about 0.1 g of PVA film into 20 g of pure water, and completely lowering the PVA film to below the liquid level of the pure water. (b) The PVA film was taken out from pure water and placed in a 0.2 wt% boric acid aqueous solution, and left to stand for 1.0 hour. After taking out the film from the 0.2 wt % boric acid aqueous solution, the PVA film is placed in a closed container and allowed to stand and mature for 16.0 hours to complete the boric acid treatment. In the present invention, a low concentration of boric acid is used, but this is because if a high concentration of boric acid is used, the high concentration of boric acid will cause the PVA film to become reactive even in areas that are originally difficult to react with. This is because uniformity is difficult to observe.

The PVA film of the present invention has a melting point (Tm) of >210°C as measured by differential scanning calorimetry (DSC) after being treated with boric acid, and has a melting point difference (ΔTm) at any two points on the PVA film. ) is <3.2°C. Specifically, the melting point (Tm) is, for example, 211°C, 212°C, 215°C, 220°C, 225°C, or 230°C, but the present invention is not limited thereto. Specifically, ΔTm at two arbitrary points on the PVA film is, for example, 1°C, 1.5°C, 2°C, 2.5°C, 3°C, or 3.1°C, but the present invention but not limited to.

When producing an optical film using PVA film, it is stretched and dyed. Taking polarizing film as an example, in the manufacturing process of PVA film, PVA is coated with a boric acid aqueous solution containing I ₃ - and I ₅ -iodide ions. Since the film is dyed, iodide ions are fixed after the boric acid has a cross-linking effect with the amorphous areas of PVA, which can prevent the elution of iodide ions. However, after the PVA molecule forms a crosslink with boric acid, its crystallinity decreases, and the melting point decreases accordingly. When the crystallinity distribution becomes uneven, the reaction between the PVA film and boric acid becomes uneven, the fixation of I ₃ - and I ₅ - iodide ions becomes uneven, and the color becomes uneven. . Therefore, the present invention performs boric acid treatment of the PVA film by boric acid dipping, which is adopted in the manufacturing process of analog polarizers, and uses a low concentration of boric acid (0.2 wt% is adopted in this specification). According to the differential scanning calorimetry (DSC) measurement, the melting point (Tm) is >210°C, and the ΔTm at any two points of the PVA film is <3.2°C, and the PVA film is moderate and uniform. It has excellent crystallinity, and when stretched and dyed, a polarizing film with uniform color can be obtained. The PVA film of the present invention can also be used to produce other optical films through stretching and dyeing, and is not limited to polarizing films.

Furthermore, the PVA film after boric acid treatment has a crystallization temperature of (T _c )<153.5° C. as measured by DSC. Specifically, the crystallization temperature (T _c ) of the PVA film after the boric acid treatment is, for example, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 150°C, or 155°C. However, the present invention is not limited thereto.

Furthermore, the ΔT _c at any two locations of the PVA film after the boric acid treatment is <3.85°C. Specifically, ΔT _c at any two locations on a PVA film subjected to boric acid treatment is, for example, 0°C, 0.5°C, 1°C, 1.5°C, 2°C, 2.5°C, 3°C, or 3.19°C, etc., but the present invention is not limited thereto.

In the present invention, in the process of the above-mentioned DSC measurement method, as shown in FIG. 1, the MD position of the PVA film is within 40 cm from the end in the TD direction, and , on the right, involves cutting out three test pieces with an area of 10 cm * 10 cm. Next, weigh out 0.1g of PVA film from each test piece, put it in a glass bottle, add 20g of pure water, make sure the PVA film is completely below the level of pure water, and add a stirring ball. The stirring speed was 250 rpm, and the mixture was stirred at room temperature for 1.0 hour. After that, the pure water in the glass bottle was taken out, 20g of 0.2wt% boric acid aqueous solution was added, and after standing for 1.0 hour, the boric acid aqueous solution was taken out and the PVA film after boric acid treatment was placed in the bottle. It was sealed and allowed to stand and mature for 16.0 hours. Finally, the PVA film treated with boric acid was taken out, spread flat on a steel plate, and dried at 105° C. for 1.0 hour, and then the PVA film treated with boric acid was taken out and subjected to DSC analysis. The DSC test temperature range is 30°C to 250°C, and each temperature is maintained for 1 minute at 30°C and 250°C, and the melting point (T _m ) at the first temperature increase and the crystallization temperature ( T _c ) and the glass transition temperature (T _g ) at the second temperature increase were recorded. The DSC measuring instrument may be a general purpose DSC measuring instrument, for example the DSC is a TA Instruments DSC 25 or the like.

Furthermore, the standard deviation of the amount of retardation that the PVA film of the present invention has is <3 nm. Specifically, the standard deviation of the delay amount of the PVA film is, for example, 0 nm, 0.5 nm, 1.0 nm, 1.5 nm, 2.0 nm, 2.5 nm, or 2.9 nm, but the present invention Not limited.

In the present invention, the amount of retardation (Retardation) refers to the amount of phase change of incident polarized light when light passes through a film, that is, the amount by which the phase is delayed, and the unit is nm. The uniformity of the delay amount value is related to the uniformity of molecular orientation and uniformity of thickness, and greatly affects the subsequent manufacturing process of the optical film. As shown in Figure 1, in the measurement of the amount of delay, the MD position of the PVA film after boric acid treatment was within 40 cm from the end in the TD direction, and the MD position was set at the left, middle, and right along the TD direction. Three test pieces with an area of 10 cm * 10 cm are cut out and measured, and the delay amount values and statistical data (maximum value, standard deviation, etc.) at all points within the range can be obtained.

Furthermore, after treating 0.12 g of the PVA film of the present invention with a 0.2 wt % boric acid aqueous solution and dissolving it in water, the boron content of the aqueous solution is 16 to 19 ppm. Specifically, it is, for example, 16 ppm, 16.5 ppm, 17 ppm, 17.5 ppm, 18 ppm, 18.5 ppm, or 19 ppm, but the present invention is not limited thereto. If the boron content is lower than the above-mentioned boron content, it is not good because the fixing force when dyeing the PVA film later is insufficient. If the boron content is higher than the above-mentioned boron content, the degree of crosslinking between boric acid and PVA becomes excessively high, which is inconvenient for the subsequent stretching process, which is not good.

In the present invention, the steps of the above-mentioned boron content measurement method include taking out the PVA film after boric acid treatment, passing it through pure water, placing it in a 30 ml glass sample collection container, and adding 20 g of pure water. This involves sufficiently dissolving the boron concentration in the solution and then analyzing it by ICP-OES.

The optical film in the present invention is completed using the PVA film of the present invention. Optical films include polarizing films, blue light cutting films, filter lenses, etc., but the present invention is not limited thereto. Preferably, the optical film is a polarizing film, and the polarizing film has a degree of polarization >99.8.

The present invention will be explained in more detail below along with examples. However, it should be understood that these examples are provided to help the invention be more easily understood and are not intended to limit the scope of the invention.

Examples 1 to 4

Production of PVA film: Polyvinyl alcohol resins with melting points (Tm) of 233.21°C, 233.24°C, 233.01°C and 233.08°C, respectively, are used as the main components of the polyvinyl alcohol casting solution to produce the PVA film of the present invention. PVA films of Examples 1 to 4 were manufactured using the manufacturing method. The operating conditions for the manufacturing method are shown in Table 1.

Comparative examples 1 to 3

Fabrication of PVA film: Polyvinyl alcohol resins with melting points (Tm) of 233.15°C and 233.17°C, respectively, were the main components of the polyvinyl alcohol casting solution, using the same manufacturing method as the PVA film of the present invention but using different operating conditions. PVA films of Comparative Examples 1 to 3 were produced. The operating conditions for the manufacturing method are shown in Table 1.

Experimental example

Property analysis of PVA films: The standard deviation of the retardation amount of each PVA film in Examples 1 to 4 and Comparative Examples 1 to 3 was measured, and after further treatment with a 0.2 wt% boric acid aqueous solution, their melting points (Tm ), melting point difference (ΔTm), crystallization temperature (T _c ), crystallization temperature difference (ΔT _c ), and boron content were measured. The measurement results are shown in Table 2.

Production of polarizing film: Each PVA film in Examples 1 to 4 and Comparative Examples 1 to 3 was immersed in water at about 30°C to swell, and then uniaxially stretched for the first time in the MD direction. The stretched length was 2.0 times the original length of the PVA film. Next, the PVA film after the first stretching was immersed in an aqueous solution at 30°C containing 0.03% by mass of iodine and 3% by mass of potassium iodide, and a second uniaxial stretching was carried out in the MD direction. The length after stretching was 3.3 times the original length of the PVA film. Next, the PVA film after the second stretching was immersed in an aqueous solution at 30°C containing 3% by mass of potassium iodide and 3% by mass of boric acid, and a third uniaxial stretching was carried out in the MD direction. The length after stretching was made 3.6 times the original length of the PVA film. Next, the PVA film after the third stretching was immersed in an aqueous solution at 60°C containing 5% by mass of potassium iodide and 4% by mass of boric acid, and the fourth uniaxial stretching was carried out in the MD direction. The length after stretching was made 6.0 times the length of the PVA film after the third stretching. Finally, the PVA film after the fourth stretching was immersed for 15 seconds in an aqueous solution containing 3% by mass of potassium iodide, and then dried at 60° C. for 4 minutes to obtain a polarizing film. The degree of polarization and color uniformity of the polarizing films obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were measured. The measurement results are shown in Table 2.

How to measure the degree of polarization: Layer two identical polarizing films with the same orientation direction and measure the light transmittance (H ₀ ) under the wavelength. The light transmittance (H ₉₀ ) under the wavelength was measured, and the degree of polarization was calculated using the formula: [(H ₀ −H ₉₀ )/(H ₀ +H ₉₀ )] ^1/2 .

Method for measuring color uniformity: Layer two identical polarizing films at right angles, irradiate one side using a lamp house with luminous flux divergence = 14,000 lx, and observe the color uniformity of the other side. , O: No non-uniform color, Δ: Slightly non-uniform color, X: Significant non-uniform color.

As shown in Table 2, the melting points (Tm) of the PVA films of Examples 1 to 4 are all >210°C, and the ΔTm at any two locations is <3.2°C, and these The polarizing film completed by this process showed good color uniformity. In comparison, the melting point (Tm) of each PVA film of Comparative Examples 1 to 3 exceeds 210°C, or the ΔTm at any two locations exceeds 3.2°C. Undesirable color uniformity was observed in the completed polarizing film, and there were situations in which the color was slightly non-uniform and the color was noticeably non-uniform. In particular, Comparative Examples 2 and 3 were found to have low crystallization temperature (T _c ) was higher than 153.5°C, the boron content was not 16-19 ppm, or the standard deviation of the amount of retardation was higher than 3 nm, and the color uniformity of those polarizing films was significantly poor (the color was not noticeable). unevenly). The melting point (Tm) of the PVA film of the present invention is >210°C, and the ΔTm at any two points is <3.2°C, and if used for manufacturing a polarizing film, it can be provided with color uniformity. This reduces the possibility of problems with uneven density and color spots.

In summary, the PVA film of the present invention has a melting point (Tm)>210°C, and ΔTm at any two locations is <3.2°C, and has color uniformity and density after stretching and dyeing. It does not cause color unevenness problems such as unevenness and color spots, and effectively improves the drawbacks of producing optical films with PVA film, and improves the yield rate in the production of optical films, especially in the production of large-sized polarizing films. will improve.

Although the present invention has been described in detail above, the above description is only a preferred embodiment of the present invention, and does not limit the scope of implementation of the present invention. All equivalent changes and modifications based on the scope of the claims of the present invention are included within the scope of the claims of the present invention.

Claims (15)

A polyvinyl alcohol (PVA) film is treated with a 0.2 wt% boric acid aqueous solution, and then the melting point ( A PVA _film , wherein the PVA film has a melting point difference (ΔT _m ) of <3.2° C. at any two points of the PVA film. 2. A PVA film according to claim 1, having a crystallization temperature ( _Tc ) of <153.5<0>C, measured by DSC, after treatment with a 0.2 wt% aqueous solution of boric acid. The PVA film according to claim 2, wherein after treatment with a 0.2 wt% aqueous boric acid solution, the crystallization temperature difference (ΔT _c ) at any two locations of the PVA film is <3.85°C. The PVA film according to any one of claims 1 to 3, wherein the standard deviation of the amount of retardation measured along the TD direction (width direction) of the PVA film is <3 nm. The PVA film according to claim 4, wherein 0.12 g of the PVA film is treated with a 0.2 wt% boric acid aqueous solution, and the boron content of the aqueous solution after dissolving in water is 16 to 19 ppm. An optical film manufactured using the PVA film according to any one of claims 1 to 5. The optical film according to claim 6, which is a polarizing film. The optical film according to claim 7, wherein the polarizing film has a degree of polarization of >99.8. (a) Stir the polyvinyl alcohol resin at a heating rate of 4 to 20°C/hour, maintain the dissolution temperature for 2 to 4 hours after reaching >100°C, and stir in the direction of stirring at least 3 times per hour. forming a polyvinyl alcohol casting solution;
(b) casting the casting solution into a casting drum;
(c) The method for producing a PVA film according to any one of claims 1 to 4, comprising the step of forming a PVA film through drying. The manufacturing method according to claim 9, wherein the rotation speed of the casting drum is 3 to 7 m/min. The manufacturing method according to claim 10, wherein the drying in the step (c) is performed using a heated roller or a floating dryer. 12. The manufacturing method of claim 11, wherein the maximum/minimum air flow rate that the film receives along its width within the floating dryer is ≦3.0. The production according to any one of claims 9 to 12, wherein the polyvinyl alcohol casting solution further comprises a plasticizer, and the plasticizer is 3 to 30 parts by weight based on 100 parts by weight of the polyvinyl alcohol resin. Method. The manufacturing method according to claim 13, wherein the polyvinyl alcohol resin content in the polyvinyl alcohol casting solution is 10 to 60 wt%. The manufacturing method according to claim 14, wherein the degree of polymerization of the polyvinyl alcohol resin is between 800 and 10,000.

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