Polyvinyl alcohol film and optical film prepared therefrom

Abstract

To provide a polyvinyl alcohol film and an optical film produced from the same.SOLUTION: The present invention relates to a polyvinyl alcohol film and an optical film prepared therefrom, wherein the polyvinyl alcohol film comprises a polyvinyl alcohol having a degree of saponification of 95 mol% or more, and the polyvinyl alcohol film has a degree of area swelling of 17.0 to 38.0% when immersed in a 4wt% aqueous boric acid solution at 55 °C. for 1 minute, and a temperature at which an inflection point of a storage modulus (E ') of the polyvinyl alcohol film in water in a range of 30 °C. to 55 °C. is 37 to 40 °C. The optical film comprising the polyvinyl alcohol film of the present invention has characteristics of good polarizing property and low polarization reduction value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to polyvinyl alcohol (PVA) products, particularly but not exclusively to polyvinyl alcohol films and optical films made therefrom. [Background technology]

[0002] Polyvinyl alcohol (PVA) film is a hydrophilic material obtained by coating and drying an aqueous solution containing polyvinyl alcohol polymer and plasticizer. It has properties such as high transparency, mechanical strength, water solubility, and good processability, and is therefore widely used in packaging materials and various optical films for electronic devices, such as polarizing films.

[0003] Polarizing films obtained by processing polyvinyl alcohol films through a polarization process have the property of blocking light from certain directions, thereby controlling the brightness of the light that passes through. Based on this property, polarizing films are used in various displays, glasses, and wearable devices, making them an essential component. The polarization process generally involves processes such as swelling, stretching, and dyeing. Specifically, polyvinyl alcohol films are placed in a solution and the above processes are carried out, causing dye molecules to diffuse into the molecules in the polyvinyl alcohol film. A relatively regular alignment is obtained through stretching, allowing the polarizing film to absorb light components parallel to the alignment direction and transmit light components perpendicular to the alignment direction, resulting in polarized light.

[0004] Polarizing films must have appropriate optical properties (e.g., high polarization degree and low polarization degree loss). To this end, prior art has improved optical properties by modifying the structure of polyvinyl alcohol or adding functional groups. Prior art, such as Patent Document 1, simultaneously controls the total necking ratio (A) during polarizing film production, the necking ratio (B) during the stretching process, and the necking ratio (C) during the drying process, providing a polarizing film with excellent polarization performance and low shrinkage stress. Controlling the total necking ratio (A) prevents a decrease in polarization performance and yield, while appropriately controlling the necking ratio (B) during the stretching process and the necking ratio (C) during the drying process allows the production of a polarizing film with excellent polarization performance and low shrinkage stress.

[0005] The polarizing films disclosed in prior art such as Patent Document 2 use a dyed cellulose triacetate film as a protective layer to prevent the polarization degree of the polarizing film from decreasing under high temperature and high humidity conditions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Taiwan Patent Publication No. 202231448 [Patent Document 2] Taiwan Patent Publication No. 202212872 Summary of the Invention [Problem to be solved by the invention]

[0007] In the prior art, when an optical film is manufactured using a polyvinyl alcohol film, it is common to find that the polarization degree significantly decreases after long-term use. In this regard, the present inventors have found that by immersing a polyvinyl alcohol film in a boric acid solution of a specific temperature, time, and concentration to achieve a specific range of surface swelling and adjusting the temperature at which the inflection point of the storage modulus (E') in a specific temperature range falls within a specific range, the manufactured polarizing film has an excellent polarization degree, thereby alleviating the problem of the polarization degree decrease of the optical film after long-term use. [Means for solving the problem]

[0008] Specifically, one aspect of the present invention provides a polyvinyl alcohol film that contains a polyvinyl alcohol resin with a degree of saponification of 95 mol % or more, has an area swelling degree of 17.0 to 38.0% obtained when immersed in a 4 wt % aqueous boric acid solution at 55°C for 1 minute, and has an inflection point of its storage modulus (E') in water at a temperature range of 30°C to 55°C of 37 to 40°C.

[0009] In one or more embodiments, the polyvinyl alcohol film has an area swelling degree of 17.5 to 35.0% and an inflection point temperature of 37.8 to 38.5°C.

[0010] In one or more embodiments, the polyvinyl alcohol film obtained by immersing it in a 4 wt % boric acid solution at 55°C for 1 minute has a swelling degree in the transverse direction (TD) of 9.0 to 18.0% and a swelling degree in the machine direction (MD) of 7.5 to 18.0%.

[0011] In one or more embodiments, the polyvinyl alcohol film has a swelling degree in the TD direction of 9.0 to 16.50%, and a swelling degree in the MD direction of 8.0 to 16.0%.

[0012] In one or more embodiments, the absolute value of the difference between the swelling degree in the TD direction and the swelling degree in the MD direction of the polyvinyl alcohol film is 5% or less.

[0013] In one or more embodiments, the ratio of the swelling degree in the TD direction to the swelling degree in the MD direction of the polyvinyl alcohol film is 0.9 to 1.7.

[0014] In one or more embodiments, the ratio of the swelling degree in the TD direction to the swelling degree in the MD direction of the polyvinyl alcohol film is 0.95 to 1.60.

[0015] In one or more embodiments, the polyvinyl alcohol film has an average degree of polymerization of 1,500 to 3,500.

[0016] In one or more embodiments, the polyvinyl alcohol film has an average degree of polymerization of 2,000 to 3,500.

[0017] In one or more embodiments, the polyvinyl alcohol film has a thickness of 30 to 75 μm.

[0018] Another aspect of the present invention provides an optical film made from the above polyvinyl alcohol film.

[0019] In one or more embodiments, the optical film is a polarizing film.

[0020] In one or more embodiments, the polarization degree of the polarizing film is 99.99% or more.

[0021] In one or more embodiments, when the polarizing film is heat-treated at 80° C. for 500 hours, the decrease in polarization degree is 0.01% or less.

[0022] In one or more embodiments, the polarizing film is heat-treated at 80° C. for 500 hours, and the area shrinkage rate is 2% or less.

[0023] In one or more embodiments, when the polarizing film is heat-treated at 80° C. for 500 hours, the shrinkage rate in the TD direction and the shrinkage rate in the MD direction are both 1% or less. [Effects of the Invention]

[0024] The polyvinyl alcohol film of the present invention is suitable for various applications, and is particularly suitable for producing optical films such as polarizing films. The polarizing film produced using the polyvinyl alcohol film obtained by the technical contents of the present invention has an excellent polarization degree and can improve the deterioration of the polarization degree after long-term use.

[0025] The inventors have found that by immersing the polyvinyl alcohol film of the present invention in a boric acid solution of a specific temperature, time, and concentration and controlling the swelling degree to a specific range, an optical film with improved shrinkage can be produced, and further, when the polyvinyl alcohol film has a specific range of TD swelling degree and a specific range of MD swelling degree, the optical film produced from the polyvinyl alcohol film has an improved shrinkage rate in the corresponding direction. As a result, the present invention can prevent warping at the corners of the optical film due to long-term use in a high-temperature environment. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a graph showing the storage modulus (E') of a polyvinyl alcohol film analyzed in water with temperature change according to an embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram illustrating a sample preparation method for analyzing the inflection point temperature of a polyvinyl alcohol film according to one embodiment of the present invention. [Figure 3] FIG. 1 is a schematic diagram illustrating a sample preparation method for analyzing the inflection point temperature of a polyvinyl alcohol film according to one embodiment of the present invention. [Figure 4]FIG. 1 is a schematic diagram illustrating a sample preparation method for analyzing the inflection point temperature of a polyvinyl alcohol film according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] To provide a more detailed and complete description of the present invention, the following provides illustrative descriptions of embodiments and specific examples of the present invention, which are not the only ways of implementing or operating the specific examples of the present invention. In this specification and the appended claims, "a" and "the" may also be construed as plural unless the context dictates otherwise.

[0028] Although the numerical ranges and parameters specifying the present invention are approximations, the relevant numerical values ​​in the specific examples are presented as precisely as possible. However, any numerical value inherently contains standard deviations resulting from each individual testing method. As used herein, the term "about" indicates that the actual value falls within an acceptable standard error of the mean and is within the understanding of one of ordinary skill in the art to which the present invention pertains.

[0029] [Polyvinyl alcohol film] One aspect of the present invention provides a polyvinyl alcohol film containing a polyvinyl alcohol resin with a degree of saponification of 95 mol% or more, which exhibits an areal swelling index of 17.0 to 38.0% after immersion in a 4 wt% aqueous boric acid solution at 55°C for 1 minute, and exhibits an inflection point of its storage modulus (E') in water at a temperature range of 30 to 55°C at a temperature of 37 to 40°C. According to some embodiments, the polyvinyl alcohol film of the present invention has a thickness of 30 to 75 μm. Specifically, the thickness is within the range between any two of the following values, such as 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, or 75 μm. These values ​​are merely examples and are not intended to be limiting.

[0030] The "swelling degree" referred to in this specification includes the "area swelling degree," "transverse direction (TD) swelling degree (hereinafter referred to as TD swelling degree)," and "machine direction (MD) swelling degree (hereinafter referred to as MD swelling degree)" of the polyvinyl alcohol film. The "area swelling degree obtained by immersion in a 4 wt% boric acid aqueous solution at 55°C for 1 minute (hereinafter referred to as MD swelling degree)" referred to in this specification is obtained by immersing a polyvinyl alcohol film in a 4 wt% boric acid solution at 55°C for 1 minute and then calculating the change in area of ​​the polyvinyl alcohol film.

[0031] Polyvinyl alcohol films have crystalline and amorphous regions. When a polyvinyl alcohol film is immersed in a boric acid solution, the aqueous solution containing boric acid penetrates the amorphous regions of the film, dissolving some of the crystals, and the boric acid in the solution further crosslinks the molecules. The inventors have found that the numerical range of the area swelling degree is related to the polarization degree, polarization degree reduction, and shrinkage rate of the subsequently manufactured optical film. Presumably, as more crystals dissolve, the number of bonds with boric acid increases, resulting in a more stable intermolecular structure. When the crystalline portion dissolves, the chain segments at the dissolved sites are arranged in a relatively orderly manner, allowing them to move freely and change the distance between chain segments, thereby creating more positions suitable for boric acid bonding. Chain segments that are themselves amorphous have irregular spacing and a disordered arrangement, resulting in fewer positions for bonding with boric acid. As described above, if the swelling degree of the polyvinyl alcohol film is too low, the dissolved crystals are reduced, and the number and regions of chain segments that can be crosslinked with boric acid are also reduced, resulting in an unstable intermolecular structure that is susceptible to change due to exposure to heat, which leads to a subsequent decrease in the polarization degree of the optical film and to in-plane shrinkage that makes the polarizing film more likely to deform. On the other hand, if the swelling degree is too high, the crystals in the polyvinyl alcohol film are dissolved too easily, resulting in an insufficient amount of crystals in the polyvinyl alcohol film, which reduces the physical strength of the film and makes the film more likely to tear.

[0032] Therefore, in order to obtain a polyvinyl alcohol film with both a stable intermolecular structure and good physical strength, the areal swelling degree must be controlled within a specific range. Specifically, the areal swelling degree obtained by immersion in a 4 wt % boric acid aqueous solution at 55°C for 1 minute is 17.0 to 38.0%, and is a value within a range between any two of the following values, for example, 17.0%, 17.5%, 18.0%, 18.5%, 19.0%, 19.5%, 20.0%, 20.5%, 21.0%, 21.5%, 22.0%, 22.5%, 23.0%, 23.5%, 24.0%, 24.5%, 25.0%, 25.5%, 26.0%, 26.5%, and 27.0%. %, 27.0%, 27.5%, 28.0%, 28.5%, 29.0%, 29.5%, 30.0%, 30.5%, 31.0%, 31.5%, 32.0%, 32.5%, 33.0%, 33.5%, 34.0%, 34.5%, 35.0%, 35.5%, 36.0%, 36.5%, 37.0%, 37.5%, or 38.0%, and preferably, the areal swelling degree of the polyvinyl alcohol film is 17.5 to 35.0% or 35%.

[0033] The "elastic modulus" mentioned in this specification is a physical property used to analyze the relationship between stress and strain in material mechanics, and is the ratio of stress to strain. The modulus of elasticity obtained when the stress applied to the material is a normal force is represented by E. The "storage modulus (E')" is the component that a material absorbs and converts applied energy and stores it within the material. It is usually used to recover from deformation and maintain the original appearance of the material. It is related to the elastic properties of the material and can also reflect the rigidity of the material. From a molecular perspective, the higher the storage modulus (E'), the stronger the intermolecular forces.

[0034] The "temperature at which the storage modulus (E') inflection point in water in the range of 30°C to 55°C (hereinafter referred to as the E' inflection point temperature)" described herein is obtained by immersing a polyvinyl alcohol film in water and measuring the change in E' of the polyvinyl alcohol film using a temperature-elevation method. After a large amount of water is absorbed into the film, the polyvinyl alcohol molecular chains swell and separate, reducing entanglement between the molecular chains. At this time, the response of the storage modulus (E') is more significantly affected by crystallization within the film, resulting in a greater response. Therefore, analysis of the storage modulus (E') of polyvinyl alcohol immersed in water can be used to evaluate changes in the crystallinity within the film at different temperatures. Furthermore, analysis of polyvinyl alcohol film in water revealed that the temperature-dependent change curve of storage modulus (E') had the characteristics shown in Figure 1, where the change curve between 30°C and 55°C had two different slopes, and the tangent intersection of the two slopes was the E' inflection point, and the corresponding temperature was the E' inflection point temperature (the point indicated by the arrow in Figure 1).

[0035] The inventors have discovered that the physical meaning of the E' inflection temperature includes the temperature at which crystals in a polyvinyl alcohol film begin to dissolve in large quantities, and that a higher E' inflection temperature indicates a larger average crystal grain size in the film, and therefore that large amounts of crystal dissolution occurs only at relatively high temperatures. Without being bound by any particular theory, it is believed that controlling the crystal grain size within an appropriate range likely allows the polyvinyl alcohol film to have an optimal amount and region of crystal dissolution for crosslinking with boric acid, and optical films (such as polarizing films) made from the polyvinyl alcohol film have the effect of preventing a decrease in polarization degree even when used at high temperatures for long periods of time.

[0036] Therefore, in order to achieve the above-mentioned effects, it is necessary to control the E' inflection point temperature within a specific range. Specifically, the temperature at which the inflection point of the storage modulus (E') of the polyvinyl alcohol film of the present invention in water in the range of 30°C to 55°C (the temperature range for the initial stretching in the production process of the polarized film) is 37 to 40°C, for example, 37°C, 37.1°C, 37.3°C, 37.5°C, 37.7°C, 37.9°C, 38°C, 38.1°C, 38.3°C, 38.5°C, 38.7°C, 38.9°C, 39°C, 39.1°C, 39.3°C, 39.5°C, 39.7°C, 39.9°C, or 40°C, but is not limited thereto. Preferably, the E' inflection point temperature of the polyvinyl alcohol film is 37.8 to 38.5°C, and is a value within a range between any two of the following values, for example, 37.8°C, 37.9°C, 38.0°C, 38.1°C, 38.2°C, 38.3°C, 38.4°C, or 38.5°C. If the measurement temperature exceeds 55°C, the crystallization of the polyvinyl alcohol film may completely disappear and the physical properties may change suddenly. Therefore, when analyzing the inflection point temperature, the analysis temperature range set should not exceed 55°C.

[0037] Regarding the method for measuring the E' inflection point temperature, since polyvinyl alcohol films absorb moisture (including moisture in the air) and this may affect their crystal morphology and other conditions, the moisture content of each polyvinyl alcohol film to be measured can be adjusted to the same value by pretreatment before measuring the E' inflection point temperature of the polyvinyl alcohol film, thereby making the state of each polyvinyl alcohol film to be measured the same. The moisture content can usually be adjusted to about 8 to 9 wt%, but the present specification is not limited to this.

[0038] The "saponification degree" referred to in this specification means a measured value obtained by the test method described in JIS K 6726 (1994). In one or more embodiments, the saponification degree of the polyvinyl alcohol resin is 95 mol% or more, for example, 95 mol% or more, 95.5 mol% or more, 96 mol% or more, 96.5 mol% or more, 97 mol% or more, 97.5 mol% or more, 98 mol% or more, 98.5 mol% or more, 99 mol% or more, or 99.5 mol% or more. According to one embodiment of the present invention, the saponification degree of the polyvinyl alcohol-based resin of the present invention is greater than 99.90 mol%, for example, greater than 99.90 mol%, greater than 99.92 mol%, greater than 99.94 mol%, greater than 99.96 mol%, or greater than 99.98 mol%. Preferably, the saponification degree of the polyvinyl alcohol-based resin is 99.95 mol% or more, for example, 99.95 mol% or more, 99.97 mol% or more, or 99.99 mol% or more. More preferably, the saponification degree of the polyvinyl alcohol-based resin is 99.97 mol% or more, for example, 99.97 mol% or more, 99.98 mol% or more, or 99.99 mol% or more.

[0039] The "TD swelling degree (hereinafter referred to as TD swelling degree) and MD swelling degree (hereinafter referred to as MD swelling degree) obtained by immersing a polyvinyl alcohol film in a 4 wt% boric acid solution at 55°C for 1 minute" described herein are calculated by immersing the polyvinyl alcohol film in a 4 wt% boric acid solution at 55°C for 1 minute and then calculating the change in length along the TD and MD of the polyvinyl alcohol film. Here, the TD direction is also called the cross direction, and the MD direction is also called the machine direction. Without being bound by any particular theory, controlling the TD swelling degree and MD swelling degree of a polyvinyl alcohol film within specific ranges can reduce the subsequent shrinkage rate of the optical film in the TD and MD directions, thereby preventing warping of the corners of the optical film when attached to a display under long-term use at high temperatures.

[0040] Therefore, in order to obtain the above-mentioned effects, the swelling degree in the TD direction of the polyvinyl alcohol film of the present invention is 9.0 to 18.0%, and is a value within a range between any two of the following values, for example, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%, 15.0%, 15.5%, 16.0%, 16.5%, 17.0, 17.5%, or 18.0%, and more preferably, the swelling degree in the TD direction of the polyvinyl alcohol film is 9.0 to 16.5%. The swelling degree of the polyvinyl alcohol film in the MD direction is 7.5 to 16.0%, and is a value within a range between any two of the following values, for example, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%, 15.0%, 15.5%, or 16.0%, and more preferably, the swelling degree of the polyvinyl alcohol film in the MD direction is 8.0 to 16.0%.

[0041] In one or more embodiments, the absolute value of the difference between the swelling degree in the TD direction and the swelling degree in the MD direction of the polyvinyl alcohol film is 5% or less, for example, 5% or less, 4.5% or less, 4% or less, 3.5% or less, 3% or less, 2.5% or less, 2% or less, 1.5% or less, 1% or less, or 0.5% or less.

[0042] In one or more embodiments, the ratio of the swelling degree in the TD direction to the swelling degree in the MD direction of the polyvinyl alcohol film is 0.95 to 1.60, and is a value within a range between any two of the following values, for example, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, or 1.60.

[0043] Without being bound by any particular theory, if the absolute value of the difference between the swelling degree in the TD direction and the swelling degree in the MD direction or the ratio is too large, peeling may occur between the polarizing film and the protective layer or between the polarizing film and the glass in a display containing the polyvinyl alcohol film produced thereafter, which may cause a problem of deterioration of the display.

[0044] The "average degree of polymerization" referred to in this specification means a measured value obtained by the test method described in JIS K 6726 (1994). In one or more embodiments, the average degree of polymerization of the polyvinyl alcohol film is 1500 to 3500, and is a value within a range between any two of the following values, for example, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100, 3200, 3300, 3400, or 3500, and preferably, the average degree of polymerization of the polyvinyl alcohol film is 2000 to 3500.

[0045] Without being bound by any particular theory, the inventors have found that the above-mentioned areal swelling degree, E' inflection point temperature, TD swelling degree, and MD swelling degree can be controlled by adjusting the average polymerization degree of the resin raw materials of the polyvinyl alcohol film, as well as by adjusting the dissolution temperature and time of the raw materials in the production method, the total drying time at a temperature exceeding 50°C, and the molecular orientation of the polyvinyl alcohol polymer in the polyvinyl alcohol film, but are not limited thereto.

[0046] [Manufacturing method of polyvinyl alcohol film] The method for producing a polyvinyl alcohol film of the present invention includes (a) a dissolving step in which a polyvinyl alcohol resin is heated and dissolved to adjust the concentration of the polyvinyl alcohol resin to form a polyvinyl alcohol casting solution, (b) a casting step in which the polyvinyl alcohol casting solution is cast onto a casting drum and peeled from the casting drum to obtain a preformed film, (c) a heating roller step in which the preformed polyvinyl alcohol film is contacted with multiple heating rollers to obtain a semi-formed polyvinyl alcohol film, (d) an oven step in which the semi-formed polyvinyl alcohol film is placed in an oven to dry and obtain a molded polyvinyl alcohol film, and (e) a temperature and humidity adjustment step in which the molded polyvinyl alcohol film is placed in a temperature and humidity controller to adjust the temperature and humidity to obtain a polyvinyl alcohol film. In the heating roller and oven steps, the total drying time at temperatures above 50°C is controlled to 10 to 20 minutes.

[0047] [Dissolution process] According to some embodiments, the dissolving step involves raising the temperature to 130-165°C while stirring the polyvinyl alcohol resin, solvent, plasticizer, etc., dissolving for 1-5 hours, and adjusting the resin to an appropriate concentration to obtain a polyvinyl alcohol casting solution. The appropriate resin concentration is approximately 20 wt%-50 wt%, for example, 20, 30, 40, or 50 wt%. If the resin concentration is too low, the subsequent drying load of the film will be high, while if the resin concentration is too high, the viscosity will be too high, making film formation difficult.

[0048] In one or more embodiments, the polyvinyl alcohol resin used in the dissolution step is polymerized with a vinyl ester resin monomer to form a polyvinyl alcohol resin with an average degree of polymerization of 1500 to 3500, followed by a saponification reaction to obtain a polyvinyl alcohol resin with a degree of saponification of greater than 95%. The average degree of polymerization likely affects the dissolution and elongation of polyvinyl alcohol molecules in water, thereby affecting the crystal grain size and crystallinity of the polyvinyl alcohol film. The higher the degree of polymerization, the larger the crystal grains and the higher the crystallinity of the polyvinyl alcohol film. However, if the degree of polymerization is too high, the crystal grains will be too large, affecting the E' inflection point temperature. If the degree of polymerization is too low, the E' inflection point will be too low, resulting in too low mechanical strength of the film.

[0049] Examples of the vinyl ester resin monomer include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, and vinyl octoate, or combinations thereof, but are not limited to these in the present invention. Vinyl acetate is preferably used. The vinyl ester resin monomer can also be modified by adding and copolymerizing an olefin compound or an acrylic ester derivative. Examples of the olefin compound include ethylene, propylene, and butene, but are not limited to these in the present invention. The amount of the olefin compound added can be 2 to 4 mol%, for example, 2 mol%, 2.5 mol%, 3 mol%, 3.5 mol%, or 4 mol%, but is not limited to these. Examples of the acrylate derivative include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate, but are not limited to these in the present invention.

[0050] In one or more embodiments, the solvent used in the dissolving step is not particularly limited in the present invention, as long as it can dissolve the polyvinyl alcohol resin. Examples of solvents include, but are not limited to, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, and diethylenetriamine. The above solvents can be used alone or in combination of two or more. In consideration of environmental and economic aspects, it is preferable to use water as the solvent in the present invention.

[0051] In one or more embodiments, the "plasticizer" referred to herein specifically includes, but is not limited to, glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trimethylolpropane, or a combination thereof. In the present invention, glycerin is preferred. In one or more embodiments, the amount of the plasticizer added is 5 to 15 wt% based on the weight of the polyvinyl alcohol resin, specifically, but not limited to, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, and 15 wt%. In a preferred embodiment, the amount of the plasticizer added is 10 wt% based on the weight of the polyvinyl alcohol resin.

[0052] In addition to the plasticizer, other additives, including but not limited to surfactants, may be added as needed in the dissolution process. The surfactants include, but are not limited to, cationic, anionic, or nonionic surfactants, and specific examples include carboxylate-type surfactants such as potassium laurate, sulfate-type surfactants such as sodium lauryl sulfate, sulfonate-type surfactants such as dodecylbenzenesulfonate, alkylphenyl ether-type surfactants such as polyoxyethylene octylphenyl ether, alcohol phenyl ether-type surfactants such as polyethylene glycol monooctylphenyl ether, alkyl ester-type surfactants such as polyethylene glycol monolaurate, alkylamine-type surfactants such as polyoxyethylene laurylamino ether, alkylamide-type surfactants such as polyoxyethylene lauramide, polypropylene glycol ether-type surfactants such as polyoxyethylene polyoxypropylene ether, alkanolamide-type surfactants such as lauric acid diethanolamide and oleic acid diethanolamide, allylphenyl ether-type surfactants such as polyoxyallylphenyl ether, and sodium lauryl alcohol polyoxyethylene ether sulfate.

[0053] In one or more embodiments, the temperature at which the polyvinyl alcohol resin aqueous solution is dissolved is preferably 130 to 165°C, and is a value within a range between any two of the following values, specifically, for example, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 160°C, or 165°C. The dissolution temperature likely affects the dissolution of the polyvinyl alcohol resin and the dispersion of additives. The higher the dissolution temperature, the better the dissolution of the polyvinyl alcohol resin and the dispersion of additives, resulting in smaller crystal particles and a lower degree of crystallinity in the resulting polyvinyl alcohol film. However, if the dissolution temperature is too high, the resulting polyvinyl alcohol film may locally yellow. Conversely, without being bound by any particular theory, if the dissolution temperature is too low, the dissolution of the polyvinyl alcohol resin and the dispersion of additives are poor, resulting in relatively large crystal particles in the resulting polyvinyl alcohol film, resulting in an excessively high E' inflection point temperature of the polyvinyl alcohol film.

[0054] In one or more embodiments, the dissolution time of the polyvinyl alcohol resin aqueous solution is preferably 1 to 5 hours, and is a value within a range between any two of the following values, specifically, 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours. The dissolution time likely affects the dissolution of the polyvinyl alcohol resin and the dispersion of the additives. The longer the dissolution time, the better the dissolution of the polyvinyl alcohol resin and the dispersion of the additives, resulting in smaller crystal particles and a lower degree of crystallinity in the polyvinyl alcohol film. Conversely, without being bound by any particular theory, if the dissolution time is too short, the dissolution of the polyvinyl alcohol resin and the dispersion of the additives are poor, resulting in relatively large crystal particles in the formed polyvinyl alcohol film, and the E' inflection point temperature of the polyvinyl alcohol film becoming too high.

[0055] [Casting process] According to some embodiments, the casting process mainly involves feeding the polyvinyl alcohol casting solution into a twin-screw extruder, where it is homogenously mixed again and degassed (for example, but not limited to, degassing using a twin-screw extruder), and then discharging it through a T-die lip and casting it onto a rotating high-temperature casting drum (also called a casting drum) or a cast drum support such as an endless belt to form a polyvinyl alcohol preformed film. In one or more embodiments, the casting process preferably involves transporting the polyvinyl alcohol casting solution at a temperature of at least 90°C or higher (for example, but not limited to, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C, or 99°C). The polyvinyl alcohol casting solution after being homogenously mixed again and degassed in the extruder must also be controlled to at least 90°C or higher (for example, but not limited to, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, or 98°C).

[0056] In one or more embodiments, when the polyvinyl alcohol casting solution is cast onto a rotating high-temperature casting drum, the temperature of the casting drum is preferably 85 to 95°C, and the residence time of the polyvinyl alcohol on the casting drum is preferably 0.6 to 1.2 minutes.

[0057] [Heating roller process] In some embodiments, the heating roller process involves contacting and drying the upper and lower surfaces of the polyvinyl alcohol preformed film peeled from the casting drum with multiple heating rollers to obtain a semi-formed polyvinyl alcohol film. The temperatures of the multiple heating rollers (e.g., 13 to 19 heating rollers) gradually decrease from high to low, with the first heating roller having the highest temperature (e.g., 90 to 99°C, but not limited to, e.g., 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99°C) and the last heating roller having the lowest temperature (e.g., 25 to 40°C, but not limited to, e.g., 25, 30, 35, or 40°C).

[0058] [Oven process] According to some embodiments, the oven process is a process of drying the upper and lower surfaces of the polyvinyl alcohol film semi-molded product peeled off from the heating roller in an oven (for example, but not limited to, drying with hot air or infrared rays) to obtain a polyvinyl alcohol film molded product. In one or more embodiments, the oven is preferably a floating oven, and the temperature is controlled in the range of 100°C to 120°C (for example, 100°C, 105°C, 110°C, 115°C, or 120°C, preferably 110°C).

[0059] In one or more embodiments, the total drying time at temperatures above 50° C. in the heated roller step and oven step is 10 to 20 minutes (eg, 10, 15, or 20 minutes). The total drying time at temperatures exceeding 50°C likely affects the area swelling, TD swelling, MD swelling, and E' inflection temperature of the polyvinyl alcohol film. Without being bound by any theory, it is likely that when the water content is 2 wt%, the glass transition temperature of a molded polyvinyl alcohol film is 40-50°C, and therefore, when the drying temperature exceeds 50°C, the polymer chains become free to move within a narrow range. Therefore, the longer the total drying time, the larger the crystal particles become and the higher the crystallinity. Conversely, the shorter the total drying time, the smaller the crystal particles become and the lower the crystallinity, which affects the swelling and E' inflection temperature of the polyvinyl alcohol film.

[0060] [Temperature and humidity adjustment process] According to a preferred embodiment of the present invention, after the above steps, the polyvinyl alcohol film molded article can be further placed in a temperature and humidity controller to adjust the temperature and humidity.

[0061] In one or more embodiments, the temperature of the temperature / humidity controller is in the range of 35 to 45°C, specifically a value within a range between any two of the following values, for example, 35°C, 40°C, or 45°C, with 45°C being preferred. In one or more embodiments, the relative humidity of the temperature / humidity controller is in the range of 40 to 60%, specifically a value within a range between any two of the following values, for example, 40%, 45%, 50%, 55%, or 60%, with 50% being preferred. In one or more embodiments, the time for which the polyvinyl alcohol film is left in the temperature / humidity controller is, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes, or 40 minutes, with 20 to 40 minutes being preferred. In one or more embodiments, for example, the polyvinyl alcohol film is left in a temperature / humidity controller at a temperature of 45°C and a relative humidity of 50% for 30 minutes.

[0062] According to a preferred embodiment of the present invention, the molecular orientation of the film can be adjusted by additionally clamping both sides of the film and separately applying a tensile force in the TD direction during the oven process to maintain the film dimensions in the TD direction. Since polyvinyl alcohol films are generally stretched in the MD direction and polyvinyl alcohol polymers tend to orient along the MD direction, the difference in swelling degree between the two directions can be reduced by separately applying a force to the film in the TD direction and then maintaining a balance in the orientation of the polyvinyl alcohol polymer.

[0063] [Optical film] Another object of the present invention is to provide an optical film manufactured from the polyvinyl alcohol film. The "optical film" referred to in this specification may be a polarizing film, a retardation film, a viewing angle widening film, a brightness enhancing film, etc., and is particularly a polarizing film.

[0064] The "degree of polarization" referred to in this specification means a measured value obtained by the test method described in JIS Z 8722. In one or more embodiments, the degree of polarization of the polarizing film of the present invention is 99.99% or more, for example, 99.990% or more, 99.992% or more, 99.994% or more, 99.996% or more, or 99.998% or more.

[0065] In one or more embodiments, when the polarized film is heat-treated at 80°C for 500 hours, the decrease in polarization degree is 0.01% or less, for example, 0.010% or less, 0.009% or less, 0.008% or less, 0.007% or less, 0.006% or less, or 0.005% or less.

[0066] In one or more embodiments, when the polarized film is heat-treated at 80°C for 500 hours, the area shrinkage rate is 2% or less, for example, 2% or less, 1.9% or less, 1.8% or less, 1.7% or less, 1.6% or less, 1.5% or less, or 1.4% or less.

[0067] In one or more embodiments, the polarized film is heat-treated at 80°C for 500 hours, and the shrinkage percentages in the TD and MD are both 1% or less, for example, both 1% or less, both 0.95% or less, both 0.90% or less, both 0.85% or less, both 0.80% or less, or both 0.75% or less.

[0068] [Optical film manufacturing method] According to some embodiments, the "method for producing an optical film" of the present invention refers to a method for producing a polarizing film, and further includes producing a polarizing film from a polyvinyl alcohol film. The method for producing a polarizing film includes a dyeing step for adsorbing iodide ions, a boric acid treatment step, and a water washing step. A uniaxial stretching step can be performed during or before the boric acid treatment step. Preferably, a swelling step for swelling the polyvinyl alcohol film with water can be performed before the dyeing step. Note that a final drying step is usually performed after the water washing step.

[0069] In the swelling step, the polyvinyl alcohol film is immersed in a treatment bath (e.g., pure water) at a temperature of 30°C to 55°C, for example, to wash the film surface and perform a swelling treatment. The swelling treatment time is usually 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, a plurality of guide rollers are arranged in a swelling tank containing the treatment bath to transport the polyvinyl alcohol film. Next, the polyvinyl alcohol film is stretched in the MD direction to 1.05 to 2.5 times its original length, and then the dyeing step is performed.

[0070] In the dyeing process, the polyvinyl alcohol film that has undergone the swelling process is immersed in a dyeing tank containing a dye bath. The dyeing conditions can be determined based on the range in which iodine is adsorbed onto the polyvinyl alcohol film without causing defects such as excessive dissolution or devitrification of the film. The dye bath used in the dyeing process can be, for example, an aqueous solution containing iodine and potassium iodide. The iodine concentration in the dye bath is preferably 0.01 to 0.5 wt% and the potassium iodide concentration is preferably 0.01 to 10 wt%. Specific examples include, but are not limited to, an aqueous solution containing 0.037 wt% iodine and 1.85 wt% potassium iodide. Alternatively, other iodides, such as zinc iodide, may be used in place of potassium iodide, or other iodides may be used in addition to potassium iodide. The temperature of the dye bath is usually 20 to 40°C, for example, 20, 30, or 40°C, and the time for the dyeing treatment (dyeing time) is usually 10 to 600 seconds, preferably 30 to 200 seconds. Next, the polyvinyl alcohol film is stretched in the machine direction to 2 to 4 times its original length, and then subjected to a boric acid treatment and a stretching step.

[0071] The boric acid treatment and stretching process involves treating a polyvinyl alcohol film dyed with iodine with a boric acid-containing aqueous solution to crosslink the film and fix the absorbed iodine to the resin. This process is typically carried out by immersing the polyvinyl alcohol film that has undergone the dyeing process in a fixing tank containing a boric acid-containing treatment bath. The boric acid treatment bath preferably contains iodide in addition to boric acid. Examples of iodide used here include potassium iodide or zinc iodide, such as an aqueous solution containing boric acid and potassium iodide at a concentration of 5.5 wt% each. The boric acid treatment bath may also contain compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, or sodium sulfate. The boric acid treatment and stretching steps are usually carried out at a temperature of 50 to 70°C, preferably 55°C, and the treatment time is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 100 seconds. Next, the polyvinyl alcohol film is stretched in the machine direction to at least three times its original length, and the subsequent steps are carried out. The upper limit of the stretching ratio is not particularly limited, but is preferably less than 8 times. The stretching ratio may be 3.3 times or more, for example, 3.3 to 8.0 times, more preferably 3.5 to 6.0 times, and particularly preferably 4.0 to 5.5 times, but is not limited thereto.

[0072] After the polyvinyl alcohol film is subjected to the above process and subsequent washing and drying processes, a polarizing film is formed. In the washing and drying processes, the iodine solution and boric acid remaining on the film surface are washed with water or an aqueous solution containing an iodide (e.g., but not limited to, a 5.5 wt% potassium iodide aqueous solution). Next, after a drying process (e.g., but not limited to, drying in an oven at 60°C for 5 minutes), a polarizing film is formed. A protective layer may be formed on at least one side of the polarizing film to manufacture a finished polarizing film (also known as a polarizer). Specifically, the protective layer has functions such as preventing wear on the polarizing film surface and is preferably a component containing a transparent resin. According to different embodiments, the protective layer is formed on only one side of the polarizing film, but is preferably formed on both sides of the polarizing film. The protective layer may be a protective film made of a transparent resin material. The transparent resin may be an acrylic resin such as a methyl methacrylate resin, an olefin resin, a polyvinyl chloride resin, a cellulose resin, a styrene resin, an acrylonitrile-butadiene-styrene copolymer resin, an acrylonitrile-styrene copolymer resin, a polyvinyl acetate resin, a polyvinylidene chloride resin, a polyamide resin, a polyacetal resin, a polycarbonate resin, a modified polyphenylene ether resin, a polyester resin (such as a polybutylene terephthalate resin or a polyethylene terephthalate resin), a polysulfone resin, a polyethersulfone resin, a polyarylate resin, a polyimideimide resin, a polyimide resin, an epoxy resin, or an oxetane resin, and is preferably a cellulose resin such as cellulose triacetate (TAC). [Example]

[0073] The present invention will be described in detail below with reference to specific examples. However, it should be understood that these specific examples are intended to aid in understanding the present invention and are not intended to limit the scope of the present invention in any way.

[0074] 1. Preparation of Polyvinyl Alcohol Film Here, the present invention provides a non-limiting method for preparing polyvinyl alcohol films. Non-limiting examples of polyvinyl alcohol films (Examples 1 to 12) and comparative examples (Comparative Examples 1 to 5) are prepared by methods similar to those disclosed below.

[0075] The following are the main steps common to the production of polyvinyl alcohol films in this example and the comparative example, and the following Table 1 details the differences in one or more process parameters between this example and the comparative example.

[0076] [Dissolution process] 1800 kg of polyvinyl alcohol resin with an average degree of polymerization of 1500-3500 (see Table 1) and a degree of saponification of 99.95 mol%, 4000 kg of water, and 180 kg of plasticizer glycerol were added to a dissolving barrel. The dissolving temperature was raised to 130-165°C (see Table 1) while stirring, and the dissolving time was controlled to 1-5 hours (see Table 1). After uniform dissolution, water was added to adjust the resin concentration to 30 wt% to obtain a polyvinyl alcohol casting solution. The polyvinyl alcohol resin used in the examples and comparative examples was unmodified (i.e., did not contain other comonomers) and polymerized with vinyl acetate. The specific average degree of polymerization, dissolving temperature, and dissolving time for the polyvinyl alcohol resin are shown in Table 1.

[0077] [Casting process] The polyvinyl alcohol casting solution is transported to a twin-screw extruder, where it is mixed uniformly again and degassed. The temperature of the casting solution is controlled to 98°C, and then the casting solution is discharged from a T-die lip and cast onto a rotating high-temperature casting drum, where it is dried and formed into a preformed film.

[0078] [Heating roller process] After the polyvinyl alcohol preformed film was peeled off from the casting drum, the top and bottom surfaces of the film were dried by contact with 13 heating rollers. The first heating roller was the hottest roller (95°C) of all the heating rollers, and the temperature of the subsequent heating rollers gradually decreased from high to low, until the temperature of the 13th heating roller reached 30°C.

[0079] [Oven process] The film is dried in a floating oven, and both the top and bottom surfaces of the film are dried with hot air and infrared rays to produce a polyvinyl alcohol film. In another example, when drying in a floating oven, clamps are further placed on both sides of the film to apply a tensile force in the TD direction to maintain the film dimensions in the TD direction.

[0080] Here, in the above casting, hot roller and oven processes, the total drying time at a temperature above 50° C. is controlled (the specific duration is shown in Table 1).

[0081] [Temperature and humidity adjustment process] Thereafter, the polyvinyl alcohol film molded article was placed in a temperature and humidity controller at a temperature of 45° C. and a relative humidity of 50% for 30 minutes to obtain a polyvinyl alcohol film.

[0082] [Table 1]

[0083] 2. Preparation of Polarizing Films Here, the present invention provides a non-limiting method for preparing an optical film from a polyvinyl alcohol film, particularly a non-limiting method for preparing a polarizing film. According to the method disclosed below, the polyvinyl alcohol films of the non-limiting examples (Examples 1 to 12) and the polyvinyl alcohol films of the comparative examples (Comparative Examples 1 to 5) are prepared into corresponding polarizers.

[0084] The main steps common to the preparation of polarized films in the examples and comparative examples of the present invention are as follows: After unwinding, the polyvinyl alcohol film was placed in a swelling bath filled with 30°C pure water to rinse and swell the film surface. The polyvinyl alcohol film was then stretched in the machine direction to 1.2 times its original length. The temperature was then adjusted to 30°C and the film was placed in a dyeing bath containing an aqueous solution of 0.037 wt% iodine and 1.85 wt% potassium iodide to dye the film while stretching it in the machine direction to 3.4 times its original length. After dyeing, the temperature was adjusted to 55°C and the film was placed in a stretching bath containing an aqueous solution of 5.5 wt% boric acid and 5.5 wt% potassium iodide to stretch the polyvinyl alcohol film in the machine direction to 6 times its original length. The iodine water and boric acid remaining on the film surface were then washed with an aqueous solution containing 5.5 wt% potassium iodide. Next, the film was dried in an oven at 60°C for 5 minutes, and cellulose triacetate protective films were attached to the top and bottom surfaces, followed by drying to produce a polarizing film.

[0085] 3.Analysis method Here, the present invention provides methods for analyzing and testing the polyvinyl alcohol films of Examples 1 to 12 and Comparative Examples 1 to 5 above.

[0086] <Saponification degree> The method for measuring the degree of saponification in the present invention is the test method described in JIS K 6726 (1994).

[0087] <Average degree of polymerization> The method for measuring the degree of polymerization in the present invention is the test method described in JIS K 6726 (1994).

[0088] <Swelling degree> 1. Test conditions: Cut the polyvinyl alcohol film into a square sample with a size of 10 cm in the MD direction and 10 cm in the TD direction.夹 the edge of the sample together with glass using a long-tail clip, immerse it in a 4 wt% boric acid solution at 55 °C for 1 minute to swell it. After the swelling is complete, take out the sample and the glass, remove the long-tail clip, flatten the swollen sample, and measure the lengths in the MD direction and TD direction of the swollen sample using a vernier caliper respectively.

[0089] 2. The calculation formulas for the swelling degree in each direction (MD direction and TD direction) are expressed as follows.

[0090]

Number

[0091] 3. The calculation formula for the area swelling degree is expressed as follows.

[0092]

Number

[0093] <E’ inflection point temperature> 1. Equipment and its brand: DMA 850 of TA Instruments 2. Sample Preparation Method: Polyvinyl alcohol film was cut into a strip measuring 5 cm in the machine direction (MD) and 5 mm in the transverse direction (TD). The moisture content of the polyvinyl alcohol film was adjusted to 8-9 wt% before loading into the machine to ensure uniformity of the polyvinyl alcohol film. Next, referring to Figure 2, one end of the polyvinyl alcohol film 2 was fixed with a dovetail clip 3 weighing approximately 1.11 g, and the other end was inserted into the immersion stretching jig 1. A portion of the polyvinyl alcohol film 2 was positioned between the upper and lower fixed shafts 1a and 1b. The bottom end 3a of the dovetail clip 3 was spaced 1 cm (distance a shown in Figure 2) from the immersion stretching jig 1, leaving a space between the upper end 2a of the polyvinyl alcohol film 2 and the upper end of the immersion stretching jig 1. Next, the upper fixed shaft 1a was tightened to fix the polyvinyl alcohol film 2. Referring to Figure 3, the entire immersion stretching jig 1, the held polyvinyl alcohol film 2, and the dovetail clip 3 were placed vertically in a 100 mL beaker 4 filled with pure water so that the liquid level in the beaker was aligned with the upper fixed shaft 1a. The beaker 4 was then kept at 30°C in a thermostatic water bath (not shown), and a load (the weight of the dovetail clip 3) was applied to the polyvinyl alcohol film 2 to allow it to swell for 20 minutes. Finally, referring to Figure 4, the immersion stretching jig 1 was removed from the beaker 4 and held vertically. The weight of the dovetail clip 3 stretched the polyvinyl alcohol film 2. After confirming that the polyvinyl alcohol film 2 was centered, the lower fixed shaft 1b of the immersion stretching jig 1 was tightened, and the excess polyvinyl alcohol film 2c (the mesh-like portion in Figure 4) was trimmed off. The sample was polyvinyl alcohol film 2b clamped in the immersion stretching jig 1 and subjected to the load and swelling.

[0094] 3. Test Conditions: While the sample is still attached to the immersion / stretching fixture, select the vibration heating mode. Place the sample and the immersion / stretching fixture 1 holding the sample in a bath filled with deionized water. Set the frequency to 1 Hz, the amplitude to 200 μm, and the force track to 200%. Set the temperature to measure from 30°C to 65°C. Before analysis, first stretch the film with a static force of 0.15 N, then begin analysis at a heating rate of 1°C / min to plot a storage modulus (E') vs. temperature curve. The thermocouple's detection tip is positioned 5 mm above the bottom of the bath.

[0095] 4. Data processing: Convert the storage modulus (E') coordinate axis to a linear coordinate axis and use the onset analysis built into the software provided with the instrument. Set the onset analysis temperature range to 30-45°C, and the software will automatically enter the inflection point temperature.

[0096] 4. Evaluation method and results Here, the present invention provides evaluation methods for polyvinyl alcohol films and polarizing films in Examples and Comparative Examples, and the results of correlation with the above-mentioned analysis contents.

[0097] <Evaluation of polarization degree> Test conditions: The polarizing film was cut into two square samples measuring 4 cm in the MD direction and 4 cm in the TD direction. The two polarizing film samples were then stacked parallel to the MD direction (i.e., the flow direction). The light transmittance (H11) was measured using a spectrophotometer under irradiation with light of a wavelength of 700 nanometers. The two polarizing film samples were then stacked perpendicular to the MD direction. The light transmittance (H1) was measured under irradiation with light of a wavelength of 700 nanometers, and calculated using the polarization formula below.

[0098]

number

[0099] <Evaluation of Decreased Polarization Degree> Test conditions: The polarizing film is placed in an oven at 80°C and heat treated for 500 hours, after which the polarization degree is measured again and the decrease in polarization degree is calculated using the following formula.

[0100]

number

[0101] Furthermore, the area swelling degree and E' inflection point temperature of the polyvinyl alcohol films of Examples 1 to 12 and Comparative Examples 1 to 5 of the present invention are shown in Table 2 together with the evaluation of the polarization degree and the evaluation of the polarization degree decrease value of the corresponding polarizing films.

[0102] [Table 2]

[0103] As can be seen from Table 2, in the analysis of the area swelling degree and E' inflection point temperature of the polyvinyl alcohol film samples of Examples 1 to 12, the measured area swelling degrees were all 17.0 to 38.0%, and the E' inflection point temperatures were all 37 to 40°C. The polarization degrees of the polarized films made from these polyvinyl alcohol films all exceeded 99.99%, and the polarization degree reduction values ​​were all 0.010% or less, resulting in excellent evaluations of the polarization degree and polarization degree reduction value. In contrast, in the analysis of the area swelling degree and E' inflection point temperature of the polyvinyl alcohol film samples of Comparative Examples 1 to 5, the measured area swelling degrees and E' inflection point temperatures did not simultaneously fall within the above-mentioned specific ranges. The polarization degree reduction values ​​of the polarized films made from these polyvinyl alcohol films all exceeded 0.010%, resulting in unsatisfactory results for both the polarization degree and the polarization degree reduction value. In light of this, by simply controlling the polyvinyl alcohol film so that the area swelling degree when immersed in a 4 wt% boric acid solution at 55°C for 1 minute is 17.0 to 38.0% and the temperature at which the inflection point of the storage modulus (E') of the polyvinyl alcohol film in water at 30°C to 55°C is 37°C to 40°C, the polarized film produced not only has a good degree of polarization, but also improves the decline in the degree of polarization.

[0104] <Evaluation of shrinkage rate> 1. Test conditions: The polarized film was cut into a square sample measuring 10cm in the MD direction x 10cm in the TD direction, and heat-treated in an oven at 80°C for 500 hours. After removing it and returning it to room temperature, the lengths in the MD and TD directions were measured with calipers, and the shrinkage rate in the MD direction, the shrinkage rate in the TD direction, and the area shrinkage rate were calculated using the following shrinkage rate formula.

[0105] 2. The shrinkage formula for each direction (MD and TD) is expressed as follows:

[0106]

number

[0107] 3. The area shrinkage rate formula is expressed as follows:

[0108]

number

[0109] Furthermore, the swelling degree in the TD direction, swelling degree in the MD direction, difference in swelling degree, and swelling degree ratio (TD / MD) of the polyvinyl alcohol films of Examples 1 to 12 of the present invention and Comparative Examples 1 to 5 are shown in Table 3, along with the area shrinkage rate, shrinkage rate in the TD direction, and shrinkage rate in the MD direction of the corresponding polarizing films.

[0110] [Table 3]

[0111] As can be seen from Table 3, the areal swelling ratios, TD swelling ratios, and MD swelling ratios of the polyvinyl alcohol films of Examples 1 to 12 were in the ranges of 17.0 to 38.0%, 9.0 to 18.0%, and 7.5 to 18.0%, respectively. Therefore, the produced polarized films had an areal shrinkage ratio of 2% or less, a TD shrinkage ratio of 1% or less, and a MD shrinkage ratio of 1% or less. Further observation revealed that Comparative Example 3, while ideal in terms of the areal shrinkage ratios, TD shrinkage ratios, and MD shrinkage ratios of the polarized film, had poor evaluation results for the polarization degree and polarization degree decrease, indicating that it did not have the effects of the present invention. Furthermore, during the experimental process, it was discovered that the polyvinyl alcohol film was prone to breakage during the polarized film production process. In short, by controlling the area swelling degree of a polyvinyl alcohol film to 17.0 to 38.0%, the swelling degree in the TD direction to 9.0 to 18.0%, and the swelling degree in the MD direction to 7.5 to 18.0%, the produced polarized film has a small shrinkage rate and is resistant to deformation even when used for long periods in high-temperature environments.

[0112] All ranges provided herein are intended to include each specific range within the stated range and combinations of subranges between the stated ranges. Also, all ranges explicitly stated herein include the endpoints unless otherwise indicated. Thus, the range 1 to 5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2 to 5, 3 to 5, 2 to 3, 2 to 4, 1 to 4, etc.

[0113] All publications and patent documents cited in this specification are hereby incorporated by reference and for all purposes are specifically and individually indicated to be incorporated by reference. In the event of a conflict between the present specification and a publication or patent document incorporated by reference herein, the present specification will control. [Explanation of symbols]

[0114] 1. Immersion stretching jig 1a Upper fixed shaft 1b Lower fixed shaft 2 Polyvinyl alcohol film 2a top end 2b Polyvinyl alcohol film after loading and swelling 2c Extra polyvinyl alcohol film 3 dovetail clips 3a bottom edge 4 beakers a distance

Claims

1. Contains polyvinyl alcohol resin with a saponification degree of 95 mol% or more, and The area swelling degree obtained after immersion in an acid aqueous solution for 1 minute was 17.0 to 38.0%, and the area swelling degree obtained after immersion in water for 3 minutes was 17.0 to 38.0%. The temperature at which the inflection point of the storage modulus (E') in the range of 0°C to 55°C is 37 to 40°C. , polyvinyl alcohol film.

2. The area swelling degree is 17.5 to 35.0%, and the inflection point temperature is 37.8 to 38.5°C. The polyvinyl alcohol film according to claim 1 .

3. The polyvinyl alcohol film obtained by immersing it in a 4 wt % boric acid solution at 55°C for 1 minute The swelling degree in the transverse direction (TD) of the film is 9.0 to 1. 8.0%, and the swelling degree in the MD (Machine Direction) direction is 7.5 to 8.0%.

10. The polyvinyl alcohol film of claim 1, wherein the viscosity is 18.0%.

4. The swelling degree in the TD direction is 9.0 to 16.50%, and the swelling degree in the MD direction is 8.0 to 1 4. The polyvinyl alcohol film of claim 3, wherein the viscosity is 6.0%.

5. The absolute value of the difference between the swelling degree in the TD direction and the swelling degree in the MD direction is 5% or less. The polyvinyl alcohol film according to claim 3.

6. Claim 3: The ratio of the swelling degree in the TD direction to the swelling degree in the MD direction is 0.9 to 1.

7. The polyvinyl alcohol film according to claim 1.

7. The ratio of the swelling degree in the TD direction to the swelling degree in the MD direction is 0.95 to 1.

60. Item 6. The polyvinyl alcohol film according to item 5.

8. The average polymerization degree of the polyvinyl alcohol film is 1500 to 3500. Item 8. The polyvinyl alcohol film according to any one of items 1 to 7.

9. The average polymerization degree of the polyvinyl alcohol film is 2000 to 3500. Item 9. The polyvinyl alcohol film according to item 8.

10. The thickness of the polyvinyl alcohol film is 30 to 75 μm. The polyvinyl alcohol film according to any one of claims 1 to 14.

11. A film made from the polyvinyl alcohol film according to any one of claims 1 to 10. Optical film.

12. The optical film according to claim 11, which is a polarizing film.

13. The optical filter according to claim 12, wherein the polarization degree of the polarizing film is 99.99% or more. Room.

14. The polarizing film was heat-treated at 80° C. for 500 hours, and the decrease in the polarization degree was 0.01 % or less.

15. The polarizing film is heat-treated at 80°C for 500 hours, and the area shrinkage rate is 2% or less. The optical film according to claim 12 .

16. The polarizing film was heat-treated at 80° C. for 500 hours, and the shrinkage rate in the TD direction and the shrinkage rate in the MD direction were measured. The optical film according to claim 12 , wherein the shrinkage rates in both directions are 1% or less.

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