JP2024169407A - Polyvinyl alcohol film and optical film produced from the same - Google Patents

Abstract

Polyvinyl alcohol films and optical films made therefrom are provided.
[Solution] The polyvinyl alcohol film contains a polyvinyl alcohol resin, and when the polyvinyl alcohol film is immersed in water at 40°C for 1 minute, the amount of polyvinyl alcohol eluted is 0.20 to 9.00 ppm/ ^m2 , and when the polyvinyl alcohol film is heated in water from 30°C to 65°C at a heating rate of 1°C/min, a storage modulus (E') vs. temperature curve is measured, and the curve has a slope between 45°C and 55°C, and the absolute value of the slope is 0.20 to 0.70 MPa/°C. Since the stretching tension of the polyvinyl alcohol film of the present invention is low, the film is unlikely to break during stretching, and an optical film manufactured from the polyvinyl alcohol film has high single-unit transmittance and low red light leakage rate.
[Selected Figure] Figure 1

Description

The present invention relates to polyvinyl alcohol (PVA) products, and in particular, but not exclusively, to polyvinyl alcohol films and optical films made therefrom.

Polyvinyl alcohol (PVA) film is a hydrophilic material obtained by coating and drying an aqueous solution containing polyvinyl alcohol polymers and plasticizers. 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.

The polarizing film obtained by processing polyvinyl alcohol film in a polarizing process has the property of blocking only light from a specific direction, so the brightness of the light that passes through can be controlled. Based on this property, polarizing film is applied to various displays, glasses, and wearable terminals, and has become an indispensable component. The polarizing process generally includes processes such as swelling, stretching, and dyeing. Specifically, the polyvinyl alcohol film is placed in a solution and the above-mentioned process is carried out to diffuse the dye molecules into the molecules in the polyvinyl alcohol film, and a relatively regular arrangement is obtained by stretching, so that the polarizing film can absorb the light component parallel to the arrangement direction and pass the light component perpendicular to the arrangement direction to produce the polarizing property. However, conventional polyvinyl alcohol films are often prone to film breakage during the production of polarizing films due to excessive stretching tension.

With regard to the prior art such as the polyvinyl alcohol thin film disclosed in Patent Document 1, the average value of the detection intensity of positive silicon fragment ions obtained by cation analysis based on time-of-flight secondary ion mass spectrometry on at least one side of the polyvinyl alcohol thin film is 0.001 to 0.01, and the average value of the detection intensity of the positive silicon fragment ions is the average value of the detection intensity of the positive silicon fragment ions obtained by cation analysis based on a time-of-flight secondary ion mass spectrometry device at five points that divide the polyvinyl alcohol thin film into six equal parts in the TD direction on any line parallel to the TD direction of the polyvinyl alcohol thin film, and it has been realized that stretching breaks are unlikely to occur during uniaxial stretching by using technical means.

The prior art also discloses that the properties of a polarizing film can be improved by modifying the structure of polyvinyl alcohol and adjusting the manufacturing process of the polyvinyl alcohol film or polarizing film. For example, Patent Document 2 discloses a polarizing film that has excellent polarization performance and has little leakage of red light in a crossed Nicol state. Prior art such as Patent Document 3 also discloses that a polyvinyl alcohol-based thin film has good stretchability during polarizing film manufacturing and is not easily cut, so that a polarizing film with high polarization properties can be obtained by stretching at a high stretch ratio, and that the elution and precipitation of polyvinyl alcohol-based resin during polarizing film manufacturing can be suppressed, thereby suppressing contamination of polarizing film manufacturing equipment and enabling production with a high yield.

Taiwan Patent Publication No. 202233739 Taiwan Patent Publication No. 201719207 Taiwan Patent Publication No. 202400694

In the prior art, when a polarizing film is manufactured using a polyvinyl alcohol film, the polyvinyl alcohol film is often prone to breaking during the manufacture of the polarizing film due to excessive stretching tension, and the polarizing film made from this polyvinyl alcohol film often has the problems of low single-piece transmittance and high red light leakage. In this regard, the present inventors have found that by adjusting the polyvinyl alcohol film so that it has a specific amount of polyvinyl alcohol eluted when immersed in water at a specific temperature and time, and the curve of storage modulus (E') vs. temperature measured under specific conditions has a specific absolute value of slope in a specific temperature range, thereby improving the problem that the polyvinyl alcohol film is prone to breaking during stretching, and further improving the problems of low single-piece transmittance and high red light leakage of the polarizing film manufactured thereafter.

Specifically, one aspect of the present invention provides a polyvinyl alcohol film that contains a polyvinyl alcohol resin, and when immersed in water at 40°C for 1 minute, the amount of polyvinyl alcohol eluted is 0.20 to 9.00 ppm/ ^m2 , and when the film is heated in water from 30°C to 65°C at a heating rate of 1°C/min, a storage modulus (E') vs. temperature curve is measured, and the curve has a slope value between 45°C and 55°C, and the absolute value of the slope is 0.20 to 0.70 MPa/°C.

According to some embodiments of the present invention, the elution amount of the polyvinyl alcohol is 1.60 to 7.20 ppm/ ^m2 , and the absolute value of the slope is 0.20 to 0.60 MPa/°C.

According to some embodiments of the present invention, the absolute value of the slope is 0.25 to 0.60 MPa/°C.

According to some embodiments of the present invention, the absolute value of the slope is 0.35 to 0.55 MPa/°C.

According to some embodiments of the present invention, the polyvinyl alcohol film has a storage modulus (E') of 5.00 to 14.00 MPa at 45°C and a storage modulus (E') of 3.00 to 7.00 MPa at 55°C.

According to some embodiments of the present invention, the polyvinyl alcohol film has a storage modulus (E') of 8.00 to 11.50 MPa at 45°C and a storage modulus (E') of 4.00 to 6.00 MPa at 55°C.

According to some embodiments of the present invention, the polyvinyl alcohol film has a stretching tension of 13.0 to 30.0 N/ ^mm2 when immersed in a 2 wt % aqueous solution of boric acid at 50° C. for 30 seconds.

According to some embodiments of the present invention, the polyvinyl alcohol resin has an average degree of polymerization of 1500 to 3500.

According to some embodiments of the present invention, the polyvinyl alcohol resin has a polydispersity index (Polymer Dispersity Index, PDI) of 3.5 or less.

According to some embodiments of the present invention, the polyvinyl alcohol film has a thickness of 45 to 75 μm.

Another aspect of the present invention provides an optical film produced from the polyvinyl alcohol film.

According to some embodiments of the present invention, the optical film is a polarizing film.

According to some embodiments of the present invention, the polarizing film has a single-piece transmittance of 42.5% or more and a red light leakage rate of less than 5.0%.

The polyvinyl alcohol film of the present invention is suitable for use in a variety of applications, and is particularly suitable for use in optical films such as polarizing films. The polyvinyl alcohol film obtained based on the technical content of the present invention is less likely to break due to the small stretching tension during polarizing film production. The polarizing film made of the polyvinyl alcohol film has a low red light leakage rate and a high single unit transmittance.

Furthermore, the inventors have also found that the red light leakage rate of a polarizing film made of the polyvinyl alcohol film can be further improved by controlling the polyvinyl alcohol film so that it has a specific range of storage modulus (E') under the conditions of temperatures of 45°C and 55°C.

To facilitate a better understanding of the above and other objects, features, advantages and embodiments of the present invention, the following description will be given with reference to the drawings.

FIG. 2 is a schematic diagram illustrating the absolute value of the slope of the storage modulus (E′) vs. temperature curve of a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the absolute value of the slope of the storage modulus (E′) vs. temperature curve of a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the absolute value of the slope of the storage modulus (E′) vs. temperature curve of a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating the absolute value of the slope of the storage modulus (E′) vs. temperature curve of a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing the analysis of the single transmittance and red light leakage rate of a polarizing film corresponding to a polyvinyl alcohol film according to one embodiment of the present invention. 6A and 6B are schematic diagrams illustrating the analysis of the single transmittance and red light leakage rate of a polarizing film equivalent to a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing sampling for analyzing the red light leakage rate of a polarizing film corresponding to a polyvinyl alcohol film according to one embodiment of the present invention.

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.

The numerical ranges and parameters specifying the present invention are approximate, but the relevant numerical values in the specific examples are presented as precisely as possible. However, any numerical value necessarily contains standard deviations that are inherent in the particular testing method. As used herein, the term "about" indicates that the actual value is within an acceptable standard error of the mean and would be determined by one of ordinary skill in the art to which the present invention pertains.

[Polyvinyl alcohol film]
One aspect of the present invention provides a polyvinyl alcohol film comprising a polyvinyl alcohol resin, in which when immersed in 40°C water for 1 minute, the amount of polyvinyl alcohol dissolved is 0.20 to 9.00 ppm/ ^m2 , and when the film is heated in water from 30°C to 65°C at a heating rate of 1°C/min, a storage modulus (E') vs. temperature curve is measured, the curve has a slope value between 45°C and 55°C, and the absolute value of the slope is 0.20 to 0.70 MPa/°C. According to some embodiments of the present invention, the polyvinyl alcohol film has a thickness of 45 to 75 μm. Specifically, the thickness is the following value or a value within a range between any two of these values, for example, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, and 75 μm, and the above values are merely examples and are not limited thereto.

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 elastic modulus 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 the applied energy and stores it inside the material, and is usually used to recover from deformation and maintain the original appearance of the material, and is related to the elastic properties of the material.

In this specification, "the polyvinyl alcohol film is heated in water from 30°C to 65°C at a heating rate of 1°C/min, and the storage modulus (E') vs. temperature curve (hereinafter referred to as the storage modulus vs. temperature curve) is measured" has a double slope characteristic as shown in Figure 1, and the absolute value of the slope after the inflection point temperature is exceeded (the dashed line in Figure 1) represents the dissolution rate of the crystals in the polyvinyl alcohol film in water during the heating process. Since the heating rate during the analysis is constant, the larger the absolute value of the slope, the faster the dissolution rate of the crystals in the same time period is, and it can be seen that the crystals in the polyvinyl alcohol film are smaller and more uniform. Without being bound by a specific theory, it is believed that the appropriate size and uniformity of the crystals in the polyvinyl alcohol film makes it easy to stretch and dye the polarizing film during production, and a polarizing film with high single-piece transmittance and low red light leakage rate is produced. Furthermore, the appropriate crystals reduce the stretching tension during stretching, reducing the occurrence of film breakage. The inventors have determined that if the absolute value of the slope is too large, the polyvinyl alcohol film will be prone to breakage due to insufficient mechanical strength, and if the absolute value of the slope is too small, the polyvinyl alcohol film will have too many undissolved crystals (i.e., crystals that remain without being completely dissolved), and the stretching tension will become too large, making the film prone to breakage.

Therefore, in order to improve the problem that the polyvinyl alcohol film is easily broken due to excessive stretching tension during stretching, and the resulting polarizing film has low single-unit transmittance and high red light leakage rate, it is necessary to control the absolute value of the slope within a specific range. Specifically, a polyvinyl alcohol film is heated in water from 30°C to 65°C at a heating rate of 1°C/min, and a storage modulus (E') vs. temperature curve is measured, and the curve has a slope between 45°C and 55°C, with the absolute value of the slope being 0.20 to 0.70 MPa/°C. Specifically, the analytical temperature is the following value or a value within a range between any two of these values, for example, 0.20 MPa/°C, 0.25 MPa/°C, 0.30 MPa/°C, 0.35 MPa/°C, 0.40 MPa/°C, 0.45 MPa/°C, 0.50 MPa/°C, 0.55 MPa/°C, 0.60 MPa/°C, 0.65 MPa/°C, and 0.70 MPa/°C, preferably 0.20 MPa/°C to 0.60 MPa/°C, more preferably 0.25 MPa/°C to 0.60 MPa/°C, and particularly preferably 0.35 MPa/°C to 0.60 MPa/°C. When the analytical temperature exceeds 55°C, the crystallization of the polyvinyl alcohol film may completely disappear and the physical properties may change rapidly, so when analyzing the absolute value of the slope, it is important to note that the analytical temperature range set does not exceed 55°C.

As for the method of analyzing the absolute value of the slope, since polyvinyl alcohol films absorb moisture (including moisture in the air) and this may affect the state of the crystal morphology, molecular arrangement, etc., the moisture content of each polyvinyl alcohol film to be measured can be adjusted to the same by pretreatment before measuring the storage modulus (E') vs. temperature curve 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% by weight, but is not limited to this in the present specification.

The "amount of polyvinyl alcohol eluted" mentioned in this specification means that after a polyvinyl alcohol film is immersed in water, polyvinyl alcohol elution occurs, and as the temperature of the water rises and the immersion time becomes longer, the amount of polyvinyl alcohol eluted increases. Since the temperature of the dye bath in the dyeing tank is generally about 20 to 45°C, polyvinyl alcohol elution also occurs during dyeing of the polyvinyl alcohol film. Without being bound by a specific theory, the amount of polyvinyl alcohol eluted is probably related to the amount of polyvinyl alcohol that can freely diffuse in the amorphous region in the polyvinyl alcohol film. In other words, the higher the amount of polyvinyl alcohol eluted, the more polyvinyl alcohol that can freely diffuse, which indicates that the structure of the amorphous region in the polyvinyl alcohol film is loose but not tight, and relatively speaking, the dyeing solution can easily diffuse into the polyvinyl alcohol film through the loose structure and adsorb around the polyvinyl alcohol. The inventors have found that if the amount of polyvinyl alcohol eluted is too large, the polyvinyl alcohol film is likely to excessively adsorb the dye solution, and the color of the polarized film produced thereafter becomes too dark, resulting in an excessively low single-piece transmittance, and if the amount of polyvinyl alcohol eluted is too small, it becomes difficult to stretch or dye the polyvinyl alcohol film, and the single-piece transmittance of the polarized film produced thereafter may become too low.

Therefore, in order to improve the single-piece transmittance of a polarizing film made of the polyvinyl alcohol film, it is necessary to control the amount of elution of the polyvinyl alcohol within a specific range. Specifically, the amount of polyvinyl alcohol eluted after immersing the polyvinyl alcohol film in water at 40° C. for 1 minute is 0.20 to 9.00 ppm/m ² , specifically, the amount of polyvinyl alcohol eluted is the following value or a value within a range between any two of these values, for example, 0.20 ppm/m ² , 1.00 ppm/m ² , 1.60 ppm/m ² , 2.40 ppm/m ² , 3.20 ppm/m ² , 4.00 ppm/m ² , 4.80 ppm/m ² , 5.60 ppm/m ² , 6.40 ppm/m ² , 7.20 ppm/m ² , 8.00 ppm/m ² , and 9.00 ppm/m ² , and preferably 1.60 ppm/m ² to 7.20 ppm/m ² . The answer is ² .

The term "storage modulus (E') of a polyvinyl alcohol film at 45°C (hereinafter referred to as storage modulus at 45°C)" described in this specification can represent the degree of undissolved crystals of the polyvinyl alcohol film in water at a temperature of 45°C. Without being bound by a specific theory, it is believed that the degree of dye adsorption in the dyeing process in the polarizing film manufacturing process is related to the storage modulus at 45°C. The present inventors have found that the higher the storage modulus at 45°C, the higher the degree of undissolved crystals of the polyvinyl alcohol film in water at 45°C, and therefore the lower the degree of dye adsorption, i.e., the smaller the amount of dye adsorption. When the amount of dye adsorption of a polyvinyl alcohol film is low, the amount of I ₅ ^- produced after stretching in a stretching tank becomes too low, and the finally manufactured polarizing film is likely to have a red light leakage problem.

Therefore, by controlling the storage modulus of the polyvinyl alcohol film at 45°C to a specific range, the red light leakage rate of the polarizing film produced can be improved. Specifically, the storage modulus (E') of the polyvinyl alcohol film at 45°C is 5.00 to 14.00 MPa, specifically, the following values or values within any two ranges thereof, for example, 5.00 MPa, 5.5 MPa, 6.00 MPa, 6.5 MPa, 7.00 MPa, 7.5 MPa, 8.00 MPa, 8.5 MPa, 9.00 MPa, 9.5 MPa, 10.00 MPa, 10.5 MPa, 11.00 MPa, 11.5 MPa, 12.00 MPa, 12.5 MPa, 13.00 MPa, 13.5 MPa, and 14.00 MPa, and is preferably 8.00 to 11.50 MPa.

The term "storage modulus (E') of the polyvinyl alcohol film at 55°C (hereinafter referred to as storage modulus at 55°C)" described in this specification can represent the degree of insoluble crystals of the polyvinyl alcohol film in water at a temperature of 55°C. During the manufacture of a polarizing film, the film is usually stretched at 55°C and immersed in a boric acid solution. Without being bound by a specific theory, the degree of insoluble crystals of the raw film in the stretching process in the manufacturing process of the polarizing film is probably related to the storage modulus at 55°C. The inventors have found that the higher the storage modulus at 55°C, the higher the degree of insoluble crystals of the raw polyvinyl alcohol film and the lower the amount of boric acid crosslinking. If the amount of boric acid crosslinking is low, the dye adsorbed on the polyvinyl alcohol film is easily washed away and lost, and the amount of I ₅ ^- generated after stretching becomes too low, which makes the finally manufactured polarizing film more likely to have a red light leakage problem.

Therefore, by controlling the storage modulus of the polyvinyl alcohol film at 55°C to a specific range, the red light leakage rate of the polarizing film produced can be improved. Specifically, the storage modulus (E') of the polyvinyl alcohol film at 55°C is 3.00 to 7.00 MPa, specifically, the following values or values within any two ranges thereof, for example, 3.00 MPa, 3.50 MPa, 4.00 MPa, 4.50 MPa, 5.00 MPa, 5.50 MPa, 6.00 MPa, 6.50 MPa, and 7.00 MPa, and preferably 4.00 to 6.00 MPa.

According to some preferred embodiments of the present invention, the polyvinyl alcohol film has a lower stretching tension, which can reduce the occurrence of film breakage due to excessive stretching tension when produced as a polarizing film. Specifically, when immersed in a 2 wt % boric acid aqueous solution at 50° C. for 30 seconds, the stretching tension of the polyvinyl alcohol film is 13.0 to 30.0 N/mm ² , specifically, the following values or values within a range between any two of these values, for example, 13.0 N/mm ² , 15.0 N/mm ² , 17.0 N/mm ² , 19.0 N/mm ² , 21.0 N/mm ² , 23.0 N/mm ² , 25.0 N/mm ² , 27.0 N/mm ² , 29.0 N/mm ² , and 30.0 N/mm ² .

The "average degree of polymerization" referred to in this specification means a measured value obtained by the test method described in the JIS K 6726 (1994) standard. Without being bound by any particular theory, it is likely that the average degree of polymerization of the polyvinyl alcohol film is related to the size of the crystal particles produced. The inventors have found that if the average degree of polymerization is too high, the chain segments of the polyvinyl alcohol become long and difficult to move, the size of the crystal particles produced becomes non-uniform, and polyvinyl alcohol becomes difficult to dissolve, and if the average degree of polymerization is too low, crystals become difficult to form and the crystal particles become relatively non-uniform, so that polyvinyl alcohol easily dissolves. Therefore, in order to form crystal particles of uniform size, it is necessary to control the average degree of polymerization of the polyvinyl alcohol film within a specific range. Specifically, the average degree of polymerization of the polyvinyl alcohol resin is 1500 to 3500, specifically, the following values or values within a range between any two of these values, for example, 1500, 1700, 1900, 2100, 2300, 2500, 2700, 2900, 3100, 3300, and 3500.

The "Polymer Dispersity Index (PDI)" described in this specification is used to describe the molecular weight distribution of a polyvinyl alcohol resin, and is the ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn). According to some preferred embodiments of the present invention, the polyvinyl alcohol resin has a polydispersity index of 3.5 or less, specifically, for example, 3.5 or less, 3.3 or less, 3.1 or less, 2.9 or less, 2.7 or less, 2.5 or less, or 2.3 or less. Without being bound by any particular theory, the inventors have found that if the polydispersity index is too high, the polyvinyl alcohol film is more likely to dissolve low molecular weight polyvinyl alcohol, and the amount of polyvinyl alcohol dissolved becomes too large.

As described above, the polyvinyl alcohol film of the present invention is a film obtained by processing and molding a film-forming material containing a polyvinyl alcohol-based resin. When producing the film, a plasticizer, a surfactant, or other known additives may be added as a film-forming material.

The polyvinyl alcohol resin used is polymerized from vinyl ester resin monomers to form a polyvinyl ester resin having a degree of polymerization of 1500 to 3500 and a polydispersity index of 3.5 or less, and then undergoes a saponification reaction to obtain a polyvinyl alcohol resin. Examples of the vinyl ester resin monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl octoate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, 2,2,4,4-tetramethyl vinyl valerate, and ethenyl 2,2-dimethylpropanoate, but the present invention is not limited to these, and examples thereof include ethylene acetate. The vinyl ester polymer is preferably a polymer obtained by using one or more vinyl ester monomers as a monomer, and more preferably a polymer obtained by using one vinyl ester monomer as a monomer. The vinyl ester polymer may be a copolymer of one or more vinyl ester monomers and other copolymerizable monomers.

The amount of the copolymerization monomer may be, for example, 2 to 4 mol%, but is not limited thereto, and may be, for example, 2.0, 2.5, 3.0, 3.5, or 4.0 mol%. Olefin compounds having 3 to 30 carbon atoms, such as ethylene, propylene, 1-butene, and isobutylene; acrylic acid or a salt thereof; acrylic acid esters, such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or a salt thereof; Salts; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, dodecyl methacrylate, and methacrylic acid; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, N-hydroxymethylacrylamide or a salt thereof, acrylamide derivatives such as derivatives of the above; methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropylsulfonic acid or a salt thereof, methacrylamidepropyldimethylamine or a salt thereof, N-hydroxymethylmethacrylamide or a derivative thereof, and other methacrylamide derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, and isobutyl vinyl ether vinyl ethers such as tert-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or its salts, esters, or anhydrides; itaconic acid or its salts, esters, or anhydrides, maleic acid and its salts or esters, propargyl acetate, and the like; vinyl silicon compounds such as vinyltrimethoxysilane; and isopropenyl acetate. The vinyl ester-based polymer may also have one or more structural units derived from these copolymerization monomers.

The degree of saponification of the polyvinyl alcohol resin used is 95 mol% or more, and the "saponification degree" is a measured value obtained by the test method described in the JIS K 6726 (1994) standard. In one or more embodiments, the degree of saponification is, for example, 95 mol% or more, 96 mol% or more, 97 mol% or more, 98 mol% or more, 99 mol% or more, or 99.5 mol% or more. Preferably, the degree of saponification is 99.95 mol% or more, more preferably 99.97 mol% or more, for example, 99.97 mol% or more, 99.98 mol% or more, or 99.99 mol% or more.

According to some embodiments of the present invention, the "plasticizer" described herein is specifically, but not limited to, glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trimethylolpropane, or the like, 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% by weight relative to the weight of the polyvinyl alcohol resin, specifically, for example, but not limited to, 6% by weight, 7% by weight, 8% by weight, 9% by weight, 10% by weight, 11% by weight, 12% by weight, 13% by weight, 14% by weight, and 15% by weight. In a preferred embodiment, the amount of the plasticizer added is 10% by weight relative to the weight of the polyvinyl alcohol resin.

In addition to the plasticizer, other additives including, but not limited to, surfactants may be added as necessary in the dissolution process. The surfactants include, but are not limited to, cationic, anionic, or nonionic surfactants, and specifically include carboxylate types such as potassium laurate, sulfate types such as sodium lauryl sulfate, sulfonate types such as dodecylbenzenesulfonate, alkylphenyl ether types such as polyoxyethylene octylphenyl ether, alcohol phenyl ether types such as polyethylene glycol monooctylphenyl ether, alkyl ester types such as polyethylene glycol monolaurate, alkylamine types such as polyoxyethylene laurylamino ether, alkylamide types such as polyoxyethylene lauramide, polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether, alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide, allyl phenyl ether types such as polyoxyallyl phenyl ether, or sodium lauryl alcohol polyoxyethylene ether sulfate. The amount of surfactant added is 600 to 3000 ppm based on the weight of the polyvinyl alcohol resin, but is not limited thereto.

In addition, the polyvinyl alcohol film may further contain components such as water-soluble polymers, defoamers, antioxidants, UV absorbers, lubricants, other polymer compounds, or combinations thereof, to the extent that the effects of the present invention are not impaired.

[Method of producing polyvinyl alcohol film]
The method for producing a polyvinyl alcohol film of the present invention includes: (a) a dissolving step of heating and dissolving a polyvinyl alcohol-based resin and adjusting the concentration of the polyvinyl alcohol-based resin to form a polyvinyl alcohol casting solution; (b) a casting step of casting the polyvinyl alcohol casting solution onto a casting drum and peeling it off from the casting drum to obtain a preformed film; (c) a heating roller step of contacting the polyvinyl alcohol preformed film with a plurality of heating rollers to obtain a polyvinyl alcohol film preform; (d) a heat treatment step of placing the polyvinyl alcohol film preformed in a heat treatment device to heat treat it, and then obtaining a polyvinyl alcohol film semi-formed product; and (e) a temperature and humidity adjustment step of placing the polyvinyl alcohol film molded product in a temperature and humidity regulator and adjusting the temperature and humidity to obtain a polyvinyl alcohol film molded product.

<Dissolving process>
According to some embodiments of the present invention, the dissolving step is performed by stirring the film-forming materials, such as polyvinyl alcohol-based resin having a degree of polymerization of 1500-3500 and a polydispersity index of 3.5 or less, solvent, plasticizer, etc., while increasing the temperature to 140° C.-160° C. (e.g., 140° C., 150° C., or 160° C., but not limited thereto), for 1-3 hours, e.g., 1 hour, 2 hours, or 3 hours. After the materials are uniformly dissolved, a polyvinyl alcohol aqueous solution is formed, and the concentration of the polyvinyl alcohol aqueous solution is adjusted to 25-35 wt % (e.g., 25 wt %, 30 wt %, or 35 wt %, but not limited thereto), to obtain a polyvinyl alcohol casting solution.

According to some embodiments of the present invention, 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 the solvent include, but are not limited to, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, etc., and 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.

<Casting process>
According to some embodiments of the present invention, the casting process mainly involves sending the polyvinyl alcohol casting solution to an extruder to be mixed uniformly again and degassed (for example, but not limited to, degassing with a double screw extruder), discharging the solution from a T-die lip, and casting the solution onto a rotating cast drum (also called a casting drum) at a temperature of 88°C to 98°C (for example, but not limited to, 90°C, 93°C, or 96°C) to obtain a polyvinyl alcohol preformed film. According to some embodiments of the present invention, in the casting process, the temperature of the polyvinyl alcohol casting solution after being mixed uniformly again and degassed in the extruder must be controlled to at least 90°C (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). According to another embodiment of the present invention, 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.

<Heating Roller Process>
According to some embodiments of the present invention, the heating roller process mainly involves contacting and drying the upper and lower surfaces of the polyvinyl alcohol preformed film peeled off from the casting drum with multiple heating rollers, and then obtaining a polyvinyl alcohol film preform. The temperatures of the multiple heating rollers (e.g., 13 heating rollers) gradually decrease from high to low, with the first heating roller being the roller with the highest temperature among all the heating rollers (e.g., 90-99°C, but not limited thereto, specifically, for example, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C, or 99°C, but not limited thereto, preferably 95°C), and the temperature of the last heating roller being the lowest among all the heating rollers (e.g., 30-40°C, but not limited thereto).

<Heat treatment process>
According to some embodiments of the present invention, the heat treatment step is a step of heating and drying the polyvinyl alcohol film preform peeled off from the heating roller in the film thickness direction by heat radiation to obtain a polyvinyl alcohol film semi-formed product. The heat treatment is preferably performed by using infrared rays, and the film surface temperature of the polyvinyl alcohol film preform when heated by infrared rays is 80°C to 120°C, specifically, the following values or values within any two of these ranges, for example, 80°C, 90°C, 100°C, 110°C, and 120°C, and the heating time is 25 to 35 seconds, specifically, the following values or values within any two of these ranges, for example, 25 seconds, 27 seconds, 29 seconds, 31 seconds, 33 seconds, and 35 seconds. Without being bound by any particular theory, the inventors have found that if the surface temperature of the polyvinyl alcohol film preform during heating is too high, the size of the crystal particles produced will be large and the crystal distribution will be non-uniform, and if the surface temperature is too low, the size of the crystal particles will be too small or it will be even more difficult to form crystals, and the above two situations will have a negative effect on the absolute value of the slope of the storage modulus vs. temperature curve of the polyvinyl alcohol film produced thereafter.

<Temperature and humidity adjustment process>
According to some embodiments of the present invention, the polyvinyl alcohol film semi-formed product is placed in a temperature and humidity controller at a specific time, and the temperature and relative humidity are adjusted to promote recrystallization. The temperature of the temperature and humidity controller can be controlled to 60°C to 80°C, specifically, the following values or any two values within the ranges thereof, for example, 60°C, 70°C, and 80°C, the relative humidity can be controlled to 60% to 80%, specifically, the following values or any two values within the ranges thereof, for example, 60%, 70%, and 80%, and the time can be controlled to 3 to 8 minutes, specifically, the following values or any two values within the ranges thereof, for example, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, and 8 minutes. Without being bound by a particular theory, the inventors have found that increasing the high temperature and relative humidity accelerates the recrystallization process and the rearrangement of polyvinyl alcohol, which improves the denseness of the structure in the polyvinyl alcohol film and affects the absolute value of the slope and the amount of polyvinyl alcohol eluted.

[Optical film and method for producing same]
Another object of the present invention is to provide an optical film made from said polyvinyl alcohol film. The "optical film" mentioned in this specification can be a polarizing film, a retardation film, a viewing angle widening film, a brightness enhancing film, etc., and is particularly a polarizing film.

According to some embodiments of the present invention, the single transmittance of the polarizing film is 42.5% or more, e.g., 42.5% or more, 43.0% or more, or 43.5% or more, and the red light leakage rate of the polarizing film is less than 5.0%, e.g., less than 5.0%, less than 4.5%, less than 4.0%, less than 3.5%, less than 3.0%, less than 2.5%, less than 2.0%, less than 1.5%, or less than 1.0%.

According to some embodiments of the present invention, the "method of manufacturing an optical film" of the present application refers to a method of manufacturing a polarizing film, and further manufactures a polyvinyl alcohol film into a polarizing film. The manufacturing method includes a dyeing step of adsorbing iodide ions, a boric acid treatment step, and a water washing step, and a stretching step of uniaxial stretching can be performed in the boric acid treatment step or at a stage prior to this. Preferably, a swelling step of swelling the polyvinyl alcohol film with water can be provided prior to the dyeing step. Note that a final drying step is usually provided after the water washing step.

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

In the dyeing process, the polyvinyl alcohol film that has been through the swelling process is immersed in a dyeing tank containing a dye bath. The dyeing conditions can be determined within a range in which iodine is adsorbed onto the polyvinyl alcohol film without causing defects such as extreme dissolution or devitrification of the film. The dye bath in the dyeing process can be, for example, an aqueous solution containing iodine and potassium iodide, and the iodine concentration in the dye bath is preferably 0.01 to 0.5% by weight, and the potassium iodide concentration is preferably 0.01 to 10% by weight. Specific examples include, but are not limited to, an aqueous solution containing 0.037% by weight of iodine and 1.85% by weight of potassium iodide. In place of potassium iodide, other iodides such as zinc iodide may be used, or other iodides may be used in addition to potassium iodide. The temperature of the dye bath is usually 20 to 45°C, for example, 40°C, and the dyeing process time (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 the boric acid treatment and stretching steps are carried out.

In the boric acid treatment and stretching process, the polyvinyl alcohol film dyed with iodine is treated with a boric acid-containing aqueous solution to crosslink and fix the adsorbed iodine to the resin. The process is usually carried out by immersing the polyvinyl alcohol film that has been subjected to 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, and examples of the 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% by weight. In addition, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, sodium sulfate, etc., may also be present in the boric acid treatment bath. The boric acid treatment and stretching steps are usually carried out at a temperature of 50 to 70°C, for example at a temperature of 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 3 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, for example. The stretching ratio includes, but is not limited to, 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.

After the polyvinyl alcohol film is subjected to the above process and the 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 iodide (for example, but not limited to, washing with an aqueous potassium iodide solution with a concentration of 5.5% by weight). Next, after a drying process (for example, but not limited to, drying in an oven at a temperature of 60°C for 5 minutes), a polarizing film is formed.

Furthermore, the polarizing film includes a protective layer attached to at least one surface of the polarizing film. The protective layer is preferably a component having a function such as preventing abrasion of the surface of the polarizing film. According to different embodiments, the protective layer may be provided on only one surface of the polarizing film, or on both surfaces 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).

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

1. Preparation of polyvinyl alcohol films Herein, the present invention provides a non-limiting method for preparing polyvinyl alcohol films. Non-limiting example polyvinyl alcohol films (Examples 1-16) and comparative polyvinyl alcohol films (Comparative Examples 1-6) are prepared in a manner similar to that disclosed below.

The following are the main steps for producing polyvinyl alcohol films in this embodiment and the comparative example, and Table 1 below details the differences in one or more process parameters between this embodiment and the comparative example.

Dissolving process: First, 1800 kg of polyvinyl alcohol resin with a specific degree of polymerization and polydispersity index (specific degrees of polymerization and polydispersity index are shown in Table 1), 4000 kg of water, and 180 kg of glycerin were added to a dissolving barrel, and the dissolving temperature was raised to 150°C while stirring, and the mixture was dissolved for 2 hours. After uniform dissolution, the concentration of the polyvinyl alcohol aqueous solution was adjusted to 30% by weight to obtain a polyvinyl alcohol casting liquid. The polyvinyl alcohol resin in the examples and comparative examples was a polyvinyl alcohol resin that was polymerized with vinyl acetate, was not modified (i.e., did not contain other comonomers), and had a saponification degree of 99.95 mol%.

Casting process: The polyvinyl alcohol casting liquid was transported to an extruder, where it was mixed uniformly again and degassed. The temperature of the polyvinyl alcohol casting liquid was then controlled to 98°C, after which the casting liquid was discharged from a T-die lip and cast onto a rotating cast drum at a temperature of 96°C to form a film, forming a polyvinyl alcohol preformed film.

Heating roller process: After the polyvinyl alcohol preformed film was peeled off from the casting drum, the upper and lower surfaces of the polyvinyl alcohol preformed film were dried by contact with 13 heating rollers, one by one, while the temperature was lowered. The first heating roller had the highest temperature of all the heating rollers (95°C), and the 13th heating roller had the lowest temperature of all the heating rollers (30°C), and a polyvinyl alcohol film preform was obtained.

Heat treatment process: Next, the polyvinyl alcohol film preform was heated and dried with infrared rays for 30 seconds, and the film surface temperature during infrared heating was controlled within a specific range (specific film surface temperatures during heating are shown in Table 1), to obtain a semi-formed polyvinyl alcohol film.

Temperature and humidity adjustment process: Next, the polyvinyl alcohol film semi-molded product was placed in a temperature and humidity controller and recrystallized at a specific temperature and relative humidity for a specific time (the specific temperature, relative humidity, and time are shown in Table 1) to obtain a polyvinyl alcohol film molded product, which had a specific thickness (the specific thickness is shown in Table 1).

2. Preparation of Polarizing Film Herein, the present invention provides a non-limiting method for preparing a polarizing film from a polyvinyl alcohol film. According to the method disclosed below, the polyvinyl alcohol films of the non-limiting examples (Examples 1-11) and the polyvinyl alcohol films of the comparative examples (Comparative Examples 1-6) are prepared into corresponding polarizers.

2 and the following are the main steps common to the manufacture of polarizers from the vinyl alcohol films of the examples and comparative examples of the present invention. After unwinding the polyvinyl alcohol film, it is placed in a swelling tank filled with pure water at 30°C to wash and swell the film surface, and the polyvinyl alcohol film is stretched in the machine direction to 1.2 times its original length, and then the temperature is controlled to 45°C and the film is placed in a dyeing tank containing 0.037% by weight iodine and 1.85% by weight potassium iodide aqueous solution to dye and stretch the polyvinyl alcohol film in the machine direction to 3.4 times its original length. After dyeing, the temperature is controlled to 55°C and the film is placed in a stretching tank containing an aqueous potassium iodide solution with a concentration of 5.5% by weight of boric acid and potassium iodide, and the polyvinyl alcohol film is stretched along the machine direction to 6 times its original length, and then washed with the iodine water remaining on the film surface and an aqueous solution containing 5.5% by weight potassium iodide of boric acid. Next, it was dried in a 60°C oven for 5 minutes, and then cellulose triacetate protective films were attached to the top and bottom surfaces and dried to produce a polarizing film.

3. Analysis Method Here, the present invention provides analysis and testing methods for the polyvinyl alcohol films of Examples 1 to 11 and Comparative Examples 1 to 6 above.

Average Degree of Polymerization In the present invention, the average degree of polymerization is measured by the test method described in JIS K 6726 (1994).

Polydispersity Index (PDI)
Gel Permeation Chromatography (GPC) is used to test the peak molecular weight and polydispersity index of the polyvinyl alcohol resin.

Equipment and brand: (1) Waters 515 HPLC pump;
(2) Waters 717plus autosampler;
(3) A Waters 2414 refractive index detector with a detection temperature of 36° C.
(4) Analytical column: Waters Ultrahydrogel Guard Column, 125 Å, 6 μm, 6 mm*40 mm, 1 K to 500 K;
Waters Ultrahydrogel 120 Column, 120 Å, 6 μm, 7.8 mm x 300 mm, 100-5 K;
Waters Ultrahydrogel 250 Column, 250 Å, 6 μm, 7.8 mm x 300 mm, 1 K to 80 K;
Waters Ultrahydrogel 500 Column, 500 Å, 10 μm, 7.8 mm x 300 mm, 5 K to 400 K,
Waters Ultrahydrogel 1000 Column, 1000 Å, 12 μm, 7.8 mm x 300 mm, 10K to 1M.

Test conditions (1) Analysis flow rate: 0.413 mL/min,
(2) Flow liquor: 0.85 wt% sodium nitrate aqueous solution;
(3) Column oven temperature: 36° C.;
(4) Standard: Polyethylene glycol (PEG), with peak molecular weight (Mp) of 1,608,000/1,039,000/545,000/117,900/66,200/28,330/16,100/3,860/1,450/610/194;
(5) Analysis sample composition: 0.08 g of polyvinyl alcohol resin to be measured, 17 g of 0.85 wt% sodium nitrate aqueous solution, and 0.45 μm PTFE syringe filter,
(6) Sample injection volume: 210 μL,
(7) Test duration: 106 minutes.

Data processing: The number average molecular weight (Mw) and weight average molecular weight (Mn) of the polyvinyl alcohol resin were calculated, and the polydispersity index (PDI) was obtained according to the following formula:

PDI=Mw/Mn

Storage modulus (E') and absolute value of the slope of its curve versus temperature. Instrument and brand: TA Instruments DMA 850
Sample preparation method: A polyvinyl alcohol film is cut into a rectangle of 5 cm in the machine direction (MD) and 5 mm in the transverse direction (TD), and the moisture content of the polyvinyl alcohol film is adjusted to 8-9% by weight before being set in the machine to make the state of the polyvinyl alcohol film consistent. Next, referring to FIG. 2, one end of the polyvinyl alcohol film 2 is fixed with a dovetail clip 3 of about 1.11 g, and the other end is inserted into the immersion stretching jig 1, and a part of the polyvinyl alcohol film 2 is placed between the upper fixed shaft 1a and the lower fixed shaft 1b, and the bottom end 3a of the dovetail clip 3 is 1 cm (distance a shown in FIG. 2) away from the immersion stretching jig 1, and a space is provided 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 is tightened to fix the polyvinyl alcohol film 2. 3, the entire immersion stretching jig 1, the polyvinyl alcohol film 2 held therein, and the dovetail clip 3 are placed vertically in a 100 mL beaker 4 filled with pure water so that the liquid level of the beaker is located at the upper fixed shaft 1a, the beaker 4 is temperature-controlled in a thermostatic water bath (not shown) at 30° C., and a load (weight of the dovetail clip 3) is applied to the polyvinyl alcohol film 2 to cause it to swell for 20 minutes. Finally, referring to FIG. 4, the immersion stretching jig 1 is removed from the beaker 4 and held vertically, the polyvinyl alcohol film 2 is stretched by the weight of the dovetail clip 3, and after it is confirmed that the polyvinyl alcohol film 2 is in the center, the lower fixed shaft 1b of the immersion stretching jig 1 is tightened and the excess polyvinyl alcohol film 2c (the mesh-like portion in FIG. 4) is cut off, and the sample is the polyvinyl alcohol film 2b clamped by the immersion stretching jig 1 and subjected to the load and swelling.

Test conditions: When the above sample is still fixed in the immersion and stretching jig, select the vibration heating mode, place the sample and the immersion stretching jig 1 holding the sample together in an immersion bath filled with deionized water, set the frequency to 1 Hz, the amplitude to 200 μm, the force track to 200%, set the temperature to measure from 30°C to 65°C, stretch the film with a static force of 0.15 N before analysis, then start the analysis at a heating rate of 1°C/min, and draw a storage modulus (E') vs. temperature change curve. Here, the detection end of the thermocouple is placed at a height of 5 mm from the bottom of the water bath.

Data processing: The storage modulus (E') coordinate axis is converted to a linear coordinate axis, and using the slope analysis built into the software provided with the instrument, the analysis temperature range is set to 30-45°C, and the software automatically inputs the calculated slope value and takes the absolute value of the slope.

Storage modulus (E') at 45°C and 55°C: The storage modulus (E') values were obtained at 45°C and 55°C on the curve of storage modulus (E') vs. temperature on the linear coordinate axis described above.

Polyvinyl alcohol elution amount reagent composition (1) Boric acid solution: 20.00 g of boric acid was weighed into a 500 mL transparent measuring bottle, and first dissolved in a small amount of deionized water (DI water), and then deionized water was added up to the 500 mL horizontal scale line.
(2) Iodine solution: 1.27 g of solid iodine and 2.50 g of potassium iodide were weighed into a 100 mL brown measuring bottle, and first dissolved in a small amount of deionized water (DI water), and then added up to the 100 mL horizontal scale line.

Preparation of calibration curve: Polyvinyl alcohol resin with a degree of polymerization of 2400, a degree of saponification of 99.9 mol%, and a polydispersity index of 2.2 was dried at 105°C for 3 hours until completely dry, and then standards with polyvinyl alcohol resin concentrations of 100 ppm, 50 ppm, 25 ppm, 12.5 ppm, 6.25 ppm, 3.125 ppm, 1.5625 ppm, and 0.78125 ppm were prepared, and the absorbance at 670 nm was analyzed to prepare a calibration curve. 20 g of the standard was taken and placed in a grinding serum bottle, and 15 mL of boric acid solution, 3 mL of iodine solution, and 12 mL of deionized water were added in that order, shaken to mix uniformly, and then left to stand for 15 minutes.

Test conditions: 1 L of deionized water was placed in a serum bottle, and the serum bottle was placed in a thermostatic water bath at a temperature of 40°C. A polyvinyl alcohol film was cut into 20 test pieces measuring 5 cm in the machine direction (MD) and 5 cm in the transverse direction (TD), and each test piece was immersed in the deionized water in the serum bottle for 1 minute and then removed, so that a total of 20 test pieces were continuously immersed. The serum bottle was then removed from the thermostatic bath, cooled to 25°C, and the weight of the solution in the serum bottle was recorded. 20 g of the solution in the serum bottle was weighed into a grinding serum bottle, and 15 mL of boric acid solution, 3 mL of iodine solution, and 12 mL of deionized water were added in that order, shaken to mix uniformly, and then left for 15 minutes. Next, the absorbance of the solution at 670 nm was analyzed using an ultraviolet-visible spectroscopy (UV-vis), and the results were applied to a calibration curve to back-calculate the concentration, giving the amount of polyvinyl alcohol eluted.

Polyvinyl alcohol film thickness Instrument and brand: Mitutoyo 543-391B
Test method: Before analysis, confirm that the scale of the contact probe on the stage is 0, then place the polyvinyl alcohol film to be tested flat on the stage, lift up the probe and gently place it on the polyvinyl alcohol film, and read the thickness value.

4. Evaluation Method and Results Here, the present invention provides the evaluation methods of the polyvinyl alcohol films and polarizers of the examples and comparative examples, and the results of correlating them with the above-mentioned analysis contents.

Evaluation of stretching tension Equipment and its brand: Tensile testing machine HT-9102 manufactured by Koda
Sample preparation method: Cut a polyvinyl alcohol film into a rectangular test piece with a machine direction (MD) of 150 mm and a transverse direction (TD) of 12.7 mm. Place the test piece in an environment with a temperature of 23° C. and a relative humidity of 50%, and after adjusting the temperature and humidity for 20 hours, immerse it in an aqueous boric acid solution with a temperature of 50° C. and a concentration of 2% by weight for 30 seconds. Immediately after immersion, set it in the device and test it.

Test conditions: The distance between the fixtures is 5 cm, the tensile speed is controlled at 240 mm/min, and the sample thickness and length along the machine and width directions are input into the computer, and the system automatically converts the stretching tension.

Film Breakage Evaluation Test Conditions: A polyvinyl alcohol film is continuously pulled for 10,000 m to produce a polarizer.

Evaluation criteria: ◯: No breakage during the manufacturing process ×: Breakage occurred once or more during the manufacturing process

Single unit transmittance Equipment and brand: JASCO spectrophotometer V-7100
Sample preparation method: Two square samples of 2 cm in the machine direction (MD) and 2 cm in the transverse direction (TD) are cut out at the center of the polarizing film's width.

Test conditions: Using a spectrophotometer V-7100 equipped with an integrating sphere, perform luminosity correction for the visible light region with a C light source and a 2° visual field according to the test method described in the JIS Z 8722 (2009) standard. As shown in Figure 5, overlapping samples are taken, and the machine direction MD1 of one sample is perpendicular to the machine direction MD2 of the other sample. As shown in Figure 6A, one of the samples is rotated clockwise so that the machine directions MD1 and MD2 of the two samples form an angle A of +45°, and the light transmittance T1 at a wavelength of 550 nm is measured, and as shown in Figure 6B, one of the samples is rotated counterclockwise so that the machine directions MD1 and MD2 of the two samples form an angle B of -45°, and the light transmittance T2 at a wavelength of 550 nm is measured. Next, calculate the single transmittance according to the following formula.

Ts (%) = (Ts1 + Ts2)/2

Red light leakage rate Equipment and brand: JASCO spectrophotometer V-7100
Sample preparation method: A polarizing film was sampled at three locations: 5 cm from the left end in the full width direction (distance x shown in Figure 7), at the center, and 5 cm from the right end (distance x shown in Figure 7). Two square samples measuring 2 cm in the machine direction (MD) and 2 mm in the width direction (TD) were cut out from each of the three locations (Figure 7).

Test conditions: Single unit transmittance Ts (%): According to the method for measuring single unit transmittance described above, the single unit transmittance Ts (%) is measured at three locations, the left end, the center, and the right end.

Transmittance T700⊥(%): Two samples are taken at the same location, stacked on top of each other, and the machine direction MD1 of one sample is perpendicular to the machine direction MD2 of the other sample, as shown in Figure 5. As shown in Figure 6A, one of the samples is rotated clockwise so that the machine directions MD1 and MD2 of the two samples form an angle A of +45°, and the light transmittance _T45 at a wavelength of 700 nm is measured, and as shown in Figure 6B, one of the samples is rotated counterclockwise so that the machine directions MD1 and MD2 of the two samples form an angle B of 45°, and the light transmittance T _-45 at a wavelength of 700 nm is measured. The average value of _T45 and T _-45 is the transmittance T700⊥(%) at the position.

Red light leakage rate: With the single transmittance Ts (%) on the horizontal axis and the transmittance T700⊥ (%) on the vertical axis, an approximate line is drawn from the analysis values at three points, and the transmittance T700⊥ (%) when the single transmittance Ts is 44% (i.e. the orthogonal transmittance of light with a wavelength of 700 Nm) is found from the approximate line, and this is the red light leakage rate.

Evaluation criteria: It is preferable that the red light leakage rate is less than 5%.

Furthermore, Table 2 shows the results of the stretching tension evaluation and film breakage evaluation of the polyvinyl alcohol films of Examples 1 to 11 and Comparative Examples 1 to 6 of the present application, the single transmittance and red light leakage rate of the corresponding polarizing films, the absolute value of the slope of the storage modulus vs. temperature curve of the polyvinyl alcohol films (abbreviated as the absolute value of the slope in Table 2) and the amount of polyvinyl alcohol eluted.

As can be seen from Table 2, the absolute value of the slope of the polyvinyl alcohol films of Examples 1 to 11 is in the range of 0.20 to 0.70 MPa/°C, the amount of polyvinyl alcohol eluted is in the range of 0.20 to 9.00 ppm/ ^m2 , these polyvinyl alcohol films have uniformly small stretching tensions of 13.0 to 30.0 N/ ^mm2 and good film breakage evaluations, and the polarizing films manufactured from these polyvinyl alcohol films all have high single-piece transmittances of 42.5% or more and low red light leakage rates of less than 5.0%. In contrast, the absolute value of the slope and the amount of polyvinyl alcohol eluted of the polyvinyl alcohol of Comparative Examples 1 to 6 are not within the ideal range, so these polyvinyl alcohol films and the corresponding polarizing films cannot simultaneously have the above-mentioned technical features. Therefore, only by simultaneously controlling the absolute value of the slope of the polyvinyl alcohol film and the amount of polyvinyl alcohol dissolved therefrom within an ideal range can the problem of film tearing occurring easily when stretched due to excessive stretching tension, and the problems of low single-unit transmittance and high red light leakage rate of polarizing films made from the polyvinyl alcohol film be improved.

Furthermore, Table 3 shows the results of the red light leakage rate of the polarizing films corresponding to the polyvinyl alcohol films of Examples 1 to 11 and Comparative Examples 1 to 6 of this application, as well as the storage modulus of the polyvinyl alcohol films at 45°C and 55°C.

As can be seen from Table 3, when the storage modulus of the polyvinyl alcohol film at 45°C is controlled to 5.00 to 14.00 MPa and the storage modulus at 55°C is controlled to 3.00 to 7.00 MPa, the polarizing film manufactured from the polyvinyl alcohol film has a low red light leakage rate of less than 5.0% as in Examples 1 to 11 and Comparative Examples 1, 3, 4, and 6. In contrast, when the storage modulus of the polyvinyl alcohol film at 45°C and the storage modulus at 55°C are not simultaneously controlled within the ideal range, the polarizing film manufactured from the polyvinyl alcohol film has a high red light leakage rate as in Comparative Examples 2 and 5. Therefore, the problem of high red light leakage rate of the polarizing film manufactured from the polyvinyl alcohol film can be improved only by simultaneously controlling the storage modulus of the polyvinyl alcohol film at 45°C and the storage modulus at 55°C within the ideal range.

All ranges provided herein are intended to include each specific range within the stated range, and combinations of subranges between the stated ranges. Also, any range explicitly stated herein includes 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.

All publications and patent applications cited in this specification are hereby incorporated by reference and for all purposes are to the extent that each individual publication or patent application is specifically and individually indicated to be incorporated by reference. In the event of a conflict between this specification and a publication or patent application incorporated by reference herein, the specification shall control.

REFERENCE SIGNS LIST 1 Immersion stretching jig 1a Upper fixed shaft 1b Lower fixed shaft 2 Polyvinyl alcohol film 2a Upper end 2b Polyvinyl alcohol film subjected to load and swelling 2c Excess polyvinyl alcohol film 3 Dovetail clip 3a Bottom end 4 Beaker a, x Distance A, B Angle MD1, MD2 Machine direction

Claims (13)

Contains polyvinyl alcohol resin,
When immersed in 40°C water for 1 minute, the amount of polyvinyl alcohol dissolved is 0.20-9.00 ppm/ ^m2 , and when the temperature is increased from 30°C to 65°C at a temperature increase rate of 1°C/min in water, a storage modulus (E') vs. temperature curve is measured, the curve has a slope between 45°C and 55°C, and the absolute value of the slope is 0.20-0.70 MPa/°C.
Polyvinyl alcohol film. 2. The polyvinyl alcohol film according to claim 1, wherein the amount of elution of the polyvinyl alcohol is 1.60 to 7.20 ppm/ ^m2 , and the absolute value of the slope is 0.20 to 0.60 MPa/°C. The polyvinyl alcohol film according to claim 2, wherein the absolute value of the slope is 0.25 to 0.60 MPa/°C. The polyvinyl alcohol film according to claim 3, wherein the absolute value of the slope is 0.35 to 0.55 MPa/°C. The polyvinyl alcohol film according to claim 1, having a storage modulus (E') of 5.00 to 14.00 MPa at 45°C and a storage modulus (E') of 3.00 to 7.00 MPa at 55°C. The polyvinyl alcohol film according to claim 2, having a storage modulus (E') of 8.00 to 11.50 MPa at 45°C and a storage modulus (E') of 4.00 to 6.00 MPa at 55°C. The polyvinyl alcohol film according to any one of claims 1 to 6, which has a stretching tension of 13.0 to 30.0 N/ ^mm2 when immersed in a 2 wt% aqueous boric acid solution at 50°C for 30 seconds. The polyvinyl alcohol film according to any one of claims 1 to 6, wherein the polyvinyl alcohol resin has an average degree of polymerization of 1500 to 3500. The polyvinyl alcohol film according to any one of claims 1 to 6, wherein the polyvinyl alcohol resin has a polydispersity index (Polymer Dispersity Index, PDI) of 3.5 or less. The polyvinyl alcohol film according to any one of claims 1 to 6, having a thickness of 45 to 75 μm. An optical film produced from the polyvinyl alcohol film according to any one of claims 1 to 10. The optical film according to claim 11, which is a polarizing film. The optical film according to claim 12, wherein the polarizing film has a single-piece transmittance of 42.5% or more and a red light leakage rate of less than 5.0%.