TWI859105B - Polyvinyl alcohol film and optical film produced therefrom - Google Patents

Abstract

The present invention relates to a polyvinyl alcohol (PVA) film and optical film produced by the same. The PVA film comprises: a polyvinyl alcohol resin, wherein the PVA film has a dissolved amount of a polyvinyl alcohol of 0.20 to 9.00 ppm/m ²when immersed in water at 40℃ for 1 minute, and a curve of storage modulus (E') relative to temperature of the PVA film measured in water at a heating rate of 1℃/min from 30℃ to 65℃ has a slope value in the temperature range of 45℃ to 55℃, wherein the absolute value of the slope value is of 0.20 to 0.70 MPa/℃. The PVA film of this invention has a low elongation tension, so that the PVA film is not easy to break when stretched, and the optical film produced from the PVA film has advantages of high unit transmittance and low red light leakage rate.

Description

Polyvinyl alcohol film and optical film made therefrom

The present invention relates to a polyvinyl alcohol (PVA) product, in particular to a polyvinyl alcohol film and an optical film made therefrom, but not limited thereto.

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

The polarizing film obtained by processing the polyvinyl alcohol film through the polarization process has the property of only allowing light in a specific direction to pass through, and can thereby control the brightness of the light passing through; based on this property, the polarizing film is used in various displays, glasses and wearable devices, becoming an indispensable component. The so-called polarizing process generally includes the steps of swelling, stretching and dyeing; specifically, the polyvinyl alcohol film is placed in a solution to perform the above steps, so that the dye molecules diffuse into the molecules in the polyvinyl alcohol film, and a relatively regular arrangement is obtained through stretching, so that the polarizing film can absorb the light component parallel to its arrangement direction and allow the light component in the vertical direction to pass through, thereby generating polarization characteristics. However, the known polyvinyl alcohol film often has excessive stretching tension, which can easily cause film breakage when preparing the polarizing film.

Prior art, such as the Republic of China Invention Patent Publication No. 202233739, discloses a polyvinyl alcohol film, in which, on at least one side of the polyvinyl alcohol film, the average value of the positive silicon fragment ion detection intensity obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry is 0.001 to 0.01; wherein, the average value of the positive silicon fragment ion detection intensity is on an arbitrary straight line parallel to the TD direction of the polyvinyl alcohol film, and at 5 points that divide the polyvinyl alcohol film into 6 equal parts in the TD direction, and the average value of the positive silicon fragment ion detection intensity obtained by positive ion analysis based on time-of-flight secondary ion mass spectrometry is used as a technical means to achieve that tensile fracture is not likely to occur during uniaxial stretching.

In addition, the prior art also discloses that the properties of the polarizing film can be improved by changing the structure of polyvinyl alcohol and adjusting the process of polyvinyl alcohol film or polarizing film; for example, the Republic of China Patent Publication No. 201719207 discloses a polarizing film with good polarization performance and less leakage of red light in the orthogonal polarization state. In addition, the prior art, such as the Republic of China Patent Publication No. 202400694, also discloses a polyvinyl alcohol-based film that has good elongation during the manufacture of the polarizing film and is not easy to break, so that a high degree of elongation can be performed to obtain a polarizing film with high polarization; and because the dissolution and precipitation of the polyvinyl alcohol-based resin during the manufacture of the polarizing film can be suppressed, the contamination of the polarizing film manufacturing equipment can be suppressed, and the manufacturing can be performed with a high yield.

In the prior art, when a polyvinyl alcohol film is used to prepare a polarizing film, the polyvinyl alcohol film is often easily broken due to excessive stretching tension during the preparation of the polarizing film, and the polarizing film prepared from the polyvinyl alcohol film also often has problems of low monomer transmittance and high red light leakage rate. In response to this, the inventors have found that by adjusting the polyvinyl alcohol film so that it has a specific amount of polyvinyl alcohol dissolved when immersed in water at a specific temperature and time, and the storage modulus (E') relative to temperature curve measured under specific conditions has an absolute value of a specific slope value within a specific temperature range, the problem of the polyvinyl alcohol film being easily broken during stretching can be improved, thereby improving the low monomer transmittance and high red light leakage rate of the polarizing film prepared subsequently.

Specifically, the present invention provides a polyvinyl alcohol film, comprising: a polyvinyl alcohol resin; wherein the polyvinyl alcohol film has a polyvinyl alcohol dissolution amount of 0.20 to 9.00 ppm/m ² when immersed in 40°C water for 1 minute; and the polyvinyl alcohol film is heated from 30°C to 65°C in water at a heating rate of 1°C/min to measure a curve of storage modulus (E') relative to temperature, the curve has a slope value in the range of 45°C to 55°C, and the absolute value of the slope value is 0.20 to 0.70 MPa/°C.

According to some embodiments of the present invention, the dissolution amount of the polyvinyl alcohol is 1.60 to 7.20 ppm/m ² ; 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 the polyvinyl alcohol film has 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 the polyvinyl alcohol film has 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 tensile strength of 13.0 to 30.0 N/mm ² when immersed in a 2 wt. % boric acid aqueous solution 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 polymer dispersity index (PDI) less than or equal to 3.5.

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, which is made 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 body transmittance greater than or equal to 42.5%, and a red light leakage rate less than 5.0%.

The polyvinyl alcohol film of the present invention can be applied to a variety of purposes, especially to optical films, such as polarizing films. The polyvinyl alcohol film obtained based on the technical content of the present invention has a small stretching tension during the polarizing film manufacturing process, so it is not easy to break the film. In addition, the polarizing film made from the polyvinyl alcohol film has a low red light leakage rate and a high single body transmittance.

Furthermore, the inventors of the present invention have also discovered that by controlling the polyvinyl alcohol film to have a specific range of storage modulus (E’) at temperatures of 45°C and 55°C, respectively, the red light leakage rate of the polarizing film made from the polyvinyl alcohol film can be further improved.

In order to make the description of the present invention more detailed and complete, the following provides an illustrative description of the implementation mode and specific embodiments of the present invention, but this is not the only form of implementing or using the specific embodiments of the present invention. In this specification and the attached patent scope, unless the context otherwise states, "a", "an" and "the" may also be interpreted as plural.

Although the numerical ranges and parameters used to define the present invention are approximate, the relevant numerical values in the specific embodiments have been presented as accurately as possible. However, any numerical value inherently inevitably contains standard deviations caused by individual testing methods. Here, the word "about" means that the actual numerical value falls within the acceptable standard error of the mean value, which is determined by a person with ordinary knowledge in the field to which the present invention belongs.

[Polyvinyl alcohol film]

The present invention provides a polyvinyl alcohol film, comprising: a polyvinyl alcohol resin; wherein the polyvinyl alcohol film has a polyvinyl alcohol dissolution amount of 0.20 to 9.00 ppm/m ² when immersed in 40°C water for 1 minute; and the polyvinyl alcohol film is heated from 30°C to 65°C in water at a heating rate of 1°C/min to obtain a curve of storage modulus (E') relative to temperature, wherein the curve has a slope value in the range of 45°C to 55°C, and the absolute value of the slope value 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, for example, the following values or the range between any two thereof, for example: 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm and 75 μm, the aforementioned values are only exemplary and not restrictive.

The "modulus" mentioned in this article is a physical property used to analyze the relationship between stress and strain in material mechanics. It is the ratio of stress to strain. If the stress applied to the material is a positive force, the modulus obtained is represented by E. The "storage modulus (E')" is the stored internal energy converted from the material absorbing the applied energy. It is usually used to recover deformation and maintain the original appearance of the material. It is related to the elastic characteristics of the material.

The "storage modulus (E') curve relative to temperature of the polyvinyl alcohol film measured in water at a heating rate of 1°C/min from 30°C to 65°C (hereinafter referred to as the storage modulus curve relative to temperature)" described in this article has a double slope feature as shown in Figure 1, wherein the absolute value of the slope value after the turning point temperature (as indicated by the dotted line in Figure 1) represents the rate at which the crystals in the polyvinyl alcohol film are dissolved in water during the heating process. Since the heating rate during the analysis is fixed, the larger the absolute value of the slope value, the faster the crystal dissolution rate in the same time, which shows that the crystals in the polyvinyl alcohol film are smaller and more uniform. Without being limited by a particular theory, it is believed that the moderate size and uniformity of the crystals in the polyvinyl alcohol film facilitates stretching and dyeing in the polarizing film manufacturing process, thereby producing a polarizing film with high monomer transmittance and low red light leakage rate; further, moderate crystals can reduce the stretching tension during stretching, thereby reducing the occurrence of film breakage. The inventors have found that if the absolute value of the slope value is too large, the polyvinyl alcohol film will be easily broken due to insufficient mechanical strength; if the absolute value of the slope value is too small, the polyvinyl alcohol film will be easily broken due to excessive residual crystal dissolution (i.e., incomplete dissolution of the crystals, the remaining crystals) and excessive stretching tension.

Therefore, in order to improve the problem that the polyvinyl alcohol film is easily broken due to excessive stretching tension during stretching and the polarizing film subsequently produced has low monomer transmittance and high red light leakage rate, the absolute value of the slope value needs to be controlled within a specific range; specifically, the polyvinyl alcohol film is heated from 30°C to 65°C in water at a heating rate of 1°C/min to measure the storage modulus (E') relative to temperature curve, and the curve has a slope value in the range of 45°C to 55°C, and the absolute value of the slope value is 0.20 to 0.70 MPa/°C, specifically, for example, the following values or the range between any two thereof: 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/℃, 0.55 MPa/℃, 0.60 MPa/℃, 0.65 MPa/℃ and 0.70 MPa/℃; preferably 0.20 MPa/℃ to 0.60 MPa/℃; more preferably 0.25 MPa/℃ to 0.60 MPa/℃; and particularly preferably 0.35 MPa/℃ to 0.60 MPa/℃. It should be noted that when the analysis temperature is greater than 55℃, the physical properties of the polyvinyl alcohol film may change drastically due to the complete disappearance of crystals. Therefore, when analyzing the absolute value of the slope value, the analysis temperature range set should avoid exceeding 55℃.

In addition, regarding the analysis method of the absolute value of the slope value, since the polyvinyl alcohol film will absorb water (even water in the air) and will affect its crystal state and molecular arrangement, before measuring the storage modulus (E') relative temperature curve of the polyvinyl alcohol film, the water content of each polyethylene film to be measured can be adjusted to be consistent through pre-treatment, so that the state to be measured of each polyethylene film is consistent. The water content can usually be adjusted to about 8 to 9wt%, but this is not limited to this in this article.

The "dissolution amount of polyvinyl alcohol" mentioned in this article refers to the phenomenon that polyvinyl alcohol will dissolve after the polyvinyl alcohol film is immersed in water; and, as the temperature of the water increases and the soaking time increases, the amount of polyvinyl alcohol dissolved will increase. The temperature of the dye bath in the dyeing tank is generally about 20 to 45°C, so polyvinyl alcohol will also dissolve when the polyvinyl alcohol film is dyed. Not limited by a specific theory, it is believed that the dissolution amount of polyvinyl alcohol is related to the amount of polyvinyl alcohol that can diffuse freely in the amorphous region of the polyvinyl alcohol film: the higher the dissolution amount of polyvinyl alcohol, the more polyvinyl alcohol that can diffuse freely, which means that the structure of the amorphous region in the polyvinyl alcohol film is loose and not compact, and the dyeing solution is relatively easier to diffuse into the polyvinyl alcohol film through the loose structure and adsorb around the polyvinyl alcohol. The inventors of the present invention have found that if the amount of polyvinyl alcohol dissolved is too high, the polyvinyl alcohol film will easily absorb too much dyeing liquid, causing the color of the subsequently produced polarizing film to be too dark and the monomer transmittance to be too low; if the amount of polyvinyl alcohol dissolved is too low, the polyvinyl alcohol film will be difficult to stretch and dye, which will easily cause the monomer transmittance of the subsequently produced polarizing film to be too low.

Therefore, in order to improve the monomer transmittance of the polarizing film made of the polyvinyl alcohol film, the dissolution amount of the polyvinyl alcohol needs to be controlled within a specific range; specifically, the polyvinyl alcohol film is immersed in 40°C water for 1 minute and has a dissolution amount of polyvinyl alcohol of 0.20 to 9.00 ppm/m ² , such as the following values or the range between any two thereof: 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 ; preferably 1.60 ppm/m ² to 7.20 ppm/m ² .

The "storage modulus (E') of the polyvinyl alcohol film at 45°C (hereinafter referred to as the storage modulus at 45°C)" described herein can characterize the degree of crystallization and dissolution of the polyvinyl alcohol film in water at a temperature of 45°C. Without being limited by a specific theory, it is believed that the degree of dye adsorption in the dyeing step in the polarizing film manufacturing process is related to the storage modulus at 45°C. The inventors have found that the larger the storage modulus at 45°C, the higher the degree of crystallization and dissolution of the polyvinyl alcohol film in water at 45°C, and therefore the lower the degree of dye adsorption, that is, the less dye adsorption. When the polyvinyl alcohol film adsorbs less dye, the more dye is generated after being stretched in the stretching tank. The amount of red light generated is too low, resulting in the final polarizing film being more susceptible to red light leakage.

Therefore, controlling the storage modulus of the polyvinyl alcohol film at 45°C within a specific range can improve the red light leakage rate of the polarizing film obtained; specifically, the polyvinyl alcohol film has a storage modulus (E') of 5.00 to 14.00 MPa at 45°C, such as the following values or the range between any two thereof: 5.00 MPa, 5.50 MPa, 6.00 MPa, 6.50 MPa, 7.00 MPa, 7.50 MPa, 8.00 MPa, 8.50 MPa, 9.00 MPa, 9.50 MPa, 10.00 MPa, 10.50 MPa, 11.00 MPa, 11.50 MPa, 12.00 MPa, 12.50 MPa, 13.00 MPa, 13.50 MPa and 14.00 MPa. MPa; preferably 8.00 to 11.50 MPa.

The "storage modulus (E') of the polyvinyl alcohol film at 55°C (hereinafter referred to as the storage modulus at 55°C)" described in this article can characterize the degree of crystal dissolution of the polyvinyl alcohol film in water at a temperature of 55°C. In the production of polarizing films, they are usually stretched at 55°C and immersed in a boric acid solution. Without being limited by a specific theory, it is believed that the degree of dissolution of the original film crystals in the extension step in the polarizing film process is related to the storage modulus at 55°C. The inventors have found that the greater the storage modulus at 55°C, the higher the degree of dissolution of the original polyvinyl alcohol film, resulting in less boric acid crosslinking. When the amount of boric acid crosslinking is less, the dye adsorbed by the polyvinyl alcohol film will be more easily washed away and lost, and the dye generated after extension will be more likely to be lost. The amount of red light generated is too low, resulting in the final polarizing film being more susceptible to red light leakage.

Therefore, controlling the storage modulus of the polyvinyl alcohol film at 55°C within a specific range can improve the red light leakage rate of the polarizing film obtained; specifically, the polyvinyl alcohol film has a storage modulus (E') of 3.00 to 7.00 MPa at 55°C, such as the following values or the range between any two of them: 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; preferably 4.00 to 6.00 MPa.

According to some preferred embodiments of the present invention, the polyvinyl alcohol film has a lower extension tension, which can reduce the situation of film breakage due to excessive extension tension when preparing a polarizing film; specifically, the polyvinyl alcohol film has an extension tension of 13.0 to 30.0 N/mm ² when immersed in a 2 wt.% boric acid aqueous solution at 50° C. for 30 seconds, such as the following values or the range between any two of them: 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" described herein is measured according to the JIS K 6726 (1994) standard test method. Without being limited by a particular theory, it is believed that the average degree of polymerization of the polyvinyl alcohol film is related to the size of the generated crystalline particles. The inventors have found that if the average degree of polymerization is too high, the polyvinyl alcohol chain segments are long and difficult to move, making the size of the generated crystalline particles uneven, and thus making it difficult for the polyvinyl alcohol to dissolve; if the average degree of polymerization is too low, it is difficult to generate crystals, and the crystalline particles are also uneven, which makes it easy for the polyvinyl alcohol to dissolve. Therefore, in order to form crystalline particles of uniform size, the average degree of polymerization of the polyvinyl alcohol film needs to be controlled within a specific range; specifically, the polyvinyl alcohol resin has an average degree of polymerization of 1500 to 3500, such as the following values or the range between any two thereof: 1500, 1700, 1900, 2100, 2300, 2500, 2700, 2900, 3100, 3300 and 3500.

The "Polymer Dispersity Index (PDI)" described herein is used to describe the molecular weight distribution of the polyvinyl alcohol resin, which 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 polymer dispersity index of less than or equal to 3.5, for example less than or equal to 3.5, less than or equal to 3.3, less than or equal to 3.1, less than or equal to 2.9, less than or equal to 2.7, less than or equal to 2.5 or less than or equal to 2.3. Without being limited by a specific theory, the inventors have found that if the polymer dispersity index is too high, the polyvinyl alcohol film is more likely to have small molecules of polyvinyl alcohol dissolved, resulting in an excessively high amount of polyvinyl alcohol dissolved.

As mentioned above, the polyvinyl alcohol film of the present invention is a film formed by processing a film-forming material containing a polyvinyl alcohol resin. During the film production, a plasticizer, a surfactant, or other known additives may be added as the film-forming material.

The polyvinyl alcohol resin used is obtained by polymerizing vinyl ester resin monomers to form a polyvinyl ester resin with an average degree of polymerization of 1500 to 3500 and a polymer dispersibility index of less than or equal to 3.5, and then undergoing a saponification reaction to obtain the polyvinyl alcohol resin. The vinyl ester monomers may be, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl octanoate, vinyl laurate, vinyl stearate, vinyl benzoate, trimethylacetic acid vinyl, 2,2,4,4-tetramethylvalerate, vinyl tert-valerate, but the present invention is not limited thereto; for example, vinyl acetate. The vinyl ester polymer is preferably a polymer obtained using one or more vinyl ester monomers as monomers; and more preferably a polymer obtained using one vinyl ester monomer as a monomer. Furthermore, the vinyl ester polymer may be a copolymer of one or more vinyl ester monomers and other copolymerizable monomers.

The copolymerizable monomer may be, for example, ethylene, propylene, 1-butene, isobutylene, or other olefinic compounds having 3 to 30 carbon atoms, and the amount of the monomer added may be, for example, but not limited to, 2 to 4 mol%, such as 2.0, 2.5, 3.0, 3.5, or 4.0 mol%. mol% etc.; acrylic acid or its salt; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tertiary butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, octadecyl acrylate, etc.; methacrylic acid or its salt; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tertiary butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, octadecyl methacrylate, etc.; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidepropanesulfonic acid or its salt, acrylamidepropyldimethylamine or its salt, N-hydroxymethylacrylamide or its derivatives Acrylamide derivatives such as biological products; methacrylamide derivatives such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamide propanesulfonic acid or its salts, methacrylamide propyl dimethylamine or its salts, N-hydroxymethyl methacrylamide or its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tertiary butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as acrylonitrile and methacrylonitrile; halogenated vinyls 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; Iconic acid or its salt, ester or anhydride; maleic acid and its salt or ester, vinyl silyl compounds such as vinyl trimethoxy silane, isopropynyl acetate, etc.; vinyl silyl compounds such as vinyl trimethoxy silane; isopropenyl acetate, etc. In addition, the vinyl ester polymer may have structural units derived from one or more of these copolymerizable monomers.

The degree of alkalization of the polyvinyl alcohol resin used is 95.00 mol% or more, wherein the "degree of alkalization" is a measured value obtained according to the JIS K 6726 (1994) standard test method. In one or more embodiments, the degree of alkalization is, for example, 95.00 mol% or more, 96.00 mol% or more, 97.00 mol% or more, 98.00 mol% or more, 99.00 mol% or more, or 99.50 mol% or more. Preferably, the degree of alkalization 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 may be, but is not limited to, glycerol, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol or trihydroxymethylpropane, or a combination of any of the aforementioned plasticizers; glycerol is preferred in the present invention. In one or more embodiments, the amount of the plasticizer added is 5 to 15 wt.% relative to the weight of the polyvinyl alcohol resin, such as, but not limited to, 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.% relative to the weight of the polyvinyl alcohol resin.

In addition to the above-mentioned plasticizers, other additives may be further added during the dissolution process as needed, including but not limited to surfactants, etc. The surfactants include but are not limited to cationic, anionic or non-ionic surfactants, and may specifically be but are not limited to: carboxylate type such as potassium laurate, sulfate type such as sodium lauryl polyether sulfate, sulfonate type such as dodecylbenzenesulfonate, alkylphenyl ether type such as polyethylene oxide octylphenyl ether, polyethylene glycol ether, etc. Alcohol phenyl ether type such as glycol monooctylphenyl ether, alkyl ester type such as polyoxyethylene laurate, alkyl amine type such as polyoxyethylene laurylamine ether, alkyl amide type such as polyoxyethylene lauryl amide, polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether, alkanolamide type such as lauric acid diethanolamide, oleyl diethanolamide, allyl phenyl ether type such as polyoxyalkylene allyl phenyl ether, or sodium lauryl alcohol polyoxyethylene ether sulfate. The amount of surfactant added is 600 to 3000 ppm relative to the weight of the polyvinyl alcohol resin, but is not limited thereto.

In addition, the polyvinyl alcohol film may further contain water-soluble polymers, defoaming agents, antioxidants, ultraviolet absorbers, lubricants, other polymer compounds or combinations thereof, without hindering the effects of the present invention.

[Production method of polyvinyl alcohol film]

The method for producing a polyvinyl alcohol film of the present invention generally comprises the following steps: (a) a dissolution process: heating a polyvinyl alcohol resin film-forming material to dissolve the film, and adjusting the concentration of the polyvinyl alcohol resin to form a polyvinyl alcohol casting solution; (b) a casting process: casting the polyvinyl alcohol casting solution onto a casting drum, and obtaining a polyvinyl alcohol primary film after peeling off the casting drum; (c) a hot roll process; : The polyvinyl alcohol preformed film is contacted with a plurality of hot rollers to obtain a polyvinyl alcohol preformed film; (d) heat treatment process: The polyvinyl alcohol preformed film is placed in a heat treatment device for heat treatment to obtain a polyvinyl alcohol semi-finished film; and (e) temperature and humidity control process: The polyvinyl alcohol semi-finished film is placed in a temperature and humidity control box to adjust the temperature and humidity to obtain a polyvinyl alcohol finished film.

＜Dissolution Process＞

According to some embodiments of the present invention, the dissolution process mainly comprises raising the temperature of a film-forming material such as a polyvinyl alcohol-based resin having an average degree of polymerization of 1500 to 3500 and a polymer dispersibility index of less than or equal to 3.5, a solvent, a plasticizer, etc. to 140° C. to 160° C., specifically but not limited to 140° C., 150° C. or 160° C., while stirring, and dissolving for 1 to 3 hours, specifically for example 1 hour, 2 hours or 3 hours; after being uniformly dissolved to form a polyvinyl alcohol aqueous solution, and adjusting the concentration of the polyvinyl alcohol aqueous solution to 25 to 35 wt.%, specifically for example but not limited to 25 wt.%, 30 wt.% or 35 wt.%, to obtain a polyvinyl alcohol casting solution.

According to some embodiments of the present invention, the solvent used in the aforementioned dissolution process can dissolve the polyvinyl alcohol resin, so it is not limited in the present invention. As a solvent, for example, but not limited to: water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, etc., the above solvents can be used alone, and two or more can be used in combination. Considering the environment and economy, water is preferably used as the solvent in the present invention.

＜Casting Process＞

According to some embodiments of the present invention, the casting process mainly involves conveying the polyvinyl alcohol casting solution to an extruder for further mixing and defoaming (for example but not limited to using a twin-screw extruder for defoaming), and then discharging it from a T-shaped narrow slit die lip and casting it onto a rotating casting drum (or casting cylinder) to form a film, and the temperature of the casting drum is 88°C to 98°C, for example but not limited to: 90°C, 93°C or 96°C, thereby obtaining a polyvinyl alcohol primary formed film. According to some embodiments of the present invention, in the casting process, the temperature of the polyvinyl alcohol casting solution after being mixed and defoamed again by the extruder must be controlled to be at least 90°C or above, 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 other embodiments of the present invention, when the polyvinyl alcohol casting solution is cast onto the rotating casting drum, the temperature of the casting drum is preferably 85 to 95°C, and the time for which the polyvinyl alcohol remains on the casting drum is preferably 0.6 to 1.2 minutes.

＜Hot Roll Process＞

According to some embodiments of the present invention, the hot roller process mainly involves drying the upper and lower surfaces of the polyvinyl alcohol preformed film peeled off from the casting drum by a plurality of hot rollers, thereby obtaining a polyvinyl alcohol preformed film; wherein the temperature of the plurality of hot rollers (for example, 13 hot rollers) gradually decreases from high to low, and the starting hot roller is the one with the highest temperature among all the hot rollers, for example but not limited to 90°C to 99°C, specifically, 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, preferably 95°C; the temperature of the final hot roller is the lowest temperature among all the hot rollers, for example but not limited to 30°C to 40°C.

＜Heat treatment process＞

According to some embodiments of the present invention, the heat treatment process mainly involves heating and drying the polyvinyl alcohol preliminary finished film after being peeled off from the heat roller by heat radiation in the film thickness direction, thereby obtaining a polyvinyl alcohol semi-finished film; wherein, the heat treatment is preferably performed using infrared rays, and the film surface temperature of the polyvinyl alcohol preliminary finished film during infrared heating is 80°C to 120°C, specifically such as the following values or the range between any two of them: 80°C, 90°C, 100°C, 110°C and 120°C, and the heating time is 25 to 35 seconds, specifically such as the following values or the range between any two of them: 25 seconds, 27 seconds, 29 seconds, 31 seconds, 33 seconds and 35 seconds. Without being limited by a specific theory, the inventors have found that if the film surface temperature of the polyvinyl alcohol primary finished film is too high during heating, the size of the generated crystallized particles will be larger and the crystal distribution will be uneven; if it is too low, the size of the crystallized particles will be smaller or even difficult to form crystals, and both of the above conditions will have a negative impact on the absolute value of the slope value of the curve of the storage modulus relative to temperature of the polyvinyl alcohol film subsequently produced.

＜Temperature and humidity control process＞

According to some embodiments of the present invention, the polyvinyl alcohol semi-finished film is placed in a temperature and humidity control box for a specific time and its temperature and relative humidity are adjusted to facilitate recrystallization; wherein the temperature of the temperature and humidity control box can be controlled to be 60°C to 80°C, specifically for example, the following values or the range between any two of them: 60°C, 70°C and 80°C; the relative humidity can be controlled to be 60% to 80%, specifically for example, the following values or the range between any two of them: 60%, 70% and 80%; and the time can be controlled to be 3 to 8 minutes, specifically for example, the following values or the range between any two of them: 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 temperature and relative humidity can accelerate the recrystallization and rearrangement of polyvinyl alcohol, thereby improving the tightness of the structure within the polyvinyl alcohol film, affecting the absolute value of its slope value and the dissolution amount of the polyvinyl alcohol.

[Optical film and its manufacturing method]

Another object of the present invention is to provide an optical film, which is made from the aforementioned polyvinyl alcohol film. The "optical film" described herein can be a polarizing film, a phase difference film, a viewing angle enhancement film or a brightness enhancement film, etc., especially a polarizing film.

According to some embodiments of the present invention, the polarizing film has a single body transmittance greater than or equal to 42.5%, specifically, for example, greater than or equal to 42.5%, greater than or equal to 43.0%, or greater than or equal to 43.5%; and has a red light leakage rate less than 5.0%, specifically, for example, 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 for manufacturing an optical film" described herein refers to a method for manufacturing a polarizing film, and more specifically, a method for manufacturing a polyvinyl alcohol film into a polarizing film. The manufacturing method includes a dyeing step for adsorbing iodine ions, a boric acid treatment step, and a water washing step; a uniaxial stretching step can be performed in the boric acid treatment step or before the step. Preferably, a swelling step for swelling the polyvinyl alcohol film with water can be provided before the dyeing step. In addition, a final drying step is usually provided after the water washing step.

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. to wash the film surface and perform swelling treatment. The swelling treatment time is generally 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, a plurality of guide rollers are arranged in the swelling tank containing the treatment bath to transport the polyvinyl alcohol film. Subsequently, the polyvinyl alcohol film is stretched in the machine direction (MD) to 1.05 to 2.5 times of the original length before performing the dyeing step.

In the dyeing step, the polyvinyl alcohol film after the swelling step is immersed in a dyeing tank containing a dyeing bath. The dyeing treatment conditions can be determined within the range of allowing iodine to be adsorbed within the polyvinyl alcohol film without causing extreme dissolution of the film, devitrification, and other undesirable conditions. The dyeing bath in the dyeing step is, for example, an aqueous solution containing iodine and potassium iodide. The concentration of iodine in the dyeing bath is preferably 0.01-0.5wt%, and the concentration of potassium iodide is preferably 0.01-10wt%. A specific example may be, but is not limited to, an aqueous solution of iodine with a concentration of 0.037wt% and potassium iodide with a concentration of 1.85wt%. In addition, other iodides such as zinc iodide may be used instead of potassium iodide, or other iodides may be used in addition to potassium iodide. The temperature of the dyeing bath is usually set to 20 to 45°C, for example, at 40°C, and the dyeing treatment time (dyeing time) is usually 10 to 600 seconds, preferably 30 to 200 seconds. Subsequently, the polyvinyl alcohol film is stretched along the mechanical direction to 2 to 4 times the original length, and then subjected to boric acid treatment and stretching steps.

In the boric acid treatment and extension step, the iodine-stained polyvinyl alcohol film is treated with an aqueous solution containing boric acid to crosslink it and fix the adsorbed iodine in the resin. This step is usually performed by immersing the polyvinyl alcohol film after the dyeing step in a fixing tank containing a treatment bath containing boric acid. The boric acid treatment bath preferably contains iodide in addition to boric acid. The iodide used can be potassium iodide or zinc iodide, for example, an aqueous solution containing boric acid and potassium iodide at a concentration of 5.5wt%. In addition, compounds other than iodide can also coexist in the boric acid treatment bath, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, sodium sulfate, etc. The boric acid treatment and stretching steps are usually carried out at 50-70°C, for example, at 55°C, and the treatment time is usually 10-600 seconds, preferably 20-300 seconds, and more preferably 20-100 seconds. Subsequently, the polyvinyl alcohol film is stretched in the mechanical direction to more than 3 times the original length before the subsequent steps; the upper limit of the stretching ratio is not particularly limited, but is preferably 8 times or less, for example, stretching to include but not limited to 3.3 times or more, such as 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 above steps and the subsequent washing and drying steps, the polyvinyl alcohol film forms a polarizing film. In the washing and drying steps, 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 a 5.5 wt% potassium iodide aqueous solution for washing. Then, after a drying step, 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 side of the polarizing film. The protective layer is preferably a component having functions such as preventing the surface of the polarizing film from being worn. According to different embodiments, the protective layer can be disposed on only one side of the polarizing film or on both sides of the polarizing film. The protective layer can be a protective film of a transparent resin material; the transparent resin can be an acrylic resin such as 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, or a polyvinyl acetate resin. The present invention can be a polyurethane resin or a nylon resin. The polyurethane resin can be a polyurethane resin or a nylon resin. The polyurethane resin can be a polyurethane resin or a nylon resin. The polyurethane resin can be a polyurethane resin or a nylon resin.

Embodiment

Hereinafter, the present invention will be further described with detailed explanations and specific embodiments. However, it should be understood that these specific embodiments are only used to help the present invention to be more easily understood and are not used to limit the scope of the present invention.

1. Polyvinyl alcohol film Preparation

Here, the present invention provides a non-limiting method for preparing polyvinyl alcohol films. According to a method similar to the method disclosed below, non-limiting example polyvinyl alcohol films (Examples 1 to 11) and comparative example polyvinyl alcohol films (Comparative Examples 1 to 6) were prepared.

The following are the main steps common to the present embodiment and the comparative example in manufacturing the polyvinyl alcohol film, and the following Table 1 will show in detail the differences in one or more process parameters between the present embodiment and the comparative example: Dissolution process: First, add 1800 kg of polyvinyl alcohol resin with a specific average degree of polymerization and polymer dispersibility index (the specific average degree of polymerization and polymer dispersibility index are shown in Table 1), 4000 kg of water, and 180 kg of glycerol into the dissolution tank, and raise the dissolution temperature to 150°C while stirring and dissolving for 2 hours. After the solution is uniformly dissolved, adjust the concentration of the polyvinyl alcohol aqueous solution to 30wt.%, and obtain a polyvinyl alcohol casting solution. The polyvinyl alcohol resin in the embodiment and the comparative example is a polyvinyl alcohol resin polymerized from vinyl acetate and unmodified (i.e., not containing other copolymer monomers) and having a alkalization degree of 99.95 mol%. Casting process: The polyvinyl alcohol casting solution is transported to the extruder for further mixing and defoaming, and the temperature of the polyvinyl alcohol casting solution is controlled to 98°C, and then discharged from the T-shaped narrow slit die lip, and cast onto a rotating casting drum for film formation, and the temperature of the casting drum is 96°C, thereby forming a polyvinyl alcohol primary film. Hot roller process: After the polyvinyl alcohol preformed film is peeled off from the casting drum, 13 hot rollers are used to contact and dry the upper and lower surfaces of the polyvinyl alcohol preformed film with decreasing temperatures; the first hot roller is the one with the highest temperature among all the hot rollers, which is 95°C; the 13th hot roller is the one with the lowest temperature among all the hot rollers, which is 30°C, to obtain a polyvinyl alcohol preformed film. Heat treatment process: Then, the polyvinyl alcohol preformed film is dried by infrared heating for 30 seconds, and the film surface temperature during infrared heating is controlled within a specific range (the specific film surface temperature during heating is shown in Table 1) to obtain a polyvinyl alcohol semi-finished film. Temperature and humidity control process: Subsequently, the polyvinyl alcohol semi-finished film is placed in a temperature and humidity control box for recrystallization 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 finished film, and the polyvinyl alcohol finished film has a specific thickness (the specific thickness is shown in Table 1).

Table 1 Project Thickness(μm) Average degree of polymerization Polymer Dispersibility Index (PDI) Membrane surface temperature during infrared heating (℃) Temperature and humidity control process Temperature(℃) Relative humidity (%) Time(minutes) Embodiment 1 75 2400 2.2 100 60 60 3 Embodiment 2 60 2400 2.2 100 60 60 3 Embodiment 3 45 2400 2.2 100 60 60 3 Embodiment 4 60 3500 2.2 100 60 60 3 Embodiment 5 60 1500 2.2 100 60 60 3 Embodiment 6 60 2400 3.5 100 60 60 3 Embodiment 7 60 2400 2.2 80 60 60 3 Embodiment 8 60 2400 2.2 120 60 60 3 Embodiment 9 60 2400 2.2 100 80 60 3 Embodiment 10 60 2400 2.2 100 60 80 3 Embodiment 11 60 2400 2.2 100 60 60 8 Comparison Example 1 60 1000 2.2 100 60 60 3 Comparison Example 2 60 4000 2.2 100 60 60 3 Comparison Example 3 60 2400 4.0 100 60 60 3 Comparison Example 4 60 2400 2.2 130 60 60 3 Comparison Example 5 60 2400 2.2 70 60 60 3 Comparison Example 6 60 2400 2.2 100 90 90 10

2. Preparation of polarizing film

Here, 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 non-limiting example polyvinyl alcohol film (Examples 1 to 11) and the comparative example polyvinyl alcohol film (Comparative Examples 1 to 6) are prepared into corresponding polarizing films.

The following are the main steps common to the preparation of polarizers from polyvinyl alcohol films in the embodiments and comparative examples of the present invention: after the polyvinyl alcohol film is unwound, it is then placed in a swelling tank filled with 30°C pure water for washing and swelling treatment of the film surface; after the polyvinyl alcohol film is stretched to 1.2 times of its original length in the mechanical direction, it is then placed in a dyeing tank controlled at 45°C and containing an aqueous solution of 0.037 wt.% iodine and 1.85 wt.% potassium iodide, and the polyvinyl alcohol film is stretched to 3.4 times of its original length in the mechanical direction; after the dyeing is completed, it is placed in a stretching tank controlled at 55°C and containing an aqueous solution of 5.5 wt.% boric acid and potassium iodide, and the polyvinyl alcohol film is stretched to 6 times of its original length in the mechanical direction, and then the film is dyed with a 5.5 wt.% potassium iodide solution. wt.% aqueous solution to clean the iodine solution and boric acid remaining on the film surface. Then, dry it in a 60℃ oven for 5 minutes, and then laminate the upper and lower surfaces with a cellulose triacetate protective film and dry them to produce a polarizing film.

3. Analytical methods

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.

Average degree of polymerization

The method for measuring the average degree of polymerization of the present invention is based on the JIS K 6726 (1994) standard method.

Polymer Dispersibility Index (PDI)

Gel Permeation Chromatography (GPC) was used to test the peak molecular weight and polymer dispersibility index of polyvinyl alcohol resin.

Instruments and brands: (1) Waters 515 HPLC pump; (2) Waters 717 plus automatic sample injector; (3) Waters 2414 refractive index detector, detection temperature is 36℃; (4) Analytical columns: Waters Ultrahydrogel Guard Column, 125Å, 6 μm, 6 mm*40 mm, 1K-500K, Waters Ultrahydrogel 120 Column, 120Å, 6 μm, 7.8 mm*300 mm, 100-5K, Waters Ultrahydrogel 250 Column, 250Å, 6 μm, 7.8 mm*300 mm, 1K-80K, Waters Ultrahydrogel 500 Column, 500Å, 10 μm, 7.8 mm*300 mm, 5K-400K, Waters' Ultrahydrogel 1000 Column, 1000Å, 12 μm, 7.8 mm*300 mm, 10K-1M.

Test conditions: (1) Analysis flow rate: 0.413 mL/min; (2) Running liquid: 0.85 wt.% sodium nitrate aqueous solution; (3) Column oven temperature: 36°C; (4) Standard: polyethylene glycol (PEG), with peak molecular weight (Mp) = 1608000/ 1039000/ 545000/ 117900/ 66200/ 28330/ 16100/ 3860/ 1450/ 610/ 194; (5) Analysis sample configuration: 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) Injection volume: 210 μL; (7) Test time: 106 minutes.

Data processing: Calculate the weight average molecular weight (Mw) and number average molecular weight (Mn) of the polyvinyl alcohol resin, and calculate the polymer dispersibility index (PDI) according to the following formula: PDI = Mw/Mn.

Storage modulus (E’) , and the absolute value of the slope of its curve relative to temperature

Instrument and brand: TA Instruments DMA 850

Sample preparation method: Cut the polyvinyl alcohol film into a rectangle with a length of 5 cm in the machine direction (MD) and 5 mm in the width direction (TD). Adjust the moisture content of the polyvinyl alcohol film to 8 to 9 wt.% before loading the machine to make the polyvinyl alcohol film uniform. Next, please refer to Figure 2. Clamp one end of the polyvinyl alcohol film 2 with a dovetail clamp 3 weighing about 1.11 g, and insert the other end into the immersion stretching fixture 1. Place a part of the polyvinyl alcohol film 2 between the fixed shaft 1a and the lower fixed shaft 1b, so that the lower end 3a of the dovetail clamp 3 is 1 cm away from the immersion stretching fixture 1 (distance a shown in Figure 2), and there is a reserved space between the upper end 2a of the polyvinyl alcohol film 2 and the upper end of the immersion stretching fixture 1; then, lock the upper fixed shaft 1a to fix the polyvinyl alcohol film 2. Next, please refer to FIG. 3 , the entire immersion stretching fixture 1 together with the polyvinyl alcohol film 2 and the dovetail clamp 3 clamped therein are vertically placed in a 100 mL beaker 4 filled with pure water so that the liquid level of the beaker is located on the upper fixed axis. The beaker 4 is temperature-controlled in a 30°C constant temperature water bath (not shown in the figure), and the polyvinyl alcohol film 2 is loaded (the weight of the dovetail clamp 3) and swells for 20 minutes. Finally, referring to FIG. 4 , the immersion and stretching fixture 1 is taken out of the beaker 4 and kept upright, and the polyvinyl alcohol film 2 is stretched and supported by the weight of the dovetail clamp 3. After ensuring that the polyvinyl alcohol film 2 is centered, the fixed axis 1b under the immersion and stretching fixture 1 is locked, and then the excess polyvinyl alcohol film 2c (i.e., the grid-like portion in FIG. 4 ) is cut off. The sample is the polyvinyl alcohol film 2b that has been swollen under the load and clamped in the immersion and stretching fixture 1.

Test conditions: The upper open sample is still fixed in the immersion and stretching fixture. The oscillation heating mode is selected. The sample and the immersion and stretching fixture holding it are placed in an immersion tank filled with deionized water. The frequency is set to 1 Hz, the amplitude is set to 200 μm, the holding pressure (force track) is set to 200%, and the temperature is set to measure from 30℃ to 65℃. Before the analysis, the film is stretched with a static force of 0.15 N before starting to analyze and draw a curve of the storage modulus (E') relative to the temperature change at a heating rate of 1℃/min. Among them, the thermocouple sensing end is placed 5 mm above the bottom of the water tank.

Data processing: Change the coordinate axis of the stored modulus (E’) to a linear coordinate axis, use the slope analysis built into the software that comes with the instrument, set the analysis temperature range to 45°C to 55°C, the software will automatically calculate the slope value, and then take the absolute value of the slope value.

Storage modulus (E’) at 45°C and 55°C: The storage modulus (E’) versus temperature curve on the aforementioned linear coordinate axis is the value of the storage modulus (E’) obtained at 45°C and 55°C.

Dissolution of polyvinyl alcohol

Reagent preparation: (1) Boric acid solution: Weigh 20.00 g of boric acid into a 500 mL transparent quantitative bottle, dissolve it with a small amount of deionized water (DI water), and then add deionized water to the horizontal scale line of 500 mL. (2) Iodine solution: Weigh 1.27 g of solid iodine and 2.50 g of potassium iodide into a 100 mL brown quantitative bottle, dissolve it with a small amount of deionized water (DI water), and then add deionized water to the horizontal scale line of 100 mL.

Preparation of calibration curve: PVA resin with an average degree of polymerization of 2400, a degree of alkalinity of 99.90 mol% and a polymer dispersibility index of 2.2 was dried at 105°C for 3 hours until absolutely dry, and then PVA resin standards with concentrations of 100ppm, 50ppm, 25ppm, 12.5ppm, 6.25ppm, 3.125ppm, 1.5625ppm and 0.78125ppm were prepared, and the absorbance at 670 nm was analyzed to establish the calibration curve. 20g of the standard was placed in a frosted serum bottle, and 15mL of boric acid solution, 3mL of iodine solution and 12mL of deionized water were added in sequence, shaken to mix evenly and then left for 15 minutes.

Test conditions: 1L of deionized water was placed in a serum bottle, and the serum bottle was placed in a constant temperature water bath at 40°C. The polyvinyl alcohol film was cut into 20 test pieces with a length of 5 cm in the machine direction (MD) and 5 cm in the width direction (TD). Each test piece was immersed in the deionized water in the serum bottle, and then taken out after immersion for 1 minute. A total of 20 test pieces were immersed continuously. Then the serum bottle was taken out from the constant temperature bath, cooled to 25°C, and the weight of the solution in the serum bottle was recorded. Weigh 20 g of the solution in the serum bottle and place it in a frosted serum bottle. Add 15 mL of boric acid solution, 3 mL of iodine solution and 12 mL of deionized water in sequence, shake to mix evenly and let stand for 15 minutes. Then, use an ultraviolet-visible spectroscopy (UV-vis) to analyze the absorbance of the solution at 670 nm. Substitute the result into the calibration curve to infer the concentration, and the dissolution amount of polyvinyl alcohol can be obtained.

Polyvinyl alcohol film thickness

Instrument and brand: Mitutoyo 543-391B

Test method: Before analysis, make sure that the scale of the contact probe on the platform is 0. Place the polyvinyl alcohol film to be tested flat on the platform, pull up the probe and gently place it on the polyvinyl alcohol film to read the thickness value.

4. Evaluation methods and results

Here, the present invention provides an evaluation method for polyvinyl alcohol film and polarizing film of the embodiment and comparative example, and the corresponding results thereof with the above analysis contents.

Extension tension evaluation

Instrument and brand: Hongda tensile testing machine HT-9102

Sample preparation method: Cut the polyvinyl alcohol film into a rectangular specimen with a length of 150 mm in the machine direction (MD) and 12.7 mm in the width direction (TD). Place the specimen in an environment with a temperature of 23°C and a relative humidity of 50% for 20 hours, then immerse it in a boric acid aqueous solution with a temperature of 50°C and a concentration of 2 wt.% for 30 seconds, and immediately test it on the machine after immersion.

Test conditions: Fix the distance between the fixtures to 5 cm, control the tension speed to 240 mm/min, input the thickness of the sample and its length along the mechanical direction and width direction into the computer, and the system will automatically convert the extension tension.

Film break evaluation

Test conditions: Continuously use 10000 m of polyvinyl alcohol film for polarizing film production.

Evaluation criteria: ○: No cracks occurred during the process ╳: There were more than 1 cracks or equal to 1 during the process

Single body transmittance

Instrument and brand: JASCO spectrophotometer V-7100

Sample preparation method: Cut two square samples with a size of 2 cm in the machine direction (MD) and 2 cm in the width direction (TD) from the middle of the polarizing film.

Test conditions: Use a spectrophotometer V-7100 with an integrating sphere to perform visual sensitivity calibration in the visible light field with a C light source and a 2° field of view according to the JIS Z 8722 (2009) standard method. As shown in Figure 5, take overlapping samples, where the mechanical direction MD1 of one sample is perpendicular to the mechanical direction MD2 of the other sample. As shown in Figure 6A, rotate one sample clockwise so that the mechanical directions MD1 and MD2 of the two samples form an angle A of +45°, and measure the transmittance T1 of light at a wavelength of 550 nm; as shown in Figure 6B, rotate one sample counterclockwise so that the mechanical directions MD1 and MD2 of the two samples form an angle B of -45°, and measure the transmittance T2 of light at a wavelength of 550 nm. Then the single body transmittance Ts(%) is calculated according to the following formula: (Ts1 + Ts2)/2.

Red light leakage rate

Instrument and brand: JASCO spectrophotometer V-7100

Sample preparation method: Take three positions of the polarizing film in the full width direction, 5 cm from the left edge (as shown in the distance x in Figure 7), the middle, and 5 cm from the right edge (as shown in the distance x in Figure 7), and cut two square samples with a length of 2 cm in the machine direction (MD) and 2 mm in the width direction (TD) at each of the three positions (as shown in Figure 7).

Test conditions:

Single body transmittance Ts(%): According to the above single body transmittance method, the single body transmittance Ts(%) of three positions, namely the left side, the middle and the right side, is tested.

Transmittance T700⊥(%): Take two samples at the same position and overlap each other, as shown in Figure 5, and the mechanical direction MD1 of one sample is perpendicular to the mechanical direction MD2 of the other sample. As shown in Figure 6A, rotate one of the samples clockwise so that the mechanical directions MD1 and MD2 of the two samples form an angle A of 45°, and measure the transmittance T ₄₅ of light at a wavelength of 700 nm; as shown in Figure 6B, rotate one of the samples counterclockwise so that the mechanical directions MD1 and MD2 of the two samples form an angle B of 45°, and measure the transmittance T _-45 of light at a wavelength of 700 nm. Take the average value of T ₄₅ and T _-45 as the transmittance T700⊥(%) at that position.

Red light leakage rate: With the single body transmittance Ts (%) as the horizontal axis and the transmittance T700⊥ (%) as the vertical axis, draw an approximate straight line from the analysis values of the three positions, and from the approximate straight line, obtain the transmittance T700⊥ (%) when the single body transmittance Ts is 44% (i.e., the orthogonal transmittance of light with a wavelength of 700 nm), which is the red light leakage rate.

Evaluation criteria: Red light leakage rate less than 5% is preferred.

Furthermore, the results of the extension tension evaluation and film break evaluation of the polyvinyl alcohol films of Examples 1 to 11 of the present invention and Comparative Examples 1 to 6, as well as the monomer transmittance and red light leakage rate of the corresponding polarizing films, are presented in Table 2 together with the absolute value of the slope value of the curve of the storage modulus of the polyvinyl alcohol film relative to the temperature (abbreviated as the absolute value of the slope value in Table 2) and the dissolution amount of polyvinyl alcohol.

Table 2 Project Absolute value of slope (MPa/℃) Dissolution amount of polyvinyl alcohol (ppm/m ² ) Elongation tension evaluation (N/mm ² ) Film break evaluation Single body transmittance (%) Red light leakage rate (%) Embodiment 1 0.45 4.16 22.9 ○ 44.0 1.0 Embodiment 2 0.48 4.00 23.3 ○ 44.1 0.9 Embodiment 3 0.46 3.52 23.4 ○ 44.1 1.3 Embodiment 4 0.61 0.32 20.7 ○ 42.8 2.7 Embodiment 5 0.34 6.40 19.6 ○ 43.6 2.2 Embodiment 6 0.51 7.68 20.5 ○ 42.5 3.1 Embodiment 7 0.69 5.12 14.8 ○ 42.6 1.9 Embodiment 8 0.23 5.76 29.7 ○ 43.3 3.4 Embodiment 9 0.29 0.80 26.3 ○ 42.5 4.7 Embodiment 10 0.36 1.28 24.8 ○ 42.7 4.4 Embodiment 11 0.42 3.04 23.6 ○ 44.2 1.2 Comparison Example 1 0.18 9.44 10.2 ╳ 41.5 3.5 Comparison Example 2 0.72 0.06 32.5 ╳ 41.8 5.7 Comparison Example 3 0.49 11.52 21.1 ○ 42.1 3.9 Comparison Example 4 0.15 5.12 33.4 ╳ 41.7 4.4 Comparison Example 5 0.80 4.16 11.6 ╳ 41.6 5.5 Comparison Example 6 0.12 0.08 37.4 ╳ 42.0 1.7

According to Table 2, the absolute values of the slope values of the polyvinyl alcohol films of Examples 1 to 11 are all within the range of 0.20 to 0.70 MPa/℃, and the dissolution amount of polyvinyl alcohol is all within the range of 0.20 to 9.00 ppm/m ^2. It is observed that these polyvinyl alcohol films all have a relatively small extension tension of 13.0 to 30.0 N/mm ² and a good film breaking evaluation; and the polarizing films prepared from these polyvinyl alcohol films all have a high monomer transmittance of greater than or equal to 42.5% and a low red light leakage rate of less than 5.0%. On the contrary, if the polyvinyl alcohol films of Comparative Examples 1 to 6 do not have the absolute value of the slope value and the dissolution amount of polyvinyl alcohol in the ideal range at the same time, then these polyvinyl alcohol films and their corresponding polarizing films cannot have the above-mentioned technical effects at the same time. In view of this, only by controlling the absolute value of the slope value of the polyvinyl alcohol film and the dissolution amount of polyvinyl alcohol within the ideal range at the same time can the problem of the polyvinyl alcohol film being easily broken during stretching due to excessive extension tension, as well as the problems of low monomer transmittance and high red light leakage rate of the polarizing film prepared therefrom, be improved.

Furthermore, 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 the present invention are shown in Table 3 together with the storage modulus of the polyvinyl alcohol films at 45° C. and 55° C.

Table 3 Project Storage modulus at 45℃(MPa) Storage modulus at 55℃(MPa) Red light leakage rate (%) Embodiment 1 9.52 5.02 1.0 Embodiment 2 10.66 5.86 0.9 Embodiment 3 9.97 5.37 1.3 Embodiment 4 12.98 6.88 2.7 Embodiment 5 9.24 5.84 2.2 Embodiment 6 11.55 6.45 3.1 Embodiment 7 12.74 5.84 1.9 Embodiment 8 5.83 3.53 3.4 Embodiment 9 7.84 4.94 4.7 Embodiment 10 8.99 5.39 4.4 Embodiment 11 8.83 4.63 1.2 Comparison Example 1 12.69 5.49 3.5 Comparison Example 2 9.28 7.48 5.7 Comparison Example 3 10.33 5.43 3.9 Comparison Example 4 5.99 4.49 4.4 Comparison Example 5 14.13 6.13 5.5 Comparison Example 6 5.78 4.58 1.7

According to Table 3, when the storage modulus of the polyvinyl alcohol film at 45°C is controlled to be 5.00 to 14.00 MPa, and the storage modulus at 55°C is controlled to be 3.00 to 7.00 MPa, the polarizing film prepared from the polyvinyl alcohol film has a low red light leakage rate of less than 5.0%, such as Examples 1 to 11 and Comparative Examples 1, 3, 4 and 6. On the contrary, when the storage modulus at 45°C and the storage modulus at 55°C of the polyvinyl alcohol film are not controlled to be within the ideal range at the same time, the polarizing film prepared from the polyvinyl alcohol film has a high red light leakage rate, such as Comparative Examples 2 and 5. In view of this, when the storage modulus at 45° C. and the storage modulus at 55° C. of the polyvinyl alcohol film are simultaneously controlled within an ideal range, the problem of high red light leakage rate of the polarizing film prepared from the polyvinyl alcohol film can be improved.

Herein, all ranges provided are intended to include each specific range within the given range and the combination of sub-ranges between the given ranges. In addition, unless otherwise specified, all ranges provided herein include the endpoints of the ranges. Thus, ranges 1 to 5 specifically include 1, 2, 3, 4 and 5, as well as sub-ranges 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 incorporated herein by reference, and each individual publication or patent application is specifically and individually indicated as incorporated herein by reference for any and all purposes. In the event of any inconsistency between this document and any publication or patent application incorporated by reference, this document controls.

1: Immersion stretching fixture 1a: Upper fixed axis 1b: Lower fixed axis 2: PVA film 2a: Upper end 2b: PVA film swollen by weight 2c: Excess PVA film 3: Dovetail clamp 3a: Bottom end 4: Flask a, x: Distance A, B: Angle MD1, MD2: Mechanical direction

In order to make the above and other purposes, features, advantages and embodiments of the present application more clearly understood, the attached drawings are described as follows:

1 to 4 are schematic diagrams showing the absolute values of the slope of the curve of the storage modulus (E') of the polyvinyl alcohol film according to one embodiment of the present invention relative to the temperature.

5, 6A and 6B are schematic diagrams of analyzing the monomer transmittance and red light leakage rate of the polarizing film corresponding to the polyvinyl alcohol film according to an embodiment of the present invention.

FIG. 7 is a schematic diagram of sampling when analyzing the red light leakage rate of the polarizing film corresponding to the polyvinyl alcohol film according to an embodiment of the present invention.

without.

Claims (13)

A polyvinyl alcohol film comprises: a polyvinyl alcohol resin; wherein the polyvinyl alcohol film has a polyvinyl alcohol dissolution amount of 0.20 to 9.00 ppm/ ^m2 when immersed in 40°C water for 1 minute; and when the polyvinyl alcohol film is heated from 30°C to 65°C in water at a heating rate of 1°C/min, a curve of storage modulus (E') relative to temperature is measured, and the curve has a slope value in the range of 45°C to 55°C, and the absolute value of the slope value is 0.20 to 0.70 MPa/°C. The polyvinyl alcohol film as claimed in claim 1, wherein the dissolution amount of the polyvinyl alcohol is 1.60 to 7.20 ppm/m ² ; and the absolute value of the slope value is 0.20 to 0.60 MPa/°C. The polyvinyl alcohol film as described in claim 2, wherein the absolute value of the slope value is 0.25 to 0.60 MPa/°C. The polyvinyl alcohol film as described in claim 3, wherein the absolute value of the slope value is 0.35 to 0.55 MPa/°C. The polyvinyl alcohol film as described in claim 1, wherein the polyvinyl alcohol film has a storage modulus (E') of 5.00 to 14.00 MPa at 45°C; and the polyvinyl alcohol film has a storage modulus (E') of 3.00 to 7.00 MPa at 55°C. The polyvinyl alcohol film as described in claim 2, wherein the polyvinyl alcohol film has a storage modulus (E') of 8.00 to 11.50 MPa at 45°C; and the polyvinyl alcohol film has a storage modulus (E') of 4.00 to 6.00 MPa at 55°C. The polyvinyl alcohol film as claimed in any one of claims 1 to 6 has an extension tension of 13.0 to 30.0 N/mm ² when immersed in a 2 wt. % boric acid aqueous solution at 50° C. for 30 seconds. The polyvinyl alcohol film as described in 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 as described in any one of claims 1 to 6, wherein the polyvinyl alcohol resin has a polymer dispersion index (PDI) less than or equal to 3.5. The polyvinyl alcohol film as described in any one of claims 1 to 6, which has a thickness of 45 to 75 μm. An optical film is made from the polyvinyl alcohol film as described in any one of claims 1 to 10. The optical film as claimed in claim 11 is a polarizing film. An optical film as claimed in claim 12, wherein the polarizing film has a single body transmittance greater than or equal to 42.5% and a red light leakage rate less than 5.0%.