Polyvinyl alcohol film, optical film comprising same, and method for manufacturing same

Abstract

The present invention provides a polyvinyl alcohol film, an optical film comprising the same, and a method for preparing the same.SOLUTION: The polyvinyl alcohol film provided by the present invention has a water vapor transmission rate (Water Vapor Transmission Rate, WVTR) of 1,500-1,900 g / m2 / 24 h, wherein the water vapor transmission rate is measured under the conditions of a temperature of 40 ± 2 °C. and a humidity of 90 ± 5% RH. The polyvinyl alcohol film of the present invention can prevent a white turbidity phenomenon on the surface. In addition, the film can have a more excellent polarization degree and is hardly broken.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol (PVA) film and a method for producing the same, and the polyvinyl alcohol film can be used to form an optical film, particularly a polarizing film, although the present invention is not limited thereto. [Background technology]

[0002] Polyvinyl alcohol (PVA) film is a hydrophilic polymer that has properties such as transparency, mechanical strength, water solubility, and good processability. It is widely used in packaging materials and optical films for electronic products, especially polarizing films.

[0003] Polyvinyl alcohol film is an important component of polarizing plates, and can exhibit polarization properties after dyeing and stretching. Polarizing films obtained through the polarization process can polarize light, making it possible to control brightness and darkness. Therefore, they are now widely used in various LCD screens, displays, eyeglasses, and wearable devices, and have become an essential component in display devices.

[0004] In recent years, with advances in screen technology, demands for the quality of polarizing films have been increasing. At the same time, greater importance has been placed on whether polyvinyl alcohol films are prone to tearing during the polarizing film manufacturing process and whether a high degree of polarization can be achieved. Summary of the Invention [Problem to be solved by the invention]

[0005] However, the present inventors have discovered that polyvinyl alcohol films used in conventional technologies are prone to developing cloudiness on the surface, resulting in poor transparency, and that when conventional polyvinyl alcohol films are used to produce polarizing films, they generally do not simultaneously achieve a good polarization degree and prevent breakage.

[0006] In view of the above-mentioned problems, the main object of the present invention is to provide a polyvinyl alcohol film that can prevent the occurrence of cloudiness on the surface. Furthermore, when a polarizing film is produced from the polyvinyl alcohol film provided by the present invention, a good polarization degree can be obtained while preventing the occurrence of tearing. [Means for solving the problem]

[0007] Specifically, the polyvinyl alcohol film provided as one embodiment of the present invention has a water vapor transmission rate (WVTR) of 1500 to 1900 g / m2 measured under an environment of a temperature of 40±2°C and a humidity of 90±5% RH. 2 / 24hr.

[0008] According to one embodiment of the present invention, the polyvinyl alcohol film has two surfaces, a first surface and a second surface, and a plurality of positions on any surface are measured by Fourier-transform infrared spectroscopy (FTIR) to determine the A 1140 cm -1 / A 1090 cm -1 Calculating the ratio, A at multiple positions 1140 cm -1 / A 1090 cm -1 The difference between the maximum and minimum values ​​of the ratio is less than 0.05, where A 1140 cm -1 / A 1090 cm -1 are 1140cm respectively -1 and 1090cm -1 is the absorption peak intensity at

[0009] According to one embodiment of the present invention, the polyvinyl alcohol film has A at a plurality of positions on the first surface. 1140 cm -1 / A 1090 cm -1 A ratio of the second surface corresponding to the plurality of positions on the first surface 1140 cm -1 / A 1090 cm -1 The difference between the ratios is less than 0.06 in average absolute value.

[0010] According to one embodiment of the present invention, the retardation relaxation rate of the polyvinyl alcohol film is 0.55 to 0.70, and the retardation relaxation rate is expressed by the following formula:

number

[0011] According to one embodiment of the present invention, the polyvinyl alcohol film has a film thickness of 45 to 75 μm.

[0012] Another aspect of the present invention provides an optical film, which is formed from the polyvinyl alcohol film described above.

[0013] According to one embodiment of the present invention, the optical film is a polarizing film.

[0014] According to one embodiment of the present invention, the polarization degree of the optical film is greater than 99.9%.

[0015] Another embodiment of the present invention provides a method for producing the above-mentioned polyvinyl alcohol film, which includes: (a) a dissolving step in which a polyvinyl alcohol resin and additives are added to water at a temperature of 95°C or less, and then the water is heated to 140°C to 150°C to dissolve the resin and additives, thereby forming a polyvinyl alcohol cast solution; (b) a cast drying step in which the polyvinyl alcohol cast solution is cast onto a casting drum and then peeled off from the casting drum to obtain a polyvinyl alcohol preliminary film; and (c) a drying step in which the polyvinyl alcohol preliminary film is dried using a plurality of heating rollers and a dryer having different temperatures, thereby obtaining the polyvinyl alcohol film.

[0016] According to one embodiment of the present invention, in step (a), the water temperature is 85°C to 95°C.

[0017] According to one embodiment of the present invention, in step (b), when the polyvinyl alcohol casting solution is cast onto the casting drum and the solid content of the casting solution reaches more than 85%, the drying time is 60 to 120 seconds.

[0018] According to one embodiment of the present invention, in step (b), after the solid content of the casting solution exceeds 85%, it is dried at a temperature higher than 100° C. for 20-30 seconds.

[0019] According to one embodiment of the present invention, in step (c), the average temperature of the plurality of heating rollers is more than 90°C, and the temperature distribution range in the width direction thereof is 2.0°C or less.

[0020] According to one embodiment of the present invention, in step (c), the dryer has an upper chamber body and a lower chamber body, and the temperature difference between the upper and lower chamber bodies is less than 5.0°C.

[0021] According to one embodiment of the present invention, the temperature difference between the upper and lower chamber bodies is 2.0°C to 4.0°C. [Effects of the Invention]

[0022] The main effect of the present invention is that the polyvinyl alcohol film provided by the present invention can prevent the occurrence of cloudiness. Furthermore, when a polarizing film is produced from the polyvinyl alcohol film of the present invention, a good polarization degree can be obtained and the occurrence of tearing can be prevented. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 2 is a diagram for explaining the measurement of retardation relaxation rate in an example. DETAILED DESCRIPTION OF THE INVENTION

[0024] In order to describe the present invention in more detail and completely, the following description is presented illustrating the embodiments and specific examples of the present invention, but they are not the only forms of specific examples for carrying out or applying the present invention. In this specification and the appended claims, the terms "a," "an," and "the" may be construed as plural unless the context dictates otherwise.

[0025] Although the numerical ranges and parameters defining the present invention are all approximate, the relevant numerical values ​​in the specific examples are shown as precisely as possible. However, any numerical value inherently contains standard deviations resulting from individual testing methods. Here, the term "about" means that the actual numerical value falls within an acceptable standard error of the mean value when considered and determined by a person skilled in the art to which the present invention pertains.

[0026] [Polyvinyl alcohol film] The polyvinyl alcohol film provided by the present invention has a water vapor transmission rate (WVTR) of 1500 to 1900 g / m2 measured under an environment of a temperature of 40±2°C and a humidity of 90±5% RH. 2 Specifically, but not limited to, the following value: 1500 g / m 2 / 24hr, 1550g / m 2 / 24hr, 1600g / m2 / 24hr, 1650g / m 2 / 24hr, 1700g / m 2 / 24hr, 1750g / m 2 / 24hr, 1800g / m 2 / 24hr, 1850g / m 2 / 24hr or 1900g / m 2 / 24hr, or any two values ​​between them. The "water vapor transmission rate" mentioned above is the amount of water vapor passing through a polyvinyl alcohol film per unit area per unit time under specific temperature and humidity conditions, and is a parameter used to evaluate the resistance of a polyvinyl alcohol film to moisture permeation. Without being limited to a particular theory, the water vapor transmission rate is related to the uniformity of dispersion of additives during the polyvinyl alcohol film manufacturing process and affects the occurrence of cloudiness on the polyvinyl alcohol film surface. While controlling the water vapor transmission rate within the specified range can prevent the cloudiness problem, an excessively high water vapor transmission rate causes cloudiness on the polyvinyl alcohol film surface, and an excessively low water vapor transmission rate is believed to result in dehydration of the polyvinyl alcohol and the formation of a double bond structure, possibly due to an excessively high drying temperature during the manufacturing process, which will adversely affect the polarized film manufactured subsequently.

[0027] According to at least one embodiment of the present invention, the polyvinyl alcohol film has two surfaces, a first surface and a second surface, and a plurality of positions on any surface are measured by Fourier-transform infrared spectroscopy (FTIR) to determine the A 1140 cm -1 / A 1090 cm -1 The ratio is calculated by multiplying the A 1140 cm -1 / A 1090 cm -1 The difference between the maximum and minimum values ​​of the ratio is less than 0.05, where A 1140 cm -1 / A 1090 cm -1 are 1140cm respectively-1 and 1090cm -1 Specific examples of the difference value are, but are not limited to, the following values: 0.01, 0.02, 0.03, and 0.04, or values ​​between any two of the above values. 1140 cm -1 / A 1090 cm -1 The ratio is used as a "crystallinity index" that can evaluate the crystallinity of the polyvinyl alcohol film surface, and the difference obtained by subtracting the minimum value from the maximum value among the individual ratios at multiple positions on a single surface is defined as the "crystallinity index range." Without being limited to a specific theory, the crystallinity index range is related to the occurrence of cloudiness on the polyvinyl alcohol film surface, and data shows that when the crystallinity index range is controlled within the above-mentioned specific range, the problem of cloudiness can be prevented.

[0028] According to at least one embodiment of the present invention, the polyvinyl alcohol film has A at a plurality of positions on the first surface. 1140 cm -1 / A 1090 cm -1 ratio and A of the second surface corresponding to the plurality of positions of the first surface 1140 cm -1 / A 1090 cm -1 The difference between the ratio and the average absolute value is less than 0.06. Specific examples of the average value include, but are not limited to, the following values: 0.01, 0.02, 0.03, 0.04, and 0.05, or a value between any two of the above values. In the present invention, the above average value is defined as the "average crystal deviation value of the two surfaces" of the polyvinyl alcohol film. Without being limited to a particular theory, it is believed that the average crystal deviation value of the two surfaces is related to the occurrence of cloudiness on the surface of the polyvinyl alcohol film, and that controlling the average crystal deviation value of the two surfaces within the above-mentioned specific range can prevent the occurrence of the above-mentioned cloudiness. More specifically, if the average crystal deviation value of the two surfaces is excessively high, the polyvinyl alcohol film may be prone to adhesion and cloudiness on the surface.

[0029] According to at least one preferred embodiment of the present invention, the retardation relaxation rate of the polyvinyl alcohol film is 0.55 to 0.70, and the retardation relaxation rate is calculated by the following formula.

number

[0030] According to at least one preferred embodiment of the present invention, the thickness of the polyvinyl alcohol film is 45 to 75 μm, including, but not limited to, the following values: 45 μm, 47 μm, 49 μm, 51 μm, 53 μm, 55 μm, 57 μm, 59 μm, 61 μm, 63 μm, 65 μm, 67 μm, 69 μm, 71 μm, 73 μm, and 75 μm, or a value between any two of the above values.

[0031] [Manufacturing method of polyvinyl alcohol film] Another embodiment of the present invention provides a method for producing the aforementioned polyvinyl alcohol film, which includes: (a) a dissolving step in which a polyvinyl alcohol resin and additives are added to water at a temperature of less than 95°C, and then the water is heated to 140°C to 150°C to dissolve the resin and additives, thereby forming a polyvinyl alcohol cast solution; (b) a cast drying step in which the polyvinyl alcohol cast solution is cast onto a casting drum and then peeled off from the casting drum to obtain a polyvinyl alcohol preliminary film; and (c) a drying step in which the polyvinyl alcohol preliminary film is dried using a plurality of heating rollers and a dryer having different temperatures, thereby obtaining the polyvinyl alcohol film.

[0032] The polyvinyl alcohol resin is obtained by polymerizing a vinyl ester resin monomer to form a polyvinyl ester resin, followed by a saponification reaction. Among these, the vinyl ester resin monomer includes vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate, although the present invention is not limited thereto. Preferably, vinyl acetate is used. Copolymers formed by copolymerizing an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used. Examples of olefin compounds include, but are not limited to, ethylene, propylene, and butylene. Examples of acrylate derivatives include, but are not limited to, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate.

[0033] The additives mentioned above include plasticizers, surfactants or alkyl ether additives, among which the surfactants mentioned above are, but are not limited to, cationic, anionic or nonionic surfactants, and specifically include, but are not limited to, carboxylate-type surfactants such as potassium laurate, sulfate-type surfactants such as sodium laureth sulfate, sulfonate-type surfactants such as dodecylbenzenesulfonate, alkylphenyl ether-type surfactants such as polyoxyethylene octylphenyl ether, alcohol-based phenyl ether-type surfactants such as polyethylene glycol monooctylphenyl ether, alkyl ester-type surfactants such as polyoxyethylene laurate, alkylamine-type surfactants such as polyoxyethylene laurylamine, alkylamide-type surfactants such as polyoxyethylene lauric acid amide, polypropylene glycol ether-type surfactants such as polyoxyethylene polyoxypropylene ether, alkanolamide-type surfactants such as lauric acid diethanolamide and oleyl diethanolamide, and allylphenyl ether-type surfactants such as polyoxyethylene allylphenyl ether. The above-mentioned plasticizers can increase the flexibility of the material or liquefy the material to improve the processability of the film. Usable plasticizers include, but are not limited to, phthalate esters (Phthalates), bis(2-ethylhexyl) phthalate (DEHP), glycerin, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane.

[0034] According to at least one preferred embodiment of the present invention, in the dissolving step (a), the polyvinyl alcohol resin and the additive are added to water (solution) at a temperature of 95°C or lower, and specific examples of the temperature include, but are not limited to, the following values: 85°C, 90°C, and 95°C, or a value between any two of the above values, preferably 85°C to 95°C. Without being limited to a particular theory, the inventors have found through experiments that if the temperature of the water (solution) when the additive is added to it is above 95°C, the polyvinyl alcohol resin likely has already begun to dissolve, increasing the viscosity of the entire solution and making it difficult for the additive to be uniformly dispersed, and the water vapor transmission rate of the polyvinyl alcohol film excessively increases, causing cloudiness on the surface.

[0035] According to at least one preferred embodiment of the present invention, during the dissolution step (a), the temperature (maximum dissolution temperature) at which the temperature is increased and the film is dissolved is controlled to 140°C to 150°C. Specific examples include, but are not limited to, a temperature between any two of the following values: 140°C, 145°C, or 150°C. Without being limited to a particular theory, the inventors have found through experiments that if the maximum dissolution temperature is too low, the additives are not uniformly dispersed, resulting in an excessively high water vapor transmission rate of the produced polyvinyl alcohol film, which can cause cloudiness on the surface. Conversely, if the maximum dissolution temperature is too high, the productivity of the polyvinyl alcohol film is adversely affected. Furthermore, according to at least one preferred embodiment of the present invention, the polyvinyl alcohol cast solution is formed by maintaining the temperature for a certain period of time after the temperature increase, preferably 2 hours.

[0036] The polyvinyl alcohol resin content in the polyvinyl alcohol casting solution is 10 to 60 weight percent. Specific examples include, but are not limited to, the following values: 10 weight percent, 15 weight percent, 20 weight percent, 25 weight percent, 30 weight percent, 35 weight percent, 40 weight percent, 45 weight percent, 50 weight percent, 55 weight percent, and 60 weight percent, or values ​​between any two of the above values, preferably 15 to 40 weight percent, and more preferably 20 to 30 weight percent. Without being limited by any particular theory, an insufficient polyvinyl alcohol resin content can result in an excessively low viscosity of the polyvinyl alcohol casting solution, excessively increasing the drying load and reducing the film formation efficiency in the production of polyvinyl alcohol films. Conversely, an excessively high polyvinyl alcohol resin content can make it difficult to dissolve the entire solution uniformly, resulting in the formation of clusters.

[0037] In accordance with at least one preferred embodiment of the present invention, after the (a) dissolution step, the polyvinyl alcohol casting solution described above is defoamed in a twin-screw extruder and then curtain coated through a T-slit die onto a rotating casting drum for the subsequent (b) cast drying step.

[0038] According to at least one preferred embodiment of the present invention, during the (b) cast drying step, when the casting solution is cast onto a casting drum and the solids content of the casting solution exceeds 85%, the drying time is 60 to 120 seconds. Specifically, the drying time can also be referred to as the "pre-drying time," and it begins when the casting solution is curtain-coated onto the casting drum from a T-shaped slit die and continues through drying on the casting drum until the solids content exceeds 85%. Specific examples of the drying time include, but are not limited to, the following values: 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds, 110 seconds, and 120 seconds, or a value between any two of the above values. Without being limited by theory, if the drying time is too short, the number of nuclei formed in the polyvinyl alcohol film will likely be too low, resulting in an excessively low overall degree of entanglement and an excessively high retardation relaxation rate. It is believed that under these conditions, polarized films produced from polyvinyl alcohol films will have insufficient polarization. If the drying time is too long, not only will it affect the productivity of the polyvinyl alcohol film, but also the number of nuclei formed in the polyvinyl alcohol film will be too large, resulting in an excessively low retardation relaxation rate, which is believed to make the polyvinyl alcohol film prone to tearing when producing a polarizing film from the film.

[0039] According to at least one preferred embodiment of the present invention, during the (b) cast-drying step, after the solids content of the cast solution exceeds 85%, the cast solution is dried at a temperature above 100°C for 20 to 30 seconds. This drying time can also be referred to as the "post-drying time." Specific examples include, but are not limited to, the following values: 20 seconds, 21 seconds, 22 seconds, 23 seconds, 24 seconds, 25 seconds, 26 seconds, 27 seconds, 28 seconds, 29 seconds, and 30 seconds, or any two values ​​between the above values. Without being limited to a particular theory, if the drying time is too short, the crystallinity of the polyvinyl alcohol film will likely be too low, resulting in an excessively low degree of molecular entanglement and an excessively high retardation relaxation rate. It is believed that under these conditions, the polarization degree of a polarized film produced from the polyvinyl alcohol film will be insufficient. If the drying time is too long, not only will the productivity of the polyvinyl alcohol film be affected, but the number of nuclei formed in the polyvinyl alcohol film will be too high, resulting in an excessively low retardation relaxation rate. We believe that in this state, the polyvinyl alcohol film will be more likely to tear when it is used to make polarizing films.

[0040] According to at least one embodiment of the present invention, during the (c) drying step, the polyvinyl alcohol preliminary film is first dried with a heating roller, and then subsequently dried in a dryer. Specifically, the number of heating rollers and the number of sections of the dryer are not particularly limited in the present invention and can be adjusted by the user as needed. Furthermore, according to at least one preferred embodiment of the present invention, the polyvinyl alcohol preliminary film is first dried on both the top and bottom surfaces of the film by contact with multiple heating rollers (for example, but not limited to, 15 heating rollers). Here, the first heating roller has the highest temperature of all the heating rollers, and the temperatures of the subsequent heating rollers are adjusted to gradually decrease, with the last heating roller having the lowest temperature of all the heating rollers.

[0041] According to at least one preferred embodiment of the present invention, during the drying step (c), the average temperature of the multiple heating rollers exceeds 90°C, and the temperature distribution range in the width direction is 2.0°C or less. Specific examples of the range include the following values: 1.0°C, 1.5°C, and 2.0°C, or a range between any two of the above values. Without being limited to a particular theory, if the range is too large, the crystallinity index range of the polyvinyl alcohol film becomes too large, which may cause clouding on the surface.

[0042] According to at least one preferred embodiment of the present invention, the rotation speed of the multiple heating rollers is 3 to 15 m / min, preferably 5 to 10 m / min. If the rotation speed of the heating rollers is too slow, productivity may decrease. Conversely, if the rotation speed of the heating rollers is too fast, the polyvinyl alcohol preliminary film may not be sufficiently dried, resulting in poor peelability.

[0043] According to at least one preferred embodiment of the present invention, during the drying step (c), the dryer has an upper chamber body and a lower chamber body, and the temperature difference between the upper and lower chamber bodies is less than 5.0°C. Specific examples of the temperature difference include, but are not limited to, the following values: 1.0°C, 2.0°C, 3.0°C, 4.0°C, and 5.0°C, or a value between any two of the above values, preferably 2.0°C to 4.0°C. Without being limited to a particular theory, if the temperature difference is too large, the average crystal deviation value on the two sides of the polyvinyl alcohol film may become too large, which may cause cloudiness on the surface.

[0044] According to at least one preferred embodiment of the present invention, the dryer may be, for example, a floating-type dryer, and the temperature may be between 80°C and 115°C, but the temperature may be, but is not limited to, the following values: 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, or 115°C, or a value between any two of the above values.

[0045] The equipment used in the above-described manufacturing method includes a dissolving tank, a filter, a coating machine, and a transport pipe connecting the dissolving tank to the front of the coating machine. These pieces of equipment are preferably covered with a heat-retaining device. The heat-retaining device may be a metal heating element (heating wire) or a jacket filled with a liquid such as oil or water. The heat-retaining device heats the metal wire or the liquid in the jacket to uniformly heat the equipment (especially the surfaces of the equipment and pipes) and maintain a warm state, thereby preventing loss of surface temperature of the equipment or pipes and the formation of gels or clusters in the polyvinyl alcohol in the polyvinyl alcohol casting solution. Furthermore, the above-described heat-retaining temperature should not be excessively high; otherwise, a portion of the polyvinyl alcohol casting solution may dehydrate or gel, forming a golden brown or black gel, which may affect the surface quality and uniformity of the polyvinyl alcohol after coating in a subsequent process. According to at least one preferred embodiment of the present invention, the incubation temperature is 80 to 120°C, including, but not limited to, the following values: 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, and 120°C, or between any two of the above values.

[0046] [Optical film and manufacturing method thereof] Another aspect of the present invention provides an optical film, which is formed from the above-mentioned polyvinyl alcohol film. The optical film may be a polarizing film, a blue light blocking film, a filter lens, or the like, but the present invention is not limited thereto. In one preferred embodiment, the optical film of the present invention is a polarizing film.

[0047] According to at least one preferred embodiment of the present invention, the polarization degree of the polarizing film is greater than 99.9, and specific examples, but not limited to, include the following values: 99.90, 99.91, 99.92, 99.93, 99.94, 99.95, 99.96, 99.97, 99.98, and 99.99, or a value between any two of the above values.

[0048] According to at least one embodiment of the present invention, the optical film manufacturing method of the present application refers to a method for manufacturing a polarizing film, and further refers to manufacturing a polarizing film from a polyvinyl alcohol film. The above-mentioned manufacturing method includes a dyeing step in which iodine is adsorbed, a boric acid treatment step, and a water washing step. A uniaxial stretching step can be performed during the boric acid treatment step or before the boric acid treatment step, and a primary drying step can be performed between the boric acid treatment step and the water washing step. Preferably, a swelling step in which the polyvinyl alcohol film is swelled with water can be performed before the dyeing step. In addition, a final drying step is generally performed after the water washing step.

[0049] According to at least one embodiment of the present invention, during the swelling step, the polyvinyl alcohol film is immersed in a treatment bath (e.g., water) at a temperature of, for example, 30°C to wash the film surface with water and perform a swelling treatment. The swelling treatment time is generally 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, the polyvinyl alcohol film is transported by multiple guide rollers arranged in a swelling tank containing the treatment bath. Subsequently, the polyvinyl alcohol film is stretched in the machine direction (MD) to 1.2 times its original length, and then the dyeing step is performed.

[0050] In the dyeing process, after the swelling process, the polyvinyl alcohol film is immersed in a dyeing tank containing a dye bath. The dyeing conditions can be determined based on the range in which iodine is adsorbed into the polyvinyl alcohol film and in which defects such as excessive dissolution or devitrification of the film do not occur. The dye bath used in the dyeing process may be an aqueous solution containing 0.003 to 0.200 parts by weight of iodine and 0.1 to 10.0 parts by weight of potassium iodide per 100 parts by weight of water. Potassium iodide may be replaced with other iodides such as zinc iodide, or other iodides may be used in combination. The temperature of the dye bath (dyeing temperature) is usually 10 to 50°C, for example, 30°C. The dyeing time (dyeing time) is usually 10 to 600 seconds, preferably 30 to 200 seconds. Subsequently, the polyvinyl alcohol film is stretched in the machine direction to 3.4 times its original length, followed by a boric acid treatment and a stretching process.

[0051] The boric acid treatment and stretching process involves treating a polyvinyl alcohol film dyed with iodine in an aqueous solution containing boric acid to crosslink the film and fix the adsorbed iodine in the resin. This process is typically carried out by immersing the polyvinyl alcohol film after the dyeing process in a fixation tank containing a treatment bath containing boric acid. The boric acid treatment bath may be an aqueous solution containing 0.5 to 15.0 parts by weight of boric acid per 100 parts by weight of water. The boric acid treatment bath preferably contains an iodide in addition to boric acid, in an amount of 5 to 20 parts by weight per 100 parts by weight of water. The iodide used may be potassium iodide or zinc iodide. Compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium hyposulfite, potassium sulfite, sodium sulfite, potassium sulfate, or sodium sulfate, may also be present in the boric acid treatment bath. The boric acid treatment and stretching steps are usually carried out at 50 to 70°C, preferably 53 to 65°C, for example, at 55°C. The treatment time is usually 10 to 600 seconds, preferably 20 to 300 seconds, more preferably 20 to 100 seconds. Subsequently, the polyvinyl alcohol film is stretched in the machine direction to 6 times its original length, and then the subsequent steps are carried out.

[0052] After the polyvinyl alcohol film undergoes the above-mentioned process, followed by water washing and drying, a polarizing film is formed. Furthermore, a protective layer may be formed on at least one side of the polarizing film to produce a polarizing film product (also called a polarizing plate). Specifically, the protective layer is preferably made of a material containing a transparent resin and capable of preventing abrasion and other damage to the surface of the polarizing film. In another embodiment, the protective layer is provided on only one side of the polarizing film, but is preferably formed on both sides of the polarizing film. The protective layer may be a protective film made of a transparent resin material, and 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 polyamideimide resin, a polyimide resin, an epoxy resin, or an oxetane resin, and preferably a cellulose resin such as triacetyl cellulose (TAC). [Example]

[0053] The non-limiting examples of each aspect of the present invention provided below are primarily intended to clarify each aspect of the present invention and the advantages achieved thereby.

[0054] Non-limiting methods for producing polyvinyl alcohol films are provided below. Based on the methods disclosed below or similar methods, 15 types of polyvinyl alcohol films of non-limiting examples (Examples 1 to 15) and 7 types of polyvinyl alcohol films of comparative examples (Comparative Examples 1 to 7) were produced. However, the specific methods for producing Examples 1 to 15 and Comparative Examples 1 to 7 differ from the methods disclosed below in one or more aspects.

[0055] 1. Production of polyvinyl alcohol film The following are the main steps common to the production of polyvinyl alcohol films in this example and the comparative example, and Table 1 below details the differences in one or more process parameters between this example and the comparative example.

[0056] (a) Melting process: 1,800 kg of polyvinyl alcohol resin, 200 kg of plasticizer glycerin, 4,000 kg of water, 500 ppm (by weight relative to the polyvinyl alcohol resin) of surfactant lauric acid diethanolamide, and 500 ppm (by weight relative to the polyvinyl alcohol resin) of alkyl ether additive were added to water having a water temperature (details shown in Table 1) and stirred uniformly.The temperature was then raised at a rate of 10°C / hour to the maximum dissolution temperature (details shown in Table 1), and the temperature was maintained for 2 hours to obtain a uniform polyvinyl alcohol cast solution (film-forming stock solution).

[0057] Casting and drying process: The polyvinyl alcohol casting solution was defoamed using a twin-screw extruder, then extruded through a T-shaped slit die and curtain-coated onto a rotating, high-temperature casting drum to form a dry film. After the solution was curtain-coated onto the rotating casting drum from the T-shaped slit die, a pre-drying period (details shown in Table 2) was allowed to pass through the casting drum until the solids content exceeded 85%. Furthermore, after the solids content of the casting solution exceeded 85%, post-drying was carried out at a temperature higher than 100°C (details shown in Table 2) to obtain a polyvinyl alcohol preliminary film.

[0058] (c) Drying process: After peeling the polyvinyl alcohol preliminary film from the casting drum, it was brought into contact with ten heated rollers to dry both the top and bottom surfaces of the film. The first heated roller was set to the highest temperature of all the heated rollers, and the temperatures of the subsequent heated rollers were adjusted to gradually decrease. The average temperature of the ten heated rollers was over 90°C, and they had a specific temperature distribution range across their width (details shown in Table 1). The polyvinyl alcohol preliminary film was then advanced to a floating-type dryer for heat treatment. The dryer had an upper chamber and a lower chamber, and the upper and lower chambers had a specific temperature difference between them (details shown in Table 1). Finally, a thin polyvinyl alcohol film product was obtained.

[0059] 2.Analysis method Here, the analysis and testing methods for the polyvinyl alcohol films of the above-mentioned Examples 1 to 15 and Comparative Examples 1 to 7 of the present invention are provided.

[0060] [Water vapor permeability] The method for measuring the water vapor transmission rate of the present invention is to measure the moisture permeability of a polyvinyl alcohol film based on the calcium chloride method of JIS L1099 A-1 (cup method), and the specific analytical method is as follows. 1. Set the temperature and humidity chamber to 40±2℃ and 90±5%RH to keep the equipment stable. 2. Dry the moisture absorbent (CaCl2) in a dryer at 120°C for 6 hours in advance. 3. Quickly place the moisture absorbent into the moisture permeable cup, ensuring that the volume is approximately 3 mm from the mouth of the cup. 4. Quickly cover the polyvinyl alcohol film test piece onto the moisture permeability cup, then attach the gasket and ring in that order and secure with a nut. 5. The moisture permeability cup containing the polyvinyl alcohol film test piece is quickly placed in a thermo-hygrostat set at a temperature of 40±2°C and a humidity of 90±5%RH, and after one hour is removed and weighed to determine its weight W0 (unit: g). 6. Next, place it in a constant temperature and humidity chamber, and take it out after 2 hours, 4 hours, and 6 hours, and measure its weight W2, W4, and W6 (unit: g) respectively. 7. Water vapor permeability is calculated as (W6 - W2) / 0.002827 (unit: g / m 2 / 24hr).

[0061] [FT-IR crystal index] During FT-IR analysis, 1140cm -1 The OCC absorption peak in the vicinity is related to the crystals in the polyvinyl alcohol film and belongs to the signal specific to the crystals. The specific analysis method is as follows. 1. Crystal index range / average crystal index of the first surface: Three positions are arbitrarily selected from any surface of a circular polyvinyl alcohol film test piece with a radius of 3 cm, and the three 1140 cm -1 The absorption peak intensity (i.e., A 1140 cm -1 ) and 1090cm -1 The absorption peak intensity (i.e., A 1090 cm -1 ) and analyze and measure the A at each of the three positions. 1140 cm-1 / A 1090 cm -1 The ratio (crystal index) is obtained. The minimum value of the crystal indexes at the three positions is subtracted from the maximum value to obtain the crystal index range of the first surface. At the same time, the average value of the crystal indexes at the three positions is calculated to obtain the average crystal index value of the first surface. 2. Average crystal deviation value of two surfaces: Three positions are arbitrarily selected from a circular polyvinyl alcohol film test piece with a radius of 3 cm, and the individual crystal indices of the two surfaces (a total of three pairs of crystal indices) are measured and calculated at the same (corresponding) positions on both surfaces. The difference values ​​of the individual crystal indices at the three positions on both surfaces are then calculated, and the average of these absolute values ​​is calculated to obtain the average crystal deviation value of the two surfaces.

[0062] [Phase difference relaxation rate] The retardation relaxation rate is measured after swelling for 2 minutes in water at 30° C. The specific method is as follows. 1.Analysis equipment: Photonic Lattice, PA series birefringence measurement system 2. Measurement range: 8cm in machine direction (MD) * 8cm in cross direction 3. Sampling position: See Figure 1. Three test pieces, each 10 cm long in the machine direction (a) and 10 cm long in the width direction (b), are sampled and analyzed within a length L (40 cm) from the two ends of the polyvinyl alcohol film inward along the width direction. The present invention does not particularly limit the sampling position. 4. Before swelling, the retardation of each test piece was analyzed over an area of ​​approximately 8 cm in the machine direction and 8 cm in the width direction, excluding the edges. The average retardation of the three test pieces was calculated to obtain the retardation of the polyvinyl alcohol film before swelling. 5. The three test pieces are immersed in water at 30°C, swollen for 2 minutes, and then removed. The retardation analysis is performed within an area of ​​8 cm in the machine direction and 8 cm in the width direction. The average retardation of the three test pieces after swelling is calculated to obtain the retardation of the polyvinyl alcohol film swollen in water for 2 minutes. It is not necessary to wipe off the water from the test piece surface during the measurement. 6. Calculation of retardation rate:

number

[0063] 3. Evaluation method and results Here, the evaluation methods for the polyvinyl alcohol films and polarizing films of the examples and comparative examples of the present invention, as well as the results corresponding to the above-mentioned analysis contents, are provided.

[0064] [Evaluation of cloudiness] The light transmittance of the polyvinyl alcohol film was analyzed using a spectrophotometer (analytical instrument) to evaluate the uniformity of its transparency. The specific analysis method is as follows. 1. Calibration of analytical equipment: The spectrophotometer is automatically calibrated before starting, so you need to insert two pieces of film into the equipment first. 2. Preparation of sample: Divide the polyvinyl alcohol film into 5 equal parts across the width and cut the test pieces in the center. Each piece should be 5cm x 5cm (machine direction x width direction) in area, for a total of 5 pieces. 3. Analysis method: The light transmittance T% of the five test pieces was analyzed at a wavelength of 550 nm, and the average was calculated to obtain the average light transmittance. The light transmittance range was obtained by subtracting the minimum value from the maximum value. 4. Evaluation criteria: If the range is less than 0.5 and the average light transmittance is greater than 90.0%, the evaluation is ⊚. If the range is 0.5 to 0.7, the evaluation is ◯. If the range is greater than 0.7, the evaluation is ×.

[0065] [Evaluation of polarization degree] 1.Analytical equipment: PerkinElmer, Lambda 365 2. Preparation of Samples: Corresponding polarizing plate samples were prepared from the polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 7 according to the above-described polarizing film production process. 3. Analysis method: Based on the standard method of JIS Z 8722:2009, a luminosity correction is performed in the visible light region at 2° using a C light source. Then, two polarized films are stacked with the same orientation direction, and the light transmittance (H0) under the wavelength is measured. Separately, two polarized films are stacked with the orientation direction perpendicular to each other, and the light transmittance (H 90 ) is measured. 4. Data processing: Degree of polarization = [(H0 - H 90 ) / (H0+H 90 )] 1 / 2

[0066] [Rating of tear] 1. Test conditions: During the polarizing film manufacturing process, 10,000m of polyvinyl alcohol film was continuously used to make polarizing film, and the presence or absence of tears during the manufacturing process was observed. 2. Evaluation criteria: If there are no tears, the evaluation is O. If there are one or more tears, the evaluation is X.

[0067] Furthermore, the above-mentioned analysis items and evaluation results for the polyvinyl alcohol films and polarizing films of Examples 1 to 15 of the present invention and Comparative Examples 1 to 7 are summarized in Tables 1 and 2.

[0068] [Table 1]

[0069] [Table 2]

[0070] According to Table 1, the water vapor transmission rate, the crystal index range of the first surface (any surface), and the average crystal deviation of the two surfaces of the polyvinyl alcohol films of Examples 1 to 15 were all controlled within specific ranges, and all of these films exhibited excellent performance in the evaluation of cloudiness. In other words, all of these Examples prevented cloudiness on the surface of the polyvinyl alcohol film. Specifically, the present invention adjusts and controls the following process conditions: the water temperature when adding additives to the polyvinyl alcohol film, the maximum dissolution temperature, the temperature distribution range in the width direction of the heating roller (average temperature exceeding 90°C), and the temperature difference between the upper and lower chambers of the dryer. By controlling the above process conditions within specific ranges, the water vapor transmission rate, the crystal index range of the first surface (any surface), and the average crystal deviation of the two surfaces of the polyvinyl alcohol film are all controlled within specific ranges, thereby achieving the goal of preventing cloudiness in the polyvinyl alcohol film.

[0071] Please refer to Table 1. For the polyvinyl alcohol films of Comparative Examples 1 to 7, at least one of the water vapor transmission rate, the crystallinity index range of the first side (any side), and the average crystallinity deviation value of the two sides was within the specified range, and therefore, the performance of each film was either fair or poor in the evaluation of cloudiness. In other words, these comparative examples were unable to reliably prevent cloudiness on the polyvinyl alcohol film surface. More specifically, in each of these comparative examples, at least one of the process conditions, such as the water temperature when adding additives to the polyvinyl alcohol film, the maximum dissolution temperature, the temperature distribution range in the width direction of the heating roller (average temperature exceeding 90°C), and the temperature difference between the upper and lower chambers of the dryer, was not well controlled within the specified range. Therefore, at least one of the water vapor transmission rate, the crystallinity index range of the first side (any side), and the average crystallinity deviation value of the two sides was not within the specified range.

[0072] As can be seen from a closer look at Table 1, if the water temperature when the additives were added increased, the water vapor permeability increased (Examples 2 and 3). If the water temperature was too high (above 95°C), the water vapor permeability exceeded the specified range (Comparative Examples 1 to 7), resulting in either passable or unsatisfactory performance in the evaluation of opacity of the polyvinyl alcohol film. If the maximum dissolution temperature decreased, the water vapor permeability increased (Examples 4 and 5). If the dissolution temperature was too low (below 140°C), the water vapor permeability exceeded the specified range (Comparative Examples 3 to 7), resulting in opacity on the surface of the polyvinyl alcohol film. If the temperature distribution range across the width of the heating roller increased, the crystallinity index range increased (Examples 10 and 11). If the temperature distribution range was too large (above 2°C), the crystallinity index range exceeded the specified range (Comparative Examples 2 to 7), resulting in opacity on the surface of the polyvinyl alcohol film. When the temperature difference between the upper and lower chambers of the dryer becomes large, the average crystallinity deviation increases (Examples 10 and 11), and when the temperature distribution range is too large (greater than 2°C), the crystallinity index range exceeds the specific range mentioned above (Comparative Examples 2 to 7), causing cloudiness on the surface of the polyvinyl alcohol film.

[0073] According to Table 2, in Examples 1 to 15, the retardation relaxation rate of the polyvinyl alcohol film was controlled within a specific range, and therefore the polyvinyl alcohol film had an excellent polarization degree and also achieved good performance in the tear evaluation. More specifically, the present invention adjusts and controls the pre-drying time and post-drying time during the process, and controls these process conditions within specific ranges, thereby achieving the objective of controlling the retardation relaxation rate of the polyvinyl alcohol film within a specific range, thereby achieving the objective of having an excellent polarization degree and preventing tearing when producing a polarizing film from the polyvinyl alcohol film.

[0074] Please refer to Table 2. In Comparative Examples 1 to 7, the retardation relaxation rates of the polyvinyl alcohol films were all outside the specific range, and therefore none of these polyvinyl alcohol films simultaneously exhibited excellent polarization and tear resistance. Specifically, in these Comparative Examples, at least one of the process conditions, i.e., the pre-drying time and post-drying time, was not well controlled within the above-mentioned specific range, and therefore the retardation relaxation rate was not within the specific range. More specifically, when the retardation relaxation rate was too high (Comparative Examples 1 to 3), the polarization degree was poor even if the film showed good performance in terms of tear resistance. When the retardation relaxation rate was too low (Comparative Examples 4 to 7), the film showed poor performance in terms of tear resistance, even if it had a good polarization degree.

[0075] As can be seen from a closer look at Table 2, shortening the pre-drying time increases the retardation relaxation rate (Examples 6 and 7), whereas too short a pre-drying time results in an excessively high retardation relaxation rate, exceeding the specific range (Comparative Examples 1 to 3). Furthermore, too long a pre-drying time results in an excessively low retardation relaxation rate (Comparative Examples 4 to 7). Shortening the post-drying time increases the retardation relaxation rate (Examples 8 and 9), whereas too short a post-drying time results in an excessively high retardation relaxation rate, exceeding the specific range (Comparative Examples 2 to 3). In Comparative Example 4, the post-drying time was too short, but the pre-drying time was too long. Due to the combined effects of these factors, the retardation relaxation rate of the polyvinyl alcohol film in this example was excessively low. Furthermore, too long a post-drying time results in an excessively low retardation relaxation rate (Comparative Examples 4 to 7).

[0076] In summary, the polyvinyl alcohol film of the present invention can prevent the occurrence of cloudiness. Furthermore, when a polarizing film is produced from the polyvinyl alcohol film of the present invention, a good polarization degree can be obtained and tearing can be prevented.

[0077] All ranges provided herein are intended to include each specific range within the assigned range and any combination of subranges between the assigned ranges. Also, unless otherwise specified, all ranges provided herein include the endpoints of the range. Thus, the range 1-5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2-5, 3-5, 2-3, 2-4, and 1-4.

[0078] All publications and patent applications referenced in this specification are hereby incorporated by reference and are specifically and individually indicated to be incorporated by reference into each and every publication or patent application for all purposes. In the event of a conflict between this specification and any publication or patent application incorporated by reference herein, the specification controls.

[0079] Except in the operating examples or where otherwise indicated, all numbers indicating quantities of ingredients and / or reaction conditions can in all instances be modified by the term "about" to mean within ±5% of the indicated number. As used herein, the terms "essentially free" or "substantially free" mean less than about 2% of a particular characteristic. Any element or characteristic explicitly recited in this specification can be negatively excluded from the claims.

[0080] Although the present invention has been described in detail above, the above description is merely a preferred embodiment of the present invention and does not limit the scope of the present invention. Any equivalent changes or modifications based on the scope of the claims of the present invention are also within the scope of the claims of the present invention. [Explanation of symbols]

[0081] a Length along the machine direction b Length along the width direction L length

Claims

1. The water vapor permeability measured under an environment of 40±2°C temperature and 90±5% RH is 1500 to 1900 g / m ２ / 24hr.

2. The polyvinyl alcohol film has two surfaces, a first surface and a second surface, and a plurality of positions on any surface are measured by Fourier transform infrared spectroscopy to determine the A １１４０ cm －１ / A １０９０ cm －１ Calculating the ratio, A １１４０ cm －１ / A １０９０ cm －１ The difference between the maximum and minimum values ​​of the ratio is less than 0.05, and １１４０ cm －１ / A １０９０ cm －１ are 1140 cm －１ and 1090 cm －１ 2. The polyvinyl alcohol film according to claim 1, wherein the absorption peak intensity is

3. The polyvinyl alcohol film has A at a plurality of positions on the first surface. １１４０ cm －１ / A １０９０ cm －１ ratio and A of the second surface corresponding to the plurality of positions of the first surface １１４０ cm －１ / A １０９０ cm －１ The polyvinyl alcohol film according to claim 2, wherein the difference between the ratios is less than 0.06, and the average of the absolute values ​​is less than 0.

06.

4. The retardation relaxation rate of the polyvinyl alcohol film is 0.55 to 0.70, and the retardation relaxation rate is expressed by the following formula: [Equation 1] The polyvinyl alcohol film according to any one of claims 1 to 3, calculated by:

5. The polyvinyl alcohol film according to claim 4, wherein the thickness of the film is 45 to 75 μm.

6. An optical film formed from the polyvinyl alcohol film according to any one of claims 1 to 6.

7. The optical film according to claim 6 , which is a polarizing film.

8. The optical film of claim 7 , wherein the polarization degree of the polarizing film is greater than 99.9%.

9. A method for producing the polyvinyl alcohol film according to any one of claims 1 to 5, (a) a dissolving step of adding a polyvinyl alcohol resin and an additive to water at a temperature of 95°C or less, and then heating the water to 140°C to 150°C to dissolve the polyvinyl alcohol resin and the additive to form a polyvinyl alcohol cast solution; (b) a cast drying step of casting the polyvinyl alcohol cast solution onto a casting drum and peeling it off from the casting drum to obtain a preliminary polyvinyl alcohol film; (c) a drying step, in which the polyvinyl alcohol preliminary film is dried in a plurality of heating rollers having different temperatures and a dryer, and then the polyvinyl alcohol film is obtained; A manufacturing method comprising:

10. The method according to claim 9, wherein the water temperature during step (a) is 85°C to 95°C.

11. 11. The method according to claim 10, wherein in step (b), when the polyvinyl alcohol casting solution is cast onto the casting drum and the solids content of the casting solution reaches more than 85%, the drying time is 60 to 120 seconds.

12. The method according to claim 11, wherein in step (b), after the solid content of the casting solution exceeds 85%, the casting solution is dried at a temperature higher than 100°C for 20 to 30 seconds.

13. The manufacturing method according to claim 12, wherein in the step (c), the average temperature of the plurality of heating rollers is higher than 90°C, and the temperature distribution range in the width direction is 2.0°C or less.

14. The manufacturing method according to any one of claims 9 to 13, wherein in step (c), the dryer has an upper chamber body and a lower chamber body, and the temperature difference between the upper and lower chamber bodies is less than 5.0°C.

15. The manufacturing method according to claim 14, wherein the temperature difference between the upper and lower chamber bodies is 2.0°C to 4.0°C.

Images