Polyvinyl alcohol film, optical film containing the same, and method for manufacturing the same

Abstract

This invention provides a polyvinyl alcohol film that is less prone to elution and has uniform transparency. [Solution] The polyvinyl alcohol film of the present invention has a weight swelling value of 2.4 ln (minutes of immersion time) + 39 during a 3-minute water immersion time.

Description

[Technical Field]

[0001] The present invention relates to polyvinyl alcohol (PVA) films, and more particularly to polyvinyl alcohol films that are less prone to elution and have uniform transparency, optical films containing the same, and methods for producing the same, but the present invention is not limited to these. [Background technology]

[0002] Polyvinyl alcohol (PVA) film is a type of hydrophilic polymer obtained by coating and drying an aqueous solution containing polyvinyl alcohol polymer and a plasticizer. Due to its properties such as transparency, mechanical strength, water solubility, and good processability, it is widely used in packaging materials and optical films for electronic products, including polarizing films.

[0003] Polarized films, obtained by processing polyvinyl alcohol films in a polarization process, have the property of allowing only light rays from a specific direction to pass through, thereby controlling the brightness of the passing light rays. Based on this property, polarized films are used in various displays, eyeglasses, and wearable devices. Good polarized films have properties such as uniform color, minimal color unevenness, and no wrinkles, and can provide excellent optical properties. In order to improve the optical properties of polarized films, conventional techniques have improved the performance of the film by changing the viscosity and degree of saponification by altering the structure of polyvinyl alcohol or by adding functional groups (e.g., cationic groups). [Overview of the project] [Problems that the invention aims to solve]

[0004] However, polyvinyl alcohol films sometimes have the problem of high polyvinyl alcohol leaching during the optical film manufacturing process, which can lead to contamination or foreign matter in subsequent finished products, resulting in them being judged as defective. In addition, if the degree of confluence of polyvinyl alcohol molecules is not uniform in different areas of the film, there is a risk that the transmittance of subsequent optical films will be uneven. [Means for solving the problem]

[0005] Regarding the above-mentioned problems, although not limited to any particular theory, the inventors have discovered that the problem of high polyvinyl alcohol leaching can be improved by controlling the numerical value of the weight swelling of a polyvinyl alcohol film within a specific water immersion time (i.e., swelling time) to a specific range. Furthermore, the inventors have also discovered that the problem of the optical film having non-uniform transmittance can be improved by controlling the standard deviation of the weight swelling of a polyvinyl alcohol film within a specific water immersion time to a specific range.

[0006] Therefore, the present invention provides a polyvinyl alcohol film in which the weight swelling value during a 3-minute water immersion time satisfies the formula 2.4ln(minutes of immersion time) + 39 < weight swelling value < 4.3ln(minutes of immersion time) + 44, and furthermore, the standard deviation of the weight swelling value during a 3-minute water immersion time measured at multiple locations is ≤ 1.70.

[0007] According to one embodiment of the present invention, the polyvinyl alcohol film of the present invention has two opposing surfaces, the X surface and the Y surface, and the difference in water content between the two surfaces is ≤0.015. Here, the difference in water content between the two surfaces is measured at multiple locations on the polyvinyl alcohol film using FTIR-ATR and is given by the following formula: |X surface (A 1654cm -1 / A 1090cm -1 ) Average value - Y plane (A1654cm -1 / A 1090cm -1 )It is calculated by the average value.

[0008] According to one embodiment of the present invention, when the water immersion time of the polyvinyl alcohol film is 30 seconds, the weight swelling degree is 37.3 wt% to 41.0 wt%.

[0009] According to one embodiment of the present invention, when the water immersion time of the polyvinyl alcohol film is 1 minute, the weight swelling degree is 39.0 wt% to 44.0 wt%.

[0010] According to one embodiment of the present invention, when the water immersion time of the polyvinyl alcohol film is 2 minutes, the weight swelling degree is 40.7 wt% to 47.0 wt%.

[0011] According to one embodiment of the present invention, when the water immersion time of the polyvinyl alcohol film is 3 minutes, the weight swelling degree is 41.6 wt% to 48.7 wt%.

[0012] According to one embodiment of the present invention, the thickness of the polyvinyl alcohol film is 45 to 75 μm.

[0013] According to one embodiment of the present invention, when a polyvinyl alcohol film with an area of 25 cm 2 is impregnated in 80 mL of water at 50 °C for 1 minute to obtain an impregnated solution, the elution amount of polyvinyl alcohol in 20 mL of the impregnated solution is ≤ 30.0 ppm.

[0014] According to one embodiment of the present invention, the elution amount of polyvinyl alcohol in 20 mL of the impregnated solution is ≤ 15.0 ppm.

[0015] According to one embodiment of the present invention, for the polyvinyl alcohol film, when the area is 10 cm in the machine direction (MD) × 5 cm in the transverse direction (TD), and it is sandwiched at the central part in the transverse direction at both ends and immersed in water for 60 seconds, the folding angle at the end in the machine direction is less than 180 degrees.

[0016] According to one embodiment of the present invention, the above-mentioned folding angle in the machine direction is less than 90 degrees.

[0017] Another object of the present invention is to provide an optical film, which is formed of the above-mentioned polyvinyl alcohol film.

[0018] Another object of the present invention is to provide a polarizing film, which is formed of the above-mentioned polyvinyl alcohol film.

[0019] According to one embodiment of the present invention, at multiple positions of the above-mentioned polarizing film, when measuring the light transmittance T1% when tilted +45 degrees with respect to the stretching direction of the uniaxial stretching and the light transmittance T2% when tilted -45 degrees with respect to the stretching direction of the uniaxial stretching, the maximum value - minimum value among the average values of T1% and T2% at each position is ≤ 0.50%.

[0020] According to one embodiment of the present invention, the above-mentioned maximum value - minimum value of the average value is ≤ 0.10%.

[0021] Yet another object of the present invention is to provide a method for manufacturing the above-mentioned polyvinyl alcohol film, (a) A dissolving step of adding a polyvinyl alcohol-based resin to a solvent and a plasticizer, then heating to 110°C to 150°C to dissolve and form a polyvinyl alcohol casting solution, where the viscosity of the polyvinyl alcohol casting solution when the solid content is 4% is 60 to 120 cps, (b) A casting step in which the polyvinyl alcohol casting solution is cast onto a casting drum, the casting drum is surrounded by a hot air dryer with a maximum temperature of 80-140°C, and the solution is peeled off the casting drum to obtain a polyvinyl alcohol preformed film. (c) A drying step of obtaining a polyvinyl alcohol film by drying the polyvinyl alcohol preformed film in a heating roller with multiple temperature differences and in a floating dryer.

[0022] According to one embodiment of the present invention, the total temperature of the heating rollers having multiple temperature differences is 700 to 1050°C.

[0023] According to one embodiment of the present invention, the temperature of each heating roller is higher than 40°C.

[0024] According to one embodiment of the present invention, the maximum temperature of the floating dryer is 105 to 130°C.

[0025] According to one embodiment of the present invention, the difference in airflow velocity between the upper and lower sides of the floating dryer is ≤1.0 m / s, and the temperature difference between the upper and lower sides of the floating dryer is ≤5°C.

[0026] According to one embodiment of the present invention, the sum of the east-west temperature difference between the heating roller and the floating dryer is ≤10°C. [Effects of the Invention]

[0027] The features of the polyvinyl alcohol film of the present invention are as follows: Not only can the problem of high polyvinyl alcohol elution be avoided, but the problem of uneven transmittance in the subsequent manufacturing of optical films can also be improved. [Brief explanation of the drawing]

[0028] [Figure 1]This shows the results of the weight swelling of a polyvinyl alcohol film at different times based on one embodiment of the present invention. [Figure 2] This is a conceptual diagram of the casting and drying processes for a polyvinyl alcohol film based on one embodiment of the present invention. [Figure 3] This is a conceptual diagram of measuring edge folds in a polyvinyl alcohol film based on one embodiment of the present invention. [Modes for carrying out the invention]

[0029] The following describes embodiments of the technology of the present invention, with reference to the accompanying drawings, only as examples. It should be understood that the embodiments of the present invention are not limited to the arrangements, means, and characteristics shown in the accompanying drawings.

[0030] The proportions of various features and components in the diagrams are not actual proportions, but rather represent the specific features and components related to the present invention in the most optimal way, drawn using a conventional drawing method based on established procedures. Furthermore, in separate diagrams, identical or similar component symbols refer to similar components or members.

[0031] The following embodiments are not intended to unduly limit the present invention. Those skilled in the art in which the present invention pertains can modify or change the embodiments discussed herein without departing from the spirit or scope of the invention, and all such modifications will fall within the scope of the present invention.

[0032] In this specification, unless otherwise stated in the context, the terms “includes,” “contains,” “have,” or “contains” are inclusive or open and do not exclude other components or methods / processes not mentioned. The terms “one” and “the” can be interpreted as singular or plural. The term “one or more” means “at least one” and therefore may include a single characteristic or mixture / combination. Also, in this specification and the attached utility model claims, unless otherwise stated, “placed on an object” can be considered as being attached to or otherwise in contact with the surface of an object, directly or indirectly, and the definition of the surface shall be determined by the implications of the preceding / following paragraphs of this specification and ordinary knowledge in the art to which this specification belongs.

[0033] Polyvinyl alcohol film

[0034] In one aspect of the present invention, a polyvinyl alcohol film is provided, wherein the weight swelling value during a 3-minute water immersion time satisfies the formula 2.4ln(minutes of immersion time) + 39 < weight swelling value < 4.3ln(minutes of immersion time) + 44, and furthermore, the standard deviation of the weight swelling value during a 3-minute water immersion time measured at multiple locations is ≤ 1.70.

[0035] As used herein, "weight swelling degree" refers to the degree to which water molecules can penetrate into a polyvinyl alcohol film. The general procedure for measuring the "weight swelling degree" is as follows: a sample of the polyvinyl alcohol film is taken in a non-moisture-absorbing state, then placed in water to swell (also called water immersion), the polyvinyl alcohol film is removed, the surface moisture is wiped off, and the weight of the film is weighed is taken as M1. The polyvinyl alcohol film is then dried in a dryer, and the weight of the film is weighed as M2. The weight swelling degree (wt%) is then obtained by calculating (M1-M2) / M1.

[0036] Furthermore, the "numerical value of weight swelling within a 3-minute immersion time" as described herein refers to the numerical value of weight swelling obtained within the 3-minute immersion time (i.e., swelling time) set during the above-described process. The so-called "numerical value" refers to the numerical portion of the weight swelling, as used herein. For example, if the weight swelling is 35 wt%, the numerical value of the weight swelling is 35. As shown in Figure 1, the inventors discovered that the weight swelling exhibits a logarithmic change with respect to the swelling time. Furthermore, with changes in swelling time, the numerical value of the weight swelling has a specific range, with a lower limit of 2.4ln(A)+39 and an upper limit of 4.3ln(A)+44. Here, A is the fraction of the immersion time. Those skilled in the art will naturally understand that the fraction of the immersion time refers to the numerical portion of the immersion time. For example, if the immersion time is 3 minutes, the fraction of the immersion time is 3. While not limited to any particular theory, the inventors have also found that if the values ​​of weight swelling during a 3-minute water immersion time are all controlled within the specified range, the problem of high polyvinyl alcohol leaching can be effectively mitigated.

[0037] Furthermore, the "standard deviation of weight swelling within a 3-minute water immersion time" as described herein is obtained by calculating the weight swelling at different positions (areas) in the width direction of the polyvinyl alcohol film at the same time point, based on the difference in drying history in the width direction of the film. More specifically, the present invention adopts the maximum value of the standard deviation within a specific time. Therefore, the "multiple positions" described herein means multiple different positions obtained in the width direction of the polyvinyl alcohol film. However, the present invention does not limit the size of each of these positions or the spacing between them. Also, although not limited to a specific theory, the inventors have shown that if the standard deviation of weight swelling within a 3-minute water immersion time is excessively large, the degree of confounding of polyvinyl alcohol molecules at different positions will differ, and have discovered that this can lead to the problem of uneven transmittance in different areas during the manufacture of polarizing films. In other words, if the standard deviation of weight swelling within a 3-minute water immersion time of a polyvinyl alcohol film is controlled within a specific range, the problem of uneven transmittance in polarizing films can be improved. Specifically, the aforementioned range is ≤1.70, preferably 0.65 to 1.70, and includes, but is not limited to, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, or 1.70.

[0038] According to at least one embodiment of the present invention, the polyvinyl alcohol film has two opposing surfaces, the X surface and the Y surface, and the difference in moisture content between the two surfaces is ≤0.015. Here, the difference in moisture content between the two surfaces is measured at multiple locations on the polyvinyl alcohol film using FTIR-ATR and is given by the following formula: |X surface (A 1654cm -1 / A 1090cm -1 ) Average value - Y plane (A 1654cm -1 / A 1090cm -1 This is calculated using the average value.

[0039] Specifically, the "two opposing surfaces" as described herein refer to the opposing front and back surfaces of the film (which may also be the top and bottom surfaces or front and back surfaces; in this specification, they are merely illustrative examples referred to as the X and Y surfaces). Furthermore, the "difference in water content between the two surfaces" as described in this invention refers to taking multiple locations on each of the X and Y surfaces of a polyvinyl alcohol film sample and measuring the water content at each of these locations using FTIR-ATR (A 1654cm -1 / A 1090cm -1 ), here, A 1654cm -1 This is the peak value of the deformation vibration of water, and A 1090cm -1 (CO) is the peak value of the stretching vibration, and next, the average value of the moisture content at the multiple locations is given by the following formula: | X plane (A 1654cm -1 / A 1090cm -1 ) Average value - Y plane (A 1654cm -1 / A 1090cm -1)By calculating using the average value, the difference in moisture content between the two surfaces mentioned above can be obtained. The inventors have shown that when the difference in moisture content between the two surfaces is excessively large, the swelling rates of the two surfaces differ when water molecules subsequently enter the polyvinyl alcohol film. In this case, they discovered that the surface with lower moisture content may curl towards the surface with higher moisture content, causing folds in the film surface. Furthermore, when manufacturing a polarizing film, if both ends of the film curl significantly due to immersion in water, folds may occur at the ends, resulting in creases. If the creases are severe, there is a risk of tearing of the film. Therefore, although not limited to a specific theory, if the difference in moisture content between the two surfaces is controlled within a specific range, the folds at the ends of the polyvinyl alcohol film can be improved. Specifically, the aforementioned range is ≤0.015, preferably 0.002 to 0.015, for example, 0.002, 0.004, 0.006, 0.008, 0.010, 0.012, 0.014, or 0.015, but is not limited to these.

[0040] According to at least one embodiment of the present invention, the thickness of the polyvinyl alcohol film is 45 to 75 μm, for example, 45, 47, 50, 52, 55, 57, 60, 62, 65, 68, 70, 71, 72, 73, 74, or 75 μm, but is not limited to these.

[0041] According to at least one embodiment of the present invention, the weight swelling of the polyvinyl alcohol film when immersed in water for 30 seconds is 37.3 wt% to 41.0 wt%, for example, 37.3, 37.4, 37.5, 37.6, 37.7, 37.8, 37.9, 38.0, 38.1, 38.2, 38.3, 38.4, 38.5, 38.6, 38.7, 38.8, 38.9, 39.0, 39.1, 39.2, 39.3, 39.4, 39.5, 39.6, 39.7, 39.8, 39.9, 40.0, 40.1, 40.2, 40.3, 40.4, 40.5, 40.6, 40.7, 40.8, 40.9, or 41.0 wt%, but is not limited to these.

[0042] According to at least one embodiment of the present invention, the weight swelling of the polyvinyl alcohol film when immersed in water for 1 minute is 39.0 wt% to 44.0 wt%, for example, 39.0, 39.1, 39.2, 39.3, 39.4, 39.5, 39.6, 39.7, 39.8, 39.9, 40.0, 40.1, 40.2, 40.3, 40.4, 40.5, 40.6, 40.7, 40.8, 40 0.9, 41.0, 41.1, 41.2, 41.3, 41.4, 41.5, 41.6, 41.7, 41.8, 41.9, 42.0, 42.1, 42.2, 42.3, 42.4, 42.5, 42.6, 42.7, 42.8, 42.9, 43.0, 43.1, 43.2, 43.3, 43.4, 43.5, 43.6, 43.7, 43.8, 43.9, or 44.0 wt%, but not limited to these.

[0043] According to at least one embodiment of the present invention, the weight swelling of the polyvinyl alcohol film when immersed in water for 2 minutes is 40.7 wt% to 47.0 wt%, for example, 40.7, 40.8, 40.9, 41.0, 41.1, 41.2, 41.3, 41.4, 41.5, 41.6, 41.7, 41.8, 41.9, 42.0, 42.1, 42.2, 42.3, 42.4, 42.5, 42.6, 42.7, 42.8, 42.9, 43.0, 43.1, 43.2 , 43.3, 43.4, 43.5, 43.6, 43.7, 43.8, 43.9, 44.0, 44.1, 44.2, 44.3, 44.4, 44.5, 44.6, 44.7, 44.8, 44.9, 45.0, 45.1, 45.2, 45.3, 45.4, 45.5, 45.6, 45.7, 45.8, 45.9, 46.0, 46.1, 46.2, 46.3, 46.4, 46.5, 46.6, 46.7, 46.8, 46.9 or 47.0 wt%, but these are black.

[0044] According to at least one embodiment of the present invention, the weight swelling of the polyvinyl alcohol film when immersed in water for 3 minutes is 41.6 wt% to 48.7 wt%, for example, 41.6, 41.7, 41.8, 41.9, 42.0, 42.1, 42.2, 42.3, 42.4, 42.5, 42.6, 42.7, 42.8, 42.9, 43.0, 43.1, 43.2, 43.3, 43.4, 43.5, 43.6, 43.7, 43.8, 43.9, 44.0, 44.1, 44.2, 44.3, 44.4, 44.5 , 44.6, 44.7, 44.8, 44.9, 45.0, 45.1, 45.2, 45.3, 45.4, 45.5, 45.6, 45.7, 45.8, 45.9, 46.0, 46.1, 46.2, 46.3, 46.4, 46.5, 46.6, 46.7, 46.8, 46.9, 47.0, 47.1, 47.2, 47.3, 47.4, 47.5, 47.6, 47.7, 47.8, 47.9, 48.0, 48.1, 48.2, 48.3, 48.4, 48.5, 48.6 or 48.7 wt%, but these are linear.

[0045] Optical film

[0046] Another aspect of the present invention provides an optical film, which is formed from the polyvinyl alcohol film described above. This optical film is, but is not limited to, a polarizing film, a blue light cut film, or a filter lens.

[0047] Polarizing film

[0048] Another embodiment of the present invention provides a polarizing film, which is formed from the polyvinyl alcohol film described above. In one preferred embodiment, the method for manufacturing the polarizing film includes the following steps: swelling, dyeing, stretching, complementary coloring, drying, and lamination of the polyvinyl alcohol film with a triacetylcellulose film (TAC). More specifically, the manufacturing process of the polarizing film can be carried out by employing the following method: immersing the polyvinyl alcohol film in water at 30°C to swell it and uniaxially stretching it in the mechanical direction (MD) to 2.0 times its original length; then stretching the polyvinyl alcohol film to 3.3 times its original length while immersing it in an aqueous solution at 30°C containing 0.03 mass percent iodine and 3 mass percent potassium iodide; and then further stretching it to 3.6 times its original length by immersing it in an aqueous solution at 30°C containing 3 mass percent potassium iodide and 3 mass percent boric acid. Next, the material is immersed in an aqueous solution containing 5% by mass potassium iodide and 4% by mass boric acid at 60°C and further stretched to 6.0 times its original length. After that, it is immersed in an aqueous solution of 3% by mass potassium iodide for 15 seconds and then dried at 60°C for 4 minutes to obtain a polarizing film.

[0049] Method for manufacturing polyvinyl alcohol film

[0050] Another embodiment of the present invention provides the above-described method for producing a polyvinyl alcohol film, which includes: (a) a dissolution step in which a polyvinyl alcohol resin is added to a solvent and a plasticizer, then heated to 110°C to 150°C to dissolve it and form a polyvinyl alcohol cast solution, wherein the viscosity of the polyvinyl alcohol cast solution is 60 cps to 120 cps when the solid content is 4%; (b) a casting step in which the polyvinyl alcohol cast solution is cast onto a cast drum, the cast drum is surrounded by a hot air dryer with a maximum temperature of 140°C, and the solution is peeled off the cast drum to obtain a polyvinyl alcohol preformed film; and (c) a drying step in which the polyvinyl alcohol preformed film is dried in a heating roller with a plurality of temperature differences and in a floating dryer, respectively, to obtain the polyvinyl alcohol film.

[0051] Specifically, the polyvinyl alcohol-based resin described above is obtained by forming a polyvinyl ester resin by polymerization of vinyl ester resin monomers, followed by a saponification reaction. Among these, the vinyl ester resin monomers include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, or vinyl octanoate, but the present invention is not limited to these. Copolymers formed by copolymerization of olefin compounds or acrylate derivatives with the above-mentioned vinyl ester resin monomers can also be used. Among these, olefin compounds include ethylene, propylene, or butylene, but the present invention is not limited to these. Acrylate derivatives include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, or n-butyl acrylate, but the present invention is not limited to these.

[0052] The “plasticizers” described herein are capable of increasing the flexibility of a material or liquefying the material, and include, but are not limited to, 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, or trimethylolpropane. Preferably, the plasticizer is glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane.

[0053] According to at least one embodiment of the present invention, (a) in the dissolution step, the above-mentioned heating temperature is, for example, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, or 150°C, but is not limited thereto. Furthermore, the viscosity of the polyvinyl alcohol cast solution when the solid content is 4% is, for example, 60 cps, 65 cps, 70 cps, 75 cps, 80 cps, 85 cps, 90 cps, 95 cps, 100 cps, 105 cps, 110 cps, 115 cps, or 120 cps, but is not limited thereto.

[0054] For the above-mentioned (b) casting process and (c) drying process, please also refer to Figure 2. In the apparatus 100, after forming the polyvinyl alcohol casting solution 170, it is discharged from the T-slit die lip 100 and cast onto the casting drum 120. Next, it is dried sequentially using multiple heating rollers (CY Roller, CY) 140 with different temperatures and a floating dryer (Floating Dryer, FD) 150. Finally, it is brought into a temperature and humidity controller 160 for adjustment and winding to obtain a polyvinyl alcohol film. More specifically, a hot air dryer (CAP) 130 is installed around the casting drum 120. Furthermore, the number of heating rollers 140 or the number of sections of the floating dryer 150 are not limited to this invention, and Figure 2 is merely an illustrative example.

[0055] While not limited to any particular theory, the inventors of this invention have discovered that the degree of weight swelling of a polyvinyl alcohol film can be controlled by adjusting and controlling the temperature at specific process stages in the manufacturing process. Specifically, by adjusting and controlling the maximum temperature of the hot air dryer 130, the sum of the temperatures of the multiple heating rollers 140, and the maximum temperature of the floating dryer 150 within a specific range, the numerical value of the degree of weight swelling of the polyvinyl alcohol film within a 3-minute immersion time can be controlled within an ideal range. According to at least one embodiment of the present invention, the maximum temperature of the hot air dryer 130 is 80 to 140°C, for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, or 140°C. According to at least one embodiment of the present invention, the total temperature of the multiple heating rollers is 700 to 1050°C, for example, 700°C, 715°C, 738°C, 750°C, 784°C, 800°C, 839°C, 850°C, 879°C, 900°C, 913°C, 935°C, 955°C, 962°C, 980°C, 999°C, 1002°C, 1005°C, 1018°C, 1023°C, 1035°C, 1036°C, 103 The temperature is 7°C, 1038°C, 1040°C, 1041°C, 1042°C, 1043°C, 1044°C, 1045°C, 1046°C, 1047°C, 1048°C, 1049°C, or 1050°C, but is not limited to these. Preferably, it is between 722°C and 1008°C, for example, 722°C, 750°C, 775°C, 800°C, 839°C, 880°C, 900°C, 950°C, or 1008°C, but is not limited to these. According to a preferred embodiment of the present invention, the temperature of a single heating roller 140 needs to be higher than 40°C. According to at least one embodiment of the present invention, the maximum temperature of the floating dryer 150 is 105 to 130°C, for example, 105°C, 106°C, 107°C, 108°C, 109°C, 110°C, 111°C, 112°C, 113°C, 114°C, 115°C, 116°C, 117°C, 118°C, 119°C, 120°C, 121°C, 122°C, 123°C, 124°C, 125°C, 126°C, 127°C, 128°C, 129°C, or 130°C, but is not limited to these.

[0056] While not limited to any particular theory, the inventors of this invention have discovered that the standard deviation of the weight swelling of a polyvinyl alcohol film can be controlled by adjusting and controlling the temperature difference between the east and west sides of the cast drum 120, heating roller 140, and floating dryer 150 during the manufacturing process. Specifically, by adjusting and controlling the sum of the east-west temperature differences between the cast drum 120, heating roller 140, and floating dryer 150 to a specific range, the standard deviation of the weight swelling of the polyvinyl alcohol film within a 3-minute immersion time can be controlled to an ideal range. Here, "east and west sides" as used herein refers to both sides along the machine direction (MD) of the apparatus, that is, both sides perpendicular to the direction of travel of the sample / product. According to at least one embodiment of the present invention, the sum of the east-west temperature difference between the cast drum 120, the heating roller 140, and the floating dryer 150 is ≤10°C, and is, for example, 10°C, 9°C, 8°C, 7°C, 6°C, 5°C, 4°C, 3°C, 2°C, or 1°C, but is not limited to these values.

[0057] While not limited to any particular theory, the inventors of this invention have discovered that the difference in moisture content between two surfaces of a polyvinyl alcohol film can be controlled by adjusting and controlling the difference in airflow velocity and temperature between the upper and lower parts of the floating dryer 150 during the manufacturing process. Specifically, by controlling the difference in airflow velocity and temperature between the upper and lower parts of the floating dryer 150 to within a specific range, the difference in moisture content between the two surfaces can be further reduced to within a specific range. According to at least one embodiment of the present invention, the difference in air velocity between the upper and lower sides of the floating dryer 150 is ≤1.0 m / s, and is not limited to, for example, 0.1 m / s, 0.2 m / s, 0.3 m / s, 0.4 m / s, 0.5 m / s, 0.6 m / s, 0.7 m / s, 0.8 m / s, 0.9 m / s, or 1.0 m / s, and the difference in temperature between the upper and lower sides of the floating dryer 150 is ≤5°C, and is not limited to, for example, 5°C, 4.5°C, 4.2°C, 4°C, 3.7°C, 3.5°C, 3.1°C, 2.9°C, 2.4°C, 2°C, 1.6°C, 1.3°C, 1.0°C, or 0.5°C.

[0058] According to at least one embodiment of the present invention, the area is 25 cm² 2 When the polyvinyl alcohol film is impregnated in 80 mL of 50°C water for 1 minute to obtain an impregnation solution, the amount of polyvinyl alcohol eluted in 20 mL of the impregnation solution is ≤30.0 ppm, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30 ppm, but is not limited to these. In one preferred embodiment, the amount of polyvinyl alcohol eluted in 20 mL of the impregnation solution is ≤15.0 ppm, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 ppm, but is not limited to these.

[0059] Furthermore, please refer to the polyvinyl alcohol film edge fold measuring device 200 shown in Figure 3. According to at least one embodiment of the present invention, the polyvinyl alcohol film 210 has an area of ​​10 cm in the machine direction (MD) × 5 cm in the transverse direction (TD), and is immersed in water 220, sandwiched at the center of the transverse direction of both ends, and after swelling in water 220 for 60 seconds, the fold angle of the machine direction edge is less than 180 degrees, for example, 175 degrees, 170 degrees, 165 degrees, 160 degrees, 150 degrees, 140 degrees, 130 degrees, 120 degrees, 110 degrees, 100 degrees, 90 degrees, 80 degrees, 70 degrees, 60 degrees, 50 degrees, 40 degrees, or 30 degrees, and is not limited to these. In one preferred embodiment, the bending angle in the mechanical direction is less than 90 degrees, and is, for example, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, or 30 degrees. Specifically, the “clamping” described herein is for fixing and tensioning the polyvinyl alcohol film 210, which can be achieved by any suitable method familiar to those skilled in the art.

[0060] According to at least one embodiment of the present invention, when the light transmittance T1% is measured at multiple positions of the polarizing film when tilted at +45 degrees with respect to the uniaxial stretching direction, and the light transmittance T2% is measured when tilted at -45 degrees with respect to the uniaxial stretching direction, the maximum - minimum value in the average values ​​of T1% and T2% at each position is ≤0.50%, and is, for example, 0.50%, 0.48%, 0.45%, 0.42%, 0.40%, 0.37%, 0.35%, 0.31%, 0.30%, 0.24%, 0.20%, 0.15%, 0.10%, 0.05%, or 0.02%. In one preferred embodiment, the maximum-minimum value of the mean is ≤0.10%, and is, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.10%. [Examples]

[0061] Examples

[0062] The present invention will be described in more detail below, along with examples. However, please understand that these examples are intended to help make the present invention easier to understand and do not limit the scope of the invention.

[0063] The following provides a non-limiting method for preparing polyvinyl alcohol films. Based on the same or similar method, non-limiting examples of polyvinyl alcohol films (Examples 1-7) and comparative examples of polyvinyl alcohol films (Comparative Examples 1-6) were prepared.

[0064] 1-1. Example: Preparation of polyvinyl alcohol film

[0065] 1800 kg of polyvinyl alcohol resin with alkalinity >99.0% and viscosity 80 cps, 4000 kg of water, and 200 kg of glycerin plasticizer were added to a dissolution tank. The mixture was heated to 140°C while stirring to dissolve the resin, then a mixer was used to ensure uniform dissolution. Water was then added to adjust the resin concentration to 30.0% to obtain a polyvinyl alcohol cast solution. The polyvinyl alcohol cast solution was defoamed using a twin-screw extruder, then discharged from a T-type slit die to form a curtain coating on a rotating cast drum (which was surrounded by a hot air dryer). The solution was then peeled off the cast drum to obtain a polyvinyl alcohol preformed film. The polyvinyl alcohol preformed film was dried in a heating roller with multiple temperature differences and in a floating dryer to obtain the polyvinyl alcohol film. The maximum temperature of the hot air dryer (CAP) was 110°C. After peeling the polyvinyl alcohol preformed film from the cast drum, it is dried on both the upper and lower surfaces by contact with a heating roller (CY), and the total temperature of the heating rollers is 839°C (the temperature of each individual heating roller is all above 40°C). Furthermore, the maximum temperature of the floating dryer (FD) is 120°C, the difference in air velocity between the upper and lower parts of the floating dryer is 0.4 m / s, the temperature difference between the upper and lower parts of the floating dryer is 2°C, and the sum of the east-west temperature differences between the cast drum, heating rollers, and floating dryer during the process (hereinafter referred to as the sum of east-west temperature differences) is 6.6°C. Details of the preparation conditions for Examples 1 to 7 are shown in Table 1.

[0066] 1-2. Comparative Example: Preparation of Polyvinyl Alcohol Film

[0067] The same process used to prepare the polyvinyl alcohol film in the aforementioned example was employed in the comparative example. However, the maximum CAP temperature, total CY temperature, maximum FD temperature, total temperature difference between the east and west sides, upper and lower FD wind speed difference, and upper and lower FD temperature difference differed from those in the example. Details of the preparation conditions are shown in Table 2.

[0068] 2. Analysis and Measurement Methods

[0069] The parameters and measurement methods for Examples 1-7 and Comparative Examples 1-6 are provided below. Tables 1 and 2 show the process conditions and parameters of the polyvinyl alcohol films for Examples 1-7 and Comparative Examples 1-6, as well as the evaluation results for the reduction in polyvinyl alcohol elution, permeability uniformity, and edge folding.

[0070] 2-1. Method for measuring the degree of weight swelling and the standard deviation of the degree of weight swelling: A non-hygroscopic polyvinyl alcohol film was taken as a sample, divided into 5 equal parts along the width direction, and the center of each divided polyvinyl alcohol film was cut. The cut area of ​​each piece was (MD5cm*TD5cm). Next, it was swelled in pure water at 30°C for 30 seconds, 1 minute, and 3 minutes. After swelling was complete, the swollen polyvinyl alcohol film was removed, the surface moisture was wiped off, and the weight at this time (M1) was recorded. The polyvinyl alcohol film was then dried in a dryer at 120°C for 120 minutes, and the weight after drying (M2) was recorded. The calculation method for the degree of weight swelling is (M1-M2) / M1.

[0071] Since five samples were measured at all different swelling times, the average value of these five samples gives the weight swelling degree for that swelling time, and simultaneously, the standard deviation of the weight swelling degree for that swelling time can be determined from these five samples. In the examples and comparative examples, the maximum value of the standard deviation within a specific time period was used for the weight swelling degree.

[0072] 2-2. Method for measuring the difference in moisture content between two surfaces: A polyvinyl alcohol film was divided into 5 equal parts along its width, and the center of each divided polyvinyl alcohol film was cut. The cut area of ​​each piece was (MD 5cm * TD 5cm). The film was placed directly on an FTIR-ATR, and the absorbance was measured at the intermediate position, and the moisture content was calculated using the formula: A 1654cm -1 / A 1090cm -1 Substituting this value into the formula, then calculating the difference between the average moisture content of five points on the X-face and the average moisture content of five points on the Y-face, and then taking the absolute value of that result, we obtain the difference in moisture content between the two faces.

[0073] 2-3. Measurement and evaluation of polyvinyl alcohol elution: a. A 5cm x 5cm polyvinyl alcohol film was taken and impregnated in 80mL of 50°C water for 1 minute (in a serum vial). Then the impregnated solution was removed, and 20mL of the impregnated solution was uniformly mixed with 15mL of 4% boric acid solution, 3mL of iodine / potassium iodide solution, and 12mL of pure aqueous solution. b. A standard solution was prepared by uniformly mixing 35 mL of pure water, 15 mL of 4% boric acid solution, and 3 mL of iodine / potassium iodide solution.

[0074] First, prepare a solution with a known polyvinyl alcohol content using the method described above, and then measure its 670 cm³. -1 By measuring the absorbance in the sample and creating a calibration curve, the amount of polyvinyl alcohol eluted can be obtained by applying the absorbance of the actual measurement solution to the calibration curve. Here, the amount of polyvinyl alcohol eluted is divided into three grades. The "◎" mark indicates that the elution amount is ≤15.0 ppm, the "〇" mark indicates that the elution amount is ≤30.0 ppm, and the "X" mark indicates that the elution amount is >30.0 ppm.

[0075] 2-4. Measurement and Evaluation of Transmittance Uniformity: First, a polarizing film was prepared using polyvinyl alcohol film. Next, five samples were taken evenly from the width direction of the polarizing film, and each sample was made into a 5cm x 5cm square. Then, the light transmittance was measured using a spectrophotometer "Perkin Elmer Lambda 750S". Specifically, during measurement, a C light source was used according to the method of JIS Z 8722 (2009), and luminous efficiency correction was performed for the visible light area of ​​a 2-degree field of view. For each sample, the light transmittance when tilted at +45 degrees relative to the uniaxial stretching direction and the light transmittance when tilted at -45 degrees relative to the uniaxial stretching direction were measured, and the average value (T1) (%) was calculated. The maximum and minimum values ​​were found from the five individual transmittances obtained, and the value of maximum - minimum was divided into three grades. The "◎" mark indicates that the difference between the maximum and minimum transmittance is ≤0.10%, the "〇" mark indicates that the difference between the maximum and minimum transmittance is ≤0.50%, and the "X" mark indicates that the difference between the maximum and minimum transmittance is >0.50%.

[0076] 2-5. Measurement and Evaluation of Edge Fold: Refer again to Figure 3 for this section. A newly opened polyvinyl alcohol film was cut to a size of MD10cm*TD5cm, and the center of both sides in the TD direction of the polyvinyl alcohol film was secured with a 2.5cm double clip. The film was then immersed in water, and the maximum curl angle of the MD direction edge within 60 seconds was observed. Here, the mark "◎" indicates that the edge fold angle of the polyvinyl alcohol film is <90 degrees, the mark "〇" indicates that the edge fold angle of the polyvinyl alcohol film is <180 degrees, and the mark "X" indicates that the edge fold angle of the polyvinyl alcohol film is >180 degrees.

[0077] [Table 1-1] [Table 1-2]

[0078] As can be seen from Examples 1 to 7 in Table 1, when the weight swelling value during a 3-minute water immersion time is within 2.4 ln (minutes of foam immersion time) + 39 and 4.3 ln (minutes of immersion time) + 44, an effect of reducing the amount of polyvinyl alcohol eluted was obtained. At the same time, when the standard deviation of the weight swelling value during a 3-minute water immersion time is ≤1.70, a polarizing film with good transmission uniformity was obtained. Furthermore, when the difference in water content between two sides of the polyvinyl alcohol film is ≤0.015, the edge fold angle of the polyvinyl alcohol film is relatively small, thereby avoiding a condition where the fold angle becomes excessively large and tears occur at the edges of the polyvinyl alcohol film.

[0079] [Table 2-1] [Table 2-2]

[0080] Examples 1-6 in Table 2 show that if the weight swelling value during a 3-minute immersion time is not between 2.4 ln (minutes of immersion time) + 39 and 4.3 ln (minutes of immersion time) + 44, the effect of reducing polyvinyl alcohol elution is not good. Specifically, we are confident that the weight swelling value during the 3-minute immersion time mentioned above can be controlled by adjusting and controlling the maximum temperature of the hot air dryer, the sum of the heating roller temperatures, and the maximum temperature of the floating dryer. While not limited to any particular theory, the inventors discovered that when the maximum temperature of the hot air dryer, the total temperature of the heating rollers, and the maximum temperature of the floating dryer are excessively high (Comparative Examples 1, 3, and 5), the weight swelling degree after 3 minutes is too low (Comparative Example 1), the weight swelling degree after 30 seconds and 1 minute is too low (Comparative Example 3), and the weight swelling degree after 30 seconds, 1 minute, 2 minutes, and 3 minutes is too low (Comparative Example 5), respectively. Even if the amount of polyvinyl alcohol eluted is reduced, the weight swelling degree of the polyvinyl alcohol film becomes excessively low. An excessively low weight swelling degree of the polyvinyl alcohol film is undesirable because it makes the edges more prone to cracking after thermal shock during the subsequent manufacturing of the polarizing film. Furthermore, if the maximum temperature of the hot air dryer, the total temperature of the heating rollers, and the maximum temperature of the floating dryer are excessively low (Comparative Examples 2, 4, and 6), the weight swelling of the resulting polyvinyl alcohol film becomes too high, resulting in an excessive amount of polyvinyl alcohol elution.

[0081] Referring again to Table 2, if the standard deviation of weight swelling within a 3-minute immersion time is not ≤ 1.70 (Comparative Examples 1, 2, 3, and 5), the polarizing film produced thereby exhibits poor transmission uniformity. Specifically, we believe that the standard deviation of weight swelling within a 3-minute immersion time mentioned above can be controlled by adjusting and controlling the sum of the east-west temperature difference. While not limited to any particular theory, the inventors have found that when the sum of the east-west temperature difference is excessively high (Comparative Examples 1, 2, 3, and 5), the standard deviation of weight swelling of the polyvinyl alcohol film becomes excessively high, resulting in poor transmission uniformity performance of the polarizing film produced thereafter.

[0082] Referring again to Table 2, when the difference in moisture content between the two sides of the polyvinyl alcohol film is not ≤0.015 (Comparative Examples 1-5), the edge fold angle of the polyvinyl alcohol film is relatively large, and there is a risk that the edges of the polyvinyl alcohol film may tear due to an excessively large fold angle. Specifically, we are confident that the difference in moisture content between the two sides mentioned above can be controlled by adjusting and controlling the difference in airflow velocity and temperature between the upper and lower parts of the floating-type dryer. Although not limited to a specific theory, the inventors have found that when the difference in airflow velocity between the upper and lower parts of the floating-type dryer is excessively large (Comparative Examples 1-4, 6) or when the temperature difference between the upper and lower parts is excessively large (Comparative Example 5), the difference in moisture content between the two sides of the polyvinyl alcohol film becomes excessively high in all cases, resulting in poor performance in edge folds.

[0083] The present invention further increased the swelling time of the polyvinyl alcohol films in Examples 1-7 to 30 minutes, 60 minutes, and 240 minutes. See Table 3 for details of the results.

[0084] [Table 3-1] [Table 3-2]

[0085] As can be seen from Table 3, the degree of weight swelling of the polyvinyl alcohol film increases with increasing swelling time, but the effect of polyvinyl alcohol reduction remained in all cases.

[0086] In summary, the polyvinyl alcohol film of the present invention not only improves the problem of high polyvinyl alcohol elution, but also enables the subsequent production of a polarizing film with uniform transmittance. Furthermore, the polyvinyl alcohol film of the present invention has a relatively small edge fold angle, avoiding a situation where the fold angle becomes excessively large, making the edges prone to tearing.

[0087] All scopes provided herein include combinations of specific scopes within an allocated scope and secondary scopes between those scopes. Furthermore, unless otherwise stated, all scopes provided herein include the scope endpoints. Therefore, scopes 1-5 specifically include 1, 2, 3, 4, and 5, as well as secondary scopes such as 2-5, 3-5, 2-3, 2-4, and 1-4.

[0088] All publications and patent applications referenced herein are incorporated herein by reference, and for all purposes, each publication or patent application is explicitly and individually indicated as being incorporated herein by reference. In the event of any inconsistency between this specification and any publication or patent application incorporated herein by reference, this specification shall prevail.

[0089] The present invention has been described in detail above, but the above is merely a preferred embodiment of the present invention and does not limit the scope of implementation of the present invention. Equivalent variations and modifications based on the claims of the present invention all fall within the scope of the claims of the present invention. [Explanation of Symbols]

[0090] 100 devices 110 T-type slit die lip 120 Cast Drums 130 Hot air dryer 140 heating rollers 150 Floating-type hair dryer 160 Temperature and humidity controller 170 Polyvinyl alcohol cast solution 200 Polyvinyl alcohol film edge fold measurement device 210 Polyvinyl alcohol film 220 water

Claims

1. A polyvinyl alcohol film, wherein the degree of weight swelling when the polyvinyl alcohol film is immersed in water at 30°C for 3 minutes is given by the following formula: The following conditions must be met: 2.4 ln (minutes of immersion time) + 39 < value of weight swelling < 4.3 ln (minutes of immersion time) + 44, and The polyvinyl alcohol film is a polyvinyl alcohol film in which the standard deviation of the degree of weight swelling during a 3-minute water immersion time measured at multiple locations is ≤1.

70.

2. The polyvinyl alcohol film according to claim 1, having two opposing surfaces, which are the X surface and the Y surface, and the difference in moisture content between the two surfaces is ≤0.015, where the difference in moisture content between the two surfaces is calculated by measuring at multiple locations on the polyvinyl alcohol film using FTIR-ATR and using the following formula: |Average value of X surface (A1654 cm⁻¹ / A1090 cm⁻¹) - Average value of Y surface (A1654 cm⁻¹ / A1090 cm⁻¹)|.

3. The polyvinyl alcohol film according to claim 2, wherein the degree of weight swelling of the polyvinyl alcohol film when immersed in water for 30 seconds is 37.3 wt% to 41.0 wt%.

4. The polyvinyl alcohol film according to claim 3, wherein the degree of weight swelling of the polyvinyl alcohol film when immersed in water for 1 minute is 39.0 wt% to 44.0 wt%.

5. The polyvinyl alcohol film according to claim 4, wherein the degree of weight swelling of the polyvinyl alcohol film when immersed in water for 2 minutes is 40.7 wt% to 47.0 wt%.

6. The polyvinyl alcohol film according to claim 5, wherein the degree of weight swelling of the polyvinyl alcohol film when immersed in water for 3 minutes is 41.6 wt% to 48.7 wt%.

7. The polyvinyl alcohol film according to claim 1, having a thickness of 45 to 75 μm.

8. The polyvinyl alcohol film according to any one of claims 1 to 7, wherein the polyvinyl alcohol film has an area of ​​10 cm in the machine direction (MD) and 5 cm in the width direction (TD), and after being clamped at the center of both ends in the width direction and immersed in water for 60 seconds, the angle of fold at the machine direction end is less than 180 degrees.

9. The polyvinyl alcohol film according to claim 8, wherein the folding angle of the end in the direction of the machine is less than 90 degrees.

10. A method for producing a polyvinyl alcohol film according to any one of claims 1 to 7, comprising: (a) a dissolution step in which a polyvinyl alcohol resin is added to a solvent and a plasticizer, and then heated to 110°C to 150°C to dissolve it, forming a polyvinyl alcohol cast solution, wherein the viscosity of the polyvinyl alcohol cast solution is 60 cps to 120 cps when the curable component is 4%; (b) a casting step in which the polyvinyl alcohol cast solution is cast onto a cast drum, the cast drum is surrounded by a hot air dryer, the maximum temperature of the hot air dryer is 80 to 140°C, and the solution is peeled off the cast drum to obtain a polyvinyl alcohol preformed film; and (c) a drying step in which the polyvinyl alcohol preformed film is dried in a plurality of heating rollers with temperature differences and in a floating dryer, respectively, to obtain the polyvinyl alcohol film. A manufacturing method wherein the sum of the east-west temperature difference between the cast drum, the heating roller, and the floating dryer is ≤ 10°C.

11. The manufacturing method according to claim 10, wherein the total temperature of the heating rollers having multiple temperature differences is 700 to 1050°C.

12. The manufacturing method according to claim 11, wherein the temperature of each of the heating rollers is higher than 40°C.

13. The manufacturing method according to claim 12, wherein the maximum temperature of the floating dryer is 105 to 130°C.

14. The manufacturing method according to claim 13, wherein the difference in air velocity between the upper and lower parts of the floating dryer is ≤ 1.0 m / s, and the temperature difference between the upper and lower parts of the floating dryer is ≤ 5°C.

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