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

Abstract

To provide a polyvinyl alcohol film which is difficult to elute and has uniform transmissivity.SOLUTION: The numeric value of weight swelling degree of a polyvinyl alcohol film according to the present invention within a water immersion time of 3 minutes satisfies an expression=2.4ln (fraction of immersion time)+39<numeric value of weight swelling degree<4.3ln (fraction of immersion time)+44. Furthermore, the standard deviation of weight swelling degree of the polyvinyl alcohol film within a water immersion time of 3 minutes as measured at a plurality of positions is smaller than or equal to 1.70. The polyvinyl alcohol film according to the present invention makes it possible to avoid an increase in the amount of elution and transmissivity of becoming ununiform in the production of polarizing films by controlling the weight swelling degree within a fixed water immersion time.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol (PVA) film, and in particular to a polyvinyl alcohol film that is resistant to leaching and has uniform transmittance, an optical film containing the same, and a method for producing the same, but the present invention is not limited thereto. [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 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] Polarizing films obtained by processing polyvinyl alcohol film through a polarization process have the property of only allowing light rays in a specific direction to pass through, thereby controlling the brightness of the light rays that pass through. Based on this property, polarizing films are used in various displays, eyeglasses, and wearable devices. Good polarizing films have properties such as uniform color, minimal color spots, and no wrinkles, and can provide excellent optical properties. To improve the optical properties of polarizing films, conventional technologies have improved the film's performance by changing the viscosity and degree of saponification through methods such as changing the structure of polyvinyl alcohol or adding functional groups (e.g., cationic groups). Summary of the Invention [Problem to be solved by the invention]

[0004] However, polyvinyl alcohol films sometimes suffer from the problem of excessive polyvinyl alcohol leaching during the optical film manufacturing process, which can lead to contamination or foreign matter in the finished product, resulting in it being deemed a defective product. Also, if the degree of entanglement of polyvinyl alcohol molecules in different areas of the film is not uniform, the transmittance of the subsequently manufactured optical film may be non-uniform. [Means for solving the problem]

[0005] Without being limited to a particular theory for the above-mentioned problem, the present inventors have discovered that the problem of high polyvinyl alcohol elution can be improved by controlling the weight swelling degree of a polyvinyl alcohol film within a particular water immersion time (i.e., swelling time) within a particular range. Furthermore, the present inventors have also discovered that the problem of non-uniform transmittance of the optical film can be improved by controlling the standard deviation of the weight swelling degree of a polyvinyl alcohol film within a particular water immersion time within a particular range.

[0006] Therefore, the present invention provides a polyvinyl alcohol film, the weight swelling degree of which satisfies the formula: 2.4 ln (minutes of immersion time) + 39 < weight swelling degree < 4.3 ln (minutes of immersion time) + 44 within 3 minutes of immersion in water, and further, the standard deviation of the weight swelling degree of which is measured at multiple positions within 3 minutes of immersion in water is ≦1.70.

[0007] According to one embodiment of the present invention, the polyvinyl alcohol film of the present invention has two opposing surfaces, an X surface and a Y surface, and the difference in moisture content between the two surfaces is ≦0.015. The difference in moisture content between the two surfaces is measured at multiple points on the polyvinyl alcohol film using FTIR-ATR and calculated using the following formula: |X surface(A 1654cm -1 / A 1090cm -1 )Average value-Y surface (A1654cm -1 / A 1090cm -1 ) average value |

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

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

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

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

[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, an area of ​​25 cm 2 When the polyvinyl alcohol film is immersed in 80 mL of water at 50°C 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.

[0014] According to one embodiment of the present invention, the amount of polyvinyl alcohol eluted in 20 mL of the impregnation solution is ≦15.0 ppm.

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

[0016] According to one embodiment of the present invention, the machine direction fold angle is less than 90 degrees.

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

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

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

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

[0021] Yet another object of the present invention is to provide a method for producing the above-mentioned polyvinyl alcohol film, (a) a dissolving step in which a polyvinyl alcohol resin is added to a solvent and a plasticizer, and then the temperature is raised to 110°C to 150°C to dissolve the resin, thereby forming a polyvinyl alcohol cast solution, the viscosity of which is 60 to 120 cps when the solid content is 4%; (b) a casting step in which the polyvinyl alcohol cast solution is cast onto a casting drum, the casting drum is surrounded by a hot air dryer, and the maximum temperature of the hot air dryer is 80 to 140°C, and the polyvinyl alcohol preformed film is peeled off from the casting drum; and (c) a drying step of drying the polyvinyl alcohol preformed film with a plurality of heating rollers having different temperatures and in a floating-type dryer, respectively, to obtain the polyvinyl alcohol film.

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

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

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

[0025] According to one embodiment of the present invention, the wind speed difference between the upper and lower sides of the floating-type dryer is ≦1.0 m / s, and the temperature difference between the upper and lower sides of the floating-type dryer is ≦5°C.

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

[0027] The polyvinyl alcohol film of the present invention has the following advantages: It can not only avoid the problem of high polyvinyl alcohol elution, but also improve the problem of non-uniform transmittance in the subsequent production of optical films. [Brief explanation of the drawings]

[0028] [Figure 1]1 shows the results of weight swelling of a polyvinyl alcohol film at different times according to one embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of a polyvinyl alcohol film casting and drying process according to one embodiment of the present invention. [Figure 3] FIG. 1 is a conceptual diagram of measuring the folding of an edge of a polyvinyl alcohol film according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] Embodiments of the present technology will now be described, by way of example only, with reference to the accompanying drawings, in which it is to be understood that aspects of the invention are not limited to the arrangements, instrumentalities and features shown in the drawings.

[0030] The proportions of various features and components in the drawings are not to scale, but are drawn in accordance with conventional working methods in order to best illustrate the specific features and components related to the present invention. In addition, the same or similar component symbols in different drawings refer to similar components or members.

[0031] The following embodiments do not unduly limit the present invention, and those skilled in the art to which the present invention pertains may make modifications and variations to the examples discussed herein without departing from the spirit or scope of the present invention, and all such modifications and variations are within the scope of the present invention.

[0032] In the specification, unless otherwise indicated in context, the terms "comprise," "include," "have," or "contain" are inclusive or open-ended and do not exclude other unrecited elements or methods or steps. The terms "one" and "the" can be interpreted as either singular or plural. The term "one or more" refers to "at least one" and can therefore include a single feature or a mixture / combination. Furthermore, in this specification and the appended claims, unless otherwise specified, "placed on an object" can be considered to be attached to or in other form of contact with the surface of an object, directly or indirectly, and the definition of the surface should be determined by the implication of the preceding and following paragraphs of the content of the specification and common knowledge in the field to which this specification pertains.

[0033] Polyvinyl alcohol film

[0034] One aspect of the present invention provides a polyvinyl alcohol film, the weight swelling degree of which satisfies the formula: 2.4 ln (minutes of immersion time) + 39 < weight swelling degree < 4.3 ln (minutes of immersion time) + 44 within 3 minutes of immersion in water, and the standard deviation of the weight swelling degree of which is measured at multiple positions within 3 minutes of immersion in water is ≦1.70.

[0035] The "weight swelling degree" described in this specification refers to the degree to which water molecules can penetrate into a polyvinyl alcohol film. The rough measurement process for the "weight swelling degree" is as follows: a polyvinyl alcohol film is sampled in a moisture-free state, placed in water to swell (also called water immersion), the polyvinyl alcohol film is then removed, the moisture on the surface is wiped off, and the weight of the film is weighed, which is designated as M1; the polyvinyl alcohol film is then placed in a dryer to dry, and the weight of the film is weighed, which is designated as M2; and the weight swelling degree (wt%) is calculated by (M1-M2) / M1.

[0036] Furthermore, the "weight swelling degree value within a 3-minute water immersion time" used herein refers to the weight swelling degree value obtained within the 3-minute water immersion time (i.e., swelling time) set during the above-mentioned process. As used herein, the so-called "value" refers to the numerical portion of the weight swelling degree. For example, if the weight swelling degree is 35 wt%, the numerical value of the weight swelling degree is 35. Furthermore, as shown in Figure 1, the inventors discovered that the weight swelling degree exhibits a logarithmic change with swelling time. Furthermore, as the swelling time changes, the numerical value of the weight swelling degree has a specific range, with the lower limit being 2.4 ln(A) + 39 and the upper limit being 4.3 ln(A) + 44. Here, A is the number of minutes of water immersion time. Those skilled in the art will understand that the number of minutes of water immersion time refers to the numerical portion of the water immersion time. For example, if the water immersion time is 3 minutes, the number of minutes of water immersion time is 3. Without being limited to a particular theory, the present inventors have also discovered that when the weight swelling values ​​within the 3-minute water immersion time are all controlled within the specific range, the problem of high polyvinyl alcohol elution can be effectively improved.

[0037] Furthermore, the "standard deviation of weight swelling within a 3-minute water immersion period" described herein is obtained by calculating the weight swelling at different positions (areas) in the width direction of a 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 employs the maximum value of the standard deviation within a specific time period. Therefore, the term "multiple positions" used herein refers to multiple different positions in the width direction of the polyvinyl alcohol film. However, the present invention does not limit the size and spacing of these positions. Furthermore, without being limited by any particular theory, the present inventors have discovered that an excessively large standard deviation of weight swelling within a 3-minute water immersion period indicates different degrees of entanglement of polyvinyl alcohol molecules at different positions, which can lead to problems such as non-uniform transmittance in different areas during the production of a polarized film. In other words, controlling the standard deviation of weight swelling within a 3-minute water immersion period of a polyvinyl alcohol film within a specific range can alleviate the problem of non-uniform transmittance in polarized films. Specifically, the above-mentioned specific range is ≦1.70, preferably 0.65 to 1.70, for example, 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, but is not limited thereto.

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

[0039] Specifically, the "two opposing surfaces" described in this specification refer to the opposing front and back surfaces of the film body (they may also refer to the upper and lower surfaces or the front and rear surfaces, and are merely referred to as the X and Y surfaces in this specification for illustrative purposes). Furthermore, the "difference in moisture content between the two surfaces" described in this specification is calculated by taking multiple points on each of the X and Y surfaces of a polyvinyl alcohol film sample, measuring the moisture content at each of the multiple points using FTIR-ATR (A 1654cm -1 / A 1090cm -1 ), where A 1654cm -1 is the peak value of the deformation vibration of water, and A 1090cm -1 is the peak value of the (CO) stretching vibration, and then the average value of the water content at the multiple points is calculated using the following formula: |X plane (A 1654cm -1 / A 1090cm -1 )Average value-Y surface (A 1654cm -1 / A 1090cm -1) average value|, the difference in moisture content between the two surfaces can be obtained. The inventors have discovered that if the difference in moisture content between the two surfaces is too large, the two surfaces will swell at different rates when water molecules subsequently enter the polyvinyl alcohol film. In this case, the surface with the lower moisture content may curl toward the surface with the higher moisture content, resulting in creases on the film surface. Furthermore, when manufacturing a polarized film, if both ends of the film curl significantly due to immersion in water, creases may occur on the edges, resulting in creases. If the creases are severe, the film may tear. Therefore, without being limited to a particular theory, it is believed that if the difference in moisture content between the two surfaces is controlled within a specific range, creases on the edges of the polyvinyl alcohol film can be reduced. Specifically, the above-mentioned specific 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 thereto.

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

[0041] According to at least one embodiment of the present invention, the weight swelling degree of the polyvinyl alcohol film when immersed in water for 30 seconds is 37.3 wt% to 41.0 wt%, for example, but not limited to, 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%.

[0042] According to at least one embodiment of the present invention, the weight swelling degree 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, 40.9, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 4 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%.

[0043] According to at least one embodiment of the present invention, the weight swelling degree 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%.

[0044] According to at least one embodiment of the present invention, the weight swelling degree 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%.

[0045] Optical Film

[0046] Another aspect of the present invention provides an optical film formed from the polyvinyl alcohol film described above, such as, but not limited to, a polarizing film, a blue light blocking film, and a filter lens.

[0047] Polarizing film

[0048] Another aspect of the present invention provides a polarizing film, which is formed from the above-mentioned polyvinyl alcohol film. In a preferred embodiment, the manufacturing method for the polarizing film includes the following steps: swelling, dyeing, stretching, color-matching, drying, and laminating a triacetyl cellulose (TAC) film to the polyvinyl alcohol film. More specifically, the manufacturing process for the polarizing film can be carried out as follows: The polyvinyl alcohol film is immersed in water at 30°C to swell it and uniaxially stretched in the machine direction (MD) to 2.0 times its original length. The polyvinyl alcohol film is then stretched to 3.3 times its original length while immersed in an aqueous solution at 30°C containing 0.03% by weight iodine and 3% by weight potassium iodide. The polyvinyl alcohol film is then further stretched to 3.6 times its original length while immersed in an aqueous solution at 30°C containing 3% by weight potassium iodide and 3% by weight boric acid. The film is then immersed in an aqueous solution containing 5% by mass of potassium iodide and 4% by mass of boric acid at 60°C and further stretched to 6.0 times its original length.Then, the film is immersed in an aqueous solution of 3% by mass of potassium iodide for 15 seconds, and then dried at 60°C for 4 minutes to obtain a polarized film.

[0049] Method for producing polyvinyl alcohol film

[0050] 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 is added to a solvent and a plasticizer, and the resulting mixture is heated to 110°C to 150°C to dissolve the resin, thereby forming a polyvinyl alcohol cast solution, the viscosity of which 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 casting drum surrounded by a hot air dryer, the maximum temperature of which is 140°C, and the polyvinyl alcohol preformed film is peeled off from the casting drum; and (c) a drying step in which the polyvinyl alcohol preformed film is dried using a plurality of heating rollers and a floating-type dryer, each having a different temperature, to obtain the polyvinyl alcohol film.

[0051] Specifically, 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, vinyl ester resin monomers include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate, but the present invention is not limited thereto. Copolymers formed by copolymerizing an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used. Among these, olefin compounds include, but are not limited to, ethylene, propylene, and butylene. Acrylate derivatives include, but are not limited to, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate.

[0052] The "plasticizer" described herein can increase the flexibility or liquefy the material, and includes, but is 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, in the (a) dissolving step, the elevated temperature may be, for example, but not limited to, 110° C., 115° C., 120° C., 125° C., 130° C., 135° C., 140° C., 145° C., or 150° C. Furthermore, the viscosity of the polyvinyl alcohol casting solution at a solids content of 4% may be, for example, but not limited to, 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.

[0054] Please also refer to Figure 2 for the above-mentioned (b) casting step and (c) drying step. In the apparatus 100, a polyvinyl alcohol casting solution 170 is formed, extruded through a T-shaped slit die lip 100, and cast onto a casting drum 120. It is then dried sequentially through multiple heating rollers (CY Roller, CY) 140 with different temperatures and a floating dryer (FD) 150. Finally, the polyvinyl alcohol film is obtained by entering a temperature and humidity controller 160 for conditioning and winding. More specifically, a hot air dryer (CAP) 130 is installed around the casting drum 120. The number of heating rollers 140 or the number of sections of the floating dryer 150 are not limited to the present application, and Figure 2 is merely illustrative.

[0055] Without being limited by any particular theory, the inventors of the present application have discovered that the weight swelling degree of a polyvinyl alcohol film can be controlled by adjusting and controlling the temperatures at specific process steps during the manufacturing process. Specifically, by adjusting and controlling the maximum temperature of the hot air dryer 130, the total temperature of the multiple heating rollers 140, and the maximum temperature of the floating-type dryer 150 within specific ranges, the weight swelling degree of the polyvinyl alcohol film within the three-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, but not limited to, 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 plurality of 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, 1037°C, 1038°C, 1039°C, 1040°C, 1041°C, 1042°C, 1043°C, 1044°C, 1045°C, 1046°C, 1047°C, 1048°C, 1049°C, 1050°C, 1051°C, 1052°C, 1053°C, 1054°C, 1055°C, 1056°C, 1057°C, 1058°C, 1059°C, 1060°C, 1061°C, 1062°C, 1063°C, 1064°C, 1065°C, 1066°C, 1067°C, 1068°C, 1069°C, 1070°C, 1071°C, 1072°C, 1073°C, 1074°C, 1075°C, The temperature range is, but is not limited to, 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, and is preferably 722 to 1008°C, for example, but not limited to, 722°C, 750°C, 775°C, 800°C, 839°C, 880°C, 900°C, 950°C, or 1008°C. According to a preferred embodiment of the present invention, the temperature of the single heated roller 140 should be higher than 40°C. According to at least one embodiment of the present invention, the maximum temperature of the floating-type dryer 150 is 105 to 130°C, for example, but not limited to, 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.

[0056] Without being limited to any particular theory, the inventors of the present application have discovered that the standard deviation of the weight swelling degree of a polyvinyl alcohol film can be controlled by adjusting and controlling the temperature difference between the east and west sides of the casting drum 120, the heating roller 140, and the floating-type dryer 150 during the manufacturing process. Specifically, by adjusting and controlling the total temperature difference between the east and west sides of the casting drum 120, the heating roller 140, and the floating-type dryer 150 within a specific range, the standard deviation of the weight swelling degree of the polyvinyl alcohol film during the three-minute immersion time can be controlled within an ideal range. Here, the "east and west sides" referred to in this specification refer to both sides along the machine direction (MD) of the apparatus, i.e., both sides perpendicular to the direction of travel of the sample or product. According to at least one embodiment of the present invention, the sum of the east-west temperature difference of the casting drum 120, the heating roller 140, and the floating-type dryer 150 is ≦10°C, for example, but not limited to, 10°C, 9°C, 8°C, 7°C, 6°C, 5°C, 4°C, 3°C, 2°C, or 1°C.

[0057] Without being limited to a particular theory, the inventors of the present application discovered that the moisture content difference between the two surfaces of a polyvinyl alcohol film can be controlled by adjusting and controlling the upper and lower air speed difference and the upper and lower temperature difference of the floating-type dryer 150 during the manufacturing process. Specifically, by controlling the upper and lower air speed difference and the upper and lower temperature difference of the floating-type dryer 150 within specific ranges, the moisture content difference between the two surfaces can be made to fall within an even more specific range. According to at least one embodiment of the present invention, the difference in wind speed between the upper and lower sides of the floating-type dryer 150 is ≦1.0 m / s, for example, but not limited to, 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-type dryer 150 is ≦5°C, for example, but not limited to, 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, an area of ​​25 cm 2 When the polyvinyl alcohol film is immersed in 80 mL of water at 50°C 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, but not limited to, 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. In a preferred embodiment, the amount of polyvinyl alcohol eluted in 20 mL of the impregnation solution is ≦15.0 ppm, for example, but not limited to, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 ppm.

[0059] Furthermore, please refer to the apparatus 200 for measuring the fold angle of the edge of a polyvinyl alcohol film shown in Figure 3. According to at least one embodiment of the present invention, a polyvinyl alcohol film 210 having an area of ​​10 cm in the machine direction (MD) and 5 cm in the transverse direction (TD) is sandwiched at the center of both ends in the width direction and immersed in water 220. After swelling in water 220 for 60 seconds, the fold angle of the edge in the machine direction is less than 180 degrees, for example, but not limited to, 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. In a preferred embodiment, the machine direction fold angle is less than 90 degrees, such as, but not limited to, 85 degrees, 80 degrees, 75 degrees, 70 degrees, 65 degrees, 60 degrees, 55 degrees, 50 degrees, 45 degrees, 40 degrees, 35 degrees, or 30 degrees. Specifically, the "sandwiching" described herein refers to securing and tensioning the polyvinyl alcohol film 210, which can be accomplished by any suitable method familiar to those skilled in the art of the present invention.

[0060] According to at least one embodiment of the present invention, when the light transmittance T1% when tilted at +45 degrees relative to the stretching direction of uniaxial stretching and the light transmittance T2% when tilted at -45 degrees relative to the stretching direction of uniaxial stretching are measured at multiple positions on the polarizing film, the maximum-minimum value of the average values ​​of T1% and T2% at each position is ≦0.50%, for example, but not limited to, 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 a preferred embodiment, the maximum-minimum value of the average value is ≦0.10%, for example, but not limited to, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09% or 0.10%. [Example]

[0061] Example

[0062] The present invention will be described in more detail below with reference to examples, but it should be understood that these examples are intended to help the present invention be more easily understood and are not intended to limit the scope of the present invention.

[0063] A non-limiting method for preparing a polyvinyl alcohol film is provided below. Non-limiting examples of polyvinyl alcohol films (Examples 1 to 7) and comparative examples (Comparative Examples 1 to 6) were prepared using the same or similar methods.

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

[0065] 1800 kg of polyvinyl alcohol resin with an alkalinity of >99.0% and a viscosity of 80 cps, 4000 kg of water, and 200 kg of plasticizer glycerin were added to a dissolution tank and heated to 140°C while stirring. The mixture was then homogenized using a mixer, and water was added to adjust the resin concentration to 30.0%, yielding a polyvinyl alcohol cast solution. The polyvinyl alcohol cast solution was defoamed using a twin-screw extruder, then discharged through a T-slit die and curtain-coated onto a rotating casting drum (surrounded by a hot air dryer). The preformed polyvinyl alcohol film was then peeled off from the casting drum. The polyvinyl alcohol preformed film was then dried using multiple temperature-differential heating rollers and a floating-type dryer, yielding a polyvinyl alcohol film. The maximum temperature of the hot air dryer (CAP) was 110°C. After peeling from the casting drum, the polyvinyl alcohol preformed film is brought into contact with a heated roller (CY) to dry both the top and bottom surfaces of the film, and the total temperature of the heated rollers is 839°C (the temperature of each individual heated roller is over 40°C). Furthermore, the maximum temperature of the floating dryer (FD) is 120°C, the difference in air speed between the top and bottom of the floating dryer is 0.4 m / s, the temperature difference between the top and bottom of the floating dryer is 2°C, and the total temperature difference between the east and west sides of the casting drum, heated roller, and floating dryer during the process (hereinafter referred to as the total east-west temperature difference) is 6.6°C. The detailed preparation conditions for Examples 1 to 7 are as shown in Table 1.

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

[0067] The same process as used to prepare the polyvinyl alcohol film in the previous example was used, but the maximum CAP temperature, total CY temperature, maximum FD temperature, total east-west temperature difference, upper and lower FD wind speed difference, and upper and lower FD temperature difference during the process were all different from those in the comparative example. The details of the preparation conditions are shown in Table 2.

[0068] 2. Analysis and measurement methods

[0069] The following provides the parameters and measurement methods for Examples 1 to 7 and Comparative Examples 1 to 6. Tables 1 and 2 provide the process conditions and parameters for the polyvinyl alcohol films of Examples 1 to 7 and Comparative Examples 1 to 6, as well as the evaluation results for the reduction in polyvinyl alcohol elution amount, permeation uniformity, and edge folding.

[0070] 2-1. Measurement method for weight swelling and standard deviation of weight swelling: A non-moisture-absorbed polyvinyl alcohol film was taken as a sample. The polyvinyl alcohol film was divided into five equal parts along the width direction, and the center of each polyvinyl alcohol film was cut. The cut area of ​​each piece was (MD 5cm x TD 5cm). Next, the polyvinyl alcohol film was placed in pure water at 30°C to swell for 30 seconds, 1 minute, and 3 minutes. After swelling was complete, the swollen polyvinyl alcohol film was removed and the surface moisture was wiped off. At this time, the weight (M1) was recorded. The polyvinyl alcohol film was then placed in a dryer and dried at 120°C for 120 minutes, and the weight (M2) after drying was recorded. The weight swelling was calculated as (M1 - M2) / M1.

[0071] Since five samples were measured at each different swelling time, the weight swelling degree for that swelling time was determined by taking the average of the five samples, and the standard deviation of the weight swelling degree for that swelling time could also be calculated from the five samples. Note that the standard deviation of the weight swelling degree in the examples and comparative examples was the maximum standard deviation within a specific time.

[0072] 2-2. Method for measuring the difference in moisture content between two surfaces: The polyvinyl alcohol film was divided into five equal parts along the width direction, and the center of each divided polyvinyl alcohol film was cut. The cut area of ​​each piece was (MD 5cm x TD 5cm). The film was placed directly on the FTIR-ATR, and the absorbance was measured at the middle position, and the moisture content was calculated using the formula: A 1654cm -1 / A 1090cm -1 Then, calculate the average moisture content of the five points on the X surface minus the average moisture content of the five points on the Y surface, and then take the absolute value to obtain the difference in moisture content between the two surfaces.

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

[0074] First, a solution with a known polyvinyl alcohol content was prepared using the method described above, and the 670 cm -1 The absorbance at each sample is measured to create a calibration curve, and the absorbance of the actual measurement solution is then applied to the calibration curve to obtain the amount of polyvinyl alcohol eluted. The amount of polyvinyl alcohol eluted is divided into three grades: a "◎" mark indicates an elution amount of ≦15.0 ppm, a "◯" mark indicates an elution amount of ≦30.0 ppm, and an "X" mark indicates an elution amount of >30.0 ppm.

[0075] 2-4. Measurement and Evaluation of Transmittance Uniformity: First, a polarizing film was prepared using polyvinyl alcohol film. Five samples were then cut evenly across the width of the polarizing film, each measuring 5 cm x 5 cm. The light transmittance was then measured using a Perkin-Elmer Lambda 750S spectrophotometer. Specifically, measurements were performed using a C-type light source in accordance with JIS Z 8722 (2009), with luminosity correction for a 2-degree visible light field. For each sample, the light transmittance was measured when tilted +45 degrees relative to the uniaxial stretching direction and when tilted -45 degrees relative to the uniaxial stretching direction. The average light transmittance (T1) (%) was calculated, and the maximum and minimum values ​​of the five single transmittances were found. The minus-maximum value was then divided into three categories. The mark "◎" indicates that the value of maximum transmittance - minimum transmittance is ≦0.10%, the mark "◯" indicates that the value of maximum transmittance - minimum transmittance is ≦0.50%, and the mark "X" indicates that the value of maximum transmittance - minimum transmittance is >0.50%.

[0076] 2-5. Measurement and Evaluation of Edge Folding: Please refer to Figure 3 again for this section. Freshly opened polyvinyl alcohol films were cut to a size of 10 cm in MD x 5 cm in TD, and the polyvinyl alcohol films were clamped at the center of both sides in the TD direction with 2.5 cm double clips. The films were then immersed in water and the maximum curl angle of the MD edge was observed within 60 seconds. Here, a "◎" mark indicates that the edge fold angle of the polyvinyl alcohol film was less than 90 degrees, a "◯" mark indicates that the edge fold angle of the polyvinyl alcohol film was less than 180 degrees, and an "X" mark indicates that the edge fold angle of the polyvinyl alcohol film was greater than 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 index within a 3-minute water immersion period was within the range of 2.4 ln (minutes of immersion time) + 39 and 4.3 ln (minutes of immersion time) + 44, the amount of polyvinyl alcohol elution was reduced. At the same time, when the standard deviation of the weight swelling index within a 3-minute water immersion period was ≦1.70, a polarizing film with good transmittance uniformity was obtained. Furthermore, when the difference in water content between the two surfaces of the polyvinyl alcohol film was ≦0.015, the folding angle at the end of the polyvinyl alcohol film was relatively small, thereby preventing excessively large folding angles and resulting in tearing of the polyvinyl alcohol film at the end.

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

[0080] From Examples 1 to 6 in Table 2, it can be seen that the effect of reducing the amount of polyvinyl alcohol eluted was poor when the weight swelling index within 3 minutes of water immersion was not between 2.4 ln (minutes of immersion time) + 39 and 4.3 ln (minutes of immersion time) + 44. Specifically, we believe that the weight swelling index within the above-mentioned 3 minutes of water immersion can be controlled by adjusting and controlling the maximum temperature of the hot air dryer, the total temperature of the heating rollers, and the maximum temperature of the floating-type dryer. Without being limited to a particular theory, the inventors discovered that when the maximum temperature of the hot air dryer, the total combined temperature of the heating rollers, and the maximum temperature of the floating-type dryer were excessively high (Comparative Examples 1, 3, and 5), the weight swelling degree after 3 minutes was too low (Comparative Example 1), the weight swelling degree after 30 seconds and 1 minute was too low (Comparative Example 3), and the weight swelling degree after 30 seconds, 1 minute, 2 minutes, and 3 minutes was too low (Comparative Example 5), resulting in an excessively low weight swelling degree of the polyvinyl alcohol film even if the amount of polyvinyl alcohol leaching was reduced. An excessively low weight swelling degree of the polyvinyl alcohol film is undesirable because it is prone to cracking at the edges when subjected to thermal shock after subsequent polarizing film production. Furthermore, if the maximum temperature of the hot air dryer, the total temperature of the heating rollers, and the maximum temperature of the floating-type 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 leaching.

[0081] Referring back to Table 2, if the standard deviation of the weight swelling index within the three-minute water immersion period is not ≦1.70 (Comparative Examples 1, 2, 3, and 5), the polarizing film produced thereby will have poor transmittance uniformity. Specifically, we believe that the standard deviation of the weight swelling index within the three-minute water immersion period can be controlled by adjusting and controlling the total temperature difference between the east and west sides. Without being limited to a particular theory, the inventors discovered that if the total temperature difference between the east and west sides is too high (Comparative Examples 1, 2, 3, and 5), the standard deviation of the weight swelling index of the polyvinyl alcohol film will be too high, resulting in poor transmittance uniformity in the subsequently produced polarizing film.

[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 to 5), the folding angle at the end of the polyvinyl alcohol film is relatively large, and an excessively large folding angle may cause tearing at the end of the polyvinyl alcohol film. Specifically, the inventors believe that the moisture content difference between the two sides can be controlled by adjusting and controlling the difference in air speed and temperature between the top and bottom of the floating-type dryer. Without being limited to a particular theory, the inventors discovered that when the difference in air speed between the top and bottom of the floating-type dryer is excessively large (Comparative Examples 1 to 4, 6) or the difference in temperature between the top and bottom is excessively large (Comparative Example 5), the moisture content difference between the two sides of the polyvinyl alcohol film becomes excessively high, resulting in poor folding performance at the end.

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

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

[0085] As can be seen from Table 3, the weight swelling degree of the polyvinyl alcohol film increased as the swelling time increased, but the effect of reducing polyvinyl alcohol remained in both cases.

[0086] In summary, the polyvinyl alcohol film of the present invention not only alleviates 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 folding angle at the edge, which prevents the edge from being easily torn due to an excessively large folding angle.

[0087] 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.

[0088] 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.

[0089] 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]

[0090] 100 devices 110 T-type slit die lip 120 Casting Drum 130 Hot air dryer 140 Heating roller 150 Floating Dryer 160 Temperature and humidity controller 170 Polyvinyl alcohol casting solution 200 Polyvinyl alcohol film edge fold measuring device 210 Polyvinyl alcohol film 220 water

Claims

1. A polyvinyl alcohol film, the polyvinyl alcohol film having a weight swelling degree within a water immersion time of 3 minutes: The formula is satisfied: 2.4 ln (minutes of immersion time) + 39 < weight swelling value < 4.3 ln (minutes of immersion time) + 44; and The polyvinyl alcohol film has a standard deviation of weight swelling within a 3-minute water immersion period measured at multiple positions of ≦1.

70.

2. The polyvinyl alcohol film has two opposing surfaces, designated as an X surface and a Y surface, and the difference in moisture content between the two surfaces is ≦0.015, wherein the difference in moisture content between the two surfaces is measured at multiple locations on the polyvinyl alcohol film using FTIR-ATR and is determined by the following formula: |X side (A １６５４ｃｍ －１ / A １０９０ｃｍ －１ ) average value - Y surface (A １６５４ｃｍ －１ / A １０９０ｃｍ －１ 2. The polyvinyl alcohol film according to claim 1, wherein the average value of the polyvinyl alcohol content is 1.

0.

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

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

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

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

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

8. The area is 25 cm ２ 8. The polyvinyl alcohol film according to claim 1, wherein, when the polyvinyl alcohol film is immersed in 80 mL of water at 50°C 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.

9. The polyvinyl alcohol film according to claim 8, wherein the amount of polyvinyl alcohol eluted in 20 mL of the impregnation solution is ≦15.0 ppm.

10. 8. The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol film has an area of ​​10 cm in the machine direction (MD) × 5 cm in the transverse direction (TD), and when the film is sandwiched at the center of both ends in the width direction and immersed in water for 60 seconds, the folding angle of the machine direction end is less than 180 degrees.

11. 11. The polyvinyl alcohol film according to claim 10, wherein the fold angle of the machine direction edge is less than 90 degrees.

12. An optical film formed from the polyvinyl alcohol film according to any one of claims 1 to 11.

13. A polarizing film formed from the polyvinyl alcohol film according to any one of claims 1 to 11.

14. The polarizing film according to claim 13, wherein, when a light transmittance T1% when tilted at +45 degrees with respect to the stretching direction of uniaxial stretching and a light transmittance T2% when tilted at −45 degrees with respect to the stretching direction of uniaxial stretching are measured at a plurality of positions on the polarizing film, the maximum value−minimum value of the average values ​​of T1% and T2% at each position is ≦0.50%.

15. The polarizing film according to claim 14, wherein the maximum value−minimum value of the average value is ≦0.10%.

16. A method for producing the polyvinyl alcohol film according to any one of claims 1 to 11, (a) a dissolving step in which a polyvinyl alcohol resin is added to a solvent and a plasticizer, and then the temperature is raised to 110°C to 150°C to dissolve the resin, thereby forming a polyvinyl alcohol cast solution, and the viscosity of the polyvinyl alcohol cast solution when the curable content is 4% is 60 cps to 120 cps; (b) a casting step in which the polyvinyl alcohol cast solution is cast onto a casting drum, the casting drum is surrounded by a hot air dryer, and the maximum temperature of the hot air dryer is 80 to 140°C, and the polyvinyl alcohol preformed film is peeled off from the casting drum; and (c) a drying step of drying the polyvinyl alcohol preformed film with a plurality of heating rollers having different temperatures and in a floating-type dryer, respectively, to obtain the polyvinyl alcohol film.

17. The manufacturing method according to claim 16, wherein the total temperature of the plurality of heating rollers having different temperatures is 700 to 1050°C.

18. The manufacturing method according to claim 17, wherein the temperature of each of the heating rollers is higher than 40°C.

19. The method according to claim 18, wherein the maximum temperature of the floating-type dryer is 105 to 130°C.

20. 20. The method according to claim 19, wherein the air speed difference between the upper and lower sides of the floating-type dryer is ≦1.0 m / s, and the temperature difference between the upper and lower sides of the floating-type dryer is ≦5°C.

21. The manufacturing method according to claim 20, wherein the sum of the temperature differences between the east and west sides of the casting drum, the heating roller, and the floating-type dryer is ≦10° C.

Images