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

Abstract

The invention provides a polyvinyl alcohol film which is not easy to dissolve out and uniform in transmittance. The weight swelling degree value of the polyvinyl alcohol film within 3 minutes of water soaking time meets the following formula: 2.4 ln (minutes of water soaking time) + 39 lt; the weight swelling degree numerical value is lt; 4.3 ln (the number of minutes of soaking time) + 44; in addition, the standard deviation of the weight swelling degree of the polyvinyl alcohol film within 3 minutes of soaking time measured at a plurality of positions is less than or equal to 1.70. By controlling the weight swelling degree of the polyvinyl alcohol film in the fixed water soaking time, the situation that the dissolution amount is too high or the transmittance is not uniform when the polyvinyl alcohol film is made into a polarizing film can be avoided.

Description

Technical Field

[0001] This invention relates to a polyvinyl alcohol (PVA) film, and more particularly to a polyvinyl alcohol film that is not easily dissolved and has uniform transmittance, an optical film comprising the same, and a method for manufacturing the same; however, this invention is not limited thereto. Background Technology

[0002] Polyvinyl alcohol (PVA) film is a hydrophilic polymer obtained by coating and drying an aqueous solution containing polyvinyl alcohol and plasticizers. It has properties such as transparency, mechanical strength, water solubility and good processability, and therefore has been widely used in packaging materials or optical films of electronic products, especially polarizing films.

[0003] Polarizing films, obtained by processing polyvinyl alcohol (PVA) films through a polarization process, possess the characteristic of allowing light to pass through only in specific directions, thereby controlling the brightness of the transmitted light. Based on this characteristic, polarizing films are used in various displays, eyeglasses, and wearable devices. A good polarizing film exhibits characteristics such as uniform color, few color spots, and no wrinkles, providing superior optical properties. To improve the optical properties of polarizing films, existing technologies modify the PVA structure or add functional groups (such as cationic groups) to alter viscosity or saponification, thereby enhancing the film's performance. Summary of the Invention

[0004] However, during the manufacturing process of optical films, there is sometimes a problem of excessive polyvinyl alcohol leaching, which can lead to contamination or foreign matter in the final product, resulting in it being deemed a defective product. In addition, when the degree of entanglement of polyvinyl alcohol molecules in different blocks within the film is not uniform, it may result in uneven transmittance in the optical films produced later.

[0005] Based on the above problems, and without being limited by any specific theory, the inventors have discovered that when the weight swelling value of the polyvinyl alcohol film is controlled within a specific range during a specific soaking time (i.e., swelling time), the problem of excessive polyvinyl alcohol dissolution can be improved. Furthermore, the inventors have also discovered that when the standard deviation of the weight swelling of the polyvinyl alcohol film is controlled within a specific range during a specific soaking time, the uneven transmittance of the optical film can be improved.

[0006] Therefore, the present invention provides a polyvinyl alcohol film whose weight swelling value within a 3-minute soaking time conforms to the following formula: 2.4ln(soaking time in minutes) + 39 < weight swelling value < 4.3ln(soaking time in minutes) + 44; and the standard deviation of the weight swelling of the polyvinyl alcohol film within a 3-minute soaking 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 sides, namely an X side and a Y side, and the difference in moisture content between the two sides is ≤0.015; wherein, the difference in moisture content between the two sides is measured at multiple locations of the polyvinyl alcohol film using FTIR-ATR and calculated by the following formula: |X side (A 1654cm -1 / A 1090cm -1 Average value - Y-plane (A) 1654cm -1 / A 1090cm -1 Average value |.

[0008] According to one embodiment of the invention, the polyvinyl alcohol film has a weight swelling of 37.3 wt% to 41.0 wt% when immersed in water for 30 seconds.

[0009] According to one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 39.0 wt% to 44.0 wt% after soaking in water for 1 minute.

[0010] According to one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 40.7 wt% to 47.0 wt% after soaking in water for 2 minutes.

[0011] According to one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 41.6 wt% to 48.7 wt% when soaked in water for 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, the polyvinyl alcohol film has an area of ​​25 cm². 2 An impregnation solution was obtained by immersing the sample in 80 mL of water at 50 °C for 1 minute. The amount of polyvinyl alcohol dissolved in 20 mL of this impregnation solution was ≤30.0 ppm.

[0014] According to one embodiment of the present invention, the amount of polyvinyl alcohol dissolved 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 ​​10cm in the machine direction (MD) × 5cm in the transverse direction (TD). After being clamped at the transverse center of both ends and soaked in water for 60 seconds, the folding angle of its machine direction edge is less than 180 degrees.

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

[0017] Another object of the present invention is to provide an optical film formed from the polyvinyl alcohol film as described above.

[0018] Another object of the present invention is to provide a polarizing film formed from the polyvinyl alcohol film as described above.

[0019] According to one embodiment of the present invention, the light transmittance T1 when the polarizing film is tilted at +45 degrees relative to the stretching direction of uniaxial stretching, and the light transmittance T2 when the film is tilted at -45 degrees relative to the stretching direction of uniaxial stretching, are measured at multiple locations. The maximum value minus the minimum value of the average values ​​of T1% and T2% at each location is ≤0.50%.

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

[0021] Another object of the present invention is to provide a method for manufacturing a polyvinyl alcohol film as described above, comprising: (a) Dissolution process: Polyvinyl alcohol resin is added to solvent and plasticizer and then heated to 110°C to 150°C to dissolve, forming a polyvinyl alcohol casting solution. The viscosity of the polyvinyl alcohol casting solution at 4% solids is 60 to 120 cps. (b) Casting process: The polyvinyl alcohol casting solution is cast into a casting drum, and a hot air dryer is provided around the casting drum, the maximum temperature of which is 80 to 140°C; a polyvinyl alcohol preform is obtained after peeling off from the casting drum; and (c) Drying process: The polyvinyl alcohol preform is dried in multiple hot rollers with temperature differences and a floating oven to obtain the polyvinyl alcohol film.

[0022] According to one embodiment of the present invention, the plurality of heated rollers with temperature differences have a total temperature of 700 to 1050°C.

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

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

[0025] According to one embodiment of the present invention, the wind speed difference between the upper and lower sides of the floating oven is ≤1.0m / s, and the temperature difference between the upper and lower sides of the floating oven is ≤5℃.

[0026] According to one embodiment of the invention, the sum of the temperature differences between the east and west sides of the casting drum, the hot roller, and the floating oven is ≤10°C.

[0027] The advantage of the polyvinyl alcohol film of the present invention is that it not only avoids the problem of excessive polyvinyl alcohol leaching, but also improves the uneven transmittance of the optical film produced in the subsequent process. Attached Figure Description

[0028] Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein:

[0029] Figure 1 The following are the weight swelling results of a polyvinyl alcohol film according to an embodiment of the present invention at different times;

[0030] Figure 2 This is a schematic diagram of the casting and drying process of a polyvinyl alcohol film according to an embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram of the edge folding measurement of a polyvinyl alcohol film according to an embodiment of the present invention;

[0032] It should be understood that various aspects of the present invention are not limited to the configurations, means and features shown in the accompanying drawings. Detailed Implementation

[0033] As is customary practice, the various features and components in the figures are not drawn to scale, but rather in a manner designed to best represent the specific features and components relevant to this invention. Furthermore, similar components and parts are referred to by the same or similar component symbols across different figures.

[0034] The following embodiments should not be considered as unduly limiting the present invention. Those skilled in the art can make modifications and variations to the embodiments discussed herein without departing from the spirit or scope of the present invention, and such modifications and variations shall still fall within the scope of the present invention.

[0035] In this document, unless the context otherwise requires, the terms “comprising,” “including,” “having,” or “containing” are inclusive or open-ended and do not exclude other elements or method steps not set forth. The terms “a” and “the” can be interpreted as either singular or plural. The term “one or more” means “at least one,” and therefore can include a single feature or a mixture / combination. Furthermore, in this specification and claims, unless otherwise stated, “deposited on something” can be considered as direct or indirect contact with a surface of something, whether attached or otherwise, and the definition of that surface should be determined in accordance with the context of the preceding / following paragraphs and common general knowledge in the art to which this specification pertains.

[0036] Polyvinyl alcohol film

[0037] The present invention provides a polyvinyl alcohol film, wherein the weight swelling value of the polyvinyl alcohol film within a soaking time of 3 minutes conforms to the following formula: 2.4ln(soaking time in minutes) + 39 < weight swelling value < 4.3ln(soaking time in minutes) + 44; and the standard deviation of the weight swelling of the polyvinyl alcohol film within a soaking time of 3 minutes measured at multiple locations is ≤1.70.

[0038] The "weight swelling" mentioned in this article refers to the degree to which water molecules can penetrate the polyvinyl alcohol (PVA) membrane. The determination procedure for "weight swelling" is roughly as follows: a sample of the PVA membrane is taken in a non-hygroscopic state and then placed in water to swell (or soak in water). Next, the PVA membrane is removed, the surface moisture is wiped off, and its weight is weighed as M1. Then, the PVA membrane is placed in an oven to dry and its weight is weighed as M2. The weight swelling (wt%) can then be calculated by (M1-M2) / M1.

[0039] Furthermore, the "weight swelling value within 3 minutes of soaking time" mentioned in this article refers to the weight swelling value obtained within 3 minutes of soaking time (i.e., swelling time) in the above steps; and the so-called "value," as used in this article, refers to the numerical part of the weight swelling, for example: when the weight swelling is 35wt%, its weight swelling value is 35. Also, as... Figure 1 As shown, the inventors discovered that the weight swelling degree exhibits a logarithmic change with respect to swelling time. Furthermore, the weight swelling degree value falls within a specific range with varying swelling time, with a lower limit of 2.4ln(A) + 39 and an upper limit of 4.3ln(A) + 44, where A represents the soaking time in minutes. It should be understood by those skilled in the art that the soaking time in minutes refers to the numerical part of the soaking time; for example, a soaking time of 3 minutes is represented by the soaking time in minutes of 3. Not limited by any specific theory, the inventors also discovered that when the weight swelling degree value is controlled within this specific range for a soaking time of 3 minutes, the problem of excessive polyvinyl alcohol dissolution can be effectively improved.

[0040] Furthermore, the "standard deviation of weight swelling within 3 minutes of soaking time" mentioned in this article is calculated based on the differences in the drying process of the polyvinyl alcohol film in the width direction, and is therefore calculated for the weight swelling of different positions (blocks) of the film in the width direction at the same time point; more specifically, this invention uses the maximum standard deviation within a specific time period. Therefore, the "multiple positions" mentioned in this article refer to multiple different positions obtained in the width direction of the polyvinyl alcohol film; however, the size and spacing of each position are not limited by this invention. Moreover, not limited by a specific theory, the inventors have discovered that when the standard deviation of weight swelling within 3 minutes of soaking time is too large, it may indicate that the degree of entanglement of polyvinyl alcohol molecules at different positions is different, leading to uneven monomer transmittance in different blocks during the manufacture of the polarizing film. In other words, when the standard deviation of weight swelling within 3 minutes of soaking time of the polyvinyl alcohol film is controlled within a specific range, the uneven transmittance of the polarizing film can be improved. Specifically, the specific range is ≤1.70, preferably 0.65 to 1.70, for example, but 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.

[0041] According to at least one embodiment of the present invention, the polyvinyl alcohol film has two opposing sides, namely an X side and a Y side, and the difference in moisture content between the two sides is ≤0.015; wherein, the difference in moisture content between the two sides is measured at multiple locations on the polyvinyl alcohol film using FTIR-ATR and calculated by the following formula: |X side (A 1654cm -1 / A 1090cm -1 Average value - Y-plane (A) 1654cm -1 / A 1090cm -1 Average value |.

[0042] In detail, the "opposite sides" mentioned herein refer to the opposite front and back sides of the membrane (which may also be top / bottom or front / back; here, it is only illustratively referred to as X / Y sides). Furthermore, the "difference in moisture content between the two sides" mentioned in this invention is determined by taking multiple locations on both the X and Y sides of a polyvinyl alcohol membrane sample and measuring the moisture content at each location using FTIR-ATR (A... 1654cm -1 / A 1090cm -1 ), where A 1654cm -1It is the peak value of the deformation vibration of water, while A 1090cm -1 It is the peak value of (CO) stretching vibration; subsequently, the average value of the moisture content at these multiple locations can be calculated using the following formula to obtain the moisture content difference between the two surfaces: |X surface (A 1654cm -1 / A 1090cm -1 Average value - Y-plane (A) 1654cm -1 / A 1090cm -1 Average value. The inventors have discovered that when the difference in moisture content between the two sides is too large, it may indicate that the swelling rate of water molecules entering the polyvinyl alcohol film is inconsistent. In this case, the side with lower moisture content will curl towards the side with higher moisture content, resulting in film folding. Furthermore, when manufacturing polarizing films, if the two ends of the film curl significantly after immersion in water, folding may occur at the edges, resulting in indentations. If the indentations are too obvious, film breakage may occur. Therefore, without being limited by a specific theory, controlling the difference in moisture content between the two sides within a specific range can improve the folding of the polyvinyl alcohol film edges. Specifically, the specific range is ≤0.015, preferably 0.002 to 0.015, for example, but not limited to: 0.002, 0.004, 0.006, 0.008, 0.010, 0.012, 0.014, or 0.015.

[0043] According to at least one embodiment of the present invention, the thickness of the polyvinyl alcohol film is 45 to 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.

[0044] According to at least one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 37.3 wt% to 41.0 wt% after immersion in water for 30 seconds, 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%.

[0045] According to at least one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 39.0 wt% to 44.0 wt% after soaking in water for 1 minute, for example, but not limited to: 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, 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%.

[0046] According to at least one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 40.7 wt% to 47.0 wt% after soaking in water for 2 minutes, for example, but not limited to: 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, 4 3.3, 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%.

[0047] According to at least one embodiment of the present invention, the polyvinyl alcohol film has a weight swelling of 41.6 wt% to 48.7 wt% after soaking in water for 3 minutes, for example, but not limited to: 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, 4 4.6, 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%.

[0048] Optical film

[0049] In another aspect, the present invention provides an optical film formed from a polyvinyl alcohol film as described above. This optical film includes, but is not limited to, polarizing films, anti-blue light films, and filters.

[0050] Polarizing film

[0051] In another aspect, the present invention provides a polarizing film formed from a polyvinyl alcohol film as described above. In a preferred embodiment, a method for manufacturing a polarizing film (or polarizing plate) includes the following steps: swelling, dyeing, stretching, color matching, drying, and laminating the polyvinyl alcohol film with a triacetate cellulose film (TAC); more specifically, the polarizing film manufacturing process can be carried out as follows: 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, while immersing it in an aqueous solution at 30°C containing 0.03% by mass of iodine and 3% by mass of potassium iodide, stretching the polyvinyl alcohol film to 3.3 times its original length; then immersing it in an aqueous solution at 30°C containing 3% by mass of potassium iodide and 3% by mass of boric acid, and further stretching it to 3.6 times its original length; subsequently immersing it in an aqueous solution at 60°C containing 5% by mass of potassium iodide and 4% by mass of boric acid, and further stretching it to 6.0 times its original length. After soaking in a 3% potassium iodide aqueous solution for 15 seconds, the film is dried at 60°C for 4 minutes to obtain a polarizing film.

[0052] Method for manufacturing polyvinyl alcohol film

[0053] Another aspect of the present invention provides a method for manufacturing a polyvinyl alcohol film as described above, comprising: (a) a dissolution process: adding a polyvinyl alcohol resin to a solvent and a plasticizer and heating the solution to 110°C to 150°C to dissolve it, forming a polyvinyl alcohol casting solution, wherein the viscosity of the polyvinyl alcohol casting solution is 60 cps to 120 cps at 4% solids; (b) a casting process: casting the polyvinyl alcohol casting solution into a casting drum, wherein a hot air dryer is provided around the casting drum, the maximum temperature of the hot air dryer being 140°C; obtaining a polyvinyl alcohol preform film after peeling it off from the casting drum; and (c) a drying process: drying the polyvinyl alcohol preform film in a plurality of hot rollers with temperature differences and a floating oven to obtain the polyvinyl alcohol film.

[0054] In detail, the polyvinyl alcohol resin is obtained by polymerizing vinyl ester-based resin monomers, followed by a saponification reaction after forming the polyvinyl ester-based resin. The vinyl ester-based resin monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, or vinyl octanoate, etc., and the invention is not limited to these. Furthermore, copolymers formed by copolymerizing olefin compounds or acrylate derivatives with the aforementioned vinyl ester-based resin monomers can also be used, wherein the olefin compounds include ethylene, propylene, or butene, etc., and the invention is not limited to these. The acrylate derivatives include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, or n-butyl acrylate, etc., and the invention is not limited to these.

[0055] The plasticizers (or plasticizers) described herein can increase the softness of materials or liquefy materials, and include, but are not limited to: phthalates, 2-ethylhexyl phthalate (DEHP), glycerin, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butylparaben (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, or trimethylolpropane, etc.; preferably, the plasticizer is glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, or trimethylolpropane.

[0056] According to at least one embodiment of the present invention, (a) in the dissolution process, the heating temperature is, 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. Additionally, the viscosity of the polyvinyl alcohol casting solution at 4% solids content is, 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.

[0057] Regarding (b) the casting process and (c) the drying process, please refer to the following: Figure 2 In apparatus 100, the polyvinyl alcohol casting solution 170, after being formed, is ejected from the T-shaped die lip 100 and cast into the casting drum 120. It then sequentially passes through multiple temperature-differential hot rollers (CY Rollers, CY) 140 and a floating dryer (FD) 150 for drying. Finally, it enters a temperature and humidity regulator 160 for adjustment and winding to obtain a polyvinyl alcohol film. Specifically, a hot air dryer (CAP) 130 is provided around the casting drum 120. Furthermore, the number of the multiple hot rollers 140 or the number of sections of the floating dryer 150 are not limited by this application. Figure 2 Used only as an example to present the content.

[0058] Without being limited by any specific theory, the inventors of this case discovered that the weight swelling of the polyvinyl alcohol film can be controlled during the manufacturing process by adjusting the temperature of a specific process segment. Specifically, by adjusting the maximum temperature of the hot air dryer 130, the sum of the temperatures of the multiple hot rollers 140, and the maximum temperature of the floating oven 150 within a specific range, the weight swelling value of the polyvinyl alcohol film during the immersion time of 3 minutes 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, such as, 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 sum of the temperatures of the plurality of hot rollers 140 is 700 to 1050°C, such as, but not limited to: 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, etc. The temperatures range from 1036°C, 1037°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; preferably, the temperatures are between 722°C and 1008°C, including, 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 invention, the temperature of a single heated roller 140 needs to be greater than 40°C. According to at least one embodiment of the present invention, the maximum temperature of the floating oven 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.

[0059] Without being limited by any specific theory, the inventors of this case discovered that the standard deviation of the weight swelling of the polyvinyl alcohol film can be controlled during the manufacturing process by adjusting the temperature difference between the east and west sides of the casting drum 120, the hot roller 140, and the floating oven 150. Specifically, by adjusting the sum of the east and west side temperature differences of the casting drum 120, the hot roller 140, and the floating oven 150 within a specific range, the standard deviation of the weight swelling of the polyvinyl alcohol film during a 3-minute soaking time can be kept within an ideal range. Here, "east and west sides" as used herein refers to both sides of the device along the machine direction (MD), 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 and west side temperature differences of the casting drum 120, the hot roller 140, and the floating oven 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.

[0060] Without being limited by any specific theory, the inventors of this case discovered that the moisture content difference between the two sides of the polyvinyl alcohol film can be controlled by adjusting the difference in air velocity and temperature between the upper and lower sides of the floating oven 150 during the manufacturing process. Specifically, by controlling both the difference in air velocity and temperature between the upper and lower sides of the floating oven 150 within a specific range, the moisture content difference between the two sides can be further kept within a specific range. According to at least one embodiment of the present invention, the wind speed difference between the upper and lower sides of the floating oven 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 temperature difference between the upper and lower sides of the floating oven 150 is ≤5℃, for example, but not limited to: 5℃, 4.5℃, 4.2℃, 4℃, 3.7℃, 3.5℃, 3.1℃, 2.9℃, 2.4℃, 2℃, 1.6℃, 1.3℃, 1.0℃ or 0.5℃.

[0061] According to at least one embodiment of the present invention, the polyvinyl alcohol film is 25 cm 2 An impregnation solution is obtained by immersing the surface area of ​​the sample in 80 mL of water at 50°C for 1 minute. The amount of polyvinyl alcohol dissolved in 20 mL of this 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 dissolved 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.

[0062] Please see further. Figure 3 The polyvinyl alcohol film edge refraction measuring device 200 shown, according to at least one embodiment of the present invention, has a polyvinyl alcohol film 210 with an area of ​​10cm in the machine direction (MD) × 5cm in the transverse direction (TD). After being clamped at the transverse center of both ends, it is immersed in water 220 and swelled in water 220 for 60 seconds. The refraction angle of its machine direction edge 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 mechanical folding angle is less than 90 degrees, for example, 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 "clamping" described in this invention is used to fix and tension the polyvinyl alcohol film 210, which can be accomplished by any suitable method known in the art to which this invention pertains.

[0063] According to at least one embodiment of the present invention, the light transmittance T1 when the polarizing film is tilted at +45 degrees relative to the stretching direction of uniaxial stretching, and the light transmittance T2 when it is tilted at -45 degrees relative to the stretching direction of uniaxial stretching, are measured at multiple locations. The maximum value minus the minimum value of the average values ​​of T1% and T2% at each location 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%.

[0064] Example

[0065] The invention will be further described below with detailed descriptions and embodiments. However, it should be understood that these embodiments are only for the purpose of helping to more readily understand the invention and are not intended to limit the scope of the invention.

[0066] The following provides a non-limiting method for preparing polyvinyl alcohol (PVA) films. Non-limiting examples of PVA films (Examples 1-7) and comparative examples of PVA films (Comparative Examples 1-6) were prepared according to the same or similar methods.

[0067] 1-1. Example: Manufacturing a polyvinyl alcohol film

[0068] 1800 kg of polyvinyl alcohol resin with an alkalinity >99.0% and a viscosity of 80 cps, 4000 kg of water, and 200 kg of plasticizer glycerin were added to a dissolving tank. The mixture was stirred and heated to 140°C for dissolution. After homogenization using a mixer, water was added to adjust the resin concentration to 30.0%, yielding a polyvinyl alcohol casting solution. This polyvinyl alcohol casting solution was defoamed by a twin-screw extruder and then extruded from a T-slit die lip onto a rotating casting drum (with a hot air dryer around its perimeter). Peeling off from the casting drum yielded a preliminary polyvinyl alcohol film. This preliminary polyvinyl alcohol film was then dried in multiple hot rollers with temperature differences and in a floating oven to obtain the final polyvinyl alcohol film. The highest temperature of the hot air dryer (CAP) is 110°C. The polyvinyl alcohol pre-formed film, after being peeled from the casting drum, contacts both the upper and lower surfaces of the drying film via hot rollers (CY), with the total temperature of the hot rollers reaching 839°C (each individual hot roller temperature is greater than 40°C). The highest temperature of the floating oven (FD) is 120°C, the vertical air velocity difference within the floating oven is 0.4 m / s, and the vertical temperature difference within the floating oven is 2°C. The total east-west temperature difference of the casting drum, hot rollers, and floating oven (hereinafter referred to as the total east-west temperature difference) during the process is 6.6°C. Further preparation conditions for Examples 1 to 7 are detailed in Table 1.

[0069] 1-2. Comparative Example: Manufacturing Polyvinyl Alcohol Film

[0070] The same steps as those used in the previous embodiments were employed to manufacture the polyvinyl alcohol film. However, the comparative example differed from the embodiments in terms of the highest CAP temperature, the sum of CY temperatures, the highest FD temperature, the sum of the temperature difference between the east and west sides, the difference in wind speed between the upper and lower sides of FD, and the difference in temperature between the upper and lower sides of FD. The preparation conditions are detailed in Table 2.

[0071] 2. Analytical and Determination Methods

[0072] The following provides measurement methods for the parameters and properties of Examples 1-7 and Comparative Examples 1-6. Tables 1 and 2 provide the process conditions and parameters of the polyvinyl alcohol films of Examples 1-7 and Comparative Examples 1-6, as well as the evaluation results for reducing polyvinyl alcohol leaching, transmission uniformity, and edge reflection.

[0073] 2-1. Measurement method of weight swelling and standard deviation of weight swelling: Take a sample of non-hygroscopic polyvinyl alcohol (PVA) film, divide the PVA film into 5 equal parts along the width direction, and cut the PVA film at the center of each part. The cut area of ​​each piece is (MD 5cm * TD 5cm). Then, place it in pure water at 30℃ for swelling for 30 seconds, 1 minute, 2 minutes or 3 minutes. After swelling is completed, take out the swollen PVA film and wipe off the surface moisture. Record the weight at this time (M1). Place the PVA film in an oven at 120℃ for 120 minutes and record the weight after drying (M2). The weight swelling is calculated as (M1-M2) / M1.

[0074] Since five samples were tested for different swelling times, the average value of the five samples is the weight swelling at that swelling time. The standard deviation of the weight swelling at that swelling time can also be calculated from the five samples. It should be noted that the standard deviation of the weight swelling in the examples and comparative examples is the maximum value of the standard deviation over a specific time period.

[0075] 2-2. Method for measuring the difference in moisture content between the two sides: Divide the polyvinyl alcohol film into 5 equal parts along its width, and cut the film in the middle of each part. The cut area of ​​each piece is (MD 5cm * TD 5cm). Place the film directly on the FTIR-ATR and measure the absorbance at the middle position. Substitute this value into the formula for calculating the moisture content: A 1654cm -1 / A 1090cm -1 Next, calculate the average moisture content of 5 points on the X side minus the average moisture content of 5 points on the Y side, and then take the absolute value to obtain the difference in moisture content between the two sides.

[0076] 2-3. Determination and evaluation of polyvinyl alcohol dissolution: a. Take a 5cm*5cm polyvinyl alcohol film and immerse it in 80mL of water at 50℃ for one minute (in a serum bottle), then remove the immersion solution; mix 20mL of the immersion solution with 15mL of 4% boric acid solution, 3mL of iodine / potassium iodide solution, and 12mL of pure water solution. b. Mix 35 mL of pure water with 15 mL of 4% boric acid solution and 3 mL of iodine / potassium iodide solution to prepare a standard solution.

[0077] First, prepare a solution with a known polyvinyl alcohol content using the method described above, and then test its concentration at 670 cm⁻¹. -1The absorbance is used to establish a calibration curve. The absorbance of the actual test solution is then applied to the calibration curve to obtain the polyvinyl alcohol (PVA) dissolution rate. Here, PVA dissolution rates are divided into three levels: "◎" indicates dissolution rate ≤ 15.0 ppm; "O" indicates dissolution rate ≤ 30.0 ppm; and "X" indicates dissolution rate > 30.0 ppm.

[0078] 2-4. Measurement and Evaluation of Transmission Uniformity: First, a polarizing film was prepared using polyvinyl alcohol film. Then, five samples were taken from the width of the polarizing film, each sample being a 5cm × 5cm square. Next, the light transmittance was measured using a Perkin Elmer Lambda 750S spectrophotometer. Specifically, the measurement was performed according to JIS Z 8722 (2009), using a C light source, and visual sensitivity correction was performed in the visible light region with a 2-degree field of view. For each sample, the light transmittance at a +45° tilt relative to the uniaxial stretching direction and at a -45° tilt relative to the uniaxial stretching direction were measured, and the average value (T1) (%) was calculated. Then, the maximum and minimum values ​​of the five individual transmittances were identified, and the maximum-minimum value was divided into three levels: those marked "◎" represent maximum transmittance minus minimum transmittance ≤ 0.10%; those marked "O" represent maximum transmittance minus minimum transmittance ≤ 0.50%; and those marked "X" represent maximum transmittance minus minimum transmittance > 0.50%.

[0079] 2-5. Measurement and evaluation of edge folding: Please refer to section 2-5 for details. Figure 3 Cut freshly unrolled polyvinyl alcohol (PVA) film into pieces measuring 10cm x 5cm (MD). Clamp the film at the center of the TD direction on both sides with 2.5cm long binder clips, and immerse it in water. Observe the maximum curl angle of the edge along the MD direction within 60 seconds. Those marked with "◎" represent a folding angle <90 degrees; those marked with "O" represent a folding angle <180 degrees; and those marked with "X" represent a folding angle >180 degrees.

[0080] Table 1 (Continued from Table 1)

[0081] As can be seen from Examples 1 to 7 in Table 1, when the weight swelling value within a 3-minute soaking time is within 2.4ln(soaking time in minutes) + 39 and 4.3ln(soaking time in minutes) + 44, the effect of reducing polyvinyl alcohol leaching can be obtained. Simultaneously, when the standard deviation of weight swelling within a 3-minute soaking time is ≤1.70, a polarizing film with better transmission uniformity can be obtained. Furthermore, when the difference in moisture content between the two sides of the polyvinyl alcohol film is ≤0.015, the edge folding angle of the polyvinyl alcohol film is smaller, thereby avoiding the situation where the edge of the polyvinyl alcohol film breaks due to an excessively large folding angle.

[0082] Table 2 (Continued from Table 2)

[0083] As can be seen from Examples 1 to 6 in Table 2, when the weight swelling value within a 3-minute soaking time is not between 2.4ln(soaking time in minutes) + 39 and 4.3ln(soaking time in minutes) + 44, the effect of reducing polyvinyl alcohol dissolution is not good. Specifically, the weight swelling value within a 3-minute soaking time is believed to be controlled by adjusting the maximum temperature of the hot air dryer, the sum of the temperatures of the hot rollers, and the maximum temperature of the floating oven. Not limited by any specific theory, the inventors have discovered that when the maximum temperature of the hot air dryer, the total temperature of the hot rollers, or the maximum temperature of the floating oven is too high (as in Comparative Examples 1, 3, and 5), the resulting weight swelling is excessively low at 3 minutes (as in Comparative Example 1), excessively low at 30 seconds and 1 minute (as in Comparative Example 3), and excessively low at 30 seconds, 1 minute, 2 minutes, and 3 minutes (as in Comparative Example 5). This results in a reduction in polyvinyl alcohol (PVA) dissolution, but the PVA film's weight swelling will be too low. This low weight swelling will cause the edges of the PVA film to crack easily after being processed into polarizing films and subjected to thermal shock, leading to poor quality. Conversely, when the maximum temperature of the hot air dryer, the total temperature of the hot rollers, or the floating oven is too low (as in Comparative Examples 2, 4, and 6), the obtained PVA film has too high a weight swelling, resulting in excessive PVA dissolution.

[0084] Referring again to Table 2, it can be seen that when the standard deviation of weight swelling within 3 minutes of soaking in water is not ≤1.70 (as in Comparative Examples 1, 2, 3, and 5), the transmission uniformity of the polarizing film produced is poor. Specifically, the standard deviation of weight swelling within 3 minutes of soaking in water is believed to be controlled by adjusting the sum of the temperature differences between the east and west sides. Not limited by any particular theory, the inventors have found that when the sum of the temperature differences between the east and west sides is too high (as in Comparative Examples 1, 2, 3, and 5), the standard deviation of weight swelling of the polyvinyl alcohol film will be too high, which in turn leads to poor transmission uniformity of the subsequently produced polarizing film.

[0085] Referring again to Table 2, it can be seen that when the moisture content difference between the two sides of the polyvinyl alcohol film is not ≤0.015 (as in Comparative Examples 1 to 5), the edge folding angle of the polyvinyl alcohol film is relatively large, which may lead to excessive folding angle and film breakage at the edge of the polyvinyl alcohol film. Specifically, the moisture content difference between the two sides is believed to be controlled by adjusting the difference in vertical air velocity and the difference in vertical temperature of the floating oven. Not limited by a specific theory, the inventors found that when the difference in vertical air velocity of the floating oven is too high (Comparative Examples 1 to 4, 6), or the difference in vertical temperature is too high (Comparative Example 5), the moisture content difference between the two sides of the polyvinyl alcohol film is too high, resulting in poor edge folding performance.

[0086] The present invention further increases the swelling time of the polyvinyl alcohol films of Examples 1 to 7 to 30 minutes, 60 minutes and 240 minutes, respectively, and the results are detailed in Table 3.

[0087] Table 3 (Continued from Table 3)

[0088] As shown in Table 3, the weight swelling of the polyvinyl alcohol film increases with the increase of swelling time, but the polyvinyl alcohol content is still reduced.

[0089] In summary, the polyvinyl alcohol film of the present invention not only improves the problem of high polyvinyl alcohol leaching, but also enables the subsequent fabrication of a polarizing film with uniform transmittance. Furthermore, the polyvinyl alcohol film of the present invention has a smaller edge folding angle, thereby avoiding the situation where the edge is prone to film breakage due to an excessively large folding angle.

[0090] In this document, all ranges provided are intended to include combinations of each specific range within a given range and of subranges between that given range. Furthermore, unless otherwise stated, all ranges provided herein include the endpoints of the range. Thus, ranges 1-5 specifically include 1, 2, 3, 4, and 5, as well as subranges such as 2-5, 3-5, 2-3, 2-4, 1-4, etc.

[0091] All publications and patent applications cited in this specification are incorporated herein by reference, and for any and all purposes, each individual publication or patent application is expressly and individually identified as incorporated herein by reference. In the event of any inconsistency between this document and any publication or patent application incorporated herein by reference, this document shall prevail.

[0092] The present invention has been described in detail above. However, the above description is only a preferred embodiment of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the claims of the present invention should still fall within the scope of the patent coverage of the present invention. [Explanation of Symbols in the Attached Drawings] 100: Device 110: T-slit die lip 120: Casting Drum 130: Hot air dryer 140: Hot roller 150: Floating Oven 160: Temperature and humidity controller 170: Polyvinyl alcohol casting solution 200: Polyvinyl alcohol film edge folding measurement device 210: Polyvinyl alcohol film 220: Water

Claims

1. A polyvinyl alcohol film, wherein the weight swelling value of the polyvinyl alcohol film after immersion in water for 3 minutes conforms to the following formula: 2.4ln (soaking time in minutes) + 39 < weight swelling value < 4.3ln (soaking time in minutes) + 44; and The standard deviation of the weight swelling of the polyvinyl alcohol film measured at multiple locations within a 3-minute immersion time was ≤1.

70.

2. The polyvinyl alcohol film as described in claim 1, having two opposing sides, namely an X side and a Y side, wherein the difference in moisture content between the two sides is ≤0.015; wherein, The moisture content difference between the two sides was measured at multiple locations on the polyvinyl alcohol film using FTIR-ATR and calculated using the following formula: |X side (A) 1654cm -1 / A 1090cm -1 Average value - Y-plane (A) 1654cm -1 / A 1090cm -1 Average value |.

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

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

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

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

7. The polyvinyl alcohol film as described in claim 1, wherein the film thickness is 45 to 75 μm.

8. The polyvinyl alcohol film according to any one of claims 1 to 7, wherein a 25 cm² area is immersed in 80 mL of water at 50 °C for 1 minute to obtain an impregnation solution, wherein the amount of polyvinyl alcohol dissolved in 20 mL of the impregnation solution is ≤30.0 ppm.

9. The polyvinyl alcohol film of claim 8, wherein the amount of polyvinyl alcohol dissolved in 20 mL of the impregnation solution is ≤15.0 ppm.

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

11. The polyvinyl alcohol film as described in claim 10, wherein the folding angle of its mechanically oriented edge is less than 90 degrees.

12. An optical film formed from a polyvinyl alcohol film as described in any one of claims 1 to 11.

13. A polarizing film formed from a polyvinyl alcohol film as described in any one of claims 1 to 11.

14. The polarizing film as claimed in claim 13, wherein 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 locations of the polarizing film, and the maximum value minus the minimum value of the average values ​​of T1% and T2% at each location is ≤0.50%.

15. The polarizing film of claim 14, wherein the maximum-minimum value of the average value is ≤0.10%.

16. A method for manufacturing a polyvinyl alcohol film as described in any one of claims 1 to 11, comprising: (a) Dissolution process: Polyvinyl alcohol resin is added to solvent and plasticizer and then heated to 110°C to 150°C to dissolve it, forming a polyvinyl alcohol casting solution, wherein the viscosity of the polyvinyl alcohol casting solution is 60cps to 120cps at 4% solids content. (b) Casting process: The polyvinyl alcohol casting solution is cast into a casting drum, and a hot air dryer is provided around the casting drum, the maximum temperature of which is 80 to 140°C; a polyvinyl alcohol preform is obtained after peeling off from the casting drum; and (c) Drying process: The polyvinyl alcohol preformed film is dried in multiple hot rollers with temperature differences and a floating oven to obtain the polyvinyl alcohol film.

17. The manufacturing method of claim 16, wherein the aggregate temperature of the plurality of hot rollers with temperature differences is 700 to 1050°C.

18. The manufacturing method of claim 17, wherein the temperature of each of the hot rollers is greater than 40°C.

19. The manufacturing method of claim 18, wherein the maximum temperature of the floating oven is 105 to 130°C.

20. The manufacturing method of claim 19, wherein the wind speed difference between the upper and lower sides of the floating oven is ≤1.0 m / s, and the temperature difference between the upper and lower sides of the floating oven is ≤5℃.

21. The manufacturing method of claim 20, wherein the sum of the temperature differences between the east and west sides of the casting drum, the hot roller, and the floating oven is ≤10°C.

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