TWI870317B - Polyvinyl alcohol film, optical film comprising the same and manufacturing method thereof - Google Patents

Abstract

The present invention provides a polyvinyl alcohol film, an optical film comprising the same, and a manufacturing method thereof. The polyvinyl alcohol film has a water vapor transmission rate (WVTR) of 1500 to 1900 g/m ²/24hr at a temperature of 40±2℃ and a humidity of 90±5%RH. The polyvinyl alcohol film of the present invention has the characteristics that the surface of the film is free from white haze and in addition, said polyvinyl alcohol film can exhibit excellent polarization and is not easy to break.

Description

Polyvinyl alcohol film, optical film containing the same and preparation method thereof

The present invention relates to a polyvinyl alcohol (PVA) film and a manufacturing method thereof; the polyvinyl alcohol film can be formed into an optical film, particularly a polarizing film, but the present invention is not limited thereto.

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

Polyvinyl alcohol film is an important component of polarizing plates. After being dyed and stretched, it can exhibit polarized properties. The polarizer obtained through the polarization process can polarize light and control brightness and darkness. Therefore, it has been widely used in various LCD screens, displays, glasses and wearable devices. It is one of the indispensable components of display devices.

In recent years, with the advancement of screen technology, the quality requirements for polarizing films have gradually increased; at the same time, more attention has been paid to whether the polyvinyl alcohol film is easy to break during the polarizing film manufacturing process and whether a high degree of polarization can be obtained.

However, the inventors have found that the surface of the polyvinyl alcohol film used in the prior art is prone to white fogging, resulting in poor transparency. In addition, when the prior polyvinyl alcohol film is prepared into a polarizing film, it is usually impossible to simultaneously obtain a good polarization degree and avoid film breakage.

In view of the above problems, the main purpose of the present invention is to provide a polyvinyl alcohol film that can avoid the generation of white mist on the surface; further, the polyvinyl alcohol film provided by the present invention can obtain good polarization degree and avoid film breakage when it is prepared into a polarizing film.

Specifically, the present invention provides a polyvinyl alcohol film having a water vapor transmission rate (WVTR) of 1500 to 1900 g/m ² /24hr measured at a temperature of 40±2° C. and a humidity of 90±5% RH.

According to one embodiment of the present invention, the polyvinyl alcohol film has two sides, a first side and a second side, and Fourier-transform infrared spectroscopy (FTIR) is measured at multiple positions on any one side and the A _1140cm ^-1 /A _1090cm ^-1 ratio is calculated, and the difference between the maximum and minimum values of the A _1140cm ^-1 /A _1090cm ^-1 ratio at the multiple positions is less than 0.05; wherein A _1140cm ^-1 and A _1090cm ^-1 are the absorption peak intensities at 1140 cm ^-1 and 1090 cm ^-1, respectively.

According to an embodiment of the present invention, the A _1140cm ^-1 /A _1090cm ^-1 ratio of the polyvinyl alcohol film at multiple locations on the first surface and the A _1140cm ^-1 /A _1090cm ^-1 ratio of the second surface corresponding to the first surface have a difference, and the average value of the absolute values is less than 0.06.

According to one embodiment of the present invention, the polyvinyl alcohol film has a phase difference relaxation rate of 0.55 to 0.70, and the phase difference relaxation rate is calculated according to the following formula: .

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

Another aspect of the present invention provides an optical film formed from the polyvinyl alcohol film.

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

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

In another aspect, the present invention provides a method for manufacturing the polyvinyl alcohol film as described above, comprising: (a) a dissolving process: adding a polyvinyl alcohol resin and an additive into water at a temperature of less than or equal to 95°C, and then heating the water to 140°C to 150°C to dissolve the water, thereby forming a polyvinyl alcohol casting solution; and (b) a casting and drying process: casting the polyvinyl alcohol casting solution onto a casting drum, and obtaining a polyvinyl alcohol preliminary film after peeling off the casting drum; and (c) a drying process: drying the polyvinyl alcohol preliminary film on a plurality of hot rollers with temperature differences and in an oven, thereby obtaining the polyvinyl alcohol film.

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

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

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

According to an embodiment of the present invention, in step (c), the average temperature of the plurality of hot rollers is greater than 90°C, and the temperature distribution in the width direction is less than or equal to 2.0°C.

According to an embodiment of the present invention, in step (c), the oven has an upper cavity and a lower cavity, and the temperature difference between the upper and lower cavities is less than 5.0°C.

According to one embodiment of the present invention, the temperature difference between the upper and lower chambers is 2.0°C to 4.0°C.

The main effect of this case is that the polyvinyl alcohol film provided by the present invention can avoid the occurrence of white mist on the surface. Secondly, when the polyvinyl alcohol film of the present invention is prepared into a polarizing film, it can obtain a good polarization degree and avoid the occurrence of film breakage.

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

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

[Polyvinyl alcohol film]

The present invention provides a polyvinyl alcohol film having a water vapor transmission rate (WVTR) of 1500 to 1900 g/m ² /24 hr measured at a temperature of 40±2° C. and a humidity of 90±5% RH; specifically, for example but not limited to the following values or values between any two thereof: 1500 g/m ² /24 hr, 1550 g/m ² /24 hr, 1600 g/m ² /24 hr, 1650 g/m ² /24 hr, 1700 g/m ² /24 hr, 1750 g/m ² /24 hr, 1800 g/m ² /24 hr, 1850 g/m ² /24 hr or 1900 g/m ² /24 hr. The "water vapor transmission rate" is a parameter that measures the degree to which the polyvinyl alcohol film resists the passage of water, based on the weight of water vapor that passes through the polyvinyl alcohol film per unit time and per unit area at a specific temperature and humidity. Not limited by a specific theory, the water vapor transmission rate is related to the dispersion uniformity of the additives in the process of the polyvinyl alcohol film, which in turn affects the generation of white fog on the surface of the polyvinyl alcohol film; it is believed that when the water vapor transmission rate is controlled within the above-mentioned specific range, the problem of generating white fog can be avoided; if the water vapor transmission rate is too high, white fog will be generated on the surface of the polyvinyl alcohol film; if the water vapor transmission rate is too low, it may be due to the high drying temperature of the process, resulting in the dehydration of the polyvinyl alcohol to form a hydrophobic double bond structure, which in turn has a negative impact on the subsequent polarizing film.

According to at least one embodiment of the present invention, the polyvinyl alcohol film has two sides, a first side and a second side, and Fourier-transform infrared spectroscopy (FTIR) is measured at a plurality of positions on any one side and the ratio of A _{1140 cm} ^-1 /A 1090 _cm ^-1 is calculated, and the difference between the maximum and minimum values of the ratio of A _{1140 cm} ^-1 /A _{1090 cm} ^-1 at the plurality of positions is less than 0.05; wherein A _{1140 cm} ^-1 and A _{1090 cm} ^-1 are absorption peak intensities at 1140 cm ^-1 and 1090 cm ^-1 , respectively. The difference is specifically, for example but not limited to, the following values or between any two thereof: 0.01, 0.02, 0.03 and 0.04. The present invention uses the ratio of A _1140cm ^-1 /A _1090cm ^-1 as the "crystallization index" for judging the degree of crystallization on the surface of the polyvinyl alcohol film, and the difference obtained by subtracting the minimum value from the maximum value of the ratio at multiple positions on a single surface is the "crystallization index range". Not limited by a specific theory, the crystallization index range is related to the occurrence of white fog on the surface of the polyvinyl alcohol film; according to data, when the crystallization index range is controlled within the above-mentioned specific range, the problem of white fog can be avoided.

According to at least one embodiment of the present invention, the A _1140cm ^-1 /A _1090cm ^-1 ratio of the polyvinyl alcohol film at multiple positions on the first side and the A _1140cm ^-1 /A _1090cm ^-1 ratio of the second side corresponding to the first side have a difference, and the average value of the absolute value is less than 0.06; the average value is specifically, for example but not limited to, the following values or between any two of them: 0.01, 0.02, 0.03, 0.04 and 0.05. The present invention uses the above average value as the "average value of the crystallization deviation on both sides" of the polyvinyl alcohol film. Without being limited to a specific theory, the average value of the crystallization deviation on both sides is related to the generation of white fog on the surface of the polyvinyl alcohol film; it is believed that when the average value of the crystallization deviation on both sides is controlled within the above specific range, the problem of generating white fog can be avoided. More specifically, when the average value of the crystallization deviation of both sides is too high, it may mean that the polyvinyl alcohol film is easy to stick and white fog is easily generated on the surface.

According to at least one preferred embodiment of the present invention, the polyvinyl alcohol film has a phase difference relaxation rate of 0.55 to 0.70, and the phase difference relaxation rate is calculated according to the following formula: . The phase difference relaxation rate is specifically, for example but not limited to, the following values or any two thereof: 0.55, 0.57, 0.59, 0.61, 0.63, 0.65, 0.67, 0.69 and 0.70. Without being limited by a specific theory, the phase difference relaxation rate is related to the degree of polarization and film breakage when the polyvinyl alcohol film is made into a polarizing film. More specifically, it is believed that when the phase difference relaxation rate is too low, it may indicate that the molecular entanglement and the degree of crystallinity are too high, resulting in the problem of film breakage when the polyvinyl alcohol film is made into a polarizing film; and when the phase difference relaxation rate is too high, it may indicate that the molecular entanglement and the degree of crystallinity are too low, resulting in the degree of polarization of the polyvinyl alcohol film after being made into a polarizing film being not ideal. In addition, since the swelling and dyeing time of the polarizing film is usually about 2 minutes when it is prepared, and then the stretching step is performed; therefore, the present invention sets 2 minutes as the measurement standard of the phase difference relaxation rate to simulate the molecular entanglement state of the film before stretching.

According to at least one preferred embodiment of the present invention, the thickness of the polyvinyl alcohol film is 45 to 75 μm; specific examples include but are not limited to the following values or between any two of them: 45 μm, 47 μm, 49 μm, 51 μm, 53 μm, 55 μm, 57 μm, 59 μm, 61 μm, 63 μm, 65 μm, 67 μm, 69 μm, 71 μm, 73 μm and 75 μm.

[Production method of polyvinyl alcohol film]

In another aspect, the present invention provides a method for manufacturing the polyvinyl alcohol film as described above, comprising: (a) a dissolving process: adding a polyvinyl alcohol resin and an additive into water at a temperature less than 95°C, and then heating the water to 140°C-150°C to dissolve the water, thereby forming a polyvinyl alcohol casting solution; and (b) a casting and drying process: casting the polyvinyl alcohol casting solution onto a casting drum, and obtaining a polyvinyl alcohol preliminary film after peeling off the casting drum; and (c) a drying process: drying the polyvinyl alcohol preliminary film on a plurality of hot rollers with temperature differences and in an oven, thereby obtaining the polyvinyl alcohol film.

The polyvinyl alcohol resin is obtained by polymerizing vinyl ester resin monomers to form polyvinyl ester resins, and then performing a saponification reaction. Among them, the vinyl ester resin monomers include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate or vinyl octanoate, and the present invention is not limited to these. Vinyl acetate is preferred. In addition, copolymers formed by copolymerization of olefin compounds or acrylate derivatives with the above-mentioned vinyl ester resin monomers can also be used; the olefin compounds include ethylene, propylene or butene, and the present 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, and the present invention is not limited to these.

The additives include plasticizers, surfactants or alkyl ether additives. Among them, the surfactant is, for example, but not limited to, a cationic, anionic or non-ionic surfactant, specifically, for example, including but not limited to carboxylate type such as potassium laurate; sulfate type such as sodium lauryl polyether sulfate; sulfonate type such as dodecylbenzenesulfonate; alkylphenyl ether type such as polyoxyethylene octylphenyl ether; alcohol phenyl ether type such as polyethylene glycol monooctylphenyl ether; alkyl ester type such as polyoxyethylene laurate; alkylamine type such as polyoxyethylene laurylamine ether; alkylamide type such as polyoxyethylene laurylamide; polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether; alkanolamide type such as lauric acid diethanolamide and oleyl diethanolamide; allylphenyl ether type such as polyoxyallylphenyl ether, etc. The plasticizer can increase the softness of the material or liquefy the material to improve the processability of the film. The plasticizer that can be used includes, but is not limited to, phthalate (Phthalate), 2-ethylhexyl phthalate (DEHP), glycerol, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butylbenzyl phthalate (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol or trihydroxymethylpropane.

According to at least one preferred embodiment of the present invention, in the dissolution process (a), a polyvinyl alcohol resin and an additive are added to water (solution) at a temperature less than or equal to 95°C. The temperature is specifically, for example but not limited to, the following values or any two thereof: 85°C, 90°C and 95°C; preferably 85°C to 95°C. Without being limited by a specific theory, the inventors have found through experiments that if the additive is added to the water (solution) when the temperature is greater than 95°C, the polyvinyl alcohol resin may have already begun to dissolve, causing the overall viscosity of the solution to increase and the additive to be difficult to disperse evenly, resulting in an excessively high water vapor permeability of the polyvinyl alcohol film, which in turn causes the generation of white mist on its surface.

According to at least one preferred embodiment of the present invention, in the (a) dissolution process, the temperature of the heating dissolution (maximum dissolution temperature) is controlled to be 140°C to 150°C, specifically, for example but not limited to, between any two of the following values: 140°C, 145°C or 150°C. Without being limited by a specific theory, the inventors have found through experiments that if the maximum dissolution temperature is too low, the additive may not be evenly dispersed, which will cause the water vapor permeability of the polyvinyl alcohol film to be too high, thereby causing white mist to be generated on its surface; on the contrary, if the maximum dissolution temperature is too high, it will affect the productivity of the polyvinyl alcohol film. In addition, according to at least one preferred embodiment of the present invention, the temperature is maintained for a period of time after the temperature is raised to form the polyvinyl alcohol casting solution; preferably, the time is 2 hours.

In the polyvinyl alcohol casting solution, the content of the polyvinyl alcohol resin is 10 to 60% by weight, specifically, for example but not limited to the following values or any two thereof: 10% by weight, 15% by weight, 20% by weight, 25% by weight, 30% by weight, 35% by weight, 40% by weight, 45% by weight, 50% by weight, 55% by weight and 60% by weight; preferably, 15 to 40% by weight; more preferably, 20 to 30% by weight. Without being limited by a specific theory, if the content of the polyvinyl alcohol resin is insufficient, the viscosity of the polyvinyl alcohol casting solution will be too low, the drying load will be too large, and the film-forming efficiency of the polyvinyl alcohol film will be poor; on the contrary, if the content of the polyvinyl alcohol resin is too high, it will be difficult to dissolve uniformly as a whole, and clusters will easily remain.

According to at least one preferred embodiment of the present invention, after the (a) dissolution process is completed, the polyvinyl alcohol casting solution is defoamed by a twin-screw extruder and then cast from a T-slit to a rotating casting drum for a subsequent (b) casting drying process.

According to at least one preferred embodiment of the present invention, in the (b) casting and drying process, the casting solution is cast onto the casting drum until the solid content of the casting solution is greater than 85%, and the drying time is 60 to 120 seconds; in detail, the above drying time can also be referred to as "pre-drying time", which is from the casting solution being cast from the T-slit onto the rotating casting drum to the solid content being greater than 85% after being dried by the casting drum. The drying time is specifically, for example but not limited to, the following values or any two thereof: 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds, 110 seconds and 120 seconds. Without being limited by a particular theory, if the drying time is too short, the number of nuclei of the polyvinyl alcohol film may be too small, resulting in an overall low degree of entanglement, which in turn makes the phase difference relaxation rate too high; in this case, it is believed that the polarization degree of the polyvinyl alcohol film after being made into a polarizing film is insufficient. If the drying time is too long, in addition to affecting the productivity of the polyvinyl alcohol film, it will also lead to an excessive number of nuclei of the polyvinyl alcohol film, which in turn makes the phase difference relaxation rate too low; in this case, it is believed that the polyvinyl alcohol film is easy to break when it is made into a polarizing film.

According to at least one preferred embodiment of the present invention, in the (b) casting drying process, after the solid content of the casting solution is greater than 85%, it is dried at a temperature greater than 100°C for 20 to 30 seconds. The drying time may also be referred to as "post-drying time", and specifically, for example but not limited to the following values or between any two of them: 20 seconds, 21 seconds, 22 seconds, 23 seconds, 24 seconds, 25 seconds, 26 seconds, 27 seconds, 28 seconds, 29 seconds and 30 seconds. Without being limited by a specific theory, if the drying time is too short, the crystallinity of the polyvinyl alcohol film may be too low, resulting in too low molecular entanglement, and thus too high a phase difference relaxation rate; in this case, it is believed that the degree of polarization of the polyvinyl alcohol film after being made into a polarizing film is insufficient. If the drying time is too long, in addition to affecting the productivity of the polyvinyl alcohol film, it will also lead to an excessive number of polyvinyl alcohol film nuclei, which in turn makes the phase difference relaxation rate too low; in this case, it is believed that the polyvinyl alcohol film is prone to film breakage when it is made into a polarizing film.

According to at least one embodiment of the present invention, in the (c) drying process, the polyvinyl alcohol preliminary film is dried by the hot rollers first, and then the subsequent drying step is carried out in the oven. In detail, the number of the hot rollers and the number of sections of the oven are not particularly limited by the present invention, and the user can adjust them as needed. In addition, according to at least one preferred embodiment of the present invention, the polyvinyl alcohol preliminary film is contacted and dried on the upper and lower surfaces of the film by a plurality of hot rollers (for example but not limited to 15 hot rollers); wherein the first hot roller has the highest temperature among all the hot rollers, the temperature of the subsequent hot rollers is gradually reduced, and the last hot roller has the lowest temperature among all the hot rollers.

According to at least one preferred embodiment of the present invention, in the drying process (c), the average temperature of the plurality of hot rollers is greater than 90°C, and the temperature distribution in the width direction is less than or equal to 2.0°C. The distribution is specifically, for example, the following values or any two thereof: 1.0°C, 1.5°C and 2.0°C. Without being limited by a specific theory, too large a distribution may result in a too large distribution of the crystallization index of the polyvinyl alcohol film, thereby causing the generation of white mist on its surface.

According to at least one preferred embodiment of the present invention, the rotation speed of the plurality of hot rollers is 3 to 15 m/min, preferably 5 to 10 m/min. It is believed that when the rotation speed of the hot rollers is too slow, there is a risk of reduced productivity. On the contrary, when the rotation speed of the hot rollers is too fast, the polyvinyl alcohol preliminary film may not be fully dried, and the release property is reduced.

According to at least one preferred embodiment of the present invention, in the drying process (c), the oven has an upper chamber and a lower chamber, and the temperature difference between the upper and lower chambers is less than 5.0°C. The temperature difference is specifically, for example but not limited to, the following values or any two thereof: 1.0°C, 2.0°C, 3.0°C, 4.0°C and 5.0°C; preferably 2.0°C to 4.0°C. Without being limited by a specific theory, if the temperature difference is too large, the average value of the crystal deviation on both sides of the polyvinyl alcohol film may be too large, thereby causing the generation of white mist on its surface.

According to at least one preferred embodiment of the present invention, the aforementioned oven can be, for example, a floating oven, and the temperature is between 80°C and 115°C; the temperature is, for example but not limited to, the following values or between any two of them: 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C or 115°C.

The equipment used in the manufacturing method includes a dissolving tank, a filter, a coating machine and a conveying pipeline connected from the dissolving tank to the coating machine. Preferably, the equipment is covered with a heat preservation device. The heat preservation device can be a metal heating wire or a jacket containing a liquid, such as oil or water. The heat preservation device heats the metal wire or the liquid in the jacket to keep the equipment in a uniformly heated heat preservation state, especially the equipment and pipeline surface, to prevent the equipment or pipeline from losing temperature on the surface, which causes the polyvinyl alcohol in the polyvinyl alcohol casting solution to form gel or clusters. In addition, the above-mentioned heat preservation temperature should not be too high, otherwise it may cause part of the polyvinyl alcohol casting solution to dehydrate or gelatinize, forming a brown or black gel, thereby affecting the surface quality and uniformity of the polyvinyl alcohol film after coating. According to at least one preferred embodiment of the present invention, the heat preservation temperature is 80 to 120°C, for example but not limited to the following values or any two thereof: 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C and 120°C.

[Optical film and its manufacturing method]

Another aspect of the present invention provides an optical film, which is formed by the polyvinyl alcohol film as described above. The optical film refers to a polarizing film, an anti-blue light film, a filter, etc., but the present invention is not limited to these. In a preferred embodiment, the optical film of the present invention is a polarizing film.

According to at least one preferred embodiment of the present invention, the polarization degree of the polarizing film is greater than 99.9, for example but not limited to the following values or between any two of them: 99.90, 99.91, 99.92, 99.93, 99.94, 99.95, 99.96, 99.97, 99.98 and 99.99.

According to at least one embodiment of the present invention, the manufacturing method of the optical film in this case refers to a manufacturing method of a polarizing film, and further refers to manufacturing a polyvinyl alcohol film into a polarizing film. The manufacturing method includes a dyeing step of iodine adsorption, a boric acid treatment step, and a water washing step; a uniaxial stretching step can be implemented in the boric acid treatment step or before the stage, and a drying step can be implemented between the boric acid treatment step and the water washing step. Preferably, a swelling step of swelling the polyvinyl alcohol film with water can be provided before the dyeing step. In addition, a final drying step is usually provided after the water washing step.

According to at least one preferred embodiment of the present invention, in the swelling step, the polyvinyl alcohol film is immersed in a treatment bath (e.g., water) at a temperature of, for example, 30°C to wash the film surface and perform swelling treatment. The swelling treatment time is generally 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, a plurality of guide rollers are arranged in the swelling tank containing the treatment bath to transport the polyvinyl alcohol film. Subsequently, the polyvinyl alcohol film is stretched in the machine direction (MD) to 1.2 times of the original length before performing the dyeing step.

In the dyeing step, the polyvinyl alcohol film after the above-mentioned swelling step is immersed in a dyeing tank containing a dyeing bath. The conditions of the dyeing treatment can be determined within the range of allowing iodine to be adsorbed within the polyvinyl alcohol film without causing adverse conditions such as extreme dissolution and devitrification of the film. The dyeing bath used in the dyeing step can be an aqueous solution containing 0.003 to 0.200 parts by weight of iodine and 0.1 to 10.0 parts by weight of potassium iodide relative to 100 parts by weight of water. In addition, other iodides such as zinc iodide can also be used instead of potassium iodide, or other iodides can be used in addition to potassium iodide. The temperature of the dyeing bath (dyeing temperature) is generally 10 to 50°C, for example 30°C; and the time of the dyeing treatment (dyeing time) is generally 10 to 600 seconds, preferably 30 to 200 seconds. Subsequently, the polyvinyl alcohol film was stretched along the mechanical direction to 3.4 times of its original length and then subjected to boric acid treatment and stretching steps.

In the boric acid treatment and extension step, the iodine-dyed polyvinyl alcohol film is treated with an aqueous solution containing boric acid to crosslink it and fix the adsorbed iodine in the resin. This step is usually performed by immersing the polyvinyl alcohol film after the dyeing step in a fixing tank containing a treatment bath containing boric acid. The boric acid treatment bath can be an aqueous solution containing 0.5 to 15.0 parts by weight of boric acid relative to 100 parts by weight of water. The boric acid treatment bath preferably contains iodide in addition to boric acid, and its amount is usually 5 to 20 parts by weight relative to 100 parts by weight of water. The iodide used for this purpose can be potassium iodide or zinc iodide, etc. In addition, compounds other than iodide may also coexist in the boric acid treatment bath, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, sodium sulfate, etc. The boric acid treatment and stretching steps are usually performed at 50 to 70°C, preferably 53 to 65°C, such as 55°C, and the treatment time is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 100 seconds. Subsequently, the polyvinyl alcohol film is stretched along the mechanical direction to 6 times the original length before performing the subsequent steps.

After the above steps and the subsequent washing and drying steps, the polyvinyl alcohol film forms a polarizing film. Furthermore, a protective layer can be formed on at least one side of the polarizing film to produce a polarizing film product (or polarizer). In detail, the protective layer is preferably a component having the function of preventing the surface of the polarizing film from being worn, and includes a transparent resin. According to different embodiments, the protective layer is only provided on one side of the polarizing film, but preferably formed on both sides of the polarizing film. The protective layer can be a protective film of a transparent resin material; the transparent resin can be an acrylic resin such as methyl methacrylate resin, an olefin resin, a polyvinyl chloride resin, a cellulose resin, a styrene resin, an acrylonitrile-butadiene-styrene copolymer resin, an acrylonitrile-styrene copolymer resin, a polyvinyl acetate resin, a polyvinylidene chloride resin, a polyamide resin, a polyacetal resin, or a polyvinyl acetate resin. Resins such as polybutylene terephthalate resins, polyethylene terephthalate resins, etc. can be used as the cellulose resins. Examples of the cellulose resins include polybutylene terephthalate resins, polyether terephthalate resins, polyarylate resins, polyamide imide resins, polyimide resins, epoxy resins, cyclohexane oxybutane resins, etc. Cellulose resins such as cellulose triacetate (TAC) are preferably used .

The following non-limiting examples of various aspects of the present invention are provided mainly to illustrate various aspects of the present invention and the benefits achieved therefrom.

The following provides a non-limiting method for preparing polyvinyl alcohol films. According to the method disclosed below or a similar method, 15 non-limiting examples of polyvinyl alcohol films (Examples 1 to 15) and 7 comparative examples of polyvinyl alcohol films (Comparative Examples 1 to 7) were prepared. However, the specific method for preparing Examples 1 to 15 and Comparative Examples 1 to 7 may differ from the method disclosed below in one or more aspects.

1. Preparation of polyvinyl alcohol film

The following are the main steps common to the present embodiment and the comparative example in manufacturing the polyvinyl alcohol film, and the following Table 1 will present in detail the differences between the present embodiment and the comparative example in one or more process parameters:

(a) Dissolution process: 1800 kg of polyvinyl alcohol resin, 200 kg of plasticizer glycerol, 4000 kg of water, 500 ppm of surfactant diethanolamide laurate (relative to the weight of polyvinyl alcohol resin) and 500 ppm of alkyl ether additive (relative to the weight of polyvinyl alcohol resin) were added to water at a certain temperature (as shown in Table 1) and stirred evenly. Then, the temperature was raised at a rate of 10°C/hour to the maximum dissolution temperature (as shown in Table 1) and the temperature was maintained for 2 hours to obtain a uniform polyvinyl alcohol casting solution (film-making stock solution).

(b) Casting and drying process: The polyvinyl alcohol casting solution is defoamed by a twin-screw extruder, discharged from a T-slit die lip and cast onto a rotating high-temperature casting drum for drying and film formation; the solution is cast from the T-slit onto the rotating casting drum and then dried on the casting drum until the solid content is greater than 85% for a pre-drying time (as shown in Table 2); in addition or alternatively, when the solid content of the casting solution is greater than 85%, it is dried at a temperature greater than 100°C for a post-drying time (as shown in Table 2) to obtain a polyvinyl alcohol preliminary film.

(c) Drying process: After the polyvinyl alcohol preliminary film is peeled off from the casting drum, it is contacted with the upper and lower surfaces of the drying film by 10 hot rollers, of which the first hot roller is the one with the highest temperature among all the hot rollers, and the temperature of the subsequent hot rollers is gradually reduced; the average temperature of the 10 hot rollers is greater than 90°C, and they have a specific temperature distribution in the width direction (as shown in Table 1). Then the polyvinyl alcohol preliminary film enters the floating oven for heat treatment; the oven has an upper cavity and a lower cavity, and there is a specific temperature difference between the upper and lower cavities (as shown in Table 1). Finally, a polyvinyl alcohol film product is obtained.

2. Analytical methods

Here, the present invention provides analysis and testing methods for the polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 7.

[ Water vapor permeability ] The water vapor permeability determination method of the present invention is based on JIS L1099 A-1 (positive cup) CaCl ₂ water vapor absorption method to test the moisture permeability of polyvinyl alcohol film; the specific analysis method is as follows: 1. First set the temperature of the constant temperature and humidity machine to 40±2℃ and the humidity to 90±5%RH, and keep the machine stable. 2. Dry the moisture absorbent (CaCl ₂ ) in an oven at 120℃ for 6 hours in advance. 3. Quickly place the moisture absorbent in the moisture permeability cup, with its capacity about 3mm away from the cup mouth. 4. Quickly cover the polyvinyl alcohol film specimen on the moisture permeability cup, and then install the liner and ring in sequence, and fix them with a nut. 5. Quickly place the moisture permeable cup containing the polyvinyl alcohol film specimen in a constant temperature and humidity machine with a temperature of 40±2℃ and a humidity of 90±5%RH for 1 hour, then take it out and weigh it directly to obtain the weight _W0 (in g); 6. Then, place it in the constant temperature and humidity machine, take it out after 2, 4 and 6 hours, and measure the weight _W2 , _W4 and _W6 (in g) respectively; 7. Calculate the water vapor transmission rate: (Unit: g/ ^m2 /24hr).

[FT-IR crystallization index ] In FT-IR analysis, the absorption peak of OCC near 1140cm ^-1 is related to the crystallization of polyvinyl alcohol and is a signal unique to crystallization. The specific analysis method is as follows: 1. The full range of crystallization index of the first surface / average value of crystallization index : Take any 3 positions on any surface of a circular polyvinyl alcohol film specimen with a radius of 3 cm (defined as the first surface here), analyze and measure the absorption peak intensity of 1140cm ^-1 (i.e. A _1140cm ^-1 ) and the absorption peak intensity of 1090cm ^-1 (i.e. A _1090cm ^-1 ) of each of the 3 positions, and obtain the A _1140cm ^-1 / A _1090cm ^-1 ratio (crystallization index) of each of the three positions. Subtract the minimum value from the maximum value of the crystallization index among the 3 positions to obtain the full range of crystallization index of the first surface. At the same time, the crystallization index of the three positions is averaged to obtain the average crystallization index of the first surface. 2. Average value of crystallization deviation of the two surfaces : Randomly select three positions on a circular polyvinyl alcohol film specimen with a radius of 3 cm, and measure and calculate the individual crystallization index of the two surfaces at the same (corresponding) positions on the two opposite surfaces (a total of 3 pairs of crystallization indexes), and then calculate the individual crystallization index difference of the three positions on the two surfaces, and then average the absolute values to obtain the average value of crystallization deviation of the two surfaces.

[ Phase difference relaxation rate ] The phase difference relaxation rate is measured after swelling in 30°C water for 2 minutes. The specific analysis method is as follows: 1. Analytical instrument: Photonic Lattice's PA series birefringence measurement system 2. Measuring range: 8cm along the mechanical direction * 8cm along the width direction 3. Sampling position: Please refer to Figure 1, and sample 3 test pieces with a length a of 10cm along the mechanical direction MD and a length b of 10cm along the width direction TD from the two ends of the polyvinyl alcohol film within a range of a length L of 40cm inward along the width direction TD for analysis; the present invention has no special restrictions on the sampling position. 4. Before swelling, perform phase difference analysis on the range of 8cm in the mechanical direction and 8cm in the width direction after deducting the edge of each test piece, and take an average of the phase differences of the three test pieces to obtain the phase difference of the polyvinyl alcohol film before swelling. 5. Soak the three test pieces in 30℃ water for swelling for 2 minutes, then take them out, and perform phase difference analysis on the range of 8cm in the mechanical direction and 8cm in the width direction, and take an average of the phase differences of the three test pieces after swelling to obtain the phase difference of the polyvinyl alcohol film after swelling in water for 2 minutes; it is not necessary to wipe off the water on the surface of the test piece during measurement. 6. Calculate the phase difference relaxation rate: .

3. Evaluation methods and results

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

[ White Mist Evaluation ] Use a spectrophotometer (analytical instrument) to analyze the light transmittance of the polyvinyl alcohol film to evaluate its transparency uniformity. The specific analysis method is as follows: 1. Calibration of analytical instrument: The spectrophotometer will automatically calibrate before turning on, so two white sheets need to be placed in the instrument first. 2. Sample preparation: Cut the polyvinyl alcohol film into five equal pieces in the width direction, and each piece is cut into an area of 5cm*5cm (mechanical direction*width direction), for a total of 5 pieces. 3. Analysis method: Analyze the light transmittance T% of the 5 test pieces at a wavelength of 550nm, and take the average to obtain the average light transmittance; subtract the minimum value from the maximum value to obtain the full range of light transmittance. 4. Evaluation standard: If the full range is less than 0.5 and the average light transmittance is greater than 90.0%, the evaluation is ◎. If the range is between 0.5 and 0.7, the evaluation is ○. If the range is greater than 0.7, the evaluation is ╳.

[ Polarization evaluation ] 1. Analytical instrument: Lambda 365 from PerkinElmer 2. Sample preparation: According to the above polarizing film process, the polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 7 are prepared into corresponding polarizing film samples. 3. Analysis method: According to the standard method of JIS Z 8722:2009, use C light source to perform 2-degree visible light sensitivity correction, then overlap two polarizing films with the same orientation direction, and measure the transmittance (H ₀ ) at the wavelength. Also overlap two polarizing films with the orientation directions perpendicular to each other, and measure the transmittance (H ₉₀ ) at the wavelength. 4. Data processing: Polarization = [(H ₀ -H ₉₀ )/(H ₀ +H ₉₀ )] ^1/2

[ Film break evaluation ] 1. Test conditions: Use 10,000 m of polyvinyl alcohol film continuously to prepare polarizing film in the polarizing film production process, and observe whether there is film break during the production process. 2. Evaluation criteria: If there is no film break, the evaluation is ○. If the film breaks more than once, the evaluation is ╳.

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

Table 1 Project Water temperature when adding additives (℃) Maximum melting temperature (℃) Temperature distribution of the hot roller in the width direction (°C) Temperature difference between upper and lower chambers of oven (℃) Water vapor transmission rate (g/m ² /24hr) Average value of the first surface crystal index First face crystal index full distance Average value of crystallization deviation on both sides White Mist Evaluation Embodiment 1 85 150 1.0 2 1550 0.65 0.02 0.01 ◎ Embodiment 2 90 150 1.0 2 1650 0.65 0.02 0.01 ◎ Embodiment 3 95 150 1.0 2 1890 0.65 0.02 0.01 ◎ Embodiment 4 85 145 1.0 2 1660 0.65 0.02 0.01 ◎ Embodiment 5 85 140 1.0 2 1880 0.65 0.02 0.01 ◎ Embodiment 6 85 150 1.0 2 1560 0.65 0.02 0.01 ◎ Embodiment 7 85 150 1.0 2 1510 0.65 0.02 0.01 ◎ Embodiment 8 85 150 1.0 2 1540 0.64 0.02 0.01 ◎ Embodiment 9 85 150 1.0 2 1550 0.63 0.02 0.01 ◎ Embodiment 10 85 150 1.5 2 1550 0.65 0.03 0.01 ◎ Embodiment 11 85 150 2.0 2 1540 0.65 0.04 0.01 ◎ Embodiment 12 85 150 1.0 3 1550 0.65 0.02 0.04 ◎ Embodiment 13 85 150 1.0 4 1530 0.65 0.02 0.05 ◎ Embodiment 14 85 150 1.0 4 1600 0.65 0.02 0.05 ◎ Embodiment 15 85 150 1.0 4 1500 0.65 0.02 0.05 ◎ Comparison Example 1 100 140 2.0 2 1910 0.65 0.04 0.01 ○ Comparison Example 2 100 140 3.0 5 1980 0.60 0.06 0.07 ╳ Comparison Example 3 100 130 3.0 5 2000 0.59 0.06 0.07 ╳ Comparison Example 4 100 130 3.0 5 2000 0.59 0.06 0.07 ╳ Comparison Example 5 100 130 3.0 5 1990 0.65 0.06 0.07 ╳ Comparison Example 6 100 130 4.0 6 1995 0.65 0.07 0.08 ╳ Comparative Example 7 100 130 4.0 6 2060 0.65 0.07 0.08 ╳

Table 2 Project Pre-drying time ( seconds ) Post drying time ( seconds ) Film thickness (μm) Phase difference relaxation rate Polarization degree (%) Film break evaluation Embodiment 1 120 30 60 0.55 99.95 ○ Embodiment 2 120 30 60 0.56 99.95 ○ Embodiment 3 120 30 60 0.55 99.96 ○ Embodiment 4 120 30 60 0.55 99.96 ○ Embodiment 5 120 30 60 0.57 99.96 ○ Embodiment 6 90 30 60 0.62 99.96 ○ Embodiment 7 60 30 60 0.68 99.95 ○ Embodiment 8 120 25 60 0.65 99.95 ○ Embodiment 9 120 20 60 0.70 99.95 ○ Embodiment 10 120 30 60 0.56 99.96 ○ Embodiment 11 120 30 60 0.58 99.96 ○ Embodiment 12 120 30 60 0.55 99.96 ○ Embodiment 13 120 30 60 0.56 99.96 ○ Embodiment 14 120 30 45 0.55 99.96 ○ Embodiment 15 120 30 75 0.56 99.96 ○ Comparison Example 1 50 30 60 0.75 99.89 ○ Comparison Example 2 50 10 60 0.78 99.85 ○ Comparison Example 3 50 10 60 0.78 99.85 ○ Comparison Example 4 130 10 60 0.50 99.96 ╳ Comparison Example 5 130 40 60 0.45 99.96 ╳ Comparison Example 6 130 40 60 0.45 99.96 ╳ Comparative Example 7 130 40 45 0.48 99.96 ╳

According to Table 1, the water vapor permeability, the full range of the crystallization index of the first side (either side), and the average value of the crystallization deviation of the two sides of the polyvinyl alcohol films of Examples 1 to 15 are all controlled within a specific range, so these films all have excellent performance in the white fog evaluation; in other words, these embodiments can avoid the generation of white fog on the surface of the polyvinyl alcohol film. In detail, the present invention adjusts the following process conditions: the water temperature when adding additives to the polyvinyl alcohol film, the maximum dissolution temperature, the temperature distribution of the hot roller (average temperature greater than 90°C) in the width direction, and the temperature difference between the upper and lower chambers of the oven, and controls the above process conditions within a specific range so that the water vapor permeability of the polyvinyl alcohol film, the first side (either side) crystallization index full range and the average value of the crystallization deviation of the two sides are all within a specific range, thereby achieving the purpose of avoiding the white fog condition of the polyvinyl alcohol film.

Please refer to Table 1. For the polyvinyl alcohol films of Comparative Examples 1 to 7, at least one of the water vapor transmission rate, the full range of the crystallization index of the first side (either side), and the average value of the crystallization deviation of the two sides is not outside the specific range. Therefore, the performance of these films in the white fog evaluation is respectively unsatisfactory or poor; in other words, these comparative examples cannot effectively avoid the white fog condition on the surface of the polyvinyl alcohol film. Specifically, in the comparative examples, at least one of the process conditions such as the water temperature when adding additives to the polyvinyl alcohol film, the maximum dissolution temperature, the temperature distribution of the hot roller (average temperature greater than 90°C) in the width direction, and the temperature difference between the upper and lower chambers of the oven was not well controlled within the above-mentioned specific range, resulting in at least one of the water vapor transmission rate, the first side (either side) crystallization index full range, and the average value of the crystallization deviation of the two sides being outside the specific range.

A detailed review of Table 1 shows that when the water temperature is increased when the additive is added, the water vapor permeability increases (Examples 2 and 3), and when the water temperature is too high (greater than 95°C), the water vapor permeability exceeds the above-mentioned specific range (Comparative Examples 1 to 7), causing the polyvinyl alcohol film to perform poorly or even poorly in the white mist evaluation. When the maximum dissolution temperature is reduced, the water vapor permeability increases (Examples 4 and 5), and when the dissolution temperature is too low (lower than 140°C), the water vapor permeability exceeds the above-mentioned specific range (Comparative Examples 3 to 7), causing the surface of the polyvinyl alcohol film to have white mist. When the temperature distribution of the hot roller in the width direction increases, the crystallization index increases (Examples 10 and 11), and when the temperature distribution is too large (higher than 2°C), the crystallization index exceeds the above-mentioned specific range (Comparative Examples 2 to 7), causing the surface of the polyvinyl alcohol film to be foggy. When the temperature difference between the upper and lower chambers of the oven increases, the average value of the crystallization deviation increases (Examples 10 and 11), and when the temperature distribution is too large (higher than 2°C), the crystallization index exceeds the above-mentioned specific range (Comparative Examples 2 to 7), causing the surface of the polyvinyl alcohol film to be foggy.

According to Table 2, the phase difference relaxation rate of the polyvinyl alcohol films of Examples 1 to 15 is controlled within a specific range, so these polyvinyl alcohol films have excellent polarization and obtain good performance in film break evaluation. In detail, the present invention adjusts the pre-drying time and post-drying time in the process and controls these process conditions within a specific range so that the phase difference relaxation rate of the polyvinyl alcohol film is within a specific range, thereby achieving the purpose of not only having excellent polarization but also avoiding film breakage when the polyvinyl alcohol film is prepared into a polarizing film.

Please refer to Table 2. The phase difference relaxation rates of the polyvinyl alcohol films of Comparative Examples 1 to 7 are all outside the specific range. Therefore, these polyvinyl alcohol films cannot have excellent polarization and avoid film breakage at the same time. In detail, in these comparative examples, at least one of the process conditions of the pre-drying time and the post-drying time is not well controlled within the above-mentioned specific range, resulting in the phase difference relaxation rate not being within the specific range. More specifically, when the phase difference relaxation rate is too high (Comparative Examples 1 to 3), although the film performs well in the film breakage evaluation, its polarization is not ideal; and when the phase difference relaxation rate is too low (Comparative Examples 4 to 7), even if the film has good polarization, its performance in the film breakage evaluation is still poor.

A detailed examination of Table 2 shows that when the pre-drying time is shortened, the phase difference relaxation rate will increase (Examples 6 and 7), and when the pre-drying time is too short (less than 60 seconds), the phase difference relaxation rate will be too high and exceed the above-mentioned specific range (Comparison Examples 1 to 3); at the same time, if the pre-drying time is too long, the phase difference relaxation rate will be too low (Comparison Examples 4 to 7). When the post-drying time is shortened, the phase difference relaxation rate will increase (Examples 8 and 9), and when the post-drying time is too short (less than 20 seconds), the phase difference relaxation rate will be too high and exceed the above-mentioned specific range (Comparative Examples 2 to 3); although the post-drying time of Comparative Example 4 is also too short, its pre-drying time is too long, resulting in the phase difference relaxation rate of the polyvinyl alcohol film of this example being low after the combined influence of the two. At the same time, if the post-drying time is too long, the phase difference relaxation rate will be too low (Comparative Examples 4 to 7).

In summary, the effect of the present invention is that the polyvinyl alcohol film can avoid the occurrence of white fog on the surface. Secondly, when the polyvinyl alcohol film of the present invention is prepared into a polarizing film, it can obtain a good polarization degree and avoid the occurrence of film breakage.

Herein, all ranges provided are intended to include each specific range within the given range and the combination of sub-ranges between the given ranges. In addition, unless otherwise specified, all ranges provided herein include the endpoints of the ranges. Thus, ranges 1 to 5 specifically include 1, 2, 3, 4 and 5, as well as sub-ranges such as 2 to 5, 3 to 5, 2 to 3, 2 to 4, 1 to 4, etc.

All publications and patent applications cited in this specification are incorporated herein by reference, and each individual publication or patent application is specifically and individually indicated as incorporated herein by reference for any and all purposes. In the event of any inconsistency between this document and any publication or patent application incorporated by reference, this document controls.

Except in the operating examples, or where otherwise indicated, all numbers expressing amounts of ingredients and/or reaction conditions may in all cases be modified by the term "about," meaning within ±5% of the indicated number. The terms "substantially free" or "substantially free" as used herein refer to less than about 2% of a particular feature. All elements or features that are affirmatively described herein may be negatively excluded from the scope of the claims.

The present invention has been described in detail above. However, what has been described above is only the preferred embodiment of the present invention and should not be used to limit the scope of implementation of the present invention. That is, all equivalent changes and modifications made according to the scope of the patent application of the present invention should still fall within the scope of the patent of the present invention.

a: Length along the mechanical direction

b: Length along the width direction

L: Length

MD: Mechanical Direction

TD: width direction

FIG. 1 is a schematic diagram showing the sampling positions of a polyvinyl alcohol film for analyzing the phase difference relaxation rate according to an embodiment of the present invention.

without.

a: Length along the mechanical direction

b: Length along the width direction

L: Length

MD: Mechanical Direction

TD: width direction

Claims (14)

A polyvinyl alcohol film has a water vapor transmission rate of 1500 to 1900 g/m ² /24 hr measured in an environment of temperature 40±2°C and humidity 90±5%RH. The polyvinyl alcohol film has two sides, a first side and a second side. Fourier transform infrared spectra are measured at multiple positions on any one side and the A 1140 cm ^-1 /A _{1090 cm} ^-1 ratio is calculated. The difference between the maximum and minimum values of the A ₁₁₄₀ cm ^-1 /A _{1090 cm} ^-1 ratio at the multiple positions is less than 0.05. The A _{1140 cm} ^-1 and A _{1090 cm} ^-1 are absorption peak intensities at 1140 cm ^-1 and 1090 cm ^-1 , respectively. The polyvinyl alcohol film as claimed in claim 1, wherein the polyvinyl alcohol film has a difference between the A _1140cm ^-1 /A _1090cm ^-1 ratio at multiple locations on the first side and the A _1140cm ^-1 /A _1090cm ^-1 ratio at multiple locations on the second side corresponding to the first side, and the average of the absolute values thereof is less than 0.06. The polyvinyl alcohol film as claimed in claim 1 or 2, wherein the polyvinyl alcohol film has a phase difference relaxation rate of 0.55 to 0.70, and the phase difference relaxation rate is calculated according to the following formula:

The polyvinyl alcohol film as described in claim 3 has a film thickness of 45 to 75 μm. An optical film formed from a polyvinyl alcohol film as described in any one of claims 1 to 4. The optical film described in claim 5 is a polarizing film. The optical film as described in claim 6, wherein the polarization degree of the polarizing film is greater than 99.9%. A method for manufacturing a polyvinyl alcohol film as described in any one of claims 1 to 4, comprising: (a) a dissolving process: adding a polyvinyl alcohol resin and an additive to water at a temperature of less than or equal to 95°C, and then heating to 140°C to 150°C to dissolve, thereby forming a polyvinyl alcohol casting solution; and (b) a casting and drying process: casting the polyvinyl alcohol casting solution onto a casting drum, and obtaining a polyvinyl alcohol preliminary film after peeling off the casting drum; and (c) a drying process: drying the polyvinyl alcohol preliminary film on a plurality of hot rollers with temperature differences and in an oven to obtain the polyvinyl alcohol film. The manufacturing method as described in claim 8, wherein in step (a), the water temperature is 85°C to 95°C. The manufacturing method as described in claim 9, wherein in step (b), the polyvinyl alcohol casting solution is cast onto the casting drum, and when the solid content of the casting solution is greater than 85%, the drying time is 60 to 120 seconds. The manufacturing method as described in claim 10, wherein in step (b), the casting solution is dried at a temperature greater than 100°C for 20 to 30 seconds after the solid content is greater than 85%. The manufacturing method as described in claim 11, wherein in step (c), the average temperature of the plurality of hot rollers is greater than 90°C, and the temperature distribution in the width direction is less than or equal to 2.0°C. A manufacturing method as described in any one of claims 8 to 12, wherein in step (c), the oven has an upper cavity and a lower cavity, and the temperature difference between the upper and lower cavities is less than 5.0°C. A manufacturing method as described in claim 13, wherein the temperature difference between the upper and lower chambers is 2.0°C to 4.0°C.