JP7490027B2 - Polyvinyl alcohol film and optical film produced therefrom - Google Patents

Description

The present invention relates to optical films, and in particular to polyvinyl alcohol (PVA) films that can be used to manufacture polarizing films.

Polyvinyl alcohol film is a hydrophilic polymer that has excellent properties such as transparency, mechanical strength, water solubility, and processability, and has therefore been widely used in packaging materials and optical films for electronic devices, particularly polarizing films.

In the manufacture of a polarizing film made of a polyvinyl alcohol film, a monolayer of polyvinyl alcohol resin formed into a film is usually dyed and stretched, so that the dichroic material is adsorbed in an oriented state to the polyvinyl alcohol resin layer within the polyvinyl alcohol resin, forming a polarizing film.

Prior art such as Patent Document 1 discloses the high stretchability required when stretching a polyvinyl alcohol film, and a method for reducing residual stress that can cause problems such as deformation after processing. Patent Document 1 discloses a polyvinyl alcohol film formed using polyvinyl alcohol (A) and a modified conjugated diene polymer (B) having a hydrophilic group, which has excellent stretchability and productivity, and can be processed to obtain a stretched film and a polarizing film with excellent formability.

In addition, in the polyvinyl alcohol film manufacturing process, uneven mixing of the raw materials causes uneven stretchability of the molded film and an increase in defects, which frequently results in film breakage in the molded film in the subsequent processing steps. Means to solve this problem are also disclosed in prior art such as Patent Document 2. Patent Document 2 discloses that in the raw material mixing sequence, a solid surfactant and an antioxidant are first added to better mix the raw materials and at the same time avoid the embedding phenomenon, and then microwave heating is used to more uniformly mix the raw materials, resulting in an optical film with stable stretchability, good thickness uniformity, few defects and excellent surface smoothness.

Chinese Patent Publication No. 114051513A Chinese Patent Publication No. CN112341636A

In addition to the fact that the polyvinyl alcohol film is known to have poor stretchability/elongation and is prone to film breakage in subsequent processing steps, the inventors of the present application have discovered that when the manufactured polyvinyl alcohol film is left for a long time without external force, morphological changes such as wrinkles or cloudiness (referred to as "mutation" or "film surface mutation" in this specification) spontaneously occur. Therefore, the problem that the present invention aims to solve is how to solve the problem of the film surface mutation of the polyvinyl alcohol film, while at the same time imparting the film with the property of being difficult to break in the subsequent preparation process, and imparting excellent polarizing ability to the polarizing film manufactured from the polyvinyl alcohol film.

Based on the above problem, the present invention presents the following technical means. That is, one of the main objects of the present invention is to provide a polyvinyl alcohol film containing a polyvinyl alcohol resin with a degree of saponification exceeding 95 mole%, and under the condition of 25°C: in shear mode, the loss factor (Tan δ) T1 measured at an oscillation frequency of 100 radians/second (rad/s) is 0.40 to 0.93, the loss factor (Tan δ) T2 measured at an oscillation frequency of 1 radian/second (rad/s) in shear mode is 0.20 to 0.46, and T2/T1 does not exceed 0.75.

In one or more embodiments, the loss factor T1 of the polyvinyl alcohol film is 0.58 to 0.85, and T1/T2 does not exceed 0.60.

In one or more embodiments, the loss factor T2 of the polyvinyl alcohol film is 0.30 to 0.45.

In one or more embodiments, the degree of saponification of the polyvinyl alcohol resin is 99.85 mole% or more.

In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is 1,300 to 5,200.

In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is 2000 to 3000.

In one or more embodiments, the thickness of the polyvinyl alcohol film is 30 to 75 μm.

Another object of the present invention is to provide an optical film produced from the polyvinyl alcohol film.

In one or more embodiments, the optical film is a polarizing film.

In one or more embodiments, the optical film has a polarization degree of 99.99%.

The technical content of the present invention makes it possible to obtain a polyvinyl alcohol film that is less likely to cause deformation of the film surface and is less likely to tear. In addition, a polarizing film manufactured from the polyvinyl alcohol film has excellent polarization properties. Therefore, the polyvinyl alcohol film of the present invention is suitable for various applications, and is particularly suitable for optical films such as polarizing films.

These and other embodiments disclosed by the present invention are described below. Below is a detailed description of the present invention and preferred embodiments.

The following embodiments should not be considered as unduly limiting the present invention. Those skilled in the art may modify and vary the examples discussed in this specification without departing from the spirit or scope of the present invention, and such modifications and variations are also included within the scope of the present invention.

One of the objects of the present invention is to provide a polyvinyl alcohol film and an optical film produced therefrom. The polyvinyl alcohol film contains a polyvinyl alcohol resin having a degree of saponification of more than 95 mole%, and under the condition of 25°C, in a shear mode, the loss factor (Tan δ) T1 measured at an oscillation frequency of 100 radians/second (rad/s) is 0.40 to 0.93, the loss factor (Tan δ) T2 measured at an oscillation frequency of 1 radian/second (rad/s) in a shear mode is 0.20 to 0.46, and T2/T1 does not exceed 0.75.

The term "loss factor (Tan δ)" used in this specification refers to the ratio (G"/G') of the loss shear modulus (G") to the storage shear modulus (G'). The storage shear modulus (G') is a measure of the energy stored in a material during the application of an oscillating shear force, and this energy can be used for deformation recovery after the external vibration is terminated. The loss shear modulus (G") is a measure of the energy lost (internal energy loss) when a material is deformed by the application of an oscillating shear force, and is related to the viscous characteristics of the material. Therefore, the loss factor (Tan δ), which represents the ratio of G" to G', can be used to indicate the viscoelastic properties of a material. The larger the loss factor (Tan δ), the higher the viscosity of the material, and the smaller the loss factor (Tan δ), the higher the elasticity of the material.

Therefore, if the measured Tan δ is large, the polyvinyl alcohol film is more likely to have surface deformation (i.e., the film becomes wrinkled or cloudy), whereas if the measured Tan δ is small, the polyvinyl alcohol film is less likely to have surface deformation, but the film is more likely to break during the subsequent manufacturing process of the polarizing film.

In this specification, "film surface deformation" means that when a polyvinyl alcohol film is left for a long time without external force, morphological changes such as wrinkles or cloudiness spontaneously occur. Without being bound by any theory, the deformation of the film surface is probably caused by molecular rearrangement in the film and local compositional non-uniformity due to aggregation of some components. It is likely that part of the shear force applied by the cast drum and heating roller in the polyvinyl alcohol film manufacturing process is absorbed and accumulated in the film, so that when the polyvinyl alcohol film is left for a long time without external force, the stress is gradually released, causing the molecules in the film to rearrange. The larger the measured Tan δ of the polyvinyl alcohol film, the easier it is for molecular rearrangement to occur, causing deformation of the film surface.

In this specification, "film break" refers to carrying out the necessary processing steps (e.g., not limited to the manufacture of polarizing films) on a certain length of polyvinyl alcohol film, and observing the presence or absence of film breakage on the certain length of polyvinyl alcohol film during the processing steps. Without being bound by any theory, it is likely that the smaller the measured Tan δ of the polyvinyl alcohol film, the greater the stored energy of the polyvinyl alcohol film, and the more regular the overall structure and molecular arrangement, which increases the deformation resistance of the polyvinyl alcohol film, causing the tensile strength to be too high in the subsequent polarizing film manufacturing process, leading to film breakage.

Therefore, in order to obtain a film that is less likely to cause deformation on the film surface and is less likely to break, it is necessary to control Tan δ within a certain range. Furthermore, the inventors of the present application have discovered that by controlling the loss factors (Tan δ) T1 and T2 of a polyvinyl alcohol film measured at oscillation frequencies of 1 and 100 radians/second (rad/s) in shear mode within specific ranges, it is possible to obtain a film that is less likely to cause deformation on the film surface and is less likely to break.

As used herein, "loss factor (Tan δ) T1" refers to the value of the loss factor measured using a rheometer in shear mode at an oscillation frequency of 100 radians/second (rad/s) under conditions of 25°C, where 25°C refers to the temperature of the rheometer fixture. In one or more embodiments, the Tan δ T1 is 0.40 to 0.93, for example 0.40 to 0.93, 0.45 to 0.92, 0.50 to 0.91, 0.55 to 0.90, 0.60 to 0.89, 0.65 to 0.87, 0.70 to 0.88, 0.75 to 0.86, 0.80 to 0.85, 0.81 to 0.84 or 0.82 to 0.83, in particular 0.40, 0.45, 0.48, 0.57, 0.59, 0.61, 0.63, 0.66, 0.75, 0.79, 0.81 or 0.93, preferably 0.58 to 0.85.

As used herein, "loss factor (Tan δ) T2" refers to the value of the loss factor measured using a rheometer in shear mode at an oscillation frequency of 1 radian/second (rad/s) at 25°C, where 25°C refers to the temperature of the rheometer fixture. In one or more embodiments, the Tan δ T2 is 0.20 to 0.46, for example 0.20 to 0.46, 0.22 to 0.46, 0.24 to 0.45, 0.25 to 0.45, 0.28 to 0.44, 0.30 to 0.44, 0.32 to 0.44, 0.35 to 0.43, 0.40 to 0.42 or 0.40 to 0.41, in particular 0.20, 0.24, 0.27, 0.31, 0.32, 0.33, 0.34, 0.41, 0.43, 0.45 or 0.46, preferably 0.30 to 0.45.

Furthermore, the measured loss coefficients T1 and T2 are converted by T2/T1 to obtain the Tan δ change rate (i.e., Tan δ change rate = Tan δ T2 measured under low frequency / Tan δ T1 measured under high frequency), which represents the amount of loss change. Without being bound by any theory, a large Tan δ change rate probably indicates that the internal consumption action (molecular chain movement, molecular bond movement, phase change, etc.) of the polyvinyl alcohol film is large, and the structure of the material is loose and the molecular arrangement is disordered, making it difficult to align the molecules of the polyvinyl alcohol film in the subsequent polarization process, which leads to a low polarization degree of the polarizing film manufactured from the polyvinyl alcohol film. When the change amount of Tan δ of the polyvinyl alcohol film is less than a certain value, the internal consumption action is small, and the overall structure and molecular arrangement are relatively regular, which allows the polarizing film manufactured from the polyvinyl alcohol film to have a good polarization degree. Therefore, in order to obtain a polarizing film with an excellent polarization degree, it is necessary to control the change amount of Tan δ of the polyvinyl alcohol film within a certain range.

Thus, in one or more embodiments, the value of "Tan δ change ratio" (which may also be expressed as "T2/T1") herein is less than 0.75, preferably less than 0.60, such as, but not limited to, 0.75, 0.74, 0.73, 0.72, 0.71, 0.70, 0.69, 0.68, 0.67, 0.66, 0.65, 0.64, 0.63, 0.62, 0.61, 0.60, 0.59, 0.58, 0.57, 0.56, 0.55, 0.54, 0.53, 0.52, 0.51, 0.50, 0.49, 0.48, 0.47, 0.46, 0.45, 0.44, 0.43, 0.42, 0.41, or 0.40.

The "polarization degree" described in this specification means a result obtained by cutting a polarizer manufactured from the polyvinyl alcohol film of the present invention into a plurality of samples of a specific size, stacking the plurality of polarizers in the same flow direction (orthogonal direction), measuring the light transmittance H ₁₁ under irradiation with a wavelength of 700 nm using a spectrophotometer, stacking the plurality of polarizers in the perpendicular flow direction, and similarly measuring the light transmittance H ₁ under irradiation with a wavelength of 700 nm using a spectrophotometer, and finally calculating the measured light transmittances H ₁₁ and H ₁ using the following formula.

Degree of polarization (%)=[(H ₁₁ −H ₁ )/(H ₁₁ +H ₁ )] ^1/2

Meanwhile, the method for producing the polyvinyl alcohol film includes: (a) a dissolving step of heating and dissolving a polyvinyl alcohol-based resin and adjusting the concentration of the polyvinyl alcohol-based resin to form a polyvinyl alcohol casting solution; (b) a casting step of casting the polyvinyl alcohol casting solution onto a casting drum and peeling it off from the casting drum to obtain a preliminary polyvinyl alcohol film; (c) a drying step of drying the preliminary polyvinyl alcohol film with a plurality of heating rollers and an oven to obtain a polyvinyl alcohol film; and (d) a temperature and humidity adjustment step of placing the polyvinyl alcohol film in a temperature and humidity adjuster to adjust the temperature and humidity.

Without being bound by any theory, it is likely that in the preparation of polyvinyl alcohol films, the temperature on the cast drum during the film casting process, the drying conditions in the drying process, the temperature, humidity and time in the temperature and humidity adjustment process, and the amount of additives added can control the magnitude of Tan δ or the rate of change in Tan δ.

[About the dissolution process]
According to at least one embodiment, the dissolving step mainly comprises raising the temperature of the polyvinyl alcohol-based resin, the solvent and the additives to at least 130° C. while stirring them to dissolve them uniformly, and then adjusting the resin concentration to 20-50% to obtain a polyvinyl alcohol casting solution.

In one or more embodiments, the polyvinyl alcohol resin used in the dissolution step is polymerized with a vinyl ester resin monomer to form a polyvinyl alcohol resin having a degree of polymerization of 1300 to 5200, and then a saponification reaction is performed to obtain a polyvinyl alcohol resin having a degree of saponification exceeding 95% mole. Examples of the vinyl ester resin monomer include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, or vinyl octoate, or combinations thereof, but are not limited thereto in the present invention. In addition, a copolymer formed by copolymerization of an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used, where the olefin compound includes ethylene, propylene, or butene, but are not limited thereto in the present invention. The amount of the olefin compound added can be 2 to 4 mol%, for example, 2 mol%, 2.5 mol%, 3 mol%, 3.5 mol%, or 4 mol%, but are not limited thereto. Examples of the acrylate derivative include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate, but the present invention is not limited to these.

The "degree of polymerization" referred to in this specification means a measured value obtained by the test method described in JIS K 6726 (1994). In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is in the range of 1300 to 5200, and may be any value within a range between any two of the following values, for example, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100, 3200, 3300, 3400, 3500, 3600, 3700, 3800, 3900, 4000, 4100, 4200, 4300, 4400, 4500, 4600, 4700, 4800, 4900, 5000, 5100, 5200, 5300, 5400, 5500, 5600, 5700, 5800, 5900, 6000, 6100, 6200, 6300, 6400, 6500, 6600, 6700, 6800, 6900, 7000, 7100, 7200, 7300, 7400, 7500, 7600, 7700, 7800, 7900, 8000, 8100, 8200, 8 800, 2900, 3000, 3100, 3200, 3300, 3400, 3500, 3600, 3700, 3800, 3900, 4000, 4100, 4200, 4300, 4400, 4500, 4600, 4700, 4800, 4900, 5000, 5100 or 5200, and in the present invention, the range of 2000 to 3000 is preferred.

The "saponification degree" referred to in this specification means a measured value obtained by the test method described in the JIS K 6726 (1994) standard. In one or more embodiments, the saponification degree of the polyvinyl alcohol-based resin is greater than 95 mole%, for example, greater than 95 mole%, greater than 96 mole%, greater than 97 mole%, greater than 98 mole%, or greater than 99 mole%, in order to obtain better optical properties. When the saponification degree of the polyvinyl alcohol-based resin exceeds 95 mole%, the strength of the resin increases, and the resin is less likely to dissolve during the subsequent dyeing and fixing treatments in the polarizing film process, making it easy to obtain a polarizing film with high polarizing performance. Furthermore, the degree of saponification of the polyvinyl alcohol resin is 99.00 mole% or more, for example, 99.00 mole% or more, 99.15 mole% or more, 99.25 mole% or more, 99.35 mole% or more, 99.45 mole% or more, 99.55 mole% or more, 99.65 mole% or more, 99.75 mole% or more, 99.85 mole% or more, or 99.95 mole% or more, and in the present invention, preferably exceeds 99.85 mole%.

In one or more embodiments, the solvent used in the dissolving step may be one capable of dissolving polyvinyl alcohol resin, and is not particularly limited in the present invention. Examples of the solvent include, but are not limited to, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, and the like, and the above solvents may be used alone or in combination of two or more. Considering environmental and economical aspects, it is preferable to use water as the solvent in the present invention.

In one or more embodiments, the additive used in the dissolving step may be at least one of a surfactant, a plasticizer/plasticizer, etc. The "plasticizer" is specifically, but not limited to, glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trimethylolpropane, etc., or a combination thereof. In the present invention, it is preferably glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, or trimethylolpropane. In one or more embodiments, the amount of the plasticizer added is 5 to 15 wt% based on the weight of the polyvinyl alcohol resin, specifically, for example, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, or 15 wt%, but is not limited thereto.

The aforementioned "surfactant" is not limited to cationic, anionic or nonionic surfactants, and specifically includes carboxylate type such as potassium laurate, sulfate type such as sodium lauryl 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 polyethylene glycol monolaurate, alkylamine type such as polyoxyethylene laurylamino ether, alkylamide type such as polyoxyethylene lauramide, polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether, alkanolamide type such as lauric acid diethanolamide, oleic acid diethanolamide, allyl phenyl ether type such as polyoxyallyl phenyl ether, or sodium lauryl alcohol polyoxyethylene ether sulfate. The amount of additive added affects the rate of change of Tan δ and Tan δ, and it is better to control the amount added within a certain range. In one or more embodiments, the amount of the surfactant added is 1500 to 3000 ppm based on the weight of the polyvinyl alcohol resin, specifically, for example, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm, 2100 ppm, 2200 ppm, 2300 ppm, 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, or 3000 ppm, but is not limited thereto.

In one or more embodiments, the temperature for dissolving the polyvinyl alcohol-based resin aqueous solution is preferably at least 130°C, specifically, for example, at least 130°C, at least 140°C, or at least 150°C, etc.

In one or more embodiments, the concentration of the aqueous polyvinyl alcohol resin solution is preferably 20 to 50%, specifically, for example, 20%, 25%, 30%, 35%, 40%, 45%, or 50%. If the resin concentration is too low, the drying load of the film will be high, and conversely, if the resin concentration is too high, the viscosity will be high, making film formation difficult.

[Casting process]
According to at least one embodiment, the casting step mainly involves degassing the polyvinyl alcohol casting solution (the degassing can be performed using a static degassing method or a multi-screw extruder having a vent hole, but is not limited to, for example, degassing using a twin-screw extruder), controlling the solution temperature to at least 90° C., and then discharging the solution from a T-die lip, and casting the solution onto a cast drum (also called a casting drum) or a cast drum serving as a support such as an endless belt to form a film, thereby obtaining a preliminary polyvinyl alcohol film.

In one or more embodiments, the temperature of the polyvinyl alcohol casting solution is preferably at least 90°C, for example, 90°C, 91°C, 92°C, 93°C, 94°C, or 95°C. In one or more embodiments, the temperature of the casting drum is preferably in the range of, for example, 85 to 95°C, and specifically, for example, but not limited to, 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, 91°C, 92°C, 93°C, 94°C, or 95°C.

[About the drying process]
According to at least one embodiment, the drying process mainly involves peeling the preliminary polyvinyl alcohol film from the casting drum, contact drying the upper and lower surfaces of the preliminary polyvinyl alcohol film with a plurality of heated rollers, and then drying the upper and lower surfaces of the preliminary polyvinyl alcohol film with hot air in an oven to obtain a polyvinyl alcohol film.

In one or more embodiments, the number of the plurality of heating rollers may be 2 to 30, for example, but not limited to, 2, 5, 10, 15, 20, 25, or 30. The temperature of the plurality of heating rollers gradually decreases from high to low, with the first heating roller being the hottest roller among all the heating rollers, and the last heating roller being the coldest among all the heating rollers. In one or more embodiments, the temperature of the first heating roller is preferably in the range of 80 to 100°C, for example, 80°C, 85°C, 90°C, 95°C, or 100°C, among any two of the following values: In one or more embodiments, the temperature of the last heating roller is preferably in the range of 30°C to 60°C, for example, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, or 60°C, among any two of the following values:

In one or more embodiments, the oven is preferably a floating oven, and the temperature is controlled to a range of 40°C to 120°C, specifically a value within a range between any two of the following values, for example, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, or 120°C. The overall average temperature of the oven is preferably in a range of 70°C to 80°C, specifically, for example, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, or 80°C. In one or more embodiments, the drying time of the polyvinyl alcohol film in the oven is a value within a range between any two of the following values, for example, 1 minute, 1.5 minutes, 2 minutes, 2.5 minutes, or 3 minutes, and is preferably in a range of 1 to 3 minutes.

[Temperature and humidity adjustment process]
According to a preferred embodiment of the present application, the polyvinyl alcohol film is further placed in a temperature and humidity controller to adjust the temperature and humidity after the drying process.

In one or more embodiments, the temperature of the temperature and humidity regulator is preferably in the range of 35 to 45°C, for example, 35°C, 40°C, or 45°C. In one or more embodiments, the relative humidity of the temperature and humidity regulator is preferably in the range of 70 to 80%, for example, 70%, 72%, 74%, 76%, 78%, or 80%, for example. In one or more embodiments, the time for which the polyvinyl alcohol film is left in the temperature and humidity regulator is preferably in the range of 5 to 20 minutes, for example, 5 minutes, 10 minutes, 15 minutes, or 20 minutes.

In one or more embodiments, the thickness of the polyvinyl alcohol film obtained by the above-mentioned preparation method is a value within a range between any two of the following values, for example, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, or 80 μm, and is preferably in the range of 30 to 75 μm, and the above values are merely examples and are not limiting. Furthermore, when the obtained polyvinyl alcohol film is used to manufacture a thin polarizing film, the thickness of the polyvinyl alcohol film may be 50 μm or less, preferably 40 μm or less, and particularly preferably 30 μm or less.

Another object of the present invention is to provide an optical film produced from the polyvinyl alcohol film. The "optical film" described in this specification may be a polarizing film, a retardation film, a viewing angle widening film, a brightness enhancing film, or the like, and is preferably a polarizing film.

Hereinafter, the present invention will be described in detail with reference to examples. However, it should be understood that these examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention in any way.

Manufacturing of polyvinyl alcohol film: First, a polyvinyl alcohol resin is heated and melted in a dissolving barrel to form a polyvinyl alcohol casting solution, then the polyvinyl alcohol casting solution is extruded and molded by a T-die lip and a casting drum to form a preliminary polyvinyl alcohol film, and finally the preliminary polyvinyl alcohol film is dried by multiple heating rollers and an oven to obtain a polyvinyl alcohol film.

The following are the main steps for producing polyvinyl alcohol films in this embodiment and the comparative example, and Table 1 below details the differences in one or more process parameters between this embodiment and the comparative example.

1. First, 1800 kg of polyvinyl alcohol resin with a degree of saponification of ≥ 99.85 mole% and a degree of polymerization of approximately 2400, 4000 kg of water, 180 kg of plasticizer glycerin, and lauryl alcohol polyoxyethylene ether (addition amounts shown in Table 1) are added to a dissolution barrel, and the temperature is raised to 130°C while stirring to dissolve uniformly. After that, water is added to adjust the resin concentration to 30.0% to obtain a polyvinyl alcohol casting solution.

2. After the polyvinyl alcohol casting solution is degassed using a twin screw extruder, the temperature of the polyvinyl alcohol casting solution is controlled to 93°C, and then the polyvinyl alcohol casting solution is discharged from a T-die lip and cast onto a rotating high-temperature casting drum, and dried to produce a preliminary polyvinyl alcohol film.

3. After the preliminary polyvinyl alcohol film is peeled off from the casting drum, the top and bottom surfaces of the film are dried by contact with 15 heating rollers. The first heating roller is the hottest roller among all the heating rollers, and the temperature of the succeeding heating rollers is gradually reduced, and the temperature of the 15th heating roller is reduced to 40°C, the lowest among all the heating rollers.

4. Next, the top and bottom surfaces of the preliminary polyvinyl alcohol film are dried with hot air in a floating oven at a drying temperature in the range of 40 to 120°C.

5. Finally, the dried preliminary polyvinyl alcohol film is placed in a temperature and humidity control device and processed to obtain a polyvinyl alcohol film product.

The polyvinyl alcohol films obtained in Examples 1 to 12 and Comparative Examples 1 to 8 were subjected to the following analytical tests, and the analytical test methods used are described in detail below.

<Analysis of Tan δ and Tan δ Change Rate>
Equipment: TA Instruments' DHR HR-20 rheometer. Sample preparation: A polyvinyl alcohol film was cut into a circle with a diameter of 2.5 cm, and then allowed to stand for 5 minutes in an environment with a temperature of 30° C. and a relative humidity of 70% to allow the sample to absorb moisture uniformly.

Test method: Measure in sweep oscillation mode and record the torque and angular displacement detected by the fixture, as well as stress and strain information. Here, sweep oscillation involves the motor rotating clockwise/counterclockwise at the zero point to shear the sample.

Test conditions Mode: Sweep oscillation Frequency range: 0.1 to 350 rad/s (0.01 to 100 Hz)
Strain: 1%
Jig: Flat clamp (diameter 2.5 cm), fixed normal force 1 N
Jig temperature: 25°C
Ambient temperature: 25-30°C
Ambient humidity: 40-60% relative humidity.

Data results: Tan δ corresponding to different frequencies is displayed directly by the instrument.

Data conversion: For the rheometer sweep results, the data results for frequency 1 and 100 rad/s are taken and calculated, and the calculation formula is shown as follows:

<Film surface evaluation>
Preparation of sample: A polyvinyl alcohol film is cut into a film sample having a size of about 40 cm ± 5% in the machine direction and 2.8 m ± 5% in the transverse direction.

Test conditions: The film sample is placed in an aluminum foil bag, sealed, and stored in a room temperature cabinet (23-27°C) for at least one month.

Evaluation criteria: Check the film surface of the film sample for wrinkles and/or cloudiness with an area exceeding 1 cm x 1 cm. If the film surface of the film sample is free of wrinkles or cloudiness, it is rated as "O", and if the film surface of the film sample has wrinkles or cloudiness, it is rated as "X".

<Film cut evaluation>
Test conditions: A polyvinyl alcohol film sample is continuously stretched for 5000 m to produce a polarizing film. The polarizing film is prepared as follows:
i. The polyvinyl alcohol film was immersed in a swelling tank filled with pure water at 30° C., the film surface was washed with water to swell the film, and the film was stretched along the machine direction (MD) to 1.2 times its original length;
ii. Dyeing the pre-stretched film in a dyeing tank containing an aqueous solution of 0.37 g/L iodine and 18.5 g/L potassium iodide at a temperature of 30° C., and stretching the film in the machine direction to 3.4 times its original length;
iii. The film after twice stretching and dyeing is immersed in a stretching tank containing an aqueous solution of boric acid and potassium iodide at 55° C. and 55 g/L, and then the film is stretched along the machine direction to 6 times its original length;
iv. The film stretched three times is immersed in an aqueous solution containing 55 g/L of potassium iodide to wash the iodine solution remaining on the film surface and boric acid;
v. The washed film was dried in an oven at 60° C. for 5 minutes to complete the preparation of the polarizing membrane.

Evaluation criteria: Observe whether the film sample has any breaks during the polarizing film preparation process. If the film is not broken during the preparation process, it is rated as "◎", if the film breaks once during the preparation process, it is rated as "O", and if the film breaks multiple times during the preparation process, it is rated as "X".

<Polarization degree evaluation>
Test conditions: a polarizer made from a polyvinyl alcohol film sample was cut into two square samples of 4 cm (MD) × 4 cm (TD), and then the multiple polarizers were stacked in the same flow direction and the light transmittance H ₁₁ was measured under irradiation with a spectrophotometer at a wavelength of 700 nm; then the multiple polarizers were stacked in the perpendicular flow direction (orthogonal direction) and the light transmittance H ₁ was similarly measured under irradiation with a spectrophotometer at a wavelength of 700 nm; finally, the measured light transmittances H ₁₁ and H ₁ were calculated, and the calculation formula was as follows:

Degree of polarization (%)=[(H ₁₁ −H ₁ )/(H ₁₁ +H ₁ )] ^1/2

The results obtained after carrying out the above analytical tests on the polyvinyl alcohol films of Examples 1 to 12 and Comparative Examples 1 to 8 are summarized in Table 2 below.

As can be seen from Table 2, the polyvinyl alcohol films of Examples 1 to 12 have a Tan δ T1 of 0.40 to 0.93 when measured at an oscillation frequency of 100 rad/s under shear mode at 25°C, a Tan δ T2 of 0.20 to 0.46 when measured at an oscillation frequency of 1 rad/s, and T2/T1 does not exceed 0.75. The polyvinyl alcohol films have an excellent film surface evaluation (i.e., no wrinkles or cloudiness on the film surface), the number of times that film breakage occurred during the polarizing film production process was limited to one, and the polarizing film produced from the film has a polarization degree of 99.9%. In particular, the films of Examples 2 to 6 and 8 did not experience film breakage during the polarizing film production process. In contrast, the polyvinyl alcohol films of Comparative Examples 1 and 4 to 7 have a Tan δ T1 of 0.40 to 0.93 measured at an oscillation frequency of 100 rad/s under a shear mode in which the temperature is not controlled at 25°C, a Tan δ T2 of 0.20 to 0.46 measured at an oscillation frequency of 1 rad/s, and T2/T1 does not exceed 0.75, so that the polyvinyl alcohol films cannot simultaneously have an excellent film surface evaluation and a film break occurrence rate of only one in the polarizing film manufacturing process. Even though the polyvinyl alcohols of Comparative Examples 2, 3, and 8 have an excellent film surface evaluation and a film break evaluation, the polarizing films manufactured from the polyvinyl alcohols cannot have an excellent polarization degree like the polarizing films manufactured in Examples 1 to 12.

In summary, according to the technical content of the present invention, a polyvinyl alcohol film containing a polyvinyl alcohol resin with a degree of saponification exceeding 95 mole% has a Tan δ T1 of 0.40 to 0.93 measured at an oscillation frequency of 100 rad/s under a shear mode at 25°C, a Tan δ T2 of 0.20 to 0.46 measured at an oscillation frequency of 1 rad/s, and a T2/T1 of not exceeding 0.75, thereby obtaining a polyvinyl alcohol film that is unlikely to cause deformation of the film surface and is unlikely to tear, and at the same time, a polarizing film manufactured from the film has excellent polarization properties.

All ranges provided herein are intended to include each specific range within the stated range, and combinations of subranges between the stated ranges. Also, every range explicitly stated herein includes its endpoints, unless otherwise indicated. Thus, the range 1 to 5 specifically includes 1, 2, 3, 4, and 5, as well as subranges 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 hereby incorporated by reference and for all purposes are to the extent that each individual publication or patent application is specifically and individually indicated to be incorporated by reference into this specification. In the event of a conflict between this specification and a publication or patent application incorporated by reference into this specification, the specification shall control.

As used herein, the terms "comprising," "having," and "including" have an open-ended, non-limiting meaning. The terms "a" and "the" may be intended to include the plural as well, unless the context clearly indicates a singular form. The term "one or more" means "at least one" and can therefore include single features or mixtures/combinations.

Other than in the examples described herein, or unless otherwise specified, all numerical values expressing amounts of ingredients and/or reaction conditions can be modified in all instances by the term "about," which means within ±5% of the stated numerical value. As used herein, the terms "essentially free" or "substantially free" mean less than about 2% of a particular feature. Any element or feature explicitly described in this specification can be negatively excluded from the claims.

Claims (10)

A composition comprising a polyvinyl alcohol resin having a degree of saponification of more than 95 mole %, and at 25° C.:
In shear mode, the loss factor (Tan δ) T1 measured at an oscillation frequency of 100 radians/second (rad/s) is 0.40 to 0.93;
The loss factor (Tan δ) T2 measured in shear mode at an oscillation frequency of 1 radian/second (rad/s) is 0.20 to 0.46;
T2/T1 does not exceed 0.75;
Polyvinyl alcohol film. 2. The polyvinyl alcohol film of claim 1, wherein the polyvinyl alcohol film has a loss factor T1 of 0.58 to 0.85 and T2 / T1 does not exceed 0.60. The polyvinyl alcohol film according to claim 2, wherein the loss coefficient T2 of the polyvinyl alcohol film is 0.30 to 0.45. The polyvinyl alcohol film according to claim 1, wherein the degree of saponification of the polyvinyl alcohol resin is 99.85 mole % or more. 2. The polyvinyl alcohol film according to claim 1 , wherein the degree of polymerization of the polyvinyl alcohol resin is 1,300 to 5,200. 2. The polyvinyl alcohol film according to claim 1 , wherein the degree of polymerization of the polyvinyl alcohol resin is 2,000 to 3,000. The polyvinyl alcohol film according to claim 1 , wherein the polyvinyl alcohol film has a thickness of 30 to 75 μm. An optical film produced from the polyvinyl alcohol film according to any one of claims 1 to 7. The optical film according to claim 8, which is a polarizing film. The optical film according to claim 9, having a polarization degree of 99.99%.