JP7688097B2 - Polyvinyl alcohol film, optical film made from the same, and method for producing the same - Google Patents

Description

The present invention relates to a polyvinyl alcohol (PVA) film that can be used as an optical film, in particular as a polarizing film.

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

The manufacturing methods for producing optical films from polyvinyl alcohol resin are divided into coating and blow molding. The coating method produces products by subjecting the film-forming solution to processes such as defoaming, coating, and drying, while the blow molding method produces products by subjecting the film-forming solution to processes such as defoaming, inflation, bidirectional pulling, and drying. Polyvinyl alcohol films prepared by the blow molding method often have problems with scratches and streaks on the surface, but polyvinyl alcohol films produced by the coating method have excellent performance (e.g. surface smoothness), so polyvinyl alcohol films are now generally produced using the coating method.

If the distribution of microstructures is uneven during the production of polarizing film, lines will appear in the polarizing film, affecting the uniformity of the degree of polarization. Therefore, in order to obtain a polyvinyl alcohol film with excellent optical properties, the surface lines are reduced as much as possible during the preparation process.

A good polarizing film has the characteristics of being free of streak defects and having a uniform degree of polarization, and can provide excellent optical properties. In order to improve the optical properties of polarizing films, conventional techniques have attempted to improve the optical properties by improving equipment technology and selecting the materials of the related components.

However, in the case of optical films manufactured using polyvinyl alcohol films in the prior art, the phenomenon of non-uniform polarization degree often occurred. The inventors of the present application realized that the phenomenon of non-uniform polarization degree occurs because the distribution of the microstructure in the original film before the polyvinyl alcohol film is prepared into a polarizing film is probably non-uniform, which causes streak defects to appear on the surface of the polarizing film that is finally prepared.

To solve the above problems, the present invention provides a polyvinyl alcohol film, which has a uniform distribution of microstructure, so that the polarizing film made from the polyvinyl alcohol film can have the characteristics of being free of streak defects and having a uniform degree of polarization.

One object of the present invention is to provide a polyvinyl alcohol film, which, when analyzed by differential scanning calorimetry (DSC) after immersion in pure water and drying, has a crystallization temperature peak value range of 0.1 to 3°C, and the difference value of the peak part after first differentiation of the relative temperature change curve graph of the heat flow is 2 to 7°C.

In a preferred embodiment, the polyvinyl alcohol film has a peak crystallization temperature of 206°C or higher.

In a preferred embodiment, the polyvinyl alcohol film has a degree of swelling in the transverse direction (TD) of 10 to 19% after immersion in water for 30 seconds.

In a preferred embodiment, the polyvinyl alcohol film has a degree of swelling in the transverse direction (TD) of greater than 22% after immersion in water for 15 minutes.

Another object of the present invention is to provide a polyvinyl alcohol film, which upon small angle X-ray scattering (SAXS) analysis after standing for 24 hours under conditions of 23° C./50% RH has a peak value range of scattering intensity (I(q)) of less than 0.06 nm ⁻¹ for scattering vectors (q) between 0.3 and 0.7 nm ⁻¹ .

In a preferred embodiment, the polyvinyl alcohol film has a degree of polymerization between 1800 and 3000.

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

In a preferred embodiment, the optical film is a polarizing film.

In a preferred embodiment, the polarizing film has a standard deviation of the degree of polarization measured at multiple positions of less than 0.006.

It is yet another object of the present invention to provide a method for producing a polyvinyl alcohol film, which includes the steps of: (a) drying and water-washing a polyvinyl alcohol-based resin; (b) stirring and heating the polyvinyl alcohol-based resin, a surfactant, a plasticizer, and water until dissolved to form a polyvinyl alcohol casting solution; (c) casting the polyvinyl alcohol casting solution into a casting drum and drying to produce a preformed film; and (d) exposing the preformed film to multiple heated rollers whose temperature is gradually decreased from high to low, and then entering a dryer with multiple sections for heat treatment.

In a preferred embodiment, in step (a) of the method for producing the polyvinyl alcohol film, the drying temperature of the polyvinyl alcohol resin is 115 to 125°C.

In a preferred embodiment, in step (a) of the method for producing the polyvinyl alcohol film, the weight ratio of the water for washing to the polyvinyl alcohol resin is 9 or more.

In a preferred embodiment, in step (c) of the method for producing the polyvinyl alcohol film, the wind speed difference in the transverse (TD) direction during drying of the casting drum is 2.5 m/s or less.

In a preferred embodiment, in step (d) of the method for producing the polyvinyl alcohol film, the total contact time between the preformed film and the heating roller having a temperature of more than 85°C is 6 to 20 seconds.

In a preferred embodiment, in step (d) of the method for producing the polyvinyl alcohol film, the standard deviation of the temperature in the width direction (TD) of the dryer is less than 2.6.

The effect of the present invention is that the polyvinyl alcohol film provided by the present invention has a uniform distribution of its microstructure, and when it is subsequently used to manufacture an optical film, it can have a high degree of polarization uniformity and is free of streak defects.

1 is a graph showing a temperature drop curve of a polyvinyl alcohol film according to one embodiment of the present invention, which is obtained by a DSC analysis method, and which is differentiated using Origin's data analysis software. FIG. 2 is a SAXS scattering diagram of a polyvinyl alcohol film according to one embodiment of the present invention. FIG. 2 is a schematic diagram of iodide ions in a polyvinyl alcohol film reacting with amorphous regions of polyvinyl alcohol during dyeing according to one embodiment of the present invention. FIG. 2 is a schematic diagram of a sample piece cut to an area of 10 cm×10 cm of polyvinyl alcohol film according to one embodiment of the present invention.

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

The terms "a" and "a kind" are used herein to refer to the presence of one or more (i.e., at least one) of the grammatical object herein.

The polyvinyl alcohol film provided by the present invention has a crystallization temperature peak value range of 0.1 to 3°C when analyzed by differential scanning calorimetry (DSC) after immersing in pure water and drying, and the difference value of the peak part after first differentiation of the relative temperature change curve of the heat flow is 2 to 7°C.

As used herein, "crystallization temperature" refers to the temperature required for a substance to transition from a liquid to a crystalline state, and "peak crystallization temperature" refers to the Tc,p (peak of crystallization temperature) value, which is the average value obtained by measuring the crystallization temperatures at multiple positions (e.g., but not limited to, three positions) on a sample. In one preferred embodiment, the peak crystallization temperature is 206°C or higher, e.g., 206°C or higher, 206.5°C or higher, 207°C or higher, or 207.5°C or higher.

The aforementioned "range" refers to the difference between the maximum and minimum numbers in a set of data, i.e., the range of the data, and the "peak value range of crystallization temperature during analysis by differential scanning calorimetry (DSC)" refers to the Tc,p deviation, which is the value obtained by subtracting the minimum value from the maximum value after measuring the crystallization temperatures at multiple positions (for example, but not limited to, three positions) on a sample during analysis by differential scanning calorimetry (DSC). In a preferred embodiment, the polyvinyl alcohol film has a peak value range of crystallization temperature of 0.1 to 3°C, for example, 0.1 to 3°C, 0.1 to 2.5°C, 0.1 to 2°C, 0.1 to 1.5°C, 0.1 to 1°C, 0.1 to 0.5°C, 0.5 to 3. ℃, 0.5-2.5℃, 0.5-2℃, 0.5-1.5℃, 0.5-1℃, 1-3℃, 1-2.5℃, 1-2℃, 1-1.5℃, 1.5-3℃, 1.5-2.5℃, 1.5-2℃, 2-3℃, 2-2.5℃, 2.5-3℃.

The aforementioned "difference value of the peak portion after first differentiation of the relative temperature change curve of that heat flow" refers to the value obtained by plotting a graph of the relative temperature change curve of heat flow using analysis by differential scanning calorimetry (DSC), the measurement concept diagram of which is as shown in Figure 1, differentiating using software (such as Origin's data analysis software) to obtain a differential line, defining the highest value near the crystallization peak as Tc,2 and the lowest value as Tc,1, calculating Tc,1-Tc,2, and then averaging Tc,1-Tc,2 at multiple positions (for example, three positions, although this is not limited thereto). In a preferred embodiment, the polyvinyl alcohol film has a peak difference value of 2 to 7°C after first differentiation of a relative temperature change curve of heat flow, for example, 2 to 7°C, 2 to 6.5°C, 2 to 6°C, 2 to 5.5°C, 2 to 5°C, 2 to 4.5°C, 2 to 4°C, 2 to 3.5°C, 2 to 3°C, 2 to 2.5°C, 2.5 to 7°C, 2.5 to 6.5°C, 2.5 to 6°C, 2.5 to 5.5°C, 2.5 to 5°C, 2.5 to 4.5°C, 2.5 to 4°C, 2.5 to 3.5°C, 2. . 5-3°C, 3-7°C, 3-6.5°C, 3-6°C, 3-5.5°C, 3-5°C, 3-4.5°C, 3-4°C, 3-3.5°C, 3.5-7°C, 3.5-6.5°C, 3.5-6°C, 3.5-5.5°C, 3.5-5°C, 3.5-4.5°C, 3.5-4°C, 4-7°C, 4-6.5°C, 4-6°C, 4-5.5°C, 4-5°C, 4-4.5°C, 5-7°C, 5-6.5°C, 5-6°C, 5-5.5°C, 6-7°C, 6-6.5°C, 6.5-7°C.

The present invention uses the crystallization peak value range (Tc,p deviation) and the peak difference value (Tc,1-Tc,2) obtained by first differentiation of the relative temperature change curve of the heat flow as indicators of the crystallization temperature distribution. The larger the value, the greater the deviation in the crystallization temperature or the greater the distribution width, indicating a more non-uniform distribution of the microstructure, and the more likely it is that lines will appear on the surface of the polarizing plate that the polyvinyl alcohol film will later form.

In one preferred embodiment of the present invention, the polyvinyl alcohol film of the present invention has a degree of swelling in the transverse direction (TD) of 10 to 19% after immersion in water for 30 seconds. In another preferred embodiment of the present invention, the polyvinyl alcohol film of the present invention has a degree of swelling in the transverse direction (TD) of greater than 22% after immersion in water for 15 minutes.

The term "swelling degree" as used herein refers to the change in length of a polymeric substance before and after it is swollen in a solvent, and is defined by the following formula:
Cross-linked polymeric materials are not dissolved in solvents, and the solvent can only penetrate into the polymer network structure, causing swelling. The higher the network density of the polymeric material, the less the solvent can penetrate into the network structure, and the smaller the swelling degree. Temperature and solvent affect the swelling degree of polymeric materials, and the greater the force of the solvent and polymer, the greater the swelling degree. "After immersing in water for 30 seconds" refers to placing a test piece with a fixed length in the transverse (TD) direction (e.g., 10 cm) in a thermostatic tank at 30°C, immersing for 30 seconds, and then measuring the transverse (TD) length after swelling. Without being limited to a particular theory, the inventors have found that if the transverse (TD) swelling degree (hereinafter referred to as the 30-second TD swelling degree) of a polyvinyl alcohol film after immersing in water for 30 seconds is too high or too low, it will cause lines to appear on the surface of the final polarizing plate. In one preferred embodiment, the polyvinyl alcohol film has a transverse (TD) swelling of 10-19% after immersion in water for 30 seconds.

In this specification, "after immersion in water for 15 minutes" refers to placing a test piece of a fixed length in the transverse direction (TD) (e.g., 10 cm) in a thermostatic tank at 30°C, immersing it for 15 minutes, and then measuring the length in the transverse direction (TD) after swelling. In one preferred embodiment, the polyvinyl alcohol film has a degree of swelling in the transverse direction (TD) after immersion in water for 15 minutes (hereinafter referred to as the 15-minute TD swelling degree) of greater than 22%.

The polyvinyl alcohol film further provided by the present invention has a peak value range of scattering intensity (I(q)) of less than 0.06 nm ⁻¹ when the scattering vector (q) is between 0.3 and 0.7 nm ⁻¹ in small angle X-ray scattering (SAXS) analysis after standing for 24 hours under conditions of 23° C./50% RH.

The aforementioned "small angle X-ray scattering (SAXS)" is one of the methods for non-destructively investigating the microstructure of a material, and utilizes the scattering phenomenon of X-rays and electrons. SAXS can accurately observe and measure the size, particle size distribution, shape, and orientation of nanoparticles and nanovoids of a material, and the scattering vector (q) is defined as 4πλ ^-1 sinθ (2θ is the scattering angle), and a calibration routine is performed using silver behenate. The "peak value range of the scattering intensity (I(q)) when the scattering vector (q) is between 0.3 and 0.7 nm ^-1 (hereinafter referred to as the scattering intensity peak value range)" refers to a range obtained by performing a SAXS analysis after leaving the sample to stand for 24 hours under conditions of 23°C/50% RH (relative humidity), plotting the scattering intensity (I(q)) on the vertical axis and the scattering vector (q) on the horizontal axis as shown in FIG. 2, obtaining the position of the highest point of the scattering intensity (I(q)) corresponding to the data interval of 0.3 to 0.7 nm ^-1 of the scattering vector (q) (since most of the scattering from the plate-like crystal structure of polyvinyl alcohol falls within this interval), and subtracting the minimum value from the maximum value of the highest point data for multiple (not limited to, for example, three) samples with a fixed area (for example, 1 cm x 1 cm). In preferred embodiments, the range is less than 0.06 nm ⁻¹ , specifically, for example, less than 0.06 nm ⁻¹ , less than 0.055 nm ⁻¹ , less than 0.05 nm ⁻¹ , less than 0.045 nm ⁻¹ , less than 0.04 nm ⁻¹ , less than 0.035 nm ⁻¹ , less than 0.03 nm ⁻¹ , less than 0.025 nm ⁻¹ , less than 0.02 nm ⁻¹ , less than 0.015 nm ⁻¹ , less than 0.01 nm ⁻¹ , less than 0.005 nm ⁻¹ .

The smaller the peak value range of the scattering intensity (I(q)) when the scattering vector (q) is between 0.3 and 0.7 nm ^-1 , the more uniform the distribution of the microstructure of the polyvinyl alcohol film is, and the smaller the standard deviation of the polarization degree of a polarizing plate manufactured using the polyvinyl alcohol film is.

The "degree of polymerization" described in this specification is an index for evaluating the molecular size of a polymer, and refers to the number of repeating units in a polymer molecule. In a preferred embodiment, the polyvinyl alcohol film has a degree of polymerization of 1800 to 3000.

The term "moisture content" used in this specification refers to the amount of water contained in a substance, and is defined by the following formula:
In a preferred embodiment, the water content of the polyvinyl alcohol film is 0.1 to 5.0 wt %.

The present invention further provides an optical film comprising the polyvinyl alcohol film of the present invention.

The term "optical film" used in this specification may refer to a polarizing film, a blue light cut film, a filter film, etc., but the present invention is not limited to these. Preferably, the polyvinyl alcohol film of the present invention is a polarizing film.

The term "degree of polarization" used in this specification refers to the ratio of the light intensity of the polarized portion to the total light intensity, and is defined by the following formula.
Here, _H0 is the light transmittance measured when two polarizing films are stacked in the same orientation direction, and _H90 is the light transmittance measured when two polarizing films are stacked in the perpendicular orientation direction. The measurement conditions of the present invention are, for example, a polarizing plate manufacturing process for a polyvinyl alcohol film, a Perkin Elmer Lambda 365 is used as a measuring instrument, and luminosity correction is performed in the visible light region of 2° based on the standard method of JIS Z 8722 (2009) using a C light source.

The "degree of polarization measured at multiple positions" described in this specification refers to multiple pieces of polarization degree data measured at multiple positions (e.g., but not limited to, three positions) on a single sample using the above-mentioned polarization degree measurement method. In a preferred embodiment, the polarizing film has a standard deviation of the degree of polarization measured at multiple positions of less than 0.006, for example, less than 0.006, less than 0.005, less than 0.004, less than 0.003, less than 0.002, or less than 0.001.

The standard deviation indicates the degree of dispersion of a set of data relative to the average value, and a larger standard deviation means that most of the values are more different from their average value, and a smaller standard deviation means that the values are less different from their average value. The present invention uses the standard deviation of the degree of polarization to evaluate the uniformity of the degree of polarization of a polarizing film, and a larger standard deviation indicates a more non-uniform degree of polarization, and a smaller standard deviation indicates a more uniform degree of polarization.

In another aspect, the present invention also provides a method for producing the polyvinyl alcohol film, which includes the steps of drying and water-rinsing the polyvinyl alcohol resin, stirring and heating the polyvinyl alcohol resin, surfactant, plasticizer, and water until dissolved to form a polyvinyl alcohol casting solution, casting the polyvinyl alcohol casting solution into a casting drum and drying to produce a preformed film, and exposing the preformed film to multiple heated rollers whose temperature is gradually reduced from high to low, followed by heat treatment in a dryer having multiple sections.

The polyvinyl alcohol resin is obtained by forming a polyvinyl ester resin by polymerization of a vinyl ester resin monomer, followed by a saponification reaction. The vinyl ester resin monomer includes vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate, but the present invention is not limited thereto, and is preferably vinyl acetate. Copolymers formed by copolymerizing an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used. The olefin compound includes ethylene, propylene, butylene, but the present invention is not limited thereto. The acrylate derivative includes acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, or n-butyl acrylate, but the present invention is not limited thereto.

The polyvinyl alcohol resin is dried at a maximum temperature of preferably 115-125°C. According to at least one embodiment, the polyvinyl alcohol resin is dried at 115-125°C, specifically, for example, 115°C, 120°C, or 125°C. If the temperature is higher than 125°C, localized dehydration or cross-linking is likely to occur, the crystallization temperature of the film becomes non-uniform, Tc,1-Tc,2 becomes large, and the deviation of Tc,p also becomes large. If the temperature is lower than 115°C, the resin is not sufficiently dried, which affects productivity.

The dried polyvinyl alcohol resin is washed with water to remove impurities (e.g., methanol, sodium acetate, etc.) in the resin, and the weight ratio of water to the polyvinyl alcohol resin is preferably 9 or more. According to at least one embodiment, the weight ratio of water to the polyvinyl alcohol resin when washing the dried polyvinyl alcohol resin with water is 9 or more, specifically, for example, >9 or >10. If the weight ratio of water to the polyvinyl alcohol resin is too low, impurities cannot be completely removed, dehydration reaction during drying of the film becomes more likely, Tc,1-Tc,2 becomes large, and the deviation of Tc,p also becomes large. If the weight ratio of water to the polyvinyl alcohol resin is too high, the productivity of the polyvinyl alcohol film is affected.

In preparing the polyvinyl alcohol casting solution, the polyvinyl alcohol resin, surfactant, plasticizer, and water are stirred and heated until dissolved. The surfactant is, for example, a cationic, anionic, or nonionic surfactant, and examples of the surfactant include, but are not limited to, carboxylate types such as potassium laurate, sulfate ester types such as sodium laureth sulfate, sulfonate types such as dodecylbenzenesulfonate, alkylphenyl ether types such as polyoxyethylene octylphenyl ether, alcohol-based phenyl ether types such as polyethylene glycol monooctylphenyl ether, alkyl ester types such as polyoxyethylene laurate, alkylamine types such as polyoxyethylene laurylamine, alkylamide types such as polyoxyethylene lauric acid amide, polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether, alkanolamide types such as lauric acid diethanolamide and oleyl diethanolamide, and allylphenyl ether types such as polyoxyethylene allylphenyl ether.

In preparing the polyvinyl alcohol casting solution, a plasticizer can enhance the processability of the film, and examples of usable plasticizers include, but are not limited to, phthalate esters (Phthalates), bis(2-ethylhexyl) phthalate (DEHP), glycerin, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. Preferably, the plasticizer is glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane.

The preformed film is prepared by casting a polyvinyl alcohol casting solution into a casting drum and drying it. The casting solution is defoamed in a twin-screw extruder, then discharged from a T-slit die lip, and curtain-coated onto a rotating, high-temperature casting drum and dried to prepare a film. Among them, the twin-screw extruder has good raw material supply performance, kneading and plasticizing performance, exhaust performance, and extrusion stability, and can obtain sufficient dispersion, stirring, compression, and kneading of the casting solution to increase the production capacity of film production, and controls the size of the extrudate to an appropriate thickness and good uniformity through the T-slit die lip. In a more excellent embodiment, the wind speed difference in the transverse (TD) direction on the air side of the casting drum is 2.5 m/s or less, preferably 1 m/s, for example, 2.5 m/s, 2.0 m/s, 1.5 m/s, or 1.0 m/s. Here, the "wind speed difference" refers to the range of wind speeds measured at multiple (not limited to, for example, three) positions in the transverse (TD) direction. The inventor discovered that the greater the difference in wind speed across the width (TD) of the air on the casting drum, the more non-uniform the distribution of long periods in the final polyvinyl alcohol film becomes, and the more non-uniform the degree of polarization also becomes. The long period is determined by the position of the peak in the SAXS data, and in the process of preparing the polyvinyl alcohol film into a polarizing plate, as shown in Figure 3, iodide ions react with the amorphous regions of polyvinyl alcohol during dyeing, so if the long period is non-uniform across the width, the degree of polarization becomes non-uniform.

The heat treatment of the preformed film involves contacting the preformed film with multiple heating rollers that gradually decrease in temperature from high to low, and then entering a dryer equipped with multiple sections for heat treatment. After the preformed film is peeled off from the casting drum, it is brought into contact with multiple heating rollers to dry both the top and bottom surfaces of the film; the number of heating rollers is not particularly limited and can be adjusted as necessary. In accordance with at least one preferred embodiment, the total contact time of the preformed film with the heated roller having a temperature above 85° C. is from 6 to 20 seconds, preferably 10 seconds, e.g., from 6 to 20 seconds, 6 to 18 seconds, 6 to 16 seconds, 6 to 14 seconds, 6 to 12 seconds, 6 to 10 seconds, 6 to 8 seconds, 8 to 20 seconds, 8 to 18 seconds, 8 to 16 seconds, 8 to 14 seconds, 8 to 12 seconds, 8 to 10 seconds, 10 to 20 seconds, 10 to 18 seconds, 10 to 16 seconds, 10 to 14 seconds, 10 to 12 seconds, 12 to 20 seconds, 12 to 18 seconds, 12 to 16 seconds, 12 to 14 seconds, 14 to 20 seconds, 14 to 18 seconds, 14 to 16 seconds, 16 to 20 seconds, 16 to 18 seconds, 18 to 20 seconds. If the total contact time is too short, less than 6 seconds, the surface crystallinity of the PVA film is insufficient, the degree of swelling in the transverse direction (TD) becomes excessive, and deep lines will appear when processed into a polarizing plate. Furthermore, after repeated research and testing, it was concluded that if the total contact time of the heating roller is too long, more than 20 seconds, the surface crystallinity of the polyvinyl alcohol film becomes too high, the degree of swelling in the transverse direction (TD) becomes insufficient, and lines will appear when processed into a polarizing plate.

In addition, in the portion where the molded film is heat-treated by entering a floating-type dryer (FD) having multiple sections, the number of dryers is not particularly limited and can be adjusted as necessary. In at least one preferred embodiment, the standard deviation of the temperature in the width (TD) direction of the dryer is less than 2.6, for example, less than 2.6, less than 2.4, less than 2.2, less than 2.0, less than 1.8, less than 1.6, less than 1.4, less than 1.2, less than 1.0, less than 0.8, less than 0.6, less than 0.4, less than 0.2. The inventors have discovered that when the standard deviation of the temperature in the width (TD) direction of the dryer is 2.6 or more, the distribution of the long period of the PVA film in the width (TD) direction becomes non-uniform, resulting in a phenomenon in which the degree of polarization becomes non-uniform when processed into a polarizing plate.

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

Below, non-limiting methods for preparing polyvinyl alcohol films are provided. Eight types of polyvinyl alcohol films (Examples 1-8) as non-limiting examples and five types of polyvinyl alcohol films (Comparative Examples 1-5) as comparative examples were prepared based on methods similar to those disclosed below. However, the specific methods for preparing Examples 1-8 and Comparative Examples 1-5 typically differ from the methods disclosed below in one or more aspects.

Specifically, the method for producing the polyvinyl alcohol film includes the following steps. The polyvinyl alcohol resin pellets are dried at the temperature process shown in the table below to obtain the raw material. Before uniformly dissolving the polyvinyl alcohol raw material, plasticizer, water, etc., the raw material polyvinyl alcohol is first washed with water to remove impurities (such as sodium acetate), and then uniformly dissolved with the plasticizer, water, etc. to obtain a film-forming stock solution. The film-forming stock solution is defoamed using a twin-screw extruder, then discharged from a T-slit die lip and curtain-coated on a rotating high-temperature casting drum for drying to prepare a film. The wind speed difference in the transverse (TD) direction on the air side of the casting drum is controlled, and after the preformed film is peeled off from the casting drum, both the upper and lower sides of the film are dried through contact with multiple heating rollers, and the total contact time between the film and the heating rollers is controlled. Next, heat treatment is performed using a floating-type dryer (FD), and the standard deviation of the temperature in the transverse (TD) direction of the dryer is controlled to finally obtain a finished polyvinyl alcohol film.

Examples 1-8 and Comparative Examples 1-5 were evaluated to determine the properties of these polyvinyl alcohol films. Tables 1 and 2 provide an overview of the processes for Examples 1-8 and Comparative Examples 1-5, the attributes of the polyvinyl alcohol films, and the state of polarization uniformity of the polyvinyl alcohol films.

The air side wind speed difference (TD direction) of the casting drum in the above table is the wind speed range in the width direction (TD, transverse direction), and is the value obtained by subtracting the minimum value from the maximum value of the wind speed measured at three positions.

The crystallization temperature peak value (Tc, p value) in the above table was obtained by analysis in the DSC measurement method, which includes pretreatment and crystallization temperature analysis. In the pretreatment, the polyvinyl alcohol film was divided into three equal parts in the width direction, and the center of the equally divided polyvinyl alcohol film was cut. The cut area of each piece was as shown in Figure 4 (machine direction (MD, Machine Direction) 10 cm x transverse direction (TD, Transverse Direction) 10 cm). The 10 cm x 10 cm test piece was immersed in 2000 mL of 30 ° C pure water, stirred with a magnet for 5 minutes, and then removed. The above process was repeated once more to ensure that the additive was thoroughly washed, and then the wet film was placed in a 105 ° C dryer to dry for 1 hour, and the removed film was used as the test sample. The crystallization temperature was analyzed by taking 3-5 milligrams of sample and performing DSC analysis by keeping the sample isothermally at 40°C for 1 minute, then increasing the temperature to 250°C at a rate of 10°C/min, keeping the sample isothermally at 250°C for 1 minute, then decreasing the temperature to 30°C at a rate of 10°C/min, and recording the crystallization temperature. The above-mentioned Tc,p value can be obtained by obtaining the crystallization temperature at three positions and averaging them.

The crystallization temperature peak value range (Tc,p deviation) in the above table was obtained by analysis during the DSC measurement method, and the Tc,p deviation is obtained by subtracting the minimum value from the maximum value of the crystallization temperatures obtained at the three positions.

The peak difference value (Tc,1-Tc,2 value) obtained by first differentiation of the heat flow relative temperature change curve in the above table was obtained by analysis during the DSC measurement method. Using Origin's data analysis software, the temperature drop curve was differentiated to obtain a differential line (as shown in Figure 1), and the lowest value of the line near the crystallization peak was defined as Tc,1, the highest value as Tc,2, and Tc,1-Tc,2 was calculated. The average value of Tc,1-Tc,2 at the three positions was then calculated to obtain the above-mentioned Tc,1-Tc,2 value, which is used as an index of crystallization temperature distribution.

The specific method for analyzing the 30-second TD swelling degree in the above table includes a sample preparation method and a test method. The sample preparation method is to divide the polyvinyl alcohol film into three equal parts in the transverse (TD) direction, and cut the center of the polyvinyl alcohol film, with the cut area of each piece being (MD 10 cm x TD 10 cm) to obtain a test sample. The test method is to put the test sample into a thermostatic tank at 30°C and start measuring the time at the same time, immerse it for 30 seconds, measure the length in the TD direction after swelling, and calculate the TD swelling degree represented by the following formula.

The specific method for analyzing the 15-minute TD swelling degree in the above table includes a sample preparation method and a test method. The sample preparation method is to divide the polyvinyl alcohol film into three equal parts in the transverse (TD) direction, and cut the center of the polyvinyl alcohol film, with the cut area of each piece being (MD 10 cm x TD 10 cm) to obtain a test sample. The test method is to put the test sample into a thermostatic tank at 30°C and start measuring the time at the same time, immerse it for 15 minutes, measure the length in the TD direction after swelling, and calculate the TD swelling degree represented by the following formula.

The SAXS peak position deviation in the above table was obtained during the SAXS analysis, and the specific analysis method includes a sample preparation method and a test method. The sample preparation method is to divide the polyvinyl alcohol film into three equal parts in the width direction, cut the central part of the equal parts into a test piece with an area of (1 cm x 1 cm), and leave the sample at rest under the condition of 23°C/50% RH (relative humidity) for 24 hours, and then perform SAXS analysis. The test method is to perform SAXS analysis using the BL23A small angle X-ray scattering beamline of the National Accelerator Radiation Research Center in Taiwan, and obtain the position of the intensity corresponding to the high point in the data range of 0.3 to 0.7 nm ^-1 . The scattering vector (q) is defined as 4πλ ^-1 sin θ (2θ is the scattering angle), and a calibration routine is further performed using silver behenate, and all data including the transmission of the sample, background, and detector sensitivity are further calibrated.

The method for evaluating the lines in the above table involves visually observing whether or not lines appear on the film surface during the polarizing plate manufacturing process.

The polarization degree in the above table is obtained in the measurement method of the polarization degree, and its specific analysis method includes sample preparation method and test method. Among them, the sample preparation method is to carry out the manufacturing process of a polarizing plate on a polyvinyl alcohol film. The test method is to use Perkin Elmer Lambda 365 as a measuring instrument, and according to the standard method of JIS Z 8722 in 2009, use C light source to perform luminosity correction in the visible light region of 2°, then stack two polarizing films with the same orientation direction, measure the light transmittance (H ₀ ) under the wavelength, and separately stack two polarizing films with the orientation direction perpendicular to the wavelength, measure the light transmittance (H ₉₀ ) under the wavelength, and calculate the polarization degree represented by the following formula:

The standard deviation of the degree of polarization in the table above was obtained during the measurement method for the degree of polarization, and can be obtained by calculating the standard deviation of the degree of polarization at three positions.

The examples in Table 1 show that when a polarizing film is prepared using a polyvinyl alcohol film manufacturing process that conforms to the present invention, the resulting polarizing film has no streaks on its surface and has a good and uniform degree of polarization.

Without being limited to a particular theory, the inventors have discovered that, among the process parameters for polyvinyl alcohol films, the indices that are primarily affected by the maximum temperature of the polyvinyl alcohol resin dryer are the Tc,p value and the Tc,1-Tc,2 value. For example, in the results of Comparative Examples 1 to 5, when the temperature is too high, the Tc,p value and the Tc,1-Tc,2 value become large, which makes it easier for localized dehydration or cross-linking to occur, leading to non-uniform crystallization temperatures of the film and the appearance of lines on the surface of the polarizing film.

The weight ratio of water to polyvinyl alcohol resin in water washing, one of the process parameters for polyvinyl alcohol film, mainly affects the Tc,p value and the Tc,1-Tc,2 value. For example, in the results of Comparative Examples 4 and 5, when the weight ratio of water to polyvinyl alcohol resin in water washing was too small, the dehydration reaction was more likely to occur, the Tc,p value and the Tc,1-Tc,2 value became larger, and lines were more likely to appear on the surface of the polarizing film.

Among the process parameters for polyvinyl alcohol film, the index that is primarily affected by the total contact time with a heating roller at over 85°C is the TD swelling degree for 30 seconds. For example, in the results of Comparative Examples 2 to 4, when the total contact time was too short, the surface crystallinity was insufficient, the TD swelling degree was excessive, and deep lines appeared on the prepared polarizing plate, whereas when the total contact time was too long, the surface crystallinity was too low, the TD swelling degree was insufficient, and lines also appeared on the prepared polarizing plate.

Among the process parameters for polyvinyl alcohol film, the indicator that is primarily affected by the dryer temperature standard deviation (TD direction) and the air speed difference on the casting drum side (TD direction) is the SAXS peak position deviation. For example, in the results of Comparative Examples 2 and 3, when the dryer temperature standard deviation (TD direction) is excessive and the air speed difference on the casting drum side (TD direction) is excessive, the long period of the polyvinyl alcohol film tends to become non-uniform in the TD direction, and the degree of polarization of the prepared polarizing plate becomes non-uniform.

All ranges provided herein are meant to include each specific range within the assigned range and any combination of subranges between the assigned ranges. Also, unless otherwise stated, all ranges provided herein include the endpoints of the range. Thus, the range 1-5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2-5, 3-5, 2-3, 2-4, 1-4, etc.

All publications and patent applications cited in this specification are hereby incorporated by reference, and each and every publication or patent application is specifically and individually indicated to be incorporated by reference into this specification for all purposes. In the event of a conflict between this specification and any publication or patent application incorporated by reference into this specification, this specification controls.

As used herein, the terms "comprise," "have," and "include" have an open, non-limiting meaning. The terms "a" and "the" should be understood to include the plural and the singular. The term "one or more" refers to "at least one" and can therefore include a single property or a mixture/combination of properties.

Except in the operating examples or where otherwise indicated, all numbers indicating amounts of ingredients and/or reaction conditions may in all cases be modified by the term "about" to mean within ±5% of the indicated number. As used herein, the terms "essentially free" or "substantially free" mean less than about 2% of a particular characteristic. Any element or characteristic expressly recited in this specification may be negatively excluded from the claims.

Claims (16)

A polyvinyl alcohol film, which, when analyzed by differential scanning calorimetry (DSC) after immersing in pure water and drying, has a peak value of crystallization temperature in the range of 0.1 to 3°C and a peak difference value after first differentiation of a relative temperature change curve of the heat flow is 2 to 7°C ,
The method for measuring the peak value range of the crystallization temperature and the difference value of the peak portion through the first derivative of the relative temperature change curve of the heat flow is
Divide the polyvinyl alcohol film into three equal parts in the transverse direction (TD), cut the center of each polyvinyl alcohol film to obtain a polyvinyl alcohol film with an area of 10 cm along the machine direction (MD) x 10 cm along the transverse direction, immerse the 10 cm x 10 cm polyvinyl alcohol film in 2000 ml of pure water at 30°C and stir with a magnet for 5 minutes to obtain a total of three wet polyvinyl alcohol films, repeat the previous step once, and then put the three wet polyvinyl alcohol films into a 105°C oven to dry for 1 hour to obtain three polyvinyl alcohol films to be measured, including pretreatment;
Polyvinyl alcohol film . The polyvinyl alcohol film according to claim 1, wherein the peak value of the crystallization temperature is 206°C or higher. The polyvinyl alcohol film according to claim 1, which has a degree of swelling in the transverse direction (TD) of 10 to 19% after immersion in water for 30 seconds. The polyvinyl alcohol film of claim 1, which has a degree of swelling in the transverse direction (TD) of greater than 22% after immersion in water for 15 minutes. A polyvinyl alcohol film, in which, in a small angle X-ray scattering (SAXS) analysis after being left at rest for 24 hours under conditions of 23° C./50% RH, the peak value range of scattering intensity (I(q)) at a scattering vector (q) between 0.3 and 0.7 nm ⁻¹ is less than 0.06 nm ⁻¹ ,
The method for measuring the peak value range of the scattering intensity (I(q)) is
The method includes a sample preparation method in which a polyvinyl alcohol film is divided into three equal parts in the transverse direction (TD), the center of each polyvinyl alcohol film is cut to obtain a polyvinyl alcohol film having an area of 1 cm x 1 cm, and the 1 cm x 1 cm polyvinyl alcohol film is then left to stand for 24 hours under conditions of 23°C/50% RH to obtain three polyvinyl alcohol films to be measured.
Polyvinyl alcohol film . The polyvinyl alcohol film according to any one of claims 1 to 5, wherein the degree of polymerization of the polyvinyl alcohol film is between 1800 and 3000. The polyvinyl alcohol film according to any one of claims 1 to 5, wherein the polyvinyl alcohol film has a water content of between 1.0 and 5.0 wt%. An optical film produced from the polyvinyl alcohol film according to claim 1 or 5. The optical film according to claim 8, which is a polarizing film. The optical film according to claim 9, wherein the standard deviation of the degree of polarization measured at multiple positions in the polarizing film is less than 0.006. A method for producing the polyvinyl alcohol film according to claim 1 or 5,
(a) drying and washing the polyvinyl alcohol-based resin with water;
(b) stirring and heating the polyvinyl alcohol-based resin, surfactant, plasticizer, and water until dissolved to form a polyvinyl alcohol casting solution;
(c) casting the polyvinyl alcohol casting solution onto a casting drum and drying to form a preformed film;
and (d) contacting the preformed film with a plurality of heated rollers whose temperature is gradually decreased from a high temperature to a low temperature, and then entering a dryer having a plurality of sections for heat treatment. The method for producing a polyvinyl alcohol film according to claim 11, wherein the drying temperature of the polyvinyl alcohol resin in step (a) is 115 to 125°C. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (a), the weight ratio of the water for washing to the polyvinyl alcohol resin is 9 or more. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (c), the wind speed difference in the width (TD) direction during drying of the casting drum is 2.5 m/s or less. The method for producing a polyvinyl alcohol film according to claim 11, wherein the total contact time between the preformed film and the heating roller having a temperature of more than 85°C during step (d) is 6 to 20 seconds. The method for producing a polyvinyl alcohol film according to claim 11, wherein during step (d), the standard deviation of the temperature in the width direction (TD) of the dryer is less than 2.6.