Polyvinyl alcohol film, optical film made therefrom, and method for producing the same

Abstract

To provide a polyvinyl alcohol film, an optical film composed of the same, and a method for manufacturing the same.SOLUTION: There are provided a polyvinyl alcohol film, an optical film composed of the same, and a method for manufacturing the same, wherein in analysis by differential scan calorimetry (DSC) after the polyvinyl alcohol film has been immersed in pure water and dried, a peak value range of a crystallization temperature is 0.1 to 3°C, and a difference value of a peak part through primary differential of a relative temperature change curve of its heat flow is 2 to 7°C. A polarization film produced from the polyvinyl alcohol film has characteristics having no defect of a streaks.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a polyvinyl alcohol (PVA) film that can be used as an optical film, particularly as a polarizing film.

Background Art

[0002] Polyvinyl alcohol (PVA) film is a kind of hydrophilic polymer, which has properties such as transparency, mechanical strength, water solubility, and good processability, and is widely used in packaging materials or optical films of electronic products, particularly polarizing films.

[0003] The manufacturing methods for producing optical films from polyvinyl alcohol resins can be divided into a coating method and a blow molding method. The coating method manufactures products through processes such as defoaming, coating, and drying of the film-forming stock solution, and the blow molding method manufactures products through processes such as defoaming, inflation, biaxial stretching, and drying of the film-forming stock solution. The polyvinyl alcohol film prepared by the blow molding method often has problems such as scratches and streaks on the surface, but the polyvinyl alcohol film manufactured by the coating method has excellent performance (such as surface smoothness), so currently, the coating method is generally used to manufacture polyvinyl alcohol films.

[0004] When the distribution of the fine structure is non-uniform during the production of a polarizing film, lines will occur on the polarizing film, which will affect the uniformity of the polarization degree. Therefore, in order to obtain a polyvinyl alcohol film with excellent optical properties, the lines on the surface are reduced as much as possible during the preparation process.

[0005] A good polarizing film has the characteristics of no streak defects and uniform polarization degree, and can provide excellent optical properties. In order to improve the optical properties of polarizing films, the prior art attempts to improve the optical properties by improving equipment technology and selecting the materials of related components.

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the case of an optical film manufactured using a polyvinyl alcohol film in the prior art, the phenomenon of uneven polarization degree often occurred. The inventor of the present application noticed that probably due to the uneven distribution of the fine structure in the original film before the polyvinyl alcohol film was prepared into a polarizing film, streak defects appeared on the surface of the finally prepared polarizing film, resulting in the phenomenon of uneven polarization degree.

Means for Solving the Problems

[0007] To solve the above problems, the present invention provides a polyvinyl alcohol film, which has a uniform distribution of fine structure, thereby enabling the polarizing film manufactured from the polyvinyl alcohol film to have the characteristics of no streak defects and uniform polarization degree.

[0008] One object of the present invention is to provide a polyvinyl alcohol film, which in the analysis by differential scanning calorimetry (DSC) after being immersed in pure water and dried, has a peak value range of crystallization temperature of 0.1 - 3°C, and the difference value of the peak part after passing through the first derivative of the relative temperature change curve graph of its heat flow is 2 - 7°C.

[0009] In a preferred embodiment, the polyvinyl alcohol film has a peak value of crystallization temperature of 206°C or higher.

[0010] In a preferred embodiment, the polyvinyl alcohol film has a swelling degree in the width (TD) direction of 10 - 19% after being immersed in water for 30 seconds.

[0011] In a preferred embodiment, the polyvinyl alcohol film has a swelling degree in the width (TD) direction greater than 22% after being immersed in water for 15 minutes.

[0012] Another object of the present invention is to provide a polyvinyl alcohol film, which has a scattering vector (q) of 0.3 to 0.7 nm in small-angle X-ray scattering (SAXS) analysis after standing still for 24 hours under the conditions of 23 °C / 50% RH. -1 The peak value range of the scattering intensity (I(q)) between is less than 0.06 nm. -1

[0013] In a preferred embodiment, the degree of polymerization of the polyvinyl alcohol film is between 1800 and 3000.

[0014] Another object of the present invention is to provide an optical film, which is made of the polyvinyl alcohol film of the present invention.

[0015] In a preferred embodiment, the optical film is a polarizing film.

[0016] In a preferred embodiment, the standard deviation of the degree of polarization measured at a plurality of positions of the polarizing film is less than 0.006.

[0017] Still another object of the present invention is to provide a method for manufacturing a polyvinyl alcohol film, which includes: (a) a step of drying and then washing with water a polyvinyl alcohol-based resin; (b) a step of stirring and heating a mixture of the polyvinyl alcohol-based resin, a surfactant, a plasticizer, and water until dissolution to form a polyvinyl alcohol casting solution; (c) a step of casting the polyvinyl alcohol casting solution onto a casting drum, drying it to form a preformed film; and (d) a step of contacting the preformed film with a plurality of heating rollers whose temperatures gradually decrease from high to low, and then entering a dryer having a plurality of sections for heat treatment.

[0018] In a preferred embodiment, in step (a) of the method for manufacturing the polyvinyl alcohol film, the drying temperature of the polyvinyl alcohol-based resin is 115 to 125 °C.

[0019] ​In a preferred embodiment, in step (a) of the method for producing the polyvinyl alcohol film, the weight ratio of the water for washing with water to the polyvinyl alcohol-based resin is 9 or more.

[0020] In a preferred embodiment, in step (c) of the method for producing the polyvinyl alcohol film, the wind speed difference in the width (TD) direction during drying of the casting drum is 2.5 m / s or less.

[0021] 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 exceeding 85°C is 6 to 20 seconds.

[0022] 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 (TD) direction of the dryer is less than 2.6.

Advantages of the Invention

[0023] As an effect of the present invention, the polyvinyl alcohol film provided by the present invention has a uniform distribution of its fine structure, and when it is later used in the production of an optical film, it can have a high uniformity of polarization degree and has no streak defects.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0025] The following embodiments do not unduly limit the present invention. Those skilled in the technical field to which the present invention pertains can make modifications and changes to the embodiments discussed in this specification without departing from the spirit or scope of the present invention, and all of them fall within the scope of the present invention.

[0026] The terms "1" and "a kind" in this specification refer to the existence of one or more (i.e., at least one) grammatical objects in this specification.

[0027] In the analysis by differential scanning calorimetry (DSC) after the polyvinyl alcohol film provided by the present invention is immersed in pure water and dried, the peak value range of the crystallization temperature is 0.1 to 3 °C, and the difference value of the peak part after passing through the first derivative of the relative temperature change curve of the heat flow is 2 to 7 °C.

[0028] The "crystallization temperature" described in this specification refers to the temperature required for a substance to transfer from a liquid to a crystal, and the "peak value of the crystallization temperature" refers to the Tc,p (peak of crystallization temperature) value, which is a value obtained by measuring the crystallization temperatures at a plurality of (not limited, for example, 3 locations) positions on the sample and calculating the average. In one preferred embodiment, the peak value of the crystallization temperature is 206 °C or higher, for example, 206 °C or higher, 206.5 °C or higher, 207 °C or higher, 207.5 °C or higher.

[0029] The "range" mentioned above refers to the difference between the maximum and minimum numbers in a set of data, which is the range of that data. The "peak value range of the crystallization temperature during analysis by differential scanning calorimetry (DSC)" is the Tc,p deviation. It refers to the value obtained by measuring the crystallization temperatures at multiple (for example, three locations, without limitation) positions on the sample during analysis by differential scanning calorimetry (DSC) and then subtracting the minimum value from the maximum value. In a preferred embodiment, the polyvinyl alcohol film has a peak value range of the 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 °C, 0.5 to 2.5 °C, 0.5 to 2 °C, 0.5 to 1.5 °C, 0.5 to 1 °C, 1 to 3 °C, 1 to 2.5 °C, 1 to 2 °C, 1 to 1.5 °C, 1.5 to 3 °C, 1.5 to 2.5 °C, 1.5 to 2 °C, 2 to 3 °C, 2 to 2.5 °C, 2.5 to 3 °C.

[0030] The "difference value of the peak part after the first derivative of the relative temperature change curve of the heat flow" mentioned above refers to plotting a relative temperature change curve graph of heat flow using analysis by differential scanning calorimetry (DSC). The measurement conceptual diagram is as shown in Figure 1. Differentiation is performed using software (such as Origin data analysis software), and a differential line is obtained. The highest value near the crystallization peak is defined as Tc,2, and the lowest value is defined as Tc,1. Then, Tc,1 - Tc,2 is calculated, and the value obtained by obtaining the average value of Tc,1 - Tc,2 at multiple (not limited, for example, 3 locations) positions is meant. In a preferred embodiment, the polyvinyl alcohol film has a difference value of the peak part after the first derivative of the relative temperature change curve of the heat flow of 2 to 7 °C. 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 to 3 °C, 3 to 7 °C, 3 to 6.5 °C, 3 to 6 °C, 3 to 5.5 °C, 3 to 5 °C, 3 to 4.5 °C, 3 to 4 °C, 3 to 3.5 °C, 3.5 to 7 °C, 3.5 to 6.5 °C, 3.5 to 6 °C, 3.5 to 5.5 °C, 3.5 to 5 °C, 3.5 to 4.5 °C, 3.5 to 4 °C, 4 to 7 °C, 4 to 6.5 °C, 4 to 6 °C, 4 to 5.5 °C, 4 to 5 °C, 4 to 4.5 °C, 5 to 7 °C, 5 to 6.5 °C, 5 to 6 °C, 5 to 5.5 °C, 6 to 7 °C, 6 to 6.5 °C, 6.5 to 7 °C.

[0031] The present invention uses the peak value range of crystallization (Tc,p deviation) and the difference value of the peak part after the first derivative of the relative temperature change curve of the heat flow (Tc,1 - Tc,2) as indicators of the crystallization temperature distribution. The larger the numerical value, the greater the deviation of the crystallization temperature or the greater the width of the distribution, indicating that the distribution of the fine structure is more uneven, which means that lines are more likely to occur on the surface of the polarizing plate formed later by the polyvinyl alcohol film.

[0032] In one preferred embodiment of the present invention, the polyvinyl alcohol film of the present invention has a swelling degree in the width (TD) direction of 10-19% after being immersed in water for 30 seconds. In another preferred embodiment of the present invention, the polyvinyl alcohol film of the present invention has a swelling degree in the width (TD) direction of more than 22% after being immersed in water for 15 minutes.

[0033] As used herein, the "swelling degree" refers to the measurement of the change in length before and after the polymer substance swells in the solvent, that is, it is defined by the following formula.

Equation

[0034] As used herein, "after being immersed in water for 15 minutes" means that a test piece with a fixed length (for example, 10 cm) in the width (TD) direction is placed in a constant temperature tank at 30°C and immersed for 15 minutes, and then the length in the width (TD) direction after swelling is measured. In one preferred embodiment, the polyvinyl alcohol film has a swelling degree in the width (TD) direction (hereinafter referred to as the 15-minute TD swelling degree) of more than 22% after being immersed in water for 15 minutes.

[0035] The polyvinyl alcohol film further provided by the present invention has a scattered intensity (I(q)) peak value range of less than 0.06 nm between a scattering vector (q) of 0.3 to 0.7 nm in a small-angle X-ray scattering (SAXS) analysis after standing still for 24 hours under the conditions of 23 °C / 50% RH. -1 -1 less than.

[0036] The aforementioned "small-angle X-ray scattering (SAXS)" is one of the methods for examining the fine structure of substances non-destructively and utilizes the scattering phenomena of X-rays and electrons. SAXS can accurately observe and measure the fine structure such as the size and particle size distribution, shape, and orientation of nanoparticles and nano voids in materials. Among them, the scattering vector (q) is defined as 4πλ -1 sinθ (where 2θ is the scattering angle), and a calibration routine is further performed with silver behenate. The "peak value range of the scattered intensity (I(q)) between a scattering vector (q) of 0.3 to 0.7 nm" (hereinafter referred to as the scattered intensity peak value range) means that after standing still for 24 hours under the conditions of 23 °C / 50% RH (relative humidity), SAXS analysis is performed. As shown in Figure 2, the scattered intensity (I(q)) is plotted on the vertical axis and the scattering vector (q) is plotted on the horizontal axis. The position of the highest point of the scattered intensity (I(q)) corresponding to the data interval of 0.3 to 0.7 nm of the scattering vector (q) (since most of the scattering of the plate-like crystal structure of polyvinyl alcohol is within this interval) is obtained. By subtracting the minimum value from the maximum value of the highest point data of a plurality of (not limited, for example, 3) samples with a fixed area (for example, 1 cm * 1 cm), the range can be obtained. In a preferred embodiment, the range is less than 0.06 nm, specifically, for example, less than 0.06 nm, less than 0.055 nm, less than 0.05 nm, less than 0.045 nm, less than 0.04 nm, less than 0.035 nm, less than 0.03 nm, less than 0.025 nm, less than 0.02 nm. -1 -1 -1 less than, -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 Is less than.

[0037] When the aforementioned scattering vector (q) is between 0.3 and 0.7 nm -1 The smaller the peak value range of the scattering intensity (I(q)), the more uniform the distribution of the fine structure of the polyvinyl alcohol film, indicating that the standard deviation of the degree of polarization of the polarizing plate manufactured by the polyvinyl alcohol film is smaller.

[0038] 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.

[0039] The "water content" described in this specification refers to the water content in a substance, which is defined by the following formula.

Equation

[0040] The present invention further provides an optical film composed of the polyvinyl alcohol film of the present invention.

[0041] The "optical film" described 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 thereto. Preferably, the polyvinyl alcohol film of the present invention is a polarizing film.

[0042] The "degree of polarization" described in this specification refers to the ratio of the light intensity of the polarized part to the total light intensity, which is defined by the following formula.

Equation

[0043] The "polarization degrees measured at a plurality of positions" described in this specification refers to a plurality of polarization degree data measured at a plurality (not limited, for example, three positions) of positions on one sample using the aforementioned polarization degree measurement method. In a preferred embodiment, the standard deviation of the polarization degrees measured at a plurality of positions of the polarizing film is 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, less than 0.001.

[0044] The standard deviation indicates the degree of dispersion with respect to the average value of a set of data. If the standard deviation becomes larger, it means that the difference between most of the numerical values and their average value is large. If the standard deviation becomes smaller, it means that the difference between those numerical values and their average value is small. The present invention evaluates the uniformity of the polarization degree of the polarizing film using the standard deviation of the polarization degree. The larger the standard deviation, the more uneven the polarization degree is, and the smaller the standard deviation, the more uniform the polarization degree is.

[0045] As another aspect, the present invention also provides a method for manufacturing the polyvinyl alcohol film. The steps include a step of drying and then washing the polyvinyl alcohol-based resin with water, a step of stirring and heating the polyvinyl alcohol-based resin, a surfactant, a plasticizer, and water until dissolution to form a polyvinyl alcohol casting solution, a step of casting the polyvinyl alcohol casting solution onto a casting drum, drying it to produce a preformed film, and a step of contacting the preformed film with a plurality of heating rollers whose temperature gradually decreases from a high temperature to a low temperature, and then entering a dryer having a plurality of sections for heat treatment.

[0046] The above-mentioned polyvinyl alcohol-based resin is obtained by forming a polyvinyl ester-based resin through polymerization of a vinyl ester-based resin monomer and then performing a saponification reaction. Among them, the vinyl ester-based resin monomer includes vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl pentanoate, and vinyl octanoate. However, the present invention is not limited thereto, and preferably vinyl acetate. In addition, a copolymer formed by copolymerizing an olefin compound or an acrylate derivative with the above-mentioned vinyl ester-based resin monomer can also be used. The olefin compound includes ethylene, propylene, or butylene, etc., 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, etc., but the present invention is not limited thereto.

[0047] The polyvinyl alcohol-based resin is dried, and its maximum temperature is preferably controlled at 115 - 125°C. According to at least one embodiment, the polyvinyl alcohol-based resin is dried at 115 - 125°C. Specifically, for example, it is 115°C, 120°C, or 125°C, etc. When the temperature is higher than 125°C, local dehydration or cross-linking is likely to occur, the crystallization temperature of the film becomes non-uniform, Tc,1 - Tc,2 becomes larger, and the deviation of Tc,p also becomes larger. When the temperature is lower than 115°C, the drying of the resin is insufficient, which affects productivity.

[0048] The dried polyvinyl alcohol-based resin is washed with water to remove impurities (such as methanol and sodium acetate) in the resin, and the weight ratio of the water to the polyvinyl alcohol-based resin is preferably 9 or more. According to at least one embodiment, the weight ratio of the water to the polyvinyl alcohol-based resin when washing the dried polyvinyl alcohol-based resin with water is 9 or more. Specifically, for example, it is >9, or >10, etc. If the weight ratio of the water to the polyvinyl alcohol-based resin is too low, impurities cannot be removed cleanly, and the dehydration reaction during film drying is more likely to occur, resulting in an increase in Tc,1 - Tc,2 and a large deviation in Tc,p. If the weight ratio of the water to the polyvinyl alcohol-based resin is too high, it will affect the productivity of the polyvinyl alcohol film.

[0049] In the preparation of the polyvinyl alcohol casting solution, the polyvinyl alcohol-based resin, surfactant, plasticizer, and water are stirred and heated to dissolve. The surfactant is, for example, but not limited to, a cationic, anionic, or nonionic surfactant. Examples of the surfactant include carboxylate types such as potassium laurate, sulfate ester salt types such as sodium lauryl sulfate, sulfonate types such as dodecylbenzene sulfonate, alkyl phenyl ether types such as polyoxyethylene octyl phenyl ether, alcohol-based phenyl ether types such as polyethylene glycol mono octyl phenyl ether, alkyl ester types such as polyoxyethylene laurate, alkyl amine types such as polyoxyethylene lauryl amine, alkyl amide 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, allyl phenyl ether types such as polyoxyethylene allyl phenyl ether, etc., but are not limited thereto.

[0050] In the preparation of the polyvinyl alcohol casting solution, the plasticizer can enhance the processability of the film. Examples of usable plasticizers include, but are not limited to, phthalates such as phthalic acid esters, 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.

[0051] The preparation of the preformed film is obtained by casting a polyvinyl alcohol casting solution onto a casting drum and then drying it. After defoaming the casting solution with a twin-screw extruder, it is discharged from a T-shaped slit die lip and curtain-coated onto a rotating high-temperature casting drum for drying to prepare a film. Among them, the twin-screw extruder has good raw material supply performance, kneading / plasticizing performance, exhaust performance, and extrusion stability, and can obtain sufficient dispersion, stirring, compression, and kneading in the casting solution to improve the production capacity of film production, and control the size of the extrudate to an appropriate thickness and good uniformity through the T-shaped slit die lip. Also, in a more excellent embodiment, the wind speed difference in the width (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, 1.0 m / s. Here, the "wind speed difference" refers to the range of wind speeds measured at a plurality (not limited, for example, 3) of positions in the width (TD) direction. The inventor has discovered that the larger the wind speed difference in the width (TD) direction on the air side of the casting drum, the more uneven the distribution of the long period in the final polyvinyl alcohol film becomes, and the more uneven the polarization degree becomes. Among them, the long period is obtained from the position of the peak in the SAXS data. In the process of preparing the polyvinyl alcohol film into a polarizing plate, as shown in FIG. 3, since iodide ions react with the amorphous region of polyvinyl alcohol during dyeing, when the long period is uneven in the width direction, a phenomenon occurs in which the polarization degree becomes uneven.

[0052] The heat treatment of the preformed film is carried out by bringing the preformed film into contact with a plurality of heating rollers whose temperature gradually decreases from a high temperature to a low temperature, and then entering a dryer having a plurality of sections for heat treatment. After peeling the preformed film from the casting drum, both the upper and lower surfaces of the film are dried through contact with a plurality of heating rollers. The number of heating rollers is not particularly limited and can be adjusted as needed. According to at least one preferred embodiment, the total contact time between the preformed film and the heating roller with a temperature exceeding 85°C is 6 to 20 seconds, preferably 10 seconds. For example, 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. When the total contact time is too short, less than 6 seconds, the surface crystallinity of the PVA film is insufficient, the swelling degree in the width (TD) direction becomes excessive, and deep lines will occur when processed into a polarizing plate. Also, as a result of repeated research and tests, when the total contact time of the heating roller is too long, exceeding 20 seconds, the surface crystallinity of the polyvinyl alcohol film becomes too high, the swelling degree in the width (TD) direction is insufficient, and lines will still occur when processed into a polarizing plate.

[0053] In addition, regarding the part where the formed film is introduced into a floating dryer (FD) having a plurality of sections for heat treatment, 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 inventor has 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 in the width (TD) direction of the PVA film becomes non-uniform, and a phenomenon occurs in which the polarization degree becomes non-uniform when processed into a polarizing plate.

Example

[0054] Hereinafter, the present invention will be described in more detail in conjunction with examples. However, it should be understood that these examples are for helping to more easily understand the present invention and do not limit the scope of the present invention.

[0055] Hereinafter, a non-limiting method for preparing a polyvinyl alcohol film is provided. Based on a method similar to the method disclosed below, eight types of polyvinyl alcohol films (Examples 1 to 8) as non-limiting examples and five types of polyvinyl alcohol films (Comparative Examples 1 to 5) as comparative examples were prepared. However, the specific methods for preparing Examples 1 to 8 and Comparative Examples 1 to 5 are usually different from the method disclosed below in one or more aspects.

[0056] Specifically, the method for manufacturing the polyvinyl alcohol film includes the following steps. Polyvinyl alcohol resin pellets are dried by a process at the temperature shown in the following table to obtain raw materials. Before uniformly dissolving the polyvinyl alcohol raw material, plasticizer, water, etc., first wash the polyvinyl alcohol of the raw material with water to remove impurities (such as sodium acetate), and then uniformly dissolve it with the plasticizer, water, etc. to obtain a film-forming stock solution. After defoaming the film-forming stock solution with a twin-screw extruder, it is discharged from a T-shaped slit die lip and curtain-coated onto a rotating high-temperature casting drum for drying to prepare a film. Control the wind speed difference in the width (TD) direction on the air side of the casting drum. After peeling the preformed film from the casting drum, dry both the upper and lower surfaces of the film through contact with a plurality of heating rollers, and control the total contact time between the film and the heating rollers. Next, perform heat treatment using a floating dryer (FD), and finally obtain a finished polyvinyl alcohol film by controlling the standard deviation of the temperature in the width (TD) direction of the dryer.

[0057] Evaluations were conducted on Examples 1 to 8 and Comparative Examples 1 to 5 to determine the properties of these polyvinyl alcohol films. Tables 1 and 2 provide an overview of the processes of Examples 1 to 8 and Comparative Examples 1 to 5 respectively, the attributes of the polyvinyl alcohol films, and the state of the polarization uniformity of the polyvinyl alcohol films.

[0058]

Table 1

[0059]

Table 2

[0060] The wind speed difference on the air side of the casting drum in the above table (in the TD direction) is the wind speed range in the width (TD, Transverse Direction) direction, and is the value obtained by subtracting the minimum value from the maximum value of the wind speeds measured at three positions.

[0061] The crystallization temperature peak value (Tc,p value) in the above table is obtained by analysis during the DSC measurement method, and the DSC measurement method includes pretreatment and analysis of the crystallization temperature. Among them, the pretreatment is to divide the polyvinyl alcohol film into three equal parts in the width direction and cut the central part of the divided polyvinyl alcohol film. The cut area of each piece is, as shown in Figure 4 , (Machine Direction (MD) 10 cm × Transverse Direction (TD) 10 cm), immerse a 10 cm × 10 cm test piece in 2000 mL of pure water at 30 °C, stir with a magnet for 5 minutes and then take it out, repeat the above process once again to ensure that the additive is thoroughly washed, then put the wet film into a dryer at 105 °C and dry for 1 hour, and the taken-out film is obtained as a test sample. The analysis of the crystallization temperature is to take a 3 - 5 milligram sample for DSC analysis. The method is to maintain the sample isothermally at 40 °C for 1 minute, then heat it up to 250 °C at a rate of 10 °C / min, maintain it isothermally at 250 °C for 1 minute, and then cool it down to 30 °C at a rate of 10 °C / min to record the crystallization temperature. The above Tc,p value can be obtained by obtaining the crystallization temperature at three positions and calculating the average value.

[0062] The crystallization temperature peak value range (Tc,p deviation) in the above table is obtained by analysis during the DSC measurement method. Subtract the minimum value from the maximum value of the crystallization temperatures obtained at three positions to obtain the Tc,p deviation.

[0063] The peak part difference value (Tc,1 - Tc,2 value) after the first derivative of the heat flow relative temperature change curve in the above table is obtained by analysis during the DSC measurement method. Using Origin's data analysis software, differentiate the cooling curve to obtain the differential line (as shown in Figure 1), define the lowest value of the line near the crystallization peak as Tc,1 and the highest value as Tc,2, calculate Tc,1 - Tc,2, and obtain the above Tc,1 - Tc,2 value by calculating the average value of Tc,1 - Tc,2 at three positions, and use this as an index for the distribution of the crystallization temperature.

[0064] The specific methods for analyzing the TD swelling degree in the above table for 30 seconds include a sample preparation method and a test method. Among them, the sample preparation method is to divide the polyvinyl alcohol film into three equal parts in the width (TD) direction, cut the central part of the divided polyvinyl alcohol film, and set the cut area of each piece to (MD 10 cm × TD 10 cm) to obtain a test sample. The test method is to put the test sample into a constant temperature tank at 30 °C and start timing 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.

Number

[0065] The specific methods for analyzing the TD swelling degree in the above table for 15 minutes include a sample preparation method and a test method. Among them, the sample preparation method is to divide the polyvinyl alcohol film into three equal parts in the width (TD) direction, cut the central part of the divided polyvinyl alcohol film, and set the cut area of each piece to (MD 10 cm × TD 10 cm) to obtain a test sample. The test method is to put the test sample into a constant temperature tank at 30 °C and start timing 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.

Number

[0066] The SAXS peak position deviation in the above table is obtained during the analysis method by SAXS. The specific analysis methods include a sample preparation method and a test method. Among them, 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 divided polyvinyl alcohol film into test pieces with an area of (1 cm * 1 cm), let the sample stand for 24 hours under the conditions of 23 °C / 50% RH (relative humidity), 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 Synchrotron Radiation Research Center in Taiwan, 0.3 - 0.7 nm -1Obtain the position of the intensity corresponding to the high points within the data interval. The scattering vector (q) is defined as 4πλ -1 sinθ (where 2θ is the scattering angle), and further perform a calibration routine with silver behenate, and further calibrate all data including the transmission of the sample, the background, and the sensitivity of the detector.

[0067] The method for evaluating the lines in the above table is to visually observe whether lines are generated on the film surface in the manufacturing process of the polarizing plate.

[0068] The degree of polarization in the above table is obtained during the measurement method of the degree of polarization, and its specific analysis methods include a sample preparation method and a test method. Among them, the sample preparation method is to execute the manufacturing process of the polarizing plate on a polyvinyl alcohol film. The test method is to use Perkin Elmer Lambda 365 as the measuring instrument, perform visual sensitivity correction in the visible light region of 2° using a C light source based on the standard method of JIS Z 8722 in 2009, and then stack two polarizing films in the same orientation state, measure the light transmittance (H 0 ) at the wavelength, separately stack two polarizing films in a perpendicular orientation state, measure the light transmittance (H 90 ) at the wavelength, and calculate the degree of polarization represented by the following formula.

Equation

[0069] The standard deviation of the degree of polarization in the above table is obtained during the measurement method of the degree of polarization, and can be obtained by calculating the standard deviation of the degree of polarization at three positions.

[0070] According to the examples in Table 1, when a polarizing film is prepared by the manufacturing process of a polyvinyl alcohol film conforming to the present invention, it can be seen that the obtained polarizing film has no streaks on the surface and has a good and uniform degree of polarization.

[0071] Although not limited to a specific theory, the inventors have discovered that among the process parameters of the polyvinyl alcohol film, the indicators that are mainly 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 - 5, when the temperature is too high, the Tc,p value and the Tc,1 - Tc,2 value increase, and local dehydration or cross-linking is likely to occur, the crystallization temperature of the film becomes non-uniform, and lines appear on the surface of the polarizing film.

[0072] Among the process parameters of the polyvinyl alcohol film, the weight ratio of water to the polyvinyl alcohol-based resin in the water washing 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 the polyvinyl alcohol-based resin in the water washing is too small, the dehydration reaction is more likely to occur, the Tc,p value and the Tc,1 - Tc,2 value increase, and lines are likely to appear on the surface of the polarizing film.

[0073] Among the process parameters of the polyvinyl alcohol film, the indicator that is mainly affected by the total contact time with a heating roller above 85°C is the TD swelling degree at 30 seconds. For example, in the results of Comparative Examples 2 - 4, when the total contact time is too short, the surface crystallinity is insufficient and the TD swelling degree is excessive, and deep lines occur in the prepared polarizing plate. When the total contact time is too long, the surface crystallinity becomes too low and the TD swelling degree is insufficient, and lines also occur in the prepared polarizing plate.

[0074] Among the process parameters of the polyvinyl alcohol film, the indicator that is mainly affected by the dryer temperature standard deviation (in the TD direction) and the casting drum air-side wind speed difference (in the 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 (in the TD direction) is excessive and the casting drum air-side wind speed difference (in the TD direction) is excessive, the long period of the polyvinyl alcohol film is likely to become non-uniform in the TD direction, and the polarization degree of the prepared polarizing plate is non-uniform.

[0075] All ranges provided in this specification are meant to include each specific range within the assigned range and combinations of secondary ranges between the assigned ranges. Also, unless otherwise stated, all ranges provided in this specification include the endpoints of the range. Thus, the range 1 - 5 includes specifically 1, 2, 3, 4, and 5, as well as secondary ranges such as 2 - 5, 3 - 5, 2 - 3, 2 - 4, 1 - 4, etc.

[0076] All publications and patent applications cited in this specification are hereby incorporated by reference, and for all purposes, each publication or patent application is hereby clearly and individually indicated as being incorporated by reference into this specification. In the event of any inconsistency between this specification and any publication or patent application incorporated by reference into this specification, this specification shall govern.

[0077] The terms "comprising", "having", and "including" as used in this specification have an open, non - limiting meaning. The terms "a" and "the" are to be understood to include the plural as well as the singular. The term "one or more" refers to "at least one", and thus can include a single property or a mixture / combination of properties.

[0078] Except in the examples of operations or where otherwise indicated, all numbers indicating amounts of components and / or reaction conditions can in all cases be modified with the term "about" and are meant to be within ±5% of the indicated number. The terms "substantially free of" or "essentially free of" as used in this specification mean that a particular property is less than about 2%. All elements or properties specifically recited in this specification can be negatively excluded from the claims.

Claims

1. In the analysis by differential scanning calorimetry (DSC) after immersion in pure water and drying, the peak value range of the crystallization temperature is 0.1 to 3 °C, and the difference value of the peak part after passing the first derivative of the relative temperature change curve of the heat flow is 2 to 7 °C, a polyvinyl alcohol film, The method for measuring the peak value range of the crystallization temperature and the difference value of the peak part after passing the first derivative of the relative temperature change curve of the heat flow is, The polyvinyl alcohol film is divided into three equal parts in the width direction (TD), the central part of each polyvinyl alcohol film is cut, and a polyvinyl alcohol film having an area of 10 cm × 10 cm along the machine direction (MD) and 10 cm in the transverse direction is obtained. The 10 cm × 10 cm polyvinyl alcohol film is immersed in 2000 ml of pure water at 30 °C and stirred with a magnet for 5 minutes to obtain a total of three wet polyvinyl alcohol films. The previous step is repeated once. Next, the three wet polyvinyl alcohol films are placed in an oven at 105 °C and dried for 1 hour to obtain three polyvinyl alcohol films to be measured, including the pretreatment, Polyvinyl alcohol film.

2. The polyvinyl alcohol film according to claim 1, wherein the peak value of the crystallization temperature is 206 °C or higher.

3. The polyvinyl alcohol film according to claim 1, wherein the degree of swelling in the width (TD) direction after immersion in water for 30 seconds is 10 to 19%.

4. The polyvinyl alcohol film according to claim 1, wherein the degree of swelling in the width (TD) direction after immersion in water for 15 minutes is greater than 22%.

5. In the small-angle X-ray scattering (SAXS) analysis after standing still for 24 hours under the conditions of 23°C / 50% RH, the peak value range of the scattering intensity (I(q)) between the scattering vectors (q) of 0.3 to 0.7 nm －１ is less than 0.06 nm, a polyvinyl alcohol film, －１ wherein The method for measuring the peak value range of the scattering intensity (I(q)) is, The polyvinyl alcohol film is divided into three equal parts in the width direction (TD), the central part of each polyvinyl alcohol film is cut, and a polyvinyl alcohol film having an area of 1 cm × 1 cm is obtained. Next, the 1 cm × 1 cm polyvinyl alcohol film is allowed to stand for 24 hours under the conditions of 23 °C / 50% RH to obtain three polyvinyl alcohol films to be measured, including the sample preparation method, Polyvinyl alcohol film.

6. 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.

7. The polyvinyl alcohol film according to any one of claims 1 to 5, wherein the water content is between 1.0 and 5.0 wt%.

8. An optical film produced by the polyvinyl alcohol film according to claim 1 or 5.

9. The optical film according to claim 8, which is a polarizing film.

10. The optical film according to claim 9, wherein the standard deviation of the degree of polarization measured at a plurality of positions of the polarizing film is less than 0.

006.

11. A method for producing a polyvinyl alcohol film according to claim 1 or 5, comprising: Step (a) of drying and then washing with water a polyvinyl alcohol-based resin; Step (b) of stirring and heating a polyvinyl alcohol-based resin, a surfactant, a plasticizer, and water until dissolution to form a polyvinyl alcohol casting solution; Step (c) of casting the polyvinyl alcohol casting solution onto a casting drum, drying it to form a preformed film; Step (d) of bringing the preformed film into contact with a plurality of heating rollers whose temperatures gradually decrease from a high temperature to a low temperature, and then entering a dryer having a plurality of sections for heat treatment. A method for producing a polyvinyl alcohol film.

12. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (a), the drying temperature of the polyvinyl alcohol-based resin is 115 to 125°C.

13. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (a), the weight ratio of the water for washing with water to the polyvinyl alcohol-based resin is 9 or more.

14. 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.

15. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (d), the total contact time between the preformed film and the heating roller having a temperature exceeding 85°C is 6 to 20 seconds.

16. The method for producing a polyvinyl alcohol film according to claim 11, wherein in step (d), the standard deviation of the temperature in the width (TD) direction of the dryer is less than 2.6.

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