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

Abstract

To provide a polyvinyl alcohol film, an optical film including the same, and a method for producing the same.SOLUTION: The polyvinyl alcohol film has a first side and a second side, wherein the difference in A1142cm - 1 / A1090cm - 1 crystallinity between the two sides is less than 0.015 as measured by FTIR-ATR, and the film has a retardation value of 55 to 95nm after being swollen and stretched in water along the machine direction by a factor of 2. The polyvinyl alcohol film of the present invention has a characteristic that folding of an end portion or breakage of the film hardly occurs.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an optical film, particularly to a polyvinyl alcohol (PVA) film that can be used as a polarizing film, and a method for producing the same. [Background technology]

[0002] Polyvinyl alcohol (PVA) film is a hydrophilic polymer that has excellent properties such as excellent transparency, mechanical strength, water solubility, and good processability, and has therefore been widely used in packaging materials and optical films for electronic devices, especially polarizing films.

[0003] Polyvinyl alcohol film is an important component of polarizing plates, and after it is dyed and stretched, it develops polarizing properties. The polarizers obtained through the polarizing manufacturing process can polarize light and control brightness, so they are currently widely used in various LCD screens and are an essential component of display devices.

[0004] When a polyvinyl alcohol film is produced into a polarizing film, the higher the stretching ratio, the better the optical properties obtained. Therefore, by stretching the polyvinyl alcohol film to a stretching ratio close to the limit at which the film breaks during stretching, a polyvinyl alcohol film with better optical properties can be obtained.

[0005] In recent years, as screen technology has become increasingly thinner, the demands for quality in the manufacturing process of polarizing films have gradually increased. In the process of making polarizing films thinner, the stretchability of polyvinyl alcohol and the uniformity of dyeing have become increasingly important. Summary of the Invention [Problem to be solved by the invention]

[0006] However, the inventors of the present application have discovered that in the prior art, when a large polarizing film is produced using a polyvinyl alcohol film, folding of the edges of the polyvinyl alcohol film or breakage of the film often occurs during stretching.

[0007] Therefore, in order to solve the above-mentioned problems, the present invention provides a polyvinyl alcohol film that is less likely to fold at the edges or break. [Means for solving the problem]

[0008] The object of the present invention is to provide a semiconductor device having a first surface and a second surface, and measuring the A of both surfaces by FTIR-ATR. 1142 cm -1 / A 1090 cm -1 The objective of the present invention is to provide a polyvinyl alcohol film having a crystallinity difference of less than 0.015 and a retardation value of 55 to 95 nm after swelling and stretching in water to twice its length along the machine direction (MD).

[0009] According to one embodiment of the present invention, the A 1142 cm -1 / A 1090 cm -1 The difference in crystallinity is 0.001 to 0.014.

[0010] According to one embodiment of the present invention, the coefficient of variation of the phase difference (standard deviation / average value×100%) is less than 13.0%.

[0011] According to one embodiment of the present invention, the maximum average value of the in-plane retardation of the film minus the minimum average value of the in-plane retardation of the film is less than 20.0 nm.

[0012] According to one embodiment of the present invention, the film width is greater than 1.5 m and the film thickness is 45-75 μm.

[0013] According to one embodiment of the present invention, the viscosity of the 4% aqueous polyvinyl alcohol solution after reconstitution is 60 cps to 120 cps.

[0014] According to one embodiment of the present invention, the composition contains a polyvinyl alcohol resin having a degree of saponification of more than 99.85%.

[0015] Another object of the present invention is to provide an optical film formed from the above-mentioned polyvinyl alcohol film.

[0016] According to one embodiment of the present invention, the optical film is a polarizing film.

[0017] Yet another object of the present invention is to provide a method for producing the above-mentioned polyvinyl alcohol film, a dissolving step (a) of adding water and a plasticizer to a polyvinyl alcohol resin, raising the temperature to 110°C to 150°C, and dissolving the resin to form a polyvinyl alcohol casting liquid; a casting step (b) of casting the polyvinyl alcohol casting solution onto a casting drum, peeling the film from the casting drum, and obtaining a preliminary polyvinyl alcohol film; a drying step (c) of drying the preliminary polyvinyl alcohol film in a plurality of heating rollers and an oven having different temperatures, and then obtaining the polyvinyl alcohol film; The above method, comprising:

[0018] According to one embodiment of the present invention, the degree of saponification of the polyvinyl alcohol-based resin is greater than 99.85%.

[0019] According to one embodiment of the present invention, the difference in heat received between the front and rear surfaces of the preliminary polyvinyl alcohol film on the heating roller is less than 1,000.

[0020] According to one embodiment of the present invention, the moisture content of the preliminary polyvinyl alcohol film after passing through the last heating roller is less than 11.0%.

[0021] According to one embodiment of the present invention, the polyvinyl alcohol casting solution is cast onto the casting drum through a T-shaped slit die, and the lip pressure of the T-shaped slit die is 25.0 to 40.0 kg / cm. 2 The range is.

[0022] It is still another object of the present invention to provide the aforementioned method for producing a polyvinyl alcohol film, wherein the average temperature difference of the heating rollers is in the range of 2.0 to 5.0°C. [Effects of the Invention]

[0023] The polyvinyl alcohol film provided by the present invention is less likely to have its edges folded or torn.

[0024] In a preferred embodiment of the present invention, the polyvinyl alcohol film exhibits even better dye uniformity. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic diagram of five test specimens having areas cut out from a polyvinyl alcohol film according to one embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of swelling and stretching of a polyvinyl alcohol film according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Implementations of the techniques of the present invention will now be described by way of example with reference to the drawings.

[0027] It will be understood that aspects of the present invention are not limited to the arrangements, instrumentalities and features shown in the drawings.

[0028] According to common practice, the various features and components in the figures are not drawn to scale, but rather to best illustrate the specific features and components relevant to the present invention. In addition, the same or similar reference numerals in different figures represent the same or similar components or parts.

[0029] The following embodiments should not be considered to excessively limit the present invention. Those skilled in the art can modify and change the examples discussed herein without departing from the spirit or scope of the present invention, and such modifications and changes are also included in the scope of the present invention.

[0030] The articles "a" and "a kind" are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0031] The present invention has a first surface and a second surface, and A1142cm on both surfaces measured by FTIR-ATR -1 / A1090cm -1 In a preferred embodiment, the difference in crystallinity between the A on both sides is less than 0.015. 1142 cm -1 / A 1090 cm -1The difference in crystallinity is 0.001 to 0.014, for example, but not limited to, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.010, 0.011, 0.012, 0.013, 0.014, or a combination of any two of the above values. After the polyvinyl alcohol film is swelled and stretched in water to twice its length along the machine direction (MD), the retardation value is 55 to 95 nm, for example, but not limited to, 55 nm, 56 nm, 59 nm, 63 nm, 65 nm, 68 nm, 71 nm, 74 nm, 78 nm, 82 nm, 85 nm, 88 nm, 91 nm, 93 nm, 95 nm, or a combination of any two of the above values. The machine direction (MD) in the present invention specifically refers to the longitudinal direction of the polyvinyl alcohol film.

[0032] The aforementioned "A 1142 cm -1 / A 1090 cm -1 The "crystallinity" of the polymer is measured by the FTIR-ATR absorption peak ratio, which is the ratio of the characteristic peak of the infrared spectrum to the positioning peak. -1 ) is the crystallinity absorption peak of polyvinyl alcohol (ν(CC) stretching vibrations), which is the main structure in the polyvinyl alcohol film composition. -1 is the characteristic peak of crystallinity, and 1085-1095 (cm -1 ) is the positioning peak, and the normal peak top is 1090 cm -1 The characteristic peak is (ν(CO) stretching vibrations), which is the main characteristic peak of polyvinyl alcohol film and is less affected by ν(CC) stretching, making it suitable as a positioning peak.

[0033] The present invention is 1142 cm -1 Absorption peak / 1090cm -1The crystallinity of a polyvinyl alcohol film can be determined using absorption peaks; the higher the characteristic peak / positioning peak ratio, the higher the crystallinity. If the difference in crystallinity between the two sides is too large, when water molecules subsequently penetrate the polyvinyl alcohol film, the swelling degrees on both sides will be inconsistent, causing the more swollen side to curl toward the less swollen side, resulting in folding of the film surface. When manufacturing a polarizing film, if significant curling occurs at both ends of the film due to immersion in water, folding will occur at the edges of the rollers, leaving pinch marks on the edges. If the pinch marks are noticeable, the film will break. Therefore, the present invention provides a method for determining the crystallinity of both sides of a polyvinyl alcohol film. 1142 cm -1 / A 1090 cm -1 By controlling the difference in crystallinity, the edge folding or film breaking situation is further improved.

[0034] Referring to FIG. 1, the "double-sided A" described herein 1142 cm -1 / A 1090 cm -1 The "difference in crystallinity" refers to the difference in crystallinity between the polyvinyl alcohol film 110 and the polyvinyl alcohol film 110, which is divided into a plurality of equal parts (5 equal parts in the figure, but not limited to 5, 10, 15 or more equal parts), cut out at the center of each of the divided polyvinyl alcohol films, and measured by FTIR-ATR the absorbance of the front surface (also called the first surface) and the back surface (also called the second surface) of the cut polyvinyl alcohol film specimen 120. 1142 cm -1 / A 1090 cm -1 This refers to calculating the difference in crystallinity between both surfaces by subtracting the average of the five crystallinity values ​​of the second surface from the average of the five crystallinity values ​​of the first surface.

[0035] Continuing with reference to Figure 1, the "retardation value after swelling and stretching" described herein refers to a measurement obtained by dividing a polyvinyl alcohol film 110 into equal parts (five parts in the figure, but not limited to this), cutting out the center of each part, and treating the resulting polyvinyl alcohol film specimen 120 in a constant temperature and humidity chamber. The polyvinyl alcohol film specimen 120 is then stretched in pure water to twice its original size in the MD direction. The stretched polyvinyl alcohol film specimen 120 is then placed directly on a retardation meter and analyzed for its retardation value. Also referring to Figure 2, a polyvinyl alcohol film 210 is swollen and stretched in the MD direction in water and then placed on the retardation meter to measure the degree of retardation. The greater the retardation, the greater the degree of unidirectional movement of molecular chains. This leads to significant entanglement of polyvinyl alcohol molecules, making it more difficult to disentangle the crystalline molecules within the polyvinyl alcohol. This allows water to penetrate between the polyvinyl alcohol molecules, making swelling more difficult. Therefore, if the retardation value of the polyvinyl alcohol film after swelling and stretching is too high, it indicates that the intermolecular forces of the polyvinyl alcohol film after stretching are large, which results in poor stretchability and, if stretched forcefully, causes the film to break. On the other hand, if the retardation value of the polyvinyl alcohol film after swelling and stretching is too low, the intermolecular forces are too small, resulting in good stretchability, but the mechanical strength of the film is low during the stretching process, which can lead to breakage from the edges or defects. Therefore, the present invention further improves the edge folding or film breakage by controlling the retardation value of the polyvinyl alcohol film after swelling and stretching.

[0036] In a preferred embodiment, the coefficient of variation (standard deviation / average value×100%) of the retardation of the polyvinyl alcohol film of the present invention is less than 13.0%, for example, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0% or 12.5%, but is not limited thereto. Referring again to FIG. 1 , the “coefficient of variation of retardation” described in this specification relates to the uniformity of dyeing of a polyvinyl alcohol film, and is obtained by dividing a polyvinyl alcohol film 110 into a plurality of equal parts (5 equal parts in the figure, but not limited to this, for example, 5, 10, 15 or more equal parts), cutting the center of each equal part of the polyvinyl alcohol film, placing the cut-out polyvinyl alcohol film specimen 120 flat on a retardation timescale at room temperature, and measuring the retardation value in a dark room. The retardation value is measured by taking an area range (e.g., 10 cm × 10 cm) in the center of the polyvinyl alcohol film specimen 120 and calculating the value by (standard deviation of retardation / average retardation value) × 100%. The inventors measured polyvinyl alcohol films using a retardation meter and found that the retardation of polyvinyl alcohol exhibited a smile curve along the width direction (the effective width of the film surface, which indicates the remaining length after subtracting a specified length on each side of the entire polyvinyl alcohol film). Therefore, it was found inappropriate to simply evaluate the stress uniformity of polyvinyl alcohol films by the average retardation value or the standard deviation of the retardation. To resolve the difference in retardation values ​​across the width direction, the present invention introduces a coefficient of variation. The larger the coefficient of variation, the greater the difference in stress experienced within a region. When the polyvinyl alcohol film swells and stretches, differences in the stress release rate within this region can result in localized dyeing unevenness. Therefore, the present invention further improves the dyeing uniformity of polyvinyl alcohol films by controlling the numerical range of the coefficient of variation of the retardation of the polyvinyl alcohol film.

[0037] In a more preferred embodiment, the polyvinyl alcohol film of the present invention has a maximum average value of the in-plane retardation minus a minimum average value of the in-plane retardation of the film of less than 20.0 nm, for example, 1.0 nm, 2.0 nm, 3.0 nm, 4.0 nm, 5.0 nm, 6.0 nm, 7.0 nm, 8.0 nm, 9.0 nm, 10.0 nm, 11.0 nm, 12.0 nm, 13.0 nm, 14.0 nm, 15.0 nm, 16.0 nm, 17.0 nm, 18.0 nm, or 19.0 nm, but is not limited thereto. Referring again to FIG. 1 , the term "maximum average value-minimum average value of retardation" used herein refers to a value obtained by dividing a polyvinyl alcohol film 110 into multiple equal parts (5 equal parts in the figure, but not limited to 5, 10, 15, or more equal parts), cutting the center of each polyvinyl alcohol film, placing the cut polyvinyl alcohol film specimen 120 flat on a retardation meter at room temperature, and measuring the retardation value in a darkroom. Using the values ​​measured by the retardation meter, a certain number of specimens (e.g., 5 or 10 or less) are selected to calculate the maximum average value of retardation and the minimum average value of retardation, and then subtracting the two values. The inventors have found that, when measuring polyvinyl alcohol films with a retardation meter, the retardation of polyvinyl alcohol exhibits a smile curve along the width direction, and the larger the smile curve, the more pronounced the overall dyeing unevenness. Therefore, the present invention improves the dyeing uniformity of polyvinyl alcohol films by controlling the numerical range of the maximum average value within the polyvinyl alcohol film and the minimum average value of in-plane retardation of the film.

[0038] In a preferred embodiment, the width of the polyvinyl alcohol film is greater than 1.5 m, for example, but not limited to, 1.6 m, 1.7 m, 1.8 m, 1.9 m, 2.0 m, 2.1 m, 2.2 m, 2.3 m, or 2.4 m, and the thickness of the polyvinyl alcohol film is 45 to 75 μm, for example, 45 μm, 46 μm, 47 μm, 48 μm, 49 μm,

[0033] The thickness may be, but is not limited to, 50 μm, 51 μm, 52 μm, 53 μm, 54 μm, 55 μm, 56 μm, 57 μm, 58 μm, 59 μm, 60 μm, 61 μm, 62 μm, 63 μm, 64 μm, 65 μm, 66 μm, 67 μm, 68 μm, 69 μm, 70 μm, 71 μm, 72 μm, 73 μm, 74 μm, 75 μm or between any two of the above values.

[0039] The present inventors have found that the viscosity of a 4% aqueous polyvinyl alcohol solution obtained after reconstitution of a polyvinyl alcohol film affects the retardation value of the resulting polyvinyl alcohol film after swelling and stretching. The lower the viscosity, the lower the retardation value of the resulting polyvinyl alcohol film after swelling and stretching. Specifically, the "viscosity of a 4% aqueous polyvinyl alcohol solution obtained after reconstitution of a polyvinyl alcohol film" described herein can also be called the "viscosity of a 4% aqueous PVA solution" or the "viscosity of the polyvinyl alcohol film," and is believed to be related to the polyvinyl alcohol resin raw material. The present invention allows for further control of the retardation value of a polyvinyl alcohol film after swelling and stretching by controlling the viscosity. In a preferred embodiment, the viscosity of the 4% aqueous polyvinyl alcohol solution after redissolution of the polyvinyl alcohol film is 60 cps to 120 cps, for example, 60 cps to 120 cps, such as, but not limited to, 60 cps, 61 cps, 65 cps, 70 cps, 75 cps, 80 cps, 85 cps, 90 cps, 95 cps, 100 cps, 105 cps, 110 cps, 115 cps, 120 cps, or a value between any two of the above values.

[0040] In a preferred embodiment, the polyvinyl alcohol resin has a degree of saponification of more than 99.85%, for example, but not limited to, 99.85% to 100.00%, 99.85% to 99.99%, 99.85% to 99.98%, 99.85% to 99.97%, 99.85% to 99.96%, 99.85% to 99.95%, 99.85% to 99.94%, 99.85% to 99.93%, 99.85% to 99.92%, 99.85% to 99.91%, 99.85% to 99.90%, 99.85% to 99.89%, 99.85% to 99.88%, 99.85% to 99.87%, or 99.85% to 99.86%.

[0041] Another object of the present invention is to provide an optical film formed from the above-mentioned polyvinyl alcohol film. The optical film may be a polarizing film, a blue light blocking film, a filter, etc., but the present invention is not limited thereto. In a preferred embodiment, the polyvinyl alcohol film of the present invention is used as a polarizing film.

[0042] When polyvinyl alcohol film is made into optical film, it is stretched and dyed. For example, when making polarizing film, I 3- , I 5- When dyeing polyvinyl alcohol film using an aqueous solution of boric acid containing iodide ions, the boric acid crosslinks with the amorphous regions of polyvinyl alcohol to fix the iodide ions, preventing them from dissolving.

[0043] In another aspect, the present invention also provides a method for producing the polyvinyl alcohol film, which includes a dissolving step (a) of adding water and a plasticizer to a polyvinyl alcohol resin in a dissolution tank and dissolving the resin at a temperature of 110°C to 150°C to form a polyvinyl alcohol casting solution; a casting step (b) of casting the polyvinyl alcohol casting solution onto a casting drum and peeling the film from the casting drum to obtain a preliminary polyvinyl alcohol film; and a drying step (c) of drying the preliminary polyvinyl alcohol film in a plurality of heating rollers and an oven having different temperatures to obtain the polyvinyl alcohol film.

[0044] After conducting experiments, the inventors further found that the dissolution temperature of polyvinyl alcohol also affects the retardation value of the polyvinyl alcohol film after subsequent swelling and stretching, and that the higher the dissolution temperature of polyvinyl alcohol, the higher the retardation value of the polyvinyl alcohol film after swelling and stretching. Therefore, in a preferred embodiment, the preparation of the polyvinyl alcohol casting solution in the dissolution step (a) involves dissolving a polyvinyl alcohol resin in a dissolution tank, and the dissolution temperature is preferably 110°C to 150°C. According to at least one embodiment, the polyvinyl alcohol resin is dissolved at a temperature of 110°C or higher for at least 180 minutes to form the polyvinyl alcohol casting solution. The temperature may be, for example, but not limited to, 110°C, 111°C, 115°C, 119°C, 121°C, 125°C, 128°C, 133°C, 137°C, 141°C, 145°C, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C, and the dissolution time may be, for example, but not limited to, at least 180 minutes, at least 240 minutes, at least 300 minutes, at least 360 minutes, or at least 400 minutes.

[0045] The polyvinyl alcohol casting solution is prepared so that the polyvinyl alcohol resin content is 10 to 60 wt%, preferably 15 to 40 wt%, more preferably 20 to 30 wt%, specifically, for example, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 wt%. If the polyvinyl alcohol resin content is insufficient, the viscosity of the polyvinyl alcohol casting solution becomes too low, and the drying load becomes too large, resulting in a decrease in the film-forming efficiency of the produced polyvinyl alcohol film. Conversely, if the polyvinyl alcohol resin content is too high, the polyvinyl alcohol resin as a whole becomes difficult to dissolve uniformly, and clusters are likely to remain.

[0046] The polyvinyl alcohol resin described above can be obtained by polymerizing a vinyl ester resin monomer to form a polyvinyl ester resin, followed by a saponification reaction. Examples of the vinyl ester resin monomer include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, and vinyl octoate. The present invention is not limited to these, but vinyl acetate is preferred. Copolymers formed by copolymerizing an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used. Examples of the olefin compound include, but are not limited to, ethylene, propylene, and butene. Examples of the acrylate derivative include, but are not limited to, acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate.

[0047] In a preferred embodiment, the saponification degree of the polyvinyl alcohol resin is greater than 99.85% to obtain better optical properties, and is, for example, 99.85% to 100.00%, 99.85% to 99.99%, 99.85% to 99.98%, 99.85% to 99.97%, 99.85% to 99.96%, 99.85% to 99.9 5%, 99.85% to 99.94%, 99.85% to 99.93%, 99.85% to 99.92%, 99.85% to 99.91%, 99.85% to 99.90%, 99.85% to 99.89%, 99.85% to 99.88%, 99.85% to 99.87% or 99.85% to 99.86%.

[0048] In a preferred embodiment, the plasticizer can increase the flexibility of the material or liquefy the material to improve the processability of film formation. Examples of 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), bis(2-ethylhexyl) phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. In a preferred embodiment, the plasticizer is glycerol, ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. The amount of the plasticizer added is typically 3 to 30 parts by weight, for example, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30 parts by weight, per 100 parts by weight of polyvinyl alcohol resin, but is not limited thereto, and is preferably 7 to 20 parts by weight. If the amount of plasticizer is insufficient, the resulting polyvinyl alcohol film is prone to crystallization, which affects dyeability in subsequent processing. Conversely, if the amount of plasticizer is too high, the mechanical properties of the polyvinyl alcohol film are impaired.

[0049] In the casting step (b), after the preparation of the polyvinyl alcohol casting solution is completed, a film-forming solution is obtained. The solution is degassed using a double-screw extruder and then cast onto the casting drum through a T-slit die. The inventors have found that the lip pressure of the T-slit die affects the coefficient of variation of the retardation of the polyvinyl alcohol film, thereby affecting the dyeing uniformity. Without being bound by any particular theory, the inventors have found that when the lip pressure is too high, the coefficient of variation of the retardation becomes too large, resulting in poor dyeing uniformity of the polyvinyl alcohol film. Specifically, the lip pressure of the T-slit die is preferably 25.0 to 40.0 kg / cm. 2 range, e.g., 25.0 kg / cm 2 , 26.0 kg / cm 2 , 27.0 kg / cm 2 , 28.0 kg / cm 2 , 29.0 kg / cm 2 , 30.0 kg / cm 2 , 31.0 kg / cm 2 , 32.0 kg / cm 2 , 33.0 kg / cm 2 , 34.0 kg / cm 2 , 35.0 kg / cm 2 , 36.0 kg / cm 2 , 37.0 kg / cm 2 , 38.0 kg / cm 2 , 39.0 kg / cm 2 , 40.0 kg / cm 2 or between any two of the above values, but not limited to these.

[0050] The polyvinyl alcohol casting solution is cast onto a casting drum, peeled off from the casting drum, and then proceeds to a drying step (c). That is, the preliminary polyvinyl alcohol film on the casting drum is peeled off from the casting drum and dried to form a polyvinyl alcohol film. The drying process is carried out first with a heated roller and then in an oven. The number of heated rollers and the number of oven sections are not particularly limited and can be adjusted as needed. The number of heated rollers is preferably 10, for example, 10, 11, 12, 15, 20, 30, or 40. The number of oven sections is preferably two or more, for example, 2, 3, 5, 6, 8, 10, 15, 20, 25, or 30. The preliminary polyvinyl alcohol film is heated by contacting multiple heating rollers on both the top and bottom surfaces of the dry film, with the first heating roller being the hottest of all the heating rollers, followed by the subsequent heating rollers whose temperatures gradually decrease until the last heating roller has the lowest temperature of all the heating rollers. The average temperature difference between the heating rollers affects the maximum average value - minimum average value of the phase difference. Preferably, the average temperature difference is in the range of 2.0 to 5.0°C, for example, 2.0°C, 2.1°C, 2.5°C, 2.7°C, 3.0°C, 3.5°C, 3.8°C, 4.2°C, 4.5°C, 4.8°C, 5.0°C, or a value between any two of the above values, but is not limited thereto.

[0051] After passing through the last heating roller, a preliminary polyvinyl alcohol film is obtained, and the moisture content of the preliminary polyvinyl alcohol film is less than 11.0%, for example, but not limited to, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 9.5%, 10%, 10.5%, or 10.9%. The inventors of the present application have determined that the difference in heat received between the front and back sides of the preliminary polyvinyl alcohol film on the heating roller and the moisture content of the preliminary polyvinyl alcohol film are determined by the difference in heat received between the front and back sides of the polyvinyl alcohol film on both sides of the polyvinyl alcohol film. 1142 cm -1 / A 1090 cm -1Without being bound by any particular theory, it is believed that the greater the difference in heat received between the front and back sides or the higher the moisture content of the preliminary polyvinyl alcohol film, the greater the crystallinity difference between both sides of the polyvinyl alcohol film. 1142 cm -1 / A 1090 cm -1 It has been found that this increases the difference in crystallinity between the two rollers, ultimately resulting in curling or folding at the edges. In a preferred embodiment, the heat reception difference between the front and back surfaces of the preliminary polyvinyl alcohol film on the heating roller is less than 1000, for example, but not limited to, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, or 990. As used herein, the term "heat reception difference between the front and back surfaces" is defined as the value of (temperature of even-numbered heating rollers x time) - (temperature of odd-numbered heating rollers x time). As used herein, the term "moisture content" refers to the moisture content calculated by cutting the preliminary polyvinyl alcohol film that has passed through the last heated roller, measuring its weight (W1), then placing this film in an oven to dry it at 105°C for 10 minutes, measuring the weight (W2) of the dried film, and calculating (W1-W2) / W1.

[0052] The rotation speed of the heating roller is about 3 to 15 m / min, preferably 5 to 10 m / min. If the rotation speed of the heating roller is too slow, the productivity may decrease. On the other hand, if the rotation speed of the heating roller is too fast, the casting liquid cannot be dried sufficiently, and the releasability may decrease.

[0053] According to at least one preferred embodiment, the drying step uses a floating oven at a temperature in the range of 80°C to 115°C, such as, but not limited to, 80°C, 81°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, or 115°C.

[0054] The equipment used in the polyvinyl alcohol film manufacturing method includes a dissolution tank, a filter, a coating machine, and a transportation pipeline connected to the coating machine. These equipment are preferably covered with a heat-retaining device, which may be a metal heating wire or a jacket filled with a liquid such as oil or water. By heating the metal wire or the liquid in the jacket, the surfaces of these equipment, particularly the equipment and pipeline, are kept uniformly heated and kept warm, preventing the polyvinyl alcohol in the polyvinyl alcohol casting solution from forming gel or clusters due to a drop in the surface temperature of the equipment or pipeline. The heat-retaining temperature should not be too high, as too high a temperature can cause partial dehydration or gelation of the polyvinyl alcohol casting solution, forming a burnt yellow or black gel, which will affect the surface quality and uniformity of the polyvinyl alcohol film after coating in the subsequent process. The temperature of the polyvinyl alcohol casting liquid at the coating and molding location is 80 to 120°C, for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, or a temperature between any two of the above values, but is not limited thereto, and is preferably 90 to 110°C, more preferably 90 to 100°C. [Example]

[0055] The following non-limiting examples of various aspects of the present invention are provided primarily to illustrate various aspects and advantages of the present invention.

[0056] Non-limiting methods for producing polyvinyl alcohol films are provided below. Seven non-limiting example polyvinyl alcohol films (Examples 1-7) and five comparative polyvinyl alcohol films (Comparative Examples 1-5) were produced according to the methods disclosed below or similar methods. However, the specific methods for producing Examples 1-7 and Comparative Examples 1-5 differ from the methods disclosed below in one or more aspects.

[0057] (Example: Production of polyvinyl alcohol film) First, a polyvinyl alcohol resin is heated and melted in a dissolving tank to form a polyvinyl alcohol casting solution, which is then extruded through a T-slit die lip and a casting drum to form a preliminary film, which is then treated with multiple heating rollers to adjust the moisture content, and then dried in an oven to obtain a polyvinyl alcohol film. Depending on user needs, the T-slit die lip can also be replaced with other flat dies such as a fishtail die and a coathanger die, but the present invention is not limited thereto.

[0058] Specifically, the process for producing polyvinyl alcohol films includes the following steps: 1,800 kg of polyvinyl alcohol resin with a saponification degree of >99.85%, 4,000 kg of water, and 200 kg of plasticizer glycerin are placed in a dissolution tank, the temperature is raised to 140°C while stirring, and the mixture is dissolved by holding at 110-150°C for 300 minutes. After uniform dissolution in a mixer, water is added to adjust the resin concentration to 30.0% to obtain a film-forming solution. The film-forming solution is degassed in a double-screw extruder and then discharged from a T-slit die lip, with the lip pressure of the T-slit die being 25-40 kg / cm. 2 The polyvinyl alcohol film is then cast onto a rotating, high-temperature casting drum and dried to form a film. After the preformed film is peeled from the casting drum, ten heated rollers are placed in contact with both the top and bottom surfaces of the dried film. The first heated roller is the hottest of all the heated rollers, and the temperatures of the subsequent heated rollers gradually decrease until the average temperature difference between the heated rollers is 2-5°C. After passing through the last heated roller, the polyvinyl alcohol film has a moisture content of less than 11% and a heat difference between the front and back surfaces of less than 1000°C. The film is then dried in a floating oven, where both the top and bottom surfaces are dried with hot air. The oven's maximum temperature is 115°C, and a polyvinyl alcohol film is finally obtained. The production conditions for Examples 1-7 are shown in Table 1.

[0059] (Comparative Example: Production of Polyvinyl Alcohol Film) The polyvinyl alcohol film manufacturing process was the same as that of the above-mentioned Examples, but the melting temperature of the polyvinyl alcohol film, the viscosity of the polyvinyl alcohol film, the lip pressure of the T-slit die, the average temperature difference between each heating roller, the heat difference between the front and back sides, and the moisture content of the polyvinyl alcohol film after passing through the last heating roller were different in each Comparative Example from those of the Examples, and the manufacturing conditions are shown in Table 2.

[0060] Examples 1-7 and Comparative Examples 1-5 were evaluated to determine the properties of these polyvinyl alcohol films. Tables 1-4 show the process control variables for Examples 1-7 and Comparative Examples 1-5, a summary of the properties of these polyvinyl alcohol films, and the breakage, edge fold, and dye uniformity of these polyvinyl alcohol films.

[0061] Measurement of viscosity of polyvinyl alcohol films: A 40 x 40 cm piece of polyvinyl alcohol film was immersed in 30°C pure water and slowly stirred for 5 minutes. The polyvinyl alcohol film was then removed and placed flat on a PP dish. It was then placed in a 105°C oven and dried for 1 hour. After drying, 4 g of polyvinyl alcohol film was added to 96 g of ultrapure water and dissolved by heating in a steam or water bath. After dissolution, the polyvinyl alcohol solution was cooled. Once the solution temperature had cooled to room temperature, excess water was wiped off around the beaker and the solution was left at room temperature for 15 minutes. The polyvinyl alcohol solution was then poured onto an iron plate (in two portions) and dried in a 105°C oven for 3 hours. After drying, the iron plate was placed on a drying dish and cooled for 15 minutes. The solids content of the iron plate (the average of the two measurements) was calculated. The solids content of the solution was accurately adjusted to 4% and placed in a mixer and stirred uniformly at low speed. The solids content was calculated as follows:

[0062] Weight of the remaining PVA solution after weighing the iron plate × volatile content / amount of water added (g) = 4.0% Then, the thermostatic water bath of the viscometer (model: LVT) was set to 20°C, the stirred solution was poured exactly up to the scale line on the jig tube wall of the viscometer, the rotation speed was set to 6.0 rpm with the 0 needle, and timing was started for 10 minutes. After the data stabilized, the viscosity value was obtained.

[0063] Moisture content measurement: First, the polyvinyl alcohol film that has passed through the last heating roller is cut and its weight (W1) is measured. Next, this film is placed in an oven and dried at 105°C for 10 minutes. The weight (W2) of the dried film is measured and substituted into (W1-W2) / W1 to determine the moisture content.

[0064] Measurement of retardation value after swelling and stretching: A polyvinyl alcohol film was divided into five equal sections widthwise, and the center of each section was cut to a cut area of ​​20 cm MD x 15 cm TD. The sections were then placed in a thermo-hygrostat at 23°C and 50% RH for 24 hours. The polyvinyl alcohol film (5 cm MD x 15 cm TD) was then fixed and stretched twice at 4.3 cm / min in pure water at 30°C. The stretched polyvinyl alcohol film was placed directly on a retardation meter PA-300, and the central region of the stretched area was flattened. The retardation value of the flattened, bubble-free region was analyzed. The transverse direction (TD) in this specification specifically refers to the horizontal direction of the polyvinyl alcohol film.

[0065] Measurement of the difference in crystallinity between the two sides: The polyvinyl alcohol film was divided into five equal parts in the width direction, and the center of each cut polyvinyl alcohol film was cut. The cut area of ​​each piece was (MD 10 cm × TD 10 cm). The film was placed directly on the FTIR-ATR and the absorbance of the center was measured. 1142 cm -1 / A 1090 cm -1 The difference in crystallinity between both surfaces was calculated by subtracting the average of the five crystallinity values ​​of the second surface from the average of the five crystallinity values ​​of the first surface.

[0066] Measurement of lip pressure in T-slit dies: The T-slit die is equipped with a Bourdon tube pressure gauge, and the pressure generated by the solution is transmitted to the central axis of the fixed end of the spring through the elasticity of the spring, and then the pointer attached to the central axis indicates the corresponding number according to the displacement, indicating the magnitude of the pressure.

[0067] Measurement of the coefficient of variation of the phase difference: The polyvinyl alcohol film was divided into 5 equal parts widthwise and cut down the center, with each cut out area being (MD 12 cm × TD 12 cm). At room temperature, the cut polyvinyl alcohol film was placed flat on a PA-300 retardation meter and the retardation was measured in a dark room. After the measurement, the retardation value was calculated by taking a 10 × 10 cm area in the center, and the mean retardation value and standard deviation of the calculated area size were substituted into (standard deviation / mean value) × 100% to calculate the coefficient of variation of the retardation.

[0068] Measurement of maximum-minimum average phase difference: The polyvinyl alcohol film was divided into five equal parts widthwise and cut down the center, with each cut out measuring 12 cm MD x 12 cm TD. The cut polyvinyl alcohol film was placed flat on a PA-300 retardation meter at room temperature, and the retardation was measured in a darkroom. After the measurement, the maximum average retardation value of the 10 films minus the minimum average retardation value of the 10 films was calculated.

[0069] Measurement and evaluation of film breaking performance: The polyvinyl alcohol film is processed into a polarizer using a stretching machine for the polarizer process. A 5,000-meter roll is used continuously in the polarization process. A "◎" mark indicates no film breakage, a "○" mark indicates one film breakage, and an "×" mark indicates two or more film breakages.

[0070] Edge tuck measurement and evaluation: Immediately after unfolding, a new polyvinyl alcohol film was cut into a size of 10 cm in MD x 5 cm in TD, and the cut sample was stored in an aluminum foil bag. A water immersion curl test was completed within 5 minutes, and the folding condition of the edge was observed. A "◎" mark indicates that the folding angle of the edge of the polyvinyl alcohol film was 90 degrees or less, a "○" mark indicates that the folding angle of the edge of the polyvinyl alcohol film was 180 degrees or less, and an "×" mark indicates that the folding angle of the edge of the polyvinyl alcohol film was more than 180 degrees.

[0071] Measurement and evaluation of color uniformity performance: After processing the polyvinyl alcohol film into a polarizer, two polyvinyl alcohol polarizing films were stacked perpendicularly, and one side was illuminated using a light box with an illuminance of 14,000 lux, and the color uniformity of the other side was observed and evaluated. A "◎" mark indicates that the film has no color unevenness, a "○" mark indicates that the film has slight color unevenness, and an "×" mark indicates that the film has obvious color unevenness.

[0072] [Table 1]

[0073] [Table 2]

[0074] [Table 3]

[0075] [Table 4]

[0076] According to Tables 1 and 2, the present inventors have found that only by controlling the retardation value of the polyvinyl alcohol film after swelling and stretching and the difference in crystallinity between both surfaces of the polyvinyl alcohol film within specific ranges can the polyvinyl alcohol film simultaneously exhibit the properties of being resistant to tearing and having its edges resistant to folding.

[0077] Furthermore, according to Tables 3 and 4, only by controlling the coefficient of variation of the retardation of the polyvinyl alcohol film and the maximum average value-minimum average value of the retardation within a specific range can the polyvinyl alcohol film exhibit good dye uniformity.

[0078] In summary, the polyvinyl alcohol film of the present invention has the effect of making film breakage and edge folding less likely to occur by controlling the retardation value of the polyvinyl alcohol film after swelling and stretching and the difference in crystallinity on both sides of the polyvinyl alcohol film within specific ranges, and furthermore, by controlling the coefficient of variation of retardation and the maximum average value-minimum average value of retardation, the dyeing uniformity of the polyvinyl alcohol film can be improved.

[0079] All ranges provided herein are intended to include each specific range within the stated range and combinations of subranges between the stated ranges. Also, any range explicitly stated herein includes its endpoints unless otherwise specified. Thus, the range 1 to 5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2 to 5, 3 to 5, 2 to 3, 2 to 4, 1 to 4, etc.

[0080] All publications and patent applications cited in this specification are herein incorporated by reference and for all purposes are specifically and individually indicated to be incorporated by reference. In the event of a conflict between the present specification and a publication or patent application incorporated by reference herein, the present specification will control.

[0081] As used herein, the terms "comprising," "having," and "including" have an open-ended, non-limiting meaning. The terms "a," "an," and "the" may be intended to include the plural as well, unless the context clearly dictates otherwise. The term "one or more" means "at least one" and can therefore include a single feature or a mixture / combination.

[0082] Other than in the examples provided herein, or unless otherwise specified, all numerical values ​​expressing amounts of ingredients and / or reaction conditions can be modified in all instances by the term "about," which means within ±5% of the stated numerical value. As used herein, the terms "essentially free" or "substantially free" mean less than about 2% of a particular characteristic. Any element or feature explicitly recited in this specification can be negatively excluded from the claims.

[0083] Although the present invention has been described in detail above, the above description is only a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. That is, any equivalent changes and modifications made based on the claims of the present invention fall within the scope of patent protection of the present invention. [Explanation of symbols]

[0084] 110 Polyvinyl alcohol film 120 Polyvinyl alcohol film test piece 210 Polyvinyl alcohol film MD machine direction TD Lateral

Claims

1. A film having a first surface and a second surface, and A of both surfaces measured by FTIR-ATR １１４２ cm －１ / A １０９０ cm －１ and a retardation value of 55 to 95 nm after swelling and stretching in water to twice its length along the machine direction (MD).

2. A on both sides １１４２ cm －１ / A １０９０ cm －１ 2. The polyvinyl alcohol film according to claim 1, wherein the difference in crystallinity between the polyvinyl alcohol film and the polyvinyl alcohol film is 0.001 to 0.

014.

3. 2. The polyvinyl alcohol film according to claim 1, wherein a coefficient of variation of retardation (standard deviation / average value×100%) is less than 13.0%.

4. 2. The polyvinyl alcohol film according to claim 1, wherein a value of the maximum average value of the in-plane retardation of the film minus a minimum average value of the in-plane retardation of the film is less than 20.0 nm.

5. 2. The polyvinyl alcohol film of claim 1, wherein the film width is greater than 1.5 m and the film thickness is 45 to 75 μm.

6. 2. The polyvinyl alcohol film according to claim 1, wherein the viscosity of a 4% aqueous solution of polyvinyl alcohol after reconstitution is 60 cps to 120 cps.

7. The polyvinyl alcohol film according to any one of claims 1 to 6, comprising a polyvinyl alcohol resin having a degree of saponification of more than 99.85%.

8. An optical film formed from the polyvinyl alcohol film according to any one of claims 1 to 7.

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

10. A method for producing the polyvinyl alcohol film according to any one of claims 1 to 6, comprising the steps of: (a) dissolving a polyvinyl alcohol resin by adding water and a plasticizer to the polyvinyl alcohol resin and raising the temperature to 110°C to 150°C to dissolve the polyvinyl alcohol resin, thereby forming a polyvinyl alcohol casting liquid; a casting step (b) of casting the polyvinyl alcohol casting solution onto a casting drum, peeling the film from the casting drum, and obtaining a preliminary polyvinyl alcohol film; a drying step (c) of drying the preliminary polyvinyl alcohol film in a plurality of heating rollers and an oven having different temperatures, and then obtaining the polyvinyl alcohol film; A manufacturing method comprising:

11. The method according to claim 10, wherein the degree of saponification of the polyvinyl alcohol-based resin is greater than 99.85%.

12. The method according to claim 10, wherein a difference in heat received between the front and rear surfaces of the preliminary polyvinyl alcohol film on the heating roller is less than 1000.

13. The method of claim 10, wherein the preliminary polyvinyl alcohol film has a moisture content of less than 11.0% after passing through the final heating roller.

14. The polyvinyl alcohol casting solution is cast onto the casting drum through a T-shaped slit die, and the lip pressure of the T-shaped slit die is 25.0 to 40.0 kg / cm ２ The method according to claim 10, wherein the range is

15. The manufacturing method according to any one of claims 10 to 14, wherein the average temperature difference of the heating rollers is in the range of 2.0 to 5.0°C.

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