Polyvinyl alcohol film and optical film manufactured from the same

Abstract

To provide a polyvinyl alcohol film and an optical film manufactured from the same.SOLUTION: The present invention relates to a polyvinyl alcohol film and an optical film manufactured from the same. The polyvinyl alcohol film contains polyvinyl alcohol resin having a saponification degree of 95% or more, and a complex viscosity (η*) measured from the polyvinyl alcohol film when sweep oscillation is applied at 25°C and an angular frequency of 10 rad / s is 400-1500 Pa s. The polyvinyl alcohol film of the present invention has improved a film surface defect to have excellent film surface characteristics.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to polyvinyl alcohol (PVA) products, and in particular, but not exclusively, to polyvinyl alcohol films and optical films made therefrom. [Background technology]

[0002] Polyvinyl alcohol (PVA) film is a hydrophilic material obtained by coating and drying an aqueous solution containing polyvinyl alcohol polymer and plasticizer. It has properties such as high transparency, mechanical strength, water solubility, and good processability, and is therefore widely used in packaging materials and various optical films for electronic devices, such as polarizing films.

[0003] A polarizing film obtained by processing a polyvinyl alcohol film through a polarizing process has the property of blocking only light from a specific direction, so that the brightness of the light that passes through can be controlled. Based on this property, polarizing films are applied to various displays, glasses, and wearable devices. The polarizing process generally includes processes such as swelling, stretching, and dyeing. Specifically, the polyvinyl alcohol film is placed in a solution and the above-mentioned process is carried out to diffuse the dye molecules into the molecules in the polyvinyl alcohol film and form a regular arrangement, so that the polarizing film can absorb the light component parallel to the arrangement direction and pass the light component perpendicular to the arrangement direction to produce the property of polarization.

[0004] In order to provide good optical properties and effects, an ideal polarizing film should have properties such as uniform color, few film surface defects, and good hue effect. Regarding the properties related to film surface defects, prior art such as Patent Document 1 discloses adjusting the surface roughness of a polyvinyl alcohol film to improve the film surface defect status of the optical film produced thereafter. Prior art such as Patent Document 2 discloses adjusting the number of cracks on the surface of a metal support during the preparation process to reduce the surface dent defects of the obtained polyvinyl alcohol film, and when the film is used as a film roll for producing a polarizing film, a polarizing film with good yield, few production defects, and meeting the required quality level can be obtained. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Taiwan Patent Application Publication No. TWI557164 [Patent Document 2] Taiwan Patent Application Publication No. TWI639636 Summary of the Invention [Problem to be solved by the invention]

[0006] Unlike the technical content of the above-mentioned prior art, which improves the film surface defects by adjusting the surface roughness of the polyvinyl alcohol film or the number of cracks on the metal support surface during the preparation process, the present inventors have found that by adjusting the complex viscosity of the polyvinyl alcohol film within a specific value range, the film surface defects of the molded product can be improved, and the optical film produced thereafter can have good optical performance. In addition, the present inventors have found that when the weight swelling degree of the polyvinyl alcohol film at a specific temperature is controlled to be greater than the lower limit, the optical film produced from the polyvinyl alcohol film has excellent color uniformity. [Means for solving the problem]

[0007] Specifically, one aspect of the present invention provides a polyvinyl alcohol film containing a polyvinyl alcohol resin having a degree of saponification of 95% or more, and having a complex viscosity (η*) of 400 to 1500 Pa s measured when subjected to sweep oscillation at 25°C and an angular frequency of 10 rad / s.

[0008] In one or more embodiments, the polyvinyl alcohol film has a weight swelling degree of more than 37% when immersed in water at 30° C. for 20 minutes, and the polyvinyl alcohol film has a weight swelling degree of more than 60% when immersed in water at 55° C. for 3 minutes.

[0009] In one or more embodiments, the polyvinyl alcohol resin has a degree of saponification of 99.95% or more.

[0010] In one or more embodiments, the complex viscosity (η*) is 410 to 1100 Pa·s.

[0011] In one or more embodiments, the polyvinyl alcohol resin has a degree of polymerization of 1,300 to 5,200.

[0012] In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is 2,000 to 3,000.

[0013] In one or more embodiments, the polyvinyl alcohol film has a thickness of 30 to 75 μm.

[0014] Another embodiment provides an optical film made from the polyvinyl alcohol film described above.

[0015] In one or more embodiments, the optical film is a polarizing film.

[0016] In one or more embodiments, the optical film further includes a protective layer attached to at least one surface of the optical film. Effect of the Invention

[0017] The polyvinyl alcohol film obtained based on the technical contents of the present invention has significantly improved film surface defects, and the polyvinyl alcohol film of the present invention is suitable for various applications, particularly optical films such as polarizing films, and further, optical films produced from the polyvinyl alcohol film have good color uniformity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In order to provide a more detailed and complete description of the present invention, the following provides an illustrative description of the embodiments and specific examples of the present invention, which are not the only ways of implementing or operating the specific examples of the present invention. In this specification and the appended claims, unless the context dictates otherwise, "a" and "the" may also be interpreted as plural. In addition, unless the specification and the appended claims are otherwise clearly stated, "provided on something" may be considered to be directly or indirectly in contact with some surface in the form of attachment or other means. The identity of the surface must be determined based on the context of the preceding and following paragraphs of the specification and the ordinary skill in the art to which the present invention pertains.

[0019] The numerical ranges and parameters specifying the present invention are approximate, but the relevant numerical values ​​in the specific examples are presented as precisely as possible. However, any numerical value necessarily contains standard deviations inherent in each individual testing method. As used herein, the term "about" indicates that the actual value is within an acceptable standard error of the mean and is determined by the belief of one of ordinary skill in the art to which the present invention pertains.

[0020] [Polyvinyl alcohol film] One aspect of the present invention provides a polyvinyl alcohol film containing a polyvinyl alcohol resin having a degree of saponification of 95% or more, and having a complex viscosity (η*) of 400 to 1500 Pa s measured when subjected to sweep oscillation at 25°C and an angular frequency of 10 rad / s.

[0021] The term "viscosity" as used herein is a rheology-based measure of the intermolecular viscosity of the constituents of a material. The "complex viscosity (η*)" as used herein is the ratio of the complex modulus (G*) to the angular frequency (ω) and is used to quantify the ability of a material to resist deformation at a specific oscillation frequency. The complex viscosity can be determined by measuring the complex modulus (G*) of a material at a specific angular frequency (ω) using a rheometer via micro-oscillation techniques. A higher complex viscosity indicates a higher ability of the material to resist deformation. The value of the "complex modulus (G*)" is calculated as the square root of the sum of the squares of the loss modulus (G'') and the storage shear modulus (G'), i.e., √(G' 2 +G'' 2 ).

[0022] The "film surface defect" mentioned in this specification includes damage to the film surface of the polyvinyl alcohol film (e.g. scratches, streaks) and wrinkles in appearance. The complex viscosity of the polyvinyl alcohol film is probably related to the film surface defect, and affects the optical performance of the optical film manufactured from the polyvinyl alcohol film. In other words, when the complex viscosity parameter of the polyvinyl alcohol film is within an appropriate range, the film surface defect can be improved. Specifically, the complex viscosity parameter has a lower limit and an upper limit. Without being bound by a particular theory, when the complex viscosity parameter exceeds the upper limit, the polyvinyl alcohol film is prone to irreversible contact friction with the manufacturing equipment during the manufacturing process, and then more serious damage such as scratches and streaks on the film surface occurs. When the complex viscosity parameter is below the lower limit, wrinkles and unevenness occur on the surface of the polyvinyl alcohol film after it is left stationary, which may affect the yield of the molded product.

[0023] Therefore, in order to improve the film surface defects of the polyethylene film, it is necessary to control the complex viscosity within a specific range. Specifically, the complex viscosity (η*) of the polyvinyl alcohol film of the present invention, measured at 25° C. and sweep oscillation at an angular frequency of 10 rad / s, is 400 to 1500 Pa·s, for example, 400 to 500, 500 to 600, 600 to 700, 700 to 800, 800 to 900, 900 to 1000, 1000 to 1100, 1100 to 1200, 1200 to 1300, 1300 to 1400, or 1400 to 1500 Pa·s, and particularly 400 , 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450 or 1500 Pa·s, preferably 410 to 1100 Pa·s, in particular 410, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050 or 1100 Pa·s. Furthermore, without being bound by any particular theory, the complex viscosity parameter is related to the moisture content of the polyvinyl alcohol film in the casting stage, the heated roller stage and the oven stage during the polyethylene film preparation process. The complex viscosity parameter is also related to the amount of additive added to the polyethylene film.

[0024] The "saponification degree" referred to in this specification means a measured value obtained by a test method described in the JIS K 6726 (1994) standard. In one or more embodiments, the saponification degree of the polyvinyl alcohol-based resin exceeds 95% in order to obtain better optical properties, for example, more than 95%, more than 96%, more than 97%, more than 98%, or more than 99%. When the saponification degree of the polyvinyl alcohol-based resin exceeds 95%, the strength of the resin increases, and the resin is less likely to dissolve during dyeing and fixing treatments in the subsequent polarizing film process, and a polarizing film with high polarizing performance can be easily obtained. Furthermore, the saponification degree of the polyvinyl alcohol resin is 99.00% or more, for example, 99.00% or more, 99.15% or more, 99.25% or more, 99.35% or more, 99.45% or more, 99.55% or more, 99.65% or more, 99.75% or more, 99.85% or more, or 99.95% or more, and in the present invention, for example, exceeds 99.85%.

[0025] According to some embodiments, the polyvinyl alcohol film of the present invention has a weight swelling degree of more than 37% when immersed in water at 30° C. for 20 minutes, and the polyvinyl alcohol film has a weight swelling degree of more than 60% when immersed in water at 55° C. for 3 minutes.

[0026] The "weight swelling degree (%)" mentioned in this specification refers to the percentage weight change ((W1-W2) / W1x100) between the polyvinyl alcohol film cut to a specific size, immersed in water at a specific temperature (e.g., but not limited to, 30°C and 55°C) in a flat state for a certain time (e.g., 20 minutes), wiped off the moisture on the film surface with dust-free paper, and then weighed to obtain the film weight (W1). The film is then dried (e.g., but not limited to, 120°C), weighed again to obtain the film weight (W2). The numerical range of the weight swelling degree is probably related to the hue uniformity of the optical film made from the polyvinyl alcohol film. In the conventional manufacturing process of the polarizing film, the polyvinyl alcohol film is swollen and dyed in water at 30°C, and stretched in an acidic solution (such as a boric acid solution) at 55°C. Therefore, without being bound by any particular theory, the weight swelling degree measured after immersion in water at 30°C and 55°C can indicate the degree of entanglement and the degree of crystal dissociation of the polymer chains of the polyvinyl alcohol film, respectively. The present inventors have found that when the weight swelling degree measured after immersion in water at 30°C and 55°C is controlled to be higher than a specific lower limit, the optical film produced thereafter can have good hue uniformity. If the measured weight swelling degree does not exceed a specific lower limit, it is presumed that the polymer chains are entangled or too many crystalline regions remain, making it difficult for the dye molecules to attach uniformly to the polymer chains, resulting in color unevenness. Specifically, after immersion in water at 30°C for 20 minutes, the weight swelling degree of the polyvinyl alcohol film of the present invention is more than 37%, for example, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, or 49%. And after the polyvinyl alcohol film is immersed in water at 55° C. for 3 minutes, the weight swelling degree is more than 60%, for example, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74 or 75%.Therefore, the present invention does not further specify the upper limit of the weight swelling parameter, but a person having ordinary skill in the art to which the present invention pertains can clearly understand the content and scope of the present invention based on the effect achieved by the weight swelling exceeding the specific lower limit. Furthermore, without being bound by a specific theory, the weight swelling parameter is related to the water content of the polyvinyl alcohol film at the casting stage, the heating roller stage and the oven stage during the preparation process, and is also related to the amount of additive added to the polyethylene film.

[0027] According to some embodiments, the degree of polymerization of the polyvinyl alcohol resin of the present invention is 1300 to 5200. The "degree of polymerization" described in this specification means a measured value obtained by a test method described in JIS K 6726 (1994). In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is in the range of 1300 to 5200, such as a value within a range between any two of the following values, for example, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100 , 3200, 3300, 3400, 3500, 3600, 3700, 3800, 3900, 4000, 4100, 4200, 4300, 4400, 4500, 4600, 4700, 4800, 4900, 5000, 5100 or 5200, preferably in the range of 2000 to 3000, for example, but not limited to, 2100, 2300, 2500, 2700 or 2900.

[0028] According to some embodiments, the thickness of the polyvinyl alcohol film of the present invention is 30 to 75 μm. Specifically, the thickness is within a range between any two of the following values, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 μm, and the above values ​​are merely examples and are not limiting. In addition, when the obtained polyvinyl alcohol film is used to manufacture a thin polarizing film, the thickness of the polyvinyl alcohol film may be 50 μm or less, preferably 40 μm or less, and particularly preferably 30 μm or less.

[0029] [Method of manufacturing polyvinyl alcohol film] The method for producing a polyvinyl alcohol film of the present invention includes: (a) a dissolving step of heating and dissolving a polyvinyl alcohol-based resin and adjusting the concentration of the polyvinyl alcohol-based resin to form a polyvinyl alcohol cast solution; (b) a casting step of casting the polyvinyl alcohol cast solution onto a casting drum and peeling it off from the casting drum to obtain a preformed film; (c) a heating roller step of contacting the polyvinyl alcohol preformed film with a plurality of heating rollers to obtain a semi-formed polyvinyl alcohol film product; and (d) an oven step of placing the semi-formed polyvinyl alcohol film product in an oven to dry it and obtain a molded polyvinyl alcohol film product, and depending on the situation, it may further include a temperature and humidity adjustment step (e) of placing the polyvinyl alcohol film product in a temperature and humidity regulator to adjust the temperature and humidity.

[0030] [Dissolution process] According to some embodiments, the dissolving step is carried out by stirring the polyvinyl alcohol-based resin, solvent, plasticizer / plasticizer and additives while raising the temperature to at least 130°C to dissolve them uniformly, and then adding additional solvent (e.g., water) to adjust the concentration of non-volatile components (e.g., resin, plasticizer, glycerol and additives) to 20-50 wt % to obtain a polyvinyl alcohol cast solution.

[0031] In one or more embodiments, the polyvinyl alcohol resin used in the dissolving step is polymerized with a vinyl ester resin monomer to form a polyvinyl alcohol resin having a degree of polymerization of 1300 to 5200, and then a saponification reaction is carried out to obtain a polyvinyl alcohol resin having a degree of saponification of more than 95%. Examples of the vinyl ester resin monomer include vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, or vinyl octoate, or combinations thereof, but are not limited to these in the present invention. In addition, a copolymer formed by copolymerization of an olefin compound or an acrylate derivative with the vinyl ester resin monomer can also be used, and examples of the olefin compound include ethylene, propylene, or butene, but are not limited to these in the present invention. The amount of the olefin compound added can be 2 to 4 mol%, for example, 2, 2.5, 3, 3.5, or 4 mol%, but are not limited to these. Examples of the acrylate derivative include acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and n-butyl acrylate, but the present invention is not limited thereto.

[0032] In one or more embodiments, the solvent used in the dissolving step is not particularly limited in the present invention, as long as it can dissolve polyvinyl alcohol resin. Examples of the solvent include, but are not limited to, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, diethylenetriamine, etc., and the above solvents can be used alone or in combination of two or more. Considering environmental and economical aspects, it is preferable to use water as the solvent in the present invention.

[0033] In one or more embodiments, the "plasticizer" is specifically, but not limited to, glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, trimethylolpropane, or the like, or a combination thereof. In the present invention, glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, or trimethylolpropane is preferable. In one or more embodiments, the amount of the plasticizer added is 5 to 15 wt% relative to the weight of the polyvinyl alcohol resin, specifically, for example, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 wt%, but is not limited thereto.

[0034] In one or more embodiments, the term "additive" as used herein refers to a surfactant. The term "surfactant" is not limited to cationic, anionic, or nonionic surfactants, and specifically includes carboxylate-type surfactants such as potassium laurate, sulfate-type surfactants such as sodium lauryl sulfate, sulfonate-type surfactants such as dodecylbenzenesulfonate, alkylphenyl ether-type surfactants such as polyoxyethylene octylphenyl ether, alcohol phenyl ether-type surfactants such as polyethylene glycol monooctylphenyl ether, alkyl ester-type surfactants such as polyethylene glycol monolaurate, alkylamine-type surfactants such as polyoxyethylene laurylamino ether, alkylamide-type surfactants such as polyoxyethylene lauramide, polypropylene glycol ether-type surfactants such as polyoxyethylene polyoxypropylene ether, alkanolamide-type surfactants such as lauric acid diethanolamide and oleic acid diethanolamide, allylphenyl ether-type surfactants such as polyoxyallylphenyl ether, or sodium lauryl alcohol polyoxyethylene ether sulfate, but is not limited thereto. Without being bound by any particular theory, it is believed that additives can reduce the amount of hydrogen bond formation between polymers, thereby reducing the degree of entanglement and crystallinity of polymer chains. Therefore, the amount of the additive affects the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at a specific temperature. In other words, by adjusting the amount of the additive added during the manufacturing process within a specific range, the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at a specific temperature can be further controlled. Here, the amount of the additive added is expressed as a concentration (ppm), and the specific calculation method is to divide the weight of the additive by the weight of all non-volatile components (e.g., resin, plasticizer glycerol, and additives) and then convert it into a ppm concentration. In one or more embodiments, the amount of the additive added is 3000 ppm or less, preferably 200 to 3000 ppm, and specifically, for example, 200, 400, 600, 800, 1000, 1200, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, or 3000 ppm, but is not limited thereto.

[0035] In one or more embodiments, the temperature for dissolving the aqueous polyvinyl alcohol resin solution is preferably at least 130°C, specifically, for example, at least 130°C, at least 140°C, or at least 150°C.

[0036] In one or more embodiments, the concentration of the aqueous polyvinyl alcohol resin solution is preferably 20 to 50%, specifically, for example, 20, 25, 30, 35, 40, 45, or 50%. If the resin concentration is too low, the drying load of the film increases, and conversely, if the resin concentration is too high, the viscosity increases, making film formation difficult.

[0037] [Casting process] According to some embodiments, the casting process mainly involves degassing the polyvinyl alcohol casting solution (the degassing can be performed using a static degassing method or a multi-screw extruder having vent holes, for example, but not limited to, a twin-screw extruder), controlling the solution temperature to at least 90°C, and then discharging the solution from a T-die lip and casting the solution onto a casting drum (also called a casting drum) or an endless belt or other supporting body to form a film, thereby obtaining a polyvinyl alcohol preformed film.

[0038] The moisture content can be controlled by methods such as, but not limited to, the temperature of the casting drum, the residence time of the polyvinyl alcohol in the casting drum, hot air drying, etc. Hot air drying is a method of controlling the moisture content by blowing hot air at a specific temperature onto the surface of the polyvinyl alcohol preformed film that contacts the air.

[0039] In one or more embodiments, the temperature of the polyvinyl alcohol casting solution is preferably at least 90°C, for example, 90, 91, 92, 93, 94, or 95°C. In one or more embodiments, the temperature of the casting drum is preferably, for example, in the range of 85 to 95°C, specifically, for example, but not limited to, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, or 95°C. In one or more embodiments, the residence time of the polyvinyl alcohol in the casting drum is preferably 0.6 to 1.2 minutes, specifically, but not limited to, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, or 1.2 minutes. In one or more embodiments, the temperature of the hot air is preferably, but not limited to, 60°C to 120°C.

[0040] Without being bound by any particular theory, it is believed that the water content of the polyvinyl alcohol preformed film affects the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at a particular temperature. Specifically, this stage is the main arrangement and formation stage of the polyvinyl alcohol polymer chains, and this stage controls the degree of entanglement of the polymer chains. The water content of the polyvinyl alcohol preformed film is related to the degree of entanglement of the polymer chains, and the lower the water content, the more entangled the polymer chains are, which affects the complex viscosity of the subsequent molded product and the weight swelling degree after immersion in water at 30°C. In other words, by adjusting the water content of the polyvinyl alcohol preformed film within a particular range, the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at 30°C can be further controlled. In one or more embodiments, the moisture content of the polyvinyl alcohol preformed film is 8 to 17%, for example, 8 to 9, 9 to 10, 10 to 11, 11 to 12, 12 to 13, 13 to 14, 14 to 15, 15 to 16, or 16 to 17%, and in particular, 8.5, 9, 10, 11, 12, 13, 14, 15, 16, or 17%.

[0041] [Heating roller process] According to at least one embodiment, the heated roller process mainly involves contacting and drying the upper and lower surfaces of the polyvinyl alcohol preformed film peeled off from the casting drum with a plurality of heated rollers, and then obtaining a semi-formed polyvinyl alcohol film.

[0042] The moisture content can be controlled by, but not limited to, adjusting the number of heating rollers, the temperature of the heating rollers, hot air drying, etc. Hot air drying is to blow hot air at a specific temperature onto the surface of the polyvinyl alcohol film semi-formed product that contacts the air to adjust the moisture content of the sample.

[0043] In one or more embodiments, the number of the plurality of heating rollers may be 2 to 30, for example, but not limited to, 2, 5, 10, 15, 20, 25, or 30. The temperature of the plurality of heating rollers is gradually decreased from high to low, with the first heating roller being the highest temperature roller among all the heating rollers, and the last heating roller being the lowest temperature among all the heating rollers. In one or more embodiments, the temperature of the first heating roller is a value within a range between any two of the following values, for example, 80, 85, 90, 95, or 100°C, and is preferably in the range of 80 to 100°C. In one or more embodiments, the temperature of the last heating roller is a value within a range between any two of the following values, for example, 30, 35, 40, 45, 50, 55, or 60°C, and is preferably in the range of 30°C to 60°C. In one or more embodiments, the temperature of the hot air is preferably, but not limited to, 50°C to 110°C.

[0044] Without being bound by any particular theory, it is believed that the water content of the polyvinyl alcohol film semi-molded product affects the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at a particular temperature. Specifically, this stage is the main stage of polyvinyl alcohol crystal formation, and the water content of the polyvinyl alcohol film semi-molded product is related to the amount of crystal formation, and the lower the water content, the more crystals are formed, which affects the complex viscosity of the subsequent molded product and the weight swelling degree after immersion in water at 55 ° C. In other words, by adjusting the water content of the polyvinyl alcohol film semi-molded product to a particular range, the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at 55 ° C. can be further controlled. In one or more embodiments, the water content of the polyvinyl alcohol film semi-molded product is 4.5 to 9%, for example, 5 to 6, 6 to 7, 7 to 8, or 8 to 9%, and particularly 4.8, 5, 6, 7, 8, or 9%.

[0045] [Oven process] According to some embodiments, the oven process mainly involves drying the upper and lower surfaces of the polyvinyl alcohol film semi-molded product peeled off from the heated roller in an oven with hot air to obtain a polyvinyl alcohol film molded product.

[0046] In one or more embodiments, the oven is preferably a floating oven, and the temperature is controlled to a range of 40° C. to 120° C., specifically a value within a range between any two of the following values, for example, 40, 50, 60, 70, 80, 90, 100, 110, or 120° C. The overall average temperature of the oven is preferably in a range of 70° C. to 80° C., specifically, for example, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80° C. In one or more embodiments, the drying time of the polyvinyl alcohol film in the oven is a value within a range between any two of the following values, for example, 1 minute, 1.5 minutes, 2 minutes, 2.5 minutes, 3 minutes, 3.5 minutes, 4 minutes, 4.5 minutes, 5 minutes, 5.5 minutes, or 6 minutes, and is preferably in the range of 1 to 6 minutes.

[0047] Without being bound by any particular theory, it is believed that the water content of the polyvinyl alcohol film molded product affects the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at a specific temperature. Specifically, the water content of the polyvinyl alcohol film molded product is related to the entanglement of polymer chains and the amount of crystal formation, and therefore affects the complex viscosity of the subsequent molded product and the weight swelling degree after immersion in water at 30°C and 55°C, respectively. In other words, by adjusting the water content of the polyvinyl alcohol film molded product within a specific range, the complex viscosity of the polyvinyl alcohol film and the weight swelling degree after immersion in water at 30°C and 55°C can be further controlled, and if the water content is too low, the entanglement and crystallization between the polymers of the polyvinyl alcohol film molded product will increase. In one or more embodiments, the moisture content of the polyvinyl alcohol film molded product is 1.8 to 5%, for example, 1.8 to 2, 2 to 3, 3 to 4, or 4 to 5%, and particularly 1.8, 2, 3, 4, or 5%.

[0048] [Temperature and humidity adjustment process] According to a preferred embodiment of the present invention, the polyvinyl alcohol film product can be further placed in a temperature and humidity regulator to adjust the temperature and humidity after the above steps.

[0049] In one or more embodiments, the temperature of the temperature and humidity regulator is a value within a range between any two of the following values, for example, 35° C., 40° C., or 45° C., and is preferably in the range of 35 to 45° C. In one or more embodiments, the relative humidity of the temperature and humidity regulator is a value within a range between any two of the following values, for example, 70%, 72%, 74%, 76%, 78%, or 80%, and is preferably in the range of 70 to 80%. In one or more embodiments, the time for which the polyvinyl alcohol film is left to stand in the temperature and humidity regulator is a value within a range between any two of the following values, for example, 5 minutes, 10 minutes, 15 minutes, or 20 minutes, and is preferably in the range of 5 to 20 minutes.

[0050] [Optical film and its manufacturing method] Another object of the present invention is to provide an optical film manufactured from the polyvinyl alcohol film. The "optical film" described in this specification may be a polarizing film, a retardation film, a viewing angle widening film, or a brightness increasing film, and is particularly a polarizing film. According to some embodiments, the "method of manufacturing an optical film" of the present invention refers to a method of manufacturing a polarizing film, and further, a polyvinyl alcohol film is manufactured into a polarizing film and the hue uniformity is evaluated. The manufacturing method includes a dyeing step of adsorbing iodine, a boric acid treatment step, and a water washing step, and a stretching step of uniaxial stretching can be performed in the boric acid treatment step or the step before this, and a drying step can be performed once between the boric acid treatment step and the water washing step. Preferably, a swelling step of swelling the polyvinyl alcohol film with water can be provided before the dyeing step. In addition, a final drying step is usually provided after the water washing step.

[0051] In the swelling step, the polyvinyl alcohol film is immersed in a treatment bath (e.g., water) at a temperature of 30°C, for example, to wash the film surface and perform a swelling treatment. The swelling treatment time is usually 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, a plurality of guide rollers are arranged in a swelling tank containing the treatment bath in order to transport the polyvinyl alcohol film. Next, the polyvinyl alcohol film is stretched in the machine direction (MD) to 1.2 times its original length, and then a dyeing step is performed.

[0052] In the dyeing step, the polyvinyl alcohol film that has been through the swelling step is immersed in a dyeing tank containing a dye bath. The dyeing conditions can be determined within a range in which iodine is adsorbed into the polyvinyl alcohol film and does not cause defects such as extreme dissolution or devitrification of the film. The dye bath used in the dyeing step can be an aqueous solution containing 0.003 to 0.2 parts by weight of iodine and 0.1 to 10 parts by weight of potassium iodide per 100 parts by weight of water. In addition, other iodides such as zinc iodide may be used instead of potassium iodide, or other iodides may be used in addition to potassium iodide. The temperature of the dye bath (dyeing temperature) is usually 10 to 50°C, for example 30°C, and the time of the dyeing treatment (dyeing time) is usually 10 to 600 seconds, preferably 30 to 200 seconds. Next, the polyvinyl alcohol film is stretched in the machine direction to 3.4 times its original length, and then the boric acid treatment and stretching steps are carried out.

[0053] In the boric acid treatment and stretching process, the polyvinyl alcohol film dyed with iodine is treated with a boric acid-containing aqueous solution to crosslink, and the adsorbed iodine is fixed to the resin. The above process is usually carried out by immersing the polyvinyl alcohol film that has been subjected to the dyeing process in a fixing tank containing a boric acid-containing treatment bath. The boric acid treatment bath may be an aqueous solution containing 0.5 to 15 parts by weight of boric acid per 100 parts by weight of water. The boric acid treatment bath preferably contains an iodide in addition to boric acid, and the amount is 5 to 20 parts by weight per 100 parts by weight of water. The iodide used for this purpose may be potassium iodide or zinc iodide. In addition, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, sodium sulfate, etc. may also be present in the boric acid treatment bath. The boric acid treatment and stretching steps are usually carried out at a temperature of 50 to 70° C., preferably 53 to 65° C., for example 55° C., and the treatment time is usually 10 to 600 seconds, preferably 20 to 300 seconds, more preferably 20 to 100 seconds. Next, the polyvinyl alcohol film is stretched in the machine direction to 6 times its original length, and the subsequent steps are carried out.

[0054] The polyvinyl alcohol film was subjected to the above process and the subsequent washing and drying processes to form a polarizing film. Furthermore, a protective layer can be formed on at least one surface of the polarizing film to manufacture a finished polarizing film (also called a polarizer). In detail, the protective layer has a function of preventing abrasion of the surface of the polarizing film, and is preferably a component containing a transparent resin. According to a different embodiment, the protective layer is provided on only one surface of the polarizing film, but is preferably formed on both surfaces of the polarizing film. The protective layer can be a protective film made of a transparent resin material. The transparent resin may be an acrylic resin such as a methyl methacrylate resin, an olefin resin, a polyvinyl chloride resin, a cellulose resin, a styrene resin, an acrylonitrile-butadiene-styrene copolymer resin, an acrylonitrile-styrene copolymer resin, a polyvinyl acetate resin, a polyvinylidene chloride resin, a polyamide resin, a polyacetal resin, a polycarbonate resin, a modified polyphenylene ether resin, a polyester resin (such as a polybutylene terephthalate resin or a polyethylene terephthalate resin), a polysulfone resin, a polyethersulfone resin, a polyarylate resin, a polyimideimide resin, a polyimide resin, an epoxy resin, an oxetane resin, or the like, and a cellulose resin such as cellulose triacetate (TAC) is preferred. EXAMPLES

[0055] The present invention will be described in detail below with reference to specific examples. However, it should be understood that these specific examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention in any way.

[0056] 1. Preparation of Polyvinyl Alcohol Film The following provides non-limiting preparation methods of polyvinyl alcohol films. Non-limiting example polyvinyl alcohol films (Examples 1-15) and comparative polyvinyl alcohol films (Comparative Examples 1-5) were prepared in a manner similar to that disclosed below.

[0057] The following are the main steps of producing polyvinyl alcohol films in this embodiment and the comparative example, and the following Table 1 details the differences in one or more process parameters between this embodiment and the comparative example.

[0058] 1. First, 1800 kg of polyvinyl alcohol resin with a saponification degree of 99.95% and a polymerization degree of 2000 to 3000 (specific polymerization degrees are shown in Table 1), 3900 kg of water, 190 kg of plasticizer glycerol, and additives (specific amounts added are shown in Table 1) were added to a dissolving barrel. Here, the additives were lauryl alcohol polyoxyethylene ether and sodium lauryl alcohol polyoxyethylene ether sulfate, and the weight ratio of lauryl alcohol polyoxyethylene ether and sodium lauryl alcohol polyoxyethylene ether sulfate contained in the additives was 1:1. Next, the temperature was raised while stirring to dissolve uniformly, and water was added to adjust the concentration of non-volatile components (polyvinyl alcohol resin, plasticizer glycerol, and additives) to 30 wt.%, to obtain a polyvinyl alcohol cast solution.

[0059] 2. The polyvinyl alcohol cast solution was degassed in a twin-screw extruder, and then discharged from a T-die lip and cast onto a rotating casting drum at 90°C, and the air-contacting surface was dried with hot air at 110°C to produce a polyvinyl alcohol preformed film having a specific moisture content (the specific moisture content is shown in Table 1). The moisture content of the preformed film was adjusted by controlling the residence time of the polyvinyl alcohol in the casting drum.

[0060] 3. After the polyvinyl alcohol preformed film was peeled off from the casting drum, the upper and lower surfaces of the film were dried by contact with seven heating rollers, while the air contact surface was dried with hot air 10°C higher than that of the heating rollers, to produce a polyvinyl alcohol film semiformed product with a specific moisture content (specific moisture content is shown in Table 1). The first heating roller was the hottest roller of all the heating rollers, with a temperature of 90°C, and the temperature of the subsequent heating rollers was gradually reduced by 5°C each time, until the temperature of the seventh heating roller was reduced to 60°C. The moisture content of the polyvinyl alcohol film semiformed product was adjusted by increasing or decreasing the number of heating rollers.

[0061] 4. Next, both the upper and lower surfaces of the polyvinyl alcohol preformed film were dried with hot air in a floating oven to produce a polyvinyl alcohol film molded product having a specific moisture content (specific moisture contents are shown in Table 1). The average drying temperature of the oven was 75°C. The moisture content of the molded product was adjusted by controlling the residence time in the oven.

[0062] The moisture content of the polyvinyl alcohol film samples of the above Examples and Comparative Examples was calculated using the following analytical method. Sample preparation: Polyvinyl alcohol film sample was cut into 10 × 10 cm 2 Cut to size. Test conditions: The weight (W1) of a film sample was measured before drying, and after drying at 105° C. for 10 minutes, it was taken out and the weight (W2) was measured. The moisture content was calculated according to the following formula. Moisture content (%)=(W1-W2) / W1×100

[0063] [Table 1]

[0064] 2. Analysis and Evaluation The following analytical tests and evaluations were carried out on the polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 5. The results are shown in Table 2.

[0065] <Complex viscosity> Instrument: TA Instruments DHR HR-20 Rheometer Sample size: Polyvinyl alcohol film samples were cut into circles with a diameter of 2.5 cm. Sample treatment: The film cut as described above was placed in a 30°C, 70% RH environment and allowed to absorb moisture until the moisture content reached 9-10% before measurement. Test method: The torque and angular displacement detected by the fixture in sweep oscillation mode were measured, and the stress and strain information was recorded. Here, sweep oscillation means that the motor rotates clockwise or counterclockwise at the zero point to generate a shear force on the sample. Test conditions: Mode: Oscillation Angular frequency: 10rad / s Distortion: 1% Sampling time: 10 points per minute, specifically, 1 point every 6 seconds per minute Fixture: Flat clamp (diameter 2.5 cm), fixed normal force 1 N Jig temperature: 25℃ Ambient temperature: 25~30℃ Data calculation: The viscosity values ​​measured at the above 10 sampling points are summed up and then the average value is taken.

[0066] <Weight swelling degree> How to prepare the sample: Polyvinyl alcohol film 5 x 5 cm 2 Cut to size. Weight swelling measured after immersion in water at 30°C (hereinafter referred to as weight swelling at 30°C): The film sample was immersed in 500ml of water at 30°C in a flat state for 20 minutes, then removed with tweezers, and both sides of the film sample were lightly wiped with four sheets of laboratory dust-free paper (GBFA-00000000001, manufactured by Cleanhorse, USA), and this was repeated twice. After removing excess surface moisture, the film sample was placed flat on one sheet of dust-free paper, covered with one sheet of dust-free paper on top, and pressed with four stacked stainless steel sample pans (approximately 30.5±1.5g each), allowing the surface of the dust-free paper to absorb the residual water for 30 seconds, then the film sample was turned over and the above method was repeated with new dust-free paper to remove the surface moisture once more. Thereafter, the weight (W1) of the film sample was measured, and then the film sample was dried in an oven at 120° C. for 2 hours, and then left to stand in a desiccator for 5 minutes, after which the weight (W2) of the film sample was measured. Weight swelling measured after immersion in water at 55°C (hereinafter referred to as 55°C weight swelling): The film was immersed in 500 ml of water at 55°C for 3 minutes, and the surface moisture was removed by the method described above, and the weight (W1) was measured. The film sample was then dried in an oven at 120°C for 2 hours, and then placed in a desiccator for 5 minutes, after which the weight (W2) of the film sample was measured. The weight swelling degree was calculated according to the following formula. Weight swelling rate (%) = (W1-W2) / W1 x 100

[0067] <Damage assessment> Test conditions: Polyvinyl alcohol film sample 30 x 30 cm 2 The test sample was placed 10 cm away from a white wall and a 1200 lumen flashlight was shone onto the test sample at a distance of 50 cm from the white wall. The reflection of the test sample on the white wall was then visually observed and the damage to the polyvinyl alcohol film was evaluated according to the following evaluation criteria. Evaluation criteria: A: Area 0.01mm for 10 sheets 2No damage exceeding 200μm or streaks exceeding 200μm in width are observed. B: All 10 pieces have an area of ​​0.01 mm 2 Damage exceeding 100 μm or a streak exceeding 200 μm in width was confirmed in one location. C: All 10 pieces have an area of ​​0.01 mm 2 Damage exceeding 100 μm or streaks exceeding 200 μm in width are confirmed in two or more places.

[0068] <Wrinkle evaluation> Sample: A polyvinyl alcohol film sample was cut to approximately 40 cm ± 5% in the machine direction and 2.8 m ± 5% in the width direction. Test conditions: Stored in an aluminum foil bag for more than one month Evaluation criteria: The surface of the polyvinyl alcohol film sample was visually inspected for the presence or absence of wrinkles, and the wrinkles were evaluated according to the following evaluation criteria. ◎: No wrinkles, film surface is smooth ○: There are slight wrinkles, but this does not affect the fabrication of the polarizer. ×: Wrinkles are noticeable and the film surface is uneven

[0069] <Color uniformity evaluation> According to the above-mentioned polarizing film process, the polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 5 were made into corresponding polarizer samples, and the hue uniformity of each sample was further analyzed and evaluated. The specific evaluation method is as follows. Sample: Polarizer sample 30 x 30 cm 2 Cut to size. Test conditions: A base polarizer with a single transmittance of 43% was placed in a 15,000 CD light box in the form of a backlight (i.e., the sample was placed between the human eye and the light source, and the naked eye was directed toward the light source to observe). The polarizer sample to be observed was then placed on top of the base polarizer in a cross-polarized state, and the polarizer sample was observed from a bird's-eye view from a position 1 m directly above it. Evaluation criteria: The polarizer sample was visually inspected for the presence or absence of areas of uneven dyeing, and was evaluated according to the following evaluation criteria. ○: No color unevenness △: Slight color unevenness ×: Color unevenness is noticeable

[0070] The polyvinyl alcohol films of Examples 1 to 15 and Comparative Examples 1 to 5 and the polarizers produced therefrom were subjected to the above-mentioned analytical tests, and the results obtained are summarized in Tables 2 and 3 below.

[0071] [Table 2]

[0072] As can be seen from Table 2, the complex viscosity (η*) parameters measured for the polyvinyl alcohol film samples of Examples 1 to 15 were all within 400 to 1500 Pa·s, and these polyvinyl alcohol film samples obtained good results in damage evaluation and wrinkle evaluation. In contrast, the complex viscosity (η*) parameters measured for the polyvinyl alcohol film samples of Comparative Examples 1 to 5 were not within the above range, and none of these polyvinyl alcohol film samples obtained ideal results in both damage evaluation and wrinkle evaluation. Therefore, by controlling the complex viscosity of the polyvinyl alcohol film only within the range of 400 to 1500 Pa·s, the damage and wrinkle conditions can be improved at the same time. In detail, when the complex viscosity exceeds 1500 Pa·s (e.g. Comparative Example 4), the polyvinyl alcohol film is prone to irreversible contact friction with the manufacturing equipment during the manufacturing process, which then leads to more serious film surface defects, resulting in non-ideal damage evaluation results. When the complex viscosity is less than 400 Pa s (e.g., Comparative Examples 1 to 3 and 5), even if damage to the polyvinyl alcohol film during the manufacturing process can be reduced, wrinkles are likely to occur on the exterior, which affects the yield of the subsequent molded products.

[0073] [Table 3]

[0074] As can be seen from Table 3, by further controlling the weight swelling degree of the polyvinyl alcohol film at 30°C to be more than 37% and the weight swelling degree at 55°C to be more than 60%, all of these polyvinyl alcohol film samples can be manufactured into polarizers with good hue uniformity (see Examples 1 to 12). In contrast, since the weight swelling degree at 30°C and the weight swelling degree at 55°C are not simultaneously controlled within the above-mentioned ideal ranges, these polyvinyl alcohol film samples cannot be manufactured into polarizers with good hue uniformity. Therefore, by further controlling the weight swelling degree at 30°C to be more than 37% and the weight swelling degree at 55°C to be more than 60%, the polyvinyl alcohol film can have the most ideal hue uniformity after being manufactured into a polarizing piece.

[0075] 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 the endpoints unless otherwise indicated. Thus, the range 1-5 specifically includes 1, 2, 3, 4, and 5, as well as subranges 2-5, 3-5, 2-3, 2-4, 1-4, etc.

[0076] The contents of all publications and patent applications cited in this specification are incorporated herein by reference and for all purposes are to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated herein by reference. In the event of a conflict between this specification and the contents of a publication or patent application incorporated herein by reference, the specification shall control.

Claims

1. A polyvinyl alcohol film containing a polyvinyl alcohol resin having a degree of saponification of 95% or more, and having a complex viscosity (η*) of 400 to 1500 Pa·s measured when subjected to sweep oscillation at 25° C. and an angular frequency of 10 rad / s.

2. 2. The polyvinyl alcohol film of claim 1, wherein the polyvinyl alcohol film has a weight swelling degree of more than 37% when immersed in water at 30° C. for 20 minutes, and the polyvinyl alcohol film has a weight swelling degree of more than 60% when immersed in water at 55° C. for 3 minutes.

3. The polyvinyl alcohol film according to claim 2 , wherein the polyvinyl alcohol resin has a degree of saponification of 99.95% or more.

4. The polyvinyl alcohol film according to claim 1, wherein the complex viscosity (η*) is 410 to 1100 Pa·s.

5. The polyvinyl alcohol film according to any one of claims 1 to 4, wherein the degree of polymerization of the polyvinyl alcohol resin is 1,300 to 5,200.

6. The polyvinyl alcohol film according to any one of claims 1 to 4, wherein the degree of polymerization of the polyvinyl alcohol resin is 2000 to 3000.

7. The polyvinyl alcohol film according to any one of claims 1 to 4, wherein the polyvinyl alcohol film has a thickness of 30 to 75 µm.

8. An optical film produced from the polyvinyl alcohol film according to any one of claims 1 to 4.

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

10. The optical film of claim 9 further comprising a protective layer laminated to at least one surface of the optical film.

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