Polyvinyl alcohol film and optical film produced therefrom

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, particularly 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] Polarizing films obtained by processing polyvinyl alcohol films through a polarization process have the property of blocking only light from a specific direction, thereby controlling the brightness of the light that passes through. Based on this property, polarizing films are used in various displays, eyeglasses, and wearable devices. The polarization process generally involves processes such as swelling, stretching, and dyeing. Specifically, the polyvinyl alcohol film is placed in a solution and the above processes are carried out to diffuse the dye molecules into the polyvinyl alcohol film, resulting in a regular alignment of the molecules. This allows the polarizing film to absorb light components parallel to the alignment direction and transmit light components perpendicular to the alignment direction, thereby producing polarized light.

[0004] 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 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 subsequently manufactured optical film. 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 surface dent defects in the resulting polyvinyl alcohol film. When the film is used as a film roll for producing polarizing films, a polarizing film that meets the required quality level can be obtained with a high yield and few manufacturing defects. [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 film surface defects by adjusting the surface roughness of the polyvinyl alcohol film or the number of cracks on the surface of the metal support 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, it is possible to improve the film surface defects of the molded product and further to impart good optical performance to the subsequently manufactured optical film. Furthermore, 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 manufactured 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 with 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 degree of saponification of the polyvinyl alcohol resin is 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 degree of polymerization of the polyvinyl alcohol resin is 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.

[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. [Effects 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 INVENTION

[0018] To provide a more detailed and complete description of the present invention, the following provides illustrative descriptions of embodiments and specific examples of the present invention, but these are not the only ways to implement or operate the specific examples of the present invention. In this specification and the appended claims, unless the context dictates otherwise, the terms "a," "an," and "the" may be interpreted as plural. Furthermore, unless otherwise specified in this specification and the appended claims, "disposed on something" may be considered to be in direct or indirect contact with a surface by adhering or otherwise. The identity of the surface must be determined based on the context of the preceding and following paragraphs of the specification and common knowledge in the art to which the present invention pertains.

[0019] Although the numerical ranges and parameters specifying the present invention are approximations, the relevant numerical values ​​in the specific examples are presented as precisely as possible. However, any numerical value inherently contains standard deviations resulting from individual testing methods. As used herein, the term "about" indicates that the actual value is within an acceptable standard error of the mean and is within the understanding 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 constituent components of a material. As used herein, "complex viscosity (η*)" is the ratio of the complex modulus (G*) to the angular frequency (ω), and is used to quantify a material's ability to resist deformation at a specific oscillation frequency. Complex viscosity can be determined by measuring the complex modulus (G*) of a material at a specific angular frequency (ω) using a rheometer using the micro-oscillation method. A higher complex viscosity indicates a material's ability to resist deformation. The value of "complex modulus (G*)" is calculated as the square root of the sum of the squares of the loss shear modulus (G'') and the storage shear modulus (G'), i.e., √(G' 2 +G'' 2 )

[0022] The term "film surface defects" used herein includes damage to the film surface of a polyvinyl alcohol film (e.g., scratches, streaks) and wrinkles in appearance. The complex viscosity of a polyvinyl alcohol film likely influences the optical performance of an optical film manufactured from the polyvinyl alcohol film by relating it to film surface defects. In other words, if the complex viscosity parameter of a polyvinyl alcohol film is within an appropriate range, film surface defects can be improved. Specifically, the complex viscosity parameter has a lower and upper limit. Without being bound by any particular theory, if the complex viscosity parameter exceeds the upper limit, the polyvinyl alcohol film is likely to experience irreversible contact friction with the manufacturing equipment during the manufacturing process, resulting in more serious damage such as scratches and streaks on the film surface. If the complex viscosity parameter is below the lower limit, wrinkles and unevenness may occur on the surface of the polyvinyl alcohol film after it has been left standing, 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, and particularly 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 herein means a measured value obtained by the test method described in JIS K 6726 (1994). In one or more embodiments, the saponification degree of the polyvinyl alcohol-based resin exceeds 95% to obtain better optical properties, for example, greater than 95%, greater than 96%, greater than 97%, greater than 98%, or greater than 99%. When the saponification degree of the polyvinyl alcohol-based resin exceeds 95%, the strength of the resin increases, making the resin less likely to dissolve during subsequent dyeing and fixing processes in the polarizing film production process, and making it easier to obtain a polarizing film with high polarizing performance. Furthermore, the degree of saponification 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 greater than 37% when immersed in water at 30°C for 20 minutes, and the polyvinyl alcohol film has a weight swelling degree of greater than 60% when immersed in water at 55°C for 3 minutes.

[0026] The "weight swelling ratio (%)" used herein refers to the percentage weight change ((W1-W2) / W1 x 100) between the weight change and the temperature of a polyvinyl alcohol film cut to a specific size. The weight change is measured by immersing the film flat in water at a specific temperature (e.g., but not limited to, 30°C and 55°C) for a certain period of time (e.g., 20 minutes). The film is then wiped dry with clean paper and weighed to obtain the film weight (W1). The film is then dried (e.g., but not limited to, 120°C) and weighed again to obtain the film weight (W2). The weight swelling ratio range is likely related to the color uniformity of optical films made from the polyvinyl alcohol film. In conventional polarizing film manufacturing processes, a polyvinyl alcohol film is swollen and dyed in water at 30°C and stretched in an acidic solution (e.g., 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 crystalline dissociation of the polymer chains of the polyvinyl alcohol film, respectively. The present inventors have discovered that controlling the weight swelling degree measured after immersion in water at 30°C and 55°C above a specific lower limit can ensure good color uniformity in the subsequently manufactured optical film. 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 uniformly attach 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 exceeds 37%, for example, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, or 49%. After immersing the polyvinyl alcohol film 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 skilled 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 when the weight swelling parameter exceeds a specific lower limit. Furthermore, without being bound by a particular theory, the weight swelling parameter is related to the moisture content of the polyvinyl alcohol film in 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" referred to in this specification means a measured value obtained by the test method described in JIS K 6726 (1994). In one or more embodiments, the degree of polymerization of the polyvinyl alcohol resin is in the range of 1300 to 5200, and is 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 a value 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. The above values ​​are merely examples and are not limiting. Furthermore, when the obtained polyvinyl alcohol film is used to produce 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] [Manufacturing method of 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; and (d) an oven step of placing the semi-formed polyvinyl alcohol film in an oven to dry it and obtain a molded polyvinyl alcohol film; and depending on the situation, it may further include (e) a temperature and humidity adjustment step of placing the molded polyvinyl alcohol film in a temperature and humidity regulator to adjust the temperature and humidity.

[0030] [Dissolution process] In some embodiments, the dissolution step involves stirring a mixture of primarily polyvinyl alcohol-based resin, solvent, plasticizer / plasticizer, and additives while raising the temperature to at least 130°C until uniform dissolution is achieved. After that, additional solvent (e.g., water) is added 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 dissolution step is polymerized with a vinyl ester resin monomer to form a polyvinyl alcohol resin with a degree of polymerization of 1300 to 5200, and then subjected to a saponification reaction to obtain a polyvinyl alcohol resin with a degree of saponification of greater than 95%. Examples of the vinyl ester resin monomer include, but are not limited to, vinyl esters such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, and vinyl octoate, or combinations thereof. 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. The amount of the olefin compound added can be 2 to 4 mol %, for example, 2, 2.5, 3, 3.5, or 4 mol %. 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.

[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 the polyvinyl alcohol resin. Examples of solvents include, but are not limited to, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylenediamine, and diethylenetriamine. 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 preferred. Furthermore, 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, but not limited to, for example, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 wt %.

[0034] In one or more embodiments, the term "additive" used herein refers to a surfactant. The term "surfactant" is not limited to cationic, anionic, or nonionic surfactants, and specifically includes, but is not limited to, 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. Without being bound by any particular theory, it is believed that additives can reduce the degree of entanglement and crystallinity of polymer chains by reducing the amount of hydrogen bonding formed between polymers. Therefore, the amount of the additive affects the complex viscosity of the polyvinyl alcohol film and its weight swelling after immersion in water at a specific temperature. In other words, by adjusting the amount of additive added during the manufacturing process within a specific range, it is possible to further control the complex viscosity of the polyvinyl alcohol film and its weight swelling after immersion in water at a specific temperature. Here, the amount of additive added is expressed as a concentration (ppm). A specific calculation method is to divide the weight of the additive by the weight of all nonvolatile components (e.g., resin, plasticizer glycerol, and additives) and then convert it to a ppm concentration. In one or more embodiments, the amount of the additive added is 3000 ppm or less, preferably 200 to 3000 ppm, 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 to these.

[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 step mainly involves degassing the polyvinyl alcohol casting solution (defoaming can be achieved by a static degassing method or by using a multi-screw extruder with 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 through a T-die lip. The solution is then cast onto a casting drum (also called a flow 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 casting drum temperature, the residence time of the polyvinyl alcohol in the casting drum, and hot air drying. 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 comes into contact with 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, 85 to 95°C, 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, for example, 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 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 its weight swelling after immersion in water at a specific temperature. Specifically, this is the main stage of polyvinyl alcohol polymer chain alignment and formation, which 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; the lower the water content, the more entangled the polymer chains become, which affects the complex viscosity of the subsequent molded product and its weight swelling after immersion in water at 30°C. In other words, by adjusting the water content of the polyvinyl alcohol preformed film within a specific range, the complex viscosity of the polyvinyl alcohol film and its weight swelling 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 particularly 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 from the casting drum with multiple 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 involves blowing hot air at a specific temperature onto the surface of the polyvinyl alcohol film semi-formed product that comes into contact with air to adjust the moisture content of the sample.

[0043] In one or more embodiments, the number of the multiple heating rollers may be 2 to 30, for example, but not limited to, 2, 5, 10, 15, 20, 25, or 30. The temperatures of the multiple heating rollers gradually decrease from high to low, with the first heating roller having the highest temperature among all the heating rollers and the last heating roller having 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 to 60°C. In one or more embodiments, the temperature of the hot air is preferably, but not limited to, 50 to 110°C.

[0044] Without being bound by any particular theory, it is believed that the water content of the semi-formed polyvinyl alcohol film affects the complex viscosity of the polyvinyl alcohol film and its weight swelling after immersion in water at a specific temperature. Specifically, this stage is the main stage of polyvinyl alcohol crystal formation, and the water content of the semi-formed polyvinyl alcohol film is related to the amount of crystal formation; the lower the water content, the more crystals are formed, which in turn affects the complex viscosity of the subsequent formed product and its weight swelling after immersion in water at 55°C. In other words, adjusting the water content of the semi-formed polyvinyl alcohol film within a specific range can further control the complex viscosity of the polyvinyl alcohol film and its weight swelling after immersion in water at 55°C. In one or more embodiments, the water content of the semi-formed polyvinyl alcohol film 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 is mainly to dry the upper and lower surfaces of the polyvinyl alcohol film semi-molded product peeled off from the heating 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 within a range of 40°C to 120°C, specifically 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 within 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 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 within a 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 its weight swelling 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 molded product and its weight swelling after immersion in water at 30°C and 55°C. 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 its weight swelling after immersion in water at 30°C and 55°C can be further controlled. If the water content is too low, the entanglement and crystallization between the polymer chains of the polyvinyl alcohol film molded product will increase. In one or more embodiments, the moisture content of the polyvinyl alcohol film molded article is 1.8 to 5%, for example, 1.8 to 2, 2 to 3, 3 to 4, or 4 to 5%, particularly 1.8, 2, 3, 4, or 5%.

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

[0049] In one or more embodiments, the temperature of the temperature and humidity controller 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 controller 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 standing in the temperature and humidity controller 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 manufacturing method thereof] Another object of the present invention is to provide an optical film manufactured from the polyvinyl alcohol film. The "optical film" referred to herein may be a polarizing film, a retardation film, a viewing angle widening film, or a brightness enhancing film, and is particularly a polarizing film. According to some embodiments, the "manufacturing method for an optical film" of the present invention refers to a manufacturing method for a polarizing film, further comprising manufacturing a polarizing film from a polyvinyl alcohol film and evaluating the color uniformity. The manufacturing method includes a dyeing step in which iodine is adsorbed, a boric acid treatment step, and a water washing step. A uniaxial stretching step can be performed during or before the boric acid treatment step, and a drying step can be performed once between the boric acid treatment step and the water washing step. Preferably, a swelling step in which the polyvinyl alcohol film is swelled with water can be performed before the dyeing step. A final drying step is usually performed 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, for example, 30°C to wash the film surface and perform a swelling treatment. The swelling treatment time is typically 5 to 300 seconds, preferably 20 to 240 seconds. According to some embodiments, multiple guide rollers are placed in a swelling tank containing the treatment bath 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 the dyeing step is performed.

[0052] In the dyeing process, the polyvinyl alcohol film that has undergone the swelling process is immersed in a dyeing tank containing a dye bath. The dyeing conditions can be determined based on the range in which iodine is adsorbed into the polyvinyl alcohol film without causing defects such as excessive dissolution or devitrification of the film. The dye bath used in the dyeing process 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. Potassium iodide may be replaced with another iodide, such as zinc iodide, or potassium iodide may be used in combination with another iodide. The temperature of the dye bath (dyeing temperature) is usually 10 to 50°C, for example 30°C, and the dyeing time (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, followed by a boric acid treatment and a stretching process.

[0053] The boric acid treatment and stretching process involves treating a polyvinyl alcohol film dyed with iodine with a boric acid-containing aqueous solution to crosslink the film and fix the absorbed iodine to the resin. This process is typically carried out by immersing the polyvinyl alcohol film that has undergone 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, in an amount of 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. Furthermore, the boric acid treatment bath may also contain compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, or sodium sulfate. 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, followed by a water washing process and a drying process, to form a polarizing film. Furthermore, a protective layer may be formed on at least one side of the polarizing film to manufacture a finished polarizing film (also called a polarizer). Specifically, the protective layer has a function of preventing abrasion of the polarizing film surface, and is preferably a component containing a transparent resin. In another embodiment, the protective layer is provided on only one side of the polarizing film, but is preferably formed on both sides of the polarizing film. The protective layer may 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. [Example]

[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 understanding 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 from polyvinyl alcohol films. Non-limiting example polyvinyl alcohol films (Examples 1-15) and comparative example polyvinyl alcohol films (Comparative Examples 1-5) were prepared in a manner similar to the methods disclosed below.

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

[0058] 1. First, 1800 kg of polyvinyl alcohol resin with a degree of saponification of 99.95% and a degree of polymerization of 2000 to 3000 (specific degrees of polymerization 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. The additives were lauryl alcohol polyoxyethylene ether and sodium lauryl alcohol polyoxyethylene ether sulfate, and the weight ratio of lauryl alcohol polyoxyethylene ether to sodium lauryl alcohol polyoxyethylene ether sulfate contained in the additive was 1:1. Next, the temperature was raised with stirring to achieve uniform dissolution. After that, water was added to adjust the concentration of nonvolatile components (polyvinyl alcohol resin, plasticizer glycerol, and additives) to 30 wt.%, yielding a polyvinyl alcohol cast solution.

[0059] 2. The polyvinyl alcohol cast solution was degassed in a twin-screw extruder, then discharged from a T-die lip and cast onto a rotating casting drum at 90°C. The air-contacting surface was dried with hot air at 110°C to produce a polyvinyl alcohol preformed film with 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 peeling the polyvinyl alcohol preformed film from the casting drum, the top and bottom surfaces of the film were dried by contact with seven heated rollers. At the same time, the air-contacting surface was dried with hot air 10°C warmer than the heated rollers, producing a polyvinyl alcohol film semi-formed product with a specific moisture content (the specific moisture content is shown in Table 1). The first heated roller was the hottest of all the heated rollers, with a temperature of 90°C. The temperature of the subsequent heated rollers was gradually reduced by 5°C each time, until the temperature of the seventh heated roller reached 60°C. The moisture content of the polyvinyl alcohol film semi-formed product was adjusted by increasing or decreasing the number of heated rollers.

[0061] 4. Next, both the top and bottom 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 10 × 10 cm 2 Cut to size. Test conditions: The weight (W1) of the 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, and the moisture content was calculated using 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 cut film was placed in an environment of 30°C and 70% RH, and the film was allowed to absorb moisture until the moisture content reached 9-10%, before measurement. Test method: The torque and angular displacement detected by the fixture were measured in sweep oscillation mode, and the stress and strain information was recorded. Here, sweep oscillation means that the motor rotates clockwise or counterclockwise from the zero point to generate shear force on the sample. Test conditions: Mode: Oscillation Angular frequency: 10rad / s Strain: 1% Sampling time: 10 points per minute, specifically, 1 point every 6 seconds per minute Jig: 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> Sample preparation method: Polyvinyl alcohol film 5 x 5 cm 2 Cut to size. Weight swelling measured after immersion in 30°C water (hereafter referred to as weight swelling at 30°C): A film sample was immersed flat in 500 ml of water at 30°C for 20 minutes, then removed with tweezers. Both sides of the film sample were gently wiped with four sheets of laboratory paper (GBFA-00000000001, CleanHose, USA). This process was repeated twice. After removing excess surface moisture, the film sample was placed flat on one sheet of paper, covered with another sheet of paper, and pressed with four stacked stainless steel sample pans (approximately 30.5 ± 1.5 g each). The remaining water was allowed to absorb into the surface of the paper for 30 seconds. The film sample was then turned over and the above process was repeated with new 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, the surface moisture was removed using 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 before 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 from a white wall, and a 1200 lumen flashlight was shone on 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 across 10 sheets 2No damage exceeding 200 μm or streaks exceeding 200 μm in width are observed. B: All 10 pieces have an area of ​​0.01 mm 2 Damage exceeding 100μm or a streak exceeding 200μm in width is confirmed in one location C: All 10 pieces have an area of ​​0.01 mm 2 Damage exceeding 100μm or streaks exceeding 200μm 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 wrinkles, and the wrinkles were evaluated according to the following evaluation criteria. ◎: No wrinkles, smooth film surface ○: 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 prepared 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 for observation). The polarizer sample to be observed was then placed on top of the base polarizer in a cross-polarized state, and observed from a bird's-eye view from a position 1 m directly above the polarizer sample. Evaluation criteria: The polarizer sample was visually inspected for the presence or absence of uneven dyeing areas, and 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 manufactured 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 between 400 and 1500 Pa·s, indicating that these polyvinyl alcohol film samples achieved good results in damage and wrinkle assessments. 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 achieved ideal results in both damage and wrinkle assessments. Therefore, by controlling the complex viscosity of the polyvinyl alcohol film to within the range of 400 to 1500 Pa·s, damage and wrinkle conditions could be simultaneously improved. Specifically, 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, resulting in more serious film surface defects, resulting in non-ideal damage assessment 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 tend to appear 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 to exceed 37% at 30°C and exceed 60% at 55°C, good color uniformity was obtained after fabricating these polyvinyl alcohol film samples into polarizers (see Examples 1 to 12). In contrast, because the weight swelling degrees at 30°C and 55°C were not simultaneously controlled within the above-mentioned ideal ranges, good color uniformity was not obtained after fabricating these polyvinyl alcohol film samples into polarizers. Therefore, by further controlling the weight swelling degree at 30°C to exceed 37% and exceed 60% at 55°C, the most ideal color uniformity was obtained after fabricating the polyvinyl alcohol film into a polarizer.

[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, all ranges explicitly stated herein include the endpoints unless otherwise indicated. Thus, the range 1 to 5 specifically includes 1, 2, 3, 4, and 5, as well as subranges such as 2 to 5, 3 to 5, 2 to 3, 2 to 4, 1 to 4, etc.

[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 were 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 present specification will 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 according to 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 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 1300 to 5200.

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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