JP7592048B2 - Polyvinyl alcohol film, optical film produced therefrom and method for producing same - Google Patents

Description

The present invention relates to a polyvinyl alcohol (PVA) film that can be used in the manufacture of optical films, particularly polarizing films.

Polyvinyl alcohol (PVA) film is a type of 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 widely used in packaging materials and optical films for electronic products such as polarizing films.

Polarizing films obtained by processing polyvinyl alcohol film through a polarizing process have the property of only passing light rays in a specific direction, which allows the brightness of the passing light to be controlled. Based on this property, polarizing films are used in various displays, glasses, and wearable devices. The process of manufacturing polarizing films includes swelling, stretching, and dyeing. Specifically, the polyvinyl alcohol film is placed in a solution and the above-mentioned process is carried out, and the dye molecules are diffused and inserted between the polyvinyl alcohol molecules of the polyvinyl alcohol film to arrange them in a regular manner, allowing the polarizing film to absorb the light components parallel to the arrangement direction and transmit the light components perpendicular to the arrangement direction, resulting in the property of having polarized light.

In order to provide good optical properties, an ideal polarizing film must have properties such as uniform color, few color spots, no wrinkles, and good hue effect. Therefore, in conventional technology, it has been common to improve optical properties by changing the viscosity or degree of saponification or adjusting the plasticizer content by using methods such as adjusting the structure of polyvinyl alcohol or adding functional groups (e.g., cationic groups).

In the prior art, when manufacturing optical films from polyvinyl alcohol films, a problem of dye spots occurring on the finished product often occurred. The inventor realized that the cause of the above problem is probably that additives must be mixed into the polyvinyl alcohol film when it is used to manufacture optical films, but the additives precipitate after the swelling and stretching process of the polyvinyl alcohol film, and some of them remain on the film. For example, if the distribution of the amount of additive precipitated is uneven, iodide ions cannot be uniformly attached to the film during the subsequent dyeing process, leading to uneven dyeing.

Furthermore, without being limited to a particular theory, the inventors have realized that many factors are involved in the above situation, such as the standard deviation of the temperature of the dryer in which the preformed film is dried after passing through the heated rollers during the polyvinyl alcohol film manufacturing process, and the temperature difference between the upper and lower surfaces of the preformed film in the dryer, all of which at least partially affect the amount of additive that precipitates and remains after the film body has swelled.

Furthermore, the inventors have noticed that when producing an optical film from a polyvinyl alcohol film, the slow axis angle after swelling and stretching is related to the polarization degree of the finished polarizing film. Furthermore, without being limited to a particular theory, the inventors believe that there are several factors that affect the above-mentioned situation during the preparation process of the polyvinyl alcohol film. For example, these factors include the stirring temperature when heating and dissolving the polyvinyl alcohol resin, plasticizer, and water, the frequency of changing the stirring direction, and the addition of a surfactant.

Therefore, in order to solve the above problems, the present invention provides a polyvinyl alcohol film by adjusting the standard deviation of the amount of additive residues after swelling and stretching of the polyvinyl alcohol film within a certain range, which is used to manufacture an optical film with uniform dyeing. Furthermore, the present invention obtains a polyvinyl alcohol film by adjusting the slow axis angle after swelling and stretching of the polyvinyl alcohol film within a certain range, and the polarizing film prepared thereby has a good polarization degree.

Specifically, in one embodiment of the present invention, the polyvinyl alcohol film has a standard deviation of 0.05 to 0.60 in the amount of residual additives after swelling and stretching of multiple polyvinyl alcohol films cut evenly in the width direction. In the above-mentioned swelling and stretching, multiple polyvinyl alcohol films were stretched in water at a speed of 4.3 cm/min to double the length. The above-mentioned amount of residual additives was calculated using the formula: (W3-W4)/W3 x 100%. In the formula, W3 is the weight of the polyvinyl alcohol film after swelling and stretching and drying, and W4 is the weight of the polyvinyl alcohol film after swelling and stretching, drying, placing in pure water, stirring for 5 minutes, and then drying again.

In a preferred embodiment, the average amount of additive remaining in multiple polyvinyl alcohol films after swelling and stretching is 1.40 to 4.15 wt %.

In a preferred embodiment, the standard deviation of the slow axis angle of multiple polyvinyl alcohol films after swelling and stretching is 0.30 to 2.79.

In a preferred embodiment, the average slow axis angle of multiple polyvinyl alcohol films after swelling and stretching is 89.00 to 91.00.

In a preferred embodiment, the polyvinyl alcohol film has an initial additive content of 6-15 wt %.

In a preferred embodiment, the degree of polymerization of the polyvinyl alcohol film is 1800 to 3000.

In a preferred embodiment, the water content of the polyvinyl alcohol film is 1.0 to 5.0 wt %.

In another embodiment of the present invention, an optical film is provided which is made from the polyvinyl alcohol film described above.

In a preferred embodiment, the optical film is a polarizing film. Preferably, the polarizing film has a polarization degree of 99.8% or more.

In yet another embodiment of the present invention, there is provided a method for producing a polyvinyl alcohol film, comprising the steps of:
(a) A polyvinyl alcohol casting solution is formed by stirring and heating a polyvinyl alcohol-based resin, a surfactant, a plasticizer and water to a dissolution temperature above 100° C. and maintaining the temperature for 2-4 hours, and reversing the stirring direction at least three times per hour.
(b) A polyvinyl alcohol casting solution is cast onto a casting drum and dried to prepare a preformed film.
(c) the preformed film is heat-treated by contacting it with a number of heated rollers whose temperatures are gradually decreased from high to low, and then entering a multi-section dryer, in which the standard deviation of the temperature (along the width) is 0.93-3.00 and the temperature difference between the upper and lower surfaces of the polyvinyl alcohol film in the dryer is 5° C. or less.

In a preferred embodiment, the dissolution temperature in step (a) is 130 to 140°C.

In a preferred embodiment, the polyvinyl alcohol resin concentration in the polyvinyl alcohol casting solution in step (b) is 20.0 to 40.0%.

In a preferred embodiment, the final surfactant content is 0.10-0.20 wt%.

In a preferred embodiment, the dryer in step (c) is a floating type dryer.

The polyvinyl alcohol film provided based on the above-mentioned characteristics of the present invention can be dyed uniformly and has a better polarization degree. In addition, the characteristics of the present invention can be used to more precisely adjust and manufacture the above-mentioned polyvinyl alcohol film.

In order to make the above and other objects, features, advantages and embodiments of the present invention more clear, reference is made to the following drawings.
FIG. 1 is a schematic diagram of a process for producing a polyvinyl alcohol film according to one embodiment of the present invention. 1 is a schematic diagram of a polyvinyl alcohol film manufacturing apparatus according to one embodiment of the present invention.

In this specification, the proportions of various features and components in the drawings are not actual proportions, but are drawn in a drawing method based on a conventional work method in order to show the specific features and components related to the present invention in an optimal manner. In addition, in different drawings, the same or similar component symbols refer to similar components or parts.

In order to describe the present invention in more detail and without omission, the following description is provided with reference to the embodiments and specific examples of the present invention, which are not the only forms of specific examples for carrying out or applying the present invention. In this specification and the appended claims, the terms "one" and "the" may be interpreted as plural unless otherwise specified in the context. In this specification and the appended claims, unless otherwise specified, "installed on an object" may be considered to be attached to or in other ways contacted with the surface of an object, directly or indirectly, and the definition of the surface shall be determined by the implication of the preceding/following paragraphs/paragraphs of the contents of the specification and the common knowledge in the field to which this specification pertains.

All numerical ranges and parameters specifying the present invention are approximate, but the relevant numerical values in the specific examples are as precise as possible. However, it is inherently inevitable that any numerical value will include standard deviations resulting from individual testing methods. In this context, "about" generally means that the actual numerical value is within ±10%, 5%, 1% or 0.5% of the specified numerical value or range. Alternatively, the term "about" means that the actual numerical value is within an acceptable standard error of the mean value, as considered and determined by a person skilled in the art to which the present invention pertains. Thus, unless otherwise stated, all numerical parameters disclosed in this specification and the appended claims are approximate and may be considered to vary as necessary. At the very least, such numerical parameters should be interpreted as being the numerical values obtained by applying the indicated number of significant digits and ordinary carryover techniques.

The polyvinyl alcohol film provided by the present invention is cut evenly in the width direction into several pieces, and the standard deviation of the additive residue amount after swelling and stretching is 0.05 to 0.60. In the above-mentioned swelling and stretching, several pieces of polyvinyl alcohol film cut evenly in the width direction were stretched in water at a speed of 4.3 cm/min to double the length. The above-mentioned additive residue amount was obtained by the calculation formula: (W3-W4)/W3 x 100%. In the formula, W3 is the weight of the polyvinyl alcohol film after swelling and stretching and drying, and W4 is the weight of the polyvinyl alcohol film after swelling and stretching, drying, placing it in pure water, stirring for 5 minutes, and then drying it again. The standard deviation of the residual amount of additives is, for example, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, or between any two of the above values, but is not limited thereto. During the swelling process of the polyvinyl alcohol film, additives are precipitated, but in areas where the amount of precipitation is small, iodide ions are less likely to form complexes with polyvinyl alcohol, and conversely, in areas where the amount of precipitation is large, a large amount of complexes are likely to be formed, so if the amount of precipitation is non-uniform, dye spots will occur in the polarizing film.

According to a preferred embodiment of the present invention, the average amount of residual additives after swelling and stretching of multiple polyvinyl alcohol films is 1.40 to 4.15 wt%, for example, 1.44, 1.71, 1.92, 1.93, 2.91, 3.89, or 4.12 wt%.

The present invention solves the problem of dyeing unevenness of polarizing films by adjusting the standard deviation of the additive residual amount of the polyvinyl alcohol film after swelling and stretching, that is, by simulating the conditions before dyeing the polarizing film and controlling the standard deviation of the additive residual amount before it enters the dyeing tank. The inventor of the present invention noticed that when actually manufacturing a polarizing film, the polyvinyl alcohol film goes through swelling and stretching treatment, during which additives and other impurities in the polyvinyl alcohol are precipitated, causing the additive content of each part of the polyvinyl alcohol film finally manufactured into a polarizing film to differ, and therefore, the residual state of the additives when the polarizing plate manufacturing process is actually performed cannot be expressed by simply measuring the initial amount of additives in the polyvinyl alcohol film or the difference in the initial amount of additives, and the problem of dyeing unevenness of the polarizing film cannot be solved. Specifically, the above parameters are specified based on the process of preparing the polyvinyl alcohol film into an optical film, particularly a polarizing film. The method of preparing a polyvinyl alcohol film into a polarizing film is not particularly limited, but preferably includes a swelling/stretching process in which the polyvinyl alcohol film is placed in water and stretched, a dyeing process in which the film is dyed with a dichroic dye, and a stretching process in which the film is uniaxially stretched. Among these, methods such as boric acid crosslinking, fixing, washing, and heating may be further performed as necessary. In this case, the order of each process is not particularly limited, but preferably, the swelling process, dyeing process, and stretching process are performed in that order. In the industry, it is common to stretch the film to twice its length in the swelling tank, which is the approximate stretching ratio.

The residual amount of the additives described above was determined by stirring the polyvinyl alcohol film after swelling, stretching, and drying in pure water for 5 minutes to precipitate the additives in the film. Specifically, the formula for calculating the residual amount of additives is (W3-W4)/W3×100%, and is preferably the average value of a plurality of films (e.g., 5 or more) cut evenly in the width direction. In addition, in order to further increase the accuracy of this parameter used in the evaluation, control, and selection of polyvinyl alcohol films, it is preferable in this specification to adopt a numerical range of the standard deviation of the residual amount of additives. The "standard deviation" used in this specification refers to the standard deviation of a sample and is used to evaluate the degree of dispersion of data in a sample group. Specifically, the calculation formula for the standard deviation s is as follows:
where x is the average value of the sample and n is the sample size.

As used herein, the term "additive" includes at least one of an initial additive, a surfactant, a clouding agent, an emulsifier, or a foaming agent, but is not limited thereto. Among them, the "plasticizer" can increase the flexibility of the material or liquefy the material, and is, for example, but not limited to, phthalate ester, bis(2-ethylhexyl) phthalate (DEHP), glycerin, dibutyl phthalate (DBP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), dioctyl phthalate (DOP), ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. Preferably, the plasticizer is glycerin, ethylene glycol, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, or trimethylolpropane. The "surfactant" is not limited to cationic, anionic or nonionic surfactants, such as, but not limited to, carboxylate type such as potassium laurate, sulfate ester type such as sodium laureth sulfate, sulfonate type such as dodecylbenzenesulfonate, alkylphenyl ether type such as polyoxyethylene octylphenyl ether, alcohol-based phenyl ether type such as polyethylene glycol monooctylphenyl ether, alkyl ester type such as polyoxyethylene laurate, alkylamine type such as polyoxyethylene laurylamine, alkylamide type such as polyoxyethylene lauric acid amide, polypropylene glycol ether type such as polyoxyethylene polyoxypropylene ether, alkanolamide type such as lauric acid diethanolamide, oleyl diethanolamide, allyl phenyl ether type such as polyoxyethylene allyl phenyl ether, or sodium polyoxyethylene lauryl ether sulfate. According to a preferred embodiment of the present invention, the polyvinyl alcohol film has an additive initial content between 6 and 15 wt%, for example, 7 to 14, 8 to 13, 9 to 12, or 10 to 11 wt%.

According to a preferred embodiment of the present invention, the additives contained in the polyvinyl alcohol film are mainly plasticizers and surfactants, or only plasticizers. Plasticizers and surfactants are low molecular weight compounds that increase the free volume of the material when blended with a polymer. These additives are particularly easy to precipitate during the swelling process, which causes the amount of additive residue to be non-uniform during the dyeing process. Specifically, during the swelling process of the polyvinyl alcohol film, some of the additives remain in the polyvinyl alcohol film. If the amount of additive residue is too large, the free cavities will be occupied by the additives, and when the polyvinyl alcohol film is placed in the dyeing tank, the iodide ions will not be able to form complexes with the polyvinyl alcohol in the free cavities. Conversely, if the amount of additive residue is too small, it will affect the molecular arrangement, causing excessive complex formation, resulting in the problem of dye spots throughout the polarizing film.

According to at least one embodiment of the present invention, the slow axis angle of multiple polyvinyl alcohol films cut evenly in the width direction after swelling and stretching is 0.30 to 2.79, for example, 0.30, 0.35, 0.43, 0.75, 0.91, 1.25, or 2.78. Preferably, the average value of the slow axis angle is 89.00 to 91.00, for example, 89.22, 89.90, 90.16, 90.17, 90.27, 90.29, or 90.64.

The term "slow axis" as used herein refers to the axial direction in which the in-plane refractive index is maximum, and is generally used to indicate the direction of molecular alignment in a sample. If there is a deviation in the direction of molecular alignment, a state occurs in which the orthogonal transmittance increases partially in the polarization degree analysis of the sample, which leads to a decrease in the polarization degree. The slow axis of a polyvinyl alcohol film refers to the axis with a large birefringence value, and is usually the direction of molecular alignment. If there is a deviation in the alignment direction, a state occurs in which the orthogonal transmittance increases partially in the polarization degree analysis, which leads to a decrease in the polarization degree. Therefore, in this application, a phase difference analyzer is used to measure the standard deviation of the slow axis angle of multiple polyvinyl alcohol film samples after swelling and stretching, and the standard deviation is controlled within a certain range to obtain a polyvinyl alcohol film with a good polarization degree.

According to at least one embodiment of the present invention, the degree of polymerization of the polyvinyl alcohol film is between 1800 and 3000, such as, but not limited to, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, or 3000. According to at least one embodiment of the present invention, the water content of the polyvinyl alcohol film is between 1.0 and 5.0 wt%, such as, but not limited to, 1, 1.1, 1.3, 1.5, 1.7, 1.9, 2.0, 2.1, 2.3, 2.5, 2.7, 2.9, 3.0, 3.1, 3.3, 3.5, 3.7, 3.9, 4.0, 4.1, 4.3, 4.5, 4.7, 4.9, or 5.0 wt%. Additionally, according to at least one embodiment of the present invention, the thickness of the polyvinyl alcohol film is 20 to 100 μm, preferably 60 to 75 μm, for example, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74 or 75 μm.

The present invention further provides an optical film, which is prepared from the polyvinyl alcohol film described above. The "optical film" described in this specification may refer to a polarizing film, a blue light cut film, a filter lens, etc., but the present invention is not limited thereto. Preferably, the polyvinyl alcohol film of the present invention is a polarizing film. Furthermore, the polarization degree of the polarizing film of the preferred embodiment provided by the present invention is 99.8% or more, and preferably 99.9%.

In another aspect, the present invention also provides a method for producing a polyvinyl alcohol film, and the steps and production apparatus thereof can be referred to in the contents shown in FIG. 1 and FIG. 2. The production method includes the following steps: Step S100: A polyvinyl alcohol resin, a surfactant, a plasticizer, and water are stirred and heated to form a polyvinyl alcohol casting solution. Step S102: The polyvinyl alcohol casting solution is cast into a casting drum and dried to prepare a preformed film. Step S104: The preformed film is brought into contact with n heating rollers whose temperature is gradually decreased from high to low, and then entered into a dryer with multiple sections for heat treatment.

Specifically, in step S100, polyvinyl alcohol resin, surfactant, plasticizer and water are stirred in a dissolution tank 110 and heated to a dissolution temperature of more than 100°C, maintained at that temperature for 2-4 hours, and the stirring direction is reversed at least three times per hour to form a polyvinyl alcohol casting solution. The dissolution temperature is preferably 130-140°C, such as 130, 131, 132, 133, 134, 135, 136, 137, 138, 139 or 140°C. The amount of plasticizer added is usually between 3 and 30 parts by weight, preferably between 7 and 20 parts by weight, for example, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 parts by weight, based on 100 parts by weight of polyvinyl alcohol resin. If the content of plasticizer is insufficient, the polyvinyl alcohol film formed is prone to crystallization, which affects the dyeing effect in the subsequent processing. On the other hand, if the content of plasticizer is too high, the mechanical properties of the polyvinyl alcohol film will be impaired. The final content of surfactant is 0.10 to 0.20 wt%, for example, 0.10 wt%, 0.15 wt%, 0.20 wt%, or between any two of the above values, but is not limited thereto. Preferably, the final content of the surfactant is 0.15 wt%. According to at least one embodiment, step S100 is performed in a dissolution tank. In addition, the polyvinyl alcohol resin concentration in preparing the polyvinyl alcohol casting solution is 10.0-60.0 wt%, preferably 15.0-40.0 wt%, more preferably 20.0-40.0 wt%, such as 10.0, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, 50.0, 55.0 or 60.0 wt%. The calculation method of the polyvinyl alcohol resin concentration mentioned above is polyvinyl alcohol resin/(polyvinyl alcohol resin+water+plasticizer+surfactant). If the content of the polyvinyl alcohol resin is insufficient, the viscosity of the polyvinyl alcohol casting solution becomes too low, the drying load becomes excessively large, and the film formation efficiency in the preparation of the PVA film becomes poor. On the other hand, if the polyvinyl alcohol resin content is too high, it becomes difficult for the polyvinyl alcohol resin to dissolve uniformly throughout, and clusters are more likely to remain.

Specifically, in step S102, the polyvinyl alcohol casting solution is selectively filtered through a filter, and the polyvinyl alcohol casting solution is introduced into a T-shaped slit die in a quantitative manner, and then discharged and cast onto the casting drum 120 to prepare the preformed film M. Specifically, the rotation speed of the casting drum 120 is about 5-30 m/min, preferably 5-7 m/min. If the speed of the casting drum 120 is too slow, the productivity may decrease. On the other hand, if the speed of the casting drum 120 is too fast, the drying of the casting solution may be insufficient, resulting in poor peelability. In addition, in a preferred embodiment, the temperature of the casting drum 120 is set to 90-95°C, specifically, for example, 90, 91, 92, 93, 94, 95°C, or between any two of the above values. If the temperature of the casting drum 120 is too high, the casting solution is prone to foaming.

Specifically, in step S104, the preformed film M is peeled off from the casting drum 120 and then contacted with a plurality of heating rollers 130 to dry both the upper and lower surfaces of the film body. Here, the first of the plurality of heating rollers 130 is the hottest among all the heating rollers 130, and the temperature of the subsequent heating rollers 130 is adjusted so as to gradually decrease. The number of the plurality of heating rollers 130 is between 10 and 20, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20. Next, heat treatment is performed in a dryer 140 having multiple sections. Among them, the standard deviation (along the width direction) between the temperatures of the dryers 140 having multiple sections is 0.93 to 3.00, preferably between 0.93 to 2.5, for example, 0.93, 1.0, 1.3, 2.3, or 2.5. The standard deviation of the temperature of the dryer having multiple sections refers to the standard deviation of the temperature (along the width direction) in the dryer space of each section being 0.93 to 3.00, the standard deviation being obtained by measuring three points evenly in the width direction, and the average temperature of the dryer being between 45 and 130°C. In addition, the dryer having multiple sections may specifically have 2 to 10 sections, preferably 3 to 8 sections, for example, 3, 4, 5, 6, 7 or 8. Furthermore, the temperature of the upper and lower cavities in the dryer 140 can be adjusted by the amount of hot air, so that the temperature difference between the upper and lower surfaces of the polyvinyl alcohol film being heat-treated in the dryer 140 is 5°C or less, preferably 0.5 to 3.4°C, for example, 0.51, 1.52, 1.87 or 3.4°C. According to at least one embodiment, the dryer in step S104 is a floating type dryer. The temperature of the dryer can be controlled, for example, by adjusting the amount of hot air in the upper and lower cavities. In another embodiment, the heat treatment can be performed using a heated roller.

The inventor of the present invention has found that by adjusting the standard deviation (along the width direction) between the temperatures of the dryer 140 having multiple sections and the temperature difference between the upper and lower surfaces of the polyvinyl alcohol film being heat-treated in the dryer 140 within a specific range, the range of the additive residue after swelling and stretching of the polyvinyl alcohol film can be effectively controlled, thereby achieving the effect of making the dyeing uniform. Furthermore, the inventor of the present invention has found that by adjusting the stirring temperature and the frequency of changing the stirring direction when heating and dissolving the polyvinyl alcohol resin, plasticizer, and water in the dissolution tank 110, the dissolution can be made more uniform, and the molecular alignment direction of the film body formed later can be made more regular, and that the presence or absence of the addition of a surfactant affects the releasability of the preformed film M, leading to the prevention of changes in the molecular alignment direction during peeling. The specific factors that affect the molecular alignment direction described above substantially control the standard deviation range of the slow axis angle after swelling and stretching of the polyvinyl alcohol film, and based on these, the polarization degree of the polyvinyl alcohol film can be improved.

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

The saponification degree/alkalinization degree of the polyvinyl alcohol resin disclosed above is preferably 99.00% or more, which provides good optical properties, but specifically, for example, 99.00% to 100.00%, 99.00% to 99.99%, 99.00% to 99.95%, 99.00% to 99.90%, 99.00% to 99.85%, 99.00% to 99.80%, 99.00% to 99.75%, 99.00% to 99.70%, 99.00% to 99.65%, 99.00% to 99.60%, 99.00% to 99.55%, 99.00% to 99.50%, 99.0 0% to 99.45%, 99.00% to 99.40%, 99.00% to 99.35%, 99.00% to 99.30%, 99.00% to 99. 25%, 99.00% to 99.20%, 99.00% to 99.15%, 99.00% to 99.10%, 99.00% to 99.05%, 99. 20% to 100.00%, 99.20% to 99.99%, 99.20% to 99.95%, 99.20% to 99.90%, 99.20% to 9 9.85%, 99.20% to 99.80%, 99.20% to 99.75%, 99.20% to 99.70%, 99.20% to 99.65%, 9 9.20% to 99.60%, 99.20% to 99.55%, 99.20% to 99.50%, 99.20% to 99.45%, 99.20% to 99.40%, 99.20% to 99.35%, 99.20% to 99.30%, 99.20% to 99.25%, 99.40% to 100.00 %, 99.40% to 99.99%, 99.40% to 99.95%, 99.40% to 99.90%, 99.40% to 99.85%, 99.4 0% to 99.80%, 99.40% to 99.75%, 99.40% to 99.70%, 99.40% to 99.65%, 99.40% to 99.6 0%, 99.40% to 99.55%, 99.40% to 99.50%, 99.40% to 99.45%, 99.60% to 100.00%, 99 .60% to 99.99%, 99.60% to 99.95%, 99.60% to 99.90%, 99.60% to 99.85%, 99.60% to 99 . 80%, 99.60% to 99.75%, 99.60% to 99.70%, 99.60% to 99.65%, 99.80% to 100.00%, 99.80% to 99.99%, 99.80% to 99.95%, 99.80% to 99.90%, or 99.80% to 99.85%, etc.

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

1. Preparation of Polyvinyl Alcohol Films Non-limiting methods for preparing polyvinyl alcohol films are provided below. Seven non-limiting example polyvinyl alcohol films (Examples 1-7) and five comparative polyvinyl alcohol films (Comparative Examples 1-5) were prepared based on methods similar to those disclosed below. However, the specific methods for preparing Examples 1-7 and Comparative Examples 1-5 generally differ from the methods disclosed below in one or more aspects.

Specifically, the method for preparing the polyvinyl alcohol film includes the following steps. 1800 kg of polyvinyl alcohol resin with an alkalinity of >99.9% and a polymerization degree of about 2400, 4000 kg of water, 207 kg of glycerin plasticizer, and a surfactant were added, and the temperature was raised to 140°C while stirring, and dissolution was performed for 180 minutes while maintaining the temperature at 140°C. Here, the stirring direction during dissolution can be changed at a certain frequency, which can increase the dissolution effect and prevent clusters from remaining. For example, the stirring direction is changed at a frequency of three times per hour. Specifically, the surfactant used in Examples 1 to 4 and Comparative Example 1 is polyoxyethylene lauryl ether, and the surfactant used in Examples 5 to 6 and Comparative Example 3 is sodium polyoxyethylene lauryl ether sulfate, and the final content of each is 0.15 wt%. Next, water was added to the uniformly dissolved polyvinyl alcohol resin solution to adjust the resin concentration to 30.0%, and a polyvinyl alcohol casting solution was obtained. After the polyvinyl alcohol casting solution had defoamed, it was discharged from a T-shaped slit die and curtain-coated onto a rotating, high-temperature casting drum and dried to prepare a preformed film. After the preformed film was peeled off from the casting drum, it was brought into contact with multiple heating rollers to dry both the top and bottom surfaces of the film, and then heat-treated using a floating-type dryer, and the standard deviation of the dryer temperature (along the width direction) and the temperature difference between the top and bottom surfaces of the film body during heat treatment in the dryer were controlled within a specific range.

2. Analysis and Measurement Methods The following provides analysis and measurement methods for Examples 1 to 7 and Comparative Examples 1 to 5 to determine the properties of the polyvinyl alcohol films.

2-1. Analysis of initial additive content The sample was prepared by first dividing a polyvinyl alcohol film into 5 equal pieces along the transverse direction (TD), and then cutting the center of the divided polyvinyl alcohol film. The cut area of each piece was 5 cm MD x 10 cm TD (MD stands for machine direction, i.e., the longitudinal or machine direction). The sample was then left in a thermo-hygrostat at 23°C and 50% relative humidity (RH) for 24 hours.

The measurement conditions were as follows: first, the polyvinyl alcohol film was dried and dehydrated at 105°C for 10 minutes, and weighed after drying (W1). Next, the polyvinyl alcohol film was stirred in 2000 ml of pure water at 30°C with a stirrer (115-120 rpm) for 5 minutes to precipitate glycerin, and after completion, the water on the surface of the polyvinyl alcohol film was dehydrated, placed in a dryer and completely dried at 105°C for 1 hour, and the weight was measured (W2). The initial additive content was then (W1-W2)/W1 x 100%.

2-2. Analysis of residual additive amount after swelling and stretching The sample was prepared by first dividing the polyvinyl alcohol film into 5 equal parts along the transverse direction, and then cutting the center of the polyvinyl alcohol film after the division. The cut area of each piece was 20 cm in MD x 15 cm in TD. Then, the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Next, the polyvinyl alcohol film (5 cm in MD x 15 cm in TD) was fixed and stretched to twice its length at 4.3 cm/min in pure water at 30°C. After completion, the moisture on the surface of the polyvinyl alcohol film was sucked out, and the film was left in a thermohygrostat for 24 hours at 23°C and 50% RH. Finally, a polyvinyl alcohol film of 5 cm in MD x 10 cm in TD was cut out from the stretched part.

Measurement conditions:
First, the stretched polyvinyl alcohol film was dried and dehydrated at 105°C for 10 minutes, and weighed after drying (W3). Next, the polyvinyl alcohol film was stirred in 2000 ml of pure water at 30°C with a stirrer (rotation speed 115-120 rpm) for 5 minutes to precipitate glycerin and surfactant, and after completion, the water on the surface of the polyvinyl alcohol film was dehydrated, placed in a dryer and completely dried at 105°C for 1 hour, and weighed (W4). Then, the residual amount of plasticizer after swelling is (W3-W4)/W3×100%.

2-3. Analysis of the slow axis angle after swelling and stretching The sample was prepared by first dividing the polyvinyl alcohol film into 5 equal parts along the transverse direction, and then cutting the center of the polyvinyl alcohol film after the division. The cut area of each piece was MD 20 cm x TD 15 cm. Then, the film was left in a thermohygrostat for 24 hours under conditions of 23°C and 50% RH. Next, the polyvinyl alcohol film (MD 5 cm x TD 15 cm) was fixed, and the polyvinyl alcohol film was stretched to twice its length at 4.3 cm/min in pure water at 30°C, and after completion, the average slow axis angle and its standard deviation within the stretching range (MD 5 cm x TD 5 cm) were measured using a Photonic Lattice PA series phase difference analyzer. Here, the sample piece was placed so that the swelling and stretching direction (MD) was the same direction as 90 degrees to the 0 degree coordinate axis in the phase difference analyzer.

2-4. Measurement of Polarization Degree Expression The sample preparation method is, first, a process of preparing a polyvinyl alcohol film into a polarizing film. The method of preparing a polyvinyl alcohol film into a polarizing film is not particularly limited, but preferably includes a swelling process in which the polyvinyl alcohol film is put in water and stretched, a dyeing process in which the film is dyed with a dichroic dye, and a stretching process in which the film body is uniaxially stretched. Among these, methods such as boric acid crosslinking, fixing, washing, and heating may be further performed as necessary. In this case, the order of each treatment is not particularly limited, but preferably, the swelling process, dyeing process, and stretching process are performed in this order.

The measuring instrument used here is Perkin Elmer Lambda 365. The measurement conditions are based on the standard method of JIS Z 8722, and a luminosity correction is performed in the visible light region at 2° using a C light source. Then, two polarizing films are stacked with the same orientation direction, and the light transmittance (H ₀ ) at a wavelength is measured. Separately, two polarizing films are stacked with the orientation direction perpendicular to the film, and the light transmittance (H ₉₀ ) at a wavelength is measured. Finally, the polarization degree data was obtained by calculating the polarization degree=[(H ₀ -H ₉₀ )/(H ₀ +H ₉₀ )] ^1/2 .

2-5. Performance evaluation of dye uniformity A sample piece of 30 cm x 30 cm was cut out from the obtained polarizing film, and sandwiched at an angle of 45° between two sample pieces in an orthogonal polarization state (single transmittance 43.5%, polarization degree 99.9%). The color uniformity of the sample piece was observed in transmission mode using a lamp house with a light source illuminance of 14,000 lx.

3. Data Contents of Examples and Comparative Examples First, parameters were set for the following variables of the Examples and Comparative Examples. Standard deviation of the temperature of the dryer during the polyvinyl alcohol film preparation process (along the width direction), temperature difference between the upper and lower surfaces of the polyvinyl alcohol film during heat treatment in the dryer, dissolution method when heating and dissolving the polyvinyl alcohol resin, plasticizer, and water (stirring temperature and frequency of stirring direction change), and presence or absence of addition of surfactant (V: added/X: not added). Please refer to Table 1 for details of the settings. Note that when 1 is written in the dissolution method part in Table 1, it means "stirring type dissolution, stirring direction change frequency is 3 times (times/hour), and maximum dissolution temperature is 130°C or more", and when 2 is written, it means "stirring type dissolution, stirring direction change frequency is 1 time (times/hour), and maximum dissolution temperature is less than 130°C".

Next, under the conditions of the above-mentioned Examples 1 to 7 and Comparative Examples 1 to 5, characteristic parameters such as the amount of residual additive (average and standard deviation values) and the slow axis angle (average and standard deviation values) after swelling and stretching were further measured, and the effects related thereto were further measured. For details, see Table 2. In the dyeing uniformity state part in Table 2, "O" indicates a state in which there are no dyeing spots, and "X" indicates a state in which there are dyeing spots in 20% or more of the area.

With reference to both Tables 1 and 2, it can be seen that during the preparation process of Examples 1 to 7, with the above-mentioned variable settings, the standard deviation of the amount of additive remaining after swelling and stretching in Examples 1 to 7 was between 0.05 and 0.60, which allowed for uniform dyeing of the polyvinyl alcohol films in Examples 1 to 7. In contrast, the standard deviation of the amount of additive remaining after swelling and stretching in Comparative Examples 1 to 5 was not desirable in any case, which led to dyeing spots.

Furthermore, referring to both Tables 1 and 2, it can be seen that, in the above-mentioned variable settings, the standard deviation of the slow axis angle after swelling and stretching in Examples 1 to 7 was between 0.30 and 2.79, which provided all of the polyvinyl alcohol films prepared according to Examples 1 to 7 with a polarization degree of 99.8 or more. The standard deviation of the slow axis angle in Examples 1 to 6 was less than 2, which provided all of the polyvinyl alcohol films prepared according to Examples 1 to 6 with an even better polarization degree of 99.9 or more. In contrast, the standard deviation of the slow axis angle after swelling and stretching in Comparative Examples 1 to 5 was greater than 2.6, and all of the polyvinyl alcohol films prepared according to these Comparative Examples had a polarization degree of 99.8.

In view of the above results, the inventors realized that by adjusting the standard deviation (along the width direction) of the temperature of the dryer and the range of the temperature difference between the upper and lower surfaces of the polyvinyl alcohol film during heat treatment in the dryer, it is possible to control the standard deviation range of the amount of additives remaining after swelling and stretching of the polyvinyl alcohol film, and the effect of making the dyeing of the prepared polyvinyl alcohol film uniform can also be achieved. Furthermore, the inventors realized that the stirring temperature when heating and dissolving the polyvinyl alcohol resin, plasticizer, and water, the frequency of changing the stirring direction, and the presence or absence of the addition of a surfactant are all substantially related to the standard deviation range of the slow axis angle after swelling and stretching of the polyvinyl alcohol film, and affect the polarization degree of the polyvinyl alcohol film.

The polyvinyl alcohol film provided based on the above-mentioned characteristics of the present invention can be dyed uniformly and has a better polarization degree. In addition, the characteristics of the present invention can be used to more accurately adjust, manufacture, and select the above-mentioned polyvinyl alcohol film.

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

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

110 Dissolution tank 120 Casting drum 130 Heating roller 140 Dryer M Preformed film S100 to S104 Processes

Claims (14)

A polyvinyl alcohol film, which is cut evenly in the width direction to form a plurality of polyvinyl alcohol film sheets, and after swelling and stretching, the standard deviation of the amount of additive remaining is 0.05 to 0.60 , and the water content is 1.0 to 5.0 wt % .
The swelling and stretching step involves stretching the polyvinyl alcohol films in water at a speed of 4.3 cm/min to double their length;
The residual amount of additive was calculated by the formula: (W3-W4)/W3x100% (W3 is the weight of the polyvinyl alcohol film after it has been swollen, stretched, and dried, and W4 is the weight of the polyvinyl alcohol film after it has been swollen, stretched, and dried, and then placed in pure water and stirred for 5 minutes, and then dried again). The polyvinyl alcohol film according to claim 1, wherein the average amount of additive remaining in the multiple polyvinyl alcohol films after swelling and stretching is 1.40 to 4.15 wt %. The polyvinyl alcohol film according to claim 1, wherein the standard deviation of the slow axis angle of the multiple polyvinyl alcohol films after swelling and stretching is 0.30 to 2.79. The polyvinyl alcohol film according to claim 1, wherein the average slow axis angle of the multiple polyvinyl alcohol films after swelling and stretching is 89.00 to 91.00. The polyvinyl alcohol film of claim 1, wherein the polyvinyl alcohol film has an initial additive content of 6 to 15 wt %. The polyvinyl alcohol film according to claim 1, wherein the degree of polymerization of the polyvinyl alcohol film is 1800 to 3000. An optical film produced from the polyvinyl alcohol film according to any one of claims 1 to 6 . The optical film of claim 7 which is a polarizing film. The optical film according to claim 8 , wherein the film has a polarization degree of 99.8% or more. A method for producing the polyvinyl alcohol film according to any one of claims 1 to 6 , comprising the steps of:
(a) stirring and heating polyvinyl alcohol-based resin, plasticizer, surfactant and water to a dissolution temperature above 100° C. and maintaining the temperature for 2-4 hours, and reversing the stirring direction at least three times per hour to form a polyvinyl alcohol casting solution;
(b) casting the polyvinyl alcohol casting solution onto a casting drum and drying to form a preformed film;
(c) exposing the preformed film to a plurality of heated rollers whose temperature is gradually decreased from high to low, and then entering a dryer having a plurality of sections for heat treatment;
Including,
The standard deviation (along the width direction) of the temperature of the dryer is 0.93 to 3.00, and the temperature difference between the upper and lower surfaces of the polyvinyl alcohol film in the dryer is 5° C. or less. The method according to claim 10 , wherein the dissolution temperature in step (a) is 130 to 140°C. The method according to claim 10 , wherein the polyvinyl alcohol-based resin concentration in the polyvinyl alcohol casting solution in step (a) is 20.0-40.0%. The method of claim 10 , wherein the final content of the surfactant is 0.10 to 0.20 wt %. The method according to claim 10 , wherein the dryer in step (c) is a floating type dryer.