Optical polyvinyl alcohol-based films, polarizing films, and polarizing plates

Abstract

This invention provides a polyvinyl alcohol-based film suitable for optical applications, capable of producing polarizing films and polarizing plates with excellent heat resistance and moisture resistance. [Solution] An optical polyvinyl alcohol-based film containing a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in its main chain and side chains, wherein the content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol resin (A) is 1.7 mol% or less, and the content of 1,2-glycol bonds in the side chains is 0.01 to 6 mol%.

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol film. More specifically, the present invention relates to an optical polyvinyl alcohol film suitable for optical applications, which is capable of providing a polarizing film and a polarizing plate excellent in heat resistance and moist heat resistance. [Background Art]

[0002] Conventionally, a polyvinyl alcohol film containing a specific amount of 1,2-glycol bonds in the main chain has been used as a raw film for producing polarizing films, since it can improve various physical properties such as water resistance, heat resistance, and moist heat resistance when used in a polarizing film.

[0003] For example, as disclosed in Patent Document 1, a polarizing film is known which uses a uniaxially stretched film made of a polyvinyl alcohol-based polymer having a 1,2-glycol bond content of 1.5 mol% or less as a base material, aiming to provide a highly durable polarizing film that is excellent in optical properties such as degree of polarization and transmittance, and has improved water resistance, moist heat resistance and heat resistance.

[0004] Also, as disclosed in Patent Document 2, a vinyl alcohol-based polymer film is known which combines various excellent properties such as high polymerization degree, excellent mechanical properties, water resistance and weather resistance, no coloration, and low elution rate when immersed in water, aiming to provide a vinyl alcohol-based polymer film that can be effectively used in production of optical films such as polarizing films and other applications. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Unexamined Patent Publication No. Hei 3-175404 [Patent Document 2] Japanese Unexamined Patent Publication No. 2009-221462 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] However, the polyvinyl alcohol-based films described in Patent Documents 1 and 2 fail to adequately provide the heat resistance and moisture resistance required for polarizing films, thus necessitating improvement.

[0007] Therefore, against this background, the present invention provides a polyvinyl alcohol-based film suitable for optical applications, capable of producing polarizing films and polarizing plates with excellent heat resistance and moisture resistance, a polarizing film using the polyvinyl alcohol-based film, and a polarizing plate. [Means for solving the problem]

[0008] Against this backdrop, the inventors discovered that by incorporating 1,2-glycol bonds not only in the main chain but also in the side chains, and by setting the content of both within a specific range, it is possible to withstand the current high requirements for heat resistance and moisture resistance, thus completing the present invention.

[0009] In other words, the present invention has the following aspects. [1] An optical polyvinyl alcohol-based film containing a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains, An optical polyvinyl alcohol-based film wherein the content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol-based resin (A) is 1.7 mol% or less, and the content of 1,2-glycol bonds in the side chains is 0.01 to 6 mol%. [2] The optical polyvinyl alcohol-based film according to [1], wherein the average degree of saponification of the polyvinyl alcohol-based resin (A) is 95 mol% or more. [3] The optical polyvinyl alcohol film according to [1] or [2], wherein the viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol resin (A) is 70 mPa·s or more. [4] An optical polyvinyl alcohol-based film according to any of [1] to [3], wherein the film thickness is 5 to 50 μm. [5] An optical polyvinyl alcohol-based film as described in any of [1] to [4], used for manufacturing polarizing films. [6] A polarizing film obtained using an optical polyvinyl alcohol-based film described in any of [1] to [5]. A polarizing plate comprising the polarizing film described in [7] [6] and a protective film provided on at least one side of the polarizing film. [Effects of the Invention]

[0010] The polyvinyl alcohol-based film of the present invention contains 1,2 glycol bonds within a specific range not only in the main chain but also in the side chains, making it possible to provide an optical polyvinyl alcohol-based film that can produce polarizing films and polarizing plates with superior heat resistance and moisture resistance. [Modes for carrying out the invention]

[0011] The present invention will be described below based on examples of embodiments for carrying out the present invention. However, the present invention is not limited to the embodiments described below.

[0012] In this specification, "x and / or y (where x, y are any configuration)" means at least one of x and y, and can mean x only, y only, or x and y. In this specification, when "X~Y" (where X and Y are any numbers) is used, unless otherwise specified, it means "X or greater and Y or less," and also includes the meanings of "preferably greater than X" or "preferably less than Y." In this specification, when we use the terms "X or more" (where X is any number) or "Y or less" (where Y is any number), we also mean "preferably greater than X" or "preferably less than Y." In this specification, the numerical ranges described in stages may be arbitrarily combined with the upper or lower limits of the numerical ranges in any stage. Furthermore, in the numerical ranges described herein, the upper or lower limits may be replaced with the values ​​shown in the examples. As used herein, the term "main component" refers to a component that has a significant impact on the properties of the target object. The content of this component is usually 50% by mass or more in the target object, preferably 55% by mass or more, more preferably 60% by mass or more, further preferably 70% by mass or more, and may be 100% by mass. As used herein, the term "film" includes "tape" and "sheet".

[0013] An optical polyvinyl alcohol-based film according to one embodiment of the present invention (hereinafter referred to as "the present polyvinyl alcohol-based film") contains a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains, wherein the content of 1,2-glycol bonds in the main chain is 1.7 mol% or less, and the content of 1,2-glycol bonds in the side chains is 0.01 to 6 mol%.

[0014] The polyvinyl alcohol-based resin (A) has a 1,2-glycol bond represented by the following chemical formula (1) in the main chain and a 1,2-glycol bond represented by the following chemical formula (2) in the side chains. By containing the polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains within a specific range, a polarizing film and a polarizing plate obtained using the present polyvinyl alcohol-based film can have both excellent heat resistance and moist heat resistance.

[0015]

Chemical Formula

[0016]

Chemical Formula

[0017] One method for obtaining a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains is to copolymerize a vinyl ester monomer with an unsaturated monomer copolymerizable thereto and having 1,2-glycol bonds, and / or a monomer obtained by protecting an unsaturated monomer having 1,2-glycol bonds with an ester or the like (hereinafter, these may be referred to as "1,2-glycol bond-containing monomers"), and then saponify the resulting vinyl ester copolymer. In other words, when a vinyl ester monomer is copolymerized with an unsaturated monomer having a 1,2-glycol bond, a 1,2-glycol bond can be directly introduced into the side chain of the polyvinyl alcohol resin (A). Furthermore, when a vinyl ester monomer is copolymerized with a monomer in which an unsaturated monomer having a 1,2-glycol bond is protected with an ester or the like, the resulting vinyl ester copolymer is saponified, which removes the ester or the like that protecting the unsaturated monomer having a 1,2-glycol bond, resulting in a polyvinyl alcohol resin (A) having a 1,2-glycol bond in its side chain.

[0018] Examples of the vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl versatate. These may be used individually or in combination of two or more. Vinyl acetate is particularly preferred.

[0019] Examples of unsaturated monomers having a 1,2-glycol bond include monomers such as 3,4-diol-1-butene, 4,5-diol-1-pentene, 4,5-diol-3-methyl-1-pentene, 5,6-diol-1-hexene, and glycerol monoallyl ether. These may be used individually or in combination of two or more.

[0020] Examples of monomers obtained by protecting the unsaturated monomer having a 1,2-glycol bond with an ester include 3,4-diacetoxy-1-butene, 4,5-diacetoxy-1-pentene, 4,5-diacetoxy-3-methyl-1-pentene, 5,6-diacetoxy-1-hexene, and 3-allyloxy-1,2-propanediol diacetate. These may be used individually or in combination of two or more.

[0021] Among these 1,2-glycol bond-containing monomers, monomers obtained by protecting unsaturated monomers having a 1,2-glycol bond with an ester or the like are preferred from the viewpoint of productivity, more preferably are diacetoxyalkyl group-containing monomers, and even more preferably are 3,4-diacetoxy-1-butene.

[0022] As a copolymerization method for the vinyl ester monomer and the 1,2-glycol bond-containing monomer, any known polymerization method such as solution polymerization, emulsion polymerization, or suspension polymerization can be used, and solution polymerization is usually used.

[0023] There are no particular restrictions on the method of charging the monomer components during copolymerization, and any method such as batch charging, divided charging, or continuous charging can be employed. However, dropwise polymerization is preferred in terms of physical properties, such as the uniform distribution of the 1,2-glycol bond-containing monomer within the molecular chain of the polyvinyl ester copolymer and the lowering of the melting point of the polyvinyl alcohol resin (A). A polymerization method based on the HANNA method is particularly preferred.

[0024] The solvents used in such copolymerization are typically lower alcohols such as methanol, ethanol, propanol, and butanol, or ketones such as acetone and methyl ethyl ketone. Industrially, methanol is preferred.

[0025] The amount of solvent used can be appropriately selected in accordance with the degree of polymerization of the target copolymer, taking into account the chain transfer constant of the solvent. For example, when the solvent is methanol, the ratio of S (solvent) to M (monomer) can be selected from a range of approximately 0.01 to 10 (mass ratio), preferably 0.05 to 3 (mass ratio).

[0026] Furthermore, polymerization catalysts are typically used in the copolymerization process. Examples of the polymerization catalysts include known radical polymerization catalysts such as 2,2'-azobisisobutyronitrile, acetyl peroxide, benzoyl peroxide, and lauryl peroxide, as well as low-temperature active radical polymerization catalysts such as 2,2'-azobis(2,4-dimethylvaleronitrile) and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0027] The amount of polymerization catalyst used varies depending on the type of catalyst and cannot be determined in general terms, but can be arbitrarily selected according to the polymerization rate. For example, when using a radical polymerization catalyst, 0.001 to 0.2 mol% of the monomer component is preferred, and more preferably 0.005 to 0.15 mol%. Furthermore, when using a low-temperature active radical polymerization catalyst, the amount is preferably 0.0005 to 0.1 mol%, and more preferably 0.001 to 0.05 mol%, relative to the monomer component.

[0028] The reaction temperature when copolymerizing the monomer components is appropriately selected depending on the solvent and pressure used, but it is preferably 30 to 55°C in order to reduce the content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol resin (A).

[0029] After copolymerizing the monomer components, it is also preferable to add polymerization inhibitors such as m-dinitrobenzene, hydroquinone, or hydroquinone monomethyl ether to ensure that the reaction is stopped.

[0030] By saponifying the vinyl ester copolymer obtained in this way, a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains can be obtained. Furthermore, it is preferable to heat the solution before saponification to reduce the content of unreacted monomer components remaining in the solution, from the viewpoint of suppressing discoloration of the resulting polyvinyl alcohol-based resin (A).

[0031] The saponification method described above can employ known methods, for example, it can be carried out using a saponification catalyst in a solution obtained by dissolving the vinyl ester copolymer obtained above in a solvent such as alcohol or aqueous alcohol.

[0032] Examples of the aforementioned alcohols include lower alcohols having 1 to 5 carbon atoms, such as methanol, ethanol, and propanol. These may be used individually or in combination of two or more. Methanol is particularly preferred.

[0033] The concentration of the vinyl ester copolymer in the solution is appropriately selected depending on the viscosity, and is usually 1 to 60% by mass.

[0034] Examples of the saponification catalyst include alkali metal hydroxides and alkoxides such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and potassium ethylate; and acid catalysts such as sulfuric acid, hydrochloric acid, nitric acid, metasulfonic acid, zeolite, and cation exchange resin.

[0035] The temperature for saponification is not limited, but a range of 20 to 70°C is preferable. As saponification progresses, particulate matter is formed, indicating that the reaction has proceeded. If a gel-like product precipitates at this point, the product can be crushed. The resulting granules can be washed and dried to obtain polyvinyl alcohol-based resin (A).

[0036] Furthermore, in order to increase the degree of saponification of the polyvinyl alcohol-based resin (A), it is also preferable to perform secondary saponification by removing by-products (for example, vinyl acetate when vinyl ester monomers are used, and methyl acetate when methanol is used as the solvent during saponification) from the granular material that has been formed by washing, etc., then disperse it again in alcohol or the like, add an alkaline catalyst, and react it further.

[0037] The saponification process converts vinyl ester units in the vinyl ester copolymer into vinyl alcohol units, yielding a polyvinyl alcohol resin. During this process, heterogeneous bonds (head-to-head bonds) of vinyl ester units formed in the main chain of the vinyl ester copolymer become 1,2-glycol bonds in the main chain, resulting in a polyvinyl alcohol resin having 1,2-glycol bonds in the main chain. Furthermore, when a monomer is used as the monomer component, which is an unsaturated monomer having a 1,2-glycol bond protected with an ester or the like, the ester or the like of the protected monomer is also deprotected by saponification, converting it back to a 1,2-glycol bond, and a polyvinyl alcohol-based resin having a 1,2-glycol bond in its side chain is obtained.

[0038] The content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol resin (A) is 1.7 mol% or less, preferably 1.6 mol% or less, and more preferably 1.5 mol% or less. There is no particular lower limit to the content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol resin (A), but it is preferably 1.0 mol% or more, more preferably 1.1 mol% or more, and even more preferably 1.2 mol% or more, and the range is, for example, 1.0 to 1.7 mol%. By setting the content of 1,2-glycol bonds in the main chain to 1.7 mol% or less, the heat resistance and heat and humidity resistance can be improved. Here, the 1,2-glycol bond content refers to the percentage (mol%) of moles of bonds between adjacent vinyl alcohol units that are linked by 1,2-glycol bonds relative to the total number of moles of bonds between adjacent vinyl alcohol units, and can be determined by NMR measurement.

[0039] On the other hand, the content of 1,2-glycol bonds in the side chains of the polyvinyl alcohol-based resin (A) is 0.01 to 6 mol%, preferably 0.02 to 4 mol%, more preferably 0.03 to 2 mol%, even more preferably 0.04 to 1 mol%, and particularly preferably 0.05 to 0.9 mol%. If the content of 1,2-glycol bonds in the side chains is within the above range, the heat resistance and heat and humidity resistance of the polarizing film can be improved.

[0040] Furthermore, the content of 1,2-glycol bonds in the polyvinyl alcohol-based resin (A) is usually greater than 1.5 mol%, preferably 1.6 mol% or more, and more preferably 1.7 mol% or more. The upper limit is usually 5.5 mol% or less, preferably 4.2 mol% or less, more preferably 3.0 mol% or less, and even more preferably 2.6 mol%, with a range such as greater than 1.5 mol% and less than or equal to 5.5 mol%. When the content of 1,2-glycol bonds is within the above range, the heat resistance and heat and humidity resistance of the polarizing film tend to be improved.

[0041] The content of 1,2-glycol bonds in the main chain and side chains of the polyvinyl alcohol-based resin (A) can be adjusted by the copolymerization conditions. For example, the content of 1,2-glycol bonds in the main chain can be increased or decreased by adjusting the reaction temperature of the copolymerization reaction. The content of 1,2-glycol bonds in the side chains can be increased or decreased by adjusting the charging ratio of vinyl ester monomers and 1,2-glycol bond-containing monomers in the copolymerization reaction.

[0042] The weight-average molecular weight of the polyvinyl alcohol-based resin (A) is typically 85,000 or more, preferably 90,000 or more, and more preferably 100,000 or more. The upper limit of the weight-average molecular weight is typically 300,000 or less, preferably 280,000 or less, and more preferably 260,000 or less, with a range of, for example, 85,000 to 300,000. If the weight-average molecular weight is too low, it tends to be difficult to obtain sufficient optical performance when using the polyvinyl alcohol-based resin as an optical film, and if it is too high, it tends to be difficult to stretch when manufacturing a polarizing film using the polyvinyl alcohol-based film. The weight-average molecular weight of the polyvinyl alcohol-based resin (A) is the weight-average molecular weight measured by the GPC-MALS method.

[0043] The average degree of saponification of the polyvinyl alcohol-based resin (A) is preferably 95 mol% or higher, more preferably 96 mol% or higher, even more preferably 98 mol% or higher, and particularly preferably 99 mol% or higher. If the average degree of saponification is too low, sufficient optical performance tends not to be obtained when the polyvinyl alcohol-based film is used as a polarizing film. Here, the average degree of saponification is measured in accordance with JIS K 6726.

[0044] In this embodiment, two or more polyvinyl alcohol-based resins (A) with different characteristics such as modified species, modification amount, weight-average molecular weight, and average degree of saponification may be used in combination. Alternatively, the polyvinyl alcohol-based resin (A) may be a mixture of polyvinyl alcohol-based resins including unmodified polyvinyl alcohol-based resins. When the polyvinyl alcohol-based resin (A) contains two or more polyvinyl alcohol-based resins (A) with different characteristics such as modified species, modification amount, weight-average molecular weight, and average degree of saponification, or an unmodified polyvinyl alcohol-based resin, it is sufficient that the content of 1,2-glycol bonds in the entire polyvinyl alcohol-based resin is within the aforementioned range.

[0045] The polyvinyl alcohol-based resin (A) is preferably the main component of the polyvinyl alcohol-based film.

[0046] The manufacturing method of this polyvinyl alcohol-based film will be described in more detail below, in order of the steps, but this polyvinyl alcohol-based film is not limited to these embodiments.

[0047] This polyvinyl alcohol-based film is preferably manufactured through the following steps (I) to (III), and preferably through step (IV) if necessary. Step (I): Step of preparing an aqueous solution of polyvinyl alcohol-based resin. Step (II): A step in which an aqueous solution of polyvinyl alcohol-based resin is cast into a mold to form a film. Step (III): A step to dry the formed film. Step (IV): A step in which the obtained film is heat-treated.

[0048] <Process (I)> Step (I) is the step of preparing an aqueous solution of polyvinyl alcohol-based resin. The polyvinyl alcohol resin used as the material for this polyvinyl alcohol film is the one described in the previously mentioned polyvinyl alcohol resin (A).

[0049] In addition to the polyvinyl alcohol resin (A), the aqueous solution of the polyvinyl alcohol resin may preferably contain, as needed, commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, as well as at least one nonionic, anionic, and cationic surfactant, for better film-forming properties. These may be used individually or in combination of two or more.

[0050] If the aqueous solution of the polyvinyl alcohol-based resin contains a plasticizer, its content is usually 1 part by mass or more, preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of the polyvinyl alcohol-based resin (A). The upper limit of the content is usually 35 parts by mass or less, preferably 30 parts by mass or less, and more preferably 25 parts by mass or less, with a range of, for example, 1 to 35 parts by mass. If the plasticizer content is too low, the stretchability during the production of the polarizing film tends to decrease, and if it is too high, the strength of the resulting polyvinyl alcohol-based film tends to decrease.

[0051] If the aqueous solution of the polyvinyl alcohol-based resin contains a surfactant, its content is preferably 0.15 parts by mass or less, more preferably 0.1 parts by mass or less, and even more preferably 0.07 parts by mass or less, per 100 parts by mass of the polyvinyl alcohol-based resin (A). The lower limit of the content is usually 0.01 parts by mass or more, and its range is, for example, 0.01 to 0.15 parts by mass.

[0052] The resin concentration of the polyvinyl alcohol-based resin aqueous solution obtained in this manner is preferably 15% by mass or more, more preferably 17% by mass or more, and even more preferably 20% by mass or more. Furthermore, the upper limit of the resin concentration is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less, with a range of, for example, 15 to 60% by mass. If the resin concentration of such an aqueous solution is too low, the drying load increases, which tends to reduce production capacity, and if it is too high, the viscosity becomes too high, which tends to make uniform dissolution difficult.

[0053] The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 70 mPa·s or higher, more preferably 80 mPa·s or higher, and even more preferably 90 mPa·s or higher, when it is a 4% by mass aqueous solution. The upper limit of the viscosity is preferably 200 mPa·s or lower, more preferably 170 mPa·s or lower, and the range is, for example, 70 to 200 mPa·s. If the viscosity of a 4% by mass aqueous solution is too low, the polarization performance of the polarizing film tends to decrease.

[0054] Next, the obtained polyvinyl alcohol-based resin aqueous solution is subjected to degassing treatment. Degassing methods include static degassing and degassing using a multi-screw extruder. Any multi-screw extruder with a vent will suffice, and a twin-screw extruder with a vent is usually used.

[0055] <Process (II)> Step (II) is a process of forming a film by casting an aqueous solution of polyvinyl alcohol-based resin into a cast mold. After the degassing treatment, a fixed amount of the polyvinyl alcohol-based resin aqueous solution is introduced into a T-type slit die, discharged and cast onto a rotating casting drum, and a film is formed by a continuous casting method.

[0056] The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-type slit die is usually 70°C or higher, preferably 80°C or higher. The upper limit of the resin temperature is usually 100°C or lower, preferably 98°C or lower, and the range is, for example, 70 to 100°C. If the resin temperature of such a polyvinyl alcohol-based resin aqueous solution is too low, it tends to flow poorly, and if it is too high, it tends to foam.

[0057] The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 Pa·s or higher, and more preferably 70 Pa·s or higher, at the time of discharge. The upper limit of the viscosity is preferably 200 Pa·s or lower, and more preferably 150 Pa·s or lower, with a range of, for example, 50 to 200 Pa·s. If the viscosity of such an aqueous solution is too high, it tends to result in poor flow, and if it is too low, it tends to make film formation by casting difficult.

[0058] The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-type slit die to the casting drum is usually 0.2 m / min or more, preferably 0.4 m / min or more, and more preferably 0.6 m / min or more. The upper limit of the discharge rate is usually 5 m / min or less, preferably 4 m / min or less, and more preferably 3 m / min or less, with a range such as 0.2 to 5 m / min. If the discharge speed is too slow, productivity tends to decrease, and if it is too fast, casting tends to become difficult.

[0059] The diameter of the cast drum is preferably 2m or more, more preferably 2.4m or more, and even more preferably 2.8m or more. The upper limit of the diameter is usually 5m or less, preferably 4.5m or less, and more preferably 4m or less, with a range of, for example, 2 to 5m. If the diameter of the cast drum is too small, the drying section on the cast drum becomes shorter, which tends to make it difficult to increase the speed. If it is too large, transportability tends to decrease.

[0060] The width of the cast drum is preferably 3m or more, more preferably 3.5m or more, even more preferably 4m or more, particularly preferably 4.5m or more, and especially preferably 5 to 8m. If the width of the cast drum is too small, productivity tends to decrease.

[0061] The surface temperature of the cast drum is preferably 99°C or lower in order to suppress foaming during drying and to obtain a film with excellent appearance, more preferably 98°C or lower, even more preferably 97°C or lower, and particularly preferably 96°C or lower. Furthermore, a lower limit of 40°C or higher is preferable in terms of excellent peelability when peeling the film obtained by film formation from the cast drum, more preferably 50°C or higher, even more preferably 60°C or higher, and particularly preferably 70°C or higher. In other words, the surface temperature of the cast drum is preferably 40 to 99°C, more preferably 50 to 98°C, even more preferably 60 to 97°C, and particularly preferably 70 to 96°C.

[0062] Furthermore, the moisture content of the film when peeled from the cast drum is preferably 20% by mass or less, more preferably 18% by mass or less, even more preferably 17% by mass or less, and particularly preferably 16% by mass or less. If the moisture content is too high, it tends to lead to poor peeling during film formation and increased risk of breakage. Furthermore, the moisture content of the film at the time of peeling is preferably 5% by mass or more, more preferably 7% by mass or more, and even more preferably 9% by mass or more. If the moisture content is too low, the film tends to curl easily. In other words, the moisture content is preferably 5 to 20% by mass, more preferably 7 to 18% by mass, and even more preferably 9 to 17% by mass.

[0063] <Process (III)> Step (III) is a step in which the film prepared above is heated and dried. In step (II) above, the film peeled from the cast mold (the manufactured film) is conveyed in the flow direction (MD direction) using a nip roll or the like, and is dried by alternately bringing the front and back surfaces of the film into contact with multiple heat rolls.

[0064] The aforementioned heat rolls are, for example, rolls with a diameter of 0.2 to 2 m whose surfaces are hard chrome plated or mirror-finished, and it is preferable to use 2 to 30 rolls, preferably 10 to 25 rolls, for drying.

[0065] The surface temperature of the heat roll is not particularly limited, but is usually 30°C or higher, preferably 40°C or higher. The upper limit of the surface temperature is usually 150°C or lower, preferably 140°C or lower, and the range is, for example, 30 to 150°C. If the surface temperature of the heat roll is too low, drying tends to be insufficient, and if it is too high, it tends to dry too much, leading to appearance defects such as waviness.

[0066] <Process (IV)> Step (IV) is a process in which the obtained film is heat-treated using hot air. The film that has undergone the above step (III) can be heat-treated, for example, in a hot air drying oven. The upper limit of the heat treatment temperature is preferably 170°C or lower, more preferably 160°C or lower, and even more preferably 155°C or lower. The lower limit of the temperature is preferably 60°C or higher, more preferably 80°C or higher, even more preferably 90°C or higher, and particularly preferably 95°C or higher. In other words, the heat treatment temperature is preferably 60 to 170°C, more preferably 80 to 160°C, even more preferably 90 to 155°C, and particularly preferably 95 to 155°C. If the heat treatment temperature is too high, the tension during polarizing film production tends to increase, and if it is too low, the amount of low molecular weight components leached out during polarizing film production tends to increase, and wrinkles and folds tend to occur in the film during the swelling process. Furthermore, the heat treatment time is preferably 20 to 100 seconds, and particularly preferably 40 to 70 seconds.

[0067] Up to this point, we have described a method for producing a polyvinyl alcohol-based film by preparing an aqueous solution of polyvinyl alcohol-based resin, casting this solution onto a rotating casting drum (drum-type roll), and then forming and drying the film by the casting method. However, it is also possible to cast the aqueous solution of polyvinyl alcohol-based resin onto a resin film or a metal belt, and then form and dry the film.

[0068] [Polyvinyl alcohol-based film] Thus, the polyvinyl alcohol-based film is obtained through the above steps (I) to (III), and optionally step (IV), and is finally wound onto a roll to become the final product.

[0069] The film thickness of this polyvinyl alcohol-based film is preferably 5 to 50 μm, more preferably 10 to 48 μm, even more preferably 15 to 45 μm, and particularly preferably 20 to 40 μm, from the viewpoint of thinning the polarizing film.

[0070] The length of this polyvinyl alcohol-based film is preferably 4 km or more from the viewpoint of increasing the area of ​​the polarizing film, and particularly preferably 5 to 50 km from the viewpoint of transport mass.

[0071] The width of this polyvinyl alcohol-based film is preferably 1 m or more, more preferably 2 m or more, even more preferably 3 m or more, and particularly preferably 3.5 m or more. The upper limit of the film width is usually 7 m or less.

[0072] This polyvinyl alcohol-based film is excellent in heat resistance and moisture resistance, making it useful in optical applications where heat and moisture resistance are required. In particular, it is very useful as a base film for polarizing films, and the following describes a method for manufacturing polarizing films and polarizing plates made from this polyvinyl alcohol-based film.

[0073] [Method for manufacturing polarizing films] A polarizing film according to one embodiment of the present invention is manufactured by unwinding the polyvinyl alcohol-based film from a roll and transporting it horizontally, followed by processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

[0074] The swelling process is performed before the dyeing process, for example, by immersing the film in a swelling solution (swelling bath). In addition to cleaning dirt from the surface of the polyvinyl alcohol-based film, the swelling process also helps prevent uneven dyeing by swelling the film.

[0075] Water is typically used as the swelling solution in the swelling process. While water is the main component of the swelling solution, it may also contain additives such as iodide compounds, surfactants, and alcohol.

[0076] The temperature of the swelling bath is usually around 10-45°C, and the immersion time in the swelling bath is usually around 0.1-10 minutes.

[0077] The dyeing process is carried out by bringing the film into contact with a liquid (dyeing solution) containing iodine or a dichroic dye, for example, by immersing it in the dyeing solution.

[0078] Typically, an aqueous solution of iodine and potassium iodide is used as the staining solution, with an appropriate iodine concentration of 0.1-2 g / L and a potassium iodide concentration of 1-100 g / L. The staining solution may also contain a small amount of a water-miscible organic solvent in addition to water. A practical staining time is approximately 30-500 seconds. A staining temperature of 5-50°C is preferred.

[0079] The boric acid crosslinking process is carried out using boric acid or boron compounds such as borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 10 to 100 g / L, and it is preferable to include potassium iodide in the solution for stabilizing the polarization performance. The processing temperature is preferably about 30 to 70°C, and the processing time is preferably about 0.1 to 20 minutes, and stretching operations may be performed during the process as needed.

[0080] The stretching process preferably involves stretching the film 3 to 10 times in a uniaxial direction, and particularly preferably 3.5 to 7 times. At this time, some stretching (to the extent necessary to prevent shrinkage in the width direction, or more) in the direction perpendicular to the stretching direction is also acceptable. The stretching temperature is preferably 40 to 170°C. Furthermore, the stretching ratio only needs to be ultimately set within the above range, and the stretching operation may be performed not only once but multiple times during the manufacturing process.

[0081] The washing process is carried out, for example, by immersing the film in an iodide aqueous solution such as water or potassium iodide, which can remove precipitates that form on the surface of the film. When using a potassium iodide aqueous solution, the potassium iodide concentration is preferably around 10 to 1000 g / L. The temperature during the washing process is usually 5 to 50°C, preferably 10 to 45°C. The processing time is usually 1 to 300 seconds, preferably 10 to 240 seconds. Note that washing with water and washing with a potassium iodide aqueous solution may be carried out in combination as appropriate.

[0082] The drying process is carried out, for example, by drying in a dryer at 40-100°C for 0.1-10 minutes.

[0083] Thus, a polarizing film is obtained, and the degree of polarization of such a polarizing film is preferably 99.90% or higher, more preferably 99.99% or higher. If the degree of polarization is too low, the contrast in the liquid crystal display tends to decrease. The degree of polarization is generally measured by the light transmittance (H) at wavelength λ when two polarizing films are superimposed so that their orientation directions are the same. 11 The light transmittance (H1) measured at wavelength λ with the two polarizing films superimposed so that their orientation directions are perpendicular to each other is used to calculate the following equation (1). Polarization degree = [(H 11 -H1) / (H 11 +H1)〕 1 / 2 ...(1)

[0084] Furthermore, the transmittance of the polarizing film in this embodiment is preferably 41.5% or higher. If the transmittance is too low, it tends not to be possible to achieve high brightness in the liquid crystal display. The transmittance of the polarizing film is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0085] Next, a method for manufacturing a polarizing plate using the polarizing film of this embodiment will be described. The polarizing film of this embodiment has excellent heat resistance and moisture resistance, as well as minimal color unevenness and superior polarization performance, making it suitable for manufacturing polarizing plates.

[0086] [Method for manufacturing polarizing plates] A polarizing plate is manufactured by laminating an optically isotropic resin film as a protective film to one or both sides of the polarizing film of this embodiment via an adhesive. Examples of protective films include films made of acetylcellulose-based resins such as triacetylcellulose and diacetylcellulose, films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate, polycarbonate-based resin films, cycloolefin-based resin films, acrylic-based resin films, and films made of chain-like olefin-based resins such as polypropylene-based resins.

[0087] The bonding method is carried out by known techniques, for example, by uniformly applying a liquid adhesive composition to the polarizing film, the protective film, or both, then bonding the two together, pressing them together, and irradiating them with heat or active energy rays.

[0088] Furthermore, a curable resin such as urethane resin, acrylic resin, or urea resin can be applied to one or both sides of the polarizing film, and cured to form a cured layer, thereby creating a polarizing plate. In this way, the cured layer replaces the protective film, allowing for a thinner film. These curable resins can be used individually or in combination of two or more types.

[0089] The polarizing films and polarizing plates obtained using the polyvinyl alcohol-based film for manufacturing polarizing films of this embodiment have excellent polarizing performance and are preferably used in portable information terminals, personal computers, televisions, projectors, signage, electronic desktop calculators, electronic clocks, word processors, electronic paper, game consoles, video players, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices such as instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, foldable displays, anti-reflective layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like. [Examples]

[0090] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist of the invention. Note that in the example, "part" refers to the mass basis.

[0091] <Manufacturing of polyvinyl alcohol-based resin (A)> [Manufacturing Example 1] In a reactor equipped with a reflux condenser, sample inlet, nitrogen inlet, and stirring blades, 800 parts vinyl acetate, 3.2 parts 3,4-diacetoxy-1-butene, and 193 parts methanol were added, and nitrogen bubbling was performed for 30 minutes to purge the reactor with nitrogen and expel dissolved oxygen from the liquid. Then, the reactor was immersed in a water bath and heated to 40°C, and polymerization was started by adding 0.08 parts 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) as an initiator. After 3.6 hours of polymerization, polymerization was terminated by adding 0.016 parts m-dinitrobenzene and 2000 parts methanol as polymerization termination agents. Next, methanol was added to displace the remaining vinyl acetate, yielding a methanol solution of polyvinyl acetate copolymerized with an unsaturated monomer capable of introducing 1,2-glycol bonds into the side chains. The methanol solution of polyvinyl acetate obtained in this manner was diluted with methanol to a concentration of 3.5% by mass, and 6000 parts of this polyvinyl acetate methanol solution were charged into the reactor as described above. The reactor was immersed in a water bath and heated to 65°C. 70.1 parts of a 3.5% by mass sodium hydroxide methanol solution were added to start the saponification reaction. 1.5 hours after adding the sodium hydroxide methanol solution, the resulting slurry was filtered to recover the saponified product. The recovered saponified product was diluted with methanol to a total volume of 2800 parts to form a reslurry, which was then charged back into the reactor and heated to 65°C. 42.1 parts of a 3.5% by mass sodium hydroxide methanol solution were added, and the reaction was carried out for 2.5 hours to complete the saponification reaction. Subsequently, the solution was neutralized with acetic acid until its pH was neutral to weakly acidic, and the saponified product was obtained by suction filtration while washing with methanol. The obtained saponified material was washed with methanol and then dried in a vacuum dryer heated to 50°C for 20 hours to obtain a polyvinyl alcohol-based resin (1) [4% by mass aqueous solution viscosity 140 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.47 mol%; content of 1,2-glycol bonds in the side chain 0.2 mol%].

[0092] [Manufacturing Example 2] By performing the polymerization reaction in the same manner as in Production Example 1, except that 16.2 parts of 3,4-diacetoxy-1-butene and 139 parts of methanol were used, a polyvinyl alcohol-based resin (2) [4% by mass aqueous solution viscosity 109 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.43 mol%; content of 1,2-glycol bonds in the side chain 1.1 mol%] was obtained.

[0093] [Manufacturing Example 3] By performing the same procedure as in Production Example 1, except that 16 parts of 3,4-diacetoxy-1-butene from Production Example 1 were used in the polymerization reaction, a polyvinyl alcohol-based resin (3) [4% by mass aqueous solution viscosity 135 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.49 mol%; content of 1,2-glycol bonds in the side chain 1.0 mol%] was obtained.

[0094] [Manufacturing Example 4] By performing the polymerization reaction in the same manner as in Production Example 1, except that 6.4 parts of 3,4-diacetoxy-1-butene from Production Example 1 and 180 parts of methanol were used, a polyvinyl alcohol-based resin (4) [4% by mass aqueous solution viscosity 156 mPa·s; degree of saponification 99.7 mol%; content of 1,2-glycol bonds in the main chain 1.46 mol%; content of 1,2-glycol bonds in the side chain 0.4 mol%] was obtained.

[0095] [Manufacturing Example 5] By performing the polymerization reaction in the same manner as in Production Example 1, except that 9.7 parts of 3,4-diacetoxy-1-butene and 166 parts of methanol were used, a polyvinyl alcohol-based resin (5) [4% by mass aqueous solution viscosity 148 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.47 mol%; content of 1,2-glycol bonds in the side chain 0.6 mol%] was obtained.

[0096] [Manufacturing Example 6] By performing the polymerization reaction in the same manner as in Production Example 1, except that 12.9 parts of 3,4-diacetoxy-1-butene from Production Example 1 and 153 parts of methanol were used, a polyvinyl alcohol-based resin (6) [4% by mass aqueous solution viscosity 152 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.46 mol%; content of 1,2-glycol bonds in the side chain 0.8 mol%] was obtained.

[0097] [Manufacturing Example 7] 650 parts vinyl acetate and 52 parts methanol were added to a reactor equipped with a reflux condenser, a sample inlet, and a stirring blade, and the reactor was immersed in a water bath and heated. With reflux occurring in the reactor, 0.0325 parts 2,2'-azobis(isobutyronitrile) were added as an initiator to start polymerization. After 3.5 hours of polymerization, 0.013 parts m-dinitrobenzene and 1000 parts methanol were added as polymerization termination agents to terminate the polymerization. Next, methanol was added to displace the remaining vinyl acetate and obtain a methanol solution of polyvinyl acetate. The subsequent synthesis steps were carried out in the same manner as in Production Example 1 to obtain a polyvinyl alcohol-based resin (7) [4% by mass aqueous solution viscosity 134 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.80 mol%].

[0098] [Manufacturing Example 8] 650 parts vinyl acetate and 91 parts methanol were added to a reactor equipped with a reflux condenser, a sample inlet, and a stirring blade, and the reactor was immersed in a water bath and heated. With reflux occurring in the reactor, 0.052 parts 2,2'-azobis(isobutyronitrile) were added as an initiator to start polymerization. After 5.5 hours of polymerization, 0.013 parts m-dinitrobenzene and 1000 parts methanol were added as polymerization termination agents to terminate the polymerization. Next, methanol was added to displace the remaining vinyl acetate and obtain a methanol solution of polyvinyl acetate. The subsequent synthesis steps were carried out in the same manner as in Production Example 1 to obtain a polyvinyl alcohol-based resin (8) [4% by mass aqueous solution viscosity 66 mPa·s; degree of saponification 99.8 mol%; content of 1,2-glycol bonds in the main chain 1.80 mol%].

[0099] [Manufacturing Example 9] 650 parts vinyl acetate, 26 parts 3,4-diacetoxy-1-butene, and 26 parts methanol were added to a reactor equipped with a reflux condenser, a sample inlet, and a stirring blade, and the reactor was immersed in a water bath and heated. With reflux occurring in the reactor, 0.039 parts 2,2'-azobis(isobutyronitrile) was added as an initiator to start polymerization. After 6 hours of polymerization, 0.013 parts m-dinitrobenzene and 1000 parts methanol were added as polymerization termination agents to terminate the polymerization. Next, methanol was added to displace the remaining vinyl acetate and obtain a methanol solution of polyvinyl acetate. The subsequent synthesis steps were carried out in the same manner as in Production Example 1 to obtain a polyvinyl alcohol-based resin (9) [4% by mass aqueous solution viscosity 68 mPa·s; degree of saponification 99.8 mol%; 1,2-glycol content of the main chain 1.90 mol%; 1,2-glycol bond content of the side chain 2.0 mol%].

[0100] [Manufacturing Example 10] 700 parts vinyl acetate, 8.45 parts 3,4-diacetoxy-1-butene, and 31.5 parts methanol were added to a reactor equipped with a reflux condenser, a sample inlet, and a stirring blade, and the reactor was immersed in a water bath and heated. With reflux occurring in the reactor, 0.021 parts 2,2'-azobis(isobutyronitrile) was added as an initiator to start polymerization. After 4 hours of polymerization, 0.014 parts m-dinitrobenzene and 1000 parts methanol were added as polymerization termination agents to terminate the polymerization. The subsequent synthesis steps were carried out in the same manner as in Production Example 1 to obtain a polyvinyl alcohol-based resin (10) [4% by mass aqueous solution viscosity 138 mPa·s; degree of saponification 99.8 mol%; main chain 1,2-glycol content 1.8 mol%; side chain 1,2-glycol bond content 0.6 mol%].

[0101] <Method for measuring the content of 1,2-glycol bonds> After removing the residual solvent from the obtained polyvinyl alcohol-based resin, 50 mg of the polyvinyl alcohol-based resin was dissolved in 1 mL of DMSO-d6, and 40 μL of trifluoroacetic acid was added to prepare the measurement sample. Under the following conditions: 1 The 1,2-glycol bond content was calculated by measuring 1H-NMR. [Measurement conditions] Equipment: Brucker Ascend400 AVANCEIII Measurement program: N Proton Total number of times: 16 Measurement temperature: 50℃ In cases where the polyvinyl alcohol-based resin (A) is a mixture obtained by mixing multiple polyvinyl alcohol-based resins, the weighted average value was calculated by multiplying the content of 1,2-glycol bonds in each resin by the mixing ratio.

[0102] [Calculation of the content of 1,2-glycol bonds in the main chain] The peaks originating from methine, a vinyl alcohol structural unit, were attributed to 3.15–4.3 ppm (integral value A), and the peak originating from one methine of the 1,2-glycol bonds was attributed to 3.15–3.35 ppm (integral value B). The content of 1,2-glycol bonds was calculated using the following formula. Main chain 1,2-glycol bond content (mol%) = (B / A) × 100

[0103] [Calculation of the content of 1,2-glycol bonds in the side chain] The peaks originating from the methylene in the vinyl alcohol structural unit, the methylene in the unsaponified vinyl acetate structural unit, and the methylene in the 1,2-butenediol structural unit are attributed to 1.05-1.88 ppm (integral value E), while the peak originating from the methine in the main chain of the 1,2-butenediol structural unit is attributed to 1.76-1.88 ppm (integral value F). Since the peak originating from the main chain methylene structure and the peak originating from the methine in the main chain of the 1,2-butenediol structural unit overlap, the 1.05-1.88 ppm (integral value E') and 1.76-1.88 ppm (integral value F') were calculated for the polyvinyl alcohol-based resin that does not contain the 1,2-butenediol structural unit [polyvinyl alcohol-based resin (7) synthesized in Production Example 7], and the content of the 1,2-glycol bonds in the side chain was calculated using the following formula. The content of 1,2-glycol bonds in the side chain (mol%) = (F / (E / 2) - F' / (E' / 2)) × 100

[0104] <Method for measuring the viscosity of a 4% by mass aqueous solution> The viscosity of a 4% by mass aqueous solution of polyvinyl alcohol-based resin was measured in accordance with JIS K 6726 3.11.1.

[0105] <Example 1> (Preparation of polyvinyl alcohol-based film) To 100 parts of the polyvinyl alcohol-based resin (1) obtained in Production Example 1, 12 parts of glycerin as a plasticizer and 0.1 parts of polyoxylaurylamine as a surfactant were added to the composition. Ion-exchanged water was added to the composition so that its concentration was 18% by mass, and the mixture was heated and dissolved in an autoclave at 130°C for 1 hour to obtain an aqueous solution of the polyvinyl alcohol-based resin. This polyvinyl alcohol-based resin aqueous solution was cast onto a polyethylene terephthalate film as a base film to a dry film thickness of 30 μm, and dried in a drying oven heated to 110°C for 7 minutes. Afterward, the dried polyvinyl alcohol-based film was peeled from the base film and heat-treated in a drying oven at 120°C for 1 minute to obtain a polyvinyl alcohol-based film. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2.

[0106] (Fabrication of polarizing films and polarizing plates) The obtained polyvinyl alcohol-based film was cut to a length of 100 mm and a width of 60 mm, fixed to a chuck stretcher (initial length in the stretching direction: 40 mm), and stretched to 1.8 times its original size over 90 seconds while immersed in a water bath at 30°C. Next, it was immersed in a staining bath (30°C) with a composition of 1.3 g / L iodine and 30 g / L potassium iodide, and stretched to 2.4 times its original size over 40 seconds. Furthermore, it was immersed in a boric acid treatment bath (40°C) with a composition of 10 g / L boric acid and 25 g / L potassium iodide, and stretched to 3.2 times its original size over 20 seconds. After that, it was immersed in a stretching bath (56°C) with a composition of 30 g / L boric acid and 40 g / L potassium iodide, and stretched to a total stretching ratio of 5.8 times over 70 seconds. Finally, it was dried in an 80°C dryer for 40 seconds to obtain a polarizing film. Furthermore, a polarizing plate was fabricated by cutting a 40 mm length from the center position in the stretching direction of the polarizing film, laminating a 40 μm thick triacetylcellulose film to both sides using an aqueous polyvinyl alcohol solution as an adhesive, and drying it at 80°C for 80 seconds.

[0107] <Example 2> A polyvinyl alcohol-based film with a thickness of 30 μm was obtained in the same manner as in Example 1, except that the polyvinyl alcohol-based resin (2) obtained in Production Example 2 was used, and the polyvinyl alcohol-based film that had undergone the drying process was peeled from the base film and heat-treated in a drying oven at 155°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 6.2 times.

[0108] <Example 3> A polyvinyl alcohol-based film with a thickness of 30 μm was obtained in the same manner as in Example 1, except that the polyvinyl alcohol-based resin (3) obtained in Production Example 3 was used, and the polyvinyl alcohol-based film that had undergone the drying process was peeled from the base film and heat-treated in a drying oven at 160°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 6.1 times.

[0109] <Example 4> A polyvinyl alcohol-based film with a thickness of 30 μm was obtained in the same manner as in Example 1, except that the polyvinyl alcohol-based resin (4) obtained in Production Example 4 was used, and the polyvinyl alcohol-based film that had undergone the drying process was peeled from the base film and heat-treated in a drying oven at 137°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, using the polyvinyl alcohol-based film described above, polarizing films and polarizing plates were prepared in the same manner as in Example 1.

[0110] <Example 5> A polyvinyl alcohol-based film with a thickness of 30 μm was obtained in the same manner as in Example 1, except that the polyvinyl alcohol-based resin (5) obtained in Production Example 5 was used, and the polyvinyl alcohol-based film that had undergone the drying process was peeled from the base film and heat-treated in a drying oven at 145°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 6.0 times.

[0111] <Example 6> A polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the polyvinyl alcohol-based resin (6) obtained in Production Example 6 was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 150°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 6.0 times.

[0112] <Example 7> A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 1, except that 100 parts of a polyvinyl alcohol-based resin, obtained by mixing the polyvinyl alcohol-based resin (7) obtained in Production Example 7 and the polyvinyl alcohol-based resin (5) obtained in Production Example 5 in a mass ratio of 50:50, was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 138°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 5.8 times.

[0113] <Example 8> A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 1, except that 100 parts of a polyvinyl alcohol-based resin, obtained by mixing the polyvinyl alcohol-based resin (7) obtained in Production Example 7 and the polyvinyl alcohol-based resin (3) obtained in Production Example 3 in a mass ratio of 50:50, was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 145°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, polarizing films and polarizing plates were prepared in the same manner as in Example 1, except that the polyvinyl alcohol-based film was stretched to a total stretching ratio of 5.8 times.

[0114] <Comparative Example 1> (Manufacturing of polyvinyl alcohol-based films) To 100 parts of the polyvinyl alcohol-based resin (7) obtained in the above production example 7, 12 parts of glycerin as a plasticizer and 0.1 parts of polyoxylaurylamine as a surfactant were added to the composition. Ion-exchanged water was added to the composition so that its concentration was 18% by mass, and the mixture was heated and dissolved in an autoclave at 130°C for 1 hour to obtain an aqueous solution of the polyvinyl alcohol-based resin. This polyvinyl alcohol-based resin aqueous solution was cast onto a polyethylene terephthalate film as a base film to a dry film thickness of 30 μm, and dried in a drying oven heated to 110°C for 7 minutes. Then, the polyvinyl alcohol-based film that had undergone the drying process was peeled from the base film and heat-treated in a drying oven at 133°C for 1 minute to obtain a polyvinyl alcohol-based film. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2.

[0115] (Fabrication of polarizing films and polarizing plates) The obtained polyvinyl alcohol-based film was cut to a length of 100 mm and a width of 60 mm, fixed to a chuck stretcher (initial length in the stretching direction: 40 mm), and stretched to 1.8 times its original size over 90 seconds while immersed in a water bath at 30°C. Next, it was immersed in a staining bath (30°C) with a composition of 1.3 g / L iodine and 30 g / L potassium iodide, and stretched to 2.4 times its original size over 40 seconds. Furthermore, it was immersed in a boric acid treatment bath (40°C) with a composition of 10 g / L boric acid and 25 g / L potassium iodide, and stretched to 3.2 times its original size over 20 seconds. After that, it was immersed in a stretching bath (56°C) with a composition of 30 g / L boric acid and 40 g / L potassium iodide, and stretched to a total stretching ratio of 5.8 times over 70 seconds. Finally, it was dried in an 80°C dryer for 40 seconds to obtain a polarizing film. Furthermore, a polarizing plate was fabricated by cutting a 40 mm length from the center position in the stretching direction of the polarizing film, laminating a 40 μm thick triacetylcellulose film to both sides using an aqueous polyvinyl alcohol solution as an adhesive, and drying it at 80°C for 80 seconds.

[0116] <Comparative Example 2> (Preparation of polyvinyl alcohol-based film) A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 1, except that 100 parts of a polyvinyl alcohol-based resin, obtained by mixing the polyvinyl alcohol-based resin (7) obtained in Production Example 7 and the polyvinyl alcohol-based resin (9) obtained in Production Example 9 in a mass ratio of 50:50, was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 165°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2.

[0117] (Fabrication of polarizing films and polarizing plates) The obtained polyvinyl alcohol-based film was cut to a length of 100 mm and a width of 60 mm, fixed to a chuck stretcher (initial length in the stretching direction: 40 mm), and stretched to 1.8 times its original size over 90 seconds while immersed in a water bath at 30°C. Next, it was immersed in a staining bath (30°C) with a composition of 1.3 g / L iodine and 30 g / L potassium iodide, and stretched to 2.4 times its original size over 40 seconds. Then, it was immersed in a boric acid treatment bath (40°C) with a composition of 10 g / L boric acid and 25 g / L potassium iodide, and stretched to 3.2 times its original size over 20 seconds. After that, it was immersed in a stretching bath (56°C) with a composition of 40 g / L boric acid and 40 g / L potassium iodide, and stretched to a total stretching ratio of 5.8 times over 70 seconds. Finally, it was dried in an 80°C dryer for 40 seconds to obtain a polarizing film. Furthermore, a polarizing plate was fabricated by cutting a 40 mm length from the center position in the stretching direction of the polarizing film, laminating a 40 μm thick triacetylcellulose film to both sides using an aqueous polyvinyl alcohol solution as an adhesive, and drying it at 80°C for 80 seconds.

[0118] <Comparative Example 3> A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 1, except that the polyvinyl alcohol-based resin (8) obtained in Production Example 8 was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 130°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, using the polyvinyl alcohol-based film described above, polarizing films and polarizing plates were prepared in the same manner as in Comparative Example 1.

[0119] <Comparative Example 4> A polyvinyl alcohol-based film was obtained in the same manner as in Comparative Example 1, except that the polyvinyl alcohol-based resin (10) obtained in Production Example 10 was used, and the polyvinyl alcohol-based film, which had undergone a drying process, was peeled from the base film and heat-treated in a drying oven at 160°C for 1 minute. The properties of the obtained polyvinyl alcohol-based film are shown in Tables 1 and 2. Furthermore, using the polyvinyl alcohol-based film described above, polarizing films and polarizing plates were prepared in the same manner as in Comparative Example 1.

[0120] The following heat resistance tests and humidity resistance tests were performed using the obtained polarizing plates.

[0121] <Heat resistance test> (Evaluation method) An acrylic adhesive sheet was bonded to one side of the obtained polarizing plate, and then bonded to a 4.0 cm x 8.0 cm glass plate (1 mm thick). The layers were stacked in the order of glass plate / adhesive / polarizing plate to create a laminate. This laminate was then compressed using a pressure degassing device at 50°C under a pressure of 0.5 MPa for 20 minutes to obtain a heat-resistant durability test sample. The orthogonal transmittance (%), polarization degree (%), and orthogonal transmittance [Tc700] (%) at a wavelength of 700 nm were measured using a spectrophotometer (automatic polarizing film measuring device VAP-7070 manufactured by JASCO Corporation). Subsequently, the heat-resistant durability test sample was placed in a 105°C dryer for 500 hours, and the orthogonal transmittance (%), polarization degree (%), and orthogonal transmittance [Tc700] (%) at a wavelength of 700 nm were measured in the same manner after placement. The change after the heat-resistant test was calculated by subtracting the measurement value before placement from the measurement value after placement, and the heat-resistant durability was evaluated by comparing the absolute values ​​of these changes according to the following evaluation criteria. The evaluation results are shown in Table 1.

[0122] (Evaluation Criteria) • Orthogonal transmittance ○: The absolute value of the change is 0-0.03% ×: The absolute value of the change is greater than 0.03% • Polarization ○: The absolute value of the change is 0-0.07% ×: The absolute value of the change is greater than 0.07% Tc700 ○: The absolute value of the change is between 0 and 5.5% ×: The absolute value of the change is greater than 5.5%

[0123] <Heat and moisture resistance test> (Evaluation method) In Examples 1, 4-7 and Comparative Examples 1-4, an acrylic adhesive sheet was bonded to one side of the polarizing plates. This sheet was then bonded to a 4.0 cm x 8.0 cm glass plate (1 mm thick), and the layers were stacked in the order of glass plate / adhesive / polarizing plate to create a laminate. This laminate was then compressed using a pressure degassing device at 50°C under a pressure of 0.5 MPa for 20 minutes to obtain a moisture and heat resistance test sample. The orthogonal transmittance (%), polarization degree (%), orthogonal transmittance at a wavelength of 700 nm [Tc700] (%), and orthogonal transmittance at a wavelength of 430 nm [Tc430] (%) of the obtained moisture and heat resistance test sample were measured using a spectrophotometer (automatic polarizing film measuring device VAP-7070 manufactured by JASCO Corporation).

[0124] Subsequently, the aforementioned moisture and heat resistance test samples were placed in a constant temperature and humidity chamber at 85°C × 85%RH for 120 hours. The orthogonal transmittance (%), polarization degree (%), orthogonal transmittance at a wavelength of 700nm [Tc700] (%), and orthogonal transmittance at a wavelength of 430nm [Tc430] (%) were measured in the same manner after placement. The change after the moisture and heat resistance test was calculated by subtracting the measured value before placement from the measured value after placement. The moisture and heat resistance was evaluated by comparing these changes in absolute value according to the following evaluation criteria. The evaluation results are shown in Table 2.

[0125] (Evaluation Criteria) • Orthogonal transmittance ○: The absolute value of the change is 0-0.08% ×: The absolute value of the change is greater than 0.08% • Polarization ○: The absolute value of the change is 0-0.2% ×: The absolute value of the change is greater than 0.2% Tc700 ○: The absolute value of the change is between 0 and 0.12% ×: The absolute value of the change is greater than 0.12% ·Tc430 ○: The absolute value of the change is between 0 and 0.35% ×: The absolute value of the change is greater than 0.35%

[0126] [Table 1]

[0127] [Table 2]

[0128] The polarizers of Examples 1 to 8, obtained from polyvinyl alcohol-based films containing a polyvinyl alcohol-based resin in which the 1,2-glycol bonds of the main chain and side chains are within the range specified in this application, all showed smaller absolute values ​​of the change in orthogonal transmittance (%), polarization degree (%), and Tc700 (%) after the heat resistance test compared to the polarizers of Comparative Examples 1 to 4. This indicates that polarizers with excellent optical performance can be obtained under harsh environmental conditions of high temperature. Furthermore, the polarizing plates of Example 1 and Examples 4-7 all showed smaller absolute values ​​for the change in orthogonal transmittance (%), degree of polarization (%), Tc700 (%), and Tc430 (%) after the humidity and heat resistance test compared to the polarizing plates of Comparative Examples 1-4. This indicates that polarizing plates with excellent optical performance can be obtained even under harsh environmental conditions of high temperature and high humidity. [Industrial applicability]

[0129] This polyvinyl alcohol-based film is preferably used as a raw material for manufacturing polarizing films used in portable information terminals, personal computers, televisions, projectors, signage, electronic calculators, electronic clocks, word processors, electronic paper, game consoles, video players, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices such as instruments in automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, foldable displays, rollable televisions, rollable displays, anti-reflective coatings for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

Claims

1. An optical polyvinyl alcohol-based film containing a polyvinyl alcohol-based resin (A) having 1,2-glycol bonds in the main chain and side chains, An optical polyvinyl alcohol-based film wherein the content of 1,2-glycol bonds in the main chain of the polyvinyl alcohol-based resin (A) is 1.7 mol% or less, and the content of 1,2-glycol bonds in the side chains is 0.01 to 6 mol%.

2. The optical polyvinyl alcohol-based film according to claim 1, wherein the average degree of saponification of the polyvinyl alcohol-based resin (A) is 95 mol% or more.

3. The optical polyvinyl alcohol-based film according to claim 1 or 2, wherein the viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol-based resin (A) is 70 mPa·s or more.

4. The optical polyvinyl alcohol-based film according to claim 1 or 2, wherein the film thickness is 5 to 50 μm.

5. An optical polyvinyl alcohol-based film according to claim 1 or 2, used for manufacturing polarizing films.

6. A polarizing film obtained using the optical polyvinyl alcohol-based film described in claim 1 or 2.

7. A polarizing plate comprising a polarizing film according to claim 6 and a protective film provided on at least one side of the polarizing film.

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