Polyvinyl alcohol-based film, method for manufacturing the same, and polarizing film and polarizing plate using the same.

Abstract

Provided are: a polyvinyl-alcohol-based film that has exceptional stretchability during manufacture of a polarizing film, that makes it possible to inhibit breakage, and that makes it possible to reduce the amount of elution of a polyvinyl-alcohol-based resin when immersed in water; and, inter alia, a method for manufacturing the polyvinyl-alcohol-based film.　The present invention provides a polyvinyl-alcohol-based film, the polyvinyl-alcohol-based film being characterized in that with regard to the thickness-direction orientation distribution MDΔn−TDΔn of the film, which is the difference between the thickness-direction birefringence distribution MDΔn of a film slice obtained by slicing the polyvinyl-alcohol-based film in an MD direction and the thickness-direction birefringence distribution TDΔn of a film slice obtained by slicing the polyvinyl-alcohol-based film in a TD direction, at least one of the following relationships (1) and (2) is satisfied, where / / s1 and / / s2 are the respective peak top values in ranges corresponding to 40% at both thickness-direction ends of the film relative to 100% representing the total thickness of the film, / / c is the average value in the range corresponding to 20% at the thickness-direction center of the film relative to 100% representing the total thickness of the film, and / / s1> / / s2.　Relationship (1): 0.00150≤| / / s1- / / c|≤0.00300 Relationship (2): 0.00045≤| / / s2- / / c|≤0.00090

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol-based film, and more specifically, to a polyvinyl alcohol-based film that is effective in reducing tension and defects during stretching in the production of polarizing films, and can be manufactured with a high yield by suppressing breakage and exceeding specifications for defects during stretching, as well as to a polarizing film and polarizing plate using the polyvinyl alcohol-based film. [Background technology]

[0002] Traditionally, polyvinyl alcohol-based films have been used in many applications due to their excellent transparency, and one of their useful applications is polarizing films. Polarizing films are used as a basic component of liquid crystal displays, and in recent years, their use has expanded to equipment requiring high quality and high reliability.

[0003] In this context, as screens in LCD televisions and multi-functional mobile devices become brighter, higher resolution, larger in area, and thinner, there is a demand for polarizing films with superior optical properties. Specifically, there is a need for further improvements in polarization degree and elimination of color unevenness.

[0004] Generally, polyvinyl alcohol-based films are manufactured by a continuous casting method using an aqueous solution of polyvinyl alcohol-based resin. Specifically, an aqueous solution of polyvinyl alcohol-based resin is poured into a casting mold such as a casting drum or endless belt, and after the resulting film is peeled from the casting mold, it is dried using a hot roll or floating dryer while being transported in the flow direction (MD direction) using a nip roll or the like. In this transport process, the film is pulled in the flow direction (MD direction), so the polyvinyl alcohol-based polymer tends to orient in the MD direction.

[0005] On the other hand, polarizing films are generally manufactured by swelling a polyvinyl alcohol-based film, which is the raw material, with water (including warm water), dyeing it with a dichroic dye such as iodine, and then stretching it. In this swelling process, it is important to swell the polyvinyl alcohol-based film quickly and to swell it uniformly so that the dye can smoothly penetrate into the film during the dyeing process. Furthermore, this stretching process involves stretching the dyed film in the flow direction (MD) to highly orient the dichroic dyes within the film. In order to improve the polarization performance of the polarizing film, it is important that the polyvinyl alcohol-based film used as the base material exhibits good stretchability in the flow direction (MD) during the stretching process.

[0006] Furthermore, in the swelling step of the polarizing film manufacturing process, if impurities leach from the polyvinyl alcohol-based film and contaminate the swelling tank, the contamination will spread throughout the subsequent processes. In the dyeing and boric acid crosslinking steps as well, if impurities leach from the polyvinyl alcohol-based film, not only will the polarization performance of the resulting polarizing film decrease, but a great deal of effort will be required to filter and replace the chemicals used in each step. Examples of such impurities include low molecular weight polyvinyl alcohol-based resins (including oligomers) present in the polyvinyl alcohol-based film. In particular, low molecular weight materials with a molecular weight of 50,000 or less tend to dissolve easily in water and tend to form low molecular weight iodine complexes that reduce the degree of polarization.

[0007] As methods to improve the aforementioned stretchability, for example, a method of controlling the speed of the cast drum and the final winding speed (see, for example, Patent Document 1), and a polyvinyl alcohol-based film in which the averaged birefringence values ​​in the thickness direction of the polyvinyl alcohol-based film satisfy a specific relational expression in the length direction and width direction, respectively (see, for example, Patent Document 2) have been proposed. Furthermore, as a method to improve contamination due to elution from polyvinyl alcohol-based films during the swelling process, for example, the amount of polyvinyl alcohol-based resin eluted when immersed in 50°C water for 1 minute is 900 ppm / m². 2 A polyvinyl alcohol-based film has been proposed (see, for example, Patent Document 3) characterized by the following and having a curl angle in the short-side direction of 135° or less, as measured under specific conditions. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2001-315141 [Patent Document 2] International Publication No. 2012 / 132984 [Patent Document 3] International Publication No. 2017 / 204271 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, even with the method described in the aforementioned patent document, it was insufficient to improve the stretchability during polarizing film production and to control the amount of polyvinyl alcohol-based resin eluted. Specifically, Patent Document 1 specifies the degree of stretching (tensile strength) in the MD direction when manufacturing a polyvinyl alcohol-based film, but it does not disclose the stretchability of thin polyvinyl alcohol-based films when manufacturing polarizing films. Furthermore, polyvinyl alcohol-based films stretched in the flow direction (MD) when the ratio of the cast drum speed to the final winding speed is greater than 1 tend to have reduced stretchability when manufacturing polarizing films. On the other hand, when the ratio of the cast drum speed to the final winding speed is reduced to less than 0.9, the transport (manufacturing) stability tends to deteriorate due to the effects of wrinkles and sagging.

[0010] While the technology disclosed in Patent Document 2 exhibits high stretchability, there is room for improvement in reducing the elution of polyvinyl alcohol-based resin during the swelling process.

[0011] While the technology disclosed in Patent Document 3 can produce a polyvinyl alcohol-based film with a low amount of polyvinyl alcohol-based resin elution, there is room for improvement in terms of stretchability required to produce polarizing films with high polarization degrees, which are currently in demand.

[0012] Therefore, against this background, the present invention provides a polyvinyl alcohol-based film that exhibits excellent stretchability during polarizing film manufacturing, suppresses breakage, and enables the production of a polarizing film with excellent polarization performance, and that also reduces the amount of polyvinyl alcohol-based resin leached out when immersed in water and does not contaminate polarizing film manufacturing equipment, as well as a method for manufacturing the same, and a polarizing film and polarizing plate using the polyvinyl alcohol-based film. [Means for solving the problem]

[0013] However, in light of these circumstances, the inventors conducted extensive research and found that a polyvinyl alcohol-based film in which the orientation of the surface layer and core layer in the thickness direction is controlled to a specific range is effective in reducing the amount of polyvinyl alcohol-based resin eluted and the tension during stretching when manufacturing polarizing films, resulting in a polarizing film with fewer defects and suppressing contamination of polarizing film manufacturing equipment.

[0014] In other words, the present invention provides the following [1] to

[13] . [1] A polyvinyl alcohol-based film, In the orientation distribution MDΔn-TDΔn of the film in the thickness direction, which is the difference between the birefringence distribution MDΔn in the thickness direction of a film piece obtained by slicing the polyvinyl alcohol-based film in the MD direction and the birefringence distribution TDΔn in the thickness direction of a film piece obtained by slicing the polyvinyl alcohol-based film in the TD direction, Let ∥s1 and ∥s2 be the peak top values ​​in the 40% range at both ends of the film's thickness direction out of the total 100% of the film's thickness, and let ∥c be the average value in the central 20% range of the film's thickness direction out of the total 100% of the film's thickness, and if ∥s1 > ∥s2, A polyvinyl alcohol-based film characterized by satisfying at least one of the following formulas (1) and (2). 0.00150 ≤ |∥s1 - ∥c| ≤ 0.00300 ···(1) 0.00045 ≤ |∥s² - ∥c| ≤ 0.00090 ···(2) [2] The polyvinyl alcohol-based film according to [1], wherein ∥c satisfies the following formula (3). -0.0005≦∥c<0.0006 ···(3) [3] The polyvinyl alcohol-based film according to [1] or [2], wherein when, with respect to 100% of the total thickness of the film, the thickness position of the film surface on the side close to ∥s1 is 0% and the thickness position of the film surface on the side close to ∥s2 is 100%, the thickness position corresponding to the value represented by |∥s1-∥c| / 2 is within a range of 4% or more and less than 7%. [4] The polyvinyl alcohol-based film according to any one of [1] to [3], wherein the polyvinyl alcohol-based film has a thickness of 5 to 70 µm, a width of 4 m or more, and a length of 4 km or more. [5] A polarizing film obtained by using the polyvinyl alcohol-based film according to any one of [1] to [4]. [6] A polarizing plate comprising: the polarizing film according to [5]; and a protective film provided on at least one surface of the polarizing film. [7] A method for producing the polyvinyl alcohol-based film according to any one of [1] to [4], wherein the polyvinyl alcohol-based film is produced through the following steps (A) to (C). Step (A): a step of preparing a polyvinyl alcohol-based resin aqueous solution. Step (B): a step of casting the polyvinyl alcohol-based resin aqueous solution onto a casting mold to form a film. Step (C): a step of heating and drying the formed film by bringing the film into contact with a plurality of heat rolls. [8] The method for producing a polyvinyl alcohol-based film according to [7], wherein drying using the plurality of heat rolls in the step (C) satisfies the following conditions (c1) and (c2). Condition (c1): The temperature of the heat rolls in contact with films with a film moisture content greater than 11% by mass is always between 50 and 90°C. Condition (c2): The film moisture content is 11% by mass or less, and the temperature of the heat roll that first contacts the film surface opposite to the surface in contact with the casting mold is 100°C or higher. [9] A method for producing a polyvinyl alcohol-based film according to [8], characterized in that the drying using multiple hot rolls in step (C) further satisfies the following condition (c3). Condition (c3): In any of the hot rolls after the hot roll that under condition (c2) has been dried, the temperature of the hot roll is set to 100°C or higher and the film surface that is in contact with the cast mold is brought into contact with it.

[10] A method for producing a polyvinyl alcohol-based film according to any one of [7] to [9], comprising the following step (D), wherein the heat treatment temperature in step (D) is less than 100°C. Process (D): A process in which the film obtained in process (C) is heat-treated using hot air.

[11] Step (A) of preparing an aqueous solution of polyvinyl alcohol-based resin, (B) A process in which an aqueous solution of polyvinyl alcohol-based resin is cast into a mold to form a film. (C) A step in which the formed film is heated and dried by bringing it into contact with multiple heat rolls. A method for manufacturing a polyvinyl alcohol-based film, which is produced via the following steps: A method for producing a polyvinyl alcohol-based film, characterized in that the drying using multiple hot rolls in step (C) satisfies the following conditions (c1) and (c2). Condition (c1): The temperature of the heat rolls in contact with films with a film moisture content greater than 11% by mass is always between 50 and 90°C. Condition (c2): The film moisture content is 11% by mass or less, and the temperature of the heat roll that first contacts the film surface opposite to the surface in contact with the casting mold is 100°C or higher.

[12] A method for producing a polyvinyl alcohol-based film according to

[11] , characterized in that the drying using multiple hot rolls in step (C) further satisfies the following condition (c3). Condition (c3): In any of the hot rolls after the hot roll that under condition (c2) has been dried, the temperature of the hot roll is set to 100°C or higher and the film surface that is in contact with the cast mold is brought into contact with it.

[13] A method for producing a polyvinyl alcohol-based film according to

[11] or

[12] , comprising a step (D) of heat-treating the film obtained in step (C) using hot air, wherein the heat treatment temperature in step (D) is less than 100°C. [Effects of the Invention]

[0015] The polyvinyl alcohol-based film of the present invention is a film that has good stretchability and is resistant to breakage during the production of polarizing films, thus enabling high stretchability and resulting in a polarizing film with high polarization performance. Furthermore, because the elution of polyvinyl alcohol-based resin during the production of polarizing films can be suppressed, contamination of polarizing film production equipment can be reduced, and a polarizing film with fewer defects can be obtained. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram illustrating an example of the orientation distribution MDΔn-TDΔn in the thickness direction of a film according to an example embodiment of the present invention. [Figure 2] This is a schematic diagram illustrating an example of the orientation distribution MDΔn-TDΔn in the thickness direction of a film according to an example embodiment of the present invention. [Modes for carrying out the invention]

[0017] The present invention will be described in detail below. In this invention, 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 meaning of "preferably greater than X" or "preferably less than Y." Furthermore, "X and / or Y (where X and Y are any combination)" means at least one of X and Y, and can mean X only, Y only, or X and Y.

[0018] A polyvinyl alcohol-based film according to an example of an embodiment of the present invention (hereinafter sometimes referred to as "this polyvinyl alcohol-based film") is characterized in that, in the orientation distribution MDΔn-TDΔn of the film in the thickness direction, which is the difference between the birefringence distribution MDΔn in the thickness direction of a film piece sliced ​​in the MD direction and the birefringence distribution TDΔn in the thickness direction of a film piece sliced ​​in the TD direction, when the peak top values ​​(maximum values) in each range of the 40% at both ends of the film in the thickness direction are ∥s1 and ∥s2 (where ∥s1 > ∥s2), and the average value in the central 20% range of the film in the thickness direction is ∥c, at least one of the following equations (1) and (2) is satisfied. 0.00150 ≤ |∥s1 - ∥c| ≤ 0.00300 ···(1) 0.00045 ≤ |∥s² - ∥c| ≤ 0.00090 ···(2)

[0019] The aforementioned central 20% range refers to the central 20% region of the film in the thickness direction (a region within ±10% of the center in the film thickness direction) when the entire film (total thickness) is considered to be 100%, and the aforementioned 40% ranges at both ends refer to the 40% regions located on either side of the central 20% region in the thickness direction when the entire film (total thickness) is considered to be 100%. In this specification, the central 20% range may be referred to as the core layer, and each of the 40% ranges at both ends may be referred to as the surface layer.

[0020] The polyvinyl alcohol-based film must satisfy at least one of the above formulas (1) and (2). A polyvinyl alcohol-based film that does not satisfy formula (1) or (2) cannot achieve the objective of the present invention because it has an inferior balance between high stretchability during polarizing film production and low elution of the polyvinyl alcohol-based resin.

[0021] This polyvinyl alcohol-based film is preferable to satisfy formula (1) above, and more preferably satisfies both formulas (1) and (2) above, in order to have an excellent balance between low elution and high stretchability.

[0022] The reason why satisfying either formula (1) or (2) above results in excellent stretchability during polarizing film production and suppression of polyvinyl alcohol resin elution is not entirely clear, but in the process of diligent research, the inventors have discovered that by controlling the difference between the orientation of the surface layer and the orientation of the core layer of the film, it is possible to provide a polyvinyl alcohol-based film with an excellent balance of low elution and high stretchability. Specifically, the inventors have found that in the orientation distribution MDΔn-TDΔn in the thickness direction of the film, if the difference between the peak top value on the surface layer and the average value of the core layer is too large compared to a predetermined value, the stretchability during polarizing film production tends to be poor and the film tends to break easily, while if the difference is too small compared to a predetermined value, the amount of polyvinyl alcohol-based film elution tends to increase. By controlling the difference within a specific range, the inventors have found that it is possible to provide a polyvinyl alcohol-based film with an excellent balance of low elution and high stretchability during polarizing film production.

[0023] Here, we will explain the relationship between birefringence Δn and orientation. Birefringence Δn is the extraordinary refractive index (n e ) and normal refractive index (n o Defined as the difference between ), Δn=n e -n o It is described as follows. Here, polymers where Δn>0 are called positive polymers, and polymers where Δn<0 are called negative polymers, and polyvinyl alcohol is known to be a positive polymer. Generally, when light is incident on a polymer, it splits into polarization parallel to the optical axis due to the orientation of the polymer and polarization perpendicular to it. Polyvinyl alcohol has an orientation direction and an optical axis (slow axis: refractive index n e ) can be considered to match, and since polyvinyl alcohol is a rectifying polymer, if it is an MD-oriented film, the MD refractive index (n e )>TD refractive index (n o), in the case of a TD-oriented film, the TD refractive index (n e )> the MD refractive index (n o ). Here, Δn is considered when the film is sliced (cut into a thin sheet) and light is incident perpendicularly to the sliced surface. When defining the distribution of MD refractive index in the thickness direction as nMD, the distribution of TD refractive index in the thickness direction as nTD, and the distribution of thickness refractive index in the thickness direction as nZ, the birefringence distribution in the thickness direction when the film is sliced in the MD direction can be described as nMD-nZ, and the birefringence distribution in the thickness direction when the film is sliced in the TD direction can be described as nTD-nZ. Here, the former is denoted as MDΔn and the latter as TDΔn. Since nZ is the same in both observation methods, taking the difference between the two gives MDΔn-TDΔn=(nMD-nZ)-(nTD-nZ)=nMD-nTD, which is the refractive index difference distribution of MD and TD in the thickness direction. That is, in the distribution, the portion where nMD-nTD>0 indicates MD orientation, and the portion where nMD-nTD<0 indicates TD orientation, therefore MDΔn-TDΔn represents the orientation distribution in the thickness direction. Accordingly, by taking the difference between MDΔn and TDΔn, which are birefringence distributions in the film thickness direction, the orientation distribution MDΔn-TDΔn in the film thickness direction can be obtained.

[0024] An example of the orientation distribution MDΔn-TDΔn in the film thickness direction according to the present invention is schematically shown in Fig. 1. As shown in the figure, in each peak top value within the 40% range at both ends of the film thickness direction (each surface layer), the relatively large peak top value is ∥s1, and the relatively small peak top value is ∥s2. Also, as shown in the figure, ∥c is the average value within the central 20% range (core layer) of the film thickness direction. This polyvinyl alcohol-based film achieves an excellent balance between low elution and high stretchability during polarizing film production by controlling the difference between the relatively large peak top value ∥s1 and the average value ∥c (|∥s1-∥c|) in the orientation distribution MDΔn-TDΔn in the film thickness direction to a range of 0.00150 or more and 0.00300 or less (Equation (1)), or by controlling the difference between the relatively small peak top value ∥s2 and the average value ∥c (|∥s2-∥c|) to a range of 0.00045 or more and 0.00090 or less (Equation (2)).

[0025] In equation (1) above, when the value of |∥s1-∥c| falls below the lower limit, there is a tendency for the amount of elution during polarizing film manufacturing to increase. Furthermore, in equation (1) above, if the value of |∥s1-∥c| exceeds the upper limit, the orientation balance between the surface layer and the core layer becomes poor, the tensile strength during polarizing film manufacturing increases, and the film tends to break easily.

[0026] This polyvinyl alcohol-based film is preferable to satisfy the following formula (1') and particularly preferable to satisfy the following formula (1'') in order to have an excellent balance of low elution and high stretchability. 0.00160 ≤ |∥s1 - ∥c| ≤ 0.00300···(1') 0.00170 ≤ |∥s1 - ∥c| ≤ 0.00250···(1'')

[0027] Furthermore, in equation (2) above, when the value of |∥s2-∥c| falls below the lower limit, there is a tendency for the amount of elution during polarizing film manufacturing to increase. Furthermore, in equation (2) above, if the value of |∥s2-∥c| exceeds the upper limit, the orientation balance between the surface layer and the core layer becomes poor, the tensile strength during polarizing film manufacturing increases, and the film tends to break easily.

[0028] This polyvinyl alcohol-based film is preferable to satisfy the following formula (2'), and particularly preferable to satisfy the following formula (2''), in order to have an excellent balance of low elution and high stretchability. 0.00050 ≤ |∥s² - ∥c| ≤ 0.00075 ···(2') 0.00050 ≤ |∥s² - ∥c| ≤ 0.00070 ···(2'')

[0029] In terms of high stretchability, it is preferable that the polyvinyl alcohol-based film satisfies the following formula (3) when ||c is present. -0.00050 ≤ ||c < 0.0005 ···(3)

[0030] In equation (3) above, if the value of ||c is too small, the material tends to spread too much in the width direction (TD) during swelling, resulting in a decrease in extensibility. Furthermore, in equation (3) above, if the value of ||c is too large, the tension becomes too high during stretching, which tends to make it prone to breaking.

[0031] This polyvinyl alcohol-based film is particularly preferably satisfied with the following formula (3'), more preferably with the following formula (3''), and especially preferably with the following formula (3''') in terms of high stretchability. -0.00050≦∥c<0 ···(3') -0.00048≦∥c≦-0.00005 ···(3'') -0.00045≦∥c≦-0.00010 ···(3''')

[0032] In this polyvinyl alcohol-based film, in order to reduce the amount of polyvinyl alcohol-based resin that elutes, it is preferable that the thickness position of the film surface on the side closer to ∥s1 and the thickness position of the film surface on the side closer to ∥s2 is 0% and the thickness position of the film surface on the side closer to ∥s2 is 100%, and that the thickness position of the value expressed as |∥s1-∥c| / 2 is within the range of 4% or more and less than 7%.

[0033] The value represented by |∥s1-∥c| / 2 is particularly preferably in the range of 4.5 to 6.8%, and more preferably in the range of 5.0 to 6.5%, from the standpoint of an excellent balance between low dissolution and high stretchability.

[0034] When the value represented by |∥s1-∥c| / 2 is less than 4%, i.e., too close to the film surface, the amount of elution tends to increase. Also, when the value represented by |∥s1-∥c| / 2 is 7% or more, i.e., too far from the film surface, the tension during stretching tends to increase, making the film more prone to breakage.

[0035] Figure 2 schematically shows an example of the orientation distribution MDΔn-TDΔn in the film thickness direction in the present invention, represented by |∥s1-∥c| / 2. As shown in the figure, when the thickness position of the film surface closer to ∥s1 is set to 0% and the thickness position of the film surface closer to ∥s2 is set to 100%, it is preferable to control the thickness position of the value represented by |∥s1-∥c| / 2 to a range of 4% or more and less than 7%, as this can further reduce the amount of polyvinyl alcohol-based resin eluted.

[0036] In the present invention, a preferred method for controlling the film to fall within the range of formulas (1) to (3) is a method used in the polyvinyl alcohol-based film manufacturing method by the continuous casting method described later, in which the film formed in the casting mold is peeled from the casting mold and then dried using a hot roll that satisfies predetermined conditions. In particular, it is preferable to set the temperature of the hot roll that contacts the film with a moisture content greater than 11% by mass to 50 to 90°C (condition (c1)), and to set the temperature of the hot roll that first contacts the film surface opposite to the surface that contacted the casting mold when the film moisture content becomes 11% by mass or less to 100°C or higher (condition (c2)). In the case of a manufacturing method that does not satisfy both conditions (c1) and (c2), it tends to be difficult to control the film to fall within the range of formula (1) or (2), making it difficult to obtain a polyvinyl alcohol-based film with an excellent balance of low elution and high stretchability, and thus difficult to achieve the objective of the present invention.

[0037] Generally, when polyvinyl alcohol-based films are continuously manufactured, drying and heat treatment are performed while the film is transported in the median-demand (MD) direction, which tends to result in a strong MD orientation. If the MD orientation is too strong, the film tension increases, making it prone to breakage during polarizing film production. On the other hand, if the temperature conditions in the film manufacturing process are lowered to weaken the MD orientation, the amount of polyvinyl alcohol-based resin that elutes tends to increase. Therefore, conventionally, it has been difficult to achieve both high stretchability and low elution during polarizing film production. The inventors focused on the fact that in the drying process of the manufacturing process of polyvinyl alcohol-based films, a gradient in moisture content occurs from the surface layer to the core layer of the film, resulting in a difference in the timing of drying and heat treatment between the surface layer and the core layer of the film. They discovered that the orientation of the surface layer and core layer of the film can be suitably controlled by adjusting the moisture content, drying conditions, and heat treatment conditions of the film. Furthermore, by controlling the orientation of the surface layer and core layer of the film to fall within the range of formulas (1) and / or (2) above, they found an optimal orientation distribution that can achieve both high stretchability and low elution during the production of polarizing films.

[0038] The method for manufacturing this polyvinyl alcohol-based film will be described in more detail below, in order of the steps, but the method for manufacturing this polyvinyl alcohol-based film is not limited to these embodiments.

[0039] This polyvinyl alcohol-based film is preferably manufactured through the following steps (A) to (C), and more preferably through a further step (D) after steps (A) to (C). Step (A): Step of preparing an aqueous solution of polyvinyl alcohol-based resin. Process (B): A process of forming a film by casting an aqueous solution of polyvinyl alcohol-based resin into a cast mold. Process (C): A process in which the prepared film is heated and dried by bringing it into contact with multiple heat rolls. Step (D): A step in which the obtained film is heat-treated using hot air.

[0040] <Process (A)> Step (A) is the step of preparing an aqueous solution of polyvinyl alcohol-based resin. First, we will explain the polyvinyl alcohol-based resin and the aqueous solution of the polyvinyl alcohol-based resin, which are the materials for this polyvinyl alcohol-based film. The polyvinyl alcohol resin used to constitute this polyvinyl alcohol film is typically an unmodified polyvinyl alcohol resin, i.e., a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of a component copolymerizable with vinyl acetate can also be used. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (e.g., salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, and unsaturated sulfonates. Modified polyvinyl alcohol resins obtained by chemically modifying the hydroxyl groups after saponification can also be used. These can be used individually or in combination of two or more.

[0041] Furthermore, a polyvinyl alcohol resin having a 1,2-diol structure in its side chain can also be used as the polyvinyl alcohol resin for this polyvinyl alcohol film. Such a polyvinyl alcohol resin having a 1,2-diol structure in its side chain can be obtained, for example, by (i) saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, (iii) saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, or (iv) saponifying a copolymer of vinyl acetate and glycerol monoallyl ether.

[0042] The weight-average molecular weight of the polyvinyl alcohol resin used in this polyvinyl alcohol film is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If the weight-average molecular weight is too low, it tends to be difficult to obtain sufficient optical performance when the polyvinyl alcohol resin is used 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 film. The weight-average molecular weight of the polyvinyl alcohol-based resin is the weight-average molecular weight measured by the GPC-MALS method.

[0043] The average degree of saponification of the polyvinyl alcohol resin used in this polyvinyl alcohol film is preferably 98 mol% or higher, particularly preferably 99 mol% or higher, even more preferably 99.5 mol% or higher, and especially preferably 99.8 mol% or higher. If the average degree of saponification is too low, sufficient optical performance tends not to be obtained when the polyvinyl alcohol film is used as a polarizing film. Here, the average degree of saponification in this invention is measured in accordance with JIS K 6726.

[0044] Two or more polyvinyl alcohol-based resins with different characteristics such as modified species, degree of modification, weight-average molecular weight, and average degree of saponification may be used in combination in this polyvinyl alcohol-based film.

[0045] In addition to the polyvinyl alcohol resin used in this polyvinyl alcohol-based film, it is more preferable from the standpoint of film-forming properties to include, as necessary, 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. These can be used individually or in combination of two or more.

[0046] The resin concentration of the polyvinyl alcohol-based resin aqueous solution obtained in this manner is preferably 15 to 60% by mass, particularly preferably 17 to 55% by mass, and even more preferably 20 to 50% by mass. If the resin concentration of the 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.

[0047] 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 typically a twin-screw extruder with a vent is used.

[0048] <Process (B)> Step (B) 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 formed into a film by a continuous casting method.

[0049] The resin temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-type slit die is preferably 80 to 100°C, and particularly preferably 85 to 98°C. If the resin temperature is too low, it tends to flow poorly, and if it is too high, it tends to foam.

[0050] The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 to 200 Pa·s, and particularly preferably 70 to 150 Pa·s, at the time of discharge. If the viscosity is too high, it tends to result in poor flow, and if it is too low, it tends to make film formation difficult.

[0051] The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-type slit die to the casting drum is preferably 0.2 to 5 m / min, particularly preferably 0.4 to 4 m / min, and even more preferably 0.6 to 3 m / min. If the discharge speed is too slow, productivity tends to decrease, and if it is too fast, casting tends to become difficult.

[0052] The diameter of the cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and even more preferably 2.8 to 4 m. If the diameter is too small, the drying section on the cast drum becomes shorter, which tends to make it difficult to increase the speed, and if it is too large, transportability tends to decrease.

[0053] The width of the casting drum is preferably 4m or more, particularly preferably 4.5m or more, even more preferably 5m or more, and especially preferably 5 to 8m. If the width of the casting drum is too small, productivity tends to decrease.

[0054] The rotational speed of the casting drum is preferably 3 to 50 m / min, particularly preferably 7 to 40 m / min, and even more preferably 10 to 35 m / min. If the rotational speed is too slow, productivity tends to decrease, and if it is too fast, drying tends to be insufficient.

[0055] The surface temperature of the cast drum is preferably 40 to 99°C, and particularly preferably 60 to 95°C. If the surface temperature is too low, drying tends to be insufficient, and if it is too high, foaming tends to occur.

[0056] <Process (C)> Step (C) is a step of heating and drying the prepared film. The film peeled from the cast drum (the prepared film) is conveyed in the flow direction (MD direction) using a nip roll or the like, and dried by alternately bringing the front and back surfaces of the film into contact with multiple heat rolls. The 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.

[0057] The surface temperature of the heat roll (hereinafter sometimes referred to as "heat roll temperature") is not particularly limited, but is usually 50 to 150°C, and more preferably 70 to 140°C. If the surface temperature is too low, drying tends to be insufficient, and if it is too high, it tends to dry too much, leading to surface defects such as waviness.

[0058] In step (C) above, it is preferable that the multiple hot roll temperatures satisfy the following conditions (c1) and (c2), and more preferably that they satisfy all of the following conditions (c1) to (c3), in order to obtain a film with an excellent balance of low elution and high stretchability. Condition (c1): The temperature of the heat rolls in contact with films with a film moisture content greater than 11% by mass is always between 50 and 90°C. Condition (c2): The film moisture content is 11% by mass or less, and the temperature of the heat roll that first contacts the film surface opposite to the surface in contact with the casting mold is 100°C or higher. Condition (c3): The temperature of any hot roll after the hot roll that under condition (c2) is set to 100°C or higher, and the film surface on the side that contacts the cast mold is brought into contact with it.

[0059] Under the above condition (c1), the heat roll temperature of the heat roll in contact with the film having a moisture content greater than 11% by mass is preferably 55 to 90°C, and particularly preferably 60 to 90°C.

[0060] In the above condition (c2), the heat roll temperature of the heat roll that first contacts the film surface opposite to the surface that contacted the cast mold when the film moisture content is 11% by mass or less is particularly preferably 102°C or higher, and more preferably 105°C or higher. An upper limit of 130°C or lower for the heat roll temperature is particularly preferable in that it suppresses the generation of film waviness.

[0061] In the above condition (c2), the "heat roll on the opposite side of the film surface that first contacts the cast mold when the film moisture content becomes 11% by mass or less" (hereinafter sometimes referred to as the "c2 heat roll") refers to any heat roll (R1) located upstream of the multiple heat rolls used in process (C), a heat roll (R2) installed adjacent to the downstream side of heat roll (R1), and a heat roll (R3) installed adjacent to the downstream side of heat roll (R2). When the film moisture content after passing through heat roll (R1) is measured, and the film moisture content for the first time becomes 11% by mass or less, if the heat roll (R2) makes contact with the film surface opposite to the side that contacted the cast mold, then heat roll (R2) corresponds to the "c2 heat roll". Furthermore, if the heat roll (R2) is in contact with the film surface that is in contact with the cast mold, and the heat roll (R3) is in contact with the film surface opposite to the surface that is in contact with the cast mold, then the heat roll (R3) corresponds to the "c2 heat roll".

[0062] Furthermore, under the conditions (c2) described above, it is preferable that the heat roll that first contacts the film after the moisture content reaches 11% by mass contacts the film surface opposite to the surface that first contacted the cast mold, thereby drying the opposite film surface.

[0063] In the above condition (c3), the heat roll temperature of the heat roll that contacts the film surface on the side in contact with the casting mold is particularly preferably 102°C or higher, and more preferably 105°C or higher. An upper limit of 130°C or lower for the heat roll temperature is particularly preferable in that it suppresses the generation of film waviness. Furthermore, in condition (c3), it is preferable that the heat roll that contacts the film surface on the side in contact with the cast mold, with a heat roll temperature of 100°C or higher, is the heat roll installed after the heat roll in condition (c2), in order to minimize the difference in drying between the front and back surfaces of the film. Furthermore, after the hot roll under condition (c3), it is preferable to further dry with a hot roll at a temperature of 50 to 95°C.

[0064] <Process (D)> Step (D) is a step in which the film obtained in step (C) is heat-treated using hot air. The film that has undergone step (C) above may be heat-treated, for example, with a floating dryer. The temperature of such heat treatment is preferably less than 100°C, particularly preferably 70 to 99°C, and even more preferably 75 to 97°C. If the heat treatment temperature is too high, excessive drying can lead to excessively high tension during stretching, making the product prone to breakage. Furthermore, the heat treatment time is preferably 20 to 100 seconds, and particularly preferably 40 to 70 seconds. If the heat treatment time is too long, excessive drying occurs, which tends to increase the tension during stretching and make the material prone to breakage.

[0065] [Film moisture content] In this invention, the moisture content of the film can be measured as follows. The mass of a sample film (before vacuum drying) taken from a drying roll and placed in the center of the width direction of the polyvinyl alcohol-based film is measured. Next, the sample film is vacuum-dried in a vacuum dryer (vacuum level: 10 mmHg or less) at 83°C for 20 minutes, and the mass of the sample film after vacuum drying is measured. The moisture content is calculated from the masses of the sample film before and after vacuum drying using the following formula. Moisture content (%) = {(Mass of film before vacuum drying) - (Mass of film after vacuum drying)} × 100 / (Mass of film before vacuum drying)

[0066] [Polyvinyl alcohol-based film] Thus, a polyvinyl alcohol-based film is obtained through the above steps (A) to (D), and is finally wound onto a roll to become the final product.

[0067] The length of the 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.

[0068] The width of the polyvinyl alcohol-based film is preferably 4 m or more, particularly preferably 5 m or more, and even more preferably 5 to 6 m, from the viewpoint of avoiding breakage during the manufacturing of the polarizing film.

[0069] The thickness of the polyvinyl alcohol-based film is preferably 5 μm or more, particularly preferably 15 μm or more, even more preferably 25 μm or more, and especially preferably 30 μm or more. Furthermore, the thickness is preferably 70 μm or less, particularly preferably 65 μm or less, and even more preferably 60 μm or less.

[0070] In polyvinyl alcohol-based films, the amount of polyvinyl alcohol-based resin eluted is preferably 60 ppm or less, and particularly preferably 50 ppm or less, from the viewpoint of avoiding contamination of polarizing film manufacturing equipment. The amount of eluted is the value measured by the method described in the examples below.

[0071] The limiting stretch ratio of the polyvinyl alcohol-based film is preferably 6.8 times or higher from the viewpoint of avoiding breakage. The limiting stretch ratio is a value measured by the method described in the examples below.

[0072] A polyvinyl alcohol-based film obtained by a manufacturing method according to one embodiment of the present invention is useful for optical applications. In particular, it is very useful as a raw material film for manufacturing polarizing films. The following describes how to manufacture a polarizing film and a polarizing plate made from this polyvinyl alcohol-based film.

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

[0074] The swelling process is performed before the dyeing process. The swelling process not only cleans dirt from the surface of the polyvinyl alcohol-based film, but also prevents uneven dyeing by swelling the film. In the swelling process, water is usually used as the treatment solution. The treatment solution may contain additives such as iodide compounds, surfactants, and alcohol, as long as the main component is water. The temperature of the swelling bath is usually around 10 to 45°C, and the immersion time in the swelling bath is usually around 0.1 to 10 minutes.

[0075] The dyeing process is carried out by contacting the film with a liquid containing iodine or a dichroic dye. Typically, an aqueous solution of iodine-potassium iodide is used, with an appropriate iodine concentration of 0.1-2 g / L and a potassium iodide concentration of 1-100 g / L. A practical dyeing time is approximately 30-500 seconds. The temperature of the treatment bath is preferably 5-50°C. In addition to water, a small amount of an organic solvent that is compatible with water may be added to the aqueous solution.

[0076] 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 may be performed during the process as needed.

[0077] The stretching process preferably involves stretching the film 3 to 10 times, more preferably 3.5 to 7 times, in a uniaxial direction. At this time, slight stretching in a direction perpendicular to the stretching direction (to the extent necessary to prevent shrinkage in the width direction, or more) 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.

[0078] 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 can be 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.

[0079] The drying process may involve drying in a dryer at 40-100°C for 0.1-10 minutes, for example.

[0080] Thus, a polarizing film is obtained, and the degree of polarization of the polarizing film is preferably 99% or higher, more preferably 99.5% or higher. If the degree of polarization is too low, the contrast in the liquid crystal display tends to decrease. Note that the degree of polarization is measured by, for example, the light transmittance (H) measured at wavelength λ when two polarizing films are generally superimposed so that their orientation directions are in the same direction. 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 (4). Polarization degree = [(H 11 -H1) / (H 11 +H1)〕 1 / 2 ...(4)

[0081] Furthermore, the transmittance of the polarizing film according to one embodiment of the present invention is preferably 43% 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 a single polarizing film is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

[0082] [Method for manufacturing polarizing plates] Next, a method for manufacturing a polarizing plate using a polarizing film according to an example of an embodiment of the present invention will be described. A polarizing film according to one embodiment of the present invention is suitable for manufacturing a polarizing plate with minimal color unevenness and excellent polarizing performance.

[0083] A polarizing plate according to one embodiment of the present invention is manufactured by laminating an optically isotropic resin film as a protective film to one or both sides of the polarizing film via an adhesive. Examples of protective films include films or sheets of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyethersulfone, polyarylene ester, poly-4-methylpentene, and polyphenylene oxide.

[0084] The bonding method is carried out by known techniques, for example, by uniformly applying a liquid adhesive composition to the polarizing film, protective film, or both, then bonding the two together, pressing them together, and irradiating them with heat or active energy rays.

[0085] Alternatively, 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.

[0086] Polarizing films and polarizing plates using polyvinyl alcohol-based films obtained by a manufacturing method according to one embodiment of the present invention have excellent polarization 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 equipment, 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 coatings for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like. [Examples]

[0087] 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 "parts" and "%" in the examples refer to mass.

[0088] <Measurement conditions> (1) Amount of polyvinyl alcohol-based resin eluted (ppm) The resulting polyvinyl alcohol-based film was conditioned at 23°C and 50% RH for 24 hours, and then rolled into 100mm x 100mm (0.01m) sheets. 2 Five test pieces were cut out, and all (total 0.05m) 2 The sample was immersed in 1 L of deionized water at 50°C for 1 minute to obtain an eluate. 10 mL of this eluate was mixed with 10 mL of a colorimetric reagent (500 g of deionized water, 7.4 g of potassium iodide, 0.65 g of iodine, and 10.6 g of boric acid) at room temperature (23°C). The absorbance at a wavelength of 690 nm was then measured using a spectrophotometer (Shimadzu UV-3100PC), and the concentration (ppm) of the polyvinyl alcohol-based resin was calculated from a pre-prepared calibration curve. (Evaluation Criteria) ○ (very good): Elution amount is 50 ppm or less. △ (good): Elution amount exceeds 50 ppm but is 60 ppm or less. × (poor): Elution amount exceeds 60 ppm

[0089] (2) Maximum extension ratio The obtained polyvinyl alcohol-based film was transported horizontally and immersed in a water bath at 30°C while being stretched 1.5 times in the flow direction (MD) relative to the original roll. Next, it was immersed for 240 seconds in a staining bath (30°C) containing 0.2 g / L iodine and 15 g / L potassium iodide while being further stretched 1.3 times. Finally, it was immersed in a boric acid treatment bath (50°C) containing 50 g / L boric acid and 30 g / L potassium iodide, and stretched until the film broke. The limiting stretch ratio was measured to evaluate the stretchability. (Evaluation Criteria) 〇(very good): Can be stretched to 6.8 times or more × (poor): Breaks at less than 6.8 times the load.

[0090] (3) Orientation distribution [Measurement of orientation distribution MDΔn-TDΔn] A small piece measuring MD × TD = 5 mm × 10 mm was cut from the center of the polyvinyl alcohol film in the width direction (TD) at an arbitrary position in the flow direction (MD) of the polyvinyl alcohol film. This piece was then sandwiched on both sides with a 100 μm thick PET film, and the whole thing was further sandwiched in a wooden frame and mounted on a microtome apparatus. Next, the previously cut segments were sliced ​​at 10 μm intervals parallel to the direction of flow (MD) of the segments to prepare observation slices (MD × TD = 5 mm × 10 μm). Next, to allow observation of the slice surface, the slice was tilted so that the slice surface was facing upwards and placed on a glass slide. It was then sealed with a coverslip and tricresil phosphate (refractive index 1.557), and retardation was measured using the two-dimensional photoelastic evaluation system "PA-micro" (manufactured by Photonic Lattice Co., Ltd.). With the retardation distribution of the slice displayed on the "PA-micro" measurement screen, a line segment X perpendicular to the surface of the original polyvinyl alcohol-based film was drawn from one surface to the other, crossing the slice. Line analysis was then performed on this line segment X to obtain retardation distribution data in the thickness direction of the slice. The observation was performed using a 40x objective lens, with a line width of 3 pixels, and the average value of the retardation was adopted. The retardation distribution data in the thickness direction of the obtained slice was divided by the slice thickness of 10 μm to obtain the birefringence distribution MDΔn in the thickness direction of the slice. Using a similar procedure, we observed a slice with MD×TD = 10 μm × 5 mm and obtained the birefringence distribution TDΔn. To correct for the difference in the number of data points due to pixel misalignment between the birefringence distribution MDΔn and the birefringence distribution TDΔn, each birefringence distribution was aligned to 1000 points using cubic spline interpolation. The difference between the interpolated MDΔn and TDΔn was taken to obtain the orientation distribution MDΔn-TDΔn. Pixel misalignment refers to the fact that even films of the same thickness will not have the same number of data points for their birefringence distributions due to subtle differences in measurement conditions when measuring MD and TD retardation distribution data.

[0091] [Measurement of ||s1, ||s2, ||c, ||s1-||c| / 2] From the orientation distribution MDΔn-TDΔn obtained above, peak top values ​​were obtained for each range within 40% of the film thickness direction, out of the total 100% of the film thickness. Comparing the obtained peak top values, the relatively larger peak top value was designated as ∥s1, and the relatively smaller peak top value as ∥s2. Furthermore, the average value ∥c was obtained by averaging the values ​​in the central 20% range of the film thickness direction out of the total 100% of the film thickness in the orientation distribution MDΔn-TDΔn obtained above. Furthermore, from the orientation distribution MDΔn-TDΔn obtained above, the thickness position of half of |∥s1-∥c| (|∥s1-∥c| / 2) was obtained when the thickness position of the film surface closer to ∥s1 is set to 0% and the thickness position of the film surface closer to ∥s2 is set to 100% of the total film thickness.

[0092] (4) Number of fractures In the process of manufacturing a polarizing film using the obtained polyvinyl alcohol-based film, the number of times the film broke within 12 hours of the start of polarizing film manufacturing was visually confirmed.

[0093] (5) Polarization degree (%) A 4cm x 4cm sample was cut from the obtained polarizing film, and the degree of polarization (%) was measured using an automated polarizing film measuring device (JASCO Corporation: VAP7070).

[0094] (6) Number of defects in the polarizing film From the obtained polarizing film, a test piece measuring 30 cm in length (stretching direction) and 20 cm in width was cut out. The number of blue foreign matter present on the surface of the polarizing film was visually observed, and the number of foreign matter defects with a longest diameter of 50 μm or more was determined. This was repeated three times, and the average number of foreign matter defects per test piece was calculated. The longest diameter of the foreign matter defects was measured using a digital microscope.

[0095] <Example 1> (Preparation of polyvinyl alcohol-based film) In a 5,000L dissolution tank, 1,000 kg of polyvinyl alcohol-based resin with a weight-average molecular weight of 142,000 and a degree of saponification of 99.8 mol%, 2,500 kg of water, 105 kg of glycerin as a plasticizer, and 0.25 kg of sodium dodecyl sulfonate as a surfactant were added. The mixture was heated to 150°C while stirring and dissolved under pressure to obtain an aqueous solution of polyvinyl alcohol-based resin with a resin concentration of 25% by mass. Next, the polyvinyl alcohol-based resin aqueous solution was supplied to a twin-screw extruder with a vent to remove air bubbles, and the aqueous solution temperature was raised to 95°C. The solution was then extruded and cast from a T-type slit die outlet onto a cast drum (surface temperature 90°C) rotating at 10 m / min to form a film. The obtained film was peeled from the cast drum and transported while alternately bringing the front and back surfaces of the film into contact with a total of 15 heat rolls. The heat roll temperature of the heat rolls (1st to 8th heat rolls) was set to 90°C to 60°C (average 75°C) until the film's moisture content reached 11% by mass. After the moisture content reached 11% by mass, the heat roll that made contact for the first time (9th heat roll) was used to dry the side of the film opposite to the side that first contacted the cast mold at 108°C. Subsequently, the side that had contacted the cast mold was dried at 108°C using the 10th heat roll, and thereafter, the 11th to 15th heat rolls were used to dry the film at 95°C to 50°C (average 80°C). Next, the film was heat-treated by blowing 95°C hot air from both sides for 50 seconds. Finally, it was slit and wound up to obtain a roll of polyvinyl alcohol-based film (film thickness 60 μm, width 5 m, length 5 km). The properties of the obtained polyvinyl alcohol-based film are shown in Table 1.

[0096] (Fabrication of polarizing films) The obtained polyvinyl alcohol-based film was unwound from the roll and transported horizontally while being immersed in a water bath at 30°C to swell, and then stretched in the flow direction (MD) to 1.7 times the original roll size. No folds or wrinkles occurred in the film during this swelling process. Next, the amount of iodine was adjusted so that the transmittance of the final polarizing film was 43.5%, and the film was immersed in an aqueous solution (30°C) containing 30 g / L potassium iodide to stain it, and then stretched in the flow direction (MD direction) to 2.7 times the original roll size. Subsequently, it was immersed in an aqueous solution (55°C) containing 40 g / L boric acid and 30 g / L potassium iodide to crosslink with boric acid, and then uniaxially stretched in the flow direction (MD direction) to 6.2 times the original roll size. Finally, the film was washed with an aqueous potassium iodide solution and dried at 70°C for 2 minutes to obtain the polarizing film. The number of fractures within 12 hours of starting polarizing film production, the polarization performance of polarizing films sampled 12 hours after starting polarizing film production, and the number of defects are shown in Table 2.

[0097] <Example 2> In Example 1, a polyvinyl alcohol-based film (width 5 m, thickness 60 μm, length 5 km) was obtained in the same manner as in Example 1, except that the side of the film opposite to the side that first contacted the cast mold was dried at 102°C using the ninth heat roll that made contact after the moisture content reached 11% by mass, and the side that contacted the cast mold was dried at 102°C using the tenth heat roll. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, a polarizing film was obtained using the obtained polyvinyl alcohol-based film in the same manner as in Example 1. The properties of the obtained polarizing film are shown in Table 2.

[0098] <Example 3> In Example 1, a polyvinyl alcohol-based film was obtained in the same manner as in Example 1, except that the film was extruded and cast from the T-type slit die discharge port onto a cast drum (surface temperature 85°C) rotating at 10 m / min, and after the moisture content reached 11% by mass, the side of the film opposite to the side that first contacted the cast mold was dried at 107°C using the first heat roll (5th heat roll), the side that contacted the cast mold was dried at 106°C using the 6th heat roll, and thereafter the film was dried at 70-50°C (average 55°C) using the 7th to 15th heat rolls, and then heat-treated by blowing hot air at 93°C from both sides of the film. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, a polarizing film was obtained using the obtained polyvinyl alcohol-based film in the same manner as in Example 1. The properties of the obtained polarizing film are shown in Table 2.

[0099] <Comparative Example 1> In Example 1, after the moisture content reached 11% by mass, the first heat roll (9th heat roll) was used to dry the side of the film opposite to the side that first contacted the cast mold at 95°C. Subsequently, the 10th heat roll was used to dry the side that had contacted the cast mold at 95°C. After that, the 11th to 15th heat rolls were used to dry the film at 95 to 50°C (average 90°C). Then, a heat treatment was performed by blowing 100°C hot air from both sides of the film. Otherwise, a polyvinyl alcohol-based film (width 5m, thickness 60μm, length 5km) was obtained in the same manner as in Example 1. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, a polarizing film was obtained using the obtained polyvinyl alcohol-based film in the same manner as in Example 1. The properties of the obtained polarizing film are shown in Table 2.

[0100] <Comparative Example 2> In Example 1, a polyvinyl alcohol-based film (width 5 m, thickness 60 μm, length 5 km) was obtained in the same manner as in Example 1, except that the side of the film opposite to the side that first contacted the cast mold was dried at 60°C using the first heat roll (9th heat roll) that came into contact with the film after the moisture content reached 11% by mass, the side that had come into contact with the cast mold was dried at 60°C using the 10th heat roll, and thereafter the film was dried at 50°C using the 11th to 15th heat rolls, and then heat treatment was performed by blowing hot air at 125°C from both sides of the film. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1. Furthermore, when a polarizing film was manufactured using the obtained polyvinyl alcohol-based film in the same manner as in Example 1, two breaks occurred within 12 hours from the start of manufacturing, so the stretching ratio was changed from 6.2 times to 6.0 times. The properties of the obtained polarizing film are shown in Table 2.

[0101] [Table 1]

[0102] In Table 1, X represents the number of the first heat roll that came into contact with the film after its moisture content reached 11% by mass. Furthermore, regarding the drying surface of the heat roll X, A represents the side opposite to the cast mold, and B represents the side on the same side as the cast mold.

[0103] [Table 2]

[0104] In Examples 1 and 2, the orientation of the surface layer and core layer in the thickness direction is within a specific range, and in Example 3, the orientation of the surface layer and core layer on one side in the thickness direction is within a specific range. As a result, the amount of elution from the film is low, the limiting stretching ratio is high, and no breakage occurs during the production of the polarizing film, resulting in a polarizing film with excellent polarization performance and a small number of defects. In contrast, the polyvinyl alcohol-based film of Comparative Example 1 had a high elution rate from the film, resulting in a polarizing film with a large number of defects. Furthermore, the polyvinyl alcohol-based film of Comparative Example 2 had a low limiting stretching ratio, leading to breakage during polarizing film production and poor productivity. Additionally, the stretching ratio was reduced to avoid breakage, resulting in a polarizing film with inferior polarization performance. Furthermore, the reason for the increased number of defects in Comparative Example 2 is not due to the amount of elution, but rather to film breakage.

[0105] While the above embodiments illustrate specific forms of the present invention, these embodiments are merely illustrative and should not be interpreted restrictively. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention. [Industrial applicability]

[0106] The polyvinyl alcohol-based film of the present invention is suitably used as a polarizing film, and this polarizing film has excellent polarizing performance and is preferably used in portable information terminals, personal computers, televisions, projectors, signage, electronic desktop calculators, electronic clocks, word processors, electronic paper, game consoles, video, cameras, photo albums, thermometers, audio equipment, liquid crystal display devices such as instruments in automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, foldable displays, rollable televisions, rollable displays, anti-reflective films for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

Claims

1. A polyvinyl alcohol-based film, In the orientation distribution MDΔn - TDΔn of the film in the thickness direction, which is the difference between the birefringence distribution MDΔn in the thickness direction of a film piece obtained by slicing the polyvinyl alcohol-based film in the MD direction and the birefringence distribution TDΔn in the thickness direction of a film piece obtained by slicing the polyvinyl alcohol-based film in the TD direction, Let ∥s1 and ∥s2 be the peak top values ​​in the 40% range at both ends of the film's thickness direction out of the total 100% of the film's thickness, and let ∥c be the average value in the central 20% range of the film's thickness direction out of the total 100% of the film's thickness, and if ∥s1 > ∥s2, A polyvinyl alcohol-based film characterized by satisfying at least one of the following formulas (1) and (2). 0.00150 ≤ ||s1 - ||c| ≤ 0.00300 ... (1) 0.00045 ≤ ||s² - ||c| ≤ 0.00090 ... (2)

2. The polyvinyl alcohol-based film according to claim 1, characterized in that the above-mentioned ||c satisfies the following formula (3). -0.00050 ≤ ||c < 0.0006 ... (3)

3. The polyvinyl alcohol-based film according to claim 1 or 2, characterized in that, when the thickness position of the film surface closer to ∥s1 is set to 0% and the thickness position of the film surface closer to ∥s2 is set to 100% of the total thickness of the film, the thickness position of the value represented by |∥s1 - ∥c| / 2 is in the range of 4% or more and less than 7%.

4. The polyvinyl alcohol-based film according to any one of claims 1 to 3, characterized in that the polyvinyl alcohol-based film has a thickness of 5 to 70 μm, a width of 4 m or more, and a length of 4 km or more.

5. A polarizing film characterized by being obtained using a polyvinyl alcohol-based film according to any one of claims 1 to 4.

6. A polarizing plate comprising a polarizing film according to claim 5 and a protective film provided on at least one side of the polarizing film.

7. A method for producing a polyvinyl alcohol-based film according to any one of claims 1 to 4, characterized in that it is produced by going through the following steps (A) to (C). Step (A): Step of preparing an aqueous solution of polyvinyl alcohol-based resin. Process (B): A process of forming a film by casting an aqueous solution of polyvinyl alcohol-based resin into a cast mold. Process (C): A process in which the formed film is heated and dried by bringing it into contact with multiple heat rolls.

8. The method for producing a polyvinyl alcohol-based film according to claim 7, characterized in that the drying using multiple hot rolls in step (C) satisfies the following conditions (c1) and (c2). Condition (c1): The temperature of the heat rolls in contact with films with a film moisture content greater than 11% by mass is always between 50 and 90°C. Condition (c2): The film moisture content is 11% by mass or less, and the temperature of the heat roll that first contacts the film surface opposite to the surface that contacted the cast mold is 100°C or higher.

9. The method for producing a polyvinyl alcohol-based film according to claim 8, characterized in that the drying using multiple hot rolls in step (C) further satisfies the following condition (c3). Condition (c3): In any of the hot rolls after the hot roll that under condition (c2) has been dried, the temperature of the hot roll is set to 100°C or higher and the film surface that is in contact with the cast mold is brought into contact with it.

10. A method for producing a polyvinyl alcohol-based film according to any one of claims 7 to 9, comprising the following step (D), wherein the heat treatment temperature in step (D) is less than 100°C. Process (D): A process in which the film obtained in process (C) is heat-treated using hot air.

11. Step (A) of preparing an aqueous solution of polyvinyl alcohol-based resin, A process (B) in which an aqueous solution of polyvinyl alcohol-based resin is cast into a mold to form a film. A step (C) in which the formed film is heated and dried by bringing it into contact with multiple heat rolls. A method for manufacturing a polyvinyl alcohol-based film, which is produced via the following steps: A method for producing a polyvinyl alcohol-based film, characterized in that the drying using multiple hot rolls in step (C) satisfies the following conditions (c1) and (c2). Condition (c1): The temperature of the heat rolls in contact with films with a film moisture content greater than 11% by mass is always between 50 and 90°C. Condition (c2): The film moisture content is 11% by mass or less, and the temperature of the heat roll that first contacts the film surface opposite to the surface that contacted the cast mold is 100°C or higher.

12. The method for producing a polyvinyl alcohol-based film according to claim 11, characterized in that the drying using multiple hot rolls in step (C) further satisfies the following condition (c3). Condition (c3): In any of the hot rolls after the hot roll that under condition (c2) has been dried, the temperature of the hot roll is set to 100°C or higher and the film surface that is in contact with the cast mold is brought into contact with it.

13. A method for producing a polyvinyl alcohol-based film according to claim 11 or 12, comprising a step (D) of heat-treating the film obtained in step (C) using hot air, wherein the heat treatment temperature in step (D) is less than 100°C.

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