Polyvinyl alcohol film and polarizing film using the same

Abstract

Even when the maximum stretching speed is high, a PVA film is provided in which it is difficult for wrinkles to form on the surface during uniaxial stretching and also in which fracture during uniaxial stretching is suppressed. A water-insoluble PVA film, wherein the crystallinity index of the first surface is set to Fd1 and Fg1 and the crystallinity index of the second surface is set to Fd2 and Fg2, and the Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (1) to (4). Fd1 ≤ 0.8 (1) Fd1 / Fg1 < 1 (2) Fd2 ≤ 0.8 (3) Fd2 / Fg2 < 1 (4) [In the above equations (1) to (4), Fd1 and Fg1 are crystallinity indices calculated using a diamond prism and a germanium prism, respectively, when FT-IR measurements by the ATR method are performed on the first surface, and Fd2 and Fg2 are crystallinity indices calculated using a diamond prism and a germanium prism, respectively, by applying the same method to the second surface.]

Description

Technology Field

[0001] The present invention relates to a polyvinyl alcohol film and a polarizing film using the same. Background Technology

[0002] Polarizers, which have light transmission and shielding functions, are fundamental components of liquid crystal displays (LCDs) along with liquid crystals, which have light switching functions. The application fields of these LCDs have also expanded from small devices such as electronic desktop calculators and wristwatches during the early stages of development to various fields recently, including notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, in-vehicle navigation systems, mobile phones, and measuring instruments used indoors and outdoors.

[0003] A polarizing plate is manufactured by laminating a protective film, such as a cellulose triacetic acid (TAC) film or a cellulose acetic acid-butyric acid (CAB) film, onto the surface of a polarizing film. Furthermore, the polarizing film is generally manufactured by dyeing a polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be referred to as "PVA") and then stretching it uniaxially, or by dyeing it while dyeing, or by dyeing it after uniaxial stretching to produce a dyed uniaxially stretched film, and then immobilizing this uniaxially stretched film with a boron compound. In addition, this immobilization treatment with a boron compound may be performed simultaneously with the uniaxial stretching or dyeing treatment.

[0004] For products equipped with large LCDs, such as liquid crystal monitors or liquid crystal televisions, high contrast and clear images are required. Consequently, high performance is required for polarizing films, specifically, increasing the polarization degree of the polarizing film. However, if the stretching ratio is increased when uniaxially stretching a PVA film to increase the polarization degree of the polarizing film, wrinkles are prone to form on the surface of the PVA film during the uniaxial stretching process. As a result, wrinkles are also prone to form on the surface of the resulting polarizing film. If many wrinkles form on the surface of the polarizing film, it is likely to cause image non-uniformity in the final product, such as a liquid crystal monitor or liquid crystal television. Furthermore, if many wrinkles form on the surface of the polarizing film, such a polarizing film cannot be used as a product, which also causes a decrease in the product yield of the polarizing film.

[0005] As a method to suppress wrinkles occurring on the surface of a polarizing film, it has been proposed to control the relaxation time or component ratio for a component with a short relaxation time (a component with low molecular mobility and rigidity) when the PVA film is measured by pulsed NMR (see Patent Document 1). Prior art literature

[0006] WO2019 / 189695 The problem to be solved

[0007] Recently, the demand for higher contrast and image clarity in LCDs has increased, and consequently, wrinkles on the surface of polarizing films, which were not previously an issue, are becoming a problem more frequently. Furthermore, to increase the production efficiency of polarizing films, it is required to perform uniaxial stretching at high speed during the stretching process when manufacturing polarizing films—that is, to set the maximum stretching speed of uniaxial stretching to high speed. However, if the maximum stretching speed during uniaxial stretching in the manufacturing of polarizing films is set to high speed, wrinkles are more likely to occur on the surface of the PVA film during uniaxial stretching. As a result, wrinkles are also more likely to occur on the surface of the resulting polarizing film. Additionally, when the maximum stretching speed is high, excessive local tension may be applied to the PVA film during uniaxial stretching. Consequently, there is also a problem in that the PVA film is prone to breakage during uniaxial stretching, leading to a decrease in the product yield of the polarizing film.

[0008] In the PVA film described in Patent Document 1, when the maximum stretching speed is set to high during uniaxial stretching to manufacture a polarizing film, wrinkles are prone to occur on the surface of the PVA film during uniaxial stretching, and there were cases where the wrinkles on the surface of the polarizing film could not be sufficiently suppressed. In addition, when the maximum stretching speed is set to high, the PVA film may break during uniaxial stretching. Furthermore, suppressing wrinkles on the surface of the PVA film when uniaxially stretching and suppressing breakage of the PVA film during uniaxial stretching are also important for suppressing wrinkles and breakage on the surface of optical films other than polarizing films.

[0009] Therefore, the present invention aims to provide a PVA film in which, even when the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film, wrinkles are unlikely to occur on the surface during uniaxial stretching, and fracture during uniaxial stretching is suppressed. means of solving the problem

[0010] As a result of repeated careful examination, the inventors discovered that the above objective could be achieved by adjusting the crystallinity index of two surfaces orthogonal to the thickness direction of the PVA film to a specific range, and based on this finding, further examination was conducted to complete the present invention.

[0011] That is, the present invention is,

[0012] [1] A water-insoluble PVA film, wherein two surfaces orthogonal to the thickness direction of the PVA film are respectively a first surface and a second surface, and the crystallinity indices of the first surface are Fd1 and Fg1, and the crystallinity indices of the second surface are Fd2 and Fg2, and the PVA film such that Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (1) to (4);

[0013] Fd1 ≤ 0.8 (1)

[0014] Fd1 / Fg1 < 1 (2)

[0015] Fd2 ≤ 0.8 (3)

[0016] Fd2 / Fg2 < 1 (4)

[0017] [In the above equations (1) to (4), Fd1 is a crystallinity index calculated using a diamond prism when performing FT-IR measurement by the ATR method on the first surface, Fg1 is a crystallinity index calculated using a germanium prism when performing FT-IR measurement by the ATR method on the first surface, Fd2 is a crystallinity index calculated using a diamond prism when performing FT-IR measurement by the ATR method on the second surface, and Fg2 is a crystallinity index calculated using a germanium prism when performing FT-IR measurement by the ATR method on the second surface.]

[0018] [2] A PVA film of [1] in which the above Fd1 and Fd2 satisfy the following equations (5) to (6);

[0019] Fd1 ≥ 0.5 (5)

[0020] Fd2 ≥ 0.5 (6)

[0021] [3] A PVA film of [1] or [2] in which the above Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (7) to (8);

[0022] Fd1 / Fg1 ≥ 0.6 (7)

[0023] Fd2 / Fg2 ≥ 0.6 (8)

[0024] [4] Any one of [1] to [3] PVA films in which Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (9) to (10);

[0025] |Fd1-Fd2| ≤ 0.07 (9)

[0026] |Fg1-Fg2| ≤ 0.07 (10)

[0027] [5] A PVA film of any one of [1] to [4] that is a film for manufacturing optical films;

[0028] [6] PVA film of [5], which is an optical film that is a polarizing film;

[0029] It is about. Effects of the invention

[0030] According to the present invention, a PVA film is provided in which, even when the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film, wrinkles are unlikely to occur on the surface during uniaxial stretching, and fracture during uniaxial stretching is suppressed. With such a PVA film, wrinkles occurring on the surface of an optical film such as a polarizing film can be suppressed. In addition, since fracture during uniaxial stretching is suppressed, an optical film such as a polarizing film can be manufactured with a high product yield. Brief explanation of the drawing

[0031] FIG. 1 is a perspective view of the PVA film of the present invention. FIG. 2 is a side view of the PVA film of the present invention. Figure 3 is a schematic diagram showing the ATR method in FT-IR measurement. Specific details for implementing the invention

[0032] The present invention will be described in detail below.

[0033] <PVA Film>

[0034] In the present invention, as shown in FIGS. 1 and 2, two surfaces orthogonal to the thickness direction (2) of the PVA film (1) are defined as the first surface (3) and the second surface (4), respectively. Accordingly, the first surface (3) and the second surface (4) of the PVA film (1) of the present invention face each other. In the present invention, FT-IR (Fourier Transform Infrared Spectroscopy) measurements according to the ATR method are performed on the first surface (3) and the second surface (4), respectively. Then, the crystallinity indices Fd1, Fg1, Fd2, and Fg2 calculated from this measurement satisfy the following equations (1) to (4).

[0035] Fd1 ≤ 0.8 (1)

[0036] Fd1 / Fg1 < 1 (2)

[0037] Fd2 ≤ 0.8 (3)

[0038] Fd2 / Fg2 < 1 (4)

[0039] Among the above equations (1) to (4), Fd1 is a crystallinity index calculated using a diamond prism when FT-IR measurement according to the ATR method is performed on the first surface (3) of the PVA film (1), and Fg1 is a crystallinity index calculated using a germanium prism when FT-IR measurement according to the ATR method is performed on the first surface (3) of the PVA film (1). Additionally, Fd2 is a crystallinity index calculated using a diamond prism when FT-IR measurement according to the ATR method is performed on the second surface (4) of the PVA film (1), and Fg2 is a crystallinity index calculated using a germanium prism when FT-IR measurement according to the ATR method is performed on the second surface (4) of the PVA film (1). In addition, in the above equation (2), Fd1 / Fg1 is the value obtained by dividing Fd1 by Fg1, and in the above equation (4), Fd2 / Fg2 is the value obtained by dividing Fd2 by Fg2.

[0040] In the PVA film of the present invention, as shown in the above formulas (1) and (3), Fd1 and Fd2 need to be 0.8 or less. If Fd1 or Fd2 exceeds 0.8, when the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film, wrinkles are likely to occur on the surface of the PVA film (1) during uniaxial stretching, and the PVA film (1) is likely to break during uniaxial stretching. Although the reason is not necessarily clear, it is presumed that if the crystallinity of the surface of the PVA film (1) is excessively high, water in the stretching treatment solution is difficult to penetrate into the interior of the PVA film (1) during uniaxial stretching, and the flexibility of the film during uniaxial stretching becomes insufficient. Fd1 and Fd2 are preferably 0.75 or less, more preferably 0.72 or less, even more preferably 0.7 or less, and particularly preferably 0.68 or less.

[0041] In the PVA film of the present invention, as shown in formulas (2) and (4) above, Fd1 / Fg1 and Fd2 / Fg2 need to be less than 1. When Fd1 / Fg1 or Fd2 / Fg2 is 1 or more, wrinkles are likely to occur on the surface of the PVA film (1) during uniaxial stretching when the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film. Fd1 / Fg1 and Fd2 / Fg2 are preferably 0.98 or less, more preferably 0.96 or less, even more preferably 0.94 or less, even more preferably 0.92 or less, and particularly preferably 0.9 or less.

[0042] In the PVA film of the present invention, as shown in formulas (1) and (3) above, Fd1 and Fd2 are 0.8 or less. Also, as shown in formulas (2) and (4) above, Fd1 / Fg1 and Fd2 / Fg2 are less than 1. As described below, Fd1 and Fd2 represent the crystallinity of the relatively deep interior of the PVA film (1), while Fg1 and Fg2 represent the crystallinity of the extreme surface layer near the surface of the PVA film (1). That is, in the PVA film of the present invention, the crystallinity of the relatively deep interior of the PVA film (1) is less than or equal to a predetermined value, and the crystallinity of the extreme surface layer near the surface of the PVA film (1) is higher than the crystallinity of the relatively deep interior of the PVA film (1). In this way, by controlling the crystallinity of the relatively deep interior and the extreme surface layer of the PVA film (1), even if the maximum stretching speed is high during uniaxial stretching when manufacturing optical films such as polarizing films, it is difficult for wrinkles to form on the surface during uniaxial stretching, and fracture during uniaxial stretching is suppressed. Although the reason is not necessarily clear, it is presumed that the high crystallinity of the extreme surface layer of the PVA film (1) suppresses the formation of wrinkles on the surface of the PVA film (1) during uniaxial stretching, and also that the low crystallinity of the relatively deep interior of the PVA film (1) relieves the stress generated during uniaxial stretching, thereby suppressing fracture.

[0043] In the PVA film of the present invention, the lower limits of Fd1 and Fd2 are not necessarily limited, but in the case where the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film, it is preferable to satisfy the following equations (5) and (6) in order to further suppress breakage when uniaxially stretching the PVA film (1).

[0044] Fd1 ≥ 0.5 (5)

[0045] Fd2 ≥ 0.5 (6)

[0046] As shown in the above equations (5) and (6), since Fd1 and Fd2 are 0.5 or higher, the degree of crystallization in the relatively deep interior of the PVA film (1) increases. As a result, the degree of crystallization in the central part of the thickness direction (2) of the PVA film (1) increases, and the mechanical strength of the PVA film (1) is improved. Therefore, by using such a PVA film (1), breakage when uniaxially stretching the PVA film (1) is further suppressed, even when the maximum stretching speed is high during uniaxial stretching when manufacturing optical films such as polarizing films. It is more preferable that Fd1 and Fd2 are 0.52 or higher, and even more preferable that they are 0.55 or higher.

[0047] In the PVA film of the present invention, the lower limit of Fd1 / Fg1 and Fd2 / Fg2 is not necessarily limited, but in the case of uniaxial stretching when manufacturing an optical film such as a polarizing film, the maximum stretching speed is high, so it is preferable to satisfy the following equations (7) and (8) in order to further suppress breakage when uniaxially stretching the PVA film (1).

[0048] Fd1 / Fg1 ≥ 0.6 (7)

[0049] Fd2 / Fg2 ≥ 0.6 (8)

[0050] As shown in the above equations (7) and (8), since Fd1 / Fg1 and Fd2 / Fg2 are 0.6 or higher, the crystallinity of the relatively deep interior of the PVA film (1) is not excessively small compared to the crystallinity of the extreme surface layer of the PVA film (1). As a result, the crystallinity of the central part in the thickness direction (2) of the PVA film (1) becomes relatively large, and the mechanical strength of the PVA film (1) is improved. Therefore, by using such a PVA film (1), breakage when uniaxially stretching the PVA film (1) is further suppressed, even when the maximum stretching speed is high during uniaxial stretching when manufacturing optical films such as polarizing films. It is more preferable that Fd1 / Fg1 or Fd2 / Fg2 be 0.65 or higher, even more preferable that it be 0.7 or higher, and particularly preferable that it be 0.75 or higher.

[0051] In the PVA film of the present invention, the absolute values ​​of the difference between Fd1 and Fd2 and the difference between Fg1 and Fg2 are not necessarily limited, but when the maximum stretching speed is high during uniaxial stretching when manufacturing an optical film such as a polarizing film, it is preferable to satisfy the following equations (9) and (10) in order to further suppress wrinkles occurring on the surface of the PVA film (1) during uniaxial stretching.

[0052] |Fd1-Fd2| ≤ 0.07 (9)

[0053] |Fg1-Fg2| ≤ 0.07 (10)

[0054] As indicated by the above equations (9) and (10), since |Fd1-Fd2| and |Fg1-Fg2| are 0.07 or less, the difference in the crystallinity index between the first surface (3) and the second surface (4) of the PVA film (1) is not excessively large, and the elastic modulus becomes nearly equal between the two surfaces (first surface (3) and second surface (4)) that are orthogonal to the thickness direction (2) of the PVA film (1). Therefore, by using such a PVA film (1), it becomes difficult for wrinkles to form on the surface of the PVA film (1) during uniaxial stretching, even if the maximum stretching speed is high during uniaxial stretching when manufacturing optical films such as polarizing films. |Fd1-Fd2| and |Fg1-Fg2| is more preferably 0.06 or less, more preferably 0.05 or less, and particularly preferably 0.04 or less.

[0055] (FT-IR measurement)

[0056] Generally, when measuring the infrared absorption spectrum (IR spectrum) of a PVA film (1), 1140 cm⁻¹ is obtained due to the PVA contained therein. -1 An absorption peak is observed. This absorption peak is generally referred to as the crystallization band of the PVA film (1) and is one of the peaks derived from the stretching vibration of the carbon bond (CC) of PVA. It is known that this crystallization band is observed to emphasize that the phase of the vibration of the PVA polymer molecular chains becomes aligned as the PVA polymer molecular chains within the PVA film (1) crystallize. In other words, as the crystallinity of the PVA film (1) increases, the peak intensity of the crystallization band becomes relatively higher. Additionally, when measuring the infrared absorption spectrum of the PVA film (1), a peak at 1425 cm⁻¹ originates from the angle-shifting vibration of the methylene (-CH₂-) main chain of PVA. -1An absorption peak is observed. The intensity of this absorption peak is believed not to depend on the crystallinity of the PVA film (1).

[0057] In the present invention, this crystallization band (1140 cm⁻¹) -1 The absorption peak intensity of ) and the angular vibration of methylene (-CH2-), the main chain of PVA (1425 cm⁻¹) -1 By calculating the intensity ratio with the absorption peak intensity of ), the crystallinity index (Fg1, Fg2, Fd1 and Fd2) of the PVA film (1) can be obtained. Specifically, 1140 cm -1 and 1425 cm -1 Draw the baseline of the infrared absorption spectrum at, and 1140 cm from the baseline. -1 and 1425 cm -1 With the height to the peak top as the respective absorption peak intensity, 1140 cm -1 The absorption peak intensity of is 1425 cm⁻¹ -1 The value divided by the peak intensity was designated as the crystallinity index (Fg1, Fg2, Fd1 and Fd2).

[0058] It is well known that the values ​​of the crystallinity indices (Fg1, Fg2, Fd1 and Fd2) obtained in this way are proportional to the crystallinity of the PVA film (1) (e.g., NAPeppas, Macromol. Chem., Vol. 178, No. 595 (1977), Japanese Patent Publication No. Hei 6-138321). Since the values ​​of these crystallinity indices vary somewhat depending on the amount of moisture absorbed by the PVA film (1), in the present invention, the PVA film (1) was stored for 24 hours in an environment of a temperature of 24.0 ℃ and a relative humidity of 45.0 % RH, and then FT-IR measurements were performed in the same environment.

[0059] In the present invention, FT-IR measurement is performed by the ATR method (total reflection absorption measurement method). As shown in FIG. 3, the ATR method is a type of reflective IR measurement method in which a sample is placed in close contact with an objective lens called an ATR prism (7), and infrared rays (8) are irradiated obliquely from within the ATR prism (7) onto the sample to measure the spectrum of the reflected light. Compared to a conventional reflective IR measurement method, it has the characteristic of obtaining a sharp spectrum with less noise. In this measurement method, when a PVA film (1) is used as a sample, the infrared rays (8) are not only reflected from the surface of the PVA film (1), but also infrared rays (8) that penetrate slightly from the ATR prism (7) side toward the PVA film (1) side are reflected. Therefore, according to the FT-IR measurement by the ATR method, it is possible to obtain information about the surface layer of the PVA film (1) (a part that has penetrated slightly in the depth direction from the surface of the PVA film (1). Here, if the depth of penetration of infrared (8) that penetrates from the ATR prism (7) side to the PVA film (1) side is d, the value is expressed by the following equation (11). As is evident from the following equation (11), if an ATR prism (7) with a different refractive index is used, it is possible to obtain a reflective infrared absorption spectrum with a different depth of penetration.

[0060] d = λ / 2Πn1×1 / {sin 2 θ-(n2 / n1) 2} 0.5 (11)

[0061] In the above equation (11), n1 represents the refractive index of the ATR prism (7), n2 represents the refractive index of the PVA film (1), λ represents the wavelength of the infrared (8), and θ represents the angle of incidence of the infrared (8).

[0062] In the present invention, as shown in FIG. 3, diamond with a refractive index of 2.4 or germanium with a refractive index of 4.0 is used as the substrate of the ATR prism (7). Since the refractive index of the PVA film (1) is 1.5, in the above equation (11), the angle of incidence of the infrared (8) is 45° and the wavenumber of the infrared (8) is 1140 cm -1 When calculating the penetration depth of infrared (8) into the surface layer of the PVA film (1) in this case, the penetration depth (5) of infrared (8) when diamond is used as the substrate of the ATR prism (7), i.e., when a diamond prism is used, is about 2 μm. On the other hand, when germanium is used as the ATR prism (7), i.e., when a germanium prism is used, the penetration depth (6) of infrared (8) is about 0.5 μm. Therefore, the crystallinity index when a diamond prism is used corresponds to the crystallinity up to the relatively deep interior of the PVA film (1). On the other hand, the crystallinity index when a germanium prism is used corresponds to the crystallinity of the extreme surface layer near the surface of the PVA film (1).

[0063] In the present invention, it is important to control the crystallinity indices Fg1 and Fg2 of the extreme surface layer of the PVA film (1) and the crystallinity indices Fd1 and Fd2 of the relatively deep interior of the PVA film (1) to the above range. Since the crystal structure of the PVA film (1) is affected by various factors in the composition or manufacturing process of the PVA film (1), methods for controlling the crystallinity indices (Fg1, Fg2, Fd1 and Fd2) may include, for example, adjusting the type of polyvinyl alcohol (degree of saponification, amount of modification, blend ratio of unmodified PVA / modified PVA, etc.), adjusting the amount of plasticizer added, adjusting the film manufacturing conditions (surface temperature of the roll support, heat treatment conditions, etc.), or adjusting a combination of these.

[0064] A method for adjusting the crystallinity indices Fd1 and Fd2 to 0.8 or less and Fd1 / Fg1 and Fd2 / Fg2 to less than 1 can be, more specifically, a method in which the degree of saponification of PVA is 90 mol% or more, the ratio of structural units derived from other monomers in the vinyl ester-based polymer that is the raw material of PVA is 15 mol% or less based on the total number of moles of structural units constituting the vinyl ester-based polymer, and the degree of polymerization of PVA is 200 to 8000. At this time, it is preferable that the amount of plasticizer added is 1 to 40 parts by mass per 100 parts by mass of PVA. In addition, at this time, it is preferable that the volatile fraction of the film-forming liquid be 50 to 90 mass%, the surface temperature of the support material that supports the film-forming liquid be 65 to 110 ℃, the temperature of the hot air sprayed on the non-contact side be 50 to 150 ℃ or lower, and the humidity of the hot air be 20 to 90 % RH. In addition, at this time, it is preferable that the temperature of the drying oven or the surface temperature of the drying roll be 45 to 110 ℃, and the surface temperature of the heat treatment roll be 60 to 135 ℃.

[0065] Methods for adjusting the crystallinity indices Fd1 and Fd2 to 0.5 or higher include setting the degree of saponification of PVA to 95 to 99.9 mol%, setting the ratio of structural units derived from other monomers in the vinyl ester-based polymer that is the raw material for PVA to 10 mol% or less based on the total number of moles of structural units constituting the vinyl ester-based polymer, and setting the degree of polymerization of PVA to 1000 to 4000. At this time, it is preferable to add 5 to 20 parts by mass per 100 parts by mass of PVA. In addition, at this time, it is preferable that the volatile fraction of the film-forming liquid be 60 to 80 mass%, the surface temperature of the support material that supports the film-forming liquid be 80 to 110 ℃, the temperature of the hot air sprayed on the non-contact side be 70 to 110 ℃ or lower, and the humidity of the hot air be 1 to 40 % RH. In addition, at this time, it is preferable that the temperature of the drying oven or the surface temperature of the drying roll be 60 to 110 ℃, and the surface temperature of the heat treatment roll be 80 to 135 ℃.

[0066] As a method for adjusting Fd1 / Fg1 and Fd2 / Fg2 to 0.6 or higher, the degree of saponification of PVA is set to 99 to 99.9 mol%, the ratio of structural units derived from other monomers in the vinyl ester-based polymer that is the raw material for PVA is set to 5 mol% or less based on the total number of moles of structural units constituting the vinyl ester-based polymer, and the degree of polymerization of PVA is set to 1000 to 3700. At this time, it is preferable that the amount of plasticizer added is 8 to 20 parts by mass per 100 parts by mass of PVA. In addition, at this time, it is preferable that the volatile fraction of the film-forming liquid be 65 to 80 mass%, the surface temperature of the support material that supports the film-forming liquid be 80 to 100 ℃, the temperature of the hot air sprayed on the non-contact side be 70 to 100 ℃, and the humidity of the hot air be 3 to 40 % RH. In addition, at this time, it is preferable that the temperature of the drying oven or the surface temperature of the drying roll be 60 to 100 ℃, and the surface temperature of the heat treatment roll be 80 to 120 ℃.

[0067] As a method for adjusting |Fd1-Fd2| and |Fg1-Fg2| to 0.07 or less, it is preferable to set the volatile fraction of the film-forming liquid to 65 to 75 mass%, the surface temperature of the support material that supports the film-forming liquid to 80 to 95 ℃, the temperature of the hot air sprayed on the non-contact side to 75 to 90 ℃, and the humidity of the hot air to 5 to 40 RH. In addition, at this time, the temperature of the drying oven or the surface temperature of the drying roll is preferably 60 to 90 ℃, and the surface temperature of the heat treatment roll is preferably 80 to 110 ℃.

[0068] (PVA)

[0069] In the PVA film of the present invention, as the PVA, a polymer prepared by saponifying a vinyl ester-based polymer obtained by polymerizing a vinyl ester-based monomer may be used. Examples of vinyl ester-based monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, etc. Among these, vinyl acetate is preferred as the vinyl ester-based monomer.

[0070] A vinyl ester-based polymer is preferably a polymer obtained using only one or two or more vinyl ester-based monomers as monomers, and more preferably a polymer obtained using only one vinyl ester-based monomer as monomers. Additionally, the vinyl ester-based polymer may be a copolymer of one or two or more vinyl ester-based monomers and another monomer capable of copolymerizing with it.

[0071] Other monomers include, for example, ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; Acrylamide derivatives such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidepropanesulfonic acid or its salt, acrylamidepropyldimethylamine or its salt, N-methylolacrylamide or its derivatives; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid or its salt, methacrylamidepropyldimethylamine or its salt, N-methylolmethacrylamide or its derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; Examples include vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanide such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or its salts, esters, or acid anhydrides; itaconic acid or its salts, esters, or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; and isopropyl acetate. Additionally, vinyl ester-based polymers may have structural units derived from one or more of these other monomers.

[0072] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol% or less, and more preferably 8 mol% or less, based on the total number of moles of structural units constituting the vinyl ester polymer. In general, the crystallization of PVA tends to become more difficult as the proportion of structural units derived from other monomers in the vinyl ester polymer increases. Therefore, by appropriately copolymerizing these other monomers in the above proportions, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted.

[0073] The degree of polymerization of PVA is preferably 200 or higher, more preferably 300 or higher, and even more preferably 500 or higher. By keeping the degree of polymerization of PVA above the above lower limit, it is possible to secure the mechanical strength of the obtained PVA film while preventing excessive crystallization of PVA. Meanwhile, the degree of polymerization of PVA is preferably 8,000 or lower, more preferably 6,000 or lower, and even more preferably 4,000 or lower. In general, crystallization of PVA tends to be difficult to proceed as the degree of polymerization of PVA increases. Therefore, by keeping the degree of polymerization of PVA below the above upper limit, crystallization of PVA can be appropriately carried out, and the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted. In addition, by keeping the degree of polymerization of PVA below the upper limit mentioned above, the viscosity of the film-making solution for the PVA film does not become excessively high, and the productivity of the PVA film can be increased.

[0074] The degree of polymerization of PVA refers to the average degree of polymerization measured in accordance with the specifications of JIS K 6726-1994. That is, the degree of polymerization (Po) is calculated by the following formula (12).

[0075] Degree of polymerization Po = ([η]×10 4 / 8.29) (1 / 0.62) (12)

[0076] In the above formula (12), η is the intrinsic viscosity (deciliter / g) measured in water at 30°C after re-saponifying and purifying PVA.

[0077] The degree of saponification of PVA is preferably 90 mol% or higher, more preferably 95 mol% or higher, even more preferably 99 mol% or higher, and particularly preferably 99.8 mol% or higher. In general, crystallization of PVA tends to proceed more easily as the degree of saponification of PVA increases. Therefore, by making the degree of saponification of PVA above the above lower limit, crystallization of PVA can be appropriately promoted, thereby increasing the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film. That is, by using PVA with a high degree of saponification in the film-forming solution for the PVA film, the crystallinity tends to increase in the extreme surface layer near the surface of the PVA film, which is susceptible to heat, and in the relatively deep interior of the PVA film during the heat treatment process of the PVA film after drying.

[0078] The degree of saponification of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of vinyl alcohol units and structural units (typically vinyl ester monomer units) that can be converted into vinyl alcohol units by saponification. The degree of saponification of PVA can be measured in accordance with the description in JIS K 6726-1994.

[0079] The PVA film of the present invention may contain one type of PVA alone, or may contain two or more types of PVA that differ from each other in degree of polymerization, degree of saponification, degree of modification, etc.

[0080] The content of PVA in the PVA film of the present invention is not necessarily limited, but it is preferably 50 mass% or more, more preferably 80 mass% or more, and even more preferably 85 mass% or more.

[0081] (Plasticizer)

[0082] The PVA film of the present invention preferably contains a plasticizer. By containing a plasticizer, the PVA film can be imparted flexibility equivalent to that of other plastic films, thereby suppressing the breaking of the PVA film during the film formation or stretching process.

[0083] Examples of plasticizers include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. These plasticizers may be used individually or in combination of two or more types. Among these, ethylene glycol or glycerin is preferred as a plasticizer, and glycerin is more preferred, for reasons such as difficulty in bleeding out on the surface of the PVA film.

[0084] In the PVA film of the present invention, the content of the plasticizer is preferably 1 part by mass or more per 100 parts by mass of PVA, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more. Meanwhile, the content of the plasticizer is preferably 40 parts by mass or less per 100 parts by mass of PVA, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less. If the content of the plasticizer is within the above range, the crystallinity index (Fg1, Fg2, Fd1 and Fd2) of the PVA film can be easily adjusted, and in addition, sufficient improvement effects on mechanical properties such as impact strength can be obtained. Furthermore, it is possible to prevent the PVA film from becoming excessively flexible, which reduces handling properties, or the plasticizer from bleeding out onto the surface of the PVA film.

[0085] Here, the reason why the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of a PVA film can be adjusted by adjusting the content of the plasticizer is as follows. Generally, when a suitable amount of plasticizer is included in a PVA film, the crystallization of PVA proceeds. This is presumed to be because the polymer molecular chains of PVA become more mobile due to the plasticizer, making it easier to adopt a more energetically stable crystalline or confined amorphous structure. On the other hand, if the PVA film contains an excess amount of plasticizer, the crystallization of PVA tends to be inhibited. This is presumed to be because the amount of plasticizer interacting with the hydroxyl groups of the polymer molecular chains of PVA increases, and the interactions between the polymer molecular chains of PVA weaken. Therefore, by adjusting the content of the plasticizer, the crystallization of PVA can proceed appropriately, thereby allowing the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film to be adjusted.

[0086] (Surfactant)

[0087] The PVA film of the present invention preferably contains a surfactant. By containing a surfactant, the handling properties of the PVA film and the peelability of the PVA film from the film-forming device during manufacturing can be improved. The surfactant is not particularly limited, and, for example, anionic surfactants and nonionic surfactants are preferably used.

[0088] Examples of anionic surfactants include carboxylic acid type surfactants such as potassium laurate; sulfate ester type surfactants such as octyl sulfate; and sulfonic acid type surfactants such as dodecylbenzene sulfonate.

[0089] Examples of nonionic surfactants include, for instance, alkyl ether type surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether type surfactants such as polyoxyethylene octylphenyl ether; alkyl ester type surfactants such as polyoxyethylene laurate; alkylamine type surfactants such as polyoxyethylene lauryl amino ether; alkylamide type surfactants such as polyoxyethylene lauric acid amide; polypropylene glycol ether type surfactants such as polyoxyethylene polyoxypropylene ether; alkanolamide type surfactants such as lauric acid diethanolamide and oleic acid diethanolamide; and allylphenyl ether type surfactants such as polyoxyalkylene allylphenyl ether.

[0090] One of these surfactants may be used alone, or two or more may be used in combination. As for the surfactant, a nonionic surfactant is preferred because it has an excellent effect of reducing surface defects during the formation of a PVA film, an alkanolamide type surfactant is more preferred, and a dialkanolamide of an aliphatic carboxylic acid (e.g., a saturated or unsaturated aliphatic carboxylic acid having 8 to 30 carbon atoms, etc.) (e.g., diethanolamide, etc.) is even more preferred.

[0091] In the PVA film of the present invention, the surfactant content is preferably 0.01 parts by mass or more per 100 parts by mass of PVA, more preferably 0.02 parts by mass or more, and even more preferably 0.05 parts by mass or more. Meanwhile, the surfactant content is preferably 10 parts by mass or less per 100 parts by mass of PVA, more preferably 1 part by mass or less, even more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less. If the surfactant content is within the above range, the peelability of the PVA film from the film-forming device during manufacturing is improved, and the occurrence of adhesion between PVA films (hereinafter referred to as "blocking") can be prevented. In addition, it is possible to prevent the surfactant from bleeding out onto the surface of the PVA film or the appearance of the PVA film from deteriorating due to the aggregation of the surfactant.

[0092] (Other ingredients)

[0093] The PVA film of the present invention may contain, in addition to PVA, components such as water-soluble polymers, moisture, antioxidants, UV absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, antifungal agents, and other polymer compounds, within a range that does not interfere with the effects of the present invention. The ratio of the total mass of PVA, surfactants, plasticizers, and other components other than PVA to the total mass of the PVA film is preferably 60 mass% or more, more preferably 80 mass% or more, and even more preferably 90 mass% or more. The ratio of the total mass of other components to the total mass of the PVA film is preferably 100 mass% or less.

[0094] (Physical properties)

[0095] The PVA film of the present invention is water-insoluble. Since the PVA film is water-insoluble, when uniaxial stretching is performed in an aqueous solution during the manufacture of optical films such as polarizing films, the PVA film can be stretched without breaking during uniaxial stretching, even if the maximum stretching speed is high. Here, in the present invention, water-insoluble refers to the following: <1> ~ <4> This refers to the case where, when a PVA film is immersed in water (deionized water) at 30°C in the order of [deionized water], the PVA film is not completely dissolved and some remains undissolved.

[0096] <1> The PVA film is placed in a constant temperature and humidity chamber adjusted to 20°C and 65% RH for at least 16 hours to adjust the humidity.

[0097] <2> After cutting out a rectangular sample with a length of 40 mm × a width of 35 mm from a humidity-adjusted PVA film, insert and fix it between two 50 mm × 50 mm plastic plates with a rectangular window (hole) with a length of 35 mm × a width of 23 mm, so that the length direction of the sample is parallel to the length direction of the window and the sample is positioned almost in the center of the width direction of the window.

[0098] <3> 300 mL of deionized water is placed in a 500 mL beaker, and the water temperature is adjusted to 30 ℃ while stirring with a magnetic stirrer equipped with a 3 cm long bar at a rotation speed of 280 rpm.

[0099] <4> The above <2> In this case, the sample fixed to the plastic plate is immersed in deionized water in a beaker for 1000 seconds, taking care not to let it come into contact with the bar of a rotating magnetic stirrer.

[0100] <PVA 필름의 제조 방법>

[0101] The method for manufacturing a PVA film according to the present invention is not particularly limited and, for example, any of the following methods may be employed. Such methods include a method of forming a film using a film-forming stock solution homogenized by adding a solvent, additives, etc. to PVA, by a flexible film-forming method, a wet film-forming method (a method of extruding into a solvent-free solution), a dry-wet film-forming method, a gel film-forming method (a method of cooling and gelling the film-forming stock solution first, and then extracting and removing the solvent), or a combination thereof; or a melt extrusion film-forming method or an inflation molding method in which a film-forming stock solution obtained using an extruder, etc. is extruded from a T-die, etc. Among these, the flexible film-forming method and the melt extrusion film-forming method are preferred as methods for manufacturing a PVA film. By using these methods, a homogeneous PVA film can be obtained with good productivity. Below, a case in which a PVA film is manufactured using the flexible film-forming method or the melt extrusion film-forming method will be described.

[0102] When manufacturing the PVA film of the present invention using a flexible film-making method or a melt extrusion film-making method, first, a film-making solution containing PVA, a solvent, and additives such as a plasticizer as needed is prepared. Next, this film-making solution is flexible (supplied) into a film shape onto a rotating support such as a metal roll or a metal belt. By doing so, a liquid film of the film-making solution is formed on the support. The liquid film is heated on the support to remove the solvent, thereby solidifying and forming a film. Examples of methods for heating the liquid film include raising the support itself to a high temperature using a heat transfer medium, or spraying hot air onto the surface opposite to the side of the liquid film in contact with the support. The solidified long film (PVA film) is peeled off from the support, dried as needed by a drying roll, a drying oven, etc., further heat-treated as needed, and wound onto a roll.

[0103] In the drying process (solvent removal process) of the liquid film flexible on a support and the subsequent drying process of the PVA film, crystallization of the PVA proceeds while it is heated. At this time, the rate of crystallization is influenced by the moisture content in the PVA, temperature, and draw (tensile elongation in the flow direction), in addition to the ratio of structural units derived from other monomers in the PVA, the degree of polymerization of the PVA, the degree of saponification of the PVA, and the content of the plasticizer. Draw is presumed to be due to the influence of orientation crystallization caused by the tension of the polymer molecular chains of the PVA.

[0104] Typically, the drying of a PVA film proceeds as volatile components evaporate from the freed film surface that is not in contact with a support or drying roll. Therefore, during the drying process, a concentration distribution of volatile components, such as moisture, occurs in the thickness direction of the PVA film, and thus, a distribution of the crystallinity index occurs in the thickness direction of the PVA film depending on the temperature and draw conditions at that time. This distribution of the crystallinity index can be adjusted by the volatile fraction of the film-making solution, the temperature of the support, the contact time with the support, the temperature and amount of hot air, and the temperatures of the drying roll and drying oven. Therefore, by appropriately adjusting each of the above factors, the crystallization of PVA can be appropriately carried out, and the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted.

[0105] The volatile fraction of the film-forming solution (the concentration of volatile components, such as solvents, that are removed by volatilization or evaporation during film formation, etc.) is preferably 50 mass% or more, and more preferably 55 mass% or more. The volatile fraction of the film-forming solution is preferably 90 mass% or less, and more preferably 80 mass% or less. If the volatile fraction is within the above range, the viscosity of the film-forming solution can be adjusted to a desirable range, thereby improving the film-forming properties of the liquid film formed on the support and making it easier to obtain a PVA film with a uniform thickness. In addition, if the volatile fraction is within the above range, the crystallization of PVA on the support proceeds appropriately, making it easier to adjust the crystallinity index and distribution of the obtained PVA film. The film-forming solution may contain a dichroic dye as needed. Also, the volatile fraction of the film-forming solution refers to the value obtained by the following formula (13).

[0106] Volatility fraction of film-forming solution (mass%) = {(Wa-Wb) / Wa}×100 (13)

[0107] In the above formula (13), Wa represents the mass (g) of the film-making stock solution, and Wb represents the mass (g) of the film-making stock solution of Wa (g) after drying it in an electric dryer at 105°C for 16 hours.

[0108] Methods for adjusting the film-making solution are not particularly limited and include, for example, a method of dissolving PVA and additives such as plasticizers and surfactants in a solvent in a dissolution tank, or a method of melt-kneading PVA in a hydrated state together with additives such as plasticizers and surfactants using a single-screw or twin-screw extruder.

[0109] The film-forming solution generally passes through the die lip of a die, such as a T-die, and is formed into a film shape on a support such as a metal roll or a metal belt. On the support, the solvent volatilizes from the side of the formed film solution that is not in contact with the support (hereinafter referred to as the free side), while substantially not volatilizing from the side in contact with the support (hereinafter referred to as the touch side). Consequently, a distribution is created in which the solvent concentration is lower on the free side and higher on the touch side in the thickness direction of the film. Therefore, the solidification of PVA proceeds first from the free side.

[0110] Crystallization of PVA proceeds in parallel with the solidification of PVA. Crystallization of PVA is difficult to proceed if the solvent concentration is excessively high or excessively low, and although it varies depending on the primary structure of the PVA molecule, it proceeds easily when the volatile fraction of the flexible film-making solution is in the range of 20 to 60 mass%. In addition, the rate of crystallization of PVA increases with increasing temperature, but the rate of solvent volatilization also increases with increasing temperature. Therefore, in order to efficiently promote crystallization of the extreme surface layer near the surface of the PVA film and to control the crystallinity index (Fg1 and Fg2) of the extreme surface layer, it is important to control the temperature of the atmosphere near the free surface and the vapor pressure of the solvent, in addition to the temperature of the support and the contact time with the support.

[0111] The PVA film of the present invention is a film in which the crystallinity of the extreme surface layer near the surface of the film is high compared to the crystallinity of the relatively deep interior of the film. Therefore, to obtain the PVA film of the present invention, conditions are selected that allow crystallization of the extreme surface layer near the surface of the film to proceed while suppressing crystallization of the relatively deep interior of the film. For example, during the initial drying stage when the volatile fraction of the extreme surface layer near the surface of the film decreases, conditions for slow drying, such as lowering the drying temperature, are adopted to increase the moisture content of the extreme surface layer during the process of crystallization. On the other hand, examples include adopting conditions that make it difficult for internal crystallization to proceed by drying rapidly at a relatively high temperature during the middle to late drying stages when crystallization proceeds in the relatively deep interior of the film.

[0112] The surface temperature of the support that softens the film-forming solution is preferably 65°C or higher, and more preferably 70°C or higher. The surface temperature of the support that softens the film-forming solution is preferably 110°C or lower, more preferably 100°C or lower, and more preferably 95°C or lower. When the surface temperature is within the above range, the drying of the liquid film softened on the support and the crystallization of the extreme surface layer near the surface of the film proceed at an appropriate rate, thereby allowing the crystallinity index (Fg1 and Fg2) of the PVA film to be adjusted.

[0113] While heating the liquid film on the support, hot air with a wind speed of 1 to 10 m / sec may be uniformly sprayed over the entire area on the non-contact side of the liquid film. The temperature of the hot air sprayed on the non-contact side is preferably 50°C or higher, and more preferably 70°C or higher. The temperature of the hot air sprayed on the non-contact side is preferably 150°C or lower, and more preferably 120°C or lower. In addition, the humidity of the hot air is preferably 1% RH or higher, more preferably 3% RH or higher, and even more preferably 5% RH or higher. The humidity of the hot air is preferably 40% RH or lower, and more preferably 30% RH or lower. If the temperature and humidity of the hot air sprayed on the non-contact side are within the above range, it becomes easier to adjust the crystallinity index (Fg1, Fg2, Fd1 and Fd2) of the PVA film.

[0114] The PVA film is preferably dried (solvent removed) on a support until the volatile fraction is 5 to 50 mass%, then peeled off from the support, and further dried as necessary. The drying method is not particularly limited and may include passing through a drying oven or contacting it with drying rolls. When drying the PVA film using multiple drying rolls, it is preferable to alternately contact one surface of the PVA film with the other surface of the drying rolls. This allows for adjusting the difference in the crystallinity index of PVA (|Fd1-Fd2| and |Fg1-Fg2|) on both sides of the PVA film (two surfaces orthogonal to the thickness direction). In this case, the number of drying rolls is preferably 3 or more, more preferably 4 or more, and even more preferably 5 to 30.

[0115] The upper limit of the temperature of the drying oven or the surface temperature of the drying roll is preferably 110°C or lower, more preferably 100°C or lower, even more preferably 90°C or lower, and particularly preferably 85°C or lower. Meanwhile, the lower limit of the temperature of the drying oven or the surface temperature of the drying roll is preferably 40°C or higher, more preferably 45°C or higher, and even more preferably 50°C or higher. By keeping the temperature of the drying oven or the surface temperature of the drying roll within the above range, it becomes easier to adjust the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film.

[0116] After drying, additional heat treatment may be performed on the PVA film as needed. By performing heat treatment, the crystallinity of the extreme surface layer near the surface of the film and the crystallinity of the relatively deep interior of the film can be increased, thereby allowing adjustment of the crystallinity index (Fg1, Fg2, Fd1 and Fd2) of the PVA film. In addition, properties such as mechanical strength and swelling properties of the PVA film can also be adjusted.

[0117] The lower limit of the surface temperature of the heat treatment roll for performing heat treatment is preferably 60°C or higher. The upper limit of the surface temperature of the heat treatment roll is preferably 135°C or lower, and more preferably 130°C or lower. By keeping the surface temperature of the heat treatment roll within the above range, it becomes easier to adjust the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film.

[0118] The PVA film manufactured in this manner is further subjected to moisture control treatment, cutting of both ends (edges) of the film as needed, then wound into a roll on a cylindrical core and packaged in a moisture-proof manner to become a product.

[0119] The volatile fraction of the PVA film finally obtained by a series of treatments is not necessarily limited. It is preferable that the volatile fraction of the PVA film be 1 mass% or more, and more preferable that it be 2 mass% or more. It is preferable that the volatile fraction of the PVA film be 5 mass% or less, and more preferable that it be 4 mass% or less.

[0120] Method for manufacturing optical film

[0121] The PVA film of the present invention is used as a base film when manufacturing an optical film. Examples of optical films include polarizing films, viewing angle enhancement films, phase difference films, brightness enhancement films, etc., but a polarizing film is preferred. Below, as an example of a method for manufacturing an optical film, a method for manufacturing a polarizing film is described in detail.

[0122] A polarizing film can typically be manufactured by using a PVA film as a base film and undergoing processing steps such as a swelling process, a dyeing process, a crosslinking process, a stretching process, and a fixing process. Specific examples of the processing solutions used in each process include a swelling treatment solution used for swelling treatment, a dyeing treatment solution (dyeing solution) used for dyeing treatment, a crosslinking treatment solution used for crosslinking treatment, a stretching treatment solution used for stretching treatment, a fixing treatment solution used for fixing treatment, and a cleaning treatment solution (cleaning solution) used for cleaning treatment.

[0123] Each processing step that can be employed in a manufacturing method for manufacturing a polarizing film is described in detail below. In addition, in a manufacturing method for a polarizing film, one or two or more of the following processing steps may be omitted, the same processing may be performed multiple times, or separate processing steps may be performed simultaneously.

[0124] (Cleaning treatment before swelling treatment)

[0125] It is preferable to perform a cleaning treatment on the PVA film before performing a swelling treatment on the PVA film. By performing this cleaning treatment prior to the swelling treatment, anti-blocking agents and the like attached to the PVA film can be removed, thereby preventing the various treatment solutions in the manufacturing process of the polarizing film from being contaminated by anti-blocking agents and the like. The cleaning treatment is preferably performed by immersing the PVA film in the cleaning solution, but it may also be performed by spraying the cleaning solution onto the PVA film. For example, water can be used as the cleaning solution. The temperature of the cleaning solution is preferably within the range of 20 to 40°C. If the temperature is 20°C or higher, it becomes easier to remove anti-blocking agents and the like attached to the PVA film. In addition, if the temperature is 40°C or lower, it is possible to prevent a portion of the surface of the PVA film from dissolving and causing the films to stick together, thereby preventing a decrease in handling performance. The temperature of the cleaning solution is more preferably 22°C or higher, more preferably 24°C or higher, and particularly preferably 26°C or higher. In addition, the temperature of the cleaning solution is more preferably 38°C or lower, more preferably 36°C or lower, and particularly preferably 34°C or lower.

[0126] (Swelling treatment)

[0127] Swelling treatment can be performed by immersing a PVA film in a swelling treatment solution such as water. The temperature of the swelling treatment solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 24°C or higher. The temperature of the swelling treatment solution is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 36°C or lower. In addition, the immersion time in the swelling treatment solution is preferably, for example, 0.1 minutes or longer, and more preferably 0.5 minutes or longer. The immersion time in the swelling treatment solution is preferably, for example, 5 minutes or less, and more preferably 3 minutes or less. Furthermore, the water used as the swelling treatment solution is not limited to pure water; it may be an aqueous solution in which various components, such as boron-containing compounds, are dissolved, or a mixture of water and an aqueous medium. The type of boron-containing compound is not particularly limited, but boric acid or borax is preferred from the perspective of handling ease. When the swelling treatment solution contains a boron-containing compound, the concentration is preferably 6 mass% or less from the perspective of improving the stretchability of the PVA film.

[0128] (Dyeing process)

[0129] It is preferable to perform the dyeing treatment using an iodine-based dye as a dichroic pigment, and the dyeing can be performed at any stage before, during, or after the stretching treatment.

[0130] The dyeing treatment is preferably carried out by using a solution containing iodine and potassium iodide (preferably an aqueous solution) as the dyeing solution and immersing a PVA film in the dyeing solution. The concentration of iodine in the dyeing solution is preferably 0.005 mass% or more. The concentration of iodine in the dyeing solution is preferably 0.2 mass% or less. The potassium iodide / iodine (mass) ratio is preferably 20 or more. The potassium iodide / iodine (mass) ratio is preferably 100 or less. The temperature of the dyeing solution is preferably 20°C or higher, and more preferably 25°C or higher. The temperature of the dyeing solution is preferably 50°C or lower, and more preferably 40°C or lower. The dyeing solution may contain a boron-containing compound, such as boric acid, as a crosslinking agent. In addition, if a dichroic dye is contained in advance in the PVA film used as a base film, the dyeing treatment can be omitted. Also, a boron-containing compound such as boric acid or borax may be contained in advance in the PVA film used as a base film.

[0131] (Cross-linking treatment)

[0132] In the manufacture of polarizing films, it is desirable to perform a crosslinking treatment after dyeing treatment for purposes such as strengthening the adsorption of dichroic pigments onto the PVA film. The crosslinking treatment can be performed by immersing the PVA film in a crosslinking treatment solution containing a crosslinking agent (preferably an aqueous solution) as the crosslinking treatment solution. As the crosslinking agent, one or more types of boron-containing compounds, such as boric acid and borax, may be used. If the concentration of the crosslinking agent in the crosslinking treatment solution is excessively high, the crosslinking reaction proceeds excessively, making it difficult to perform sufficient stretching in the subsequent stretching treatment; conversely, if it is excessively low, the effect of the crosslinking treatment tends to be reduced. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 1 mass% or more, more preferably 1.5 mass% or more, and even more preferably 2 mass% or more. The concentration of the crosslinking agent in the crosslinking treatment solution is preferably 6 mass% or less, more preferably 5.5 mass% or less, and even more preferably 5 mass% or less.

[0133] To suppress the leaching of dichroic pigments from the PVA film after dyeing treatment, the crosslinking solution may contain an iodine-containing compound, such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking solution is excessively high, the heat resistance of the resulting polarizing film tends to decrease, although the reason is unclear. Furthermore, if the concentration of the iodine-containing compound in the crosslinking solution is excessively low, the effect of suppressing the leaching of dichroic pigments tends to be reduced. The concentration of the iodine-containing compound in the crosslinking solution is preferably 1 mass% or more, more preferably 1.5 mass% or more, and even more preferably 2 mass% or more. The concentration of the iodine-containing compound in the crosslinking solution is preferably 6 mass% or less, more preferably 5.5 mass% or less, and even more preferably 5 mass% or less.

[0134] If the temperature of the crosslinking solution is excessively high, the dichroic pigment is eluted, and the resulting polarizing film tends to be prone to dyeing non-uniformity; furthermore, if the temperature is excessively low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking solution is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 25°C or higher. The temperature of the crosslinking solution is preferably 45°C or lower, more preferably 40°C or lower, and even more preferably 35°C or lower.

[0135] Apart from the stretching treatment described below, the PVA film may be stretched during or between each of the aforementioned treatments. By performing such stretching (full stretching), the occurrence of wrinkles on the surface of the PVA film can be prevented. The total stretching ratio of full stretching (a ratio obtained by multiplying the stretching ratio in each treatment) is preferably 4 times or less, and more preferably 3.5 times or less, based on the original length of the PVA film of the base material before stretching, from the perspective of the polarization performance of the obtained polarizing film. The total stretching ratio of full stretching is preferably 1.5 times or more, based on the original length of the PVA film of the base material before stretching, from the perspective of the polarization performance of the obtained polarizing film. The stretching ratio in the swelling treatment is preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.4 times or more. In the swelling treatment, the elongation ratio is preferably 3 times or less, more preferably 2.5 times or less, and even more preferably 2.3 times or less. In the dyeing treatment, the elongation ratio is preferably 2 times or less, more preferably 1.8 times or less, and even more preferably 1.5 times or less. In the dyeing treatment, the elongation ratio is more preferably 1.1 times or more. In the crosslinking treatment, the elongation ratio is preferably 2 times or less, more preferably 1.5 times or less, and even more preferably 1.3 times or less. In the crosslinking treatment, the elongation ratio is more preferably 1.05 times or more.

[0136] (Stretching process)

[0137] Stretching treatment may be performed using either a wet stretching method or a dry stretching method. In the case of the wet stretching method, a solution containing a boron-containing compound such as boric acid (preferably an aqueous solution) may be used as the stretching treatment solution, and the treatment may be performed in the stretching treatment solution, or in a dyeing treatment solution or a fixing treatment solution described later. In addition, in the case of the dry stretching method, the treatment may be performed in air using a PVA film after absorption. Among these, the wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is more preferred. When the stretching treatment solution contains a boron-containing compound, the concentration of the boron-containing compound in the stretching treatment solution is preferably 1.5 mass% or more, more preferably 2.0 mass% or more, and even more preferably 2.5 mass% or more, in order to improve the stretchability of the PVA film. The concentration of the boron-containing compound in the stretching treatment solution is preferably 7 mass% or less, more preferably 6.5 mass% or less, and even more preferably 6 mass% or less, in order to improve the stretchability of the PVA film.

[0138] It is preferable to include an iodine-containing compound, such as potassium iodide, in the stretching treatment solution. If the concentration of the iodine-containing compound in the stretching treatment solution is excessively high, the color of the resulting polarizing film tends to have a strong blue tint; furthermore, if it is excessively low, the heat resistance of the resulting polarizing film tends to decrease, although the reason is unclear. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 2 mass% or more, more preferably 2.5 mass% or more, and even more preferably 3 mass% or more. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 8 mass% or less, more preferably 7.5 mass% or less, and even more preferably 7 mass% or less.

[0139] If the temperature of the stretching solution is too high, the PVA film tends to melt, soften, and become prone to breakage, and if it is too low, the stretchability tends to decrease. The temperature of the stretching solution is preferably 50°C or higher, more preferably 52.5°C or higher, and even more preferably 55°C or higher. The temperature of the stretching solution is preferably 70°C or lower, more preferably 67.5°C or lower, and even more preferably 65°C or lower. In addition, the preferred range of the stretching temperature when the stretching treatment is performed by a dry stretching method is as described above.

[0140] In the stretching process, the stretching ratio is preferably 1.2 times or more, more preferably 1.5 times or more, and even more preferably 2 times or more, as a higher ratio results in a polarizing film with superior polarization performance. Furthermore, the total stretching ratio (a ratio obtained by multiplying the stretching ratios in each process), which includes the stretching ratio of the aforementioned pre-stretching, is preferably 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more, in terms of the polarization performance of the polarizing film obtained based on the original length of the raw material PVA film before stretching. Although there is no particular upper limit for the stretching ratio, it is preferably 8 times or less, as excessively high values ​​make stretching breakage more likely to occur.

[0141] There are no particular restrictions on the method of performing the stretching treatment as uniaxial stretching, and uniaxial stretching in the length direction or transverse uniaxial stretching in the width direction may be adopted. When manufacturing a polarizing film, uniaxial stretching in the length direction is preferred in that excellent polarization performance is obtained. Uniaxial stretching in the length direction can be performed by using a stretching device equipped with a plurality of rolls parallel to each other and changing the peripheral speed between each roll.

[0142] In the present invention, there is no particular limitation on the maximum stretching speed (% / min) when the stretching treatment is performed as uniaxial stretching, but it is preferable that it be 200 % / min or higher, more preferable that it be 300 % / min or higher, and even more preferable that it be 400 % / min or higher. Here, the maximum stretching speed refers to the fastest stretching speed among the stages when the stretching treatment of a PVA film is performed in two or more stages using three or more rolls with different main speeds. In addition, if the stretching treatment of a PVA film is performed in one stage without being divided into two or more stages, the stretching speed in that stage becomes the maximum stretching speed. Furthermore, the stretching speed refers to the increase in the length of the PVA film increased by stretching relative to the length of the PVA film before stretching per unit time. For example, a stretching speed of 100 % / min is the speed at which the PVA film is deformed to twice its length in one minute from its length before stretching. As the maximum stretching speed increases, the stretching process (uniaxial stretching) of the PVA film can be performed at a high speed, which is desirable because it improves the productivity of the polarizing film. On the other hand, if the maximum stretching speed becomes excessively large, excessive local tension may be applied to the PVA film during the stretching process (uniaxial stretching), making it prone to stretching breakage. From this perspective, it is desirable that the maximum stretching speed does not exceed 900% / min.

[0143] (Fixed processing)

[0144] In the manufacture of a polarizing film, it is desirable to perform a fixation treatment to strengthen the adsorption of dichroic pigments onto the PVA film. The fixation treatment can be performed by using a solution (preferably an aqueous solution) containing one or more boron-containing compounds, such as boric acid or borax, as the fixation treatment solution, and immersing the PVA film (preferably a PVA film after stretching treatment) in the fixation treatment solution. Additionally, if necessary, the fixation treatment solution may contain an iodine-containing compound or a metal compound. The concentration of the boron-containing compound in the fixation treatment solution is preferably 2 mass% or more, and more preferably 3 mass% or more. The concentration of the boron-containing compound in the fixation treatment solution is preferably 15 mass% or less, and more preferably 10 mass% or less. The temperature of the fixation treatment solution is preferably 15°C or higher, and more preferably 25°C or higher. The temperature of the fixing treatment solution is preferably 60 ℃ or lower, and more preferably 40 ℃ or lower.

[0145] (Cleaning treatment after dyeing)

[0146] It is preferable to perform a cleaning treatment on the PVA film after dyeing, preferably after stretching. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning solution, but it may also be performed by spraying the cleaning solution onto the PVA film. For example, water may be used as the cleaning solution. The water is not limited to pure water and may contain an iodine-containing compound, such as potassium iodide. In addition, the cleaning solution may contain a boron-containing compound, but in that case, the concentration of the boron-containing compound is preferably 2.0 mass% or less.

[0147] It is preferable that the temperature of the cleaning solution be within the range of 5 to 40°C. By keeping the temperature of the cleaning solution at 5°C or higher, the breakage of the PVA film due to the freezing of moisture can be suppressed. In addition, by keeping the temperature of the cleaning solution at 40°C or lower, the optical properties of the resulting polarizing film are improved. It is more preferable that the temperature of the cleaning solution be 7°C or higher, and even more preferable that it be 10°C or higher. Furthermore, it is more preferable that the temperature of the cleaning solution be 38°C or lower, and even more preferable that it be 35°C or lower.

[0148] Specific methods for manufacturing a polarizing film include dyeing, stretching, and crosslinking and / or fixing treatments on a PVA film. A preferred example is a method of performing swelling, dyeing, crosslinking, stretching (especially uniaxial stretching), and washing treatments on a PVA film in this order. In addition, the stretching treatment may be performed at any of the preceding treatment processes, or it may be performed in two or more stages.

[0149] A polarizing film can be obtained by performing a drying treatment on the PVA film after undergoing each of the above treatments. There are no particular restrictions on the method of drying treatment; examples include a contact method in which the film is brought into contact with a heating roll, a method of drying in a hot air dryer, and a floating method in which the film is dried by hot air while floating.

[0150] Polarizer

[0151] The polarizing film obtained as described above is preferably used as a polarizing plate by laminating a protective film that is optically transparent and also has mechanical strength onto both or one side thereof.

[0152] Cellulose triacetic acid (TAC) film, cycloolefin polymer (COP) film, cellulose acetic acid-butyric acid (CAB) film, acrylic film, polyester film, etc. are used as protective films. In addition, PVA-based adhesives or urethane-based adhesives can be used as adhesives for lamination, but PVA-based adhesives are preferred.

[0153] The polarizing plate obtained as described above can be used as a component of an LCD by laminating an adhesive such as an acrylic-based adhesive onto a glass substrate. At the same time, it may also be laminated with a phase difference film, a viewing angle enhancement film, a brightness enhancement film, etc.

[0154] Examples

[0155] The present invention will be specifically described below by way of examples, but the present invention is not limited in any way by the following examples.

[0156] <FT-IR 측정에 의한 결정도 지수의 산출>

[0157] From the PVA films obtained in the following examples or comparative examples, a PVA film with a width of 30 mm × a length of 30 mm was cut and used as a measurement sample. Since the value of the crystallinity index of the PVA film varies slightly depending on the moisture absorption amount of the PVA film, this measurement sample was stored for 24 hours under an environment of a temperature of 24.0 ℃ and a relative humidity of 45.0 % RH, and FT-IR measurements were performed using a measuring device installed in a room with the same environment. FT-IR measurements were performed on both sides of the PVA film (two surfaces orthogonal to the thickness direction of the PVA film, namely the first surface and the second surface) under the following conditions.

[0158] Measuring device: NICOLET is 10 (Manufactured by Thermo Fisher)

[0159] Measurement conditions: 1 reflection ATR method, angle of incidence 45°

[0160] Resolution: 4.0 cm-1

[0161] Accumulated count: 32 times

[0162] Measured temperature: 24.0 ℃ (ambient temperature)

[0163] Measured humidity: 45.0 % RH (Ambient relative humidity)

[0164] ATR Prism: Diamond Prism or Germanium Prism

[0165] From the infrared absorption spectrum obtained from the FT-IR measurement of the PVA film, the crystallinity index of both sides of the PVA film (two surfaces orthogonal to the thickness direction of the PVA film, namely the first surface and the second surface) was calculated using the above method.

[0166] Evaluation of Wrinkles on the Surface of Polarizing Films

[0167] For the polarizing films obtained in the following examples or comparative examples, light was irradiated obliquely onto the surface of the polarizing film with a fluorescent lamp, and the appearance of the reflected light was observed visually to confirm the wrinkle condition of the surface of the polarizing film and evaluated according to the following criteria.

[0168] Evaluation Criteria:

[0169] A: No wrinkles were confirmed.

[0170] B: Slight wrinkles were observed to the extent that they do not pose a practical problem.

[0171] C: Wrinkles significant enough to cause practical problems were clearly identified.

[0172] <Evaluation of Stretch Break Frequency in Polarizing Film Manufacturing>

[0173] In the following examples or comparative examples, uniaxial stretching was continuously performed for 20 minutes during the stretching treatment when manufacturing a polarizing film. The number of stretching breakage occurrences during this 20-minute continuous stretching was measured, and the stretching breakage frequency (times / 20 mim) was evaluated.

[0174] <Example 1>

[0175] <PVA 필름의 제조 및 평가>

[0176] A film-forming stock solution (volatile fraction 66 mass%) was prepared by melt-mixing 100 parts by mass of PVA (degree of saponification 99 mol%, degree of polymerization 2400), 12 parts by mass of glycerin plasticizer as a plasticizer, 0.1 parts by mass of lauric acid diethanolamide as a surfactant, and 217.6 parts by mass of water using a melt extruder. Next, this film-forming stock solution was extruded in the form of a film onto a support (surface temperature 80 ℃) from a T-die to form a liquid film on the support. On the support, hot air at 85 ℃ and 3% RH was sprayed at a speed of 5 m / sec over the entire non-contact surface of the liquid film with the support to dry it, thereby obtaining a PVA film (moisture content 32 mass%). Next, the PVA film was peeled off from the support, and further dried between the first drying roll and the final drying roll (the 19th drying roll) immediately preceding the heat treatment roll, so that one side and the other side of the PVA film alternately contacted each drying roll, and then peeled off from the final drying roll. At this time, the surface temperature of each drying roll from the first drying roll to the final drying roll was set to 75°C. In addition, the PVA film was peeled off from the final drying roll, and heat treatment was performed so that one side and the other side of the PVA film alternately contacted each heat treatment roll. At this time, the heat treatment was performed using two heat treatment rolls, and the surface temperature of the heat treatment rolls was set to 90°C. FT-IR measurements were performed on the obtained PVA film (thickness 30 μm, width 1200 mm) using the method described above, and the crystallinity indices (Fg1, Fg2, Fd1, and Fd2) were calculated. The results are shown in Table 1.

[0177] <Manufacturing and Evaluation of Polarizing Films>

[0178] The obtained PVA film was slit to a width of 650 mm, and a polarizing film was continuously manufactured by performing swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, washing treatment, and drying treatment on the film in this order. The swelling treatment was performed by uniaxially stretching the film 2.00 times in the longitudinal direction while immersing it in pure water (swelling treatment solution) at 25 ℃. The dyeing treatment was performed by uniaxially stretching the film 1.26 times in the longitudinal direction while immersing it in a potassium iodide / iodine dyeing solution (dyeing treatment solution) at a temperature of 32 ℃ (potassium iodide / iodine (mass ratio) 23, iodine concentration 0.03 to 0.05 mass%). In this dyeing treatment, the iodine concentration in the dyeing solution was adjusted within the range of 0.03 to 0.05 mass% so that the single transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was in the range of 43.5% ± 0.2%. The crosslinking treatment was carried out by uniaxial stretching 1.19 times in the longitudinal direction while immersing in an aqueous boric acid solution (crosslinking treatment solution) (boric acid concentration 2.6 mass%) at 32°C. The stretching treatment was carried out by uniaxial stretching 2.00 times in the longitudinal direction while immersing in an aqueous boric acid / potassium iodide solution (stretching treatment solution) (boric acid concentration 2.8 mass%, potassium iodide concentration 5 mass%) at 55°C. The maximum stretching speed of uniaxial stretching in this stretching treatment was 400% / min. The cleaning treatment was carried out by immersing the polarizing film in an aqueous potassium iodide / boric acid solution (cleaning solution) at 22°C (potassium iodide concentration 3–6 mass%, boric acid concentration 1.5 mass%) for 12 seconds without stretching. The drying treatment was carried out by drying with hot air at 80°C for 1.5 minutes without stretching to obtain a polarizing film. For the obtained polarizing film, wrinkles on the surface of the polarizing film and the frequency of stretching breakage during manufacturing were evaluated using the above method. The results are shown in Table 2.

[0179] <Example 2>

[0180] of Example 1<PVA 필름의 제조 및 평가> In this case, a PVA film and a polarizing film were obtained in the same manner as in Example 1, except that the PVA used in the preparation of the film-making solution was changed to PVA (degree of saponification 99 mol%, degree of polymerization 2400, ethylene modification 2.5 mol%) and the surface temperature of both heat treatment rolls was changed to 85 ℃. Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0181] <Example 3>

[0182] of Example 1<PVA 필름의 제조 및 평가> In this case, a PVA film and a polarizing film were obtained in the same manner as in Example 1, except that the surface temperature of the support was changed to 100 ℃, the temperature of the hot air sprayed over the entire non-contact surface of the liquid film with the support was changed to 105 ℃, the surface temperature of each drying roll from the first drying roll to the final drying roll (19th drying roll) immediately preceding the heat treatment roll was changed to 90 ℃, and the surface temperature of both heat treatment rolls was changed to 80 ℃. Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0183] <Example 4>

[0184] of Example 1<PVA 필름의 제조 및 평가> In this case, a PVA film and a polarizing film were obtained in the same manner as in Example 1, except that the drying from the first drying roll to the final drying roll (the 19th drying roll) immediately preceding the heat treatment roll was performed by contacting each drying roll only with one side of the PVA film (the side where the liquid film on the support was in contact with the support). Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0185] <Comparative Example 1>

[0186] of Example 1<PVA 필름의 제조 및 평가> In this case, a PVA film and a polarizing film were obtained in the same manner as in Example 1, except that the surface temperature of the support was changed to 115 ℃, the temperature of the hot air sprayed over the entire non-contact surface of the liquid film with the support was changed to 120 ℃, the surface temperature of each drying roll from the first drying roll to the final drying roll (19th drying roll) immediately preceding the heat treatment roll was changed to 65 ℃, and the surface temperature of both heat treatment rolls was changed to 65 ℃. Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0187] <Comparative Example 2>

[0188] of Example 1<PVA 필름의 제조 및 평가> In this case, a PVA film and a polarizing film were obtained in the same manner as in Example 1, except that the surface temperature of the support was changed to 60°C, the temperature of the hot air sprayed over the entire non-contact surface of the liquid film with the support was changed to 70°C, and the surface temperature of each drying roll from the first drying roll to the final drying roll (the 19th drying roll) immediately preceding the heat treatment roll was changed to 90°C. Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0189] <Reference Example 1>

[0190] In the <Preparation and Evaluation of Polarizing Film> of Example 1, a PVA film and a polarizing film were obtained in the same manner as Comparative Example 1, except that the maximum stretching speed of uniaxial stretching in the stretching treatment was changed to 190%. Measurements and evaluations were performed on the obtained PVA film and polarizing film in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0191]

[0192]

[0193] As shown in Tables 1 and 2, when a polarizing film was manufactured using the PVA films of Examples 1 to 4, no wrinkles were observed on the surface of the obtained polarizing film, or only slight wrinkles were observed to the extent that they did not cause practical problems. Here, the wrinkles occurring on the surface of the polarizing film are caused by wrinkles on the surface of the PVA film, that is, wrinkles on the surface of the PVA film that occur during uniaxial stretching in the stretching treatment when manufacturing the polarizing film. Therefore, it can be said that the PVA films of Examples 1 to 4 are unlikely to develop wrinkles on their surface during uniaxial stretching.

[0194] As shown in Tables 1 and 2, when a polarizing film was manufactured using the PVA films of Examples 1 to 4, the frequency of stretching breakage was 0 to 2 times / 20 min when uniaxial stretching was performed continuously for 20 minutes during the stretching treatment. Therefore, it can be said that the PVA films of Examples 1 to 4 suppressed breakage during stretching (uniaxial stretching).

[0195] In addition, as shown in Reference Example 1, when the maximum stretching speed of uniaxial stretching in the stretching treatment when manufacturing a polarizing film is relatively low (190 % / min), even when using the PVA film of Comparative Example 1, no wrinkles were observed on the surface of the polarizing film obtained, and the fracture frequency during stretching (uniaxial stretching) was 0 times / 20 min. On the other hand, as shown in Comparative Example 1, when the maximum stretching speed is high (400 % / min), when using the PVA film of Comparative Example 1, wrinkles of a degree that is practically problematic were clearly observed on the surface of the polarizing film obtained, and the fracture frequency during stretching (uniaxial stretching) was 5 times / 20 min.

[0196] That is, the PVA film of Comparative Example 1 can be said to be prone to wrinkles on the surface and prone to breakage during stretching (uniaxial stretching) when the maximum stretching speed is high (400 % / min). On the other hand, the PVA films of Examples 1 to 4 can be said to be prone to wrinkles on the surface and breakage during stretching (uniaxial stretching) even when the maximum stretching speed is high (400 % / min). Explanation of the symbols

[0197] 1 : PVA film 2: Thickness direction of the PVA film 3: First Surface 4: Second Surface 5: Infrared penetration depth when using a diamond prism (approx. 2 μm) 6: Infrared penetration depth when using a germanium prism (approx. 0.5 μm) 7 : ATR Prism (Diamond Prism or Germanium Prism) 8: Infrared

Claims

Claim 1 A water-insoluble polyvinyl alcohol film, wherein two surfaces perpendicular to the thickness direction of the polyvinyl alcohol film are respectively designated as a first surface and a second surface, and when the crystallinity indices of the first surface are set as Fd1 and Fg1 and the crystallinity indices of the second surface are set as Fd2 and Fg2, the polyvinyl alcohol film such that Fd1, Fg1, Fd2, and Fg2 satisfy the following equations (1) to (4). Fd1 ≤ 0.8 (1) Fd1 / Fg1 < 1 (2) Fd2 ≤ 0.8 (3) Fd2 / Fg2 < 1 (4) [Among the above equations (1) to (4), Fd1 is a crystallinity index calculated using a diamond prism when FT-IR measurement by the ATR method is performed on the first surface, and Fg1 is the ATR Fd2 is a crystallinity index calculated using a germanium prism when performing FT-IR measurements by the ATR method, Fd2 is a crystallinity index calculated using a diamond prism when performing FT-IR measurements by the ATR method on the second surface, and Fg2 is a crystallinity index calculated using a germanium prism when performing FT-IR measurements by the ATR method on the second surface. Claim 2 A polyvinyl alcohol film according to claim 1, wherein Fd1 and Fd2 satisfy the following formulas (5) to (6). Fd1 ≥ 0.5 (5) Fd2 ≥ 0.5 (6) Claim 3 A polyvinyl alcohol film according to claim 1, wherein Fd1, Fg1, Fd2 and Fg2 satisfy the following formulas (7) to (8). Fd1 / Fg1 ≥ 0.6 (7) Fd2 / Fg2 ≥ 0.6 (8) Claim 4 A polyvinyl alcohol film according to claim 1, wherein Fd1, Fg1, Fd2 and Fg2 satisfy the following formulas (9) to (10). |Fd1-Fd2| ≤ 0.07 (9) |Fg1-Fg2| ≤ 0.07 (10) Claim 5 In claim 1, a polyvinyl alcohol film, which is a film for manufacturing an optical film. Claim 6 In claim 5, a polyvinyl alcohol film in which the optical film is a polarizing film. Claim 7 A method for manufacturing a polyvinyl alcohol film as described in any one of claims 1 to 6, comprising the step of forming a liquid film of the film-making solution on a support by bending the film-making solution containing polyvinyl alcohol and a solvent into a film shape on a rotating support, and heating the liquid film on the support to form a film, wherein when heating the liquid film on the support, the humidity of the hot air sprayed on the non-contact surface of the liquid film with the support is 1% RH or higher.

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