Polyvinyl alcohol film and polarizing film using the same

Abstract

Provided is a PVA film in which wrinkles are less likely to occur on the surface even when the maximum stretching speed is high, and the breakage during uniaxial stretching is suppressed. The PVA film, which is a non-water-soluble PVA film, has a first surface, and when the crystallinity indices of the first surface are set as Fd1 and Fg1, and the crystallinity indices of a second surface are set as Fd2 and Fg2, the aforementioned Fd1, Fg1, Fd2, and Fg2 satisfy the following formulas (1) to (4), Fd1 ≤ 0.8 (1) Fd1 / Fg1 < 1 (2) Fd2 ≤ 0.8 (3) Fd2 / Fg2 < 1 (4) [In the aforementioned formulas (1) to (4), Fd1 and Fg1 are the crystallinity indices calculated using a diamond prism and a germanium prism, respectively, when the aforementioned first surface is subjected to FT-IR measurement by the ATR method, and Fd2 and Fg2 are similarly the crystallinity indices calculated using a diamond prism and a germanium prism, respectively, for the aforementioned second surface].

Description

Technical Field

[0001] This invention relates to polyvinyl alcohol films and polarizing films using the same. Background Technology

[0002] A polarizing plate, which has both light-transmitting and light-blocking functions, and a liquid crystal, which has light-switching functions, are both fundamental components of a liquid crystal display (LCD). In recent years, the application areas of LCDs have expanded from small devices such as calculators and wristwatches at the beginning of their development to various fields such as laptops, LCD monitors, LCD color projectors, LCD TVs, in-vehicle navigation systems, mobile phones, and measuring equipment used indoors and outdoors.

[0003] Polarizing plates are manufactured by laminating a protective film, such as a cellulose triacetate (TAC) film or a cellulose acetate-butyrate (CAB) film, onto the surface of a polarizing film. The polarizing film is typically manufactured as follows: a polyvinyl alcohol (PVA) film (hereinafter sometimes referred to as "PVA") is dyed and then uniaxially stretched; or, dyed and then uniaxially stretched; or uniaxially stretched and then dyed, to create a dyed uniaxially stretched film. This uniaxially stretched film is then immobilized using a boron compound. It should be noted that the immobilization treatment using the boron compound is sometimes performed simultaneously with the uniaxial stretching or dyeing process.

[0004] In large LCD products such as LCD monitors and LCD TVs, high contrast and clear images are required. Consequently, there is a demand for high-performance polarizing films, specifically, an increase in the polarization degree of the polarizing film. However, when increasing the stretching ratio of the PVA film during uniaxial stretching to improve the polarization degree, wrinkles are easily generated on the surface of the PVA film during the uniaxial stretching process. As a result, wrinkles are also easily generated on the surface of the resulting polarizing film. If a large number of wrinkles are generated on the surface of the polarizing film, it can easily become a cause of image unevenness in the final products such as LCD monitors and LCD TVs. Furthermore, if a large number of wrinkles are generated on the surface of the polarizing film, such a polarizing film cannot be used as a finished product, thus reducing the product yield (finished product yield).

[0005] As a method to suppress wrinkles on the surface of polarizing films, it is proposed to control the tempering time and component ratio for components with short tempering times (components with low molecular mobility and rigidity) during pulsed NMR measurements of PVA films (see Patent Document 1).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: WO2019 / 189695 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] In recent years, the demands for higher contrast and clearer images in LCDs have increased, leading to a rise in surface wrinkles in polarizing films, which were previously not a problem. Furthermore, to improve the production efficiency of polarizing films, efforts are being made to perform uniaxial stretching during the manufacturing process at high speeds—that is, setting the maximum uniaxial stretching speed to high. However, setting the maximum stretching speed to high during uniaxial stretching makes it easier for wrinkles to form on the PVA film surface. Consequently, wrinkles are more likely to form on the surface of the resulting polarizing film. Additionally, at high maximum stretching speeds, excessive tension can sometimes be applied to localized areas of the PVA film during uniaxial stretching. This results in problems such as PVA film breakage and reduced yield of polarizing film products during uniaxial stretching.

[0011] In the PVA film described in Patent Document 1, if the maximum stretching speed is set to high during uniaxial stretching in the manufacture of the polarizing film, wrinkles may easily form on the surface of the PVA film during uniaxial stretching, making it impossible to sufficiently suppress surface wrinkles in the polarizing film. Furthermore, if the maximum stretching speed is set to high, the PVA film may sometimes break during uniaxial stretching. It should be noted that suppressing surface wrinkles and breaking of the PVA film during uniaxial stretching is also important for suppressing surface wrinkles and breaking in optical films other than polarizing films.

[0012] Therefore, the object of the present invention is to provide a PVA film that does not easily wrinkle on the surface during uniaxial stretching and whose fracture during uniaxial stretching is suppressed, even when the maximum stretching speed during uniaxial stretching is high during the manufacture of optical films such as polarizing films.

[0013] means for solving problems

[0014] Through repeated and in-depth research, the inventors discovered that by adjusting the crystallinity index of the two surfaces of the PVA film orthogonal to the thickness direction to a specific range, the above-mentioned problem can be achieved. Based on this insight, they conducted further and repeated research, thereby completing this invention.

[0015] That is, the present invention relates to:

[0016] [1] PVA film, which is a non-water-soluble PVA film, with the two surfaces of the aforementioned PVA film orthogonal to the thickness direction respectively designated as the first surface and the second surface, the crystallinity index of the aforementioned first surface designated as Fd1 and Fg1, and the crystallinity index of the aforementioned second surface designated as Fd2 and Fg2, the aforementioned Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (1) to (4);

[0017] Fd1≤0.8 (1)

[0018] Fd1 / Fg1<1 (2)

[0019] Fd2≤0.8 (3)

[0020] Fd2 / Fg2<1 (4)

[0021] [In the aforementioned formulas (1) to (4), Fd1 is the crystallinity index calculated using a diamond prism when performing FT-IR measurement on the aforementioned first surface using the ATR method, Fg1 is the crystallinity index calculated using a germanium prism when performing FT-IR measurement on the aforementioned first surface using the ATR method, Fd2 is the crystallinity index calculated using a diamond prism when performing FT-IR measurement on the aforementioned second surface using the ATR method, and Fg2 is the crystallinity index calculated using a germanium prism when performing FT-IR measurement on the aforementioned second surface using the ATR method.]

[0022] [2] According to the PVA film of [1], the aforementioned Fd1 and Fd2 satisfy the following equations (5) to (6);

[0023] Fd1≥0.5 (5)

[0024] Fd2≥0.5 (6)

[0025] [3] According to the PVA film of [1] or [2], wherein the aforementioned Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (7) to (8);

[0026] Fd1 / Fg1≥0.6 (7)

[0027] Fd2 / Fg2≥0.6 (8)

[0028] [4] According to any one of [1] to [3], the aforementioned Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (9) to (10);

[0029] |Fd1-Fd2|≤0.07 (9)

[0030] |Fg1-Fg2|≤0.07 (10)

[0031] [5] The PVA film according to any one of [1] to [4] is a film for manufacturing optical films;

[0032] [6] According to [5], the PVA film is a polarizing film.

[0033] Invention Effects

[0034] According to the present invention, there is provided a PVA film which is not likely to generate wrinkles on the surface during uniaxial stretching even when the maximum stretching speed during uniaxial stretching in the production of an optical film such as a polarizing film is high, and the breakage during uniaxial stretching is suppressed. With such a PVA film, the generation of wrinkles on the surface of an optical film such as a polarizing film can be suppressed. In addition, since the breakage during uniaxial stretching is suppressed, an optical film such as a polarizing film can be produced with a high product yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a perspective view of the PVA film of the present invention.

[0036] Figure 2 is a view of the PVA film of the present invention as observed from the side.

[0037] Figure 3 is a diagram schematically showing the ATR method in FT-IR measurement. DETAILED DESCRIPTION OF THE INVENTION

[0038] Hereinafter, the present invention will be described in detail.

[0039] <PVA Film>

[0040] In the present invention, as Figure 1 , 2 shown, the two surfaces of the PVA film 1 orthogonal to the thickness direction 2 are respectively defined as the first surface 3 and the second surface 4. Therefore, 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, for the first surface 3 and the second surface 4, FT-IR (Fourier transform infrared spectroscopy) measurement is respectively performed by the ATR method. And the crystallinity indices Fd1, Fg1, Fd2 and Fg2 calculated by this measurement satisfy the following formulas (1) to (4).

[0041] Fd1 ≤ 0.8 (1)

[0042] Fd1 / Fg1 < 1 (2) <所ID=

[0043] Fd2 ≤ 0.8 (3)

[0044] Fd2 / Fg2 < 1 (4)

[0045] In equations (1) to (4) above, Fd1 is the crystallinity index calculated using a diamond prism when performing FT-IR measurement on the first surface 3 of PVA film 1 using the ATR method, and Fg1 is the crystallinity index calculated using a germanium prism when performing FT-IR measurement on the first surface 3 of PVA film 1 using the ATR method. Additionally, Fd2 is the crystallinity index calculated using a diamond prism when performing FT-IR measurement on the second surface 4 of PVA film 1 using the ATR method, and Fg2 is the crystallinity index calculated using a germanium prism when performing FT-IR measurement on the second surface 4 of PVA film 1 using the ATR method. It should be noted that in equation (2) above, Fd1 / Fg1 is the value obtained by dividing Fd1 by Fg1, and in equation (4) above, Fd2 / Fg2 is the value obtained by dividing Fd2 by Fg2.

[0046] In the PVA film of the present invention, as shown in formulas (1) and (3) above, Fd1 and Fd2 need to be 0.8 or less. When Fd1 or Fd2 exceeds 0.8, during uniaxial stretching in the manufacture of optical films such as polarizing films, at high speeds, wrinkles easily form on the surface of the PVA film 1 during uniaxial stretching, and the PVA film 1 is prone to breakage during uniaxial stretching. The reason may not be clear, but it can be speculated that if the crystallinity of the PVA film 1 surface is too high, water in the stretching solution will have difficulty penetrating into the interior of the PVA film 1 during uniaxial stretching, and the flexibility of the film during uniaxial stretching will be insufficient. Fd1 and Fd2 are preferably 0.75 or less, more preferably 0.72 or less, further preferably 0.7 or less, and particularly preferably 0.68 or less.

[0047] 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, during uniaxial stretching in the manufacture of optical films such as polarizing films, when the maximum stretching speed is high, wrinkles are easily generated on the surface of the PVA film 1 during uniaxial stretching. Fd1 / Fg1 and Fd2 / Fg2 are preferably 0.98 or less, more preferably 0.96 or less, further preferably 0.94 or less, even more preferably 0.92 or less, and particularly preferably 0.9 or less.

[0048] In the PVA film of the present invention, as shown in formulas (1) and (3) above, Fd1 and Fd2 are 0.8 or less. Furthermore, as shown in formulas (2) and (4) above, Fd1 / Fg1 and Fd2 / Fg2 are less than 1. As will be explained later, Fd1 and Fd2 represent the crystallinity of the deeper interior of the PVA film 1, while Fg1 and Fg2 represent the crystallinity near the surface of the PVA film 1, i.e., the polar surface portion. That is, regarding the PVA film of the present invention, the crystallinity of the deeper interior of the PVA film 1 is below a predetermined value, and the crystallinity near the surface of the PVA film 1, i.e., the polar surface portion, is higher than the crystallinity of the deeper interior of the PVA film 1. In this way, by controlling the crystallinity of the deeper interior and the polar surface portion of the PVA film 1, wrinkles are less likely to form on the surface during uniaxial stretching during the manufacture of optical films such as polarizing films, even at high maximum stretching speeds, and breakage during uniaxial stretching is suppressed. The reason may not be clear, but it can be speculated that: because the crystallinity of the outermost layer of PVA film 1 is high, wrinkles are suppressed on the surface of PVA film 1 during uniaxial stretching; and because the crystallinity of the deeper interior of PVA film 1 is low, the stress generated during uniaxial stretching is mitigated, and fracture is suppressed.

[0049] In the PVA film of the present invention, the lower limit values ​​of Fd1 and Fd2 are not necessarily limited, but when the maximum stretching speed is high during the uniaxial stretching of optical films such as polarizing films, the breakage of PVA film 1 during uniaxial stretching can be further suppressed. Therefore, it is preferable to satisfy the following formulas (5) and (6).

[0050] Fd1≥0.5 (5)

[0051] Fd2≥0.5 (6)

[0052] As shown in equations (5) and (6) above, by making Fd1 and Fd2 0.5 or more, the crystallinity of the deeper interior of the PVA film 1 increases. As a result, the crystallinity of the central portion of the PVA film 1 in the thickness direction 2 increases, and the mechanical strength of the PVA film 1 is improved. Therefore, by using this PVA film 1, even at high speeds when the maximum stretching speed is high, breakage during uniaxial stretching of the PVA film 1 during the manufacture of optical films such as polarizing films can be further suppressed. Fd1 and Fd2 are more preferably 0.52 or more, and even more preferably 0.55 or more.

[0053] In the PVA film of the present invention, the lower limit values ​​of Fd1 / Fg1 and Fd2 / Fg2 are not necessarily limited. In the uniaxial stretching of optical films such as polarizing films, when the maximum stretching speed is high, the breakage of PVA film 1 during uniaxial stretching can be further suppressed. Therefore, it is preferable to satisfy the following formulas (7) and (8).

[0054] Fd1 / Fg1≥0.6 (7)

[0055] Fd2 / Fg2≥0.6 (8)

[0056] As shown in equations (7) and (8) above, by making Fd1 / Fg1 and Fd2 / Fg2 0.6 or more, the crystallinity of the deeper interior of the PVA film 1 will not become too small compared to the crystallinity of the outermost layer of the PVA film 1. As a result, the crystallinity of the central portion of the thickness direction 2 of the PVA film 1 becomes larger, and the mechanical strength of the PVA film 1 is improved. Therefore, by using this PVA film 1, even when the maximum stretching speed is high, breakage during uniaxial stretching of the PVA film 1 during the manufacture of optical films such as polarizing films can be further suppressed. Fd1 / Fg1 or Fd2 / Fg2 is more preferably 0.65 or more, more preferably 0.7 or more, and particularly preferably 0.75 or more.

[0057] 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. In the uniaxial stretching during the manufacture of optical films such as polarizing films, when the maximum stretching speed is high, wrinkles generated on the surface of PVA film 1 can be further suppressed during uniaxial stretching. Therefore, it is preferable to satisfy the following formulas (9) and (10).

[0058] |Fd1-Fd2|≤0.07 (9)

[0059] |Fg1-Fg2|≤0.07 (10)

[0060] As shown in equations (9) and (10) above, by setting |Fd1-Fd2| and |Fg1-Fg2| to 0.07 or less, the difference in crystallinity index between the first surface 3 and the second surface 4 of the PVA film 1 will not be too large, and the elastic modulus of the two surfaces (first surface 3 and second surface 4) of the PVA film 1 orthogonal to the thickness direction 2 will be substantially equal. Therefore, by using such a PVA film 1, wrinkles are less likely to form on the surface of the PVA film 1 during uniaxial stretching in the manufacture of optical films such as polarizing films, even at high maximum stretching speeds. |Fd1-Fd2| and |Fg1-Fg2| are more preferably 0.06 or less, more preferably 0.05 or less, and particularly preferably 0.04 or less.

[0061] (FT-IR measurement)

[0062] Generally, if the infrared absorption spectrum (IR spectrum) of PVA film 1 is measured, the PVA content will result in an absorption rate of 1140 cm⁻¹. -1An absorption peak was observed. This absorption peak is generally referred to as the crystallization band of PVA film 1, which is a peak of the stretching vibration of PVA originating from carbon bonds (CC). It is known that this crystallization band is observed because the polymer molecular chains of PVA in PVA film 1 undergo crystallization, and the vibrations of the PVA polymer molecular chains are enhanced due to phase consistency. That is, the higher the crystallinity of PVA film 1, the higher the peak intensity of the crystallization band will be. In addition, if the infrared absorption spectrum of PVA film 1 is measured, the angle-shifting vibration originating from the methylene (-CH2-) group, which is the main chain of PVA, is observed at 1425 cm⁻¹. -1 An absorption peak was observed. The intensity of this absorption peak is independent of the crystallinity of PVA film 1.

[0063] In this invention, the crystallization zone (1140 cm⁻¹) is calculated. -1 The absorption peak intensity of ) is related to the angle vibration of the methylene group (-CH2-) in the main chain of PVA (1425 cm⁻¹). -1 The intensity ratio of the absorption peak intensities of the PVA film 1 can be used to obtain the crystallinity indices (Fg1, Fg2, Fd1, and Fd2). Specifically, the 1140 cm⁻¹ peak intensity is plotted. -1 and 1425cm -1 The baseline of the infrared absorption spectrum at that location will be from the baseline to 1140 cm⁻¹. -1 and 1425cm -1 The height up to the peak is taken as the absorption peak intensity, which will be obtained through 1140 cm⁻¹. -1 Divide the absorption peak intensity by 1425 cm⁻¹ -1 The values ​​obtained from the peak intensity are used as crystallinity indices (Fg1, Fg2, Fd1, and Fd2).

[0064] It is well known that the values ​​of the crystallinity indices (Fg1, Fg2, Fd1, and Fd2) obtained by such operation are proportional to the crystallinity of the PVA film 1 (e.g., NAPeppas, Macromol. Chem., Vol. 178, 595 (1977), Japanese Patent Application Publication No. 6-138321). The values ​​of these crystallinity indices vary slightly depending on the moisture absorption of the PVA film 1. Therefore, in this invention, after storing the PVA film 1 for 24 hours at a temperature of 24.0°C and a relative humidity of 45.0% RH, FT-IR measurements were performed under the same conditions.

[0065] In this invention, FT-IR measurements are performed using the ATR method (total reflectance absorption measurement). For example... Figure 3As shown, the ATR method refers to a reflectance-type IR measurement method in which the sample is tightly fitted to an objective lens called an ATR prism 7, infrared radiation 8 is incident on the sample at an angle from inside the ATR prism 7, and the spectrum of the reflected light is measured. Compared with the usual reflectance-type IR measurement method, it has the characteristic of obtaining a sharp spectrum with less noise. When using a PVA film 1 as the sample in this measurement method, the infrared radiation 8 is not only reflected on the surface of the PVA film 1, but also reflected as infrared radiation 8 penetrates slightly from the ATR prism 7 side to the PVA film 1 side. Therefore, according to the FT-IR measurement based on the ATR method, information about the surface layer of the PVA film 1 (the part that penetrates slightly from the surface of the PVA film 1 in the depth direction) can be obtained. Here, if the penetration depth of the infrared radiation 8 penetrating from the ATR prism 7 side to the PVA film 1 side is denoted as d, then this value is expressed by the following formula (11). It can be clearly seen from the following formula (11) that if ATR prisms 7 with different refractive indices are used, reflectance-type infrared absorption spectra with different penetration depths can be obtained.

[0066] d=λ / 2Πn1×1 / {sin 2 θ-(n2 / n1) 2} 0.5 (11)

[0067] In the above formula (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 incident angle of the infrared 8.

[0068] In this invention, such as Figure 3 As shown, diamond with a refractive index of 2.4 or germanium with a refractive index of 4.0 is used as the substrate for the ATR prism 7. Since the refractive index of the PVA film 1 is 1.5, the incident angle of the infrared 8 is calculated to be 45° and the wavenumber of the infrared 8 is 1140 cm⁻¹ in the above equation (11). -1 When considering the penetration depth of infrared light 8 penetrating towards the surface of the PVA film 1, when diamond is used as the substrate of the ATR prism 7, the penetration depth 5 of infrared light 8 is approximately 2 μm. On the other hand, when germanium is used as the ATR prism 7, the penetration depth 6 of infrared light 8 is approximately 0.5 μm. Therefore, the crystallinity index when using a diamond prism corresponds to the crystallinity up to the deeper interior of the PVA film 1. Conversely, the crystallinity index when using a germanium prism corresponds to the crystallinity near the surface of the PVA film 1, i.e., the outermost layer.

[0069] In this invention, it is important to control the crystallinity indices, namely Fg1 and Fg2, of the outermost layer of the PVA film 1, and the crystallinity indices, namely Fd1 and Fd2, of the deeper interior layer of the PVA film 1, to the aforementioned ranges. The crystal structure of the PVA film 1 is affected by the composition of the PVA film 1 and various factors in the manufacturing process. Therefore, methods for controlling the crystallinity indices (Fg1, Fg2, Fd1, and Fd2) include, for example, adjusting the type of polyvinyl alcohol (degree of saponification, amount of modification, blending ratio of unmodified PVA / modified PVA, etc.); adjusting the amount of plasticizer added; adjusting the film-forming conditions (surface temperature of the roller support, heat treatment conditions, etc.); or combining and adjusting these methods.

[0070] More specifically, methods for adjusting the crystallinity indices Fd1 and Fd2 to below 0.8 and the ratios of Fd1 / Fg1 and Fd2 / Fg2 to less than 1 can be exemplified by: setting the degree of saponification of PVA to 90 mol% or more; setting the proportion of structural units derived from other monomers in the raw material of PVA, i.e., the vinyl ester polymer, to 15 mol% or less based on the total number of moles of all structural units constituting the vinyl ester polymer; and setting the degree of polymerization of PVA to 200 to 8000. In this case, the amount of plasticizer added is preferably 1 to 40 parts by mass relative to 100 parts by mass of PVA. Furthermore, the volatile fraction of the film-forming solution is preferably 50 to 90% by mass, the surface temperature of the support for casting the film-forming solution is preferably 65 to 110°C, the temperature of the hot air blown towards the non-contact side is preferably 50 to 150°C or less, and the humidity of the hot air is preferably 20 to 90% RH. Therefore, the temperature of the drying oven or the surface temperature of the drying roller is preferably 45-110°C, and the surface temperature of the heat treatment roller is preferably 60-135°C.

[0071] One method for adjusting the crystallinity indices Fd1 and Fd2 to 0.5 or higher is to set the saponification degree of PVA to 95–99.9 mol%, set the proportion of structural units derived from other monomers in the PVA raw material (i.e., vinyl ester polymer) to 10 mol% or less based on the total number of moles of all structural units constituting the vinyl ester polymer, and set the degree of polymerization of PVA to 1000–4000. In this case, the amount of plasticizer added is preferably 5–20 parts by mass relative to 100 parts by mass of PVA. Furthermore, the volatile fraction of the film-forming solution is preferably 60–80% by mass, the surface temperature of the support for casting the film-forming solution is preferably 80–110°C, the temperature of the hot air blown onto the non-contact side is preferably 70–110°C or less, and the humidity of the hot air is preferably 1–40% RH. Furthermore, the temperature of the drying oven or the surface temperature of the drying roller is preferably 60–110°C, and the surface temperature of the heat treatment roller is preferably 80–135°C.

[0072] As a method for adjusting Fd1 / Fg1 and Fd2 / Fg2 to 0.6 or higher, the following methods can be used: setting the saponification degree of PVA to 99-99.9 mol%, setting the proportion of structural units derived from other monomers in the PVA raw material (i.e., vinyl ester polymer) to 5 mol% or less based on the total number of moles of all structural units constituting the vinyl ester polymer), and setting the degree of polymerization of PVA to 1000-3700. In this case, the amount of plasticizer added is preferably 8-20 parts by mass relative to 100 parts by mass of PVA. Furthermore, the volatile fraction of the film-forming solution is preferably 65-80% by mass, the surface temperature of the support for casting the film-forming solution is preferably 80-100°C, the temperature of the hot air blown onto the non-contact side is preferably 70-100°C, and the humidity of the hot air is preferably 3-40% RH. Furthermore, the temperature of the drying oven or the surface temperature of the drying roller is preferably 60-100°C, and the surface temperature of the heat treatment roller is preferably 80-120°C.

[0073] As a method for adjusting |Fd1-Fd2| and |Fg1-Fg2| to below 0.07, it is preferable to set the volatile fraction of the film-forming solution to 65-75% by mass, the surface temperature of the support for casting the film-forming solution to be set to 80-95°C, the temperature of the hot air blown onto the non-contact side to be set to 75-90°C, and the humidity of the hot air to be set to 5-40% RH. Furthermore, at this time, the temperature of the drying oven or the surface temperature of the drying roller is preferably 60-90°C, and the surface temperature of the heat treatment roller is preferably 80-110°C.

[0074] (PVA)

[0075] In the PVA film of the present invention, the PVA can be a polymer manufactured by saponifying a vinyl ester polymer obtained by polymerizing vinyl ester monomers. Examples of vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl neovalerate, and vinyl tert-carboxylate. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0076] Vinyl ester polymers are preferably polymers obtained using only one or more vinyl ester monomers as monomers, and more preferably polymers obtained using only one vinyl ester monomer as monomer. It should be noted that vinyl ester polymers can be copolymers of one or more vinyl ester monomers with other monomers capable of copolymerizing with them.

[0077] Other monomers include, for example, ethylene; olefins with 3 to 30 carbon atoms such as propylene, 1-butene, and isobutene; acrylic acid or its salts; acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or its salts; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamide propanesulfonic acid or its salts, acrylamide propyl dimethylamine or its salts, and N-hydroxymethylacrylamide or its derivatives. Biological acrylamide derivatives; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamide propanesulfonic acid or its salts, methacrylamide propyl dimethylamine or its salts, N-hydroxymethylmethacrylamide or its derivatives; N-vinyl amides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; cyanide such as acrylonitrile and methacrylonitrile; halogenated vinylides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl acetate, allyl chloride and other allyl compounds; maleic acid or its salts, esters or anhydrides; itaconic acid or its salts, esters or anhydrides; vinyl silyl compounds such as vinyltrimethoxysilane; isopropyl acetate, etc. It should be noted that vinyl ester polymers can have one or more structural units derived from these other monomers.

[0078] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 mol% or less, more preferably 8 mol% or less, depending on the total molar number of all structural units constituting the vinyl ester polymer. Generally, there is a tendency for PVA crystallization to occur less frequently as the proportion of structural units derived from other monomers in the vinyl ester polymer increases. Therefore, by copolymerizing these other monomers in the aforementioned proportions, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted.

[0079] The degree of polymerization of PVA is preferably 200 or higher, more preferably 300 or higher, and even more preferably 500 or higher. By setting the degree of polymerization of PVA to the above lower limit or higher, over-crystallization of PVA can be prevented and the mechanical strength of the resulting PVA film can be ensured. On the other hand, 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. Generally speaking, there is a tendency that the higher the degree of polymerization of PVA, the more difficult it is to crystallize PVA. Therefore, by setting the degree of polymerization of PVA to the above upper limit or lower, PVA crystallization can be carried out appropriately, and the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted. In addition, by setting the degree of polymerization of PVA to the above upper limit or lower, the viscosity of the PVA film-forming solution will not be too high, and the productivity of PVA film can be improved.

[0080] The degree of polymerization of PVA refers to the average degree of polymerization as recorded in JIS K 6726-1994. That is, the degree of polymerization (Po) is calculated using the following formula (12).

[0081] Degree of polymerization Po = ([η] × 10) 4 / 8.29) (1 / 0.62) (12)

[0082] In the above formula (12), η is the intrinsic viscosity (decL / g) of PVA after resaponification and refining in water at 30°C.

[0083] The degree of saponification of PVA is preferably 90 mol% or more, more preferably 95 mol% or more, further preferably 99 mol% or more, and particularly preferably 99.8 mol% or more. Generally, there is a tendency for higher PVA saponification to facilitate PVA crystallization. Therefore, by setting the degree of PVA saponification to the above-mentioned lower limit or above, PVA crystallization can be appropriately carried out, 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 PVA film-forming solution, the crystallinity near the surface of the PVA film, i.e., the outermost layer, and the deeper interior of the PVA film are more likely to increase during the heat treatment process of the dried PVA film.

[0084] The degree of saponification of PVA refers to the proportion (mol%) of the number of moles of vinyl alcohol units relative to the total number of moles of structural units (typically vinyl ester monomer units) and vinyl alcohol units that can be converted into vinyl alcohol units through saponification. The degree of saponification of PVA can be determined according to JIS K 6726-1994.

[0085] The PVA film of the present invention may contain only one type of PVA, or it may contain two or more types of PVA with different degrees of polymerization, saponification, and modification.

[0086] The PVA content in the PVA film of the present invention is not necessarily limited, but is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0087] (Plasticizer)

[0088] The PVA film of the present invention preferably contains a plasticizer. By containing a plasticizer, the PVA film can be given the same softness as other plastic films, and the breakage of the PVA film can be suppressed during the film-forming and stretching processes.

[0089] Examples of plasticizers include ethylene glycol, glycerol, diglycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, sorbitol, and other polyols. These plasticizers can be used alone or in combination of two or more. Among these, ethylene glycol or glycerol are preferred as plasticizers, and glycerol is more preferred, considering reasons such as difficulty in seeping into the surface of the PVA film.

[0090] The plasticizer content in the PVA film of the present invention is preferably 1 part by weight or more, more preferably 3 parts by weight or more, and even more preferably 5 parts by weight or more, relative to 100 parts by weight of PVA. On the other hand, the plasticizer content is preferably 40 parts by weight or less, more preferably 30 parts by weight or less, and even more preferably 20 parts by weight or less, relative to 100 parts by weight of PVA. If the plasticizer content is within the above range, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be easily adjusted, and the effect of improving mechanical properties such as impact strength can be sufficiently obtained. In addition, it is possible to prevent the PVA film from becoming too soft and reducing its processability, or to prevent the plasticizer from seeping into the surface of the PVA film.

[0091] Here, the reason why the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted by changing the plasticizer content is as follows. Generally, if the PVA film contains an appropriate amount of plasticizer, PVA crystallization will occur. This is presumably because the plasticizer makes the polymer molecular chains of PVA more mobile, easily resulting in a more energy-stable crystalline or confined amorphous structure. On the other hand, if the PVA film contains excessive plasticizer, PVA crystallization is easily hindered. This is presumably because the amount of plasticizer that interacts with the hydroxyl groups of the PVA polymer molecular chains increases, weakening the interaction between the PVA polymer molecular chains. Therefore, by adjusting the plasticizer content, and thus appropriately crystallizing the PVA, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted.

[0092] (surfactant)

[0093] The PVA film of the present invention preferably contains a surfactant. By including a surfactant, the processability of the PVA film and the peelability of the PVA film during manufacturing can be improved. There are no particular limitations on the surfactant used, but anionic surfactants and nonionic surfactants are preferred.

[0094] Examples of anionic surfactants include carboxylic acid surfactants such as potassium laurate; sulfate ester surfactants such as octyl sulfate; and sulfonic acid surfactants such as dodecylbenzene sulfonate.

[0095] As nonionic surfactants, examples include alkyl ether surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether surfactants such as polyoxyethylene octylphenyl ether; alkyl ester surfactants such as polyoxyethylene laurate; alkylamine surfactants such as polyoxyethylene lauryl amino ether; alkylamide surfactants such as polyoxyethylene lauryl amide; polypropylene glycol ether surfactants such as polyoxyethylene polyoxypropylene ether; alkanolamide surfactants such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ether surfactants such as polyoxyalkylene allyl phenyl ether.

[0096] This surfactant can be used alone or in combination of two or more. As a surfactant, nonionic surfactants are preferred, more preferably alkanolamide type surfactants, and even more preferably dialkylolamides (e.g., diethanolamides) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids with 8 to 30 carbon atoms).

[0097] The surfactant content in the PVA film of the present invention is preferably 0.01 parts by weight or more, more preferably 0.02 parts by weight or more, and even more preferably 0.05 parts by weight or more, relative to 100 parts by weight of PVA. On the other hand, the surfactant content is preferably 10 parts by weight or less, more preferably 1 part by weight or less, even more preferably 0.5 parts by weight or less, and particularly preferably 0.3 parts by weight or less, relative to 100 parts by weight of PVA. If the surfactant content is within the above range, the peelability of the PVA film self-made film device during manufacturing becomes good, and adhesion (hereinafter sometimes referred to as "adhesion") between PVA films can be prevented. In addition, it is possible to prevent surfactant from seeping onto the surface of the PVA film or causing deterioration of the appearance of the PVA film due to surfactant aggregation.

[0098] (Other ingredients)

[0099] In the PVA film of the present invention, in addition to containing PVA, it may also contain components such as water-soluble polymers, water, antioxidants, ultraviolet absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, mildew-proof agents, and other polymer compounds within the range that does not hinder the effects of the present invention. The proportion of the total mass of PVA, surfactants, plasticizers, and other components other than PVA in the total mass of the PVA film is preferably 60% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. The proportion of the total mass of other components in the total mass of the PVA film is preferably 100% by mass or less.

[0100] (Physical properties)

[0101] The PVA film of the present invention is water-insoluble. By making the PVA film water-insoluble, when performing uniaxial stretching during the manufacture of optical films such as polarizing films in an aqueous solution, even if the maximum stretching speed is high, stretching can be performed during uniaxial stretching without causing the PVA film to break. Here, in the present invention, water-insoluble means that when the PVA film is immersed in water (deionized water) at 30 °C according to the following steps <1> to <4>, the PVA film is not completely dissolved but partially dissolved and remains.

[0102] <1> Place the PVA film in a thermo-hygrostat adjusted to 20 °C and 65% RH for 16 hours or more for humidity conditioning.

[0103] <2> After cutting out a rectangular sample with a length of 40 mm × a width of 35 mm from the humidity-conditioned PVA film, fix it between two 50 mm × 50 mm plastic plates with a rectangular window (hole) of a length of 35 mm × a width of 23 mm, such that the length direction of the sample is parallel to the length direction of the window and the sample is located approximately in the center of the width direction of the window.

[0104] <3> Pour 300 mL of deionized water into a 500 mL beaker, stir it with a magnetic stirrer equipped with a 3 cm long rod at a rotation speed of 280 rpm, and adjust the water temperature to 30 °C.

[0105] <4> While paying attention to ensuring that the sample fixed to the plastic plate in <2> does not contact the rotating rod of the magnetic stirrer, immerse it in the deionized water in the beaker for 1000 seconds.

[0106] <Manufacturing method of PVA film>

[0107] The manufacturing method of the PVA film of the present invention is not particularly limited, and any method such as the following can be used. Examples of such methods include: a film-forming method using a casting method, a wet film-forming method (a method of spraying in a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method of temporarily cooling and gelling the film-forming solution and then extracting and removing the solvent), or a combination thereof, to form a film-forming solution obtained by adding solvents, additives, etc., to a T-die or similar die, thereby forming a film; and a melt extrusion film-forming method, a blow molding method, etc., to form a film by extruding the film-forming solution obtained using an extruder or similar device. Among these, casting and melt extrusion film-forming methods are preferred as manufacturing methods for PVA films. Using these methods, a homogeneous PVA film can be obtained with good productivity. The following describes the case of manufacturing a PVA film using casting or melt extrusion film-forming methods.

[0108] When manufacturing the PVA film of the present invention using a casting method or a melt extrusion method, firstly, a film-forming stock solution containing PVA, a solvent, and additives such as plasticizers as needed is prepared. Next, the film-forming stock solution is cast (supplyed) into a film shape on a rotating support such as a metal roller or metal belt. Thus, a liquid coating of the film-forming stock solution is formed on the support. The liquid coating is cured by heating on the support and removing the solvent, thereby achieving film formation. Examples of methods for heating the liquid coating include: heating the support itself to a high temperature using a heat medium, or blowing hot air onto the side of the liquid coating opposite to the side in contact with the support. The cured strip film (PVA film) is peeled off from the support, dried as needed using a drying roller or drying oven, and then heat-treated as needed before being wound into a roll.

[0109] In the drying process (solvent removal process) of the liquid coating cast on the support, and the subsequent drying process of the PVA film, PVA crystallizes during heating. The crystallization rate at this time is influenced not only by the proportion 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 plasticizer, but also by the moisture content, temperature, and stretching (elongation in the flow direction) of the PVA. Regarding stretching, it can be inferred that it is influenced by the oriented crystallization caused by the stretching of the PVA polymer molecular chains.

[0110] Typically, the drying of PVA films involves the gradual evaporation of volatile components from the film surface, which are not in contact with the support or drying rollers. Therefore, during the drying process, a concentration distribution of volatile components such as moisture occurs along the thickness direction of the PVA film. Consequently, depending on the temperature and stretching conditions, a crystallinity index distribution occurs along the thickness direction of the PVA film. This crystallinity index distribution can be adjusted using factors such as the volatile fraction of the film-forming solution, the support temperature, the contact time with the support, the hot air temperature and flow rate, and the temperatures of the drying rollers and drying oven. Therefore, by appropriately adjusting these factors, PVA crystallization can be moderately achieved, and the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be adjusted.

[0111] The volatile fraction (concentration of volatile components such as solvents removed by evaporation or volatilization during film formation) of the film-forming solution is preferably 50% by mass or more, more preferably 55% by mass or more. The volatile fraction of the film-forming solution is preferably 90% by mass or less, more preferably 80% by mass or less. If the volatile fraction is within the above range, the viscosity of the film-forming solution can be adjusted to a suitable range, thus improving the film-forming properties of the liquid coating cast on the support and easily obtaining a PVA film with a uniform thickness. In addition, if the volatile fraction is within the above range, PVA crystallization is appropriately carried out on the support, thus making it easy to adjust the crystallinity index and its distribution of the obtained PVA film. The film-forming solution may contain dichroic dyes as needed. Furthermore, the volatile fraction of the film-forming solution refers to the value obtained using the following formula (13).

[0112] The volatile fraction (mass%) of the film-forming solution = {(Wa-Wb) / Wa) × 100 (13)

[0113] In the above formula (13), Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of Wa(g) film-forming stock solution after drying in an electric dryer at 105°C for 16 hours.

[0114] There are no particular limitations on the method of adjusting the film-forming solution. Examples include dissolving PVA and additives such as plasticizers and surfactants in a solvent such as a dissolving tank; or using a single-screw extruder or a twin-screw extruder to melt and mix PVA in an aqueous state with additives such as plasticizers and surfactants.

[0115] The film-forming solution typically flows through the die lip of a T-die or similar die onto a support such as a metal roller or strip to form a film. On the support, the solvent gradually evaporates from the surface of the cast film that is not in contact with the support (hereinafter sometimes referred to as the free surface), while it does not evaporate from the surface that is substantially in contact with the support (hereinafter sometimes referred to as the contact surface). Therefore, relative to the thickness direction of the film, a distribution occurs where the solvent concentration is low on the free surface side and high on the contact surface side. Consequently, the curing of PVA also begins from the free surface.

[0116] PVA crystallization also occurs simultaneously with PVA curing. Crystallization of PVA is difficult regardless of whether the solvent concentration is too high or too low, and it also depends on the primary structure of the PVA molecules. However, it is easier to crystallize when the volatile fraction of the casting solution is in the range of 20-60% by mass. Furthermore, the higher the temperature, the faster the crystallization rate of PVA, but the higher the temperature, the faster the solvent evaporates. Therefore, in order to effectively crystallize near the surface of the PVA film, i.e., the electrode layer, it is important to control the crystallinity index (Fg1 and Fg2) of the electrode layer. In addition to controlling the temperature of the support and the contact time with the support, it is also important to control the atmosphere temperature near the free surface and the vapor pressure of the solvent.

[0117] The PVA film of the present invention has a higher crystallinity near the surface of the film, i.e., the outermost layer, compared to the crystallinity of the deeper interior of the film. Therefore, to obtain the PVA film of the present invention, it is only necessary to select conditions that allow crystallization to occur near the surface of the film, i.e., the outermost layer, while suppressing crystallization in the deeper interior of the film. For example, at the initial stage of drying, when the volatile fraction near the surface of the film, i.e., the outermost layer, is reduced, conditions such as lowering the drying temperature or other methods to slow down the drying process are used to pre-increase the moisture content of the outermost layer during crystallization. On the other hand, from the middle stage of drying, when crystallization occurs in the deeper interior of the film, to the later stage, conditions such as rapid drying at a higher temperature can be used to prevent internal crystallization.

[0118] The surface temperature of the support for casting the film-forming solution is preferably 65°C or higher, more preferably 70°C or higher. The surface temperature of the support for casting the film-forming solution is preferably 110°C or lower, more preferably 100°C or lower, and even more preferably 95°C or lower. If the surface temperature is within the above range, the drying of the liquid coating cast on the support and the crystallization near the surface of the film, i.e., the electrode surface portion, will proceed at a suitable rate, thereby allowing adjustment of the crystallinity index (Fg1 and Fg2) of the PVA film.

[0119] While heating the liquid coating on the support, hot air with a velocity of 1 to 10 m / s can be uniformly blown onto the entire area of ​​the non-contact side of the liquid coating. The temperature of the hot air blown onto the non-contact side is preferably 50°C or higher, more preferably 70°C or higher. The temperature of the hot air blown onto the non-contact side is preferably 150°C or lower, more preferably 120°C or lower. Furthermore, 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, more preferably 30% RH or lower. If the temperature and humidity of the hot air blown onto the non-contact side are within the above ranges, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be easily adjusted.

[0120] The PVA film is preferably dried (solvent removed) on a support until the volatile content is 5-50% by mass, then peeled off from the support and further dried as needed. The drying method is not particularly limited; examples include passing the film through a drying oven and contacting it with drying rollers. When using multiple drying rollers to dry the PVA film, it is preferable to alternately contact one surface of the PVA film with the drying rollers. This allows adjustment of the difference in crystallinity indices (|Fd1-Fd2| and |Fg1-Fg2|) of the PVA on both sides of the PVA film (the two surfaces orthogonal to the thickness direction). In this case, the number of drying rollers is preferably 3 or more, more preferably 4 or more, and even more preferably 5 to 30.

[0121] The upper limit of the temperature of the drying oven or the surface temperature of the drying roller is preferably 110°C or less, more preferably 100°C or less, further preferably 90°C or less, and particularly preferably 85°C or less. On the other hand, the lower limit of the temperature of the drying oven or the surface temperature of the drying roller is preferably 40°C or more, more preferably 45°C or more, and further preferably 50°C or more. By setting the temperature of the drying oven or the surface temperature of the drying roller within the above range, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be easily adjusted.

[0122] The dried PVA film can be further heat-treated as needed. Heat treatment can improve the crystallinity near the surface (the outermost layer) and the deeper interior of the film, thus adjusting the crystallinity indices (Fg1, Fg2, Fd1, and Fd2) of the PVA film. Furthermore, it can also adjust the mechanical strength, swelling properties, and other characteristics of the PVA film.

[0123] The lower limit of the surface temperature of the heat treatment roller used for heat treatment is preferably 60°C or higher. The upper limit of the surface temperature of the heat treatment roller is preferably 135°C or lower, more preferably 130°C or lower. By setting the surface temperature of the heat treatment roller within the above range, the crystallinity index (Fg1, Fg2, Fd1, and Fd2) of the PVA film can be easily adjusted.

[0124] The PVA film manufactured in this way is further subjected to moisture conditioning treatment and cutting of both ends (edges) of the film as needed, and then rolled into a roll on a cylindrical core for moisture-proof packaging to form a finished product.

[0125] The volatile content of the PVA film obtained through a series of processes is not necessarily limited. The volatile content of the PVA film is preferably 1% by mass or more, more preferably 2% by mass or more. The volatile content of the PVA film is preferably 5% by mass or less, more preferably 4% by mass or less.

[0126] <Methods for manufacturing optical films>

[0127] The PVA film of the present invention is used as a raw material film in the manufacture of optical films. Examples of optical films include polarizing films, field-of-view improvement films, phase difference films, and brightness enhancement films, with polarizing films being preferred. Hereinafter, as an example of a method for manufacturing an optical film, a method for manufacturing a polarizing film will be specifically described.

[0128] Polarizing films are typically manufactured by using PVA film as the base material and undergoing processing steps such as swelling, dyeing, crosslinking, stretching, and fixation. Specific examples of the processing solutions used in each step include swelling solutions, dyeing solutions, crosslinking solutions, stretching solutions, fixation solutions, and cleaning solutions.

[0129] The following details the various processing steps that can be used in the manufacturing method for polarizing films. It should be noted that in the manufacturing method for polarizing films, one or more of the following processes may be omitted, the same process may be performed multiple times, and other processes may be performed simultaneously.

[0130] (Cleaning treatment before swelling)

[0131] Before performing swelling treatment on the PVA film, it is preferable to clean the PVA film. This pre-swelling cleaning removes anti-adhesion agents and other contaminants adhering to the PVA film, preventing contamination of the processing solutions in the polarization film manufacturing process by these agents. Cleaning is preferably performed by immersing the PVA film in a cleaning solution, or by blowing the cleaning solution onto the PVA film. Water, for example, can be used as the cleaning solution. The temperature of the cleaning solution is preferably in the range of 20–40°C. Maintaining a temperature of 20°C or higher facilitates the removal of anti-adhesion agents and other contaminants adhering to the PVA film. Furthermore, maintaining a temperature of 40°C or lower prevents partial dissolution of the PVA film surface, film adhesion, and reduced processability. The temperature of the cleaning solution is more preferably 22°C or higher, further preferably 24°C or higher, and particularly preferably 26°C or higher. Additionally, the temperature of the cleaning solution is more preferably 38°C or lower, further preferably 36°C or lower, and particularly preferably 34°C or lower.

[0132] (Swelling treatment)

[0133] Swelling treatment can be performed by immersing the 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. Furthermore, the immersion time in the swelling treatment solution is preferably 0.1 minutes or higher, more preferably 0.5 minutes or higher. The immersion time in the swelling treatment solution is preferably 5 minutes or lower, more preferably 3 minutes or lower. It should be noted that the water used as the swelling treatment solution is not limited to pure water; it can be an aqueous solution containing various components such as boron compounds, or a mixture of water and an aqueous medium. The type of boron compound is not particularly limited; from a processability point of view, boric acid or borax is preferred. When the swelling treatment solution contains a boron compound, from the viewpoint of improving the stretchability of the PVA film, its concentration is preferably 6% by mass or lower.

[0134] (Staining treatment)

[0135] The dyeing process can be performed using iodine-based dyes as dichroic dyes. The dyeing stage can be any period before, during, or after the stretching treatment. Preferably, the dyeing process is performed by immersing the PVA film in a solution containing iodine and potassium iodide (suitably an aqueous solution). The iodine concentration in the dyeing solution is preferably 0.005% by mass or more. The iodine concentration in the dyeing solution is preferably 0.2% by mass or less. The potassium iodide / iodine (by mass) ratio is preferably 20 or more. The potassium iodide / iodine (by mass) ratio is preferably 100 or less. The temperature of the dyeing solution is preferably 20°C or more, more preferably 25°C or more. The temperature of the dyeing solution is preferably 50°C or less, more preferably 40°C or less. The dyeing solution may contain boron-containing compounds such as boric acid as crosslinking agents. It should be noted that if the PVA film used as the base film already contains a dichroic dye, the dyeing process can be omitted. Alternatively, the PVA film used as the base film may also contain boron-containing compounds such as boric acid or borax.

[0136] (Cross-linking treatment)

[0137] In manufacturing polarizing films, for the purpose of firmly adsorbing dichroic dyes onto the PVA film, it is preferable to perform a crosslinking treatment after dyeing. The crosslinking treatment can be performed by using a solution containing a crosslinking agent (suitably an aqueous solution) as the crosslinking treatment liquid, and immersing the PVA film in the crosslinking treatment liquid. As the crosslinking agent, one or more boron-containing compounds such as boric acid and borax can be used. If the concentration of the crosslinking agent in the crosslinking treatment liquid is too high, there is a tendency for excessive crosslinking reaction to occur, making it difficult to achieve sufficient stretching during subsequent stretching treatment; conversely, if the concentration is too low, there is a tendency for the crosslinking treatment effect to decrease. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0138] To suppress the dissolution of dichroic dyes from the dyed PVA film, the crosslinking treatment solution may contain iodine-containing compounds such as potassium iodide. If the concentration of the iodine-containing compound in the crosslinking treatment solution is too high, the heat resistance of the resulting polarized film tends to decrease, although the reason is unclear. Conversely, if the concentration of the iodine-containing compound in the crosslinking treatment solution is too low, the effect of suppressing the dissolution of dichroic dyes tends to decrease. The concentration of the iodine-containing compound in the crosslinking treatment solution is preferably 1% by mass or more, more preferably 1.5% by mass or more, and even more preferably 2% by mass or more. The concentration of the iodine-containing compound in the crosslinking treatment solution is preferably 6% by mass or less, more preferably 5.5% by mass or less, and even more preferably 5% by mass or less.

[0139] If the temperature of the crosslinking treatment solution is too high, the polarizing film obtained may be prone to uneven dyeing due to the dissolution of dichroic dyes. Conversely, if the temperature is too low, the crosslinking effect may sometimes be reduced. The temperature of the crosslinking treatment 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 treatment solution is preferably 45°C or lower, more preferably 40°C or lower, and even more preferably 35°C or lower.

[0140] In and between the aforementioned processes, the PVA film can be stretched differently than in the stretching process described later. This stretching (pre-stretching) prevents wrinkles from forming on the surface of the PVA film. From the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio of the pre-stretch (the ratio obtained by multiplying by the stretching ratio in each process) is preferably 4 times or less, more preferably 3.5 times or less, based on the original length of the PVA film before stretching. From the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio of the pre-stretch is preferably 1.5 times or more, based on the original length of the PVA film before stretching. The stretching ratio in the swelling process is preferably 1.1 times or more, more preferably 1.2 times or more, and even more preferably 1.4 times or more. The stretching ratio in the swelling process is preferably 3 times or less, more preferably 2.5 times or less, and even more preferably 2.3 times or less. The stretching ratio in the dyeing process is preferably 2 times or less, more preferably 1.8 times or less, and even more preferably 1.5 times or less. The stretching ratio in the dyeing process is even more preferably 1.1 times or more. The stretching ratio during the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and even more preferably 1.3 times or less. The stretching ratio during the crosslinking treatment is even more preferably 1.05 times or more.

[0141] (Stretching treatment)

[0142] The stretching treatment can be performed using either a wet stretching method or a dry stretching method. In the case of a wet stretching method, a solution containing a boron-containing compound such as boric acid (suitably an aqueous solution) can be used as the stretching treatment solution, and the stretching treatment can be performed in the stretching treatment solution, or in a dyeing treatment solution or a fixation treatment solution described later. Alternatively, in the case of a dry stretching method, a water-absorbed PVA film can be used and the stretching can be performed in air. Of these, a 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% by mass or more, more preferably 2.0% by mass or more, and even more preferably 2.5% by mass or more, from the perspective of improving the stretchability of the PVA film. The concentration of the boron-containing compound in the stretching treatment solution is preferably 7% by mass or less, more preferably 6.5% by mass or less, and even more preferably 6% by mass or less, from the perspective of improving the stretchability of the PVA film.

[0143] The stretching treatment solution preferably contains iodine-containing compounds such as potassium iodide. If the concentration of the iodine-containing compound in the stretching treatment solution is too high, the resulting polarized film tends to have a noticeably bluish hue. Conversely, if the concentration is too low, although the reason is unclear, the heat resistance of the resulting polarized film tends to decrease. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 2% by mass or more, more preferably 2.5% by mass or more, and even more preferably 3% by mass or more. The concentration of the iodine-containing compound in the stretching treatment solution is preferably 8% by mass or less, more preferably 7.5% by mass or less, and even more preferably 7% by mass or less.

[0144] If the temperature of the stretching treatment solution is too high, the PVA film tends to dissolve, soften, and easily break. Conversely, if the temperature is too low, the tensile properties tend to decrease. The temperature of the stretching treatment 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 treatment solution is preferably 70°C or lower, more preferably 67.5°C or lower, and even more preferably 65°C or lower. It should be noted that the preferred range of stretching temperature when using the dry stretching method is also as described above.

[0145] Regarding the stretching ratio in the stretching process, considering that a higher ratio yields a polarizing film with superior polarization performance, a ratio of 1.2 times or more is preferred, more preferably 1.5 times or more, and even more preferably 2 times or more. Furthermore, based on the original length of the PVA film of the raw material before stretching, from the viewpoint of the polarization performance of the resulting polarizing film, the total stretching ratio (the ratio obtained by multiplying the stretching ratio in each process, including the aforementioned stretching ratio before stretching) is preferably 5.5 times or more, more preferably 5.7 times or more, and even more preferably 5.9 times or more. There is no particular upper limit to the stretching ratio; however, if it is too high, stretching breakage is likely to occur, therefore, a ratio of 8 times or less is preferred.

[0146] There is no particular limitation on the method of stretching by uniaxial stretching; uniaxial stretching in the length direction or transverse uniaxial stretching in the width direction can be used. In the case of manufacturing polarizing films, from the viewpoint of obtaining polarizing films with excellent polarization performance, uniaxial stretching in the length direction is preferred. Uniaxial stretching in the length direction can be performed using a stretching device with multiple parallel rollers, and the circumferential speed between each roller can be varied.

[0147] In this invention, the maximum stretching speed (% / min) during uniaxial stretching is not particularly limited, but is preferably 200% / min or higher, more preferably 300% / min or higher, and even more preferably 400% / min or higher. Here, maximum stretching speed refers to the fastest stretching speed in a given stage when the PVA film is stretched in two or more stages using three or more rollers with different circumferential speeds. It should be noted that if the PVA film stretching is performed in one stage instead of two or more stages, the stretching speed in that stage is considered the maximum stretching speed. Furthermore, stretching speed refers to the increase in the length of the PVA film due to stretching, relative to the original length of the PVA film per unit time. For example, a stretching speed of 100% / min means the speed at which the PVA film doubles in length from its original length in one minute. A higher maximum stretching speed allows for faster PVA film stretching (uniaxial stretching), resulting in increased productivity of the polarizing film, which is therefore preferable. On the other hand, if the maximum stretching speed becomes too high, excessive local tension on the PVA film during the stretching process (uniaxial stretching) can easily lead to tensile fracture. From this perspective, the maximum stretching speed is preferably no more than 900% / min.

[0148] (Fixed treatment)

[0149] In manufacturing polarizing films, a fixation treatment is preferably performed to ensure that dichroic dyes are firmly adsorbed onto the PVA film. The fixation treatment can be performed by using a solution (suitably an aqueous solution) containing one or more boron-containing compounds such as boric acid and borax as the fixation solution, and immersing the PVA film (suitably a stretched PVA film) in the fixation solution. Alternatively, the fixation solution may contain iodine-containing compounds or metal compounds as needed. The concentration of the boron-containing compound in the fixation solution is preferably 2% by mass or more, more preferably 3% by mass or more. The concentration of the boron-containing compound in the fixation solution is preferably 15% by mass or less, more preferably 10% by mass or less. The temperature of the fixation solution is preferably 15°C or more, more preferably 25°C or more. The temperature of the fixation solution is preferably 60°C or less, more preferably 40°C or less.

[0150] (Cleaning process after dyeing)

[0151] After dyeing, it is preferable to clean the stretched PVA film. Cleaning is preferably performed by immersing the PVA film in a cleaning solution, or by blowing the cleaning solution onto the PVA film. Water, for example, can be used as the cleaning solution. The water is not limited to pure water and may contain iodine-containing compounds such as potassium iodide. It should be noted that the cleaning solution may contain boron-containing compounds; in this case, the concentration of the boron-containing compound is preferably 2.0% by mass or less.

[0152] The temperature of the cleaning solution is preferably in 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 caused by water freezing can be suppressed. Furthermore, by keeping the temperature of the cleaning solution at 40°C or lower, the optical properties of the resulting polarizing film are improved. The temperature of the cleaning solution is more preferably 7°C or higher, and even more preferably 10°C or higher. Furthermore, the temperature of the cleaning solution is more preferably 38°C or lower, and even more preferably 35°C or lower.

[0153] Specific methods for manufacturing polarizing films include dyeing, stretching, crosslinking, and / or fixing treatments on a PVA film. As a preferred example, a method may be described that sequentially performs swelling, dyeing, crosslinking, stretching (especially uniaxial stretching), and cleaning treatments on the PVA film. Furthermore, the stretching treatment can be performed in any processing step preceding the aforementioned steps, or it can be performed in multiple stages (two or more).

[0154] By drying the PVA film after the aforementioned treatments, a polarizing film can be obtained. The drying method is not particularly limited; examples include contact drying (where the film is brought into contact with heated rollers), drying in a hot air dryer, and floating drying (where the film is floated while being dried with hot air).

[0155] <Polarizing plate>

[0156] The polarizing film obtained by the above operation is preferably laminated with an optically transparent and mechanically strong protective film on both sides or one side to form a polarizing plate for use. As the protective film, cellulose triacetate (TAC) film, cyclic olefin polymer (COP) film, cellulose acetate butyrate (CAB) film, acrylic film, polyester film, etc., can be used. Furthermore, as the adhesive used for lamination, PVA-based adhesives, urethane-based adhesives, etc., are examples, with PVA-based adhesives being preferred.

[0157] The polarizing plate obtained by the above operation can be used as a component of an LCD by being bonded to a glass substrate after being laminated with an acrylic adhesive. Phase retardation film, viewing angle improvement film, brightness enhancement film, etc. can also be bonded at the same time.

[0158] Example

[0159] The present invention will be specifically described below through examples, but the present invention is not limited to the following examples.

[0160] <Calculation of Crystallinity Index Based on FT-IR Measurement>

[0161] A 30mm wide × 30mm long PVA film was cut from the PVA film obtained in the following examples or comparative examples and used as a test sample. The crystallinity index of the PVA film varies slightly due to the moisture absorption of the PVA film; therefore, the test sample was stored at 24.0°C and 45.0% RH for 24 hours, and FT-IR measurements were performed using a testing apparatus set up in a room with the same environment. In the FT-IR measurement, measurements were taken on both sides of the PVA film (the two surfaces of the PVA film orthogonal to the thickness direction, the first surface and the second surface) under the following conditions.

[0162] Measuring apparatus: NICOLET is 10 (manufactured by Thermo Fisher Scientific)

[0163] Measurement conditions: Incident angle of 45° for single-reflection ATR method.

[0164] Resolution: 4.0cm -1

[0165] Total number of times: 32

[0166] Measurement temperature: 24.0℃ (ambient temperature)

[0167] Humidity measured: 45.0% RH (ambient relative humidity)

[0168] ATR prism: diamond prism or germanium prism

[0169] Based on the infrared absorption spectrum obtained by FT-IR measurement of the PVA film, the crystallinity index of the two surfaces of the PVA film (the first surface and the second surface) is calculated using the aforementioned method.

[0170] <Evaluation of Surface Wrinkles in Polarizing Films>

[0171] For the polarizing film obtained in the following embodiments or comparative examples, the surface of the polarizing film was illuminated with a fluorescent lamp at an angle, and the reflected light was visually observed to confirm the wrinkle state of the surface of the polarizing film, and evaluated according to the following criteria.

[0172] Evaluation Criteria:

[0173] A: No wrinkles can be identified.

[0174] B: A small amount of wrinkles that are confirmed to have no practical problems.

[0175] C: Wrinkles that are clearly confirmed to be a problem in practical use.

[0176] <Evaluation of the stretching fracture frequency during the production of a polarizing film>

[0177] In the following examples or comparative examples, uniaxial stretching in the stretching process during the production of a polarizing film was continuously performed for 20 minutes. The number of stretching fractures that occurred during this 20-minute continuous stretching was measured, and the stretching fracture frequency (times / 20 min) was evaluated.

[0178] <Example 1>

[0179] <Production and evaluation of a PVA film>

[0180] Using 100 parts by mass of PVA (saponification degree: 99 mol%, polymerization degree: 2400), 12 parts by mass of glycerol plasticizer as a plasticizer, 0.1 parts by mass of diethanolamide laurate as a surfactant, and 217.6 parts by mass of water, melt mixing was performed using a melt extruder to prepare a film-forming stock solution (volatile fraction: 66% by mass). Then, this film-forming stock solution was ejected from a T-die onto a support (surface temperature: 80°C) to form a film-like shape, and a liquid coating film was formed on the support. On the support, hot air at 85°C and 3% RH was blown onto the entire surface of the liquid coating film that was not in contact with the support at a speed of 5 m / s for drying to obtain a PVA film (moisture content: 32% by mass). Then, this PVA film was peeled off from the support, and further dried between the first drying roller and the final drying roller (the 19th drying roller) immediately before the heat treatment roller in such a manner that one side and the other side of the PVA film alternately contacted each drying roller, and then peeled off from the final drying roller. At this time, the surface temperature of each drying roller from the first drying roller to the final drying roller was set to 75°C. Furthermore, the PVA film was peeled off from the final drying roller, and heat treatment was performed in such a manner that one side and the other side of the PVA film alternately contacted each heat treatment roller. At this time, the heat treatment was performed using 2 heat treatment rollers, and the surface temperature of each heat treatment roller was set to 90°C. For the obtained PVA film (thickness: 30 μm, width: 1200 mm), FT-IR measurement was performed using the above method, and the crystallinity index (Fg1, Fg2, Fd1, and Fd2) was calculated. The results are shown in Table 1.

[0181] <Production and evaluation of a polarizing film>

[0182] The obtained PVA film was cut into a width of 650 mm, and the film was successively subjected to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, cleaning treatment, and drying treatment to continuously manufacture a polarizing film. The swelling treatment was carried out by uniaxially stretching along the longitudinal direction to 2.00 times while immersing in pure water (swelling treatment liquid) at 25 °C. The dyeing treatment was carried out by uniaxially stretching along the longitudinal direction to 1.26 times while immersing in a potassium iodide / iodine dyeing solution (dyeing treatment liquid) at 32 °C (the mass ratio of potassium iodide / iodine was 23, and the iodine concentration was 0.03 to 0.05 mass%). In this dyeing treatment, the iodine concentration in the dyeing treatment liquid was adjusted within the range of 0.03 to 0.05 mass% so that the monomer 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 uniaxially stretching along the longitudinal direction to 1.19 times while immersing in an aqueous boric acid solution (crosslinking treatment liquid) at 32 °C (the boric acid concentration was 2.6 mass%). The stretching treatment was carried out by uniaxially stretching along the longitudinal direction to 2.00 times while immersing in an aqueous boric acid / potassium iodide solution (stretching treatment liquid) at 55 °C (the boric acid concentration was 2.8 mass%, and the potassium iodide concentration was 5 mass%). The maximum stretching speed of the uniaxial stretching in this stretching treatment was 400% / min. The cleaning treatment was carried out by immersing in an aqueous potassium iodide / boric acid solution (cleaning treatment liquid) at 22 °C (the potassium iodide concentration was 3 to 6 mass%, and the boric acid concentration was 1.5 mass%) for 12 seconds without stretching. The drying treatment was carried out by hot air drying at 80 °C for 1.5 minutes without stretching to obtain a polarizing film. For the obtained polarizing film, the surface wrinkles of the polarizing film and the stretching fracture frequency during the manufacture of the polarizing film were evaluated by the above method. The results are shown in Table 2.

[0183] <Example 2>

[0184] In <Manufacture and Evaluation of PVA Film> of Example 1, the PVA used for preparing the film-forming stock solution was changed to PVA (saponification degree: 99 mol%, polymerization degree: 2400, ethylene modification: 2.5 mol%), and the surface temperatures of the two heat treatment rollers were both changed to 85 °C. Except for this, the same operations as in Example 1 were carried out to obtain a PVA film and a polarizing film. For the obtained PVA film and polarizing film, the same measurements and evaluations as in Example 1 were carried out. The results are shown in Tables 1 and 2 respectively.

[0185] <Example 3>

[0186] In the <Manufacture and Evaluation of PVA Film> of Example 1, the surface temperature of the support was changed to 100°C, the temperature of the hot air blown onto the entire surface of the liquid film that does not contact the support was changed to 105°C, the surface temperatures of each drying roller from the first drying roller to the final drying roller (the 19th drying roller) immediately before the heat treatment roller were changed to 90°C, and the surface temperatures of both heat treatment rollers were changed to 80°C. Except for this, the same operations as in Example 1 were performed to obtain a PVA film and a polarizing film. For the obtained PVA film and polarizing film, measurements and evaluations were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0187] <Example 4>

[0188] In the <Manufacture and Evaluation of PVA Film> of Example 1, drying from the first drying roller to the final drying roller (the 19th drying roller) immediately before the heat treatment roller was performed by only bringing each drying roller into contact with one side of the PVA film (on the support, the side where the liquid film contacts the support). Except for this, the same operations as in Example 1 were performed to obtain a PVA film and a polarizing film. For the obtained PVA film and polarizing film, measurements and evaluations were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0189] <Comparative Example 1>

[0190] In the <Manufacture and Evaluation of PVA Film> of Example 1, the surface temperature of the support was set to 115°C, the temperature of the hot air blown onto the entire surface of the liquid film that does not contact the support was changed to 120°C, the surface temperatures of each drying roller from the first drying roller to the final drying roller (the 19th drying roller) immediately before the heat treatment roller were changed to 65°C, and the surface temperatures of both heat treatment rollers were changed to 65°C. Except for this, the same operations as in Example 1 were performed to obtain a PVA film and a polarizing film. For the obtained PVA film and polarizing film, measurements and evaluations were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0191] <Comparative Example 2>

[0192] In the <Manufacture and Evaluation of PVA Film> of Example 1, the surface temperature of the support was changed to 60°C, the temperature of the hot air blown onto the entire surface of the liquid film that does not contact the support was changed to 70°C, and the surface temperatures of each drying roller from the first drying roller to the final drying roller (the 19th drying roller) immediately before the heat treatment roller were changed to 90°C. Except for this, the same operations as in Example 1 were performed to obtain a PVA film and a polarizing film. For the obtained PVA film and polarizing film, measurements and evaluations were performed in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0193] <Reference Example 1>

[0194] In Example 1, "Manufacturing and Evaluation of Polarizing Film," the maximum uniaxial stretching speed during the stretching process was changed to 190%. Otherwise, the same procedures as in Comparative Example 1 were followed to obtain a PVA film and a polarizing film. The resulting PVA film and polarizing film were measured and evaluated in the same manner as in Example 1. The results are shown in Tables 1 and 2, respectively.

[0195]

[0196] [Table 2]

[0197]

[0198] As shown in Tables 1 and 2, when using the PVA films of Examples 1 to 4 to manufacture polarizing films, a small number of wrinkles were observed on the surface of the resulting polarizing films, to the point that they posed no practical problem. Here, the wrinkles on the surface of the polarizing film are caused by surface wrinkles in the PVA film, specifically, surface wrinkles in the PVA film generated during uniaxial stretching in the stretching process during the manufacture of the polarizing film. Therefore, it can be said that the PVA films of Examples 1 to 4 are less prone to surface wrinkling during uniaxial stretching.

[0199] As shown in Tables 1 and 2, when using the PVA films of Examples 1 to 4 to manufacture polarizing films, the frequency of tensile fracture during uniaxial stretching in a continuous 20-minute stretching process was 0 to 2 times / 20 minutes. Therefore, it can be said that the fracture of the PVA films of Examples 1 to 4 during stretching (uniaxial stretching) was suppressed.

[0200] Furthermore, as shown in Reference Example 1, when the maximum uniaxial stretching speed during the stretching process in manufacturing the polarizing film is a low speed (190% / min), even when using the PVA film of Comparative Example 1, no wrinkles were observed on the surface of the resulting polarizing film, and the breakage 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 a high speed (400% / min), when using the PVA film of Comparative Example 1, wrinkles of a practically problematic degree were clearly observed on the surface of the resulting polarizing film, and the breakage frequency during stretching (uniaxial stretching) was 5 times / 20 min.

[0201] That is, the PVA film of Comparative Example 1, when stretched at a high maximum speed (400% / min), is prone to wrinkling on its surface during uniaxial stretching, and can be said to be prone to breakage during stretching (uniaxial stretching). On the other hand, the PVA films of Examples 1 to 4, even when stretched at a high maximum speed (400% / min), are not prone to wrinkling on their surface during uniaxial stretching, and can be said to have their breakage during stretching (uniaxial stretching) suppressed.

[0202] Explanation of reference numerals in the attached figures

[0203] 1 PVA membrane

[0204] 2. Thickness direction of PVA film

[0205] 3 First Surface

[0206] 4 Second Surface

[0207] 5. Infrared penetration depth when using a diamond prism (approximately 2 μm)

[0208] 6. Infrared penetration depth when using a germanium prism (approximately 0.5 μm)

[0209] 7. ATR prism (diamond prism or germanium prism)

[0210] 8 Infrared

Claims

1. Polyvinyl alcohol film, which is a water-insoluble polyvinyl alcohol film. The two surfaces of the polyvinyl alcohol film orthogonal to the thickness direction are designated as the first surface and the second surface, respectively. The crystallinity indices of the first surface are set as Fd1 and Fg1. When the crystallinity indices of the second surface are set to Fd2 and Fg2, The Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (1) to (8): Fd1≤0.8 (1) Fd1 / Fg1<1 (2) Fd2≤0.8 (3) Fd2 / Fg2<1 (4) Fd1≥0.5 (5) Fd2≥0.5 (6) Fd1 / Fg1≥0.6 (7) Fd2 / Fg2≥0.6 (8) In equations (1) to (8), Fd1 is the crystallinity index calculated using a diamond prism when performing FT-IR measurements on the first surface using the ATR method; Fg1 is the crystallinity index calculated using a germanium prism when performing FT-IR measurements on the first surface using the ATR method; Fd2 is the crystallinity index calculated using a diamond prism when performing FT-IR measurements on the second surface using the ATR method; and Fg2 is the crystallinity index calculated using a germanium prism when performing FT-IR measurements on the second surface using the ATR method. The polyvinyl alcohol film is obtained as follows: a film-forming stock solution containing polyvinyl alcohol and solvent is cast into a film on a rotating support, thereby forming a liquid coating of the film-forming stock solution on the support. The liquid coating is filmified by heating on the support. When heating the liquid coating on the support, the humidity of the hot air blown onto the non-contact surface of the liquid coating at a wind speed of 1~10m / s is 1%RH or higher and 40%RH or lower, and the surface temperature of the support is 65~110℃.

2. The polyvinyl alcohol film according to claim 1, wherein, The value of Fd1 is greater than or equal to 0.52, and the value of Fd2 is greater than or equal to 0.

52.

3. The polyvinyl alcohol film according to claim 1 or 2, wherein, The ratio of Fd1 / Fg1 is 0.65 or higher, and the ratio of Fd2 / Fg2 is 0.65 or higher.

4. The polyvinyl alcohol film according to claim 1 or 2, wherein, The Fd1, Fg1, Fd2 and Fg2 satisfy the following equations (9)~(10): |Fd1-Fd2|≤0.07 (9) |Fg1-Fg2|≤0.07 (10).

5. The polyvinyl alcohol film according to claim 1 or 2, wherein it is a film for manufacturing optical films.

6. The polyvinyl alcohol film according to claim 5, wherein, The optical film is a polarizing film.

Images