Polyvinyl alcohol film

Abstract

A PVA film with excellent polarization performance and resistance to damp heat is provided, which can be manufactured without adding a step in the manufacturing of polarization film. A polyvinyl alcohol film, which is a water-insoluble polyvinyl alcohol film, wherein the composite elastic coefficient of the cross section obtained by perpendicularly cutting the first surface and the second surface of the aforementioned polyvinyl alcohol film is subjected to a nano-indentation test at 23°C and 40%RH, when the composite elastic coefficient near the first surface is set as Er1 (GPa), the composite elastic coefficient near the second surface is set as Er2 (GPa), and the composite elastic coefficient at the center of the thickness direction is set as Er3 (GPa), the aforementioned Er1, Er2 and Er3 satisfy the following equations (1) and (2): 0.40≦Er1-Er3≦1.20 (1) 0.40≦Er2-Er3≦1.20 (2) [In the aforementioned formulas (1) to (2), Er1 is the composite elastic coefficient of the region within 5 μm from the first surface to the center in the thickness direction, Er2 is the composite elastic coefficient of the region within 5 μm from the second surface to the center in the thickness direction, and Er3 is the composite elastic coefficient of the region in the center in the thickness direction.]

Description

Technical Field

[0001] This invention relates to polyvinyl alcohol films. Prior Technology

[0002] Polarizing plates, which have the function of transmitting and blocking light, and liquid crystals, which change the polarization state of light, are both basic components of liquid crystal displays (LCDs). Many polarizing plates have a structure in which a protective film, such as a cellulose triacetate (TAC) film, is laminated onto the surface of the polarizing film. As for the polarizing film constituting the polarizing plate, the mainstream method is to uniaxially stretch a polyvinyl alcohol film (hereinafter, sometimes abbreviated as "PVA") so that the aligned stretched film adsorbs dichroic pigments such as iodine-based pigments (I3-, I5-, etc.) and dichroic organic dyes. This type of polarizing film is manufactured by uniaxially stretching a PVA film that has been pre-containing dichroic pigments, or by adsorbing dichroic pigments during the uniaxial stretching of the PVA film, or by adsorbing dichroic pigments after uniaxial stretching of the PVA film.

[0003] LCDs have gradually been adapted for use in a wide range of applications, including small devices such as computers and watches, laptops, LCD TVs, car navigation systems, mobile phones, tablet terminals, and measuring instruments that can be used indoors and outdoors. In recent years, there has been a demand for polarizing films with durability superior to existing products, especially excellent resistance to damp heat under high temperature and high humidity conditions.

[0004] As polarizing films with excellent resistance to damp heat, those containing PVA, dichroic pigments, and specific organic acids are known (see Patent Document 1). Furthermore, a method has been proposed to contact the surface of the polarizing film with a processing solution containing a specific proportion of a reducing agent (see Patent Document 2). [Previous Technical Documents] [Patent Literature]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2011-237580 [Patent Document 2] International Publication No. 2016 / 056600 Summary of the Invention

[0006] [The problem the invention aims to solve]

[0007] However, based on the inventors' research, in the method described in Patent Document 1, because a specific organic acid is contained in the PVA film, the orientation of the dichroic pigments adsorbed on the PVA film is disturbed during the uniaxial stretching of the PVA film to manufacture a polarizing film, resulting in a decrease in the polarization performance of the obtained polarizing film. Furthermore, in the method described in Patent Document 2, it is necessary to increase the steps in manufacturing the polarizing film, and the cost increases due to the need to use a reducing agent.

[0008] The present invention was made based on the above circumstances, and its purpose is to provide a PVA film that can produce a polarizing film with excellent polarization performance and resistance to damp heat without adding steps to the manufacturing of the polarizing film. [Methods used to solve problems]

[0009] In order to achieve the above-mentioned objective, the inventors discovered that by adjusting the difference between the composite elastic coefficient near the surface of the cross-section formed by perpendicularly cutting the two surfaces of the PVA film and the composite elastic coefficient at the center of the thickness direction to a specific range, a polarizing film with excellent polarization performance and resistance to damp heat can be manufactured. Based on these insights, the inventors conducted further in-depth research and completed the present invention.

[0010] That is, this invention relates to: [1] A polyvinyl alcohol film, which is a non-water-soluble polyvinyl alcohol film, wherein the composite elastic coefficient of the cross section obtained by perpendicularly cutting the first surface and the second surface of the aforementioned polyvinyl alcohol film is subjected to a nano-indentation test at 23°C and 40%RH, when the composite elastic coefficient near the first surface is set as Er1 (GPa), the composite elastic coefficient near the second surface is set as Er2 (GPa), and the composite elastic coefficient at the center of the thickness direction is set as Er3 (GPa), the aforementioned Er1, Er2 and Er3 satisfy the following equations (1) and (2): 0.40≦Er1-Er3≦1.20 (1) 0.40≦Er2-Er3≦1.20 (2) [In the aforementioned equations (1) and (2), Er1 is the composite elastic coefficient of the region within 5 μm from the first surface to the center of the thickness direction, Er2 is the composite elastic coefficient of the region within 5 μm from the second surface to the center of the thickness direction, and Er3 is the composite elastic coefficient of the region at the center of the thickness direction]; [2] As described in [1] above, regarding the polyvinyl alcohol film, when the crystallinity index is measured by FT-IR using the ATR method on the first and second surfaces of the aforementioned polyvinyl alcohol film and the central portion of the cross section obtained by perpendicularly cutting the first and second surfaces, when the crystallinity index of the first surface is set as Fg1, the crystallinity index of the second surface is set as Fg2, and the crystallinity index of the central portion of the thickness direction is set as Fg3, the aforementioned Fg1, Fg2, and Fg3 satisfy the following equations (3) to (5): 0.70≦Fg1≦0.90 (3) 0.70≦Fg2≦0.90 (4) 0.30≦Fg3≦0.50 (5) [In the aforementioned formulas (3) to (5), Fg1 is the crystallinity index calculated when performing FT-IR measurement on the first surface, Fg2 is the crystallinity index calculated when performing FT-IR measurement on the second surface, and Fg3 is the crystallinity index calculated when performing FT-IR measurement on the central part of the cross-section in the thickness direction]; [3] The polyvinyl alcohol film as described in claim 2, wherein the aforementioned Fg1 and Fg2 satisfy the following formula (6): |Fg1-Fg2|≦0.10 (6); [4] The polyvinyl alcohol film described in any of [1] to [3] above is a film used for manufacturing optical films; [5] The polyvinyl alcohol film described in [4] above, wherein the aforementioned optical film is a polarizing film. [Effects of the Invention]

[0011] According to the present invention, a PVA film capable of producing a polarizing film with excellent polarization performance and resistance to damp heat can be provided. Simple Explanation of the Diagram

[0012] Figure 1 is a perspective view of the PVA film of the present invention. Figure 2 is a view of the PVA film of the present invention viewed from the thickness direction. Figure 3 is a cross-section of the PVA film of the present invention when the first and second surfaces are cut perpendicularly, and shows the position of the PVA film for nano-indentation test and FT-IR measurement. Figure 4 is a schematic diagram illustrating the ATR method in FT-IR determination. Implementation

[0013] [Mode for Carrying Out the Invention]

[0014] Hereinafter, the present invention will be described in detail.

[0015] <PVA Film> For the PVA film of the present invention, when performing a nanoindentation test on the cross-section obtained by vertically cutting the first surface and the second surface in the atmosphere of 23°C and 40% RH, regarding the complex elastic modulus, when the complex elastic modulus near the first surface is set as Er1 (GPa), the complex elastic modulus near the second surface is set as Er2 (GPa), and the complex elastic modulus at the central part in the thickness direction is set as Er3 (GPa), the aforementioned Er1, Er2, and Er3 satisfy the following formulas (1) and (2). 0.40≦Er1-Er3≦1.20 (1) 0.40≦Er2-Er3≦1.20 (2)

[0016] In the above formulas (1) to (2), Er1 is the complex elastic modulus of the region within 5 μm from the first surface toward the central part in the thickness direction, Er2 is the complex elastic modulus of the region within 5 μm from the second surface toward the central part in the thickness direction, and Er3 is the complex elastic modulus of the region at the central part in the thickness direction.

[0017] Here, 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 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 face each other. In the present invention, a sample is made by vertically cutting the PVA film 1 with respect to the first surface 3 and the second surface 4, and a cross-section 5 appears. Moreover, as shown in FIG. 3, nanoindentation tests are performed at each position of the vicinity 6 of the first surface, the vicinity 7 of the second surface, and the central part 8 in the thickness direction. The vicinity 6 of the first surface is the region within 5 μm from the first surface 3 toward the central part in the thickness direction. The vicinity 7 of the second surface is the region within 5 μm from the second surface 4 toward the central part in the thickness direction. The central part 8 in the thickness direction refers to the position on the straight line that is equidistant from and parallel to the first surface 3 and the second surface 4 in the thickness direction 2.

[0018] [[ID=2,6]] The complex elastic modulus Er measured by the nanoindentation method can be calculated from the rigidity S and the contact projected area Ac using the following formula (7).

[0019] The rigidity S is calculated from the slope of the unloading curve in the load-displacement curve. Furthermore, the aforementioned slope of the unloading curve refers to the slope of the unloading curve at high displacement, that is, the slope of the unloading curve at the moment unloading begins after the probe is pressed in. The contact projected area Ac is obtained using a specified formula from the contact depth (the difference in surface displacement at the surface surrounding the contact point, obtained from the gradient of the unloading curve at the specified indentation depth and maximum load) and the inherent constants and correction terms of the probe's geometry.

[0020] The composite elastic coefficient of PVA film is affected by the crystalline elastic coefficient, amorphous elastic coefficient, degree of crystallinity, and crystal size of PVA. Furthermore, the composite elastic coefficient of PVA film also varies depending on the moisture absorption of the PVA film. Therefore, in this invention, nano-indentation tests are conducted at 23°C and 40% RH. Specifically, in the embodiments, the composite elastic coefficient is calculated using the method described later.

[0021] The composite elastic modulus of PVA films is affected by the crystallinity of PVA. PVA films contain both crystalline and amorphous regions. Since the composite elastic modulus of crystalline regions is larger than that of amorphous regions, there is a tendency for higher crystallinity of PVA in a PVA film to result in a larger composite elastic modulus. Furthermore, small crystal size, high alignment, and high degree of polymerization of PVA are also factors contributing to an improved composite elastic modulus of PVA films.

[0022] Furthermore, PVA films exhibit structural and property differences along their thickness. In the PVA film of this invention, the composite elastic coefficients Er1 and Er2 near the first and second surfaces are greater than the composite elastic coefficient Er3 at the center of the thickness direction, and it is necessary to adjust these differences to a specific range. If the difference between the composite elastic coefficients Er1 and Er2 near the first and second surfaces of a PVA film and the composite elastic coefficient Er3 at the center of the thickness direction exceeds a certain range—that is, if the composite elastic coefficients Er1 and Er2 near the first and second surfaces are too large, and the composite elastic coefficient Er3 at the center of the thickness direction is too small—during the uniaxial stretching of the PVA film to manufacture a polarizing film, dichroic pigments such as iodine-based pigments are difficult to absorb near the surface, and dyeing cannot reach the center of the thickness direction. This results in reduced resistance to damp heat in the obtained polarizing film and polarizing plate. On the other hand, if the difference between the composite elastic coefficients Er1 and Er2 near the first and second surfaces of the PVA film and the composite elastic coefficient Er3 at the center of the thickness direction is less than a certain range—that is, if the composite elastic coefficients Er1 and Er2 near the first and second surfaces are too small, and the composite elastic coefficient Er3 at the center of the thickness direction is too large—then when manufacturing a polarizing film by uniaxial stretching the PVA film, if the stretching temperature is set to ensure that the stress of the PVA film during stretching is the same, the alignment of PVA near the surface will be suppressed, making it difficult to form iodine complexes that exhibit polarizing properties. The proportion of iodine complexes tends to be higher near the surface than at the center of the thickness direction, thus causing a significant reduction in the polarizing performance of the obtained polarizing film.

[0023] In the PVA film of the present invention, as shown in formulas (1) and (2) above, Er1-Er3 and Er2-Er3 need to be 0.40 GPa or more and 1.20 GPa or less. By setting Er1-Er3 and Er2-Er3 to 0.40 GPa or more, when the PVA film is uniaxially stretched to manufacture a polarizing film, the alignment of PVA near the first and second surfaces, where the proportion of iodine complexes exhibiting polarizing performance is high, is promoted, thus obtaining a polarizing film with excellent polarizing performance. On the other hand, by setting Er1-Er3 and Er2-Er3 to 1.20 GPa or less, when the PVA film is uniaxially stretched to manufacture a polarizing film, the dyeing reaches the central part in the thickness direction and forms iodine complexes, thus obtaining a polarizing film with excellent resistance to damp heat. The preferred values ​​for Er1-Er3 and Er2-Er3 are 0.42 GPa or higher, more preferably 0.45 GPa or higher, even more preferably 0.47 GPa or higher, and particularly preferably 0.50 GPa or higher. The preferred values ​​for Er1-Er3 and Er2-Er3 are 1.15 GPa or lower, more preferably 1.10 GPa or lower, even more preferably 1.05 GPa or lower, and particularly preferably 0.98 GPa or lower.

[0024] Regarding methods for achieving a PVA film forming pressure of 0.40 GPa to 1.20 GPa for Er1-Er3 and Er2-Er3, the following methods can be used: appropriately adjusting the film forming conditions of the PVA film. As methods for adjusting the film forming conditions of the PVA film, the following methods can be used: adjusting the temperature of the support for casting the film forming solution, the temperature of the hot air blown onto the liquid film on the support, the volatile content of the liquid film when peeled from the support, the volatile content / thickness of the liquid film when peeled from the support, the surface temperature of the drying roller when drying the film after peeling from the support, the volatile content of the film when peeled from the drying roller (i.e., the volatile content of the film when in contact with the initial heat treatment roller), and the surface temperature of the heat treatment roller used for heat treatment, etc. The volatile content of the liquid film upon peeling from the support can be adjusted by factors such as the volatile content of the film-forming solution, the temperature of the support, the temperature or speed of the hot air blowing onto the liquid film on the support, and the circumferential speed of the support, i.e., the contact time between the liquid film and the support. The volatile content of the film upon peeling from the drying roller can be adjusted by factors such as the method of passing the film through the drying oven, the method of contacting the film with the drying roller, the temperature of the drying oven or the surface temperature of the drying roller, and the drying time. More specifically, the higher the temperature of the support, the temperature of the hot air, the volatile content of the liquid film upon peeling from the support, the surface temperature of the drying roller, the volatile content of the film upon peeling from the drying roller, or the surface temperature of the heat treatment roller used for heat treatment, the greater the potential for Er1-Er3 or Er2-Er3 values. On the other hand, the lower the temperature of the support, the temperature of the hot air, the volatile content of the liquid film when it is peeled off from the support, the surface temperature of the drying roller, the volatile content of the film when it is peeled off from the drying roller, or the surface temperature of the heat treatment roller used for heat treatment, the smaller Er1-Er3 or Er2-Er3 tends to be.

[0025] Er1 is preferably 2.50 or higher, more preferably 3.00 or higher, and even more preferably 3.50 or higher. Er1 is preferably 7.00 or lower, more preferably 6.50 or lower, and even more preferably 6.00 or lower. Er2 is preferably 2.50 or higher, more preferably 3.00 or higher, and even more preferably 3.50 or higher. Er2 is preferably 7.00 or lower, more preferably 6.50 or lower, and even more preferably 6.00 or lower. Er3 is preferably 1.50 or higher, more preferably 2.00 or higher, and even more preferably 2.50 or higher. Er3 is preferably 6.00 or lower, more preferably 5.50 or lower, and even more preferably 5.00 or lower.

[0026] As mentioned earlier, the composite elastic modulus of PVA films is affected by the degree of crystallinity. Methods for evaluating crystallinity include X-ray diffraction, density, heat of fusion, FT-IR, and NMR. Since the composite elastic modulus is evaluated at room temperature and under atmospheric conditions, FT-IR is chosen as the evaluation method, as it operates at a position equivalent to that used for evaluating the composite elastic modulus.

[0027] In the PVA film of the present invention, regarding the crystallinity index calculated by FT-IR measurement of the thickness direction of the first surface and the second surface, and the cross section formed by perpendicularly cutting the first surface and the second surface, when the crystallinity index of the first surface is set as Fg1, the crystallinity index of the second surface is set as Fg2, and the crystallinity index of the thickness direction center is set as Fg3, it is preferable that the aforementioned Fg1, Fg2 and Fg3 satisfy the following formulas (3) to (5). 0.70≦Fg1≦0.90 (3) 0.70≦Fg2≦0.90 (4) 0.30≦Fg3≦0.50 (5)

[0028] In the above formulas (3) to (5), Fg1 is the crystallinity index calculated when the first surface is subjected to FT-IR measurement, Fg2 is the crystallinity index calculated when the second surface is subjected to FT-IR measurement, and Fg3 is the crystallinity index calculated when the central part of the cross section in the thickness direction is subjected to FT-IR measurement.

[0029] In this invention, as shown in FIG2, the first surface 3 and the second surface 4 are each subjected to FT-IR measurement. Furthermore, as shown in FIG3, in the same manner as the nano-indentation test, a sample is prepared by perpendicularly cutting the PVA film 1 through the aforementioned first surface 3 and second surface 4 to form a cross section 5, and the FT-IR measurement of the central portion 8 in the thickness direction of the cross section 5 is performed.

[0030] The FT-IR measurement performed in this invention is conducted using the ATR (Total Reflectance Absorption Spectrometry) method. As shown in Figure 4, the ATR method involves placing the sample (here, a PVA film 1) in close contact with an objective lens called an ATR prism 10, obliquely irradiating infrared light 11 onto the sample through the ATR prism 10, and measuring the spectrum of its reflected light. It is a type of reflectance-type IR measurement method. Compared with conventional reflectance-type IR measurement methods, it has the characteristic of obtaining a clear spectrum with less noise. In this measurement method, when using PVA film 1 as the sample, infrared light 11 is reflected not only on the surface of PVA film 1, but also infrared light that slightly penetrates into the film from the ATR prism 10 side. Therefore, based on the FT-IR measurement using the ATR method, information about the surface layer of the film (the portion that slightly penetrates into the depth direction from the surface of the film) can be obtained. Here, when the penetration depth 9 of the infrared rays penetrating from the ATR prism 10 side to the PVA film 1 side is set as d, this value is expressed by the following equation (8). As can be seen from the following equation (8), if ATR prisms 10 with different refractive indices are used, infrared absorption spectra with different penetration depths 9 can be obtained.

[0031] d=λ / 2Πn 1×1 / {sin 2θ-(n 2 / n 1) 2} 0.5(8)

[0032] In the above formula (8), n1 represents the refractive index of the ATR prism, n2 represents the refractive index of the thin film, λ represents the wavelength of infrared radiation, and θ represents the incident angle of infrared radiation.

[0033] In this invention, germanium with a refractive index of 4.0 is used as the substrate of the ATR prism. Since the refractive index of the PVA film is 1.5, when calculating the depth of infrared penetration into the surface of the film in the case of an infrared incident angle of 45° and an infrared wavenumber of 1140 cm⁻¹ in the above formula (8), the infrared penetration depth is about 0.5 μm when germanium is used as the substrate of the ATR prism, and the polar surface portion near the surface of the PVA film can be measured.

[0034] In this invention, the crystallinity index calculated during FT-IR measurement using the ATR method is determined. Generally, when measuring the infrared absorption spectrum (IR spectrum) of a PVA film, an absorption peak at 1140 cm⁻¹ can be observed due to the presence of PVA. This absorption peak is generally referred to as the crystallization band of the PVA film, and is one of the peaks originating from the stretching vibrations of the carbon bonds (CC) of PVA. This crystallization band is known to be observed because the vibrational phases of the PVA polymer molecular chains are emphasized and aligned due to crystallization of the PVA polymer molecular chains in the PVA film. That is, the higher the crystallinity of the PVA film, the higher the peak intensity of the crystallization band. Furthermore, when measuring the infrared absorption spectrum of a PVA film, an absorption peak can be observed at 1425 cm⁻¹ due to the angular vibrations of the methylene (-CH₂-) groups in the PVA backbone. The intensity of this absorption peak is considered to be independent of the crystallinity of the PVA film.

[0035] In this invention, the crystallinity index (Fg1, Fg2) of the PVA film can be obtained by calculating the ratio of the absorption peak intensity of the crystallization band (1140 cm⁻¹) to the absorption peak intensity of the angular vibration of the methylene group (-CH₂-) in the PVA main chain (1425 cm⁻¹). Specifically, the baselines of the infrared absorption spectra in 1140 cm⁻¹ and 1425 cm⁻¹ are drawn, and the height from the baseline to the peak of 1140 cm⁻¹ and 1425 cm⁻¹ is taken as the respective absorption peak intensity. The value of dividing the absorption peak intensity of 1140 cm⁻¹ by the peak intensity of 1425 cm⁻¹ is set as the crystallinity index (Fg1, Fg2, Fg3).

[0036] It is well known that the crystallinity indices (Fg1, Fg2, Fg3) obtained in this way are proportional to the crystallinity of the PVA film (e.g., NAPeppas, Macromol. Chem., 178 595 (1977), Japanese Patent Application Publication No. 6-138321). The value of these crystallinity indices varies somewhat depending on the moisture absorption of the PVA film; therefore, in this invention, the PVA film was stored at 25°C and 45% RH for 24 hours, and then FT-IR measurements were performed under the same conditions.

[0037] In the PVA film of the present invention, the crystallinity indices Fg1 and Fg2 of the first and second surfaces are greater than the crystallinity index Fg3 of the central part in the thickness direction, and it is preferable to adjust these to a specific range. When the crystallinity indices Fg1 and Fg2 of the first and second surfaces of a PVA film are too large, due to the scarcity of amorphous regions in the PVA film, dichroic pigments such as iodine-based pigments are difficult to absorb during uniaxial stretching of the PVA film to manufacture a polarizing film, resulting in the dyeing failing to reach the center of the thickness direction. Consequently, the damp heat resistance of the obtained polarizing film or polarizing plate decreases. On the other hand, when the crystallinity indices Fg1 and Fg2 of the first and second surfaces of the PVA film are too small, the alignment of the PVA on the surface is suppressed during uniaxial stretching of the PVA film to manufacture a polarizing film, making it difficult to form iodine complexes that exhibit polarization properties. Therefore, the polarization performance of the obtained polarizing film becomes significantly reduced. Furthermore, when the crystallinity index Fg3 in the central part of the thickness direction of the PVA film is too large, dichroic pigments such as iodine-based pigments are difficult to absorb during uniaxial stretching of the PVA film to manufacture a polarizing film, resulting in the central part of the thickness direction being unable to be dyed. This reduces the resistance to damp heat of the obtained polarizing film or polarizing plate. On the other hand, when the crystallinity index Fg3 in the central part of the thickness direction of the PVA film is too small, the ends of the PVA film are prone to breakage in the processing solutions during uniaxial stretching of the PVA film to manufacture a polarizing film, leading to poor processability.

[0038] As shown in equations (3) to (5) above, Fg1 and Fg2 are preferably 0.70 or higher. Fg3 is preferably 0.30 or higher. Furthermore, Fg1 and Fg2 are preferably 0.90 or lower. Fg3 is preferably 0.50 or lower. By having Fg1, Fg2, and Fg3 fall within the above ranges, when a polarizing film is manufactured by uniaxially stretching a PVA film, dyeing can reach the central part in the thickness direction and an iodine complex can be formed, thus obtaining a polarizing film with excellent resistance to damp heat. Furthermore, a polarizing film with excellent polarization performance can also be obtained. Fg1 and Fg2 are more preferably 0.72 or higher, more preferably 0.74 or higher, and especially preferably 0.76 or higher. Fg1 and Fg2 are more preferably 0.88 or lower, more preferably 0.86 or lower, and especially preferably 0.84 or lower. Furthermore, Fg3 is preferably 0.31 or higher, even better is 0.32 or higher, and ideally 0.33 or higher. Furthermore, Fg3 is preferably 0.49 or lower, even better is 0.48 or lower, and ideally 0.47 or lower.

[0039] Regarding the method of making Fg1 and Fg2 ≥ 0.70 ≤ 0.90 and Fg3 ≥ 0.30 ≤ 0.50, methods for appropriately adjusting the film-forming conditions of PVA film can be listed. Regarding the method of adjusting the film-forming conditions of PVA film, the following methods can be listed: the temperature of the support for casting the film-forming solution, the temperature of the hot air blown onto the liquid film on the support, the volatile content of the liquid film when peeled from the support, the volatile content / thickness of the liquid film when peeled from the support, the surface temperature of the drying roller when drying the film after peeling from the support, the volatile content of the film when peeled from the drying roller (i.e., the volatile content of the film when in contact with the initial heat treatment roller), and the surface temperature of the heat treatment roller used for heat treatment, etc. More specifically, the higher the temperature of the support, the hot air temperature, the volatile content of the liquid film peeling off from the support, the surface temperature of the drying roller, the volatile content of the film peeling off from the drying roller, or the surface temperature of the heat treatment roller used for heat treatment, the larger the values ​​of Fg1 and Fg2 tend to be. On the other hand, the lower the temperature of the support, the hot air temperature, the volatile content of the liquid film peeling off from the support, the surface temperature of the drying roller, the volatile content of the film peeling off from the drying roller, or the surface temperature of the heat treatment roller used for heat treatment, the smaller the values ​​of Fg1 and Fg2 tend to be.

[0040] In the PVA film of the present invention, the absolute value of the difference between Fg1 and Fg2 need not be limited, but Fg1 and Fg2 are preferably satisfied by the following formula (6). |Fg1-Fg2|≦0.10 (6)

[0041] As shown in equation (6) above, by having |Fg1-Fg2| less than 0.10, the difference in crystallinity index between the first and second surfaces of the PVA film will not be too large. Therefore, when the PVA film is uniaxially stretched to manufacture a polarizing film, it will be uniformly dyed from both surfaces to the center of the thickness direction, thus obtaining a polarizing film with excellent resistance to damp heat. |Fg1-Fg2| is more preferably less than 0.08, more preferably less than 0.05, and most preferably less than 0.03.

[0042] Regarding methods to make |Fg1-Fg2| less than 0.10, methods for appropriately adjusting the film-forming conditions of PVA film can be listed. Methods for adjusting the film-forming conditions of PVA film include adjusting the following: the temperature of the support on which the film-forming solution is cast, the temperature of the hot air blown onto the liquid film on the support, the volatile content of the liquid film when peeled from the support, the volatile content / thickness of the liquid film when peeled from the support, the surface temperature of the drying roller when drying the film after peeling from the support, the volatile content of the film when peeled from the drying roller (i.e., the volatile content of the film when in contact with the initial heat treatment roller), and the surface temperature of the heat treatment roller used for heat treatment, etc.

[0043] (PVA) 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, trimethylvinyl acetate, vinyl versatate, etc. Among these, vinyl acetate is preferred as the vinyl ester monomer.

[0044] The vinyl ester polymer is preferably a polymer obtained by using only one or more vinyl ester monomers as monomers, and more preferably a polymer obtained by using only one vinyl ester monomer as monomer. Furthermore, the vinyl ester polymer may also be a copolymer of one or more vinyl ester monomers and other monomers that can be copolymerized with them.

[0045] 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, tributyl 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, tributyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide, N-methacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetone acrylamide, acrylamide propanesulfonic acid or its salts, acrylamide propyl dimethylamine or its salts, N-hydroxymethylacrylamide or its derivatives, etc. Enylamine 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-vinylamines such as N-vinylmethacrylamide, N-vinylacetamide, N-vinylpyrrolidone; methyl vinyl ethers, ethyl vinyl ethers, n-propyl vinyl ethers, isopropyl vinyl ethers. Vinyl ethers such as ethers, n-butyl vinyl ether, isobutyl vinyl ether, tributyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc.; vinyl cyanide such as acrylonitrile and methacrylonitrile; halogenated vinyl groups 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 anhydrides; isocynic acid or its salts, esters, or anhydrides; vinyl silyl compounds such as vinyltrimethoxysilane; isopropyl acetate, etc. Furthermore, vinyl ester polymers may have structural units derived from one or more of these other monomers.

[0046] The proportion of structural units derived from other monomers in the vinyl ester polymer is preferably 15 moles or less, and more preferably 8 moles or less, based on the total mole count of all structural units constituting the vinyl ester polymer. Generally, the crystallization of PVA tends to be more difficult as the proportion of structural units derived from other monomers in the vinyl ester polymer increases. Therefore, the composite elastic modulus (Er1, Er2, and Er3) and crystallinity index (Fg1, Fg2, and Fg3) of the PVA film can be adjusted by appropriately copolymerizing these other monomers in the above-mentioned proportions.

[0047] The degree of polymerization of PVA is preferably 1,000 or higher, more preferably 1,500 or higher, and even more preferably 1,700 or higher. By setting the degree of polymerization of PVA to the lower limit mentioned above, the flexibility of the PVA film can be improved. On the other hand, the degree of polymerization of PVA is preferably 10,000 or lower, more preferably 8,000 or lower, and even more preferably 5,000 or lower. By setting the degree of polymerization of PVA to the upper limit mentioned above, the increase in manufacturing costs of PVA or the occurrence of defects during film formation can be suppressed. Furthermore, the degree of polymerization of PVA refers to the average degree of polymerization measured according to the description in JIS K6726-1994.

[0048] Judging from the improved resistance to damp heat of the obtained polarizing film, the saponification degree of PVA is preferably 90 mol% or higher, more preferably 95 mol% or higher, even more preferably 99 mol% or higher, and particularly preferably 99.3 mol% or higher. Furthermore, the saponification degree of PVA refers to the ratio (mol%) of the number of vinyl alcohol units to the total number of moles (of structural units that can be converted into vinyl alcohol units through saponification, typically ethylene ester units) and vinyl alcohol units. The saponification degree can be determined according to the description in JIS K6726-1994.

[0049] 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.

[0050] Regarding modified PVA, for example, ethylene-modified PVA can be used. The degree of modification of PVA is preferably 0.01 mol% or more, more preferably 0.1 mol% or more, and even more preferably 1 mol% or more. The degree of modification of PVA is preferably 15 mol% or less, more preferably 8 mol% or less, and even more preferably 5 mol% or less.

[0051] The PVA content in the PVA film of the present invention is not necessarily limited, but it is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0052] (Plasticizer) The PVA film of this invention preferably contains a plasticizer. By including a plasticizer, the processability and tensile strength of the PVA film can be improved. Examples of plasticizers include polyols such as ethylene glycol, glycerol, diglycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. One of these plasticizers can be used alone, or two or more can be used in combination. Among these, ethylene glycol or glycerol are preferred plasticizers, especially glycerol, due to reasons such as difficulty in seeping to the surface of the PVA film.

[0053] The plasticizer content in the PVA film of the present invention is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 4 parts by mass or more, relative to 100 parts by mass of PVA. By setting the plasticizer content to the above-mentioned lower limit or above, the tensile strength of the PVA film is improved. On the other hand, the plasticizer content is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, relative to 100 parts by mass of PVA. By setting the plasticizer content to the above-mentioned upper limit or below, the exudation of plasticizer to the surface of the PVA film can be suppressed, thereby reducing the processability of the PVA film.

[0054] (surfactants) The PVA film of this invention preferably contains a surfactant. By containing a surfactant, the processability of the PVA film and its peelability from the film-forming apparatus during manufacturing can be improved. There are no particular limitations on the surfactant used; for example, anionic surfactants and nonionic surfactants are preferred.

[0055] Examples of anionic surfactants include: carboxylic acid surfactants such as potassium laurate; sulfate surfactants such as octyl sulfate; and sulfonic acid surfactants such as dodecyl benzenesulfonate.

[0056] For nonionic surfactants, examples include: alkyl ether surfactants such as polyoxyethylene ethyl lauryl ether and polyoxyethylene ethyl oleyl ether; alkyl phenyl ether surfactants such as polyoxyethylene ethyl octylphenyl ether; alkyl ester surfactants such as polyoxyethylene ethyl lauryl ester; alkylamine surfactants such as polyoxyethylene ethyl lauryl amino ether; alkyl amide surfactants such as polyoxyethylene ethyl lauryl amide; polypropylene glycol ether surfactants such as polyoxyethylene ethyl polyoxypropylene ether; alkylol amide surfactants such as laurate diethanolamide and oleate diethanolamide; and allyl phenyl ether surfactants such as polyoxyethylene alkyl allyl phenyl ether.

[0057] This type of surfactant can be used alone or in combination with two or more. Regarding surfactants, considering their excellent effect in reducing surface anomalies during PVA film formation, nonionic surfactants are preferred, alkanolamide-type surfactants are more preferred, and dialkylamides (e.g., diethanolamides) of aliphatic carboxylic acids (e.g., saturated or unsaturated aliphatic carboxylic acids with 8 to 30 carbon atoms) are even more preferred.

[0058] The surfactant content in the PVA film of the present invention is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, and even more preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA. On the other hand, the surfactant content is preferably 10 parts by mass or less, more preferably 1 part by mass or less, and even more preferably 0.5 parts by mass or less, and particularly preferably 0.3 parts by mass or less, relative to 100 parts by mass of PVA. When the surfactant content is within the above range, the peelability of the PVA film forming apparatus during manufacturing is good, and adhesion (hereinafter also referred to as "bonding") between PVA films can be prevented. Furthermore, it can prevent the surfactant from seeping onto the surface of the PVA film or the appearance of the PVA film from deteriorating due to the aggregation of surfactant.

[0059] (Other ingredients) In addition to PVA, the PVA film of the present invention may also contain water-soluble polymers, water, antioxidants, ultraviolet absorbers, lubricants, crosslinking agents, colorants, fillers, preservatives, fungicides, and other polymeric compounds, within a range that does not impair the effects of the present invention. The total mass of the components other than PVA, surfactants, plasticizers, and PVA in the total mass of the PVA film is preferably 60-100% by mass, more preferably 80-100% by mass, and even more preferably 90-100% by mass.

[0060] (Thickness and shape of PVA film) The upper limit of the average thickness of the PVA film of the present invention is not particularly limited, but is preferably 100 μm, more preferably 80 μm, even more preferably 60 μm, and particularly preferably 50 μm. On the other hand, regarding the lower limit of this average thickness, from the viewpoint that a PVA film satisfying the above formulas (1) to (2) can be obtained, it is preferably 10 μm, more preferably 15 μm, and even more preferably 20 μm. The thicker the average thickness of the PVA film, the larger the value of Er1-Er3 or Er2-Er3 tends to be. The thinner the average thickness of the PVA film, the smaller the value of Er1-Er3 or Er2-Er3 tends to be.

[0061] The shape of the PVA film of this invention is not particularly limited, but a long and thin film is preferred for the purpose of continuous and efficient production of polarizing films. The length of the long and thin film is not particularly limited and can be appropriately set according to the intended use of the polarizing film; for example, it can be made in the range of 5m to 20,000m. The width of the long and thin film is not particularly limited; for example, it can be made 50cm or more. Since wide polarizing films are needed in recent years, a width of 1m or more is preferred, more preferably 2m or more, and more preferably 4m or more. The upper limit of the width of the long and thin film is not particularly limited. However, if the width is too wide, it tends to become difficult to stretch uniformly when manufacturing polarizing films with practical equipment; therefore, the width of the PVA film is preferably 7m or less.

[0062] The shape of the PVA film of the present invention is not particularly limited. It can be a single-layer film or a multi-layer film (laminated body). From the point of view of the complexity and cost of lamination (coating, etc.) operations, a single-layer film is preferred.

[0063] The PVA film of this invention is typically an unstretched film. Because the PVA film of this invention is an unstretched film, it is suitable for use as a film for manufacturing optical films such as polarizing films. However, stretched PVA films are also within the scope of this invention.

[0064] (Physical properties of PVA film) The PVA film of this invention is water-insoluble. Because the PVA film is water-insoluble, during uniaxial stretching in an aqueous solution when manufacturing optical films such as polarizing films, the PVA film can be stretched without breaking. Here, in this invention, "water-insoluble" refers to the following... <1> ~ <4> When a PVA film is immersed in water (deionized water) at 30°C in sequence, the PVA film is not completely dissolved, or only a portion of it dissolves and remains.

[0065] <1> Place the PVA film in a constant temperature and humidity chamber set to 20°C and 65%RH for more than 16 hours to adjust the humidity. <2> After cutting a rectangular sample with a length of 40mm and a width of 35mm from a moisture-conditioned PVA film, clamp and fix it between two 50mm × 50mm plastic plates with a rectangular window (hole) of 35mm in length and 23mm in width, with the length of the sample parallel to the length of the window and the sample located almost in the center of the width of the window. <3> Add 300 mL of deionized water to a 500 mL beaker and stir with an electromagnetic stirrer with a 3 cm long stirring rod at 280 rpm while adjusting the water temperature to 30°C. <4> In the above <2> The sample, fixed in a plastic plate, is immersed in deionized water in a beaker for 1000 seconds while being careful not to let it come into contact with the stirring rod of the rotating electromagnetic stirrer.

[0066] From the viewpoints of productivity and polarization performance of polarizing films, the swelling degree of the PVA film of the present invention is preferably 130% or more, more preferably 150% or more, and even more preferably 170% or more. From the viewpoints of productivity and polarization performance of polarizing films, the swelling degree of the PVA film is preferably 300% or less, more preferably 280% or less, and even more preferably 260% or less. In particular, by making the swelling degree 130% or more, sufficient swelling in the stretching bath makes it easier to adsorb dichroic pigments, resulting in a polarizing film with high polarization performance. The swelling degree of the PVA film can be adjusted by, for example, heat treatment conditions. Specifically, by strengthening the heat treatment conditions, the swelling degree can be adjusted to a smaller value.

[0067] The swelling degree of the PVA film can be obtained as a percentage by dividing the mass of a PVA film sample after immersion in distilled water at 30°C for 15 minutes by the mass after drying at 105°C for 16 hours. Specifically, this swelling degree can be measured using the method described later in the examples. Furthermore, the PVA film sample can be taken from the center of the width direction of the PVA film at the representative location.

[0068] From the viewpoints of the manufacturability and polarization performance of polarizing films, the softening point of the PVA film of the present invention is preferably 60°C or higher, and more preferably 65°C or higher. From the viewpoints of the manufacturability and polarization performance of polarizing films, the softening point of the PVA film of the present invention is preferably 80°C or lower, and more preferably 75°C or lower.

[0069] The softening point of the PVA film can be determined by measuring the hot water deformation temperature when a sample of the PVA film is placed in distilled water at 25°C and heated at a rate of 5°C / min. Specifically, this softening point can be measured using the method described later in the examples. Furthermore, the PVA film sample can be taken from the center of the width direction of the PVA film at the representative location.

[0070] From the viewpoints of optical properties, the polarization performance of the obtained polarized film, etc., the haze of the PVA film of the present invention is preferably 2.0% or less, more preferably 1.5% or less. Also, regarding the lower limit of this haze, for example, it is preferably 0.1%, more preferably 0.3%. The haze can be obtained as the ratio (percentage) of the diffusion transmittance to the total transmittance when visible light is irradiated on the PVA film.

[0071] The PVA film of the present invention becomes a film having good water swelling properties by adjusting the swelling degree to an appropriate range by using heat treatment, etc. Therefore, from this PVA film, a polarized film having high polarization performance can be manufactured, and it can be suitably used as an original film for optical films for manufacturing polarized films, etc. Also, the use of the PVA film of the present invention is not particularly limited, and it can also be used as an original film for stretched films, agricultural films, packaging films, etc.

[0072] <Manufacturing method of PVA film> The manufacturing method of the PVA film of the present invention is not particularly limited. For example, any method as described below can be adopted. Regarding this method, examples include: a method of forming a film-forming stock solution obtained by adding a solvent, an additive, etc. to PVA and making it uniform by a casting film-forming method, a wet film-forming method (a method of discharging into a poor solvent), a dry-wet film-forming method, a gel film-forming method (a method of temporarily cooling and gelifying the film-forming stock solution and then extracting and removing the solvent), or a combination of these; a melt extrusion film-forming method of extruding the film-forming stock solution obtained by using an extruder, etc. from a T die, etc. to form a film; a blown film forming method, etc. Among these, as the manufacturing method of the PVA film, the casting film-forming method and the melt extrusion film-forming method are particularly preferred. By using these methods, a homogeneous PVA film can be obtained with good productivity. Hereinafter, the case of manufacturing PVA by using the casting film-forming method or the melt extrusion film-forming method will be described.

[0073] In the case of manufacturing the PVA film of the present invention by using the casting film-forming method or the melt extrusion film-forming method, first, a film-forming stock solution containing PVA, a solvent, additives such as a plasticizer as necessary, etc. is prepared. Then, this film-forming stock solution is cast (supplied) in a film shape on a rotating support such as a metal roller, a metal belt, etc. Thereby, a liquid film of the film-forming stock solution is formed on the support. The liquid film is heated on the support to remove the solvent, and thereby it is solidified and made into a film. Examples of the method of heating the liquid film include a method of heating the support itself to a high temperature with a heat medium, etc., a method of blowing hot air on the opposite side of the surface of the liquid film in contact with the support, etc. The solidified elongated film (PVA film) is peeled off from the support, and if necessary, it is dried by a drying roller, a drying furnace, etc., and further heat-treated as necessary, and wound into a roll shape.

[0074] The drying steps (solvent removal step) of the liquid film cast on the support, followed by the drying step of the PVA film, involve the PVA film being heated and then drawn outwards (stretching in the flow direction), causing the PVA to crystallize and align. The rate of crystallization and the degree of alignment at this stage are 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 plasticizers, but also by the moisture content and temperature of the PVA.

[0075] PVA films are typically dried by allowing volatile components to gradually evaporate from the open film surface that has never been in contact with the support and drying rollers. Therefore, during the drying process, the concentration distribution of volatile components such as moisture is generated along the thickness direction of the PVA film. Consequently, the distribution of the composite elastic modulus and crystallinity index along the thickness direction of the PVA film will vary depending on the temperature and shrinkage conditions. This distribution of the composite elastic modulus and crystallinity index can be adjusted using factors such as support temperature, hot air temperature, the volatile content of the PVA film at the time of peeling from the support, the heat treatment method, the volatile content of the PVA film during heat treatment, and the heat treatment temperature. Therefore, by appropriately adjusting the above factors, the crystallization and alignment of PVA can be carried out properly, and the composite elastic modulus and crystallinity index of the PVA film can be adjusted.

[0076] The volatile content (concentration of volatile components such as solvents removed by evaporation or volatilization during film formation) of the film-forming solution is preferably 60% by mass or more, and more preferably 65% ​​by mass or more. The volatile content of the film-forming solution is preferably 90% by mass or less, and more preferably 80% by mass or less. When the volatile content 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 film cast on the support and making it easier to obtain a PVA film with a uniform thickness. Furthermore, when the volatile content is within the above range, PVA crystallization on the support occurs appropriately, thus making it easier to adjust the crystallinity index and its distribution of the obtained PVA film. The film-forming solution may also contain dichroic dyes if necessary.

[0077] Here, the volatile fraction of the film-forming solution in this specification refers to the value obtained by the following formula (9). Volatility fraction (mass %) of film-forming solution = {(Wa-Wb) / Wa} × 100 (9) In the above formula (9), Wa represents the mass (g) of the film-forming stock solution, and Wb represents the mass (g) of Wa (g) of the film-forming stock solution after drying in an electric dryer at 105°C for 16 hours.

[0078] There are no particular restrictions on the method of adjusting the film-forming solution. Examples include: dissolving additives such as PVA, plasticizers, and surfactants in a solvent in a dissolving tank; or using a single-shaft or bi-shaft extruder to melt and knead PVA in a water-containing state together with additives such as plasticizers and surfactants.

[0079] The film-forming solution is typically cast into a film onto a support such as a metal roller or metal strip using the die lip of a mold, such as a T-die. On the support, the solvent evaporates from the side of the cast film that is not in contact with the support (hereinafter, sometimes referred to as the free side), while it does not actually evaporate from the side that is in contact with the support (hereinafter, sometimes referred to as the contact side). Therefore, in the thickness direction of the film, a distribution occurs where the solvent concentration is low on the free side and high on the contact side. Consequently, the curing of PVA also begins from the free side.

[0080] PVA crystallization occurs concurrently with PVA curing. PVA crystallization is difficult to achieve with excessively high or low solvent concentrations. While it is also related to the primary structure of the PVA molecule, it is easier to crystallize when the volatile content of the cast film-forming solution is between 10% and 50% by mass. Furthermore, the crystallization rate of PVA increases with higher temperatures, but the solvent evaporation rate also increases at higher temperatures. Therefore, to control the composite elastic modulus and crystallinity index near the surface of the PVA film, in addition to controlling the temperature of the support and the contact time with the support, it is also important to control the ambient gas temperature and solvent vapor pressure near the surface not in contact with the support.

[0081] In the PVA film of this invention, the cross-section obtained by perpendicularly cutting the two surfaces of the PVA film is designed to adjust the difference between the composite elastic coefficient near the surface and the composite elastic coefficient at the center of the thickness direction to a specific range. Therefore, to obtain the PVA film of this invention, it is only necessary to select a condition that increases the composite elastic coefficient near the surface of the film while suppressing the composite elastic coefficient at the center of the thickness direction. For example, when the difference between the volatile content near the surface of the PVA film and the volatile content at the center of the thickness direction is large, i.e., when the volatile content of the PVA film is high, it is preferable to apply conditions of stretching by peeling from the support and performing heat treatment. Furthermore, it is also preferable to apply conditions of drying and heat treatment simultaneously by having rollers or the like directly contact the surface of the PVA film.

[0082] To obtain the PVA film of the present invention, the surface temperature of the support for casting the film-forming solution is preferably 65°C or higher, more preferably 70°C or higher, and even more preferably 75°C or higher. To obtain the PVA film of the present invention, the surface temperature of the support for casting the film-forming solution is preferably 120°C or lower, more preferably 105°C or lower, and even more preferably 99°C or lower. When the surface temperature is within the above range, the drying of the liquid film cast on the support is carried out at a suitable rate. Furthermore, the composite elastic modulus and crystallinity index near the non-contact side of the support that is not in contact with the PVA film can be adjusted.

[0083] While heating the liquid film on the support, hot air at a velocity of 1 to 10 m / s can be uniformly blown onto the entire area of ​​the non-contact side of the liquid film. 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. When the temperature of the hot air blown onto the non-contact side is within the above range, it becomes easy to adjust the composite elastic coefficient and crystallinity index near the surface of the non-contact side of the PVA film.

[0084] To obtain the PVA film of the present invention, it is preferable to adjust the volatile content of the PVA film at the time of peeling from the support. The value of dividing the volatile content (mass %) at the time of peeling from the support by the thickness (μm) of the dried PVA film is preferably 0.1 mass % / μm or more, more preferably 0.2 mass % / μm or more, and even more preferably 0.3 mass % / μm or more. The value of dividing the volatile content (mass %) at the time of peeling from the support by the thickness (μm) of the dried PVA film is preferably 1.0 mass % / μm or less, more preferably 0.8 mass % / μm or less, and even more preferably 0.5 mass % / μm or less. If the volatile content of the PVA film at the time of peeling from the support is too low, the difference between the composite elastic modulus near the surface of the obtained PVA film and the composite elastic modulus at the center in the thickness direction will become too small. On the other hand, if the volatile content of the PVA film is too high, the difference between the composite elastic coefficient near the surface of the obtained PVA film and the composite elastic coefficient at the center in the thickness direction will become too large, making it difficult to peel off from the PVA film support, which is undesirable.

[0085] Here, the volatile content of liquid coating or PVA film in this specification refers to the value obtained by the following formula (10). Volatile content (mass %) of liquid coatings or PVA films ={(Wc-Wd) / Wc}×100 (10) In the above formula (10), Wc represents the mass (g) of the sample taken from the liquid film or PVA film, and Wd represents the mass (g) of the sample Wc (g) after being placed in a vacuum dryer at a temperature of 50°C and a pressure of 0.1 kPa or less for 4 hours.

[0086] Furthermore, the volatile content of the PVA film during peeling from the support can be adjusted by changing the circumferential speed of the support, such as a metal roller or metal belt. For example, when the circumferential speed of the support is slow, the contact time between the liquid film and the support is longer, thus reducing the volatile content of the PVA film during peeling from the support. The circumferential speed of the support is preferably 8 m / min or more, more preferably 11 m / min or more, and even more preferably 14 m / min or more. The circumferential speed of the support is preferably 30 m / min or less, more preferably 27 m / min or less, and even more preferably 24 m / min or less. When the circumferential speed of the support is within the above range, the productivity of the PVA film can be maintained while producing a PVA film with excellent in-plane uniformity and minimal thickness unevenness.

[0087] The PVA film peeled from the support can be further dried as necessary. There are no particular limitations on the drying method; methods such as passing it through a drying oven or contacting it with drying rollers can be listed. However, to ensure that the difference between the composite elastic modulus near the surface of the obtained PVA film and the composite elastic modulus at the center in the thickness direction, as well as the crystallinity index of the surface and the crystallinity index at the center in the thickness direction, are within a specified range, it is preferable to have the surface of the PVA film directly contact the drying rollers. Furthermore, when using multiple drying rollers to dry the PVA film, it is preferable to have one surface of the PVA film alternately contact the drying rollers with the other surface. This allows adjustment of the composite elastic modulus and crystallinity index of the PVA film on both sides (the first surface and the second surface). In this case, the number of drying rollers is preferably three or more, more preferably four or more, and preferably five or more. The number of drying rollers is preferably 30 or less.

[0088] The temperature of the drying oven or the surface temperature of the drying rollers is preferably between 40°C and 110°C. The upper limit of the temperature of the drying oven or the surface temperature of the drying rollers is preferably 110°C, more preferably 100°C, even more preferably 90°C, and particularly preferably 85°C. On the other hand, the lower limit of the temperature of the drying oven or the surface temperature of the drying rollers is preferably 45°C, and even more preferably 50°C. By setting the temperature of the drying oven or the surface temperature of the drying rollers within the above ranges, it becomes easier to adjust the composite elastic modulus and crystallinity index of the PVA film.

[0089] After drying, the PVA film can be further heat-treated as needed. Heat treatment can adjust the composite elastic modulus and crystallinity index of the PVA film. Furthermore, it can also adjust the mechanical properties of the PVA film, such as strength and swelling.

[0090] The heat treatment method is not particularly limited; methods such as using a floating dryer to blow hot air, irradiating near-infrared light with infrared lamps, and contacting the film with heat treatment rollers are examples. However, in order to specify the difference between the composite elastic modulus near the surface of the obtained PVA film and the composite elastic modulus at the center of the thickness direction, as well as the crystallinity index of the surface or the crystallinity index at the center of the thickness direction, it is preferable to have the surface of the PVA film directly contact the heat treatment rollers. Furthermore, when using multiple heat treatment rollers to dry the PVA film, it is preferable to have one surface of the PVA film alternately contact the heat treatment rollers with the other surface. This allows adjustment of the composite elastic modulus and crystallinity index of the PVA film on both sides (the first surface and the second surface). In this case, the number of heat treatment rollers is preferably two or more, more preferably four or more.

[0091] The volatile content of the PVA film upon contact with the initial heat treatment roller is preferably 2% by mass or more, and more preferably 4% by mass or more. The volatile content of the PVA film upon contact with the initial heat treatment roller is preferably 15% by mass or less, and more preferably 12% by mass or less. By achieving a volatile content of the PVA film upon contact with the initial heat treatment roller within the above range, it becomes easier to adjust the composite elastic modulus and crystallinity index of the PVA film.

[0092] The surface temperature of the heat treatment roller used for heat treatment is preferably 80°C or higher, more preferably 90°C or higher. The surface temperature of the heat treatment roller used for heat treatment is preferably 140°C or lower, more preferably 130°C or lower. By making the surface temperature of the heat treatment roller within the above range, it becomes easier to adjust the composite elastic modulus and crystallinity index of the PVA film.

[0093] The PVA film manufactured in this way undergoes further moisture conditioning and cutting of both ends (ears) as needed, and is then wound into a roller shape on a cylindrical core, and packaged in a moisture-proof manner to become a finished product.

[0094] The volatile content of the PVA film finally obtained through a series of processes is not necessarily limited, but it is preferably 1% by mass or more, and more preferably 2% by mass or more. The volatile content of the final PVA film is not necessarily limited, but is preferably below 5% by mass, and more preferably below 4% by mass.

[0095] <Optical Thin Films> By using the PVA film of this invention as a preform for optical films, optical films with excellent polarization performance and resistance to damp heat can be manufactured, making it suitable as a film for optical film manufacturing. Examples of optical films include polarizing films, viewing angle enhancement films, phase difference films, and brightness enhancement films, with polarizing films being preferred. Hereinafter, as one example of an optical film manufacturing method, a method for manufacturing a polarizing film will be specifically described.

[0096] The method for manufacturing the polarizing film is not particularly limited, and any conventional method can be used. For example, the polarizing film can be manufactured by subjecting the PVA film of the present invention to swelling treatment, dyeing treatment, uniaxial stretching, and, as necessary, further crosslinking treatment, fixing treatment, drying treatment, and heat treatment. In this case, the order of the swelling treatment, dyeing treatment, uniaxial stretching treatment, fixing treatment, etc., is not particularly limited, and one or more treatments can be performed simultaneously. Furthermore, one or more of the treatments can be performed two or more times.

[0097] The swelling treatment can be carried out by immersing the PVA film in water. The lower limit of the water temperature during immersion is preferably 20°C, more preferably 22°C, and even more preferably 25°C. On the other hand, the upper limit is preferably 40°C, more preferably 38°C, and even more preferably 35°C. Furthermore, the immersion time is, for example, 0.1 minutes or more, preferably 5 minutes or less. Also, the water used for immersion is not limited to pure water; it can be an aqueous solution containing various dissolved components, or a mixture of water and an aqueous medium.

[0098] The dyeing process is carried out using dichroic pigments such as iodine-based pigments and dyes. The dyeing can be performed at any stage: before uniaxial stretching, during uniaxial stretching, or after uniaxial stretching. Dyeing is generally carried out by immersing the PVA film in a solution (especially an aqueous solution) containing iodine and potassium iodide, which serves as the dyeing bath. The concentration of iodine in the dyeing bath is preferably 0.01% by mass or more. The concentration of potassium iodide in the dyeing bath is preferably 0.5% by mass or less. The concentration of potassium iodide is preferably 0.01% by mass or more. The concentration of potassium iodide is preferably 10% by mass or less. Furthermore, the temperature of the dyeing bath is preferably 20°C or more. The temperature of the dyeing bath is preferably 60°C or less.

[0099] Crosslinking treatment can be performed by immersing a PVA film in an aqueous solution containing a crosslinking agent. During crosslinking treatment, crosslinking can be introduced into the PVA film, and uniaxial stretching can be performed at a relatively high temperature. Regarding the crosslinking agent used, one or more boron compounds such as boric acid and borax borates can be used. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 1% by mass or more, more preferably 2% by mass or more. The concentration of the crosslinking agent in the aqueous solution containing the crosslinking agent is preferably 15% by mass or less, more preferably 7% by mass or less. The temperature of the aqueous solution containing the crosslinking agent is preferably 20°C or higher. The temperature of the aqueous solution containing the crosslinking agent is preferably 60°C or lower.

[0100] Uniaxial stretching can be performed using either a wet stretching method or a dry stretching method. In the case of a wet stretching method, it can be performed in an aqueous solution containing boric acid, or in the aforementioned staining bath or the fixation bath described later. In the case of a dry stretching method, it can be performed in air. Of these, a wet stretching method is preferred, and uniaxial stretching in an aqueous solution containing boric acid is even more preferred. The concentration of boric acid in the aqueous solution is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and further preferably 1.5% by mass or more. The concentration of boric acid in the aqueous solution is preferably 6.0% by mass or less, more preferably 5.0% by mass or less, and further preferably 4.0% by mass or less. Furthermore, the aqueous solution of boric acid may contain potassium iodide, with a concentration preferably 0.01% by mass or more. The concentration of potassium iodide is preferably 10% by mass or less.

[0101] For wet tensile testing, the tensile temperature in uniaxial tensile testing is preferably above 30°C, more preferably above 40°C, and even more preferably above 50°C. The tensile temperature in uniaxial tensile testing is preferably below 90°C, more preferably below 80°C, and even more preferably below 70°C.

[0102] In uniaxial stretching, the stretching ratio, from the perspective of the polarization performance of the obtained polarizing film, is preferably 5 times or more, and more preferably 5.5 times or more. There is no particular upper limit to the stretching ratio, but the stretching ratio is preferably below 8 times.

[0103] During the manufacture of polarizing films, a fixation treatment is preferably performed to ensure strong adsorption of dichroic pigments onto the PVA film. The fixation bath used for this treatment can be an aqueous solution containing one or more boron compounds such as boric acid or borax. Furthermore, iodine compounds or metal compounds may be added to the fixation bath if necessary. The concentration of the boron compound in the fixation bath is generally preferably 2% by mass or more, more preferably 3% by mass or more. The concentration of the boron compound in the fixation bath is generally preferably 15% by mass or less, more preferably 10% by mass or less. The temperature of the fixation bath is preferably 15°C or more, more preferably 25°C or more. The temperature of the fixation bath is preferably 60°C or less, more preferably 40°C or less.

[0104] The drying process is preferably carried out at a temperature above 30°C, and more preferably at a temperature above 50°C. The drying process is also preferably carried out at a temperature below 150°C, and more preferably at a temperature below 130°C. By drying at temperatures within the above range, polarizing films with excellent dimensional stability can be easily obtained.

[0105] <Polarizing plate> The polarizing film obtained as described above is preferably used as a polarizing plate by laminating an optically transparent and mechanically strong protective film onto both or one side of the film. For 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, for the adhesive used for lamination, PVA-based adhesives, carbamate-based adhesives, etc., can be listed, but PVA-based adhesives are preferred.

[0106] The polarizing plate obtained as described above can be laminated onto a glass substrate after being coated with an acrylic adhesive, and used as a component of an LCD. It can also be used in conjunction with phase retardation films, viewing angle enhancement films, brightness enhancement films, etc. [Example]

[0107] The present invention will be specifically described below by way of examples, but the present invention is not limited to the following examples.

[0108] <Determination of composite elastic modulus (Er1, Er2, Er3) using nanoindentation testing> (Preparation of cross-sectional samples) After embedding the PVA film obtained in the following examples or comparative examples in epoxy resin, the cross-section of the PVA film was revealed using a slicing machine.

[0109] (Determination of Er1, Er2, and Er3) By setting the scanning probe microscope so that the cross-section of the PVA film obtained above faces upward, the composite elastic coefficient is calculated when the tip of the indenter is inserted from the surface of the film to a depth of about 250 nm. As shown in Figure 3, the composite elastic coefficient measured within 5 μm of the center in the thickness direction from the first surface is designated as Er1, the composite elastic coefficient measured within 5 μm of the center in the thickness direction from the second surface is designated as Er2, and the composite elastic coefficient of the region in the thickness direction at the center is designated as Er3. Er1, Er2, and Er3 are calculated using the analysis software attached to the device based on the load-displacement curves at insertion. Furthermore, the average value of any 10 points is used during measurement. Detailed measurement conditions are shown below.

[0110] Measurement apparatus: Multifunctional SPM device (manufactured by Hitachi High-Tech), TriboScope (manufactured by HYSITRON) Parsing software: Parsing software attached to the device Measurement temperature: 23℃ Relative humidity measured: 40% Indenter insertion conditions: Insert at 200μN for 3 seconds and withdraw for 3 seconds (insertion depth approximately 250nm). Indenter: Berkovich indenter (Model: TI-0039, manufactured by HYSITRON)

[0111] <Calculation of crystallinity indices (Fg1, Fg2, Fg3) determined by FT-IR> (Calculation of Fg1 and Fg2) The PVA films obtained in the following examples or comparative examples were stored at 25°C and 45%RH for 24 hours, and then FT-IR measurements were performed in a measuring apparatus set in a room with the same environment. The FT-IR measurements were performed on both sides of the PVA film (two surfaces orthogonal to the thickness direction of the PVA film, the first surface and the second surface) under the following conditions.

[0112] Measuring apparatus: Nicolet iS 10 (Thermo Fisher Scientific) Measurement conditions: Single-reflection ATR method, incident angle 45° Decomposition energy: 4.0 cm⁻¹ Number of calculations: 32 Measurement temperature: 25℃ (ambient temperature) Humidity measured: 45%RH (ambient relative humidity) ATR Prism: Germanium Prism

[0113] The crystallinity index Fg1 of the first surface and the crystallinity index Fg2 of the second surface of the PVA film were calculated from the infrared absorption spectrum obtained by FT-IR measurement of the PVA film using the aforementioned method.

[0114] (Preparation of cross-sectional samples) After the PVA films obtained in the following examples or comparative examples were stored at 25°C and 45%RH for 24 hours, the cross-section of the PVA films was revealed using a slicing cutter (JASCO HW-1 type, FEATHER FA-10 0.1mm blade).

[0115] (Calculation of Fg3) The central portion of the cross-section of the PVA film produced above is pressed against the germanium prism surface of an FT-IR instrument, and FT-IR measurements are performed. The FT-IR measurements are performed under the same conditions as described above (calculation of Fg1 and Fg2).

[0116] The crystallinity index Fg3 of the central part of the PVA film in the thickness direction was calculated using the aforementioned method, based on the infrared absorption spectrum obtained from the FT-IR measurement of the PVA film.

[0117] <Polarization performance> (a) Determination of penetration rate Ts Two square samples, each 3 cm long and 3 cm wide, were taken from the polarizing films obtained in the following examples or comparative examples. Using a spectrophotometer with an integrating sphere (V7100 manufactured by Japan Spectrophotometer Co., Ltd.), visual sensitivity correction was performed in the visible light region with a C light source and a 2° field of view, according to JIS Z8722:2009 (Method for Determination of Object Color). For one sample, the transmittance at a 45° tilt relative to the length direction and the transmittance at a -45° tilt were measured, and the average value Ts1 (%) was obtained. The same procedure was performed on the other sample, measuring the transmittance at a 45° tilt and the transmittance at a -45° tilt, and obtaining the average value Ts2 (%). The average of Ts1 and Ts2 was obtained using the following formula (11) as the transmittance Ts (%) of the polarizing film. Ts=(Ts1+Ts2) / 2 (11) In the following examples or comparative examples, samples were prepared by adjusting the dyeing treatment conditions to make the transmittance Ts 44.0%, and the polarization V was measured.

[0118] (b) Measurement of polarization V Two samples used for the above-mentioned transmittance Ts measurement were overlapped so that their length directions were parallel. The transmittance of light when tilted at 45° and -45° relative to the length direction was measured, and the average value T∥ (%) was obtained. Then, the samples were overlapped so that their length directions were orthogonal, and the transmittance of light when tilted at 45° and -45° relative to the length direction was measured, and the average value T⊥ (%) was obtained. The polarization V (%) was obtained by the following formula (12). V={(T∥-T⊥) / (T∥+T⊥)} 1 / 2×100 (12) Furthermore, polarization performance is judged based on the following criteria. Also, A and B are judged as good because they are practically feasible and usable, while C is judged as poor. A: When the transmittance Ts is 44%, the polarization V is above 99.95%. B: When the transmittance Ts is 44%, the polarization V is 99.9% or higher but less than 99.95%. C: When the transmittance Ts is 44%, the polarization V is less than 99.9%.

[0119] <Damp heat resistance test> The polarizing plates obtained in the following embodiments or comparative examples were cut into 4cm squares, and the light transmittance Ts(I) before the damp heat resistance test was measured using the method described above. Then, the plates were placed in a constant temperature and humidity chamber at 60°C and 90%RH for 24 hours, and the light transmittance Ts(II) after the damp heat resistance test was measured. Furthermore, as an indicator of damp heat resistance, the change in light transmittance ΔTs before and after the damp heat resistance test was obtained by the following formula (13). ΔTs=Ts(I)-Ts(II) (13) Furthermore, resistance to damp heat is determined based on the following criteria. Also, A and B are judged as good because they can be used practically without problems, while C is judged as poor. A: ΔTs is less than 0.7% B: ΔTs is greater than 0.7% and less than 0.8%. C: ΔTs is above 0.8%

[0120] <Example 1> <Manufacture and Evaluation of PVA Film> A film-forming stock solution (volatile fraction: 66% by mass) containing 100 parts by mass of PVA (saponification degree: 99 mol%, degree of polymerization: 2400), 12 parts by mass of glycerol as a plasticizer, 0.1 part by mass of diethanolamide laurate as a surfactant, and water was extruded in a film shape from a T die onto a support (surface temperature: 90°C, peripheral speed: 16.7 m / min) to form a liquid film on the support. On the support, hot air at 90°C was blown at a speed of 5 m / s over the entire non-contact surface of the liquid film with the support to dry it. The volatile fraction of the PVA film when peeled off from the support was made 23% by mass, and further drying was performed during the period until the final drying roller (the 19th drying roller) immediately before the heat treatment roller by alternately bringing the non-contact surface and the contact surface of the PVA film with the subsequent drying rollers into contact with them. At this time, the surface temperature of each drying roller from the drying roller after peeling off from the support to the final drying roller was 75°C, and the volatile fraction of the PVA film after peeling off from the final drying roller, that is, the volatile fraction of the PVA film when contacting the initial heat treatment roller, was 6% by mass. Thereafter, heat treatment was performed by alternately bringing one side and the other side of the PVA film into contact with each heat treatment roller. At this time, the heat treatment was performed using two heat treatment rollers, and the surface temperature of either of the heat treatment rollers was made 105°C. For the obtained PVA film (thickness: 60 μm, width: 1200 mm), nanoindentation tests and FT-IR measurements were performed by the above method, and the composite elastic coefficients (Er1, Er2, Er3) and the crystallinity indices (Fg1, Fg2, Fg3) were calculated. The results are shown in Table 1.

[0121] <Manufacture and Evaluation of Polarizing Film> The obtained PVA film was cut into 650 mm wide pieces, and the film was continuously manufactured into a polarizing film by sequentially performing swelling, dyeing, crosslinking, stretching, washing, and drying treatments. The swelling treatment was performed by immersing the film in pure water at 25°C (swelling solution) and stretching it uniaxially by 2.00 times in the length direction. The dyeing treatment was performed by immersing the film in a potassium iodide / iodine dyeing solution (dyeing solution) at 32°C (potassium iodide / iodine (mass ratio) 2:3, iodine concentration 0.03~0.05% by mass) and stretching it uniaxially by 1.26 times in the length direction. In this dyeing treatment, the monomer transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment was adjusted to be 44%, with the iodine concentration in the dyeing solution adjusted within the range of 0.02% by mass to 0.05% by mass. The crosslinking treatment was performed by immersing the film in a boric acid aqueous solution (crosslinking solution) at 32°C (boric acid concentration 2.6% by mass) and stretching it uniaxially by 1.19 times in the length direction. The stretching treatment was performed by uniaxially stretching the film 2.00 times in the length direction while immersing it in a boric acid / potassium iodide aqueous solution (stretching treatment solution) (boric acid concentration 2.8 wt%, potassium iodide concentration 5 wt%). The stretching treatment temperature was set at a stretching temperature where the tensile stress (tensile tension (N) / (PVA film width (mm) × PVA film thickness (mm)) in the stretching tank was between 10 N / mm² and 12 N / mm². The washing treatment was performed by immersing the film unstretched in a potassium iodide / boric acid aqueous solution (washing treatment solution) at 22°C (potassium iodide concentration 3-6 wt%, boric acid concentration 1.5 wt%) for 12 seconds. The drying treatment was performed by hot air drying unstretched at 80°C for 1.5 minutes, and a polarizing film was obtained. The polarizing performance of the obtained polarizing film was evaluated using the above method. The results are shown in Table 1.

[0122] <Manufacturing and Evaluation of Polarizing Plates> A triacetyl cellulose film was laminated onto both sides of the obtained polarizing film using a PVA adhesive (a 3.5% by mass aqueous solution of PVA), and then dried at 70°C to manufacture a polarizing plate. Furthermore, the PVA in the PVA adhesive used is a saponified homopolymer of vinyl acetate, with a degree of polymerization of 2400 and a degree of saponification of 99 moles.

[0123] Using the obtained polarizing plate, a damp heat resistance test was conducted based on the above method, and the light transmittance was measured. The results are shown in Table 1.

[0124] <Example 2> Except for changing the PVA used in the film-forming solution to PVA (saponification degree 99 mol%, polymerization degree 2400, ethylene modified 2.5 mol%), the process was the same as in Example 1 to obtain PVA films, polarizing films, and polarizing plates. The obtained polarizing films and polarizing plates were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0125] <Example 3> Except for changing the volatile content of the film-forming solution to 72% by mass (adjusting the water content from Example 1), the temperature of the support to 85°C, the peripheral speed of the support to 19.0 m / min, the volatile content of the PVA film peeled from the support to 13% by mass, the volatile content of the PVA film peeled from the final drying roller to 5% by mass, the surface temperature of the heat treatment roller to 120°C, and the thickness of the obtained PVA film to 30 μm, the process was the same as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0126] <Example 4> Except for changing the PVA used in the film-forming solution to PVA (saponification degree 99 moles, polymerization degree 4000), changing the volatile content of the film-forming solution to 72% by mass (the water content was adjusted according to Example 1), changing the temperature of the support to 85°C, changing the peripheral speed of the support to 19.0 m / min, changing the volatile content of the PVA film peeled from the support to 12% by mass, changing the volatile content of the PVA film peeled from the final drying roller to 5% by mass, changing the surface temperature of the heat treatment roller to 120°C, and changing the thickness of the obtained PVA film to 30 μm, the process was the same as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 3. The results are shown in Table 1.

[0127] <Example 5> Except for changing the temperature of the support to 95°C, the temperature of the hot air sprayed onto the non-contact surfaces of the liquid film and the support to 95°C, the volatile content of the PVA film at the time of peeling from the support to 18% by mass, the surface temperature of each drying roller from the drying roller after peeling from the support to the final drying roller to 60°C, the volatile content of the PVA film at the time of peeling from the final drying roller to 10% by mass, and the surface temperature of the heat treatment roller to 120°C, the process was the same as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0128] <Example 6> Except for changing the temperature of the support to 80°C, the temperature of the hot air sprayed onto the non-contact surfaces of the liquid film and the support to 80°C, the volatile content of the PVA film at the time of peeling from the support to 27% by mass, the surface temperature of each drying roller from the drying roller after peeling from the support to the final drying roller to 80°C, and the surface temperature of the heat treatment roller to 90°C, the process was carried out in the same manner as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0129] <Comparative Example 1> Except for the following changes: the temperature of the support was changed to 80°C, the circumferential speed of the support was changed to 9.0 m / min, the temperature of the hot air sprayed onto the non-contact surfaces of the liquid film and the support was changed to 80°C, the volatile content of the PVA film at the time of peeling from the support was changed to 17% by mass, the volatile content of the PVA film at the time of peeling from the final drying roller was changed to 5% by mass, and the surface temperature of the heat treatment roller was changed to 90°C, the same procedures were followed as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0130] <Comparative Example 2> Except that a suspension dryer was used instead of heat treatment rollers, and hot air at 120°C was blown for heat treatment, the process was the same as in Example 1 to obtain PVA film, polarizing film, and polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0131] <Comparative Example 3> Except for changing the temperature of the support to 95°C, the circumferential speed of the support to 22.0 m / min, the temperature of the hot air sprayed onto the non-contact surfaces of the liquid film and the support to 95°C, the volatile content of the PVA film at the time of peeling from the support to 32% by mass, and the surface temperature of the heat treatment roller to 120°C, the process was the same as in Example 1 to obtain a PVA film, a polarizing film, and a polarizing plate. The obtained polarizing film and polarizing plate were measured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0132] [Table 1] PVA PVA film Manufacturing conditions of PVA film Composite elastic modulus of PVA film Crystallinity index of PVA film polarized light performance Moist heat resistance Degree of aggregation Modification thickness Volatile components of film-forming solution Rate Support body temperature Support body's weekly velocity hot air temperature When the support is peeled off Zhihui Sending points Rate Volatile content during support peeling / thickness dry Roller temperature Heat treatment methods Volatile content during final drying roller peeling Heat treatment temperature Er1 Er2 Er3 Er1- Er3 Er2- Er3 Fg1 Fg2 Fg3 |Fg1- Fg2| evaluate Polarization evaluate ⊿Ty (Initial Ty43%) unit [Moll%] [μm] [%] [℃] [m / minute] [℃] [%] [% / μm] [℃] roller [%] [℃] [GPa] [GPa] [GPa] [GPa] [GPa] [%] [%] Example 1 2400 none 60 66 90 16.7 90 twenty three 0.38 75 roller 6 105 3.64 3.91 2.95 0.69 0.96 0.832 0.820 0.471 0.012 A 99.96 A 0.60 Example 2 2400 ethylene 2.5 60 66 90 16.7 90 twenty three 0.38 75 roller 6 105 4.37 4.40 3.39 0.98 1.01 0.772 0.762 0.354 0.01 A 99.95 A 0.61 Example 3 2400 none 30 72 85 19.0 90 13 0.43 75 roller 5 120 4.81 5.04 4.34 0.47 0.70 0.868 0.832 0.377 0.036 B 99.94 A 0.68 Example 4 4000 none 30 72 85 19.0 90 12 0.40 75 roller 5 120 5.90 6.14 5.45 0.45 0.69 0.897 0.814 0.385 0.083 A 99.96 A 0.65 Example 5 2400 none 60 66 95 16.7 95 18 0.38 60 roller 10 120 4.33 4.52 3.53 0.80 0.99 0.960 0.948 0.651 0.012 B 99.94 B 0.72 Example 6 2400 none 60 66 80 16.7 80 27 0.37 80 roller 6 90 3.35 3.41 2.80 0.55 0.61 0.631 0.622 0.292 0.009 B 99.91 B 0.73 Comparative example 1 2400 none 60 66 80 9.0 80 17 0.28 75 roller 5 90 3.27 3.30 3.01 0.26 0.29 0.688 0.686 0.470 0.002 C 99.85 C 0.81 Comparative example 2 2400 none 60 66 90 16.7 90 twenty three 0.38 75 Floating 6 120 3.66 3.81 3.47 0.19 0.34 0.852 0.833 0.562 0.019 C 99.78 C 0.85 Comparative example 3 2400 none 60 66 95 22.0 95 32 0.53 75 roller 6 120 4.12 4.10 2.85 1.27 1.25 0.904 0.905 0.289 0.001 B 99.94 C 0.90

[0133] As shown in Table 1, when polarizing films were manufactured using the PVA films of Examples 1-6, the polarizing performance of the obtained polarizing films and the resistance to damp heat of the polarizing plates were good. On the other hand, when polarizing films were manufactured using the PVA films of Comparative Examples 1-3, the polarizing performance of the obtained polarizing films was low, and the resistance to damp heat of the polarizing plates was also poor.

[0134] 1: PVA film 2: Thickness direction of PVA film 3: First surface 4: Surface 2 5: The cross-section formed by perpendicularly cutting the first and second surfaces. 6: The vicinity of the first surface (the region within 5 μm from the first surface toward the center in the thickness direction) 7: The vicinity of the second surface (the region within 5 μm from the second surface toward the center in the thickness direction) 8: Central part in the thickness direction 9: Infrared penetration depth (approximately 0.5 μm) 10: ATR Prism 11: Infrared

Claims

1. A polyvinyl alcohol film, which is a non-water-soluble polyvinyl alcohol film, wherein the composite elastic coefficient of the cross section obtained by perpendicularly cutting the first surface and the second surface of the aforementioned polyvinyl alcohol film is subjected to a nano-indentation test at 23°C and 40%RH, when the composite elastic coefficient near the first surface is set as Er1 (GPa), the composite elastic coefficient near the second surface is set as Er2 (GPa), and the composite elastic coefficient at the center of the thickness direction is set as Er3 (GPa), the aforementioned Er1, Er2 and Er3 satisfy the following equations (1) and (2): 0.40≦Er1-Er3≦1.20 (1) 0.40≦Er2-Er3≦1.20 (2) [In the aforementioned formulas (1) to (2), Er1 is the composite elastic coefficient of the region within 5 μm from the first surface to the center in the thickness direction, Er2 is the composite elastic coefficient of the region within 5 μm from the second surface to the center in the thickness direction, and Er3 is the composite elastic coefficient of the region in the center in the thickness direction.] 2. The polyvinyl alcohol film of claim 1, wherein the crystallinity index calculated by FT-IR measurement of the thickness direction of the first surface and the second surface of the aforementioned polyvinyl alcohol film, and the central portion of the cross section obtained by perpendicularly cutting the first surface and the second surface, is given by ATR method. When the crystallinity index of the first surface is set as Fg1, the crystallinity index of the second surface is set as Fg2, and the crystallinity index of the central portion of the thickness direction is set as Fg3, the aforementioned Fg1, Fg2 and Fg3 satisfy the following formulas (3) to (5): 0.70≦Fg1≦0.90 (3) 0.70≦Fg2≦0.90 (4) 0.30≦Fg3≦0.50 (5) [In the aforementioned formulas (3) to (5), Fg1 is the crystallinity index calculated when the first surface is subjected to FT-IR measurement, Fg2 is the crystallinity index calculated when the second surface is subjected to FT-IR measurement, and Fg3 is the crystallinity index calculated when the central part of the cross-section in the thickness direction is subjected to FT-IR measurement.] 3. The polyvinyl alcohol film of claim 2, wherein the aforementioned Fg1 and Fg2 satisfy the following formula (6): |Fg1-Fg2|≦0.10 (6).

4. The polyvinyl alcohol film of any one of claims 1 to 3 is a film for manufacturing optical films.

5. The polyvinyl alcohol film of claim 4, wherein the aforementioned optical film is a polarizing film.

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