CN115777075A

CN115777075A - Polyvinyl alcohol film and polarizing film using the same

Info

Publication number: CN115777075A
Application number: CN202180047201.5A
Authority: CN
Inventors: 田边裕史; 冈本稔; 风藤修
Original assignee: Kuraray Co Ltd
Current assignee: Kuraray Co Ltd
Priority date: 2020-06-30
Filing date: 2021-06-23
Publication date: 2023-03-10
Also published as: TW202216806A; KR20230027003A; JPWO2022004537A1; WO2022004537A1

Abstract

Provided is a PVA film which is less likely to wrinkle on the surface during uniaxial stretching and which is suppressed in breakage during uniaxial stretching even when the maximum stretching speed is high. A PVA film, which is a water-insoluble PVA film, wherein the crystallinity indexes of the first surface are Fd1 and Fg1, and the crystallinity indexes of the second surface are Fd2 and Fg2, wherein Fd1, fg1, fd2 and Fg2 satisfy the following expressions (1) to (4), fd1 is not more than 0.8 (1) Fd1/Fg1 <1 (2) Fd2 is not more than 0.8 (3) Fd2/Fg2 <1 (4) [ in the expressions (1) to (4), fd1 and Fg1 are crystallinity indexes calculated by using a diamond prism and a germanium prism, respectively, when FT-IR measurement is performed on the first surface by ATR method, fd2 and Fg2 are similarly crystallinity indexes calculated by using a diamond prism and a germanium prism, respectively, on the second surface ].

Description

Polyvinyl alcohol film and polarizing film using the same

Technical Field

The present invention relates to a polyvinyl alcohol film and a polarizing film using the same.

Background

A polarizing plate having light transmitting and light blocking functions and a liquid crystal having a light switching function are both basic components of a Liquid Crystal Display (LCD). In recent years, the application field of the LCD has been expanded from small-sized devices such as calculators and wristwatches at the beginning of development to various fields such as notebook computers, liquid crystal monitors, liquid crystal color projectors, liquid crystal televisions, car navigation systems, cellular phones, and measuring devices used indoors and outdoors.

Polarizing plates are manufactured by attaching a protective film such as a Triacetylcellulose (TAC) film or a Cellulose Acetate Butyrate (CAB) film to the surface of a polarizing film. Also, the polarizing film is generally manufactured as follows: a method for producing a dyed uniaxially stretched film by dyeing a polyvinyl alcohol film (hereinafter, referred to as "polyvinyl alcohol" in some cases) and then uniaxially stretching the dyed film, or uniaxially stretching the dyed film while dyeing the film, or dyeing the dyed film after uniaxially stretching the film, and then fixing the dyed uniaxially stretched film with a boron compound. The immobilization treatment with the boron compound may be performed simultaneously with the uniaxial stretching or dyeing treatment.

Large-sized products having LCDs, such as liquid crystal monitors and liquid crystal televisions, are required to have high contrast and sharp images. Accordingly, polarizing films are also required to have higher performance, and specifically, the polarization degree of polarizing films is required to be improved. However, when the stretching ratio in the uniaxial stretching of the PVA film is increased in order to increase the degree of polarization of the polarizing film, wrinkles are likely to be generated on the surface of the PVA film in the uniaxial stretching process. As a result, wrinkles are also likely to occur on the surface of the obtained polarizing film. When a large amount of wrinkles are generated on the surface of the polarizing film, the wrinkles tend to cause image unevenness in a liquid crystal monitor, a liquid crystal television, or the like, which is a final product. In addition, if a large amount of wrinkles occur on the surface of the polarizing film, the polarizing film cannot be used as a product, and thus, the product yield (product yield) of the polarizing film is also reduced.

As a method for suppressing the occurrence of wrinkles on the surface of a polarizing film, there are proposed: the relaxation time and the composition ratio are controlled for a component having a short relaxation time (a component having low molecular mobility and hardness) when the PVA film is subjected to pulse NMR measurement (see patent document 1).

Documents of the prior art

Patent document

Patent document 1: WO2019/189695

Disclosure of Invention

Problems to be solved by the invention

In recent years, demands for higher contrast and image sharpness of LCDs have been further increased, and with this, surface wrinkles of a polarizing film, which have not been a problem in the past, have become a problem in many cases. In order to improve the production efficiency of polarizing films, it is desired to perform uniaxial stretching in the stretching process in the production of polarizing films at high speed, that is, to set the maximum stretching speed of the uniaxial stretching to high speed. However, if the maximum stretching speed is set to a high speed in the uniaxial stretching in the production of the polarizing film, wrinkles are more likely to occur on the surface of the PVA film in the uniaxial stretching. As a result, wrinkles are more likely to be generated on the surface of the polarizing film obtained. In addition, when the maximum stretching speed is high, an excessive tension may be applied to a part of the PVA film during uniaxial stretching. As a result, there is also a problem that the PVA film is likely to be broken during uniaxial stretching, and the product yield of the polarizing film is reduced.

In the PVA film described in patent document 1, if the maximum stretching speed is set to a high speed in the uniaxial stretching in the production of the polarizing film, wrinkles are likely to be generated on the surface of the PVA film in the uniaxial stretching, and the surface wrinkles of the polarizing film cannot be sufficiently suppressed in some cases. When the maximum stretching speed is set to a high speed, the PVA film may be broken during uniaxial stretching. It is also important to suppress surface wrinkles of the PVA film during uniaxial stretching and to suppress breakage of the PVA film during uniaxial stretching, for suppressing surface wrinkles and breakage of the optical film other than the polarizing film.

Accordingly, an object of the present invention is to provide a PVA film in which wrinkles are not easily generated on the surface during uniaxial stretching and the breaking during uniaxial stretching is suppressed even when the maximum stretching speed in uniaxial stretching in producing an optical film such as a polarizing film is high.

Means for solving the problems

The present inventors have conducted extensive studies and, as a result, have found that: the above-described problems can be achieved by adjusting the crystallinity indexes of both surfaces of the PVA film orthogonal to the thickness direction to a specific range, and further studies have been repeated based on this finding, thereby completing the present invention.

Namely, the present invention relates to:

[1] a PVA film that is water-insoluble, wherein when two surfaces of the PVA film that are orthogonal to a thickness direction are respectively a first surface and a second surface, a crystallinity index of the first surface is Fd1 and Fg1, and a crystallinity index of the second surface is Fd2 and Fg2, the Fd1, fg1, fd2, and Fg2 satisfy the following expressions (1) to (4);

Fd1≤0.8 (1)

Fd1/Fg1＜1 (2)

Fd2≤0.8 (3)

Fd2/Fg2＜1 (4)

in the formulae (1) to (4), fd1 is a crystallinity index calculated by using a diamond prism when FT-IR measurement is performed on the first surface by the ATR method, fg1 is a crystallinity index calculated by using a germanium prism when FT-IR measurement is performed on the first surface by the ATR method, fd2 is a crystallinity index calculated by using a diamond prism when FT-IR measurement is performed on the second surface by the ATR method, and Fg2 is a crystallinity index calculated by using a germanium prism when FT-IR measurement is performed on the second surface by the ATR method.

[2] The PVA film according to [1], wherein Fd1 and Fd2 satisfy the following formulas (5) to (6);

Fd1≥0.5 (5)

Fd2≥0.5 (6)

[3] the PVA film according to [1] or [2], wherein Fd1, fg1, fd2 and Fg2 satisfy the following formulae (7) to (8);

Fd1/Fg1≥0.6 (7)

Fd2/Fg2≥0.6 (8)

[4] the PVA film according to any one of [1] to [3], wherein Fd1, fg1, fd2 and Fg2 satisfy the following formulae (9) to (10);

|Fd1-Fd2|≤0.07 (9)

|Fg1-Fg2|≤0.07 (10)

[5] the PVA film according to any one of [1] to [4], which is a film for producing an optical film;

[6] the PVA film according to [5], wherein the optical film is a polarizing film.

Effects of the invention

According to the present invention, there is provided a PVA film in which wrinkles are not easily generated on the surface during uniaxial stretching and the breakage during uniaxial stretching is suppressed even when the maximum stretching speed in the uniaxial stretching in the production of an optical film such as a polarizing film is high. Such a PVA film can suppress wrinkles generated on the surface of an optical film such as a polarizing film. In addition, since the breakage during uniaxial stretching is suppressed, an optical film such as a polarizing film can be produced with a high product yield.

Drawings

FIG. 1 is a perspective view of a PVA film of the present invention.

FIG. 2 is a side view of the PVA film of the present invention.

Fig. 3 is a diagram schematically showing the ATR method in the FT-IR measurement.

Detailed Description

The present invention will be described in detail below.

< PVA film >

In the present invention, as shown in fig. 1 and 2, two surfaces of the PVA film 1 orthogonal to the thickness direction 2 are defined as a first surface 3 and a second surface 4, respectively. Thus, the first surface 3 and the second surface 4 of the PVA film 1 of the present invention are opposed to each other. In the present invention, FT-IR (fourier transform infrared spectroscopy) measurement is performed on the first surface 3 and the second surface 4 by ATR method, respectively. The crystallinity indices Fd1, fg1, fd2, and Fg2 calculated by the measurement satisfy the following expressions (1) to (4).

Fd1≤0.8 (1)

Fd1/Fg1＜1 (2)

Fd2≤0.8 (3)

Fd2/Fg2＜1 (4)

In the above expressions (1) to (4), fd1 is the crystallinity index calculated using a diamond prism when FT-IR measurement is performed on the first surface 3 of the PVA film 1 by the ATR method, and Fg1 is the crystallinity index calculated using a germanium prism when FT-IR measurement is performed on the first surface 3 of the PVA film 1 by the ATR method. Fd2 is a crystallinity index calculated by using a diamond prism when FT-IR measurement is performed on the second surface 4 of the PVA film 1 by the ATR method, and Fg2 is a crystallinity index calculated by using a germanium prism when FT-IR measurement is performed on the second surface 4 of the PVA film 1 by the ATR method. In the formula (2), fd1/Fg1 is a value obtained by dividing Fd1 by Fg1, and in the formula (4), fd2/Fg2 is a value obtained by dividing Fd2 by Fg 2.

In the PVA film of the present invention, as shown in the above formulas (1) and (3), fd1 and Fd2 need to be 0.8 or less. When Fd1 or Fd2 exceeds 0.8, when the maximum stretching speed is high in the uniaxial stretching in the production of an optical film such as a polarizing film, wrinkles are likely to occur on the surface of PVA film 1 in the uniaxial stretching, and PVA film 1 is likely to be broken in the uniaxial stretching. The reason is not necessarily clear, but is presumed to be that: if the crystallinity of the surface of the PVA film 1 is too high, water in the stretching treatment liquid is less likely to penetrate into the PVA film 1 during uniaxial stretching, and the flexibility of the film during uniaxial stretching becomes insufficient. Fd1 and Fd2 are preferably 0.75 or less, more preferably 0.72 or less, further preferably 0.7 or less, and particularly preferably 0.68 or less.

In the PVA film of the present invention, fd1/Fg1 and Fd2/Fg2 need to be less than 1 as shown in the above formulas (2) and (4). When Fd1/Fg1 or Fd2/Fg2 is 1 or more, the surface of the PVA film 1 is likely to be wrinkled during uniaxial stretching at a high maximum stretching speed in the uniaxial stretching in the production of an optical film such as a polarizing film. Fd1/Fg1 and Fd2/Fg2 are preferably 0.98 or less, more preferably 0.96 or less, still more preferably 0.94 or less, yet more preferably 0.92 or less, and particularly preferably 0.9 or less.

In the PVA film of the present invention, fd1 and Fd2 are 0.8 or less as shown in the above formulas (1) and (3). In addition, fd1/Fg1 and Fd2/Fg2 are less than 1, as shown in the above formulas (2) and (4). As described later, fd1 and Fd2 represent the crystallinity of the deeper inside of the PVA film 1, while Fg1 and Fg2 represent the crystallinity of the near surface portion, i.e., the very surface layer portion, of the PVA film 1. That is, in the PVA film of the present invention, the crystallinity in the deeper interior of the PVA film 1 is equal to or lower than the predetermined value, and the crystallinity in the vicinity of the surface of the PVA film 1, that is, in the extreme surface layer portion is higher than the crystallinity in the deeper interior of the PVA film 1. By controlling the crystallinity of the deeper inner portion and the extremely surface portion of the PVA film 1 in this manner, wrinkles are less likely to occur on the surface during uniaxial stretching even at a high maximum stretching speed in the uniaxial stretching during the production of an optical film such as a polarizing film, and the occurrence of cracks during the uniaxial stretching is suppressed. The reason is not necessarily clear, but is presumed to be: since the crystallinity of the very surface layer portion of the PVA film 1 is high, wrinkles are suppressed from being generated on the surface of the PVA film 1 during uniaxial stretching, and since the crystallinity of the deeper inside of the PVA film 1 is low, stress generated during uniaxial stretching is relaxed, and breaking is suppressed.

In the PVA film of the present invention, the lower limit values of Fd1 and Fd2 are not necessarily limited, but in the uniaxial stretching in the production of an optical film such as a polarizing film, since the fracture in the uniaxial stretching of the PVA film 1 can be further suppressed when the maximum stretching speed is high, it is preferable that the following formulas (5) and (6) are satisfied.

Fd1≥0.5 (5)

Fd2≥0.5 (6)

As shown in the above equations (5) and (6), if Fd1 and Fd2 are set to 0.5 or more, the crystallinity of the PVA film 1 in the deep interior becomes large. As a result, the crystallinity of the central portion in the thickness direction 2 of the PVA film 1 increases, and the mechanical strength of the PVA film 1 improves. Therefore, by using such a PVA film 1, in the uniaxial stretching in the production of an optical film such as a polarizing film, even when the maximum stretching speed is high, the fracture in the uniaxial stretching of the PVA film 1 can be further suppressed. Fd1 and Fd2 are more preferably 0.52 or more, and still more preferably 0.55 or more.

In the PVA film of the present invention, the lower limit values of Fd1/Fg1 and Fd2/Fg2 are not necessarily limited, and it is preferable that the following formulas (7) and (8) are satisfied because the fracture of the PVA film 1 can be further suppressed when the PVA film is uniaxially stretched at a high maximum stretching speed in the case of uniaxial stretching at the time of producing an optical film such as a polarizing film.

Fd1/Fg1≥0.6 (7)

Fd2/Fg2≥0.6 (8)

As shown in the above-mentioned formulas (7) and (8), by setting Fd1/Fg1 and Fd2/Fg2 to 0.6 or more, the crystallinity from the deep inside of the PVA film 1 does not become too small as compared with the crystallinity of the extreme surface layer portion of the PVA film 1. As a result, the crystallinity of the central portion in the thickness direction 2 of the PVA film 1 becomes large, and the mechanical strength of the PVA film 1 is improved. Therefore, by using such a PVA film 1, in the uniaxial stretching in the production of an optical film such as a polarizing film, even when the maximum stretching speed is high, the fracture in the uniaxial stretching of the PVA film 1 can be further suppressed. Fd1/Fg1 or Fd2/Fg2 is more preferably 0.65 or more, still more preferably 0.7 or more, and particularly preferably 0.75 or more.

In the PVA film of the present invention, the absolute values of the difference between Fd1 and Fd2 and the difference between Fg1 and Fg2 are not necessarily limited, and in the uniaxial stretching in the production of an optical film such as a polarizing film, since wrinkles generated on the surface of the PVA film 1 can be further suppressed in the uniaxial stretching when the maximum stretching speed is high, it is preferable that the following formulas (9) and (10) are satisfied.

|Fd1-Fd2|≤0.07 (9)

|Fg1-Fg2|≤0.07 (10)

As shown in the above equations (9) and (10), by setting | Fd1-Fd2| and | Fg1-Fg2| to 0.07 or less, the difference in crystallinity index from the first surface 3 and the second surface 4 of the PVA film 1 is not excessively large, and the elastic modulus is substantially equal in both surfaces (the first surface 3 and the second surface 4) orthogonal to the thickness direction 2 of the PVA film 1. Therefore, by using such a PVA film 1, in the uniaxial stretching in the production of an optical film such as a polarizing film, even when the maximum stretching speed is high, wrinkles are less likely to occur on the surface of the PVA film 1 in the uniaxial stretching. The | Fd1-Fd2| and | Fg1-Fg2| are more preferably 0.06 or less, still more preferably 0.05 or less, and particularly preferably 0.04 or less.

(FT-IR measurement)

Generally, when the infrared absorption spectrum (IR spectrum) of the PVA film 1 is measured, 1140cm is observed depending on the PVA contained therein ^-1 An absorption peak was observed. This absorption peak is generally called the crystallization band of the PVA film 1, and is a peak of PVA derived from stretching vibration of carbon bond (C — C). This crystallization zone is observed when the polymer molecular chains of the PVA in the PVA film 1 are crystallized, etc., and the phases of the vibrations of the polymer molecular chains of the PVA are known to be aligned and strengthened. That is, the higher the crystallinity of the PVA film 1, the higher the peak intensity of the crystallization band is. When the infrared absorption spectrum of the PVA film 1 is measured, it is derived from methylene (-CH) which is the main chain of PVA ₂ -) variable angle vibration at 1425cm ^-1 An absorption peak was observed. The intensity of this absorption peak is independent of the crystallinity of the PVA film 1.

In the present invention, the crystallization band (1140 cm) is calculated ^-1 ) And the absorption peak intensity of (2) with methylene group (-CH) as the main chain of PVA ₂ -) variable angle vibration (1425 cm ^-1 ) The crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film 1 can be obtained by the intensity ratio of the absorption peak intensities of (a). Specifically, 1140cm is drawn ^-1 And 1425cm ^-1 The base line of the infrared absorption spectrum of (A) is from the base line to 1140cm ^-1 And 1425cm ^-1 Will pass through 1140cm as the respective absorption peak intensity ^-1 Divided by 1425cm ^-1 The values obtained from the peak intensities of (a) are regarded as crystallinity indexes (Fg 1, fg2, fd1 and Fd 2).

It is well known that: the values of the crystallinity indices (Fg 1, fg2, fd1, and Fd 2) thus obtained are proportional to the crystallinity of the PVA film 1 (for example, n.a. peppas, macromol. Chem., volume 178, 595 (1977), japanese patent application laid-open No. 6-138321). Since the value of the crystallinity index slightly varies depending on the moisture absorption amount of the PVA film 1, in the present invention, the FT-IR measurement was performed in the same environment after the PVA film 1 was stored for 24 hours in the environment of 24.0 ℃ and 45.0% rh in relative humidity.

In the present invention, the FT-IR measurement is performed by ATR method (total reflection absorptiometry). As shown in fig. 3, the ATR method is: one of reflection-type IR measurement methods is a method in which a sample is closely contacted with an objective lens called ATR prism 7, infrared rays 8 are obliquely irradiated from inside the ATR prism 7 toward the sample, and the spectrum of the reflected light is measured. This method is characterized in that a sharp spectrum with less noise can be obtained as compared with a conventional reflection-type IR measurement method. In the case where the PVA film 1 is used as a sample in this measurement method, the infrared ray 8 is not only reflected only on the surface of the PVA film 1, but also the infrared ray 8 slightly transmitted from the ATR prism 7 side to the PVA film 1 side is reflected. Therefore, information on the surface layer (a portion slightly penetrating from the surface of the PVA film 1 in the depth direction) of the PVA film 1 can be obtained by FT-IR measurement by the ATR method. Here, when the penetration depth of the infrared ray 8 penetrating from the ATR prism 7 side toward the PVA film 1 side is denoted by d, the value is expressed by the following formula (11). As is clear from the following formula (11): by using the ATR prism 7 having a different refractive index, reflection-type infrared absorption spectra having different penetration depths can be obtained.

d＝λ/2Πn ₁ ×1/{sin ² θ-(n ₂ /n ₁ ) ² } ^0.5 (11)

In the above formula (11), n ₁ Representing the refractive index, n, of the ATR prism 7 ₂ Denotes the refractive index of the PVA film 1, λ denotes the wavelength of the infrared ray 8, and θ denotes the incident angle of the infrared ray 8.

In the present invention, as shown in fig. 3, diamond having a refractive index of 2.4 or germanium having a refractive index of 4.0 is used as the base material of the ATR prism 7. Since the refractive index of the PVA film 1 was 1.5, the incident angle of the infrared ray 8 was 45 ° and the wave number of the infrared ray 8 was 1140cm in the above equation (11) ^-1 When the penetration depth of the infrared ray 8 penetrating toward the surface layer of the PVA film 1 is large, the penetration depth of the infrared ray 8 when diamond is used as the base material of the ATR prism 7, that is, when a diamond prism is used5 is about 2 μm. On the other hand, in the case of using germanium as the ATR prism 7, that is, the penetration depth 6 of the infrared ray 8 in the case of a germanium prism is about 0.5 μm. Therefore, the crystallinity index when using the diamond prism corresponds to the crystallinity up to the deeper inside of the PVA film 1. On the other hand, the crystallinity index when the germanium prism is used corresponds to the crystallinity in the vicinity of the surface of the PVA film 1, that is, in the very surface layer portion.

In the present invention, it is important: the crystallinity indexes Fg1 and Fg2 of the extreme surface layer portion of the PVA film 1 and the crystallinity indexes Fd1 and Fd2 of the deeper inside of the PVA film 1 are controlled to the above ranges. The crystal structure of the PVA film 1 is affected by various factors in the composition and production process of the PVA film 1, and thus, as a method for controlling the crystallinity index (Fg 1, fg2, fd1, and Fd 2), for example, a method of adjusting the kind of polyvinyl alcohol (saponification degree, modification amount, blending ratio of unmodified PVA/modified PVA, etc.); a method of adjusting the amount of plasticizer added; a method of adjusting film forming conditions (surface temperature of a roll support, heat treatment conditions, etc.); or a method of combining them and adjusting them.

More specifically, the crystallinity indices Fd1 and Fd2 are adjusted to 0.8 or less and Fd1/Fg1 and Fd2/Fg2 are adjusted to less than 1, and the examples include: a method in which the saponification degree of PVA is 90 mol% or more, the proportion of structural units derived from other monomers in the vinyl ester polymer that is the raw material of PVA is 15 mol% or less based on the number of moles of all the structural units constituting the vinyl ester polymer, and the degree of polymerization of PVA is 200 to 8000. In this case, the amount of the plasticizer to be added is preferably 1 to 40 parts by mass with respect to 100 parts by mass of PVA. In this case, the evaporation fraction of the film-forming dope is preferably 50 to 90 mass%, the surface temperature of the support from which the film-forming dope is cast is preferably 65 to 110 ℃, the temperature of the hot air blown to the non-contact surface side is preferably 50 to 150 ℃ or less, and the humidity of the hot air is preferably 20 to 90% rh. In this case, the temperature of the drying furnace or the surface temperature of the drying roll is preferably 45 to 110 ℃, and the surface temperature of the heat treatment roll is preferably 60 to 135 ℃.

Examples of the method for adjusting the crystallinity indices Fd1 and Fd2 to 0.5 or more include: the method is a method in which the degree of saponification of PVA is 95 to 99.9 mol%, and the proportion of structural units derived from other monomers in the vinyl ester polymer that is the raw material of PVA is 10 mol% or less based on the number of moles of all structural units constituting the vinyl ester polymer, and the degree of polymerization of PVA is 1000 to 4000. In this case, the amount of the plasticizer to be added is preferably 5 to 20 parts by mass with respect to 100 parts by mass of PVA. In this case, the volatile fraction of the film-forming dope is preferably 60 to 80 mass%, the surface temperature of the support on which the film-forming dope is cast is preferably 80 to 110 ℃, the temperature of hot air blown to the non-contact surface side is preferably 70 to 110 ℃ or less, and the humidity of the hot air is preferably 1 to 40% rh. Further, in this case, the temperature of the drying furnace or the surface temperature of the drying roll is preferably 60 to 110 ℃, and the surface temperature of the heat treatment roll is preferably 80 to 135 ℃.

Examples of the method for adjusting Fd1/Fg1 and Fd2/Fg2 to 0.6 or more include: a method in which the saponification degree of PVA is 99 to 99.9 mol%, the proportion of structural units derived from other monomers in the vinyl ester polymer that is the raw material of PVA is 5 mol% or less based on the number of moles of all the structural units constituting the vinyl ester polymer, and the degree of polymerization of PVA is 1000 to 3700. In this case, the amount of the plasticizer added is preferably 8 to 20 parts by mass with respect to 100 parts by mass of PVA. In this case, the volatile fraction of the film-forming dope is preferably 65 to 80 mass%, the surface temperature of the support on which the film-forming dope is cast is preferably 80 to 100 ℃, the temperature of hot air blown to the non-contact surface side is preferably 70 to 100 ℃, and the humidity of the hot air is preferably 3 to 40% rh. Further, in this case, the temperature of the drying furnace or the surface temperature of the drying roll is preferably 60 to 100 ℃, and the surface temperature of the heat treatment roll is preferably 80 to 120 ℃.

As a method for adjusting | Fd1-Fd2| and | Fg1-Fg2| to 0.07 or less, it is preferable that the evaporation fraction of the film-forming dope is 65 to 75 mass%, the surface temperature of the support body on which the film-forming dope is to be cast is 80 to 95 ℃, the temperature of hot air blown to the non-contact surface side is 75 to 90 ℃, and the humidity of the hot air is 5 to 40% rh. Further, in this case, the temperature of the drying furnace or the surface temperature of the drying roll is preferably 60 to 90 ℃, and the surface temperature of the heat treatment roll is preferably 80 to 110 ℃.

(PVA)

In the PVA film of the present invention, a polymer produced by saponifying a vinyl ester polymer obtained by polymerizing a vinyl ester monomer can be used as the PVA. Examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate. Among these, vinyl acetate is preferable as the vinyl ester monomer.

The vinyl ester polymer is preferably a polymer obtained by using only 1 or 2 or more vinyl ester monomers as monomers, and more preferably a polymer obtained by using only 1 vinyl ester monomer as a monomer. The vinyl ester polymer may be a copolymer of 1 or 2 or more vinyl ester monomers and another monomer copolymerizable therewith.

Examples of the other monomer include ethylene; olefins having 3 to 30 carbon atoms such as propylene, 1-butene and isobutylene; acrylic acid or a salt thereof; acrylic esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, and octadecyl acrylate; methacrylic acid or a salt thereof; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, and octadecyl methacrylate; acrylamide derivatives such as acrylamide, N-methylacrylamide, N-ethylacrylamide, N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid or a salt thereof, acrylamidopropyldimethylamine or a salt thereof, and N-methylolacrylamide or a derivative thereof; methacrylamide derivatives such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid or a salt thereof, methacrylamidopropyldimethylamine or a salt thereof, and N-methylolmethacrylamide or a derivative thereof; n-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; vinyl cyanide such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or a salt, ester or anhydride thereof; itaconic acid or a salt, ester or anhydride thereof; vinyl silyl compounds such as vinyltrimethoxysilane; isopropenyl acetate, and the like. The vinyl ester polymer may have 1 or 2 or more structural units derived from these other monomers.

The proportion of the structural unit derived from another monomer in the vinyl ester polymer is preferably 15 mol% or less, and more preferably 8 mol% or less, based on the number of moles of all the structural units constituting the vinyl ester polymer. In general, the higher the proportion of the structural unit derived from another monomer in the vinyl ester polymer, the more likely the crystallization of PVA becomes to occur. Therefore, the crystallinity indices (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted by appropriately copolymerizing these other monomers in the above-described ratio.

The polymerization degree of PVA is preferably 200 or more, more preferably 300 or more, and further preferably 500 or more. When the polymerization degree of PVA is not less than the lower limit, excessive crystallization of PVA can be prevented and the mechanical strength of the PVA film obtained can be ensured. On the other hand, the polymerization degree of PVA is preferably 8,000 or less, more preferably 6,000 or less, and further preferably 4,000 or less. Generally, the higher the polymerization degree of PVA, the more difficult crystallization of PVA tends to proceed. Therefore, by setting the polymerization degree of PVA to the upper limit or less, crystallization of PVA can be appropriately performed, and the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted. Further, by setting the polymerization degree of PVA to the upper limit or less, the viscosity of the film-forming dope of the PVA film is not excessively high, and the productivity of the PVA film can be improved.

The polymerization degree of PVA is an average polymerization degree measured according to JIS K6726-1994. That is, the polymerization degree (Po) is determined by the following formula (12).

Polymerization degree Po = ([ eta.) ]]×10 ⁴ /8.29) ^(1/0.62) (12)

In the above formula (12), η is an intrinsic viscosity (deciliter/g) measured in water at 30 ℃ after the PVA is re-saponified and purified.

The saponification degree of PVA is preferably 90 mol% or more, more preferably 95 mol% or more, still more preferably 99 mol% or more, and particularly preferably 99.8 mol% or more. In general, the higher the saponification degree of PVA, the more likely the PVA is crystallized. Therefore, by setting the saponification degree of the PVA to the lower limit or more, crystallization of the PVA can be appropriately performed, and the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be improved. That is, by using PVA having a high saponification degree in the film-forming stock solution of the PVA film, the crystallinity tends to increase in the vicinity of the surface of the PVA film, which is likely to be heated, that is, in the extreme surface layer portion and the deeper inside of the PVA film in the step of heat-treating the PVA film after the drying treatment.

The saponification degree of PVA means: the proportion (% by mole) of the number of moles of the vinyl alcohol unit relative to the total number of moles of the structural unit (typically, vinyl ester monomer unit) which can be converted into the vinyl alcohol unit by saponification and the vinyl alcohol unit. The degree of saponification of PVA can be measured according to JIS K6726-1994.

The PVA film of the present invention may contain 1 PVA alone, or may contain 2 or more kinds of PVA different in polymerization degree, saponification degree, modification degree, and the like.

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

(plasticizer)

The PVA film of the present invention preferably contains a plasticizer. By containing the plasticizer, the PVA film can be provided with flexibility equivalent to that of other plastic films, and the PVA film can be prevented from breaking in the film forming and stretching steps of the PVA film.

Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, diglycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, and sorbitol. These plasticizers may be used alone in 1 kind, or may be used in combination in 2 or more kinds. Among these, ethylene glycol or glycerin is preferable, and glycerin is more preferable as the plasticizer, for the reason that bleeding out to the surface of the PVA film is difficult.

The content of the plasticizer 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 further preferably 5 parts by mass or more, based on 100 parts by mass of PVA. On the other hand, the content of the plasticizer is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and further preferably 20 parts by mass or less with respect to 100 parts by mass of PVA. When the content of the plasticizer is in the above range, the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be easily adjusted, and the effect of improving mechanical properties such as impact strength can be sufficiently obtained. Further, the PVA film can be prevented from becoming too soft and reducing the handleability, or the plasticizer can be prevented from bleeding out to the surface of the PVA film.

The reason why the crystallinity indices (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted by adjusting the content of the plasticizer is as follows. Generally, when a proper amount of plasticizer is contained in the PVA film, crystallization of the PVA proceeds. This is presumed to be because: the plasticizer easily moves the polymer molecular chain of PVA, and thus the PVA easily has a more energy-stable crystalline or amorphous-constrained structure. On the other hand, when the PVA film contains an excessive amount of the plasticizer, crystallization of the PVA is easily hindered. This is presumably because: the amount of the plasticizer interacting with the hydroxyl groups of the polymer molecular chains of PVA increases, and the interaction between the polymer molecular chains of PVA decreases. Therefore, the crystallinity index (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted by adjusting the content of the plasticizer to appropriately crystallize the PVA.

(surfactant)

The PVA film of the present invention preferably contains a surfactant. By including the surfactant, the handleability of the PVA film and the peelability of the PVA film from a film-making apparatus during production can be improved. The surfactant is not particularly limited, and for example, an anionic surfactant or a nonionic surfactant is preferably used.

Examples of the anionic surfactant include carboxylic acid type surfactants such as potassium laurate; sulfate ester type surfactants such as octyl sulfate; sulfonic acid surfactants such as dodecylbenzenesulfonate, and the like.

Examples of the nonionic surfactant include alkyl ether type surfactants such as polyoxyethylene lauryl ether and polyoxyethylene oleyl ether; alkyl phenyl ether type surfactants such as polyoxyethylene octyl phenyl ether; alkyl ester surfactants such as polyoxyethylene laurate; alkylamine type surfactants such as polyoxyethylene lauryl amino ether; alkylamide surfactants such as polyoxyethylene laurylamides; polypropylene glycol ether type surfactants such as polyoxyethylene polyoxypropylene ether; alkanolamide type surfactants such as lauric acid diethanolamide and oleic acid diethanolamide; and an allyl phenyl ether type surfactant such as polyoxyalkylene allyl phenyl ether.

These surfactants may be used alone in 1 kind, or in combination of 2 or more kinds. The surfactant is preferably a nonionic surfactant, more preferably an alkanolamide surfactant, and still more preferably a dialkanolamide (e.g., diethanolamide) of an aliphatic carboxylic acid (e.g., a saturated or unsaturated aliphatic carboxylic acid having 8 to 30 carbon atoms), from the viewpoint of having an excellent effect of reducing surface irregularities in the production of a PVA film.

The content of the surfactant in the PVA film of the present invention is preferably 0.01 part by mass or more, more preferably 0.02 part by mass or more, and further preferably 0.05 part by mass or more, based on 100 parts by mass of PVA. On the other hand, the content of the surfactant is preferably 10 parts by mass or less, more preferably 1 part by mass or less, further preferably 0.5 part by mass or less, and particularly preferably 0.3 part by mass or less, relative to 100 parts by mass of PVA. When the content of the surfactant is in the above range, the PVA film can be favorably peeled off from the film-making apparatus during production, and the occurrence of sticking (hereinafter, also referred to as "blocking") between the PVA films can be prevented. In addition, it is possible to prevent the surfactant from bleeding out to the surface of the PVA film or the PVA film from deteriorating in appearance due to aggregation of the surfactant.

(other Components)

The PVA film of the present invention may contain, in addition to PVA, components such as a water-soluble polymer, moisture, an antioxidant, an ultraviolet absorber, a lubricant, a crosslinking agent, a colorant, a filler, a preservative, a fungicide, and other polymer compounds, within a range not to impair the effects of the present invention. The ratio of the total mass of the PVA, the surfactant, the plasticizer, and the other components except the PVA to the total mass of the PVA film is preferably 60 mass% or more, more preferably 80 mass% or more, and still more preferably 90 mass% or more. The ratio of the total mass of the other components to the total mass of the PVA film is preferably 100 mass% or less.

(Properties)

The PVA film of the present invention is water insoluble. When the PVA film is made water-insoluble, uniaxial stretching in the production of an optical film such as a polarizing film in an aqueous solution can be performed without breaking the PVA film during the uniaxial stretching even at a high maximum stretching speed. Here, in the present invention, water-insoluble means: when the PVA film is immersed in water (deionized water) at 30 ℃ according to the following <1> to <4>, the PVA film is completely dissolved and remains partially dissolved.

<1> the PVA film was allowed to stand in a constant temperature and humidity apparatus adjusted to 20 ℃ and 65% RH for 16 hours or more to adjust the humidity.

<2> after a rectangular sample having a length of 40mm x a width of 35mm was cut out from a conditioned PVA film, the sample was sandwiched and fixed between two plastic plates having a length of 50mm x 50mm, each of which had a rectangular window (hole) having a length of 35mm x a width of 23mm, in such a manner that the longitudinal direction of the sample was parallel to the longitudinal direction of the window and the sample was positioned at the approximate center in the width direction of the window.

<3> 300mL of deionized water was put into a 500mL beaker, stirred at 280rpm with a magnetic stirrer equipped with a 3cm long rod, and the water temperature was adjusted to 30 ℃.

<4> the sample fixed to the plastic plate in <2> above was immersed in deionized water in a beaker for 1000 seconds without contacting the rod of the rotating magnetic stirrer.

< method for producing PVA film >

The method for producing the PVA film of the present invention is not particularly limited, and any method such as the following may be employed. Examples of the method include: a method of forming a film by a casting film-forming method, a wet film-forming method (a method of spraying a solvent), a dry-wet film-forming method, a gel film-forming method (a method of once cooling and gelling a film-forming dope, and then extracting and removing the solvent), or a combination thereof, on a film-forming dope obtained by adding a solvent, an additive, or the like to PVA and homogenizing the dope; a melt extrusion film-forming method, an inflation molding method, and the like, in which a film-forming dope obtained by using an extruder or the like is extruded from a T die or the like to form a film. Among these, the production method of the PVA film is preferably a casting film formation method or a melt extrusion film formation method. By using these methods, a homogeneous PVA film can be obtained with good productivity. Hereinafter, a case of producing a PVA film by a casting film forming method or a melt extrusion film forming method will be described.

When the PVA film of the present invention is produced by a casting film forming method or a melt extrusion film forming method, first, a film forming dope containing PVA, a solvent, and, if necessary, an additive such as a plasticizer is prepared. Then, the film-forming dope is cast (supplied) in a film form on a support that rotates such as a metal roll or a metal belt. Thereby, a liquid coating of the film forming dope is formed on the support. The liquid coating film is cured by heating the support and removing the solvent, and is formed into a film. The method of heating the liquid coating film can be exemplified by: a method of raising the temperature of the support itself with a heat medium or the like, a method of blowing hot air to the surface of the liquid coating opposite to the surface in contact with the support, or the like. The cured long film (PVA film) is peeled off from the support, dried by a drying roll, a drying furnace, or the like as necessary, and further subjected to heat treatment as necessary, and wound into a roll.

In the drying step (solvent removal step) of the liquid coating film cast on the support and the subsequent drying step of the PVA film, the PVA is crystallized while being heated. The crystallization rate at this time is influenced by the proportion of the structural unit derived from another monomer in the PVA, the degree of polymerization of the PVA, the degree of saponification of the PVA, and the content of the plasticizer, as well as the moisture percentage, temperature, and draft (tensile elongation in the flow direction) in the PVA. The drawing is presumably an influence of oriented crystallization caused by the stretching of the polymer molecular chain of the PVA.

Generally, drying of the PVA film is performed by gradually volatilizing volatile components from the surface of the film that is released without coming into contact with a support, a drying roller, or the like. Therefore, in the step in the drying process, since the concentration distribution of volatile components such as water is generated in the thickness direction of the PVA film, a distribution of crystallinity index is generated in the thickness direction of the PVA film depending on the temperature and the draft condition at that time. The distribution of the crystallinity index can be adjusted by adjusting the volatile fraction of the film-forming dope, the temperature of the support, the time of contact with the support, the temperature and amount of hot air, the temperature of the drying roll and the temperature of the drying furnace, and the like. Therefore, by appropriately adjusting the above factors, crystallization of the PVA proceeds appropriately, and the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted.

The evaporation fraction of the film-forming dope (concentration of volatile components such as a solvent removed by evaporation or evaporation during film formation or the like) is preferably 50 mass% or more, and more preferably 55 mass% or more. The volatile fraction of the film-forming dope is preferably 90 mass% or less, more preferably 80 mass% or less. When the volatile fraction is in the above range, the viscosity of the film-forming dope can be adjusted to an appropriate range, and therefore, the film-forming property of the liquid coating film cast on the support is improved, and a PVA film having a uniform thickness can be easily obtained. When the volatile fraction is in the above range, crystallization of the PVA proceeds moderately on the support, and therefore, the crystallinity index and the distribution of the PVA film obtained are easily adjusted. The film-forming dope may contain a dichroic dye as necessary. The volatile fraction of the film-forming dope is a value obtained by the following formula (13).

Film-forming stock solution volatile fraction (% by mass) = { (Wa-Wb)/Wa) × 100 (13)

In the above formula (13), wa represents the mass (g) of the film-forming dope, and Wb represents the mass (g) of the film-forming dope after drying the Wa (g) in an electrothermal dryer at 105 ℃ for 16 hours.

The method for adjusting the film-forming dope is not particularly limited, and examples thereof include a method of dissolving PVA, additives such as a plasticizer and a surfactant in a solvent in a dissolving tank; and a method of melt-kneading the PVA in a water-containing state together with additives such as a plasticizer and a surfactant using a single-screw extruder or a twin-screw extruder.

The film-forming dope is generally cast into a film shape on a support such as a metal roll or a metal belt through a die lip of a die such as a T-die. On the support, the solvent gradually volatilizes from the surface of the cast film-like dope that does not come into contact with the support (hereinafter, sometimes referred to as a free surface), while the solvent does not substantially volatilize from the surface that comes into contact with the support (hereinafter, sometimes referred to as a contact surface), and therefore, a distribution occurs in which the solvent concentration on the free surface side is low and the solvent concentration on the contact surface side is high with respect to the thickness direction of the film. Therefore, the PVA is cured from the free surface.

Crystallization of the PVA also proceeds together with solidification of the PVA. Even if the solvent concentration is too high or too low, crystallization of PVA is difficult, and the volatile fraction of the casting dope is easily in the range of 20 to 60 mass%, depending on the primary structure of PVA molecules. The crystallization rate of PVA increases as the temperature increases, but the volatilization rate of the solvent increases as the temperature increases. Therefore, in order to efficiently crystallize the surface portion, which is the vicinity of the surface of the PVA film, it is important to control the crystallinity indexes (Fg 1 and Fg 2) of the surface portion, control the temperature of the support, the time of contact with the support, and the like, and also control the temperature of the atmosphere in the vicinity of the free surface, the vapor pressure of the solvent, and the like.

The PVA film of the present invention has higher crystallinity in the vicinity of the surface of the film, that is, in the very surface layer portion, than in the deeper inside of the film. Therefore, in order to obtain the PVA film of the present invention, conditions may be selected such that crystallization proceeds near the surface of the film, i.e., in the outermost layer portion, and crystallization in the deeper inside of the film is suppressed. For example, in the process of crystallization, the moisture percentage of the electrode surface portion is increased in advance by using a condition of gradually drying the film by lowering the drying temperature or the like at the beginning of drying in which the volatilization fraction of the electrode surface portion in the vicinity of the surface of the film is reduced. On the other hand, from the middle stage to the later stage of drying in which crystallization progresses in the deep inside of the film, the following conditions and the like are used: by rapidly drying at a high temperature, the inside is less likely to be crystallized.

The surface temperature of the support to be cast into the film forming dope is preferably 65 ℃ or higher, and more preferably 70 ℃ or higher. The surface temperature of the support to be cast into the film forming dope is preferably 110 ℃ or lower, more preferably 100 ℃ or lower, and further preferably 95 ℃ or lower. When the surface temperature is in the above range, the drying of the liquid coating film cast on the support and the crystallization of the extreme surface portion near the surface of the film can be performed at an appropriate rate, whereby the crystallinity indexes (Fg 1 and Fg 2) of the PVA film can be adjusted.

The liquid coating is heated on the support, and hot air with an air speed of 1 to 10 m/sec can be uniformly blown to the whole area of the non-contact surface side of the liquid coating. The temperature of the hot air blown to the non-contact surface side is preferably 50 ℃ or higher, more preferably 70 ℃ or higher. The temperature of the hot air blown to the non-contact surface side is preferably 150 ℃ or lower, more preferably 120 ℃ or lower. The humidity of the hot air is preferably 1% rh or more, more preferably 3% rh or more, and still more preferably 5% rh or more. The humidity of the hot air is preferably 40% RH or less, more preferably 30% RH or less. When the temperature and humidity of the hot air blown to the non-contact surface side are within the above ranges, the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film are easily adjusted.

The PVA film is preferably dried (solvent removed) on the support until the volatile fraction is 5 to 50 mass%, then peeled off from the support, and further dried as necessary. The drying method is not particularly limited, and may be a method of passing the sheet through a drying oven or a method of bringing the sheet into contact with a drying roller. In the case where the PVA film is dried using a plurality of drying rollers, it is preferable that one surface and the other surface of the PVA film are alternately brought into contact with the drying rollers. Thus, the difference (| Fd1-Fd2| and | Fg1-Fg2 |) in the crystallinity index of the PVA on both sides (both surfaces orthogonal to the thickness direction) of the PVA film can be adjusted. In this case, the number of drying rollers is preferably 3 or more, more preferably 4 or more, and further preferably 5 to 30.

The upper limit of the temperature of the drying furnace or the surface temperature of the drying roll is preferably 110 ℃ or less, more preferably 100 ℃ or less, further preferably 90 ℃ or less, and particularly preferably 85 ℃ or less. On the other hand, the lower limit of the temperature of the drying furnace or the surface temperature of the drying roll is preferably 40 ℃ or more, more preferably 45 ℃ or more, and still more preferably 50 ℃ or more. By setting the temperature of the drying oven or the surface temperature of the drying roller within the above range, the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be easily adjusted.

The PVA film after drying may be further subjected to heat treatment as necessary. By performing the heat treatment, the crystallinity of the PVA film near the surface, that is, in the very surface layer portion and the crystallinity of the PVA film in the deep interior can be improved, and the crystallinity indexes (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be adjusted. In addition, the properties of the PVA film such as mechanical strength and swelling property can be adjusted.

The lower limit of the surface temperature of the heat treatment roll for performing heat treatment is preferably 60 ℃ or higher. The upper limit of the surface temperature of the heat treatment roller is preferably 135 ℃ or less, more preferably 130 ℃ or less. By setting the surface temperature of the heat treatment roller within the above range, the crystallinity indices (Fg 1, fg2, fd1, and Fd 2) of the PVA film can be easily adjusted.

The PVA film produced in this manner is subjected to a humidity conditioning treatment, cutting of both ends (edges) of the film, and the like as necessary, and then wound into a roll on a cylindrical core, and is moisture-proof packaged to form a product.

The volatilization fraction of the PVA film finally obtained by a series of treatments is not necessarily limited. The volatile fraction of the PVA film is preferably 1 mass% or more, and more preferably 2 mass% or more. The volatile fraction of the PVA film is preferably 5 mass% or less, and more preferably 4 mass% or less.

< method for producing optical film >

The PVA film of the present invention is used as a raw material film in the production of an optical film. Examples of the optical film include a polarizing film, a viewing angle improving film, a retardation film, and a brightness enhancing film, and a polarizing film is preferable. Hereinafter, a method for producing a polarizing film will be specifically described as an example of a method for producing an optical film.

The polarizing film is usually produced by using a PVA film as a raw material film and subjecting the PVA film to a treatment step such as a swelling step, a dyeing step, a crosslinking step, a stretching step, and a fixing step. Specific examples of the treatment liquid used in each step include a swelling treatment liquid used in a swelling treatment, a dyeing treatment liquid (dyeing liquid) used in a dyeing treatment, a crosslinking treatment liquid used in a crosslinking treatment, a stretching treatment liquid used in a stretching treatment, a fixing treatment liquid used in a fixing treatment, and a cleaning treatment liquid (cleaning liquid) used in a cleaning treatment.

The respective processing steps that can be employed in the manufacturing method for manufacturing the polarizing film will be described in detail below. In the method for producing a polarizing film, 1 or 2 or more of the following processes may be omitted, the same process may be performed a plurality of times, or other processes may be performed simultaneously.

(cleaning treatment before swelling treatment)

Before the swelling treatment is performed on the PVA film, it is preferable to perform a washing treatment on the PVA film. By the cleaning treatment before the swelling treatment, the anti-blocking agent and the like adhering to the PVA film can be removed, and each treatment liquid in the polarizing film production process can be prevented from being contaminated with the anti-blocking agent and the like. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning treatment liquid, or may be performed by blowing the cleaning treatment liquid onto the PVA film. As the cleaning treatment liquid, water, for example, can be used. The temperature of the cleaning solution is preferably in the range of 20 to 40 ℃. By setting the temperature to 20 ℃ or higher, the anti-blocking agent and the like adhering to the PVA film can be easily removed. Further, by setting the temperature to 40 ℃ or lower, it is possible to prevent a part of the surface of the PVA film from being dissolved, the films from sticking to each other, and the handleability from being lowered. The temperature of the cleaning treatment liquid is more preferably 22 ℃ or higher, still more preferably 24 ℃ or higher, and particularly preferably 26 ℃ or higher. The temperature of the cleaning treatment liquid is more preferably 38 ℃ or lower, still more preferably 36 ℃ or lower, and particularly preferably 34 ℃ or lower.

(swelling treatment)

The swelling treatment can be performed by immersing the PVA film in a swelling treatment liquid such as water. The temperature of the swelling solution is preferably 20 ℃ or higher, more preferably 22 ℃ or higher, and still more preferably 24 ℃ or higher. The temperature of the swelling treatment liquid is preferably 40 ℃ or lower, more preferably 38 ℃ or lower, and still more preferably 36 ℃ or lower. The time for immersion in the swelling treatment liquid is, for example, preferably 0.1 minute or more, and more preferably 0.5 minute or more. The time for immersion in the swelling treatment liquid is, for example, preferably 5 minutes or less, and more preferably 3 minutes or less. The water used as the swelling treatment liquid is not limited to pure water, and may be an aqueous solution in which various components such as a boron-containing compound are dissolved, or may be a mixture of water and an aqueous medium. The type of the boron-containing compound is not particularly limited, but boric acid or borax is preferable from the viewpoint of handling property. When the swelling treatment liquid contains a boron-containing compound, the concentration thereof is preferably 6 mass% or less from the viewpoint of improving the stretchability of the PVA film.

(dyeing treatment)

The dyeing treatment may be performed using an iodine-based dye as a dichroic dye, and the dyeing time may be any of before, during, and after the stretching treatment. The dyeing treatment is preferably performed by immersing the PVA film in a dyeing treatment liquid using a solution (suitably an aqueous solution) containing iodine-potassium iodide as the dyeing treatment liquid. The iodine concentration in the dyeing treatment liquid is preferably 0.005 mass% or more. The iodine concentration in the dyeing treatment liquid is preferably 0.2% by mass or less. The potassium iodide/iodine (by mass) is preferably 20 or more. The ratio of potassium iodide/iodine (by mass) is preferably 100 or less. The temperature of the dyeing treatment liquid is preferably 20 ℃ or higher, more preferably 25 ℃ or higher. The temperature of the dyeing treatment liquid is preferably 50 ℃ or lower, more preferably 40 ℃ or lower. The dyeing treatment liquid may contain a boron-containing compound such as boric acid as a crosslinking agent. Note that, if the PVA film used as the raw material film contains a dichroic dye in advance, the dyeing process can be omitted. Further, the PVA film used as the raw material film may contain a boron-containing compound such as boric acid or borax in advance.

(crosslinking treatment)

In the production of a polarizing film, it is preferable to perform a crosslinking treatment after a dyeing treatment for the purpose of strongly adsorbing a dichroic dye to a PVA film or the like. The crosslinking treatment can be performed by using a solution (suitably an aqueous solution) containing a crosslinking agent as a crosslinking treatment liquid and immersing the PVA film in the crosslinking treatment liquid. As the crosslinking agent, 1 or 2 or more kinds of boron-containing compounds such as boric acid and borax can be used. If the concentration of the crosslinking agent in the crosslinking treatment liquid is too high, the following tendency is present: if the crosslinking reaction is excessively progressed, sufficient stretching is difficult to be performed in the subsequent stretching treatment, and if too little, the effect of the crosslinking treatment tends to be reduced. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and further preferably 2% by mass or more. The concentration of the crosslinking agent in the crosslinking treatment liquid is preferably 6% by mass or less, more preferably 5.5% by mass or less, and further preferably 5% by mass or less.

The crosslinking treatment liquid may contain an iodine-containing compound such as potassium iodide in order to suppress elution of the dichroic dye from the dyed PVA film. If the concentration of the iodine-containing compound in the crosslinking treatment liquid is too high, the heat resistance of the polarizing film obtained tends to be lowered although the reason is not clear. When the concentration of the iodine-containing compound in the crosslinking treatment liquid is too low, the effect of suppressing elution of the dichroic dye tends to decrease. The concentration of the iodine-containing compound in the crosslinking treatment liquid is preferably 1% by mass or more, more preferably 1.5% by mass or more, and further preferably 2% by mass or more. The concentration of the iodine-containing compound in the crosslinking treatment liquid is preferably 6% by mass or less, more preferably 5.5% by mass or less, and still more preferably 5% by mass or less.

When the temperature of the crosslinking treatment liquid is too high, the polarizing film obtained by elution of the dichroic dye tends to be uneven in dyeing, and when it is too low, the effect of the crosslinking treatment may be reduced. The temperature of the crosslinking treatment liquid is preferably 20 ℃ or higher, more preferably 22 ℃ or higher, and still more preferably 25 ℃ or higher. The temperature of the crosslinking treatment liquid is preferably 45 ℃ or lower, more preferably 40 ℃ or lower, and further preferably 35 ℃ or lower.

In the above-described respective processes, the PVA film may be stretched differently from the stretching process described later. By performing such stretching (pre-stretching), wrinkles can be prevented from occurring on the surface of the PVA film. From the viewpoint of polarization performance of the obtained polarizing film, etc., the total stretch ratio of the pre-stretching (the ratio obtained by multiplying the stretch ratio in each process) is preferably 4 times or less, more preferably 3.5 times or less, depending on the original length of the PVA film of the original material before stretching. From the viewpoint of polarization performance of the polarizing film obtained, etc., the total stretch ratio of the pre-stretching is preferably 1.5 times or more, depending on the original length of the PVA film as a raw material before stretching. The stretch ratio in the swelling treatment is preferably 1.1 times or more, more preferably 1.2 times or more, and still more preferably 1.4 times or more. The stretch ratio in the swelling treatment is preferably 3 times or less, more preferably 2.5 times or less, and further preferably 2.3 times or less. The stretch ratio in the dyeing treatment is preferably 2 times or less, more preferably 1.8 times or less, and still more preferably 1.5 times or less. The stretch ratio in the dyeing treatment is more preferably 1.1 times or more. The stretching ratio in the crosslinking treatment is preferably 2 times or less, more preferably 1.5 times or less, and further preferably 1.3 times or less. The stretching ratio in the crosslinking treatment is more preferably 1.05 times or more.

(stretching treatment)

The stretching treatment may be performed by either a wet stretching method or a dry stretching method. In the case of the wet stretching method, the stretching treatment may be carried out in a stretching treatment liquid using a solution (preferably an aqueous solution) containing a boron-containing compound such as boric acid, or may be carried out in a dyeing treatment liquid or a fixing treatment liquid described later. In the case of the dry stretching method, the stretching can be performed in air using a PVA film after water absorption. Among these, wet stretching is preferable, and uniaxial stretching in an aqueous solution containing boric acid is more preferable. When the stretching treatment liquid contains a boron-containing compound, the concentration of the boron-containing compound in the stretching treatment liquid is preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and further preferably 2.5% by mass or more, from the viewpoint of improving the stretchability of the PVA film. The concentration of the boron-containing compound in the stretching treatment liquid is preferably 7% by mass or less, more preferably 6.5% by mass or less, and still more preferably 6% by mass or less, from the viewpoint of being able to improve the stretchability of the PVA film.

The stretching treatment liquid preferably contains an iodine-containing compound such as potassium iodide. If the concentration of the iodine-containing compound in the stretching treatment liquid is too high, the hue of the obtained polarizing film tends to be bluish, and if it is too low, the reason is not clear, but the heat resistance of the obtained polarizing film tends to be lowered. The concentration of the iodine-containing compound in the stretching treatment liquid is preferably 2% by mass or more, more preferably 2.5% by mass or more, and further preferably 3% by mass or more. The concentration of the iodine-containing compound in the stretching treatment liquid is preferably 8% by mass or less, more preferably 7.5% by mass or less, and still more preferably 7% by mass or less.

When the temperature of the stretching treatment liquid is too high, the PVA film tends to be dissolved and softened, and tends to be easily broken, and when it is too low, the stretchability tends to be reduced. The temperature of the stretching treatment liquid is preferably 50 ℃ or higher, more preferably 52.5 ℃ or higher, and still more preferably 55 ℃ or higher. The temperature of the stretching treatment liquid is preferably 70 ℃ or lower, more preferably 67.5 ℃ or lower, and further preferably 65 ℃ or lower. The preferable range of the stretching temperature in the stretching treatment by the dry stretching method is also as described above.

The stretching ratio in the stretching treatment is preferably 1.2 times or more, more preferably 1.5 times or more, and further preferably 2 times or more, from the viewpoint that a polarizing film having more excellent polarizing performance can be obtained when the stretching ratio is high. Further, the total stretching ratio (ratio obtained by multiplying the stretching ratio in each step) including the stretching ratio of the pre-stretching is preferably 5.5 times or more, more preferably 5.7 times or more, and still more preferably 5.9 times or more, from the viewpoint of the polarization performance of the obtained polarizing film, depending on the original length of the PVA film as a raw material before stretching. The upper limit of the stretch ratio is not particularly limited, and if it is too high, the stretch breaking is likely to occur, and therefore, it is preferably 8 times or less.

The method of stretching by uniaxial stretching is not particularly limited, and uniaxial stretching in the longitudinal direction and transverse uniaxial stretching in the width direction may be employed. In the case of producing a polarizing film, uniaxial stretching in the longitudinal direction is preferable from the viewpoint of obtaining a polarizing film excellent in polarizing performance. Uniaxial stretching in the longitudinal direction can be performed by using a stretching apparatus including a plurality of rollers parallel to each other and changing the peripheral speed between the rollers.

In the present invention, the maximum stretching speed (%/min) in the case of stretching treatment by uniaxial stretching is not particularly limited, but is preferably 200%/min or more, more preferably 300%/min or more, and still more preferably 400%/min or more. Here, the maximum drawing speed means: when the PVA film is stretched in two or more stages using 3 or more rolls having different peripheral speeds, the stretching speed is the highest in the stage. When the PVA film is stretched in 1 stage without being divided into two or more stages, the stretching speed in this stage is the maximum stretching speed. The drawing speed is: the amount of increase in the length of the PVA film that is increased by stretching with respect to the length of the PVA film before stretching per unit time. For example, a drawing speed of 100%/min means: the speed at which the PVA film was changed from the length before stretching to 2-fold length in 1 minute. The larger the maximum stretching speed is, the more the stretching treatment (uniaxial stretching) of the PVA film can be performed at a high speed, and as a result, the productivity of the polarizing film is improved, which is preferable. On the other hand, if the maximum stretching speed is too high, an excessive tension may be applied to a part of the PVA film during the stretching process (uniaxial stretching) of the PVA film, and thus the PVA film may be easily subjected to stretch breaking. From this viewpoint, the maximum drawing speed is preferably not more than 900%/min.

(fixation treatment)

In order to firmly adsorb the dichroic dye to the PVA film in the production of the polarizing film, it is preferable to perform a fixing treatment. The fixing treatment can be performed by using a solution (suitably an aqueous solution) containing 1 or 2 or more kinds of boron-containing compounds such as boric acid and borax as a fixing treatment liquid, and immersing a PVA film (suitably a PVA film after the stretching treatment) in the fixing treatment liquid. The fixing treatment liquid may contain an iodine-containing compound or a metal compound as needed. The concentration of the boron-containing compound in the fixation treatment liquid is preferably 2% by mass or more, and more preferably 3% by mass or more. The concentration of the boron-containing compound in the fixation treatment liquid is preferably 15% by mass or less, and more preferably 10% by mass or less. The temperature of the fixing treatment liquid is preferably 15 ℃ or higher, more preferably 25 ℃ or higher. The temperature of the fixing treatment liquid is preferably 60 ℃ or lower, more preferably 40 ℃ or lower.

(cleaning treatment after dyeing treatment)

After the dyeing treatment, the PVA film after the stretching treatment is preferably subjected to a washing treatment. The cleaning treatment is preferably performed by immersing the PVA film in a cleaning treatment liquid, or may be performed by blowing the cleaning treatment liquid onto the PVA film. As the cleaning treatment liquid, water, for example, can be used. The water is not limited to pure water, and may contain an iodine-containing compound such as potassium iodide. The cleaning treatment liquid may contain a boron-containing compound, and in this case, the concentration of the boron-containing compound is preferably 2.0 mass% or less.

The temperature of the cleaning solution is preferably in the range of 5 to 40 ℃. By setting the temperature of the cleaning treatment liquid to 5 ℃ or higher, the PVA film can be prevented from being broken due to freezing of moisture. In addition, the temperature of the cleaning treatment liquid is set to 40 ℃ or lower, so that the optical properties of the obtained polarizing film are improved. The temperature of the cleaning treatment liquid is more preferably 7 ℃ or higher, and still more preferably 10 ℃ or higher. The temperature of the cleaning treatment liquid is more preferably 38 ℃ or lower, and still more preferably 35 ℃ or lower.

Specific examples of the method for producing the polarizing film include a method of subjecting a PVA film to dyeing treatment, stretching treatment, and crosslinking treatment and/or fixing treatment. A preferable example is a method in which the PVA film is subjected to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment (in particular, uniaxial stretching treatment), and washing treatment in this order. The stretching treatment may be performed in any treatment step prior to the above-described step, or may be performed in multiple stages of two or more stages.

The polarizing film can be obtained by drying the PVA film subjected to each of the above treatments. The method of the drying treatment is not particularly limited, and examples thereof include a contact method in which the film is brought into contact with a heated roller, a method of drying the film in a hot air dryer, and a floating method in which the film is dried by hot air while floating.

< polarizing plate >

The polarizing film obtained in the above manner is preferably used as a polarizing plate by laminating optically transparent protective films having mechanical strength on both surfaces or one surface thereof. As the protective film, a cellulose Triacetate (TAC) film, a cycloolefin polymer (COP) film, a Cellulose Acetate Butyrate (CAB) film, an acrylic film, a polyester film, or the like can be used. The adhesive used for bonding may be a PVA adhesive, a urethane adhesive, or the like, and is preferably a PVA adhesive.

The polarizing plate obtained in the above manner can be used as a member of an LCD by laminating an adhesive such as an acrylic adhesive and then bonding the laminate to a glass substrate. A retardation film, a viewing angle improving film, a brightness enhancing film, and the like may be simultaneously laminated.

Examples

The present invention will be specifically described below with reference to examples and the like, but the present invention is not limited to the following examples at all.

< calculation of crystallinity index based on FT-IR measurement >

PVA films having a width of 30 mm. Times.30 mm in length were cut out from the PVA films obtained in the following examples and comparative examples to prepare measurement samples. Since the value of the crystallinity index of the PVA film slightly fluctuates depending on the amount of moisture absorption of the PVA film, the measurement sample was stored for 24 hours in an environment in which the temperature was 24.0 ℃ and the relative humidity was 45.0%rh, and FT-IR measurement was performed using a measuring apparatus installed in a room in the same environment. In the FT-IR measurement, both surfaces of the PVA film (both surfaces orthogonal to the thickness direction of the PVA film, the first surface and the second surface) were measured under the following conditions.

Measurement device: NICOLET is 10 (manufactured by Thermo Fisher Co., ltd.)

The measurement conditions were as follows: 1-reflection ATR method with incident angle of 45 °

Resolution ratio: 4.0cm ^-1

Cumulative number of times: 32 times (twice)

Measuring temperature: 24.0 deg.C (ambient temperature)

And (3) measuring humidity: 45.0% RH (relative humidity of the environment)

ATR prism: diamond prism or germanium prism

From the infrared absorption spectrum obtained by FT-IR measurement of the PVA film, the crystallinity indexes of both surfaces (both surfaces orthogonal to the thickness direction of the PVA film, the first surface and the second surface) of the PVA film were calculated by the aforementioned method.

< evaluation of wrinkles on the surface of polarizing film >

The polarizing films obtained in the following examples and comparative examples were evaluated by irradiating the surface of the polarizing film with light obliquely from a fluorescent lamp and visually observing the appearance of the reflected light, thereby confirming the state of wrinkles on the surface of the polarizing film and following criteria.

Evaluation criteria:

a: no wrinkles were observed.

B: a small amount of wrinkles was observed to the extent that no practical problems were observed.

C: wrinkles were clearly observed to such an extent that they were practically problematic.

< evaluation of tensile failure frequency in producing polarizing film >

In the following examples or comparative examples, uniaxial stretching in the stretching treatment in the production of a polarizing film was continuously performed for 20 minutes. The number of times of tensile failure occurred in the 20-minute continuous stretching was measured, and the tensile failure frequency (times/20 mm) was evaluated.

< example 1>

< production and evaluation of PVA film >

A film-forming raw solution (a volatile fraction of 66 mass%) was prepared by melt-mixing 100 parts by mass of PVA (a degree of saponification of 99 mol% and a degree of polymerization of 2400), 12 parts by mass of a glycerin plasticizer as a plasticizer, 0.1 part by mass of lauric acid diethanolamide as a surfactant, and 217.6 parts by mass of water in a melt extruder. Then, the film-forming dope was discharged from the T-die onto a support (surface temperature: 80 ℃) in a film form, and a liquid coating film was formed on the support. On the support, hot air of 85 ℃ and 3% RH was blown to the entire surface of the liquid coating film not in contact with the support at a rate of 5 m/sec, and the PVA film was obtained by drying (moisture content: 32 mass%). Next, the PVA film was peeled from the support, and further dried between the first drying roller and the final drying roller (19 th drying roller) immediately before the heat treatment roller so that one surface and the other surface of the PVA film alternately contacted the respective drying rollers, and then peeled from the final drying roller. At this time, the surface temperature of each drying roller from the first drying roller to the final drying roller was set to 75 ℃. Further, the PVA film was peeled off from the final drying roll, and heat treatment was performed so that one surface and the other surface of the PVA film alternately contacted each heat treatment roll. At this time, the heat treatment was performed using 2 heat treatment rolls, and the surface temperatures of the heat treatment rolls were all set to 90 ℃. With respect to the obtained PVA film (thickness: 30 μm, width: 1200 mm), FT-IR measurement was performed by the above-mentioned method, and the crystallinity indexes (Fg 1, fg2, fd1 and Fd 2) were calculated. The results are shown in Table 1.

< production and evaluation of polarizing film >

The obtained PVA film was cut into 650mm wide, and the film was subjected to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, washing treatment, and drying treatment in this order to continuously produce a polarizing film. The swelling treatment was performed by uniaxially stretching to 2.00 times in the longitudinal direction while immersing in pure water (swelling treatment liquid) at 25 ℃. The dyeing treatment is carried out by uniaxially stretching to 1.26 times in the longitudinal direction while immersing in a potassium iodide/iodine dyeing solution (dyeing treatment solution) having a temperature of 32 ℃ (potassium iodide/iodine (mass ratio) of 23 and an iodine concentration of 0.03 to 0.05 mass%). In the dyeing treatment, the iodine concentration in the dyeing treatment liquid is adjusted within a range of 0.03 to 0.05 mass% so that the monomer transmittance of the polarizing film obtained after uniaxial stretching in the stretching treatment is within a range of 43.5% ± 0.2%. The crosslinking treatment was carried out by uniaxially stretching to 1.19 times in the longitudinal direction while immersing in an aqueous boric acid solution (crosslinking treatment liquid) at 32 ℃ (boric acid concentration of 2.6 mass%). The stretching treatment was performed by uniaxially stretching to 2.00 times in the longitudinal direction while immersing in an aqueous boric acid/potassium iodide solution (stretching treatment liquid) at 55 ℃ (boric acid concentration of 2.8 mass%, potassium iodide concentration of 5 mass%). The maximum stretching speed of the uniaxial stretching in the stretching treatment was 400%/min. The cleaning treatment was performed by immersing in an aqueous potassium iodide/boric acid solution (cleaning treatment solution) at 22 ℃ (potassium iodide concentration of 3 to 6 mass%, boric acid concentration of 1.5 mass%) for 12 seconds without stretching. The drying treatment was performed by hot air drying at 80 ℃ for 1.5 minutes without stretching, to obtain a polarizing film. With respect to the obtained polarizing film, the surface wrinkle of the polarizing film and the tensile failure frequency at the time of manufacturing the polarizing film were evaluated by the above-described methods. The results are shown in Table 2.

< example 2>

A PVA film and a polarizing film were obtained in the same manner as in example 1 except that the PVA used for preparing the film forming stock solution was changed to PVA (degree of saponification was 99 mol%, degree of polymerization was 2400, and ethylene modification was 2.5 mol%) and the surface temperatures of the 2 heat treatment rolls were all changed to 85 ℃. The measurement and evaluation of the PVA film and the polarizing film obtained were carried out in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

< example 3>

In < production and evaluation of PVA film > in example 1, a PVA film and a polarizing film were obtained in the same manner as in example 1 except that the surface temperature of the support was changed to 100 ℃, the temperature of hot air blown to the entire surface of the liquid coating film not in contact with the support was changed to 105 ℃, the surface temperature of each drying roller from the first drying roller to the final drying roller (19 th drying roller) immediately before the heat treatment roller was changed to 90 ℃, and the surface temperature of all the 2 heat treatment rollers was changed to 80 ℃. The measurement and evaluation of the PVA film and the polarizing film obtained were carried out in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

< example 4>

In < production and evaluation of PVA film > in example 1, a PVA film and a polarizing film were obtained in the same manner as in example 1 except that drying from the first drying roller to the final drying roller (19 th drying roller) immediately before the heat treatment roller was performed by bringing only one surface of the PVA film (on the support, the surface of the support in contact with the liquid coating film) into contact with each drying roller. The measurement and evaluation of the PVA film and the polarizing film obtained were carried out in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

< comparative example 1>

In < production and evaluation of PVA film > in example 1, a PVA film and a polarizing film were obtained in the same manner as in example 1 except that the surface temperature of the support was set to 115 ℃, the temperature of hot air blown to the entire surface of the liquid coating film not in contact with the support was changed to 120 ℃, the surface temperatures of the drying rollers from the first drying roller to the final drying roller (19 th drying roller) immediately before the heat treatment roller were changed to 65 ℃, and the surface temperatures of all the 2 heat treatment rollers were changed to 65 ℃. The PVA film and the polarizing film thus obtained were measured and evaluated in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

< comparative example 2>

A PVA film and a polarizing film were obtained in the same manner as in example 1 except that the surface temperature of the support was changed to 60 ℃, the temperature of hot air blown to the entire surface of the liquid coating film not in contact with the support was changed to 70 ℃, and the surface temperatures of the drying rolls from the first drying roll to the final drying roll (19 th drying roll) immediately before the heat treatment roll were changed to 90 ℃ in < production and evaluation of PVA film > in example 1. The measurement and evaluation of the PVA film and the polarizing film obtained were carried out in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

< reference example 1>

A PVA film and a polarizing film were obtained in the same manner as in comparative example 1 except that the maximum stretching speed of uniaxial stretching in the stretching treatment was changed to 190% in < production and evaluation of polarizing film > in example 1. The measurement and evaluation of the PVA film and the polarizing film obtained were carried out in the same manner as in example 1. The results are shown in tables 1 and 2, respectively.

[ Table 2]

As shown in tables 1 and 2, when polarizing films were produced using the PVA films of examples 1 to 4, wrinkles were not observed on the surfaces of the obtained polarizing films or a small amount of wrinkles was observed to the extent that no practical problems were observed. Here, the wrinkles generated on the surface of the polarizing film are caused by surface wrinkles of the PVA film, that is, surface wrinkles of the PVA film generated at the time of uniaxial stretching in the stretching process at the time of manufacturing the polarizing film. Thus, it can be said that: the PVA films of examples 1 to 4 were less likely to wrinkle on the surface thereof during uniaxial stretching.

As shown in tables 1 and 2, when uniaxial stretching in the stretching treatment was continuously performed for 20 minutes in the case of producing a polarizing film using the PVA films of examples 1 to 4, the frequency of tensile failure was 0 to 2 times/20 min. Therefore, it can be said that the PVA films of examples 1 to 4 were suppressed in breakage at the time of stretching (at the time of uniaxial stretching).

As shown in reference example 1, when the maximum stretching speed of uniaxial stretching in the stretching treatment in the production of the polarizing film was a low speed (190%/min), wrinkles were not observed in the surface of the obtained polarizing film even in the case of using the PVA film of comparative example 1, and the frequency of breakage in stretching (uniaxial stretching) was 0 times/20 min. On the other hand, as shown in comparative example 1, when the PVA film of comparative example 1 was used at a high maximum stretching speed (400%/min), wrinkles were clearly observed on the surface of the obtained polarizing film to such an extent that the wrinkles became practically problematic, and the breaking frequency during stretching (uniaxial stretching) was 5 times/20 min.

That is, the PVA film of comparative example 1 is likely to have wrinkles on the surface thereof during uniaxial stretching and to break during stretching (uniaxial stretching) when the maximum stretching speed is high (400%/min). On the other hand, the PVA films of examples 1 to 4 were less likely to wrinkle on the surface thereof during uniaxial stretching even at a high maximum stretching speed (400%/min), and it can be said that the fracture during stretching (uniaxial stretching) was suppressed.

Description of the reference numerals

1 PVA film

2 thickness direction of PVA film

3. First surface

4. Second surface

5. Penetration depth of infrared ray (about 2 μm) when a diamond prism is used

6. Depth of penetration of infrared ray (about 0.5 μm) when germanium prism is used

7 ATR prism (Diamond prism or germanium prism)

8. Infrared ray

Claims

1. A polyvinyl alcohol film which is a water-insoluble polyvinyl alcohol film,

setting two surfaces of the polyvinyl alcohol film, which are orthogonal to the thickness direction, as a first surface and a second surface respectively,

setting the crystallinity index of the first surface to Fd1 and Fg1,

when the crystallinity indexes of the second surface are set to Fd2 and Fg2,

the Fd1, fg1, fd2 and Fg2 satisfy the following formulas (1) to (4):

Fd1≤0.8 (1)

Fd1/Fg1＜1 (2)

Fd2≤0.8 (3)

Fd2/Fg2＜1 (4)

in the above formulas (1) to (4), fd1 is a crystallinity index calculated by using a diamond prism when FT-IR measurement is performed on the first surface by ATR method, fg1 is a crystallinity index calculated by using a germanium prism when FT-IR measurement is performed on the first surface by ATR method, fd2 is a crystallinity index calculated by using a diamond prism when FT-IR measurement is performed on the second surface by ATR method, and Fg2 is a crystallinity index calculated by using a germanium prism when FT-IR measurement is performed on the second surface by ATR method.

2. The polyvinyl alcohol film according to claim 1, wherein the Fd1 and Fd2 satisfy the following formulas (5) to (6):

Fd1≥0.5 (5)

Fd2≥0.5 (6)。

3. the polyvinyl alcohol film according to claim 1 or 2, wherein the Fd1, fg1, fd2, and Fg2 satisfy the following formulas (7) to (8):

Fd1/Fg1≥0.6 (7)

Fd2/Fg2≥0.6 (8)。

4. the polyvinyl alcohol film according to any one of claims 1 to 3, wherein the Fd1, fg1, fd2 and Fg2 satisfy the following formulae (9) to (10):

|Fd1-Fd2|≤0.07 (9)

|Fg1-Fg2|≤0.07 (10)。

5. the polyvinyl alcohol film according to any one of claims 1 to 4, which is a film for producing an optical film.

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