Method for manufacturing polarizing film and polarizing film

Abstract

A method for producing a polarizing film, comprising: a dyeing step of dyeing a PVA film with a dichroic dye; a stretching step of performing uniaxial stretching in an aqueous solution containing boric acid; and a drying step, the concentration of boric acid in the aqueous solution of the stretching step being 1 to 3 mass%, the total stretching ratio being 5.5 to 7.4, the total neck-in ratio (A) represented by formula (1) being 57.5 to 61.0%, the neck-in ratio (B) represented by formula (2) in the stretching step being 31.0 to 38.0%, and the neck-in ratio (C) represented by formula (3) in the drying step being 9.8 to 16.5%.(A) = ((X1 - X2) / X1) x 100 (1)(B) = {(Y1 - Y2) / Y1} x 100 (2)(C) = {(Z1 - X2) / Z1} x 100 (3)X1 represents the width (m) before the dyeing step; X2 represents the width (m) after the drying step; Y1 represents the width (m) before the stretching step; Y2 represents the width (m) after the stretching step; and Z1 represents the width (m) before the drying step.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a polarizing film and to a polarizing film. Background Technology

[0002] Polarizing plates, which transmit and block polarized light, and liquid crystals, which change the polarization state of light, are both fundamental components of liquid crystal displays (LCDs). Many polarizing plates have a structure in which a protective film, such as a triacetate cellulose (TAC) film, is adhered to the surface of the polarizing film. As the polarizing film, an iodine-based dye (I3) is adsorbed onto a substrate (a stretched film obtained by uniaxially stretching and orienting a polyvinyl alcohol film (hereinafter sometimes abbreviated as "PVA") formed by uniaxial stretching. - I5 - Polarizing films obtained through methods such as (e.g., uniaxial stretching of PVA films pre-containing dichroic dyes, or adsorbing dichroic dyes during uniaxial stretching of PVA films, or adsorbing dichroic dyes after uniaxial stretching of PVA films) have become the mainstream.

[0003] LCDs are widely used in a wide range of applications, including calculators, watches, smartphones, laptops, liquid crystal displays, LCD color projectors, LCD TVs, car navigation systems, mobile phones, and indoor and outdoor measuring devices. To meet the demands for higher performance in displays in recent years, there is a need for polarizing films with excellent optical properties. Furthermore, the trend towards thinner displays necessitates polarizing films with low shrinkage stress.

[0004] Patent Document 1 describes a method for manufacturing a polarizing film with excellent polarization performance by irradiating a specified electromagnetic wave during the manufacturing process of the polarizing film. Patent Document 2 describes a method for manufacturing a polarizing film with excellent polarization performance and low shrinkage stress by stretching a PVA film to 1.8 to 3.0 times its original length in a boric acid aqueous solution at 60 to 70°C, with the total stretching ratio set to 6 to 8 times.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-032025

[0008] Patent Document 2: International Publication No. 2017 / 138551 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, in the method described in Patent Document 1, the existing manufacturing process requires additional steps, or, if the intensity of electromagnetic waves is not strictly managed, the orientation of PVA will become excessively relaxed, reducing the polarization performance of the polarization film, thus making industrial implementation difficult. Furthermore, in the method described in Patent Document 2, due to its overly stringent stretching conditions, the PVA film sometimes dissolves during the stretching process, resulting in a reduced yield of the polarization film.

[0011] The present invention was made to solve the above-mentioned problems, and its purpose is to provide a method for manufacturing a polarizing film with excellent polarization performance and low shrinkage stress.

[0012] means for solving problems

[0013] Through in-depth research, the inventors have discovered that, for manufacturing polarizing films with excellent polarization performance and low shrinkage stress, it is important to appropriately control the shrinkage phenomenon of the PVA film width during the stretching and drying processes in the polarizing film manufacturing process (hereinafter sometimes referred to as "necking phenomenon"), and to control the shrinkage rate of the film width (hereinafter sometimes referred to as "necking rate") within a certain range.

[0014] That is, the present invention relates to:

[0015] [1] A method for manufacturing a polarizing film, comprising: a dyeing step of dyeing a polyvinyl alcohol film with a dichroic dye; a stretching step of uniaxially stretching the dyed polyvinyl alcohol film in an aqueous solution containing boric acid; and a drying step of drying the stretched polyvinyl alcohol film, wherein the concentration of boric acid in the aqueous solution of the stretching step is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total necking ratio (A) shown in the following formula (1) is 57.5% to 61.0%, the necking ratio (B) shown in the following formula (2) of the stretching step is 31.0% to 38.0%, and the necking ratio (C) shown in the following formula (3) of the drying step is 9.8% to 16.5%.

[0016] Total necking ratio (A) = {(X1-X2) / X1} × 100 (1)

[0017] Necking ratio (B) = {(Y1-Y2) / Y1} × 100 (2)

[0018] Necking ratio (C) = {(Z1-X2) / Z1} × 100 (3)

[0019] [X1 represents the length (m) of the polyvinyl alcohol film before the dyeing process; X2 represents the length (m) of the polyvinyl alcohol film after the drying process; Y1 represents the length (m) of the polyvinyl alcohol film before the stretching process; Y2 represents the length (m) of the polyvinyl alcohol film after the stretching process; Z1 represents the length (m) of the polyvinyl alcohol film before the drying process.]

[0020] [2] According to the method for manufacturing polarizing film described above [1], the necking ratio (D) shown in the following formula (4) up to the aforementioned stretching process is 46.0% to 54.0%;

[0021] Necking ratio (D) = {(X1-Y2) / X1} × 100 (4)

[0022] [3] According to the manufacturing method of the polarizing film described in [2] above, the difference between the total necking ratio (A) and the necking ratio (D) ((A)-(D)), i.e. the necking ratio difference (W), is 8.0 to 11.0%.

[0023] [4] In the method for manufacturing the polarizing film described in [3] above, the ratio of the aforementioned necking difference (W) to the aforementioned total necking (A) ((W) / (A)) is 0.14 to 0.19;

[0024] [5] The method for manufacturing a polarizing film according to any one of [1] to [4] above, wherein the total boron content in the polarizing film is 2.0 to 4.0% by mass;

[0025] [6] The method for manufacturing a polarizing film according to any one of [1] to [5] above, wherein the stretching temperature in the stretching process is 53°C to 70°C;

[0026] [7] The method for manufacturing a polarizing film according to any one of [1] to [6] above, wherein the drying temperature in the aforementioned drying process is 60°C to 100°C;

[0027] [8] The method for manufacturing a polarizing film according to any one of [1] to [7] above, wherein the polarization degree of the polarizing film is 99.963% or more when the monomer transmittance is 44% and the shrinkage stress is 100 N / mm. 2 the following;

[0028] [9] A polarizing film formed of polyvinyl alcohol film, with a polarization degree of over 99.963% when the monomer transmittance is 44% and a shrinkage stress of 100 N / mm. 2 the following;

[0029]

[10] According to the polarizing film described above [9], the total boron content in the polarizing film is less than 4.0% by mass.

[0030] Invention Effects

[0031] According to the method for manufacturing polarizing films of the present invention, it is possible to manufacture polarizing films with excellent polarization performance and low shrinkage stress. Therefore, the resulting polarizing films are suitable for high-performance liquid crystal displays, especially liquid crystal displays that are sometimes used at high temperatures. Attached Figure Description

[0032] Figure 1 This is a graph plotting the degree of polarization of the polarizing films obtained in Examples 1-5 and Comparative Examples 1-9 when the monomer transmittance relative to the shrinkage stress is 44%.

[0033] Figure 2 This is a graph plotting the tensile tension of the polarizing films obtained in Examples 1-5 and Comparative Examples 1-9 relative to the shrinkage stress. Detailed Implementation

[0034] The method for manufacturing the polarizing film of the present invention includes: a dyeing step of dyeing a PVA film with a dichroic dye; a stretching step of uniaxially stretching the dyed PVA film in an aqueous solution containing boric acid; and a drying step of drying the stretched PVA film, wherein the boric acid concentration in the aqueous solution of the stretching step is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total necking ratio (A) shown in formula (1) below is 57.5% to 61.0%, the necking ratio (B) shown in formula (2) below in the stretching step is 31.0% to 38.0%, and the necking ratio (C) shown in formula (3) below in the drying step is 9.8% to 16.5%.

[0035] Total necking ratio (A) = {(X1-X2) / X1} × 100 (1)

[0036] Necking ratio (B) = {(Y1-Y2) / Y1} × 100 (2)

[0037] Necking ratio (C) = {(Z1-X2) / Z1} × 100 (3)

[0038] In the aforementioned formulas (1) to (3), X1 represents the length of the width of the PVA film before the aforementioned dyeing process (the length of the width of the unstretched PVA film used to manufacture the polarizing film) (m); X2 represents the length of the width of the PVA film after the aforementioned drying process (m); Y1 represents the length of the width of the PVA film before the aforementioned stretching process (m); Y2 represents the length of the width of the PVA film after the aforementioned stretching process (m); Z1 represents the length of the width of the PVA film before the aforementioned drying process (m). It should be noted that if a heat treatment process is set after the drying process, the length of the width of the PVA film after the heat treatment (m) can be set as X2. If a crosslinking process is set before the stretching process, the length of the width of the PVA film after the crosslinking process and before the stretching process (m) can be set as Y1. If a cleaning process is set after the stretching process, the length of the width of the PVA film after the stretching process and before the cleaning process (m) can be set as Y2.

[0039] To manufacture polarizing films with excellent polarization performance and low shrinkage stress, it is necessary to simultaneously control the total necking ratio (A), the necking ratio during the stretching process (B), and the necking ratio during the drying process (C). By controlling the total necking ratio (A), it is possible to suppress the reduction in polarization performance and yield.

[0040] In the stretching process, PVA is highly oriented, thereby causing the iodine-based dyes to be highly oriented or generating residual stress. Furthermore, in the drying process, the residual stress is released through orientation relaxation and crystallization of the PVA. Simultaneously, there are situations where the polarization performance improves due to the thermal decomposition of unwanted iodine-based dyes, and where the polarization performance decreases due to the thermal decomposition of even necessary iodine-based dyes. Therefore, by appropriately controlling the necking ratio (B) in the stretching process and the necking ratio (C) in the drying process, it is possible to manufacture a polarizing film with excellent polarization performance and low shrinkage stress.

[0041] The total necking rate (A) must be 57.5% or more, preferably 58.5% or more, and more preferably 59.0% or more. The total necking rate (A) must be 61.0% or less, preferably 60.8% or less, and more preferably 60.5% or less. When the total necking rate (A) is less than 57.5%, the necking phenomenon up to the stretching process is insufficient, and the film width becomes too wide, resulting in wrinkles on the surface of the polarizing film. This can easily reduce the polarization performance or the yield of the polarizing film, so it is not preferred. On the other hand, when the total necking rate (A) exceeds 61.0%, the necking phenomenon proceeds excessively up to the stretching process. Sometimes, the PVA film breaks during the stretching and drying processes, leading to a decrease in yield. Alternatively, unwanted iodine-based dyes may be stabilized, resulting in a decrease in polarization performance, so it is not preferred. There are no particular limitations on the methods for adjusting the total necking ratio (A) to these ranges. Examples include, for instance, appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the swelling, dyeing, cross-linking, stretching, and cleaning processes, the stretching ratio in each of the swelling, dyeing, cross-linking, stretching, and cleaning processes, and the drying temperature and drying time in the drying process.

[0042] The necking rate (B) in the stretching process must be 31.0% or more, preferably 32.0% or more, and more preferably 33.0% or more. The necking rate (B) in the stretching process must be 38.0% or less, preferably 37.5% or less, more preferably 37.0% or less, and particularly preferably 36.0% or less. When the necking rate (B) in the stretching process is less than 31.0%, it is difficult to achieve high orientation of the iodine-based dyes, making it difficult to obtain a polarizing film with excellent polarization properties, which is not preferred. On the other hand, when the necking rate (B) in the stretching process exceeds 38.0%, although the reason is not yet certain, the interaction between PVA molecular chains and the boric acid-based crosslinking become strong. Therefore, necking cannot occur in the drying process, and the release of residual stress achieved by the decomposition of unwanted iodine-based dyes, orientation relaxation of PVA, and crystallization becomes insufficient, making it difficult to obtain a polarizing film with excellent polarization properties and low shrinkage stress, which is also not preferred. There are no particular limitations on the methods for adjusting the necking ratio (B) to these ranges. Examples include, for instance, appropriately adjusting the boric acid concentration and temperature of the aqueous solution in the stretching process, the stretching ratio, etc.

[0043] The necking ratio (C) during the drying process must be 9.8% or more, preferably 11.5% or more, more preferably 12.0% or more, and particularly preferably 12.5% ​​or more. The necking ratio (C) during the drying process must be 16.5% or less, preferably 16.3% or less, and more preferably 16.1% or less. When the necking ratio (C) during the drying process is less than 9.8%, the release of residual stress achieved by the orientation relaxation and crystallization of PVA becomes insufficient, making it difficult to obtain a polarizing film with low shrinkage stress, which is therefore undesirable. On the other hand, when the necking ratio (C) during the drying process exceeds 16.5%, excessive orientation relaxation of PVA occurs, even causing the necessary iodine-based dyes to decompose, making it difficult to obtain a polarizing film with excellent polarization properties, which is also undesirable. Methods for adjusting the necking ratio (C) to these ranges are not particularly limited, and examples include appropriately adjusting the drying temperature and drying time during the drying process.

[0044] The total stretching ratio must be 5.5 times or more, preferably 5.8 times or more, more preferably 5.9 times or more, and particularly preferably 6.0 times or more. The total stretching ratio must be 7.4 times or less, preferably 7.3 times or less, more preferably 7.2 times or less, and particularly preferably 6.8 times or less. The total stretching ratio refers to the ratio of the length of the polarizing film after all processes has been stretched to the original length of the unstretched PVA film used to manufacture the polarizing film. When the total stretching ratio is less than 5.5 times, necking cannot occur sufficiently, and the iodine dyes cannot be highly oriented. Therefore, it is difficult to obtain a polarizing film with excellent polarization performance, which is not preferred. On the other hand, when the total stretching ratio exceeds 7.4 times, necking occurs excessively, and the release of residual stress achieved by the orientation relaxation and crystallization of PVA cannot occur sufficiently. Therefore, it is difficult to obtain a polarizing film with low shrinkage stress, which is also not preferred. In addition, stretching can sometimes become uneven, reducing the yield of the polarizing film, which is also not preferred from a productivity point of view. There are no particular limitations on the methods for adjusting the total stretch ratio to these ranges. Examples of methods include appropriately adjusting the stretch ratio in each of the swelling, dyeing, cross-linking, stretching, and cleaning processes.

[0045] The boric acid concentration in the aqueous solution of the stretching process is preferably 1.0% by mass or more, more preferably 1.1% by mass or more, particularly preferably 1.2% by mass or more, and even more preferably 1.4% by mass or more. The boric acid concentration in the aqueous solution of the stretching process is preferably 3.0% by mass or less, more preferably 2.9% by mass or less, particularly preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less. When the boric acid concentration in the aqueous solution of the stretching process is less than 1.0% by mass, insufficient cross-linking of boric acid results in insufficient necking during the stretching process, making it difficult to achieve high orientation of iodine-based dyes and obtain a polarizing film with excellent polarization performance; therefore, this is not preferred. On the other hand, when the boric acid concentration in the aqueous solution of the stretching process exceeds 3.0% by mass, excessive cross-linking based on boric acid occurs, leading to the formation of unwanted iodine-based dyes during the stretching process; or, insufficient necking occurs during the drying process, preventing sufficient release of residual stress achieved by orientation relaxation and crystallization of PVA; or, it is difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress; therefore, this is also not preferred.

[0046] The necking rate (D) shown in formula (4) up to the aforementioned stretching process is preferably 46.0% or more, more preferably 48.5% or more, particularly preferably 49.0% or more, and even more preferably 49.3% or more. The necking rate (D) shown in formula (4) up to the aforementioned stretching process is preferably 54.0% or less, more preferably 53.0% or less, particularly preferably 52.0% or less, and even more preferably 51.2% or less.

[0047] Necking ratio (D) = {(X1-Y2) / X1} × 100 (4)

[0048] When the necking ratio (D) up to the stretching process is less than 46.0%, there is a tendency that the necking phenomenon occurring up to the stretching process is insufficient among all the necking phenomena occurring in all processes, making it difficult to achieve high orientation of the iodine-based dyes and obtain a polarizing film with excellent polarization properties. On the other hand, when the necking ratio (D) up to the stretching process exceeds 54.0%, there is a tendency that the necking phenomenon occurring up to the stretching process is excessive among all the necking phenomena occurring in all processes, making it difficult to release the residual stress achieved by the orientation relaxation and crystallization of PVA in the stretching process and beyond without reducing the yield of the polarizing film, making it difficult to obtain a polarizing film with low shrinkage stress. As a method to adjust the necking ratio (D) to these ranges, there are no particular limitations, and examples include, for instance, appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the dyeing, crosslinking, and stretching processes, the stretching ratio in each of the swelling, dyeing, crosslinking, and stretching processes, and the drying temperature and drying time in the drying process.

[0049] The difference between the aforementioned total necking ratio (A) and the aforementioned necking ratio (D) ((A)-(D)), i.e., the necking ratio difference (W), is preferably 8.0% or more, more preferably 8.5% or more, and particularly preferably 9.0% or more. The aforementioned necking ratio difference (W) is preferably 11.0% or less, more preferably 10.5% or less, and particularly preferably 10.0% or less. When the aforementioned total necking ratio (A) and the aforementioned necking ratio (D) are less than 8.0%, there is a tendency that the proportion of necking phenomena occurring in the drying process is insufficient among all the necking phenomena occurring in the process, and the release of residual stress achieved by the decomposition of unwanted iodine-based dyes, orientation relaxation of PVA, and crystallization is easily insufficient, making it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress. In particular, there is a tendency that it is difficult to reduce shrinkage stress. On the other hand, when the necking difference (W) exceeds 11.0%, there is a tendency that the proportion of necking that occurs in the drying process is excessive among all the processes, and the process continues until the necessary decomposition of the iodine dye is achieved, making it difficult to obtain a polarizing film with excellent polarization performance.

[0050] The ratio of the aforementioned necking difference (W) to the aforementioned total necking (A) ((W) / (A)) is preferably 0.14 or more, more preferably 0.15 or more, and particularly preferably 0.16 or more. The ratio of the aforementioned necking difference (W) to the aforementioned total necking (A) ((W) / (A)) is preferably 0.19 or less, more preferably 0.18 or less, and particularly preferably 0.17 or less. When the ratio of the aforementioned necking difference (W) to the aforementioned total necking (A) ((W) / (A)) is less than 0.14, there is a tendency that the proportion of necking occurring in the drying process is insufficient among all the necking phenomena occurring in the process. The release of residual stress achieved by the decomposition of unwanted iodine-based dyes, orientation relaxation of PVA, and crystallization is easily insufficient, making it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress. In particular, there is a tendency to make it difficult to reduce shrinkage stress. On the other hand, when the ratio ((W) / (A)) exceeds 0.19, there is a tendency that the proportion of necking that occurs in the drying process is excessive among all the processes, and the process continues until the necessary decomposition of the iodine dye is achieved, making it difficult to obtain a polarizing film with excellent polarization performance.

[0051] The total boron content in the polarizing film is preferably 2.0% by mass or more, more preferably 2.5% by mass or more, and particularly preferably 2.8% by mass or more. The total boron content in the polarizing film is preferably 4.0% by mass or less, more preferably 3.8% by mass or less, and particularly preferably 3.7% by mass or less. When the total boron content in the polarizing film is less than 2.0% by mass, insufficient crosslinking of boric acid makes it difficult to control necking during the stretching and drying processes to achieve high orientation of the iodine-based dyes, resulting in a polarizing film with excellent polarization performance; therefore, this is not preferred. On the other hand, when the total boron content in the polarizing film exceeds 4.0% by mass, excessive crosslinking of boric acid makes it difficult to control necking during the drying process to achieve the release of residual stress through orientation relaxation and crystallization of PVA; therefore, a polarizing film with low shrinkage stress is difficult to obtain; therefore, this is also not preferred. The total boron content in the polarizing film can be determined by ICP luminescence analysis, etc. Specifically, it can be determined by the method described in the examples.

[0052] Thus, the preferred polarizing film obtained in this invention has the following characteristics: a polarization degree of 99.963% or higher when the monomer transmittance is 44%, and a shrinkage stress of 100 N / mm. 2 The following is applicable to high-performance liquid crystal displays, especially liquid crystal displays that are sometimes used at high temperatures. The degree of polarization when the single-cell transmittance is 44% is preferably 99.965% or more, more preferably 99.967% or more, particularly preferably 99.970% or more, and even more preferably 99.975% or more.

[0053] The thickness of the polarizing film obtained by the manufacturing method of the present invention is preferably 5 μm or more, more preferably 10 μm or more, particularly preferably 12 μm or more, and even more preferably 14 μm or more. The thickness of the polarizing film obtained by the manufacturing method of the present invention is preferably 60 μm or less, more preferably 45 μm or less, particularly preferably 30 μm or less, and even more preferably 25 μm or less. When the thickness is less than 5 μm, tensile breakage is likely to occur during manufacturing, potentially reducing productivity. On the other hand, when the thickness exceeds 60 μm, the performance requirements for polarizing plates, such as thin-film properties and lightweight design, may not be met.

[0054] <pva>

[0055] The PVA film used in the manufacturing method of the present invention contains PVA. PVA is a polymer having vinyl alcohol units (-CH2-CH(OH)-) as the main structural unit.

[0056] The degree of polymerization of PVA is preferably 1,500 or more, more preferably 1,800 or more, and even more preferably 2,000 or more. The degree of polymerization of PVA is preferably 6,000 or less, more preferably 5,000 or less, and even more preferably 4,000 or less. By setting the degree of polymerization to 1,500 or more, the durability of the polarizing film obtained by uniaxial stretching the film can be improved. On the other hand, by setting the degree of polymerization to 6,000 or less, the increase in manufacturing costs and the poor quality of the film-forming process can be suppressed. It should be noted that the degree of polymerization of PVA(A) in this specification refers to the average degree of polymerization measured according to JIS K6726-1994.

[0057] From the viewpoint of the water resistance of polarizing films obtained by uniaxial stretching of the film, the degree of saponification of PVA is preferably 95 mol% or more, more preferably 96 mol% or more, and even more preferably 98 mol% or more. It should be noted that the degree of saponification of PVA in this specification refers to the proportion (mol%) of the number of moles of vinyl alcohol units relative to the total number of moles of structural units (typically vinyl ester units) and vinyl alcohol units in PVA that can be converted into vinyl alcohol units (-CH2-CH(OH)-) through saponification. This degree of saponification can be measured according to the description in JIS K6726-1994.

[0058] The manufacturing method of PVA is not particularly limited. Examples include methods that convert the vinyl ester units of polyvinyl ester obtained by polymerizing vinyl ester monomers into vinyl alcohol units. The vinyl ester monomers used to manufacture PVA(A) are not particularly limited, and examples include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl neopentanoate, vinyl tert-carboxylate, vinyl hexanoate, vinyl octanoate, vinyl decanate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, etc. From an economic point of view, vinyl acetate is preferred.

[0059] Additionally, PVA can be a substance obtained by converting the vinyl ester units of a vinyl ester copolymer, obtained by copolymerizing a vinyl ester monomer with other monomers capable of copolymerizing it, into vinyl alcohol units. Examples of other monomers capable of copolymerizing with vinyl ester monomers include, for instance, α-olefins with 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutene; (meth)acrylic acid or its salts; (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide, N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamide propanesulfonic acid or its salts; (Methacrylamide)propyldimethylamine or its salts, N-hydroxymethyl (meth)acrylamide or its derivatives, and other (meth)acrylamide derivatives; N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, and other N-vinylamides; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, and other vinyl ethers; (meth)acrylonitrile or other cyanide; vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, and other haloethylenes; allyl acetate, allyl chloride, and other allyl compounds; maleic acid or its salts, esters, or anhydrides; itaconic acid or its salts, esters, or anhydrides; vinyltrimethoxysilane and other vinylsilyl compounds; unsaturated sulfonic acids, etc. The above vinyl ester copolymers may have structural units derived from one or more of the aforementioned monomers. The other monomers can be used by pre-existing in the reaction vessel when supplying the vinyl ester monomer to the polymerization reaction, or by adding them to the reaction vessel during the polymerization reaction. From the viewpoint of optical properties, the content of units derived from other monomers relative to the total number of moles of the structural units constituting PVA(A) is preferably 10 mol% or less, more preferably 5 mol% or less, and even more preferably 2 mol% or less.

[0060] Some of the hydroxyl groups in PVA may be cross-linked or not. Furthermore, some of the hydroxyl groups in the aforementioned PVA may react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure, or they may not react with these compounds to form an acetal structure.

[0061] From the perspectives of improved stretchability, enabling stretching at a higher temperature, reducing the occurrence of failures such as tensile breakage, and further improving the productivity of polarizing films, ethylene is preferred as a monomer that can be copolymerized with the above-mentioned vinyl ester monomer. When the PVA contains ethylene units, from the above viewpoints of stretchability and stretchable temperature, the content of ethylene units is preferably 1 mol% or more, more preferably 2 mol% or more, relative to the total moles of structural units constituting PVA. When the PVA contains ethylene units, from the viewpoints of stretchability and stretchable temperature, the content of ethylene units is preferably 10 mol% or less, more preferably 6 mol% or less, relative to the total moles of structural units constituting PVA.

[0062] <PVA film>

[0063] The PVA film used in the production method of the present invention may further contain a plasticizer in addition to the above-mentioned PVA. Polyols can be cited as preferred plasticizers, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, trimethylolpropane, etc. Furthermore, one or two or more of these plasticizers can be contained. Among these, glycerin is preferred from the viewpoint of the effect of improving stretchability.

[0064] The content of the plasticizer in the PVA film is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and still more preferably 5 parts by mass or more, relative to 100 parts by mass of PVA (A). The content of the plasticizer in the PVA film is preferably 20 parts by mass or less, more preferably 17 parts by mass or less, and still more preferably 15 parts by mass or less, relative to 100 parts by mass of PVA (A). Setting the content to 1 part by mass or more improves the stretchability of the film. On the other hand, setting the content to 20 parts by mass or less can suppress the decrease in handleability caused by the film becoming too soft.

[0065] The PVA membrane may further be appropriately mixed with other additives other than PVA (A) and plasticizers as required, such as fillers, processing stabilizers such as copper compounds, weather resistance stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, defoamers, deodorants, extenders, release agents, mold release agents, reinforcing agents, crosslinking agents, mildew inhibitors, preservatives, crystallization rate retarders, and the like. The content of other additives in the aforementioned PVA film is generally preferably 10 mass% or less, more preferably 5 mass% or less.

[0066] The swelling degree of the PVA film is preferably 160% or more, more preferably 170% or more, and particularly preferably 180% or more. The swelling degree of the PVA film is preferably 240% or less, more preferably 230% or less, and particularly preferably 220% or less. By maintaining a swelling degree of 160% or more, extreme crystallization can be suppressed, allowing for stable stretching to high magnification. On the other hand, by maintaining a swelling degree of 240% or less, dissolution during stretching is suppressed, enabling stretching even at higher temperatures.

[0067] The thickness of PVA film is not particularly limited, but it is typically 1 μm or more, suitable for 5 μm or more, and particularly suitable for 10 μm or more. The thickness of PVA film is typically 100 μm or less, suitable for 60 μm or less, and particularly suitable for 45 μm or less. If the aforementioned PVA film is too thin, it tends to break easily during the stretching process used in manufacturing polarizing films. Conversely, if the aforementioned PVA film is too thick, it tends to produce uneven stretching during the stretching process used in manufacturing polarizing films, resulting in a thicker polarizing film. Therefore, it tends to be difficult to use in thin and compact devices such as smartphones and laptops.

[0068] The width of the PVA film is not particularly limited and can be determined according to the intended use of the polarizing film. Considering the increasing trend towards larger screen sizes in LCD TVs and LCD displays, a PVA film width of 3 μm or more is suitable for these applications. On the other hand, if the PVA film used for polarizing film manufacturing is too wide, it becomes difficult to stretch it uniformly when using commercially available equipment. Therefore, a PVA film width of 10 μm or less is preferably preferred for manufacturing polarizing films.

[0069] There are no particular limitations on the manufacturing method of PVA membranes, but a preferred method is one that results in a uniform thickness and width of the membrane after film formation. The film can be manufactured using, for example, the following film-forming stock solutions: a film-forming stock solution obtained by dissolving PVA(A) and, if necessary, one or more of the aforementioned plasticizers, other additives, and surfactants described later, in a liquid medium; or a film-forming stock solution containing PVA(A) and, if necessary, one or more of the plasticizers, other additives, surfactants, and liquid medium, wherein PVA(A) has melted. When the film-forming stock solution contains at least one of the plasticizers, other additives, and surfactants, it is preferable that these components are uniformly mixed.

[0070] Examples of liquid media used for preparing the film-forming stock solution include, for instance, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine. One or more of these may be used. From the viewpoint of environmental impact and recyclability, water is preferred.

[0071] The volatile content (the proportion of volatile components such as liquid media removed through evaporation or volatilization during membrane formation) of the membrane-forming stock solution varies depending on the membrane formation method and conditions, and is generally preferably 50% by mass or more, more preferably 55% by mass or more. The volatile content of the membrane-forming stock solution also varies depending on the membrane formation method and conditions, and is generally preferably 95% by mass or less, more preferably 90% by mass or less. By ensuring that the volatile content of the membrane-forming stock solution is 50% by mass or more, the viscosity of the membrane-forming stock solution is not too high, allowing for smooth filtration and degassing during preparation, and facilitating the manufacture of membranes with fewer foreign matter and defects. On the other hand, by ensuring that the volatile content of the membrane-forming stock solution is 95% by mass or less, the concentration of the membrane-forming stock solution is not too low, facilitating industrial membrane manufacturing.

[0072] The film-forming solution preferably contains a surfactant. By including a surfactant, film-forming properties are improved, uneven film thickness is suppressed, and the film is easily peeled off from the metal rollers or belts used for film formation. When manufacturing PVA films from a film-forming solution containing a surfactant, the film may contain the surfactant. The type of surfactant is not particularly limited, but from the viewpoint of peelability from metal rollers or belts, anionic or nonionic surfactants are preferred.

[0073] As anionic surfactants, suitable types include carboxylic acid surfactants such as potassium lauryl ether sulfate; sulfates such as polyoxyethylene lauryl ether sulfate, sodium alkyl sulfate, potassium alkyl sulfate, ammonium alkyl sulfate, triethanolamine alkyl sulfate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxypropylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, and octyl sulfate; sulfonic acid surfactants such as sodium alkyl sulfonate, potassium alkyl sulfonate, ammonium alkyl sulfonate, triethanolamine alkyl sulfonate, sodium alkylbenzene sulfonate, disodium dodecyl diphenyl ether disulfonate, sodium alkylnaphthalene sulfonate, disodium alkyl sulfosuccinate, disodium polyoxyethylene alkyl sulfosuccinate, and dodecylbenzene sulfonate; and phosphate surfactants such as sodium alkyl phosphate, potassium alkyl phosphate, ammonium alkyl phosphate, triethanolamine alkyl phosphate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxypropylene alkyl ether phosphate, and sodium polyoxyethylene alkylphenyl ether phosphate.

[0074] As a nonionic surfactant, it is suitable for, for example, alkyl ethers such as polyoxyethylene oleyl ether; alkyl phenyl ethers such as polyoxyethylene octylphenyl ether; alkyl esters such as polyoxyethylene laurate; alkylamines such as polyoxyethylene lauryl amino ether; alkylamides such as polyoxyethylene laurylamide; polypropylene glycol ethers such as polyoxyethylene polyoxypropylene ether; alkanolamides such as lauric acid diethanolamide and oleic acid diethanolamide; and allyl phenyl ethers such as polyoxyalkylene allylphenyl ether.

[0075] These surfactants can be used alone, or in combination of two or more.

[0076] When the film-forming solution contains a surfactant, its content is preferably 0.01 parts by mass or more, more preferably 0.02 parts by mass or more, and particularly preferably 0.05 parts by mass or more, relative to 100 parts by mass of PVA(A) contained in the film-forming solution. When the film-forming solution contains a surfactant, its content is preferably 0.5 parts by mass or less, more preferably 0.3 parts by mass or less, and particularly preferably 0.2 parts by mass or less, relative to 100 parts by mass of PVA(A) contained in the film-forming solution. By setting this content to 0.01 parts by mass or more, film-forming properties and peelability are further improved. On the other hand, by setting this content to 0.5 parts by mass or less, it is possible to suppress the exudation of surfactant to the surface of the PVA film, which could lead to adhesion and reduced processability.

[0077] Examples of film-forming methods for manufacturing PVA films using the aforementioned film-forming solution include casting, extrusion, wet film-forming, and gel film-forming. One or more of these methods may be used. Among these methods, casting and extrusion are preferred from the perspective of obtaining a PVA film with uniform thickness and width and good physical properties for manufacturing polarizing films. The resulting PVA film can be dried or heat-treated as needed.

[0078] As an example of a specific manufacturing method for the PVA film used in the manufacturing method of the present invention, the following method is preferably adopted industrially: For example, using a T-die, hopper plate, I-die head, lip coating machine die head, etc., the film-forming stock solution is uniformly sprayed or cast onto the circumferential surface of a rotating and heated first roller (or belt) located on the upstream side. Volatile components evaporate from one side of the film sprayed or cast onto the circumferential surface of the first roller (or belt), thus drying it. Then, it is further dried on the circumferential surface of one or more rotating and heated rollers located downstream, or further dried by passing it through a hot air drying apparatus, and then wound up using a winding device. Drying based on heated rollers and drying based on hot air drying apparatus can be appropriately combined. Alternatively, a multilayer PVA film can also be produced by forming a layer containing PVA(A) on one side of a substrate film composed of a single resin layer. The thickness of the substrate film in the multilayer film is typically 20–500 μm.

[0079] When using a multilayer film as the PVA film, the substrate film must be able to be stretched together with the PVA(A). Polyester, polyolefin resins, etc., can be used. Among these, amorphous polyester resins are preferred, and amorphous polyester resins obtained by copolymerizing polyethylene terephthalate, isophthalic acid, 1,4-cyclohexanediol, etc., are suitable. It is preferable to manufacture the multilayer film by coating the PVA solution onto the substrate film. In this case, to improve the adhesion between the PVA(A) layer and the substrate film, the surface of the substrate film can be modified or an adhesive layer can be formed between the two layers.

[0080] <Methods for manufacturing polarizing films>

[0081] The method for manufacturing the polarizing film of the present invention uses the PVA film described above as raw material. Specifically, the method for manufacturing the polarizing film includes: a dyeing step of dyeing the PVA film with a dichroic dye, a stretching step of uniaxially stretching the dyed film, and a drying step of drying the stretched film. A suitable method for manufacturing the polarizing film also includes: a stretching step of stretching the PVA film in a boric acid aqueous solution. Based on the aforementioned dyeing, stretching, and drying steps, a swelling step, a crosslinking step, a cleaning step, a heat treatment step, etc., can be further performed on the PVA film as needed. The order of each step is not particularly limited, and one or more processes can be performed simultaneously. Furthermore, one or more of each step can be performed two or more times, preferably the swelling step, dyeing step, crosslinking step, stretching step, and drying step are performed sequentially to manufacture the polarizing film. It is also preferable to perform a cleaning step after the aforementioned stretching step. Hereinafter, each step will be described in detail.

[0082] The swelling treatment can be performed by immersing the PVA membrane in water. The temperature of the water used for immersion is preferably 20°C or higher, more preferably 22°C or higher, and even more preferably 25°C or higher. The temperature of the water used for immersion is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 35°C or lower. Furthermore, the immersion time in water is preferably 0.1 minutes or higher, more preferably 0.2 minutes or higher. The immersion time in water is preferably 5 minutes or lower, more preferably 3 minutes or lower. It should be noted that the water used for immersion is not limited to pure water; it can be an aqueous solution containing various dissolved components, or a mixture of water and a hydrophilic medium. By setting this immersion time, the PVA membrane can swell efficiently and uniformly.

[0083] The dyeing process is performed by contacting the PVA film with a dichroic dye. Iodine-based dyes and dichroic dyes are commonly used as dichroic dyes, and iodine-based dyes are preferred in the manufacturing method of this invention. The dyeing process can be performed at any stage before, during, or after the stretching process; from the viewpoint of achieving high orientation of the iodine-based dye, it is preferred to perform the dyeing process before the stretching process. The dyeing process is generally performed by immersing the PVA film in a solution containing iodine and potassium iodide (especially an aqueous solution) or a solution containing various dichroic dyes (especially an aqueous solution) as a dyeing bath. The concentration of iodine in the dyeing bath is preferably in the range of 0.01 to 0.5% by mass. The concentration of potassium iodide in the dyeing bath is preferably in the range of 0.01 to 15% by mass. Furthermore, the temperature of the dyeing bath is preferably 20°C or higher, more preferably 25°C or higher. The temperature of the dyeing bath is preferably 50°C or lower, more preferably 40°C or lower. A suitable dyeing time is 0.2 to 5 minutes.

[0084] When using dichroic dyes, aqueous dyes are preferred. Furthermore, the dye concentration in the dyeing bath is preferably 0.001 to 10% by mass. Dyeing auxiliaries can be used as needed. As dyeing auxiliaries, inorganic salts such as sodium sulfate and surfactants can be used. When using sodium sulfate, the dye concentration in the dyeing bath is preferably 0.1 to 10% by mass. The dyeing temperature is preferably 30 to 80°C.

[0085] Specific dichroic dyes include CI Direct Yellow 28, CI Direct Orange 39, CI Direct Yellow 12, CI Direct Yellow 44, CI Direct Orange 26, CI Direct Orange 71, CI Direct Orange 107, CI Direct Red 2, CI Direct Red 31, CI Direct Red 79, CI Direct Red 81, CI Direct Red 247, CI Direct Green 80, CI Direct Green 59, etc., with dichroic dyes developed for the manufacture of polarizing plates being preferred.

[0086] The staining bath may contain boric acid crosslinking agents such as boric acid and borax. The content of boric acid crosslinking agents is usually less than 5% by mass based on boric acid conversion, and is preferably less than 1% by mass.

[0087] The crosslinking process can be performed by immersing the PVA film in an aqueous solution containing a boric acid crosslinking agent. By performing the crosslinking process on the PVA film, the PVA molecular chains are crosslinked with the aid of boric acid, thereby increasing the orientation of the PVA molecular chains. As a result, the orientation of dichroic dyes adsorbed on the PVA film is improved, thus improving the optical properties of the resulting polarizing film. From this viewpoint, the crosslinking process is more preferably performed after the dyeing process and before the stretching process. As the aforementioned boric acid crosslinking agent, one or more boron-containing inorganic compounds such as boric acid and borax can be used; from the perspective of ease of processing, boric acid is preferred. From the viewpoint of maintaining sufficient stretchability, the concentration of the boric acid crosslinking agent in the aqueous solution containing the boric acid crosslinking agent is preferably 1% by mass or more, more preferably 2% by mass or more. Similarly, from the viewpoint of maintaining sufficient stretchability, the concentration of the boric acid crosslinking agent in the aqueous solution containing the boric acid crosslinking agent is preferably 10% by mass or less, more preferably 7% by mass or less. If the concentration of the boric acid crosslinking agent exceeds 10% by mass, over-crosslinking may occur, reducing stretchability. Furthermore, if the concentration of the boric acid crosslinking agent is less than 1% by mass, the orientation of the dichroic dyes adsorbed on the PVA film may not be sufficiently improved, thus failing to adequately improve the polarization performance of the resulting polarizing film. The aqueous solution containing the boric acid crosslinking agent may contain iodide additives such as potassium iodide. From the viewpoint of enabling efficient boric acid crosslinking, the temperature of the aqueous solution containing the boric acid crosslinking agent is preferably 20°C or higher, and particularly preferably 25°C or higher. Similarly, from the viewpoint of enabling efficient boric acid crosslinking, the temperature of the aqueous solution containing the boric acid crosslinking agent is preferably 50°C or lower, and particularly preferably 40°C or lower.

[0088] In addition to the stretching process described later, the PVA film can be stretched (pre-stretched) separately during or between the aforementioned processes. From the viewpoint of the optical properties of the resulting polarizing film, the stretching ratio of the pre-stretching performed before the stretching process (the ratio obtained by multiplying the stretching ratios in each process) is preferably 1.5 times or more, more preferably 2.0 times or more, and even more preferably 2.5 times or more, depending on the original length of the unstretched PVA film used to manufacture the polarizing film. On the other hand, this pre-stretching stretching ratio is preferably 3.6 times or less, more preferably 3.4 times or less. The stretching ratio in the swelling process is preferably 1.05 to 2.5 times. The stretching ratio in the dyeing process is preferably 1.1 to 2.5 times. The stretching ratio in the crosslinking process is preferably 1.1 to 2.5 times.

[0089] The stretching process is preferably carried out in an aqueous solution containing boric acid. By carrying out the stretching process in an aqueous solution containing boric acid, necking during the stretching and drying processes can be easily controlled. The concentration of boric acid in the aqueous solution containing boric acid is preferably 1% by mass or more, more preferably 1.1% by mass or more, particularly preferably 1.2% by mass or more, and even more preferably 1.4% by mass or more. The concentration of boric acid in the aqueous solution containing boric acid is preferably 3% by mass or less, more preferably 3.0% by mass or less, particularly preferably 2.9% by mass or less, and even more preferably 2.5% by mass or less. In addition, the aqueous solution containing boric acid may contain iodide auxiliaries such as potassium iodide, and the concentration of these auxiliaries is preferably set in the range of 0.01% to 10% by mass.

[0090] The stretching temperature in the stretching process is preferably 53°C or higher, more preferably 55°C or higher, and particularly preferably 55°C or higher. The stretching temperature in the stretching process is preferably 70°C or lower, 65°C or lower, and particularly preferably 60°C or lower. It should be noted that here, the stretching temperature refers to the temperature of the aqueous solution containing boric acid. By setting this temperature range, the moisture content of the boric acid-based crosslinking and the polarizing film before drying can be controlled within a suitable range, making it easier to control necking during the stretching and drying processes. Specifically, when the stretching temperature exceeds 70°C, there is a tendency for the reactivity of PVA with boric acid to decrease, resulting in insufficient crosslinking based on boric acid. This makes it difficult to control necking during the stretching and drying processes, leading to high orientation of iodine-based dyes and making it difficult to obtain a polarizing film with excellent polarization properties. Furthermore, there is a risk that the PVA film may dissolve during the stretching process, reducing the yield of the polarizing film; therefore, there is a tendency for reduced productivity. On the other hand, when the stretching temperature is less than 53°C, there is a tendency for the following: the moisture content of the polarization film before drying is insufficient, making it difficult to control the necking phenomenon during the drying process and release the residual stress achieved by the orientation relaxation and crystallization of PVA, thus making it difficult to obtain a polarization film with low shrinkage stress.

[0091] Furthermore, the stretching ratio in the stretching process (in other words, the stretching ratio obtained based on the length of the PVA film before and after the stretching process) is preferably 2.0 times or more, and from the viewpoint of the optical performance of the obtained polarizing film, this stretching ratio is more preferably 2.2 times or more. The stretching ratio in the stretching process is preferably 4.0 times or less, and more preferably 3.5 times or less.

[0092] The total stretch ratio, obtained based on the original length of the unstretched PVA film supplied for manufacturing the polarizing film, must be at least 5.5 times, preferably at least 5.8 times, more preferably at least 5.9 times, and particularly preferably at least 6.0 times. The total stretch ratio must be at least 7.4 times, preferably at least 7.3 times, more preferably at least 7.2 times, and particularly preferably at least 6.8 times. By adjusting the total stretch ratio in this way, necking phenomena in all processes can be easily and appropriately controlled.

[0093] The tensile tension in the stretching process can be determined by measuring the tension between adjacent rollers using tension rollers positioned between them. The tensile tension in the stretching process is preferably 200 N or more, more preferably 300 N or more, and particularly preferably 450 N or more. The tensile tension in the stretching process is preferably 1100 N or less, more preferably 1000 N or less, and particularly preferably 700 N or less.

[0094] When stretching long strips of PVA film, the stretching direction is not particularly limited; uniaxial stretching along the strip direction, transverse uniaxial stretching, and so-called inclined stretching can be used. From the viewpoint of obtaining a polarizing film with excellent optical performance, uniaxial stretching along the strip direction is preferred. Uniaxial stretching along the strip direction can be performed using a stretching device with multiple parallel rollers and varying the circumferential speed between each roller. On the other hand, transverse uniaxial stretching can be performed using a stretching machine of the spreading type.

[0095] The cleaning process is preferably performed after the aforementioned stretching process. The cleaning process can remove unwanted chemicals and foreign matter from the surface of the PVA film, or adjust the optical properties of the polarizing film. The cleaning process can be performed by immersing the PVA film in a cleaning bath or by distributing a cleaning solution over the PVA film. Water can be used as the cleaning solution, and it may contain iodide additives such as potassium iodide and boric acid crosslinking agents. From the viewpoint of the optical properties of the polarizing film, it is preferable to contain iodide additives such as potassium iodide, and the content of these additives is preferably 0.1 to 10% by mass. Furthermore, from the viewpoint of improving the appearance of the polarizing film, it is also preferable to contain boric acid crosslinking agents, and when boric acid crosslinking agents are included, the content of the boric acid crosslinking agent is preferably 0.1% by mass or more, more preferably 0.5% by mass or more. When the polarizing film contains boric acid crosslinking agents, the content of the boric acid crosslinking agent in the cleaning solution is preferably 10% by mass or less, more preferably 5% by mass or less.

[0096] The temperature of the cleaning process is typically above 10°C, preferably above 15°C, and particularly preferably above 20°C. The temperature of the cleaning process is typically below 50°C, preferably below 40°C, and particularly preferably below 35°C. When the temperature of the cleaning process exceeds 50°C, wrinkles may sometimes form on the polarizing film, deteriorating its appearance, which is therefore undesirable. On the other hand, setting the temperature of the cleaning process below 20°C is not preferred from an economic point of view. It should be noted that the temperature of the cleaning process refers to the temperature of the cleaning solution. It should also be noted that the stretching ratio in the cleaning process is preferably below 1.3 times, more preferably below 1.2 times, and even more preferably below 1.1 times.

[0097] However, sometimes the PVA film is immersed in an aqueous solution containing boric acid crosslinking agent and stretched during the crosslinking, stretching, and cleaning processes, respectively. Among these processes, the process with the highest stretching ratio can be defined as the stretching process, the process performed before the stretching process can be defined as the crosslinking process, and the process performed after the stretching process can be defined as the cleaning process. In the cleaning process, the PVA film is typically impregnated with an aqueous solution of lower boric acid concentration compared to the stretching process.

[0098] The drying process is not particularly limited, but the drying temperature is preferably 60°C or higher, more preferably 70°C or higher, and particularly preferably 75°C or higher. The drying temperature is preferably 100°C or lower, more preferably 90°C or lower, and particularly preferably 85°C or lower. By performing drying at temperatures within the aforementioned range, it is easier to appropriately control the necking phenomenon during the drying process. Specifically, when the drying temperature is below 60°C, there is a tendency for insufficient necking during the drying process, resulting in inadequate release of residual stress from the orientation relaxation and crystallization of PVA, making it difficult to obtain a polarizing film with low shrinkage stress. On the other hand, when the drying temperature exceeds 100°C, there is a tendency for excessive necking to occur during the drying process, sometimes even causing the necessary iodine-based dyes to decompose, making it difficult to obtain a polarizing film with excellent polarization performance. Furthermore, red discoloration of the polarizing film sometimes occurs, leading to a decrease in the yield of the polarizing film and a tendency to reduce productivity.

[0099] The drying time in the drying process is not particularly limited, but is preferably 10 seconds or more, more preferably 25 seconds or more, and particularly preferably 40 seconds or more. The drying time in the drying process is preferably 120 seconds or less, more preferably 110 seconds or less, and particularly preferably 95 seconds or less. By setting the drying time within this range, it is easier to properly control necking during the drying process.

[0100] From the viewpoint of easily obtaining polarizing films with low shrinkage stress, the stretching ratio in the drying process is preferably 1.3 times or less, more preferably 1.2 times or less, and even more preferably less than 1.1 times. In order not to hinder the release of residual stress, it is also preferable not to perform stretching in the drying process.

[0101] The drying tension in the drying process can be determined by measuring the tension between adjacent rollers using tension rollers installed between them. The drying tension in the drying process is preferably 100 N or more, more preferably 200 N or more, and particularly preferably 260 N or more. The drying tension in the drying process is preferably 600 N or less, more preferably 500 N or less, and particularly preferably 400 N or less.

[0102] By performing heat treatment after the drying process, a polarizing film with excellent dimensional stability can be further obtained. Here, heat treatment refers to further heating the polarizing film with a moisture content of less than 5% after drying to improve the dimensional stability of the polarizing film. The heat treatment conditions are not particularly limited, but heat treatment is preferably performed at 60°C or higher, particularly preferably at 70°C or higher. Heat treatment is preferably performed at 150°C or lower, particularly preferably at 100°C or lower. If heat treatment is performed at a temperature lower than 60°C, the dimensional stabilization effect achieved by heat treatment tends to be insufficient. If heat treatment is performed at a temperature higher than 150°C, the polarizing film may sometimes undergo severe red discoloration. The heat treatment time is preferably 5 seconds or more, more preferably 15 seconds or more. The heat treatment time is preferably 100 seconds or less, more preferably 60 seconds or less. If the heat treatment time is less than 5 seconds, the dimensional stabilization effect achieved by heat treatment may not be sufficient. If the heat treatment time is longer than 100 seconds, the polarizing film may sometimes undergo red discoloration.

[0103] <Polarizing film>

[0104] The polarization degree of the polarizing film manufactured using the manufacturing method of the present invention is preferably 99.963% or higher when the monomer transmittance is 44%. If the polarization degree is less than 99.963% when the monomer transmittance is 44%, a high-quality LCD panel may not be obtainable. The polarization degree when the monomer transmittance is 44% is preferably 99.965% or higher, more preferably 99.967% or higher, particularly preferably 99.970% or higher, and even more preferably 99.975% or higher. As a method for setting the polarization degree of the polarizing film to 99.963% or higher when the monomer transmittance is 44%, examples include setting the total necking ratio (A) to 57.5% or higher and 61.0% or lower, setting the necking ratio (B) to 31.0% or higher and 38.0% or lower, and setting the necking ratio (C) to 16.5% or lower. In this case, it is preferable to set the boric acid concentration in the aqueous solution of the stretching process to 1.0% by mass or higher and 3.0% by mass or lower. In this case, it is preferable to set the total stretch ratio to 5.5 times or more.

[0105] The shrinkage stress of the polarizing film manufactured by the manufacturing method of the present invention is preferably 100 N / mm. 2 The following, and more preferably, is 90 N / mm 2 The following is particularly preferred: 85 N / mm 2 The following is an example where the shrinkage stress of the polarizing film is set to 100 N / mm. 2 Examples of methods include setting the necking ratio (B) to 38.0% or less and the necking ratio (C) to 9.8% or more. Furthermore, in this case, it is preferable to set the boric acid concentration in the aqueous solution of the stretching process to 3.0% by mass or less. Additionally, in this case, it is preferable to set the total stretching ratio to 7.4 times or less.

[0106] The polarizing film manufactured by the method of the present invention is typically made into a polarizing plate by bonding an optically transparent and mechanically strong protective film to one or both sides. As the protective film, cellulose triacetate (TAC) film, cellulose acetate butyrate (CAB) film, acrylic film, polyester film, etc., are used. Furthermore, as the adhesive used for bonding, PVA-based adhesives, UV-curing adhesives, etc., are examples.

[0107] The polarizing plate obtained through the above operations can be bonded to phase retardation films, viewing angle improvement films, brightness enhancement films, etc. Alternatively, after coating the polarizing plate with an acrylic adhesive, it can be bonded to a glass substrate to be used as a component of an LCD.

[0108] Example

[0109] The present invention is illustrated by the following embodiments, but the present invention is not limited to these embodiments at all. It should be noted that the evaluation methods used in the following embodiments and comparative examples are shown below.

[0110] [Calculation of total neck retraction rate (A)]

[0111] In the following examples or comparative examples, the length X1 (m) of the unstretched PVA film used to manufacture the polarizing film and the length X2 (m) of the PVA film after the drying process were measured. The total necking ratio (A) was calculated by substituting the obtained measured values ​​into the following equation (1).

[0112] Total necking ratio (A) = {(X1-X2) / X1} × 100 (1)

[0113] [Calculation of necking ratio (B) in the stretching process]

[0114] In the following examples or comparative examples, the length Y1 (m) of the width of the PVA film after the crosslinking process and before the stretching process, and the length Y2 (m) of the width of the PVA film after the stretching process and before the cleaning process were measured. The necking ratio (B) in the stretching process was calculated by substituting the obtained measured values ​​into the following equation (2).

[0115] Necking ratio (B) = {(Y1-Y2) / Y1} × 100 (2)

[0116] [Calculation of necking rate (C) in the drying process]

[0117] In the following examples or comparative examples, the length Z1 (m) of the width of the PVA film after the cleaning process and before the drying process, and the length X2 (m) of the width of the PVA film after the drying process were measured. The necking rate (C) in the drying process was calculated by substituting the obtained measured values ​​into the following equation (3).

[0118] Necking ratio (C) = {(Z1-X2) / Z1} × 100 (3)

[0119] [Calculation of necking ratio (D) up to the stretching process]

[0120] In the following examples or comparative examples, the length X1 (m) of the width of the unstretched PVA film used to manufacture the polarizing film and the length Y2 (m) of the width of the PVA film before the cleaning process and after the stretching process were measured. The necking ratio (D) up to the stretching process was calculated by substituting the obtained measured values ​​into the following equation (4).

[0121] Necking ratio (D) = {(X1-Y2) / X1} × 100 (4)

[0122] The difference between the total neck retraction rate (A) and the neck retraction rate (D) ((A)-(D)) is calculated, and this difference is taken as the neck retraction rate difference (W). In addition, the ratio of the neck retraction rate difference (W) to the total neck retraction rate (A) ((W) / (A)) is calculated by dividing the neck retraction rate difference (W) by the total neck retraction rate (A).

[0123] Optical properties of polarizing films

[0124] In the following examples and comparative examples, a rectangular sample of the polarizing film, measuring 4 cm in length and 2 cm in width, was taken from the center of both the width and length directions. The parallel transmittance and cross transmittance of the polarizing film were measured using a V-7100 spectrophotometer with an integrating sphere (manufactured by Nippon Spectrophotometer Co., Ltd.) and an automatic polarizing film measurement device VAP-7070S (manufactured by Nippon Spectrophotometer Co., Ltd.) equipped with a Glan Taylor polarizer. Here, the measurement wavelength range was set to 380–780 nm. The transmittance when the vibration direction of the polarized light incident on the polarizing film through the Glan Taylor polarizer is parallel to the transmission axis of the polarizing film was defined as the parallel transmittance, and the transmittance when it is perpendicular to the transmission axis of the polarizing film was defined as the cross transmittance. Subsequently, using the "Polarizing Film Evaluation Procedure" (manufactured by Japan Spectrophotometer Co., Ltd.), and following JIS Z 8722 (Method for Measurement of Object Color), visual sensitivity correction was performed in the visible light region with a C-light source and a 2° field of view using the aforementioned parallel transmittance and cross transmittance. The individual transmittance and degree of polarization of the polarizing film were then calculated, and these values ​​were used as the optical characteristics of the polarizing film. More specifically, the degree of polarization was calculated when the individual transmittance was 44%.

[0125] [Contraction stress of polarizing film]

[0126] In the following examples and comparative examples, the shrinkage stress of the polarizing film was measured using an Autograph AG-X thermostat with a constant temperature bath and a camera-type stretch meter TRViewX120S manufactured by Shimadzu Corporation. The polarizing film was conditioned at 20°C / 20% RH for 18 hours. After setting the thermostat of the Autograph AG-X to 20°C, the polarizing film (15 cm in length and 1.5 cm in width) was mounted on a fixture (5 cm spacing between fixtures). Simultaneously with the start of stretching, the thermostat was heated to 80°C. The polarizing film was stretched at a speed of 1 mm / min, and stretching was stopped when the tension reached 2 N. The tension was measured up to 4 hours later under this condition. Since the distance between the fixtures changed due to thermal expansion, marking labels were affixed to the fixtures, and the TRViewX120S camera-type stretch meter was used to measure the tension in a way that allowed the distance between the fixtures to be precisely adjusted to the amount of movement of the marking labels affixed to the fixtures. It should be noted that the value obtained by subtracting the initial tension of 2N from the tension measured after 4 hours is taken as the shrinkage force of the polarizing film, and the value obtained by dividing this value by the cross-sectional area of ​​the polarizing film is defined as the shrinkage stress (N / mm²). 2 ).

[0127] [Calculation of the total boron content in the polarization film]

[0128] In the following examples and comparative examples, the obtained polarizing film was conditioned at 23°C and 50% RH for 16 hours. After measuring the mass [E(g)] of the polarizing film, it was dissolved in 20 mL of distilled water to a concentration of 0.005% by mass. The aqueous solution containing the polarizing film was used as the test sample, and its mass [F(g)] was measured. The boron concentration [G(ppm)] of the test sample was measured using a multi-functional ICP luminescence analyzer (ICP) manufactured by Shimadzu Corporation. Subsequently, the value calculated by subtracting the measured value from the following formula was taken as the total boron content (mass%) in the polarizing film.

[0129] Total boron content (mass%) in polarization film = [(G×10 -6 [×F) / E]×100

[0130] Tensile tension in the stretching process

[0131] In the following embodiments and comparative examples, regarding the tensile tension in the stretching process, the tension carried between adjacent rollers is measured using tension rollers disposed between adjacent rollers. When using three or more rollers, the maximum tensile tension among them is used.

[0132] Drying tension in the drying process

[0133] In the following embodiments and comparative examples, regarding the drying tension in the drying process, the tension carried between adjacent rollers is measured using tension rollers arranged between adjacent rollers. When using three or more rollers, the maximum drying tension among them is used.

[0134] [Example 1]

[0135] A film roll with a thickness of 45 μm was obtained by casting a film-forming stock solution containing 100 parts by mass of PVA (a saponified product of vinyl acetate polymer, with a degree of polymerization of 2400 and a degree of saponification of 99.9 mol%), 10 parts by mass of glycerol as a plasticizer, 0.1 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and water. A polarizing film was then manufactured by sequentially performing swelling, dyeing, crosslinking, stretching, cleaning, and drying processes on the PVA film. It should be noted that the length X1 (m) of the unstretched PVA film used for manufacturing the polarizing film is 0.65 m.

[0136] Specifically, the polarizing film is manufactured as follows: First, in the swelling process, the PVA film is uniaxially stretched (first stage stretching) along the length direction (MD direction) to twice its original length during a period of immersion in water at 25°C for 90 seconds. Next, in the dyeing process, the PVA film is uniaxially stretched (second stage stretching) along the length direction (MD direction) to 2.4 times its original length during a period of immersion in an aqueous solution containing 0.093% by mass of iodine and 2.14% by mass of potassium iodide at 32°C (iodine to potassium iodide weight ratio of 1:23) for 163 seconds. Next, in the crosslinking process, the PVA film is uniaxially stretched (third stage stretching) along the length direction (MD direction) to three times its original length during a period of immersion in an aqueous solution containing 2.6% by mass of boric acid at 32°C for 135 seconds. Next, in the stretching process, the PVA film is uniaxially stretched (fourth-stage stretching) along its length (MD direction) to 6.8 times its original length while immersed in an aqueous solution containing 1.5% by mass boric acid and 5% by mass potassium iodide at 56°C. The maximum tensile tension in the stretching process is 679 N. Next, in the cleaning process, the film is cleaned by immersion in an aqueous solution containing 1.5% by mass boric acid and 5.4% by mass potassium iodide at 22°C for 10 seconds. Next, in the drying process, a polarizing film with a thickness of 15.6 μm is produced by drying in a dryer at 80°C for 90 seconds. It should be noted that the maximum drying tension in the drying process is 382.5 N.

[0137] Using the obtained polarizing film, the monomer transmittance, degree of polarization, shrinkage stress, and total boron content in the polarizing film were determined using the methods described above. The evaluation results are shown in Table 1. Figure 1 The relationship between shrinkage stress and polarization degree is shown in the figure. Additionally, in... Figure 2 The figure shows the relationship between maximum tensile tension and shrinkage stress.

[0138] [Examples 2-5 and Comparative Examples 1-9]

[0139] As shown in Table 1, the concentration of boric acid aqueous solution, total stretching ratio, and temperature of boric acid aqueous solution in the stretching process were changed. The drying temperature and drying time in the drying process were also changed as shown in Table 1. The iodine concentration in the dyeing bath was appropriately adjusted so that the monomer transmittance of the polarizing film was 44%. Otherwise, the stretching tension and drying tension were measured using the same method as in Example 1, and the polarizing film was manufactured using the same method as in Example 1. It should be noted that in Examples 2-5 and Comparative Examples 1-9, the stretching ratio from the first stage stretching to the third stage stretching was set to the same stretching ratio as in Example 1. The fourth stage stretching was changed compared to Example 1, and thus, the total stretching ratio was adjusted so that it was the value shown in Table 1. Subsequently, the monomer transmittance, degree of polarization, shrinkage stress, and total boron content in the polarizing film were evaluated using the methods described above. The results are shown in Table 1. Figure 1 The relationship between shrinkage stress and polarization degree is shown in the figure. Additionally, in... Figure 2 The figure shows the relationship between maximum tensile tension and shrinkage stress.

[0140] [Table 1]

[0141]

[0142] like Figure 1 As shown, for embodiments 1 to 5 that meet the requirements of this invention, the polarizing film exhibits low shrinkage force and excellent optical performance at high temperatures.

[0143] In addition, such as Figure 2 As shown, for Examples 1-3, where the total amount of boron in the polarizing film is low, the relationship between the maximum tensile tension and the shrinkage stress changes significantly towards the low shrinkage stress side.< / pva>

Claims

1. A method for manufacturing a polarizing film, comprising: A dyeing process for dyeing polyvinyl alcohol films using dichroic dyes; A stretching process involves uniaxially stretching a dyed polyvinyl alcohol film in an aqueous solution containing boric acid; and a drying process involves drying the stretched polyvinyl alcohol film. The boric acid concentration in the aqueous solution of the stretching process is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total necking ratio (A) shown in formula (1) is 57.5% to 61.0%, the necking ratio (B) shown in formula (2) of the stretching process is 31.0% to 38.0%, and the necking ratio (C) shown in formula (3) of the drying process is 9.8% to 16.5%. Total necking ratio (A) = {(X1-X2) / X1} × 100 (1) Necking ratio (B) = {(Y1-Y2) / Y1}×100 (2) Necking ratio (C) = {(Z1-X2) / Z1}×100 (3) X1 represents the length (m) of the polyvinyl alcohol film before the dyeing process; X2 represents the length (m) of the polyvinyl alcohol film after the drying process; Y1 represents the length (m) of the polyvinyl alcohol film before the stretching process; Y2 represents the length (m) of the polyvinyl alcohol film after the stretching process; Z1 represents the length (m) of the polyvinyl alcohol film before the drying process.

2. The method for manufacturing a polarizing film according to claim 1, wherein, The necking ratio (D) shown in the following formula (4) up to the stretching process is 46.0%~54.0%. The necking ratio (D) = {(X1-Y2) / X1} × 100 (4).

3. The method for manufacturing a polarizing film according to claim 2, wherein, The difference between the total necking ratio (A) and the necking ratio (D) ((A)-(D)), i.e. the necking ratio difference (W), is 8.0~11.0%.

4. The method for manufacturing a polarizing film according to claim 3, wherein, The ratio of the necking rate difference (W) to the total necking rate (A) ((W) / (A)) is 0.14 to 0.

19.

5. The method for manufacturing a polarizing film according to any one of claims 1 to 4, wherein, The total boron content in the polarization film is 2.0~4.0 by mass.

6. The method for manufacturing a polarizing film according to any one of claims 1 to 4, wherein, The stretching temperature in the stretching process is 53℃~70℃.

7. The method for manufacturing a polarizing film according to any one of claims 1 to 4, wherein, The drying temperature in the drying process is 60℃~100℃.

8. The method for manufacturing a polarizing film according to any one of claims 1 to 4, wherein, The polarization degree of the polarizing film is above 99.963% when the monomer transmittance is 44% and the shrinkage stress is 100 N / mm. 2 the following.

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