JP7839743B2 - Method for manufacturing polarizing film and polarizing film - Google Patents

Description

This invention relates to a method for manufacturing a polarizing film and to a polarizing film.

Polarizing plates, which have the function of transmitting and blocking polarized light, are a basic component of liquid crystal displays (LCDs) along with liquid crystals that change the polarization state of light. Many polarizing plates have a structure in which a protective film such as cellulose triacetate (TAC) film is laminated to the surface of a polarizing film. As for polarizing films, the mainstream is one in which an iodine-based dye ( _I3- or ^{I5- ,} etc.) is adsorbed onto a matrix (a stretched film that has been uniaxially stretched and oriented) made of polyvinyl alcohol film (hereinafter, "polyvinyl alcohol" may be abbreviated as " _PVA "). Such polarizing films are manufactured by uniaxially stretching a PVA film that already contains a dichroic dye, adsorbing a dichroic dye at the same time as uniaxial stretching of a PVA film, or adsorbing a dichroic dye after uniaxial stretching of a PVA film.

LCDs are used in a wide range of applications, including small devices such as calculators, watches, and smartphones, as well as laptop computers, LCD monitors, LCD color projectors, LCD televisions, in-car navigation systems, mobile phones, and measuring instruments used both indoors and outdoors. In response to the increasing performance of displays in recent years, there is a demand for polarizing films with superior optical performance. Furthermore, as displays become thinner, there is also a need for polarizing films with low shrinkage stress.

Patent Document 1 describes a method for manufacturing a polarizing film with excellent polarization performance, which involves irradiating a predetermined 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, which involves stretching a PVA film to 1.8 to 3.0 times its original size in a boric acid aqueous solution at 60 to 70°C, with a total stretching ratio of 6 to 8 times.

Japanese Patent Publication No. 2018-032025 International Publication No. 2017/138551

However, the method described in Patent Document 1 required additional steps to the conventional manufacturing process, and if the intensity of electromagnetic waves was not strictly controlled, the orientation of the PVA would be excessively relaxed, reducing the polarization performance of the polarizing film, making industrial implementation difficult. Furthermore, the method described in Patent Document 2 had the problem that the PVA film would melt during stretching due to the harsh stretching conditions, resulting in a decrease in the yield of the polarizing film.

The present invention was made to solve the above problems, and aims to provide a method for manufacturing a polarizing film that has excellent polarization performance and low shrinkage stress.

Through diligent research, the inventors of this invention have found that in order to manufacture a polarizing film with excellent polarization performance and low shrinkage stress, it is important to appropriately control the shrinkage phenomenon of the film width of the PVA film during the stretching and drying processes in the polarizing film manufacturing process (hereinafter, this phenomenon may be referred to as the "neck-in phenomenon") and control the shrinkage rate of the film width (hereinafter, this shrinkage rate may be referred to as the "neck-in rate") to be within a certain range.

In other words, the present invention is
[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 boric acid concentration of the aqueous solution in the stretching step is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total neck-in ratio (A) represented by the following formula (1) is 57.5% to 61.0%, the neck-in ratio (B) represented by the following formula (2) in the stretching step is 31.0% to 38.0%, and the neck-in ratio (C) represented by the following formula (3) in the drying step is 9.8% to 16.5%.
Total neck-in ratio (A) = {(X1 - X2) / X1} × 100 (1)
Neck-in ratio (B) = {(Y1 - Y2) / Y1} × 100 (2)
Neck-in ratio (C) = {(Z1 - X2) / Z1} × 100 (3)
[X1 represents the width (m) of the polyvinyl alcohol film before the dyeing process, X2 represents the width (m) of the polyvinyl alcohol film after the drying process, Y1 represents the width (m) of the polyvinyl alcohol film before the stretching process, Y2 represents the width (m) of the polyvinyl alcohol film after the stretching process, and Z1 represents the width (m) of the polyvinyl alcohol film before the drying process.]
[2] The method for manufacturing a polarizing film according to [1], wherein the neck-in ratio (D) represented by the following formula (4) up to the stretching step is 46.0% to 54.0%.
Neck-in ratio (D) = {(X1 - Y2) / X1} × 100 (4)
[3] A method for manufacturing a polarizing film according to [2], wherein the difference in the neck-in rate (W), which is the difference between the total neck-in rate (A) and the neck-in rate (D) ((A) - (D)), is 8.0 to 11.0%;
[4] A method for manufacturing a polarizing film according to [3], wherein the ratio of the difference in neck-in ratio (W) to the total neck-in ratio (A) ((W)/(A)) is 0.14 to 0.19;
[5] A method for producing 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;
[6] A method for manufacturing a polarizing film according to any one of [1] to [5], wherein the stretching temperature in the stretching step is 53°C to 70°C;
[7] A method for manufacturing a polarizing film according to any one of [1] to [6], wherein the drying temperature in the drying step is 60°C to 100°C;
[8] A method for manufacturing a polarizing film according to any one of [1] to [7] ^above , wherein the degree of polarization of the polarizing film is 99.963% or more when the transmittance of the polarizing film alone is 44%, and the shrinkage stress is 100 N/mm² or less;
[9] A polarizing film made of polyvinyl alcohol film, having a polarization degree of 99.963% or higher when the single-layer transmittance is 44%, and a shrinkage stress of 100 N/ ^mm² or less;
[10] The polarizing film according to [9], wherein the total boron content in the polarizing film is 4.0% by mass or less;
Regarding.

The present invention provides a method for manufacturing polarizing films that exhibits excellent polarization performance and low shrinkage stress. Therefore, the resulting polarizing film is suitable for use in high-performance liquid crystal displays, particularly those used at high temperatures.

This graph plots the degree of polarization at a single-layer transmittance of 44% against the shrinkage stress of the polarizing films obtained in Examples 1-5 and Comparative Examples 1-9. This graph plots the tensile tension against the shrinkage stress of the polarizing films obtained in Examples 1-5 and Comparative Examples 1-9.

The present invention relates to a method for manufacturing a polarizing film, comprising 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 of the aqueous solution in the stretching step is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total neck-in ratio (A) represented by the following formula (1) is 57.5% to 61.0%, the neck-in ratio (B) represented by the following formula (2) in the stretching step is 31.0% to 38.0%, and the neck-in ratio (C) represented by the following formula (3) in the drying step is 9.8% to 16.5%.
Total neck-in ratio (A) = {(X1 - X2) / X1} × 100 (1)
Neck-in ratio (B) = {(Y1 - Y2) / Y1} × 100 (2)
Neck-in ratio (C) = {(Z1 - X2) / Z1} × 100 (3)

In formulas (1) to (3) above, X1 represents the width of the PVA film before the dyeing process (the width of the unstretched PVA film used for manufacturing the polarizing film) (m), X2 represents the width of the PVA film after the drying process (m), Y1 represents the width of the PVA film before the stretching process (m), Y2 represents the width of the PVA film after the stretching process (m), and Z1 represents the width of the PVA film before the drying process (m). If a heat treatment process, described later, is provided after the drying process, the width of the PVA film after the heat treatment (m) can be set to X2. If a crosslinking process is provided before the stretching process, the width of the PVA film after the crosslinking process and before the stretching process (m) can be set to Y1. If a washing process, described later, is provided after the stretching process, the width of the PVA film after the stretching process and before the washing process (m) can be set to Y2.

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

In the stretching process, the PVA becomes highly oriented, which can lead to the highly oriented iodine-based dyes and the generation of residual stress. In the drying process, the residual stress is released through the relaxation of PVA orientation and crystallization. At the same time, unwanted iodine-based dyes are decomposed by heat, which can improve polarization performance, or it can decompose necessary iodine-based dyes, which can decrease polarization performance. Therefore, by appropriately controlling the neck-in ratio (B) in the stretching process and the neck-in ratio (C) in the drying process, it is possible to manufacture a polarizing film with excellent polarization performance and low shrinkage stress.

The total neck-in rate (A) must be 57.5% or higher, preferably 58.5% or higher, and more preferably 59.0% or higher. The total neck-in rate (A) must be 61.0% or lower, preferably 60.8% or lower, and more preferably 60.5% or lower. If the total neck-in rate (A) is less than 57.5%, the neck-in phenomenon up to the stretching process is insufficient, often resulting in an excessively wide film width. This is undesirable because it can lead to wrinkles on the surface of the polarizing film, reducing polarization performance and decreasing the yield of the polarizing film. On the other hand, if the total neck-in rate (A) exceeds 61.0%, the neck-in phenomenon has progressed excessively up to the stretching process, which can lead to PVA film breakage during the stretching and drying processes, reducing yield, and stabilizing unwanted iodine-based dyes, further reducing polarization performance. There are no particular limitations on the method for adjusting the total neck-in ratio (A) to these ranges, but examples include appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the swelling, dyeing, crosslinking, stretching, and washing steps, the stretching ratio in each of the swelling, dyeing, crosslinking, stretching, and washing steps, and the drying temperature and drying time in the drying step.

The neck-in ratio (B) in the stretching process must be 31.0% or higher, preferably 32.0% or higher, and more preferably 33.0% or higher. The neck-in ratio (B) in the stretching process must be 38.0% or lower, preferably 37.5% or lower, more preferably 37.0% or lower, and particularly preferably 36.0% or lower. If the neck-in ratio (B) in the stretching process is less than 31.0%, the iodine-based dye cannot be highly oriented, making it difficult to obtain a polarizing film with excellent polarization performance, which is undesirable. On the other hand, if the neck-in ratio (B) in the stretching process exceeds 38.0%, although the reason is unclear, the interactions between PVA molecular chains and crosslinking by boric acid become stronger, making it impossible to promote the neck-in phenomenon in the drying process. This results in insufficient decomposition of unwanted iodine-based dyes and release of residual stress through relaxation of PVA orientation and crystallization, making it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress, which is undesirable. There are no particular limitations on the method for adjusting the neck-in ratio (B) to these ranges, but examples include appropriately adjusting the boric acid concentration and temperature of the aqueous solution in the stretching process, the stretching ratio, etc.

The neck-in ratio (C) in the drying process must be 9.8% or higher, preferably 11.5% or higher, more preferably 12.0% or higher, and particularly preferably 12.5% or higher. The neck-in ratio (C) in the drying process must be 16.5% or lower, preferably 16.3% or lower, and more preferably 16.1% or lower. If the neck-in ratio (C) in the drying process is less than 9.8%, the relaxation of PVA orientation and the release of residual stress due to crystallization will be insufficient, making it difficult to obtain a polarizing film with low shrinkage stress, which is undesirable. On the other hand, if the neck-in ratio (C) in the drying process exceeds 16.5%, the relaxation of PVA orientation will proceed excessively, decomposing even the necessary iodine-based dyes, making it difficult to obtain a polarizing film with excellent polarizing performance, which is undesirable. There are no particular limitations on the method for adjusting the neck-in ratio (C) to these ranges, but for example, one method is to appropriately adjust the drying temperature and drying time in the drying process.

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 stretched through all processes to the original length of the unstretched PVA film used in the manufacture of the polarizing film. If the total stretching ratio is less than 5.5 times, it is not possible to sufficiently advance the neck-in phenomenon and highly orient the iodine-based dye, making it difficult to obtain a polarizing film with excellent polarizing performance, which is undesirable. On the other hand, if the total stretching ratio exceeds 7.4 times, the neck-in phenomenon progresses excessively, making it difficult to sufficiently relax the orientation of the PVA and release residual stress through crystallization, making it difficult to obtain a polarizing film with low shrinkage stress, which is undesirable. Furthermore, uneven stretching is likely to occur, which can reduce the yield of polarizing films and is undesirable from a productivity standpoint. There are no particular limitations on the method for adjusting the total stretching ratio to these ranges, but for example, one method is to appropriately adjust the stretching ratio in each step of the swelling process, dyeing process, crosslinking process, stretching process, and washing process.

The boric acid concentration in the aqueous solution used in 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 used in 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. If the boric acid concentration in the aqueous solution used in the stretching process is less than 1.0% by mass, the crosslinking by boric acid is insufficient, resulting in insufficient neck-in phenomenon in the stretching process, making it difficult to highly orient the iodine-based dye, and thus making it difficult to obtain a polarizing film with excellent polarizing performance, which is undesirable. On the other hand, if the boric acid concentration in the aqueous solution during the stretching process exceeds 3.0% by mass, excessive crosslinking by boric acid may occur, leading to the formation of unwanted iodine-based dyes during the stretching process. Alternatively, the neck-in phenomenon during the drying process may be insufficient, preventing adequate relaxation of PVA orientation and release of residual stress through crystallization. This makes it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress, which is undesirable.

The neck-in ratio (D) represented by the following formula (4) up to the stretching step 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 neck-in ratio (D) represented by the following formula (4) up to the stretching step 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.
Neck-in ratio (D) = {(X1 - Y2) / X1} × 100 (4)

If the neck-in ratio (D) up to the stretching step is less than 46.0%, the neck-in phenomenon that progresses throughout the entire process up to the stretching step is insufficient, making it difficult to highly orient the iodine-based dye and thus making it difficult to obtain a polarizing film with excellent polarization performance. On the other hand, if the neck-in ratio (D) up to the stretching step exceeds 54.0%, the neck-in phenomenon that progresses throughout the entire process up to the stretching step is excessive, making it difficult to relax the orientation of PVA or release residual stress through crystallization in a way that does not reduce the yield of the polarizing film after the stretching step, thus making it difficult to obtain a polarizing film with low shrinkage stress. There are no particular limitations on how to adjust the neck-in ratio (D) to these ranges, but examples include appropriately adjusting the boric acid concentration and temperature of the aqueous solution in each of the dyeing, crosslinking, and stretching steps, the stretching ratio in each of the swelling, dyeing, crosslinking, and stretching steps, and the drying temperature and drying time in the drying step.

The difference in the neck-in rate (W), which is the difference between the total neck-in rate (A) and the neck-in rate (D) ((A) - (D)), is preferably 8.0% or more, more preferably 8.5% or more, and particularly preferably 9.0% or more. The difference in the neck-in rate (W) is preferably 11.0% or less, more preferably 10.5% or less, and particularly preferably 10.0% or less. If the difference in the neck-in rate (W) between the total neck-in rate (A) and the neck-in rate (D) is less than 8.0%, the proportion of the neck-in phenomenon that occurs in the drying process is insufficient in the neck-in phenomenon that occurs throughout the entire process, making it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress, as the decomposition of unwanted iodine-based dyes and the relaxation of PVA orientation and release of residual stress by crystallization tend to be insufficient. In particular, it tends to be difficult to reduce the shrinkage stress. On the other hand, if the neck-in rate difference (W) exceeds 11.0%, the proportion of the neck-in phenomenon occurring during the drying process is excessive compared to the neck-in phenomenon that occurs throughout the entire process. This often leads to the decomposition of necessary iodine-based dyes, making it difficult to obtain a polarizing film with excellent polarizing performance.

The ratio of the difference in neck-in rate (W) to the total neck-in rate (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 difference in neck-in rate (W) to the total neck-in rate (A) ((W)/(A)) is preferably 0.19 or less, more preferably 0.18 or less, and particularly preferably 0.17 or less. If the ratio of the difference in neck-in rate (W) to the total neck-in rate (A) ((W)/(A)) is less than 0.14, the proportion of the neck-in phenomenon that occurs in the drying process is insufficient in the neck-in phenomenon that occurs throughout the entire process, making it difficult to obtain a polarizing film with excellent polarization performance and low shrinkage stress, as the decomposition of unwanted iodine-based dyes and the release of residual stress due to the relaxation of PVA orientation and crystallization tend to be insufficient. In particular, it tends to be difficult to reduce shrinkage stress. On the other hand, if the ratio ((W)/(A)) exceeds 0.19, the proportion of the neck-in phenomenon occurring during the drying process is excessive compared to the neck-in phenomenon that occurs throughout the entire process. This often leads to the decomposition of necessary iodine-based dyes, making it difficult to obtain a polarizing film with excellent polarizing performance.

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. If the total boron content in the polarizing film is less than 2.0% by mass, there is insufficient crosslinking with boric acid, making it difficult to control the neck-in phenomenon in the stretching and drying processes to highly orient the iodine-based dye, and thus it is undesirable to obtain a polarizing film with excellent polarization performance. On the other hand, if the total boron content in the polarizing film exceeds 4.0% by mass, there is excessive crosslinking with boric acid, making it difficult to control the neck-in phenomenon in the drying process to relax the orientation of PVA and release residual stress through crystallization, and thus it is undesirable to obtain a polarizing film with low shrinkage stress. The total boron content in the polarizing film can be determined by ICP emission spectrometry or the like. Specifically, it can be determined by the method described in the examples.

The polarizing film obtained in this manner preferably has a polarization degree of 99.963% or higher when the single-layer transmittance is 44%, and a shrinkage stress of 100 N/ ^mm² or less, making it suitable for use in high-performance liquid crystal displays, especially liquid crystal displays that may be used at high temperatures. The polarization degree when the single-layer 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.

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. If the thickness is less than 5 μm, stretching breakage is more likely to occur during manufacturing, which may reduce productivity. On the other hand, if the thickness exceeds 60 μm, the performance required of the polarizing plate, such as thinness and weight reduction, may not be met.

<PVA>
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 its main structural units.

The degree of polymerization of PVA is preferably 1,500 or higher, more preferably 1,800 or higher, and even more preferably 2,000 or higher. The degree of polymerization of PVA is preferably 6,000 or lower, more preferably 5,000 or lower, and even more preferably 4,000 or lower. A degree of polymerization of 1,500 or higher can improve the durability of the polarizing film obtained by uniaxial stretching of the film. On the other hand, a degree of polymerization of 6,000 or lower can suppress increases in manufacturing costs and poor process passability during film formation. In this specification, the degree of polymerization of PVA(A) refers to the average degree of polymerization measured in accordance with the description in JIS K6726-1994.

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, from the viewpoint of the water resistance of the polarizing film obtained by uniaxial stretching of the film. In this specification, the degree of saponification of PVA refers to the ratio (mol%) of the number of moles of vinyl alcohol units to the total number of moles of vinyl alcohol units ( _-CH2 -CH(OH)-) that can be converted to vinyl alcohol units by saponification (typically vinyl ester units) and vinyl alcohol units in PVA. The degree of saponification can be measured in accordance with the description in JIS K6726-1994.

The method for producing PVA is not particularly limited. For example, one method involves polymerizing vinyl ester monomers to obtain polyvinyl esters, and then converting the vinyl ester units of these polyvinyl esters into vinyl alcohol units. The vinyl ester monomers used in the production of PVA(A) are not particularly limited, but examples include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, and vinyl benzoate. From an economic standpoint, vinyl acetate is preferred.

Furthermore, PVA may be obtained by copolymerizing a vinyl ester monomer with another monomer copolymerizable thereto, and then converting the vinyl ester units of the vinyl ester copolymer to vinyl alcohol units. Other monomers copolymerizable with vinyl ester monomers include, for example, α-olefins having 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutene; (meth)acrylic acid or its salts; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-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)acrylamidepropanesulfonic acid or its salts, (meth)acrylamidepropyldimethylamine or so Examples include (meth)acrylamide derivatives such as salts of N-methylol(meth)acrylamide or derivatives 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, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, and stearyl vinyl ether; vinyl cyanides such as (meth)acrylonitrile; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid or its salts, esters, or acid anhydrides; itaconic acid or its salts, esters, or acid anhydrides; vinylsilyl compounds such as vinyltrimethoxysilane; and unsaturated sulfonic acids. The vinyl ester copolymers described above may have structural units derived from one or more of the other monomers mentioned above. The other monomer can be used by pre-adding it to the reaction vessel when the vinyl ester monomer is subjected to the polymerization reaction, or by adding it to the reaction vessel during the polymerization reaction. From the viewpoint of optical performance, the content of units derived from the other monomer is preferably 10 mol% or less, more preferably 5 mol% or less, and even more preferably 2 mol% or less, relative to the total number of moles of structural units constituting PVA(A).

PVA may or may not have some of its hydroxyl groups cross-linked. Furthermore, some of the hydroxyl groups of the above-mentioned 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.

Ethylene is preferred as a monomer copolymerizable with the vinyl ester monomer, as it improves stretchability, allows stretching at higher temperatures, reduces the occurrence of problems such as stretch breakage, and further improves the productivity of polarizing films. When PVA contains ethylene units, the ethylene unit content is preferably 1 mol% or more, and more preferably 2 mol% or more, relative to the total number of moles of structural units constituting PVA, from the viewpoint of stretchability and stretchable temperature as described above. When PVA contains ethylene units, the ethylene unit content is preferably 10 mol% or less, and more preferably 6 mol% or less, relative to the total number of moles of structural units constituting PVA, from the viewpoint of stretchability and stretchable temperature.

<PVA film>
The PVA film used in the manufacturing method of the present invention may contain a plasticizer in addition to the PVA mentioned above. Preferred plasticizers include polyhydric alcohols, and specific examples include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. Furthermore, one or more of these plasticizers may be included. Among these, glycerin is preferred in terms of its effect on improving stretchability.

The plasticizer content in the PVA film is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of PVA(A). The plasticizer content in the PVA film is preferably 20 parts by mass or less, more preferably 17 parts by mass or less, and even more preferably 15 parts by mass or less, per 100 parts by mass of PVA(A). A content of 1 part by mass or more improves the stretchability of the film. On the other hand, a content of 20 parts by mass or less can prevent the film from becoming too flexible and reducing its handling properties.

The PVA film may also contain, as needed, other additives besides PVA(A) and plasticizers, such as fillers, processing stabilizers including copper compounds, weather stabilizers, colorants, UV absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, defoamers, deodorizers, bulking agents, release agents, mold release agents, reinforcing agents, crosslinking agents, antifungal agents, preservatives, and crystallization rate retarders. The content of these other additives in the PVA film is usually preferably 10% by mass or less, and more preferably 5% by mass or less.

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. A swelling degree of 160% or more suppresses the progression of extreme crystallization, allowing for stable stretching to high magnification. On the other hand, a swelling degree of 240% or less suppresses dissolution during stretching, making it possible to stretch even under higher temperature conditions.

The thickness of the PVA film is not particularly limited, but is generally 1 μm or more, preferably 5 μm or more, and particularly preferably 10 μm or more. The thickness of the PVA film is generally 100 μm or less, preferably 60 μm or less, and particularly preferably 45 μm or less. If the PVA film is too thin, stretching breakage tends to occur easily during the stretching process for manufacturing the polarizing film. Also, if the PVA film is too thick, stretching unevenness tends to occur easily during the stretching process for manufacturing the polarizing film, and the resulting polarizing film will also be thicker. For this reason, it tends to be difficult to use in thin and small devices such as smartphones and laptop computers.

The width of the PVA film is not particularly limited and can be determined according to the intended use of the polarizing film being manufactured. In recent years, given the increasing size of LCD televisions and monitors, a PVA film width of 3m or more used in the manufacture of polarizing films is suitable for these applications. On the other hand, if the width of the PVA film used in the manufacture of polarizing films is too large, it becomes difficult to perform uniform stretching when manufacturing polarizing films using commercially available equipment. Therefore, it is preferable that the width of the PVA film used in the manufacture of polarizing films be 10m or less.

The method for manufacturing a PVA film is not particularly limited, but a manufacturing method that results in a uniform thickness and width of the film after film formation is preferably employed. For example, it can be manufactured using a film-forming stock solution in which PVA(A) and, if necessary, one or more of the plasticizer, other additives, and surfactants described later are dissolved in a liquid medium, or a film-forming stock solution containing PVA(A) and, if necessary, one or more of the plasticizer, other additives, surfactants, and liquid medium, in which PVA(A) is dissolved. If the film-forming stock solution contains at least one of the plasticizer, other additives, and surfactants, it is preferable that these components are uniformly mixed.

Examples of the liquid medium used in preparing the film-forming stock solution include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, glycerin, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, trimethylolpropane, ethylenediamine, and diethylenetriamine. One or more of these can be used. Of these, water is preferred in terms of environmental impact and recoverability.

The volatile content of the film-forming solution (the percentage of volatile components, such as liquid media, that are removed by volatilization or evaporation during film formation) varies depending on the film formation method and conditions, but is generally preferable to be 50% by mass or more, and more preferable to be 55% by mass or more. The volatile content of the film-forming solution varies depending on the film formation method and conditions, but is generally preferable to be 95% by mass or less, and more preferable to be 90% by mass or less. When the volatile content of the film-forming solution is 50% by mass or more, the viscosity of the film-forming solution does not become too high, allowing for smooth filtration and degassing during the preparation of the film-forming solution, and facilitating the production of films with fewer foreign matter and defects. On the other hand, when the volatile content of the film-forming solution is 95% by mass or less, the concentration of the film-forming solution does not become too low, facilitating the production of industrial films.

The film-forming stock solution preferably contains a surfactant. The inclusion of a surfactant improves film-forming properties, suppresses variations in film thickness, and facilitates the peeling of the film from the metal rolls and belts used in film formation. When a PVA film is produced from a film-forming stock solution containing a surfactant, the film may contain the surfactant. While the type of surfactant is not particularly limited, anionic or nonionic surfactants are preferred from the viewpoint of peelability from metal rolls and belts.

Suitable anionic surfactants include, for example, carboxylic acid types such as potassium laurate; sulfate ester types 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 types 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 ester types 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.

Suitable nonionic surfactants include, for example, alkyl ether types such as polyoxyethylene oleyl ether; alkylphenyl ether types such as polyoxyethylene octylphenyl ether; alkyl ester types such as polyoxyethylene laurate; alkylamine types such as polyoxyethylene laurylamino ether; alkylamide types such as polyoxyethylene lauric acid amide; polypropylene glycol ether types such as polyoxyethylene polyoxypropylene ether; alkanolamide types such as lauric acid diethanolamide and oleic acid diethanolamide; and allylphenyl ether types such as polyoxyalkylene allylphenyl ether.

These surfactants can be used individually or in combination of two or more.

If the film-forming stock 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, based on 100 parts by mass of PVA(A) contained in the film-forming stock solution. If the film-forming stock 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, based on 100 parts by mass of PVA(A) contained in the film-forming stock solution. A content of 0.01 parts by mass or more further improves film-forming properties and release properties. On the other hand, a content of 0.5 parts by mass or less can suppress the surfactant from bleeding out onto the surface of the PVA film, causing blocking and reducing handling properties.

Examples of film-forming methods for producing PVA films using the above-mentioned film-forming stock solution include casting, extrusion, wet film-forming, and gel film-forming. These film-forming methods may be used individually or in combination of two or more. Among these film-forming methods, casting and extrusion are preferred because they yield PVA films with uniform thickness and width and good physical properties suitable for producing polarizing films. The produced PVA film can be dried or heat-treated as needed.

As an example of a specific manufacturing method for the PVA film used in the manufacturing method of the present invention, for example, a T-type slit die, hopper plate, I-die, lip coater die, etc., can be used to uniformly discharge or cast the above-mentioned film-forming stock onto the circumferential surface of a rotating, heated first roll (or belt) located at the upstream end, and the volatile components of the film discharged or cast onto the circumferential surface of this first roll (or belt) can be evaporated and dried from one side, and then further dried on the circumferential surfaces of one or more rotating, heated rolls located downstream, or further dried by passing it through a hot air drying device, before being wound up by a winding device. This method can be industrially preferred. Drying with heated rolls and drying with a hot air drying device may be carried out in appropriate combinations. Alternatively, a multilayer PVA film can be manufactured by forming a layer made of PVA(A) on one side of a base film made of a single resin layer. The thickness of the base film in a multilayer film is usually 20 to 500 μm.

When using a multilayer film as the PVA film, the base film must be stretchable together with the PVA(A), and materials such as polyester and polyolefin resins can be used. Among these, amorphous polyester resins are preferred, and amorphous polyester resins obtained by copolymerizing polyethylene terephthalate with copolymer components such as isophthalic acid and 1,4-cyclohexanedimethanol are suitably used. It is preferable to manufacture the multilayer film by applying the PVA solution to the base film. At this time, in order to improve the adhesion between the PVA(A) layer and the base film, the surface of the base film may be modified or an adhesive layer may be formed between the two layers.

<Method for manufacturing polarizing film>
The present invention provides a method for manufacturing a polarizing film using the PVA film described above as a raw material. Specifically, the method 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. Preferably, the method includes a stretching step of stretching the PVA film in an aqueous boric acid solution. In addition to the dyeing, stretching, and drying steps, the PVA film may be subjected to further steps such as swelling, crosslinking, washing, and heat treatment as needed. The order of each step is not particularly limited, and one or more processes may be performed simultaneously. Furthermore, one or more of each step may be performed two or more times, but it is preferable to manufacture the polarizing film by performing the swelling, dyeing, crosslinking, stretching, and drying steps in this order. It is also preferable to perform a washing step after the stretching step. Each step will be described in detail below.

The swelling treatment can be carried out by immersing the PVA film in water. The temperature of the water used to immerse the film 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 to immerse the film is preferably 40°C or lower, more preferably 38°C or lower, and even more preferably 35°C or lower. The immersion time is preferably, for example, 0.1 minutes or more, more preferably 0.2 minutes or more. The immersion time is preferably, for example, 5 minutes or less, and even more preferably 3 minutes or less. The water used to immerse the film is not limited to pure water; it may be an aqueous solution containing various components, or a mixture of water and a hydrophilic medium. By using such an immersion time, the PVA film can be swollen efficiently and uniformly.

The dyeing process can be carried out by contacting the PVA film with a dichroic dye. Iodine-based dyes and dichroic dyes are commonly used as dichroic dyes, and in the manufacturing method of the present invention, it is preferable to use iodine-based dyes. The dyeing process can be carried out at any stage before, during, or after the stretching process, but it is preferable to carry it out before the stretching process in order to allow for a high degree of orientation of the iodine-based dye. The dyeing process is generally carried out by immersing the PVA film in a dyeing bath containing a solution (especially an aqueous solution) containing iodine-potassium iodide, or a solution (especially an aqueous solution) containing multiple dichroic dyes. 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, and more preferably 25°C or higher. The temperature of the dyeing bath is preferably 50°C or lower, and more preferably 40°C or lower. The preferred staining time is 0.2 to 5 minutes.

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 may be used as needed. Inorganic salts such as sodium sulfate or surfactants may be used as dyeing auxiliaries. 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.

Specific examples of dichroic dyes include C.I. Direct Yellow 28, C.I. Direct Orange 39, C.I. Direct Yellow 12, C.I. Direct Yellow 44, C.I. Direct Orange 26, C.I. Direct Orange 71, C.I. Direct Orange 107, C.I. Direct Red 2, C.I. Direct Red 31, C.I. Direct Red 79, C.I. Direct Red 81, C.I. Direct Red 247, C.I. Direct Green 80, and C.I. Direct Green 59, but dichroic dyes developed for polarizing plate manufacturing are preferred.

The dyeing bath may contain boric acid, borates such as borax, or other boric acid crosslinking agents, but the content of the boric acid crosslinking agent is usually less than 5% by mass in terms of boric acid, and preferably 1% by mass or less.

The crosslinking process can be carried out 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 boric acid, improving the orientation of the PVA molecular chains. As a result, the orientation of the dichroic dye adsorbed on the PVA film is improved, thus improving the optical performance of the resulting polarizing film. From this viewpoint, it is more preferable to perform the crosslinking process after the dyeing process and before the stretching process. As the boric acid crosslinking agent, one or more boron-containing inorganic compounds such as boric acid and borates such as borax can be used, and for ease of handling, boric acid is preferred as the boric acid crosslinking agent. In order to maintain 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, and more preferably 2% by mass or more. Similarly, in order to maintain 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, and more preferably 7% by mass or less. If the concentration of the boric acid crosslinking agent exceeds 10% by mass, excessive crosslinking may occur, potentially reducing the stretchability. Furthermore, if the concentration of the boric acid crosslinking agent is less than 1% by mass, the orientation of the dichroic dye adsorbed on the PVA film may not be sufficiently improved, potentially resulting in insufficient improvement in the polarization performance of the resulting polarizing film. The aqueous solution containing the boric acid crosslinking agent may also contain iodide additives such as potassium iodide. From the viewpoint of 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 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.

In addition to the stretching process described later, the PVA film may be stretched (pre-stretched) during or between each of the processes described above. In this way, 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, based on the original length of the unstretched PVA film used to manufacture the polarizing film, from the viewpoint of the optical performance of the resulting polarizing film. On the other hand, the stretching ratio of the pre-stretching is preferably 3.6 times or less, and 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.

The stretching process is preferably carried out in an aqueous solution containing boric acid. Performing the stretching process in an aqueous solution containing boric acid makes it easier to control the neck-in phenomenon during the stretching and drying processes. 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. The aqueous solution containing boric acid may also contain iodide additives such as potassium iodide, and its concentration is preferably in the range of 0.01 to 10% by mass.

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. Here, the stretching temperature refers to the temperature of the aqueous solution containing boric acid. By setting the temperature within this range, it is possible to control the crosslinking by boric acid and the water content of the polarizing film before drying to a suitable range, and it becomes easier to control the neck-in phenomenon in the stretching and drying processes. In other words, if the stretching temperature exceeds 70°C, the reactivity of PVA and boric acid decreases, resulting in insufficient crosslinking by boric acid. This makes it difficult to control the neck-in phenomenon in the stretching and drying processes to highly orient the iodine-based dyes, and tends to make it difficult to obtain a polarizing film with excellent polarizing performance. In addition, there is a risk that the PVA film will dissolve during the stretching process, reducing the yield of the polarizing film, which tends to decrease productivity. On the other hand, if the stretching temperature is below 53°C, the moisture content of the polarizing film before drying is insufficient, making it difficult to control the neck-in phenomenon during the drying process to relax the orientation of PVA and release residual stress through crystallization. This tends to make it difficult to obtain a polarizing film with low shrinkage stress.

Furthermore, the stretching ratio in the stretching process (i.e., the stretching ratio after pre-stretching, based on the length of the PVA film before the stretching process) is preferably 2.0 times or more, and more preferably 2.2 times or more from the viewpoint of the optical performance of the resulting polarizing film. The stretching ratio in the stretching process is preferably 4.0 times or less, and more preferably 3.5 times or less.

The total stretching ratio, based on the original length of the unstretched PVA film used in the manufacture of polarizing films, 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. By adjusting the total stretching ratio in this way, it becomes easier to appropriately control the neck-in phenomenon throughout the entire process.

The stretch tension in the stretching process can be determined by measuring the tension between adjacent rolls using a tension roll installed between them. The stretch 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 stretch tension in the stretching process is preferably 1100 N or less, more preferably 1000 N or less, and particularly preferably 700 N or less.

When stretching a long PVA film, there are no particular restrictions on the stretching direction; uniaxial stretching in the longitudinal direction, transverse uniaxial stretching, and so-called oblique stretching can be employed. However, uniaxial stretching in the longitudinal direction is preferred because it yields a polarizing film with superior optical performance. Uniaxial stretching in the longitudinal direction can be performed using a stretching device equipped with multiple rolls parallel to each other, by changing the peripheral speed between each roll. On the other hand, transverse uniaxial stretching can be performed using a tenter-type stretcher.

It is preferable to perform a washing step after the stretching step. In the washing step, unwanted chemicals and foreign matter can be removed from the surface of the PVA film, and the optical properties of the polarizing film can be adjusted. The washing step can be performed by immersing the PVA film in a washing bath or by spraying a washing solution onto the PVA film. Water can be used as the washing solution, but it may also contain iodide additives such as potassium iodide or boric acid crosslinking agents. From the viewpoint of the optical properties of the polarizing film, it is preferable to include iodide additives such as potassium iodide, and the content is preferably 0.1 to 10% by mass. It is also preferable to include a boric acid crosslinking agent from the viewpoint of improving the appearance of the polarizing film, and when a boric acid crosslinking agent is included, the content of the boric acid crosslinking agent is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more. When a boric acid crosslinking agent is included in the polarizing film, the content of the boric acid crosslinking agent in the washing solution is preferably 10% by mass or less, and more preferably 5% by mass or less.

The temperature of the washing process is usually 10°C or higher, preferably 15°C or higher, and particularly preferably 20°C or higher. The temperature of the washing process is usually 50°C or lower, preferably 40°C or lower, and particularly preferably 35°C or lower. If the temperature of the washing process exceeds 50°C, wrinkles may form in the polarizing film, and the appearance of the polarizing film may deteriorate, which is undesirable. On the other hand, if the temperature of the washing process is below 20°C, it is undesirable from an economic standpoint. Note that the temperature of the washing process refers to the temperature of the washing solution. Note that the stretching ratio in the washing process is preferably 1.3 times or less, more preferably 1.2 times or less, and even more preferably less than 1.1 times.

Incidentally, in each of the crosslinking, stretching, and washing processes, the PVA film may be immersed in an aqueous solution containing a boric acid crosslinking agent and stretched. Of 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 washing process. In the washing process, the PVA film is usually immersed in an aqueous solution with a lower boric acid concentration than in the stretching process.

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. Drying within the above temperature range makes it easier to appropriately control the neck-in phenomenon during the drying process. That is, if the drying temperature is below 60°C, the neck-in phenomenon during the drying process is insufficient, and the relaxation of PVA orientation and the release of residual stress due to crystallization are insufficient, making it difficult to obtain a polarizing film with low shrinkage stress. On the other hand, if the drying temperature exceeds 100°C, the neck-in phenomenon tends to progress excessively during the drying process, which can decompose even the necessary iodine-based dyes, making it difficult to obtain a polarizing film with excellent polarizing performance. In addition, reddening of the polarizing film may occur, reducing the yield of the polarizing film and tending to decrease productivity.

The drying time in the drying process is not particularly limited, but it is preferably 10 seconds or more, more preferably 25 seconds or more, and especially 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 especially preferably 95 seconds or less. By keeping the drying time within this range, it becomes easier to appropriately control the neck-in phenomenon in the drying process.

In order to obtain a polarizing film 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 preferable not to stretch the film substantially during the drying process.

The drying tension in the drying process can be determined by measuring the tension between adjacent rolls using a tension roll 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.

By performing a heat treatment after the drying process, a polarizing film with even better dimensional stability can be obtained. Here, heat treatment refers to a process in which a polarizing film with a moisture content of 5% or less after drying is further heated to improve the dimensional stability of the polarizing film. The conditions for heat treatment are not particularly limited, but it is preferable to perform the heat treatment at 60°C or higher, and especially at 70°C or higher. It is preferable to perform the heat treatment at 150°C or lower, and especially at 100°C or lower. If the heat treatment is performed at a temperature lower than 60°C, the dimensional stabilization effect of the heat treatment tends to be insufficient. If the heat treatment is performed at a temperature higher than 150°C, severe reddening may occur in the polarizing film. The heat treatment time is preferably 5 seconds or more, and more preferably 15 seconds or more. The heat treatment time is preferably 100 seconds or less, and more preferably 60 seconds or less. If the heat treatment time is shorter than 5 seconds, the dimensional stabilization effect of the heat treatment may not be sufficiently obtained. If the heat treatment time is longer than 100 seconds, reddening may occur in the polarizing film.

<Polarizing film>
The polarizing film produced by the manufacturing method of the present invention preferably has a polarization degree of 99.963% or higher when the single-layer transmittance is 44%. If the polarization degree when the single-layer transmittance is 44% is less than 99.963%, there is a risk that a high-quality LCD panel cannot be obtained. The polarization degree when the single-layer 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 making the polarization degree of the polarizing film 99.963% or higher when the single-layer transmittance is 44%, for example, a method in which the total neck-in rate (A) is set to 57.5% or higher and 61.0% or lower, the neck-in rate (B) is set to 31.0% or higher and 38.0% or lower, and the neck-in rate (C) is set to 16.5% or lower. In this case, it is preferable that the boric acid concentration of the aqueous solution in the stretching process be 1.0% by mass or more and 3.0% by mass or less. In this case, it is preferable that the total stretching ratio be 5.5 times or more.

The shrinkage stress of the polarizing film produced by the manufacturing method of the present invention is preferably 100 N/ ^mm² or less, more preferably 90 N/ ^mm² or less, and particularly preferably 85 N/ ^mm² or less. A method for achieving a shrinkage stress of 100 N/ ^mm² or less for the polarizing film is, for example, to set the neck-in ratio (B) to 38.0% or less and the neck-in ratio (C) to 9.8% or more. In this case, it is preferable to set the boric acid concentration of the aqueous solution in the stretching process to 3.0% by mass or less. Furthermore, in this case, it is preferable to set the total stretching ratio to 7.4 times or less.

The polarizing film produced by the manufacturing method of the present invention is typically used as a polarizing plate by laminating an optically transparent and mechanically strong protective film to one or both sides of it. Examples of protective films include cellulose triacetate (TAC) film, cellulose acetate/butyrate (CAB) film, acrylic film, and polyester film. Examples of adhesives used for lamination include PVA-based adhesives and UV-curing adhesives.

The polarizing plate obtained as described above may be laminated with a phase difference film, a viewing angle enhancement film, a brightness enhancement film, etc. Alternatively, the polarizing plate can be coated with an acrylic-based adhesive and then laminated with a glass substrate to be used as a component of an LCD.

The present invention will be specifically described by the following examples, but the present invention is not limited in any way by these examples. The evaluation methods used in the following examples and comparative examples are shown below.

[Calculation of Total Neck-In Rate (A)]
In the following examples or comparative examples, the width length X1 (m) of the unstretched PVA film used for manufacturing the polarizing film and the width length X2 (m) of the PVA film after the drying process were measured. The total neck-in ratio (A) was calculated by substituting the obtained measurement values into the following formula (1).
Total neck-in ratio (A) = {(X1 - X2) / X1} × 100 (1)

[Calculation of neck-in ratio (B) in the stretching process]
In the following examples or comparative examples, the width length Y1 (m) of the PVA film after the crosslinking process and before the stretching process, and the width length Y2 (m) of the PVA film after the stretching process and before the washing process were measured. The neck-in ratio (B) in the stretching process was calculated by substituting the obtained measurement values into the following formula (2).
Neck-in ratio (B) = {(Y1 - Y2) / Y1} × 100 (2)

[Calculation of neck-in ratio (C) in the drying process]
In the following examples or comparative examples, the width length Z1 (m) of the PVA film after the washing process and before the drying process, and the width length X2 (m) of the PVA film after the drying process were measured. The neck-in rate (C) in the drying process was calculated by substituting the obtained measurement values into the following formula (3).
Neck-in ratio (C) = {(Z1 - X2) / Z1} × 100 (3)

[Calculation of neck-in ratio (D) up to the stretching process]
In the following examples or comparative examples, the width length X1 (m) of the unstretched PVA film used for manufacturing polarizing films and the width length Y2 (m) of the PVA film before the washing process and after the stretching process were measured. The neck-in ratio (D) up to the stretching process was calculated by substituting the obtained measurement values into the following formula (4).
Neck-in ratio (D) = {(X1 - Y2) / X1} × 100 (4)

The difference between the obtained total neck-in rate (A) and the neck-in rate (D) ((A) - (D)) was calculated and defined as the neck-in rate difference (W). Furthermore, the ratio of the neck-in rate difference (W) to the total neck-in rate (A) ((W) / (A)) was calculated by dividing the neck-in rate difference (W) by the total neck-in rate (A).

[Optical properties of polarizing films]
In the following examples and comparative examples, rectangular samples measuring 4 cm in the length direction and 2 cm in the width direction were taken from the center of the obtained polarizing film in both the width and length directions. The parallel transmittance and crossed nicol transmittance of the polarizing film were measured using a V-7100 spectrophotometer with an integrating sphere (manufactured by JASCO Corporation) and an automatic polarizing film measuring device VAP-7070S (manufactured by JASCO Corporation) equipped with a GranTerra polarizer. Here, the measurement wavelength range was set to 380 to 780 nm. Parallel transmittance was defined as the transmittance when the vibration direction of the polarization incident on the polarizing film through the GranTerra polarizer was parallel to the transmission axis of the polarizing film, and crossed nicol transmittance was defined as the transmittance when the vibration direction was perpendicular to the transmission axis of the polarizing film. Subsequently, using the "Polarizing Film Evaluation Program" (manufactured by JASCO Corporation), and in accordance with JIS Z 8722 (Method for Measuring Object Color), the aforementioned parallel transmittance and crossed nicol transmittance were used to perform luminous efficiency correction in the visible light region with a C light source and a 2° field of view. The single transmittance and polarization degree of the polarizing film were then calculated, and these two values were obtained as the optical properties of the polarizing film. More specifically, the polarization degree was calculated when the single transmittance was 44%.

[Shrinkage stress of polarizing film]
In the following examples and comparative examples, the shrinkage stress of the polarizing film was measured using an Autograph AG-X with a constant temperature chamber and a TRViewX120S video extensometer manufactured by Shimadzu Corporation. For the measurement, a polarizing film that had been conditioned at 20°C/20%RH for 18 hours was used. After setting the Autograph AG-X constant temperature chamber to 20°C, a polarizing film (15 cm in length and 1.5 cm in width) was attached to a chuck (chuck spacing 5 cm), and the constant temperature chamber was started to rise to 80°C at the same time as the tension was started. The polarizing film was pulled at a speed of 1 mm/min, and the tension was stopped when the tension reached 2 N. The tension was then measured for up to 4 hours after that. At this time, since the distance between the chucks changes due to thermal expansion, marking stickers were attached to the chucks, and the measurement was performed using the TRViewX120S video extensometer so that the distance between the chucks could be corrected by the amount that the marking stickers attached to the chucks moved. Furthermore, the shrinkage force of the polarizing film was defined as the value obtained by subtracting the initial tension of 2N from the tension measurement after 4 hours, and the shrinkage stress (N/ ^mm² ) was defined as the value obtained by dividing this value by the cross-sectional area of the polarizing film.

[Calculation of total boron content in polarizing film]
In the following examples and comparative examples, the obtained polarizing films were conditioned at 23°C and 50% RH for 16 hours, and the mass of the polarizing film [E (g)] was measured. Then, the polarizing film was dissolved in 20 mL of distilled water to a concentration of 0.005% by mass. The aqueous solution containing the dissolved polarizing film was used as the measurement sample, and its mass [F (g)] was measured. The boron concentration [G (ppm)] of the measurement sample was measured using a multi-type ICP emission spectrometer (ICP) manufactured by Shimadzu Corporation. Subsequently, the value calculated by substituting the measured value into the following formula was defined as the total boron element content (mass%) in the polarizing film.
Total boron content (mass%) in polarizing film = [(G × ^10⁻⁶ × F) / E] × 100

[Stretching tension in the stretching process]
In the following examples and comparative examples, the stretch tension during the stretching process was measured by the tension between adjacent rolls using a tension roll installed between them. When three or more rolls were used, the maximum stretch tension among them was adopted.

[Drying tension in the drying process]
In the following examples and comparative examples, the drying tension during the drying process was measured by the tension between adjacent rolls using a tension roll installed between them. When three or more rolls were used, the maximum drying tension among them was adopted.

[Example 1]
A roll of PVA film with a thickness of 45 μm was obtained by casting a film-forming stock solution consisting of 100 parts by mass of PVA (saponified vinyl acetate polymer, degree of polymerization 2400, degree of saponification 99.9 mol%), 10 parts by mass of glycerin as a plasticizer, 0.1 parts by mass of sodium polyoxyethylene lauryl ether sulfate as a surfactant, and water. A polarizing film was manufactured by sequentially performing swelling, dyeing, crosslinking, stretching, washing, and drying processes on this PVA film. The width X1 (m) of the unstretched PVA film used to manufacture the polarizing film was 0.65 m.

Specifically, the polarizing film was manufactured as follows: First, in the swelling process, the PVA film was uniaxially stretched in the longitudinal direction (MD direction) to twice its original length (first stage stretching) while immersed in water at 25°C for 90 seconds. Subsequently, in the dyeing process, the PVA film was uniaxially stretched in the longitudinal direction (MD direction) to 2.4 times its original length (second stage stretching) while immersed in an aqueous solution at 32°C containing 0.093% by mass of iodine and 2.14% by mass of potassium iodide (weight ratio of iodine to potassium iodide is 1:23) for 163 seconds. Subsequently, in the crosslinking process, the PVA film was uniaxially stretched in the longitudinal direction (MD direction) to three times its original length (third stage stretching) while immersed in an aqueous solution at 32°C containing 2.6% by mass of boric acid for 135 seconds. In the subsequent stretching process, the PVA film was uniaxially stretched in the longitudinal direction (MD direction) to 6.8 times its original length (fourth stage stretching) while immersed in an aqueous solution at 56°C containing 1.5% by mass of boric acid and 5% by mass of potassium iodide. The maximum stretch tension during the stretching process was 679 N. In the subsequent washing process, the film was washed by immersing it for 10 seconds in an aqueous solution at 22°C containing 1.5% by mass of boric acid and 5.4% by mass of potassium iodide. In the subsequent drying process, a polarizing film with a thickness of 15.6 μm was produced by drying in a dryer at 80°C for 90 seconds. The maximum drying tension during the drying process was 382.5 N.

Using the obtained polarizing film, the transmittance, degree of polarization, shrinkage stress, and total boron content in the polarizing film were measured using the method described above. These evaluation results are shown in Table 1, and the relationship between shrinkage stress and degree of polarization is shown in Figure 1. Furthermore, the relationship between maximum tensile strength and shrinkage stress is shown in Figure 2.

[Examples 2-5 and Comparative Examples 1-9]
Except for changing the boric acid aqueous solution concentration, total stretching ratio, and boric acid aqueous solution temperature in the stretching process as shown in Table 1, changing the drying temperature and drying time in the drying process as shown in Table 1, and appropriately adjusting the iodine concentration in the dyeing bath so that the transmittance of the polarizing film as a single unit was 44%, the stretching tension and drying tension were measured in the same manner as in Example 1, and the polarizing film was manufactured in the same manner as in Example 1. In Examples 2 to 5 and Comparative Examples 1 to 9, the stretching ratio from the first to the third stretching stage was the same as in Example 1, and the fourth stretching stage was changed from Example 1 so that the total stretching ratio was the value shown in Table 1. Subsequently, the transmittance of the single unit, degree of polarization, shrinkage stress, and total amount of boron elements in the polarizing film were evaluated using the method described above. The results are shown in Table 1, and the relationship between shrinkage stress and degree of polarization is shown in Figure 1. Also, the relationship between maximum stretching tension and shrinkage stress is shown in Figure 2.

As shown in Figure 1, in Examples 1 to 5 that satisfy the provisions of the present invention, the polarizing films exhibited low shrinkage force at high temperatures and excellent optical performance.

Furthermore, as shown in Figure 2, in Examples 1 to 3, where the total amount of boron in the polarizing film was low, the relationship between maximum tensile strength and shrinkage stress clearly shifted towards the low shrinkage stress side.

Claims (8)

A method for producing 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,
A method for manufacturing a polarizing film, wherein the boric acid concentration of the aqueous solution in the stretching step is 1% to 3% by mass, the total stretching ratio is 5.5 to 7.4 times, the total neck-in ratio (A) represented by the following formula (1) is 57.5% to 61.0%, the neck-in ratio (B) represented by the following formula (2) in the stretching step is 31.0% to 38.0%, and the neck-in ratio (C) represented by the following formula (3) in the drying step is 9.8% to 16.5%.
Total neck-in ratio (A) = {(X1 - X2) / X1} × 100 (1)
Neck-in ratio (B) = {(Y1 - Y2) / Y1} × 100 (2)
Neck-in ratio (C) = {(Z1 - X2) / Z1} × 100 (3)
[X1 represents the width (m) of the polyvinyl alcohol film before the dyeing process, X2 represents the width (m) of the polyvinyl alcohol film after the drying process, Y1 represents the width (m) of the polyvinyl alcohol film before the stretching process, Y2 represents the width (m) of the polyvinyl alcohol film after the stretching process, and Z1 represents the width (m) of the polyvinyl alcohol film before the drying process.] A method for manufacturing a polarizing film according to claim 1, wherein the neck-in ratio (D) represented by the following formula (4) up to the stretching step is 46.0% to 54.0%.
Neck-in ratio (D) = {(X1 - Y2) / X1} × 100 (4) The method for manufacturing a polarizing film according to claim 2, wherein the difference in neck-in ratio (W), which is the difference between the total neck-in ratio (A) and the neck-in ratio (D) ((A) - (D)), is 8.0 to 11.0%. The method for manufacturing a polarizing film according to claim 3, wherein the ratio of the difference in neck-in ratio (W) to the total neck-in ratio (A) ((W)/(A)) is 0.14 to 0.19. A method for producing a polarizing film according to any one of claims 1 to 4, wherein the total boron content in the polarizing film is 2.0 to 4.0% by mass. A method for manufacturing a polarizing film according to any one of claims 1 to 5, wherein the stretching temperature in the stretching step is 53°C to 70°C. A method for manufacturing a polarizing film according to any one of claims 1 to 6, wherein the drying temperature in the drying step is 60°C to 100°C. A method for manufacturing a polarizing film according to any one of claims 1 to 7, wherein the degree of polarization of the polarizing film is 99.963% or more when the transmittance of the polarizing film alone is 44%, and the shrinkage stress is 100 N/ ^mm² or less.