KR20230073227A - Manufacturing method of polarizing film - Google Patents

Abstract

A method for producing a polarizing film in which at least a swelling process, a dyeing process, a first bridge-stretching process, a second bridge-stretching process, and a third bridge-stretching process are performed in this order on a polyvinyl alcohol film, wherein the polyvinyl alcohol film The thickness is 5 to 100 μm, the average degree of polymerization of the polyvinyl alcohol contained in the polyvinyl alcohol film is 2000 to 4000, and in the swelling step, immersion in water at 10 to 50 ° C. to swell the polyvinyl alcohol film In the dyeing step, immersed in an aqueous solution at 10 to 50 ° C. containing 0.5 to 3 mass% of iodine and potassium iodide in total, and impregnating the polyvinyl alcohol film with an iodine-based dichroic dye, total It is uniaxially stretched so that the draw ratio is 2 to 3 times, and in the first bridge stretching step, the draw ratio during the step is 1.1 to 1.3 times in an aqueous solution at a temperature T ₁ including 1 to 5% by mass of boric acid. It is uniaxially stretched so that the total draw ratio is 2.5 to 3.5 times, and in the second bridge stretching step, the draw ratio during the step is 1.3 to 1.8 in an aqueous solution containing 1 to 5 mass% of boric acid and at a temperature T ₂ . It is also uniaxially stretched so that the total draw ratio is 4 to 6 times, and in the third bridge stretching step, the draw ratio during the step is 1.1 in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₃ . It is a manufacturing method in which uniaxial stretching is performed so that the total stretch ratio is 4.5 to 7 times, and T ₁ , T ₂ and T ₃ satisfy the following formulas (1) and (2). Thereby, even if it is a case where average degree of polymerization of PVA is low, a polarizing film with small shrinkage stress can be obtained, maintaining production stability and maintaining excellent polarization performance.
25 ≤ T ₁ ≤ 45 (1)
T ₁ < T ₂ < T ₃ ≤ 75 (2)

Description

Manufacturing method of polarizing film

The present invention relates to a method for producing a polarizing film having low shrinkage stress made of a polyvinyl alcohol film containing an iodine-based dichroic dye.

A polarizing film used for a polarizing plate having functions of transmitting and shielding light is a basic component of a liquid crystal display (LCD). Most polarizing plates have a structure in which a protective film such as a cellulose triacetate (TAC) film is bonded to the surface of a polarizing film, and as a polarizing film, a polyvinyl alcohol film (hereinafter “polyvinyl alcohol” (sometimes referred to as "PVA") is mainly a stretched film in which a dichroic dye such as iodine-based dye (I ₃ ^- or I ₅ ^- ) is adsorbed to a stretched film obtained by uniaxially stretching and orienting the film. Such a polarizing film is manufactured by the method of performing a swelling process, a dyeing process, a crosslinking process, an extending process, a fixation process, and a drying process etc. with respect to a PVA film.

In recent years, LCDs have been widely used in mobile applications such as notebook computers and mobile phones. Such LCDs for mobile devices are used under various environments. Therefore, a polarizing film having low shrinkage stress at high temperatures and excellent dimensional stability is required.

In Patent Document 1, in order to reduce the shrinkage stress of the polarizing film, in the first cross-linking stretching step, after stretching in a 50 ° C. aqueous solution containing boric acid, in the second cross-linking stretching step, containing boric acid and potassium iodide What was stretched in a 65 degreeC aqueous solution is described.

WO 2017/138551

However, in the method described in Patent Literature 1, since the first cross-linking and stretching step is performed at a high temperature of 50 ° C. after the dyeing step, when the average degree of polymerization of PVA contained in the PVA film is low, the stretching temperature is increased in the second cross-linking and stretching step. When raised, breakage of the film occurred, and it was sometimes difficult to lower the shrinkage stress while maintaining the polarization performance. In addition, iodine adsorbed to the film in the dyeing step is eluted in the first cross-linking and stretching step, and it is difficult to adjust the transmittance to a desired level in some cases. In addition, as a result of the eluted iodine coloring the boric acid aqueous solution in the first bridge-stretching step, the PVA film was colored in the first bridge-stretching step, making it difficult to maintain production stability in some cases.

The present invention has been made in order to solve the above problems, and even when the average degree of polymerization of PVA is low, while maintaining production stability and maintaining excellent polarization performance, it is an object of providing a method for producing a polarizing film with small shrinkage stress will be.

The manufacturing method of the polarizing film of the present invention,

[1] A method for producing a polarizing film in which at least a swelling step, a dyeing step, a first cross-linking stretching step, a second cross-linking stretching step, and a third cross-linking stretching step are performed in this order on a polyvinyl alcohol film,

The thickness of the polyvinyl alcohol film is 5 to 100 μm,

The average degree of polymerization of the polyvinyl alcohol contained in the polyvinyl alcohol film is 2000 to 4000,

In the swelling step, the polyvinyl alcohol film is immersed in water at 10 to 50 ° C. to swell,

In the above dyeing step, while immersing in an aqueous solution at 10 to 50 ° C. containing 0.5 to 3 mass% of iodine and potassium iodide in total, and impregnating the polyvinyl alcohol film with an iodine-based dichroic dye, total draw ratio uniaxial stretching so that this is 2 to 3 times,

In the first crosslinking stretching step, uniaxially stretching so that the draw ratio during the step is 1.1 to 1.3 times and the total draw ratio is 2.5 to 3.5 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₁ . ,

In the second bridge stretching step, uniaxial stretching is performed so that the draw ratio during the step is 1.3 to 1.8 times and the total draw ratio is 4 to 6 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₂ . do,

In the third crosslinking and stretching step, uniaxial stretching is performed so that the draw ratio in the step is 1.1 to 1.3 times and the total draw ratio is 4.5 to 7 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature of T ₃ . do,

A method for producing a polarizing film in which T ₁ , T ₂ and T ₃ satisfy the following formulas (1) and (2);

25 ≤ T ₁ ≤ 45 (1)

T ₁ < T ₂ < T ₃ ≤ 75 (2)

[2] The method for producing a polarizing film according to [1], wherein T ₂ and T ₃ satisfy the following formulas (3) and (4);

50 ≤ T ₂ ≤ 65 (3)

55 ≤ T ₃ ≤ 75 (4)

[3] A fourth cross-linking stretching step is performed after the third cross-linking stretching step, and in the fourth cross-linking stretching step, in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₄ , stretching during the step It is uniaxially stretched so that the magnification is 1.1 to 1.3 times and the total draw ratio is 5 to 8 times, and T ₁ , T ₂ , T ₃ and T ₄ satisfy the following formula (5), [1] or [2] A method for producing a polarizing film of

T ₁ < T ₂ < T ₃ ≤ T ₄ ≤ 75 (5)

[4] The method for producing a polarizing film according to [3], wherein T ₄ satisfies the following formula (6);

60 ≤ T ₄ ≤ 75 (6)

[5] The method for producing a polarizing film according to [3] or [4], wherein the maximum stretching stress is 10 N/mm ² or less in the fourth bridge-stretching step;

[6] The method for producing a polarizing film according to [1] to [5], wherein a polarizing film having a shrinkage stress of 50 N/mm ² or less is obtained;

[7] The method for producing a polarizing film according to [1] to [6], wherein a polarizing film having a polarization degree of 99.80% or more when the single transmittance is 43.5% is obtained.

By providing, it is achieved.

According to the production method of the present invention, even when the average degree of polymerization of PVA is low, a polarizing film having a small shrinkage stress can be obtained while maintaining production stability and maintaining excellent polarization performance.

1 : is a schematic diagram which shows an example of a polarizing film manufacturing apparatus.
2 : is a schematic diagram which shows another example of a polarizing film manufacturing apparatus.
3 : is a schematic diagram which shows another example of a polarizing film manufacturing apparatus.
4 : is a schematic diagram which shows another example of a polarizing film manufacturing apparatus.

This invention is a manufacturing method of the polarizing film which performs at least a swelling process, a dyeing process, a 1st crosslinking-stretching process, a 2nd crosslinking-stretching process, and a 3rd crosslinking-stretching process in this order with respect to a PVA film. Among them, the temperature T ₁ in the first cross-linking and stretching step, the temperature T ₂ in the second cross-linking and stretching step, and the temperature T ₃ in the third cross-linking and stretching step are expressed by the following formulas (1) and (2) It is particularly important to satisfy

25 ≤ T ₁ ≤ 45 (1)

T ₁ < T ₂ < T ₃ ≤ 75 (2)

As can be seen from the comparison between Examples described later and Comparative Examples, in Comparative Example 3 in which the temperature T ₁ is 50° C. and does not satisfy Formula (1), iodine adsorbed to the PVA film by the dyeing step is first bridged and stretched. Since it was eluted in the step, the boric acid aqueous solution in the first cross-linking and stretching step was colored, and it was difficult to maintain production stability. Moreover, in Comparative Examples 1-3 in which temperature _T2 and temperature _T3 do not satisfy Formula (2) which is the same temperature, the fall of the shrinkage stress of the polarizing film obtained was insufficient. In contrast, in Examples 1 to 3 satisfying equations (1) and (2) in which the temperature T ₁ is 32°C, the temperature T ₂ is approximately 61°C, and the temperature T ₃ is approximately 64°C, excellent polarization performance is maintained. It became clear that a polarizing film having a small shrinkage stress can be manufactured while doing so. Therefore, the significance of the present invention employing such a method is great.

PVA contained in the PVA film of the fabric used for manufacture of the polarizing film of this invention can use what was obtained by saponifying polyvinyl ester obtained by superposing|polymerizing 1 type(s) or 2 or more types of vinyl esters. Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropenyl acetate. Among these, , From the viewpoints of ease of manufacture of PVA, ease of availability, cost and the like, vinyl acetate is preferred.

The polyvinyl ester may be obtained by using only one or two or more vinyl esters as monomers, but within a range that does not impair the effect of the present invention, one or two or more vinyl esters and copolymerizable with this. A copolymer with another monomer may be sufficient.

As another monomer copolymerizable with vinyl ester, For example, C2-C30 alpha-olefins, such as ethylene, propylene, 1-butene, and isobutene; (meth)acrylic acid or its salt; Methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t- (meth)acrylate (meth)acrylic acid esters such as butyl, 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 (meth)acrylamide derivatives such as salts thereof, (meth)acrylamide propyldimethylamine or salts thereof, N-methylol (meth)acrylamide or derivatives thereof; N-vinylamides such as N-vinylformamide, N-vinylacetamide, and N-vinylpyrrolidone; Vinyl 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 ether; Vinyl cyanide, 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 salt, ester or acid anhydride; itaconic acid or its salt, ester or acid anhydride; vinylsilyl compounds such as vinyltrimethoxysilane; An unsaturated sulfonic acid etc. are mentioned. Said polyvinyl ester may have a structural unit derived from 1 type, or 2 or more types of said other monomers. As said other monomer, alpha-olefin is preferable, and ethylene is especially preferable.

The proportion of structural units derived from other monomers in the polyvinyl ester is preferably 15 mol% or less, more preferably 10 mol% or less, based on the number of moles of all structural units constituting the polyvinyl ester. More preferably, it is mol% or less.

In particular, when the other monomer is a monomer likely to promote the water solubility of the obtained PVA, such as (meth)acrylic acid and unsaturated sulfonic acid, in order to prevent the PVA from dissolving in the manufacturing process of the polarizing film, polyvinyl The ratio of structural units derived from these monomers in the ester is preferably 5 mol% or less, more preferably 3 mol% or less, based on the number of moles of all structural units constituting the polyvinyl ester.

As long as the PVA used in the present invention is within a range that does not impair the effects of the present invention, it may be modified by one type or two or more types of graft-copolymerizable monomers. Examples of the monomer capable of graft copolymerization include unsaturated carboxylic acids or derivatives thereof; Unsaturated sulfonic acid or its derivative(s); C2-C30 alpha-olefin etc. are mentioned. The proportion of structural units (structural units in the graft-modified portion) derived from monomers capable of graft copolymerization in PVA is preferably 5 mol% or less based on the number of moles of all structural units constituting PVA.

In the PVA, a part of the hydroxyl groups may or may not be crosslinked. Moreover, in said PVA, a part of the hydroxyl group may react with aldehyde compounds, such as acetaldehyde and butyraldehyde, etc., and may form an acetal structure.

The polymerization degree of the said PVA exists in the range of 2000-4000. When the degree of polymerization is 2000 or more, it is possible to extend the PVA film without breaking even if it extends at a high temperature in the second crosslinking stretching step. As for the said degree of polymerization, it is more preferable that it is 2200 or more. On the other hand, when the said degree of polymerization is 4000 or less, the shrinkage stress of the polarizing film obtained can be made small. The degree of polymerization is more preferably 3500 or less, still more preferably 3000 or less, and particularly preferably less than 2500. The polymerization degree of PVA in this specification means the average polymerization degree measured according to description of JISK6726-1994. Moreover, although the PVA in a polarizing film contains the crosslinked structure by boron compounds, such as boric acid, when it dissociates by hydrolyzing a boric acid ester, there is no substantial change in the average polymerization degree itself of PVA.

The degree of saponification of PVA is preferably 98 mol% or more, more preferably 98.5 mol% or more, and still more preferably 99 mol% or more, from the viewpoint of the polarization performance of the polarizing film and the like. When the degree of saponification is less than 98 mol%, PVA is easily eluted during the production process of the polarizing film, and the eluted PVA adheres to the film to deteriorate the polarizing performance of the polarizing film in some cases. In addition, the saponification degree of PVA in this specification is the vinyl alcohol unit relative to the total number of moles of the structural unit (typically a vinyl ester unit) and the vinyl alcohol unit that can be converted into a vinyl alcohol unit by saponification, which the PVA has. refers to the ratio (mol%) occupied by the number of moles of The degree of saponification can be measured according to the description of JIS K6726-1994. In addition, the saponification degree is substantially the same as the PVA in a raw film and the PVA in the obtained polarizing film.

It is preferable that the content rate of PVA in the PVA film used by this invention exists in the range of 50-99 mass % from the ease of manufacture of a desired polarizing film, etc. As for the said content rate, it is more preferable that it is 75 mass % or more, it is still more preferable that it is 80 mass % or more, and it is especially preferable that it is 85 mass % or more. Moreover, it is more preferable that it is 98 mass % or less, it is more preferable that it is 96 mass % or less, and it is especially preferable that it is 95 mass % or less.

It is preferable that a PVA film contains a plasticizer from a viewpoint of improving the ductility at the time of extending|stretching it. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, tetraethylene glycol, and trimethylolpropane. One or two or more of may be included. Among these, glycerol is preferable from the viewpoint of the effect of improving the stretchability.

It is preferable that content of the plasticizer in a PVA film exists in the range of 1-20 mass parts with respect to 100 mass parts of PVA contained therein. When the said content is 1 mass part or more, the ductility of a PVA film can be improved more. On the other hand, when the said content is 20 parts by mass or less, it can prevent that a PVA film becomes too soft and handling property falls. The content of the plasticizer in the PVA film is more preferably 2 parts by mass or more, more preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more with respect to 100 parts by mass of PVA. Moreover, it is more preferable that it is 15 parts by mass or less, and, as for content of a plasticizer, it is still more preferable that it is 12 parts by mass or less. In addition, although it differs according to the manufacturing conditions of a polarizing film, etc., since the plasticizer contained in a PVA film may elute when manufacturing a polarizing film, it cannot be said that the whole amount remains in a polarizing film.

The PVA film may further contain components such as an antioxidant, an antifreezing agent, a pH adjuster, a masking agent, an anticoloring agent, an oil agent, and a surfactant, as needed.

The thickness of the PVA film used in the manufacturing method of this invention is 5-100 micrometers. When thickness is 100 micrometers or less, a thin polarizing film is easily obtained. It is preferable that the thickness of a PVA film is 60 micrometers or less. In addition, the thickness of the PVA film is preferably 20 μm or more, and more preferably 30 μm or more, in order to ensure handleability. On the other hand, when thickness is less than 5 micrometers, manufacture of a polarizing film becomes difficult, and dyeing nonuniformity becomes easy to generate|occur|produce. It is preferable that the thickness of the PVA film is 7 μm or more. The thickness here refers to the thickness of the PVA layer in the case of a multilayer film.

A single layer film may be sufficient as a PVA film, and a multilayer film which has a PVA layer and a base resin layer may be used. In the case of a single-layer film, the thickness of the film is preferably within the above range. On the other hand, in the case of a multilayer film, the thickness of the PVA layer may be 20 μm or less, or 15 μm or less. The thickness of the base resin layer in the multilayer film is usually 20 to 500 µm.

As a PVA film, when using the multilayer film which has a PVA layer and a base resin layer, base resin must be what can be stretched together with PVA. Polyester, polyolefin resin, etc. can be used. Especially, an amorphous polyester resin is preferable, and a polyethylene terephthalate or an amorphous polyester resin obtained by copolymerizing a copolymerization component such as isophthalic acid or 1,4-cyclohexanedimethanol therein is preferably used. It is preferable to prepare a multilayer film by applying a PVA solution to a base resin film. At this time, in order to improve the adhesiveness between the PVA layer and the base resin layer, the surface of the base resin film may be modified or an adhesive layer may be formed between the two layers.

Although the shape of the PVA film is not particularly limited, it is preferably a long PVA film from the viewpoint of continuously supplying when manufacturing a polarizing film. The length of the long PVA film (length in the direction of a long picture) is not particularly limited, and can be appropriately set depending on the use of the polarizing film to be produced, for example, within the range of 5 to 20,000 m.

The width of the PVA film is not particularly limited and can be appropriately set according to the purpose of the polarizing film to be produced. In recent years, since the screen size of liquid crystal televisions and liquid crystal monitors is progressing, it is optimal for these uses when the width|variety of a PVA film is 0.5 m or more, More preferably, it is 1 m or more. On the other hand, if the width of the PVA film is too wide, it is preferred that the width of the PVA film is 7 m or less from the point in which it tends to be difficult to stretch uniformly when producing a polarizing film with a device that has been put into practical use.

The polarizing film of this invention is manufactured using the PVA film demonstrated above as a raw material. Specifically, a polarizing film is manufactured by performing at least a swelling process, a dyeing process, a 1st bridge-stretching process, a 2nd bridge-stretching process, and a 3rd bridge-stretching process in this order. It is also preferable to perform a washing process or a drying process after the said 3rd bridge|crosslinking extending|stretching process. Hereinafter, each process is demonstrated in detail.

In the production method of the present invention, the PVA film is first subjected to a swelling step. In a swelling process, it immerses in 10-50 degreeC water and swells a PVA film. It is preferable that the temperature of water is 20 degreeC or more, and it is also preferable that it is 40 degreeC or less. By immersing in water within such a temperature range, the PVA film can be efficiently and uniformly swollen. It is preferable that it is the range for 0.1 to 5 minutes, and, as for the time to immerse a PVA film in water, it is more preferable that it is the range for 0.5 to 3 minutes. By setting it as such an immersion time, a PVA film can be swollen efficiently and uniformly. In addition, the water in which the PVA film is immersed is not limited to pure water, and may be an aqueous solution in which various components are dissolved, or a mixture of water and a water-soluble organic solvent. In the swelling process, it is preferable to uniaxially stretch the PVA film. Although the draw ratio in that case is not specifically limited, It is preferable that it is 1.2 to 2.8 times. As for the said draw ratio, it is more preferable that it is 1.5 times or more, and it is also more preferable that it is 2.5 times or less.

In the production method of the present invention, the dyeing step is provided after the swelling step. In the dyeing step, while immersing in a 10 to 50°C aqueous solution containing 0.5 to 3 mass% of iodine and potassium iodide in total and impregnating the PVA film with an iodine dichroic dye, the total draw ratio is 2 to 3 times It is uniaxially stretched as much as possible. Thereby, while dyeing a PVA film with an iodine dichroic dye, the molecular chain of PVA in a film is orientated, and an iodine dichroic dye is also orientated.

Dyeing is performed by immersing the PVA film in a dyeing bath containing an iodine dichroic dye. A dyeing bath is prepared by mixing iodine (I ₂ ) and potassium iodide (KI) with water. By mixing iodine and potassium iodide with water, iodine dichroic dyes such as I ₃ ^- and I ₅ ^- can be generated. The total content of iodine and potassium iodide in the dyeing bath is 0.5 to 3% by mass in their total. It is preferable that it is 0.8 mass % or more, and, as for the total content of iodine and potassium iodide, it is also preferable that it is 2.5 mass % or less. By dyeing in such a concentration range, it is possible to efficiently and uniformly dye. The mass ratio of potassium iodide to iodine (KI/I ₂ ) is preferably from 10 to 200, and more preferably from 15 to 150. The dyeing bath may contain boron compounds such as borates such as boric acid and borax, but the content thereof is usually less than 5% by mass in terms of boric acid, and preferably 1% by mass or less.

The temperature of the dyeing bath is 10 to 50°C. The temperature is preferably 15°C or higher, and more preferably 20°C or higher. Moreover, it is preferable that it is 40 degrees C or less, and, as for the said temperature, it is more preferable that it is 35 degrees C or less. By dyeing within such a temperature range, the PVA film can be efficiently and uniformly dyed. Moreover, as time to immerse a PVA film in a dyeing bath, it is preferable that it is the range for 0.1 to 10 minutes, and it is more preferable that it is the range for 0.2 to 5 minutes. By setting it as such a range of time, a PVA film can be dyed unevenly.

In the dyeing step, the PVA film is dyed and uniaxially stretched so that the total draw ratio is 2 to 3 times. With respect to the PVA film which has such a total draw ratio, it is possible to obtain a polarizing film having excellent polarization performance and low shrinkage stress by subsequently performing at least three stages of crosslinking and stretching steps. What is necessary is just to make it 2-3 times the total draw ratio which passed through the process so far including a swelling process and a dyeing process. The draw ratio in the dyeing process should just exceed 1 time, and it is more preferable that it is 1.05 times or more.

In the production method of the present invention, after the dyeing step, the first cross-linking and stretching step, the second cross-linking and stretching step, and the third cross-linking and stretching step are provided. By carrying out the cross-linking and stretching process in three steps under different conditions, the crystal state and orientation state of the obtained polarizing film can be controlled, and a polarizing film having excellent polarization performance and low shrinkage stress can be obtained. As will be described later, it is a preferred embodiment to perform the fourth bridge-stretching process after the third bridge-stretching process. Hereinafter, these four cross-linking and stretching steps are described.

In the first bridge stretching step, in an aqueous solution containing 1 to 5% by mass of boric acid and having a temperature T ₁ of 25 to 45°C, the draw ratio during the step is 1.1 to 1.3 times and the total draw ratio is 2.5 to 3.5 times. axial stretching The boric acid aqueous solution in which the PVA film is immersed contains 1 to 5% by mass of boric acid. It is preferable that the concentration of boric acid is 1.5 mass % or more, and it is also preferable that it is 4 mass % or less. By setting such a concentration, the intermolecular crosslinking reaction by boric acid can proceed at an appropriate rate. In addition, boric acid should just be what can become boric acid or a boric acid ion in aqueous solution, and either boric acid or borate may be used, but boric acid is preferable. The concentration in the case of using a borate is calculated in terms of mass of boric acid (H ₃ BO ₃ ). The boric acid aqueous solution may contain potassium iodide, and the concentration in that case is preferably in the range of 0.01 to 10% by mass. By containing potassium iodide, the polarization performance of the polarizing film obtained can be adjusted. Potassium iodide may be included in the first bridge-stretching step, potassium iodide may be included in the second to fourth bridge-stretching steps described later, or may be included in all the steps.

The temperature T ₁ of the aqueous solution containing boric acid in the first bridge-stretching step is 25 to 45°C. That is, the temperature T ₁ satisfies the following expression (1).

25 ≤ T ₁ ≤ 45 (1)

As can be seen from the comparison between Examples described later and Comparative Examples, in Comparative Example 3 in which the temperature T ₁ is 50° C. and does not satisfy Formula (1), iodine adsorbed to the PVA film by the dyeing step is first bridged and stretched. Since it was eluted in the step, the boric acid aqueous solution in the first cross-linking and stretching step was colored, and it was difficult to maintain production stability. On the other hand, in the present invention in which the temperature T ₁ is within this range, even when the average degree of polymerization of PVA is low, a polarizing film having a small shrinkage stress can be produced while maintaining excellent polarization performance. When the temperature T ₁ is less than 25°C, progress of the crosslinking reaction by boric acid becomes insufficient, and there is a possibility that the polarization performance of the polarizing film obtained may decrease. On the other hand, when the temperature T ₁ exceeds 45 ° C., iodine adsorbed on the PVA film in the dyeing step is eluted, so that the boric acid aqueous solution in the first cross-linking and stretching step is colored, and it may be difficult to maintain production stability. . The temperature T ₁ is preferably 28°C or higher, and more preferably 30°C or higher. Further, the temperature T ₁ is preferably 40°C or less, and more preferably 38°C or less. And in the said temperature range, it uniaxially stretches so that a draw ratio may become 1.1 to 1.3 times and a total draw ratio of 2.5 to 3.5 times. It is preferable that it is 2.6 times or more, and, as for total draw ratio, it is also preferable that it is 3.4 times or less. Thus, in the 1st bridge|crosslinking extending|stretching process, a boric acid crosslinking reaction advances, carrying out uniaxial stretching only a little and orientating suitably, maintaining temperature _T1 in the said range. As a result, even when immersed in a high-temperature boric acid aqueous solution after the subsequent second cross-linking stretching step, PVA is not eluted from the film in the boric acid aqueous solution or the strength of the film is not greatly reduced, and it can be stretched at a higher magnification.

Subsequently, in the second cross-linking and stretching step, the draw ratio during the step is in an aqueous solution containing 1 to 5% by mass of boric acid and having a temperature T ₂ that exceeds the temperature T ₁ of the first cross-linking and stretching step and is less than 75°C. 1.3 to 1.8 times, and uniaxial stretching is performed so that the total stretching ratio is 4 to 6 times. The composition of the aqueous solution containing boric acid to be used may be the same as that within the range used in the first cross-linking and stretching step.

In the second bridge-stretching step, the temperature T ₂ of the aqueous solution containing boric acid exceeds the temperature T ₁ in the first bridge-stretching step and is less than 75°C. The temperature T ₂ is preferably 50°C or higher. Moreover, it is preferable that it is 70 degrees C or less, and, as for the said temperature _T2 , it is more preferable that it is 65 degrees C or less. When the temperature is too low, the shrinkage stress increases, while when the temperature is too high, PVA is eluted from the film in an aqueous solution of boric acid, or the degree of polarization is reduced. And in the said temperature range, it uniaxially stretches so that a draw ratio may become 1.3 to 1.8 times and a total draw ratio to 4 to 6 times. It is preferable that it is 1.4 times or more, and, as for the draw ratio in a 2nd bridge|crosslinking process, it is also preferable that it is 1.7 times or less. Moreover, as for total draw ratio, it is preferable that it is 4.1 times or more, and it is also preferable that it is 5.9 times or less. That is, in an aqueous solution containing boric acid at high temperature, the boric acid crosslinking reaction proceeds while stretching at a relatively high magnification, and as a result, in the next third crosslinking stretching step, elution of PVA from the film into the aqueous solution of boric acid or breakage can be prevented. can

Subsequently, in the third cross-linking and stretching step, in an aqueous solution containing 1 to 5% by mass of boric acid and having a temperature T ₃ that exceeds the temperature T ₂ of the second cross-linking and stretching step and is 75° C. or less, the draw ratio during the step is 1.1 to 1.3 times, and uniaxial stretching is performed so that the total stretching ratio is 4.5 to 7 times. The composition of the aqueous solution containing boric acid to be used may be the same as that within the range used in the first cross-linking and stretching step. Further, as shown in FIG. 1 , a partition plate or the like may be formed in one tank to the extent that the temperature T ₂ and the temperature T ₃ can be set, and the second cross-linking and stretching step and the third cross-linking and stretching step may be performed. In FIG. 3 As shown, you may implement the 2nd bridge|crosslinking extension process and the 3rd bridge|crosslinking extension process in another tank.

In the third bridge-stretching step, the temperature T ₃ of the aqueous solution containing boric acid exceeds the temperature T ₂ in the second bridge-stretching step and is 75°C or lower. That is, temperatures T ₁ , T ₂ and T ₃ satisfy the following formula (2).

T ₁ < T ₂ < T ₃ ≤ 75 (2)

As can be seen from the comparison between Examples and Comparative Examples described later, in Comparative Examples 1 to 3 in which the temperature T ₂ and the temperature T ₃ do not satisfy the equation (2) at the same temperature, the decrease in shrinkage stress of the polarizing film obtained is was insufficient. In contrast, in Examples 1 to 3 satisfying equation (2) in which the temperature T ₁ is 32°C, the temperature T ₂ is approximately 61°C, and the temperature T ₃ is approximately 64°C, the shrinkage stress is reduced while maintaining excellent polarization performance. It has become clear that small polarizing films can be produced. Therefore, in the manufacturing method of the present invention, it is important to satisfy the above formula (2).

The temperature T ₃ is preferably 55°C or higher, and more preferably 58°C or higher. Moreover, it is preferable that it is 75 degrees C or less, and, as for the said temperature _T3 , it is more preferable that it is 72 degrees C or less. When the temperature is too low, the shrinkage stress increases, while when the temperature is too high, PVA is eluted from the film in an aqueous solution containing boric acid, or the degree of polarization is reduced. And in the said temperature range, it uniaxially stretches so that a draw ratio may become 1.1 to 1.3 times and a total draw ratio of 4.5 to 7 times. The film stretched in the third cross-linking stretching step is subjected to a boric acid cross-linking reaction while being stretched at a relatively high magnification in a high-temperature boric acid-containing aqueous solution, thereby preventing PVA from eluting from the film into the boric acid-containing aqueous solution or breaking. can do.

As the above T ₂ and T ₃ , those satisfying the following formulas (3) and (4) are preferred embodiments.

50 ≤ T ₂ ≤ 65 (3)

55 ≤ T ₃ ≤ 75 (4)

In the production method of the present invention, it is a preferred embodiment to perform a fourth cross-linking and stretching step after the third cross-linking and stretching step, and in the fourth cross-linking and extending step, 1 to 5% by mass of boric acid is included, and the temperature T In the aqueous solution of ₄ , uniaxial stretching is performed so that the draw ratio during the step is 1.1 to 1.3 times and the total draw ratio is 5 to 8 times, and T ₁ , T ₂ , T ₃ and T ₄ satisfy the following formula (5) It is desirable to do The composition of the aqueous solution containing boric acid to be used may be the same as that within the range used in the first cross-linking and stretching step. Further, as shown in FIG. 2 , a partition plate or the like is formed in one tank to such an extent that the temperature T ₂ , the temperature T ₃ , and the temperature T ₄ can be set, and the second bridge stretching step, the third bridge stretching step, and the fourth bridge bridge An extending process may be performed, and as shown in FIG. 4, the 2nd cross-linking and extending process, the 3rd bridge-stretching process, and the 4th bridge-stretching process may be performed in another tank.

T ₁ < T ₂ < T ₃ ≤ T ₄ ≤ 75 (5)

In the fourth cross-linking and stretching step, the temperature T ₄ of the aqueous solution containing boric acid is preferably equal to or higher than the temperature T ₃ and 75°C or lower in the third cross-linking and extending step. It may be the same temperature as the temperature T ₃ of the third bridge-stretching step. The temperature T ₄ is preferably 60°C or higher, and more preferably 62°C or higher. Moreover, as for the said temperature _T4 , it is more preferable that it is 74 degreeC or less. When the temperature is too low, the shrinkage stress increases, while when the temperature is too high, PVA is eluted from the film in an aqueous solution containing boric acid, or the degree of polarization is reduced. And in the said temperature range, it uniaxially stretches so that it may become 1.1 to 1.3 times and a total draw ratio of 5 to 8 times. The film stretched in the fourth cross-linking stretching step is subjected to a boric acid cross-linking reaction while being stretched at a relatively high magnification in an aqueous solution containing boric acid at a high temperature, thereby preventing PVA from eluting from the film into an aqueous solution containing boric acid or breaking. can do.

As the above T ₄ , one that satisfies the following formula (6) is a preferable embodiment.

60 ≤ T ₄ ≤ 75 (6)

In the fourth bridge stretching step, it is preferable that the maximum stretching stress is 10 N/mm ² or less. Here, the maximum stretching stress is a value obtained by dividing the tensile stress applied between adjacent rolls by the cross-sectional area of the PVA film as a raw material in the fourth cross-linking stretching step. By making the maximum stretching stress small, a polarizing film having a small shrinkage stress can be obtained. The maximum stretching stress is preferably 8 N/mm ² or less, more preferably 5 N/mm ² or less, still more preferably 4 N/mm ² or less. Also, usually, the maximum stretching stress is 1 N/mm ² or more.

In the first to fourth cross-linking stretching steps, when the PVA film is uniaxially stretched, the boric acid aqueous solution in the first to fourth cross-stretching steps is prepared by using a stretching device equipped with a plurality of rolls parallel to each other, It can be implemented by changing the circumferential speed between rolls.

It is preferable to use in a washing|cleaning process after the said 3rd bridge|crosslinking extension process or the said 4th bridge|crosslinking extension process. In the cleaning process, unnecessary chemicals and foreign substances on the surface of the film are removed, or the optical performance of the finally obtained polarizing film is adjusted. The washing process can be performed by immersing the PVA film in a washing bath or spraying a washing liquid on the PVA film. Although water can be used as a cleaning liquid, you may contain potassium iodide in these. When potassium iodide is contained, the color tone of a polarizing film can be adjusted. It is preferable that content of potassium iodide is 0.1-10 mass %. The temperature of the washing liquid is usually 10 to 40°C, preferably 15 to 30°C. The washing bath may be used not only in one layer but also in multiple baths. In the case of using a plurality of tanks, the composition of the washing liquid in each tank can be adjusted according to the purpose.

Subsequent to the washing process, it is preferable to use the drying process. The temperature in the drying step is not particularly limited, but is preferably 30 to 150°C, more preferably 50 to 130°C. A polarizing film excellent in dimensional stability is easily obtained by drying at a temperature within the above range.

It is preferable that the thickness of the polarizing film obtained by this invention is 1-30 micrometers. When the thickness is less than 1 µm, it is sometimes difficult to produce at high speed, and more preferably 3 µm or more. On the other hand, when the thickness exceeds 30 µm, the stretching tension at the time of the stretching process increases and the device may be damaged. More preferably, it is 25 µm or less. The thickness here refers to the thickness of the PVA layer in the case of a multilayer film.

When the obtained polarizing film is a single layer film of PVA, in order to ensure handling properties, it is preferable that the thickness of the polarizing film is 5 μm or more, and it is more preferable that it is 7 μm or more. On the other hand, in the case of a polarizing film made of a multilayer film, the thickness of the polarizing film layer may be 5 μm or less, or may be 3 μm or less. The thickness of the base resin layer in the multilayer film is usually 10 to 250 µm.

It is preferable that the single transmittance of the polarizing film obtained by this invention is 42 to 45 %. When the single transmittance is less than 42%, the brightness of the liquid crystal display decreases. The single transmittance is more preferably 42.5% or more. On the other hand, in a polarizing film having a single transmittance of more than 45%, it is difficult to obtain a polarizing film having a high polarization degree, and the single transmittance is more preferably 44.5% or less. Moreover, it is preferable that the polarization degree of the polarizing film of this invention is 99.80 % or more. The degree of polarization is more preferably 99.90% or more.

It is preferable that the shrinkage stress of the polarizing film obtained by this invention is 50 N/mm ^<2> or less. When shrinkage stress is small, even when it is used under high temperature, it becomes a thing excellent in dimensional stability. The shrinkage stress is more preferably 42 N/mm ² or less, still more preferably 38 N/mm ² or less, and particularly preferably 35 N/mm ² or less. Here, shrinkage stress refers to a value obtained by dividing the tension obtained by fixing a polarizing film serving as a sample and holding at 80°C for 4 hours by the cross-sectional area of the sample.

Further, the polarizing film obtained in the present invention preferably has a "polarization degree when the single transmittance is 43.5%" of 99.80% or more, more preferably 99.90% or more, still more preferably 99.95% or more, and particularly preferably 99.98% or more. desirable. This value calculates the polarization degree at the time of assuming that it is 43.5 %, when single transmittance (T) of a polarizing film is not 43.5 %.

The calculation method of "polarization degree when single transmittance is 43.5%" is as follows. First, the relationship between the transmittance (T') excluding surface reflection and the single transmittance (T) is expressed by equation (7). At this time, the refractive index of PVA was assumed to be 1.5, and the reflectance on the surface was assumed to be 4%. The relationship between transmittance (T'), polarization degree (V), and dichroic ratio (R) is expressed by formula (8), and formula (9) is a modification of formula (8). Here, the dichroic ratio (R) hardly fluctuates with the dye concentration in the range where the single transmittance (T) does not fluctuate greatly, for example, in the range of 42 to 45%, so it can be treated as a constant. . Therefore, after measuring the single transmittance (T) and the degree of polarization (V), the dichroic ratio (R) of the polarizing film can be calculated as an integer by solving equations (7) and (8) using these values. The degree of polarization (V) at T = 43.5 (%) can be obtained from equation (9) and equation (7) in which R is substituted.

T' = T/(1 - 0.04) ² (7)

R = {-ln[T'(1 - V)]}/{-ln[T'(1 + V)]} (8)

T' = [1 - V] ^1/(R-1) /[1 + V] ^R/(R-1) (9)

The polarizing film obtained in the present invention is usually used as a polarizing plate by bonding a protective film to both surfaces or single side thereof. Examples of the protective film include those that are optically transparent and have mechanical strength. Specifically, for example, cellulose triacetate (TAC) film, cellulose acetate-butyrate (CAB) film, acrylic film, polyester film, etc. can be used. Moreover, as an adhesive agent for bonding, a PVA type adhesive agent, a urethane type adhesive agent, or an ultraviolet curable adhesive agent etc. are mentioned.

The polarizing plate obtained in this way can be used for a high-performance liquid crystal display (LCD). It is possible to provide a polarizing plate that is bright, has good polarization properties, and has excellent dimensional stability even when used under high temperature conditions. Therefore, it can be preferably used as a polarizing plate for various high-performance LCDs, especially LCDs for mobile applications.

Example

Hereinafter, the present invention will be specifically described by examples, but the present invention is not limited at all by these examples. Evaluation methods and the like were implemented according to the methods shown below.

[Optical Performance of Polarizing Film]

A rectangular measurement sample of 4 cm in the machine flow direction (MD direction) of the polarizing film and 1.5 cm in the width direction (TD direction) of the polarizing film obtained in the following Examples and Comparative Examples from the center of the polarizing film in the width direction (TD direction) was collected. For this measurement sample, a spectrophotometer with an integrating sphere ("V7100" manufactured by Nippon Spectroscopy Co., Ltd.) was used to correct the visibility in the visible region of a C light source and a 2° field of view in accordance with JIS Z8722 (method for measuring object color). Next, single transmittance and polarization were measured. The degree of polarization at a single transmittance of 43.5% was calculated by the method described above.

[Shrinkage stress of polarizing film]

Shrinkage stress was measured using Autograph AG-X with a thermostat manufactured by Shimadzu Corporation. For the measurement, a polarizing film (length direction 15 cm, width direction 1.5 cm) humidified at 20°C/20%RH for 18 hours was mounted on a chuck (chuck interval 5 cm), and the temperature of the thermostat was raised to 80 ° C. at the same time as the start of tension. Initiated. The polarizing film was stretched at a rate of 1 mm/min, the tension was stopped when the tension reached 2 N, and the tension was measured from that state until 4 hours later. At this time, since the distance between chucks changes due to the thermal expansion of the shaft, the distance between chucks can be corrected by attaching a gauge seal to the chuck and moving the gauge seal attached to the chuck using the video-type extensometer TR ViewX120S. The measurement was carried out in such a way that In addition, the value obtained by subtracting the initial tension 2 N from the measured tension value after 4 hours was used as the shrinkage force of the polarizing film, and the value obtained by dividing the value by the cross-sectional area of the sample was defined as the shrinkage stress (N/mm ² ).

[Maximum Stretching Stress in the Fourth Bridge-stretching Step]

In the following Examples and Comparative Examples, the film width (mm) and film thickness (mm) of the PVA film (unprocessed raw film) used for production of the polarizing film are obtained in advance, and the cross-sectional area (mm ² ) is calculated by multiplying them. did Next, in the 4th bridge|crosslinking and stretching process at the time of manufacturing a polarizing film, the tensile stress (N) applied between adjacent rolls was measured with the tensiometer. By dividing the obtained tensile stress (N) by the cross-sectional area (mm ² ) obtained above, the maximum stretching stress (N/mm ² ) in the fourth bridge-stretching step was obtained.

[Coloration of boric acid aqueous solution in the first cross-linking and stretching step]

Regarding the coloration of the boric acid aqueous solution in the first cross-linking stretching step, 20 cc of the boric acid aqueous solution 1 hour after starting the continuous stretching was sampled in a 30 cc screw pipe, visually confirmed, and the color change compared to the boric acid aqueous solution before continuous stretching The case where it was not was marked as ○, and the case where the color was changed and there was iodine staining was marked as ×.

[Example 1]

100 parts by mass of PVA (saponified product of vinyl acetate polymer, degree of polymerization 2400, degree of saponification 99.9 mol%, ethylene modification 2.0 mol%), 10 parts by mass of glycerin as a plasticizer, 0.1 part by mass of sodium polyoxyethylene lauryl sulfate as a surfactant, and water A roll of a PVA film having a thickness of 45 μm was obtained by performing cast film formation using the film forming stock solution. With respect to this PVA film, a swelling process, a dyeing process, a 1st cross-linking-stretching process, a 2nd bridge-stretching process, a 3rd bridge-stretching process, a 4th bridge-stretching process, a washing|cleaning process, and a drying process are sequentially performed, A polarizing film is manufactured did The schematic diagram of a polarizing film manufacturing apparatus is shown to FIGS. 1-4.

Specifically, a polarizing film was manufactured as follows. First, in the swelling step, while the PVA film was immersed in water at a temperature of 30°C for 1.6 minutes, it was uniaxially stretched (first stage stretching) in the longitudinal direction (MD) twice the original length. Subsequently, in the dyeing step, uniaxial stretching in the longitudinal direction (MD) up to 2.5 times the original length while being immersed in an aqueous solution at a temperature of 32 ° C. containing 0.06 mass% of iodine and 1.4 mass% of potassium iodide for 2.3 minutes ( 2nd stage stretching). Subsequently, in the first bridge stretching step, uniaxial stretching (3 first stretch). Subsequently, in the second cross-linking stretching step, while being immersed in an aqueous solution containing 2.8% by mass of boric acid and 5% by mass of potassium iodide at a temperature of 60.6°C, it is stretched to 4.46 times the original length in the longitudinal direction (MD). uniaxial stretching (4th stage stretching) was performed. Subsequently, in the third cross-linking stretching step, while being immersed in an aqueous solution containing boric acid at a concentration of 2.8% by mass and potassium iodide at a concentration of 5% by mass, at a temperature of 63.6°C, it is stretched to 5.27 times the original length in the longitudinal direction (MD). uniaxial stretching (fifth stage stretching) was carried out. Subsequently, in the fourth bridge stretching step, while immersed in an aqueous solution containing boric acid at a concentration of 2.8% by mass and potassium iodide at a concentration of 5% by mass and having a temperature of 64.6°C, up to 6 times the original length in the longitudinal direction (MD). uniaxial stretching (stretching in the sixth stage) was performed. The maximum stretching stress in the fourth bridge stretching step was 1.4 N/mm ² . Subsequently, in the washing step, the film was washed by immersing for 10 seconds in an aqueous solution at a temperature of 22°C containing 1.5% by mass of boric acid and 5% by mass of potassium iodide. Subsequently, in the drying process, a polarizing film having a thickness of 14.0 μm was manufactured by drying for 120 seconds with an 80°C dryer. Table 1 shows the conditions of each bridge-stretching process and the value of the maximum stretching stress in the fourth bridge-stretching process, and Table 2 shows the evaluation results.

[Examples 2 to 3, Comparative Examples 1 to 3]

The temperature of the aqueous solution containing boric acid in the first cross-linking and stretching step, the temperature and total draw ratio of the aqueous solution containing boric acid in the second cross-linking and stretching step, and the aqueous solution containing boric acid in the third cross-linking and stretching step A polarizing film was manufactured in the same manner as in Example 1, except that the temperature, the total draw ratio, and the temperature and total draw ratio of the aqueous solution containing boric acid in the fourth bridge stretching step were changed as shown in Table 1. Table 1 shows the conditions of each bridge-stretching process and the value of the maximum stretching stress in the fourth bridge-stretching process, and Table 2 shows the evaluation results. Moreover, Table 3 shows the evaluation result of Example 3 and Comparative Example 2 whose total draw ratio is 6.6 times. In addition, Examples 1 and 2 and Comparative Examples 1 and 3 in Table 2 are all examples in which the total draw ratio is 6 times.

1: Polarizing film manufacturing device
2: PVA film roll
3: Swelling process
4: Dyeing process
5: 1st Bridge Stretching Step
6: 2nd Bridge Stretching Step
7: 3rd Bridge Stretching Step
8: 4th Bridge-stretching Step
9: cleaning process
10: Drying process

Claims (7)

A method for producing a polarizing film in which at least a swelling step, a dyeing step, a first bridge-stretching process, a second bridge-stretching process, and a third bridge-stretching process are performed in this order on a polyvinyl alcohol film,
The thickness of the polyvinyl alcohol film is 5 to 100 μm,
The average degree of polymerization of the polyvinyl alcohol contained in the polyvinyl alcohol film is 2000 to 4000,
In the swelling step, the polyvinyl alcohol film is immersed in water at 10 to 50 ° C. to swell,
In the above dyeing step, while immersing in an aqueous solution at 10 to 50 ° C. containing 0.5 to 3 mass% of iodine and potassium iodide in total, and impregnating the polyvinyl alcohol film with an iodine-based dichroic dye, total draw ratio uniaxial stretching so that this is 2 to 3 times,
In the first cross-linking stretching step, uniaxially stretching so that the draw ratio during the step is 1.1 to 1.3 times and the total draw ratio is 2.5 to 3.5 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₁ . ,
In the second bridge stretching step, uniaxial stretching is performed so that the draw ratio during the step is 1.3 to 1.8 times and the total draw ratio is 4 to 6 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₂ . do,
In the third crosslinking and stretching step, uniaxial stretching is performed so that the draw ratio in the step is 1.1 to 1.3 times and the total draw ratio is 4.5 to 7 times in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature of T ₃ . do,
The manufacturing method of the polarizing film in which said _T1 , _T2, and _T3 satisfy Formula (1) and (2) below.
25 ≤ T ₁ ≤ 45 (1)
T ₁ < T ₂ < T ₃ ≤ 75 (2) According to claim 1,
The manufacturing method of the polarizing film in which the said _T2 and _T3 satisfy the following formulas (3) and (4).
50 ≤ T ₂ ≤ 65 (3)
55 ≤ T ₃ ≤ 75 (4) According to claim 1 or 2,
A fourth cross-linking and stretching step is performed after the third cross-linking and stretching step, and in the fourth cross-linking and stretching step, a draw ratio in the step is 1.1 in an aqueous solution containing 1 to 5% by mass of boric acid at a temperature T ₄ . to 1.3 times and uniaxially stretching so that the total stretch ratio is 5 to 8 times, and T ₁ , T ₂ , T ₃ and T ₄ satisfy the following formula (5), a method for producing a polarizing film.
T ₁ < T ₂ < T ₃ ≤ T ₄ ≤ 75 (5) According to claim 3,
The manufacturing method of the polarizing film in which said _T4 satisfies the following formula (6).
60 ≤ T ₄ ≤ 75 (6) According to claim 3 or 4,
A method for producing a polarizing film having a maximum stretching stress of 10 N/mm ² or less in the fourth bridge-stretching step. According to any one of claims 1 to 5,
A method for producing a polarizing film, wherein a polarizing film having a shrinkage stress of 50 N/mm ² or less is obtained. According to any one of claims 1 to 6,
The manufacturing method of the polarizing film which obtains the polarizing film whose polarization degree in case single transmittance is 43.5 % is 99.80 % or more.

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