Method for manufacturing a polarizing film

Abstract

A method for manufacturing a polarizing film, comprising at least the following steps sequentially: a swelling step, a dyeing step, a first crosslinking and stretching step, a second crosslinking and stretching step, and a third crosslinking and stretching step for a polyvinyl alcohol film. The polyvinyl alcohol film has a thickness of 5–100 μm and an average degree of polymerization of 2000–4000 for the polyvinyl alcohol contained in the film. In the swelling step, the polyvinyl alcohol film is swelled by immersion in water at 10–50°C. In the dyeing step, the film is immersed in an aqueous solution containing a total of 0.5–3% by mass of iodine and potassium iodide at 10–50°C to allow the iodine-based dichroic pigment to penetrate the polyvinyl alcohol film. Simultaneously, uniaxial stretching is performed to achieve a total stretch ratio of 2–3 times. In the first crosslinking and stretching step… Uniaxial stretching is performed in an aqueous solution containing 1-5% by mass boric acid at temperature T1, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 2.5-3.5 times. In the aforementioned second crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass boric acid at temperature T2, so that the stretching ratio in this step is 1.3-1.8 times and the total stretching ratio is 4-6 times. In the aforementioned third crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass boric acid at temperature T3, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 4.5-7 times. The above T1, T2 and T3 satisfy the following equations (1) and (2). Thus, even when the average degree of polymerization of PVA is low, production stability can be maintained, and a polarizing film with excellent polarization performance and low shrinkage stress can be obtained. 25≤T1≤45 (1), T1<T2<T3≤75 (2).

Description

Technical Field

[0001] This invention relates to a method for manufacturing a polarizing film with low shrinkage stress formed from a polyvinyl alcohol film containing an iodine-based dichroic pigment. Background Technology

[0002] The polarizing film used in polarizing plates, which have both light transmission and shielding functions, is a fundamental component of liquid crystal displays (LCDs). Many polarizing plates have a structure in which a protective film, such as a triacetate cellulose (TAC) film, is laminated onto the surface of the polarizing film. The mainstream polarizing film is a stretched film made by uniaxially stretching a polyvinyl alcohol (PVA) film (hereinafter sometimes referred to as "PVA") to orient it, onto which iodine-based pigments (I3) are adsorbed. - I5 - Polarizing films obtained by using dichroic pigments such as (etc.). Such polarizing films can be manufactured by performing swelling, dyeing, cross-linking, stretching, immobilization and drying processes on PVA films.

[0003] In recent years, LCDs have been widely used in mobile applications such as laptops and mobile phones. LCDs used in these mobile devices are used in various environments. Therefore, there is a demand for polarizing films with low shrinkage stress at high temperatures and excellent dimensional stability.

[0004] Patent Document 1 describes a process in which, in order to reduce the shrinkage stress of the polarizing film, the film is stretched in an aqueous solution containing boric acid at 50°C during the first crosslinking stretching step, and then stretched in an aqueous solution containing boric acid and potassium iodide at 65°C during the second crosslinking stretching step.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: WO2017 / 138551 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] However, in the method described in Patent Document 1, since the first crosslinking and stretching process is performed at a high temperature of 50°C after the dyeing process, if the average degree of polymerization of the PVA contained in the PVA film is low, the following situation occurs: when the stretching temperature is increased in the second crosslinking and stretching process, the film breaks, making it difficult to maintain polarization properties and reduce shrinkage stress. Furthermore, there is a possibility that iodine adsorbed on the film during the dyeing process dissolves in the first crosslinking and stretching process, making it difficult to adjust to the desired transmittance. Moreover, the dissolved iodine causes coloring of the boric acid aqueous solution in the first crosslinking and stretching process, resulting in coloring of the PVA film in the first crosslinking and stretching process, making it difficult to maintain production stability.

[0010] This invention was made to solve the above-mentioned problems, and its purpose is to provide a method for manufacturing a polarizing film that maintains production stability and excellent polarization performance, while having low shrinkage stress, even when the average degree of polymerization of PVA is low.

[0011] Solution for solving the problem

[0012] The method for manufacturing the polarizing film of the present invention is achieved by providing the following manufacturing method.

[0013] [1] A method for manufacturing a polarizing film, wherein the polyvinyl alcohol film is subjected to at least the following steps in sequence: swelling step, dyeing step, first crosslinking stretching step, second crosslinking stretching step, and third crosslinking stretching step, wherein,

[0014] The thickness of the aforementioned polyvinyl alcohol film is 5–100 μm.

[0015] The polyvinyl alcohol contained in the aforementioned polyvinyl alcohol film has an average degree of polymerization of 2000–4000.

[0016] In the aforementioned swelling process, the polyvinyl alcohol film is swelled by immersion in water at 10–50°C.

[0017] In the aforementioned dyeing process, the polyvinyl alcohol film is immersed in an aqueous solution containing a total of 0.5–3% by mass of iodine and potassium iodide at a temperature of 10–50°C, allowing the iodine-based dichroic pigment to penetrate the film. Simultaneously, uniaxial stretching is performed to achieve a total stretch ratio of 2–3 times.

[0018] In the aforementioned first crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T1, so that the stretching ratio in this step is 1.1 to 1.3 times and the total stretching ratio is 2.5 to 3.5 times.

[0019] In the aforementioned second crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T2, so that the stretching ratio in this step is 1.3-1.8 times and the total stretching ratio is 4-6 times.

[0020] In the aforementioned third crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T3, so that the stretching ratio in this step is 1.1 to 1.3 times and the total stretching ratio is 4.5 to 7 times.

[0021] The above T1, T2 and T3 satisfy the following equations (1) and (2),

[0022] 25≤T1≤45 (1)

[0023] T1<T2<T3≤75 (2);

[0024] [2] According to the manufacturing method of the polarizing film in [1], wherein T2 and T3 satisfy the following equations (3) and (4),

[0025] 50≤T2≤65 (3)

[0026] 55≤T3≤75 (4);

[0027] [3] According to the manufacturing method of polarizing film of [1] or [2], a fourth cross-linking stretching step is performed after the aforementioned third cross-linking stretching step. In the aforementioned fourth cross-linking stretching step, uniaxial stretching is performed in an aqueous solution containing 1 to 5% by mass of boric acid at temperature T4, so that the stretching ratio in this step is 1.1 to 1.3 times and the total stretching ratio is 5 to 8 times. T1, T2, T3 and T4 satisfy the following formula (5).

[0028] T1<T2<T3≤T4≤75 (5);

[0029] [4] According to the manufacturing method of the polarizing film in [3], wherein the above T4 satisfies the following equation (6),

[0030] 60≤T4≤75 (6);

[0031] [5] According to the manufacturing method of the polarizing film of [3] or [4], wherein, in the aforementioned fourth crosslinking stretching step, the maximum tensile stress is 10 N / mm. 2 the following;

[0032] [6] According to the manufacturing method of the polarizing film in [1] to [5], the shrinkage stress is obtained to be 50 N / mm. 2 The following polarizing films;

[0033] [7] According to the manufacturing method of polarizing film in [1] to [6], a polarizing film with a polarization degree of 99.80% or more when the monomer transmittance is 43.5% is obtained.

[0034] The effects of the invention

[0035] According to the manufacturing method of the present invention, even when the average degree of polymerization of PVA is low, production stability can be maintained, and a polarizing film with low shrinkage stress while maintaining excellent polarization performance can be obtained.

[0036] Brief description of the attached diagram

[0037] Figure 1 This is a schematic diagram illustrating an example of a polarizing film manufacturing apparatus.

[0038] Figure 2This is a schematic diagram illustrating another example of a polarizing film manufacturing apparatus.

[0039] Figure 3 This is a schematic diagram illustrating another example of a polarizing film manufacturing apparatus.

[0040] Figure 4 This is a schematic diagram illustrating another example of a polarizing film manufacturing apparatus. Detailed Implementation

[0041] This invention is a method for manufacturing a polarizing film, wherein for a PVA film, at least the following steps are performed sequentially: a swelling step, a dyeing step, a first crosslinking stretching step, a second crosslinking stretching step, and a third crosslinking stretching step. Particularly important is that the temperatures T1 in the first crosslinking stretching step, T2 in the second crosslinking stretching step, and T3 in the third crosslinking stretching step satisfy the following equations (1) and (2).

[0042] 25≤T1≤45 (1)

[0043] T1<T2<T3≤75 (2)

[0044] As can be seen from the comparison of the examples and comparative examples described later, in Comparative Example 3, where temperature T1 is 50°C and formula (1) is not satisfied, the iodine adsorbed on the PVA film during the dyeing process dissolves in the first crosslinking and stretching process, resulting in coloring of the boric acid aqueous solution in the first crosslinking and stretching process, making it difficult to maintain production stability. Furthermore, in Comparative Examples 1-3, where temperatures T2 and T3 are the same and formula (2) is not satisfied, the reduction in shrinkage stress of the obtained polarizing film is insufficient. In contrast, in Examples 1-3, where temperatures T1 is 32°C, T2 is approximately 61°C, and T3 is approximately 64°C, formulas (1) and (2) are satisfied, it is evident that polarizing films maintaining excellent polarization performance while exhibiting low shrinkage stress can be manufactured. Therefore, the present invention employing such a method is of great significance.

[0045] The PVA contained in the PVA film used in the manufacture of the polarizing film of the present invention can be PVA obtained by saponifying polyethylene ester obtained by polymerizing one or more ethylene esters. Examples of such ethylene esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl tert-carbonate, vinyl laurate, vinyl stearate, vinyl benzoate, and isopropylene acetate. Among these ethylene esters, vinyl acetate is preferred from the viewpoints of ease of manufacturing, ease of obtaining, and cost of PVA.

[0046] Polyethylene ester can be a polyethylene ester obtained by using only one or two or more ethylene esters as monomers, but it can also be a copolymer of one or more ethylene esters with other monomers that can copolymerize therewith, provided that it does not impair the effects of the present invention.

[0047] Other monomers that can copolymerize with vinyl esters include, for example, α-olefins with 2 to 30 carbon atoms such as ethylene, propylene, 1-butene, and isobutene; (meth)acrylic acid or its salts; (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, dodecyl (meth)acrylate, and octadecyl (meth)acrylate; (meth)acrylamide; N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, diacetone (meth)acrylamide, (meth)acrylamide propanesulfonic acid or its salts; and (meth)acrylic acid. The following are types of vinyl esters: (methyl)acrylamide propyl dimethylamine or its salts, N-hydroxymethyl (meth)acrylamide or its derivatives, etc. (meth)acrylamide derivatives; N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, etc., N-vinylamides; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, tert-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc., vinyl ethers; (meth)acrylonitrile or other cyanide; vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, etc., haloethylene; allyl acetate, allyl chloride, etc., allyl compounds; maleic acid or its salts, esters or anhydrides; itaconic acid or its salts, esters or anhydrides; vinyltrimethoxysilane or other vinylsilyl compounds; unsaturated sulfonic acids, etc. The above-mentioned polyethylene esters may have one or more structural units derived from the aforementioned other monomers. As the aforementioned other monomers, α-olefins are preferred, with ethylene being particularly preferred.

[0048] The proportion of structural units derived from other monomers in polyethylene ester is based on the total number of moles of all structural units constituting polyethylene ester, preferably 15 mol% or less, more preferably 10 mol% or less, and even more preferably 5 mol% or less.

[0049] In particular, when the other monomer is a monomer such as (meth)acrylic acid or unsaturated sulfonic acid that has the potential to promote the water solubility of the resulting PVA, in order to prevent the PVA from dissolving during the manufacturing process of the polarizing film, the proportion of structural units derived from these monomers in the polyethylene ester is preferably 5 mol% or less, more preferably 3 mol% or less, based on the total number of moles of all structural units constituting the polyethylene ester.

[0050] The PVA used in this invention can be a substance modified by one or more graft copolymerizable monomers, provided that it does not impair the effects of this invention. Examples of such graft copolymerizable monomers include, for instance, unsaturated carboxylic acids or their derivatives; unsaturated sulfonic acids or their derivatives; and α-olefins with 2 to 30 carbon atoms. The proportion of structural units derived from graft copolymerizable monomers in the PVA (structural units in the graft-modified portion) is based on the total number of moles of all structural units constituting the PVA, and is preferably 5 mol% or less.

[0051] Regarding the aforementioned PVA, some of its hydroxyl groups may be cross-linked or not. Furthermore, some of the hydroxyl groups in the aforementioned PVA can react with aldehyde compounds such as acetaldehyde and butyraldehyde to form an acetal structure.

[0052] The aforementioned degree of polymerization of PVA is in the range of 2000 to 4000. By setting this degree of polymerization to 2000 or higher, the PVA film can be stretched without breaking even when stretched at high temperatures during the second crosslinking stretching process. This degree of polymerization is more preferably 2200 or higher. On the other hand, by setting this degree of polymerization to 4000 or lower, the shrinkage stress of the resulting polarizing film can be reduced. This degree of polymerization is more preferably 3500 or lower, further preferably 3000 or lower, and particularly preferably less than 2500. The degree of polymerization of PVA in this specification refers to the average degree of polymerization measured according to JIS K6726-1994. It should be noted that the PVA in the polarizing film contains a crosslinked structure formed by boron compounds such as boric acid, but if it is dissociated by hydrolyzing borate esters, etc., there is no substantial change to the average degree of polymerization of the PVA itself.

[0053] From the viewpoint of polarization performance of the polarizing film, the degree of saponification of PVA is preferably 98 mol% or more, more preferably 98.5 mol% or more, and even more preferably 99 mol% or more. When the degree of saponification is less than 98 mol%, PVA is easily dissolved during the manufacturing process of the polarizing film. The dissolved PVA adheres to the film, potentially reducing the polarization performance of the polarizing film. It should be noted that the degree of saponification of PVA in this specification refers to the proportion (mol%) of the total number of moles of the structural units (typically ethylene ester units) and ethylene alcohol units that can be converted into ethylene alcohol units through saponification in PVA. The degree of saponification can be measured according to JIS K6726-1994. It should be noted that the degree of saponification is substantially the same for PVA in the original reflective film and PVA in the obtained polarizing film.

[0054] From the viewpoint of ease of manufacturing the desired polarizing film, the PVA content in the PVA film used in this invention is preferably in the range of 50 to 99% by mass. More preferably, it is 75% by mass or more, further preferably 80% by mass or more, and particularly preferably 85% by mass or more. Furthermore, it is more preferably 98% by mass or less, further preferably 96% by mass or less, and particularly preferably 95% by mass or less.

[0055] From the viewpoint of improving the tensile strength of PVA film when stretched, it is preferable to include a plasticizer. Examples of such plasticizers include polyols such as ethylene glycol, glycerol, propylene glycol, diethylene glycol, diglycerol, triethylene glycol, tetraethylene glycol, and trimethylolpropane. The PVA film may contain one or more of these plasticizers. Of these, glycerol is preferred from the viewpoint of improving tensile strength.

[0056] The content of plasticizer in the PVA film is preferably in the range of 1 to 20 parts by mass relative to the 100 parts by mass of PVA contained therein. By setting this content to 1 part by mass or more, the tensile strength of the PVA film can be further improved. On the other hand, by setting this content to 20 parts by mass or less, the PVA film can be prevented from becoming too soft and reducing its operability. The content of plasticizer in the PVA film is more preferably 2 parts by mass or more relative to the 100 parts by mass of PVA, further preferably 4 parts by mass or more, and particularly preferably 5 parts by mass or more. Furthermore, the content of plasticizer is more preferably 15 parts by mass or less, and further preferably 12 parts by mass or less. It should be noted that, although depending on the manufacturing conditions of the polarizing film, the plasticizer contained in the PVA film dissolves during the manufacturing of the polarizing film, so not all of it remains in the polarizing film.

[0057] PVA membranes may also contain antioxidants, antifreeze, pH adjusters, masking agents, colorants, oils, surfactants, etc., as needed.

[0058] The thickness of the PVA film used in the manufacturing method of the present invention is 5 to 100 μm. By making the thickness 100 μm or less, a thin polarizing film can be easily obtained. The thickness of the PVA film is preferably 60 μm or less. In addition, to ensure processability, the thickness of the PVA film is preferably 20 μm or more, more preferably 30 μm or more. On the other hand, when the thickness is less than 5 μm, it is difficult to manufacture the polarizing film, and uneven dyeing is easily generated. The thickness of the PVA film is preferably 7 μm or more. Here, the thickness refers to the thickness of the PVA layer in the case of multilayer films.

[0059] The PVA film can be a single-layer film or a multilayer film having a PVA layer and a substrate resin layer. In the case of a single-layer film, the film thickness is preferably within the range described above. On the other hand, in the case of a multilayer film, the thickness of the PVA layer can be 20 μm or less, or even 15 μm or less. The thickness of the substrate resin layer in a multilayer film is typically 20–500 μm.

[0060] When using a multilayer film having a PVA layer and a substrate resin layer as a PVA film, the substrate resin must be a material capable of being stretched together with the PVA. Polyester, polyolefin resins, etc., can be used. Among these, amorphous polyester resin is preferred, and polyethylene terephthalate (PET) with copolymers of isophthalic acid, 1,4-cyclohexanediol, etc., is suitable. It is preferable to manufacture the multilayer film by coating a PVA solution onto the substrate resin film. In this case, to improve the adhesion between the PVA layer and the substrate resin layer, the surface of the substrate resin film may be modified, or an adhesive layer may be formed between the two layers.

[0061] There are no particular limitations on the shape of the PVA film, but a long strip of PVA film is preferred from the perspective of continuous supply during the manufacture of polarizing film. The length (length in the longitudinal direction) of the long strip of PVA film is not particularly limited and can be appropriately set according to the application of the polarizing film being manufactured, for example, it can be set in the range of 5 to 20,000 m.

[0062] There is no particular limitation on the width of the PVA film, and it can be appropriately set according to the intended use of the polarizing film. In recent years, LCD TVs and LCD monitors have been moving towards larger screen sizes, so setting the width of the PVA film to 0.5m or more, preferably 1m or more, is most suitable for these applications. On the other hand, if the width of the PVA film is too wide, there is a tendency for it to be difficult to stretch the polarizing film evenly when manufacturing it in a practical device. Therefore, a width of 7m or less for the PVA film is preferred.

[0063] The polarizing film of the present invention is manufactured using the PVA film described above as a raw material. Specifically, the polarizing film is manufactured by performing at least the following steps in sequence: swelling, dyeing, first crosslinking and stretching, second crosslinking and stretching, and third crosslinking and stretching. After the aforementioned third crosslinking and stretching step, a washing step and a drying step are preferably performed. Each step will be described in detail below.

[0064] In the manufacturing method of the present invention, a PVA film is first subjected to a swelling process. In the swelling process, the PVA film is swelled by immersion in water at a temperature of 10–50°C. The water temperature is preferably 20°C or higher, and more preferably 40°C or lower. Immersion in water within this temperature range allows the PVA film to swell efficiently and uniformly. The immersion time of the PVA film in water is preferably in the range of 0.1–5 minutes, more preferably 0.5–3 minutes. This immersion time allows the PVA film to swell efficiently and uniformly. It should be noted that the water used to immerse the PVA film is not limited to pure water, but can be an aqueous solution containing various dissolved components, or a mixture of water and a water-soluble organic solvent. In the swelling process, uniaxial stretching of the PVA film is preferably performed. The stretching ratio is not particularly limited in this case, but is preferably 1.2–2.8 times. This stretching ratio is more preferably 1.5 times or higher, and even more preferably 2.5 times or less.

[0065] In the manufacturing method of the present invention, the material is fed into a dyeing step after the aforementioned swelling step. In the dyeing step, the PVA film is immersed in an aqueous solution containing a total of 0.5 to 3% by mass of iodine and potassium iodide at 10 to 50°C, allowing the iodine-based dichroic dye to permeate the PVA film. Simultaneously, uniaxial stretching is performed to achieve a total stretch ratio of 2 to 3 times. This dyes the PVA film with the iodine-based dichroic dye, simultaneously orienting the PVA molecular chains in the film and also orienting the iodine-based dichroic dye.

[0066] Staining is performed by immersing the PVA film in a staining bath containing an iodine-based dichroic dye. The staining bath is prepared by mixing iodine (I₂) and potassium iodide (KI) with water. By mixing iodine and potassium iodide with water, I₃⁻ can be generated. - I5 - Iodine-based dichroic pigments, etc. The total content of iodine and potassium iodide in the staining bath is 0.5% to 3% by mass. The total content of iodine and potassium iodide is preferably 0.8% by mass or more, and more preferably 2.5% by mass or less. Staining within this concentration range allows for efficient and uniform staining. The mass ratio of potassium iodide to iodine (KI / I₂) is preferably 10 to 200, more preferably 15 to 150. The staining bath may contain boron compounds such as borates, borax, etc., but their content is generally less than 5% by mass (converted to boric acid), and preferably less than 1% by mass.

[0067] The temperature of the dyeing bath is 10–50°C. This temperature is preferably 15°C or higher, more preferably 20°C or higher. Furthermore, this temperature is preferably 40°C or lower, more preferably 35°C or lower. By performing dyeing within this temperature range, the PVA film can be dyed efficiently and uniformly. Additionally, the immersion time of the PVA film in the dyeing bath is preferably in the range of 0.1–10 minutes, more preferably 0.2–5 minutes. By setting this time range, the PVA film can be dyed without unevenness.

[0068] In the dyeing process, the PVA film is dyed while simultaneously undergoing uniaxial stretching to achieve a total stretch ratio of 2 to 3 times. By subsequently performing at least three stages of crosslinking and stretching processes on the PVA film with such a total stretch ratio, a polarizing film with excellent polarization properties and low shrinkage stress can be obtained. A total stretch ratio of 2 to 3 times is sufficient for the processes up to this point, including the swelling and dyeing processes. The stretch ratio in the dyeing process only needs to exceed 1 time, and more preferably 1.05 times or more.

[0069] In the manufacturing method of the present invention, the film is subjected to a first crosslinking stretching step, a second crosslinking stretching step, and a third crosslinking stretching step after the aforementioned dyeing step. By performing the three crosslinking stretching steps with different conditions, the crystallization state and orientation state of the obtained polarizing film can be controlled, and a polarizing film with excellent polarization performance and low shrinkage stress can be obtained. As described below, a suitable embodiment is to perform a fourth crosslinking stretching step after the third crosslinking stretching step. These four crosslinking stretching steps will be described below.

[0070] In the first crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at a temperature T1 of 25-45°C, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 2.5-3.5 times. The boric acid aqueous solution impregnating the PVA film contains 1-5% by mass of boric acid. The concentration of boric acid is preferably 1.5% by mass or more, and more preferably 4% by mass or less. By setting such a concentration, the intermolecular crosslinking reaction generated by boric acid can proceed at an appropriate rate. It should be noted that any substance that can be boric acid or borate ions in aqueous solution can be used; either boric acid or borate can be used, but boric acid is suitable. When using borate, the concentration is calculated using the mass conversion of boric acid (H3BO3). The boric acid aqueous solution may contain potassium iodide, and in this case, the concentration is preferably in the range of 0.01-10% by mass. By containing potassium iodide, the polarization properties of the resulting polarization film can be adjusted. Potassium iodide may be included in the first crosslinking stretching step, or in the second to fourth crosslinking stretching steps described later, or in all the steps.

[0071] The temperature T1 of the aqueous solution containing boric acid in the first crosslinking stretching process is 25–45°C. That is, the temperature T1 satisfies the following equation (1).

[0072] 25≤T1≤45 (1)

[0073] As can be seen from the comparison of the examples and comparative examples described later, in Comparative Example 3, where the temperature T1 is 50°C and formula (1) is not satisfied, the iodine adsorbed on the PVA film during the dyeing process dissolves in the first crosslinking stretching process, causing the boric acid aqueous solution in the first crosslinking stretching process to become colored, making it difficult to maintain production stability. In contrast, in the present invention where the temperature T1 is within this range, even when the average degree of polymerization of PVA is low, it is possible to manufacture a polarizing film that maintains excellent polarization performance while having low shrinkage stress. When the temperature T1 is less than 25°C, there is concern that the crosslinking reaction generated by boric acid may become insufficient, resulting in a decrease in the polarization performance of the obtained polarizing film. On the other hand, when the temperature T1 exceeds 45°C, there is concern that the iodine adsorbed on the PVA film during the dyeing process may dissolve, causing the boric acid aqueous solution in the first crosslinking stretching process to become colored, making it difficult to maintain production stability. The temperature T1 is preferably 28°C or higher, more preferably 30°C or higher. In addition, the temperature T1 is preferably 40°C or lower, more preferably 38°C or lower. Furthermore, uniaxial stretching is performed within the aforementioned temperature range to achieve a stretching ratio of 1.1 to 1.3 times and a total stretching ratio of 2.5 to 3.5 times. The total stretching ratio is preferably 2.6 times or more, and more preferably 3.4 times or less. Thus, in the first crosslinking stretching step, the temperature T1 is maintained within the aforementioned range, and uniaxial stretching is performed slightly to allow for proper orientation while the boric acid crosslinking reaction occurs. Consequently, even after immersion in a high-temperature boric acid aqueous solution in the subsequent second crosslinking stretching step, PVA will not dissolve from the film into the boric acid aqueous solution, nor will the film strength decrease significantly, enabling high-ratio stretching.

[0074] Next, in the second crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1 to 5% by mass of boric acid, at a temperature T2 exceeding the temperature T1 of the first crosslinking stretching step but less than 75°C, so that the stretching ratio in this step is 1.3 to 1.8 times and the total stretching ratio is 4 to 6 times. The composition of the aqueous solution containing boric acid can be the same as that used in the first crosslinking stretching step.

[0075] In the second crosslinking stretching step, the temperature T2 of the aqueous solution containing boric acid is higher than the temperature T1 of the first crosslinking stretching step but lower than 75°C. This temperature T2 is preferably 50°C or higher. Furthermore, this temperature T2 is preferably 70°C or lower, more preferably 65°C or lower. If the temperature is too low, the shrinkage stress increases; on the other hand, if the temperature is too high, PVA dissolves from the film into the boric acid aqueous solution, or the polarization degree decreases. Moreover, uniaxial stretching is performed within the aforementioned temperature range to achieve a stretching ratio of 1.3 to 1.8 times and a total stretching ratio of 4 to 6 times. The stretching ratio in the second crosslinking step is preferably 1.4 times or higher, and more preferably 1.7 times or lower. Furthermore, the total stretching ratio is preferably 4.1 times or higher, and more preferably 5.9 times or lower. That is, by performing the boric acid crosslinking reaction while stretching at a high ratio in a high-temperature aqueous solution containing boric acid, the PVA can be prevented from dissolving from the film into the boric acid aqueous solution or from breaking during the subsequent third crosslinking stretching step.

[0076] Following this, in the third crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid, at a temperature T3 exceeding the temperature T2 of the second crosslinking stretching step but below 75°C, to achieve a stretching ratio of 1.1-1.3 times and a total stretching ratio of 4.5-7 times. The composition of the boric acid-containing aqueous solution can be the same as that used in the first crosslinking stretching step. It should be noted that... Figure 1 As shown, the second and third crosslinking stretching processes are performed by setting partitions or the like in one tank to allow setting temperatures T2 and T3, or as shown... Figure 3 As shown, the second and third crosslinking stretching processes are carried out in their respective tanks.

[0077] In the third crosslinking stretching process, the temperature T3 of the aqueous solution containing boric acid exceeds the temperature T2 of the second crosslinking stretching process but is below 75°C. That is, temperatures T1, T2, and T3 satisfy the following equation (2).

[0078] T1<T2<T3≤75 (2)

[0079] As can be seen from the comparison between the embodiments and comparative examples described later, in Comparative Examples 1 to 3, where temperature T2 and temperature T3 are the same and equation (2) is not satisfied, the reduction in shrinkage stress of the obtained polarizing film is insufficient. In contrast, in Examples 1 to 3, where equation (2) is satisfied at temperature T1 of 32°C, temperature T2 of approximately 61°C, and temperature T3 of approximately 64°C, it is clearly possible to manufacture a polarizing film that maintains excellent polarization performance while having low shrinkage stress. Therefore, satisfying the above equation (2) is important in the manufacturing method of the present invention.

[0080] The temperature T3 is preferably 55°C or higher, more preferably 58°C or higher. Furthermore, the temperature T3 is preferably 75°C or lower, more preferably 72°C or lower. If the temperature is too low, the shrinkage stress increases; on the other hand, if the temperature is too high, PVA dissolves from the film into the aqueous solution containing boric acid, or the polarization degree decreases. Moreover, uniaxial stretching is performed within the aforementioned temperature range to achieve a stretching ratio of 1.1 to 1.3 times and a total stretching ratio of 4.5 to 7 times. For films stretched in the third crosslinking stretching step, performing the boric acid crosslinking reaction while stretching at a high ratio in a high-temperature aqueous solution containing boric acid can prevent PVA from dissolving from the film into the aqueous solution containing boric acid or from breaking.

[0081] As for T2 and T3 above, a suitable implementation is to satisfy the following equations (3) and (4).

[0082] 50≤T2≤65 (3)

[0083] 55≤T3≤75 (4)

[0084] In the manufacturing method of the present invention, a suitable embodiment is to perform a fourth crosslinking stretching step after the aforementioned third crosslinking stretching step. In the aforementioned fourth crosslinking stretching step, uniaxial stretching is preferably performed in an aqueous solution containing 1-5% by mass of boric acid at a temperature T4, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 5-8 times, and T1, T2, T3, and T4 satisfy the following formula (5). The composition of the aqueous solution containing boric acid used can be the same as the composition used in the first crosslinking stretching step. It should be noted that it can be as follows... Figure 2 As shown, by setting partitions in one tank to allow setting temperatures T2, T3, and T4, the second, third, and fourth crosslinking stretching processes are performed, or as shown... Figure 4 As shown, the second, third, and fourth cross-linking stretching processes are carried out in separate tanks.

[0085] T1<T2<T3≤T4≤75 (5)

[0086] In the fourth crosslinking stretching step, the temperature T4 of the aqueous solution containing boric acid is preferably above the temperature T3 of the third crosslinking stretching step and below 75°C. It can also be the same temperature as the temperature T3 of the third crosslinking stretching step. This temperature T4 is preferably above 60°C, more preferably above 62°C. Furthermore, this temperature T4 is more preferably below 74°C. If the temperature is too low, the shrinkage stress increases; on the other hand, if the temperature is too high, PVA dissolves from the film into the aqueous solution containing boric acid, or the polarization degree decreases. Moreover, uniaxial stretching is performed within the aforementioned temperature range to achieve a stretching ratio of 1.1 to 1.3 times and a total stretching ratio of 5 to 8 times. For the film stretched in the fourth crosslinking stretching step, conducting the boric acid crosslinking reaction while stretching at a high ratio in a high-temperature aqueous solution containing boric acid can prevent PVA from dissolving from the film into the aqueous solution containing boric acid or from breaking.

[0087] As for the above T4, a suitable implementation is one that satisfies the following equation (6).

[0088] 60≤T4≤75 (6)

[0089] In the fourth crosslinking stretching process, the maximum tensile stress is preferably 10 N / mm. 2 Below. Here, the maximum tensile stress refers to the value obtained by dividing the tensile stress applied between adjacent rollers in the fourth cross-linking stretching process by the cross-sectional area of ​​the PVA film in the raw material. By reducing the maximum tensile stress, a polarizing film with low shrinkage stress can be obtained. A suitable maximum tensile stress is 8 N / mm². 2 The following is more suitable for 5N / mm 2 The following is further adapted to 4N / mm 2 The following is a summary. Additionally, the maximum tensile stress is typically 1 N / mm². 2 above.

[0090] In the aforementioned first to fourth crosslinking stretching processes, when the PVA film is uniaxially stretched, it can be done in the boric acid aqueous solution of the first to fourth crosslinking stretching processes by using a stretching device with multiple parallel rollers and changing the circumferential speed between each roller.

[0091] After the aforementioned third or fourth crosslinking stretching step, a washing step is preferably performed. In the washing step, unwanted chemical reagents and impurities are removed from the film surface, or the optical properties of the final polarizing film are adjusted. The washing step can be performed by immersing the PVA film in a washing bath or by spreading a washing solution on the PVA film. Water can be used as the washing solution, or it can contain potassium iodide. When potassium iodide is present, the hue of the polarizing film can be adjusted. The potassium iodide content is preferably 0.1 to 10% by mass. The temperature of the washing solution is typically 10 to 40°C, and preferably 15 to 30°C. Multiple washing baths can be used instead of just one. Furthermore, when multiple baths are used, the composition of the washing solution in each bath can be adjusted according to the purpose.

[0092] Preferably, the material is supplied to the drying process immediately following the aforementioned washing process. The temperature in the drying process is not particularly limited, but is preferably 30–150°C, more preferably 50–130°C. By drying at temperatures within the above range, a polarizing film with excellent dimensional stability can be easily obtained.

[0093] The thickness of the polarizing film obtained in this invention is preferably 1 to 30 μm. A thickness less than 1 μm makes high-speed production difficult, and a thickness of 3 μm or more is more suitable. On the other hand, a thickness exceeding 30 μm leads to increased tensile tension during stretching processes and potential equipment damage, making a thickness of 25 μm or less more suitable. Here, the thickness referred to in the case of multilayer films refers to the thickness of the PVA layer.

[0094] When the resulting polarizing film is a single-layer PVA film, the thickness of the polarizing film is preferably 5 μm or more, and more preferably 7 μm or more, to ensure processability. On the other hand, in the case of a polarizing film formed by multiple layers, the thickness of the polarizing film layer can be set to 5 μm or less, or even 3 μm or less. The thickness of the substrate resin layer in the multilayer film is typically 10 to 250 μm.

[0095] The polarizing film obtained in this invention preferably has a single-unit transmittance of 42-45%. When the single-unit transmittance is less than 42%, the brightness of the liquid crystal display decreases. A single-unit transmittance of 42.5% or higher is more suitable. On the other hand, for polarizing films with a single-unit transmittance exceeding 45%, it is difficult to obtain polarizing films with a high degree of polarization; a single-unit transmittance of 44.5% or lower is more suitable. Furthermore, the degree of polarization of the polarizing film of this invention is preferably 99.80% or higher. A degree of polarization of 99.90% or higher is more suitable.

[0096] The shrinkage stress of the polarizing film obtained in this invention is preferably 50 N / mm. 2 The following method achieves excellent dimensional stability even when used at high temperatures by minimizing shrinkage stress. A shrinkage stress of 42 N / mm is ideally suited. 2The following is further adapted to 38 N / mm 2 The following is particularly suitable for 35N / mm 2 Below. Here, shrinkage stress refers to the value obtained by dividing the tension of the polarizing film that will become the specimen, when fixed at 80°C for 4 hours, by the cross-sectional area of ​​the specimen.

[0097] Furthermore, the polarizing film obtained in this invention preferably has a polarization degree of 99.80% or more when the monomer transmittance is 43.5%, more preferably 99.90% or more, even more preferably 99.95% or more, and particularly preferably 99.98% or more. This value is calculated by assuming the monomer transmittance (T) of the polarizing film is 43.5% when it is not.

[0098] The method for calculating the degree of polarization when the monomer transmittance is 43.5% is as follows. First, the relationship between the transmittance (T') and the monomer transmittance (T) excluding surface reflection is expressed by equation (7). At this time, the refractive index of PVA is set to 1.5, and the surface reflectance is set to 4%. The relationship between transmittance (T'), degree of polarization (V), and dichroism ratio (R) is expressed by equation (8), which is transformed into equation (9). Among them, the dichroism ratio (R) hardly changes depending on the dye concentration in a range where the monomer transmittance (T) does not change much, for example, in the range of 42-45%. Therefore, it can be operated as a constant. Therefore, after measuring the monomer transmittance (T) and degree of polarization (V), the dichroism ratio (R) of the polarizing film can be calculated as a constant by using these values ​​and solving equations (7) and (8). According to equations (9) and (7) with this R substituted, the degree of polarization (V) when T = 43.5 (%) can be obtained.

[0099] T' = T / (1-0.04) 2 (7)

[0100] R={-ln[T'(1-V)]} / {-ln[T'(1+V)]} (8)

[0101] T' = [1-V] 1 / (R-1) / [1+V] R / (R-1) (9)

[0102] The polarizing film obtained in this invention is typically used to form a polarizing plate by laminating a protective film onto both or one side of the plate. Examples of protective films include optically transparent films with mechanical strength; specifically, cellulose triacetate (TAC) films, cellulose acetate / butyrate (CAB) films, acrylic films, and polyester films can be used. Furthermore, examples of adhesives used for lamination include PVA-based adhesives, urethane-based adhesives, and UV-curable adhesives.

[0103] The resulting polarizing plate can be used in high-performance liquid crystal displays (LCDs). It provides a bright polarizing plate with excellent polarization characteristics, superior dimensional stability even under high-temperature conditions. Therefore, it is suitable for use as a polarizing plate for various high-performance LCDs, especially mobile LCDs.

[0104] Example

[0105] The present invention will now be specifically described through examples, but the present invention is not limited to these examples in any way. Evaluation methods, etc., are performed according to the methods shown below.

[0106] [Optical properties of polarizing films]

[0107] A rectangular measurement sample, 4 cm in length along the mechanical flow direction (MD direction) and 1.5 cm in length along the width direction (TD direction), was collected from the center of the polarizing film obtained in the following examples and comparative examples. For this measurement sample, a spectrophotometer with an integrating sphere (Nippon Spectrophotometer Co., Ltd., "V7100") was used. After visibility correction in the visible light region with a C-light source and a 2° field of view, according to JIS Z8722 (Method for Determination of Object Color), the single-cell transmittance and degree of polarization were measured. Using the above method, the degree of polarization at a single-cell transmittance of 43.5% was calculated.

[0108] [Contraction stress of polarizing film]

[0109] Shrinkage stress was measured using an Autograph AG-X thermostat with a temperature control bath manufactured by Shimadzu Corporation. During the measurement, a polarizing film (15 cm in length and 1.5 cm in width) conditioned for 18 hours at 20°C / 20% RH was mounted on a fixture (5 cm spacing). Simultaneously with the start of stretching, the thermostat was heated to 80°C. The polarizing film was stretched at 1 mm / min, and stretching was stopped when the tension reached 2 N. The tension was measured up to 4 hours later under this condition. Since the distance between the fixtures changed due to thermal expansion of the axis, marking labels were affixed to the fixtures. A TR ViewX120S camera-type tensile tester was used to correct for the distance between the fixtures based solely on the movement of the marking labels. It should be noted that the shrinkage force of the polarizing film is defined as the value obtained by subtracting the initial tension of 2 N from the measured tension after 4 hours, and the shrinkage stress (N / mm²) is defined as the value obtained by dividing this value by the sample cross-sectional area. 2 ).

[0110] [Maximum tensile stress during the fourth cross-linking stretching process]

[0111] In the following examples and comparative examples, the width (mm) and thickness (mm) of the PVA film (unprocessed raw material film) supplied for the manufacture of the polarizing film are determined in advance, and the cross-sectional area (mm²) is calculated by multiplying them together. 2 Next, in the fourth cross-linking stretching process during the manufacture of the polarizing film, the tensile stress (N) applied between adjacent rollers is measured using a tension meter. The obtained tensile stress (N) is then divided by the cross-sectional area (mm²) calculated above. 2 ), calculate the maximum tensile stress (N / mm) in the fourth crosslinking stretching process. 2 ).

[0112] [Coloring of boric acid aqueous solution in the first cross-linking stretching process]

[0113] For the coloring of the boric acid aqueous solution in the first cross-linking stretching process, 20cc of the boric acid aqueous solution was collected in a 30cc spiral tube after continuous stretching for 1 hour. Visual inspection was performed to confirm that the color did not change compared with the boric acid aqueous solution before continuous stretching, and the color changed or showed iodine coloring, which was marked as ×.

[0114] [Example 1]

[0115] A film-forming stock solution consisting of 100 parts by weight of PVA (a saponified product of vinyl acetate polymer, degree of polymerization 2400, degree of saponification 99.9 mol%, ethylene modified 2.0 mol%), 10 parts by weight of glycerol as a plasticizer, 0.1 parts by weight of sodium polyoxyethylene dodecyl sulfate as a surfactant, and water is used for casting to obtain a roll of PVA film with a thickness of 45 μm. This PVA film is then subjected to a swelling process, a dyeing process, a first crosslinking stretching process, a second crosslinking stretching process, a third crosslinking stretching process, a fourth crosslinking stretching process, a washing process, and a drying process to manufacture a polarizing film. A schematic diagram of the polarizing film manufacturing apparatus is shown below. Figures 1-4 .

[0116] Specifically, the polarizing film is manufactured as follows: First, in the swelling process, the PVA film is immersed in water at 30°C for 1.6 minutes, and then uniaxially stretched (stage 1 stretching) along the length direction (MD) to twice its original length. Next, in the dyeing process, the film is immersed in an aqueous solution containing 0.06% by mass of iodine and 1.4% by mass of potassium iodide at 32°C for 2.3 minutes, and then uniaxially stretched (stage 2 stretching) along the length direction (MD) to 2.5 times its original length. Next, in the first crosslinking stretching process, the film is immersed in an aqueous solution containing 2.6% by mass of boric acid at 32°C for 2 minutes, and then uniaxially stretched (stage 3 stretching) along the length direction (MD) to three times its original length. Following the second crosslinking and stretching step, during immersion in an aqueous solution containing 2.8% by mass boric acid and 5% by mass potassium iodide at a temperature of 60.6°C, uniaxial stretching (stage 4 stretching) is performed along the length direction (MD) to 4.46 times the original length. Following the third crosslinking and stretching step, during immersion in an aqueous solution containing 2.8% by mass boric acid and 5% by mass potassium iodide at a temperature of 63.6°C, uniaxial stretching (stage 5 stretching) is performed along the length direction (MD) to 5.27 times the original length. Following the fourth crosslinking and stretching step, during immersion in an aqueous solution containing 2.8% by mass boric acid and 5% by mass potassium iodide at a temperature of 64.6°C, uniaxial stretching (stage 6 stretching) is performed along the length direction (MD) to 6 times the original length. The maximum tensile stress in the fourth crosslinking and stretching step is 1.4 N / mm. 2 The film was then washed in a washing process by immersing it in an aqueous solution containing 1.5% by mass boric acid and 5% by mass potassium iodide at 22°C for 10 seconds. Following this, a polarizing film with a thickness of 14.0 μm was produced by drying it in a dryer at 80°C for 120 seconds. The conditions for each crosslinking and stretching process and the maximum tensile stress value in the fourth crosslinking and stretching process are shown in Table 1, and the evaluation results are shown in Table 2.

[0117] [Examples 2-3, Comparative Examples 1-3]

[0118] Except for changing the temperature of the aqueous solution containing boric acid in the first crosslinking and stretching step, the temperature and total stretching ratio of the aqueous solution containing boric acid in the second crosslinking and stretching step, the temperature and total stretching ratio of the aqueous solution containing boric acid in the third crosslinking and stretching step, and the temperature and total stretching ratio of the aqueous solution containing boric acid in the fourth crosslinking and stretching step as shown in Table 1, the polarizing film was manufactured in the same manner as in Example 1. The conditions of each crosslinking and stretching step and the value of the maximum tensile stress in the fourth crosslinking and stretching step are shown in Table 1, and the evaluation results are shown in Table 2. In addition, the evaluation results of Example 3 and Comparative Example 2 with a total stretching ratio of 6.6 times are shown in Table 3. It should be noted that Examples 1 and 2 and Comparative Examples 1 and 3 in Table 2 are all examples with a total stretching ratio of 6 times.

[0119] Table 1

[0120]

[0121] Table 2

[0122]

[0123] ○: No change after 1 hour; ×: Iodine staining appears after 1 hour.

[0124] *1) Polarization degree when the single-cell transmittance is 43.5%

[0125] Table 3

[0126]

[0127] ○: No change after 1 hour; ×: Iodine staining appears after 1 hour.

[0128] *1) Polarization degree when the single-cell transmittance is 43.5%

[0129] Explanation of symbols

[0130] 1. Polarizing film manufacturing apparatus

[0131] 2 PVA film rollers

[0132] 3. Swelling process

[0133] 4. Dyeing process

[0134] 5. First cross-linking stretching process

[0135] 6. Second cross-linking stretching process

[0136] 7. Third cross-linking stretching process

[0137] 8. Fourth cross-linking stretching process

[0138] 9 Washing process

[0139] 10. Drying process

Claims

1. A method for manufacturing a polarizing film, wherein the polyvinyl alcohol film is subjected to at least the following steps in sequence: swelling step, dyeing step, first crosslinking stretching step, second crosslinking stretching step, and third crosslinking stretching step, wherein, The thickness of the aforementioned polyvinyl alcohol film is 5–100 μm. The polyvinyl alcohol contained in the aforementioned polyvinyl alcohol film has an average degree of polymerization of 2000–4000. In the aforementioned swelling process, the polyvinyl alcohol film is swelled by immersion in water at 10–50°C. In the aforementioned dyeing process, the polyvinyl alcohol film is immersed in an aqueous solution containing a total of 0.5–3% by mass of iodine and potassium iodide at a temperature of 10–50°C, allowing the iodine-based dichroic pigment to penetrate the film. Simultaneously, uniaxial stretching is performed to achieve a total stretch ratio of 2–3 times. In the aforementioned first crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T1, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 2.5-3.5 times. In the aforementioned second crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T2, so that the stretching ratio in this step is 1.3-1.8 times and the total stretching ratio is 4-6 times. In the aforementioned third crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1-5% by mass of boric acid at temperature T3, so that the stretching ratio in this step is 1.1-1.3 times and the total stretching ratio is 4.5-7 times. The above T1, T2 and T3 satisfy the following equations (1) and (2). 25≤T1≤45　　　　　　（1） T1<T2<T3≤75 (2).

2. The method for manufacturing a polarizing film according to claim 1, wherein, The above T2 and T3 satisfy the following equations (3) and (4). 50≤T2≤65　　　　　　（3） 55≤T3≤75　　　　　　（4）。 3. The method for manufacturing a polarizing film according to claim 1 or 2, wherein, After the aforementioned third crosslinking stretching step, a fourth crosslinking stretching step is performed. In the aforementioned fourth crosslinking stretching step, uniaxial stretching is performed in an aqueous solution containing 1 to 5% by mass of boric acid at temperature T4, so that the stretching ratio in this step is 1.1 to 1.3 times and the total stretching ratio is 5 to 8 times. T1, T2, T3 and T4 satisfy the following equation (5). T1<T2<T3≤T4≤75 (5).

4. The method for manufacturing a polarizing film according to claim 3, wherein, The above T4 satisfies the following equation (6). 60≤T4≤75　　　　　　（6）。 5. The method for manufacturing a polarizing film according to claim 3, wherein, In the aforementioned fourth cross-linking stretching process, the maximum tensile stress is 10 N / mm. 2 the following.

6. The method for manufacturing a polarizing film according to claim 1 or 2, wherein, The shrinkage stress was obtained as 50 N / mm. 2 The following are polarizing films.

7. The method for manufacturing a polarizing film according to claim 1 or 2, wherein, A polarizing film with a polarization degree of over 99.80% was obtained when the single-unit transmittance was 43.5%.

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