TWI764696B - Polarizing film - Google Patents

Abstract

The present invention provides a method for manufacturing polarizing film capable of obtaining a polarizing film having high optical characteristics despite even if the boron content is reduced, and a manufacturing apparatus thereof.

The method for manufacturing a polarizing film having a boron content of 2.0 to 3.5% by weight from a polyvinyl alcohol-based resin film, which comprises: a dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye; a crosslinking step of immersing the polyvinyl alcohol-based resin film after the dyeing step in a crosslinking bath composed of an aqueous solution of a crosslinking agent containing a boron compound to crosslink the polyvinyl alcohol resin film; an electromagnetic wave irradiation step of irradiating the polyvinyl alcohol-based resin film after the crosslinking step with an electromagnetic wave in which the ratio of radiant energy of infrared radiation with a wavelength of more than 2 μm to 4 μm or less with respect to the total radiant energy is 25% or more; and a washing step of washing the polyvinyl alcohol-based resin film having been irradiated with the electromagnetic wave.

Description

polarizing film

The present invention relates to a method for producing a polarizing film from a polyvinyl alcohol-based resin film, and a production apparatus.

Polarizing plates are widely used as polarizing elements and the like in video display devices such as liquid crystal display devices. Generally, as a polarizing plate, a transparent resin film (protective film etc.) is laminated on one side or both sides of a polarizing film using an adhesive or the like.

The polarizing film is mainly immersed in a dyeing bath containing dichroic dyes such as iodine, and then immersed in a crosslinking bath containing a crosslinking agent such as boric acid, etc. The film is produced by uniaxially extending the film at any stage. The uniaxial stretching includes dry stretching in which stretching is performed in air, and wet stretching in which stretching is performed in liquids such as the above-mentioned dyeing bath and cross-linking bath.

The crosslinked polarizing film tends to shrink when heated, and the durability may be insufficient. Japanese Patent Laid-Open No. 2013-148806 (Patent Document 1) discloses that a polarizing film having excellent durability can be provided when the boron content of the polarizing film is as low as 1 to 3.5% by weight.

[Prior Art Literature]

[Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2013-148806.

However, when the boron content in the polarizing film is lowered, a sufficient degree of crosslinking cannot be obtained, and the optical properties may be lowered. The objective of this invention is to provide the manufacturing method of the polarizing film which has high optical characteristics even if the boron content rate is reduced, and its manufacturing apparatus. Furthermore, it aims at providing the novel polarizing film obtained by the manufacturing method of such a polarizing film.

The present invention provides a manufacturing method, a manufacturing apparatus, and a polarizing film of the polarizing film shown below.

[1] A method for producing a polarizing film, comprising producing a polarizing film having a boron content of 2.0 to 3.5 wt % from a polyvinyl alcohol-based resin film, and comprising:

The dyeing step is to dye the polyvinyl alcohol-based resin film with a dichroic pigment;

In the cross-linking step, the polyvinyl alcohol-based resin film after the dyeing step is immersed in a cross-linking bath composed of an aqueous solution of a cross-linking agent containing a boron compound for cross-linking treatment;

The electromagnetic wave irradiation step is to irradiate the polyvinyl alcohol-based resin film with electromagnetic waves after the cross-linking step, and the ratio of the radiation energy of infrared rays with wavelengths exceeding 2 μm and 4 μm or less in the electromagnetic waves is 25% of the total radiation energy. above; and

In the cleaning step, the polyvinyl alcohol-based resin film after being irradiated with the electromagnetic wave is cleaned.

[2] The method for producing a polarizing film according to [1], wherein, in the electromagnetic wave irradiation step, the irradiation energy of the electromagnetic wave is 100 J/cm ³ or more and 50 kJ per unit volume of the polyvinyl alcohol-based resin film. /cm ³ or less.

[3] The method for producing a polarizing film as described in [1] or [2], further comprising a liquid removal step for removing the liquid adhered to the polyvinyl alcohol system before irradiating the electromagnetic wave after the crosslinking treatment. The aforementioned aqueous solution on the surface of the resin film is removed.

[4] The method for producing a polarizing film according to any one of [1] to [3], wherein the electromagnetic wave irradiation step is performed 5 seconds after the polyvinyl alcohol-based resin film is pulled out from the crosslinking bath carried out within.

[5] The method for producing a polarizing film according to any one of [1] to [4], wherein, in the crosslinking step, the crosslinking bath contains 1.5 parts by weight or more of 3 parts by weight relative to 100 parts by weight of water; It is composed of an aqueous solution of the crosslinking agent of the aforementioned boron compound in parts by weight or less.

[6] An apparatus for producing a polarizing film, which produces a polarizing film with a boron content of 2.0 to 3.5 wt % from a polyvinyl alcohol-based resin film, and includes:

The dyeing part is to dye the polyvinyl alcohol-based resin film with a dichroic dye;

The cross-linked part is the polyvinyl alcohol-based resin after the dyeing treatment. The film is immersed in a cross-linking bath composed of an aqueous solution of a cross-linking agent containing a boron compound for cross-linking treatment;

The electromagnetic wave irradiation section irradiates the polyvinyl alcohol-based resin film after the crosslinking treatment with electromagnetic waves, and the ratio of the radiation energy of infrared rays with wavelengths exceeding 2 μm and 4 μm or less in the electromagnetic waves is 25% or more of the total radiation energy; and

The cleaning section cleans the polyvinyl alcohol-based resin film after being irradiated with the electromagnetic wave.

[7] A polarizing film obtained by dyeing a polyvinyl alcohol-based resin film with a dichroic dye, wherein

The boron content is 2.0 to 3.5% by weight,

Visual sensitivity correction monomer penetration rate is more than 42.0%,

Sensitivity correction polarization is more than 99.990%,

The ratio A ₄₈₀ /A ₆₀₀ of the absorbance A ₄₈₀ in the direction of the absorption axis at a wavelength of 480 nm and the absorbance A 600 in the direction of the absorption axis at a wavelength of ₆₀₀ nm is 0.75 or more and 1.00 or less.

According to the present invention, it is possible to provide a manufacturing method and a manufacturing apparatus of a polarizing film having high optical properties even if the boron content is reduced. Furthermore, according to the present invention, a polarizing film having a low boron content and excellent optical properties can be provided.

10: Raw material film composed of polyvinyl alcohol-based resin

11: Raw Rolls

13: Swelling bath

15: Dye Bath

17a: 1st Crosslinking Bath

17b: 2nd crosslinking bath

19: wash bath

21: Drying oven

23: polarizing film

30 to 48, 60, 61: Guide rollers

50 to 52, 53a, 53b, 54, 55: nip rolls

71: Electromagnetic wave irradiation section

Fig. 1 is a cross-sectional view schematically showing an example of a method for producing a polarizing film of the present invention and an example of a polarizing film producing apparatus used therefor.

Fig. 2 shows the radiation energy spectrum of various types of electromagnetic wave irradiators.

<Manufacturing method of polarizing film>

The polarizing film in the present invention is a dichroic dye (iodine or dichroic dye) adsorbed and aligned on a uniaxially stretched polyvinyl alcohol-based resin film. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is usually about 85 mol% or more, preferably about 90 mol% or more, more preferably about 99 mol% or more. The polyvinyl acetate-based resin may be, for example, a copolymer of vinyl acetate and other monomers copolymerizable therewith, in addition to polyvinyl acetate which is a vinyl acetate homopolymer. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is usually about 1,000 to 10,000, preferably about 1,500 to 5,000.

These polyvinyl alcohol-based resins can be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc. modified with aldehydes can also be used.

In the present invention, as a starting material for producing a polarizing film, unstretched polyvinyl alcohol having a thickness of 65 μm or less (for example, 60 μm or less), preferably 50 μm or less, more preferably 35 μm or less, and still more preferably 30 μm or less is used. Resin film (raw material film).

Thereby, a thin-film polarizing film with increasing market requirements can be obtained. The width of the raw material film is not particularly limited, for example, it may be about 400 to 6000 mm. raw material For the film, for example, a roll (raw material roll) of a long unstretched polyvinyl alcohol-based resin film can be prepared.

In addition, the polyvinyl alcohol-based resin film used in the present invention may be one that is laminated on a supporting base film, that is, a laminated film of a base film and a polyvinyl alcohol-based resin film laminated thereon may be prepared, and as the polyvinyl alcohol-based resin film. In this case, the polyvinyl alcohol-based resin film can be produced, for example, by applying a coating liquid containing a polyvinyl alcohol-based resin on at least one side of the base film, and then drying.

As the base film, for example, a film made of a thermoplastic resin can be used. As a specific example, it is a light-transmitting thermoplastic resin, preferably a film made of an optically transparent thermoplastic resin, such as a chain polyolefin-based resin (polypropylene-based resin, etc.), a cyclic polyolefin-based resin Polyolefin-based resins such as (norbornene-based resins, etc.); cellulose-based resins such as cellulose triacetate and cellulose diacetate; polyethylene terephthalate, polybutylene terephthalate Polycarbonate resins; (meth)acrylic resins such as methyl methacrylate resins; Polystyrene resins; Polyvinyl chloride resins; Acrylonitrile/butadiene / Styrenic resin; Acrylonitrile / Styrenic resin; Polyvinyl acetate resin; Polyvinylidene chloride resin; Polyamide resin; Polyacetal resin; Modified polyphenylene ether resin; Polyethylene Polyether-based resin; Polyether-based resin; Polyarylate resin; Polyamide-imide-based resin; Polyamide-imide-based resin, etc.

The polarizing film is continuously conveyed along the film conveying path of the polarizing film manufacturing apparatus while pulling out the above-mentioned long raw material film from the raw material roll, and is immersed in a treatment liquid (hereinafter also referred to as a "treatment bath") accommodated in a treatment tank. After pulling out, the drying step is carried out after the predetermined processing step is carried out, whereby it is possible to Continuous production of long polarizing films. In addition, the treatment step is not limited to a method of immersing the film in a treatment bath as long as the treatment step is a method of contacting the film with the treatment liquid, and may be a method of treating the film by attaching the treatment liquid to the film surface by spraying, flowing, dripping, or the like. method. When the treatment step is performed by immersing the film in the treatment bath, the treatment bath for performing one treatment step is not limited to one, and one treatment step may be completed by sequentially immersing the film in two or more treatment baths.

As said process liquid, a swelling liquid, a dyeing liquid, a crosslinking liquid, a washing|cleaning liquid etc. are mentioned, for example. Next, the above-mentioned treatment steps include, for example, a swelling step in which a raw film is brought into contact with a swelling liquid to perform a swelling treatment, a dyeing step in which the film after the swelling treatment is brought into contact with a dyeing liquid to perform a dyeing treatment, and a film after the dyeing treatment is brought into contact and crosslinking A cross-linking step for performing cross-linking treatment with a liquid, and a cleaning step for performing a cleaning treatment by contacting the film after the cross-linking treatment with a cleaning liquid. Furthermore, wet or dry uniaxial stretching can be performed between the series of processing steps (ie, before and after any one or more processing steps and/or during any one or more processing steps). Other processing steps may be added as required.

In the present invention, after the cross-linking treatment and before the washing treatment, an electromagnetic wave irradiation step, which will be described later, is performed on the film by irradiating the film with electromagnetic waves. By performing the electromagnetic wave irradiation step, a polarizing film having excellent optical properties can be obtained even if the boron content is low.

Hereinafter, an example of the manufacturing method of the polarizing film of this invention is demonstrated in detail, referring FIG. 1. FIG. FIG. 1 is a cross-sectional view schematically showing an example of a manufacturing method of a polarizing film of the present invention and an example of a polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in Fig. 1 is as follows Formula structure: The raw material (unstretched) film 10 composed of polyvinyl alcohol-based resin is continuously drawn from the raw material roll 11, and is conveyed along the film conveying path, thereby sequentially passing through the film conveying path provided on the film conveying path. Swelling bath (swelling liquid stored in the swelling tank) 13, dyeing bath (staining liquid stored in the staining tank) 15, first cross-linking bath (first cross-linking liquid stored in the cross-linking tank) 17a, first 2. The cross-linking bath (the second cross-linking liquid contained in the cross-linking tank) 17b and the cleaning bath (the cleaning liquid contained in the cleaning tank) 19 are finally passed through the drying furnace 21 . The obtained polarizing film 23 can be directly conveyed, for example, to the following polarizing plate manufacturing step (step of attaching a protective film to one side or both sides of the polarizing film 23 ). Arrows in Fig. 1 indicate the film conveying direction.

In the description of Fig. 1, "treatment tank" is a general term including swelling tank, dyeing tank, cross-linking tank and cleaning tank, and "treatment liquid" is a general term including swelling liquid, dyeing liquid, cross-linking liquid and cleaning liquid , "Treatment bath" is a general term including swelling bath, dyeing bath, cross-linking bath and cleaning bath. The swelling bath, the dyeing bath, the crosslinking bath, and the washing bath constitute the swelling part, the dyeing part, the crosslinking part, and the washing part, respectively, in the manufacturing apparatus of the present invention.

The film conveyance path of the polarizing film manufacturing apparatus can be constituted by, in addition to the above-mentioned treatment bath, the following modes: guide rollers 30 to 48, 60, 61 which can support the conveyed film or further change the film conveying direction; The nip rolls 50 to 55 which hold the film to be conveyed and can give a driving force to the film through its rotation, or which can further change the direction of film conveyance, are arranged at appropriate positions. Guide rolls or nip rolls can be arranged before and after each treatment bath or in the treatment bath, whereby the film can be introduced into and immersed in the treatment bath, and pulled out from the treatment bath [see Fig. 1]. For example, one or more guide rolls may be provided in each treatment bath and along the The guide roll conveys the film, whereby the film can be immersed in each treatment bath.

In the polarizing film manufacturing apparatus shown in Fig. 1, nip rolls (nip rolls 50 to 54) are arranged before and after each treatment bath, whereby stretching between rolls can be performed in any one or more treatment baths. A peripheral speed difference was applied between the nip rolls before and after the treatment bath, and vertical uniaxial stretching was performed.

In the polarizing film manufacturing apparatus shown in FIG. 1, the electromagnetic wave irradiation part 71 is arrange|positioned in the conveyance path downstream of the 2nd crosslinking bath 17b and the upstream of the cleaning bath 19, and the electromagnetic wave irradiation process is performed. Each step is explained below.

(swelling step)

The purpose of performing the swelling step is to remove foreign matter on the surface of the raw material film 10 , remove the plasticizer in the raw material film 10 , impart easy dyeability, and plasticize the raw material film 10 . The processing conditions are determined in a range in which the object can be achieved, and in a range in which the raw material film 10 does not suffer from extreme dissolution or devitrification.

Referring to FIG. 1, the swelling step can be carried out by: immersing the raw material film 10 in the swelling bath 13 for a predetermined time while continuously pulling out the raw material film 10 from the raw material roll 11 and conveying it along the film conveying path, Then pull out. In the example of FIG. 1, the raw material film 10 is conveyed along the film conveyance path constructed by the guide rolls 60 and 61 and the nip roll 50 between the extraction of the raw material film 10 and the immersion in the swelling bath 13. In the swelling treatment, the film is transported along the film transport path constructed by the guide rolls 30 to 32 and the nip roll 51 .

As the swelling liquid of the swelling bath 13, besides pure water, boric acid (Japanese Unexamined Patent Publication No. 10-153709), chlorides (Japanese Unexamined Patent Publication No. 06-281816 ), Aqueous solutions of inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc.

The temperature of the swelling bath 13 is, for example, about 10 to 50°C, preferably about 10 to 40°C, and more preferably about 15 to 30°C. The immersion time of the raw material film 10 is preferably about 10 to 300 seconds, more preferably about 20 to 200 seconds. In addition, when the raw material film 10 is a polyvinyl alcohol-based resin film previously stretched in gas, the temperature of the swelling bath 13 is, for example, about 20 to 70°C, preferably about 30 to 60°C. The immersion time of the raw material film 10 is preferably about 30 to 300 seconds, more preferably about 60 to 240 seconds.

During the swelling treatment, the raw material film 10 is likely to swell in the width direction to form wrinkles in the film. As one method for removing the wrinkles and conveying the film, the guide rollers 30, 31, and/or 32 may use rolls having a widening function, such as expansion rolls, spiral rolls, and crown rolls. Or use other widening devices such as cross guiders, curved bars, tenter clips, or the like. Another method for suppressing wrinkle generation is to perform a stretching treatment. For example, the uniaxial stretching treatment can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip roll 50 and the nip roll 51 .

In the swelling treatment, the film also swells and expands in the film conveying direction. Therefore, in order to eliminate film slack in the conveying direction when the film is not actively stretched, it is preferable to employ, for example, control the nip rolls 50 arranged before and after the swelling bath 13. , 51 speed and other methods. In addition, for the purpose of stabilizing the film transfer in the swelling bath 13, the water flow in the swelling bath 13 is controlled by bathing in water, or an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents the film from meandering) is used in combination. is useful.

In the example shown in FIG. 1 , the film drawn from the swelling bath 13 passes through the guide roll 32 , the nip roll 51 , and the guide roll 33 in this order, and is introduced into the dyeing bath 15 .

(Dyeing step)

The purpose of the dyeing step is to adsorb, align the dichroic dye, and the like to the polyvinyl alcohol-based resin film after the swelling treatment. The treatment conditions are determined within a range within which the purpose can be achieved, and within a range where defects such as extreme dissolution and devitrification of the film do not occur. Referring to FIG. 1, the dyeing step can be carried out along the film conveying path constructed by the nip rolls 51, the guide rolls 33 to 36, and the nip rolls 52, and the film after the swelling treatment can be placed in the dyeing bath 15 (treatment in the dyeing tank). liquid) is immersed for a predetermined time, and then pulled out. In order to improve the dyeability of the dichroic dye, the film to be used in the dyeing step is preferably a film subjected to at least a certain degree of uniaxial stretching treatment, or is preferably a uniaxial stretching treatment in place of the dyeing treatment, or in addition to the dyeing treatment. In addition to the uniaxial stretching treatment, the uniaxial stretching treatment is performed during the dyeing treatment.

When iodine is used as a dichroic dye, for example, an aqueous solution having a concentration of iodine/potassium iodide/water=about 0.003 to 0.3/about 0.1 to 10/100 by weight can be used in the dyeing solution of the dyeing bath 15 . Other iodides such as zinc iodide may be used in place of potassium iodide, or potassium iodide and other iodides may be used in combination. Moreover, compounds other than iodide, for example, boric acid, zinc chloride, cobalt chloride, etc. may coexist. When adding boric acid, it can be distinguished from the crosslinking treatment described later by the point containing iodine, and an aqueous solution containing about 0.003 parts by weight or more of iodine can be regarded as dyeing bath 15 with respect to 100 parts by weight of water. The temperature of the dyeing bath 15 when dipping the film is usually about 10 to 45°C, preferably 10 to 40°C, more preferably 20 to 35°C, and the immersion time of the film is usually about 30 to 600 seconds, preferably 60 to 60°C 300 seconds.

Use of water-soluble dichroic dyes as dichroic dyes In the dyeing bath 15, for example, an aqueous solution having a concentration of dichroic dye/water=about 0.001 to 0.1/100 by weight can be used. The dyeing bath 15 may coexist with dyeing auxiliaries and the like, and may contain, for example, inorganic salts such as sodium sulfate, surfactants, and the like. Only one type of dichroic dye may be used alone, or two or more types of dichroic dye may be used in combination. The temperature of the dyeing bath 15 when dipping the film is, for example, about 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

In the dyeing step as described above, the uniaxial extension of the film may be performed in the dyeing bath 15 . The uniaxial stretching of the film can be performed by a method such as applying a peripheral speed difference between the nip rolls 51 and 52 disposed before and after the dyeing bath 15 .

In the dyeing treatment, in the same manner as in the swelling treatment, in order to remove the wrinkles of the film and convey the polyvinyl alcohol-based resin film, an expansion roll, a spiral roll, a convex roll, etc. For rolls with widening function, other widening devices such as cross guide rolls, bending rods, tenter clips, etc. can also be used. Similar to the swelling treatment, another method for suppressing the occurrence of wrinkles is to perform a stretching treatment.

In the example shown in FIG. 1 , the film drawn from the dyeing bath 15 passes through the guide roll 36 , the nip roll 52 , and the guide roll 37 in this order, and is introduced into the crosslinking bath 17 .

(crosslinking step)

The purpose of the cross-linking step is to improve water resistance or adjust the color tone (prevent the film from appearing blue, etc.) by cross-linking. In the example shown in Fig. 1, two cross-linking baths are arranged as the cross-linking baths for the cross-linking step. The first cross-linking step for water resistance is performed in the first cross-linking bath 17a, and the color tone is adjusted as The second cross-linking step performed for the purpose is performed in the second cross-linking bath 17b. refer to In FIG. 1, the first cross-linking step can be conveyed along the film conveying path constructed by the nip rolls 52, the guide rolls 37 to 40, and the nip roll 53a, and the dyed film is placed in the first cross-linking bath 17a ( The 1st crosslinking liquid) accommodated in a crosslinking tank is immersed for a predetermined time, and it pulls out and implements. In the second cross-linking step, the film after the first cross-linking step can be transferred to the second cross-linking bath 17b (stored in The second cross-linking liquid of the cross-linking tank) is immersed for a predetermined time, and then pulled out and implemented. Hereinafter, when it calls a crosslinking bath, it contains both the 1st crosslinking bath 17a and the 2nd crosslinking bath 17b, When it calls a crosslinking liquid, it also contains both a 1st crosslinking liquid and a 2nd crosslinking liquid.

As a cross-linking liquid, a solution in which a cross-linking agent is dissolved in a solvent can be used. As a crosslinking agent, boron compounds, such as boric acid and borax, glyoxal, glutaraldehyde, etc. are mentioned, for example. These may be one type or two or more types may be used in combination. As a solvent, for example, water can be used, and an organic solvent having compatibility with water may also be contained. The concentration of the cross-linking agent in the cross-linking liquid, the temperature of the cross-linking bath, the film immersion time, and the number of cross-linking baths for immersing the film are not particularly limited, and the boron content can be obtained by appropriately selecting from these. 2.0 to 3.5 wt% polarizing film. As the cross-linking liquid, for example, an aqueous solution containing 1 to 10 parts by weight of a boron compound such as boric acid can be used with respect to 100 parts by weight of water, and an aqueous solution containing 1.5 to 3 parts by weight is preferably used. When the dichroic dye used in the dyeing treatment is iodine, the crosslinking solution preferably contains iodide in addition to boric acid, and the amount of iodide can be, for example, 1 to 30 parts by weight relative to 100 parts by weight of water. Examples of the iodide include potassium iodide, zinc iodide, and the like. Moreover, compounds other than iodide, for example, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc. may coexist.

In the crosslinking treatment, the concentrations of boric acid and iodide and the temperature of the crosslinking bath 17 can be appropriately changed according to the purpose. For example, in the case of the first cross-linking liquid whose purpose of cross-linking is to achieve water resistance by cross-linking, the concentration may be an aqueous solution of boric acid/iodide/water=1 to 10/1 to 30/100 in weight ratio. The temperature of the first cross-linking bath 17a when the film is dipped is usually about 50 to 70°C, preferably 53 to 65°C, and the immersion time of the film is usually about 10 to 600 seconds, preferably 20 to 300 seconds, more preferably for 20 to 200 seconds. Moreover, when the dyeing treatment and the first cross-linking treatment are sequentially performed on the polyvinyl alcohol-based resin film stretched before the swelling treatment, the temperature of the first cross-linking bath 17a is usually about 50 to 85° C., preferably 55 to 85° C. 80°C.

For example, when iodine is used as a dichroic dye in the second cross-linking liquid for the purpose of adjusting the color tone, the concentration of boric acid/iodide/water=1 to 5/3 to 30/100 by weight can be used. The temperature of the second crosslinking bath 17b when the film is dipped is usually about 10 to 45°C, and the immersion time of the film is usually about 1 to 300 seconds, preferably 2 to 100 seconds.

The cross-linking treatment may be performed a plurality of times, usually 2 to 5 times. At this time, the composition and temperature of each of the crosslinking baths used may be the same or different. The cross-linking treatment for achieving water resistance through cross-linking and the cross-linking treatment for adjusting the color tone may be performed in plural steps, respectively.

The uniaxial stretching process can be performed in the 1st crosslinking bath 17a using the peripheral speed difference of the nip roll 52 and the nip roll 53a. Moreover, the uniaxial stretching process may be performed in the 2nd crosslinking bath 17b using the peripheral speed difference of the nip roll 53a and the nip roll 53b.

In the cross-linking treatment, as in the swelling treatment, in order to remove To remove the wrinkles of the film and convey the polyvinyl alcohol-based resin film, rolls with a widening function such as expansion rolls, spiral rolls, convex rolls, etc. Other widening devices such as cross guide rolls, curved bars, tenter clips, etc. may also be used. Similar to the swelling treatment, another method for suppressing the occurrence of wrinkles is to perform a stretching treatment.

In the example shown in FIG. 1, the film drawn from the second crosslinking bath 17b passes through the guide roll 44 and the nip roll 53b in this order, and is introduced into the cleaning bath 19.

(cleaning step)

The example shown in Fig. 1 includes a washing step after the crosslinking step. The purpose of the cleaning treatment is to remove chemicals such as excess boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step is performed by, for example, immersing the cross-linked polyvinyl alcohol-based resin film in the cleaning bath 19 . Furthermore, instead of the step of immersing the film in the cleaning bath 19, the cleaning step may be performed by spraying the film with the cleaning solution as a shower, or by using the immersion in the cleaning bath 19 and the cleaning solution in combination. by spraying.

FIG. 1 shows an example in which a polyvinyl alcohol-based resin film is immersed in a cleaning bath 19 and subjected to a cleaning process. The temperature of the cleaning bath 19 in the cleaning treatment is usually about 2 to 40° C., and the immersion time of the film is usually about 2 to 120 seconds. The conditions of the cleaning treatment may be appropriately selected so that the boron content of the polarizing film finally obtained is 2.0 to 3.5 mass %.

In addition, in the cleaning process, for the purpose of removing wrinkles and conveying the polyvinyl alcohol-based resin film, the guide rollers 45 , 46 , 47 and/or 48 may use expansion rollers, helical rollers, convex rollers, etc. Functional rolls, and other widening devices such as cross guide rolls, curved bars, tenter clips, etc. can also be used. In addition, a stretching treatment for suppressing the occurrence of wrinkles may be performed in the film cleaning treatment.

(extension step)

As described above, the raw material film 10 is subjected to wet or dry uniaxial stretching treatment between the above-mentioned series of processing steps (ie, before and after any one or more processing steps and/or during any one or more processing steps). The specific method of the uniaxial stretching treatment can be, for example, applying a peripheral speed difference between two nip rolls (for example, two nip rolls arranged before and after the treatment bath) that constitute the film conveying path to perform longitudinal uniaxial stretching between the rolls, such as: The hot roll stretching, tenter stretching, and the like described in Japanese Patent No. 2731813 are preferably inter-roll stretching. The uniaxial stretching step may be performed a plurality of times between obtaining the polarizing film 23 from the raw material film 10 . As described above, the stretching treatment is also advantageous for suppressing the occurrence of wrinkles in the film.

Based on the raw material film 10 , the final cumulative stretching ratio of the polarizing film 23 is usually about 4.5 to 7 times, preferably 5 to 6.5 times. The extension step may be performed in any of the treatment steps, and when the extension treatment is performed in two or more treatment steps, the extension treatment may be performed in any of the treatment steps.

(Electromagnetic wave irradiation step)

In the apparatus shown in FIG. 1, after the film is pulled out from the second crosslinking step 17b and passed through the nip rolls 53b, the film is irradiated with electromagnetic waves before being immersed in the cleaning bath 19 (electromagnetic wave irradiation step). In the apparatus shown in FIG. 1 , the electromagnetic wave is irradiated by the electromagnetic wave irradiation unit 71 . In the electromagnetic wave irradiation step of the present invention, the proportion of infrared radiation energy with wavelengths exceeding 2 μm and below 4 μm is 25% or more of the total radiation energy, preferably 28% or more, more preferably 35% or more. By irradiating the film with such electromagnetic waves, the optical properties of the obtained polarizing film can be improved. In addition, regarding the electromagnetic wave used in the present invention, it exceeds 2 μm In addition, the upper limit of the ratio of infrared radiation energy of wavelengths of 4 μm or less is not particularly limited, but is, for example, 80% or less. Usually electromagnetic waves with wavelengths from 0.75μm to 1000μm are called infrared rays.

In the electromagnetic wave irradiation step, by irradiating electromagnetic waves with wavelengths exceeding 2 μm and below 4 μm, the ratio of infrared radiation energy is 25% or more of the total radiation energy, the mechanism by which the optical properties of the polarizing film can be improved is not clear, but it is presumed that by exceeding 25% of the total radiation energy. Molecular movement in the film excited by infrared rays with a wavelength of 2 μm and below 4 μm promotes the immobilization of dichroic dyes such as iodine in the cross-linked film, thereby improving the optical properties of the polarizing film.

Figure 2 shows the radiation energy spectrum of various types of electromagnetic wave irradiators. In addition, Table 1 shows the ratio of the electromagnetic wave radiation energy to the total radiation energy in each wavelength region (represented by the range of wavelength x μm) of various types of electromagnetic wave irradiators. The electromagnetic wave irradiators shown in Fig. 2 and Table 1 are halogen heaters (heat source temperature 2600°C), short-wavelength infrared heaters (heat source temperature 2200°C), fast-reaction medium-wavelength infrared heaters (heat source temperature 1600°C), carbon Heater (heat source temperature 1200°C), carbon heater (heat source temperature 950°C), and mid-wavelength infrared heater (heat source temperature 900°C).

[Table 1]

As shown in Table 1, due to the short-wavelength infrared heater (heat source temperature of 2200 ° C), the rapid response medium-wavelength infrared heater (heat source temperature 1600°C), carbon heaters (heat source temperature 1200°C), carbon heaters (heat source temperature 950°C), and mid-wavelength infrared heaters (heat source temperature 900°C), the proportion of infrared radiation energy with wavelengths exceeding 2μm and below 4μm is all Since the radiation energy is 25% or more, it can be used as an electromagnetic wave irradiator constituting the electromagnetic wave irradiation part 71 . The electromagnetic wave irradiation unit 71 may be constituted by one electromagnetic wave irradiator, or may be constituted by a plurality of electromagnetic wave irradiators. When it is composed of a plurality of electromagnetic wave irradiators, the plurality of electromagnetic wave irradiators are selected so that the infrared radiation energy of wavelengths exceeding 2 μm and 4 μm or less radiated by the plurality of electromagnetic wave irradiators is the total amount of electromagnetic waves radiated by the plurality of electromagnetic wave irradiators. More than 25% of the radiant energy. In addition, although the electromagnetic wave irradiation part 71 is comprised so that only one surface of a film may irradiate an electromagnetic wave in FIG. 1, a plurality of electromagnetic wave irradiators may be arrange|positioned so that an electromagnetic wave may be irradiated to both surfaces of a film. It is preferable that the electromagnetic wave irradiation part 71 is comprised so that the whole width direction of the polyvinyl-alcohol-type resin film of irradiation object may be irradiated with an electromagnetic wave.

In the electromagnetic wave irradiation step, the electromagnetic wave is preferably irradiated from the upper side in the vertical direction with respect to the film surface. Moreover, in the electromagnetic wave irradiation part 71, the distance between the electromagnetic wave radiation opening of the electromagnetic wave irradiator and the film is preferably 2 to 40 cm, more preferably 5 to 20 cm. However, it is preferable that this distance is appropriately selected and performed in consideration of the radiation energy amount of the electromagnetic wave radiated from the electromagnetic wave irradiator, the temperature of the film surface, and the like. The film surface temperature at the time of electromagnetic wave irradiation is preferably maintained at 30 to 90°C, and more preferably maintained at 40 to 80°C.

In the electromagnetic wave irradiation step, the electromagnetic wave irradiation heat amount per unit volume of the film is usually 100 J/cm ³ or more and 50 kJ/cm ³ or less. From the viewpoint of improving the optical properties of the polarizing film, it is preferably 100 J/cm ³ or more, more preferably 500 J/cm ³ or more, and still more preferably 1000 J/cm ³ or more. In addition, from the viewpoint of suppressing deterioration of the film due to temperature rise, the radiation heat amount of the electromagnetic wave per unit volume of the film is preferably 10 kJ/cm ³ or less, more preferably 5000 J/cm ³ or less, and still more preferably 3000 J/cm 3 ³ or less. Usually, the reduction of the moisture content of the film is proportional to the heat of irradiation of electromagnetic waves, but the purpose of the electromagnetic wave irradiation step of the present invention is not to reduce the moisture content of the film, so the heat of irradiation can be appropriately selected, preferably within the above-mentioned range. .

In the present invention, by performing the electromagnetic wave irradiation step before the cleaning treatment, the optical properties of the obtained polarizing film can be improved. The electromagnetic wave irradiation step may be performed only on the film after immersion in at least one cross-linking bath, and as shown in FIG. 1 , it is not limited to be performed on the film after immersion in all cross-linking baths. That is, in the example shown in FIG. 1, the electromagnetic wave irradiation step may be performed on the film after immersion in the first crosslinking bath and before the immersion in the second crosslinking bath, and the electromagnetic wave irradiation may be performed on the film after immersion in the second crosslinking bath. step. However, through the electromagnetic wave irradiation step, the boric acid entering the film due to immersion in the crosslinking bath can be crosslinked. Therefore, the electromagnetic wave irradiation step for the film immersed in all the crosslinking baths can more effectively carry out the boric acid crosslinking. better.

The irradiation of electromagnetic waves is preferably performed within 10 seconds after pulling out the film from the crosslinking bath, and more preferably within 5 seconds. The optical properties of the polarizing film by electromagnetic wave irradiation can be further improved as the time taken from the crosslinking bath to irradiate the electromagnetic wave is shorter. Moreover, in the electromagnetic wave irradiation process, it is preferable that there are few water molecules adhering to the film surface. If there are water molecules on the surface of the film, the water molecules on the surface of the film will absorb infrared rays, thus reducing the effect of stimulating the movement of molecules in the film by electromagnetic wave irradiation. After pulling from the crosslinking bath Since the cross-linking liquid adheres to the surface of the film, it is preferable to provide a liquid removal method for removing it before the electromagnetic wave irradiation step. In Fig. 1, the nip roll 53b also functions as a liquid removal method for removing the crosslinking liquid adhering to the film surface. In addition to nip rolls, there is a method of removing liquid by blowing air on the film, and a doctor blade or the like that is in contact with the film to remove liquid can also be used.

From an economical point of view, if the film processing speed is high, specifically, when the processing speed is 10 to 100 m/min as the high processing speed, the electromagnetic wave irradiation time is short and the irradiation heat is insufficient. In this corresponding method, a plurality of electromagnetic wave irradiators are installed in parallel, whereby sufficient irradiation heat can be obtained.

(drying step)

It is preferable to perform the process of drying a polyvinyl-alcohol-type resin film after a washing|cleaning process. The drying of the film is not particularly limited, and can be performed using a drying furnace 21 as shown in FIG. 1 . As the drying furnace 21, for example, a hot air dryer can be used. The drying temperature is, for example, about 30 to 100° C., and the drying time is, for example, about 30 to 600 seconds. The treatment of drying the polyvinyl alcohol-based resin film can be performed using a far-infrared heater. The thickness of the polarizing film 23 obtained in the above manner is, for example, about 5 to 30 μm.

The obtained polarizing film can be sequentially wound into a roll to form a roll, or it can be directly supplied to the polarizing plate manufacturing step (steps such as layering a protective film on one side or two areas of the polarizing film) without being wound.

(Other processing steps for polyvinyl alcohol-based resin film)

Processing other than the above processing can be added. Examples of additional treatments include: immersion in a boric acid-free iodide aqueous solution after the cross-linking step treatment (color correction treatment), and treatment (zinc treatment) immersed in an aqueous solution that does not contain boric acid but contains zinc chloride or the like.

<Polarizing film>

By manufacturing the polarizing film by the above method, a polarizing film satisfying the following can be obtained:

i) a boron content of 2.0 to 3.5% by weight,

ii) Visual sensitivity correction monomer transmittance (Ty) is 42.0% or more,

iii) Visual sensitivity correction polarization degree (Py) is more than 99.990%,

iv) The ratio A ₄₈₀ /A ₆₀₀ of the absorbance A ₄₈₀ in the absorption axis direction of the polarizing film at a wavelength of 480 nm and the absorbance A 600 in the absorption axis direction of the polarizing film at a wavelength of ₆₀₀ nm is 0.75 or more and 1.00 or less.

Since the boron content of the polarizing film is as described in i) above, the shrinkage force can be suppressed. The boron content of the polarizing film is more preferably 2.0 to 3.0% by weight. The shrinkage force of the polarizing film is preferably 3.8N/2mm or less, more preferably 3.5N/2mm or less. The boron content and shrinkage force of the polarizing film here are measured according to the methods described in the examples described later.

In general, high optical characteristics can be obtained by making the boron content of the polarizing film as high as, for example, 3.8% by weight or more. However, it is difficult to obtain excellent optical characteristics while the boron content of the polarizing film is as low as i) above. In the production method of the present invention, by having an electromagnetic wave irradiation step, a polarizing film excellent in optical properties satisfying the above i) and satisfying the above ii) and the above iii) can be obtained. The optical sensitivity correction monomer transmittance (Ty) and the optical sensitivity correction polarization degree (Py) of the polarizing film here are measured according to the methods described in the following examples.

Furthermore, in the production method of the present invention, as the electromagnetic wave used in the electromagnetic wave irradiation step, by using electromagnetic waves whose radiation energy ratio of infrared rays having a wavelength exceeding 2 μm and 4 μm or less is 25% or more of the total radiation energy, it is possible to A polarizing film with an excellent hue close to neutral gray is obtained that satisfies the properties of iv) above. When A ₄₈₀ /A ₆₀₀ is less than 0.75, blue is strong, and when A ₄₈₀ /A ₆₀₀ exceeds 1.00, red is strong. In order to suppress red, A ₄₈₀ /A ₆₀₀ is preferably 0.90 or less. That is, according to the production method of the present invention, a polarizing film having excellent optical properties can be obtained which can reduce the boron content of the polarizing film to the above i) and suppress the shrinkage force, and simultaneously satisfy the properties ii) to iv) above. .

<Polarizer>

A polarizing plate can be obtained by bonding a protective film to at least one side of the polarizing film manufactured by the above method via an adhesive. Examples of the protective film include films made of cellulose acetate-based resins such as cellulose triacetate or cellulose diacetate; polyethylene terephthalate, polyethylene naphthalate, and polyethylene terephthalate. Films composed of polyester resins such as butylene formate; polycarbonate resin films, cycloolefin resin films; acrylic resin films; polypropylene resins and chain olefin resins.

In order to improve the adhesion between the polarizing film and the protective film, corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. can be performed on the bonding surface of the polarizing film and/or protective film deal with. Examples of the adhesive used for bonding the polarizing film and the protective film include active energy ray-curable adhesives such as ultraviolet curable adhesives, aqueous solutions of polyvinyl alcohol-based resins, or aqueous solutions in which crosslinking agents are blended, and Carbamate Water-based adhesives such as ethyl ester emulsion adhesives. The ultraviolet curable adhesive can be a mixture of an acrylic compound and a photo-radical polymerization initiator, or a mixture of an epoxy compound and a photo-cationic polymerization initiator, and the like. Moreover, a cationically polymerizable epoxy compound and a radically polymerizable acrylic compound may be used together, and a photocationic polymerization initiator and a photoradical polymerization initiator as initiators may be used together.

[Example]

Hereinafter, the present invention will be further specifically described with reference to examples, but the present invention is not limited to these examples.

<Example 1>

Using the manufacturing apparatus shown in FIG. 1, the polarizing film of Example 1 was manufactured from the polyvinyl alcohol-type resin film. Specifically, a long polyvinyl alcohol (PVA) raw material film with a thickness of 60 μm [trade name "Kuraray Vinylon VF-PE#6000" manufactured by Kuraray Co., Ltd., average degree of polymerization 2400, degree of saponification 99.9 mol% or more) It was continuously conveyed while being pulled out from the roll, and was immersed in a swelling bath containing pure water at 30°C for a residence time of 79 seconds (swelling step). After that, the film pulled out from the swelling bath was dipped in a dyeing bath containing iodine at 30° C. containing potassium iodide/boric acid/water of 1/0.3/100 (weight ratio) with a residence time of 123 seconds (dyeing step). Next, the film pulled out from the dyeing bath was immersed in a first crosslinking bath at 53°C with a potassium iodide/boric acid/water ratio of 11/2.0/100 (weight ratio) for a residence time of 44 seconds, followed by potassium iodide/boric acid/water as a The 40°C second crosslinking bath of 11/2.0/100 (weight ratio) was immersed with a residence time of 6 seconds (crosslinking step). In the dyeing step and the cross-linking step, longitudinal uniaxial stretching is performed by stretching between rolls in a bath. The total stretching ratio based on the raw material film was 5.65 times.

Next, an electromagnetic wave irradiator (medium wavelength infrared heater (MW heater), product name: Golden 8 Medium-wave twin tube emitter, manufactured by Heraeus Corporation) was used for the film drawn from the second crosslinking bath 17b and passed through the nip roll 53b. , heat source temperature 900℃, maximum energy density 60kW/m ² ), the electromagnetic wave radiation port is arranged at a position 5cm away from the film surface, and the electromagnetic wave is irradiated at 50% of the maximum radiation output of the electromagnetic wave irradiator. The radiation heat of electromagnetic waves per unit volume of the film was 490 J/cm ³ . In addition, the irradiated heat amount of the electromagnetic wave per unit volume of the film was calculated by the following formula.

(Electromagnetic wave irradiation heat per unit volume of film)={(maximum energy density)×(heater heating part surface area)×output (%)/(electromagnetic wave irradiation area)}×(electromagnetic wave irradiation time)÷(film thickness)

Output (%) refers to the ratio (%) of the actual irradiated output to the maximum irradiated output of the electromagnetic wave irradiator.

After pulling out from the 2nd crosslinking bath 17b, the time required from conveying a film until reaching the irradiation position of an electromagnetic wave irradiator and irradiating an electromagnetic wave is 5 seconds.

The film irradiated with electromagnetic waves was immersed in a cleaning bath 19 containing pure water at 5°C for a residence time of 3 seconds (cleaning step). Then, the film was dried in the drying furnace 21 at a temperature of 60° C. and an absolute humidity of 11 g/cm ³ to obtain a polarizing film. The thickness of the obtained polarizing film was 23 μm.

<Examples 2 to 6, Comparative Examples 3, 4>

In the electromagnetic wave irradiation step, the type of electromagnetic wave irradiator, output (%), the amount of electromagnetic wave irradiation heat per unit volume of the film, and the weight parts of boric acid relative to 100 parts by weight of water in the first crosslinking bath and the second crosslinking bath are set. As shown in Table 2, except this point, it carried out similarly to Example 1, and obtained the polarizing film. The thicknesses of the obtained polarizing films were all 23 μm. As an electromagnetic wave irradiator, halogen heaters (product name: straight tube halogen heater lamp QIR, manufactured by Ushio Lighting Co., Ltd., heat source temperature 2600°C, maximum energy density 300kW/m ² ), short-wavelength infrared rays Heater (SW heater) (product name: Golden 8 Short-wave twin tube emitter, manufactured by Heraeus, heat source temperature 2200°C, maximum energy density 200kW/m ² ), high-speed reaction mid-wavelength infrared heater (FRMW heater) (Product name: Golden 8 Medium-wave fast response twin tubu emitter, manufactured by Heraeus, heat source temperature 1600°C, maximum energy density 150kW/m ² ), Mid-wavelength infrared heater (MW heater) (Product name: Golden 8 Medium -wave twin tube emitter, manufactured by Heraeus, heat source temperature 900°C, maximum energy density 60kW/m ² ).

<Comparative Examples 1, 2, 5>

The procedure was the same as that of Example 1, except that the electromagnetic wave irradiation step was not performed, and the parts by weight of boric acid relative to 100 parts by weight of water in the first cross-linking bath and the second cross-linking bath were set as shown in Table 2. A polarizing film is obtained by applying it in the same way. The thickness of the obtained polarizing film was 23 μm.

[Evaluation of polarizing film]

(a) Measurement of monomer transmittance and polarization degree

The MD transmittance and TD transmittance in the wavelength range of 380 to 780 nm were measured using a spectrophotometer with an integrating sphere (“V7100” manufactured by JASCO Corporation) for the polarizing films obtained in the respective Examples and Comparative Examples. , and the monomer transmittance and polarization degree at each wavelength are calculated according to the following formula.

Monomer penetration rate (%)=(MD+TD)/2

Polarization (%)={(MD-TD)/(MD+TD)}×100

"MD transmittance" refers to the transmittance when the direction of polarized light emitted by the Glan-Thomson fluoride is parallel to the transmission axis of the polarizing film sample, and is expressed as "MD" in the above formula. In addition, the "TD transmittance" refers to the transmittance when the polarizing direction of the Glan-Thomson ion beam is perpendicular to the transmission axis of the polarizing film sample, and is expressed as "TD" in the above formula. The obtained monomer transmittance and polarization degree were corrected by the 2-degree field of view (C light source) of JIS Z 8701:1999 "Color Representation Method - XYZ Color System and X ₁₀ Y ₁₀ Z ₁₀ Color System", and The sensitivity-corrected single transmittance (Ty) and the sensitivity-corrected polarization degree (Py) were obtained. Table 2 shows the calculation results of the sensitivity-corrected single transmittance (Ty) and the sensitivity-corrected polarization (Py).

(b) Measurement of absorbance of polarizing film

The measurement was performed using a spectrophotometer with an integrating sphere (“V7100” manufactured by JSC Co., Ltd.). Specifically, the TD transmittances TD ₄₈₀ and TD ₆₀₀ in the wavelengths of 480 nm and 600 nm are used, according to the following formula:

A ₄₈₀ =-log ₁₀ (TD ₄₈₀ /100)

A ₆₀₀ =-log ₁₀ (TD ₆₀₀ /100)

The absorbance A ₄₈₀ at a wavelength of 480 nm and the absorbance A ₆₀₀ at a wavelength of 600 nm were calculated. Table 2 shows the calculated values of the absorbance A ₄₈₀ , the absorbance A ₆₀₀ , and the ratio A ₄₈₀ /A ₆₀₀ calculated from these values.

(c) MD shrinkage force

From the obtained polarizing film, a measurement sample having a width of 2 mm and a length of 10 mm with the absorption axis direction (MD, extending direction) as the long side was cut out. This sample was set in a thermomechanical analyzer made by SII NanoTechnology Co., Ltd. (TMA) "EXSTAR-6000", the shrinkage force (MD shrinkage force) in the longitudinal direction (absorption axis direction, MD) generated when kept at 80° C. for 4 hours while maintaining a fixed size was measured. Table 2 shows the measured contractile force values.

(d) Boron content of polarizing film

0.2 g of the polarizing film was added to 170 ml of pure water, and after completely dissolving at 95° C., 30 g of a mannitol aqueous solution (12.5 wt %) was added to obtain a sample solution for measurement. A sodium hydroxide aqueous solution (1 mol/l) was dropped until the sample solution for measurement reached the neutralization point, and the boron content (% by weight) in the polyvinyl alcohol-based resin film was calculated from the drop amount according to the following formula.

Boron content rate (% by weight)=1.08×dropping amount of sodium hydroxide aqueous solution (ml)/weight of polarizing film (g)

Table 2 shows the measured values of the boron content.

[Table 2]

As shown in Table 2, the polarizing film systems of Examples 1 to 6 satisfy:

i) a boron content of 2.0 to 3.5% by weight,

ii) Visual sensitivity correction monomer penetration rate Ty is 42.0% or more,

iii) Visual sensitivity correction polarization degree Py is 99.990% or more,

iv) The ratio A ₄₈₀ /A ₆₀₀ of the absorbance A ₄₈₀ in the absorption axis direction of the polarizing film at a wavelength of 480 nm and the absorbance A 600 in the absorption axis direction of the polarizing film at a wavelength of ₆₀₀ nm is 0.75 or more and 1 or less. Further, by satisfying the above i), the shrinkage force can be suppressed, and by satisfying the above iv), there is no problem with the hue. The polarizing film of Comparative Example 1 did not perform the electromagnetic wave irradiation step, and was the one with the lower value of the correction polarization degree (Py) of the visual sensitivity.

The polarizing film system of Comparative Example 2 does not satisfy the above i), so the shrinkage force is large. The value of A ₄₈₀ /A ₆₀₀ of the polarizing film of Comparative Example 3 exceeds 1, so the red color is stronger. In Comparative Examples 4 and 5, the boron content rate was insufficient, so the value of the correction polarization degree (Py) of the visual sensitivity was lower.

10: Raw material film composed of polyvinyl alcohol-based resin

11: Raw Rolls

13: Swelling bath

15: Dye Bath

17a: 1st Crosslinking Bath

17b: 2nd crosslinking bath

19: wash bath

21: Drying oven

23: polarizing film

30 to 48, 60, 61: Guide rollers

50 to 52, 53a, 53b, 54, 55: nip rolls

71: Electromagnetic wave irradiation section

Claims (1)

A polarizing film, which is a polarizing film obtained by dyeing a polyvinyl alcohol-based resin film with a dichroic dye, wherein the boron content is 2.0 to 3.5% by weight, the transmittance of the visual sensitivity correction monomer is 42.0% or more, and the visual sensitivity correction monomer transmittance is more than 42.0%. The sensitivity correction polarization is 99.990% or more, and the ratio _A480 /A600 of the absorbance A480 in the direction of the absorption axis at a wavelength of ₄₈₀ nm and the absorbance A600 of the polarizing film at a wavelength of ₆₀₀ nm is 0.75 or _more and 1.00 or less.

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