KR102572466B1 - Method and apparatus of manufacturing polarizing film - Google Patents

Abstract

(Problem) To provide a manufacturing method and manufacturing apparatus for a polarizing film that does not contract easily even when exposed to a high temperature environment and has excellent optical properties.
(Solution) A dyeing step of dyeing a polyvinyl alcohol-based resin film with a dichroic dye, and sequentially placing the polyvinyl alcohol-based resin film after the dyeing step in n (n is an integer of 2 or more) crosslinking baths. A crosslinking step of immersion and crosslinking treatment, and the polyvinyl alcohol-based resin film pulled out after being immersed in the xth crosslinking bath disposed at the xth (x is an integer less than or equal to n) from the upstream side toward the downstream side, 2 μm An electromagnetic wave irradiation step of irradiating electromagnetic waves in which the ratio of infrared radiant energy with a wavelength of more than 4 μm or less is 25% or more of the total radiant energy, wherein each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more; , The method for producing a polarizing film in which the nth crosslinking bath disposed at the nth position from the upstream side toward the downstream side is composed of a solution having a concentration of the boron compound of 2.4% by mass or less.

Description

Manufacturing method and manufacturing apparatus of polarizing film {METHOD AND APPARATUS OF MANUFACTURING POLARIZING FILM}

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

A polarizing plate is widely used as a polarizing element or the like in image display devices such as liquid crystal display devices. As a polarizing plate, the thing of the structure which bonded the transparent resin film (protective film etc.) using an adhesive etc. to one side or both sides of a polarizing film is common.

The polarizing film is mainly immersed in a dyeing bath containing dichroic dyes such as iodine, and subsequently immersed in a crosslinking bath containing a crosslinking agent such as boric acid, etc. While performing, it is manufactured by uniaxially stretching a film at a certain stage. Uniaxial stretching includes dry stretching in which stretching is performed in air, and wet stretching in which stretching is performed in a liquid such as the dye bath and crosslinking bath described above.

A crosslinked polarizing film tends to shrink when exposed to a high-temperature environment, and may not have sufficient durability. In Unexamined-Japanese-Patent No. 2013-148806 (Patent Document 1), it is described that the boron content of the polarizing film is reduced to 1 to 3.5% by weight to provide a polarizing film excellent in durability.

Patent Document 1: Japanese Unexamined Patent Publication No. 2013-148806

An object of the present invention is to provide a method and apparatus for producing a polarizing film that does not contract easily even when exposed to a high-temperature environment and has excellent optical properties.

This invention provides the manufacturing method and manufacturing apparatus of the polarizing film shown below.

[1] As a method for producing a polarizing film from a polyvinyl alcohol-based resin film,

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 order in n (n is an integer of 2 or more) crosslinking baths for crosslinking treatment;

On the polyvinyl alcohol-based resin film taken out after being immersed in the x-th crosslinking bath disposed at the x-th (x is an integer of n or less) from the upstream side toward the downstream side, infrared rays having a wavelength of more than 2 μm and less than 4 μm Including an electromagnetic wave irradiation step of irradiating electromagnetic waves in which the ratio of radiant energy is 25% or more of the total radiant energy,

Each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more, and the nth crosslinking bath disposed at the nth position from the upstream side toward the downstream side is made of a solution having a boron compound concentration of 2.4% by mass or less. , A method for producing a polarizing film.

[2] Production of the polarizing film according to [1], wherein in the electromagnetic wave irradiation step, the irradiation heat amount of the electromagnetic wave is 100 J/cm ³ or more and 50 kJ/cm ^{3 or} less per unit volume of the polyvinyl alcohol-based resin film. method.

[3] The method for producing a polarizing film according to [1] or [2], wherein x is n.

[4] The method for producing a polarizing film according to any one of [1] to [3], wherein the temperature of the nth crosslinking bath is 30°C or higher.

[5] The method for producing a polarizing film according to [4], wherein the temperature of the nth crosslinking bath is 35°C or higher.

[6] Among the crosslinking baths arranged from the 1st to the n−1th positions from the upstream side toward the downstream side, at least one of the crosslinking baths is composed of a solution having a concentration of the boron compound of 2.5% by mass or more, [1] to [ 5] The manufacturing method of the polarizing film described in any one of them.

[7] The method for producing a polarizing film according to [6], wherein the temperature of the crosslinking bath made of a solution having a boron compound concentration of 2.5% by mass or higher is 45°C or higher.

[8] The method for producing a polarizing film according to [7], wherein the temperature of the crosslinking bath made of a solution having a concentration of the boron compound of 2.5% by mass or higher is 55°C or higher.

[9] The method for producing a polarizing film according to any one of [1] to [8], further comprising a washing step of washing the polyvinyl alcohol-based resin film after the crosslinking step and the electromagnetic wave irradiation step.

[10] With regard to the polyvinyl alcohol-based resin film taken out after being immersed in the xth cross-linking bath, [1] to [[1] to [ 9] The manufacturing method of the polarizing film described in any one of them.

[11] The method for producing a polarizing film according to any one of [1] to [10], wherein the electromagnetic wave irradiation step is performed within 5 seconds after taking out the xth crosslinking bath.

[12] A manufacturing apparatus for producing a polarizing film from a polyvinyl alcohol-based resin film,

A dyeing unit for dyeing the polyvinyl alcohol-based resin film with a dichroic dye;

A crosslinking portion for crosslinking treatment by immersing the polyvinyl alcohol-based resin film after the dyeing treatment in n (n is an integer of 2 or more) crosslinking baths;

On the polyvinyl alcohol-based resin film taken out after being immersed in the x-th crosslinking bath disposed at the x-th (x is an integer of n or less) from the upstream side toward the downstream side, infrared rays having a wavelength of more than 2 μm and less than 4 μm An electromagnetic wave irradiation unit for irradiating electromagnetic waves having a ratio of radiant energy of 25% or more of total radiant energy,

Each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more, and the nth crosslinking bath disposed at the nth position from the upstream side toward the downstream side is made of a solution having a boron compound concentration of 2.4% by mass or less. , A device for producing a polarizing film.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide the manufacturing method and manufacturing apparatus of the polarizing film which does not shrink easily even when exposed to a high-temperature environment, and also has excellent optical characteristics.

1 is a diagram showing a radiant energy spectrum for each type of electromagnetic wave irradiator.
Fig. 2 is a cross-sectional view schematically showing an example of a method for manufacturing a polarizing film of a first embodiment according to the present invention and a polarizing film manufacturing apparatus used therein.

[Method for producing polarizing film]

In the polarizing film of the present invention, a dichroic dye (iodine or dichroic dye) is adsorbed and oriented on a polyvinyl alcohol-based resin film stretched uniaxially. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying polyvinyl acetate-based resin. The saponification degree is usually about 85 mol% or more, preferably about 90 mol% or more, and more preferably about 99 mol% or more. The polyvinyl acetate-based resin may be, for example, a copolymer of vinyl acetate and another monomer copolymerizable therewith other than polyvinyl acetate which is a homopolymer of vinyl acetate. As another monomer which can be copolymerized, unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids etc. are mentioned, for example. The degree of polymerization of the polyvinyl alcohol-based resin is usually about 1000 to 10000, preferably about 1500 to 5000.

These polyvinyl alcohol-based resins may be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral and the like modified with aldehydes can also be used. In the present specification, "polyvinyl alcohol-based resin" means a resin in which the structural unit of vinyl alcohol (-CH ₂ -CH(OH)-) is 50 mol% or more among all the structural units included in the resin.

In the present invention, as a starting material for producing a polarizing film, unstretched polyvinyl 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, still more preferably 30 μm or less An alcohol-based resin film (original film) is used. Accordingly, it is possible to obtain a thin polarizing film for which market demand is gradually increasing. The width of the raw film is not particularly limited, and may be, for example, about 400 to 6000 mm. The raw film is prepared, for example, as a roll (original roll) of a long unstretched polyvinyl alcohol-based resin film.

The polarizing film is continuously conveyed along the film conveyance path of the polarizing film manufacturing apparatus while unwinding the above-described elongated raw film from a raw film roll, and immersed in a treatment liquid (hereinafter also referred to as a “treatment bath”) accommodated in a treatment tank. It can manufacture continuously as a long polarizing film by implementing a drying process, after performing the predetermined|prescribed processing process to take out later. In addition, the treatment step is not limited to a method in which the film is immersed in a treatment bath as long as the treatment is performed by contacting the treatment solution to the film, except for the crosslinking step described later, and the treatment solution is adhered to the film surface by spraying, dripping, dripping, or the like. A method of treating the film by making it do so may be used. When the treatment step is performed by immersing the film in a treatment bath, the number of treatment baths in which one treatment step is performed is not limited to one, and the film is sequentially immersed in two or more treatment baths to complete one treatment step. You can do it.

Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a crosslinking liquid, and a washing liquid. As the treatment step, a swelling step in which a swelling liquid is brought into contact with a raw film to perform a swelling treatment, a dyeing step in which a dyeing solution is brought into contact with the film after swelling treatment to perform a dyeing treatment, and a crosslinking solution is brought into contact with the film after the dyeing treatment A crosslinking step of performing a crosslinking treatment and a washing step of performing a washing treatment by bringing a washing solution into contact with the film after the crosslinking treatment are exemplified. In addition, during these series of treatment steps (ie, before and after any one or more treatment steps and/or during any one or more treatment steps), a wet or dry uniaxial stretching treatment is performed. You may add another treatment process as needed.

<Crosslinking process>

The crosslinking step is a treatment performed for purposes such as water resistance by crosslinking or color adjustment (preventing a film from becoming bluish). In the crosslinking step of the present invention, the crosslinking treatment is performed by sequentially immersing the film in n (n is an integer of 2 or more) crosslinking baths. Each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more. A boron compound acts as a crosslinking agent, and examples thereof include boric acid and borax. The crosslinking liquid may contain a crosslinking agent such as glyoxal or glutaraldehyde in addition to the boron compound. As the solvent for the crosslinking solution, water can be used, for example, but an organic solvent compatible with water may be further included. The temperature of each crosslinking bath is preferably 30° C. or higher from the viewpoint of promoting crosslinking of the polyvinyl alcohol-based resin film and equilibration of the amount of the crosslinking agent in the polyvinyl alcohol film. In addition, the temperature of each crosslinking bath is preferably 65°C or less, and more preferably 60°C or less, from the viewpoint of preventing the elution of the polyvinyl alcohol-based resin film. The immersion time of the film in each crosslinking bath is about 10 to 600 seconds, preferably 20 to 300 seconds, more preferably 20 to 200 seconds.

The n-th cross-linking bath (lowest-most cross-linking bath) disposed at the nth position from the upstream side toward the downstream side is made of a solution having a boron compound concentration of 2.4% by mass or less. When n is 2, the second crosslinking bath (downstream crosslinking bath) is made of a solution having a boron compound concentration of 2.4% by mass or less. In the present invention, the crosslinking step is performed using n crosslinking baths, and a polarizing film with suppressed shrinkage force can be obtained by making the nth crosslinking bath a solution having a boron compound concentration of 2.4% by mass or less.

The temperature of the nth crosslinking bath is preferably 30°C or higher, more preferably 35°C or higher, and still more preferably 42°C or higher, from the viewpoint of accelerating the equilibration of the amount of crosslinking agent in the polyvinyl alcohol film.

In the present invention, it is preferable that at least one of the cross-linking baths, among the cross-linking baths disposed at the 1st to n-1th positions from the upstream side to the downstream side, is made of a solution having a boron compound concentration of 2.5% by mass or more, and boron It is more preferable to consist of a solution in which the concentration of the compound is 2.8% by mass or more. When n is 2, the first crosslinking bath (uppermost crosslinking bath) is preferably composed of a solution having a boron compound concentration of 2.5% by mass or more. When n is 3, it is preferable that at least one of the first crosslinking bath or the second crosslinking bath is made of a solution having a boron compound concentration of 2.5% by mass or more. In the present invention, by having a crosslinking bath comprising a solution having a boron concentration of 2.5% by mass or more, water resistance and color adjustment by crosslinking can be more effectively performed through the entire crosslinking step. Further, by immersing the polyvinyl alcohol-based resin film in a crosslinking bath having a boron concentration of 2.5% by mass or more, folding of the ends of the polyvinyl alcohol-based resin film during conveyance can be suppressed. When the first crosslinking bath (uppermost crosslinking bath) is a crosslinking bath made of a solution having a boron concentration of 2.5% by mass or more, in all the crosslinking baths, it is preferable because folding of the ends during conveyance can be effectively suppressed. The number of crosslinking baths comprising a solution having a boron compound concentration of 2.5% by mass or more may be one or two or more.

The temperature of the crosslinking bath comprising a solution having a boron compound concentration of 2.5% by mass or higher is preferably 45°C or higher, and more preferably 55°C or higher, from the viewpoint of promoting crosslinking of the polyvinyl alcohol-based resin film and effectively making it water resistant.

<Electromagnetic wave irradiation process>

In the present invention, an electromagnetic wave irradiation step of irradiating electromagnetic waves to a polyvinyl alcohol-based resin film drawn after being immersed in the x-th crosslinking bath disposed at the x-th (x is an integer less than or equal to n) from the upstream side toward the downstream side do

In the electromagnetic wave used in the electromagnetic wave irradiation step of the present invention, the ratio of infrared radiant energy having a wavelength of more than 2 μm to 4 μm or less is 25% or more of the total radiant energy, preferably 28% or more, and more preferably 35% or more. more than 1% By irradiating the film with such electromagnetic waves, the optical properties of the resulting polarizing film can be improved. In addition, with respect to electromagnetic waves used in the present invention, the upper limit of the ratio of infrared radiant energy having a wavelength of more than 2 μm to 4 μm or less is not particularly limited, but is, for example, 80% or less. Usually, electromagnetic waves with a wavelength of 0.75 μm to 1000 μm are called infrared rays.

In the electromagnetic wave irradiation step, the mechanism by which the optical properties of the polarizing film can be improved by irradiating electromagnetic waves in which the ratio of infrared radiant energy of a wavelength of more than 2 μm to 4 μm or less is 25% or more of the total radiant energy is not clear. It is speculated that the optical properties of the polarizing film can be improved by promoting the immobilization of dichroic dyes such as iodine in the crosslinked film by molecular motion in the film excited by infrared rays with a wavelength of more than 2 μm and not more than 4 μm. do.

1 shows a radiation energy spectrum for each type of electromagnetic wave irradiator. In addition, Table 1 shows the ratio of the radiant energy of electromagnetic waves in each wavelength region (expressed in the range of wavelength x μm) to the total radiant energy for each type of electromagnetic wave irradiator. The electromagnetic wave irradiator shown in FIG. 1 and Table 1 includes a halogen heater (heat source temperature: 2600° C.), a short-wavelength infrared heater (heat source temperature: 2200° C.), a high-speed response mid-wavelength infrared heater (heat source temperature: 1600° C.), a carbon heater (heat source temperature: 1200° C.) ), a carbon heater (heat source temperature: 950°C), and a mid-wavelength infrared heater (heat source temperature: 900°C).

As shown in Table 1, short-wave infrared heater (heat source temperature 2200 ° C.), high-speed response mid-wavelength infrared heater (heat source temperature 1600 ° C.), carbon heater (heat source temperature 1200 ° C.), carbon heater (heat source temperature 950 ° C.), medium-wavelength An infrared heater (heat source temperature: 900° C.) is suitable as an electromagnetic wave irradiator for performing the electromagnetic wave irradiation step according to the present invention, since the ratio of infrared radiant energy with a wavelength of more than 2 μm to 4 μm or less is 25% or more of the total radiant energy. available.

In the electromagnetic wave irradiation step, the irradiation heat amount of electromagnetic waves per unit volume of the film can be 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. From the viewpoint of suppressing deterioration of the film due to temperature rise, the irradiation heat amount of electromagnetic waves per unit volume of the film is preferably 10 kJ/cm ³ or less, more preferably 5000 J/cm ³ or less, and 3000 J/cm 3 or less. It is more preferable that it is cm ³ or less. Usually, the moisture content of the film decreases in proportion to the amount of heat irradiated with electromagnetic waves. However, since the purpose of the electromagnetic wave irradiation step of the present invention is not to reduce the amount of moisture in the film, the amount of irradiation heat can be appropriately selected, and preferably It is appropriately selected within the above range.

In this invention, the optical characteristic of the polarizing film obtained can be improved by performing an electromagnetic wave irradiation process before a washing process. The electromagnetic wave irradiation step may be performed on the film after being immersed in at least one cross-linking bath, and is not limited to being performed on the film after being immersed in all n cross-linking baths. However, since crosslinking of the boric acid introduced into the film can be promoted by being immersed in a crosslinking bath in the electromagnetic wave irradiation step, performing the electromagnetic wave irradiation step on the film after immersion in all the crosslinking baths reduces the crosslinking of the boron compound. It is preferable because it can advance more effectively. That is, x is n, and it is preferable to apply electromagnetic wave to the polyvinyl alcohol-based resin film taken out from the nth crosslinking bath.

The electromagnetic wave irradiation is preferably performed within 10 seconds, and more preferably within 5 seconds after the film is taken out of the xth crosslinking bath. The shorter the time from being taken out of the xth crosslinking bath to the electromagnetic wave irradiation, the more the optical properties of the polarizing film by electromagnetic wave irradiation can be improved. Further, in the electromagnetic wave irradiation step, it is preferable that the amount of moisture adhering to the surface of the film is small. This is because when moisture exists on the surface of the film, the water molecules contained in the moisture on the film surface absorb infrared rays, thereby reducing the effect of excitation of molecular motion in the film by electromagnetic wave irradiation. Since the crosslinking liquid adheres to the surface of the film immediately after being taken out of the x-th crosslinking bath, it is preferable to perform a liquid removal process to remove it before the electromagnetic wave irradiation process. Examples of the liquid removing means for performing the liquid removing step include a nip roll, a means for spraying air to a film to remove liquid, a scraper for contacting the film to perform liquid removal, and the like.

<First Embodiment>

Referring to Fig. 2, one embodiment of a method for manufacturing a polarizing film according to the present invention will be described in detail. Fig. 2 is a cross-sectional view schematically showing an example of a method for manufacturing a polarizing film according to the present invention and a polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in FIG. 2 conveys along the film conveyance route while unwinding the raw (unstretched) film 10 made of polyvinyl alcohol-based resin continuously from the original roll 11, thereby conveying the film along the film conveyance path. A swelling bath (swelling liquid accommodated in the swelling tank) 13, a dyeing bath (dyeing liquid accommodated in the dyeing tank) 15, a first crosslinking bath (first crosslinking liquid accommodated in the crosslinking tank) 17a, The second cross-linking bath (second cross-linking liquid accommodated in the cross-linking tank) 17b, the third cross-linking bath (third cross-linking liquid accommodated in the cross-linking tank) 17c, and the washing bath (washing liquid accommodated in the washing tank) 19 are sequentially supplied. It is comprised so that it may pass, and finally the drying furnace 21 may pass. The obtained polarizing film 23 can be conveyed to the following polarizing plate manufacturing process (process of bonding a protective film on one side or both surfaces of the polarizing film 23) as it is, for example. Arrows in Fig. 2 indicate the transport direction of the film.

In the description of FIG. 2, the "treatment tank" is a general term including a swelling tank, a dyeing tank, a crosslinking tank, and a washing tank, and a "treatment liquid" is a general term including a swelling liquid, a dyeing liquid, a crosslinking liquid, and a washing liquid. And "treatment bath" is a general term including a swelling bath, a dyeing bath, a crosslinking bath, and a washing bath. The swelling bath, the dyeing bath, the crosslinking bath, and the washing bath respectively constitute a swelling portion, a dyeing portion, a crosslinking portion, and a washing portion in the manufacturing apparatus of the present invention.

In addition to the treatment bath, the film conveyance route of the polarizing film manufacturing device includes guide rolls 30 to 48 and 56 to 61 that support the conveyed film or can further change the film conveyance direction, and press/press the conveyed film. It can be constructed by arranging at appropriate positions nip rolls 50 to 55 capable of being clamped and providing a driving force by rotation thereof to the film or further changing the film transport direction. Guide rolls and nip rolls can be placed 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 taken out of the treatment bath (see FIG. 2 ). For example, the film can be immersed in each treatment bath by providing one or more guide rolls in each treatment bath and transporting the film along these guide rolls.

In the polarizing film manufacturing apparatus shown in FIG. 2, nip rolls are disposed before and after each treatment bath (nip rolls 50 to 54), and accordingly, among one or more treatment baths, nip rolls disposed before and after that It is possible to perform stretching between rolls in which a circumferential speed difference is provided therebetween and longitudinal uniaxial stretching is performed.

In the polarizing film manufacturing apparatus shown in FIG. 2 , an electromagnetic wave irradiation unit 71 is disposed on a transport path upstream of the cleaning bath 19 as a downstream side of the third crosslinking bath 17c, and the electromagnetic wave irradiation step is performed. It is done. In the apparatus for producing a polarizing film shown in FIG. 2 , there are three crosslinking baths, that is, n is 3, and the crosslinking bath immediately before the electromagnetic wave irradiation step is the third crosslinking bath from the upstream side, that is, x is 3. Hereinafter, each process is demonstrated.

(swelling process)

The swelling step is performed for purposes such as removal of foreign matter on the surface of the raw film 10, removal of plasticizers in the raw film 10, provision of dyeability, plasticization of the raw film 10, and the like. The processing conditions are determined within a range in which the above object can be achieved and in a range in which problems such as extreme dissolution or devitrification of the raw film 10 do not occur.

Referring to FIG. 2 , in the swelling step, the raw film 10 is continuously unwound from the raw roll 11, conveyed along the film transport path, and the raw film 10 is immersed in the swelling bath 13 for a predetermined time And it can be carried out by continuously withdrawing. In the example of FIG. 2 , the raw film 10 is immersed in the swelling bath 13 after unwinding the raw film 10 by the guide rolls 60 and 61 and the nip roll 50. It is conveyed along the constructed film conveyance path. In the swelling process, it is conveyed along the film conveyance path constructed by the guide rolls 30 to 32 and the nip rolls 51 .

As the swelling liquid of the swelling bath 13, in addition to pure water, boric acid (Japanese Unexamined Patent Publication No. 10-153709), chloride (Japanese Unexamined Patent Publication No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols It is also possible to use an aqueous solution in which eg.

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 film 10 is preferably about 10 to 300 seconds, more preferably about 20 to 200 seconds. In the case where the raw film 10 is a polyvinyl alcohol-based resin film previously stretched in a substrate, 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 film 10 is preferably about 30 to 300 seconds, more preferably about 60 to 240 seconds.

In the swelling treatment, the problem that the raw film 10 swells in the width direction and causes wrinkles in the film tends to occur. As one means for conveying the film while removing these wrinkles, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used for the guide rolls 30, 31 and/or 32, or a cloth guider or bend The use of other widening devices such as bars and tenter clips is exemplified. Another means for suppressing the occurrence of wrinkles is to perform stretching treatment. For example, the uniaxial stretching process can be performed in the swelling bath 13 using the circumferential speed difference between the nip rolls 50 and the nip rolls 51 .

In the swelling treatment, since the film swells and expands also in the conveying direction of the film, when the film is not actively stretched, in order to eliminate the sagging of the film in the conveying direction, for example, a nip disposed before and after the swelling bath 13 It is preferable to provide means such as controlling the speed of the rolls 50 and 51. In addition, for the purpose of stabilizing the conveyance of the film in the swelling bath 13, the water flow in the swelling bath 13 is controlled by an underwater shower, or an EPC device (Edge Position Control device: detects the end of the film and meanders the film) It is also useful to use a device to prevent it) or the like in combination.

In the example shown in FIG. 2, the film taken out from the swelling bath 13 is introduced into the dyeing bath 15 by passing through guide rolls 32, nip rolls 51, and guide rolls 33 in order. .

(dyeing process)

The dyeing step is performed for the purpose of adsorbing and orienting the dichroic dye to the polyvinyl alcohol-based resin film after the swelling treatment. Treatment conditions are determined within a range capable of achieving the above object and within a range in which problems such as extreme dissolution or devitrification of the film do not occur. Referring to FIG. 2, in the dyeing process, the film after the swelling treatment is conveyed along the film conveyance path constructed by the nip rolls 51, guide rolls 33 to 36, and nip rolls 52, and the dyeing bath 15 ) (processing liquid contained in the dyeing tank) for a predetermined time, and then drawn out. In order to improve the dyeability of the dichroic dye, the film used in the dyeing step is preferably a film subjected to at least a certain degree of uniaxial stretching treatment, or instead of uniaxial stretching treatment before dyeing treatment, or uniaxial stretching treatment before dyeing treatment. In addition to the stretching treatment, it is preferable to perform a uniaxial stretching treatment at the time of the dyeing treatment.

In the case of using iodine as the 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 in weight ratio can be used for the dyeing solution of the dyeing bath 15. Instead of potassium iodide, other iodides such as zinc iodide may be used, or potassium iodide and other iodides may be used in combination. In addition, compounds other than iodide, such as boric acid, zinc chloride, cobalt chloride, and the like may coexist. When the dyeing solution of the dyeing bath contains boric acid, it is distinguished from the crosslinking bath in that it contains iodine, and if the aqueous solution contains about 0.003 parts by weight or more of iodine with respect to 100 parts by weight of water, the dyeing bath (15 ) to be considered. The temperature of the dyeing bath 15 when the film is immersed 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 30 to 40°C. It is about 600 seconds, Preferably it is 60 to 300 seconds.

In the case of using a water-soluble dichroic dye as the dichroic dye, for example, an aqueous solution having a concentration of dichroic dye/water = about 0.001 to 0.1/100 in weight ratio can be used for the dyeing solution of the dyeing bath 15. The dyeing bath 15 may coexist with a dyeing aid or the like, and may contain, for example, an inorganic salt such as sodium sulfate or a surfactant. As for the dichroic dye, only 1 type may be used independently, and two or more types of dichroic dye may be used together. The temperature of the dyeing bath 15 at the time of immersing the film is, for example, about 20 to 80°C, preferably about 30 to 70°C, and the immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300°C. It's about a second.

As described above, in the dyeing step, the film can be uniaxially stretched in the dyeing bath 15. Uniaxial stretching of the film can be performed by a method such as providing a difference in circumferential speed between the nip rolls 51 and 52 disposed before and after the dyeing bath 15 .

Also in the dyeing treatment, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles of the film in the same way as in the swelling treatment, guide rolls 33, 34, 35 and/or 36 are provided with expander rolls, spiral rolls, and crown rolls. A roll having the same widening function may be used, or other widening devices such as cloth guiders, bend bars, and tenter clips may be used. Another means for suppressing the occurrence of wrinkles is to perform the stretching treatment in the same way as the swelling treatment.

In the example shown in FIG. 2, the film taken out from the dyeing bath 15 passes through the guide rolls 36, nip rolls 52, and guide rolls 37 in order to form a first crosslinking bath 17a introduced into

(Crosslinking process)

The crosslinking step is a treatment performed for purposes such as water resistance by crosslinking or color adjustment (preventing a film from becoming bluish). In the crosslinking step, n (n is an integer of 2 or more) crosslinking baths are used. In the example shown in Fig. 2, three (n is 3) cross-linking baths are arranged as cross-linking baths for performing the cross-linking step, and the first cross-linking step for water resistance is carried out in the first cross-linking bath 17a and the second cross-linking bath 17a. 2. The third crosslinking step for the purpose of color adjustment, which is carried out in the crosslinking bath 17b, is carried out in the third crosslinking bath 17c. Referring to FIG. 2 , in the first crosslinking step, the first crosslinking bath 17a is formed by transporting the film along the transport path constructed by the nip rolls 52, the guide rolls 37 to 40, and the nip rolls 53a. It can be carried out by immersing the film after dyeing treatment in (the first crosslinking liquid contained in the crosslinking tank) for a predetermined time, and then pulling it out. In the second cross-linking step, the second cross-linking bath 17 b (the second It can be implemented by immersing the film after the first crosslinking step in a crosslinking liquid) for a predetermined period of time and subsequently pulling it out. In the third crosslinking step, the third crosslinking bath 17c (the third It can be implemented by immersing the film after the 2nd bridge|crosslinking process in bridge|crosslinking liquid) for a predetermined time, and then taking it out.

The crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more. When the dichroic dye used in the dyeing treatment is iodine, the crosslinking bath preferably contains iodide in addition to the boron compound, and the concentration of iodide in the crosslinking bath can be, for example, 1 to 30% by mass. Potassium iodide, zinc iodide, etc. are mentioned as an iodide. In addition, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate and the like may coexist.

A uniaxial stretching process can be performed in the 1st crosslinking bath 17a using the circumferential speed difference of the nip roll 52 and the nip roll 53a. The uniaxial stretching process can be performed in the second crosslinking bath 17b using the difference in circumferential speed between the nip rolls 53a and the nip rolls 53b. The uniaxial stretching process can be performed in the third crosslinking bath 17c using the difference in circumferential speed between the nip rolls 53b and the nip rolls 53c.

Also in the crosslinking treatment, guide rolls 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 and In 48), a roll having a widening function such as an expander roll, a spiral roll, or a crown roll may be used, or another widening device such as a cloth guider, a bend bar, or a tenter clip may be used. Another means for suppressing the occurrence of wrinkles is to perform the stretching treatment in the same way as the swelling treatment.

In the example shown in FIG. 2 , the film taken out from the third crosslinking bath 17c passes through the guide rolls 48 and nip rolls 53c in this order and is introduced into the washing bath 19 .

(cleaning process)

In the example shown in Fig. 2, a cleaning step is included after the crosslinking step. The washing treatment is performed for the purpose of removing excess boric acid, iodine, or other chemicals adhering to the polyvinyl alcohol-based resin film. The cleaning step is performed by immersing the crosslinked polyvinyl alcohol-based resin film in the cleaning bath 19, for example. Further, the cleaning step may be performed by spraying the film with a cleaning liquid as a shower instead of immersing the film in the cleaning bath 19, or by using a combination of immersion in the cleaning bath 19 and spraying of the cleaning liquid.

FIG. 2 shows an example in which a polyvinyl alcohol-based resin film is immersed in a cleaning bath 19 to perform cleaning treatment. The temperature of the washing bath 19 in the washing treatment is usually about 2 to 40°C, and the immersion time of the film is usually about 2 to 120 seconds.

Also in the cleaning treatment, for the purpose of transporting the polyvinyl alcohol-based resin film while removing wrinkles, the guide rolls 56, 57, 58 and/or 59 are provided with a widening function such as an expander roll, a spiral roll, and a crown roll. It is possible to use a roll having a width, or to use another widening device such as a cloth guider, a bend bar, or a tenter clip. Further, in the film cleaning treatment, stretching treatment may be performed in order to suppress the occurrence of wrinkles.

(stretching process)

As described above, the raw film 10 is uniaxially stretched in a wet or dry manner during the series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps). A specific method of the uniaxial stretching treatment is, for example, between two nip rolls constituting the film conveyance path (eg, two nip rolls disposed before and after the treatment bath) to perform longitudinal uniaxial stretching by providing a difference in circumferential speed between the rolls. It may be stretching, hot roll stretching as described in Japanese Patent Publication No. 2731813, tenter stretching, or the like, and inter-roll stretching is preferable. The uniaxial stretching process can be performed over a plurality of times between obtaining the polarizing film 23 from the raw film 10 . As described above, the stretching treatment is also advantageous in suppressing the occurrence of wrinkles in the film.

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

(electromagnetic wave irradiation process)

In the apparatus shown in FIG. 2 , after the film is taken out from the third crosslinking step 17c and passed through the nip rolls 53c, and before being immersed in the washing bath 19, the film is subjected to the electromagnetic wave irradiation step described above this is done

In the electromagnetic wave irradiation step, the electromagnetic wave is preferably irradiated from above in a direction perpendicular to the film surface. Further, the distance between the electromagnetic wave spinneret of the electromagnetic wave irradiator in the electromagnetic wave irradiator 71 and the film is preferably 2 to 40 cm, more preferably 5 to 20 cm. However, it is preferable to select this distance appropriately in consideration of the amount of radiant energy of electromagnetic waves emitted from the electromagnetic wave irradiator, the temperature of the surface of the film, and the like. The temperature of the surface of the film during electromagnetic wave irradiation is preferably maintained at 30 to 90°C, and more preferably at 40 to 80°C.

The electromagnetic wave irradiation unit 71 may be composed of one electromagnetic wave irradiator or may be composed of a plurality of electromagnetic wave irradiators. In the case of a plurality of electromagnetic wave irradiators, 25% or more of the total radiation energy of electromagnetic waves emitted from the plurality of electromagnetic wave irradiators is Select as many electromagnetic wave irradiators as possible. In Fig. 2, the electromagnetic wave irradiation unit 71 is configured so that only one side of the film is irradiated with electromagnetic waves, but a plurality of electromagnetic wave irradiators may be arranged so that electromagnetic waves are irradiated from both sides of the film.

(drying process)

After the washing step, it is preferable to perform a treatment for drying the polyvinyl alcohol-based resin film. Drying of the film is not particularly limited, but may be performed using a drying furnace 21 as in the example shown in FIG. 2 . The drying furnace 21 can be provided with a hot air dryer, for example. 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 also be performed using a far-infrared heater. The thickness of the polarizing film 23 obtained by performing it above is about 5-30 micrometers, for example.

The obtained polarizing film may be rolled up sequentially on a winding roll to form a roll, or may be directly used for a polarizing plate production step (a step of laminating a protective film or the like on one side or both sides of a polarizing film) without winding up.

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

Processing other than the above processing may be added. Examples of treatments that can be added include immersion treatment in an iodide aqueous solution not containing boric acid (complementary color treatment), which is performed after the crosslinking step, and immersion treatment in an aqueous solution containing zinc chloride and the like without boric acid (zinc processing) included.

<Polarizing film>

By producing a polarizing film by the above manufacturing method, for example,

i) the contractile force is 2.8 N/2 mm or less;

ii) the visibility correction single transmittance (Ty) is 42.0% or more;

iii) a visibility correction polarization degree (Py) of 99.985% or more;

iv) A polarizing film having an orthogonal color b value of -1.9 or more can be obtained.

In the present invention, the crosslinking step is performed using n crosslinking baths, and the nth crosslinking bath is a solution having a boron compound concentration of 2.4% by mass or less, so that a polarizing film having shrinkage force of the above i) can be obtained. The shrinking force of the polarizing film is more preferably 2.6 N/2 mm or less, and a polarizing film having such a shrinking force can be obtained in the method of the present invention. The shrinking force here is measured according to the description in the section of Examples described later.

In the present invention, by having the electromagnetic wave irradiation step, even if the shrinkage force is low as in the above i), the visibility corrected single transmittance (Ty) satisfies the above ii), and the visibility corrected polarization degree (Py) is the above iii) It is possible to obtain a polarizing film having excellent optical properties that satisfies. The visibility corrected single transmittance (Ty) and the visibility corrected polarization degree (Py) of the polarizing film here are measured according to the description in the section of Examples described later.

In the present invention, by having the electromagnetic wave irradiation step, even if the shrinkage force is low as in the above i), the b value of the orthogonal color satisfies the above iv), and a polarizing film with excellent optical properties can be obtained. The b value of the orthogonal color is appropriately designed in combination with the color of the color filter of the liquid crystal display device, but by satisfying the above iv), it can be set within the color design range of the color filter of the liquid crystal display device.

The above-described orthogonal color means a color of light passing through the other surface when light is emitted from one surface of the polarizing plate. The color here can be expressed as a value and b value in the Lab color system, and is measured using standard light. In the present invention, measurement of the orthogonal color of the polarizing film is performed in a state in which an adhesive layer is formed on one side of the polarizing film and the adhesive layer is bonded to the glass plate. The Lab color system is represented by Hunter's lightness index L and colors a and b, as described in "5.5 Accelerated Weatherability Test" of JIS K 5981: 2006 "Synthetic Resin Powder Coating Film". As a concept similar to the Lab color system, there is an L ^* a ^* b ^* color system defined in JIS Z 8781-4:2013 "Colorimetry - Part 4: CIE 1976 L ^* a ^* b ^* Space", but in the present invention, the Lab color system Hire. The lightness index L and the values of hues a and b are calculated by the following formula from three stimulus values X, Y, and Z specified in JIS Z 8722:2009 "Method for measuring color - reflective and transmitted object color".

^L = 10Y ^1/2

a = 17.5(10.2XY)/Y ^1/2

b = 7.0(Y-0.847Z)/Y ^1/2 .

In the Lab color system, the color a and b values may represent positions corresponding to saturation, and when the color a value increases, the color changes to red, and when the color b value increases, the color changes to yellow. Further, the closer to 0, the closer all are to achromatic colors.

<Polarizer>

A polarizing plate can be obtained by bonding a protective film to at least one surface of the polarizing film manufactured as described above through an adhesive agent. Examples of protective films include films made of acetyl cellulose-based resins such as triacetyl cellulose and diacetyl cellulose; films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; polycarbonate-based resin films, cycloolefin-based resin films; acrylic resin film; A film made of a chain olefin resin of a polypropylene resin is exemplified.

In order to improve the adhesion between the polarizing film and the protective film, surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, and saponification treatment may be performed on the bonding surface of the polarizing film and/or the protective film. good night. As the adhesive used for bonding the polarizing film and the protective film, an active energy ray curable adhesive such as an ultraviolet curable adhesive, an aqueous solution of a polyvinyl alcohol-based resin, or an aqueous solution in which a crosslinking agent is mixed therein, and a water-based adhesive such as a urethane emulsion adhesive are used. can be heard The ultraviolet curable adhesive may be a mixture of an acrylic compound and a photo-radical polymerization initiator, a mixture of an epoxy compound and a photo-cationic polymerization initiator, or 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 may be used together as an initiator.

Example

Hereinafter, the present invention will be described more specifically by disclosing test examples, but the present invention is not limited by these examples.

<Test Example 1>

A polarizing film was manufactured from the polyvinyl alcohol-based resin film on the conditions shown in Table 2 using the manufacturing apparatus shown in FIG. 2 . Specifically, a long polyvinyl alcohol (PVA) raw 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 conveyed continuously while unwinding from the roll, and was immersed in a swelling bath made of pure water at 25°C for a residence time of 80 seconds (swelling step). Thereafter, the film taken out from the swelling bath was immersed in a 30°C dyeing bath containing 1.25% by mass of potassium iodide, 0.3% by mass of boric acid, and 1.25 mM/L of iodine at a residence time of 150 seconds (dyeing step). Subsequently, the film taken out from the dyeing bath was immersed in a first crosslinking bath containing 11% by mass of potassium iodide and 3.0% by mass of boric acid for a residence time of 26 seconds, followed by 11% by mass of potassium iodide and 3.0% by mass of boric acid. It was immersed in the second crosslinking bath containing mass% for a residence time of 20 seconds, and then immersed in a third crosslinking bath containing 5% by mass of potassium iodide and 2.4% by mass of boric acid for a residence time of 10 seconds (crosslinking step). In the swelling step, the dyeing step, and the crosslinking step, longitudinal uniaxial stretching was performed at a draw ratio shown in Table 2 by inter-roll stretching in a bath. The total draw ratio based on the raw film was 5.89 times.

Next, with respect to the film taken out from the third crosslinking bath and passed through the nip roll, an electromagnetic wave irradiator (high-speed response medium-wave infrared heater (FRMW heater) (product name: Golden 8 Medium-wave fast response twin tubu emitter, manufactured by Heraeus) , using a heat source temperature of 1600°C and a maximum energy density of 150 kW/m ² )), an electromagnetic wave spinneret was placed at a distance of 5 cm from the surface of the film, and electromagnetic waves were irradiated. The irradiation heat amount of electromagnetic waves per unit volume of the film was 3000 J/cm ³ . In addition, the irradiation heat amount of electromagnetic waves per unit volume of the film was calculated by the following formula.

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

The output represents the ratio of the output actually irradiated with respect to the maximum irradiation power of the electromagnetic wave irradiator.

In addition, in the FRMW heater used in Test Example 1, the ratio of infrared radiant energy having a wavelength of more than 2 μm and 4 μm or less was 40% of the total radiant energy.

After being taken out of the third crosslinking bath, the time required until the film was conveyed and reached the irradiation position of the electromagnetic wave irradiator and was irradiated with electromagnetic waves was 5 seconds.

The film irradiated with electromagnetic waves was immersed in a cleaning bath made of pure water at 6° C. with a residence time of 7 seconds (cleaning step). After that, the film was dried for 300 seconds in a drying furnace at a temperature of 90°C to obtain a polarizing film. The thickness of the obtained polarizing film was 23 micrometers.

<Test Examples 2 to 4, 6, 8, 9>

The mass % of boric acid in the first cross-linking bath and the second cross-linking bath, the output (%), and the electromagnetic wave irradiation heat amount per film unit volume in the electromagnetic wave irradiation step are as shown in Table 3, Test Example 1 and A polarizing film was obtained under the same conditions. All of the thicknesses of the obtained polarizing films were 23 micrometers.

<Test Example 5, 7, 10>

A polarizing film was obtained under the same conditions as in Test Example 1 except that the electromagnetic wave irradiation step was not performed and the mass% of boric acid in the first crosslinking bath and the second crosslinking bath was set as shown in Table 3. The thickness of all the obtained polarizing films was 23 micrometers.

[Evaluation of polarizing film]

(a) MD contraction force

From the obtained polarizing film, a measurement sample having a width of 2 mm and a length of 15 mm with the long side in the direction of the absorption axis (MD, stretching direction) was cut out. The shrinkage force in the long side direction (absorption axis direction, MD) generated when this sample is set in a thermomechanical analyzer (DMA) "Q800" manufactured by TA and maintained at 80°C for 1 hour while maintaining constant dimensions. (MD contractility) was measured. Table 3 shows the measured contractile force values.

(b) measurement of single transmittance and degree of polarization

About the obtained polarizing film, MD transmittance and TD transmittance in the range of wavelength 380-780 nm were measured using the spectrophotometer with an integrating sphere ("V7100" manufactured by Nippon Bunko Co., Ltd.), and the following formula:

Single transmittance (%) = (MD+TD)/2

Degree of polarization (%) = {(MD-TD)/(MD+TD)}×100

Based on this, the single transmittance and polarization degree at each wavelength were calculated.

The "MD transmittance" is the transmittance when the direction of polarized light emitted from the Glan Thomson prism and the transmittance axis of the polarizing film sample are made parallel, and is represented by "MD" in the above formula. In addition, the "TD transmittance" is the transmittance when the direction of polarized light emitted from the Glan Thomson prism and the transmission axis of the polarizing film sample are orthogonal, and is represented by "TD" in the above formula. Regarding the obtained single transmittance and polarization degree, visibility correction was performed by a 2-degree field of view (C light source) of JIS Z 8701: 1999 "Color display method-XYZ color system and X ₁₀ Y ₁₀ Z ₁₀ color system", and visibility corrected single transmittance ( Ty) and visibility correction polarization degree (Py) were obtained. Table 3 shows the calculation results of the visibility corrected single transmittance (Ty) and the visibility corrected polarization degree (Py).

(c) orthogonal color b value

Regarding the obtained polarizing film, the b value of the orthogonal hue was determined according to the method described above. Table 3 shows the calculation results of the b values of orthogonal colors.

(d) Workability (folding of film)

For each test example, in the second cross-linking bath and the third cross-linking bath, the presence or absence of folding and/or breakage of the end portion of the film was visually confirmed. Table 3 shows the confirmation results.

10: original film made of polyvinyl alcohol-based resin, 11: original roll, 13: swelling bath, 15: dyeing bath, 17a: first crosslinking bath, 17b: second crosslinking bath, 17c: third crosslinking bath, 19: Washing bath, 21: drying furnace, 23: polarizing film, 30 to 48, 56 to 61: guide roll, 50 to 52, 53a, 53b, 53c, 54, 55: nip roll, 71: electromagnetic wave irradiation unit.

Claims (12)

A method for producing a polarizing film from a polyvinyl alcohol-based resin film,
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 order in n (n is an integer of 2 or more) crosslinking baths for crosslinking treatment;
With respect to the polyvinyl alcohol-based resin film drawn out after being immersed in the x-th crosslinking bath disposed at the x-th (x is an integer of n or less) from the upstream side toward the downstream side, removing moisture adhering to the surface liquid preparation process,
An electromagnetic wave irradiation step of irradiating the polyvinyl alcohol-based resin film with electromagnetic waves having an infrared radiant energy ratio of 25% or more of the total radiant energy having a wavelength of more than 2 μm and less than 4 μm,
Each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more, and the nth crosslinking bath disposed at the nth position from the upstream side toward the downstream side is made of a solution having a boron compound concentration of 2.4% by mass or less. , A method for producing a polarizing film. The method of claim 1, wherein in the electromagnetic wave irradiation step, the irradiation heat amount of the electromagnetic wave is 100 J/cm ³ or more and 50 kJ/cm ^{3 or} less per unit volume of the polyvinyl alcohol-based resin film. The method for producing a polarizing film according to claim 1 or 2, wherein x is n. The method for producing a polarizing film according to claim 1 or 2, wherein the nth crosslinking bath has a temperature of 30°C or higher. The method for producing a polarizing film according to claim 4, wherein the nth crosslinking bath has a temperature of 35°C or higher. The cross-linking bath according to claim 1 or 2, wherein at least one of the cross-linking baths disposed at the 1st to n-1th positions from the upstream side to the downstream side is composed of a solution having a concentration of the boron compound of 2.5% by mass or more. , A method for producing a polarizing film. The method for producing a polarizing film according to claim 6, wherein the temperature of the crosslinking bath made of a solution having a concentration of the boron compound of 2.5% by mass or higher is 45°C or higher. The method for producing a polarizing film according to claim 7, wherein the temperature of the crosslinking bath made of a solution having a concentration of the boron compound of 2.5% by mass or higher is 55°C or higher. The method for producing a polarizing film according to claim 1 or 2, further comprising a cleaning step of washing the polyvinyl alcohol-based resin film after the crosslinking step and the electromagnetic wave irradiation step. delete The method for producing a polarizing film according to claim 1 or 2, wherein the electromagnetic wave irradiation step is performed within 5 seconds after taking out the xth crosslinking bath. A manufacturing apparatus for manufacturing a polarizing film from a polyvinyl alcohol-based resin film,
A dyeing unit for dyeing the polyvinyl alcohol-based resin film with a dichroic dye;
A crosslinking portion for crosslinking treatment by immersing the polyvinyl alcohol-based resin film after the dyeing treatment in n (n is an integer of 2 or more) crosslinking baths;
With respect to the polyvinyl alcohol-based resin film drawn out after being immersed in the x-th crosslinking bath disposed at the x-th (x is an integer of n or less) from the upstream side toward the downstream side, removing moisture adhering to the surface liquidation means;
An electromagnetic wave irradiation unit for irradiating the polyvinyl alcohol-based resin film with electromagnetic waves having an infrared radiation energy ratio of 25% or more of total radiation energy having a wavelength of more than 2 μm and less than 4 μm,
Each crosslinking bath is made of a solution having a boron compound concentration of 0.5% by mass or more, and the nth crosslinking bath disposed at the nth position from the upstream side toward the downstream side is made of a solution having a boron compound concentration of 2.4% by mass or less. , A device for producing a polarizing film.