TWI782046B - Method and apparatus for manufacturing polarizing film - Google Patents

Abstract

The present invention provides a method and an apparatus for manufacturing a polarizing film which is hard to shrink even when exposed to a high temperature environment and has excellent optical characteristics.

The method for manufacturing polarizing film of the present invention includes: a dyeing step of dyeing a 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 crosslinking baths with a quantity of n (n is an integer of 2 or more) in sequence, and an electromagnetic wave irradiation step of irradiating an electromagnetic wave, in which the ratio of radiant energy of infrared rays having wavelengths from more than 2 μm to 4 μm or less is 25% or more of the total radiant energy, to the polyvinyl alcohol-based resin film which was drawn out after immersed in a x^th crosslinking bath disposed at the x^th (x is an integer equal to or less than n) position from the upstream side toward the downstream side; wherein each crosslinking bath is composed of a solution having a concentration of boron compound of 0.5 mass% or more, and the n^th crosslinking bath disposed at the n^th position from the upstream side toward the downstream side is composed of a solution having a concentration of the boron compound of 2.4 mass% or less.

Description

Polarizing film manufacturing method and manufacturing device

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

Polarizing plates are widely used as polarizing elements in image display devices such as liquid crystal display devices and the like. A polarizing plate is generally constructed by bonding a transparent resin film (protective film, etc.) to one or both sides of a polarizing film using an adhesive or the like.

Polarizing film is mainly treated by immersing the base material film composed of polyvinyl alcohol-based resin in a dyeing bath containing dichroic pigments such as iodine, and then dipping in a cross-linking bath containing a cross-linking agent such as boric acid, etc. Simultaneously, the film is uniaxially stretched at any stage to produce it. Uniaxial stretching includes dry stretching in which stretching is carried out in air, and wet stretching in which stretching is carried out in liquids such as the above-mentioned dyeing bath and crosslinking bath.

The cross-linked polarizing film tends to shrink easily when exposed to high temperature environment, and the durability may be insufficient. Japanese Unexamined Patent Publication No. 2013-148806 (Patent Document 1) describes that the boron content rate of the polarizing film is reduced to 1 to 3.5% by weight to provide a polarizing film with excellent durability.

[Prior Art Literature] [Patent Document]

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

The object of the present invention is to provide a manufacturing method and manufacturing apparatus of a polarizing film that is not easily shrunk 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] A method for producing a polarizing film, which is a method for producing a polarizing film from a polyvinyl alcohol-based resin film, the production method comprising: a dyeing process of dyeing the polyvinyl alcohol-based resin film with a dichroic dye, wherein The above-mentioned polyvinyl alcohol-based resin film after the above-mentioned dyeing process is immersed in n (n is an integer of 2 or more) cross-linking baths in order to carry out the cross-linking process of cross-linking treatment, and for immersion in the arrangement from the upstream side to the downstream Ratio of radiant energy of the above-mentioned polyvinyl alcohol-based resin film pulled out after the xth cross-linking bath on the xth side (x is an integer of n or less) irradiated with infrared rays having a wavelength of more than 2 μm and 4 μm or less The electromagnetic wave irradiation process of electromagnetic waves with 25% or more of the total radiation energy; each cross-linking bath is composed of a solution with a boron compound concentration of 0.5% by mass or more, and is arranged at the nth number nth from the upstream side to the downstream side The crosslinking bath is composed of a solution having a concentration of the aforementioned boron compound of 2.4% by mass or less.

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

[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 n-th crosslinking bath is 30° C. or higher.

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

[6] The method for producing a polarizing film according to any one of [1] to [5], wherein it is arranged in No. 1 to No. n-1 crosslinking baths from the upstream side to the downstream side, and at least One cross-linking bath is composed of a solution of the aforementioned boron compound having a concentration of 2.5% by mass or more.

[7] The method for producing a polarizing film according to [6], wherein the temperature of the crosslinking bath composed of a solution having a concentration of the boron compound 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 composed 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 washing the polyvinyl alcohol-based resin film after the crosslinking step and the electromagnetic wave irradiation step. net process.

[10] The method for producing a polarizing film according to any one of [1] to [9], wherein the polyvinyl alcohol-based resin film pulled out after being immersed in the x-th crosslinking bath is, before the electromagnetic wave irradiation step, , further including a liquid removal process for removing moisture adhering to its surface.

[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 pulling out from the x-th crosslinking bath.

[12] A polarizing film manufacturing apparatus, which is a manufacturing apparatus for manufacturing a polarizing film from a polyvinyl alcohol-based resin film, the manufacturing apparatus comprising: a dyeing section for dyeing the polyvinyl alcohol-based resin film with a dichroic dye, The above-mentioned polyvinyl alcohol-based resin film after the above-mentioned dyeing treatment is sequentially immersed in n (n is an integer of 2 or more) cross-linking baths to carry out the cross-linking part of the cross-linking treatment, and for immersion in the cross-linking part arranged from the upstream side toward The above-mentioned polyvinyl alcohol-based resin film pulled out after the x-th (x is an integer of n or less) cross-linking bath on the downstream side is irradiated with radiation energy of infrared rays having a wavelength of more than 2 μm and 4 μm or less The electromagnetic wave irradiation part of the electromagnetic wave with a ratio of 25% or more of the total radiant energy; each cross-linking bath is composed of a solution with a boron compound concentration of 0.5% by mass or more, and is arranged in the nth number from the upstream side to the downstream side n The cross-linking bath is composed of a solution having a concentration of the aforementioned boron compound of 2.4% by mass or less.

According to the present invention, it is possible to provide a method and apparatus for manufacturing a polarizing film that does not easily shrink even when exposed to a high-temperature environment and that has excellent optical properties.

10‧‧‧Basic film made of polyvinyl alcohol resin

11‧‧‧Blade material roll

13‧‧‧Swelling bath

15‧‧‧dye bath

17a‧‧‧The first cross-linking bath

17b‧‧‧The second cross-linking bath

17c‧‧‧The third cross-linking bath

19‧‧‧Cleansing bath

21‧‧‧Drying furnace

23‧‧‧Polarizing film

30 to 48, 56 to 61‧‧‧guide roller

50 to 52, 53a, 53b, 53c, 54, 55‧‧‧roll

71‧‧‧Electromagnetic wave irradiation department

Fig. 1 is a graph showing radiant energy spectra of different types of electromagnetic wave irradiators.

Fig. 2 is a cross-sectional view schematically showing an example of a method for producing a polarizing film and an apparatus for producing a polarizing film used in the production method according to the first embodiment of the present invention.

[Manufacturing method of polarizing film]

In the present invention, the polarizing film is one in which a dichroic dye (iodine or a dichroic dye) is adsorbed and aligned on a uniaxially stretched polyvinyl alcohol-based resin film. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film can generally be obtained by saponifying polyvinyl acetate-based resin. The degree of saponification is usually above 85 mol%, preferably above 90 mol%, more preferably above 99 mol%. The polyvinyl acetate-based resin may be, for example, a copolymer of vinyl acetate and other monomers copolymerizable with the vinyl acetate other than polyvinyl acetate which is a homopolymer of vinyl acetate. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and the like. 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 may be modified, for example, polyvinyl formaldehyde, polyvinyl acetal, polyvinyl butyral modified with aldehydes, etc. may also be used. The term "polyvinyl alcohol-based resin" in this specification means a resin in which the structural unit of vinyl alcohol (-CH ₂ -CH(OH)-) accounts for 50 mol% or more of all structural units contained in the resin.

In the present invention, the starting material for the manufacture of the polarizing film is used with 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 more preferably 30 μm or less An unstretched polyvinyl alcohol-based resin film (base material film). In this way, the polarizing film of the thin film that the market demand is increasing day by day can be obtained. The width of the base material film is not particularly limited, for example, it may be about 400 to 6000 mm. The raw material film is prepared in the form of a long unstretched polyvinyl alcohol-based resin film roll (raw material roll), for example.

The polarizing film can be continuously conveyed along the film conveying path of the polarizing film manufacturing device by rolling out the above-mentioned strip-shaped substrate film from the substrate roll, and dipped in the treatment liquid contained in the treatment tank (hereinafter also referred to as After the predetermined treatment process of pulling out after the "treatment bath"), a drying process is performed to continuously manufacture a long polarizing film. The treatment process is not limited to the method of immersing the film in a treatment bath, as long as it is a method of making the treatment solution contact the membrane except for the cross-linking process described later, it may be performed by spraying, inflowing, or dripping. A method of treating a membrane by attaching a treatment solution to the surface of the membrane. When the treatment process is implemented by immersing the membrane in a treatment bath, the treatment bath for a treatment process is not limited to one, and the membrane can also be immersed in two or more treatment baths in order to complete a process. Processing procedure.

Examples of the above treatment liquid include swelling liquid, dyeing liquid, crosslinking liquid, washing liquid and the like. The above-mentioned treatment process can be exemplified: the swelling process of making the swelling liquid contact the embryo material film to perform the swelling treatment; A cross-linking step of performing a cross-linking treatment on the membrane, and a washing step of performing a washing treatment by contacting the cross-linked membrane with a cleaning solution. In addition, between such a 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 uniaxial stretching treatment is applied by a wet method or a dry method. Other processing steps may also be added as required.

〈Crosslinking process〉

The cross-linking process is a treatment for the purpose of achieving water resistance or hue adjustment (preventing the film from bluish, etc.) through cross-linking. In the cross-linking step of the present invention, the film is sequentially immersed in n (n is an integer of 2 or more) cross-linking baths to perform cross-linking treatment. Each crosslinking bath is composed of a solution having a boron compound concentration of 0.5% by mass or more. A boron compound functions as a crosslinking agent, for example, boric acid, borax, etc. can be illustrated. The crosslinking solution may contain crosslinking agents such as glyoxal and glutaraldehyde in addition to the boron compound. As a solvent for the crosslinking liquid, for example, water may be used, and an organic solvent compatible with water may be further contained. 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 or balancing the amount of crosslinking in the polyvinyl alcohol film. In addition, the temperature of each crosslinking bath is preferably 65° C. or lower, more preferably 60° C. or lower, from the viewpoint of preventing 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 (the most downstream cross-linking bath) arranged on the n-th cross-linking bath from the upstream side to the downstream side is composed of a solution having a boron compound concentration of 2.4% by mass or less. When n is 2, the second crosslinking bath (the most downstream crosslinking bath) is composed of a solution having a boron compound concentration of 2.4% by mass or less. In the present invention, by performing the crosslinking step using n crosslinking baths and configuring the nth crosslinking bath as a solution having a boron compound concentration of 2.4% by mass or less, a polarizing film with suppressed shrinkage force can be obtained.

The n-th crosslinking bath is preferably at least 30°C, more preferably at least 35°C, and still more preferably at least 42°C, from the viewpoint of promoting the balance of the amount of crosslinking in the polyvinyl alcohol film.

In the present invention, it is preferred that at least one crosslinking bath is composed of a solution having a boron compound concentration of 2.5% by mass or more in the No. 1 to No. n-1 crosslinking baths from the upstream side to the downstream side. , more preferably composed of a solution with a boron compound concentration of 2.8% by mass or more. When n is 2, the first crosslinking bath (the most upstream crosslinking bath) is preferably composed of a solution having a boron compound concentration of 2.5% by mass or more. When n is 3, at least one of the first crosslinking bath and the second crosslinking bath is preferably composed of a solution having a boron compound concentration of 2.5% by mass or more. In the present invention, since there is a crosslinking bath composed of a solution having a boron compound concentration of 2.5% by mass or more, water resistance or color adjustment can be more effectively performed by crosslinking through the entire crosslinking process. In addition, by immersing the polyvinyl alcohol-based resin film in a cross-linking bath in which the concentration of the boron compound is 2.5% by mass or more, folding of the polyvinyl alcohol-based resin film at the edge during transportation can be suppressed. When the first cross-linking bath (the most upstream cross-linking bath) is configured as a cross-linking bath composed of a solution with a boron compound concentration of 2.5% by mass or more, it is possible to effectively suppress the end of the transport time in all the cross-linking baths The part is folded in, so it is better. The number of crosslinking baths composed of a solution having a boron compound concentration of 2.5% by mass or more may be one or two or more.

The crosslinking bath composed of a solution having a boron compound concentration of 2.5% by mass or more is preferably 45°C or higher, more preferably Above 55°C.

<Electromagnetic wave irradiation process>

In the present invention, the polyvinyl alcohol-based resin film pulled out after dipping in the x-th cross-linking bath arranged on the x-th (x is an integer less than or equal to n) arranged from the upstream side to the downstream side is irradiated with electromagnetic waves by irradiating electromagnetic waves. process.

The electromagnetic wave that can be used in the electromagnetic wave irradiation process of the present invention is more than 2 μm and the ratio of the radiant energy of infrared rays with a wavelength of 4 μm or less is 25% or more of the total radiant energy, preferably 28% or more, more preferably more than 35%. By irradiating the film with this electromagnetic wave, the optical properties of the obtained polarizing film can be improved. In addition, the upper limit of the ratio of the radiated energy of infrared rays with a wavelength of more than 2 μm to 4 μm for electromagnetic waves usable in the present invention is not particularly limited, for example, it is 80% or less. Electromagnetic waves with a wavelength of 0.75 μm to 1000 μm are usually called infrared rays.

In the electromagnetic wave irradiation process, the mechanism by which the optical properties of the polarizing film can be improved by irradiating electromagnetic waves with a ratio of radiant energy of infrared rays having a wavelength of more than 2 μm and 4 μm or less is 25% or more of the total radiated energy is not yet clear. However, it is presumed that the molecular movement in the film is excited by infrared rays with a wavelength of more than 2 μm and less than 4 μm , which promotes the immobilization of dichroic pigments such as iodine in the film after crosslinking treatment, and improves polarization. Optical properties of the film.

Fig. 1 is a diagram showing radiant energy spectra of different types of electromagnetic wave irradiators. In addition, Table 1 shows the ratio of the radiated energy of electromagnetic waves to the total radiated energy in each wavelength region (expressed in the range of wavelength x μm ) of different types of electromagnetic wave irradiators. The electromagnetic wave irradiators shown in Figure 1 and Table 1 are halogen heaters (heat source temperature 2600°C), short-wavelength infrared heaters (heat source temperature 2200°C), and high-speed response mid-wavelength infrared heaters (heat source temperature 1600°C) , Carbon heater (heat source temperature 1200°C), carbon heater (heat source temperature 950°C), mid-wavelength infrared heater (heat source temperature 900°C).

As shown in Table 1, short-wavelength infrared heaters (heat source temperature 2200°C), high-speed response mid-wavelength infrared heaters (heat source temperature 1600°C), carbon heaters (heat source temperature 1200°C), carbon heaters (heat source temperature 950°C) ℃), mid-wavelength infrared heater (heat source temperature 900 ℃), since the ratio of the radiated energy of infrared rays with a wavelength of more than 2 μm and 4 μm or less is more than 25% of the total radiated energy, it can be preferably used as An electromagnetic wave irradiator for performing the electromagnetic wave irradiating step of the present invention.

In the electromagnetic wave irradiation step, the heat quantity of electromagnetic wave irradiation per unit volume of the film can be generally set at 100 J/cm ³ or more and 50 kJ/cm ³ or less. From the viewpoint of enhancing the optical properties of the polarizing film, it is preferably at least 100 J/cm ³ , more preferably at least 500 J/cm ³ , and more preferably at least 1000 J/cm ³ . In addition, the electromagnetic wave irradiation heat 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 Below 3000J/ ^cm3 . Usually, the water content of the film is reduced in proportion to the radiation heat of electromagnetic waves, but since the electromagnetic wave irradiation process of the present invention is not aimed at reducing the water content of the film, the radiation heat can be appropriately selected, preferably within the above range Choose appropriately.

In the present invention, the optical properties of the obtained polarizing film can be improved by performing the electromagnetic wave irradiation process before the cleaning treatment. The electromagnetic wave irradiation step may be performed on a film immersed in at least one crosslinking bath, and is not limited to being performed on a film immersed in all n crosslinking baths. However, through this electromagnetic wave irradiation process, the boric acid incorporated into the film by immersing in the cross-linking bath can be cross-linked, so the electromagnetic wave irradiation process can be performed more effectively on the film after immersion in the cross-linking bath. Cross-linking is preferred. That is, it is preferable to irradiate the polyvinyl alcohol-based resin film pulled out from the n-th crosslinking bath where x is n and is irradiated with electromagnetic waves.

The irradiation of electromagnetic waves is preferably performed within 10 seconds, more preferably within 5 seconds after the film is pulled out from the x-th crosslinking bath. The shorter the time between taking out from the x-th crosslinking bath and irradiating electromagnetic waves is, the more the optical properties of the polarizing film can be improved by irradiating electromagnetic waves. In the electromagnetic wave irradiation step, it is preferable to have less moisture adhering to the surface of the film. This is because when water exists on the surface of the film, the water molecules contained in the water on the film surface absorb infrared rays, which reduces the effect of electromagnetic wave irradiation on exciting molecular motion in the film. Immediately after being pulled out from the x-th crosslinking bath, since the crosslinking liquid adheres to the surface of the film, it is preferable to provide a liquid removal step for removing it before the electromagnetic wave irradiation step. The liquid removal means in the liquid removal step may, for example, be a roll, a means for blowing air to the film to remove the liquid, or a scraper for contacting the film to remove the liquid, or the like.

<First Embodiment>

An embodiment of the method for manufacturing a polarizing film of the present invention will be described in more detail below with reference to FIG. 2 . Fig. 2 is a cross-sectional view schematically showing an example of a method for manufacturing a polarizing film of the present invention and a device for manufacturing a polarizing film used in the manufacturing method. The polarizing film manufacturing device shown in Fig. 2 is to continuously unwind the blank (unstretched) film 10 made of polyvinyl alcohol-based resin from the blank roll 11 and transport it along the film delivery path, thereby Swelling bath (swelling solution contained in the swelling tank) 13, dyeing bath (staining solution contained in the dyeing tank) 15, first cross-linking bath (contained in the cross-linking tank The first cross-linking liquid in the cross-linking bath) 17a, the second cross-linking bath (the second cross-linking liquid contained in the cross-linking tank) 17b, the third cross-linking bath (the third cross-linking liquid contained in the cross-linking tank) 17c and the cleaning bath (cleaning solution contained in the cleaning tank) 19, and finally pass through the drying furnace 21 to form. The obtained polarizing film 23 can be directly transported to the next polarizing plate production process (process of attaching a protective film to one side or both sides of the polarizing film 23 ), for example. Arrows in Fig. 2 show the transport direction of the film.

In the description of Figure 2, "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 washing 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 cross-linking bath, and the cleaning bath respectively constitute the swelling section, the dyeing section, the cross-linking section, and the cleaning section in the production apparatus of the present invention.

The film transport path of the polarizing film manufacturing device, in addition to the above-mentioned treatment baths, can also be configured at appropriate positions: guide rollers 30 to 48, 56 to 61 that can support the transported film or further change the film transport direction, or It is constructed with rolls 50 to 55 that can press and hold the conveyed film and impart driving force to the film due to its rotation, or can further change the conveying direction of the film. Guide rolls or nip rolls can be arranged before and after each treatment bath or in the treatment bath, so that the film can be introduced into and immersed in the treatment bath and pulled out from the treatment bath [see Fig. 2]. For example, one or more guide rolls are provided in each treatment bath, and the film can be immersed in each treatment bath by conveying the film along these guide rolls.

The polarizing film manufacturing device shown in Fig. 2 is configured with rolls (rolls 50 to 54) before and after each treatment bath, so that in any one or more treatment baths, a circle can be formed between the rolls arranged before and after it. The inter-roll stretching of longitudinal uniaxial stretching is carried out by speed difference.

In the polarizing film manufacturing apparatus shown in FIG. 2 , an electromagnetic wave irradiation unit 71 is disposed on the transport path downstream of the third crosslinking bath 17 c and upstream of the cleaning bath 19 to perform the electromagnetic wave irradiation process. In the manufacturing device of the polarizing film shown in Figure 2, there are three cross-linking baths, that is, n is 3, and the cross-linking bath before the electromagnetic wave irradiation process is the third cross-linking bath from the upstream side, that is, x for 3. Each step will be described below.

(swelling process)

The swelling process is performed for the purpose of removing impurities on the surface of the base material film 10, removing plasticizers in the base material film 10, imparting easy dyeability, and achieving plasticization of the base material film 10, and the like. The treatment conditions are determined within the range that achieves the purpose and within the range that does not cause defects such as extreme dissolution or devitrification of the base material film 10 .

Referring to FIG. 2 , the swelling process is carried out along the film delivery path while continuously rolling out the raw material film 10 from the raw material reel 11 , dipping the raw material film 10 in the swelling bath 13 for a predetermined time, and then pulling it out. In the example of FIG. 2 , the raw material film 10 is conveyed along the film conveyance path constructed by the guide rollers 60 , 61 and the nip roller 50 from the time when the raw material film 10 is unwound until it is immersed in the swelling bath 13 . In the swelling process, the film is conveyed along the film conveyance path constructed by the guide rollers 30 to 32 and the nip roller 51 .

The swelling liquid of the swelling bath 13 can also be used in addition to pure water: adding boric acid (Japanese Patent Application Laid-Open No. 10-153709 ), chloride (Japanese Patent Laid-Open No. 06-281816 ) in the range of about 0.01 to 10% by weight. Gazette), 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, 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 base material film 10 is a polyvinyl alcohol-based resin film stretched in air, 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.

In the swelling treatment, the problem that the raw material film 10 is swelled in the width direction and the film is wrinkled easily occurs. One of the means to remove the wrinkle and transport the film at the same time, you can use the roll with the expansion function like the expansion roll, the spiral roll, and the convex roll, or use the cloth guide device, the bending roll opener, the tenter clip Other expansion devices like pincers are used as guide rollers 30, 31 and/or 32. As another means for suppressing the generation of wrinkles, stretching treatment can be applied. For example, the uniaxial stretching process can be performed in the swelling bath 13 by utilizing the difference in circumferential speed between the roll 50 and the roll 51 .

In the swelling treatment, since the film also swells and expands in the conveying direction of the film, when the film is not actively stretched, in order to eliminate the slack of the film in the conveying direction, for example, it is preferable to use a control device arranged before and after the swelling bath 13. The means such as the speed of the roll 50,51. In addition, for the purpose of stabilizing the transport of the membrane in the swelling bath 13, the water flow in the swelling bath 13 is controlled by showering in water, or an EPC device (Edge Position Control device: detecting the end of the membrane and preventing the meandering of the membrane) is used together. devices), etc., are also useful.

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

(dyeing process)

The dyeing step is performed for the purpose of adsorbing and aligning the dichroic dye on the polyvinyl alcohol-based resin film or the like after the swelling treatment. The treatment conditions are determined within the range that achieves the purpose and within the range that does not cause defects such as extreme dissolution or devitrification of the film. With reference to Fig. 2, the dyeing process can be transported along the film transport path constructed by the roll 51, the guide rolls 33 to 36 and the roll 52, and the film after the swelling process is immersed in the dyeing bath 15 (accommodated in the dyeing tank). Treatment liquid) for a predetermined time, and then pulled out and implemented. In order to improve the dyeability of dichroic pigments, the film provided to the dyeing process is preferably a film that has been subjected to at least a certain degree of uniaxial stretching treatment, or, preferably, it is uniaxially stretched during the dyeing process. Instead of the uniaxial stretching treatment before the dyeing treatment, or in addition to the uniaxial stretching treatment before the dyeing treatment, the uniaxial stretching treatment is also performed during the dyeing treatment.

When iodine is used as a dichroic pigment, the dyeing solution of the dyeing bath 15 can be, 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. Other iodides such as zinc iodide may be used instead of potassium iodide, or potassium iodide and other iodides may be used in combination. In addition, compounds other than iodide, for example, 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 by the point of containing boron. If the aqueous solution contains more than about 0.003 parts by weight of iodine relative to 100 parts by weight of water, it can be regarded as the dyeing bath 15. The temperature of the dyeing bath 15 when dipping the film is usually about 10 to 45°C, preferably about 10 to 40°C, more preferably about 20 to 35°C, and the dipping time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds Second.

When a water-soluble dichroic dye is used as the dichroic pigment, the dyeing liquid in the dyeing bath 15 may be an aqueous solution having a concentration of dichroic dye/water=about 0.001 to 0.1/100 in weight ratio. In this dyeing bath 15, a dyeing auxiliary agent etc. can be made to coexist, For example, inorganic salts, such as sodium sulfate, a surfactant, etc. can be contained. The dichroic dye may be used alone or in combination of two or more. The temperature of the dyeing bath 15 when immersing the membrane is, for example, about 20 to 80°C, preferably about 30 to 70°C, and the immersion time of the membrane is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

As described above, in the dyeing step, the film can be uniaxially stretched in the dyeing bath 15 . The uniaxial stretching of the film can be performed by a method such as forming a peripheral speed difference between the roll 51 and the roll 52 arranged at the front and back of the dyeing bath 15 .

In the dyeing process, similar to the swelling process, in order to transport the polyvinyl alcohol-based resin film while removing the wrinkles of the film, it is also possible to use a roll with a widening function such as an expanding roll, a spiral roll, and a convex roll, or use a cloth guide Other expanding devices such as device, bending roll spreader, tenter clamp are used as guide rollers 33, 34, 35 and/or 36. As another means for suppressing the generation of wrinkles, stretching treatment can be applied in the same way as swelling treatment.

In the example shown in FIG. 2, the film pulled out from the dyeing bath 15 passes through the guide roll 36, the nip roll 52, and the guide roll 37 in order, and is introduced into the first cross-linking bath 17a.

(cross-linking process)

The cross-linking process is a treatment for the purpose of achieving water resistance or hue adjustment (preventing the film from bluish, etc.) based on cross-linking. In the crosslinking step, n (n is an integer of 2 or more) crosslinking baths are used. In the example shown in Figure 2, three (n is 3) cross-linking baths are configured as the cross-linking baths for the cross-linking process, and are carried out in the first cross-linking bath 17a and the second cross-linking bath 17b to ensure water resistance. The 1st crosslinking process performed for the purpose, and the 3rd crosslinking process performed for the purpose of hue adjustment are performed in the 3rd crosslinking bath 17c. With reference to Fig. 2, the first cross-linking process is carried along the film delivery path constructed by the rollers 52, guide rollers 37 to 40 and rollers 53a, and the dyed film is immersed in the first cross-linking bath 17a (the first cross-linking solution contained in the cross-linking tank) for a predetermined time, and then pulled out for implementation. The 2nd cross-linking process is transported along the film delivery path constructed by the roll 53a, guide rolls 41 to 44 and roll 53b, and the film after the 1st cross-linking process is immersed in the 2nd cross-linking bath 17b ( The second cross-linking solution held in the cross-linking tank) for a predetermined time, and then pulled out for implementation. The 3rd cross-linking process is transported along the film transport path constructed by the roll 53b, guide rolls 45 to 48 and roll 53c, and the film after the 2nd cross-linking process is immersed in the 3rd cross-linking bath 17c ( The third cross-linking solution held in the cross-linking tank) for a predetermined time, and then pulled out for implementation.

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

The uniaxial stretching treatment can be performed in the first crosslinking bath 17a by utilizing the difference in circumferential speed between the roll 52 and the roll 53a. The uniaxial stretching treatment can be performed in the second crosslinking bath 17b by utilizing the difference in circumferential speed between the roll 53a and the roll 53b. The uniaxial stretching treatment can be performed in the third crosslinking bath 17c by utilizing the difference in circumferential speed between the roll 53b and the roll 53c.

In the cross-linking treatment, similar to the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing the wrinkles of the film, a roll with a stretching function such as an expanding roll, a spiral roll, and a convex roll can be used, or a guide can be used. Other spreading devices such as cloth device, bending roll spreader, and tenter clamps are used as guide rollers 38, 39, 40, 41, 42, 43, 44, 45, 46, 47 and/or 48. As another means for suppressing the generation of wrinkles, stretching treatment can be applied in the same way as swelling treatment.

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

(cleaning process)

In the example shown in Fig. 2, a washing step after the crosslinking step is included. The cleaning treatment is performed for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step can be performed by immersing the cross-linked polyvinyl alcohol-based resin film in the cleaning bath 19 , for example. The cleaning process can also be performed by spraying the cleaning solution onto the membrane by showering instead of immersing the membrane in the cleaning bath 19, or by using both immersion in the cleaning bath 19 and spraying of the cleaning solution. .

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

In the washing process, for the purpose of removing the wrinkles of the film while conveying the polyvinyl alcohol-based resin film, a roll with a widening function such as an expansion roll, a spiral roll, and a convex roll can be used, or a cloth guide device, a bender, etc. can be used. Other expanding devices such as roll spreaders and tenter clamps are used as guide rollers 56, 57, 58 and/or 59. In addition, in the membrane cleaning treatment, in order to suppress generation of wrinkles, stretching treatment may be applied.

(stretching process)

As mentioned above, the base material film 10 is between the above-mentioned series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps), by wet or dry uniaxial Stretch treatment. The specific method of the uniaxial stretching treatment may be, for example, an inter-roll stretching method in which a circumferential speed difference is formed between two rolls constituting the film conveying path (for example, two rolls arranged in front of and behind the treatment bath) to perform longitudinal uniaxial stretching. Stretching, or hot roll stretching, tenter stretching, etc. described in Japanese Patent No. 2731813, preferably inter-roll stretching. The uniaxial stretching process can be implemented plural times from the base material film 10 to obtaining the polarizing film 23 . As described above, the stretching treatment is also advantageous for suppressing the occurrence of wrinkles in the film.

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

(Electromagnetic wave irradiation process)

In the apparatus shown in FIG. 2, the film is pulled out from the third cross-linking bath 17c, passed through the roll 53c and before being immersed in the cleaning bath 19, the above-mentioned electromagnetic wave irradiation process is performed on the film.

In the electromagnetic wave irradiation step, the electromagnetic wave is preferably irradiated from above in a direction perpendicular to the film surface. In addition, the distance between the electromagnetic wave radiation opening of the electromagnetic wave irradiator in the electromagnetic wave irradiating unit 71 and the film is preferably 2 to 40 cm, more preferably 5 to 20 cm. However, this distance is preferably selected while taking into consideration the radiation energy of the electromagnetic wave radiated from the electromagnetic wave irradiator, the surface temperature of the film, and the like. The temperature of the film surface when irradiated with electromagnetic waves is preferably maintained at 30 to 90°C, more preferably at 40 to 80°C.

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 composed of a plurality of electromagnetic wave irradiators, it is preferable to make the radiated energy of infrared rays with a wavelength of more than 2 μm and 4 μm or less radiated from the plurality of electromagnetic wave irradiators be irradiated from the plurality of electromagnetic wave irradiators. Select a plurality of electromagnetic wave irradiators in such a way that more than 25% of the total radiated energy of the radiated electromagnetic waves is used. In addition, in FIG. 2 , the electromagnetic wave irradiation unit 71 is configured so that electromagnetic waves are irradiated on only one surface of the film, but a plurality of electromagnetic wave irradiators may be arranged to irradiate electromagnetic waves from both sides of the film.

(drying process)

After the cleaning step, it is preferable to perform a process of drying the polyvinyl alcohol-based resin film. The drying of the film is not particularly limited, and can be performed using a drying oven 21 as in the example shown in FIG. 2 . The drying furnace 21 can be comprised including the 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 process of drying a polyvinyl-alcohol-type resin film can also 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 formed into a roll that is sequentially wound on a take-up drum, or it can be directly provided to the polarizing plate manufacturing process without being wound up (the process of laminating a protective film on one or both sides of the polarizing film ).

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

Processing other than the above-mentioned processing may also be added. Examples of additional treatments include: dipping in an iodide aqueous solution not containing boric acid after the crosslinking process (color correction treatment), and dipping in an aqueous solution not containing boric acid but containing zinc chloride, etc. treatment (zinc treatment).

<Polarizing film>

Polarizing film can be produced by the above method, for example: i) the shrinkage force is 2.8N/2mm or less, ii) the transmittance (Ty) of the light sensitivity correction monomer is 42.0% or more, iii) the light sensitivity correction polarization degree ( Py) is 99.985% or more, and iv) the b value of the orthogonal hue is -1.9 or more.

In the present invention, n cross-linking baths are used for the cross-linking process, and the n-th cross-linking bath is configured as a solution having a boron compound concentration of 2.4% by mass or less, thereby obtaining a polarized light having a shrinkage force of i) above. membrane. The shrinkage force of the polarizing film is more preferably 2.6 N/2mm or less. In the method of the present invention, a polarizing film with such shrinkage force can be obtained. The contraction force here was measured in accordance with the descriptions in the items of the Examples described later.

In the present invention, by having the electromagnetic wave irradiation process, even if the shrinkage force is as low as the above i), the transmittance (Ty) of the light sensitivity correction monomer can be obtained to satisfy the above ii) and the light sensitivity correction polarization degree (Py ) A polarizing film satisfying the excellent optical characteristics of the above iii). The sensitivity-corrected monomer transmittance (Ty) and the sensitivity-corrected polarization degree (Py) of the polarizing film here were measured in accordance with the descriptions in the items of the examples described later.

In the present invention, by having the electromagnetic wave irradiation process, even if the shrinkage force is as low as the above i), a polarizing film having an orthochromatic phase b value satisfying the excellent optical characteristics of the above iv) can be obtained. The b value of the orthogonal hue can be appropriately designed by combining with the hue of the color filter of the liquid crystal display device. By satisfying the above iv), it can be configured as a hue design in the color filter of a general liquid crystal display device. within range.

The above-mentioned orthogonal hue refers to the hue of light passing through the other surface when light enters from one surface of the polarizing plate. The hue here can be represented by a value and b value in the Lab colorimetric system, and can be measured using standard light. In the present invention, the actual measurement of the orthogonal hue of the polarizing film is carried out by disposing an adhesive layer on one side of the polarizing film and attaching the adhesive layer side to a glass plate. Lab color system, as recorded in "5.5 Accelerated Weather Resistance Test" of JIS K 5981:2006 "Synthetic Resin Powder Coating Film", is expressed by Hunter brightness index L and hue a and b. The concept similar to the Lab color system is the L*a*b* color system stipulated in JIS Z 8781-4:2013 "Color Measurement-Part 4: CIE 1976 L*a*b* Space", but In the present invention, the Lab color system is adopted. The values of luminance index L and hue a and b are calculated from the tristimulus values X, Y and Z stipulated in JIS Z 8722:2009 "Measurement method of color - reflected and penetrating object color" by the following formula .

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 hue a value and b value can show the position corresponding to the chroma. When the hue a value increases, the hue changes to red, and when the hue b increases, the hue changes to yellow. In addition, the closer to 0, the closer to no color.

<Polarizing plate>

A polarizing plate can be obtained by affixing a protective film to at least one surface of the polarizing film produced above through an adhesive. Examples of protective films include films made of cellulose acetate-based resins such as cellulose triacetate or cellulose diacetate; films made of polyethylene terephthalate, polyethylene naphthalate, and polyethylene terephthalate; Films made of polyester-based resins such as butylene formate; polycarbonate-based resin films; cycloolefin-based resin films; acrylic resin films; films made of chain olefin-based resins such as polypropylene-based 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 applied to the bonding surface of the polarizing film and/or protective film. surface treatment. Adhesives used for bonding the polarizing film and the protective film include active energy ray-curable adhesives such as UV-curable adhesives, or aqueous solutions of polyvinyl alcohol-based resins, or those prepared with a crosslinking agent. Water-based adhesives such as aqueous solutions and urethane-based emulsified adhesives. The ultraviolet curable adhesive can be a mixture of acrylic compound and photo-radical polymerization initiator, or a mixture of epoxy compound and photo-cationic polymerization initiator. In addition, 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]

The following shows test examples to describe the present invention more specifically, but the present invention is not limited to these examples.

<Test Example 1>

Using the manufacturing apparatus shown in FIG. 2, a polarizing film was manufactured from a polyvinyl alcohol-based resin film under the conditions shown in Table 2. Specifically, a strip-shaped polyvinyl alcohol (PVA) blank film [trade name "Kuraray Vinylon VF-PE #6000" manufactured by Kuraray Co., Ltd., with an average degree of polymerization of 2400) was rolled out from a roll with a thickness of 60 μm . , saponification degree of 99.9 mole% or more], one side is continuously transported, and immersed in a swelling bath composed of pure water at 25°C for a residence time of 80 seconds (swelling process). Then, the film pulled out from the swelling bath was immersed in a 30° C. dyeing bath containing 1.25 mass % potassium iodide, 0.3 mass % boric acid, and 1.25 mM/L iodine for a residence time of 150 seconds (dyeing process). Next, the film pulled out from the dyeing bath was immersed in the first crosslinking bath containing 11 mass % of potassium iodide and 3.0 mass % of boric acid with a residence time of 26 seconds, and then immersed in the first crosslinking bath containing 11 mass % of potassium iodide and boric acid with a residence time of 20 seconds. 3.0% by mass of the second crosslinking bath, and then immersed in the third crosslinking bath containing 5% by mass of potassium iodide and 2.4% by mass of boric acid with a residence time of 10 seconds (crosslinking process). In the swelling step, the dyeing step, and the crosslinking step, longitudinal uniaxial stretching was carried out at the draw ratios shown in Table 2 by stretching between rolls in a bath. The total draw ratio based on the raw material film was set to 5.89 times.

Next, an electromagnetic wave irradiator (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/cm ² ) was used. An electromagnetic wave radiation port was arranged at a distance of 5 cm from the surface of the film, and electromagnetic waves were irradiated on the film pulled out from the third crosslinking bath and passed through the rollers. The heat of electromagnetic wave irradiation per unit volume of the film was 3000 J/cm ³ . The electromagnetic wave irradiation heat per unit volume of the film was calculated by the following formula.

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

The so-called output refers to the ratio of the actual irradiated output to the maximum irradiating power of the electromagnetic wave irradiator.

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

The time required for the film to be conveyed from the third crosslinking bath to the irradiation position of the electromagnetic wave irradiator to be 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. for a residence time of 7 seconds (cleaning step). Then, the film was dried for 300 seconds in a drying oven at a temperature of 90° C. to obtain a polarizing film. The obtained polarizing film had a thickness of 23 μm .

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

In addition to setting 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 per unit volume of the film in the electromagnetic wave irradiation process as shown in Table 3, Others obtained a polarizing film under the same conditions as in Test Example 1. The thicknesses of the obtained polarizing films were all 23 μm .

<Test examples 5, 7, 10>

A polarizing film was obtained under the same conditions as in Test Example 1, except that the electromagnetic wave irradiation process 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 thicknesses of the obtained polarizing films were all 23 μm .

[Evaluation of polarizing film]

(a) MD contractility

A measurement sample having a width of 2 mm and a length of 15 mm with the absorption axis direction (MD, stretching direction) as the long side was cut out from the obtained polarizing film. This sample was set in a thermomechanical analyzer (DMA) "Q800" manufactured by TA Co., Ltd., and kept at 80°C for 1 hour while keeping the dimensions constant, and the longitudinal direction (absorption axis direction, absorption axis direction, MD) contraction force (MD contraction force). Table 3 shows the measured contraction force values.

(b) Determination of monomer transmittance and degree of polarization

For the obtained polarizing film, the MD transmittance and TD transmittance in the wavelength range of 380 to 780 nm were measured using a spectrophotometer with an integrating sphere [manufactured by JASCO Corporation "V7100"], and according to the following Formula: monomer transmittance (%)=(MD+TD)/2 degree of polarization (%)={(MD-TD)/(MD+TD)}×100 to calculate the monomer transmittance in each wavelength rate and polarization.

The so-called "MD transmittance" is the transmittance when the direction of the polarized light emitted from the Glan-Thompson (Glan-Thompson) beam is parallel to the transmission axis of the polarizing film sample. The above formula is expressed as "MD". In addition, the so-called "TD transmittance" refers to the transmittance when the direction of the polarized light emitted from the Glan-Thomson's polarized film is perpendicular to the transmission axis of the polarizing film sample. In the above formula, it is expressed as "TD ". For the obtained monomer transmittance and polarization degree, it can be viewed by JIS Z8701: 1999 color display method - XYZ color system and X ₁₀ Y ₁₀ Z ₁₀ color system" with a 2-degree field of view (C light source) Sensitivity correction, and obtain the sensitivity correction monomer transmittance (Ty) and the sensitivity correction polarization degree (Py). Table 3 shows the calculation results of the sensitivity-corrected monomer transmittance (Ty) and the sensitivity-corrected degree of polarization (Py).

(c) The b value of the orthogonal hue

For the obtained polarizing film, the b value of the orthogonal hue was obtained by following the above-mentioned method. Table 3 shows the calculation results of the b values of the orthogonal hues.

(d) Workability (film folding)

For each test example, it was checked visually whether or not the edge of the film was bent and/or broken in the second crosslinking bath and the third crosslinking bath. Table 3 shows the confirmation results.

10‧‧‧Basic film made of polyvinyl alcohol resin

11‧‧‧Blade material reel

13‧‧‧Swelling bath

15‧‧‧dye bath

17a‧‧‧The first cross-linking bath

17b‧‧‧The second cross-linking bath

17c‧‧‧The third cross-linking bath

19‧‧‧Cleansing bath

21‧‧‧Drying furnace

23‧‧‧Polarizing film

30 to 48, 56 to 61‧‧‧guide roller

50 to 52, 53a, 53b, 53c, 54, 55‧‧‧roll

71‧‧‧Electromagnetic wave irradiation department

Claims (10)

A method of manufacturing a polarizing film, which is a method of manufacturing a polarizing film from a polyvinyl alcohol-based resin film, the manufacturing method comprising: a dyeing process of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment, and converting the aforementioned dyeing step to The last aforementioned polyvinyl alcohol-based resin film is sequentially immersed in n (n is an integer greater than 2) cross-linking baths to carry out the cross-linking process of cross-linking treatment. No. (x is an integer less than or equal to n), the liquid removal process of removing the moisture adhering to the surface of the polyvinyl alcohol-based resin film pulled out after the x-th crosslinking bath, and the aforementioned polyvinyl alcohol-based resin film after the liquid removal process Electromagnetic wave irradiation process in which the resin film is irradiated with infrared rays with a wavelength of more than 2 μm and less than 4 μm, and the ratio of the radiant energy of the electromagnetic wave is 25% or more of the total radiant energy; each crosslinking bath is a solution with a boron compound concentration of 0.5% by mass or more constituted, wherein the n-th cross-linking bath arranged from the upstream side to the downstream side is composed of a solution with a concentration of the aforementioned boron compound of 2.4% by mass or less, and is arranged from the upstream side to the downstream side At least one of the cross-linking baths in No. 1 to No. n-1 cross-linking baths on the side is composed of a solution with a concentration of the aforementioned boron compound of 2.5% by mass or more. The method for producing a polarizing film according to claim 1, wherein in the electromagnetic wave irradiation step, the heat of electromagnetic wave irradiation per unit volume of the polyvinyl alcohol-based resin film is 100J/ ^cm3 or more and 50kJ/ ^cm3 the following. The method for manufacturing a polarizing film as described in claim 1 or 2 of the patent claims, wherein x is n. The method for manufacturing a polarizing film as described in claim 1 or 2 of the patent claims, wherein the temperature of the nth cross-linking bath is above 30°C. The method for manufacturing a polarizing film as described in claim 4, wherein the temperature of the nth cross-linking bath is above 35°C. The method for producing a polarizing film according to claim 1 or 2, wherein the temperature of the crosslinking bath composed 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 1 or 2, wherein the temperature of the crosslinking bath composed 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 washing step of washing the polyvinyl alcohol-based resin film after the crosslinking step and the electromagnetic wave irradiation step. The method of manufacturing a polarizing film as described in claim 1 or 2 of the patent claims, wherein the electromagnetic wave irradiation step is carried out within 5 seconds after being pulled out from the xth cross-linking bath. A polarizing film manufacturing device, which is a manufacturing device for manufacturing a polarizing film from a polyvinyl alcohol-based resin film, the manufacturing device includes: a dyeing section that dyes the polyvinyl alcohol-based resin film with a dichroic dye, and the dyed polyvinyl alcohol-based resin film is dyed. Impregnation of the treated polyvinyl alcohol-based resin film For the cross-linking part that is subjected to cross-linking treatment in n (n is an integer of 2 or more) cross-linking baths, for the x-th (x is an integer of n or less) that is immersed in the x-th (x is an integer of n or less) arranged from the upstream side to the downstream side Liquid removal means for removing moisture adhering to the surface of the polyvinyl alcohol-based resin film pulled out after the cross-linking bath, and irradiating the polyvinyl alcohol-based resin film after the liquid removal with infrared rays having a wavelength of more than 2 μm and 4 μm or less The electromagnetic wave irradiation part of the electromagnetic wave whose radiant energy ratio is 25% or more of the total radiant energy; each cross-linking bath is composed of a solution with a boron compound concentration of 0.5% by mass or more, and is arranged from the upstream side to the downstream side The nth cross-linking bath of the nth number is composed of a solution having a concentration of the aforementioned boron compound of 2.4% by mass or less, and is arranged at the intersection of No. 1 to No. n-1 from the upstream side to the downstream side. At least one cross-linking bath in the joint bath is composed of a solution having a concentration of the aforementioned boron compound of 2.5% by mass or more.

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