Polarizing film manufacturing method and polarizing film

Abstract

To provide a method for manufacturing a polarization film, capable of obtaining a polarization film having a high iodine content while having high visibility corrected single body transmittance, and the polarization film excellent in optical properties.SOLUTION: A method for manufacturing a polarization film from a polyvinyl alcohol based resin film comprises the dyeing and crosslinking step of dyeing and crosslinking the polyvinyl alcohol based resin film. The dyeing and crosslinking step comprises: the first dyeing step of dyeing the polyvinyl alcohol based resin film with a dichroic dye; the first crosslinking step of crosslinking the polyvinyl alcohol based resin film with a cross linking agent after the first dyeing step; and the second dyeing step of dyeing the polyvinyl alcohol based resin film with a dichroic dye after the first crosslinking step.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a polarizing film from a polyvinyl alcohol-based resin film, and to a polarizing film. [Background technology]

[0002] Polarizing plates are widely used as polarizing elements in image display devices such as liquid crystal display devices. Polarizing plates generally have a configuration in which a transparent resin film (such as a protective film) is attached to one or both sides of a polarizing film using an adhesive or the like.

[0003] Polarizing films are mainly produced by subjecting raw film made of polyvinyl alcohol-based resin to a process such as immersion in a dye bath containing a dichroic dye such as iodine, followed by immersion in a crosslinking bath containing a crosslinking agent such as boric acid, and uniaxially stretching the film at one of these stages. 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 or crosslinking bath. Japanese Patent Laid-Open Publication No. 2013-148806 (Patent Document 1) describes a polarizing film with excellent durability, which has a low boron content of 1 to 3.5 wt %.

[0004] The luminosity-corrected transmittance and luminosity-corrected polarization degree are known as indicators of the optical properties of polarizing films. As the iodine content of polarizing films increases, the luminosity-corrected polarization degree tends to increase, while the luminosity-corrected transmittance tends to decrease. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-148806 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to provide a method for producing a polarizing film that has a high luminosity-corrected single transmittance and a high iodine content, and a polarizing film that has excellent optical properties. [Means for solving the problem]

[0007] The present invention provides the following method for producing a polarizing film and the polarizing film. [1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing and crosslinking step of dyeing and crosslinking the polyvinyl alcohol-based resin film, The dyeing and crosslinking step a first dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye; a first crosslinking step of crosslinking the polyvinyl alcohol-based resin film with a crosslinking agent after the first dyeing step; a second dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye after the first crosslinking step. [2] The method for producing a polarized film according to [1], wherein the dyeing and crosslinking step further includes a second crosslinking step of crosslinking the polyvinyl alcohol-based resin film with a crosslinking agent after the second dyeing step. [3] a washing step of washing the polyvinyl alcohol-based resin film after the dyeing and crosslinking step; The method for producing a polarized film according to [1] or [2], further comprising a drying step of drying the polyvinyl alcohol-based resin film after the washing step. [4] The method according to any one of [1] to [3], wherein in the drying step, the polyvinyl alcohol-based resin film is uniaxially stretched at a stretch ratio of 1.10 or more. [5] A polarizing film having a luminous efficiency corrected single transmittance of 43.0% or more and an iodine content of 4.0% by mass or more. [6] The polarizing film according to [5], which has a crosslinking efficiency of 5.0 or more. [Effects of the Invention]

[0008] According to the manufacturing method of the present invention, a polarizing film having a high luminous efficiency-corrected single transmittance and a high iodine content can be obtained. Furthermore, according to the present invention, a polarizing film having a high luminous efficiency-corrected single transmittance and a high iodine content can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view schematically illustrating an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used in the method according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a schematic cross-sectional view of a polarizing plate including a polarizing film. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Polarizing film manufacturing method] In the present invention, the polarizing film is a uniaxially stretched polyvinyl alcohol resin film to which a dichroic pigment (iodine or a dichroic dye) is adsorbed and aligned. The polyvinyl alcohol resin constituting the polyvinyl alcohol resin film is typically obtained by saponifying a polyvinyl acetate resin. The degree of saponification is typically 85 mol % or more, preferably 90 mol % or more, and more preferably 99 mol % or more. Examples of polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as copolymers of vinyl acetate with other monomers copolymerizable therewith. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol resin is typically about 1,000 to 10,000, and preferably about 1,500 to 5,000.

[0011] These polyvinyl alcohol-based resins may be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc., modified with aldehydes may also be used. In this specification, the term "polyvinyl alcohol-based resin" refers to a resin in which the vinyl alcohol structural unit (-CH-CH(OH)-) accounts for 50 mol % or more of all structural units contained in the resin.

[0012] In the present invention, an unstretched polyvinyl alcohol-based resin film (raw film) having a thickness of 65 μm or less (e.g., 60 μm or less), preferably 50 μm or less, and more preferably 45 μm or less, is used as the starting material for producing a polarizing film. This makes it possible to obtain thin polarizing films, for which market demand is increasing. The width of the raw film is not particularly limited and can be, for example, about 400 to 6000 mm. The raw film is prepared, for example, as a roll (raw film roll) of a long unstretched polyvinyl alcohol-based resin film.

[0013] Polarized film can be continuously produced as a long polarized film by unwinding the long raw film from the raw film roll and continuously transporting it along a film transport path in a polarized film manufacturing apparatus, carrying out a predetermined treatment step in which the film is immersed in a treatment liquid (hereinafter also referred to as a "treatment bath") contained in a treatment tank and then withdrawn, followed by a drying step. Note that the treatment step is not limited to a method of immersing the film in a treatment bath as long as it is a method of treating the film by contacting the film with the treatment liquid, and may be a method of treating the film by attaching the treatment liquid to the film surface by spraying, flowing, dropping, or the like. When the treatment step is performed by a method of immersing the film in a treatment bath, the number of treatment baths used in one treatment step is not limited to one, and one treatment step may be completed by sequentially immersing the film in two or more treatment baths.

[0014] Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a crosslinking liquid, and a cleaning liquid. Examples of the treatment process include a swelling process in which a raw film is brought into contact with a swelling liquid to perform a swelling treatment; a dyeing process in which the film after the swelling treatment is brought into contact with a dyeing liquid to perform a dyeing treatment; a crosslinking process in which the film after the dyeing treatment is brought into contact with a crosslinking liquid to perform a crosslinking treatment; and a cleaning process in which the film after the crosslinking treatment is brought into contact with a cleaning liquid to perform a cleaning treatment. In addition, between these series of treatment processes (i.e., before, after, and / or during any one or more treatment processes), a wet or dry uniaxial stretching treatment is performed. Other treatment processes may be added as necessary. In this specification, the dyeing process and crosslinking process are collectively referred to as the dyeing and crosslinking process.

[0015] <Dyeing cross-linking process> The dyeing and crosslinking process includes a first dyeing process in which the polyvinyl alcohol-based resin film is dyed with iodine, a first crosslinking process in which the polyvinyl alcohol-based resin film is crosslinked with a crosslinking agent after the first dyeing process, and a second dyeing process in which the polyvinyl alcohol-based resin film is dyed with iodine after the first crosslinking process. The dyeing and crosslinking process preferably further includes a second crosslinking process in which the polyvinyl alcohol-based resin film is crosslinked with a crosslinking agent after the second dyeing process. The dyeing and crosslinking process may further include a second crosslinking process and a third crosslinking process in which the polyvinyl alcohol-based resin film is crosslinked after the second dyeing process. The dyeing and crosslinking process may further include another dyeing process or crosslinking process after the second crosslinking process. After the dyeing and crosslinking process, the polyvinyl alcohol-based resin film is sent to, for example, a washing process. The final process in the dyeing and crosslinking process, i.e., the process immediately before the washing process, is preferably a crosslinking process for adjusting the hue.

[0016] By performing two or more dyeing steps in the dyeing and crosslinking process and performing a second dyeing step after the first crosslinking step, it is possible to obtain a polarizing film with a high luminous efficacy-corrected single transmittance and a high iodine content. This is presumably because the crosslinking agent incorporated into the polyvinyl alcohol-based resin film in the first crosslinking step is easily released in the dye bath of the second dyeing step, which has a low crosslinking agent concentration. The release of the crosslinking agent facilitates the incorporation of iodine in the second dyeing step. Furthermore, it is presumed that the iodine incorporated into the polyvinyl alcohol-based resin film in the second dyeing step is unlikely to cause a decrease in luminous efficacy-corrected single transmittance. While the reason why iodine incorporated into the polyvinyl alcohol-based resin film in the second dyeing step is unlikely to cause a decrease in luminous efficacy-corrected single transmittance is unclear, it is presumed to be related to the release of the crosslinking agent and its incorporation into the space previously occupied by the crosslinking agent. Increasing the iodine content in polarizing films allows for the production of polarizing films with a neutral hue.

[0017] [First embodiment] One embodiment of the polarizing film manufacturing method according to the present invention will be described in detail with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically illustrating an example of the polarizing film manufacturing method according to the present invention and a polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in FIG. 1 is configured such that raw (unstretched) film 10 made of a polyvinyl alcohol resin is continuously unwound from raw roll 11 and conveyed along a film conveyance path, whereby the film passes sequentially through a swelling bath (swelling liquid contained in a swelling tank) 13, a dyeing and crosslinking step 18, and a washing bath (washing liquid contained in a washing tank) 19, all of which are provided along the film conveyance path, and finally passes through a drying oven 21. The resulting polarizing film 23 can be conveyed directly to the next polarizing plate manufacturing process (a process of laminating a protective film to one or both sides of the polarizing film 23), for example. The arrows in FIG. 1 indicate the film conveyance direction.

[0018] The dyeing and crosslinking step 18 is a step in which the film removed from the swelling bath 13 is sequentially passed through a first dye bath (dyeing liquid contained in a dyeing tank) 15a, a first crosslinking bath (first crosslinking liquid contained in a crosslinking tank) 17a, a second dye bath (second dyeing liquid contained in a dyeing tank) 15b, a second crosslinking bath (second crosslinking liquid contained in a crosslinking tank) 17b, and a third crosslinking bath (third crosslinking liquid contained in a crosslinking tank) 17c. The step of immersing the film in the first dye bath 15a is referred to as the first dyeing step, and the step of immersing the film in the second dye bath 15b is referred to as the first dyeing step, and these steps are collectively referred to as the dyeing step. The step of immersing the film in the first crosslinking bath 17a is referred to as the first crosslinking step, the step of immersing the film in the second crosslinking bath 17b is referred to as the second crosslinking step, and the step of immersing the film in the third crosslinking bath 17c is referred to as the third crosslinking step, and these steps are collectively referred to as the crosslinking step.

[0019] In the description of Fig. 1, "treatment tank" is a general term including a swelling tank, a dyeing tank, a crosslinking tank, and a washing tank, "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, dyeing bath, crosslinking bath, and washing bath respectively constitute the swelling section, the dyeing section, the crosslinking section, and the washing section in the production apparatus of the present invention.

[0020] The film transport path of the polarizing film manufacturing equipment can be constructed by appropriately arranging, in addition to the treatment baths described above, guide rolls 30-49, 56-61 that support the film being transported or can even change the film transport direction, and nip rolls 50-55 that press and clamp the film being transported, imparting driving force to the film through their rotation, or even changing the film transport direction. Guide rolls and nip rolls can be placed before, after, or within each treatment bath, allowing the film to be introduced into, immersed in, and withdrawn from the treatment bath (see Figure 1). For example, one or more guide rolls can be provided in each treatment bath, and the film can be immersed in each treatment bath by transporting it along these guide rolls.

[0021] The polarizing film manufacturing apparatus shown in Fig. 1 has nip rolls arranged before and after each treatment bath (nip rolls 50 to 54), which makes it possible to perform inter-roll stretching in one or more treatment baths, in which longitudinal uniaxial stretching is performed by creating a peripheral speed difference between the nip rolls arranged before and after the treatment bath. In the polarizing film manufacturing apparatus shown in Fig. 1, longitudinal uniaxial stretching is preferably also performed in the drying oven 21, and longitudinal uniaxial stretching in the drying oven 21 can be performed, for example, by creating a peripheral speed difference between the nip rolls 54 and 55.

[0022] <Swelling process> The swelling process is carried out for the purposes of removing foreign matter from the surface of the raw film 10, removing plasticizers in the raw film 10, imparting ease of dyeing, plasticizing the raw film 10, etc. The treatment conditions are determined within a range in which the above-mentioned purposes can be achieved and in which defects such as extreme dissolution or devitrification of the raw film 10 do not occur.

[0023] 1, the swelling step can be carried out by continuously unwinding raw film 10 from raw film roll 11 and transporting it along a film transport path, immersing raw film 10 in swelling bath 13 for a predetermined time, and then pulling it out. In the example of FIG. 1, from the time raw film 10 is unwound until it is immersed in swelling bath 13, raw film 10 is transported along a film transport path formed by guide rolls 60, 61 and nip roll 50. During the swelling treatment, raw film 10 is transported along a film transport path formed by guide rolls 30 to 32 and nip roll 51.

[0024] As the swelling liquid for the swelling bath 13, in addition to pure water, it is also possible to use an aqueous solution containing boric acid (Japanese Patent Application Laid-Open No. 10-153709), chlorides (Japanese Patent Application Laid-Open No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc. in an amount of approximately 0.01 to 10% by weight.

[0025] 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, and more preferably about 20 to 200 seconds. When the raw film 10 is a polyvinyl alcohol resin film that has been stretched in a gas in advance, the temperature of the swelling bath 13 is, for example, about 20 to 70°C, and preferably about 30 to 60°C. The immersion time of the raw film 10 is preferably about 30 to 300 seconds, and more preferably about 60 to 240 seconds.

[0026] The swelling treatment can easily cause problems such as the raw film 10 swelling in the width direction and wrinkling. One method for conveying the film while removing these wrinkles is to use rolls with a width-expanding function, such as expander rolls, spiral rolls, or crown rolls, as the guide rolls 30, 31, and / or 32, or to use other width-expanding devices, such as cross guiders, bend bars, or tenter clips. Another method for suppressing wrinkles is to perform a stretching treatment. For example, uniaxial stretching can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip rolls 50 and 51.

[0027] During the swelling treatment, the film also swells and expands in the film transport direction, so if the film is not actively stretched, it is preferable to take measures to eliminate slack in the film in the transport direction, such as controlling the speed of nip rolls 50, 51 placed before and after the swelling bath 13. Furthermore, for the purpose of stabilizing the film transport in the swelling bath 13, it is also useful to control the water flow in the swelling bath 13 with an underwater shower or to use an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents the film from meandering) in combination.

[0028] In the example shown in FIG. 1, the film drawn out from the swelling bath 13 passes through a guide roll 32, a nip roll 51 and a guide roll 33 in this order before being introduced into the dye bath 15.

[0029] <Dyeing cross-linking process> (1st dyeing process) The first dyeing step is performed for the purpose of adsorbing and orienting a dichroic dye such as iodine to the polyvinyl alcohol-based resin film after swelling treatment. The treatment conditions are determined within a range that achieves the purpose and does not cause defects such as excessive dissolution or devitrification of the film. Referring to FIG. 1 , the first dyeing step can be performed by conveying the film along a film conveyance path formed by nip roll 51, guide rolls 33-36, and nip roll 52, immersing the swelling film in first dye bath 15a (a treatment solution contained in a dyeing tank) for a predetermined time, and then withdrawing the film. To enhance the dyeability of the dichroic dye, the film subjected to the first dyeing step is preferably a film that has been subjected to at least some degree of uniaxial stretching treatment. Alternatively, uniaxial stretching treatment is preferably performed during the first dyeing step instead of or in addition to the uniaxial stretching treatment before the first dyeing step.

[0030] When the dichroic dye is iodine, the dye solution for the first dye bath 15 can be, for example, an aqueous solution with a weight ratio of iodine / potassium iodide / water of 0.003-0.3 / 0.1-10 / 100. Potassium iodide can be replaced with other iodides, such as zinc iodide, or potassium iodide can be used in combination with other iodides. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, or cobalt chloride, can also be present. When the dye solution for the dye bath contains boric acid, it is distinguished from a crosslinking bath in that it contains iodine at a higher concentration than boric acid. The iodine concentration in the dye bath is preferably 0.6 mM or higher, more preferably 1.0 mM or higher, and even more preferably 2.0 mM or higher. The temperature of the dye bath 15 when the film is immersed is usually about 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C, and the immersion time of the film is usually about 30 to 600 seconds, and preferably 60 to 300 seconds.

[0031] As described above, in the first dyeing step, the film can be uniaxially stretched in the first dye bath 15. The film can be uniaxially stretched by, for example, creating a difference in peripheral speed between the nip rolls 51 and 52 disposed before and after the first dye bath 15.

[0032] In the first dyeing step, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, similar to the swelling treatment, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls may be used as the guide rolls 33, 34, 35, and / or 36, or other width-expanding devices such as cross guiders, bend bars, and tenter clips may be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, similar to the swelling treatment.

[0033] In the example shown in FIG. 1, the film drawn out from the first dyeing bath 15 passes through a guide roll 36, a nip roll 52, and a guide roll 37 in this order, and is introduced into a first crosslinking bath 17a.

[0034] (1st crosslinking step) The first crosslinking step is a treatment carried out for the purpose of crosslinking the film, and this treatment can improve water resistance and heat resistance.

[0035] Referring to FIG. 1, the first crosslinking step can be carried out by transporting the film after the first dyeing step along a film transport path formed by nip roll 52, guide rolls 37 to 40, and nip roll 53a, immersing the film in first crosslinking bath 17a for a predetermined time, and then pulling it out.

[0036] The difference in peripheral speed between the nip roll 52 and the nip roll 53a can be used to perform uniaxial stretching in the first crosslinking bath 17a.

[0037] In the first crosslinking step, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles from the film, similar to the swelling treatment, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls may be used as the guide rolls 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49a, 49b, and / or 49d, or other width-expanding devices such as cross guiders, bend bars, and tenter clips may be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, similar to the swelling treatment.

[0038] The temperature of the first crosslinking bath 17a is preferably 30°C or higher, more preferably 35°C or higher, and even more preferably 42°C or higher, from the viewpoint of promoting crosslinking of the polyvinyl alcohol-based resin film and equilibration of the amount of crosslinking agent in the polyvinyl alcohol film. The temperature of the first crosslinking bath 17a is preferably 65°C or lower, more preferably 64°C or lower, from the viewpoint of preventing elution of the polyvinyl alcohol-based resin film. The immersion time of the film in the first crosslinking bath 17a is about 3 to 600 seconds, preferably 4 to 300 seconds, and more preferably 5 to 200 seconds.

[0039] The first crosslinking bath 17a contains a crosslinking agent, preferably a boron compound. The first crosslinking bath 17a preferably has a boron compound concentration of 2.5% by mass or more, more preferably 2.8% by mass or more. The first crosslinking bath 17a has a boron compound concentration of, for example, 4.0% by mass or less. The boron compound acts as a crosslinking agent, and examples thereof include boric acid and borax. The first crosslinking bath 17a may contain a crosslinking agent such as glyoxal or glutaraldehyde in addition to the boron compound. The solvent for the crosslinking bath may be water, but may also contain an organic solvent compatible with water. The first crosslinking bath 17a preferably contains an iodide in addition to the boron compound, and the concentration of the iodide in the crosslinking bath may be, for example, 1 to 30% by mass. Examples of iodides include potassium iodide and zinc iodide. Furthermore, compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present.

[0040] (Second dyeing process) The above description of the first dyeing step applies to the second dyeing step. In the second dyeing step, uniaxial stretching can be performed in the second dyeing bath 15b by, for example, creating a difference in peripheral speed between the nip rolls 53a and 53b. The second dyeing bath 15b may be the same as or different from the first dyeing bath 15a in terms of composition, temperature, and immersion time. The temperature of the second dyeing bath 15b may be higher than the temperature of the first dyeing bath 15a, and may be 5°C or more higher, or 10°C or more higher. By making the temperature of the second dyeing bath 15a higher than the temperature of the first dyeing bath, the stretch ratio in the stretching treatment in the drying step can be increased.

[0041] (Second crosslinking step) The second crosslinking step is the same as the first crosslinking step. In the second crosslinking step, uniaxial stretching can be performed in the second crosslinking bath 17b by, for example, creating a difference in peripheral speed between the nip rolls 53b and 53c. The second crosslinking bath 17b may be the same as or different from the first crosslinking bath 17a in terms of composition, temperature, and immersion time.

[0042] (Third crosslinking step) In FIG. 1, the third crosslinking step is a treatment performed for the purpose of adjusting the hue of the film (e.g., preventing the film from becoming bluish). In the third crosslinking step, uniaxial stretching can be performed in the third crosslinking bath 17c by, for example, creating a difference in peripheral speed between the nip rolls 53c and 53d. The third crosslinking bath 17c may be the same as or different from the first crosslinking bath 17a or the second crosslinking bath 17b in terms of temperature, composition, and immersion time. The temperature of the crosslinking bath in the third crosslinking step, which is intended to adjust the hue of the film, is preferably lower than the temperature of the crosslinking bath in the first crosslinking step or the temperature of the crosslinking bath in the second crosslinking step, which are intended to crosslink the film. The temperature of the third crosslinking bath 17c can be, for example, 45°C or lower.

[0043] <Cleaning process> In the example shown in FIG. 1, a washing step is included after the dyeing and crosslinking step. The washing treatment is carried out for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The washing step is carried out, for example, by immersing the crosslinked polyvinyl alcohol-based resin film in a washing bath 19. Note that the washing step can also be carried out by spraying a washing solution onto the film as a shower instead of immersing the film in the washing bath 19, or by combining immersion in the washing bath 19 with spraying the washing solution.

[0044] 1 shows an example of a case where a polyvinyl alcohol-based resin film is subjected to a cleaning treatment by immersing it in a cleaning bath 19. The temperature of the cleaning bath 19 in the cleaning treatment is usually about 2 to 40°C, and the immersion time of the film is usually about 2 to 120 seconds.

[0045] In addition, in the washing treatment, for the purpose of transporting the polyvinyl alcohol-based resin film while removing wrinkles, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls can be used as the guide rolls 56, 57, 58, and / or 59, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used. In addition, in the film washing treatment, a stretching treatment may be performed to suppress the occurrence of wrinkles.

[0046] <Stretching process> As described above, the raw film 10 is subjected to a wet or dry uniaxial stretching process during the series of processing steps (i.e., before, after, and / or during any one or more processing steps). Specific methods for the uniaxial stretching process include, for example, inter-roll stretching, in which longitudinal uniaxial stretching is performed by applying a peripheral speed difference between two nip rolls (e.g., two nip rolls disposed before and after a processing bath) that constitute a film transport path, hot roll stretching as described in Japanese Patent No. 2731813, tenter stretching, etc., with inter-roll stretching being preferred. The uniaxial stretching process can be performed multiple times from the raw film 10 until the polarizing film 23 is obtained. As described above, the stretching process is also advantageous in suppressing the occurrence of wrinkles in the film.

[0047] <Drying process> After the washing step, it is preferable to carry out a treatment to dry the polyvinyl alcohol-based resin film. There are no particular restrictions on the drying of the film, but it can be carried out using a drying oven 21 as shown in the example of FIG. 1. The drying oven 21 can be equipped with, for example, a hot air dryer. 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 to dry the polyvinyl alcohol-based resin film can also be carried out using a far-infrared heater. The polarizing film 23 obtained in this manner has a thickness of, for example, about 5 to 30 μm.

[0048] In the production method of the present invention, the polyvinyl alcohol-based resin film may be stretched in the drying step. According to the production method of the present invention, the tension of the polyvinyl alcohol-based resin film generated in the crosslinking step is alleviated, so that the polyvinyl alcohol-based resin film can be stretched in the drying step without being broken.

[0049] In the drying step, stretching can be performed, for example, by setting a difference in rotation speed between a nip roll provided near the entrance of the drying oven and a nip roll provided near the exit of the drying oven. In such stretching, if the ratio of the rotation speed of the nip roll provided near the exit of the drying oven to the rotation speed of the nip roll provided near the entrance of the drying oven is defined as the stretch ratio in the drying step, the stretch ratio of the polyvinyl alcohol-based resin film in the drying step is, for example, preferably 1.05 times or more, more preferably 1.10 times or more. Stretching at a stretch ratio of 1.05 times or more in the drying step can improve the optical properties of the resulting polarized film.

[0050] <Other treatment processes for polyvinyl alcohol-based resin films> Treatments other than those described above can also be added. Examples of treatments that can be added include immersion treatment in an aqueous solution containing zinc chloride or the like but not a boron compound (zinc treatment).

[0051] The obtained polarizing film may be wound up onto a winding roll to form a roll, or may be used directly without being wound up in the polarizing plate production process (a process of laminating a protective film or the like onto one or both sides of the polarizing film).

[0052] The final cumulative stretching ratio of the polarizing film 23 based on the raw film 10 is usually about 4.5 to 8 times, and preferably 6 to 8 times. The stretching step may be carried out in any of the processing steps or in the drying step, and even when the stretching treatment is carried out in two or more processing steps, the stretching treatment may be carried out in any of the processing steps.

[0053] [Polarizing film] The thickness of the polarizing film is usually 65 μm or less, preferably 50 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less. The thickness of the polarizing film is usually 2 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. The thickness of the polarizing film can be controlled, for example, by selecting a polyvinyl alcohol-based resin film or adjusting the stretching ratio.

[0054] The luminosity-corrected polarization degree of the polarizing film may be, for example, 99.9800% or more, preferably 99.9900% or more, more preferably 99.9910% or more, and even more preferably 99.9915% or more. The polarization degree of the polarizing film is usually 100% or less, and in practical use, it is less than 100%, and may be 99.9950% or less. The luminosity-corrected polarization degree of the polarizing film can be measured by the measurement method described in the Examples section below.

[0055] The luminosity-corrected single transmittance of the polarizing film may be, for example, 43.0% or more, preferably 43.5% or more, and more preferably 43.9% or more. The luminosity-corrected single transmittance is usually 50.0% or less, and may be, for example, less than 50.0%, or may be 49.0% or less. The transmittance of the polarizing film can be measured by the measurement method described in the Examples section below.

[0056] The orthogonal hue b value of the polarizing film may be, for example, in the range of -3.0 to 3.0, preferably -2.5 to 2.6, more preferably -2.0 to 2.0, even more preferably -1.5 to 1.5, and particularly preferably -1.2 to 1.2. The orthogonal hue b value of the polarizing film when positioned orthogonally can be obtained by calculating the chromaticity in the L*a*b* (CIE) color system using a spectrophotometer with an integrating sphere ("V7100" manufactured by JASCO Corporation) and the color matching function of Illuminant C for the obtained transmittance. The orthogonal hue b value of the polarizing film can be measured using the measurement method described in the Examples section below.

[0057] The iodine content of the polarizing film may be, for example, 3.5% by mass or more, preferably 4.0% by mass or more, and more preferably 4.3% by mass or more. For practical purposes, the iodine content is 6.0% by mass or less, and may be 5.5% by mass or less. The iodine content of the polarizing film can be measured by the measurement method described in the Examples section below.

[0058] The boron content of the polarizing film is preferably 4.0% by mass or less, and more preferably 3.5% by mass or less. The boron content may be, for example, 3.0% by mass or more, or 3.2% by mass or more. The boron content of the polarizing film can be measured by the measurement method described in the Examples section below.

[0059] The crosslinking efficiency of the polarizing film may be, for example, in the range of 5.0 to 6.0, and preferably in the range of 5.2 to 5.8. The crosslinking efficiency of the polarizing film can be calculated by the calculation method explained in the Examples section below. The higher the crosslinking efficiency of the polarizing film, the more efficiently the boron in the polarizing film is used for crosslinking. It is preferable that the boron content in the polarizing film can be reduced while maintaining a desired degree of crosslinking.

[0060] Generally, in polarizing films, the luminous-corrected single transmittance tends to decrease as the iodine content increases. According to the manufacturing method of the present invention, a polarizing film having a high luminous-corrected single transmittance and a high iodine content can be obtained. For example, a polarizing film having a luminous-corrected single transmittance of 43.0% or more and an iodine content of 4.0% by mass or more can be obtained. With such a polarizing film, a high luminous-corrected polarization degree can be obtained due to the high iodine content. For example, a polarizer having a luminous-corrected polarization degree of 99.9800% or more can be obtained.

[0061] [Polarizing plate] A polarizing plate can be obtained by laminating a protective film to at least one surface of the polarizing film via an adhesive layer. Examples of the protective film include films made of acetylcellulose-based resins such as triacetylcellulose and diacetylcellulose, films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, polycarbonate-based resin films, cycloolefin-based resin films, acrylic-based resin films, and films made of linear olefin-based resins such as polypropylene-based resins.

[0062] In order to improve the adhesion between the polarizing film and the protective film, the bonding surfaces of the polarizing film and / or the protective film may be subjected to surface treatments such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. The pressure-sensitive adhesive layer can be composed of an adhesive or a pressure-sensitive adhesive.

[0063] The polarizing plate 60 shown in FIG. 2 has a polarizing film 23, an adhesive layer 70, and a protective film 80, in this order. The polarizing plate may be a linear polarizing plate or a circular polarizing plate used in a display device. The display device may be any type of display device, such as a liquid crystal display device or an organic EL display device. The display device may be a display device used in a mobile device such as a television, personal computer, mobile phone, or tablet terminal. [Example]

[0064] The present invention will be explained in more detail below by showing test examples, but the present invention is not limited to these examples.

[0065] Example 1 A polarizing film was produced from a polyvinyl alcohol-based resin film using the production apparatus shown in Figure 1 under the conditions shown in Table 1. Specifically, a 45 μm-thick, long polyvinyl alcohol (PVA) raw film (trade name "TS45" manufactured by Kuraray Co., Ltd., average degree of polymerization 2400, saponification degree 99.9 mol% or more) was continuously transported while being unwound from a roll and immersed in a swelling bath of water (deionized water) at 30°C for 1 minute 20 seconds, whereby it was stretched 1.56 times (swelling step). The film was then pulled out of the swelling bath and immersed in a first dyeing bath at 30°C containing 1.0 mass% potassium iodide and 1.4 mM iodine for 100 seconds, whereby it was stretched 1.15 times (first dyeing step). The film was then drawn out of the first dye bath and immersed in a first crosslinking bath containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid at 60°C for 37 seconds, where it was stretched 1.46 times (first crosslinking step). The film was then drawn out of the first crosslinking bath and immersed in a second dye bath containing 1.0% by mass of potassium iodide and 1.4 mM of iodine at 30°C for 20 seconds, where it was stretched 1.70 times (second dyeing step). The film was then drawn out of the second dye bath and immersed in a second crosslinking bath containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid at 63°C for 6 seconds, where it was stretched 1.34 times (second crosslinking step). The film was then drawn out of the second crosslinking bath and immersed in a third crosslinking bath containing 10.0% by mass of potassium iodide and 3.0% by mass of boric acid at 57°C for 7 seconds, where it was stretched 1.00 times (third crosslinking step). By undergoing the swelling step and the dyeing and crosslinking step (in Example 1, the dyeing and crosslinking step consisted of a first dyeing step, a first crosslinking step, a second dyeing step, a second crosslinking step, and a third crosslinking step), the total stretching ratio based on the original film was set to 6.0 times by stretching between rolls in the bath.

[0066] The film was then immersed in a 13°C washing bath for 2 seconds to remove any foreign matter adhering to the film surface (washing step). After the washing step, the film was held in a drying oven at a tension of 470 N and then dried with hot air at 92°C while being stretched at the stretch ratio shown in Table 1 (drying step). A polarized film was thus obtained. The thickness of the resulting polarized film was 16.6 μm. In the drying step, uniaxial stretching was performed by setting a difference between the rotation speed of the nip rolls located near the entrance and the rotation speed of the nip rolls located near the exit of the drying oven. The stretch ratio shown in Table 1 is the ratio of the rotation speed of the nip rolls located near the entrance to the rotation speed of the nip rolls located near the exit.

[0067] <Examples 2 to 7> Polarized films were obtained under the same conditions as in Example 1, except that the temperature of the second dye bath and the stretch ratio in the drying step were set as shown in Table 1. The thickness of each of the obtained polarized films was 16.4 μm.

[0068] <Comparative Example 1> A polarized film was obtained under the same conditions as in Example 1, except that the order of each step in the dyeing and crosslinking step was set as shown in Table 1 and the stretch ratio in the drying step was set as shown in Table 1. The thickness of the obtained polarized film was 17.1 μm.

[0069] [Evaluation of Polarizing Film] (a) Measurement of single unit transmittance and polarization degree The MD transmittance and TD transmittance of the polarizing film were measured in the wavelength range of 380 to 780 nm using a spectrophotometer equipped with an integrating sphere ("V7100" manufactured by JASCO Corporation), and the transmittance was calculated using the following formula: Single transmittance (%) = (MD + TD) / 2 Degree of polarization (%)={(MD-TD) / (MD+TD)}×100 The single transmittance and polarization degree at each wavelength were calculated based on the above.

[0070] "MD transmittance" is the transmittance when the direction of polarized light emerging from the Glan-Thompson prism is parallel to the transmission axis of the polarizing film sample, and is represented as "MD" in the above formula. "TD transmittance" is the transmittance when the direction of polarized light emerging from the Glan-Thompson prism is perpendicular to the transmission axis of the polarizing film sample, and is represented as "TD" in the above formula. The obtained single transmittance and polarization degree were measured in accordance with JIS Z 8701:1999 "Method of representing color - XYZ color system and XYZ color system". 10 Y 10 Z 10 Luminosity correction was performed using a 2-degree visual field (C light source) of the "Color Space" to determine the luminosity-corrected single transmittance (Ty) and luminosity-corrected degree of polarization (Py). Table 1 shows the calculated luminosity-corrected single transmittance (Ty) and luminosity-corrected degree of polarization (Py).

[0071] (b) Orthogonal hue b value The polarizing film was cut into a size of 4 cm x 4 cm, and then the orthogonal hue b value was measured using an ultraviolet-visible spectrometer (V-7100, manufactured by JASCO Corporation).

[0072] (c) Measurement of iodine content 0.2 g of polarized film (sample) was placed in a 30 ml vial, and deionized water was added to make the total amount of sample and deionized water 25 g. The vial was then immersed in a 90°C water bath and left for at least 3 hours to dissolve the sample, after which it was cooled. The iodine content of the dissolved sample was measured using an XRF (Terkin Elmer Optima 5300DV). The measured value, X (ppm), was used to calculate the iodine content (mass%) of the polarized film using the following formula: Iodine content (mass%) = X × (25 / 1,000,000) × [100 / (mass of polarizing film, g)]

[0073] (d) Measurement of boron content 0.2 g of polarized film (sample) was placed in a 30 ml vial, and deionized water was added until the total weight of the sample and deionized water reached 25 g. The vial was then immersed in a 90°C water bath for at least 3 hours to dissolve the sample, after which it was cooled. The dissolved sample was placed in a 100 ml vial, and 50 ml of mannitol solution (D-MANNITOL CAS NO. 69065-8, manufactured by OCI Corporation) was added. The solution thus prepared was titrated with 0.1 N NaOH solution, and the boron content (% by mass) of the polarized film was calculated using the following formula: Boron content (mass%) = [(NaOH added amount, ml) x 61.8 x 1.001 x 0.1 x 0.1749] / [(polarizing film mass, g) x 1000] x 100

[0074] (e) Calculation of cross-linking efficiency To calculate the crosslinking efficiency, the degree of crosslinking was first calculated. After cutting the central part of the polarizing film into a size of 10 cm x 10 cm, the degree of crosslinking was measured using a Fourier transform infrared spectrophotometer (FT-IR) (Nicolet 5700, manufactured by Thermo Fisher Scientific). The measurement was performed using a VeeMAX III (ATR) manufactured by Pike Technologies as the FT-IR chip, with 16 scans and a wavenumber resolution of 4 cm. -1 The measured IR data wavenumber was 1200-1360 cm -1 The sum of the areas (a) when the regions in the range of wavenumbers 2850 to 3000 cm are combined according to the standard of 3.2 -1 The area of ​​each region was divided by the sum of the areas (b) obtained by combining the regions in the same standard, and the average value of three measurements was taken as the degree of crosslinking.

[0075] The crosslinking efficiency was calculated by the following formula using the boron content measured in (d) above and the degree of crosslinking calculated above. Crosslinking efficiency = [degree of crosslinking × 10.812] / [(boron content, mass%) × 3]

[0076] [Durability test] (Water immersion test) Two saponified triacetyl cellulose films (40 μm thick) were prepared and attached to both sides of the polarizer produced in Example 1 using an adhesive containing polyvinyl alcohol resin, resulting in a polarizing plate with a layer structure of triacetyl cellulose film / adhesive / polarizer / adhesive / triacetyl cellulose film. The resulting polarizing plate was cut into a 4 cm × 4 cm piece to serve as an evaluation sample. The evaluation sample was immersed in water at 60°C for 30 minutes, then removed and left to stand for approximately 12 hours in an environment of 23°C and 55% relative humidity. The luminosity-corrected polarization index Py of the evaluation sample after the water immersion test was measured in the same manner as above. The luminosity-corrected polarization index Py of the evaluation sample was also measured before the water immersion test. The absolute values ​​of Py after the water immersion test minus Py before the water immersion test are listed in Table 1.

[0077] (Heat and humidity resistance test) The evaluation sample was placed in an oven conditioned to a temperature and humidity of 85°C and 85% for 24 hours, then removed and left to stand for approximately 12 hours in an environment of 23°C and 55% relative humidity. The luminosity-corrected polarization degree Py of the evaluation sample after the heat resistance test was then measured using the same method as above. The table also lists the absolute value of Py after the water immersion test minus Py before the water immersion test.

[0078] [Table 1]

[0079] In Comparative Example 1, the second dyeing step is carried out before the first crosslinking step. The results shown in Table 1 show that the polarized film obtained in Comparative Example 1 has a lower iodine content and a larger change in luminosity-corrected polarization degree due to the water immersion test and moist heat resistance test than the polarized films obtained in Examples 1 to 7. [Explanation of symbols]

[0080] 10 Raw film made of polyvinyl alcohol resin, 11 Raw film roll, 13 Swelling bath, 15a First dyeing bath, 17a First crosslinking bath, 15b Second dyeing bath, 17b Second crosslinking bath, 17c Third crosslinking bath, 18 Dyeing and crosslinking step, 19 Cleaning bath, 21 Drying oven, 23 Polarizing film, 30-49, 56-61 Guide rolls, 50-52, 53a, 53b, 53c, 53d, 54, 55 Nip rolls, 60 Polarizing plate, 70 Pressure-sensitive adhesive layer, 80 Protective film.

Claims

1. A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing and crosslinking step of dyeing and crosslinking the polyvinyl alcohol-based resin film, The dyeing and crosslinking step a first dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye; a first crosslinking step of crosslinking the polyvinyl alcohol-based resin film with a crosslinking agent after the first dyeing step; a second dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye after the first crosslinking step; a second crosslinking step of crosslinking the polyvinyl alcohol-based resin film with a crosslinking agent after the second dyeing step.

2. A method for manufacturing a polarized film as described in claim 1, wherein the polarized film has an iodine content of 4.0 mass% or more and a crosslinking efficiency of 5.0 or more.

3. a washing step of washing the polyvinyl alcohol-based resin film after the dyeing and crosslinking step; The method for producing a polarized film according to claim 1 or 2, further comprising a drying step of drying the polyvinyl alcohol-based resin film after the washing step.

4. The method according to claim 3 , wherein the polyvinyl alcohol-based resin film is uniaxially stretched at a stretch ratio of 1.10 or more in the drying step.

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