Polarizer manufacturing method

Abstract

To provide a method for manufacturing a polarizer having an excellent visual sensitivity correction polarization degree and a neutral color phase (an orthogonal color phase is nearly 0).SOLUTION: The method for manufacturing a polarizer includes a processing step of immersing a polyvinyl alcohol-based resin film in a processing liquid and performing processing on the film. In the processing method, the polyvinyl alcohol-based resin film is subjected to humidifying processing immediately before the polyvinyl alcohol-based resin film is immersed in the processing liquid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a polarizer. [Background technology]

[0002] Patent Document 1 discloses a method for manufacturing a polarizer, characterized by controlling the boron content in the polarizer and the shrinkage force during heating, in order to improve the durability of the polarizing plate in a heat cycle test and to make the orthogonal hue neutral gray. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-148806 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a method for manufacturing a polarizer having a good luminosity-corrected polarization degree (hereinafter simply referred to as polarization degree) and a neutral hue (crossed hue b value close to 0). [Means for solving the problem]

[0005] The present invention provides the following method for producing a polarizer. [1] A method for manufacturing a polarizer, comprising: The method includes a treatment step of immersing a polyvinyl alcohol-based resin film in a treatment solution, In the treating step, the polyvinyl alcohol-based resin film is subjected to a humidifying treatment before being immersed in the treating liquid. [2] The method for producing a polarizer according to [1], wherein the treatment step is at least one selected from the group consisting of a dyeing step, a crosslinking step, and a complementary color step. [3] The method for producing a polarizer according to [1], wherein the treatment step is at least one step selected from the group consisting of a dyeing step and a crosslinking step. [4] The method for producing a polarizer according to any one of [1] to [3], wherein the humidification treatment is carried out by spraying air at a temperature of 25° C. and a humidity of 90% RH or more onto the polyvinyl alcohol-based resin film. [Effects of the Invention]

[0006] According to the present invention, a method for producing a polarizer having a good degree of polarization and a neutral hue can be provided. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is a cross-sectional view schematically illustrating an example of an arrangement of devices in a method for producing a polarizer. [Figure 2] FIG. 1 is a schematic cross-sectional view of a polarizing plate including a polarizer. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In all of the drawings, the scales of the components are appropriately adjusted to make them easier to understand, and the scales of the components shown in the drawings do not necessarily match the scales of the actual components.

[0009] <Polarizer manufacturing method> A method for manufacturing a polarizer according to one aspect of the present invention includes a treatment step of immersing a polyvinyl alcohol-based resin film in a treatment liquid, and in the treatment step, the polyvinyl alcohol-based resin film is subjected to a humidification treatment before being immersed in the treatment liquid.

[0010] The polarizer can be a polyvinyl alcohol-based resin film in which a dichroic dye such as iodine is adsorbed and aligned. The polarizer can be produced, for example, by a method including a swelling step in which the polyvinyl alcohol-based resin film is immersed in a treatment solution containing water to swell the polyvinyl alcohol-based resin film, a dyeing step in which the polyvinyl alcohol-based resin film is immersed in a treatment solution containing a dichroic dye to adsorb the dichroic dye to the polyvinyl alcohol-based resin film, a crosslinking step in which the polyvinyl alcohol-based resin film is immersed in a treatment solution containing a crosslinking agent to fix the adsorbed dichroic dye in the polyvinyl alcohol-based resin film and crosslink the polyvinyl alcohol-based resin, a cleaning step in which the polyvinyl alcohol-based resin film is immersed in a treatment solution containing water to clean and remove chemicals and foreign matter adhering to the polyvinyl alcohol-based resin film, a drying step in which the polyvinyl alcohol-based resin film is dried, a stretching step in which the polyvinyl alcohol-based resin film is uniaxially stretched in the crosslinking step or a step prior to the crosslinking step, and a primary drying step in which the polyvinyl alcohol-based resin film is dried between the crosslinking step and the cleaning step. Each step will be described later.

[0011] FIG. 1 is a cross-sectional schematic diagram showing an example of an apparatus layout for the polarizer manufacturing method of the present invention. The apparatus shown in FIG. 1 is configured such that raw film 10 made of a polyvinyl alcohol-based resin is unwound from a feed roll 11 and passes sequentially through a swelling tank 13 for a swelling treatment, a dyeing tank 15 for a dyeing treatment, and a crosslinking tank 17 for a crosslinking treatment. After passing through crosslinking tank 17, the film passes through primary drying oven 21 for primary drying, followed by a washing tank 19 to wash away unreacted dichroic dyes such as iodine and boric acid. Finally, the film passes through final drying oven 23 for drying, thereby obtaining polarizer 30. Although not shown, uniaxial stretching is performed in or before crosslinking tank 17. While the resulting polarizer 30 is shown wound around winding roll 27, it can also be subjected to the subsequent process of attaching a protective film without being wound. Although Fig. 1 shows an example in which one swelling tank 13, one dyeing tank 15, one crosslinking tank 17, and one washing tank 19 are provided, multiple tanks may be provided for a single treatment. The arrows in Fig. 1 indicate the film transport direction.

[0012] The apparatus for manufacturing a polarizer can be constructed by arranging nip rolls 12, guide rolls 14, and the like at appropriate positions along the film transport path. For example, guide rolls 14 can be placed before, after, or within a treatment tank containing a treatment solution, thereby enabling the film to be introduced into, immersed in, and withdrawn from the treatment tank. More specifically, nip rolls 12 can be placed at the entrance and exit of the treatment tank, allowing the film to be transported while applying tension to it. Alternatively, two or more guide rolls 14 can be provided in the treatment tank, and the film can be immersed in the treatment tank by transporting the film along these guide rolls 14.

[0013] The treatment process in which the humidification treatment can be carried out includes at least one selected from the group consisting of a dyeing process, a crosslinking process, and a complementary color process. In the apparatus shown in Figure 1, a device 16 for carrying out the humidification treatment described below is disposed between a nip roll 12 on the inlet side (upstream side) of a crosslinking tank 17 and a liquid surface 18 of the treatment liquid contained in the crosslinking tank 17.

[0014] As shown in the figure, raw film 10 made of polyvinyl alcohol-based resin, which is the raw material for polarizers, is usually wound in a roll shape around a payout roll 11 and is unwound in a continuous length from this payout roll 11. The thickness of raw film 10 made of polyvinyl alcohol-based resin is usually within the range of 20 to 100 μm, preferably within the range of 30 to 80 μm, and the industrially practical width is within the range of 1500 to 6000 mm.

[0015] (Polyvinyl alcohol resin film) A polyvinyl alcohol resin film is a resin film that serves as the base material of a polarizer, and specifically, is a film made of a resin obtained by saponifying a polyvinyl acetate resin. 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 with vinyl acetate. Examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and acrylamides having an ammonium group.

[0016] The saponification degree of the polyvinyl alcohol resin is usually about 85 to 100 mol%, preferably 98 mol% or more. The polyvinyl alcohol resin may be modified, and for example, polyvinyl formal or polyvinyl acetal modified with aldehydes can also be used. The polymerization degree of the polyvinyl alcohol resin is usually about 1,000 to 10,000, preferably about 1,500 to 5,000.

[0017] The polyvinyl alcohol resin is formed into a film to be used as the raw sheet of the polarizer. The method for forming the polyvinyl alcohol resin into a film is not particularly limited, and the film can be formed by a known method. The thickness of the polyvinyl alcohol resin raw film is also not particularly limited, and may be appropriately selected from the range of, for example, about 20 to 150 μm.

[0018] A polarizer is formed by adsorbing and aligning a dichroic dye on the polyvinyl alcohol-based resin film.

[0019] (humidification treatment) In the method for producing a polarizer, in the treatment step, a polyvinyl alcohol-based resin film is subjected to a humidification treatment before being immersed in a treatment liquid. The treatment step will be described later. The humidification treatment can be a treatment for adding moisture to the polyvinyl alcohol-based resin film. The humidification treatment tends to improve the degree of polarization of the polarizer and tends to make the film more likely to exhibit a neutral hue. This is because the humidification treatment increases the amount of moisture contained in the polyvinyl alcohol-based resin film, further promoting swelling and facilitating penetration of a treatment liquid containing iodine and a crosslinking agent, which will be described later, into the polyvinyl alcohol-based resin film. As a result, it is presumed that further stress relaxation of the polyvinyl alcohol-based resin film progresses, making the film more likely to be stretched by a stretching treatment, which will be described later.

[0020] Methods for providing moisture to a polyvinyl alcohol-based resin film include, for example, spraying humidity-adjusted air onto the polyvinyl alcohol-based resin film and passing the polyvinyl alcohol-based resin film through a humidity-adjusted atmosphere. Among these, the method of spraying humidity-adjusted air onto the polyvinyl alcohol-based resin film is preferred. The air sprayed onto the polyvinyl alcohol-based resin film may be, for example, at a temperature of 25°C and a humidity of 90% RH or higher, and the spray rate may be, for example, 0.1 L / h to 10 L / h, preferably 1 L / h to 5 L / h. Examples of devices for performing the humidification treatment include a humidifier and a high-humidity furnace.

[0021] By carrying out the humidification treatment before immersing the polyvinyl alcohol-based resin film in the treatment liquid, the polyvinyl alcohol-based resin film can be immersed in the treatment liquid without reducing the water content in the polyvinyl alcohol-based resin film that increases due to the humidification treatment. The position where the humidification treatment is carried out may be, for example, between a nip roll attached to the entrance of a treatment tank containing the treatment liquid and the liquid surface of the treatment liquid contained in the treatment tank, preferably 10 cm to 1 m upstream from the liquid surface. The humidification treatment can be carried out, for example, 0.5 to 4 seconds, more preferably 1 to 2 seconds, before immersing the film in the treatment liquid.

[0022] When the humidification treatment is performed by spraying humidity-adjusted air onto the above-mentioned polyvinyl alcohol-based resin film, a humidifier is installed between a nip roll attached to the inlet side (upstream side) of a treatment tank containing the treatment liquid and the liquid surface of the treatment liquid contained in the treatment tank, and humidity-adjusted air is sprayed onto one or both sides of the polyvinyl alcohol-based resin film, thereby allowing the humidification treatment to be performed before the polyvinyl alcohol-based resin film is immersed in the treatment liquid.

[0023] (Processing process) The treatment step can be at least one selected from the group consisting of a dyeing step in which the polyvinyl alcohol-based resin film is immersed in a treatment liquid containing a dichroic dye, a crosslinking step in which the polyvinyl alcohol-based resin film is immersed in a treatment liquid containing a crosslinking agent, and a color complementing step in which the polyvinyl alcohol-based resin film is immersed in a treatment liquid containing a color complementing agent. The treatment step is preferably at least one selected from the group consisting of a dyeing step and a crosslinking step.

[0024] In the processing step, when processing is performed using multiple processing tanks containing processing liquid, a device for performing humidification processing may be provided for each processing tank, or a device for performing humidification processing may be provided only for the first processing tank.

[0025] (Swelling process) The swelling step in which swelling treatment is performed can be a process in which a raw film made of a polyvinyl alcohol-based resin is brought into contact with a treatment liquid containing water and swelled (swelling treatment). This swelling treatment is performed for purposes such as removing foreign matter attached to the film surface, removing plasticizers such as glycerin contained in the film, imparting ease of dyeing in subsequent processes, and plasticizing the film. The conditions for the swelling treatment are determined within a range that achieves these purposes and does not cause problems such as excessive dissolution or devitrification of the polyvinyl alcohol-based resin film. Specifically, the swelling treatment is performed by immersing the raw film made of a polyvinyl alcohol-based resin in a treatment liquid containing water at a temperature of, for example, 10 to 50°C, preferably 20 to 50°C. The swelling treatment time is usually 5 to 300 seconds, preferably 20 to 240 seconds.

[0026] Typically, in the swelling step, as shown in the figure, multiple guide rollers are placed in a swelling tank 13 containing a water-containing treatment solution, and the polyvinyl alcohol-based resin film is transported through this. Furthermore, since the film is prone to problems such as wrinkling due to swelling in the width direction, it is preferable to transport the film while removing wrinkles using a known width-expanding device such as an expander roll, spiral roll, crown roll, cross guider, tenter clip, or bend bar. Furthermore, to stabilize the film transport during immersion, it is also useful to control the water flow in the swelling tank 13 with an underwater shower or to use an EPC device (Edge Position Control device: a device that detects the edge of the film to prevent it from meandering).

[0027] The water-containing treatment liquid used in the swelling tank 13 may be pure water or an aqueous solution containing boric acid, chlorides, other inorganic salts, water-soluble organic solvents, alcohols, etc., added in an amount ranging from 0.01 to 10% by mass. However, for the above-mentioned purposes, pure water that does not substantially contain dissolved components is preferably used. Pure water that does not contain dissolved components can be obtained by, for example, subjecting ordinary water to reverse osmosis membrane treatment.

[0028] In the swelling process, only one swelling tank may be provided as shown in Figure 1, or two or more tanks may be provided. When the swelling tank is composed of two tanks, they are called the first swelling tank and the second swelling tank from the upstream side. The same applies when the swelling tank is composed of three or more tanks.

[0029] Humidification can be performed by placing a device for humidification on the inlet side (upstream side) of the swelling tank 13. If the swelling tank is composed of two or more tanks, a device for humidification may be placed in each tank, or a device for humidification may be placed only in the first swelling tank.

[0030] In the swelling step, the film swells in both the width direction and the conveying direction. Since the film also swells and expands in the conveying direction during the swelling step, it is preferable to take measures such as controlling the speed of the conveying rolls before and after the swelling tank 13 to eliminate slack in the film in the conveying direction. Specifically, it is preferable to set the ratio of the peripheral speed of the conveying roll on the exit side of the swelling tank 13 to the peripheral speed of the conveying roll on the entrance side of the swelling tank 13 to about 1.2 to 2 times, depending on the temperature of the treatment bath. Furthermore, if desired, uniaxial stretching can also be performed in this step.

[0031] (dying process) The dyeing process is performed by immersing a polyvinyl alcohol-based resin film in a treatment liquid (hereinafter also referred to as a dyeing liquid) containing a dichroic dye to dye the polyvinyl alcohol-based resin film and allow the dichroic dye to adsorb into the polyvinyl alcohol-based resin film (dyeing process). This dyeing process is usually performed by immersing the polyvinyl alcohol-based resin film, which has undergone a swelling process and, in some cases, a water immersion process, in a dyeing tank 15 containing a dyeing liquid, as shown in the figure. The dyeing process conditions can be determined within a range that allows the dichroic dye to be adsorbed into the polyvinyl alcohol-based resin film and does not cause problems such as excessive dissolution or devitrification of the film. Examples of dichroic dyes include iodine.

[0032] When the dichroic dye used in the dyeing process is iodine, the dye solution can be an aqueous solution containing 0.003 to 0.2 parts by mass of iodine and 0.1 to 10 parts by mass of potassium iodide per 100 parts by mass of water. Potassium iodide may be replaced with other iodides, such as zinc iodide, or other iodides may be used in addition to potassium iodide. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, may also be present. In the present invention, even if a component other than iodine is contained, an aqueous solution containing 0.003 parts by mass or more of iodine per 100 parts by mass of water can be considered a dyebath. The temperature of the dyebath (dyeing temperature) is usually 10 to 50°C, preferably 20 to 40°C, and the dyeing time (dyeing time) is usually 10 to 600 seconds, preferably 30 to 200 seconds.

[0033] In the dyeing process, there may be one dyeing tank as shown in Figure 1, or two or more dyeing tanks. When there are two dyeing tanks, they are called the first dyeing tank and the second dyeing tank from the upstream side. The same applies when there are three or more dyeing tanks.

[0034] A humidifying device can be placed on the inlet side (upstream side) of the dyeing tank 15 to perform the humidifying treatment. If the dyeing tank is composed of two or more tanks, a humidifying device may be placed in each tank, or a humidifying device may be placed only in the first crosslinking tank. By performing a humidifying treatment in the dyeing process, spaces are secured between the molecules in the swollen polyvinyl alcohol-based resin film for the dye solution to be impregnated, the amount of adsorption of the dichroic dye increases, the dyeability improves, and the hue of the polarizer (single hue b value and crossed hue b value) tends to be improved.

[0035] In the dyeing step, as in the swelling step, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, a widening device such as an expander roll, a spiral roll, a crown roll, a cross guider, or a bend bar can be appropriately arranged. When using these devices, they may be installed inside the dyeing tank 15 or at least one of the inlet and outlet thereof.

[0036] (Crosslinking process) The crosslinking step is performed by treating the dyed polyvinyl alcohol-based resin film with a treatment liquid containing a crosslinking agent to crosslink the polyvinyl alcohol-based resin and fix the adsorbed dichroic dye such as iodine in the resin (crosslinking treatment). This step is usually performed by immersing the polyvinyl alcohol-based resin film after the dyeing step in a crosslinking tank 17 containing a treatment liquid containing a crosslinking agent.

[0037] The crosslinking agent can be, for example, boric acid. The treatment liquid containing the crosslinking agent can be an aqueous solution containing boric acid (boric acid treatment liquid). The boric acid treatment liquid can be an aqueous solution containing 0.5 to 15 parts by mass of boric acid per 100 parts by mass of water. If the content of boric acid in the boric acid treatment liquid is too low, it tends to be difficult to obtain a sufficient crosslinking effect, and dichroic dyes such as iodine may be eluted from the polyvinyl alcohol-based resin film during the water washing process described below, which may cause the orthogonal hue of the polarizer to shift to blue. On the other hand, if the content of boric acid is too high, the contraction force in the absorption axis direction tends to increase under heating conditions, which may reduce the durability of the polarizing plate, especially in heat cycle tests. In the present invention, the content of boric acid in the boric acid treatment liquid is preferably 1 to 3.5 parts by mass, more preferably 2 to 3.5 parts by mass, per 100 parts by mass of water.

[0038] When the dichroic dye is iodine, the boric acid treatment liquid preferably contains an iodide in addition to boric acid, typically in an amount of 5 to 20 parts by mass, preferably 8 to 15 parts by mass, per 100 parts by mass of water. If the iodide content in the boric acid treatment liquid is low, the orthogonal hue of the polarizer tends to shift to blue. On the other hand, if the iodide content is high, the crosslinking reaction by boric acid may be inhibited, and the orthogonal hue of the polarizer also tends to shift to blue.

[0039] Iodides that can be contained in the boric acid treatment solution can be potassium iodide, zinc iodide, etc. Compounds other than iodides can also be present in the boric acid treatment solution, and examples of such compounds include zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, and sodium sulfate. Furthermore, if necessary, crosslinkers other than boric acid, such as glyoxal and glutaraldehyde, can be used together with boric acid.

[0040] The crosslinking treatment is usually carried out at a temperature of 50 to 70°C, preferably 53 to 65°C. If the temperature is too low, the crosslinking reaction tends to proceed insufficiently, while if the temperature is too high, the film is more likely to break during the crosslinking treatment, resulting in a significant decrease in processing stability. The crosslinking treatment time is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 100 seconds.

[0041] In the crosslinking step, only one crosslinking tank may be provided as shown in Figure 1, or two or more tanks may be provided. When the crosslinking tank is composed of two tanks, they are referred to as the first crosslinking tank and the second crosslinking tank from the upstream side. The same applies when the crosslinking tank is composed of three or more tanks. When the crosslinking tank is composed of two or more tanks, it is preferable that the boric acid concentration in the upstream crosslinking tank is higher than the concentration in the crosslinking tank installed after it.

[0042] A device for humidifying treatment can be disposed on the inlet side (upstream side) of the crosslinking tank 17 to perform the humidifying treatment. When the crosslinking tank is composed of two or more tanks, a device for humidifying treatment may be disposed in each tank, or a device for humidifying treatment may be disposed only in the first crosslinking tank. By performing the humidifying treatment in the crosslinking step, stress relaxation proceeds due to swelling before crosslinking, making further stretching possible and facilitating an improvement in the degree of orientation. In addition, spaces are secured between the molecules in the swollen polyvinyl alcohol-based resin film for the crosslinking agent to be impregnated, which tends to facilitate an improvement in the degree of crosslinking.

[0043] The polyvinyl alcohol-based resin film may be uniaxially stretched during the crosslinking process. The stretching process is described in detail below, but is usually performed along the mechanical flow direction (transport direction). When uniaxial stretching is performed during the crosslinking process, the stretching ratio is preferably within the range of, for example, 1.2 to 3 times. In this case, the uniaxial stretching may be performed in multiple stages using multiple pairs of rolls arranged at intervals.

[0044] (complementary color process) Although not shown, a complementary color treatment for adjusting the hue (complementary color) may be performed. The complementary color treatment step can usually be performed after the crosslinking step. The complementary color treatment can be performed by immersing the polyvinyl alcohol resin film in a complementary color tank containing a treatment liquid containing a complementary color agent. The temperature of the treatment liquid contained in the complementary color tank is, for example, 10°C or higher and 65°C or lower, and preferably 20°C or higher and lower than 60°C. When the dichroic dye is iodine, iodide can be used as the complementary color agent. The content of the crosslinking agent in the treatment liquid containing the complementary color agent is, for example, 1 to 5 parts by mass per 100 parts by mass of water. The content of the iodide in the treatment liquid containing the complementary color agent is, for example, 3 to 30 parts by mass per 100 parts by mass of water. The temperature of the complementary color liquid in the complementary color treatment step is preferably lower than the temperature of the crosslinking liquid in the crosslinking step.

[0045] In the complementary color process, there may be one complementary color tank or two or more tanks. When there are two complementary color tanks, they are called the first complementary color tank and the second complementary color tank from the upstream side. The same applies when there are three or more complementary color tanks.

[0046] Humidification can be performed by placing a device for humidification on the inlet side (upstream side) of the complementary color tank 17. If the complementary color tank is made up of two or more tanks, a device for humidification may be placed for each tank, or a device for humidification may be placed only in the first complementary color tank.

[0047] (Cleaning process) The washing step is performed to wash the polyvinyl alcohol-based resin film (washing treatment). Specifically, excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film are removed by this washing treatment.

[0048] The cleaning treatment is carried out by immersing the polyvinyl alcohol-based resin film in a treatment liquid containing water contained in a cleaning tank 19. Alternatively, the treatment liquid containing water may be sprayed onto the polyvinyl alcohol-based resin film as a shower in the cleaning tank. The treatment liquid containing water is preferably pure water that does not substantially contain dissolved components. The conditions for the cleaning treatment are that the temperature of the treatment liquid is usually 2 to 40°C and the treatment time is usually 2 to 120 seconds.

[0049] The cleaning process may be carried out in a single stage using a single cleaning tank, as shown in Figure 1, or in multiple stages using several cleaning tanks arranged in series. When the cleaning process is carried out in multiple stages, an aqueous solution of an inorganic salt may be used in one of the upstream treatment solutions. This inorganic salt may be selected from, for example, potassium iodide, sodium iodide, zinc iodide, zinc chloride, sodium sulfate, sodium sulfite, etc. Furthermore, these inorganic salts may be used alone or in combination.

[0050] Humidification can be performed by placing a device for humidification on the inlet side (upstream side) of the cleaning tank 19. If the cleaning tank is made up of two or more tanks, a device for humidification may be placed in each tank, or a device for humidification may be placed only in the first cleaning tank.

[0051] In the washing step, since the polyvinyl alcohol-based resin film is transported while removing wrinkles from the film, similarly to the swelling step, it is preferable to apply tension to the polyvinyl alcohol-based resin film along its mechanical flow direction, and the tension at that time is preferably, for example, 300 to 1000 N / m.

[0052] The transport speed of the polyvinyl alcohol-based resin film in the washing step can be appropriately selected from the optimum speeds, and can be, for example, 5 to 30 m / min. If the transport speed of the polyvinyl alcohol-based resin film is faster than 30 m / min, the film tends to slip easily on the rolls, which tends to cause problems such as difficulty in stable stretching.

[0053] The film may be uniaxially stretched during the washing step. If stretched, the stretching ratio may be, for example, 1.05 to 1.2 times.

[0054] (Other processes) In addition to the above-mentioned processing steps, the method for producing a polarizer may further include, for example, a stretching step, a primary drying step, and a final drying step.

[0055] (Stretching process) The stretching process is intended to uniaxially stretch and orient the polyvinyl alcohol-based resin film and orient iodine along the orientation direction, and is carried out during or before the crosslinking process described above. Specifically, this stretching process is carried out during at least one of the swelling, dyeing, and crosslinking processes, or before any of these processes. When uniaxial stretching is carried out during any of the swelling, dyeing, and crosslinking processes, it can be carried out, for example, by a method of imparting a difference in peripheral speed between the conveying roll at the tank inlet side and the conveying roll at the tank outlet side. On the other hand, when uniaxial stretching is carried out before the swelling, dyeing, or crosslinking process, wet stretching may be carried out using a stretching tank before each process, or dry stretching may be carried out, such as a method of stretching in air or a method of stretching while in contact with a heated roll.

[0056] The stretching treatment is preferably carried out at least in the crosslinking step, and more preferably in both the dyeing step and the crosslinking step, and particularly preferably in each of the swelling step, the dyeing step, and the crosslinking step. When stretching is carried out during these steps, the polyvinyl alcohol-based resin film is uniaxially stretched while immersed in the treatment liquid in a tank. Referring to FIG. 1, uniaxial stretching in the crosslinking step is carried out in the treatment liquid in the crosslinking tank 17, while uniaxial stretching in the dyeing step is carried out in the treatment liquid in the dyeing tank 15, and uniaxial stretching in the swelling step is carried out in the treatment liquid in the swelling tank 13.

[0057] The polyvinyl alcohol-based resin film that has undergone all stretching steps preferably has a final cumulative stretch ratio (total cumulative stretch ratio) of 4.5 to 8 times, more preferably 5 to 7 times. Here, the cumulative stretch ratio refers to the reference length in the stretching axis direction of the raw film 10 wound around the payout roll 11 after all stretching steps have been completed. If stretching is performed in the swelling step or dyeing step in addition to the boric acid treatment step, the value includes those stretches. For example, if a portion of the raw film that was 1 m long in the stretching axis direction becomes 5 m after all stretching steps have been completed, the cumulative stretch ratio is 5 times.

[0058] (Primary drying process) A primary drying step may be carried out between the crosslinking step and the washing step as shown in Fig. 1. This primary drying step is carried out for the purpose of adjusting the proportion of water contained in the crosslinked polyvinyl alcohol-based resin film, i.e., the moisture content.

[0059] 1, the film leaving the crosslinking tank 17 is introduced into a primary drying furnace 21, where it is heated and subjected to a primary drying treatment. The heating can be performed by, for example, blowing hot air onto the polyvinyl alcohol-based resin film, bringing the polyvinyl alcohol-based resin film into direct contact with a heat-generating member, or irradiating the polyvinyl alcohol-based resin film with radiant energy.

[0060] When hot air is blown onto the polyvinyl alcohol-based resin film, for example, a hot air nozzle that sprays hot air serves as the heating means, and the hot air may be sprayed directly onto the polyvinyl alcohol-based resin film from there. This method allows the polyvinyl alcohol-based resin film to be dried while removing moisture from the surface with hot air, making it possible to dry the film surface particularly efficiently.

[0061] When the polyvinyl alcohol-based resin film is brought into direct contact with a heat-generating member, for example, a heated roll (heat roll) serves as the heating means, and the polyvinyl alcohol-based resin film is wrapped around it to heat the film. According to this method, since the heat-generating member is brought into direct contact with the polyvinyl alcohol-based resin film, it is possible to make the heating temperature of the film uniform, and uneven drying is less likely to occur.

[0062] In the case of irradiating radiant energy, for example, an infrared heater serves as a heating means, and the radiant energy is irradiated from the infrared heater to the polyvinyl alcohol-based resin film, thereby causing the film itself to generate heat and dry it. According to this method, the polyvinyl alcohol-based resin film itself is heated and dried, so that the entire film, including the interior thereof, can be heated uniformly.

[0063] The above-mentioned methods may be carried out alone or in combination. It is also preferable to arrange the above-mentioned heating means on both sides of the polyvinyl alcohol-based resin film so that both sides of the film can be dried.

[0064] Primary drying can be performed in an ambient atmosphere (outside air), for example, by applying heat from a heater to the moving film. However, in this case, the outside air can reduce heating efficiency and cause uneven drying due to disturbances in the outside air. To prevent this, it is preferable to perform the primary drying inside a primary drying oven 21, as shown in FIG. 1, and it is particularly preferable to perform the drying inside a closed system. In particular, methods that use hot air or radiant energy are likely to be significantly affected by outside air, so it is preferable to perform the drying process in a closed system. However, in cases where the influence of outside air is small, such as in methods in which the film is directly brought into contact with heated rolls, it is also possible to perform drying in an open system in which the inside of the drying oven is not closed or no drying oven is provided at all.

[0065] The drying temperature in the primary drying step is preferably 40 to 300°C, and particularly preferably 50 to 100°C. This drying temperature can be defined as the temperature measured inside the drying oven in the case of a closed system such as the one described above. Alternatively, in the case of an open system, it can be defined as the temperature of the heating means (such as a heated roll) itself.

[0066] The drying time in the primary drying step is about 1 to 100 seconds, preferably 3 to 30 seconds. This drying time can be defined as the time from when the polyvinyl alcohol-based resin film enters the drying oven until when it leaves the oven in a closed system, or the time from when the polyvinyl alcohol-based resin film comes close to a position where it can be exposed to the heat of the heating means or comes into contact with the heating means until when it moves away from a position where it is less exposed to the heat of the heating means or comes out of contact with the heating means in an open system.

[0067] The primary drying step may be carried out in one stage using one heating means, or in multiple stages using several heating means connected in series. When drying is carried out in multiple stages, the drying temperatures in each stage may be the same or different, but it is preferable to create a temperature gradient so that the drying temperature is higher in the later drying stages.

[0068] (Final drying process) The final drying step is performed to heat and dry the polyvinyl alcohol-based resin film after the water-washing step in a drying oven 23. This results in the production of the target polarizer 30. As the drying method performed in the final drying treatment, the methods listed in the above-mentioned primary drying step can be used.

[0069] The final drying conditions are preferably a drying oven maintained at a temperature of 40 to 100°C, preferably 50 to 100°C, for a treatment time of about 30 to 600 seconds. The drying may be performed using multiple drying ovens. In this case, the temperatures of the drying ovens may be the same or different, but it is preferable to create a temperature gradient so that the temperature inside the oven increases as the oven advances.

[0070] After the final drying step, the polarizer 30 is wound onto the winding roll 27 shown in Figure 1 and stored as needed, or is not wound here and is directly supplied to the next step of applying a protective film, thereby producing a polarizing plate in which a protective layer is formed on the surface of the polarizer 30.

[0071] (polarizer) The thickness of the polarizer 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 polarizer 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 polarizer can be controlled, for example, by selecting a polyvinyl alcohol-based resin film, adjusting the stretching ratio, etc.

[0072] The degree of polarization of the polarizer 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 degree of polarization of the polarizer is usually 100% or less, and in practical use, it is less than 100%, and may be 99.9950% or less. The degree of polarization of the polarizer can be measured by the measurement method described in the Examples section below.

[0073] The luminosity-corrected single transmittance (hereinafter simply referred to as transmittance) of the polarizer may be, for example, 43.0% or more, preferably 43.5% or more, and more preferably 43.9% or more. The 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 polarizer can be measured by the measurement method described in the Examples section below.

[0074] The single hue b value of the polarizer may be, for example, in the range of 2.50 to 4.0, and preferably 2.55 to 3.0. The crossed hue b value of the polarizer may be, for example, in the range of -3 to 3, preferably in the range of -2 to 2, and more preferably in the range of -1.5 to 1.5. Using a spectrophotometer with an integrating sphere ("V7100" manufactured by JASCO Corporation), the chromaticity in the L*a*b* (CIE) color system is calculated using the color matching function of Illuminant C for the obtained transmittance, thereby obtaining the single hue b value of the polarizer itself and the crossed hue b value when the polarizers are arranged crosswise. The single hue b value and crossed hue b value of the polarizer can be measured by the measurement methods described in the Examples section below.

[0075] The degree of orientation of the polarizer may be, for example, in the range of 0.100 to 0.300, preferably in the range of 0.150 to 0.250, and more preferably in the range of 0.200 to 0.220. The degree of orientation of the polarizer can be measured by the measurement method described in the Examples section below.

[0076] The degree of crosslinking of the polarizer may be, for example, in the range of 5.00 to 6.00, and preferably in the range of 5.05 to 5.5. The degree of crosslinking of the polarizer can be measured by the measurement method described in the Examples section below.

[0077] A polarizing plate can be obtained by laminating a protective film to at least one surface of a polarizer 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.

[0078] In order to improve the adhesion between the polarizer and the protective film, the bonding surfaces of the polarizer and / or the protective film may be subjected to surface treatment 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.

[0079] The polarizing plate 60 shown in FIG. 2 has a polarizer 30, a pressure-sensitive adhesive layer 40, and a protective film 50, 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 electroluminescence (EL) display device. The display device may be a display device used in a mobile device such as a television, a personal computer, a mobile phone, or a tablet terminal. [Example]

[0080] The present invention will be described in more detail below with reference to examples. In the examples, "%" and "parts" are by mass % and mass parts unless otherwise specified.

[0081] <Evaluation test> (1) Measurement of the degree of polarization The polarizers prepared in the examples and comparative examples were cut into pieces measuring 4 cm x 4 cm, and the polarization degree was evaluated using an ultraviolet-visible spectrometer (V-7100, manufactured by JASCO Corporation). The polarization degree (P) was calculated using the following formula: Degree of polarization (P) = [(T1-T2) / (T1+T2)] 1 / 2 ×100 In the formula, T1 is the parallel transmittance obtained when a pair of polarizers are arranged with their absorption axes parallel to each other, and T2 is the crossed transmittance obtained when a pair of polarizers are arranged with their absorption axes perpendicular to each other.

[0082] (2) Transmittance measurement The polarizers produced in the examples and comparative examples were cut into pieces of 4 cm x 4 cm, and then the transmittance was evaluated using an ultraviolet-visible spectrometer (V-7100, manufactured by JASCO Corporation).

[0083] (3) Measurement of hue (single b value and orthogonal hue b value) The polarizers produced in the examples and comparative examples were cut into pieces of 4 cm x 4 cm, and then the single b value and orthogonal hue b value were measured using an ultraviolet-visible spectrometer (V-7100, manufactured by JASCO Corporation).

[0084] (4) Measurement of cross-linking degree The central portion of each polarizer prepared in the Examples and Comparative Examples was cut into a size of 10 cm x 10 cm, and 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 crosslinking degree was determined by dividing the area of ​​the region in the graph by the sum of the areas (b) obtained by combining the regions in the graph based on the same standard, and averaging the values ​​obtained three times. -1 The region in the figure represents the absorption due to the stretching vibration of the BOC bond formed by combining polyvinyl alcohol resin and boric acid, and is in the wavenumber range of 2850 to 3000 cm. -1 The region in represents the absorption due to the stretching vibration of the C-H bond contained in the polyvinyl alcohol resin. The higher the crosslinking degree value, the more efficiently the boron contained in the polarizer is crosslinked.

[0085] (5) Measurement of the degree of orientation The central portion of each polarizer prepared in the examples and comparative examples was cut into a 10 cm × 10 cm piece, and the degree of orientation was measured using a Fourier transform infrared spectrophotometer (FT-IR) (Nicolet 5700, manufactured by Thermo Fisher Scientific). The degree of orientation (S) was calculated using the following formula: Orientation degree (S)=(A / / -A ⊥ ) / (A / / +2A ⊥ ) In the formula, A / / is the wavenumber 1290 cm obtained when a pair of polarizers are arranged with their absorption axes parallel. -1 represents the absorbance at ⊥ is the wavenumber 1290 cm obtained when a pair of polarizers are arranged with their absorption axes perpendicular to each other. -1 represents the absorbance at

[0086] Example 1 A transparent, unstretched polyvinyl alcohol film (TS4500, manufactured by Kuraray Co., Ltd.) with a thickness of 45 μm and a saponification degree of 99.9% or more was immersed in 25°C water (deionized water) for 1 minute 20 seconds to swell (swelling step), and then dyed by immersion in a 30°C aqueous dye solution containing 1.25 mM / L iodine, 1.25 mass% potassium iodide, and 0.3 mass% boric acid for 2 minutes 30 seconds (dyeing step). The film was stretched at stretch ratios of 1.56 and 1.96 in the swelling and dyeing steps, respectively, and stretching was continued until the cumulative stretch ratio up to the dyeing tank was 3.05.

[0087] Next, the film was immersed in a 63°C aqueous crosslinking solution containing 9% by mass of potassium iodide and 3% by mass of boric acid for 26 seconds to crosslink the film (first crosslinking step), while being stretched at a draw ratio of 1.46. Furthermore, in the first crosslinking step, a humidifier (JINBANG Industrial Humidifier JB-620A) was installed at the inlet of the first crosslinking tank, and moisture at a temperature of 25°C and humidity of 90% RH or more was sprayed at a rate of 3 L / h.

[0088] Thereafter, the film was immersed in a crosslinking aqueous solution containing 9% by mass of potassium iodide and 3% by mass of boric acid at 63° C. for 20 seconds to crosslink it (second crosslinking step), while being stretched at a stretch ratio of 1.34.

[0089] Next, in the color-complementing step, the film was stretched 1.00 times while immersed in a 57°C aqueous color-complementing solution containing 9% by mass of potassium iodide and 3% by mass of boric acid for 10 seconds. The total stretch ratio during the swelling, dyeing, crosslinking, and color-complementing steps was adjusted to 6.0 times. The film was then washed with water for 2 seconds in a 13°C aqueous solution to remove any foreign matter adhering to the film surface. The film was then dried at 92°C for 110 seconds to obtain a polarizer. The resulting polarizer was then evaluated. The results are shown in Table 1.

[0090] <Example 2 and Comparative Example 1> A polarizer was produced in the same manner as in Example 1, except that the humidifier used in Example 1 was installed at the position shown in Table 1. The obtained polarizer was subjected to an evaluation test. The results are shown in Table 1.

[0091] [Table 1] [Explanation of symbols]

[0092] 10 raw film of polyvinyl alcohol resin, 11 payout roll, 12 nip roll, 13 swelling tank, 14 guide roll, 15 dyeing tank, 16 humidification treatment device, 17 crosslinking tank, 18 liquid surface, 19 washing tank, 21 primary drying oven, 23 final drying oven, 27 take-up roll, 30 polarizer, 40 adhesive layer, 50 protective film, 60 polarizing plate.

Claims

1. A method for manufacturing a polarizer, comprising: The method includes a treatment step of immersing a polyvinyl alcohol-based resin film in a treatment solution, In the treatment step, the polyvinyl alcohol-based resin film is subjected to a humidification treatment before being immersed in the treatment liquid; In the processing step, the humidification treatment is performed at a position between a nip roll attached to an inlet of a processing tank containing the processing liquid and a liquid surface of the processing liquid contained in the processing tank; and The method for producing a polarizer, wherein the treatment step is a crosslinking treatment step.

2. A method for manufacturing a polarizer as described in claim 1, further comprising a humidification treatment in at least one processing step selected from the group consisting of a dyeing process and a complementary color process.

3. The method for producing a polarizer according to claim 1 , further comprising a humidification treatment before the treatment step.

4. 4. The method for producing a polarizer according to claim 1, wherein the humidification treatment is performed by spraying air at a temperature of 25° C. and a humidity of 90% RH or more onto the polyvinyl alcohol-based resin film.

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