Method for manufacturing polarizers

Abstract

To provide a manufacturing method of a polarizer having a favorable hue (an orthogonal hue b value) and an improved degree of cross-linking.SOLUTION: A manufacturing method of a polarizer includes: a dyeing step of dyeing a polyvinyl alcohol resin film with a dichroic dye; a cross-linking step of cross-linking the polyvinyl alcohol resin film after the dyeing step with a cross-linking agent; a complementary color processing step of applying complementary color processing to the polyvinyl alcohol resin film after the cross-linking step using a complementary color processing liquid; and a washing step of washing the polyvinyl alcohol resin film after the complementary color processing using a washing solution. The manufacturing method of the polarizer includes after performing any one of the complementary color processing and the washing processing and before removing the complementary color processing liquid or the washing solution attached to the polyvinyl alcohol resin film, performing a cooling process of exposing the polyvinyl alcohol resin film to an atmosphere at a temperature of 20°C or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing a polarizer.

Background Art

[0002] Patent Document 1 discloses a method for producing a polarizer by performing a dyeing treatment, a crosslinking treatment, and a stretching treatment on a long polyvinyl alcohol-based resin film, and then performing a washing treatment and a drying treatment.

Prior Art Document

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a method for producing a polarizer having a neutral hue (orthogonal hue b value close to 0).

Means for Solving the Problems

[0005] The present invention provides the following method for producing a polarizer. [1] A method for producing a polarizer, comprising: a dyeing treatment step of dyeing a polyvinyl alcohol-based resin film with a dichroic dye; Nil a crosslinking treatment step of crosslinking the polyvinyl alcohol-based resin after the dyeing treatment step with a crosslinking agent; a complementary color treatment step of performing a complementary color treatment on the polyvinyl alcohol-based resin film after the crosslinking treatment step using a complementary color treatment liquid; a washing treatment step of performing a washing treatment on the polyvinyl alcohol-based resin film after the complementary color treatment using a washing treatment liquid; and including A method for manufacturing a polarizer, comprising performing at least one of the aforementioned color correction treatment and cleaning treatment, and then, before removing the color correction treatment solution or the cleaning treatment solution adhering to the polyvinyl alcohol-based resin film, a cooling treatment in which the polyvinyl alcohol-based resin film is exposed to an atmosphere with a temperature of 20°C or lower. [2] The method for manufacturing a polarizer according to [1], wherein the cooling treatment is performed after the color correction treatment but before the color correction treatment solution is removed. [3] The method for manufacturing a polarizer according to [1] or [2], wherein the cooling treatment is performed after the cleaning treatment but before the cleaning treatment liquid is removed. [4] The method for manufacturing a polarizer according to any one of [1] to [3], wherein the color-correcting treatment solution or the cleaning treatment solution is removed by passing the polyvinyl alcohol-based resin film through a nip roll. [Effects of the Invention]

[0006] According to the present invention, a method for manufacturing a polarizer having a neutral hue can be provided. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic cross-sectional diagram showing an example of the arrangement of equipment in a polarizer manufacturing method. [Figure 2] This is a schematic cross-sectional view of a polarizing plate containing a polarizer. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to the following embodiments. In all the following drawings, the scale has been adjusted as appropriate to make each component easier to understand, and the scale of each component shown in the drawings does not necessarily match the scale of the actual component.

[0009] <Method for manufacturing polarizers> The method for manufacturing polarizers is polyvinyl NilThe process includes a dyeing step of dyeing an alcohol-based resin film with a dichroic dye, a crosslinking step of crosslinking the polyvinyl alcohol-based resin after the dyeing step with a crosslinking agent, a color correction step of applying a color correction solution to the polyvinyl alcohol-based resin film after the crosslinking step, and a cleaning step of applying a cleaning solution to the polyvinyl alcohol-based resin film after the color correction step, wherein after performing at least one of the color correction and cleaning steps, a cooling process is performed in which the polyvinyl alcohol-based resin film is exposed to an atmosphere at a temperature of 20°C or lower before removing the color correction solution or cleaning solution adhering to the polyvinyl alcohol-based resin film.

[0010] A polarizer is a uniaxially stretched polyvinyl alcohol-based resin film on which a dichroic dye (iodine or dichroic dye) is adsorbed and oriented. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is usually 85 mol% or more, preferably 90 mol% or more, and more preferably 99 mol% or more. The polyvinyl acetate-based resin can be, for example, polyvinyl acetate, which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable thereto. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is usually 1000 to 10000, preferably 1500 to 5000.

[0011] These polyvinyl alcohol-based resins may be modified; for example, polyvinyl formal, polyvinyl acetal, and polyvinyl butyral modified with aldehydes can also be used.

[0012] In the present invention, an unstretched polyvinyl alcohol-based resin film (raw film) with a thickness of 65 μm or less (for example, 60 μm or less), preferably 50 μm or less, is used as a starting material for producing a polarizer. Thereby, a polarizer of a thin film, for which market requirements are increasing more and more, can be obtained. The raw film may be a polyvinyl alcohol-based resin film that has been previously stretched in the gas phase.

[0013] The width of the raw film is not particularly limited and can be, for example, 300 to 6000 mm. However, the larger the film width, the more likely the film is to break during the stretching process.

[0014] In the present invention, the raw film is prepared as a roll of a long unstretched polyvinyl alcohol-based resin film (raw roll).

[0015] The method for producing a polarizer is to continuously convey a long unstretched polyvinyl alcohol-based resin film along the film conveyance path of a polarizer production apparatus, and to continuously produce a long polarizer by performing a dyeing treatment step, a crosslinking treatment step, a complementary color treatment step, and a washing treatment step, which will be described later. The method for producing a polarizer may further include an unwinding step of unwinding the above-mentioned long raw film from the raw roll, a swelling treatment step of immersing the raw film in a swelling bath and then pulling it out, and a drying treatment step of drying the polarizer after the washing treatment. Further, a uniaxial stretching treatment can be performed wet or dry during each step (that is, before any one or more steps, after any one or more steps, and / or during any one or more steps).

[0016] While referring to FIG. 1, the manufacturing method of a polarizer will be described in more detail. FIG. 1 is a cross-sectional view schematically showing an example of a manufacturing method of a polarizer and a polarizer manufacturing device used therefor. The polarizer manufacturing device shown in FIG. 1 is configured to pass a raw film (un-stretched) 10 made of a polyvinyl alcohol-based resin through a swelling bath 13, a dyeing bath 15, a cross-linking bath 17, a complementary color bath 19, a washing bath 21, and a drying furnace 23 provided on a film conveyance path while continuously unwinding the raw film from a raw film roll 11. The arrow in FIG. 1 indicates the conveyance direction of the film.

[0017] In the polarizer manufacturing device shown in FIG. 1, a device 16 for performing a cooling process is arranged between the liquid surface 18 of the complementary color treatment liquid contained in the complementary color bath 19 and a nip roll 12 installed on the outlet side (downstream side) of the complementary color bath 19. Although not shown, a device for performing a cooling process may be arranged between the liquid surface of the washing treatment liquid contained in the washing bath 21 and the nip roll 12 installed on the outlet side (downstream side) of the washing bath 21 together with or instead of the device 16 for performing the cooling process shown in FIG. 1. The cooling process will be described later.

[0018] In FIG. 1, the obtained polarizer 30 can be conveyed, for example, directly to the next polarizing plate manufacturing process (the process of laminating a protective film on one or both sides of the polarizer 30).

[0019] Note that FIG. 1 shows an example in which one tank each of the swelling bath 13, the dyeing bath 15, the cross-linking bath 17, the complementary color bath 19, the washing bath 21, and the drying furnace 23 are provided. However, if necessary, two or more of any one or more treatment baths (a bath that stores a treatment liquid for treating a polyvinyl alcohol-based resin film provided on a film conveyance path, such as the swelling bath 13, the dyeing bath 15, the cross-linking bath 17, the complementary color bath 19, and the washing bath 21, is also collectively referred to as a "treatment bath".) and drying furnaces may be provided. In this specification, the liquid contained in the treatment bath is also referred to as a treatment liquid.

[0020] The processing bath may be equipped with a means for removing the processing liquid from the polyvinyl alcohol-based resin film that has been processed with the processing liquid, in order to prevent the processing liquid from being carried over to the next process. Examples of the means for removing the processing liquid include a nip roll. If the means for removing the processing liquid is a nip roll, for example, a nip roll can be installed on the outlet side (downstream side) of the processing bath, and the processing liquid adhering to the polyvinyl alcohol-based resin film can be removed by pressing and clamping the polyvinyl alcohol-based resin film that has been drawn out of the processing liquid with the nip roll.

[0021] The film transport path of the polarizer manufacturing apparatus can be constructed by placing guide rolls 14, which support the transported film or can further change the film transport direction, and nip rolls 12, which press and grip the transported film and can provide driving force to the film through rotation or can further change the film transport direction, at appropriate positions, in addition to the processing bath and drying oven. Guide rolls and nip rolls can be placed before, after, or within each processing bath and drying oven, thereby enabling the introduction, immersion, and withdrawal of the film into the processing bath. For example, by providing one or more guide rolls in each processing bath and transporting the film along these guide rolls, the film can be immersed in each processing bath.

[0022] The polarizer manufacturing apparatus shown in Figure 1 has nip rolls (nip rolls 12) positioned at the inlet (upstream) and outlet (downstream) of each processing bath, which allows for the removal of processing liquid adhering to the film as it is drawn out of the processing bath. Furthermore, it is possible to perform inter-roll stretching in any one or more processing baths by creating a difference in peripheral speed between the nip rolls positioned before and after the bath. The following describes each process.

[0023] (Swelling treatment process) The swelling treatment process is performed for purposes such as removing foreign matter from the surface of the raw film 10, removing plasticizers from the raw film 10, imparting dyeability, and plasticizing the raw film 10. The treatment conditions are determined within a range that achieves these objectives and does not cause defects such as extreme dissolution or devitrification of the raw film 10.

[0024] Referring to Figure 1, the swelling process can be carried out by continuously unwinding the raw film 10 from the raw film roll 11 and transporting it along the film transport path, immersing the raw film 10 in the swelling treatment liquid contained in the swelling bath 13 for a predetermined time, and then unwinding it. In the example in Figure 1, from the time the raw film 10 is unwound until it is immersed in the swelling bath 13, the raw film 10 is transported along the film transport path constructed by the guide roll 14 and the nip roll 12. During the swelling process, the film is transported along the film transport path constructed by the guide roll 14.

[0025] As a swelling treatment solution, in addition to pure water, it is also possible to use aqueous solutions to which boric acid (Japanese Patent Publication No. 10-153709), chloride (Japanese Patent Publication No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc. are added in a range of 0.01 to 10% by mass.

[0026] The temperature of the swelling treatment solution is, for example, 10 to 50°C, preferably 10 to 40°C, and more preferably 15 to 30°C. The immersion time of the raw film 10 is preferably 10 to 300 seconds, and more preferably 20 to 200 seconds. If the raw film 10 is a polyvinyl alcohol-based resin film that has been stretched in gas beforehand, the temperature of the swelling treatment solution is, for example, 20 to 70°C, preferably 30 to 60°C. The immersion time of the raw film 10 is preferably 30 to 300 seconds, and more preferably 60 to 240 seconds.

[0027] During the swelling process, problems such as the raw film 10 swelling in the width direction and developing wrinkles are likely to occur. One way to transport the film while removing these wrinkles is to use rolls with a widening function, such as expander rolls, spiral rolls, or crown rolls, as guide rolls, or to use other widening devices such as cross guiders, bend bars, or tenter clips. Another way to suppress the occurrence of wrinkles is to perform a stretching process. For example, uniaxial stretching can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between two nip rolls.

[0028] In the swelling process, the film swells and expands in the direction of film transport. Therefore, 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 the nip rolls placed before and after the swelling bath 13. Furthermore, to stabilize 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 edges of the film and prevents the film from meandering).

[0029] In the example shown in Figure 1, the film withdrawn from the swelling bath 13 passes through the nip roll of the swelling bath 13 and is introduced into the staining bath 15.

[0030] (Dyeing process) The dyeing process is performed for purposes such as adsorbing and oriented a dichroic dye onto a polyvinyl alcohol-based resin film after swelling. The dyeing conditions are determined within a range that achieves the above objective and does not cause problems such as extreme dissolution or devitrification of the film. Referring to Figure 1, the dyeing process can be carried out by transporting the film along a film transport path constructed by guide rolls and nip rolls, immersing the swollen film in a dyeing solution containing a dichroic dye in a dyeing bath 15 for a predetermined time, and then withdrawing it. To improve the dyeability of the dichroic dye, it is preferable that the film used in the dyeing process is a film that has undergone at least some degree of uniaxial stretching, or, instead of or in addition to uniaxial stretching before dyeing, uniaxial stretching is performed during the dyeing process.

[0031] When iodine is used as a dichroic dye, the dyeing solution can be an aqueous solution with a concentration of iodine / potassium iodide / water = 0.003~0.3 / 0.1~10 / 100 by mass ratio. Other iodides, such as zinc iodide, may be used instead of potassium iodide, or potassium iodide and other iodides may be used in combination. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, or cobalt chloride, may be present. When boric acid is added, it is distinguished from the crosslinking treatment described later in that it contains iodine, and an aqueous solution containing 0.003 parts by mass or more of iodine per 100 parts by mass of water can be considered a dyeing bath 15. The temperature of the dyeing bath 15 when immersing the film is usually 10~45°C, preferably 10~40°C, more preferably 20~35°C, and the immersion time of the film is usually 30~600 seconds, preferably 60~300 seconds.

[0032] When using a water-soluble dichroic dye as the dichroic pigment, the dyeing solution can be an aqueous solution with a concentration of dichroic dye / water = 0.001 to 0.1 / 100 by mass ratio. The dyeing bath 15 may also contain dyeing aids, such as inorganic salts like sodium sulfate or surfactants. Only one type of dichroic dye may be used alone, or two or more types of dichroic dyes may be used in combination. The temperature of the dyeing solution when immersing the film is, for example, 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

[0033] As described above, in the dyeing process, the film can be uniaxially stretched in the dyeing bath 15. Uniaxial stretching of the film can be performed by methods such as creating a difference in peripheral speed between nip rolls placed before and after the dyeing bath 15.

[0034] In the dyeing process, as in the swelling process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles from the film, guide rolls with a widening function such as expander rolls, spiral rolls, and crown rolls, or other widening devices such as cross guiders, bend bars, and tenter clips can be used. Another means of suppressing the occurrence of wrinkles is to perform a stretching treatment, similar to the swelling treatment.

[0035] In the example shown in Figure 1, the film withdrawn from the staining bath 15 passes through the nip roll 12 and is introduced into the crosslinking bath 17.

[0036] (Crosslinking process) The crosslinking process is a treatment performed for purposes such as water resistance through crosslinking. Referring to Figure 1, the crosslinking process can be carried out by transporting the film along a film transport path constructed by guide rolls and nip rolls, immersing the dyed film in a crosslinking treatment solution containing a crosslinking agent contained in a crosslinking bath 17 for a predetermined time, and then withdrawing it. Examples of crosslinking agents include boric acid.

[0037] The crosslinking treatment solution can be an aqueous solution containing, for example, 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When the dichroic dye used in the dyeing treatment is iodine, the crosslinking treatment solution preferably contains iodide in addition to boric acid, and the amount of iodide can be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodide include potassium iodide and zinc iodide. In addition, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present.

[0038] In the crosslinking treatment, the concentrations of boric acid and iodide, as well as the temperature of the crosslinking treatment solution, can be appropriately changed depending on the purpose. For example, if the purpose of the crosslinking treatment is to make the film water-resistant by crosslinking, and a polyvinyl alcohol-based resin film is subjected to swelling, dyeing, and crosslinking treatments in that order, the crosslinking agent-containing solution in the crosslinking bath can be an aqueous solution with a mass ratio of boric acid / iodide / water = 3-10 / 1-20 / 100. If necessary, other crosslinking agents such as glyoxal or glutaraldehyde may be used instead of boric acid, or boric acid may be used in combination with other crosslinking agents. The temperature of the crosslinking bath when immersing the film is usually 50-70°C, preferably 53-65°C, and the immersion time of the film is usually 10-600 seconds, preferably 20-300 seconds, more preferably 20-200 seconds. Furthermore, when dyeing and crosslinking treatments are applied in this order to a polyvinyl alcohol-based resin film that has been stretched before the swelling treatment, the temperature of the crosslinking bath 17 is usually 50 to 85°C, preferably 55 to 80°C.

[0039] The crosslinking treatment may be performed multiple times, usually 2 to 5 times. In this case, the composition and temperature of each crosslinking bath used may be the same or different, as long as they are within the above range. Each crosslinking treatment may be performed in multiple steps.

[0040] By utilizing the difference in peripheral speed between the two nip rolls, uniaxial stretching can also be performed in the crosslinking bath 17.

[0041] In the crosslinking process, as in the swelling process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles from the film, guide rolls with a widening function such as expander rolls, spiral rolls, and crown rolls, or other widening devices such as cross guiders, bend bars, and tenter clips can be used. Another means of suppressing the occurrence of wrinkles is to perform a stretching process, as in the swelling process.

[0042] In the example shown in Figure 1, the film drawn out of the crosslinking bath 17 passes through the nip roll 12 and is introduced into the complementary color bath 19.

[0043] (Complementary color treatment process) The complementary color treatment process is a process for performing a treatment to adjust the hue (complementary color). The complementary color treatment can be performed by immersing a polyvinyl alcohol-based resin film in a complementary color treatment solution contained in a complementary color bath 19. Iodide and boric acid can be used as complementary agents. For example, when iodine is used as a dichroic dye, a color-correcting solution with a mass ratio of boric acid / iodide / water = 1-5 / 3-30 / 100 can be used. The temperature of the color-correcting solution when immersing the film is usually 10-45°C, and the immersion time is usually 1-300 seconds, preferably 2-100 seconds. It is preferable that the temperature of the color-correcting solution is lower than the temperature of the crosslinking solution.

[0044] As shown in Figure 1, a cooling device is placed between the liquid surface 18 of the complementary color treatment solution and the nip roll 12 on the outlet side (downstream side) of the complementary color bath 19, allowing the polyvinyl alcohol-based resin film to be cooled between the liquid surface 18 and the nip roll 12. If the complementary color bath consists of two or more, the cooling treatment may be performed for each complementary color bath, or only for the first complementary color bath, or only for the last complementary color bath. Performing the cooling treatment after the complementary color treatment but before removing the complementary color treatment solution tends to result in a better orthogonal hue b value. It also tends to make it easier to obtain polarizers with improved crosslinking. It is preferable to perform the cooling treatment after the complementary color treatment but before removing the complementary color treatment solution.

[0045] The complementary color treatment may be performed multiple times, for example, 2 to 5 times. In this case, the composition and temperature of each complementary color treatment solution used may be the same or different, as long as they are within the above range.

[0046] By utilizing the difference in peripheral speed between the two nip rolls, uniaxial stretching can also be performed in the complementary color bath 19.

[0047] In the color correction process, as in the swelling process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles from the film, rolls with a widening function such as expander rolls, spiral rolls, and crown rolls can be used as guide rolls, or other widening devices such as cross guiders, bend bars, and tenter clips can be used. Another means of suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling process.

[0048] In the example shown in Figure 1, the film drawn out of the complementary color bath 19 passes through the nip roll 12 and is introduced into the washing bath 21.

[0049] (Washing process) A method for manufacturing polarizers may include a cleaning step after a color-correcting step. The cleaning step is performed to remove excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step can be performed, for example, by immersing the polyvinyl alcohol-based resin film after color-correcting in a cleaning solution contained in a cleaning bath 21, or by spraying the cleaning solution onto the film as a shower in the cleaning bath 21, or by using a combination of these methods.

[0050] Figure 1 shows an example of a cleaning process in which a polyvinyl alcohol-based resin film is immersed in a cleaning bath 21. The temperature of the cleaning solution during the cleaning process is typically 2 to 40°C, and the cleaning process time is typically 2 to 120 seconds.

[0051] Although not shown in the diagram, a cooling device can be placed between the surface of the cleaning solution and the nip roll 12 on the outlet side (downstream side) of the cleaning bath 21, allowing the polyvinyl alcohol-based resin film to be cooled between the liquid surface and the nip roll 12. If the cleaning bath consists of two or more baths, the cooling treatment may be performed on each bath, or only on the first bath, or only on the last bath.

[0052] Furthermore, in the washing process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles, guide rolls with a widening function such as expander rolls, spiral rolls, and crown rolls may be used, or other widening devices such as cross guiders, bend bars, and tenter clips may be used. In addition, stretching treatment may be performed in the washing process to suppress the occurrence of wrinkles.

[0053] (Cooling process) In the polarizer manufacturing method, after performing at least one of a complementary color treatment and a cleaning treatment, a cooling treatment is performed by exposing the polyvinyl alcohol-based resin film to an atmosphere at a temperature of 20°C or lower before removing the complementary color treatment solution or cleaning treatment solution adhering to the polyvinyl alcohol-based resin film. By performing such a cooling treatment, the hue (orthogonal hue b value) of the resulting polarizer tends to become neutral. Furthermore, the degree of crosslinking of the resulting polarizer tends to improve.

[0054] Conventionally, in polarizer manufacturing methods, the degree of crosslinking in the polyvinyl alcohol-based resin film increases due to the heat drying process performed after the washing treatment, and polyvinyl alcohol and polyiodide ions (I3) - , I5 - A complex was formed with ), and the hue (orthogonal hue b value) of the polarizer tended to become neutral. The inventors found that even when the above cooling treatment was performed after at least one of the complementary color treatment and the cleaning treatment, and before removing the complementary color treatment solution or cleaning treatment solution adhering to the polyvinyl alcohol-based resin film, the degree of crosslinking of the polarizer increased, and the hue (orthogonal hue b value) of the polarizer tended to become neutral.

[0055] The cooling treatment can be a process that lowers the temperature of the polyvinyl alcohol-based resin film immediately after performing at least one of the color correction treatment and the washing treatment, for example, immediately after removing it from the treatment solution. Methods for applying the cooling treatment include, for example, blowing temperature-controlled air onto the polyvinyl alcohol-based resin film or passing it through a temperature-controlled furnace. Examples of equipment for applying the cooling treatment include blowers and cooling furnaces. Among these, blowers are preferred from the viewpoint of ease of installation and handling. When performing a cooling treatment in which the polyvinyl alcohol-based resin film is exposed to an atmosphere at a temperature of 20°C or lower, it is possible to prevent the outflow of iodine from the polyvinyl alcohol-based resin film more easily than, for example, when cooling the polyvinyl alcohol-based resin film by spraying it with cold water.

[0056] The cooling device can be positioned, for example, between the liquid surface of the processing liquid contained in the processing bath and the means for removing the processing liquid. The means for removing the processing liquid can be the nip roll described above.

[0057] In the cooling process, the temperature of the atmosphere to which the polyvinyl alcohol-based resin film is exposed (hereinafter also referred to as the exposure temperature) may be lower than the temperature of the polyvinyl alcohol-based resin film immediately after at least one of the color correction treatment and the washing treatment, preferably 19°C or lower, more preferably 10°C or lower, and may be, for example, 3°C or higher. The exposure temperature may be, for example, 0.5°C or more lower than the temperature of the polyvinyl alcohol-based resin film immediately after at least one of the color correction treatment and the washing treatment, preferably 1°C or more lower, more preferably 2°C or more lower, and even more preferably 5°C or more lower.

[0058] When the cooling device is a blower, the air velocity blown onto the polyvinyl alcohol-based resin film is preferably 0.1 m / s or more, more preferably 0.5 m / s or more, and even more preferably 1.5 m / s or more, for example, 5 m / s or less. The angle between the air blown out from the blower and the direction of transport of the polyvinyl alcohol-based resin film is preferably 0° to 90°, and more preferably 0° to 60°, from the viewpoint of minimizing damage to the polyvinyl alcohol-based resin film caused by wind pressure.

[0059] If the cooling device is a blower, the temperature of the air blown out from the blower can be considered the exposure temperature.

[0060] The cooling treatment time is preferably 10 seconds or more, more preferably 30 seconds or more, for example, 100 seconds or less, and preferably 50 seconds or less.

[0061] (Stretching process) As described above, the raw film 10 is subjected to wet or dry uniaxial stretching during the series of processing steps described above (i.e., before any one or more processing steps, after any one or more processing steps, and / or during any one or more processing steps). Specific methods of uniaxial stretching include, for example, inter-roll stretching, which involves performing longitudinal uniaxial stretching with a peripheral speed difference between two nip rolls constituting the film transport path (e.g., two nip rolls positioned before and after the processing bath), hot roll stretching as described in Japanese Patent Publication No. 2731813, tenter stretching, etc., with inter-roll stretching being preferred. The uniaxial stretching process can be carried out multiple times from the raw film 10 until the polarizer 30 is obtained. As described above, the stretching process is also advantageous in suppressing the occurrence of wrinkles in the film.

[0062] The final cumulative stretching ratio of the polarizer 30, relative to the original film 10, is typically 4.5 to 7.0 times, preferably 5.0 to 6.5 times.

[0063] The stretching process may be performed in any of the above-mentioned processes as long as it is performed before the drying process, and even if stretching is performed in two or more processes, the stretching process may be performed in any of the processes.

[0064] (Drying process) In the drying process, after the washing process, the polyvinyl alcohol-based resin film is dried in a drying oven 23. The drying temperature is, for example, 80°C or higher, preferably 85°C or higher, and more preferably 90°C or higher. The drying temperature is, for example, 120°C or lower.

[0065] Examples of drying methods include using a hot air dryer, an infrared heater, or a combination of these. The drying time is preferably 10 seconds or more, more preferably 30 seconds or more, for example, 200 seconds or less, and preferably 150 seconds or less.

[0066] After the drying process, the polarizer 30 is either wound onto the winding roll 27 shown in Figure 1 for storage as needed, or supplied directly to the next protective film application process without being wound up, until a polarizing plate with a protective layer formed on the surface of the polarizer 30 is manufactured.

[0067] (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, and more preferably 10 μm or more. The thickness of the polarizer can be controlled, for example, by selecting a polyvinyl alcohol-based resin film or adjusting the stretching ratio.

[0068] The orthogonal hue b value of the polarizing film may be in the range of -3.0 to 3.0, preferably -2.5 to 2.6 or less, more preferably in the range of -2.0 to 2.0, even more preferably in the range of -1.5 to 1.5, and particularly preferably in the range of -1.2 to 1.2. The orthogonal hue b value of the polarizing film arranged orthogonally is 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 the C light source for the obtained transmittance. The orthogonal hue b value of the polarizer is determined by the measurement method described in the Examples section below. If the orthogonal hue b value of the polarizer is within the above range, the orthogonal hue of the polarizer tends to approach neutral gray.

[0069] The boric acid content in the polarizer may be, for example, 3 to 4% by mass based on the mass of the polarizer, preferably 3.4 to 3.8% by mass, more preferably 3.5 to 3.75% by mass, and even more preferably 3.55 to 3.7% by mass. The boric acid content in the polarizer is measured by the method described in the Examples section below.

[0070] The degree of crosslinking of the polarizer may be in the range of 5 to 6, preferably in the range of 5.05 to 5.5, and more preferably in the range of 5.1 to 5.3. The degree of crosslinking of the polarizer is measured by the measurement method described in the Examples section below.

[0071] A polarizing plate can be obtained by laminating a protective film to at least one side of a polarizer via an adhesive layer. Examples of protective films include films made of acetylcellulose resins such as triacetylcellulose and diacetylcellulose; films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate resin films, cycloolefin resin films; acrylic resin films; and films made of chain-like olefin resins such as polypropylene resins.

[0072] To improve the adhesion between the polarizer and the protective film, surface treatments such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer application, and saponification may be applied to the bonding surfaces of the polarizer and / or protective film. The adhesive layer can consist of an adhesive or a bonding agent.

[0073] The polarizing plate 60 shown in Figure 2 has a polarizer 30, an adhesive layer 40, and a protective film 50 in that 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, such as a liquid crystal display device or an organic EL display device. The display device may be a display device used in televisions, personal computers, mobile phones, tablet terminals, and other mobile devices. [Examples]

[0074] The present invention will be described in more detail below with reference to examples. In the examples, "%" and "parts" refer to mass%, and parts, respectively, unless otherwise specified.

[0075] <Evaluation Test> (1) Measurement of hue (orthogonal hue b value) The polarizers prepared in the examples and comparative examples were cut to a size of 4 cm x 4 cm, and then the orthogonal hue b values ​​were measured using a UV-Vis spectrometer (V-7100, manufactured by JASCO Corporation).

[0076] (2) Measurement of the boron content in the polarizer 0.2 g of polarizer was dissolved in 200 g of 1.9 mass% mannitol aqueous solution. The resulting aqueous solution was titrated with 1 mol / L NaOH aqueous solution, and the boron content (mass%) of the polarizer was calculated by comparing the amount of NaOH solution required for neutralization with the calibration curve. The results are shown in Table 1.

[0077] (3) Measurement of the degree of crosslinking The central portion of the polarizers fabricated in the examples and comparative examples was cut to a size of 10 cm × 10 cm, and the degree of crosslinking was measured using a Fourier transform infrared spectrophotometer (FT-IR) (Nicolet 5700, Thermo Fisher Scientific). The measurement was performed using a VeeMAX III (ATR) FT-IR chip from Pike Technologies, with 16 scans and a wavenumber resolution of 4 cm. -1 The measurement was performed using wavenumbers of 1200-1360 cm² in the measured IR data. -1 The sum of the areas (a) when the regions in the region are combined according to the 3.2 standard is calculated for wavenumbers 2850-3000 cm². -1 The degree of bridge crossing was defined as the average of three values ​​obtained by dividing the sum of the areas (b) when the regions in the specified area were combined according to the same criteria. (Wavenumber 1200-1360 cm) -1 The region in this area represents absorption originating from the stretching vibrations of the BOC bond formed by the bonding of polyvinyl alcohol resin and boric acid, at wavenumbers 2850-3000 cm⁻¹. -1 The region shown represents absorption originating from the stretching vibrations of the CH bonds contained in the polyvinyl alcohol resin. A higher degree of crosslinking indicates that more boron contained in the polarizer is being used for crosslinking, and that the crosslinking is proceeding more efficiently.

[0078] <Example 1> A transparent, unstretched polyvinyl alcohol-based film (VF-TS#4500, manufactured by Kuraray Co., Ltd.) with a thickness of 45 μm and a saponification degree of 99.9% or higher was immersed in 25°C pure water (deionized water) for 1 minute and 20 seconds to swell (swelling treatment step). Subsequently, it was stained by immersing it in a 30°C dyeing aqueous solution containing 1.25 mM / L iodine, 1.25 mass% potassium iodide, and 0.3 mass% boric acid for 2 minutes and 30 seconds (staining treatment step). During this process, the film was stretched to a stretch ratio of 1.56 times and 1.96 times in the swelling treatment step and the staining treatment step, respectively, so that the cumulative stretch ratio up to the dyeing bath was 3.05 times.

[0079] Next, the material was immersed in a 63°C crosslinking aqueous solution containing 9% by mass of potassium iodide and 3% by mass of boric acid for 26 seconds to crosslink (first crosslinking treatment step), and then stretched to a stretch ratio of 1.46. Subsequently, it was immersed in a 63°C crosslinking aqueous solution containing 9% by mass of potassium iodide and 3% by mass of boric acid for 20 seconds to crosslink (second crosslinking treatment step), and stretched to a stretch ratio of 1.34.

[0080] Next, in the color correction process, the film was stretched 1.00 times while being immersed for 10 seconds in a color correction aqueous solution containing 9% by mass of potassium iodide and 4% by mass of boric acid at 43°C. At this time, the total volume stretch ratio in the swelling treatment, dyeing treatment, crosslinking treatment, and color correction treatment was set to 6.0 times. After immersing the film in the color correction aqueous solution and removing the film from the solution when it reached a temperature close to the temperature of the solution, air at 5°C was blown onto the film using a blower before removing the color correction aqueous solution from the film using a nip roll. Then, the film was washed with 13°C pure water for 2 seconds to remove any foreign matter adhering to the film surface (washing treatment process). After that, the film was dried at 92°C for 110 seconds to obtain a polarizer. Evaluation tests were performed on the obtained polarizer. The results are shown in Table 1.

[0081] <Examples 2-7 and Comparative Examples 1-4> A polarizer was fabricated in the same manner as in Example 1, except that the location and temperature of the cooling process performed in Example 1 were changed to those shown in Table 1. An evaluation test was performed on the obtained polarizer. The results are shown in Table 1.

[0082] [Table 1] [Explanation of symbols]

[0083] 10. Polyvinyl alcohol-based resin raw film, 11. Feed roll, 12. Nip roll, 13. Swelling tank, 14. Guide roll, 15. Dyeing tank, 16. Cooling device, 17. Crosslinking tank, 18. Liquid level, 19. Color correction tank, 21. Washing tank, 23. Drying oven, 27. Winding roll, 30. Polarizer, 40. Adhesive layer, 50. Protective film, 60. Polarizing plate.

Claims

1. A method for manufacturing polarizers, A dyeing process in which a polyvinyl alcohol-based resin film is dyed with a dichroic dye, A crosslinking process in which the polyvinyl alcohol-based resin film after the dyeing process is crosslinked with a crosslinking agent, A color correction treatment step is performed on the polyvinyl alcohol-based resin film after the crosslinking treatment step using a color correction treatment solution. A cleaning process is performed on the polyvinyl alcohol-based resin film after the color correction treatment using a cleaning solution. Includes, After performing at least one of the aforementioned color correction treatment and the aforementioned cleaning treatment, and before removing the color correction treatment solution or the cleaning treatment solution adhering to the polyvinyl alcohol-based resin film, the polyvinyl alcohol-based resin film is subjected to a cooling treatment by exposing it to an atmosphere with a temperature of 3°C to 15°C. The color-correcting treatment solution or the cleaning treatment solution is removed by passing the polyvinyl alcohol-based resin film through a nip roll. The method for performing the cooling treatment is to blow temperature-controlled air onto the polyvinyl alcohol-based resin film or to pass it through a temperature-controlled furnace. A method for manufacturing a polarizer, wherein the cooling treatment is performed after the cleaning treatment but before the cleaning solution is removed.

2. The method for manufacturing a polarizer according to claim 1, wherein the cooling treatment is performed after the color correction treatment but before the color correction treatment solution is removed.

3. The method for manufacturing a polarizer according to claim 1, wherein the cooling treatment is performed only after the cleaning treatment and before the cleaning treatment liquid is removed.

4. The method for manufacturing a polarizer according to any one of claims 1 to 3, wherein in the cooling process, the temperature of the atmosphere to which the polyvinyl alcohol-based resin film is exposed is 10°C or lower.

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