Polarizing film manufacturing method and manufacturing device

Abstract

To provide a method and a device for manufacturing a polarization film which has a neutral color phase and excellent optical characteristics.SOLUTION: A method for manufacturing a polarization film from a polyvinyl alcohol resin film includes: a staining step of staining the polyvinyl alcohol resin film with a dichroic dye; and a cross-linking step of immersing the stained polyvinyl alcohol resin film into n number (n is an integer of at least 4) of cross-link tanks and performing cross-link processing. The temeperature of the second or third cross-link tank from the top is higher than the temperature of the first cross-link tank from the top.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

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

[0003] Polarizing films are mainly produced by subjecting a raw film made of a polyvinyl alcohol-based resin to a treatment such as immersion in a dye bath containing a dichroic dye such as iodine, followed by a treatment such as immersion in a crosslinking bath containing a crosslinking agent such as boric acid, and uniaxially stretching the film at one of these stages. Uniaxial stretching can be classified into dry stretching, in which stretching is performed in air, and wet stretching, in which stretching is performed in a liquid such as the dye bath or crosslinking bath.

[0004] Crosslinked polarizing films tend to shrink when exposed to high-temperature environments and may not have sufficient durability. JP 2013-148806 A (Patent Document 1) describes a polarizing film with excellent durability that has a low boron content of 1 to 3.5 wt %. [Prior art documents] [Patent documents]

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

[0006] An object of the present invention is to provide a method and apparatus for producing a polarizing film that has a neutral hue and excellent optical properties. [Means for solving the problem]

[0007] The present invention provides the following method and apparatus for producing a polarizing film. [1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye; a crosslinking step of immersing the polyvinyl alcohol-based resin film after the dyeing step in n crosslinking baths in sequence (n is an integer of 4 or more), A method for producing a polarizing film, wherein the temperatures of the second and third crosslinking baths from the upstream side are higher than the temperature of the first crosslinking bath from the upstream side. [2] The method for producing a polarized film according to [1], wherein each crosslinking bath has a boron compound concentration of 2.5% by mass or more. [3] The method for producing a polarized film according to [1] or [2], wherein the temperature of each crosslinking bath is 65°C or less. [4] A manufacturing apparatus for manufacturing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing section that dyes the polyvinyl alcohol-based resin film with a dichroic dye; a crosslinking unit that performs a crosslinking treatment by sequentially immersing the polyvinyl alcohol-based resin film after the dyeing treatment in n crosslinking baths (n is an integer of 4 or more), An apparatus for producing a polarizing film, wherein the temperatures of the second and third crosslinking baths from the upstream side are higher than the temperature of the first crosslinking bath from the upstream side. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method and an apparatus for producing a polarizing film that has a neutral hue and excellent optical properties. [Brief explanation of the drawings]

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

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

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

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

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

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

[0015] <Crosslinking process> The crosslinking step is a treatment carried out for the purpose of crosslinking the film, which can improve water resistance and heat resistance, and further adjust the hue (prevent the film from becoming bluish, etc.). In the crosslinking step of the present invention, the film is immersed in n crosslinking baths in sequence (n is an integer of 4 or more) to carry out the crosslinking treatment.

[0016] The temperatures of the second and third crosslinking baths from the upstream side are higher than the temperature of the first crosslinking bath from the upstream side, preferably by at least 1°C higher, more preferably by at least 2°C higher, and even more preferably by at least 3°C ​​higher. By arranging at least two consecutive crosslinking baths with a temperature higher than that of the first crosslinking bath, a polarized film with a neutral hue, specifically a small absolute value of the orthogonal b-value, can be obtained, and the polarized film can also have excellent optical properties. This is thought to be due to the fact that crosslinking treatment in two or more crosslinking baths with a higher temperature than the first crosslinking bath immediately after crosslinking treatment in the first crosslinking bath efficiently crosslinks the film, increasing the amount of iodine complex and improving the hue, while also alleviating the stress caused by crosslinking and allowing for a higher stretch ratio (especially in the drying process). Increasing the stretch ratio allows for a polarized film with excellent optical properties. The temperature of the fourth or subsequent crosslinking bath may be higher, lower, or the same as the temperature of the first crosslinking bath. If the fourth crosslinking bath is intended to adjust the hue, it is preferably the same as or lower than the temperature of the first crosslinking bath. If the temperature of the fourth or subsequent crosslinking bath is lower than the temperature of the first crosslinking bath, the difference in temperature between the fourth or subsequent crosslinking bath and the first crosslinking bath is preferably 5°C or less. In this specification, the film is transported from the upstream side to the downstream side in the transport direction, and when comparing two points, the upstream side means the upstream side along the film transport direction.

[0017] The temperature of each crosslinking bath is preferably 40° C. or higher, more preferably 45° C. or higher, from the viewpoint of efficient crosslinking. Furthermore, the temperature of each crosslinking bath is preferably 65° C. or lower, more preferably 64° C. or lower, from the viewpoint of preventing elution of the polyvinyl alcohol-based resin film. The immersion time of the film in each crosslinking bath is about 3 to 600 seconds, preferably 4 to 300 seconds, and more preferably 5 to 200 seconds.

[0018] The concentration of the boron compound in each crosslinking bath is preferably 2.5% by mass or more, more preferably 2.8% by mass or more. The concentration of the boron compound in each crosslinking bath is, for example, 4.0% by mass or less. The boron compound acts as a crosslinking agent, and examples thereof include boric acid and borax. The crosslinking bath may contain a crosslinking agent such as glyoxal or glutaraldehyde in addition to the boron compound. The solvent for the crosslinking bath may be, for example, water, or may further contain an organic solvent compatible with water. The temperature of each crosslinking bath is preferably 30°C or more, more preferably 35°C or more, and even more preferably 42°C or more, from the viewpoint of promoting crosslinking of the polyvinyl alcohol resin film and equilibration of the amount of crosslinking agent in the polyvinyl alcohol film.

[0019] First Embodiment One embodiment of a polarizing film manufacturing method according to the present invention will be described in detail with reference to Figure 1. Figure 1 is a cross-sectional view schematically illustrating an example of a polarizing film manufacturing method according to the present invention and a polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in Figure 1 is configured so that raw (unstretched) film 10 made of a polyvinyl alcohol resin is continuously unwound from raw roll 11 and transported along a film transport path, causing the film to pass sequentially through a swelling bath (swelling liquid contained in a swelling tank) 13, a dyeing bath (dyeing liquid contained in a dyeing tank) 15, a first crosslinking bath (first crosslinking liquid contained in a crosslinking tank) 17a, a second crosslinking bath (second crosslinking liquid contained in a crosslinking tank) 17b, a third crosslinking bath (third crosslinking liquid contained in a crosslinking tank) 17c, a fourth crosslinking bath (fourth crosslinking liquid contained in a crosslinking tank) 17d, and a washing bath (washing liquid contained in a washing tank) 19, all of which are provided on the film transport path, and finally passes through a drying oven 21. The obtained polarizing film 23 can be transported directly to the next polarizing plate production process (a process of laminating a protective film to one or both sides of the polarizing film 23). The arrow in Fig. 2 indicates the transport direction of the film.

[0020] In the description of Fig. 1, "treatment tank" is a general term including a swelling tank, a dyeing tank, a crosslinking tank, and a washing tank, "treatment liquid" is a general term including a swelling liquid, a dyeing liquid, a crosslinking liquid, and a washing liquid, and "treatment bath" is a general term including a swelling bath, a dyeing bath, a crosslinking bath, and a washing bath. The swelling bath, dyeing bath, crosslinking bath, and washing bath respectively constitute the swelling section, the dyeing section, the crosslinking section, and the washing section in the production apparatus of the present invention.

[0021] The film transport path of the polarizing film manufacturing equipment can be constructed by appropriately arranging, in addition to the treatment baths described above, guide rolls 30-49, 56-61 that support the film being transported or can even change the film transport direction, and nip rolls 50-55 that press and clamp the film being transported, imparting driving force to the film through their rotation, or even changing the film transport direction. Guide rolls and nip rolls can be placed before, after, or within each treatment bath, allowing the film to be introduced into, immersed in, and withdrawn from the treatment bath (see Figure 1). For example, one or more guide rolls can be provided in each treatment bath, and the film can be immersed in each treatment bath by transporting it along these guide rolls.

[0022] The polarized film manufacturing apparatus shown in FIG. 1 has nip rolls arranged before and after each treatment bath (nip rolls 50 to 54), which makes it possible to perform inter-roll stretching in one or more treatment baths, in which longitudinal uniaxial stretching is performed by creating a peripheral speed difference between the nip rolls arranged before and after the treatment bath.

[0023] 1, there are four crosslinking baths, that is, n is 4. Each step will be described below.

[0024] (Swelling process) The swelling process is carried out for the purposes of removing foreign matter from the surface of the raw film 10, removing plasticizers in the raw film 10, imparting ease of dyeing, plasticizing the raw film 10, etc. The treatment conditions are determined within a range in which the above-mentioned purposes can be achieved and in which defects such as extreme dissolution or devitrification of the raw film 10 do not occur.

[0025] 1, the swelling step can be carried out by continuously unwinding raw film 10 from raw film roll 11 and transporting it along a film transport path, immersing raw film 10 in swelling bath 13 for a predetermined time, and then pulling it out. In the example of FIG. 1, from the time raw film 10 is unwound until it is immersed in swelling bath 13, raw film 10 is transported along a film transport path formed by guide rolls 60, 61 and nip roll 50. During the swelling treatment, raw film 10 is transported along a film transport path formed by guide rolls 30 to 32 and nip roll 51.

[0026] As the swelling liquid for the swelling bath 13, in addition to pure water, it is also possible to use an aqueous solution containing boric acid (Japanese Patent Application Laid-Open No. 10-153709), chlorides (Japanese Patent Application Laid-Open No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc. in an amount of approximately 0.01 to 10% by weight.

[0027] The temperature of the swelling bath 13 is, for example, about 10 to 50°C, preferably about 10 to 40°C, and more preferably about 15 to 30°C. The immersion time of the raw film 10 is preferably about 10 to 300 seconds, and more preferably about 20 to 200 seconds. When the raw film 10 is a polyvinyl alcohol resin film that has been stretched in a gas in advance, the temperature of the swelling bath 13 is, for example, about 20 to 70°C, and preferably about 30 to 60°C. The immersion time of the raw film 10 is preferably about 30 to 300 seconds, and more preferably about 60 to 240 seconds.

[0028] The swelling treatment can easily cause problems such as the raw film 10 swelling in the width direction and wrinkling. One method for conveying the film while removing these wrinkles is to use rolls with a width-expanding function, such as expander rolls, spiral rolls, or crown rolls, as the guide rolls 30, 31, and / or 32, or to use other width-expanding devices, such as cross guiders, bend bars, or tenter clips. Another method for suppressing wrinkles is to perform a stretching treatment. For example, uniaxial stretching can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip rolls 50 and 51.

[0029] During the swelling treatment, the film also swells and expands in the film transport direction, so if the film is not actively stretched, it is preferable to take measures to eliminate slack in the film in the transport direction, such as controlling the speed of nip rolls 50, 51 placed before and after the swelling bath 13. Furthermore, for the purpose of stabilizing the film transport in the swelling bath 13, it is also useful to control the water flow in the swelling bath 13 with an underwater shower or to use an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents the film from meandering) in combination.

[0030] In the example shown in FIG. 1, the film drawn out from the swelling bath 13 passes through a guide roll 32, a nip roll 51 and a guide roll 33 in this order before being introduced into the dye bath 15.

[0031] (dying process) The dyeing process is carried out for the purpose of adsorbing and orienting a dichroic dye into the polyvinyl alcohol-based resin film after swelling treatment. The treatment conditions are determined within a range that achieves the purpose and does not cause defects such as extreme dissolution or devitrification of the film. Referring to FIG. 1, the dyeing process can be carried out by conveying the film along a film conveyance path formed by nip roll 51, guide rolls 33 to 36, and nip roll 52, immersing the swelling film in dye bath 15 (a treatment solution contained in a dyeing tank) for a predetermined period of time, and then withdrawing the film. To enhance the dyeability of the dichroic dye, the film used in the dyeing process is preferably a film that has been subjected to at least some degree of uniaxial stretching treatment. Alternatively, it is preferable to perform uniaxial stretching during the dyeing process instead of or in addition to the uniaxial stretching treatment before the dyeing process.

[0032] When iodine is used as the dichroic dye, the dyeing solution of the dye bath 15 can be, for example, an aqueous solution with a weight ratio of iodine / potassium iodide / water of 0.003-0.3 / 0.1-10 / 100. Potassium iodide can be replaced with other iodides, such as zinc iodide, or potassium iodide can be used in combination with other iodides. Compounds other than iodides, such as boric acid, zinc chloride, or cobalt chloride, can also be present. When the dyeing solution of the dye bath contains boric acid, it is distinguished from a crosslinking bath in that it contains iodine at a higher concentration than boric acid. The iodine concentration of the dye bath is preferably 0.6 mM or higher, more preferably 1.0 mM or higher, and even more preferably 2.0 mM or higher. The temperature of the dye bath 15 when the film is immersed is usually about 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C, and the immersion time of the film is usually about 30 to 600 seconds, and preferably 60 to 300 seconds.

[0033] When a water-soluble dichroic dye is used as the dichroic pigment, the dyeing solution of the dye bath 15 can be, for example, an aqueous solution having a weight ratio of dichroic dye / water of 0.001 to 0.1 / 100. This dye bath 15 may contain a dyeing assistant, for example, an inorganic salt such as sodium sulfate or a surfactant. Only one dichroic dye may be used alone, or two or more dichroic dyes may be used in combination. The temperature of the dye bath 15 when the film is immersed is, for example, about 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

[0034] As described above, in the dyeing step, the film can be uniaxially stretched in the dye bath 15. The uniaxial stretching of the film can be performed by, for example, creating a difference in peripheral speed between the nip rolls 51 and 52 arranged before and after the dye bath 15.

[0035] In the dyeing treatment, as in the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls can be used as the guide rolls 33, 34, 35, and / or 36, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling treatment.

[0036] In the example shown in FIG. 1, the film drawn out from the dye bath 15 passes through a guide roll 36, a nip roll 52, and a guide roll 37 in this order, and is introduced into a first crosslinking bath 17a.

[0037] (Crosslinking process) The crosslinking step is a treatment carried out for the purpose of crosslinking the film, which can improve water resistance and heat resistance, and even adjust the hue (prevent the film from becoming bluish, for example). In the crosslinking step, n crosslinking baths (n is an integer of 4 or more) are used. In the example shown in FIG. 1, four crosslinking baths (n is 4) are arranged as crosslinking baths for carrying out the crosslinking step, and crosslinking treatment is carried out in the first crosslinking bath 17a, the second crosslinking bath 17b, and the third crosslinking bath 17c for the purpose of water resistance, and crosslinking treatment is carried out in the fourth crosslinking bath 17d for the purpose of adjusting the hue.

[0038] Referring to FIG. 1, the first crosslinking step can be carried out by conveying the film along a film conveying path formed by nip roll 52, guide rolls 37-40, and nip roll 53a, immersing the dyed film in first crosslinking bath 17a (first crosslinking bath) for a predetermined time, and then withdrawing it. The second crosslinking step can be carried out by conveying the film along a film conveying path formed by nip roll 53a, guide rolls 41-44, and nip roll 53b, immersing the film after the first crosslinking step in second crosslinking bath 17b (second crosslinking bath) for a predetermined time, and then withdrawing it. The third crosslinking step can be carried out by conveying the film along a film conveying path formed by nip roll 53b, guide rolls 45-48, and nip roll 53c, immersing the film after the second crosslinking step in third crosslinking bath 17c (third crosslinking bath) for a predetermined time, and then withdrawing it. The fourth crosslinking step can be carried out by transporting the film along a film transport path formed by nip roll 53c, guide rolls 49a to 49d, and nip roll 53d, immersing the film after the third crosslinking step in fourth crosslinking bath 17d (fourth crosslinking bath) for a predetermined period of time, and then pulling it out.

[0039] When the dichroic dye used in the dyeing process is iodine, the crosslinking bath preferably contains an iodide in addition to a boron compound, and the concentration of the iodide in the crosslinking bath can be, for example, 1 to 30 mass%. Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present.

[0040] The difference in peripheral speed between nip roll 52 and nip roll 53a can be used to perform uniaxial stretching in the first crosslinking bath 17a. The difference in peripheral speed between nip roll 53a and nip roll 53b can be used to perform uniaxial stretching in the second crosslinking bath 17b. The difference in peripheral speed between nip roll 53b and nip roll 53c can be used to perform uniaxial stretching in the third crosslinking bath 17c. The difference in peripheral speed between nip roll 53c and nip roll 53d can be used to perform uniaxial stretching in the fourth crosslinking bath 17d.

[0041] In the crosslinking treatment, as in the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls can be used as the guide rolls 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49a, 49b, and / or 49d, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling treatment.

[0042] In the example shown in FIG. 1, the film drawn out from the fourth crosslinking bath 17d passes through a guide roll 49d and a nip roll 53d in this order and is introduced into the washing bath 19.

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

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

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

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

[0047] (drying process) After the washing step, it is preferable to carry out a process of drying the polyvinyl alcohol-based resin film. There are no particular restrictions on the drying of the film, but it can be carried out using a drying oven 21 as shown in the example of FIG. 2. The drying oven 21 can be equipped with, for example, a hot air dryer. The drying temperature is, for example, about 30 to 100°C, and the drying time is, for example, about 30 to 600 seconds. The process of drying the polyvinyl alcohol-based resin film can also be carried out using a far-infrared heater. The polarizing film 23 obtained in this manner has a thickness of, for example, about 5 to 30 μm.

[0048] The obtained polarizing film may be wound up onto a winding roll to form a roll, or may be used directly without being wound up in the polarizing plate production process (a process of laminating a protective film or the like on one or both sides of the polarizing film).

[0049] The final cumulative stretching ratio of the polarizing film 23, based on the raw film 10, is usually about 4.5 to 8 times, and preferably 6 to 8 times. The stretching step may be performed in any of the processing steps, and even when stretching is performed in two or more processing steps, the stretching step may be performed in any of the processing steps. In order to achieve a final cumulative stretching ratio of 6 to 8 times for the polarizing film 23, it is desirable to perform stretching in the drying step as well. The stretching ratio in the drying step is preferably 1.05 times or more, and more preferably 1.10 times or more. Stretching by 1.05 times or more in the drying step can improve the optical properties of the resulting polarizing film. According to the present invention, the tension generated in the crosslinking step is alleviated, so that the film can be stretched by 1.05 times or more in the drying step without breaking.

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

[0051] <Polarizing film> By producing a polarizing film by the above-mentioned production method, the following i) to iv): i) The absolute value of the b value of the orthogonal hue is 0.9 or less; ii) The luminous efficiency corrected single transmittance (Ty) is 42.0% or more; iii) The luminosity-corrected polarization (Py) is 99.986% or more; iv) The absorbance at a wavelength of 700 nm (A700) is 3.0 or more; It is possible to obtain a polarizing film that satisfies the above.

[0052] In the present invention, the crosslinking step is performed using n crosslinking baths, and the temperatures of the second and third crosslinking baths from the upstream side are set higher than that of the first crosslinking bath from the upstream side, thereby making it possible to obtain the polarizing film of i) above, i.e., a polarizing film with a neutral hue. The b value of the orthogonal hue is appropriately designed in combination with the hue of the color filter of the liquid crystal display device, and by satisfying i) above, the hue can be within the hue design range of the color filter of a general liquid crystal display device.

[0053] The orthogonal hue mentioned above refers to the hue of light transmitted through one side of a polarizing plate when light is irradiated through the other side. The hue here can be expressed by the a and b values ​​in the Lab color system, and is measured using standard light. In the present invention, the orthogonal hue of a polarizing film is measured by providing an adhesive layer on one side of the polarizing film and pasting the adhesive layer side to a glass plate. The Lab color system is expressed by Hunter's lightness index L and hues a and b, as described in "5.5 Accelerated Weathering Test" of JIS K 5981:2006 "Synthetic Resin Powder Coating Films." A similar concept to the Lab color system is the "Colorimetry - Part 4: CIE 1976 L" in JIS Z 8781-4:2013 "Colorimetry - Part 4: CIE 1976 L" * a * b * L defined in "Space" * a * b * There are various color systems, but the present invention uses the Lab color system. The lightness index L and hues a and b are calculated from the tristimulus values ​​X, Y, and Z defined in JIS Z 8722:2009 "Methods for measuring color - Reflected and transmitted object color" using the following formula:

[0054] L=10Y 1 / 2 a=17.5(10.2XY) / Y 1 / 2 b=7.0(Y-0.847Z) / Y 1 / 2

[0055] In the Lab color system, the hue a and b values ​​indicate the position equivalent to saturation, and as the hue a value increases, the hue changes to reddish, and as the hue b value increases, the hue changes to yellowish. In addition, the closer to 0, the closer the hue is to achromatic color.

[0056] In the present invention, the crosslinking step is performed using n crosslinking baths, and the temperatures of the second and third crosslinking baths from the upstream side are set higher than that of the first crosslinking bath from the upstream side, so that the film can be stretched by 1.05 times or more in the drying step, thereby obtaining a polarized film with excellent optical properties that satisfies the above ii) and iii).The luminous efficiency-corrected single transmittance (Ty) and luminous efficiency-corrected polarization degree (Py) of the polarized film are measured as described in the Examples section below.

[0057] The polarizing film preferably has an absorbance (A700) of 3.0 or more at a wavelength of 700 nm. The higher this absorbance value, the more the deterioration of optical properties in high-temperature environments is suppressed.

[0058] <Polarizing plate> A polarizing plate can be obtained by laminating a protective film via an adhesive to at least one surface of the polarizing film produced as described above. 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.

[0059] To improve the adhesion between the polarizing film and the protective film, the bonding surface of the polarizing film and / or the protective film may be subjected to a surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating, or saponification. Examples of adhesives used to bond the polarizing film and the protective film include active energy ray-curable adhesives such as ultraviolet-curable adhesives, aqueous solutions of polyvinyl alcohol resins or aqueous solutions containing crosslinkers, and water-based adhesives such as urethane emulsion adhesives. The ultraviolet-curable adhesive may be a mixture of an acrylic compound and a photoradical polymerization initiator, or a mixture of an epoxy compound and a photocationic polymerization initiator. Alternatively, a cationically polymerizable epoxy compound and a radically polymerizable acrylic compound may be used in combination, and a photocationic polymerization initiator and a photoradical polymerization initiator may be used in combination as initiators. [Example]

[0060] The present invention will be explained in more detail below by showing test examples, but the present invention is not limited to these examples.

[0061] Example 1 A polarizing film was produced from a polyvinyl alcohol-based resin film using the production apparatus shown in Figure 1 under the conditions shown in Table 1. Specifically, a 45 μm-thick, long polyvinyl alcohol (PVA) raw film (trade name "TS45" manufactured by Kuraray Co., Ltd., average polymerization degree 2400, saponification degree 99.9 mol% or more) was continuously transported while being unwound from a roll and immersed in a swelling bath of water (deionized water) at 30°C for 1 minute 20 seconds, whereby it was stretched 1.56 times (swelling step). The film was then pulled out of the swelling bath and immersed in a dye bath containing 1.0 mass% potassium iodide and 3.6 mM iodine at 30°C for 100 seconds, whereby it was stretched 1.15 times (dyeing step). The film was then removed from the dye bath and immersed in a first crosslinking bath (first crosslinking bath from the upstream side) containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid at 59°C for 37 seconds, whereby it was stretched 1.46 times (first crosslinking step). The film was then immersed in a second crosslinking bath (second crosslinking bath from the upstream side) containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid at 63°C for 6 seconds, whereby it was stretched 1.70 times (second crosslinking step). The film was then immersed in a third crosslinking bath (third crosslinking bath from the upstream side) containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid at 63°C for 6 seconds, whereby it was stretched 1.34 times (third crosslinking step). The film was then immersed in a fourth crosslinking bath (fourth crosslinking bath from the upstream side) containing 10.0% by mass of potassium iodide and 3.0% by mass of boric acid at 57°C for 7 seconds and stretched 1.00 times (fourth crosslinking step). Through the swelling step, dyeing step, and crosslinking step, the film was stretched between rolls in the bath, resulting in a total stretching ratio of 6.0 times based on the original film.

[0062] The film was then immersed in a 13°C washing bath for 2 seconds to remove any foreign matter adhering to the film surface (washing step). After the washing step was completed, the film was dried in a drying oven at 90°C for 300 seconds and stretched 1.039 times (drying step). In this way, a polarized film was obtained. The thickness of the resulting polarized film was 18 μm.

[0063] <Examples 2 and 3, Comparative Example 1> Polarized films were obtained under the same conditions as in Example 1, except that the iodine concentration in the dye bath, the boric acid concentration, the temperature of the second crosslinking bath, the amount of iodine added to the second crosslinking bath, and the stretching ratio in the drying step were set as shown in Table 1. The thickness of each of the obtained polarized films was 18 μm.

[0064] [Evaluation of Polarizing Film] (a) Measurement of single unit transmittance and polarization degree The MD transmittance and TD transmittance of the obtained polarizing film were measured in the wavelength range of 380 to 780 nm using a spectrophotometer equipped with an integrating sphere ("V7100" manufactured by JASCO Corporation), and the transmittance was calculated using the following formula: Single transmittance (%) = (MD + TD) / 2 Degree of polarization (%)={(MD-TD) / (MD+TD)}×100 The single transmittance and polarization degree at each wavelength were calculated based on the above.

[0065] "MD transmittance" is the transmittance when the direction of polarized light emerging from the Glan-Thompson prism is parallel to the transmission axis of the polarizing film sample, and is represented as "MD" in the above formula. "TD transmittance" is the transmittance when the direction of polarized light emerging from the Glan-Thompson prism is perpendicular to the transmission axis of the polarizing film sample, and is represented as "TD" in the above formula. The obtained single transmittance and polarization degree were measured in accordance with JIS Z 8701:1999 "Method of representing color - XYZ color system and XYZ color system". 10 Y 10 Z 10 Luminosity correction was performed using a 2-degree visual field (C light source) of the "Color Space" to determine the luminosity-corrected single transmittance (Ty) and luminosity-corrected degree of polarization (Py). Table 1 shows the calculated luminosity-corrected single transmittance (Ty) and luminosity-corrected degree of polarization (Py).

[0066] (b) Orthogonal hue b value For the obtained polarizing film, the b value of the orthogonal hue was determined according to the method described above. Table 1 shows the calculated b value of the orthogonal hue.

[0067] (c) A700 measurement The absorbance at a wavelength of 700 nm (A700) can be determined from the crossed transmittance (Tc) of the polarizing film, and can be calculated using the following formula by measuring the MD transmittance and TD transmittance of the polarizing film. Tc(700nm)(%)=[MD(700nm) / 100]×[TD(700nm) / 100]×100 A700=-log〔Tc(700nm) / 100〕 Table 1 shows the measured values ​​of A700.

[0068] [Table 1]

[0069] The temperature of the second crosslinking bath is lower than that of the first crosslinking bath in Comparative Example 1. The results shown in Table 1 show that the polarized film obtained in Comparative Example 1 has a larger absolute value of the orthogonal hue b value and a lower degree of polarization than the polarized films obtained in Examples 1 to 3. [Explanation of symbols]

[0070] 10 Raw film made of polyvinyl alcohol resin, 11 Raw film roll, 13 Swelling bath, 15 Dyeing bath, 17a First crosslinking bath, 17b Second crosslinking bath, 17c Third crosslinking bath, 17d Fourth crosslinking bath, 19 Washing bath, 21 Drying oven, 23 Polarizing film, 30-49, 56-61 Guide rolls, 50-52, 53a, 53b, 53c, 53d, 54, 55 Nip rolls.

Claims

1. A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye; a crosslinking step of immersing the polyvinyl alcohol-based resin film after the dyeing step in n crosslinking baths (n is an integer of 4 or more) in turn to perform a crosslinking treatment, the temperatures of the second and third crosslinking baths from the upstream side are higher than the temperature of the first crosslinking bath from the upstream side; A method for producing a polarizing film, wherein the temperature of the fourth crosslinking bath from the upstream side is lower than the temperature of the first crosslinking bath from the upstream side.

2. A method for manufacturing a polarized film as described in claim 1, wherein the temperature difference between the crosslinking bath located fourth from the upstream and the crosslinking bath located first from the upstream is 5°C or less.

3. The method for producing a polarized film according to claim 1 or 2, wherein each crosslinking bath has a boron compound concentration of 2.5% by mass or more.

4. The method for producing a polarized film according to claim 1 , wherein the temperature of each crosslinking bath is 65° C. or lower.

5. A manufacturing apparatus for manufacturing a polarizing film from a polyvinyl alcohol-based resin film, a dyeing section that dyes the polyvinyl alcohol-based resin film with a dichroic dye; a crosslinking unit that performs a crosslinking treatment by sequentially immersing the polyvinyl alcohol-based resin film after the dyeing treatment in n crosslinking baths (n is an integer of 4 or more), the temperatures of the second and third crosslinking baths from the upstream side are higher than the temperature of the first crosslinking bath from the upstream side; An apparatus for producing a polarizing film, wherein the temperature of the fourth crosslinking bath from the upstream side is lower than the temperature of the first crosslinking bath from the upstream side.

6. A polarized film manufacturing apparatus as described in Claim 5, wherein the temperature difference between the crosslinking bath located fourth from the upstream and the crosslinking bath located first from the upstream is 5°C or less.

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