Method for manufacturing polarizing film and manufacturing device

Abstract

The present invention provides a method and apparatus for manufacturing a polarizing film with neutral hue and excellent optical properties. The present invention provides a method for manufacturing a polarizing film, which is made from a polyvinyl alcohol-based resin film, comprising: a dyeing step of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment; and a crosslinking step of sequentially immersing the aforementioned polyvinyl alcohol-based resin film in n (n is an integer of 4 or more) crosslinking baths after the dyeing step for crosslinking treatment; wherein the temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side.

Description

[Technical Field]

[0001] This invention relates to a method and apparatus for manufacturing a polarizing film from a polyvinyl alcohol-based resin film. [Previous Technology]

[0002] Polarizing plates are widely used as polarizing elements in image display devices such as liquid crystal displays. As a polarizing plate, it is generally composed of a transparent resin film (protective film, etc.) bonded to one or both sides of a polarizing film using an adhesive.

[0003] Polarizing films are mainly manufactured by immersing a raw film containing polyvinyl alcohol resin in a dyeing bath containing dichroic pigments such as iodine, followed by immersion in a crosslinking bath containing crosslinking agents such as boric acid, and then uniaxially stretching the film at any stage. Uniaxial stretching can be performed in the air (dry stretching) or in the aforementioned dyeing and crosslinking baths (wet stretching).

[0004] Cross-linked polarizing films are prone to shrinkage when exposed to high temperatures, resulting in insufficient durability. Japanese Patent Publication No. 2013-148806 (Patent Document 1) discloses a polarizing film that reduces the boron content of the polarizing film to 1 to 3.5% by weight and has excellent durability.

[0004] [Previous Technical Documents]

[0004] [Patent Document 1: Patent Document]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2013-148806

[0006] The purpose of this invention is to provide a method and apparatus for manufacturing a polarizing film with a neutral hue and excellent optical properties.

[0007] The present invention provides a method and apparatus for manufacturing a polarizing film as shown below.

[0007] [1] A method for manufacturing a polarizing film, wherein the polarizing film is manufactured from a polyvinyl alcohol resin film, comprising

[0007] The dyeing step of dyeing the aforementioned polyvinyl alcohol resin film with a dichroic pigment, and

[0007] The crosslinking step involves sequentially immersing the aforementioned polyvinyl alcohol resin film in n (n being an integer greater than or equal to 4) crosslinking baths after the aforementioned dyeing step for crosslinking treatment.

[0007] The temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side.

[0007] [2] The method for manufacturing polarizing film as described in [1], wherein the concentration of boron compound in each crosslinking bath is 2.5% by mass or more.

[0007] [3] The method for manufacturing polarizing film as described in [1] or [2], wherein the temperature of each crosslinking bath is below 65°C.

[0007] [4] An apparatus for manufacturing a polarizing film, which manufactures a polarizing film from a polyvinyl alcohol resin film, comprising:

[0007] The dyed portion of the aforementioned polyvinyl alcohol resin film treated with dichroic pigments, and

[0007] The cross-linked portion is formed by sequentially immersing the aforementioned polyvinyl alcohol resin film, after the aforementioned dyeing treatment, in n (n is an integer of 4 or more) cross-linking baths for cross-linking treatment.

[0007] The temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side.

[0008] According to the present invention, a method and apparatus for manufacturing a polarizing film having a neutral hue and excellent optical properties can be provided. [Simplified Explanation of the Diagram]

[0009] FIG1 is a cross-sectional view schematically showing a method for manufacturing a polarizing film according to a first embodiment of the present invention and an example of a polarizing film manufacturing apparatus applicable to the method.

Implementation Method

[0010] [Manufacturing Method of Polarizing Film]

[0010] In this invention, the polarizing film is a polyvinyl alcohol-based resin film that has adsorbed and aligned a dichroic pigment (iodine or dichroic dye) onto it. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. Its degree of saponification is usually 85 mol% or more, preferably 90 mol% or more, and even more preferably 99 mol% or more. The polyvinyl acetate-based resin can be, for example, polyvinyl acetate as a homopolymer of vinyl acetate, or a copolymer of vinyl acetate with other monomers that can be copolymerized therewith. Examples of other monomers that can be copolymerized include: unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, etc. The degree of polymerization of the polyvinyl alcohol-based resin is usually 1000 to 10000, preferably around 1500 to 5000.

[0011] These polyvinyl alcohol-based resins can also be modified, for example, aldehyde-modified polyvinyl alcohol formaldehyde, polyvinyl alcohol acetal, polyvinyl alcohol butyral, etc. In this specification, "polyvinyl alcohol-based resin" refers to a resin in which the structural unit of vinyl alcohol (-CH2-CH(OH)-) is 50 mol% or more of all structural units contained in the resin.

[0012] In this invention, an unstretched polyvinyl alcohol resin film (raw material film) with 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 manufacturing the polarizing film. This allows for the production of polarizing films with increasingly demanding market requirements. The width of the raw material film is not particularly limited, and can be, for example, around 400 mm to 6000 mm. The raw material film is prepared as, for example, a roll of unstretched polyvinyl alcohol resin film (raw material film roll).

[0013] A polarizing film can be continuously manufactured by rolling out the aforementioned long strip of raw material film while continuously conveying it along the film conveying path of a polarizing film manufacturing apparatus. After immersing it in a treatment solution contained in a treatment tank (hereinafter also referred to as a "treatment bath"), a specific treatment step of extraction is performed, followed by a drying step, thereby continuously manufacturing long strips of polarizing film. Furthermore, apart from the crosslinking step described below, any method of treatment that brings the treatment solution into contact with the film is not limited to immersing the film in a treatment bath. It can also be a method of film treatment by spraying, flowing, dripping, or other methods that allow the treatment solution to adhere to the film surface. When the treatment step is performed by immersing the film in a treatment bath, the treatment bath for performing one treatment step is not limited to one. The film can also be immersed in two or more treatment baths in sequence to complete one treatment step.

[0014] Examples of the above-mentioned processing liquids include swelling solutions, dyeing solutions, crosslinking solutions, and washing solutions. Examples of the above-mentioned processing steps include: a swelling step in which the swelling solution is brought into contact with the raw film to perform swelling treatment; a dyeing step in which the dyeing solution is brought into contact with the swollen film to perform dyeing treatment; a crosslinking step in which the crosslinking solution is brought into contact with the dyed film to perform crosslinking treatment; and a washing step in which the washing solution is brought into contact with the crosslinked film to perform washing treatment. Furthermore, uniaxial stretching treatment is performed wet or dry between these series of processing steps (i.e., before and after any one or more processing steps and / or in any one or more processing steps). Other processing steps may also be added as needed.

[0015] <Crosslinking Steps>

[0015] The crosslinking step is a treatment performed for the purpose of crosslinking the membrane. This treatment can improve water resistance or heat resistance, and further adjust the color (preventing the membrane from turning blue, etc.). In the crosslinking step of the present invention, the membrane is sequentially immersed in n (n is an integer of 4 or more) crosslinking baths to perform crosslinking treatment.

[0016] The temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side, preferably by 1°C or more, more preferably by 2°C or more, and even more preferably by 3°C or more. After the second bath, by continuously arranging at least two crosslinking baths with temperatures higher than the first crosslinking bath, a polarizing film with a neutral hue, specifically a smaller absolute value of the orthogonal b-value, can be obtained, thereby obtaining a polarizing film with excellent optical properties. This can be attributed to the fact that by immediately performing crosslinking treatment in two or more crosslinking baths with temperatures higher than the first crosslinking bath after the crosslinking treatment in the first crosslinking bath, the film can be crosslinked efficiently, increasing the iodine complex and improving the hue, and the stress generated by crosslinking can be mitigated, thereby increasing the elongation ratio (including increasing the elongation ratio in the drying step). By increasing the elongation ratio, a polarizing film with excellent optical properties can be obtained.

[0016] Furthermore, the temperature of the crosslinking baths configured in the fourth and subsequent baths can be higher, lower, or the same as the temperature of the first crosslinking bath. If the fourth crosslinking bath is configured for hue adjustment, it is preferable to have the same or lower temperature as the first crosslinking bath. When the temperature of the fourth and subsequent crosslinking baths is lower than the temperature of the first crosslinking bath, the temperature difference between the fourth and subsequent crosslinking baths and the first crosslinking bath is preferably less than 5°C.

[0016] In this specification, the membrane conveying direction is set to convey from the upstream side to the downstream side. Comparing the two locations, the so-called upstream side refers to the location located upstream along the membrane conveying direction.

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

[0018] Preferably, the concentration of boron compound in each crosslinking bath is 2.5% by mass or more, and more preferably 2.8% by mass or more. In each crosslinking bath, the concentration of boron compound is, for example, 4.0% by mass or less. The boron compound acts as a crosslinking agent, and examples include boric acid and borax. In addition to boron compounds, the crosslinking bath may also contain crosslinking agents such as glyoxal and glutaraldehyde. As a solvent for the crosslinking bath, water may be used, and further, an organic solvent that is compatible with water may be included. From the viewpoint of promoting the crosslinking of polyvinyl alcohol-based resin films or balancing the crosslinking dosage in polyvinyl alcohol films, the temperature of each crosslinking bath is preferably 30°C or more, more preferably 35°C or more, and more preferably 42°C or more.

[0019] <First Implementation Form>

[0019] One aspect of the method for manufacturing the polarizing film of the present invention will be described in detail with reference to FIG1. ​​FIG1 is a cross-sectional view schematically showing an example of the method for manufacturing the polarizing film of the present invention and an apparatus for manufacturing the polarizing film therefor. The polarizing film manufacturing apparatus shown in Figure 1 is constructed by continuously winding a raw material (unstretched) film 10 made of polyvinyl alcohol resin from a raw material film roll 11 along a film transport path. The film is constructed by sequentially passing through a swelling bath (containing swelling liquid in a swelling tank) 13, a dyeing bath (containing dyeing liquid in a dyeing tank) 15, a first crosslinking bath (containing first crosslinking liquid in a crosslinking tank) 17a, a second crosslinking bath (containing second crosslinking liquid in a crosslinking tank) 17b, a third crosslinking bath (containing third crosslinking liquid in a crosslinking tank) 17c, a fourth crosslinking bath (containing fourth crosslinking liquid in a crosslinking tank) 17d, and a washing bath (containing washing liquid in a washing tank) 19, and finally through a drying oven 21. The obtained polarizing film 23 can be directly transported to the next polarizing plate manufacturing step (the step of laminating a protective film to one or both sides of the polarizing film 23). The arrows in Figure 2 indicate the direction of membrane transport.

[0020] In the description of Figure 1, "processing tank" is a general term encompassing a swelling tank, a dyeing tank, a crosslinking tank, and a washing tank; "processing liquid" is a general term encompassing a swelling liquid, a dyeing liquid, a crosslinking liquid, and a washing liquid; and "processing bath" is a general term encompassing 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, dyeing section, crosslinking section, and washing section of the manufacturing apparatus of the present invention.

[0021] In addition to the aforementioned treatment baths, the film conveying path of the polarizing film manufacturing apparatus can also be constructed by arranging guide rollers 30-49, 56-61, which can support the conveyed film or change the film conveying direction, or clamping rollers 50-55, which press, clamp, and rotate the conveyed film to impart driving force to the film or change the film conveying direction, in appropriate positions. The guide rollers or clamping rollers can be arranged before, after, or in each treatment bath, thereby enabling the film to be introduced into and pulled out of the treatment bath [see Figure 1]. For example, by providing one or more guide rollers in each treatment bath and conveying the film along these guide rollers, the film can be immersed in each treatment bath.

[0022] The polarizing film manufacturing apparatus shown in Figure 1 has clamping rollers (clamping rollers 50 to 54) arranged before and after each processing bath, thereby enabling longitudinal uniaxial extension to be performed in any one or more processing baths by imparting a circumferential speed difference between the clamping rollers arranged before and after them.

[0023] In the polarizing film manufacturing apparatus shown in Figure 1, there are 4 crosslinking baths, i.e., n is 4. The following describes each step.

[0024] (Swelling step)

[0024] The swelling step is performed for the purposes of removing foreign matter from the surface of the raw material film 10, removing plasticizers from the raw material film 10, imparting dyeability, and plasticizing the raw material film 10. The processing conditions are determined within the range that achieves the purpose and does not produce adverse conditions such as extreme dissolution or devitrification of the raw material film 10.

[0025] Referring to Figure 1, the swelling step can be performed by continuously winding the raw film 10 from the preform roll 11 while conveying it along the film conveying path, immersing the raw film 10 in the swelling bath 13 for a specific time, and then pulling it out. In the example of Figure 1, from the time the raw film 10 is wound out until it is immersed in the swelling bath 13, the raw film 10 is conveyed along the film conveying path constructed by guide rollers 60, 61 and clamping roller 50. During the swelling process, it is conveyed along the film conveying path constructed by guide rollers 30-32 and clamping roller 51.

[0026] As the swelling solution of the swelling bath 13, in addition to pure water, an aqueous solution of boric acid (Japanese Patent Publication No. 10-153709), chloride (Japanese Patent Publication No. 06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol, etc., can be added in the range of about 0.01 to 10% by weight.

[0027] The temperature of the swelling bath 13 is, for example, about 10~50°C, preferably about 10~40°C, and more preferably about 15~30°C. The immersion time of the raw material membrane 10 is preferably about 10~300 seconds, and more preferably about 20~200 seconds. Furthermore, when the raw material membrane 10 is a polyvinyl alcohol-based resin membrane pre-stretched in a gas, the temperature of the swelling bath 13 is, for example, about 20~70°C, and more preferably about 30~60°C. The immersion time of the raw material membrane 10 is preferably about 30~300 seconds, and more preferably about 60~240 seconds.

[0028] During the swelling process, problems such as wrinkles forming in the film due to the swelling of the raw material film 10 in the width direction can easily occur. As a means to obtain such wrinkles while conveying the film, rollers with a widening function, such as spreading rollers, spiral rollers, and crown rollers, can be used on guide rollers 30, 31, and / or 32, or other widening devices such as transverse guides, bending bars, and tenter clamps can be used. Another method to suppress the formation of wrinkles is to perform a stretching process. For example, a uniaxial stretching process can be performed in the swelling bath 13 by utilizing the difference in circumferential speed between clamping rollers 50 and 51.

[0029] During the swelling process, since the film also swells and expands in the film transport direction, in cases where the film is not actively stretched, to eliminate film slack in the transport direction, it is preferable to control the speed of the pinch rollers 50 and 51 positioned before and after the swelling bath 13, for example. Furthermore, to stabilize film transport in the swelling bath 13, the water flow in the swelling bath 13 can be controlled by water showering, or an EPC (Edge Position) device can be used.

[0029] Control device: Devices that detect the end of the membrane and prevent the membrane from serpentinizing are also useful.

[0030] In the example shown in Figure 1, the film pulled out from the swelling bath 13 is sequentially guided into the dyeing bath 15 through the guide roller 32, the clamping roller 51, and the guide roller 33.

[0031] (Staining steps)

[0031] The dyeing step is performed to adsorb and align the dichroic pigment onto the swollen polyvinyl alcohol resin membrane, etc. The processing conditions are determined within the range that achieves this purpose and without causing adverse conditions such as extreme dissolution or depermeation of the membrane. Referring to FIG1, the dyeing step can be performed as follows: the swollen membrane is conveyed along the membrane conveying path constructed by the pinch roller 51, guide rollers 33-36 and pinch roller 52, immersed in the dyeing bath 15 (the processing liquid contained in the dyeing tank) for a specific time, and then pulled out. In order to improve the dyeability of the dichroic pigment, the membrane supplied for the dyeing step is preferably a membrane that has undergone at least some degree of uniaxial stretching treatment, or preferably a membrane that replaces the uniaxial stretching treatment before the dyeing treatment, or a membrane that is subjected to uniaxial stretching treatment during the dyeing treatment in addition to the uniaxial stretching treatment before the dyeing treatment.

[0032] When iodine is used as a dichroic pigment, the staining solution in the staining bath 15 may be an aqueous solution with a concentration of iodine / potassium iodide / water ratio of 0.003~0.3 / 0.1~10 / 100 by weight. Other iodides such as zinc iodide may be used instead of potassium iodide, or potassium iodide may be used in combination with other iodides. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, may coexist. When the staining solution in the staining bath contains boric acid, it differs from the crosslinking bath in that the concentration of iodine contained in the staining bath is higher than that of boric acid. Preferably, the iodine concentration in the staining bath is 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 dyeing bath 15 during membrane impregnation is usually around 10~45℃, preferably 10~40℃, and even more preferably 20~35℃. The membrane impregnation time is usually around 30~600 seconds, preferably 60~300 seconds.

[0033] When using water-soluble dichroic dyes as dichroic pigments, the dyeing solution in the dyeing bath 15 can be, for example, an aqueous solution with a concentration of dichroic dye / water ratio of 0.001 to 0.1 / 100 by weight. Dyeing auxiliaries can also coexist in the dyeing bath 15, such as inorganic salts like sodium sulfate or surfactants. Only one dichroic dye can be used, or two or more dichroic dyes can be used together. The temperature of the dyeing bath 15 during membrane impregnation is, for example, around 20 to 80°C, preferably 30 to 70°C, and the membrane impregnation time is typically around 30 to 600 seconds, preferably around 60 to 300 seconds.

[0034] As described above, during the dyeing step, the uniaxial stretching of the membrane can be performed using the dyeing bath 15. The uniaxial stretching of the membrane can be performed by methods such as imparting a difference in circumferential speed between the clamping rollers 51 and 52 disposed before and after the dyeing bath 15.

[0035] In the dyeing process, similarly to the swelling process, in order to remove wrinkles from the film while conveying the polyvinyl alcohol-based resin film, rollers with a spreading function, such as spreading rollers, spiral rollers, and crown rollers, can be used on guide rollers 33, 34, 35 and / or 36, or other spreading devices such as transverse guides, bending bars, and tenter clamps can be used. Another means to suppress the formation of wrinkles is to perform a stretching process, similar to the swelling process.

[0036] In the example shown in Figure 1, the film pulled out from the dyeing bath 15 is sequentially guided through the guide roller 36, the clamping roller 52 and the guide roller 37 to the first crosslinking bath 17a.

[0037] (Cross-linking step)

[0037] The crosslinking step is a treatment performed for the purpose of crosslinking the membrane. This treatment can improve water resistance or heat resistance, and further adjust the color (preventing the membrane from turning blue, etc.). In the crosslinking step, n (n is an integer of 4 or more) crosslinking baths are used. In the example shown in Figure 1, four (n is 4) crosslinking baths are configured as the crosslinking baths for the crosslinking step. Crosslinking treatment is performed 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 performed in the fourth crosslinking bath 17d for the purpose of color adjustment.

[0038] Referring to Figure 1, the first crosslinking step can be implemented as follows: the dyed film is conveyed along a film conveying path constructed by clamping rollers 52, guide rollers 37-40 and clamping roller 53a, and immersed in the first crosslinking bath 17a (the first crosslinking bath) for a specific time, and then pulled out. The second crosslinking step can be implemented as follows: the film after the first crosslinking step is conveyed along a film conveying path constructed by clamping rollers 53a, guide rollers 41-44 and clamping roller 53b, and immersed in the second crosslinking bath 17b (the second crosslinking bath) for a specific time, and then pulled out. The third crosslinking step can be implemented as follows: the film after the second crosslinking step is conveyed along a film conveying path constructed by clamping rollers 53b, guide rollers 45-48 and clamping roller 53c, and immersed in the third crosslinking bath 17c (the third crosslinking bath) for a specific time, and then pulled out. The fourth crosslinking step can be carried out as follows: the membrane after the third crosslinking step is transported along the membrane transport path constructed by the clamping roller 53c, guide rollers 49a~49d and clamping roller 53d, and immersed in the fourth crosslinking bath 17d (the fourth crosslinking bath) for a specific time, and then pulled out.

[0039] When the dichroic pigment used in the staining process is iodine, the crosslinking bath preferably contains iodides in addition to boron compounds, and the concentration of iodides in the crosslinking bath can be set, for example, to 1-30% by mass. Examples of iodides include potassium iodide and zinc iodide. Furthermore, compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, can also coexist.

[0040] Uniaxial stretching processing can be performed in the first crosslinking bath 17a by utilizing the difference in circumferential speed between clamping roller 52 and clamping roller 53a. Uniaxial stretching processing can be performed in the second crosslinking bath 17b by utilizing the difference in circumferential speed between clamping roller 53a and clamping roller 53b. Uniaxial stretching processing can be performed in the third crosslinking bath 17c by utilizing the difference in circumferential speed between clamping roller 53b and clamping roller 53c. Uniaxial processing can be performed in the fourth crosslinking bath 17d by utilizing the difference in circumferential speed between clamping roller 53c and clamping roller 53d.

[0041] In the crosslinking process, similarly to the swelling process, in order to remove wrinkles from the film while conveying the polyvinyl alcohol resin film, rollers with a spreading function, such as spreading rollers, spiral rollers, and crown rollers, or other spreading devices such as transverse guides, bending bars, and tenter clamps, can be used on guide rollers 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49a, 49b and / or 49d. Another means to suppress the formation of wrinkles is to perform a stretching process, similar to the swelling process.

[0042] In the example shown in Figure 1, the film pulled out from the fourth crosslinking bath 17d is sequentially guided into the washing bath 19 through the guide roller 49d and the clamping roller 53d.

[0043] (Washing steps)

[0043] In the example shown in Figure 1, a washing step is included after the crosslinking step. The washing treatment is performed to remove excess boric acid or iodine and other agents adhering to the polyvinyl alcohol resin film. The washing step is performed, for example, by immersing the crosslinked polyvinyl alcohol resin film in a washing bath 19. Alternatively, the washing step can be performed instead of immersing the film in the washing bath 19 by spraying the film with a washing solution as a shower, or by combining immersion in the washing bath 19 with spraying of the washing solution.

[0044] Figure 1 shows an example of cleaning a polyvinyl alcohol resin film by immersing it in a washing bath 19. The temperature of the washing bath 19 during the cleaning process is usually around 2 to 40°C, and the immersion time of the film is usually around 2 to 120 seconds.

[0045] Furthermore, in the washing process, in order to remove wrinkles while conveying the polyvinyl alcohol-based resin film, rollers with spreading functions such as spreading rollers, spiral rollers, and crown rollers can be used in guide rollers 56, 57, 58 and / or 59, or other spreading devices such as transverse guides, bending bars, and tenter clamps can be used. Also, in the film washing process, in order to suppress the formation of wrinkles, an stretching process can be implemented.

[0046] (Extended Steps)

[0046] As described above, the raw material film 10 undergoes uniaxial stretching processing in a wet or dry manner between the aforementioned series of processing steps (i.e., before, after, and / or during any one or more processing steps). Specific methods for uniaxial stretching processing may include, for example, inter-roll stretching where a circumferential speed difference is applied between two clamping rollers constituting the film transport path (e.g., two clamping rollers positioned before and after the processing bath) to perform longitudinal uniaxial stretching, hot roller stretching as described in Japanese Patent No. 2731813, stretching stretching, etc., with inter-roll stretching being preferred. The uniaxial stretching step may be performed multiple times during the period from the raw material film 10 to obtaining the polarizing film 23. As described above, the stretching processing also helps to suppress the formation of wrinkles in the film.

[0047] (Drying Step)

[0047] After the washing step, it is preferable to perform a drying process on the polyvinyl alcohol-based resin film. There are no particular limitations on the drying method; it can be performed using a drying oven 21, as shown in Figure 2. The drying oven 21 can be, for example, equipped with a hot air dryer. The drying temperature is, for example, around 30~100°C, and the drying time is, for example, around 30~600 seconds. The drying process on the polyvinyl alcohol-based resin film can also be performed using a far-infrared heater. The thickness of the polarizing film 23 obtained in the above manner is, for example, approximately 5~30 μm.

[0048] The obtained polarizing film can be sequentially wound onto a winding roller to form a roll, or it can be directly supplied to the polarizing plate manufacturing process (such as the process of applying a protective film on one or both sides of the polarizing film).

[0049] The final cumulative elongation of the polarizing film 23, based on the original film 10, is typically around 4.5 to 8 times, preferably 6 to 8 times. The elongation step can be performed in any processing step, and when the elongation process is performed in two or more processing steps, the elongation process can also be performed in any processing step. In order to achieve a final cumulative elongation of 6 to 8 times for the polarizing film 23, it is ideal to also perform the elongation process in the drying step. The elongation rate in the drying step is preferably 1.05 times or more, more preferably 1.10 times or more. By performing an elongation of 1.05 times or more in the drying step, the optical properties of the obtained polarizing film can be improved. According to the present invention, since the tension generated in the crosslinking step is relieved, an elongation of 1.05 times or more can be performed in the drying step without cutting the film.

[0050] (Other processing steps for polyvinyl alcohol-based resin films)

[0050] Additional treatments other than those described above may also be added. Examples of additional treatments include immersion treatment in an aqueous solution containing zinc chloride, etc., without boron compounds (zinc treatment).

[0051] <Polarizing film>

[0051] By using the above manufacturing method to produce a polarizing film, a polarizing film satisfying the following i) to iv is obtained:

[0051] i) The absolute value of the b-value of the orthogonal hue is less than 0.9.

[0051] ii) The visual sensitivity correction for a single-cell transmittance (Ty) is 42.0% or higher.

[0051] iii) The visual sensitivity correction polarization (Py) is above 99.986%.

[0051] iv) The absorbance (A700) at a wavelength of 700 nm is 3.0 or higher.

[0052] In this invention, n crosslinking baths are used for the crosslinking step, and the temperature of the second and third crosslinking baths disposed from the upstream side is higher than the temperature of the first crosslinking bath disposed from the upstream side, thereby obtaining the polarizing film i) above, i.e., the neutral hue polarizing film. The b value of the orthogonal hue is appropriately designed based on the combination with the hue of the color filter of the liquid crystal display device, but by satisfying i) above, it can be set within the hue design range of the color filter of a general liquid crystal display device.

[0053] The aforementioned orthogonal hue refers to the hue of light transmitted from one side of a polarizing plate when light is irradiated from one side. This hue can be expressed as a and b values ​​in the Lab colorimetric system and measured using standard light. Furthermore, in this invention, the actual measurement of the orthogonal hue of the polarizing film is performed with an adhesive layer applied to one side of the polarizing film and the adhesive layer side bonded to a glass plate. The Lab colorimetric system is as described in JISK 5981:2006 "Synthetic Resin Powder Coatings" "5.5 Accelerating Weather Resistance Test", where Hunter's brightness index L corresponds to the hue a and b.

[0053] This refers to the L*a*b* color system, which is similar to the Lab color system in JIS Z8781-4:2013 "Color Measurement - Part 4: CIE 1976 L*a*b* Space". However, the Lab color system is used in this invention. The values ​​of the luminance index L and the hues a and b are based on the tristimulus values ​​specified in JIS Z8722:2009 "Methods for Determining Color - Colors of Reflected and Transmitted Objects".

[0053] Calculations are performed based on the values ​​X, Y and Z using the following formula.

[0054] L=10Y1 / 2

[0054] a=17.5(10.2XY) / Y1 / 2

[0054] b=7.0(Y-0.847Z) / Y1 / 2

[0055] In the Lab color system, hue values ​​a and b represent the corresponding chroma positions. Increasing the hue a value shifts the hue to the red family, while increasing the hue b value shifts the hue to the yellow family. Furthermore, values ​​closer to 0 indicate achromaticity.

[0056] Furthermore, in this invention, by using n crosslinking baths for the crosslinking step, and by setting the temperature of the second and third crosslinking baths disposed from the upstream side to be higher than the temperature of the first crosslinking bath disposed from the upstream side, an extension of more than 1.05 times can be performed in the drying step, thereby obtaining a polarizing film that satisfies the excellent optical properties described in ii) and iii) above. The sensitivity correction monomer transmittance (Ty) and sensitivity correction polarization (Py) of the polarizing film here were measured according to the description in the following examples.

[0057] The absorbance (A700) of the polarizing film at a wavelength of 700 nm should preferably be 3.0 or higher. The higher the absorbance value, the better the reduction of optical properties under high temperature conditions can be suppressed.

[0058] <Polarizing plate>

[0058] A polarizing plate can be obtained by bonding a protective film to at least one side of the polarizing film manufactured in the above manner via an adhesive. The protective film may include, for example, a film made of acetylcellulose-based resins such as triacetylcellulose or diacetylcellulose; a film made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; a polycarbonate-based resin film; a cyclic olefin-based resin film; an acrylic resin film; and, for example, a film made of chain olefin-based resins including polypropylene-based resins.

[0059] To improve the adhesion between the polarizing film and the protective film, surface treatments such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating, and saponification can be applied to the bonding surfaces of the polarizing film and / or the protective film. Examples of adhesives used for bonding the polarizing film and the protective film include UV-curable adhesives such as active energy line-curable adhesives, aqueous solutions of polyvinyl alcohol resins, aqueous solutions containing crosslinking agents, and aqueous adhesives such as urethane emulsion adhesives. UV-curable adhesives can be mixtures of acrylic compounds and photoradical polymerization initiators, or mixtures of epoxy compounds and photocationic polymerization initiators. Furthermore, cationicly polymerizable epoxy compounds and radically polymerizable acrylic compounds can be used together, with photocationic polymerization initiators and photoradical polymerization initiators used as initiators.

[0059] [Example]

[0060] Hereinafter, the present invention will be described in more detail with reference to test examples, but the present invention is not limited to these examples.

[0061] <Example 1>

[0061] Using the manufacturing apparatus shown in Figure 1, a polarizing film was manufactured from a polyvinyl alcohol-based resin film under the conditions shown in Table 1. Specifically, a long strip of polyvinyl alcohol (PVA) preform film with a thickness of 45 μm [trade name "TS45" manufactured by Kuraray (stock), average degree of polymerization 2400, degree of saponification 99.9 moles or higher] was continuously conveyed while being wound out of a roll, and immersed in a swelling bath composed of water (deionized water) at 30°C for 1 minute and 20 seconds to stretch it by 1.56 times (swelling step). Subsequently, the film drawn from the swelling bath was immersed in a dyeing bath at 30°C containing 1.0% by mass potassium iodide and 3.6 mM iodine for 100 seconds to stretch it by 1.15 times (dyeing step). Next, the membrane drawn from the self-dyeing bath was immersed in a first crosslinking bath (the first crosslinking bath from the upstream side) at 59°C containing 9.0% by mass potassium iodide and 3.0% by mass boric acid for 37 seconds, resulting in a stretch of 1.46 times (first crosslinking step). Then, it was immersed in a second crosslinking bath (the second crosslinking bath from the upstream side) at 63°C containing 9.0% by mass potassium iodide and 3.0% by mass boric acid for 6 seconds, resulting in a stretch of 1.70 times (second crosslinking step). Next, it was immersed in a third crosslinking bath (the third crosslinking bath from the upstream side) at 63°C containing 9.0% by mass potassium iodide and 3.0% by mass boric acid for 6 seconds, resulting in a stretch of 1.34 times (third crosslinking step). Subsequently, the film is immersed for 7 seconds in a fourth crosslinking bath (the 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 to achieve a stretch of 1.00 times (fourth crosslinking step). Through the swelling, dyeing, and crosslinking steps, and by roller stretching in the bath, the total stretch ratio based on the original film is set to 6.0 times.

[0062] Subsequently, the membrane is immersed in a washing bath at 13°C for 2 seconds to remove foreign matter adhering to the membrane surface (washing step). After the washing step, the membrane is dried in a drying oven at 90°C for 300 seconds and stretched by 1.039 times (drying step). Thus, a polarizing film is obtained. The thickness of the obtained polarizing film is 18 μm.

[0063] <Example 2, 3, Comparative Example 1>

[0063] The iodine concentration in the dyeing bath, the boric acid concentration, the temperature of the second crosslinking bath, the amount of iodine added in the second crosslinking bath, and the stretching ratio in the drying step were set as shown in Table 1. Except for these aspects, polarizing films were obtained under the same conditions as in Example 1. The thickness of the obtained polarizing films was 18 μm.

[0064] [Evaluation of Polarizing Film]

[0064] (a) Measurement of monomer transmittance and polarization

[0064] For the obtained polarizing film, the MD transmittance and TD transmittance in the wavelength range of 380~780nm were measured using a spectrophotometer with an integrating sphere [Nippon Spectrophotometer Co., Ltd. "V7100"], as shown in the following formula:

[0064] Single-cell penetration rate (%) = (MD TD) / 2

[0064] Polarization (%) = {(MD-TD) / (MD TD)} × 100

[0064] Calculate the transmittance and polarization of the individual cells at each wavelength.

[0065] The term "MD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glan-Thomson prism is parallel to the transmission axis of the polarizing film sample, and is expressed as "MD" in the above formula. Similarly, the term "TD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glan-Thomson prism is orthogonal to the transmission axis of the polarizing film sample, and is expressed as "TD" in the above formula. For the obtained individual transmittance and polarization, visual sensitivity correction was performed using a 2-degree field of view (C light source) according to JIS Z8701:1999 "Methods for displaying colors - XYZ color system and X10Y10Z10 color system" to calculate the visual sensitivity corrected individual transmittance (Ty) and visual sensitivity corrected polarization (Py). Table 1 shows the calculated results of the visual sensitivity corrected individual transmittance (Ty) and visual sensitivity corrected polarization (Py).

[0066] (b) Orthographic hue b value

[0066] For the obtained polarizing film, the b-value of the orthogonal hue is calculated according to the above method. Table 1 shows the calculation results of the b-value of the orthogonal hue.

[0067] (c) Measurement of A700

[0067] The absorbance (A700) at a wavelength of 700nm can be obtained from the cross transmittance (Tc) of the polarizing film. The MD transmittance and TD transmittance of the polarizing film can be measured and calculated by the following formula.

[0067] Tc(700nm)(%)=[MD(700nm) / 100]×[TD(700nm) / 100]×100

[0067] A700=-log[Tc(700nm) / 100]

[0067] Table 1 shows the measured values ​​of A700.

[0068] [Table 1]

[0069] In Comparative Example 1, the temperature of the second crosslinking bath was lower than that of the first crosslinking bath. According to the results shown in Table 1, compared with the polarizing films obtained in Examples 1 to 3, the polarizing film obtained in Comparative Example 1 has a larger absolute value of the orthogonal hue b value and a lower polarization degree.

Claims

1. A method for manufacturing a polarizing film, comprising a dyeing step of dyeing the aforementioned polyvinyl alcohol resin film with a dichroic pigment, and a crosslinking step of sequentially immersing the aforementioned polyvinyl alcohol resin film in n (n being an integer of 4 or more) crosslinking baths after the dyeing step for crosslinking treatment, wherein... The temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side, and the temperature of the fourth crosslinking bath arranged from the upstream side is lower than the temperature of the first crosslinking bath arranged from the upstream side. The temperature of each crosslinking bath is below 65°C.

2. The method for manufacturing a polarizing film as described in claim 1, wherein the concentration of the boron compound in each crosslinking bath is 2.5% by mass or more.

3. The method for manufacturing a polarizing thin film as described in claim 1, wherein, The temperature difference between the fourth crosslinking bath located from the upstream side and the first crosslinking bath located from the upstream side is less than 5°C.

4. An apparatus for manufacturing a polarizing film, comprising a dyeing section for dyeing the polyvinyl alcohol resin film with a dichroic pigment, and a crosslinking section for sequentially immersing the dyed polyvinyl alcohol resin film in n (n being an integer of 4 or more) crosslinking baths for crosslinking treatment, wherein... The temperature of the second and third crosslinking baths arranged from the upstream side is higher than the temperature of the first crosslinking bath arranged from the upstream side, and the temperature of the fourth crosslinking bath arranged from the upstream side is lower than the temperature of the first crosslinking bath arranged from the upstream side. The temperature of each crosslinking bath is below 65°C.

5. The apparatus for manufacturing a polarizing film as described in claim 4, wherein, The temperature difference between the fourth crosslinking bath located from the upstream side and the first crosslinking bath located from the upstream side is less than 5°C.

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