Method of manufacturing polarizing film, and polarizing film

Abstract

[The technical problem to be solved] The present invention provides a method for manufacturing a polarizing film, which can obtain a polarizing film with high visual sensitivity correction monomer transmittance and high iodine content, and provides a polarizing film with excellent optical properties. Technical means to solve the problem The solution to the above problem is a method for manufacturing a polarizing film, which is a method for manufacturing a polarizing film from a polyvinyl alcohol-based resin film. The manufacturing method includes a dyeing and crosslinking step of dyeing and crosslinking the aforementioned polyvinyl alcohol-based resin film. The aforementioned dyeing and cross-linking steps include the following steps: The first dyeing step involves dyeing the aforementioned polyvinyl alcohol resin film with a dichroic pigment. The first crosslinking step is performed after the aforementioned first dyeing step, by crosslinking the aforementioned polyvinyl alcohol resin film with a crosslinking agent; and The second dyeing step is performed after the first crosslinking step, by dyeing the aforementioned polyvinyl alcohol resin film with a dichroic pigment.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a polarizing film from a polyvinyl alcohol-based resin film and a polarizing film. Prior Art

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

[0003] For a polarizing film, it is mainly manufactured by treating a film formed from a polyvinyl alcohol-based resin by immersing it in a dyeing bath containing a dichroic pigment such as iodine, and then treating it by immersing it in a crosslinking bath containing a crosslinking agent such as boric acid, etc., and uniaxially stretching the film at any stage. For uniaxial stretching, there are dry stretching performed in the air and wet stretching performed in a liquid such as the above-mentioned dyeing bath and crosslinking bath. Japanese Patent Application Laid-Open No. 2013-148806 (Patent Document 1) has described providing a polarizing film with excellent durability by reducing the boron content of the polarizing film to 1 to 3.5% by weight

[0004] It is known that the visual sensitivity corrected monomer transmittance and the visual sensitivity corrected degree of polarization are indicators of the optical properties of a polarizing film. Regarding a polarizing film, if the iodine content is increased, the visual sensitivity corrected degree of polarization tends to increase, but the visual sensitivity corrected monomer transmittance tends to decrease. [Prior Art Documents] [Patent Documents]

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

[0006] An object of the present invention is to provide a method for manufacturing a polarizing film that can obtain a polarizing film having a high visual sensitivity corrected monomer transmittance and a high iodine content, and a polarizing film having excellent optical properties. Means for Solving the Problems

[0007] The present invention provides a method for manufacturing a polarizing film and a polarizing film as shown below. [1] A method for manufacturing a polarizing film, which is a method for manufacturing a polarizing film from a polyvinyl alcohol-based resin film. This manufacturing method has a dyeing and crosslinking step of dyeing and crosslinking the aforementioned polyvinyl alcohol-based resin film; The aforementioned dyeing and crosslinking step includes: A first dyeing step of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment; A first crosslinking step of crosslinking the aforementioned polyvinyl alcohol-based resin film with a crosslinking agent after the aforementioned first dyeing step; and A second dyeing step of dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment after the aforementioned first crosslinking step. [2] The method for manufacturing a polarizing film as described in [1], wherein the aforementioned dyeing and crosslinking step further includes a second crosslinking step; this second crosslinking step is to perform a crosslinking treatment on the aforementioned polyvinyl alcohol-based resin film with a crosslinking agent after the aforementioned second dyeing step. [3] The method for manufacturing a polarizing film as described in [1] or [2] further has: A washing step of washing the aforementioned polyvinyl alcohol-based resin film after the aforementioned dyeing and crosslinking step; and A drying step of drying the aforementioned polyvinyl alcohol-based resin film after the aforementioned washing step. [4] The manufacturing method as described in any one of [1] to [3], wherein in the aforementioned drying step, the aforementioned polyvinyl alcohol-based resin film is uniaxially stretched at a stretching ratio of 1.10 times or more. [5] A polarizing film having a corrected monomer transmittance of visual sensitivity of 43.0% or more and an iodine content of 4.0 mass% or more. [6] The polarizing film as described in [5], having a crosslinking efficiency of 5.0 or more. Effects of the Invention

[0008] According to the manufacturing method of the present invention, a polarizing film with a high corrected monomer transmittance of visual sensitivity and a high iodine content can be obtained. Also, according to the present invention, a polarizing film with a high corrected monomer transmittance of visual sensitivity and a high iodine content can be provided. Brief Explanation of Drawings

[0009] Fig. 1 is a schematic cross-sectional view showing an example of the manufacturing method of the polarizing film of the first embodiment of the present invention and the polarizing film manufacturing apparatus used therein. Fig. 2 is a schematic cross-sectional view of a polarizing plate including a polarizing film. Embodiments

[0010] [Manufacturing Method of Polarizing Film] In this invention, the polarizing film is a polyvinyl alcohol-based resin film that has adsorbed oriented dichroic pigments (iodine or dichroic dyes) in a uniaxially extended polyvinyl alcohol-based resin film. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is typically obtained by saponifying a polyvinyl acetate-based resin. Its degree of saponification is typically 85 moles or more, preferably 90 moles or more, and even more preferably 99 moles or more. The polyvinyl acetate-based resin can be, for example, polyvinyl acetate homopolymers, and copolymers 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, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is typically 1000 to 10000, preferably around 1500 to 5000.

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

[0012] In this invention, the starting material for manufacturing the polarizing film is an unstretched polyvinyl alcohol resin film (embryo film) with a thickness of 65 μm or less (e.g., 60 μm or less, preferably 50 μm or less, more preferably 45 μm or less). This allows for the production of polarizing films with increasing market demand. The width of the embryo film is not particularly limited, and can be, for example, around 400 to 6000 mm. The embryo film is, for example, prepared as a long roll of unstretched polyvinyl alcohol resin film (embryo film roll).

[0013] Regarding polarizing films, the process involves winding the aforementioned long strip of preform film from a preform film roll and continuously conveying it along the film transport path of the polarizing film manufacturing apparatus. After immersing it in a treatment solution (hereinafter also referred to as a "treatment bath") contained in a treatment tank, it is pulled out to perform predetermined treatment steps. Then, a drying step is performed to continuously manufacture long strips of polarizing films. Furthermore, regarding the treatment steps, any method that involves contacting the film with the treatment solution is acceptable. It is not limited to immersing the film in a treatment bath; methods such as spraying, flowing, or dripping can also be used to adhere the treatment solution to the film surface for treatment. When the treatment step is performed by immersing the film in a treatment bath, the treatment bath used for one treatment step is not limited to one. The film can also be sequentially immersed in two or more treatment baths to complete one treatment step.

[0014] Examples of the aforementioned processing solutions include swelling solutions, dyeing solutions, crosslinking solutions, and washing solutions. Examples of the aforementioned processing steps include: a swelling step where the swelling solution contacts the embryo membrane to perform swelling treatment; a dyeing step where the dyeing solution contacts the swollen membrane to perform dyeing treatment; a crosslinking step where the crosslinking solution contacts the dyed membrane to perform crosslinking treatment; and a washing step where the washing solution contacts the crosslinked membrane to perform washing treatment. Furthermore, between these series of processing steps (i.e., before and after any one or more processing steps, and / or within any one or more processing steps), uniaxial stretching treatment is performed using either a wet or dry method. Additional processing steps may be added as needed. In this specification, the dyeing step and the crosslinking step are collectively referred to as the dyeing-crosslinking step.

[0015] <Dyeing and Crosslinking Steps> The dyeing and crosslinking step includes the following steps: a first dyeing step of dyeing the aforementioned polyvinyl alcohol (PVA) resin film with iodine; a first crosslinking step of crosslinking the aforementioned PVA resin film with a crosslinking agent after the first dyeing step; and a second dyeing step of dyeing the aforementioned PVA resin film with iodine after the first crosslinking step. Preferably, the dyeing and crosslinking step is after the second dyeing step and further includes a second crosslinking step of crosslinking the aforementioned PVA resin film with a crosslinking agent. The dyeing and crosslinking step may be after the second dyeing step and further include a second crosslinking step and a third crosslinking step of crosslinking the aforementioned PVA resin film with a crosslinking agent. The dyeing and crosslinking step may be after the second crosslinking step and further include other dyeing or crosslinking steps. After the dyeing and crosslinking step, the PVA resin film is, for example, transferred to a washing step. The final step in the dyeing and crosslinking step (i.e., the step before the washing step) is preferably a crosslinking step for the purpose of hue adjustment.

[0016] In a polarizing film with two or more dyeing steps, and a second dyeing step following the first, a polarizing process is performed to obtain a polarizing film with high transmittance of the visual sensitivity correction monomer and high iodine content. This is presumably because the crosslinking agent absorbed into the polyvinyl alcohol resin film in the first crosslinking step is easily released in the dyeing bath of the second dyeing step, where the crosslinking agent concentration is low. This release of the crosslinking agent leads to the easy absorption of iodine in the second dyeing step. Furthermore, it is presumed that the iodine absorbed into the polyvinyl alcohol resin film in the second dyeing step is less likely to cause a decrease in the transmittance of the visual sensitivity correction monomer. The reason why "iodine absorbed into the polyvinyl alcohol resin film in the second dyeing step is less likely to cause a decrease in the transmittance of the visual sensitivity correction monomer" remains unclear, but it is presumed to be related to "the crosslinking agent being released, and iodine being absorbed into the space originally occupied by the crosslinking agent." In polarizing films, by increasing the iodine content, a neutral hue polarizing film can be obtained.

[0017] [Implementation Type 1] Referring to FIG1, one embodiment of the method for manufacturing the polarizing film of the present invention will be described in detail. FIG1 is a cross-sectional view schematically showing an example of the method for manufacturing the polarizing film of the present invention and the polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in FIG1 is configured as follows: a preform film (unstretched) 10 made of polyvinyl alcohol resin is continuously wound out from a preform film roll 11 and conveyed along a film conveying path, thereby sequentially passing through a swelling bath (swelling liquid contained in a swelling tank) 13 provided on the film conveying path, a dyeing and crosslinking step 18, and a washing bath (washing liquid contained in a washing tank) 19, and finally passing through a drying oven 21. The resulting polarizing film 23 can be directly conveyed, for example, to a subsequent polarizing plate manufacturing step (a step of laminating a protective film on one or both sides of the polarizing film 23). The arrows in FIG1 indicate the conveying direction of the film.

[0018] The so-called dyeing and crosslinking step 18 refers to the process of sequentially passing the membrane taken out from the swelling bath 13 through a first dyeing bath (dyeing solution contained in the dyeing tank) 15a, a first crosslinking bath (first crosslinking solution contained in the crosslinking tank) 17a, a second dyeing bath (second dyeing solution contained in the dyeing tank) 15b, a second crosslinking bath (second crosslinking solution contained in the crosslinking tank) 17b, and a third crosslinking bath (third crosslinking solution contained in the crosslinking tank) 17c. The step of immersing the membrane in the first dyeing bath 15a is called the first dyeing step, and the step of immersing the membrane in the second dyeing bath 15b is called the second dyeing step; these are collectively referred to as dyeing steps. The step of immersing the membrane in the first crosslinking bath 17a is called the first crosslinking step, the step of immersing the membrane in the second crosslinking bath 17b is called the second crosslinking step, and the step of immersing the membrane in the third crosslinking bath 17c is called the third crosslinking step. These are also collectively referred to as crosslinking steps.

[0019] In the description of Figure 1, "processing tank" refers to the collective term including the swelling tank, dyeing tank, crosslinking tank, and washing tank; "processing liquid" refers to the collective term including the swelling liquid, dyeing liquid, crosslinking liquid, and washing liquid; and "processing bath" refers to the collective term including the swelling bath, dyeing bath, crosslinking bath, and washing bath. The swelling bath, dyeing bath, crosslinking bath, and washing bath respectively constitute the swelling section, dyeing section, crosslinking section, and washing section in the manufacturing apparatus of the present invention.

[0020] Regarding the film transport path of the polarizing film manufacturing apparatus, in addition to the aforementioned processing baths, it can be constructed by arranging guide rollers 30 to 49, 56 to 61 and clamping rollers 50 to 55 in appropriate positions. The guide rollers 30 to 49 and 56 to 61 can support the transported film or further change the film transport direction. The clamping rollers 50 to 55 can squeeze / clamp the transported film and apply the driving force generated by its rotation to the film, or further change the film transport direction. The guide rollers or clamping rollers can be arranged before, after, or within each processing bath, thereby allowing the film to be introduced / immersed in the processing bath and pulled out of the processing bath (see Figure 1). For example, by providing one or more guide rollers in each processing bath and transporting the film along these guide rollers, the film can be immersed in each processing bath.

[0021] In the polarizing film manufacturing apparatus shown in Figure 1, clamping rollers (clamping rollers 50 to 54) are arranged before and after each processing bath. This allows for a circumferential speed difference between the clamping rollers arranged before and after each processing bath, thereby performing longitudinal uniaxial extension between rollers. In the polarizing film manufacturing apparatus shown in Figure 1, longitudinal uniaxial extension is preferably also performed in the drying oven 21. The longitudinal uniaxial extension in the drying oven 21 can be performed, for example, by applying a circumferential speed difference between clamping rollers 54 and 55.

[0022] <Swelling Steps> Regarding the swelling step, it is performed with the aim of "removing foreign matter from the surface of the embryo 10, removing plasticizers from the embryo 10, imparting dyeability, and plasticizing the embryo 10". The processing conditions are determined within the range that achieves the purpose and within the range that will not cause undesirable conditions such as extreme dissolution or depermeability of the embryo 10.

[0023] Referring to Figure 1, the swelling step can be performed by "continuously winding the preform film 10 out of the preform film roll 11 while conveying it along the film conveying path, immersing the preform film 10 in the swelling bath 13 for a predetermined time, and then pulling it out." In the example of Figure 1, from the time the preform film 10 is wound out until it is immersed in the swelling bath 13, the preform film 10 is conveyed along the film conveying path formed by the guide rollers 60, 61 and the clamping roller 50. During the swelling process, it is conveyed along the film conveying path formed by the guide rollers 30 to 32 and the clamping roller 51.

[0024] In the swelling solution used as the swelling bath 13, in addition to pure water, an aqueous solution containing boric acid (Japanese Patent Application Publication No. 10-153709), chloride (Japanese Patent Application Publication No. 06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol, etc., can be used in a range of 0.01 to 10% by mass.

[0025] The temperature of the swelling bath 13 is, for example, 10 to 50°C, preferably 10 to 40°C, and more preferably 15 to 30°C. The immersion time of the preform 10 is preferably 10 to 300 seconds, and more preferably 20 to 200 seconds. Furthermore, when the preform 10 is a polyvinyl alcohol-based resin film pre-stretched in a gas, the temperature of the swelling bath 13 is, for example, 20 to 70°C, preferably 30 to 60°C. The immersion time of the preform 10 is preferably 30 to 300 seconds, and more preferably 60 to 240 seconds.

[0026] During the swelling process, a problem can easily occur where the preform film 10 swells in the width direction, resulting in wrinkles in the film. One method to eliminate these wrinkles and transport the film includes using rollers with stretching functions, such as expander rolls, spiral rolls, and crown rolls, in the guide rollers 30, 31, and / or 32, or using other stretching devices such as fabric guides, bending bars, and tenter frame clamps. Another method to suppress wrinkle formation is to perform a stretching process. For example, a uniaxial stretching process can be performed in the swelling bath 13 using the difference in circumferential speed between the clamping rollers 50 and 51.

[0027] During the swelling process, since the membrane also swells and expands in the direction of membrane transport, in order to eliminate membrane slack in the transport direction when the membrane is not actively stretched, it is preferable to take measures such as "controlling the speed of the clamping rollers 50 and 51 arranged before and after the swelling bath 13". Furthermore, in order to stabilize the membrane transport in the swelling bath 13, it is also useful to take measures such as "controlling the water flow in the swelling bath 13 by rinsing with water" or "using an EPC device (Edge Position Control device: a device used to detect the end of the membrane and prevent the membrane from snagging)".

[0028] In the example shown in Figure 1, the film system 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.

[0029] <Dyeing and Crosslinking Steps> (Step 1: Staining) The first dyeing step is performed with the aim of "adsorbing dichroic pigments such as directional iodine onto the swollen polyvinyl alcohol resin membrane". The processing conditions are determined within a range that achieves this objective and does not result in extreme dissolution or depermeability of the membrane. Referring to Figure 1, the first dyeing step can be performed by "transporting the swollen membrane along a membrane transport path formed by the clamping roller 51, guide rollers 33 to 36, and clamping roller 52, immersing the swollen membrane in the first dyeing bath 15a (the processing liquid contained in the dyeing tank) for a predetermined time, and then pulling it out". To improve the dyeability of dichroic pigments, the membrane supplied to the first dyeing step is preferably a membrane that has undergone at least some degree of uniaxial stretching treatment, or preferably undergoes uniaxial stretching treatment during the first dyeing step instead of uniaxial stretching treatment before the first dyeing step, or undergoes uniaxial stretching treatment both before and during the first dyeing step.

[0030] In the staining solution of the first staining bath 15, when the dichroic dye is iodine, an aqueous solution with a mass ratio of iodine / potassium iodide / water of 0.003 to 3 / 0.1 to 10 / 100 can be used. Other iodides such as zinc iodide can be used instead of potassium iodide, or potassium iodide can be used in combination with other iodides. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, can also coexist. When the staining solution of the staining bath contains boric acid, the difference from the crosslinking bath is that it contains iodine at a higher concentration than 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 staining bath 15 during membrane impregnation is typically 10 to 45°C, preferably 10 to 40°C, more preferably 20 to 35°C, and the membrane impregnation time is typically 30 to 600 seconds, preferably 60 to 300 seconds.

[0031] As described above, in the first dyeing step, uniaxial stretching of the membrane can be performed in the first dyeing bath 15. Uniaxial stretching of the membrane can be performed by applying a difference in circumferential speed between the clamping rollers 51 and 52 arranged before and after the first dyeing bath 15.

[0032] In the first dyeing step, similar to the swelling treatment step, in order to remove wrinkles in the film and transport the polyvinyl alcohol-based resin film, rollers with stretching functions such as spreading rollers, spiral rollers, and convex rollers can be used in guide rollers 33, 34, 35 and / or 36, or other stretching devices such as fabric guides, bending bars, and stretching machine clamps can be used. Another means to suppress wrinkle formation is to perform a stretching treatment, similar to the swelling treatment.

[0033] In the example shown in Figure 1, the membrane system pulled out from the first dyeing bath 15 is sequentially guided into the first crosslinking bath 17a through the guide roller 36, the clamping roller 52 and the guide roller 37.

[0034] (First crosslinking step) The first crosslinking step is a process performed for the purpose of "membrane crosslinking", which can improve water resistance or heat resistance.

[0035] Referring to Figure 1, the first crosslinking process can be carried out by "transporting the film along the film transport path formed by the clamping roller 52, the guide rollers 37 to 40 and the clamping roller 53a, so that the film after the first dyeing step is immersed in the first crosslinking bath 17a for a predetermined time and then pulled out".

[0036] The uniaxial stretching process can be performed in the first crosslinking bath 17a by utilizing the difference in circumferential speed between the clamping roller 52 and the clamping roller 53a.

[0037] In the first crosslinking step, similar to the swelling treatment, in order to remove wrinkles in the film and transport the polyvinyl alcohol-based resin film, rollers with a stretching function, such as spreading rollers, spiral rollers, and convex rollers, can be used in guide rollers 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49a, 49b, and / or 49d, or other stretching devices such as fabric guides, bending bars, and stretching machine clamps can be used. Another means to suppress wrinkle formation is to perform a stretching treatment, similar to the swelling treatment.

[0038] From the viewpoint of promoting crosslinking of polyvinyl alcohol (PVA) resin films or balancing the crosslinking dosage in PVA films, the temperature of the first crosslinking bath 17a is preferably 30°C or higher, more preferably 35°C or higher, and even more preferably 42°C or higher. From the viewpoint of preventing the dissolution of PVA resin films, the temperature of the first crosslinking bath 17a is preferably 65°C or lower, and more preferably 64°C or lower. The immersion time of the film in the first crosslinking bath 17a is approximately 3 to 600 seconds, preferably 4 to 300 seconds, and more preferably 5 to 200 seconds.

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

[0040] (Second staining step) The second dyeing step follows the same explanation as the first dyeing step. In the second dyeing step, uniaxial stretching treatment can be performed in the second dyeing bath 15b by applying a difference in circumferential speed between the clamping rollers 53a and 53b. Regarding the second dyeing bath 15b, its composition, temperature, and immersion time can be the same as or different from those of the first dyeing bath 15a. The temperature of the second dyeing bath 15b can be higher than that of the first dyeing bath 15a, by at least 5°C or at least 10°C. By setting the temperature of the second dyeing bath 15b to be higher than that of the first dyeing bath, the stretching ratio of the stretching treatment in the drying step can be further improved.

[0041] (Second crosslinking step) The second crosslinking step follows the same description as the first dyeing step. In the second crosslinking step, uniaxial stretching treatment can be performed in the second crosslinking bath 17b by applying a difference in circumferential speed between the clamping rollers 53b and 53c. Regarding the second crosslinking bath 17b, the composition, temperature, and immersion time can be the same as or different from those of the first crosslinking bath 17a.

[0042] (Third crosslinking step) In Figure 1, the third crosslinking step is a process performed for the purpose of "adjusting the hue of the membrane (preventing the membrane from having a bluish tint, etc.)". In the third crosslinking step, uniaxial stretching treatment can be performed in the third crosslinking bath 17c by applying a difference in circumferential speed between the clamping rollers 53c and 53d. Regarding the third crosslinking bath 17c, the composition, temperature, and immersion time can be the same as or different from the first crosslinking bath 17a or the second crosslinking bath 17b. The temperature of the crosslinking bath in the third crosslinking step, which aims to "adjust the hue of the membrane", is preferably lower than the temperature of the crosslinking bath in the first crosslinking step or the crosslinking bath in the second crosslinking step, which aims to "crosslink the membrane". The temperature of the third crosslinking bath 17c can be set to, for example, 45°C or below.

[0043] <Washing Steps> In the example shown in Figure 1, a washing step is included after the dyeing and cross-linking step. The washing process aims to remove excess boric acid or iodine and other agents adhering to the polyvinyl alcohol (PVA) resin film. The washing step is performed, for example, by immersing the cross-linked PVA resin film in a washing bath 19. Alternatively, the washing step can be performed by spraying the film with a cleaning solution in a rinsing manner instead of immersing the film in the washing bath 19, or by combining immersion in the washing bath 19 with spraying the cleaning solution.

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

[0045] Furthermore, during the washing process, to remove wrinkles and transport the polyvinyl alcohol-based resin film, rollers with stretching functions, such as extension rollers, spiral rollers, and convex rollers, can be used in guide rollers 56, 57, 58, and / or 59, or other stretching devices such as fabric guides, bending bars, and tenter clamps can be used. Additionally, during the film washing process, a stretching treatment can also be performed to suppress the occurrence of wrinkles.

[0046] <Extended Steps> As described above, the preform 10 undergoes uniaxial stretching treatment in a wet or dry manner between the aforementioned series of processing steps (i.e., before and after any one or more processing steps, and / or during any one or more processing steps). Specific methods for uniaxial stretching treatment include, for example, inter-roll stretching by imparting a circumferential speed difference between two clamping rollers constituting the film transport path (e.g., two clamping rollers arranged before and after the treatment bath) to perform longitudinal uniaxial stretching, hot roller stretching as described in Japanese Patent No. 2731813, and tenter stretching, with inter-roll stretching being preferred. The uniaxial stretching step can be performed multiple times from the preform 10 until the polarizing film 23 is obtained. As described above, the stretching treatment also helps to suppress the occurrence of film wrinkles.

[0047] <Drying Steps> After the washing step, it is preferable to dry the polyvinyl alcohol (PVA) resin film. There are no particular limitations on the drying method, but as shown in the example in Figure 1, a drying oven 21 can be used. The drying oven 21 can be, for example, equipped with a hot air dryer. The drying temperature is, for example, around 30 to 100°C, and the drying time is, for example, around 30 to 600 seconds. The drying of the PVA 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 to 30 μm.

[0048] In the manufacturing method of the present invention, the polyvinyl alcohol-based resin film can be stretched during the drying step. According to the manufacturing method of the present invention, the tension of the polyvinyl alcohol-based resin film generated during the crosslinking step is mitigated; therefore, the polyvinyl alcohol-based resin film can be stretched without cutting it during the drying step.

[0049] In the drying step, stretching can be achieved, for example, by setting a difference in the rotational speeds of the clamping rollers positioned near the inlet of the drying oven and the clamping rollers positioned near the outlet of the drying oven. In such stretching, if the ratio of the rotational speed of the clamping rollers positioned near the outlet of the drying oven to the rotational speed of the clamping rollers positioned near the inlet of the drying oven is defined as the stretching ratio in the drying step, then the stretching ratio of the polyvinyl alcohol-based resin film in the drying step is preferably, for example, 1.05 times or more, and more preferably 1.10 times or more. In the drying step, stretching with a stretching ratio of 1.05 times or more can improve the optical properties of the obtained polarizing film.

[0050] <Other processing steps for polyvinyl alcohol-based resin films> Additional treatments beyond those described above may also be added. Examples of such additional treatments include: immersion treatment in an aqueous solution containing zinc chloride, etc., without boron compounds (zinc treatment).

[0051] The obtained polarizing film can be rolled into a roll by sequentially winding it onto a winding roller, or it can be fed directly to the polarizing plate manufacturing process (such as applying a protective film to one or both sides of the polarizing film) without being wound.

[0052] The final cumulative elongation of the polarizing film 23, based on the preform 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 or in the drying step. When the elongation process is performed in two or more processing steps, the elongation process can be performed in any processing step.

[0053] [Polarizing film] The thickness of the polarizing film is typically below 65 μm, preferably below 50 μm, more preferably below 35 μm, and even more preferably below 30 μm. The thickness of the polarizing film is typically above 2 μm, preferably above 5 μm, more preferably above 10 μm, and even more preferably above 15 μm. The thickness of the polarizing film can be controlled, for example, by selecting the polyvinyl alcohol-based resin film and adjusting the elongation.

[0054] The polarization degree of the polarizing film, for example, can be 99.9800% or higher, preferably 99.9900% or higher, more preferably 99.9910% or higher, and even more preferably 99.9915% or higher. The polarization degree of the polarizing film is typically below 100%, and practically, it is less than 100%, typically below 99.9950%. The polarization degree of the polarizing film, which is corrected for visual sensitivity, can be measured according to the measurement methods described in the examples section below.

[0055] The transmittance of the polarizing film's sensitivity correction monomer can be, for example, 43.0% or higher, preferably 43.5% or higher, and even more preferably 43.9% or higher. The transmittance of the sensitivity correction monomer is typically 50.0% or lower, for example, it can be less than 50.0% or lower, or even 49.0% or lower. The transmittance of the polarizing film can be measured according to the measurement method described in the following examples.

[0056] The orthogonal hue b-value of the polarizing film can be, for example, in the range of -3.0 to 3.0, preferably -2.5 to 2.6 or lower, more preferably in the range of -2.0 to 2.0, even more preferably in the range of -1.5 to 1.5, and particularly preferably in the range of -1.2 to 1.2. Using a spectrophotometer with an integrating sphere ("V7100" manufactured by Japan Spectrophotometer Co., Ltd.), the chromaticity in the L*a*b* (CIE) color system is calculated using the isochromatic function of the C light source for the obtained transmittance, thereby obtaining the orthogonal hue b-value of the polarizing film with orthogonal configuration. The orthogonal hue b-value of the polarizing film can be measured according to the measurement method described in the section of the embodiments described later.

[0057] The iodine content of the polarizing film can be, for example, 3.5% by mass or more, preferably 4.0% by mass or more, and even more preferably 4.3% by mass or more. Practically, the iodine content is 6.0% by mass or less, and can be 5.5% by mass or less. The iodine content of the polarizing film can be determined according to the measurement method described in the section of the embodiments described later.

[0058] The boron content of the polarizing film is preferably 4.0% by mass or less, and more preferably 3.5% by mass or less. For example, the boron content can be 3.0% by mass or more, or 3.2% by mass or more. The boron content of the polarizing film can be determined according to the measurement method described in the section of the embodiments described later.

[0059] The crosslinking efficiency of the polarizing film can be, for example, in the range of 5.0 to 6.0, preferably in the range of 5.2 to 5.8. The crosslinking efficiency of the polarizing film can be calculated according to the calculation method described in the following examples. A higher crosslinking efficiency of the polarizing film means that the boron in the polarizing film is used more efficiently for crosslinking. In the polarizing film, it is preferable to maintain the desired degree of crosslinking while reducing the boron content.

[0060] Generally, in polarizing films, a higher iodine content tends to decrease the transmittance of the vision correction monomer. According to the manufacturing method of this invention, a polarizing film with high vision correction monomer transmittance and high iodine content can be obtained. For example, a polarizing film with a vision correction monomer transmittance of 43.0% or more and an iodine content of 4.0% by mass or more can be obtained. With such a polarizing film, a high vision correction polarization can be obtained due to the high iodine content. For example, a polarizing element with a vision correction polarization of 99.9800% or more can be obtained.

[0061] [Polarizing plate] A polarizing plate can be obtained by laminating a protective film onto at least one side of a polarizing film through an adhesive layer. Examples of protective films include: films made of acetylcellulose resins such as triacetylcellulose or diacetylcellulose; films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate resin films; cyclic olefin resin films; acrylic resin films; and films made of chain olefin resins from polypropylene resins.

[0062] 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. The adhesive layer can be composed of an adhesive or a bonding agent.

[0063] The polarizing plate 60 shown in Figure 2 sequentially comprises a polarizing film 23, an adhesive layer 70, and a protective film 80. The polarizing plate can be a linear polarizing plate or a circular polarizing plate used in a display device. The display device can be any of a liquid crystal display device or an organic EL display device. The display device can be used in mobile devices such as televisions, personal computers, mobile phones, or tablet computers. [Example]

[0064] The present invention will be described in more detail below with reference to experimental examples, but the present invention is not limited to these examples.

[0065] <Example 1> 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 45 μm thick strip of polyvinyl alcohol (PVA) preform (trade name "TS45" manufactured by KURARAY Co., Ltd., with an average degree of polymerization of 2400 and a saponification degree of 99.9 moles or higher) was continuously conveyed from a roll and immersed in a swelling bath containing water (deionized water) at 30°C for 1 minute and 20 seconds, stretching 1.56 times (swelling step). Then, the film drawn from the swelling bath was immersed in a first dyeing bath at 30°C containing 1.0% by mass potassium iodide and 1.4 mM iodine for 100 seconds, stretching 1.15 times (first dyeing step). Next, the film drawn from the first dyeing bath was immersed in a first crosslinking bath at 60°C containing 9.0% by mass potassium iodide and 3.0% by mass boric acid for 37 seconds, stretching 1.46 times (first crosslinking step). Next, the film drawn from the first crosslinking bath was immersed in a second dyeing bath at 30°C containing 1.0% by mass of potassium iodide and 1.4 mM of iodine for 20 seconds, resulting in a stretch of 1.70 times (second dyeing step). Then, the film drawn from the second dyeing bath was immersed in a second crosslinking bath at 63°C containing 9.0% by mass of potassium iodide and 3.0% by mass of boric acid for 6 seconds, resulting in a stretch of 1.34 times (second crosslinking step). Then, it was immersed in a third crosslinking bath at 57°C containing 10.0% by mass of potassium iodide and 3.0% by mass of boric acid for 7 seconds, resulting in a stretch of 1.00 times (third crosslinking step). After the swelling step and the dyeing and crosslinking step (in Example 1, the dyeing and crosslinking step consisted of the first dyeing step, the first crosslinking step, the second dyeing step, the second crosslinking step, and the third crosslinking step), and by roller stretching in the bath, the total stretching ratio based on the preform film was set to 6.0 times.

[0066] Subsequently, the film was immersed in a 13°C washing bath for 2 seconds to remove foreign matter adhering to the film surface (washing step). After the washing step, the film, held at a tension of 470N, was dried with hot air at 92°C in a drying oven and stretched according to the stretch ratio listed in Table 1 (drying step). A polarizing film was obtained by operating in this manner. The thickness of the obtained polarizing film was 16.6 μm. In the drying step, uniaxial stretching was performed by setting a difference between the rotation speed of the clamping rollers set near the inlet of the drying oven and the rotation speed of the clamping rollers set near the outlet. The stretch ratio listed in Table 1 is set as the ratio of the rotation speed of the clamping rollers set near the outlet to the rotation speed of the clamping rollers set near the inlet.

[0067] <Examples 2 to 7> Except for setting the temperature of the second dyeing bath and the stretch ratio of the drying step as shown in Table 1, the polarizing films were obtained under the same conditions as in Example 1. The thickness of the obtained polarizing films was 16.4 μm.

[0068] <Comparative Example 1> Except for setting the order of the dyeing and crosslinking steps as shown in Table 1 and setting the extension ratio of the drying step as shown in Table 1, the polarizing film was obtained under the same conditions as in Example 1. The thickness of the obtained polarizing film was 17.1 μm.

[0069] [Evaluation of polarizing film] (a) Measurement of monomer transmittance and polarization For the polarizing film, a spectrophotometer with an integrating sphere (V7100 manufactured by Nippon Spectrophotometer Co., Ltd.) was used to measure the MD transmittance and TD transmittance in the wavelength range of 380 to 780 nm. The single-cell transmittance and polarization at each wavelength were calculated according to the following formula: Single-cell penetration rate (%) = (MD + TD) / 2 Polarization (%) = {(MD-TD) / (MD+TD)} × 100.

[0070] The term "MD transmittance" refers to the transmittance when the direction of polarized light emitted 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. Similarly, the term "TD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glan-Thompson prism is orthogonal to the transmission axis of the polarizing film sample, and is represented as "TD" in the above formula. For the obtained individual transmittance and polarization, visual sensitivity correction is performed using a 2D viewing angle (C light source) according to JIS Z 8701:1999 "Methods for displaying colors—XYZ color system and X10Y10Z10 color system" to determine the visual sensitivity corrected individual transmittance (Ty) and visual sensitivity corrected polarization (Py). Table 1 shows the calculated results of the sensitivity-corrected single-cell transmittance (Ty) and sensitivity-corrected polarization (Py).

[0071] (b) Orthorhombic hue b-value After the polarizing film was cut into 4cm×4cm pieces, the cross hue b value was measured using a UV-Vis spectrometer (V-7100, manufactured by Japan Spectrophotometer Co., Ltd.).

[0072] (c) Determination of iodine content 0.2 g of polarizing film (sample) was placed in a 30 ml vial, and deionized water was added to bring the total amount of sample and deionized water to 25 g. The vial was then immersed in a 90°C water bath for at least 3 hours to dissolve the sample, followed by cooling. The iodine content of the dissolved sample was determined using XRF (Terkin Elmer Optima 5300DV). The iodine content (mass %) of the polarizing film was calculated using X (ppm), the measured value, from the following formula. Iodine content (mass %) = X × (25 / 1000000) × [100 / (mass of polarizing film, g)]

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

[0074] (e) Calculation of crosslinking efficiency To calculate the crosslinking efficiency, the degree of crosslinking was first calculated. The central portion of the polarizing film was cut into 10cm × 10cm pieces, and the degree of crosslinking was measured using a Fourier transform infrared spectrophotometer (FT-IR) (Nicolet 5700, Thermo Fisher Scientific). The measurement was performed using a Pike Technologies VeeMAX III (ATR) FT-IR wafer, with 16 scans and a wavenumber resolution of 4 cm⁻¹. The degree of crosslinking was calculated by dividing the sum of the areas in the wavenumber range of 1200 to 1360 cm⁻¹ aligned with a 3.2 reference (a) by the sum of the areas in the wavenumber range of 2850 to 3000 cm⁻¹ aligned with the same reference (b). The three-fold average of the resulting value was taken as the degree of crosslinking.

[0075] Regarding the crosslinking efficiency, it is calculated using the boron content determined in (d) above and the degree of crosslinking calculated above, and according to the following formula. Crosslinking efficiency = [degree of crosslinking × 10.812] / [(boron content, mass%) × 3]

[0076] [Durability Testing] (Immersion test) Two saponified triacetyl cellulose membranes (40 μm thick) were prepared and bonded together on both sides of the polarizer manufactured in Example 1 using an adhesive containing polyvinyl alcohol resin to obtain a polarizing plate composed of layers of triacetyl cellulose membrane / adhesive / polarizer / adhesive / triacetyl cellulose membrane. The obtained polarizing plate was cut into 4 cm × 4 cm pieces as evaluation samples. The aforementioned evaluation samples were immersed in water at 60°C for 30 minutes, then the samples were lifted and left to stand at 23°C and 55% relative humidity for about 12 hours. After such an immersion test, the visual sensitivity-corrected polarization Py of the evaluation samples after the immersion test was measured using the same method as above. In addition, the visual sensitivity-corrected polarization Py of the evaluation samples was also measured before the immersion test. Table 1 records the absolute values ​​of Py after the immersion test and Py before the immersion test.

[0077] (Damp heat resistance test) After placing the evaluation samples in an oven at 85°C and 85% relative humidity for 24 hours, the samples were removed and left to stand for approximately 12 hours at 23°C and 55% relative humidity. The visual sensitivity-corrected polarization Py of the evaluation samples after the heat resistance test was then measured using the same method as described above. The table records the absolute values ​​of Py after the water immersion test versus Py before the water immersion test.

[0078] [Table 1]

[0079] In Comparative Example 1, the second dyeing step was performed before the first crosslinking step. As can be seen from the results shown in Table 1, compared with the polarizing films obtained in Examples 1 to 7, the polarizing film obtained in Comparative Example 1 had a lower iodine content and a larger change in the visual sensitivity-corrected polarization measured in the immersion test and the damp heat resistance test.

[0080] 10: Embryo membrane (embryo membrane) composed of polyvinyl alcohol-based resins. 11: Embryonic membrane roll 13: Bentonite Bath 15a: First staining bath 17a: First cross-linking bath 15b: Second staining bath 17b: Second cross-linking bath 17c: Third cross-linking bath 18: Dyeing and Crosslinking Steps 19: Cleansing bath 21: Drying Oven 23:Polarizing film 30 to 49, 56 to 61: Guide rollers 50 to 52, 53a, 53b, 53c, 53d, 54, 55: Clamping rollers 60:Polarizing plate 70: Adhesive layer 80: Protective film

Claims

1. A method for manufacturing a polarizing film, comprising a method for manufacturing a polarizing film from a polyvinyl alcohol-based resin film, wherein, The manufacturing method includes a dyeing and crosslinking step for dyeing and crosslinking the aforementioned polyvinyl alcohol-based resin film; the aforementioned dyeing and crosslinking step includes: a first dyeing step, which involves dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment; a first crosslinking step, which involves crosslinking the aforementioned polyvinyl alcohol-based resin film with a crosslinking agent after the aforementioned first dyeing step; and a second dyeing step, which involves dyeing the aforementioned polyvinyl alcohol-based resin film with a dichroic pigment after the aforementioned first crosslinking step. The second crosslinking step is performed after the aforementioned second dyeing step, by crosslinking the aforementioned polyvinyl alcohol resin film with a crosslinking agent.

2. The method for manufacturing the polarizing film as described in claim 1 further comprises: a washing step, which is performed after the aforementioned dyeing and crosslinking step, to wash the aforementioned polyvinyl alcohol resin film; and a drying step, which is performed after the aforementioned washing step, to dry the aforementioned polyvinyl alcohol resin film.

3. The manufacturing method as described in claim 2, wherein, In the aforementioned drying step, the aforementioned polyvinyl alcohol resin film is uniaxially stretched at an elongation ratio of 1.10 or higher.

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