Method and apparatus for manufacturing polarizing film

Abstract

To provide a method and device for manufacturing a polarizing film which offer an increased degree of cross-linking and yet is less susceptible to shrinkage caused by temperature change.SOLUTION: A method of manufacturing a polarizing film from a polyvinyl alcohol-based resin film is provided, the method comprising a supersonic wave treatment step for immersing the polyvinyl alcohol-based resin film in a supersonic treatment bath having a supersonic wave propagating therein and containing potassium iodide and boron compound.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

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

[0003] A polarizing film is usually produced by subjecting a raw film made of a polyvinyl alcohol-based resin to a dyeing treatment in which it is immersed in a dyeing bath containing a dichroic dye such as iodine, and then a crosslinking treatment in which it is immersed in a crosslinking bath containing a crosslinking agent such as boric acid, and uniaxially stretching the film at any stage. Uniaxial stretching includes dry stretching in which stretching is performed in air and wet stretching in which stretching is performed in a liquid such as the above dyeing bath and crosslinking bath.

[0004] A polarizing film can obtain high optical properties by increasing the degree of crosslinking. Although the degree of crosslinking can be increased by increasing the concentration of the crosslinking agent in the crosslinking bath, there is a problem that it is likely to shrink due to temperature changes.

[0005] JP-A-59-094706 (Patent Document 1) describes that a polarizing film having a small shrinkage rate and good flatness can be obtained by performing an immobilization treatment by irradiating far-infrared rays having a wavelength of 1 μm or more.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

[0007] The present invention aims to provide a method and apparatus for manufacturing a polarizing film that increases the degree of crosslinking while being less susceptible to shrinkage due to temperature changes. [Means for solving the problem]

[0008] The present invention provides a method and apparatus for manufacturing a polarizing film as described below. [1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, The process includes an ultrasonic treatment step in which the polyvinyl alcohol-based resin film is immersed in an ultrasonic treatment bath in which ultrasonic waves are propagated. The ultrasonic treatment bath is a method for producing a polarizing film, comprising potassium iodide and a boron compound. [2] The manufacturing method according to [1], wherein in the ultrasonic treatment step, the temperature of the ultrasonic treatment bath is 40°C or higher and 50°C or lower. [3] The method for manufacturing a polarizing film according to [1] or [2], wherein in the ultrasonic treatment step, the polyvinyl alcohol-based resin film is transported in the ultrasonic treatment bath, and the angle between the transport direction of the polyvinyl alcohol-based resin film in the ultrasonic treatment bath and the propagation direction of the ultrasonic waves is 0° or more and 90° or less. [4] The process further includes a dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic dye, The ultrasonic treatment step is performed after the dyeing step, and is a method for manufacturing a polarizing film according to any one of items [1] to [3]. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method and apparatus for manufacturing a polarizing film that increases the degree of crosslinking while being less susceptible to shrinkage due to temperature changes. [Brief explanation of the drawing]

[0010] [Figure 1]This is a schematic cross-sectional view showing an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used therein according to the present invention. [Figure 2] This is a schematic cross-sectional view showing another embodiment of an ultrasonic treatment bath. [Figure 3] This is a schematic cross-sectional view showing another embodiment of an ultrasonic treatment bath. [Modes for carrying out the invention]

[0011] <Method for manufacturing polarizing film> In the present invention, the polarizing film is a uniaxially oriented polyvinyl alcohol-based resin film on which a dichroic dye (iodine or dichroic dye) is adsorbed and oriented. The polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is usually 85 mol% or more, preferably 90 mol% or more, and more preferably 99 mol% or more. The polyvinyl acetate-based resin can be, for example, polyvinyl acetate, which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable thereto. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is usually 1000 to 10000, preferably 1500 to 5000.

[0012] These polyvinyl alcohol-based resins may be modified; for example, polyvinyl formal, polyvinyl acetal, and polyvinyl butyral modified with aldehydes can also be used.

[0013] In this invention, an unstretched polyvinyl alcohol-based resin film (raw material film) with a thickness of 65 μm or less (e.g., 60 μm or less), preferably 50 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less is used as the starting material for manufacturing polarizing films. This makes it possible to obtain thin-film polarizing films, which are in increasingly high market demand. The width of the raw material film is not particularly limited and can be, for example, 300 to 6000 mm. The raw material film is prepared, for example, as a long roll of unstretched polyvinyl alcohol-based resin film (raw material roll).

[0014] Polarizing films can be continuously manufactured as long polarizing films by unwinding the long roll of raw film described above from the raw film roll, continuously transporting it along the film transport path of a polarizing film manufacturing apparatus, immersing it in a processing liquid (hereinafter also referred to as the "processing bath") contained in a processing tank, and then performing a predetermined processing step of drawing it out, followed by a drying step. The processing step is not limited to immersing the film in a processing bath, as long as it is a method of processing the film by bringing the processing liquid into contact with the film. It may also be a method of applying the processing liquid to the film surface by spraying, flowing, dropping, etc. If the processing step is performed by immersing the film in a processing bath, it is not limited to using only one processing bath for one processing step. The film may be sequentially immersed in two or more processing baths to complete one processing step.

[0015] Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a crosslinking liquid, a complementary color liquid, a cleaning liquid, and the like. Examples of the treatment process include a swelling process of bringing a swelling liquid into contact with a raw film to perform a swelling treatment, a dyeing process of bringing a dyeing liquid into contact with the film after the swelling treatment to perform a dyeing treatment, a crosslinking process of bringing a crosslinking liquid into contact with the film after the dyeing treatment to perform a crosslinking treatment, a complementary color process of bringing a complementary color liquid into contact with the film after the crosslinking treatment to perform a complementary color treatment, and a cleaning process of bringing a cleaning liquid into contact with the film after the complementary color treatment to perform a cleaning treatment. Further, a uniaxial stretching treatment is performed wet or dry during these series of treatment steps (that is, before and / or after any one or more of the treatment steps and / or during any one or more of the treatment steps). Other treatment steps may be added as necessary.

[0016] In the method for producing a polarizing film of the present invention, an ultrasonic treatment step of immersing the film in an ultrasonic treatment bath through which ultrasonic waves are propagating is performed. The ultrasonic treatment step is performed in an ultrasonic treatment bath containing potassium iodide and a boron compound. By performing the ultrasonic treatment step, it is possible to improve the crosslinking degree while suppressing the shrinkage due to temperature change of the obtained polarizing film. By improving the crosslinking degree, a polarizing film having excellent optical properties can be obtained. As a polarizing film having excellent optical properties, for example, a polarizing film having a high single transmittance, a high degree of polarization, and a small absolute value of the orthogonal hue b value can be obtained.

[0017] According to the method for producing a polarizing film of the present invention, for example, a polarizing film having a shrinkage force of 2.82 N / 2 mm or less and a crosslinking degree of 5.52 or more can be obtained, a polarizing film having a shrinkage force of 2.5 N / 2 mm or less and a crosslinking degree of 6.5 or more can also be obtained, and a polarizing film having a shrinkage force of 2.04 N / 2 mm or less and a crosslinking degree of 6.65 or more can also be obtained. The shrinkage force and the crosslinking degree referred to here are based on the values measured by the measurement methods described in the examples.

[0018] By performing an ultrasonic treatment step, it is possible to improve the crosslinking degree while suppressing the shrinkage of the obtained polarizing film due to temperature changes. The reason is that cavitation occurs in the ultrasonic treatment bath by generating ultrasonic waves, which causes fine vibrations in the ultrasonic treatment bath and the film, improving the penetration power of chemical solutions such as the dyeing solution into the film, and presumably improving the dyeability and crosslinking degree. The ultrasonic treatment bath is not limited as long as it is a bath containing potassium iodide and a boron compound. When the crosslinking solution contains potassium iodide and a boron compound, it may be a crosslinking bath containing the crosslinking solution, and when the complementary color solution contains potassium iodide and a boron compound, it may be a complementary color bath containing the complementary color solution. In this case, the ultrasonic treatment step is performed in the crosslinking step or the ultrasonic treatment step is performed in the complementary color step. The ultrasonic treatment step may be performed multiple times in multiple baths without being limited to once. Examples of the boron compound contained in the ultrasonic treatment bath include boric acid and borax.

[0019] Hereinafter, an example of the method for manufacturing a polarizing film according to the present invention will be described in detail while referring to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the method for manufacturing a polarizing film according to the present invention and a polarizing film manufacturing apparatus used therefor. The polarizing film manufacturing apparatus shown in FIG. 1 continuously unwinds an original web (unstretched) film 10 made of a polyvinyl alcohol-based resin from an original web roll 11 and conveys it along a film conveyance path, so that a swelling bath (a swelling liquid contained in a swelling tank) 13, a dyeing bath (a dyeing liquid contained in a dyeing tank) 15, a crosslinking bath (a crosslinking liquid contained in a crosslinking tank) 17a, a complementary color bath (a complementary color liquid contained in a complementary color tank) 17b, and a washing bath (a washing liquid contained in a washing tank) 19 provided on the film conveyance path are sequentially passed through, and finally, it is configured to pass through a drying furnace 21. The obtained polarizing film 23 can be conveyed, for example, directly to the next polarizing plate manufacturing step (a step of laminating a protective film on one or both sides of the polarizing film 23). The arrows in FIG. 1 indicate the conveyance direction of the film.

[0020] In the description of Figure 1, "processing tank" is a general term including the swelling tank, dyeing tank, crosslinking tank, color restoration tank, and washing tank; "processing solution" is a general term including the swelling solution, dyeing solution, crosslinking solution, color restoration solution, and washing solution; and "processing bath" is a general term including the swelling bath, dyeing bath, crosslinking bath, color restoration bath, and washing bath. The swelling bath, dyeing bath, crosslinking bath, color restoration bath, and washing bath constitute the swelling section, dyeing section, crosslinking section, color restoration section, and washing section, respectively, in the manufacturing apparatus of the present invention.

[0021] The film transport path of the polarizing film manufacturing apparatus can be constructed by placing guide rolls 30-48, 60, 61 (which support the transported film or can further change the film transport direction) and nip rolls 50-55 (which press and grip the transported film and can provide driving force to the film through rotation or can further change the film transport direction) at appropriate positions, in addition to the processing baths mentioned above. Guide rolls and nip rolls can be placed before, after, or within each processing bath, thereby enabling the introduction, immersion, and withdrawal of the film into the processing baths (see Figure 1). For example, by providing one or more guide rolls in each processing bath and transporting the film along these guide rolls, the film can be immersed in each processing bath.

[0022] The polarizing film manufacturing apparatus shown in Figure 1 has nip rolls positioned before and after each processing bath (nip rolls 50-54), which makes it possible to perform inter-roll stretching in any one or more processing baths by creating a difference in peripheral speed between the nip rolls positioned before and after the bath, thereby performing longitudinal uniaxial stretching.

[0023] In the polarizing film manufacturing apparatus shown in Figure 1, an ultrasonic transducer is placed in the complementary color bath 17b; that is, the complementary color bath 17b also serves as an ultrasonic treatment bath, and the ultrasonic treatment process is performed in the complementary color bath 17b along with the complementary color treatment. Each process will be described below.

[0024] (Swelling process) The swelling process is performed for purposes such as removing foreign matter from the surface of the raw film 10, removing plasticizers from the raw film 10, imparting dyeability, and plasticizing the raw film 10. The processing conditions are determined within a range that achieves these objectives and does not cause defects such as extreme dissolution or devitrification of the raw film 10.

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

[0026] In addition to pure water, the swelling solution for the swelling bath 13 can also be an aqueous solution to which boric acid (Japanese Patent Publication No. 10-153709), chloride (Japanese Patent Publication No. 06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohols, etc. are added in an amount ranging from 0.01 to 10% by weight.

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

[0028] During the swelling process, problems such as the raw film 10 swelling in the width direction and developing wrinkles are likely to occur. One way to transport the film while removing these wrinkles is to use rolls with a widening function, such as expander rolls, spiral rolls, or crown rolls, on the guide rolls 30, 31 and / or 32, or to use other widening devices such as cross guiders, bend bars, or tenter clips. Another way to suppress the occurrence of wrinkles is to perform a stretching process. For example, uniaxial stretching can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip roll 50 and the nip roll 51.

[0029] In the swelling process, the film swells and expands in the direction of film transport. Therefore, if the film is not actively stretched, it is preferable to take measures to eliminate slack in the film in the transport direction, such as controlling the speed of the nip rolls 50 and 51 placed before and after the swelling bath 13. In addition, to stabilize the film transport in the swelling bath 13, the water flow in the swelling bath 13 may be controlled by an underwater shower, or an EPC (Edge Position Control) device may be used. It is also useful to use a control device (a device that detects the edges of the film and prevents the film from meandering) in conjunction with this method.

[0030] In the example shown in Figure 1, the film withdrawn from the swelling bath 13 passes through the guide roll 32, the nip roll 51, and the guide roll 33 in order before being introduced into the staining bath 15.

[0031] (dying process) The dyeing process is performed for purposes such as adsorbing and oriented dichroic dyes onto a polyvinyl alcohol-based resin film after swelling. The processing conditions are determined within a range that achieves the above objective and does not cause problems such as extreme dissolution or devitrification of the film. Referring to Figure 1, the dyeing process can be carried out by transporting the film along a film transport path constructed by nip roll 51, guide rolls 33-36 and nip roll 52, immersing the swollen film in a dyeing bath 15 (processing liquid contained in a dyeing tank) for a predetermined time, and then withdrawing it. To improve the dyeability of the dichroic dye, it is preferable that the film used in the dyeing process is a film that has undergone at least some degree of uniaxial stretching, or, instead of or in addition to uniaxial stretching before the dyeing process, uniaxial stretching is performed during the dyeing process.

[0032] When iodine is used as a dichroic dye, the staining solution in the staining bath 15 can be, for example, an aqueous solution with a mass ratio of iodine / potassium iodide / water = 0.003~0.3 / 0.1~10 / 100. Other iodides, such as zinc iodide, may be used instead of potassium iodide, or potassium iodide and other iodides may be used in combination. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, or cobalt chloride, may be present. When boric acid is added, it is distinguished from the crosslinking treatment described later in that it contains iodine, and an aqueous solution containing 0.003 parts by mass or more of iodine per 100 parts by mass of water can be considered the staining bath 15. The temperature of the dyeing bath 15 when immersing the film is usually 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C, and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

[0033] When a water-soluble dichroic dye is used as the dichroic dye, the dyeing solution in the dyeing bath 15 can be an aqueous solution with a concentration of dichroic dye / water = 0.001 to 0.1 / 100 by mass ratio. Dyeing aids may be present in the dyeing bath 15, and may contain, for example, inorganic salts such as sodium sulfate or surfactants. Only one type of dichroic dye may be used alone, or two or more types of dichroic dyes may be used in combination. The temperature of the dyeing bath 15 when immersing the film is, for example, 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually about 30 to 600 seconds, preferably about 60 to 300 seconds.

[0034] As described above, in the dyeing process, the film can be uniaxially stretched in the dyeing bath 15. Uniaxial stretching of the film can be performed by methods such as creating a difference in peripheral speed between the nip rolls 51 and 52 positioned before and after the dyeing bath 15.

[0035] In the dyeing process, as with the swelling process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles from the film, guide rolls 33, 34, 35 and / or 36 may be used, which have a widening function such as expander rolls, spiral rolls, or crown rolls, or other widening devices such as cross guiders, bend bars, or tenter clips may be used. Another means of suppressing the occurrence of wrinkles is to perform a stretching process, similar to the swelling process.

[0036] In the example shown in Figure 1, the film withdrawn from the staining bath 15 passes through the guide roll 36, the nip roll 52, and the guide roll 37 in sequence before being introduced into the crosslinking bath 17a.

[0037] (Crosslinking process) The crosslinking process aims to make the film water-resistant through crosslinking. Referring to Figure 1, the crosslinking process can be carried out by transporting the film along a film transport path constructed by nip roll 52, guide rolls 37-40 and nip roll 53a, immersing the dyed film in the crosslinking bath 17a (crosslinking liquid contained in the crosslinking tank) for a predetermined time, and then withdrawing it.

[0038] As the crosslinking solution, a solution in which the crosslinking agent is dissolved in a solvent can be used. Examples of crosslinking agents include boron compounds, glyoxal, and glutaraldehyde. Examples of boron compounds include boric acid and borax. One type may be used, or two or more types may be used in combination. As the solvent, water may be used, but an organic solvent that is compatible with water may also be included. As the crosslinking solution, for example, an aqueous solution containing 1 to 10 parts by mass of a boron compound such as boric acid per 100 parts by mass of water can be used, and it is preferable to use an aqueous solution containing 1.5 to 3 parts by mass. When the dichroic dye used in the dyeing treatment is iodine, the crosslinking solution preferably contains iodide in addition to boric acid, and the amount of iodide can be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodide include potassium iodide and zinc iodide. Furthermore, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may be present in the same mixture.

[0039] In the crosslinking treatment, the concentrations of boric acid and iodide in the crosslinking solution, and the temperature of the crosslinking bath 17a can be appropriately selected. The crosslinking solution can be, for example, an aqueous solution with a mass ratio of boric acid / potassium iodide / water = 1-10 / 1-30 / 100. The temperature of the crosslinking bath 17a when immersing the film is usually 50-70°C, preferably 53-65°C, and the immersion time of the film is usually 10-600 seconds, preferably 20-300 seconds, more preferably 20-200 seconds. Furthermore, when dyeing and crosslinking treatments are performed in this order on a polyvinyl alcohol-based resin film that has been stretched before the swelling treatment, the temperature of the crosslinking bath 17a is usually around 50-85°C, preferably 55-80°C.

[0040] (complementary color process) The complementary color process has the objective of adjusting the hue. Referring to Figure 1, the complementary color process can be carried out by transporting the film along a film transport path constructed by nip roll 53a, guide rolls 41-44 and nip roll 53b, immersing the film after the crosslinking process in the complementary color bath 17b (complementary color solution contained in the complementary color tank) for a predetermined time, and then withdrawing it.

[0041] As a color-correcting solution, a solution in which a crosslinking agent is dissolved in a solvent can be used, similar to the crosslinking solution. Examples of crosslinking agents include boron compounds, glyoxal, and glutaraldehyde. Examples of boron compounds include boric acid and borax. One type may be used, or two or more types may be used in combination. As a solvent, water may be used, but an organic solvent that is compatible with water may also be included. As a color-correcting solution, for example, an aqueous solution containing 1 to 10 parts by mass of a boron compound such as boric acid per 100 parts by mass of water can be used, and it is preferable to use an aqueous solution containing 1.5 to 3 parts by mass. When the dichroic dye used in the dyeing process is iodine, it is preferable that the color-correcting solution contains iodide in addition to boric acid, and the amount of iodide can be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodide include potassium iodide and zinc iodide. Furthermore, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may be present in the same mixture.

[0042] In the color correction treatment, the concentrations of boric acid and iodide in the color correction solution, and the temperature of the color correction bath 17b can be appropriately selected. For example, the color correction solution can be a boric acid / potassium iodide / water mixture in a mass ratio of 1-5 / 3-30 / 100. The temperature of the color correction bath 17b when immersing the film is usually 10-55°C, preferably 40-50°C, and the immersion time of the film is usually 1-300 seconds, preferably 2-100 seconds.

[0043] The crosslinking and color-correcting treatments may be performed multiple times, usually 2 to 5 times. In this case, the composition and temperature of each crosslinking bath and color-correcting bath used may be the same or different.

[0044] The difference in peripheral speed between the nip roll 52 and the nip roll 53a can be used to perform uniaxial stretching in the crosslinking bath 17a. Alternatively, the difference in peripheral speed between the nip roll 53a and the nip roll 53b can be used to perform uniaxial stretching in the complementary color bath 17b.

[0045] In crosslinking and color correction treatments, as with swelling treatments, guide rolls 38, 39, 40, 41, 42, 43 and / or 44 can be used to transport the polyvinyl alcohol-based resin film while removing wrinkles, by using rolls with a widening function such as expander rolls, spiral rolls, or crown rolls, or by using other widening devices such as cross guiders, bend bars, or tenter clips. Another means of suppressing wrinkle formation is to perform a stretching treatment, similar to the swelling treatment.

[0046] In the example shown in Figure 1, the film drawn out of the complementary color bath 17b passes through the guide roll 44 and the nip roll 53b in order before being introduced into the washing bath 19.

[0047] (Washing process) In the example shown in Figure 1, a cleaning step is included after the crosslinking process. The cleaning process is performed to remove excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol resin film. The cleaning process is carried out, for example, by immersing the crosslinked polyvinyl alcohol resin film in a cleaning bath 19. Alternatively, the cleaning process can be performed by spraying the cleaning solution onto the film as a shower, or by combining immersion in the cleaning bath 19 and spraying the cleaning solution.

[0048] Figure 1 shows an example of a cleaning process in which a polyvinyl alcohol-based resin film is immersed 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 2 to 120 seconds.

[0049] Furthermore, in the washing process, in order to convey the polyvinyl alcohol-based resin film while removing wrinkles, guide rolls 45, 46, 47 and / or 48 may be used, which have a widening function such as expander rolls, spiral rolls, or crown rolls, or other widening devices such as cross guiders, bend bars, or tenter clips may be used. In addition, stretching treatment may be performed in the film washing process to suppress the occurrence of wrinkles.

[0050] (Stretching process) As described above, the raw film 10 is subjected to wet or dry uniaxial stretching during the 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 of uniaxial stretching include, for example, inter-roll stretching, which involves performing longitudinal uniaxial stretching with a peripheral speed difference between two nip rolls constituting the film transport path (e.g., two nip rolls positioned before and after the processing bath), hot roll stretching as described in Japanese Patent Publication No. 2731813, tenter stretching, etc., with inter-roll stretching being preferred. The uniaxial stretching process can be carried out multiple times from the raw film 10 to the polarizing film 23. As described above, the stretching process is also advantageous in suppressing the occurrence of wrinkles in the film.

[0051] The final cumulative stretch ratio of the polarizing film 23, relative to the original film roll 10, is typically around 4.5 to 7 times, preferably 5 to 6.5 times. The stretching process may be performed in any of the processing steps, and even when stretching is performed in two or more processing steps, the stretching process may be performed in any of the processing steps.

[0052] (Ultrasonic treatment process) In the apparatus shown in Figure 1, the ultrasonic treatment process is performed within the complementary color bath 17b. That is, the complementary color bath 17b also serves as the ultrasonic treatment bath. In the apparatus shown in Figure 1, an ultrasonic transducer 71 is placed within the complementary color bath (ultrasonic treatment bath) 17b, and ultrasonic waves emitted from the ultrasonic transducer 71 propagate within the complementary color bath 17b, irradiating the film immersed in the complementary color bath 17b with ultrasonic waves from the ultrasonic transducer 71. In the apparatus shown in Figure 1, the ultrasonic transducer 71 is positioned on the inlet side of the complementary color bath 17b, and the angle between the film transport direction (parallel to the bottom surface of the complementary color bath 17b) and the vibration plane of the ultrasonic transducer 71 is 90°, meaning the angle between the film transport direction and the propagation direction of the ultrasonic waves emitted from the ultrasonic transducer 71 is 0°. The ultrasonic transducer 71 is connected to an ultrasonic oscillator (not shown), and is configured to irradiate ultrasonic waves using the output from the ultrasonic oscillator.

[0053] The ultrasonic treatment process is not limited to being performed in the complementary color bath 17b together with the complementary color process, as long as it is carried out in a bath containing potassium iodide and a boron compound, as shown in Figure 1. If the crosslinking solution contains potassium iodide and a boron compound, it may also be carried out in the crosslinking bath 17a containing the crosslinking solution together with the crosslinking process. Furthermore, the ultrasonic treatment process may be performed separately from the complementary color process or the crosslinking process.

[0054] The frequency of the ultrasound emitted from the ultrasonic transducer 71 is not particularly limited and may be in the range of 10 kHz to 300 kHz, preferably in the range of 20 kHz to 300 kHz. The output of the ultrasonic oscillator to which the ultrasonic transducer 71 is connected is not particularly limited and may be in the range of 0.6 kW to 10 kW, for example. The output of the ultrasonic oscillator can be appropriately selected according to the size of the ultrasonic treatment bath, etc.

[0055] The angle between the film transport direction and the propagation direction of the ultrasonic waves emitted from the ultrasonic transducer (parallel to the bottom surface of the complementary color bath 17b) is not limited to 0° as shown in Figure 1, but is preferably between 0° and 90°. Furthermore, the angle between the film transport direction and the vibration plane of the ultrasonic transducer is not limited to 90° as shown in Figure 1, but is preferably between 0° and 90°.

[0056] The position of the ultrasonic transducer within the ultrasonic treatment bath is not limited to the inlet side as shown in Figure 1, but may also be on the bottom or outlet side. Figure 2 schematically shows the case where the ultrasonic transducer 71 is positioned on the bottom side within the ultrasonic treatment bath. Figure 3 schematically shows the case where the ultrasonic transducer 71 is positioned on the outlet side within the ultrasonic treatment bath. From the viewpoint of enabling the film to efficiently receive ultrasound and increasing the area over which the film receives ultrasound, it is preferable to position the ultrasonic transducer on the inlet side or the bottom side within the ultrasonic treatment bath.

[0057] The ultrasonic treatment process is not limited to a single step, but may be performed multiple times or in multiple baths. Examples of boron compounds included in the ultrasonic treatment bath include boric acid and borax. For example, the ultrasonic treatment bath can be an aqueous solution containing 1 to 10 parts by mass of a boron compound per 100 parts by mass of water, and it is preferable to use an aqueous solution containing 1.5 to 3 parts by mass of a boron compound.

[0058] The concentration and temperature of the ultrasonic treatment bath can be selected as appropriate. For example, a mass ratio of boric acid / potassium iodide / water = 1-5 / 3-30 / 100 can be used for the ultrasonic treatment bath. The temperature of the ultrasonic treatment bath in the ultrasonic treatment process is usually 10-55°C, preferably 40-50°C, and the immersion time of the film in the ultrasonic treatment bath is usually 1-300 seconds, preferably 2-100 seconds.

[0059] (drying process) After the washing process, it is preferable to dry the polyvinyl alcohol-based resin film. The drying of the film is not particularly limited, but it can be done using a drying oven 21 as shown in the example in Figure 1. The drying oven 21 can be equipped with, for example, a hot air dryer. The drying temperature is, for example, 30 to 100°C, and the drying time is, for example, 30 to 600 seconds. The drying of the polyvinyl alcohol-based resin film can also be done using a far-infrared heater. The thickness of the polarizing film 23 obtained in this way is, for example, about 5 to 30 μm.

[0060] The obtained polarizing film may be sequentially wound onto a winding roll to form a roll, or it may be used directly in the polarizing plate manufacturing process (a process in which a protective film or the like is laminated onto one or both sides of the polarizing film) without being wound.

[0061] (Other processing steps for polyvinyl alcohol-based resin films) Other treatments besides those described above can also be added. Examples of additional treatments include immersion in an iodide aqueous solution that does not contain boric acid, and immersion in an aqueous solution that does not contain boric acid but contains zinc chloride, etc. (zinc treatment).

[0062] <Polarizing film> By producing a polarizing film using the above method, it is possible to obtain a polarizing film that exhibits minimal shrinkage due to temperature changes and has optical properties that simultaneously satisfy the following i) to iii). i) The luminous sensitivity correction single-unit transmittance (Ty) is 42.0% or higher. ii) The degree of polarization corrected for visual sensitivity (Py) is 99.980% or higher. iii) The absolute value of the b-value of the orthogonal hue is 0.6 or less. The luminous efficiency-corrected single-unit transmittance (Ty), luminous efficiency-corrected polarization degree (Py), and the b-value of the orthogonal hue are measured according to the description in the "Actual Practices" section below.

[0063] The obtained polarizing film may be sequentially wound onto a winding roll to form a roll, or it may be used directly in the polarizing plate manufacturing process (a process in which a protective film or the like is laminated onto one or both sides of the polarizing film) without being wound.

[0064] <Polarizing plate> A polarizing plate can be obtained by laminating a protective film to at least one side of the polarizing film manufactured as described above via an adhesive. Examples of protective films include films made of acetylcellulose resins such as triacetylcellulose and diacetylcellulose; films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; polycarbonate resin films, cycloolefin resin films; acrylic resin films; and films made of chain-like olefin resins such as polypropylene resins.

[0065] 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 application, and saponification may 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 active energy ray curable adhesives such as ultraviolet curable adhesives, aqueous solutions of polyvinyl alcohol resins or aqueous solutions thereof containing a crosslinking agent, and water-based adhesives such as urethane emulsion adhesives. The ultraviolet curable adhesive can be a mixture of an acrylic compound and a photoradical polymerization initiator, or a mixture of an epoxy compound and a photocationic polymerization initiator. Alternatively, a cationic epoxy compound and a radical polymerizable acrylic compound can be used in combination, and a photocationic polymerization initiator and a photoradical polymerization initiator can be used in combination as initiators. [Examples]

[0066] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0067] <Example 1> Using the manufacturing apparatus shown in Figure 1 (which differs from Figure 1 in that the ultrasonic transducer in the complementary color bath is positioned at the bottom as shown in Figure 2, and it has two crosslinking baths), the polarizing film of Example 1 was manufactured from a polyvinyl alcohol-based resin film. Specifically, a long roll of polyvinyl alcohol (PVA) raw material film with a thickness of 45 μm [product name "VF-TS#4500" manufactured by Kuraray Co., Ltd., average degree of polymerization 2400, degree of saponification 99.9 mol% or more] was continuously conveyed while being unwound from a roll and immersed in a swelling bath 13 consisting of pure water at 25°C for a residence time of 1 minute and 20 seconds (swelling process).

[0068] Subsequently, the film removed from the swelling bath 13 was immersed in a 30°C staining bath 15 containing 1.25 mmML of iodine, 1.25% by mass of potassium iodide, and 0.3% by mass of boric acid for a residence time of 2 minutes and 30 seconds (staining process). At this time, the film was stretched at a stretching ratio of 2.15 times and 1.56 times in the swelling process and the staining process, respectively, until the cumulative stretching ratio by the end of the staining process was 3.5 times. Next, the film removed from the staining bath 15 was immersed in a 59°C first crosslinking bath containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 26 seconds, and stretched at a stretching ratio of 1.4 times while crosslinking (first crosslinking process). Subsequently, the film was immersed in a 59°C second crosslinking bath containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 20 seconds, and stretched at a stretching ratio of 1.19 times while crosslinking (second crosslinking process).

[0069] Next, the film was immersed in a complementary color bath 17b at 43°C containing 8% by mass of potassium iodide and 4% by mass of boric acid for a residence time of 10 seconds and stretched to 1.00 times its original size (complementary color step). In the complementary color step, ultrasonic waves of 20 kHz or higher were generated from an ultrasonic transducer 71 positioned at the bottom of the complementary color bath 17b (where the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves is 90°) (ultrasonic treatment step). After the swelling step, dyeing step, first crosslinking step, second crosslinking step, and complementary color step, the film was stretched to a total stretching ratio of 5.7 times that of the original film.

[0070] The film removed from the complementary color bath 17b was immersed in a washing bath 19 consisting of pure water at 13°C for a residence time of 2 seconds (washing step). Subsequently, the film removed from the washing bath 19 was dried in a drying oven 21 at a temperature of 100°C for 110 seconds to obtain a polarizing film. The thickness of the obtained polarizing film was 18 μm.

[0071] An ultrasonic transducer 71 of a multi-frequency ultrasonic device (manufactured by Korea Ultrasonics Co., Ltd., operating frequency 164kHz, output 900W) was placed inside the complementary color bath 17b.

[0072] <Example 2> A polarizing film was manufactured in the same manner as in Example 1, except that the ultrasonic transducer 71 in the complementary color bath 17b was positioned on the entrance side (where the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves is 0°), as shown in Figure 1.

[0073] <Example 3> A polarizing film was manufactured in the same manner as in Example 1, except that the ultrasonic transducer 71 in the complementary color bath 17b was positioned on the outlet side as shown in Figure 3 (the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves was 180°).

[0074] <Example 4> A polarizing film was manufactured in the same manner as in Example 1, except that the ultrasonic treatment process was performed in the first crosslinking process, rather than in the complementary color process, with the ultrasonic transducer placed on the outlet side of the first crosslinking bath.

[0075] <Example 5> A polarizing film was manufactured in the same manner as in Example 1, except that the ultrasonic treatment process was performed during the dyeing process, rather than the complementary color process, and the ultrasonic transducer was placed on the side of the outlet of the dyeing bath.

[0076] <Comparative Example 1> A polarizing film was manufactured in the same manner as in Example 1, except that the ultrasonic treatment process was omitted.

[0077] [Evaluation of polarizing films] (a) Measurement of the b-value of the single-element transmittance, polarization degree, and orthogonal hue. For each example and comparative example, a 4cm x 4cm sample was cut from the polarizing film obtained. The MD transmittance and TD transmittance of this sample were measured in the wavelength range of 380-780nm using a spectrophotometer with an integrating sphere (JASCO Corporation's "V7100"), and the following formula was used: Single-unit transmittance (%) = (MD + TD) / 2 Degree of polarization (%)={(MD-TD) / (MD+TD)}×100 Based on this, the transmittance and polarization degree at each wavelength were calculated.

[0078] "MD transmittance" is the transmittance when the direction of polarization emitted from the Grant-Thomson prism is parallel to the transmission axis of the polarizing film sample, and is represented as "MD" in the above formula. "TD transmittance" is the transmittance when the direction of polarization emitted from the Grant-Thomson prism is perpendicular to the transmission axis of the polarizing film sample, and is represented as "TD" in the above formula. The obtained individual transmittance and degree of polarization are as follows: JIS Z 8701:1999 "Method of indicating color - XYZ color system and X 10 Y 10 Z 10 The luminous efficiency was corrected using a 2-degree field of view (C light source) of the "color system," and the luminous efficiency corrected single-lens transmittance (Ty), luminous efficiency corrected polarization degree (Py), and the b value of the orthogonal hue were determined. Table 1 shows the calculation results for the luminous efficiency corrected single-lens transmittance (Ty), luminous efficiency corrected polarization degree (Py), and the b value of the orthogonal hue.

[0079] (b) MD contraction force From the polarizing films obtained in each example and comparative example, a measurement sample measuring 2 mm in width and 30 mm in length was cut out, with the absorption axis direction (MD, stretching direction) as the longer side. This sample was set in a TA Corporation Dynamic Mechanical Analyzer and held at 80°C for 4 hours while maintaining constant dimensions. The shrinkage force in the direction of the longer side (absorption axis direction, MD) (MD shrinkage force) was measured. Table 1 shows the measured shrinkage force values.

[0080] (c) Degree of crosslinking For each example and comparative example, a 10cm x 10cm sample was cut from the center of the polarizing film obtained. The degree of crosslinking of this sample was measured using a Thermo Fisher Scientific Nicolet 5700 (FT-IR) instrument.

[0081] The FT-IR chip used was Pike Technologies' VeeMAX III (ATR), with 16 scans and a resolution of 4 cm. -1 The measurement was performed at 2850-3000 cm². -1 After setting the area of ​​the region (a) as the reference peak area (a) (measured value: 3.2), the range is 1200~1360 cm². -1 Divide the area by the standard peak area (a).

[0082] Crosslinking degree=(1200~1360cm -1 (Area of) / (Base Peak Area (a)) The above method for measuring the degree of crosslinking was performed three times, and the average value was calculated. Table 1 shows the average degree of crosslinking.

[0083] [Table 1]

[0084] <Example 6> A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol (PVA) raw material film was changed to a thickness of 60 μm (product name VF-PE#6000, manufactured by Kuraray Co., Ltd.), the ultrasonic transducer was positioned on the inlet side as shown in Figure 1 (the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves was 0°), and the ultrasonic frequency used was 35 kHz. The thickness of the obtained polarizing film was 23 μm.

[0085] <Example 7> A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol (PVA) raw material film was changed to a thickness of 60 μm (product name VF-PE#6000, manufactured by Kuraray Co., Ltd.), and the position of the ultrasonic transducer was set to the inlet side as shown in Figure 1 (the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves was 0°). The thickness of the obtained polarizing film was 23 μm.

[0086] <Example 8> A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol (PVA) raw material film was changed to a thickness of 20 μm (product name VF-TS#2000, manufactured by Kuraray Co., Ltd.), the ultrasonic transducer was positioned on the inlet side as shown in Figure 1 (the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves was 0°), and the ultrasonic frequency used was 35 kHz. The thickness of the obtained polarizing film was 8 μm.

[0087] <Example 9> A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol (PVA) raw material film was changed to a thickness of 20 μm (product name VF-TS#2000, manufactured by Kuraray Co., Ltd.), and the position of the ultrasonic transducer was set to the inlet side as shown in Figure 1 (the angle between the transport direction of the polyvinyl alcohol resin film and the propagation direction of the ultrasonic waves was 0°). The thickness of the obtained polarizing film was 8 μm.

[0088] The polarizing films produced in Examples 6 to 9 were evaluated in the same manner as described above, and the results are shown in Table 2.

[0089] [Table 2] [Explanation of Symbols]

[0090] 10. Raw material film made of polyvinyl alcohol resin, 11. Raw material roll, 13. Swelling bath, 15. Dyeing bath, 17a. Crosslinking bath, 17b. Complementary color bath, 19. Washing bath, 21. Drying oven, 23. Polarizing film, 30-48, 60, 61. Guide rolls, 50-52, 53a, 53b, 54, 55. Nip rolls, 71. Ultrasonic transducer.

Claims

1. A method for producing a polarizing film from a polyvinyl alcohol-based resin film, The polyvinyl alcohol-based resin film is subjected to a swelling bath, a dyeing bath, a crosslinking bath, a color-correcting bath, and a washing bath in that order. The process includes an ultrasonic treatment step in which the polyvinyl alcohol-based resin film is immersed in an ultrasonic treatment bath in which ultrasonic waves are propagated. A method for producing a polarizing film, wherein the ultrasonic treatment bath is the complementary color bath containing potassium iodide and a boron compound.

2. The method for manufacturing a polarizing film according to claim 1, wherein in the ultrasonic treatment step, the temperature of the ultrasonic treatment bath is 40°C or higher and 50°C or lower.

3. A method for manufacturing a polarizing film according to claim 1 or 2, wherein in the ultrasonic treatment step, the polyvinyl alcohol-based resin film is transported in the ultrasonic treatment bath, and the angle between the transport direction of the polyvinyl alcohol-based resin film in the ultrasonic treatment bath and the propagation direction of the ultrasonic waves is 0° or more and 90° or less.

4. The process further includes a dyeing step of dyeing the polyvinyl alcohol-based resin film with the dyeing bath containing a dichroic dye, The method for manufacturing a polarizing film according to any one of claims 1 to 3, wherein the ultrasonic treatment step is performed after the dyeing step.

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