TWI892995B - Method and equipment for manufacturing polarizing film - Google Patents

Abstract

An object of the present invention is to provide a novel method for manufacturing polarizing film which is capable of reducing variation in optical characteristics in width direction.

A method for manufacturing polarizing film from a polyvinyl alcohol-based resin film comprises a plurality of treatment steps for bringing a polyvinyl alcohol-based resin film into contact with a treatment liquid, wherein at least one of the plurality of treatment steps is a multi-stage treatment step which includes a uniform contact step of bringing the polyvinyl alcohol-based resin film into contact with the treatment liquid in a uniform amount over the entire width, and an adjusting contact step of bringing the polyvinyl alcohol-based resin film into contact with the treatment liquid in an amount adjusted according to the position in the width direction of the entire width; the treatment liquid used in the multi-step treatment step contains at least one of a dichroic dye and a boron compound.

Description

Polarizing film manufacturing method and manufacturing device

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

Polarizing plates are widely used as polarizing elements in image display devices such as liquid crystal displays. Polarizing plates are typically constructed by laminating a transparent resin film (such as a protective film) to one or both sides of a polarizing film using an adhesive.

Polarizing films are primarily manufactured by immersing a polyvinyl alcohol-based resin membrane in a dye bath containing a dichroic dye such as iodine, followed by immersion in a crosslinking bath containing a crosslinking agent such as boric acid. Uniaxial stretching is performed at any stage. Uniaxial stretching can be performed dry in air or wet in liquids such as the dye bath and crosslinking bath.

Conventionally, methods for manufacturing polarizing films have required considerable effort to suppress variations in optical properties across the width. Japanese Patent Publication No. 2018-32026 (Patent Document 1) describes a method for suppressing variations in optical properties across the width by irradiating a polyvinyl alcohol-based resin film with electromagnetic waves containing infrared light at a distributed heat intensity across the width.

[Prior Art Literature]

[Patent Literature]

[Patent Document 1] Japanese Patent Publication No. 2018-32026

The object of the present invention is to provide a novel method and apparatus for manufacturing a polarizing film that can reduce variations in optical properties in the width direction.

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

[1] A method for producing a polarizing film, comprising producing a polarizing film from a polyvinyl alcohol resin film,

The manufacturing method comprises a plurality of treatment steps of bringing the aforementioned polyvinyl alcohol-based resin film into contact with a treatment liquid, wherein:

At least one of the plurality of treatment steps is a multi-stage treatment step, comprising a uniform contact step for contacting the polyvinyl alcohol-based resin film with a uniform amount of treatment liquid across its entire width, and an adjusted contact step for contacting the polyvinyl alcohol-based resin film with an amount of treatment liquid adjusted in widthwise position according to the entire width.

The treatment solution used in the multi-stage treatment step contains at least one of a dichroic pigment and a boron compound.

[2] The method for producing a polarizing film as described in [1], wherein, in the multi-stage treatment step, the treatment liquid is a dyeing liquid, a crosslinking liquid, or a toner.

[3] The method for producing a polarizing film according to [1] or [2], wherein in the contact adjustment step, the polyvinyl alcohol-based resin film is divided into a central portion and both end portions across its entire width, and the amount of the treatment liquid in contact with the central portion is adjusted to be greater than the amount of the treatment liquid in contact with the both end portions.

[4] The method for producing a polarizing film as described in [3], wherein, in the contact adjustment step, the central portion of the polyvinyl alcohol-based resin film occupies 5 to 80% of the total width.

[5] The method for producing a polarizing film as described in [3] or [4], wherein in the contact adjustment step, the central portion of the polyvinyl alcohol-based resin film is adjusted so as to contact the treatment liquid, while the end portions are not in contact with the treatment liquid.

[6] The method for producing a polarizing film as described in any one of [1] to [5], wherein the uniform contact step is a step of immersing the polyvinyl alcohol-based resin film in the treatment liquid contained in a treatment tank.

[7] The method for producing a polarizing film as described in any one of [1] to [6], wherein the contact adjustment step is a step of rinsing the polyvinyl alcohol-based resin film with a spray of the treatment liquid.

[8] The method for producing a polarizing film according to any one of [1] to [7], wherein the total width of the polyvinyl alcohol-based resin film is not less than 400 mm and not more than 8000 mm.

[9] A polarizing film manufacturing device, which is a polarizing film manufacturing device for manufacturing a polarizing film from a polyvinyl alcohol resin film.

The manufacturing apparatus is equipped with a plurality of processing sections for bringing the polyvinyl alcohol-based resin film into contact with a processing liquid, wherein:

At least one of the plurality of treatment sections is a multi-stage treatment section, comprising a uniform contact section for contacting the polyvinyl alcohol-based resin film with a uniform amount of treatment liquid across its entire width, and an adjustment contact section for contacting the polyvinyl alcohol-based resin film with the treatment liquid in an amount adjusted in the widthwise direction according to the entire width.

The processing liquid used in the multi-stage processing section contains at least one of a dichroic dye and a boron compound.

According to the present invention, a method and apparatus for manufacturing a polarizing film can be provided that reduces the variation in optical properties of the polarizing film in its width direction.

10: Embryonic membrane, embryonic membrane, membrane composed of polyvinyl alcohol resin

11: Raw material reels

13: Swelling bath

15:Dyeing bath

16: Spray device

16a, 16b: Spray stick

17a: Crosslinking bath

17b: Toning Bath

19: Cleansing Bath

21: Drying furnace

23:Polarizing film

30 to 35, 37 to 48, 60, 61: Guide rollers

50 to 52, 53a, 53b, 54, 55, 56: Clamping rollers

161,161a,161b: Shower nozzle

Figure 1 is a cross-sectional view schematically showing an example of the polarizing film manufacturing method of the present invention and the polarizing film manufacturing apparatus used therein.

Figure 2 is a top view of the dyeing section of the manufacturing apparatus shown in Figure 1.

Figure 3 is a cross-sectional view illustrating a method for adjusting the spray amount of dye solution from a spray device.

Figure 4 is a graph plotting the luminescence factor-corrected polarization degree Py for Examples 1 and 2 and Comparative Example 1.

Figure 5 is a graph plotting the luminescence factor-corrected polarization degree Py for Examples 3 and 4 and Comparative Example 2.

<Polarizing Film Manufacturing Method>

The present invention relates to a method for producing a polarizing film from a polyvinyl alcohol-based resin film. The method comprises a plurality of treatment steps in which the polyvinyl alcohol-based resin film is contacted with a treatment liquid. At least one of the treatment steps is a multi-stage treatment step, comprising a uniform contact step in which a uniform amount of treatment liquid is contacted across the entire width of the polyvinyl alcohol-based resin film, and an adjusted contact step in which the polyvinyl alcohol-based resin film is contacted with an amount of the treatment liquid adjusted in the widthwise direction according to the entire width. The treatment liquid used in the multi-stage treatment step contains at least one of a dichroic dye and a boron compound, such as a dyeing solution, a crosslinking solution, or a color-correcting solution.

In the present invention, the polarizing film is a film in which a dichroic pigment (iodine, dichroic dye, etc.) is adsorbed and aligned on a uniaxially stretched polyvinyl alcohol resin film. The polyvinyl alcohol resin constituting the polyvinyl alcohol resin film is usually obtained by saponifying a polyvinyl acetate resin. The degree of saponification is usually above about 85 mol%, preferably above about 90 mol%, and more preferably above about 99 mol%. The polyvinyl acetate resin may be, for example, polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate and other monomers copolymerizable with the vinyl acetate. Examples of other copolymerizable monomers include unsaturated carboxylic acids, alkenes, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of polyvinyl alcohol-based resins is generally about 1,000 to 10,000, preferably about 1,500 to 5,000.

These polyvinyl alcohol-based resins may be modified. For example, aldehyde-modified polyvinyl formaldehyde, polyvinyl acetal, and polyvinyl butyral may also be used.

In the present invention, the starting material for producing a polarizing film is an unstretched polyvinyl alcohol-based resin film (embryo) 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. This allows for the production of a polarizing film of the type for which market demand is increasing. The width of the embryo is not particularly limited, and can be, for example, from 400 mm to 8000 mm, preferably from 2000 mm to 5500 mm. In particular, when the embryo width is greater than 2000 mm, the polarizing film's properties in the width direction tend to vary. However, the production method of the present invention can reduce this variation. The embryo is, for example, used as a roll (stock roll) of long, unstretched polyvinyl alcohol-based resin film.

Furthermore, the polyvinyl alcohol-based resin film used in the present invention may be laminated onto a substrate film that supports the polyvinyl alcohol-based resin film. In other words, the polyvinyl alcohol-based resin film may serve as a laminated film comprising a substrate film and a polyvinyl alcohol-based resin film laminated onto the substrate film. In this case, the polyvinyl alcohol-based resin film can be produced, for example, by applying a coating liquid containing a polyvinyl alcohol-based resin to at least one surface of a substrate film and then drying the coating liquid.

The substrate film may be, for example, a film made of a thermoplastic resin. Specific examples include films made of light-transmitting thermoplastic resins (preferably optically transparent thermoplastic resins), such as: polyolefin resins such as chain polyolefin resins (polypropylene resins, etc.), cyclic polyolefin resins (norbornene resins, etc.); cellulose resins such as cellulose triacetate and cellulose diacetate; polyester resins such as polyethylene terephthalate and polybutylene terephthalate; polycarbonate resins; and Methyl methacrylate resins, (meth)acrylate resins; polystyrene resins; polyvinyl chloride resins; acrylonitrile/butadiene/styrene resins; acrylonitrile/styrene resins; polyvinyl acetate resins; polyvinylidene chloride resins; polyamide resins; polyacetal resins; modified polyphenylene ether resins; polysulfone resins; polyethersulfone resins; polyarylate resins; polyamide imide resins; polyimide resins, etc.

By rolling the long strip of film from a raw material reel and continuously conveying it along a film conveying path of a polarizing film manufacturing apparatus, immersing it in a treatment liquid (hereinafter referred to as a "treatment bath") contained in a treatment tank, and then pulling it out through a predetermined treatment step, followed by a drying step, a polarizing film can be continuously manufactured into a long strip of polarizing film. Furthermore, the treatment step can be any method that treats the film by contacting it with the treatment liquid. The method of immersing the film in the treatment bath is not limited; the film surface may also be rinsed with the treatment liquid. For rinsing the film surface with the treatment liquid, a method of spraying the treatment liquid onto the film surface is suitable. Here, the term "spraying" of the treatment liquid refers to a stream of the treatment liquid ejected from a plurality of nozzles. The stream may contain the treatment liquid intermittently (e.g., in the form of particles or a mist) or continuously. A spraying device is a device having a plurality of nozzles through which liquid is sprayed to produce a liquid stream. A spraying device is typically used to rinse the membrane surface with the spray of the treatment liquid. When the treatment step is performed by immersing the membrane in a treatment bath, the treatment bath is not limited to a single bath; the membrane may be immersed in two or more treatment baths sequentially to complete the treatment step.

The treatment liquids include swelling liquids, staining liquids, crosslinking liquids, color-correcting liquids, and washing liquids. Furthermore, the treatment steps include a swelling step of contacting the embryonic membrane with a swelling liquid to perform a swelling treatment, a staining step of contacting the membrane after the swelling treatment with a staining liquid to perform a staining treatment, a crosslinking step of contacting the membrane after the staining treatment with a crosslinking liquid to perform a crosslinking treatment, a color-correcting step of contacting the crosslinked membrane with a color-correcting liquid to perform a color-correcting treatment, and a washing step of contacting the membrane after the color-correcting treatment with a washing liquid to perform a washing treatment. Furthermore, between the series of treatment steps (i.e., before or after any one or more treatment steps and/or during any one or more treatment steps), uniaxial elongation treatment is performed using a wet or dry method. Other treatment steps may also be added as needed.

The present inventors have completed the present invention based on the following knowledge: From the perspective of suppressing variations in optical properties in the widthwise direction, a multi-stage treatment step is preferred in a treatment step using a treatment solution containing at least one of a dichroic dye and a boron compound. This multi-stage treatment step includes a uniform contact step for uniformly contacting the film with the treatment solution across its entire width, and an adjustment contact step for contacting the film with an excess amount of treatment solution adjusted in the widthwise direction according to the film's entire width. Examples of treatment steps using a treatment solution containing at least one of a dichroic dye and a boron compound include a dyeing step, a crosslinking step, and a color-correction step.

An example of a method for manufacturing a polarizing film according to the present invention will be described below with reference to Figure 1. Figure 1 is a cross-sectional view schematically showing an example of the method for manufacturing a polarizing film according to the present invention and the polarizing film manufacturing apparatus used therein. The polarizing film manufacturing apparatus shown in FIG1 is constructed as follows: a raw (unstretched) film 10 made of a polyvinyl alcohol-based resin is continuously rolled off a raw material reel 11 while being transported along a film conveying path, and is sequentially passed through a swelling bath (swelling liquid contained in a swelling tank) 13, a dyeing bath (dyeing liquid contained in a dyeing tank) 15, a crosslinking bath (crosslinking liquid contained in a crosslinking tank) 17a, a color-correcting bath (color-correcting liquid contained in a crosslinking tank) 17b, and a cleaning bath (cleaning liquid contained in a cleaning tank) 19 provided on the film conveying path, and finally passes through a drying furnace 21. The resulting polarizing film 23 can be directly transported to a subsequent polarizing plate production step (a step of laminating a protective film to one or both sides of the polarizing film 23). The arrows in Figure 1 indicate the direction of film transport. A spray device 16 is located in the dye bath 15, and the film 10, after passing through the dye bath 15, is then passed through the spray device 16.

In the description of Figure 1, "processing tank" is a collective term encompassing the swelling tank, dyeing tank, crosslinking tank, color-correcting tank, and cleaning tank; "processing liquid" is a collective term encompassing the swelling liquid, dyeing liquid, crosslinking liquid, color-correcting tank liquid, and cleaning liquid; and "processing bath" is a collective term encompassing the swelling bath, dyeing bath, crosslinking bath, color-correcting bath, and cleaning bath. The swelling bath, dyeing bath, spraying device, crosslinking bath, color-correcting bath, and cleaning bath respectively constitute the swelling section, dyeing section, crosslinking section, color-correcting section, and cleaning section of the manufacturing apparatus of the present invention.

In the manufacturing apparatus shown in Figure 1, the dyeing section is configured as follows: after a uniform contact step in a dye bath 15, the dye passes through a spray device 16 to adjust the contact, thereby performing a multi-stage processing step. In other words, the dyeing section comprises a multi-stage processing section comprising a dye bath 15 as a uniform contact section and a spray device 16 as a contact adjustment section.

In addition to the aforementioned treatment baths and spray system, the film transport path of the polarizing film manufacturing apparatus is constructed by appropriately positioning guide rollers 30 to 35, 37 to 48, 60, and 61, which support the film during transport or further redirect its transport direction, and clamp rollers 50 to 56, which press/clamp the film during transport and use their rotation to impart driving force to the film or further redirect its transport direction. The guide rollers and clamp rollers can be positioned before, after, or within each treatment bath, enabling the film to be introduced into, immersed in, and removed from the treatment baths (see Figure 1). For example, by installing one or more guide rollers in each treatment bath and conveying the film along these guide rollers, the film can be immersed in each treatment bath.

The polarizing film manufacturing apparatus shown in Figure 1 features nip rollers (nip rollers 50 to 54, 56) positioned before and after each treatment bath. This allows for inter-roller stretching. Specifically, within one or more treatment baths, a rotational speed difference is created between the nip rollers positioned before and after the bath, enabling longitudinal uniaxial stretching. The following describes each step.

(Swelling step)

The swelling step is performed to remove foreign matter from the surface of the embryonic membrane 10, remove plasticizers from the embryonic membrane 10, impart dyeability, and plasticize the embryonic membrane 10. Treatment conditions are determined to achieve these objectives without causing adverse effects such as extreme dissolution or devitrification of the embryonic membrane 10.

Referring to Figure 1 , the swelling step can be performed by continuously unwinding the embryonic membrane 10 from the raw material reel 11 while conveying it along a membrane conveying path, immersing the embryonic membrane 10 in the swelling bath 13 for a predetermined period of time, and then withdrawing it. In the example of Figure 1 , from the time the embryonic membrane 10 is unwound to the time it is immersed in the swelling bath 13 , the embryonic membrane 10 is conveyed along the membrane conveying path defined by guide rollers 60 and 61 and the nip roller 50 . During the swelling process, the membrane is conveyed along the membrane conveying path defined by guide rollers 30 to 32 and the nip roller 51 .

The swelling liquid in swelling bath 13 may be pure water or an aqueous solution containing approximately 0.01 to 10% by mass of boric acid (Japanese Patent Publication No. 10-153709), chloride (Japanese Patent Publication No. 06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol, or the like.

The temperature of the swelling bath 13 is, for example, approximately 10 to 50°C, preferably approximately 10 to 40°C, and more preferably approximately 15 to 30°C. The immersion time of the embryonic membrane 10 is preferably approximately 10 to 300 seconds, more preferably approximately 20 to 200 seconds. When the embryonic membrane 10 is a polyvinyl alcohol-based resin membrane that has been previously stretched in air, the temperature of the swelling bath 13 is, for example, approximately 20 to 70°C, preferably approximately 30 to 60°C. The immersion time of the embryonic membrane 10 is preferably approximately 30 to 300 seconds, more preferably approximately 60 to 240 seconds.

During the expansion process, the embryonic membrane 10 can easily expand in its width, causing wrinkles. One method for conveying the membrane while eliminating these wrinkles includes using expansion rollers, spiral rollers, or crown rollers with expansion functions on the guide rollers 30, 31, and/or 32, or using other expansion devices such as cloth guides, bending rods, and tenter clamps. Another method for suppressing wrinkles is to implement a stretching process. For example, a uniaxial stretching process can be performed in the expansion bath 13 by utilizing the speed difference between the nip rollers 50 and 51.

During the expansion treatment, the film will also expand in the direction of transport. Therefore, if the film is not actively stretched, it is desirable to eliminate film slack in the transport direction by, for example, controlling the speed of the nip rollers 50 and 51 positioned before and after the expansion bath 13. Furthermore, to stabilize the transport of the film in the expansion bath 13, controlling the water flow in the expansion bath 13 with an underwater spray system or using an EPC system (Edge Position Control: a device that detects the membrane's edges to prevent meandering) are also effective.

In the example shown in Figure 1, the film drawn from the swelling bath 13 passes through the guide roller 32, the clamping roller 51, and the guide roller 33 in sequence and is introduced into the dyeing bath 15.

(Dyeing step)

The dyeing step is performed to allow the swollen polyvinyl alcohol-based resin film to adsorb and align a dichroic dye such as iodine. Treatment conditions are determined to achieve this objective without causing undesirable effects such as extreme dissolution or devitrification of the film. Referring to Figure 1 , the dyeing step is performed by immersing the swollen film in a dye bath 15 (the treatment solution contained in a dye tank) for a predetermined period of time, conveying the film along a film conveying path defined by a nip roller 51, guide rollers 33 to 35, and nip rollers 56 and 52. The film is then drawn out and passed through a spray device 16. To improve the dyeability of dichroic dyes, the film to be dyed is preferably one that has been subjected to at least some degree of uniaxial stretching. Alternatively, uniaxial stretching can be performed during the dyeing process, either instead of or in addition to the uniaxial stretching prior to dyeing.

Iodine is preferably used as the dichroic dye. For example, the dye bath 15 may contain an aqueous solution having a mass ratio of iodine/potassium iodide/water of 0.003 to 0.3/0.1 to 10/100. Other iodides such as zinc iodide may be used in place 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, and cobalt chloride, may also be present. When boric acid is added, the addition of dichroic dye differs from the crosslinking solution described later in that it contains dichroic dye. If the aqueous solution contains at least 0.003 parts by mass of dichroic dye per 100 parts by mass of water, it can be considered a dye solution. The temperature of the dye bath 15 during film immersion is typically between 10 and 45°C, preferably between 10 and 40°C, and more preferably between 20 and 35°C. The film immersion time is typically between 30 and 600 seconds, preferably between 60 and 300 seconds.

A water-soluble dichroic dye can be used together with iodine as a dichroic dye. The dye bath 15 may contain an aqueous solution having a mass ratio of dichroic dye to water of approximately 0.001 to 0.1/100. This dye bath may also contain dyeing auxiliaries, such as inorganic salts such as sodium sulfate or surfactants. A single dichroic dye may be used, or two or more types may be used in combination.

As described above, during the dyeing step, the film can be uniaxially stretched in the dye bath 15. This uniaxial stretching of the film can be achieved by creating a rotational speed difference between the nip rollers 51 and 56 disposed before and after the dye bath 15.

During the dyeing process, similar to the swelling process, to convey the polyvinyl alcohol-based resin film while eliminating wrinkles, rollers with a widening function, such as expansion rollers, spiral rollers, or crown rollers, may be used in conjunction with the guide rollers 33, 34, and/or 35. Alternatively, other widening devices such as cloth guides, bending rods, and tenter clamps may be used. Another method for suppressing wrinkles, similar to the swelling process, is to perform a stretching process.

In the example shown in Figure 1 , the film drawn from the dye bath 15 passes through the spray device 16 and then sequentially through the nip roller 56 , nip roller 52 , and guide roller 37 before being introduced into the crosslinking bath 17a . Thus, in the example shown in Figure 1 , the dyeing step becomes a multi-stage process. Details of the multi-stage process are described below.

(Cross-linking step)

The crosslinking step is a treatment performed to achieve water resistance and other effects through crosslinking. This step can be performed multiple times. Referring to Figure 1 , the dyed membrane is transported along a membrane transport path formed by nip roller 52, guide rollers 37 to 40, and nip roller 53a. The membrane is immersed in a crosslinking bath 17a (a crosslinking solution contained in a crosslinking tank) for a predetermined period of time and then withdrawn.

The crosslinking solution can be a solution of a crosslinking agent dissolved in a solvent. Examples of crosslinking agents include boric acid, borax, and other boron compounds, glyoxal, glutaraldehyde, and the like. These agents may be used singly or in combination. The solvent may be water, for example, but may further contain a water-compatible organic solvent. The concentration of the crosslinking agent in the crosslinking solution is not limited thereto, but is preferably in the range of 1 to 20 mass %, more preferably 4 to 15 mass %.

The crosslinking solution can be an aqueous solution containing, for example, approximately 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When iodine is used as the dichroic pigment in the dyeing process, the crosslinking solution preferably contains an iodide in addition to boric acid, preferably in an amount of 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodides, such as zinc chloride, cobalt chloride, zirconium tetrachloride, sodium thiosulfate, potassium zincate, and sodium sulfate, may also be present.

During the crosslinking treatment, the concentrations of boric acid and iodide, as well as the temperature of crosslinking bath 17, can be appropriately adjusted depending on the intended purpose. For example, an aqueous solution can be used with a mass ratio of boric acid/iodide/water = 3 to 10/1 to 20/100. Other crosslinking agents can be used in place of boric acid, or in combination with other crosslinking agents, as needed. The temperature of crosslinking bath 17a during membrane immersion is typically around 50 to 70°C, preferably 53 to 65°C. The membrane immersion time is typically around 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 200 seconds.

The crosslinking treatment can be performed multiple times, typically 2 to 5 times. The composition and temperature of each crosslinking bath used can be the same or different, as long as they are within the above ranges. Furthermore, a uniaxial stretching treatment can be performed in the crosslinking bath 17a by utilizing the speed difference between the nip rollers 52 and 53a.

During the crosslinking process, similar to the swelling process, to smooth out wrinkles while conveying the polyvinyl alcohol-based resin film, guide rollers 38, 39, 40, 41, 42, 43, and/or 44 may be equipped with widening rollers such as expansion rollers, spiral rollers, or crown rollers, or other widening devices such as cloth guides, bending rods, or tenter clamps. Another method for suppressing wrinkles is to perform a stretching process, similar to the swelling process.

(Coloring step)

For tinting solutions used for hue adjustment, for example, when iodine is used as the dichroic dye in the dyeing solution, a concentration of boric acid/iodide/water (by mass ratio) of 1 to 5/3 to 30/100 can be used. The temperature of the tinting bath 17b during film immersion is typically between 20 and 65°C, and the film immersion time is typically between 1 and 300 seconds, preferably between 2 and 100 seconds.

A uniaxial stretching treatment can also be performed in the crosslinking bath 17a by utilizing the speed difference between the nip rollers 52 and 53a. Furthermore, a uniaxial stretching treatment can also be performed in the color-correcting bath 17b by utilizing the speed difference between the nip rollers 53a and 53b.

In the example shown in Figure 1, the film drawn from the toner bath 17b passes sequentially through the guide roller 44 and the clamping roller 53b and is introduced into the cleaning bath 19.

(Washing step)

The example shown in Figure 1 includes a cleaning step after the crosslinking step. The cleaning process is performed to remove excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step is performed, for example, by immersing the crosslinked polyvinyl alcohol-based resin film in a cleaning bath 19. Alternatively, the cleaning step can be performed by spraying the film with a cleaning solution instead of immersing it in the cleaning bath 19, or by combining immersion in the cleaning bath 19 with rinsing and spraying with the cleaning solution.

Figure 1 shows an example of a polyvinyl alcohol-based resin film being immersed in a cleaning bath 19 for cleaning. The temperature of the cleaning bath 19 during the cleaning process is typically between 2 and 50°C, and the film is typically immersed for between 2 and 120 seconds.

Furthermore, to remove wrinkles while conveying the polyvinyl alcohol-based resin film during the cleaning process, guide rollers 45, 46, 47, and/or 48 may be equipped with widening rollers, such as expansion rollers, spiral rollers, or crown rollers, or other widening devices, such as cloth guides, bending rods, and tenter clamps. Furthermore, a stretching process may be performed during the film cleaning process to suppress wrinkles.

(Extended steps)

As described above, the embryonic membrane 10 is subjected to a uniaxial stretching treatment, either wet or dry, between the aforementioned series of treatment steps (i.e., before or after any one or more treatment steps and/or during any one or more treatment steps). Specific methods for uniaxial stretching include, for example, inter-roller stretching, which creates a rotational speed difference between two nip rollers forming a film transport path (e.g., two nip rollers positioned before and after the treatment bath) to perform longitudinal uniaxial stretching, hot roller stretching as described in Japanese Patent No. 2731813, or tenter frame stretching. Inter-roller stretching is preferred. The uniaxial stretching step can be performed multiple times until the polarizing film 23 is obtained from the embryonic membrane 10. As mentioned above, stretching treatment is also helpful in suppressing membrane wrinkling.

Based on the embryonic membrane 10, the final cumulative stretch ratio of the polarizing film 23 is typically around 4.5 to 7 times, preferably 5 to 6.5 times. The stretching step can be performed at any processing step. If stretching is performed in two or more processing steps, the stretching step can be performed at any processing step.

(Drying step)

After the washing step, the polyvinyl alcohol resin film is preferably dried. While there are no particular limitations on the method for drying the film, it can be performed using a drying oven 21, as shown in Figure 1. The drying oven 21 can be, for example, a hot air dryer. The drying temperature is, for example, approximately 30 to 100°C, and the drying time is, for example, approximately 30 to 600 seconds. The polyvinyl alcohol resin film can also be dried using a far-infrared heater. The resulting polarizing film 23 has a thickness of, for example, approximately 5 to 30 μm.

(Multi-stage processing steps)

In the polarizing film manufacturing apparatus shown in Figure 1, the dyeing step is a multi-stage process, with a uniform contact treatment step performed in a dye bath 15 and a controlled contact step performed in a spray device 16. The dyeing liquid in the dye bath 15 and the dyeing liquid sprayed from the spray device 16 preferably have the same or similar composition. Furthermore, the temperature of the dyeing liquid sprayed from the spray device 16 is preferably adjusted within the temperature range of the dye bath. The dyeing liquid sprayed from the spray device 16 is preferably the same dyeing liquid as that discharged from the dye bath 15 and is preferably recovered from the dye bath 15 after rinsing the film.

The spraying device 16 is configured to rinse the film being treated by adjusting the excess dyeing liquid across its entire width. This multi-stage treatment process, which combines a dyeing step with a uniform contact step and a contact adjustment step, suppresses uneven dyeing across the film's width and reduces variations in the optical properties of the resulting polarizing film.

Figure 2 shows a top view of the dyeing section of the polarizing film manufacturing apparatus shown in Figure 1 , where the dyeing process is performed. As shown in Figure 2 , film 10 is transported in the direction of the arrow by transport guide rollers 33 . After being immersed in a dye bath 15 , film 10 passes through a spray device 16 . Spray device 16 consists of two spray bars 16a and 16b positioned opposite each other on opposite sides of film 10 . Each spray bar 16a and 16b has a plurality of spray nozzles 161 arranged along the width of the film, and the dye solution is sprayed from each nozzle 161 . By adjusting the amount of dye sprayed from each spray nozzle 161, excess dye can be brought into contact with the film 10 in a widthwise positional manner consistent with the film's entire width. After passing through the spray device 16, the film 10 is transferred to the nip roller 56. The nip roller 56 also serves as a liquid remover, removing any dye adhering to the surface of the film 10.

Figure 3 is a cross-sectional view illustrating a method for adjusting the amount of dyeing liquid sprayed from the spray device 16. The method for adjusting the amount of dyeing liquid sprayed from the spray device 16 can be exemplified as follows: the spray nozzles 161 arranged along the width of the film 10 are divided into a spray nozzle 161a corresponding to the center portion of the film 10 and a spray nozzle 161b corresponding to the ends of the film. The spray amount of the spray nozzle 161a corresponding to the center portion of the film 10 and the spray nozzle 161b corresponding to the ends of the film 10 are adjusted.

For example, as shown in FIG3 , the following specific embodiment can be enumerated: In the spraying device 16 , the dye liquid is sprayed only from the spray nozzle 161a corresponding to the center portion of the film 10 , and the dye liquid is not sprayed from the spray nozzles 161b corresponding to the ends of the film 10 .

The central portion of the film preferably accounts for 5 to 80% of the total film width, more preferably 20 to 70%, and even more preferably 40 to 60%. The ratio of the central portion to the total film width can be selected based on the variation in optical properties of the object being improved. The film's end portions are preferably located at the center of the film, with the width of one end being the same as the width of the other end.

In each of the aforementioned treatment steps, the treatment liquid more readily penetrates the end surfaces than the surface of the polyvinyl alcohol resin film. This results in differences in the amount of treatment liquid penetrating near the end surfaces and near the center, leading to differences in the degree of treatment. This is presumed to be one of the factors contributing to variations in the optical properties of the polarizing film. Furthermore, variations in thickness across the width due to stretching are also presumed to contribute to differences in the degree of treatment in each treatment step. Based on these factors, the method of dividing the polyvinyl alcohol resin film along the width direction into the "end/center/end" pattern as described above is not limited. Alternatively, one or more regions between the end surfaces and the center can be separated, and a contact adjustment step can be employed to adjust the amount of dye solution applied to each region.

The order of the uniform contact step and the contact adjustment step in the multi-stage treatment process is not limited. Therefore, the placement of the spray device is not limited to the outlet side of the dye bath 15 as shown in Figure 1; it can be located on the inlet side or both. Furthermore, in a configuration where the spray device is located on the outlet side, the spray device can be located upstream or downstream of the clamping roller 56, which also serves as a liquid removal device. To ensure specific area rinsing, the spray device can be located downstream of the clamping roller 56, which also serves as a liquid removal device. Furthermore, to prevent a clear boundary in optical properties from forming between two areas with different amounts of dye exposure, a contact adjustment step can be performed upstream of the nip roller 56, which acts as a liquid removal device, to remove residual dye from the membrane surface immediately after immersion.

The spray can rinse both sides of the membrane or only one side. When the membrane is transported at a predetermined angle relative to the vertical direction of the lead, that is, when the membrane is transported with both sides of the membrane separated into upper and lower sides relative to the vertical direction of the lead, it is preferable to spray at least the upper surface of the membrane from the perspective of liquid removal and uniformity. The total spray volume per unit area of the membrane within the spray area is, for example, 0.05 to 20 L/ ^m² , preferably 0.1 to 10 L/ ^m² . The total amount of spray used to flush the membrane can be adjusted by adjusting the spray rate of the treatment liquid from the spray device, adjusting the transport speed of the membrane, etc.

As for the liquid removal method for removing the dye liquid from the surface of the film, in addition to the nip roller 56 shown in Figure 1, a method of blowing air onto the film to remove the liquid, a scraper that contacts the film to remove the liquid, etc. can also be used.

At least one of the dyeing, crosslinking, and touch-up steps is preferably a multi-stage treatment step, or all of the dyeing, crosslinking, and touch-up steps may be multi-stage treatment steps. When the crosslinking and/or touch-up steps are multi-stage treatment steps, the configuration of the crosslinking and touch-up sections can be adapted from the configuration of the dyeing section described above. For treatment steps employing a multi-stage treatment step in which the membrane is sequentially immersed in two or more treatment baths, a spray device may be provided in any one of the treatment baths.

Furthermore, the multi-stage treatment step is not limited to the dyeing step, the crosslinking step, and the color-correcting step; the cleaning step may also be a multi-stage treatment step. When the cleaning step is a multi-stage treatment step, it is preferable to adjust the contacting step so that the amount of cleaning solution contacting the ends of the membrane is greater than the amount contacting the center of the membrane.

When multiple processing steps are multi-stage processing steps, the contact adjustment step in each multi-stage processing step can adopt different adjustment methods. For example, the width of the central portion can be a different width in each contact adjustment step.

(Other treatment steps for polyvinyl alcohol-based resin films)

Additional treatments other than those described above may also be performed. Examples of additional treatments include immersion in an aqueous solution containing zinc chloride (zinc treatment) after the crosslinking step that does not contain a boron compound.

<Polarizing film>

The above method can produce a polarizing film with suppressed optical property variation in the width direction. Considering the balance with the luminous factor correction polarization Py, the luminous factor correction monomer transmittance Ty of the resulting polarizing film is preferably 40 to 47%, more preferably 41 to 45%. The luminous factor correction polarization Py is preferably 99.9% or greater, more preferably 99.95% or greater, at any position in the width direction. Furthermore, the difference between the maximum and minimum values of the luminous factor correction polarization Py in the width direction is preferably 0.0015% or less, with a smaller difference being preferred.

The MD and TD transmittances of polarizing films were measured using a spectrophotometer with an integrating sphere (V7100 manufactured by JASCO Corporation) within the wavelength range of 380 to 780 nm. The single-element transmittance and degree of polarization at each wavelength were calculated using the following formula:

Single-body penetration rate (%) = (MD + TD) / 2

Polarization degree (%) = {(MD-TD)/(MD+TD)}×100.

"MD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glen-Thomson prism is parallel to the transmission axis of the polarizing film sample, and is represented by "MD" in the above formula. Furthermore, "TD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glen-Thomson prism is orthogonal to the transmission axis of the polarizing film sample, and is represented by "TD" in the above formula. The obtained single-unit transmittance and polarization degree are subjected to luminescence factor correction at a 2-degree viewing angle (illuminant C) according to JIS Z 8701:1999 "Methods of color display - XYZ colorimetric system and _{X10Y10Z10 colorimetric} system" _to determine the luminescence factor-corrected single-unit transmittance (Ty) and luminescence factor-corrected polarization degree (Py).

The width of the polarizing film of the present invention is, for example, 50 mm to 5000 mm, preferably 500 mm to 4000 mm. The resulting polarizing film can be wound onto a take-up roller to form a roller, or it can be directly used for polarizing plate production (such as laminating a protective film on one or both sides of the polarizing film) without being wound up.

<Polarizing plate>

A polarizing plate can be obtained by laminating a protective film to at least one side of the polarizing film produced in the above manner via an adhesive. Examples of protective films include: films made of cellulose acetate resins such as cellulose triacetate or cellulose diacetate; 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 olefin resins such as polypropylene resins.

To improve the adhesion between the polarizing film and the protective film, the bonding surfaces of the polarizing film and/or protective film can be subjected to surface treatments such as corona treatment, flame treatment, plasma treatment, UV irradiation, primer coating, and saponification. Adhesives used for bonding the polarizing film and protective film include UV-curable adhesives, active energy ray-curable adhesives, aqueous solutions of polyvinyl alcohol-based resins, and aqueous solutions containing crosslinking agents, such as urethane emulsion adhesives. UV-curable adhesives can be mixtures of acrylic compounds and photoradical polymerization initiators, or mixtures of epoxy compounds and photocationic polymerization initiators. Alternatively, a cationic polymerizable epoxy compound and a radically polymerizable acrylic compound may be used in combination, and both a photo-cationic polymerization initiator and a photo-radical polymerization initiator may be used as initiators.

(Example)

The present invention is described in more detail with the following examples, but the present invention is not limited to these examples.

[Manufacturing of polarizing film]

<Example 1>

The polarizing film of Example 1 was produced using the following manufacturing apparatus: From the upstream side, it comprises, in order, a swelling bath, two dyeing baths (a first dyeing bath and a second dyeing bath), two crosslinking baths (a first crosslinking bath and a second crosslinking bath), a toner bath, and a cleaning bath. Sprayers were located at the inlet and outlet of the first dyeing bath, the inlet of the second dyeing bath, and the inlet and outlet of the toner bath. The remaining configuration was identical to that of the manufacturing apparatus shown in Figure 1.

Specifically, a long strip of polyvinyl alcohol (PVA) membrane (Mitsubishi Chemical's "M6000," with a saponification degree of 99.9 mol% or higher) with a width of 4260 mm and a thickness of 60 μm was continuously transported from a roller and immersed in a swelling bath (26°C) of pure water for a retention time of 15 seconds (swelling step). The membrane pulled out of the swelling bath was then immersed in a first dye bath (100 parts by mass of water, 0.028 parts by mass of iodine, 1.3 parts by mass of potassium iodide) at 30°C and a second dye bath (100 parts by mass of water, 0.023 parts by mass of iodine, 1.7 parts by mass of potassium iodide, 0.4 parts by mass of boric acid) at 30°C, containing iodine, so that the total residence time in the first and second dye baths was 58 seconds (dyeing step). The membrane pulled out of the second dye bath was then immersed in a first crosslinking bath (potassium iodide/boric acid/water in a mass ratio of 12/5/100) at 56°C for 20 seconds, and then immersed in a second crosslinking bath (58°C) with the same liquid composition for 38 seconds. Next, the membrane was immersed in a 40°C color-correcting bath containing potassium iodide/boric acid/water (mass ratio: 13/5/100) for a retention time of 17 seconds (color-correcting step). The membrane was then immersed in a 5°C cleansing bath consisting of pure water (cleaning step).

In the first dye bath, a dye solution with the same composition as the first dye bath was sprayed only over the central 2420mm (50% of the total width) of the membrane, relative to a width of 4803mm, using spray devices installed at the inlet and outlet of the first dye bath. In the second dye bath, a dye solution with the same composition as the second dye bath was sprayed only over the central 2310mm (50% of the total width) of the membrane, relative to a width of 4646mm, using a spray device installed at the inlet. In the toning bath, spray devices installed at the inlet and outlet sides rinsed only the central 1210mm of the 2406mm film width (the central portion represented 50% of the total width) with a toning solution of the same composition as the toning bath. Furthermore, during the dyeing, crosslinking, and toning steps, the film was stretched longitudinally uniaxially in the bath between rollers, with a total stretching ratio of 6x based on the embryonic membrane. Following the washing step, drying was initially performed at 48°C and then at 91°C, resulting in a polarizing film with a thickness of 24μm and a width of 1920mm.

<Example 2>

In Example 1, during the step of flushing the treatment solution with the spraying device, only the spraying devices installed at the inlet and outlet sides of the first dye bath were used to spray the dye solution with the same composition as the first dye bath in the central portion (2420 mm) of the film width of 4803 mm (the central portion's ratio to the total width was 50%). The spraying devices installed in the second dye bath and the touch-up bath were not operated. Otherwise, the polarizing film was produced using the same method as in Example 1.

<Comparative Example 1>

In Example 1, the polarizing film was produced using the same method as in Example 1, except that the spraying device was not operated in any of the treatment baths, and the step of spraying the treatment liquid to rinse the film was omitted.

<Measurement of Polarization>

On the resulting 1920 mm wide polarizing films of Examples 1 and 2, and Comparative Example 1, the luminescence factor-corrected polarization (Py) was measured using the above method at seven locations along three straight lines extending in the width direction: 20 mm (position 1), 100 mm (position 2), 500 mm (position 3), 960 mm (position 4), 1420 mm (position 5), 1820 mm (position 6), and 1900 mm (position 7) from the left end. Table 1 shows the average of the luminescence factor-corrected polarization (Py) along the three straight lines at these seven locations (this average is referred to as the luminescence factor-corrected polarization (Py) at each location). Table 1 also shows the calculated difference between the maximum and minimum values of the luminescence factor-corrected polarization at the seven locations. Figure 4 is a graph of the polarization values shown in Table 1.

[Table 1]

As shown in Table 1 and Figure 4, the difference between the maximum and minimum values of the luminous factor-corrected polarization degree in the width direction of the polarizing films of Examples 1 and 2 is smaller than that of Example 1, and the variation of the luminous factor-corrected polarization degree in the width direction is suppressed.

<Example 3>

In Example 1, a polyvinyl alcohol film with a width of 3390 mm was used as the polarizing film.

In the first dye bath, the spray devices installed at the inlet and outlet of the first dye bath respectively sprayed a dye solution with the same composition as that of the first dye bath only in the central 1925mm portion relative to the membrane width of 3859mm (the ratio of the central portion to the total width was 50%). In the second dye bath, the spray device installed at the inlet side sprayed a dye solution with the same composition as that of the second dye bath only in the central 1760mm portion relative to the membrane width of 3751mm (the ratio of the central portion to the total width was 47%). In the toner bath, spraying devices installed at the inlet and outlet sides spray a toner liquid with the same composition as the toner bath, only in the central 990mm of the membrane width (the central area accounts for 50% of the total width) relative to the membrane width of 1962mm.

During the dyeing, crosslinking, and color-correcting steps, longitudinal uniaxial stretching was performed in a bath between rollers, with the total stretching ratio set to 6x based on the embryonic membrane. After the washing step, drying was initially performed at 46°C and then at 92°C, resulting in a polarizing film with a thickness of 24 μm and a width of 1460 mm. Aside from the above, the polarizing film was produced using the same manufacturing method as in Example 1.

<Example 4>

In Example 3, the step of spraying the treatment solution with the spraying device was performed by spraying only the central 1925 mm portion of the film width (3859 mm) with the spraying device installed at the inlet and outlet of the first dye bath (the central portion accounts for 50% of the total width). The spraying device installed in the second dye bath and the touch-up bath was not operated. Otherwise, the polarizing film was produced using the same method as in Example 3.

<Comparative example 2>

In Example 3, a polarizing film was produced in the same manner as in Example 1, except that the step of rinsing the treatment solution with a spraying device was not performed in any of the treatment baths.

<Measurement of Polarization>

In the resulting 1460 mm wide polarizing films of Examples 3 and 4 and Comparative Example 2, the luminescence factor-corrected polarization (Py) was calculated using the above method at five locations along four straight lines extending in the width direction: 20 mm (position 1), 100 mm (position 2), 730 mm (position 3), 1360 mm (position 4), and 1440 mm (position 5) from the left end. Table 2 shows the average value of the luminescence factor-corrected polarization (Py) along the four straight lines at these five locations (this average value is used as the luminescence factor-corrected polarization (Py) at each location). Table 2 also shows the calculated difference between the maximum and minimum values of the luminescence factor-corrected polarization (Py) at the five locations. Figure 5 is a graph of the luminescence factor-corrected polarization (Py) values shown in Table 2.

[Table 2]

The results shown in Table 2 and Figure 5 indicate that the difference between the maximum and minimum values of the luminous factor-corrected polarization degree in the width direction of the polarizing films of Examples 3 and 4 is smaller than that of Example 2, thus suppressing the variation in the luminous factor-corrected polarization degree in the width direction.

10: Embryonic membrane, embryonic membrane, membrane composed of polyvinyl alcohol resin

11: Raw material reels

13: Swelling bath

15:Dyeing bath

16: Spray device

16a, 16b: Spray stick

17a: Crosslinking bath

17b: Toning Bath

19: Cleansing Bath

21: Drying furnace

23:Polarizing film

30 to 35, 37 to 48, 60, 61: Guide rollers

50 to 52, 53a, 53b, 54, 55, 56: Clamping rollers

Claims (7)

A method for producing a polarizing film is provided, wherein the polarizing film is produced from a polyvinyl alcohol resin film. The method comprises a plurality of treatment steps for contacting the polyvinyl alcohol resin film with a treatment liquid. At least one of the plurality of treatment steps is a multi-stage treatment step, wherein the multi-stage treatment step comprises a uniform contact step for contacting the polyvinyl alcohol resin film with a uniform amount of treatment liquid over the entire width, and a step for contacting the polyvinyl alcohol resin film with the treatment liquid in an amount adjusted in a width direction according to the entire width. The polyvinyl alcohol-based resin film is divided into a central portion and two end portions across its entire width. The amount of the treatment liquid contacting the central portion is adjusted so that a greater amount of the treatment liquid contacts the end portions. In the contact adjustment step, the central portion of the polyvinyl alcohol-based resin film occupies 5 to 80% of the total width. The treatment liquid used in the multi-stage treatment step contains at least one of a dichroic dye and a boron compound. The method for manufacturing a polarizing film as described in claim 1, wherein, in the multi-stage treatment step, the treatment liquid is a dyeing liquid, a crosslinking liquid, or a toner. In the method for manufacturing a polarizing film as described in claim 1, in the contact adjustment step, the polyvinyl alcohol-based resin film is adjusted so that the central portion contacts the treatment liquid, while the end portions do not contact the treatment liquid. The method for producing a polarizing film as described in any one of claims 1 to 3, wherein the uniform contact step is a step of immersing the polyvinyl alcohol-based resin film in the treatment liquid contained in a treatment tank. The method for producing a polarizing film as described in any one of claims 1 to 3, wherein the contact adjustment step comprises rinsing the polyvinyl alcohol-based resin film with a spray of the treatment liquid. The method for producing a polarizing film according to any one of claims 1 to 3, wherein the total width of the polyvinyl alcohol-based resin film is not less than 400 mm and not more than 8000 mm. A polarizing film manufacturing apparatus is provided for manufacturing a polarizing film from a polyvinyl alcohol resin film. The polarizing film is provided with a plurality of processing sections for contacting the polyvinyl alcohol resin film with a processing liquid, wherein at least one of the plurality of processing sections is a multi-stage processing section having a uniform contact section for contacting the polyvinyl alcohol resin film with a uniform amount of processing liquid over the entire width, and a contact section for contacting the polyvinyl alcohol resin film with the processing liquid in an amount adjusted in a width direction according to the entire width. The polyvinyl alcohol-based resin film is divided into a central portion and two end portions across its entire width in the adjusted contact portion. The amount of the treatment liquid contacting the central portion is adjusted so that a greater amount of the treatment liquid contacts the end portions. In the adjusted contact portion, the central portion of the polyvinyl alcohol-based resin film occupies 5 to 80% of the total width. The treatment liquid used in the multi-stage treatment portion contains at least one of a dichroic dye and a boron compound.