TWI884211B - Method for manufacturing polarizing film - Google Patents

Abstract

An object of the present invention is to provide a method for manufacturing polarizing film, which is capable of providing a polarizing film having an excellent effect of suppressing yellowing even when exposed to a high temperature environment exceeding 95℃.

As a solution, the present invention provides a method for manufacturing polarizing film from a polyvinyl alcohol-based resin film, the method including a first treatment step of bringing the polyvinyl alcohol-based resin film into contact with a first treatment liquid, and the first treatment liquid containing boric acid and an iodide compound and having a pH of 5.0 or more at 25℃.

Description

Manufacturing method of polarizing film

The present invention relates to a method for manufacturing a polarizing film, and also to a method for manufacturing a polarizing plate, a polarizing film, a polarizing plate, and an image display device.

In recent years, image display devices containing polarizing plates have been used in mobile devices such as mobile phones and tablet computers, as well as in-vehicle image display devices such as car navigation devices and rear-view cameras, and their uses are expanding. As a result, image display devices are required to have higher durability than ever before in more severe environments (for example, high-temperature environments).

Patent document 1 discloses a method of lowering the pH of a polarizer and providing a polarizing plate having excellent optical properties and wet-heat durability. Patent document 1 describes a method of lowering the pH of a polarizer by setting the pH of a hard coating liquid to 1.5 to 3.2.

[Prior Art Literature]

[Patent Literature]

[Patent Document 1] Japanese Patent Publication No. 2005-62458

When the polarizing film contained in the polarizing plate is exposed to a high temperature environment, yellowing may occur due to the progress of polyene transformation of polyvinyl alcohol. In particular, under severe temperature conditions exceeding 95°C (such as 105°C), the yellowing tends to progress more significantly.

The purpose of the present invention is to provide a method for manufacturing a polarizing film, which can provide a polarizing film that has excellent yellowing inhibition effect even when exposed to a high temperature environment exceeding 95°C.

Another object of the present invention is to provide a method for manufacturing a polarizing plate, the polarizing film, the polarizing plate, and an image display device having the polarizing plate, wherein the polarizing plate has an excellent yellowing inhibition effect even in a high temperature environment exceeding 95°C.

The present invention provides a method for manufacturing a polarizing film, a method for manufacturing a polarizing plate, a polarizing film, a polarizing plate, and an image display device as shown below.

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

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

The aforementioned first treatment solution contains boric acid and iodide, and the pH at 25°C is above 5.0.

[2] The method for producing a polarizing film as described in [1] comprises a second treatment step of bringing the polyvinyl alcohol-based resin film into contact with a second treatment liquid,

The aforementioned second treatment solution contains boric acid and iodide, and the pH at 25°C is less than 5.0.

The aforementioned second processing step is performed before the aforementioned first processing step.

[3] The method for producing a polarizing film as described in [1] or [2], wherein the pH of the first treatment solution at 25°C is below 6.5.

[4] A method for producing a polarizing film as described in any one of [1] to [3], wherein a test piece having an area of 1500 ^cm2 cut from the polarizing film is immersed in 10 mL of pure water at 25°C for 10 minutes, and the pH of the immersion liquid after taking out the test piece at 25°C is 5.3 or more.

[5] A method for manufacturing a polarizing plate, comprising:

The step of manufacturing a polarizing film by the method for manufacturing a polarizing film described in any one of [1] to [4], and

The step of laminating the protective film on one or both sides of the aforementioned polarizing film via a laminating layer.

[6] A polarizing film having an area of 1500 ^cm2 is immersed in 10 mL of pure water at 25°C for 10 minutes, and the pH of the immersion liquid after taking out the polarizing film is 5.3 or more at 25°C.

[7] The polarizing film as described in [6] is obtained by dyeing a polyvinyl alcohol resin film with a dichroic pigment.

[8] A polarizing plate comprising the polarizing film described in [6] or [7], and a protective film laminated on one or both sides of the polarizing film via a lamination layer.

[9] An image display device, which is a stacked polarizing plate as described in [8] and an image display element.

[10] A polarizing film with a bonding layer, wherein bonding layers are laminated on both sides of the polarizing film.

A polarizing film with a lamination layer of 1500 ^cm2 was immersed in 10 mL of pure water at 25°C for 10 minutes. The pH of the immersion solution after taking out the polarizing film with a lamination layer was greater than 3.4 at 25°C.

[11] A polarizing plate, wherein a protective film is laminated on the laminating layer of the polarizing film with laminating layer described in [10].

[12] An image display device, which is a laminated polarizing plate as described in [11] and an image display element.

According to the present invention, a method for manufacturing a polarizing film that can achieve excellent yellowing suppression effect even in a high temperature environment exceeding 95°C, a method for manufacturing a polarizing plate having the polarizing film, the polarizing film, the polarizing plate, and an image display device having the polarizing plate can be provided.

10: Raw material film composed of polyvinyl alcohol resin (raw material film)

11: Raw material roll

13: Swelling bath

15:Dyeing bath

17: Cross-linking bath

18: Color touch up bath

19: Cleansing bath

21: Drying furnace

23:Polarizing film

30 to 48, 60, 61: guide roller

50 to 52,53a,53b,54,55: Roller

FIG1 is a cross-sectional view schematically showing a method for manufacturing a polarizing film of the present invention and an example of a polarizing film manufacturing device used in the method.

The following describes the implementation of the present invention with reference to the drawings, but the present invention is not limited to the following implementation. In the drawings, the scale is appropriately adjusted to make each element of the invention easier to understand, and the scale of each component shown in the drawings is not necessarily consistent with the scale of the actual component.

<Polarizing film manufacturing method>

In the present embodiment, the polarizing film is a film of polyvinyl alcohol resin having a dichroic pigment (iodine or dichroic dye) adsorbed and aligned on a uniaxially stretched 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 is, for example, a copolymer of vinyl acetate and other monomers copolymerizable with vinyl acetate, in addition to polyvinyl acetate which is a homopolymer of vinyl acetate. Examples of other copolymerizable monomers include unsaturated fatty acids, olefins, vinyl ethers, unsaturated sulfonic acids, and the like. The degree of polymerization of polyvinyl alcohol-based resins is generally about 1000 to 10000, preferably about 1500 to 5000.

These polyvinyl alcohol resins can be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc. modified with aldehydes can also be used.

In this embodiment, as the starting material for manufacturing the polarizing film, an unstretched polyvinyl alcohol resin film (raw film) with a thickness of less than 65 μm (for example, less than 60 μm), preferably less than 35 μm, and more preferably less than 30 μm is used. In this way, a thin film polarizing film with a gradually increasing market demand can be obtained. The width of the raw film is not particularly limited, and can be, for example, 400 to 6000 mm. The raw film is prepared, for example, in the form of a long roll of unstretched polyvinyl alcohol resin film (raw roll).

The polarizing film is continuously manufactured as a long polarizing film by unrolling the above-mentioned long raw material film from the raw material roll, continuously conveying it along the film conveying line of the polarizing film manufacturing device, and performing a specified treatment step of immersing it in a treatment liquid (hereinafter, also referred to as a "treatment bath") contained in a treatment tank and then taking it out, and then performing a drying treatment step. In addition, the treatment step is not limited to the method of immersing the film in the treatment bath as long as the method of contacting the film with the treatment liquid is a method of treating the film by spraying, flowing, dripping, etc. so that the treatment liquid adheres to the surface of the film. When the treatment step is implemented by immersing the membrane in a treatment bath, the treatment bath for implementing one treatment step is not limited to one type, and the membrane may be immersed in two or more treatment baths sequentially to complete one treatment step.

Examples of the treatment liquid include swelling liquid, dyeing liquid, crosslinking liquid, color-replenishing liquid, and cleaning liquid. Examples of the treatment step include a swelling step in which a swelling liquid is brought into contact with a raw film to perform swelling treatment, a dyeing step in which a dyeing liquid is brought into contact with a film after the swelling step to perform dyeing treatment, a crosslinking step in which a crosslinking liquid is brought into contact with a film after the dyeing treatment to perform crosslinking treatment, a color-replenishing step in which a color-replenishing liquid is brought into contact with a film after the crosslinking treatment to perform color-replenishing treatment, and a cleaning step in which a cleaning liquid is brought into contact with a film after the color-replenishing treatment to perform cleaning treatment. Furthermore, between such a series of treatment steps (i.e., before or after any of the above treatment steps and/or during any of the above treatment steps), uniaxial stretching treatment is performed in a wet or dry manner. Other treatment steps may be added as necessary.

In this specification, a treatment liquid containing boric acid and iodine compounds and having a pH of 5.0 or more at 25°C is referred to as the first treatment liquid, and a treatment liquid containing boric acid and iodine compounds and having a pH of less than 5.0 at 25°C is referred to as the second treatment liquid. Since both the first treatment liquid and the second treatment liquid contain boric acid and iodine compounds, they can be used for crosslinking treatment of crosslinking the film after dyeing treatment, and can also be used for color correction treatment of adjusting the hue of the film. In addition, since the pH value range of the first treatment liquid and the second treatment liquid is limited, they can also be used for pH adjustment treatment of adjusting the pH.

In this embodiment, the first treatment step of contacting the membrane with the first treatment solution is included, and the second treatment step of contacting the membrane with the second treatment solution may be included. When the second treatment step is included, it is preferably performed before the first treatment step, and the treatment is preferably performed in the order of the second treatment step and the first treatment step. When the washing step is included, the treatment is preferably performed in the order of the second treatment step, the first treatment step, and the washing step. The second processing step and the first processing step are, for example, a crosslinking step and a color correction step, a first color correction step and a second color correction step, a first crosslinking step and a second crosslinking step, etc. In this embodiment, since the first processing step is included, a polarizing film having an excellent yellowing inhibition effect can be manufactured even in a high temperature environment exceeding 95°C.

Hereinafter, an example of the method for producing the polarizing film of the present invention will be described in detail with reference to Fig. 1. Fig. 1 is a cross-sectional view schematically showing an example of the method for producing the polarizing film of the present invention and a polarizing film producing apparatus used in the method. The polarizing film manufacturing device shown in FIG1 is configured as follows: a raw material (unstretched) film 10 made of a polyvinyl alcohol resin is continuously rolled out from a raw material roll 11 and conveyed along a film conveying route, and sequentially passes 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) 17, a coloring bath (coloring liquid contained in a coloring tank) 18, and a cleaning bath (cleaning liquid contained in a cleaning tank) 19 arranged on the film conveying route, and finally passes through a drying furnace 21. The arrow in FIG1 indicates the conveying direction of the film.

In the description of FIG1, "treatment tank" is a general term including swelling tank, dyeing tank, crosslinking tank, color-replenishing tank and cleaning tank, "treatment liquid" is a general term including swelling liquid, dyeing liquid, crosslinking liquid, color-replenishing liquid and cleaning liquid, and "treatment bath" is a general term including swelling bath, dyeing bath, crosslinking bath, color-replenishing bath and cleaning bath. The swelling bath, dyeing bath, crosslinking bath, color-replenishing bath and cleaning bath respectively constitute the swelling part, dyeing part, crosslinking part and cleaning part in the manufacturing device of this embodiment.

In addition to the above-mentioned treatment baths, the following can be arranged at appropriate positions on the film conveying route of the polarizing film manufacturing device: guide rollers 30 to 48, 60, and 61 that support the film to be conveyed or can further change the film conveying direction, and rollers 50 to 55 that squeeze and clamp the film to be conveyed and can give a driving force to the film due to its rotation or can further change the film conveying direction. The guide rollers and rollers can be arranged before and after each treatment bath or in the treatment bath, thereby allowing the film to be introduced into the treatment bath, immersed in the treatment bath, and taken out of the treatment bath [refer to Figure 1]. For example, by setting 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 FIG1 is equipped with rollers (rollers 50 to 54) before and after each treatment bath, so that uniaxial stretching can be performed in any one or more treatment baths at the peripheral speed difference between the rollers arranged before and after, and stretching can be performed between the rollers. Each step is described below.

(Swelling step)

The swelling step is carried out for the purpose of removing foreign matter on the surface of the raw material film 10, removing plasticizer in the raw material film 10, imparting easy dyeing properties, and plasticizing the raw material film 10. The treatment conditions are determined within the range that can achieve the purpose and within the range of problems such as extreme dissolution or devitrification of the raw material film 10.

Referring to FIG1 , the swelling step is implemented by continuously rolling out the raw material film 10 from the raw material roll 11 while conveying it along the film conveying route, immersing the raw material film 10 in the swelling bath 13 for a specified time, and then taking it out. In the example of FIG1 , the raw material film 10 is conveyed along the film conveying route constructed by the guide rollers 60, 61 and the roller 50 from the time the raw material film 10 is rolled out to the time the raw material film 10 is immersed in the swelling bath 13. During the swelling treatment, the raw material film 10 is conveyed along the film conveying route constructed by the guide rollers 30 to 32 and the roller 51.

As the swelling liquid of the swelling bath 13, in addition to pure water, 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, alcohol, etc. are added in a range of about 0.01 to 10 mass % can be used.

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 about 10 to 300 seconds, and more preferably 20 to 200 seconds. In addition, when the raw film 10 is a polyvinyl alcohol resin film pre-stretched in gas, the temperature of the swelling bath 13 is, for example, 20 to 70°C, and preferably 30 to 60°C. The immersion time of the raw film 10 is preferably about 30 to 300 seconds, and more preferably about 60 to 240 seconds.

During the swelling treatment, there is a problem that the raw film 10 swells in the width direction and wrinkles are easily generated in the film. As one of the means to transport the film while removing the wrinkles, it can be listed that a roller with a widening function such as an expansion roller, a spiral roller, a convex roller, etc. is used in the guide rollers 30, 31 and/or 32, or other widening devices such as a cross guide, a bending roller, and a tentering cloth are used. Another means to suppress the generation of wrinkles is to perform a stretching treatment. For example, a uniaxial stretching treatment can be performed in the swelling bath 13 by utilizing the peripheral speed difference between the rollers 50 and 51.

During the swelling treatment, the membrane will also swell and expand in the transport direction of the membrane. Therefore, when the membrane is not actively stretched, in order to eliminate the sagging of the membrane in the transport direction, it is better to take measures such as controlling the speed of the rollers 50 and 51 arranged in front and behind the swelling bath 13. In addition, in order to stabilize the transport of the membrane in the swelling bath 13, it is also useful to control the water flow in the swelling bath 13 with an underwater shower or use an EPC device (Edge Position Control device: a device that detects the tip of the membrane and prevents the membrane from bending).

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

(Dyeing step)

The dyeing step is carried out for the purpose of adsorbing and aligning the dichroic dye on the polyvinyl alcohol resin film after the swelling treatment. The treatment conditions are determined within the range that can achieve the purpose and does not cause problems such as extreme dissolution and devitrification of the film. Referring to Figure 1, the dyeing step can be carried out as follows: the film after the swelling treatment is transported along the film transport route constructed by the roller 51, the guide rollers 33 to 36 and the roller 52, and the film is immersed in the dyeing bath 15 (the treatment liquid contained in the dyeing tank) for a specified time, and then taken out. In order to improve the dyeability of the dichroic dye, the film for the dyeing step is preferably a film that has been subjected to at least a certain degree of uniaxial stretching treatment, or instead of the uniaxial stretching treatment before the dyeing treatment, or in addition to the uniaxial stretching treatment before the dyeing treatment, it is preferred to perform uniaxial stretching treatment during the dyeing treatment.

When iodine is used as a dichroic pigment, an aqueous solution having a concentration of iodine/potassium iodide/water = about 0.003 to 0.3/about 0.1 to 10/100 in a mass ratio can be used in the dyeing solution of the dyeing bath 15. Instead of potassium iodide, other iodides such as zinc iodide can be used, and potassium iodide and other iodides can also be used in combination. In addition, compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, can coexist. When boric acid is added, in terms of the presence of iodine, in contrast to the crosslinking treatment and color correction treatment described later, an aqueous solution containing about 0.003 parts by mass or more of iodine relative to 100% by mass of water can be used as the dyeing bath 15. The temperature of the dyeing bath 15 during the immersion of the film is usually 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C. The immersion time of the film is usually 30 to 600 seconds, and preferably 60 to 300 seconds.

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

As described above, in the dyeing step, the film can be uniaxially stretched in the dyeing bath 15. The uniaxial stretching of the film can be performed by a method such as the peripheral speed difference between the rollers 51 and 52 arranged before and after the dyeing bath 15.

In the dyeing process, similar to the swelling process, in order to eliminate the wrinkles of the film while conveying the polyvinyl alcohol resin film, a roller with a widening function such as a widening roller, a spiral roller, a convex roller, or other widening devices such as a cross guide, a bending roller, and a tentering cloth can be used in the guide rollers 33, 34, 35 and/or 36. Another means to suppress the occurrence of wrinkles is to perform a stretching process similar to the swelling process.

In the example shown in FIG. 1 , the film drawn out from the dye bath 15 is introduced into the crosslinking bath 17 through the guide roller 36, the roller 52, and the guide roller 37 in sequence.

(Cross-linking step)

The crosslinking step is a process of crosslinking the membrane for the purpose of improving water resistance, etc. Referring to FIG1 , the crosslinking step can be implemented as follows: the membrane is transported along the membrane transport route constructed by roller 52, guide rollers 37 to 40 and roller 53a, and the membrane after dyeing treatment is immersed in the crosslinking bath 17 (the first crosslinking liquid contained in the crosslinking tank) for a specified time, and then drawn out.

As a crosslinking liquid, a solution in which a crosslinking agent is dissolved in a solvent can be used. As a crosslinking agent, for example, boric acid, borax and other boron compounds, glyoxal, glutaraldehyde and the like can be listed. These can be one type, or two or more types can be used in combination. As a solvent, for example, water can be used, and furthermore, an organic solvent that is compatible with water can also be contained. 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 %, and more preferably in the range of 6 to 15 mass %.

As a crosslinking liquid, for example, it can be an aqueous solution containing about 1 to 10 parts by mass of boric acid relative to 100 parts by mass of water. When the dichroic pigment used in the dyeing process is iodine, it is preferred that the crosslinking liquid contains iodide in addition to boric acid, and its amount can be, for example, 1 to 30 parts by mass relative to 100 parts by mass of water. As iodide, potassium iodide, zinc iodide, etc. can be listed. In addition, compounds other than iodide, for example, zinc chloride, cobalt chloride, zinc chloride, etc. can be listed. In addition, compounds other than iodide, for example, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, etc. can coexist.

During the crosslinking treatment, the concentrations of boric acid and iodide, and the temperature of the crosslinking bath 17 can be changed appropriately according to the purpose. The crosslinking solution can be, for example, an aqueous solution having a mass ratio of boric acid/iodide/water = 3 to 10/1 to 20/100. If necessary, other crosslinking agents can be used instead of boric acid, or boric acid and other crosslinking agents can be used in combination. The temperature of the crosslinking bath 17 during the impregnation of the membrane is usually 50 to 70° C., preferably 53 to 65° C., and the membrane impregnation time is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 200 seconds. Furthermore, when dyeing and crosslinking are sequentially performed on the polyvinyl alcohol-based resin film that has been stretched before the swelling treatment, the temperature of the crosslinking bath 17 is usually 50 to 85°C, preferably 55 to 80°C.

The crosslinking treatment can be performed multiple times, usually 2 to 5 times. At this time, the composition and temperature of each crosslinking bath used can be the same or different within the above range. The uniaxial stretching treatment can be performed in the crosslinking bath 17 by utilizing the peripheral speed difference between the roller 52 and the roller 53a.

In the crosslinking treatment, similar to the swelling treatment, in order to eliminate the wrinkles of the film while conveying the polyvinyl alcohol resin film, a roller with a widening function such as a widening roller, a spiral roller, a convex roller, or other widening devices such as a cross guide, a bending roller, and a tentering cloth can be used in the guide rollers 37, 38, 39, and/or 40. Another means to suppress the occurrence of wrinkles is to implement a stretching treatment similar to the swelling treatment.

(Coloring step)

The color correction step is a process for adjusting the hue of the film. Referring to FIG1 , the color correction step can be implemented as follows: the film after the crosslinking step is transported along the film transport route constructed by roller 53a, guide rollers 41 to 44 and roller 53b, immersed in the color correction bath 18 (color correction liquid contained in the color correction tank) for a specified time, and then drawn out.

As a color-replenishing solution, for example, it can be an aqueous solution containing about 1 to 10 parts by mass of boric acid relative to 100 parts by mass of water. When the dichroic pigment used in the color-replenishing solution in the dyeing process is iodine, it is preferred to contain an iodide other than boric acid, and its amount relative to 100 parts by mass of water can be, for example, 1 to 30 parts by mass. As iodides, for example, potassium iodide, zinc iodide, etc. can be listed. In addition, compounds other than iodides, for example, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc. can coexist.

In the color-replenishing solution, for example, when iodine is used as a dichroic pigment, the concentration can be used at a mass ratio of boric acid/iodide/water = 1 to 5/3 to 30/100. The temperature of the color-replenishing bath 18 when the film is immersed is usually 10 to 45°C, and the immersion time of the film is usually 1 to 300 seconds, preferably 2 to 100 seconds.

The color correction treatment can be performed multiple times, usually 2 to 5 times. At this time, the composition and temperature of each color correction bath used can be the same or different within the above range. In addition, the uniaxial stretching treatment can be performed in the color correction bath 18 by utilizing the peripheral speed difference between the roller 53a and the roller 53b.

In the color touch-up treatment, similar to the swelling treatment, in order to eliminate the wrinkles of the film while conveying the polyvinyl alcohol-based resin film, a roller with a width expansion function such as a spreading roller, a spiral roller, or a convex roller can be used in the guide rollers 41, 42, 43, and/or 44, or other width expansion devices such as a cross guide, a bending roller, and a tentering cloth can be used. Another means to suppress the occurrence of wrinkles is to perform a stretching treatment similar to the swelling treatment.

In the example shown in FIG1 , the film drawn out from the color correction bath 18 is introduced into the cleaning bath 19 through the guide roller 44 and the roller 53b in sequence.

(Cleaning steps)

The example shown in FIG1 includes a cleaning step after the color correction step. The cleaning treatment is performed for the purpose of removing excess boric acid and iodine and other chemicals attached to the polyvinyl alcohol resin film. The cleaning step is performed, for example, by immersing the polyvinyl alcohol resin film that has undergone the color correction treatment in a cleaning bath 19. In addition, the cleaning step can also be implemented as follows: instead of immersing the film in the cleaning bath 19, the cleaning liquid is sprayed on the film in a shower, or the immersion in the cleaning bath 19 and the spraying of the cleaning liquid are combined.

FIG1 shows an example of a polyvinyl alcohol resin film immersed in a cleaning bath 19 for cleaning treatment. The temperature of the cleaning bath 19 during the cleaning treatment is usually 2 to 40°C, and the immersion time of the film is usually 2 to 120 seconds.

Furthermore, during the cleaning process, in order to remove wrinkles while conveying the polyvinyl alcohol-based resin film, rollers with a widening function such as a widening roller, a spiral roller, or a convex roller may be used as guide rollers 45, 46, 47, and/or 48, or other widening devices such as a cross guide, a bending roller, and a tentering cloth may be used. Furthermore, during the film cleaning process, a stretching process may be performed to suppress the occurrence of wrinkles.

(Extended steps)

As described above, the raw film 10 is subjected to uniaxial stretching treatment in a wet or dry manner between the above series of treatment steps (i.e., before or after any one or more treatment steps and/or in any one or more treatment steps). A specific method of uniaxial stretching may be, for example, inter-roll stretching in which longitudinal uniaxial stretching is performed by using a peripheral speed difference between two rolls constituting a film conveying path (e.g., two rolls arranged before and after a treatment bath), hot roll stretching, tenter stretching, etc. described in Japanese Patent No. 2731813, preferably inter-roll stretching. The uniaxial stretching step may be performed multiple times between the raw film 10 and the polarizing film 23. As mentioned above, stretching treatment is also helpful in suppressing the occurrence of wrinkling of the film.

Taking the raw material film 10 as a reference, the final cumulative stretching ratio of the polarizing film 23 is usually 4.5 to 7 times, preferably 5 to 6.5 times. The stretching step can be performed using any processing step. When the stretching treatment is performed using more than 2 processing steps, the stretching treatment can also be performed in any processing step.

(1st processing step)

The first treatment step is a step of bringing the membrane into contact with a first treatment solution, the first treatment solution contains boric acid and iodide, and has a pH of 5.0 or more at 25°C. The pH of the first treatment solution at 25°C is preferably 5.2 or more, preferably 5.3 or more, and more preferably 5.5 or more. Furthermore, the pH of the first treatment solution at 25°C is preferably 6.8 or less, more preferably 6.5 or less, and even more preferably 6.3 or less. The first treatment step is, for example, the above-mentioned color correction step. When the first treatment step is a color correction step, the description of the above-mentioned color correction step can be directly applied to the description of the first treatment step. The first treatment solution contains boric acid and iodide, and has a pH of 5.0 or more. As the first treatment solution, an aqueous solution with a mass ratio of boric acid/iodide/water = 1 to 5/3 to 30/100 can be used. The first treatment solution further contains a pH adjuster. Examples of the pH adjuster include potassium hydroxide, sodium hydroxide, etc. The first treatment solution is adjusted in such a way that the pH at 25°C is adjusted to 5.0 or more by adding the pH adjuster. In this embodiment, since there is a first treatment step, a polarizing film having an excellent yellowing inhibition effect can be provided even in a high temperature environment exceeding 95°C.

(Processing step 2)

The second treatment step is a step of contacting the membrane with a second treatment solution, the second treatment solution contains iodide and has a pH of less than 5.0 at 25°C. The second treatment step is, for example, the above-mentioned crosslinking step. When the second treatment step is a crosslinking step, the above-mentioned description of the crosslinking step can be directly applied to the description of the second treatment step. The second treatment solution contains boric acid and iodide and has a pH of less than 5.0 at 25°C. As the second treatment solution, an aqueous solution with a mass ratio of boric acid/iodide/water = 3 to 10/1 to 20/100 can be used. For example, the second treatment liquid can adjust the amount of boric acid and iodide added so that the pH is less than 5.0, or an acid pH adjuster can be added so that the pH at 25°C is less than 5.0. The second treatment liquid is preferably adjusted in such a way that the amount of boric acid added is adjusted so that the pH at 25°C is less than 5.0.

(Drying step)

After the second treatment step, or after the washing step when the washing step is included, it is preferred to dry the polyvinyl alcohol resin film. There is no particular limitation on the drying of the film, but as shown in the example of FIG. 1 , it can be carried out using a drying furnace 21. The drying furnace 21 can be, 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 treatment of drying the polyvinyl alcohol resin film can also be carried out using a far infrared heater.

(Other treatment steps for polyvinyl alcohol resin film)

Additional treatments other than the above treatments may also be added. Examples of additional treatments include immersion treatment in an aqueous iodide solution that does not contain boric acid, and immersion treatment in an aqueous solution that does not contain boric acid but contains zinc chloride or the like (zinc treatment).

The polarizing film obtained as described above can be rolled up on a winding roll in sequence to form a roll shape, or can be directly provided to the manufacturing method of the polarizing plate without being rolled up. One aspect of the manufacturing method of the polarizing plate has a laminating step of laminating a protective film on one or both sides of the polarizing film via a laminating layer.

<Polarizing film>

The polarizing film of this embodiment is preferably a polyvinyl alcohol-based resin film dyed with a dichroic pigment. The thickness of the polarizing film is preferably 5 μm to 60 μm, and more preferably 7 μm to 30 μm. A test piece of 1500 ^cm2 obtained by cutting the polarizing film is immersed in 10 mL of pure water at 25°C, and placed at 25°C for 10 minutes. The pH of the immersion liquid after taking out the test piece at 25°C is preferably 5.3 or more, more preferably 5.4 or more, and more preferably 5.6 or more. The upper limit is preferably 6.8 or less, more preferably 6.5 or less, and more preferably 6.2 or less. In addition, the details of the pH measurement of the immersion liquid of the polarizing film are carried out according to the method described in the embodiments described later. The sensitivity compensation monomer transmittance Ty of the polarizing film is preferably 40 to 47%, more preferably 41 to 45%, in consideration of the balance with the sensitivity compensation polarization Py. The sensitivity compensation polarization Py is preferably 99.9% or more, more preferably 99.95% or more. Such a polarizing film can be obtained by the above-mentioned manufacturing method.

<Polarizing plate>

The polarizing plate of this embodiment can be obtained by laminating a protective film on one or both sides of the above-mentioned polarizing film via a laminating layer. As examples of the protective film, there can be cited films made of acetic acid cellulose such as cellulose triacetate and cellulose diacetate; films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; polycarbonate resins, cyclic olefin resin films; acrylic resin films; films made of chain olefin resins such as polypropylene resins.

In order to improve the adhesion between the polarizing film and the protective film, the bonding surface of the polarizing film and/or the protective film may be subjected to surface treatments such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, and saponification treatment. The bonding layer between the polarizing film and the protective film may be formed with a bonding agent or an adhesive. As bonding agents, there may be listed active energy line curing bonding agents such as ultraviolet curing bonding agents, aqueous solutions of polyvinyl alcohol bonding agent resins, or aqueous solutions containing crosslinking agents, and water-based bonding agents such as urethane emulsion bonding agents. In the water-based bonding agent, a zinc compound such as zinc nitrate may be added. UV curable adhesives can be mixtures of acrylic compounds and photo-radical polymerization initiators, and mixtures of epoxy compounds and photo-cationic polymerization initiators. In addition, cationic polymerizable epoxy compounds and photo-radical polymerizable acrylic compounds can be used together, and photo-cationic polymerization initiators and photo-radical polymerization initiators can also be used together as initiators.

The bonding layer is preferably a layer formed by a water-based adhesive, more preferably a layer formed by a water-based adhesive containing a zinc compound, and even more preferably a layer formed by a water-based adhesive containing a zinc compound and a polyvinyl alcohol resin.

A polarizing film with a laminating layer on both sides of the polarizing film is immersed in 10 mL of pure water at 25°C for a test piece of 1500 ^cm2 of the polarizing film with a laminating layer, and placed at 25°C for 10 minutes. The pH of the immersion solution after taking out the test piece at 25°C is preferably greater than 3.4, more preferably greater than 3.5, more preferably greater than 4.0, and more preferably greater than 4.4. The upper limit is preferably less than 6.8, more preferably less than 6.5, more preferably less than 6.2, and particularly preferably less than 5.4. In addition, the details of the pH measurement of the immersion solution of the polarizing film with a laminating layer are carried out according to the method described in the embodiments described below.

The thickness of the laminating layer of the polarizing film with laminating layer is preferably 50 to 200 nm, more preferably 70 to 150 nm.

<Image display device>

Polarizing plates can be used in image display devices. As image display elements used in image display devices, for example, liquid crystal display elements, organic EL display elements, etc. can be listed. When constructing a liquid crystal display device, the polarizing plate of the present invention can be configured and used on the viewing side, can be configured and used on the backlight side, or can be used on both the viewing side and the backlight side. In addition to being used in mobile devices such as televisions, personal computers, mobile phones, and tablet computers, the image display device of the present invention is particularly suitable for use in vehicles that are easily exposed to more severe temperature conditions because it has a high inhibitory effect on yellowing in high-temperature environments and can present a long-term stable image display function. For example, the image display device can be used in car navigation devices, speedometers, air conditioning touch panels, rear monitors, and tail monitors.

[Implementation example]

The present invention is described in more detail below by way of examples, but the present invention is not limited to these examples.

<Implementation Example 1>

The polarizing film of Example 1 was produced from a polyvinyl alcohol resin film using the production apparatus shown in FIG1 . Specifically, a 60 μm thick long polyvinyl alcohol (PVA) raw material film [trade name "Poval Film OPL Film M-6000, saponification degree 99.9 mol% or more" manufactured by Mitsubishi Chemical Co., Ltd.] was continuously conveyed while being unrolled from a roll, and was immersed in a swelling bath composed of pure water at 28° C. for an immersion time of 140 seconds (swelling step). Thereafter, the film drawn out from the swelling bath was immersed in a 30° C. dyeing solution composed of an iodine-containing dyeing solution of iodine/potassium iodide/boric acid/water at a mass ratio of 0.03/1.3/0.3/100 for an immersion time of 130 seconds (dyeing step). Next, the membrane drawn out from the dyeing bath was immersed in a crosslinking bath composed of a crosslinking solution of potassium iodide/boric acid/water at a ratio of 13.9/3.0/100 (mass ratio) at 56°C for 50 seconds (crosslinking step, second treatment step). The pH of the crosslinking bath at 25°C was 3.8.

Next, the membrane taken out from the crosslinking bath is immersed in a 40°C color correction bath composed of a solution of potassium iodide/boric acid/water at 9.0/3.0/100 (mass ratio) and a potassium hydroxide aqueous solution added thereto for 10 seconds (color correction step, first treatment step). Next, the membrane taken out from the color correction bath is retained in a drying oven at a temperature of 80°C for 150 seconds and dried (drying step). In addition, the pH of the color correction solution (first treatment solution) at 25°C is 6.1.

The thickness of the obtained polarizing film was 23 μm. When the pH of the immersion liquid of the obtained polarizing film was measured by the method described below, the result was 6.14.

<Implementation Example 2>

For the color-correcting solution used in the color-correcting step, the polarizing film was prepared in the same manner as in Example 1, except that potassium hydroxide was added to adjust the pH at 25°C to 5.7.

The thickness of the obtained polarizing film was 23 μm. When the pH of the immersion liquid of the obtained polarizing film was measured by the method described below, the result was 5.92.

<Comparative Example 1>

For the color-correcting solution used in the color-correcting step, the polarizing film was prepared in the same manner as in Example 1, except that sulfuric acid was added instead of potassium hydroxide. In addition, the pH of the color-correcting solution at 25°C was 3.8.

The thickness of the obtained polarizing film was 23 μm. In addition, when the pH of the immersion liquid of the obtained polarizing film was measured by the method described below, the result was 5.16.

<Implementation Example 3>

(Preparation of water-based adhesive composition)

Dissolve acetoacetyl modified polyvinyl alcohol [trade name "Gohsenol Z-200" manufactured by Nippon Synthetic Chemical Industry Co., Ltd., 4% aqueous solution viscosity = 12.4mPa‧sec, saponification degree = 99.1 mol%] in pure water to prepare a 10% aqueous solution. Mix this acetoacetyl modified polyvinyl alcohol aqueous solution with sodium glyoxylate as a crosslinking agent in a solid mass ratio of 1:0.1, and then dilute with water in a ratio of 1.5 mass parts of zinc nitrate and 2.5 mass parts of acetoacetyl modified polyvinyl alcohol to 100 mass parts of water to prepare a water-based adhesive composition.

A saponification treatment was applied to a triacetyl cellulose film with a thickness of 40 μm. A saponification treatment was also applied to a low phase difference TAC film with a thickness of 40 μm. The two sides of the polarizing film of Example 1 were respectively laminated with the above-mentioned aqueous adhesive to obtain a laminate having a layer structure of TAC/adhesive/polarizing film/adhesive layer/low phase difference TAC film. The obtained laminate was heated at 80°C for 140 seconds in a hot air dryer to produce a polarizing plate having a layer structure of TAC film/adhesive layer/polarizing film/adhesive layer/low phase difference TAC film. The obtained polarizing plate was subjected to the yellowing evaluation test described later. The results are shown in Table 1.

<Implementation Example 4>

A polarizing plate was prepared in the same manner as in Example 3, except that the polarizing film was the polarizing film prepared in Example 2. The obtained polarizing plate was subjected to the yellowing evaluation test described later, and the results are shown in Table 1.

<Comparative example 2>

A polarizing plate was prepared in the same manner as Example 3, except that the polarizing film was the polarizing film prepared in Comparative Example 1. The obtained polarizing plate was subjected to the yellowing evaluation test described later, and the results are shown in Table 1.

[pH measurement of polarizing film immersion solution]

The polarizing films of Examples 1, 2 and Comparative Example 1 were cut into test pieces of 1500 ^cm2 (30 cm×50 cm). The test pieces were further cut into 5 cm×5 cm pieces to obtain 60 polarizing films of 5 cm×5 cm each. All the 60 polarizing films of 5 cm×5 cm each were placed in a 500 mL beaker, and 10 mL of pure water at 25°C was added thereto using a pipette. The pure water was stirred by repeatedly sucking it in and out of the pipette several times, and then allowed to stand at 25°C for 10 minutes. Afterwards, the pH of the immersion liquid after taking out the test piece was measured at 25°C using a pH meter (HORIBA D-54). The results are shown in Table 1.

[pH measurement of immersion liquid of polarizing film with bonding layer]

A laminate having a layer structure of 40 μm thick TAC film/water-based adhesive layer/polarizing film of Example 1/water-based adhesive layer/40 μm thick low-retardation TAC film was obtained in the same manner as in Example 3 except that the 40 μm thick TAC film and the 40 μm thick low-retardation TAC film were not subjected to saponification treatment. The 40 μm thick TAC film and the 40 μm thick low-retardation TAC film were peeled off from the obtained laminate to obtain a polarizing film with a water-based adhesive layer (polarizing film with a laminating layer). The obtained polarizing film with a water-based adhesive layer was cut into a test piece of 1500 cm ² (30 cm×50 cm). Cut the test piece into 5cm×5cm size, and get 60 polarizing plates of 5cm×5cm size. Put all the test pieces in a 500mL beaker, and add 10mL of pure water at 25℃ with a pipette. Stir the pure water by repeatedly sucking it in and out of the pipette, and then let it stand at 25℃ for 10 minutes. Then, take out the polarizing film with the bonding layer, and measure the pH of the immersion liquid at 25℃ with a pH meter (HORIBA D-54).

In addition, except for using the polarizing film of Example 2 or the polarizing film of Comparative Example 1, the Ph measurement of the polarizing film immersion liquid with each water-based adhesive layer was carried out in the same manner as above. The results are shown in Table 1.

[Yellowing evaluation (105℃)]

The polarizing plates of Examples 3, 4 and Comparative Example 2 were cut into 40 mm × 40 mm sizes, and 40 mm × 40 mm alkali-free glass ("EAGLE XG" manufactured by Corning Incorporated) was bonded to both sides using a 25 μm thick acrylic adhesive to prepare evaluation samples.

After the sample was placed at 50°C and 0.5Mpa (about 5kg/cm ² ) for 20 minutes, it was placed in an environment of 23°C and 55% relative humidity for 24 hours. Furthermore, it was heated at 95°C for 3 hours. The sample was heated at 105°C for 50 hours to visually check the color change before and after heating. The sample with almost no change was designated as A, the sample with slight but noticeable color change was designated as B, the sample with color change that could be confirmed in about half of the polarizing plate surface was designated as C, and the sample with a large color change almost entirely on the polarizing plate surface was designated as D. When the evaluation result is A, a heating test is performed again in a heating environment at a temperature of 105°C for 50 hours to visually confirm the color change before and after heating. The heating test is terminated at any time point from B to D when the evaluation result is obtained. The total storage time in the heating test until the end is shown in Table 1 as the result of the yellowing evaluation test.

[Table 1]

As shown in Table 1, in the polarizing plates of Examples 3 and 4, the color change before and after heating could not be confirmed until 1000 hours. On the other hand, in the polarizing plate of Comparative Example 2, the color change could not be confirmed until 300 hours, but after the total storage time in the heating experiment reached 300 hours, the color change before and after heating was visually confirmed after 50 hours of storage.

10: Raw material film composed of polyvinyl alcohol resin

11: Raw material roll

13: Swelling bath

15:Dyeing bath

17: Cross-linking bath

18: Color touch up bath

19: Cleansing bath

21: Drying furnace

23:Polarizing film

30 to 48, 60, 61: guide roller

50 to 52,53a,53b,54,55: Roller

Claims (11)

A method for manufacturing a polarizing film is a method for manufacturing a polarizing film from a polyvinyl alcohol resin film; the manufacturing method comprises: a first treatment step of contacting the polyvinyl alcohol resin film with a first treatment liquid; the first treatment liquid contains boric acid, iodide and a pH adjuster, and the pH at 25°C is 5.0 or more and 6.5 or less; the pH adjuster is potassium hydroxide or sodium hydroxide. The method for manufacturing a polarizing film as described in claim 1 comprises a second treatment step of contacting the polyvinyl alcohol-based resin film with a second treatment liquid, wherein the second treatment liquid contains boric acid and iodide and has a pH of less than 5.0 at 25°C, and the second treatment step is performed before the first treatment step. A method for producing a polarizing film as claimed in claim 1 or 2, wherein a test piece with an area of 1500 ^cm2 obtained by cutting the polarizing film is immersed in 10 mL of pure water at 25°C and left for 10 minutes, and the pH of the immersion liquid after taking out the test piece at 25°C is 5.3 or more and 6.8 or less. A method for manufacturing a polarizing plate comprises: a step of manufacturing a polarizing film by the method for manufacturing a polarizing film described in any one of claims 1 to 3, and a step of laminating a protective film on one or both sides of the polarizing film via a laminating layer. A polarizing film having an area of 1500 ^cm2 is immersed in 10 mL of pure water at 25°C for 10 minutes, and the pH of the immersion liquid after taking out the polarizing film is 5.3 or more and 6.8 or less at 25°C. The polarizing film as described in claim 5 is obtained by dyeing a polyvinyl alcohol resin film with a dichroic pigment. A polarizing plate comprises a polarizing film as described in claim 5 or 6, and a protective film laminated on one or both sides of the polarizing film via a lamination layer. An image display device is a laminated polarizing plate and an image display element as described in claim 7. A polarizing film with a laminating layer is provided, wherein the laminating layer is provided on both sides of the polarizing film. The polarizing film with a laminating layer having an area of 1500 ^cm2 is immersed in 10 mL of pure water at 25°C and left for 10 minutes. After the polarizing film with the laminating layer is taken out, the pH of the immersion liquid at 25°C is greater than 3.4 and less than 6.2. A polarizing plate, wherein a protective film is laminated on the laminating layer of the polarizing film with laminating layer as described in claim 9. An image display device is a laminated polarizing plate and an image display element as described in claim 10.

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