JP2022150490A - Polarizing film and manufacturing method of polarizing plate - Google Patents

Abstract

To provide a polarizing film manufacturing method capable of providing a polarizing film which has an excellent yellowing inhibitory effect even when exposed to a high-temperature environment exceeding 95°C.SOLUTION: A polarizing film manufacturing method for manufacturing a polarizing film from a polyvinylalcohol resin film includes a treatment step of making the polyvinylalcohol resin film contact with a treatment liquid. The treatment liquid contains nitrate, boric acid, and an iodide compound and has a pH value of 4.5 or more at 25°C.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a polarizing film and a polarizing plate.

In recent years, image display devices containing polarizing plates have been used in mobile devices such as mobile phones and tablet terminals, as well as in-vehicle image display devices such as car navigation devices and back monitors. there is Along with this, image display devices are required to have higher durability in harsher environments (for example, in high-temperature environments) than has been conventionally required.

Patent Literature 1 discloses that the pH of the polarizer is lowered to provide a polarizing plate having excellent optical properties and wet heat durability. Patent Document 1 describes a method for lowering the pH of the polarizer by adjusting the pH of the hardener to 1.5 to 3.2.

Also known is a method of adding a metal halide such as zinc chloride to a polarizer treatment solution to improve the durability of the polarizer. However, in order to reduce the impact on the environment, it is required to be halogen-free (for example, chlorine is 900 ppm or less).

JP-A-2005-62458

A polarizing film included in a polarizing plate has a problem of yellowing when exposed to a high-temperature environment because polyvinyl alcohol undergoes polyene conversion. In particular, under severe temperature conditions exceeding 95°C (for example, 105°C), the progress of yellowing tends to become more pronounced.

An object of the present invention is to provide a method for producing a polarizing film that can provide a polarizing film that exhibits an excellent effect of suppressing yellowing even when exposed to a high temperature environment exceeding 95°C.

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

The present invention provides a method for producing a polarizing film, a method for producing a polarizing plate, a polarizing film, a polarizing plate, and an image display device described below.

[1] A method for producing a polarizing film from a polyvinyl alcohol resin film, comprising:
A treatment step of bringing the polyvinyl alcohol resin film into contact with a treatment liquid,
The method for producing a polarizing film, wherein the treatment liquid contains nitrate, boric acid and an iodide compound, and has a pH of 4.5 or more at 25°C.
[2] The method for producing a polarizing film according to [1], wherein the treatment liquid has a pH of 6.5 or less at 25°C.
[3] A step of producing a polarizing film by the method for producing a polarizing film according to any one of [1] or [2];
A method for producing a polarizing plate, comprising a bonding step of bonding a protective film to one or both sides of the polarizing film via a bonding layer.

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

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows typically an example of the manufacturing method of the polarizing film which concerns on this invention, and a polarizing film manufacturing apparatus used therewith.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In the drawings, the scale of each component is appropriately adjusted to facilitate understanding, and the scale of each component shown in the drawing does not necessarily match the scale of the actual component.

<Method for producing polarizing film>
In the present embodiment, the polarizing film is a uniaxially stretched polyvinyl alcohol resin film in which a dichroic dye (iodine or dichroic dye) is adsorbed and oriented. The polyvinyl alcohol-based resin that constitutes the polyvinyl alcohol-based resin film is usually obtained by saponifying a polyvinyl acetate-based resin. The degree of saponification is generally about 85 mol% or more, preferably about 90 mol% or more, more preferably about 99 mol% or more. The polyvinyl acetate-based resin may be, for example, polyvinyl acetate, which is a homopolymer of vinyl acetate, or a copolymer of vinyl acetate and other monomers copolymerizable therewith. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids and the like. The degree of polymerization of the polyvinyl alcohol resin is generally about 1000-10000, preferably about 1500-5000.

These polyvinyl alcohol-based resins may be modified, and for example, aldehyde-modified polyvinyl formal, polyvinyl acetal, polyvinyl butyral and the like may be used.

In the present embodiment, a polyvinyl alcohol-based resin film (original film) having a thickness of 65 μm or less (for example, 60 μm or less), preferably 50 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less is used as a starting material for polarizing film production. Use This makes it possible to obtain a thin polarizing film for which market demand is increasing. The width of the original film is not particularly limited, and can be, for example, 400 to 6000 mm. The original film is prepared, for example, as a roll of a long polyvinyl alcohol-based resin film (original roll).

The polarizing film is produced by unwinding the long raw film from the raw roll and continuously conveying it along the film conveying route of the polarizing film manufacturing apparatus, and processing liquid contained in the processing tank (hereinafter referred to as A long polarizing film can be continuously produced by carrying out a drying step after carrying out a predetermined treatment step of immersing the film in a "treatment bath" and drawing it out. The treatment process is not limited to the method of immersing the film in the treatment bath as long as it is a method of contacting the film with the treatment solution, and the treatment solution is attached to the film surface by spraying, flowing, dripping, or the like. It may also be a method of processing the film by allowing the When the treatment step is performed by immersing the film in a treatment bath, the treatment bath in which one treatment step is performed is not limited to one. One processing step may be completed.

Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a cross-linking liquid, a complementary color liquid, and a washing liquid. Then, as the above-mentioned treatment steps, a swelling step of contacting a swelling liquid to the raw film to perform a swelling treatment, a dyeing step of contacting a dyeing solution to the film after the swelling treatment and performing a dyeing process, and a dyeing process after the dyeing process A cross-linking step in which the film is brought into contact with a cross-linking solution to carry out cross-linking treatment, a complementary color step in which the complementary color solution is brought into contact with the film after cross-linking treatment to carry out complementary color treatment, and a cleaning solution is brought into contact with the film after the complementary color treatment. washing steps to be performed are exemplified. In addition, between these series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps), wet or dry uniaxial stretching treatment is performed. Other processing steps may be added as desired.

The production method of the present invention includes a step of treating with a treatment solution containing nitrate, boric acid and an iodide compound and having a pH of 4.5 or higher at 25°C. Since the treatment liquid contains nitrate, boric acid and an iodide compound, it can be used for a cross-linking treatment for cross-linking the film after the dyeing treatment, and can also be used for a complementary color treatment for adjusting the hue of the film. Further, since the pH range of the treatment liquid is limited, it can also be used for pH adjustment treatment for adjusting the pH of the film. Through such a treatment process, it is possible to obtain an effect of suppressing yellowing even in a high-temperature environment exceeding 95°C. can reduce the environmental impact of

The nitrate can contain at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. Preferably, the nitrate comprises zinc nitrate.

The concentration of nitrate is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, relative to 100 parts by mass of water. The concentration of nitrate is preferably 30 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less with respect to 100 parts by mass of water.

The concentration of the halide in the treatment liquid is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 1 part by mass or less with respect to 100 parts by mass of water. The processing liquid may be halide-free.

Hereinafter, an example of the method for producing a polarizing film according to the present invention will be described in detail with reference to FIG. FIG. 1 is a cross-sectional view schematically showing an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used therein according to the present invention. In the polarizing film manufacturing apparatus shown in FIG. 1, a raw film 10 made of a polyvinyl alcohol-based resin is continuously unwound from a raw roll 11 and conveyed along the film conveying route. Provided swelling bath (swelling liquid contained in the swelling tank) 13, dyeing bath (dyeing liquid contained in the dyeing tank) 15, cross-linking bath (cross-linking liquid contained in the cross-linking tank) 17, complementary color bath ( A complementary color liquid contained in a complementary color tank) 18 and a cleaning bath (a cleaning liquid contained in a cleaning tank) 19 are sequentially passed through, and finally passed through a drying oven 21 . The arrows in FIG. 1 indicate the transport direction of the film.

In the description of FIG. 1, "treatment tank" is a general term including a swelling tank, a dyeing tank, a cross-linking tank, a complementary color tank and a washing tank, and a "treatment liquid" is a swelling liquid, a dyeing liquid, a cross-linking liquid, a complementary color liquid and It is a general term including washing liquid, and "processing bath" is a generic term including swelling bath, dyeing bath, cross-linking bath, complementary color bath and washing bath. The swelling bath, the dyeing bath, the cross-linking bath, the complementary color bath, and the cleaning bath respectively constitute the swelling section, the dyeing section, the cross-linking section, and the cleaning section in the manufacturing apparatus of the present embodiment.

In addition to the treatment bath, the film transport path of the polarizing film manufacturing apparatus includes guide rolls 30 to 48, 60, 61 that support the film to be transported or can change the film transport direction, and the film to be transported. It can be constructed by arranging nip rolls 50 to 55 at appropriate positions that can press and hold the and can apply a driving force to the film by its rotation, or can further change the film conveying direction. Guide rolls and nip rolls can be arranged before, after, or in each treatment bath, so that the film can be introduced into, immersed in, and pulled out of the treatment bath (see FIG. 1). For example, the film can be immersed in each treatment bath by providing one or more guide rolls in each treatment bath and transporting the film along these guide rolls.

In the polarizing film manufacturing apparatus shown in FIG. 1, nip rolls are arranged before and after each treatment bath (nip rolls 50 to 54). It is possible to perform inter-roll stretching in which longitudinal uniaxial stretching is performed with a peripheral speed difference between rolls. Each step will be described below.

(Swelling process)
The swelling process is performed for the purpose of removing foreign matter from the surface of the raw film 10, removing the plasticizer in the raw film 10, imparting easy dyeability, plasticizing the raw film 10, and the like. The processing conditions are determined within a range in which the object can be achieved and in which problems such as extreme dissolution or devitrification of the original film 10 do not occur.

Referring to FIG. 1, in the swelling step, raw film 10 is continuously unwound from raw film roll 11 and conveyed along a film conveying path, and raw film 10 is immersed in swelling bath 13 for a predetermined time. , and then withdrawing. In the example of FIG. 1 , the raw film 10 is conveyed along the film conveying path constructed by the guide rolls 60 and 61 and the nip rolls 50 from when the raw film 10 is unwound until it is immersed in the swelling bath 13. be done. In the swelling process, the film is conveyed along a film conveying path constructed by guide rolls 30 to 32 and nip rolls 51 .

As the swelling liquid of the swelling bath 13, in addition to pure water, boric acid (JP-A-10-153709), chloride (JP-A-06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol It is also possible to use an aqueous solution to which a range of about 0.01 to 10% by mass of such as is added.

The temperature of the swelling bath 13 is, for example, 10-50°C, preferably 10-40°C, more preferably 15-30°C. The immersion time of the original film 10 is preferably about 10 to 300 seconds, more preferably 20 to 200 seconds. Further, when the raw film 10 is a polyvinyl alcohol resin film stretched in a gas in advance, the temperature of the swelling bath 13 is, for example, 20 to 70.degree. C., preferably 30 to 60.degree. The immersion time of the original film 10 is preferably 30 to 300 seconds, more preferably 60 to 240 seconds.

In the swelling treatment, the raw film 10 tends to swell in the width direction, causing wrinkles in the film. As one means for transporting the film while removing the wrinkles, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used as the guide rolls 30, 31 and/or 32, or a cross guider or bend bar is used. , using other widening devices such as tenter clips. Another means for suppressing the occurrence of wrinkles is to apply a stretching treatment. For example, the uniaxial stretching process can be performed in the swelling bath 13 by utilizing the peripheral speed difference between the nip rolls 50 and 51 .

In the swelling process, the film swells and expands also in the transport direction of the film. Therefore, if the film is not actively stretched, it is placed, for example, before and after the swelling bath 13 in order to eliminate the slack of the film in the transport direction. It is preferable to take measures such as controlling the speed of the nip rolls 50 and 51 . In addition, for the purpose of stabilizing the film transport in the swelling bath 13, the water flow in the swelling bath 13 is controlled by a water shower, or an EPC device (Edge Position Control device: detects the edge of the film and controls the meandering of the film. It is also useful to use a device that prevents

In the example shown in FIG. 1, the film pulled out from the swelling bath 13 passes through the guide roll 32, the nip roll 51 and the guide roll 33 in order and is introduced into the dyeing bath 15. FIG.

(Dyeing process)
The dyeing step is performed for the purpose of adsorbing and orienting the dichroic dye on the polyvinyl alcohol-based resin film after the swelling treatment. The processing conditions are determined within a range in which the object can be achieved and in which problems such as extreme dissolution or devitrification of the film do not occur. Referring to FIG. 1, in the dyeing process, the film is conveyed along a film conveying path constructed by nip roll 51, guide rolls 33 to 36 and nip roll 52, and the film after swelling is stored in dyeing bath 15 (dyeing tank). It can be carried out by immersing the substrate in the treatment solution for a predetermined period of time and then withdrawing it. In order to enhance the dyeability of the dichroic dye, the film subjected to the dyeing process is preferably a film that has been uniaxially stretched to at least some degree, or instead of the uniaxial stretching process before the dyeing process, or In addition to the uniaxial stretching treatment before the dyeing treatment, it is preferable to perform the uniaxial stretching treatment during the dyeing treatment.

When iodine is used as the dichroic dye, the dyeing liquid in the dyeing bath 15 has a concentration by mass of iodine/potassium iodide/water=approximately 0.003 to 3/approximately 0.1 to 10/100, for example. Certain aqueous solutions can be used. Instead of potassium iodide, other iodides such as zinc iodide may be used, or potassium iodide and other iodides may be used in combination. Compounds other than iodide, such as boric acid, zinc chloride, and cobalt chloride, may coexist. When boric acid is added, it is distinguished from the cross-linking treatment and complementary color treatment described later in that it contains iodine. It can be regarded as a dyeing bath 15 . The temperature of the dyeing bath 15 when the film is immersed is usually 10 to 45° C., preferably 10 to 40° C., more preferably 20 to 35° C., and the immersion time of the film is usually 30 to 600 seconds. It is preferably 60 to 300 seconds.

When a water-soluble dichroic dye is used as the dichroic dye, the dyeing solution in the dyeing bath 15 has a concentration of, for example, a dichroic dye/water in mass ratio of about 0.001 to 0.1/100. Aqueous solutions can be used. The dyeing bath 15 may contain a dyeing assistant or the like, and may contain, for example, an inorganic salt such as sodium sulfate or a surfactant. Only one type of dichroic dye may be used alone, or two or more types of dichroic dyes may be used in combination. The temperature of the dyeing bath 15 when the film is immersed is, for example, 20 to 80° C., preferably 30 to 70° C., and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

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

In the dyeing process as well as in the swelling process, the guide rolls 33, 34, 35 and/or 36 are provided with expander rolls, spiral rolls, crown rolls, etc., in order to convey the polyvinyl alcohol resin film while removing wrinkles of the film. Rolls with a widening function can be used, or other widening devices such as cross guiders, bend bars, tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to apply stretching treatment, as in the case of swelling treatment.

In the example shown in FIG. 1, the film drawn from dyeing bath 15 passes through guide roll 36, nip roll 52, and guide roll 37 in sequence and is introduced into cross-linking bath 17. In the example shown in FIG.

(Crosslinking step)
The cross-linking step is a process of cross-linking the film to improve water resistance and the like. Referring to FIG. 1, in the cross-linking step, the film is transported along a film transport path constructed by nip roll 52, guide rolls 37 to 40, and nip roll 53a, and dyed in cross-linking bath 17 (cross-linking liquid contained in cross-linking bath). It can be carried out by immersing the treated film for a predetermined time and then withdrawing it.

As the cross-linking liquid, a solution obtained by dissolving a cross-linking agent in a solvent can be used. Examples of cross-linking agents include boron compounds such as boric acid and borax, glyoxal, and glutaraldehyde. One of these may be used, or two or more of them may be used in combination. As a solvent, for example, water can be used, but an organic solvent compatible with water may be included. Although the concentration of the cross-linking agent in the cross-linking solution is not limited to this, it is preferably in the range of 1 to 20% by mass, more preferably 6 to 15% by mass.

The cross-linking liquid may be an aqueous solution containing, for example, about 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When the dichroic dye used in the dyeing treatment is iodine, the cross-linking liquid preferably contains iodide in addition to boric acid, and the amount thereof is, for example, 1 to 30 parts by weight per 100 parts by weight of water. can be Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodide, such as sodium thiosulfate, potassium sulfite, and sodium sulfate, may also coexist. Moreover, you may coexist nitrate. The nitrate can contain at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. Preferably, the nitrate comprises zinc nitrate.

In the cross-linking treatment, the concentrations of boric acid and iodide and the temperature of the cross-linking bath 17 can be appropriately changed depending on the purpose. The cross-linking liquid can be, for example, an aqueous solution having a concentration by weight of boric acid/iodide/water=3 to 10/1 to 20/100. If necessary, other cross-linking agents may be used instead of boric acid, or boric acid and other cross-linking agents may be used in combination. The temperature of the cross-linking bath 17 when the film is immersed is usually 50 to 70° C., preferably 53 to 65° C., and the film immersion time is usually 10 to 600 seconds, preferably 20 to 300 seconds, more preferably 20 to 200 seconds. When dyeing treatment and cross-linking treatment are performed in this order on a polyvinyl alcohol resin film that has been stretched in advance before swelling treatment, the temperature of the cross-linking bath 17 is usually 50 to 85°C, preferably 55 to 80°C.

The cross-linking treatment may be performed multiple times, usually 2 to 5 times. In this case, the composition and temperature of each cross-linking bath used may be the same or different within the above ranges. The uniaxial stretching treatment can also be performed in the cross-linking bath 17 by utilizing the peripheral speed difference between the nip roll 52 and the nip roll 53a.

In the cross-linking treatment, as in the swelling treatment, the guide rolls 37, 38, 39 and/or 40 are provided with expander rolls, spiral rolls, crown rolls, etc., in order to convey the polyvinyl alcohol resin film while removing wrinkles of the film. Other widening devices such as cross guiders, bend bars, tenter clips, etc. can be used. Another means for suppressing the occurrence of wrinkles is to apply stretching treatment, as in the case of swelling treatment.

(Complementary color process)
The complementary color process is a process for adjusting the hue of the film. Referring to FIG. 1, in the complementary color process, the film is transported along a film transport path constructed by nip roll 53a, guide rolls 41 to 44, and nip roll 53b, and is crosslinked in complementary color bath 18 (complementary color liquid stored in complementary color tank). It can be carried out by immersing the post-processed film for a predetermined period of time and then withdrawing it.

The complementary color liquid may be an aqueous solution containing, for example, about 1 to 10 parts by weight of boric acid per 100 parts by weight of water. Complementary color liquid, when the dichroic dye used in the dyeing treatment is iodine, preferably contains iodide in addition to boric acid, and the amount thereof is, for example, 1 to 30 parts by weight with respect to 100 parts by weight of water. can be Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodide, such as sodium thiosulfate, potassium sulfite, and sodium sulfate, may also coexist. Moreover, you may coexist nitrate. The nitrate can contain at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. Preferably, the nitrate comprises zinc nitrate.

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

The complementary color processing may be performed multiple times, and is normally performed 2 to 5 times. In this case, the composition and temperature of each complementary color bath to be used may be the same or different within the above ranges. Further, the uniaxial stretching treatment can be applied in the complementary color bath 18 by utilizing the peripheral speed difference between the nip rolls 53a and 53b.

In the complementary color treatment, as in the swelling treatment, the guide rolls 41, 42, 43 and/or 44 are provided with expander rolls, spiral rolls, crown rolls, etc., in order to convey the polyvinyl alcohol resin film while removing wrinkles of the film. Rolls with a widening function can be used, or other widening devices such as cross guiders, bend bars, tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to apply stretching treatment, as in the case of swelling treatment.

In the example shown in FIG. 1, the film drawn out from the complementary color bath 18 passes through the guide roll 44 and the nip roll 53b in order and is introduced into the cleaning bath 19. In the example shown in FIG.

(Washing process)
The example shown in FIG. 1 includes a washing step after the complementary color step. The cleaning treatment is performed for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step is performed by, for example, immersing the polyvinyl alcohol-based resin film that has undergone complementary color treatment in the cleaning bath 19 . The cleaning step is performed by spraying the film with the cleaning solution as a shower instead of immersing the film in the cleaning bath 19, or by combining immersion in the cleaning bath 19 and spraying the cleaning solution. can also

FIG. 1 shows an example in which a polyvinyl alcohol-based resin film is immersed in a cleaning bath 19 for cleaning treatment. The temperature of the washing bath 19 in the washing treatment is usually 2 to 40° C., and the immersion time of the film is usually 2 to 120 seconds.

Also in the cleaning process, for the purpose of conveying the polyvinyl alcohol resin film while removing wrinkles, the guide rolls 45, 46, 47 and/or 48 are rolls having a widening function such as expander rolls, spiral rolls and crown rolls. can be used, or other widening devices such as cross guiders, bend bars, tenter clips can be used. Further, in the film washing treatment, a stretching treatment may be applied to suppress the generation of wrinkles.

(Stretching process)
As described above, the raw film 10 is uniaxially stretched wet or dry during the series of treatment steps (that is, before and after any one or more treatment steps and/or during any one or more treatment steps). It is processed. A specific method of uniaxial stretching treatment is, for example, between two nip rolls (for example, two nip rolls arranged before and after the treatment bath) that constitute the film conveying path, and longitudinal uniaxial stretching is performed by applying a peripheral speed difference between the rolls. Stretching can be hot roll stretching or tenter stretching as described in Japanese Patent No. 2731813, preferably stretching between rolls. The uniaxial stretching process can be performed multiple times from raw film 10 to polarizing film 23 . As described above, the stretching treatment is also advantageous in suppressing the generation of wrinkles in the film.

The final cumulative draw ratio of the polarizing film 23 based on the raw film 10 is usually 4.5 to 7 times, preferably 5 to 6.5 times. The stretching process may be carried out in any treatment process, and even when the stretching treatment is carried out in two or more treatment processes, the stretching treatment may be carried out in any treatment process.

(Processing process)
The treatment step is a step of bringing the film into contact with a treatment liquid, the treatment liquid containing nitrate, boric acid and an iodide compound and having a pH of 4.5 or higher at 25°C. The pH of the treatment liquid at 25° C. is preferably 4.6 or higher, preferably 4.7 or higher, and more preferably 4.8 or higher. Also, the pH of the treatment liquid at 25° C. is preferably 6.8 or less, more preferably 6.5 or less, and even more preferably 6.3 or less. The processing step is, for example, the complementary color step described above. When the treatment process is a complementary color process, the above description of the complementary color process can be directly applied to the description of the process process. The treatment liquid contains nitrate, boric acid and an iodide compound and has a pH of 4.5 or higher. As the treatment liquid, an aqueous solution having a concentration by mass of nitrate/boric acid/iodide/water=0.5 to 2.2/1 to 5/3 to 30/100 can be used. The treatment liquid further contains a pH adjuster. Potassium hydroxide, sodium hydroxide, etc. are illustrated as a pH adjuster. The treatment liquid is adjusted to have a pH of 4.5 or more at 45° C. by adjusting the amount of the pH adjuster added. In this embodiment, a polarizing film having an excellent effect of suppressing yellowing even in a high-temperature environment exceeding 95° C. can be provided by including the treatment step.

(Second treatment step)
The present invention may include a second treatment step. This is the step of contacting the film with a second processing liquid containing nitrates, boric acid and iodide compounds and having a pH of less than 4.5 at 25°C. The second treatment step is, for example, the cross-linking step described above. When the second treatment step is a cross-linking step, the above description of the cross-linking step can be directly applied to the description of the second treatment step. The second treatment liquid contains nitrate, boric acid and an iodide compound, and has a pH of less than 4.5 at 25°C. As the second treatment liquid, an aqueous solution having a concentration by mass of nitrate/boric acid/iodide/water=0.5 to 2.2/3 to 10/1 to 20/100 can be used. The second treatment liquid may have a pH of less than 5.0 by adjusting the amounts of nitrate, boric acid, and iodine compound added, or may be adjusted to pH 5.0 at 25° C. by adding an acid pH adjuster. It may be less than 0. The second treatment liquid is preferably adjusted to have a pH of less than 5.0 at 25° C. by adjusting the amount of boric acid added.

When the second treatment step is included, it is preferably performed before the treatment step, preferably in the order of the second treatment step and the treatment step, and when the washing step is included, the second treatment step , treatment step, and washing step. The second treatment process and the treatment process are, for example, a cross-linking process and a complementary color process, a first complementary color process and a second complementary color process, a first cross-linking process and a second cross-linking process, and the like.

(Drying process)
Finally, it is preferable to dry the polyvinyl alcohol resin film. Drying of the film is not particularly limited, but can be performed using a drying oven 21 as in the example shown in FIG. The drying oven 21 may be equipped with, for example, a hot air dryer. The drying temperature is, for example, 30 to 100° C., and the drying time is, for example, 30 to 600 seconds. The treatment for drying the polyvinyl alcohol-based resin film can also be performed using a far-infrared heater.

(Other processing steps for polyvinyl alcohol-based resin film)
Processing other than the processing described above can also be added. Examples of treatments that may be added include immersion in aqueous iodide solutions without boric acid.

The polarizing film obtained as described above may be successively wound on a winding roll to form a roll, or may be directly subjected to a polarizing plate manufacturing method without being wound. The roll-shaped polarizing film can then be subjected to a method for producing a polarizing plate. One aspect of the method for producing a polarizing plate has a bonding step of bonding a protective film to one side or both sides of a polarizing film via a bonding layer.

<Polarizing film>
Preferably, the polarizing film according to the present embodiment is obtained by dyeing a polyvinyl alcohol-based resin film with a dichroic dye. The thickness of the polarizing film is preferably 5 μm or more and 60 μm or less, more preferably 7 μm or more and 30 μm or less.

The luminosity correction single transmittance Ty of the polarizing plate is preferably 35 to 47%, more preferably 36 to 45%, in consideration of the balance with the luminosity correction polarization degree Py. The visibility correction polarization degree Py is preferably 99.9% or more, more preferably 99.95% or more. A polarizing film constituting such a polarizing plate can be obtained by the manufacturing method described above.

The chlorine content of the polarizing film can be, for example, 900 ppm or less. Although the lower limit is not limited, the chlorine content of the polarizing film can be 0 ppm or more. The chlorine content of the polarizing film can be reduced by reducing the amount of chloride in the processing solution used in the manufacturing process of the polarizing film.

<Polarizing plate>
The polarizing plate according to this embodiment can be obtained by laminating a protective film on one or both sides of the polarizing film via a bonding layer. Protective films include, for example, films made of acetylcellulose-based resins such as triacetylcellulose and diacetylcellulose; films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; polycarbonate-based resin films, cyclo Olefin-based resin films; acrylic-based resin films; and films made of linear olefin-based resins such as polypropylene-based 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 is subjected to corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. may be treated. The bonding layer interposed between the polarizing film and the protective film can be formed using an adhesive or pressure-sensitive adhesive. As the adhesive, an active energy ray-curable adhesive such as an ultraviolet-curable adhesive, an aqueous solution of a polyvinyl alcohol-based resin, or an aqueous solution in which a cross-linking agent is added thereto, and a water-based adhesive such as a urethane-based emulsion adhesive. agents can be mentioned. A zinc compound such as zinc nitrate may be added to the water-based adhesive. The UV-curable adhesive may be a mixture of an acrylic compound and a photoradical polymerization initiator, a mixture of an epoxy compound and a photocationic polymerization initiator, or the like. Moreover, a cationic polymerizable epoxy compound and a radically polymerizable acrylic compound can be used together, and a photocationic polymerization initiator and a photoradical polymerization initiator can be used together as initiators.

<Image display device>
A polarizing plate can be used in an image display device. Examples of image display elements used in image display devices include liquid crystal display elements and organic EL display elements. In constructing a liquid crystal display device, the polarizing plate according to the present invention may be used by being placed on the viewing side, may be used by being placed on the backlight side, or may be used on both the viewing side and the backlight side. may be used for The image display device of the present invention can be used for mobile devices such as televisions, personal computers, mobile phones and tablet terminals, and in addition, it has a high inhibitory effect on yellowing in a high temperature environment, and provides stable images for a long period of time. Since it can exhibit a display function, it can be used particularly suitably for in-vehicle applications that are likely to be exposed to more severe temperature conditions. Vehicle applications include, for example, car navigation devices, speedometers, touch panels for air conditioners, image display devices used for back monitors and rear monitors, and the like.

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

(1) Measurement of thickness of polarizing film:
It was measured using a digital micrometer "MH-15M" manufactured by Nikon Corporation.

(2) Measurement of luminosity correction single transmittance of polarizing plate:
Measurement was performed using a spectrophotometer with an integrating sphere ["V7100" manufactured by JASCO Corporation, 2-degree field of view; C light source].

(3) Measurement of pH value:
It was measured using a pH meter (HORIBA D-54).

(4) Measurement of yellow index (yellowness):
A spectrophotometer CM-3700A manufactured by Konica Minolta was used. Reflected light was measured in SCI mode, tristimulus values (X, Y, Z) were obtained, and yellow index (YI) was calculated according to the following formula based on ASTM E 313-73 standards.
YI = 100 x (1.000 - 0.847 x Z/Y)

<Example 1>
A polarizing film of Example 1 was manufactured from a polyvinyl alcohol-based resin film using the manufacturing apparatus shown in FIG. Specifically, a long polyvinyl alcohol (PVA) raw film with a thickness of 45 μm [trade name “VF-PE #4500” manufactured by Kuraray Co., Ltd., saponification degree of 99.9 mol% or more] was wound from a roll. The film was continuously transported while being discharged, immersed in a swelling bath made of pure water at 23° C. for 110 seconds, and uniaxially stretched by 2.1 times (swelling step). After that, the film pulled out from the swelling bath was immersed in a 23° C. dyeing bath containing iodine with a ratio of iodine/boric acid/water of 1.0/0.5/100 (mass ratio) for 163 seconds. It was immersed and uniaxially stretched to 1.22 times (dyeing step). Then, the film pulled out from the dyeing bath is immersed in a cross-linking bath at 59° C. containing a cross-linking solution of potassium iodide/boric acid/water of 2.3/3.7/100 (mass ratio) for an immersion time of 92 seconds. and uniaxially stretched to 2.24 times (stretching/crosslinking step).

Subsequently, the film pulled out from the cross-linking bath was mixed with zinc nitrate hexahydrate/potassium iodide/boric acid/water/potassium hydroxide at 1.3/2.7/5.0/100/0.35 (mass The film was immersed in a complementary color bath at 45° C. for 14 seconds and uniaxially stretched to 1.02 times (complementary color step, treatment step). Subsequently, the pulled out film was dried by staying in a drying oven at a temperature of 55° C. for 90 seconds (drying step). The complementary color liquid had a pH of 4.95 at 25°C. The thickness of the obtained polarizing film was 19 μm.

(Preparation of PVA solution A for adhesive)
50 g of a modified PVA resin containing an acetoacetyl group (“Gohsenex Z-410” manufactured by Mitsubishi Chemical Corporation) is dissolved in 950 g of pure water, heated at 90° C. for 2 hours and then cooled to room temperature to obtain a PVA solution for adhesives. (hereinafter referred to as "PVA solution A").

(Preparation of adhesive)
The PVA solution A prepared above, pure water, a 40% by mass glyoxal solution, and methanol were blended so as to have the contents shown below per 100 parts by mass of the adhesive, and the adhesive (hereinafter, "adhesive 1") was added. ) was produced.
PVA content: 3 parts by mass Methanol content: 36 parts by mass Glyoxal content: 0.3 parts by mass Pure water: 60.7 parts by mass

<Preparation of transparent protective film>
A commercially available cellulose acylate film TD40N (manufactured by Fuji Film Co., Ltd., film thickness 40 μm) was immersed in a 1.5 mol/L NaOH aqueous solution (saponification solution) maintained at 55° C. for 2 minutes, and then washed with water. Thereafter, the film was immersed in a 0.05 mol/L sulfuric acid aqueous solution at 25° C. for 30 seconds, and then passed through a washing bath under running water for 30 seconds to neutralize the film. After removing the water by repeating water removal with an air knife three times, the film was dried by staying in a drying zone at 70° C. for 15 seconds to prepare a saponified film to obtain a transparent protective film.

[Preparation of polarizing plate]
Transparent protective films are laminated on both sides of the polarizing film with the adhesive 1 interposed therebetween using a roll laminating machine, and then subjected to heat treatment at 65° C. for 12 minutes to dry the adhesive and form the polarizing plate 1. Obtained. The single transmittance of the polarizing plate was 40.8%.

(Adjustment of moisture content of polarizing plate (polarizing film))
The polarizing plate 1 obtained above was stored at a temperature of 20° C. and a relative humidity of 40% for 72 hours. Moisture content was measured using the Karl Fischer method at 66, 69 and 72 hours of storage. Under any humidity conditions, the moisture content did not change after storage for 66 hours, 69 hours, and 72 hours. Therefore, it can be considered that the moisture content of the polarizing plate 1 is the same as the equilibrium moisture content of the 72-hour storage environment used in this experimental example. When the moisture content of the polarizing plate 1 reaches equilibrium in a certain storage environment, it can be considered that the moisture content of the polarizing film in the polarizing plate 1 also reaches equilibrium in that storage environment. Moreover, when the moisture content of the polarizing film in the polarizing plate 1 reaches equilibrium in a certain storage environment, it can be considered that the moisture content of the polarizing plate 1 also reaches equilibrium in that storage environment.

The moisture content of the polarizing plate 1 was adjusted by storing it for 72 hours under conditions of a temperature of 20° C. and a relative humidity of 40%.

<High temperature durability evaluation>
An acrylic pressure-sensitive adhesive (manufactured by Lintec Corporation, product number: #7) was formed on both sides of the polarizing plate 1 whose water content was adjusted in this manner. Furthermore, it is cut into a size of 110 mm × 60 mm so that the absorption axis of the polarizing plate is parallel to the long side. An evaluation sample was prepared by laminating. The samples for evaluation thus obtained were autoclaved for 15 minutes at 50° C. and 5 atm, and then exposed to an environment of 105° C. for 168 hours to perform a high temperature durability test. The YI after 168 hours was 45. Since the YI after 168 hours was 50 or less, it was found that the polarizing plate 1 exhibits excellent durability while being halogen-free.

10 raw film made of polyvinyl alcohol-based resin, 11 raw roll, 13 swelling bath, 15 dyeing bath, 17 cross-linking bath, 18 complementary color bath, 19 cleaning bath, 21 drying furnace, 23 polarizing film, 30 to 48, 60, 61 guide rolls, 50-52, 53a, 53b, 54, 55 nip rolls.

Claims (3)

A method for producing a polarizing film from a polyvinyl alcohol resin film,
A treatment step of bringing the polyvinyl alcohol resin film into contact with a treatment liquid,
The method for producing a polarizing film, wherein the treatment liquid contains nitrate, boric acid and an iodide compound, and has a pH of 4.5 or more at 25°C. The method for producing a polarizing film according to claim 1, wherein the treatment liquid has a pH of 6.5 or less at 25°C. A step of producing a polarizing film by the method for producing a polarizing film according to claim 1 or 2;
A method for producing a polarizing plate, comprising a bonding step of bonding a protective film to one or both sides of the polarizing film via a bonding layer.

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