CN119065047A

CN119065047A - Method for manufacturing polarizing plate

Info

Publication number: CN119065047A
Application number: CN202411088324.5A
Authority: CN
Inventors: 岛津亮; 后藤周作; 森崎真由美
Original assignee: Nitto Denko Corp
Current assignee: Nitto Denko Corp
Priority date: 2019-06-25
Filing date: 2020-06-19
Publication date: 2024-12-03
Also published as: JP2021004946A; JP7312038B2; CN114096898A; KR20220023352A; WO2020262213A1

Abstract

The present invention provides a polarizing plate having high single-sheet transmittance and excellent durability under high-temperature and high-humidity environments, and a method for manufacturing the polarizing plate. The method for producing a polarizing plate comprises applying an acidic aqueous solution containing an acid component and having a pH of 2.5 or less to at least one surface of a polarizing film.

Description

Method for manufacturing polarizing plate

The present application is a divisional application of application number 202080046164.1, application number 2020, application name "method for manufacturing polarizing plate".

Technical Field

The present invention relates to a method for manufacturing a polarizing plate.

Background

In a liquid crystal display device, which is a typical image display device, polarizing films are disposed on both sides of a liquid crystal cell due to an image forming method thereof. As a method for producing a polarizing film, for example, a method has been proposed in which a laminate having a resin substrate and a polyvinyl alcohol resin layer is stretched and then subjected to dyeing treatment to obtain a polarizing film on the resin substrate (for example, patent document 1). According to this method, a polarizing film having a small thickness can be obtained, and thus attention has been paid to contributing to the reduction in thickness of image display devices in recent years. However, a thin polarizing film is required to have further improved durability under high-temperature and high-humidity environments.

Prior art literature

Patent literature

Patent document 1 Japanese patent laid-open No. 2001-343521

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above-described conventional problems, and a main object of the present invention is to provide a method for producing a polarizing plate having high single-sheet transmittance and excellent durability under high-temperature and high-humidity environments.

Solution for solving the problem

According to one embodiment of the present invention, there is provided a method for producing a polarizing plate, comprising applying an acidic aqueous solution containing an acid component and having a pH of 2.5 or less to at least one surface of a polarizing film.

In one embodiment, the acid component includes hydrogen chloride.

In one embodiment, the acid component includes sulfuric acid.

In one embodiment, the acidic aqueous solution further includes a water-soluble resin, which is applied to at least one surface of the polarizing film and dried to form a functional film.

In one embodiment, the water-soluble resin includes a polyvinyl alcohol resin.

In one embodiment, the polymerization degree of the polyvinyl alcohol resin is 1500 to 4500.

In one embodiment, the viscosity of the acidic aqueous solution is 350mpa·sec or less.

ADVANTAGEOUS EFFECTS OF INVENTION

The production method of the present invention comprises applying an acidic aqueous solution having a predetermined pH to the surface of a polarizing film. According to this manufacturing method, a polarizing plate having high single-sheet transmittance and excellent durability in a high-temperature and high-humidity environment can be obtained.

Drawings

Fig. 1 is a schematic diagram showing an example of a drying shrinkage process using a heating roller.

Fig. 2 is a schematic cross-sectional view of an example of a polarizing plate obtained by the production method of the present invention.

Fig. 3 is a schematic cross-sectional view of another example of a polarizing plate obtained by the production method of the present invention.

Detailed Description

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

A. method for manufacturing polarizing plate

The method for producing a polarizing plate according to an embodiment of the present invention comprises applying an acidic aqueous solution containing an acid component and having a pH of 2.5 or less to at least one surface of a polarizing film. By applying the acidic aqueous solution to the surface, the durability of the polarizing film in a high-temperature and high-humidity environment can be improved.

The polarizing film included in the polarizing plate may be formed using a single polyvinyl alcohol (PVA) -based resin film, or may be formed using a laminate of two or more layers including PVA-based resin layers. Specific examples of the polarizing film obtained by using the laminate include a polarizing film obtained by using a laminate of a thermoplastic resin substrate and a PVA-based resin layer formed on the thermoplastic resin substrate.

A-1 Process for producing polarizing plate Using laminate

A method for producing a polarizing plate according to one embodiment of the present invention comprises forming a PVA based resin layer on one side of a long thermoplastic resin substrate to form a laminate, stretching and dyeing the laminate to form a polarizing film from the PVA based resin layer, and applying an acidic aqueous solution containing an acid component and having a pH of 2.5 or less to at least one surface of the polarizing film. In the present embodiment, it is preferable that the PVA-based resin layer is formed into a polarizing film by sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, and an in-water stretching treatment. Further, the PVA-based resin layer preferably contains a halide and a PVA-based resin. The method may further include a drying shrinkage treatment of heating the laminate after the underwater stretching treatment while conveying the laminate along the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction. The drying shrinkage treatment is preferably performed by using a heating roller, and the temperature of the heating roller is preferably 60 ℃ to 120 ℃. In particular, a laminate including a PVA-based resin layer including a halide and a PVA-based resin is produced, and stretching of the laminate is performed in multiple stages including air-assisted stretching and in-water stretching, and the stretched laminate is heated by a heating roller, whereby a polarizing film having excellent optical characteristics (typically, single-sheet transmittance and polarization degree) can be obtained. The method for producing the polarizing plate includes, when the drying shrinkage treatment is performed, the application of the acidic aqueous solution may be performed after the stretching treatment in water and before the drying shrinkage treatment or after the drying shrinkage treatment.

A-1-1. Production of laminate

As a method for producing a laminate of the thermoplastic resin base material and the PVA-based resin layer, any suitable method can be used. It is preferable that the PVA-based resin layer is formed on the thermoplastic resin substrate by coating a coating liquid containing a PVA-based resin on the surface of the thermoplastic resin substrate and drying.

As a coating method of the coating liquid, any suitable method can be used. Examples thereof include roll coating, spin coating, bar coating, dip coating, die coating, curtain coating, spray coating, knife coating (comma coating, etc.), and the like. The coating/drying temperature of the coating liquid is preferably 50 ℃ or higher.

The thickness of the PVA resin layer is preferably 3 μm to 40 μm, more preferably 3 μm to 20 μm.

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be subjected to a surface treatment (for example, corona treatment or the like), or an easy-to-adhere layer may be formed on the thermoplastic resin substrate. By performing such a treatment, the adhesion between the thermoplastic resin base material and the PVA-based resin layer can be improved.

As the thermoplastic resin base material, any suitable thermoplastic resin film can be used. Details of the thermoplastic resin base material are described in, for example, japanese patent application laid-open No. 2012-73580. The entire disclosure of this publication is incorporated by reference into this specification. Preferably, a polyester resin is used, and more preferably, a polyethylene terephthalate resin is used.

The coating liquid contains a PVA-based resin, preferably further contains a halide. The coating liquid may be, for example, a solution obtained by dissolving a halide and a PVA-based resin in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various diols, polyols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These may be used singly or in combination of two or more. Among these, the solvent is preferably water.

As the PVA-based resin, any suitable resin may be used. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer can be obtained by saponifying an ethylene-vinyl acetate copolymer. The saponification degree of the PVA based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The saponification degree can be determined in accordance with JIS K6726-1994. By using the PVA-based resin having such a saponification degree, a polarizing film excellent in durability can be obtained. If the saponification degree is too high, gelation may occur.

The average polymerization degree of the PVA-based resin may be appropriately selected according to the purpose. The average polymerization degree is usually 1000 to 10000, preferably 1200 to 4500, more preferably 1500 to 4300. The average polymerization degree can be determined in accordance with JIS K6726-1994.

The PVA-based resin preferably contains an acetoacetyl-modified PVA-based resin. With such a constitution, a polarizing film having a desired mechanical strength can be obtained. When the total amount of the PVA based resin is 100% by weight, the blending amount of the acetoacetyl-modified PVA based resin is preferably 5% by weight to 20% by weight, more preferably 8% by weight to 12% by weight. If the compounding amount is in such a range, a polarizing film having more excellent mechanical strength can be obtained.

The concentration of the PVA-based resin in the coating liquid is preferably 3 parts by weight to 20 parts by weight relative to 100 parts by weight of the solvent. If the resin concentration is such, a uniform coating film can be formed by adhering to the thermoplastic resin substrate.

Any suitable halide may be used as the halide. Examples thereof include iodide and sodium chloride. Examples of the iodide include potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferable.

The amount of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight relative to 100 parts by weight of the PVA based resin, more preferably 10 parts by weight to 15 parts by weight relative to 100 parts by weight of the PVA based resin. If the amount of the halide exceeds 20 parts by weight relative to 100 parts by weight of the PVA-based resin, the halide may ooze out, and the finally obtained polarizing film may be clouded.

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin is increased by stretching the PVA-based resin layer, but when the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules may be disturbed, and the orientation may be reduced. In particular, when a laminate of a thermoplastic resin substrate and a PVA-based resin layer is stretched in boric acid water, the degree of orientation tends to be significantly reduced when the laminate is stretched in boric acid water at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin substrate. For example, stretching of a PVA film alone in boric acid water is usually performed at 60 ℃, whereas stretching of a laminate of a-PET (thermoplastic resin base) and a PVA-based resin layer is performed at a high temperature of about 70 ℃, and in this case, the orientation of PVA at the initial stage of stretching may be reduced at a stage before the stretching in water increases. In contrast, by producing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate and stretching the laminate in water at a high temperature in air (auxiliary stretching) before stretching the laminate in boric acid water, crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after the auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, disturbance of orientation and decrease of orientation of polyvinyl alcohol molecules can be suppressed as compared with the case where the PVA-based resin layer does not contain a halide. This can improve the optical characteristics of the polarizing film obtained by the treatment step of immersing the laminate in a liquid, such as dyeing treatment or stretching treatment in water.

Additives may be further compounded into the coating liquid. Examples of the additive include plasticizers and surfactants. Examples of the plasticizer include polyols such as ethylene glycol and glycerin. Examples of the surfactant include nonionic surfactants. They can be used for the purpose of further improving the uniformity, dyeing property, stretchability of the resulting PVA-based resin layer.

A-1-2 air assisted stretching treatment

In particular, in order to obtain high optical properties, a two-stage stretching method in which dry stretching (auxiliary stretching) and in-water stretching (preferably in-boric acid water stretching) are combined may be selected. By introducing the auxiliary stretching as in the two-stage stretching, the stretching can be performed while suppressing crystallization of the thermoplastic resin base material, and the problem of the decrease in stretchability due to excessive crystallization of the thermoplastic resin base material in the subsequent in-water stretching (preferably in-boric acid water stretching) can be solved, whereby the laminate can be stretched to a higher magnification. Further, when the PVA-based resin is coated on the thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, it is necessary to lower the coating temperature as compared with the case of coating the PVA-based resin on a usual metal roll, and as a result, there is a possibility that crystallization of the PVA-based resin is relatively lowered, and sufficient optical characteristics cannot be obtained. In contrast, by introducing the auxiliary stretching, even when the PVA-based resin is coated on the thermoplastic resin substrate, the crystallinity of the PVA-based resin can be improved, and high optical characteristics can be realized. Further, by increasing the orientation of the PVA-based resin in advance, it is possible to prevent problems such as a decrease in the orientation and dissolution of the PVA-based resin when immersed in water in the subsequent dyeing step and stretching step, and to realize high optical characteristics.

The stretching method of the air-assisted stretching may be fixed-end stretching (for example, stretching using a tenter), or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls having different peripheral speeds), and free-end stretching may be positively employed in order to obtain high optical characteristics. In one embodiment, the air stretching process includes a heated roll stretching step of stretching the laminate while conveying the laminate in the longitudinal direction thereof by using a peripheral speed difference between heated rolls. Typically, the air stretching process includes a zone stretching process and a heated roll stretching process. The order of the region stretching step and the heat roller stretching step is not limited, and the region stretching step may be performed first, or the heat roller stretching step may be performed first. The zone stretching process may also be omitted. In one embodiment, the zone stretching process and the heated roll stretching process are sequentially performed. In another embodiment, the stretching is performed by grasping the film end and widening the distance between the tenters in the flow direction in the tenter stretching machine (the extension of the distance between the tenters becomes the stretching magnification). At this time, the distance of the tenter in the width direction (the direction perpendicular to the flow direction) is set to be arbitrarily close. The stretching ratio in the flow direction can be preferably set so as to be closer to the free end stretching. In the case of the free end stretching, it is calculated by a shrinkage ratio= (1/stretching ratio) ^1/2 in the width direction.

The air-assisted stretching may be performed in one stage or in a plurality of stages. When the stretching is performed in a plurality of stages, the stretching ratio is a product of the stretching ratios of the respective stages. The stretching direction in the air-assisted stretching is preferably substantially the same as the stretching direction in the water stretching.

The stretching ratio in the air auxiliary stretching is preferably 2.0 times to 3.5 times. The maximum stretching ratio when combining the air-assist stretching and the underwater stretching is preferably 5.0 times or more, more preferably 5.5 times or more, and still more preferably 6.0 times or more the original length of the laminate. In the present specification, "maximum stretch ratio" means a stretch ratio immediately before the laminate breaks, and means a stretch ratio at which the laminate breaks is confirmed separately and is lower than the value by 0.2.

The stretching temperature of the air-assisted stretching may be set to any suitable value depending on the material forming the thermoplastic resin base material, the stretching method, and the like. The stretching temperature is preferably not less than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably not less than the glass transition temperature (Tg) +10 ℃, particularly preferably not less than tg+15 ℃. On the other hand, the upper limit of the stretching temperature is preferably 170 ℃. By stretching at such a temperature, crystallization of the PVA-based resin can be suppressed from proceeding rapidly, and defects caused by the crystallization (e.g., impeding orientation of the PVA-based resin layer by stretching) can be suppressed.

A-1-3 insolubilization treatment, dyeing treatment and crosslinking treatment

If necessary, the insolubilization treatment is performed after the air-assisted stretching treatment and before the underwater stretching treatment and dyeing treatment. Typically, the insolubilization treatment is performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Typically, the dyeing treatment is performed by dyeing the PVA-based resin layer with iodine. If necessary, after the dyeing treatment and before the stretching treatment in water, a crosslinking treatment is performed. Typically, the crosslinking treatment is performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Details of the insolubilization treatment, dyeing treatment, and crosslinking treatment are described in, for example, japanese patent application laid-open No. 2012-73580.

A-1-4 in-water stretching treatment

The stretching treatment in water is performed by immersing the laminate in a stretching bath. According to the in-water stretching treatment, the stretching can be performed at a temperature lower than the glass transition temperature (typically, about 80 ℃) of the thermoplastic resin base material and the PVA-based resin layer, and the PVA-based resin layer can be stretched to a high magnification while suppressing crystallization. As a result, a polarizing film having excellent optical characteristics can be produced.

Any suitable method may be used for stretching the laminate. Specifically, the stretching may be performed at a fixed end or at a free end (for example, a method in which a laminate is uniaxially stretched by passing it between rolls having different peripheral speeds). The free end stretch is preferably selected. Stretching of the laminate may be performed in one stage or in a plurality of stages. When the stretching is performed in a plurality of stages, the stretching ratio (maximum stretching ratio) of the laminate to be described later is a product of the stretching ratios of the respective stages.

The stretching in water is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in boric acid water). By using an aqueous boric acid solution as the stretching bath, rigidity that can withstand tension applied at the time of stretching and water-insoluble water resistance can be imparted to the PVA-based resin layer. Specifically, boric acid is capable of generating a tetrahydroxyborate anion in an aqueous solution and crosslinking with the PVA-based resin via hydrogen bonds. As a result, the PVA-based resin layer can be given rigidity and water resistance, and stretched well, and a polarizing film having excellent optical characteristics can be produced.

The aqueous boric acid solution is preferably obtained by dissolving boric acid and/or a borate in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, particularly preferably 3 to 5 parts by weight, based on 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film having higher characteristics can be produced. In addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent may be used.

Preferably, iodide is added to the stretching bath (boric acid aqueous solution). By adding iodide, elution of iodine adsorbed to the PVA-based resin layer can be suppressed. Specific examples of iodides are described above. The concentration of the iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, based on 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40 ℃ to 85 ℃, more preferably 60 ℃ to 75 ℃. If the temperature is such, the PVA-based resin layer can be stretched to a high magnification while suppressing dissolution. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60 ℃ or higher, in terms of the relationship with the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40 ℃, the thermoplastic resin base material may not be stretched well in consideration of plasticization by water. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer becomes, and there is a possibility that excellent optical characteristics cannot be obtained. The dipping time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio based on stretching in water is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretch ratio of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more, relative to the original length of the laminate. By realizing such a high stretching ratio, a polarizing film extremely excellent in optical characteristics can be produced. Such a high stretching ratio can be achieved by employing an in-water stretching method (boric acid in-water stretching).

A-1-5 cleaning treatment

The washing treatment is preferably performed after the stretching treatment in water and before the drying shrinkage treatment. Typically, the washing treatment is performed by immersing the PVA-based resin layer in an aqueous solution containing an iodide such as potassium iodide. The iodide concentration in the cleaning liquid is preferably 0.5 to 10 wt%, more preferably 0.5 to 5wt%, and still more preferably 1 to 4 wt%. The temperature of the cleaning liquid is usually 10 to 50 ℃, preferably 20 to 35 ℃. The dipping time is usually 1 second to 1 minute, preferably 10 seconds to 1 minute.

A-1-6 drying shrinkage treatment

The drying shrinkage treatment may be performed by zone heating in which the entire zone is heated, or may be performed by heating a conveying roller (using a so-called heating roller) (heating roller drying method). Both are preferably used. By drying the laminate using a heating roller, heating warpage of the laminate can be effectively suppressed, and a polarizing film excellent in appearance can be produced. Specifically, by drying the laminate in a state of being brought along the heating roller, the crystallization of the thermoplastic resin base material can be effectively promoted to increase the crystallinity, and even at a low drying temperature, the crystallinity of the thermoplastic resin base material can be satisfactorily increased. As a result, the rigidity of the thermoplastic resin base material increases, and the PVA-based resin layer is allowed to shrink due to drying, so that warping can be suppressed. Further, since the laminate can be dried while maintaining a flat state by using the heating roller, not only warpage but also occurrence of wrinkles can be suppressed. At this time, the laminate is shrunk in the width direction by the drying shrinkage treatment, whereby the optical characteristics can be improved. This is because the orientation of PVA and PVA/iodine complex can be effectively improved. The shrinkage in the width direction of the laminate by the drying shrinkage treatment is preferably 2% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using the heating roller, the laminate can be continuously contracted in the width direction while being conveyed, and high productivity can be achieved.

Fig. 1 is a schematic diagram showing an example of the drying shrinkage treatment. In the drying shrinkage process, the laminate 200 is dried while being conveyed by conveying rollers R1 to R6 and guide rollers G1 to G4 heated to a predetermined temperature. In the illustrated example, the conveying rollers R1 to R6 are disposed so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but the conveying rollers R1 to R6 may be disposed so as to continuously heat only one surface (for example, the surface of the thermoplastic resin substrate) of the laminate 200, for example.

The drying condition can be controlled by adjusting the heating temperature of the conveying roller (temperature of the heating roller), the number of heating rollers, the time of contact with the heating roller, and the like. The temperature of the heating roller is preferably 60 to 120 ℃, more preferably 65 to 100 ℃, and particularly preferably 70 to 80 ℃. The crystallinity of the thermoplastic resin can be increased well, warpage can be suppressed well, and an optical laminate extremely excellent in durability can be produced. The temperature of the heating roller may be measured by a contact thermometer. In the example of the figure, 6 conveying rollers are provided, but there is no particular limitation as long as the conveying rollers are plural. The number of the conveying rollers is usually 2 to 40, preferably 4 to 30. The contact time (total contact time) between the laminate and the heating roller is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, and still more preferably 1 to 10 seconds.

The heating roller may be provided in a heating furnace (for example, an oven) or may be provided in a general production line (in a room temperature environment). Preferably, the air blower is provided in a heating furnace provided with an air blowing unit. By using the combination of the drying by the heating roller and the hot air drying, abrupt temperature changes between the heating rollers can be suppressed, and the shrinkage in the width direction can be easily controlled. The temperature of the hot air drying is preferably 30-100 ℃. In addition, the hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10m/s to 30 m/s. The wind speed is the wind speed in the heating furnace, and can be measured by a mini-blade type digital anemometer.

A-1-7. Coating of acidic aqueous solutions

In one embodiment, an acidic aqueous solution containing an acid component and having a pH of 2.5 or less is applied to the polarizing film surface of the laminate (polarizing plate) of the thermoplastic resin substrate and the polarizing film obtained in the above-described manner. In this embodiment, the thermoplastic resin substrate may be directly used as a protective layer of the polarizing film. In another embodiment, a resin film (to be a protective layer) is bonded to the polarizing film surface of a laminate of a thermoplastic resin substrate and a polarizing film, a laminate of a protective layer/polarizing film/thermoplastic resin substrate is produced, the thermoplastic resin substrate is peeled from the laminate, a laminate of a protective layer/polarizing film (polarizing plate) is produced, and the acidic aqueous solution is applied to the polarizing film surface of the polarizing plate obtained. After the application of the acidic aqueous solution, a separate protective layer may be attached to the side of the polarizing film on which the protective layer (thermoplastic resin substrate) is not provided (the side of the acidic aqueous solution on which the protective layer is applied), if necessary.

As the acid component contained in the acidic aqueous solution, any suitable acid may be used as long as the pH of the acidic aqueous solution can be adjusted to 2.5 or less. Specific examples of the acid component include hydrogen chloride, sulfuric acid, nitric acid, phosphoric acid, and citric acid, and among these, strong acids (for example, acids having pKa < 3) such as hydrogen chloride, sulfuric acid, and nitric acid are preferable, and hydrogen chloride and sulfuric acid are more preferable. The acid component may be used alone or in combination of two or more.

The concentration of the acid component in the acidic aqueous solution may be appropriately set in consideration of a desired pH. Typically, the pH (20 ℃) of the acidic aqueous solution is 2.5 or less, preferably 2.3 or less, more preferably 2.1 or less. By applying an acidic aqueous solution having a pH of 2.5 or less, preferably 2.3 or less, to the surface of the polarizing film, when the polarizing film is exposed to a high-temperature and high-humidity environment, the absorbance at 600nm (the absorption of PVA-I ₅ ^-) which would be reduced if the polarizing film were in general can be increased, and the decrease in absorbance at 470nm (the absorption of PVA-I ₃ ^-) can be suppressed, so that the durability of the polarizing film in a high-temperature and high-humidity environment can be improved. The lower limit of the pH of the acidic aqueous solution is not particularly limited, and may be 0.9 or more.

The acidic aqueous solution may further contain a water-soluble resin in addition to the acid component. In this case, the acidic aqueous solution may be an aqueous solution obtained by dissolving an acid component and a water-soluble resin in water. An acidic aqueous solution containing a water-soluble resin is used, and the acidic aqueous solution is applied to the surface of the polarizing film and dried, whereby a functional film containing an acid component and having the water-soluble resin as a base resin is formed on the surface of the polarizing film. By providing such a functional film adjacent to the polarizing film, the acid component can be more suitably oozed from the functional film to the polarizing film under a humidified environment to exert an effect of improving the durability of the polarizing film.

Examples of the water-soluble resin include PVA-based resins, acrylic resins, polysulfone-based resins, and polystyrene-based resins. Among them, PVA-based resins can be preferably used. The PVA-based resin has excellent adhesion to the polarizing film, and furthermore, an aqueous solution having excellent handleability is easily formed, and an appropriate mechanical strength can be imparted to the obtained functional film. The water-soluble resin may be used alone or in combination of two or more.

As the PVA-based resin, any suitable resin may be used. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. The polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer can be obtained by saponifying an ethylene-vinyl acetate copolymer. The saponification degree of the PVA based resin is preferably 98.00 mol% to 99.99 mol%, more preferably 99.50 mol% to 99.99 mol%. By using the PVA-based resin having such a saponification degree, an effect of improving durability under a high-temperature and high-humidity environment can be suitably obtained.

The average polymerization degree of the PVA-based resin may be appropriately selected according to the purpose. The average polymerization degree is preferably 1500 to 4500, more preferably 1800 to 4000, and even more preferably 2000 to 3500.

The concentration of the water-soluble resin (for example, the concentration of the PVA based resin) in the acidic aqueous solution is preferably 0.02 to 6.0 wt%, more preferably 0.04 to 5.0 wt%, and still more preferably 0.1 to 4.0 wt%. In the case of such a resin concentration, a uniform functional film can be formed in close contact with the polarizing film.

The viscosity (20 ℃) of the acidic aqueous solution containing the water-soluble resin is preferably 0 mPas to 350 mPas, more preferably 10 mPas to 200 mPas, and still more preferably 30 mPas to 120 mPas. If the viscosity is such, the handleability (coatability) is good.

As a method for applying the acidic aqueous solution, any suitable method can be used. Specific examples thereof include the method described in item A-1-1 as a method for applying a coating liquid for forming a PVA based resin layer (polarizing film).

The coating amount of the acidic aqueous solution containing the water-soluble resin can be appropriately set according to the thickness of the functional film as a target. The coating amount of the acidic aqueous solution containing no water-soluble resin is, for example, 7g/m ²～15g/m², preferably 9g/m ²～11g/m².

Typically, after the acidic aqueous solution is applied, the applied layer (coated surface) is dried. The drying temperature is, for example, 40 ℃ to 100 ℃, and the drying time is, for example, 1 minute to 20 minutes.

A-2 method for producing polarizing plate Using Single PVA resin film

In item a-1, a method of manufacturing a polarizing plate using a laminate of a thermoplastic resin substrate and a PVA-based resin layer formed on the thermoplastic resin substrate is described, but the present invention is also applicable to a method of manufacturing a polarizing plate using a single PVA-based resin film. Typically, the method comprises stretching and dyeing a self-supporting PVA resin film, forming the PVA resin film into a polarizing film, and coating at least one surface of the polarizing film with an acidic aqueous solution containing an acid component and having a pH of 2.5 or less. More specifically, a polarizing film is produced by uniaxially stretching a long PVA-based resin film in the long direction by a roll stretcher, swelling, dyeing, crosslinking, and cleaning, and an acidic aqueous solution containing an acid component and having a pH of 2.5 or less is applied to at least one surface of the cleaned polarizing film. The application of the acidic aqueous solution can be carried out in the same manner as in items A-1 to 7. The protective layer may be attached to one or both surfaces of the polarizing film as needed.

B. Polarizing plate obtained by the method of the present invention

The polarizing plate obtained by the production method described in the above item A includes a polarizing film, and preferably further includes a protective layer provided on at least one side thereof. In the above-described production method, when the acidic aqueous solution applied to the surface of the polarizing film contains a water-soluble resin, a polarizing plate having a functional film formed on the surface of the coating layer can be obtained.

Fig. 2 is a schematic cross-sectional view of an example of a polarizing plate obtained by the method for producing a polarizing plate according to item a-1, using an acidic aqueous solution containing a water-soluble resin. The polarizing plate 10a includes a polarizing film 12, a functional film 14 provided adjacent to one surface of the polarizing film 12, a first protective layer 16 provided on the side of the polarizing film 12 opposite to the side on which the functional film 14 is provided, and a second protective layer 18 provided on the side of the functional film 14 opposite to the side on which the polarizing film 12 is provided. One of the first protective layer 16 and the second protective layer 18 may be omitted. When one protective layer is omitted, the protective layer on the functional film side (the second protective layer 18 in the illustrated example) may be omitted as a representative. The first protective layer 16 may be a thermoplastic resin base material used for producing the laminate described in item a-1 to 1.

FIG. 3 is a schematic cross-sectional view of an example of a polarizing plate obtained by the method for producing a polarizing plate according to item A-1, using an acidic aqueous solution containing no water-soluble resin. The polarizing plate 10b includes a polarizing film 12, a first protective layer 16 disposed on one side of the polarizing film 12, and a second protective layer 18 disposed on the other side of the polarizing film 12. One of the first protective layer 16 and the second protective layer 18 may be omitted. When one of the protective layers is omitted, the protective layer on the coated surface side of the acidic aqueous solution may be omitted. One of the first protective layer and the second protective layer (the protective layer on the side opposite to the coated surface of the acidic aqueous solution) may be a thermoplastic resin substrate for producing the laminate of item A-1-1.

B-1 polarizing film

As described above, the polarizing film is composed of the PVA-based resin film containing iodine. The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, further preferably 5 μm or less, particularly preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in one embodiment, and may be 2 μm in another embodiment. Such thickness can be achieved, for example, by producing a polarizing film using a laminate of a thermoplastic resin substrate and a PVA-based resin layer formed on the thermoplastic resin substrate. When a polarizing film is produced from a single PVA-based resin film, the thickness of the polarizing film may be, for example, 12 μm to 35 μm.

The iodine concentration in the polarizing film is preferably 3 wt% or more, more preferably 4 wt% to 10 wt%, and still more preferably 4 wt% to 8 wt%. In the present specification, "iodine concentration" means the amount of all iodine contained in the polarizing film. More specifically, iodine exists in the polarizing film in the form of an I ^-、I₂、I₃ ^-、PVA/I₃ ^- complex, PVA/I ₅ ^- complex, or the like, and the iodine concentration in this specification means the concentration of iodine including all of these forms. The iodine concentration is calculated, for example, from the fluorescent X-ray intensity and the film (polarizing film) thickness based on fluorescent X-ray analysis.

The single-sheet transmittance of the polarizing film is preferably 42.0% or more, more preferably 42.5% or more, further preferably 43.5% or more, and particularly preferably 45.0% or more. On the other hand, the single sheet transmittance is preferably 48.0% or less, more preferably 46.0% or less. Although the durability of the thin polarizing film having a high single-sheet transmittance may sometimes be reduced in a high-temperature and high-humidity environment, according to the embodiment of the present invention, excellent durability in a high-temperature and high-humidity environment can be achieved even in the case where the thin polarizing film has such a high single-sheet transmittance. In the present specification, the terms "single-sheet transmittance, orthogonal transmittance, and polarization degree" refer to the single-sheet transmittance, orthogonal transmittance, and polarization degree before the endurance test. The polarization degree of the polarizing film is preferably 98.0% or more, more preferably 99.0% or more. On the other hand, the degree of polarization is preferably 99.998% or less. According to the embodiment of the present invention, it is possible to achieve both high monolithic transmittance and high polarization degree, and to achieve excellent durability in a high-temperature and high-humidity environment as described later. Typically, the single-sheet transmittance is a Y value obtained by measuring with an ultraviolet-visible spectrophotometer and performing visibility correction. The single-sheet transmittance is a value obtained by converting the refractive index of one surface of the polarizing plate into 1.50 and converting the refractive index of the other surface into 1.53. Typically, the polarization degree is obtained by measuring the polarization degree using an ultraviolet-visible spectrophotometer and correcting the visibility based on the parallel transmittance Tp and the orthogonal transmittance Tc, and by using the following equation.

The polarization degree (%) = { (Tp-Tc)/(tp+tc) } ^1/2 ×100

In one embodiment, the transmittance (single-sheet transmittance) of a thin polarizing film of 8 μm or less is measured by using an ultraviolet-visible spectrophotometer with a laminate of a polarizing film (surface refractive index: 1.53) and a protective layer (protective film) (refractive index: 1.50) as a measurement target. The reflectance at the interface of each layer may vary depending on the surface refractive index of the polarizing film and/or the refractive index of the surface of the protective layer in contact with the air interface, and as a result, the measured value of the transmittance may vary. Therefore, for example, when a protective layer having a refractive index of not 1.50 is used, the measured value of the transmittance can be corrected based on the refractive index of the surface of the protective layer in contact with the air interface. Specifically, the correction value C of the transmittance is expressed by the following expression using the reflectance R ₁ (transmission axis reflectance) of polarized light parallel to the transmission axis at the interface of the protective layer and the air layer.

C=R₁-R₀

R₀＝((1.50-1)²/(1.50+1)²)×(T₁/100)

R₁＝((n₁-1)²/(n₁+1)²)×(T₁/100)

Here, R ₀ is the transmission axis reflectance when a protective layer having a refractive index of 1.50 is used, n ₁ is the refractive index of the protective layer used, and T ₁ is the transmittance of the polarizing film. For example, when a base material having a surface refractive index of 1.53 (cycloolefin film, hard-coated film, or the like) is used as the protective layer, the correction amount C becomes about 0.2%. In this case, by adding 0.2% to the transmittance obtained by measurement, the transmittance of the case where the protective layer having a refractive index of 1.50 is used can be converted into a polarizing film having a refractive index of 1.53 on the surface. The change amount of the correction value C when the transmittance T ₁ of the polarizing film was changed by 2% was 0.03% or less based on the calculation based on the above equation, and the influence of the transmittance of the polarizing film on the value of the correction value C was limited. Further, when the protective layer has absorption other than surface reflection, appropriate correction can be made according to the absorption amount.

In one embodiment, the variation Δp of the polarization degree of the polarizing film after 240 hours endurance test at 60 ℃ and 95% relative humidity may be-0.05% or more, preferably-0.04% to 2.00%, and more preferably-0.03% to 1.50%. The amount of change Δp in the polarization degree is expressed by the following formula.

ΔP=P₂₄₀-P₀

(Wherein P ₂₄₀ is the degree of polarization after the endurance test, P ₀ is the degree of polarization before the endurance test (degree of polarization described above))

B-2. Functional film

The functional film contains an acid component and a water-soluble resin. The thickness of the functional film is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. If the thickness of the functional film is not less than the above value, the effect of improving durability under a high-temperature and high-humidity environment can be suitably obtained. On the other hand, the thickness of the functional film may be 3.0 μm or less.

B-3 protective layer

The first protective layer and the second protective layer are formed of any appropriate thin film that can be used as a protective layer of the polarizing film. Specific examples of the material that is the main component of the film include cellulose resins such as triacetyl cellulose (TAC), transparent resins such as polyester resins, polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyether sulfone resins, polysulfone resins, polystyrene resins, polynorbornene resins, polyolefin resins, (meth) acrylic resins, and acetate resins. Further, a thermosetting resin such as a (meth) acrylic resin, a urethane resin, a (meth) acrylic urethane resin, an epoxy resin, or a silicone resin, an ultraviolet curable resin, and the like can be mentioned. In addition, for example, a vitreous polymer such as a silicone polymer can be used. In addition, a polymer film described in Japanese patent application laid-open No. 2001-343529 (WO 01/37007) can also be used. As a material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in a side chain, for example, a resin composition having an alternating copolymer of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer can be used. The polymer film may be, for example, an extrusion molded product of the above resin composition.

Typically, when the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the opposite side of the display panel is 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and still more preferably 10 μm to 60 μm. When the surface treatment is performed, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

When the polarizing plate is applied to an image display device, the thickness of the protective layer (inner protective layer) disposed on the display panel side is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, still more preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a phase difference layer having an arbitrary and appropriate phase difference value. In this case, the in-plane retardation Re (550) of the retardation layer is, for example, 110nm to 150nm. "Re (550)" is an in-plane phase difference measured by light having a wavelength of 550nm at 23℃and is obtained by the formula Re= (nx-ny). Times.d. Here, "nx" is a refractive index in a direction (i.e., a slow axis direction) in which the in-plane refractive index reaches a maximum, "ny" is a refractive index in a direction (i.e., a fast axis direction) orthogonal to the slow axis in the plane, "nz" is a refractive index in a thickness direction, and "d" is a thickness (nm) of the layer (thin film).

Examples

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. The measurement methods of the respective characteristics are as follows. Unless explicitly stated otherwise, "parts" and "%" in examples and comparative examples are weight basis.

(1) Thickness of (L)

The polarizing plates of examples and comparative examples were cut, and the cross section of the polarizing plate was observed with a scanning electron microscope ("JSM 7100F" manufactured by japan electronics corporation), and the thickness of the functional film was measured. The thickness of the polarizing film was measured using an interferometer film thickness meter (product name "MCPD-3000" manufactured by Otsuka electronics).

(2) Monolithic transmittance, orthogonal transmittance, and polarization degree

The single sheet transmittance Ts, the parallel transmittance Tp, and the orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer (LPF 200 manufactured by the large tsuka electronics company) for the polarizing plates of examples and comparative examples (for the single sheet transmittance, for the laminate having the configuration of the protective layer/the polarizing film before the application of the acidic aqueous solution) were respectively set as Ts, tp, and Tc of the polarizing film. These Ts, tp, and Tc are Y values obtained by measurement and visibility correction using a2 degree field of view (C light source) of JIS Z8701. The following formula was used to determine the polarization degree from the obtained Tp and Tc.

The polarization degree (%) = { (Tp-Tc)/(tp+tc) } ^1/2 ×100

Next, glass (alkali-free glass) from which alkali components were removed was stuck to the coated surface side of the polarizing plate with an adhesive, and the resultant was left to stand in an oven at a temperature of 60 ℃ and a relative humidity of 95% for 240 hours, and a durability test was performed, and the degree of polarization P ₂₄₀ after the durability test was obtained in the same manner as described above.

(3) Viscosity of the mixture

The viscosity of the acidic aqueous solution was measured using a VISCO METER R85 type viscometer (RE 85L manufactured by eastern machine industry company) under conditions of a measurement temperature of 25 ℃ and a rotation speed of 0.5 to 100 rpm.

Example 1

As the thermoplastic resin substrate, an amorphous isophthalic acid copolymerized polyethylene terephthalate film (thickness: 100 μm) having a long shape and a Tg of about 75℃was used, and one side of the resin substrate was subjected to corona treatment.

To 100 parts by weight of a PVA-based resin obtained by mixing polyvinyl alcohol (polymerization degree: 4200, saponification degree: 99.2 mol%) and acetoacetyl-modified PVA (trade name: GOHSEFIMER, manufactured by japan chemical industries, co.) at a ratio of 9:1, 13 parts by weight of potassium iodide was added, and the product thus obtained was dissolved in water to prepare a PVA aqueous solution (coating liquid).

The PVA aqueous solution was applied to the corona treated surface of the resin substrate, and dried at 60 ℃ to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.

The resulting laminate was uniaxially stretched to 2.4 times in the machine direction (lengthwise direction) in an oven at 130 ℃ (air-assisted stretching treatment).

Next, the laminate was immersed in an insolubilization bath (an aqueous boric acid solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40 ℃ for 30 seconds (insolubilization treatment).

Next, in a dyeing bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with respect to 100 parts by weight of water) at a liquid temperature of 30 ℃, the resulting polarizing plate was immersed for 60 seconds while adjusting the concentration so that the single-sheet transmittance (Ts) of the resulting polarizing plate became 44.0% (dyeing treatment).

Then, the resultant solution was immersed in a crosslinking bath (aqueous boric acid solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with respect to 100 parts by weight of water) at a liquid temperature of 40℃for 30 seconds (crosslinking treatment).

Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4 wt% and potassium iodide concentration: 5 wt%) at a liquid temperature of 70 ℃ and uniaxially stretched (in-water stretching treatment) between rolls having different peripheral speeds so that the total stretching ratio became 5.5 times in the longitudinal direction (longitudinal direction).

Thereafter, the laminate was immersed in a washing bath (an aqueous solution obtained by mixing 100 parts by weight of water with 4 parts by weight of potassium iodide) at a liquid temperature of 20 ℃ (washing treatment).

Thereafter, while drying in an oven maintained at about 90 ℃, the sheet was brought into contact with a SUS-made heating roller maintained at a surface temperature of about 75 ℃ (drying shrinkage treatment). The shrinkage in the width direction of the laminate based on the drying shrinkage treatment was 2%.

In this manner, a polarizing Film having a thickness of 5.0 μm was formed on a resin substrate, a cycloolefin Film (product name "G-Film" manufactured by ZEON corporation) as a protective layer (protective Film) was adhered to the surface of the polarizing Film by a UV curable adhesive (thickness: 1.0 μm), and then the resin substrate was peeled off to obtain a laminate having a structure of protective layer/polarizing Film. The single-sheet transmittance (Ts) of the obtained laminate was 44.0%, and the surface refractive index of the polarizing film/protective layer constituting the laminate was 1.53/1.53, so that the single-sheet transmittance was obtained by correcting the actual measured value by +0.2% and converting it to a state of 1.53/1.50.

Then, an aqueous solution containing hydrogen chloride and containing no water-soluble resin (aqueous solution containing HCl) was applied to the polarizing film surface of the laminate in a coating amount of 10g/m ². Then, the coated surface of the aqueous solution containing HCl was dried at 60 ℃ for 5 minutes to obtain a polarizing plate having a structure in which an acidic aqueous solution was coated on the surface of the side where the protective layer was not provided and the protective layer/polarizing film was provided. The pH of the aqueous HCl-containing solution was 1.3.

Examples 2 to 1

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 1, except that an aqueous solution (aqueous solution of PVA containing HCl (PVA concentration: 3.5 wt%) including hydrogen chloride and polyvinyl alcohol (polymerization degree: 2600, saponification degree: 99.98 mol%) was used as the acidic aqueous solution, and the aqueous solution of PVA containing HCl was applied so that the thickness of the functional film became 0.4 μm. The aqueous solution of PVA containing HCl had a pH of 0.9 and a viscosity of 72mPa sec.

Examples 2 to 2

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 2-1, except that an aqueous PVA solution containing HCl was applied so that the thickness of the functional film became 0.8 μm.

Examples 2 to 3

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 2-1, except that an aqueous PVA solution containing HCl was applied so that the thickness of the functional film became 1.2 μm.

Examples 3 to 1

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 1, except that an aqueous solution containing sulfuric acid and the above-mentioned polyvinyl alcohol (aqueous solution of PVA containing H ₂SO₄ (PVA concentration: 3.5 wt%)) was used as the acidic aqueous solution, and the aqueous solution of PVA containing H ₂SO₄ was applied so that the thickness of the functional film became 0.6 μm. The aqueous PVA solution containing H ₂SO₄ had a pH of 0.9 and a viscosity of 70mPa sec.

Examples 3 to 2

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 3-1, except that an aqueous PVA solution containing H ₂SO₄ was applied so that the thickness of the functional film became 1.2 μm.

Examples 4 to 1

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 1, except that an aqueous solution containing hydrogen chloride and the above-mentioned polyvinyl alcohol (aqueous solution of PVA containing HCl (PVA concentration: 3.5 wt%)) was used as the acidic aqueous solution, and the aqueous solution of PVA containing HCl was applied so that the thickness of the functional film became 1.2 μm. The aqueous solution of PVA containing HCl had a pH of 0.9 and a viscosity of 70 mPa.sec.

Examples 4 to 2

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 4-1, except that an aqueous solution of PVA containing HCl at pH 1.2 was used.

Examples 4 to 3

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 4-1, except that the aqueous solution of PVA containing HCl was used at a pH of 1.6, and the aqueous solution of PVA containing HCl was applied so that the thickness of the functional film became 1.1 μm.

Examples 4 to 4

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 4-1, except that the aqueous solution of PVA containing HCl was used at a pH of 1.8 and applied so that the thickness of the functional film became 1.5 μm.

Examples 4 to 5

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 4-1, except that the aqueous solution of PVA containing HCl was used at a pH of 2.0 and applied so that the thickness of the functional film became 1.2 μm.

Examples 5 to 1

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 1, except that an aqueous solution containing sulfuric acid and the above-mentioned polyvinyl alcohol (aqueous solution of PVA containing H ₂SO₄ (PVA concentration: 3.5 wt%)) was used as the acidic aqueous solution, and the aqueous solution of PVA containing H ₂SO₄ was applied so that the thickness of the functional film became 0.7 μm. The aqueous PVA solution containing H ₂SO₄ had a pH of 1.0 and a viscosity of 70 mPas.

Examples 5 to 2

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 5-1, except that an aqueous PVA solution containing H ₂SO₄ was used at a pH of 1.2.

Examples 5 to 3

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 5-1, except that an aqueous PVA solution containing H ₂SO₄ was used at a pH of 1.5.

Examples 5 to 4

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 5-1, except that an aqueous PVA solution containing H ₂SO₄ was used at a pH of 1.7.

Examples 5 to 5

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 5-1, except that an aqueous PVA solution containing H ₂SO₄ was used at a pH of 2.0.

Examples 6 to 1

A polarizing plate having a structure of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 1, except that an aqueous solution containing nitric acid and the above-mentioned polyvinyl alcohol (PVA aqueous solution containing HNO ₃ (PVA concentration: 3.5 wt%)) was used as the acidic aqueous solution, and the PVA aqueous solution containing HNO ₃ was applied so that the thickness of the functional film became 0.7 μm. The aqueous PVA solution containing HNO ₃ had a pH of 1.1 and a viscosity of 72 mPa.sec.

Examples 6 to 2

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 6-1, except that an aqueous solution containing citric acid and the above-mentioned polyvinyl alcohol (aqueous solution of PVA containing citric acid (PVA concentration: 3.5 wt%) was used as the acidic aqueous solution. The aqueous solution of PVA containing citric acid had a pH of 1.3 and a viscosity of 75mPa sec.

Examples 6 to 3

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 6-1, except that an aqueous solution containing phosphoric acid and the above-mentioned polyvinyl alcohol (an aqueous solution of PVA containing phosphoric acid (PVA concentration: 3.5 wt%) was used as the acidic aqueous solution. The aqueous solution of PVA containing phosphoric acid had a pH of 1.1 and a viscosity of 68mPa sec.

Comparative example 1

A polarizing plate having a protective layer/polarizing film structure was obtained in the same manner as in example 1, except that an aqueous solution containing HCl having a pH of 4.0 was used.

Comparative example 2

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 4-1, except that an aqueous solution of PVA containing HCl at pH 3.1 was used.

Comparative example 3

A polarizing plate having a configuration of a protective layer, a polarizing film, and a functional film was obtained in the same manner as in example 5-1, except that an aqueous PVA solution containing H ₂SO₄ was used at a pH of 2.6.

Reference example

A polarizing plate (Ts: 44.0%) having a structure of a protective layer/polarizing film was obtained in the same manner as in example 1, except that the acidic aqueous solution was not applied. The polarization degree P ₀ of the obtained polarizing plate (substantially polarizing film) before the durability test was 99.8%, which was substantially the same as that of P ₀ of the polarizing plates of examples and comparative examples to which the above-mentioned acidic aqueous solution was applied.

The polarizing plates (substantially polarizing films) obtained in the above examples, comparative examples and reference examples are shown in table 1 as Δp. In table 1, Δp is a value larger than Δp of the reference example (in other words, the degree of degradation of the optical characteristic by the durability test is smaller than that of the reference example (including the case where the optical characteristic has been improved)) and "good", and Δp is a value smaller than Δp of the reference example (in other words, the degree of degradation of the optical characteristic by the durability test is larger than that of the reference example) and "x", and the determination result is shown.

TABLE 1

As is clear from Table 1, the polarizing plate of the examples of the present invention is excellent in durability under a high-temperature and high-humidity environment.

Industrial applicability

The polarizing plate obtained by the production method of the present invention is suitable for use in a liquid crystal display device.

Description of the reference numerals

10. Polarizing plate

12. Polarizing film

14. Functional film

16. First protective layer

18. A second protective layer

Claims

1. A method for manufacturing a polarizing plate, comprising: applying an acidic aqueous solution containing an acid component and having a pH of 2.5 or less to at least one surface of a polarizing film, wherein the acidic aqueous solution does not include: an acidic aqueous solution containing a water-soluble crosslinking agent of a vinyl alcohol polymer and an acidic aqueous solution containing an oxazoline-containing polymer,

The polarizing film is composed of a polyvinyl alcohol-based resin film containing iodine.

The acidic aqueous solution further includes a water-soluble resin, which is coated on at least one surface of the polarizing film and dried to form a functional film.

2 . The production method according to claim 1 , wherein the acid component contains hydrogen chloride.

The production method according to claim 1 , wherein the acid component contains sulfuric acid.

4 . The production method according to claim 1 , wherein the water-soluble resin comprises a polyvinyl alcohol-based resin.

5 . The production method according to claim 4 , wherein the degree of polymerization of the polyvinyl alcohol-based resin is 1500 to 4500.

6 . The production method according to claim 4 , wherein the acidic aqueous solution has a viscosity of 350 mPa·sec or less.