KR101606594B1 - A method for producing a polarizing film, a polarizing plate and an optical laminate - Google Patents

Abstract

The present invention relates to a method for producing a polarizing film excellent in optical durability under high temperature conditions, especially under dry heat conditions, a polarizing plate using a polarizing film obtained by the method, and an optical laminate using the polarizing plate.

Description

TECHNICAL FIELD [0001] The present invention relates to a polarizing plate and a polarizing plate, and more particularly to a polarizing plate and an optical laminate,

The present invention relates to a method for producing a polarizing film excellent in optical durability under high temperature conditions, especially under dry heat conditions, a polarizing plate using a polarizing film obtained by the method, and an optical laminate using the polarizing plate.

As a polarizing film, conventionally, a polyvinyl alcohol film having a dichroic dye adsorbed and oriented has been used. That is, an iodine-based polarizing film using iodine as a dichroic dye or a dye-based polarizing film using a dichroic dye as a dichroic dye are known. A protective film such as triacetyl cellulose is bonded to at least one surface, preferably both surfaces, of these polarizing films through an adhesive made of an aqueous solution of polyvinyl alcohol resin to form a polarizing plate.

As a method for producing a polarizing film, for example, Patent Document 1 discloses a method for producing a polarizing film by dipping (swelling) a polyvinyl alcohol-based film in water and then dyeing the film with a dichroic dye (dyeing treatment) To fix iodine on a film, a polyvinyl alcohol-based film is treated with boric acid (i.e., water resistance treatment by crosslinking) in a crosslinking treatment tank, followed by washing with water, followed by drying.

[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-153709

However, the polarizing film described in Patent Document 1 has a problem that its optical durability is lowered under a high temperature condition, especially under a dry heat condition.

A principal object of the present invention is to provide a method for producing a polarizing film having excellent optical durability even under high temperature conditions, and to provide a polarizing plate and an optical laminated body using the polarizing film.

These and other objects of the present invention will become apparent from the following description.

The present invention relates to the following description.

(1) A method for producing a polarizing film in which a polyvinyl alcohol film is dyed with a dichroic dye and then subjected to a crosslinking treatment with boric acid, wherein the crosslinking treatment is carried out so that the pH of the aqueous solution of boric acid in the aqueous solution .

(2) The method according to (1), wherein the concentration of boric acid in the aqueous solution of boric acid in the crosslinking treatment is 4 parts by weight or more and 6 parts by weight or less based on 100 parts by weight of water.

(3) The method according to (1) or (2), wherein the crosslinking treatment is carried out while continuously or intermittently activating the activated carbon treatment of the boric acid aqueous solution.

(4) Crosslinking treatment is carried out in the treatment tank. In the activated carbon treatment, part of the aqueous boric acid solution is extracted from the treatment tank, a part or all of the extracted boric acid-containing aqueous solution is brought into contact with the activated carbon and an aqueous boric acid solution containing the activated carbon- (3) The method according to (3), which is carried out by returning to a treatment tank.

(5) The method according to any one of (1) to (4), wherein the pH of the aqueous solution of boric acid in the crosslinking treatment is adjusted by adding an acid.

(6) The method according to (5), wherein the acid to be added is an inorganic acid.

(7) The method according to (6), wherein the inorganic acid is sulfuric acid or hydrochloric acid.

(8) A polarizing film obtained by dyeing a polyvinyl alcohol-based film with a dichroic dye and then carrying out a crosslinking treatment with an aqueous solution of boric acid adjusted to a pH of 4 or less,

And a protective film bonded to the polarizing film.

(9) The polarizing plate according to (8), wherein the protective film is a film having any one function selected from a retardation film, a brightness enhancement film, a viewing angle improving film and a transflective film.

(10) A polarizing plate obtained by bonding a protective film to a polarizing film obtained by dyeing a polyvinyl alcohol film with a dichroic dye and then crosslinking the film with an aqueous solution of boric acid adjusted to a pH of 4 or less,

And at least one film selected from a retardation film, a brightness enhancement film, a viewing angle improving film and a transflective film bonded to the polarizing plate.

In the method for producing a polarizing film according to the present invention, it is preferable that the boric acid concentration in the crosslinking treatment is 4 parts by weight or more and 6 parts by weight or less based on 100 parts by weight of water.

Further, in the method for producing a polarizing film according to the present invention, it is preferable that the cross-linking treatment is carried out while continuously or intermittently activating the aqueous boric acid solution in the treatment tank. In the activated carbon treatment, it is preferable that a part of the aqueous boric acid solution is extracted from the crosslinking treatment tank and a part or all of the extracted aqueous boric acid solution is brought into contact with the activated carbon and then returned to the treatment tank.

A polarizing plate according to the present invention is obtained by bonding a protective film to a polarizing film obtained as described above. The protective film may be for the purpose of protecting the polarizing film only, and may have any one of the function of the retardation film function, the function of improving the luminance, the function of improving the viewing angle, and the function of the transflective film.

The optical laminate according to the present invention is obtained by bonding at least one selected from a polarizing plate obtained as described above and a retardation film, a brightness enhancement film, a viewing angle improving film and a transflective film.

According to the present invention, it is possible to obtain a polarizing film excellent in optical durability under high temperature conditions, especially under dry heat conditions.

Further, by using a polarizing plate or an optical laminate obtained by using the obtained polarizing film in a liquid crystal display device, a thin and high quality liquid crystal display device can be obtained.

Hereinafter, an embodiment of the present invention will be described.

The polarizing plate according to this embodiment is a polarizing plate in which a film such as a polyvinyl alcohol film useful as a raw film of a polarizing film is subjected to a swelling treatment, a dyeing treatment with a dichroic dye, a crosslinking with boric acid in a crosslinking treatment tank Washing, washing and drying to prepare a polarizing film, and thereafter bonding the protective film to at least one surface of the polarizing film obtained by the above-mentioned process using an adhesive.

Examples of the polyvinyl alcohol-based resin forming the polyvinyl alcohol-based film in the present invention include resins obtained by saponifying a polyvinyl acetate-based resin. The saponification degree is usually 85 mol% or more, preferably 90 mol% or more, and more preferably 99 mol% to 100 mol%. Examples of the polyvinyl acetate resin include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate and other monomers copolymerizable therewith. Examples of copolymers of vinyl acetate and other monomers copolymerizable therewith include ethylene-vinyl acetate copolymers and the like. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids and the like. The polymerization degree of the polyvinyl alcohol-based resin is usually 1000 to 10000, preferably 1500 to 5000.

Examples of the modified resin include polyvinyl formal, polyvinyl acetal, polyvinyl butyral, and the like modified with aldehydes. Usually, an unoriented film of a polyvinyl alcohol-based resin film having a thickness of 20 m to 100 m, and preferably 30 m to 80 m can be used as a starting material for polarizing film production. From a technical point of view, the width of the film is practically 1500 mm to 4000 mm. The thickness of the polyvinyl alcohol polarizing film obtained by the swelling treatment, the stretching treatment, the dyeing treatment, the boric acid treatment, the water washing treatment, and the finally dried non-stretched film is, for example, about 5 to 50 μm.

The swelling treatment is carried out by immersing the undrawn original film in water for the purpose of removing foreign substances on the film surface, removing the plasticizer in the film, imparting easiness of dyeing in the next treatment, plasticizing the film, and the like. The processing conditions are determined within a range in which these objects can be achieved and within a range in which problems such as extreme melting and devitrification of the original film do not occur. For example, by immersing an undrawn original film in water at 10 ° C to 50 ° C, preferably 20 ° C to 40 ° C. The immersion time of the film is 30 seconds to 300 seconds, and more preferably 60 seconds to 240 seconds.

In the swelling process, the film tends to swell in the width direction and wrinkles are likely to occur in the film. Therefore, the wideness (width) of the wipes such as an expander roll, a spiral roll, a crown roll, a cross guider, a bend bar, ) Apparatus, it is desirable to transport the film in a water bath. In order to stabilize the film transportation in the water tank, it is also possible to control the water flow in the water tank by an underwater shower or to use an EPC apparatus (Edge Position Control device: detecting the end of the film and preventing the film from skewing) useful. In the swelling process, since the film swells and expands in the running direction of the film, it is preferable to take measures such as controlling the speed of the transport roll before and after the water tank, for example, in order to eliminate the looseness of the film in the transport direction. In addition to pure water, boric acid (described in JP-A No. 10-153709), chloride (JP-A No. 6-281816), inorganic acid, inorganic salt, water-soluble An aqueous solution containing an organic solvent, an alcohol or the like in an amount of 0.01 to 10 wt% can be used, but water is preferred.

After the swelling treatment, the polyvinyl alcohol film is subjected to wet stretching treatment. In this stretching treatment, the film is uniaxially stretched at a draw ratio of 1.1 times or more and less than 3 times in an aqueous solution containing 0.01 to 2 parts by weight of boric acid per 100 parts by weight of water. If the stretching magnification in the wet stretching process is out of the above range, uneven dyeing can not be reduced or the optical properties of the final polarizing film become insufficient. The wet stretching treatment may be performed not only in a single aqueous solution of boric acid but also continuously in a plurality of steps using a plurality of baths. In this case, however, the wet stretching treatment may be performed in a range of 1.1 times or more and 3 times or less It is good to do.

As the stretching method, for example, an inter-roll stretching method in which the film is stretched with a difference in peripheral speed of the nip rolls provided before and after the aqueous boric acid solution bath is suitably employed, but the present invention is not limited to this, Various stretching methods can be adopted.

The boric acid aqueous solution used in the wet stretching treatment is an aqueous solution containing 0.01 to 2 parts by weight of boric acid per 100 parts by weight of water. However, the use of such an aqueous solution of boric acid also has an advantage that wrinkles are not readily generated. The temperature of the boric acid aqueous solution used in the wet stretching treatment is preferably from 10 캜 to 40 캜. In order to maintain the temperature of the boric acid aqueous solution at a temperature lower than 10 ° C, a large-scale cooling facility is required for temperature control, which is uneconomical, and if it exceeds 40 ° C, the original film may be dissolved.

After the stretching treatment, the film is subjected to a dyeing treatment. A conventional dyeing treatment with a dichroic dye is carried out by immersing the film in an aqueous solution of a dichroic dye for the purpose of adsorbing and orienting the dichroic dye to the film. The treatment conditions are determined within a range in which these objects can be achieved and within a range in which problems such as extreme melting of the substrate film, When iodine is used as the dichroic dye, the stretched film is subjected to stretching treatment at a temperature of, for example, 10 ° C to 45 ° C, preferably 20 ° C to 35 ° C, and an iodine / KI / water ratio of 0.003 to 0.2 / / 100 concentration aqueous solution for 30 to 600 seconds, preferably 60 to 300 seconds. Other iodides, such as zinc iodide, may be used instead of potassium iodide. In addition, other iodides may be used in combination with potassium iodide. Further, compounds other than iodide such as boric acid, zinc chloride, cobalt chloride, etc. may be coexistent. When boric acid is added, the aqueous solution for dyeing treatment is distinguished from the later-described aqueous solution for crosslinking treatment in that it contains iodine. Even in the case of containing boric acid, an aqueous solution of a dichroic dye for dyeing treatment can be used if it is an aqueous solution containing 0.003 part by weight or more of iodine relative to 100 parts by weight of water.

When a water-soluble dichroic dye is used as the dichroic dye, the stretched film is dyed at a temperature of 20 占 폚 to 80 占 폚, preferably 30 占 폚 to 70 占 폚, and a dichroic dye / water ratio of 0.001 to 0.1 / 100, for 30 to 600 seconds, preferably 60 to 300 seconds. The aqueous solution of the dichroic dye to be used may contain a dyeing aid or the like, and may contain an inorganic salt such as sodium sulfate, a surfactant, or the like. Each of the dichroic dyes may be used alone or in combination with at least one of them.

Further, like the swelling process, the expander roll, the spiral roll, the crown roll, the cross guider, the bent bar, and the like may be provided in the aqueous solution tank of the dichroic dye and / or at the entrance / exit.

After the dyeing treatment, a crosslinking treatment with an aqueous solution of boric acid is carried out. Concretely, this is carried out by immersing a polyvinyl alcohol-based film obtained by dyeing in an aqueous solution in a crosslinking treatment tank containing usually not more than 6 parts by weight, preferably 4 to 6 parts by weight, of boric acid per 100 parts by weight of water. When the dichroic dye is iodine, 15 parts by weight or less, preferably 15 to 7 parts by weight, of iodide is preferably contained. Examples of the iodide include potassium iodide and zinc iodide.

Such crosslinking treatment is carried out for water resistance or color adjustment (such as prevention of blueness) by crosslinking or the like. When water resistance due to crosslinking is the main target, a crosslinking agent such as glyoxal or glutaraldehyde may be used together with boric acid, if necessary.

When the boric acid aqueous solution is used for a comparatively long period of time in the crosslinking treatment, the polarization degree of the polarizing film may be lowered. It is preferable to carry out treatment for bringing at least a part of the aqueous boric acid solution used to prevent this into contact with the activated carbon.

The activated carbon treatment is preferably carried out continuously or intermittently during the crosslinking treatment of the polyvinyl alcohol film in which the iodine is adsorbed and oriented. In order to perform the crosslinking treatment while carrying out the activated carbon treatment, for example, a part of the boric acid-containing aqueous solution is extracted from the crosslinking treatment tank by using a pump or the like, passed through an activated carbon filling column, and then returned to the crosslinking treatment tank. The crosslinking treatment can be carried out by dividing the crosslinking treatment tank into a plurality of times by using a plurality of crosslinking treatment tanks. In this case, it is preferable to perform the activated carbon treatment of the aqueous solution in each treatment tank or each extracted boric acid aqueous solution tank. At this time, the pH of the aqueous boric acid solution in the crosslinking treatment tank may rise due to the elution of ash in the activated carbon. The increase in the pH of the aqueous boric acid solution in the crosslinking treatment tank lowers the optical durability of the resultant polarizing film under high temperature conditions.

Therefore, the pH of the aqueous solution of boric acid in the crosslinking treatment tank may be monitored with a pH meter or the like to adjust the pH to 4 or lower, preferably 3.5 or lower.

In the crosslinking treatment, the concentration of boric acid is usually not more than 6 parts by weight based on 100 parts by weight of water as described above. Therefore, pH adjustment of the aqueous solution of boric acid in the crosslinking treatment tank is preferably carried out by addition of an acid other than boric acid . Examples of the acid to be added include inorganic acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid and perchloric acid, and organic acids such as formic acid, acetic acid and propionic acid. Preferably sulfuric acid and hydrochloric acid.

The cross-linking treatment can be carried out by appropriately changing the concentration of boric acid and iodide and the temperature of the treatment tank, depending on its main purpose, but is usually carried out under the following conditions.

When the original film is subjected to swelling, dyeing or cross-linking treatment, when the cross-linking treatment is primarily aimed at water resistance by cross-linking, boric acid is usually used in an amount of about 3 to 6 parts by weight based on 100 parts by weight of water, The treatment temperature is usually 50 to 70 캜, preferably 55 to 65 캜, and the immersing time is usually 30 to 600 sec, preferably 60 to 420 sec, more preferably 60 to 40 sec. Is performed in 90 to 300 seconds.

After the crosslinking treatment primarily for hydration, the crosslinking treatment for color adjustment may be performed. For example, when the dichroic dye is iodine, an aqueous boric acid solution containing boric acid in an amount of usually about 1 to 5 parts by weight and iodide in an amount of usually about 3 to 30 parts by weight is used for 100 parts by weight of water, And the immersion time is usually about 3 to 300 seconds, preferably 10 to 240 seconds.

The crosslinking treatment mainly for color adjustment is usually carried out at a low boric acid concentration, a high iodide concentration, and a low temperature in comparison with a crosslinking treatment which is mainly for water resistance.

The cross-linking treatment may be carried out in a plurality of steps, or in two to five steps. In this case, the composition and temperature of the aqueous solution of each crosslinking treatment tank to be used may be the same or different. A cross-linking treatment mainly for water resistance, and a cross-linking treatment for mainly color adjustment may be separately performed. It is necessary to adjust the pH of the aqueous solution of boric acid in each crosslinking treatment tank to 4 or less.

In the crosslinking treatment, stretching of the film may be performed as in the dyeing treatment. When stretching is carried out, the final integrated stretching magnification ratio of the original film (unstretched film) is usually about 4.5 to 7 times, preferably 5 to 6.5 times.

After the cross-linking treatment, it is washed with water. The water washing treatment is carried out, for example, by immersing a crosslinked polyvinyl alcohol-based film in water, spraying water as a shower, or using both immersion and spraying. The temperature of the water in the water washing treatment is usually about 2 to 40 캜, and the immersing time is usually about 2 to 120 seconds. Drying after washing is carried out, for example, at about 40 to 100 DEG C for about 60 to 600 seconds in a drying furnace.

A polarizing plate can be obtained by bonding a protective film to at least one surface of the thus prepared polarizing film with an adhesive.

The protective film laminated on one side or both sides of the polarizing film may be of the same kind or different types depending on the purpose when laminated on both sides. In the case of using different types of protective films, examples of the protective film include amorphous polyolefin-based resin films, polyester-based resin films such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate, acrylic resin films, polycarbonate- A low-moisture-permeability resin film such as a resin film, a polysulfone-based resin film, and an alicyclic polyimide-based resin film. Examples of the amorphous polyolefin-based resin film include "Topaz" (registered trademark) manufactured by Ticona of Germany, "Aton" (registered trademark) manufactured by JSR Corporation, ZEONOR "and" ZEONEX "(trademarks all registered) manufactured by Mitsui Chemicals, Inc., and" Apel "(registered trademark) manufactured by Mitsui Chemicals, Inc.

Other examples of the protective film include cellulose acetate based resin films such as triacetylcellulose film and diacetylcellulose film in addition to these films.

Examples of the triacetylcellulose film include "Fujitack TD80", "Fujitack TD80UF" and "Fujitack TD80UZ" manufactured by Fuji Photo Film Co., Ltd., "KC8UX2M" and "KC8UY" manufactured by Konica Corporation And the like.

The method of applying the adhesive to the protective film is not particularly limited, but various coating methods such as a doctor blade, a wire bar, a die coater, a comma coater, and a gravure coater can be used. Further, the protective film may be subjected to a treatment for easy bonding such as saponification treatment, corona treatment, primer treatment, anchor coating treatment and the like on the bonding surface prior to bonding to the polarizing film. The surface opposite to the protective film surface to be bonded to the polarizing film may have various treated layers such as a hard coat layer, an antireflection layer, and an antiglare layer.

The thickness of the protective film is preferably small, but if it is too thin, the strength is lowered and the workability is poor. On the other hand, if it is too thick, transparency deteriorates or the curing time required after lamination becomes long . Therefore, the suitable thickness of the protective film is, for example, about 5 to 200 mu m, preferably 10 to 150 mu m, more preferably 20 to 100 mu m.

The polarizing film may be subjected to surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet ray irradiation, primer coating treatment and saponification treatment to improve the adhesion between the adhesive and the polarizing film.

The protective film may be subjected to surface treatment such as an anti-glare treatment, an anti-reflection treatment, a hard coat treatment, an antistatic treatment, a contamination treatment or the like, either singly or in combination. The surface-treated layer of the protective film and / or the protective film may contain an ultraviolet absorber such as a benzophenone-based compound or a benzotriazole-based compound, or a plasticizer such as a phenylphosphate-based compound or a phthalic acid ester compound. Such a protective film may be bonded to one surface of the polarizing film or to both surfaces thereof.

The polarizing film and the protective film are laminated using an adhesive such as a water-soluble adhesive, an organic solvent adhesive, a hot-melt adhesive, a UV curable adhesive, or a solvent-based adhesive. Examples of the water-based adhesives include a polyvinyl alcohol-based resin aqueous solution, a water-based liquid-based urethane emulsion adhesive and the like. Examples of the organic solvent-based adhesive include a liquid type urethane-based adhesive and the like. Examples of the UV- , An alicyclic epoxy resin, an aliphatic epoxy resin, etc., and an epoxy resin adhesive containing no aromatic ring in the molecule. Examples of the solventless adhesive include a one-component urethane adhesive. When an acetylcellulose-based film having a hydrophilicized saponification treatment such as a saponification treatment is used as a protective film, a polyvinyl alcohol-based resin aqueous solution is suitably used as an adhesive.

The polyvinyl alcohol-based resin used as an adhesive includes a vinyl alcohol homopolymer obtained by saponifying a polyvinyl acetate which is a homopolymer of vinyl acetate, a vinyl alcohol homopolymer obtained by saponifying a copolymer of vinyl acetate and another monomer copolymerizable with the vinyl alcohol homopolymer, A modified polyvinyl alcohol polymer in which these hydroxyl groups are partially modified, and the like. A polyaldehyde, a water-soluble epoxy compound, a melamine compound, or the like may be added to the adhesive. The epoxy resin adhesive containing no aromatic ring in the molecule used as the adhesive may be a polymerization initiator such as a photo cationic polymerization initiator for polymerization by irradiation with active energy rays, a thermal cationic polymerization initiator for polymerization by heating, Etc.) may be further added.

A method of bonding the polarizing film and the protective film is not particularly limited, and for example, a method of uniformly applying an adhesive to the surface of a polarizing film or a protective film and bonding the other film to the applied surface by rolls, have.

After the preparation, the adhesive is usually applied at a temperature of 15 to 40 캜, and the bonding temperature is usually in the range of about 15 to 30 캜. After the bonding, the drying treatment is performed to remove the solvent such as water contained in the adhesive. The drying temperature at this time is usually 30 to 85 캜, preferably 40 to 80 캜. Thereafter, the adhesive may be cured by curing for about 1 to 90 days under a temperature environment of 15 to 85 캜, preferably 20 to 50 캜, more preferably 35 to 45 캜. Since the productivity is deteriorated when the curing period is long, the curing period is preferably about 1 to 30 days, and more preferably 1 to 7 days. Thus, a polarizing plate having a protective film bonded to one or both surfaces of the polarizing film through the adhesive layer can be obtained.

In the present invention, the protective film may have optical functions such as a retardation film function, a brightness enhancement film function, a reflection film function, a transflective film function, a diffusion film function, and an optical compensation film function. In this case, such functions can be obtained by laminating an optical functional film such as a retardation film, a luminance enhancement film, a reflection film, a transflective film, a diffusion film, or an optical compensation film on the surface of a protective film. In addition, this function may be imparted to the protective film itself. Further, the protective film itself may have a plurality of functions such as a diffusion film having a brightness enhancement film function.

For example, the stretching treatment described in Japanese Patent No. 2841377 and Japanese Patent No. 3094113 or the like described in Japanese Patent No. 3168850 can be given to the above-mentioned protective film to give a function as a retardation film have.

By forming fine holes by the method described in JP-A-2002-169025 or JP-A-2003-29030 in the protective film described above, it is possible to provide a protective film having two or more layers having different central wavelengths of selective reflection By superimposing the cholesteric liquid crystal layer, the brightness enhancement film function can be given. By forming a metal thin film on the protective film by vapor deposition, sputtering or the like, a reflective film function or a transflective film function can be imparted. By coating the above-mentioned protective film with a resin solution containing fine particles, a diffusion film function can be given. Further, by coating a liquid crystal compound such as a discotic liquid crystal compound and orienting the above protective film, an optical compensation film function can be imparted. Further, a luminance improvement film such as a luminance enhancement film such as a trade name: DBEF (manufactured by 3M Co., Ltd.) using a suitable adhesive, a viewing angle improving film such as a trade name: WV film (manufactured by Fuji Photo Film Co., Ltd.) (Manufactured by Sumitomo Chemical Co., Ltd.) or the like may be directly bonded to the polarizing film.

Further, the optical laminate can be produced by bonding at least one selected from the polarizing plate having a protective film bonded to at least one surface thereof, and a retardation film, a brightness enhancement film, a viewing angle improving film and a transflective plate.

Hereinafter, the present invention will be described in detail by way of examples, but the present invention is not limited to the following examples.

First Embodiment

The film was immersed in pure water for about 60 seconds while keeping the tension so that the film did not become loose, and the film was immersed in pure water to a thickness of 75 탆 (polyvinyl alcohol film having a thickness of 75 탆 (Gurarevinylon VF-PS # 7500, degree of polymerization 2400, degree of saponification 99.9 mol% Swollen enough. Next, the film was dipped in an aqueous solution of iodine / potassium iodide / water at a weight ratio of 0.04 / 1.5 / 100 for 98 seconds. Next, an aqueous solution (pH of about 3.85) containing 54 parts by weight of boric acid and 12 parts by weight of potassium iodide was adjusted to pH 2.5 with sulfuric acid and then dyed in this aqueous solution for about 55 seconds. One film was stretched (stretching magnification 1.89 times) and boric acid was adsorbed and crosslinked, followed by washing with pure water and drying to obtain a polarizing film. Subsequently, a polarizing film was coated with a polyvinyl alcohol-based adhesive on both sides of the polarizing film, and a protective film (triacetylcellulose film (saponification treatment on the surface) (thickness 80 占 퐉)) was bonded to both sides to form a polarizing plate.

Second Embodiment

Boric acid, 5.0 parts by weight of boric acid, and 12 parts by weight of potassium iodide was adjusted to about 3.5 by using a concentrated sulfuric acid solution to an aqueous solution (pH of about 3.85) A film was obtained. Subsequently, a polyvinyl alcohol-based adhesive was applied to both sides of the polarizing film, and the same protective film as in Example 1 was bonded to both sides to form a polarizing plate.

Third Embodiment

Boric acid, 5.0 parts by weight of boric acid, and 12 parts by weight of potassium iodide was adjusted to about 3.5 by using concentrated hydrochloric acid with respect to an aqueous solution (pH of about 3.85) A film is obtained. Then, a polarizing plate can be obtained by applying a polyvinyl alcohol-based adhesive to both surfaces of the polarizing film and bonding the same protective film to both surfaces of the first embodiment.

Comparative Example

A polarizing film was obtained in the same manner as in Example 1 and Example 2 except that potassium hydroxide was used to adjust the pH to about 4.5 with respect to an aqueous solution (pH of about 3.85) containing 5.0 parts by weight of boric acid and 12 parts by weight of potassium iodide . Subsequently, a polarizing film was coated with a polyvinyl alcohol-based adhesive on both sides thereof, and the same protective film as in Example 1 was bonded to both sides to form a polarizing plate.

The initial optical performance and the optical durability under the dry heat conditions (100 占 폚, 3 hours) of the polarizing plates obtained in the first, second and comparative examples were evaluated. The optical performance was determined by setting the polarizer on an ultraviolet visible spectrophotometer V7100 manufactured by Bunko Co., Ltd. and measuring the ultraviolet visible spectrum of each polarizer plate in the transmission direction and the absorption direction. The initial properties (a simple transmittance, an orthogonal color (orthogonal a, orthogonal b)) at the center of the bonded product of each polarizing plate were calculated by JIS-Z8729J. In the durability test, each polarizing plate was bonded to sodium glass having a size of 40 mm x 40 mm and a thickness of 11 mm and treated in an oven at 100 deg. C for 3 hours. As a evaluation index of optical durability, a change amount orthogonal? E of the orthogonal colors (orthogonal a and orthogonal b) before and after the durability test was obtained by the following formula. The results are shown in Table 1.

[Mathematical Expression]

(Initial orthogonality b) - ² (initial orthogonality a)} ² + {(orthogonal b after durability) - (initial orthogonality b)} ² ] ^0.5

Initial Characteristics
(Central portion of the bonded article) durability
(100 DEG C, 3 hours) PH of crosslinking bath
Unit transmittance
(weight%) Orthogonal a Orthogonal b Cartesian ΔE Example 1 41.46 0.04 -0.13 0.33 2.5 Example 2 41.47 0.02 -0.12 0.27 3.5 Comparative Example 41.64 0.05 -0.05 1.31 4.5

As shown in Table 1, the amounts of change (orthogonal? E) of the polarizing plates in the first and second embodiments were 0.33 and 0.27, respectively, and the change amounts (orthogonal? E1 and 31) The value decreases to a meaningful value. As a result, in the first and second embodiments, by adding an acid other than boric acid in the crosslinking treatment of the polarizing film to lower the pH in the crosslinking treatment tank, the change of the orthogonal color of the polarizing plate before and after the durability test was significantly Can be mitigated. Thus, it can be seen that a polarizing film excellent in optical durability can be obtained under high temperature conditions.

Claims (10)

A method for producing a polarizing film in which a polyvinyl alcohol film is dyed with a dichroic dye and then subjected to a crosslinking treatment with boric acid is characterized in that the crosslinking treatment is carried out such that the pH of the aqueous solution of boric acid in the aqueous solution of boric acid is changed from more than 0 to 4 And the cross-linking treatment is carried out while continuously or intermittently activating the activated carbon treatment with the boric acid aqueous solution. The method according to claim 1, wherein the concentration of boric acid in the aqueous solution of boric acid in the crosslinking treatment is 4 parts by weight or more and 6 parts by weight or less based on 100 parts by weight of water. The method according to claim 1, wherein the cross-linking treatment is carried out in a treatment tank, and the activated carbon treatment is carried out by extracting a part of the aqueous boric acid solution from the treatment tank, bringing a part or all of the extracted boric acid-containing aqueous solution into contact with the activated carbon, And the boric acid aqueous solution is returned to the treatment tank. The method according to claim 1, wherein the pH of the aqueous solution of boric acid in the crosslinking treatment is adjusted by adding an acid. 5. The process of claim 4 wherein the acid added is an inorganic acid. 6. The method of claim 5, wherein the inorganic acid is sulfuric acid or hydrochloric acid. delete delete delete delete