KR101483793B1 - Method for producing polarizing film and method for producing polarizing plate - Google Patents

Abstract

The original film (1) of the polyvinyl alcohol resin is subjected to a swelling treatment in the swelling bath (3), a water immersion treatment in the water immersion tank (4), a dyeing treatment in the dye bath (5) And the boric acid treatment step in the step (6), and the uniaxially stretched film is continuously uniaxially stretched in at least one of the dyeing bath (5) and the boric acid bath (6) to produce the polarizing film (9) In the tank 4, the polyvinyl alcohol-based resin film is processed so as to have a stretching magnification of 1 time or more and 1.05 times or less with respect to the machine direction. It is preferable that the bath of the water immersion tank 4 is substantially pure water free of dissolved components. A polarizing plate is produced by bonding a transparent protective film to at least one surface of the resulting polarizing film (9).

Polarizing film, polarizing plate, polyvinyl alcohol

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a polarizing film,

The present invention relates to a polarizing film production method which is manufactured as a roll type elongate product and in which fluctuation of optical characteristics in the width direction is small, and a polarizing plate production method in which a transparent protective film is laminated on at least one surface of the polarizing film.

The polarizing film is a polyvinyl alcohol-based resin film, which is obtained by adsorbing and orienting a dichromatic dye, and is widely used. An iodine-based polarizing film using iodine as a dichromatic dye or a dye-based polarizing film using a dichromatic direct dye as a dichromatic dye is known. These polarizing films can usually be formed as a polarizing plate by bonding a transparent protective film such as triacetyl cellulose or the like via an adhesive containing an aqueous solution of a polyvinyl alcohol resin to one or both sides thereof.

Japanese Patent Application Laid-Open No. 10-153709 (Patent Document 1) discloses a method for producing a polarizing film, for example, a method in which a polyvinyl alcohol resin film is dipped in water to swell, followed by dyeing with iodine, , And then boric acid treatment (in other words, water resistance treatment by crosslinking) is further carried out to fix iodine, followed by washing with water and drying. The swelling treatment with water is carried out for the purpose of uniformly swelling the film prior to dyeing, shortening the dyeing time, and improving the uneven dyeing. At this time, in view of uneven dyeing and the like, Patent Document 1 contains boric acid in the swelling treatment bath. In this document 1, after dyeing, the film is immersed in an aqueous solution containing boric acid to perform stretching, and thereafter further immersed in an aqueous solution of boric acid to perform water resistance treatment (referred to as fixing or fixing in this document) by crosslinking I am doing.

Japanese Patent Laid-Open No. 6-281816 (Patent Document 2) discloses adding a chloride such as lithium chloride or zinc chloride to a swelling treatment bath to sufficiently swell the film. Japanese Patent Application Laid-Open No. 2001-141926 (Patent Document 3) discloses a method in which a polyvinyl alcohol-based resin film is dipped in a water bath to swell the polyvinyl alcohol-based resin film to reduce the uneven dyeing, And then performing a dyeing treatment and a crosslinking treatment with a dichroic dye to prepare a polarizing film.

Japanese Patent Application Laid-Open No. 2005-114990 (Patent Document 4) discloses a method of continuously treating a substrate in a bath in the order of a swelling treatment, a dyeing treatment and a boric acid treatment, uniaxially stretching at least one of them, , A wet stretching process is provided after the swelling process and before the dyeing process, and in the wet stretching process, uniaxial stretching is performed in an aqueous boric acid solution at a draw ratio of 1.1 times or more and less than 3 times, thereby further improving the color unevenness .

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

[Patent Document 2] Japanese Unexamined Patent Application Publication No. 6-281816

[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-141926

[Patent Document 4] Japanese Patent Application Laid-Open No. 2005-114990

The present inventors have conducted further studies on a method for producing a polarizing film after the swelling treatment and before the dyeing treatment as described in Patent Document 4 by preparing a wet stretching process. As a result, it has been found that the polarizing film obtained by this method is less prone to color unevenness, but easily changes in color in the widthwise middle portion and the end portion of the roll film. Accordingly, it is an object of the present invention to provide a polarizing film and a method of producing a polarizing plate having high optical properties in the width direction, particularly, high uniformity of hue.

DISCLOSURE OF THE INVENTION The inventors of the present invention conducted intensive studies to solve the above problems and found that a polyvinyl alcohol resin film was continuously passed through a swelling treatment step, a water immersion treatment step, a dyeing treatment step and a boric acid treatment step, In a water immersion treatment step between the swelling treatment step and the dyeing treatment step, when the polarizing film is uniaxially stretched in at least one of the treatment step and the boric acid treatment step, A polarizing film having high optical uniformity can be obtained, and thus the present invention has been accomplished.

That is, in the method of producing a polarizing film according to the present invention, the polyvinyl alcohol based resin film is continuously passed through the swelling treatment step, the water immersion treatment step, the dyeing treatment step and the boric acid treatment step, When the polarizing film is uniaxially stretched in at least one step in the boric acid treatment step, the polyvinyl alcohol based resin film is treated to have a stretching magnification of at least 1.05 times the machine direction in the water immersion treatment step will be.

The water immersion treatment is preferably performed at a temperature of 10 to 50 캜. The water immersion treatment is preferably carried out in pure water substantially free of dissolved components.

The method for producing a polarizing plate according to the present invention is a method for producing a polarizing film by the above method and then joining a transparent protective film to at least one surface of the obtained polarizing film. The transparent protective film may have any of the functions of a retardation film, a luminance enhancement film, a viewing angle improving film, and a transflective film. Alternatively, at least one selected from a retardation film, a brightness enhancement film, a viewing angle improving film and a transflective film may be bonded to the polarizing plate having a transparent protective film bonded to at least one surface thereof to produce an optical laminate.

According to the present invention, a water immersion treatment step is provided between the swelling treatment step and the dyeing treatment step, and the stretching magnification with respect to the machine direction of the film is reduced in the water immersion treatment step, It is possible to obtain an advantageous effect that a polarizing film having high optical properties in the width direction of the polarizing film, in particular, high uniformity of color.

Hereinafter, the present invention will be described in detail. In the present invention, the polyvinyl alcohol-based resin film is continuously passed through a swelling treatment step, a water immersion treatment step, a dyeing treatment step and a boric acid treatment step in this order, and is subjected to at least one of a dyeing treatment step and a boric acid treatment step To obtain a polarizing film. As a result of such continuous processing, the original film of polyvinyl alcohol resin is usually supplied in a roll form. Fig. 1 schematically shows an arrangement example of an apparatus suitable for carrying out the method for producing a polarizing film according to the present invention.

In the example shown in Fig. 1, the polyvinyl alcohol original film 1 drawn out from the original roll 2 is divided into a swelling tank 3 for swelling treatment, a water immersion tank 4 for performing water immersion treatment ), A dyeing tank 5 for dyeing, and a boric acid tank 6 for carrying out a boric acid treatment. The polyvinyl alcohol resin film after leaving the boric acid bath 6 is usually passed through the water bath 7 to wash the unreacted aqueous boric acid solution adhered in the previous bath and then passes through the drying furnace 8 And dried to obtain the polarizing film 9. Although not shown, a transparent protective film is usually bonded to at least one surface of the resulting polarizing film to form a polarizing plate.

[Polyvinyl alcohol-based resin]

The polyvinyl alcohol-based resin to be the disc of the polarizing film is specifically a saponified polyvinyl acetate-based resin. The saponification degree of the polyvinyl alcohol-based resin is 85 mol% or more, preferably 90 mol% or more, and more preferably 99 to 100 mol%. Polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as copolymers of vinyl acetate and other monomers copolymerizable therewith. Examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, unsaturated amines and the like. The polymerization degree of the polyvinyl alcohol-based resin is usually about 1,000 to 10,000, preferably about 1,500 to 5,000.

These polyvinyl alcohol resins may be modified, and for example, polyvinylformal modified with aldehydes, polyvinyl acetal, polyvinyl butyral and the like may be used. Normally, an unoriented film of a polyvinyl alcohol-based resin having a thickness of 20 to 100 mu m, preferably 30 to 80 mu m is used as the disk of the polarizing film.

The width of the film from 1,500 to 6,000 mm is industrially practical.

[Production of polarizing film]

The original film 1 passes through the swelling tank 3, the water immersion tank 4, the dyeing tank 5, and the boric acid tank 6 in order, and the respective treatments are performed. After the boric acid bath 6 is discharged, it is washed with water in the water tank 7, and finally dried in the drying furnace 8. The thickness of the polyvinyl alcohol polarizing film thus obtained is, for example, about 1 to 50 mu m. 1 shows an example in which one swatch tank 3, a water immersion tank 4, a dyeing tank 5, a boric acid tank 6 and a water tank 7 are provided, respectively. A plurality of tanks can be provided.

[Swelling Treatment]

In the original film (1), swelling treatment is first carried out in the swelling tank (3). This swelling treatment is carried out for the purpose of removing foreign substances on the film surface, removing the plasticizer in the film, imparting the dyeability in the subsequent process, plasticizing the film, and the like. The conditions of the swelling treatment are determined within a range capable of achieving these purposes and within a range that does not cause drawbacks such as extreme dissolution of the base film and loss of transparency. The undrawn original film is immersed in a treating bath at a temperature of, for example, 10 to 50 占 폚, preferably 20 to 50 占 폚 to perform swelling treatment. The swelling time is about 5 to 300 seconds, preferably about 20 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, in a known width expansion device such as an expander roll, a spiral roll, a crown roll, a cross gauge, a bend bar, It is preferable to transport the film while removing wrinkles of the film. For the purpose of stabilizing the film transportation in the bath, the water stream in the swelling bath may be controlled by an underwater shower, or an EPC apparatus (edge position control apparatus: a device for detecting the end of the film to prevent meandering of the film) useful. In this process, since the film swells and expands even in the running direction of the film, means for controlling the speed of the transport rolls before and after the swelling tank 3, for example, It is desirable to take a look. Specifically, the ratio of the main speed of the exit-side transport roll to the peripheral speed of the inlet-side transport roll of the swelling tank 3 (hereinafter sometimes referred to as "roll speed ratio") is set to 1.2 To about two times. In addition, if desired, uniaxial stretching may be performed in this step.

The treatment bath used in the swelling bath 3 is not limited to pure water but also includes boric acid as described in the above Patent Document 1 (Japanese Patent Application Laid-open No. Hei 10-153709), Patent Document 2 (Japanese Patent Application Laid- And the other inorganic acid, other inorganic salts, water-soluble organic solvents, alcohols and the like in the range of 0.01 to 10% by weight. However, in this swelling tank 3, pure water substantially free from dissolved components is preferably used.

[Water immersion treatment]

After the swelling treatment, the polyvinyl alcohol resin film is dewatered, and then subjected to water immersion treatment in the water immersion tank (4). This water immersion treatment is performed to adjust the absorption state in the film width direction so as to increase the mechanical properties of the film and further the uniformity of the optical characteristics of the finally obtained polarizing film. In this step, the film is processed at a stretching magnification of 1 time or more and 1.05 times or less with respect to the machine direction (MD, that is, the traveling direction of the film). When the stretching magnification is 1, it means that the film is neither stretched nor reduced in the machine direction.

The stretching magnification in this step is not usually lower than 1 time because a series of processes are performed by stretching the film so that the film does not become loose in the running direction. However, when the stretching magnification exceeds 1.05, The uniformity of the optical characteristics of the light-emitting layer tends to deteriorate.

The temperature of the treatment bath used in the water immersion tank 4 is preferably 10 to 50 캜. If the temperature of the treatment bath is lower than 10 캜, a large-scale cooling facility is required for temperature control, which is uneconomical. Conversely, if it exceeds 50 캜, the film may be dissolved. It is preferable that the bath used in the water immersion treatment is pure water substantially free of dissolved components. When this treatment bath contains a medicine such as boric acid, the uniformity of the film tends to be impaired.

[Dyeing treatment]

After the water immersion treatment, the film is dyed in the dyeing tank 5. The dyeing treatment with a conventional dichroic dye is carried out for the purpose of adsorbing a dichroic dye to the film or the like. The treatment conditions are within a range that can achieve such a purpose, but are determined within a range in which defects such as extreme dissolution of the base film and loss of transparency do not occur.

When iodine is used as the dichroic dye, it is used at a temperature of, for example, 10 to 50 ° C, preferably 20 to 40 ° C, and furthermore, 0.003 to 0.2 part by weight of iodine and 0.1 to 10 parts by weight of potassium iodide Is dipped in an aqueous solution containing 10 to 600 seconds, preferably 30 to 200 seconds. Instead of potassium iodide, other iodides, such as zinc iodide, may be used. Other iodides may also be used in combination with potassium iodide. Further, compounds other than iodide, for example, boric acid, zinc chloride, cobalt chloride, and the like may be coexistent. Even when boric acid is added, it is distinguished from the subsequent boric acid treatment in that it contains iodine. A bath containing iodine in an amount of 0.003 part by weight or more based on 100 parts by weight of water can be regarded as a dyeing bath.

On the other hand, when a water-soluble dichroic dye is used as the dichroic dye, the dichroic dye is added in an amount of 0.001 to 0.1 part by weight, for example, at a temperature of 20 to 80 캜, preferably 30 to 60 캜, For 10 to 600 seconds, preferably 20 to 300 seconds. The aqueous solution of the dichroic dye to be used may contain a dyeing aid or the like, or may contain an inorganic salt such as sodium sulfate, a surfactant or the like. Only one kind of dichroism dye may be used, or two or more kinds of dichroism dyes may be used together according to a desired color.

Also, in the dyeing treatment step, an expander roll, a spiral roll, a crown roll, a cross guider, a bend bar and the like may be appropriately provided in the dyeing bath and / or the bath entrance as well as the swelling treatment step. In this dyeing treatment step, uniaxial stretching can be simultaneously performed in the machine direction.

[Treatment with boric acid]

After the dyeing treatment, boric acid treatment is carried out in the boric acid bath (6). This boric acid treatment is carried out by immersing a polyvinyl alcohol-based resin film dyed with a dichroic dye into an aqueous solution containing about 1 to 10 parts by weight of boric acid per 100 parts by weight of water. When iodine is used as the dichroism dye, it is preferable that the boric acid treatment bath contains iodide in an amount of about 0.1 to 30 parts by weight per 100 parts by weight of water in addition to boric acid.

Examples of the iodide include potassium iodide and zinc iodide. In addition, compounds other than iodide, for example, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfite, potassium sulfate, sodium sulfate and the like may be coexistent.

This boric acid treatment is carried out for water resistance and color adjustment (to prevent blue coloration) by cross-linking. When it is desired to attain water resistance by crosslinking, a crosslinking agent such as glyoxal or glutaraldehyde may be used together with boric acid as necessary. On the other hand, the boric acid treatment for moisture resistance may be referred to as water resistance treatment, crosslinking treatment, immobilization treatment and the like. In addition, the boric acid treatment may be regarded as color adjustment, and may be referred to as complementary coloring treatment, coloring treatment or the like.

This boric acid treatment is carried out by appropriately changing the concentration of boric acid and iodide or the temperature of the treatment bath, depending on the purpose. The boric acid treatment for moisture resistance and the boric acid treatment for color adjustment are not particularly discriminated but are preferably carried out under the following conditions.

In the case where the original film is subjected to swelling, water immersion, dyeing and boric acid treatment in order, and when the boric acid treatment is intended for water resistance by crosslinking, about 3 to 10 parts by weight of boric acid and 100 parts by weight of iodide In an amount of about 1 to 20 parts by weight is used as a boric acid treatment bath, usually at a temperature of 50 to 70 占 폚, preferably 53 to 65 占 폚. The treatment time is usually about 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 100 seconds.

After the boric acid treatment for moisture resistance is performed, a boric acid treatment for color adjustment may be additionally performed. For example, when the dichroic dye is iodine, for this purpose, an aqueous solution containing about 1 to 5 parts by weight of boric acid and about 3 to 30 parts by weight of iodide relative to 100 parts by weight of water is used as the boric acid treatment bath, And is usually carried out at a temperature of about 10 to 45 占 폚. The immersion time is usually about 1 to 300 seconds, preferably 2 to 100 seconds. The boric acid treatment for color adjustment is usually performed at a lower temperature than the boric acid treatment for water resistance.

Therefore, this boric acid treatment may be carried out in a plurality of tanks, and in many cases, 1 to 5 tanks are often arranged. When a plurality of tanks are arranged, the films sequentially pass through the tanks, and the film is subjected to boric acid treatment. In the case of arranging a plurality of tanks, the composition and temperature of the aqueous solution of each of the boric acid treatment tanks to be used may be the same or different within the above range. The boric acid treatment for the water resistance and the boric acid treatment for the color adjustment can be performed in a plurality of tanks, respectively. Also in the boric acid treatment step, uniaxial stretching can be performed in the machine direction of the film as in the dyeing treatment step.

The final cumulative stretching magnification from the swelling treatment step to the boric acid treatment step is about 4.5 to 8 times, preferably 5 to 7 times.

[Post-treatment after boric acid treatment]

After the boric acid treatment, washing treatment is carried out in the water treatment tank (7). The water washing treatment may be carried out, for example, by a method of immersing a boric acid-treated polyvinyl alcohol resin film for water resistance and / or color adjustment in water, a method of spraying water with a shower, Is done. The temperature of the water in the water washing treatment is usually about 2 to 40 占 폚, and the treating time is usually about 2 to 120 seconds. After washing with water, the film is led to the drying furnace 8, and the film is dried. This drying is performed for about 30 to 600 seconds in the drying furnace 8 maintained at a temperature of about 40 to 100 캜.

[Uniaxial stretching]

In the present invention, uniaxial stretching is performed in at least one of the dyeing treatment step and the boric acid treatment step. This uniaxial stretching can be performed in one step or both steps, but it is preferable to perform uniaxial stretching in both steps, that is, both the dyeing step and the boric acid treatment step. The stretching can be performed by, for example, a method of imparting a peripheral speed difference to the conveying roll on the side of the bowl inlet and the conveying roll on the side of the emitter. On the other hand, the final cumulative stretching magnification is preferably about 4.5 to 8 times as described above, and more preferably about 5 to 7 times. The term "total stretching magnification" as used herein means the length of the original film in the lengthwise direction of the film after the stretching process has been completed. When the film is stretched in the swelling treatment step or the water immersion treatment step, . For example, if the portion of 1 m in the original film is 5 m in the film after all of the stretching treatment, then the total stretching magnification at this time is 5 times.

[Production of polarizing plate]

The thus prepared polarizing film can be used as a polarizing plate by bonding a transparent protective film to at least one side thereof with an adhesive. As the transparent protective film, for example, a film comprising an acetylcellulose resin such as triacetylcellulose or diacetylcellulose, a film containing a polyester resin such as polyethylene terephthalate, polyethylene naphthalate or polybutylene terephthalate, A film containing a polycarbonate resin, a film containing a cycloolefin resin containing a cyclic olefin such as norbornene as a main monomer, and the like. Examples of the commercially available thermoplastic cycloolefin resins include "Topas" sold by Ticona of Germany, "Aton" sold by JSR Co., Ltd., and sold by Nippon Zeon Co., Ltd. "Zeonor" and "Zeonex" which are commercially available from Mitsui Chemicals, Inc. and "Arpels" (all trade names) sold by Mitsui Chemicals, Inc. When a protective film is formed from such a cycloolefin-based resin, known methods such as solvent casting method and melt extrusion method are suitably used for film formation. The cycloolefin resin film formed is also commercially available, and examples thereof include "ESCINA" and "SCA40" sold by Sekisui Chemical Co.,

Though the thickness of the transparent protective film is preferably thin, if it is too thin, the strength is lowered and the workability is lowered. On the other hand, if it is too thick, transparency is lowered or a curing time required after lamination is prolonged. Therefore, the appropriate thickness of the protective film is, for example, about 5 to 200 占 퐉, preferably 10 to 150 占 퐉, and more preferably 10 to 100 占 퐉.

The polarizing film and / or the protective film is subjected to surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet ray irradiation, primer coating treatment and saponification treatment to improve adhesion between the adhesive and the polarizing film and / or the protective film can do.

The transparent protective film may be subjected to surface treatment such as antiglare treatment, antireflection treatment, hard coating treatment, antistatic treatment and antifouling treatment, either individually or in combination of two or more. The protective film and / or protective film surface protective layer may also have a plasticizer such as an ultraviolet absorber such as a benzophenone-based compound or a benzotriazole-based compound, or a phenylphosphate-based compound or a phthalic acid ester compound. The transparent protective film may be bonded to one side of the polarizing film or may be bonded to both sides of the polarizing film.

The polarizing film and the transparent protective film are laminated using an adhesive such as a water-based adhesive, an organic solvent adhesive, a hot melt adhesive, or a solventless adhesive. As the solvent-based adhesive, for example, a polyvinyl alcohol-based resin aqueous solution, an aqueous two-part type urethane emulsion adhesive, etc. may be used. As the organic solvent-based adhesive, for example, A liquid type urethane adhesive, and an epoxy adhesive.

When an acetylcellulose-based film having a hydrophilicized hydrophilic surface is used as a protective film on the adhesion surface of the polarizing film, a polyvinyl alcohol-based resin aqueous solution is preferably used as an adhesive. Examples of the polyvinyl alcohol-based resin used as an adhesive include a vinyl alcohol homopolymer obtained by saponifying a polyvinyl acetate which is a homopolymer of vinyl acetate, a vinyl alcohol copolymer obtained by saponifying a copolymer of vinyl acetate and another monomer copolymerizable therewith Alcohol-based copolymers, and further modified polyvinyl alcohol polymers obtained by partially modifying hydroxyl groups thereof. This adhesive may be blended with a polyaldehyde, a water-soluble epoxy compound, a melamine compound or the like as a crosslinking agent.

The method of bonding the polarizing film and the transparent protective film is not particularly limited. For example, an adhesive is uniformly applied to the surface of a polarizing film or a protective film, and the other film is overlaid on the coated surface, And the like.

Usually, the adhesive is applied at a temperature of 15 to 40 캜 after production, 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 100 占 폚, preferably 40 to 80 占 폚. Thereafter, the adhesive can 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. In the case where the protective film is an acetylcellulose-based resin, sufficient adhesion strength is exhibited even without such curing. In the case where a protective film containing other resin is used and bonded to a polarizing film via a water-based adhesive, It is preferable to perform the same curing. Since the productivity is deteriorated when the curing period is long, the curing period is preferably about 1 to 14 days, more preferably about 1 to 7 days.

Thus, a polarizing plate having a protective film bonded to one side or both sides of the polarizing film through the adhesive layer is obtained.

In the present invention, the transparent protective film may have an optical function such as a function as a retardation film, a function as a luminance enhancement film, a function as a reflection film, a function as a transflective film, a function as a diffusion film, have. In this case, for example, by laminating an optically functional film such as a retardation film, a luminance enhancement film, a reflection film, a transflective film, a diffusion film, and an optical compensation film on the surface of a transparent protective film, In addition, the transparent protective film itself may be provided with such a function. In addition, a plurality of functions such as a diffusion film having the function of a brightness enhancement film can be provided in the transparent protective film itself.

For example, a stretching treatment described in Japanese Patent No. 2841377, Japanese Patent No. 3094113, etc. is performed on the above-mentioned transparent protective film, or a treatment described in Japanese Patent No. 3168850 is performed to give a function as a retardation film . The retardation characteristics in the retardation film can be appropriately selected, for example, in the range of the front retardation value of 5 to 100 nm and the thickness retardation value of 40 to 300 nm. Further, by forming fine holes in the above-mentioned transparent protective film by the method described in Japanese Patent Laid-Open Nos. 2002-169025 and 2003-29030, or by forming fine holes having a central wavelength of selective reflection, By superimposing the steric liquid crystal layers, a function as a brightness enhancement film can be provided.

When a metal thin film is formed on the transparent protective film by vapor deposition, sputtering or the like, a function as a reflective film or a transflective film can be given. By coating the transparent protective film with a resin solution containing fine particles, a function as a diffusion film can be given. In addition, a function as an optical compensation film can be imparted by coating a liquid crystal compound such as a discotic liquid crystal compound and orienting the transparent protective film. Further, various optical functional films can be directly bonded to the polarizing film by using a suitable adhesive. As a commercially available product of the optically functional film, for example, a luminance enhancement film such as "DBEF " (trade name) sold by 3M Company (Sumitomo 3M Co., Ltd.)," WV film "sold by Fuji Film Co., (Trade name), a retardation film such as "SUMIKA LIGHT" (trade name) sold by Sumitomo Chemical Co., Ltd., and the like.

<Examples>

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples. The following examples and comparative examples are schematically shown in Fig. 1, wherein the swelling tank 3, the water immersion tank 4, the dyeing tank 5, the boric acid tank 6, the water tank 7, (8) were successively arranged in a continuous conveying apparatus in which a long film of polyvinyl alcohol was combined with a nip roll and a pre-roll, and various treatments were carried out. As the original film (1), a polyvinyl alcohol film having a degree of polymerization of 2,400, a degree of saponification of 99.9 mol% or more, a thickness of 75 mu m and a width of 3,000 mm ("Kurarabinetron VF-PS # 7500" manufactured by Kuraray Co., Respectively. Stretching was performed by applying a peripheral speed difference to the driving nip roll before and after the treatment tank.

The uniformity of the optical characteristics of the obtained polarizing film was evaluated by the b ^* value of the parallel color. The parallel color means a color when two polarizing films are overlapped so that their absorption axes are equal to each other and natural light is irradiated. The b ^* value thereof is a parallel spectral transmittance Tp (?) At a wavelength? Is calculated with respect to one wavelength region or at a predetermined wavelength interval, and is calculated from these in accordance with JIS Z 8729.

The parallel transmittance means the transmittance when the two polarizing films are superimposed so that their absorption axes are the same, and thus the parallel spectral transmittance Tp (?) At an arbitrary wavelength? Is 2 Means the transmittance when the natural light of wavelength? Is irradiated in a state in which the polarizing film of the sheet is overlapped so that the respective absorption axes become the same, but this is equivalent to the value obtained by the method described below. Namely, one polarizing film is irradiated with linearly polarized light having a wavelength? In a direction parallel to its absorption axis, and the spectral transmittance k1 (?) At this time is measured. Next, linearly polarized light having a wavelength? The spectral transmittance k2 (?) At this time is measured. The transmittance (parallel spectral transmittance) when the two polarizing films are superposed so that their absorption axes are equal to each other and the natural light of wavelength? Is irradiated is the transmittance {the parallel spectral transmittance) after passing through the film twice with the transmittance k1 since k1 (λ)} ² and a transmittance transmittance {k2 (λ)} ² the average value with after passing through the twice loaded with k2 (λ) is the same, the Tp (λ) parallel to the spectral transmittance at a wavelength λ from these values Is obtained by the following equation (1).

Tp (λ) = ^{[{k1 (λ)} 2 +} {k2 (λ)} 2 ] / 2

These measurements and calculations are made for successive wavelength regions or at predetermined wavelength intervals, and the b ^* value of the parallel color is calculated from the parallel spectral transmittance Tp (?) At each wavelength? According to JIS Z 8729. The spectral transmittances k1 (?) And k2 (?) And the parallel spectral transmittances Tp (?) Are all displayed as the maximum value, not the% display. It is good.

[Example 1]

First, the original film 1 was immersed in the swelling bath 3 containing pure water at 30 DEG C for about 80 seconds while keeping the tension of the film so that the original film 1 was not loosened, thereby sufficiently swelling the film. The roll speed ratio of the inlet and the outlet due to the swelling in the swelling tank 3 was 1.2. After dewatering by a nip roll, it was dipped in a water immersion tank (4) containing pure water at 30 DEG C for about 160 seconds. The draw ratio in the machine direction in this tank was 1.04 times. Next, uniaxial stretching was carried out at a stretching magnification of about 1.5 times while immersing in a dyeing bath 5 containing an aqueous solution of 0.02 / 1.5 / 100 by weight of iodine / potassium iodide / water. Thereafter, while immersing in a boric acid bath (6) containing an aqueous solution of 12/5/100 by weight of potassium iodide / boric acid / water in a weight ratio of about 130 seconds at 56.5 ° C, And stretching was carried out. Finally, in the water bath 7, the film was washed with pure water at 9 DEG C for about 16 seconds, and then dried at about 60 DEG C for 160 seconds in the drying furnace 8 to obtain an iodine polarizing film.

It was determined for the b ^* value of the color of the parallel in the parallel b ^* color value to the width ends of the central portion in the width direction of the obtained polarizing film. As a result, the difference between them was 2.1.

A polarizing plate is obtained by bonding a protective film containing triacetyl cellulose to each side of the polarizing film via an adhesive containing an aqueous solution of a polyvinyl alcohol-based resin. In addition, a protective film containing triacetyl cellulose on one side of the polarizing film and a protective film containing a norbornene-based resin on the other side are interposed with an adhesive containing an aqueous solution of a polyvinyl alcohol resin, respectively, or A polarizing plate can be obtained even by bonding via a solvent-free adhesive.

[Comparative Example 1]

First, the original film 1 was immersed in the swelling bath 3 containing pure water at 30 DEG C for about 50 seconds while keeping the tension of the film so that the original film 1 did not become loose, thereby sufficiently swelling the film. Dipped in a nip roll, immersed in a water immersion tank (4) containing pure water at 30 DEG C for about 160 seconds, and drawn in the machine direction at a draw ratio of 1.20 times in the machine direction. Thereafter, uniaxial stretching in the dyeing bath 5, uniaxial stretching in the boric acid bath 6, washing with water and drying were carried out in the same manner as in Example 1 to obtain a polarizing film.

It was determined for the b ^* value of the color of the parallel in the parallel b ^* color value to the width ends of the central portion in the width direction of the obtained polarizing film. As a result, the difference between them was 4.9.

According to the method of the present invention, a polarizing film or a polarizing plate having high optical property uniformity in the width direction of a film produced as a roll-like elongate product is obtained. The polarizing plate chips cut to various sizes from the polarizing plate exhibit almost the same optical characteristics between the respective sheets, and thus can be effectively applied to various display devices including a liquid crystal display device.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram schematically showing an arrangement example of a device suitable for carrying out the method of the present invention. Fig.

DESCRIPTION OF THE REFERENCE NUMERALS

1: original film of polyvinyl alcohol-based resin

2: disk roll

3: swelling tank

4: Water immersion tank

5: dyeing tank

6: Boric acid bath

7:

8: Drying furnace

9: polarizing film

Claims (5)

The polyvinyl alcohol based resin film is continuously passed through the swelling treatment step, the water immersion treatment step, the dyeing treatment step and the boric acid treatment step in this order, and is uniaxially stretched in at least one of the dyeing treatment step and the boric acid treatment step A method for producing a polarizing film, Wherein the swelling treatment step is carried out so that the ratio of the main stream of the outlet-side transport roll to the main stream of the inlet-side transport roll of the swelling tank for performing the treatment is 1.2 to 2 times, Wherein the polyvinyl alcohol-based resin film is treated to have a stretching magnification of at least 1.05 times the machine direction in the water immersion treatment step. The method according to claim 1, wherein the water immersion treatment is performed at a temperature of 10 to 50 캜. The method according to claim 1 or 2, wherein the water immersion treatment step is performed in pure water substantially free of dissolved components. A process for producing a polarizing plate, comprising the steps of: preparing a polarizing film by the method according to claim 1 or 2, and then bonding a transparent protective film to at least one surface of the polarizing film obtained. A process for producing a polarizing plate, wherein a polarizing film is produced by the method according to claim 3, and then a transparent protective film is bonded to at least one surface of the obtained polarizing film.

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