JP2005084505A - Polarizing film and method for producing the same, polarizing plate and optical laminate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarizing film free from hue unevenness.
SOLUTION: A polyvinyl alcohol film is treated in the order of swelling, dyeing, boric acid treatment and water washing treatment, uniaxial stretching is carried out in the boric acid treatment step and / or the preceding step, and further in each step after the stretching treatment. It is a polarizing film obtained by maintaining the tension applied to the film substantially constant, and the polyvinyl alcohol film is continuously treated in the order of swelling treatment, dyeing treatment, boric acid treatment, and water washing treatment, boric acid treatment. Uniaxial stretching is performed in the step and / or the previous step, and the film is manufactured by controlling the tension of the film in each step after the stretching treatment. As a result, the running of the film is stabilized and hue unevenness can be reduced.
[Selection] Figure 1

Description

  The present invention relates to a polarizing film with little hue unevenness and a method for producing the same, a polarizing plate in which a protective film is laminated on at least one side of the polarizing film, a retardation film, a brightness enhancement film, a viewing angle improvement film, and a transflective film. It is related with the optical laminated body by which kaga is individual or multiple bonded.

  Conventionally, a polarizing film in which a dichroic dye is adsorbed and oriented on a polyvinyl alcohol film has been used. That is, an iodine polarizing film using iodine as a dichroic dye, a dye polarizing film using a dichroic dye as a dichroic dye, and the like are known. These polarizing films are usually used as polarizing plates by attaching a protective film such as triacetyl cellulose to at least one surface, preferably both surfaces, with an adhesive made of an aqueous solution of a polyvinyl alcohol resin.

  As a method for producing a polarizing film, for example, in Patent Document 1, after a polyvinyl alcohol film is immersed in water and swollen, it is dyed with the dichroic dye, stretched, and then fixed with iodine to the film. For this purpose, a method is described in which a polyvinyl alcohol film is treated with boric acid, washed with water and then dried.

  Patent Document 2 describes that a dry uniaxially stretched polyvinyl alcohol film is relaxed while being dyed with the dichroic dye-containing aqueous solution, and is further stretched in a boric acid-containing aqueous solution to obtain a polarizing film. Has been.

  Uniaxial stretching of the polyvinyl alcohol film is important for obtaining a polarizing film having a high degree of polarization. Of the stretching, wet stretching is usually performed by changing the rotation speed of the nip roll before and after immersing the film in the treatment bath, specifically, by making the rotation speed of the nip roll after immersion higher than before the immersion. Stretching is performed. At this time, in order to perform stretching with high accuracy, so-called ratio control is performed in which the rotational speed ratio of the nip rolls before and after immersion in the treatment bath is controlled.

  However, even when such ratio control is accurately performed, hue unevenness may occur in the manufactured polarizing film. Such hue unevenness deteriorates the quality of the polarizing film.

JP-A-10-153709 JP 2002-182035 A

  The main problem to be solved by the present invention is to provide a polarizing film with less hue unevenness.

  As a result of intensive studies to find out the cause of the occurrence of such hue unevenness as described above, the tension of the polyvinyl alcohol film fluctuates in the step after the stretching treatment, and the film running is not stable. In addition, as a result of further examination, it is considered that uneven hues may occur, and when the tension control is adopted in the process after the stretching process instead of the ratio control that has been adopted throughout the entire process, a polyvinyl alcohol film As a result, the present inventors have found a new fact that the fluctuation of the tension applied to the film is suppressed, the film running is stabilized, and the hue unevenness can be reduced, thereby completing the present invention.

  That is, the polarizing film of the present invention is obtained by treating a polyvinyl alcohol film in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, performing uniaxial stretching in the boric acid treatment step and / or the previous step, and further stretching. In each step after the treatment, the tension applied to the film is maintained substantially constant.

  Further, in the method for producing a polarizing film of the present invention, a polyvinyl alcohol film is continuously treated in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, and the uniaxial process is performed in the boric acid treatment step and / or the previous step. A method for producing a polarizing film for stretching, wherein the tension of the film is controlled in each step after the stretching treatment.

  The tension per unit width of the film in each step after the stretching treatment is preferably in the range of 150 N / m to 2000 N / m.

  In the present invention, the boric acid treatment step may comprise a plurality of boric acid treatment steps, in which case, the film is stretched in the boric acid treatment step from the first or the first to the second step, It is preferable to control the tension so that the tension of the film becomes substantially constant in each step from the boric acid treatment step to the water washing step next to the boric acid treatment step after the stretching treatment.

  Moreover, the polarizing plate of the present invention is obtained by bonding a protective film to at least one surface of the polarizing film. This protective film may have any function of a retardation film, a brightness enhancement film, a viewing angle improvement film, and a transflective film. Or even if it is the optical laminated body which bonded at least 1 sort (s) chosen from a phase difference plate, a brightness improvement film, a viewing angle improvement film, and a transflective plate to the said polarizing plate which bonded the protective film on the single side | surface at least. Good.

  According to the present invention, by performing tension control in each step after the stretching treatment, the tension applied to the film is maintained substantially constant, and fluctuations in tension are suppressed. There is an effect that the hue unevenness of the film can be reduced. By using the polarizing plate and the optical laminate of the present invention in a liquid crystal display device, a thin and high-quality liquid crystal display can be obtained.

Hereinafter, the present invention will be described in detail.
Examples of the polyvinyl alcohol-based resin that forms the polyvinyl alcohol-based film in the present invention include those obtained by saponifying a polyvinyl acetate-based resin. As a saponification degree, it is 85 mol% or more, Preferably it is 90 mol% or more, More preferably, it is 99 mol%-100 mol%. Polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate and other monomers copolymerizable therewith, such as ethylene-vinyl acetate copolymer. Examples include coalescence. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is about 1000 to 10,000, preferably about 1500 to 5,000.

These polyvinyl alcohol resins may be modified. For example, polyvinyl formal modified with aldehydes, polyvinyl acetal, polyvinyl butyral, and the like may be used. Usually, as a starting material for producing a polarizing film, an unstretched film of a polyvinyl alcohol-based resin film having a thickness of 20 μm to 100 μm, preferably 30 μm to 80 μm is used. Industrially, the width of the film is practically 1500 mm to 4000 mm.
The unstretched film is treated in the order of swelling treatment, dyeing treatment, boric acid treatment, and water washing treatment, and finally dried to obtain a polyvinyl alcohol polarizing film having a thickness of, for example, about 5 to 50 μm.

  The polarizing film of the present invention is a polyvinyl alcohol uniaxially stretched film obtained by adsorbing and orienting a dichroic dye, and there are roughly two production methods as the production method. One is a method in which a polyvinyl alcohol film is uniaxially stretched in air or an inert gas, followed by solution treatment in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, followed by drying. Secondly, an unstretched polyvinyl alcohol film is treated with an aqueous solution in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, and uniaxial stretching is performed in a wet manner in the boric acid treatment step and / or the previous step. This is a method of performing drying and finally drying.

In any method, the uniaxial stretching may be performed in one step or in two or more steps, but is preferably performed in a plurality of steps. As a stretching method, a known method can be adopted. For example, stretching between rolls in which stretching is performed with a difference in peripheral speed between two nip rolls for transporting a film, hot roll stretching as described in Japanese Patent No. 2731813 Method and tenter stretching method. The order of the steps is basically as described above, but there are no restrictions on the number of treatment baths or treatment conditions.
Needless to say, a process not described in the above process can be freely inserted for another purpose. As an example of this process, after boric acid treatment, immersion treatment (iodide treatment) with an aqueous iodide solution not containing boric acid or immersion treatment (zinc treatment) step with an aqueous solution containing zinc chloride not containing boric acid, etc. Can be mentioned.

  The swelling step is performed for the purpose of removing foreign matter from the film surface, removing the plasticizer in the film, imparting easy dyeability in the next step, and plasticizing 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 dissolution and devitrification of the base film do not occur. When the film previously stretched in the gas is swollen, for example, the film is immersed in an aqueous solution at 20 ° C to 70 ° C, preferably 30 ° C to 60 ° C. The immersion time of the film is about 30 seconds to 300 seconds, more preferably about 60 seconds to 240 seconds. In order to swell an unstretched raw film from the beginning, the film is immersed in an aqueous solution of, for example, 10 ° C to 50 ° C, preferably 20 ° C to 40 ° C. The immersion time of the film is about 30 seconds to 300 seconds, more preferably about 60 seconds to 240 seconds.

  In the swelling treatment process, the film is likely to swell in the width direction and wrinkles into the film, so that the film may be spread with a known widening device such as an expander roll, a spiral roll, a crown roll, a cross guider, a bend bar, or a tenter clip. It is preferable to transport the film while removing wrinkles. In order to stabilize the film transport in the bath, the water flow in the swelling bath is controlled with an underwater shower, or an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents meandering of the film) It is also useful to use these together. In this step, 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 treatment tank in order to eliminate the slack of the film in the transport direction. In addition to pure water, the swelling treatment bath used is boric acid (described in JP-A-10-153709), chloride (described in JP-A-06-281816), inorganic acid, inorganic salt, water-soluble An aqueous solution to which an organic solvent, alcohol or the like is added in an amount of 0.01 to 10% by weight can also be used.

  The dyeing step with the dichroic dye is performed for the purpose of adsorbing and orienting the dichroic dye on 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 dissolution and devitrification of the base film do not occur. When iodine is used as the dichroic dye, for example, iodine / KI / water = 0.003 to 0.2 / 0.1 at a temperature of 10 to 45 ° C., preferably 20 to 35 ° C. and in a weight ratio. The immersion treatment is performed at a concentration of 10/100 for 30 seconds to 600 seconds, preferably 60 seconds to 300 seconds. Instead of potassium iodide, other iodides such as zinc iodide may be used. 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 coexist. When boric acid is added, it is distinguished from the following boric acid treatment in that it contains iodine. Any dye containing 0.003 parts by weight or more of iodine with respect to 100 parts by weight of water can be regarded as a dyeing tank.

  When a water-soluble dichroic dye is used as the dichroic dye, for example, a temperature of 20 ° C. to 80 ° C., preferably 30 ° C. to 70 ° C., and a weight ratio of dichroic dye / water = 0.001 to 0.00. An immersion treatment is performed at a concentration of 1/100 for 30 seconds to 600 seconds, preferably 60 seconds to 300 seconds. The aqueous solution of the dichroic dye to be used may have a dyeing assistant or the like, and may contain, for example, an inorganic salt such as sodium sulfate, a surfactant or the like. The dichroic dye may be used alone, or two or more dichroic dyes may be used at the same time.

  As described above, the film may be stretched in a dyeing tank. Stretching is performed by a method of giving a peripheral speed difference between the nip rolls before and after the dyeing tank. Similarly to the swelling step, an expander roll, a spiral roll, a crown roll, a cross guider, a bend bar, and the like can be installed in the dyeing bath and / or at the bath entrance / exit.

The boric acid treatment is performed by immersing a polyvinyl alcohol film dyed with a dichroic dye in an aqueous solution containing about 1 to 10 parts by weight of boric acid with respect to 100 parts by weight of water. When the dichroic dye is iodine, it is preferable to contain about 1 to 30 parts by weight of iodide.
Examples of iodide include potassium iodide and zinc iodide. Further, compounds other than iodide, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc. may coexist.
This boric acid treatment is carried out for water resistance, hue adjustment (preventing bluishness, etc.) by crosslinking, and the like. In the case of water resistance by cross-linking, a cross-linking agent such as glyoxal or glutaraldehyde can be used in addition to or together with boric acid, if necessary.
In addition, the boric acid treatment for water resistance may be referred to by names such as water resistance treatment, crosslinking treatment, and immobilization treatment. In addition, boric acid treatment for hue adjustment may be referred to by a name such as complementary color treatment or re-dyeing treatment.

This boric acid treatment is performed by appropriately changing the concentrations of boric acid and iodide and the temperature of the treatment bath according to the purpose.
The boric acid treatment for water resistance and the boric acid treatment for hue adjustment are not particularly distinguished, but are carried out under the following conditions.
When the raw film is swollen, dyed or treated with boric acid, and the boric acid treatment is aimed at water resistance by crosslinking, about 3 to 10 parts by weight of boric acid and 100% by weight of iodide, iodide Is carried out at a temperature of usually 50 ° C to 70 ° C, preferably 55 ° C to 65 ° C. The immersion time is usually about 30 to 600 seconds, preferably 60 to 420 seconds, and more preferably 90 to 300 seconds.
In addition, when dye | staining and boric-acid-treat the film extended | stretched previously, the temperature of a boric-acid treatment bath is 50 to 85 degreeC normally, Preferably it is 55 to 80 degreeC.

After boric acid treatment for water resistance, boric acid treatment for hue adjustment may be performed. For example, when the dichroic dye is iodine, a boric acid treatment bath containing about 1 to 5 parts by weight of boric acid and about 3 to 30 parts by weight of iodide for 100 parts by weight of water is used for this purpose. It is usually used at a temperature of 10 ° C to 45 ° C. The immersion time is usually about 3 to 300 seconds, preferably 10 to 240 seconds.
The boric acid treatment for adjusting the hue is usually performed at a lower boric acid concentration, a higher iodide concentration, and a lower temperature than the boric acid treatment for water resistance.

These boric acid treatments may be performed in a plurality of steps and are usually performed in 2 to 5 steps. In this case, the aqueous solution composition and temperature of each boric acid treatment tank to be used may be the same or different within the above range. The boric acid treatment for water resistance and the boric acid treatment for hue adjustment may be performed in a plurality of steps, respectively.
In the boric acid treatment process, the film may be stretched in the same manner as in the dyeing process. The final cumulative draw ratio is about 4.5 to 7.0 times, preferably 5.0 to 6.5 times.

  After boric acid treatment, it is washed with water. The water washing treatment is performed, for example, by immersing a polyvinyl alcohol film treated with boric acid for water resistance and / or color tone adjustment in water, spraying water as a shower, or combining immersion and spraying. The temperature of the water in the washing treatment is usually about 2 to 40 ° C., and the immersion time is preferably about 2 to 120 seconds. Drying after washing with water is performed in a drying furnace at a temperature of about 40 to 100 ° C. for about 60 to 600 seconds.

In the present invention, tension control is performed so that the tension of the film becomes substantially constant in each step after the stretching treatment.
When stretching is completed in the dyeing process, tension control is performed in the subsequent boric acid treatment process and the water washing process. When stretching is completed in the previous process of the dyeing process, tension control is performed in subsequent processes including the dyeing process and the boric acid process.
When the boric acid treatment step is composed of a plurality of boric acid treatment steps, the film is stretched in the boric acid treatment step from the first or the first to the second step, and the next boric acid treatment step after the boric acid treatment step in which the stretching treatment is performed. Tension control is performed in each step from the acid treatment step to the water washing step, or the film is stretched in the boric acid treatment step from the first to the third step, and the boric acid next to the boric acid treatment step in which the stretching treatment is performed. Although it is preferable to perform tension control in each process from the treatment process to the water washing process, industrially. The film is stretched in the boric acid treatment process from the first or the first to the second stage, and tension control is performed in each process from the boric acid treatment process to the water washing process following the boric acid treatment process. Is more preferable.
When the above-described iodide treatment or zinc treatment is performed after the boric acid treatment, tension control is also performed for these steps.

The tension in each step may be the same or different. The tension to the film in the tension control is not particularly limited, and is appropriately set within a range of 150 N / m to 2000 N / m, preferably 600 N / m to 1500 N / m per unit width. When the tension is less than 150 N / m, the film is likely to be wrinkled. On the other hand, when the tension exceeds 2000 N / m, problems such as film breakage and life reduction due to bearing wear occur. The tension per unit width is calculated from the film width near the entrance of the process and the tension value of the tension detector.
In addition, when tension control is performed, there are cases where the film is inevitably slightly stretched or shrunk, but in the present invention, this is not included in the stretching process.

  FIG. 1 shows an example of such tension control. That is, as shown in FIG. 1, nip rolls 2 and 3 are respectively provided on the inlet side and the outlet side of the treatment tank 1, and the polyvinyl alcohol film 4 is conveyed in the direction of the arrow by these nip rolls 2 and 3. A plurality of guide rolls are arranged inside and outside the treatment tank 1. Among these, the free roll 5 provided immediately after the entrance side nip roll 2 is provided with a tension detector.

  As a tension detector, a commercially available one can be used, and the tension is detected by a principle such as a differential transformer method or a strain gauge method. Thereby, the tension is continuously monitored. For example, the detected tension value is sent to the control system 6. In the control system 6, when the detected tension value deviates from the set value, the amount is measured, and a signal is sent to a driving device (for example, a servo motor) that drives the outlet side nip roll 3 to control the driving, and the tension is controlled. Try to keep the value constant.

  In the boric acid treatment process or a process before that, ratio control is performed to control the ratio of the rotational speed of the nip rolls provided before and after the treatment tank in consideration of uniaxial stretching. Therefore, in the present invention, the ratio control and the tension control are combined to stabilize the stretching and the running of the film, thereby obtaining a polarizing film free from color unevenness.

A polarizing plate is obtained by bonding a protective film with an adhesive on at least one side of the polarizing film thus produced.
As the protective film, for example, a film made of an acetyl cellulose resin such as triacetyl cellulose or diacetyl cellulose, a film made of a polyester resin such as polyethylene terephthalate, polyethylene naphthalate or polybutylene terephthalate, or a film made of a polycarbonate resin And a film made of a cycloolefin resin. Examples of commercially available thermoplastic cycloolefin resins include “Topas” (registered trademark) sold by Ticona of Germany and “Arton” (trademark) sold by JSR Co., Ltd. Registered), “ZEONOR” and “ZEONEX” (both registered trademarks) sold by Nippon Zeon Co., Ltd., and “APEL” (trademark registered) sold by Mitsui Chemicals, Inc. A film formed from such a cycloolefin-based resin is used as a protective film. For the film formation, a known method such as a solvent casting method or a melt extrusion method is appropriately used. The formed cycloolefin resin film is also commercially available, for example, “Essina” and “SCA40” sold by Sekisui Chemical Co., Ltd.

  The thickness of the protective film is preferably thin. However, if it is too thin, the strength is lowered and the processability is inferior. On the other hand, if it is too thick, the transparency is lowered and the curing time required after lamination is increased. Problems occur. Therefore, the suitable thickness of a protective film is about 5-200 micrometers, for example, Preferably it is 10-150 micrometers, More preferably, it is 20-100 micrometers.

  In order to improve the adhesiveness between the adhesive and the polarizing film and / or protective film, the polarizing film and / or protective film may be subjected to corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. A surface treatment may be applied.

The protective film may be subjected to surface treatment such as anti-glare treatment, anti-reflection treatment, hard coat treatment, antistatic treatment, and antifouling treatment alone or in combination. The protective film and / or the protective film surface protective layer may have a UV absorber such as a benzophenone compound or a benzotriazole compound, or a plasticizer such as a phenyl phosphate compound or a phthalate compound.
Such a protective film may be bonded to one side of the polarizing film or may be bonded to both sides.

  The polarizing film and the protective film are laminated using an adhesive such as a water solvent adhesive, an organic solvent adhesive, a hot melt adhesive, or a solventless adhesive. Examples of aqueous solvent adhesives include aqueous polyvinyl alcohol resins and aqueous two-component urethane emulsion adhesives, and examples of organic solvent adhesives include two-component urethane adhesives as solvent-free adhesives. For example, a one-pack type urethane adhesive may be used. When an acetylcellulose-based film whose surface to be bonded to the polarizing film is hydrophilized by saponification or the like is used as a protective film, a polyvinyl alcohol-based resin aqueous solution is suitably used as an adhesive. Polyvinyl alcohol resins used as adhesives include vinyl alcohol homopolymers obtained by saponifying polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as other single quantities copolymerizable with vinyl acetate. And vinyl alcohol copolymers obtained by saponifying the copolymer with the polymer, and modified polyvinyl alcohol polymers obtained by partially modifying the hydroxyl groups. For this adhesive, a polyvalent aldehyde, a water-soluble epoxy compound, a melamine compound or the like may be used as an additive.

  The method for laminating the polarizing film and the protective film is not particularly limited. For example, an adhesive is uniformly applied to the surface of the polarizing film or the protective film, and the other film is stacked on the coated surface by a roll or the like. The method of pasting and drying is mentioned.

Usually, an adhesive agent is apply | coated at the temperature of 15-40 degreeC after preparation, and the bonding temperature is the range of about 15-30 degreeC normally. After pasting, a drying treatment is performed to remove a solvent such as water contained in the adhesive, and the drying temperature at this time is usually in the range of 30 to 85 ° C, preferably 40 to 80 ° C. Thereafter, the adhesive may be cured by curing for about 1 to 90 days under a temperature environment of 15 to 85 ° C, preferably 20 to 50 ° C, more preferably 35 to 45 ° C. When this curing period is long, productivity is deteriorated, so the curing period is about 1 to 30 days, preferably 1 to 7 days.
Thus, a polarizing plate in which the protective film is bonded to one side or both sides of the polarizing film through the adhesive layer is obtained.

  In the present invention, the protective film has a function as a retardation film, a function as a brightness enhancement film, a function as a reflection film, a function as a transflective film, a function as a diffusion film, a function as an optical compensation film, etc. It can also have an optical function. In this case, for example, by laminating an optical functional film such as a retardation film, a brightness enhancement film, a reflection film, a transflective film, a diffusion film, an optical compensation film on the surface of the protective film, it has such a function. In addition, the protective film itself can be given such a function. Further, the protective film itself may have a plurality of functions such as a diffusion film having the function of a brightness enhancement film.

  For example, the protective film is subjected to a stretching process described in Japanese Patent No. 2841377, Japanese Patent No. 3094113, etc., or a process described in Japanese Patent No. 3168850, etc. The function as can be provided. In addition, by forming micropores in the above protective film by a method as described in JP 2002-169025 A or JP 2003-29030 A, two or more layers having different central wavelengths of selective reflection are formed. By superimposing these cholesteric liquid crystal layers, a function as a brightness enhancement film can be imparted. By forming a metal thin film on the above protective film by vapor deposition or sputtering, a function as a reflective film or a transflective film can be imparted. By coating the protective film with a resin solution containing fine particles, a function as a diffusion film can be imparted. Moreover, the function as an optical compensation film can be provided by coating and aligning liquid crystalline compounds, such as a discotic liquid crystalline compound, on said protective film. In addition, using an appropriate adhesive, a brightness enhancement film such as a trade name: DBEF (manufactured by 3M Co., Ltd.), a trade name: a viewing angle improving film such as a WV film (manufactured by Fuji Photo Film Co., Ltd.), a trade name : Commercially available optical functional films such as retardation films such as Sumikalite (registered trademark) (Sumitomo Chemical Co., Ltd.) may be directly bonded to the polarizing film.

EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated further more concretely, this invention is not limited by these examples. In the examples, “%” and “part” representing the content or amount used are based on weight unless otherwise specified.
The optical performance was measured by measuring the UV-visible spectrum of the polarizing plate in the transmission direction and absorption direction by setting the optional film holder with a polarizer on the UV2450 UV-Vis spectrophotometer manufactured by Shimadzu Corporation. The hue when the two polarizing plates are arranged in crossed Nicols, the orthogonal hue, the orthogonal L ^* , the orthogonal a ^* , and the orthogonal b ^*, were calculated and calculated according to JIS-Z8729.

A 75 μm-thick polyvinyl alcohol film (Kurarevinylon VF-PS # 7500, polymerization degree 2,400, saponification degree 99.9 mol% or more) is kept in 30 ° C. pure water so that the film does not loosen. The film was sufficiently swollen by immersion for about 130 seconds.
Next, uniaxial stretching was performed while immersing in an aqueous solution of iodine / potassium iodide / water in a weight ratio of 0.02 / 1.5 / 100 and dyeing. After that, it is immersed in a 60 ° C. aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/5/100, and uniaxially stretched until the total draw ratio from the raw fabric reaches 5.9 times while boric acid treatment. Went. Further, a second boric acid treatment was performed by immersing in an aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/3/100 at 40 ° C. for approximately 30 seconds. After the second boric acid treatment, it was washed with pure water at 10 ° C. for about 10 seconds.
In the second boric acid treatment and water washing treatment steps, the film tension was controlled while detecting the tension (tension control), and the tensions of the two tanks were both maintained at 1250 N / m. After washing with water and drying at 60 ° C. for 2 minutes, an iodine-based polarizing film having a thickness of 28 μm was obtained.
Polyvinyl alcohol-based adhesive is applied to both sides of this polarizing film, and a protective film [Triacetyl cellulose film with surface saponification treatment, “Fujitac (registered trademark) T80UNL”, manufactured by Fuji Photo Film Co., Ltd.], thickness 80 μm] was bonded to both sides and dried at 60 ° C. for 5 minutes to obtain a polarizing plate.
The orthogonal hue of the obtained polarizing plate was found offline at 31 points equally in the film flow direction, and the standard deviation of the orthogonal b ^* value was obtained. As a result, the standard deviation was 0.2, and the polarizing plate had little hue unevenness.

The same procedure as in Example 1 was performed except that two-stage boric acid treatment and tension control were performed as follows.
It is immersed in a 60 ° C aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/5/100, and uniaxially stretched until the total draw ratio from the raw fabric reaches 5.9 times while boric acid treatment. It was. Further, a second boric acid treatment was performed by immersing in an aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/5/100 at 60 ° C. for about 30 seconds.
In the second boric acid treatment and water washing treatment steps, the film tension was controlled while detecting the tension.
As a result of obtaining the standard deviation of the orthogonal b ^* value, the standard deviation was 0.2, and the polarizing plate had little hue unevenness.

The same procedure as in Example 1 was performed except that the three-stage boric acid treatment and tension control were performed as follows.
Uniaxial stretching was performed while dipping in a 55 ° C. aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/5/100, and further potassium iodide / boric acid / water at a weight ratio of 10/5/100. While being immersed in a 60 ° C. aqueous solution and subjected to a second water resistance treatment, uniaxial stretching was performed until the cumulative stretch ratio from the original fabric reached 5.9 times. Further, a third boric acid treatment was performed by immersing in an aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/3/100 at 40 ° C. for about 30 seconds. In the third boric acid treatment and water washing treatment steps, the film tension was controlled while detecting the tension.
As a result of obtaining the standard deviation of the orthogonal b ^* value, the standard deviation was 0.2, and the polarizing plate had little hue unevenness.

The same procedure as in Example 1 was performed except that the three-stage boric acid treatment and tension control were performed as follows.
The film was immersed in a 45 ° C. aqueous solution of potassium iodide / boric acid / water at a weight ratio of 1.5 / 2/100 and uniaxially stretched while being treated with boric acid. Thereafter, the total draw ratio from the original fabric becomes 5.9 times while being immersed in a 60 ° C. aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/5/100 and treated with the second boric acid. Until uniaxial stretching. Further, a third boric acid treatment was performed by immersing in an aqueous solution of potassium iodide / boric acid / water at a weight ratio of 10/3/100 at 40 ° C. for about 30 seconds. In the third boric acid treatment and water washing treatment steps, the film tension was controlled while detecting the tension.
As a result of obtaining the standard deviation of the orthogonal b ^* value, the standard deviation was 0.2, and the polarizing plate had little hue unevenness.
[Comparative Example 1]

Example 2 except that the second boric acid treatment step and the water washing treatment step were also ratio controlled, and the draw ratio of the nip rolls on the inlet side and outlet side in each bath was set to 1, that is, no stretching or relaxation occurred. A polarizing film was obtained in the same manner. However, the film tension fluctuated randomly during processing. The polarizing plate obtained using the polarizing film had a standard deviation of the orthogonal b ^* value of 0.7, and the hue unevenness was larger than those of Examples 1 to 4.

It is explanatory drawing which shows an example of the tension control in this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Processing tank 2 Nip roll 3 Nip roll 4 Polyvinyl alcohol-type film 5 Free roll 6 Control system

Claims (7)

  The polyvinyl alcohol film is treated in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, uniaxial stretching is performed in the boric acid treatment step and / or the preceding step, and the film is further processed in each step after the stretching treatment. A polarizing film obtained by maintaining the tension applied to each substantially constant.   A method for producing a polarizing film in which a polyvinyl alcohol film is continuously treated in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment, and uniaxial stretching is performed in the boric acid treatment step and / or the previous step, A method for producing a polarizing film, wherein the tension of the film is controlled in each step after the stretching treatment.   The method for producing a polarizing film according to claim 2, wherein the tension per unit width of the film in each step after the stretching treatment is in a range of 150 N / m to 2000 N / m.   The boric acid treatment step comprises a plurality of boric acid treatment steps, and the boric acid treatment following the boric acid treatment step in which the film is stretched and stretched in the boric acid treatment step from the first stage to the second stage. The method for producing a polarizing film according to any one of claims 2 to 3, wherein tension control is performed so that the tension of the film is substantially constant in each step from the step to the water washing step.   A polarizing plate comprising a protective film bonded to at least one surface of the polarizing film according to claim 1.   The polarizing plate according to claim 5, wherein the protective film has a function of any one of a retardation film, a brightness enhancement film, a viewing angle improvement film, and a transflective film. An optical laminate comprising the polarizing plate according to claim 5 and at least one selected from a retardation film, a brightness enhancement film, a viewing angle improvement film, and a transflective film.

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