JP2009069375A - Method of manufacturing polarizing plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a polarizing plate which has reduced blueness of hue quadrature and does not cause cracking even when being exposed under a thermal shock environment, in regard to the polarizing plate prepared by laminating protective films on both surfaces of a polyvinyl alcohol-based polarizing film by using an aqueous adhesive. <P>SOLUTION: The manufacturing method of polarizing plate comprises: a process of laminating a polarizing film and a protective film; and a process of drying the laminated film through N drying furnaces (N≥2). In the drying process, temperature in the first stage of drying furnace is less than 60°C, temperature in at least one drying furnace among the second to N-th stage of drying furnaces is 60°C or more and the laminated film is dried in such a condition as to satisfy Y>-0.3X+16,000. X is obtained by adding products of the temperatures (°C) in respective drying furnaces and residence time (second) of the laminated film in the respective drying furnaces, and Y is obtained by adding products of the temperatures (°C) in drying furnaces which have a temperature of 60°C or more and residence time (second) of the laminated film in the respective drying furnaces. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a polarizing plate in which protective films are laminated on both sides of a polarizing film made of a polyvinyl alcohol-based resin.

  A polarizing plate is usually a protective film, for example, cellulose acetate-based transparent material represented by triacetylcellulose, on one or both sides of a polarizing film made of a polyvinyl alcohol-based resin adsorbed and oriented with a dichroic dye. It is the structure which laminated | stacked the resin film. This is bonded to a liquid crystal cell with an adhesive via another optical film as necessary to obtain a component of a liquid crystal display device.

  Usually, a polarizing plate is manufactured by bonding a protective film on a polarizing film using an adhesive and then drying it. For example, Patent Document 1 describes that a laminated film of a polarizing film and a protective film is heat-treated, and specific conditions relating to the heat treatment are disclosed. However, under the heat treatment conditions described in this document, there is a problem that bluishness may occur in the orthogonal hue of the obtained polarizing plate.

On the other hand, in recent years, liquid crystal display devices for TVs, particularly large screen TVs, have been expanded, and the polarizing plates used therefor are also required to be enlarged. However, when the size of the polarizing plate is increased, there is a problem that cracks are likely to occur in the polarizing film when exposed to an environment where heat shock is applied, that is, an environment where cold heat is repeated.
JP 2006-313205 A

  An object of the present invention is a polarizing plate in which a protective film is laminated on both surfaces of a polyvinyl alcohol polarizing film using a water-based adhesive, and the bluish of an orthogonal hue is reduced. An object of the present invention is to provide a method for producing a polarizing plate in which cracks do not occur in the polarizing film even when exposed.

  As a result of intensive studies under these objectives, the present inventors have conducted the drying step in drying a laminated film obtained by laminating a protective film with a water-based adhesive on a polyvinyl alcohol polarizing film. The temperature of the first-stage drying furnace that is multi-staged and first passes through the laminated film is less than 60 ° C., and the temperature of at least one of the second-stage and subsequent drying furnaces is 60 ° C. or more. As a result, it has been found that the bluishness of the orthogonal hue is reduced or prevented. On the other hand, even when drying is performed under such conditions, when the obtained polarizing plate is exposed to a thermal shock environment in a state of being bonded to a substrate such as glass via an adhesive or the like, two sheets of A crack may occur in the polarizing film sandwiched between the protective films.

  Therefore, as a result of further research to effectively prevent the occurrence of such cracks, drying is performed so that the temperature of each drying furnace and the residence time of the laminated film in the drying furnace satisfy specific conditions. As a result, it has been found that the generation of cracks in the polarizing film in a cold shock environment can be prevented. That is, the present invention is as follows.

The present invention is a method for producing a polarizing plate in which protective films are laminated on both sides of a polarizing film made of a polyvinyl alcohol resin, and the polarizing film and the protective film are bonded using a water-based adhesive, and the laminated film And a drying step of drying the laminated film by passing it through N (N ≧ 2) drying furnaces, and in the drying step, the laminated film is first passed. The temperature of the stage drying oven is less than 60 ° C., the temperature of at least one of the 2nd to Nth stage drying ovens is 60 ° C. or more, and in the drying step, the laminated film is This is a method for producing a polarizing plate, which is dried under conditions satisfying the relationship of the following formula (1).
Y> −0.3X + 16000 (1)
Here, X (° C. · second) is the sum of the product of the temperature (° C.) of each drying furnace and the residence time (second) of the laminated film in the drying furnace for all drying furnaces. Y (° C. · sec) is a product of the temperature of the drying furnace (° C.) having a temperature of 60 ° C. or more and the residence time (seconds) of the laminated film in the drying furnace, and the temperature is 60 ° C. or more. The sum for the drying oven.

  Here, the temperature of the first-stage drying furnace is preferably 30 ° C. or higher and lower than 60 ° C. Moreover, it is preferable that the temperature of the drying furnace whose said temperature is 60 degreeC or more is 60 degreeC or more and 100 degrees C or less. Furthermore, in the drying step, the laminated film is preferably dried in a state where tension is applied.

  According to the present invention, there is provided a polarizing plate in which a protective film is laminated on both sides of a polarizing film made of a polyvinyl alcohol resin, and bluish of an orthogonal hue is reduced or prevented, and the polarizing film in a thermal shock environment is applied to the polarizing film. A polarizing plate in which cracks are also prevented can be provided. Such a polarizing plate can be suitably used for a large-screen liquid crystal display device, for example.

Hereinafter, the present invention will be described in detail. The polarizing plate of this invention is manufactured by laminating | stacking a protective film on both surfaces of the polarizing film which consists of polyvinyl alcohol-type resin through an aqueous adhesive layer. The method for producing a polarizing plate of the present invention basically includes the following steps (1) to (3).
(1) A polarizing film production process for producing a polarizing film using a polyvinyl alcohol-based resin film,
(2) A laminating step of laminating a polarizing film and a protective film using an aqueous adhesive to obtain a laminated film, and
(3) A drying step of drying the laminated film by passing it through N (N ≧ 2) drying furnaces. Hereinafter, each step will be described in detail.

(1) Polarizing film preparation process The polyvinyl alcohol-type resin which comprises a polarizing film is normally obtained by saponifying a polyvinyl acetate type resin. The saponification degree of the polyvinyl alcohol-based resin is usually about 85 mol% or more, preferably about 90 mol% or more, more preferably about 99 mol% to 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 monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is usually about 1000 to 10000, preferably about 1500 to 5000.

  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, an unstretched film of a polyvinyl alcohol-based resin film having a thickness of about 20 μm to 100 μm, preferably about 30 μm to 80 μm is used as a starting material for manufacturing a polarizing film. Industrially, the width of the film is practically about 1500 mm to 4000 mm. This unstretched film is processed in the order of swelling treatment, dyeing treatment, boric acid treatment, water washing treatment, uniaxial stretching is performed in the process up to boric acid treatment, and finally the thickness of the polyvinyl alcohol polarizing film obtained by drying is For example, it is about 5 μm to 50 μm.

  As the polarizing film used in the present invention, a polyvinyl alcohol uniaxially stretched film in which a dichroic dye is adsorbed and oriented can be preferably exemplified. There are roughly two production methods for the production. The first method 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, and finally drying. In the second method, an unstretched polyvinyl alcohol film is subjected to a solution treatment in the order of swelling treatment, dyeing treatment, boric acid treatment and water washing treatment with an aqueous solution, and is uniaxially wetted in the boric acid treatment step and / or the previous step. This is a method of performing stretching and finally drying.

  In any method, uniaxial stretching may be performed in one step or in two or more steps, but it is preferably performed in a plurality of steps. As the 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 that transport a film, for example, heat described in Japanese Patent No. 2731813. Examples thereof include a roll stretching method and a tenter stretching method. The order of the steps is basically as described above, but there is no restriction on the number of treatment baths, treatment conditions, and the like. Moreover, you may add the process which is not described in the said 1st and 2nd method for another objective. Examples of such steps include immersion treatment with an aqueous iodide solution not containing boric acid (iodide treatment) or immersion treatment with an aqueous solution containing zinc chloride not containing boric acid (zinc treatment) after boric acid treatment, etc. Is mentioned.

  The swelling treatment 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 processing conditions are determined within a range in which these objects can be achieved, and in a range in which problems such as extreme dissolution and devitrification of the base film do not occur. When a film previously stretched in a gas is swollen, for example, the film is immersed in an aqueous solution of about 20 ° C. to 70 ° C., preferably about 30 ° C. to 60 ° C. The immersion time of the film is about 30 seconds to 300 seconds, preferably about 60 seconds to 240 seconds. In order to swell the unstretched raw film from the beginning, the film is immersed in an aqueous solution of about 10 ° C. to 50 ° C., preferably about 20 ° C. to 40 ° C., for example. The immersion time of the film is about 30 seconds to 300 seconds, preferably about 60 seconds to 240 seconds.

  In the swelling process, problems such as swelling of the film in the width direction and wrinkling of the film are likely to occur, so known widening rolls such as widening rolls (expander rolls), spiral rolls, crown rolls, cross guiders, bend bars, tenter clips, etc. It is preferable to transport the film while removing wrinkles of the film with an apparatus. For the purpose of stabilizing film transport in the bath, the water flow in the swelling bath is controlled by 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 the range of about 0.01% by mass to 10% by mass 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 processing conditions are determined within a range in which these objects can be achieved, and in 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 / potassium iodide / water = about 0.003 to about 0.003 to about 0.13 in a mass ratio under a temperature condition of about 10 ° C. to 45 ° C., preferably about 20 ° C. to 35 ° C. The immersion treatment is performed for about 30 seconds to 600 seconds, preferably about 60 seconds to 300 seconds, using an aqueous solution having a concentration of 2 / about 0.1 to 10/100. Instead of potassium iodide, other iodides such as zinc iodide may be used. Other iodides may be used in combination with potassium iodide. Furthermore, 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 about 0.003 parts by mass or more of iodine with respect to 100 parts by mass of water can be regarded as a dyeing tank.

  When a water-soluble dichroic dye is used as the dichroic dye, for example, dichroic dye / water = about 0.001 in a mass ratio under a temperature condition of about 20 ° C. to 80 ° C., preferably about 30 ° C. to 70 ° C. Using an aqueous solution having a concentration of ˜0.1 / 100, an immersion treatment is performed for about 30 seconds to 600 seconds, preferably about 60 seconds to 300 seconds. The aqueous solution of the dichroic dye to be used may contain 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 in combination.

  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 treatment step, a widening roll (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 mass of boric acid with respect to 100 parts by mass of water. When the dichroic dye is iodine, it is preferable to contain about 1 to 30 parts by mass 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.

  The boric acid treatment is carried out for the purpose of water resistance by cross-linking, hue adjustment (preventing bluishness, etc.) and the like. When boric acid treatment is performed for water resistance by cross-linking, a cross-linking agent such as glyoxal or glutaraldehyde other than boric acid or together with boric acid can be used as 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 can be carried out under the following conditions. When the raw film is swollen, dyed and treated with boric acid, and the boric acid treatment is aimed at water resistance by cross-linking, about 3 to 10 parts by mass of boric acid with respect to 100 parts by mass of water, A boric acid treatment bath containing about 1 to 20 parts by mass of iodide is used, and it is usually performed at a temperature of about 50 to 70 ° C., preferably about 55 to 65 ° C. The immersion time is usually about 30 to 600 seconds, preferably about 60 to 420 seconds, and more preferably about 90 to 300 seconds. In addition, when dye | staining and boric-acid treatment the film extended | stretched previously, the temperature of a boric-acid treatment bath is about 50 to 85 degreeC normally, Preferably it is about 55 to 80 degreeC.

  You may make it perform the boric-acid process for hue adjustment after the boric-acid process for water resistance. For example, when the dichroic dye is iodine, for this purpose, 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 with respect to 100 parts by weight of water. Is usually performed at a temperature of about 10 ° C to 45 ° C. The immersion time is usually about 3 to 300 seconds, preferably about 10 to 240 seconds. Subsequent boric acid treatment for hue adjustment is usually performed at a lower boric acid concentration, a higher iodide concentration, and a lower temperature than boric acid treatment for water resistance.

  These boric acid treatments may consist of a plurality of steps and are usually carried out 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 step, the film may be stretched as in the dyeing step. The final cumulative draw ratio is about 4.5 to 7.0 times, preferably about 5.0 to 6.5 times. The cumulative stretching ratio here means how long the reference length in the length direction of the original film is in all the films after the completion of the stretching process. For example, it is 1 m in the original film. If the portion is 5 m in all the films after the stretching treatment, the cumulative stretching ratio at that time is 5 times.

  After the boric acid treatment, a water washing treatment is performed. The water washing treatment is performed, for example, by immersing a polyvinyl alcohol film treated with boric acid for water resistance and / or color adjustment in water, spraying water as a shower, or using both immersion and spraying. The water temperature in the water washing treatment is usually about 2 to 40 ° C., and the immersion time is preferably about 2 to 120 seconds.

  Here, in each step after the stretching treatment, tension control may be performed so that the tension of the film becomes substantially constant. Specifically, 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 stage, and the next boric acid treatment step after the boric acid treatment step in which the stretching treatment has been 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 stage, 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 was stretched in the boric acid treatment process from the first or the first to the second stage, and the stretching process was performed. It is more preferable to perform tension control in each step from the boric acid treatment step next to the boric acid treatment step to the water washing step. In addition, when performing the above-described iodide treatment or zinc treatment after the boric acid treatment, tension control can also be performed for these steps.

  The tension in each step from the swelling treatment to the water washing treatment 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 about 150 N / m to 2000 N / m, preferably about 600 N / m to 1500 N / m per unit width. When the tension is less than about 150 N / m, the film is likely to be wrinkled. On the other hand, if the tension exceeds about 2000 N / m, problems such as film breakage and shortened life 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, it may be inevitably slightly stretched or shrunk, but in the present invention, this is not included in the stretching process.

  Finally, a drying process is performed. The drying process is preferably performed in a large number of stages by changing the tension little by little. However, the drying process is usually performed in two or three stages because of restrictions on equipment. When performed in two stages, the tension in the front stage is preferably set in the range of 600 to 1500 N / m, and the tension in the rear stage is preferably set in the range of 300 to 1200 N / m. If the tension is too high, the film will break more, and if it is too low, wrinkles will increase, which is not preferable. Moreover, it is preferable to set the drying temperature of a front | former stage from the range of 30-90 degreeC, and the drying temperature of a back | latter stage from the range of 70-100 degreeC. If the temperature is too high, the film will be ruptured and the optical properties will be deteriorated. If the temperature is too low, streaks will increase, which is not preferable. The drying processing time can be, for example, 60 to 600 seconds, and the drying time in each stage may be the same or different. If the time is too long, the optical properties are deteriorated, and if the time is too short, the drying becomes insufficient.

  As the nip roll for controlling the tension and the guide roll for controlling the film conveyance direction, a rubber roll, a stainless steel polishing roll, a sponge rubber roll, and the like can be used. The rubber roll is made of NBR or the like, and its hardness is about 60 to 90 degrees, preferably about 70 to 80 degrees on the JIS Shore C scale measured by the test method of JIS K 6301, and the surface roughness is JIS B 0601 (surface roughness). It is preferably about 0.1 to 5S, preferably about 0.5 to 1S in terms of the average interval S of the local peaks of the roughness curve.

  The stainless steel polishing roll is made of SUS304, SUS316 or the like, and in order to make the film thickness uniform, the surface roughness is the average distance S between the local peaks of the roughness curve of JIS B 0601 (surface roughness). And about 0.2 to 1.0 S.

As the sponge rubber roll, the hardness of the sponge is about 20 to 60 degrees, preferably about 25 to 50 degrees, and the density is about 0.4 to 0.6 g / cm ³ on the JIS Shore C scale measured by the test method of JIS K 6301. , Preferably about 0.42 to 0.57 g / cm ³ and a surface roughness of about 10 to 30 S, preferably about 15 in terms of the average spacing S of the local peaks of the roughness curve of JIS B 0601 (surface roughness). It is preferably ˜25S.

(2) Bonding process A protective film is bonded on both surfaces of the polarizing film manufactured as described above using an aqueous adhesive to obtain a laminated film. Examples of protective films include films made of acetyl cellulose resins such as triacetyl cellulose and diacetyl cellulose, films made of polyester resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate, and films made of polycarbonate resins. And a film made of a cycloolefin resin. Examples of commercially available thermoplastic cycloolefin resins include “Topas (registered trademark)” (Topas) sold by Ticona in Germany and “Arton (registered trademark)” sold by JSR Corporation. ) "," Zeonor (registered trademark) "and" Zeonex (registered trademark) "sold by Nippon Zeon Co., Ltd., and" Apel (registered trademark) "sold by Mitsui Chemicals, Inc. A film formed from such a cycloolefin resin is used as a protective film. For film formation, known methods such as a solvent casting method and a melt extrusion method are 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, but if it is too thin, the strength is lowered and the processability is poor. On the other hand, when it is too thick, problems such as a decrease in transparency and a longer curing time after lamination occur. Therefore, a suitable thickness of the protective film is, for example, about 5 to 200 μm, preferably about 10 to 150 μm, more preferably about 20 to 100 μm.

  In order to improve the adhesion between the water-based adhesive and the polarizing film and / or protective film, the polarizing film and / or protective film is subjected to corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. The surface treatment may be performed.

  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 an ultraviolet absorber such as a benzophenone compound or a benzotriazole compound, or a plasticizer such as a phenyl phosphate compound or a phthalate compound.

  A protective film is bonded on both surfaces of a polarizing film. These protective films bonded to both surfaces may be made of the same resin or different resins.

  In the present invention, the polarizing film and the protective film are bonded using an aqueous adhesive. Examples of the water-based adhesive include a polyvinyl alcohol-based resin aqueous solution and a water-based two-component urethane emulsion adhesive. 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. A polyhydric aldehyde, a water-soluble epoxy compound, a melamine compound, a zirconia compound, or the like may be added as an additive to the water-based adhesive.

  The adhesive used for adhesion of the protective film bonded to both surfaces of the polarizing film may be the same or different.

  The method for laminating the polarizing film and the protective film is not particularly limited. For example, an aqueous adhesive is uniformly applied to the surface of the polarizing film and / or the protective film, and the other film is stacked on the coated surface. The method of pasting with a roll etc. and drying is mentioned. Usually, the water-based adhesive is applied at a temperature of about 15 to 40 ° C. after the preparation, and the bonding temperature is usually in the range of about 15 to 30 ° C.

(3) Drying process After laminating the polarizing film and the protective film, the laminated film is dried to remove water contained in the aqueous adhesive. In the present invention, drying is performed by continuously passing the laminated film through a drying furnace maintained at a suitable temperature in a total of N (N ≧ 2). Although such drying is not particularly limited, for example, the laminated film is wound into a roll while the laminated film is continuously passed through N drying furnaces that are continuously installed. Can be done.

  The temperature of the first stage drying oven through which the laminated film first passes needs to be less than 60 ° C. When it exceeds 60 ° C., the blue hue of the orthogonal hue of the polarizing plate becomes strong, and it becomes difficult to obtain a polarizing plate exhibiting a neutral hue. The temperature of the first stage drying furnace is preferably 30 ° C. or higher. If it is lower than 30 ° C., it tends to be easily peeled between the polarizing film and the protective film. More preferably, it is 40 ° C. or higher. By setting the temperature of the first-stage drying furnace to less than 60 ° C., a polarizing plate having a neutral hue with reduced bluishness of orthogonal hues can be obtained. The residence time of the laminated film in the first-stage drying furnace can be, for example, 1 to 300 seconds, and is preferably 5 to 150 seconds from the viewpoint of the hue and productivity of the obtained polarizing plate.

  In addition, for the second-stage drying furnace and the subsequent drying furnaces, the temperature of at least one of these drying furnaces is 60 ° C. or higher, and the drying furnace whose temperature is 60 ° C. or higher is preferably 60 ° C. It is set in the range of -100 ° C. When it exceeds 100 ° C., the parallel hue of the polarizing plate tends to yellow due to deterioration. When the second-stage drying furnace and the subsequent drying furnaces are set to less than 60 ° C., the drying furnace temperature is not particularly limited, and can be, for example, 30 ° C. or more and less than 60 ° C.

Furthermore, in the present invention, the multi-stage drying of the laminated film is performed under conditions satisfying the relationship of the following formula (1) in addition to the above conditions.
Y> −0.3X + 16000 (1)
Here, X (° C. · second) is the sum of the product of the temperature (° C.) of each drying furnace and the residence time (second) of the laminated film in the drying furnace for all drying furnaces. Y (° C. · sec) is a product of the temperature of the drying furnace (° C.) having a temperature of 60 ° C. or more and the residence time (seconds) of the laminated film in the drying furnace, and the temperature is 60 ° C. or more. The sum for the drying oven. That is, if the drying temperature × the residence time in the drying furnace is referred to as “amount of heat”, it can be said that X is the total amount of heat applied to the laminated film in the entire drying process. Y is the total amount of heat applied by a drying furnace having a temperature of 60 ° C. or higher. Therefore, the above formula (1) means that the total amount of heat applied by the drying furnace having a temperature of 60 ° C. or higher must occupy a certain amount or more of the total amount of heat. As described above, since the temperature of the first-stage drying furnace is less than 60 ° C., X> Y is necessarily satisfied.

  The temperature of the first-stage drying furnace is set to less than 60 ° C., the temperature of at least one of the second to N-th stage drying furnaces is set to 60 ° C. or more, and the above formula (1) is satisfied By drying under such conditions, it is possible to obtain a polarizing plate that reduces the bluishness of the orthogonal hue and does not generate cracks in the polarizing film even when exposed to a thermal shock environment.

  When the above equation (1) is plotted with the horizontal axis as X and the vertical axis as Y, a straight line descending to the right is obtained. This means that the smaller the X (total amount of heat applied to the laminated film), the larger the minimum value of Y that satisfies the above formula (1), and the larger the X, the smaller the minimum value of Y. In view of this tendency, that is, from the viewpoint of the amount of heat applied to the laminated film during drying, the boundary of the occurrence of cracks in the polarizing film is a straight line that goes downward to the right. It can be said.

  That is, since a polyvinyl alcohol-based resin film (polarizing film) exhibits a large shrinkage behavior in a temperature range exceeding about 60 ° C. in a dry heat environment, a drying furnace maintained at a temperature of 60 ° C. or higher. It is necessary to cause the contraction by passing the. In this way, if the polarizing film is not shrunk in the drying step, a polarizing plate formed by bonding protective films on both sides of the polarizing film was bonded to a substrate such as glass via an adhesive or the like. When exposed to a cold shock environment in this state, this shrinkage occurs and cracks are likely to occur in the polarizing film. On the other hand, when the polarizing plate is not sufficiently dried and moisture remains in the polarizing film, the polarizing plate formed by bonding protective films on both sides of the polarizing film is made of glass or the like via an adhesive or the like. When exposed to a cold shock environment in a state of being bonded to the substrate, the moisture evaporates, which also causes cracks. When X is large, the total amount of heat applied in the drying process is large, so moisture is sufficiently removed by the amount of heat, and the amount of heat Y applied at a temperature of 60 ° C. or higher causes the shrinkage as described above. It is considered that the shrinkage and the accompanying crack generation can be suppressed even if Y is small to some extent. On the other hand, when X is small, the total amount of heat applied in the drying step is small, so it is considered necessary to increase the amount of heat Y applied at a temperature of 60 ° C. or higher to accelerate moisture removal. It is done.

  FIG. 1 is a diagram showing the correlation between the relationship between X and Y in Reference Examples, Examples, and Comparative Examples and the presence or absence of cracks in a thermal shock test (heat shock test), as will be described later. A straight line Y = −0.3X + 16000 representing a boundary line is represented by a solid line A. As described above, since X> Y, a straight line Y = X representing the boundary line is represented by a broken line B. Therefore, the region defined by the present invention is the region on the right side in the figure surrounded by the straight line A representing Y = −0.3X + 16000 and the straight line B representing Y = X (however, on the straight line of Y = X) Never). Since the intersection of these two straight lines is X = Y = 16000 / 1.3≈12308, X takes a value exceeding about 12308 ° C. · sec.

  The upper limit of the total heat amount X applied to the laminated film is not particularly limited, but is usually 53000 ° C. · second or less, preferably 50000 ° C. · second or less, more preferably 45000 ° C. · second or less. When X exceeds 53000 ° C. · sec, a long time is spent in the drying process, resulting in a decrease in productivity. Further, as described above, the total amount of heat X takes a value exceeding about 12308 ° C. · second, preferably 12,500 ° C. · second or more, more preferably 13000 ° C. · second or more. If X is less than 12500 ° C. · sec, cracks may occur in the polarizing film under a thermal shock environment. Since the total amount of heat Y applied by the drying furnace having a temperature of 60 ° C. or higher is X> Y, it is usually less than 53000 ° C. · second, preferably less than 50000 ° C. · second, more preferably less than 45000 ° C. · second. On the other hand, the lower limit of Y is naturally determined according to the value of X from the relationship of X> Y and the relationship of the above formula (1).

  The total residence time in the second to Nth stage drying furnaces is usually about 60 to 750 seconds. If it exceeds 750 seconds, a long time is spent in the drying process, and productivity is lowered.

  The temperature difference between the n-th stage drying furnace (n = 1, 2, 3,..., N <N) and the (n + 1) -th stage drying furnace is 40 ° C. or less, preferably 30 ° C. or less. If the temperature difference is large, wrinkles or irregularities are likely to occur in the laminated film, which tends to be unfavorable in appearance. The temperature of the (n + 1) th stage drying furnace may be higher or lower than the temperature of the nth stage drying furnace.

  The number N of drying ovens used for drying is not particularly limited as long as it is 2 or more, but is preferably 3 or more. When the number of drying furnaces is two, in order to satisfy the relationship of the above formula (1), the residence time in the second stage drying furnace must be increased, but the number of drying furnaces is three or more. And it is preferable because the temperature and residence time in the second and subsequent drying furnaces are adjusted to easily satisfy the relationship of the above formula (1).

  In the drying step, the laminated film is preferably dried in a state where tension is applied. The tension is preferably 100 to 1500 N / m, more preferably 100 to 1000 N / m. The tension is applied to the laminated film by adjusting the tensile force between the feeding side nip roll and the outlet side nip roll when the laminated film is continuously passed through a drying furnace that is continuously installed. Can be done.

  After the drying step, in order to cure the adhesive, it is usually about 1 to 90 days in a temperature environment of about 15 to 85 ° C, preferably about 20 to 50 ° C, more preferably about 35 to 45 ° C. The laminated film may be additionally dried (cured). The humidity during curing is usually 70% RH. If it exceeds 70% RH, problems such as condensation may occur. The curing period is usually about 1 to 30 days, and preferably about 1 to 7 days in consideration of productivity. Usually, curing is often performed in a state of being wound on a roll. 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, and an optical compensation film. An optical function such as a function can also be imparted. 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, etc. In addition, the protective film itself can be provided with 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.

  More specifically, the protective film is protected by subjecting the protective film to a stretching process described in Japanese Patent No. 2841377, Japanese Patent No. 3094113, or the like, or a process described in Japanese Patent No. 3168850. A function as a retardation film can be imparted to the film. Further, two or more layers having different central wavelengths of selective reflection are formed by forming micropores in the protective film by a method as described in JP-A No. 2002-169025 and JP-A No. 2003-29030. By superimposing these cholesteric liquid crystal layers, a function as a brightness enhancement film can be imparted. A function as a reflective film or a transflective film can be imparted to the protective film by forming a metal thin film by vapor deposition or sputtering. A function as a diffusion film can be imparted by coating the protective film with a resin solution containing fine particles. 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 a suitable adhesive, a brightness enhancement film such as trade name: DBEF (manufactured by 3M, available from Sumitomo 3M Co., Ltd. in Japan), trade name: WV film (manufactured by Fuji Film Co., Ltd.), etc. A commercially available optical functional film such as a retardation film such as a viewing angle improving film, a trade name: Sumikalite (registered trademark) (manufactured by Sumitomo Chemical Co., Ltd.), or the like may be directly bonded to a protective film.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these examples. In the examples, “%” and “part” representing the content or amount used are based on mass unless otherwise specified.

  First, a reference showing the importance of setting the temperature of the first-stage drying furnace defined in the present invention to less than 60 ° C., and setting at least one of the second-stage and subsequent drying furnaces to 60 ° C. or more. Take an example.

<Reference Example 1>
A polyvinyl alcohol film having an average degree of polymerization of about 2,400 and a degree of saponification of 99.9 mol% or more and a thickness of 75 μm is uniaxially stretched about 5 times in a dry manner and further maintained at 60 ° C. with pure water. And then immersed in an aqueous solution having a mass ratio of iodine / potassium iodide / water of 0.05 / 5/100 at 28 ° C. for 60 seconds. Thereafter, it was immersed in an aqueous solution having a mass ratio of potassium iodide / boric acid / water of 8.5 / 8.5 / 100 at 72 ° C. for 300 seconds. Subsequently, the film was washed with pure water at 26 ° C. for 20 seconds and then dried at 65 ° C. to obtain a polarizing film in which iodine was adsorbed and oriented on polyvinyl alcohol.

  Separately, in 100 parts of water, 3 parts of carboxyl group-modified polyvinyl alcohol (“Kuraray Poval KL318” manufactured by Kuraray Co., Ltd.) and water-soluble polyamide epoxy resin (“Smiles Resin 650” manufactured by Sumika Chemtex Co., Ltd.) An aqueous solution having a solid content concentration of 30%)) 1.5 parts was dissolved to prepare a polyvinyl alcohol-based adhesive.

  One side of the previously obtained polarizing film is a 80 μm-thick film made of norbornene-based resin that has been subjected to corona treatment as the first protective film (“Zeonor film” manufactured by Optes Co., Ltd.) On the surface, a 80 μm-thick film (“KC8UX2M” manufactured by Konica Minolta Opto Co., Ltd.) made of triacetyl cellulose subjected to saponification treatment as a second protective film was passed through the above adhesives, respectively. And laminated with a nip roll to obtain a laminated film.

The laminated film was passed through a 55 ° C. first stage drying furnace and a 65 ° C. second stage drying furnace in this order while maintaining a tension of 450 N / m to obtain a polarizing plate. At this time, the residence time in each drying furnace is obtained from the film residence length and the line speed of each drying furnace, and X and Y defined above are obtained from this and the above temperature of each drying furnace. It was shown to. The orthogonal hue of the obtained polarizing plate was orthogonal a ^* = 0.05, orthogonal b ^* = − 0.08, and was a neutral hue.

The polarizing plate was subjected to a thermal shock test (heat shock test). In this test, a pressure-sensitive adhesive layer was provided on the surface of the first protective film of the polarizing plate, cut into a size of 277 × 345 mm, and bonded to glass as a test sample. The thermal shock test was performed by holding this sample at -35 ° C for 1 hour, then raising the temperature to 70 ° C and holding it for 1 hour as one cycle, and repeating this for a total of 240 cycles. As a result, cracks occurred in the polarizing film. Table 1 shows the results of the thermal shock test (heat shock test) and the values of a ^* and b ^* of orthogonal hues together with the drying conditions of the laminated film including the values of X and Y.

<Reference Example 2>
A polarizing plate was obtained in the same manner as in Reference Example 1 except that it was passed through a 65 ° C. first stage drying oven and a 75 ° C. second stage drying oven in that order and dried. The orthogonal hue of the obtained polarizing plate was orthogonal a ^* = 0.22, orthogonal b ^* = − 0.55, and the value of orthogonal b ^* was large on the minus side, and it had a strong bluishness visually. . The polarizing plate was subjected to the same thermal shock test as in Reference Example 1. As a result, in this polarizing plate, no crack was observed in the polarizing film. Table 1 shows the drying conditions of the laminated film including the values of X and Y, the results of the thermal shock test (heat shock test), and the values of a ^* and b ^* of the orthogonal hue.

  Next, examples and comparative examples showing the importance of satisfying the above formula (1) in relation to the drying furnace in the second and subsequent stages while setting the temperature of the first drying furnace to less than 60 ° C. Is raised.

<Example 1>
A polyvinyl alcohol film having an average polymerization degree of about 2,400 and a saponification degree of 99.9 mol% or more and a thickness of 75 μm is stretched while being immersed and swollen in pure water at 30 ° C. while maintaining a tension state. The film was stretched in the longitudinal direction up to a magnification of 1.3. The polyvinyl alcohol film was dyed with an aqueous solution having a mass ratio of iodine / potassium iodide / water at 30 ° C. of 0.05 / 2/100 while maintaining the above draw ratio. While performing a crosslinking treatment with an aqueous solution having a potassium fluoride / boric acid / water mass ratio of 12/5/100, the film was stretched to a total magnification of 5.6 times and then washed with pure water at 12 ° C. The polyvinyl alcohol film after washing was dried at 65 ° C. to obtain a polarizing film in which iodine was adsorbed and oriented on polyvinyl alcohol.

  Separately, in 100 parts of water, 1.8 parts of carboxyl group-modified polyvinyl alcohol (“Kuraray Poval KL318” manufactured by Kuraray Co., Ltd.) and a water-soluble polyamide epoxy resin (“Smiles Resin” manufactured by Sumika Chemtex Co., Ltd.) 650 "(30% solid content aqueous solution)) 0.9 parts was dissolved to prepare a polyvinyl alcohol-based adhesive.

  On one side of the previously obtained polarizing film, a 43 μm thick film (“KC4FR-1” manufactured by Konica Minolta Opto Co., Ltd.) made of triacetyl cellulose as the first protective film, and on the other side As a second protective film, a 83 μm-thick surface treatment film (“Matte Hard Coat TAC Film DS-LR2” manufactured by Dai Nippon Printing Co., Ltd.) with an antiglare treatment layer provided on the surface of triacetyl cellulose was used. Then, each was laminated by a nip roll through the above adhesive to obtain a laminated film.

The laminated film was passed through a drying furnace (three stages in total) maintained at the temperature shown in Table 1 while being kept at a tension of 450 N / m, and dried to obtain a polarizing plate. The obtained polarizing plate was subjected to the same thermal shock test as in Reference Example 1. As a result, in this polarizing plate, no crack was observed in the polarizing film. Table 1 shows the drying conditions of the laminated film including the values of X and Y, the results of the thermal shock test (heat shock test), and the values of a ^* and b ^* of the orthogonal hue.

<Examples 2-4, Comparative Examples 1-3>
A polarizing plate was obtained in the same manner as in Example 1 except that the temperature setting of each drying furnace and the type of protective film were changed as shown in Table 1 and the residence time of the laminated film in each drying furnace was changed. Table 1 shows the drying conditions of the laminated film including the values of X and Y in each Example and Comparative Example, the results of the thermal shock test (heat shock test), and the values of a ^* and b ^* of orthogonal hues. However, in Comparative Examples 1 and 2, since the data of the orthogonal hue is not taken, the value of the orthogonal hue in these examples is not shown.

In Table 1, “A” in the column of “Heat Shock Test Results” indicates that no crack occurred in the polarizing film sandwiched between the two protective films after the test, and “B” indicates that after the test. Indicates that a crack was observed. In addition, names (product names) in the “protective film” column are as follows.
ZEONOR: “ZEONOR film” which is a norbornene-based resin film manufactured by Optes Co., Ltd., which is the same as that used as the first protective film in Reference Example 1.
KC8UX2M: “KC8UX2M”, which is a triacetylcellulose film manufactured by Konica Minolta Opto Co., Ltd., which was used as the second protective film in Reference Example 1.
KC4FR-1: “KC4FR-1”, which is the same triacetyl cellulose film manufactured by Konica Minolta Opto Co., Ltd. as used in Example 1 as the first protective film.
DS-LR2: “Matte Hard Coat TAC Film DS-LR2” which is the same surface-treated triacetyl cellulose film manufactured by Dai Nippon Printing Co., Ltd. as used in Example 1 as the second protective film.

  FIG. 1 is a diagram showing a correlation between the relationship between X and Y in the reference examples, examples, and comparative examples, and the presence or absence of occurrence of cracks in the thermal shock test (heat shock test). In FIG. 1, a point indicated by “◯” (Reference Example 2 and Example) means that no crack was generated, and a point indicated by “X” (Reference Example 1 and Comparative Example) This means that cracks were observed after the impact test. As shown in FIG. 1, it can be seen that the condition of the above formula (1) must be satisfied in order to prevent the occurrence of cracks.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the correlation with the relationship between X and Y in a reference example, an Example, and a comparative example, and the presence or absence of crack generation.

Claims (4)

A method for producing a polarizing plate in which protective films are laminated on both sides of a polarizing film made of a polyvinyl alcohol-based resin,
A polarizing film and a protective film are bonded using an aqueous adhesive to obtain a laminated film, and a drying process in which the laminated film is dried by passing through N (N ≧ 2) drying furnaces. With
In the drying step, the temperature of the first stage drying furnace through which the laminated film first passes is less than 60 ° C., and the temperature of at least one of the second to Nth stage drying furnaces is 60 ° C or higher, and
In the drying step, the laminated film is dried under conditions that satisfy the relationship of the following formula (1).
Y> −0.3X + 16000 (1)
Here, X (° C. · second) is the sum of the product of the temperature (° C.) of each drying furnace and the residence time (seconds) of the laminated film in the drying furnace for all drying furnaces. Y (° C. · sec) is the product of the temperature (° C.) of the drying furnace having a temperature of 60 ° C. or more and the residence time (seconds) of the laminated film in the drying oven, and the temperature is 60 ° C. or more. The sum for a drying oven.   The method for producing a polarizing plate according to claim 1, wherein the temperature of the first-stage drying furnace is 30 ° C. or higher and lower than 60 ° C.   The method for producing a polarizing plate according to claim 1 or 2, wherein the temperature of the drying furnace having a temperature of 60 ° C or higher is 60 ° C or higher and 100 ° C or lower.   In the said drying process, the said laminated | multilayer film is a manufacturing method of the polarizing plate in any one of Claims 1-3 dried in the state to which tension | tensile_strength was added.

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