JP2017106968A - Method for manufacturing polarizing film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new manufacturing method of a polarizing film performing dry stretching that reduces contractile force.SOLUTION: A method for manufacturing a polarizing film comprises, in this order: a stretching step of uniaxially stretching a polyvinyl alcohol resin film in a dry process; a dyeing step of dyeing the polyvinyl alcohol resin film by making it come into contact with a dyeing solution including a dichroic dye; a cross-linking step of cross-linking the polyvinyl alcohol resin film by making it come into contact with a cross-linking solution including boric acid and uniaxially stretching it at the stretching rate of a times; and a cleaning step of cleaning the polyvinyl alcohol resin film by making it come into contact with a cleaning solution and uniaxially stretching it at the stretching rate of b times. The stretching rate a and the stretching rate b satisfy relations between (1)-(3) expressions: a≥1.01 expression(1), b≥1.01 expression (2), and b≥0.8a expression (3).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a polarizing film from a polyvinyl alcohol-based resin film.

  A polarizing plate is widely used as a polarizing element in an image display device such as a liquid crystal display device. As a polarizing plate, the thing of the structure which bonded the transparent resin film (protective film etc.) on the single side | surface or both surfaces of the polarizing film using the adhesive agent etc. is common.

  The polarizing film is mainly immersed in a dye bath containing a dichroic dye such as iodine, and then immersed in a cross-linking bath containing a cross-linking agent such as boric acid, with respect to the raw film made of polyvinyl alcohol resin. The film is produced by uniaxially stretching the film at any stage. Uniaxial stretching includes dry stretching in which stretching is performed in the air before the immersion treatment and wet stretching in which stretching is performed in a liquid such as the dyeing bath and the crosslinking bath.

  Since the polarizing film is cross-linked, it tends to shrink when heated, and the durability may not be sufficient. Japanese Patent Application Laid-Open No. 2013-148806 (Patent Document 1) describes providing a polarizing film having excellent durability with a boron content as low as 1 to 3.5% by weight.

JP 2013-148806 A

  However, in the polarizing film, when the boron content is reduced, a sufficient degree of crosslinking cannot be obtained, and the optical characteristics particularly in a high temperature and high humidity environment may be deteriorated. An object of this invention is to provide the new manufacturing method which reduces shrinkage force in the manufacturing method of the polarizing film which performs dry drawing.

This invention provides the manufacturing method of the polarizing film shown below.
[1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, the stretching step of uniaxially stretching the polyvinyl alcohol-based resin film in a dry manner;
A dyeing step of dyeing the polyvinyl alcohol-based resin film by contacting a dye solution containing a dichroic dye; and
A cross-linking step in which a cross-linking liquid containing boric acid is brought into contact with the polyvinyl alcohol-based resin film for cross-linking and uniaxially stretched at a stretch ratio of a;
A cleaning step of contacting the polyvinyl alcohol resin film with a cleaning solution and uniaxially stretching at a stretching ratio of b times, and in this order,
The draw ratio a and the draw ratio b are the manufacturing methods of a polarizing film which satisfy | fill the relationship of Formula (1)-Formula (3).

a ≧ 1.01 Formula (1),
b ≧ 1.01 Formula (2),
b ≧ 0.8a Formula (3).

  [2] The ratio of uniaxial stretching in the stretching step is 3.0 to 4.5 times, and the cumulative stretching ratio of uniaxial stretching up to the washing step is 4.0 to 6.0 times. [1] The manufacturing method of the polarizing film of description.

  [3] The method for producing a polarizing film according to [1] or [2], wherein the polarizing film has a boron content of 4.0 to 4.2% by weight.

  According to the production method of the present invention, a polarizing film having a low shrinkage force can be obtained.

It is a flowchart which shows one Embodiment of the manufacturing method of the polarizing film of this invention.

<Production method of polarizing film>
In the present invention, the polarizing film is one in which a dichroic dye (iodine or dichroic dye) is adsorbed and oriented on a uniaxially stretched polyvinyl alcohol resin film. As the polyvinyl alcohol resin constituting the polyvinyl alcohol resin film, a saponified polyvinyl acetate resin can be used. Examples of the polyvinyl acetate-based resin include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate and other monomers copolymerizable therewith. Examples of other monomers copolymerizable with vinyl acetate include unsaturated carboxylic acids, olefins, vinyl ethers, unsaturated sulfonic acids, and (meth) acrylamides having an ammonium group. “(Meth) acryl” represents at least one selected from the group consisting of acryl and methacryl. The same applies to other terms with “(meta)”.

  The degree of saponification of the polyvinyl alcohol-based resin can be in the range of 80.0 to 100.0 mol%, preferably in the range of 90.0 to 100.0 mol%, more preferably 98.0. It is the range of -100.0 mol%. When the degree of saponification is less than 80.0 mol%, the water resistance and heat-and-moisture resistance of the polarizing plate including the polarizing film to be obtained can be lowered.

The degree of saponification is the unit ratio (mol%) of the proportion of acetate groups (acetoxy groups: —OCOCH ₃ ) contained in polyvinyl acetate resin, which is a raw material for polyvinyl alcohol resins, changed to hydroxyl groups by the saponification step. The following formula:
Degree of saponification (mol%) = 100 × (number of hydroxyl groups) / (number of hydroxyl groups + number of acetate groups)
Defined by The saponification degree can be determined according to JIS K 6726 (1994). The higher the degree of saponification, the higher the proportion of hydroxyl groups, and thus the lower the proportion of acetate groups that inhibit crystallization.

  The average degree of polymerization of the polyvinyl alcohol-based resin is preferably 100 to 10,000, more preferably 1500 to 8000, and further preferably 2000 to 5000. The average degree of polymerization of the polyvinyl alcohol resin can also be determined according to JIS K 6726 (1994). When the average degree of polymerization is less than 100, when the obtained stretched film is used as a raw material for the polarizing film, it is difficult to obtain a polarizing film having preferable polarizing performance. Formation of an alcohol-based resin film can be difficult.

  The polyvinyl alcohol-based resin film is obtained by forming the above-described polyvinyl alcohol-based resin into a film. The film forming method is not particularly limited, and a known method such as a melt extrusion method or a solvent casting method can be employed. The thickness of the polyvinyl alcohol-based resin film is, for example, about 10 to 150 μm, preferably 100 μm or less, more preferably 65 μm or less, further preferably 50 μm or less, and particularly preferably 35 μm or less (for example, 30 μm or less, further 20 μm or less). is there.

  The polyvinyl alcohol-based resin film can contain additives such as a plasticizer. Preferred examples of the plasticizer are polyhydric alcohols, and specific examples thereof include ethylene glycol, glycerin, propylene glycol, diethylene glycol, diglycerin, triethylene glycol, triglycerin, tetraethylene glycol, trimethylolpropane, polyethylene glycol and the like. . The PVA film can contain one or more plasticizers. Content of a plasticizer is 5-20 weight part normally with respect to 100 weight part of polyvinyl alcohol-type resin which comprises a PVA film, Preferably it is 7-15 weight part. The width | variety in particular of a polyvinyl alcohol-type resin film is not restrict | limited, For example, it can be about 400-6000 mm.

  FIG. 1 is a flowchart showing an embodiment of a method for producing a polarizing film of the present invention. In the embodiment shown in FIG. 1, while unwinding a long polyvinyl alcohol resin film from a raw roll, it is continuously transported along a film transport path of a polarizing film manufacturing apparatus and stretched uniaxially in a dry process. Step (S10), dyeing step (S20) for dyeing by contacting a dye solution containing a dichroic dye, and crosslinking by bringing a cross-linking solution containing boric acid into contact with each other and uniaxially drawing at a draw ratio of a The cleaning step (S30) and the cleaning step (S40) in which the cleaning liquid is brought into contact with the cleaning liquid and uniaxially stretched at a stretch ratio of b are included in this order.

  In the above, the draw ratio “a” in the crosslinking step (S30) and the draw ratio “b” in the washing step (S40) satisfy the relationships of the expressions (1) to (3).

a ≧ 1.01 Formula (1),
b ≧ 1.01 Formula (2),
b ≧ 0.8a Formula (3).

  When the draw ratio “a” and the draw ratio “b” satisfy the relationships of the above formulas (1) to (3), a polarizing film with reduced shrinkage can be produced. The draw ratio a in the crosslinking step (S30) is preferably 2.0 times or less, more preferably 1.3 times or less. The draw ratio b in the washing step (S40) is preferably 1.9 times or less, more preferably 1.2 times or less.

  In the manufacturing method of the polarizing film of this embodiment, you may further have a swelling process which makes a swelling liquid contact and performs a swelling process before a dyeing process (S20). Moreover, you may further have a drying process after a washing | cleaning process (S40). In the drying step, the moisture content is reduced to, for example, less than 15% by weight. In the swelling step, the dyeing step (S20), the crosslinking step (S30), and the washing step (S40), the step of bringing the swelling solution, the dyeing solution, the crosslinking solution, and the washing solution into contact with each other, for example, is filled with these treatment solutions. It can be set as the process which immerses a film in a bath. In this case, the bath used for immersion in each step may be one or plural.

  As described above, in the present invention, the stretching process is performed at least in the stretching step (S10), the crosslinking step (S30), and the washing step (S40). The uniaxial stretching performed in the stretching step (S10) is dry stretching, and the uniaxial stretching performed in the crosslinking step (S30) and the washing step (S40) is wet stretching. In addition, if necessary, a stretching process other than the above may be performed anywhere on the transport path. Hereinafter, each step will be described in detail.

(Stretching process)
The stretching process in the present embodiment is a stretching process in which a polyvinyl alcohol-based resin film is uniaxially stretched in a dry manner. In this embodiment, the polyvinyl alcohol-type resin film used for an extending process is prepared as a roll (raw fabric roll) of a long unstretched polyvinyl alcohol-type resin film. Dry stretching refers to stretching performed in the air, and uniaxial stretching is usually longitudinal uniaxial stretching. As dry stretching, a film was passed between a hot roll having a heated surface and a guide roll (or a hot roll) having a different peripheral speed from the hot roll, and a hot roll was used. Hot roll stretching that performs longitudinal stretching under heating; roll that performs longitudinal stretching due to the difference in peripheral speed between these two nip rolls while passing heating means (such as an oven) between two nip rolls installed at a distance. Interstretching; Tenter stretching; Compression stretching and the like. The stretching temperature (the surface temperature of the hot roll, the temperature in the oven, etc.) is, for example, 80 to 150 ° C., preferably 100 to 135 ° C.

  The magnification of the uniaxial stretching of the polyvinyl alcohol-based resin film in the stretching step is preferably 3.0 to 4.5 times, more preferably 3.5 to 4 from the viewpoint of optical properties (particularly polarization properties) of the polarizing film. .2 times, more preferably 3.6 to 4.1 times.

  The stretched film after the stretching step that is continuously obtained may be wound up at one end to form a film roll, or may be continuously supplied to the next step without being wound up.

(Swelling process)
The swelling step is performed for the purpose of removing foreign matter on the surface of the stretched film, removing the plasticizer in the stretched film, imparting easy dyeability, and plasticizing the stretched film. The processing conditions are determined within a range in which the object can be achieved and within a range in which problems such as extreme dissolution and devitrification of the stretched film do not occur.

  The swelling step can be performed by immersing the stretched film in a swelling bath for a predetermined time and then pulling it out. As the swelling liquid of the swelling bath, in addition to pure water, boric acid (JP-A-10-153709), chloride (JP-A-06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohols It is also possible to use an aqueous solution to which, for example, about 0.01 to 10% by weight is added.

  The temperature of the swelling bath is, for example, about 20 to 70 ° C., preferably about 30 to 60 ° C. The immersion time of the stretched film is preferably about 30 to 300 seconds, more preferably about 45 to 240 seconds.

  In the swelling treatment, a problem that the stretched film swells in the width direction and the film is wrinkled easily occurs. One means for conveying the film while removing the wrinkles is to use a roll having a widening function such as an expander roll, a spiral roll, or a crown roll as a guide roll, or another such as a cross guider, a bend bar, or a tenter clip. Or using a widening apparatus. Another means for suppressing the generation of wrinkles is to perform stretching. For example, the uniaxial stretching process can be performed in the swelling bath using the difference in the peripheral speed of the nip rolls disposed before and after the swelling treatment bath.

  In the swelling treatment, the film swells and expands in the film conveyance direction. Therefore, when the film is not actively stretched, for example, a nip roll disposed before and after the swelling bath in order to eliminate sagging of the film in the conveyance direction. It is preferable to take measures such as controlling the speed of Also, for the purpose of stabilizing the film transport in the swelling bath, the water flow in the swelling bath is controlled with an underwater shower, or the EPC device (Edge Position Control device: detects the edge of the film to prevent the film from meandering. It is also useful to use a combination of such devices.

(Dyeing process)
The dyeing step is performed for the purpose of adsorbing and orienting the dichroic dye on the polyvinyl alcohol-based resin film after the swelling step. The dyeing process conditions are determined within a range in which the object can be achieved and in a range in which defects such as extreme dissolution and devitrification of the film do not occur. The film after the swelling treatment can be carried out by immersing the film in a dyeing bath (treatment liquid contained in a dyeing tank) for a predetermined time and then pulling it out.

  When iodine is used as the dichroic dye, the dyeing solution in the dyeing bath contains, for example, a concentration of iodine / potassium iodide / water by weight ratio of about 0.003 to 0.3 / about 0.1 to 10/100. An aqueous solution can be used. Instead of potassium iodide, other iodides such as zinc iodide may be used, or potassium iodide and other iodides may be used in combination. In addition, compounds other than iodide, for example, boric acid, zinc chloride, cobalt chloride and the like may coexist. When boric acid is added, it is distinguished from the crosslinking treatment described later in that it contains iodine. If the aqueous solution contains about 0.003 parts by weight or more of iodine with respect to 100 parts by weight of water, Can be considered. The temperature of the dyeing bath when dipping the film is usually about 10 to 45 ° C., preferably 10 to 40 ° C., more preferably 20 to 35 ° C., and the dipping time of the film is usually about 20 to 600 seconds. , Preferably 30 to 300 seconds.

  When a water-soluble dichroic dye is used as the dichroic dye, for example, an aqueous solution having a concentration of dichroic dye / water by weight ratio of about 0.001 to 0.1 / 100 is used as the dyeing solution of the dyeing bath. Can be used. This dyeing bath 15 may contain a dyeing assistant or the like, and may contain, for example, an inorganic salt such as sodium sulfate or a surfactant. Only one dichroic dye may be used alone, or two or more dichroic dyes may be used in combination. The temperature of the dyeing bath 15 when dipping the film is, for example, about 20 to 80 ° C., preferably 30 to 70 ° C., and the dipping time of the film is usually about 20 to 600 seconds, preferably about 30 to 300 seconds. is there.

  The film may be uniaxially stretched in a dyeing bath. When uniaxial stretching is performed, it can be carried out by a method such as making a peripheral speed difference between nip rolls arranged before and after the dyeing bath.

  In the dyeing process, as in the swelling process, in order to convey the polyvinyl alcohol resin film while removing the wrinkles of the film, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used as a guide roll. Other widening devices such as cross guiders, bend bars, tenter clips can be used. Another means for suppressing the generation of wrinkles is to perform a stretching treatment as in the swelling step.

(Crosslinking process)
The crosslinking step is a treatment performed for the purpose of water resistance and hue adjustment (such as preventing the film from being bluish) by crosslinking. The cross-linking step can be performed by immersing the film after the dyeing step in a cross-linking bath (cross-linking solution accommodated in a cross-linking tank) for a predetermined time and then pulling it out.

  The crosslinking solution for the crosslinking bath can be an aqueous solution containing, for example, about 1 to 10 parts by weight of boric acid with respect to 100 parts by weight of water. When the dichroic dye used in the dyeing treatment is iodine, the crosslinking liquid preferably contains iodide in addition to boric acid. The amount is, for example, 1 to 30 parts by weight with respect to 100 parts by weight of water. It can be. Examples of iodide include potassium iodide and zinc iodide. In addition, compounds other than iodide, for example, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate and the like may coexist.

  In the crosslinking step, the concentration of boric acid and iodide and the temperature of the crosslinking bath can be appropriately changed depending on the purpose. For example, when the purpose of the crosslinking step is water resistance by crosslinking, and the polyvinyl alcohol resin film is subjected to a swelling step, a dyeing step, and a crosslinking step in this order, the crosslinking solution in the crosslinking bath has a boric acid concentration by weight ratio. / Iodide / water = 3-10 / 1-20 / 100 aqueous solution. As needed, it may replace with boric acid and may use other crosslinking agents, such as a glyoxal or glutaraldehyde, and may use boric acid and another crosslinking agent together. The temperature of the crosslinking bath when dipping the film is usually about 50 to 85 ° C., preferably 55 to 80 ° C., and the dipping time of the film is usually about 10 to 600 seconds, preferably 20 to 300 seconds, more preferably. Is 40 to 200 seconds.

  In the crosslinking treatment for the purpose of adjusting the hue, for example, when iodine is used as the dichroic dye, the concentration of boric acid / iodide / water is 1 to 5/3 to 30/100 in terms of weight ratio. Liquid can be used. The temperature of the crosslinking bath when dipping the film is usually about 10 to 45 ° C., and the dipping time of the film is usually about 1 to 300 seconds, preferably 2 to 100 seconds. The boron content in the polarizing film can be adjusted by adjusting the composition of the crosslinking liquid, the temperature of the crosslinking bath, the immersion time, and the like.

  The crosslinking treatment may be performed a plurality of times, and is usually performed 2 to 5 times. In this case, the composition and temperature of each crosslinking bath used may be the same or different as long as they are within the above range. The cross-linking treatment for water resistance by cross-linking and the cross-linking treatment for hue adjustment may be performed in a plurality of steps, respectively. In the crosslinking step, wet stretching is performed to uniaxially stretch the film. In the wet stretching, in at least one crosslinking bath, a uniaxial stretching treatment can be performed in the crosslinking bath using a difference in peripheral speed between nip rolls disposed before and after the stretching bath.

  In the crosslinking step, in order to convey the polyvinyl alcohol-based resin film while removing the wrinkles of the film as in the swelling step, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used as a guide roll. Other widening devices such as cross guiders, bend bars, tenter clips can be used.

(Washing process)
In the manufacturing method of this embodiment, a washing process is performed after a bridge | crosslinking process. The washing process is performed for the purpose of removing excess chemicals such as boric acid and iodine attached to the polyvinyl alcohol-based resin film. The washing step can be performed, for example, by immersing the polyvinyl alcohol-based resin film after the crosslinking step in a washing bath, spraying a washing solution as a shower on the film, or using these in combination.

  When the polyvinyl alcohol resin film is immersed in a cleaning bath for cleaning treatment, the temperature of the cleaning bath is usually about 2 to 40 ° C., and the immersion time of the film is usually about 2 to 120 seconds. In the washing step, wet stretching is performed to uniaxially stretch the film. In the uniaxial stretching, the uniaxial stretching treatment can be performed in the cleaning bath using the difference in the peripheral speed of the nip rolls disposed before and after the cleaning bath.

  In the cleaning process, a roll having a widening function such as an expander roll, a spiral roll, or a crown roll is used as a guide roll, a cross guider, a bend bar, Other widening devices such as tenter clips can be used.

(Drying process)
After the cleaning process, a drying process is performed in which the polyvinyl alcohol-based resin film is subjected to a drying process. A drying process is a process of reducing the moisture content of a polyvinyl alcohol-type resin film, for example, is a process of reducing to a moisture content of less than 15 weight%. Examples of the drying method include a hot air dryer, an infrared heater, or a method of drying using a combination of these.

  In the drying step, the maximum temperature reached by the polyvinyl alcohol-based resin film is, for example, 30 to 100 ° C, preferably 50 to 90 ° C. The time for the drying step is, for example, 30 to 600 seconds, preferably 60 to 300 seconds.

(Other processing steps for polyvinyl alcohol resin film)
Processing other than the processing described above can also be added. Examples of treatments that can be added include immersion treatment (complementary color treatment) in an aqueous iodide solution that does not contain boric acid, and immersion treatment in an aqueous solution that does not contain boric acid and contains zinc chloride, etc. Zinc treatment).

<Polarizing film>
The thickness of the polarizing film obtained as described above is, for example, about 5 to 30 μm. And the shrinkage stress measured with the method of the below-mentioned is 114 N / mm < ² > or less, for example. When the shrinkage stress is 114 N / mm ² or less, a polarizing film having excellent durability can be obtained. In addition, although the polarizing film whose shrinkage stress is 114 N / mm ² or less can be realized by, for example, lowering the cumulative draw ratio of the polarizing film or lowering the boron content of the polarizing film, According to the invention, the cumulative draw ratio of the polarizing film is lowered by producing the draw ratio a in the crosslinking process and the draw ratio b in the washing process so as to satisfy the relationships of the above formulas (1) to (3). In addition, a polarizing film having a shrinkage stress of 114 N / mm ² or less can be produced without reducing the boron content of the polarizing film. In addition, when the cumulative draw ratio of the polarizing film is lowered or the boron content of the polarizing film is lowered, the optical properties of the polarizing film may be lowered.

  The cumulative stretching ratio of uniaxial stretching up to the washing step is preferably 4.0 to 6.0 times, more preferably 4.2 to 5.0 times, and still more preferably 4.3 to 0.3 times based on the unstretched film. 4.8 times. Except for the case where uniaxial stretching is performed after the washing process, the cumulative stretching ratio of the uniaxial stretching up to the cleaning process matches the final cumulative stretching ratio of the polarizing film. The boron content of the polarizing film is preferably 2.0 to 5.0% by weight, more preferably 3.0 to 4.8% by weight, and still more preferably 4.0 to 4.2% by weight.

<Polarizing plate>
A polarizing plate can be obtained by bonding a protective film via an adhesive on at least one surface of the polarizing film produced as described above. 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 and polybutylene terephthalate; a polycarbonate resin film, a cyclo Examples include olefin resin films; acrylic resin films; and films made of chain olefin resins such as polypropylene resins.

  Surfaces such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating treatment, saponification treatment, etc. on the polarizing film and / or protective film bonding surface in order to improve the adhesion between the polarizing film and the protective film Processing may be performed. As an adhesive used for laminating a polarizing film and a protective film, an active energy ray curable adhesive such as an ultraviolet curable adhesive, an aqueous solution of a polyvinyl alcohol resin, or an aqueous solution in which a crosslinking agent is blended. And water-based adhesives such as urethane emulsion adhesives. The ultraviolet curable adhesive may be a mixture of an acrylic compound and a photo radical polymerization initiator, a mixture of an epoxy compound and a photo cationic polymerization initiator, or the like. Alternatively, a cationic polymerizable epoxy compound and a radical polymerizable acrylic compound may be used in combination, and a photo cationic polymerization initiator and a photo radical polymerization initiator may be used in combination as an initiator.

  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 following examples, the boron content, film thickness, and shrinkage stress were measured by the following methods.

[Boron content of polarizing film]
After adding 0.2 g of a polarizing film to 170 ml of pure water and completely dissolving it at 95 ° C., 30 g of an aqueous mannitol solution (12.5% by weight) was added to obtain a sample solution for measurement. A sodium hydroxide aqueous solution (1 mol / l) is dropped until the sample solution for measurement reaches the neutralization point, and the boron content (% by weight) in the polyvinyl alcohol-based resin film is calculated from the amount added by the following formula:
Boron content (% by weight) = 1.08 × amount of sodium hydroxide aqueous solution dropped (ml) / weight of polarizing film (g)
Calculated from

[Measurement of film thickness]
The thickness of the polarizing film was measured with a contact-type film thickness meter (trade name “DIGIMICRO MH-15M” manufactured by Nikon Corporation). In addition, the film thickness of the polarizing film was measured at 15 points at different positions in the width direction, and the average value was calculated.

[Shrinkage stress of polarizing film]
An evaluation sample having a width of 2 mm and a length of 50 mm was cut out with a super cutter (manufactured by Sugano Seiki Co., Ltd.) so that the absorption axis direction (stretching direction) of the polarizing film was the major axis. The contraction force of the obtained strip-shaped evaluation sample was measured using a thermomechanical analyzer (model TMA / 6100, manufactured by SII Nano Technology Co., Ltd.). This measurement was performed in a constant dimension mode (the distance between chucks was set to 10 mm), and the specimen was left in the room at 20 ° C. for a sufficient period of time, and then the temperature setting in the sample room was changed from 20 ° C. to 80 ° C. in 1 minute. The temperature was raised, and the temperature inside the sample chamber was set to be maintained at 80 ° C. after the temperature was raised. After allowing the temperature to rise for another 4 hours, the contraction force in the long side direction of the test piece was measured in an environment at 80 ° C. In this measurement, the static load was 0 mN, and a SUS probe was used as the jig. The value obtained by dividing the measured shrinkage force by the fragment area of the evaluation sample was taken as the shrinkage stress.

<Example 1>
The polarizing film was manufactured with the manufacturing method of the polarizing film shown by FIG.

(1) Stretching process A surface temperature of a 30 μm-thick polyvinyl alcohol resin film (trade name “Kuraray Poval Film VF-PE # 3000” manufactured by Kuraray Co., Ltd., polymerization degree 2400, saponification degree 99.9 mol% or more) Was uniaxially stretched in the machine direction in a dry manner using a 120 ° C. hot roll. Table 1 shows the draw ratio in the drawing step.

(2) Swelling Step The obtained stretched polyvinyl alcohol-based resin film was immersed in a swelling bath containing pure water at 40 ° C. for 60 seconds while maintaining a tension state so that the film does not loosen.

(3) Dyeing process The polyvinyl alcohol-type resin film which passed through the swelling process was immersed in the dyeing bath of 28 degreeC whose iodine / potassium iodide / water (weight ratio) is 0.1 / 6/100 for 60 seconds. Table 1 shows the draw ratio of the swelling process and the dyeing process. In the swelling process and the dyeing process, no positive stretching or shrinking treatment was performed, but as a result, the polyvinyl alcohol resin film contracted due to the influence of immersion in the swelling bath and the dyeing bath.

(4) Crosslinking process The polyvinyl alcohol-type resin film which passed through the dyeing | staining process was immersed for 130 second in the 68 degreeC crosslinking bath whose potassium iodide / boric acid / water (weight ratio) is 15 / 5.5 / 100. Along with the immersion in the crosslinking bath, the film was uniaxially stretched in the machine direction by utilizing the peripheral speed difference of the nip rolls arranged before and after the crosslinking bath. Table 1 shows the draw ratio a in the crosslinking step.

(5) Washing process The polyvinyl alcohol-based resin film that had undergone the crosslinking process was immersed in a washing bath containing pure water at 20 ° C. Along with the immersion in the cleaning bath, the film was uniaxially stretched in the longitudinal direction using the difference in the peripheral speed of the nip rolls arranged before and after the cleaning bath. Table 1 shows the draw ratio b in the washing bath.

(6) Drying process The polyvinyl alcohol-type resin film which passed through the washing | cleaning process was dried for 90 second in 60 degreeC atmosphere, and the polarizing film was produced.

<Examples 2-8, Comparative Examples 1-4>
About the polarizing film of Examples 2-8 and Comparative Examples 1-4, it is the same as the method of Example 1 except the point which adjusted the draw ratio a in a bridge | crosslinking process, and the draw ratio b in a washing | cleaning process as shown in Table 1. Produced by the method. In addition, the draw ratio which combined the swelling process and the dyeing | staining process had some fluctuation | variations with atmospheric temperature, humidity, etc. at the time of manufacture.

<Evaluation>
About the polarizing film of obtained Examples 1-8 and Comparative Examples 1-4, the boron content rate and the film thickness were measured. The results are shown in Table 1. In addition, the shrinkage stress calculated based on the measurement values of the shrinkage force and the film thickness, the stretch ratio (stretch ratio b / stretch ratio a), the cumulative stretch ratio up to the washing process, and each process (stretching process, swelling process, and Table 1 shows the calculation results of the stretching ratio of uniaxial stretching in the dyeing step, the crosslinking step, and the washing step.

  As shown in Table 1, the polarizing films of Examples 4 and 6 and Comparative Examples 1 and 2 having the same cumulative draw ratio of 4.55 times, Example 5 and Comparative Example having the same cumulative draw ratio of 4.51 times When the polarizing film of Example 4 and the polarizing films of Examples 7 and 8 and Comparative Example 3 having the same cumulative stretch ratio of 4.53 times were compared, the stretch ratio (stretch ratio b / stretch ratio a) was 0.8. The comparative example in which the polarizing film of the example which satisfies the above (satisfying the above formula (3)) has a stretch ratio (stretch ratio b / stretch ratio a) of less than 0.8 (does not satisfy the above formula (3)). The shrinkage stress was lower than that of the polarizing film of the example.

Claims (3)

A method for producing a polarizing film from a polyvinyl alcohol-based resin film,
A stretching step of uniaxially stretching the polyvinyl alcohol-based resin film in a dry manner;
A dyeing step of dyeing the polyvinyl alcohol-based resin film by contacting a dye solution containing a dichroic dye; and
A cross-linking step in which a cross-linking liquid containing boric acid is brought into contact with the polyvinyl alcohol-based resin film for cross-linking and uniaxially stretched at a stretch ratio of a;
A cleaning step of contacting the polyvinyl alcohol resin film with a cleaning solution and uniaxially stretching at a stretching ratio of b times, and in this order,
The draw ratio a and the draw ratio b are the manufacturing methods of a polarizing film which satisfy | fill the relationship of Formula (1)-Formula (3).
a ≧ 1.01 Formula (1)
b ≧ 1.01 Formula (2)
b ≧ 0.8a Formula (3)   2. The uniaxial stretching ratio in the stretching step is 3.0 to 4.5 times, and the cumulative stretching ratio of uniaxial stretching up to the washing step is 4.0 to 6.0 times. A method for producing a polarizing film.   The manufacturing method of the polarizing film of Claim 1 or 2 whose boron content rate of the said polarizing film is 4.0 to 4.2 weight%.

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