JP2013037222A - Polarizer and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarizer which exhibits both high transmission and high polarization, i.e., high white luminance and high display contrast, and also provide a method of manufacturing the same.SOLUTION: The method of manufacturing the polarizer at least comprises a swelling process, a dyeing process, a crosslinking process, and an extending process which are applied to a polyvinyl alcohol film. In the swelling process, the polyvinyl alcohol film is extended by a factor of 1.4 to 2.4, thereby controlling the degree of crystallinity of the polyvinyl alcohol film to be 25 to 32%.

Description

  The present invention relates to a polarizer and a method for manufacturing the same. The present invention also relates to a polarizing plate using the polarizer. The polarizer and the polarizing plate can be used alone or as an optical film laminated thereon to form an image display device such as a flat panel display such as a liquid crystal display device or an organic EL display device.

  Conventionally, as a polarizer used for a liquid crystal display device or the like, an absorbing dichroic polarizer formed by uniaxially stretching a polyvinyl alcohol film dyed with iodine or a dichroic dye has been widely used. . Further, the polarizer is used as a polarizing plate having a strength enhanced by bonding a saponified transparent protective film such as triacetyl cellulose to both sides or one side of the polarizer.

  In particular, in recent years, from the viewpoint of reducing the power consumption of a liquid crystal display device, it is desired to improve the white luminance of the liquid crystal display device, and accordingly, development of a polarizer having a high transmittance is desired. However, simply increasing the single transmittance causes a problem in that the degree of polarization decreases, and consequently the display contrast decreases. On the other hand, when the degree of polarization is to be increased, the single transmittance is lowered, and as a result, white brightness is lowered. Thus, the single transmittance of the polarizer and the degree of polarization are in a trade-off relationship. Therefore, development of a polarizer that achieves both high transmittance and high degree of polarization is required.

  For example, Patent Document 1 discloses a polarizing plate having a transmittance of 41.1 to 44.3 and a degree of polarization of 99% or more.

  However, the polarizing plate is not satisfactory with respect to transmittance and degree of polarization.

JP 2009-92847 A

  An object of the present invention is to provide a polarizer that achieves both high transmittance and high degree of polarization, that is, high white luminance and high display contrast, and a method for manufacturing the same.

  As a result of intensive studies to solve the above problems, the present inventors have found that the object can be achieved by the following method for producing a polarizer, and have completed the present invention.

That is, the present invention provides a method for producing a polarizer in which a polyvinyl alcohol film is subjected to at least a swelling process, a dyeing process, a crosslinking process, and a stretching process.
In the swelling step, the polyvinyl alcohol film is stretched by 1.4 to 2.4 times to control the crystallinity of the polyvinyl alcohol film to 25 to 32%. About.

  In the raw film formed of polyvinyl alcohol (hereinafter referred to as “PVA”) before stretching, crystal parts (lamellar layers) of PVA are present randomly. When such a raw film is stretched, the crystal part collapses and extends. That is, it is considered that the crystallinity of the raw film decreases with stretching.

  After the swelling process, iodine is adsorbed on the PVA in the dyeing process, and an iodine-PVA complex is formed to develop dichroism. Iodine can be adsorbed only on the amorphous part of PVA and not adsorbed on the crystal part. If the crystallinity of the PVA film is too high when the polyvinyl alcohol film (hereinafter referred to as “PVA film”) is stained with iodine, the adsorption site of iodine is insufficient, and the iodine-PVA complex is hardly formed. On the other hand, if the degree of crystallinity of the PVA film is too low, the stress applied during stretching decreases, the orientation of PVA does not increase, and the iodine-PVA complex becomes difficult to form.

  The inventors pay attention to the degree of crystallinity of the PVA film, and in the swelling step, which is the previous step of the dyeing step, the PVA film is stretched by 1.4 to 2.4 times, and the degree of crystallinity of the PVA film is 25. It was found that a polarizer having both high transmittance and high degree of polarization can be obtained by controlling to ˜32% and then performing a dyeing step.

  The polarizer obtained by the manufacturing method has a boron content of 4 to 5.5% by weight, a single transmittance of 43.0% or more, and a degree of polarization of 99.99% or more. It is a polarizer with a high degree of polarization. When the boron content is less than 4% by weight, the optical properties tend to deteriorate. When the boron content exceeds 5.5% by weight, the stretching breakage tends to occur in the stretching step.

  The present invention also relates to a polarizing plate in which a transparent protective film is laminated on at least one surface of the polarizer.

  The present invention also relates to an optical film in which at least one polarizer or the polarizing plate is laminated.

  Furthermore, this invention relates to the image display apparatus containing the said optical film.

  The polarizer of the present invention has both a high transmittance and a high degree of polarization, and by using the polarizer, it is possible to simultaneously improve white luminance and display contrast of a liquid crystal display device.

  As a PVA film (raw film) that is a raw material of a polarizer, a film having a crystallinity of 35 to 50% is usually used. If the degree of crystallinity is too high, the stretchability is lowered and the adsorbing sites such as iodine are insufficient, so that the polarization characteristics are lowered. On the other hand, if the degree of crystallinity is too low, no tension is applied to the film during stretching, and PVA is difficult to orient, resulting in a decrease in polarization characteristics. For example, when a PVA film having a crystallinity of 30% is used and the PVA film is not stretched in the swelling process, the crystallinity of the PVA film after the swelling process falls within the range of 25 to 32%. Characteristics cannot be obtained. That is, the initial crystallinity of the PVA film is lowered by stretching in the swelling process, and the polarization characteristics of the present invention can be obtained by controlling the crystallinity to 25 to 32% by stretching.

  You may add additives, such as a plasticizer and surfactant, in a PVA film. Examples of the plasticizer include polyols and condensates thereof, and examples thereof include glycerin, diglycerin, triglycerin, ethylene glycol, propylene glycol, and polyethylene glycol. The amount of the plasticizer used is not particularly limited, but is preferably 20% by weight or less in the PVA film.

  The polarizer of the present invention is produced by subjecting the PVA film to at least a swelling process, a dyeing process, a crosslinking process, and a stretching process.

  The swelling process is performed before the dyeing process. In addition to washing the surface of the PVA film and the anti-blocking agent by the swelling step, there is also an effect of preventing unevenness such as uneven dyeing by swelling the PVA film.

  In the swelling step, water, distilled water, or pure water is usually used as the treatment liquid. If the main component is water, the treatment solution may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol, or the like. Moreover, when making the said processing liquid contain the iodide compound, the density | concentration of the iodide compound is about 0.1 to 10 weight%, Preferably it is 0.2 to 5 weight%.

  The treatment temperature in the swelling step is usually preferably adjusted to about 20 to 45 ° C, more preferably 25 to 40 ° C. In addition, if there is swelling unevenness, the portion becomes uneven coloring in the dyeing process, so that swelling unevenness is not generated. The immersion time is usually about 10 to 300 seconds, preferably 20 to 240 seconds.

  In the method for producing a polarizer of the present invention, the crystallinity of the PVA film is controlled to 25 to 32% by stretching the PVA film by 1.4 to 2.4 times the original length in the swelling step. It is necessary. The draw ratio is preferably 1.4 to 2.2 times. Thereby, the crystallinity of the PVA film can be controlled to 27 to 32%. The draw ratio is particularly preferably 1.6 to 2.0 times. Thereby, the crystallinity of the PVA film can be controlled to 28 to 31%.

  The dyeing step is performed by adsorbing and orienting iodine or a dichroic dye on the swollen PVA film. In the method for producing a polarizer according to the present invention, the dyeing step has a crystallinity of 25 PVA film in order to dye in a state where there are many adsorption sites such as iodine and the orientation of the amorphous part of PVA is high. It is performed so as to pass through a state of ˜32%.

  Dyeing is usually performed by immersing the film in a dyeing solution. As the staining solution, an iodine solution is generally used. As the iodine aqueous solution used as the iodine solution, an aqueous solution containing iodine ions with, for example, potassium iodide or the like as iodine and a dissolution aid is used. Other aids such as iodides such as lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide and titanium iodide. Can be used. The iodine concentration is about 0.01 to 0.5% by weight, preferably 0.02 to 0.4% by weight, and the potassium iodide concentration is about 0.01 to 10% by weight, preferably 0.02 to 8% by weight. %. In iodine staining, the temperature of the iodine solution is usually about 20 to 50 ° C., preferably 25 to 40 ° C. The immersion time is usually about 10 to 300 seconds, preferably 20 to 240 seconds.

  In the crosslinking step, a boron compound is usually used as a crosslinking agent. You may perform a bridge | crosslinking process with an extending process. The crosslinking step can be performed multiple times. Examples of the boron compound include boric acid and borax. The boron compound is generally used in the form of an aqueous solution or a water-organic solvent mixed solution. Usually, an aqueous boric acid solution is used. In order to make the boron content in the polarizer 4 to 5.5% by weight, the boric acid concentration of the boric acid aqueous solution is preferably about 2 to 10% by weight, more preferably 3 to 8% by weight. is there. The boric acid aqueous solution or the like can contain an iodide compound such as potassium iodide. When the iodide compound is contained in the boric acid aqueous solution, the iodide compound concentration is about 0.1 to 10% by weight, preferably 0.2 to 5% by weight.

  The crosslinking step can be performed by immersing the dyed PVA film in a boric acid aqueous solution or the like. In addition, it can be performed by applying or spraying a boron compound or the like to the PVA film. The processing temperature in a bridge | crosslinking process is 25 degreeC or more normally, Preferably it is 30-85 degreeC, More preferably, it is 30-60 degreeC. The treatment time is usually 10 to 800 seconds, preferably 30 to 500 seconds.

  In the stretching process, a uniaxial stretching process is usually performed. The stretching method is not particularly limited, and either a wet stretching method or a dry stretching method can be employed. Examples of the stretching means of the dry stretching method include an inter-roll stretching method, a heated roll stretching method, and a compression stretching method. Stretching can also be performed in multiple stages. The stretch ratio of the stretched film can be appropriately set according to the purpose, but the total stretch ratio is about 2 to 7 times, preferably 3 to 6.8 times, more preferably 3.5 to 6.5 times.

  Thereafter, the PVA film may be subjected to a cleaning process. The precipitate generated on the surface of the stretched film can be removed by the washing step.

  The washing step can be performed by washing with water, distilled water, pure water or the like, for example. The water washing step is usually performed by immersing the PVA film in a water washing bath. Moreover, a washing | cleaning process can be performed by immersing in the aqueous solution containing iodides, such as potassium iodide. For example, the aqueous solution preferably has a potassium iodide concentration of about 0.5 to 10% by weight, more preferably 1 to 8% by weight. The temperature of the washing bath in the washing step is usually 5 to 50 ° C, preferably 10 to 45 ° C, more preferably 15 to 40 ° C. The immersion time is usually 1 to 300 seconds, preferably 10 to 240 seconds. The cleaning with the aqueous solution can be performed in combination with water cleaning, and can be performed before or after the water cleaning.

  Then, you may give a drying process to the said PVA film.

  The polarizer produced by the above method has a boron content of 4 to 5.5% by weight, a single transmittance of 43.0% or more, and a polarization degree of 99.99% or more.

  The obtained polarizer can be made into the polarizing plate which provided the transparent protective film in the at least single side | surface according to the conventional method. The transparent protective film can be provided as a coating layer made of a polymer or a laminate layer of the film. As the transparent polymer or film material for forming the transparent protective film, an appropriate transparent material can be used, but a material excellent in transparency, mechanical strength, thermal stability, moisture barrier property and the like is preferably used. Examples of the material for forming the transparent protective film include polyester polymers such as polyethylene terephthalate and polyethylene naphthalate, cellulose polymers such as cellulose diacetate and cellulose triacetate, acrylic polymers such as polymethyl methacrylate, polystyrene, acrylonitrile, Examples thereof include styrene polymers such as styrene copolymers (AS resins), polycarbonate polymers, and the like. In addition, polyethylene, polypropylene, polyolefins having a cyclo or norbornene structure, polyolefin polymers such as ethylene / propylene copolymers, vinyl chloride polymers, amide polymers such as nylon and aromatic polyamide, imide polymers, sulfone polymers , Polyether sulfone polymer, polyether ether ketone polymer, polyphenylene sulfide polymer, vinyl alcohol polymer, vinylidene chloride polymer, vinyl butyral polymer, arylate polymer, polyoxymethylene polymer, epoxy polymer, or the above Polymer blends and the like are also examples of polymers that form the transparent protective film. The transparent protective film can also be formed as a cured layer of thermosetting or ultraviolet curable resin such as acrylic, urethane, acrylurethane, epoxy, and silicone.

  Moreover, the polymer film described in JP-A-2001-343529 (WO01 / 37007), for example, (A) a thermoplastic resin having a substituted and / or unsubstituted imide group in the side chain, and (B) a substitution in the side chain And / or a resin composition containing a thermoplastic resin having unsubstituted phenyl and a nitrile group. A specific example is a film of a resin composition containing an alternating copolymer composed of isobutylene and N-methylmaleimide and an acrylonitrile / styrene copolymer. As the film, a film made of a mixed extruded product of the resin composition or the like can be used. Since these films have a small phase difference and a small photoelastic coefficient, problems such as unevenness due to the distortion of the polarizing plate can be eliminated, and since the moisture permeability is small, the humidification durability is excellent.

  Although the thickness of a transparent protective film can be determined suitably, generally it is about 1-500 micrometers from points, such as workability | operativity, such as intensity | strength and handleability, and thin layer property. 1-300 micrometers is especially preferable, and 5-200 micrometers is more preferable.

  Moreover, it is preferable that a transparent protective film has as little color as possible. Therefore, Rth = (nx−nz) · d (where nx is the refractive index in the slow axis direction in the film plane, nz is the refractive index in the film thickness direction, and d is the film thickness). A transparent protective film having a direction retardation value of −90 nm to +75 nm is preferably used. By using a film having a thickness direction retardation value (Rth) of −90 nm to +75 nm, the coloring (optical coloring) of the polarizing plate caused by the protective film can be almost eliminated. The thickness direction retardation value (Rth) is more preferably −80 nm to +60 nm, and particularly preferably −70 nm to +45 nm.

  As the protective film, a triacetyl cellulose film, a norbornene-based film, a cycloolefin-based film, and an acrylic resin film are preferable from the viewpoints of polarization characteristics and durability. A triacetyl cellulose film is particularly preferable. In addition, when providing a protective film in the both sides of a polarizer, the protective film which consists of the same polymer material may be used by the front and back, and the protective film which consists of a different polymer material etc. may be used.

  The surface of the transparent protective film to which the polarizer is not adhered may be subjected to a hard coat layer, an antireflection treatment, an antisticking treatment, or a treatment for diffusion or antiglare.

  The antireflection layer, the antisticking layer, the diffusion layer, the antiglare layer, and the like can be provided on the transparent protective film itself, or can be provided as a separate optical layer from the transparent protective film.

  An adhesive is used for the adhesion treatment between the polarizer and the transparent protective film. Examples of the adhesive include isocyanate adhesives, polyvinyl alcohol adhesives, gelatin adhesives, vinyl latexes, and water-based polyesters. As the adhesive, an adhesive made of an aqueous solution is usually used.

  The polarizing plate of the present invention is produced by bonding the transparent protective film and the polarizer using the adhesive. The adhesive may be applied to either the transparent protective film or the polarizer, or to both. After the bonding, a drying process is performed to form an adhesive layer composed of a coating dry layer. Bonding of a polarizer and a transparent protective film can be performed with a roll laminator or the like. The thickness of the adhesive layer is not particularly limited, but is usually about 0.1 to 5 μm.

  The polarizing plate of the present invention can be used as an optical film laminated with another optical layer in practical use. The optical layer is not particularly limited. For example, for forming a liquid crystal display device such as a reflection plate, a semi-transmission plate, a retardation plate (including wavelength plates such as 1/2 and 1/4), and a viewing angle compensation film. One or more optical layers that may be used can be used. In particular, a reflective polarizing plate or a semi-transmissive polarizing plate in which a polarizing plate or a semi-transmissive reflecting plate is further laminated on the polarizing plate of the present invention, an elliptical polarizing plate or a circularly polarizing plate in which a retardation plate is further laminated on the polarizing plate. A wide viewing angle polarizing plate obtained by further laminating a viewing angle compensation film on a plate or a polarizing plate, or a polarizing plate obtained by further laminating a brightness enhancement film on the polarizing plate is preferable.

  The polarizing plate or the optical film of the present invention can be preferably used for forming various devices such as a liquid crystal display device. The liquid crystal display device can be formed according to the conventional method. That is, a liquid crystal display device is generally formed by appropriately assembling components such as a liquid crystal cell, a polarizing plate or an optical film, and an illumination system as necessary, and incorporating a drive circuit. There is no limitation in particular except the point which uses the polarizing plate or optical film by invention, and it can apply according to the former. As the liquid crystal cell, any type such as a TN type, an STN type, or a π type can be used.

  Examples and the like specifically showing the configuration and effects of the present invention will be described below. In each example, parts and% are based on weight unless otherwise specified.

(Measurement of crystallinity of PVA)
A small-angle X-ray scattering is performed by setting a PVA film (trade name: VF-PS # 7500, manufactured by Kuraray Co., Ltd.) having a width of 20 mm, a length of 30 mm, and a thickness of 75 μm in a stretching machine and stretching the PVA film in water at 25 ° C. The crystallinity was measured by the method described above (Spring-8 beam line, BL40B2; X-ray wavelength λ = 1Å). Table 1 shows the crystallinity at each draw ratio.

(Measurement of boron content in polarizer)
The produced polarizer was dried at 120 ° C., and the weight of the polarizer was measured. Next, mannitol and bromothymol blue as a titration indicator were added to a solution obtained by completely dissolving the polarizer in pure water, and an aqueous NaOH (0.1 mol / L) solution was dropped. When the color of the solution changed from orange to blue, dropping of the NaOH aqueous solution was stopped and the amount of dripping was measured. The boron content in the polarizer was calculated by the following formula.
Boron content (% by weight) = 0.1 × {Drop amount of NaOH aqueous solution (ml) / 1000} ×
10.81 × {1 / weight of the polarizer (g)} × 100

Example 1
(Production of polarizer)
While stretching a 75 μm thick PVA film (trade name: VF-PS # 7500, manufactured by Kuraray Co., Ltd.) in warm water (swelling bath) at 25 ° C., the draw ratio is 1.4 times the original length. It extended | stretched in the flow direction so that it might become. Thereafter, the PVA film was immersed in a 30 ° C. aqueous iodine solution (dyeing bath) containing an iodine concentration of 0.04% and a potassium iodide concentration of 0.4% for 60 seconds. Was stretched in the flow direction so as to be 3.3 times. Next, the film was immersed in an aqueous solution at 30 ° C. containing 4% by weight of boric acid and 3% by weight of potassium iodide for 30 seconds. Thereafter, the film was immersed in a 60 ° C. aqueous solution (stretching bath) containing 4% by weight boric acid and 5% by weight potassium iodide for 40 seconds while flowing so that the stretching ratio was 6 times the original length. Stretched in the direction. Thereafter, the film was immersed in a 30 ° C. aqueous solution containing 3% by weight of potassium iodide for 10 seconds for washing, and further dried at 50 ° C. for 4 minutes to obtain a polarizer.

Examples 2 to 4, Comparative Examples 1 and 2
A polarizer was produced in the same manner as in Example 1 except that, in the swelling step of Example 1, the draw ratio was changed as shown in Table 1.

(Evaluation)
The polarizers obtained in the examples and comparative examples were evaluated for the following optical characteristics. The results are shown in Table 1.

<Optical characteristics>
The spectral transmittance of the polarizer at a wavelength of 380 to 780 nm was measured using a spectrophotometer with an integrating sphere (manufactured by JASCO Corporation, trade name: V7100). The transmittance for each linearly polarized light was measured with 100% of the completely polarized light obtained through the Grantera prism polarizer. The Y value in the C light source 2 ° field of view was calculated from the measured spectral transmittance according to the CIE1931 Yxy color system. These were designated as single transmittance (Ts (Y)), parallel transmittance (Tp (Y)), and orthogonal transmittance (Tc (Y)).

The degree of polarization (P) was calculated by {(parallel transmittance−orthogonal transmittance) / (parallel transmittance + orthogonal transmittance)} ^1/2 × 100 (%).

  The polarizer and polarizing plate of the present invention are used for the production of image display devices such as flat panel displays such as liquid crystal display devices and organic EL display devices.

Claims (5)

In the method for producing a polarizer, the polyvinyl alcohol film is subjected to at least a swelling process, a dyeing process, a crosslinking process, and a stretching process.
In the swelling step, the polyvinyl alcohol film is stretched by 1.4 to 2.4 times, thereby controlling the crystallinity of the polyvinyl alcohol film to 25 to 32%.   A polarizer obtained by the production method according to claim 1, having a boron content of 4 to 5.5% by weight, a single transmittance of 43.0% or more, and a polarization degree of 99.99% or more.   A polarizing plate in which a transparent protective film is laminated on at least one surface of the polarizer according to claim 2.   An optical film in which at least one polarizer according to claim 2 or at least one polarizing plate according to claim 3 is laminated.   An image display device comprising the optical film according to claim 4.