JPWO2019189718A1 - Polarizer and polarizing plate - Google Patents

Abstract

Provided is a polarizer that is thin and has excellent heat resistance. The polarizer of the present invention is composed of a polyvinyl alcohol-based resin film containing iodine, and has a free boric acid content of 0.4% by weight or less.

Description

The present invention relates to a polarizer and a polarizing plate.

A liquid crystal display device, which is a typical image display device, has polarizing plates (substantially, polarizing plates including the polarizing elements) arranged on both sides of the liquid crystal cell due to the image forming method. The polarizer is typically produced by dyeing a polyvinyl alcohol (PVA) -based resin film with a dichroic substance such as iodine (for example, Patent Documents 1 and 2). In recent years, there has been an increasing demand for thinner image display devices. Therefore, the polarizer is also required to be further thinned. However, the thinner the polarizer, the lower the heat resistance, and there is a problem that the optical characteristics are likely to change in a high temperature environment.

Japanese Patent No. 5048120 Japanese Unexamined Patent Publication No. 2013-156391

The present invention has been made to solve the above-mentioned conventional problems, and a main object thereof is to provide a polarizing element which is thin and has extremely excellent heat resistance.

The polarizer of the present invention is composed of a polyvinyl alcohol-based resin film containing iodine.
The free boric acid content is 0.4% by weight or less.
In one embodiment, the thickness of the polarizer is 7 μm or less.
In one embodiment, the polarizer has an iodine content of 10% to 25% by weight.
According to another aspect of the present invention, a polarizing plate is provided. The polarizing plate includes the above-mentioned polarizing element and a protective film laminated on one side or both sides of the polarizing element.

According to the present invention, by setting the content of free boric acid contained in the polarizer to 0.4% by weight or less, it is thin and has very excellent heat resistance, which has long been desired but could not be realized. It was possible to realize a polarizer having a property.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

A. Polarizer A-1. Outline of Polarizer The polarizer according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA) -based resin film containing iodine, and has a free boric acid content of 0.4% by weight or less.

The free boric acid content of the polarizer is preferably 0.39% by weight or less, and more preferably 0.38% by weight or less. The lower limit of the free boric acid content is, for example, 0.01% by weight. As described above, one of the features of the present invention is to pay attention to the content of free boric acid rather than the content of all boric acid contained in the polarizer. When the free boric acid content of the polarizer is within the above range, the heat resistance is excellent, and changes in optical characteristics (for example, single transmittance) in a high temperature environment can be suppressed. The free boric acid content of the polarizer can typically be determined by the following procedure using inductively coupled plasma emission spectroscopy (ICP-AES). That is, the measurement sample obtained by freeze-grinding the polarizer was mixed with a mixed solution of 2-ethyl-1,3-hexanediol / chloroform (volume ratio: 10/90), and this mixture was filtered. The boron content in the filtrate is quantified using an ICP emission spectrometer. The obtained boron content is all derived from boric acid, and all boric acid in the filtrate is considered to be derived from the free boric acid of the polarizer, and the boron content is converted into the free boric acid content of the polarizer. In one embodiment, the free boric acid content of the polarizer is dried at a temperature lower than the conventional heating temperature (preferably 50 ° C. or lower) in the drying step in the method for producing a polarizer as described later. Thereby, it can be adjusted within the above range.

The upper limit of the thickness of the polarizer is 7 μm in one embodiment, 3 μm in another embodiment, and 2 μm in yet another embodiment. The lower limit of the thickness is 0.5 μm in one embodiment, 0.6 μm in another embodiment, and 0.8 μm in yet another embodiment. According to the embodiment of the present invention, it is possible to realize a desired single transmittance as described later even with a polarizing element having a thin thickness.

The iodine content of the polarizer can be appropriately set so as to achieve both sufficient polarization performance and optimum single transmittance. The iodine content is preferably 10% to 25% by weight, more preferably 15% to 25% by weight. According to the embodiment of the present invention, in a polarizer having such an extremely high iodine content, it is possible to realize extremely excellent heat resistance, which has been difficult in the past. More specifically, in a polarizer having an extremely high iodine content, changes in optical properties in a high temperature environment can be significantly suppressed. As used herein, the term "iodine content" means the amount of all iodine contained in the polarizer (PVA-based resin film). More specifically, iodine during polarizers iodide ion (I ^-), polyiodine ion _{^{(I 3 -, I 5 -}} ) where present in the form of such as iodine content in the present specification, these It means the amount of iodine including all forms. The iodine content can be calculated, for example, by the calibration curve method of fluorescent X-ray analysis. The polyiodine ion exists in a state in which a PVA-iodine complex is formed in the polarizer. By forming such a complex, absorption dichroism can be exhibited in the wavelength range of visible light. Specifically, a complex of PVA and tri-iodide ion (PVA · _{I 3} ^-) has a light absorption peak around 470 nm, a complex of PVA and five iodide ion (PVA · _{I 5} ^-) is 600nm near Has an absorptive peak. As a result, polyiodine ions can absorb light in a wide range of visible light, depending on their morphology. On the other hand, iodine ion (I ⁻ ) has an absorption peak near 230 nm and is not substantially involved in the absorption of visible light. Therefore, polyiodine ions present in the form of a complex with PVA may be mainly involved in the absorption performance of the polarizer.

The simple substance transmittance (Ts) of the polarizer is preferably 30.0% to 43.0%, more preferably 35.0% to 41.0%. The degree of polarization of the polarizer is preferably 99.9% or more, more preferably 99.95% or more, and further preferably 99.98% or more. By setting the single transmittance to be low and increasing the degree of polarization, the contrast can be increased and the black display can be displayed in black, so that an image display device having excellent image quality can be realized. The single transmittance is a value measured by a spectrophotometer with an integrating sphere. The single transmittance is a Y value measured by a two-degree field of view (C light source) of JIS Z8701 and corrected for luminosity factor. For example, a spectrophotometer with an integrating sphere (manufactured by JASCO Corporation, product name: V7100). Can be measured using.

A-2. PVA-based resin film Examples of the PVA-based resin forming the PVA-based resin film include polyvinyl alcohol and an ethylene-vinyl alcohol copolymer. Polyvinyl alcohol is obtained by saponification of polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying the ethylene-vinyl acetate copolymer. The degree of saponification of the PVA-based resin is usually 85 mol% or more and less than 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. is there. The degree of saponification can be determined according to JIS K 6726-1994. By using a PVA-based resin having such a degree of saponification, a polarizer having excellent durability can be obtained. If the degree of saponification is too high, gelation may occur.

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the intended purpose. The average degree of polymerization is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average degree of polymerization can be determined according to JIS K 6726-1994.

The thickness of the PVA-based resin film is not particularly limited and can be set according to the desired thickness of the polarizer. The thickness of the PVA-based resin film is, for example, 10 μm to 200 μm.

In one embodiment, the PVA-based resin film may be a PVA-based resin layer formed on a substrate. The laminate of the base material and the PVA-based resin layer can be obtained, for example, by a method of applying the coating liquid containing the PVA-based resin to the base material, a method of laminating a PVA-based resin film on the base material, or the like.

B. Method for manufacturing a polarizer B-1. Outline of Method for Producing Polarizer The method for producing a polarizer according to the embodiment of the present invention includes at least stretching and dyeing a PVA-based resin film. Typically, the production method includes a step of preparing a PVA-based resin film, a stretching step, a dyeing step, a cross-linking step, a washing step, and a drying step. Further, if necessary, a swelling step may be provided before the stretching step. Each step in which the PVA-based resin film is provided can be performed in any suitable order and timing. Therefore, each step may be performed in the above order, or may be performed in a different order from the above. If necessary, one step may be performed a plurality of times. Further, a step other than the above (for example, an insolubilization step) may be performed at an arbitrary appropriate timing. When the PVA-based resin film is a PVA-based resin layer formed on a base material, a laminate of the base material and the PVA-based resin layer is subjected to the above step.

Hereinafter, each step will be described, but as described above, each step can be performed in any appropriate order, and is not limited to the description order.

B-2. Stretching Step In the stretching step, the PVA-based resin film is typically uniaxially stretched 3 to 7 times. The stretching direction may be the longitudinal direction of the film (MD direction) or the width direction of the film (TD direction). The stretching method may be dry stretching, wet stretching, or a combination of these. Further, the PVA-based resin film may be stretched when performing a crosslinking step, a swelling step, a dyeing step, or the like. The stretching direction can correspond to the absorption axis direction of the obtained polarizer.

B-3. Swelling step The swelling step is usually performed before the dyeing step. The swelling step is performed, for example, by immersing a PVA-based resin film in a swelling bath. As the swelling bath, water such as distilled water or pure water is usually used. The swelling bath may contain any suitable other ingredients other than water. Examples of other components include solvents such as alcohol, additives such as surfactants, and iodides. Examples of iodide include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. And so on. Preferably, potassium iodide is used. The temperature of the swelling bath is, for example, 20 ° C to 45 ° C. The immersion time is, for example, 10 seconds to 300 seconds.

B-4. Dyeing step The dyeing step is a step of dyeing a PVA-based resin film with a dichroic substance. It is preferably carried out by adsorbing a dichroic substance. Examples of the adsorption method include a method of immersing a PVA-based resin film in a dyeing solution containing a bicolor substance, a method of applying the dyeing solution to the PVA-based resin film, and a method of spraying the dyeing solution onto the PVA-based resin film. The method of doing this can be mentioned. A method of immersing the PVA-based resin film in the dyeing solution is preferable. This is because the dichroic substance can be adsorbed well.

Examples of the dichroic substance include iodine and a dichroic dye. Iodine is preferred. When iodine is used as the dichroic substance, an aqueous iodine solution is preferably used as the staining solution. The iodine content of the iodine aqueous solution is preferably 0.04 parts by weight to 5.0 parts by weight with respect to 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add iodide to the aqueous iodine solution. Potassium iodide is preferably used as the iodide. The content of iodide is preferably 0.3 to 15 parts by weight with respect to 100 parts by weight of water.

The liquid temperature at the time of dyeing the dyeing liquid can be set to an arbitrary appropriate value, for example, 20 ° C. to 50 ° C. When the PVA-based resin film is immersed in the dyeing solution, the immersion time is, for example, 5 seconds to 5 minutes.

B-5. Cross-linking step In the cross-linking step, a boron compound is usually used as a cross-linking agent. Examples of the boron compound include boric acid and borax. Preferably, it is boric acid. In the cross-linking step, the boron compound is usually used in the form of an aqueous solution.

When an aqueous boric acid solution is used, the boric acid concentration of the aqueous boric acid solution is, for example, 1% by weight to 15% by weight, preferably 1% by weight to 10% by weight. The boric acid aqueous solution may further contain an iodide such as potassium iodide and a zinc compound such as zinc sulfate and zinc chloride.

The cross-linking step can be performed by any suitable method. For example, a method of immersing a PVA-based resin film in an aqueous solution containing a boron compound, a method of applying an aqueous solution containing a boron compound to a PVA-based resin film, or a method of spraying an aqueous solution containing a boron compound onto a PVA-based resin film can be mentioned. Be done. It is preferable to immerse in an aqueous solution containing a boron compound.

The temperature of the solution used for crosslinking is, for example, 25 ° C. or higher, preferably 30 ° C. to 85 ° C., and more preferably 40 ° C. to 70 ° C. The immersion time is, for example, 5 seconds to 800 seconds, preferably 8 seconds to 500 seconds.

B-6. Cleaning Step The cleaning step can typically be performed after the crosslinking step. The cleaning step is typically performed by immersing a PVA-based resin film in a cleaning liquid. Pure water is a typical example of the cleaning liquid. Potassium iodide may be added to pure water.

The temperature of the cleaning liquid is, for example, 5 ° C to 50 ° C. The immersion time is, for example, 1 second to 300 seconds.

B-7. Drying Step The drying step can be carried out by any suitable method. Examples of the drying method include natural drying, blast drying, vacuum drying, heat drying and the like. Heat drying is preferably used. From the viewpoint of shortening the drying time, when heat-drying is performed, the heating temperature is preferably 50 ° C. or lower, more preferably 45 ° C. or lower, and particularly preferably 40 ° C. or lower. The lower limit of the heating temperature is not particularly limited, but is a lower limit temperature that can be set by the heating / drying device. For example, 30 ° C. The drying time is, for example, 20 seconds to 10 minutes. In one embodiment, heat drying is performed in two or more steps. In this case, preferably, the heating temperature at at least one stage is a temperature within the above range. By setting the heating temperature within the above range in the case of heat drying, a polarizer having extremely excellent heat resistance can be obtained.

C. Polarizing Plate The polarizer according to the embodiment of the present invention is typically used in a state where a protective film is laminated on one side or both sides thereof (that is, as a polarizing plate). Practically, the polarizing plate has an adhesive layer as the outermost layer. The pressure-sensitive adhesive layer is typically the outermost layer on the image display device side. A separator is temporarily attached to the pressure-sensitive adhesive layer so that it can be peeled off, protecting the pressure-sensitive adhesive layer until actual use and enabling roll formation.

As the protective film, any suitable resin film is used. Examples of the resin film forming material include (meth) acrylic resin, cellulose-based resin such as diacetyl cellulose and triacetyl cellulose, cycloolefin-based resin such as norbornene-based resin, olefin-based resin such as polypropylene, and polyethylene terephthalate-based resin. Etc., ester-based resins, polyamide-based resins, polycarbonate-based resins, and copolymer resins thereof. The "(meth) acrylic resin" refers to an acrylic resin and / or a methacrylic resin.

In one embodiment, as the (meth) acrylic resin, a (meth) acrylic resin having a glutarimide structure is used. Examples of the (meth) acrylic resin having a glutarimide structure (hereinafter, also referred to as glutarimide resin) include JP-A-2006-309033, JP-A-2006-317560, JP-A-2006-328329, and JP-A. 2006-328334, 2006-337491, 2006-337492, 2006-337493, 2006-337569, 2007-009182, 2009- It is described in JP-A-161744 and JP-A-2010-284840. These statements are incorporated herein by reference.

When a polarizer is produced using a laminate of a base material and a PVA-based resin layer, the base material may be used as it is as a protective film without peeling. Alternatively, the base material may be peeled off and a protective film may be attached to the polarizer.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. The measurement method of each characteristic is as follows.
(1) Single transmittance change amount Ts
A reflective polarizer (manufactured by 3M, trade name "DBEF") was attached to the polarizer side of the laminates obtained in Examples and Comparative Examples via an adhesive layer having a thickness of 1.0 μm. Next, the thermoplastic resin base material was peeled off, and non-alkali glass having a thickness of 1.3 mm was attached to the peeled surface via an acrylic pressure-sensitive adhesive layer having a thickness of 20 μm to prepare a test sample. This test sample was heated at 80 ° C. for 200 hours (heating test). The single transmittance of the polarizer before the test and after the heating test was measured using a spectrophotometer with an integrating sphere (manufactured by JASCO Corporation, product name: V7100). From the simple substance transmittance Ts0 before heating and the simple substance transmittance Ts1 after the heating test, the amount of change in simple substance transmittance Ts was determined using the following formula.
Ts (%) = Ts1-Ts0
(2) Iodine content The polarizers of the laminates obtained in Examples and Comparative Examples are fluorescent using a fluorescent X-ray analyzer (manufactured by Rigaku, trade name "ZSX-PRIMUS II", measurement diameter: ψ20 mm). The X-ray intensity (kcps) was measured. On the other hand, the thickness (μm) of the polarizer was measured using a spectroscopic film thickness meter (manufactured by Otsuka Electronics Co., Ltd., trade name “MCPD-3000”). The iodine content (% by weight) was determined from the obtained fluorescent X-ray intensity and thickness using the following formula.
(Iodine concentration) = 20.5 x (fluorescent X-ray intensity) / (film thickness)
The coefficient for calculating the iodine content differs depending on the measuring device, but the coefficient can be obtained by using an appropriate calibration curve.
(3) Free boric acid content 50 mg of the polarizers obtained in Examples and Comparative Examples cut into about 5 mm squares with scissors was filled in a sample container together with steel balls. Then, the measurement sample was freeze-ground using a freeze-crusher JFC-300 (manufactured by Nippon Analytical Industry Co., Ltd.) under the conditions of pre-cooling 7 minutes and vibration 5 minutes using liquid nitrogen as a refrigerant. The pulverized measurement sample was left at room temperature for about 30 minutes. The obtained measurement sample was mixed with 3.5 mL of a mixed solution of 2-ethyl-1,3-hexanediol / chloroform (volume ratio: 10/90) and left at room temperature for 24 hours. The resulting mixture was filtered using a 0.45 μm filter. Chloroform was removed from the obtained filtrate on a hot plate, and the residue was transferred to a Teflon (registered trademark) decomposition vessel, acid was added, and the mixture was sealed. The decomposition vessel was irradiated with microwaves to decompose the acid under pressure. After decomposition, ultrapure water was added to the volume to 25 mL, and the volume was analyzed under the following quantification conditions for boric acid.
<Boric acid quantification conditions>
Device name: ICP emission spectrometer SPS-3520UV (manufactured by Hitachi High-Tech Science Corporation)
Measurement wavelength: B 249.848 nm
All the obtained boron contents were considered to be derived from boric acid and converted into boric acid content. This boric acid content was defined as the free boric acid content of the polarizer.

[Example 1]
As the thermoplastic resin base material, an amorphous isophthalic acid copolymer polyethylene terephthalate (IPA copolymer PET) film (thickness: 100 μm) having a water absorption rate of 0.75% and a Tg of 75 ° C. was used. One side of the base material is corona-treated, and polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (polymerization degree 1200, acetoacetyl modification degree 4.6) are applied to the corona-treated surface. %, Degree of polymerization of 99.0 mol% or more, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "Gosefimer Z200") in a ratio of 9: 1 is applied and dried at 25 ° C. to a thickness of 11 μm. A PVA-based resin layer was formed to prepare a laminate.
The obtained laminate was stretched 4.5 times in the air at 140 ° C. in a direction orthogonal to the longitudinal direction of the laminate using a tenter stretching machine (stretching treatment).
Next, the laminate was immersed in a dyeing bath at a liquid temperature of 25 ° C. (an aqueous solution having an iodine concentration of 1.4% by weight and a potassium iodide concentration of 9.8% by weight) for 12 seconds for dyeing (dyeing treatment).
Next, the laminate was immersed in a washing bath (pure water) having a liquid temperature of 25 ° C. for 6 seconds (first washing treatment).
Then, it was immersed in a cross-linking bath (an aqueous solution having a boron concentration of 1% by weight and a potassium iodide concentration of 1% by weight) at a liquid temperature of 60 ° C. for 16 seconds (crosslinking treatment).
Next, the laminate was immersed in a washing bath (an aqueous solution having a potassium iodide concentration of 1% by weight) at a liquid temperature of 25 ° C. for 3 seconds (second washing treatment).
The laminate was then dried in an oven at 25 ° C. for 8 seconds (first drying process).
Finally, the laminate was dried in an oven at 25 ° C. for 13 seconds (second drying treatment) to obtain a laminate having a PVA-based resin layer (polarizer) having a thickness of 1.2 μm. The iodine content of the obtained polarizer was 18.5% by weight, the free boric acid content was 0.32% by weight, and the single transmittance was 40.0%.
The obtained laminate was subjected to the evaluation of (1) above. The results are shown in Table 1.

[Example 2]
In the first drying treatment and the second drying treatment, a laminate having a polarizer was obtained in the same manner as in Example 1 except that the laminates were each dried in an oven at 30 ° C. The iodine content of the obtained polarizer was 18.8% by weight, the free boric acid content was 0.32% by weight, and the single transmittance was 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Example 3]
In the first drying treatment and the second drying treatment, a laminate having a polarizer was obtained in the same manner as in Example 1 except that the laminates were each dried in an oven at 40 ° C. The iodine content of the obtained polarizer was 18.6% by weight, the free boric acid content was 0.39% by weight, and the single transmittance was 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Comparative Example 1]
In the second drying treatment, a laminate having a polarizer was obtained in the same manner as in Example 3 except that the laminate was dried in an oven at 60 ° C. The iodine content of the obtained polarizer was 19.1% by weight, the free boric acid content was 0.45% by weight, and the single transmittance was 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Comparative Example 2]
In the first drying treatment, a laminate having a polarizer was obtained in the same manner as in Comparative Example 1 except that the laminate was dried in an oven at 50 ° C. The iodine content of the obtained polarizer was 19.2% by weight, the free boric acid content was 0.48% by weight, and the single transmittance was 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Comparative Example 3]
In the first drying treatment, a laminate having a polarizer was obtained in the same manner as in Comparative Example 1 except that the laminate was dried in an oven at 60 ° C. The iodine content of the obtained polarizer was 19.3% by weight, the free boric acid content was 0.57% by weight, and the single transmittance was 40.0%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

As is clear from Table 1, it can be seen that the polarizer of the embodiment of the present invention has a smaller amount of change in the simple substance transmittance after the heating test and has extremely excellent heat resistance as compared with the polarizer of the comparative example. .. Specifically, the polarizer of the example has a Ts of 0.75% to 0.80%, and if the Ts is a value of this level, it is said that there is no problem of heat resistance in practical use. The polarizer of the example is thin, and the amount of change in the single transmittance in a high temperature environment is remarkably suppressed. It is presumed that such an excellent effect is realized by suppressing the free boric acid content of the obtained polarizer to prevent polyene formation in a high temperature environment. This solves a new problem found by actually producing a very thin (for example, 7 μm or less) polarizer, which was difficult to produce in the past, and is unexpectedly excellent. It is an effect.

The polarizer of the present invention can be widely applied to liquid crystal panels of liquid crystal televisions, liquid crystal displays, mobile phones, digital cameras, video cameras, portable game machines, car navigation systems, copiers, printers, fax machines, watches, microwave ovens, and the like. it can.

Claims (4)

Consists of a polyvinyl alcohol-based resin film containing iodine,
A polarizer having a free boric acid content of 0.4% by weight or less. The polarizer according to claim 1, wherein the iodine content is 10% by weight to 25% by weight. The polarizer according to claim 1 or 2, wherein the thickness is 7 μm or less. A polarizing plate comprising the polarizing element according to any one of claims 1 to 3 and a protective film laminated on one side or both sides of the polarizing element.