Polarizing plate

Abstract

To enhance the durability of a polarizing plate in a high-temperature environment.SOLUTION: A polarizing plate 30 includes a polarizer 31, the polarizing plate 30 being provided with a protective film 33 that is laminated on at least one side of the polarizer 31 through an adhesive layer 32, wherein the adhesive layer 32 contains an alkali metal nitrate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polarizing plate. [Background technology]

[0002] Polarizing plates widely used in image display devices, such as liquid crystal display devices, typically have a configuration in which a thermoplastic resin film, such as a protective film, is laminated to one or both sides of a polarizer. An adhesive is typically used to bond the polarizer and the thermoplastic resin film. Known examples of such adhesives include active energy ray-curable adhesives and water-based adhesives. Such polarizing plates are disclosed, for example, in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-008860 Summary of the Invention [Problem to be solved by the invention]

[0004] The polarizing plate described in Patent Document 1 may not necessarily have sufficient durability when used in harsher environments (e.g., high-temperature environments) such as in-vehicle image display devices such as car navigation devices and rearview monitors. [Means for solving the problem]

[0005] The present invention includes the following inventions. [Invention 1] A polarizer; a protective film laminated on at least one surface of the polarizer via an adhesive layer, The adhesive layer of the polarizing plate contains an alkali metal nitrate.

[0006] [Invention 2] The polarizing plate according to [Invention 1], wherein the adhesive layer is an adhesive layer. [Invention 3] The polarizing plate according to [Invention 2], wherein the adhesive layer is a layer of a cured product of an aqueous adhesive composition containing a polyvinyl alcohol-based resin.

[0007] [Invention 4] The polarizing plate according to [Invention 3], wherein the content of the alkali metal nitrate in the adhesive layer is 50 parts by mass or more and 90 parts by mass or less with respect to 100 parts by mass of the polyvinyl alcohol-based resin.

[0008] [Invention 5] The polarizing plate according to any one of [Invention 1] to [Invention 4], wherein the polarizer contains zinc element. [Effects of the Invention]

[0009] The polarizing plate of the present invention exhibits good durability even in a high-temperature environment. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a polarizing plate. DETAILED DESCRIPTION OF THE INVENTION

[0011] The polarizing plate of the present invention will be described below. <Polarizing plate> The polarizing plate 30 includes a polarizer 31 .

[0012] As shown in FIG. 1, the polarizing plate 30 has a polarizer 31 and a first protective film 33 laminated on one surface thereof with an adhesive layer 32 interposed therebetween. As shown in FIG. 1, the polarizing plate 30 may have a second protective film 34 laminated on the other surface of the polarizer 31 with an adhesive layer 32 interposed therebetween.

[0013] The adhesive layer 32 interposed between the polarizer 31 and the protective film contains an alkali metal nitrate, as will be described later. In the polarizing plate 30, it is sufficient that a protective film is laminated on at least one surface of the polarizer 31 via the adhesive layer 32. That is, only the first protective film 33 may be laminated, or only the second protective film 34 may be laminated. The protective film can be laminated to the polarizing plate 30 using, for example, a roll laminating machine.

[0014] <Polarizer> A polarizer is a film that selectively transmits linearly polarized light in one direction from natural light. The polarizer is preferably a uniaxially stretched polyvinyl alcohol resin film in which a dichroic dye is adsorbed and aligned. For example, a saponified polyvinyl acetate resin can be used as the polyvinyl alcohol resin. The saponification degree of the polyvinyl acetate resin is preferably 85 mol % or more, more preferably 90 mol % or more, and even more preferably 99 mol % or more.

[0015] Examples of polyvinyl acetate resins that can be used include polyvinyl acetate, which is a homopolymer of vinyl acetate, and copolymers of vinyl acetate with other copolymerizable monomers. Examples of the copolymerizable other monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids.

[0016] The degree of polymerization of the polyvinyl alcohol resin is preferably 1,000 or more and 10,000 or less, and more preferably 1,500 or more and 5,000 or less. The polyvinyl alcohol resin may be modified. Examples of the modified polyvinyl alcohol resin include polyvinyl formal, polyvinyl acetal, and polyvinyl butyral modified with aldehydes.

[0017] Examples of the dichroic dye include iodine and water-soluble dichroic dyes. The thickness of the polarizer is not particularly limited, but is, for example, 1 μm or more, preferably 5 μm or more, more preferably 8 μm or more, and 50 μm or less, preferably 30 μm or less.

[0018] The polarizer may contain elemental zinc, which can improve durability in high-temperature environments. The zinc compound that supplies zinc element to the polarizer is not particularly limited, and examples thereof include zinc aldonic acid, zinc chloride, zinc nitrate, and zinc sulfate. Examples of aldonic acids include gluconic acid, galactonic acid, mannonic acid, talonic acid, gulonic acid, idonic acid, allonic acid, and altronic acid. Examples of a method for incorporating zinc element into a polarizer include a method in which the zinc compound is incorporated into each treatment solution during the production of a polarizer, which will be described later.

[0019] A known method can be used to manufacture a polarizer. For example, this manufacturing method involves sequentially performing a swelling step, a dyeing step, a crosslinking step, a washing step, and a drying step using a polyvinyl alcohol-based resin film as a raw film. The swelling step is a treatment step in which the raw film is immersed in a swelling liquid to swell it. The dyeing step is a treatment step in which the film after the swelling step is immersed in a dyeing liquid containing a dichroic dye to adsorb and align the dichroic dye in the film. The crosslinking step is a treatment step in which a crosslinking treatment is performed by bringing the film into contact with a crosslinking liquid. Between each step, i.e., before, after, or during any one or more of the treatment steps, a uniaxial stretching treatment can be performed as a stretching step.

[0020] The crosslinking step is a treatment step carried out for the purposes of imparting water resistance through crosslinking, adjusting hue (complementary color), etc. The crosslinking treatment may be carried out multiple times. When the crosslinking treatment is carried out multiple times, the crosslinking treatment for the purpose of imparting water resistance through crosslinking may be carried out multiple times, or the crosslinking treatment for the purpose of adjusting hue may be carried out multiple times. However, it is preferable to carry out the crosslinking treatment for the purpose of imparting water resistance through crosslinking at least once and the crosslinking treatment for the purpose of adjusting hue at least once, and it is more preferable to carry out the crosslinking step for the purpose of imparting water resistance through crosslinking after the crosslinking step for the purpose of adjusting hue.

[0021] When at least one of the swelling liquid, dyeing liquid, and crosslinking liquid used in the method for producing a polarizer contains the zinc compound, a polarizer containing elemental zinc can be produced. The expression "at least one" used in this specification means "one or more" of the desired options. For example, when the number of options is three or more, the expression "at least one" used in this specification means "only one option" or "any combination of two or more options."

[0022] <Protective film> The protective film is not particularly limited, and various transparent protective films that can be used in polarizing plates can be employed. Examples of materials that can be used to form the protective film include thermoplastic resins that are excellent in transparency, mechanical strength, thermal stability, moisture barrier properties, and isotropy. Examples of the thermoplastic resins include cellulose ester resins such as triacetyl cellulose, polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyethersulfone resins, polysulfone resins, polycarbonate resins, polyamide resins such as nylon and aromatic polyamide, polyimide resins, polyolefin resins such as polyethylene, polypropylene, and ethylene-propylene copolymers, (meth)acrylic resins, cyclic polyolefin resins (norbornene resins) having a cyclo- or norbornene structure, polyarylate resins, polystyrene resins, polyvinyl alcohol resins, and mixtures thereof. The protective film can also be a cured layer formed from a thermosetting resin or ultraviolet-curable resin, such as a (meth)acrylic, urethane, acrylic urethane, epoxy, or silicone resin. Among these, cellulose ester resins, polycarbonate resins, (meth)acrylic resins, cyclic polyolefin resins, and polyester resins are preferred.

[0023] The surface of the protective film to which the polarizer is not attached may be provided with a functional layer such as a hard coat layer, an antireflection layer, an antisticking layer, a diffusion layer, an antiglare layer, etc. The functional layer such as the hard coat layer, the antireflection layer, the antisticking layer, the diffusion layer, or the antiglare layer may be provided in the protective film itself, or may be provided separately as a layer separate from the protective film.

[0024] Either or both of the surface of the protective film to which the polarizer is attached and the surface of the polarizer to which the protective film is attached may be subjected to a surface treatment, such as a corona treatment, a plasma treatment, a primer treatment, or a saponification treatment.

[0025] <Adhesive layer> The adhesive layer 32 may be a pressure-sensitive adhesive layer or an adhesive layer, and is preferably an adhesive layer.

[0026] The adhesive composition forming the adhesive layer 32 may be any of various adhesive compositions that can be used in polarizing plates, including aqueous adhesive compositions in which a curable adhesive component is dissolved or dispersed in water, and active energy ray-curable adhesive compositions containing an active energy ray-curable compound. Examples of aqueous adhesive compositions include isocyanate-based adhesives, polyvinyl alcohol-based adhesives, gelatin-based adhesives, vinyl latex-based adhesives, and aqueous polyesters. The adhesive composition is preferably an aqueous solution in which the adhesive component is dissolved in water, and the solids concentration of the adhesive composition is preferably 0.5 to 20% by mass, more preferably 1 to 15% by mass.

[0027] When a polyvinyl alcohol resin is used as the main component of the adhesive composition, the polyvinyl alcohol resin may be a polyvinyl alcohol resin such as a partially saponified polyvinyl alcohol or a fully saponified polyvinyl alcohol, or a modified polyvinyl alcohol resin. Examples of the modified polyvinyl alcohol resin include a carboxyl group-modified polyvinyl alcohol resin and an acetoacetyl group-modified polyvinyl alcohol resin.

[0028] The average degree of polymerization of the polyvinyl alcohol resin (preferably an acetoacetyl-modified polyvinyl alcohol resin) is preferably from 100 to 5,500, more preferably from 500 to 4,500, from the viewpoint of adhesiveness.

[0029] The degree of saponification of the polyvinyl alcohol resin (preferably an acetoacetyl-modified polyvinyl alcohol resin) is usually 80 mol % to 100 mol %, and preferably 85 mol % or more.

[0030] The degree of modification (amount of modification) with acetoacetyl groups in the acetoacetyl group-modified polyvinyl alcohol resin is usually 0.1 mol % to 40 mol %, and preferably 0.5 mol % to 20 mol %, from the viewpoint of adhesiveness.

[0031] Among the aqueous adhesive compositions, a polyvinyl alcohol-based adhesive composition is preferred, and an acetoacetyl-modified polyvinyl alcohol-based adhesive composition is more preferred. That is, the adhesive layer 32 is preferably a cured product layer of an aqueous adhesive composition containing a polyvinyl alcohol-based resin.

[0032] The aqueous adhesive composition may contain a crosslinking agent. The crosslinking agent is typically a compound having at least two functional groups per molecule that are reactive with the polymer or other components that make up the adhesive composition. Examples of the crosslinking agent include alkylenediamines, isocyanates, epoxies, aldehydes, amino-formaldehydes such as methylol urea and methylol melamine, glyoxal, and glyoxal derivatives. The amount of crosslinking agent in the adhesive composition is typically about 5 to 60 parts by mass per 100 parts by mass of the polymer or other components that make up the adhesive composition.

[0033] The aqueous adhesive composition may also contain an organic solvent. The organic solvent is preferably an alcohol-based solvent because it is miscible with water, and more preferably methanol or ethanol.

[0034] Examples of adhesive compositions include the aqueous adhesive compositions described above, as well as active energy ray-curable adhesive compositions such as ultraviolet-curable adhesive compositions and electron beam-curable adhesive compositions. Examples of active energy ray-curable adhesive compositions include (meth)acrylate adhesives. Examples of curable components in (meth)acrylate adhesives include compounds having a (meth)acryloyl group and compounds having a vinyl group. Examples of compounds having a (meth)acryloyl group include alkyl (meth)acrylates such as linear alkyl (meth)acrylates, alicyclic alkyl (meth)acrylates, and polycyclic alkyl (meth)acrylates having 1 to 20 carbon atoms; hydroxyl group-containing (meth)acrylates; and epoxy group-containing (meth)acrylates such as glycidyl (meth)acrylate. The (meth)acrylate adhesive may contain a nitrogen-containing monomer such as hydroxyethyl (meth)acrylamide, N-methylol (meth)acrylamide, N-methoxymethyl (meth)acrylamide, N-ethoxymethyl (meth)acrylamide, (meth)acrylamide, or (meth)acryloylmorpholine. The (meth)acrylate adhesive may contain a polyfunctional monomer as a crosslinking component, such as tripropylene glycol diacrylate, 1,9-nonanediol diacrylate, tricyclodecane dimethanol diacrylate, cyclic trimethylolpropane formal acrylate, dioxane glycol diacrylate, or EO-modified diglycerin tetraacrylate. Furthermore, compounds containing epoxy groups or oxetanyl groups can also be used as cationic polymerization-curable adhesives. The epoxy group-containing compound is not particularly limited as long as it has at least two epoxy groups in the molecule, and various commonly known curable epoxy compounds can be used.

[0035] The active energy ray-curable adhesive composition may contain appropriate additives as needed, such as coupling agents (e.g., silane coupling agents, titanium coupling agents), adhesion promoters (e.g., ethylene oxide), ultraviolet absorbers, antidegradants, dyes, processing aids, ion trapping agents, antioxidants, tackifiers, fillers, plasticizers, leveling agents, foam inhibitors, antistatic agents, heat stabilizers, and hydrolysis stabilizers.

[0036] The adhesive composition may be applied to either the protective film side or the polarizer side, or to both. After lamination, a drying step is performed to form an adhesive layer consisting of a dried coated layer. After the drying step, ultraviolet light or electron beams may be irradiated as necessary.

[0037] As the pressure-sensitive adhesive composition for forming the pressure-sensitive adhesive layer, any conventionally known pressure-sensitive adhesive composition having excellent optical transparency can be used without particular limitation, and for example, a pressure-sensitive adhesive composition having a base polymer such as an acrylic resin, a urethane resin, a silicone resin, or a polyvinyl ether resin can be used. Among these, a pressure-sensitive adhesive composition having an acrylic resin as a base polymer is preferred, as it is excellent in transparency, adhesive strength, removability, weather resistance, heat resistance, etc. The pressure-sensitive adhesive composition may further contain a crosslinking agent, a silane compound, an antistatic agent, etc.

[0038] The thickness of the adhesive layer 32 is not particularly limited. For example, when an aqueous adhesive composition or the like is used, the thickness is preferably about 1 to 5000 nm, and more preferably about 10 to 1000 nm. For example, when an active energy ray-curable adhesive composition or the like is used, the thickness is preferably about 0.1 to 10 μm, and more preferably about 0.5 to 5 μm. For example, when a pressure-sensitive adhesive composition is used, the thickness is usually preferably 0.1 to 30 μm, more preferably 3 to 30 μm, and even more preferably 5 to 25 μm.

[0039] Specific examples of the alkali metal nitrate contained in the adhesive layer 32 include lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, and francium nitrate. The alkali metal nitrate is preferably potassium nitrate.

[0040] The content of the alkali metal nitrate in the adhesive layer is not particularly limited. For example, when the adhesive layer is an aqueous adhesive composition containing a polyvinyl alcohol-based resin, the content of the alkali metal nitrate is preferably 50 parts by mass or more and 90 parts by mass or less per 100 parts by mass of the polyvinyl alcohol-based resin.

[0041] <Effects of this embodiment> In a high temperature environment, for example, polyenization of polyvinyl alcohol progresses, which causes a problem of yellowing.

[0042] In this regard, the polarizing plate of the present embodiment can reduce the yellow index (yellowness degree) even in a high-temperature environment because the adhesive layer contains an alkali metal nitrate. According to the present embodiment, the progression of yellowing can be suppressed, and therefore durability in a high-temperature environment is good.

[0043] A polarizing plate having excellent high-temperature durability can also be suitably used in image display devices that are exposed to high-temperature environments for long periods of time. For example, image display devices for vehicle use, such as car navigation systems and rearview monitors, are sometimes exposed to high-temperature environments for long periods of time. The polarizing plate of this embodiment can also be suitably used in image display devices for vehicle use.

[0044] In this embodiment, "high temperature environment" means an environment of 100°C or higher, for example, 105°C or higher. Furthermore, "long period of time" means 100 hours or longer, for example, 120 hours. [Example]

[0045] The polarizing plate of the present invention will be described in more detail based on the following examples, but the present invention is not limited to the configurations described in the examples. (1) Polarizer thickness measurement: Measurements were made using a Nikon digital micrometer "MH-15M."

[0046] (2) Measurement of the luminosity-corrected single transmittance of the polarizing plate: Measurement was carried out using a spectrophotometer with an integrating sphere ("V-7100" manufactured by JASCO Corporation, 2-degree field of view; C light source).

[0047] (3) Yellow index (yellowness) measurement: A Konica Minolta CM-3700A spectrophotometer was used. Reflected light was measured in SCI mode to determine the tristimulus values ​​(X, Y, Z), and the yellow index (YI) was calculated using the following formula based on the ASTM E 313-73 standard.

[0048] YI=100×(1.000-0.847×Z / Y) (Fabrication of polarizer) A 40 μm-thick, long polyvinyl alcohol (PVA) raw film (manufactured by Kuraray Co., Ltd. under the trade name "VF-PE#4000," with a saponification degree of 99.9 mol% or higher) was continuously transported while being unwound from a roll, immersed in a swelling solution of pure water at 22°C for 79 seconds, and uniaxially stretched to 2.00 times its original size (swelling step).The film was then pulled out of the swelling solution and immersed in a dyeing solution (23°C) containing iodine and a 0.2 / 0.2 / 2.0 / 100 (mass ratio) of iodine / potassium iodide / boric acid / water for 143 seconds, and uniaxially stretched to 1.22 times its original size (dyeing step). Next, the film removed from the dyeing solution was immersed in a first crosslinking liquid (61°C) containing potassium iodide, boric acid, and water in a 2.3 / 3.7 / 100 (mass ratio) for 77 seconds, and uniaxially stretched to 2.33 times its original size (first crosslinking step). The film removed from the first crosslinking liquid was then immersed in a second crosslinking liquid (45°C) containing zinc chloride, potassium iodide, boric acid, and water in a 1.0 / 2.4 / 5.0 / 100 (mass ratio) for 12 seconds, and uniaxially stretched to 1.03 times its original size (second crosslinking step). The film removed from the second crosslinking liquid was then allowed to dwell in a drying oven at 38°C for 92 seconds and dried (drying step). The resulting polarizer had a thickness of 15 μm.

[0049] (Preparation of PVA solution A for adhesive) 50 g of an acetoacetyl group-modified PVA resin ("Gohsenex Z-410" manufactured by Mitsubishi Chemical Corporation) was dissolved in 950 g of pure water, heated at 90°C for 2 hours, and then cooled to room temperature to obtain a PVA solution for adhesive (hereinafter referred to as "PVA solution A").

[0050] (Preparation of PVA solution B for adhesive) The PVA solution A prepared above, pure water, a 40% by mass solution of glyoxal, ethanol, and methanol were mixed together to the contents shown below per 100 parts by mass of adhesive to prepare an adhesive (hereinafter referred to as "PVA solution B").

[0051] PVA content: 2.8 parts by mass Methanol content: 35.0 parts by mass Ethanol content: 3.0 parts by mass Glyoxal content: 0.2 parts by mass Pure water: 59.0 parts by mass <Comparative Example 1> (Preparation of adhesive 1) Pure water was added to the PVA solution B prepared above to prepare adhesive 1 (PVA concentration 1.9% by mass).

[0052] [Preparation of polarizing plates] Using a roll laminator, transparent protective films made of saponified cellulose acylate were laminated on both sides of the polarizer via an adhesive (adhesive 1), and then the adhesive was dried by heat treatment at 65°C for 720 seconds to obtain polarizing plate 1. The single transmittance of polarizing plate 1 was 41.2±0.2%.

[0053] <Comparative Example 2> (Preparation of adhesive 2) The PVA solution B prepared above was mixed with the following contents per 100 parts by mass of PVA to prepare adhesive 2 (PVA concentration 1.9% by mass).

[0054] KCl content: 166.3 parts by mass [Preparation of polarizing plates] Polarizing plate 2 was obtained in the same manner as in Comparative Example 1, except that adhesive 2 was used instead of adhesive 1. The single transmittance of polarizing plate 2 was 41.2±0.2%.

[0055] <Comparative Example 3> (Preparation of adhesive 3) The PVA solution B prepared above was mixed with the following contents per 100 parts by mass of PVA to prepare adhesive 3 (PVA concentration 1.9% by mass).

[0056] KI content: 82.8 parts by mass [Preparation of polarizing plates] Polarizing plate 3 was obtained in the same manner as in Comparative Example 1, except that adhesive 3 was used instead of adhesive 1. The single transmittance of polarizing plate 3 was 41.2±0.2%.

[0057] Example 1 (Preparation of adhesive 4) The PVA solution B prepared above was mixed with the following contents per 100 parts by mass of PVA to prepare adhesive 4 (PVA concentration 1.9% by mass).

[0058] KNO3 content: 50.4 parts by mass [Preparation of polarizing plates] Polarizing plate 4 was obtained in the same manner as in Comparative Example 1, except that adhesive 4 was used instead of adhesive 1. The single transmittance of polarizing plate 4 was 41.2±0.2%.

[0059] <Example 2> (Preparation of adhesive 5) The PVA solution B prepared above was mixed with the following contents per 100 parts by mass of PVA to prepare adhesive 5 (PVA concentration 1.9% by mass).

[0060] KNO3 content: 89.7 parts by mass [Preparation of polarizing plates] Polarizing plate 5 was obtained in the same manner as in Comparative Example 1, except that adhesive 5 was used instead of adhesive 1. The single transmittance of polarizing plate 5 was 41.2±0.2%.

[0061] <High temperature durability evaluation> The polarizing plates 1, 2, 3, 4, and 5 obtained above were stored for 7 days at a temperature of 23°C and a relative humidity of 53% so that the moisture content of the polarizing plates reached an equilibrium moisture content. An acrylic pressure-sensitive adhesive was then formed on both sides of each of the polarizing plates 1, 2, 3, 4, and 5 whose moisture contents had been adjusted in this manner.

[0062] Furthermore, the polarizing plate was cut into a size of 110 mm x 60 mm so that the absorption axis was parallel to the long side, and an alkali-free glass (Corning "EAGLE XG", size 120 mm x 70 mm) was attached to the adhesive surface of each plate to prepare an evaluation sample.

[0063] The evaluation sample thus obtained was subjected to autoclave treatment at 50°C and 5 atmospheres for 15 minutes, and then exposed to an environment at 105°C for 120 hours to carry out a high temperature durability test. As shown in Table 1, the YI of polarizing plate 1 after 120 hours was 67. The YI of polarizing plate 2 was 56, the YI of polarizing plate 3 was 65, the YI of polarizing plate 4 was 33, and the YI of polarizing plate 5 was 33. When the YIs after 120 hours were compared, the YIs of polarizing plates 4 and 5 were all lower than those of polarizing plates 1 to 3, being 40 or less, indicating improved durability.

[0064] [Table 1] [Explanation of symbols]

[0065] 30...Polarizing plate 31...Polarizer 32…Adhesive layer 33...First protective film 34...Second protective film

Claims

1. A polarizer; a protective film laminated on at least one surface of the polarizer via an adhesive layer, The adhesive layer of the polarizing plate contains an alkali metal nitrate.

2. The polarizing plate according to claim 1 , wherein the adhesive layer is an adhesive layer.

3. 3. The polarizing plate according to claim 2, wherein the adhesive layer is a layer of a cured product of an aqueous adhesive composition containing a polyvinyl alcohol-based resin.

4. 4. The polarizing plate according to claim 3, wherein the content of the alkali metal nitrate in the adhesive layer is 50 parts by mass or more and 90 parts by mass or less with respect to 100 parts by mass of the polyvinyl alcohol-based resin.

5. 5. The polarizing plate according to claim 1, wherein the polarizer contains zinc element.

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