Anisotropic light-diffusing adhesive layer and a display device comprising the anisotropic light-diffusing adhesive layer

Abstract

Provided is an anisotropic light-diffusing adhesive layer that achieves excellent viewing angle expansion. The anisotropic light-diffusing adhesive layer contains an adhesive and an acicular filler. The acicular filler is dispersed in the anisotropic light-diffusing adhesive layer so as to be oriented in one direction. When the anisotropic light-diffusing adhesive layer is layered between two polarizing plates that have absorption axes that differ by 90° such that the absorption axis of one of the polarizing plates and the orientation axis of the acicular filler are parallel, the degree of orientation indicated by [(the total transmittance of the anisotropic light-diffusing adhesive layer for light emitted by the other polarizing plate that has the absorption axis that is not parallel to the orientation direction of the acicular filler) / (the percentage content of the acicular filler in the anisotropic light-diffusing adhesive layer)] is no more than 30%.

Description

Technical Field

[0001] The present invention relates to a display device including an anisotropic light-diffusing adhesive layer and an anisotropic light-diffusing adhesive layer.

Background Art

[0002] In recent years, as a light-diffusing element used in a liquid crystal display device, an anisotropic light-diffusing element in which transmitted light is diffused while being polarized in a specific direction instead of being isotropic has been proposed. For example, Patent Document 1 discloses an anisotropic light-diffusing adhesive laminate including at least one adhesive layer in which needle-like fillers are dispersed so as to be oriented in substantially the same direction, and the adhesive layer has two or more layers. According to such an anisotropic light-diffusing adhesive laminate, when linearly polarized light is perpendicularly incident on the anisotropic light-diffusing adhesive laminate, the projected image of the transmitted light has an elliptical shape extending in a direction orthogonal to the major axis direction of the needle-like fillers.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The above-described anisotropic light-diffusing adhesive laminate is a useful technique because it can achieve both an anisotropic light-diffusing function and an adhesive function.

[0005] However, with the diversification of the uses of liquid crystal display devices, there is a demand for an anisotropic light-diffusing element with a further enhanced viewing angle expansion effect.

[0006] Therefore, an object of the present invention is to provide an anisotropic light-diffusing adhesive layer excellent in viewing angle expansion.

Means for Solving the Problems

[0007] The inventors conducted diligent research and discovered that the above problems can be solved by having an anisotropic light-diffusing adhesive layer satisfy specific physical properties, thus completing the present invention.

[0008] The present invention An anisotropic light-diffusing adhesive layer containing an adhesive and needle-shaped fillers, The needle-shaped fillers are dispersed in the anisotropic light-diffusing adhesive layer, oriented in one direction. The anisotropic light-diffusing adhesive layer is characterized in that, when the anisotropic light-diffusing adhesive layer is laminated between two polarizing plates whose absorption axes are 90° apart from each other, such that the absorption axis of one polarizing plate and the orientation axis of the needle-shaped filler are parallel, the degree of orientation shown by the following formula (1) is 30% or less. Degree of orientation = (Total light transmittance emitted from the other polarizer having an absorption axis not parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer) / (Percentage of needle-shaped fillers in the anisotropic light-diffusing adhesive layer) ... (1)

[0009] Preferably, the needle-shaped filler has a short diameter of 0.1 μm to 20 μm and a long diameter of 2 μm to 5000 μm. Preferably, the adhesive is an acrylic resin adhesive made of a copolymer containing an (meth)acrylic acid ester monomer having an aromatic ring. It is preferable that the absolute value of the difference between the refractive index of the needle-shaped filler in the longitudinal direction and the refractive index of the acrylic resin is 0.07 or less. The weight-average molecular weight of the adhesive is preferably between 200,000 and 1,000,000. The anisotropic light-diffusing adhesive layer preferably has a film thickness of 5 μm to 100 μm.

[0010] Furthermore, the present invention, The display device is characterized in that the anisotropic light-diffusing adhesive layer is laminated on an upper polarizing plate of the liquid crystal display device such that the orientation axis of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer is parallel to the absorption axis of the upper polarizing plate. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide an anisotropic light-diffusing adhesive layer that is excellent in expanding the viewing angle. [Brief explanation of the drawing]

[0012] [Figure 1] This graph shows the relationship between the front and 60° contrast ratios in an embodiment of the present invention. [Modes for carrying out the invention]

[0013] The anisotropic light-diffusing adhesive layer according to the present invention will be described in detail below, but the present invention is not limited to these. Furthermore, the present invention may be not only an anisotropic light-diffusing adhesive layer, but also an adhesive coating for obtaining an anisotropic light-diffusing adhesive layer, a display device to which the anisotropic light-diffusing adhesive layer is applied, or a method for manufacturing these. In addition, the present invention may be a laminate comprising the anisotropic light-diffusing adhesive layer according to the present invention and other predetermined layers (substrate or other adhesive layers).

[0014] In the following descriptions, if upper and lower limits are listed separately, all combinations of upper and lower limits shall be those specified herein.

[0015] In the following explanation, the anisotropic light-diffusing adhesive layer may sometimes be simply referred to as the "adhesive layer."

[0016] <<<Composition of the anisotropic light-diffusing adhesive layer>>> The anisotropic light-diffusing adhesive layer contains an adhesive and needle-shaped fillers. The anisotropic light-diffusing adhesive layer may also contain other components.

[0017] <<Ingredients>> <Adhesive> The adhesive is not particularly limited. For example, when used in a liquid crystal display device or the like, it is preferably one that satisfies the following requirements: (a) having high optical transparency, (b) having a refractive index close to that of an adjacent base material (e.g., a TAC film which is a protective film of a polarizing plate) when laminated, (c) having high reliability and a large number of achievements as an adhesive for a polarizing element, (d) being relatively inexpensive, etc. Examples of those satisfying such requirements include adhesives made of acrylic resins (adhesives containing acrylic resins / acrylic adhesives).

[0018] As the main component of the acrylic adhesive, there may be mentioned homopolymers of acrylic monomers such as acrylic acid and its esters, methacrylic acid and its esters, acrylamide, acrylonitrile, etc., or copolymers thereof, and copolymers of at least one of the acrylic monomers and vinyl monomers such as vinyl acetate, maleic anhydride, styrene, etc. Also, as the monomer, those having an aromatic ring can be used.

[0019] For example, the acrylic adhesive is preferably a copolymer composed of monomers such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc., and monomers having an aromatic ring such as 2-phenoxyethyl acrylate. In this case, as the monomer, it is further preferable to contain functional group-containing monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic anhydride, hydroxyethyl methacrylate, hydroxypropyl methacrylate, dimethylaminoethyl methacrylate, methylol acrylamide, glycidyl methacrylate, etc.

[0020] Also, as the acrylic adhesive, one can be used which is formed by blending an oligomer having an acrylic group at its terminal or side chain and an acrylic monomer with a photoinitiator or the like, and after coating on a base material, irradiating with ultraviolet rays or the like to make the coating layer into an adhesive.

[0021] The weight-average molecular weight (Mw) of the adhesive is not particularly limited as long as it does not hinder the effects of the present invention, but is preferably, for example, 200,000 to 1,000,000. By setting the weight-average molecular weight within this range, an adhesive with good hardness and resistance to peeling can be obtained. The weight-average molecular weight can be measured using a known measurement method, for example, by a method conforming to JIS K7252-1:2008 "Plastics - Method for determining the average molecular weight and molecular weight distribution of polymers by size exclusion chromatography".

[0022] The refractive index of the adhesive is preferably 1.48 to 1.58. The refractive index of the adhesive can be measured by a method in accordance with Method A described in JIS K-7142 (1996).

[0023] The glass transition temperature (Tg) of the adhesive is preferably -50°C to -10°C, and more preferably -40°C to -30°C. By setting the Tg within this range, the adhesive can be made harder and its adhesion to the polarizing plate surface of the display device can be increased. This makes it possible to obtain an adhesive that is less prone to blistering and peeling. The Tg can be measured using a known measurement method, for example, by a method conforming to JIS K7121-1987 "Method for measuring the transition temperature of plastics".

[0024] (Crosslinking agent) The adhesive of the present invention can be further crosslinked using a crosslinking agent for the purpose of appropriately adjusting the gel fraction. The crosslinking agent is not particularly limited as long as it does not hinder the effects of the present invention, and for example, isocyanate-based crosslinking agents, aziridine-based crosslinking agents, epoxy-based crosslinking agents, metal chelate-type crosslinking agents, etc., can be used. These can be used alone or in combination. Of these, isocyanate-based crosslinking agents have appropriate reactivity and are preferred from the viewpoint of productivity.

[0025] The isocyanate crosslinking agent is not particularly limited as long as it does not inhibit the effects of the present invention, but examples include aromatic polyisocyanates such as toluene diisocyanate (TDI), methylenediphenyl diisocyanate (MDI), naphthalene diisocyanate, xylylene diisocyanate, and polymeric polyisocyanate (crude MDI); aliphatic polyisocyanates such as hexamethylene diisocyanate (HDI), isophorone diisocyanate, and dicyclohexamethane diisocyanate; alicyclic polyisocyanates such as cyclohexane 1,4-diisocyanate, isophorone diisocyanate (IPDI), hydrogenated xylylene diisocyanate, hydrogenated bis(isocyanatophenyl)methane, and bicycloheptane triisocyanate; isocyanurate compounds; burette-type compounds; and modified versions thereof. These can be used individually or in combination.

[0026] <Needle-shaped filler> (Material) The needle-shaped filler is not particularly limited as long as it is a high aspect ratio filler exhibiting a needle-like (including fibrous) shape. However, when the anisotropic light-diffusing adhesive layer of the present invention is used in liquid crystal display devices and the like, a colorless or white filler is preferred in order to prevent discoloration of the transmitted light.

[0027] Specifically, needle-shaped or fibrous materials made of metal oxides such as titanium dioxide, zirconium oxide, and zinc oxide; metal compounds such as boehmite, aluminum borate, calcium silicate, basic magnesium sulfate, calcium carbonate, and potassium titanate; glass; synthetic resins, etc., are preferably used.

[0028] To enhance the effects of the present invention, it is preferable that the needle-shaped filler be calcium carbonate.

[0029] (shape) The short diameter of the needle-shaped filler is preferably 0.1 μm to 20 μm, more preferably 0.3 μm to 5 μm, and even more preferably 0.5 μm to 1 μm.

[0030] The major axis of the needle-shaped filler is preferably 2 μm to 5000 μm, and more preferably 10 μm to 300 μm.

[0031] By setting the short and long axes of the needle-shaped fillers within this range, the diffusion of light by the needle-shaped fillers within the anisotropic light-diffusing layer is optimized, making it possible to enhance the viewing angle expansion effect of the anisotropic light-diffusing adhesive layer.

[0032] The aspect ratio (major axis / minor axis) of the needle-shaped filler is preferably 10 to 60, more preferably 20 to 60, and even more preferably 30 to 60. By setting the aspect ratio of the needle-shaped filler within this range, the diffusion of light by the needle-shaped filler within the anisotropic light diffusion layer is optimized, and the viewing angle expansion effect of the anisotropic light diffusion adhesive layer can be enhanced.

[0033] The needle-shaped filler preferably has a refractive index of 1.48 to 1.58 in the direction of its major axis. Furthermore, the absolute value of the difference between the refractive index of the needle-shaped filler in the longitudinal direction and the refractive index of the adhesive is preferably 0.07 or less, more preferably 0.05 or less, and even more preferably 0.03 or less. The needle-shaped filler preferably has a refractive index of 1.60 to 1.70 in the short-axis direction. Furthermore, the absolute value of the difference between the refractive index of the needle-shaped filler in the short-axis direction and the refractive index of the adhesive is preferably 0.1 to 0.2, and more preferably 0.15 to 0.2. By setting the difference between the refractive index of the needle-shaped filler in the short-axis or long-axis direction and the refractive index of the adhesive within this range, the diffusion of light by the needle-shaped filler within the anisotropic light-diffusing layer is optimized, and the viewing angle expansion effect of the anisotropic light-diffusing adhesive layer can be enhanced.

[0034] (Content) The content of the needle-shaped filler is preferably 1 to 50 parts by weight, more preferably 15 to 40 parts by weight, and even more preferably 15 to 30 parts by weight, when the total solid content of the paint is set to 100. By setting the content of the needle-shaped filler within this range, the diffusion of light by the needle-shaped filler in the anisotropic light diffusion layer is optimized, and the viewing angle expansion effect of the anisotropic light diffusion adhesive layer can be enhanced.

[0035] <Other ingredients> The anisotropic light-diffusing adhesive layer of the present invention may contain other components. These other components are not particularly limited as long as they do not hinder the effects of the present invention, and may include, for example, various additives common in the field of adhesive compositions, such as coupling agents, leveling agents, crosslinking aids, plasticizers, softeners, fillers, antistatic agents, anti-aging agents, ultraviolet absorbers, antioxidants, light stabilizers, and surfactants. Such additives can be conventionally used by common methods and do not particularly characterize the present invention; therefore, a detailed explanation is omitted.

[0036] <<<Structure / Properties of Anisotropic Light-Diffusing Adhesive Layer>>> <<Orientation of needle-shaped fillers>> It is preferable that the needle-shaped fillers are dispersed in the adhesive layer in a unidirectional orientation.

[0037] In this invention, when an anisotropic light-diffusing adhesive layer is laminated between two polarizing plates whose absorption axes are 90° apart from each other, such that the absorption axis of one polarizing plate and the orientation axis of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer are parallel, the following expression is used to define the "degree of orientation," which is an indicator of orientation: (total light transmittance emitted from the other polarizing plate having an absorption axis not parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer) / (ratio of needle-shaped fillers in the anisotropic light-diffusing adhesive layer).

[0038] In this case, the smaller the degree of orientation, the less influence the anisotropic light-diffusing adhesive layer has on the fact that the transmission axes of the two polarizing plates are at a 90° angle to each other (crossed nicols), meaning that the orientation of the needle-shaped fillers is good. The degree of orientation is 30% or less, but a smaller degree of orientation is preferable, preferably 25% or less, and more preferably 15% or less. By setting the degree of orientation within this range, the diffusion of light by the needle-shaped fillers in the anisotropic light-diffusing layer is optimized, and the viewing angle expansion effect of the anisotropic light-diffusing adhesive layer can be enhanced. The lower limit of the degree of orientation is preferably 10%.

[0039] The total light transmittance (specific total light transmittance) of an anisotropic light-diffusing adhesive layer is measured by laminating the anisotropic light-diffusing adhesive layer between two crossed nicol polarizers. At this time, the absorption axis, which is perpendicular to the transmission axis of one polarizer, is positioned parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer. The measurement is then performed according to the method compliant with JIS K 7361. In this case, light is irradiated from the light source from one polarizer side, and the light transmittance is obtained by measuring the light transmittance of the other polarizer (the polarizer whose absorption axis and the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer are not parallel).

[0040] The specific total light transmittance is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less.

[0041] The specific total light transmittance can be adjusted by the degree of orientation of the filler, the viscosity of the paint, the thickness of the paint application, the application speed, and the type of adhesive resin and needle-shaped filler.

[0042] <<Thickness>> The thickness of the anisotropic light-diffusing adhesive layer is preferably 5 μm to 100 μm. By setting the thickness of the anisotropic light-diffusing adhesive layer within this range, the diffusion of light by the needle-shaped fillers within the anisotropic light-diffusing layer is optimized, making it possible to enhance the viewing angle expansion effect of the anisotropic light-diffusing adhesive layer.

[0043] <<<Method for manufacturing an anisotropic light-diffusing adhesive layer>>> The following is an example of a method for manufacturing an anisotropic light-diffusing adhesive layer.

[0044] First, a paint is prepared by stirring a solution containing the components that make up the anisotropic light-diffusing adhesive layer described above, and optionally containing a volatile solvent. Next, the paint is applied to a substrate such as a release film or various optical elements, and the solvent is dried and removed to form a composition. The release film or various optical elements are then laminated onto this composition, and if necessary, the composition is cured at room temperature or in a temperature environment of about 30°C to 60°C for about 1 day to 2 weeks to cure or stabilize it, thereby producing an anisotropic light-diffusing adhesive layer.

[0045] When applying a coating containing needle-shaped fillers, the shear force applied to the coating causes the needle-shaped fillers to orient themselves so that their major axis is almost aligned with the coating direction. This allows for the production of an anisotropic light-diffusing adhesive layer in which the needle-shaped fillers are dispersed and oriented in approximately the same direction. The degree of orientation of the needle-shaped fillers can be adjusted by the size of the needle-shaped fillers, the viscosity of the coating containing the needle-shaped fillers, the coating method, the coating speed, etc. Furthermore, the film thickness of the formed anisotropic light-diffusing adhesive layer can be adjusted by the amount of solvent in the coating, etc.

[0046] <<<Usage>>> The anisotropic light-diffusing adhesive layer can be applied to bonding the surface of the viewing-side polarizing plate (upper polarizing plate) of a display device to an optical functional layer such as a hard coat layer.

[0047] For example, if the display device is a liquid crystal display device, a display device with an excellent viewing angle expansion effect can be obtained by laminating a polarizing plate (the polarizing plate on the viewing side) of the liquid crystal display device such that the absorption axis of the upper polarizing plate is parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer.

[0048] The method for applying the anisotropic light-diffusing adhesive layer to a member to be adhered to may be to directly form the anisotropic light-diffusing adhesive layer on the member, or to form the anisotropic light-diffusing adhesive layer in a sheet-like manner on a release substrate (e.g., a film), adhere the anisotropic light-diffusing adhesive layer on the release substrate to the member, and then peel off the release substrate. [Examples]

[0049] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0050] <<Fabrication of anisotropic light-diffusing adhesive layer>> <Example 1> A paint 1 was prepared by mixing 100 parts by weight of acrylic resin 1 (solids concentration 40%, weight-average molecular weight 700,000, solvent: ethyl acetate), which is a copolymer containing an aromatic ring-containing (meth)acrylic acid ester monomer, 15 parts by weight of calcium carbonate whiskers (major axis 10 μm to 30 μm, minor axis 0.5 μm to 1.0 μm, refractive index in the major axis direction 1.49) as needle-shaped fillers, and 0.275 parts by weight of hexamethylene diisocyanate as a crosslinking agent. After mixing these, the mixture was diluted with ethyl acetate and stirred for 60 minutes to prepare a paint 1 with a viscosity of 3010 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped fillers in the major axis direction was 0.03. Next, paint 1 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 1. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 1, was laminated onto the composition 1, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 1 of Example 1, in which the anisotropic light-diffusing adhesive layer 1 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 1 was 15 μm.

[0051] <Example 2> Paint 2, with a viscosity of 1530 mPa·s, was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 1, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 1500 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the longitudinal direction was 0.03. Next, the paint 2 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 2. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 2, was laminated onto the composition 2, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 2 of Example 2, in which the anisotropic light-diffusing adhesive layer 2 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 2 was 15 μm.

[0052] <Example 3> A paint 3 was prepared by mixing 100 parts by weight of acrylic resin 2 (solids concentration 40%, weight-average molecular weight 600,000, solvent: ethyl acetate), which is a copolymer containing an aromatic ring-containing (meth)acrylic acid ester monomer, 20 parts by weight of calcium carbonate whiskers (major axis 10 μm to 30 μm, minor axis 0.5 μm to 1.0 μm, refractive index in the major axis direction 1.49) as needle-shaped fillers, and 0.125 parts by weight of hexamethylene diisocyanate as a crosslinking agent. After mixing these, the mixture was diluted with ethyl acetate and stirred for 60 minutes to prepare a paint 3 with a viscosity of 3040 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped fillers in the major axis direction was 0. Next, the paint 3 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 3. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 3, was laminated onto the composition 3, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 3 of Example 3, in which the anisotropic light-diffusing adhesive layer 3 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 3 was 15 μm.

[0053] <Example 4> Paint 4, with a viscosity of 1490 mPa·s, was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 3, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 1500 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the longitudinal direction was 0. Next, the paint 4 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 4. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 4, was laminated onto the composition 4, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 4 of Example 4, in which the anisotropic light-diffusing adhesive layer 4 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 4 was 15 μm.

[0054] <Example 5> A paint 5 was prepared by mixing 100 parts by weight of an acrylic resin 3 (solids concentration 40%, weight-average molecular weight 800,000, solvent: ethyl acetate) consisting of a copolymer containing an aromatic ring-containing (meth)acrylic acid ester monomer, 25 parts by weight of calcium carbonate whiskers (major axis 10 μm to 30 μm, minor axis 0.5 μm to 1.0 μm, refractive index in the major axis direction 1.49) as a needle-shaped filler, and 0.7 parts by weight of hexamethylene diisocyanate as a crosslinking agent. After mixing, the mixture was diluted with ethyl acetate and stirred for 60 minutes to prepare a paint 5 with a viscosity of 2950 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the major axis direction was 0.04. Next, the paint 5 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 5. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 5, was laminated onto the composition 5, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 5 of Example 5, in which the anisotropic light-diffusing adhesive layer 5 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 5 was 15 μm.

[0055] <Example 6> Paint 6 with a viscosity of 1510 mPa·s was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 5, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 1500 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the longitudinal direction was 0.04. Next, the paint 6 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 6. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 6, was laminated onto the composition 6, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 6 of Example 6, in which the anisotropic light-diffusing adhesive layer 6 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 6 was 15 μm.

[0056] <Comparative Example 1> Paint 7, with a viscosity of 790 mPa·s, was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 1, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 800 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the longitudinal direction was 0.03. Next, the paint 7 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 7. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 7, was laminated onto the composition 7, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 7 of Comparative Example 1, in which the anisotropic light-diffusing adhesive layer 7 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 7 was 15 μm.

[0057] <Comparative Example 2> Paint 8 with a viscosity of 805 mPa·s was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 3, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 800 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the major axis direction was 0. Next, the paint 8 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 8. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 8, was laminated onto the composition 8, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 8 of Comparative Example 2, in which the anisotropic light-diffusing adhesive layer 8 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 8 was 15 μm.

[0058] <Comparative Example 3> Paint 9 with a viscosity of 3010 mPa·s was prepared using the same method as for the preparation of the double-sided film-attached anisotropic light-diffusing adhesive layer 5, except that the amount of dilution with ethyl acetate and the stirring time of the paint material were changed so that the paint viscosity was 3000 mPa·s. At this time, the absolute value of the refractive index difference between the acrylic resin and the needle-shaped filler in the longitudinal direction was 0.04. Next, the paint 9 was applied to a 38 μm thick release PET film using an applicator, and then dried at 80°C for 2 minutes to form composition 9. A 38 μm thick release PET film, which was easier to peel than the release PET film applied to composition 9, was laminated onto the composition 9, and then cured at room temperature for 1 week to obtain the double-sided film-attached anisotropic light-diffusing adhesive layer 9 of Comparative Example 3, in which the anisotropic light-diffusing adhesive layer 9 was sandwiched between a pair of release PET films. The film thickness of the anisotropic light-diffusing adhesive layer 9 was 15 μm.

[0059] <<Rating>> The anisotropic light-diffusing adhesive layers with double-sided films obtained in Examples 1-6 and Comparative Examples 1-3 were evaluated as follows.

[0060] <Orientation evaluation by measuring the degree of orientation> Between two polarizing plates with absorption axes differing by 90° from each other (crossed nicol configuration), the anisotropic light-diffusing adhesive layers with double-sided films of the examples and comparative examples were laminated after peeling off the double-sided films, such that the absorption axis of one polarizing plate and the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer were parallel. Next, in accordance with JIS K 7361, the total light transmittance was measured using a haze meter (NDH7000, manufactured by Nippon Denshoku Industries Co., Ltd.). At this time, light from the light source was irradiated from the side where the absorption axis of one polarizer plate was parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer, and the total light transmittance of the other polarizer plate (a polarizer plate where the absorption axis of the polarizer plate and the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer were not parallel) was measured. From the obtained total light transmittance, the ratio of total light transmittance to (percentage of needle-shaped fillers in the anisotropic light-diffusing adhesive layer) was calculated, and this value was defined as the degree of orientation. Furthermore, a smaller degree of orientation value was considered to indicate better orientation of the needle-shaped filler.

[0061] <Evaluation of viewing angle characteristics by measuring contrast ratio in the front and 60° directions> After peeling off the release PET film, which is easily peelable, from the double-sided film-attached anisotropic light-diffusing adhesive layer of the examples and comparative examples, a TAC film was laminated. Subsequently, after peeling off the release PET film on the opposite side from the TAC film laminate, the anisotropic light-diffusing adhesive layer surface of the TAC film-attached anisotropic light-diffusing adhesive layer was laminated onto the surface of a VA-type liquid crystal display so that the absorption axis of the polarizing plate (viewing side) on the liquid crystal display and the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer were parallel. Using a luminance viewing angle meter (CONOMETER 80, manufactured by TEM Co., Ltd.) on the viewing side of a liquid crystal display with an anisotropic light-diffusing adhesive layer, the luminance for white display and the luminance for black display were measured in the range of -80° to 80° in the direction perpendicular to the orientation direction of the needle-shaped filler. The contrast ratio (white luminance / black luminance) was calculated from the measured values ​​of white luminance and black luminance at each angle. The contrast ratio in the front direction was calculated as (contrast ratio at 0°) / (contrast ratio at 0° without the TAC film anisotropic light-diffusing adhesive layer), and the contrast ratio in the 60° direction was calculated as (contrast ratio at 60°) / (contrast ratio at 0°). The relationship between the contrast ratios in each direction was graphed with the 60° direction contrast ratio on the X axis and the front direction contrast ratio on the Y axis, and is shown in Figure 1. Furthermore, the viewing angle characteristics were determined as follows, taking into account the balance of contrast ratios in each direction in Figure 1: "○" when the 60° direction contrast ratio (X) value is "X>15" and the front direction contrast ratio (Y) value is "Y>-2.67X+110", and "×" when the 60° direction contrast ratio (X) value is "X>15" and the front direction contrast ratio (Y) value is "Y≦-2.67X+110".

[0062] <Evaluation of reflectivity by reflectance measurement> After peeling off the release PET film, which is lightly peelable, from the double-sided film-attached anisotropic light-diffusing adhesive layer of the Examples and Comparative Examples, it was laminated to the TAC side of an LR-TAC film prepared according to Example 1 in Japanese Patent Publication No. 663471. Subsequently, after peeling off the release PET film on the opposite side of the LR-TAC film laminate, the anisotropic light-diffusing adhesive layer side of the LR-TAC film-attached anisotropic light-diffusing adhesive layer was laminated to the adhesive side of an adhesive-backed black PET film (Tomoegawa Paper Co., Ltd., "Kukkiri Mieru"). The reflectance was measured from the LR-TAC film side of the anisotropic light-diffusing adhesive layer with the LR-TAC film and adhesive-backed black PET film attached, using a spectrophotometer (Konica Minolta, CM-700d) in accordance with JIS Z 8722, and the resulting value was used as an indicator of the reflectivity characteristics.

[0063] The evaluation results are summarized in Table 1.

[0064] [Table 1]

[0065] As shown in Table 1, the anisotropic light-diffusing adhesive layer, which is an embodiment of the present invention, exhibited excellent viewing angle characteristics. In contrast, the anisotropic light-diffusing adhesive layer of the comparative example had inferior viewing angle characteristics because the degree of orientation of the needle-shaped fillers was inferior.

[0066] Based on the evaluation results of the examples, it was found that the anisotropic light-diffusing adhesive layer of the present invention is an anisotropic light-diffusing adhesive layer that is excellent in expanding the viewing angle.

[0067] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above. That is, other embodiments or various modifications that a person skilled in the art could conceive of within the scope of the invention described in the claims are also understood to fall within the technical scope of the present invention.

Claims

1. An anisotropic light-diffusing adhesive layer containing an adhesive and needle-shaped fillers, The needle-shaped fillers are dispersed in the anisotropic light-diffusing adhesive layer, oriented in one direction. When the anisotropic light-diffusing adhesive layer is laminated between two polarizing plates whose absorption axes are 90° apart from each other, such that the absorption axis of one polarizing plate and the orientation axis of the needle-shaped filler are parallel, the degree of orientation shown by the following formula (1) is 30% or less. The aspect ratio (major axis / minor axis) of the needle-shaped filler is 10 to 60. An anisotropic light-diffusing adhesive layer characterized in that, when the total solid content of the anisotropic light-diffusing adhesive layer is 100 parts by weight, the content of the needle-shaped filler is 1 to 50 parts by weight. Degree of orientation = (Total light transmittance emitted from the other polarizer having an absorption axis not parallel to the orientation direction of the needle-shaped fillers in the anisotropic light-diffusing adhesive layer) / (Percentage of needle-shaped fillers in the anisotropic light-diffusing adhesive layer) ... (1)

2. The anisotropic light-diffusing adhesive layer according to claim 1, characterized in that the needle-shaped filler has a short diameter of 0.1 μm to 20 μm and a long diameter of 2 μm to 5000 μm.

3. The anisotropic light-diffusing adhesive layer according to claim 1 or 2, characterized in that the adhesive is an acrylic resin adhesive comprising a copolymer containing an aromatic ring-containing (meth)acrylic acid ester monomer.

4. The anisotropic light-diffusing adhesive layer according to claim 3, characterized in that the absolute value of the difference between the refractive index of the needle-shaped filler in the longitudinal direction and the refractive index of the acrylic resin is 0.07 or less.

5. The anisotropic light-diffusing adhesive layer according to any one of claims 1 to 4, characterized in that the weight-average molecular weight of the adhesive is 200,000 to 1,000,000.

6. An anisotropic light-diffusing adhesive layer according to any one of claims 1 to 5, characterized in that the film thickness is 5 μm to 100 μm.

7. The anisotropic light-diffusing adhesive layer according to any one of claims 1 to 6, characterized in that the refractive index of the needle-shaped filler in the major axis direction is 1.48 to 1.58 and the refractive index in the minor axis direction is 1.60 to 1.

70.

8. A display device characterized in that an anisotropic light-diffusing adhesive layer according to any one of claims 1 to 7 is laminated on an upper polarizing plate of a liquid crystal display device such that the orientation axis of the needle-shaped filler in the anisotropic light-diffusing adhesive layer is parallel to the absorption axis of the upper polarizing plate.

Images