Backlight unit and image display device

Abstract

The present application provides a backlight unit in which bright light is emitted and light leakage in a slanting direction is suppressed. The backlight unit of the present application includes a light source, a light guide plate disposed opposite the light source and having an end surface on which light from the light source is incident and an exit surface from which the light is emitted after being incident, a housing that houses the light source and the light guide plate and has a front surface portion and a back surface portion, and an optical adhesive film disposed between the front surface portion of the housing on the light source side and the light guide plate and adhered to the light guide plate. The optical adhesive film has, in order from the front surface portion side of the housing, a light absorbing layer, a substrate, a low refractive index layer, and an adhesive layer.

Description

TECHNICAL FIELD

[0001] The present application relates to a backlight unit. BACKGROUND

[0002] Liquid crystal display devices are used in a wide range of applications such as mobile terminals, personal computers, car navigation systems, and televisions. In liquid crystal display devices, a planar backlight that is disposed on the back side of a liquid crystal panel and emits light in a planar shape is used most often. In the planar backlight, a light source is disposed opposite an end surface of a light guide plate, and a side light method in which light incident from the end surface is emitted from an emission surface of the light guide plate is used most often. In the backlight of the side light method, the back side of the light guide plate is typically attached to a flexible printed circuit board (FPC) with a white diffusion tape interposed therebetween. On the other hand, the front side of the light guide plate is typically supported / fixed by a front surface portion of a housing of the light guide plate, as a result of which the entire light guide plate is fixed inside the housing. According to such a structure, there is a problem of causing a loss of luminance. In order to suppress such a loss of luminance, a technique of attaching the light guide plate to the FPC with a double-coated adhesive film having a low refractive index layer having a refractive index of 1.25 or less has been proposed (Patent Literature 1). However, in the backlight unit, further improvement in performance (e.g., loss of luminance (brightness), light leakage) is continuously required.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: International Publication No. 2019 / 151073 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] The present application has been achieved in order to solve the above-described conventional problems, and a main object thereof is to provide a backlight unit that is bright and in which light leakage in a tilted direction is suppressed.

[0008] MEANS FOR SOLVING PROBLEMS

[0009] The backlight unit of the embodiment of the present application includes a light source, a light guide plate disposed opposite the light source and having an end surface into which light from the light source is incident and an emission surface from which the light after the incidence is emitted, a housing that houses the light source and the light guide plate and has a front surface portion and a back surface portion, and an optical adhesive film disposed between the front surface portion of the housing on the light source side and the light guide plate and attached to the light guide plate. The optical adhesive film has, in order from the front surface portion side of the housing, a light absorbing layer, a substrate, a low refractive index layer, and an adhesive layer.

[0010] In one embodiment, the backlight unit described above further has a reflection plate disposed on the back surface side of the light guide plate in the housing. In this case, the reflection plate can be disposed together with the light guide plate with an air layer therebetween; the reflection plate and the light guide plate can be attached together with a double-coated adhesive film having a low refractive index layer therebetween.

[0011] In one embodiment, the light source is an LED light source.

[0012] Effects of the Invention

[0013] According to the present application, by disposing the optical adhesive film having the light absorbing layer and the low refractive index layer in a given positional relationship between the front surface portion of the housing on the light source side and the light guide plate in the backlight unit, a bright backlight unit in which the light leakage in the oblique direction is suppressed can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a cross-sectional schematic view of a backlight unit according to one embodiment of the present application.

[0015] Figure 2 is an enlarged cross-sectional schematic view of the II portion of the backlight unit of Figure 1

[0016] SYMBOL EXPLANATION

[0017] 10 light source

[0018] 20 light guide plate

[0019] 30 housing

[0020] 31 front surface portion

[0021] 32 back surface portion

[0022] 40 optical adhesive film

[0023] 41 light absorbing layer

[0024] 42 substrate

[0025] 43 low refractive index layer

[0026] 44 adhesive layer

[0027] 100 backlight unit DETAILED DESCRIPTION

[0028] Hereinafter, embodiments of the present application will be described, but the present application is not limited to these embodiments.

[0029] A. Overall structure of the backlight unit

[0030] Figure 1 ​FIG. 1 is a cross-sectional view of a backlight unit according to an embodiment of the present application. Figure 2 Figure 1 FIG. 2 is an enlarged cross-sectional view of a portion II of the backlight unit of FIG. 1. The illustrated backlight unit 100 includes a light source 10, a light guide plate 20, a housing 30, and an optical adhesive film 40. The housing 30 has a front surface portion 31 and a back surface portion 32, and houses the light source 10 and the light guide plate 20. The back surface portion 32 is a box-shaped portion that is open at the top (front surface side), and in the illustrated example, the front surface portion 31 protrudes inward from the back surface portion at both ends opposite the back surface portion 32. Note that the front surface portion 31 can not protrude inward at these ends. Typically, the front surface portion 31 is bonded to the back surface portion 32 to form a single unit. Typically, the housing 30 can have the light guide plate 20 fitted therein. Typically, an arbitrary optical member (not shown) can be placed (crossed) on the step portion (portion protruding inward) of the front surface portion. Note that the step portion can not be provided on the front surface portion. As the optical member, for example, a diffusion sheet, a prism sheet, or the like can be given. A plurality of diffusion sheets and / or prism sheets can be placed. Note that the housing 30 of the illustrated example is a back surface side portion of a housing that houses an entire image display device.

[0031] The light source 10 is disposed on one end portion side of the light guide plate 20 so as to face the end surface 20a of the light guide plate. Thus, the backlight unit is typically an edge light type. Typically, the light source 10 is surrounded by a reflector (not shown). As the light source, an arbitrary appropriate configuration can be used. As specific examples, a light emitting diode (LED), a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and a hot cathode fluorescent lamp (HCFL) can be given. In one embodiment, an LED light source can be used. Typically, the LED light source can be configured by arranging LEDs (point light sources) in a line.

[0032] ​The light guide plate 20 has an end surface 20a on which light from the light source is incident, and an exit surface 20b from which the light after the incidence is emitted. The end surface 20a of the light guide plate 20 is disposed opposite to the light source 10. More specifically, the light guide plate 20 guides the light incident to the end surface 20a from the light source 10 to the side of the end surface 20a opposite to the side of the end surface 20a on which the light is incident, while internally receiving a reflection action or the like, and slowly emits the light from the exit surface 20b in this light guiding process. Typically, an exit pattern is provided on the exit surface 20b. As the exit pattern, for example, a concave-convex shape can be given. Further, typically, a light extraction pattern is provided on the side of the light guide plate opposite to the exit surface. As the light extraction pattern, for example, a white dot can be given. Note that any appropriate configuration can be employed as the light guide plate. In one embodiment, the back surface side of the light guide plate 20 can be attached to the flexible printed circuit board for the light source via any appropriate mechanism, such as an adhesive tape.

[0033] In the embodiment of the present application, the optical adhesive film 40 is disposed between the front surface portion 31 of the housing 30 on the side of the light source 10 and the light guide plate 20. The optical film 40 has, in order from the side of the front surface portion 31 of the housing 30, a light absorbing layer 41, a substrate 42, a low refractive index layer 43, and an adhesive layer 44. A portion of the optical adhesive film 40 is attached to the light guide plate 20 with the adhesive layer 44 interposed therebetween. If such a configuration is employed, by the total reflection function of the low refractive index layer of the optical adhesive film, the light guide plate can guide the light by reflection even in a portion not in contact with the space (air portion) (i.e., a portion in contact with the front surface portion of the housing), and can absorb the light incident to the low refractive index layer at an incident angle of equal to or more than the critical angle and not reflected in the low refractive index layer by the light absorbing layer. As a result, the light use efficiency can be further improved, and thus the brightness is improved, and the light that cannot be used and leaks out can be absorbed and blocked, and thus the light leakage (particularly, the light leakage in the oblique direction) can be suppressed. Note that in the illustrated example, the optical adhesive film protrudes from the light guide plate toward the side of the light source, but the end surface of the optical adhesive film can be flush with the end surface of the light guide plate.

[0034] The backlight unit 100 can further have a reflection plate 50 disposed on the back surface side of the light guide plate 20 within the housing 30. The reflection plate 50 can be disposed together with the light guide plate 20 with an air layer therebetween (i.e., can be separately disposed with the light guide plate and housed in the housing); the reflection plate 50 and the light guide plate 20 can be bonded together with a double-sided adhesive film (not shown) having a low refractive index layer therebetween. The illustrated example shows a configuration in which an air layer is interposed between the reflection plate 50 and the light guide plate 20. As the reflection plate 50, any appropriate reflection plate can be used. For example, the reflection plate can be a specular reflection plate or a diffuse reflection plate. As specific examples of the reflection plate, a resin sheet (e.g., an acrylic plate) having a high reflectance, a metal sheet or foil such as aluminum, stainless steel, or the like, an evaporation sheet in which aluminum, silver, or the like is evaporated on a substrate such as a resin film, a laminate of a substrate such as a resin film and a metal foil such as aluminum, and a resin film having voids (pores) formed therein can be given. By providing the reflection plate 50, the light utilization efficiency of the backlight unit can be improved.

[0035] Hereinafter, the optical adhesive film 40 and the double-sided adhesive film will be described in detail.

[0036] B. Optical adhesive film

[0037] B-1. Outline of optical adhesive film

[0038] As described in item A above, the optical adhesive film 40 has, in order from the front surface portion 31 side of the housing 30, a light absorbing layer 41, a substrate 42, a low refractive index layer 43, and an adhesive layer 44.

[0039] B-2. Light absorbing layer

[0040] The light absorbing layer 41 can be an appearance design layer in which a given design is implemented, or can be a solid colored layer. The light absorbing layer is preferably a solid colored layer, and more preferably a black colored layer. By providing such a light absorbing layer closer to the front surface side than the low refractive index layer, light that is incident to the low refractive index layer at an incident angle of 45 degrees or more and does not reflect in the low refractive index layer can be well absorbed and blocked.

[0041] The total light transmittance of the light absorbing layer at a thickness of 3 μm to 5 μm is preferably 0.01% or less, and more preferably 0.008% or less. The total light transmittance of the light absorbing layer is more preferably smaller, and the lower limit thereof can be, for example, 0.001%. When the total light transmittance of the light absorbing layer is in such a range, a more excellent light absorbing and blocking function can be achieved.

[0042] The thickness of the light absorbing layer is preferably 0.1 μm to 300 μm. If the thickness is such, a desired total light transmittance (as a result of the light absorbing and blocking functions) can be easily achieved by appropriately selecting the constituent material.

[0043] The light-absorbing layer can be formed by any suitable printing method using any suitable ink or paint. As specific examples of the printing method, there are inkjet printing, gravure printing, offset printing, screen printing, and transfer printing from a transfer sheet.

[0044] Typically, the ink or paint used contains a binder, a colorant, a solvent, and any suitable additive that can be used as needed. As the binder, there are chlorinated polyolefins (e.g., chlorinated polyethylene, chlorinated polypropylene), polyester-based resins, urethane-based resins, acrylic-based resins, vinyl acetate resins, vinyl chloride-vinyl acetate copolymers, and cellulose-based resins. The binder resin can be used alone or in combination with two or more. The binder resin can be a thermally polymerizable resin or a photopolymerizable resin. In one embodiment, the binder resin is an (meth)acrylic-based resin, preferably an acrylic-based resin containing a multifunctional monomer (e.g., pentaerythritol triacrylate) as a copolymerization component. By using an acrylic-based resin containing a multifunctional monomer as a copolymerization component, a colored layer having a suitable modulus of elasticity can be formed.

[0045] As the colorant, any suitable colorant can be used as needed. As specific examples of the colorant, there are inorganic pigments such as titanium white, zinc white, carbon black, iron black, red iron oxide, molybdate red, ultramarine blue, cobalt blue, chromium yellow, and titanium yellow; organic pigments or dyes such as phthalocyanine blue, indanthrone blue, isoindolinone yellow, benzidine yellow, quinacridone red, disazo red, perylene red, and aniline black; metallic pigments including flaky foils of aluminum, brass, and the like; and pearlescent pigments (pearl pigments) including titanium dioxide-coated mica, basic lead carbonate, and the like. In the case of forming a black colored layer, carbon black, iron black, and aniline black can be suitably used. In this case, it is preferable to use the colorants in combination. The reason is that they can form a colored layer that uniformly absorbs visible light in a wide range without color, i.e., a full black colored layer. For example, in addition to the above-described colorants, an azo compound and / or a quinone compound can be used. In one embodiment, the colorant contains carbon black as a main component and another colorant (e.g., an azo compound and / or a quinone compound). If configured in this way, a colored layer that is colorless and has excellent stability over time can be formed. In the case of forming a black colored layer, the colorant can be used in a proportion of preferably 1 part by weight to 200 parts by weight, relative to 100 parts by weight of the binder resin. In this case, the proportion of carbon black in the colorant is preferably 1% to 100%. By using the colorant (particularly carbon black) in such a proportion, a colored layer that has a very small total light transmittance and has excellent stability over time can be formed.

[0046] B-3. Substrate

[0047] The substrate 42 is typically composed of a film or a plate of a resin, preferably a transparent resin. As representative examples of such a resin, thermoplastic resins, reactive resins (e.g., ionizing radiation-curable resins) can be cited. As specific examples of the thermoplastic resins, polymethyl methacrylate (PMMA), polyacrylonitrile, and the like (meth)acrylic resins, polycarbonate (PC) resins, polyester resins such as PET, cellulose-based resins such as triacetyl cellulose (TAC), cyclic polyolefin-based resins, styrene-based resins can be cited. As specific examples of the ionizing radiation-curable resins, epoxy acrylate-based resins, urethane acrylate-based resins can be cited. These resins can be used alone or in combination of two or more.

[0048] The thickness of the substrate is, for example, 10 μm to 100 μm, preferably 10 μm to 50 μm.

[0049] The refractive index of the substrate is preferably 1.47 or greater, more preferably 1.47 to 1.60, further preferably 1.47 to 1.55. If it is in this range, the image display unit can be introduced without adversely affecting the light guided from the light guide plate.

[0050] B-4. Low-refractive-index layer

[0051] The low-refractive-index layer typically has voids inside. The void fraction of the low-refractive-index layer is, as described above, 40 vol% or greater, typically 50 vol% or greater, preferably 70 vol% or greater, more preferably 80 vol% or greater. On the other hand, the void fraction is, for example, 90 vol% or less, preferably 85 vol% or less. By making the void fraction within the above range, the refractive index of the low-refractive-index layer can be brought to an appropriate range. The void fraction is a value calculated from the value of the refractive index measured using an ellipsometer, using the Lorentz-Lorenz's equation.

[0052] The refractive index of the low-refractive-index layer is preferably 1.30 or less, more preferably 1.20 or less, further preferably 1.15 or less. The lower limit of the refractive index can be, for example, 1.01. If it is in this range, very excellent light use efficiency can be achieved in the backlight unit. The refractive index refers to the refractive index measured at a wavelength of 550 nm, unless otherwise specified. The refractive index is a value measured by the method described in "(1) Refractive index of low-refractive-index layer" of the following Examples.

[0053] The low-refractive-index layer can employ any appropriate configuration as long as it has the above-described desired void ratio and refractive index. The low-refractive-index layer can be preferably formed by coating or printing, or the like. As a material constituting the low-refractive-index layer, for example, the materials described in International Publication No. 2004 / 113966, Japanese Patent Application Publication No. 2013-254183, and Japanese Patent Application Publication No. 2012-189802 can be employed. Specifically, for example, the following can be listed: a silicon dioxide-based compound; a hydrolyzable silane-based compound, and a partial hydrolyzate and a dehydration condensate thereof; an organic polymer; a silane alcohol group-containing silicon compound; active silica obtained by bringing a silicate with an acid, an ion exchange resin; a polymerizable monomer (for example, a (meth)acrylic monomer, and a styrene monomer); a curable resin (for example, a (meth)acrylic resin, a fluorine-containing resin, and a urethane resin); and a combination thereof. The low-refractive-index layer can be formed by coating or printing, or the like, of a solution or a dispersion liquid of such a material.

[0054] The size of the voids (pores) in the low-refractive-index layer refers to the diameter of the long axis of the voids (pores) and the diameter of the short axis. The size of the voids (pores) is, for example, 2 nm to 500 nm. The size of the voids (pores) is, for example, 2 nm or more, preferably 5 nm or more, more preferably 10 nm or more, and further preferably 20 nm or more. On the other hand, the size of the voids (pores) is, for example, 500 nm or less, preferably 200 nm or less, and more preferably 100 nm or less. The size of the voids (pores) is, for example, in the range of 2 nm to 500 nm, preferably 5 nm to 500 nm, more preferably 10 nm to 200 nm, and further preferably 20 nm to 100 nm. The size of the voids (pores) can be adjusted to a desired size according to the purpose and use, or the like. The size of the voids (pores) can be quantified by the BET test method.

[0055] The size of the voids (pores) can be quantified by the BET test method. Specifically, 0.1 g of a sample (the formed void layer) is put in a capillary of a specific surface area measuring device (manufactured by Micromeritics Corporation: ASAP2020), and the gas in the void structure is degassed by performing 24-hour reduced pressure drying at room temperature. Then, the adsorption isotherm is plotted by adsorbing nitrogen to the above-described sample, and the fine pore distribution is calculated. Thus, the void size can be evaluated.

[0056] The haze of the low-refractive-index layer is, for example, less than 5%, and preferably less than 3%. On the other hand, the haze is, for example, 0.1% or more, and preferably 0.2% or more. The range of the haze is, for example, 0.1% or more and less than 5%, and preferably 0.2% or more and less than 3%. The haze can be measured by the method described below, for example. Note that the haze is an index of the transparency of the low-refractive-index layer.

[0057] The void layer (low refractive index layer) was cut to a size of 50 mm x 50 mm, and set in a haze meter (manufactured by Murakami Color Research Laboratory Co., Ltd.: HM-150) to measure the haze. The haze value was calculated by the following equation.

[0058] Haze (%) = [Diffuse transmittance (%) / Total light transmittance (%)] x 100 (%)

[0059] As the low refractive index layer having a void inside as described above, for example, a low refractive index layer having a porous layer and / or an air layer in at least a part thereof can be exemplified. The porous layer typically contains an aerogel, and / or a particle (for example, a hollow particle and / or a porous particle). The low refractive index layer can preferably be a nanoporous layer (specifically, a porous layer in which 90% or more of the micropores have a diameter in the range of 10 -1 nm to 10 3 nm).

[0060] As the particle, any appropriate particle can be used. The particle is typically formed of a silica-based compound. As the shape of the particle, for example, a spherical shape, a plate shape, a needle shape, a string shape, and a grape cluster shape can be exemplified. As the particle of the string shape, for example, a particle in which a plurality of particles having a spherical shape, a plate shape, or a needle shape are connected in a rosary shape; a short fiber-like particle (for example, a short fiber-like particle described in Japanese Patent Application Laid-Open No. 2001-188104), and a combination thereof can be exemplified. The particle of the string shape can be a straight chain shape or a branched shape. As the particle of the grape cluster shape, for example, a particle in which a plurality of spherical, plate, and needle-shaped particles are agglomerated into a grape cluster shape can be exemplified. The shape of the particle can be confirmed by observation using, for example, a transmission electron microscope.

[0061] The thickness of the low refractive index layer is preferably in the range of 0.2 μm to 5 μm, and more preferably in the range of 0.3 μm to 3 μm. If the thickness of the low refractive index layer is in this range, the damage prevention effect of the present application becomes significant. In addition, the above-described desired thickness ratio can be easily achieved.

[0062] Hereinafter, an example of a specific configuration of the low refractive index layer will be described. The low refractive index layer of the present embodiment contains one or more structural units that form a fine void structure, which are chemically bound together by catalytic action. As the shape of the structural unit, for example, a particle shape, a fiber shape, a rod shape, and a flat plate shape can be exemplified. The structural unit can have only one shape, or can have two or more shapes in combination. Hereinafter, the case in which the low refractive index layer is a void layer of a porous body in which the above-described microporous particles are chemically bound together will be mainly described.

[0063] Such a void layer can be formed by, for example, causing the microporous particles to chemically bond to each other in the void layer forming step. Note that in the embodiments of the present application, the shape of the "particles" (e.g., the microporous particles described above) is not particularly limited, and can be, for example, spherical or another shape. In addition, in the embodiments of the present application, the microporous particles described above can be, for example, sol-gel bead-like particles, nanoparticles (hollow nanosilica / nano hollow sphere particles), nanofibers, or the like. The microporous particles typically include an inorganic substance. As specific examples of the inorganic substance, the following can be given: silicon (Si), magnesium (Mg), aluminum (Al), titanium (Ti), zinc (Zn), and zirconium (Zr). These can be used alone or in combination with two or more. In one embodiment, the microporous particles described above are, for example, microporous particles of a silicon compound, and the porous body described above is, for example, a silicone porous body. The microporous particles of the silicon compound include, for example, a pulverized body of a gel-like silica compound. In addition, as other forms of the low-refractive layer having a porous layer and / or an air layer in at least a portion thereof, the following can be given: a void layer formed of a fibrous substance such as a nanofiber, the fibrous substance being entangled with each other to form voids and form a layer. The manufacturing method of such a void layer is not particularly limited, and is the same as in the case of the void layer of the porous body in which the microporous particles described above are chemically bonded to each other, for example. Further, as other forms, the following can be given: a void layer using hollow nanoparticles, a nanoclay, a void layer formed using hollow nanohollow spheres, magnesium fluoride. The void layer can be a void layer formed of a single constituent substance or a void layer formed of a plurality of constituent substances. The void layer can be constituted of a single form described above or can be constituted of a plurality of forms described above.

[0064] In the present embodiment, the porous structure of the porous body can be, for example, a continuous bubble structure in which the pore structures are continuous. In the case of a continuous bubble structure, for example, in the above-described silicone porous body, it refers to a state in which the pore structures are three-dimensionally connected, and can also be said to be a state in which the internal voids of the pore structures are continuous. By providing the porous body with a continuous bubble structure, the void ratio can be increased. On the other hand, in the case of using individual bubble particles (particles each having a pore structure) such as hollow silica, it is not possible to form a continuous bubble structure. On the other hand, in the case of using, for example, silica gel particles (pulverized products of a gel-like silicon compound that forms a sol), since the particles have a three-dimensional dendritic structure, the dendritic particles can settle and deposit in a coating film (a coating film of a sol containing pulverized products of a gel-like silicon compound), and thus a continuous bubble structure can be easily formed. The low-refractive-index layer more preferably has a monolithic structure in which a plurality of pore distributions are included. The monolithic structure refers to a hierarchical structure, which includes, for example, a structure in which nano-sized fine voids are present, and a continuous bubble structure in which such nano voids are aggregated. In the case of forming a monolithic structure, for example, the fine voids can be used to impart strength to the film, and the coarse continuous bubble voids can be used to impart a high void ratio, so that both the film strength and the high void ratio can be achieved. Such a monolithic structure is preferably formed by controlling the pore distribution of the void structure generated in a gel (a gel-like silicon compound) in a stage prior to pulverization into silica gel particles. In addition, for example, by controlling the particle size distribution of the silica gel particles after pulverization to be a desired size when the gel-like silicon compound is pulverized, a monolithic structure can be formed.

[0065] The low-refractive-index layer contains, for example, as described above, pulverized products of a gel-like compound, and the pulverized products are chemically bound to each other. The form of the chemical binding (chemical bond) of the pulverized products to each other in the low-refractive-index layer is not particularly limited, and examples include cross-linking bonds, covalent bonds, and hydrogen bonds.

[0066] The volume average particle diameter of the above-described pulverized products in the low-refractive-index layer is, for example, 0.10 μm or more, preferably 0.20 μm or more, and more preferably 0.40 μm or more. On the other hand, the volume average particle diameter is, for example, 2.00 μm or less, preferably 1.50 μm or less, and more preferably 1.00 μm or less. The range of the volume average particle diameter is, for example, 0.10 μm to 2.00 μm, preferably 0.20 μm to 1.50 μm, and more preferably 0.40 μm to 1.00 μm. The particle size distribution can be measured by, for example, a particle size distribution evaluation device such as a dynamic light scattering method, a laser diffraction method, and the like, and an electron microscope such as a scanning electron microscope (SEM), a transmission electron microscope (TEM), and the like. Note that the volume average particle diameter is an index of the deviation of the particle size of the pulverized products.

[0067] The type of the gel-like compound is not particularly limited. As the gel-like compound, a gel-like silicon compound can be exemplified.

[0068] In addition, in the low refractive index layer (void layer), for example, it is preferable that silicon atoms contained form siloxane bonds. As a specific example, the proportion of unbound silicon atoms (i.e., residual silanol) among all silicon atoms contained in the void layer is, for example, less than 50%, preferably 30% or less, and more preferably 15% or less.

[0069] Hereinafter, an example of a method of forming such a low refractive index layer will be described.

[0070] Typically, the method includes a precursor forming step of forming a precursor of a void structure as a low refractive index layer (void layer) on a resin film, and a cross-linking reaction step of inducing a cross-linking reaction inside the precursor after the precursor forming step. The method further includes a containing liquid preparation step of preparing a containing liquid (hereinafter sometimes referred to as a "microparticle-containing liquid" or simply a "containing liquid") containing microparticles, and a drying step of drying the containing liquid, in which the microparticles in the dried body are chemically bonded to each other to form the precursor in the precursor forming step. The containing liquid is not particularly limited, and is, for example, a suspension containing microparticles. Note that, hereinafter, a case in which the microparticles are a pulverized product of a gel-like compound and the void layer contains a porous body (preferably, a silicone porous body) of the pulverized product of the gel-like compound will be mainly described. Note that, in a case other than the case in which the microparticles are a pulverized product of a gel-like compound, a low refractive index layer can also be formed in the same manner.

[0071] According to the above-described method, a low refractive index layer (void layer) having, for example, a very low refractive index can be formed. The reason therefor can be inferred as follows, for example. However, this inference is not limiting on the method of forming a low refractive index layer.

[0072] The above-described pulverized product is obtained by pulverizing a gel-like silicon compound, and thus the three-dimensional structure of the gel-like silicon compound before pulverization is formed in a state of being dispersed in a three-dimensional basic structure. Further, in the above-described method, by applying the pulverized product of the gel-like silicon compound to a resin film, a precursor of a porous structure based on the three-dimensional basic structure can be formed. That is, according to the above-described method, a new porous structure (three-dimensional basic structure) based on the application of the pulverized product, which is different from the three-dimensional structure of the gel-like silicon compound, can be formed. Thus, in the finally obtained void layer, a low refractive index that functions to the same extent as an air layer, for example, can be achieved. In addition, in the above-described method, the pulverized products are chemically bonded to each other, and thus the three-dimensional basic structure can be immobilized. Thus, although the finally obtained void layer is a structure having voids, sufficient strength and flexibility can be maintained.

[0073] The detailed circumstances of the specific configuration of the low refractive index layer and the method of forming the same are described in, for example, International Publication No. 2019 / 151073. The description of this publication is incorporated herein by reference.

[0074] B-5. Adhesive layer

[0075] The adhesive layer has a hardness such that, under normal conditions, the adhesive constituting the adhesive layer will not penetrate into the voids of the low-refractive-index layer. The storage modulus of the adhesive layer at 23°C is, for example, 1.0 × 10⁻⁶. 5 (Pa)~1.0×10 7 (Pa), for example, 1.1 × 10 5 (Pa) or above, 1.2×10 5 (Pa) or above, 1.3×10 5 (Pa) or above, 1.4×10 5 (Pa) or above, 1.5×10 5 (Pa) or above, 1.6×10 5 (Pa) or above, 1.7×10 5 (Pa) or above, 1.8×10 5 (Pa) or above, 1.9×10 5 (Pa) or above or 2.0×10 5 (Pa) or higher, and 1.0 × 10 7 (Pa) below, 5.0×10 6 (Pa) or less, 1.0×10 6 (Pa) or below or 5.0×10 5 (Pa) or less, preferably 1.3 × 10 5 (Pa)~1.0×10 6 (Pa), more preferably 1.5 × 10 5 (Pa)~5.0×10 5 (Pa). The storage modulus was determined as follows: based on the method described in JIS K7244-1 "Plastics - Test Method for Dynamic Mechanical Properties", the measurement was performed at a frequency of 1 Hz, within a range of -50°C to 150°C at a heating rate of 5°C / min, and the value at 23°C was recorded. By setting the storage modulus of the adhesive layer adjacent to the low-refractive-index layer to the range described above, it is possible to prevent the adhesive from entering the voids of the low-refractive-index layer. Therefore, the refractive index of the low-refractive-index layer can be kept at a low level, and this effect can be maintained.

[0076] As the adhesive constituting the adhesive layer, any appropriate adhesive having the characteristics as described above can be used. As the adhesive, an acrylic adhesive (acrylic adhesive composition) can be typically exemplified. The acrylic adhesive composition typically contains a (meth)acrylic polymer as a main component (base polymer). In the solid content of the adhesive composition, the (meth)acrylic polymer can be contained in the adhesive composition at a ratio of 50% by weight or more, preferably 70% by weight or more, and more preferably 90% by weight or more, for example. The (meth)acrylic polymer contains an alkyl (meth)acrylate as a monomer unit as a main component. Note that the (meth)acrylate refers to an acrylate and / or a methacrylate. As the alkyl group of the alkyl (meth)acrylate, a linear or branched alkyl group having 1 to 18 carbon atoms can be exemplified. The average number of carbon atoms of the alkyl group is preferably 3 to 9. As the monomer constituting the (meth)acrylic polymer, in addition to the alkyl (meth)acrylate, a carboxyl group-containing monomer, a hydroxyl group-containing monomer, an amide group-containing monomer, an aromatic ring-containing (meth)acrylate, a heterocyclic ring-containing (meth)acrylate, and the like can be exemplified as a copolymerizable monomer. The copolymerizable monomer is preferably a hydroxyl group-containing monomer and / or a heterocyclic ring-containing (meth)acrylate, and more preferably an N-acryloylmorpholine. The acrylic adhesive composition can preferably contain a silane coupling agent and / or a crosslinking agent. As the silane coupling agent, an epoxy group-containing silane coupling agent can be exemplified, for example. As the crosslinking agent, an isocyanate-based crosslinking agent, a peroxide-based crosslinking agent, and the like can be exemplified, for example. Details of such an adhesive layer or acrylic adhesive composition are described in, for example, Japanese Patent No. 4140736, the disclosure of which is incorporated herein by reference.

[0077] The thickness of the adhesive layer is preferably 3 μm to 30 μm, and more preferably 5 μm to 10 μm. If the thickness of the adhesive layer is in this range, the following advantages are obtained: sufficient adhesion is obtained, and the influence of the thickness of the adhesive layer on the overall thickness is small.

[0078] C. Double-Sided Adhesive Film

[0079] Typically, the double-sided adhesive film has, in this order, a first adhesive layer, a substrate, a low refractive index layer, and a second adhesive layer. The substrate and the low refractive index layer are the same as those of the optical adhesive film described in the above items B-3 and B-4. The second adhesive layer is the same as the adhesive layer of the optical adhesive film described in the above item B-5. The first adhesive layer is composed of any appropriate adhesive. The first adhesive layer can be the same as or different from the second adhesive layer.

[0080] D. Image Display Device

[0081] The backlight unit described above can be applied to an image display device (e.g., liquid crystal display, etc.). Therefore, embodiments of the present application also include such an image display device. The image display device has the backlight unit described above, and an image display panel disposed on the exit surface side of the light guide plate.

[0082] Embodiments

[0083] Hereinafter, the present application will be specifically described by way of examples, but the present application is not limited to these examples. Note that, unless otherwise specified, "%" and "parts" in the examples are on a weight basis.

[0084] (1) Refractive index of low-refractive layer

[0085] After forming the low-refractive layer on the acrylic film, it was cut into a size of 50 mm x 50 mm, and was attached to the surface of a glass plate (thickness: 3 mm) via an adhesive layer. The back surface central portion (diameter of about 20 mm) of the glass plate was fully painted with a black marker, and a sample was obtained in which no reflection occurred on the back surface of the glass plate. The sample was set in an ellipsometer (manufactured by J. A. Woollam Japan Co.: VASE), and the refractive index was measured at a wavelength of 550 nm and an incident angle of 50 to 80 degrees.

[0086] (2) White brightness

[0087] A liquid crystal display device into which the backlight unit obtained in the examples and comparative examples was incorporated was set to white display of the entire screen, and the brightness in the front direction was measured using a cone polariscope manufactured by AUTRONIC MELCHERS Co.

[0088] (3) Light leakage in oblique direction

[0089] A liquid crystal display device into which the backlight unit obtained in the examples and comparative examples was incorporated was set to black display of the entire screen, and the light leakage in the oblique direction was observed by the naked eye.

[0090] [Production Example 1] Preparation of coating liquid for forming low-refractive layer

[0091] (1) Gelation of silicon compound

[0092] Methyltrimethoxysilane (MTMS) 0.95 g as a precursor of a silicon compound was dissolved in 2.2 g of dimethyl sulfoxide (DMSO), and a mixed solution A was prepared. To the mixed solution A, 0.5 g of an oxalic acid aqueous solution of 0.01 mol / L was added, and stirring was performed at room temperature for 30 minutes, whereby MTMS was hydrolyzed, and a mixed solution B containing trihydroxymethylsilane was produced.

[0093] After adding 28% by weight of aqueous ammonia 0.38 g and pure water 0.2 g to 5.5 g of DMSO, the above-described mixed solution B was further added, and stirring was performed at room temperature for 15 minutes, whereby gelation of trihydroxymethylsilane was performed, and a mixed solution C containing a gel-like silicon compound was obtained.

[0094] (2) Ripening treatment

[0095] Ripening treatment was performed by directly incubating the mixed solution C containing a gel-like silicon compound prepared as described above at 40°C for 20 hours.

[0096] (3) Pulverization treatment

[0097] Next, the gel-like silicon compound subjected to the ripening treatment as described above was crushed into a granular shape of several mm to several cm in size using a spatula. Next, isopropyl alcohol (IPA) 40 g was added to the mixed solution C, and after gentle stirring, the gel was allowed to stand at room temperature for 6 hours, whereby the solvent and catalyst in the gel were subjected to decantation. Solvent replacement was performed by performing the same decantation treatment three times, and a mixed solution D was obtained. Next, the gel-like silicon compound in the mixed solution D was subjected to pulverization treatment (high-pressure medium-free pulverization). The pulverization treatment (high-pressure medium-free pulverization) was performed using a homogenizer (SMT Co., Ltd., trade name "UH-50") under the conditions of 50 W and 20 kHz for 2 minutes after weighing the gel-like compound 1.85 g and IPA 1.15 g in a 5 cc screw bottle.

[0098] By this pulverization treatment, the gel-like silicon compound in the above-described mixed solution D was pulverized, and thus the mixed solution D' became a sol liquid of the pulverized product. The volume average particle size representing the particle size deviation of the pulverized product contained in the mixed solution D' was confirmed by dynamic light scattering type nano-trace particle size analyzer (Upatras Co., Ltd., UPA-EX150 type), and the result was 0.50 to 0.70. Further, 0.062 g of a 1.5% by weight concentration MEK (methyl ethyl ketone) solution of photobase generator (Wako Pure Chemical Industries, Ltd.: trade name WPBG266) and 0.036 g of a 5% concentration MEK solution of bis(trimethoxysilyl)ethane were added at a ratio of 0.75 g of the sol liquid (mixed solution C') to 1.5% by weight of the photobase generator and 5% by weight of bis(trimethoxysilyl)ethane, and a coating liquid for a low-refractive layer was obtained.

[0099] [Manufacturing Example 2] Preparation of an adhesive constituting an adhesive layer

[0100] In a four-necked flask equipped with a stirring blade, a thermometer, a nitrogen gas introduction tube, and a condenser, 90.7 parts of butyl acrylate, 6 parts of N-acryloylmorpholine, 3 parts of acrylic acid, 0.3 parts of 2-hydroxybutyl acrylate, and 0.1 part by weight of 2,2'-azobisisobutyronitrile as a polymerization initiator were added together with 100 g of ethyl acetate, and nitrogen gas replacement was performed while slowly stirring and introducing nitrogen gas. After the liquid temperature in the flask was maintained at around 55°C, a polymerization reaction was performed for 8 hours to produce an acrylic polymer solution. With respect to 100 parts of the solid content of the obtained acrylic polymer solution, 0.2 parts of an isocyanate crosslinking agent (CORONATE L manufactured by Japan Polyurethane Industries, Inc., an adduct of trimethylolpropane with toluene diisocyanate), 0.3 parts of benzoyl peroxide (NYPER BMT manufactured by NOF Corporation), and 0.2 parts of γ-glycidoxypropylmethoxysilane (KBM-403 manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed to produce an acrylic adhesive solution. Next, the acrylic adhesive solution was applied to one side of a polyethylene terephthalate (PET) film (manufactured by Mitsubishi Chemical Polyester Film Co., Ltd., thickness: 38 μm) on which a silicone treatment had been performed, in such a manner that the thickness of the dried adhesive layer would be 20 μm, and drying was performed at 150°C for 3 minutes to form an adhesive layer. The storage modulus of the obtained adhesive was 1.3 x 104Pa. 5 Pa.

[0101] [Manufacturing Example 3] Preparation of black ink constituting light absorbing layer

[0102] An adhesive resin (acrylic resin: LIGHT ACRYLATE PE-3A manufactured by Kyoeisha Chemical Co., Ltd.), 100 parts of carbon black, and a solvent (methyl ethyl ketone: MEK) for adjusting the viscosity, 200 parts were mixed, and the mixture was subjected to high dispersing treatment using ultrasonic waves to produce a black ink.

[0103] [Manufacturing Example 4] Production of optical adhesive film

[0104] The coating solution for low refractive index layer formation prepared in Production Example 1 was applied to a substrate (acrylic film) having a thickness of 30 μm. The wet thickness (thickness before drying) of the applied layer was about 27 μm. The applied layer was dried by performing a treatment at a temperature of 100°C for 1 minute, and a low refractive index layer (thickness 0.9 μm) was formed on the substrate. The void ratio of the obtained low refractive index layer was 58 vol%, and the refractive index was 1.18. Next, an adhesive layer (thickness 10 μm) composed of the adhesive prepared in Production Example 2 was formed on the surface of the low refractive index layer, and further a black ink prepared in Production Example 3 was printed on the surface of the substrate by gravure printing, and a light absorbing layer (thickness 3 μm) was formed. In this way, an optical adhesive film 1 having a structure of light absorbing layer / substrate / low refractive index layer / adhesive layer was produced.

[0105] [Production Example 5] Production of Optical Adhesive Film

[0106] An optical adhesive film 2 having a structure of substrate / low refractive index layer / adhesive layer was obtained in the same manner as in Production Example 4, except that a light absorbing layer was not formed.

[0107] [Production Example 6] Production of Optical Adhesive Film

[0108] An optical adhesive film 3 having a structure of light absorbing layer / substrate / adhesive layer was produced in the same manner as in Production Example 4, except that a low refractive index layer was not formed.

[0109] [Example 1]

[0110] A backlight unit was taken out from a commercially available liquid crystal display device (manufactured by KING JIM Co., trade name "XMC10"). A light guide plate was taken out from the taken-out backlight unit (essentially a case), and the optical adhesive film 1 obtained in Production Example 4 was attached to the part of the light guide plate which contacted with the front surface portion of the case. The light guide plate to which the optical adhesive film 1 was attached was again loaded into the backlight unit, and the backlight unit was again loaded into the liquid crystal display device. The obtained liquid crystal display device was subjected to the evaluations of (2) and (3) described above. The results are shown in Table 1.

[0111] [Comparative Example 1]

[0112] The optical adhesive film 2 obtained in Production Example 5 was used instead of the optical adhesive film 1, and the backlight unit was loaded into the liquid crystal display device in the same manner as in Example 1. The obtained liquid crystal display device was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0113] [Comparative Example 2]

[0114] A liquid crystal display device was assembled with a backlight unit in the same manner as in Example 1, using the optical adhesive film 3 obtained in Production Example 6 in place of the optical adhesive film 1. The obtained liquid crystal display device was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0115] [Comparative Example 3]

[0116] A liquid crystal display device was assembled with a backlight unit in the same manner as in Example 1, without using an optical adhesive film. That is, a commercially available backlight unit for a liquid crystal display device was used as it was. The liquid crystal display device was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0117] [Table 1]

[0118]

[0119] From Table 1, it is clear that according to the embodiment of the present application, a backlight unit that is bright and in which light leakage in the oblique direction is suppressed can be achieved. It is also clear that the liquid crystal display device using the backlight unit of the embodiment of the present application is brighter than the liquid crystal display device of Comparative Example 3, which does not have a light absorbing layer. It is further clear that the combination of a light absorbing layer and a low refractive index layer exerts a synergistic effect far exceeding the simple sum, considering that Comparative Example 2, which has only a light absorbing layer, is extremely dark.

[0120] Industrial Applicability

[0121] The backlight unit of the present application can be suitably used for a backlight unit of an image display device (particularly, a liquid crystal display device).

Claims

1. A backlight unit comprising: a light source; a light guide plate disposed opposite the light source and having an end surface on which light from the light source is incident and an exit surface from which the light after the incidence is emitted; a housing having a front surface portion and a back surface portion and accommodating the light source and the light guide plate; and an optical adhesive film disposed between the front surface portion of the housing on the light source side and the light guide plate and adhered to the light guide plate, the optical adhesive film sequentially having a light absorbing layer, a substrate, a low refractive index layer having a refractive index of 1.30 or less at a wavelength of 550 nm, and an adhesive layer from the front surface portion side of the housing.

2. The backlight unit according to claim 1, wherein a reflection plate disposed on the back surface side of the light guide plate is further provided in the housing.

3. The backlight unit according to claim 2, wherein the reflection plate is disposed with an air layer interposed therebetween.

4. The backlight unit according to claim 2, wherein the reflection plate is adhered to the light guide plate with a double-sided adhesive film having a low refractive index layer interposed therebetween.

5. The backlight unit according to any one of claims 1 to 4, wherein the light source is an LED light source.

6. An image display device comprising: the backlight unit according to any one of claims 1 to 5, and an image display panel disposed on the exit surface side of the light guide plate. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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