JP2005128140A - Optical sheet and backlight unit using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical sheet for effectively utilizing light which is emitted by a light source, without increasing power consumption of a light source, in particular, to provide an optical sheet for a backlight unit for improving brightness of a liquid crystal display device, without increasing power consumption of the backlight unit of the liquid crystal display device, and to provide the backlight unit which uses the sheet. <P>SOLUTION: The optical sheet contains an organic compound emitting phosphorescence, by absorbing ultraviolet light and converts ultraviolet light, emitted by the light source, into visible light. The optical sheet is encapsulated by resin, in which the organic compound will not dissolve in a binder resin of the optical sheet. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical sheet for converting ultraviolet light emitted from a light source into visible light, and particularly to an optical sheet incorporated in a backlight unit of a liquid crystal display device and a backlight unit using the same.

  Examples of the optical sheet include a hologram sheet, a polarizing sheet, a light reflection preventing sheet, a light reflecting sheet, a light partially reflecting partially transmissive sheet, a diffraction grating sheet, an interference filter sheet, a color filter sheet, a light wavelength conversion sheet, and a light diffusion sheet. is there. Examples of the optical sheet incorporated in the backlight unit of the liquid crystal display device include a light guide plate, a light diffusion sheet, and a prism sheet.

  In a liquid crystal display device, a backlight method in which a liquid crystal layer is illuminated from the back surface is widely used. In such a backlight method, a backlight unit is provided on the lower surface side of the liquid crystal layer. A schematic diagram of the backlight unit is shown in FIG. In general, the backlight unit 10 includes a light source 11 such as a cold cathode tube or an LED, and a plurality of optical sheets 12 such as a light guide plate 13, a light diffusion sheet 14, and a prism sheet 15.

  Light rays that are incident on the light guide plate 13 from the light source 11 provided on the side surface of the backlight unit 10 are microdots disposed on the bottom surface of the light guide plate or a reflection sheet disposed on the bottom surface and side surfaces of the light guide plate. Reflected by (not shown) and emitted from the surface of the light guide plate 13. The light beam emitted from the light guide plate 13 enters the light diffusion sheet 14, diffuses, and is emitted from the surface of the light diffusion sheet 14. Thereafter, the light beam emitted from the light diffusion sheet 14 enters the prism sheet 15 and is emitted substantially upward by the prism portion 15 a formed on the surface of the prism sheet 15.

  Some backlight units 10 are provided with a plurality of prism sheets 15 and a light diffusion sheet 14 in order to improve the light collecting property.

  By the way, in order to display a still image and a moving image clearly, it is necessary to improve the brightness | luminance of a liquid crystal display device. Means for that purpose include improving the light quantity of the light source 11 and improving various optical properties of the light diffusion sheet 14 and the prism sheet 15. However, portable electronic devices such as mobile phones and notebook computers that use liquid crystal display devices have limited power consumption in order to enable long-term use, and limit the amount of light emitted from the light source 11. There is.

As a means for improving the brightness of the liquid crystal display device without increasing the power consumption of the backlight unit, a fluorescent dye is added to the resin layer of the light diffusion sheet (see, for example, Patent Document 1), and the backlight unit is equipped. Various proposals have been made such as containing an optically active ion for converting light other than visible wavelengths into visible light (for example, see Patent Document 2). However, even with these technologies, it has not been possible to sufficiently improve the luminance of the liquid crystal display device to the level currently required.
Japanese Patent No. 2698898 Japanese Patent Laid-Open No. 2001-21885

  Accordingly, an object of the present invention is to provide an optical sheet for effectively using light emitted from a light source without increasing the power consumption of the light source. In particular, an object of the present invention is to provide an optical sheet for a backlight unit capable of improving the luminance of the liquid crystal display device without increasing the power consumption of the backlight unit of the liquid crystal display device, and a backlight unit using the same. Is to provide.

  As a result of intensive studies, the present inventors have increased the amount of light in the visible region by absorbing the ultraviolet light contained in the light emitted from the light source into the phosphorescent substance from the excited triplet state and releasing it as visible light. It was found that wavelength conversion to visible light can be performed more efficiently by encapsulating a phosphorescent substance with a resin, and the present invention has been completed.

  That is, the present invention is an optical sheet containing an organic compound that emits phosphorescence by absorbing ultraviolet light, and converts ultraviolet light emitted from a light source into visible light, wherein the organic compound is a binder resin of the optical sheet. Provides an optical sheet that is encapsulated with an incompatible resin.

  When the optical sheet of the present invention is used, for example, in a backlight unit of a liquid crystal display device, it is possible to convert ultraviolet rays contained in the light emitted from the light source into visible light with high efficiency. Even without increasing the brightness, the brightness of the liquid crystal display device can be improved by the amount of visible light converted from ultraviolet rays. Therefore, it is not necessary to increase the power consumption to improve the brightness of the liquid crystal display device, use a portable electronic device such as a mobile phone or a notebook computer using the backlight unit for a long time, and still images and moving images. It becomes possible to display more clearly.

  When the optical sheet of the present invention is used, the ultraviolet rays contained in the light emitted from the light source can be efficiently converted into visible light, and high-luminance display can be achieved without increasing the power consumption of the light source. Therefore, when the optical sheet of the present invention is used for a backlight unit of a liquid crystal display element, ultraviolet rays contained in light rays emitted from a light source such as a cold cathode tube or an LED can be converted into visible rays. Only the brightness of the liquid crystal display device can be improved. Therefore, it is possible to display still images and moving images more clearly. Moreover, in the optical sheet of this invention, since the resin particle which encapsulated the organic compound which emits phosphorescence is disperse | distributing, it has the function to scatter light uniformly. Therefore, the optical sheet of the present invention has performance suitable as a light diffusion sheet.

  The organic compound that emits phosphorescence by absorbing ultraviolet rays used in the present invention is a substance that emits light when electrons transition from the excited triplet state to the ground singlet state. Specific examples include transition metal complexes, Examples include benzophenone, 4-methyl-benzoic acid, dibenzothiophene and the like. Moreover, as a transition metal used for a transition metal complex, Cr, Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt etc. can be illustrated, for example. Examples of the ligand used in the transition metal complex include acetylacetonate, 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 1,10-phenanthroline, and 2-phenylpyridine. And porphyrin, phthalocyanine, and the like. One kind or a plurality of kinds of these ligands may be coordinated with respect to one complex.

  Among these, the organic compound that emits phosphorescence by absorbing the ultraviolet rays used in the present invention is preferably a platinum or iridium complex. Furthermore, a compound represented by the following formula (I), formula (II), formula (III), formula (IV), formula (V), or formula (VI) is more preferable. In particular, a compound represented by Formula (II), Formula (V), or Formula (VI) is preferable.

The converted visible light is preferably white light when used in a backlight unit or the like. The emission colors of the above compounds are as follows.
Compounds of formula (I); green compounds of formula (II); green compounds of formula (III); compounds of red formula (IV); compounds of yellow formula (V); compounds of red formula (VI); In the optical sheet of the present invention, it is preferable to mix an appropriate amount of the compound of the formula (V) and the compound of the formula (VI), since the color of visible light obtained by wavelength conversion of ultraviolet light becomes white light.

  In the optical sheet of the present invention, an organic compound that emits phosphorescence by absorbing ultraviolet rays causes non-radiative transition, and the phosphorescence emission efficiency is prevented from decreasing. Therefore, a resin that is incompatible with the binder resin of the optical sheet is used. It encapsulates phosphorescent organic compounds. One of the causes of inactivation of the excited triplet state and causing non-radiative transition is an interaction between excitons called concentration quenching that occurs when the concentration of a phosphorescent substance in the medium is increased. Another cause of the non-radiative transition caused by the deactivation of the excited triplet state is the presence of oxygen. In the present invention, by adopting the above method, it is possible to suppress concentration quenching or non-radiative transition due to the presence of oxygen. The shape of the phosphorescent organic compound encapsulated particles (hereinafter referred to as encapsulated resin particles) used in the present invention is preferably spherical. By adopting such a shape, the light scattering property of the optical sheet of the present invention is improved, and the performance when used as a light diffusion sheet is improved.

  The resin encapsulating the phosphorescent organic compound is not particularly limited, and examples thereof include acrylic resin, polyurethane resin, polyvinyl chloride resin, polystyrene resin, polyacrylonitrile resin, polyamide resin, and polyester resin. It is done. The encapsulated resin particles of an organic compound that emits phosphorescence are preferably transparent in order to increase the amount of transmitted light, and are particularly preferably colorless and transparent. Among the above resins, it is preferable to use a hydrophobic resin having no oxygen permeability in order to enclose a substance that emits phosphorescence from an excited triplet state in the encapsulated resin particles.

  The resin encapsulating the phosphorescent organic compound used in the present invention needs to be a resin that is incompatible with the binder resin of the optical sheet, and needs to be selected according to the type of the binder resin of the optical sheet. is there. In this case, the binder resin of the optical sheet and the encapsulating resin are preferably different types of resins, but when using the same type of resin, for example, one of them is a linear resin. It is preferable to select a combination of resins having different structures such that the other is a resin having a crosslinked structure.

  As a method of encapsulating a phosphorescent substance in a resin, a method of kneading and pulverizing a resin and a phosphorescent substance, a method of dispersing and mixing a phosphorescent substance in a resin solution, and then drying and granulating by spray drying or the like, a monomer Various conventional methods such as emulsion polymerization method, suspension polymerization method, dispersion polymerization method or precipitation polymerization method, in which a phosphorescent substance is added and resin particles are formed by emulsification, dispersion or dissolution in a medium for polymerization. Known methods can be applied. In the present invention, among the above production methods, it is preferable to produce encapsulated resin particles encapsulated with an acrylic resin or a polystyrene resin using an emulsion polymerization method. In this case, an organic compound that emits phosphorescence is dispersed in an acrylic monomer or a styrene monomer, and encapsulated resin particles of the organic compound that emit phosphorescence are produced by a known emulsion polymerization method. According to the emulsion polymerization method, spherical encapsulated resin particles can be obtained. The particle diameter of the encapsulated resin particles used in the present invention is preferably 0.5 μm to 100 μm, more preferably 1 μm to 50 μm.

  Examples of the binder resin used in the optical sheet of the present invention include ionizing radiation such as acrylic resin, polyurethane resin, polyester resin, fluorine resin, silicone resin, polyamideimide resin, epoxy resin, or ultraviolet curable resin. Examples thereof include curable resins. In addition to the above resin, for example, a plasticizer, a stabilizer, a dispersant, and the like may be blended in the binder resin. The binder resin is preferably transparent in order to transmit light, and particularly colorless and transparent. Examples of the method for producing the optical sheet of the present invention in which the encapsulated resin particles are dispersed include, for example, a method in which the encapsulated resin particles are dispersed in a resin solution of a binder resin, coated in a thin film, and then dried. There is a method in which resin and encapsulated resin particles are mixed, melt-kneaded, and then rolled into a thin film.

  Hereinafter, an example in which the optical sheet of the present invention is used for a light diffusion sheet of a backlight unit of a liquid crystal display element will be described. FIG. 1 is a schematic cross-sectional view showing a light diffusion sheet for a backlight unit according to an embodiment of the present invention.

  FIG. 1 shows a light diffusing sheet having an optical property of uniformly diffusing transmitted light. Specifically, the light diffusion sheet 1 includes a base material layer 2, a light diffusion layer 3 laminated on the front side of the base material layer 2, and an anti-sticking layer 4 laminated on the back surface of the base material layer 2. It is composed of

  Since the base material layer 2 needs to transmit light, it is made of a synthetic resin that is transparent, particularly colorless and transparent. The synthetic resin used for the base material layer 2 is not particularly limited, and examples thereof include polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, and weather resistant vinyl chloride. .

  The light diffusion layer 3 includes a binder resin 5 and resin beads 6 dispersed in the binder resin 5. By dispersing the resin beads 6 in this manner, the light beam that passes through the light diffusion layer 3 can be uniformly diffused. Further, by providing a resin bead 6 whose upper end protrudes from the binder resin 5 or embedded in the binder resin 5, the light beam can be diffused better.

  Examples of the resin used for the binder resin 5 include an ionizing radiation curable resin such as an acrylic resin, a polyurethane resin, a polyester resin, a fluorine resin, a silicone resin, a polyamideimide resin, an epoxy resin, or an ultraviolet curable resin. Etc. In addition to the above resin, for example, a plasticizer, a stabilizer, a dispersant, and the like may be blended in the binder resin 5. Since the binder resin needs to transmit light, it is transparent, and colorless and transparent is particularly preferable. In addition, a hydrophobic material having no oxygen permeability is preferable.

  The anti-sticking layer 4 is composed of a binder resin 7 and resin beads 8 dispersed in the binder resin 7. The material of the binder resin 7 and the resin beads 8 may be the same as that used for the light diffusion layer 3. Since the anti-sticking layer 4 is intended to prevent sticking with the light guide plate (see FIG. 2), the amount of the resin beads 8 is relatively small. Therefore, the resin beads are separated from each other in the binder resin 7. Are distributed.

  Wavelength conversion for converting ultraviolet light into visible light by using resin particles 6 or resin beads 8 of the light diffusion sheet 1 or both as resin particles (encapsulated resin particles) encapsulating the phosphorescent organic compound. The function is expressed in the light diffusion sheet 1. Therefore, by using the light diffusing sheet 1 as a light diffusing sheet of the backlight unit, the amount of visible light transmitted to the liquid crystal surface can be increased, and as a result, the liquid crystal display device can be used without increasing power consumption. Brightness can be improved.

  The optical sheet of the present invention may be used not only for the light diffusion sheet as described above but also for the light guide plate 13 and the prism sheet 15 shown in FIG. Further, the optical sheet of the present invention is not limited to use as an optical sheet incorporated in a backlight unit of a liquid crystal display device. For example, a hologram sheet, a polarizing sheet, a light reflection preventing sheet, a light reflection sheet, It can be used for applications such as a light partial reflection partial transmission sheet, a diffraction grating sheet, an interference filter sheet, a color filter sheet, a light wavelength conversion sheet, and a light diffusion sheet.

(Example 1)
(Method for producing encapsulated resin particles)
In a flask equipped with a stirrer, an inert gas introduction tube, a reflux condenser and a thermometer, 0.5 part by mass of polyoxyethylene alkylphenyl ether sulfoammonium (Hightenol N-08, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was dissolved. 900 parts by mass of deionized water was charged. 85 parts by mass of methyl methacrylate, 10 parts by mass of trimethylolpropane trimethacrylate, and 5 parts by mass of a mixture of two types of Ir complexes (4.9 parts by mass of the compound of the formula (VI) and the above formula) A mixture containing 0.1 part by mass of the compound (V) was charged and stirred with a stirrer at 3000 rpm to obtain a uniform suspension.

  Subsequently, it heated at 75 degreeC, blowing nitrogen gas from an inert gas introduction pipe | tube, and it cooled, after performing a polymerization reaction at this temperature for 5 hours. The suspension was filtered, washed and dried to obtain encapsulated resin particles containing an Ir complex having an average particle size of 5 μm. Using the encapsulated resin particles containing the Ir complex, a light diffusion sheet was prepared by the following method.

A capsule containing acrylic polyol (Acridic A-807, manufactured by Dainippon Ink & Chemicals, Inc.) as a resin and the above Ir complex as encapsulated resin particles on the surface of a 100 μm thick polyester film (Lumilar T60 manufactured by Toray Industries, Inc.) A light diffusion layer having a dry film thickness of 15 μm in which 150 parts by mass of the encapsulated resin particles were dispersed with respect to 100 parts by mass of the resin was prepared. Next, an anti-sticking layer having a dry film thickness of 2 μm in which 10 parts by mass of silica (Nippu Kagaku Nib Seal E-220A) is dispersed with respect to 100 parts by mass of the resin is formed on the back surface of the polyester film. A light diffusing sheet with an anti-sticking layer was prepared.
(Example 2)
In Example 1, a light diffusion sheet with a sticking prevention layer was prepared in the same manner as in Example 1 except that 5 parts by mass of the compound of the formula (II) was used as the Ir complex.

(Comparative Example 1)
Instead of the encapsulated resin particles in Example 1, commercially available polymethyl methacrylate particles (Techpolymer MBX-5 manufactured by Sekisui Plastics Co., Ltd.) are used as resin beads, and a fluorescent dye (manufactured by Nippon Chemical Industry Co., Ltd.) is used for the light diffusion layer. A light diffusion sheet with a sticking prevention layer was prepared in the same manner as in Example 1 except that 1 part by mass of Nikka Flow OB) was added.
(Comparative Example 2)
In place of the encapsulated resin particles in Example 1, a commercially available polymethyl methacrylate particle (Techpolymer MBX-5 manufactured by Sekisui Plastics Kogyo Co., Ltd.) was used as a resin bead with a sticking prevention layer in the same manner as in Example 1. A light diffusing sheet was prepared.
The obtained four types of light diffusion sheets were incorporated into the backlight unit, and the brightness was visually determined from the upper surface of the backlight unit.
Example 1: ○ (good)
Example 2: Good (good)
Comparative Example 1: Δ (somewhat inferior)
Comparative Example 2: X (Inferior)

It is typical sectional drawing which shows the light-diffusion sheet for backlight units which concerns on one Embodiment of this invention. It is a typical perspective view which shows a general backlight unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light diffusing sheet 2 Base material layer 3 Light diffusing layer 4 Anti-sticking layer 5 Resin binder 6 Resin beads (encapsulated resin particles)
7 Resin binder 8 Resin beads (encapsulated resin particles)
DESCRIPTION OF SYMBOLS 10 Backlight unit 11 Light source 12 Optical sheet 13 Light guide plate 14 Light diffusion sheet 15 Prism sheet 15a Prism part

Claims (6)

An optical sheet that contains an organic compound that emits phosphorescence by absorbing ultraviolet light, and that converts ultraviolet light emitted from a light source into visible light, wherein the organic compound is incompatible with the binder resin of the optical sheet. An optical sheet encapsulated by The optical sheet according to claim 1, wherein the phosphorescent organic compound is a platinum or iridium complex. The phosphorescent organic compound has the formula (I),

Formula (II),

Formula (III),

Formula (IV),

Formula (V),

Or formula (VI)

The optical sheet according to claim 1, which is a compound represented by the formula: The optical sheet according to claim 1, wherein the optical sheet is used for a backlight unit of a liquid crystal display device. A light diffusing sheet for a backlight unit of a liquid crystal display device, wherein the optical sheet according to claim 1 is used. A backlight unit for a liquid crystal display device, wherein the optical sheet according to claim 1 is used.

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