JP2012230364A - Light diffusion sheet, optical unit, backlight unit and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light diffusion sheet realizing further increase in high luminance and in thinning of a liquid crystal display device, and an optical unit, a backlight unit and a liquid crystal display device comprising this diffusion sheet.SOLUTION: A light diffusion sheet comprises a transparent base material layer and a light diffusion layer disposed on the surface of this base material layer and having resin beads in a binder, and beads having a high refractive index are used as the resin beads. The refractive index of the binder may be 1.49 or more and 1.51 or less, and the refractive index of the resin beads may be 1.52 or more and 1.59 or less. The value calculated by subtracting the refractive index of the binder from the refractive index of the resin beads may be 0.06 or more and 0.08 or less. The main polymer of the resin beads may be a copolymer of a (meth)acryl-based monomer and a styrene-based monomer.

Description

  The present invention relates to a light diffusion sheet, an optical unit including the light diffusion sheet, a backlight unit including the optical unit, and a liquid crystal display device including the backlight unit.

  Liquid crystal display devices are widely used as flat panel displays, taking advantage of their features such as thinness, light weight, and low power consumption, and their uses are expanding year by year as display devices such as mobile phones, personal digital assistants, personal computers, and televisions. Yes. In recent years, characteristics required for liquid crystal display devices include higher brightness, wider viewing angle, energy saving, thinner and lighter, and the demand for higher brightness is particularly high.

  A conventional general liquid crystal display device includes a liquid crystal display element and a backlight unit disposed on the back side of the liquid crystal display element (see, for example, JP-A-2005-106959). The liquid crystal display element has a structure in which a liquid crystal cell is sandwiched between a front surface side polarizing plate and a back surface side polarizing plate, and is driven by various display modes. The backlight unit illuminates the liquid crystal display element from the back side, and edge light type and direct type backlight units are widely used. These conventional backlight units include a light source, a light guide plate that guides light from the light source to the liquid crystal display element side, and an optical unit that is disposed on the surface of the light guide plate on the liquid crystal display element side. The optical unit is for making the light from the light guide plate efficiently and uniformly incident on the entire back surface of the liquid crystal display element. For example, the optical unit diffuses the light from the light guide plate and transmits the transmitted light. A prism sheet that refracts in the normal direction side is provided.

JP 2005-106959 A

  Since the conventional liquid crystal display device has a light diffusion sheet, a prism sheet, and the like as described above, a certain level of brightness has been achieved, but today, there is a demand for higher brightness of the liquid crystal display device. . High gain prism sheets have been developed for the purpose of improving the luminance of liquid crystal display devices. For example, a high gain prism sheet with the product name “BEF2-G2-MR-155” manufactured by Sumitomo 3M Limited is commercialized. The high gain prism sheet is a prism sheet made of a high refractive index material. Thus, although the high gain prism sheet has been commercialized, a light diffusion sheet that matches the high gain prism sheet has not been developed, and the effect of improving the luminance by the high gain prism sheet has not been brought out. In addition, today, there is a demand for further thinning of the liquid crystal display device.

  The present invention relates to a light diffusing sheet for realizing further increase in luminance and thickness of a liquid crystal display device, an optical unit including the light diffusing sheet, a backlight unit including the optical unit, and further increasing luminance. An object of the present invention is to provide a liquid crystal display device that is reduced in size and thickness.

The light diffusion sheet of the present invention is
A transparent substrate layer;
A light diffusing sheet comprising a light diffusing layer laminated on the surface of the base material layer and having resin beads in a binder,
The resin beads have a refractive index of 1.50 or more.

  Since the refractive index of the resin beads of the light diffusing sheet is higher than the refractive index of the resin beads of the conventional light diffusing sheet, the reflection and refraction of light rays by the resin beads are larger than before. Therefore, if the light diffusing sheet is incorporated in a liquid crystal display device, the luminance of the liquid crystal display device can be made higher than before. As a result, the backlight unit can be reduced in thickness, and consequently the liquid crystal display device can be reduced in thickness. In addition, by adjusting the refractive index of the binder and resin beads of the light diffusion sheet, the high gain prism sheet can be used to raise the light beam in the vertical direction regardless of the refractive index. can do.

  The refractive index of the binder may be 1.49 to 1.51, and the refractive index of the resin beads may be 1.52 to 1.59. In that case, since the refractive index of the resin beads of the light diffusing sheet is higher than the refractive index of the resin beads of the conventional light diffusing sheet, the reflection and refraction of light rays by the resin beads are larger than before, and the brightness of the liquid crystal display device is high. Become.

  In the light diffusion sheet, the value obtained by subtracting the refractive index of the binder from the refractive index of the resin beads may be 0.06 or more and 0.08 or less. In that case, since the reflection and refraction of the light rays by the resin beads are increased, the luminance of the liquid crystal display device is further increased.

  The main polymer of the resin beads may be a copolymer of a (meth) acrylic monomer and a styrene monomer. In that case, the refractive index of the resin beads is easily set to 1.52 or more and 1.59 or less, or the value obtained by subtracting the refractive index of the binder from the refractive index of the resin beads is easily set to 0.06 or more and 0.08 or less. be able to.

  40 mass parts or more and 230 mass parts or less may be sufficient as the compounding quantity with respect to 100 mass parts of binders of the said resin bead. In that case, since the reflection and refraction of the light rays by the resin beads are increased, the luminance of the liquid crystal display device is further increased.

  The haze value of the light diffusion sheet may be 78% or more and 93% or less. In that case, since the reflection and refraction of the light rays by the resin beads are increased, the luminance of the liquid crystal display device is further increased.

  The average particle diameter of the resin beads may be 2 μm or more and 5 μm or less. In that case, since the reflection and refraction of the light rays by the resin beads are increased, the luminance of the liquid crystal display device is further increased.

  The resin beads may be polydisperse beads. In that case, since the reflection and refraction of the light rays by the resin beads are increased, the luminance of the liquid crystal display device is further increased.

  In order to achieve further increase in brightness and thickness of the liquid crystal display device, the light diffusion sheet and a single prism sheet superimposed on the surface side of the light diffusion sheet are provided, and the refractive index of the prism sheet An optical unit having a value of 1.50 or more can be used as one component of the liquid crystal display device. Similarly, in order to further increase the brightness and thickness of the liquid crystal display device, the light diffusion sheet and the two prism sheets superimposed on the surface side of the light diffusion sheet are provided. These prism sheets are arranged so that their ridge lines intersect with each other, and an optical unit in which the refractive index of the two prism sheets is 1.50 or more can be used as one component of the liquid crystal display device. In that case, if the refractive index of each prism sheet is 1.55 or more and 1.70 or less, the luminance of the liquid crystal display device is further increased. Further, the liquid crystal display device can be thinned.

  In addition, in order to realize further increase in brightness and thickness of the liquid crystal display device, a light source, a light guide plate from which the light beam from the light source is incident, and a light guide plate that emits the incident light beam are laminated on the upper side of the light guide plate. A backlight unit having the optical unit can be used as one component of the liquid crystal display device.

  Furthermore, a liquid crystal display device including the backlight unit and a liquid crystal display element disposed on a prism sheet of the backlight unit is also an aspect of the present invention.

  The present invention relates to a light diffusing sheet for realizing further increase in luminance and thickness of a liquid crystal display device, an optical unit including the light diffusing sheet, a backlight unit including the optical unit, and further increasing luminance. It is possible to provide a liquid crystal display device that has been reduced in size and thickness.

It is a figure which shows typically the cross section of the liquid crystal display device of embodiment of this invention. It is a figure for demonstrating the progress of the light ray in the liquid crystal display device of FIG.

  Embodiments of the present invention will be described below with reference to the drawings.

(Configuration of liquid crystal display device)
The liquid crystal display device 1 of FIG. 1 includes a liquid crystal display element 10 and a backlight unit 20. The liquid crystal display element 10 is an element for displaying information using light, and has a structure in which a liquid crystal cell 12 is sandwiched between a front surface side polarizing plate 11 and a back surface side reflective polarizing plate 13. The back surface side reflection polarizing plate 13 transmits the polarization component in one direction of light and reflects the remaining polarization component.

  The backlight unit 20 is an edge light type backlight unit that is disposed on the back side of the liquid crystal display element 10 and illuminates the liquid crystal display element 10 from the back side, and includes a light source 21, a light guide plate 22, an optical unit 23, and the like. Have

  The light source 21 has a plurality of light emitting diodes arranged in a straight line. The light guide plate 22 has a wedge shape and has a rectangular plane when viewed from the surface of the liquid crystal display device 1. The light guide plate 22 is disposed so that one side of the rectangle faces the entire light source 21. Yes. Although not shown, a light reflecting member is provided on the side surface of the light guide plate 22 that does not face the light source 21. Although not shown, a member that reflects light rays is also provided on the back surface of the light guide plate 22. The light guide plate 22 emits the light beam incident from the light source 21 to the optical unit 23 by a member that reflects the light beam.

  The optical unit 23 is disposed on the surface of the light guide plate 22 on the liquid crystal display element 10 side, and diffuses the light incident from the light guide plate 22 so that the light is incident on the entire back surface of the liquid crystal display element 10 vertically. To the liquid crystal display element 10. Specifically, the optical unit 23 diffracts the light beam incident from the light guide plate 22 and diffracts the light beam diffused by the light diffusion sheet 31 to travel substantially vertically to the liquid crystal display element 10. The first prism sheet 32 and the second prism sheet 33 are included. As shown in FIG. 1, each of the light diffusion sheet 31, the first prism sheet 32, and the second prism sheet 33 is superimposed in order from the back surface side to the front surface side of the liquid crystal display device 1.

  The light diffusion sheet 31 diffuses the light incident from the light guide plate 22 and emits the light to the first prism sheet 32 so that the light is incident on the entire back surface of the liquid crystal display element 10. The light diffusion sheet 31 includes a base material layer 41, a light diffusion layer 42 laminated on the front surface side of the base material layer 41, and a sticking prevention layer 43 laminated on the back surface side of the base material layer 41. The detailed configuration of the light diffusion sheet 31 will be described later.

  The first prism sheet 32 is a high gain prism sheet, and refracts the light incident from the light diffusion sheet 31 and outputs it to the second prism sheet 33. The second prism sheet 33 is a high gain prism sheet, and refracts the light incident from the first prism sheet 32 so that the emitted light travels substantially perpendicularly to the back surface of the liquid crystal display element 10 to display the liquid crystal. The light is emitted to the element 10. The 1st prism sheet 32 and the 2nd prism sheet 33 are provided with the base material layer and the projection row | line | column which consists of a some protrusion prism part laminated | stacked on the surface of this base material layer.

  The thickness of the first prism sheet 32 and the second prism sheet 33 (height from the back surface to the apex of the protruding prism portion) is preferably 50 μm or more and 200 μm or less, and more preferably 100 μm or more and 180 μm or less. Moreover, as a space | interval (pitch) between the protruding prism parts in the 1st prism sheet 32 and the 2nd prism sheet 33, 30 micrometers or more and 100 micrometers or less are preferable, and 40 micrometers or more and 60 micrometers or less are more preferable. Further, the apex angle of the protruding prism portion is preferably 85 ° or more and 95 ° or less. The refractive indexes of the first prism sheet 32 and the second prism sheet 33 are 1.50 or more, preferably 1.55 or more and 1.70 or less. In addition, the refractive index of a prism sheet means the refractive index of a prism part. By using the prism sheets 32 and 33 having such a structure in combination with the light diffusion sheet 31, further increase in luminance can be achieved. Further, both the first prism sheet 32 and the second prism sheet 33 refract the light rays from the direction forming an angle of about 33 to 34 ° with the normal line of the substrate on the back surface side to the surface side of the normal line. The conventional prism sheet refracts light rays from the direction that forms an angle of about 30 to 31 ° with the normal line of the substrate toward the surface side of the normal line. In other words, the first prism sheet 32 and the second prism sheet 33 have a higher capability than the conventional prism sheet with respect to the function of raising the light incident in an oblique direction with respect to the back surface of the substrate. . Each of the first prism sheet 32 and the second prism sheet 33 is, for example, a high gain prism sheet having a product name “BEF2-G2-MR-155” manufactured by Sumitomo 3M Limited.

(Configuration of light diffusion sheet)
The light diffusion sheet 31 is an optical sheet that diffuses and emits incident light. As described above, the base material layer 41, the light diffusion layer 42 laminated on the surface side of the base material layer 41, and the base And an anti-sticking layer 43 laminated on the back side of the material layer 41.

  The base material layer 41 is a transparent layer that can transmit light, and is formed of a resin. The resin is, for example, acrylic resin, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polystyrene, polyolefin, cellulose acetate, or weather-resistant vinyl chloride. The average thickness of the base material layer 41 is preferably 10 μm or more and 400 μm or less, and more preferably 20 μm or more and 130 μm or less. By setting the average thickness to 10 μm or more, the base material layer 41 can have a strength that does not cause bending. Further, by setting the average thickness to 400 μm or less, the liquid crystal display device 1 can be thinned and the luminance of the liquid crystal display device 1 can be sufficiently secured.

  The light diffusion layer 42 includes resin beads 51 having a high refractive index and a binder 52 for fixing the resin beads 51 in a dispersed state. The resin beads 51 are also dispersed and arranged on the surface of the light diffusion layer 42, whereby minute and disordered irregularities are provided on the surface of the light diffusion layer 42. Due to the unevenness and the interface between the resin beads 51 and the binder 52, the function of diffusing light rays is greatly enhanced.

  The refractive index of the resin beads 51 is 1.50 or more, preferably 1.52 or more and 1.59 or less, and more preferably 1.55 or more and 1.57 or less. Since the refractive index of the resin beads in the conventional light diffusion sheet is about 1.49, the refractive index of the resin beads 51 of the present embodiment is larger than the conventional one.

  The main polymer that forms the resin beads 51 is not particularly limited, and acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile, polyamide, and the like can be used. (Meth) acrylic monomers and styrene A copolymer with a monomer is preferable, and a crosslinked (meth) acryl-styrene copolymer obtained from a (meth) acrylic monomer, a styrene monomer, and a crosslinkable monomer is more preferable. By using a main polymer in which a styrene skeleton is introduced into a (meth) acrylic skeleton, the refractive index of the resin beads 51 is increased, and the refractive index can be further increased by forming a crosslinked structure. Moreover, when acrylic resin (acrylic polyol etc.) is used for the binder 52, since it has the same acrylic frame | skeleton, the resin bead 51 is uniformly disperse | distributed in the binder 52, and front brightness can be improved more.

  Examples of the (meth) acrylic monomer include acrylic acid and derivatives of acrylic acid, such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate , Nonyl acrylate, decyl acrylate, undecyl acrylate, dodecyl acrylate, glycidyl acrylate, methoxyethyl acrylate, propoxyethyl acrylate, butoxyethyl acrylate, methoxydiethylene glycol acrylate, ethoxydiethylene glycol acrylate, methoxyethylene acrylate Glycol, butoxytriethylene glycol acrylate, methoxydipropylene glycol acrylate, phenoxyethyl acrylate, phenoxydie acrylate Lenglycol, phenoxytetraethylene glycol acrylate, benzyl acrylate, cyclohexyl acrylate, tetrahydrofurfuryl acrylate, dicyclopentenyl acrylate, dicyclopentenyloxyethyl acrylate, N-vinyl-2-pyrrolidone acrylate, acrylic acid Hydroxyethyl, hydroxypropyl acrylate, hydroxybutyl acrylate, 2-hydroxy-3-phenyloxypropyl acrylate, glycidyl acrylate, acrylonitrile, acrylamide, N-methylol acrylamide, diacetone acrylamide, or derivatives of methacrylic acid, methacrylic acid , For example, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, pentyl methacrylate, meta Hexyl rilate, heptyl methacrylate, octyl methacrylate, nonyl methacrylate, decyl methacrylate, undecyl methacrylate, dodecyl methacrylate, glycidyl methacrylate, methoxyethyl methacrylate, propoxyethyl methacrylate, butoxyethyl methacrylate, methoxy methacrylate Diethylene glycol, ethoxydiethylene glycol methacrylate, methoxyethylene glycol methacrylate, butoxytriethylene glycol methacrylate, methoxydipropylene glycol methacrylate, phenoxyethyl methacrylate, phenoxydiethylene glycol methacrylate, phenoxytetraethylene glycol methacrylate, benzyl methacrylate, methacrylic acid Cyclohexyl, tetrahydrophthal methacrylate Rufuryl, dicyclopentenyl methacrylate, dicyclopentenyloxyethyl methacrylate, N-vinyl-2-pyrrolidone methacrylate, methacrylonitrile, methacrylamide, N-methylolmethacrylamide, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate , Hydroxybutyl methacrylate, 2-hydroxy-3-phenyloxypropyl methacrylate, and the like. These can be used alone or in combination.

  Examples of the (meth) acrylic monomer include styrene derivatives such as styrene, α-methylstyrene, vinyltoluene, and pt-butylstyrene. These can be used alone or in combination.

  Examples of the crosslinkable monomer include divinylbenzene, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butanediol dimethacrylate, 1,5-pentanediol methacrylate, 1,6-hexanediol dimethacrylate, neo Examples include pentyl glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, and tripropylene glycol dimethacrylate. These can be used alone or in combination.

  The main polymer may further contain other monomers in addition to the (meth) acrylic monomer, the styrene monomer and the crosslinkable monomer. Examples of other monomers include vinyl acetate, vinyl chloride, maleic anhydride, maleic acid, maleic acid ester, fumaric acid, fumaric acid ester, and triaryl isocyanurate.

  The resin beads having the copolymer as the main polymer can be obtained, for example, by suspension polymerization of a mixture of monomers in the presence of a polymerization initiator. This suspension polymerization can usually be carried out in an aqueous medium such as water or a mixture of water and alcohol.

  The shape of the resin beads 51 is not particularly limited, and examples thereof include a spherical shape, a cubic shape, a needle shape, a rod shape, a spindle shape, a plate shape, a scale shape, and a fiber shape, and among them, a spherical shape having excellent light diffusibility. Is preferred.

  The average particle size of the resin beads 51 is preferably 2 μm to 5 μm, and more preferably 2.5 μm to 4 μm. By using particles having such an average particle diameter, light diffusibility and the like can be improved. In addition, the average particle diameter in this invention means what averaged the particle diameter of 30 particle | grains extracted from the particle | grains observed in the electron microscope of 1000 times magnification. The particle diameter is defined by the ferret diameter (interval when the projected image is sandwiched between parallel lines in a certain direction).

  The resin beads 51 are preferably polydisperse beads, and the coefficient of variation in particle diameter is preferably 20% or more and 40% or less. By using such polydisperse beads, light diffusibility, and hence front luminance and the like can be enhanced.

  Furthermore, the blending amount of the resin beads 51 contained in the light diffusion layer 42 with respect to 100 parts by mass of the binder 52 is preferably 40 parts by mass or more and 230 parts by mass or less, more preferably 45 parts by mass or more and 100 parts by mass or less, and 60 parts by mass. The following is more preferable. The blending amount of the resin beads 51 is determined according to the average particle size so that the light diffusibility is more greatly exhibited.

  The binder 52 is usually obtained by curing a base polymer with a curing agent. In addition, a plasticizer, a dispersant, various leveling agents, an ultraviolet absorber, an antioxidant, a viscosity modifier, a lubricant, a light stabilizer, and the like may be appropriately added to the binder 52. This binder 52 arranges and fixes the resin beads 51 at substantially equal density on the entire surface of the base material layer 41 by the base polymer as the main constituent element.

  The base polymer is not particularly limited, and examples thereof include acrylic resins, polyurethanes, polyesters, fluorine resins, silicone resins, polyamideimides, epoxy resins, ultraviolet curable resins, and the like. Can be used singly or in combination of two or more. In particular, the base polymer is preferably a polyol that has high processability and can easily form the light diffusion layer 42 by means such as coating. The base polymer used for the binder 52 is transparent because it is necessary to transmit light, and colorless and transparent is particularly preferable.

  As the polyol, polyester polyol or (meth) acryl polyol is preferable, and (meth) acryl polyol is more preferable. The binder 52 having such a polyester polyol or acrylic polyol as a base polymer has high weather resistance, and can suppress yellowing of the light diffusion layer 42 and the like. Moreover, when the copolymer containing the (meth) acrylic-type monomer mentioned above is used as the resin bead 51, dispersibility can be increased by using the (meth) acrylic polyol, and the front luminance can be increased. In addition, any one of this polyester polyol and acrylic polyol may be used, and both may be used.

  The number of hydroxyl groups in the polyester polyol and (meth) acryl polyol is not particularly limited as long as it is 2 or more per molecule, but if the hydroxyl value in the solid content is 10 or less, the number of crosslinking points decreases, and the solvent resistance Film properties such as heat resistance, water resistance, heat resistance and surface hardness tend to decrease.

  As described above, when a polyol is used as the base polymer, hexamethylene diisocyanate, isofurone diisocyanate and xylene diisocyanate are preferable as the curing agent, hexamethylene diisocyanate is more preferable, and adduct type hexamethylene diisocyanate is further preferable. When these curing agents are used, the curing reaction rate of the polymer composition increases. The improvement in the curing reaction rate contributes to the uniform dispersibility of the resin beads in the binder. In order to further improve the uniform dispersibility and the like, it is more preferable to use a combination of (meth) acrylic polyol and hexamethylene diisocyanate.

  The refractive index of the binder 52 is preferably 1.49 or more and 1.51 or less. The value obtained by subtracting the refractive index of the binder 52 from the refractive index of the resin beads 51 is preferably 0.06 or more and 0.08 or less.

  The anti-sticking layer 43 is composed of a binder and beads dispersed in the binder. As the component of this binder, the same ones as the binder 52 of the light diffusion layer 42 can be exemplified. In addition, examples of the material of the beads include inorganic beads and organic beads. The thickness of the anti-sticking layer 43 (the thickness of the binder part excluding the beads) is not particularly limited, but can be, for example, about 1 μm or more and 10 μm or less.

  The blending amount of the beads in the anti-sticking layer 43 is relatively small, the beads are separated from each other and dispersed in the binder, and most of the beads protrude from the binder at a very small amount. Therefore, when the light diffusion sheet 31 is laminated with the light guide plate 22, the lower end of the protruding beads contacts the surface of the light guide plate 22, and the entire back surface of the light diffusion sheet 31 does not contact the light guide plate 22. As a result, sticking between the light diffusion seam 31 and the light guide plate 22 is prevented, and uneven brightness on the screen of the liquid crystal display device is suppressed.

  Next, a method for manufacturing the light diffusion sheet 31 will be described. The light diffusing sheet 31 is produced by, for example, (a) a step of producing a light diffusing layer coating liquid by mixing resin beads 51 with a composition (base polymer, curing agent, etc.) constituting the binder 52. And (b) a step of laminating the light diffusing layer 42 by applying the coating solution for the light diffusing layer on the surface of the base material layer 41, and (c) mixing the beads with the polymer composition constituting the binder. A step of producing the anti-sticking layer coating solution, and (d) a step of laminating the anti-sticking layer 43 by applying the anti-sticking layer coating solution to the back surface of the base material layer 41. Have.

(Progress of rays)
Next, the progression of light rays in the liquid crystal display device 1 of the present embodiment will be described. The light beam is generated by the light source 21 emitting light. As shown in FIG. 2, the light beam A from the light source 21 is guided to the optical unit 23 by the light guide plate 22 and scattered by the light diffusion sheet 31 of the optical unit 23. The light enters the first prism sheet 32 from the entire back surface of the one prism sheet 32. The light beam A incident on the first prism sheet 32 is refracted by the first prism sheet 32, and thereby rises in a direction perpendicular to the back surface of the liquid crystal display element 10 and proceeds to the second prism sheet 33. The light beam A that has traveled to the second prism sheet 33 is refracted by the second prism sheet 33 and further rises, and enters the liquid crystal display element 10 substantially perpendicularly to the back surface. In this way, the light beam A from the backlight unit 20 enters the liquid crystal display element 10.

  Since the back side reflective polarizing plate 13 is provided on the back side of the liquid crystal display element 10, a part of the light beam A incident on the liquid crystal display element 10 is transmitted through the back side reflective polarizing plate 13 and the remaining part is a backlight unit. Reflected to the 20 side. The light beam A that has passed through the back-side reflective polarizing plate 13 passes through the liquid crystal cell 12 and the front-side polarizing plate 11 in order, and is emitted to the front surface side of the liquid crystal display device 1. The light beam A reflected to the backlight unit 20 side is reflected by the reflecting member of the light guide plate 22, passes through the optical unit 23 and enters the liquid crystal display element 10 again as described above.

(effect)
As described above, the light diffusion sheet 31 of the present embodiment includes the light diffusion layer 42 including the resin beads 51 having a high refractive index and the binder 52 having a refractive index of preferably 1.49 to 1.51. Have. Since the refractive index of the resin beads in the conventional light diffusion sheet is about 1.49 as described above, the refractive index of the resin beads 51 of the present embodiment is larger than the conventional one. For this reason, the reflection or refraction of the light beam by the resin beads 51 of the present embodiment is greater than in the past. As a result, the light diffusion sheet 31 of the present embodiment matches the first prism sheet 32 and the second prism sheet 33 that are high gain prism sheets, and improves the luminance by the first prism sheet 32 and the second prism sheet 33. The effect can be brought out. Therefore, the brightness of the liquid crystal display device 1 of the present embodiment is higher than before, and the high brightness required by society is realized.

  In addition, since the first prism sheet 32 and the second prism sheet 33 are high gain prisms made of a high refractive index material, the ratio of the reflected light is increased in the light rays traveling on the back surface of the prism sheet. However, according to the light diffusion sheet 31, since the reflection of the light beam by the resin beads 51 is large, the light beam reflected to the light diffusion layer 42 side by the prism sheet is transmitted to the first prism sheet 32 and the first prism by the light diffusion layer 42. The light is reflected again on the two prism sheet 33 side. Therefore, the light use efficiency is improved as compared with the conventional case, and the liquid crystal display device 1 is further increased in luminance.

  Since the liquid crystal display device can be increased in luminance, the backlight unit 20 can be thinned, and thus the liquid crystal display device can be thinned. Further, the light diffusion sheet 31 of the present embodiment makes the luminance of the liquid crystal display device 1 of the present embodiment uniform within the screen while maintaining a high state. Therefore, the backlight unit 20 can be concealed from the surface side. In addition, by adjusting the refractive index of the binder and resin beads of the light diffusion sheet 31, the high gain prism sheet can raise the light beam in the vertical direction regardless of the refractive index. can do.

  The haze value of the light diffusion sheet is preferably from 78% to 93%, and more preferably from 80% to 90%. By having such a relatively high haze value, the front luminance of the liquid crystal display device 1 is further increased. The “haze value” is a value measured according to JIS-K-7105.

  As described above, the liquid crystal display device 1 according to the present embodiment has the light diffusion sheet 31 including the resin beads 51 having a higher refractive index than that of the conventional resin beads. The higher brightness is achieved. Therefore, the liquid crystal display device 1 of the present embodiment is useful as a high-luminance display device of various sizes including, for example, 2 inches to 22 inches. That is, the liquid crystal display device 1 of the present embodiment is useful as a display device such as a mobile phone, a smart phone, an in-vehicle device, a game machine, a tablet personal computer, a mobile personal computer, a notebook personal computer, and a monitor.

(Modification)
In the embodiment described above, the optical unit 23 includes the first prism sheet 32 and the second prism sheet 33. However, the optical unit 23 may have only the first prism sheet 32 as a prism sheet. In addition, the light diffusion sheet may not have a sticking prevention layer.

  In the embodiment described above, the backlight unit 20 is an edge light type backlight unit. However, the backlight unit 20 may be a direct type backlight unit. When the backlight unit 20 is a direct type backlight unit, even if the optical unit 23 has only the first prism sheet 32 as a prism sheet, the light beam can be diffused very greatly. As a result, it is possible to achieve high brightness of the liquid crystal display device demanded by society.

  Further, in the above-described embodiment, the light source 21 has a plurality of light emitting diodes arranged in a straight line, but the light source 21 may be a rod-shaped cold cathode tube. In the above-described embodiment, the light guide plate 22 has a wedge shape, but the light guide plate 22 may have a flat plate shape. Thus, the form of the backlight unit 20 including the light source 21 and the light guide plate 22 is not limited to the form described above.

  Further, in the above-described embodiment, the liquid crystal display element 10 has the back side reflective polarizing plate 13, but the liquid crystal display element 10 is a polarizing plate that does not have a reflecting function instead of the back side reflective polarizing plate 13. You may have.

  EXAMPLES Hereinafter, although this invention is explained in full detail based on an Example, this invention is not interpreted limitedly based on description of this Example.

[Example 1]
A transparent polyethylene terephthalate film having a thickness of 100 μm was used as the base material layer. As the polymer composition for the light diffusion layer, a polymer composition comprising 100 parts of a binder (acrylic polyol and adduct type hexamethylene diisocyanate curing agent), 50 parts of resin beads and a solvent was used. The number of parts indicating the blending amount of each composition is a mass ratio in terms of solid content. The light diffusion sheet of Example 1 was obtained by laminating 5 g / m ² (in terms of solid content) of the polymer composition for the light diffusion layer on the surface of the base material layer by the gravure coating method. The refractive index of the binder is 1.49. The resin beads used include a crosslinked (meth) acryl-styrene copolymer obtained by suspension polymerization as a main polymer. The refractive index of the resin beads was 1.56, and the average particle size was 3.0 μm (coefficient of variation 30%).

[Examples 2 to 10]
The light diffusion sheets of Examples 2 to 10 were obtained in the same manner as in Example 1 except that the refractive index binder shown in Table 1 and the refractive index, average particle diameter and number of resin beads shown in Table 1 were used. .

[Comparative Example 1]
A light diffusion sheet of Comparative Example 1 was obtained in the same manner as in Example 1 except that acrylic resin beads (refractive index: 1.49) formed by suspension polymerization were used as the resin beads.

[Comparative Example 2]
A light diffusing sheet of Comparative Example 2 was obtained in the same manner as Comparative Example 1 except that the amount of the resin beads was 100 parts by mass.

[Comparative Example 3]
A light diffusing sheet of Comparative Example 3 was obtained in the same manner as Comparative Example 1 except that a binder having a refractive index of 1.42 was used.

[Evaluation of characteristics]
Using the light diffusion sheets of Examples 1 to 10 and the light diffusion sheets of Comparative Examples 1 to 3, the haze values of these light diffusion sheets were measured. Further, these light diffusion sheets were actually incorporated into an edge light type backlight unit, and the front luminance was measured. The results are shown in Table 1 below. The backlight unit has a structure in which two prism sheets are superposed on the surface side of the light diffusion sheet, and the two prism sheets are arranged so that their ridge lines intersect with each other. In addition, “BEF2-G2-MR-155” manufactured by Sumitomo 3M Limited was used as the prism sheet. The evaluation results are shown in Table 1. The front luminance is shown as a relative value with Example 1 as 100.0%.

  As shown in Table 1 above, the light diffusing sheets of Examples 1 to 10 show a high haze value and front luminance as compared with the light diffusing sheets of comparative examples using conventional beads, and are excellent in optical properties. It has characteristics.

  The light diffusion sheet, optical unit and backlight unit of the present invention are useful as members constituting a liquid crystal display device. In addition, the liquid crystal display device of the present invention is useful as a display device for mobile phones, personal computers, and the like.

DESCRIPTION OF SYMBOLS 1 Liquid crystal display device 10 Liquid crystal display element 11 Front surface side polarizing plate 12 Liquid crystal cell 13 Back surface side reflective polarizing plate 20 Backlight unit 21 Light source 22 Light guide plate 23 Optical unit 31 Light diffusion sheet 32 First prism sheet 33 Second prism sheet 41 Material layer 42 Light diffusion layer 43 Anti-sticking layer 51 Resin beads 52 Binder

Claims (13)

A transparent substrate layer;
A light diffusing sheet comprising a light diffusing layer laminated on the surface of the base material layer and having resin beads in a binder,
A light diffusion sheet, wherein the resin beads have a refractive index of 1.50 or more. The refractive index of the binder is 1.49 or more and 1.51 or less,
The light diffusion sheet according to claim 1, wherein the resin beads have a refractive index of 1.52 or more and 1.59 or less.   The light diffusion sheet according to claim 1 or 2, wherein a value obtained by subtracting the refractive index of the binder from the refractive index of the resin beads is 0.06 or more and 0.08 or less.   The light diffusion sheet according to claim 1, wherein the main polymer of the resin beads is a copolymer of a (meth) acrylic monomer and a styrene monomer.   The light-diffusion sheet of any one of Claims 1-4 whose compounding quantity with respect to 100 mass parts of binders of the said resin bead is 40 mass parts or more and 230 mass parts or less.   The light diffusion sheet according to any one of claims 1 to 5, wherein a haze value is 78% or more and 93% or less.   The light diffusion sheet according to any one of claims 1 to 6, wherein an average particle diameter of the resin beads is 2 µm or more and 5 µm or less.   The light diffusion sheet according to any one of claims 1 to 7, wherein the resin beads are polydisperse beads. The light diffusion sheet according to any one of claims 1 to 8,
A prism sheet superimposed on the surface side of the light diffusion sheet,
An optical unit in which the prism sheet has a refractive index of 1.50 or more. The light diffusion sheet according to any one of claims 1 to 8,
Two prism sheets superimposed on the surface side of the light diffusion sheet,
These two prism sheets are arranged so that their ridges intersect,
An optical unit in which the refractive index of the two prism sheets is 1.50 or more.   The optical unit according to claim 9 or 10, wherein a refractive index of the prism sheet is 1.55 or more and 1.70 or less. A light source;
A light guide plate that receives light from the light source and emits the incident light;
A backlight unit comprising: the optical unit according to claim 9, 10, or 11, wherein the optical unit is laminated on an upper side of the light guide plate. The backlight unit according to claim 12,
A liquid crystal display device comprising: a liquid crystal display element disposed on the prism sheet of the backlight unit.

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