JP4686016B2 - Light diffusion sheet and backlight unit using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light diffusion sheet incorporated in a backlight unit of a liquid crystal display device and a backlight unit using the same.
[0002]
[Prior art]
In the liquid crystal display device, a backlight system in which a liquid crystal layer is illuminated from the back surface is widely used, and a backlight unit is provided on the lower surface side of the liquid crystal layer. As shown in FIG. 4A, the backlight unit 50 generally includes a rod-shaped lamp 51 serving as a light source, and a rectangular plate-shaped light guide plate 52 that is disposed so that an end thereof is along the lamp 51. And a plurality of optical sheets 53 stacked on the surface side of the light guide plate 52. Each of the optical sheets 53 has specific optical properties such as refraction and diffusion. Specifically, the light diffusion sheet 54 disposed on the surface side of the light guide plate 52 and the surface of the light diffusion sheet 54 For example, the prism sheet 55 disposed on the side corresponds to this.
[0003]
The function of the backlight unit 50 will be described. First, a light beam incident on the light guide plate 52 from the lamp 51 is reflected by a reflection dot or a reflection sheet (not shown) on the back surface of the light guide plate 52 and each side surface. Emitted from the surface. Light rays emitted from the light guide plate 52 enter the light diffusion sheet 54, are diffused, and are emitted from the surface of the light diffusion sheet 54. Thereafter, the light beam emitted from the light diffusion sheet 54 enters the prism sheet 55 and is emitted as a light beam having a distribution having a peak in a substantially upward direction by the prism portion 55 a formed on the surface of the prism sheet 55. In this way, the light beam emitted from the lamp 51 is diffused by the light diffusion sheet 54 and refracted by the prism sheet 55 so as to show a peak in a substantially upward direction, and further the entire liquid crystal layer not shown above is Illuminate.
[0004]
Although not shown, there is also a backlight unit in which a light diffusion sheet and a prism sheet are further arranged on the surface side of the prism sheet 55 in consideration of the light condensing characteristics of the prism sheet 55 described above.
[0005]
As the light diffusion sheet 54 of the backlight unit 50 having the above structure, a laminate of a base material layer 56 and a light diffusion layer 57 as shown in FIG. 4B is generally used. The light diffusion layer 57 has a structure in which beads 59 made of synthetic resin, glass or the like are dispersed in a binder 58 made of synthetic resin. In the light diffusing layer 57, the beads 59 are usually buried in the binder 58 and may protrude from the surface of the binder 58, but the degree of protrusion is not so large.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a light diffusing sheet having a large light diffusing ability and a small loss of light and a backlight unit using the same.
[0007]
[Means for Solving the Problems]
As a result of intensive studies on which part of the light diffusion layer in the conventional light diffusion sheet contributes to the light diffusibility, the present inventors have found that the part of the beads embedded in the binder is less light diffusing. It has been found that the portion protruding from the binder surface mainly contributes to the light diffusivity without contributing to the properties.
[0008]
The resulting invention is a light diffusing sheet comprising a transparent base layer and a light diffusing layer in which beads are dispersed in a binder, wherein the particle size of the beads is larger than the thickness of the binder. It is what. Here, the “binder thickness” means the average thickness of the binder in a portion where no beads exist.
[0009]
According to the light diffusing sheet, since the bead particle size is larger than the thickness of the binder, all the beads protrude from the surface of the binder, so that all the beads exhibit effective light diffusibility as a whole. Light diffusibility can be significantly improved. In addition, the amount of beads per unit area required to exhibit the same level of light diffusibility as conventional light diffusion sheets can be reduced, resulting in a reduction in light loss and improved economics. Can be made. Furthermore, since relatively hard beads protrude from the surface, damage to the surface of the light diffusion sheet can be reduced, and quality, handleability, and product yield can be improved.
[0010]
In the light diffusion sheet, the ratio of the difference between the bead particle size and the binder thickness (that is, the protruding distance of the beads from the binder surface) to the bead particle size (hereinafter referred to as the protruding ratio) is 5% or more, and The following formula (Equation 1)
X = 0.5 × (1-1 / n) × 100 [%] (Equation 1)
It is good to make it less than X% shown by. Here, n represents the refractive index of the synthetic resin constituting the beads.
[0011]
According to such means, by setting the protrusion ratio to 5% or more, sufficient light diffusibility can be exhibited as a light diffusion sheet, and by setting the protrusion ratio to less than X%, beads Thus, the total reflection at the interface of the protruding portion can be reduced, and the reduction of the total light transmittance due to the reflection at the interface of the beads can be suppressed.
[0012]
The protrusion ratio may be not less than X% and not more than 50% indicated by the above formula (Equation 1). According to this means, since the protrusion ratio is set to X% or more, reflection occurs at the interface of the protruding portion of the bead and the total light transmittance is reduced, but the light diffusibility can be greatly improved. Moreover, since the protrusion ratio is set to 50% or less, it is possible to prevent a decrease in strength due to bead dropping or the like.
[0013]
Further, a sticking prevention layer in which beads are dispersed in a binder can be laminated on the surface of the base material layer opposite to the surface on which the light diffusion layer is laminated. The light diffusing layer is laminated on one surface of the light diffusing sheet, and this light diffusing layer has an anti-sticking action because the beads protrude as described above. By laminating the anti-sticking layer on the other surface of the light diffusion sheet, the light diffusion sheet can exhibit an anti-sticking action on both surfaces, and can prevent sticking with a light guide plate or the like disposed in an overlapping manner.
[0014]
Therefore, in the backlight unit for a liquid crystal display device provided with a lamp and a light diffusion sheet, when the light diffusion sheet of the present invention is used, uneven brightness of the liquid crystal display device can be suppressed due to high light diffusibility, and bright lines And the effect of reducing the generation of the lamp image. Further, the luminance can be improved due to the small light loss.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. FIG. 1 is a schematic cross-sectional view showing a light diffusion sheet according to an embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view showing a light diffusion sheet according to a different form from the light diffusion sheet of FIG.
[0016]
The light diffusing sheet 1 in FIG. 1 includes a base material layer 3 and a light diffusing layer 5 provided on the surface of the base material layer 3.
[0017]
The base material layer 3 is made of, for example, a synthetic resin such as polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, and weather resistant vinyl chloride. The base material layer 3 is transparent because it is necessary to transmit light, and colorless and transparent is particularly preferable. Although the thickness of the base material layer 3 is not specifically limited, For example, it is 50 micrometers or more and 250 micrometers or less. When the thickness of the base material layer 3 is less than the above range, curling may easily occur when the resin composition forming the light diffusion layer 5 is applied. On the contrary, if the thickness of the base material layer 3 exceeds the above range, the luminance of the liquid crystal display device may decrease, and the thickness of the backlight unit becomes large, which is contrary to the demand for thinning of the liquid crystal display device. It will also be a thing.
[0018]
The light diffusion layer 5 is composed of a binder 7 and beads 9 dispersed in the binder 7. Examples of the polymer used for the binder 7 include acrylic resin, polyurethane, polyester, fluorine resin, silicone resin, polyamideimide, and epoxy resin. In the binder 7, in addition to the above-mentioned polymer, for example, a plasticizer, a stabilizer, a deterioration preventing agent, a dispersing agent, an antistatic agent and the like may be blended. Since the binder 7 needs to transmit light, it is transparent, and colorless and transparent is particularly preferable.
[0019]
The beads 9 are substantially spherical, and resin beads or glass beads are used. Examples of the material of the resin beads include acrylic resin, polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile, polyamide and the like. The beads 9 are preferably transparent in order to increase the amount of light transmitted through the light diffusing sheet 1, and particularly preferably colorless and transparent.
[0020]
In the light diffusion sheet 1, the particle diameter D of the beads 9 is larger than the thickness T of the binder 7, and the upper parts of all the beads 9 are protruded from the surface of the binder 7. As described above, since the protruding portion of the beads 9 from the surface of the binder 7 contributes to the light diffusibility, the light diffusing sheet 1 exhibits all the light diffusibility, and the light diffusibility is remarkably enhanced. it can. The term “protrusion” means that the upper part of the bead 9 protrudes from the average surface of the binder 7 in addition to the case where the upper part of the bead 9 protrudes completely from the surface of the binder 7. This is a concept including a case where the polymer constituting the binder 7 is coated.
[0021]
Next, the protrusion ratio of the beads 9 will be described with reference to FIG. The protruding distance Y of the beads 9 from the surface of the binder 7 is the difference between the particle diameter D of the beads 9 and the thickness T of the binder 7 (DT), and the protruding ratio is (Y / D) × 100 = {(D -T) / D} × 100. In order to exhibit a sufficient light diffusion action as a light diffusion sheet, the protrusion ratio needs to be 5% or more.
[0022]
On the other hand, as shown in FIG. 2, when a parallel light beam is incident from the back surface side of the light diffusing layer 5 perpendicular to the sheet surface, the light beam passing through the side of the bead 9 has an incident angle to the interface of the bead 9. If the incident angle is smaller and the incident angle is smaller than the critical angle, the light beam is reflected at the interface of the beads 9 and hardly transmitted to the surface side, which causes a decrease in transmittance. Therefore, considering the prevention of reduction of the total light transmittance, it is necessary to determine the protrusion ratio so that the incident angle does not exceed the critical angle. The minimum value of the incident angle of the light beam on the upper interface of the protruding portion of the bead 9 is an angle α formed by a radial direction passing through a point in contact with the binder 7 surface of the bead 9 surface with respect to the sheet surface. By defining the protrusion ratio so that the angle α is equal to or greater than the critical angle, total reflection at the interface of the beads 9 is prevented. When the radial direction of the contact point between the bead 9 and the binder 7 is an angle α, the protrusion ratio X is X = 0.5 × (1-sin α) × 100 [%]
When α is a critical angle, sin α = 1 / n. Therefore, the above formula is X = 0.5 × (1-1 / n) × 100 [%]
And can be transformed. Here, n is the refractive index of the synthetic resin constituting the beads 9. Therefore, by setting the protrusion ratio to less than X% calculated by the above equation, reflection on the surface of the beads 9 can be reduced, and a decrease in the total light transmittance can be prevented.
[0023]
Further, when it is desired to improve the light diffusibility even at the sacrifice of the transmittance, the protrusion ratio is preferably set to X% or more calculated by the above formula. However, if the beads 9 are protruded by more than half from the surface of the binder 7, the beads 9 may be lost. Therefore, the protruding ratio of the beads 9 needs to be 5% or less.
[0024]
If the bead 9 is an acrylic bead, the protrusion ratio X% is calculated to be about 16% when the general refractive index of acrylic is n = 1.49. Therefore, when promoting light diffusibility while maintaining the total light transmittance, the protrusion ratio of the beads 9 may be 5% or more and less than 16%. When importance is placed on light diffusivity, the protrusion ratio is 16% or more and 50%. % Or less.
[0025]
The particle size of the beads 9 is preferably 1 μm or more and 50 μm or less, and particularly preferably 2 μm or more and 20 μm or less. This is because when the particle size is less than the above range, it is difficult to adjust the protrusion ratio of the beads 9 described above, and when the particle size exceeds the above range, uniform diffusion becomes difficult.
[0026]
The particle size distribution of the beads 9 needs to be uniform so that the width thereof is small and a sharp curve is obtained. This is because the beads 9 cannot be projected from the surface of the binder 7 as described above unless the particle size of the beads 9 is determined. In addition, since it is actually impossible to make the particle diameters of the beads 9 exactly, beads 9 embedded in the binder 7 may be generated, but the expression of the embedded beads 9 is within an error range, and the present invention. include.
[0027]
The blending amount of the beads 9 in the light diffusion layer 5 is determined by the relationship between the protruding ratio of the beads 9 and the density of the beads 9 per unit area, and is 10 parts or more and 300 parts or less with respect to 100 parts of the polymer in the binder 7. It is recommended to select as appropriate within the range.
[0028]
In the light diffusing sheet 1, the back surface of the base material layer 3 is a smooth surface. However, the light diffusing performance and the anti-sticking performance may be improved by embossing the back surface.
[0029]
The light diffusion sheet 11 of FIG. 3 includes a base material layer 3, a light diffusion layer 5 provided on the front side of the base material layer 3, and a sticking prevention layer 13 provided on the back surface of the base material layer 3. The structure of the base material layer 3 and the light diffusion layer 5 is the same as that of the embodiment shown in FIG.
[0030]
The anti-sticking layer 13 is composed of a binder 15 and beads 17 dispersed in the binder 15. The material of the binder 15 and the beads 17 is the same as that used for the light diffusion layer 5. The thickness of the anti-sticking layer 13 (the thickness of the binder 15 portion excluding the beads 17) is not particularly limited, but is, for example, about 1 μm to 10 μm.
[0031]
Since the blending amount of the beads 17 is relatively small, the beads 17 are separated from each other and dispersed in the binder 15. Many of the beads 17 have their lower ends protruding from the binder 15. When the light diffusing sheet 11 is laminated with the light guide plate (see FIG. 4), the protruding lower end of the beads 17 comes into contact with the surface of the light guide plate. Therefore, the entire back surface of the light diffusion sheet 11 does not come into contact with the light guide plate. Thereby, sticking between the light diffusion sheet 11 and the light guide plate is prevented, and uneven brightness of the screen of the liquid crystal display device is suppressed.
[0032]
Therefore, when the light diffusing sheets 1 and 11 of FIG. 1 or FIG. 3 are used in the backlight unit as shown in FIG. 4, even if the light diffusing sheets 1 and 11 exhibit a predetermined light diffusibility, a loss of light is caused. Since there are few, it can improve a brightness | luminance. Further, since the light diffusion sheets 1 and 11 can remarkably improve the light diffusibility, generation of bright lines and lamp images can be prevented, and the quality is improved.
[0033]
【The invention's effect】
As described above, according to the light diffusing sheet of the present invention, light diffusibility can be remarkably improved while suppressing loss of light, and the luminance of the backlight unit can be improved and luminance unevenness can be prevented. it can. Moreover, handling property and product yield can be improved, and cost reduction can be achieved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a light diffusion sheet according to an embodiment of the present invention.
FIG. 2 is a schematic partial cross-sectional view illustrating a protruding ratio of beads in the light diffusion sheet of FIG.
FIG. 3 is a schematic cross-sectional view showing a light diffusion sheet according to a different form from the light diffusion sheet of FIG.
4A is a schematic perspective view showing a general backlight unit, and FIG. 4B is a schematic cross-sectional view showing a conventional general light diffusion sheet.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light diffusion sheet 3 Base material layer 5 Light diffusion layer 7 Binder 9 Bead 11 Light diffusion sheet 13 Sticking prevention layer 15 Binder 17 Bead

Claims (2)

A light diffusion sheet comprising a transparent base material layer and a light diffusion layer in which beads are dispersed in a binder,
The particle size of the beads is larger than the thickness of the binder, the beads are substantially in contact with the base material layer,
The ratio with respect to the particle diameter of a bead of the difference between the thickness of the grain size and the binder of the bead 5% or more and Ri X% less der represented by the following formula (Formula 1),
A light diffusion sheet, wherein a sticking prevention layer in which beads are dispersed in a binder is laminated on a surface of the base material layer opposite to a surface on which a light diffusion layer is laminated .
X = 0.5 × (1-1 / n) × 100 [%] (Equation 1)
Here, n represents the refractive index of the synthetic resin constituting the beads. The backlight unit for liquid crystal display devices provided with the lamp | ramp and the light-diffusion sheet | seat of Claim 1 .

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